9

411

9 High current switchgear

High current switchgear is understood to form the link, required in power stations of alltypes, between the generator and the unit transformer, through which the electricalenergy generated is fed into the transmission or distribution network (see section 11,High voltage switchgear).

9.1 Generator circuit-breakers

Generator circuit-breakers are switchgear in the high-current connection between thegenerator and unit (generator) transformer. They must be capable of handling the veryhigh operating currents (up to 50 kA) on the one hand, and the extremely high short-circuit currents occurring power station operation on the other hand. These require-ments for generator breakers are much more stringent than those for breakers in net-work service and are specified in detail in the (unique in the world) “IEEE”C37.013-1997 standard (ANSI). The following list summarizes the most importantareas of application and the advantages.

Functions Advantages

In specific cases there can be economic, operational and technical reasons for usingsuch breakers.

Synchronization of the generator on thegenerator voltage level

Securing station service supply on gener-ator shut-down

Disconnection of a fault in the unit trans-former or station service transformer

Disconnection of a fault in the generator

Only 1 switching operation requiredinstead of 5-7 switching operations forsynchronization on the HV level, thusavoiding maloperation

Uninterruptible supply to station servicesystem from network via unit trans-former

Restriction of the effects of faults in thetransformer, as the opening time ismuch shorter than generator shut-downby high-speed de-excitation

Secure disconnection of the short-cir-cuit current in spite of missing currentzeroes. Station service remains on thenetwork without interruption, increasingreliability of power plant operation.

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412

Fig. 9-1

Unit connections of power plants a) Basic circuit diagram, b) and c) Large generatorswith part-load transformers, d) Pumped storage power station, e) Hydro power plant;

1 Generator, 2 Generator breaker, 2a 5-pole generator breaker for switchoverbetween motor and generator operation, 3 High-voltage circuit-breaker 4 Main trans-former, 5 Station service transformer, 6 Starting transformer, 7 Starting motor

Fig. 9-1 shows examples of unit connections with generator circuit-breakers. The var-ious types show how these breakers ensure maximum possible availability of stationservices in the event of a fault for large units with several unit and station transformers.

Conventional and gas turbine power plants, hydro-electric plants and nuclear powerplants with high unit capacity and special requirements for safety and availability arepreferred areas of application for generator circuit-breakers.

The use of generator circuit-breakers must be considered in the early stages ofdesigning a power plant. The following requirements are important when designing thestructure:

a) Space required for breakerBreaker dimensions, phase spacing (note minimum clearances), accessibility foroperation and subsequent maintenance.

b) Space required for additional auxiliary equipment such as an external cooling sys-tem.(The auxiliary equipment should be located in the immediate vicinity of the genera-tor circuit-breaker.)

c) Structural requirementsStable foundation (account is to be taken of reaction forces during switching oper-ations), lifting gear for installation and maintenance, distance to be traveled to thepoint of installation.

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413

Modern generator circuit-breakers are not generally offered as single unit but as afunctional unit which contains the disconnectors, shorting links, earthing switches andstart-up disconnectors inside single-phase enclosures.

In addition, the current and voltage transformers, surge arresters and protectioncapacitors required for generator and unit protection and synchronization are accom-modated inside the enclosures (see figure 9-2).

Fig. 9-2

Single-line diagram of a generator circuit-breaker system

1 Circuit-breaker, 2 Disconnector, 3 Earthing switch, 4 Start-up disconnector, 5 Short-circuiting link (manual), 6 Surge arrester, 7 Current transformer, 8 Voltage transformer,9 Protection capacitor , 10 Short-circuiting link (motorized), 11 Breaker enclosure (sin-gle phase) 12 Earth

9.1.1 Selection criteria for generator circuit-breakers

Apart from the rated voltage, the most important criteria are the rated current and therated breaking current of the power station. ABB supplies several types of generatorcircuit-breaker, which can be used depending on the generator capacity. These are onthe one hand the SF6 breakers of types HECS and HEC 7/8 (single-pole enclosed) andtypes HGI and HECI (unenclosed), and on the other hand the vacuum circuit-breakerof type VD4 G.

