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TEST GRIDS FOR RELIABILITY ANALYSIS
Analysing interruptions and developing test grids based on Mälarenergi Elnät distribution grid
JONAS ELD
JENS MELIN
School of Business, Society and Engineering Energy Engineering Degree Project 30 hp Master program in engineering, Energy System ERA403
Supervisor: Jan Sandberg Examiner: Maher Azaza Customer: Stefan Svensson, Mälarenergi Elnät AB Date: 2017-06-08 Email: jed12004@student.mdh.se jmn12008@student.mdh.se
ABSTRACT
The purpose of this degree project is to examine power outages that has occurred during the
years 2009 and 2016 on Mälarenergi Elnät distribution grid. Interruptions that has occurred
on 10 kV voltage level and on overhead lines or underground cable was examined. The
examined interruptions are based on the DARWin data from Mälarenergi Elnät. The examined
interruptions resulted in four different test grids. Test grid A are overhead line grid in rural
area, test grid B are mixed grid in rural area, test grid C are underground cable grid in semi
urban area and test grid D are underground cable grid in urban area. Test grid C and D which
have the highest customer density were then used for reliability analysis. The reliability
analysis was performed using Microsoft Excel. The main focus for the reliability analysis were
the impact of different sectioning times. Another focus for the reliability analysis was
introducing breakers at key-points in test grid C. The primarily cause of interruptions on
overhead line grids are weather related events and on underground cable grids it is digging and
fabrication or material errors. It is concluded that with quicker sectioning times, the reliability
of the grid increases linearly. The introduction of breakers at key-points in the grid results in
increased reliability.
Keywords: Sectioning time, disconnector, breaker, distribution system,
reliability, underground cable, overhead line, test grid
Nyckelord: Sektioneringstid, frånskiljare, brytare, distributionssystem,
tillförlitlighet, markkabel, friledning, typnät
PREFACE
This degree project in Master of Science energy system have been carried out at Mälarenergi
Elnät AB. We would like to thank the people at Mälarenergi Elnät AB who has helped us with
the project. Special thanks to our supervisor Stefan Svensson for providing support and
technical knowledge throughout the degree project. We would also like to thank Johanna
Rosenlind for the support throughout the project. Also special thanks to Marcus Nilsson for
his expertise regarding the operations of the grid.
We would furthermore like to thank our supervisor Jan Sandberg at Mälardalen University
who has contributed throughout this degree project.
Västerås in June 2017
Jonas Eld & Jens Melin
…………………………….… …………………………….…
SUMMARY
Power distribution with a high reliability is necessary for the modern society. The distribution
grid plays a fundamental role in order to supply reliable electricity to the consumers.
Interruptions in the delivery of electricity comes with a high societal cost. Different sectors and
customers have different costs for an interruption.
The purpose of this degree project were to examine the interruptions that have occurred on the
voltage level of 10 kV affecting the overhead lines and underground cables within the
Mälarenergi Elnät distribution grid. From the examined interruptions different test grids were
developed. Further aspects that was considered were the cause of the outage and what day they
occurred on.
DARWin data for the years 2009 to 2016 was used to conduct the degree project. The software
Trimble NIS was used to examine the grid and collect the information of the cable types,
substations, customers and the structure of the grid. Based on the data collected from DARWin
and Trimble different test grids were created with different criteria’s. The criteria’s for the
construction of the test grids were the amount of underground cable compared to the amount
of overhead line and as sub criteria the customer density was used.
The test grids with the highest customer density was then used for reliability analysis. The cost
for an interruption on these test grids was also calculated, based on the customer composition
on these grids. Since the grids were structured as loops with multiple possible back-up feeds,
different sectioning times were used to examine how the reliability indicators varied with
shorter or longer sectioning times.
One additional scenario was also considered. This scenario consisted of installing breakers at
key-points in the grid. For the base scenarios, breakers were only installed in the primary
substation.
The examined components in the distribution grid, overhead line and underground cable, have
the most number of interruptions. The cause of the interruptions are on overhead line mainly
weather related and for underground cable it is digging and fabrications or material failures
that are the most common causes. The spread on the numbers of interruptions during the week
are quite even, except for the underground cable which has a clear drop during the weekends.
The reason for the drop are that the interruptions that is caused by digging is not as frequent
during the weekends. It is concluded that the interruption cost changes linearly with the
sectioning time. Installing breakers at key-points in the grid increases the reliability.
SAMMANFATTNING
Eldistribution med en hög tillförlitlighet är en viktig aspekt för det moderna samhället.
Kostnaderna för samhället vid strömavbrott är höga och olika sektorer samt kunder har olika
kostnader vid ett eventuellt avbrott. Distributionsnätet spelar en stor roll i tillförlitligheten för
distributionen av elektricitet.
Syftet med detta examensarbete är att undersöka de avbrott som har skett på 10 kV’s
friledningar och markkablar inom Mälarenergi Elnäts distributionsnät. Sedan skapades typnät
utifrån de mest drabbade nätsektionerna. Vidare så undersöktes också orsaken till avbrotten
även vilken dag i veckan avbrotten inträffade.
DARWin data från åren 2009 till och med 2016 användes som grund för detta arbete.
Programmet TRIMBLE NIS användes för att studera strukturen på distributionsnätet och för
att sammanställa data om markkablar, friledningar, nätstationer och kunder. Typnäten som
sedan konstruerades baserades på denna data. Typnäten delades in beroende på andelen
markkabel och kundtätheten.
De typnäten med störst kundtäthet användes sedan för att genomföra tillförlighetsanalyser.
Kundkostnaderna för ett eventuellt avbrott beräknades också för dessa typnät. Typnäten var
uppbyggda som slingade nät med flera möjliga omkopplingar. Detta gjorde att
sektioneringstiden för dessa omkopplingar användes som variabel för tillförlighetsanalysen.
Ytterligare ett beräkningsscenario genomfördes. Detta scenario byggde på att installera extra
brytare i vissa viktiga punkter i nätet. Som standard var endast mottagningsstationerna
installerade med brytare.
De studerade komponenterna, friledning och markkabel, är de komponenterna i
distributionsnätet som har flest avbrott. De vanligaste orsakerna för dessa avbrott är
väderrelaterade på friledningar och avgrävning samt fabrikationsfel på markkablar. Avbrotten
är jämt fördelade över veckan, förutom för markkablar där antalet avbrott minskar tydligt på
helger. Anledningen till denna minskning på helger är att antalet grävningar och då också
antalet avgrävningar minskar. Avbrottskostnaderna ändrades linjärt med sektioneringstiden.
Att installera brytare in i viktiga punkter i nätet höjde tillförlitligheten.
TABLE OF CONTENT
1 INTRODUCTION .............................................................................................................1
1.1 Background ............................................................................................................. 1
1.1.1 Mälarenergi Elnät AB ....................................................................................... 2
1.1.2 Previous research ............................................................................................ 2
1.2 Problem formulation................................................................................................ 3
1.3 Purpose of degree project ...................................................................................... 3
1.4 Research questions ................................................................................................ 4
1.5 Delimitation .............................................................................................................. 4
2 METHOD .........................................................................................................................5
2.1 The literature review ................................................................................................ 5
2.2 The empirical research ............................................................................................ 5
3 THE SWEDISH ELECTRICITY GRID ..............................................................................6
3.1 Grid structure .......................................................................................................... 6
3.2 The Swedish electricity market reform 1996 ......................................................... 8
3.2.1 Reliability of delivery ........................................................................................ 9
3.2.2 The impact of Gudrun ...................................................................................... 9
3.2.3 DARWin ..........................................................................................................10
3.2.4 Interruption cost ..............................................................................................12
3.3 Overhead line and underground cable..................................................................13
3.3.1 Overhead line .................................................................................................13
3.3.2 Underground cable .........................................................................................15
3.4 Control system .......................................................................................................17
4 DISTRIBUTION SYSTEM RELIABILITY ....................................................................... 19
4.1 DARWin-file .............................................................................................................19
4.1.1 Overhead line .................................................................................................19
4.1.2 Underground cable .........................................................................................19
4.1.3 Evaluation of DARWin-file ...............................................................................20
4.2 Modelling of test grids ...........................................................................................20
4.2.1 Data collection ................................................................................................20
4.2.2 Classification of the test grids ..........................................................................20
4.2.3 Description of the structure of the test grids ....................................................21
4.2.4 Specifications of the different test grids ...........................................................22
4.3 Analytical reliability analysis .................................................................................26
4.3.1 Failure rate ......................................................................................................26
4.3.2 Unavailability ...................................................................................................27
4.3.3 Reliability indicators ........................................................................................27
4.3.4 Base scenario .................................................................................................28
4.3.5 Different sectioning times ................................................................................28
4.3.6 Design suggestion for Test grid C ...................................................................28
4.3.7 Calculations ....................................................................................................29
5 RESULTS ...................................................................................................................... 31
5.1 Histograms of the interruptions ............................................................................31
5.2 Test grid A ...............................................................................................................33
5.3 Test grid B ...............................................................................................................34
5.4 Test grid C ...............................................................................................................35
5.5 Test grid D ...............................................................................................................37
5.6 Design suggestion for test grid C .........................................................................39
5.7 Sensitivity analysis ................................................................................................40
6 DISCUSSION................................................................................................................. 43
6.1 Empirical research .................................................................................................43
6.2 Result ......................................................................................................................43
7 CONCLUSIONS ............................................................................................................ 45
8 FUTURE WORK ............................................................................................................ 45
REFERENCES ..................................................................................................................... 46
APPENDIX 1 – CABLE DESIGNATION STANDARDS
APPENDIX 2 – TEST GRID A
APPENDIX 3 – TEST GRID B
APPENDIX 4 – TEST GRID C
APPENDIX 5 – TEST GRID D
APPENDIX 6 – DESIGN SUGGESTION TEST GRID C
APPENDIX 7 – DESCRIPTIONS OF CALCULATIONS
LIST OF FIGURES
