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
10. Protective relay testing: Test relays of all
1.introduction to Protective relay
Protectiverelay works in the way of sensing
and control devices to accomplish its
function. Under normal power system
operation, a protective relay remains idleand
serves no active function.
whe n fault o r unde sirable co nditio n arrive s
Pro te ctive Re lay m ust be o pe rate d and
functio n co rre ctly.
Failure o f a pro te ctive re lay can re sult in
de vastating e q uipm e nt dam ag e and
pro lo ng e d do wntim e .
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 se nse d and
the trip circuit is actuate d by
the re lay and the faulty part is
iso late d.
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 isolateor disconnect faulted circuits or
To localizetheeffect of fault to improve
system stability, service continuity and
To minimize hazards to personnel.
5. Desirable Qualities of Protective
Speed & Time
6.Terminology of protective
Pickuplevel of actuatingsignal: value of actuating
quantity voltage or current
Reset level: relay opens its contacts and comes in
OperatingTimeof Relay: The time which elapses
between the instant
Reset timeof Relay: relay contacts returns to its
Reachof 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
Static Relay Digital Relay Numerical Relay
Type Relay (Single
Relay (All Purpose)
Type Relay (All
with self checking
A. Types of Relays: Based on
Definite time Relays.
Inverse definite minimum time Relays
IDMT with Instantaneous.
Voltage restraint over current relay
B. Based on logic:
Restricted Earth Fault
Bus bar Protection
Reverse Power Relays
Loss of excitation
Negative Phase Sequence Relays etc.
C. Based on Actuating parameter:
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
• OT Trip (O ilTe m pe rature Trip )
• WT Trip (Winding Te m pe rature Trip )
• Bearing Temp Trip etc.
Based on Operation Mechanism
• OSR (oil surge relay)
• PRV(Pressure Relief
• Water level Controls etc.
Pole discrepancy Relay
E. Based on Applications
This Relay works
This Relay works
Limitations of Electromagnetic relays
Low speedof operation.
Change in characteristics over a period due to
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
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
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
ANSI(American National Standards Institute),
- 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