Event-Triggered Non-Linear Control Strategy for Stabilizing Offshore MMC Grids During Asymmetrical AC Faults

Grunnleggende konsepter

An event-triggered control strategy using linear and non-linear controllers is proposed to stabilize an offshore MMC-HVDC grid during asymmetrical AC faults by suppressing negative sequence currents.

Sammendrag

The paper presents an event-triggered control strategy to stabilize an offshore MMC-HVDC grid during asymmetrical AC faults. The system consists of an offshore wind plant, an offshore MMC converter, and two onshore MMC converters in a three-terminal meshed configuration.

The key highlights are:

1. Asymmetrical AC faults, such as single-phase-to-ground, phase-to-phase, and phase-to-phase-to-ground faults, can generate large negative sequence currents in the system, leading to instability.

2. The proposed control strategy is activated upon fault detection, which is based on monitoring the sudden increase in negative sequence current.

3. Two control approaches are evaluated - a conventional PI controller and a non-linear Super-Twisted Sliding Mode Controller (STSMC) for the inner current control loop of the offshore MMC converter.

4. The STSMC controller demonstrates faster restoration of system stability compared to the PI controller, by quickly converging the system to a new steady-state operating point after the fault is cleared.

5. Simulation results on a ±525 kV, 2 GW, three-terminal meshed MMC-HVDC system model in RTDS validate the effectiveness of the proposed event-triggered non-linear control strategy in stabilizing the system during various asymmetrical AC fault scenarios.

Statistikk

The system has a rated power of 2 GW and a rated voltage of ±525 kV.
The offshore wind plant is connected to the DC grid through a 300 km cable, while the onshore converters are connected via a 400 km cable.
The MMC converters have 200 submodules per arm.

Sitater

"An event-triggered control strategy is activated to regulate the sequence components of voltage and current signals during a fault."
"The non-linear STSMC controller achieves faster restoration of system stability than the conventional PI controller."

Viktige innsikter hentet fra

Event-Triggered Non-Linear Control of Offshore MMC Grids for Asymmetrical AC Faults

by Naaj... klokken arxiv.org 09-26-2024

https://arxiv.org/pdf/2409.16743.pdf

Dypere Spørsmål

How can the proposed control strategy be extended to handle symmetrical AC faults in offshore MMC-HVDC grids?

The proposed control strategy, which focuses on event-triggered suppression of negative sequence currents during asymmetrical AC faults, can be extended to handle symmetrical AC faults by incorporating additional control mechanisms that address the unique characteristics of symmetrical faults. Symmetrical faults, such as three-phase short circuits, typically result in a balanced reduction of voltage across all phases, leading to a significant drop in system voltage and potential overcurrent conditions.
To adapt the existing control strategy for symmetrical faults, the following enhancements can be considered:

Enhanced Fault Detection: Implement a more comprehensive fault detection algorithm that can identify symmetrical faults by monitoring the phase voltages and currents. This could involve using a combination of sequence component analysis and real-time monitoring of voltage levels across all phases.

Dynamic Control Adjustments: Develop a control scheme that dynamically adjusts the control parameters based on the type of fault detected. For symmetrical faults, the control strategy could focus on maintaining voltage stability and managing the overall power flow rather than solely suppressing negative sequence currents.

Coordination with Protection Systems: Integrate the control strategy with existing protection systems to ensure that the MMC-HVDC system can respond effectively to symmetrical faults. This could involve coordinating the control actions with circuit breaker operations to isolate faulted sections of the grid while maintaining stability in the unaffected areas.

Utilization of Advanced Control Techniques: Explore the use of advanced control techniques, such as Model Predictive Control (MPC) or Adaptive Control, which can provide more robust responses to varying fault conditions, including symmetrical faults. These techniques can optimize the control actions based on predicted system behavior during fault conditions.

By implementing these enhancements, the control strategy can effectively manage both symmetrical and asymmetrical AC faults, ensuring the stability and reliability of offshore MMC-HVDC grids.

What are the potential challenges in implementing the STSMC controller in a real-world offshore MMC-HVDC system, and how can they be addressed?

