EE2402 – PROTECTION AND SWITCHGEAR
IAE-2 Answer key
1. What are the types of graded used in line of radial relay feeder?
Definite time relay and inverse-definite time relay.
2. Define CPMC.
It is combined protection, monitoring and control system incorporated in the static system.
3. Describe about programmable relay.
A static relay may have one or more programmable units such as microprocessors or microcomputers in
its circuit.
4. Why bus bar protection is needed?
a) Fault level at bus bar is high
b) The stability of the system is affected by the faults in the bus zone.
c) A fault in the bus bar causes interruption of supply to a large portion of the system network.
5. Mention any two disadvantage of carrier current scheme for transmission line only.
 The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile);

The response time of band pass filter;
 Capacitance phase-shift of the transmission line
6. State static relay.
It is a relay in which measurement or comparison of electrical quantities is made in a static network which
is designed to give an output signal when a threshold condition is passed which operates a tripping device.
7. Classify the causes of bus zone faults.
 Failure of support insulator resulting in earth fault
 Flashover across support insulator during over voltage
 Heavily polluted insulator causing flashover
 Earthquake, mechanical damage etc.
8. a. Enumerate the relaying schemes which are employed for the protection of a modern alternator.
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b. What type of a protective scheme is employed for the protection of the field winding of the alternator
against ground faults?
A coupling transformer is connected between the earth and the earthing resistance R.
explain each elements.
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9. a. Illustrate type of pilot protection is used for EHV and UHV transmission lines.
Translay Scheme:
The translay relay is another type of differential relay. The arrangement is similar to overcurrent
relay but the secondary winding is not closed on itself. Additionally copper ring or copper shading bands
are provided on the central limb as shown in the Fig. 1.
Fig. 1 Translay relay
These type of relays are used in the feeder protection and the scheme is called Translay scheme. In
this scheme, two such relays are employed at the two ends of feeder as shown in the Fig. 3.
Fig. 3 Translay scheme of feeder protection
The secondaries of the two relays are connected to each other using pilot wires. The connection is
such that the voltages induced in the two secondaries oppose each other. The copper coils are used to
compensate the effect of pilot wire capacitance currents and unbalance between two currents
transformers.
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Under normal operating conditions, the current at the two ends of the feeder is same. The primaries
of the two relays carry the same currents inducing the same voltage in the secondaries. As these two
voltages are in opposition, no current flows through the two secondaries circuits and no torque is exerted
on the discs of both the relays.
When the fault occurs, the currents at the two ends of the feeder are different. Hence unequal
voltages are induced in the secondaries. Hence the circulating current flows in the secondary circuit
causing torque to be exerted on the disc of each relay. But as the secondaries are in opposition, hence
torque in one relay operates so as to close the trip circuit while in other relay the torque just holds the
movement in unoperated position. The care is taken that at least one relay operates under the fault
condition.
Role of copper ring : Mainly relays may operate because of unbalance in the current transformers.
The copper rings are so adjusted that the torque due to current induced in the copper ring due to primary
winding of relay is restraining and do not allow the disc to rotate. It is adjusted just to neutralise the effect
of unbalance between current transformers. The copper rings also neutralise the effect of pilot capacitive
currents. Though the feeder current is same at two ends, the pilot capacitive currents may allow in the
pilots. This current leads the secondary voltage by 90o. The copper rings are adjusted such that no
torque is exerted on the disc, due to such capacitive pilot currents, by adjusting the angle between the
induced current in the disc and secondary current to be 90o.
The advantages of this scheme are,
1. Only two pilot wires are required.
2. The cost is very low.
3. The current transformers with normal design can be employed.
4. The capacitive effects of pilot wire currents do not affect the operation of the relays.
b. (i) Discuss the permissive under-reach transfer tripping scheme of protection.
(ii) Explain microprocessor based inverse time over current relay.
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A proposed scheme of over current relays dealing with programming of the single-board
microcomputer TK-85, built by NEC around Intel's 8-bit 8085 microprocessor, is described.
Experimental results with better relay characteristic performances and high flexibility for
implementing the desired value of n, including noninteger values, are obtained.
Explain each block.
10. a. Derive a generalized mathematical model of distance relays for digital protection
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b. Describe the construction and principle of operation of an induction type directional over current
relay.
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It is essentially directional power relay which operates when power flows in a specific direction.
explain about each element.
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