comprehensive “300+ Practical Power Transformer Interview Questions and Answers” guide with clear explanations (focused on real-world transformer operation, testing, protection, and maintenance).
all practical, interview-style transformer questions with concise explanations (ideal for field engineers, testing engineers, and maintenance professionals).
⚙️ Power Transformer Practical Interview Questions & Answers (Part 1: Q1–Q60)
🔹 Section 1: Transformer Fundamentals
1. What is the main function of a power transformer?
➡️ To transfer electrical energy between two circuits at different voltage levels without changing frequency.
2. Why is a transformer called a static device?
➡️ It has no moving parts; energy transfer occurs through electromagnetic induction.
3. What happens if DC supply is given to a transformer?
➡️ Core saturates → heavy current → overheating and winding damage.
4. Why transformer rating is in kVA not kW?
➡️ Losses depend on voltage & current, not power factor.
5. What are the main parts of a power transformer?
➡️ Core, windings, oil, tank, conservator, breather, bushings, tap changer, and protection devices.
6. Why is silicon steel used in transformer cores?
➡️ Reduces hysteresis loss due to high magnetic permeability.
7. Why laminations are provided in core?
➡️ To minimize eddy current loss by reducing loop area.
8. What is the purpose of transformer oil?
➡️ Cooling and insulation between winding and tank.
9. What is BDV test of oil?
➡️ Breakdown Voltage test – measures dielectric strength of oil.
10. What is a conservator tank?
➡️ Provides space for expansion and contraction of oil during temperature changes.
🔹 Section 2: Transformer Cooling & Oil System
11. What does ONAN mean?
➡️ Oil Natural Air Natural – both oil and air circulate naturally for cooling.
12. What is ONAF?
➡️ Oil Natural Air Forced – fans blow air for better cooling.
13. What is OFAF?
➡️ Oil Forced Air Forced – pumps circulate oil and fans cool it.
14. Why silica gel breather is used?
➡️ Absorbs moisture from air entering the conservator tank.
15. What does the color of silica gel indicate?
➡️ Blue = dry, Pink = moist → needs replacement/regeneration.
16. Why Buchholz relay is used?
➡️ Detects internal faults by gas accumulation in oil (gas-operated relay).
17. What types of faults are detected by Buchholz relay?
➡️ Inter-turn faults, insulation failure, core heating, oil leakage.
18. Why nitrogen gas sometimes used in transformer?
➡️ In sealed-type transformers to prevent oxidation of oil.
19. How is transformer oil purified?
➡️ By filtration and dehydration under vacuum.
20. What is the minimum BDV value of good oil?
➡️ Above 60 kV (for new oil); below 40 kV indicates deterioration.
🔹 Section 3: Transformer Testing
21. What is Open Circuit (OC) Test?
➡️ Determines iron loss and no-load current by keeping secondary open.
22. Why OC test done on LV side?
➡️ Easy to apply rated voltage and measure low current.
23. What is Short Circuit (SC) Test?
➡️ Determines copper loss and equivalent impedance by shorting secondary.
24. Why SC test done on HV side?
➡️ Easier to apply low voltage to get rated current.
25. What is polarity test in transformer?
➡️ Determines relative polarity of primary and secondary for parallel operation.
26. What is vector group test?
➡️ Checks phase displacement between primary and secondary windings.
27. What is insulation resistance test?
➡️ Measures insulation quality using a megger (should be >1000 MΩ for large units).
28. What is the purpose of DGA (Dissolved Gas Analysis)?
➡️ Detects internal faults by analyzing gases dissolved in oil.
29. What gases indicate arcing?
➡️ Acetylene (C₂H₂) and Hydrogen (H₂).
30. What is the purpose of the turns ratio test?
➡️ Confirms correct voltage ratio and winding connections.
🔹 Section 4: Operation & Load
31. What is voltage regulation?
➡️ The change in secondary voltage from no-load to full-load in % of full-load voltage.
32. Why voltage regulation is important?
➡️ It affects load voltage stability and performance.
33. What are transformer losses?
➡️ Core (iron) loss and copper (winding) loss.
