Virtual Capacitance in Modular Multi-Level Converters
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Solution Overview
Problem
Existing modular multi-level converters (MMC) for high voltage direct current (HVDC) transmission networks face challenges in effectively regulating internal energy and maintaining stability of the DC power supply network, leading to inadequate decoupling between submodule voltages and DC power supply voltages, and insufficient robustness in responding to voltage fluctuations.
Innovation Solution
A modular multi-level converter with a converter control module that regulates voltage across modeled capacitors and adjusts an internal energy setpoint using an adjustable virtual inertia coefficient, allowing for real-time adaptation to voltage fluctuations and improved stability of the DC power supply network.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a real capacitor is connected in parallel with the DC power supply network to stabilize voltage, then the voltage stability is improved, but the device complexity and physical space requirements increase
Solution Approach 1:
The patent creates a virtual copy of a capacitor through control algorithms that simulate capacitive behavior. The virtual capacitor is implemented via software control of the MMC submodules, replicating the voltage-stabilizing function of a physical capacitor without requiring actual capacitive components in parallel with the DC network.
Solution Approach 2:
The patent replaces the physical electrostatic field-based capacitor system with a controlled electronic system using semiconductor switches and energy management algorithms. The virtual capacitor uses active power exchange and energy storage in MMC submodules to achieve voltage stabilization, substituting physical capacitor mechanics with electronic control mechanisms.
2Adaptability or versatility
If the internal energy of the converter is increased to improve response to voltage fluctuations, then the adaptability is improved, but the energy management complexity increases
Solution Approach 1:
The patent implements a dynamic virtual capacitor where the effective capacitance value can be adjusted in real-time through control parameters. The virtual capacitance is not fixed but adapts dynamically based on operating conditions, allowing the system to optimize its energy response characteristics without physical reconfiguration.
Solution Approach 2:
The patent changes the operational parameters of the converter to achieve virtual capacitance effects. By adjusting switching patterns, duty cycles, and power exchange rates with the DC network, the system modifies its effective energy storage and release characteristics, creating variable virtual capacitance without physical component changes.
3Adaptability or versatility
If a virtual capacitor with adjustable capacity is implemented through control algorithms, then the adaptability to different operating conditions is improved, but the control system complexity increases
Solution Approach 1:
The patent makes the MMC converter perform multiple functions simultaneously: power conversion, voltage stabilization, and virtual capacitance provision. The same converter structure and control system that handle AC-DC conversion also provide the virtual capacitor function, eliminating the need for separate stabilization equipment and reducing overall system complexity.
Solution Approach 2:
The converter uses its own internal energy storage capabilities and power exchange mechanisms to provide voltage stabilization services to the DC network. Rather than requiring external capacitors or separate stabilization systems, the converter serves itself by regulating its internal energy to maintain DC voltage stability.
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
The invention relates to a modular multi-level converter (2) provided with a control module (4) and a calculator (10) for calculating a set value for the internal energy of the converter, stored in the capacitances of the sub-modules of the half-arms. The control module is designed to deduce, from said set value for the internal energy of the converter, a set value for the voltage at the terminals of each modelled capacitor used to regulate the voltage at the points of connection of the converter to the continuous power supply network and the voltage at the terminals of each modelled capacitor.