Autotransformer. 20MVA (115/69kV) McGraw-Edison Substation Auto-Transformer (Y-Y) (Source: 25

Transcription

1 Autotransformer A conventional twowinding transformer can be changed to an autotransformer by connecting its two coils in series (note: four different ways to connect). The connection may use a sliding contact to providing variable output voltage. (Source: EPRI Power System Dynamic Tutorial) 20MVA (115/69kV) McGrawEdison Substation AutoTransformer (YY) (Source: 25

2 N 1 The primary and secondary windings have a common terminal A, and hence are not isolated from each other E 2 /E 1 =E CA /E BA =N 2 /N 1 (There is always N 1 >N 2 ) I 1 N 1 =I 2 N 2 I 1 (N 1 N 2 )=(I 2 I 1 )N 2 E 1 I 1 =E 2 I 2 An autotransformer has a smaller size (or equivalently, a higher kva rating for the same size) but loses insulation between primary and secondary windings 26

3 Conventional transformer connected as an autotransformer (Example 11 2) Reconnect the transformer to obtain 600V / 480V 600V / 720V 120V / 480V 120V / 720V I 1 =15000/600=25A I 2 =15000/120=125A When a transformer connects a source (on the primary side) with a load (on the secondary side), it is the load (current flows into terminal ) of the primary circuit and is the source (current flows out of terminal ) of the secondary circuit, so two winding currents (I 1 and I 2 ) have opposite directions. 27

4 I 2 =125A 600V H 1 X 1 120V 600V I 2 I 1 =100A X 2 X 1 H 1 120V I 1 =25A 480V H 2 X 2 H 2 I 2 =125A S i = =60kVA S o = =60kVA I 1 =25A 600V I 2 I 1 =150A X 1 X 2 H 1 H 2 120V I 1 =25A 720V 120V I 2 I 1 =100A S i = =90kVA S i = =12kVA S o = =90kVA S o = =12kVA H 2 H 1 X 1 X 2 600V I 2 =125A 480V The maximum apparent power S max =max( E 1, E 2 ) ( I 1 I 2 )= ( E 1 E 2 ) max( I 1, I 2 ) 28

5 Three Phase Transformers A bank of three singlephase transformers connected in Y or configurations Four possible combinations: YY,, Y and Y Y: lower insulation costs, with neutral for grounding, 3 rd harmonics : more insulation costs, no neutral, no 3 rd harmonics 29

6 3 rd harmonics problem with three phase transformers A configuration provides a closed path for 3 rd harmonics, or in other words, all triple harmonics are trapped in the loop. The 120 o phase difference between the fundamental harmonic of I 1 an I 2 and I 3 becomes 360 o (in phase) for the 3 rd harmonic Then by KCL, I a (3 )=I 1 (3 )I 3 (3 )=0; also I b (3 )=I c (3 )=0 Distorted waveform Fundamental 3 rd harmonic, 5, 7, 3, 5, , 3, 5, 7 9, 5, 7, 3, 5, 7, 9, 5, 7 30

7 Three phase transformers: Y Y connection No phase shift between the incoming and outgoing line voltages Without a configuration to trap the 3 rd harmonic, it has seriously distorted voltages due to the 3 rd harmonics. Two solutions: Ground the neutrals of the primary winding and the source Provide each transformer with a tertiary winding, and connect 3 tertiary windings in delta 31

8 Three phase transformers: connection Each transformer, considered alone, acts as if it were placed in a singlephase circuit. Because E H1H2 and E X1X2 are in phase for each single transformer, line voltages E AB with E 12, E BC with E 23, and E CA with E 31 are in phase. A balanced threephase load produces balanced line currents in line 123 and lines ABC The power rating is 3x of a single transformer. Under maintenance or emergency conditions, one transformer may be absent (i.e. opendelta connection, whose power rating becomes 86.7% of the original; see Exp 123) 3 I p 3 I s 32

9 Three phase transformers: Y(or Y ) connection X 2 terminals of 3 secondary windings are connected to a common neutral N. Because E H1H2 and E X1X2 are in phase for each single transformer, E AB with E 1N, E BC with E 2N, and E CA with E 3N are in phase. There is a 30 o phase shift between the line voltages of the incoming (E AB ) and outgoing (E 12 ) lines Y is commonly used for voltage stepup transformers while Y is commonly used for voltage stepdown transformers 3 I p E 3N E 2N 30 o E N2 E 1N 33

10 Calculations involving 3 phase transformers For any 3phase transformer bank, assume that the primary and secondary windings are both connected in Y configuration (if not, transform it to Y by Z Y = Z /3) Consider only one transformer (single phase) of the assumed YY transformer bank Its primary and secondary voltages are both the linetoneutral voltages of the incoming and outgoing lines Its nominal power rating and load are 1/3 of the 3phase transformer bank 34

11 Examples 12 1, 12 2, 12 3, 12 6 &

12 Phase shifting Principle A 3phase system can easily shift the phase angle of a voltage to create 2, 6, and 12phase systems used in power electronic converters and controllers Phase shifting (performed by phaseshifting transformers) is also used in power flow control over long transmission lines 36

13 Phase shifting transformers A special type of 3phase autotransformer that shits the phase angle between the incoming and outgoing lines without changing the voltage ratio. Twist all incoming line voltages by angle (0~ 20 o ) without changing their magnitudes 37

14 Voltage Transformers and Current Transformers Voltage transformers (VTs), also called potential transformers (PTs), are installed on HV lines to measure linetoneutral voltage 7kVA 80.5kV 50/60Hz PT Current transformers (CTs) are used to measure or monitor the current in a line for system measurement and protection 5kVA, 100A/5A 60Hz CT 38