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Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

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main connections

and teed

System descriptions

1 32 KV

1 32KV

EHV GRID

1 ")

CONNECTIONS

( 1

3cs

400KV

(

GENERATOR

TRANSFORMER 31

TRANSFORMER 32

GENERATOR

UNIT

TRANSFORMER 31

TRANSFORMER 32

STATION
TRANSFORMER
	31
11KV	(	(
2)		i3Cw
2)	2 ) ))))
L_LD__J		jPEEiDs

3 3XV

TURBINE

AUXILIARIES

LTURBINE HOUSE

415V

OTHER AREAS

ESSENTIAL

DIESEL

ELECTRICAL

GENERATORS

SYSTEM

()

3XV

3 3K V)

BATTERY CHARGERS AND LOW VOLTAGE SYSTEMS

FIG. 1.11 PWR electrical system showing inter-relationship with off-site systems

Earthing of the generator system is achieved in the normal manner using generator neutral earthing modules. To cater for the period when the generator circuit-breaker is open, system earthing, i.e., on the generator transformer side of the disconnector, is achieved by earthing the unit transformer HV winding rather than using a dedicated earthing transformer or

a system earthing module connected to the 18 kV System.

The number and rating of the unit auxiliaries led to the adoption of 415 V as the unit system voltage. On the station system however, because of some larger motors (up to about 850 kW), a station system voltage level of 3.3 kV was chosen.

The system design incorporates three station transformers, each rated at 10 MVA off the 18 kV to three of the six machines. In this way, station transformer capacity is available such that

Electrical system design					Chapter 1

		400Av	23 5.4001,V GENERATOR	STATION
			23 5.4001,V GENERATOR	STATION
			7pANsFoRmER	TRANSFORMER I
	5 " JR! ,
TRANSFORMER
NEDTRAL EARTH
RESISTOR B				1 I kV STATION BOARD)
				1 I kV STATION BOARD)

UNIT AUX

TRANS A

							?C	UNIT
					PUMPCWMAIN		?C	ALIT TRANS
					PUMPCWMAIN	PUMPFEEDMAIN	MAIN	ALIT TRANS
		NEUTRAL EARTH ‘-,-'					FEED PUMP
							FEED PUMP

		RESISTOR IA		REACTOR
				COOLANT	0
				PUMPS	0
L-r1 3 3KV UNIT AUX BOARD 1					,t	33W UNIT AUX BOARD
	6 iL		,t	A	,t
Y			S EC T	A	`111		SECT

CONDENSATE

TURB HSE MAKEUP

CWAUX

EXTRACTION

C.)

PUMPS

SERV

TRANS CA 30%

TuRB HSE.

SERv TRANS 18

415V TUBE HSE

415V TURB. HSE

SERVICES

LOAD CENTRE ■ A

WAD CENTRE 1B y'

AUX BLDG

ALIT BLDG

TRANS 2

TRANS

415V PLNT PROTN

BOARD

415V

NT'

'6r1

415V AUX

‘T''

BLDG TOAD CENTRE I

BLDG LOAD CENTRE 2

1 B

1,1

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				STATION
				Aux TRANS 1
NEUTRAL			Cr		NEUTRAL
			Cr		NEUTRAL
						EARTH
EARTH						EARTH
EARTH					RESISTOR
RESISTOR.•„					RESISTOR
RESISTOR.•„						IC
18						IC
18
	3 3X.V STATION AUX BOARD				A,			L‘r:1

		A AAAA
		A AAAA					Ef)
				0 0 0 C}			Ef)

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CHILLER

147 GEN PLNT

CENTRAL

BLDG TRANS 1

RADWASTE

BLDG

FUEL

TRANS I

BLDG

415v

TRANS I

pH, GEN. PLNT BLDG

LOAD CENTRE 1

WATER

TREATMENT

415v

PLNT

RADWASTE BLDG

TRANS

LOAD CENTRE 1 y

CW HOUSE

y 4,5v FUEL BLDG LOAD CENTRE y

HYPO

c:3

CHLORITE

PLNT

y 415V WATER TREATMENT PLANT LOAD CENTRE I y

TRANS ,

(1, 1>

94p5v cw PUmPHOUSE HYPOCHLORITE PLNT LOAD CENTRE I

FIG. 1.12 PWR unit/station electrical auxiliaries system

the total station services requirements can be met, even with a 3.3 kV tailworks feeder outage, using only two transformers and leaving the third transformer to act as a standby. Each is therefore rated to meet the simultaneous duties of supplying half the cavern 3.3 kV and 415 V station services, the headworks and tailworks services and, when the normal Area Board supply is unavailable, the 400 kV

cable cooling plant.