34. When does maximum efficiency occur?
➡️ When copper loss = iron loss.
35. What is all-day efficiency?
➡️ Ratio of energy output to input over 24 hours — for distribution transformers.
36. What is percentage impedance?
➡️ The voltage drop across transformer under full load current, expressed as % of rated voltage.
37. Why impedance important for parallel operation?
➡️ Equal impedance ensures proper load sharing.
38. What is inrush current?
➡️ High magnetizing current during energization due to residual flux.
39. What is tap changer?
➡️ Device to regulate output voltage by changing turns ratio.
40. What is the difference between OLTC and Off-load tap changer?
➡️ OLTC can operate under load; off-load must be operated after de-energizing.
🔹 Section 5: Protection & Faults
41. What is differential protection?
➡️ Compares input and output currents — trips if difference exceeds preset value.
42. What is REF protection?
➡️ Restricted Earth Fault — detects earth faults near neutral end of winding.
43. What is over-fluxing?
➡️ When V/f exceeds design limit → core saturation and heating.
44. What is overload protection?
➡️ Trips transformer when load current exceeds rated capacity for a period.
45. What is oil temperature protection?
➡️ Trips the transformer when oil temperature exceeds safe limit (~90°C).
46. What is winding temperature protection?
➡️ Uses embedded RTDs to measure internal temperature.
47. What is sudden pressure relay?
➡️ Detects sudden rise in pressure inside tank during severe faults.
48. What is the function of surge arrester?
➡️ Protects transformer against lightning and switching surges.
49. What is the purpose of neutral grounding?
➡️ Limits fault current and stabilizes system voltage.
50. What happens if neutral is not grounded?
➡️ Unbalanced voltages and risk of insulation failure.
🔹 Section 6: Maintenance & Field Operation
51. How is transformer insulation tested?
➡️ Megger test, Tan-Delta test, and Partial Discharge (PD) test.
52. What is Tan Delta test?
➡️ Measures insulation quality — lower value indicates healthy insulation.
53. What is IR value trend indication?
➡️ Increasing = good insulation; decreasing = moisture ingress.
54. Why oil level must be checked regularly?
➡️ Low oil → exposed windings → flashover or overheating.
55. What is oil leakage effect?
➡️ Moisture entry and reduced insulation strength.
56. Why transformer makes humming noise?
➡️ Magnetostriction in core laminations due to alternating flux.
57. What is vector group Dyn11?
➡️ Delta primary, star secondary, 30° lead — common in distribution transformers.
58. Why earthing of tank is important?
➡️ Ensures operator safety and fault current path to ground.
59. What is a temperature indicator (OTI/WTI)?
➡️ Shows oil and winding temperature readings with alarm/trip contacts.
60. Why nitrogen blanketing used?
➡️ Prevents oil oxidation and moisture absorption in sealed transformers.
Excellent ⚡ — here’s your Part 2 (Q61–Q150) of the
👉 “300+ Practical Power Transformer Interview Questions & Answers (with brief explanations)”
This section covers Testing, Parallel Operation, Faults, Troubleshooting, and Design Insights — very useful for field engineers, commissioning staff, and interview prep.
⚙️ Part 2: Advanced Power Transformer Questions (Q61–Q150)
🔹 Section 7: Advanced Testing & Performance
61. What is a Temperature Rise Test?
➡️ Checks the temperature increase of oil and winding under rated load conditions.
62. What is permissible temperature rise for oil and winding?
➡️ Oil: 50–55°C, Winding: 60–65°C above ambient temperature.
63. What is percentage impedance used for?
➡️ To calculate short-circuit current and voltage drop during load.
64. How is percentage impedance measured?
➡️ From Short Circuit Test by applying voltage to get rated current.
65. What happens if two transformers with different impedance are paralleled?
➡️ Unequal load sharing and circulating currents.
66. What is open-circuit loss also called?
➡️ Iron or core loss (constant loss).
67. What is short-circuit loss also called?
➡️ Copper loss (variable loss).
68. What test checks insulation condition?
➡️ Megger Test (Insulation Resistance Test).
