Dynamic Line Rating

Dynamic Transmission Line Rating

Technology Review

208478-CR-001

30 July 2009

Dynamic Transmission Line Rating - Technology Review Revision No: 1 208478-CR-001 30 July 2009

The concepts and information contained in this document are the property of Hydro Tasmania Consulting. This document may only be used

for the purposes for which, and upon the conditions, the report is supplied. Use or copying of this document in whole or in part for any other

purpose without the written permission of Hydro Tasmania Consulting constitutes an infringement of copyright.

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Document information

Title Dynamic Transmission Line Rating

Technology Review

Client organisation Electricity Commission of New Zealand

Client contact John Gleadow

Document number 208478-CR-001

Project manager Angus Ketley / Geoff McDougall

Project reference 208478

Revision history Revision 1

Revision description

Prepared by Adonis Dino/Angus Ketley

Reviewed by Geoff McDougall

Approved by Donald Vaughan (name) (signature) (date)

Distributed to John Gleadow Electricity Commission of

New Zealand

(name) (organisation) (date)


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Executive Summary

The line rating represents the line current which corresponds to the maximum allowable

conductor temperature for a particular line without clearance infringements or significant loss

in conductor tensile strength due to annealing. This report deals primarily with the provision

of ratings which ensure that the required clearances are not violated. However, this can be

extended to other applications such as the management of annealing.

Transmission of electric power has traditionally been limited by conductor thermal capacity

defined in terms of a static line rating, which is based on a predetermined set of conditions.

These conditions are incorporated into the design of the line to take into account statutory

vertical clearances. In the last two decades, technologies and strategies have emerged to

allow the real-time or pseudo-real-time measurement of transmission line characteristics and

environmental conditions which enabled calculation of a real-time rating.

Transmission line ratings are determined using the conductor�s heat balance and are

dependent on the cooling effect of wind, warming due to line current, air temperature and

solar heating. By accurately monitoring these conditions, a corresponding line current limit

can be determined, thereby enabling the system operator to ensure that conductor

temperature does not exceed the design limit, and maximises line utilisation under all

conditions.

Dynamic line rating methods can be broken down into two main categories:

Weather based (indirect)

The line rating is determined by measurement of ambient climatic conditions, and by means

of the heat balance equation, to obtain the instantaneous conductor temperature rise

available, and hence, the allowable current that can be transferred.

Field data includes the following

(a) Wind speed measured by an anemometer;

(b) Wind direction;

(c) Air temperature;

(d) Solar heat intensity; and

(e) Conductor parameters


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Sag based (direct)

The line rating is determined by direct measurement of the conductor�s state. The actual

conductor temperature can be calculated using the predetermined relationship between

conductor position/tension and temperature. The heat balance equation is used to determine

the additional current that can be transferred before the conductor�s maximum operating

temperature is achieved.

This method still requires the indirect parameters to be measured. However, it has the added

benefit of providing a direct alarm should the conductor sag exceed or tension fall below a

predetermined point that represents a violation of the required statutory ground clearance.

Field data includes the following:

(a) Conductor position and or tension;

(b) Air temperature;

(c) Wind speed measured by an anemometer;

(d) Wind direction;

(e) Solar heat intensity;

(f) Line current; and

(g) Conductor parameters

Benefits and Considerations

The benefits of dynamic line rating include but are not limited to; improved system reliability

and safety, reduced and or deferred capital expenditure, increased efficiency of generation

resources, and lower rates for utility customers. However, the following issues need to be

considered, clearly understood, and evaluated accurately when applying either direct or

indirect rating strategies:

In line rating calculations, it is normally assumed that the conductors are in their �as

designed� and �as installed� condition. The line to which dynamic rating is to be

implemented should be inspected and its actual state confirmed prior to the application

of available strategies.


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While in general new connectors and line hardware runs cooler than the conductor,

older connectors and hardware may not. Unless periodically inspected, the use of

system-wide line ratings could lead to line failures.

It is common for measuring devices to be in a remote location. A high standard of

reliability and durability is vital for effective real-time rating determination, as well as a

robust power source and communication links. Orders of backup adjacent monitoring

stations are important and �watchdogs� are necessary to monitor the integrity of the

telemetered data elements.

Dynamic rating does not eliminate the risk of violating statutory ground clearances.

However, it does arm the network controller with much more information in order to

better manage contingent events and periods of uncharacteristic weather conditions.

The magnetic field of a transmission line increases with line ampacity, and increases at

ground level with increased conductor sag. Even though transmission line voltage is

managed to remain at the rated level, electric fields also need to be evaluated as its

strength at ground level is affected by conductor sag.

Overhead conductors elongate with time, temperature and tension. Despite the effects

of weather and loading, the conductors have to remain at a safe distance from

buildings, objects, and people or vehicles passing beneath and near the line at all

times.