Figure 1: Double cable structure. ............................................................................................... 7
Figure 2: Loop structure. ........................................................................................................... 7
Figure 3: Radial structure. ......................................................................................................... 8
Figure 4: Free overhead line (isolated and uninsulated) in use at the voltage level of 10 kV...14
Figure 5: Line street overhead line. .......................................................................................... 15
Figure 6: Underground cable types in use at the voltage level of 10kV in the MEE grid. ........16
Figure 7: Schematic over Test grid A. ...................................................................................... 23
Figure 8: Schematic over Test grid B. ...................................................................................... 23
Figure 9: Schematic over Test grid C. ...................................................................................... 24
Figure 10: Schematic over Test grid D. .................................................................................... 24
Figure 11: Schematic over the design suggestion for Test grid C. ............................................ 29
Figure 12: Interruptions on components at the voltage level of 10 kV during the years 2009-
2016. .......................................................................................................................................... 31
Figure 13: Interruption causes on overhead line and underground cable 10 kV 2009-2016. . 32
Figure 14: Number of interruptions per day at the voltage level of 10 kV and on overhead line
and underground cable during the years 2009-2016. ............................................................. 32
Figure 15: Interruptions on overhead line sorted by day. ........................................................ 33
Figure 16: Interruptions on underground cable sorted by day. ............................................... 33
Figure 17: Interruptions by category on Test grid A. ............................................................... 34
Figure 18: Interruptions by cause on Test grid A. .................................................................... 34
Figure 19: Interruptions by category on Test grid B. ............................................................... 35
Figure 20: Interruptions by cause on Test grid B. ................................................................... 35
Figure 21: Interruption by category on Test grid C. ................................................................. 36
Figure 22: Interruptions by cause on Test grid C. ................................................................... 36
Figure 23: Interruption costs Test grid C. ................................................................................ 37
Figure 24: Interruptions by category on Test grid D. .............................................................. 38
Figure 25: Interruptions by cause on Test grid D. ................................................................... 38
Figure 26: Interruption costs Test grid D. ............................................................................... 39
Figure 27: SAIFI & SAIDI for the design suggestion for Test grid C, fault occurring in the east
feed. .......................................................................................................................................... 39
Figure 28: SAIFI & SAIDI for the design suggestion for Test grid C, fault occurring in the west
feed. .......................................................................................................................................... 40
Figure 29: Interruption cost for the design suggestion for Test grid C. .................................. 40
Figure 30: SAIDI of the MEE grid, based on all the examined interruptions. .........................41
Figure 31: SAIFI for MEE grid, based on all the examined interruptions. ...............................41
LIST OF TABLES
Table 1: The affected component causing the interruption. ..................................................... 11
Table 2: The cause of the interruption. .....................................................................................12
Table 3: SNI-codes and interruption costs. .............................................................................. 13
Table 4: Advantages and disadvantages with overhead line. .................................................... 15
Table 5: Advantages and disadvantages with underground cables. ......................................... 17
Table 6: K-factors and T-factors. ..............................................................................................21
Table 7: Technical data for the different test grids. ................................................................. 22
Table 8: Line and Cable composition of the different test grids. ............................................. 22
Table 9: Number of customers of each category on each test grid. ......................................... 25
Table 10: Annual electricity consumption per customer category and test grid. .................... 25
Table 11: Length & number of interruptions on underground cables & overhead lines. ......... 26
Table 12: Failure rates for overhead lines and underground cables. ....................................... 26
Table 13: Sectioning times for the different scenarios. ............................................................ 28
Table 14: Example of calculations. ........................................................................................... 29
Table 15: Reliability indicators Test grid C. ............................................................................. 36
Table 16: Reliability indicators Test grid D. ............................................................................. 38
Table 17: Test grid C sensitivity analysis. ..................................................................................41
Table 18: Test grid D sensitivity analysis. ................................................................................ 42
ABBREVIATIONS AND NOTATIONS
Abbreviation/Notation Unit Description
ASAI Interruptions/Customer, Year
Average Service Availability Index
ASUI Hours/Customer, Year Average Service Unavailability Index
CAIDI Hours/Interruption Customer Average Interruption Duration Index
DSO - Distribution System Operator
Ei - Energy Market Inspectorate
ENS kWh/Year Energy Not Supplied
Abbreviation/Notation Unit Description
MEE - Mälarenergi Elnät
r h Repair time
SAIDI - System Average Interruption Duration Index
SAIFI h System Average Interruption Frequency Index
U h Unavailability
λ Failures/km, year Failure rate
1
1 INTRODUCTION
Power distribution with a high availability is necessary for the modern society and the future
development of the modern world. (Wallnerström & Grahn, 2016)
In order to supply reliable electricity to the consumer, the power distribution grid plays a
fundamental role (Cadini, et al., 2017). The baseline of this degree project is to examine the
current most affected overhead line and underground cable grids, at 10 kV, and by different
criteria construct them into several test grids. Then, the test grids could be used for analytical
reliability analysis.
1.1 Background
Power outages generates a high societal cost. In the case of an outage, all the different parts of
the society is affected. Sectors and customers are affected differently; some industries and
services are affected independently of the length of the outage, and some are more affected by
longer outages. Hospitals and other crucial public services often have backup generators, but
the vulnerability of operation is significantly increased following power outages.
(Wallnerström & Grahn, 2016)
The electricity act (SFS 1997:857) specifies that the transfer of electricity should be of good
quality (3rd Chapter, 9§)
The Energy market inspectorate (Ei) in Sweden also specifies the requirements for power
distribution quality. Those requirements include for instance, the maximum number of power
outages per year, and voltage quality, among others. The Distribution System Operator (DSO)
is responsible for the distribution grid in their area. (Morén, 2013)
Each year, the DSOs is responsible for submitting information about the power outages in their
grid to Ei, through a system called DARWin. DARWin is a statistical database that makes it
possible for the DSOs and Ei to steer investments and maintenance to those parts in the grid
where the investments are needed mostly. (Wallnerström & Grahn, 2016)
The distribution grid is a local monopoly and therefore there is a need for some control on how
much the customer will have to pay to the DSO to be connected to the grid. In the year 2009
the government in Sweden decided to change the electricity act (SFS 1997:857) so that the grid
tariffs’ fairness should be examined beforehand in a so called ex-ante regulation. This
regulation means that Ei in advance, decides the allowed revenue for each DSO depending on
the reliability of the grid. (Ström, 2015)
2
The reason for this regulation is to counteract the potential risk of decreasing quality due to
the monopoly situation for the DSOs. There is a risk that the reliability is decreasing in the grid
in order to maximize the profit for the DSOs. (Ström, 2015)
To steer towards increased grid reliability cost effectively it is helpful to construct test grids,
which can be a normalization of the actual grid for the current DSO. Using these test grids, it
is possible to calculate the effects of different improvement efforts before implementing the
new changes (Engblom & Ueda, 2007). One way to measure and compare the effects of power
outages is to calculate the SAIFI and SAIDI for each outage. SAIFI is the number of customers
affected and SAIDI is how long each customer is affected by the power outage.
The more precise the test grids can be, the more helpful they are for the DSO. In this degree
project, different test grids are constructed based on the current grid at MEE. The DARWin
report from the DSO Mälarenergi Elnät AB is used to search for certain areas in the grid where
power outages are more or less clustered and moreover if there are any distinct similarities or
differences between these areas.
1.1.1 Mälarenergi Elnät AB
Mälarenergi Elnät AB is the DSO for the grid in the municipalities of Arboga, Hallstahammar,
Kungsör, Köping and Västerås. The grid stretches over the western part of Mälardalen. MEE
distributes electricity to over 100 000 customers through the local distribution grid. As the
DSO for this area, MEE is responsible for operation and maintenance of the distribution grid.
According to the regulations stated by Ei, MEE is accountable for the quality of distribution.
(Mälarenergi AB, 2015)
1.1.2 Previous research
Previous work has been done when constructing test grids that is representative for Sweden as
a whole. The purpose of that project was to construct different test grids for the countryside
and urban areas. These grids were used for reliability and interruption cost calculations.
(Engblom & Ueda, 2007)
Other studies have created reliability test systems for learning purposes, based on statistics
from other studies and then combined into a test system. The test system contains all the
components found in a real system but it is small enough for students and others to perform
calculations by hand. (Allan, et al., 1991)
The test system constructed by (Ueda, et al., 2009) describes the Swedish distribution grid as
two different type grids, one rural and one urban. This is a simplification of Swedish grid.
All these test systems have been created to fit many different grids due to an underlying
purpose of generalisation. In this degree project test grids are constructed with the specific grid
at MEE as a base system, due to a purpose of specificity.
3
1.2 Problem formulation
Overhead lines are often replaced with underground cables to get rid of weather-related power
outages. Decreasing outages has a direct correlation to increased grid reliability. There are
disadvantages with underground cables, compared to overhead lines. Underground cables are
2 – 10 times more expensive than overhead line and it is harder to localize faults along the
length of the cable (Svensson, 2012).
Replacing overhead line with underground cable is a measure to decrease the number of
outages. For the duration of an outage, the sectioning time is of importance. The sectioning
time is the time it takes to isolate the fault and reconnect the customers. Longer sectioning
time leads to an increased cost. The sectioning is mainly done manually by fitters on sight. The
sectioning time often depends on the experience of the operators and fitters.
There will be both overhead lines and underground cables in the future electricity grid, hence
creating test grids comprising both of them is of significant importance.
The most important aspect of using test grids, involves and understanding on how to tailor the
model to suit the particular needs of the MEE grid. Previously-developed test grids could be
considered too general and the underlying assumptions could be too restrictive, prohibiting a
valuable analysis. In reality, the grid is a complex entity and looks different depending on
aspects such as: amount of underground cables, overhead lines, rural and urban areas and the
geographical location. All of this points to the fact that a tailored test grid, representable for
the MEE grid, should be considered superior when it comes down to examining and analysing
the grid reliability.
1.3 Purpose of degree project
The purpose of this degree project is to examine the power outages that has occurred on
overhead lines and underground cables at the voltage level of 10 kV within the MEE grid.
Further aspects that will be considered are: the cause of the power outages and the day of the
outages. This degree project will be performed by using test grids constructed by relevant
criteria with a relevant connection to the MEE grid. Then, the test grids are used for an
investigation of the different types of outages and their corresponding consequence. The
outcome of the analysis will be the interruption costs that occurs in the event of an
interruption.