To meet the requirements of the STPs, diesel generators are provided in order that the station can be started in the generating mode in the absence of grid supplies. In addition, the diesel generators maintain essential services in the cavern such as lighting, heating and ventilation plant to the personnel areas, battery chargers and normal drainage.

The electrical

System descriptions

4204V

STATION

GENERATOR

TRANSFORMER 2

400kV

- 23 5ioe

EARTHING

235 • •

MODULE 24

T RANSP cdP ME 2

GENERATOR

NEUTRAL EARTH

CIRCUIT

BREAKER

RESISTOR 2A

MAIN

21 5 ,, v

INDUCTOR 2 GENERATOR 2

NEUTRAL EARTHING

EAR'.

TRANSFORMER 2

RES 1SfOR

Ilk's( STATION BOARD 2

¶IkVUNITBOAF102

STATION AuX

TRANS 2

UNIT

(1 Aux TRANS 28

UNIT ADO

TRANS 2A

CUE

O w

0 0.1

NEUTRAL EARTH

Lii 04

RESISTOR 2C EL'

ILL

U- a.

T. 1

> CC

3.2 W STATION ADO BOARD 2

NEUTRAL EARTH

1 NEUTRAL

RESISTOR 28

REACTOR

A AA A AAA

(I-, RESISTOR

COOLANT

33KV UNIT AUX

6,--•

3310/ U NIT AUx BOARD 2

BOARD

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A sEcrkAT.B

,..c

SECT A

rr-I

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1 1

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LLI t (.6

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PRESS HTR

A A A A A261i

41-

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—0-

H, DEN PLNT

(.2`L'

BLDG.

... 0›-

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CENTRAL

L„..

. .—.._,.—

Luc,.

pRESSURISER HTPI

f', S

I.)

CONDENSATE

LOAD CENTRE

RAD WASTE

EXTRACTION

a a.

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TRANS. 2

TURD. HSE

s.LI

AUX

SERV

415V

FUEL BLDG

TRANS 28

H2 ON PLANT BLDG

TRANS 2

TURB HSE

LOAD CENTRE 2

SERV TRANS 2A

415V TUFiB.

415V TURD.

0„ RAIDWA4 T5E4' BLDG

WATER

HSE SERVICES

LOAD CENTRE 2B

LOAD CENTRE 2A

TREATMENT

PLNT

y LOAD CENTRE 2 1-

TRANS. 2

AUX. BLDG.

TRANS

ADO BLDG

Y 05v FUEL BLDG LOAD CENTRE 2

TRANS 3 -

A4:>—'1

AdA.

2., 415v PLNT PROTN

415V AUX. BLDG

BO A R D 2

4 i 5v WATER TREATMENT PLANT LOAD CENTRE 2

Cr-

4ISv A1.1%. BLDG

LOAD CENTRE 4

LOAD CENTRE 3

CW HOUSE

HYPOCHLORITE

PLNT TRANS. 2

y 415vcvo PUmPHOUSE HYPOCHLORITE PLNT LOAD CENTRE 2

Flo. 1,12 (cont'd) PWR unit/station electrical auxiliaries system

system design has to recognise the function of the pumped-storage scheme to provide very rapid response to extra load demand. The start-up supplies are arranged such that a fault on any machine

being started will not affect a unit which is operating in a generating or pumping mode by the provision of section isolators. This also enables starting busbar

maintenance to be carried out with minimum affect on

machine availability.

