Question

The Differential protection is primarily used for:

• A. Long Transmission Lines
• B. Transformers and Generators
• C. Distribution Feeders
• D. Lightning Protection

Hint:

Differential protection provides "Unit Protection".
This means it only operates for faults within its strictly defined zone (between the current transformers) and remains completely unaffected by faults outside this zone.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary equipment applications of differential protection schemes in power system protection.

Step 2: Detailed Explanation:

• Differential protection is based on the Merz-Price circulating current principle. It compares the currents entering and leaving a protected zone.

• Under normal conditions or external (through) faults, the current entering the zone equals the current leaving it, resulting in zero differential operating current:
\[ I_{\text{diff}} = I_{\text{in}} - I_{\text{out}} = 0 \]
• During an internal fault within the protected zone, the balance is disrupted, producing a net differential operating current that causes the relay to trip instantly.

• This scheme is highly sensitive, selective, and extremely fast, but it requires pilot wires to connect the current transformers at both ends of the protected zone.

• For long transmission lines, transmitting current signals over long distances becomes expensive and prone to signal attenuation. Therefore, differential protection is not typically used as primary protection for long lines (instead, distance or carrier-current protection is preferred).

• However, for localized, high-value equipment like power transformers, alternators (generators), and busbars, the physical distance between terminals is small. This makes it straightforward and highly effective to apply differential protection.

• Thus, differential protection is primarily used for transformers and generators.

Step 3: Final Answer:
Differential protection is primarily used for transformers and generators.