Modular Multilevel Converter Arm Voltage Control
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Solution Overview
Problem
Conventional power conversion devices using modular multilevel converters assume equal inductance components in the positive and negative arms, leading to voltage fluctuations and DC circuit oscillations when this assumption is not met, especially in systems with high inductance and capacitance and low resonance frequencies.
Innovation Solution
A power conversion device with a control system that generates separate voltage commands for the positive and negative arms, using PWM control and circulating current control to suppress voltage fluctuations and oscillations, including double-frequency and quadruple-frequency components, even when the arms have different inductance components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If separate voltage commands for positive and negative arms are generated, then voltage fluctuations and current oscillations are suppressed, but control system complexity increases
Solution Approach 1:
The control system is segmented into separate control paths for the positive arm and negative arm, with independent voltage command generation for each arm. This allows targeted suppression of fluctuations in each arm without requiring complete redesign of the entire control system, thereby improving reliability while limiting complexity increase to specific control modules.
Solution Approach 2:
The control system dynamically adjusts voltage command parameters (Vp_cmd and Vn_cmd) based on detected arm currents and voltage fluctuations. By changing these control parameters adaptively, the system suppresses oscillations without requiring additional hardware components, thus improving reliability while maintaining relatively simple control architecture.
2Object-generated harmful factors
If circulating current control is implemented, then harmonic components are reduced, but control processing time increases
Solution Approach 1:
The control system implements periodic detection and suppression of circulating current components at specific frequencies (double-frequency and quadruple-frequency components). By targeting only these periodic harmonic components rather than all frequency components, the system effectively reduces harmonics while minimizing the additional processing time required.
Solution Approach 2:
The control system extracts and separately processes only the harmful circulating current components (double-frequency and quadruple-frequency) from the total arm current. By isolating and addressing only these specific harmful components rather than processing the entire current waveform, harmonic reduction is achieved with reduced computational burden and processing time.
Data Source
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AI summary
This power conversion device performs power conversion between three-phase AC, and DC, and comprises a positive arm (5) and a negative arm (6) for each phase, each of which comprises one or more converter cells (10) connected in series. A voltage command generating unit (21) in a control device (20) comprises an AC current control unit (23), a circulating current control unit (24), and a command distributing unit (25), and generates a positive arm voltage command Vp+ and a negative arm voltage command Vp-for each phase. In the circulating current control unit (24), a compensator (27) calculates a first control command ΔVzp so that circulating current izp for each phase approaches a circulating current command value, a suppression unit (28) generates a second control command ΔVfzp so as to suppress a 2f component and a 4f component contained in the first control command ΔVzp, and then the first control command ΔVzp and the second control command ΔVfzp are combined to generate a circulation control command Vzp.