4
1.4 Research questions
The research questions in this degree project are stated in the following bulleted list.
What impact have the day of the week on the number of interruptions?
What causes the interruptions that occurs in the different grid sections?
How does the sectioning time affect the interruption cost in semi urban and urban grids?
How does more breakers in the grid influence the reliability?
1.5 Delimitation
The examined power outages in this degree project are the ones that occurs on underground
cables and overhead lines. These two components are seen as the major criteria’s when creating
test grids. The examined interruptions are spread throughout the MEE concession area. The
voltage level examined is 10 kV. Planned outages are not considered in this degree project. The
outage data that is examined originated from events occurring during the years 2009-2016,
but the grid could, in some, locations be altered with new equipment and/or changes from
overhead lines to underground cable could have occurred, for example. This factor has not
been taken into consideration, and the grid “as is” right now is assumed to be the same as 2009.
For the reliability and cost calculations, only one interruption at a time will be considered. The
reliability and cost calculations will be performed on the test grids with the highest customer
density.
5
2 METHOD
The method to achieve the objectives in this degree project is divided in to two parts: literature
reviewing and empirical researching.
2.1 The literature review
A literature study was carried out by reading articles journals, books and degree projects within
the subject. Electrical power distribution systems were examined. Theories about reliability
was also examined. The Swedish electricity markets rules and regulations were examined. The
article journals have been found on different databases through the university library at
Mälardalen högskola. The construction of overhead lines and underground cables have been
examined and is described in chapter 3. Construction of test grids has been examined from
different articles and discussed with the supervisor at MEE.
2.2 The empirical research
The empirical research includes collecting information through discussions with employees at
MEE and gathering data from historical records on power outages. The research is mainly on
gathering data from historical records on power outages inside MEE concession area. Data
comes from the database DARWin (managed by Energiföretagen Sverige). The empirical
research include collecting information from discussions. The discussions have been
performed with employees at MEE with extensive experience and technical knowledge on the
electricity grid, within their own concession area.
In order to examine the structure of the grid along with geography, the software Trimble NIS
is used. Using this software, the grid structure and different parts of the system could be
examined. The geography of the area surrounding the grid could also be analysed in this
software.
The ten most affected grid sections were examined and classified into different grid types
(overhead line, underground cable or mixed) and customer density. For each grid section,
certain data was collected.
The data collected during the empirical research were used for modelling the test grids and for
reliability calculations. The analytical reliability analysis was conducted using Microsoft Excel.
6
3 THE SWEDISH ELECTRICITY GRID
The Swedish electricity grid is divided into three different levels depending on the voltage, the
national grid, regional grid and the local grid (also called the distribution grid). The national
grid is the backbone of the electricity grid in Sweden (Pettersson, 2017). It transfers the
electricity from the large hydro power plants in the northern part of Sweden, to the south where
most of the consumers are accommodated. The nuclear power plants are also connected to the
national grid (Wallnerström & Grahn, 2016). The voltage in the national grid is 220kV or
400kV, the reason for the high voltage is to minimize the losses when transferring the power
over long distances (Konsumenternas Energimarknadsbyrå, 2017).
The national grid is connected to the regional grid, where the voltage is between 40kV to 130kV
(Konsumenternas Energimarknadsbyrå, 2017). Wind power plants and other smaller power
plants are connected to the regional grid. The regional grid transfers the power from the
national grid and the power plants directly connected to the regional grid to the local grid and
also to the customers directly connected to the regional grid (Konsumenternas
Energimarknadsbyrå, 2017; Wallnerström & Grahn, 2016).
The last level is the local grid (or the distribution grid), this is where most of the customers are
connected. The distribution grid is divided into two different voltage levels, 10kV and 0,4kV.
Industries and other high consuming customers are connected to the 10kV grid. The
distribution grid is constructed in different ways depending on the density of the customers
and the need for redundancy (Söder & Amelin, 2011).
3.1 Grid structure
The structure of the distribution grid is mainly dependent on economic aspects. Constructing
redundant distribution grids is not economically viable in all places. In Sweden, the
distribution grid is mainly structured in one of the three following ways (Engblom & Ueda,
2007).
Double cable structure
Loop structure
Radial structure
Normally, these three structures are used and combined into a single grid entity. Double cable
structure is used in the city center where a high reliability level is required and economically
viable. In this structure, each substation is connected to two parallel cables. This structure has
a fast reconnection in case of a power outage, the reconnection from the faulty component is
done automatically and therefore the average outage time is less than 1 minute (Engblom &
Ueda, 2007). An illustration of a double cable structure is presented in Figure 1 below.
7
Figure 1: Double cable structure.
The looped structure is the most common way of structuring a distribution grid. The
characteristic of a looped structure is that each substation has the possibility to be feed from a
different primary substation (Engblom & Ueda, 2007). The looped grid is almost always run
as a radial grid, which means that the loop is split into two radial sections by a disconnector,
fed from two different slots from a primary substation. Sometimes the grid is fed from two
different primary substations (Lexholm, 2016).
The breakers in the primary substation will, in case of a fault, disconnect all the substations
fed from that slot. Before the customers can be reconnected, by closing the disconnector that
splits the loop, the fault location needs to be found and this is often done by personnel on site
(Lexholm, 2016). Also, the reconnection is done manually, this leads to an average down time
equal to 0,5 to 2,5 hours (Engblom & Ueda, 2007). In Figure 2 below, an illustration of a looped
structure is presented.
Figure 2: Loop structure.
The radial structure is most commonly used at the countryside, where the consequence of a
power outage is limited as compared to more densely populated areas. In this structure, there
is no redundancy in the grid. In the case of a power outage, each substation after the fault
location will be out of power until the fault is fixed or until a mobile backup power is connected.
8
The average down time for a radial grid is approximately between 1 and 12 hours (Engblom &
Ueda, 2007).
In a radial containing overhead lines, there is often an installed delayed reconnection on the
breaker in the primary substation slot. In case of a transient fault, the customers will have their
power back after a short period without any work done by personnel (Lexholm, 2016).
Transient errors on overhead lines is often caused by tree branches touching the lines or
lightning strikes (Torstensson & Bollen, 2012). An illustration of a radial structure is presented
in Figure 3 below.
Figure 3: Radial structure.
3.2 The Swedish electricity market reform 1996
The electricity market was at the beginning a monopoly business, the DSOs was given an
obligation to deliver electricity to its customers inside their concession area. Then, the DSOs
had a sort of monopoly position of selling electricity to the connected customers (Banefelt &
Larsson, 2006).
To make the electricity market more rational and effective, a new electricity market reform was
made in 1996 (Banefelt & Larsson, 2006). In this reform, it is stated that production or selling
of electricity cannot be conducted by a legal person that is also the concession owner (SFS
1997:857). The reform implies that the DSOs only have an obligation to distribute the
electricity and not deliver to its connected customers. This allows the customer to choose where
to buy their electricity. The purpose of the reform was to make the electricity market more
competitive and to get rid of the monopoly business. Distribution of electricity is still
conducted on a monopoly market, but the monitoring and control is done by a new authority
(Banefelt & Larsson, 2006).
The supervision of the grid operations is done by the so called “Network authority”. The tasks
of the network authority is in the hands of the Energy Market Inspectorate (Ei). There are two
parts that the Ei is not in charge of, the safety of the grid - which is in the hands of the
9
“Elsäkerhetsverket”, and the competitive part (production and trade) – which is in the hands
of the Swedish Competition Authority (Banefelt & Larsson, 2006).
3.2.1 Reliability of delivery
The electricity act (SFS 1997:857) specifies that the transfer of electricity should be of good
quality (3rd Chapter, 9§). Quality, in this case, involves delivery reliability and voltage quality.
The reliability of the delivery means the probability that the power will be distributed to the
customer. The voltage quality of the delivery implies that the power can be distributed to the
customer without disruption in the voltage (excluding power outages).
The distribution to low-voltage consumers, such as household, the delivery is of good quality
if there are no more than three unannounced outages that exceeds three minutes per year. If
the number of unannounced outages exceeds eleven per year, the quality is not of good quality
(Morén, 2013).
The general voltage quality regulations are specified in the EIFS 2013:1 7th chapter (Morén,
2013).
The distribution companies have incentives for maintaining a good quality of the distribution
provided by the Ei. These incentives were introduced in order to motivate the distribution
companies to, on their own, increase the reliability of delivery and to make investments and
conduct maintenance on the electricity grid, according to best practice. The delivery reliability
affects the income frame for the company, increasing the reliability of the grid can lead to a
bigger income frame for the company and vice versa (Wallnerström & Grahn, 2016).
3.2.2 The impact of Gudrun
In 2005, the southern part of Sweden was hit by the storm Gudrun. The storm gave rise to a
numerous amount of outages that, not only affected the electricity grid, but also the railway
network, telecommunication network and the mobile communication network (Banefelt &
Larsson, 2006). For the electricity grid, the local grid was affected the most but the regional
grid was also affected (Heden & Johansson, 2005). The impact of the storm was enormous, it
left people power less for days, and for some, even weeks (Energimyndigheten, 2015). The
direct costs for the DSOs, as a result of the storm, were also extremely high, the highest cost
affected the DSO Sydkraft that had an estimated cost of 1 690 000 000 SEK (Heden &
Johansson, 2005).
Gudrun was a tipping point when it comes to reliable electricity distribution, after the storm
the Swedish government gave a mission to the Ei. The mission was to leave suggestions on how
the reliability for the power distribution can be obtained, and regulations that keeps the grid
companies to follow these suggestions (Banefelt & Larsson, 2006). The Ei came up with
suggestions that could be added into the already consisted electricity act. Two of the
suggestions that was added to the electricity act were (Engblom & Ueda, 2007):
10
The duration of an outage should not exceed 24 hours.
If the duration of an outage lasts between 12 and 24 hours, the DSO will pay a fee of 12,5
% of the costumers calculated yearly grid fee.
The compensation fee, mentioned above, for outages longer than 12 hours is a percentage of
the yearly cost for the grid for the customer but no less than 900kr and it increases for every
24-hour period to a maximum of 300 percent of the annual cost for the customer (Engblom &
Ueda, 2007).
