MMC Converter Current Limiting During Grid Transients
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Solution Overview
Problem
Modular Multilevel Converters (MMC) face challenges in controlling current during transient events like grid failures or load steps, leading to overcurrent protection activation and converter disconnection, as they conventionally operate in voltage control mode without current control loops.
Innovation Solution
A method and device that limit current above a certain threshold using a converter controller to calculate a compounded voltage reference based on real-time grid current and voltage measurements, allowing the MMC to operate as a voltage source without switching control modes, thereby preventing overcurrent protection activation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the MMC operates in conventional voltage control mode without current control loops, then the control strategy is simplified and the converter can operate as a voltage source, but during grid failures or load steps the current may exceed the maximum allowed value activating overcurrent protection and disconnecting the converter
Solution Approach 1:
The control system dynamically adapts its behavior by monitoring current magnitude and automatically switching between conventional voltage control mode and current-limited mode. The controller adjusts the control strategy in real-time based on operating conditions, enabling the system to maintain simplicity during normal operation while providing current protection during transient events
Solution Approach 2:
A current monitoring and control mechanism is introduced as an intermediary layer between the voltage control reference and the converter output. This intermediary continuously monitors the converter current and intervenes by limiting the output current when thresholds are exceeded, without requiring complete mode switching or complex control architecture changes
2Reliability
If the MMC switches between voltage control mode and current control mode to prevent overcurrent, then the current can be controlled during transient events, but the switching between control modes takes time during which fast transient episodes may still trip the converter
Solution Approach 1:
The control system maintains continuous voltage control action throughout all operating conditions. Instead of switching between discrete control modes, the system continuously monitors current and applies current limiting within the same unified control framework, ensuring uninterrupted control action during transient events and eliminating switching delays
Solution Approach 2:
The control system continuously monitors converter current and prepares current limiting action in advance before overcurrent conditions occur. By detecting current trends and anticipating potential overcurrent events, the controller can preemptively adjust the control output to prevent tripping, rather than reacting after the fault has already occurred
Data Source
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AI summary
The present disclosure relates to a method for controlling a converter (10) connected to a first AC grid (17) via a first transformer (14a) and to a second AC grid (18) via a second transformer (14b). The method comprises calculating a first voltage reference based on real power (P) and reactive power (Q) of the second grid and on a reference for the real power and for the reactive power. The method also comprises calculating a second voltage reference comprising a current-dependent part. The current-dependent part is dependent on whether an absolute value of grid current (IR) of the second grid measured in real-time is above a predefined current threshold. The method also comprises combining the first and second voltage references to a compounded reference voltage. The method also comprises controlling output converter voltage (uR) of the MMC to the second transformer based on the compounded voltage reference.