Wind energy-harvesting technologies and recent research progresses in wind farm control models

The ESS is considered as an effective tool for enhancing the flexibility and controllability of a wind farm, and the optimal control scheme of a wind farm with distributed ESSs is vital to the stable operation of wind power generation. In this paper, a coordinated active and reactive power control strategy based on model predictive control (MPC) is proposed for doubly fed induction generator (DFIG)-based wind farm (WF) with distributed energy storage systems (ESSs). The proposed control scheme coordinates the active and reactive power output among DFIG wind turbines (WTs), grid-side converters (GSCs), and distributed ESSs inside the WF, and the aim is to decrease fatigue loads of WTs, make the WT terminal voltage inside the extent practicable, and take the WF economic operation into consideration. Moreover, the best reactive power references of DFIG stator and GSC are produced independently based on their dynamics. At last, the control scheme generates optimal power references for all ESS to make the SOC of each ESS converge to their average state. With the distributed ESSs, the WF controller regulates the WTs inside WF more flexibly. A WF composed of 10 DFIG WTs was utilized to verify the control performance of the proposed coordinated active and reactive power control strategy.

This study aims to conduct the sub‐synchronous (SS) resonance analysis of a multi‐machine power system incorporating doubly fed induction generator (DFIG)‐ or permanent magnetic synchronous generator (PMSG)‐based wind farm. The linearised equations involving the dynamics of steam turbine generator, wind farm, shunt capacitor, transmission line, and load are derived, and the detailed state matrix formation process pertinent to each component is presented. Taking the IEEE 16‐machine‐68‐bus test system as example, the eigenvalue analysis along with participation factor is applied to investigate the impacts of the series compensation level of transmission line on the modal frequencies and damping ratios of two groups of oscillatory modes of interest: namely, the SS mode of the series‐compensated transmission line and the shaft modes of the studied steam turbine generator before and after one of the un‐studied steam turbine generators is replaced by the DFIG‐ or PMSG‐based wind farm. Some useful conclusions and comments are drawn.

The transient stability analysis incorporating wind power intermittency and volatility is studied in this study. The wind turbines with doubly fed induction generator (DFIG) and direct‐driven permanent magnet synchronous generator (PMSG) are considered, respectively, during analysis. In modelling of DFIG and PMSG, the default GE wind turbine model and the Western Electricity Coordinating Council generic wind turbine model are employed, respectively. Based on the Jensen model, which is applied to describe the wake effect in a wind farm located on flat terrain, an equivalent simplified model of a wind farm is established regarding wind farm layout. On the basis of the built wind farm model, the Monte Carlo simulation technique combined with two evaluation indices, namely angle‐based margin index and critical clearing time (CCT) is applied on the IEEE 10‐generator‐39‐bus test system and a real‐sized China‐Jiangxi power grid as a benchmark to exploit and explore the effects of wind power intermittency and volatility on power system transient stability. The frequency distributions of transient stability evaluation index, CCT, probability of system transient instability and the range of wind speeds causing system transient instability are simulated. In addition, the influence of wake effect on transient stability is discussed as well.

In last few years, the most accepted and prominent choice in wind farm technology is Doubly Fed Induction Generators (DFIGs). But it is vulnerable to faults, especially which relate to grid, as its stator winding is directly attached to the grid. For uninterrupted supply of power, the DFIG requires to be connected to the grid during the faults according to the grid code. Hence, improving the Fault Ride Through (FRT) capability is one of the major concerns to ensure the stable operation of DFIG based wind farms. This paper is focused on the performance of various converter-inverter topologies for transient stability analysis of DFIG. Thus far, two level and three level neutral point clamped converter-inverter topologies have been implemented in FRT analysis. However, the analysis has not yet been performed for the two newly introduced advanced topologies i.e. flying capacitor and Zsource model. Simulations had been executed using PSCAD/EMTDC simulation platform. The effect of different converter-inverter topologies on the terminal voltage and the DC-link voltage were demonstrated and compared. Simulation results show that flying capacitor model outperforms all the other converter-inverter topologies by all aspects.

