Assets.pdf

7/27/2019 Assets.pdf

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Brandenburg University of Technology Cottbus

Department of Power Distribution and High-Voltage Technology

Electrical Distribution Systems I

Dr. Klaus Pfeiffer

LG 3

Walther-Pauer-Strae 503046 Cottbus

Phone: (0355) [email protected]

September, 2005

Brandenburg University of Technology

Department of Power Distribution and High Voltage Technology Pfeiffer 1

Script

M.V. and L.V. electrical equipment

Contents

1 M.V. switching devices

2 L.V. switching devices

3 M.V switchgears

4 L.V. switchgears

5 Example short-circuit calculation


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Electrical Distribution Systems Part I M.V. and L.V. electrical equipment



1. Medium-voltage switching devices

1.1 Overview

Switching capacity

Short-circuit current

Switching device Operating

current

Breaking

capacity

Making

capacity

Isolating

distance

Symbol

Circuit breaker X X X X

Switch X ___ X ___ X

Switch disconnector X___

X X

Fuse-switch disconnector X X X X

Disconnector ___ ___ ___ X

Vacuum contactor X ___ ___ ___

Earthing switch ___ ___ ___ ___

H.V. HRC fuse ___ X ___ ___

1.2 Medium-voltage circuit breaker

Rated values

English German

Rated operating voltage Bemessungsbetriebsspannung Ue

Rated operational current Bemessungsbetriebsstrom Ie

Rated short-circuit breaking capacity Bemessungskurzschlussausschaltstrom Iar

Rated short-circuit making capacity Bemessungskurzschlusseinschaltvermgen Icm

Rated short-time withstand current Bemessungskurzzeitstrom fr 1s Icw(1s)

Rated short-circuit duration Bemessungskurzschlussdauer tkr


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Standard medium-voltage circuit breaker (commercially available)

Rated operating voltage Ue = (3,6 36) kV

Rated operational current Ie = (630 2500) A

Rated short-circuit breaking capacity Iar= (16 50) kA

Rated short-circuit making capacity Icm = (40 125) kA

Rated short-time withstand current Icw(1s) = Iar

Rated short-circuit duration tkr= (1 3) s

Stress values

Ib operational current

ip prospective peak short-circuit current

kI prospective initial short-circuit alternating current

Ia prospective symmetrical short-circuit current at breaking time

Ith(1s) thermal equivalent short-circuit current for 1s

tk short-circuit duration

ka II = Factor according VDE 0102

kkth(1s) tnmII += m , n Factors according VDE 0102


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Selection of M.V. circuit breakers

Withstand > Stress

Ue > UbIe > Ib

Iar > Ia

Icm > ip

Icw(1s) > Ith(1s)

tkr > tk

Illustration of isolating distance

At Umax no flash-over across the isolating distance between open contacts has to occur.

Arc quenching

Vacuum circuit breaker

- vacuum is the arc quenching medium

- hermetically closed arc quenching chamber

- pressure: (10-3 10-6) Pa

- distance between the contact elements: about 6 mm

- recovery strength of the isolating distance between contacts of about 50 kV in 10 s

after current zero

- current chopping (some Amps) before current zero possible in that case switching

overvoltages occur

2grid1grid UUU YY = Umax in case of phase opposition


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Oil-blast circuit breaker

- thermal decomposition of the oil due to the arc hydrogen is produced (gas-vapour-

bubble)

- hydrogen has a very good thermal conductivity cooling of the arc and heatdissipation from the arc column

- several designs: a) arc columns is blown crosswise

b) arc column is blown lengthwise

Main difference between the two arc quenching principles:

An arc needs a plasma and therewith an ionized gas. This ionized gas isnt available in a

vacuum. The vacuum arc is a complete metal vapour arc (metal vapour from the contact

material surface).

1.3 M.V. HRC fuses (HRC = High Rupturing Capacity)

Rated values

English German

Nominal current Nennstrom In

Minimum breaking current Mindestausschaltstrom Iamin

Nominal breaking current Nennausschaltstrom Ia

Note:

The term rated (Bemessung) instead of nominal (Nenn) is not yet adopted for fuses.