Fig. 9-3 Selection table for generator circuit-breakers

HEC 7/8

HECS

HECI

HGI

VD4G VD4Gwith fan

160

130

120

63

25

101.000 4.5003.400 8.000 9.000 18.000 >28.000

Rated voltages (IEC)

VD4G: 17,5 kVHGI: 17,5/21 kVHECI: 25 kVHECS: 23/25 kVHEC 7/8: 30 kV

Rated current (A)

Rat

ed b

reak

ing

cur

rent

(kA

)

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414

9.1.2. Generator circuit-breaker series HECS and HEC 7 / 8 (SF6 gas breakers)

These breaker systems are designed for generator capacities of 100-2000 MVA and –depending on the type – can be used for generator voltages of up to 30 kV. They aresingle-pole metal-enclosed and suitable for both indoor and outdoor installation.

The power-interrupter chambers of these breakers are filled with SF6 gas as thequenching and insulation material. The arc is interrupted by the proven ABB self-blast-ing principle: The arc that is generated when the contacts open heats the SF6 gas,increases the pressure and generates a stronger gas flow, which blasts the arc andextinguishes it.

In addition, the arc is set in rotation and thus reduces contact burn-off.

The contacts, which carry current continuously, are placed separately from the inter-rupting contacts, guaranteeing optimum current transfer at all times.

The voltage-carrying components are air-insulated against earth.

The 3-pole design on a common base frame makes installation very simple. Specialfoundations are not required.

The power chambers are actuated by the proven ABB type HMB spring mechanism.The energy storage capacity is rated for 2 ON-OFF switching cycles. Disconnector,earthing switch and start-up disconnector have electric motor-operated mechanisms.They are controlled in accordance with the current requirements in power plant designby an integrated control cabinet with conventional relay technology.

The modular design makes it possible to expand these enclosed generator circuit-breakers into very compact functional systems with disconnectors, earth switches,transformers etc. (see figures 9-2, 9-4). Production and testing of the complete systemin the manufacturer’s works greatly reduces the time and expense of assembly andtesting at the construction site.

The single-line breaker enclosure is welded to the bus duct enclosure.

The live parts are bolted to the high-current bus duct conductor by way of flexible cop-per extension straps.

The service intervals, in accordance with the demands of modern power plant design,have been extended to 15 years service life or 10,000 operating cycles* for series HEC7/8 and 20 years service life or 20,000 operating cycles* for series HECS.

* mechanical operating cycles

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9

415415

Tab

le 9

-1

Tech

nica

l dat

a of

gen

erat

or c

ircui

t-b

reak

ers

typ

e H

EC

S /

HE

C 7

/8 (s

ingl

e-p

ole

encl

osed

circ

uit-

bre

aker

s)

Typ

e d

esig

natio

nkV

HE

CS

HE

CS

HE

CS

HE

CS

HE

CS

HE

CS

HE

CS

HE

CH

EC

7H

EC

8

- 80

S-

80 M

- 10

0 M

- 10

0 L

- 10

0 X

L-

130

L-

30 X

L7

S

Max

. op

erat

ing

volta

gekV

2323

25,3

25,3

25,3

25,3

25,3

3030

30

Rat

ed s

hort

-tim

e p

ower

freq

uenc

y w

ithst

and

vol

tage

50/6

0 H

z 1

min

, aga

inst

ear

thkV

6060

6060

6060

6080

8080

over

isol

atin

g d

ista

nce1)

kV70

7070

7070

7070

8888

88R

ated

ligh

ting

imp

ulse

with

stan

d v

olta

ge1,

2/50

µs

agai

nst

eart

hkV

125

125

125

125

125

125

125

150

150

150

over

isol

atin

g d

ista

nce

1)kV

145

145

145

145

145

145

145

165

165

165

Rat

ed c

urre

nt,

natu

ral c

oolin

g, 5

0 H

zA

8.50

010

.500

10.5

0013

.000

18.0

00 1)

13.0

0018

.000

1)23

.000

24.0

00 2)

28.0

00na

tura

l coo

ling,

60

Hz

A8.

