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Medium Voltage Switchgear (2) Selection of Switching Devices

http://electr ical-engineering-portal.com/medium-voltage-switchgear-selection-of-switching-devices February 15, 2013

Medium Voltage Switchgear (2) Selection of SwitchingDevices

Emmanouil Angeladas

Continued from first part:Medium Voltage Switchgear (1) Basics of Switching Devices

3. Selection of Switching Devices

Switching devices are selected both according to t heir ratings and according to t he switchingduties to be performed, which also includes the switching rates.

1. Select ion according to rat ings

2. Select ion according to endurance and switching rates

1. Switches (general, sf6, air-break, vacuum)

2. Circuit Breakers

3. Disconnectors

4. Earthing Switches
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5. Contactors

3.1 Selection according to ratings

The system conditions, that is, the propert ies of the primary circuit , determine the requiredparameters.

The most important of these are:

Rated voltage

The upper limit of the system voltage t he device is designed for. Because all high voltageswitching devices are zero-current interrupters, except for some for fuses the system voltageis the most important dimensioning criterion.

It determines the dielectric stress of the switching device by means of the transient

recovery voltage and the recovery voltage, especially while switching of f .

Rated insulation level

The dielectric strength f rom phase to earth, between phases and across the open contact gap,or across the isolat ing distance. The dielectric strength is the capability of an electricalcomponent to withstand all voltages with a specific time sequence up to the magnitude ofthe corresponding withstand voltages.

These can be operating voltages or higher frequency voltages caused by switching operat ions,

earth faults (internal overvoltages) orlightning strikes (external overvoltages). The dielectricstrength is verif ied by a lightning impulse withstand voltage test with t he standard impulse waveof 1.2/50s and apower frequency withstand voltage test (50Hz/1min).

Rated normal current

The current t hat the main circuit of a device can continuously carry under defined conditions.The temperature increase of components especially contacts must not exceed definedvalues.

Permissible temperature increases always refer to t he ambient air temperature. If a device ismounted in an enclosure, it may be advisable to load it below its f ull rated current, dependingon t he quality of heat dissipation.

Rated peak withstand current

The peak value of the major loop of the short circuit current during a compensation processafter the beginning of the current f low, which the device can carry in closed state.

It is a measure fo r the electrodynamic(mechanical) loadof an electrical component . For

devices with full making capacity, this value is not relevant.

Rated short circuit making current

The peak value of the making current in case of short circuit at the terminals of the switching
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device. This stress is greater than that of the rated peak withstand current, because dynamicforces may work against the contact movement.

Rated breaking current

The load breaking current in normal operation. For devices with full breaking capacity andwithout a crit ical current range, this value is not relevant.

Rated short circuit breaking current

The root mean square value of the breaking current in case of short circuit at the terminalsof the switching device.

3.2 Selection according to endurance and switching rates

If several devices satisfy the electrical requirements and no addit ional criteria have to be takeninto account, the required switching rate can be used as an additional selection criterion.

Table 1 through table 5 show the endurance of the switching devices, providing arecommendation for their appropriate use. The respect ive device standards distinguishbetween classes ofmechanical (M) and electrical (E) endurance, whereby they can also beused together on the same switching device.

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Switches:

Standard IEC 62271 103/VDE 0671 103 only specifies classes for the so called general-purpose switches. There are also special switches and switches for limited applications.

Important note:Disconnectors up to 52kV may only switch negligible currents up to 500mA (e.g., voltagetransformer), or larger currents only when there is an insignificant voltage diff erence (e.g., duringbusbar transfer when the bus coupler is closed).

Generalpurpose switches

General-purpose switches must be able to break dif ferent types of operat ing currents (loadcurrents, ring currents, currents of unloaded transformers, charging currents of unloaded cablesand overhead lines), as well as to make on short circuit currents.

General-purpose switches that are intended fo r use in systems with isolated neut ral or withearth fault compensat ion must also be able to switch under earth f ault condit ions.

SF6 (Sulfur hexafluoride) switches
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Table 1 - Classes f or switches

SF6 switches are appropriate when the switching rate is not more than once a month. Theseswitches are usually classified as E3 with regard to their electrical endurance.

Air-break or hard-gas switches

Air-break or hard-gas switches are appropriate when the switching rate is not more than oncea year. These switches are simpler and usually belong to the E1 class. There are also E2versions available.

Vacuum switches

The switching capacity ofvacuum switches is significantly higher than that of the M2/E2classes. Vacuum switches are used for special tasks: mostly in industrial power supply systems,

or when the switching rate is at least once a week.

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Circuit breakers

Whereas the number of

mechanical operatingcycles is specificallystated in the Mclasses, the circuitbreaker standard IEC62271-100/VDE 0671-100does not define theelectrical endurance ofthe E classes byspecific numbers ofoperat ing cycles; the

standard remains veryvague on this.

The test duties of theshort circuit type testsprovide an orientationas to what is meant bynormal electricalendurance and extended
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VD4 medium voltage circuit breaker - ABB

electrical endurance. Thenumber ofmake andbreak operations (Close,Open) is specif ied in Table2.

Modern vacuum circuitbreakers can generally

make and break the ratednormal current up to thenumber of mechanicaloperating cycles.

The switching rate is not adetermining selectioncriterion, because circuitbreakers are always usedwhere short circuitbreaking capacity is

required to protectequipment.

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Disconnectors

Disconnectors do nothave any switching

capacity(switches forlimited applications mustonly control some of theswitching duties of a

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Table 2 - Classes f or circuit breakers

Table 3 - Endurance classes f or disconnectors

switch). Switches forspecial applicat ions areprovided for switchingduties such asswitching of singlecapacitor banks,switching of ring circuitsformed by transformers

connected in parallel, orswitching of motors innormal and lockedcondition.

Therefore, classes areonly specified for thenumber of mechanicaloperating cycles.

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Earthing switches

With earthing switches,the E classesdesignate the shortcircuit making capacity(earthing on appliedvoltage). E

0corresponds to anormal earthingswitch; switches of theE1 and E2 classes arealso called make-proofor high-speed earthingswitches.

The standard does notspecify how often an

earthing switch can beactuated purelymechanically; thereare no M classes for these switches.


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Table 4 - Endurance classes f or earthing switches

Toshiba vacuum contactors, 400A. Current limiting, highinterrupting power fuses.

Table 5 - Classes f or contactors

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Contactors

The standard has not

specified anyendurance classes forconductors yet.Commonly usedconductors today havea mechanical and electricalendurance in the range of250,000 to1,000,000 operating cycles.

They are used wherever switchingoperat ions are performed very

frequently, e.g., more than once perhour.

References:

- SIEMENS Power Engineering Guide,Switchgear and Substations

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