Coordinated frequency regulation by doubly fed induction generator-based wind power plants

Wind turbines (WTs) participate in frequency regulation, which is one of the means to solve the problem of inadequate regulation capacity in power systems with a high proportion of renewable energy. The doubly fed induction generator (DFIG) can reserve part of power to achieve bidirectional regulation capability through rotor over-speed and increasing pitch angle. In this paper, it is pointed out that the available bidirectional regulation power of the WT is constrained by the maximum regulation power under the rotor speed regulation. The regulation power constraints under the pitch regulation considering the time scale are calculated. The adjustment coefficient of WT participating in frequency regulation is designed. Considering the regulation power constraints, the frequency difference interval in which the WT can provide the regulation power according to the adjustment coefficient is analyzed. The rotor speed and pitch coordinated control strategy of DFIG with different wind speeds is designed. Based on 24-hour measured data from a wind farm, the power constraints and their effects of WTs in the wind farm participating in frequency regulation are verified by simulation. The regulation power of the wind farm, frequency quality, and wind power utilization under the different control strategies are analyzed. The results show that the effects of bidirectional power constraints must be taken into account when evaluating the effectiveness of WTs in continuous frequency regulation.

When wind generator is integrated into the power system, it will not only change the system's inertia but also introduce a lot of randomness to the active power output of wind farms. This is due to the wind speed disturbance which wind farms has always suffered from. Hence the large-scale wind farms integration into the power system will results in profound influence on frequency stability. This paper focuses on the study of frequency stability of a power system integrated with large-scale wind farms under different wind speed disturbance scenarios. A comparison of dynamic frequency response characteristics during the period when the wind turbines experience random noise disturbance, ramp events and generator trips in the power grid with two different wind turbines which are doubly fed induction generator (DFIG) and direct drive synchronous generator (DDSG) is made. The simulation results indicate the wind speed disturbance significantly influence the frequency of power grid. The advantages of DDSGs over DFIGs are that they reduce the impact of wind speed random noise disturbance and ramp events are significant. Nonetheless, the DFIGs are less likely to tripping from power grid when the wind speed is high.

Doubly fed induction generator (DFIG) with over-speed control has the ability to adjust output quickly, which is suitable for maintaining frequency stability in power system. Under the condition that the system suffers sudden power shortage, emergency frequency support can be provided to the system temporarily by it, which is more economical than relying on storage devices. In the designed system, synchronous generator (SG) and wind turbines are parallel connected to support the load, and the wind turbines play the role of providing emergency frequency support, cooperating with synchronous machines in an orderly manner. Finally, the studied system with 60% wind power penetration is established, and the effectiveness of the proposed strategy is verified by MATLAB/Simulink simulation. This operation strategy does not need to be equipped with large capacity energy storage device, thus optimizing the economy of the system operation. At the same time, excavating deeply the potential for frequency regulation of wind turbines helps system improve the frequency regulation performance.

With the development of complex renewable energy systems, the frequency control and regulation of the power grid powered by such renewable energies (e.g., wind turbine) are more critical, since the adopted different power generators can lead to frequency variations. To address the frequency regulation of such power grids, we will present a variable coefficient coordinated primary frequency regulation scheme for synchronous generator (SG) and doubly fed induction generator (DFIG). The variable adjustment coefficient of DFIG is defined according to the current reserve capacity, which can be applied to adjust different operation conditions to regulate the frequency variation within a predefined allowable range. Since the DFIG can make full use of the reserve wind power in the system frequency regulation, the proposed method can address both the frequency regulation response and the economic performance. Simulation results indicate that the proposed coordinated control scheme can achieve satisfactory frequency regulation response and lead to reduced demand for frequency regulation of SG.

