In recent years, the demand for renewable energy systems has surged, with photovoltaic (PV) power generation playing a pivotal role. Among various PV system configurations, transformerless solar inverters have gained significant attention due to their compact size, lightweight design, high efficiency, and cost-effectiveness. However, the absence of galvanic isolation in transformerless solar inverters introduces a critical issue: common-mode leakage currents. These currents arise due to parasitic capacitances between the PV panels and the ground, leading to safety hazards, electromagnetic interference (EMI), and distortion in grid-connected currents. Therefore, mitigating leakage currents in transformerless solar inverters is a paramount research focus.
Numerous topologies and modulation strategies have been proposed to address leakage current issues. For instance, conventional H-bridge inverters with unipolar sinusoidal pulse width modulation (SPWM) exhibit high common-mode voltage variations, resulting in substantial leakage currents. Modified topologies, such as H5, H6, and HERIC inverters, incorporate additional switches to decouple the PV array from the grid during freewheeling periods, thereby reducing common-mode voltage fluctuations. While effective, these solutions often increase component count, complexity, and cost, and may suffer from reliability concerns like shoot-through risks in bridge legs. Alternatively, multilevel solar inverters offer advantages such as reduced voltage stress on power devices, improved output waveform quality, and lower EMI. Yet, traditional multilevel inverters require numerous components, which can compromise system reliability and economic viability.
In this context, we propose a novel single-phase four-switch multilevel solar inverter derived from the full-bridge topology. The proposed inverter integrates coupled inductors and diodes to achieve five-level output voltage while eliminating the need for dead-time insertion, thus enhancing reliability. We analyze the operating principles, establish a high-frequency common-mode model, and evaluate common-mode voltage characteristics under various switching states. Furthermore, we present two modulation strategies tailored for different applications: one for five-level output voltage and another for constant common-mode voltage to suppress leakage currents. Simulation and experimental results validate the effectiveness of the proposed solar inverter topology and modulation techniques.
The core of our proposed solar inverter is illustrated in Figure 1, which shows a single-phase four-switch configuration with coupled inductors on each bridge arm. The topology comprises switches S1 to S4, diodes VD1 to VD4, coupled inductors, filter inductors L, and the parasitic capacitance CPV from the PV panels to ground. Unlike traditional H-bridge solar inverters, this topology allows simultaneous conduction of switches in the same arm without shoot-through risk, as the coupled inductors and diodes provide alternative current paths. This design enables five-level output voltage, reducing harmonic distortion and EMI.
To understand the operation, consider the switching states of arm a (switches S1 and S2). Let “1” denote switch ON and “0” denote switch OFF. The four possible states are “10”, “01”, “11”, and “00”. For state “10” (S1 ON, S2 OFF), the output voltage VaN equals the DC link voltage VPN. For state “01” (S1 OFF, S2 ON), VaN is zero. For state “11” (both S1 and S2 ON), VaN equals VPN/2 due to the voltage division across the coupled inductor. Similarly, for state “00” (both S1 and S2 OFF), VaN also equals VPN/2 as the current freewheels through diodes. The same logic applies to arm b (switches S3 and S4). Consequently, the inverter can produce output voltage levels of +VPN, +VPN/2, 0, -VPN/2, and -VPN, forming a five-level waveform. The voltage stress on all power devices remains at VPN, comparable to conventional topologies.
The high-frequency common-mode behavior is crucial for leakage current analysis. The common-mode voltage VCM is defined as the average of the two bridge arm voltages relative to the DC link midpoint N:
$$V_{CM} = \frac{V_{aN} + V_{bN}}{2}$$
The differential-mode voltage VDM, which drives the grid current, is:
$$V_{DM} = V_{aN} – V_{bN}$$
From these equations, we can express VaN and VbN in terms of VCM and VDM:
$$V_{aN} = V_{CM} + \frac{V_{DM}}{2}$$
$$V_{bN} = V_{CM} – \frac{V_{DM}}{2}$$
Figure 2 depicts the high-frequency common-mode equivalent circuit, where Lg represents the parasitic inductance between the PV system and ground. When the filter inductors L are balanced, the differential-mode voltage does not influence the common-mode loop. Thus, the leakage current primarily depends on VCM. If VCM remains constant, the voltage across CPV lacks high-frequency components, effectively suppressing leakage current.
