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System Operator Report: Automatic Under-Frequency Load Shedding (AUFLS) Economic and Provision of 43

Automatic Under-Frequency Load Shedding (AUFLS)

Rate of Change of Frequency Testing & Recommendation

System Operator

20/07/2012

System Operator Report: Automatic Under-Frequency Load Shedding (AUFLS) RoCoF Testing of 43

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Date

Prepared by: System Operator July 2012

This report and the appendices are available to download from the System Operator website at www.systemoperator.co.nz


TABLE OF CONTENTS 1 EXECUTIVE SUMMARY ................................................................................................................................. 4 2 INTRODUCTION ............................................................................................................................................ 6

2.1 Background and Purpose .................................................................................................. 6 2.1.1 Providing comments to the System Operator ................................................................... 8

2.2 Survey of North Island Distributors ................................................................................... 9 2.2.1 The AUFLS testing process (current and future) .............................................................. 9 2.2.2 Cost recovery of scheme changes.................................................................................. 10 2.2.3 The reliable use and implementation of the RoCoF scheme .......................................... 10

2.3 Objective of the AUFLS Scheme .................................................................................... 11 2.3.1 The challenges of achieving block discrimination ........................................................... 11 2.3.2 The benefit of faster AUFLS operation ........................................................................... 12 2.3.3 Proposed benefit of RoCoF ............................................................................................ 13

3 INTERNATIONAL USE OF ROCOF FOR AUFLS ............................................................................................. 14 3.1.1 Use of RoCoF in Tasmania ............................................................................................ 15

4 CALCULATING THE RATE OF CHANGE OF FREQUENCY .................................................................................... 16 4.1.1 Frequency Calculation .................................................................................................... 16 4.1.2 Rate-of-Change-of-Frequency Calculation ..................................................................... 17

5 ROCOF BENCH TESTING - METHODOLOGY .................................................................................................. 18 5.1 Initial RoCoF relay performance requirements ............................................................... 18 5.2 Test equipment arrangement and process ..................................................................... 20 5.3 Test Cases ...................................................................................................................... 21 5.4 Settings Used .................................................................................................................. 22

5.4.1 The role of the frequency guard ...................................................................................... 22 6 BENCH TESTING RESULTS ......................................................................................................................... 23

6.1 Bench Testing Results Summary .................................................................................... 23 6.1.1 Logic ............................................................................................................................... 23 6.1.2 Stability ........................................................................................................................... 23 6.1.3 Uniformity and Response Time ...................................................................................... 24 6.1.4 Accuracy ......................................................................................................................... 25

7 INACCURACY FROM POWER SYSTEM OSCILLATIONS ..................................................................................... 26 7.1 Limitation in calculating frequency and RoCoF ............................................................... 26 7.2 Impact of system oscillations on RoCoF Calculations .................................................... 27 7.3 Oscillations during Under Frequency Events .................................................................. 29 7.4 Mitigating the effects of power system oscillations ......................................................... 31

7.4.1 The Frequency Guard ..................................................................................................... 31 7.4.2 Increasing the RoCoF calculation time ........................................................................... 32 7.4.3 Low-Pass Filtering .......................................................................................................... 34

8 ROCOF RECOMMENDATION SUMMARY ....................................................................................................... 37 9 SOUTH ISLAND BLOCK 2 INCREASE ............................................................................................................ 38

9.1 Background ..................................................................................................................... 38 9.2 Cost Benefit Analysis Methodology ................................................................................. 39 9.3 Cost Benefit Analysis Results ......................................................................................... 41

10 NEXT STEPS ............................................................................................................................................. 42 11 SUMMARY OF QUESTIONS FOR CONSIDERATION ........................................................................................... 42 12 ACKNOWLEDGEMENTS ............................................................................................................................... 43 13 REFERENCES ............................................................................................................................................ 43


1 Executive Summary

Automatic Under-Frequency Load Shedding (AUFLS) is vital to managing power system security and acts as a safety net to prevent power system collapse and blackout following large, rare system events.

New Zealand’s AUFLS scheme is made up of a minimum of two 16% blocks in each island. This means that 32% of customer demand can be automatically disconnected to restore stability to the power system. In 2009, the System Operator determined that a technical review of these arrangements was required.

The results of the technical review concluded that the Reserve Management Tool (RMT) and the under-frequency products available for use by the System Operator should prevent system collapse from large defined risks (such as the sudden disconnection of HVDC bi-pole) at all times1.

However, the results of the technical review demonstrated that the operation of the current AUFLS scheme could result in over-frequency and potentially system collapse from some of the defined risks. Further, the current AUFLS scheme does not provide the System Operator with sufficient confidence that it will prevent the system from collapsing following undefined larger risks.

The System Operator identified technically feasible options for improving the performance of the AUFLS system and under took a cost-benefit analysis of those options. One of the options proposed included the use of rate-of-change-of-frequency (RoCoF) relays. The analysis revealed the scheme, using RoCoF elements, resulted in the largest benefit for North Island.

Following the cost-benefit analysis the System Operator conducted a literature review which revealed the use of RoCoF relays for under-frequency load shedding schemes is rare. In addition, some North Island distributors voiced concern whether the RoCoF relays would increase risk of trip due to mal-operation.

The System Operator tested several RoCoF relays to address the issue of relay reliability and to identify the Code requirements to ensure the implementation of a reliable AUFLS scheme. This report presents the findings and conclusions of the RoCoF relay bench testing.

The testing process aimed to verify the logic, stability, uniformity, response time, and accuracy of the relays to identify the requirements for reliable use of RoCoF relays on the New Zealand power system.

Using RoCoF technology for the proposed AUFLS scheme requires a level of logic to implement a frequency guard and backup under-frequency settings. All of the relays tested had some form of logic programming that allowed users to implement the required combinations of elements. Each manufacturer has a different way of arranging and setting their logic elements and care is required to ensure the desired result is achieved when programming and setting the various relays.

The bench testing aimed primarily at ensuring the RoCoF relays remain stable during a range of system disturbances that would not normally result in AUFLS operation. The tests results indicate that the sample RoCoF relays were stable under all of the disturbances to which they were subjected. The guard frequency used in these tests likely plays a major role in preventing the undesired operation of RoCoF.

1 This could require the transfer on the HVDC link to be limited to below its maximum capability under certain

system conditions to ensure power system security.


A level of uniformity is required to ensure the installed AUFLS system achieves the desired levels of operation should a range of different relays be used. The tests demonstrated that the sampled RoCoF relays do not behave uniformly when subjected to identical inputs. This is not necessarily a problem provided that relays comply within a specified maximum allowed response time.

The RoCoF relays must be able to accurately calculate the rate of system frequency decay when subjected to a variety of grid conditions to be used for load shedding. The consequences of inaccuracy could result in the relay mal-operating by tripping inappropriately. The relays performed within the specified accuracy margins for clean frequency decays. However, the testing results did verify that RoCoF relays are susceptible to calculation inaccuracy due to system oscillations.

RoCoF relays use low pass filters to manage the impact of power oscillations on the RoCoF calculation. Ideally, the low pass filters would attenuate the noise and oscillations while leaving the slower-changing underlying RoCoF relatively unaffected. It is recommended that a filtering standard be required for all relays, regardless of the RoCoF settings.

The System Operator has concluded the use of RoCoF relays is feasible on the New Zealand system provided the following requirements are specified in the code:

1. The required RoCoF calculation filtering to mitigate the inaccuracy caused by an identified level of oscillations during under-frequency events.

2. The maximum response time required for the RoCoF settings proposed so that the various relays provide a degree of uniformity.

3. A test regime to verify the logic elements of the installed relays meet the performance requirements.

The System Operator requests that Industry reviews the tests results and provides feedback on the level of confidence in the stability of the relays.

For the South Island, the System Operator believes it is prudent to hold off proposing new AUFLS schemes until there is further clarity of the future of the frequency band and AUFLS provision at the Tiwai grid exit point.

