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14-Oct-11
1Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Example of Transformer Connections and Ground
Currents
14-Oct-11
2Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Y-Y transformer and ground currents
Assume an unbalanced current only in phase “a”and no current in phases “b”and “c”
This could lead to high unwanted earth currents on the primary side of a Y-Y transformer
14-Oct-11
3Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Y-D transformer and ground currents
The Y-D transformer will not either have any ground current on the D side in the same unbalanced situation
14-Oct-11
4Lecture 09 Power Engineering - Egill Benedikt HreinssonY-Y transformer with a D-tertiary and ground currents
The extra D-tertiary winding will prevent the ground current in the same unbalanced situation
A
B
C
A
B
C
c
ab
14-Oct-11
5Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The Autotransformer
14-Oct-11
6Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The Autotransformer
VV11 +V+V22
VV22
NN11
NN22
II11
II11+I+I22
II11 II11+I+I22
II22
II11 VV11NN11
NN22
II11
II12
II22
II11
VV22
“Self” “Conventional”
Rating of windings 1 is
Rating of windings 2 is
22
2
VZ
I=
1 1
2 2
V N aV N
= =1 1 1S V I=
2 2 2S V I=
14-Oct-11
7Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Autotransformer rating
VV11+V+V22
VV22
NN11
NN22
II11
II11+I+I22
II11 II11+I+I22
II22
II11 VV11NN11
NN22
II11
II12
II22
II11
VV22
“Auto” “Conventional”
The capacity of the Auto transformer is: ( )1 1 2 1 1 1 1
1 1(1 ) (1 )S V V I I V Sa a
′ = + ⋅ = + = +
The turns ration: 1 2
21
V Va a
V+′ = = +
For instance if a = 1, the rating is doubled!
14-Oct-11
8Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The Auto - Transformer - Pros/cons -
•No “galvanic” isolation between primary and secondary windings
•More power transformation capacity with the same size of the transformer
•Possibilities to control voltage and reactive power flow
•Widespread applications in power systems
14-Oct-11
9Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Transformer Application and Design Issues
14-Oct-11
10Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Examples of applications of transformers in power systems
• Generator “step up” transformer 20/220 kV
• Transmission 132/220 kV• Station supply transformers 20/4 kV• Control transformers 400/220kV• Substations transformers 132/6kV• Distribution transformers 6/0,4 kV• Measurement transformers• Multi winding transformers
14-Oct-11
11Lecture 09 Power Engineering - Egill Benedikt HreinssonTransformers in Hrauneyjafoss hydro plant
Generator “step up” transformer
14-Oct-11
12Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Transformers in distribution stations
A 3 phase power system
Circuit breakers
Low voltage terminals, 400 V
Disconnectswitches
14-Oct-11
13Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Cooling methods for transformers (OFAF)
OFAF = Oil Forced,Air Forced
14-Oct-11
14Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Cooling Methods for Transformers (ONAN)
ONAN = Oil Natural,Air Natural
14-Oct-11
15Lecture 09 Power Engineering - Egill Benedikt HreinssonCooling Methods for Transformers (ONAF)
ONAF = Oil Natural,Air Forced
14-Oct-11
16Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Different Designs of Transformers
Core Shell
14-Oct-11
17Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The flux in a 1 phase transformer
14-Oct-11
18Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Windings in a “Core”- Transformer
14-Oct-11
19Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Windings in a “Shell”- Transformer
14-Oct-11
20Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The development of transformers 1880-2000
The figure shows how the rating of transformers and maximum voltage has increase during the last century. Note the logarithmic scale
Source: R. Baehr: “Transformer technology. State of the art and trends of future developments”, Electra nr 198, October 2001
14-Oct-11
21Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The development of transformers 1880-2000
The figure show how the specific losses (per kg) of the ferromagnetic core has developed during the last century
Source: R. Baehr: “Transformer technology. State of the art and trends of future developments”, Electra nr 198, October 2001
14-Oct-11
22Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Transformer Purchasing Issues
•Efficiency •Audible Noise •Installation Costs •Manufacturing Facilities •Performance Record
14-Oct-11
23Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Control Devices and Transformers
Controlling the flow of real and reactive power in the network
