Three-Level Power Converter Neutral-Point Balancing via Harmonic Injection
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Power converters with three-level topology face potential imbalance issues at the neutral point due to inconsistent switching transistor characteristics and drive delay differences, leading to increased stress on transistors, distorted output voltage, and reduced system reliability, which existing software algorithms struggle to effectively address, especially under complex operating conditions.
Innovation Solution
A power converter with a controller that injects a second harmonic current into the alternating current power grid when a voltage difference between the positive and negative bus capacitors exceeds a threshold, with the frequency of the second harmonic current being twice the fundamental frequency and an initial phase angle within a specific interval, to balance the neutral point potential, adaptable to various modulation schemes and power factors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If software control algorithms are used to balance the neutral point potential, then the complexity of control is increased, but the effectiveness in resolving potential imbalance under complex operating conditions remains insufficient
Solution Approach 1:
The patent changes the operating parameters of the power converter by injecting a second harmonic current with specific amplitude and phase angle into the AC grid. This parameter change modifies the current waveform to create a compensating effect that balances the neutral point potential, transforming the control approach from complex multi-variable algorithms to a more manageable parameter-based solution
Solution Approach 2:
The second harmonic current acts as an intermediary element that mediates between the unbalanced capacitor voltages and the grid connection. By introducing this intermediate current component, the patent creates a feedback mechanism that automatically compensates for potential imbalance without requiring complex control algorithms
2Adaptability or versatility
If the power converter operates under complex operating conditions with varying power factors, then the adaptability is improved, but the existing algorithms cannot effectively resolve the potential imbalance problem
Solution Approach 1:
The patent implements a dynamic control strategy where the amplitude and phase angle of the injected second harmonic current are continuously adjusted based on the actual operating conditions and power factor. This dynamic adaptation allows the system to maintain neutral point balance across varying operating conditions without requiring multiple fixed algorithms
Solution Approach 2:
By dynamically changing the parameters (amplitude and phase angle) of the injected second harmonic current according to the power factor and operating conditions, the system achieves adaptability while maintaining effectiveness in resolving potential imbalance
Data Source
Figure 1
Figure 2
Figure 3
AI summary
This application provides a power converter, an energy storage system, and a control method. The power converter includes a power conversion circuit, a bus capacitor, and a controller. A direct current input end of the power conversion circuit is configured to connect to a direct current power supply through a direct current bus. An alternating current output end of the power conversion circuit is configured to connect to an alternating current power grid through a point of common coupling PCC. The bus capacitor includes a positive bus capacitor and a negative bus capacitor that are connected in series. A series connection point of the positive bus capacitor and the negative bus capacitor is a neutral point. The controller is configured to: in response to that a difference between a voltage across two ends of the positive bus capacitor and a voltage across two ends of the negative bus capacitor is greater than a specified threshold, control the power converter to inject a second harmonic current into the alternating current power grid, to reduce the difference between the voltage across the two ends of the positive bus capacitor and the voltage across the two ends of the negative bus capacitor. An initial phase angle of the second harmonic current is within a preset interval. This application can resolve a problem of potential imbalance of the neutral point of the power converter.