# Multiple Choice Questions on Transformer | Page – 5

1. Minimum allowable dielectric strength or break down voltage of power transformer insulating oil is generally taken as

As per IS 1866 of 2000 the minimum allowable dielectric strength or break down voltage of transformer oil is 30 KV.

2. SFRA or sweep frequency response analysis is carried out on a transformer to monitor the

Transformer can be considered as a combination of multiple complex networks of capacitance, inductance and resistance that can generate a unique signature when tested at discrete frequencies and plotted as a curve. If any deformation occurs in any internal part of transformer, the values of the capacitance inductance and resistances may be changed hence the signature generated will not match with the signature taken before. SFRA or sweep frequency response analysis is modern technique for monitoring internal condition of transformer.

3. Flash point of transformer insulating oil should be more than

Flash point is the temperature at which oil gives so much vapour that this vapour, when mixed with air, forms an ignitable mixture and gives a momentary flash on application of flame. A minimum flash point is specified in order to prevent the risk of fire hazard. Minimum Flash Point of transformer insulating oil should be 140°C.

4. A transformer has R2 secondary resistance, k is its transformation ratio, then the resistance referred to primary will be

Let’s secondary circuit has resistance R2 ohm, primary and secondary current I1 and I2 respectively. If k is the transformation ratio then approximately I2 = kI1.
Now, voltage drop across secondary resistance is I2R2.
This voltage drop referred to primary is kI2.R2 = k.
( kI1R2 ) = I1[ k2R2 ]
Therefore, secondary resistance referred to primary R2 = k2R2.

5. A transformer has R1 primary resistance, k is its transformation ratio, this resistance referred to secondary will be

Let’s primary circuit has resistance R1 ohm, primary current and secondary current I1 and I2 respectively. If k is the transformation ratio then approximately I1 = I2 / k.
Now, voltage drop across primary resistance is I1.R1.
This voltage drop referred to secondary is ( I1R1 ) / k = { ( I2 / k )R1} / k = I2[ R1/k2 ].
Therefore, primary resistance referred to secondary is R1 = R1/k2.

6. What is the thickness of laminated CRGO steel sheet used for core of transformer ?

0.35 mm thick laminated sheet of CRGO steel is generally used in core of transformer. If lamination thickness is reduced, the eddy current loss in the core will be reduced significantly but very thin sheet is difficult to manufacture and handle hence we go for optimum thickness i.e. 0.18 mm to 0.35 mm.

7. Insulated surface coating of laminated sheet in the core of transformer helps to reduce

If insulation in between two adjacent laminated sheets in a transformer core is not provided, sheets would comes under direct contact, and behave as one single sheet of double thickness. We know that if thickness of lamination is increased, eddy current is increased which causes increase in eddy current loss.

8. If supply frequency of a transformer is 50 Hz, its magnetostriction frequency would be

Magnetostriction is the change in configuration of a magnetizable body in a magnetic field, leads to periodic changes in the length of the body in alternating magnetic field. The frequency of this magnetostriction is twice of frequency of alternating magnetic field. When magnetizing of an electrical sheet with 50 Hz, the fundamental frequency of magnetostriction is 100 Hz.

9. Noise of transformer mainly due to

Transformer noise is mainly produced due to magnetostriction in core which leads to periodical change of dimension of laminated sheet. Noise also generated in transformer due to cooling fans and vibration of core but magnetostriction is the main cause of transformer noise.

10. The ideal shape for the cross section of core is

Circular cross – section would waste no space beyond that taken up by insulation between lamination.

11. Percentage leakage reactance in core type construction, compared to shell type construction of core, is

The percentage leakage reactance for core type transformer is comparatively higher compared to shell type construction due to distributed nature of the windings in the two limbs separately.

12. The tap changer in an electrical power transformer is provided on

Generally tap changer connections are provided with HV winding mainly due to two reasons:
1. HV Winding is placed above LV windig on core so tapping terminals can easily accessible.
2. In HV Winding current is less compared to LV Winding hence arcing would be less during tap changing.

13. Why HV winding is always placed above the LV winding in a core type transformer?

As the voltage difference between core and LV winding is less than that of core and HV winding, amount of insulation required between core and LV winding is less than that of core and HV winding, it is economical to place LV winding nearer to core inside HV winding.

14. If silica gel breather of a transformer is blocked then

If breather of a transformer is blocked, breathing action of transformer stops. Hence oil inside the transformer will not get enough space of expansion hence convectional flow of oil stop which badly affects the cooling of transformer.

15. Buchholz relay is provided on transformer to

Buchholz relay is provided on electrical power transformer to give alarm and remote indication during minor fault inside the transformer and it trips the circuit breakers of both sides of transformer with alarm indication during major internal fault of transformer.

16. Full load copper loss in transformer is 1600 W. At half load the loss will be

Say the transformer has equivalent resistance Re and full load current IL then full load copper loss WL = IL2Re. At half load, IL/2 = IL / 2. Therefore copper loss at half load, WL/2 = IL/22Re = (IL / 2 )2Re = 0.25 × IL2Re = 0.25 × WL.

17. Iron loss for a transformer is 400 W. If the transformer is working at its maximum efficiency then copper loss will be

At maximum efficiency of transformer iron loss = copper loss in a transformer.

18. Which test on a transformer provides information about regulation, efficiency and heating under load condition ?

The regulation, efficiency and heating can be done by full load test. Back to back test is one kind of full load test.

19. If supply frequency of a transformer increases, the secondary output voltage of the transformer

The secondary voltage of a transformer depends upon the input or primary voltage and its voltage or turns ratio. So if primary voltage is fixed, then whatever may be the frequency of the primary input voltage, the output or secondary voltage will also be fixed.

20. If the cross – sectional area of magnetic core of a transformer increases, the secondary output voltage of the transformer

The secondary voltage of a transformer depends upon the input or primary voltage and its voltage or turns ratio. So if primary voltage is fixed, then whatever may be the cross – sectional area of magnetic core of a transformer, the output or secondary voltage will also be fixed.