14-Oct-11
24Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Control Devices and Transformers
•Classical control transformers•Facts devices
– FACTS=Flexible AC Transmission Systems– Using power electronic devices to control the
power flow and operation
14-Oct-11
25Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Control transformers control the flow of either P or Q
Control transformers will change the phase or magnitude of a voltage depending on the circumstances and hence control the real or reactive power flow:
Controltrans-former
aV
bV
cV
b bV V+ Δ
a aV V+ Δ
c cV V+ Δ
14-Oct-11
26Lecture 09 Power Engineering - Egill Benedikt Hreinsson
How is the input connected to the output
• We define the input, output and increment
• In summary:
a a a a
b b b b
c c c c
V V V VV V V VV V V V
Δ + Δ⎡ ⎤ ⎡ ⎤ ⎡ ⎤⎢ ⎥ ⎢ ⎥ ⎢ ⎥= Δ = Δ = + Δ⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎢ ⎥Δ + Δ⎣ ⎦ ⎣ ⎦ ⎣ ⎦
i uV V V
Δu iV = V + V
14-Oct-11
27Lecture 09 Power Engineering - Egill Benedikt Hreinsson
A control transformer or P (δ ⇔ P)
This control transformer adds incremental voltage to the voltage vector The added voltage has a 90 degrees phase difference from the original voltage. Therefore this control transformer controls primarily the real power flow
14-Oct-11
28Lecture 09 Power Engineering - Egill Benedikt HreinssonA control transformer for controlling Q(|V| ⇔ Q)
This control transformer adds incremental voltage vector to the original voltage vector. The incremental voltage vector has the same phase as the original voltage vector. Therefore this incremental change affects primarily the reactive power flow
14-Oct-11
29Lecture 09 Power Engineering - Egill Benedikt Hreinsson3 phase autotransformer with a D tertiary and tap changer
A
B
C
a
b
c
14-Oct-11
30Lecture 09 Power Engineering - Egill Benedikt Hreinsson
FACTS
•FACTS=Flexible AC Transmission Systems– =>Using electronic devices to control the power
flow in an AC system•SVC=Static Var compensators•VSC/CSC=Voltage or Current Source
Converters•Static Synchronous Series Compensator
(SSSC),
14-Oct-11
31Lecture 09 Power Engineering - Egill Benedikt Hreinsson
FACTS (2)
•Static Synchronous compensator (STATCOM)•Static Synchronous Series Compensator (SSSC), •Unified Power Flow Controller (UPFC), •Interline Power Flow Controller (IPFC).•Thyristor-controlled Series Capacitor (TCSC),
14-Oct-11
32Lecture 09 Power Engineering - Egill Benedikt Hreinsson
An example: Interline Power Flow Controller (IPFC).
http://www.pedc.am.gdynia.pl/pedc/materialy/artykuly/journal/j.30.pdf
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01015231
The IPFC interjects a voltage VC in series and the N-1links exchange active power (N = 3 in the picture)
primary line
14-Oct-11
33Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Multi Winding TransformersTransformers with more than 2 windings in each
phase
14-Oct-11
34Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Multi winding transformer
1 1
2 2
n n
dV NdtdV Ndt
dV Ndt
Φ=
Φ=
Φ=
1 1
2 2
n n
V j NV j N
V j N
ωω
ω
= Φ= Φ
= Φ
,i iij
j j
V Na i j
V N= = ∀
14-Oct-11
35Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The per unit system and multi winding transformer
• We assume that the ratio of voltage bases is always equal to the nominal turns ratio for each pair of windings
• Also we assume that the power base is the same for all windings
• Then we get the following circuit model for the ideal multi-winding transformer in the p.u system
,bi iij
jbj
V N a i jNV
= = ∀
,bi bjS S i j= ∀
14-Oct-11
36Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The ideal multi-winding transformer in P.U.
The sum of the currents circulating the flux of a multi-winding transformer must be “almost” zero. This is the Kirchoff’s “like” law
V1
I1
II
2
3
InV3
V2
Vn_ _ _ _
++
++
1
2
3
n1
2
n
kk
I I=
=∑
14-Oct-11
37Lecture 09 Power Engineering - Egill Benedikt Hreinsson
The real multi-winding transformer in P.U.
The sum of the current which links the flux in a multi-winding transformer almost zero. The deviation is the current I0 that magnetizes the transformer. This is almost like the Kirchoffs law
V1
I
I
1
0
II2
3
InV3
V2
Vn
R1 +jX1
G1 +jB1
R2 +jX2
R3 +jX3
Rn +jXn
_ _ _ _
++
++
1
2
3
n
1 02 2
n n
k kk k
I I I I= =
= +∑ ∑
14-Oct-11
38Lecture 09 Power Engineering - Egill Benedikt Hreinsson
Transformers Summary
• Transformer circuit model introduced as a T-link with the leakage/short circuit reactance dominant
• The p.u. system introduced. It must have the same system wide S-base and V-base according to turns ratios
• Then in p.u. all ideal transformers can be removed from model
• 3 phase transformer connections, auto, multi-winding, control transformers introduced