69. What is Polarization Index (PI)?
➡️ Ratio of 10-minute IR to 1-minute IR → shows insulation dryness (>1.5 good).
70. What is sweep frequency response analysis (SFRA)?
➡️ Checks winding deformation by comparing frequency response curves.
🔹 Section 8: Parallel Operation
71. Why transformers are connected in parallel?
➡️ For load sharing, redundancy, and maintenance flexibility.
72. What conditions must be satisfied for parallel operation?
➡️ Same voltage ratio, phase sequence, polarity, and % impedance.
73. What happens if polarity not matched?
➡️ Short circuit at connection instant.
74. What if phase sequence is wrong?
➡️ Large circulating current and voltage unbalance.
75. Why same % impedance important?
➡️ To ensure load sharing proportional to kVA ratings.
76. How is load shared between two transformers?
➡️ According to their kVA ratings and impedance ratio.
77. Can delta–star and delta–delta transformers operate in parallel?
➡️ Yes, if phase displacement is compensated (e.g., Dyn11 with Dd0).
78. Why neutral grounding needed in star secondary?
➡️ Provides path for zero-sequence currents and system stability.
79. What is circulating current in parallel transformers?
➡️ Current flow due to unequal voltage ratios or phase shifts.
80. How to avoid circulating current?
➡️ Match voltage ratio and vector group before paralleling.
🔹 Section 9: Faults & Abnormal Conditions
81. What are the main causes of transformer faults?
➡️ Overload, insulation failure, moisture, oil contamination, short circuit.
82. What is inter-turn fault?
➡️ Short circuit between turns of the same winding.
83. What is earth fault?
➡️ Connection of winding or core to earth due to insulation breakdown.
84. What is differential protection relay principle?
➡️ Difference between incoming and outgoing currents triggers tripping.
85. What is Buchholz relay function?
➡️ Detects gas accumulation and oil surge caused by internal faults.
86. What type of gas is generated during arcing?
➡️ Hydrogen (H₂) and acetylene (C₂H₂).
87. What fault generates CO and CO₂ gases?
➡️ Overheating of insulation and cellulose degradation.
88. What does DGA test identify?
➡️ Internal faults based on dissolved gas composition.
89. What happens during transformer overloading?
➡️ Increased copper loss → overheating → insulation failure.
90. How is transformer fault location identified?
➡️ Using DGA, thermal imaging, or SFRA tests.
🔹 Section 10: Troubleshooting in Field
91. Transformer hums loudly — probable cause?
➡️ Loose core laminations or magnetostriction.
92. Oil level low in conservator — cause?
➡️ Leakage or loss due to breathing.
93. Oil color dark — meaning?
➡️ Aging or contamination; oil needs filtration/replacement.
94. Silica gel turned pink — action?
➡️ Replace or regenerate silica gel crystals.
95. Buchholz relay gas accumulation but no trip?
➡️ Minor fault or air ingress; perform gas analysis.
96. Unequal phase voltages in secondary — reason?
➡️ Tap changer contact issue or unbalanced load.
97. Sudden pressure relay tripped — reason?
➡️ Severe internal arcing or mechanical failure.
98. Winding temperature high but oil normal — why?
➡️ Circulation issue inside winding or RTD fault.
99. Transformer tripped during load increase — cause?
➡️ Overcurrent, overload, or OLTC malfunction.
100. Repeated tripping without fault — possible cause?
➡️ Relay calibration error or CT polarity issue.
🔹 Section 11: Transformer Design & Parameters
101. Why copper used for winding?
➡️ High conductivity and mechanical strength.
102. Why aluminum used in some transformers?
➡️ Lower cost and lighter weight.
103. What is core cross-section shape?
➡️ Generally stepped-laminated rectangular or circular for flux uniformity.
104. What is flux density in core?
➡️ Usually 1.5–1.7 Tesla for CRGO steel.
105. What is window space factor?
➡️ Ratio of copper area to total window area; affects efficiency.
106. Why CRGO steel used in core?
➡️ High permeability, low hysteresis loss.
107. Why insulation provided between layers of winding?
➡️ To prevent inter-turn short circuit.
