Balancing and Discharge Resistor Arrangements for MMC Sub-Modules
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Solution Overview
Problem
In high voltage converters, such as Modular Multilevel Converters (MMCs), balancing and discharge resistor arrangements face challenges in minimizing operational losses while ensuring effective voltage sharing and safe discharge of energy storage devices.
Innovation Solution
The solution involves selectively switching in a sub-module resistor, allowing it to be optimized for its function when needed, using a series connection of semiconductor switches and a potential divider to manage voltage sharing and discharge, enabling the use of high-voltage-rated switches and reducing losses by minimizing the resistor's presence in the circuit during non-essential states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a discharge resistor is provided in parallel with an energy storage device to enable discharge, then the energy storage device can be discharged for maintenance access, but operational losses increase during normal use
Solution Approach 1:
The discharge resistor is made dynamically controllable through a switching device that can connect or disconnect the resistor from the energy storage device based on operational requirements. This allows the system to optimize between discharge capability and operational efficiency by activating the resistor only when discharge is needed.
Solution Approach 2:
The discharge resistor is activated periodically or on-demand rather than continuously. The switching device enables the resistor to be connected temporarily for discharge operations and then disconnected to eliminate continuous operational losses, implementing a periodic rather than continuous discharge mechanism.
2Speed
If a low resistance value is chosen for the discharge resistor to enable faster discharge, then discharge speed increases, but operational losses during normal use increase
Solution Approach 1:
The resistance value is made dynamically adjustable rather than fixed. The switching device can connect different resistance values in series or parallel configurations, allowing the system to use low resistance for fast discharge when needed and high resistance (or disconnected state) during normal operation to minimize losses.
Solution Approach 2:
The discharge resistor is divided into multiple resistive elements that can be selectively connected in series or parallel through the switching device. This segmentation allows the effective resistance to be adjusted: low resistance for fast discharge by connecting in parallel, high resistance for minimal losses by connecting in series or disconnecting entirely.
3Loss of energy
If a discharge resistor is provided outside the circuit to eliminate operational losses, then operational losses are minimized, but the resistor cannot provide voltage balancing during normal operation
Solution Approach 1:
The discharge resistor is designed to serve multiple functions: voltage balancing during normal operation and discharge operation when needed. The switching device enables the same resistor to be connected in parallel for voltage balancing and disconnected or reconfigured for discharge operations, eliminating the need for separate resistors for each function.
4Stress or pressure
If high voltage-rated switches are used to handle high voltage environments, then voltage handling capability increases, but device complexity and cost increase
Solution Approach 1:
The high voltage switching function is segmented into multiple lower voltage-rated switches connected in series. Each switch handles a portion of the total voltage, allowing the use of simpler, lower-voltage-rated semiconductor devices while still achieving the required overall voltage handling capability. This segmentation reduces complexity and cost compared to using a single high-voltage switch.
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
This approach allows for efficient voltage balancing and discharge without incurring high operational losses, enabling the use of high-voltage-rated switches and ensuring safe and efficient operation of MMCs.
Implementation Method 1
each switch comprising a series connection of semiconductor switches arranged to block voltage across each switch when off
Implementation Method 2
A potential divider may be provided across the switch to distribute the voltage across the switching elements
Implementation Method 3
a discharge resistor and a discharging switch... allowing the capacitor to discharge
Data Source
Figure 1A~1B
Figure 2
Figure 3
AI summary
Circuits comprising at least one energy storage device (120), a resistor (206) and switch (204) arranged in series with the resistor (206) are described. The energy storage device is arranged in parallel with the series connection of the switch (204) and the resistor (206), and the switch (204) is arranged to selectively switch the resistor (206) into a parallel connection to the energy storage device (120). In some examples, the switch (204) comprises a series connection of semiconductor switching elements (202). The circuit may comprise a sub-module (200) for use in a multilevel modular converter (300).