Energy storage (ES) has a flexible regulation performance to improve the frequency stability of the wind turbine system. However, the doubly-fed induction generator (DFIG) has different frequency regulation capabilities operating at different wind speeds, and how to collaborate the operation of DFIG with ES for frequency regulation is a very critical problem. A combined wind-storage frequency regulation control strategy considering full wind speed is proposed in this paper. Firstly, the equivalent model of the combined wind-storage system is established, and the virtual inertia control of DFIG and the droop control of ES are analyzed for the system frequency regulation. Secondly, propose the combined wind-storage frequency regulation control strategy based on the operation characteristics of the DFIG in different wind speed zones. At low wind speed, the frequency regulation capability of DFIG is fully considered, and ES is allowed to start first for frequency regulation. Improving frequency fluctuations at medium wind speeds using a combined wind-storage system operating in concert. In order to allow DFIG to operate safely at high wind speeds only frequency regulation is performed by ES. Finally, the effectiveness of the proposed strategy is verified by simulating the sudden increase of system load under different wind speed zones on MATLAB/Simulink platform. The results indicate that the strategy has better frequency regulation effect and effectively improves the stability of the system frequency.

The increasing level of wind power integration using doubly fed induction generators (DFIGs) has implications for local frequency support as a consequence of decoupling between DFIG rotor speed and grid frequency. To ensure reliable and stable power system operation with DFIG integration, supplementary inertial control strategies are required. Conventional inertial control algorithms which use the rate of change of frequency (RoCoF) and frequency deviation loops (droop loop) require great effort to determine appropriate gains suitable for all power grid and wind speeds. In this paper, the influence of supplementary inertial control loop parameters on the inertial response and power system frequency are analysed. An active control strategy is proposed for frequency regulation using variable gains in the frequency deviation loop for the inertial controller. The variable gain control approach is shown to actively respond to system changes to improve the performance. The controller is compared with the widely used PID method. The proposed method is shown to enhance the frequency nadir and guarantee steady DFIG operation.

Background: With the large-scale Doubly Fed Induction Generator (DFIG) wind turbine integrated into the power system, the DFIG inertia response of the wind turbine should be provided. Also, the frequency response should be similar to the conventional generation technologies. This paper investigated the influence of frequency response term and wind speed conditions on system frequency control. Methods: The specific operating conditions of four control strategies, including inertia control, droop control, over speed control and pitch angle control were researched in this paper. Multi-factor coordinated frequency control strategy of DFIG wind turbine was established based on the above researches. The strategy was composed of wind speed ranging from low to high. Results: According to the simulation results, the DFIG wind turbine, which was based on multifactor coordinated frequency control strategy, could respond to the system’s frequency change of power grid, effectively. Conclusion: It helps system frequency return to stable states better and faster than DFIG wind turbine and also could reduce the fluctuation of system frequency.

Wind power as a renewable energy has the potential to become a clean energy source in almost all countries of the world but, there are lots of technical challenges that need to be addressed in advance. Wind speed variation (WSV) is one of the most important issues. Separate from its mechanical effects on wind turbines (WTs), it causes electrical power variations on WT as well. Doubly Fed Induction Generators (DFIGs) and Maximum Power Point Tracking (MPPT) system have been helpful so far but some problems such as inertia still needed to be solved. Inertia in the WTs causes a delay on MPPT. This paper proposes a novel method to increase WTs output power in which a DFIG equipped with an energy storage system such as Ultra-capacitor helps MPPT system to track the wind variations rapidly by absorption or injection energy during the wind changes. The ultra-capacitor and control system are modeled and simulated in Matlab/Simulink® environment. The simulation results proofs that the proposed system can improve electrical power and increase electrical energy during WSVs. The simulation model will lead to more penetration of wind power and also enables engineers to optimize the system.

The integration of wind farms into the electricity grid has become an important challenge for the utilization and control of electric power systems, because of the fluctuating and intermittent behavior of wind power generation. Doubly fed induction generators (DFIG) are commonly used wind turbine generators (WTG) in electricity networks. In DFIG there are three control loops which should be implemented in order to appropriate performance of WTG. These control loops are pitch angle control, rotor speed control and voltage control. In this paper a unified scheme is used to adjust these controllers in the same time. Particle Swarm Optimization (PSO) is used to adjustment the proposed controllers. The results of wind power on the system performance are compared with a conventional thermal power plant.

