Transformer Interview Questions

Q1. What is the principle of operation of a transformer?

A transformer works on the principle of mutual electromagnetic induction: a time-varying current in the primary winding induces an EMF in the secondary winding through the shared magnetic flux in the core. In a 230V/12V step-down transformer used in mobile chargers, the turns ratio is about 19:1. The key insight is that energy transfer is purely magnetic — no electrical connection exists between primary and secondary, which is what makes galvanic isolation possible.

Follow-up: Why does a transformer not work on DC supply?

Q2. What are the different types of losses in a transformer and how do you minimise them?

Transformer losses are of two types: core losses (hysteresis and eddy current) and copper losses (I²R in windings). In a 100 kVA distribution transformer, core losses are roughly 300–400 W and are nearly constant regardless of load. Eddy current losses are minimised by using laminated silicon steel cores with 0.3–0.5 mm thick sheets, while copper losses are reduced by using larger conductor cross-sections in high-current windings.

Follow-up: Which loss dominates at no-load and which at full-load?

Q3. How do you determine transformer efficiency, and at what load is it maximum?

Efficiency equals (output power / input power) × 100%, and it is maximum when copper losses equal core losses. For a standard 25 kVA distribution transformer, maximum efficiency typically occurs around 70–80% of full load. This matters in practical installations — utility companies deliberately design distribution transformers for peak efficiency at average daily load rather than rated load to reduce energy waste over 24 hours.

Follow-up: What is all-day efficiency and why is it different from commercial efficiency?

Q4. Explain the open-circuit test and short-circuit test on a transformer.

The open-circuit test, performed at rated voltage on the LV side with HV open, measures core loss and no-load current — typically 2–5% of rated current. The short-circuit test, done at rated current on the HV side with LV shorted, measures copper loss and leakage impedance at a much reduced voltage, usually 5–10% of rated voltage. Together, these two tests give all four parameters of the equivalent circuit without loading the transformer to full power.

Follow-up: Why is the OC test done on the LV side and SC test on the HV side?

Q5. What is voltage regulation of a transformer and what causes it?

Voltage regulation is the change in secondary terminal voltage from no-load to full-load, expressed as a percentage of full-load voltage. It is caused by the resistive and leakage reactance voltage drops in the windings — a typical 11 kV/415 V distribution transformer has regulation of 3–5%. Transformers supplying motor loads see worse regulation because motor starting draws 5–7× rated current, causing a momentary voltage dip that can trip sensitive controls.

Follow-up: How does the power factor of the load affect voltage regulation?

Q6. What is the difference between core-type and shell-type transformers?

In a core-type transformer the windings surround the magnetic core, while in a shell-type transformer the core surrounds the windings on three sides. Shell-type designs, used in large power transformers above 100 MVA, provide better mechanical support to windings against short-circuit forces and give lower leakage flux. Core-type is preferred for high-voltage applications because the winding can be split into concentric layers for easier insulation grading.

Follow-up: Which type has better leakage reactance control and why?

Q7. Why is the transformer core made of silicon steel and not plain iron?

Silicon steel (about 3–4% silicon) has higher electrical resistivity than plain iron, which reduces eddy current losses significantly, and its narrow hysteresis loop means lower hysteresis loss per cycle. In a 50 Hz power transformer running 8760 hours a year, this difference compounds to hundreds of kilowatt-hours of savings. The silicon also increases permeability at moderate flux densities, allowing the core to be operated at flux densities of 1.5–1.7 T without excessive magnetising current.

Follow-up: Why can you not increase silicon content beyond about 4–5%?

Q8. What is an auto-transformer and when is it preferred over a two-winding transformer?

An auto-transformer has a single winding that serves as both primary and secondary, with the secondary being a tapped portion of the primary — there is no electrical isolation between input and output. A 415 V to 380 V auto-transformer for driving imported European motors is far smaller and cheaper than a two-winding unit because only the voltage difference (35 V) needs electromagnetic transformation. The saving in copper and core material makes auto-transformers preferred when the turns ratio is close to 1 and isolation is not required.

Follow-up: Why is an auto-transformer not used for stepping up 230 V to 11 kV?