It further states that the DSOs should every year conduct a risk and vulnerability analysis of its
electrical distribution, as another suggestion of the Ei (Banefelt & Larsson, 2006). This will
allow the DSOs to have an overview of the electrical distribution system and to see where
investment could be allocated to achieve an increased reliability level. The Ei also announced
that it will impose on the DSOs to report any outages that has a duration of 3 minutes or higher.
3.2.3 DARWin
During the years 1988 – 1990 a system for interruption and outage reporting was developed
that was called DAR. DAR was used by approximately 40 DSOs, DAR was an upgrade from the
previous system FAR-81. The new electricity reform in 1996 lead to higher demand for
interruption and outage reporting (Jansson, 2000). This followed that the DAR system was
upgraded to DARWin.
DARWin has 156 DSOs represented, which equals over 90 % of Sweden’s electricity customers
(Tapper, 2016). DARWin is the database that the DSOs reports their yearly interruptions (>3
minutes). Energiföretagen Sverige is responsible for the database (Energiforsk, 2016). The
DSOs that submits their data to DARWin reports the following data (Tapper, 2016):
• Interruption (announced or unannounced)
• Date of interruption
• Time of interruption
• Number of affected customers
• Breaking unit
• Voltage level of interruption
• Affected component of the interruption
• Cause of interruption
This degree project focuses on the affected component and the cause of the interruption.
The affected component of the interruption is divided into categories. In each of the different
categories, there are subcategories where the affected component is further specified. This
keeps the DSOs aware of the number of interruptions categorized by component in the
electrical grid. The categories and the sub categories are presented in Table 1 (Jansson, 2000):
11
Table 1: The affected component causing the interruption.
Stations Regional station
Distribution station
Switching station
Rectifier station
Substations Pole station
Concrete station (Indoor operated)
Concrete station (Outdoor operated)
Sheet metal station
Satellite station
Transformer
Housed station
Other station type
Overhead line Free overhead line (Uninsulated)
Free overhead line (Isolated)
Hang line
Hang spiral line
Other type of line
Fuse box
Underground cable Cable in ground
Cable in water
Cable cabinet
Fuse box
The affected component examined in this degree project are free overhead line (uninsulated
and isolated), hereinafter referred to as overhead line, and cable in ground, hereinafter referred
to as underground cable.
The categories are associated with the cause of the interruption. In the categories, the sub
categories specify the cause furthermore. The different categories are a list from weather-based
to, unknown. The different categories and subcategories are presented in Table 2 below
(Jansson, 2000):
12
Table 2: The cause of the interruption.
Weather Falling tree, snow
Falling tree, wind
Rain
Salt
Snow
Thunder
Weather
Wind
Damage Animals
Damage
Digging
Sabotage
Traffic
Wood cutting
Material/method Manufacturing or material failure
Inaccurate method
Insufficient maintenance
Material/method
Sizing failure
Personal Inaccurate montage
Incorrect operation
Personal
Testing
Other Fuse failure
Other
Overload
Returning load
Unknown
3.2.4 Interruption cost
When calculating the cost of a power outage it is common to calculate it from the approximated
costs for the affected customer. Different customers have different costs for an outage. The
figures presented in Table 3 comes from an update of a previous survey conducted by the
University of Gothenburg in 2005, SINTEF was responsible for this update upon the request
of Ei (Vefsnmo & Kjolle, 2015). The costs are adjusted to the consumer price index of 2015
(Wallnerström & Grahn, 2016). The cost “SEK/kW” is for every interruption independent of
the duration, while “SEK/kWh” depends on the duration of the interruption.
13
Table 3: SNI-codes and interruption costs.
Unannounced interruptions SNI code
Customer category SEK/kW SEK/kWh Range 1 Range 2
Agriculture 8 44 01110 - 03220 Industry 23 71 05100 - 43999 Businesses & services 62 148 45110 - 82990 94111 - 96090
Public sector 5 39 84111 - 93290 Household 1 2 97000 - 98200 111111
To calculate the cost for an interruption the following equation is used. Were C is the customer
category, P is the total power for that customer category and t is the duration of the
interruption.
𝐶𝑜𝑠𝑡 = ∑ 𝐶𝑖 ∗ 𝑃𝑖𝑆𝐸𝐾/𝑘𝑊 + ∑ 𝐶𝑖 ∗ 𝑃𝑖 ∗ 𝑡𝑆𝐸𝐾/𝑘𝑊ℎ Equation 3.1
The different customer categories are divided according to the standard called “Svensk
Näringsgrensindelning” (SNI). The SNI-code for household is developed by Ei (Ström, 2015).
3.3 Overhead line and underground cable
For the transfer of electricity in the distribution grid, the two most common techniques are
overhead lines and underground cables. This section covers the differences between overhead
lines and underground cables. Furthermore, it covers the techniques for constructing both
overhead lines and underground cable, and pros and cons with overhead line and underground
cable.
3.3.1 Overhead line
Poles support overhead lines. The design of each pole differ and it is dependent on the voltage
level. The most common poles are the one made out of impregnated wood. At the highest
voltage levels, the poles are made from galvanized steel (Hildenwall & Westberg, 1999).
General the overhead lines on the voltage level 10 kV are the types: Free overhead-line (isolated
or uninsulated), hang line and hang spiral line. The most common on the 10 kV voltage level
are the free overhead line (isolated or uninsulated). A free overhead line is constructed by
metal-lines hanging free from each other (Svenska Kraftnät, 2014). The metal lines are made
from aluminium alloy or copper, were the aluminium alloy is the more common type. The line
is either isolated or uninsulated. Please regard the following examples of overhead lines:
• FeAl 62/0 12 kV
o Uninsulated overhead line with an iron-aluminium alloy, the cross sectional area is 62
mm2.
• BLX 99/0 12 kV
o Plastic isolated line with aluminium conductor, the cross sectional area is 99 mm2.
14
The isolated overhead line is less sensitive, than the uninsulated. For example, any contact on
the uninsulated line could cause an outage (Svenska Kraftnät, 2014). The different types of
overhead lines within MEE concession areas are presented in Figure 4.
Figure 4: Free overhead line (isolated and uninsulated) in use at the voltage level of 10 kV.
The spacing area needed for the overhead line depends on the voltage level. The higher the
voltage level, the more area is needed. The area surrounding the overhead line is called a forest
street. At the end of the forest street the side area starts. The forest street plus the side area is
called a line street. The forest street should be constructed so the tree closest to the line cannot
fall on it hence it is dependent on the expected height of the trees. The forest street should be
kept maintained during the overhead lines lifetime (Svenska Kraftnät, 2014). An illustration of
the spacing area is presented in Figure 5 below (Svensson, 2012):
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
AlMgSi BLX CU FeAl
Overhead line (isolated & uninsulated)
15
Figure 5: Line street overhead line.
Overhead lines affects the surroundings visually and the agriculture and forestry during its
lifetime. The distance between the poles is very flexible. This makes the overhead lines less
sensitive to the type of construction area (Svensson, 2012).
The advantages and disadvantages with overhead line are presented in Table 4 (Svensson,
2012).
Table 4: Advantages and disadvantages with overhead line.
Advantages Robust and well-tested technique that is rarely damaged if the surrounding is tree safe.
Easy to identify damage
Less expensive technique compared to underground cable
Disadvantages Big effect on the landscape
Affects the agriculture and forestry more than underground cable
3.3.2 Underground cable
Underground cables are most common on the lower voltage levels, but they do occur on the
higher voltage levels as well. Underground cables are either one phase cable or a three-phase
cable. The underground cable consists of a conductor in the middle, and an isolation material
surrounding it. Underground cables consists of different types of conductor and isolation
materials, to keep the different sorts of cables apart there is a designation system (Svenska
Kraftnät, 2014).
The different cables are defined by different designations that consists of letters (Svenska
Kraftnät, 2014). Power Cables, control cables and installation cables are defined by Swedish
standard by two different systems (Svenska Elektriska Komissionen, 2003):
• SS 424 17 01, Edition 5
16
o This consists of a designation system that has been long used in Sweden and is applied
on national cable types. It applies also on CENELEC harmonized cable types that is not
covered in SS 424 07 02.
• SS 424 17 01, Edition 3
o This which fully reflects the designation system of CENELEC HD 361 S3 (System for
cable designation). It applies on CENELEC harmonized cable types with a rated voltage
level up to 450/750 V.
For a complete cable designation on a cable, the number of conductors, conductor area and in
some cases the area of the concentric conductor should be defined. The area of the concentric
conductor is separated from the conductor by a slash (/). Below, an example of the cable
designation be demonstrated (Svenska Elektriska Komissionen, 2003).
• AKKJ 3x150/41
o The cable have three isolated aluminium phase-conductor and one concentric
conductor with the area 41 mm. The isolation and the concentric conductor consists of
PVC.
The cable design is defined by different letters that range from A-Z, each letter has different
meanings, the letters also have different meaning depending on order (Svenska Elektriska
Komissionen, 2003). See Appendix 1 for the cable standard. The different type of underground
cable on MEE concession area are presented in Figure 6.
Figure 6: Underground cable types in use at the voltage level of 10kV in the MEE grid.
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
Underground cable
17
The advantages and disadvantages with underground cable are presented in Table 5 (Svensson,
2012):
Table 5: Advantages and disadvantages with underground cables.
Advantages Less exposed to weather
Less impact on the landscape
Disadvantages Expensive technique compared to overhead line
Harder to localize faults
3.4 Control system
The demand for reliable distribution have increased continuously since the start of the
electrical era. In order to increase the reliability of the grid, new technologies were invented
and implemented to the grid. This technology goes under the name SCADA, “Supervisory
Control and Data Acquisition”. SCADA enables remote monitoring of the grid and the different
components, such as substations. SCADA can also be used for remote operations of breakers
and disconnectors (Nordell, 2008).
When a fault occurs on the 10 kV voltage level the breaker that detects the fault will break. The
installed SCADA system will register this event and alert the grid operator. Fault occurring on
lower voltage levels will not be detected by the SCADA system and therefore the DSO is
dependent on the customer to call in the interruption.
