10 Multiple and Relay Protection

10% multiple and relay protection involves configuring protective relays to detect faults near the generator neutral and coordinating multiple generators in parallel using settings like PSM, TSM, and earth fault relays.

10% Winding Fault Protection

In generator protection, a fault occurring 10% from the neutral refers to a short-circuit or ground fault located close to the neutral point of the generator winding. The fault current in this case is proportional to the distance from the neutral, meaning a fault at 10% of the winding produces a relatively low current. For example, with a 1000/5 CT, a 10% winding fault may result in only 0.5 A secondary current, requiring sensitive earth fault relays (51N) or differential relays (87G) to detect and isolate the fault effectively . This ensures that even small faults near the neutral are cleared before causing damage.

Multiple Generator Protection

When multiple generators operate in parallel, protection schemes must account for shared transformers and system grounding. Each generator may have its own overcurrent or differential relay, but coordination is essential to prevent unnecessary tripping. The transformer can serve as an earthing point, providing sufficient ground current (typically at least 50% of generator rated current) to cover most of the winding for earth fault detection . High-resistance grounding may be used to limit fault currents to 5–10 A, reducing the risk of iron damage while still allowing relays to operate.

Relay Settings: PSM, TSM, and MF

• Plug Setting Multiplier (PSM): Indicates how many times the relay secondary current exceeds the pickup current. Higher PSM results in faster relay operation and is critical for IDMT (inverse definite minimum time) relays .
• Time Setting Multiplier (TSM): Scales the base operating time of the relay. Proper TSM selection ensures coordination between upstream and downstream relays, allowing backup protection without unnecessary tripping .
• Multiplying Factor (MF): Compensates for CT/PT ratios and measurement losses, ensuring the relay calculates the correct primary current for pickup and tripping . For medium-voltage relays, earth fault sensitivity is typically set between 10%–40% of the CT primary to detect low-level faults that may be hidden in phase overcurrent measurements .

Practical Considerations

• Earth Fault Relays (EL): Detect ground currents and prevent insulation failure or fires.
• Differential Relays (87G): Provide sensitive protection for generator windings, especially for faults near the neutral.
• Coordination: In multiple generator setups, relays must be coordinated to ensure selective tripping, avoiding unnecessary shutdowns of healthy units .
• High-Resistance Grounding: Limits fault current while maintaining relay sensitivity, particularly in unit-connected schemes without low-side breakers . By combining sensitive earth fault detection, differential protection, and properly coordinated PSM/TSM settings, engineers can protect generators and parallel systems effectively, even for faults occurring close to the neutral or in complex multi-generator configurations.