In the interim, the System Operator recommends the trip setting of the second AUFLS block is increased to 46.5 Hz to improve the capability of the current South Island AUFLS scheme.

The implementation cost for the change is assessed as low so a high level cost-benefit analysis was undertaken to avoid costly studies. The cost-benefit analysis suggests the setting increase will be economically beneficially.

The testing results and South Island benefit analysis were presented and discussed with industry at System Operator workshops commencing on 9th July 2012. Following the workshops, the System Operator will consider industry feedback before making recommendations to the Electricity Authority regarding the future AUFLS arrangements in both islands.


2 Introduction

2.1 Background and Purpose

AUFLS is the acronym for Automatic Under-Frequency Load Shedding and describes the set of relays in New Zealand which automatically trip blocks of load following a severe under-frequency event to seek to restore the system frequency.

These relays are relied upon by the System Operator to prevent the collapse of the system from under-frequency following events which have the potential to cause a system blackout.

New Zealand’s current AUFLS scheme is made up of a minimum of two 16% blocks in each island. This means that 32% of customer demand can be automatically disconnected to restore stability to the power system.

The AUFLS obligations are set out in Part 8 Schedule 8.3 Technical Code B of the Electricity Industry Participation Code (“the Code”). Distributors2 in the North Island and Grid Owners in the South Island are required to provide a minimum of 2 x 16% blocks of AUFLS as described in Table 1 below:

Table 1 Required AUFLS settings

North Island South Island

Block 1 Block 2 Block 1 Block 2

Trip Frequency (Hz) 47.8 47.8 47.5 47.5

Time Delay (sec) 0.4 15 0.4 15

2nd

Trip Frequency (Hz) - 47.5 - 45.5

Time Delay (sec) - 0.4 - 0.4

The current AUFLS arrangements are largely based on historical practice. In 2009, the System Operator determined that a technical review of these arrangements was required. This technical review was completed in 20103.

The results of the technical review concluded that the Reserve Management Tool (RMT) and the under-frequency products available for dispatch should prevent system collapse from large defined risks, such as the sudden disconnection of HVDC bi-pole, at all times4.

However, the results of the technical review demonstrated that the operation of the current AUFLS scheme could result in over-frequency and potentially system collapse from some of the defined risks.

There is also concern that the current AUFLS scheme does not provide the System Operator with sufficient confidence that it will be effective to prevent the system from collapsing following a range of undefined larger risks.

The technical review found the lack of discrimination between AUFLS blocks can result in more AUFLS tripping than is required which leads to dangerous over-frequency conditions and potential combined cycle generators tripping, which then can cause another under-frequency event and, without any AUFLS remaining, system collapse.

To address the issues identified in the technical review, the System Operator worked through a process of identifying technical feasible options aimed at improving the performance of the AUFLS system and undertaking cost-benefit

2 The obligations includes consumers connected directly to the grid, also known as ‘direct connects’

3 The technical report can be found at http://www.systemoperator.co.nz/n3210,521.html.

4 This could require the transfer on the HVDC link to be limited to below its maximum capability under certain

system conditions to ensure power system security.

http://www.systemoperator.co.nz/n3210,521.html


analyses of those options. One of the options proposed included the use of rate-of-change-of-frequency (RoCoF) relays.

The economic analysis revealed the scheme using RoCoF elements resulted in the largest benefit range for North Island5. The results of the economic analysis were presented to industry in August 2011 and industry feedback was received on the proposed changes.

Any changes to the AUFLS scheme will require changes to the Electricity Industry Participation 2010 Code (Code). In November 2011, the Electricity Authority agreed that the System Operator would complete the RoCoF testing work required to compile code change recommendations. The System Operator finalised the scope of the project after collecting feedback from the Electricity Authority and industry participants.

The purpose of RoCoF testing work is to determine the requirements that will provide a reliable, secure, and efficient RoCoF AUFLS system to deliver greater certainty on system integrity during major under-frequency events. To make the code recommendations the RoCoF testing work aimed at the following:

evaluating the technical requirements for the reliable use of RoCoF relays; outlining the required installation design outcomes to provide flexibility in

meeting the requirements; and developing a proposed implementation plan to maintain system security

during rollout.

The work also included completing a high level cost-benefit analysis of proposed South Island setting changes. The purpose of this report is to set out the System Operator’s findings on the above6 and to seek industry and stakeholder views on such findings. A high level overview will be presented here while a detailed technical report is attached as Appendix B. Once the System Operator has considered any comments received in respect of this report’s findings, it expects to make any code recommendations to the Electricity Authority.

5 The economic report can be found at http://www.systemoperator.co.nz/aufls

6 This excludes the code recommendations themselves and the implementation plan which will be published

at a later date.


2.1.1 Providing comments to the System Operator

The System Operator requests comments on this report’s findings by Friday 17 August 2012 so that it can continue the next phase of work.

The System Operator’s preference is to receive submissions in electronic form. The electronic version should be emailed with the phrase “Submissions on AUFLS scheme options” in the subject header to [email protected]. Hard copies can be posted to the following address:

Mr Justin Blass Transpower New Zealand Limited Level 7, Transpower House 96 The Terrace PO Box 1021 Wellington New Zealand

The System Operator will acknowledge receipt of all submissions electronically.

The System Operator values openness and transparency and therefore comments will be published on the System Operator’s website. Those providing comments should discuss with the System Operator any intended provision of confidential information, prior to sending the information.

mailto:[email protected]


2.2 Survey of North Island Distributors

Following the economic review the System Operator engaged in discussions with North Island distributors7 to ensure the concerns of the obligated parties were captured prior to completing the next scope. The discussions were grouped into the following categories:

Load allocation (specifically the management of critical load) Relay ownership (specifically management of relays at the Grid Exit Point

Level) RoCoF Feasibility and Implementation Market Arrangements and load optimisation

The discussions helped to shape the work being carried out in the AUFLS programme. The views presented during these discussions were varied. However, the commonly re-occurring themes were8:

1. The AUFLS testing process (current and future) 2. Cost recovery of required scheme changes 3. The reliable use and implementation of the RoCoF scheme

In light of the industry discussion and the present scope of work the System Operator invited industry participants to participate in an advisory group. The purpose of the advisory group was to provide additional input into the testing of the RoCoF relays.

Participation in the advisory group included providing comment on the testing methodology and discussing these comments via teleconferences9. The advisory group had nine members from different distribution companies that provided input into the initial testing process.

The System Operator would like to thank the participants for their input and co-operation with the development of the RoCoF relay bench testing process.

2.2.1 The AUFLS testing process (current and future)

The Code currently requires the testing of AUFLS operation every 4 years regardless of relay type10. However, various parts of the Code stipulate the testing requirements for analogue, non-self-monitoring digital, and self-monitoring digital protection systems11. The self-monitoring digital protection systems are required to be tested every 10 years while the others are every 4.

Several of the distributors suggested bringing consistency to the code requirements by allowing AUFLS provided on self-monitoring digital protection systems to be tested every 10 years.

In addition, some expressed concern over the clarity of roles when AUFLS operation is provided at the Grid Exit Point (GXP) level. Distributors had a range of experiences managing the testing and status of AUFLS relays installed inside Transpower controlled sites.

Although the obligation for testing and operation of the relays remains with the distributor; there appears to be no clear process to enable the coordination required. It was suggested that a process be developed to ensure consistency

7 Due to availability and timing constraints the following parties were not surveyed; Centralines, Scanpower,

and Electra. 8 Other concerns included proper incentives for scheme development, .4s IL trip time proposal, and visibility of

load aggregation and AUFLS provision. 9 Minutes from the advisory group teleconferences can be found here http://www.systemoperator.co.nz/aufls

10 Schedule 8.3, Technical Code A, Appendix B, clause 6 and 7

11 Schedule 8.3, Technical Code A, Appendix B, clause 13(a),(b), and (c)


and clarity in managing GXP level AUFLS relays. The Grid Owner has been notified and is establishing a person responsible for this coordination.

