MMC Capacitor Pre-Charging via Alternative Current Path Boosting
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Solution Overview
Problem
The existing methods for pre-charging capacitors in modular multilevel converters (MMCs) are slow and often require additional equipment or complex algorithms to boost capacitor voltages to the necessary level for operation, leading to inefficiencies and increased costs.
Innovation Solution
A method and arrangement for pre-charging MMC capacitors using an alternative current flow path with a current limiting resistor, where the existing valve control unit modulates the converter to achieve a voltage reference lower than the grid peak, allowing capacitors to be boosted to the required level without additional equipment, using existing control algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a passive charging process with blocked converter valve is used, then inrush currents are prevented during capacitor charging, but the charging process becomes slow and capacitor voltages remain insufficient
Solution Approach 1:
The converter valve switching state is dynamically changed from blocked to unblocked during the charging process. The method transitions the converter from a passive charging mode (blocked valve) to an active boosting mode (unblocked valve with PWM control), enabling the system to adapt its charging characteristics and achieve both safe current limiting and sufficient voltage boosting.
Solution Approach 2:
The control parameter of the converter valve is changed from a fixed blocked state to a controllable unblocked state with PWM modulation. By adjusting the switching duty cycle and control signals, the system can regulate the charging current while simultaneously boosting the capacitor voltage to the required level for normal operation.
2Device complexity
If the converter valve remains in blocking mode during charging, then simple control is maintained, but additional boosting equipment or complex algorithms are required to reach sufficient voltage levels
Solution Approach 1:
The converter valve, originally designed for power conversion, is made to serve a dual function: it acts as a simple switch during blocked mode for basic charging, and as an active PWM-controlled device during unblocked mode for voltage boosting. This eliminates the need for separate boosting equipment while maintaining control simplicity through existing control infrastructure.
Solution Approach 2:
The converter valve itself performs the voltage boosting function that would otherwise require external equipment. By utilizing the valve's inherent switching capability and controlling it with PWM signals, the system generates the necessary voltage increase internally, making the system self-sufficient and eliminating additional hardware requirements.
3Ease of manufacture
If capacitor voltages are not sufficiently charged before grid connection, then simpler disconnectors can be used, but the charging time increases and operational readiness is delayed
Solution Approach 1:
The system performs preliminary voltage boosting action by unblocking the converter valve and applying PWM control before the actual grid connection is established. This pre-charging phase ensures that capacitor voltages reach the necessary level in advance, allowing the use of simpler disconnectors while minimizing the overall charging time and operational delay.
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 reduces the time and cost associated with capacitor charging, avoids inrush currents, and eliminates the need for complex disconnector devices by leveraging existing control mechanisms to synchronize MMC voltages with the grid.
Implementation Method 1
Energy is usually taken from capacitors in the converter cells of the converter valves of the MMC
Implementation Method 2
In order to prevent high inrush currents during the charging of the capacitors, a converter valve may be connected to the power supply via a charging resistor
Data Source
Figure 1a~1c
Figure 2
AI summary
A method of pre-charging a modular multilevel converter (MMC) is provided, including interrupting a normal, first current flow path between the MMC and the electric grid, providing an alternative, second current flow path between the MMC and a power source, pre-charging, via the second current flow path, the capacitors in the converter cells of the converter valve to a first voltage level below a peak voltage magnitude of the electric grid but sufficient to power the valve control unit, after reaching the first voltage level, setting the voltage reference to a finite voltage having a magnitude lower than the peak voltage magnitude of the power source and thereby, using the valve control unit, boosting the capacitors in the converter cells of the converter valve to a second voltage level having a magnitude at least equal to or higher than the peak voltage magnitude of the electric grid.