Fields of application


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Time-current-characteristics

Current limitation characteristics


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Nominal current of M.V. HRC fuses in dependency of the rated apparent transformer power

rated percentage impedance: ukr= 4% (with exception of SrT = 1000 kVA: ukr= 6%)

Rated apparent transformer power SrT [kVA]

100 250 630 1000

Um [kV] Nominal current of the M.V. HRC fuse In [A]

12 16 40 100 160

24 10 25 50 80

Nominal current of H.R.C.-fuses in dependency on the motor parameters

Maximum permissible motor start-up current [A]Motor start-up

duration at nominal current of M.V. HRC fuse

ta [s]

Number of

start-upsper hour 50 A 160 A 250 A

15 2 85 310 635

15 8 70 260 530

15 16 60 235 475

2. Low-voltage switching devices

2.1 Overview

Switching capacity

Short-circuit current

Operating

current

Breaking

capacity

Making

capacity

Symbol

Air circuit breaker X X X

Moulded-case circuit breaker X X X

Vacuum circuit breaker X X X

Switch disconnector X ___ X

Fuse-switch disconnector X X X

L.V. HRC fuse ___ X ___

Note:

The term air circuit breaker is derived from the arc chamber, which is not completely closed

but opened to the ambient.


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2.2 Differences between air circuit breaker and moulded-case circuit breaker

Air circuit breaker

- circuit breaking after a certain time delay is possible

- no short-circuit limitation

- category B (according to EN 60947-2) particularly suitable for selectivity

Moulded-case circuit breaker

- undelayed circuit breaking

- short-circuit limitation

- category A (according to EN 60947-2) unsuitable or only restricted suitable for

selectivity

Category A B

Circuit breaker type Moulded-case circuit breaker Air circuit breaker

Breaking capacity high lower than at category A

Short-time withstand current 0 breaking capacity

Time delay not possible (or only few ms) possible

Note:

Moulded-case circuit breaker for category B are also available. For these circuit breakers

Icw(1s) = 12 Iu applies.

2.3 Air circuit breakers

Rated values

English German

Rated continuous current (at 40C) Bemessungsdauerstrom (bei 40C) Iu

Rated ultimate short-circuit breakingcapacitiy

Bemessungsgrenzkurzschluss-ausschaltvermgen

Icu

Rated service short-circuit breakingcapacity

Bemessungsbetriebskurzschluss-ausschaltvermgen

Ics

Rated short-time withstand currentfor 0,3s or 1s or 3s

Bemessungskurzzeitstrom for 0,3s oder 1soder 3s

Icw

Rated short-circuit making capacity(peak value)

Bemessungskurzschlusseinschaltvermgen Icm

Note:

Number of circuit breakings - at ultimate short-circuit breaking capacity Icu 1- at service short-circuit breaking capacity Ics 3


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Dependence of the short-circuit breaking capacity on the rated operation voltage

Ue = 400 V Ue = 690 V

Air circuit breaker slight decrease

Moulded-case circuit breaker strong decrease

Standard L.V. circuit breakers (commercially available)

Rated operating voltage Ue = (230 , 400 , 690) kV

Rated continuous current Iu = (100 6300) A

Rated service short-circuit breaking capacity Ics = (16 150) kA

Rated ultimate short-circuit breaking capacity Icu Ics

Rated short-circuit making capacity Icm = (55 300) kA

Rated short-time withstand current Icw = (5 100) kA (for category B)

Selection of L.V. circuit breakers

Selection of category (A or B)

Iu > Ib

Ics > kI or Icu > kI

Icm > ip

Icw(1s) > Ith(1s)

2.6 L.V. HRC fuses

Current limitation

ts pre-arcing time (melting time)

tLi arcing time

ta clearing time (total operation time)

iD cut-off current


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Cut-off characteristics

Utilization category gL/gG for cable and line protection (general power supply application)

full range breaking capacity

for overload protection and short-circuit protection

Utilization category aM for protection of motor circuits

partial range breaking capacity

only short-circuit protection

no operation at motor start-up currents

Rated voltages

AC 400 V , 500 V , 690 V

DC 250 V , 440 V , 750 V

Sizes

Nominal current [A]

Size 500 V AC / 440 V DC 690 V AC

00 6 100 6 100

0 6 160 1) ----

1 80 250 80 200

2 125 400 125 315

3 315 630 315 500

4a 500 1250 500 - 800

1) Not permitted for new plants.


Note:

In time-current-characteristics the

manufacturer always gives the pre-

arcing time.


