Protection

CURRENT SITUATION IN UK POWER E'LAhTS R W Oldham Scottish Hydro-Electric plc

Abstract Before Privatisation of the UK Electricity Supply Indushy, mosi! protection design and applicaiion work on generating assets WQS carried out in-house. The protection schemes were qecijled in detail and the setrings were calculated and qplied by utili@st& Under the impetus of Privatisation and the move to turnkey constmetion iofnou generating plant, the repomibilipfor some of this work has shrfrd to the contrrrctor m d the consultant. Relay marmjacturers have brought several multi-finction generator protection reiays to the market, some of which suggest that a complete generator protection packge is available in one bar. n i s has prompted protection engineers to re-evaluate their requirements and re-awess their funciional requirements. 7hispaper does not purport to offer the answers: mere& to examine some of the issues. 1,

INTRODUCTION

Prior to Privatisation of the UK electricity supply industry (ESI), the North of Scotland Hydro-Electric Board (NSHEB)owned generating assets only withiin it'i geographical area. These assets consisted of over sixty conventional hydro-electric powar stations, two pumped storage hydro-electric power stations, Pcterhead oivgas fired steam turbine power station and several small diesel stations. The electrical protection of these generating assets have trsditionafly been designed in-house and the associated settings have been determined and applied in-house. The work associated with their installation has normally been carried out by contractors. Privatisation saw the creation of Scottish Hydro-Electric plc (Hydro-Electric) which took over all the generating assets of NSHEB with the exception of Cruachan pumped storage power station which was transferred to Scottish Power. Since Privatisation, Hydro-Electric has embarked upon a strategy to build up generating assets outwith it's inherited geographical area. In the main, this has taken the form of new Combined Cycle Gas Turbine (CCGT) power stations and new Combined Heat and Power (CHP) stations. 2.

EXISTING HYDRO-ELECTRIC POWER STATIONS

The majority of Hydro-Electric's portfolio of conventional hydra-electric power stations were built between 1946 and 1963 and were designed to be permanently marmed. Consequently the "turbine driver" was looked upon as providing the "back-up" protection in the went of a failure of the "main" generator protection. The situation today is somewhat different. All these power stations are controlled from two remote control centres and there is no longer a permanent presence at these stations. To

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accommodate this change, protection suppjy and trip circuit supervision schemes were installed where they had previously been absent or designed for site annunciation only. 2.1

The Application of Discrete Relrys.

For mmy years Hydro-Electric have been carrying out refurbishment of the protective schemes associated with these hydro-electric stations and usually this has entailed replacing the generator protection with new relays. Often it has also involved the installation of protection functions such as negative phase sequence protection and field failure protection which were not part of the original scheme.

These schemes generally employed a single tripping system and back-up protection was normally limited to the use of voltage restrained overcurrent protection to ensure generator stator phase faults were cleared in the event of the stator dserential protection failing to operate. Refurbishment projects have also been used as an appropriate opportunity to replace existing generator system voltage low resistance neutral earthing installations with the high impedance distribution transformer method of earthing. This has been done to reduce the incidence of stator core damage in the event of stator insulation breakdown. In common with other generator electrical and mechanical faults, earth faults are not detectable from another external circuit breaker position as would usually be the case on a distribution or transmission system. Consequently, the associated protection system is duplicated through to the trip relays and on 8 r e m t project this principle was extended to include duplicate circuit breaker trip coils.

It is recognised that generator protection schemes installed before the appearance of mufti-findon relays were not filly dupIicated but they did provide "dispersed intelligence" because they used discrete relays with dedicated functions. It is also true that the condition of many components of the protection scheme would be unknown between maintenance visits. Nevenhdess such schemes have proven reliability and dependability and were considered sufficient for hydro-electric generators of less than 50 MW rating. 2.2

The Appiication of Multi-Function Relays

The introduction of multi-finction, numerical integrated generator protection relays offers many advantages, e.g.:-

Continuous supervision from the remote control centre. Disturbance record capture. Scheme logic flexibility. The use of a single multi-function relay for generator protection which employs the traditional "energisc to trip" philosophy suffers from the obvious risk that the generator could be completely denuded of electrical protection shouId the single relay fail. This may be considered to be an aeceptable level of risk if the control room communications can be relied upon. If a "ffailsafe tripping" philosophy was adopted instead, the single relay principle may be more acceptable ES it should always tend to protect the plant. This is an attractive solution

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for small generator plant but one which requires the multi-fbnction relay to trip the plant upon relay intemal failure.

The generating plant under consideration are nomrally connected to Hydro-Electric's 132kV system and as such the generator protection scheme has to interface with the transmission protection schemes designed on the "energise to trip" philosophy. In the interests of compatibility the generator electrical protection scheme uses the same philosophy.

The risks associated with the possibility of a failure of the single relay protection scheme to operate in the event of a fault are considered to be unacceptibly high when employed in an "energise to trip" scheme so the basic principle adopted was that no one failure of M element of the protection scheme should prevent successful fault clearance. Relay manufacturers provide multi-hnction relays which can be configured to comply, in principle, with this clause and can be loosely classified into two basic schemes:a)

Duplicated relays with all protective fumtions duplicated.

b)

Several separate relays which between them provide the main function and the back-up function for each typo of fault in different relays.