This study deals with non‐linear interaction problems of DFIG (doubly fed induction generator) based wind farm in a stressed power system. The study is performed on an IEEE modified test system using the non‐linear analysis based on the modal series (MS) method. As the installed capacity of renewable energy, especially the wind‐energy increases, the wind farm connection to the grid should be studied from different viewpoints. The dynamic performance of this composite power system is an important issue and should be investigated carefully especially in stressed conditions and different operating points of system controllers. The non‐linear interactions will complicate the dynamic behaviour of power systems. If such interaction exists between wind farm and other system controllers and is quantitatively significant, the stability of wind plant is jeopardised. To verify the analytical results of MS method, the necessary comparisons have been made with the results of the non‐linear simulation and approximate linear methods.

Participation of PMSG-based wind farms to the grid ancillary services

During recent years, wind energy has moved from an emerging technology to a nearly competitive technology. This fact, coupled with an increasing global focus on environmental concern and a political desire of a certain level of diversification in the energy supply, ensures wind energy an important role in the future electricity market. For this challenge to be met in a cost-efficient way, a substantial part of new wind turbine installations is foreseen to be erected in big onshore or offshore wind farms. This fact makes the production, loading and reliability of turbines operating under such conditions of particular interest.

The paper considers a previously unreported control approach for the control of off-shore DFIG-based wind farm grid with line commutated HVDC connection. The method is applicable for wind farms based-on doubly fed induction generators (DFIGs) or generators interfaced with full-rated power electronic converters. The paper focused on DFIG- based generation. The proposed control scheme allows each wind turbine to share in the voltage and frequency control of the offshore grid. The classical approach of using droop characteristics for adjusting the voltage and frequency according to the power flow of each turbine is employed. In result, the offshore grid is controlled by each turbine using only the electrical variables of its local connection point. The proposed control approach is validated via PSCAD simulations of normal operation regime and of ride-through operation under loss of the main onshore AC grid. The results confirm very good active and reactive power sharing between wind-farm DFIGs while frequency and voltage deviations in the off-shore grid are very small.

This paper proposes a grouped, reactive power optimization control strategy to maximize the active power output of a doubly-fed induction generator (DFIG) based on a large wind farm (WF). Optimization problems are formulated based on established grouped loss models and the reactive power limits of the wind turbines (WTs). The WTs in the WF are grouped to relieve computational burden. The particle swarm optimization (PSO) algorithm is applied to optimize the distribution of reactive power among groups, and a proportional control strategy is used to distribute the reactive power requirements in each group. Furthermore, the proposed control strategy optimizes the reactive power distribution between the stator and the grid side converter (GSC) in each WT. The proposed control strategy greatly reduces the number of variables for optimization, and increases the calculation speed of the algorithm. Thus, the control strategy can not only increase the active power output of the WF but also enable the WF to track the reactive power dispatching instruction of the power grid. A simulation of the DFIG WF is given to verify the effectiveness of the proposed control strategy at different wind speeds and reactive power references.

This paper examines the small-signal oscillation stability of a grid-connected wind farm affected collectively by multiple doubly fed induction generators (DFIGs) with similar dynamics. An analysis of this issue, which is referred to as the collective impact of similar DFIGs, revealed that the interconnection of a standalone DFIG with the rest of the wind farm determines whether the collective impact of similar DFIGs is beneficial or detrimental to the oscillation stability of the wind farm. The network topology and parameters of the wind farm affect the scale of the collective impact in several respects. First, increasing the length of the transmission line connecting the wind farm to the AC grid leads to an increase of the collective impact. Second, when the transmission line is much longer than the cables within the farm, the collective impact is approximately proportional to the number of DFIGs. Accordingly, increasing the transmission line length and/or number of DFIGs may magnify the detrimental collective impact, resulting in growing oscillations in the wind farm in the worst case. The analysis of the collective impact of similar DFIGs in this study revealed a new mechanism through which grid-connected DFIG wind farms can lose oscillation stability.

As installed capacity of wind power increases significantly around the world, the power from wind energy starts to replace conventional generation and the active power control of wind farms has become a major concern to the stability and security of the entire power system. It is expected that the wind farm should operate upon operator's request and control its active power according to the dispatch command, which means the wind farm should behavior in some ways more similar to the conventional power plants. This paper mainly focuses on the control system and control capability of the double fed induction generator (DFIG)-based wind farm as well as the wind turbine. Besides, the requirements and different control modes of active power in wind farm are also presented. The control performance demonstrated by the simulation result validates the feasibility of active power control methods and control system presented in this paper.

