Article Overview
The Relay Protection Balance Coefficient is calculated as the ratio of actual fault current to the relay pickup current, often expressed using the Plug Setting Multiplier (PSM) and Time Setting Multiplier (TSM) in IDMT relays.
Definition and Purpose
The Relay Protection Balance Coefficient is used to quantify the relationship between the fault current flowing through a relay and the relay's pickup setting. It ensures proper coordination between upstream and downstream relays, allowing selective tripping and minimizing unnecessary outages . This coefficient is particularly important in overcurrent protection schemes and IDMT relays.
Formula
The coefficient can be expressed as: Balance Coefficient (BC) = Fault Current / Relay Pickup Current Where:
- Fault Current is the actual current at the relay location during a fault.
- Relay Pickup Current is the minimum current at which the relay begins to operate (also called Plug Setting). In terms of relay settings using PSM and TSM: PSM = Fault Current / Plug Setting Current Operating Time = Base Time × TSM (Time Setting Multiplier) The Balance Coefficient is effectively equivalent to the PSM, which is used in the IDMT relay characteristic curves to determine the operating time . For numerical relays, the operating time can be calculated as: t = TSM × f(PSM) Where f(PSM) is the time-current characteristic function of the relay, which depends on the type of inverse curve (standard inverse, very inverse, extremely inverse).
Practical Application
- Select Relay Pickup: Determine the minimum current that should trigger the relay.
- Calculate PSM: Divide the expected fault current by the relay pickup.
- Set TSM: Adjust the time multiplier to ensure proper grading between upstream and downstream relays.
- Compute Operating Time: Use the relay's IDMT curve to find the trip time for the given PSM and TSM. This approach ensures selectivity, reliability, and coordination in the protection system, preventing upstream relays from tripping before downstream relays clear the fault . By using the Balance Coefficient, engineers can optimize relay settings to achieve fast fault clearance while maintaining system stability.
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