108. Why core clamped tightly?
➡️ To reduce vibration and humming noise.
109. What is the purpose of core earthing?
➡️ Prevents static charge buildup and safe fault path.
110. What are typical insulation classes?
➡️ Class A (105°C), B (130°C), F (155°C), H (180°C).
🔹 Section 12: Tap Changer Operation
111. What is purpose of tap changer?
➡️ Regulate secondary voltage by altering turns ratio.
112. What is step size in OLTC?
➡️ Usually ±1.25% per tap; ±10% overall range.
113. How many tap positions usually provided?
➡️ 17 (from –8 to +8 and one neutral).
114. What is diverter switch in OLTC?
➡️ Transfers load current from one tap to another without interruption.
115. What maintenance needed for OLTC?
➡️ Oil replacement, contact inspection, and motor drive check.
116. What is “pumping” in OLTC mechanism?
➡️ Frequent tap changes due to voltage fluctuations.
117. Why is tap changing done on HV side?
➡️ Lower current → smaller contacts and easier switching.
118. What causes OLTC contact pitting?
➡️ Arcing during tap transition under load.
119. Why separate oil chamber used in OLTC?
➡️ Prevents contamination of main transformer oil.
120. What is AVR (Automatic Voltage Regulator)?
➡️ Controls OLTC motor based on voltage variation.
🔹 Section 13: Earthing, Protection & Safety
121. Why neutral earthing important?
➡️ Limits fault current and ensures stable operation.
122. What is double earthing of transformer body?
➡️ Two separate earth connections for safety redundancy.
123. What is step potential near transformer yard?
➡️ Voltage difference between two feet during fault → can be dangerous.
124. What is touch potential?
➡️ Voltage between hand and foot when touching an energized object during fault.
125. What is marshalling box?
➡️ Houses indicators, alarms, terminal blocks, and control wiring.
126. Why current transformers (CTs) used in transformer protection?
➡️ To measure and relay secondary current safely.
127. What are typical CT ratios in large transformers?
➡️ 800/1A, 1600/1A, etc., depending on rating.
128. What happens if CT polarity reversed?
➡️ Differential protection malfunctions → false tripping.
129. What is NGR (Neutral Grounding Resistor)?
➡️ Limits earth fault current to safe value.
130. What is step-up transformer used for?
➡️ In power plants to increase generator voltage for transmission.
🔹 Section 14: Field Maintenance & Diagnostics
131. What routine checks are done on transformers?
➡️ Oil level, temperature, load current, BDV, silica gel color.
132. What periodic tests done yearly?
➡️ IR test, Tan-Delta, DGA, oil filtration, OLTC inspection.
133. What are major maintenance intervals?
➡️ Typically every 3–5 years for large power transformers.
134. How often should DGA be done?
➡️ Every 6 months or immediately after faults.
135. Why bushing maintenance critical?
➡️ Bushings carry high voltage — failure can cause explosions.
136. What is partial discharge test?
➡️ Detects insulation voids and surface discharges.
137. What is “neutral displacement” indication?
➡️ Neutral voltage shift due to unbalanced phase loading.
138. What is LV bushing crack effect?
➡️ Leakage current and partial discharges leading to flashover.
139. What is the safe oil temperature limit?
➡️ Normally below 90°C for mineral oil.
140. Why nitrogen pressure monitored in sealed units?
➡️ Pressure drop indicates gas leakage or oil shrinkage.
🔹 Section 15: Miscellaneous & Practical
141. Why transformer efficiency high?
➡️ No rotating parts → minimal friction and mechanical losses.
142. What is harmonics effect on transformer?
➡️ Causes additional heating and losses due to non-sinusoidal current.
143. What causes magnetizing inrush current?
➡️ Core residual flux and saturation at energization.
144. How to reduce inrush current?
➡️ Controlled switching or pre-insertion resistors.
145. What is transformer vector group importance?
➡️ Ensures correct phase relationship for parallel operation.
146. Why Dyn11 common in distribution systems?
➡️ Provides neutral and 30° phase lead to reduce harmonics.
147. What is tertiary winding used for?
➡️ For harmonics suppression or station service load.
