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A network protector is a device that monitors the flow of electric power between interconnected power systems, automatically disconnecting it if the power starts flowing in the reverse direction. It is a protective device that's used in grid and spot networks to prevent the backward flow of electrical energy from the secondary network. Network protector relays consist of circuit breakers, relay setups, and housing mechanisms. They are mostly used in underground power distribution networks to provide power reliably in high-density population load areas. These areas may be industrial sites, large buildings, or even sections of a city.
Secondary power distribution networks typically contain interlaced grids whose power is supplied by a minimum of two or more power sources. It is structured in this way to allow the power distribution network to function uninterrupted even if one power source is lost. Every power source contains a switch, a multiphase bus, and a transformer. The network protector connects the multiphase feeder bus to the network and is typically located in dustproof housing mechanisms. The housing cases are also moisture proof because of the devices' locations; they are mostly located in underground passages in large urban areas.
The housing mechanism protects the relay and circuit breaker from exposure to the elements and tampering, ultimately preventing it from damage. The circuit breaker has contacts that toggle between open and closed positions. The relay acts as the brains of the device and monitors the line currents, transformer, and network voltages with the help of sensors. Power flows through the network protector when the main contacts within it are closed. If the relay detects a reverse flow of power or an overcurrent situation, it executes algorithms to initiate breaker tripping and trips the system.
Even though it may seem so, the network protector doesn't protect the secondary network but stops power from flowing away from it to the primary network. It maintains the dependency and stability of the secondary system. The relays detect faults in the primary feeder, and the circuit breaker opens to disconnect the primary feeder from the secondary network. This is done because the primary cable is connected to the secondary network through the network transformer. If power is allowed to flow in reverse, it energizes the primary feeder through the process of magnetic induction.
This is a hazardous situation because the fault will continue to be energized through power being supplied by the secondary network. The relay in the network protector senses reverse-flowing power and trips the system to prevent this. If a fault exists in the secondary network, the relay does not trip, and the fault will continue to be energized by the primary feeder. In such cases, the networks rely on cable limiters to act like fuses, melting to disconnect the secondary fault. Sometimes, the the cables are allowed to burn clear, and the fault is isolated. This can be dangerous because the cable may fail to burn and the secondary network becomes damaged due to excessive overloading in the long term.
The control relays have reclosers that close the circuit breaker after it has been tripped and the fault has been rectified. Earlier network protectors were electromechanical systems, while more modern ones are entirely electronic. Electronic network protectors calculate power flow or use currents and sequence voltages to make tripping decisions. Digital, sequence-based relays are even capable of metering power flows and can communicate this data to remote stations.