Modular Multilevel Converter Circulating Current Suppression
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Solution Overview
Problem
Modular multi-level converters for HVDC systems face challenges in suppressing circulating currents, which lead to increased material costs, unstable sub-module voltage, and reduced efficiency due to unnecessary AC components and harmonic components in the current waveform.
Innovation Solution
A method that completely removes the AC component of the circulating current by modeling and controlling all sequence components (positive, negative, and zero sequence) of the circulating current using a PIR controller, allowing for the suppression of harmonic components in the 3-phase stationary reference frame, thereby improving the efficiency and stability of the converter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If circulating current suppression control is applied in modular multi-level converter, then sub-module voltage stability is improved, but device complexity increases due to additional control algorithms and components
Solution Approach 1:
The circulating current suppression control is segmented into three independent sequence component controls: positive sequence, negative sequence, and zero sequence. Each sequence component is controlled separately through dedicated control algorithms that calculate and suppress specific circulating current components, thereby stabilizing sub-module voltage while maintaining manageable control complexity through modularization of the control strategy.
Solution Approach 2:
The control method transforms the circulating current suppression problem by changing the parameter domain from time-domain current control to frequency-domain sequence component control. By decomposing circulating currents into positive, negative, and zero sequence components and applying specific suppression controls for each, the system achieves voltage stability without requiring overly complex time-domain control algorithms.
2Loss of energy
If AC components and harmonic components are eliminated from circulating current, then converter efficiency is improved, but control difficulty increases due to need for precise sequence component management
Solution Approach 1:
The control strategy segments circulating current into distinct sequence components (positive, negative, zero sequence) and applies targeted suppression for each. This segmentation allows systematic elimination of AC components and harmonics through separate control channels, improving converter efficiency while making the control process more structured and manageable compared to attempting to suppress all circulating current components simultaneously.
Solution Approach 2:
The patent introduces sequence component decomposition as an intermediary step between current measurement and control action. By using Fourier series expansion to decompose circulating currents into positive, negative, and zero sequence components, the system creates a structured intermediate representation that simplifies the control process and enables precise elimination of unwanted AC components and harmonics.
3Manufacturing precision
If all sequence components of circulating current are controlled, then waveform quality is improved, but manufacturing cost increases due to higher specification components
Solution Approach 1:
The control method applies partial action by selectively suppressing specific sequence components of circulating current rather than attempting to eliminate all circulating current. By focusing control efforts on positive, negative, and zero sequence components that most affect waveform quality, the system achieves improved waveform quality without requiring excessive current suppression that would necessitate higher specification and more expensive components.
Solution Approach 2:
The patent changes the control parameter from raw circulating current magnitude to sequence component decomposition using Fourier series. This parameter transformation enables precise control of waveform quality by targeting specific harmonic components, thereby achieving high manufacturing precision without requiring over-engineered components that would increase manufacturing cost.
Data Source
AI summary
Provided is a method for suppressing a circulating current in a modular multi-level converter for a high voltage direction-current (HVDC) transmission system. The HVDC transmission system converts an alternating current (AC) into a direct current (DC) and vice versa, transmits energy using a DC cable, and including a modular multilevel converter generating a high voltage source by stacking a plurality of sub-modules in series. In the circulating current suppression method, a circulating current (idiffj; j=a,b,c) of a,b,c phase in an abc 3-phase stationary reference frame, a DC current (idc) flowing in a DC cable, a current reference value (i*dc) of a DC component that needs to flow in the DC cable are inputted. The circulating current (idiffj) of the a,b,c phase is controlled to become zero. A compensation value (V*diffj) for suppressing a harmonic component of the circulating current is outputted.