MMC Voltage Balancing via Active Discharge Sorting
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Solution Overview
Problem
MMC converters require voltage balancing in submodules before reconnecting to the grid, as incomplete balancing leads to prolonged discharging times and potential semiconductor overvoltage issues, causing operational delays and inefficiencies.
Innovation Solution
A method using a sorting algorithm to actively discharge and balance submodule voltages by switching H-bridge submodules to plus, minus, or zero states, ensuring all submodules are energized and within a predefined voltage range, allowing for rapid reconnection to the grid.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If passive charging is used to charge converter cell capacitors, then inrush current is limited and capacitors are charged safely, but the charging process takes between ten seconds to several minutes which causes operational delays
Solution Approach 1:
The patent applies preliminary action by pre-charging the converter cell capacitors using the grid voltage rectification through freewheeling diodes before the main breaker closes. This preliminary charging action reduces the voltage difference that would otherwise cause inrush current, while the blocking of firing pulses during this phase ensures safe charging without immediate power conversion operation.
2Reliability
If the MMC is disconnected from the grid and energy storages are discharged completely by component losses, then voltage balancing is achieved, but the discharging time takes approximately 20 minutes causing prolonged operational unavailability
Solution Approach 1:
The patent applies periodic action by implementing controlled switching of H-bridge submodules between plus, minus, and zero states in a periodic sequence. This periodic switching creates circulating currents that actively discharge energy storages and balance voltages across submodules, replacing the passive 20-minute discharge process with an active controlled process that achieves balancing in seconds.
Solution Approach 2:
The patent applies self-service by using the MMC's own internal submodules and circulating currents to perform the voltage balancing operation. The system uses its inherent H-bridge submodule structure to generate the balancing currents without requiring external balancing equipment, making the system self-sufficient for voltage regulation during grid disconnection.
3Productivity
If submodule voltages are not properly balanced before grid reconnection, then operational delays occur, but implementing active balancing requires complex control algorithms and switching operations
Solution Approach 1:
The patent applies feedback by continuously monitoring the voltage across each energy storage element and using this information to determine the switching state of submodules. The control algorithm measures voltage deviations and adjusts the switching pattern of H-bridges accordingly, creating a closed-loop system that automatically balances voltages without requiring complex external control infrastructure.
Solution Approach 2:
The patent applies parameter changes by dynamically altering the switching states of H-bridge submodules between plus, minus, and zero states. This changes the effective impedance and current flow paths in the circuit, enabling active control of energy storage discharge rates and achieving voltage balancing through parameter modulation rather than fixed resistance discharge.
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
Enables quick reconnection of the MMC to the grid by actively balancing submodule voltages, reducing discharging time from 20 minutes to seconds, preventing semiconductor overvoltage, and ensuring safe operation.
Implementation Method 1
each submodule comprising an energy storage
Implementation Method 2
discharging the energy storages by means of a circulating current
Data Source
AI summary
A method of discharging a Modular Multilevel Converter (MMC) includes a plurality of phase legs connected in delta configuration. Each leg includes a plurality of series connected submodules, each submodule including an energy storage. The method includes disconnecting the MMC from an electrical grid, discharging the energy storages by means of a circulating current, and reconnecting the MMC to the electrical grid. The discharging includes, for each phase leg, setting a voltage reference, and sequentially selecting submodules in zero state by means of a sorting algorithm for switching each of the selected submodules to plus or minus state until the voltage deviation from the set voltage reference of the energy storage of each submodule in the phase leg is within a predefined range.