Considering the miscellaneous forms of power generation, the CEGB has considered several renewable energy sources. Of these sources, the most cost effective being pursued is wind turbine-generators (WTGs). These are mostly located in very remote locations and as such are generally unmanned. The CEGB has tried several sites, including the Orkneys, Carmarthen Bay

Electrical system design

Chapter 1

FROM ilkV

FROM likV

STATION BOARD 1

STATION BOARD 2

ESSENTIAL

DIESEL

GENERATOR

GENERATOR 4

ESSENTIAL

TRANSFORMER 4

NEUTRAL

EARTHING

EARTHiNG

EARTHING

RESISTOR

NER

NE R

y),,„v,„ENT BOARD 1 Ti

j:f

yA„,,,, ESSENTIAL

$$1666$1

< O. uJ

X Q. 4[..x

op <72

LLp ,JJ CE CC

415V ESS DIESEL LOAD CENTRE ,y

415V ESS DIESEL LOAD CENTRE 4y

1/6

RUHSLOADCENTRE1

RUHS LOAD CENTRE 4

415V ESS DIESEL MCC 1

(

415V ESS. DIESEL MCC 4

PRESS HTRS LOAD CENTRE 4

415V ESSENTIAL LOAD CENTRE 4

6 6

415V ESSENTIAL MCC lE

415V ESSENTIAL MCC 4E

415V ESSENTIAL MCC 1C

415V ESSENTIAL MCC 4C

415V ESSENTIAL MCC

I A

e415V ESSENTIAL MCC 4A

d 5V ESSENTIAL MDC 1B

415V ESSENTIAL MCC 4B

FIG. 1.13 PWR essential electrical AC systems

System descriptions

E 55

5E Rv TRANS

FROM 11EV

FROM I IkV

UNIT BOARD

UNIT BOARD 2

ESSENTIAL

DIESEL

GENERATOR 3

GENERATOR 2

ESSENTIAL

EsSENTiAL

TRANSFORMER 2

NEUTRAL

TRANSFORMER 3

NEUTRAL

EARTHING

NEUTRAL

SART H INC

RESISTOR

EARTHING

RESISTOR

NER L

óo

NER

3 „ V E „ ENTIAL „ARD 2 T

3. ESSENTIAL BOARD

E!I

500 500

Li_

Wa-

< 0

WuJ

(.7

tn ,„

4,_1„,,,.0.2E2L

...N„.E3,

RUHS LOAD CENTRE 2

RuH$ LOAD CENTRE 3

ESS

415V ESS DIESEL MCC 2

SERV.

415V ESS. DIESEL MCC 3

TRANS

PRESS HTRS LOAD CENTRE 2

PRESS. IITRS.

