Partial Discharge Test ofHigh-frequency Transformers
with Plastic Mold for SST
R. Yonetomi1, K. Kusaka1, N. Koike2, and S. Nagai2
1

Nagaoka University of Technology, Japan
2
PONY ELECTRIC CO., LTD, Japan

Abstract—This paper reports on a fundamental evaluationof high-frequency molded transformers for solid-statetransformers (SSTs). The high-frequency transformers ofDC/DC converters for SST require high-voltage isolation
between the primary and secondary windings. In this paper,
a molded transformer with high insulation reliability isemployed and evaluated by a partial discharge test, which
estimates the lifetime of the insulation material.
Experimental results show that the mold transformer has adischarge starting voltage of 9.67 kV for the moldedtransformer and 2.66 kV for the unmolded transformer,
which is advantageous for the insulation of the moldedtransformer.
Index Terms—High-frequency transformer, Moldedtransformer, Partial discharge, Solid-State Transformer.

I. INTRODUCTION
In recent years, solid-state transformers (SSTs) haveattracted attention as a replacement for power transformers
in DC smart grids and commercial frequency transformers
in ultra-rapid electric vehicle chargers. SSTs reducetransformer size by operating at medium or highfrequencies. Moreover, additional features, such as voltage
regulation, reactive power compensation, and connectionof energy storage systems in multi-cell configurations,which can not be realized with the conventionalcommercial-frequency transformer, will be realized [1–2].
As a topology for SST, a multi-cell system withmultiple cells connected in series on the input side and inparallel on the output side (input-series, output-parallel:ISOP) has been proposed. Each cell consists of an isolated
DC/DC converter with a transformer operating at medium
and high frequencies, which plays an important role inproviding the desired voltage to the load through inputoutput isolation and voltage conversion [3–4]. The ISOP
connection reduces the voltage applied to the primary andsecondary sides of the high-frequency transformer. It
enables miniaturization of transformers. However, highinsulation performance is required for the transformers
with a 6.6-kV AC grid between the primary and secondary
windings because a maximum of 6.6 kV is applied
between the primary and secondary [5].
Transformer insulation design is one of the mostsignificant constraints in downsizing a circuit. The
insulation of a transformer affects not only its volume but

also the reliability of the entire equipment [6]. Thus, it isdesirable to ensure the reliability of the transformer
insulation and to make the transformer both compact andreliable.
Partial discharge is one of the causes of long-term
insulation performance degradation in transformers towhich high voltages are applied. Partial discharges cause
deterioration of the transformer’s insulation material,
shortening the life of the insulation and leading topremature failure [7–9]. However, partial discharges
cannot be seen because they occur inside the insulationmaterials. For this reason, partial discharge tests are used
to improve the insulation reliability of equipment. This test
detects minute electrical charges when high voltages areapplied to insulators to estimate the risk of deterioration of
the insulating materials [10–11].
Oil-filled transformers using insulating oil have been
conventionally used in high-voltage transformers withcommercial frequency to avoid electrical discharges due tohigh voltage. However, oil-filled transformers have therisk of fire and reduced maintainability, requiring oilchanges. These features make them unsuitable for use inhigh-frequency transformers for SSTs, which are expected
to have a long service life.
Another insulation method is the molded transformer.
Molded materials have high safety and insulation
performance due to the use of flame-retardant materials.
Thus, molded transformers can be smaller and lighter thanconventional transformers, such as oil-filled transformers,and require less maintenance [4]. However, the use ofmolded transformers has been limited to commercial
frequencies, and the insulation performance of molded
high-frequency transformers has not been evaluated.
In this paper, a high-frequency molded transformer for
SSTs is designed and developed to improve the insulation
performance of high-frequency transformers. As acomparison, a transformer with the same volume without
the mold is developed. Partial discharge tests areconducted on the transformer, and the partial discharge
performance’s superiority in withstand voltage is reported.

