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DESIGN, DEVELOPMENT AND ANALYSIS OF HIGH VOLTAGE, HIGH
FREQUENCY TRANSFORMER FOR DC ACCELERATOR APPLICATION
R. Patel, R. I. Bhaktsingh, D. K. Sharma, S. Dewangan, R. N. Rajan, S. Gond, N. B. Thakur,
A. Waghmare, K. C. Mittal and L. M. Gantayet
APPD, BARC, Trombay, Mumbai – 400085, India.
Abstract
This paper covers the design, development and analysis
of High Voltage, High Frequency Transformer for DC
Accelerator application. Distributed capacitance, leakage
inductance, skin effect and HV Insulation are major
design challenges for this type of Transformer. A
prototype of 30 kV – 0 – 30 kV, 10 kHz, 500 W output
power, Ferrite Core Transformer have been designed,
fabricated and tested. Spice simulations have been done
for estimating transformer parameters. Effect of high
frequency and requirement of HV Insulation have been
studied and analyzed. The effects of Magnetic Core
behaviour and its losses have been studied. Based on
study and test result, distributed capacitance, leakage
inductance, and Transformer scheme has been optimized
for 30 kV – 0 – 30 kV, 10 kHz, 10 kW output power.
INTRODUCTION
High Voltage, High Frequency Transformer is
providing reliable solution for conversation of AC mains
to high voltage, high power DC which has special
requirement for DC Accelerator application. 500 keV DC
accelerator is based on Cockcroft Walton voltage
multiplier scheme. The secondary winding of a
transformer supplies a high frequency alternating voltage
input to a 10 stage voltage multiplier of 500 keV DC
Accelerator. For n identical stages in the Voltage
Multiplier of DC Accelerator, a constant output voltage
2𝑛𝑉 is generated at no load. If the Voltage Multiplier of
DC Accelerator is connected to accelerating tube through
which beam current 𝑖 flows, the capacitor of Voltage
Multiplier partially discharges. The output voltage
regulation is given by
∆𝑉 =
𝑖
𝑓𝐶
×(
2𝑛3
3
+
and the ripple voltage is given by
𝑖
𝑛(𝑛+1)
𝛿𝑉 =
×
𝑓𝐶
2
𝑛2
2
𝑛
− )
6
(1)
(2)
To reduce the regulation and ripple, the product 𝑓𝐶
must be large. Typically, the capacitances of several nano
Farads and frequencies of several tens of kHz are used.
To minimize ripple and regulation of high voltage
multiplier, they have to be powered by a high frequency
source of 10 – 100 kHz. Therefore, for 500 kV multiplier
the optimized input voltage and frequency are 30 kV –0–
30 kV and 10 kHz respectively, which are the output
parameters of the transformer. Similarly, three phase 415
V input supply fixed the input voltage of 500Vp for
transformer. Hence we get high voltage transformation
ratio of 120:1 for the transformer [1].
The design of High Voltage, High frequency Transformer
differs widely from the standard transformer design
methodology. It is necessary to put sufficient insulation
between the primary and secondary windings in order to
avoid electrical breakdown which reduces the
electromagnetic coupling between primary and secondary
windings and increases leakage inductance. Core
selection is a major design criterion to obtain the best
performance from the core; it is well understood that
hysteresis and eddy current losses increase with
frequency. Similarly the skin effect also increases the
winding losses because the conductor effective AC
resistance (RAC) is higher than DC resistance (RDC) at
high frequency. Furthermore, the distributed capacitance
(CD) of secondary winding becomes very large since it is
reflected to the primary side due to multiplication effect
of square of the turns ratio. Therefore the combined effect
of magnetizing inductance and distributed capacitance
can cause the resonance effect at lower frequency than
operating frequency of transformer, which is undesirable.
TRANSFORMER DESIGN
The transformer design requires consideration of the
insulation materials, tracking distance, the magnetic
material and design of winding scheme.
Selection of Core
Core is main contributor for losses and reduces the
efficiency of the entire system. Several cores materials
such as Silicon Steel (CRGO), Ferrite and Metglas
2605SA1 has been compared in Table No. 1. Most
commonly used materials are alloys of iron and they
contain some amounts of other elements, such as silicon
(Si), nickel (Ni), chrome (Cr), and cobalt (Co).These
materials are referred to as ferromagnetic materials. The
values of saturation flux density begin at 1.4T, and for
some of the materials the values are nearly 1.9 T. The
electrical resistivity of these alloys is only slightly higher
than good conductors, such as copper or aluminium.
Ferrites are ceramic materials, basically soft magnetic
oxide mixtures of iron and other magnetic elements, such
as manganese (Mn), zinc (Zn), nickel and cobalt. They
are characterized by a high resistivity. The order of
magnitude of the resistivity is at least 10 6 higher than the
Silicon Steel. Metglas amorphous alloy 2605SA1 has
good saturation flux density compare to Ferrites but
higher core losses compared to ferrites. MnZn Ferrite core
has been selected for designing the transformer because
of its high permeability and low corecivity, which is a
measure design criteria in high frequency applications [2].
Table 1: Comparison of Cores
Material
Silicon Steel
Ferrites
Met-Glas
Contents
3-6% Si
MnZn bulk
alloy2605SA1
Permeability μi
1000-10000
100-20000
2000
Bpeak, T
1.9
0.3-0.45
1.5
ρ, μΩm
0.4-0.7
102-104
1.3 × 104
Ploss, W/Kg
0.3-3 at
12 at
12.8 at
1.5T/50 Hz
0.2T/20 kHz
0.2T/10 kHz
720
125-450
399
Curie temp. Tc,
0C
Insulating Media
Windings in a high voltage transformer are subjected to
high electric field stresses. The insulation system should
be designed to reliably withstand these stresses under
normal and fault conditions. A good dielectric should
have low dielectric loss, high mechanical strength, should
be free from gaseous inclusions, and moisture, and be
resistant to thermal and chemical deterioration. Solid
insulating materials used for transformer are Kapton,
Mylar, Nomex Paper, enamel, etc. Liquid dielectrics
possess much higher dielectric strength of the order of
107 V/cm. Also, liquids, like gases, fill the complete
volume to be insulated and simultaneously will dissipate
heat by convection. Oil is about 10 times more efficient
than air or nitrogen in its heat transfer capability when
used in transformers [3].
