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Selecting CB for 6.6 kV, 650 MVA

Short answer

Rated current is 175 A (standard 630 A), breaking current 56.9 kA, making current 145 kA. Specify a 7.2 kV, 630 A, 63 kA vacuum circuit breaker.

Select a circuit breaker for the following application: 6.6 kV, 650 MVA (breaking capacity) breaker. It is supposed to be connected to primary of 6600/433 V, 2000 kVA transformer. Justify your choice and prepare the specifications for the breaker.

The breaker controls the 6.6 kV primary of a 2000 kVA, 6600/433 V distribution transformer and must interrupt 650 MVA at 6.6 kV.

Calculations

Rated (normal) current:
I_n = 2000 × 10³ / (√3 × 6600) = 174.95 A

Symmetrical breaking current:
I_b = 650 × 10⁶ / (√3 × 6600) = 56 860 A ≈ 56.86 kA

Making current (peak):
I_m = 2.55 × I_b = 2.55 × 56.86 = 144.99 kA ≈ 145 kA

Short-time current (1 s or 3 s) = I_b ≈ 56.86 kA

Choice of breaker and justification

A vacuum circuit breaker (indoor, withdrawable type) is chosen:

  • 6.6 kV is in the medium voltage range, where vacuum breakers are standard and have the lowest lifetime cost.
  • Vacuum gives fast recovery of dielectric strength, very low contact erosion and long life (10,000+ operations) with almost no maintenance.
  • No fire hazard (no oil) and no gas handling (unlike SF6), so it suits indoor switchgear near the transformer.
  • Compact size; suitable for transformer switching. An SF6 breaker of the same rating is an acceptable alternative; older practice would have used a minimum oil breaker.
  • Transformer magnetising current is small; modern vacuum interrupters have a low chopping current, and surge arresters at the transformer terminals take care of switching surges.

Specification of the breaker (as per IEC 62271-100 standard values)

ParameterValue
TypeIndoor, 3-pole vacuum CB, withdrawable
Rated voltage7.2 kV (system 6.6 kV)
Rated frequency50 Hz
Rated normal current630 A (required 175 A)
Rated short-circuit breaking current63 kA symmetrical (required 56.86 kA)
Rated breaking capacity≥ 650 MVA at 6.6 kV
Rated making current157.5 kA peak (2.5 × 63 kA; required 145 kA)
Rated short-time withstand current63 kA for 3 s
Rated operating sequenceO – 0.3 s – CO – 3 min – CO
Power frequency withstand20 kV rms for 1 min
Lightning impulse withstand60 kV peak
Opening / break timeabout 3 cycles (60 ms)
Operating mechanismMotor-wound spring, 110 V DC trip and close coils
ProtectionIDMT overcurrent + earth fault relay through CTs 200/5 A

Answer: rated current 174.95 A (630 A standard), breaking current 56.86 kA, making current 145 kA; a 7.2 kV, 630 A, 63 kA vacuum circuit breaker is specified.

✓ Answer checked by Claude Opus 5.5 (high effort)

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