Design and Validation of a SiC-Based Single-to-Three-Phase Converter for Low-Voltage Distribution Systems

Abstract

In areas such as remote, rural, and mountainous regions, supplying low-voltage three-phase power has traditionally required distribution line extension and transformer installation. However, these areas often yield low electricity revenues, making cost recovery difficult for utilities. To address this challenge, this paper proposes a Single-to-Three-Phase Converter (STPC) capable of converting single-phase low-voltage input into three-phase output for use in low-voltage distribution systems. The STPC topology employs a single-phase half-bridge AC–DC stage and a three-phase full-bridge inverter stage using SiC-MOSFETs. To validate the system, simulations and experiments were conducted under various load conditions, including unbalanced, nonlinear, and motor loads. The results show that STPC maintains output stability while minimizing impact on the existing grid. The findings demonstrate STPC’s feasibility as an alternative to conventional line extension and transformer installation, with potential for application in grid-forming and low-voltage distribution current (LVDC) systems.

1. Introduction

The topology of distribution equipment and its influence on power distribution systems (PDSs) were analyzed. There are three methods for single-phase to three-phase power conversion: static phase converter, rotary phase converter, and electronic phase converter. However, the static and rotary methods are not suitable for use in PDS and not in customer facilities; therefore, the electronic phase converter topology is proposed. The electronic conversion method ultimately relies on power semiconductors. Power semiconductors include diodes, IGBTs, and MOSFETs. In this paper, MOSFETs are selected due to their suitability for low-voltage distribution systems (LVDSs) and their capability to operate at high switching frequencies. The power electronic equipment proposed in the paper is called a Single-to-Three-Phase Converter (STPC). The proposed topology comprises an input converter, a DC-link, and an output inverter. The Korea Electric Power Corporation (KEPCO) stipulates that the three-phase line-to-line voltage should remain within 380 ± 38 V. Therefore, the output voltage of the STPC must also output a voltage that conforms to the reference voltage. To achieve this, the STPC monitors both the output voltage and the DC link voltage to regulate the PWM signal effectively.

1.1. Background

Unlike Power Transmission Systems (PTSs), PDSs accommodate various loads, including both single-phase and three-phase loads. Single-phase loads are predominantly residential and consist mainly of household appliances such as televisions and refrigerators. Additionally, small-scale agricultural and industrial equipment also operates on single-phase voltage. In contrast, three-phase loads primarily comprise electric motor loads, which are extensively utilized in industrial facilities such as factories, large shopping malls, and agricultural applications. When electric motors are controlled via inverters, issues such as inrush current are mitigated. However, in the absence of such control, as seen in Direct-On-Line (DOL) motors and certain agricultural machinery such as crushers, the substantial inrush current can adversely affect the STPC.

In order to supply a three-phase voltage, KEPCO currently installs three conductors on Concrete Poles (CPs) to supply a three-phase voltage distribution. Additionally, for low-load applications, three transformers are installed to ensure power supply. However, this conventional power distribution method incurs substantial construction costs, even for low-power loads. Since electricity consumption is low, the corresponding electricity bills are also minimal, making cost recovery virtually impossible. Furthermore, the ongoing maintenance of extensive power infrastructure results in a continuous accumulation of operational costs.

In addition, in mountainous regions, both high-voltage and low-voltage lines must be installed to deliver three-phase voltage to end-use loads, raising concerns about forest fires due to potential faults such as short circuits. To address these issues, this paper analyzes the topology of a power semiconductor-based STPC for converting single-phase to three-phase voltage, and evaluates its applicability for effective for integration with connected loads. Since the loads in the PDS are directly connected to the Power Transmission System (PTS), a comprehensive PDS analysis is essential when integrating Power Distribution Equipment (PDE). The variables of the PDS were simulated through computer program and subsequently validated via experimental demonstration.

The main objective of this paper was to develop and validate a SiC-MOSFET-based STPC suitable for LVDSs. Specifically, the research aims to achieve the following:

• Design a compact and modular STPC topology capable of generating a stable three-phase voltage from a single-phase input.
• Evaluate the performance of the STPC under various load conditions (nonlinear, unbalanced, and motor loads) through both simulation and field testing.
• Assess the applicability of the STPC in real PDS, especially in rural areas where a conventional three-phase supply is economically constrained.

