MPC Based Coordinated Active and Reactive Power Control Strategy of DFIG Wind Farm with Distributed ESSs

Since the doubly fed induction generator (DFIG) converters are designed by the slip power (a fraction 25-30% of the total power) to achieve full control of the generator, there is not enough capacity to provide reactive power when the voltage dips below the tolerable grid voltage level at the point of common coupling (PCC). We propose in this paper to use the static synchronous compensator (STATCOM) for compensating the grid voltage dip to improve the power quality in a weak grid or asymmetrical load. The simplified active and reactive power control strategies of a 6 MVA wind farm with DFIG are described. The maximal power point tracking (MPPT) algorithm is implemented. The simulation results using the platform between Matlab/Simulink and PLECS demonstrate the good performance of the wind farm with DFIG and STATCOM, validating the analytical conclusions.

Doubly-fed induction generator (DFIG) wind turbines connected to capacitive series-compensated transmission lines are prone to exhibit oscillatory behavior. The phenomena is called sub-synchronous resonances (SSRs), as these oscillations occur at frequencies below the fundamental component. This paper first develops a modeling methodology for DFIG wind turbines, based on impedance matrices, that is applied to model a real wind farm where SSRs were reported. The stability analysis performed shows how the interaction between the grid-side converter and the rotor-side converter contribute to the instability of DFIG wind energy conversion systems connected to series compensated grids. With this model, we propose a simple sub-synchronous resonance control strategy based on an orthogonal proportional action applied to the rotor currents, and a variable gain in the PI controller adjusted as a function of the DFIG rotational speed. This control strategy depends only on the rotor currents, which are local and already measured variables in any DFIG wind turbine, and is implemented in the rotor side converter, so it does not imply an additional cost at wind farm or wind turbine level and can be applied to any DFIG wind energy conversion system (WECS). Additionally, it proves to be robust for any line impedance series compensation level, and it does not need real-time information concerning the grid at which the wind turbine is connected, or its parameters. A real case study is considered, where the sub-synchronous resonance damping strategy presented in this work is able to stabilize the system for every possible line impedance compensation level.

With the increasing penetration of wind power into electric power grids, energy storage devices will be required to dynamically match the intermittency of wind energy. This paper proposes a novel two-layer constant power control scheme for a wind farm equipped with doubly fed induction generator (DFIG) wind turbines. Each DFIG wind turbine is equipped with a supercapacitor energy storage system (ESS) and is controlled by the low-layer wind turbine generator (WTG) controllers and coordinated by a high-layer wind farm supervisory controller (WFSC). The WFSC generates the active power references for the low-layer WTG controllers according to the active power demand from or generation commitment to the grid operator; the low-layer WTG controllers then regulate each DFIG wind turbine to generate the desired amount of active power, where the deviations between the available wind energy input and desired active power output are compensated by the ESS. Simulation studies are carried out in PSCAD/EMTDC on a wind farm equipped with 15 DFIG wind turbines to verify the effectiveness of the proposed control scheme.

Many wind turbine generators (WTGs) are installed in remote, rural areas, where the power grids are usually weak, characterized by unbalanced voltage conditions. If the voltage unbalance is not taken into account in the control system, it will cause poor power quality and poor operating performance of the WTG systems. This paper proposes a novel control scheme to improve the operating performance of a wind turbine equipped with a doubly fed induction generator (DFIG) under unbalanced network voltage conditions. The rotor-side converter (RSC) and grid-side converter (GSC) of the DFIG are controlled in a positive (dq)+ reference frame as well as in a negative (dq)- reference frame. Control of the RSC and GSC in the (dq)+ reference frame is the same as the regular control under balanced network conditions. The supplementary control of the RSC in the (dq)- reference frame minimizes the electromagnetic torque pulsations of the DFIG caused by the unbalanced network voltage; the supplementary control of the GSC in the (dq)- reference frame contributes to balance the total output currents of the DFIG. The proposed control scheme is implemented in PSCAD/EMTDC on a 3.6 MW DFIG wind turbine connected to a power network with unbalanced voltage. Results show that it improves the operating performance of the DFIG wind turbine system.

The growth rate of penetration of wind energy has addressed many challenges and impacts related to the power quality and power system stability because of its intermittent nature. Once a doubly fed induction generator (DFIG) is connected to a power system, it absorbs lagging reactive power during the grid operation causing instability issues to the grid voltages. In this research paper, a DFIG based wind turbine is being modeled considering the optimal power coefficient and reactive power control strategy. The optimal power coefficient correlates the power law exponent with the respect of velocity and elevation. The reactive power control strategy is modeled using back-to-back converter system. The paper also investigates the voltage improvement of a large distribution network connected with DFIG system. By using steady state analysis, the voltage magnitude profiles are determined during DFIG at point of common coupling. The proposed control shows that DFIG based wind-turbine integration to the large distribution network enhances the weak bus voltage.

