Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
Practical Impact of Very Small Power Producers
(VSPP) on Control and Protection System in
Distribution Networks
Noppatee Sabpayakom and Somporn Sirisumrannukul Department of Electrical and Computer Engineering, Faculty of Engineering, King Mongkut’s University of
Technology North Bangkok (KMUTNB), Bangkok, Thailand
Email: {noppatees, spss}@kmutnb.ac.th
Abstract—Due to incentive policies to promote renewable
energy and energy efficiency, high penetration levels of very
small power producers (VSPP) located in distribution
networks have imposed technical barriers and established
new requirements for protection and control of the
networks. Although VSPPs have economic and
environmental benefit, they may introduce negative effects
and cause several challenges on the issue of control and
protection system. This paper presents comprehensive
studies of possible impacts on control and protection system
based on real distribution systems located in a metropolitan
area. A number of scenarios were examined primarily
focusing on state of islanding, and undisconnected VSPP
during faults. It is shown that without proper measures to
address the issues, the system would be unable to maintain
its integrity of electricity power supply for disturbance
incidents.
Index Terms—control and protection system, distributed
generation, renewable energy, very small power producers
I. INTRODUCTION
Traditionally, much of the electricity generated has
been produced by large-scale, centralized power plants
using fossil fuels (e.g., coal, oil, and gas), hydropower, or
nuclear power. The electrical energy is transmitted over
long distances by high voltage (HV) or extra high voltage
(EHV) transmission lines and from there, the high voltage
levels are converted to medium voltage (MV) and low
voltage (LV) levels through distribution lines in the
distribution system to end-use customers.
Such a centralized generation pattern, however, suffers
a number of drawbacks, such as a high level of
dependence on imported fuels that are price-vulnerable,
transmission losses, the necessity for continuous
upgrading and replacement of the transmission and
distribution facilities and therefore high operating cost, as
well as environmental impact. In addition, as electric
demand is substantially increasing as a result of economic
and social growths, the construction of a large sized
power plant is running into financial and technical
difficulties because it is capital intensive and needs
considerable amount of time to complete.
Manuscript received June 10, 2015; revised August 25, 2015.
Alternatively, an ideal alternative on electric power
supply to electric users is the installation of a small sized
generator or commonly known as distributed generator
(DG). For the last decade, distribution systems have seen
a large number of small sized generators due to incentive
policies to promote renewable energy and energy
efficiency. DGs can be powered by conventional and
renewable energy resources and in Thailand, a DG with a
net injected capacity less than or equal to 10MW is
commonly known as very small power producer (VSPP).
VSPPs are different in types of generating plant ranging
from well established technology such as combined heat
and power (CHP) units to more recent types of generation
technology like photovoltaics.
Although VSPPs have gained many positive effects,
they still have some specific, technical issues that need to
be addressed before their applications in the distribution
system driven by two fundamental goals can be fully
realized: 1) delivering an acceptable quality of supply to
consumers under normal conditions and 2) protecting the
integrity of the system when disturbed by faults.
There have been a number of problems reported in
literature that create complexity in control and protection
system of existing distribution networks such as failed
reclosing due to temporary fault current, out of
synchronous, protection coordination, false tripping,
protection under reach, and islanding operation [1]-[3].
It is obviously seen that while many of the problems are
shared in common, some are system-dependent.
Consequently, system operators and planners have to
carefully review and probably revise, if necessary, their
existing protection schemes to accommodate high
penetration levels of distributed generation.
This paper presents comprehensive studies on
technical impacts of VSPP on distribution systems in
metropolitan area, where the power delivery
infrastructure to the consumers is covered in three
provinces: Bangkok, Samutprakarn, and Nontaburi. The
distribution electricity infrastructure is of the type of
overhead and underground systems with different voltage
levels of 400V, 12kV, 24kV, 69kV, 115kV, and 230kV
[4].
