Introduction to prefabricated metal-clad equipment

oltage, current, frequencyshort circuit power areare the four main bases

which define the electricalequipment.

he insulation function,the control functionand the protection

function of electricalnetworks, are carried outthrough the switchgears.

T

VWe are not giving you hereafter,the exact standard terms.

The definitions are issued by theInternational Electrical Commission.

e need to consider5 criteria to specifythe design of the

compartments of ametal-enclosed equipment

ll the documents,standards, regulations,recommendations

and local customs are inreference to the electricalequipment and will not havethe same legal impact.

A

W

Are you sure you knowMedium Voltage termsperfectly ?

We are reminding youof the main definitionsin usual terms.

Keep in mindthe following

points

date

08/91

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revised

05/2004

Introduction to prefabricated metal-enclosed equipment

■ Merlin Gerin ■ Square D ■ Telemecanique

2

Presentation
factory-built equipment
nd cy

Voltage

Operating voltage U (kV)This is applied across the equipment terminals.

Rated voltage Ur (kV)

The rated voltage is the upper limit of the highest voltage of system for which the equipment is intended

The rated voltage is always greater than the operating voltage and, is associated with an rated insulation level.

Rated insulation level Ud (kV rms. 1 mn) and Up (kV peak)

The rated insulation level is specified by the rated lighning impulse withstavoltage phase to earth (Up). It is linked to rated short duration power-frequen

� Ud: rated short duration power-frequency withstand voltageIt is simulated in a laboratory by the rated power-frequency withstandfor one minute.� Us: switching impulse voltage

Up: rated lightning impulse withstand voltage The voltage wave that results is simulated in a laboratory

N.B.: IEC 60694, article 4 sets the various voltage values together with, in article 6,the dielectric testing conditions.

Example:

� Operating voltage: 20 kV

� Rated voltage: 24 kV

� Power frequency voltage 50 Hz 1 mn: 50 kV rms.

� Lighnting impulse voltage125 kV peak.

date

08/91

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05/2004

Introduction

In order to design a medium-voltage cubicle, you need toknow the following basic magnitudes: � Voltage� Current� Frequency� Short-circuit power.

The voltage, the rated current and the rated frequency are often known orcan easily be defined, but how can we calculate the short-circuit power orcurrent at a given point in an installation?

Knowing the short-circuit power of the network allows us to choose thevarious parts of a switchboard which must withstand significanttemperature rises and electrodynamic constraints. Knowing the voltage (kV) will allow us to define the dielectric withstand of the components.E.g.: circuit breakers, insulators, CT.

withstand voltage (Ud)

Presentation
0Metal-enclosed,factory-built equipment
Apart from special cases, MERLIN GERIN equipment is in conformity with list 2 hereunder

Ratedvoltage

Rated lightning impulse withstand

voltage

Rated power-frequencywithstand voltage

Normal operating voltage

UpkV rms. kV peak 1 minute kV rms. kV rms.

list 1 list 27.2 40 60 20 3.3 to 6.612 60 75 28 10 to 1117.5 75 95 38 13.8 to 1524 95 125 50 20 to 2236 145 170 70 25.8 to 36

Insulation levels apply to switchgear under operating conditions in accordance with IEC 60694 §2.1If not, derating should be considered.

Each insulation level corresponds to a distance in air which guarantees equipment withstand without a test certificate.(IEC60071-2)

Rated voltage kV rms.

Rated impulse withstand voltage

Distance/earth in air cm

1.2/50 µs kV peak7.2 60 1012 75 1217.5 95 1624 125 2236 170 32

rated withstand voltage valuesfor lightning impulse voltage

U

t

Um

0,5 Um

0

2028

3850

70

7,212

17,52436

7560

95125

170

STANDARD IEC VOLTAGE

rated voltage

Ud Ur Up

rated short duration power-frequencywithstand voltage

Ur Up

NOTE:different "across the isolating distance" values are mentioned for the switchning device to meet the safety requirements specified for disconnectors

date

08/91

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05/2004

3

4

Presentation

0Metal-enclosed, factory-built equipment

Current

Rated normal current: Ir (A)This is the rms. value of current that equipment can withstand when closed, without exceeding the temperature rise allowed in standards.The table below gives the temperature rises authorised by the IEC according to the type of contacts.

