MMC Converter DC Component Removal via Voltage Detection
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Solution Overview
Problem
Existing methods for removing direct current components from the output terminal of a modular multilevel converter (MMC) are costly and inefficient, as they require additional hardware like current transformers and struggle to precisely detect and eliminate direct current components affecting HVDC or STATCOM systems.
Innovation Solution
A method that detects charging voltages in sub-modules of the MMC converter, calculates an average voltage, delays it, and uses dq conversion to extract the q-axis component, then outputs an offset voltage through PI control to remove the direct current component, eliminating the need for additional hardware by integrating this process within the MMC converter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current transformer (CT) is provided to detect current flowing through converter arms, then direct current component can be detected and removed, but the cost increases significantly
Solution Approach 1:
The patent uses voltage detection as a copy/alternative approach to current detection. Instead of directly measuring current with expensive CTs, the system detects voltage across known impedance elements (submodule capacitors) and derives current information from these voltage measurements, achieving the same detection goal at lower cost
Solution Approach 2:
The patent replaces the mechanical/electromagnetic current transformer system with an electronic voltage measurement and calculation system. By measuring voltage and using computational methods (dq conversion, coordinate transformation), the system achieves current component detection without physical current transformers
2Reliability
If current transformer (CT) is used to detect direct current component, then direct current removal is achieved, but device complexity increases
Solution Approach 1:
The patent extracts the direct current component detection function from the physical current measurement domain and relocates it to the voltage measurement and computational processing domain. This separation allows the system to achieve DC component removal using existing voltage sensing infrastructure combined with digital signal processing
Solution Approach 2:
The voltage detection system serves multiple functions: it monitors submodule capacitor voltages for normal operation and simultaneously provides the data needed for direct current component detection. This multi-functionality eliminates the need for separate dedicated DC detection hardware
3Reliability
If offset detection is performed from input terminal current to remove direct current at output terminal, then direct current component can be removed, but measurement precision deteriorates due to offset affecting the current
Solution Approach 1:
The patent performs preliminary voltage detection and processing at the input side to calculate the expected voltage waveform, then compares this with actual measurements to identify and remove DC components before they affect output current. This proactive approach prevents DC offset propagation
Data Source
AI summary
A method of removing a direct current component at an output terminal of an MMC converter according to the present invention includes a detection step of individually detecting charging voltages charged in capacitors of a plurality of sub-modules connected in series to each other in the MMC converter; outputting an average value of the individually detected charging voltages; delaying the outputted average value by a predetermined phase to output a phase-delayed average value; outputting the average value and the phase-delayed average value as a q-axis component voltage by using a predetermined dq conversion unit; calculating an error between the q-axis component voltage and a three-phase average voltage for the q-axis component voltage; and outputting, through a pre-determined first PI control unit, an offset voltage for reducing the error.