insight - Wind power system - # Voltage oscillation caused by repeated low voltage ride through of wind turbine

Core Concepts

Voltage oscillations at the grid-connected point of wind turbine are caused by the different stable equilibrium points of the normal operation and low voltage ride through subsystems, leading to repeated switching between the two subsystems under reactive power control.

Abstract

The paper analyzes the conditions for voltage oscillations caused by repeated low voltage ride through (LVRT) of wind turbines through steady-state power flow calculation. It then introduces the switched system theory to model the grid-side converter (GSC) of the wind turbine, considering the external connected impedance and internal control dynamics.
The key highlights are:
The necessary conditions for voltage oscillations are that the initial voltage of the wind turbine's P-V curve is greater than the exiting LVRT threshold, and the static voltage stability limit is less than the entering LVRT threshold.
The voltage oscillations are caused by the different stable equilibrium points of the normal operation and LVRT subsystems, leading to repeated switching between the two subsystems under reactive power control.
The theoretical calculations and simulation results of the WT-GSC switched system model match well, verifying the correctness and effectiveness of the model.
The voltage oscillation frequency, amplitude and other dynamic characteristics are mainly affected by the grid strength, wind turbine active power output, control delay, and dynamic reactive current proportional coefficient.

Stats

The grid-connected point voltage can be expressed as Uw = f(U, X, Pw, Qw).
The necessary conditions for voltage oscillations are: -0.09p.u. < QwX < 0.14p.u.
The critical active power output before entering LVRT is Pw1 > PwLVRT = f^-1(U, X, Qw1, 0.80).
The active and reactive power output during the LVRT period are Pw2 and Qw2, satisfying Uw = f(U, X, Pw2, Qw2) > 0.90.

Quotes

"The electrical distance between the wind power collection sending end grid and the main grid is relatively long, lacking synchronous power supply support, showing the characteristics of weak grid. Therefore, the voltage oscillation phenomenon is easy to happen, threatening the safe and stable operation of the grid."
"The voltage fluctuations can be suppressed by optimizing the active power control strategy and LVRT thresholds."
"The reactive power control switching and reactive power recovery strategy during the LVRT recovery process of the wind turbine are the root of repeated voltage fluctuations."

Key Insights Distilled From

by Qiping Lai,C... at **arxiv.org** 04-02-2024

Deeper Inquiries

To suppress or eliminate the voltage oscillation phenomenon caused by repeated LVRT of the wind turbine, advanced control strategies can be implemented:
Optimized Active Power Control: By optimizing the active power control strategy of the wind turbine, the fluctuations in active power output can be minimized, reducing the impact on the grid-connected voltage.
Improved Reactive Power Control: Enhancing the reactive power control switching and reactive power recovery strategy during the LVRT recovery process can help stabilize the grid-connected voltage and prevent oscillations.
Fast Voltage Regulation: Implementing fast and efficient voltage regulation mechanisms can help maintain the grid-connected voltage within the desired limits, reducing the likelihood of oscillations.
Advanced Control Algorithms: Utilizing advanced control algorithms, such as model predictive control or adaptive control, can provide more precise and responsive control over the wind turbine's operation, mitigating voltage oscillations.
Coordination with Grid Support Devices: Coordinating the operation of the wind turbine with grid support devices, such as energy storage systems or FACTS devices, can help stabilize the grid voltage and prevent oscillations.

The potential impacts of repeated LVRT-induced voltage oscillations on power system stability and reliability can be mitigated through the following measures:
Dynamic Voltage Control: Implementing dynamic voltage control mechanisms to regulate the grid voltage and prevent excessive fluctuations caused by wind turbine LVRT events.
Enhanced Fault Ride Through Capability: Improving the fault ride-through capability of wind turbines to ensure smooth recovery after grid disturbances, reducing the likelihood of voltage oscillations.
Grid Reinforcement: Strengthening the grid infrastructure and enhancing its stability to better accommodate the intermittent nature of wind power generation and mitigate voltage oscillations.
Advanced Monitoring and Control Systems: Deploying advanced monitoring and control systems to detect voltage oscillations early and take corrective actions to stabilize the grid.
Collaborative Operation: Promoting collaborative operation between wind farms and grid operators to ensure coordinated responses to voltage oscillations and maintain system stability.

The insights from the study on switched system modeling can be extended to analyze other types of hybrid power electronic systems in power grids by:
Model Adaptation: Adapting the switched system modeling approach to capture the dynamics and interactions of different components in hybrid power systems, such as solar PV systems, energy storage systems, and converters.
Parameter Estimation: Estimating the parameters of the switched system model for specific hybrid power electronic systems based on their characteristics and control strategies.
Dynamic Stability Analysis: Conducting dynamic stability analysis of hybrid power systems using switched system theory to assess their transient and steady-state behavior under varying operating conditions.
Control Strategy Optimization: Optimizing control strategies for hybrid power systems based on the insights gained from switched system modeling to enhance system performance and stability.
Integration with Grid Models: Integrating the switched system models of hybrid power systems with grid models to simulate and analyze their impact on overall power system operation and stability.

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