R
base_p
23.04
Ω
=
R
base_s
208 volt
⋅
3
I
rat_s
:=
R
base_s
4.326
Ω
=
R
wp
0.01 R
base_p
⋅
:=
R
wp
0.23
Ω
=
R
ws
0.01 R
base_s
⋅
:=
R
ws
0.043
Ω
=
Problem 3: A threephase load is drawing 250 kW real power at a power factor of 0.75 lagging.
a) Find the apparent power S drawn by the load in kVA
333.3
.
b) Find the reactive power Q drawn by the load in kVAR
220.5
.
c) Find the capacitor bank rating in kVAR to increase the combined power factor to 0.95 lagging.
S
V I
⋅
=
P
V I
⋅
pf
⋅
=
Q
V I
⋅
sin acos pf
(
)
(
)
⋅
=
S
P
2
Q
2
+
=
acos 0.75
(
)
41.41 deg
=
S
P
pf
=
S
250 kW
⋅
0.75
:=
S
333.3 kVA
=
Q
075
S sin acos 0.75
(
)
(
)
⋅
:=
Q
075
220.5 kVAR
=
EE 5343, Test 1, Spring 2002:
Dr. M. Giesselmann, February 14, 2002
Definitions:
freq
60 Hz
⋅
:=
ω
2
π
⋅
freq
⋅
:=
MW
1000 kW
⋅
≡
kVA
kW
≡
kVAR
kW
≡
m
Ω
Ω
1000
≡
MVA
1000 kW
⋅
≡
MVAR
1000 kW
⋅
≡
Problem 1:
a) What is the principal advantage of AC power systems compared to DC power systems?
The ability to step voltages up and down for long distance transmission.
b) Give two solutions to eliminate oscillations of the instantaneous power in AC power systems.
Use a balanced twophase system with voltages 90 deg apart, or a balanced threephase system
with voltages 120 deg apart.
c) Which of the solutions listed in (b) is better for long distance power distribution and why?
Threephase systems transmit more power per line since the currents add up to zero.
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 Spring '08
 GIESSELMANN
 Power, Electric power transmission, Iload Rload, Hz kVAR kW

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