000

10.0

0010

.000

12.0

0017

.000

1)12

.500

17.0

00 1)

22.0

0022

.000

26.0

00

Rat

ed b

reak

ing

curr

ent

kAef

f80

8010

010

012

013

013

014

016

0 3)

160

3)

Mak

ing

curr

ent

kAsw

220

220

280

280

280

360

360

390

440

3)44

0 3)

1)O

nly

valid

for

mod

els

with

dis

conn

ecto

r2)

Rat

ed c

urre

nt in

form

atio

n co

rres

pon

din

g to

am

bie

nt t

emp

erat

ure:

max

. 40

°C3)

Tem

per

atur

e of

the

hig

h-cu

rren

t b

us d

ucts

at

the

bre

aker

ter

min

als:

con

duc

tor

max

. 90

°C; e

ncap

sula

tion

max

. 65

°C

ABB_11_E_09 13.08.2007 9:12 Uhr Seite 415

Fig. 9-4

Generators circuit-breaker system of type HECS / Dimensional chart 1 Control cubicle,2 Breaker enclosure, 3 Assembly lid, 4 Breaker feet, 5 Generator bus duct connection- enclosure, 6 Generator bus duct connection - conductor, 7 Foundation / Steel plat-form, 8 Circuit-breaker, 9 Disconnector, 10 Current transformer, 11 Voltage trans-former, 12 Surge arrester, 13 Capacitor, 14 Space for installation and servicing, 15Opening for control cables, N Variable phase spacing

416

Generator circuit-breaker Type HECS

Weight max. 6.500 kg

Transformer side

Generator side

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417

9.1.3. Generator circuit-breaker series HGI / HECI (SF6-gas breakers )

These generator circuit-breakers are designed for generator capacities of 100-400MVA and are usable – depending on their type – for rated generator voltages up to 25kV.

They are unenclosed and only suitable for indoor installation.

The areas of application for these breakers are power stations with open-type genera-tor bus ducts (without phase insulation).

They are especially suitable as replacements for older air-blast circuit-breakers incomparatively old power stations in the course of retrofitting.

Table 9-2

Technical data of generator circuit-breakers type HGI / HECI (unenclosed breakers)

Type designation HGI 2 HGI 3 HECI HECI 3/R 5/R

Max. operating voltage kV 17,5 21 25,3 25,3Rated short-time power frequency withstand voltage50 Hz, 1 min against earth kV 50 60 60 60across open isolating distance kv 55 70 70 70Rated lightning impulse withstand voltage 1.2/50 µs against earth kV 110 125 125 125across open isolating distance kV 121 145 145 145Rated currentNaturally cooled, 50 / 60 Hz A 6.300 8.000 9.000 9.000Rated breaking current kAeff 50 63 100 120Making current kAsw 138 190 300 360

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418

9.1.4 Generator circuit-breaker series VD 4 G (vacuum circuit-breakers)

Vacuum circuit-breakers from standard ranges can also be used as generator circuit-breakers with smaller generators (up to 100 MW). These breakers allow very compactsolutions. They are used as a fixed-mounted single unit or as a draw-out device with-in a functional system with metallic compartment walls, earthing switch and discon-nector function (segregation) (figure 9-5). Current and voltage transformers and surgearresters can also be integrated.

Fig. 9-5Generator circuit-breaker type HGI / Dimensional chart

1 Circuit-breaker poles, 2 Terminal surfaces for generator bus duct, 3 Operating mech-anism, 4 Position indicator, 5 Cable connection to control cubicle, 6 Current direction,7 Min. space for servicing

Generator circuit-breaker Type HGI 2

Weight ca. 500 kg

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419

Fig. 9-6 Generators circuit-breaker VD4G

A1 Upper terminal compartment(e.g. transformer)

A2 Lower terminal compartment(e.g. generator)

B Circuit-breaker compartmentC Low voltage compartment

1 Terminal lead2 Isolating contact3 Circuit-breaker4 Earthing switch5 Bay control and protection unit REF 542

Table 9-3

Technical data of generator circuit-breaker type VD4 G

Type designation VD4G

Rated voltage to IEC kV 17,5Rated voltage to ANSI/IEEE kV 15,8Rated short-time power frequency withstand voltage kV 50Rated lightning impulse withstand voltage kV (95) 110

Rated current (at max. 40°C) without fan A 3400with fan A 5000

Rated breaking current system source (symm.) kA 40generator source kA 25/18,5

Rated making current kA 110

The technical data listed in the following table are based on testing in accordance withANSI standard IEEE C 37.013-1997.

