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Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

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<<< < Предыдущая 82 83 84 85 86 87 88 89 90 91 92 93 94 9596 / 10696 97 98 99 100 101 102 103 104 105 106 > Следующая >>>

z	MECHANICAL PLANT & ELECTRICAL
	TRIP INITIATIONS

TURBINE

2POWER FLUID PRESSURE SWITCHES TRIP ON LOW PRESSURE

LOW VACUUM TRIP

1_-.1fs f_f,fSR:CA TING OIL PRESSURE 2 CONDENSATE CONDUCTIVITY HIGH

2 LOW INLET STEAM TEMPERATURE & PRESSURE ) 2k.GF1 ONLY HIGH WATER LEVEL IN STEAM GENERATOR .:PwR ONLY)

2 OvERSPEED TRIP

2 LOCAL TURBINE TRIP LEVER

21 LOSS OF SPEED GOVERNOR

REACTOR FOR NUCLEAR STATION ONLY/ TRIPS ACCORDING TO REACTOR TYPE

GENERATOR

2 LOSS OF EXCITATION 1 & 2

2 EXCITATION FAIL TRIP (ALSO STANDBY EXCiT FAIL)

2 STATOR EARTH FAULT INVERSE HIGH RESISTANCE 1 & 2 2 NEGATIVE PHASE SEQUENCE I & 2

2 STATOR DIFFERENTIAL 1 & 2

2 STATOR EARTH FAULT INSTANTANEOUS

2 STATOR COOLANT FLOW LOW DIRECTIONAL OVERCURRENT

GENERATOR TRANSFORMER

OVERALL PROTECTION BUCHHOLZ SURGE

Hv OVERCURRENT 10mT

2 Hy RESTRICTED EARTH FAULT

2 HV HIGH SET OVERCuRRENT

2 OVERFLUXING 1 & 2

WINCING TEMPERATURE (-IV & LV)

UNIT TRANSFORMER(S)

2 HS OVERCURRENT OVERALL PROTECTION BUCHHOLZ SURGE

-IV IDMT OVERCUR RENT (2nd STAGE) 2 LV RESTRICTED EARTH FAULT

2 LV STANDBY EARTH FAULT (2nd STAGE)

NV IDMT OVER CURRENT (1st STAGE)

2 LV STANDBY EARTH FAULT (1s1 STAGE)

WINDING TEMP (NOT REOD ON AN TRANSFORMERS)

Hi/ CONNECTIONS (POWER STATION)

FIRST MAIN FEEDER PROTECTION

2 SECOND MAIN FEEDER PROTECTION FIRST INTERTRIP RECEIVE

2 SECOND INTERTRIP RECEIVE

GENERATOR HV FEEDER (TRANSMISSION STN) 2 HV BUSBAR PROTECTION

2 HV BUSBAR BACK TRIP RECEIVE FIRST MAIN FEEDER PROTECTION

2 SECOND MAIN FEEDER PROTECTION

FIRST INTERTRIP RECEIVE

2 SECOND INTER TRIP RECEIVE

2 TRIP INITIATION HV CIRCUIT BREAKER FAIL EARTHING TRANSFORMER

1 EARTHING TRANSFORMER BUCHHOLZ

EARTH FAULT INVERSE HIGH RESISTANCE 1 & 2

GENERATOR VOLTAGE CIRCUIT BREAKER

2 FAIL PROTECTION

COMMON EQUIPMENT

I 2 EMERGENCY STOP BUTTON CCR

1 2 EMERGENCY STOP BUTTON LOCAL LV CONNECTIONS (POWER STATION)

2 LV CONNECTION PROTECTION 1 & 2

DC tripping systems

SEE		GROUP
SEE
NOTE fa) ; 1	2	4	5	6	7

7-1

Flo. 11.35 (cont'd) Overall protection logic diagram for main generating units

917

-	-
-

Protection

PPC	r__	S_PoL	,
	r__	S_PoL	,	PIE PP SELF RESET

				rvTh RJP API AS

REACTOR ESEENFAL PLAN: EROTP1- TiON GAS IICSLArIP - RIP cA.L '31- AGE 2.

