Delta-Connected MMC Control Decoupling Transient Stability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The control mechanisms for delta-connected modular multilevel converters (MMC) interfere with each other during transient fluctuations, leading to unstable operation due to interference between average voltage control and phase-specific voltage control of DC capacitors.
Innovation Solution
A control configuration that separates the control of output currents and circulating currents by removing the representative voltage deviation of all capacitors from the circulating current control computation, preventing interference between output current control and circulating current control, and using specific control units to manage interphase balance and voltage commands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If control means for average voltage of all DC capacitors and control means for average voltage of DC capacitors in each phase are both implemented, then voltage balance is improved, but control stability deteriorates during transient fluctuations
Solution Approach 1:
The control means is segmented into two independent parts: (1) control means for the average value of voltages across all DC capacitors, and (2) control means for the average value of voltages across DC capacitors in each phase. These are implemented as separate control loops that operate independently, preventing mutual interference while achieving comprehensive voltage balance control.
Solution Approach 2:
The circulating current is introduced as an intermediary variable to decouple the control of output current and DC capacitor voltage. By controlling the circulating current separately, the patent enables independent management of power distribution among cells without interfering with the output current control, thus maintaining stability during transient fluctuations.
2Strength
If delta-connected MMC configuration is used, then high voltage capability is improved, but control complexity increases due to circulation current
Solution Approach 1:
The circulation current component is extracted and controlled separately from the output current. By identifying and isolating the circulation current path in the delta-connected configuration, the patent enables independent control of this additional current component, simplifying the overall control strategy despite the complex topology.
Solution Approach 2:
The control strategy changes the approach by introducing circulating current as a controllable parameter rather than treating it as an unwanted artifact. This parameter change allows the system to actively manage the circulation current to maintain voltage balance while preserving the high voltage capability of the delta-connected MMC.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A control device (3) generates a voltage command value for controlling a current flowing between a three-phase AC power supply (1) and a power converter (2) such that a full voltage representative value representing voltage values of all power storage devices agrees with a DC voltage command value. The control device (3) generates a zero-phase voltage command value for controlling a circulating current flowing in a delta connection such that the voltage values of the power storage devices are balanced among first to third arms (A1 to A3). The control device (3) combines the voltage command value and the zero-phase current command value to generate an output voltage command value for controlling an output voltage of each unit converter (5). The control device (3) removes a control amount of the full voltage representative value from a computation of the zero-phase voltage command value to cause output current control and circulating current control not to interfere with each other.