Rated normal current:

Type of mechanism of material

Max. values

Max. temperature

of conductor (°C)

Max. temp. rise

= tmax°- 40 °Ccontacts in air

bare copper or copper alloy 75 35silver or nickel plated 105 65tin-plated 90 50bolted connections or equivalent devices

bare copper, bare copper alloy or aluminium alloy 90 50

silver or nickel plated 115 75tin-plated 105 65

N.B.: rated currents usually used by Merlin Gerin are:400, 630, 1 250, 2 500 and 3 150 A.

Operating current: I (A)This is calculated from the consumption of the devices connected to the circuit in question. It is the current that really passes through the equipment. If we do not have the information to calculate it, the customer has to provide us with its value. The operating current can be calculated when we know the power of the current consumers.

date

08/91

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revised

05/2004

Examples:

� For a switchboard with a 630 kW motor feeder and a 1 250 kVA transformer feeder at 5.5 kV operating voltage.

� calculating the operating current of the transformer feeder:

Apparent power:

� calculating the operating current of the motor feeder:

cosϕ = power factor = 0.9η = motor efficiency = 0.9

S UI 3=

I S

U 3----------- 1 250

5 5 / 1 732,,--------------------------= 130A= =

I P

U 3 ϕηcos-----------------------------=

630----------------------------------------------------------- 82A==5.5 • 1.732 • 0.9 • 0.9

Presentation
0Metal-enclosed, factory-built equipment
Minimal short-circuit current: Isc (kA rms.)(see explanation in "Short-circuit currents" chapter.)

Rated short-time withstand current (Ik) (kA rms. 1 s or 3 s)the r.m.s value of the current which the switchgear can carry in the closed

Rated peak withstand current (Ip) (kA peak) value of the initial peak in the transient periodfor a rated frequency of 50Hz and below it is equal to 2.5 Ik (2.6 for 60Hz)

Rated frequency fr (Hz)# Two frequencies are usually used throughout the world: � 50 Hz in Europe� 60 Hz in America.For other value of frequency, contact the technical department

Switchgear functions

Designation function operating current fault current and symbol switching breakingDisconnecter

isolates

Earthing disconnecter

earthes

Switchswitches,does not isolate

✔

Disconnecter switchswitchesisolates ✔

Fixed circuit breakerswitchesprotectsdoes not isolate

✔ ✔

Withdrawable circuit breakerswitches / protectsisolates in some case ✔ ✔

Fixed contactorswitchesdoes not isolate ✔

Withdrawable contactorswitchesisolates in some case ✔

Fuseprotectsdoes not isolate ✔ (once)

✔ = YES

(the IEC standard values of the rated frequency are 16 2/3; 25; 50; 60 Hz)

position during a specified rated duration (generally 1s or 3s)

date

08/91

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revised

05/2004

Note: some installations,close generator for example, can have higher

peak coefficient.

if withdrawn

if withdrawn

5

6

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THE INSULATIONFUNCTION

The insulation function, the control function andthe protection function of electrical networks,are carried out through the switchgear.

he term “Switchgear” is a general one. It is applied to apparatus withmechanical contacts and their arrangement with devices, of control,measurement and protection.

It equally concerns the grouping of such apparatus with connections,accessories and enclosures…

THE DISCONNECTOR

The disconnector is a mechanical switching device which provides in theopen position, an isolating distance which ensures the 2 sub-installationsto run separatly.

The disconnector has two stable positions : open and closed.

As the disconnector does not have breaking and closing capacity, theopening and closing must be unloaded.In the “open” position, the isolating distance allows the dielectric withstandbetween input and output.To ensure the dielectrical independence of the 2 sub-installations, the input/output distance is superior to the phase/earth distance.

In the “closed” position, it has to withstand the service current and the shortcircuit current during the specified duration.

The two positions must have a locking device forbidding the manœuvrewhen the circuit is on load.

