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Power system protection is a set of techniques andpower grid equipment used to limit the damage caused by anelectrical fault and safeguard other components of the grid, likegenerators andtransmission lines.[1] The term is also used for a branch ofelectrical power engineering that deals with the protection.[2] There is an overlap between the power system protection andpower system operations, as the protection equipment, like otherswitchgear, can be used for operations.[3]
Theprotection devices are used to protect the power systems from faults by detecting the faults and takingaction ("tripping"). P. M. Anderson distinguishes thereactionary devices, likeprotective relays, that "clear" a fault by isolating it from the rest of system andsafeguard devices that address the source of the hazard (for example, anemergency core cooling system of a nuclear reactor). As a discipline, power system protection mostly deals with the reactionary devices.[4]
Power system protection relies on few basic elements:[5]
Protective devices include,[7] under a common label of "switchgear":[8]
Connecting the protective devices to the grid usually involves additional hardware:
With the exception of the breaker, the components of the protective device are frequently deployed in aredundant fashion.[13]
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 theloss of load.[14] This property of the protection system is calledselectivity.[6] To achieve selectivity, the power system is subdivided intoprotective 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 ofinstrument 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.[15] For example, unless special arrangements are made, ashort circuit above the relay A, but still within the blue zone on the diagram, might causeovercurrent 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 ordirectional ones) or by coordinating the relay actions using acommunication 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.[16]
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, thebackup 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)[17] or through coordination via a pilot.[citation needed] The termlocal backup is used when the backup relays are within the same zone as the "primary" one being duplicated.[18]
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.
The faults can be classified by their level of permanence that affects the possibility ofautoreclosing:[19]
Many pieces of the grid equipment can develop internal problems. For these devices, the faults can be classified intointernal andexternal. As an example of the internal fault, a transformer might develop overpressure inside its containment vessel with the root cause (for example, local overheating) not triggering any other alarms. For the same transformer, an overload condition would represent an external fault.[20]
The relays can be classified by their sensitivity to the location of a fault:
In the 21st century a lot of testing for abnormal conditions is performed by multifunctionalnumerical relays that use computers for calculations. Individual measurements use cryptic identifiers from theANSI device numbers list, like "50" for the "Instantaneous Overcurrent Relay" or "87L" for the "Segregated Line Current Differential".
Historically, the power industry went through multiple generations of sensors and comparators, retaining the terminology and some of the devices:
High-voltage transmission lines typically form a mesh-like grid, so the current might be flowing into the fault from either direction, making the non-directional relays mostly unsuitable for protection, so the distance and pilot relays are typically used.[27] The use of directional relays is not always possible.[28]
In agrounded neutral transmission line, the phase relays might detect and clear aground fault. However, since almost all faults on high-voltage lines are of the one-phase-to-ground variety, specializedground relays are used for quick reaction.[29] These relay utilize thezero-sequence current for detection. During the normal operation, the zero-sequence current is very small, so a high current value that depends on the network configuration, not on the (varying) load, is a convenient and reliable indicator of a ground fault.[30]
Generators are expensive and complex pieces of the grid equipment, thus the larger machines use tens of types of protection devices. Practically every generator has the stator protection, usually using differential relays.[31] The internal failures of generators are unusual, so the protective relays trips are very rare.
Not only the transformers are expensive, but it might take a long time to replace a failed large transformer.[32] Electrical protection of a transformer mostly uses the differential relays.[33] This protection can be combined with the one of thebusbar[34] or generator.[35]
Thelow-voltage network generally relies upon fuses or low-voltage circuit breakers to remove both overload and earth faults.
Disturbance-monitoring equipment (DME) monitors and records system data pertaining to afault. DME accomplish three main purposes:
DME devices include:[37]
Protection engineers define dependability as the tendency of the protection system to operate correctly for in-zone faults. They define security as the tendency not to operate for out-of-zone faults. Both dependability and security are reliability issues. Fault tree analysis is one tool with which a protection engineer can compare the relative reliability of proposed protection schemes. Quantifying protection reliability is important for making the best decisions on improving a protection system, managing dependability versus security tradeoffs, and getting the best results for the least money. A quantitative understanding is essential in the competitive utility industry.[38][39]
Reliability: Dependability vs Security
There are two aspects of reliable operation of protection systems: dependability and security.[40] Dependability is the ability of the protection system to operate when called upon to remove a faulted element from the power system. Security is the ability of the protection system to restrain itself from operating during an external fault. Choosing the appropriate balance between security and dependability in designing the protection system requires engineering judgement and varies on a case-by-case basis.
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