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Protective Relaying Fundamental

Topic objective to know
1. What is protective relaying
2. What is its purpose
3. How does protective relaying works.
4.To know the basic parts of a relay and its function
5.To know what are the types of relays.
6.What is and instrument trasformer.
7. To know the different types of Instrument transformer.

- is a collective term which defines all equipment necessary for:
- Initiating
- Recognizing
- Locating
- the removal of a fault or abnormal condition from the power system.
- is a system employed in a power circuit basically used for:
Public safety
*Service reliability
*Equipment protection
a.Power lines

Protect from what?
b.Short Circuit of Fault
c.Abnormal Phenomenon

What is a RELAY?
- A device designed With precise measuring instruments
To interpret input conditions in a prescribed manner and specified conditions
To operates when the quantities flowing through them exceed or go below some predetermined value

- Equipped with auxiliary contacts and are made to respond by closing the contacts.




1.Electromechanical relay

2.Solid-state / static relay

3.Digital / Numeric relay

-A device used to close or open a power circuit which is live and loaded.
-Also used to interrupt a power circuit under fault condition.
-it serves as the muscle in the protection system which does the work of isolating the faulty component, while the relay is the brain, which gives order to the breaker.

Circuit breakers are strategically placed in a power system so that each generator, transformer, bus and transmission line etc., can be completely disconnected from the rest of the system

- Are usually step down transformers used to keep values of voltage or current to levels within the capabilities of relays.

- They provide insulation against the high voltage of the power circuit, and supply the relays with accurate quantities proportional to that of the power circuit, but sufficiently reduced in magnitude so that the relays can be made relatively small and inexpensive.

- Allow relays and meters to measure high values of current and voltage accurately.

- In protective relaying, they give the relay the necessary input, may it be current or voltage, for the relay to evaluate and then decide to what action to be taken.


-Standard voltage transformers are single-phase units designed and constructed so that the secondary voltage maintains a fixed relationship with primary voltage.
-The primary are made to suit high voltage, while the secondary are usually 120 volts

-A device used to transform line current into values suitable for standard instruments and protective relays.
-This device serves two purposes:
1.Insulates the relay from primary voltage.
2.Allows a 0-5 amp output to be used for relays.

How do protective relaying operate?
- Basically protective relaying has three major components namely:

Is the sensor of the system
It usually detects either current, voltage or frequency
It feed the information to the relay, which is the brain of the system

is the brain of the system
it receives input signal from the instrument transformer
it then evaluate the input condition and then decide what to do.
it gives advice to the breaker to trip, if the relay decides to do so.

is the muscle of the system
is always ready to implement whatever the relay want him to do.

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