II. DESIGN OF HIGH-FREQUENCY TRANSFORMER FOR SST
A. Electrical DesignFig. 1 shows the circuit in which the transformer
designed in this paper will be installed. The SST with ISOP
connection is designed to connect to a 6.6-kV grid with 9cells per phase. The DC/DC converters in the cells are thecurrent-resonant isolated converters. Thus the current
flowing through the transformers is sinusoidal.
Table 1 shows the transformer design conditions for theSST. The transformer structure is of the outer iron type due
to concerns about its size, and the primary and secondary
windings are wound separately from the perspective ofinsulation distance. Single-phase winding is chosen for thehigh-frequency transformer because layered winding
causes a reduction in efficiency due to the proximity effect.
The core is made of ferrite, and UU80×150×30N PC40
material is used for high frequency and large size. Since
the saturation magnetic flux density of the ferrite used is
380 mT at 100 °C, the windings are designed not to exceed
200 mT with the margin.
The number of primary windings N1 that does notexceed a maximum flux density B of 200 mT is

N1 V14 fBAe(1)

where V1 is the primary voltage, f is the frequency, and Aeis the effective cross-sectional area. The secondary
winding N2 is determined by the voltage ratio. Although
the transmission power is 37.5 kW, the voltage across thetransformer is a square wave. Thus, it is necessary toconvert it to a sine wave when designing the power. The
output resistance Ro obtained from this condition isRo 

8 V22
2 P

AN 1 N1  AN 2 N 2AwFig. 1. ISOP connection assuming 6.6 kV system.
Table 1. Design conditions of SST.

(2)

In addition, the effective cross-sectional area Ae is
decided by the occupancy ratio. Occupancy ratio is thearea of the winding per window area of the core and is
obtained byf Ni Fig. 3 shows the results of FEM analysis based on thisdesign value. The result shows that the designed magnetic
flux density of 200 mT has been achieved except thecorners of cores.

(3)

where Aw is the window area. Here, the design must
consider the occupancy factor because a bobbin is inserted
between the core and windings, as well as the primary andsecondary windings. The insulation distance must belarger than 10 mm. This causes an increase in theoccupancy ratio. Fig. 2 shows the plot of the occupancy
factor considering the occupancy factor per number ofcores. In this consideration, the EE core is formed withfour UU cores with ferrite (TDK, PC40). Thus, four is theleast common multiple for the cores. In this design, thetransformer was formed by 16 cores, which has a smaller
variation in occupancy factor. The effective crosssectional area has been determined by the design to thispoint. The parameters of the transformer when 16 cores areused are shown in Table 2.

Parameter

ValueFrequencyf30 kHzTransmission powerP

37.5 kWPrimary voltage

V1750 V

Secondary voltage

V2380 V

Magnetic flux density

B

200 mT

Current density

J4 A/mm2

Fig. 2. Cross-sectional area per core.

Table 2. Design parameters.
Parameter

Value

Number of cores-16

Primary winding

N18 Turn

Secondary winding

N24 Turn

Effective cross-sectional area

Ae

4800 mm2

Window area

Aw

4400 mm2

Occupancy factor

fNi

0.64(a)Fig. 3. FEM analysis of magnetic flux density.

B. Insulation DesignThere is no clear regulation on the isolation distance forhigh-frequency transformers. Thus, the isolation distanceis defined by a similar standard. The rated impulse voltage
is determined by IEC61800-5-1 based on the system
voltage of 6.6 kVrms, and the insulation distance at theimpulse voltage is determined by IEC60664-1. The
insulation distance between the primary and secondary
windings for the transformer is designed according to thestandard. IEC61800-5-1 standard does not have a rated
impulse voltage of 6.6 kVrms. Since linear completionshould not be performed for the spatial distance, a higher
value of 7.2 kVrms shall be used. The rated impulse
voltage of 7.2 kVrms is 40 kV for overvoltage category III.
If the rated impulse voltage is 40 kV and the electric field
is unequal, the isolation distance at pollution degree II is60.0 mm. Considering the isolation distance in anotherstandard, 60.0 mm is the isolation distance at a repetitive
peak voltage when the voltage peak exceeds 30 kV. In thiscircuit, the voltage does not exceed 30 kV, even if surges
are taken into account. Therefore, the criteria for repetitive
peak voltage need not be considered, and the design isbased on an insulation distance of 60.0 mm, which is thestandard for rated impulse voltage.
From the above insulation design, a 3D printer is used
to produce a bobbin that satisfies the insulation distance of
60 mm. Since thermosetting resin urethane (UF-110-1A,UF-110B mixture, curing conditions: 80 °C/1 h) is used for

w/o mold.

(b) w/ mold.
Fig. 4. Prototype of the transformer.

the mold, the bobbin is printed with ABS material (glasstransition temperature: 105 °C) so that it would not melt
due to the heat required for curing. Fig. 4 shows atransformer with the designed bobbin applied. (a) is thetransformer without mold, and (b) is the transformer withmold.
III. PARTIAL DISCHARGE TEST
A. Measuring Principle
Fig. 5 shows the equivalent circuit of the insulator.
When a high voltage is applied between a and b, C3 is thecapacitance of the void, which causes a partial discharge,
and C2a and C2b are the capacitances C2 and C1 of theinsulator in series with C3. When the voltage applied to thevoid is ΔV, the discharge charge Q isCC Q  V  C3  1 2 
C1  C2 (4)

In addition, since partial discharge occurs at the point
where the capacitance is small. Thus, C1 >> C2 and C2 >>C3 can be approximated byQ  V  C 2(5)

However, it is impossible to measure the capacitanceaccurately. Thus, the charge between the terminals a and bis evaluated. First, the voltage drop between a and b isgiven by

Fig. 6. Circuit of partial discharge test.

