High Voltage Engineering MCQs 2026

Bundled conductors on EHV lines reduce corona by

1. A increasing surface gradient ✓
2. B increasing effective radius and lowering surface voltage gradient ✓
3. C removing all electric field ✓
4. D using only DC without any AC ✓

Corona produces

1. A only useful heating of load ✓
2. B only negative resistance in cables ✓
3. C only improved power factor always ✓
4. D ozone, audible noise, light emission and power loss ✓

Radio interference (RI) from corona on HV lines is mitigated by

1. A increasing sharp edges on conductors ✓
2. B removing earth wire ✓
3. C smooth conductor surfaces, bundled conductors and optimized line design ✓
4. D operating at DC zero only ✓

Positive and negative corona differ in that

1. A positive corona is often more continuous while negative may be patchy (Trichel pulses) ✓
2. B they are identical in all conditions ✓
3. C only negative exists in AC ✓
4. D only positive exists in vacuum ✓

Corona ring (grading ring) on hardware reduces corona at

1. A transformer core laminations only ✓
2. B conductor fittings, clamps and sharp edges ✓
3. C motor rotor bars only ✓
4. D battery terminals only ✓

Fair weather corona loss estimation uses

1. A only transformer OC test data ✓
2. B only cable Murray loop ✓
3. C only PID tuning constants ✓
4. D empirical formulas based on conductor size, voltage and weather ✓

Under foul weather, corona and flashover risk

1. A decreases to zero always ✓
2. B is unrelated to pollution ✓
3. C increases due to water films and contamination on insulators ✓
4. D eliminates need for creepage ✓

DC corona on HVDC lines differs from AC because

1. A DC has no electric field ✓
2. B space charge accumulates with one polarity affecting field distribution ✓
3. C AC has no corona ever ✓
4. D DC eliminates ionization ✓

Corona inception voltage can be raised by

1. A polishing conductor surface and increasing conductor diameter ✓
2. B adding sharp points to conductors ✓
3. C reducing clearance to ground ✓
4. D using thinner single small conductor at same voltage ✓

Insulation coordination ensures

1. A equipment insulation levels are matched to expected overvoltages and protective devices ✓
2. B zero need for surge arresters ✓
3. C identical BIL for all equipment regardless of role ✓
4. D only mechanical strength of towers ✓

Basic Insulation Level (BIL) of equipment refers to

1. A continuous RMS voltage only ✓
2. B short-circuit MVA only ✓
3. C specified impulse withstand voltage capability ✓
4. D no-load current only ✓

Metal-oxide surge arrester (MOA) protects equipment by

1. A increasing fault current intentionally ✓
2. B opening series breakers only ✓
3. C clamping overvoltages and diverting surge current to earth ✓
4. D raising BIL of transformers permanently ✓

Protective margin in insulation coordination is

1. A always zero by definition ✓
2. B difference between equipment withstand and protective level of arrester ✓
3. C equal to line length only ✓
4. D same as power factor ✓

Switching surge is significant for insulation coordination on

1. A only 11 kV distribution ✓
2. B EHV and UHV systems above about 300 kV ✓
3. C only DC control circuits ✓
4. D only motor starter panels ✓

CFO (critical flashover) voltage of an insulator string is used in

1. A statistical insulation coordination studies ✓
2. B only DC resistance measurement ✓
3. C only motor slip calculation ✓
4. D only cable ampacity tables ✓

Temporary overvoltage (TOV) following fault or load rejection can

1. A stress arrester and insulation if not coordinated ✓
2. B never occur on transmission lines ✓
3. C only affect LV fuses ✓
4. D eliminate Ferranti rise ✓

Shielding angle and earth wire design on transmission lines improve

1. A only corona loss without shielding effect ✓
2. B only cable charging current ✓
3. C lightning protection by reducing strikes to phase conductors ✓
4. D only transformer impedance ✓

Standard lightning impulse test wave shape is commonly

1. A DC ramp only ✓
2. B 50 Hz sine only ✓
3. C square wave of 1 ms only ✓
4. D 1.2/50 μs voltage wave ✓

Pollution level of an installation site affects

1. A required creepage distance of outdoor insulators ✓
2. B only conductor ampacity ✓
3. C only rotor resistance ✓
4. D only battery capacity ✓

Coordination between arrester rating and transformer insulation considers

1. A maximum continuous operating voltage and temporary overvoltages ✓
2. B only no-load loss ✓
3. C only motor starting current ✓
4. D only cable burial depth ✓

Silhouette factor and specific creepage distance (SCD) are used to select

1. A only underground cable size ✓
2. B insulator profiles for polluted environments ✓
3. C only VFD carrier frequency ✓
4. D only train pantograph pressure ✓

Breakdown strength of air at atmospheric pressure in uniform field is approximately

1. A 30 kV/cm order of magnitude ✓
2. B 1 V/cm ✓
3. C 1000 kV/cm always ✓
4. D independent of gap ✓

Capacitance voltage divider (CVD) is used for

1. A measuring high voltages by ratio of capacitors ✓
2. B starting synchronous motors only ✓
3. C biogas methane measurement only ✓
4. D yaw control of wind turbines only ✓

Vacuum breakdown at very small gaps can be worsened by

1. A only high humidity of air ✓
2. B only oil viscosity ✓
3. C only cable capacitance ✓
4. D microparticles and field emission at electrode surfaces ✓

Thermal breakdown in solid cable insulation occurs when

1. A dielectric losses cause heating exceeding heat removal capability ✓
2. B only at zero voltage ✓
3. C only during DC corona ✓
4. D only in vacuum gaps ✓