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Contents

1. Introduction 1

2. Current State of the Art 3

2.1 Background 3 2.2 Line Rating Parameters 4

2.2.1 Measuring Climatic Conditions 4 2.2.2 Monitoring Conductor State 5

2.3 Dynamic Line Rating Advantages and Disadvantages 6 2.4 Consequences of too Optimistic Rating Assumptions 8

2.4.1 Clearance Violations 8 2.4.2 Conductor Annealing 8 2.4.3 Elevated Temperature Creep 8

3. Australian Experience 9

3.1 NEMMCO Requirements 11 3.1.1 Selection of Short Time Ratings 11

3.2 Tasmanian Rating Scheme 11 3.2.1 Background 11 3.2.2 Current Practice 12

4. New Zealand Opportunities 15

4.1 Background 15 4.2 The New Zealand Electricity Market 15

4.2.1 Market Design and Operation 15 4.2.2 Right of Capacity or Dispatch 16

4.3 Transpower�s Line Rating Process 16 4.4 Possible Opportunities 17 4.5 Implementation Considerations 19

5. References 20

Appendices

Appendix. A Existing Dynamic Line Rating Technologies


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List of figures

Figure 3-1 Typical Daily Operation of the NEM [11] 10

Figure 3-2 Completed Installation of a Load Cell to Monitor Conductor Tension in Tasmania 13

Figure 3-3 Completed Installation of a Weather Station in Tasmania 14

Figure A-1 Power DonutTM (© Courtesy of Underground Systems, Inc) A-1

Figure A-2 On-line temperature monitoring system comprised of Power DonutTM temperature sensors, and weather station and ground station RTU (© Courtesy of Nitech, Inc) A-2

Figure A-3 ThermalRateTM Sensor A-3

Figure A-4 Completed Installation ThermalRateTM Sensor A-3

Figure A-5 A weather station with a 3D ultrasonic anemometer mounted next to a standard propeller-type anemometer A-4

Figure A-6 Load Cells to measure tension A-5

Figure A-7 Completed Installation of Load Cells to measure tension A-5

Figure A-8 EPRI�s Video Sagometer mounted on a wood pole A-6

Figure A-9 Sagometer Target A-6

Figure A-10 Proposed Basic Configuration of Differential GPS A-7

List of tables

Table 2-1 Simplified Comparison of Common Dynamic Line Rating Methods 6

Table 2-2 Dynamic Line Rating System Advantages and Disadvantages 7

Table 3-1 Seasonal Alternate Rating in Tasmania 12


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1. Introduction

The Electricity Commission of New Zealand (EC) engaged Hydro Tasmania Consulting

(HTC) to prepare a review of the application of real-time thermal rating to overhead lines in

New Zealand, in an attempt to remove existing restrictions to the development of renewable

generation sources. This review includes a description of possible strategies, the present

state-of-the-art in Australia, specifically Tasmania, and a discussion of their possible

application within the New Zealand network.

This review does not provide a full assessment of the actual opportunities and benefits to

New Zealand. This review should form part of an overall feasibility study into the use of

dynamic rating technology in New Zealand, including a cost/benefit analysis.


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2. Current State of the Art

2.1 Background

The steady state thermal current rating of a conductor is the level of current that induces the

maximum steady state temperature for a given ambient condition. This temperature is based

on the heat balance equation where conductor heat gain equals conductor heat loss:

Pj + Ps = Pr + Pc

The heat gain term, Pj, represents the joule heating due to resistance of the conductor. Ps is

the solar heat gain (air temp and solar radiation). The heat loss term, Pc, represents natural

and forced convection cooling (wind) while Pr denotes radiation cooling. Secondary terms of

heat gain such as eddy currents and corona are not considered. Likewise, the evaporative

cooling heat loss term is also discounted.

Line resistance determines a level of Pj which, when combined with the remaining terms,

results in a conductor temperature that ensures required clearances are maintained for safe

operation of the line. This also ensures integrity of the conductor is not compromised through

annealing or loss of grease.

Rating strategies can be divided in two main groups; static and dynamic.

Static line ratings in their simplest form involve the predetermination of �worst-case� weather

conditions (the highest expected ambient temperature, a low wind speed, and a low

conductor emissivity) along the entire line for an extended period of time. This assures the

network operator that the conductor will not sag below the required vertical clearances at any

point in the line�s life. However, this assurance may be misguided during extreme climatic

conditions over and above the predetermined criteria. The static rating approach also has the

effect of underutilising the lines potential during climatic conditions below the predetermined

criteria. This is especially important for the wind developers, as the simplified static model

does not take ambient wind strengths or wind direction into account which would greatly

increase the possible rating of the line.

The Static rating model can be enhanced by the introduction of the line�s geography, the time

of year and the time of day; with ratings based on long term average conditions over the


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required periods. This approach results in a workbook or lookup table for each line and will

provide for better utilisation of the line during times where the actual climatic condition match

the long term averages. However, the possibility of missed opportunities is still present and

the probability of falling below the required clearances on days where the ambient

temperature exceeds the long term average is increased.

