# Development of Medium-Frequency Molded Transformer for SST with Simultaneous Insulation and Forced Air-Cooling capabilities

Ritsuki Yonetomi, Keisuke Kusaka

► To cite this version:

Ritsuki Yonetomi, Keisuke Kusaka. Development of Medium-Frequency Molded Transformer for SST with Simultaneous Insulation and Forced Air-Cooling capabilities. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0159. hal-05108304

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# Development of Medium-Frequency Molded Transformer for SST with Simultaneous Insulation and Forced Air-Cooling capabilities

Ritsuki Yonetomi, Keisuke Kusaka

Nagaoka University of Technology

1603-1, Kamitomioka-machi, Nagaoka

Niigata, Japan

Tel.: +81 / (258) - 47 - 9622

E-Mail: s213194@stn.nagaokaut.ac.jp, kusaka@vos.nagaokaut.ac.jp

URL: https://eclab01.nagaokaut.ac.jp/

## Acknowledgements

This paper is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) (JPNP14004).

## Keywords

«Solid-state transformer», «Transformer», «DC/DC converter», «Partial discharge», «Insulation».

## Abstract

This paper proposes a new plastic mold structure for medium-frequency molded transformers used in solid-state transformers (SSTs), aiming to enhance both the cooling performance of windings and insulation capabilities. In medium-frequency transformers for 6.6-kV SST, high-voltage insulation performance is required between the primary and secondary windings. Therefore, insulation should be provided by molding around the windings. In contrast, an effective cooling method for windings is necessary because copper losses in the windings also increase as the operating frequency rises.

In this paper, temperature rise in the windings, which is a problem in medium-frequency molded transformers, is addressed by applying plastic molding only to the high electric field generating part in a structure that allows efficient cooling. Experimental results show that the developed transformer, which has a power density of 12.6 kW/L, continuously transmit 37.5 kW with a maximum transmission efficiency of 99.2%. In addition, partial discharge tests of the developed transformer show that it has sufficient insulation performance for SST with a 6.6-kV input.

## Introduction

In recent years, solid-state transformers (SSTs) with the ability to reduce transformer volume by operating at medium or high frequency, have gained attention as a replacement for transformers in DC smart grids or commercial-frequency transformers in ultra-rapid electric vehicle chargers. SSTs decrease transformer size by operating at medium or high frequencies. In addition, features such voltage regulation, reactive power compensation, and the integration of energy storage systems in multi-cell configurations—capabilities that conventional commercial-frequency transformers cannot provide—will also be implemented [1–2].

Regarding SST topology, a multi-cell system with multiple cells connected in series on the input side and in parallel on the output side — known as input-series, output parallel connection (ISOP) — has been proposed. Each cell contains a DC/DC converter with a transformer that operates at medium or high frequency. Consequently, this configuration plays a crucial role in delivering the required voltage to the load through isolation and voltage conversion [3]. The ISOP connection reduces the voltage applied to the primary and secondary sides of a high-frequency transformer, making it possible to reduce transformer volume significantly. However, a maximum voltage of 6.6 kV is applied between the transformer's primary and secondary windings for a 6.6-kV AC grid. This means high insulation performance between the windings and demands must be ensured [4].

As an insulation degradation phenomenon in equipment subjected to high voltage, partial discharge (PD) is known. PD is one of the causes of long-term degradation of the insulation performance of transformers. When there are minute defects called voids in the insulation material installed between the windings to which
high voltage is applied, PD occurs due to the transfer of minute electrical charges through these voids [5]. PD causes deterioration of the transformer's insulation material, shortening the life of the insulation and leading to premature failure [6-8]. PD testing is used to improve the insulation reliability of high-voltage equipment [9]. PD testing detects minute electrical charges when high voltages are applied to insulators and can estimate the risk of deterioration of insulating materials [10-11].

Therefore, insulation performance must be improved to reduce the risk of partial discharge. One method to ensure the insulation performance of a transformer is to encapsulate the windings with a plastic mold structure to improve reliability. Mold materials provide higher safety and insulation performance in comparison with conventional transformers, such as oil-filled transformers, due to the use of flame-retardant materials. In addition, the molded transformers are compact and lightweight and, eliminating the requirements of maintenance [5].

