Protective Relay

-Bhagawan Prasad

Chartered Engineer

amieclub@gmail.com

What Components (Equipment) Do We Protect?

Content

1. Introduction to Protective Relay2. Working Principle of Protective Scheme3. What is Relay? 4. Functions of Protective Relay5. Desirable qualities of protective relaying6. Terminology of protective relay7. History of Protective Relay8. Types of Relays9. Types of Relay based on Relay Operation

Mechanism10. Protective relay testing: Test relays of all

generations

1.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.

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

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

2.Working Principle of Protective Scheme

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

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

4.Functions of Protective Relay

To sound an alarm or to close the trip circuit

To disconnect the abnormally operating part so as to prevent subsequent faults

To isolate or disconnect faulted circuits or equipment

To localize the effect of fault to improve system stability, service continuity and system performance.

To minimize hazards to personnel.

5. Desirable Qualities of Protective Relaying

Selectivity, Discrimination Stability Sensitivity, Power consumption System Security Reliability Adequateness Speed & Time

6.Terminology of protective relay

Pickup level of actuating signal: value of actuating quantity voltage or current

Reset level: relay opens its contacts and comes in original position.

Operating Time of Relay: The time which elapses between the instant

Reset time of Relay: relay contacts returns to its normal position.

Reach of Relay: This impedance or corresponding distance is called reach of the relay.

7.History of Protective Relay

1900 to 1963 1963 to 1972 1972 to 1980 1980 to 1990

Electromechanical Relay

Static Relay Digital Relay Numerical 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)

A. Types of Relays: Based on Characteristic:

Definite time Relays. Inverse definite minimum time

Relays (IDMT) Instantaneous Relays IDMT with Instantaneous.Stepped CharacteristicProgrammed SwitchesVoltage restraint over current relay

B. Based on logic: Differential Unbalance Neutral Displacement Directional Restricted Earth Fault Over Fluxing Distance Schemes Bus bar Protection Reverse Power Relays Loss of excitation Negative Phase Sequence Relays etc.

C. Based on Actuating parameter:

Current RelaysVoltage RelaysFrequency RelaysPower Relays etc.

D. Based on Operation Mechanism: 1. Electro Magnetic Relay

2. Static Relay……• Analog Relay……• Digital Relay……• Numerical /Microprocessor Relay3. Mechanical relay

Thermal• OT Trip (Oil Temperature Trip)• WT Trip (Winding Temperature Trip)• Bearing Temp Trip etc.

Based on Operation Mechanism

Float Type• Buchholz• OSR (oil surge relay)• PRV(Pressure Relief Valve )• Water level Controls etc.

Pressure Switches Mechanical Interlocks Pole discrepancy Relay

E. Based on Applications

Primary RelaysBackup Relays

Electromagnetic Attraction Relay

1.1 Electromagnetic Attraction RelayThis Relay works on Electromagnetic Attraction Principle

Electromagnetic Induction Relay

1.2 Electromagnetic Induction RelayThis Relay works on Electromagnetic Induction Principle

IDMT =inverse definite minimum time

Limitations of Electromagnetic relays

Low speed of operation. Change in characteristics over a period due to

ageing effect. Component failure leading to relay failure. Relay is Bulky Imposes high burden on CT No fault data available except phase indication. Inherent in its design, When an electromechanical Relay is activated,

bounce occurs at the contact site.

2.Solid State (Static) Relay

2.1 Analog Relay: measured quantities are converted into lower voltage but similar signals, which are then combined or compared directly to reference values

2.2 Digital Relay: measured ac quantities are manipulated in analogue form and subsequently converted into square-wave (binary) voltages.

2.3 Numerical Relay: 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.

Standard lead No.Sr. Alphabet Series Purpose Example

1 J Series D.C Incoming J1, J2, etc.

2 K Series Control – Closing, Tripping, etc. K1, K2, K3 etc.

3 L Series Alarms, indications and annunciations L1, L2, L3, etc.

4 M Series Motor Circuit M1, M2, etc.

5 E Series Potential transformer secondary E1, E2, E3, etc.

6 H Series LT A.C Supply H1, H2, H3, etc..

7 A Series C.T secondary for special protection A1, A2, A3, etc.

8 B Series Bus bar protection B1, B2, B3, etc..

9 C Series Protection Circuits C1, C2, C3, etc.

10 D Series Metering Circuit D1, D2, D3, etc

Ferrule on CT/PT Metering CT secondary’s – D11,

D31, D51, D71 etc. Protection CT secondary – C11,

C31, C51, C71 etc. Special Protection CT secondary –

A11, A31, A51, A71 etc. PT secondary – E11, E31, E51, E71

etc.

ANSI(American National Standards Institute), numbering

EXAMPLE - Lockout Relay 87 - Differential Protective Relay 88 - Auxiliary Motor or Motor Generator 89 - Line Switch 90 - Regulating Device 91 - Voltage Directional Relay 92 - Voltage and Power Directional Relay 93 - Field Changing Contactor

Company AREVA (ALSTHOM) ABB SIEMENS L&T

CT COPNNECTION STAR -

STAR

CT CONNECTION DELTA-

STAR

CT CONNECTION CT WITH 3 NOS.

O/L

2 O/C +1 E/F OVER

CURRENT AND EARTH FAULT COMBINED

Core balance CT

THANK YOU