1.2. Literature Review

Previous papers can be categorized into three major research areas: power semiconductor technology, phase conversion methods, and Power Quality (PQ) issues in PDSs caused by unbalanced loads. First, several studies have explored the application of power semiconductors in PDSs. These studies highlight the flexibility of power semiconductors in power distribution, emphasizing that power electronics technology enhances energy transmission efficiency, improves overall system performance, and enables the efficient management of distributed energy resources (DERs) in smart grids. Furthermore, power electronics technology has been widely recognized for its role in optimizing PDS operations [1,2,3,4].

Some papers specifically analyze power semiconductor applications in PDSs. For instance, one study examines solid-state transformers applicable to PDSs [5]. Another study discussed modular converter transformers, which are combinations of several converters [6].

The second category relates to other phase conversion methods. Prior research includes a study on converting three phases to single phases for traction power supply [7], and another about controlling DC/AC converters in unbalanced three-phase systems [8]. Previous papers that discuss a topology most similar to the one proposed in this paper can also be found [9,10]. Despite the similar topology, the control method is different, and this paper differs in that it uniquely investigates experiments on various actual loads (nonlinear, unbalanced, and motor loads), fault current tests, and field tests of the PDS. Therefore, further verification is required to assess its applicability to an actual PDS. The specific differences are presented in the comparative analysis shown in

Table 1. Comparison with previous papers.

Feature	Ref. [9]	Ref. [10]	This Paper
Input Type	Single-phase AC	Single-wire earth return	Single-phase 220 V AC
DC Link Usage	No	Yes	Yes, 750 V DC link
Control Strategy	Passive rectification + inverter	Unified PQ controller	PI + PR control with notch filter
Validation Method	Simulation + partial experiment	Simulation	Simulation + field experiment
Load Conditions Tested	Fixed frequency motor loads	Unbalanced voltage systems	Nonlinear, unbalanced, and motor loads
Target System	Small industrial systems	Rural electrification	Low-voltage distribution system
Semiconductor Devices	Si-based devices	IGBT	SiC-MOSFET
Application Focus	Economic converter topology	PQ compensation in SWER	Three-phase supply for LVDS with PQ assurance

.

There are other methods besides those utilizing power semiconductors for phase conversion. These include static phase converter and rotary phase converter methods [11]. The two methods are more economic feasibility than the approach proposed in this paper, but they are primarily suitable for single-load applications within PDS. However, they convert to a three-phase 220 V input, not three-phase 380 V. Therefore, they require an additional transformer when converting a three-phase 220 V input to three-phase 380 V.

The final previously published paper to mention discusses the impact of power semiconductors on a PDS. In an STPC, not only are three-phase loads connected, but also single-phase loads, which can either be evenly distributed across phases or concentrated on a specific phase. Among the previous papers, there was a paper on improving PQ due to load imbalance in LVDSs [12]. In addition to the aspect of imbalance, there are numerous papers that have focused on PQ. Unlike PTSs, various loads are connected to the PDS, so research on harmonics is essential when studying new distribution equipment. Previous studies include papers on PQ compensation methods according to DER interconnection [13,14], papers on Total Harmonic Distortion (THD) caused by power semiconductor equipment [15], and papers on control methods for PQ in microgrids [16]. Unlike previous papers that primarily focused on laboratory-level validation or theoretical modeling, this paper conducts practical testing on actual PDSs under diverse load conditions.

Several studies have addressed short-circuit behavior and PQ issues in PDS with inverter-based distributed generation, including the influence of transformer topology and temporary overvoltage phenomena [17,18]. These studies offer valuable insights into protection coordination strategies and grid behavior under abnormal operating conditions; however, the methodologies adopted are based on theoretical modeling and simulation frameworks focused on distributed generation systems. Moreover, recent studies have focused on enhancing the accuracy of dynamic state estimation in PDSs by modeling nonlinear phenomena such as transformer core saturation, which can support a more precise assessment of system conditions under transient events [19]. Based on the aforementioned studies, it is necessary to investigate further how a practical power semiconductor-based converter interacts with actual LVDSs under real operating conditions.

2. Research Questions and Hypothesis

2.1. Research Questions

This paper proposes a topology for implementing the functions of an STPC and examines its integration into PDSs. Like other power electronics equipment, an STPC generates harmonics due to its use of semiconductor switching devices. Therefore, it is necessary to study PQ, including harmonics. In addition, the topology and control method of the STPC should be designed, and both computer simulations and field demonstrations are required for on-site demonstration. The detailed research questions of this paper are presented in

Table 2. Research questions.

