This paper presents a novel three-phase solar inverter topology incorporating flying capacitors to eliminate leakage currents while maintaining high efficiency. The proposed design addresses critical challenges in transformerless photovoltaic systems by combining three modified single-phase H5 inverters with shared DC-link architecture.
Topology and Operational Principles
The three-phase solar inverter configuration consists of three independent H5-derived legs, each containing five power switches and one flying capacitor ($C_{fly}$). The DC negative terminal connects directly to ground, shorting parasitic capacitances ($C_{PV}$) between solar panels and earth. Key equations governing common-mode behavior include:
$$U_{CM} = \frac{U_{AN} + U_{BN} + U_{CN}}{3}$$
$$U_{DM} = U_{AN} – U_{BN}$$
Table 1 shows the switching states for leakage current suppression:
| Mode | S1 | S2 | S3 | S4 | S5 |
|---|---|---|---|---|---|
| 1 | 1 | 0 | 1 | 0 | 1 |
| 2 | 1 | 0 | 0 | 0 | 1 |
| … (8 operating modes total) | |||||
Control Strategy and Capacitor Design
The solar inverter employs synchronized unipolar SPWM with voltage-balancing control for flying capacitors. Critical design equations for capacitor selection include:
$$C_{fly} = \frac{P_{out}T_s}{2\Delta U_{C_{fly}}}$$
$$\Delta U_{C_{fly}} = \sqrt{U_{DC}^2 – U_{out}^2}$$
Where $T_s$ represents switching period and $\Delta U_{C_{fly}}$ limits voltage ripple to <5V. For 200W/phase operation:
$$C_{fly} = \frac{200 \times 50\mu s}{2 \times 5} = 1000\mu F$$
Experimental Verification
A 600W prototype solar inverter demonstrated:
| Parameter | Value |
|---|---|
| Leakage Current (Full Load) | 9mA peak |
| Efficiency @ 200W | 98.2% |
| THD (Balanced Load) | 2.1% |
The three-phase solar inverter topology achieves complete decoupling between phases through independent operation of each H5 leg. This enables superior unbalanced load capability compared to conventional three-phase designs, with less than 2% voltage imbalance under 100% asymmetric loading.
$$U_{imbalance} = \frac{\max(|U_a – U_{avg}|, |U_b – U_{avg}|, |U_c – U_{avg}|)}{U_{avg}} \times 100\%$$
Experimental waveforms confirm stable operation with leakage currents below 10mA under all loading conditions, complying with VDE-AR-N 4105 standards for grid-connected solar inverters.
Comparative Analysis
Key advantages over existing solar inverter topologies:
| Topology | Component Count | Leakage Current | Efficiency |
|---|---|---|---|
| Proposed | 15 switches + 3 caps | <10mA | 98.2% |
| Conventional H6 | 18 switches | 35mA | 97.1% |
| T-Type NPC | 24 switches | 22mA | 97.8% |
The proposed solar inverter architecture demonstrates significant improvements in leakage current suppression while maintaining high power conversion efficiency. The flying capacitor implementation provides inherent voltage balancing without additional complex control algorithms, making it particularly suitable for three-phase photovoltaic systems requiring high reliability and safety.
$$ \eta = \frac{P_{AC}}{P_{DC}} = \frac{3U_{phase}I_{phase}\cos\phi}{U_{DC}I_{DC}} $$
Future work will focus on optimizing the solar inverter for medium-voltage applications and integrating maximum power point tracking (MPPT) algorithms specifically tailored for three-phase flying capacitor topologies.