The technical impacts are emphasized on control and
protection system. The studies were carried out through
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 162doi: 10.18178/ijoee.3.3.162-168
power system analysis and simulation using real, existing
systems connected with VSPPs in a metropolitan service
area. The VSPPs are connected to a MV distribution
system, the main equipment of which consists of power
transformer, MV busbar, circuit breakers, MV feeders,
fuses, and DGs.
II. CONTROL AND PROTECTION SYSTEM OF
DISTRIBUTION NETWORK
The details of protective devices, protection
coordination, and restoration functions for interruption
events in the distribution system are described as follows
[4].
A. Protective Devices
A circuit breaker (CB) installed at the beginning of an
outgoing 12 or 24kV feeder (i.e., at the MV busbar of a
substation) is used to protect the feeder. It is a common
practice to utilize a fuse to protect a lateral MV feeder or
a downstream LV feeder behind a distribution
transformer. Fuse Type K (fast clearing time) is widely
used with a rating of 200A or 400A.
RR
FuseOC EF
DS
Figure 1. Operation of OC EF relay, RR, and fuse.
B. Protection Coordination
The non-directional overcurrent (OC) relay is
coordinately functioned with fuses and a reclosing relay
(RR) based on the principle of “fuse saving scheme”
using a microprocessor-based OC relay. A typical
protection system configuration of feeder is shown in Fig.
1. The fuse saving scheme strategy temporarily interrupts
an entire feeder for all faults occurring on the feeder with
the main objective to save expensive fuse replacement
and to reduce outage time of customers. The operation of
an OC relay is of two types: fast and slow, with details as
follows:
Instantaneous operation: At a current of 1,200 A
(for 24 kV) or at 1,800 A (for 12 kV) when a fault
is detected, the associated relay will disconnect the
CB at once to avoid damage on the fuse. After the
CB has been opened, this disconnecting mode will
be blocked. Then, the first step of reclosing relay
(RR) will come to operation by closing the CB.
Therefore, if the fault is temporary, the system can
return to normal without realizing a sustained
interruption.
Inverse definite minimum time (IDMT) operation:
With a rating of 600 A, 0.05 TMS (Time
Multiplier Setting), if the fault still remains, the
relay will not open the CB immediately, waiting
for the fuse to blow and isolate the faulty part. If
this is the case, the relay will not trip the CB;
therefore the customers in the healthy areas can
still be electrically supplied. On the other hand, for
example, a fault on the main feeder, for a short
period of time, the OC relay will trip the CB and
then the second step of the RR will be activated
instead.
The earth fault (EF) relay will monitor and detect earth
fault, with a rating of 120A 0.5 TMS.
The under-frequency (UF) relay monitors the system
frequency at an MV busbar. For an under-frequency
event mainly because of demand greater than supply, the
UF relay will shed some loads to keep the system balance
and prevent wide area outage such as cascade tripping. At
the frequency lower than 49Hz with a duration longer
than 0.15 second, the UF relay will start to trip feeders’
CBs for 5 different levels of the system frequencies (49,
48.8, 48.6, 48.3 and 47.9Hz).
Note that transmission lines are protected by distance
relay, directional overcurrent earth fault (OCEF) relays,
and current differential relays.
C. Restoration Functions
Automatic reclosing relay (RR) is a mechanism that
can automatically close the breaker after it has been
opened due to a fault. Because most of the faults on
overhead feeders or overhead transmission lines are
temporary (70-80%), reclosing relays play an important
role to increase system availability of supply (i.e.,
reducing outage time from sustained interruption to
momentary interruption) in an overhead feeder or an
overhead transmission line.
The control system of a recloser allows a preselected
number of attempts for service restoration after adjustable
sequential time delays. For example, a recloser may have
2 or 3 “fast” reclose operations with a few short time
delays, then a longer delay and one reclose; if the last
attempt does not successfully re-energize the line, the
recloser will lock out (for permanent faults) and require
human intervention to reset. In metropolitan distribution
systems, two pre-programmed attempts to re-energize the
feeders: first at 3 seconds and the other at 1 minute and
only one attempt at 4 seconds for the transmission lines.
