In recent years, the rapid development of alternative energy technologies has been driven by growing concerns over climate change, rising oil prices, and global warming caused by non-renewable energy sources such as coal, natural gas, and petroleum. Among various renewable energy sources, solar photovoltaic (PV) systems stand out as one of the most environmentally friendly, easily deployable, and widely adopted technologies. With continuous advancements in material and manufacturing processes, the cost of PV systems has steadily decreased, making them a preferred choice for widespread energy deployment. The utility interactive inverter, as the core component of a solar PV power generation system, plays a decisive role in the system’s cost-effectiveness, lifespan, and efficiency performance. Isolated inverters achieve electrical isolation between the DC and AC sides by incorporating a transformer. While the transformer enhances safety, it also increases the volume and cost of the equipment, no longer meeting market demands for high efficiency and low-cost technologies. In contrast, transformerless utility interactive inverters have gained popularity in the renewable energy market due to their high efficiency, compact size, and light weight. However, in transformerless utility interactive inverters, the removal of the transformer can lead to leakage current issues. Therefore, researching how to suppress or eliminate leakage current has become a key focus in the field of transformerless utility interactive inverters.
Existing topologies, such as the H5 topology and the HERIC topology, attempt to eliminate leakage current by isolating the DC side from the AC side, but due to the practical structure of switching devices, leakage current cannot be completely eliminated. Some scholars have proposed using neutral-point clamped structures to address leakage current, but this method also fails to achieve complete elimination and increases costs and reduces efficiency due to the addition of more components. Recently, common-ground topologies have emerged as a promising solution that can thoroughly eliminate leakage current. However, some proposed circuits, such as those combining Boost circuits with switched-capacitor structures, only support bipolar modulation, leading to poor grid-connected current waveform quality and reduced efficiency due to significant reactive power exchange. Other two-level common-ground topologies suffer from high output harmonic content. To overcome these limitations, I propose a novel single-phase transformerless utility interactive inverter topology. This topology consists of a new front-end interleaved buck-boost converter and a half-bridge inverter. During grid connection, this structure directly connects the negative terminal of the PV panel to the grid neutral point, effectively short-circuiting the parasitic capacitance of the PV panel and thereby solving the leakage current problem. This utility interactive inverter design aims to provide a compact, high-gain, and leakage-current-free solution for PV systems.
The proposed two-stage utility interactive inverter topology is illustrated above. It comprises a novel front-end DC-DC circuit and a traditional half-bridge inverter. The front-end structure includes one switch, two diodes, two inductors, and three capacitors. This configuration is designed to achieve high voltage gain while maintaining a simple structure. The half-bridge inverter stage enables common-ground operation, which is crucial for eliminating leakage current in transformerless utility interactive inverters. The integration of these two stages results in a compact and efficient utility interactive inverter suitable for single-phase grid-tied PV applications.
To understand the operation of the proposed utility interactive inverter, I analyze the working modes of the front-end DC-DC converter. The converter operates in two primary modes based on the switching state of the switch S1. In Mode 1, when S1 is turned on, the input voltage charges inductor L1, and the input voltage along with capacitor C1 supplies power to the load through inductor L2. The voltage and current loop equations for this mode are given by:
$$ V_{L1} = V_{in} $$
$$ V_{L2} = V_{in} + V_{C1} – V_{O1} $$
$$ I_{L1} = I_{L2} $$
where \( V_{L1} \) and \( V_{L2} \) are the voltages across inductors L1 and L2, respectively, \( V_{C1} \) is the voltage across capacitor C1, \( V_{in} \) is the input voltage, \( V_{O1} \) is the output voltage across capacitor \( C_{O1} \), and \( I_{L1} \) and \( I_{L2} \) are the currents through inductors L1 and L2, respectively.
In Mode 2, when S1 is turned off, inductor L1 discharges through diode D1 to charge capacitor C1 and supply power to load \( C_{O2} \). Inductor L2 also discharges through diode D1 to supply power to load \( C_{O1} \). The equations for this mode are:
$$ V_{L1} = V_{C1} – V_{O2} $$
$$ V_{L2} = -V_{O1} $$
$$ I_{C1} = I_{L1} $$
where \( V_{O2} \) is the output voltage across capacitor \( C_{O2} \), and \( I_{C1} \) is the current through capacitor C1.
Applying the volt-second balance principle to inductors L1 and L2, I derive the output voltages and the overall voltage gain of the front-end converter. Assuming constant capacitor voltages due to high switching frequency, the average voltage across each inductor over a switching period is zero. For inductor L1:
$$ \int_0^{DT} V_{L1,on} \, dt + \int_{DT}^T V_{L1,off} \, dt = 0 $$
Substituting the expressions from Modes 1 and 2, and solving for the output voltages, I obtain:
$$ V_{O1} = \frac{D}{1-D} V_{in} $$
$$ V_{O2} = \frac{1}{1-D} V_{in} $$
where \( D \) is the duty cycle of switch S1, ranging from 0 to 1. The total output voltage of the front-end converter is the sum of \( V_{O1} \) and \( V_{O2} \):
$$ V_{out} = V_{O1} + V_{O2} = \frac{2D}{1-D} V_{in} $$
This voltage gain formula shows that the proposed utility interactive inverter can achieve buck-boost functionality, making it adaptable to a wide range of input voltages from different PV panel configurations. The gain of \( \frac{2D}{1-D} \) allows for significant step-up or step-down operation, enhancing the flexibility of the utility interactive inverter in various PV systems.