148. What is no-load current composed of?
➡️ Magnetizing and core loss components.
149. Why bushing potential tap provided?
➡️ For testing or potential measurement.
150. Why core earthing done at one point only?
➡️ To avoid circulating currents in the core.
Excellent ⚡ — here’s your Part 3 (Q151–Q300) of the
👉 “300 + Practical Power Transformer Interview Questions & Answers with Brief Explanations.”
This final part focuses on commissioning, protection logic, fault case studies, advanced diagnostics, and field troubleshooting.
Perfect for maintenance, commissioning, protection, and testing engineer interviews.
⚙️ Part 3: Commissioning, Protection & Field Case Questions (Q151–Q300)
🔹 Section 16 – Commissioning & Pre-Startup Checks
151. Why pre-commissioning tests are performed?
➡️ To verify insulation, polarity, ratio, and correct installation before energizing.
152. What test ensures correct polarity?
➡️ Polarity test or voltage comparison test.
153. Why insulation resistance (IR) test done before ratio test?
➡️ Confirms insulation health before applying any voltage.
154. Minimum IR value for power transformer (in MΩ)?
➡️ ≥ 1000 MΩ for ≤ 11 kV; ≥ 2000 MΩ for ≥ 33 kV at ambient temp.
155. What is tan-delta test purpose?
➡️ Measures dielectric losses to assess insulation condition.
156. What is polarization index (PI)?
➡️ Ratio = IR₁₀min / IR₁min; > 1.5 is healthy insulation.
157. What checks done before oil filling?
➡️ Vacuum dehydration, BDV > 60 kV, moisture < 20 ppm.
158. What is vacuum filling?
➡️ Filling oil under vacuum to remove trapped air in windings.
159. Why neutral grounding resistance checked before energizing?
➡️ Ensures correct fault-current limiting.
160. Why transformer kept under soaking period after oil filling?
➡️ Allows insulation to absorb oil evenly before applying voltage.
🔹 Section 17 – Protection Systems
161. Why differential protection is most reliable?
➡️ Detects internal faults rapidly by current difference method.
162. What CT connection used for differential protection?
➡️ Star on delta side and delta on star side to compensate phase shift.
163. What is restricted earth fault (REF) protection?
➡️ Detects earth faults within transformer zone using residual current.
164. Why REF more sensitive than differential?
➡️ Responds to small earth faults near neutral point.
165. What is Buchholz relay function?
➡️ Detects internal faults by gas accumulation and oil surge.
166. How many contacts Buchholz relay has?
➡️ Two — alarm (upper float) and trip (lower float).
167. What gas volume triggers alarm contact?
➡️ About 250 cc of gas accumulation.
168. What gas analysis indicates arcing?
➡️ Acetylene (C₂H₂).
169. What indicates overheating of oil?
➡️ Hydrogen + methane increase in DGA.
170. Why pressure relief valve used?
➡️ Protects tank from explosion by releasing excess pressure.
🔹 Section 18 – Relay Coordination and Settings
171. Why CT secondary earthed?
➡️ Prevents dangerous voltages under open-circuit fault.
172. What is biasing in differential relay?
➡️ Avoids mal-operation during through-faults and CT errors.
173. Typical differential relay setting?
➡️ 20–30 % of rated current.
174. Why inrush current blocked by harmonic restraint?
➡️ Contains 2nd harmonic component; relay ignores that condition.
175. Why REF protection not provided in delta winding?
➡️ No neutral point → no return path for earth current.
176. What is over-fluxing protection?
➡️ Trips transformer if V/f ratio exceeds design limit.
177. What is 87T relay code?
➡️ ANSI code for transformer differential protection.
178. What is 49 relay?
➡️ Thermal overload protection.
179. What is 63 relay?
➡️ Pressure or Buchholz relay.
180. What is 64 relay?
➡️ Ground fault protection.
🔹 Section 19 – Oil & Insulation Monitoring
181. What is dielectric strength (BDV) of new oil?
➡️ > 60 kV / 2.5 mm gap.
182. What happens if BDV low?
➡️ Oil contaminated → risk of flashover.