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420

9.2 High-current bus ducts (generator bus ducts)

9.2.1 General requirements

The high-current bus ducts with all their branches are a component of the electricalinstallation in the power plant.

The high-current bus duct and switchgear generally serve the following functions (Fig.9-7). – Connection between generator and main transformer(s) including generator neutral.

– Branch connections to station services and excitation transformers as well as volt-age transformer cubicles.

– Design and connection of measuring, signalling and protection devices for current,voltage and other operating data.

– Installation and connection of high-current switching devices such as generator cir-cuit-breakers with high-current disconnectors and earth disconnectors.

– Additional facilities, e.g. for protection and working earthing, pressure-retaining sys-tems or forced cooling.

Fig. 9-7 High-current switchgear installation

1 High-current bus duct, 2 Generator, 3 Generator neutral point, 4 Neutral earthingcubicle, 5 Short-circuiting facility (temporary), 6 Voltage transformer cubicle, 7 Excita-tion transformer, 8 Generator circuit-breaker, HEC type with 8.1 control cubicle, 9Voltage and capacitor cubicle, 10 Expansion joint, 11 Station auxiliary transformer,Main transformer, 14 Current transformer / feeder side, 15 Current transformer/neu-tral side

Note: The voltage transformers (6 and 9), the capacitors (9), the earth switch, theshort-circuiting facility (5) and the surge arrester (12) can also be installed in the gen-erator terminals. The configuration of the current transformers (14) must be specified: a) at the genera-tor feed, b) in the busbar run or c) in the generator circuit-breaker, to enable theshort-circuiting facility to be located at the proper position. Consultation with the supplier of the generator circuit-breaker is required.

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421

Technical requirements

The design of the largest generators with nominal power up to 1700 MVA yields oper-ating currents of up to 50 kA. For the high-current bus duct, this means that the gen-erated heat in conductors and enclosure and the significant magnetic field effects inthe installation and its environment must be controlled.With the stated unit capacities and the high network outputs, short-circuit currents ofup to approximately 750 kA peak value may occur in the high-current bus ducts andhigh-current switchgear. In the branches, peak short-circuit currents of more than1000 kA may occur. And of course the safety and availability of a high-current bus ductmust correspond with the high standard of the other power-plant components.The high-current bus ducts must therefore comply with specified requirements: – Adherence to preset temperature limits, – Adequate short-circuit current carrying capability, (thermal and mechanical strength

with short-circuits),– Adequate magnetic shielding,– Safe insulation, i.e. protection against overvoltages, moisture and pollution.

9.2.2 Types, features, system selectionTypesIn smaller power plants (hydropower, CHP stations) with a load current of up toapproximately 2.5 kA (5 kA), the bus ducts can still have the "classic" busbar design.The simplest designs are flat and U-shaped busbars of Al or Cu (sometimes also tubu-lar conductors, in Al only). Exposed busbars are used with small generator ratings onlybecause they require locked electrical equipment rooms. In contrast, laying the bus-bars in a common rectangular aluminium duct provides protection against contact andpollution. Aluminium partitions between the phases provide additional protection. Thisprevents direct short-circuits between the phases. In the event of short-circuit currentsflowing, the compartment walls reduce the short-circuit forces (shielding) on insulatorsand busbars.Single-phase systems can be supplied in single-insulator or triple-insulator designs. Typical single-phase variations: – up to 5.5 kA in single-insulator design (type HS 5500)– up to 50 kA in triple-insulator design (type HA)

FeaturesThe single-phase enclosure is the most commonly supplied and the most technicallyadvanced model. The conductors and the concentrically arranged enclosure aroundthe conductor consist of aluminium tubes and are insulated from each other by an airgap and resin insulators (Fig. 9-8)

Fig. 9-8

a) Single-phase design withthree insulators Type A

b) Single-phase design withone insulator Type B

a) b)

