MMC Capacitor Discharge Using Breaker Resistor and Short-Circuit Path
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Solution Overview
Problem
Existing modular multilevel converters (MMC) require additional physical discharge mechanisms, such as resistor elements, which increase size and cost.
Innovation Solution
A power conversion device incorporating a current-limiting resistor, bypass switch, and short circuit switch, controlled by a control device to facilitate rapid discharge of capacitors in converter cells without the need for additional physical discharge mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a resistor element is provided in parallel with each switching element for rapid discharge, then the discharge speed of the capacitor is improved, but the device size and cost increase
Solution Approach 1:
The patent combines the discharge function with the AC circuit breaker by utilizing the existing current-limiting resistor and switching mechanisms. Instead of adding separate discharge resistors for each converter cell, the system uses the AC circuit breaker's current-limiting resistor to discharge all capacitors simultaneously when the breaker opens, thereby achieving rapid discharge without increasing device complexity
Solution Approach 2:
The current-limiting resistor in the AC circuit breaker is given a dual function: it limits inrush current during normal operation and serves as a discharge path for capacitor energy when the breaker opens. This multi-functionality eliminates the need for dedicated discharge resistors, reducing both device size and cost while maintaining rapid discharge capability
2Productivity
If additional physical discharge mechanisms are added to each converter cell, then the discharge performance is improved, but the device complexity and component count increase
Solution Approach 1:
The patent merges the discharge function into the AC circuit breaker system by utilizing the existing current-limiting resistor and breaker switching action. When the AC circuit breaker opens, the capacitors discharge through the current-limiting resistor, eliminating the need for separate discharge mechanisms in each converter cell and reducing overall component count
Solution Approach 2:
The system uses its own existing components (AC circuit breaker and current-limiting resistor) to perform the discharge function. The AC circuit breaker's opening action automatically initiates the discharge process through the current-limiting resistor, eliminating the need for additional dedicated discharge components and reducing device complexity
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 allows for rapid discharge of capacitors in MMCs, minimizing additional physical components and reducing costs while effectively managing discharge currents.
Implementation Method 1
the current-limiting resistor is connected to a path of AC output current of the power converter
Implementation Method 2
the bypass switch is connected in parallel with the current-limiting resistor. The short circuit switch short-circuits an AC line between the AC circuit breaker and the current-limiting resistor
Data Source
AI summary
In a power conversion device, each of a plurality of converter cells includes a pair of input/output terminals, a plurality of switching elements, and a power storage element electrically connected to the input/output terminals through the switching elements. A current-limiting resistor is connected to a path of AC output current of the power converter. A bypass switch is connected in parallel with the current-limiting resistor. A short circuit switch short-circuits an AC line between an AC circuit breaker and the current-limiting resistor. In a discharge operation mode, a control device performs power conversion operation by allowing switching of each switching element of each of the converter cells in a state in which the AC circuit breaker and the bypass switch are opened and the short circuit switch is closed.