Further concern was expressed with the development of the testing process for the proposed use of RoCoF elements. Many distributors suggested further work is done to develop the testing process and to ensure the practicality of testing is considered. The System Operator is considering the different technical requirements and the testing process, and expects to publish guidelines with code recommendations.

2.2.2 Cost recovery of scheme changes

Another common concern raised by the distributors was their ability to budget for and recover the costs associated with any code changes. A two year lead time is considered necessary to roll out changes to the AUFLS scheme.

It was suggested that any implementation plan would need to consider the cost recovery process. Further consideration will be included as part of the code recommendation and implementation plan.

2.2.3 The reliable use and implementation of the RoCoF scheme

The third common concern raised by distributors revolved around the use of RoCoF elements to trigger load. Several distributors queried whether the use of RoCoF elements would increase the risk of mal-operation of AUFLS.

In addition to mal-operation, other expressed concerns about their ability to implement the additional RoCoF blocks. This was of particular concern where recloses are currently used to provide AUFLS.

The System Operator completed testing work, which is outlined in this report, to specifically address the concern of mal-operation of the RoCoF relays. The testing methodology and results are highlighted in a further section. Concern over implementation will be considered as part of the code design process.


2.3 Objective of the AUFLS Scheme

Before determining the performance requirements of the RoCoF elements for under-frequency load shedding it is important to consider the primary objective of the AUFLS scheme; to perform load reduction to match significant generation loss to arrest the frequency decay and return the frequency to the normal operating while maintaining the frequency within the safe operating range.

The performance criteria of the AUFLS scheme are:

Arrest the system frequency within the safe operating range Return the system frequency to the normal range while remaining within the

safe operating range.

Achieving the stated objected of the AUFLS scheme relies upon two key factors: block discrimination and speed of operation.

2.3.1 The challenges of achieving block discrimination

Ideally the AUFLS scheme will shed only the amount of load required to arrest the system frequency and return it to normal. Excessive load shedding can cause the frequency response to ‘overshoot’. This is of special concern for the North Island, as there are significant amounts of thermal generation which will trip on over-frequency protection when operating above 52 Hz.

If an over-frequency event occurred following AUFLS operation, then it is possible some thermal units may trip and there will be a second dip in system frequency which may not be recoverable as there may be insufficient AUFLS remaining to balance this subsequent loss of generation.

Having a number of small blocks enables better “matching” of load shedding to generation loss which reduces the potential for over-frequency and over-voltage12.

An effective AUFLS scheme made up of a number of small load shedding blocks requires the blocks to be able to each operate before the next block triggers; this is known as discrimination. In general, discrimination ensures the scheme sheds only the load required to arrest and return the frequency to the normal range13. As the rate of frequency decay increases following major system disturbances, so does the difficulty in maintaining discrimination between AUFLS blocks.

The New Zealand system currently has only 1 Hz range (48 Hz – 47 Hz) within which AUFLS blocks must operate within. The limitation is imposed to ensure that the AUFLS scheme does not operate for a contingent event. This limits the ability to obtain block discrimination. The current scheme will not achieve discrimination for most AUFLS events.

12

System Operator, “Appendix C - Calculating the df/dt settings,” found here: http://www.systemoperator.co.nz/f5573,67414593/AUFLS_Stage_II_Appendix_C.pdf 13

Alternate scheme designs are feasible that may not require discrimination between blocks provided there are numerous blocks that are small enough to achieve some level of discrimination (i.e. between every other block). See Ireland’s and Tasmania’s systems.

http://www.systemoperator.co.nz/f5573,67414593/AUFLS_Stage_II_Appendix_C.pdf


Figure 1 – Overview of requirements for block discrimination

Figure 1 demonstrates the challenge for the current AUFLS scheme to achieve discrimination. Relays equipped for AUFLS utilise a time delay to ensure the calculated frequency stays at or below the trip point for a set amount of time; this is represented in Figure 1 by td1. The time delay is often adjustable and is chosen by the obligated parties when tested (test reports show it can range from 120-280ms). Once the time delay expires, and the conditions are met, the relay sends a trip signal. Time taken for the signal to be processed and the circuit breaker to open is represented by td2.

The time from the relay signal sent to circuit breaker operation is not tested and so assumptions are made by obligated parties to ensure the code requirements are met (anywhere from 80ms -120ms has been assumed).

For an event that causes the frequency to fall at a rate of 1 Hz/s the second block of AUFLS has already initiated its time delay before the first block of AUFLS has tripped. If the frequency does not increase above the block 2 trip point (47.5 Hz) before the time delay has expired block 2 will trip. The system has very little time to recover and prevent the second block of AUFLS from triggering, which will likely result in frequency over-shoot.

2.3.2 The benefit of faster AUFLS operation

In addition to block discrimination, the speed of operation was shown in the AUFLS technical review to be critical to arrest the system frequency following major system disturbances.

Increasing the speed of operation enables additional time for generators to respond which will assist in arresting the frequency. Tripping load at higher frequencies increases the minimal frequency reached following disturbances and enables the system to respond to larger disturbances.


2.3.3 Proposed benefit of RoCoF

The existing AUFLS scheme uses under-frequency elements to trigger the AUFLS blocks. This means the AUFLS will trip once the frequency has dropped below a set frequency for a set period of time.

Another way to trigger AUFLS is to use rate of change of frequency (RoCoF) elements, often referred to as df/dt. These elements will trip the AUFLS blocks once the frequency fall has reached a certain speed.

One of the benefits of utilising the RoCoF elements is that it enables AUFLS to be triggered at frequencies higher than the current contingent event target frequency; 48 Hz14. Triggering above 48 Hz increases the speed of operation and will raise the minimum frequency following most large events. This enables the AUFLS scheme to cover for larger events while maintaining the system frequency above 47 Hz limit.

Triggering AUFLS blocks above 48 Hz also enables additional blocks to be added and maintain block discrimination. A critical factor in block discrimination is the rate of frequency fall. RoCoF elements enable the AUFLS blocks to be arranged to better match of load shed and generation loss15. This should reduce the amount of unnecessary load shed and also reduce the amount of frequency over-shoot that can follow AUFLS operation.

The cost benefit considered the reduction in load shed as the key benefit for using RoCoF relays and resulted in a net benefit range of $16 million to $89 million over 15 years. In addition increasing the minimum frequency following AUFLS operation enables greater levels of DC transfer, without the procurement of additional reserves.

14

This ability requires discrimination from CE events. Previous recommendations suggest increasing the speed of interruptible load to ensure greater difference between the speed of frequency fall for CE and ECE/Other events.

15 Articles in the literature review outline that the initial rate of frequency fall after an event is an indicator of

the power system imbalance. If factors such as the system inertia can be calculated there is potential future of AUFLS scheme to have an adaptable method to load shedding [1][2].


3 International Use of RoCoF for AUFLS

An investigation was carried out to identify systems utilising RoCoF elements in their under-frequency load shedding schemes and to learn from their experience.

The systems investigated revealed that the use of RoCoF elements for under-frequency load shedding schemes is rare. The majority of systems rely on under-frequency triggering elements and do not report the same drivers (i.e. low system inertia and narrow AUFLS frequency band) that would necessitate the use of RoCoF elements.

A more common use of RoCoF elements is for generator unit islanding protection. However, there are systems that have employed RoCoF for under-frequency load shedding and others that are investigating RoCoF as a potential option16. Most notable, for the current investigation, is the Tasmanian power system.

The Tasmanian system provides an ideal case study as their grid is similar to the New Zealand grid. They have been using RoCoF in their AUFLS system since the late 1980’s17.

The literature review did reveal that Transpower considered the use of RoCoF elements in the 1990’s. However, the reasons the relays were not utilised could not be found [3].

16

Other systems investigating or using RoCoF for under-frequency load shedding include Sri Lanka, Qatar, and regions of India.