PICOT	Research Questions
Population	① A substantial financial investment is required for constructing power facilities to supply a three-phase voltage (challenge). ② Target population: Phase conversion equipment, PDS, fault analysis (target population).
Intervention	Unbalanced loads, nonlinear loads, and motor load.
Comparison/Control	Maintaining the standard voltage range, generating three-phase voltage (expected outcome).
Timeframe	① Review of research. ② Analysis of PDS challenges. ③ Assertive hypothesis. ④ Derivation of results.

.

There are few instances where power conversion equipment is directly connected to PDSs. Regarding the equipment described above, an Energy Storage System (ESS), Power Conditioning System (PCS), and Uninterruptible Power Supply (UPS) are used for load equipment, a Static Synchronous Compensator (STATCOM) is utilized in PTSs, and solid-state transformers are employed in PDSs. However, they have not yet been widely adopted in PDSs due to high production costs and size constraints.

2.2. Hypothesis

An STPC is a type of semiconductor-based equipment. Power semiconductors are known to generate harmonics due to their switching function, which enable the device to turn ON and OFF. Harmonics can distort the waveform and interfere with the normal operation of the loads. Therefore, utility companies such as KEPCO regulate the harmonic level to ensure that the harmonic distortion rate in the PDS remains below 5%. Accordingly, STPCs must comply with this standard [14].

The switching function of power semiconductors is implemented through a closed-loop control mechanism that continuously analyzes the output signal and adjusts the switching period. As a result, voltage fluctuations can occur depending on the number of connected loads and the amount of electricity consumed. These variations are detected in the output signal, and the switching period is modified. Further details on the control method are provided in Section 3.2 (Topology).

Basically, a PTS is based on a balanced three-phase voltage, and the system analysis is performed with concentrated loads, such as a single load. However, a PDS is an unbalanced system that accommodates both single-phase and three-phase loads, and the load types are very diverse. Therefore, a more comprehensive system analysis is required.

3. Methodology

Three-phase voltages have the same frequency and magnitude, but their phases are shifted by 120 degrees relative to each other. The phase difference means that they are 120 degrees apart in time, so in a three-phase voltage system with a frequency of 60 Hz, one cycle is 1/60 s (16.67 ms), so 120 degrees corresponds to a time difference of about 5.56 ms. The concept is as illustrated in Figure 1 below.

There are three methods to obtain three-phase voltages. The static phase converter method uses a capacitor to create an imaginary phase, the rotary phase converter method utilizes a single-phase voltage supply to rotate an induction motor to generate three-phase voltages, and the third method employs power semiconductors.

The first methodology uses a capacitor to create a phase difference in a single-phase power supply to create a second phase, and the remaining one phase is processed as an imaginary phase. This method operates stably under light loads; however, when the load increases, the output voltage balance and phase stability may be compromised, making it unsuitable for use in a PDS. The second methodology rotates an induction motor with a single-phase power supply to create three phases. The voltage and phase quality are superior to the first method, and it operates stably under heavier loads. However, since this method requires mechanical rotation, it generates noise and incurs maintenance costs. Additionally, its efficiency is relatively low. Furthermore, both the first and second methods convert three-phase 220 V into output voltage, not three-phase 380 V line-to-line voltage. Therefore, if 380 V is required, an additional transformer is also required, making these unsuitable for use with multiple loads connected to the PDS. The principles for the first and second phase conversion methodology are as illustrated in Figure 2 and Figure 3 below.

The last third method is the electronic phase converter method using power semi-conductors selected in this paper. It converts a single-phase AC voltage to DC using the switching function of power semiconductors and then converts it to a three-phase AC voltage again.

The conversion of single-phase to three-phase power using power semiconductors can be achieved in two ways: by converting the high-voltage single-phase 13,200 V to three-phase 22,900 V, or by converting the single-phase 13,200 V input to a low voltage through a transformer and converting it to three-phase 380 V. The common characteristic of the two methods is that they receive the high voltage of 13,200 V as the input voltage. However, this approach has the disadvantage of increased hardware size and costs due to the need for high insulation performance and a larger quantity of power converters. To address these challenges, this paper adopted a method of converting a low-voltage 220 V input to DC and then converting it to a three-phase line-to-line voltage of 380 V.

The strength and weakness of each method are as shown in

Table 3. Comparison of three methods.

Methods	Strength	Weakness
Static Phase	Simple circuit configuration Low implementation cost	Unstable performance under heavy load conditions Limited power capacity Dedicated transformer required
Rotary Phase	High reliability Resistant to load changes	Loudness, noise Maintenance Dedicated transformer required
Electronic Phase	High-quality three-phase voltage output Three-phase voltage balance High conversion efficiency Voltage stabilization	High initial cost Complex electronic circuits

.