The application of a Dynamic line rating strategy is quite different since the predetermination

of the �worst-case� weather condition need only apply for the next 5 to 15 minutes. Clearly,

dynamic ratings are likely to be higher than the static rating but have the disadvantages of

volatility and unpredictability. [2]

Quasi-dynamic line ratings are typically calculated for longer time periods than dynamic

ratings and, while typically higher than static ratings, they can be more predictable and less

variable than dynamic ratings. [2]

2.2 Line Rating Parameters

The following section discusses the parameters which may be measured in order to

determine the rating of an overhead line. The current devices and technologies used to

measure these parameters have been detailed in Appendix B.

2.2.1 Measuring Climatic Conditions

Transmission network operators typically prepare load capacity limits based on known

seasonal conditions and wind speeds. These can be measured at locations throughout the

network and provide the representative conditions for the region in question and, hence, the

possible line rating.

(a) Ambient Air Temperature

Typically, during winter-time, a line can be rated at higher capacity because at low ambient

temperatures and the conductors will have a higher nominal load carrying capacity, due

largely to their ability to dissipate heat. During summer, the line rating may decrease due to

hotter ambient temperatures.

(b) Wind Speed and Direction

Wind exerts a very strong cooling influence on the conductors irrespective of the ambient

temperature and is, therefore, an important parameter in the determination of line capacity.

However, the volatility of wind data can create some problems. Management strategies may


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include discounting wind direction, averaging the wind speed over a time period and or

setting upper and lower bounds.

(c) Solar Radiation

The level of direct and diffuse solar radiation can be reliably measured but is also volatile due

to variable cloud cover. The real-time measurement of this parameter is generally discounted

in favour of established solar radiation charts for the particular region.

(d) Conductor Condition

The emissivity and absorptivity of a conductor depends on the conductor�s surface

properties. Emissivity is a measure of the thermal radiation from the conductor while

absorptivity is a measure of how much the conductor is heated by solar radiation, and hence

both influence the heat balance equation.

Conductor emissivity and absorptivity evolve over time as the conductor ages, collects dust

and other particles, or is affected by pollution.

Due to significant uncertainty in the current measure of emissivity and absorptivity, conductor

manufacturers are generally satisfied with assigning conservative emissivity and absorptivity

values to be used in dynamic rating of transmission lines. However, determining emissivity

and absorptivity values more accurately could reap significant savings in the form of reduced

conductor size or higher transmission line ratings.

2.2.2 Monitoring Conductor State

The actual state of the conductor can be measured to determine the amount of additional

current that can be transferred by the line before the predetermined maximum conductor

temperature is exceeded. The selection of the span to be measured is important, as it

should be representative of the whole line.

(a) Conductor Sag

Conductor sag is defined as the vertical distance between any point on a conductor and a

straight line between the two attachment points. The conductor sag at any point in a given

span is a function of the conductor�s mass per unit length (ùc), tension and the span length.

Therefore, using measured sag, the conductor tension can be determined. This tension,

when combined with the conductor parameters, can be converted into a corresponding

conductor temperature. The heat balance equation is then used to determine the additional


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current that can be transferred before the conductor�s maximum operating temperature is

achieved.

(b) Conductor Tension

Conductor tension can be measured directly using a load cell within the tension insulator

string. This value is then used in the same manner as described above.

This process is complicated by the presence of wind, which has the dual effect of cooling the

conductor and also effectively adding a component to the conductor�s unit load (ùc +ùh). To

establish the line rating, direct measurements of the conductor must be made as well as the

ambient conditions of wind and air temperature.

(c) Conductor Temperature

The simplest strategy is to measure conductor temperature directly. This inherently takes into

account all external influences. However, it does introduce the added complication of

maintaining sensor isolation from the telemetry device.

2.3 Dynamic Line Rating Advantages and Disadvantages

Table 2-1 shows a simplified comparison of the common methods available for dynamic line

ratings. A detailed assessment, including site specific considerations must be carried out for

individual applications of dynamic line ratings. [3].

Table 2-1 Simplified Comparison of Common Dynamic Line Rating Methods

Cost Accuracy

Monitor Purchase Cost

Install Cost

Maintain Cost

Line Outage

Measurement Reach

Normal Wind High Load

Normal Wind Low Load

High Load Low Wind

High Load High Wind

Weather low low low no variable good good low good

Conductor Replica low low low no variable good good good good

Temperature high medium high no point good low good good

Tension high high high yes multi span good low high good

Sag high medium high no multi span good low high good

Combined low medium medium yes multi span good good high good


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Table 2-2 summarises the possible advantages and disadvantages of the available systems

for dynamic line rating.

Table 2-2 Dynamic Line Rating System Advantages and Disadvantages

Dynamic Line Rating System Advantage Disadvantage

Weather Station Simplest method to implement;

Ideal for lines with relatively low current density of less than about 0.5 amps/mm2 [1]

Can be maintenance intensive;

At low wind speeds, the prediction of wind direction and persistence is nearly impossible.

Conductor Replica A simple method to implement Measured conductor temperature may or may not be a good estimate of the average conductor temperature along the line.

