Operation: Under normal working condition pull due to voltage element is greater than current element so relay contacts are open. When fault occurs V decreases and I increases so Z decreases below predetermined value so the beam tilts towards C.T. magnet thus closing trip circuit contacts.

The pull on current element is proportional to I2 and that of voltage element to V2. So relay will operate when

K₁V² < K₂I²,  V²/I² < K₂/K₁

V/I < √K₂/K₁

The value of constants K₁ and K₂ depends on ampere turns of two electromagnets. So by providing tapings on coils this value can be changed.

Operating characteristic:

                                   

The general characteristic of this relay on R-X diagram is as shown in fig. Z = √R² +X²  so locus is a circle as shown. Any value of impedance less than Z causes relay operation.

Time Distance Impedance Relay:                                                                                                                                        

This relay automatically adjusts the time of operation according to distances of relay from the fault. Under normal working condition pull of voltage restraining magnet is more so trip contact does not close. When fault occurs V decreases. I increases so trip contact close. The force produced by lower relay current should over come the spiral spring force plus force due to voltage restraining magnet.

Disadvantages of plain impedance relay:

1.    Non-directional so can not discriminate between internal and external fault.

2.    Sensitive to power swings.

3.    It is affected by arc resistance.

MHO RELAYS/ADMITTANCE RELAYS

The impedance relay is not normally used because of need of separate directional relay. So mho type relays are normally used.

                                                                               Working is similar to impedance relay. It is made directional by providing polarizing winding so torque produced is polarizing flux times fluxes from (opposed) I and V poles as shown in fig. Consider that mho relay is located at point A. So relay operates only for fault occurring in line AB and it doesn’t operate for faults occurring in AC. Hence, from this operation it is clear that this relay is made inherently directional

.                                                             

REACTANCE RELAY

                                                             Reactance relay is over current relay with directional restraint.

The current polarizing flux goes from upper polarizing coil magnet to lower electromagnet through cylinder and iron core. So due to this flux current is induced in the cylinder whose direction is perpendicular to the plane of this paper. And it flows up and down at the sides of cylinder which are opposite of the other two poles (i.e. operating coil magnet pole and restraining coil magnet pole).

The current in cylinder portion which is opposite operating coil pole reacts with the flux of that pole and produces torque is proportional to I² tending to close the relay contacts. And current in the cylinder portion opposite to restraining coil pole reacts with the flux of that pole and produce a torque is proportional to VI tending to open the relay contacts..

K₁/K₂ = X so relay operate on reactance only.

So operating characteristic is a vector whose head lie on the characteristic such that it has a constant magnitude of X as shown above.

STATIC RELAYS

                       

Static relay is the relay in which there is no moving part. The comparison or measurement of electrical quantities is done by stationary network and gives the tripping signal when threshold condition is reached.Above figure shows essential components of a static relay. The rectified output is fed to relay measuring circuit so output is produced when threshold condition is reached which is amplified by amplifier. The amplified output is given to output device which energizes the trip coil.

Advantages of Static Relays:

1.    Low power consumption: This relays provide less burden on CT s and PTs compared to conventional relays. So power consumption in measuring circuit of static relay is lower.

2.    No moving contacts: As, there are no moving contact there is no problem of contact bounce, arcing, erosion, replacement of contacts etc.

3.    There is no effect of gravity on the operation of static relays so can be used in aircrafts.

4.    Resetting time and overshoot: By using special circuits a single static relay can be used instead of using several relays.

5.    Less operating time: (1 cycle, ½ cycle, ¼ cycle)

6.    Single relay for several functions: By combining various functional circuits a single static relay can be used instead of using several relays.

7.    High torque to weight ratio.

8.    Compactness: Static relays are compact. Single relay can perform several functions.

9.    Better selectivity, stability and adequateness can be achieved. The relays area reliable and have excellent discriminating property if logic circuits are applied.

10.  Transducers; Non-electrical quantities can be converted to electrical quantities by means of transducers and then fed to static relays hence reliability is increased and cost is also reduced.

11.  Remote backup and monitoring: Static relays assisted with PLCC can be used for remote backup and network monitoring.

12.  Repeated operations possible: Static relays can be designed for repeated operations because there is no problem of moving parts.

13.  Testing and servicing can be done easily. Defective part can be replaced quickly.

14.  Self supervision of relay.

15.  Programmable operation.

Disadvantages:

1.    Auxiliary voltage requirement.

2.    Electrostatic discharge: Semiconductor components are sensitive to electrostatic discharge. Electrostatic charge is developed by rubbing of two insulating materials. Even small discharge can damage components which withstand 100 volt. So precautions have to be taken during manufacturing.

3.    Voltage transients: The static relays are sensitive to voltage spikes or voltage transient. Such transient occur due to operation of breaker in primary circuit of CTs and PTs.  Serious over voltages are produced due to breaking of control circuit, relay contacts etc. Such voltage spikes of small duration can damage semiconductor components.

4.    Temperature dependence: The characteristic of semiconductors is influenced by ambient temperature. E.g. the forward voltage drop of a diode, current amplification factor of transistor changes with temperature.

Temperature compensation is provided by means of thermister circuits, digital measuring techniques etc.

Individual component in circuit of relay is used in such a way that change in characteristic of component due to temperature change will not affect characteristic of whole relay.

5.    Price: Higher than electromechanical relays.

6.    The characteristic is likely to be affected by the operation of output device.

7.    To introduce these relays in power system enough experience should be available. Trained and skilled maintenance personal are required.

