Protective relaying is necessary for electrical systems to detect abnormal conditions and isolate faulty parts to protect equipment and maintain system stability. Relays sense faults and trigger circuit breakers to disconnect the faulty part from the system automatically and quickly. The choice of protective devices depends on factors like the equipment type and location. Between generators and loads, various equipment needs protection provided by a coordinated system of relays, circuit breakers, and other components working together.

1. www.systemprotection.in +91-265-2225137
Basics of Protective
Relaying in Power System
ISO 9001:2015
OHSAS 18001:2007
Certified Company

2. Protective relaying is necessary with almost every electrical
plant, and no part of the power system is left unprotected. The
choice of protection depends upon several aspects such as type
and rating of the protected equipment, its importance, location,
probable abnormal conditions, cost, etc.
Between generators and the final load points, there are several
electrical equipment and machines of various ratings. Each needs
certain adequate protection.
The protective relaying senses the abnormal conditions in a part
of the power system and gives an alarm or isolates that part from
the health system.
www.systemprotection.in +91-265-2225137

3. The relays are compact, self-contained devices that respond to an abnormal condition. The relays
distinguish between normal and abnormal conditions. Whenever an abnormal condition develops,
the relays close its contacts. Thereby the trip circuit of the circuit breaker is closed. Current from
the battery supply flows in the trip-coil of the circuit-breaker and the circuit breaker opens and the
faulty part is disconnected from the supply.
The entire process, ‘occurrence of a fault, operation of the relay, the opening of the circuit
breaker, removal of the faulty part from the system’, is automatic and fast. Circuit breakers are
switching devices that can interrupt normal currents and fault currents.
Besides relays and circuit-breakers, there are several other important components in the
protective, relaying scheme, these include protective current transformers and voltage
transformers, protective relays, time-delay relays, auxiliary relays, secondary circuits, trip circuits,
auxiliaries, and accessories, etc. Each component is important. Protective relaying is teamwork of
those components.
H O W P R O T E C T I V E R E L A Y I N G W O R K S ?
ISO 9001:2015
OHSAS 18001:2007
Certified Company

4. To sound an alarm or to close the trip circuit of circuit-breaker so as to disconnect a
component during an abnormal condition in the component, which includes overload, under-
voltage, temperature rise, unbalanced load, reverse power, under-frequency, short circuits, etc.
To disconnect the abnormally operating part so as to prevent the subsequent faults, e.g.over-
load protection of a machine protects the machine and prevents insulation failure.
To disconnect the faulty part quickly so as to minimize the damage to the faulty part, e.g. If a
machine is disconnected immediately after a winding fault, only a few coils may need
replacement. If the fault is sustained, the entire winding may get damaged and the machine
may be beyond repairs.
To localize the effect of fault by disconnecting the faulty part, from the healthy part, causing
the least disturbance to the health system.
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.
F U N C T I O N O F P R O T E C T I V E R E L A Y I N G
ISO 9001:2015
OHSAS 18001:2007
Certified Company

5. CONTACT INFORMATION
301/2/306, Silverline Complex,Opposite BBC
Tower,Sayajigunj,Vadodara - 390 005,
Gujarat, India.
bdm@systemprotection.in
www.systemprotection.in
+91-265-2225137