A star connection provides
Answer: A
The neutral is what makes a four-wire distribution system possible and gives a reference for earthing and protection. A delta winding gives the closed path for triple harmonics, which is why the two are often combined.
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A delta connection is valuable because it
Answer: B
Triple harmonics circulate within the delta rather than distorting the voltage or flowing in the neutral, which is why a delta winding is usual on at least one side. The winding current is \( 1/\sqrt{3} \) of the line current.
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A star-star connected transformer without a delta or tertiary winding suffers from
Answer: C
Correct answer: distortion of the phase voltages by triple harmonics
The magnetising current contains a strong third harmonic, and in a star-star connection with no delta winding and no neutral path that harmonic current cannot circulate. It is therefore forced into the flux instead, and the phase voltages are distorted by a large third harmonic. A delta tertiary provides the circulating path and cures it.
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A tertiary or stabilising winding on a three-phase transformer is provided to
Answer: D
Correct answer: give a path for triple harmonic and unbalance currents
A closed delta tertiary lets triple harmonic magnetising currents circulate within it rather than distorting the flux, and it also provides a path for zero sequence current under unbalanced loading, so the neutral is stabilised. It is often used to supply station auxiliaries as well, but that is a convenience rather than its purpose.
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A delta-delta connected transformer can continue in service with one phase removed as
Answer: A
Two units can supply a three-phase load at about fifty-eight per cent of the bank's rating, which keeps supply going while the third is repaired. That resilience is one reason the connection was favoured for industrial supplies.
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A Scott connection is used to obtain
Answer: B
Two specially proportioned transformers give two single-phase outputs in quadrature that load the three-phase system evenly, which suits electric traction. Without such an arrangement a single-phase traction load unbalances the supply badly.
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A zigzag connected winding is used to
Answer: C
Its geometry gives a low impedance to zero-sequence current and a high one to positive sequence, which makes it an effective earthing transformer on a delta system. It is a special-purpose connection rather than a general one.
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Two transformers may be operated in parallel only if they have
Answer: D
A ratio mismatch circulates current between them even at no load, and a polarity or phase sequence error is a short circuit. Equal per unit impedances are desirable for proportional sharing but are not an absolute requirement.
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If two paralleled transformers have unequal per unit impedances, they will
Answer: A
Correct answer: share the load in inverse proportion to those impedances
Paralleled transformers see the same terminal voltage, so the one with the lower impedance takes the larger share of current: the split is in inverse proportion to the per unit impedances. Unequal impedances therefore mean one unit reaches its rating before the other, and the pair cannot deliver the sum of their ratings.
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Paralleling two transformers of different vector group would
Answer: B
A thirty degree difference in secondary phase leaves a large voltage between the paralleled terminals, and only the winding impedances limit the resulting current. It is a fault condition rather than a poor arrangement.
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