# Development of a 1MW Multi-Channel Fast Charging Station based on Solid State Transformer

Byengjoo Byen, Byunghwan Jeong, Haewon Seo

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Byengjoo Byen, Byunghwan Jeong, Haewon Seo. Development of a 1MW Multi-Channel Fast Charging Station based on Solid State Transformer. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0130. hal-05097874

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# Development of a 1MW Multi-Channel Fast Charging Station based on Solid State Transformer

Byengjoo Byen, Byunghwan Jeong, Haewon Seo
HYOSUNG

74, Simin-daero, Dongan-gu
Anayng-si, Gyeonggi-do, South Korea

Tel.: +82 / 2-707-7000

E-Mail: cooks321@hyosung.com

URL: http://www.hyosung.com

## Keywords

«Cascaded H-Bridge», « Charging infrastructure for EV's», «Power converters for EV», «Solid-State Transformer», «Quad-Active-Bridge».

## Abstract

This paper presents a novel solid-state transformer-based fast charging infrastructure for electric vehicles (EVs) with direct medium voltage connectivity. The proposed system integrates multiple charging ports, enabling simultaneous power delivery to several EVs while maintaining independent control. By eliminating the conventional low-frequency transformer and associated power conversion stages, this configuration achieves significant footprint reduction and enhanced cost-effectiveness compared to traditional low-voltage systems. The operating principles and control algorithms are thoroughly validated through experimental results, demonstrating rated power operation and successful interconnected operation with commercial EVs.

## Introduction

The growing adoption of electric vehicles (EVs) over the past 20 years has driven significant demand for charging infrastructure development across urban and rural environments. Limited battery storage capacity of current EV technology necessitates widespread deployment of distributed charging stations throughout transportation networks to maintain operational reliability and support long-distance travel capabilities. This has resulted in widespread installation of charging infrastructure globally [1]-[3].

Fast charging infrastructure has emerged as a critical solution towards minimizing EV charging duration. These charging stations

demand high power ratings (up to 400kW) for rapid power transfer to EV batteries. However, the immediate connection/disconnection of such high-power loads may result in grid stability issues, particularly when the local distribution capacity is insufficient. To address these challenges, Battery Energy Storage Systems (BESS) have been proposed as an effective solution for decoupling the charging load from grid-side variations [4]-[6]. Integration of BESS provides multiple benefits including: (a) grid stabilization during fast charging operation, (b) peak load management, and (c) potential for ancillary services through bidirectional power flow capability.

In conventional utility systems, low voltage distribution networks are fed through medium voltage transmission systems utilizing line frequency transformers. This configuration, when applied to high-power EV charging stations, results in significant space requirements and increased infrastructure costs, hindering widespread deployment. Recent research has focused on Solid-State Transformer (SST) technology as a promising alternative [7]-[11]. The SST enables direct medium-to-low voltage conversion through high-frequency operation, offering advantages such as: (a) compact footprint through elimination of bulky line-frequency transformers, (b) improved power quality control, and (c) bidirectional power flow capability. Major benefits of SST integration include reduced installation complexity and optimized power conversion efficiency compared to traditional transformer-based systems.

In this study, a novel 1MW multi-channel fast charging station based on Solid-State Transformer (SST) technology is proposed. The architecture employs a cascaded configuration integrating Cascaded H-Bridge (CHB) and Quadruple Active Bridge (QAB) converters.
Major benefits of this topology are: (a) direct medium voltage connectivity eliminating the need for additional power conversion stages, (b) simultaneous power delivery capability to five independent EV charging ports and Energy Storage System (ESS), and (c) bidirectional power flow management. System validation was performed through two methodologies: (1) rated power verification using Power Conversion System (PCS) to demonstrate the power handling capability under controlled conditions, and (2) practical implementation tests with commercial EVs to validate real-world charging performance. Experimental results demonstrate successful interconnected operation across various loading conditions, confirming the viability of the proposed architecture for high-power EV charging applications.

