Converter Cell Voltage Balancing in Cascaded Power Converters
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Solution Overview
Problem
Existing power conversion devices face challenges in stabilizing capacitor voltages of converter cells due to imbalances caused by harmonic components in system voltage, leading to potential overvoltage or low-voltage protection issues and operational stoppages.
Innovation Solution
A power conversion device with a control system that includes a balance control unit, circulating-current control unit, and oscillation control unit to stabilize capacitor voltages by oscillating a balance command value at a frequency lower than the fundamental frequency, and stopping oscillation when voltage variations fall within a set range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional balance control is used to maintain capacitor voltages, then capacitor voltage balance is maintained under normal conditions, but voltage imbalance occurs when harmonics component of system voltage increases
Solution Approach 1:
The patent applies dynamics by making the balance command value oscillate at a specific frequency (twice the fundamental frequency) in response to detected voltage imbalances. This dynamic adjustment allows the control system to actively counteract harmonic effects and maintain capacitor voltage balance under varying operating conditions, rather than using a static control approach.
Solution Approach 2:
The patent implements feedback control by detecting the actual capacitor voltages, comparing them against the oscillating balance command value, and adjusting the circulating current based on the deviation. This closed-loop feedback mechanism enables the system to respond to and correct voltage imbalances caused by harmonic components in real-time.
2Reliability
If balance control is continuously applied to maintain capacitor voltages, then voltage stability is improved, but control complexity increases
Solution Approach 1:
The patent uses periodic action by oscillating the balance command value at twice the fundamental frequency of the AC system. This periodic oscillation is deliberately introduced to counteract the ripple components in capacitor voltages, providing a systematic and predictable control approach that simplifies the overall control strategy compared to continuous complex adjustments.
Solution Approach 2:
The patent changes the frequency parameter of the balance command value to twice the fundamental frequency, which aligns with the ripple frequency in capacitor voltages. This parameter change transforms the control approach, enabling effective voltage balance maintenance through a simplified oscillating reference rather than complex continuous modulation.
Data Source
AI summary
A power conversion device includes: a power converter which includes a plurality of arms for each phase of an alternating-current system; and a control device. Each arm, among the plurality of arms, has a plurality of converter cells that are cascade-connected. The plurality of converter cells each have a plurality of switching elements, and a power storage element. The control device includes: a balance control unit to control a balance of voltages of power storage elements between predetermined groups, based on a balance command value; a circulating-current control unit to control a circulating current circulating through the power converter, based on a circulating-current command value; and an oscillation control unit to cause the balance command value to oscillate at a first frequency lower than a fundamental frequency of the alternating-current system, based on the voltages of the power storage elements included in the arm.