The purpose of the electrical protection system or protection relay is:
Firstly, to detect faults and determine their location (such as overcurrent or overvoltage faults) through voltage and current transformers (CT & VT) that monitor the voltage and current values. These data are sent to the protection relay, which then sends a trip signal to the circuit breakers in the fault zone or a block signal to prevent the connection of certain breakers during the fault (i.e., if a breaker is already disconnected, a block signal is sent to prevent its connection).
Secondly, the protection relay identifies the affected areas of the fault and isolates them from the rest of the network. It disconnects only the area with the fault to ensure the highest level of network continuity and minimize the downtime for users.
Thirdly, the protection relay needs to differentiate between genuine faults (abnormal changes in voltage or current) and normal variations in the network. For example, it should distinguish inrush current, which is a temporary high current resulting from the switching of transformers, and starting current, which refers to the high currents required by motor loads during startup. These high currents resemble fault currents but are temporary and normal in the network. Therefore, the protection relay should distinguish them and avoid disconnecting the network.
Fourthly, the protection relay determines whether the fault is within its protection zone or outside it. Each protection relay is responsible for protecting a specific part of the network and controlling specific circuit breakers, such as transformer protection relay, generator protection relay, busbar protection relay, and transmission line (e.g., cable) protection relay. When a short circuit occurs at a certain location in the network, all the protection relays on the feeders supplying the fault will sense it. However, only the relay responsible for the nearby breakers should disconnect them, while the other relays remain connected to prevent power outage in the rest of the network.
What happens if the relay responsible for the fault zone fails to disconnect? In that case, there will be a time delay for the backup protection relays to disconnect and cover the delay or failure of the original relay.
The purpose of the protection system is to safeguard equipment from the damaging effects of faults, such as excessive temperature caused by prolonged overcurrent exceeding the allowed limit, leading to equipment, cable, and network element damage. It also protects individuals from electrical shock. Similarly, high voltage can cause insulation breakdown between phases, resulting in electrical arcing or short circuits, leading to increased temperature, fires, equipment destruction, and posing a risk to individuals.
I hope this information is helpful.
SparkED
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