17 The information summarised comes from D. E. Clarke, “Tasmanian experience with the use of df/dt

triggering of UFLSS.” See reference [4] for further information.


3.1.1 Use of RoCoF in Tasmania

Tasmania began developing an under-frequency load shedding scheme following a system wide black out in 1979 caused by the mal-operation of protection equipment. They sought to develop a scheme that would protect their system from a collapse following what they classified as “reasonably foreseeable contingencies” while not operating for events covered by reserves.

In 1986, they formally defined reasonable foreseeable contingencies, which included system disturbances up to 60% of total system load and events that result in system splits creating small sub-systems with 50% island load to generation deficit. The initial AUFLS scheme designed to provide this level of protection included the use of RoCoF elements.

The initial RoCoF settings were identified by:

investigating the worst case RoCoF following events covered by reserves to ensure discrimination; and

identifying a RoCoF measurement initiation frequency (similar to a frequency guard) to reduce the possibility of mal-operation.

identifying back-up under-frequency settings to cater for slow decaying disturbances.

The resulting design of the AUFLS scheme included six load shedding steps18. RoCoF elements were utilised on the first two steps and were connected to major industrial load, as their load profiles are relatively constant.

In 2003, the AUFLS scheme was reviewed for changes to the Tasmanian frequency standards to incorporate modern gas turbines and wind generation. The operating range of AUFLS was reduced from 2.2 Hz to 1.5 Hz.

Tasmania noted that the reduction was only achieved with acceptable discrimination and acceptable over-shoot by using rate of change of frequency settings to operate early and slow down the frequency decline for large disturbances.

Further re-design of the AUFLS system took place with the installation of the Basslink (HVDC connection to Australia). The review found that the Tasmanian power system is prone to oscillations during large disturbances and the use of average RoCoF was required. This concept will be discussed in more detail later in this report.

Tasmania reported the benefit of using of RoCoF elements together with higher under-frequency settings improved the minimum system frequency reached for AUFLS events and resulted in at least one less block of load shed.

The review of the RoCoF scheme in 2008 reported that there have been no recorded instances of the RoCoF elements of relays not operating correctly [4].

18

The load shedding steps are not a set percentage of load as in New Zealand. The size of load shed in each step is different and appears to be organised around load type (major industrial and retail load) with the majority of load provided by major industrials.


4 Calculating the rate of change of frequency

The current AUFLS scheme uses relays to calculate the system frequency in real time and then shed load when a set frequency level has been met. The system frequency is not a measurable value; it must be calculated from another waveform (generally voltage). The rate of change of frequency is also a calculated value.

The RoCoF calculation methods are important as they can impact the reliability of the relays. To better understand the calculation limitations of RoCoF relays the methods were investigated and necessary tests were constructed to ensure their reliability.

4.1.1 Frequency Calculation

There are many methods to calculate the system frequency. A more detailed analysis of calculation methods exists in Appendix C to illustrate the concept. The following section will provide a very brief summary of a common method.

Most relays calculate the system frequency from the voltage waveform. The method each relay uses to calculate frequency is often not specified. A simple method is called the ‘zero crossing’ method. The relays measure the time between the points where the voltage sine wave transitions through the zero value, as displayed in Figure 2.

Figure 2 –Example of zero crossing calculation method

No matter what method a relays uses to calculate the frequency it is important to note that frequency is a calculated value. Every calculation method will have a degree of inaccuracy associated with it.


4.1.2 Rate-of-Change-of-Frequency Calculation

The rate of change of frequency is generally calculated from the frequency value rather than directly from the original voltage waveform. Similar to the frequency measurement, relay manufacturers do not make it transparent how they calculate the rate of change of frequency values.

The simplest form of RoCoF calculation is based on two successive frequency calculations with a set time difference between the frequency calculations. The difference between frequency calculations is divided by the time difference to calculate the RoCoF. However, relay manufacturers do not make it clear how they calculate RoCoF and it is unlikely any would rely on the crude method describe above.

Frequency is calculated quantity so the RoCoF is derived from a calculated quantity. Further, it is well known that differentiation is generally an unstable operation, especially in the presence of noise. This can make the calculation of RoCoF unstable and filtering is required for it to work well. The limitation of the calculation method is outlined in greater detail in Section 7.1.


5 RoCoF Bench Testing - Methodology

The objective of the RoCoF relay bench testing is to evaluate the technical requirements for the reliable use of RoCoF relays. The testing methodology was developed to verify the following performance criteria:

the relays are capable of operating the required logic; the RoCoF calculation is stable during a range of expected system

disturbances; the variety of RoCoF calculation methods used deliver a predictable response

within a range of uniformity when subjected to the same inputs; the RoCoF calculation responds within a sufficient timeframe to frequency

changes; and the RoCoF calculation is accurate when subjected to a wide variety of input

waveforms.

The testing work is not seeking to approve the use of any particular relay but to better understand what requirements need to be specified to ensure reliable operation of RoCoF relays.

This section outlines the steps taken and assumptions made to complete the RoCoF relay testing. The bench testing included the following steps:

1. Establish initial RoCoF performance requirements to create a sample relay selection

2. Develop test equipment arrangement and process 3. Design test cases objectives and generate required files 4. Carry out bench testing 5. Review results and identify next steps 6. Complete additional testing iterations as required

5.1 Initial RoCoF relay performance requirements

The first step was to propose a set of initial RoCoF relay performance requirements that would enable selection of sample RoCoF relays to be used in the bench testing process.

The investigation included an analysis of additional user requirements (i.e. Transpower and lines companies). The initial performance requirements are set out in Appendix A. A selection of the requirements for the sample relays is provided below in Table 2.


Table 2 – Selection of the RoCoF Relay Performance Requirements

Feature Minimum requirement Purpose

Fixed f + t (81U) element

Setting range 45.0 to 50.0 Hz

At least 4 stages or levels or elements

df/dt (81R) element Setting range 0.0 to 2.0 Hz/s

At least 4 stages or levels or elements

Traditional RoCoF

Trip logic Boolean internal, programmable To enable ‘frequency guard’

To combine 81x with 27 elements

Operating range 45 to 55 Hz

Accuracy (81U) ±0.05Hz of reference frequency* Precision of trip settings

Accuracy (81R) ±0.1mHz/s of reference frequency* Precision of trip settings

Repeatability Trip within ±20.0 ms over three tests using the same input waveforms and trip settings

Predictability of trip

From the list of relay requirements a survey was done of major manufacturers. The investigation resulted in the final sample set of relays provided in Table 3.

Table 3 – Relays used in RoCoF Bench Testing

Manufacturer Relay model

Description Target markets*

SEL 751 Multifunctional feeder protection Transmission / Distribution

SEL 351-7 PS Multifunctional feeder protection Transmission / Distribution

GE SR760 Multifunctional feeder protection Transmission / Distribution

Siemens 7SJ64 Multifunctional feeder protection Transmission

Siemens 7SJ80 Feeder protection Distribution

ABB RED670 Line differential protection Transmission

ABB REU615 Voltage protection Distribution

These are all multifunctional numerical relays that can be programmed to provide a range of different protection functions in addition to frequency protection.

Two additional relays were also provided for testing, namely a RMS 2H34-S frequency relay and an Alstom P145 feeder protection relay. The RMS relay was found to have a bug that prevented settings from being changed reliably, and the Alstom relay arrived half way through the second round of testing. Neither of these relays were tested as a result. However, the Alstom relay will likely be tested at a later date since, unlike all the other relays tested, its averaging window can be explicitly set.


5.2 Test equipment arrangement and process

To identify the requirements for reliable use of RoCoF relays on the New Zealand power system, a testing process was developed to verify each element of the performance criteria. The testing simulated specific system events and conditions through the relays and observed the effect on the relay’s RoCoF calculation and behaviour.

Most of the relays tested do not provide a readily accessible output of their calculated frequency or rate of change of frequency. Therefore, determining these calculated values requires some reverse engineering.

