three phase circuits - third...
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1ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Three Phase Circuits
2ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Three Phase Circuits
Chapter Objectives: Be familiar with different three-phase configurations and how
to analyze them.
Know the difference between balanced and unbalanced circuits
Learn about power in a balanced three-phase system
Know how to analyze unbalanced three-phase systems
Be able to use PSpice to analyze three-phase circuits
Apply what is learnt to three-phase measurement and residential wiring
3ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Three phase Circuits An AC generator designed to develop a single sinusoidal voltage for each rotation
of the shaft (rotor) is referred to as a single-phase AC generator.
If the number of coils on the rotor is increased in a specified manner, the result is a
Polyphase AC generator, which develops more than one AC phase voltage per
rotation of the rotor
In general, three-phase systems are preferred over single-phase systems for the
transmission of power for many reasons.
1. Thinner conductors can be used to transmit the same kVA at the same voltage, which
reduces the amount of copper required (typically about 25% less).
2. The lighter lines are easier to install, and the supporting structures can be less
massive and farther apart.
3. Three-phase equipment and motors have preferred running and starting
characteristics compared to single-phase systems because of a more even flow of power
to the transducer than can be delivered with a single-phase supply.
4. In general, most larger motors are three phase because they are essentially self-
starting and do not require a special design or additional starting circuitry.
4ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
a) Single phase systems two-wire typeb) Single phase systems three-wire type.
Allows connection to both 120 V and
240 V.
Two-phase three-wire system. The AC sources
operate at different phases.
Single Phase, Three phase Circuits
5ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Three-phase Generator The three-phase generator has three induction coils placed 120° apart on the stator.
The three coils have an equal number of turns, the voltage induced across each coil
will have the same peak value, shape and frequency.
6ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Three-phase Voltages
Three-phase four-wire system
Neutral Wire
A Three-phase Generator
Voltages having 120° phase difference
7ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Three phase Voltages
a) Wye Connected Source b) Delta Connected Source
a) abc or positive sequence b) acb or negative sequence
0
120
240
an p
bn p
cn p
V V
V V
V V
= ∠ °
= ∠ − °
= ∠ − °
0
120
240
an p
bn p
cn p
V V
V V
V V
= ∠ °
= ∠ + °
= ∠ + °
Neutral Wire
8ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Three phase Loads
a) Wye-connected load b) Delta-connected load
1 2 3
Conversion of Delta circuit to Wye or Wye to Delta. Balanced Impedance Conversion:
Y
a b c
Z Z Z Z
Z Z Z Z∆
= = =
= = =
1Z 3 Z
3Y YZ Z
∆ ∆= =
A Balanced load has equal impedances on all the phases
9ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Three phase Connections
Both the three phase source and the three phase load can be
connected either Wye or DELTA.
We have 4 possible connection types.
• Y-Y connection
• Y-∆ connection
• ∆-∆ connection
• ∆-Y connection
Balanced ∆ connected load is more common.
Y connected sources are more common.
10ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Wye-wye Connection
A balanced Y-Y system, showing the source, line and load impedances.
Source ImpedanceLine Impedance
Load Impedance
11ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Wye-wye Connection
Phase voltages are: Van, Vbn and Vcn.
The three conductors connected from a to A, b to B and c to C are called LINES.
The voltage from one line to another is called a LINE voltage
Line voltages are: Vab, Vbc and Vca
Magnitude of line voltages is √3 times the magnitude of phase voltages. VL= √3 Vp
Line current In add up to zero.
Neutral current is zero:
In= -(Ia+ Ib+ Ic)= 0
12ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Wye-wye Connection
Magnitude of line voltages is √3 times the magnitude of phase voltages. VL= √3 Vp
3
0 , 120 ,
30
3 90
3 21
120
0
an p bn p cn p
ab an nb an bn
bc bn cn
ca cn an
p
p
an bn p
V
V V V V V V
V V V V V
V V V
V V V
V
V VV
= ∠ ° = ∠ − ° = ∠ + °
= + = − =
= − =
∠ °
∠ − °
= + ∠ −= − = °
Line current In add up to zero.
Neutral current is zero:
In= -(Ia+ Ib+ Ic)= 0
13ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Wye-wye Connection Phasor diagram of phase and line voltages
= 3 3 3 = 3
L ab bc ca
an bn cn p
p an bn cn
V V V V
V V V V
V V V V
= = =
= =
= = =
14ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Single Phase Equivalent of Balanced Y-Y Connection
Balanced three phase circuits can be analyzed on “per phase “ basis..
We look at one phase, say phase a and analyze the single phase equivalent circuit.
Because the circuıit is balanced, we can easily obtain other phase values using their
phase relationships.
ana
Y
VI
Z=
15ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
16ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Wye-delta Connection
ABAB
BCBC
CACA
VI
Z
VI
Z
VI
Z
∆
∆
∆
=
=
=
Line currents are obtained from the phase currents IAB, IBC and ICA
3 30
3 30
3 30
a AB CA
b BC AB
c CA BC
AB
BC
CA
I I I
I I I
I
I
I II
I
= − =
= − =
∠ − °
∠ − °
∠− −= = ° 3
L a b c
p AB BC CA
L p
I I I I
I I I I
I I
= = =
= = =
=
Three phase sources are usually Wye connected and three phase loads are Delta
connected.
There is no neutral connection for the Y-∆ system.
17ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Wye-delta Connection
3
Z∆
Single phase equivalent circuit of the balanced Wye-delta connection
Phasor diagram of phase and line currents
3
L a b c
p AB BC CA
L p
I I I I
I I I I
I I
= = =
= = =
=
18ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Delta-delta Connection
Both the source and load are Delta connected and balanced.
, ,a AB CA b BC AB c CA BC
I I I I I I I I I= − = − = −
, ,BC CAABAB BC CA
V VVI I I
Z Z Z∆ ∆ ∆
= = =
19ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Balanced Delta-wye Connection
30
3
pV∠− °
Transforming a Delta connected source
to an equivalent Wye connection Single phase equivalent of Delta Wye connection
20ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Power in a Balanced System The total instantaneous power in a balanced three phase system is constant.
2 cos( ) 2 cos( 120 ) 2 cos( 120 )
2 cos( ) 2 cos( 120 ) 2 cos( 120 )
2 cos( )cos( ) cos( 120 ) cos( 120 ) cos( 120 )co
AN p BN p CN p
a p b p b p
a b c AN a BN b CN c
p p
v V t v V t v V t
i I t i I t i I t
p p p p v i v i v i
p V I t t t t t
ω ω ω
ω θ ω θ ω θ
ω ω θ ω ω θ ω
= = − ° = + °
= − = − − ° = − + °
= + + = + +
= − + − ° − − ° + + °[ ]s( 120 )
1cos cos [cos( ) cos( )] Using the identity and simplif
The instantenous power is not function of time.
The total power behav
n2
e
yi g
3 cos p p
t
A B A B A B
p V I
ω θ
θ
− + °
= + + −
=
s similar to DC power.
This result is true whether the load is Y or connected.
The average power per phase .3
cos 3
p p p
p
p
pP I
P
V θ
=
= =
∆
21ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Power in a Balanced System
The complex power per phase is Sp. The total complex power for all phases is S.
p p
p p p p p
2
p p p p
Complex power for each phas
3 cos
1 1= cos = sin
3 3
S V I
3 3 cos 3 cos
3 3 sin 3 sin
S=3S 3V
e
I 3
p p
p p p p p p p
a b c p p p L L
p p p L L
p
p V I
P p V I Q p V I S V I
P jQ
P P P P P V I V I
Q Q V I V I
I Z
θ
θ θ
θ θ
θ θ
∗
∗
=
= = =
= + =
= + + = = =
= = =
= = =
p L
2
p
p , , and are
Total
all rm
complex powe
s values, is the load impedance angl
3
3
r
eL
p
L L
V
Z
P jQ
I
V
V I
I
V θ
θ
∗
= + = ∠S
22ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Power in a Balanced System
Notice the values of Vp, VL, Ip, IL for different load connections.
2
p2
p p p
p
p
p L, , and are all rms values, is the load impe
To
da
3S=3S 3V I 3
nce
al c
an
3
omplex ower
gle
p
L
p
p
L L
V
VI Z
Z
P
V
V I
I
jQ
I θ
θ
∗
∗= = =
= + = ∠S
VLVL
VL
Vp Vp
VpIp
Ip Ip
VL
Vp
Ip
VL
VL Vp
Vp
Ip
Ip
Y connected load. ∆ connected load.
3L p L p
V V I I= = 3L p L p
V V I I= =
23ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Power in a Balanced System
24ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Single versus Three phase systems Three phase systems uses lesser amount of wire than single phase systems for the
same line voltage VL and same power delivered.
a) Single phase system b) Three phase system
2 2
'2 '2
Wire Material for Single phase 2( ) 2 2(2) 1.33
Wire Material for Three phase 3( ) 3 3
r l r
r l r
π
π= = = =
If same power loss is tolerated in both system, three-phase system use
only 75% of materials of a single-phase system
25ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
26ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
VL=840 V (Rms)
Capacitors for pf
Correction
IL
27ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
7365050.68A
3 3 840
Without Pf Correction
L
L
SI
V= = =
28ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Unbalanced Three Phase Systems An unbalanced system is due to unbalanced voltage sources or unbalanced load.
In a unbalanced system the neutral current is NOT zero.
Unbalanced three phase Y connected load.
Line currents DO NOT add up to zero.
In= -(Ia+ Ib+ Ic) ≠ 0
29ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
30ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Three Phase Power Measurement
Two-meter method for measuring three-phase power
31ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Residential Wiring
Single phase three-wire residential wiring
32ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Problem 12.10 Determine the current in the neutral line.
UNBALANCED LOAD
NEUTRAL CURRENT IS NOT
ZERO
33ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Problem 12.12 Solve for the line currents in the Y-∆∆∆∆ circuit. Take Z∆∆∆∆
=
60∠∠∠∠45°Ω°Ω°Ω°Ω.
SINGLE PHASE EQUIVALENT
CIRCUIT
34ELEC 24409: Circuit Theory 2 Dr. Kalyana Veluvolu
Problem 12.22 Find the line currents Ia, I
b, and I
cin the three-phase network
below. Take Z∆
= 12 - j15Ω, ZY
= 4 + j6 Ω, and Zl
= 2 Ω.
ONE DELTA AND ONE Y
CONNECTED LOAD IS
CONNECTED
TWO Loads are parallel if they are
converted to same type.
Delta connected load is converted to
Y connection.