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THREEPHASE SYSTEMSImprovement exercises
1. A balanced Yconnected source with supplies power to abalanced
Yconnected load with a perphase impedance of . If , find the
perphase impedance of the powerdistribution line.
2. The powerdistribution line of a balanced YY system has a
perphase impedance of . If the perphase load impedance is and the
perphase powerloss on the line is 60 W, find the perphase power
absorbed by the load as well as theperphase complex power supplied
by the source.
3. A balanced, positivephasesequence Yconnected source with is
connected by four perfect conductor lines to an unbalanced Y
connected load having
, , y . Find the current on the neutral line.
4. If the neutral line in the YY system of Problem 3 is removed,
find , the voltage ofthe load neutral node referenced to that of
the source neutral node.
5. A balanced, negativephasesequence Y system has a perphase
distributionlineimpedance of and a per phase load impedance of . If
, find the line currents and the phase voltages at the load.
6. A balanced Ysystem has and . If the load absorbs400W per
phase with a lagging power factor of 0.875 and the distribution
line absorbs20W per phase, find , , , y .
7. A balanced load draws a complex power of at a line voltage of
4kVrms, 60Hz.
a. Assuming a connected load, find a simple parallel equivalent
for its phaseimpedance.
b. Assuming a Yconnected load, find a simple series equivalent
for its phaseimpedance.
8.a. What connected capacitor bank must be connected in parallel
with the load of
Problem 7 to achievepf= 1
? Assume the line voltage remains unchangedb. Repeat, but for a
Yconnected bank
9. A balanced load absorbs 360kW at pf = 0.8, lagging. If the
load is fed by a balancedsource via a powerdistribution line with a
perphase impedance of and the line voltage at the load es 660Vrms,
find the power factor at the source.
10.A balanced 3 load draws a total of 20kW from a balanced
3source operating at60Hz. If the line voltage and current at the
load are, respectively, 220Vrms and 70Arms,find
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a. The complex power drawn by the load,b. The power factor of
the load, andc. A connected capacitor bank that will raise the
power factor of the load 0.95,
lagging.
11. A balanced source supplies 30kVA at pf = 0.8, lagging, to a
balanced load. If the linevoltage at the source is 240Vrms and the
distribution line has a per
phase impedance
of , find
a. The rms value of the line currentb. The rms value of the line
voltage at the load, andc. The complex power at the load.
12.A balanced source supplies power to three balanced loads via
a powerdistribution linehaving a perphase impedance of . Load 1
absorbs 100kVA at pf = 0.85,lagging; Load 2 absorbs 25kVAR atpf =
0.75, leading; Load 3 dissipates 20kW at pf =1. If the line voltage
at the load is 240Vrms, find the line voltage andpfat the
source.
13. A balanced source supplies power to three balanced loads via
a powerdistribution linehaving a perphase impedance of . Load 1
absorbs 100kVA at pf = 0.95,leading; Load 2 absorbs 125kW at pf =
0.8, lagging; Load 3 draws . Ifthe line voltage at the load is
3kVrms, find the power factor at the source and the load,as well as
the efficiency of the system.
14.A balanced Yconnected load is powered by a balanced source
and draws a total of120kW atpf = 0.8, lagging. The line voltage at
the load is 208Vrms, 60Hz.
a. If three 5mF capacitances are connected in parallel with the
load in a Yconfiguration, find the power factor of the combined
load.
b. Repeat, but with the capacitances connected in a
configuration. Compare andcomment.