LOAD CENTRE 3

415V ESSENTIAL LOAD CENTRE 2

415V ESSENTIAL iOAD CENTRE 3

415V ESSENTIAL MCC 2F

(4415v ESSENTIAL MCC 3F

415V ESSENTIAL MCC 2E

(415V ESSENTIAL MCC 3E

415V ESSENTIAL mCC 2C

(45V ESSENTIAL MCC 3C

(415v ESSENTIAL MCC 2A

415V ESSENTIAL MCC 3A

( 415v ESSENTIAL MCC 28	415V ESSENTIAL MCC 38

NOTES

MOTOR RATINGS (SHOWN IN SHAFT kWI

ARE INDICATIVE ONLY

FIG. 1.13 (coned) PWR essential electrical AC systems

Electrical system design	Chapter 1

NG		STARTING
EQUiPMENT 1		EQUIPMENT 2

400,18k V GENERATOR-MOTOR TRANSFORMER 1

18kVn 1kV

STARTING

TRANSFORMER 1

18kV/3 3kV STATION

TRANSFORMER 1

18kV/41SV

UNIT SERVICES TRANSFORMER 1

18kW717V

GENERATOR-MOTOR EXCITATION

A A TRANSFORMER 1

	1 M3PP 1 M3G				2M3PP
			18kV GENERATOR-MOTOR 1			18kV GENERATOR MOTOR 2
			CIRCUIT BREAKER			CIRCUIT BREAKER

11.07			I MO		2M7 2M0		GENERATOR-MOTOR 2
			18kV GENERATOR-MOTOR 1				GENERATOR-MOTOR 2
		•STARTING ISOLATORS					STARTING ISOLATORS
	1 M2	•STARTING ISOLATORS					18kV GENERATOR-
	1 M2				18kV GENERATOR -		MOTOR 2
			1 A3		MOTOR 1	2A3	MOTOR 2
			1 A3		BRAKING SWITCH	2A3	BRAKING SWITCH
			1 M		GENERATOR.	2M1	18kV GENERATOR-
			1 M		MOTOR 1	2M1	MOTOR 2
	GENERATOR.				EARTHING SWITCH		EARTHING SWITCH
	OO			1 S4			2S4
			NV STARTING
			NV STARTING			18kV STARTING
			BUSBAR			BUSBAR
			SECTION ISOLATORS			SECTION ISOLATORS
					Y3
					Y3	STARTING EQUIPMENT
						STARTING EQUIPMENT
						OUTPUT TRANSFORMER 1
			18kV STARTING			OUTPUT TRANSFORMER 1
			18kV STARTING			BY-PASS ISOLATOR
			EQUIPMENT 1			BY-PASS ISOLATOR

OUTPUT ISOLATOR

Fic. 1.14 Dinorwig electrical system

System performance

and Richborough. In all these sites, the wind turbines have had ratings of between 1 and 3 MW. Present thinking is to develop 'wind parks', each wind park will have about 15-20 machines feeding into the Area Board system at 33 kV and 66 kV.

The 'needs' for an electrical system for such generators are very much less onerous than for a conventional power station, especially as they are intended to be unn nned. The machines will be self-starting and will syl,:hronise themselves onto the Area Board network automatically. A simple switching system is envisaged, with tee-off connections to supply 'unit' services, if required. Battery capacity will be installed in a limited form to provide essential lighting, heating and any control supplies which may be required.

The main WTG is mounted on top of a cylindrical steel pole which, at the bottom, houses the switchgear, etc. Power is transmitted to the wind park distribution network from the machine via a 'twisting' cable, necessary as the rotor and machine assembly follows the wind around.

4 System performance

4.1 Station and unit start-up

For all types of power stations, start-up can only be achieved if there is sufficient electrical power available from either a grid connection or a large on-site power source such as a gas-turbine generator.

Grid connections will be either 132 kV, 275 kV or 400 kV depending on the availability at the site. 132 kV would be preferred, due to the cost savings in switchgear cables and transformers. However this decision, made at the planning stage, must take into account factors such as:

•Is 132 kV available at the proposed site?

•Will the 132 kV system need to be reinforced to supply the power station requirements?

•If present, is it intended to be kept for the life of the new station (due for example to Area Board bulk supply requirements)?

•Is the 132 kV in the correct geographical location on the proposed site to avoid long and complicated cable runs?

•Are there sufficient spare circuit-breakers or bays available or the capacity for extension to feed the station requirements?

Another consideration is the phase angle difference

between the 132 kV system and the 400 kV system, since the unit system will be connected to 400 kV via the

generator transformer and there may be some electrical distance between the two voltage levels. In some cases this can be quite excessive and may well be outside

the range of the approved check synchronising equipment (see Chapter 12). In addition, large phase angle differences cause large circulating currents when the unit and station systems are paralleled. This will be reflected in a short time requirement for a high rating of the unit transformer which may well lead to unacceptable constraints. Phase angle differences of about 10 ° are considered acceptable.

Whichever primary voltage levels are chosen, startup power is usually supplied to the power station via station transformers. However, where, generators are connected to the system via generator voltage switchgear, start-up power may also be provided via the generator transformer/unit transformer route. Although stations have been built with generator voltage switchgear and no station transformers, it is not a practice which would be recommended today. This is because a complete loss of supplies to half the station could occur following the tripping of a generator

[-I V circuit-breaker as a result of a major fault	in
the zone protected by the overall unit protection	(see

Chapter 11 on Protection), causing immediate and longer term operational restrictions.

