THE SPECIFICATION AND CONTROL OF THE PHASE SHIFTING TRANSFORMERS FOR THE ENHANCED INTERCONNECTION BETWEEN NORTHERN IRELAND AND THE REPUBLIC OF IRELAND

摘要

NIE is the electricity transmission and distributioncompany for Northern Ireland and the ESB is theelectricity generating, transmission and distributioncompany for the Republic of Ireland. Theinterconnection of both electricity networks bringsbenefits to each company in the form of sharing ofgeneration reserves facilitating the liberalising ofthe electricity market in the Island of Ireland andgenerally improving the quality and continuity ofelectricity to their respective customers.At present there is a 275kV AC double circuitinterconnection and two standby 110kV ACinterconnections between the NIE and ESBnetworks. The 110kV AC interconnections are runnormally open and are only closed in the event of asystem emerging on either network. The 110kVAC interconnections need to run normally openbecause it is currently not possible to control thepower flows through them and under certainnetwork conditions it is possible that theseinterconnections could be overloaded. Theinstallation of phase shifting transformers on theselines will control power flows and will enable the110kV interconnections to run normally closed andthus enable both NIE and ESB to accrue thebenefits of full interconnection.Phase shifting transformers can be considered‘special transformers’ as only a relatively smallnumber exist world-wide and they are alsogenerally designed to be site specific. The paperreviews the application of these specialtransformers to interconnect the two separateelectricity networks on the Island of Ireland.The paper reviews the network studies necessary tospecify specific parameters of phase shiftingtransformers. This includes an understanding ofthe key operating principles and how alternativedesigns influence the performance of the devices.Many phase shift transformers only provide aquadrature type of voltage tap-change controlsystem to control MW flows. The phase shiftingtransformers used for interconnecting the NIE andESB networks also have a series type of voltagetap-change control system to control MVAR flows.The paper describes how the interaction betweenthese two tap-changer control systems affects theoutput of the advance and retard angles of thedevices.Phase shifting transformer control schemes are sitespecific and may be simple or very complex. Thepaper reviews the control systems for the phaseshifting transformers under normal and abnormalsystem conditions. The control systems for thesedevices are generally not available off the shelf.They generally need to be specifically developed tomeet the requirements of the network operators.As described earlier, the phase shiftingtransformers will have both MW and MVAR tapchangecontrol systems. The paper describes thedevelopment of a control system to achieve optimalconvergence from one operating point of MVAflow and MVAR flow towards a new operatingpoint, during normal system operating conditions.The paper then goes on to describe how the controlsystem operates to support each network duringabnormal system conditions.The aim of this paper is to provide guidance toother electricity utilities who may be consideringthe application of phase shifting transformers ontheir networks, based on the experience ofNorthern Ireland Electricity and the ElectricitySupply Board on the Island of Ireland.