MICROPROCESSOR BASED PROTECTIVE RELAYS (μ{ BASED RELAY)

Microprocessor Based Protective Relay: (μP based relay)

While studying the electromagnetic relays and static relays and static relays the student might have noted the drawbacks of these relays such as:

i)        Electromagnetic relays: High burden on instrument transformer. High operating time and contact problems.

ii)       Static relays: Inflexibility, inadaptability to changing system conditions and complexity.

iii)      In early computer digital protection system, the drawbacks of the above two have been removed to some extend but the cost of computers used was much more up to 15 to 20 times as compared to the above two types.

But today the computer hardware technology has tremendously advanced and new generations of computers tend to make digital computer relaying most reliable, convenient and adaptable than the conventional systems.

The advent of microprocessor (μP) in the relaying system have proved to be the best system of protection and adopted invariably now-a-days with the development of powerful, economical and sophisticated microprocessors it becomes possible to replace old systems of protection and introduce microprocessor based relaying to the same power system structures.

Advantages/features of  μP based system:

1.    Programmable features.

2.    Superior.

3.    Economical.

4.    Compact units.

5.    More reliable in operation.

6.    More flexible.

7.    Improved performance over conventional types.

8.    The relay can display value of current, voltage etc. digitally.

9.    Ability to combine more protective and monitoring factions in a single relay.

10.  High speed.

Using the same interface the number of desired relaying characteristics such as over current, directional, reactance, impedance, mho, etc are obtainable by μP based system.

Prior to study the μP based relaying system let us know some basic things of it.

Fault current/voltage i.e. relaying signals contain harmonics and D.C. offset components. It therefore requires to filter them and then to be fed to the relay.

μP BASED OVER CURRENT RELAY

This type of relaying system is extensively used in the industrial motors/equipment and also in distribution lines. This is the simplest form of protection shown in the fig.

Main components are:

1.    Current to voltage (I to V) converter.

2.    Rectifier.

3.    C.Ts.

4.    Multiplexer.

5.    A/D converter.

6.    Microprocessor kit.

Using the multiplexer, the microprocessor can sense the fault currents of a number of circuits. If the fault current in any circuit exceeds the pickup value, the microprocessor sends a tripping signal to the circuit breaker of that faulty circuit (for converting signal from the transform is converted into voltage signal by I to V converter and then it is accepted by microprocessor and it sends it to tripping for C.B.).

The A.C. voltage proportional to the load current is converted DC by a precision rectifier. Thus, the microprocessor accepts DC voltage proportional to load current.

                       

Output of rectifier is fed to the multiplexer. Microprocessor sends a command to switch-on the desired channel of the multiplexer to obtain the rectified voltage proportional to current in the particular circuit. The output of multiplexer is fed to A/D converter to obtain the signal in digital form the microcomputer sends a signal to the ADC for starting the conversion. Microcomputer reads the end of conversion signal to examine whether the conversion is over or not. As soon as the conversion is over the microcomputer reads, the current signal in digital form and then compare it with the pickup value. In the case of a definite time over current relay, the microcomputer sends the tripping signal to the circuit breaker after  predetermined time delay if the fault current exceeds the pickup value. In case of instantaneous over current relay there is no intentional time delay.

Similar circuit with some alterations are used as μP based distance relay, static relays and so on.

MICROPROCESSOR BASED STATIC RELAY

The main components are:

1.    Converter

2.    Analog processing circuit

3.    Relay interface

4.    A/D converter

5.    Current and voltage supervision

6.    Time lag relay

7.    Digital processing

8.    Tripping output

9.    Indication and signaling.

These components are represents in the following circuit diagram.

                                   

Functions:

1.    A.C./D.C. Converter: Separation between auxiliary station battery and static relay.

2.    Analog processing circuit: 3-phase A.C. inputs include secondary current of CT and secondary voltage of PT.

3.    Relay interface with external digital signals: To receive external digital inputs and to feed to block No. 7 of digital processing.

4.    A.D. Converter: Conversion of analog digital signals into digital square wave signals.

5.    Current and voltage supervision: For controlling digital processes in block No. 7.

6.    Time lag relay: Through block No. 7 to determining operating time of backup relays.

7.    Digital processing: As per the required logic to process the digital signals received from A/D converter (block 4) and digital input interface (block No.3).

8.    Tripping out put: To give trip command to circuit breakers.

9.    Indication and signaling:

a)    To indicate operation of relay,

b)    To provide signals to remote terminals.

EXERCISE

1.    Give the classification of relays on various basis.

2.    State the merits and demerits of induction relays and static relays.

3.    What is µP based relay? Explain with block diagram the working of over-current relay.

4.    What are the requirements and qualities of protective relaying?

5.    Explain the terms: (i0 Selectivity, (ii) Reliability, (iii) Speed, (iv) Sensitivity w.r.t. protection relaying.

6.    Explain the terms: (i) Current setting, (ii) Plug setting multiplier, (iii) time setting multiplier, (iv) Relay timing, (v0 Time/PSM curve.

7.    Explain in brief protective-transformers.

8.    Explain any one type of  induction relay.

9.    Explain fully the “Static relay”.

10.  How static relay is superior over induction relays?

11.  Derive the relation of torque developed in induction type relay.

12.  State the applications of: (i) Static relay, (ii) µP based relay, (iii) Induction relays, (iv) mho relay.

********************************