Find the making current for a circuit breaker rated at 1000 A, 3000 MVA, 66 kV, 3 sec, 3 – phase, oil circuit breaker

Concept:

Breaking current: It expresses the highest number of short-circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Symmetrical breaking capacity \(= \sqrt 3 {V_{L}}{I_{B}}\)

Rated symmetrical breaking current \({I_B} = \frac{S}{{\sqrt 3 {V_L}}} \)

Making current of a circuit breaker is the peak value of the maximum current loop during sub transient condition including the DC component when the breaker closes.

Symmetrical making current = 2.55 × symmetrical breaking current

Calculation:

Symmetrical breaking capacity = 3000 MVA

\(\begin{array}{l} {V_{L}} = 66kV\\ \Rightarrow {I_{{B}}} = \frac{{3000 \times {{10}^6}}}{{\sqrt 3 \times 66 \times {{10}^3}}} \end{array}\)

IB = 26.24 kA  

Making current = 2.55 × Symmetrical breaking current

= 2.55 × I_B

= 2.55 × 26.24 kA

= 66.92 kA