A protection system protects the power system from the deleterious effects of a sustained fault. A fault (meaning in most cases a short circuit, but more gener-ally an abnormal system condition) occurs as a random event. If some faulted power system component (line, bus, transformer, etc.) is not isolated from the sys-tem quickly, it may lead to power syssys-tem instability or break-up of the syssys-tem through the action of other automatic protective devices. A protection system must
CT
CVT
Battery Breaker
Relay
Figure 2.1 Subsystems of a protection system. Besides relays, the protection system con-sists of transducers, circuit breakers and station battery
therefore remove the faulted element from the rest of the power system as quickly as possible.
Although a protection system is usually understood to mean relays, it consists of many other subsystems which contribute to the fault removal process. These subsystems are identified in Figure 2.1. The circuit breaker actually isolates the faulted circuit by interrupting the current at or near current zero. A modern Extra High Voltage (EHV) circuit breaker can interrupt fault currents of the order of 100 000 amperes at system voltages of up to 800 kV. It can do this as quickly as the first current zero after the initiation of the fault, although more often it does so at the second or third current zero. The breaker is operated by energizing its trip coil from the station battery, and the relay(s) do this job by closing contacts between the battery and the trip-coil. Very often other relays (reclosing relays) are used to reclose the circuit breaker after a suitable time interval.
The transducers (current and voltage transformers, or CTs and CVTs) constitute another major component of the protection system. They are necessary because the high magnitude currents and voltages of the power system must be reduced to more manageable levels in order to drive low energy (and hence safe for human access) devices such as relays. We will consider the current and voltage transformers in some detail in Section 2.6. For the present, it is sufficient to note that certain features of the transducers have been standardized. Current transformer secondary rating has been standardized at 5 amperes or 1 ampere, the latter standard being more common in Europe. (A few other standard ratings also exist but they are not very common). This implies that the maximum load current in the primary winding of the current transformer would produce 5 amperes (1 ampere) or less in its secondary winding. This leads to a desired CT winding ratio, which is then approximated by one of the standard CT ratios available. The voltage transformers have the secondary windings rated at 67 volts phase-to-neutral. Within certain limits (as discussed in Section 2.6), the current and voltage transformers reproduce the primary current and voltage waveforms faithfully on their secondary side. The relay
thus sees a scaled down version of currents and voltages that exist on the power system.
The last and most important component for our discussion of the protection system is the relay. This is a device which responds to the condition of its inputs (voltages, currents, or contact status) in such a manner that it provides appropriate output signals to trip circuit breakers when input conditions correspond to faults for which the relay is designed to operate. Relays are the logic elements in the entire protection system. The design of a relay (whether analog or digital) must be such that all fault conditions for which it is responsible produce a trip output, while no other conditions should. Much of this book will be dedicated to design techniques for relaying algorithms such that these requirements are met.
This is a good point to discuss the concept of reliability as understood in relaying literature. To a relay engineer, a reliable relay has two attributes: it is dependable, and it is secure. Dependability implies that the relay will always operate for con-ditions for which it is designed to operate. A relay is said to be secure if the relay will not operate for any other power system disturbance. Of the two attributes (dependability and security), the latter is more difficult to achieve. Every fault in the neighborhood of a relay will disturb its input voltages and currents. However, the relay should disregard those voltage and current conditions that are produced by faults which are not the responsibility of the relay.
The responsibility for protection of a portion of the power system is defined by a zone of protection. A zone of protection is a region clearly defined by an imaginary boundary line on the power system one line diagram. A protection system – consisting of one or several relays – is made responsible for all faults occurring within the zone of protection. When such a fault occurs, the protection system will activate trip coils of circuit breakers thereby isolating the faulty portion of the power system inside the zone boundary. Usually – though not always – the zones of protection are defined by circuit breakers. If the zone of protection does not have a circuit breaker at its boundary, the protection system must trip some remote circuit breakers (transfer the trip command through a communication channel) to de-energize the faulted zone. Figure 2.2 shows a portion of the power system divided into various zones of protection. Zones 1, 2, and 3 are transmission line protection zones for the various lines. A fault on any of these lines would be detected by their corresponding protection systems, and trip appropriate breakers at zone boundaries.