Electrode conditioning and humidity can

1. A never affect results ✓
2. B fix Paschen minimum to zero ✓
3. C significantly alter measured breakdown voltage in laboratory gaps ✓
4. D eliminate need for standards ✓

Corona discharge in HV systems is

1. A full insulation puncture through solid dielectric ✓
2. B partial ionization of air near conductors where local field exceeds ionization threshold ✓
3. C only DC motor commutation sparking ✓
4. D only transformer copper loss ✓

Corona onset on a smooth conductor in air is related to

1. A surface voltage gradient exceeding corona inception threshold ✓
2. B only conductor length in km ✓
3. C only power factor of load ✓
4. D only frequency below 1 Hz only ✓

Visual corona on HV lines is more noticeable

1. A only in dry desert at noon exclusively ✓
2. B only inside oil-filled transformers visibly ✓
3. C during wet or foggy conditions and at night ✓
4. D only at zero voltage ✓

Corona power loss on transmission lines

1. A is always zero on EHV lines ✓
2. B increases with voltage above corona inception and with conductor surface conditions ✓
3. C decreases as voltage rises without limit ✓
4. D depends only on tower height ✓

Vacuum interrupter in HV breakers extinguishes arc by

1. A oil blast only ✓
2. B air at atmospheric pressure only ✓
3. C water jet only ✓
4. D rapid reduction of plasma in high vacuum with metal vapour condensation ✓

High-voltage AC test transformers are used to

1. A only measure earth resistance ✓
2. B only start induction motors ✓
3. C produce test voltages for dielectric withstand and partial discharge tests ✓
4. D only regulate turbine governors ✓

Partial discharge (PD) measurement detects

1. A only copper losses in windings ✓
2. B only rotor speed deviation ✓
3. C localized dielectric stress that may precede insulation failure ✓
4. D only solar irradiance ✓

Tan delta (dielectric loss angle) test on insulation indicates

1. A only conductor cross-section ✓
2. B dielectric quality and moisture or ageing ✓
3. C only gearbox ratio ✓
4. D only wind cut-in speed ✓

Impulse generator Marx circuit produces

1. A only DC ripple free voltage ✓
2. B high lightning or switching impulse voltages for testing ✓
3. C only 50 Hz unregulated supply ✓
4. D only biogas pressure ✓

Sphere gap is used in HV laboratories to

1. A measure only current transformer ratio ✓
2. B calibrate only watt-hour meters ✓
3. C measure peak voltage by calibrated breakdown spacing ✓
4. D test only soil pH ✓

SF6 gas in HV switchgear and GIS provides

1. A lower dielectric strength than air at same pressure ✓
2. B only lubrication of contacts ✓
3. C only cooling of solar panels ✓
4. D high dielectric strength and arc quenching capability ✓

Townsend breakdown mechanism in gases describes

1. A avalanche multiplication of electrons leading to ionization breakdown ✓
2. B only liquid dielectric polarization ✓
3. C only solid lattice vibration ✓
4. D only magnetic saturation ✓

Paschen law relates breakdown voltage in uniform field gaps to

1. A only temperature of conductor ✓
2. B only humidity of oil alone ✓
3. C only cable length in km ✓
4. D product of pressure and gap distance (pd) ✓

Streamer breakdown in long gaps at high pd is characterized by

1. A purely uniform Townsend only at all gaps ✓
2. B no ionization whatsoever ✓
3. C breakdown independent of electrode geometry ✓
4. D leader/streamer development bridging the gap rapidly ✓

Earthing of HV test areas and labs is critical to

1. A increase corona on purpose ✓
2. B eliminate all insulation ✓
3. C measure only solar Voc ✓
4. D personnel safety and correct fault current paths during tests ✓

Type tests on HV equipment demonstrate

1. A only paint colour compliance ✓
2. B only nameplate font size ✓
3. C only shipping weight ✓
4. D design meets standard requirements including dielectric and thermal limits ✓

Routine tests on HV transformers include

1. A only blade pitch angle ✓
2. B only train braking distance ✓
3. C measurement of ratio, polarity, winding resistance and insulation resistance ✓
4. D only MPPT algorithm code ✓

Field intensity on an HV bushing is controlled by

1. A single uniform paper layer only ✓
2. B removing all capacitive grading ✓
3. C using only air gaps internally ✓
4. D graded insulation and condenser layers ✓

Dielectric breakdown in solids is often initiated by

1. A perfect homogeneous lattice only always ✓
2. B zero electric field ✓
3. C impurities, voids or thermal hot spots under high stress ✓
4. D only magnetic flux density ✓

Liquid dielectric breakdown may involve

1. A only electron avalanche in vacuum always ✓
2. B bubble formation and electrohydrodynamic instability ✓
3. C no role of impurities ✓
4. D infinite viscosity preventing motion ✓

Impulse breakdown voltage of an insulator is typically

1. A always lower than DC breakdown ✓
2. B higher than power-frequency breakdown for short duration stresses ✓
3. C equal to corona inception only ✓
4. D zero for lightning ✓

Withstand voltage test on HV equipment verifies

1. A only contact resistance of terminals ✓
2. B only nameplate MVA ✓
3. C only rotor inertia ✓
4. D insulation can endure specified test voltage without failure ✓

Creepage and clearance distances on insulators prevent

1. A only conductor heating ✓
2. B flashover along surfaces and through air ✓
3. C only skin effect ✓
4. D only transformer magnetizing current ✓