User:Marshelec/Power-System Protection

For the protection of low voltage networks or low voltage electrical wiring, see Circuit breaker § low-voltage, Residual-current device, and Fuse (electrical).

The purpose of power system protection is to initiate the prompt removal from service of any element of an electric power system when it suffers an electrical fault, or when it starts to operate in any a manner that might cause abnormal safety hazards, damage to equipment or property, or otherwise interfere with the effective operation of the rest of the system.

Role of power system protection

The role of protection in an electrical power system is to manage and minimise the consequences of electrical faults. The overall design objective is to minimise the extent of safety hazards, power outages and equipment damage that may result from a fault, and as far as practicable, to ensure the stability and reliability of the rest of the power system.

Key terms

Elementary protection system

Some key terms used in describing power system protection are:

Protection system: Protective relays, communication systems necessary for correct operation of protective functions, voltage and current sensing inputs to protective relays and associated circuitry from the voltage and current sensing devices, station dc supply, and control circuitry associated with protective functions from the station dc supply through the trip coil(s) of the circuit breakers or other interrupting devices.

Protection scheme: a collection of protection equipment providing a defined function and including all equipment required to make the scheme work (i.e. circuit breakers, measuring equipment such as current transformers, protective relays, batteries, etc.)

Unit protection scheme: a protection scheme that is designed to operate only for abnormal conditions within a protected zone. A typical example of unit protection is a differential protection scheme that measures and compares current flowing at the boundaries of the protected zone.

Electrical faults in power systems

Main article: Electrical fault

Electrical power systems are typically complex networks that connect generation sources to end users, via transmission and distribution systems.

Electrical faults in a power system can arise from time to time from a wide range of causes. The main causes are equipment breakdown, damage during storms, lightning strikes, vegetation or animals contacting with overhead lines, vandalism, accidental contact or damage (such as during excavation around buried cables, or vehicle crashes into power poles, or inadvertent contact from machinery or other activity beneath lines), and human error during operation and maintenance of the network.

Depending on the circumstances, these electrical faults can cause serious safety hazards to workers or members of the public. They can also cause equipment damage, and may trigger fires. Faults in the electricity network are likely to cause transient disturbance to the supply, or a loss of supply. Frequent or prolonged power outages result in severe disruption to normal economic and social activity, and this can lead to significant costs and a wide range of other impacts on end users and society.

To minimise these risks, protection systems are provided in electricity networks to detect faults and interrupt the supply in the shortest practical time, whilst only removing the minimum amount of power system equipment from service.

Design requirements

Reliability

To achieve the overall objective for power system protection, the protection equipment and schemes must be reliable.

For most power system equipment, the main reliability criteria is that it should operate when called upon to do so. However, for protection equipment, another vitally important reliability characteristic includes that the scheme should not operate when it is not expected to. Some authorities refer to these two aspects of reliability as dependability and security respectively.

• Dependability is the ability of the protection system to operate, when it is expected to do so, to remove a faulted element from the power system.
• Security is the ability of the protection system to restrain itself from operating during a fault that is outside the boundaries of the protected zone

Stability

Stability is similar to the 'security' criteria above, but is used by some reference sources to describe the required performance of unit protection systems. To ensure reliability against unwanted tripping, unit protection schemes must be stable and not operate during faults or other conditions that occur outside the unit protection zone.

Redundancy

For situations where particularly high reliability is required, duplicate protection schemes may be installed to provide redundancy. In duplicate protection systems the tripping signal can be provided in a number of different ways. The most common methods are:

• all protection systems must operate for a tripping to occur (e.g. ‘two-out-of-two’ arrangement)
• only one protection system needs to operate to cause a trip (e.g. ‘one-out-of two’ arrangement)

The former method guards against false tripping due to maloperation of one of the protection systems. The latter method guards against failure of one of the protection systems to operate when required to do so.

Sensitivity

The sensitivity of a protection scheme is the threshold in the measured parameter where it will just detect a fault condition and operate to initiate tripping. Protection schemes must be able to distinguish between healthy and fault conditions, i.e., to permit normal loads to be flow, but to detect, operate and initiate tripping before a fault reaches a dangerous condition.

Some electrical faults, particularly ground faults, may have high resistance, leading to a fault current that may be only a small proportion of the normal maximum load current. The sensitivity of the ground fault protection system must be sufficient to detect and interrupt potentially hazardous faults, but without being vulnerable to unwanted trippings when normal load current is flowing.

Speed

When electrical faults or short circuits occur, the safety hazards and damage produced increase rapidly with the time that the fault persists. Therefore, it is desirable that electrical faults be interrupted as quickly as possible. High-speed fault detecting relays can operate in as 10 milliseconds and initiate an order to open a circuit breaker in 2 milliseconds..

However, the costs of high speed protection systems must be weighed against the associated safety risks, the criticality of the assets being protected, and the risks to the integrity of the remaining network. For electricity distribution systems, high speed protection systems are not usually justified, whereas they are standard for generation and transmission systems.

Selectivity

Selectivity describes the ability of the protection system to limit the extent of the power system that is removed from service to isolate a fault. The placement of circuit breakers in the main high voltage circuit is critical to achieving selectivity, because this determines the boundaries of the network that can be removed from service while leaving other parts of the network in service. When a fault occurs, the protection scheme is required to trip only those circuit breakers whose operation is required to isolate the fault, to minimise extent of the disturbance to the rest of the power system. This property of selective tripping is also called 'discrimination' and is achieved by several methods:

• grading of the sensitivity and time settings of the protection scheme at each circuit breaker position
• unit protection that provides circuit breakers and measuring equipment at the boundaries of portion of the main high voltage system to specifically protect that portion

Cost-effectiveness

Duplication of protection systems is one option to achieve higher reliability. However, the high costs of duplicate high voltage transmission circuit breakers and measuring equipment may outweigh the potential lifetime benefits of improved reliability. For lower voltage distribution systems, even duplication of the protection relays may not be cost-effective. In these cases, if the primary protection fails, in the event of a fault, the back protection will be from a remote location, leading to a slower fault clearance and more equipment removed from service and the backup is called upon to operate.

Unit protection schemes

A "unit protection" protection scheme is intended to operate only for faults within a specific zone.

Overlapping zones of protection

Overlapping zones of electrical protection

To limit the extent of the power system that is disconnected when a fault occurs, protection is usually implemented in zones.

There will typically be separate zones of protection for generators, transformers, busbars, transmission lines and feeders. This means that a fault in power system equipment within one zone will be detected by the protection scheme for that zone, and only the minimum equipment will be removed from service. The zones of protection will generally overlap, so that no part of the power system is left unprotected.

Time-current grading

Time-graded co-ordination

Applications

General layout of electricity networks. The voltages and loadings are typical of a European network.

See also

• Electrical fault
• Protective relay
• Prospective short-circuit current
• Circuit breaker
• Three-phase electric power
• Electric power system
• Electricity generation
• Electric power transmission
• Electric power distribution
• Power outage