Important MCQ on Transformers with Explanations

Hint: A transformer has a core on which the primary and secondary windings are wound. When the primary winding is energized, it draws a magnetizing current and produces magnetic flux in the core. This flux links with both the primary and the secondary windings. As a result, EMF is induced in the windings. We can take power from the secondary winding due to the induced EMF produced by the mutual flux linkage between the primary and secondary windings. Therefore, a transformer works on the principle of mutual induction.

Hint: Lamination of the transformer core increases the resistance and therefore reduces the eddy current. The eddy current loss is expressed as I²R, where I represents the eddy current in this case. Obviously, the increase in resistance due to lamination has much less influence than the decrease in eddy current. Therefore, the overall eddy current loss of the core decreases due to lamination.

Hint: The design of a transformer, especially its conducting portion along with its insulation system, mainly depends on two factors. The conducting portion depends on the current limit, while the insulation portion depends on the voltage limit. Therefore, a transformer is designed based on the current it can carry and the voltage it can withstand. That is why transformers are rated in kVA rather than kW.

Hint: In a transformer, the voltage per turn is fixed and is the same for both the primary and secondary windings. Therefore, in a step-up transformer, the secondary winding has more turns than the primary winding. As a result, the secondary voltage becomes higher than the primary voltage.

Hint: The copper loss of a transformer is also referred to as the ohmic loss of the transformer. It is nothing but the I²R loss in the windings. Therefore, it depends on the magnitude of the current flowing through the transformer windings.

Hint: When the secondary winding is open-circuited, the transformer still draws a small current from the source to keep the core magnetized. This current is known as the magnetizing current and is typically about 2 to 5 percent of the rated primary current.

Hint: Both hysteresis loss and eddy current loss depend on frequency and flux density. Since flux density depends on applied voltage, the core loss ultimately depends on supply voltage and frequency.

Hint: An ideal transformer assumes zero copper loss, zero core loss, and no leakage flux. It represents perfect magnetic coupling between primary and secondary windings.

Hint: The condition for maximum efficiency of a transformer is that the copper loss equals the core loss.

Hint: The open-circuit test is normally conducted from the low-voltage side because it is more practical and safer to apply the rated voltage to the LV winding than to the HV winding.

Hint: In the short-circuit test, we apply a very small voltage to the high-voltage winding of the transformer to produce the rated current in both the primary and secondary windings. This rated current produces the required copper losses in the transformer. However, since the applied voltage is very low, the core loss is negligible. Therefore, during the short-circuit test, the measured power mainly represents the copper loss.

Hint: The core loss of a transformer consists of hysteresis loss and eddy current loss. Both of these losses depend on the supply frequency. Therefore, if the frequency increases, both hysteresis loss and eddy current loss increase. As a result, the total core loss of the transformer also increases.

Hint: A transformer works on the principle of Faraday’s law of electromagnetic induction. According to this law, whenever a changing magnetic flux links with a conductor, an EMF is induced across it. In a static device like a transformer, changing flux is possible only when an alternating supply is connected to it. Therefore, a transformer operates only on an alternating supply.

Hint: In a core-type transformer, the low-voltage winding is first wound around the limb of the core. Then, the high-voltage winding is wound over the low-voltage winding. Therefore, in a core-type transformer, the windings surround the core limb.

Hint: In a shell-type transformer construction, two parallel closed magnetic paths are formed on both sides of the central limb. The windings are wound on the central limb in a sandwich form. Therefore, a shell-type transformer always has a double parallel magnetic path in its core.

Hint: The voltage regulation of a transformer contains both resistive and reactive voltage drop components. At a slightly leading power factor, the negative reactive voltage drop can offset the positive resistive voltage drop. As a result, the overall voltage regulation can become zero or even negative.

Hint: Transformer oil performs two major functions: insulation and cooling. It also has arc-quenching properties. However, in transformers, the arc-quenching property is not normally required. Therefore, transformer oil is mainly used for insulation and cooling.

Hint: Buchholz relay is a purely oil-actuated mechanical relay. Therefore, it can be used only in oil-filled transformers.

Hint: The output voltage of a transformer depends on its turns ratio. An on-load tap changer adjusts the tap position while the transformer is in operation. It changes the effective turns ratio to control and regulate the output voltage.

Hint: The turn ratio of a transformer is defined as the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. For example, in a 132 kV / 33 kV transformer, the turn ratio is the ratio of primary turns to secondary turns.

Hint: The transformer core must be magnetized during operation. CRGO silicon steel laminations are used because they significantly reduce eddy current loss and hysteresis loss compared with ordinary magnetic materials.

Hint: The leakage reactance of a transformer is produced due to leakage flux. This leakage flux links only with its own winding and produces a self-induced EMF. The self-induced EMF opposes the mutually induced EMF and causes an internal voltage drop.

Hint: A practical transformer always has losses. Only an ideal transformer is assumed to be completely lossless and therefore has 100 percent efficiency.

Hint: In the BDV test, a gradually increasing voltage is applied across electrodes immersed in the oil. The breakdown voltage indicates the insulation strength of the transformer oil.

Hint: ONAN means Oil Natural Air Natural. Heat is carried by natural circulation of oil and dissipated naturally to the surrounding air.

Hint: The fault MVA of a transformer depends on its per-unit impedance. Therefore, per-unit impedance plays an important role in fault current calculations.

Hint: Transformer impedance contains a resistance component which varies with temperature. Therefore, impedance is generally guaranteed at the maximum operating winding temperature, commonly 75°C.

Hint: The short-circuit current of a transformer is determined by the ratio of voltage to impedance. Therefore, it is inversely proportional to transformer impedance.

Hint: For satisfactory parallel operation and to avoid circulating current, transformers should have matching voltage ratios and percentage impedances.

Hint: D indicates HV delta connection. y indicates LV star connection. n indicates neutral brought out. The clock number 11 represents a 30-degree phase displacement according to clock notation.

Hint: An auto-transformer uses a common winding for both primary and secondary circuits. This shared winding arrangement reduces copper requirement.

Hint: A delta winding does not provide a neutral point for zero-sequence current to flow. Therefore, zero-sequence current cannot flow through a delta-connected winding.

Hint: Magnetizing current is a purely inductive current and therefore lags the applied voltage by 90 degrees.

Hint: Eddy current loss is directly proportional to the square of frequency. Therefore, increasing frequency significantly increases eddy current loss.

Hint: Hysteresis loss is directly proportional to frequency. Therefore, an increase in supply frequency increases hysteresis loss.

Hint: Voltage regulation depends on resistive and reactive voltage drops. At lagging power factor, both components contribute positively, resulting in maximum regulation.

Hint: During the short-circuit test, only a small voltage is required to circulate rated current because the secondary is short-circuited. This voltage is typically about 5–10% of rated voltage.

Hint: During the short-circuit test, the applied voltage is very low. Since core loss depends mainly on voltage, it becomes negligible.

Hint: In the open-circuit test, the secondary remains open and only a small no-load current flows. Therefore, the measured power mainly represents core loss.

Hint: The transformer tank is earthed to keep the touch voltage within safe limits and to protect personnel from electric shock during fault conditions.