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422

An important technical feature is the single-phase enclosure short-circuited over thethree phases at both ends. This enables the enclosures to form a transformer sec-ondary circuit to the conductors. The current flowing in the enclosure – opposite to theconductor current – reaches approximately 95% of the conductor current dependingon the system configuration and the impedance of the short-circuit connectionbetween the enclosures (Fig. 9-10)

Fig. 9-9

Principle of the high-current busduct with electrically continuousenclosure,

1 Enclosure current,2 Conductor current,3 Enclosure connection

The magnetic field outside the enclosure is almost completely eliminated, therebyeliminating the ambient losses.

This type has the following important features:– Proof against contact, making locked electrical equipment rooms unnecessary, – Protection against pollution and moisture, maintenance limited to visual checks,– No magnetic field outside the enclosure (no induction losses in adjacent conductive

material such as screens, railings, concrete reinforcement, pipes etc.),– Reduced likelihood of ground faults and short-circuits, – Single-phase high-current switching devices can be incorporated in the bus duct.

The type range A includes 5 voltage levels – 01 to 05 – for rated current intensities of3 to 31 kA in self-cooling design (Table 9-5) and currents of up to about 50 kA withforced cooling.

The types range B includes 2 voltage levels, rated currents intensities up to 5.5 kA(Table 9-4)

Table 9-4 Single-phase high-current bus ducts type B

General table for system selection based on current and voltage (natural cooling)

Rated Conductor Enclosure Conductor/Enclosure current dia. mm dia. mm Rated short-time Rated lightning

p.-f. withstand voltage impulse withstand volt-age

50 (60) Hz 1 min in kV 1.2/50 µs in kVType B Type B Type B Type B

kA 01 and 02 01 and 02 01 02 01 02

(36) (95)5.5 150 480 28 38 75 95

Notes: For explanations, see Table 9-5For main dimensions, see Table 9-6

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9

423

Tab

le 9

-5 S

ingl

e-p

hase

hig

h-cu

rren

t b

us d

ucts

typ

e A

Gen

eral

tab

le fo

r sy

stem

sel

ectio

n b

ased

on

curr

ent

and

vol

tage

(nat

ural

coo

ling)

Rat

edC

ond

ucto

rE

nclo

sure

Con

duc

tor/

encl

osur

ecu

rren

t�

mm

�m

mR

ated

sho

rt-t

ime

p.-

f. w

ithst

and

R

ated

ligh

tnin

g im

pul

se w

ith-

stan

dvo

ltage

50

(60)

Hz

1 m

in in

kV

volta

ge 1

.2/5

0 m

s, in

kV

Volta

ge le

vels

Volta

ge le

vels

Volta

ge le

vels

kA01

to

0501

0203

0405

0102

0304

0501

0203

0405

310

046

046

055

064

073

05

190

550

550

640

730

820

828

064

064

073

082

091

010

370

730

730

820

910

1 00

0(3

6)(6

0)(8

0)(8

0)(9

5)(1

10)

(150

)(1

50)

1246

082

082

091

01

000

1 09

028

3850

7070

7595

125

145

1

7015

550

—91

01

000

1 09

01

180

1764

0—

1 00

01

090

1 18

01

270

2073

0—

1 09

01

180

1 27

01

360

2282

0—

—1

270

1 36

01

450

2491

0—

—1

360

1 45

01

540

261

000

——

1 45

01

540

1 63

030

1 00

0—

——

—1

720

Not

e: t

est

volta

ges

as I

EC

600

71-

1, T

able

2;

( ) v

alue

s in

par

enth

eses

acc

ord

ing

to A

NS

I C 3

7.23

.A

coo

ling

syst

em is

req

uire

d fo

r cu

rren

ts o

ver

31 k

A.

Tab

le 9

-6 is

ap

plia

ble

for

stru

ctur

al p

lann

ing.

