Three-Level Circuit Neutral Point Voltage Balance Control
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Solution Overview
Problem
Three-level circuits, particularly Neutral Point Clamped (NPC) inverters, face challenges in balancing neutral point voltage, leading to unbalanced output voltages and currents, especially under reactive, unbalanced, and non-linear AC loads, which can result in DC overvoltage, AC overcurrent, and system shutdowns.
Innovation Solution
A control method that adjusts the duty ratios of switch devices in the three-level circuit to balance neutral point voltage by modulating the zero, positive, and negative levels, using a capacitance bridge arm with upper and lower capacitors connected in series, and three switch bridge arms, where the duty ratios are adjusted based on the neutral point voltage and current flow direction to maintain balanced output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If conventional NPC three-level topology is used, then large output capacity and high output voltage are achieved, but neutral point voltage unbalance occurs under reactive and unbalanced loads
Solution Approach 1:
The patent applies dynamics principle by making the neutral point voltage balance control adaptive and dynamic. The control method continuously monitors the neutral point voltage and dynamically adjusts the duty ratios of switch devices based on real-time voltage differences, enabling the system to maintain balance under varying load conditions including reactive and unbalanced loads.
Solution Approach 2:
The patent implements parameter changes by modifying the duty ratio parameters of the switch devices. When neutral point voltage unbalance is detected, the control method changes the duty ratio values of specific switch devices to redirect current flow and restore voltage balance, thereby resolving the contradiction between maintaining high power output and ensuring voltage balance.
2Productivity
If neutral point voltage unbalance is not suppressed, then circuit operation continues, but DC overvoltage and AC overcurrent occur causing system shutdown
Solution Approach 1:
The patent applies feedback principle by continuously monitoring the neutral point voltage and using this information to adjust the switch device duty ratios. The control method establishes a closed-loop feedback mechanism where voltage unbalance detection triggers corrective duty ratio adjustments, preventing DC overvoltage and AC overcurrent conditions that would cause system shutdown.
Solution Approach 2:
The patent implements preliminary anti-action by proactively detecting neutral point voltage unbalance before it reaches critical levels. The control method takes preventive action by adjusting duty ratios to counteract developing unbalance conditions, thereby preventing the occurrence of DC overvoltage and AC overcurrent that would otherwise force system shutdown.
3Reliability
If duty ratio adjustment is applied to balance neutral point voltage, then voltage balance is improved, but control complexity increases
Solution Approach 1:
The patent applies local quality principle by focusing control actions on specific switch devices that have direct impact on neutral point voltage balance. Rather than uniformly adjusting all switch devices, the control method selectively modifies duty ratios of particular switches based on their position in the circuit and their influence on voltage balance, thereby simplifying the overall control strategy.
Solution Approach 2:
The patent implements segmentation by dividing the control strategy into distinct operational modes and phases. The control method segments the duty ratio adjustment into specific steps: detecting voltage unbalance, determining which switch devices to adjust, calculating appropriate duty ratio modifications, and implementing corrections. This segmented approach reduces control complexity compared to a monolithic control scheme.
Data Source
Figure 1A~1B
Figure 1C~2
Figure 3A~3B
AI summary
The present invention relates to a control method for balancing a neutral point voltage of a three-level circuit, comprising obtaining the neutral point voltage through sampling, adjusting a duty ratio of a zero level in one phase to maintain balance of the neutral point voltage when an absolute value of the neutral point voltage is larger than a threshold value, and distributing duty ratios of positive and negative levels in the phase, such that an output voltage of switch bridge arm in the phase includes positive, negative and zero levels in at least one switch period. The present invention can be carried out in a three-phase four-wire system and a three-phase three-wire system. In the three-phase three-wire system, it may also be combined with zero sequence voltage injection to form a mixed modulation method, thereby acquiring more effective neutral point balance control performance, and reducing switch loss.