If a station requires start-up power when external grid supplies are not available, i.e., black start, it will be necessary to provide power sources at the II kV voltage level by means of on-site generation of sufficient capacity diesels or gas-turbines, or off-site generation, e.g., gas-turbines at another adjacent generating station with local interconnection. It -should be noted that GTs may have other duties such as 'peak lopping' or 'frequency support' or emergency generation. This is discussed more fully in Section 6.1 of this chapter.

For start-up conditions, the station electrical system is interconnected to the unit electrical system usually at 11 kV to provide power from the station transformer to the unit system. The generator transformer HV circuit-breaker and unit transformer LV circuitbreakers are open at this time. This arrangement is shown in Fig 1.15.

The method of achieving this differs from station to station, but the principle is the same. Most fossilfired stations have unit boards interconnected by cable, but nuclear stations have a variety of arrangements. Early magnox and AGRs differed widely in the method of achieving start-up supplies, but AGRs such as Hartlepool and Heysham / and 2 introduced a generator voltage switch, allowing the generator/unit transformer combination to provide starting power (see Fig 1.10).

An electrical auxiliaries system arrangement for start-up with a generator voltage switch is shown on Fig 1.16.

4.1.1 Plant required

All power stations require at least one CW pump and one 50 07a electric boiler feed pump available and running to start up a unit. In addition, fossil plant

Electrical system design

Chapter 1

GRID'

GRID

GENERATOR

TRANSFORMER

UNIT

STATION

TRANSFORMER

11 kV

11kV STATION

UNIT BOARD

BOARD

UNIT AUXILIARIES

STATION AUXILIARIES

CIRCUIT BREAKER CLOSED

# CIRCUIT BREAKER NORMALLY OPEN

FIG. 1.15 Arrangement for station start-up (direct connected generator)

GRID

GENERATOR

TRANSFORMER

GENERATOR

VOLTAGE

SWITCH

UNIT

STATION

TRANSFORMER

INTERCONNECTOR

likV STATION

1 1 kV

UNIT BOARD

BOARD


UNIT AUXILIARIES		STATION AUXILIARIES

(13	CIRCUIT BREAKER CLOSED		Iti t) CIRCUIT BREAKER NORMALLY OPEN

FIG. 1.16 Arrangement for station start-up (generator connected by a generator voltage switch)

requires either coal mills or oil pumps and draught plant, e.g., FD and ID fans, PA fans, etc. Gas-cooled nuclear plant requires gas circulators running on main motors or pony motors at approximately 15% speed, whereas water reactors require reactor coolant pumps. Both nuclear types require various supporting aux-

iliaries to be available during the run-up stages, the poor quality steam being dumped until the correct quality is achieved.

When steam of correct quality is being produced, the turbine-generator will be run up to speed with all the unit supporting auxiliaries being powered from

1.18).

run-up equipment

System performance

the station transformers via the unit/station inter-

connectors.

4.1.2 Synchronising to the grid

When the turbine-generator is run up to the correct

speed, it is synchronised to the grid via the generator transform - r HV circuit-breaker (see Fig 1, 17) if the

generator is directly connected to the generator transforme , or at the generator voltage switch if one

is provided (see Fig This is normally achieved using dedicated automatic synchronising equipment,

GRID (TYPICALLY 400kV)

UNIT

BOARD

control of which is located in the main control room. However, the turbine automatic

could well contain an automatic synchroniser and in this case the synchronising equipment mentioned above would no longer be required. Manual check synchronising may be achieved using portable check synchronising trolleys in the main control room.

4.1.3 Synchronising unit to station

When the generator is synchronised to the grid and lightly loaded, it is appropriate to transfer the unit

GRID (TYPICALLY 132kV)

11kV STATION BOARD

MANUAL CHECK SYNCHRONISING

VIA SYNCHRONISING TROLLEYS

AUTO SYNCHRONISING VIA

DEDICATED EQUIPMENT

FIG. 1.17 Synchronising points for a direct connected generator

GRID (TYPICALLY 132kV)

Fs] F_

Fi 71 MANUAL CHECK SYNCHRONISING

VIA SYNCHRONISING TROLLEYS

AS AUTO SYNCHRONISING VIA

DEDICATED EQUIPMENT

FIG. 1.18 Synchronising points for a generator connected via a generator voltage switch

Electrical system design	Chapter 1

loads to the unit transformer. This is achieved by paralleling the unit and station boards for a short time via the 11 kV interconnector. Check synchronising facilities, by means of control room located synchronising trolleys, will be necessary, as the two sources may be out of phase and frequency. The check synchronising relay has limits of ±20 0 within which successful paralleling will be achieved.

