MMC Full-Bridge Control for DC Line Disturbance Ride-Through
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Solution Overview
Problem
Modular multilevel converters (MMCs) face challenges in quickly detecting DC faults without triggering false alarms, leading to potential shutdowns and reduced robustness due to overly sensitive fault detection algorithms.
Innovation Solution
A method for controlling MMCs that reduces AC active current and DC voltage references based on the magnitude of the DC voltage falling below a threshold, allowing autonomous handling of DC faults until traditional detection algorithms can verify the fault, thereby relaxing the speed requirements of fault detection and reducing false triggers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional DC fault detection algorithms are used with high sensitivity, then DC faults can be detected quickly, but false triggering and MMC tripping increase
Solution Approach 1:
The control method performs preliminary action by proactively reducing the DC voltage reference and AC active current reference when DC voltage drops below the upper threshold, before a DC fault is detected. This preemptive control action prepares the system to handle potential faults gracefully, reducing the risk of false tripping while maintaining reliable fault detection capability
Solution Approach 2:
The invention changes operating parameters dynamically by adjusting the DC voltage reference and AC active current reference based on the measured DC voltage level. When DC voltage falls below the upper threshold, these parameters are reduced proportionally, allowing the system to adapt to changing conditions and avoid false fault detection while maintaining operational reliability
2Speed
If DC voltage is reduced quickly upon fault detection, then DC fault isolation is achieved, but system stability may be compromised due to abrupt changes
Solution Approach 1:
The control method implements dynamic adjustment of the DC voltage reference and AC active current reference based on the actual DC voltage level. Rather than abrupt reduction, the parameters are scaled proportionally according to the DC voltage threshold ratio, creating a smooth transition that maintains system stability while achieving fault isolation
Solution Approach 2:
The system continuously monitors the DC voltage level and uses this feedback to adjust the DC voltage reference and AC active current reference in real-time. This closed-loop control ensures that voltage reduction follows the actual system state, preventing abrupt changes that could compromise stability while maintaining fast fault response
Data Source
Figure 1~2c
Figure 3a~3c
AI summary
A method (100) of controlling a modular multilevel converter, MMC, of a full-bridge or mixed arm type in case of a DC line disturbance is provided. The method includes determining (102, 104) whether a magnitude of a DC voltage (Udp) of the MMC has fallen below an upper voltage threshold (Ud_max_lim), and, if determining that the magnitude of the DC voltage has fallen below the upper voltage threshold, reducing (106) both a magnitude of an AC active current reference (IVD_ORD) and a magnitude of a DC voltage reference (UDC_REF) for the MMC based on the DC pole voltage. An MMC with a controller implementing the method, a converter station including at least one such MMC, and a power transfer system including at least one such converter station, are also provided.