183. Why tan-delta of oil checked?
➡️ Indicates dissipation losses and contamination.
184. How to remove moisture from oil?
➡️ Filtration and vacuum dehydration.
185. Why DGA used periodically?
➡️ Detects developing faults by gas trends.
186. What gases show cellulose overheating?
➡️ CO and CO₂.
187. Why oxygen content monitored?
➡️ Promotes oxidation and sludge formation.
188. What indicates sudden rise in hydrogen?
➡️ Partial discharge or corona.
189. What indicates presence of acetylene (C₂H₂)?
➡️ Severe arcing inside transformer.
190. What test checks furan content?
➡️ Furan analysis → indicates paper insulation aging.
🔹 Section 20 – Cooling & Temperature Control
191. What is ONAN cooling?
➡️ Oil Natural, Air Natural – self-cooled.
192. What is ONAF?
➡️ Oil Natural, Air Forced – fans assist air circulation.
193. What is OFAF?
➡️ Oil Forced, Air Forced – oil pumps and fans used.
194. Why temperature indicators used?
➡️ Monitor winding and top-oil temperatures.
195. What is purpose of winding temperature indicator (WTI)?
➡️ Simulates hot-spot temperature using CT heating.
196. What triggers cooling fans?
➡️ Thermostat or WTI contacts.
197. What is maximum top-oil temperature limit?
➡️ About 90 °C for mineral oil.
198. Why cooling fans interlocked with load current?
➡️ Start automatically during higher load.
199. What is cold-oil circulation time?
➡️ Delay before oil circulation stabilizes (20–30 min after start).
200. What is effect of high ambient temperature?
➡️ Reduced cooling → derating of transformer capacity.
🔹 Section 21 – Fault Analysis & Case Studies
201. Transformer trips on differential after inrush — cause?
➡️ Relay not harmonic-restrained or CT saturation.
202. Buchholz alarm but no trip — interpretation?
➡️ Minor internal fault or air ingress.
203. Repeated oil leakage at flange — cause?
➡️ Gasket aging or tank pressure pulsation.
204. Frequent oil level drop — reason?
➡️ Leak or loss due to breathing and temperature variations.
205. High winding temperature but normal oil — cause?
➡️ Cooling oil path blocked in windings.
206. Unequal phase current — cause?
➡️ Unequal loading or winding short.
207. Abnormal humming sound — cause?
➡️ Loose core laminations or saturation.
208. Silica gel frequent color change — cause?
➡️ Breather seal leak or humid atmosphere.
209. Slow pressure rise after shutdown — cause?
➡️ Residual heating or gas generation.
210. DGA shows high CO₂ — meaning?
➡️ Insulation paper overheating.
🔹 Section 22 – Advanced Diagnostics
211. What is Frequency Response Analysis (FRA)?
➡️ Detects winding movement by frequency response comparison.
212. What is tan-delta of winding?
➡️ Measures dielectric loss in winding insulation.
213. What is partial discharge test?
➡️ Detects microscopic insulation voids and discharges.
214. What is sweep frequency range for FRA?
➡️ 20 Hz to 2 MHz.
215. What is typical PD value limit?
➡️ < 500 pC for new power transformer.
216. What is recovery voltage test?
➡️ Evaluates moisture and polarization properties of insulation.
217. Why capacitance unbalance measured?
➡️ Detects deformation or shorts in windings.
218. Why leakage reactance measured?
➡️ Checks magnetic coupling and winding geometry integrity.
219. What is ratio error tolerance (IEC 60076)?
➡️ ±0.5 %.
220. What test checks core insulation?
➡️ Core-to-earth resistance test.
🔹 Section 23 – Installation & Site Practices
221. Why foundation isolated from structure?
➡️ Prevents vibration transmission and earth loops.
222. Why rollers provided under tank?
➡️ For easy movement during installation.
223. Why nitrogen filled in sealed transformer?
➡️ Maintains pressure and prevents moisture ingress.
224. Why explosion vent used?
➡️ Directs gases safely during internal faults.
225. What precautions during oil filling?
➡️ Fill under dry nitrogen and low humidity conditions.
