In recent years, the global solar photovoltaic (PV) industry has experienced rapid growth and large-scale deployment. Many countries have prioritized the integration of grid-connected PV systems as a strategic pathway for energy structure transformation. As a key component in these systems, solar inverters play a crucial role in converting DC power from PV panels into AC power for grid injection. However, transformerless solar inverters, while offering higher efficiency and lower cost, face significant technical challenges, particularly regarding common-mode leakage currents. These currents can lead to electromagnetic interference, safety hazards, and non-compliance with grid standards such as VDE-0126-1-1, which mandates disconnection if leakage currents exceed 300 mA. In this paper, I investigate the leakage current issue in conventional current-source solar inverters and propose a novel CH5 topology with an improved modulation strategy to effectively suppress common-mode leakage currents.
The leakage current in transformerless solar inverters arises due to the parasitic capacitance between PV panels and ground, forming a common-mode loop. The common-mode voltage, which drives this current, is influenced by the switching states of the inverter. For single-phase systems, traditional topologies like the current-source H4 (CH4) solar inverter exhibit high-frequency variations in common-mode voltage, leading to substantial leakage currents. I begin by analyzing the CH4 solar inverter to understand its limitations. The CH4 topology consists of four switches (typically IGBTs with series diodes or RB-IGBTs) and an inductive DC link, as shown in the common-mode model. The common-mode voltage \(V_{CM}\) is defined as:
$$V_{CM} = \frac{V_{PO} + V_{NO}}{2}$$
where \(V_{PO}\) and \(V_{NO}\) are the voltages from the positive and negative DC rails to the neutral point of the grid. Based on the switching states, I derive the common-mode voltage for the CH4 solar inverter. The table below summarizes the switching states and corresponding common-mode voltages:
| Switching State (S1, S2, S3, S4) | Output Current \(i(t)\) | Common-Mode Voltage \(V_{CM}\) |
|---|---|---|
| 1, 0, 0, 1 | \(I_{dc}\) | \(0.5V_g\) |
| 1, 1, 0, 0 | 0 | \(V_g\) |
| 0, 1, 1, 0 | \(-I_{dc}\) | \(0.5V_g\) |
| 0, 0, 1, 1 | 0 | 0 |
From this analysis, I observe that during the positive half-cycle, \(V_{CM}\) switches between \(0.5V_g\) and \(V_g\) at high frequency, while during the negative half-cycle, it switches between \(0.5V_g\) and 0. This high-frequency variation excites the common-mode loop, resulting in large leakage currents. Therefore, the CH4 solar inverter fails to meet leakage current suppression requirements, especially at higher switching frequencies or with smaller DC-link inductances, as seen in modern designs using wide-bandgap devices like SiC or GaN.
To address this issue, I propose a novel current-source H5 (CH5) solar inverter topology. This topology adds an auxiliary switch (S5) to the conventional CH4 structure, providing an additional degree of freedom for control. The schematic of the CH5 solar inverter is illustrated below, along with its common-mode model. The addition of S5 enables new switching states that can stabilize the common-mode voltage.
The operation of the CH5 solar inverter involves five distinct switching states, corresponding to five current space vectors. When S5 is turned on while S1 to S4 are off, the system enters a zero-current state with a common-mode voltage of \(0.5V_g\). I analyze the circuit to verify this. Let \(V_{s1}\) to \(V_{s4}\) represent the voltages across switches S1 to S4. When S5 is on, the DC-link voltage \(V_{PN} = 0\), leading to:
$$V_{s1} + V_{s2} = 0, \quad V_{s3} + V_{s4} = 0$$
Combined with the grid voltage relation \(V_{AO} + V_{BO} = V_g\), where \(V_{AO}\) and \(V_{BO}\) are the inverter output voltages, I derive that \(V_{PO} + V_{NO} = 0\). Thus, the common-mode voltage becomes:
$$V_{CM} = \frac{V_{PO} + V_{NO}}{2} = \frac{V_g}{2}$$
The complete switching states for the CH5 solar inverter are summarized in the following table:
| Vector | Switching State (S1, S2, S3, S4, S5) | Output Current \(i(t)\) | Common-Mode Voltage \(V_{CM}\) |
|---|---|---|---|
| I1 | 1, 0, 0, 1, 0 | \(I_{dc}\) | \(0.5V_g\) |
| I2 | 1, 1, 0, 0, 0 | 0 | \(V_g\) |
| I3 | 0, 1, 1, 0, 0 | \(-I_{dc}\) | \(0.5V_g\) |
| I4 | 0, 0, 1, 1, 0 | 0 | 0 |
| I5 | 0, 0, 0, 0, 1 | 0 | \(0.5V_g\) |
By strategically selecting switching states, I can achieve a three-level output current while maintaining a constant common-mode voltage. Specifically, during the positive half-cycle, I use states I1 and I5 to generate positive and zero current levels, both with \(V_{CM} = 0.5V_g\). During the negative half-cycle, I use states I3 and I5 to generate negative and zero current levels, also with \(V_{CM} = 0.5V_g\). This ensures that the common-mode voltage contains no high-frequency components, thereby suppressing leakage currents in the solar inverter system.
