Suppressing common mode leakage current is a key technical challenge that must be addressed in non-isolated photovoltaic (PV) grid-connected systems. This article delves into this critical issue, starting from an analysis of conventional topologies and culminating in the proposal and validation of a novel, effective solution for multi-level solar inverters.
1. Introduction and Background
The global demand for clean energy has propelled solar power to the forefront of renewable energy generation. Traditional PV generation systems employ line-frequency or high-frequency isolation transformers to provide galvanic isolation and voltage matching. However, these transformers add significant cost, weight, volume, and reduce the overall system efficiency. Non-isolated grid-connected solar inverters offer a compelling alternative by eliminating the transformer, thereby achieving higher efficiency, lower cost, and reduced size.
Nevertheless, the absence of galvanic isolation creates a direct electrical connection between the PV array and the utility grid. This leads to the generation of a common-mode (CM) leakage current flowing through the parasitic capacitance that inherently exists between the PV panels and the earth ground. This capacitance, denoted as Cpv, arises due to the physical structure of the panels and their mounting frames and typically ranges from 50 to 150 nF/kW.
High-frequency CM leakage current is highly undesirable. It can cause severe electromagnetic interference (EMI), distort the grid-injected current, increase system losses, and pose potential safety hazards. Consequently, strict safety standards like VDE-0126-1-1 mandate that a non-isolated PV system must disconnect from the grid within 0.3 seconds if the RMS value of the leakage current exceeds 30 mA. Therefore, designing solar inverters with inherently low leakage current is paramount.
Single-phase full-bridge (H-bridge) solar inverters using unipolar sinusoidal pulse-width modulation (SPWM) offer excellent differential-mode performance with a three-level output voltage and reduced inductor current ripple. However, their common-mode performance is poor because the CM voltage contains high-frequency components, which excite the parasitic capacitance and generate large leakage currents. Considerable research has been dedicated to solving this problem for single H-bridge solar inverters. The fundamental principle is to decouple the DC side from the AC side during freewheeling periods, ensuring that the CM voltage seen by the parasitic capacitance remains constant, thus suppressing the high-frequency leakage current. Topologies like the H5, H6, HERIC, and others have been proposed based on this principle, often by adding auxiliary switches to the standard H-bridge.
Meanwhile, multilevel solar inverters are gaining traction due to their advantages over standard two-level inverters, such as reduced voltage stress on power devices, improved output waveform quality, lower switching losses, and diminished EMI. Among various multilevel structures, the cascaded H-bridge (CHB) inverter is particularly attractive for photovoltaic applications because it features a modular structure, requires no clamping diodes or flying capacitors, and simplifies the control of DC-link voltage balancing when multiple independent PV sources are available.
However, applying the concept of transformerless operation to cascaded multilevel solar inverters introduces a new layer of complexity. Instead of a single CM loop, multiple independent CM paths are created due to the multiple independent DC sources. This presents a significant technical challenge in effectively suppressing the total system leakage current. This article systematically addresses this challenge for single-phase cascaded systems.
2. Analysis of the Conventional Single-Phase Cascaded H4 Solar Inverter
We begin by analyzing the conventional approach: cascading two standard H-bridge modules (often termed H4 topology). The circuit diagram is shown in the referenced figure. Each module is powered by an independent PV source, and their AC outputs are connected in series. The parasitic capacitances for the upper and lower PV arrays are combined and represented as Cpva and Cpvb, respectively. An L-type filter (La, Lb) is used at the output.
To understand the leakage current mechanism, we establish the common-mode model of this system. The CM and differential-mode (DM) voltages for the upper (a) and lower (b) H-bridge units are defined as:
$$U_{cma} = \frac{U_{AN} + U_{BN}}{2}, \quad U_{dma} = U_{AN} – U_{BN}$$
$$U_{cmb} = \frac{U_{A’N’} + U_{B’N’}}{2}, \quad U_{dmb} = U_{A’N’} – U_{B’N’}$$
where UAN, UBN, UA’N’, UB’N’ are the switching node voltages relative to the negative DC bus of their respective units. Applying circuit theory to the CM model, we can derive the mathematical expressions for the leakage currents flowing through Cpva and Cpvb, denoted as Icma and Icmb. The derivation shows that these currents are complex functions of both the CM and DM voltages of both H-bridge units, as well as the circuit impedances:
$$I_{cma} = \frac{A U_{cma} + B U_{dma} + C U_{cmb} + D U_{dmb}}{Z_{Cpva}Z_{Cpvb} + Z_{La}Z_{Lb} + (Z_{La}+Z_{Lb})(Z_{Cpva}+Z_{Cpvb})}$$
$$I_{cmb} = \frac{A’ U_{cma} + B’ U_{dma} + C’ U_{cmb} + D’ U_{dmb}}{Z_{Cpva}Z_{Cpvb} + Z_{La}Z_{Lb} + (Z_{La}+Z_{Lb})(Z_{Cpva}+Z_{Cpvb})}$$
where A, B, C, D, A’, B’, C’, D’ are coefficients composed of the impedance terms ZLa=sLa, ZLb=sLb, ZCpva=1/(sCpva), and ZCpvb=1/(sCpvb). The key takeaway is that the leakage current in each unit is excited by the high-frequency components present in both its own CM/DM voltages and the CM/DM voltages of the other unit.