„Rs,

- .•, NT-ERT-PP

Sc ID

SECOND

SEND

NIACA 1 -. , S

•

S ,

RELAy

1-t

-0-Auro qEsE ,

0 0

ILLR_ST MAN G ENERA

FEEDER

C NJ

TRIP RELAYS #5 .8

AuX RELAY

GEN TRANSF RES7PIT'EL:

—0 0

MI, RELAY IA

UT C OVERALL PRO rEC , ioN

4- - - - - - - - - - - - - - - - - - - - - - - - - - -

UT ID OVERALL PROTEC OLIN

3 I

C----w

-C 1

°--j..-

JrU BUCHHOLZ SU

r -1-

. TRIP

=i-

TURBINE TRIP SOLENOID 1

.I, RELAY

Av-

AUTO

—

0 UT 0 BUCHHOLZ SURGE

re+

. TURBINE TRIP SOLENOID 2

RESET

STATOR DIFFERENTIAL

7M5

II.

—

STATOR

EF - NvER SE HIGH

,) RESISTANCE

ALARM A

7.0

▪

LIEN SW DISC3NNEcT0p

NEGATIvE PHASE SECUENCE

I INDICA DON

—

GEN TRANSF OVERALL PROTECTION

LAST FMFP

UT c. LV CIRCUIT BREAKER

GEN

TRANsrUcHHotz

-1-

7,- ROuP •

SuRc.EONE PER PHA SFI

LAST SmFP r°-4—

—

LIEN SWITCH DISCONNECTOR FAIL

4I EEWeICIJIT

EARTHING TRANSF BUCHHOLZ SURGE .4

r-°-4—

GEN MAIN EXCITER FIELD CB

D-

CONNECTIONS PROT I

▪

FIELD CB

C HA OvERCuRRENT SEC3ND STAGE

7 Ton Jo.

• TJJ

0 SEc To

OPERATE)

UT O HS OVERCURRENT SECOND STAGE

•

)—

•-)

—C:=

r*,717T77:Is

CLC1

TO OPERATE)

EARTHING TRANSF E.F

INVERSE HIGH RES'S 1

0W FORWARD

- ----.

lo■ TRIP RELAY

POWER RELAY

TRIP RELAY 3 ,

AUTO RESET

MIX RELAY

IP R7

IP RA

IF R9

TRIP RELAY 3

cry

C-0

GROUP 21

—r'

LOW

VACUUM TRIP

8 STATOR

_L9

COOLANT TEMP

HIGH

n19-1". TRIP

TURBINE TRIP SOLENOID I

1 RELAY

TaiT

1-0.4_

TUR BINE TRIP SOLENOID 2

1 =4,—

ALARM 8

GEN SW DSCONNECTOR

4I.2_4.

INDICATION

...GEN MAIN EXCITER FIELD CB

1-9-C)---

F L

2IV1. D CB

r.. gala N?gEA

140

TRIP RELAY

AUTO RESET

TRIP RELAY 5

ALIX RELAY

0 LOSS OF EXCITATION I

TRIP RELAY 5

rYI7v-N

—

RV GENERATOR TRANSF

WT ONE PER PHASE)

4-- - - - - - - - - - - - - - - - - - -

• ROL,, P

LV GENERATOR TRANSF

▪ WT ONE PER PHASES

10-

TRIP

TURBINE TRIP SOLENOID

RELAY

7M5

US2T

TURBINE TRIP SOLENOID 2

P. ALARM AND

1-40

C.-1----.10SS OF ExCITATION RELAY

1--°-17°--1.-

0--1.. INDICATION

FIG. 11.36 Overall protection schematic diagram for main generating units

Chapter 11

TrznP RELAY B

•

4-11.

TO TRIP CONTACTS

918

DC tripping systems

GROUPS

GROUP ..]<

NOTE GROUP 7

					rPrP RE_Ax
				I	- itur0 .E SE
.7.c		RIP RELAY	I
		AUX RELAY
1 1.E., :.ENC7 PLS., 3L.T.7	1:1. NS	1- RIP RELAY :

1—()-

E AX

AUTO

1—n

ALARM

•477.1p

7, L

NOICATION

Nir AT NO

ANE

BY "s5

BTJE

rem

.PES .5_.PE

ra,c_,RE

•oI

,EACTCR QUADRANTS

TRIPP,NG

SYSI- EUS AI A2 El 92 ETC

RELATE r000ADRANTS

A 9

C AD

B ARE CZNNEC TEC 2 OUT OF ]

cor,F . GuRATION al THE REACTOR

op—. Al

-;2

0.7

11. SW:7CHGEAR ALA SN

GENERATOR SWITCH

0 0

— —

7,P,TE c,, ,E

Av TELEX

DISCONNECTOR

e-vThrTh

ME RELAY :2 secs,

: SW:7CHGEAR

RELAY

POSITION RELAY

175---

3vE RF Lui.4x;Nzpi_ AR T, EcTION

„--,---,-,

4— — —

-4I

i --

j... FIELD CB

...f.....0

:]?. R,-FErAy I '-'-1--

_ 1

(1)

-P"-GENERATOR MAIN EXCITER

C---1. GENERATOR SwITCN

-°

--1-

j aaT

r _l_...