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Current limitation characteristics


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Breaking capacity

Un 690 V AC Ics = 120 kA (minimum value Ics = 50 kA)

Un 750 V DC Ics = 25 kA

Fields of application

3. Medium-voltage switchgears

Rated values

English German



Rated peak short-circuit current Bemessungsstostrom Ipk

Standard M.V. switchgear (commercially available)

Rated operational current Ie = (630 1250) A

Rated short-time withstand current Icw(1s) = (16 50) kA

Rated peak short-circuit current Ipk = (40 125) kA

Note:

These parameters are applied for busbars and outgoing feeders.


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Selection of M.V. switchgears

Ie > Ib

Icw(1s) > Ith(1s)

Ipk > ip

3.1 Air-insulated medium-voltage switchgears

Unit design

The units consist of functional compartments, segregated from each other by means of metal

partitions:

- Busbar compartment

- Apparatus compartment

- Feeder compartment

(the feeder compartment sometimes is subdivided into two partitions, so that an

additional transformer compartment results)

- Auxiliary compartment or Low-voltage compartment

(for protection devices, control and measurement equipment)

The pressure-resistant compartments have been created as barriers to avoid the movement

of an internal arc, which means to avoid the arc, pass over from one compartment into

another. This internal subdivision reduces the effect of arc faults outside their point of origin

to a minimum.


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Switching devices

- Circuit breaker

- Fuse-switch disconnector

- Switch-disconnector-fuse- Switch disconnector

Example:

Outgoing feeder to a transformer in a wind turbine

- Fuse-switch disconnector or

- Switch-disconnector-fuse

Busbar bushing

Switchgear unit (circuit breaker unit) without

compartments

1

2

4

5

7

8

9

10

11

Low-voltage compartment

Circuit breaker

Withdrawable unit for moving thecircuit breaker in disconnectedposition

Measuring sockets for capacitivevoltage indicator system

Bar connection from busbar tobreak contact

Circuit breaker

Earthing switch

Current transformer

Cable termination


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Switchgear (circuit breaker units) with removed circuit breaker.

In this figure the circuit breaker is placed onto a handling truck. The truck is provided with a

wheel system which makes the operations for racking the circuit breaker into and out of the

switchgear unit possible.

3.2 Gas-insulated medium-voltage switchgears

All live parts (busbar, apparatus, current- and voltage transformer etc.) are arranged in a

gas-filled chamber. This chamber has to be hermetically sealed and gas-tight. SF6 (sulphur

hexafluoride) is used as insulating gas with a slight overpressure (p (2 3) bar; for

comparison: air pressure: p 1 bar).

Advantages of the SF6-insulation

- higher withstand voltage compared with air

switchgear units can be designed with smaller dimensions

- protection against moisture and contamination

the risk of arc occurrence is reduced


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Unit design

Switchgear unit (switch disconnector)

Switchgear unit (Circuit breaker)


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3.3 Arc protection

The outer enclosure has to resist the very high pressures due to arcing faults. Arc tests are

carried out to proof the following issues:- the compartment doors will remain closed

- no components will be ejected from the switchgear

- no flames or toxic gases will come out

- no holes caused by a burning arc will appear in the outer enclosure

Otherwise the requirements for operator protection are not met.

Approximate value for arc power in a 10-kV-switchgear at kI = 15 kA: PB = 13 MW

For stressing the switchgear the arc energy WB is decisive:

agBB tPW =

For limiting the arc energy the total fault duration has to be minimised as low as possible.

With special arc detection devices the total fault duration can be decreased to tag 100 ms.

These arc detection devices respond to pressure or light due to the arc.

For our example the arc energy is WB = 13 MW 100 ms = 1,3 MWs. This arc energy is still

unacceptable high.

3.4 Some information about arcs

Core temperature (15.000 20.000)C

Possible ambient temperature 5000C

Speed (15 50) m/s

(Assumption: 15 m/s in 100 ms a distance of 1,5 m)

Illuminance 106 Lux

(for comparison the illuminance in a office is about 500 Lux)

Possible arc energy some Megawattseconds

Possible forces onto the cubicle enclosure 100 kN

Pressure maximum (5 10) ms after fault begin


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4. Low-voltage switchgears

In L.V.-switchgears several function modules (outgoing feeders) are placed into one cubicle.

