Three-Level Converter Neutral Point Voltage Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Three-level power conversion apparatuses face challenges in suppressing neutral point voltage fluctuations, especially when operating as a reactive power regulator, due to delays in control mechanisms and the need for additional circuits like reactors.
Innovation Solution
The apparatus incorporates a circulating current correction circuit that adjusts active and reactive current references to regulate neutral point voltage by redistributing DC charges between capacitors, using detectors and controllers to generate PWM signals for the converters, thereby reducing delays and fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a three-level power conversion apparatus is operated as a reactive power regulator and neutral point voltage fluctuation is suppressed by supplying current from the converter, then the neutral point voltage fluctuation can be suppressed, but a delay occurs in the fluctuation suppressing control
Solution Approach 1:
The control circuit detects neutral point voltage fluctuation and generates a circulating current reference signal in advance to compensate for the delay. By performing preliminary detection and generating the compensation signal before the full control action is needed, the system reduces the effective delay in suppressing voltage fluctuation when operating as a reactive power regulator.
Solution Approach 2:
The control circuit continuously detects the neutral point voltage and uses this feedback to generate a circulating current reference signal that compensates for voltage fluctuation. The feedback mechanism allows the system to respond to actual voltage conditions and adjust the circulating current accordingly, maintaining stability while minimizing delay through real-time adjustment.
2Stability of the object's composition
If positive side and negative side switching devices are added with a common reactor to suppress neutral point voltage fluctuation, then the voltage fluctuation can be suppressed, but the circuit complexity increases
Solution Approach 1:
The invention extracts the voltage fluctuation suppression function from the main power conversion control and implements it through a dedicated circulating current control circuit. By separating this function and implementing it through control signals rather than additional power circuit components, the system achieves voltage stability without increasing circuit complexity with reactors and switching devices.
Solution Approach 2:
The invention replaces the mechanical/electrical approach of using physical reactors and switching devices with a control-based approach. The control circuit generates circulating current reference signals that achieve voltage fluctuation suppression through electronic control, substituting physical circuit additions with a control signal mechanism.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
To provide a three-level power conversion apparatus which can suppress fluctuation in a neutral point voltage even when operated as a reactive power regulator. The three-level power conversion apparatus is composed of first and second three-level converters 2A, 2B connected to an AC power supply system, positive side and negative side DC capacitors 3P, 3N, a three-level inverter 4 to drive an AC motor 5, and converter control means 10 for controlling the three-level converters 2A, 2B. The converter control means 10 has first and second reactive current control means 15A, 15B for controlling so that reactive portions of input currents of the three-level converters 2A, 2B become a prescribed reactive current reference, respectively, first and second neutral point voltage fluctuation suppressing means for controlling PWM control means 17A, 17B, respectively, so as to make a difference between voltages applied to DC capacitors 3P, 3N to be zero, and active current control means for supplying a prescribed circulating active current from the three-level converter 2A to the three-level converter 2B.