However, the molded transformer has a challenge on the cooling performance. In the conventional molded transformer, the windings are enclosed by the plastic mold. Thus, the temperature of the winding rises due to the increased copper loss due to the skin effect and the proximity effect.

This paper proposes providing plastic mold on only in regions with a concentrated electric field around the windings. The experimental results indicate that the temperature rise in the windings is reduced in comparison with the conventional fully molded transformer. Furthermore, the PD test of the transformer demonstrates that the developed transformer possesses sufficient voltage to withstand capability for SST with a 6.6-kV input.

# Design of high-frequency transformer for SST

## Design of circuit

Fig. 1 illustrates the entire circuit of the assuming SST, featuring the main focus of this paper: a high-frequency transformer. In the paper, ISOP connection with nine cells per phase is assumed for utilization in a 6.6-kV grid. The DC/DC converters in the cell are the current resonant isolated converters. The resonance capacitor is connected in series at the primary side of the transformer. Thus, the current flowing

through the transformer is almost sinusoidal. In this paper, the developed transformer is evaluated with a current resonant DC/DC converter, as shown in Fig. 2.

## Electric field distribution

Insulation design in a transformer assuming 6.6-kV input is considered through electric field analysis in FEM. The result of the analysis clarify where insulation materials are required. In this consideration, split structure for the windings is adopted in order to increase the insulation distance. The magnetic core used in the transformer is ferrite. The 16 cores of UU80×150×30N (PC40, TDK) are used from the viewpoint of transformer volume and iron loss.

<figure><img src="image_2.png" /><figcaption>Fig. 1. ISOP connection assuming 6.6 kV system.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2. Current resonant DC/DC converter.</figcaption></figure>
then heated and cured in a thermostatic bath at 85°C or above, which indicates the curing process. The transformer completed through the above process is shown in Fig. 8.

Fig. 8 shows a transformer with a conventional mold structure. The windings are fully enclosed by the plastic mold. Comparative tests will be conducted using this transformer and the transformer of the proposed structure.

Table I shows the parameters of the proposed structure transformer and the conventional structure. The difference in winding resistance

between the conventional and proposed structures is the difference in the type of winding. The conventional structure uses copper plates and the proposed structure uses litz wires.

<figure><img src="image_4.png" /><figcaption>Fig. 5. Proposed transformer's insulation structure.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. The proposed transformer with plastic molding only in the electric field concentration area.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8. The conventional transformer with plastic molding over entire winding.</figcaption></figure>

<table><caption>Table I. Transformer parameters.</caption><thead><tr><th></th><th></th><th>Conventional structure</th><th>Proposed structure</th></tr></thead><tbody><tr><td rowspan="2">Primary</td><td>Inductance</td><td>1.93 mH</td><td>1.79 mH</td></tr><tr><td>Winding resistance</td><td>160 mΩ</td><td>25 mΩ</td></tr><tr><td rowspan="2">Secondary</td><td>Inductance</td><td>484 μH</td><td>447 μH</td></tr><tr><td>Winding resistance</td><td>40.1 mΩ</td><td>7.0 mΩ</td></tr><tr><td colspan="2">Coupling coefficient</td><td>0.99</td><td>0.95</td></tr><tr><td colspan="2">Winding ratio</td><td colspan="2">8:4</td></tr><tr><td colspan="2">Power density</td><td colspan="2">12.6 kW/L</td></tr></tbody></table>
# Experimental results

## Test results

Table II lists the parameters of the experiment and Fig. 9 shows the prototype of the DC/DC converter. In this paper, the rated power is 37.5 kVA, but this time it is operated at 0.5 p.u. In this test, the design frequency of the transformer is 30 kHz; however, the experiment was conducted at the resonant frequency of 29 kHz. In addition, the current is delayed from the inverter output voltage by approximately 15 deg. to achieve zero-voltage switching.

Fig. 10 shows the operating waveforms of the inverter and transformer. Fig. 10 (a) presents the test results of the conventional structure and (b) shows the test results of the proposed structure. Both waveforms (a) and (b) indicate that the phase of the primary current is delayed in comparison to that of the voltage. These results confirm that zero-voltage switching has been successfully achieved.

