Stability analysis of large wind farms connected to weak AC networks incorporating PLL dynamics

Voltage source converter interfaced wind turbines connected to weak grids can induce system instability. A state space model is used in this paper to study the stability of large wind farms. Dynamics of the phase locked loop is integrated to the state space model. The advantage of this model is that it allows study the stability of systems with parallel wind turbines. Studies on the dynamic responses of the system show the importance of including phase locked loop as its existence can induce system instability especially under weak network conditions. This model can be used to study the stability of converter interfaced wind farms and helps to design the converter controller to ensure wind farm system stability when connected to weak grids

Machine learning based impedance estimation in power system

A passive machine learning based technique to estimate the impedance of the power grid at the point of common coupling of a converter interfaced distributed generation source is proposed. The proposed method is based on supervised learning and provides a fast and accurate estimation of the grid impedance without adversely impacting the power quality of the system. This method does not need an injection of additional signals to the grid and provides an accurate estimation of the grid impedance. Multi-objective NSGA-II algorithm is used for optimisation and tuning the random forest model for accurate estimation of both R and X The resistive and inductive reactance of grid is estimated using Random Forest model due to its capability in the prediction of multiple output values simultaneously

Estimation of the power electronics lifetime for a wind turbine

A comparison has been made of the converter lifetime for a 3MW horizontal axis wind turbine for different wind turbulence levels. Torque and speed of the turbine shaft were used to calculate voltage and current time series that those were used to calculate the junction temperatures of diode and IGBT in the generator-side converter by a thermal-electrical model. A rainflow counting algorithm of the junction temperature in combination with an empirical model of the lifetime estimation has been used to calculate the lifetime of the power electronic module in the turbine. The number of parallel converters for each wind condition to achieve 20 years life time also has been found. it is found greater turbulence levels will lead to less lifetime of the converter in the wind turbine

An alternative current-error based control for VSC integration to weak grid

An enhanced current control strategy is proposed for voltage source converters for the integration to weak grids. The control derives from the current-error based vector control. By imple menting simple close-loop compensations of both angle and magnitude inputs to the pulse width modulation, the damping of vector control in the weak grid can be significantly improved hence able to deliver full rated power to very weak grid. Due to the presence of the current loop, the fault-ride-through capability can be maintained with no need for mode switching. A comprehensive frequency domain model is employed to analyze the stability. Time domain simulations are further carried out to validate its effectiveness and robustness of integrating to the weak grid with fault-ride-through capability

Current Error Based Compensations for VSC Current Control in Weak Grid for Wind Farm Applications

A novel current control strategy is proposed for voltage source converter connecting to weak grid using conventional current vector control with additional current error based voltage angle and magnitude compensations. For connecting to very weak AC network, conventional vector control is proved to be unstable, whereas the proposed current error based compensations can significantly improve system stability. In this way, the proposed control can still benefit from the presence of current closed-loop control without the need for control switching during large AC voltage variations. Comprehensive frequency domain model is established to analyze stability performance. Comprehensive time domain simulations are further carried out to validate its effectiveness and robustness by demonstrating its current control performance during a three-phase fault, multiple-converter situation and various grid strength conditions

A Less Intrusive Solution To Stablize VSC Transmission Against Highly Variable Grid Strength

A less-intrusive solution to stabilize a Voltage Source Converter (VSC) over an unknown grid strength is presented in this paper. The existence of equilibrium point is investigated as a prerequisite to stabilization. By partially imposing grid forming control, a simple auxiliary outer loop is proposed to exhaust the physical limit of power delivery in steady state and provide support to fault-ride-through operations over a wide range of grid strength. The proposed control can be used to upgrade a commissioned VSC with inner current loop intact; it also offers a non-intrusive solution to stabilize VSCs externally. The effectiveness of the proposed approach and schemes are verified by analysis in frequency domain and case studies in time domain including change of grid strength and fault-ride-through

Different options for multi‐rotor wind turbine grid connection

In this paper, different options for integration of the multi-rotor wind turbine concept is investigated. The intra-turbine collection network of the multi-rotor wind turbine can be designed in either AC or DC. The reliability and costs associated with each connection option are compared. It is presented that the multi-rotor wind turbine can be considered as wind farm with the same number of turbines and the same configuration for collection network as for wind farms can be used for multi rotor wind turbine

