MMC Capacitor Discharge Using Circulating Current Control
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Solution Overview
Problem
The existing modular multilevel converters (MMC) require additional resistor elements for capacitor discharge, which increase the size and cost of the power conversion device.
Innovation Solution
A power conversion device with a control device that controls AC and circulating currents to manage capacitor discharge by increasing the effective value of circulating current and reducing the AC current amplitude during a discharge mode, minimizing the need for additional physical discharge mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If a resistor element is provided in parallel with each switching element for capacitor discharge, then the capacitor can be discharged quickly, but the size and cost of the power conversion device increase
Solution Approach 1:
The patent combines the capacitor discharge function with the circulating current control function. Instead of using separate resistor elements for discharge, the control device utilizes the circulating current that already flows through the converter cells to achieve discharge. This merging of functions eliminates the need for additional physical discharge components while maintaining the power conversion device's operational capabilities.
Solution Approach 2:
The circulating current controller enables the circulating current to serve multiple purposes: maintaining voltage balance between arms during normal operation and discharging capacitors when needed. By making the circulating current multi-functional, the patent eliminates the need for dedicated discharge resistor elements, thereby reducing device complexity and cost while preserving rapid discharge capability.
2Temperature
If the number of converter cells is increased to handle high voltage, then the voltage handling capability improves, but the overall size and cost of the device increases
Solution Approach 1:
The patent changes the operational parameters of the existing converter cells by dynamically adjusting the circulating current effective value or amplitude value based on operational mode. During discharge mode, the controller increases the circulating current parameters beyond normal operating levels to achieve rapid capacitor discharge. This parameter change approach allows the system to achieve discharge functionality without adding physical components, thereby avoiding the complexity increase that would result from adding more converter cells.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables rapid capacitor discharge in MMCs while reducing the physical components and costs associated with resistor elements.
Implementation Method 1
The circulating current controller lowers a voltage of the power storage element of each of the converter cells constituting the power converter by increasing an effective value or amplitude value of the circulating current in a second operation mode
Data Source
AI summary
A power conversion device includes a power converter including a plurality of arms each having a plurality of converter cells cascaded to each other. A control device includes an AC current controller to control AC current flowing between the power converter and an AC circuit and a circulating current controller to control circulating current flowing between the arms of the power converter. The AC current controller reduces an effective value or amplitude value of AC current in a discharge operation mode in which a voltage of a power storage element of each converter cell is discharged, compared with in a normal operation mode. The circulating current controller increases an effective value or amplitude value of circulating current in the discharge operation mode, compared with in the normal operation mode.