For power transformer failures, restoration is
succeeded by the bus throw over (BTO), which is an
automatic function installed in the MV levels as shown in
Fig. 2. For a substation, the MV buses are separated by a
CB and each of the buses receives power from different
power transformers. When a fault occurs on one of the
transformers and after the HV-side CB disconnects the
transformer, the function of BTO will disconnect the
MV-side CB of the transformer and close the bus coupler
CB so that the two busbars are able to receive power from
the same power transformer. The function of BTO is
completed within 4 seconds after power transformer
failures.
A similar automated mechanism can be found in
transmission systems (69 or 115kV) for the line throw
over (LTO) or the coupler throw over (CTO). The former
is an automated function for line switching operation and
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 163
the other for bus switching. For transmission line failures,
with the function of a LTO or a CTO will switch
substation’s CBs to receive power from another
transmission line within 0.2 second. The LTO function is
normally used in H–scheme substations while the CTO
function in double-bus single-breaker substations. Note
that the distribution systems do not have a syncheck relay.
LTO
BTO
Auto
Auto
RR RR RR RR
Figure 2. Operation of LTO, BTO, and RR for different fault locations.
III. STATE OF ISLANDING
The existing grid code for VSPP connection in the
network indicates that a VSPP is obliged to detect a fault
and to be self-disconnected within 0.1 second for a
machine-based VSPP [5] and 0.3 second for an inverter-
based VSPP [6]. This requirement prevents any damage
that may occur on VSPPs and electrical appliances, as
well as for safety to personal staff working onsite. To be
specific, islanding operation is currently not allowed.
Note that the self-disconnecting time for the inverter-
based VSPP in the previous grid code was 0.1 second.
Islanding situation is formed when associated
protective devices isolate small part from the main grid.
Because this small part contains DGs, it is possible that
some loads in the part can be served, causing positive and
negative impacts on the system [3].
For positive point of view, the system sees higher
power system reliability and therefore reduced electricity
outage cost of customers because VSPPs can serve some
loads while being waiting for the faulty part to be
repaired. Such intentionally islanded operation is useful,
for example, during scheduled maintenance. However, it
should be bear in mind that doing so needs to make sure
that the total capacity of the VSPPs is large enough to
supply the remaining load; otherwise, disconnection of
nonpriority loads, switching operation, and special
protection setting are required. In addition, incidents
while in islanding operation (e.g., changing demand,
short circuit, and personal staff working onsite) and while
in the process of reconnection to the main system at the
end of operation have to be taken into consideration.
Negative effects could happen because the isolated
network becomes a weak grid compared with the original
strong network (i.e., infinite bus). Therefore, problems on
stability and power quality (e.g., voltage regulation and
harmonics) can be expected, as well as protection
problems (e.g., coordination) and cause damage to
equipment in the network. To guard against unintentional
islanding operation, small generators are obliged to detect
isolation events and to be self-disconnected from the
network within suitable time frame.
IV. SMALL GENERATION AND SYMMETRICAL FAULTS
Let us consider a balanced fault shown in Fig. 3,
assuming that no load current is present in the system.
The fault level can be calculated using an equivalent
network shown in Fig. 4, where the generator model is
represented by an ideal voltage source serried with an
internal impedance ZS and feeder impedance ZL [7].
GDistance to fault = d
Line length = l
Figure 3. Symmetrical fault.
ZS ZL
E
Figure 4. Network equivalent for symmetrical fault.
For a three phase fault occurring at the end of feeder as
shown in Fig. 3, the fault current is given by:
f
S L
EI
Z Z
(1)
When a three phase fault is away from the substation
with a distance of d, the fault current becomes:
f
S L
EI
dZ Z
l
(2)
We can conclude that the impedance between the fault
position and the source limits the magnitude of fault
current. Namely, the fault current at the end of feeder is
less than that near the substation.
