Static Relays Interview Questions and Answers:

1. What is static relay ?

Ans. A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic or other components without mechanical motion.

A relay using combination of both static and electromagnetic units is also called a static relay provided that the response is accomplished by static units.

2. How are static relays classified ?

Ans. Static relays are classified according to the type of the measuring unit or comparator as follows:

  • Electronic relays
  • Transductor or magnetic amplifier relays
  • Rectifier bridge relays
  • Transistor relays
  • Hall effect relays and
  • Gauss effect relays.

3. Why electronic relays could not be adopted commercially ?

Ans. Inspite of the advantages of fast operation, low maintenance, low burden on CTs and PTs, absence of mechanical inertia and bouncing contacts, they suffered inherently from the requirements of ht supply, short life, large power consumption, It supply for the heater elements. These relays could not meet practical requirements and hence never reached the commercial stages.

4. Why are transconductor relays not employed for protection applications ?

Ans. The transductor relays are mechanically very simple and are quite reliable. Due to smoothing and rectifying a signal, a delay is introduced because of the time constant of the smoothing circuit and hence the relays are slow in operation and, therefore, are discarded for protection applications.

5. What is comparator ?

Ans. The function of a protective relay is to sense any abnormal condition in the system and send a signal to the breaker which in turn isolates the faulty section of the feeder from the healthy one. The relay does all this by comparing two quantities either in amplitude or in phase. The amplitude or phase relation depends on the conditions of the system and for a predetermined value of this relation, indicative of a particular type and location of fault, the relay operates. Except in relays, such as overcurrent relays, where only one electrical quantity overcomes a mechanical quantity such as the restraint from a spring, usually two electrical quantities are compared. The device that makes such comparison is called the comparator and forms the heart of a protective relay. The operating characteristics of any relay are governed by the comparator.

6. Differentiate between amplitude comparators and phase comparators.

Ans. Amplitude comparator compares the amplitudes of two (or more) input quantities. The phase angle between the quantities under comparison (inputs) is not recognized or noticed by the amplitude comparator.

If the two input signals are S1 and S2 (say S1 the operating and S2 restraining), the amplitude comparator gives positive output only if

S2/S1 ≤ K.

The function is represented by a circle in the complex plane with its centre at the origin. This defines the boundary of the marginal operation.

The main purpose of amplitude comparators is to provide direction and distance protection.

Phase comparison technique is the most widely used technique for all practical directional, distance, differential and carrier relays.

In a phase comparator, the operation of the relay takes place when the phase relation between two inputs S1 and S2 varies within certain specified limits. Both inputs must exist for an output to occur; ideally, operation is independent of their amplitudes, and depends only on their phase relationship. The function, as defined by the boundary of marginal operation, is represented by two straight lines from the origin of the complex plane.

Mathematically, the condition of operation is given as

where θ is the angle by which S1 lags behind S2. If α1 = α2 = 90° the comparator is called the cosine comparator and if α1 = 0 and α2 = 180°, it is called the sine comparator.

7. What is hybrid comparator ?

Ans. The hybrid comparator is a combination of amplitude comparator and phase comparator. The hybrid comparators are usually multi-input comparators. The three (or more) inputs are derived from output of CT and PT by means of replica impedances, mixing transformers, auxiliary CT’s and PT’s. The characteristic of hybrid comparator depends upon the three inputs.

8. What are advantages of microprocessor base relays ?

Ans. Microprocessor based relays have numerous advantages. They have a very small burden on the CTs and PTs. Saturation can be avoided by using air-gap CT having a limited output. They can process and display the signals very efficiently, accurately and in a fastest possible manner. Due to their programmable characteristics, they can be applied extensively in the protection of electrical power systems. Moreover, one microprocessor unit may be able to perform relaying function of several systems. Microprocessor relays are more reliable and secure to relay engineers because they can alert the user to a malfunction before a false trip or failure to trip occurs. However, the microprocessors should be properly shielded from external influences and the system earthing must be very good from which they receive their control voltage.

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