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Designing a 1600kVA/35kVA, 50Hz Distribution Oil Transformer

General Information

Technical Specification

Input voltage

3 x 35000/20230V, star
sine wave

Transformer output voltage

3 x 690/400V, star

Line output current

3 x 1340A,
continuous operating mode



Average oil temperature


Max. temperature rise

max. Cu-winding losses
 at 75C


18000W => 1.125%

Short-circuit voltage


Short-circuit voltage


Max. core losses

3200W => 0.2%

Max. no-load current


Test Voltage at 50Hz, 1 minute

Primary 85kV, outside
Secondary 4kV, inside

Steel & Core Assembly M5, annealed, strips for alternated stacking
(4x45+3x90 per shape),
"round" cross section with 8 steps
Core Size

Optimized for minimal material price for:

Cu_Price/Fe_Price = 2

with Cu-winding



Creating Input

4 input screens are used to set the input parameters for designing a transformer:

         Winding parameters per limb




and 3 screens for selection and set up of material :




Criteria and Parameters of Design

 The design of a distribution transformer is always framed by 5 criteria which have to be put into effect simultaneously:

        Short-circuit voltage

        Winding losses at 75 C

        Winding temperature rise

        Core losses

        No-load output current

Under this condition the first step is the optimizing the core size to match the above mentioned prescribed design criteria for the optimal material price using some additional parameters such as:

        Cooling media

        Testing voltages

        Steel quality and core assembly

        Winding type and wire type & material

        Cu/Al and Fe price relationship

Normally the user of this software will create an optimized core family for a typical design criteria and parameters and select a desired core per click. In order to demonstrate the procedure for core optimization, note that the following parameters of optimization are a summery of 5-6 versions:

        Max. winding losses at 75 C = 18000W

        Inductive short-circuit voltage = 6.4 %

        Max. temperature rise 25 K
For 18000W @ 25
K you need a very big cooling surface  using the vertical and horizontal cooling channels in both windings. The optimal windings construction is presented in the next picture. Note that the secondary winding can be realized by using foil with 4 cooling channels within the winding and approx. 40% more Cu material for the outside primary winding.

        Max. core losses = 3200 W

        Max, no-load current = 1.3%
These 2 criteria of design can be easy realized with annealed strips of M111 (M6) grain oriented steel at the induction 1.6T with the following shape and 8 steps "round" cross-section:

        For 85kV, 50Hz, 1 minute  and the power 1600kVA test voltage the following min. spacing is recommended:

a01=17mm; a12=27mm ;a22=30mm
δ01=δ12=5mm (tubes)
δ22=3mm ; δш=2mm ( 2 x overlaped to increase the creaping distance to the yoke)
lц1=lц2=50mm (tube width over the windings))

Note that the creeping distances between the windings and the HV-winding and the core have to be bigger than 125mm.


Windings parameters per limb


The primary is created in star connection. The sine wave input voltage is 20230V .
There are no voltage harmonics and there is no duty cycle operation mode.
The primary will be manufactured with Cu-flat wire in disc winding technology (view picture above) with the horizontal cooling channel of h=5mm. The advantage of the disc windings is low voltage per turn without any partial discharging problems. In order to suppress the high line voltage  discharge the turns of the first and last disc can easily add stronger insulation.

The following picture describes the manufacturing of a continuous disc winding:



The secondary winding is set inside. It is wound with 2 parallel connected "bifilar screw" strands  (view picture above). Between each turn there are horizontal cooling channels. h=5mm. In order to avoid the circulating currents in parallel connected wires per strand you have to use the transposition through the rotation of the wire position in the strand in accordance with some rules:


The sine wave output voltage is 399V.
The rms output current  is 1336Arms.  There are no current harmonics:
Also, there is no duty cycle operation mode on the secondary.
With the eddy current losses factor (RacRdc) 1.4 the number of parallel connected flat wires per strand will be limited . Note that at this point of the design you cannot prescribe the wire size  . You can select only the wire or family which the program must use in order to select the suitable wires for your application.


