reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

protect io n systems are run in short-time rated fire- proo f cables. These cables are designed to remain

'functional for a minimum of twenty minutes in a test fi re of 1000°C. Special attention is paid to the location an d installation of these cables to prevent accidental damage, and preserve their integrity in a fire, thereby ens uring that the operation of fire protection systems i s not jeopardised.

13.9 Batteries and chargers

In keeping with the high integrity requirements of fire protection systems, control supplies are derived from ", ..1 V lead-acid batteries operating in conjunction with aut omatic charging equipment. The chargers are sup- p lie d at 240 V single-phase 50 Hz and equipped with trickleand boost-charging manual selection facilities, battery charging current and voltage indicators, local alarms and repeat contacts for raising alarms on the central fire panel.

14 References

[II BS5000: Rotating electrical machines of particular types for particular applications

[21 554999: General requirements for rotating electrical machines

(31 BS5490: Classification of degrees of protection provided by enclosures

1 4 1 BS4683: Electrical apparatus for explosive atmospheres

151 555345: Selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres

161 CEGB Specification EES (1980): General specification for electronic equipment — this specification succeeds EES (1970): and EES (1962) bearing the same title

[ 7 ! BS800: Specification for radio interference limits and measurement for equipment embodying small motors, contacts, control and other devices causing similar interference

References

181 BS466: Electric overhead travelling cranes

[9]BS88: Cartridge fuses for voltages up to and including 1000 V AC and 1500 V DC

[10]BS5424: Control gear for voltages up to and including 1000 V AC and 1200 V DC

[11]ESI Standard 37-1: 415 V AC switchgear, control gear and fusegear

[12]BS3579: Heavy duty electric overhead travelling and special cranes for use in steelworks

[131 BS2655: General requirements for electrical, hydraulic or hand powered lifts

(141 BS5655: Lifts and service lifts

[15]BSCP407: Electrical, hydraulic and hand powered lifts

[16]BSCP326: The protection of structures against lightning

[17]CEGB Specification US/76/10: Control and instrumentation; General technical requirements

[18]BS417: Semiconductor rectifier equipment including transformers

[19]BS 171: Power transformers

(20]British Electricity Board Standard BEBST2: Specification for transformers and rectifiers

[211 BS2757: Classification of insulating material for electrical machinery and apparatus on the basis of thermal stability in service

[22]135159: Busbars and busbar connections

[23]BSI58: Marking and arrangement of switchgear busbars, main connections and small wiring

[24]BS6346: PVC insulated cables for electricity supply

[25]BSCPI003: Electrical apparatus for use in explosive atmospheres of gas or vapour

[26]BS4417: Semiconductor rectifier equipment including transformers

[27]BS5967: Operating conditions for industrial process measurement and control equipment

[28]CEGB Specification US/I2/50: General technical requirements for instrument and control equipment

[29]BS6007: Specification for rubber insulated cables for electric power and lighting

1 Introduction

2 Design criteria

3 Overall protection logic

4 Boiler protection

4.1General

4.2Low drum level or loss of boiler water 4.2.1 Steam/water mixture mass velocity

4.2.2 Steam/water mixture quality

4.3Loss of feedwater flow

4.4Loss of electric load

4.5Methods of protection

4.5.1Low drum level protection

4.5.2Loss of feedwater protection

4.5.3Boiler circulating pumps - unconditional signal 4.5,4 Sudden loss of steam demand {turbine trip)

5 Turbine protection

5.1Turbine trips

5.2Loss of lubricating oil pressure

5.3Condenser vacuum low {exhaust pressure high)

5.4Condensate conductivity high

5.5Manual trip lever

5.6Overspeed trip

5.6.1Choice of interlock

5.6.2Setting of the low forward power relay

5.7LP exhaust steam temperature high

5.8Loss of electric governor

5.9Low steam inlet temperature and pressure

6 Generator protection

6.1Stator earth faults (low impedance earthing)

6.2Stator earth faults (high resistance earthing)