( }jp.. OISCONNECTOR

OVERFLUXING

r_o I

il■

.1., ALARM &

RELAY !

' 7

PROTECTION

- 4

' C

7 j

INDICATION

-L-

UNIT TRANSF C .

UNIT TRANSr'C' NV FIRST STAGE OVERCURSENT

LX TRIP RELAY 1

--- UNIT TRANSF C WINGING-

- - - - - - - - - - -

-4

0 TEMPERATURE

1 ALARM d.

TRIP RELAY

no.4..I 0-0:i

INDICATION no.....L01

--p. i

AUTO RESET

II. J

FO_LO-11' UNIT TPANSF 'C'

-10. IN CB

UNIT TRANSF

0N11 TRANSF OHV FIRST STAGE OVERCURRENT

LV TRIP RELAY I

UNIT TRANSF WINDING

TEMPERATURE

ALARM &

P. TRIP RELAY

INDICATION

AUTO RESET

UNIT TRANSF 'Cy

, H1, CB FAIL! NOT SHOWN

Ir-LVC8

FIG. 11.36 (cont'd) Overall protection schematic diagram for main generating units

919

Protection	Chapter 11

TRIP

COIL

cv-Y-Ym—s—c=

SUPERVISION

RELAY

FR. 11.37 Trip supply supervision

FIG. 11.38 Trip circuit supervision

trip. For security, each of the smaller blocks Al, A2, etc., is separately cabled to the unit protection relay panel.

12 Auxiliaries systems

12.1 Operating criteria

Before considering the detailed protection requirements for the auxiliaries systems, it is necessary to define the relevant operating criteria. These will generally be the same as those defined in Section 2 of this chapter for the main plant systems. It is, of course, important to maintain continuity of supply to auxiliary plant as far as possible and also the stability of the unfaulted parts of the systems. The selection and setting of protection devices for the auxiliaries systems should therefore be based upon the following major requirements:

•Faults external to major power sources, i.e., unit transformers and diesel-driven generators, must only open the circuit-breakers controlling these power sources after all other protection nearer the fault has failed to clear the fault.

•Faults internal to a major power source shall cause its circuit-breaker to open as fast as possible to

ensure that the distribution system can restore itself within the limits of stability.

•The protection must be stable in transient conditions, such as motor starting, and shall not operate for current surges caused by faults external to the auxiliaries systems, which the main generator can safely withstand and which do not damage the protected plant.

•The characteristics of protection equipment must match the operating characteristics of the plant it is protecting and provide discrimination with the protection of other plant connected to the auxiliary system.

•All auxiliaries system faults must be cleared before the short-circuit capability of the cables, connections and switchgear supplying the faulted plant is exceeded.

12.2 Protection requirements

This section deals with the protection aspects of plant and equipment connected to the 11 kV, 3.3 kV and 415 V auxiliaries systems. The various protection schemes used to cover the different protection require - ments for the various kinds of auxiliary plant normally

920

Auxiliaries systems

d in a power station are reviewed. The overall

protection were not provided, phase to phase faults

,tin

as defined above,i s to provide safe and

would be cleared by the inverse time overcurrent relay

protection to ensure that the faulted element

on the HV side of the transformer and, since both

''' `

ec.1 as quickly as possible, thus minimising

these relays would 'see' the same fault current, both

r cn

transformers would be tripped.

ANion to the remainder of the system.

:,..

With the low cost of current transformers at 11/

„nownou s Wi al this requirement is the need to

dc protection which will be highly selective and

3.3 kV and no restrictions on space for mounting

,,j!i[iiinaiive in its action, and that the operation of

them in the circuit-breakers, the restricted earth fault

relays is ,:o-ordinated to give complete protec-

protection is fitted as a separate protection scheme.