Direct Temperature Measurement

Effectively prevents the loss of conductor strength at high temperature

Measured conductor temperature may or may not be a good estimate of the average conductor temperature along the line.

Tension Ideal for heavily loaded transmission lines with current density greater than approximately 1 amp/mm2. [1];

High accuracy for use in a line section with multiple suspension spans having nearly the same tension.

Can be maintenance intensive.

Sag Ideal for heavily loaded transmission lines with current density greater than approximately 1 amp/mm2. [1];

High accuracy for use in a line section with multiple suspension spans having nearly the same tension.

Wind on conductor may affect the measured sag

Combined Safest and an ideal method of dynamically rating transmission lines

High premium cost


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Combinations of both weather and physical line monitoring is clearly more expensive but will

enable improved dynamic rating at both high and low conductor current densities. This is

particularly important for lines requiring dynamic rating that may be lightly loaded with

periods of near rated load.

2.4 Consequences of too Optimistic Rating Assumptions

When weather monitoring equipment produces a poor representation of weather conditions

for a dynamically rated conductor, the conductor rated temperature may be significantly

exceeded, which may result in:

2.4.1 Clearance Violations

During line design, the location and height of each structure in a pre-selected line route is

determined using an assumed maximum conductor temperature and required statutory

clearance to which the line is expected to operate. Transmission lines operated at

temperatures higher than the designed temperature may result in clearance violations.

2.4.2 Conductor Annealing

When hard drawn aluminum conductors are operated at temperatures beyond their rated

temperature, the aluminum starts to anneal. Annealing weakens the conductor and can

potentially cause the conductor to break under wind or ice conditions. [17]

2.4.3 Elevated Temperature Creep

Permanent or irreversible elongation of the conductor is known to occur due to operation of

the conductor at elevated temperature. The elongation is a result of molecular realignment of

the conductor�s base material. Elongation increases conductor sag and reduces the

conductor clearance to ground.


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3. Australian Experience

Australia�s National Electricity Market (NEM) consists of six (6) regions - Queensland, New

South Wales, the Australian Capital Territory, Victoria, South Australia and Tasmania. It is

the mechanism through which electricity is physically traded between registered generators

and market customers. Trade in electricity takes place dynamically, 24-hours-a-day and

seven-days-a-week.

The National Electricity Market Management Company Limited (NEMMCO)1 is responsible

for managing and operating NEM in accordance with the National Electricity Rules (NER).

NEMMCO is required to balance supply with demand by scheduling the most cost-effective

generators into production. Each Transmission Network Service Provider (TNSP) provides

the necessary information to NEMMCO to allow it to forecast electricity consumption in each

of the 6 regions, identify the capability of the transmission network to transmit electricity, and

to capture the present state of the power system. NEMMCO�s system issues dispatch

instructions every five minutes after a set of rules (referred to as the dispatch algorithm) is

applied to all dispatch bids, to determine the most cost-effective way to satisfy demand. The

calculation takes into account any loss factors to be applied, the technical limitations of the

system, and any bottlenecks or physical constraint that exist [11].

While forecasting demand and managing congestions on the network rely on the input and

monitoring of operators, many of the processes that constitute NEM trade are automated

through sophisticated information technology systems.

Figure 3-1 shows that each day, NEMMCO prepares and publishes a pre-dispatch schedule

that covers the supply and projected demand for all periods from the next trading interval to

the final trading interval of the next trading day.

1 From 1 July 2009 NEMMCO ceased operations. NEMMCO's roles and responsibilities have transitioned to the

Australian Energy Market Operator (AEMO).


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Figure 3-1 Typical Daily Operation of the NEM [11]

3.1 Line Rating Principles

NEMMCO�s ability to schedule generators to meet demand within NEM regions is sometimes

limited by the physical transfer capacity of the transmission lines used to facilitate importing

electricity. Each transmission line in the network has its own limit (line rating) which is

dictated by its location, design, and operator. To determine this limit, all TNSPs apply the

same secondary environmental parameters (except for wind speed and ambient temperature

� prevailing conditions in each geographic area is used) to the agreed equation in rating

transmission lines. Each TNSP provides Static rating and Real-Time rating (if available).

3.1.1 Static ratings

Static ratings of lines are used for network planning and operation planning purposes and for

operational circumstances where another form of rating is not available or necessary. This

should include a Day-time and a Night-time rating for at least each month of the year which

reflects the average-worse-weather conditions for the particular period that it is applied [3].

NEMMCO may request a continuous current rating and one or more short term current

ratings as defined in NER. NEMMCO may require a TNSP to advise different current ratings

to be applied under nominated conditions including, without limitation:


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1. Ambient weather conditions;

2. Seasons and/or times of day;

3. Ratios of the current during an emergency to the current prior to the emergency (taking

into account pre-contingent loading history where applicable); and

4. The period of loading at the nominated level.

3.1.2 Real-Time ratings

Real-Time rating (if available) is used over Static rating during favourable weather conditions

to maximise the use of the transmission system. To provide a stable value, this rating should

be calculated using the averaged measured data of weather conditions (accurate and timely

measurements of wind speed and air temperature) which characterise the entire length of the

line over a 10-minute period. The average measured data and calculated rating are provided

to an energy control centre in a suitable format to realise any potential benefits [3].