Wind farms experience significant power losses due to wake interactions between turbines. Research shows that wake steering can alleviate these losses by redirecting the flow through the farm. However, dynamic closed‐loop implementations of wake steering are rarely presented. We present a model‐free closed‐loop control method using reinforcement learning methodology known as policy gradients in combination with recursive least squares to perform real‐time wake steering in a wind farm. We present dynamic simulations of a four‐turbine wind farm row using HAWC2Farm, implementing the reinforcement learning control method for various inflow conditions and controller configurations. By controlling the three most upstream turbines, mean power gains of and (95% confidence interval) are observed in partial wake and full wake conditions respectively at 7.5% turbulence intensity. The study helps to bridge the gap between theoretical wind farm control and real‐world wind farm systems.

The analysis of reactive power for offshore and onshore wind farms connected to the grid through high-voltage alternating-current transmission systems is considered in this paper. The considered wind farm is made up with doubly fed induction generators (DFIGs). Modeling and improved analysis of the effective reactive power capability of DFIGs are provided. Particularly, the optimal power-tracking constraints and other operational variables are considered in the modeling and analysis of the DFIG reactive power capability. Reactive power requirements for both overhead and cable transmission systems are modeled and compared with each other as well as with the reactive power capability of the wind farms. Possibility of unity power factor operation suggested by the German Electricity Association (VDEW) is investigated for both types of installations. Aggregate reactive power demands on both wind farms are assessed such that the bus voltages remain within an acceptable bandwidth considering various operational limits. The reactive power settings for both types of wind farm installations are determined. In addition, the minimum capacity and reactive power settings for reactive power compensation required for cable-based installations are determined. Several numerical examples are given to illustrate the reactive power characteristics and capability of DFIGs, performance of transmission lines and reactive power analysis for DFIG-based grid-connected wind farms. A summary of the main outcomes of the work presented in this paper is provided in the conclusions section. Copyright © 2012 John Wiley & Sons, Ltd.

In wind farm (WF) systems, the set-points of wind turbine generators (WTGs) is determined by the wind farm operator. A change in the value of set-point in two consecutive intervals leads to power fluctuations in the WF system, i.e. power deviation. A large amount of power deviation may adversely affect the operation of the WF system. Therefore, this paper proposes a strategy for operation of a WF system to minimize the power deviation of WTGs during operation time. By using the proposed strategy, the change in set-points of WTGs is minimized and the output power of WTGs is therefore smoother and avoids unnecessary fluctuations. Several grid-code constraints are also considered for operation of the WF system. This ensures the WF system to operate in compliance with the requirements from transmission system operator. Besides, an additional strategy is also proposed to monitor all events in the WF system. Whenever an event occurs in the system, the set-points of WTGs are rescheduled considering the event. Therefore, the proposed operation strategy is also capable of handling events in the WF system. Finally, the simulation results with and without the proposed method are compared to show the effectiveness of the proposed method.

In a power system with wind farms, the point of common coupling (PCC) usually suffers from voltage instability under large wind speed variations and the load impact. Using the internal converter of a doubly fed induction generator (DFIG)-based wind turbine to provide voltage support auxiliary service is an effective scheme to suppress the voltage fluctuation at PCC. To satisfy the reactive power demand of the connected grid, an active voltage coordinate control strategy with the hybrid energy storage system of the wind farm is proposed. The dynamic reactive power balance model is established to show the interaction between the reactive power limitation of the wind farm and the reactive power compensation demand of the grid. This indicates the initial conditions of the active voltage coordinate control strategy. According to the critical operating point and the operation state of the DFIG, the active and reactive power coordinate control strategy composed of active ω-β coordinate control and active β control is proposed to enhance the reactive power support capability and stabilize the grid voltage. To compensate the active power shortage, an auxiliary control strategy based on the hybrid energy storage system is introduced. The simulation results show that the proposed strategy can suppress the voltage fluctuation effectively and make full use of primary energy.