IEEMA-Basics of Dual Ratio Transformers

T

Article

lntroducti0n

Nr: Total number of turns in secondary

Dual ratio transformer is used both in utilities as well

Nr3: Total number of turns connection

in primary for 33

kV

Nr2: Total number of turns connection

in primary lor 22

kY

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. ln these cases, dual ratio transformer is specified, with primary having two voltage levels. The article explains the concepts behind functioning of dual ratio transformer.

The secondary voltage (11 kV) is same for both 33 kV and 22 kY primary connections. AT balance

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. Ir3 x N.3

/

= Ir2x

Nr2

= I, x N.

= (lp2 lIr3) =

(525 / 350)

= 1.5 ---- (1)

Pfimary Gonnection

(Np3

For illustration purposes, typicaltransformer used in Mumbai Distribution System is considered. The rating of transformer is 33 - 22 I 11 kV 20MVA, Dzn10.

Series Connection: Primary AT at 33 kV

Np'?)

= 20 / ({3 x 1 1) = 19594 Primary current at 33 kV: Ir. = 20 / ({3 x 33) =

=

Ip3

x 6N

Secondary current Is

N

350A

Primary current al 22 kV: 5254

Ir, = 20 I (S x

ZZ1 =

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

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Fig. 1: Selector switch for winding connection

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Article

Series - Parallel Connection: Primary AT at 22 12 xrr2 x N + 4 x {(Ir' x /2) N}I

ky =

=4xIp2xN =4x1.5Ir3xN = Ip3x6N Thus, primary AT is the same for both 33 kV and 22 kV connections.

U/I equality As shown in Fig. 2, the series connection or series

-

parallel connection satisfies the Volts

/

Turn

equality.

ln both cases, flux q

--r

il

=

U/1

= 5.5/

N

ssxv

---T-

-+-

N

5.5 KV

5.5 KV

-+--

N

N

ssKv

5.5 KV

-rh

xv

,h

s.s

N

-+-

5.5 KV

-+N-+-+N

t

5.5 KV

Fig. 2: VIT equality for both types

of connections

Tap $ten $ize

primary winding has taps, the step size as a

percentage of rated voltage will be different for both connections. Assume AN turns are shorted in both cases. Refer Fig. 3. Step size for 33 kV

= Step size tor 22 kY = A2 = 1.5 As

= L2 = A3

(AN / 6N) x 100% (AN

/ 4N) x

=

1.8o/o

This is further illustrated with an example in Table 1. The transformer rating is 20 MVA, gg- 22111kV. The onload tap changer (OLTC) has 10 taps (+5.4% to -10.8/" tor 22 kV and t-8.6o/o to -2.2/o for 33 kV). The number of turns shorted is same tor 33 I 22 ky. Voltage change for each tap is 396V.

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 al22 kY will be higher than at 33 kV. For example, test results for a 20 MVA transformer are: Ratio

No load loss (kW)

33/11 kV 22l11kV

12.17

74.81

12.17

86.59

Load loss (kW)

For normal design, load loss can be guaranteed at one ratio only..The loss at other ratio will change

1OO/"

correspondingly. Refer Fig. 4.

---TN

lf A3 is 1.2o/", L2

[osses

+

lf

Secondary voltage at all taps - 1l t4/ Voltage change for one tap = 396V at ail taps and for 22 kV and 33 kV.

----r

55KV

---*I

55KV

I

N

55KV

---*I

I

---*N I

5.5 Kv

lr

I

N

5,5 KV

----*I

N

5,5 KV

--+I

N

N

55KV

-+-

N

J

I ,r"

5.5 KV

N

--TI

a2

I

5.5 KV

.,

5,5 KV

+

Flg. 3; Sfep size as percentage of rated voltage

$6

Fig. 4: Load /oss ratlos

February

2014 Ig*95't'tLE UtJS

ffi{--

Article

Scrutineer's comments

The author should discuss the advantages and disadvantages of dual ratio transformer including cost,

space required.for installation etc., when compared to single ratio units.

Authors'Reply Dual ratio transformer is procured only in cases the applied voltage could be from any of the two sources with different voltage levels. ln 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. 5; Fesrstance levels

Let R be the resistance per N turns. Load loss at 33 kV = P. = I.2 x 6N x R

Load loss (1212)2

al22kV

x N x Rl

-

P,

There are not any disadvantages that are specific only to dual ratio transformer.

= l2xIrz x N x Rl + l4x

=lr2x3NxR PzlPs= (lrl\)2 l2 = 1.52 l2 = 1.125 Load loss al22 kY will be 12.50/" higher than at 33 kv.

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 TOyo (0.7R). The corresponding increase in cross section is 43o/o (110.7 Load loss at 33 kV

= P, =

I.2 x 4N

= 1.43).

xR+2x

I.2 x

N x 0.7R

Load loss

Fig. 6: Voltage selection can be done using rotating wheel

5.4Is2 x N x

R

al22kY = P"= l2xlrz x N x 0.7R1 +

x(Irl2)2 x N x

[4

Rl

=2.4xlr2xNxR =2.4x(1.sls)rxNxR

-

5.4Ig2 x N x R

Load losses at 33 kV and 22 kV are the same. This is achieved by increasing the cross section of

a part of the winding. This will result in increased

Regarding the cost, the cost ol 33-221 11 kV,2Ol25 MVA dual ratio transformer is about 15% higher than the cost of 33 / 11 kV, 2OIZS MVA single ratio transformer.

The foot prints for both transformers are nearly the same and there is no significant ditference. Approximate overall dimensions of the above transformers are 6.5 x 5.5 x S.2 M (L x B x H).

cost of transformer, Unless both ratios are used for approximately the same amount of time, it is not recommended to specify the same load losses at both the voltage ratios.

Gonclusion 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.

a€le

OUfnCa February

2014

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