Leakage current is a critical issue in transformerless utility interactive inverters. In the proposed topology, the negative terminal of the PV panel is directly connected to the grid neutral point, creating a common-ground configuration. This effectively shorts the parasitic capacitance between the PV panel and ground, eliminating the path for leakage current. To analyze this mathematically, I consider the common-mode voltage (\( V_{CM} \)) and differential-mode voltage (\( V_{DM} \)) defined as:
$$ V_{CM} = \frac{V_{AN} + V_{BN}}{2} $$
$$ V_{DM} = V_{AN} – V_{BN} $$
where node A is the midpoint of the series connection of switches S2 and S3, node B is the midpoint of the series connection of capacitors \( C_{O1} \) and \( C_{O2} \), and N is the common reference point. The total common-mode voltage (\( V_{TCM} \)) is given by:
$$ V_{TCM} = V_{CM} – \frac{V_{DM}}{2} $$
When switch S2 is turned on, \( V_{AN} = V_{P} \) and \( V_{BN} = 0 \), where \( V_{P} \) is the positive voltage. Then:
$$ V_{CM} = \frac{V_{P}}{2}, \quad V_{DM} = V_{P}, \quad V_{TCM} = 0 $$
When switch S3 is turned on, \( V_{AN} = 0 \) and \( V_{BN} = V_{N} \), where \( V_{N} \) is the negative voltage. Then:
$$ V_{CM} = \frac{V_{N}}{2}, \quad V_{DM} = -V_{N}, \quad V_{TCM} = 0 $$
Since \( V_{TCM} \) is always zero during operation, the common-mode voltage is eliminated, and consequently, the leakage current is zero. This proves that the proposed utility interactive inverter topology effectively solves the leakage current problem, making it a safe and reliable choice for grid-tied PV systems.
To validate the performance of the proposed utility interactive inverter, I conducted simulations using MATLAB/Simulink software. The circuit parameters for the simulation are summarized in the following table:
| Parameter | Value |
|---|---|
| Input Voltage \( U_{in} \) | 48 V to 600 V |
| Grid Voltage \( U_g \) | 220 V |
| Switching Frequency \( f_s \) | 28 kHz |
| Bus Capacitors \( C_{O1}, C_{O2} \) | 1000 μF |
| Inductor \( L1 \) | 22 μH |
| Inductor \( L2 \) | 500 μH |
| Capacitor \( C1 \) | 100 μF |
| Filter Inductor \( L_{fg} \) | 10 mH |
| Filter Capacitor \( C \) | 100 μF |
The simulation results demonstrate the effectiveness of the proposed utility interactive inverter. The grid-connected current and voltage waveforms show smooth synchronization with the grid, indicating proper operation. The common-mode voltage and leakage current waveforms confirm that the common-mode voltage is zero and the leakage current is negligible, aligning with the theoretical analysis. Additionally, a total harmonic distortion (THD) analysis of the grid current was performed. The THD was found to be less than 5%, which complies with standards such as GB/T 30427-2013 for grid-connected currents. This highlights the high power quality achieved by this utility interactive inverter, ensuring efficient and reliable grid integration.
The proposed utility interactive inverter offers several advantages over traditional topologies. First, it provides high voltage gain through the front-end interleaved buck-boost converter, enabling operation over a wide input voltage range. This is particularly beneficial for PV systems where the output voltage can vary due to environmental conditions. Second, the common-ground configuration of the half-bridge inverter stage completely eliminates leakage current, addressing a major safety concern in transformerless utility interactive inverters. Third, the topology uses a reduced number of power semiconductor devices compared to some existing solutions, which can lower costs and improve reliability. Finally, the two-stage design integrates DC/AC conversion seamlessly, resulting in a compact and efficient utility interactive inverter suitable for residential and commercial PV applications.
In conclusion, I have presented a novel single-phase transformerless utility interactive inverter that effectively eliminates leakage current while providing high voltage gain. The topology combines a new front-end DC-DC converter with a half-bridge inverter, leveraging common-ground technology to short-circuit the PV panel’s parasitic capacitance. Theoretical analysis and simulation results validate the inverter’s performance, showing zero leakage current, low THD, and robust grid connection. This utility interactive inverter represents a significant advancement in transformerless PV inverter design, offering a safe, efficient, and cost-effective solution for modern solar energy systems. Future work may involve hardware implementation and experimental validation to further assess its practical performance and scalability for larger utility interactive inverter applications.
Throughout this discussion, the term “utility interactive inverter” has been emphasized to highlight the grid-tied functionality of the proposed system. This utility interactive inverter is designed not only to convert DC power from PV panels to AC power but also to interact seamlessly with the utility grid, ensuring stable and efficient energy transfer. The elimination of leakage current enhances the safety of this utility interactive inverter, making it suitable for widespread adoption in residential and commercial settings. As renewable energy penetration increases, the development of advanced utility interactive inverters like this one will play a crucial role in optimizing grid integration and maximizing the benefits of solar power. By addressing key challenges such as leakage current and voltage gain, this utility interactive inverter contributes to the evolution of smarter and more reliable PV systems for a sustainable energy future.
In summary, the innovative design of this utility interactive inverter focuses on achieving high performance while maintaining simplicity and safety. The integration of buck-boost capabilities and leakage current elimination in a single topology sets it apart from conventional utility interactive inverters. With continued research and development, such utility interactive inverters can drive the adoption of transformerless technologies in the solar industry, ultimately reducing costs and improving efficiency for end-users. The proposed utility interactive inverter thus represents a step forward in the quest for optimal renewable energy solutions, underscoring the importance of advanced power electronics in the transition to a cleaner energy landscape.