For a distribution system with a VSPP, the magnitude
of fault contribution from the VSPP depends on a few
factors. The first factor is the rating of the machine; that
is, the greater MVA, the higher the fault current. The
second factor is type of machines. The directly coupled
VSPP such as a synchronous generator or an induction
generator tends to supply more fault current, compared
with the converter interfaced VSPP [1]. Therefore, the
impact of protection is primarily focused on directly
coupled VSPP. Last, the fault current is location-specific.
A VSPP located near the fault position has a greater
contribution to the fault current.
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 164
V. IMPACT OF CONTROL AND PROTECTION SYSTEM
FOR UNDISCONNECTED VSPP DURING FAULTS
When there is a small generator installed in a
distribution system, power flows on the feeder become
multidirectional and so are fault currents. Those fault
currents may affect detecting operation of protective
devices. When a fault occurs and a VSPP is not self-
disconnected, the fault contribution from the VSPP or
islanding operation of the VSPP may affect protection
system and protection coordination. The following
impacts are foreseen.
A. Impact of RR: VSPP Supplies Arc to Temporary
Fault
Let us consider Fig. 5, where there is a temporary fault
on an overhead feeder. The OC instantaneous relay sends
a command to trip the CB to stop supplying the arc to the
temporary fault. After the fault has been cleared, the CB
is reclosed. However, if the VSPP has been continuously
supplying the fault, the arc might not have been
distinguished and may cause a problem for the first
attempt of the CB reclose. The network is still supplying
the fault and therefore the CB will be tripped by the OC
IDMT relay; otherwise, the fuse will blow. Such a
situation degrades the efficiency of the operation of the
RR for temporary faults, resulting in unnecessary opening
of the CB and therefore degrading system reliability as a
whole.
Figure 5. Impact of OC EF relay, RR, and fuse in overhead feeder.
B. Impact of RR to VSPP for Islanding Operation
Let us revisit Fig. 5 for a temporary fault on a feeder.
After tripping the CB by the OC relay, the temporary
fault disappears from the system. While some of the
system demand is being covered by the VSPP through the
feeder, the VSPP is working islandingly. Afterwards, the
RR sends a command to reclose the CB without syncheck
relay (as in the case of metropolitan distribution systems).
This reconnection process would introduce voltage
difference between the grid and the VSPP. The worst
case magnitude voltage of ˆ2P
V would be seen across
equipment, causing damage the equipment and also the
VSPP.
C. Miscoordination between OC Relay and Fuse
As already detailed, in a distribution system with an
overhead feeder, the coordination between the OC relay
and the fuses follows the fuse saving scheme. That is,
each of the fuses will interrupt only sustained faults. In
Fig. 5 for a fault on a feeder, the OC (IDMT) relay will
trip the CB before the fuse operates (i.e., before minimum
melting time). As in the case of temporary fault, after the
CB is closed by RR, the feeder is able to continue to
supply the loads as normal.
However, a feeder with a VSPP connected, when there
is a temporary fault, the fault contribution from the VSPP
may be added to the fault current from the grid, resulting
significantly high fault current seen by the fuse compared
with the fault current seen by the current transformer (CT)
of the OC relay, which detects the fault current from the
grid only. This excessive high current condition may
result in the fuse to blow before the OC (IDMT) relay to
trip the CB. Therefore, the protection coordination
between the OC relay and the fuses no longer follows the
fuse saving scheme. The negative effect is that fuses are
forced to operate more often than necessary (interrupting
both temporary and sustained faults), which worsens
system reliability as well as increases maintenance cost.
This problem could be avoided if the microprocessor
based OC relays were able to be programmed in two
modes of operation: instantaneous and IDMT as in the
case of the protection scheme in the metropolitan
distribution systems.