ABB_11_E_09 13.08.2007 9:12 Uhr Seite 423

424

Table 9-6

Main dimensions of the high-current bus duct

Dimensions must be clarified with supplier

Current Type A Type AkA 01 to 05 01 02 03 04 0505E

D A B E H H H H H Hmm mm mm mm mm mm mm mm mm mm

0- 3 460 750 700 500 600 600 — — — —3- 5 550 850 750 550 650 650 650 — — —3- 8 640 950 800 600 650 650 700 750 — —3-10 730 1 000 900 650 700 700 750 800 850 —3-10 820 1 100 950 700 750 750 800 850 900 9505-12 910 1 200 1 000 750 800 800 850 900 950 1 0008-15 1 000 1 300 1 050 800 850 850 900 950 1 000 1 050

10-17 1 090 1 400 1 100 850 900 900 950 1 000 1 050 1 10012-17 1 180 1 500 1 150 900 950 950 1 000 1 050 1 050 1 10015-20 1 270 1 600 1 200 950 1 000 1 000 1 050 1 100 1 100 1 15017-22 1 360 1 700 1 250 1 000 1 050 1 050 1 050 1 100 1 150 1 20020-24 1 450 1 800 1 300 1 050 1 100 1 100 1 100 1 150 1 200 1 25022-26 1 540 2 000 1 400 1 100 1 100 1 100 1 150 1 200 1 250 1 30024-26 1 630 2 100 1 450 1 150 1 150 1 150 1 200 1 250 1 300 1 35026-30 1 720 2 300 1 500 1 200 1 200 1 200 1 250 1 300 1 350 1 400

Typeto 5.5 480 600 700 500 650 650 — — — —

9.2.3 Design dimensions

Criteria for rating a high-current bus duct:– service voltage – short-circuit current carrying capabili-ty – load current – supplementary requirements for – operating temperatures installed components and equipment– insulation level – climatic conditions

The dielectric strength (rated short-time p.-f. withstand and rated lightning impulsewithstand voltage) is assured by standardized type-sized air clearances between con-ductor and enclosure, and by standard insulators as per VDE, DIN and IEC and theassigned voltage levels with the test voltages as per IEC 600 71-1 (VDE 0111 Part 1).

The test voltages for BS and ANSI are covered by the clearances provided (Table 9-4,9-5).

The standardized type range and the connections at components of the power plantsuch as generator and transformer are rated for minimum clearances as per VDE andIEC. Verification by test is not required.

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425

Computers are used for optimum and economical design of sizes and wall thickness-es for conductor and enclosure on the basis of a comprehensive heat network. Thestandard rating is based on maximum limit temperatures with an ambient temperatureof 40°C:

Enclosure 65 °C – 80 °C; conductor 90 °C – 105 °C.

These values comply with all corresponding VDE, IEC and ANSI standards.

The short-circuit current carrying capability of the bus duct includes adequate provi-sion for peak short-circuit and short-time current. Only one short-circuit current –either from the generator or from the system side – can occur on the main conductor,but in the branches, the sum of the two short-circuit currents must be taken intoaccount. The single-phase enclosure design reduces the likelihood of a short-circuitby many times.

The main duct design for the rated current inevitably has a short-circuit current carry-ing capability by that far exceeds the rated value dynamically and thermally.

However, the branch ducts are dimensioned primarily for peak and short-time currentwithstand in compliance with the short-circuit calculations and the requirements of therelevant standards (Section 3 and 4). This automatically ensures compliance with thepermissible temperatures at load current.

9.2.4 Structural design of typical High-current bus ducts

Conductors and enclosure are of Al 99.5% sheet (DIN 40501), which is rolled and sub-merged-arc welded. To improve thermal dissipation, the conductors are painted on theoutside and the enclosures inside and outside.

The length of prefabricated assemblies depends on the feasibility of transport and theaccess and installation conditions on the construction site.

Each support of the conductor consists of one or three post insulators – in exception-al cases of four –, which are mounted from outside. Sliding surfaces or fixed pins onall insulators of each support and a spring arrangement on one insulator per supportallow relative axial movements between the conductor and the enclosure.

The enclosure supports are independent of the support of the conductor and aredesigned as sliding or fixed-point, fastened directly to the support structure. The tubeprofile allows distances of enclosure supports of 10-20 m depending on the system.