The relay’s trip signal was used as it is a readily accessible output and directly relates to the function of the relay. The point at which the relay signals to trip can be referenced against the voltage waveform to better understand the calculated RoCoF value.

The testing work involved injecting voltage waveforms into the sample relays to allow the relay to calculate the RoCoF and then observing if the relay responds with a trip signal. Figure 3 provides an overview of the arrangement used to test the relays.

Figure 3- Relay bench testing configuration19

19

The Omicron test set is used to convert data files into actual voltage waveforms injected into the relay.


A typical test will involve:

Creating a voltage waveform to inject into the test relay Loading the selected voltage waveform into the Omicron Programming the test relay with the desired settings Playing the selected voltage waveform into the test relay Recording when the test relay operates “Running” the same voltage waveform through Matlab to calculate frequency

and RoCoF using a known algorithm Comparing the operating point with the voltage waveform and Matlab

calculated frequency and RoCoF on the same time axis.

5.3 Test Cases

Several test cases were created specifically to understand each aspect of the performance criteria (logic, stability, uniformity, response time, and accuracy). Each test case required a voltage waveform and, depending on the objective of the test, a waveform was either generated or taken from real system events.

A sample waveform used to test the logic of the relays can be seen in Figure 4.

Figure 4 - Example waveform used to test the relay logic elements

One of the areas of greatest concerned outlined by the Distribution Companies was around the stability of the relay. Several test cases were created to verify the stability of the relay during system disturbances which included;

a noisy bus waveform (Glenbrook); HVDC Commutation failures; loss of phase; no volts; abnormal frequencies; and islanding.

The literature review gave particular focus to the effect of power system oscillations effect on the accuracy of the RoCoF calculation. Multiple test cases were developed to further investigate this potential issue.


5.4 Settings Used

The testing work utilised the AUFLS settings proposed in the last stage of the AUFLS review20 for the majority of test conducted21. The settings are in Table 4.

Table 4 - Proposed NI AUFLS operation criterion

Accelerated element Under frequency element 1

Under frequency element 2

Block Size

Df/dtset fguard Td3 fset1 Td1 fset2 Td2

Block 1 N/A N/A N/A 47.8 Hz 0.3s N/A N/A 8%

Block 2 N/A N/A N/A 47.5 Hz 0.3s 47.8 Hz 15s 8%

Block 3 -0.8 Hz/s 48.5 Hz 0.4s 47.3 Hz 0.3s 47.5 Hz 15s 8%

Block 4 -1.5 Hz/s 48.8 Hz 0.4s 47.3 Hz 0.3s 47.5 Hz 15s 8%

5.4.1 The role of the frequency guard

The proposed AUFLS scheme utilises different elements to achieve the designed logic output. One of the key elements is the frequency guard. The frequency guard prevents the relay from tripping on the calculated RoCoF value alone. The relay will only operate if the RoCoF is greater than or equal to the set level AND the frequency measured is less than or equal a set level for specified duration. A high level logic diagram is displayed in Figure 5.

Figure 5 – High Level Logic Overview

The frequency guard helps prevent spurious operation from frequency fluctuations not resulting from large system disturbances. Therefore, the frequency guard helps to minimize the risk of mal-operation.

20

See Appendix C of the AUFLS Stage II review. 21

It is noted that some test utilised other settings that were being investigated for possible scheme improvements prior to Pole 3 commissioning.

RoCoF Calculation

Guard frequency

and time delay

AND

OR

Backup frequency

and time delay

Trip


6 Bench Testing Results

The purpose of the testing work was to provide greater visibility of the strengths and weakness of RoCoF relays for the purpose of triggering AUFLS load. Seven numerical relays fitted with RoCoF elements were tested to help determine their suitability. The tests devised focussed on understanding the logic capability, stability, uniformity, response time, and accuracy of the RoCoF relays.

This section highlights the observations made from the test results and makes recommendations to ensure the installed RoCoF relays deliver the desired outcomes. The detailed tests and results are in Section 6 of Appendix B; the main points are summarised below.

6.1 Bench Testing Results Summary

6.1.1 Logic

The proposed AUFLS scheme using RoCoF requires a level of logic including a frequency guard and backup under-frequency settings. The testing aimed to ensure the relays are capable of operating the required logic.

All of the relays tested had some form of logic programming that allowed users to implement various combinations of RoCoF, frequency, and time delay elements to achieve a desired tripping response.

Each manufacturer has a different way of arranging and setting their logic elements and care is required to ensure the desired result is achieved when programming and setting the various relays. This was illustrated in test 3 where the two of the relays exhibited an unexpected response to a simple logic test.

It is suspected that this could be addressed by adjusting the logic arrangements used in these relays.

It is recommended that the Code specifies a test outcome to verify that the logic elements of the installed relays meet the performance requirements.

6.1.2 Stability

A primary aim of the bench testing was to ensure the RoCoF relays remain stable during a range of system disturbances that would not normally result in AUFLS operation.

The tests results indicate that the sample RoCoF relays were stable under all of the disturbances to which they were subjected. This indicates that the RoCoF algorithms are designed to reject noise and fast transient events which will help to avoid undesired operation of the RoCoF.

The guard frequency used in these tests is likely to play a major role in preventing the undesired operation of RoCoF. The above stability tests were carried out using the RoCoF settings that were available at the time of testing. The results of these tests may change if different settings are used (i.e. if the frequency guard is set to a higher frequency).

It is recommended that RoCoF relays are stable for use. It is suggested that Industry reviews the tests results and provides feedback on the level of confidence in the stability of the relays.


6.1.3 Uniformity and Response Time

A level of uniformity is required to ensure the installed AUFLS system achieves the desired levels of operation with a range of different relays.

The tests demonstrated that the sampled RoCoF relays do not behave uniformly when subjected to identical inputs. This lack of uniformity was reflected in all of the tests, and it indicates that different relay manufacturers use different algorithms for calculating RoCoF.

The nature of the algorithms used in these relays means that it will always take a finite amount of time to detect a change in frequency because it involves calculating over a set number of cycles. The response times of the relays are dependent on the RoCoF settings.

The tests demonstrate that the closer the rate of change is to the set point, the longer the response time, the greater the difference, and the faster the relay operates. Depending on the RoCoF settings used, the relays use different lengths of time (numbers of cycles) for the RoCoF calculations, as shown in Figure 6.

Figure 6 – Example of response time

The lack of uniformity means relays operate over a range of different response times during the frequency excursions. This results in AUFLS load tripping at different times even though all relays have identical settings. This is not necessarily a problem provided that relays comply within a maximum allowed response time.

It is recommended that the Code specifies the maximum response time allowed for RoCoF relay operation.


6.1.4 Accuracy

The ability to rely on RoCoF relays for load shedding requires the relay to be able to accurately calculate the rate of system frequency decay when subjected to a variety of grid conditions. The consequences of inaccuracy could result in the relay mal-operating by tripping inappropriately or not tripping when needed.

As stated previously, frequency is a calculated value from which the RoCoF is derived. The RoCoF is derived (differentiated) from a calculated value. It was assumed that most relays would simply calculate the RoCoF from two successive frequency calculations with a set time difference between the frequency calculations. However, the testing revealed the calculation method from most RoCoF relays are more complicated and it is difficult deduce the exact method used.

The relays performed within the specified accuracy margins (±0.05 Hz/s) for clean frequency decays. However, the test results did verify that RoCoF relays are susceptible to calculation inaccuracy due to system oscillations. The impact of power system oscillations is dependent on the relays settings used.


7 Inaccuracy from Power System Oscillations

7.1 Limitation in calculating frequency and RoCoF

Ideally, the frequency measured at each bus across the North Island system would be identical. However, in practice this is unlikely to be the case. The frequency measured at a specific bus may vary from the average system frequency.

