Selection of Switchgear

For selecting switchgears, in addition to normal factors current, voltage and frequency, the fault withstanding capacity is of prime importance. The allowable fault current depends on the capacity of supply company as well as the load and nature of fault. The fault level withstand capacity is known as rupturing capacity (Typical values of rupturing capacity are 250 MVA at 11 kV and 350 MVA at 22 kV).

The rupturing capacity of HV switchgear is taken to be equal to the rupturing capacity of supplier's HV switchgears, if the interconnecting cable between these two points is not large.

The withstand capacity of MV switchgear is calculated as follows:

  Short circuit MVA = (transformer capacity in kVA × 100) / (1000 × % impedance of transformer).

Typical MV switchgear withstand capacities are 25 - 35 MVA at 415 V.

Technical Data of Typical HV Oil Circuit Breaker
Lower rated voltage(KV)Upper rated voltage (kV)Current Rating (A)Rated breaking capacity (MVA)Rated making current (peak) (kA)Rated impulse current (peak) (kA)Rated short time current (1 s) (kA)
1012400150253020
  630250405025
   350556040
   5008010060
  1250350556040
   5008010060
30361250750356030
   1500657550
Typical Dimensional Details of Truck Type Oil Immersed 11 kV, 400 A, 250 MVA Circuit Breaker
Width 610 mm
Height 2565 mm (Including relay panel)
Depth 1625 mm (In closed position)
Depth 1625 + 711 mm (In service draw out position)
Rear access required610 mm (min.)
Technical Particulars of Indoor Minimum Oil Circuit Breakers
Rated Voltage 11 kV22 kV33 kV
Insulation level 27 kV50 kV70 kV
Rated current1600 A1600 A1600 A
Short time withstand current (3 s) 28 kA 20 kA16 kA
Symmetrical breaking capacity40 kA 25 kA16 kA
Making capacity (Peak) 100 kA63 kA40 kA
Oil content per breaker in litres 1010 18
Closing mechanism ----------------Manual/Motor driven----------------
Opening time ----------------Less than 3 cycles----------------
Closing time----------------Less than 5 cycles----------------

Courtesy: M/s. Eswaran and Sons Engineers Ltd.

Technical Details of Typical MV Air Circuit Breakers
Lower rated voltage (kV)Upper rated voltage (kV)Current Rating (A)Rated breaking capacity (MVA)Rated making current (peak) (kA)Rated impulse current (peak) (kA)Rated short time current (1 s) (kA)
1012400150253020
  630250405025
   350556040
   5008010060
  1250350556040
   5008010060
30361250750356030
   1500657550

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