D. False Tripping / Sympathetic Tripping
For the sake of discrimination, the protection scheme is
required to response to only faults within their clearly
designated zone for fault isolation. However, this may not
be always the case for the presence of VSPP. As an
example, let us see Fig. 6. The VSPP connected at feeder
2 supplies part of the fault current to the adjacent feeder
via the MV busbar, with the remaining fault current
coming from the grid (Igrid). As already detailed, the
contribution of fault current magnitude of a VSPP
generally depends on its type, rating, and connecting
location. The fault current from the VSPP may exceed the
pickup level of the OC relay of feeder 2. Therefore, it is
possible that CB2, which is located at the healthy feeder,
is forced to open before CB1 (disturbed feeder) will be
disconnected on time to clear the fault.
Figure 6. False tripping of protection system.
This case can happen particularly in the weak grid with
long feeders and with a definite-time OC relay because it
is to make sure that the relay can detect low short circuit
current and hence low pick up current is usually set. An
effective way to tackle this problem is to install a
directional OC relay. However, such events are unlikely
to happen in the system because
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 165
feeders are not too long (typically less than 10 km),
maximum penetration of VSPP is limited at
4MW/circuit (for 12kV) and 8MW/circuit (for
24kV),
in the event as shown in Fig. 6, CB1 will be
disconnected at once by Instantaneous OC relay at
1,200A (24kV) or 1,800A (12kV).
The false tripping described above can be simulated by
a CHP-based VSPP in a distribution network. The
cogeneration system for heat and electricity receives
natural gas from a pipeline network. The installed
electricity generation capacity is 9.6MW, 6.4 of which is
offered to sell under a contractual agreement with the
network operator. Note that under the regulation of
purchasing electric power from cogeneration, the
measured Primary Energy Saving (PES) has to be greater
than 10% a year for the minimum requirement in the
regulation. CHP-based VSPPs should regularly be
audited for PES measurement after be granted licenses
[8].
A single line diagram of the substation with the
connected DGs is shown in Fig. 7. Two DGs are
connected closed to the substation, to be specific, at the
beginning of two of 24 kV feeders: F415 and F426. The
maximum short circuit current and Thevenin impedance
of the substation is shown in Table I. From Table I, the
MVA short circuit of the substation is calculated by (3).
3 ×24kV ×6.6kA = 274.357MVA (3)
System parameters of the VSPP are given in Table II.
SingleBusbar(7)/BB3.4130.142
-23.971
SingleBusbar(6)/BB3.4130.142
-23.971
SingleBusbar(5)/BB
3.4130.142
-23.971
Sin
gle
Bu
sb
ar(
4)/
BB
3.41
30.
142
-23.
971
Sin
gle
Bu
sb
ar(
3)/
BB
3.41
30.
142
-23.
971
Sin
gle
Bu
sb
ar(
37
)/..
3.41
30.
142
-23.
971
Sin
gle
Bu
sb
ar(
36
)/..
0.00
00.
000
0.00
0
Sin
gle
Bu
sb
ar(
35
)/..
0.00
00.
000
0.00
0
Sin
gle
Bu
sb
ar(
34
)/..
0.00
00.
000
0.00
0
Sin
gle
Bu
sb
ar(
33
)/..
0.00
00.
000
0.00
0
Sin
gle
Bu
sb
ar(
32
)/..
0.00
00.
000
0.00
0
Sin
gle
Bu
sb
ar(
31
)/..
300.
777.
236
16.3
78
Sin
gle
Bu
sb
ar(
30
)/..
3.44
30.
143
-23.
985
Sin
gle
Bu
sb
ar(
27
)/..
3.41
20.
142
-23.
971
Sin
gle
Bu
sb
ar/
BB
3.41
30.
142
-23.
971
SingleBusbar(28)/..2.1660.32823.126
SingleBusbar(25)/..
3.4130.142
-23.971
SingleBusbar(29)/..2.1660.32823.124
Sin
gle
Bu
sb
ar(
2)/
BB
3.41
30.
142
-23.