3.1. Three-Phase Voltage Conversion Method Using Power Semiconductors

This paper explains the theoretical method of converting a single-phase voltage to three-phase voltage and the rationale for using semiconductor switching devices. It was stated that most three-phase loads consist of motor loads, as three-phase induction motors are utilized to achieve greater power output. The motor load used in the test in the PDS was a DOL motor without a separate control function. In addition, a pump motor used for farming was manufactured separately. Since the test must assume the worst-case scenario, it is not possible to identify the problems of an STPC in the case of motors that include inverter control functions or have a low inrush current. In this paper, tests are conducted under various load conditions and fault scenarios to assess whether there are any difficulties in using an STPC in the PDS where actual loads are connected, and the results are derived. A brief explanation of the STPC topology is a combination of a single-phase converter and a three-phase inverter. The single-phase voltage was first converted to DC voltage through the single-phase converter and then converted to a three-phase AC voltage using the three-phase inverter. A power utility company must supply power to the load by maintaining the standard voltage and frequency. The three-phase voltage generated by the STPC must conform to the standard voltage and frequency, so the topology and control logic must be configured accordingly. To convert a single-phase voltage to a three-phase voltage, it must be converted to DC and then converted back to AC. Each of the three phases must have the same magnitude, with a phase difference of 120 degrees. To convert to three phases, an AC-DC-AC conversion process must be performed. To directly perform AC-AC conversion, the voltage must be changed like a transformer, and to change the frequency or phase, a DC conversion process must be performed in the middle. This concept is as illustrated in Figure 4.

3.2. Topology

The topology is divided into input and output sections. The input section consists of a single-phase half-bridge, and the output section consists of a three-phase full-bridge. Figure 5 shows a simplified topology of STPC. Here, the source is assumed to be a single-phase Pole-mounted Transformer (P.TR), while the LV Grid represents a three-phase LVDS.

The power semiconductor devices used in an STPC are SiC-MOSFETs. Although Si-IGBTs could be used, SiC devices are more efficient and support higher switching frequencies, which allows for a reduction in the LC-filter size. In addition, the recent increase in SiC device production has made them more economically viable than before. The STPC topology can be divided into a single-phase converter as the input and a three-phase inverter as the output. There is a DC-link in the middle, which acts as a smoothing circuit. A simplified circuit representation of this is illustrated in Figure 5. While not identical, it is conceptually similar to the Back-to-Back Converter used in wind power generation. Since it can be implemented using power semiconductors, the STPC benefits from this advantage.

Table 4. Parameter settings for the construction of the STPC topology.

	Converter (Input)	Inverter (Output)
Applied topology	Single-phase Half-Bridge	Three-phase Full-Bridge
Element	SiC-MOSFET	SiC-MOSFET
Switching frequency	30 kHz	30 kHz
PWM	SPWM	SPWM
Method of control	DQ PI+PR	DQ PI+PR

presents the parameter settings applied to the topology of the STPC.

The power semiconductors used in the topology are calculated using the following equation. Since an STPC consists of a combination of 10 kVA modular units, the input power is calculated as 10 kVA. Additionally, the input voltage is calculated to be 220 V for a single-phase system, and the output voltage is calculated as 380 V/$\sqrt{2}$. The margin is assumed to be 2.

$${\displaystyle \frac{p_{input}}{v_{input}\times pf}}\times \sqrt{2}\times margin={\displaystyle \frac{10 \mathrm{k}\mathrm{V}\mathrm{A}}{220 \mathrm{V}\times 0.9}}\times \sqrt{2}\times 2=142.8\to 150[A]$$

(1)

$${\displaystyle \frac{p_{ouput}}{v_{ouput}\times 3}}\times \sqrt{2}\times margin={\displaystyle \frac{10 \mathrm{k}\mathrm{V}\mathrm{A}}{380 \mathrm{V}/\sqrt{3}\times 3}}\times \sqrt{2}\times 2=42.97\to 50[A]$$

(2)

In a PDS, the RMS voltage is commonly used, but power semiconductor devices must be rated based on peak voltage; hence, $\sqrt{2}$ is applied. A design margin is also included to operate below rated limits, reducing thermal stress and extending component lifespan. Based on the calculation, it was determined that the input converter uses a 150A SiC-MOSFET (Infineon, Neubiberg, Germany), and the output inverter uses a 50A SiC-MOSFET (Infineon, Neubiberg, Germany).

