Variable Speed Wind Turbine Generator Research Articles

DC-Bus Voltage Control of Self-Excited Induction Generator for Variable-Speed Wind Turbine Generation Including Dynamic Saturation

DC-Bus Voltage Control of Self-Excited Induction Generator for Variable-Speed Wind Turbine Generation Including Dynamic Saturation

Modelling and utilisation of frequency responsive TCSC for enhancing the frequency response of a low inertia grid

The objective of this paper is to enhance the frequency resilience and wind penetration level of a low inertia power system by utilising frequency responsive Thyristor Controlled Series Capacitor (TCSC). In recent years, wind power integration has become prolific in numerous power grids around the world. As such, Synchronous Generators (SGs) are being substituted from the generation mix. Unlike conventional synchronous generators, variable speed Wind Turbine Generators (WTGs) generally do not participate in system frequency control without additional control strategy. When subjected to prolific wind generation and interconnection import from an adjacent grid, number of online SGs in a grid can be scant. During such a situation, a large contingency in the power importing zone may overload an AC interconnection. Consequently, the interconnection could trip due to loss of synchronism. It eventually may cause a system-wide blackout. In addition, due to the risk of network separation, non-synchronous wind penetration level is kept restricted in many power systems. To overcome these challenges, this paper proposes a frequency responsive TCSC model to increase interconnection transfer capability by reducing the line reactance. To this end, for determining optimal TCSC parameters, a strategy is outlined. Furthermore, a generalised expression is developed to find the maximum wind penetration level while maintaining the frequency response adequacy. The developed techniques are applied to a low inertia test system. Simulation results reveal that the risk of blackout is averted, and the maximum wind penetration level is significantly increased after deploying the frequency responsive TCSC.

A novel scheme for control by active and reactive power utilized in gearless variable speed wind turbine system with PMSG connected to the grid

Introduction. As a result of increasing fossil fuel price and state-of-the-art technology, more and more residential and commercial consumers of electricity have been installing wind turbines. The motivation being to cut energy bills and carbon dioxide emissions. Purpose. The main goal of this work is developing a control scheme for a variable speed wind turbine generator in order to produce utmost power from varying wind types, and variable wind speed. Novelty. This research paper presents an IGBT power converter control scheme for active power in relation to wind speed and reactive power by adjusting Q-reference (Qref) value in a gearless variable speed wind turbine with permanent magnet synchronous generator. Methods. An effective modelling and control of the wind turbine with the suggested power converter is executed by utilizing MATLAB/Simulink software. The control scheme consists of both the wind turbine control and the power converter control. Simulation results are utilized in the analysis and deliberation of the ability of the control scheme, which reveals that the wind turbine generator has the capability to actively sustain an electric power grid network, owing to its ability to independently control active and reactive power according to applied reference values at variable wind speed. Practical value. This research can be utilized for assessing the control methodology, the dynamic capabilities and influence of a gearless variable-speed wind energy conversion system on electric power grids. A case study has been presented with a (3×10 MW = 30 MW) wind farm scheme.

Improvement of system strength under high wind penetration: A techno-economic assessment using synchronous condenser and SVC

Variable speed wind turbine generators (WTGs) are replacing traditional fossil fuel based synchronous generators in the generation fleet as wind penetration increases. These variable speed WTGs are decoupled from the grid by power electronics inverters and usually generate less fault current compared to synchronous generators. As a result, system strength (indicates the fault recovery capability) at the point of common coupling (PCC) of wind power plants may reduce. A minimum value level of system strength is essential for wind power plants to properly identify and overcome a fault. To enhance system strength at wind PCC buses, supplementary devices such as synchronous condenser and Static VAR Compensator (SVC) can be deployed to have additional fault current. Since these devices are expensive, their optimal allocation considering economic feasibility is a major concern. In the existing literature, locations and ratings of the optimally allocated synchronous condenser are reported. However, this approach may cause oversizing of synchronous condensers. In addition, a SVC can be utilised instead of a synchronous condenser to enhance system strength. However, no detailed study is carried out yet to find the optimal size of SVCs. Evidently, both synchronous condenser and SVC can be installed to improve system strength. However, no comprehensive techno-economic assessment is performed for selecting an appropriate choice between them. To address these research gaps, this paper develops a framework to find the optimal sizes of synchronous condensers and SVCs by taking into account the long-term financial viability. Also, to ensure the maximum possible fault contribution, synchronous condensers/SVCs are connected directly to those wind PCC buses where system strength is below the acceptable limit. Afterwards, a detailed techno-economic assessment using synchronous condenser and SVC is performed in a wind dominated grid. In addition, post-fault voltage recovery characteristics with optimally sized synchronous condenser and SVC are explored. The developed methodology is applied to the IEEE 39 bus test system considering 45% wind power penetration scenario. Also, the performance of the proposed approach is compared to that of an existing technique. Finally, based on the technical and economic aspects, a better choice between synchronous condenser and SVC is recommended to improve system strength under high wind penetration.