In micro grid, the voltage and frequency will fluctuate with the switching of large-power electric load. With inverter control of energy storage systems,both active and reactive power pumped into the utility grid can be controlled which could achieve the stabilizing effect of voltage and frequency. This paper addresses active and reactive power control strategies used in energy storage system to eliminate the fluctuation. The active power is controlled based on a frequency-droop characteristic and a complimentary frequency restoration strategy. The reactive power is controlled based on a voltage-droop characteristic and a load reactive power compensation strategy. The experimental results demonstrate the effectiveness of the proposed control strategy under various micro grid operating conditions.

With large wind energy integration into power systems, wind farms begin to influence power systems in a much more significant manner. As wind energy systems utilize different generator technologies from the one utilized in the conventional power plants, the steady-state, transient and small-signal dynamics, as well as, power system stability will thus be significantly affected. The impact of wind energy systems on the power system dynamics and stability is thus of practical importance. As there is a significant increase in installation of wind turbines equipped with doubly-fed induction generator (DFIG) in recent years, a dynamic model of the DFIG wind turbine is firstly developed in this thesis. The model is validated against field measurement data, and the validation gives confidence about the accuracy and applicability of the developed model. DFIG wind farms consist of tens to hundreds of identical DFIG wind turbines increasing the complexity of the wind farm model and simulation time. A novel aggregation technique is developed in this thesis that incorporates a multiplication factor, namely mechanical torque compensation factor (MTCF), to the mechanical torque of the full aggregated wind farm model. The MTCF is initially constructed to approximate a Gaussian function by using fuzzy logic method. By optimizing the MTCF on a trial and error basis, less than 10 percent discrepancy is then achieved between the proposed aggregated model and the complete model. The proposed aggregation technique is then applied to a 120 MVA offshore wind farm comprising of 72 DFIG wind turbines and shows higher accuracy in approximating the wind farm dynamics as it appears at the point of common coupling (PCC) as compared to the full aggregated model. The proposed aggregated model computes faster than the complete wind farm model by 90.3 percent during normal operations and 87 percent during grid disturbances. To overcome the adverse effects due to the fluctuating nature of wind, two wind power smoothing methods are proposed using a fuzzy logic pitch angle controller for a smooth performance with a minimum drop in output power. One method performs partial smoothing with only 4.74 percent drop in the output power while the other method offers complete smoothing with a 8.28 percent drop in output power. The impact of wind energy integration on power system transient stability (PSTS) is studied quantitatively with the transient energy margin (TEM), which is calculated through the evaluation of the transient energy function (TEF). This study is carried out in two ways in the thesis. One is to analyse the impact of transient fault on the DFIG wind turbine as compared to SGs for different factors, like the fault clearing time, the grid coupling, the inertia constant and the voltage sag. The study reveals that transient stability of the DFIG wind turbine is hardly affected by the grid coupling, the inertia constant and the generator terminal voltage sag variations indicating its consistent transient performance within a wide range of these factors. The fault clearing time should be almost 11 percent faster for the system with the DFIG than the synchronous generator (SG). The other is to investigate the impact of the DFIG wind farm on the PSTS with the variation of different factors, which are the voltage sag, the fault clearing time, the load and the wind power penetration level. The study reveals that power systems integrated with DFIG wind farms are sensitive to transient events with high voltage sag, high fault clearing time, low load operation and high wind power penetration level. Machines at different locations individually possess distinct fault response and suffer from a large-scale power imbalance. As a result, reliable operation of DFIG wind farm integrated power systems demands upgraded equipment, such as advanced switchgear, fast breakers/isolators, efficient power reserve systems and advanced reactive power compensating device, etc.

This paper presents a hybrid method of active power and reactive power control strategy with Quasi Z-source inverter (QZSI) in single phase grid connected photovoltaic (PV) systems. The maximum power from the PV panel is extracted and most of it is supplied as active power to the grid. The major portion of required reactive power exchange to the grid is done by means of thyristor switched capacitor (TSC) and thyristor switched reactor (TSR) to improve the generation capacity of the PV system. In the proposed topology inverter shares the minimum reactive power and large change in reactive power will be delivered/absorbed by the TSC-TSR to get smoother operation in reactive power control. The DC voltage controller and the AC current controller is discussed in brief. The mathematical analysis for grid synchronization method is presented in detail. The control algorithm for active power and reactive power is analyzed and discussed in the paper. The simulation results are shown for validating the proposed concept.