3.2 Tasmanian Rating Scheme

3.2.1 Background

Hydro Tasmania commenced a dynamic rating strategy for transmission lines in the mid

1990s. This was brought on by the need to maximize the N-1 transfer capacity of a radial

double circuit 110kV line in the event of a single circuit outage on that line during a major

outage of a local power station. As this maximised transfer capacity was required for only a

few months, this was achieved by the installation of an earthed �safety span� under the most

critical 110kV span. In the event of a trip, the remaining circuit would pick-up the entire load

until the sag approached the statutory ground clearance limit at which point the circuit would

trip. This was a relatively crude strategy and fortunately was not engaged. This short term

strategy led to the introduction of the CAT-1 system onto a number of lines deemed to be

system critical either under normal or contingent operation. Initially the scheme measured

tension only and used the span geometry to determine conductor temperature. The system

would issue an alarm should the tension drop below a designated limit. This system has

been augmented, and in some cases superseded, by the present day use of regional

weather stations to determine ambient conditions and hence available rating. The present

use of tension monitors is restricted, to provide a check for the main rating scheme and input

into the anti-icing scheme, as they signal the commencement of conductor icing.


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In 2005, ownership of these weather stations was transferred to Transend Networks Pty Ltd

(Transend), Tasmania�s network asset owner and system operator.

3.2.2 Static Rating of Transmission lines

The selection of a particular rating is time based identified as follows:

(a) 2 Season (Seasonal Rating)

Table 3-1 Seasonal Alternate Rating in Tasmania

Season Start End

Summer (TASSUM) Start of December End of February

Winter (TASWIN) Start of March End of November

(b) Temperature Based (Locational Rating)

To accommodate the temperature based ratings, temperature zones were defined

(CENTRAL, NORTH, NORTH WEST, SOUTH, WEST and FAR NORTH WEST) and each

end of all lines is allocated a temperature zone. Ratings for transmission lines within each

zone are provided on a 5 degree incremental basis from 0 degrees to 40 degrees as Static

rating data (historical static rating2).

3.2.3 Real-Time Rating Implementation

In Tasmania, the use of weather stations to support real time transmission line ratings is a

valuable and cost effective means of maximising the utilisation of, and minimising constraints

on, the existing transmission network.

Transend maintains 15 weather stations and has 19 transmission line conductor tension

monitors on 12 transmission circuits. The real-time weather information and measured

conductor tension from these devices is telemetered from remote weather stations to

Network Operation Centres (NOCs) via Remote Terminal Units (RTUs) at substations [9].

Rating software, �TRCalc�, uses circuit and conductor definition files, solar characteristics,

and the worst measured data (lowest wind and highest ambient) from the primary and

co-primary weather stations to calculate the real-time transmission circuit ratings using an

enhanced Cigré method [9] [3]. The wind speed used in the calculation is de-rated and must 2 The historical static rating is stored in a workbook which serves as the last level of back-up for TRCalc.


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be between 0.5 and 2m/s to produce a conservative result. Together with the actual bus

voltage, the Real-Time MVA rating is provided to NEMMCO for use in the 5 minute dispatch

process.

Transend has implemented a bi annual calibration, servicing and fault ratification program for

the Circuit Rating & Weather Monitoring Systems and associated communication links. In the

event of faults in the primary weather station, �TRCalc� uses the nearest among the two

weather station backups. If all else fails, �TRCalc� will then use the historical static rating

stored in a workbook. The fault is attended to at the earliest possible opportunity subject to

weather conditions and resource availability. Where faulty, custom or obsolete components

are identified, they are replaced with the latest commercially available parts where possible,

thus, gradually upgrading the weather and tension monitoring stations.

Figure 3-2 Completed Installation of a Load Cell to Monitor Conductor Tension in Tasmania


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Figure 3-3 Completed Installation of a Weather Station in Tasmania


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4. New Zealand Opportunities

4.1 Background

New Zealand�s transmission networks can be viewed as narrow and longitudinal, with areas

of demand (load) commonly some distance from the areas of significant generation.

Consequently, an extensive transmission system that allows generators to transfer energy

with minimum losses has to be developed and maintained. The National Grid consists of:

A high voltage AC transmission network made up of a grid back bone of 220kV and

110kV transmission lines that supply most of the major load centres; and

An inter-island 350kV high voltage DC link that runs from Benmore in the South Island

to Haywards in the North Island. This link includes 535km of double circuit overhead

line from Benmore to Fighting Bay, a 40 km submarine cable between Fighting Bay

and Oteranga Bay across the Cook Strait, and a further 37km double circuit overhead

line between Oteranga Bay and Haywards (ref. North & South Islands Network Maps).

