- Network protector
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A network protector (also reverse-power relay) is a type of electric protective device used in electricity distribution systems. The function of the network protector is to automatically connect and disconnect its associated power transformer from the secondary network when the power starts flowing in reverse direction. They are applied on both spot networks and grid networks.
Typically the network protector is set to close when the voltage difference and phase angle are such that the transformer will supply power to the secondary grid. Conversely the network protector is set to open when the voltage difference and phasing angle is such that the secondary grid would back-feed through the transformer and supply power to the primary circuit.
A network protector structurally consists of three main parts: the housing, the mechanism, and a relay setup. The housing holds the mechanism in place, encloses the protector mechanism and relay to prevent damage and tampering with the network protector. The mechanism contains electrical and mechanical parts to switch open and close the secondary contacts. Finally, the relay is the 'brains' behind the network protector, monitoring various electrical conditions using sensors, and controlling the mechanism through electrical signals. The device uses a power/time curve so that benign back-flows (e.g. elevators) do not trip out circuits.
Although its name implies otherwise, the network protector does not actually protect the (secondary) network cable from failure. The network protector does, however, protect the stability and dependency of the secondary grid by preventing current to travel away from the customer and towards the primary feeders. If there is a fault on the primary feeder, the substation circuit-breaker is meant to open, disconnecting the primary feeder from one side. The issue is that this primary cable is also connected to a network transformer. This network transformer is also connected to the secondary grid. By magnetic induction, the secondary network will energize the primary feeder, through the network transformer. This can be very dangerous, because a fault will continue to be 'fed' from the secondary network side of transformer. Even without a fault, if the electric utility wants to perform maintenance on that primary cable, they must have a way to fully disconnect that primary cable, without worrying about the cable being energized by the secondary network through the network transformer. Thus, the network protector is designed to open its contacts if the relay senses backwards flowing current.
However, if there is a fault on the secondary grid, the network protector is not designed to open its contacts up. The secondary fault will continue to be fed from the primary side of the system. In some cases, networks are designed with cable limiters (like fuses) to melt and disconnect the secondary fault under the right conditions. In other cases, the utility lets cable 'burn clear', in which case the fault is allowed to remain fed until the cables fuse, then the fault is isolated. Analysis of the system is required to ensure that the system can, indeed, supply enough current to fuse the cables, wherever the fault is. This method tends to works well at 120 volts but it is less reliable at higher voltages. The danger in depending on the cable to 'burn clear' is that some conditions will not cause the cable to burn in this manner and instead, the entire section of cable can be damaged from excessive, long-term overloading, causing fires and damage to the secondary network.
Typically, network protectors are contained inside a submersible enclosure which is bolted to the throat of the network transformer and placed in underground vaults. IEEE standard C57.12.44 covers network protectors.
See also
References
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