Research on the Impacts of the Inertia and Droop Control Gains from a Variable-Speed Wind Turbine Generator on the Frequency Response

System frequency must be kept very close to its nominal range to ensure the stability of an electric power grid. Excessive system frequency variations are able to result in load shedding, frequency instability, and even generator damage. With increasing wind power penetration, there is rising concern about the reduction in inertia response and primary frequency control in the electric power grid. Converter-based wind generation is capable of providing inertia response and primary frequency response; nevertheless, the primary frequency and inertia responses of wind generation are different from those of conventional synchronous fleets; it is not completely understood how the primary frequency and inertia responses affect the given system under various disturbances and available kinetic energy levels. Simulations are used to investigate the influences of inertia and droop control strategies on the dynamic frequency responses, particularly the index of the second frequency drop under various disturbance and wind conditions. A quantitative analysis provides insight into setting of inertia and droop control coefficients for various wind and disturbance conditions to facilitate adequate dynamic frequency responses during frequency events.

Third-Order Sliding Mode Applied to the Direct Field-Oriented Control of the Asynchronous Generator for Variable-Speed Contra-Rotating Wind Turbine Generation Systems

Traditional direct field-oriented control (DFOC) techniques with integral-proportional (PI) controllers have undesirable effects on the power quality and performance of variable speed contra-rotating wind power (CRWP) plants based on asynchronous generators (ASGs). In this work, a commanding technique based on the DFOC technique for ASG is presented on variable speed conditions to minimize the output power ripples and the total harmonic distortion (THD) of the grid current. A new DFOC strategy was designed based on third-order sliding mode (TOSM) control to minimize oscillations and the THD value of the current and active power of the ASG; the designed technique decreases the current THD from ASG and does not impose any additional undulations in different parts of ASG. The designed technique is simply implemented on traditional DFOC techniques in variable speed DRWP systems to ameliorate its effectiveness. Also, the results show that by using the designed TOSM controllers, in addition to regulating the active and reactive powers of the ASG-based variable speed CRWP system, the THD current and active power undulations of the traditional inverters can be minimized simultaneously, and the stator current became more like a sinusoidal form.

Stability Enhancement of DFIG Based Wind Turbine Under Three Phase Fault with STATCOM

The main aim of this paper is to improve the transient stability of Doubly Fed Induction Generator (DFIG) based wind turbine using Static Synchronous Compensator (STATCOM). In this paper, a multi-objective problem is formulated to minimize voltage deviations at the point of common coupling (PCC) and mitigate low frequency oscillations after clearing the faults. Genetic algorithm based STATCOM controller is used to enhance the stability of a grid-connected wind farm composed of a variable-speed wind turbine generator system (WTGS). The transient performance of the proposed control scheme is analyzed with a three-phase fault. The Simulations are carried out in MATLAB/Simulink environment.