4.2 The New Zealand Electricity Market

In 1996, a deregulated electricity market was introduced to New Zealand. Transpower, as the

system operator, is responsible for scheduling and dispatching generation at minimum cost

according to market rules. In addition, Transpower is also responsible for providing ancillary

services to the market including the management of reserves.

The New Zealand Electricity Market (NZEM) was previously a self regulated market

overseen by M-Co and the market surveillance committee. Since September 2003, market

regulation has been conducted by the Electricity Commission (Commission). The

Commission regulates the operation of the electricity industry and markets (wholesale and

retail) in accordance with the Electricity Act and government energy policy.

4.2.1 Market Design and Operation

The operation of NZEM relies on its Scheduling, Pricing, and Dispatch (SPD) process to take

generation offers and load bids that are used to determine the optimum clearing price for

electricity at each of approximately 250 nodes in the power system.


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The Scheduling mode of SPD is used to forecast a schedule of generation for 13 to 35 hours

ahead of time. Market participants (Generators and Loads) will make offers and bids to inject

or take off power at each node in the power system. Reserves are also offered by

Generators and contracted interruptible Loads. The offers and bids are associated with half

hour trading periods.

The SPD solver takes the offers and bids for a trading period and employs a linear

programming solution to match generation to load at minimum cost, subject to constraints in

the network. Forecast information on cleared generation and load (those that have had

successful offers and bids) as well as nodal prices is then fed back to the market

participants. The participants may then choose to alter their offers and bids and resubmit

these for a subsequent scheduling solution, up to two hours ahead of dispatch.

Dispatch occurs every five minutes through formal dispatch instructions sent electronically.

There is no Automatic Governor Control (AGC) in New Zealand.

4.2.2 Right of Capacity or Dispatch

A connection to the National Grid does not result in any capacity rights for that connected

party. Existing or future congestion on the national grid will affect the ability of a generator to

deliver its energy to the market.

Connection to the grid and participation in the electricity market does not guarantee that a

generator will be dispatched at all times by the System Operator. A generator will not be

dispatched where:

o There is sufficient lower priced generation to meed demand;

o There are constraints on the power system that limit the amount of electricity that

the generating unit can produce; or

o The generator is non-compliant with the Asset Owner Performance Obligations

(AOPOs) and technical codes.

4.3 Transpower�s Line Rating Process

Transpower currently applies a fixed summer and winter ambient temperature of, 30°C and

20°C, respectively, in a long-standing proprietary formula (Latta) developed by the

New Zealand Electricity Department (NZED) to establish the seasonal ratings

(Summer/Winter) of its transmission lines [18]. Transpower reported that the results of the


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Latta rating method are slightly higher compared to the results of modern rating methods,

such as IEEE [19].

In 1996, Transpower began exploring the available DLR systems to increase the capacities

of its major lines. Transpower have investigated the use of direct temperature monitoring and

tension monitoring systems. The latter was selected to be installed on two of its lines as a

trial setup utilising the hardware and software provided by Valley Group, USA. The analysis

of the results provided by the experiment demonstrated the real capabilities of Transpower's

transmission lines. The study concluded that dynamic line rating can contribute towards

efficient control of transmission lines [14]. However, Transpower was not able to advance

further due to the inflexibility of their legacy market system software [20].

In 2010, Transpower will be introducing additional shoulder seasonal (spring/autumn) ratings

across all transmission lines. Region specific line ratings are also planned to be applied.

Dynamic line rating will be trialled in one of its transmission routes in 2012.

4.4 Possible Opportunities

Dynamic line ratings are normally higher than static ratings and have the following

advantages:

1. Although it may require accurate prediction of values up to a day ahead, dynamic

ratings allow considerable market benefit due to more optimal generation dispatch

calculations.

2. When dynamic normal ratings are available, Transpower may easily ascertain that

normal load levels higher than the static rating do not require action, since the load

does not surpass the dynamic rating.

3. When dynamic emergency ratings are available, low probability, post-contingency

loads that exceed static emergency ratings may be less than dynamic emergency

ratings, thus avoiding the need for Transpower�s intervention to reduce load.

To gain the benefits of implementing dynamic line ratings, Transpower would need to focus

on overcoming constraints introduced by the current market processes and related software.

Some of the issues that need to be addressed would include:

1. Market process


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a. For pre-dispatch schedules, forecast weather information could be used by

employing a suitable tool for wind forecasting (Similar to wind generation

dispatch)

b. Temperature is telemetered to Transpower for use in dispatch time frame.

Therefore, for the 2-hour gate closure, combinations of forecast and

telemetered/local temperatures could be used.

It should be noted that the only case for weather input would be to change risk/security

criteria during storms. This is probably best done manually as at present.

2. Market software

Processing of dynamic rating information was not possible in the previously used market

software. This issue may now be addressed with the aid of the new market software, as

it can be designed to accommodate variable line rating data.

