Three-Level Power Converter Neutral Point Balancing by 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.
Innovation Solution
A power converter with a controller that injects a second harmonic current into the AC power grid when a voltage difference between the positive and negative bus capacitors exceeds a threshold, using a specific initial phase angle within a preset interval to balance the neutral point, 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 potential balance can be achieved under simple operating conditions, but the algorithms cannot meet requirements of complex operating conditions and cannot effectively resolve the problem of potential imbalance
Solution Approach 1:
The patent changes the operating parameters of the power converter by injecting a second harmonic current with specific frequency (twice the fundamental frequency) and controllable initial phase angle. This parameter change enables the system to adapt to complex operating conditions and effectively balance the neutral point potential under various scenarios, resolving the limitation of fixed software control algorithms.
Solution Approach 2:
The patent employs periodic injection of second harmonic current at twice the fundamental frequency of the AC power grid. This periodic action creates a controlled oscillating current that systematically adjusts the neutral point potential over each cycle, enabling effective potential balance that adapts to varying operating conditions throughout the grid cycle.
2Reliability
If second harmonic current is injected with improper initial phase angle, then the control complexity increases, but the potential balance effect deteriorates or may worsen the imbalance
Solution Approach 1:
The patent implements feedback control by monitoring the neutral point potential difference and using this information to determine the appropriate initial phase angle for the second harmonic current injection. The controller continuously adjusts the injection parameters based on the measured potential imbalance, creating a closed-loop system that automatically adapts to maintain optimal balance without requiring manual calibration or complex preset configurations.
Data Source
AI summary
A 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 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.