0 the circuits concerned and ensure that. as far

Figure 11.39 shows how the protection is applied. The

high voltage restricted earth fault protection is con-

pc—ible, only faulted plant is disconnected.

nected in the residual circuit of the overcurrertt pro-

Auxiliary transformers

tection and because the HV side of the transformer is

12.3

delta connected, earth fault currents do not appear in

the relay coil for faults on the LV side. The relay is

pr inciples adopted to protect the generator and unit

therefore of the instantaneous high impedance type

-.,:isformers, described in Section 7 of this chapter,

e qually to the auxiliary transformer circuits. Due

as used on the main supply transformers and described

in Section 7 of this chapter.

main to the smaller ratings, there are the

differences in the methods adopted when apply-

The LV restricted earth fault protection uses the

die protection.

standard scheme of three CTs in the LV circuit-breaker

balancing against one current transformer in the trans-

123.1

Phase to phase and earth fault protection

former neutral. This arrangement and the overall biased

differential protection arrangement is shown in Fig

practice on auxiliary transformer circuits is not

(1•0[3

11.39.

;)ro%

ide biased differential protection on 3.3/0.415

:ransformers, and below 10 MVA on 11/3.3 kV

12.3.2 Winding faults and transformer overloads

ormers. However, if two auxiliary 11/3.3 kV

-. : ;formers are required to operate in parallel, then

Winding faults in all oil-filled auxiliary transformers

, ,,ed differential protection is fitted to both, irre-

are detected in the same fashion as for the major

of rating: this is to avoid both transformers

transformers described in Section 7.5 of this chapter.

nping

for a phase to phase fault on either trans-

However, since all auxiliary transformers are naturally

• • aier. Referring to Fig 11.39, if biased differential

cooled, winding temperature protection gives an alarm

WHEN SPECIFIED

PEOTELTICN

HV PROTECTION

BIASED

DIFFERENTIAL

PROTECTION

7-1

AUXILIARY

TRANSFORMER

RESTRICTED

3 - POLE

OVERCU PRE NT

EARTH

RELAY

FAULT

STANDBY

7 7

EARTH

FAULT

FIG. 11.39 Auxiliary transformer protection

921

Protection	Chapter

only. The winding temperature device is similar to that used on oil-filled transformers and explained in Chapter 3. On the smaller oil-filled transformers (3,3/ 0.415 kV), without a conservator oil tank, the Buchholz relay is replaced by a top-oil temperature device which is set to alarm for an overload condition. This is more Cully described in Chapter 3.

These 3.3/0..415 kV transformers and all air insulated transformers rely on the earth fault protection for fast clearance of winding faults.

12.3.3 HV inverse time and high set instantaneous overcurrent

The protection is applied in the same way as for the unit transformer described in Section 7.3 of this chapter and shown in Fig 11.39.

The auxiliary transformer back-up protection for line to line faults on both the HV and the LV transformer connections and all plant connected to the LV side of the transformer, is provided by a relay which has an inverse characteristic, except on 3.3/0.415 kV transformers where the relay has to operate in conjunction with a 415 V fuse. In this case, an extremely inverse relay is used, since its characteristic is very si milar to that of a fuse. This gives better co-ordination between relay and fuse and is explained fully in Section 12.9 of this chapter. This relay is connected to current transformers which are located on the HV side of the transformer (Fig 11.39). The three-pole elements ensure the same clearance times for all line to line faults on the LV side of the transformer.

On 3.3/0.415 kV transformers of 1 MVA and below, a switching device is used on the 3.3 kV side of the transformer circuit. This consists of a circuit-breaker and a fuse (see Chapter 5). Faults above the circuitbreaker fault interrupting capability are cleared by the fuse and the combination, as far as the protection is concerned, can be treated as a circuit-breaker, allowing the use of instantaneous relays. Figure 11.40 shows that the instantaneous relay reduces protection clearance times for cable faults, the time reduction being between 50 and 100 ms.

All high set instantaneous overcurrent relays have a low transient over-reach to prevent operation due to faults on the LV side of the transformer. This is explained in Sections 7.3 and 12.9 of this chapter.

12.3.4 Standby earth fault

Back-up earth fault protection against all earth faults occurring on the LV side of the transformer is provided by a single-pole inverse time relay. This relay is operated from the neutral CT which is located in the neutral of the LV winding. The protection application is described in Section 7.4 of this chapter.