Q9. What happens if a transformer designed for 50 Hz is connected to a 60 Hz supply at the same voltage?

At the same voltage but higher frequency, the peak flux in the core drops because EMF = 4.44 × f × N × Φ_max, so Φ_max decreases by the ratio 50/60. Lower flux means lower magnetising current and lower core losses, so the transformer actually runs cooler. However, the leakage reactance increases proportionally with frequency, which slightly increases voltage regulation — a minor effect usually acceptable for equipment rated 50/60 Hz.

Follow-up: What would happen if the same 50 Hz transformer is connected to a 25 Hz supply at the same voltage?

Q10. What is the purpose of Buchholz relay in a transformer?

A Buchholz relay is a gas-actuated protection device fitted in the pipeline between the main tank and conservator tank of oil-immersed transformers rated above 1 MVA. When an internal fault generates gas or oil surge, the relay triggers an alarm at low severity or trips the transformer at high severity. It is particularly sensitive to slow developing faults like inter-turn short circuits that produce small gas quantities before other protection systems can detect the fault.

Follow-up: What type of faults does Buchholz relay NOT protect against?

Q11. Explain the concept of transformer vector group and give an example.

Vector group indicates the phase relationship between primary and secondary voltages and the type of winding connection — Dy11, for example, means delta primary, star secondary with 30° phase shift (11 × 30°). In India, the standard 11 kV/415 V distribution transformer uses Dyn11, with the neutral of the secondary star brought out for single-phase loads. Vector group matching is critical when paralleling transformers — mismatched groups create a circulating current that can damage both units instantly.

Follow-up: Can a Dyn11 transformer be paralleled with a Dyn1 transformer?

Q12. What is inrush current in a transformer and why does it occur?

Inrush current is the large transient current drawn when a transformer is energised — it can be 8–12 times rated current for the first few cycles. It occurs because the core may start from residual flux that adds to the flux demanded by the applied voltage, driving the core into deep saturation where magnetising inductance collapses. A 100 kVA, 415 V transformer can draw over 1000 A inrush even though its rated current is about 139 A — this is why transformer protection relays use a second harmonic restraint to distinguish inrush from a genuine fault current.

Follow-up: How does second harmonic restraint work in differential protection to block operation during inrush?

Q13. What is the significance of the per-unit system in transformer analysis?

The per-unit system expresses all quantities as fractions of a chosen base value, eliminating the actual voltage levels and making the equivalent circuit the same whether you are on the primary or secondary side. A transformer with 5% per-unit impedance means 5% voltage drop at full-load current regardless of whether the transformer is 415 V or 11 kV. This is especially useful in power system fault calculations where you might have 5 voltage levels — converting everything to per-unit collapses all ideal transformers and allows Kirchhoff's laws to be applied directly.

Follow-up: How do you convert per-unit impedance from one MVA base to another?

Q14. Why do large power transformers use oil for cooling and insulation?

Transformer oil serves dual purpose: it has a dielectric strength of about 30–50 kV per mm, providing insulation between HV windings, and its high thermal capacity (specific heat ~1.7 kJ/kg·K) carries heat from the core and windings to the radiators by natural or forced convection. A 100 MVA, 220 kV power transformer may hold 20,000–30,000 litres of oil. Modern ester-based fluids are increasingly used because they are biodegradable and have higher fire points (~300°C) compared to mineral oil (~160°C), reducing fire risk in urban substations.

Follow-up: What is the ONAN, ONAF, OFAF cooling classification system?

Q15. What is the condition for paralleling two transformers, and what happens if the voltage ratios are slightly different?

Transformers can be paralleled if they have the same voltage ratio, same per-unit impedance, same vector group, and same frequency — these four conditions must all be met. If voltage ratios differ slightly, say 415 V vs 420 V secondary, a circulating current flows even at no load: Ic = (ΔV) / (Z1 + Z2), where Z1 and Z2 are the winding impedances. This circulating current heats both transformers and reduces their available load capacity — a 2% voltage mismatch in a 1000 kVA transformer pair can cause 10–15% derating.

Follow-up: Why must per-unit impedances be equal for proper load sharing between parallel transformers?