This study presents a bi‐level decentralised active power control (DAPC) for a large‐scale wind farm cluster (WFC), consisting of several wind farms for better active power dispatch. In the upper level, a distributed active power control scheme based on the distributed consensus is designed to achieve fair active power sharing among multiple wind farms, which generates the power reference for each wind farm. A distributed estimator is used to estimate the total available power of all wind farms. In the lower level, a centralised control scheme based on the model predictive control is proposed to regulate active power outputs of all wind turbines (WTs) within a wind farm, which reduces the fatigue loads of WTs while tracking the power reference obtained from the upper‐level control. A WFC with 8 wind farms and totally 160 WTs, was used to test the control performance of the proposed DAPC scheme.

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.

Hundreds of megawatt level variable speed Doubly Fed Induction Generator (DFIG) pumped storage turbine will play an important role in voltage and frequency regulation, system stability, active and reactive power control etc.. However, similar but different from the DFIG wind turbines, such power rating and reversible pump-turbine with small capacity converter to control the whole variable speed system is challenging. In this paper, mathematical analysis and modeling of variable speed DFIG pumped storage turbine including reversible pumped turbine, DFIG and back to back converter is build first. On this basis, the control method is presented. The grid side converter stabilizes the dc-side voltage and control the grid-connected current sinusoidal. The rotor-side converter can control and decouple the active power and reactive power independently. Finally, the real time simulation results of 300MW pumped storage turbine based on Real Time Digital Simulator platform indicate the proposed control strategy can achieve good performance in different working conditions of pumping and generation. The system has fast response and good stability.

This work presents the overall model of the doubly fed induction generator (DFIG) and proposes a new control design for the grid side converter of the concerned DFIG plant. The system under study consists of two DFIG wind turbines which are connected to the grid by a bi-directional Z-source inverter. In order to ameliorate the DC link voltage regulation of the DFIG generation plant; particularly, a new supervised control method is introduced and is directly applied to the grid side converter (GSC). In this way, initially, by tuning two control coefficients (CCs) which forms the footprint of uncertainty (FOU), a double-input interval type-2 fuzzy logic (DIT2-FL) is designed for the GSC system. The best setting of the control coefficients for the desired performance of the concerned system is recommended and a new DIT2-FL supervised online adaptive neuro fuzzy inference system (ANFIS) is then developed accordingly. By implementing the supervised controller, the dynamic behavior of the tested wind turbines under two distinct operations for low/high wind speeds is investigated. Detailed experimental outcomes have been presented to ascertain the correctness and accuracy of the suggested control methodology. Compared with the prevalent state-of-the-art techniques, the suggested controller leads to remarkable transient improvement of closed-loop DFIG system.<br/>

The doubly-fed induction generator (DFIG) has received increasing attention in offshore wind farms (OWFs) due to its low cost advantage. Meanwhile, the diode rectifier unit (DRU)-based high-voltage direct current (HVDC) system is an emerging offshore wind power transmission technique, also for its low cost. Hence, it is economically optimized to combine the two low cost techniques together, namely, integrating the DFIG-based wind farms with DRU-HVDC to the onshore grid. However, since the DRU cannot transmit active power to OWFs, DFIG-based wind farms cannot autonomously start. To address this issue, this paper proposes a black start strategy based on distributed energy storage systems (ESS) connected to the DC-links of DFIG back-to-back converters. In order to maximize the startup efficiency of the wind farm under varying wind conditions, a dual-path startup strategy is further devised, involving both rotor-side and grid-side routes. The system topology is introduced, and the control strategies for both storage-equipped and non-equipped wind turbines (WTs) are presented. Finally, a simulation model is developed using Matlab/Simulink, and the simulation results verify the feasibility and effectiveness of the proposed strategy.

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.

Based on the stator flux linkage oriented control theory of doubly fed induction generator (DFIG), the control system of the megawatt level variable speed constant frequency (VSCF) wind turbine generator system, which adopts back-to-back PWM converters, is proposed. The Dual-PWM converter is divided into two parts: grid side converter and rotor side converter. The grid side converter adopts the voltage and current dual close-loop control strategy. The rotor side converter adopts rotor current open-loop control strategy and the active and reactive current close-loop control strategy. The full-load experiment is conducted when DFIG is operating under sub-synchronization or super-synchronization condition. The control system can also respectively regulate the active and reactive power output under variable wind speed condition. The experiment results validate the feasibility of the control systems.