(SEM VII) THEORY EXAMINATION 2024-25 POWER QUALITY AND FACTS

KEE074 – POWER QUALITY AND FACTS

Complete Solved Question Paper

SECTION A

(Attempt all questions – 2 × 10 = 20 marks)

Q1(a) Long-duration voltage variations

Long-duration voltage variations are sustained deviations of RMS voltage from the nominal value lasting more than 1 minute, such as overvoltage and undervoltage.

Q1(b) Voltage fluctuation

Voltage fluctuation is a series of rapid voltage changes that cause visible light flicker in lamps.

Q1(c) Common sources of voltage sag

Motor starting                                                           Short-circuit faults

Transformer energization                                          Arc furnaces

Q1(d) Voltage sag mitigation (end-user level)

Uninterruptible Power Supply (UPS)                         Dynamic Voltage Restorer (DVR)

Q1(e) Atmospheric transients

Atmospheric transients are high-magnitude, short-duration overvoltages caused mainly by lightning strokes.

Q1(f) Overvoltage protection device

Surge arrester

Q1(g) FACT full form

FACTS stands for Flexible AC Transmission Systems.

Q1(h) Types of FACT devices

Static VAR Compensator (SVC)                               Thyristor Controlled Series Capacitor (TCSC)

Q1(i) Effects of harmonics on transformers

Increased losses                                                       Overheating

Reduced efficiency                                                   Noise and vibration

Q1(j) Harmonic mitigation technique

Passive filter

SECTION B

(Attempt any three – 10 × 3 = 30 marks)

Q2(a) Power quality terms

Transients: Sudden, short-duration voltage changes

Long-duration variations: Overvoltage/undervoltage

Short-duration variations: Voltage sag, swell, interruption

Waveform distortion: Harmonics, notching, noise

These affect system reliability and equipment performance.

Q2(b) Sources of voltage sag

Motor starting: High inrush current              Arc furnaces: Irregular current draw

Fault clearing: Temporary voltage drop        Impact: Malfunction of sensitive loads like PLCs and drives.

Q2(c) Sources of transient overvoltages

Atmospheric transients: Lightning

Switching transients: Circuit breaker operation, capacitor switching

Cause insulation stress and equipment damage.

Q2(d) FACTS definition & controllers

FACTS are power electronic-based controllers that improve power flow control and stability.

Controllers:                                                    SVC

STATCOM                                                        TCSC

SSC

Q2(e) Harmonics causes & measurement

Causes:

Power electronic devices                                 Non-linear loads

Current harmonics: Generated by load         Voltage harmonics: Caused by system impedance

Measurement: Harmonic analyzers, FFT-based meters

SECTION C

Q3(a) Impact of voltage fluctuations & waveform distortion

Flicker in lighting                                            Overheating of motors

Malfunction of electronic equipment             Reduced system stability

Q3(b) Power frequency variations

Causes:                                                          Load-generation imbalance

System disturbances

Effects:                                                          Speed variation in motors

Malfunction of sensitive equipment              Industrial process instability

Q4(a) Voltage sag performance & protection

Estimation methods:                                   Sag magnitude-duration analysis

Event monitoring

Protection principles:                                  Voltage compensation

Ride-through capability                                 Tools: Power quality analyzers, sag indices

Q4(b) Voltage sag mitigation solutions

Isolation transformers                                    Voltage regulators

Static & rotary UPS                                        Active series compensators (DVR)

Q5(a) Neutral voltage swings & UPS transients

Effects:                                                          Equipment malfunction

Data loss                                                        Reduced system stability

Mitigation:                                                   Proper grounding

Isolation transformers                                   Improved UPS design

Q5(b) Overvoltage protection devices

Surge arresters                                              Metal oxide varistors (MOV)

Spark gaps                                                    Used to divert excess voltage safely to ground.

Q6(a) FACT devices & applications

SSC: Voltage control                                    SVC: Reactive power compensation

TCSC: Power flow control and stability improvement

Q6(b) Power flow control using FACTS

Power flow in a transmission line:                P=V1V2Xsin⁡δP = \frac{V_1 V_2}{X} \sin \deltaP=XV1​V2​​sinδ

FACTS devices control X, V, and δ, thereby controlling power flow.

Q7(a) Effects of harmonics

Transformers: Heating, losses                    Motors: Torque pulsations

Capacitors: Resonance                                Cables: Overheating

Communication lines: Interference

Q7(b) Harmonic mitigation techniques

Passive:                                                       LC filters                              Tuned filters

Active:                                                         Active power filters             Hybrid filters

Used in modern industrial power systems.