0195136047.pdf

502 MAGNETIC CIRCUITS AND TRANSFORMERS

8 hours full load at unity power factor

8 hours no load

8 hours one-half full load at unity power

factor

11.4.17A 10-kVA transformer is known to have an iron
loss of 150 W and a full-load copper loss of
250 W. If the transformer has the following load
cycle, compute its all-day efficiency:
4 hours full load at 0.8 power factor
8 hours 75% full load at 1.0 power factor
12 hours 50% full load at 0.6 power factor
11.4.18Show polarity markings for a single-phase trans-
former for (a) subtractive polarity, and (b) addi-
tive polarity.
11.4.19Two single-phase transformers, each rated 2400:
120-V, are to be interconnected for (a) 4800:240-
V operation, and (b) 2400:120-V operation.
Draw circuit diagrams and show polarity mark-
ings.
11.4.20Two 1150:115-V transformers are to be inter-
connected for (a) 2300:230-V operation, and (b)
1150:230-V operation. Show the interconnec-
tions and appropriate polarity markings.
11.5.1A one-line diagram of a three-phase distribution
system is given in Figure P11.5.1. Determine
the line-to-line voltage at the sending end of the
high-voltage feeder when the transformer deliv-
ers rated load at 240 V (line-to-line) and 0.8
lagging power factor. Neglect the exciting current
of the transformer.
11.5.2A 5-MVA, 66:13.2 kV, three-phase transformer
supplies a three-phase resistive load of 4500 kW
at 13.2 kV. What is the load resistance in ohms as
measured from line to neutral on the high-voltage
side of the transformer, if it is:
(a) Connected in Y–Y?

(b) Reconnected in Y– , with the same high-

tension voltage supplied and the same load

resistors connected?

11.5.3A three-phase transformer bank consisting of

three 10-kVA, 2300:230-V, 60-Hz, single-phase

transformers connected in Y– is used to step

down the voltage. The loads are connected to the

transformers by means of a common three-phase

low-voltage feeder whose series impedance is

0. 005 +j 0. 01 per phase. The transform-

ers themselves are supplied by means of a

three-phase high-voltage feeder whose series

impedance is 0. 5 +j 5. 0 per phase. The

equivalent series impedance of the single-phase

transformer referred to the low-voltage side is

0. 12 +j 0. 24 . The star point on the primary side

of the transformer bank is grounded. The load

consists of a heating load of 2 kW per phase and

a three-phase induction-motor load of 20 kVA

with a lagging power factor of 0.8, supplied at

230 V line-to-line.

(a) Draw a one-line diagram of this three-phase

distribution system.

(b) Neglecting the exciting current of the trans-

former bank, draw the per-phase equivalent

circuit of the distribution system.

(c) Determine the line current and the line-to-

line voltage at the sending end of the high-

voltage feeder.

11.5.4A single-line diagram of a three-phase trans-

former bank connected to a load is given in

Figure P11.5.4. Find the magnitudes of the line-

to-line voltages, line currents, phase voltages,

and phase currents on either side of the trans-

former bank. Determine the primary to secondary

ratio of the line-to-line voltages and the line cur-

rents.

Load

3 single-phase transformers

each rated 50 kVA, 2400:240 V

identical with that of Example 11.4.1

3-phase low-voltage feeder

series impedance

0.00083 + j 0.0033 Ω/ph

3-phase high-voltage feeder

series impedance

0.25 + j 1.0 Ω/ph

Figure P11.5.1