Dynamic Modeling and Performance Analysis of PMSG- based Variable Speed WTG: Case Study of Adama Wind Farm I, Ethiopia

In this paper, the performance of Permanent Magnet Synchronous Generator (PMSG) -based Variable Speed Wind Turbine Generator (WTG) at Adama Wind Farm I (WTG), connected to a grid is studied. To study the performance of the WTG, both machine and grid side converters are modeled and analyzed very well. On the machine side, maximum power point tracking (MPPT) for maximum energy extraction is done using the direct speed control (DSC) technique, which is linked with the optimal tip speed ratio for each wind speed value considered. On the grid side, dc-link voltage and reactive power flow to the grid are controlled. For this purpose, first, the simulation model of the system is prepared in MATLAB Simulink considering the dynamic mathematical model of the PMSG, and Wind Turbine Aerodynamic model using the user-defined function blocks. Then, the PI regulators designed for direct speed, torque (current) control, and dc-link voltage are employed in the model. Moreover, to study and analyze the behavior of the system in a variable speed operation, a wind speed starting from cut-in wind speed (3m/s) to the rated wind speed (11m/s) is applied in 4s. The simulation result of the existing system model shows that the actual values of performance variables correspond well with the analytical values of the system. In addition, the chosen control algorithms applied in the control system of the generator-side converter are hence verified.

Novel Coordinated Control Strategy of BESS and PMSG-WTG for Fast Frequency Response

An increase in inverter-based resources (IBRs) can lower the inertia of a power system, which may adversely affect the power system by causing changes such as a frequency nadir reduction or an increased initial rate of change of frequency (RoCoF). To prevent this, an ancillary service called fast frequency response (FFR) helps the inertia response by using IBRs. The main resources used in FFR are variable-speed wind turbine generators (VSWTGs) or energy storage systems (ESSs), which can respond quickly through converter control. The control is applied to the frequency regulation service faster than the primary frequency response, so the second frequency nadir may fall below the first frequency nadir. This study proposed a novel coordinated control strategy to efficiently utilize energy to improve the frequency nadir through coordinated control of wind turbines based on permanent magnetic synchronous generators (PMSGs) and battery energy storage systems (BESSs). The simulation results confirmed that the two-bus test system was composed of PSCAD/EMTDC, and the frequency nadir increased by utilizing the same amount of energy as in traditional control systems.

Novel insight on temporary frequency support of variable speed wind energy conversion system

AbstractIn the recent decade, temporary frequency support derived from variable speed wind turbine generator has been drawing increasing attention in the electronic‐growing power system. Frequency nadir and rate of change of frequency are two key indicators temporary frequency support strives to improve. This paper proposes a novel insight on frequency regulation of wind power. Note that considerable part of load is shared by synchronous generators and wind turbine generator and the system frequency still will be dominated by the former for a period of time. Simultaneously taking the frequency regulation characteristic of synchronous and power balance into account, a novel frequency regulation scheme aiming at improving system's frequency regulation performance is proposed and tested in a 7‐machine power system with an aggregated wind farm on MATLAB/Simulink to investigate what inherently determines the performance of frequency regulation. Results show that with differential cooperation of synchronous generator and doubly fed induction generator, frequency nadir and rate of change of frequency can be partially improved. Particularly, regarding to handling the issue of control delay, an appropriately slower action upon detecting a frequency event may not screw up it but contribute to a higher frequency nadir, which benefits from the spontaneous cooperation of synchronous generator and wind power with constraint of power balance.

Adaptive Control of Wind Turbine Generators for Power Capture Optimization by Using Integral Backstepping Approach During Partial-Load Operation

The paper proposes an effective nonlinear integral backstepping approach strategy (IBS) through the use of a new adaptive power control algorithm for variable-speed wind turbine generators (VS-WTGs) to optimize the power extracted from the WTGs. The suggested approach is applied to one of the most frequently used maximum power control methods called as tip speed ratio (TSR) during partial-load operation considering the effects of the variations in wind velocity profile and unmodeled system dynamics such as external disturbances. The proposed method of control is known to scale-back the mechanical stress at the level of the GT shafts (generator and turbine). Furthermore, the IBS approach is relatively simple, which significantly reduces the online computational time and cost. Some numerical simulation results prove, under different wind speed models, that the proposed AC-TSR-IBS method works well on the level of the improved efficiency and the rapid system response compared to conventional TSR-PI method.