Part of the frequency variance is caused by the addition of several sine waves as local and remote generators adjust for the varying load. Synchronous generators will vary their speed up and down because of load variations. The turbines are not always fast enough to maintain an exact frequency. When additional power is needed, the energy may initially come from the stored energy of the rotating mass of the generator and the turbine, which will decrease the speed of the generator. The characteristics of each unit will respond differently to these load variations and so units may “rock back and forth” with respect to each other due to the distance to the load [5].

The additional sine waves created by the generators rocking modulate the frequency measured on the bus, varying it from the average system frequency [6]. These additional sine waves are known as oscillations. Other spinning devices synchronised to the system, such as synchronous condensers, will also contribute to frequency oscillations.

Studies conducted elsewhere have shown that remote buses, further away from the core system, are more susceptible to oscillations [1]. Other work has suggested that the introduction of variable renewable energy resources, i.e. wind energy, may also lead to further frequency deviation and add oscillations [2]. Larger oscillations can also be expected if the system is stressed, e.g. with many lines out of service.

Studies completed on the Tasmanian power system found that their system is prone to oscillations during large system disturbances [4]. Further work was completed to understand the impact of oscillations on the calculation of rate-of-change-of-frequency during large system disturbances that would be expected to cause AUFLS response.


7.2 Impact of system oscillations on RoCoF Calculations

Oscillations can result in the relay incorrectly calculating the RoCoF depending on the settings and filtering of the relay. Figure 7 displays a simple graphical representation of a 0.5 Hz/s frequency decay with a 1.5 Hz sinusoidal oscillation added onto the average system frequency.

Figure 7 – Illustration of a .5 Hz/s decay with a 1.5 Hz sinusoidal oscillation included

If the relay employs a calculation window that averages the decay on either side of the peak of the oscillation, the RoCoF calculated will vary from the average frequency decay as demonstrated in Figure 8.

Figure 8 illustrates the change in the RoCoF during the oscillation. If the relay employs a calculation window that averages the RoCoF (or the frequency), the change (error) in the RoCoF will be reduced.

Figure 8 – Simple illustration of the impact of oscillations and calculation time on accuracy

The relay trips when the RoCoF frequency exceeds the set value; so in general, only the maximum positive error in the RoCoF is of any consequence. That is, the oscillations can cause false tripping, but will not prevent the relay from operating.


Testing work was completed to verify the relays susceptibility to power system oscillations. Test case 15 simulated a frequency decay of .35 Hz/s with a superimposed ‘noise’ of 1.5 Hz at an amplitude 2 degrees peak-to-peak. Figure 9 shows the result when the relay was armed with a set point of .4 Hz/s.

Figure 9 - Test Case 14 results for Relay A

The relay in Figure 9 continues to send trip signals for the duration of the frequency decay demonstrating the relay continues to incorrectly calculate the RoCoF as steeper than the set point.


7.3 Oscillations during Under Frequency Events

The previous example assumed the oscillation occurred at the same amplitude and frequency for the duration of the decay. An investigation was undertaken to observe the presence of oscillations during under-frequency events of the last five years22.

The investigation found that oscillations, especially those with high peak to peak values, can result in the relay calculating relatively high RoCoF values for short durations, as displayed in Figure 10

Figure 10 – Haywards RoCoF calculated following Pole 2 trip on 9th December 2011

Figure 1023 shows the calculated frequency at the Haywards 220kV bus during an under-frequency event following the tripping of HVDC Pole 2 on 9 December 2011. The impact of the oscillations can be seen in the large spikes of the RoCoF (df/dt) calculation. The initial magnitude of the oscillations decreases as the system responds to the frequency decay, which helps to reduce the inaccuracy.

These same oscillations were not present at the Huntly 220kV bus as shown in Figure 11.

22

The investigation relied upon frequency and RoCoF data calculated by the PMU’s installed at different buses.

23 The df/dt averaging window from Figure 11 is assumed to be around .2s. Further information and more

events are analysed in Appendix C.


Figure 11 – RoCoF measured at Huntly following Pole 2 tripping on 9th December 2011

The investigation indicated that at specific buses, the calculated RoCoF may be considerably different from the average system RoCoF24. In the North Island it is considered reasonable for an oscillation to occur with a frequency in the range of 0.5 Hz to 2.5 Hz25.

In practice, it appears that the duration and magnitude of the oscillations does not occur continuously during the frequency decay. Over time the system’s response to the disturbance appears to dampen the initial oscillation. However, not enough detail is known about the system to estimate the magnitude or likelihood of oscillations occurring during an AUFLS events.

In light of the uncertainty and exposure to inaccuracy the System Operator believes it prudent to mitigate against oscillations, as the consequences of mal-operation could be significant.

24

The effects of installing the RoCoF relays in the zone substations may increase the susceptibility to oscillation however this has not been analysed.

25 Work done in Tasmania found that at some buses they experienced oscillation frequency around 2.2 Hz.


7.4 Mitigating the effects of power system oscillations

Although it has been shown that the RoCoF elements are susceptible to the impact of power system oscillations, there are steps that can be taken to minimize their impact on the RoCoF calculation. Three areas will be explored; the frequency guard, filtering, and calculation window time.

7.4.1 The Frequency Guard

The frequency guard prevents the relay from tripping on the calculated RoCoF value alone. The relay will only operate if the RoCoF is greater than or equal to the set level AND the frequency measured is less than or equal a set level for specified duration.

The use of a frequency guard helps protect the system from spurious operation of RoCoF relays from frequency fluctuations not resulting from large system disturbances that would necessitate an AUFLS response.

Figure 12 – Stratford RoCoF measured following Pole 2 tripping on 27 April, 2009

Figure 12, for example, is the frequency and RoCoF measured at the Stratford 220kV bus following the loss of the HVDC Pole 2 on 27 April 2009. Although the calculated RoCoF is initially greater than any proposed RoCoF setting the frequency guard (e.g. at 48.5 Hz) will ensure that the RoCoF relays will not operate as the frequency has not decayed below the trigger level.

The risk occurs when the power system oscillations are present and the frequency has fallen below the frequency guard level as may be the case during a large tripping.

One of the main benefits of utilising RoCoF for AUFLS relies on the ability to shed load above the 48 Hz contingent event frequency limit while not tripping for single contingent events. However, the presence of power system oscillations, and their effect on the RoCoF calculation, may occur during contingent events which take the system frequency below the frequency guard limit. The spikes would then result in the operation of RoCoF relays, unneeded load shedding, and the potential for an over-frequency event.


Moving the frequency guard limit to 48 Hz and below may provide the RoCoF scheme discrimination against single contingent events but it won’t mitigate the inaccuracy which oscillations cause on the RoCoF calculation. The inaccuracy may result in RoCoF tripping improperly in response to the average system frequency. This minimizes the benefit to be gained by using RoCoF to improve better matching of load shed to generation loss.

While the frequency guard improves the performance of RoCoF calculation it alone will not provide a robust deterrent to the effects of oscillation on the RoCoF calculation.

7.4.2 Increasing the RoCoF calculation time

One way to minimize the limitations to using RoCoF as a load shedding trigger can be done by having a longer duration calculation window when calculating the RoCoF.

The longer RoCoF averaging time reduces the magnitude of the RoCoF spikes and provides a smoother RoCoF calculation that is closer to the average system RoCoF. In light of the presence of oscillations on the Tasmanian system, they required a RoCoF calculation time of .34s.

Figure 13 – Illustration of the impact of increasing RoCoF calculation time

Figure 13 provides a simplified example of the benefit of increasing the calculation time used by the RoCoF relays. As stated previously, the calculation of RoCoF is assumed to be more complicated than taking successive frequency measurements with a set time difference between measurements, perhaps utilising a type of moving average. However, increasing the calculation time will help to reduce the impact of oscillations on the RoCoF calculation.

The impact of longer calculation window on relay response time

Increasing the speed of AUFLS operation was a key benefit of utilising RoCoF elements. One concern is that increasing the calculation time of the RoCoF relays will eliminate any benefit of increased speed of operation. This may be the case when utilising an initiation frequency set point (i.e. the RoCoF will not begin calculating until a set frequency has been reached). However, the use of the frequency guard may reduce the delay caused by increasing the RoCoF calculation time.