Examining the switching states of the cascaded H4 topology under typical unipolar modulation reveals the root cause of the problem. The following table summarizes the key voltages for different active and freewheeling states (Ud is the DC-link voltage of one module):
| State | S11,S12,S13,S14,S21,S22,S23,S24 | Ucma | Udma | Ucmb | Udmb |
|---|---|---|---|---|---|
| 1 | 1,0,0,1,1,0,0,1 | Ud/2 | Ud | Ud/2 | Ud |
| 2 | 1,0,0,1,1,0,1,0 | Ud/2 | Ud | Ud | 0 |
| 3 | 1,0,1,0,1,0,0,1 | Ud | 0 | Ud/2 | Ud |
| 4 | 1,0,1,0,1,0,1,0 | Ud | 0 | Ud | 0 |
| 5 | 0,1,1,0,0,1,1,0 | Ud/2 | -Ud | Ud/2 | -Ud |
| 6 | 0,1,1,0,0,1,0,1 | Ud/2 | -Ud | 0 | 0 |
| 7 | 0,1,0,1,0,1,1,0 | 0 | 0 | Ud/2 | -Ud |
| 8 | 0,1,0,1,0,1,0,1 | 0 | 0 | 0 | 0 |
It is evident that both Ucma and Ucmb switch between 0, Ud/2, and Ud at the switching frequency. Similarly, Udma and Udmb switch between -Ud, 0, and Ud. None of these voltages remains constant. According to the derived equations, these high-frequency voltage variations directly drive the CM leakage currents. Therefore, the conventional cascaded H4 topology is incapable of effectively suppressing leakage current in transformerless PV applications, making it unsuitable for safety-compliant non-isolated solar inverters.
3. Proposed Single-Phase Cascaded H5 Solar Inverter
To solve this problem, we propose a novel topology: the single-phase cascaded H5 solar inverter. This topology replaces each standard H4 bridge in the cascade with an H5 bridge module. The H5 bridge, known for its excellent leakage current suppression in single-module solar inverters, adds one auxiliary switch to the standard H-bridge. The proposed system connects these H5 modules in series, as shown in the referenced figure.
We analyze its CM behavior by constructing and simplifying its CM model. A critical step in the analysis is to ensure the filter inductors on both sides of the series connection are equal: La = Lb = L’a = L’b = L. Under this condition, a simplified CM model can be derived where the influence of the differential-mode voltages on the leakage current paths is nullified. The leakage currents then depend solely on the CM voltages of each module and the impedances:
$$I_{cma} = \frac{U_{cma}}{Z_{Cpva} + Z_{Lab}}, \quad I_{cmb} = \frac{U_{cmb}}{Z_{Cpvb} + Z_{L’a’b’}}$$
where ZLab = s(La+Lb)/2. This is a pivotal result. It indicates that if we can maintain Ucma and Ucmb constant, the high-frequency components of Icma and Icmb will be effectively eliminated.
The next step is to design a modulation strategy for the cascaded H5 topology that achieves constant CM voltages. The proposed modulation uses four phase-shifted triangular carriers. Two carriers (vcr1, vcr1-) are 180° out-of-phase and control the upper H5 module. Another two carriers (vcr2, vcr2-), shifted by 180° from the first pair, control the lower H5 module. Both modules share the same sinusoidal modulation wave vm.
The switching logic for one H5 module (e.g., the upper one) is as follows:
- During the positive half-cycle of vm: S11 is ON, S12 is OFF.
- When vm > vcr1: S14 and S15 are ON, S13 is OFF. The output voltage UAN=+Ud, UBN=0, leading to Udma=+Ud and Ucma=Ud/2.
- When vm < vcr1: S14 and S15 are OFF, S13 is ON. The output is shorted through S11 and S13, UAN=UBN=+Ud, leading to Udma=0 and Ucma=Ud/2.
- During the negative half-cycle of vm: S13 is ON, S14 is OFF.
- When vm > vcr1-: S11 is ON, S12 and S15 are OFF. The output is shorted through S11 and S13, UAN=UBN=0, leading to Udma=0 and Ucma=Ud/2.
- When vm < vcr1-: S11 is OFF, S12 and S15 are ON. The output voltage UAN=0, UBN=+Ud, leading to Udma=-Ud and Ucma=Ud/2.
An identical logic, using carriers vcr2 and vcr2-, applies to the lower H5 module. The key outcome is that regardless of the switching state, the CM voltage for each H5 module remains constant at Ud/2. The DM voltages switch to synthesize the desired multilevel output waveform. The series connection of the two modules produces a five-level voltage at the phase leg: ±2Ud, ±Ud, and 0.