12.4 Auxiliary generators

As part of the design philosophy or the maintenance

ovERcuRRENT-	XX
PROTECT1CN RELAY	XXX
	XXX

4ceA

FUSE

HIGH SET

NSTANTANEGUS

OVERCLIRRENT

PROTECrION RELAY

0 31

FIG. 11.40 Protection co-ordination for a fused switching device

of secure electrical supplies to both station and unit auxiliaries boards, it has been the practice for many years to provide additional local generation at strategic points within the power station network, so that the safe operation and control of certain essential plant can be carried out during periods of enforced disconnection from the grid system (see Chapter 1). This is achieved by connecting diesel or gas turbine driven generators directly to the 11 kV, 3.3 kV or 415 V busbars of selected auxiliary switchboards. In this context, the term 'local generation' refers to generators which are connected to these switchboards as compared with the main generator which provides generation to the national grid system.

The protection scheme for local generation plant must take into account, and cater for, the mechanical failures of the prime mover.

The protection scheme divides faults into those which must trip circuits and those which alarm. The distinction is made on the basis that an operator cannot he expected to correct a fault in less than five minutes from it happening. Any fault which cannot be sustained for five minutes without becoming a danger to personnel or causing serious damage must be arranged to initiate an automatic trip. The philosophy is similar to that adopted for the protection functions of the main generator unit.

To illustrate the protection philosophy for local generating plant, a diesel generator system is described to show how the electrical and mechanical protection requirements of the generator set are met and how

922

				Auxiliaries systems

Ho fit into the overall protection scheme. From a				but at a reduced generated output, typically between
			point of view, the tripping functions cart	20 and 45 07o of the generator full load capability.
		,idej into t\.vo main categories — mechanical		It follows that when fuel oil is available from the
	ii			gravity tank, it is not necessary to provide a trip signal
		elec[rical.		to the generator circuit-breaker for fuel system faults.
				to the generator circuit-breaker for fuel system faults.
			Mechanical trips	Under these conditions, sufficient time is available for
	2 41		Mechanical trips	the operator to take remedial action and synchronous
	2 41		of mechanical faults will be confined	the operator to take remedial action and synchronous
			of mechanical faults will be confined	motor operation of the generator is neither harmful to
			v.hich necessitate [ripping of the generator	the generator nor to the auxiliary supply system.
		Hbreaker. The protection requirements in respect		With this fuel system design it is only necessary from
	,.		pes of mechanical fault are dealt with	a protection point of view to provide an alarm to
		Hi	pes of mechanical fault are dealt with	a protection point of view to provide an alarm to
		Hi	other	indicate the loss of one fuel oil pump, alerting the
	L hapter 9.			indicate the loss of one fuel oil pump, alerting the
			memioned previously, the essence of any pro-	plant operator to monitor the situation and take the
			system is that it must be capable of detecting	appropriate action.
		Abnormal condition in the equipment it is protec-
	:2 at an early stage and of providing means of			12.4.2 Electrical protection
	:	[irlg the faulty element before any serious damage		12.4.2 Electrical protection
		[irlg the faulty element before any serious damage		A typical electrical protection scheme for the generator
				A typical electrical protection scheme for the generator
	Fhb, philosophy is achieved by measuring the flow,			and connected loads is shown in Fig 11.41. The protec-
•.::lirerature and pressure of the fluids which are essen-				tion scheme illustrated is designed to give phase and
..11 io the healthy operation of the engine. It is normal				earth fault protection, using a combination of main and
	,Jio ose lubricating oil and cooling water for this			back-up systems to ensure that a fault on the generator
; , „rpose, since their behaviour accurately reflects the				circuit disconnects the generator as quickly as possible.
		of the engine.		The discrimination necessary can only be obtained by
	Hie		potentially damaging situations in which pro-	correct relay co-ordination. It is necessary to disconnect
			is needed, are listed below. The detection of any	the generator rapidly in order to minimise damage, to
‘: !hese faults results in the tripping of the circuit-				maintain continuity of supply to other connected loads,
sr{:.11.0.[r and eventually the shut down of the engine:				and also to maintain a stable auxiliary supply system.
•	Lubricating oil pressure low.			The network shown is a single busbar system feeding
•	Lubricating oil pressure low.			two outgoing feeders, each of which is controlled at
	Lubricating oil temperature low.			two outgoing feeders, each of which is controlled at
•	Lubricating oil temperature low.			both ends by circuit-breakers.
• Cooling water temperature high.				Any phase or earth faults occurring on the generator
• Cooling water temperature high.				windings will be detected by a differential protection
• Cooling water flow low.				windings will be detected by a differential protection
• Cooling water flow low.				system, as described in Section 6.3 of this chapter.
•	Emline overspeed.			The zone of protection extends beyond the feeder side
				of the circuit-breakers, which means that the generator
1. 'Ain be apparent from this list, that faults associated				protection system also caters for phase and earth faults
		the fuel system supplying the diesel engine do not		on the busbars as well as the circuit-breaker itself
	qui re		circuit-breaker tripping. Serious consequences	and part of the feeder circuit-breakers. The protection
•ould ensue if the generator is not disconnected quickly				system is arranged to trip the generator and the two
	un the electrical system when fuel supply is lost,			feeder circuit-breakers.
		the tripping of the circuit-breaker for failure of		The feeder protection is described in Section 12.6
	fuel supply system is unnecessary if a supply of			of this chapter.
	ILl	to the engine can be maintained for a finite period		Standby earth fault (two-stage)
.1 time. The design of the fuel system is intended to				Standby earth fault (two-stage)
- .th:r		for this. As described in Chapter 9, the fuel oil		Should an earth fault on the system not be cleared
‘.ipply arrangements on site consist of a main tank,				by any of the protection schemes described above,
H. tank and a gravity tank. The main tank supplies				then the two standby earth fault protection relays fitted
1	c Jay tank, which in turn supplies the fuel to the			to the generator neutral will clear the fault. The pro-
-[::41ne via the normal fuel feed system. The gravity				tection scheme is described in Section 7.4 of this
. .4.111, acts as a back-up supply in the event of failure				chapter. The first stage trips the generator circuit
	he		day tank supply system for any reason.	breaker only: if this fails to clear the fault in 0.3 s,
	Llider normal operating conditions, fuel oil pumps			the second stage trips the generator. Thus the protec-
.-;! hued in duplicate to feed the fuel oil to the engine				tion acts as a back-up to the generator protection
..nder			pressure from the day tank. Should either or	but is principally for the protection of the busbars,
	Jai of these pumps be lost, then the system transfers			circuit-breakers and uncleared faults on the 3.3 kV
"lorna.tically to the gravity tank system whereby fuel				system. It therefore must co-ordinate with the 3.3 kV
,	gravity fed to the engine. The diesel engine will			auxiliary system protection. This is dealt with in Section
orninue to run for several hours under this regime				12.9 of this chapter.

