Question 1

Q.1. What are aliasing issues?

Accepted Answer

Aliasing is the distortion that occurs when a continuous analog signal is sampled at a rate lower than twice its highest frequency component (below the Nyquist rate), causing high-frequency components to be misrepresented as false lower-frequency components in the digitized signal.

Question 2

Q.2. What do you mean by CT saturation?

Accepted Answer

CT saturation occurs when a current transformer's magnetic core reaches its maximum flux density and can no longer accurately reproduce the primary current waveform in the secondary, causing severe distortion and reduced magnitude of the secondary current, particularly during high fault currents with a DC offset component.

Question 3

Q.3. What is frame leakage protection of Bus Bar?

Accepted Answer

Frame leakage protection of a bus bar detects faults by insulating the switchgear frame/enclosure from ground (except at a single point through a current-sensing relay), so that any fault current flowing to ground through the frame is detected by this relay, providing sensitive protection independent of the number of bus sections or connected circuits.

Question 4

Q.4. What are the applications of WAMS for improving protection systems?

Accepted Answer

Wide Area Measurement Systems (WAMS), using synchrophasor data from Phasor Measurement Units, improve protection systems through wide-area backup protection, adaptive relay setting adjustment, out-of-step/power-swing detection and blocking, and post-disturbance event analysis across the interconnected grid.

Question 5

Q.5. Explain the phenomenon of inrush current.

Accepted Answer

Inrush current is the large, transient magnetizing current drawn by a transformer when it is first energized, caused by core flux offset and possible saturation, which can be several times the transformer's rated current and rich in second-harmonic content.

Question 6

Q.6. What are the advantages of microprocessor based relay over electromechan…

Accepted Answer

Microprocessor-based relays offer advantages including higher accuracy and flexibility, multi-function capability within a single device, self-monitoring and diagnostic capability, event recording and communication capability, and adaptive settings, compared to bulkier, single-function, non-communicating electromechanical relays.

Question 7

Q.7. Define the term pick-up value in a protective relay.

Accepted Answer

The pick-up value of a protective relay is the minimum value of the actuating quantity (current, voltage, or other measured parameter) at which the relay just begins to operate, transitioning from its non-operated to operated state.

Question 8

Q.8. Distinguish between unit and non unit protection.

Accepted Answer

Unit protection provides absolute selectivity by comparing quantities only at the two ends of a clearly-defined protected zone (such as differential protection), operating only for faults within that zone, while non-unit protection (such as overcurrent or distance relaying) responds to conditions measured at a single location without inherent zone boundaries, relying on time or impedance grading for selectivity.

Question 9

Q.9. Distinguish between symmetrical and unsymmetrical faults.

Accepted Answer

Symmetrical faults (three-phase faults) affect all three phases equally, maintaining balanced conditions and requiring only positive-sequence network analysis, while unsymmetrical faults (line-to-ground, line-to-line, double line-to-ground) affect the phases unequally, creating unbalance and requiring symmetrical component analysis using positive, negative, and zero sequence networks.

Question 10

Q.10. What are the essential qualities of protective relay?

Accepted Answer

The essential qualities of a protective relay are selectivity, speed, sensitivity, reliability, and stability, ensuring the relay correctly discriminates faulted from healthy equipment, operates quickly enough to limit damage, detects faults reliably even at low magnitude, and remains stable (non-operating) during normal or through-fault conditions.

Question 11

Q.1. What do you understand by primary and back-up protection? What is the ro…

Accepted Answer

Primary protection is the first line of defense specifically assigned to clear a fault within its own designated zone, while back-up protection provides a second, independent means of fault clearance if primary protection fails to operate, serving the critical role of preventing a single relay/breaker failure from causing extended, undetected fault persistence; back-up protection is provided through relay back-up (a second relay at the same or remote location), breaker back-up (a separate breaker-failure protection scheme), or remote back-up (upstream relays at a different substation).

Question 12

Q.2. Explain different types of over-current relay co-ordination in detail.

Accepted Answer

Over-current relay coordination is achieved through current grading (using different pick-up settings for relays at different network locations), time grading (using progressively longer time delays for relays closer to the source), and a combination of both (using inverse-time characteristics), ensuring that the relay closest to a fault operates first while relays further upstream provide graded back-up.

Question 13

Q.3. Explain stepped time-distance characteristics of three distance characte…

Accepted Answer

Distance relaying uses stepped time-distance characteristics with three (or more) zones of protection - Zone I set to reach approximately 80-90% of the protected line's impedance with instantaneous (no intentional delay) operation, Zone II set to reach beyond the remote bus with a short time delay for back-up, and Zone III set to reach further into adjacent lines with a longer time delay for remote back-up - ensuring fast primary clearance for faults within the line while safely coordinating with adjacent line protection.