971
SingleBusbar(18)/..3.4130.142
-23.971
SingleBusbar(17)/..3.4130.142
-23.971
Sin
gle
Bu
sb
ar(
1)/
BB
3.44
20.
143
-23.
984
Lin
e(2
9)
0.00
0.00
00.
000
0.00
0.00
00.
000
79
10
93
29
Lin
e(2
8)0.
000.
000
0.00
0
0.00
0.00
00.
000
Lin
e(2
7)
0.00
0.00
00.
000
0.00
0.00
00.
000
Line(26)
300.
777.
236
0.00
0
300.
777.
236
16.3
78
Line
29.2
10.
703
0.00
029
.21
0.70
30.
000
Line(25)
29.2
10.
703
0.00
0
29.2
10.
703
0.00
0
Line(24)
0.00
0.00
00.
000
0.000.0000.000
Line(1)
0.00
0.00
00.
000
0.00
0.00
00.
000
79109272 315Y
Lin
e(1
7) 0.00
0.0000.0000.000.0000.000
Line(16)
0.00
0.00
00.
000
0.000.0000.000
Su
bst
atio
n L
B
242.
385.
831
0.00
0
315Y
79109264 1000Y
7910927 2000Y
Lin
e(7
) 0.000.0000.0000.000.0000.000
Lin
e(6
) 0.000.0000.0000.000.0000.000
Lin
e(5
) 0.000.0000.0000.000.0000.000
Line(4)
0.00
0.00
00.
000
0.00
0.00
00.
000
2-Winding..
29.2
10.
703
0.00
0
29.212.5550.000
2-Winding..
29.2
10.
703
0.00
0
29.212.5550.000
G~
Gen2
29.212.5550.000
G~
Gen1
29.212.5550.000
Line(3)
0.00
0.00
00.
000
0.00
0.00
00.
000
Line(33)
29.2
10.
703
0.00
0
29.2
10.
703
0.00
0
Line(31)
0.00
0.00
00.
000
0.00
0.00
00.
000
Line(32)
271.
576.
533
0.00
0
271.
576.
533
0.00
0
Lin
e(3
0)
0.00
0.00
00.
000
0.00
0.00
00.
000
Lin
e(2
)
0.00
0.00
00.
000
0.00
0.00
00.
000
DIg
SIL
EN
T
Figure 7. Single line diagram of the substation with connected DGs.
Fig. 7 shows a simulated network and balanced fault at
the beginning of feeder F415 by DIgSILENT [9],
whereas Table III shows the simulation results of
balanced short circuit current.
Considering Table III, we can see that from the real
case of each DG with the rating of 6.75MVA, there will
be in total a fault current from the network and from both
DGs flow through feeder F415 at 6.533kA. Therefore, the
OC (instantaneous) relay, which operates at a pickup
current of 1.2kA, will trip the CB of feeder F415.
Because a short circuit current from the second DG flow
through the feeder F426 is only 0.703kA, the CB of
feeder F426 will be idle in this case. Another case study
of interest is with each of the DGs having a modified
capacity of 8MVA (maximum allowable capacity in one
feeder). The simulation result indicate that both DGs
generate fault current not significantly different from the
base case. Likewise, when a fault occurs on feeder F426
near the substation, the simulation gives a similar
tendency. This simulation of the worst case scenario
confirms that the system will not be affected by false
tripping.
TABLE I. MAXIMUM SHORT CIRCUIT CURRENT AND THEVENIN
IMPEDANCES OF SUBSTATION
Base
Voltage
(kV)
ISC,3Ph
(kA)
Thevenin Impedance (Ohms) X/R Ratio
R1 X1 R0 X0 X1/R1 X0/R0
24 6.60 0.249 1.989 0.208 1.846 7.993 8.874
1st DG
F415
F426
Balanced Fault
2nd DG
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 166
TABLE II. SYSTEM PARAMETERS OF VSPP
Parameter Unit
Type of machine Synchronous
Connection Star
Rated output 6750 kVA
Voltage 6600 V
Frequency 50 Hz
Power factor 0.8
Direct axis synchronous reactance 2.32 p.u.
