Question

As for protection relays and overcurrent. Why do we have to inform which specific norm to the use of its curve of actuation ?

Answer 1

Operation principle of protective relays :- The main principle employed in the operation of the relay is either electromagnetic attraction or electromagnetic induction. In an electro magnetic attraction relay, a plunger is drawn into a solenoid or an armature is attracted to the poles of an electromagnet. Such relays can be operated either by DC ( direct current) or AC(alternate current). In the case of electromagnetic induction relays, the principle of induction motor is used and the torque is developed by electromagnetic induction. Such relays are operated by the AC quantities only.

Example :- Buchholz relay is an example of physicoelectric relay.

According to their functions in the relay protective scheme, relays must be divided into main , auxiliary ( or supplementary) and signal relays. The main relays are the protective elements which respond to any change in the actuating quantity. e.g; current, voltage, power etc. The auxiliary relays are those which are controlled by other relays to perform some auxiliary or supplementary functions such as introduction of a time delay, multiplying the number of contacts, increasing the making or breaking capacity of the contacts of another relay, passing a command pulse from one relay to another relay, acting upon a circuit breaker closing or opening coil, energizing a signal or an alarm, etc. Signal relays function to register ( by flag or target) the operation of some relay or relay protection and control warning (visible) and alarm ( audible) signal devices. The choice of signal relay is governed by the impedance of the associated switch gear, the method of control and the number of alarm indications to be displayed.

According to the applications the relays may be classified as :-

1) OVER VOLTAGE, OVER CURRENT AND OVER POWER RELAY :- The relay operates when the voltage, current or power rises above a specified value.

2) UNDER VOLTAGE, UNDER CURRENT ANS UNDER POWER RELAY :- The relay operates when the voltage, current or power falls below a specified value.

3) DIRECTIONAL OR REVERSE CURRENT RELAY :- The relay operates when the applied current assumes a specified phase displacement with respect to the applied voltage and the relay is compensated for fall in voltage.

4) DIRECTIONAL OR REVERSE POWER RELAY :- The relay operates when the applied current and voltage assume specified phase displacement and no compensation is allowed for fall in voltage.

5) DIFFERENTIAL RELAY :- The relay operates when some specified phase or magnitude difference between two or more electrical quantities occurs.

6) DISTANCE RELAY :- In this relay the operation depends upon the ratio of the voltage to the current.

According to the timing characteristics the relays can be divided into following classes:-

1) INSTANTANEOUS RELAYS :- In these relays complete operation takes place after a very short ( negligible) time duration from the incidence of the current or other quantity resulting in operation. The time of operation of such relays is lesser than 0.2 second.

2) DEFINITE TIME LAG RELAYS :- In these relays the time of operation is sensible independent of the magnitude of the current or of other quantity causing operation.

3) INVERSE TIME LAG RELAYS :- In these relays the time of operation is approximately inversely proportional to the magnitude of the current or other quantity causing operation.

4) INVERSE DEFINITE MINIMUM TIME ( IDMT) LAG RELAY :- In these relays the time of operation is approximately inversely proportional to the smaller values of current or other quantity causing operation and tends to be a definite minimum time as the value increases without limit.

OVERCURRENT RELAYS :- Depending upon the time of operation over current relays may be categorized as 1) instantaneous over current relays, 2) inverse time over current relay, 3) definite time over current relays, 4) inverse definite minimum time (IDMT) over current relays, 5) very inverse over current relays and 6) extremely inverse over current relays.

1) INSTANTANEOUS OVER CURRENT RELAY :- An instantaneous over current relay is one in which no intentional time delay is provided for operation. In such a relay, the relay contacts close immediately after the current in the relay coil exceeds that for which it is set. Although it will be short time interval between the instant of pick up and the closing of the relay contacts, no intentional time delay is provided. This characteristics can be achieved with the help of hinged armature relays. Such relay has a unique advantage of reducing the time of operation to a minimum for faults very close to the source where the fault current is the greatest. The instantaneous relay is effective only where the impedance between the relay and source is small compared with the impedance of the section to be protected.

One of the most important consideration in over current and over voltage protection is the speed of the operation. With hinged armature relays, the time of operation of 0.01 second at three times the setting can be obtained. Such relays are employed for restricted earth faults and other types of circulating current protection.

2) INVERSE TIME OVER CURRENT RELAY :- An inverse time relay is one in which the operating time is approximately inversely proportional to the magnitude of the actuating quantity. The operating time of all over current relays tends to become asymptotic to a definite minimum value with increase in the value of actuating quantity. This is inherent in electromagnetic relays due to saturation of the magnetic circuit. So by varying the point of saturation different characteristics are obtained. These are a) definite time , b) inverse definite minimum time , c) very inverse and d) extremely inverse as shown in figure below.

These characteristics can be obtained by induction disc and induction cup relays.

3) If the core is made to saturate at very early stage, the time of operation remains same over the working range. This characteristics is shown by curve "a" in below figure and is know as definite time characteristic. Such a relay operates after a specified time irrespective of the magnitude of the fault current.

The definite time relay are used in radial or loop circuits having a few seconds, as backup protection for other types of protection and on systems with wide variations of fault current due to source impedance.

Selectivity amongst such relays is obtained if there is difference of 0.5 seconds in the time settings of the two successive relays.

4) IDMT RELAYS :- Such a relay is one in which operating time is approximately inversely proportional to fault current near pick up value and becomes substantially constant slightly above the pickup value of the relay, as illustrated by curve "b" in figure below. This is achieved by using a core of the electromagnet which gets saturated for currents slightly greater than the pick up current.

5) VERY INVERSE RELAY :- In such relay the saturation of the core occurs at a still later stage, as illustrated by curve "c" in figure below. This curve is known as very inverse characteristic curve. The time current characteristic is inverse over a greater range and after saturation tends to definite time. Relays with very inverse time current characteristic are employed on feeders and long sub transmission lines.

6) EXTREMELY INVERSE RELAY :- The curve "d" in figure below illustrate extremely inverse characteristic I.e; core saturation occurs at a very late stage. The equation describing the curve "d" is approximately of the form I^2 *t =K where I is the operating current and t is the operating time. Such relays are quite suitable for the protection of transformers, cables etc, as it is possible to achieve accurate discrimination with fuses and auto re closures in their case, which can seldom be made selective with standard IDMT relays. This is because of their ability to ride through starting currents and surges providing at the sane time fast operation under fault conditions. They are, thus more suitable for installation with large inrush currents after an outage.

Relays with inverse time current characteristic are widely employed in distribution networks and industrial plant systems. Their relatively flat time current characteristic permits them to achieve reasonably fast operation over a wide range of short circuit currents.

CHARACTERISTICS OF VARIOUS OVER CURRENT RELAYS.