Stray Capacitances(pf)
Stray Capacitances(pF)
2
3
4
Table 2: Electric Field of Windings
Field
Location
HV terminal
of secondary
winding
Inter layer of
secondary
winding
Between
primary and
secondary
winding
Maximum
Electric
Field
38.3 kV/cm
Average
Electric
Field
3 kV/cm
Insulation
provided
& Strength
Transformer
Oil, 150 kV/cm
7.2 kV/mm
5 kV/mm
12.8 kV/mm
3 kV/mm
Kapton, 7.7
kV/mil with
50% over lap
Mylar, 11
kV/mil,
20 layers
There is a 60 kV potential difference between H.V.
terminals; the maximum electric field between H.V
terminals is 38.3 kV/cm. The transformer oil has the
breakdown strength of 15kV/mm at 20 0C on 2.5 mm
standard sphere gap. Therefore, Transformer oil
breakdown strength is (150 kV/cm) 5 times higher than
the air breakdown strength and almost 4 times higher than
maximum electric field at 60 kV terminal potential.
Hence Transformer oil is preferred insulation at the rated
voltage of 60 kV. Table 2 has the detail of electric field
and required insulation provided at various locations.
40
35
30
25
20
15
10
5
0
1
at frequency of 10 kHz. It is having regulation of 5% and
transformer oil is provided for cooling and insulation. 16
nos. of primary turn and 1920 nos. of secondary can be
accommodated in core cross section area of 25 cm2 and
window area of 172 cm2. Primary and secondary winding
having bare diameter of copper wire are 4.6 mm and 0.42
mm respectively and the winding losses are 3 watt and 4
watt for primary and secondary respectively. The
estimated core loss is 100 watt. The required minimum
magnetizing inductance is 2.8 mH and the reflected
secondary capacitance referred to primary is 351.4 nF
which is not acceptable. Figure No. 1 shows the graph
between stray capacitance and number of winding section
in the transformer. It is clear from the graph that stray
capacitance decrease with increasing number of winding
section. With one section, the stray capacitance is 35pF,
which reduces to 5pF with six section of winding. So,
sectionalized winding scheme has to be adopted to reduce
the stray capacitance of the transformer.
5
6
No. of section
Figure 1: Stray Capacitance vs. No. of section in
transformer
Detail Design of Transformer
Rated primary, secondary voltages and output power of
transformer are 353 V, 21 kV and 10 kW respectively
TEST RESULT
A prototype of the 10 kHz, 0.5 kVA Manganese Zinc
based Ferrite core transformer has designed and
fabricated. It consists of 8nos. of U100 hundred cores and
10 nos. of I10025 and arranged in a way such that crosssection area of 2500 mm2 and window area of 108 cm2
has achieved. The Bobbin having outer dimension of 113
mm length, 37 mm width and 180 mm height with 3 mm
thick of Acrylic sheet has fabricated for prototype. Figure
No. 2 shows the photo of the prototype transformer.
Voltage transformation of 120 is achieved and Figure No.
3 shows the comparison of designed and measured of
output voltage. The measured primary inductance and
distributed capacitance of prototype are 1.42 mH and 882
nF respectively. The efficiency of transformer is 86.1 %
at input voltage, current, frequency and output load of
145.2 V, 2.73 A, 9.6 kHz and 355 k𝛺 respectively.
Figure 4: Measured Parameters
CH2(blue): Output Voltage and CH1(red): Input Current.
CONCLUSION
Figure 2: Prototype Transformer
A prototype of step up transformer been designed for
500 W output power and the scheme has been
successfully tested up to a power level of 400W. A
voltage level up to 17.6 kV has been achieved. The
transformation ratio was found in close match with
designed values. Reflected capacitance has been observed
as 2.5 times higher than expected value. This results in
resonance at 4.5 kHz which is lower than 10 kHz
operating frequency. The efficiency can be improved by
reducing the distributed capacitances. A full scale version
of the 60kV, 10 kHz, 10 kW power source will be
assembled and tested.
REFERENCES
[1] Mittal K.C., Majumder R., Nanu K., Jain A., Acharya
Figure 3: Designed and Measured output voltage
S., Agarwal R., Bakhtsingh R.I., Chindarkar A.R.,
Ghodke S.R., Jayaprakash D., Khole S., Kumar M.,
Mishra R.L., Puthran G.P., Rajan R.N., Raul S.R., Saroj
P.C., Sharma D.K., Sharma V., Srivastava S.K., and Sethi
R.C., “Design, Development and Operating experience of
500keV, 10kW DC Industrial Accelerator at BRIT
Vashi”, proceedings, Indian Particle Accelerator
Conference (InPAC-2003), held at CAT Indore on 02-06
Feb. 2003, p.(238-240).
[2] Snelling, E.C., Soft Ferrites, Properties and
Applications, 2nd ed., Butterworths, London, 1988.
[3] Fothergill, Jhon C., Devine, Philip W., and Lefley,
Paul W., A Novel Prototype Design for a Transformer for
High Voltage, High Frequency, High Power Use, IEEE
TRANSCATION ON POWER DELIVERY, Vol. 16,
NO. 1, January 2001.