Question 14

Q.4. How transients are simulated using electro-magnetic transients (EMT) pro…

Accepted Answer

Electromagnetic Transients (EMT) programs simulate fast transient phenomena in power systems by numerically solving the differential equations governing the network's inductances and capacitances in the time domain, most commonly using the trapezoidal-rule-based companion-circuit (Dommel/EMTP-type) method, discretizing each network element into an equivalent resistive circuit with a history-dependent current source updated at each small time step.

Question 15

Q.5. Explain the percentage differential relay with harmonic restraint in tra…

Accepted Answer

Percentage differential relaying for transformer protection compares the currents entering and leaving the transformer (referred through appropriate CT ratios and vector-group/phase-shift correction), operating when the differential (unbalance) current exceeds a percentage of the average restraining (through) current, with harmonic restraint added specifically to block operation during transformer energization inrush, which is rich in second-harmonic content that a genuine internal fault current does not contain.

Question 16

Q.6. Explain the effectiveness of different fault detection algorithms used i…

Accepted Answer

Fault detection algorithms used in computer-aided protection include sample-comparison (waveform) algorithms, Fourier-transform-based phasor estimation algorithms, and least-squares/Kalman-filter-based estimation algorithms, differing in their trade-off between computational speed, accuracy under transient/harmonic-distorted conditions, and immunity to noise and CT/VT errors.

Question 17

Q.7. For a 132kV system, the reactance and capacitance up to the location of …

Accepted Answer

For the given 132kV system (X=6 ohm, C=0.030 microF), the calculated results are: natural frequency of oscillation approximately 6649 Hz (6.65 kHz), maximum restriking voltage approximately 215.56 kV, and maximum rate of rise of restriking voltage (RRRV) approximately 2.87 kV per microsecond.

Question 18

Q.1. Explain by help of neat diagram working of Buchholz relay.

Accepted Answer

The Buchholz relay is a gas-actuated protective device mounted in the pipe connecting a transformer's main tank to its conservator, using two hinged floats with mercury switches (or, in modern designs, reed switches) that respond to gas accumulation from a minor internal fault (raising an alarm) and to the rapid oil surge caused by a major internal fault (initiating tripping), providing sensitive, early-warning protection for incipient transformer faults that other protection schemes cannot detect.

Question 19

Q.2. Explain with suitable diagram, how Definite Time Over-Current (DTOC) rel…

Accepted Answer

Definite Time Over-Current (DTOC) relays protect a radial distribution feeder by using a fixed (definite) time delay setting for each relay along the feeder, with the time delay progressively increased for relays located closer to the source, ensuring the relay nearest a fault clears it first while relays further upstream provide correctly-graded time-delayed back-up, at the practical cost of longer fault-clearing times for faults nearer the source where fault current (and hence severity) is actually greatest.

Question 20

Q.3. Discuss the effect of power swing on distance relays.

Accepted Answer

A power swing (a slow, large-amplitude oscillation in power flow and the corresponding apparent impedance seen by a distance relay, arising from a system stability disturbance rather than an actual fault) can cause the apparent impedance trajectory to enter a distance relay's tripping zones, risking unwanted tripping during a stable swing and thereby potentially precipitating an actual system separation or instability; power-swing blocking schemes distinguish a swing (characterized by a slow rate of change of impedance) from a genuine fault (characterized by an abrupt, near-instantaneous impedance change) to prevent this unwanted operation while still permitting tripping for a genuine fault occurring during a swing (out-of-step tripping logic).

Question 21

Q.4. Evaluate the challenges associated with implementing digital relay prote…

Accepted Answer

Implementing digital relay protection for transformers and generators faces challenges including cybersecurity vulnerability, dependence on precise time synchronization, complex configuration/testing requirements, electromagnetic interference susceptibility, and software/firmware obsolescence management, though digital relays offer substantial merits (multi-function integration, self-monitoring, event recording, communication capability, adaptive settings) over traditional electromechanical relays, at the cost of certain demerits (cybersecurity exposure, greater configuration complexity, and dependence on auxiliary DC/communication infrastructure).

Question 22

Q.5. Write a short note on the following:

Accepted Answer

Synchrophasors are time-synchronized, GPS-referenced phasor measurements of voltage/current taken simultaneously across widely-separated locations by Phasor Measurement Units, enabling real-time wide-area system visibility; a bus bar arrangement scheme (single bus, double bus, main-and-transfer bus, or ring/breaker-and-a-half arrangements) defines how generators, transformers, and feeders are connected to a substation's bus bars, balancing reliability, flexibility for maintenance, and cost.

Power System Protection