The relay will continuously calculate the RoCoF and once the frequency guard level has been maintained for the time delay, the trip signal is sent and the relay will operate (Figure 14). So long as the time taken for the frequency to fall from 50Hz is longer than the calculation time, no additional delay as a result of the calculation time will be introduced.

Figure 14 – Example of response of RoCoF calculation and frequency guard

Availability of adjustable calculation time within relay

Most RoCoF relay manufactures specify the number of cycles for calculating the RoCoF. The length of time can vary depending on the relay and RoCoF settings used. The length of the calculation window cannot be adjusted for any of the relays that were sampled.

A review of the technical manual for the Alstom P145 relay shows that it includes a setting that allows the user to adjust the length of the time window used by the RoCoF algorithm. The Alstom P145 relay is utilised by Tasmania to accomplish its .34s calculation time window. The Alstom P145 arrived too late to be included in the testing. The Alstom P145 will be tested at a later date to determine if the relay is suitable for use in New Zealand.

The System Operator will pursue conversations with other major relay manufacturers to understand their ability to enable manual adjustment of the RoCoF calculation time and the timeframe associated with any changes.


7.4.3 Low-Pass Filtering

RoCoF relays use low pass filters to manage the impact of power oscillations on the RoCoF calculation. Ideally, the low pass filters would attenuate the noise and oscillations while leaving the slower-changing underlying RoCoF relatively unaffected.

In practice, it can be difficult to distinguish between the low-frequency power oscillations and the underlying signal. The error in the RoCoF calculation is proportional to the square of the oscillation frequency; the higher the oscillation frequency, the higher the calculation error. The relay filters were originally designed for islanding applications rather than as a general AUFLS scheme. Therefore, their cut-off frequencies tend to be too high i.e. not “enough” filtering.

The test using the 13 December event demonstrates the functionality of the low pass filters. When the event waveform was injected though the sample relays during testing, some of the relays operated once the average system frequency was increasing (Figure 15). The blue line in Figure 15 is the trip signal sent by the relay.

Figure 15 Frequency and trip plot trace from Test Case 2 – Relay G.

During the event an oscillation initially occurred at a frequency of 7 Hz while the average system frequency decayed at about 0.5 Hz/s. As the oscillation frequency began to decrease, the relay filter started to pass through some of the oscillation, which in turn began to affect the RoCoF calculation.

The frequency of the oscillation eventually decreased to a level where the filtering allows a sufficient magnitude to affect the RoCoF calculation, causing the relay to operate on a downward swing of the oscillation, even though the underlying frequency is increasing.

While the AUFLS scheme is not designed to cover transient instability events, the test provided useful information into how the filters operate.

The majority of the sample relays sufficiently filter out the high frequency oscillation to prevent the relays from operating. However, as the frequency


decreases, the filtering used by the relay becomes important and some filtering, such as the relay in Figure 15, was insufficient to prevent operation.

Test case 14 in Appendix B specifically sought to investigate the low pass filters associated with the RoCoF calculation. The test confirmed that low frequency oscillations will be “passed through” the RoCoF filters and therefore affect the RoCoF calculated by the relay.

Figure 16 displays the filtering response of one the sample relays when set to trip at RoCoF greater than or equal to 0.4 Hz/s.

Figure 16 – Filtering response of the Relay F with RoCoF elements set to .4 Hz/s

The gain values of Figure 16 display how much of the oscillation’s RoCoF will be scaled down, or attenuated, before affecting the overall RoCoF calculation.

For example, if a steady frequency decline of 0.1 Hz/s were to occur with an oscillation of 1.6 Hz present at the point of measurement26. The maximum expected RoCoF from the oscillation would be attenuated by the filtering of the sample relay from Figure 16 by approximately 0.67 before being added onto the calculated RoCoF value.

The maximum expected rate of change of frequency from the oscillation depends on the oscillation magnitude and frequency. For example, if the oscillation’s maximum RoCoF was 0.56 Hz/s this would be multiplied by 0.67 to result in 0.38Hz/s that would be added to the calculated system frequency RoCoF.

This means that although the system frequency is falling at 0.1Hz/s the relay, because of the oscillation, calculates the decay as 0.48 Hz/s.

If the oscillation frequency was greater than 4 Hz the relay will filter out the oscillation preventing it from having an impact on the overall RoCoF calculation. The amount of pass through for unfiltered oscillation frequencies changes depending on the relay and the RoCoF settings.

26

This example is more thoroughly explored in Appendix C.


It is considered impractical to apply additional filtering to improve the performance of the RoCoF calculation. It is suggested that any requirements for use of RoCoF relays include a required range of filtering.

Availability of adjustable filtering within relay

Most RoCoF relay manufactures specify the filtering for calculating the RoCoF. The filtering can vary depending on the relay and RoCoF settings used. In general the larger RoCoF settings (i.e. -1.5 Hz/s) allow more frequencies to pass through.

It is recommended that a filtering standard be required for all relays regardless of the RoCoF settings. Any code recommendation should specify the required df/dt calculation filter response to mitigate the inaccuracy caused by the identified level of oscillations to be designed against during under-frequency events.

Figure 17 – Draft envelope for the frequency response

Figure 17 provides a draft of the kind of filtering standard that would be required for all RoCoF settings27. The filtering does not remove the effect of oscillations on the RoCoF calculation. The filtering ensures the magnitude of the oscillations is attenuated. Inaccuracy is still possible. However, with the filtering requirement, it has been mitigated to a significant level of coverage.

The System Operator will pursue conversations with major relay manufacturers to understand their ability to adjust of the RoCoF filtering and the timeframe associated with any changes.

27

To see a draft df/dt filter requirement see Appendix C.

Gain

Frequency (Hz)

0 1 2 3 4 5 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1


8 RoCoF Recommendation Summary

In light of the testing results it is recommended that the following are specified in the code (summarised from above):

1. a required RoCoF calculation filtering standard

2. a maximum response time required for the RoCoF settings proposed so that the various relays provide a degree of uniformity.

3. a test outcome to verify that the logic elements of the installed relays meet the performance requirements.

It is also suggested that Industry reviews the tests results and provides feedback on the level of confidence in the stability of the relays.

The System Operator will pursue conversations with major relay manufacturers to understand their ability to adjust of the RoCoF filtering and calculation time.


9 South Island Block 2 Increase

9.1 Background

The System Operator has undertaken work to determine technically feasible options to improve the South Island AUFLS scheme. However, the inclusion of AUFLS provision at the Tiwai Grid Exit Point (GXP) and the potential narrowing of the South Island frequency band have significant implications for the future of the South Island AUFLS design. Further clarity is needed before a significant redesign occurs.

Impact of provision at Tiwai GXP

The System Operator relies on the AUFLS scheme to prevent system collapse following an extended contingent event (ECE) or other undefined events which have the potential to cause a system blackout.

Currently the ECE risks are defined as the loss of single bus bar, an interconnecting transformer, or the loss of the HVDC bipole. When the HVDC is transferring south following the commissioning of Pole 3, the loss of the bipole can cause a much larger disturbance relative to the total size of South Island load.

The larger disturbance requires a higher quantum of AUFLS and reserves to ensure the system frequency remains within the requirements. Currently, there is

no AUFLS response at the South Island’s largest source of demand, the Tiwai grid exit point. However, the Grid Owner is progressing the installation of an AUFLS response at Tiwai GXP with the cooperation of the New Zealand Aluminium Smelter.

The most significant improvement to the capability of the South Island AUFLS scheme is considered to be the inclusion of an AUFLS response at the Tiwai GXP. The inclusion of an AUFLS response at Tiwai will decrease the amount of reserve required to cover high HVDC south and improve the South Island AUFLS safety net.

