Understanding Symmetrical Breaking Current

The symmetrical breaking current is a crucial parameter for an oil-circuit breaker. It represents the RMS value of the AC component of the current that the circuit breaker is expected to interrupt during a symmetrical short circuit. This value is typically derived from the circuit breaker's MVA rating and the system voltage.

Circuit Breaker Ratings Analysis

The provided details for the three-phase oil-circuit breaker are:

• System Voltage: 33 kV (Line Voltage)
• Rated Current: 1500 A
• MVA Rating: 2000 MVA
• Rated Time: 2 s

The symmetrical breaking current calculation primarily relies on the MVA rating and the system's line voltage.

Calculating Symmetrical Breaking Current

The formula to calculate the symmetrical breaking current ($I_{sc}$) in Amperes (A) is:

$$ I_{sc} = \frac{MVA \times 1000}{\sqrt{3} \times V_{L}} $$

Where:

• MVA is the apparent power rating in Megavolt-Amperes.
• $V_{L}$ is the system line voltage in kilovolts (kV).
• The factor 1000 is used to convert MVA to kVA.
• The factor $\sqrt{3}$ is used because the system is three-phase.

Step-by-Step Calculation

1. Substitute the given values into the formula:

   MVA = 2000

   $V_{L}$ = 33 kV

   $$ I_{sc} = \frac{2000 \times 1000}{\sqrt{3} \times 33} $$

2. Calculate the denominator:

   $\sqrt{3} \times 33 \approx 1.732 \times 33$

   $\sqrt{3} \times 33 \approx 57.156$

3. Calculate the current:

   $$ I_{sc} = \frac{2,000,000}{57.156} $$

   $$ I_{sc} \approx 35000 \text{ A} $$

4. Convert the current to kiloamperes (kA):

   $$ I_{sc} \approx \frac{35000}{1000} \text{ kA} $$

   $$ I_{sc} \approx 35 \text{ kA} $$

Conclusion

Based on the calculation using the MVA rating and system voltage, the symmetrical breaking current for the three-phase oil-circuit breaker is approximately 35 kA. This value is essential for ensuring the circuit breaker can safely interrupt fault currents within its specified limits.