A three phase breaker is rated at 2000 MVA, 33 kV, its making current is

Explanation:

Three-Phase Breaker Making Current

Problem Statement: A three-phase breaker is rated at 2000 MVA and 33 kV. The task is to calculate its making current.

Solution:

To calculate the making current of the three-phase breaker, we use the standard formula:

Making Current = √2 × Symmetrical Breaking Current

The symmetrical breaking current can be determined using the formula:

Symmetrical Breaking Current (I_sym) = Rated MVA / (√3 × Rated Voltage)

Given data:

• Rated MVA (S) = 2000 MVA
• Rated Voltage (V) = 33 kV = 33 × 10³ V

Step 1: Calculate the symmetrical breaking current:

I_sym = S / (√3 × V)

Substituting the values:

I_sym = 2000 × 10⁶ / (√3 × 33 × 10³)

We know that √3 ≈ 1.732, so:

I_sym = 2000 × 10⁶ / (1.732 × 33 × 10³)

I_sym = 2000 × 10⁶ / 57.156 × 10³

I_sym = 35,000 A or 35 kA

Step 2: Calculate the making current:

Making Current = √2 × I_sym

We know that √2 ≈ 1.414, so:

Making Current = 1.414 × 35 kA

Making Current = 49.49 kA

Since the making current is typically rounded off to the nearest whole number, the making current is approximately 49 kA.

Correct Answer: Option 3) 49 kA

Important Information

To analyze the other options, let us evaluate them:

Option 1: 70 kA

This value is significantly higher than the calculated making current of 49 kA. A making current of 70 kA would correspond to a much higher symmetrical breaking current or a different breaker rating.

Option 2: 89 kA

This value is even higher than option 1 and is unrealistic for the given breaker ratings. A making current of 89 kA would require a symmetrical breaking current of over 63 kA, which does not match the given MVA and voltage ratings.

Option 4: 35 kA

This value represents the symmetrical breaking current, not the making current. The making current is always higher than the symmetrical breaking current due to the factor of √2.

Option 5: Not Provided

This option is irrelevant as it does not present any numerical value for consideration.

Conclusion:

The making current of the given three-phase breaker is accurately calculated to be approximately 49 kA, as derived from the rated MVA and voltage. Understanding the relationship between the symmetrical breaking current and the making current is crucial for accurately determining the breaker specifications and ensuring proper system protection.