Dual Ratio Transformer Monograph

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Basics of Dual Dual Ratio Transformer Dr K Rajamani and Bina Mitra, Reliance Infrastructure Ltd., Mumbai 1.0

Introduction Dual ratio transformer has been used both in utilities as well as industries where the input can be from two sources at different voltage levels. As an example, some power transformers in Reliance Mumbai Distribution System are either connected to upstream 33 kV or 22 kV supply. These voltage levels have evolved over time. Similarly in industries, the old connection to grid can be at one voltage level and the additional (new) connection can be at different voltage level. However the user may like to use either of connection as per availability of supply. In these cases, dual ratio transformer is specified, with primary having two voltage levels. The article explains the concepts behind functioning of dual ratio transformer.

2.0 2.0

Primary conn ection For illustration purposes, typical transformer used in Mumbai Distribution System is considered. The rating of transformer is 33 – 22 / 11 kV, 20MVA, Dzn10. Secondary current,  I s =

20

√  3 x 11) ( √   = 1050 A Primary current at 33 kV, Ι  P 3 =

20

√  3 x 33) ( √   = 350 A Primary current at 22 kV, Ι  P 2  =

20

√  3 x 22) ( √   = 525 A Transformer theory demands the following: (a) Ampere Turns (AT) balance: Primary AT = Secondary AT (b) Volts per turn (V/T) equality: V/T of Primary = V/T of Secondary  N S  S:  Total number of turns in secondary 3

 N P : Total number of turns in primary for 33 kV connection 2

 N P : Total number of turns in primary for 22 kV connection

The secondary voltage (11 kV) is same for both 33 kV and 22 kV primary connections. 1 of 7 February 2014, IEEMA Journal, Page 95 to 97

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2.1

AT Balanc e Primary is connected either in series connection or series – parallel connection. The selector switch for winding connection is mounted on tank and is operated off line. Refer Fig 1.

Fig. 1

Ι  P3 x N P3 = Ι  P2 x N P2 = Ι  S  x N S  (N P  / N P ) = (Ι  P  / Ι  P ) 3

2

2

3

= (525 / 350) = 1.5

…………….(1)

Series Connection: Primary AT at 33 kV = Ι  P3 x 6  N  Series – Parallel Connectio n: Primary AT at 22 kV = [2 x Ι  P2 x N + 4 x {(Ι  P2 / 2 ) x N}] = 4  x Ι  P  x N 2

= 4 x 1.5 Ι  P  x N 3

= Ι  P  x 6 N 3

Thus Primary AT is same for both 33 kV and 22 kV connections.

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2.2

V/T equali ty  As shown in Fig 2, the series connection or series – parallel connection satisfies the Volts / Turn equality. In both cases, flux Φ = V  = 5.5 T

N

Fig. 2 3.0

Tap Step Size If primary winding has taps, the step size as percentage of rated voltage will be different for both connections. Assume  ∆N turns are shorted in both cases. Refer Fig 3.

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Fig. 3 Step size for 33 kV,  ∆3 = (  ∆N / 6N)  x  100% Step size for 22 kV,  ∆2  = (  ∆N / 4N)  x  100% 2 

 ∆

 = 1.5  ∆3

If  ∆3 is 1.2%,  ∆2  = 1.8% This is further illustrated with an example in Table-I. The transformer rating is 20 MVA, 33-22/11kV. The on load tap changer (OLTC) has 10 taps (+5.4% to 10.8% for 22 kV and +3.6% to -.7.2% for 33 kV). The number of turns shorted is same for 33 / 22 kV. Voltage change for each tap is 396V. Table-I Tap No

Tap Step (%)

Primary Voltage 22 kV

Tap Step (%)

Primary Voltage 33 kV

1

+5.4

23,188

+3.6

34,188

2

+3.6

22,792

+2.4

33,792

3

+1.8

22,396

+1.2

33,396

4(N)

0

22,000

0

33,000

5

-1.8

21,604

-1.2

32,604

6

-3.6

21,208

-2.4

32,208

7

-5.4

20,812

-3.6

31,812

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Table-I Tap No

Tap Step (%)

Primary Voltage 22 kV

Tap Step (%)

Primary Voltage 33 kV

8

-7.2

20,416

-4.8

31,416

9

-9.0

20,020

-6.0

31,020

10

-10.8

19,624

-7.2

30,624

Secondary Voltage at all taps – 11 kV Voltage change for one tap = 396V at all taps and for 22 kV and 33 kV. 4.0

Losses No load loss will be same for 33 kV and 22 kV since volts per turn (flux) is same in both cases. However load loss at 22 kV will be higher than at 33 kV. For example, test results for a 20 MVA transformer are given below: Ratio

No load loss (KW)

Load loss (KW)

33 / 11 kV

12.17

74.81

22 / 11 kV

12.17

86.59

For normal design, load loss can be guaranteed at one ratio only. The loss at other ratio will change correspondingly. Refer Fig 4. Let R be the resistance per N turns. Load loss at 33 kV, P3 = Ι  32 x 6 N x R Load loss at 22 kV, P2 = [2 x Ι  2  x N x R] + [4 x (Ι  2 /2)  x N x R] 2

2

= Ι  2  x 3N x R 2

P2 / P3 = (Ι  2 / Ι  3)  / 2 2

= 1.5 2 / 2 = 1.125 Load loss at 22 kV will be 12.5% higher than at 33 kV. Fig. 4

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It is possible to design a transformer with same load loss at 33 kV and 22 kV. This will be a special design in which part of the winding will have different cross section. Refer Fig 5. The resistance of N turns of part winding is only 70% (0.7R). corresponding

increase

in

The cross

section is 43% (1/0.7 = 1.43). Load loss at 33 kV, P 3 = Ι  3  x 4N x R + 2 x Ι  3  x N x 0.7R 2

2

= 5.4 Ι  3  x N x R 2

Load loss at 22 kV, P 2  = [2 x Ι  2  x N x 0.7R] 2

+ [4 x (Ι  2 /2)  x N x R] 2

= 2.4 x Ι  2  x N x R 2

= 2.4 x (1.5Ι  3)  x N x R 2

= 5.4 Ι  3  x N x R 2

Fig. 5

Load losses at 33 kV and 22 kV are same. This is achieved by increasing the cross section of part of the winding. This will result in increased cost of transformer. Unless both ratios will be used for approximately same amount of time, it is not recommended to specify same load losses at both the voltage ratios. 5.0

Conclusion Dual winding transformers are used only in special circumstances and intricacies involved are not generally known. The concepts behind dual winding transformer operation are explained here. This article will be very helpful to practicing engineers during specification stage.

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Comments from Scrutineers’ and Aut hor’s Replies

1.0

Scrutin eers’ Comment The author should discuss the advantages and dis advantages of dual ratio transformer including cost, space required for installation etc; when compared to single ratio units.

 Au th or s’ Repl y: Dual ratio transformer is procured only in cases the applied voltage could be from any of two sources with different voltage levels. In this case, the same transformer could be used to connect two different voltage sources. The voltage selection (carried out in off line mode) is done easily, for example, using a rotating wheel (see Fig 6).

Fig. 6 There are no disadvantages that are specific only to dual ratio transformer. Regarding the cost, the cost of 33-22/ 11 kV, 20/25 MVA dual ratio transformer is about 15% higher than the cost of 33 / 11 kV, 20/25 MVA single ratio transformer. The foot prints for both transformers are nearly same and there is no significant difference. Approximate overall dimensions of the above transformers are 6.5 x 5.5 x 5.2 M (L x B x H).

7 of 7 February 2014, IEEMA Journal, Page 95 to 97