For stations with generator voltage switchgear, the unit transformer usually provides start-up supplies. However, the station transformer still supplies the station loads and the unit to station interconnection at 11 kV can provide a standby facility to the unit transformer (if it is out of service). Again, check synchronising facilities will be required to enable the supplies to be transferred without interruption. Details of the synchronising equipment are described in Chapter 12.

4.2 Shutdown and power trip

There are basically two types of shutdown:

•A controlled shutdown, due to a request for a reduction in generation or prior to an outage.

•An emergency shutdown, following an internal or external fault requiring disconnection of the unit from the grid.

4.2.1 Controlled shutdown

A controlled shutdown is basically the reverse of a start-up sequence. The unit power output is reduced to a level appropriate to the design, when the unit and station supplies may be paralleled and all unit auxiliaries transferred to a station transformer source. In the case of a generator voltage switch arrangement, the supplies do not need to be transferred, providing a grid connection is maintained. Section 4.3 of this chapter describes what is the expected sequence of events following a loss of grid supplies.

The AGR nuclear plant is arranged to have one generating unit associated with one reactor. The PWR differs in as much as present UK designs have two generators with one reactor. All nuclear plant requires post-trip cooling to remove the fission product decay heat, but due to differing reactor/turbine configurations, the functional requirements will vary from station to station. As post-trip cooling forms part of the safety function, it is quite normal to arrange for the emergency or essential system prime-movers to start for every reactor trip whether accompanied by loss of grid or not.

4.2.2 Power trip

There are a number of types of trip which may be experienced on power station plant. These are discussed briefly below and a more exhaustive treatment is given in Chapter 11:

(a)Grid fault disturbances on the national grid system may cause a loss of connection to a single generating unit, or all power station connections. This is discussed in more detail in Section 4.3 of this chapter. An electrical fault may occur which requires the power station to grid substation circuitbreaker to open.

(b)Generator system electrical faults in the generator unit zone, i.e., the generator, unit transformers and the main connections, will require both the generator HV and unit transformer LV circuitbreakers to open instantaneously.

(c)Generator mechanical faults on the generator or turbine such as loss of lubricating oil or control fluid, loss of condenser vacuum, require shutdown of the unit, but not necessarily instantaneous electrical disconnection. Indeed some benefit can be gained in limiting turbine-generator overspeed if the generator is left connected to the off-site power system for a short time.

(d)Electrical auxiliaries system faults on the electrical system at 11 kV or lower, may cause an unacceptable reduction in the plant necessary to continue running the unit. It should be noted here that the design philosophy is such that one fault should not cause the loss of more than one unit.

(e)Consequential trip due to loss of steam generation, i.e., failure of the steam raising plant will require the unit to be tripped.

Faults in group (a) above, normally subject the generator to 100 07o load rejection. The turbine-generator unit will accelerate due to the excess of input energy over demand. The steam (governor) and excitation (AVR) control mechanisms are designed to cope with this situation but some overspeed will occur, depending largely on the inertia constant of the unit and the speed of the control systems. Back-up overspeed protection, in the form of centrifugal force-driven bolts, is provided to trip the steam valves, should the control systems fail to control the unit below about 10 07o overspeed.

Faults in groups (c) above, leave the generator electrically connected to the grid and the unit load, so that the overspeed due to entrained steam is limited. Completion of unit trip by disconnection from the electrical system is achieved by a power measurement relay detecting low forward power, see Chapter 11.

In certain of the fault trip cases considered here, the auxiliaries system will be subjected to voltage and frequency transients. The safeguards are designed such that the mechanical systems are not subjected to transients outside their design codes.

For stations with direct connected generators, faults in groups (a)-(e) will generally cause a loss of supply to the whole unit electrical system. Essential plant is supplied from battery-backed systems or by local generation using diesel generators or gas turbines.

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