Potential of narrowing the SI Frequency Band

The South Island frequency range is currently 55 Hz to 45 Hz as stated in the Principle Performance Objectives (PPO). The current frequency envelope is ‘wide’ by international standards and reflects the physical nature of the hydro generation plant that currently dominates the South Island.

The South Island AUFLS range is currently 48 Hz to 45 Hz, this allows for a greater margin between AUFLS block trip settings which enables block discrimination.

The System Operator expects most future generation installed in the South Island will be non-compliant with the frequency requirements specified in the Asset Owner Performance Obligations (AOPO).

A recent investigation28 found that despite the current level of non-compliance, there is no technical reason for changing the existing frequency range; nor is there evidence that an economic threshold has been reached that would warrant making a change. However, there will be a point at which the level of installed non-compliant plant may make it economical to change the South Island frequency band. It is uncertain when this point will occur or the quantity of non-compliant generation which will incentive the change.

28

See the AOPO Report at http://www.systemoperator.co.nz/ufm

http://www.systemoperator.co.nz/ufm


Interim Proposal

In light of the variables previously stated, the System Operator believes it is prudent to refrain from further analysing AUFLS scheme options until there is greater clarity of the future of AUFLS provision at the Tiwai GXP and frequency requirements.

In the interim, the System Operator has identified that increasing the trip setting of the second AUFLS block to 46.5 Hz will improve the current South Island AUFLS scheme. The second AUFLS block is currently set to trip at 45.5 Hz which is close to the 45 Hz standard.

An increase in the trip setting would raise the minimum frequency reached following most of the AUFLS events studied and so provide additional protection against larger events. Further information can be found in the technical review report.

To increase the second block trip setting in the Code requires a cost benefit analysis be completed to ensure the change is for the long term benefit for New Zealand consumers.

The objective of this section of the report is to summarise the results of the cost-benefit analysis completed on the proposals to raise block 2 of AUFLS in the South Island. A high level summary will be provided here while the work completed by Concept Consulting can be seen in Appendix D.

9.2 Cost Benefit Analysis Methodology

The previous cost-benefit assessment of the proposed AUFLS scheme changes separated the potential cost and benefits of an AUFLS scheme into four main areas;

1. implementation cost of proposal 2. altered energy and instantaneous reserves (IR) costs 3. altered quantity of load shed ‘unnecessarily’ in an AUFLS event 4. altered risk of system collapse

The System Operator has estimated that the cost of sending individuals to the various GXPs to manually change the relay settings could be of the order of $9,00029.

In light of the low implementation cost a high level cost-benefit evaluation was completed to demonstrate the proposal would deliver a present net benefit.

The high level analysis enabled the System Operator to analyse the scheme without having to undertake lengthy studies which would likely be more costly than the implementation.

The approach aims to consider what benefit would need to be seen to justify the cost.

Any event where the scheme using the 46.5 Hz trip setting prevents system collapse and the current scheme does not, the avoided cost can be attributed as a benefit.

The benefit is scaled by the return period or the likelihood of the event. The annual benefit is estimated by the cost of system collapse multiplied by the likelihood of the event in years.

It is considered impossible to accurately estimate the likelihood of such an event. However, it is considered possible to estimate the threshold or the probability of

29

The cost is variable depending on if the initiative was spread out over a longer period, such that technicians made changes when they were anyway due to visit a sub-station for another reason.


the event for where it would no longer be beneficial to spend the implementation cost.

For the proposal, to be considered beneficial the proposed settings must protect against an event which occurs more regularly than the threshold and that would result in system collapse with the current settings.

The following assumptions are crucial to the methodology:

1. Increasing the second block trip setting will not result in any material risk of system collapse due to over-frequency following operation of the AUFLS scheme. This assumption is based on the wide frequency range of the South Island (up to 55 Hz).

2. Increasing the second block trip settings will not result increase risk of unnecessary load shedding. There is still 1 Hz difference between the first (47.5 Hz) and second (46.5 Hz) blocks. The System Operator believes this will provide significant discrimination when needed to not increase the risk of unneeded shedding.


9.3 Cost Benefit Analysis Results

The cost of a system collapse event has been estimated to be approximately $500m – representing an estimated average South Island load of 1,400 MW being blacked-out for approximately 18 hours30, and with an average value of such load of $20,000/MWh. The cost of interruption estimation is considered conservative not accounting for the impact of outage time on cost.

The results of this back-calculation are presented for both the expected implementation cost of $9,000, and a sensitivity of $50,000 in Table 5. The net present value (NPV) calculation has been undertaken using a discount rate of 8%, and calculated over both a 10 year period and with a sensitivity of a 2 year period. The results in Table 5 ignore any possible benefit of reducing energy and IR cost.

Table 5 – Threshold estimation

Benefit Period Implementation cost

$9,000 $50,000

10 year NPV 1 in 380,000 years 1 in 70,000 years

2 year NPV 1 in 100,000 years 1 in 19,000 years

As stated previously, the South Island frequency band may change in the future to adjust for the increasing number of non-compliant generators. To analyse the benefit of implementing the proposed changes if the South Island scheme requires redesigning in two years, the 2 year net present threshold was calculated.

The 2 year net present value indicates the proposed scheme would need to at least increase protection against events more likely than a 1 in 19,000 year event to be beneficial.

The previous studies conducted in the South Island gives the System Operator confidence the proposed block settings will give the scheme resilience to events which could occur measured in tens rather than thousands of years.

The System Operator believes increasing the second block of AUFLS to 46.5 Hz demonstrates a net economic benefit.

30

The actual restoration duration is variable depending on several factors including awareness of cause. The restoration of load occurs progressively and estimation does not attempt to account for this. Previous work has estimated minimum restoration time following system collapse to be 12 hours, this is considered to be optimistic so 18 hours was used.


10 Next Steps

The System Operator will be engaging with industry participants to ensure the findings from this paper are well understood and that industry feedback has been thoroughly considered. Once the System Operator has analysed feedback, it will finalise recommendations and present them to the Electricity Authority.

In parallel, the System Operator will be conducting further work in the following areas:

additional bench testing of relay filtering requirements pursuing discussions with major relay manufacturers developing an AUFLS technical standard developing an AUFLS exemptions, equivalence, and dispensation framework developing an implementation plan for the RoCoF proposal developing relevant code change recommendations in light of work

completed on the RoCoF requirement and AUFLS standard

11 Summary of Questions for Consideration

The System Operator seeks industry feedback on the following questions:

Does the bench testing (outlined in Appendix B) provide sufficient confidence in the stability of the relays?

Are there additional tests that should be considered before proposing the use of RoCoF relays?

Does industry support the RoCoF requirements recommended in this report? Are there other factors that need consideration in the Code requirements for

the testing and use of RoCoF relays? Does industry support the cost benefit analysis methodology used for the

South Island second block setting? What obligation level is appropriate for the future AUFLS requirements (i.e.

GXP or network or other)?


12 Acknowledgements

The System Operator wishes to acknowledge and express appreciation to Victor Lo, Daniel Mulholland, Robert Derks, and Simon Coates for their significant input to the material included in this report.

13 References

[1] IEEE Std C37.117-2007, IEEE Guide for the Application of Protective Relays Used for Abnormal Frequency Load Shedding and Restoration, 2007.

[2] H. Bervani, et al., “On the Use of df/dt in Power System Emergency Control,” In Proc. Of IEEE Power Systems Conference & Exposition, Seattle, Washington, USA, 2009

[3] B.W. Leyland, et al., “A New Rate of Change of Frequency Relay,” 1997

[4] D. E. Clarke, “Tasmanian experience with the use of df/dt triggering of UFLSS,” Final Report, Transend Networks PTY LTD, No. D08/22185, 2008.

[5] M. Hemmingsson, “Power System Oscillations – Detection Estimation & Control,” Lund University, 2003

[6] D.J. Finley, et al., “Load Shedding for Utility and Industrial Power System Reliability,” Basler Electric Company

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