Three-Level Inverter Neutral Voltage Control
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Solution Overview
Problem
Three-level inverters with neutral voltage controllers struggle to effectively suppress variations in neutral voltage when one phase is grounded, leading to direct-current components in output currents.
Innovation Solution
A three-level inverter configuration with series-connected capacitors and a control section that includes a neutral voltage controller and coordinate converter, which determines a three-phase voltage correction command based on the neutral point voltage and direct-current power supply voltage, correcting voltage commands on the d-q axis to suppress neutral voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a neutral voltage controller is implemented in a three-level inverter with one phase grounded, then neutral voltage control is attempted, but the control effectiveness is insufficient and direct-current components appear in output currents
Solution Approach 1:
The patent segments the neutral voltage control into two independent parts: (1) a neutral voltage controller that generates a three-phase voltage correction command based on neutral point voltage and DC power supply voltage, and (2) a coordinate converter that converts this correction command to d-q axis voltage correction commands. This segmentation allows each part to specialize in its function, improving overall control effectiveness and eliminating DC components in output currents.
2Stability of the object's composition
If a three-level inverter configuration with series-connected capacitors is used, then neutral point voltage stabilization is attempted, but variations in neutral voltage cannot be effectively suppressed
Solution Approach 1:
The patent implements a feedback mechanism where the neutral voltage controller continuously monitors the neutral point voltage and DC power supply voltage, compares them with reference values, and generates a three-phase voltage correction command based on the detected variations. This closed-loop feedback enables dynamic compensation and effective suppression of neutral voltage variations.
Solution Approach 2:
The patent changes the control parameters by introducing a three-phase voltage correction command and its d-q axis equivalent, which are dynamically adjusted based on neutral point voltage and DC power supply voltage variations. This parameter adaptation allows the system to maintain stable neutral point voltage under varying operating conditions.
3Reliability
If voltage correction on d-q axis is implemented, then neutral voltage variations are suppressed, but system complexity increases
Solution Approach 1:
The coordinate converter serves multiple functions: it converts the three-phase voltage correction command to d-q axis voltage correction commands, and this d-q axis correction is then applied to control the inverter switching. This multi-functionality reduces the need for separate control mechanisms, thereby limiting the increase in system complexity while achieving effective neutral voltage suppression.
Data Source
AI summary
A three-level inverter includes a set of series-connected capacitors connected in parallel to a direct-current power supply; two arms connected in parallel to an output of the direct-current power supply and each having an alternating-current output terminal; and a control section having a neutral voltage controller determining a three-phase voltage correction command based on a neutral voltage and voltage of the direct-current power supply, having a coordinate converter converting the three-phase voltage correction command to a voltage correction command on d-q axis, and suppressing voltage variations at the neutral point by correcting a voltage command on d-q axis based on the d-q voltage correction command. The neutral point is connected to one grounded phase of a three-phase grounded power system. The alternating-current output terminals are connected to respective non-grounded phases of the power system. A power conditioner includes the three-level inverter. A power generating system includes the power conditioner.