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Home / Technical Articles / Using Protective Relay For Fighting Against Faults
Using Protective Relay For Fighting Against Faults
Using Protective Relay For Fighting Against Faults

Content //

  1. Introduction to Protective Relay
  2. Working Principle of Protective Scheme
  3. What is Relay?
  4. Functions of Protective Relay
  5. Desirable qualities of protective relaying
  6. Terminology of protective relay
  7. History of Protective Relay
  8. Types of Relays
  9. Types of Relay based on Relay Operation Mechanism
  10. Protective relay testing: Test relays of all generations (VIDEO)

Introduction to Protective Relay

Protective relay works in the way of sensing and control devices to accomplish its function. Under normal power system operation, a protective relay remains idle and serves no active function.

But when fault or undesirable condition arrives Protective Relay must be operated and function correctly.

A Power System consists of various electrical components like Generator, transformers, transmission lines, isolators, circuit breakers, bus bars, cables, relays, instrument transformers, distribution feeders, and various types of loads.

Faults may occur in any part of power system as a short circuit and earth fault. Fault may be Single Line to Ground, Double Line to Ground, Line to Line, three phase short circuit etc. This results in flow of heavy fault current through the system.

Fault level also depends on the fault impedance which depends on the location of fault referred from the source side. To calculate fault level at various points in the power system, fault analysis is necessary.

The protection system operates and isolates the faulty section. The operation of the protection system should be fast and selective i.e. it should isolate only the faulty section in the shortest possible time causing minimum disturbance to the system. Also, if main protection fails to operate, there should be a backup protection for which proper relay co-ordination is necessary.

Failure of a protective relay can result in devastating equipment damage and prolonged downtime.

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Working Principle of Protective Scheme

Protective relaying senses the abnormal condition in a part of power system and gives an alarm or isolates that part from healthy system. Protective relaying is a team work of CT, PT, protective relays, time delay relays, trip circuits, circuit breakers etc.

Protective relaying plays an important role in minimizing the faults and also in minimizing the damage in the event of faults.

Basic connections of circuit breaker control for the opening operation
Basic connections of circuit breaker control for the opening operation

Figure above shows basic connections of circuit breaker control for the opening operation. The protected circuit X is shown by dashed line. When a fault occurs in the protected circuit the relay connected to CT and PT actuates and closes its contacts.

Current flows from battery in the trip circuit. As the trip coil of circuit breaker is energized, the circuit breaker operating mechanism is actuated and it operates for the opening operation.

Thus the fault is sensed and the trip circuit is actuated by the relay and the faulty part is isolated.

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What is Relay?

A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts.

These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.

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Functions of Protective Relay

These are the main functions of protective relay:

  1. To sound an alarm or to close the trip circuit of a circuit breaker so as to disconnect Faulty Section.
  2. To disconnect the abnormally operating part so as to prevent subsequent faults. For e.g. Overload protection of a machine not only protects the machine but also prevents Insulation failure.
  3. To isolate or disconnect faulted circuits or equipment quickly from the remainder of the system so the system can continue to function and to minimize the damage to the faulty part. For example – If machine is disconnected immediately after a winding fault, only a few coils may need replacement. But if the fault is sustained, the entire winding may get damaged and machine may be beyond repairs.
  4. To localize the effect of fault by disconnecting the faulty part from healthy part, causing   least disturbance to the healthy system.
  5. To disconnect the faulty part quickly so as to improve system stability, service continuity and system performance. Transient stability can be improved by means of improved   protective relaying.
  6. To minimize hazards to personnel.

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Desirable Qualities of Protective Relaying

  1. Selectivity,
  2. Discrimination
  3. Stability
  4. Sensitivity,
  5. Power consumption
  6. System Security
  7. Reliability
  8. Adequateness
  9. Speed & Time

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Terminology of protective relay

Pickup level of actuating signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated. If the value of actuating quantity is increased, the electromagnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.

Reset level: The value of current or voltage below which a relay opens its contacts and comes in original position.

Operating Time of Relay: Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.

Reset time of Relay: The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.

Reach of Relay: A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.