3.3. Control

An STPC control employs proportional-integral (PI) control and proportional-resonant (PR) control to ensure compliance with the standard values for voltage, frequency, and Total Harmonic Distortion (THD) in the PDS. Additionally, due to its control function, the STPC not only performs three-phase conversion but also provides voltage regulation. The STPC utilizes phase-by-phase control, where the single-phase input voltage and output voltage are independently regulated for each phase. Figure 6 and Figure 7 illustrate the control block diagrams, which include DC link voltage control, DC link unbalance control, and input current control [20].

The error is determined by setting the DC link voltage, which is the output voltage of the single-phase converter on the input side, to 750 V.

$$e_{dc}=750V-V_{dc}$$

(3)

$$I_{dc-ref}=K_p{e}_{dc}+K_i\int e_{dc}dt$$

(4)

To determine the error, the measured $V_{dc}$ value is subtracted from the target 750 V, and $I_{dc-ref}$ is generated using a PI controller.

$$H_{notch}\left(s\right)={\displaystyle \frac{S^2+w_0^2}{s^{2+}2\xi {\omega }_0+{\omega }_0^2}}$$

(5)

A notch filter is used to control the 2nd to 7th harmonic components in the output signal of the PI controller. The transfer function of the filter is as presented in Equation (5).

$$I_{dc\_ref\_filtered}=H_{notch}\left(s\right)I_{dc\_ref}$$

(6)

$$I_{d\_ref}=I_{dc\_ref\_filtered}·\sin \theta$$

(7)

After applying the filter, the current reference value is as in Equation (6), and to obtain the d-axis current considering the d-q coordinate system, multiply by sine as in Equation (7).

$$I_{error}=I_{d-ref}-I_{in\_d}$$

(8)

The input current $I_{in\_d}$ is measured and subtracted from $I_{d-ref}$.

$$G_{PR}\left(s\right)=K_p+{\displaystyle \frac{K_{r}s}{s^2+{\omega }_{res}^2}}$$

(9)

The current error is then compensated using a PR controller.

$$V_{PR}=G_{PR}\left(s\right)·I_{error}$$

(10)

$$V_{final}=V_{PR}+V_{in}$$

(11)

Finally, the input voltage is added, and the voltage is compared with the carrier signal to generate PWM.

Figure 7 illustrates the output three-phase voltage control block diagram. A PR controller is utilized to regulate the output voltage and attenuate high-frequency components.

$$V_{ref}=220\sqrt{2}·\sin \theta$$

(12)

$$e_v=V_{ref}-V_{out\_LG}$$

(13)

The difference between the target output phase voltage and the actual output phase voltage is calculated.

$$G_{PR}\left(s\right)=K_p+{\displaystyle \frac{K_{r}s}{s^2+{\omega }_{res}^2}}$$

(14)

$$V_{PR1}=G_{PR}\left(s\right)·e_v$$

(15)

A PR controller is employed to compensate for the voltage error.

$$I_{error}=V_{PR1}-I_{out}$$

(16)

$$V_{PR2}=G_{PR}\left(s\right)·I_{error}$$

(17)

To mitigate the voltage drop caused by the output current, the output current is measured and subtracted from the voltage controller.

$$V_{final}=V_{PR2}+V_{phase}$$

(18)

Subsequently, the output phase voltage is added, and the voltage is compared with the carrier signal to generate a PWM signal. Since the output voltage is subject to phase-by-phase control, the control process is performed phase by phase. The detailed topology of the STPC, consisting of the input and output sections, is as illustrated in Figure 8 below. Assuming a transformer as the source, a single-phase 220 V as the input voltage from the source, converted to DC through a single-phase converter, and the DC voltage is converted back to a three-phase voltage. The output voltage and THD, etc., are controlled through the control section. Since the STPC is connected not only to single-phase loads but also to multiple unpredictable single and three-phase loads, these aspects must be considered in the control strategy.

3.4. Connection to PDS

The STPC is connected to the output side of the P.TR in the PDS of the overhead distribution line. Both three-phase and single-phase loads can be connected to the output side of the STPC. Since the LVDS is a radial system, in addition to the STPC line, a separate single-phase line using the existing transformer as a power source can be drawn from the output side of the transformer.

The input voltage of the STPC is identical to the output voltage of the transformer. The LV three-phase load is connected to the output terminal of the STPC. However, single-phase loads not related to the STPC can be connected to the output terminal of the transformer. Therefore, the harmonics generated by the STPC should not have a negative effect on other LVDSs.

