Example of Transformer Connections and Ground Currents

Transcription

1 1 Example of Transformer Connections and Ground Currents

2 2 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

3 3 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

4 Y-Y transformer with a D-tertiary and 4 ground currents A C c C A b a B The extra D-tertiary winding will prevent the ground current in the same unbalanced situation B

5 5 The Autotransformer

6 6 The Autotransformer Self Conventional I 1 I 1 V 1 +V 2 N 1 I 1 V I 1 +I 2 1 I 1 N 1 I 2 N 2 I 1 I 1 +I 2 Rating of windings 1 is Rating of windings 2 is V 2 S S = V 2 V I = V I I 12 I 2 N 2 V V V I 2 2 = Z N = = N a

7 7 Autotransformer rating Auto Conventional I 1 I 1 V 1 +V 2 N 1 I 1 V I 1 +I 2 1 I 1 N 1 I 2 N 2 V 2 V 2 I 2 N 2 I 1 I 1 +I 2 I 12 The capacity of the Auto transformer is: ( ) 1 1 S1 = V1 + V2 I1 = I1V1(1 + ) = S1(1 + ) a a The turns ration: a V + V 1 2 = = a + V2 1 For instance if a = 1, the rating is doubled!

8 8 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

9 9 Transformer Application and Design Issues

10 Examples of applications of transformers in power systems 10 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

11 Transformers in Hrauneyjafoss Power Engineering - Egill Benedikt hydro Hreinsson 11 plant Generator step up transformer

12 12 Transformers in distribution stations Disconnect switches A 3 phase power system Circuit breakers Low voltage terminals, 400 V

13 13 Cooling methods for transformers (OFAF) OFAF = Oil Forced, Air Forced

14 14 Cooling Methods for Transformers (ONAN) ONAN = Oil Natural, Air Natural

15 Cooling Methods for Transformers 15 (ONAF) ONAF = Oil Natural, Air Forced

16 16 Different Designs of Transformers Core Shell

17 17 The flux in a 1 phase transformer

18 18 Windings in a Core - Transformer

19 19 Windings in a Shell - Transformer

20 The development of 20 transformers 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

21 The development of 21 transformers 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

22 22 Transformer Purchasing Issues Efficiency Audible Noise Installation Costs Manufacturing Facilities Performance Record

23 23 Control Devices and Transformers Controlling the flow of real and reactive power in the network

24 24 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

25 Control transformers control the flow of 25 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: V a V b V c Control transformer V V V a b c + ΔV + ΔV + ΔV a b c

26 26 How is the input connected to the output We define the input, output and increment V Δ V V +ΔV a a a a V = V Δ V = Δ V V = V +ΔV i b b u b b V Δ V V +ΔV c c c c In summary: V = V + u i ΔV

27 27 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

28 A control transformer for Power controlling Engineering - Egill Benedikt Hreinsson Q 28 ( 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

29 3 phase autotransformer with Power Engineering a - Egill D Benedikt tertiary Hreinsson 29 and tap changer A C c a B b

30 30 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),

31 31 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),

32 An example: Interline Power Flow Controller (IPFC). 32 The IPFC interjects a voltage V C in series and the N-1 links exchange active power (N = 3 in the picture) g/stamp/stamp.jsp?arnu mber= dynia.pl/pedc/material y/artykuly/journal/j.30.pdf primary line

33 33 Multi Winding Transformers Transformers with more than 2 windings in each phase

34 34 Multi winding transformer = dφ V1 N1 dt = dφ V2 N2 dt = dφ Vn Nn dt V = jω N Φ 1 1 V = jω N Φ 2 2 V = jω N Φ n n V V i j Ni = = aij i, j N j

35 The per unit system and multi winding 35 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 Vbi Ni aij i, j V = N = bj S = S i, j bi bj j

36 36 The ideal multi-winding transformer in P.U. I I V 1 I 1 V n 2 I V n + 3 V n I 1 n = I k = 2 k The sum of the currents circulating the flux of a multi-winding transformer must be almost zero. This is the Kirchoff s like law

37 37 The real multi-winding transformer in P.U. I I 2 3 R 2 +jx 2 R 3 +jx I 1 V 1 R 1 +jx 1 I 0 G 1 +jb 1 R n +jx n V n 2 I n + V 3 V n 1 0 n I I I I = + n k k= 2 k= 2 k The sum of the current which links the flux in a multi-winding transformer almost zero. The deviation is the current I 0 that magnetizes the transformer. This is almost like the Kirchoffs law

38 38 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