Four-Pole NPC Converter Eliminates Common Mode Voltage
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Solution Overview
Problem
Conventional power converters, such as three-phase inverters, produce common mode voltage, which can cause voltage stress on AC loads and contribute to electromagnetic interference (EMI), leading to insulation breakdown and increased costs due to the need for additional filtering components.
Innovation Solution
A four-pole neutral-point clamped (NPC) three-phase converter is designed to produce virtually zero common mode voltage by constraining switch states and using a fourth pole to balance DC link voltages, enabling the utilization of all NPC switch states for zero common mode voltage production without additional filtering components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional three-phase inverters are used, then power conversion is achieved, but common mode voltage is produced causing voltage stress on AC loads and electromagnetic interference
Solution Approach 1:
The converter is divided into four independent poles instead of the conventional three poles. Each pole operates independently with its own switching devices, allowing the common mode voltage from each pole to be controlled separately so that their sum equals zero, thereby eliminating common mode voltage while maintaining power conversion functionality
Solution Approach 2:
The invention changes the fundamental parameter of pole configuration from three to four. This parameter change enables the system to produce zero common mode voltage by appropriate selection of switching states, while also providing an additional degree of freedom for controlling the neutral point voltage and balancing DC link capacitors
2Object-affected harmful factors
If switch states are constrained to produce zero common mode voltage, then common mode voltage is eliminated, but DC link voltage balance control is lost
Solution Approach 1:
The fourth pole serves multiple functions simultaneously: it provides an additional degree of freedom for zero common mode voltage production, enables active balancing of the DC link capacitor voltages through its switching devices, and maintains compatibility with all standard NPC switching states, making the system more versatile and controllable
Solution Approach 2:
The control system uses feedback from the DC link voltage measurements to actively regulate the switching states of the fourth pole. This feedback mechanism ensures that the DC link voltages remain balanced while the other poles maintain zero common mode voltage output, resolving the control challenge
3Object-affected harmful factors
If additional filtering components are added to reduce EMI, then electromagnetic interference is reduced, but system cost and complexity increase
Solution Approach 1:
The invention converts the traditionally harmful common mode voltage into a controllable parameter by using it as a design constraint. By designing the four-pole configuration to inherently produce zero common mode voltage, the system eliminates EMI at its source rather than requiring additional filtering components, thus reducing system complexity and cost
Data Source
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AI summary
A four pole, three-phase, NPC converter that produces virtually no common mode voltage. The low common mode voltage output is achieved by constraining the switch states of the NPC converter. A fourth pole and associated control balance the upper and lower DC link voltages. The converter may be an inverter or a rectifier.