923

Protection	Chapter ii

DIESEL

GENERATOR

2 STAGE sTA.,:m3Y EARTL.

FAULT 14 .Lxv


			3 POLE
	3 POLE		3 POLE
	CVERCURRENT		OVERCURRENT
	RELAY		RELAY

DIFFERENTIAL

PROTECTION

DIFFERENTIAL

PROTECTION

ESSENTIAL AUXILIARIES

SWITCHBOARD

FIG. 11.41 Electrical protection scheme for an auxiliary diesel generator

No reverse power relays have been fitted to guard against the generator running as an induction generator. The mechanical protection already outlined, adequately protects the diesel generator against loss of drive and the fitting of circulating current protection to the generator removes the need for fast operating reverse power protection for generator faults. The overcurrent protection of the feeder circuits provides back-up protection for uncleared generator faults, as is the case for the main generator at fossil-fired power stations.

12.4.3 Gas turbines

Gas turbines, as well as being used as prime movers for the generation of emergency electrical auxiliary supplies, are also used for grid supply system support and black start (loss of grid supply). If the last two functions are not required, diesels have the advantage of faster, more reliable starting for emergency supplies to auxiliaries. Gas turbine units are very much larger than diesel sets, sizes varying between 17.5 MW and 70 MW. The protection requirements are the same as for diesel generators, except that the gas turbines has a unit transformer directly connected to supply its auxiliaries. This modifies the protection to that shown in Fig 11.42.

