Protective Relay

The power system is covered by several protective zones. Each protective zone covers one or two components of the system. The neighboring protective zones overlap so that no part of the system is left unprotected. Each component of the power system is protected by a protective system comprising protective transformers, protective relays, all-or-nothing relays, auxiliaries, trip-circuit, trip coil, etc. During the abnormal condition, the protective relaying senses the condition and closes the trip circuit of the circuit-breaker. Thereby the circuit- breaker opens and the faulty part of the system is disconnected from the remaining system.

The various power system elements include generators, transformers, bus-bars, transmission lines, motors, etc. The protective relaying requirements of the various elements differ. Various types of protective systems have been developed to satisfy these requirements. For example, the over-current protection responds to increased currents. The differential protection responds to the vector difference between two or more similar electrical quantities.

The protective schemes for large electrical equipment comprise several types of protective systems. For low voltage equipment of relatively small ratings, fuses and thermal relays are generally adequate. The protective schemes of large power system-equipment are generally designed with due regards to power swings, power system stability and associated problems.

THE NATURE OF A RELAY 

 Protective relays have been called sentinels and electric brains. From the economic point of view, relays are akin to insurance; they protect the power utility from financial loss due to damage to equipment . From the underwriters' point of view they prevent accidents to personnel and minimize damage to equipment. From the customers' point of view good service depends more upon adequate relaying than upon any other equipment. The cost of this protection is between 1 and 2 % of the total cost of the power system, i.e. equivalent to an insurance premium costing about 0·1 % per year, assuming 15 years before replacement due to obsolescence . In the dictionary, four definitions of relays will be found which deal with foot races, post coaches, etc., but none even remotely fits this application. A protective relay is a device which responds to abnormal conditions on an electrical power system to control a circuit breaker, so as to isolate the faulty section of the system with the minimum interruption to service. To do this, relays must be able to decide promptly which circuit breakers are to trip in order to isolate only the faulted section(s). These relays must be designed, therefore, to be responsive to electrical quantities which are different during normal and abnormal conditions. 

The basic electrical quantities which may change in the transition from healthy to faulty conditions are current, voltage, direction, power factor (phase angle) and frequency. It is generally necessary to provide relays responsive to more than one of these quantities because, for instance, the current in a fault during minimum generation conditions may be less than the normal load current during maximum generation. As another example, the power factor measured by the relay may be as low during a power swing as during a fault. Sometimes all of the above quantities may have to be used to obtain selectivity; furthermore, in the case of an a.c. railway, several heavy trains starting up together may present current, voltage and power factor so similar to that of a fault that an additional function is necessary, the rate-of-rise of current, which is instantaneous for a fault but incremental or slower for normal service conditions. Whereas the main requirement of -instrumentation is sustained accuracy, the most important requisite of protective relays is reliability since they may supervise a circuit for years before a fault occurs; if a fault then happens, the relay must respond instantly and correctly. For this reason the designers should always attempt to use simple constructions and simple connections of relays. In spite of good intentions in this respect, there is a tendency to extend the operation of relay schemes by adding additional features until complexity results .and then it becomes necessary to re-design. In other words, a graph of the progress of relay engineering as regards complexity tends to follow a sawtooth shape. For example, a simple way to protect a circuit is to compare the current entering the circuit with the current leaving it by means of a relay in which torques corresponding to the two currents are opposed so that, if either exceeds the other, it indicates diversion of the current through a shortcircuit and hence warrants relay operation. This simple principle soon be-comes complicated because of transient magnetic conditions, such as the inrush of exciting current to a power transformer, which appear on one side of the circuit only and would cause relay operation if discriminatory blocking features were not added. Such a blocking feature, called harmonic restraint, sometimes has to be unblocked because harmonics may appear during fault conditions which demand tripping. Where possible, a principle is chosen to avoid such complications.

FUNCTION AND MODE OF OPERATION OF A RELAY

From the foregoing it can be seen that protective relays act very much like watchdogs, continuously measuring electrical quantities in the protected circuit and immediately disconnecting when the value of one becomes abnormal. For example, a reactance type relay for a transmission line closes the line connection very quickly if it detects a fault within its protection zone and if the fault does not occur outside its protection zone. To do this it measures the line reactance between itself and the fault, i.e. it measures the current, voltage and phase angle and calculates WI sin tP accurately within ± 2% and closes its contacts (or not, depending on the location of fault), and a modem relay (Figure 1.2) will do this within 20-40 milliseconds. To keep the size and cost of relays to reasonable values, the large currents and voltages of the actual primary circuit are reduced to relatively small values by current transformers (c.t's) and potential transformers (p.t's). In British practice p.t's are often referred to as v.t.s, voltage transformers. Relays measure these secondary electrical quantities and operate when the magnitude of one of them is abnormal or when the ratio between the two is abnormal. In electromagnetic relays, the measurement is made by means of an electromagnet that exerts a force on the contacts carrying the armature; Static circuits using semiconductor, thermionic and cold-cathode tubes or magnetic amplifiers can also be used although these are not all equally attractive.

 Purpose of protective Relays and Relaying

Purpose of protective Relays and Relaying system is to operate the correct CBs so as to disconnect only the fault equipment/portion from the system as quickly as possible, thus minimizing the trouble and damage caused by faults when they do occur.

 IMPORTANT REQUIREMENTS

The primary requirements for relays are reliability and selectively.

In order to have complete protection, the zones of protection given by each relay must overlap so as to leave no unprotected areas.

Furthermore, these must be a first and second line of defiance to cope with the possibility of failure of the relay or the CB at any location.