How to adjust the relay protection current

The current in relay protection is adjusted by setting the relay's pickup current using plug settings and coordinating it with the system's current transformer ratio and time settings.

Understanding Pickup Current

The pickup current is the minimum current at which a relay begins to operate. It is determined by the deflecting force of the relay coil, which is proportional to the current and the number of active turns in the coil. By changing the number of active turns using taps and a plug bridge, you can adjust the pickup current.

Increasing active turns reduces the required current, while decreasing turns increases it. The pickup current is usually expressed as a percentage of the CT's rated secondary current, for example, 125% of rated current for an overcurrent relay. 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 a key factor in determining the relay's operating time according to inverse time characteristics. A higher PSM results in faster tripping.

PS

M is calculated as: PSM = Fault Current in Relay / Relay Pickup Current For example, if a CT has a ratio of 200/1 A, the relay pickup is set at 150%, and the primary fault current is 1000 A, the secondary current in the relay is 5 A, which is used to determine the PSM.

Time Multiplier and Coordination

The Time Multiplier Setting (TMS or TSM) scales the relay's operating time based on its characteristic curve.

Adjusting TM

S ensures proper coordination between upstream and downstream relays, allowing the nearest relay to trip first while upstream relays provide backup.

Lower TM

S values result in faster tripping, while higher values delay operation to maintain selectivity.

Practical Steps to Adjust Relay Current

Determine the system's rated current and CT ratio. Select the desired pickup current as a percentage of the CT secondary rating. Set the plug on the relay coil to achieve the required number of active turns corresponding to the pickup current.

Calculate PS

M using the expected fault current and relay pickup. Adjust TMS/TSM to coordinate with other relays in the system. Verify settings using relay characteristic curves or an overcurrent relay calculator to ensure proper operation and selectivity.

By carefully adjusting the pickup current, PSM, and TMS, you ensure that the relay operates reliably during faults, protects equipment, and maintains system stability without unnecessary tripping.