Given that each bridge arm has four states, the entire inverter has 16 possible switching states. Table 1 summarizes the corresponding output voltages and common-mode voltages for each state. It reveals that VCM can vary among VPN/4, VPN/2, and 3VPN/4, while the output voltage Vab exhibits five levels. To achieve five-level modulation, we employ a carrier phase-shift PWM technique. As shown in Figure 3, two triangular carriers (c1, c4 and c2, c3) are phase-shifted by 180°, compared with a sinusoidal modulation wave m. The switching signals are generated based on comparisons: S1 and S4 are ON when c1 and c4 are below m; S2 and S3 are ON when c2 and c3 are above m. This scheme yields five-level Vab with reduced common-mode voltage jumps (VPN/4 compared to VPN/2 in unipolar H-bridge solar inverters), thereby partially mitigating leakage current.
| S1 | S2 | S3 | S4 | VaN | VbN | Vab | VCM |
|---|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 1 | VPN | 0 | VPN | VPN/2 |
| 0 | 1 | 1 | 0 | 0 | VPN | -VPN | VPN/2 |
| 1 | 0 | 0 | 0 | VPN | VPN/2 | VPN/2 | 3VPN/4 |
| 1 | 0 | 1 | 1 | VPN | VPN/2 | VPN/2 | 3VPN/4 |
| 1 | 1 | 0 | 1 | VPN/2 | 0 | VPN/2 | VPN/4 |
| 0 | 0 | 0 | 1 | VPN/2 | 0 | VPN/2 | VPN/4 |
| 0 | 1 | 0 | 0 | 0 | VPN/2 | -VPN/2 | VPN/4 |
| 0 | 1 | 1 | 1 | 0 | VPN/2 | -VPN/2 | VPN/4 |
| 0 | 0 | 1 | 0 | VPN/2 | VPN | -VPN/2 | 3VPN/4 |
| 1 | 1 | 1 | 0 | VPN/2 | VPN | -VPN/2 | 3VPN/4 |
| 0 | 0 | 0 | 0 | VPN/2 | VPN/2 | 0 | VPN/2 |
| 0 | 0 | 1 | 1 | VPN/2 | VPN/2 | 0 | VPN/2 |
| 1 | 1 | 1 | 1 | VPN/2 | VPN/2 | 0 | VPN/2 |
| 1 | 1 | 0 | 0 | VPN/2 | VPN/2 | 0 | VPN/2 |
| 1 | 0 | 1 | 0 | VPN | VPN | 0 | VPN |
| 0 | 1 | 0 | 1 | 0 | 0 | 0 | 0 |
For applications requiring stringent leakage current suppression, we propose a constant common-mode voltage modulation strategy. From Table 1, selecting switching states where VCM equals VPN/2 ensures a steady common-mode voltage. Figure 4 illustrates this approach: two sinusoidal modulation waves, phase-shifted by 180°, are compared with triangular carriers to generate gate signals. This modulation confines VCM to VPN/2, eliminating high-frequency variations and drastically reducing leakage current.
To validate the proposed solar inverter topology and modulation strategies, we conducted simulations in MATLAB/Simulink. The parameters are: DC link voltage VPN = 380 V, grid voltage 311 V (50 Hz), rated power 1 kW, filter inductance L = 5 mH, coupled inductance 2 mH, and parasitic capacitance CPV = 300 nF. For comparison, we first simulated a conventional H4 solar inverter with unipolar PWM. The results show high-magnitude, high-frequency VCM fluctuations, leading to significant leakage current (effective value over 300 mA) and grid current distortion with total harmonic distortion (THD) exceeding 5%.
Next, we simulated the proposed four-switch five-level solar inverter with carrier phase-shift modulation. The output voltage Vab displays five distinct levels, and VCM varies between VPN/4 and 3VPN/4, reducing the jump magnitude by half compared to the H4 inverter. Consequently, leakage current decreases noticeably, and grid current THD drops below 5%, demonstrating improved performance.
Finally, we simulated the four-switch inverter with constant common-mode voltage modulation. Here, Vab becomes three-level, but VCM remains fixed at VPN/2. The leakage current is effectively suppressed, with an RMS value below 30 mA, complying with the VDE-0126-1-1 safety standard for solar inverters.
Experimental verification was performed on a hardware platform controlled by a TMS320F28335 DSP and XC3S400 FPGA. The test conditions were: VPN = 120 V, grid voltage 90 V (50 Hz), L = 5 mH, coupled inductance 2 mH, and CPV = 300 nF. For the traditional H4 solar inverter, measurements confirm high-frequency VCM variations between 0, VPN/2, and VPN, causing large leakage current (RMS 372 mA). For the four-switch five-level solar inverter, the output voltage shows five levels, and VCM ranges are reduced, leading to lower leakage current (RMS 71.1 mA). For the constant VCM modulation, the leakage current RMS is merely 13.6 mA, meeting the VDE standard. Additionally, load step tests under no-load and full-load conditions confirm stable five-level output and sinusoidal grid current, proving the solar inverter’s robustness.
