Modular Multilevel Converter Circulating Current Control
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Solution Overview
Problem
Modular multilevel voltage source converters face challenges in accurately handling circulating currents without requiring special measurements, particularly in reducing or eliminating 2nd harmonics, which are not efficiently addressed by existing methods that rely on filters or conventional control based on current measurements.
Innovation Solution
A control unit determines a circulating current control signal contribution based on the sum of cell voltages in the upper and lower phase arms and combines it with a modulation reference to generate a modified voltage reference for controlling the phase arm, effectively handling circulating currents during waveshape formation using standard voltage and current measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional control based on current measurements is used, then circulating current handling is achieved, but measurement precision and control accuracy are insufficient for 2nd harmonics
Solution Approach 1:
The patent uses cell voltage measurements as an intermediary parameter to indirectly determine circulating current. Instead of directly measuring circulating current with special sensors, the control unit calculates it from the sum of cell voltages in upper and lower phase arms, which are already measured for waveshape formation. This intermediary approach achieves precise circulating current detection without additional measurement devices.
Solution Approach 2:
The existing voltage measurement infrastructure serves dual purposes: both waveshape formation and circulating current detection. The cell voltage measurements already taken for converter operation automatically provide the data needed for circulating current control, eliminating the need for separate measurement systems and achieving high precision without added complexity.
2Object-generated harmful factors
If filters are introduced to remove circulating currents, then 2nd harmonics are reduced, but device complexity and cost increase
Solution Approach 1:
The patent converts the harmful 2nd harmonic circulating current into a controllable parameter by calculating it from existing voltage measurements. Instead of passively filtering the harmful current, the control system actively detects and compensates for it through modified voltage references, transforming a structural problem into a controllable one that requires no additional hardware.
Solution Approach 2:
The patent replaces the mechanical/filter-based approach to circulating current reduction with a control-based electronic solution. Instead of using physical filters to block 2nd harmonics, the system uses computational methods to generate compensating voltage references that actively cancel circulating currents, substituting hardware complexity with software control.
3Measurement precision
If special measurements are implemented for circulating current, then control accuracy improves, but ease of operation and implementation simplicity deteriorate
Solution Approach 1:
The patent makes the existing voltage measurement system universal by having it serve both waveshape formation and circulating current detection functions simultaneously. The same cell voltage measurements used for basic converter operation are also utilized for precise circulating current control, achieving multi-functionality without requiring separate measurement infrastructure.
Solution Approach 2:
The control system serves itself by using its own existing voltage measurements for circulating current detection. No external or special measurement systems are needed because the converter's operational measurements automatically provide the circulating current information, making the system self-sufficient and easy to implement.
4Measurement precision
If conventional control without circulating current compensation is used, then device complexity is low, but circulating current control accuracy is insufficient
Solution Approach 1:
The patent applies preliminary action by calculating the circulating current control signal contribution before it is needed for compensation. The control unit proactively determines the circulating current based on cell voltages and generates the compensating signal in advance, allowing precise control without requiring complex real-time measurement and reaction systems.
Data Source
AI summary
A modular multilevel voltage source converter and a method and computer program product for handling circulating currents in such a modular multilevel voltage source converter are described. A control unit of the voltage source converter determines a circulating current control signal contribution (ER1−ER2+2VCM) for a phase leg based on the sum of cell voltages in the upper phase arm of the phase leg, the sum of cell voltages in the lower phase arm of the phase leg and at least one modulation reference and combines the circulating control signal contribution with a modulation reference used for one of the phase arms in order to obtain a modified voltage reference for controlling the phase arm to handle a circulating current while a waveshape is formed.