MMC DC Fault Ride-Through Using Voltage Threshold Control
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Solution Overview
Problem
Modular multilevel converters (MMCs) face challenges in robustly handling DC faults due to sensitive fault detection algorithms that can lead to false triggering and tripping, necessitating a method to reduce DC voltage and AC active current references based on DC voltage thresholds to manage DC line disturbances effectively.
Innovation Solution
A method for controlling MMCs that involves determining if the DC voltage has fallen below an upper threshold, reducing both the AC active current and DC voltage references, and using a DC current controller to limit upper DC current levels, allowing autonomous handling of DC faults without immediate detection by traditional algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If traditional fault detection algorithms are used to detect DC faults quickly, then the response speed is improved, but the sensitivity becomes too high causing false triggering and tripping
Solution Approach 1:
The system performs preliminary actions by reducing the DC voltage reference and AC active current reference as soon as DC voltage falls below the upper threshold, before traditional fault detection algorithms trigger. This preliminary response prevents the need for highly sensitive fault detection, allowing slower but more reliable detection algorithms to be used without risking false tripping.
Solution Approach 2:
The control method applies preliminary anti-action by proactively reducing power output when DC voltage drops, counteracting the potential harmful effects of DC faults before they can cause damage. This preemptive measure reduces the urgency for fast fault detection, thereby降低ing the detection sensitivity requirement and false triggering risk.
2Reliability
If DC voltage reference and AC active current reference are reduced based on DC voltage threshold, then the DC fault ride-through capability is improved, but the control complexity increases
Solution Approach 1:
The system changes control parameters (DC voltage reference and AC active current reference) based on the DC voltage threshold condition. When DC voltage falls below the upper threshold, these parameters are reduced according to the DC voltage level, enabling the MMC to ride through DC faults. This parameter-based control approach achieves fault ride-through capability through straightforward threshold comparison and proportional reduction, avoiding excessive control complexity.
3Reliability
If the MMC reduces DC voltage quickly in response to DC fault, then the fault isolation capability is improved, but the risk of false tripping increases
Solution Approach 1:
The control method implements dynamic response by continuously monitoring DC voltage and adjusting the DC voltage reference and AC active current reference in real-time based on the DC voltage level. This dynamic adjustment allows the MMC to reduce DC voltage progressively and safely without abrupt changes that could trigger false tripping, while still achieving effective fault isolation.
Data Source
AI summary
A method 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 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 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.