From the high-frequency CM model perspective, with constant Ucma and Ucmb, the voltages across the parasitic capacitors are also constant DC values (Ud/2 minus the DC bus potential), containing no high-frequency content to drive leakage current. One must also consider the low-frequency influence of the grid voltage. A separate low-frequency analysis shows that the grid voltage introduces only low-frequency (50/60 Hz) sinusoidal components onto the DC bias across the parasitic capacitors. Therefore, the total voltage across Cpva and Cpvb is a low-frequency AC signal with a DC offset, devoid of damaging high-frequency components. Thus, the proposed cascaded H5 solar inverter topology, with its specific modulation, successfully suppresses high-frequency common-mode leakage current.
The concept is naturally extensible to n-level solar inverters. For a system with n cascaded H5 modules, the modulation wave is compared with 2n phase-shifted carriers. The output voltage level count becomes N=2n+1. The core principle remains: the modulation strategy for each individual H5 module ensures its CM voltage is clamped to Ud/2. Consequently, even in a generalized multilevel configuration, the parasitic capacitor voltages lack high-frequency components, and the system leakage current is effectively mitigated. This makes the proposed approach highly scalable for medium-voltage or high-power non-isolated solar inverter applications.
4. Experimental Verification
To validate the theoretical analysis and the effectiveness of the proposed solar inverter topology, we constructed a digital-controlled laboratory prototype. The system parameters were as follows: each H-bridge module DC input voltage Ud = 120 V (total equivalent DC input 240 V), grid voltage 180 Vrms/50Hz, switching frequency 10 kHz. For a fair comparison, the cascaded H4 topology used filter inductors La=Lb=5 mH. The proposed cascaded H5 topology used inductors La=Lb=L’a=L’b=2.5 mH to maintain the same total inductance. The filter capacitor was 9.4 µF, and the parasitic capacitance for each array was set to 150 nF.
Results for Cascaded H4 Topology: The phase leg voltage exhibited the expected five levels (±240V, ±120V, 0V). However, the measured voltages across the parasitic capacitors Cpva and Cpvb contained significant high-frequency switching noise, confirming the theoretical prediction. This resulted in large high-frequency leakage currents. The measured RMS values of the leakage currents for the upper and lower units were 479 mA and 439 mA, respectively. These values far exceed the 30 mA safety limit specified by the VDE standard, rendering this topology non-compliant for transformerless solar inverters.
Results for Cascaded H5 Topology: The phase leg voltage also correctly showed a five-level waveform. The measurements confirmed the theoretical CM voltage analysis. The voltage across Cpva was a low-frequency sine wave with an amplitude roughly three times larger than that across Cpvb, superimposed on a DC bias, and critically, it was free of high-frequency switching components. Consequently, the measured leakage currents were drastically reduced. The RMS values were 12.2 mA and 9.76 mA for the upper and lower units, respectively. Both values are well below the 30 mA threshold, demonstrating successful leakage current suppression and compliance with safety standards. A closer inspection of the leakage current waveform near the grid voltage zero-crossing showed small spikes due to the high dv/dt of the grid voltage and the very short duration of the active switching states in that region, but this did not impact the RMS value significantly.
The experimental results provide conclusive evidence. The conventional cascaded H4 solar inverter cannot limit leakage current due to its inherent high-frequency CM voltage variations. In contrast, the proposed cascaded H5 solar inverter, with its dedicated modulation strategy, maintains a constant CM voltage in each module, thereby reducing the high-frequency leakage current to safe, compliant levels.
5. Conclusion
This article has presented a comprehensive analysis and solution for the common-mode leakage current problem in single-phase non-isolated cascaded solar inverters. The key conclusions are as follows:
- Deficiency of Conventional Topology: The CM leakage current in a traditional cascaded H4 solar inverter is determined by the high-frequency variations of both the common-mode and differential-mode voltages from all cascaded modules. Since none of these voltages is constant, the topology inherently fails to suppress leakage current, making it unsuitable for safe, transformerless PV applications.
- Proposed Solution and Its Merits: We have proposed a novel cascaded H5 solar inverter topology. Its operational principle ensures that the common-mode voltage of each individual H5 module remains constant at half of its DC-link voltage, irrespective of the switching state. This is achieved through a specific phase-shifted carrier modulation strategy. As a result, the high-frequency excitation of the parasitic capacitances is eliminated, and the system leakage current is effectively suppressed to meet stringent safety standards.
- Scalability and Practical Value: The proposed concept is straightforward, easy to implement with modern digital controllers, and naturally scalable to multi-level configurations for higher voltage or power solar inverter systems. It offers a practical and effective engineering solution for developing high-efficiency, safe, and reliable non-isolated cascaded photovoltaic inverters.
The transition towards transformerless solar inverters is crucial for improving the efficiency and reducing the cost of PV systems. Addressing the leakage current challenge in advanced multilevel topologies like the cascaded H-bridge is a vital step in this direction. The cascaded H5 topology presented here represents a significant contribution to this field, enabling the benefits of multilevel conversion—such as improved waveform quality and reduced device stress—to be safely realized in non-isolated photovoltaic applications.