Transformers Questions and Answers

Answer: B

At unity power factor the reactive drop is in quadrature and contributes only to the second order, so the regulation is nearly the resistive component. It is a useful check on whether a regulation calculation has been done correctly.

Answer: C

The reciprocal of the per unit impedance gives the multiple, so a low impedance means a stiff supply and a large fault current. Switchgear has to be rated for that current, which is one reason impedance is not made smaller than it needs to be.

Answer: D

Correct answer: 30 degrees between primary and secondary line voltages

In a delta-star connection the primary line voltage is a phase voltage of the delta while the secondary line voltage is the difference of two star phase voltages, and the two differ in phase by 30 degrees. The shift matters when transformers are paralleled: two units must belong to the same vector group or a large circulating current will flow.

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.

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.

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.

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.

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.

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.

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.