0195136047.pdf

PROBLEMS 503

Load

2400 V 208 V

3 single-phase transformers

each rated 10 kVA, 2400:120 V, 60 Hz

30 kVA balanced

Figure P11.5.4

11.5.5Three single-phase 100-kVA, 2400:240-V, 60-Hz
transformers (each of which has an equivalent se-
ries impedance of 0. 045 +j 0. 16 referred to its
low-voltage side) are connected to form a three-
phase, 4160:240-V transformer bank, which in
turn is connected to a three-phase feeder with
an impedance of 0. 5 +j 1. 5 /phase. When the
three-phase transformer bank delivers 250 kW at
240 V and 0.866 lagging power factor, determine:
(a) The transformer winding currents.
(b) The sending-end voltage (line to line) at the
source.
11.5.6Three single-phase, 10-kVA, 2400/120-V, 60-
Hz transformers are connected to form a three-
phase, 4160/208-V transformer bank. Each of
the single-phase transformers has an equivalent
series impedance of 10+j 25 referred to the
high-voltage side. The transformer bank is said
to deliver 27 kW at 208 V and 0.9 power factor
leading.
(a) Draw a schematic diagram of transformer
connections, and develop a per-phase equiv-
alent circuit.
(b) Determine the primary current, primary volt-
age, and power factor.
(c) Compute the voltage regulation.
*11.5.7A three-phase, 600-kVA, 2300:230-V, Y–Y trans-
former bank has an iron loss of 4400 W and a full-
load copper loss of 7600 W. Find the efficiency
of the transformer for 70% full load at 230 V and
0.85 power factor.
11.5.8Three identical single-phase transformers are to
be connected to form a three-phase bank rated at
300 MVA, 230:34.5 kV. For the following con-
figurations, determine the voltage, current, and
kVA ratings of each single-phase transformer: (a)

• , (b) Y– , (c) Y–Y, (d) –Y.
  11.6.1A single-phase, 10-kVA, 2300:230-V, 60-Hz,
  two-winding distribution transformer is con-
  nected as an autotransformer to step up the volt-
  age from 2300 V to 2530 V.

(a) Draw a schematic diagram of the arrange-

ment showing all the voltages and currents

while delivering full load.

(b) Find the permissible kVA rating of the auto-

transformer if the winding currents are not to

exceed those for full-load operation as a two-

winding transformer. How much of that is

transformed by electromagnetic induction?

(c) Based on the data given for the two-winding

transformer in Problem 11.4.1, compute

the efficiency of the autotransformer corre-

sponding to full load and 0.8 lagging power

factor. Comment on why the efficiency of

the autotransformer is higher than that of the

two-winding transformer.

11.6.2A two-winding, single-phase transformer rated

3 kVA, 220:110 V, 60 Hz is connected as an

autotransformer to transform a line input voltage

of 330 V to a line output voltage of 110 V and to

deliver a load of 2 kW at 0.8 lagging power factor.

Draw the schematic diagram of the arrangement,

label all the currents and voltages, and calculate

all the quantities involved.

11.6.3A two-winding, 15-kVA, 2300:115-V, 60-Hz,

single-phase transformer, which is known to have

a core loss of 75 W and a copper loss of 250 W, is

connected as an autotransformer to step up 2300

V to 2415 V. With a load of 0.8 power factor

lagging, what kVA load can be supplied without

exceeding the current rating of any winding?

Determine the efficiency at this load.

11.6.4A single-phase, two-winding, 10-kVA, 440:110-

V, 60-Hz transformer is to be connected as an

autotransformer to supply a load at 550 V from a

440-V supply. Draw a schematic diagram of the

connections and determine: (a) The maximum

kVA rating as an autotransformer. (b) The maxi-

mum apparent power transferred by conduction.

(c) The maximum apparent power transferred by

electromagnetic induction.

*11.6.5A 15-kVA, 2200:220-V, two-winding, single-

phase transformer is connected as an autotrans-