## System of Multi-Channel Fast Charging Station

The proposed system architecture, depicted in Fig. 1, implements a Solid-State Transformer (SST) topology comprising a Cascaded H-Bridge (CHB) converter for AC/DC conversion and a Quadruple Active Bridge (QAB) converter for DC/DC power transfer. This configuration enables direct connection to 22.9kVac medium voltage transmission lines without intermediate transformation stages. The system integrates five independent EV charging ports and an Energy Storage System (ESS), facilitating simultaneous charging operations and bidirectional power flow capability. For achieving robust control and protection functions, high-speed EtherCAT communication protocol operating at 100Mbps is implemented. The EtherCAT communication achieves high reliability through redundant ring topology implementation. Major advantages of this communication architecture include: (a) minimal control latency matching system control bandwidth, (b) reliable data exchange between converter modules, (c) enhanced protection coordination, (d) synchronization between sub-modules. The integrated control structure enables seamless power flow management between the utility grid, EV loads, and ESS while maintaining optimal system performance. This modular system usually needs to have redundancy modules about 10% for the stability of continuous operation despite of abnormal operation of some sub-modules. However, The absence of redundant sub-modules in the

<figure><img src="image_2.png" /><figcaption>Fig. 1: System configuration of the multi-channel fast charging station</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2: Sub-Module topology</figcaption></figure>

<table><caption>Table I: System Specification</caption><thead><tr><td>Parameter</td><td>Values</td></tr></thead><tbody><tr><td>Rated Power</td><td>1MW</td></tr><tr><td>Phase</td><td>3Phase</td></tr><tr><td>Input Type</td><td>Wye</td></tr><tr><td>Input Voltage</td><td>22.9kV<sub>ac</sub></td></tr><tr><td>Frequency</td><td>60Hz</td></tr><tr><td>EV Voltage</td><td>1,050(±2%)V<sub>dc</sub></td></tr><tr><td>ESS Voltage</td><td>900(±12.5%)V<sub>dc</sub></td></tr><tr><td>Multi-channel</td><td>5</td></tr><tr><td>Cooling Type</td><td>Liquid Cooling</td></tr><tr><td>SM Efficiency</td><td>97%</td></tr></tbody></table>

proposed system has been identified as a limitation, and implementation of such redundancy is planned for future works.

The system specifications are summarized in Table I. The converter operates at a rated power of 1 MW with a three-phase 22.9-kV AC input at 60 Hz in wye configuration. The DC output specifications comprise 1050 V (±2%) for the charging ports and 900 V (±12.5%) for the
energy storage system (ESS). The design incorporates five EV charging output ports and employs a water-cooled thermal management system.

Fig. 2 shows the sub-module topology. The sub-module topology features a full-bridge structure on the AC side, enabling efficient AC-DC conversion. On the DC side, a Quad-Active Bridge (QAB) converter incorporates two bridges connected in series on the high-voltage side to effectively manage high voltage levels. The QAB converter dedicates its remaining two bridges as ports for the electric vehicle (EV) and energy storage system (ESS). This configuration reduces voltage stress on the DC-side switching devices while allowing simultaneous EV and ESS connections. The integrated design enhances system performance and reliability, providing a robust solution for modern EV charging infrastructure.

## Operation of Multi-Channel Fast Charging Station

Unlike conventional charging systems, the proposed topology enables bidirectional power flow between the grid, electric vehicles (EVs), and energy storage system (ESS). During standard operation, the control system maintains zero power transfer to the ESS while delivering grid power directly to the EV charging ports. Fig. 3 illustrates operating modes of our systems.

Fig. 3(a)-(c) illustrates the grid-powered operation mode, where the primary power flow is directed from the grid to the EV charging ports. During this mode, the ESS power is controlled to be zero. Also The system is capable of power delivery to the battery side in either independent or simultaneous operation modes if operator requires.

Fig. 3(d)-(f) depicts the ESS-powered operation modes. During periods without EV charging demand, the system functions as a grid-support ESS, providing peak load reduction capabilities. The pre-charged ESS can also operate in parallel with the grid to meet EV charging demands, enabling hybrid power delivery.

The system additionally supports multiple operational modes, including vehicle-to-grid (V2G) power flow, electric vehicle-to-energy storage system (EV-to-ESS) energy transfer, and grid forming (GFM) capabilities, although these are not depicted in Fig. 3. These bidirectional power capabilities enhance grid stability while maintaining primary EV charging functionality.