The electrical protection requirements of gas turbine units include both generator and unit transformer pro-

tection. A unit scheme is used to detect phase and earth faults in the machine generator windings and its connections at 11 kV. The zone of protection indicated includes the interconnecting cables between the alternator and incoming side of the 11 kV switchboard as well as between the alternator and the HV connections of the 11 kV/415 V gas turbine unit transformer. Backup overcurrent protection is provided using a voltage controlled overcurrent relay. Referring to Fig 11.43, the relay has two operating characteristics determined by the operation of the instantaneous undervoltage unit. On overloads, its characteristic matches the thermal characteristic of the generator. Under fault conditions, the undervoltage element operates, changing the characteristic to the fault characteristic and ensures positive operation on the low value of sustained fault current encountered on synchronous machines. The fault characteristic is the standard IDMT and therefore allows close grading with the overcurrent protection on the outgoing circuits of the 11 kV switchboard.

The gas turbine unit has negative phase sequence protection fitted to protect the machine against uncleared faults external to the machine. It is omitted on emergency diesels in nuclear power stations in the interests of safety and continuous running during a reactor post-trip situation.

A two-stage standby earth fault protection is provided as back-up protection for the uncleared earth faults in the gas-turbine generator circuit and Ii kV

924

Au x ka ries systems

VOLTAGE

CONTROLLED

OVERCURRENT

RELAY GAS TURBINE

GENERATOR

	2	STAGE
	STANDBY
BIASED		EARTH
DIFFERENTIAL		FALLT
PROTECTION

DIFFERENTIAL

PROTECTION

GT UNIT

TRANSFORMER

3 POLE

OVERCURRENT

RELAY

FIG. 11.42 Electrical protection scheme for an auxiliary gas-turbine generator

•.■ .!iarv system faults. It also acts as protection to he eenerator for all 11 kV busbar faults. The first

.•	of the protection is designed therefore to open
•	i1 kV circuit-breaker only and, if this fails to
	he fault, the second stage trips the gas turbine.

1 he protection fitted to the 11/0.415 kV gas tur-

. :le unit trarr*ormer is the same as that described in 12.4.2 of this chapter.

ironical trips

rollowing is a list of the gas turbine mechanical

eenerator

•I aw vent temperature high.

•lubricatLng oil pressure low.

•\ nti-icing system failure.

•[Lurie failure.

•t. efl temperature high.

•lir intake differential pressure high.

I ll ■eessive.vibration.

8 bud inlet pressure low.

Gus pressure unbalance.

.i. cr turbine

•F\cessive vibration.

•ubricating oil pressure low.

•Stator temperature high.

•Bearing temperature high.

•Rotor temperature high.

•Gas inlet temperature high.

•Overspeed trip.

Electric generator

•Outlet cooling air temperature high.

•Lubricating oil tank level low.

•Excessive vibration.

•Exciter bearing temperature high.

•Air inlet differential pressure high.

•Lubricating oil pressure low.

•Generator reading temperature high.

All the above protective devices trip the generator 11 kV circuit-breaker and shut down the gas turbine by tripping the fuel valves.

12.5 Motors

The protection scheme for a motor circuit is based on a thermal overload relay which, in addition to the thermal overload, protects against open-circuit unbalanced phases and stalling conditions. Phase to phase and phase to earth fault protection is provided, depending on whether the circuit is controlled by a circuitbreaker or a motor switching device which can interrupt the maximum fault current for the switchboard, or a

925

Protection	Chapter 11

• TRIP2ALARM• 3 (tCOT

-0 ()-

AUX

UNIT

F--

OPERATINC, TIME s

30 -

26 -

22 -

20 -

18 -

16 -

14 -

12-

8 -

6 -

4 - FAULT CHARACTER/Sflc

2 -

VOLTAGE

UNIT

NOTE -

ALL THREE ELEMENTS

ARE THE SAME AND ONLY

ONE IS SHOWN ALARM AND

TRIP CIRCUITS ARE PARALLELED

OVERLOAD CHARACTERISTIC

2 4 6 8 10 12 14 1 6 18 210 CURRENT IN MULTIPLES OF PLUG SETTING

Time - current Characteristics al trre multiplier Setfing 1 0

Fic. 11.43 Voltage-controlled overcurrent relay

contactor with a limited fault interruption capability. At II kV, all circuits have circuit-breakers and at 3.3 kV all circuits have either circuit-breakers or motor switching devices. At 415 V, motor circuits are controlled by a fused contactor which is limited to inter-

rupting the motor starting currents; this restriction prevents the use of an instantaneous relay to provide instantaneous protection for phase to phase faults.

The protection philosophy adopted by the CEGB on motor circuits is discussed in the following sections.

926

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