The length of an interruption depends mainly on three events, the sectioning time, the time it
takes to find the exact location of the fault, and the repair time (Wallnerström, et al., 2012).
In a looped structured grid, when a fault occurs on an underground cable or overhead line, the
duration of the outage is only dependent on the sectioning time.
Sectioning shortens the interruption time for some of the affected customers. Isolating the fault
in smallest possible area, will decrease the interruption time for many customers but some
might still be affected. There are three main ways of sectioning, breakers, remote-controlled
disconnectors and manually operated disconnectors.
Remote breakers can be used to automatically disconnect a part of the grid if an interruption
occur. Customers closer to the primary substation will not be affected by the interruptions.
However, the fault tolerance on the remote breaker must be correct, it must be lower than the
tolerance on the primary substation breaker, but too low and it might break even though there
is no fault (Wallnerström, et al., 2012).
Remote-controlled disconnectors are used to isolate the fault. In case of an interruption the
operator opens all the disconnectors. The disconnectors are then closed one at a time until the
power goes out again. When the faulted segment is found the operator connects the grid
through a secondary feed. Then the repair crew is sent to the fault location to repair the fault
(Wallnerström, et al., 2012).
18
Manual disconnectors are operated in a similar way as remote-controlled disconnectors
described in the section above. However, since these disconnectors are manually operated a
fitter needs to be sent out directly to the affected grid section. Once on sight the fitter can start
to open and close disconnectors until the fault is located. Skilled and experienced fitters will
find the fault location quicker (Wallnerström, et al., 2012).
19
4 DISTRIBUTION SYSTEM RELIABILITY
4.1 DARWin-file
The original DARWin-file consisted of over 5000 interruptions, both announced and
unannounced. Excluding the interruptions stated by the constraints in the limitation, the
number of interruptions to examine was over 1000.
4.1.1 Overhead line
Approximately 750 interruptions occurred on both uninsulated and isolated overhead line.
Except for the information given in the DARWin-file was the following data collected using the
software Trimble NIS.
• Total length of overhead line and underground cable
• Start location of interruption (substation or disconnector)
• Overhead line
o Type
o Length
• Underground cable
o Type
o Length
• End location of interruption (substation or disconnector)
Grids containing overhead line often includes underground cable as well.
4.1.2 Underground cable
The remaining 300 outages occurred on underground cables. The description of the fault never
specified exactly which part of the cable that was the cause of the interruption. A cable section
between two substations usually consisted of several different types of underground cable. The
following data were collected with Trimble NIS.
• Total length
• Start location of the interruption (substation or disconnector)
• Number of joints
• Underground cable
o Type
o Length
• End location of interruption (substation or disconnector)
20
4.1.3 Evaluation of DARWin-file
Histograms over the interruptions on the examined components were made to see if there was
any relationship between the time of the day and the day of the week and the number of
interruptions.
4.2 Modelling of test grids
The primary substation that had the most number of interruptions reported for overhead line
and underground cables were examined. The most representative feeds when it comes to
reported outages were analysed for those primary substations.
4.2.1 Data collection
For constructing the test grids input data is needed, the input data is collected from the
software Trimble NIS. The following data were collected:
• Structure
o Radial
o Looped
o Double cable
• Overhead line and or underground cable
o Type
o Length
• Substation
o Type
o Number of customers
o Annual energy delivered
• Sectioning points
o Number of possible back-up feeds
• Interruptions
o Number of interruptions
o Cause of interruption
o Duration of interruption
o Affected customers
4.2.2 Classification of the test grids
To construct the test grids, the examined feeds are classified by two criteria’s. The main criteria
is the K-factor, and the sub criteria is the T-factor.
The K-factor states the composition of the grid.
𝐾 =𝑇𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑢𝑛𝑑𝑒𝑟𝑔𝑟𝑜𝑢𝑛𝑑 𝑐𝑎𝑏𝑙𝑒
𝑇𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑜𝑣𝑒𝑟ℎ𝑒𝑎𝑑 𝑙𝑖𝑛𝑒[−] Equation 4.1
The T-factor states the customer density of the grid.
21
𝑇 =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑢𝑠𝑡𝑜𝑚𝑒𝑟𝑠
𝑇𝑜𝑡𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑔𝑟𝑖𝑑 [
𝐶𝑢𝑠𝑡𝑜𝑚𝑒𝑟𝑠
𝑘𝑚] Equation 4.2
The terms for the K-factor and the T-factor are displayed in Table 6 below.
Table 6: K-factors and T-factors.
K- Factor T-Factor
K < 0,1 Overhead line grid T << 10 Sparsely populated area
K ≈ 1 Mixed grid T < 10 Rural area
K > 10 Underground cable grid 10 < T < 20 Semi Urban area
T > 20 Urban area
To construct the test grids, the ten primary substation feeders with the highest frequency of
interruptions were examined and classified according to the above-mentioned K and T-factors.
After the classification, there were two overhead line grids in rural area, three mixed grids in
rural area, two underground cable grids in semi urban area and three underground cable grids
in urban area. From these classified feeds, four different test grids were constructed.
• Test grid A: Overhead line grid, Rural area
• Test grid B: Mixed grid, Rural area
• Test grid C: Underground cable, Semi urban area
• Test grid D: Underground cable, Urban area
4.2.3 Description of the structure of the test grids
The structure for the different test grids have been decided from observations of the different
feeds in Trimble NIS.
The two overhead line grids in rural area follows the same structure. The main feed in both of
the grids contains overhead line, except a short section closest to the primary substation, which
is underground cable. Along the main feed there are branches which in turn has branches and
substations connected to it. In both grids, the structure is looped with several manually
operated disconnectors connected to possible back-up feeds at different branches. The breaker
in the primary substation are remotely operated. The structure of test grid A are presented in
Figure 7 and Appendix 2.
The three mixed grids in rural area have all different structures. One of the three grids have a
looped structure with branches along the main feed, the grid has three manually operated
disconnectors connected to the branches these disconnectors are used for back-up feeds. The
main feed contains of overhead line, except a short section closest to the primary substation.
The next grid has almost the same structure except for one difference. The main feed goes as a
loop, and on this loop, there are branches and substations connected to it. The last grid has the
same structure as the previous grid, but in this case, the main feed is divided into two loops.
The structure of test grid B is presented in Figure 8 and Appendix 3.
For the grids that were classified as underground cable in semi-urban area, two of them were
very similar when it comes to size and structure, the last grid is much larger when it comes to
22
the length of cable and the number of substations. The first two mentioned grids are more
similar and the structure from these two will decide the structure of the test grid. The sectional
points in this grid consists of manually operated disconnectors and also a remote-controlled
breaker in the primary substation. The structure of test grid C is presented in Figure 9 and
Appendix 4.
The two grids that were classified as an underground cable grid in urban areas, had differences
in the structure. In one of the grids, the main supply is split into two sections. The other grid
has only one main feed. The structure for the test grid in this degree project will consist of one
main feed. The primary substation are fitted with remote controlled breakers and the other
substations are fitted with manually operated disconnectors. The structure of test grid D is
presented in Figure 10 and Appendix 5.
All the test grids are considered as looped structured grids, but they are operated as radial
grids.
4.2.4 Specifications of the different test grids
The specifications for the test grids are presented below. The data for the test grids are mean
values from the examined feeds in the same classifications. Technical data as well as line and
cable composition and customers in those areas are presented. The technical data for all the
test grids are presented in Table 7 below.
Table 7: Technical data for the different test grids. Test grid A Test grid B Test grid C Test grid D
Length of line & cable [m] 52 722 49 067 13 108 5 077
K-factor 0,25 4,4 >>10 >>10
T-factor 6,3 6,6 15,5 91,2
Structure of the grid Looped Looped Looped Looped
Possible back-up feeds 3 2 4 4
Customers 335 325 204 464
Electricity consumption [MWh/year] 4 919,2 4 774,9 7 872,5 3 656,1
Substations 53 46 16 5
The line and cable composition for all the test grids are presented in Table 8 below.
Table 8: Line and Cable composition of the different test grids.
Test grid A Test grid B Test grid C Test grid D
Overhead line 80 % 50 % 0 % 10 %
Underground cable 20 % 50 % 100 % 90 %
The schematics over the different test grids are presented in Figure 7 to 10. Test grid C and test
grid D are more detailed because those two test grids were used for the reliability analysis.
25
The number of customers of each category are presented in Table 9 below.
Table 9: Number of customers of each category on each test grid.
Customer category
Number of customers
Test grid A Test grid B Test grid C Test grid D
Agriculture 16 10 1 0
Industry 7 6 27 3
Businesses & services 17 17 65 30
Public sector 13 10 23 14
Household 282 282 88 417
Total number of customers 335 325 204 464
The annual electricity consumption for the different customer categories in each test grid are
presented in Table 10.
Table 10: Annual electricity consumption per customer category and test grid.
Customer category
Annual electricity consumption [kWh]
Test grid A Test grid B Test grid C Test grid D
Agriculture 3 566 823 3 402 207 83 972 0
Industry 244 993 206 701 3 236 793 47 043
Businesses & services 561 395 405 285 3 160 606 1 275 671
Public sector 304 095 556 980 677 525 270 863
Household 241 859 204 752 713 573 2 062 517
26
4.3 Analytical reliability analysis
Test grids C and D will be used for the analytical reliability analysis. Test grid C and D has a
higher customer density than test grid A and B, this is the reason why they were chosen.
Analytical reliability analysis method is applicable when there is a high frequency of faults,
which makes it preferable on reliability calculations for the electrical grid (Wallnerström,
2011).
4.3.1 Failure rate
The number of interruptions on overhead line and underground cable between the years 2009-
2016 is taken from the DARWin-file. The length of the cables and overhead lines within the
MEE grid is also taken from the DARWin file. The data for calculation of the failure rate for
both the overhead line and underground cable are presented in Table 11 below.
Table 11: Length & number of interruptions on underground cables & overhead lines.