3. Existing line rating calculation tools in SCADA/EMS (Overload Calculation Tools)

a. Tools need updating to incorporate variable dynamic inputs of temperature

b. The calculations have to be circuit specific

c. Instead of 2 seasons or 4 seasons, the tools should be able to handle dynamic

data � e.g. temperature data of every time interval

d. Engines suitable for handling large amounts of data

e. Error checking / data validation

4. Accommodation of variable line rating during dispatch

In dealing with variable line rating during the dispatch time frame and sudden changes in

wind, fast acting monitoring tools in the DLR hardware, and similar processes as used

for wind generation need to be considered for the implementation of dynamic line rating.

5. Implications for market participants over longer time scales.

Variable transmission line ratings would require market participants to develop a more

sophisticated understanding of available transmission capacity, as ratings would have to


19

be predicted using static climate data. The degree of variability would be similar to wind

generation and demand variation, and at times be coincident with it.

4.5 Implementation Considerations

Where required, existing transmission lines should be surveyed and studied to find out the

critical span upon where the sensors are to be installed.

Installation of tension monitors on selected tension structures are to be properly coordinated

and scheduled as this requires a line outage.

The cost of weather monitoring stations using standard propeller-type or cup-type

anemometers is relatively low but they can be maintenance and calibration intensive. In fact,

use of such anemometers is generally discouraged, as its reliability and accuracy decreases

with time due to wear and tear of its mechanical components. Its use is further discouraged

in areas where ice is expected to build up. The 3D ultrasonic anemometer (an expensive

alternative) offers a more reliable and accurate readings.

In general, transmission lines in rugged terrain, like that of New Zealand, may have many

changes in direction. Thus, two or more sag or tension line monitors combined with two or

more weather stations may be necessary to achieve monitoring data that is a good

representation of the entire line. Whereas, when monitoring a straight section of transmission

line using either sag or tension line monitors and a weather station, only one monitoring

location is considered necessary. [1]

In general, for short lines less than 10km, a single line section should be monitored if

weather monitoring is the only form of field data used for dynamic rating. If the station used

does not observe wind direction, the line rating should be calculated assuming a fixed wind

heading angle of between 20 and 30 degrees from the line direction [1]. For longer lines

using weather monitoring, at least two line sections should have operating weather stations.

Ideally wind direction and wind speed would be measured simultaneously at several

locations within each line section.


20

5. References

1 Probabilistic and Predictive Circuit Thermal Rating Technology. Electric Power Research

Institute (EPRI), Palo Alto, CA: 2006. 1012409

2 Increased Power Flow Through Transmission Circuits: Overhead Line Case Studies and

Quasi-Dynamic Rating. Electric Power Research Institute (EPRI), Palo Alto, CA: 2006.

1012533.

3 TNSP Operational Line Ratings (Draft). February 2009

4 Transpower Transmission Code (Preliminary Issue). 4 March 2009

5 Guide for Selection of Weather Parameters for Bare Overhead Conductor Ratings. CIGRE

WG.B2.12, 2006. No.299

6 The ThermalRate System: A Solution for Thermal Uprating of Overhead Transmission Lines.

Power Technology, April 2004, Newsletter Issue 95.

7 Deb, Anjan K., Powerline Ampacity System � Theory, Modeling, and Applications. CRC

Press, 2000

8 Strategic Energy Research - Dynamic Circuit Thermal Line Rating. Public Interest Energy

Research (PIER), California Energy Commission, 1999. P600-00-036

9 Circuit Monitoring and Weather Monitoring System. Transend Networks Pty, Hobart

Tasmania, 2007. TNM-SY-808-0221

10 Holbert, K.E. and Heydt GT, Prospects for Dynamic Transmission Circuit Ratings. Arizona

State University, Arizona, USA

11 An Introduction to Australia�s National Electricity Market. National Electricity Market

Management Company Limited (NEMMCO), 2005. Australia

12 2008 Statement of Opportunities for National Electricity Market. National Electricity Market

Management Company Limited (NEMMCO), 2007. Australia

13 National Electricity Rules Version 28. Australian Energy Market Commission (AEMC), 2009.

Australia


21

14 Raniga, J.K.; Rayudu, R.K. Dynamic rating of transmission lines-a New Zealand experience.

Power Engineering Society Winter Meeting, 2000. IEEE Volume 4, Issue , 2000

Page(s):2403 - 2409 vol.4

15 Increased Power Flow Guidebook � Increasing Power Flow in Transmission and Substation

Circuits. Electric Power Research Institute (EPRI), Palo Alto, CA: 2005. 1010627.

16 Instrumentation for Increasing Power Flow: Needs, Concepts, Feasibility, and Benefits.

Electric Power Research Institute (EPRI), Palo Alto, CA: 2005. 1012534.

17 Morgan, V.T., The Loss of Tensile Strength of Hard-drawn conductors by Annealing in

Service. IEEE Transactions, Power Engineering Society. 1978.

18 Khot, Milind. Use of Probabilistic Temperatures in the Calculation of Transmission Line

Ratings using PLS-CADD. Transpower.

19 Transpower Response to EC Request for Information (EC letter dated 12 October 2005).

Transpower. 10 November 2005.

