From Practical Power System Protection

13.1 Protective Relay Systems

The basic function of protection is to detect faults and to clear them as soon as possible. It is also important that in the process the minimum amount of equipment should be disconnected. The ability of the protection (i.e. relays and circuit breakers) to accomplish the latter requirement is referred to as 'selectivity'.

Speed and selectivity may be considered technically as figures of merit for a protection scheme. In general; however greater the speed and/or selectivity, the greater is the cost. Hence, the degree of speed or selectivity in any scheme is not purely a technical matter, it is also an economic one.

13.2 Main or Unit Protection

The graded overcurrent systems described earlier do not meet the protection requirements of a power system. As seen in Chapter 10, the grading is not possible to be achieved in long and thin networks and also it can be noticed that grading of settings may lead to longer tripping times closer to the sources, which are not always desired. These problems have given way to the concept of 'unit protection' where the circuits are divided into discrete sections without reference to the other sections.

Ideally, to realize complete selectivity of protection, the power system is divided into discrete zones. Each zone is provided with relays and circuit breakers to allow for the detection and isolation of its own internal faults.

This ideal selective zoning is illustrated in Figure 13.1. The protection used in this manner -essentially for internal...

Products & Services
Protective Relays and Monitoring Relays
Protective relays and monitoring relays detect or monitor for abnormal power system conditions.
Circuit Breakers
Circuit breakers are mechanical switching devices capable of breaking currents under specified abnormal circuit conditions.
Motor Control Centers (MCC)
Motor Control Centers (MCC) are a modular assembly which motor control units called buckets are installed, each bucket is intended to control a motor. It acts as a central switchgear panel for the control of multiple motors in an industrial facility.
Resettable PTCs
Resettable (polymeric or positive temperature coefficient -  PTC) fuses are unlike traditional fuses that have to be replaced after a fault condition, this may not be necessary when using resettable fuses. The internal resistance of the component increases in response to an overcurrent or a short-circuit condition, limiting subsequent disturbances. After the fault condition is removed, the device cools and the internal resistance drops to nearly its initial value, permitting normal equipment operation to resume.
Circuit Tracers
Circuit tracers or signal tracers are used to troubleshoot circuits. They are relatively simple electrical testing instruments that enable electricians and other users to locate, trace and identify common electrical problems such as breakers, shorts, and open circuits.

Topics of Interest

14.1 Introduction Two commercially available systems have been chosen as typical examples to illustrate the concepts. However there are other similar products on the market using essentially the same...

Overview Protective devices form a coherent whole in relation to the network structure and its earthing system. They should be looked upon as a system based on the principle of selectivity that...

Interest in complete overcurrent device selectivity has increased due to the addition of selectivity requirements to articles 700, 701, and 708 of the National Electrical Code (NFPA70). Many users,...

INTRODUCTION Figure 25.1 illustrates the overall protection scheme in a generating station. The auxiliary equipment inside the plant requires 5 to 10 percent of the power generated. The remainder...

5.1 General Very high current levels in electrical power systems are usually caused by faults on the system. These currents can be used to determine the presence of faults and operate protection...