Primary protection is designed to operate when any fault or abnormal operating condition occurs in the power system.
The provision of adequate protection involves detecting and disconnecting elements of the power system in the event of a fault, and is an integral part of power system design.
Separation of the faulty section of the power system from the healthy section and minimizing the isolated area prevents a widespread system disturbance and minimizes shutdown of the power supply.
The protection scheme actions will prevent the expansion of the fault or minimize the consequences of the fault, including effects arising from the failure of a relay or breaker. The abnormal phenomenon originated by failure to operate a relay or a breaker to operate correctly, a sudden change of power flow, or splitting of the system during the process of fault clearance, any of which may cause the expansion of the fault, and the deterioration of the power system stability.
Requirements for the protection relays
1 Fault clearance performance in respect of Speed, Selectivity, and Sensitivity
2 Reliability
3 Cost
4 SENSITIVITY: Sensitivity is a term used when referring to the minimum operating level of relays or complete protection schemes. Relays or protection schemes are said to be sensitive if their primary operating parameters are low.
5 SELECTIVITY: When a fault occurs, the protection scheme is required to trip only those circuit breakers whose operation is required to isolate the fault. This property of selective tripping is also called ‘discrimination’.
6 SPEED: The function of protection systems is to isolate faults on the power system as rapidly as possible.
One of the main objectives is to safeguard continuity of supply by removing each disturbance before it leads to widespread loss of synchronism and consequent collapse of the power system.
The additional functions that have become available with the numerical technology:
- metering, oscillography, sequence of events capture with time tagging,
- remote setting and monitoring through communications,
- user configurability of tripping schemes and other control logic.
- Multiple setting groups for easy adaptability to network changes
- In spite of these additional functions, the required panel space and wiring are less than needed with the previous technologies.
- The burden on the VTs and CTs is substantially reduced
- The systems have the ability for continuous self-checking.
Protective Zone
Protective zone is an important factor that determines selectivity among the performances in which a protection relay should be required.
Unit protection provides a protective zone around the location of the CTs to detect faults only between the CT locations as a well-defined zone according to the sensitivity of the settings.
Non-unit protection (excluding directional comparison) has a zone that changes in accordance with the setting values and is not constrained by other CT locations.
The coordination between protection relays is the procedure to ensure that all the protection relays operate systematically to minimize the power system outage area against any fault, considering the operation limits or restriction conditions. On the other hand, when a fault occurs in equipment that the protective zone does not cover, it may not be cleared, or it may take a longer time for the fault to develop into the protective zone and then be cleared. Therefore, a fundamental principle of protection is that at least two different devices are capable of detecting any fault anywhere on the power system.
Primary or Main Protection: Main protection is installed for every equipment unit, such as a transmission line, a busbar, a transformer, etc.
When a fault occurs on any part of the power system, the main protection closest to the fault must operate faster than the other protection to minimize the extent of the power system that must be isolated to clear the fault. As the protection zones must overlap, consideration must be given to how the selectivity is achieved not to cause both zones to be tripped. Main protection is generally provided as independent duplicate protection at higher Power system voltages
where the risk of one system failing to operate correctly in the intended high speed would cause widespread consequential damage or power system instability.
A back-up protection is intended to operate when a power system fault is not cleared, or an abnormal condition is not detected, in the required time because of the failure or inability of main protections to operate, or failure of the appropriate circuit-breakers to trip. The backup protection, by definition, is slower than the main protection.
Back-up protection is installed to improve the dependability of the fault clearance system. Here, dependability is the probability of not failing to clear a power system fault or abnormality.
Back-up protection shall operate when the main protection fails to clear a fault. In such a case, the protection may not operate correctly, the circuit breaker may not receive any tripping command, or the circuit breaker may fail to open and interrupt the fault current. Such failures of a protective relay or a switching device may prevent proper clearance of the fault.
Sometimes, a second main protection or duplicate protection, intended to operate if the main protection system fails to operate or is temporarily out of service, is provided. This, however, should not be mixed up with back-up protection. The second main protection is there to increase the dependability of the normal fault-clearing mechanism, and it must always operate very selectively, while the back-up protection may operate with less selectivity because it operates after a time delay.
The requirements on backup protection cannot be independent of the requirements on the entire fault clearance system.
The use of an elementary form of the single-failure criterion is often done while planning the protection system arrangement. It requires that the failure of any one component in a fault clearance system should not result in a complete failure to clear a power system fault or abnormality.
Back-up protection is an important function of the protection system, and its design needs to be coordinated with the design of the main protection. In this process, it is suggested that the protection engineer should work closely with the power system planners and designers.
The system planner should inform the protection engineer regarding assumptions made during system design and requirements for the fault clearance system at various voltage levels in the system. He must inform him of the needs of the protection system that must be fulfilled, as for example, the total fault clearance time. The protection engineer must also be familiar with the following:
- System requirements and the system design criteria
- The plant specifications
- The failure rates of the protected plant
- The requirements of the performance indices of the fault clearance system
- The requirements for the reliability of protection equipment
- The probability that a switching device fails to interrupt the fault current.
The power system planner should help the protection engineer in formulating the requirements of the fault clearance system. Points to be considered are the stability of the power system and the type of bus bar arrangement, and the switching scheme. Protection engineers and the power system designers must coordinate the demands on the fault clearance system, as well as the performance of station equipment, and strike a balance between technical and economic benefits and the risks associated with making the protection system more complex.