The mathematical analysis of leakage current can be extended using the common-mode equivalent circuit. The leakage current i_leak is governed by:
$$i_{leak} = C_{PV} \frac{d v_{CPV}}{dt}$$
where v_CPV is the voltage across CPV. In the high-frequency model, v_CPV relates to VCM and the ground impedance. Assuming a simplified RLC circuit, the transfer function between VCM and i_leak can be derived. For constant VCM, d v_CPV/dt is zero, idealizing i_leak to zero. In practice, parasitic elements cause residual leakage, but it remains minimal.
Furthermore, the output voltage harmonic spectrum for the five-level solar inverter can be analyzed using Fourier series. The stepped waveform reduces lower-order harmonics compared to two-level inverters, easing filter design. The harmonic distortion factor (HDF) for a five-level inverter with modulation index m_a is given approximately by:
$$HDF \approx \frac{\sqrt{\sum_{n=2}^{\infty} V_n^2}}{V_1}$$
where V_n is the nth harmonic voltage magnitude and V_1 is the fundamental. For m_a = 0.8, the HDF is typically below 5%, as observed in simulations.
Efficiency is another critical metric for solar inverters. The proposed topology minimizes switching losses because only two switches toggle at high frequency per cycle, similar to unipolar modulation. Conduction losses depend on the number of active components. Table 2 compares key parameters of the proposed solar inverter with conventional topologies.
| Topology | Switch Count | Output Levels | Common-Mode Voltage | Leakage Current | Reliability |
|---|---|---|---|---|---|
| Traditional H4 | 4 | 3 | High-frequency variations | High | Moderate (dead-time needed) |
| H5/H6 | 5/6 | 3 | Reduced variations | Moderate | Moderate |
| Proposed (five-level) | 4 | 5 | Reduced variations (VPN/4 jumps) | Low | High (no dead-time) |
| Proposed (constant VCM) | 4 | 3 | Constant (VPN/2) | Very Low | High |
The design of coupled inductors is essential for proper operation. The inductance value affects voltage division and current ripple. For a given switching frequency f_sw, the inductance L_c should satisfy:
$$L_c \geq \frac{V_{PN}}{4 \cdot f_sw \cdot \Delta I}$$
where ΔI is the desired current ripple. In our design, L_c = 2 mH ensures less than 20% ripple at f_sw = 10 kHz.
Control strategies for grid-connected solar inverters often incorporate phase-locked loops (PLLs) for synchronization and current regulators like proportional-resonant (PR) or synchronous reference frame (SRF) controllers. The block diagram in Figure 5 shows a typical control scheme for the proposed solar inverter. The grid voltage is sensed for PLL, and the grid current is regulated to follow a sinusoidal reference in phase with the grid voltage. The modulation block generates switching signals based on the selected strategy.
In conclusion, the single-phase four-switch multilevel solar inverter presented here offers a compelling solution for transformerless PV systems. By leveraging coupled inductors and innovative modulation, it achieves five-level output with reduced common-mode voltage jumps or constant common-mode voltage for leakage current suppression. The topology eliminates shoot-through risks, enhancing reliability. Simulation and experimental results confirm significant leakage current reduction and improved grid current quality. Future work may explore three-phase extensions, advanced control algorithms, and integration with energy storage systems for hybrid solar inverters. As solar energy penetration grows, such efficient and reliable solar inverters will be crucial for sustainable power generation.
The proposed solar inverter topology aligns with global trends toward higher efficiency and power density in renewable energy conversion. With ongoing advancements in wide-bandgap semiconductors like SiC and GaN, switching frequencies can be increased, further shrinking passive components and improving performance. Moreover, smart grid functionalities, such as reactive power support and fault ride-through, can be incorporated into the control platform, making these solar inverters versatile for modern grid requirements.
In summary, this research contributes to the development of next-generation solar inverters that balance cost, performance, and safety. The four-switch multilevel approach demonstrates that innovative circuit design and modulation can address persistent issues like leakage currents without adding excessive components. We believe this work will inspire further innovations in transformerless solar inverter technology, accelerating the adoption of solar power worldwide.