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Designing a 500 kVA, 60Hz, K-Factor, three phase dry
transformer with cooling channels?
General Information
Technical Specification
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Input voltage
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3 x 480V, delta
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Output voltage
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3 x 208/120V,
star
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Output power
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500kVA,
K-Factor=20, continuous operating mode
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Frequency
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60Hz
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Ambient
temperature
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40°C, in a
cabinet
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Temperature rise
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Max. 120°K,
insulation class H
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°Short-circuit
voltage
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4-5%
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Steel&core
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M6, not
annealed, strips for alternated stacking (90°)
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Creating Input
There are 4 input screens to set the input parameters for the designing of
a transformer:
·
Winding parameters per limb
·
Core
·
Environment
·
Other parameters
and 3 screens for selection and set up of material :
·
wires
·
steels
·
cores.
Windings parameters per limb
Primary
The primary is created with 2 windings connected in series . The sine
wave input voltage is 480V (240V per winding).
There is no duty cycle operation mode and there are no sectors.
Note that a 500kVA transformer for K-factor=20 and the short-circuit voltage
5% can be optimal designed and manufactured only with litz wire. The round
wires within the litz are not insulated but in transposition.
The first winding has the thermal contact with the center leg of the core
within the core window via 0.1" tube thickness and 0.1" air gap
(stomach). All other surfaces of the primary are cooled via the cooling
channels of 0.6" (inside of the core window) and 0.8" (outside of
the core window). The space between the yoke and the primary windings is
1.2" With the eddy current losses factor (RacRdc) 1.1 and 1.25
shell be limited the number of the round wires per litz.
Secondary
The secondary is created with 2 windings connected in parallel . The
sine wave output voltage is 120V.
The rms current per winding is 696Arms. Normally there is no explicit
information about the current harmonics in a K-factor transformers. The
designer has to create a combination of the first current harmonic and 7.
current harmonic in order to satisfy the following conditions:
I1^2 + I7^2 = Irms^2
1^1*I1^2 + 7^2*I7^2 = K-factor* Irms^2
For Irms = 696Arms and K-factor = 20 the calculated current harmonics I1 and
I7 are:
I1= 540Arms
I7 = 437Arms
Also there is no duty cycle operation mode on the secondary side.
In order to avoid the circulating current between 2 parallel connected
secondary windings each of them is created with 2 cross connected sectors.
Once again, a 500kVA transformer for K-factor = 20 and the short-circuit
voltage 5% can be optimal designed and manufactured only with litz wire. With
the eddy current losses factor (RacRdc) 1.1 and 1.25 the number of the round
wires per litz shall be limited. Note that at this point of design you can
not prescribe the wire size. You can select only the wire family which the
program has to use in order to select the suitable wires for your
application.
The first secondary winding is cooled via the cooling channels of 0.6"
(inside of the core window) and 0.8" (outside of the core window). The
second secondary winding has only two 0.8" cooling channels outside of
the core window. It is better cooled than the first secondary winding and
therefore it is in a good thermal connection within the core window with the
first secondary winding.
The space between the yoke and the secondary windings is 1.2"
Core
On this input screen you can :
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select and manipulate the selected steel M6,
14mil
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set the operating induction (1.65T) and the
frequency (60Hz)
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select the core assembly
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and prescribe the core selection.
The core was created by program and by the designer in order to satisfy
the following conditions:
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Ucc=5%
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Optimal price and weight
Normally for this application you use M6, 14mil, non annealed after
stamping, grain oriented strips.
Environment
The cooling medium is air with the ambient temperature 40°C. The
cooling factors for the convection and the emission are set to 80% because
the transformer will be placed in a cabinet.
The cooling surface of the core is increased by using 4 L-brackets on the
core.
The impregnation is practically "dry" because there is only 10%
varnish (90% air) in the windings and in all the gaps between the insulations
and the layers of the windings
Other...
The selected criterion of the design is the temperature rise of 120°K for
insulation class H. The oval space between the first winding and the tube
(stomach), all gaps between the insulation, the windings and the varnish fill
factor of the windings play very important roll from the thermal point of
view.
Output
The first step is the presentation of the output screen DIAGNOSIS: it is
the summery 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.
In the following picture are presented the magnetic field outside of the core
window.
Finally here are 4 printed pages with all the results of the design
Input
Core
Windings
Nominal operating mode
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 manually:
·
Turns
·
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
Note that the program could not create full layer windings at the
prescribed temperature rise of 120°K. In order to get the full layer windings
you have to select the wire size in the litz and the number in parallel
connected wires manually.
Let us now change some technology parameters of this
transformer:
Impregnation
In the following table you can compare the temperature rise of our
"dry" transformer and the full impregnated (no air in the windings,
the gaps and the stomach) version.
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Core
°K
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Primary1
°K
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Primary2
°K
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Secondary1
°K
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Secondary2
°K
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Current version:
10% varnish
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105
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114
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113
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121
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117
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New version:
100% varnish
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105
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109
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109
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113
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109
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Core assembly
If you want to use 45°core
assembly with the annealed M6
strips then:
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Core
°K
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Primary1
°K
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Primary2
°K
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Secondary1
°K
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Secondary2
°K
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Current version:
90° core assembly,non annealed
strips
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105
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114
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113
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121
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117
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New version:
45° core assembly, annealed
strips
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90
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107
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109
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120
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116
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Impregnation
In the following table you can compare the temperature rise of our
"dry" transformer and the full impregnated (no air in the windings,
the gaps and the stomach) version.
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Core
°K
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Primary1
°K
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Primary2
°K
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Secondary1
°K
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Secondary2
°K
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Current version:
in the cabinet
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105
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114
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113
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121
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117
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New version:
no cabinet
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96
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104
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104
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109
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103
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