Types of Protective Relays and Design
Requirements. PART
2(C) This is the third part of topic 2 in the series of
“Types of Relays.” In this part, we will be discussing: DISTANCE OR IMPEDANCE RELAY and
REVERSE POWER FLOW RELAYS. So, the first one is, DISTANCE PROTECTION IMPEDENCE RELAYS:
Distance protection relays are those in which the operation is governed by the ratio of applied voltage to current and therefore to impedance
or a component of impedance in the protected circuit. According to the IEEE Buff Book, Distance
relay may have the following features: It has the ability to detect a fault within a pre-set distance along a transmission line from its
location. A distance relay therefore looks at current and voltage and compares these two quantities on
the basis of Ohm’s law. Moving on to the working MECHANISM of these relays:
Here we consider that the impedance relay is placed on the transmission line for the protection of the line AB. Z is the impedance of
the line in normal operating condition. If the impedances of the line falls below the normal impedance set point: Z then the relay starts
working. Let’s, suppose a fault F1 occurs in the line AB. This fault sensed by the relay has lower
impedance compared to the line impedance and therefore, falls below the relay set point. The fault impedance is Zf. The relay starts
operating, and it send the tripping command to the circuit breaker. If the fault reaches beyond the protective zone, the contacts of the relay
remain unclosed. The voltage and the current operating elements are the two important components of the
impedance relay. The deflecting torque is produced by secondary current of CT and restoring torque is produced by voltage of
potential transformer. In normal operating condition, restoring torque is greater than deflecting torque. Hence, relay will not operate.
But in faulty condition, the current becomes quite large whereas voltage becomes less. Consequently, deflecting torque becomes more
than restoring torque and dynamic parts of the relay starts moving which ultimately close the normally open contact of relay.
Now, Let’s have a look on their applications: Distance Relays are widely used for the protection of Transmission Lines by measuring
the impedance between the fault location and the protection relay. These relays provide the primary or backup
protection for AC Transmission lines and distribution lines against three phase faults, phase to phase and phase to ground faults.
The next type of relays are the Reverse Power flow Relays. Acc to IEEE Buff Book, the reverse power flow
relay is a sensitive high-speed and directional protective relay that can be used to detect line to ground faults on the delta side of the
transformer bank by detecting the in-phase component of the transformer magnetizing current and prevents power from flowing in the
reverse direction. This occurs when another relay in the system trips the transformer’s primary breaker and the transformer is
energized through its secondary circuit. In applying this relay, the relay operation should be delayed to prevent undesired operations
resulting from generator swings relative to the utility. The relay only needs to be fast enough to permit successful re-closure from the remote
end. Moving on to their working mechanism:
The relay monitors the power from the generator and in case the generator output falls below a preset value, it quickly disconnects the
generator to avoid power from flowing into the stator coil. The generator output can fail due to problems with the prime mover, turbine or
engine that drives the generator or issues with speed controller.
When the prime mover fails, the generator stops producing power and may instead start drawing power from the other parallel sources
and start motoring. The reverse power relay senses any reverse direction of power flow and disconnects the generator to avoid any possible
damage. Looking at the applications:
Reverse power flow relays may be used where secondary generators of a facility are operating in synch with a larger utility grid, such that if the
utility supply fails, the reverse power flow relay prevents the power from the generators from flowing into the grid by quickly disconnecting
the point of common coupling. The relay is used in facilities where a generator runs in parallel with the utility or another
generator so as to prevent power from the bus bar or the other active generator from flowing towards the inactive generator when its output
fails. That’s all form this part
Now in the next video we will be discussing the next two different types of Relays that are the Differential relays and Time Delay Relays used
in Electrical systems and their applications as well We hope that you have a continued interest in
this topic and in this series as either a student or a professional. And we also hope that you find this content useful and enlightening, please
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