On this input screen you can :

         select and manipulate the selected steel M97, 030mm (M5l)

         set the operating induction (1.6T) and the frequency (50Hz)

         select the core assembly

         and prescribe the core selection out of an input file. This option will not be used because the core size has to be optimized .


The cooling medium is oil with the average  temperature  55C. The cooling surface of the core is increased by using 4 L-brackets on the core. The minimum distance between the primary windings of 2 phases is 30mm. There is no flange but both windings have to be fixed in order to suppress the axial forces during the short circuit operation mode.
There is no air in the transformer!


The selected criteria of the design and core optimization are the winding losses (18000W => 1.125%) at 75 C  and the inductive short-circuit voltage 6.4%. If you prescribe also the temperature rise then the program has to use the criterion which is more critical: either the winding losses or the temperature rise with the prescribed short circuit voltage.
The core losses and the no-load input current can be manipulated only with steel quality, core assembly and  induction

Core optimization

After you have set all input screens you need to select a core family and a core as template: 3 phase core family with 8 steps "round" cross section

Click Core to open the input screen for reading the parameters of the selected core

Click Optimize to optimize the core.

The yellow output fields are optimal results. Both other columns have a higher material price for 2%.
Here you can round off the core diameter (260mm instead 261.1) and click Create. This is the optimized core after the setting X = Y = 1050 (in order to use only 3 strip sizes per shape).



The first step of the presentation of the output screen is DIAGNOSIS: it is the summary of the most important calculated parameters of your transformer.

Note that the program uses the numerical calculation of the magnetic fields and the temperature rises. Due to this technology the calculations of the eddy current losses, the steel losses, the short-circuit voltage, the circulating current and the transposition are very powerful.
The following picture shows the magnetic field outside&inside  the core window.


Note that  the criterion of design is the winding losses. With this criterion, the program optimizes the relationship of the primary and secondary losses. Due to the higher eddy current losses in the secondary winding and better cooling of the primary winding the temperature rise of the secondary is higher  than the temperature  rise of the primary winding.
A very important detail is the max. oil temperature in the cooling channel (points 2 ,5, 7 and 10)

Finally here are 4 printed pages showing the design results




The secondary winding (2 x Scr) is wound with 2 parallel strands. Each strand has 6 parallel flat wires. The transposition (rotation)  of the wires in these 2 strands has to be done after 1., 3., 5., 7., 9., 11., 13., 15., 17., 19., 21. and 23. turn. The horizontal cooling channel between these 2 strands is 5mm

The calculated number of the discs of the primary winding is 64 discs. In order to set the -5.0%, -2.5%, +2.5% and +5.0% taps for voltage regulation, the primary winding is normally cut in the middle. At this point there should be a horizontal cooling channel 12-15 mm instead 5mm.

Due to high voltage line discharge each turn in the 2 first and the 2 last discs have to be additionally insulated with approx 0.75 - 1.00mm one-side insulation. For these two reasons, the number of the discs should be set to 62, wound as follows:

        Discs 1&2 &61&62  => 10

        28&29&30&31&32&33&34&35 => 15 turns

        Other =>20&21 turns


Nominal operating mode

On this page you can check the prescribed parameter:

        winding losses at 75 C :0.99%<1.125%

        short voltage:6.49% (instead of 6.4%)

        core losses:2920W < 3200W

        No-load current : 1.2% < 1.3%

        Max temperature rise :24.8 K < 25 K

        Max.radial tension in short-circuit: 18.22N/mm^2 < 60 N/mm^2

        Max temp. rises during 4s in short-circuit:59.99K

Test Mode

If you are not satisfied with the solution made by the program you can switch into the Test Mode and change your transformer by hand:

         Turns 24.8

         Wire size

         Material (Cu or Al)

         Number parallel connected wires and their order in strand

         Cooling channels and insulations



         Technology parameter (impregnation, gaps,...)

and then you can set it under an operation mode changing:

         Input voltage


         Loads and their K-factors

         Duty cycle of each winding

         Ambient temperature

         Air flow

In order to optimize the material costs you need to reduce the very high eddy current losses. From a material costs point of view, here is a better version with secondary 2 x 12 flat wires 8mm x 2mm and primary wires 1mm x 8mm in only 48 discs.