20 Simpson, Bob. Line Rating Review. Transpower. 12 June 2009.

The information contained in this document has been carefully compiled but Hydro Tasmania Consulting takes no responsibility for any loss or liability of any kind suffered by any party, not being the intended recipient of this document, in reliance upon its contents whether arising from any error or inaccuracy in the information or any default, negligence or lack of care in relation to the preparation of the information in this document.

23

Appendices


A-1

Appendix. A Existing Dynamic Line Rating Technologies

There is a distinction to be made between the real-time monitoring and dynamic rating of

overhead lines. Real-time monitoring is easy, but less useful in guiding operator actions than

providing dynamic ratings.

The following are examples of line monitors typically used by various utilities in facilitating

their dynamic rating schemes:

A.1 Conductor Temperature Sensors

The accuracy of conductor temperature sensors depends on how close the measured

conductor temperature at one spot is to the average line section temperature. It has been

observed that conductor temperature can vary significantly along its length due to large

variations in wind speed and direction.

A.1.1 Direct Conductor Temperature Monitors

Power DonutTM incorporate a clamp-on thermocouple attached directly to the energized

conductor (see Figure A-1) and linked to a ground station by radio (see Figure A-2).

Figure A-1 Power DonutTM (© Courtesy of Underground Systems, Inc)


B-2

Figure A-2 On-line temperature monitoring system comprised of Power DonutTM temperature sensors, and

weather station and ground station RTU (© Courtesy of Nitech, Inc)

Real-time conductor temperature, meteorological data, and line current provide continuous

input to a computer system where line ampacity is calculated. The computer system requires

special hardware and software for data acquisition from remote sensor locations via special

telecommunication networks.

A.1.2 Indirect Conductor Temperature Monitors

ThermalRateTM Sensor (see Figure A-3) is basically made up of two aluminium rods which

function as simple conductor replicas. The replica, chosen to be the same material and

diameter as the line conductor, is used to determine the line capacity by measuring how the

weather conditions heat and cool the conductor. [6]

The heated and unheated replicas are relatively long in order to minimize the affect of

thermal boundary losses. An internal thermocouple near the longitudinal centre measures

the temperature of each replica. The ThermalRate Sensor is located near the line and

pointed in the same direction as the line conductors (see Figure A-4) in order to experience

the same weather conditions as the line itself. [6]


A-3

Figure A-3 ThermalRateTM Sensor

Figure A-4 Completed Installation ThermalRateTM Sensor


B-4

A.2 Weather stations

Weather stations generally measure wind speed and direction, air temperature, solar

intensity, and rain. Knowing the line current and weather conditions in real-time, the

conductor temperature near the weather monitor can be calculated in real-time. [2]

Weather stations can include standard propeller-type anemometers, or the more

sophisticated 3D ultrasonic units. The latter are quite expensive, but have no moving parts

and are therefore very reliable, are very accurate even at low wind speeds, and can measure

vertical air movement. Figure A-5 shows a photograph of a weather station with both

anemometer types. [2]

Figure A-5 A weather station with a 3D ultrasonic anemometer mounted next to a standard propeller-type

anemometer

A.3 Line Tension Monitors

A load cell (see Figure A-6) placed on the grounded side of dead-end insulator string

(see Figure A-7) can be used to monitor line tension in real-time. The measured line tension

is then converted to average conductor temperature of the line section.


A-5

A base station is mounted on the structure and connected to the load cells by cable.

Communication to a base station is usually by spread spectrum radio and the units can be

solar powered. The line is normally de-energized when the load cells are installed. [2]

Figure A-6 Load Cells to measure tension

Figure A-7 Completed Installation of Load Cells to measure tension


B-6

A.4 Sag Monitors

Figure A-8 EPRI�s Video Sagometer mounted on a wood pole

The camera unit is typically mounted on one of the structures, as shown in (Figure A-8) of

the line being monitored, but it could be mounted on any appropriate structure in the vicinity.

A small, passive, reflective target (see Figure A-9) is placed on the conductor being

monitored. A low-power solid-state illuminator (diode laser or LED-based device) is mounted

with the camera to illuminate the target at night or when ambient light is not sufficient. [2]

Figure A-9 Sagometer Target

Image recognition algorithms residing in local firmware determine the position of the target

within the camera�s field of view. The target�s ground clearance is determined from that

position through a calibration procedure performed during installation. The conductor�s sag


A-7

and/or ground clearance is determined at any point along the span from the catenary

equation. [2]

The systems can be powered by solar-cell/battery arrangements, or by standard AC

distribution power if available at the site. The systems, including the targets, can readily be

installed on energised EHV transmission lines, or readily removed and relocated. [2]

A.5 Global Positioning System (GPS) Based

A recently proposed method to measure conductor sag known as Differential Global

Positioning System (DGPS) uses two GPS measurements with one at a precisely known

point which makes it possible to correct errors in the remote measurement. Only one phase

of a circuit would be instrumented in a critical span. From the base station, hard-wire is used

to bring position data to power system operators.

Figure A-10 Proposed Basic Configuration of Differential GPS

Documents

Dynamic Line Rating