UK past practice has been generally to use first and second protection schemes based upon either different measurement principles or different manufacturer's equipment. However several manufacturers only market multi-function relays which employ identical duplicated channels. There must be some conccm that a common mode hardware or software failure could occur with such equipment. If different manufacturer's equipment are used will they be compatible? Will separate instrument transformers be required for finctions such as stator diffcrtntial protection? Furthermore, there are the problems associated with different communication interface requirements. These problems should not exist where only one manufacturer is providing the complete scheme. New power stations, constructed under turnkey contract, will generally only employ one manufacturer's relays for the generator protection. These are powefil arguments against perpetuating past practice. 3.

NEW CCGT POWER STATIOSS

Hydro-Electric has an equity stake in two new CCGT stations; Keadby and Seabank. Both stations are based upon a single module consisting of two gas turbine-generators and a single steam turbine-generator with rated station outputs of 68OMW and 7 5 5 M W respectively. The basic system configuration of Keadby is shown in Figure 1. This station entered revenue earning service earIier this year.

The generator protection schemes for both stations comply with the specified requirement that no one single failure should prevent tripping in the event of a fault. The Keadby schemes use duplicated discrete relays, duplicated tripping systems and duplicate circuit breaker trip coils to achieve this. The contract for Seabank was only placed with the contractor earlier this year but it is known that the generator protection scheme will be based upon separate multi-function numerical

relays; all from one manufacturer, which between them will provide the main function and the back-up fbnction for each type of fault in different relays. In common with several other UK CCGT stations, a turnkey contractor has been employed on both sites. This form of contract SMS the responsibility for the design and application of electrical protection schemes from the purchaser or the "Engineer" to the contractor. Naturally the contractor is still required to comply with the contract specification but there has been a general movement since Privatisation towards the use of finctional rather than detailed specifications. In part, this movement has been motivated by a desire by the purchasers to comply with European Procurement Directives . It should be evident from the questions posed by the application of generator protedon to existing hydro-electric plant (see 2.1 above) that a functional specification of requirements

may be the most sensible way to proceed with multi-function relays.

Both stations are connected to the National Grid Company (NGC) 400kV system and are required to comply with the NGC Grid Code [I] and the Supplemental Agreement between the power station owner and NGC. In keeping with the "key contract concept, the owner appoints the contractor as his agent to carry out the work under the turnkey contract and the requirements are included in the contract specification.

The experience of Keadby was that there were few instances where NGC's protection That may be because detailed functional requirements were disputed by any party. requirements were set out in the contract specification and a conseruative interpretation was made of the clauses within CC6.2.2 of the NGC Grid Code. "Detailed functional requirements" should not be seen as specifjing acceptable cable sizes but more stating clearly what protective and supewisory functions are required. 4.

NEW CID' POWER STATIONS

New CHP power stations are normally developed in conjunction with a host process plant which has a large requirement for steam. Obvious candidates include process chemical plants and paper mills. Whilst the electrical generating capacity of these plants rarely exceeds 100 M W , these plants often present problems with interfacing to the host plant,

System design problems can include the relatively IOW fault withsrand rating of the host plant's electrical switchgear and the use of low resistance neutral earthing on the system which one would wish to connect the new generating plant. These power stations are usually connected to the RegionaI Electricity Company (REC) distribution system at either 13tkV or 33kV where the requirements for connection are the Distribution Code [2], the Connection Agreement between the REC and the power station oyner and Electricity Association documents G59/1 [3 J , Em113 [4)and G75 [SI. The main distincrion benveen NGCs requirements and the REC's requirements is that with the former, power stations are expected to help to support the Grid whilst in the latter case they are not. Two contract strategies have been pursued in CHP power station construction. Some contracts have been tumkey with a consultant retained to act as the "Engineer" and the others

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have been multi-contract with the consultant co-ordinating activities, Hydro-Electric involvement is not focussed at the detailed engineering.

In both cases

All generator protection schemes installed to date in these CHP stations have been single tripping schemes using discrete relays. 5.

CONCLUSIONS

Hydro-Electric has placed it's first contract for the installation of ten nnulti-fbnction generator protection relays at two hydro-electric power stations. These are arranged as duplicated protection relays with all protective functions duplicated. In this iristance, both relays arc provided by the same manufacturer but the original intention was to use different manufacturers' equipment for the first and secand protection schemes. This could not be achieved uithh the project programme constraints. Whilst Hydro-Electric has a preference for using different manufacturers' equipment, closer inspection of the equipment available on the market reveals how difficult it can be to achieve this objective without introducing significant complexity and without being forced to exclude some of the available designs on the grounds of incompatibility. 6.

ACKNOWLEDGEMENT

The puthat wishes to express his rhanks to Scottish Hydro-Electricplcj'or granting permission to publish thispaper, References

The Grid Code, The National Grid Company plc The Distribution Code of the Public Electricity Suppliers of England and Wales Engineering Recommendation GS9/1, Recommendations for the connection of embedded generating plant to the Regional Electricity Companies' distribution systems. Published by the Electricity Association. Engineering Technical Report ETRl13, Notes of Guidance for the protection of embedded generating plant up to 5 Mu' for operation in parallel with Public Electricity Suppliers' distribution systems. Published by the Electricity Association. Engineering Recommendation G75, Recommendations for the connection of embedded generating plant with outputs over 5 MW to Regional Electricity distribution systems above 20 kV. Published by the Electricity Association.

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C 1996 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WC2R OBL. UK