Zone 4 is the bus protection zone. Zone 5 is the zone for transformer protection.
Note that there is no circuit breaker at one end of this zone, and consequently the transformer protection system must trip the breaker at bus A, and through a communication channel remotely trip the breaker at bus C .
Note also that the zones of protection always overlap. This is in order to ensure that no portion of the system is left without primary high speed protection (i.e.
there are no blind spots in the protection system). Although overlap is achieved in Figure 2.2 by including the circuit breaker in each neighboring zone, in reality this may not be possible under all circumstances. Zone overlap is achieved through the
A B
C
1 2
3
4 5
Figure 2.2 Zones of protection. Zone 2 defines the boundary for protection of transmission line A-B. Zone 4 defines bus-A protection. Zone 5 is a transformer protection zone
To A
To B To A
To B B
A B A
(a) (b)
Figure 2.3 Principle of zone overlap. (a) When current transformers are available on either side of the breaker. (b) When a single current transformer with multiple secondary windings is available
proper choice of current transformers dedicated to each protection system. Consider the arrangement shown in Figure 2.3(a). A current transformer is assumed to exist on either side of the circuit breaker. In this case, the protection systems on either side of the breaker use current transformers from opposite sides of the circuit breaker.
When current transformers are not available on both sides of the circuit breakers, an overlap is achieved by using current transformer secondary windings on the far side as illustrated in Figure 2.3(b). In this case, although there is no blind spot in relaying, tripping for faults between the circuit breaker and the CT requires special consideration. It is desirable to keep the region of overlap as small as possible.
Occasionally, a zone will be protected by several protection systems in order to make sure that failure of the protection system itself will not leave the power system unprotected. This reinforces the dependability of the overall protection sys-tem. In such cases, it is desirable to have as much independence between the two protection systems as possible. It would be prohibitively expensive to duplicate the circuit breaker, the current transformer, the voltage transformer, or even the station battery. However, some degree of separateness may be obtained by using differ-ent secondary windings of a currdiffer-ent transformer for the two protection systems, by using separate fuses in the voltage transformer circuit, by providing separate trip coils for the circuit breaker, and in exceptional cases by providing separate
batteries for relays and breaker trip circuits. These attempts are made in order to avoid common failure modes among the different protection systems and thereby to improve the dependability of the entire protection system.
Almost all the relays in use on power systems may be classified as follows:
1. Magnitude Relays: These relays respond to the magnitude of the input quantity.
An example is the overcurrent relay which responds to changes in the magnitude (either the peak value or the rms value) of the input current.
2. Directional Relays: These relays respond to the phase angle between two AC inputs. A commonly used directional relay may compare the phase angle of a current with a voltage. Or, the phase angle of one current may be compared with that of another current.
3. Ratio Relays: These relays respond to the ratio of two input signals expressed as phasors. Ratio of two phasors is a complex number, and a ratio relay may be designed to respond to the magnitude of this complex number or to the complex number itself. The most common ratio relays are the several versions of impedance or distance relays.
4. Differential Relays: These relays respond to the magnitude of the algebraic sum of two or more inputs. In their most common form, the relays respond to the algebraic sum of currents entering a zone of protection. This algebraic sum may be made to represent the current in any fault (if it exists) inside the zone of protection.
5. Pilot Relays: These relays utilize communicated information from remote loca-tions as an input signal. This type of protection generally communicates the decision made by a local relay of one of the four types described above to relays at the remote terminals of a transmission line.
A functional description of input-output relationships of the five relay classes described above will give us sufficient basis for the design of relays – all else is application of these designs to a given protection problem. As far as digital relaying is concerned, we may then proceed directly to a discussion of relaying algorithms.
However, we will give a very brief overview of the application of these relays to system protection problems. As mentioned in Section 2.1, the reader will do well to refer to several excellent books on application of relays to power systems. The material in Sections 2.4, 2.5, and 2.6 is offered in order to provide a degree of continuity in our development, as it is not advisable to develop a theory of relay algorithm design without some note being taken of the application principles.