Integration of large wind farms to weak power grids

Power grids are changing significantly with the introduction of large amounts of renewable energy (especially wind) into the system. Integration of wind energy into the grid is challenging as, firstly it increases penetration stresses when compared to conventional generation as the wind is intermittent and fluctuates in power output. Secondly, most of the wind farms are located in offshore or rural areas which have good wind conditions. The grid in these regions is not normally strong. Most of the modern variable speed wind turbines use voltage source converters (VSCs) for grid integration. However, integrating VSCs to weak power grids will cause instability when a large amount of active power is transferred to the grid. In this thesis, the integration of wind farms to very weak power grids is investigated. A multiple input, multiple output (MIMO) model of the grid side VSC of a wind turbine is developed in the frequency domain in which the d-axis of the synchronous reference frame (SRF) is aligned with the grid voltage. Then, this model has been used as the basis for modelling the multiple parallel converters in the frequency domain. In this thesis, to improve the stability of the very weak grid connected of VSCs, a control method based on the d- and q- axis current error is introduced. This controller compensates the output angle of the phase locked loop (PLL) and the voltage amplitude of the converter. Using this controller, full rated active power transfer and fault ride-through are achieved under very weak grid connection. Furthermore, a stabiliser controller based on virtual impedance is proposed in this thesis to achieve stable operation of a very weak grid connected VSC. This stabilising control method enables the VSC to operate at full power and to ride-through faults under very weak grid conditions. Based on this principle, an external device is proposed that can be utilised and connected to a weak point of the grid to allow a large amount of VSC interfaced power generation (e.g. wind power) to be connected to the grid without introducing stability issues.Power grids are changing significantly with the introduction of large amounts of renewable energy (especially wind) into the system. Integration of wind energy into the grid is challenging as, firstly it increases penetration stresses when compared to conventional generation as the wind is intermittent and fluctuates in power output. Secondly, most of the wind farms are located in offshore or rural areas which have good wind conditions. The grid in these regions is not normally strong. Most of the modern variable speed wind turbines use voltage source converters (VSCs) for grid integration. However, integrating VSCs to weak power grids will cause instability when a large amount of active power is transferred to the grid. In this thesis, the integration of wind farms to very weak power grids is investigated. A multiple input, multiple output (MIMO) model of the grid side VSC of a wind turbine is developed in the frequency domain in which the d-axis of the synchronous reference frame (SRF) is aligned with the grid voltage. Then, this model has been used as the basis for modelling the multiple parallel converters in the frequency domain. In this thesis, to improve the stability of the very weak grid connected of VSCs, a control method based on the d- and q- axis current error is introduced. This controller compensates the output angle of the phase locked loop (PLL) and the voltage amplitude of the converter. Using this controller, full rated active power transfer and fault ride-through are achieved under very weak grid connection. Furthermore, a stabiliser controller based on virtual impedance is proposed in this thesis to achieve stable operation of a very weak grid connected VSC. This stabilising control method enables the VSC to operate at full power and to ride-through faults under very weak grid conditions. Based on this principle, an external device is proposed that can be utilised and connected to a weak point of the grid to allow a large amount of VSC interfaced power generation (e.g. wind power) to be connected to the grid without introducing stability issues

Analysis of integration of multi‐terminal HVDC network to weak grids

In this paper, the integration of multi-terminal HVDC systems to weak power grids is analysed under different grid strengths and control methods. In this paper, a modular multi-level converter based MTDC network is considered. The analysis showed the importance of voltage control of DC network. The voltage control of DC network needs to be performed from the converter station connected to the strongest AC network. Furthermore, the analysis shows that either droop or PI control can be used for controlling the DC voltage of the MTDC network connected to one or more weak grids. However, the droop control of DC voltage provides faster response compared to PI with larger steady state error. Finally, the analysis showed that the MMC-based MTDC is capable of ride-through unbalanced AC side faults even if the faults occur in a weak grid