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Frequency Changes During Large Disturbances and Their Impact on the Total System

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Frequency Changes During Large Disturbances and Their Impact on the Total System Frequency Changes during Large Disturbances and their Impact on the Total System This document presents proposals to modify the Distribution Code and Engineering Recommendation G59 for Industry Consultation. Any interested party is able to make a response in line with the guidance set out in Section 7 of this document Published on: 15 August 2013 Length of Consultation: 30 Working Days Response by: 27 September 2013 Ошибка! Recommendation: Неизвестное имя свойства Rate of Change of Frequency (RoCoF) protection settings should be changed документа. at new and existing distributed generators in stations of registered capacity of -1 5MW and above to 1Hzs measured over 500ms Frequency Changes during Large High Impact: Disturbances and their Impact on the Owners of synchronous generators at stations of registered capacity of 5MW Total System and above where, subject to a site specific risk assessment, mitigation Ошибка! measures may need to be implemented before protection setting changes can Неизвестное имя be applied свойства документа. Medium Impact: Version Ошибка! Неизвестное имя свойства Owners of non-synchronous generators at stations of registered capacity of документа. 5MW and above Page 1 of 47 Low Impact: © ELEXON Limited 2013 None identified Ошибка! Неизвестное имя свойства документа. Frequency Changes during Large Disturbances and their Impact on the Total System Contents 1 Executive Summary ................................................................................ 3 2 Why Change? .......................................................................................... 7 3 Solution ................................................................................................. 14 4 Purpose & Scope of Workgroup .......................................................... 15 5 Workgroup Discussions ....................................................................... 17 6 Impact & Assessment........................................................................... 27 7 Consultation Responses ...................................................................... 34 ANNEX 1 - Workgroup Terms of Reference ......................................... 36 ANNEX 2 - Proposed Legal Text............................................................ 39 ANNEX 3 - Workshop Questions and Responses ............................... 42 ANNEX 4 - Background for RoCoF Forecast Analysis ........................ 47 Addendum Assessment of Risks Resulting from the Adjustment of ROCOF Based Loss of Mains Protection Settings About this document This Industry Consultation outlines the information required for interested parties to form an understanding of an issue within the Distribution Code and Engineering Recommendation G59 relating to the security of the total electricity supply system in Great Britain and seeks the views of interested parties in relation to the issues raised by this document Document Control Version Date Change Reference 1.0 15 August 2013 First Issue of consultation Any Questions? Contact: David Spillett, Distribution Code, Code Administrator [email protected] Contact: Robyn Jenkins, Workgroup Technical Secretary [email protected] 1 Executive Summary Background 1.1 The electricity supply system in Great Britain is designed to operate as a single synchronised system. In the event of a network fault, it is possible for part of the network to be isolated from the rest of the system forming an islanded system. In these circumstances it is possible for a distributed generator, or a group of distributed generators, located within this island to supply the local distribution network and its customer demand. 1.2 Such an island would not be controlled to normal quality of supply standards and is potentially unsafe to people in the proximity of the energised equipment. Historically, smaller distributed generators have been required to have Loss of Mains protection which would, in the event of an island being formed, shut down the generator(s), and hence the island, safely. 1.3 One technique used to detect a Loss of Mains condition is to measure the Rate of Change of Frequency (RoCoF). This technique works because it is likely there will be an imbalance between electricity demand and supply within the island when an islanded system forms, meaning that frequency within the island changes at a rate higher than that experienced under normal system conditions. However, high RoCoF can occur over the whole of the electricity supply system in the event of a large infeed (generation or import) or off-take (demand or export) loss. If the RoCoF is high enough, RoCoF based Loss of Mains protection can operate which could cause distributed generation to stop generating leading to a further disturbance and a possible cascade effect. The current minimum recommended RoCoF setting is 0.125Hzs-1. 1.4 If enough distributed generation were to cease generating (there is currently over 9GW of installed capacity), the result of this cascade effect would be the operation of Low Frequency Demand Disconnection (LFDD). A large number of electricity consumers would suffer an involuntary loss of electricity supply. National Grid has a statutory obligation to ensure that unacceptable frequency conditions do not occur under situations specified in the National Electricity Transmissions System Security and Quality of Supply Standard (the NETS SQSS1). 1.5 LFDD has only operated once since privatisation in 1990. This occurred on the 27th May 2008 after the loss of two large generators in rapid succession. There have been no occurrences of LFDD operation because of RoCoF to date. 1.6 National Grid has been working with the electricity supply industry to develop new frequency control services in response to the changing electricity generation and import mix. "Non-synchronous" technologies offer many benefits but do not provide the natural damping or "inertia" of the more conventional "synchronous" type of generation which is directly coupled to the network. This means that under high import or windy conditions, frequency will change at a faster rate than it does today, meaning more rapid frequency control capability is likely to be required. The Workgroup examining these requirements recommended that RoCoF settings should be reviewed for their future suitability. 1http://www.nationalgrid.com/uk/Electricity/Codes/gbsqsscode/DocLibrary/ Page 3 of 47 1.7 National Grid monitors frequency on the electricity supply system continuously and analyses frequency deviations in detail when they occur. Large frequency deviations do not occur very often, but when they do they can provide new information on system behaviour. Recent frequency deviations have allowed National Grid to re-assess system characteristics and take a view of future performance. The conclusion of this assessment is that there is a need to take action to ensure the minimum RoCoF protection setting of 0.125Hzs-1 will not be exceeded. 1.8 National Grid currently takes actions to ensure that the minimum RoCoF protection setting is not exceeded by procuring Balancing Services. The actions taken are either to pay for additional generators to run (these must be of a type which can limit the rate of change of frequency) or to limit the size of disturbance the system can be exposed to by reducing generator or interconnector output (or demand as the case may be). These Balancing actions are currently being taken during light load periods for more than half the weekends and some weekdays in the year. The costs of these actions are estimated at £10m pa with an upper value of £100m pa, rising into the future. In the future, fast acting control systems such as those described as Synthetic Inertia may provide an alternative solution but there is some uncertainty over whether this is feasible. The Purpose of the Workgroup 1.9 A joint Distribution Code and Grid Code Workgroup was established to examine: Whether there is still a need for RoCoF based Loss of Mains protection; The costs, benefits and risks of change to recommended RoCoF settings; and The need for a requirement for equipment to withstand a specified Rate of Change of Frequency. 1.10 The Workgroup developed a staged workplan and this industry consultation documents Workgroup discussions and presents proposals for the first phase of work. The proposals in this industry consultation apply to the protection settings for existing and new distributed generators with a registered capacity of 5MW and larger. Workgroup Activities 1.11 The Workgroup examined the potential RoCoF over the next decade and has concluded that rates of change in excess of 0.5Hzs-1 would occur for an event secured under the SQSS in a range of plausible operating conditions. 1.12 The Workgroup reviewed recent frequency deviations and observed that the measured rate of change during a frequency deviation can vary across the system. This is due to the transient nature of frequency deviations and the natural phase shifting effects of the impedances making up the total system. Amongst other things, this means that actual rate of change measured by a protection relay may be higher than the rate of change predicted by simulation. Page 4 of 47 1.13 Given the number and diversity of the stakeholders affected by the Workgroup's possible recommendations, an open letter was published2 setting out the potential need for change, what the change could be and how the decision making process would work. Two stakeholder workshops were also held, one in Glasgow and one in
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