Power sector exhibits interest in renewable energy sources to meet the ever increasing power requirement. Growth of wind power is outstanding among all renewable energy sources which make essential to provide frequency regulation to the grid from wind turbines. The inertial control and droop control techniques are employed for sharing of wind turbines in grid frequency regulation. This article presents the study of frequency response of DFIG based wind turbine for ARMA model of wind speed. Additionally, it objectives to examine the impact on frequency response with increasing wind power potential in cases of wind power plants operated without frequency control and both inertial and droop controllers. Also, the impression of droop parameter has simulated for different wind power penetration. Novelty of this article derived from the fact that it presents influence of increasing wind power potential without and with frequency regulation techniques along with varying droop parameter in totality. Present cases has simulated with the help of MATLAB/SIMULINK. The simulation results support the beneficial impact of wind turbine equipped with inertial control and droop control.

Due to a large number of grid-connected wind turbines don't have the ability to respond to system frequency changes, problems on frequency stability of power system have become increasingly prominent. At present, the primary frequency regulation of wind turbines is mostly based on the method of additional frequency regulation control to change the reference value of electromagnetic power, so as to realize that wind turbines can participate in the system frequency regulation. However, the method of additional frequency regulation control does not t consider the conflict of power increment between the power tracking controller and the frequency regulation controller, which lead to the power increment required for the primary frequency regulation can't reach the target value. Based on this issue, the paper combines the original power tracking technology of wind turbines with the existing frequency regulation technology, and change the signal of frequency deviation by the power tracking curve of the wind turbine, which realize the inertial response and primary frequency regulation of the unit through the fast active power control of the wind turbine. That the purpose of frequency regulation is realized and the power tracking curve is changed, by converting the active output required for the frequency regulation control into the increment load reduction. The simulation results show that, based on the control method in the paper, the double-fed wind turbine can fully achieve the target increment needed for primary frequency regulation, which makes the wind turbines more reliable and effective to participate in system of frequency regulation.

Sequential frequency regulation strategy for DFIG and battery energy storage system considering artificial deadbands

With the penetration of wind power increasing continuously, the inertia and the capability of primary frequency regulation of grid are declining. To deal with this problem effectively, additional primary frequency regulation control and inertia control adopted in wind turbines are necessary. However, at present, the functional orientation of primary frequency regulation control and inertia control is still unclear. It is urgent to clarify the difference between the two kinds of control methods and to make clear the demand of power grid for them. In this paper, based on analysis of main methods of primary frequency regulation and inertia control of doubly-fed induction generators (DFIG), the functional orientation distinction between inertia support function and primary frequency regulation function is studied from perspective of control principles and control results. The different effectiveness of different control functions on the frequency regulation process in different scenarios are analyzed by simulations.

Against the backdrop of continuously increasing renewable energy penetration, the large-scale integration of wind power has significantly altered the output characteristics of generation units. The growing volatility of wind power output exacerbates the complexity of frequency regulation, potentially rendering traditional under frequency load shedding (UFLS) strategies inadequate. Investigating methods that leverage the frequency regulation capabilities of doubly-fed induction generators (DFIGs) and coordinated demand-side load clusters to participate in UFLS holds significant research value for enhancing power system frequency stability, reducing load curtailment, and improving power supply reliability. Therefore, this paper proposes an optimized UFLS strategy considering large-scale wind power integration. The frequency response models of wind turbines and thermostatically controlled loads (TCLs) in grid frequency regulation are analyzed, and the configuration principles for implementing UFLS in power systems are elaborated. A simulation model incorporating both wind turbines and TCLs in frequency regulation is developed, and UFLS simulations are conducted based on the operational conditions of a regional power grid. The results demonstrate that the participation of wind power and TCLs in frequency regulation not only ensures grid security but also significantly reduces load curtailment.

A neural network controller is presented for the direct power control of a doubly fed induction generator (DFIG) in a wind turbine. Due to the variability of wind speed, aerodynamic control of the pitch angle of the turbine blades is necessary to capture maximum power from the wind. An alternative control strategy for tracking the maximum power that can be extracted is by accurate control of the turbine rotational speed. However, due to the nonlinearities inherent in a wind energy conversion system (WECS) and wind speed unpredictability, exact control of this speed becomes challenging. This is addressed by the use of an adaptive learning rate to guarantee convergence. Hence, the controller is robust to uncertainties in wind speed. The superior performance of the proposed control technique, under varying nominal wind speeds, is established by simulation results.