Current source grid-connected inverters (CSGCIs) have gained attention in solar energy systems due to their inherent shoot-through immunity and boost capabilities. However, conventional CSGCIs face challenges such as mandatory overlap time settings and significant high-frequency common-mode leakage currents. This paper proposes a transformerless current source grid-connected inverter (TCSGCI-NT) topology that eliminates overlap time requirements while suppressing leakage currents. The design leverages a unipolar SPWM modulation strategy and introduces bridge-arm capacitors to ensure safe commutation.
1. Topology and Modulation Strategy
The proposed TCSGCI-NT topology (Figure 1) modifies conventional CSGCIs by:
- Adding switch S5 in the DC-link path
- Implementing bridge capacitors C1 and C2
- Adopting split DC inductors L1 and L2
The unipolar SPWM modulation operates with:
$$u_c > 0 \rightarrow S_1/S_4 \text{ active}, S_5 = \overline{S_1}$$
$$u_c < 0 \rightarrow S_2/S_3 \text{ active}, S_5 = \overline{S_3}$$
2. Operational Analysis
The solar inverter demonstrates four operating modes with distinct switching states:
| Mode | Active Switches | Output Current | Common-Mode Voltage |
|---|---|---|---|
| 1 | S1, S4 | +Idc | $$u_{cmv} = \frac{u_g}{2}$$ |
| 2 | S5 | 0 | |
| 3 | S2, S3 | -Idc | |
| 4 | S5 | 0 |
The common-mode voltage remains constant at:
$$u_{cmv} = \frac{u_{PO} + u_{NO}}{2} = \frac{u_g}{2}$$
where uPO and uNO represent DC-link voltages relative to ground.
3. Leakage Current Suppression
The solar inverter achieves leakage current mitigation through:
$$i_{tcm} = 2C_{pv}\frac{du_{cmv}}{dt}$$
With fixed ucmv containing only fundamental frequency components, high-frequency leakage currents are eliminated. Experimental measurements show leakage currents below 20mA, complying with VDE-0126-1-1 standards.
4. Commutation Analysis
The bridge capacitors C1 and C2 enable overlap-free commutation through three-phase resonance:
$$L\frac{di_{dc}}{dt} = \frac{1}{C_{eq}}\int i_{dc}dt$$
where Ceq = C1||C2. This resonance maintains continuous current flow during switching transitions, eliminating voltage spikes without dead-time compensation.
5. Experimental Validation
A 770W prototype demonstrates the solar inverter’s performance:
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| DC Current (Idc) | 10.5A | Grid Voltage | 220V |
| Switching Frequency | 5kHz | Filter Capacitance | 80μF |
| DC Inductors | 8mH | Leakage Current | <20mA |
Key waveforms confirm:
- Sinusoidal grid current (THD < 3%)
- Stable DC-link current (10.5A ± 2%)
- Fixed common-mode voltage at ug/2
6. Conclusion
The proposed solar inverter topology achieves:
- 98.2% peak efficiency
- Zero overlap time requirement
- Leakage current reduction by 89% vs conventional CSGCIs
- Three-level output with reduced filtering needs
This design enables compact, reliable photovoltaic systems without isolation transformers, particularly suitable for 1-5kW residential applications. Future work will explore three-phase implementations and MPPT integration.