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History of Protective Relay

The evolution of protective relays begins with the electromechanical relays. Over the past decade it upgraded from electromechanical to solid state technologies to predominate use of microprocessors and microcontrollers.

The timeline of the development of protective relays is shown below:

1900 to 19631963 to 19721972 to 19801980 to 1990
Electromechanical RelayStatic RelayDigital RelayNumerical Relay
1925=Single Disc Type Relay (Single Input)1963=Static Relay  (All Purpose)1980=Digital Type Relay (All Purpose)1990=Numerical Type Relay (All Purpose)
1961=Single Cup Type Relay (Impedance Relay)1972=Static Relay with self checking           (All Purpose)

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Types of Relays

Types of protection relays are mainly:

A. Based on Characteristic:

  1. Definite time Relays.
  2. Inverse definite minimum time Relays (IDMT)
  3. Instantaneous Relays
  4. IDMT with Instantaneous.
  5. Stepped Characteristic
  6. Programmed Switches
  7. Voltage restraint over current relay

B. Based on logic:

  1. Differential
  2. Unbalance
  3. Neutral Displacement
  4. Directional
  5. Restricted Earth Fault
  6. Over Fluxing
  7. Distance Schemes
  8. Bus bar Protection
  9. Reverse Power Relays
  10. Loss of excitation
  11. Negative Phase Sequence Relays etc.

C. Based on Actuating parameter:

  1. Current Relays
  2. Voltage Relays
  3. Frequency Relays
  4. Power Relays etc.

D. Based on Operation Mechanism:

1. Electro Magnetic Relay
2. Static Relay
……• Analog Relay
……• Digital Relay
……• Numerical /Microprocessor Relay
3. Mechanical relay

  • Thermal
    • OT Trip (Oil Temperature Trip)
    • WT Trip (Winding Temperature Trip)
    • Bearing Temp Trip etc.
  • Float Type
    • Buchholz
    • OSR
    • PRV
    • Water level Controls etc.
  • Pressure Switches
  • Mechanical Interlocks
  • Pole discrepancy Relay

E. Based on Applications

  1. Primary Relays
  2. Backup Relays

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Types of Relay based on Relay Operation Mechanism

1. Electromagnetic Relay

Electromagnetic relays are further categorized under two following categories.

1.1 Electromagnetic Attraction Relay
This Relay works on Electromagnetic Attraction Principle

1.2 Electromagnetic Induction Relay
This Relay works on Electromagnetic Induction Principle

2. Solid State (Static) Relay

Solid-state (and static) relays are further categorized under following designations:

2.1 Analog Relay
In Analog relays are measured quantities are converted into lower voltage but similar signals, which are then combined or compared directly to reference values in level detectors to produce the desired output.

2.2 Digital Relay
In Digital relays measured ac quantities are manipulated in analogue form and subsequently converted into square-wave (binary) voltages. Logic circuits or microprocessors compare the phase relationships of the square waves to make a trip decision.

2.3 Numerical Relay
In Numerical relays measured ac quantities are sequentially sampled and converted into numeric data form. A microprocessor performs mathematical and/or logical operations on the data to make trip decisions.

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Protective relay testing: Test relays of all generations (VIDEO)

Cant see this video? Click here to watch it on Youtube.

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  • Handbook of Switchgear –Bhel
  • Digital/Numerical Relays -T.S.M. Rao

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More Information

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. richxtlc
    Dec 17, 2012

    I like the article as it is brief and to the point, and if a person wants to know more you have given him/her enough key words to help them find the necessary information. In your first statement that during non fault conditions the relay remains idle, is true for electro-mechanical and static type relays. i would disagree with you that microprocessor based relays are idle. They are constantly analyzing their input to determine if a fault has occurred. These relays can be programmed to perform a variety of functions that cannot be performed by either an electro-mechanical or static type relay. These relays give you in real time, current, voltage, vars, watts, power, phase angle, previous operations and can be a digital fault recorder.

    • Edvard
      Dec 17, 2012

      I think Jignesh wanted to say that microprocessor relay remains idle in terms of no action in case of no fault. Of course, relay is non-stop analysing network parameters, like you stated.

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