Modular Multilevel Converter Control for Grid Unbalance and STATCOM Mode
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Solution Overview
Problem
Existing converter technologies face challenges in efficiently operating during special modes such as static synchronous compensator (STATCOM) mode and grid unbalance mode, particularly in maintaining voltage balance and handling grid faults and asymmetries.
Innovation Solution
A method for operating a modular-multilevel converter (MMC) that includes detecting the need to switch to specific operating modes, generating mode control signals, and using a control arrangement to generate balance voltage reference signals and phase-leg control signals, thereby enabling the converter to automatically switch between normal, STATCOM, and grid unbalance modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the converter operates in normal mode without special mode control, then the control system is simpler, but the converter cannot handle grid faults, asymmetries, or STATCOM mode operations
Solution Approach 1:
The control system dynamically adapts its behavior based on operating conditions by detecting voltage imbalances and automatically switching between normal operation and special mode control strategies, allowing the converter to handle multiple operating scenarios with a single unified control architecture
Solution Approach 2:
The control arrangement is designed to perform multiple functions: normal power conversion, STATCOM mode operation, and grid fault handling, all through a single control system that selects appropriate control strategies based on real-time grid conditions
2Reliability
If the converter uses traditional control methods during grid unbalance, then the control algorithm is simpler, but the cell voltages diverge and the converter becomes unstable
Solution Approach 1:
The control system continuously monitors phase-leg capacitor voltages and uses this feedback information to generate balance voltage reference signals that actively compensate for voltage imbalances, ensuring stable operation during grid unbalance conditions
Solution Approach 2:
The control algorithm changes its behavior by generating mode control signals that adjust the balance voltage reference signals based on detected operating modes, transforming the control parameters to maintain stability under different grid conditions
3Extent of automation
If the converter automatically switches between operating modes, then the operation is more autonomous and responsive to grid conditions, but the control logic becomes more complex
Solution Approach 1:
The converter performs self-diagnosis by detecting its own operating conditions (voltage imbalances, grid faults) and automatically selects appropriate control modes without external intervention, making the system self-regulating and autonomous
Data Source
AI summary
A method is configured for operating a converter (10) which is implemented as a modular-multilevel converter and comprises a control arrangement (38) and a number M of phase-legs (21 to 29). The method comprises detecting whether the converter (10) has to be set into one mode of a group comprising a static synchronous compensator mode or a grid unbalance mode, generating mode control signals (MCS) depending on the detected mode, generating balance voltage reference signals (ubal,ref) depending on a first side frequency (ωg), a second side frequency (ωm), second side current reference signals (im,ref) and the mode control signals (MCS), generating a phase-leg control signal (uref), generating cell control signals (51 to S4) and providing the cell control signals (51 to S4) to semiconductor switches (41 to 44) of cells (31) of the phase-legs (21 to 29).