Before conducting load experiments with actual loads, various parameters are configured through computer simulations to analyze their effects on each PDS. The simulation results are presented in Section 4.

3.5. LV Loads on Distribution Lines

Unlike a PTS, a PDS has various LV loads connected to it. Since a STPC is used in an LVDS, this paper focuses exclusively on loads connected to the LVDS. The loads connected to the LVDS include single-phase household appliances such as televisions, refrigerators, washing machines, dryers, grinders, motors, and heaters used in agricultural applications. Among those, a three-phase voltage is primarily used in applications requiring high capacity, including motor-driven loads and resistive heating loads. Additionally, nonlinear loads based on switching elements are increasingly being integrated into the PDS. The loads and variables used for testing are illustrated in Figure 9.

Nonlinear loads generate harmonics in the PDS, leading to PQ degradation. Harmonics contribute to increased heating in motors, electrical equipment, and conductors, resulting in operational inefficiencies. In the case of electric motors, inrush currents can cause a significant voltage drop—up to four times the normal current—or lead to the malfunction of other connected equipment [21,22]. Additionally, unbalanced loads result in inefficient power utilization, inducing unbalanced currents in the neutral conductor [23,24].

Consequently, experimental investigations of these low-voltage loads under on-site conditions are essential. Figure 10 shows an experimental system to demonstrate these problems.

3.6. Test of LV Distribution Line

Figure 10 presents a schematic diagram of the test setup used to apply the STPC to a PDS. The test involved connecting various loads—including single-phase household loads, single-phase and three-phase motor loads, and three-phase heating loads—to the three-phase output of the STPC. Additionally, a separate single-phase line, directly connected to the main transformer and not to the STPC output, was included in the setup to evaluate whether the operation of the STPC influenced the PQ of other lines.

4. Result

4.1. Result of Converting Three-Phase Voltage with Computer Simulation

A computer simulation was conducted using PSIM, based on the topology described in Section 3.2. As shown in Figure 11, the input voltage was observed to be 220 V, and the output voltage comprised three waveforms, each with a phase voltage of 220 V.

Computer simulations were performed under four different conditions: (A) normal load conditions, (B) DER connection, (C) nonlinear load connection, and (D) short-circuit fault occurrence.

(A)

Normal Load Conditions

Under normal load conditions, as shown in Figure 11, both the input and output voltages remained within their reference ranges, and the THD was maintained below 5%. In addition, it was confirmed that the voltage, current, and THD of the line branched separately from the secondary side of the transformer—regardless of the STPC—remained within acceptable limits.

(B)

DER Connection

In the second scenario, the DER was connected to the branch line. Since the STPC operates based on output current sensing, it does not support bidirectional power flow. However, a DER can still be connected to the branch line. The simulation observed whether the DER’s power injection affected the input voltage or current of the STPC. As a result, the output voltage of the STPC remained stable, while the magnitude of the branch line current was observed to change.

(C)

Short-Circuit Fault

The third condition involved the occurrence of a short-circuit fault either on the secondary side of the STPC or on the branch line. In both cases, faults were simulated individually to ensure that one fault would not impact other lines. The simulation results indicated that when a fault occurred on the secondary side of the STPC, a fault current flowed through the STPC, but the branch line was unaffected. Conversely, when a fault occurred on the branch line, a high fault current was observed in that line, and voltage and current were not measurable at the STPC.

(D)

Nonlinear Load Connection

In the final condition, a nonlinear load was applied. Due to the characteristics of the load, only the output current was distorted, while the output voltage and the branch line remained unaffected.

The simulation results for the short-circuit fault scenario are shown in Figure 12.

4.2. Test Results of LV Loads

The load test for the STPC was configured as shown in Figure 13. Although the test system in Figure 13 produces a three-phase output, the analysis primarily focused on the unbalance caused by single-phase loads distributed across the phases, as well as the impact on other lines [25].

Household appliances are typically single-phase loads. While the output of the STPC provides a three-phase voltage, it can also accommodate single-phase loads; therefore, household loads were connected to each phase separately. The test loads included televisions, lighting devices, and heating equipment. In addition to residential appliances, single-phase industrial loads such as compressors and welders were also connected.

Although welders are not typically considered household appliances, and guidelines from KEPCO recommend connecting them through a dedicated transformer, they were included in the test to examine their impact on other loads [24].

Each phase of the STPC was connected to a single-phase load. Specifically, electric heater and compressor loads were connected to the R-N phase, the welding machine was connected to the S-N phase, and the electronic heater, television, and lighting loads were connected to the T-N phase. The test results are illustrated in Figure 14.