Year
Length [km] Number of interruptions
Cables OH-lines Cables OH-line
2009 1 358,1 466,5 39 118
2010 1 554,6 432,7 43 104
2011 1 565,8 418,0 48 114
2012 1 585,2 415,8 31 93
2013 1 616,2 398,7 44 84
2014 1 635,8 375,0 32 88
2015 1 720,2 347,4 33 67
2016 1 742,1 333,8 43 70
The average failure rate for underground cable and overhead line during the examined year
are calculated with equation 4.3 & 4.4. According to the data in Table 11 above, F is the number
of interruptions on each component for one year, and L is the total length for the same year.
𝜆𝐶𝑎𝑏𝑙𝑒 =∑ (
𝐹𝑌,𝐶𝑎𝑏𝑙𝑒𝐿𝑌,𝐶𝑎𝑏𝑙𝑒
)𝑌
∑ 𝑌𝑒𝑎𝑟𝑠 [
𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠
𝑘𝑚,𝑌𝑒𝑎𝑟] Equation 4.3
𝜆𝑂𝐻 𝐿𝑖𝑛𝑒 =∑ (
𝐹𝑌,𝑂𝐻 𝐿𝑖𝑛𝑒𝐿𝑌,𝑂𝐻 𝐿𝑖𝑛𝑒
)𝑌
∑ 𝑌𝑒𝑎𝑟𝑠 [
𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠
𝑘𝑚,𝑌𝑒𝑎𝑟] Equation 4.4
The calculated failure rates are presented in Table 12.
Table 12: Failure rates for overhead lines and underground cables. Overhead line Underground cable
Failure rate 0,2297 0,0247
Standard deviation ±0,024198 ±0,00433
27
4.3.2 Unavailability
The average repair time, 𝑟𝑖, for overhead line is according to Allan, et al, (1991), 5 hours and
for underground cable it is 30 hours. The average unavailability, 𝑈𝑖, in case of a failure on either
of these components is calculated using Equation 4.5.
𝑈𝑖 = 𝜆𝑖 ∗ 𝑟𝑖 [𝐻𝑜𝑢𝑟𝑠
𝑦𝑒𝑎𝑟] Equation 4.5
Since this degree project only handles the interruptions on overhead line and underground
cable the unavailability, 𝑈𝑖, can also be calculated with Equation 4.6, if the grid has the
possibility to be fed from a different primary substation feed. The unavailability time is then
dependent on the time it takes to isolate the faulted line and section the grid.
𝑈𝑖 = 𝜆𝑖 ∗ 𝑆𝑒𝑐𝑡𝑖𝑜𝑛𝑖𝑛𝑔 𝑡𝑖𝑚𝑒 [𝐻𝑜𝑢𝑟𝑠
𝑦𝑒𝑎𝑟] Equation 4.6
4.3.3 Reliability indicators
Reliability indicators are used for comparing the effect of interruptions between different grids
or grid sections. These reliability indicators are either weighted on number of customers
affected or the electric power affected. (Wallnerström, 2011)
The reliability indicators used in this degree project are presented in Equation 4.7 – 4.12 below,
where 𝑁𝑖 is the number of affected customers in each substation, 𝑈𝑖 is the unavailability
calculated from Equation 4.6.
System Average Interruption Frequency Index
𝑆𝐴𝐼𝐹𝐼 =∑ 𝑁𝑖𝜆𝑖𝑖
∑ 𝑁𝑖𝑖 [
𝐼𝑛𝑡𝑒𝑟𝑟𝑢𝑝𝑡𝑖𝑜𝑛𝑠
𝐶𝑢𝑠𝑡𝑜𝑚𝑒𝑟,𝑌𝑒𝑎𝑟] Equation 4.7
System Average Interruption Duration Index
𝑆𝐴𝐼𝐷𝐼 =∑ 𝑈𝑖𝑁𝑖𝑖
∑ 𝑁𝑖𝑖[
𝐻𝑜𝑢𝑟𝑠
𝐶𝑢𝑡𝑜𝑚𝑒𝑟,𝑌𝑒𝑎𝑟] Equation 4.8
Customer Average Interruption Duration Index
𝐶𝐴𝐼𝐷𝐼 =𝑆𝐴𝐼𝐷𝐼
𝑆𝐴𝐼𝐹𝐼[
𝐻𝑜𝑢𝑟𝑠
𝐼𝑛𝑡𝑒𝑟𝑟𝑢𝑝𝑡𝑖𝑜𝑛] Equation 4.9
Average Service Availability Index
𝐴𝑆𝐴𝐼 =8760∗∑ 𝑁𝑖−∑ 𝑁𝑖∗𝑈𝑖𝑖𝑖
8760∗∑ 𝑁𝑖𝑖 [𝑃𝑟𝑜𝑏𝑎𝑏𝑖𝑙𝑖𝑡𝑦] Equation 4.10
Average Service Unavailability Index
𝐴𝑆𝑈𝐼 = 1 − 𝐴𝑆𝐴𝐼 [𝑃𝑟𝑜𝑏𝑎𝑏𝑖𝑙𝑖𝑡𝑦] Equation 4.11
Energy Not Supplied
28
𝐸𝑁𝑆 =∑ 𝐴𝑛𝑛𝑢𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦𝑖∗𝑈𝑖𝑖
8760 [
𝑘𝑊ℎ
𝑦𝑒𝑎𝑟] Equation 4.12
4.3.4 Base scenario
In the base scenario, there is only one breaker in the grid. This breaker is installed at the
primary substation that feeds the grid. All the substations have manually operated
disconnectors. In event of a failure in the grid, the breaker in the primary substation will break,
and all the customers in the grid will have a power outage initially. The SCADA system will
alert the grid operator that a fault has occurred. The fitter is called to the grid section and
decides, based on information from SCADA and experience, where to start the sectioning. The
fitter then manually operates the disconnectors until the fault location is isolated and the
customers have their power back on. The sectioning time for an interruption is set to 2 hours
according to section 3.1.
4.3.5 Different sectioning times
The sectioning time at MEE is dependent on the experience of the operators and the fitters.
Therefore, the sectioning time is varied between 50 and 150 % of the original sectioning time.
This is done to see how much the sectioning time affects the cost and the reliability indicators.
The sectioning time for the different scenarios are presented in Table 13 below.
Table 13: Sectioning times for the different scenarios.
Scenario 1 (Base) 100% 2 hours
Scenario 2 50% 1 hour
Scenario 3 75% 1,5 hours
Scenario 4 125% 2,5 hours
Scenario 5 150% 3 hours
4.3.6 Design suggestion for Test grid C
There are several techniques and technologies to minimize the number of customers affected
by an interruption, SAIFI, as mentioned before removing overhead line and replacing it with
underground cable is one way of decreasing SAIFI. One other technology is to replace the
disconnectors in some of the substations with breakers that will break automatically in case of
an interruption. In this design suggestion, the substation LP3 in Test grid C is fitted with
breakers on the outgoing feeds. The purpose of the suggested design is to increase the overall
reliability of the test grid. The reason for the chosen design is because the grid splits into two
sections at LP3, and therefore, the potential for a reliability increase is assumed to be the
biggest at this point. In Figure 11, the schematics for this design are presented.
29
Figure 11: Schematic over the design suggestion for Test grid C.
A failure on line L1 – L3, will still cause a power outage on all the connected substations LP1 –
LP16. However, a failure on line section L4 – L10, or the “west” part of the test grid, will only
affect substation LP4 – LP10, because of the breaker in LP3. In the same way, if a failure occurs
on line L11 – L16, or the “east” part of the test grid, substation LP11 – LP16 will experience an
interruption.
4.3.7 Calculations
The first step of the calculations were to distribute the cables and lines between the substations.
This was done by randomization, with the original feeds as background for reasonable
distribution. The customers were distributed as demonstrated in Table 9. Since the test grids
is operated radially but with several possible back-up feeds, the failure rate for the whole grid
is the sum of each cable and line failure rate. This includes the unavailability as well. To
calculate the specific failure rate for one underground cable or overhead line section, the length
of that section is multiplied with the average failure rate for the cables or overhead lines. The
unavailability is calculated by multiplying the specific failure rate with the sectioning time. An
example is shown in Table 14 for substation n in test grid D. Were L1-L5 is cables except L2
which is overhead line.
Table 14: Example of calculations. Substation n Test grid D λ [F/Y] Sectioning time[h] U [h]
L1 0,032 2,0 0,064
L2 0,129 2,0 0,258
L3 0,028 2,0 0,055
30
L4 0,025 2,0 0,049
L5 0,028 2,0 0,057
Σ 0,242
0,483
From the above presented example, the total failure rate, λ, and unavailability, U, is now
known. The reliability indicators were then calculated according to Equation 4.7 – 4.12.
The cost for an interruption in the test grids was calculated according to Equation 3.1 and the
annual average power, P, mentioned in Equation 3.1 was calculated from the annual average
electricity from Table 10. The duration, t, in Equation 3.1 is the sectioning time.
Further description of the calculations is presented in Appendix 7.
31
5 RESULTS
The results from this degree project are presented in the sections below. In section 5.1 different
histograms of the examined interruptions are presented. Sections 5.2 to 5.5 presents the results
from the test grids and calculations.
5.1 Histograms of the interruptions
Free overhead line, uninsulated, and underground cable are the two components that are
affected by the most numbers of interruptions. Free overhead line is undoubtedly the most
dominating part when it comes to the number of interruptions. The other parts have fairly the
same amount of interruptions, except for pole station and distribution station. The
interruptions are presented in Figure 12.
Figure 12: Interruptions on components at the voltage level of 10 kV during the years 2009-2016.
Fabrications or material failure is the most common cause for an interruption on overhead
line and underground cable. For free overhead line, uninsulated, falling trees due to wind, and
thunder are the most common cause for interruptions. Fabrication or material failure and
digging causes the most interruptions on underground cable. The causes for an interruption
are presented in Figure 13.
0
100
200
300
400
500
600
700
800
Nu
mb
er o
f in
terr
up
tio
ns
Interruptions on components at the voltage level of 10 kV during the years 2009-2016
32
Figure 13: Interruption causes on overhead line and underground cable 10 kV 2009-2016.
The most interruptions on both overhead line and underground cables occurs on Mondays.
