Introduction
In Relay Coordination – Part 1, basic feeder protection devices were discussed. In this part, overcurrent relays and coordination principles are explained in detail.
Relay coordination is required to ensure that only the faulty section is isolated. As a result, the healthy part of the system remains in service.
Overcurrent Relays
An overcurrent relay operates when current exceeds a preset value.
Once it operates, it sends a trip command to the circuit breaker.
Therefore, overcurrent relays are widely used in:
Distribution feeders
Transformers
Motors
Substation protection schemes
Types of Overcurrent Relays
Instantaneous Overcurrent Relay
This relay operates without intentional delay. It trips when current exceeds the pickup setting.
Hence, it is mainly used for:
Close-in faults
High short-circuit current conditions
However, the setting must be chosen carefully to avoid unwanted tripping.
Definite Time Overcurrent Relay
In this relay, the operating time is fixed. Once pickup current is exceeded, the relay trips after a set time.
Therefore, it is suitable where simple coordination is required.
IDMT Overcurrent Relay
IDMT relays are commonly used in distribution systems. Here, operating time decreases as fault current increases.
However, a minimum operating time is always maintained. As a result, better selectivity is achieved.
Time–Current Characteristics
Relay operation is represented using time–current curves.
These curves are plotted on a log–log scale.
X-axis represents current
Y-axis represents operating time
Thus, relay behaviour becomes easy to compare and coordinate.
Relay Coordination Principle
Relay coordination follows a simple rule:
Downstream relay trips first
Upstream relay trips with time delay
Therefore, selective fault clearance is achieved.
This also improves system reliability.
Conclusion
Relay Coordination – Part 2 explains overcurrent relays and coordination principles.
It also covers IDMT characteristics and time–current curves.
