NPC Photovoltaic Inverter Control for Neutral Point Voltage Balancing
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Solution Overview
Problem
Existing NPC inverter circuits face issues with unbalanced capacitor voltages due to asymmetric switching transistor states, leading to distorted output voltages and potential system damage, with existing voltage balancing methods being complex, costly, and limited in adaptability.
Innovation Solution
A photovoltaic inverter system that adjusts the turn-on and turn-off times of switching transistors using even-order harmonic voltage regulation signals based on bus voltages and output phase to balance the neutral point voltage, applicable to various modulation types.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing voltage balancing methods calculate and adjust redundant small vectors based on direct current bus voltage difference, then the neutral point voltage can be adjusted, but the control becomes complex and costly with poor adaptability
Solution Approach 1:
The patent changes the control parameter from complex redundant small vector adjustment to simple switching transistor turn-on and turn-off time adjustment. By modifying the timing parameters of existing switching transistors, the neutral point voltage is balanced without requiring additional complex control algorithms or calculations.
Solution Approach 2:
The existing switching transistors in the NPC inverter circuit perform dual functions: both their primary switching function and the additional function of neutral point voltage balancing. The switching transistors adjust their own turn-on and turn-off times based on detected voltage differences, eliminating the need for separate balancing mechanisms.
2Reliability
If existing methods inject zero-sequence voltage into modulated wave to balance neutral point voltage, then voltage balancing is achieved, but the method imposes many restrictions on algorithm and load type with poor adaptability
Solution Approach 1:
The patent creates a universal voltage balancing method that works with various modulation types (SPWM, SVPWM, DPWM) and load types without requiring algorithm modifications. The method uses the inherent switching transistors to perform balancing through time adjustment, making it applicable across different inverter configurations and operating conditions.
3Ease of operation
If switching transistor working states are asymmetric due to different models, losses, or loads, then the inverter operates under real-world conditions, but the neutral point voltage becomes unbalanced causing output distortion
Solution Approach 1:
The patent implements a feedback mechanism where the neutral point voltage difference is detected and used to adjust the turn-on and turn-off times of switching transistors. This closed-loop control continuously compensates for voltage imbalances caused by asymmetric switching states, different transistor models, losses, or load variations, maintaining high output voltage quality.
Data Source
AI summary
A photovoltaic inverter includes a conversion circuit, a collection circuit, and a controller. The collection circuit is configured to obtain a positive direct current bus voltage and a negative direct current bus voltage of the conversion circuit. The controller is configured to generate an even-order harmonic voltage regulation signal based on the positive direct current bus voltage and the negative direct current bus voltage of the conversion circuit and a phase of an output voltage of the photovoltaic inverter, generate a drive control signal based on the even-order harmonic voltage regulation signal, and control a switching transistor in the conversion circuit to be turned on or off to control the conversion circuit to output a target voltage and reduce a difference between the positive direct current bus voltage and the negative direct current bus voltage of the conversion circuit.