Transformer Interview Question Part 3

Q. What is the significance of vector grouping in Power Transformers?

A. Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage.
Example: DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.

Q. What is meant by regulation in a transformer?

A. Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usually expressed in terms of percentage.
Example: A Transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load,
The regulation capacity varies depending on the size and the application for which they are used.

Q. Can a Single Phase Transformer be used on a Three Phase source?

A. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4 wire. The transformer output will be single phase.

Q. Can Transformers develop Three Phase power from a Single Phase source?

A. NO. Phase converters or phase shifting devices such as reactors and capacitors are required to convert single phase power to three phase.

Q. Can Single Phase Transformers be used for Three Phase applications?

A. Yes. Three phase transformers are sometimes not readily available whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer.

Example: Three 10 K VA single phase transformers can be used to accommodate a 30 KVA three phase load.

Q. What is polarity, when associated with a transformer?

A. Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage winding on the left hand side are connected together. This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings. This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.

Q. What is meant by “impedance” in transformers? Why is it important?

A. Impedance is the current limiting characteristic of a transformer and is expressed in percentage. It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.

Q. What is tertiary winding? What is Three Winding Transformer? What are its advantages?

A. In some high rating transformers, one winding, in addition to the primary and secondary winding, is used. This is known as Tertiary Winding of Transformer. Because of this third winding, the transformer is called Three Winding Transformer.
Advantages:
1. It reduces the unbalancing in the primary due to unbalancing in three phase load.
2. It redistributes the flow of fault current
3. To supply an auxiliary load in different voltage level in addition to its main secondary load.(station lighting and power)
4. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

Q. Can the transformers be used at higher frequencies than rated frequency?

A. Yes. Transformers can be used at higher frequencies than rated frequency. However as the frequency increases voltage regulation decreases.

Q. EMF equation of the transformer?

A. E = 4.44ΦmfN    Volts.
If E1 & E2 are primary and secondary emf’s and N1 & N2 are primary and secondary turns then,
 Voltage ratio= E1/E2 and
 Turns ratio of transformer = N1/N2
E1/E2 = N1/N2

Q. Explain about the open and short circuit tests on transformer?

A. Open circuit test (No load test) and short circuit test (full load test) are performed on a transformer to determine
(i) Equivalent circuit of transformer
(ii) Voltage regulation of transformer
 (iii) Efficiency of transformer.
The power required for these tests on transformer is equal to the power loss occurring in the transformer.
 Open circuit test on transformer is used to determine core losses in transformer and parameters of shunt branch of the equivalent circuit of transformer.
Short Circuit test on transformer is used to determine copper loss in transformer at full load and parameters of approximate equivalent circuit of transformer.