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

General Information

Technical Specification

Creating Input

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

• Winding parameters per limb
• Core
• Environment
• Other

and 3 screens for selection and set up of material:

• wires
• steels
• cores.

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)

l01=l02=75mm

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

Primary

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:

Secondary

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.

Core

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.

Environment

The cooling medium is oil with the average temperature 55°C. 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!

Other...

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).

Output

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

Input

Core

Windings

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:59.99°K

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
• Margin
• Steel
• Technology parameter (impregnation, gaps,...)

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

• Input voltage
• Frequency
• 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.