Power system protection
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. 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. 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 of a value ;
- a comparator checks the test result against a threshold that the result is not supposed to cross during normal operation. Ability to identify an abnormal condition is called sensitivity;
- a timing element that checks for the persistence of the condition ;
- action element.
- 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.
- instrument transformers, both current and voltage, are used to isolate the devices from the transmission levels;
- electric batteries 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.
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 that should be protected. Each zone has its own protection device 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. 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 or by coordinating the relay actions using a communication channel. 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 zone will trip. This can be arranged without coordination 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 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:- a transient fault clears quickly once the line is opened. A quick reclosing with not cause the fault to reappear. The transient faults represent the vast majority of faults on the overhead transmission lines;
- a semipermanent fault, like tree contact, are faults that might clear themselves if allowed to burn for a short time. Semipermanent faults occur more frequently in the subtransmission lines and electric distribution;
- a permanent fault must be repaired. Almost all the faults on the underground power cable are permanent.
Relay types
The relays can be classified by their sensitivity to the location of a fault:- a nondirectional relay does not provide an information on which side of it the fault is located, this is the simplest form of the overcurrent relay. For example, in a radial system of electrical power distribution, the current always flows to the load spokes, so there is no need to sense its direction, as an overcurrent condition always indicates a short circuit on the load side;
- a directional relay compares the current phase with the reference and performs action only if the direction to the fault matches the selected one;
- a differential relay compares the values of electrical measurements on the input and output of a protected device. For example, in an electrical transformer, input and output voltage and current values are related through the transformer ratio, and large deviations from this constant indicate a fault;
- a distance relay determines the distance of the fault by calculating the line electrical impedance as seen by the relay based on observe current, voltage, and the phase difference between them. The knowledge of distance can be used to prevent tripping when the fault is in the wrong zone;
- a pilot protection relay senses the conditions on the other end of the line through a communication channel.
Historically, the power industry went through multiple generations of sensors and comparators, retaining the terminology and some of the devices:
- electromechanical relays are the simplest devices used for protection since the early days of electrical power industry. For example, an induction disk overcurrent relay is an overcurrent relay that uses an induction disk as a sensor and timing element;
- electronic relays, with comparators and level detectors were introduced slowly over a long period of time starting in the 1930s. The process accelerated with the arrival of transistor in 1950s.
- digital relays were proposed in 1969, but became widespread only with the introduction of microprocessor in the early 1970s.
Types of protection