Direct axis transient reactance 0.304 p.u.
Direct axis subtransient reactance 0.19 p.u.
Negative sequence reactance 0.214 p.u.
Zero sequence reactance 0.133 p.u.
TABLE III. SIMULATION RESULTS OF BALANCED FAULTS
Case CB ''
kS ( MVA) ''
kI (kA)
Base case
(VSPP 6.75 MVA)
F415 271.57 6.533
F426 29.21 0.703
Worst case
(VSPP 8 MVA)
F415 274.85 6.612
F426 33.26 0.800
E. Impact of UF Relay and Function of LTO/CTO for
Islanding Operation of DG
Let us revisit Fig. 2. The substation is supplied from
two transmission lines and has a LTO (or a CTO)
function. When one of the lines is faulted and causes the
entire substation without electricity, the LTO will
perform switching operation within 0.2 second to recover
power from the other transmission line. Therefore, the
customers connected to this substation will experience
momentary interruptions.
If there is a VSPP connected to a MV busbar (or a
feeder), when a fault occurs at one of the two
transmission lines, the VSPP, which is assumed to have
enough capacity to supply almost the customer demands,
is not disconnected. With such a weaker network than the
original grid, the system frequency may go so low that
the UF relay can detect the falling frequency and trip
some (or all) the CBs protecting the outgoing feeders at
0.15 second (after the first transmission line outage). At
0.2 second, the LTO will try to energize another
transmission line to the substation. However, because the
feeders’ CBs have been opened, the customers of those
feeders will experience sustained interruption instead of
momentary interruption. Eventually, the customers have
to wait until the system operators coordinate with the
owner’s VSPP and close the CBs of the feeders. It can be
seen from this case that the presence of undisconnected
VSPPs degrade the efficiency of the operation of LTO
and CTO.
F. Impact of Distance Relay with Fault Contribution
from VSPP
Let us consider Fig. 8. Without a VSPP connected in
the network, when a fault occurs, the distance relay can
see the impedance defined by (4), which is exactly the
same as the fault location. However, with the presence of
the VSPP, the fault impedance is changed to (5).
Comparing two equations, we observe that the value of
the second term of (5) is greater than that of (4) due to the
fault contribution from the VSPP (I2). Without the VSPP,
I2 is zero and (4) and (5) become equal. It can be
concluded that the distance relay will see the distance
longer than actual and the relay decides not to trip the CB
as the fault stays outside its protective zone. In fact, it
should have been operated and this situation is known as
“Protection Under Reach”. In the metropolitan
distribution systems, distance relays are commonly used
to protect the transmission lines. Therefore, with high
penetration levels of VSPPs connected to transmission
lines, accuracy of the distance relay becomes problematic.
1 2 1Relay, without DG 1 2
1
( )L LL L
Z + Z IZ = = Z + Z
I (4)
1 1 1 2 2 2Relay, with DG 1 2
1 1
( )+ (1 + )L L
L L
I Z + I + I Z IZ = = Z Z
I I (5)
DG
Distance Relay
ZL1 ZL2
I1
I2
I1+I2
Figure 8. Impact of VSPP on distance relay.
VI. SUMMARY OF POTENTIAL PROBLEMS ON CONTROL
AND PROTECTION SYSTEM FOR VSPP
CONNECTION
For the metropolitan distribution systems, when there
is a fault in its networks and VSPPs are obliged to be
self-disconnected with a specified time frame (namely
within 0.1 second), it will not pose any threat on the
associated control and protection system. However, going
beyond this time frame may affect control and protection
system in the following aspects summarized in Table IV.