Except for the S phase, to which the welding machine was connected, the voltage and current in the other phases remained stable. In the S phase, a significant change in current was observed, while the voltage remained within the specified limits, and the connected loads operated normally. The THD also showed a significant increase only in the S phase, with no noticeable impact on the other phases. These results confirm that the load unbalance and distortion in one phase had a minimal effect on the performance of the other phases in the STPC. The detailed THD measurement results for voltage and current are presented in

Table 5. THD measurements for each phase.

	VTHD-R	VTHD-S	VTHD-T	ITHD-R	ITHD-S	ITHD-T
THD	0.66%	4.62%	0.33%	8.32%	92.63%	11.10%

.

Figure 15 presents the input and output voltage and current waveforms for a nonlinear load rated at 3 kVA. As shown in the figure, the voltage waveform remains sinusoidal, while the current waveform varies according to the characteristics of the load. In the case of nonlinear loads, only the current waveform is affected, with no significant impact observed on other loads or the STPC. These results indicate that the STPC maintains a stable operation even under nonlinear loading conditions.

For the induction motor load, a DOL motor without inverter control functionality was considered. Induction motors typically exhibit a high inrush current during startup, which can significantly affect upstream power equipment and the connected PDS, often resulting in voltage drops. Therefore, an experimental analysis of motor loads was necessary [21]. Although methods such as soft starters and capacitor banks are commonly used to mitigate motor inrush current, these techniques are not suitable for the STPC, as it operates as a grid-connected power conversion system supplying multiple loads [26]. While some large manufacturers provide inrush current specifications for motor loads, such data are not always available.

Table 6. Induction motor load characteristics.

HP	Rated Power	Pole	Rated Speed	Efficiency	Power Factor	Rated Current		Inrush Current
						220 V	380 V	220 V	380 V
1	0.75 kW	2	3400 rpm	73%	84	3.2	1.9	19.3	11.1
		4	1710 rpm	74%	77	3.5	2.0	20.7	12.0
2	1.5 kW	2	3400 rpm	77%	81	6.3	3.7	37.9	21.9
		4	1720 rpm	80%	77	6.4	3.7	38.3	22.2
3	2.2 kW	2	3420 rpm	80%	85	8.5	4.9	50.9	29.5
		4	1730 rpm	82%	81	8.7	5.0	52.2	30.2

presents typical starting current values based on motor ratings. It should be noted that efficiency, power factor, and rated current values are based on 100% load conditions, and these characteristics may vary depending on the manufacturer.

A motor load was required for the test. To avoid no-load operation, a separate water tank system was constructed to circulate water during motor operation.

Power semiconductors have specified rated voltage and current values, which represent their absolute maximum ratings. While AC voltage and current are typically expressed in root mean square (RMS) values, the ratings of power semiconductors refer to peak values; therefore, the actual usable range is lower than the rated values.

Similarly to conventional distribution equipment, power semiconductors can tolerate a short-duration overcurrent. However, this allowable short-time current is limited to pulse-level durations, not in seconds, and thus cannot withstand the high inrush current of induction motors. As a result, the capacity of the motor that can be used with the STPC must be carefully limited, taking into account the expected inrush current.

The motors used in the experiment were three-phase induction motors rated at 3 HP and 5 HP. The test results are presented in Figure 16 and Figure 17.

The test results indicated that both the input and output currents of the STPC exhibited an inrush current during motor startup. Specifically, the output current of the R phase reached a peak value of up to 75 and settled to a steady state after approximately eight cycles. Considering the maximum allowable current of the MOSFET elements used in the STPC’s parallel structure, the motor’s inrush current must remain below this limit to ensure safe operation. As shown in the input/output voltage waveforms on the right side of Figure 16, the voltage maintained a relatively stable sinusoidal waveform without noticeable disturbances.

5. Discussion

In the results section, the three-phase voltage and current waveforms of the STPC were verified through both simulation and experimental testing. Beyond performance under typical low-voltage load conditions, it is also important to consider fault scenarios that may occur in real PDS.

Because the STPC is installed on overhead distribution lines, it may be exposed to environmental hazards such as strong winds, tree falls, and vehicle collisions. In such cases, the STPC must disconnect immediately to protect adjacent PDS assets and loads. To support this, a Molded Case Circuit Breaker (MCCB) is embedded in the enclosure as a protective device.