Fig. 11 shows the efficiency characteristic of the transformers developed in this study. During

light-load operation, no-load losses dominate, resulting in an efficiency of only about 92%. However, as transmission power increases, the ratio of no-load loss to total loss decreases, and the efficiency near the 0.5 p.u. power reaches up to 99.2% with an output power of 16.5 kW. Note that, the maximum efficiency of the conventional transformer is 98.6% with an output power of 16.8 kW.

Fig. 12 shows a comparison of the experimental and theoretical losses for the conventional structure. In Fig. 12(b), the measured loss of the proposed structure is 138.5 W, while the theoretical value is 134.6 W, resulting in an error rate of 2.9%. Both results demonstrate that the error rate is below 10%, highlighting the high accuracy of the loss calculation.

## Partial discharge test

Fig. 13 shows a circuit diagram of the PD test. The PD measurement setup is shown in Fig. 14. In this test, the discharge threshold is set at 50 pC. The test is then conducted by shorting each winding of the primary and secondary windings. At this time, the insulation performance between

<table><caption>Table II. Circuit parameters.</caption><thead><tr><td>Parameter</td><td></td><td>Value</td></tr></thead><tbody><tr><td>Input voltage</td><td>V<sub>in</sub></td><td>750 V</td></tr><tr><td>Output voltage</td><td>V<sub>o</sub></td><td>375 V</td></tr><tr><td>Rated power</td><td>S</td><td>37.5 kVA</td></tr><tr><td>Frequency</td><td>f</td><td>29 kHz</td></tr><tr><td>Resonant inductor</td><td>L<sub>r</sub></td><td>487 μH</td></tr><tr><td>Resonant capacitor</td><td>C<sub>r</sub></td><td>56 nF</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>Fig. 9. Test environment of the transformer.</figcaption></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Operating waveforms of the transformer during 0.5 p.u. operation.</figcaption></figure>

(a) Conventional structure.

(b) Proposed structure.
the windings was evaluated at the discharge initiation voltage above the threshold value. The PD tester used in this test is DAC-PD-3 (SOKEN ELECTRIC CO., LTD).

Fig. 15 shows the test results of the PD test, in which Fig. 15 (a) demonstrates the measured result of the proposed mold structure and Fig. 15 (b) demonstrates the measured result of the conventional molded transformer. The results indicate that the proposed structure exhibits a discharge initiation voltage of 7.32 kV, while that of the conventional mold structure reaches up to 9.81 kV. Even though the insulation performance

<figure><img src="image_14.png" /><figcaption>Fig. 11. Efficiency characteristic.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>(b)</figcaption></figure>

<caption>Fig. 12. Loss Comparisons.</caption>

(a) Conventional molded transformer.

(b) Proposed molded transformer.

<figure><img src="image_17.png" /><figcaption>Fig. 13. PD test circuit</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 14. Test environment for PD test of transformer's prototype.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>(a)</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>(b)</figcaption></figure>

<caption>Fig. 15. Test results of partial discharge.</caption>

(a) Conventional molded transformer.

(b) Proposed molded transformer.
of the conventional molded transformer is more effective, as the windings are well-insulated by the plastic encasement, the proposed mold structure also achieves excellent insulation performance at 6.6 kV, confirming that it has sufficient insulation performance for the AC grid's specification.

## Conclusion

This paper outlines the development of a high-frequency molded transformer for DC/DC converters used in SSTs with an ISOP configuration. The challenge on the conventional high-frequency molded transformers is the increase in the winding temperature caused by higher AC resistance. The mold structure to achieve the forced air-cooling and high-insulation performance is proposed in this paper. The proposed transformer is developed and demonstrated in a DC/DC converter. Based on the test results, it is confirmed that an efficiency of 99.2% can be achieved with a power of 16.5 kW. PD tests are also conducted on the developed transformer, showing that the insulation performance required for a 6.6 kV SST can be achieved. However, the PD test confirmed that the insulation performance is slightly lower than that of the conventional structure.

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