Other days that have almost the same amount of interruptions are Wednesday and Friday.
Fewest interruptions occurs on Tuesday and on the weekends. The interruptions during the
week are presented in Figure 14.
Figure 14: Number of interruptions per day at the voltage level of 10 kV and on overhead line and underground
cable during the years 2009-2016.
0
20
40
60
80
100
120
140
160
180
200
Nu
mb
er o
f in
terr
up
tio
ns
The cause of the interruptions on the studied components
Overhead line & Underground cable Underground cable
Overhead line uninsulated Overhead line isolated
0
20
40
60
80
100
120
140
160
180
200
Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Nu
mb
er o
f in
terr
up
tio
ns
Interruptions on the examined components sorted by day
33
Examining the interruptions on overhead line and underground cable separately, the pattern
are similar, except that, more interruptions occurs on the weekends for overhead line than for
underground cable. The amount of interruptions is also significantly larger for overhead line
than for underground cable. Figure 15 and Figure 16 presents these above-mentioned
interruptions.
Figure 15: Interruptions on overhead line sorted by day.
Figure 16: Interruptions on underground cable sorted by day.
5.2 Test grid A
The majority of the faults in this test grid is caused by the weather. The weather category stands
for 80 % of the interruptions that has occurred during the years 2009 - 2016. The other
interruptions are caused by damage and material/method. They stand for 7 and 14 % of the
remaining interruptions. The number of interruptions on each category are presented in Figure
17 below.
0
20
40
60
80
100
120
140
Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Nu
mb
er o
f in
terr
up
tio
ns
Number of interruptions on overhead line
0
10
20
30
40
50
60
Monday Tuesday Wednesday Thursday Friday Saturday Sunday
Nu
mb
er o
f in
terr
up
tio
ns
Number of interruptions on underground cable
34
Figure 17: Interruptions by category on Test grid A.
Under the weather category, falling tree, wind and thunder, causes the most amount of
interruptions. The rest of the weather-related interruptions are evenly distributed between the
causes, wind, falling tree, snow, snow and rain. The third most frequent cause of the
interruptions are Fabrications or material. All the interruption causes on test grid A are
presented in Figure 18 below.
Figure 18: Interruptions by cause on Test grid A.
5.3 Test grid B
Weather is the most dominating category when it comes to interruptions on test grid B.
Weather stands for 65 % of the interruptions. The rest of the interruptions are under the
categories material/method, other and damage. The category other is the biggest category after
0
5
10
15
20
25
Weather Damage Material/Method
Nu
mb
er o
f in
terr
up
tio
ns
Interruption by category on Test grid A
0123456789
10
Nu
mb
er o
f in
terr
up
tio
ns
The cause of interruptions onTest grid A
35
weather with 22 % of the interruptions. Material/method and damage stands for 4 and 9 % of
the interruptions. The number of interruptions on each category are presented in Figure 19
below.
Figure 19: Interruptions by category on Test grid B.
In Figure 20 below, the interruptions are further subdivided into the cause of the failure.
Figure 20: Interruptions by cause on Test grid B.
5.4 Test grid C
Material/method is the most common category with 76% of the interruptions. The rest of the
interruptions are evenly distributed on the categories damage and other. All the interruptions
on test grid C are presented in Figure 21 below.
0
2
4
6
8
10
12
14
16
Weather Material/Method Other Damage
Nu
mb
er o
f in
terr
up
tio
ns
Interruptions by category on Test grid B
0
1
2
3
4
5
6
Nu
mb
er o
f in
terr
up
tio
ns
The cause of interruptions on Test grid B
36
Figure 21: Interruption by category on Test grid C.
Manufacturing or material failure is undoubtedly the most common cause of interruptions on
test grid C. The rest of the causes are evenly distributed on digging, material/method and
unknown. All the causes of the interruptions are presented in the Figure 22 below.
Figure 22: Interruptions by cause on Test grid C.
The results from the calculation, described in section 4.3 and 4.4, on test grid C are presented
in Table 15 and Figure 23 below.
Table 15: Reliability indicators Test grid C.
Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5
SAIFI [Outages/Customer, Year] 0,3232 0,3232 0,3232 0,3232 0,3232
SAIDI [Hours/Customer, Year] 0,6464 0,3232 0,4848 0,8079 0,9695
SAIDI [Min/Customer, Year] 38,7815 19,3907 29,0861 48,4768 58,1722
CAIDI [Hours/outage] 2,0000 1,0000 1,5000 2,5000 3,0000
0
1
2
3
4
5
6
7
8
Damage Material/Method Other
Nu
mb
er o
f in
terr
up
tio
ns
Interruptions by category on Test grid C
0
1
2
3
4
5
6
7
Digging Manufacturing ormaterial failure
Material/Method Unknow
Nu
mb
er o
f in
terr
up
tio
ns
The cause of interruptions on Test grid C
37
ASAI [Availability] 99,9926% 99,9963% 99,9945% 99,9908% 99,9889%
ASUI [Unavailability] 0,0074% 0,0037% 0,0055% 0,0092% 0,0111%
ENS [kWh/year] 581,8 290,9 436,4 727,3 872,7
The interruption cost for the base scenario is almost 200 000 SEK. The interruption cost
changes linearly with the sectioning time. This follows that for scenario 2, the interruption cost
is around 110 000 SEK and for scenario 5, the interruption is around 280 000 SEK. The
interruption costs for the five scenarios for test grid C are presented in Figure 23 below.
Figure 23: Interruption costs Test grid C.
5.5 Test grid D
The interruptions that occurred on test grid D is divided into two categories, material/method
and other. The dominating category is material/method that stands for 85 % of the
interruptions. The interruptions on test grid D are presented in Figure 24 below.
0 kr
50 000 kr
100 000 kr
150 000 kr
200 000 kr
250 000 kr
300 000 kr
Scenario 2 Scenario 3 Scenario 1 Scenario 4 Scenario 5
Interruption costs for Test grid C
38
Figure 24: Interruptions by category on Test grid D.
The causes of the interruption are presented in Figure 25 below.
Figure 25: Interruptions by cause on Test grid D.
The results for the different sectioning time on test grid D are presented in Table 16 below.
Table 16: Reliability indicators Test grid D.
Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5
SAIFI [Outages/Customer, Year] 0,2293 0,2293 0,2293 0,2293 0,2293
SAIDI [Hours/Customer, Year] 0,4587 0,2293 0,3440 0,5733 0,6880
SAIDI [Min/Customer, Year] 27,5201 13,7600 20,6401 34,4001 41,2801
CAIDI [Hours/outage] 2,0000 1,0000 1,5000 2,5000 3,0000
ASAI [Availability] 99,9948% 99,9974% 99,9961% 99,9935% 99,9921%
ASUI [Unavailability] 0,0052% 0,0026% 0,0039% 0,0065% 0,0079%
ENS [kWh/year] 191,4 95,7 143,6 239,3 287,1
0
1
2
3
4
5
6
Material/Method Other
Nu
mb
er o
f in
terr
up
tio
ns
Interruptions by category on Test grid D
0
1
2
3
4
5
6
Fabrications or material failure Unknown
Nu
mb
er o
f in
terr
up
tio
ns
The cause of interruptions on Test grid D
39
The annual interruption cost for the base scenario is around 55 000 SEK. The interruption
costs for test grid D are linearly dependent. For scenario 1, the interruption cost is around
30 000 SEK and for scenario 5, the cost is around 80 000 SEK. The interruption cost for all
the scenarios for test grid D are presented in Figure 26 below.
Figure 26: Interruption costs Test grid D.
5.6 Design suggestion for test grid C
The results for the calculations made on the design suggestion on test grid C described in
section 4.3.6 are presented in this section. Figure 27 below presents the SAIFI and SAIDI for
an interruption occurring on the east feed compared to the base scenario.
Figure 27: SAIFI & SAIDI for the design suggestion for Test grid C, fault occurring in the east feed.
Figure 28 below demonstrates the difference in SAFI and SAIDI between the base scenario and
the suggested design if a fault occurs in the west feed.
0 kr
10 000 kr
20 000 kr
30 000 kr
40 000 kr
50 000 kr
60 000 kr
70 000 kr
80 000 kr
90 000 kr
Scenario 2 Scenario 3 Scenario 1 Scenario 4 Scenario 5
Interruption costs for Test grid D
0,000,050,100,150,200,250,300,350,400,450,500,550,600,650,70
SAIFI SAIDI
SAIFI & SAIDI for the design suggestion for Test grid C, fault occurring in the east feed.
Standard Base scenario
40
Figure 28: SAIFI & SAIDI for the design suggestion for Test grid C, fault occurring in the west feed.
The difference in the interruption cost for the base scenario and the suggested design are
presented in Figure 29 below.
Figure 29: Interruption cost for the design suggestion for Test grid C.
5.7 Sensitivity analysis
SAIFI for the test grids C and D are lower than for the whole grid this can be explained by the
small amount of overhead line in test grid C and D. The original sectioning time of 2 hours
might be a bit long when comparing the SAIDI numbers.
In Figure 30 below, SAIDI extracted from the DARWin-file and SAIDI for the base scenario for
test grid C & D are presented.
0,000,050,100,150,200,250,300,350,400,450,500,550,600,650,70
SAIFI SAIDI
SAIFI & SAIDI for the design suggestion for Test grid C, fault occurring in the west feed.
Standard Base Scenario
0 kr
50 000 kr
100 000 kr
150 000 kr
200 000 kr
250 000 kr
Interruption costs for the design suggestion
Interruption in east Base Scenario Interruption in west
41
Figure 30: SAIDI of the MEE grid, based on all the examined interruptions.
Figure 31 below compares the SAIFI of all the examined interruptions compared to the base
scenario on test grid C & D.
Figure 31: SAIFI for MEE grid, based on all the examined interruptions.
A sensitivity analysis on the results from test grid C & D are conducted by varying the failure
rate for underground cable and overhead line within the standard deviation of the failure rates.
The results from the sensitivity analysis are presented in Table 17 and Table 18 below.
Table 17: Test grid C sensitivity analysis.
