Modular Multilevel Converter High-Frequency Drive System
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Solution Overview
Problem
High-power low-speed machines driven by modular multilevel converters (MMC) face significant capacitor voltage fluctuations, leading to increased volume, weight, and maintenance costs, limiting their application in medium and high-voltage machine drives, and existing solutions either worsen harmonics or require complex control adjustments.
Innovation Solution
A high-power machine drive system combining a modular multilevel converter with a multi-pulse cycloconverter, operating at high frequency to reduce capacitor voltage fluctuations, and using a zero-type cycloconverter topology to minimize costs and complexity, while ensuring high-quality output waveforms through speed vector control and multi-pulse modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the MMC is used to directly drive a high-power low-speed machine, then the system structure is simplified, but large fluctuations of the capacitor voltage of the sub-module are caused, affecting the quality of the AC side output waveform and potentially affecting normal operation
Solution Approach 1:
The patent introduces a multi-pulse cycloconverter as an intermediary device between the MMC and the high-power low-speed machine. The cycloconverter converts the high-frequency AC output of the MMC into low-frequency AC suitable for driving the machine, thereby mediating the incompatibility between MMC operating characteristics and machine drive requirements. This intermediary structure allows the MMC to operate at high frequency while the machine receives appropriate low-frequency power.
Solution Approach 2:
The patent changes the operating frequency parameter of the MMC from traditional low frequency to high frequency (several hundred Hz). This parameter change fundamentally alters the capacitor voltage fluctuation characteristics, reducing fluctuations to acceptable levels without requiring excessive capacitor capacity, thus resolving the voltage stability issue while maintaining system simplicity.
2Reliability
If the capacity of the energy storage capacitor is increased to suppress the fluctuation of the capacitor voltage, then the voltage stability is improved, but the volume and weight of the capacitor increase, and system construction and maintenance costs increase
Solution Approach 1:
By changing the operating frequency parameter to high frequency, the patent fundamentally alters the relationship between capacitor capacity and voltage fluctuation. At high frequency, the fluctuation magnitude is inherently reduced, allowing adequate voltage stability with much smaller capacitor capacity, thus avoiding the penalty of increased weight and cost.
Solution Approach 2:
The high-frequency periodic operation of the MMC creates a faster charging-discharging cycle for the capacitors, which naturally limits the voltage swing amplitude. This periodic high-frequency action reduces the required energy storage capacity while maintaining voltage stability during the machine start-up phase.
3Reliability
If traditional solutions are adopted to improve capacitor voltage fluctuation, then the fluctuation is reduced, but harmonics, voltage stress, and loss of the MMC system worsen, adversely affecting the machine
Solution Approach 1:
The multi-pulse cycloconverter serves as an intermediary that isolates the MMC from the machine, allowing the MMC to operate at high frequency without directly exposing it to the machine's low-speed requirements. This intermediary structure enables the use of high-frequency switching benefits while filtering out harmful harmonics before they reach the machine.
Solution Approach 2:
By operating the MMC at high frequency rather than using traditional low-frequency solutions or direct drive, the patent achieves voltage fluctuation suppression without the harmful side effects. The high-frequency operation, combined with the cycloconverter's multi-pulse structure, naturally filters harmonics and reduces voltage stress and losses compared to traditional approaches.
4Reliability
If existing MMC drive circuits are used, then the fluctuation during machine start-up is improved, but after start-up at low frequency (50 Hz), the capacity, volume, and weight of the capacitor are not effectively reduced
Solution Approach 1:
The patent implements continuous high-frequency periodic operation of the MMC throughout both start-up and steady-state running, rather than switching between high frequency during start-up and low frequency during operation. This continuous high-frequency action ensures that capacitor voltage fluctuations remain suppressed at all times, allowing for reduced capacitor capacity and weight without compromising performance during any phase of operation.
Data Source
AI summary
The present invention discloses a high-power machine drive system based on a modular multilevel converter (MMC), which belongs to the technical field of power generation, power transformation, or power distribution. The high-power machine drive system consists of a modular multilevel converter and a multi-pulse cycloconverter. The MMC outputs k phases of high-frequency AC voltages with a phase difference of 2π/k, and the multi-pulse cycloconverter outputs a low-frequency voltage to drive a corresponding machine. According to the present invention, the MMC is combined with the multi-pulse cycloconverter, and by adopting the MMC that operates at a high frequency, the capacity, the volume, and the weight of the energy storage capacitor of the MMC are reduced, the voltage level at the DC side of the MMC is increased, and the capacity of the drive system is increased. By adopting the multi-pulse cycloconverter, quality of an output waveform at a machine side can be guaranteed, thereby implementing low frequency control on the machine. The present invention may be adapted to drive a high-power low-speed machine.