Types of Protective Relays and Design
Requirements. PART
2(D) This is the fourth part of Topic 2 in the series
of “Types of Relays.” DIFFERENTIAL RELAYS and TIME DELAY RELAYS
(Now, the fifth type of relay we would be discussing here in this part are the differential relays)
A differential relay is one that operates when the phasor difference of two or more similar electrical quantities exceed a pre-determined
value. And to explain this we would move further the relay would operate a protective device or a CB for example, when we say
phasor, we are talking about the magnitude difference or the angle difference or both magnitude and angle. So, a phasor difference
of two or more similar electrical quantities that exceed a predetermined value, a set point or some settings of those two phasor quantities
could be current, they could be voltage, they could be angle or so forth. Now moving on to the mechanism,
MECHANISM: Thus, a current differential relay is one that compares the current entering a section of the
system with the current leaving the section. Under normal operating conditions, the two currents
are equal but as soon as a fault occurs, this condition no longer applies. The difference between the
incoming and outgoing currents is arranged to flow through the operating coil of the relay. If this
differential current is equal to or greater than the pickup value, the relay will operate and open the circuit
breaker to isolate the faulty section. Some of the applications for a current
differential relay is to: Use to protect large power transformers, for the protection of generators or for the protection of
motors against faults in short circuits. Another application is the Protection of feeder (transmission line) by pilot wire differential
protection. Or another would communication assisted differential protection. Another application is Bus-bar protection or bus
differential protection. *Now the next type of relays are Time delay Relays*
6)TIME DELAY RELAYS: Time delay relay circuit contains an electromechanical relay and driver circuit, this
circuit decides the time delay to give power supply to the electromechanical relay coil can be energized by the relay.
In a working mechanism there is, a resistor as shown… The resistor R1, a potentiometer and R2 are
connected in series with a 12V DC input supply, the other end of the potentiometer is connected to the capacitor and a Zener diode that is
reverse biased. We can use the circuit to turn on or turn off some sensitive time delay required electrical applications.
Moving on to the applications of time delay relays, Some industrial control systems need timing
operation. Time delay relays are used to provide the desired time delay and timing function.
*That’s all from today’s part. In the next part, part 2e we will continue our discussion on the last two remaining types of Relays*
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