<figure><img src="image_4.png" /><figcaption>Fig. 3: System operation mode</figcaption></figure>

## Experimental Results and Analysis

Fig. 4 illustrates the proposed multi-channel fast charging station architecture, including cooling management system and 2-MW AC/DC converter for rated load test. The experimental test configuration is shown in Fig. 5, where Fig. 5(a) depicts the 1-MW load test setup and Fig. 5(b) shows the EV charging configuration. An inductive voltage regulator (0-760V<sub>AC</sub>) controls the grid input voltage, which TR1 steps up to 22.9 kV<sub>AC</sub> for medium-voltage system input. The test setup incorporates a 2-MW AC/DC converter to compensate for system power losses under full-load conditions. And Voltage measurements on the medium-voltage side were obtained through step-down transformation via a
power transformer, whereas current measurements were conducted directly using a measurement probe.

Fig. 6 presents the system's operational waveforms during 1-MW load testing. Fig. 6(a) shows the no-load operation at rated voltage, demonstrating near-zero grid current and regulated output voltages. The EV voltage and ESS voltage was well-controlled without any distortion. The grid voltage measurement was conducted through a power transformer with a step-down ratio of 22.9kV/110V. Under rated-voltage power delivery, Fig. 6(b) indicates stable sinusoidal grid current with well-regulated output voltages, exhibiting minimal ripple and transient effects between every ports. Fig. 6(c) illustrates system performance during power ramp-up from no-load to rated conditions, showing effective control of both grid current and output voltage waveforms. Fig. 6(d) depicts ESS power control waveforms during 200-kW

<figure><img src="image_5.png" /><figcaption>Fig. 4: Experimental setup</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5: Test configuration</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6: Experimental waveform</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 7: Validation of EV charging performance</figcaption></figure>

bidirectional operation, demonstrating stable grid current maintenance during both charging and discharging modes.

Fig. 7 illustrates the implementation of the DC-DC converter and charging station configuration for EV charging validation. Fig.7(a) indicates the DC-DC converter for interfacing between EV charging station and EV charger. Because proposed charging system controlled contant output voltage and EV charger is needed controlled voltage for battery. So, the DC-DC converter delivers regulated power to the charging station, which establishes communication protocols with the vehicles to determine their required charging parameters. Experimental results demonstrate successful simultaneous charging of three EVs, validating the multiport charging system's performance under real-world conditions.

Results demonstrate the proposed multi-channel fast charging station's capability for direct medium-voltage connection and EV charging. However, system enhancements are required in two areas to improve economic feasibility and reliability: minimization of system footprint for optimal spatial efficiency, and implementation of redundancy features for enhanced operational stability.

The compact container-based charging system is shown in Fig. 8. The integrated system, housed in a 20-ft container, comprises a medium-voltage gas circuit breaker, a multi-channel fast charging station, and a liquid cooling system. The implementation framework for container-based charging systems in future EV charging infrastructure is presented in Fig. 9.

## Conclusion

This paper presents a novel multichannel fast charging station architecture capable of direct connection to a 22.9-kVAC grid. A comprehensive experimental setup was established to verify the proposed system's functionality, and the fundamental operating principles were explained through theoretical

<figure><img src="image_9.png" /><figcaption>Fig. 8: Compact container-based charging system</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 9: Future EV charging infrastructure</figcaption></figure>

analysis, were verified through experimental testing at rated load operation. Experimental validation with commercial electric vehicles demonstrates the converter's capability for simultaneous multiport charging operation.

The key contribution is the development and verification of a medium-voltage-connected system that efficiently delivers power to energy storage systems and vehicle chargers.

Future research directions encompass multiple critical aspects of system enhancement. One major focus involves improving system reliability by incorporating redundant sub-modules and developing advanced redundancy management strategies for fault-tolerant operation. Another significant research direction aims to increase system capacity through modular expansion while simultaneously implementing advanced grid-support capabilities, including reactive power compensation, frequency regulation, and dynamic grid stability enhancement. These developments will contribute to both system reliability and grid integration effectiveness.

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