226. What is typical oil expansion space in conservator?
➡️ About 10 % of total oil volume.
227. Why magnetic oil gauge (MOG) used?
➡️ Provides remote oil level indication and alarm.
228. Why breather mounted below conservator?
➡️ To dry incoming air during oil breathing.
229. What color silica gel shows moisture absorption?
➡️ Pink or light purple (from blue).
230. Why drain valve provided at bottom?
➡️ For sampling and oil replacement.
🔹 Section 24 – Performance & Load Behavior
231. What is voltage regulation?
➡️ Change in secondary voltage from no-load to full-load.
232. Typical voltage regulation value?
➡️ 2–5 % for power transformer.
233. Why efficiency decreases at low load?
➡️ Core loss constant but output reduced.
234. At what load efficiency maximum?
➡️ When Cu loss = iron loss.
235. What is all-day efficiency?
➡️ Energy efficiency considering load variation over 24 h.
236. Why copper loss increases with load?
➡️ ∝ I².
237. Why iron loss constant?
➡️ Depends on voltage & frequency only.
238. Why power factor affects regulation?
➡️ Causes voltage drop due to reactive component.
239. What is leading PF effect on regulation?
➡️ Voltage may rise at no load.
240. What happens under over-excitation?
➡️ Core saturation → high magnetizing current & overheating.
🔹 Section 25 – Standards & Ratings
241. Main design standard for transformers?
➡️ IEC 60076 / IS 2026.
242. Why nameplate data important?
➡️ For rating, vector group, tap range, impedance info.
243. What is temperature rise limit per IS 2026?
➡️ 55 °C for oil, 60 °C for winding.
244. Why lightning impulse test done?
➡️ Ensures surge withstand capability.
245. What is standard impulse voltage for 132 kV unit?
➡️ 650 kVp.
246. Why short-circuit test done at factory?
➡️ Verifies mechanical strength of windings.
247. What is induced over-voltage test value?
➡️ 2 × rated voltage, double frequency.
248. Duration of induced test?
➡️ 7200 cycles (1 min approx).
249. What is routine vs type test?
➡️ Routine = every unit; Type = sample unit.
250. Why temperature-rise test limited to rated load?
➡️ To simulate actual operating condition safely.
🔹 Section 26 – Special Transformers
251. Why auto-transformer economical?
➡️ Common winding → less copper and core material.
252. Main disadvantage of auto-transformer?
➡️ No electrical isolation between circuits.
253. Why furnace transformer has low voltage?
➡️ To supply high current for heating.
254. Why rectifier transformer used?
➡️ Designed for DC supply to rectifiers with harmonic control.
255. What is traction transformer?
➡️ Supplies 25 kV AC for railway traction.
256. Why phase-shifting transformer used?
➡️ Controls power flow between parallel lines.
257. Why earthing transformer used?
➡️ Provides neutral in un-earthed system.
258. What is current transformer (CT)?
➡️ Measures high current by scaling to 1 A or 5 A.
259. What is potential transformer (PT)?
➡️ Steps down voltage for metering and protection.
260. Why cast-resin transformer preferred indoors?
➡️ Fire safe and low maintenance.
🔹 Section 27 – Operation & Safety Practices
261. Why transformer isolated before maintenance?
➡️ For personal safety and zero voltage condition.
262. What is “LOTO”?
➡️ Lock-Out Tag-Out procedure to prevent energizing.
263. Why earthing discharge rod used?
➡️ Discharges residual charge after isolation.
264. Why fire extinguishers near transformer?
➡️ For oil fire emergency control.
265. What extinguisher type used for oil fires?
➡️ CO₂ or dry-chemical powder.
266. Why hot-line tagging prohibited?
➡️ Live work without isolation can be fatal.
267. What is safe distance for 132 kV yard?
➡️ About 2.1 m from live part (minimum clearance).
268. Why transformer neutral earthed twice?
➡️ Redundancy in fault path.
269. Why control cables separated from power cables?
➡️ To reduce induced voltage and noise.
270. Why bushing kept clean?
➡️ Prevents tracking and flashover.