Scenario 1 -σ +σ
SAIFI [Outages/Customer, Year] 0,3232 0,2664 0,3799
SAIDI [Hours/Customer, Year] 0,6464 0,5328 0,7599
SAIDI [Min/Customer, Year] 38,7815 31,9705 45,5924
CAIDI [Hours/outage] 2,0000 2,0000 2,0000
ASAI [Availability] 99,9926% 99,9939% 99,9913%
ASUI [Unavailability] 0,0074 % 0,0061 % 0,0087 %
0
5
10
15
20
25
30
35
40
45
2009 2010 2011 2012 2013 2014 2015 2016 Test gridC
Test gridD
SAIDI Minutes per Year
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
2009 2010 2011 2012 2013 2014 2015 2016 Test gridC
Test gridD
SAIFI
42
ENS [kWh/year] 581,8 479,6 684,0
Table 18: Test grid D sensitivity analysis.
Scenario 1 -σ +σ
SAIFI [Outages/Customer, Year] 0,2293 0,1973 0,2614
SAIDI [Hours/Customer, Year] 0,4587 0,3945 0,5228
SAIDI [Min/Customer, Year] 27,5201 23,6709 31,3692
CAIDI [Hours/outage] 2,0000 2,0000 2,0000
ASAI [Availability] 99,9948% 99,9955% 99,9940%
ASUI [Unavailability] 0,0052% 0,0045% 0,0060%
ENS [kWh/year] 191,4 164,7 218,2
43
6 DISCUSSION
6.1 Empirical research
The empirical research conducted in order to collect the data used for the test grids and
calculations were quite hard due to the fact that the DARWin file used were not as detailed as
one had hoped. Problems that occurred was due to the fact that all the examined interruption
was not properly reported. This lead to difficulties when searching for the affected part in
TRIMBLE. The examined interruptions occurred during the years 2009 to 2016, however,
during this period of time the grid have had some changes and the main changes are overhead
line that had been replaced with underground cable.
6.2 Result
From the histograms presented in section 5.1 it is clear that uninsulated overhead lines have a
lot more interruptions than isolated overhead line. This is due to the fact that the weather has
a significantly smaller impact on isolated overhead line, but also the fact that uninsulated is a
lot more common in the grid, which is shown in Figure 4.
The main reason for the dip in the number of interruptions on weekends on underground cable
is the fact that digging is primarily done during the weekdays. The reason for the dip on
Tuesdays on both overhead line and underground cable are a lot harder to explain.
The test grids constructed in the degree project are well suited for the reliability analysis
calculations. The faults occurring on the test grids are representable for what actually happens
on these kind of grids. The reliability of test grid A and B are heavily influenced by weather
related interruptions. This is because of the amount of overhead lines which are overall very
weather affected.
Test grid C and D which are almost completely constructed with underground cable are mainly
affected by material and methods related interruption. This category are the overall main
interruption cause on underground cable. However, only test grid C of these test grids are
affected by digging damages, which is very unusual for underground cable grids.
The reliability calculations computed on test grid C and D are showing reasonable numbers
compared to different similar studies. Quicker sectioning times decreases the SAIDI and
increases the availability ASAI in the grid. Shorter downtimes and therefore higher reliability
is something all DSOs are reaching for, because of the possibilities of bigger income frames in
future ex-ante regulations.
Since the interruption cost have a strict linear relationship to the duration of the interruption
the sectioning time have a big effect on the interruption cost. However, if the duration of the
interruption exceeds 12 hours the fee described in section 3.2.2 will take a huge part of the
interruption cost. These long outages are extremely rare when focusing on faults occurring on
44
overhead line and underground cables in looped structured grids, therefore, it is not handled
as a scenario for the models.
The design suggestion for Test grid C, where breakers are installed in substation LP3. The
reliability is increased a lot compared to the base scenario for the same test grid. Installing
breakers in an old substation may not be feasible depending on the structure of the substation.
Also, in reality the breakers will also have a failure rate, and therefore the real reliability
increase, might not be as significant as the result shown in this degree project.
45
7 CONCLUSIONS
The purpose of the degree project was to analyse interruption that has occurred on overhead
line and underground cable at voltage level at 10 kV within MEE distribution grid. Test grid
was developed for reliability analysis.
The conclusions drawn from the results are that the components that have the most amount of
interruptions are overhead line uninsulated and underground cable. This shows that the need
for studies like this one are significant and relevant.
The number of interruptions on overhead line and underground cable are evenly distributed
during the weekdays.
The primarily cause of an interruption on overhead line are weather based events. The main
causes of interruptions on underground cable grids are digging damages and fabrications or
material failures. The big share of digging failures are noticeable in Figure 16 where the number
of interruptions decreases during the weekends.
It is concluded that faster sectioning will lead to lower interruption costs. Installing breakers
at key points in the grid will help to isolate the fault, and decrease the number of affected
customers.
8 FUTURE WORK
Suggestions for future work concerning the outcome of this degree project are presented below.
Include more grid sections for more specified test grids. More underlying data for the
test grids will increase the accuracy of the test grids.
Including more data for the reliability calculations will help to improve the
understanding of the costs in case of an interruption. For example, including failure
rates for breakers, disconnectors and substations etc. will also increase the accuracy of
the test grids.
Different sectioning times dependent on where the fault occurs. For example, a fault on
a cable or overhead line far out on the grid may take longer time to find than a fault
close to the primary substation.
By introducing technology and automation to distribution grid the duration of an
interruption can be decreased. How much will it cost to decrease SAIDI by a minute in
different types of grids?
46
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1
APPENDIX 1: CABLE DESIGNATION STANDARDS
Letter First Letter - Conductor
Second letter - Isolation
Third letter - Mantle or other construction detail
Fourth letter - Construction detail or use
Fifth letter - Construction detail or use
A Aluminium
Screen of aluminium foil and/or aluminium thread
B Aluminium alloy
Flame protected thermoplastic polyolefin
Flame protected thermoplastic polyolefin/lead mantle
Vehicle cable/Connection thread/lead mantle
C Impregnated paper Concentric copper thread screen
Concentric copper screen
D
Rubber with outer rubber mantle
E Copper, one thread
Ethylene-propylene rubber Reinforcement Reinforcement
F Copper, stranded Twine of copper thread Twine of copper/steel thread
H Silicon rubber Elevator cable Suspension cable
I Urethane plastic Urethane plastic
J Steal thread
Reinforcement of steel band Laying in ground
K PVC PVC PVC PVC
L Polyethylene (PE)
Screen of plastic laid aluminium strip ev together with an copper screen / Polyethylene (PE) PE PE
M Copper, Stranded O Chloroprene rubber Chloroprene rubber Oil cable
P
Reinforcement of galvanized steel band
Reinforcement of galvanized steel band
Q
Flame protected thermoplastic polyolefin
R Copper, multi stranded
Reinforcement of plastic laid aluminium band Control cable
S Copper, fine-threaded Self-supporting
T Copper, Extra fine-threaded Fluorescent
Reinforcement of steal thread
Heavy connection cable or reinforcement of steal thread
Reinforcement of steal thread
U No outside mantle
V
Rubber without outside mantle Ethylene-propylene rubber Laying in water Laying in water
X
Crosslinked polyethylene (PEX) PVC, oval cross section
Z
Flame protected crosslinked polyolefin
Flame protected crosslinked polyolefin Cable for neon-facility
1
APPENDIX 7: DESCRIPTIONS OF CALCULATIONS
The calculations made on the test grids are explained in this appendix. The following
description are for test grid D but the same calculations are made for test grid C.
For test grid D, there was five substations according to table12 in section 4.2.2.2. The length
of the feeders L1 to L5 are distributed as follows:
Feeders OH line & UG cable Failures/Year
L1 [m] 1302 0,032101
L2 [m] 508 0,128977
L3 [m] 1123 0,027688
L4 [m] 996 0,024556
L5 [m] 1148 0,028304
L2 was overhead line with a higher failure rate than the underground cable sections. To
calculate the failures per year on each line section, the equation 4.2 were used.
The failure rate for each substation is the sum of the failure rates for all the line sections.
Substation 1 Substation 2 Substation 3 Substation 4 Substation 5
λ F/Y U [h] λ F/Y U [h] λ F/Y U [h] λ F/Y U [h] λ F/Y U [h]
L1 0,0321 0,0642 0,0321 0,0642 0,0321 0,0642 0,0321 0,0642 0,0321 0,0642
L2 0,1290 0,2580 0,1290 0,2580 0,1290 0,2580 0,1290 0,2580 0,1290 0,2580
L3 0,0277 0,0554 0,0277 0,0554 0,0277 0,0554 0,0277 0,0554 0,0277 0,0554
L4 0,0246 0,0491 0,0246 0,0491 0,0246 0,0491 0,0246 0,0491 0,0246 0,0491
L5 0,0283 0,0566 0,0283 0,0566 0,0283 0,0566 0,0283 0,0566 0,0283 0,0566
Σ 0,2416 0,4833 0,2416 0,4833 0,2416 0,4833 0,2416 0,4833 0,2416 0,4833
The number of customers on each substation and their annual electricity consumption are
presented below.
Substation
Total number of customers
Total annual consumption
[kWh]
LP1 127 1 000 697
LP2 59 464 891
LP3 148 1 166 168
LP4 54 425 494
LP5 76 598 843
Total 464 3 656 093
For calculating the reliability indicators, SAIFI, SAIDI, CAIDI, ASAI, ASUI, Equation 4.7 –
4.11 were used.
2
For calculating the last reliability indicator, Energy not Supplied (ENS), Equation 4.12 were
used. For this equation, the annual energy for each substation were used.
For the cost of the interruption Equation 3.1 were used, with the duration of the interruption
set to the sectioning time. Because in this way the cost for an interruption was calculated,
setting the duration to U, will calculate the average annual cost for interruptions.
For the design suggestion for Test grid C, the same calculations and equations were used.
However, the failure rate and unavailability for each substation is dependent on were the
failure occurs. For example, if a failure occurs on line L4 – L10, the breaker in substation LP3
will open and substation LP4 – LP10 will experience an interruption. If a failure occurs on
L1-L3, the breaker in the primary substation will open and therefore all the substations in the
test grid will experience an interruption until the fault section is isolated.
Recommended