How to set the relay protection current

Relay protection current is adjusted by setting the relay's pickup current using plug settings (PSM) and coordinating with CT ratios and time multiplier settings (TMS) to ensure proper fault detection and selective tripping.

Understanding Relay Current Settings

The current setting of a relay, also called the pickup current, determines the minimum current at which the relay will operate. It is expressed as a percentage of the CT secondary rated current. For example, if an overcurrent relay is rated at 1 A and you want it to operate at 125% of the system current, the pickup current is set to 1.25 A .

• Overcurrent relays typically have a current setting range of 50% to 200% in steps of 25%.
• Earth fault relays usually range from 10% to 70% in steps of 10% .

Using Plug Setting Multiplier (PSM)

The Plug Setting Multiplier (PSM) indicates how many times the actual current exceeds the relay's pickup current. It is calculated as: PSM = Fault Current / (Relay Pickup × CT Ratio) Adjusting the PSM involves changing the number of active turns in the relay coil using a plug bridge. Moving the plug to different taps alters the pickup current . Higher PSM values result in faster relay operation according to the IDMT (Inverse Definite Minimum Time) curve .

Example:

If a CT has a ratio of 200/1 A and the relay pickup is set at 150%:

• Fault current in CT secondary = 1000 × 1/200 = 5 A
• Relay pickup = 1 × 1.5 = 1.5 A
• PSM = 5 / 1.5 ≈ 3.33 This means the relay sees the fault current as 3.33 times its pickup, which determines the operating time on the IDMT curve .

Time Multiplier Setting (TMS)

The Time Multiplier Setting (TMS) adjusts the operating time of the relay. It scales the base time from the relay's characteristic curve: Operating Time = Base Time × TMS

• Increasing TMS delays tripping, useful for coordination with upstream relays.
• Decreasing TMS speeds up tripping for faster fault isolation .

Practical Steps to Adjust Relay Current

1. Determine system parameters: Full-load current, CT ratio, and expected fault currents.
2. Select pickup current: Above maximum load but below minimum fault current.
3. Set PSM: Adjust the plug on the relay coil to achieve the desired pickup current.
4. Adjust TMS: Set the time multiplier to coordinate with other relays in the system.
5. Verify settings: Check relay characteristic curves and ensure selective coordination.
6. Update for CT changes: If CTs are replaced, recalculate PSM to maintain the same pickup current . By carefully adjusting pickup current, PSM, and TMS, relays can reliably detect faults, prevent nuisance tripping, and maintain proper coordination across the electrical system .