Power system protection is a set of techniques and power grid equipment used to limit the damage caused by an electrical fault and safeguard other components of the grid, like generators and transmission lines. The term is also used for a branch of electrical power engineering that deals with the protection. There is an overlap between the power system protection and power system operations, as the protection equipment, like other switchgear, can be used for operations. The protection devices are used to protect the power systems from faults by detecting the faults and taking action ("tripping"). P. M. Anderson distinguishes the reactionary devices, like protective relays, that "clear" a fault by isolating it from the rest of system and safeguard devices that address the source of the hazard (for example, an emergency core cooling system of a nuclear reactor). As a discipline, power system protection mostly deals with the reactionary devices.
Protection devices Power system protection relies on few basic elements:
a sensor performs a measurement (test) of a value (for example, of electric current in a transmission line); a comparator checks the test result against a threshold that the result is not supposed to cross during normal operation (for example, the maximum acceptable current value when testing for the overcurrent condition). Ability to identify an abnormal condition is called sensitivity; a timing element (delay) that checks for the persistence of the condition (for example, if a fault had been cleared by another protection device with a smaller delay setting, this device should not take any action); action element (typically circuit-opening). Protective devices include, under a common label of "switchgear":
fuses are the simplest protection devices combining overcurrent sensing, delay, and action in a single circuit-opening fusible part; protective relays sense the fault and initiate a trip, or disconnection, command; power circuit breakers use commands from relays and autoreclosers to open/close the electric circuit. The breakers for the protective system are safe to open under a fault current; reclosers and sectionalizers. Connecting the protective devices to the grid usually involves additional hardware:
instrument transformers, both current and voltage, are used to isolate the (mostly low-voltage) devices from the transmission levels; electric batteries (with chargers) ensure operation in case of power outage; data communications to obtain the current and voltage at remote terminals of a line and to allow remote tripping of equipment. With the exception of the breaker, the components of the protective device are frequently deployed in a redundant fashion.
Protective zones
The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible in operation, thus minimizing the loss of load. This property of the protection system is called selectivity. To achieve selectivity, the power system is subdivided into protective zones, each containing a power system component (generator, bus, transformer, transmission or distribution line, motor) that should be protected. Each zone has its own protection device(s) and provides sensitivity to faults within its boundaries. If a fault were to occur in a given zone, necessary actions will be executed to isolate that zone from the entire system (all circuit breakers in a given zone with a fault will open in order to isolate the fault). The boundaries of zones overlap to leave no part of grid without protection, overlapped regions usually surround circuit breakers with two sets of instrument transformers and relays for each circuit breaker. The overlapping regions of sensitivity have a drawback of multiple relays possibly tripping when the fault is in the overlapped area. For example, unless special arrangements are made, a short circuit above the relay A, but still within the blue zone on the diagram, might cause overcurrent conditions in relays A, C, and D and cause them to trip, with the two latter trips being redundant. This can be avoided by using specialized relays (distance or directional ones) or by coordinating the relay actions using a communication channel ("pilot"). In any case, overlapped regions are designed to be as small as possible such that when a fault occurs in an overlap region and the two zones which encompass the fault are isolated, the sector of the power system.
Backup The power protection system needs to be resilient to its own malfunctions. Thus it includes backup protection devices. For example if the fault is in the top left red zone, but outside the blue zone, it is expected to be handled by the "primary" relay A. If the relay A malfunctions and cannot clear the fault, the backup relays C and D in the adjacent (blue) zone will trip. This can be arranged without coordination (for example, the delay setting of C and D can be higher so they do not act if A succeeds in clearing the fault) or through coordination via a pilot. The term local backup is used when the backup relays are within the same zone as the "primary" one being duplicated. Local back-up protection, like the primary protective device, will isolate the elements of the plant affected by the fault to clear the latter. Adjacent-zone ("remote") back-up protection will generally isolate both the affected and unaffected items of plant to clear the fault.
Fault types
The faults can be classified by their level of permanence that affects the possibility of autoreclosing:
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