How do you coordinate protective relays in a power system?

1 Why is coordination important?

Coordination is important because it ensures that only the faulty section of the power system is isolated by the relays, while the rest of the system remains intact and operational. This minimizes the impact of the fault on the system performance and reliability, and reduces the restoration time and cost. Coordination also prevents unnecessary tripping of healthy circuits, which could cause cascading failures and instability. Moreover, coordination protects the equipment from thermal and mechanical stresses caused by excessive fault currents and durations.

3 What are the coordination methods?

There are two main methods of coordination: time coordination and current coordination. Time coordination means that the relays are set to operate with different time delays, depending on their distance from the fault. The time delays are usually determined by using time-current curves, which show the relationship between the fault current and the relay operating time. The curves are plotted on a log-log scale, and the coordination is achieved by ensuring that there is enough separation between the curves of adjacent relays. Current coordination means that the relays are set to operate with different current thresholds, depending on the direction of the fault current. The current thresholds are usually determined by using directional relays, which sense the angle of the current with respect to a reference voltage. The coordination is achieved by ensuring that the relay operates only when the current is flowing towards the fault.

4 What are the coordination challenges?

Coordination can be a difficult task due to the various factors that can affect fault current and relay operation. For example, fault current can vary based on system configuration, load level, generation output, and fault type and location. Additionally, fault clearing time is dependent on the type and speed of circuit breakers, communication and control systems, and backup protection schemes. System conditions can also change over time due to load growth, network expansion, equipment aging, or maintenance. Furthermore, complex topologies such as loops, ties, or parallel lines can create multiple paths for fault current and complicate coordination. As such, relay settings must be able to accommodate a range of possible scenarios and contingencies, account for the worst-case clearing time and coordination margin, and consider the effects of mutual coupling, zero sequence impedance, and load flow direction. It is important to review and update relay settings periodically to ensure they remain valid and optimal.