Implementing the Super-Twisted Sliding Mode Controller (STSMC) in a real-world offshore Modular Multilevel Converter (MMC)-HVDC system presents several challenges, including:

Chattering Phenomenon: Although STSMC is designed to reduce chattering compared to traditional Sliding Mode Control (SMC), some level of chattering may still occur due to the discontinuous nature of the control action. This can lead to wear and tear on the converter components. To mitigate this, a higher-order sliding mode control approach or a continuous approximation of the control law can be employed to smoothen the control action.

Parameter Sensitivity: The performance of STSMC is highly sensitive to the tuning of its parameters (e.g., gains α and β). In a real-world application, variations in system dynamics and uncertainties can affect these parameters. To address this, adaptive tuning methods can be implemented, allowing the controller to adjust its parameters in real-time based on system performance and operating conditions.

Implementation Complexity: The mathematical complexity of STSMC may pose challenges in terms of implementation and integration with existing control systems. To overcome this, a modular approach can be adopted, where the STSMC is implemented in stages, allowing for gradual integration and testing within the existing control architecture.

Real-Time Computation: The computational demands of STSMC may be significant, especially in a fast-changing environment like an offshore HVDC system. Utilizing advanced digital signal processors (DSPs) or field-programmable gate arrays (FPGAs) can enhance the real-time computational capabilities, ensuring that the controller can respond promptly to system changes.

Robustness to Disturbances: While STSMC is robust against uncertainties, real-world systems may experience unexpected disturbances. Implementing a robust control framework that combines STSMC with other control strategies (e.g., PI controllers for outer loops) can enhance overall system resilience.

By addressing these challenges through careful design, adaptive strategies, and advanced computational techniques, the STSMC can be effectively implemented in offshore MMC-HVDC systems, enhancing their fault ride-through capabilities.

What other advanced control techniques could be explored to further enhance the fault ride-through capabilities of offshore MMC-HVDC systems?

To further enhance the fault ride-through capabilities of offshore Modular Multilevel Converter (MMC)-HVDC systems, several advanced control techniques can be explored:

Model Predictive Control (MPC): MPC is an advanced control strategy that utilizes a model of the system to predict future behavior and optimize control actions accordingly. By incorporating constraints related to voltage and current limits during fault conditions, MPC can provide a more proactive approach to managing system stability and performance during faults.

Back-Stepping Control (BSC): BSC is a non-linear control technique that can be particularly effective in systems with complex dynamics. By systematically designing control laws that stabilize the system step-by-step, BSC can enhance the robustness of the MMC-HVDC system during fault conditions, ensuring smoother transitions back to stable operating points.

Adaptive Control: Adaptive control techniques can adjust control parameters in real-time based on changing system dynamics and operating conditions. This adaptability can be crucial during fault scenarios, where system behavior may deviate significantly from nominal conditions. Implementing adaptive algorithms can improve the system's resilience to disturbances and enhance fault ride-through capabilities.

Fuzzy Logic Control: Fuzzy logic controllers can handle uncertainties and nonlinearities in the system effectively. By incorporating expert knowledge and rules, fuzzy logic can provide a flexible control strategy that adapts to varying fault conditions, improving the overall stability and performance of the MMC-HVDC system.

Neural Network Control: Leveraging artificial intelligence, neural networks can be trained to predict system behavior and optimize control actions based on historical data. This approach can enhance the fault detection and response capabilities of the MMC-HVDC system, allowing for quicker and more accurate adjustments during fault conditions.

Distributed Control Strategies: In multi-terminal HVDC systems, distributed control strategies can enhance coordination among different converters. By enabling local controllers to communicate and share information, the overall system can respond more effectively to faults, improving fault ride-through performance.

Hybrid Control Approaches: Combining multiple control strategies (e.g., integrating STSMC with MPC or BSC) can leverage the strengths of each method, providing a more robust and flexible control solution. Hybrid approaches can enhance fault detection, response time, and overall system stability during fault conditions.

By exploring and implementing these advanced control techniques, offshore MMC-HVDC systems can achieve improved fault ride-through capabilities, ensuring reliable operation in the face of disturbances and enhancing the integration of renewable energy sources.