Each cubicle with outgoing feeders is connected to the main busbar and has its owndistribution bar. The distribution bar provides the connection link between the main busbar

and the function modules, which contains the electrical components belonging to one

function unit.

Three versions of function modules are available:

- Fixed Technique

- Plug-in-technique

- Withdrawable technique

Rated values

Busbars, circuit breakers and function modules (withdrawable-technique or fixed-technique)

have to be rated according to the following parameters:

English German



Rated peak current Bemessungsstostrom Ipk


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Standard L.V. switchgear Maximum rated values for busbars

Distribution bar for connection ofMain

busbarCircuitbreaker

Withdraw.technique

Plug-intechnique

R. operational current Ie [A] 6300 6300 1000 2000

R. short-time withstand current Icw(1s) [kA] 100 100 50 50

R. peak current Ipk [kA] 250 220 110 110

Maximum permissible operational currents of

- Distribution modules Ie = 800 A

- Motor starter modules Ie = 630 A

Selection of L.V. switchgears

Ie > Ib

Icw(1s) > Ith(1s)

Ipk > ip


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4.1 Plug-in-technique

Basic elements for this technique are supporting plates, where the electrical components are

placed. Such a unit is called function module. Depending on the application the componentsin the module are installed in various combinations. The module height depends on the

equipment (components) and the rated power.

The modules are installed horizontally at the module frame in the equipment compartment of

the cubicle. The removable modules have plug-in connections to the incoming supply from

the distribution bar, whereas the outgoing cables are connected permanently direct to the

module terminals. The auxiliary circuits are connected via multi-pole plug-in contact units.

The modules can be combined with front modules for indicating, measuring, signalling and

operating equipment.

The distribution bars are arranged vertically.When modules will be replaced, retrofitted, or a module extension is carried out (e.g.

subsequent installation in spare modules), the cubicle must be disconnected from the mains.

Standard plug-in modules as motor starters,

above two fuseless modules with current-

limiting circuit breakers,

below two modules with fuse-switch

disconnector

Replacing of a plug-in module with

distribution bars dead


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Strip-type outgoing energy modules

Strip-type modules consist of a switch disconnector and L.V. HRC fuses. The switch

disconnector is equipped with a spring-assisted mechanism, and the switching speed doesnot depend on the operation speed of the handle at the front. The switch is found on both

sides of the fuses so that the fuses can be replaced under dead conditions.

The modules are installed horizontally in the switchgear cubicle. The complete unit is

mounted directly on the frame and connected through its own contact elements to the

distribution bar. The outgoing cable connection is made with brackets or cable terminals.

The switching state can be observed from outside through a transparent front cover and by

the position of the handle. An interlocking device between the switch-disconnector and the

front cover prevents the cover from being opened when the switch is closed.

Switchgear cubicle with strip-type

energy modules


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4.2 Withdrawable technique

In this technique, components belonging to one functional group are assembled to form a

single mechanical and electrical module as withdrawable type.

Depending on the requirements or design the cubicles are divided into functional

compartments.

Cubicle with incoming feeder


- Circuit breaker compartment

- Cable compartment

Cubicle with outgoing feeders


The busbar compartment contains

- busbars

- distribution bars

- Equipment compartment

The withdrawable function modules are situated there. Each module is a

compartment themselves.

- Cable compartment

The cable compartment contains

- incoming and outgoing cables

- appropriate accessories for interconnecting the modules

- auxiliary accessories (cable clamps, cable connectors, wiring ducts, etc.

The busbars are arranged horizontally in the rear section of the cubicle.

Busbar system with four conductors per phase


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The distribution bar is embedded into a multi-function separator made of insulating material

and held in place and covered by distribution bar covers. The multi-functional separator is

resistant to accidental arcs and thus constitutes a partition between the equipment

compartment and the busbar compartment.

Withdrawable module compartment with

multi-function separator and distribution bar

covers

Multi-functional separator with distribution

bar covers and cable connection units (right

side)

Withdrawable modules consist of a compartment bottom plate, guide rails, front posts and

the contacts. These modules have plug-in contact units at both, the incoming (from

distribution bar) and outgoing sides. The module size depends on the rated power and the

equipment.

Standardized withdrawable modules are:

- Energy distribution by means of switch disconnector or moulded-case circuit breaker

- Motor starter with fuses

- Motor starter without fuses

The maximum rated current for withdrawable modules is Iemax = 630 A.


