MMC DC Fault Control Using Arm Current and DC Link Voltage
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Solution Overview
Problem
Current methods for sensing DC failures in Modular Multilevel Converters (MMC) are inefficient due to reliance on arm current measurements, leading to delayed protection and low reliability, as they do not simultaneously consider DC link voltage, and do not allow arm controllers to control sub-module operations effectively.
Innovation Solution
A control device and method that includes an arm controller to detect arm currents, a sub-module controller to bypass DC failure currents, and a main controller to determine temporal or permanent failures by monitoring both arm currents and DC link voltage, enabling quick and reliable protection by transmitting appropriate control signals to manage sub-module operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If DC failure is sensed by measuring all six arm currents, then sensing accuracy is improved, but response time deteriorates due to the complexity of measuring all currents
Solution Approach 1:
The patent extracts the essential detection function from the complex six-arm current measurement system by selecting only the upper arm currents of two specific converters for monitoring. This extraction maintains adequate failure detection capability while dramatically simplifying the measurement burden and enabling faster response.
Solution Approach 2:
Instead of implementing complete six-arm current measurement (excessive action), the patent applies partial action by monitoring only a subset of arm currents (upper arms of two converters). This partial measurement provides sufficient information for DC failure detection without the time penalty of comprehensive measurement.
2Device complexity
If DC failure determination relies on arm current measurement only, then device complexity is reduced, but reliability of failure sensing deteriorates
Solution Approach 1:
The patent merges multiple detection parameters (arm current and DC link voltage) into a unified failure determination logic. This combination of measurement approaches compensates for the limitations of individual methods and significantly improves the reliability of DC failure sensing without adding substantial system complexity.
3Device complexity
If only main controller monitors DC link voltage, then control structure is simplified, but protection speed deteriorates due to centralized control delay
Solution Approach 1:
The patent segments the control function by distributing the DC link voltage monitoring capability to individual arm controllers. Each arm controller independently monitors its own DC link voltage and can immediately detect failures without waiting for centralized main controller processing, thereby significantly improving protection speed.
Solution Approach 2:
The arm controllers perform self-service by autonomously monitoring their respective DC link voltages and making immediate protection decisions. This eliminates the communication delay inherent in centralized monitoring and enables faster local response to DC failures.
4Reliability
If sub-module bypasses DC failure current, then sub-module protection is improved, but system flexibility deteriorates due to inability to distinguish temporary from permanent failures
Solution Approach 1:
The patent implements feedback mechanisms where arm controllers continuously monitor arm currents and DC link voltages to determine whether DC failures are temporary or permanent. Based on this feedback information, the system can dynamically adjust its response - bypassing for permanent failures while allowing continued operation for temporary failures, thus maintaining system flexibility.
Data Source
AI summary
A control device for an MMC is disclosed. The control device for an MMC including a plurality of converter arms that include a plurality of sub-modules connected in series and that are connected to a DC link includes: an arm controller, which detects the arm current of a converter arm so as to determine whether a DC failure has occurred, and, if it is determined that the DC failure has occurred, transmits a bypass control signal for protecting a sub-module and notifies of the DC failure; a sub-module controller for controlling the sub-module so as to bypass a DC failure current according to the bypass control signal received from the arm controller; and a main controller, which detects, in real-time, the arm current of the converter arm and a voltage (DC link voltage) of the DC link, determines whether the DC failure is a temporary DC failure or a permanent DC failure on the basis of the detected arm current and DC link voltage if the occurrence of the DC failure is notified by the arm controller, and transmits, to the arm controller, a normal operation control signal for normal operation of the sub-module or a bypass control signal for bypassing of the DC failure current.