THE EARTHING SWITCH

The earthing switch is a disconnector used to earth the circuit.

It is capable of supporting the short circuit currents within one second andto feed the discharged current of the cables. The earthing switch can have, in certain cases, closing capacity.The earthing switch must have a locking device in the “closed” position.

The insulation function establishes or isolates an unloaded circuit.

T

Metal-enclosed,

Electrote chnical voca bula ry is

defined by the IEC 60694

factory-built equipment Presentation

THE CONTROL FUNCTION

here are three types of control devices : the switch, the switch disconnector and the contactor. All this equipment has to withstand the short-circuit currents within a

specified time.

THE SWITCH

The switch is a control device with two stable positions, open and closed,which allows the feeding of a circuit (transformer, distribution cables…) making.and breaking the service current.

THE SWITCH DISCONNECTOR

The switch disconnector is a switch which, in the “open” position, complieswith the insulation conditions of the disconnector.

THE CONTACTOR

The contactor is an apparatus, electrically controlled, able to establish or tobreak a circuit will a strong rhythm of manœuvres.

In general, it is used in motor feeder cubicles.

It is often associated with fuses to limit the peak and to break the short-circuit current.

T

The control function of the switchgear is used toclose and to open the circuits on load.

7

date

08/91

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05/2004

Presentationfactory-built equipment Metal-enclosed,

THE PROTECTIONFUNCTION

e use fuses or circuit breakers in association with protectivesystems.

FUSES

The fuses are protective devices which have to open the circuit by blowingup one or several of its elements, when the current passes within aspecified time, to a determined value.

A fuse is designed to eliminate the short-circuits.

If the overcurrent is too low, the fuse may fail and not break properly. A fuse is specified by its melting curve (see technical leaflets).

☞ remarks : the choice of fuses depends on the voltage, the servicecurrent, the type of feeder, the starting currents and its manufacturer.

In general, the service current =

THE CIRCUIT BREAKER

The circuit breaker is an apparatus which assures the control and theprotection of a network. The circuit breaket is establish, to withstand and break the service currentsas well as the short-circuit currents.

The tripping is automatically released by means of a protection chain (TC, TP, relay, tripping coil).

When the circuit breaker is in a withdrawn position, it assures the insulationfunction and is similar to a disconnector when tested in accordance.

W

The protection function has to quickly eliminate allovercurrent, overload or short-circuit, in a networkto assure the safety of the personnel and to limit alldamages.

8

date

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revised

05/2004

nominal current2


METAL-ENCLOSEDCOMPARTMENTCHARACTERISTICS

5 criteria have to be considered tospecify compartments in accordancewith the customer's need

-architecture and accessibility s-service continuity of the switchgear-partition class-degree of protection (IP) of the enclosure-internal arc classification

.

ARCHITECTURE AND ACCESSIBILITY: four types ofcompartmentThree accessible to the user, one non-accessible

non-accessible compartment: the opening maydestroy the integrity of the compartment. For example: welded tank of a RMU. ; completely weldedGIS tank....

Accessible compartments - "interlock-controlled accessible compartment" :use of interlocks to ensure that all live parts inside are deadand earthed before opening

- "procedure-based accessible compartment":relies on user procedure and locking , use of padlocks orequivalent

- "tool-based accessible compartment":no built-in feature to ensure electrical safety before opening.They need tools to be opened


9

date

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revised

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SERVICE CONTINUITY OF THE SWITCHGEAR (LSC1,2A,2B) ONLY FOR ACCESSIBLE COMPARTMENTFor switchgear and controlgear, the Loss of ServiceContinuity (LSC) describes the extent to which the othercompartments and/or functional units may remain energized when the main circuit is opened

Category LSC1: this form does not provide servicecontinuity during maintenance and require completedisconnection of the swithgear from the system and makingdead before accessing the interior of the enclosure.

Ex: the complete installation (HV cables; busbars..) must be shut down before opening the CB enclosure

Category LSC2A: when accessing components of onefunctional unit ,the other functional units of the switchgearmay be kept in service.

Ex: the busbar may be kept alive (also the other cubicles of theswitchgear) during the maintenance of the circuit breaker.