Another consideration is economic feasibility. Unlike conventional transformers that use iron cores and coils, the STPC includes power semiconductor devices, LC filters, and cooling systems. While this enables advanced control and voltage regulation, it also introduces potential complexity in fault diagnosis and maintenance. Therefore, in real-world applications—especially in areas with limited three-phase access or wildfire risks—it is essential to perform a cost–benefit analysis when considering the STPC installation versus conventional infrastructure.

5.1. Limitations of STPC

Although the proposed STPC demonstrates a stable three-phase output under various load conditions, it has several inherent limitations that should be addressed in future work:

5.1.1. Inrush Current Sensitivity

For motors controlled via inverters, inrush current is not a critical issue because the motor speed is gradually ramped up through soft-start algorithms inherent in the inverter control. However, for DOL motors without inverter control, the initial inrush current during startup can be several times higher than the rated operating current. Therefore, it is important to limit the motor capacity such that the inrush current does not exceed the rated current of the STPC, to ensure safe and reliable operation.

5.1.2. Component Lifetime Concerns

Key components such as DC link capacitors and cooling fans are subject to degradation over time, particularly under continuous operation or in harsh environments. This may affect the long-term reliability of STPCs compared to passive transformer-based systems [27].

5.1.3. Initial Cost and Development Overhead

While STPCs can reduce line installation and transformer costs in certain cases, the initial hardware and development costs may be higher due to the need for high-performance power semiconductors, control circuitry, and thermal management. These costs are especially significant at the early deployment stage before mass production.

5.2. Future Directions

The proposed STPC also holds potential for further enhancement and integration in advanced distribution networks. Key future directions include the following:

5.2.1. Grid-Forming Inverter Function

As described in this paper, the STPC topology has the capability to regulate both voltage and frequency through closed-loop control. This makes it suitable for grid-forming inverter applications, particularly in isolated or weak grids where power converters must establish the voltage reference [28,29].

5.2.2. Modular and Scalable Design

The proposed STPC has already been developed with a modular architecture, allowing for flexible capacity scaling. Multiple units can be paralleled to support larger loads or to provide redundancy, making the system adaptable to a wide range of applications in the low-voltage distribution network [30].

5.2.3. Application to LVDC Systems

LVDC distribution systems are typically designed under the assumption that both the power lines and the connected loads operate on DC. However, in practical deployments, the need to supply conventional AC loads is expected to arise. In this context, the proposed STPC can serve as a viable interface solution. It is therefore suggested that the control and conversion principles presented in this paper may be applicable to future LVDC systems requiring AC load compatibility [31,32].

6. Conclusions

This paper presented a simulation and experimental evaluation of a method for converting a single-phase 220 V AC input to three-phase 380 V AC using power semiconductor devices, specifically SiC-MOSFETs. The proposed system topology consists of a single-phase half-bridge AC–DC converter followed by a three-phase full-bridge inverter. The 220 V input is converted to 750 V DC and then inverted to produce a 380 V three-phase AC output.

To reflect the characteristics of real-world PDSs, which are more complex than PTSs, various load conditions were considered, including unbalanced loads, nonlinear loads, and motor loads. The experimental results verified that single-phase household loads operated stably, with normal current and voltage waveforms. For nonlinear loads, although the current waveform was distorted due to load characteristics, the output voltage remained stable. In the case of motor loads, the STPC functioned normally when the system capacity was set appropriately to account for the inrush current.

Considering the nature of PDSs—where multiple diverse loads are connected—it was confirmed that converting single-phase AC to DC, and then from DC back to three-phase AC using power semiconductors, is an effective approach for application in low-voltage distribution networks. Moreover, by utilizing an STPC, it is possible to reduce the need for additional line construction, transformer installation, and pole reinforcement, as outlined in

Table 7. Comparison between conventional method and proposed STPC.

Comparison Criteria	Conventional Method	Proposed STPC
Installation Method	Line extension + transformer installation	Modular installation at load site
Conversion Efficiency	High (typically >98%)	Moderate (>96%)
Voltage Regulation	Passive, no regulation capability	Stable regulation of the output voltage
Maintenance Complexity	Low (simple structure)	Moderate (semiconductor components, cooling)
Initial Installation Cost	High (due to line and transformer construction)	Lower
MTBF (Expected Reliability)	Very high (few active components)	Moderate (semiconductor aging, fan/capacitor life)
Harmonic Mitigation	Not available	Built-in through filters and PR controller

. Thus, an STPC is a promising solution for supplying a three-phase voltage efficiently to low-voltage consumers.