Non-isolated Inverter Common-mode Current Suppression
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Solution Overview
Problem
Transformer-less non-isolated PV grid-connected systems experience increased electromagnetic interference and safety hazards due to common-mode currents generated by parasitic capacitors between the PV array and ground, which existing technologies fail to adequately suppress.
Innovation Solution
A non-isolated inverter design featuring a DC input-side, a capacitor, and two bridge-arm units with switch-elements and inductors, where the switch-elements are controlled to suppress common-mode currents through a clamping action, and a control circuit manages the switching based on input and output signals to maintain efficient operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a transformer-less non-isolated topology is used, then system efficiency is improved and volume is reduced, but common-mode currents are generated causing EMI and safety hazards
Solution Approach 1:
A common-mode choke is introduced as an intermediary component in the circuit path between the bridge-arm units and the grid connection. This choke acts as a mediator that blocks common-mode currents while allowing differential-mode power transmission to continue efficiently, thus eliminating EMI and safety hazards without sacrificing the high efficiency benefits of the transformer-less topology
Solution Approach 2:
The inverter is divided into functionally independent bridge-arm units (first bridge-arm unit for positive half-cycle, second bridge-arm unit for negative half-cycle) that can be independently controlled. This segmentation allows selective activation of appropriate bridge-arm units based on current direction, enabling precise control over common-mode current generation while maintaining high efficiency operation
2Object-affected harmful factors
If the first upper switch-element and first lower switch-element are controlled with opposite states, then common-mode currents are suppressed, but control complexity increases
Solution Approach 1:
The control system operates in periodic cycles, alternating between positive half-cycle mode (first bridge-arm unit active) and negative half-cycle mode (second bridge-arm unit active). During each half-cycle, the relevant upper and lower switch-elements are controlled with opposite states to suppress common-mode currents. This periodic control pattern simplifies the overall control strategy compared to continuous complex modulation schemes
Data Source
AI summary
A non-isolated inverter including a DC input-side, a capacitor connected in parallel with the DC input-side, an AC output-side connected in parallel with a load, and first and second bridge-arm units is provided. The first and second bridge-arm units are connected in parallel with the capacitor. The first bridge-arm unit includes a series forward-connection of upper and lower switch-elements, where a common-node of upper and lower switch-elements and a supplying terminal of the second bridge-arm unit are respectively connected to two terminals of the AC output-side. The upper and lower switch-elements are respectively turned on in positive and negative half cycles of an output current of the non-isolated inverter, and the generation of common-mode currents in the non-isolated inverter is suppressed under a clamping action between the upper and lower switch-elements due to there are no high-frequency voltages on the parasitic-capacitors from the non-isolated inverter to the ground.