To implement this, I propose a novel one-dimensional space vector modulation (SVM) strategy for the CH5 solar inverter. The current space vectors are represented in a one-dimensional diagram, where the reference vector \(I_{ref}\) is synthesized using active vectors and the zero vector I5. For the positive half-cycle (\(0 < \theta < \pi\)), the reference vector is composed of I1 and I5:
$$I_{ref} T_s = I_1 T_1 + I_5 T_5$$
where \(T_s\) is the switching period, \(T_1\) and \(T_5\) are the dwell times for vectors I1 and I5, respectively. Assuming \(I_{ref} = m \sin \theta\) with modulation index \(m\), the dwell times are calculated as:
$$T_1 = T_s \cdot m \cdot \sin \theta, \quad T_5 = T_s – T_1$$
For the negative half-cycle (\(\pi < \theta < 2\pi\)), the reference vector is synthesized using I3 and I5:
$$I_{ref} T_s = I_3 T_3 + I_5 T_5$$
with dwell times:
$$T_3 = T_s \cdot m \cdot \sin(\theta – \pi), \quad T_5 = T_s – T_3$$
The vector sequence is arranged symmetrically within each switching period to minimize switching frequency and harmonic distortion. For example, in the positive half-cycle, the sequence is I1-I5-I5-I1, ensuring smooth transitions and constant common-mode voltage. This modulation strategy is crucial for enhancing the performance of solar inverters by reducing leakage currents.
To validate the proposed CH5 solar inverter and modulation strategy, I conducted comparative experiments with the traditional CH4 solar inverter. The experimental setup used a digital control platform based on a DSP and FPGA, with a switching frequency of 5 kHz. The DC-link inductance was 8 mH, rated DC current was 8 A, AC-side capacitance was 44 μF, and the PV parasitic capacitance was 56 nF. The results demonstrated that the CH4 solar inverter exhibited high-frequency variations in common-mode voltage, leading to leakage currents exceeding 300 mA. In contrast, the CH5 solar inverter maintained a nearly constant common-mode voltage at \(0.5V_g\), with leakage currents well below the limit. Dynamic switching tests further confirmed the superiority of the CH5 topology, as it effectively reduced common-mode voltage fluctuations when transitioning from CH4 to CH5 mode.
In terms of efficiency and power density, the proposed CH5 solar inverter offers advantages over voltage-source inverters, which often require bulky electrolytic capacitors and face bridge-through risks. While the use of IGBTs with series diodes in current-source solar inverters may increase conduction losses, the adoption of wide-bandgap devices like SiC or GaN can mitigate this. Future work will focus on optimizing the design for higher efficiency and reliability in solar inverter applications.
In conclusion, I have investigated the common-mode leakage current issue in transformerless solar inverters and proposed a novel CH5 topology with an advanced space vector modulation strategy. The CH5 solar inverter achieves three-level output current while maintaining a constant common-mode voltage, effectively suppressing leakage currents to meet grid standards. This makes it a promising solution for modern PV systems, contributing to safer and more efficient solar energy conversion. The integration of such innovative topologies is essential for advancing the deployment of solar inverters in renewable energy infrastructures.