POLYPHASE CIRCUITS LEARNING GOALS Three Phase Circuits Advantages of polyphase circuits Three Phase Connections Basic configurations for three phase circuits Source/Load Connections Delta-Y connections Power Relationships Study power delivered.
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Transcript POLYPHASE CIRCUITS LEARNING GOALS Three Phase Circuits Advantages of polyphase circuits Three Phase Connections Basic configurations for three phase circuits Source/Load Connections Delta-Y connections Power Relationships Study power delivered.
POLYPHASE CIRCUITS
LEARNING GOALS
Three Phase Circuits
Advantages of polyphase circuits
Three Phase Connections
Basic configurations for three phase circuits
Source/Load Connections
Delta-Y connections
Power Relationships
Study power delivered by three phase circuits
Power Factor Correction
Improving power factor for three phase circuits
THREE PHASE CIRCUITS
Van
3 phase
voltage
Vbn
Vcn
0
120
240
Instantane ous Phase Voltages
van (t ) Vm cos( t )(V )
vbn (t ) Vm cos( t 120)(V )
vc (t ) Vm cos( t 240)(V )
Vm 120 2
a
a
+
_
V0
Wye Connected
Source
n
_
V-240
+
c
_
V-120
+
b
b
c
Delta Source
a
a
_
+
+
Delta
Source
_
Vbc = Vab -120
b
c
_
Vab = | Vab | 0
b
+
c
Vca = Vab -240
Wye – Wye System
a
A
Zl
ZL
n
N
ZL
c
b
B
Zl
Zl
ZL
C
Delta – Delta System
a
A
Zl
+
ZL
ZL
_
+
c
_
_
+
b
B
Zl
Zl
C
ZL
Delta – Wye System
a
A
Zl
_
ZL
+
+
_
ZL
c
_
+
b
B
Zl
Zl
ZL
C
a
a
A
I aA
I AB
+
_
I CA
V0
Z
Z
n
_
V-240
+
_
I BC
V-120
+
b
b
c
C
B
Z
c
Vcn
- Vbn
Vab
30 o
Van
Vbn
Vab = Van - Vbn
Vab = 3 Van 30o
Vab Van Vbn
| V p | 0 | V p | 120
| V p | 1 (cos120 j sin 120)
1
3
| V | p | V p | j
2
2
3 | V p | 30
Vbc 3 | V p | 90
Vca 3 | V p | 210
VL 3 | V p | Line Voltage
I CA
I aA = 3 I AB -30o
I AB
I aA
I BC
- I CA
INSTANTANEOUS POWER
Instantane ous Phase Voltages
van (t ) Vm cos( t )(V )
Balanced Phase Currents
ia (t ) I m cos( t )
vbn (t ) Vm cos( t 120)(V )
ib (t ) I m cos( t 120)
vc (t ) Vm cos( t 240)(V )
ic (t ) I m cos( t 240)
Instantane ous power
p(t ) van (t )ia (t ) vbn (t )ib (t ) vcn (t )ic (t )
Theorem
For a balanced three phase circuit the instantaneous power is constant
p( t ) 3
Vm I m
cos (W )
2
REVIEW OF
Y
Transformations
Ra
R1R2
R1 R2 R3
Rb
R2 R3
R1 R2 R3
Rc
R3 R1
R1 R2 R3
Y
R1
Ra Rb Rb Rc Rc Ra
Rb
R2
Ra Rb Rb Rc Rc Ra
Rc
R3
Ra Rb Rb Rc Rc Ra
Ra
Y
Rab R2 || ( R1 R3 )
REVIEW OF
Y
Transformations
Y
Rab Ra Rb
Y
Ra R1
Rb R1
Rb R2
Rb R1
R1R2
R
R
3
2
Ra
Rc R1
Rc
R2 ( R1 R3 ) Ra R R R Rb R3
1
2
3
Ra Rb
REPLACE IN THE THIRD AND SOLVE FOR R1
R1 R2 R3
R2 R3
Rb
R1 R2 R3
Ra Rb Rb Rc Rc Ra
R3 ( R1 R2 )
R
1
Rb Rc
Rb
R
R
3 1
R1 R2 R3 Rc
R1 R2 R3
R R Rb Rc Rc Ra
R2 a b
Rc
Y
R (R R )
Rc Ra
1
2
3
R1 R2 R3
SUBTRACT THE FIRST TWO THEN ADD
TO THE THIRD TO GET Ra
R3
Ra Rb Rb Rc Rc Ra
Ra
Y
POWER FACTOR CORRECTION
Similar to single phase case.
Use capacitors to increase the
power factor
Balanced
load
Low pf
lagging
Keep clear about total/phase
power, line/phase voltages
Q Qnew Qold
Reactive Power to be added
To use capacitors this value
should be negative
pf
pf cos f sin f 1 pf 2 tan
f
2
1
pf
Q P tan f
lagging Q 0
LEARNING EXAMPLE
f 60 Hz , | Vline | 34.5kV rms . Required : pf 0.94 leading
Pold 18.72 MW
S P jQ
P | S | cos f
Q | S | sin f
pf cos f
Q P tan f
tan f
pf
1 pf 2
lagging Qold 0
pf cos f sin f 1 pf 2 0.626
| Qold | 15.02 MVA
Pold 18.72 MW
Qnew 6.8 MVA
pf new 0.94 leading
Q 6.8 15.02 21.82 MVA
Qper capacitor 7.273MVA
Y connection Vcapacitor
34.5
kV rms
3
34.5 103
6
7.273 10 2 60 C
3
C 48.6 F
2
LEARNING BY DESIGN
Proposed new store
#4ACSR wire rated at 170 A rms
S1 70036.9
S2 100060kVA
S3 80025.8kVA
560 j 420 kVA 500 j866 kVA
720 j 349 kVA
Stotal 1780 j1635 kVA 241742.57 kVA
| Stotal |
2.417 106
| I line |
101.1A rms
3
3 Vline
3 13.8 10
Wire is OK
Pold
Qnew P tan f ( new) 758.28kVA
pf new
Q Qnew Qold 876.72kVA
|Q
per capacitor
| CV
2
876.72 10 / 3
C
2 60 13.8 10 / 3
3
3 2
12.2 F