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About creating  a cabinet for a  transformer

Transformer design in a cabinet

First what you need to know is the ambient t temperature Ta ( normally 40C) outside of the cabinet. On the next step you have to set the average air temperature rise dTc.av  around the transformer. Tthis temperature rise can be calculated as follows:

dTc.av = (dTc.max + dTc.min)/2

where:
dTc.min => min. temperature rise on the bottom of the transformer (normally on the bottom of the cabinet,  5-7C)
dTc.max => max. temperature rise on the top  of the transformer. If the transformer insulation class is F or higher it is optimal to set dTc.max = 30C. Note that the cabinet design calculates dTc.max but in the transformer design you have to set dTc.av.

 Now you know the range of the value dTc.av = 10 - 18 C. This value has to be set in the Large Transformers Program:

 The cabinet design supports 5 different cabinet types:

In order to select a cabinet type for your application you can use the following diagram:

Assume you would like to have dTc.max = 25C at  the loss dansity q = 50W/m2 on the cabinet surface then you can use the cabinet type 0. For q = 300W/n2 you need to use the cabinet type 4.

The loss density q  is calculated:

q = P/A

where:
P => total losses in the cabinet in W
A=> Cabinet cooling surface in m2 (without the bottom side)

t is recommended to design the transformer by using the design criterion Active Short Circuit Voltage Ucca. With this criterion you can presribe the value of the transformer losses. Also, to keep the total transformer losses low it is recommended to use a good steel quality.

Cabinet design

Here is the input form for the design of a cabinet

In the yellow output fields are the resulta of the transformer design:

Wt =>Ttransformer width
Ht => Transformer height
Dt => Transformer depth
P => Total transformer losses
dTc.av => Prescribed, average air temperature rise around the transformer: If the ambient temperature is
 40C then the air temperature aroubd the transdformer is Ta + dTc.av = 55C..

Note that the min. dimensions of the cabinet must be bigger than:
Wc>Wt/0.7
Hc>Ht/0.8
Dc>Dt/0.7

Also, note that the cabinet design supports the max. loss density q.max as follows:

Cabinet type

dTc.max

q.max

0

35

90

1

35

120

2

35

120

3

35

150

4

20

450

After you have selected the cabinet type and set the cabinet dimensions Wc,Hc and Dc the program will calculate the current loss dancity q with the transformer total losses P. If the calculated current loss dancity q is smaller than q.max then you can increase the loss density if needed  If q>q.max then the program will stopp running and return to the start position..

Cabinet type 0

This cabinet type is completly closed. The heat transfer runs only over the cabinet surface. Its thermal properties depends on the size of the survace and the fill factot Kv = Wt Ht Dt/Wc Hc Dc.  For the optimal thermal property of the cabinet the max. fill factor Kv.max is limited to the value 0.4.

Here is the typical field of the temperature rise at q=90W/m2 and Kv = 0.4:

  0 Wc 0.15 Wc and 0.85 Wc 0.5 Wc
=.9 Hc 12 C 29 C 35 C
0.1 Hc 5 C 7 C 10 C

Cabinet type 1 and 2

This cabinet type has the oprming on the top and botton of the cabinet surface. The heat transfer runs over the cabinet surface and over the heated air flow through the openings (holes, gaps,shots, gill clefts) . Its thermal properties depends on the size of the cabinet survace and the opening factot Ka = Ao/Ac , where:

Ac = Wc Dc + 2 Hc (Wc+Dc)
Ao => survace of the opening on the top of the cabinet  surface .

Note that the cabinet design supports Ka.max 0.5%.

Here is the typical field of the temperature rise at q=120W/m2 and Ka = 0.5%:

  0 Wc 0.15 Wc and 0.85 Wc 0.5 Wc
=.9 Hc 10 C 20 C 30 C
0.1 Hc 3 C 5 C 10 C

If the bottom side is open, the transformer distance to the vertical cabinet walls bigger than 50mm and the distance of the cabinet roof to the vertical cabinet walls bigger than 30mm thenthe cooling condition of the transformer are very similat to the case without the cabinet.

Cabinet type 3

This cabinet type is completly closed. The heat transfer runs only over the cabinet surface. Its thermal properties depend on the size of the survace and the air flow m  = number of cabinet volumens per hour. In this cabinet type the transformer cooling and the heat transfer to the inside surface of the cabinet are better than in the cabinet type 0. Note that you have to set the air speed in the transformer design,

If the ambient temperature Ta is 40C, do not use the max. air temperatur rise higher than 35C  because the temperature of the cabinet survace can be higher than 55C.

Cabinet type 4

The heat transfer runs  over the air flow. Its thermal properties depend on the air flow m => number of cabinet volumens per hour. In this cabinet type the transformer cooling and the heat transfer to the inside surface of the cabinet are better than in the cabinet type 3.  Note that you have to set the air speed in the transformer design,

Air speed in m/s = m Hc Wc Dc /(Wc Dc - Wt Dt)/3600

Note that the cabinet design calculates the max. air temperature rise in the cabinet dTc.max. In order to compare the calculated dTc.av use the formula:

dTc.av = (dTc.max+dTc.min)/2

 

 

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