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The withdrawable modules can be withdrawn

when connected to mains.

Single-line diagram for motor starter with fuses

Description of the operating handle positions of a module

Position ofswitch

Position of module Main- and control circuits

ON in cubicle All main- and control-circuits are connected

OFF in cubicle All main- and control-circuits are disconnected

TEST in cubicleAll main-circuits are disconnected, the control-circuitsare connected

MOVE

in cubicle--

Isolated position--

not in cubicle

All main- and control-circuits are disconnected

ISOLATEDThe module is 30 mm

drawn out of thecubicle

All main- and control-circuits are disconnected andthe isolated requirements are met


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4.3 Arc protection

In low-voltage switchgears very high arc energies occur. The arc energy depends on the

prospective short-circuit current and the total fault duration (see chapter 3.3).

Limit values for permissible arc energy

- for switchgear protection WB = 100 kWs

- for operator protection WB = 250 kWs

For decreasing the arc energy it is necessary to reduce the total fault duration at least to

tag 100 ms. For fault locations onto the busbar it is impossible to achieve this very short

fault duration when using time selectivity (time staggering). Only application of the reversed

interlocking selectivity (zone selectivity) yields to total fault durations of about 100 ms for allfault locations.

To reduce the effects of arc faults outside to their point of origin, several versions of

compartments / internal subdivision of the cubicle are suggested in German Standard

VDE 0660 Part 500.


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Version 1 Version 2

Version 3a Version 3b

Version 4

Version 1 doesnt have any compartments. This version should not be applied.


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5. Example short-circuit calculation

Given:

Grid Q Nominal voltage Un [kV] 20

Short-circuit power kS [MVA] 6286

R/X 0,035

Transformer Rated voltage (H.V.).V.H

rTU [kV] 20

Rated voltage (L.V.).V.L

rTU [kV] 10,5

Rated apparent power SrT [MVA] 45

Vector group Yy0

Rated percentage impedance ukr [%] 10,8

Rated percentage resistance uRr [%] 0,4

R0/R1 1

X0/X1 1

Cable Number of parallel systems 5

Length [m] 150

R [/km] 0,056

X [/km] 0,099

bC [nF/km] 613

0C [nF/km] 613

R0/R1 10

X0/X1 4

Find:

(according to standard DIN EN 60909-0 / VDE 0102:2002-07)

- Maximum three-phase initial short-circuit current max3kI

- Peak short-circuit current ip

Fault location: Busbar at the end of the cable


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Solution:

Grid: ( )m69,99

MVA6286

kV201,1

S

UcZ

2

kQ

2

nQQ =

=

=

m69,9470,0351

m69,99

X

R1

ZX

22

Q

Q

QQ =

+=

+

=

( ) ( ) m2,4569,947m69,99mXZR 222

Q

2

QQ ===

Transformer:( )

0,2646MVA45

kV10,50,108

S

U

100

uZ

2

rT

2

rTkr10T ===

VDE 0102: correction factor kT for transformer impedances

TTTk ZkZ = T

maxT

x0,61

c0,95k

+=

c-factors according to VDE 0102

Nominal voltage cmax cmin

(100 1000) V1,05 1)1,10 2)

0,95

> 1kV 1,10 1,0

1) Tolerance of nominal voltage: 6%2) Tolerance of nominal voltage: 10%

0,9810,108)(0,61

1,10,95

x0,61

c0,95k

T

maxT =+

=+

=

0,25970,26460,981Z10Tk ==

( )0,0098

MVA45

kV10,50,004R

210

T=

=

( ) ( ) 0,259570,00980,2597RZX 22210

T

210Tk

10T ===

Cable:

m1,68km0,15km

0,0565

1RK ==

m2km0,15km

0,0995

1XK 97,==


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R X

Grid, referred to 20-kV-side 2,45 m 69,95 m

905,15,10

20 == 2 = 3,628

Grid, referred to 10-kV-side 0,67 m 19,28 m

Transformer, referred to 10-kV-side 9,80 m 259,57 m

Cable 1,68 m 2,97 m

12,15 m 281,82 m

Zk = 282,1 m

kA22,5m282,13

kV101,1

Z3

UcI

k

nmaxk3max =

=

=

k3maxp I2i =

+= XR

3

e0,981,02

1,88e0,981,02281,82

12,153

=+=

kA60,1kA22,61,882ip ==

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