6.2.1Current transformer requirements for protection using

relay R1

6.2.2 Matching transformer

6.3Stator phase to phase faults

6.4Stator turn to turn faults

6.5Negative phase sequence

6.6Loss of generator excitation

6.7Pole slipping

6.8Loss of stator water flow

6.9Hydrogen temperature high

6.10Hydrogen/stator water cooling flow

6.11Excitation failure

6.12Motoring of the generator

6.13Emergency pushbutton

7 Generator transformer and unit transformer protection

7.1Phase to phase and earth fault protection

7.2Generator transformer HV inverse time and high set

instantaneous overcurrent

7.3Unit transformer HV inverse time and high set instantaneous overcurrent

7.4Standby earth fault

7,5 Generator transformer and unit transformer internal faults

7.6Winding temperature

7.7Conservator 'low oil level' alarm

7.8Pressure relief device alarm

7.9Freezer air drier alarm

7.10Overfluxing

8 Station transformer protection

9 HV/LV connections and generator voltage/HV circuit-breaker protection

9.1Phase to phase and earth faults

9.2HV circuit-breaker faults

9.3Generator voltage circuit-breaker or switch disconnector

10 Pumped-storage plant protection

10.1 Dynamic braking overcurrent protection

10,2 Under frequency protection

10.3Over frequency protection

10.4Overspeed in excess of 10%

10.5Loss of pumping power

10.6Emergency stop pushbuttons

10.7Overvoltage

10.8Excitation equipment protection

10.9Stator cooling air over-temperature

10.10Bearing temperatures and oil levels

10.11Back to back starting protection

10.11.1Generator runaway

10.11.2Incorrect excitation levels on the generator-motor

10.11.3Excess heating of the stationary field winding in

11 DC tripping systems

11.1Logic diagram

11.2Tripping schematic diagram

11.3Trip supply and circuit supervision

11.4General comments on the tripping arrangements

12 Auxiliaries systems

overcurrent

1 2.3.4 Standby earth fault

1 2.4 Auxiliary generators

12.4.1Mechanical trips

12.4.2Electrical protection

12.4.3Gas turbines

12.5 Motors

12.5.1Motor circuits at 415 V (contactor circuits)

12.5.2Motor circuits at 11 kV and 3.3 kV

12.5.3Thermal overload relay

12.6Cables

12.7Busbar protection

12.8High breaking capacity (NBC) fuses

12.9Protection co-ordination

1 Introduction

I he poNer station generating system basically comprises cowmain plant areas (Fig 11.1):

seam raising plant (steam generator) or, for hydro

•,:ations , a water supply and/or storage system.

•A steam or water turbine.

•3, g enerator.

•Step-up and step-down transformers, switchgear and connections.

The last equipment connects the generator to its loads. A small percentage of the power (5 to I0 07o, approximately) provides services inside the power station (this auxiliary system is dealt with in Chapter 2), the remainder goes to the transmission network. It can be seen from Fig 11.1 that the power station system is

very closely interconnected, so that a single failure requires more than the disconnection of the faulted plant, both electrically and mechanically. This chapter deals with:

•The electrical and mechanical protection of the plant items for which faults result in the tripping of one of the main plant items (main unit protection).

•The protection against electrical faults in the auxiliary system (auxiliary system protection).

•The methods adopted to initiate the tripping of the other associated main plant items.

For completeness, individual plant protection systems, both mechanical and electrical, are explained and, where explanations are to be found in other chapters concerned with that plant item, reference is made to those chapters.

Included in the sections dealing with the protection of transformers, are the generator transformer, the station transformer, the unit transformers, together with their main electrical connections, switchgear and disconnectors (isolators). In nuclear power stations, the station transformer has become another unit transformer and the level of protection has been raised to that of the unit and generator transformers, irrespective of the supply voltage.

Details of the relays used today are given; in particular, where electromagnetic types have been replaced by digital, but with the developments in digital relay design, protective systems employed by the CEGB are continually changing. When digital unit protection systems were introduced in the late 1980s, protection schemes changed from two trip channels (1 out of 2) to three independent protection systems, each providing protection such that failure of one would neither cause nor prevent a trip. This improved reliability and provided the facility for on-load testing from the electrical trip initiating device to the turbine stop valves. This is explained in Section 11 of this chapter on DC tripping.

2 Design criteria

Before describing the individual protective systems, certain design criteria have to be established. Generally the protection system is designed so that if faults occur, the faulty plant is disconnected, whilst continuity of supply from the generators is maintained, consistent with system stability. Listed below are major requirements on which protection selection and settings are based:

•Faults on plant items must be disconnected as quickly as possible to minimise damage.

•The protective systems shall be stable for faults outside the protective zone.

•Faults which are not cleared by the faulty item's ow n protection, will be cleared by secondary or backup protection.

•Protection of plant is designed to match as clo se ly as possible the plant operating characteristics, e.g,,

negative phase sequence protection is designed to

match the generator thermal withstand to negati ve phase sequence currents.

•in general, protection systems should be designed

so that no single failure of a protective device cause s a trip or permits a fault to remain connected to

the system. The exception is where the reliability of the protective system is such that failure to trip is not considered credible, i.e., where equipment is installed in controlled temperature and humidity conditions and is fully dust-proofed. This applies particularly to electrical protection systems employ. ing electromagnetic relays where risk of malfunction is very low. All other protection devices, such as plant-mounted tripping devices, employ systems of at least 'two out of three'. It is likely that the compactness of digital relays will encourage full redundancy to be built into electrical protection as well.

•To facilitate testing or fault investigation with the generating unit on-load and one of the tripping systems isolated, the allocation of the output contacts of the protective relays must be such that the operation of any one relay does not cause tripping of more than one tripping system.

•Standing trip conditions when the generator unit is out of service must be avoided. For example, turbine and generator mechanical trips which would remain operated after the unit has been shut down are removed by normally open pressure switches mounted in the turbine hydraulic fluid pressure system.

•The facilities associated with a tripping system shall be physically segregated from the other tripping systems as far as is practicable, by using separate relay panels, separate terminal blocks in marshalling and terminating cubicles, and diverse routing of secondary cables.

•The detailed electrical connections for the protective relay circuits have to be in accordance with the appropriate CEGB Standards.

3 Overall protection logic

An overall protection logic diagram typical of nuclear stations is shown and described fully in Section 11 of this chapter, together with the tripping schematic developed from it.

The role of the unit protection is:

•To accept protection tripping signals from each of the main plant items numbered 1 to 4 in Fig 11.1.