How does cold load pickup affect protection schemes during substation restoration after a blackout?

Cold load pickup (CLPU) refers to the phenomenon where a significantly higher than normal current is drawn when power is restored to a distribution circuit after an extended outage, or blackout. This inrush current occurs because many loads that are normally operating at steady-state conditions are simultaneously attempting to restart after being de-energized for a prolonged period. These loads include motors, which draw high starting currents, and appliances with heating elements, which have a low resistance when cold. The magnitude and duration of the CLPU current depend on factors such as the duration of the outage, the diversity of the loads, and the ambient temperature. CLPU significantly affects protection schemes during substation restoration by causing overcurrent relays to operate, potentially preventing the successful restoration of the circuit. The high inrush current can trip overcurrent relays that are set to protect against faults. This can lead to repeated tripping and make it difficult to restore power to the circuit. Distance relays, which measure impedance to detect faults, can also be affected by CLPU. The high current and low voltage during CLPU can cause the relay to misinterpret the situation as a fault and trip unnecessarily. Reclosers, which automatically attempt to restore power after a temporary fault, can also be affected. The CLPU current can cause the recloser to trip repeatedly, preventing the circuit from being successfully restored. Several techniques can be used to mitigate the effects of CLPU on protection schemes. Time delay settings can be increased on overcurrent relays to allow the CLPU current to subside before the relay operates. This requires careful coordination with other protective devices to ensure that faults are still cleared quickly. Reduced voltage starting can be used to limit the magnitude of the inrush current. This can be achieved by using autotransformers or solid-state starters. Load shedding involves intentionally disconnecting some loads during the restoration process to reduce the overall inrush current. After the circuit has been successfully restored, the shed loads can be gradually reconnected. Voltage or current supervision can be used to block or modify relay settings during the initial restoration period. Intelligent reclosers with CLPU detection capabilities can be used to adjust their settings automatically based on the detected load conditions. Therefore, understanding and mitigating the effects of CLPU is crucial for successful substation restoration after a blackout.