# Transformers Questions and Answers – Three Winding Transformers

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This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “Three Winding Transformers”.

1. Three winding and two winding transformers work on same working principle.
a) True
b) False

Explanation: Both the transformers work on the same principle of electromagnetic induction. Transformers may be built with a third winding in addition to the primary and secondary winding called the tertiary. There are various purposes which dictate the use of a tertiary winding.

2. Why tertiary winding is used?
a) To make circuit stable
b) To provide voltage supply to the substations different from the primary and secondary voltage levels
c) To connect static capacitors for reactive power injection
d) Stable circuit, different voltage output, static capacitor connection

Explanation: Tertiary winding can be used to supply the substation auxiliaries at a voltage different from those of the primary and secondary windings. Static capacitors or synchronous condensers may be connected to the tertiary winding of a transformer for reactive power injection into the system for voltage control.

3. Tertiary winding is generally _________
a) star connected
b) delta connected
c) open-delta connected
d) star or open-delta connected

Explanation: Tertiary winding of the three-winding transformer is generally delta connected which is helpful in reducing the impedance offered to the zero sequence currents thus, it allows a larger earth-fault current to flow for proper operation of protective equipment.

4. Tertiary winding allows the flow of _______________
a) third harmonic currents
b) fifth harmonic currents
c) seventh harmonic currents
d) eleventh harmonic currents

Explanation: Delta-connected tertiary winding limits voltage imbalance when the load is unbalanced. It also permits the third harmonic current to flow which ultimately reduces third-harmonic voltage. Thus, performs as a stabilizing winding to the transformer.

5. Tertiary winding can be used for ____________
a) to maximize the core area
b) to measure voltage during HV testing of transformer
c) to fill up the space between primary and secondary windings
d) to measure voltage during open circuit testing of transformer

Explanation: There are many uses of tertiary winding, such as, three windings may be used for interconnecting three transmission lines at different voltages. Tertiary winding can serve the purpose of measuring voltage of an HV testing transformer.

6. Any unbalanced load can be divided into _______________
a) positive and negative sequence components
b) positive, negative and zero sequence components
c) only in positive and zero sequence components
d) only in negative and zero sequence components

Explanation: Any unbalanced load can be divided into three 3-phase sets (positive, negative and zero sequence components). The zero-sequence component (co-phasal currents on three lines, I0 = In/3) caused by a line-to-neutral load on the secondary side cannot be balanced by primary currents as the zero-sequence currents cannot flow in the isolated neutral star connected primary.

7. Which currents flow through the delta-connected tertiary winding?
a) Positive sequence currents
b) Negative sequence currents
c) Zero sequence currents
d) All sequence currents

Explanation: Iron path is available for the zero-sequence flux in a bank of single-phase units and in the 5-limb core and as a consequence the impedance offered to the zero-sequence currents is very high (0.5 to 5 pu) inhibiting the flow of these currents. The provision of a delta-connected tertiary permits the circulation of zero-sequence currents in it, thereby considerably reducing the zero-sequence impedance.

8. Simple phase equivalent circuit of three winding transformer can be shown as ____________
a) simple star network
b) simple parallel network
c) simple series network
d) can be shown accordingly

Explanation: Particular winding can be represented as equivalent resistor and reactance. For simplicity, the effect of the exciting current can be ignored in the equivalent circuit. It may be noted that the load division between the secondary and tertiary is completely arbitrary.

9. How impedance of particular winding can be calculated?
a) Z1= Z12+Z13+Z23
b) Z1= 1/2½ (Z12+Z13-Z23)
c) Z1= Z12-Z13+Z23
d) Z1= Z12+Z13-Z23

Explanation: The single-phase equivalent of three-winding transformer can be shown. According to the short circuit test one can easily find the short circuit impedance between winding 1 and 2 as Z12 and thus, formula for primary impedance can be calculated.

Sanfoundry Global Education & Learning Series – Transformers.

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