Presented by Byoung-Kuk Lee, Ph. D., Senior IEEE
Energy Mechatronics Lab.
College of Information and Communication Eng.
Sungkyunkwan University Tel: +82-31-299-4581 Fax: +82-31-299-4612 http://seml.skku.ac.kr EML: bkleeskku@skku.edu
11.1 Balanced Three-Phase Voltages
Three-Phase
A basic three-phase circuit
11.1 Balanced Three-Phase Voltages
Three-Phase
Phasor diagrams of a balanced set of three-phase voltages The abc
(positive) sequence
The acb
(negative) sequence
a b c
V 0
V 120
V 120
m m m
V V V
a b c
V 0
V 120
V 120
m m m
V V V
11.1 Balanced Three-Phase Voltages
Three-Phase
a b c
a b c
V V V 0 0
v v v
Va: phasor representation of a-phase voltage va: a-phase instantaneous voltage
If Va or va (Ia or ia) is obtained,
remaining other phase voltages (Vb, Vc, vb, vc) can also be found using the sequence !!
11.1 Balanced Three-Phase Voltages
Three-Phase
A sketch of a three-phase voltage source
Generated phase voltages have the same amplitudes
Their phases are different by 120
11.2 Three-Phase Voltage Sources
Three-Phase
Y-connected source ∆-connected source
• The two basic connections of an ideal three-phase source
11.1 Balanced Three-Phase Voltages
Three-Phase
A model of a three-phase source with winding impedance:
(a) A Y-connected source (b) A ∆-connected source
11.3 Analysis of the Wye-Wye Circuit
Three-Phase
A three-phase Y-Y system
11.3 Analysis of the Wye-Wye Circuit
Assume n is ground node
▶ 9
N N a n N b n N c n
0 ga la A gb lb B gc lc C
V V V V V V V
+ + 0
Z Z Z Z Z Z Z Z Z Z
In a balanced three-phase circuit,
(1) Va'n, Vb'n, and Vc'n comprise a set of balanced three-phase voltages.
(2) Same source impedances: Zga = Zgb = Zgc (3) Same line impedances: Zla = Zlb = Zlc (4) Same load impedances: ZA = ZB = ZC
N a n b
N
0
N
n c n
N 0
3V V V V V
V 1 3
+ 0 + V 0 0
Z Z
Z Z
ga la A gb lb B gc lc C
Z
Z Z Z Z Z Z Z Z Z
11.3 Analysis of the Wye-Wye Circuit
balanced three-phase circuit
In a balanced three-phase circuit,
a n N a n
aA
ga la A
b n N b n
bB
gb lb B
c n N c n
cC
gc lc C
a n b n c n
aA bB
0 cC
V -V V
I =
V -V V
I =
V -V V
I =
V V V
I I I I 0
Z Z Z Z
Z Z Z Z
Z Z Z Z
Z
11.3 Analysis of the Wye-Wye Circuit
balanced three-phase circuit
A single-phase equivalent circuit
A single-phase equivalent circuit is used to calculate the line current and the phase voltage in one phase of the Y-Y structure.
[Caution] Current flowing through neutral line (n-N), I0, is 0 not IaA.
11.3 Analysis of the Wye-Wye Circuit
balanced three-phase circuit
AN BN
B AB
BC N CN
CN N
CA A
V V V V V V
V
V V
Line-to-line and line-to-neutral voltages 선(간) 전압 상전압
Line voltage
Phase voltage
Line current is the same as phase current in a Y load
11.3 Analysis of the Wye-Wye Circuit
balanced three-phase circuit
• Phasor diagrams showing the relationship between line-to-line and line-to-neutral voltages in a balanced system.
The abc sequence
AB
BC
C
AN BN CN
AN BN
AN
BN CN BN
30
30
1
CN AN CN
0
0
A
2
3
0 120 1
3 3 3 1
V 0 , V 120 , V
3 3 30
2 2 2 2
V
V V
120
V V
V
V V V
V V V
3
3 3 j
j
j
V V V e j
V j V j V
e
e e
V V V