All connections to the generator, to transformers and switchgear not only ensuresecure electrical connection but also allow adjustment, accommodation of thermalmovements and access to the junction points. The enclosure structure is particularlyimportant at the generator terminals because of the small spaces between them. Insmall and medium-sized installations, three-phase terminal and neutral compartmentswith hatches and viewing windows allow inspection and access to the connections. Athigher rated currents, only the single-phase enclosed bus duct construction providessufficient magnetic field compensation, prevents eddy currents and therefore ensurescontrolled temperature conditions.

The conductors are connected to the generator, transformers and switchgear termi-nals with flexible press-welded copper straps fastened with bolts. Spring washersguarantee the required contact pressure and prevent unacceptable temperature rise.The contact surfaces are silver-coated if required by the conductor limit temperature(IEC and ANSI).

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426

Current transformers for measurement and protection of the toroidal core type areeither installed at the generator terminal bushings or integrated into the bus duct at asuitable point. Detachable connections are then to be integrated into the main con-ductor for installation and removal. Voltage transformers can be incorporated into thebus duct or installed in separate instrument cubicles connected by branch ducts. Thesame applies for protection capacitors for limiting capacitively transmitted voltages.

Surge arresters protect bus duct and generator, even in the event of flashover in thetransformer, but are then usually overstressed. The use of housings with pressure reliefwill ensure the safety of personnel and the installation.

9.2.5 Earthing system

The design of earthing systems for high-current bus ducts is based on DIN VDE 0101,which also comply with the other national and international standards (such as IEC,ANSI, BS). The maximum anticipated double ground-fault current can be calculated asfollows:

��3I"K EE = –— · I"K 32

The minimum cross section AE for the main earthing conductor as per VDE 0103 is cal-culated as follows:

I"K EE · 103 · �m + nAE min. = ———————–————

1Sthn · ���—TK

The typical earthing system of high-current bus duct uses the enclosure of the threephases as the earthing conductor. The separate conductors are restricted to connect-ing the enclosure to the earth terminals on the generator, the transformers and theconnection to the power plant earthing system. All components outside the busbar runsuch as cubicles etc. are connected to the enclosure and so are earthed "by spurs".See Section 5.3 for additional information on earthing.

Note:

When installing generator circuit-breakers, the earth switch and the short-circuitingfacility can be integrated into the generator circuit-breaker.For detailed information, see generator circuit-breakers in Section 9.1!

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9.2.6 Air pressure/Cooling system

Operational reliability can be further improved by supplying the high-current bus ductwith filtered dry air. The resulting overpressure of 500 Pa (max. 2000 Pa) allows air inthe bus duct to pass from inside to outside only, preventing contamination. The dry airalso prevents the formation of condensation. The incoming air is drawn through areducing valve and a gas meter from the power plant compressed-air system with orwithout a dryer and water separator.

Forced ventilation of the high-current bus duct at 31 to max. 50 kA is of the closedloop type with an air-water heat exchanger for cooling. The cooling unit is normallyinstalled under the bus duct as close to the middle as possible. The air is blown intothe outer phases by fans and diverted to the middle phase at the end by controldampers and deionizing screens via a connecting duct, in which it flows back to thecooling unit at twice the speed. The closed circuit air-cooling system is 100% redun-dant, allowing the system to be switched to the standby fan and cooler immediatelywhen necessary. If the cooling system fails, the availability of the high-current bus ductis still 50–70%, depending on the design. Fig. 9-11 shows the air flow diagram of ahigh-current bus duct.

The limited space in the generator terminal area and the requirement to be able to workwith smaller dimensions may require cooling with a single-pass airflow below 31 kA.

Fig. 9-10

Cooling-air flow diagram for a high-current bus duct, 1 High-current bus duct, 2 Cool-ing unit with fans a; Standby fans b; Dampers on standby fan c; Cooler d and standbycooler e; 3 Damper valves for flow distribution, deionization screens, 4 Cooling watercirculation with motor-operated valves f for cooler and standby cooler (flow and return)with safety valves g; Vent and discharge valves h; 5 Make-up air with filter-dryer ele-ment i; 6 Alternative to 5: Make-up air from the compressed air system via reducingvalve k and air meter I.

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