Category LSC2B:this form is intended to allow the maximum continuity of service.

Ex: during the maintenance of the circuit breaker , MV cables andbusbar may stay alive.

PARTITION CLASSES

criteria. The opened compartment is surrounded by earthed metallic partition and shutters

There are 2 types of partitioning class

Class PM:when an accessible compartment is open the equipment is always in accordance with the metal-enclose

No electric field is present in the opening compartment and no electrical field change may occur in the surrounding compartments.

non-metallic partitions or shutters between openedaccessible compartments and live parts of the main circuit.


10

date

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05/2004

Class PI:is not a PM class, there are one or more

DEGREE OF PROTECTION OF THE ENCLOSUREsee 5.13 of IEC 60694

The degree of protection of persons provided by an eenclosure against access to hazardous parts of the main circuit is indicated in the here under table

INTERNAL ARC CLASSIFICATIONMetal-enclosed swithgear that satisfies the requirements of the IECstandard is designed to prevent the occurrence of internal fault. Experience has shown that faults are more likely to occur in some locations inside an enclosure .Designs which have been successfully tested according to Annex A of IEC 62271-200 are classified as IAC

.

Where the risk is considered negligible, IAC class is not necessaryWhere the risk is considered to be relevant, only metal-enclosedswitchgear IAC classified should be used.


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date

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05/2004

Metal-enclosed swithgear satisfies as a minimum IP2X

THE REGULATIONS

All the documents, standards, regulations,recommendations, and local customs do not havethe same legal value.

The objective of these regulations is to guarantee the safety of people andto limit the damage.

THE REGULATIONS

The regulations, either fixed a circular, must be respected imperatively bythe constructors.

THE STANDARDS

The standard has a contractual character and acts as a reference. Onlywhen a fixed or a decree imposes it, must it be respected imperatively bythe supplier.The standards define the switchgear and controlgear and their field ofapplication.

THE RECOMMENDATIONS

The standards are nationwide and are often inspired by internationalrecommendations (eg. the IEC).

THE LOCAL CUSTOMS

The local customs, though theoretically negociable, are difficult to pass.We have to work with these.

In order to describe the equipment, each contractrefers to a gathering of regular texts, standards, recommendations and local customs.


date

08/91

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revised

05/2004

12

THE REGULATIONS(cont’d)

MAIN NATIONAL STANDARDS

* Comitato Electrotecnico Italiano.

INTERNATIONAL REFERENCES

Three recommendations often act as international references :

■ I.E.C. : International Electrical Commission.

■ A.N.S.I. : American National Standard Institute (US).

■ N.E.M.A. : National Electrical Manufacturer Association (US).

MAIN IEC PUBLICATIONS ON MEDIUM VOLTAGE

■ Publication 60050: International Electrotechnical Vocabulary ■ Publication 60265: high voltage switches.■ Publication 60470: high-voltage alternating current contactors

and contactors-based motor-starters■ Publication 60529: degrees of protection provided by enclosures (IP Code)

■ Publication 60694 :common clauses for high voltage switchgear andcontrolgear standards.

■ Publication 62271: high-voltage switchgear and controlgearpart 100: high-voltage alternating current circuit breakerspart 102: alternating current disconnectors and earthing switchespart 105: alternating current switch-fuse combinationspart 200: AC metal-enclosed switchgear and controlgear for

.

WHERE CAN YOU ORDER THE IEC PUBLICATIONS ?

Central Office at the International Electrotechnical Commission,3 rue de Varembé PO Box 131 CH-1211Geneva 20- Switzerland

you may find the IEC standard on Intranet or Email: [email protected]

France Germany Great Britain Belgium Spain USA Italy

UTE DIN-VDE-TGL BS NBN UNE ANSI CEI*

298

THE

publications

IEC

13

date

08/91

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revised

05/2004


part 203: gas-insulation metal-enclosed swithgear for rated voltages > 52kVrated voltage above 1kV and up and including 52kV

part 308: guide for asymmetrical short circuit breaking test duty T100a

Documents

Introduction to prefabricated metal-clad equipment