Example of Transformer Connections and Ground Currents · Example of Transformer Connections and...

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