Fig. 5. Equivalent circuit of insulator.

C2  C1  C2  V  V (6)Approximating C1 >> C2 is given by V  VC2C1(7)where the amount of charge in the total capacity of thesample is
CC Cq   C1  2 3   V  2CCC1
23 (8)

Approximating C1 >> C2 and C2 >> C3 are given byq  V  C 2

voltage side and the secondary winding to the GND. It is
possible to evaluate the onset and extinction voltages ofthe discharge when the voltage rises and falls. In this case,only the discharge start voltage will be evaluated.
Before starting the test, the calibrator is connected tothe DUT, calibrating the instrument with the amount ofcharge expected to be discharged. The voltage at which theset amount of charge is reached becomes the discharge
start voltage, which is the threshold for determining thedischarge. In this study, the threshold is set at 50 pC, which
will significantly impact the insulation material’s life.
After calibration is completed, the input voltage isgradually increased, and the discharge charge anddischarge pulse are monitored.

(9)Q ≈ q by (5) and (9). Therefore, since the discharge charge
at the terminal between a and b are approximated as theamount of charge at the part where the partial discharge
test occurs, it is possible to measure the discharge charge
by measuring the amount of charge with a detector.
B. Measurement Method
The partial discharge measurement device is DAC-PD3 (SOKEN ELECTRIC CO., LTD.), the detector is DACCP-2 (SOKEN ELECTRIC CO., LTD.), the calibrator
DAC-CP-2 (SOKEN ELECTRIC CO., LTD.), and thetransformer for high voltage output is YT-0110 (SOKEN
ELECTRIC CO., LTD.).
Fig. 6 shows the test circuit for the partial discharge test.
Partial discharge tests measure the amount of discharged
charge in pC units, so noise is significant depending on themeasurement environment. Thus, a blocking coil in thedetector prevents partial discharge pulses from flowing out
from the sample to the power supply, and a filter is inserted
to reject the DC component with a coupling capacitor. This
test is conducted with the primary winding of thetransformer made for the sample connected to the highC. Pressure ExperimentFig. 7 shows the experiment environment. At this time,
if the ground resistance is high, a closed circuit is formed
between the stray capacitance of the measurement circuitand the ground, resulting in a noise source. Thus, it isnecessary to install a low-impedance installation, and in
this case, an aluminum plate was laid on the measurement
to form a capacitor with the ground to improve thegrounding effect.
Fig. 8(a) shows the results of a partial discharge test
between the primary and secondary windings of an
unmolded transformer. The results show that the discharge
initiation voltage is 2.66 kVrms, and the discharge
extinction voltage is 2.14 kVrms without the insulation.
From these results, it is necessary to change the corematerial and increase the insulation distance for theunmolded transformer. Fig. 8(b) shows the results of apartial discharge test between the primary and secondary
windings of a molded transformer. The results show thatthe discharge initiation voltage is 9.67 kV and thedischarge extinction voltage is 9.32 kV. From these results,
it is confirmed that 6.6 kV insulation between the primaryand secondary is achieved by molding.

and that of the transformer with the mold is 9.67 kVrms.
The results indicates that the mold improves the insulation
performance. Therefore, the required insulation
performance for the SST DC/DC converter connected to
the 6.6kV system has been confirmed.
In future work, a structure of molded transformers withcooling will be developed and demonstrated.
ACKNOWLEDGMENT
This paper is based on results obtained from a projectcommissioned by the New Energy and Industrial
Technology Development Organization (NEDO).
REFERENCES
Fig. 7. Experiment Environment.

(a)w/o mold.

(b) w/ mold.
Fig. 8. Partial discharge between primary and secondary windings.

IV. CONCLUSIONSIn this paper, a high-frequency molded transformer isdeveloped for a DC/DC converter to be used in an SST
with an ISOP connection. In this study, a transformer for
SST with mold is designed and tested in comparison withan unmolded transformer of the same geometry. The
experimental results showed that the start-of-discharge
voltage of the transformer without the mold is 2.66 kVrms

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