TABLE IV. POSSIBLE NEGATIVE IMPACTS ON CONTROL AND
PROTECTION SYSTEM IN METROPOLITAN DISTRIBUTION NETWORKS
FOR LATE DISCONNECTION OF VSPP AFTER FAULT EVENT
Causes
Control &
Protection
System Affected
Possible Consequences
Contribution of
VSPP to temporary
fault current
RR Failed reclosing
(If temporary fault is not
cleared, the event will be changed from momentary to
sustained interruption)
Islanding operation of VSPP
RR Out of synchronous (Equipment and VSPP may be
damaged)
Islanding operation of VSPP
UF relay, LTO or CTO
function
Activation of UF relay (If LTO or CTO function
cannot be achieved, the event will changed from momentary
to sustained interruption)
Contribution of VSPP to fault
current
Distance relay Protection under reach
Note that for ungrounded or neutral grounded
impedance systems, when there is a fault, islanding
operation of VSPPs may affect the EF relays. However,
because the distribution systems are normally solidly
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 167
grounded, there is no problem attached to the EF relays
for this situation.
VII. CONCLUSION
Because different distribution systems have different
protection schemes and operating principles, impacts of
small DGs are, in fact, system-specific and therefore it is
difficult to generalize effective ways to solve all control
and protection related problems. A number of scenarios
for possible impacts of VSPPs on control and protection
system have been comprehensively investigated in this
paper based on the protection and control schemes of
metropolitan distribution systems. To accommodate the
government policies to promote VSPPs for renewable
energy and energy efficiency, the implementation of
adaptive protection system become challenging for the
system operators and planners to retain system reliability.
New functionalities and communication technologies
such as smart grid or substation automation would be
required to arrive at optimal solutions that are
compromised between government policies and technical
issues.
ACKNOWLEDGMENT
The authors would like to express their sincere
gratitude to Metropolitan Electricity Authority (MEA) for
the financial and technical support of this research work.
REFERENCES
[1] J. Coster, “Integration issues of distributed generation in
distribution grids,” Proc. of the IEEE, vol. 99, no. 1, pp. 28-39, Jan. 2011.
[2] A. Girgis and S. Brahma, “Effect of distributed generation on
protective device coordination in distribution system,” in Proc.
IEEE Large Engineering Systems Conf. in Distribution System, 2001.
[3] P. Fuangfoo, et al., “PEA guidelines of impact study and operation
of DG for islanding operation,” in Proc. IEEE Industrial & Commercial Power Systems Technical Conference, 2007, pp. 1-5.
[4] Metropolitan Electricity Authority. [Online]. Available: www.mea.or.th
[5] Grid Code, Metropolitan Electricity Authority, Thailand, 2008.
[6] Grid Code for Solar PV Rooftop, Metropolitan Electricity Authority, Thailand, 2008.
[7] N. Jenkins, R. Allan, P. Crossley, D. Kirschen, and G. Strbac, Embedded Generation, IET, 2008, pp. 65-67.
[8] Regulations for CHP-Based VSPP, Metropolitan Electricity
Authority, Thailand, 2008. [9] DIgSILENT PowerFactory - User Manual, DIgSILENT GmbH,
2013, ch. 22.
Noppatee Sabpayakom is a Ph.D. student at
the King Mongkut’s University of Technology North Bangkok (KMUTNB),
Bangkok, Thailand, Faculty of Engineering, Department of Electrical and Computer
Engineering. He holds a Bachelor of
Engineering Program in Electrical Engineering (KMUTNB, 2004) and a Master
of Science Program in Electrical Power Engineering (KMUTNB, 2008).
He is a lecturer at the Department of Electrical Engineering Technology,
College of Industrial Technology, KMUTNB. His main research interests are power system operation, power system reliability, and
distributed generation.
Somporn Sirisumrannukul is an associate
professor at the Department of Electrical and Computer Engineering, Faculty of
Engineering, King Mongkut’s University of Technology North Bangkok (KMUTNB),
Bangkok, Thailand. His main teaching
responsibility includes power system-related courses for undergraduate and graduate
programs. His research interests are power system operation and optimization, reliability,
and renewable energy.
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 168