Contribution of Variable Speed Wind Turbine Generator based on DFIG using ADRC and RST Controllers to Frequency Regulation

In the interconnected power systems, there must always be a balance between the amount of electricity produced and the amount of electricity consumed in order to keep the grid frequency stable and nearby to its nominal value. Therefore, any change in consumption can lead to imbalance, instability, load shedding and system frequency disturbances. With the increase of wind energy penetration level in the electrical network, it has become necessary to make wind generators participate in frequency regulation. The fundamental objective of this research paper is to investigate the ability of wind turbine at variable speed equipped with doubly fed induction generator (DFIG) to contribute in the grid frequency regulation. For the purpose of the contribution in the frequency regulation, the wind turbine system must inject a supplementary power support into the electrical network. To do that, two control strategies are adopted: the first one is the synthetic inertia control which uses the kinetic energy reserved and stocked in the turbine and generator rotors of the wind system. The second one is the droop frequency control which includes the deloading method that allows to the VSWT to create a certain reserve active power. Then, each of these control techniques is implemented and their performances are evaluated in case of frequency variation. Moreover, the control structure that uses the combination of these two techniques is also implemented and studied. The active disturbance rejection control strategy (ADRC), the classical PI controller and the polynomial RST controller have been adopted to command the rotor side converter (RSC) of the DFIG which allows it to provide, in case of frequency fault, a support for the power system into the grid by adjusting the rotor speed. The performance of these controllers are tested and compared and the simulation is performed by MATLAB/Simulink simulation software.

A Frequency and Voltage Stability-Based Load Shedding Technique for Low Inertia Power Systems

The aim of this research work is to develop a load shedding methodology to improve the frequency response of low inertia grids by attaining satisfactory voltage stability. In recent times, wind energy integration has considerably increased in many power grids. Consequently, conventional synchronous machines are being replaced from dispatch. Unlike traditional synchronous machines, variable speed wind turbine generators usually do not take part in frequency regulation without supplementary control mechanism. During substantial wind penetration, a power system may have a small number of online synchronous machines. As a result, synchronous inertia and governor responsive reserve significantly reduce. Under such situation, a system has to rely on load shedding as a last line of defense to rescue the system frequency following a large contingency. However, the conventional Under-Frequency Load Shedding (UFLS) strategy may lead to larger frequency deviation and higher amount of load cut in certain cases. A new load shedding methodology is presented in this paper to overcome this challenge. Unlike conventional UFLS technique, higher proportion of load shedding is applied to relatively weaker buses in terms of voltage stability in the proposed mechanism. Based on reactive power margin, which is an index to specify voltage stability, a general expression to quantify load shedding is derived. Also, the adaptability of the proposed strategy to various load levels is ensured. Later on, performances of the developed strategy are explored in a low inertia wind dominated test network. Simulations are executed considering various penetration levels of wind power and for two severe contingencies - loss of 550 MW interconnection and loss of 650 MW interconnection. Investigations reveal that the proposed load shedding methodology ensures satisfactory frequency response in all simulation cases. Also, the developed technique yields less frequency deviation and load cut compared to the conventional UFLS mechanism. Therefore, the reported load shedding scheme is found to be more competent to concurrently maintain frequency and voltage stabilities in renewable dominated power systems.

Modelling and control coordination scheme of a wind‐to‐hydrogen set for future renewable‐based power systems

New challenges regarding system stability and efficiency arise when power systems operate with a high penetration level of inverter-based renewable sources (IBRSs) and few synchronous generators. Since IBRSs have been on the rise, to secure the stable operation of future power systems, IBRSs will be required to support systems without having to rely on remaining synchronous generators. Also, to efficiently manage the uncertainty of renewable production, power-to-gas technology can provide the required flexibility. This study proposes modelling and a control coordination scheme (CCS) of a wind-to-hydrogen (W2H) set to optimise electricity production from a variable-speed wind turbine generator (WTG) while helping balance between supply and demand in a system. To achieve this, a grid-forming (GFM) inverter-based WTG is modelled and a set of electrolyser and fuel cell is integrated at the DC circuit of a GFM-WTG to be coordinated. Furthermore, the CCS offers an opportunity to reduce the investment cost for deploying a W2H set by utilising the control capabilities of a WTG and reducing the need for an additional device. The performance of the proposed W2H set with the CCS was verified considering the variations in system load and wind speed by using Power System Computer Aided Design (PSCAD)/ElectroMagnetic Transients including Direct Current (EMTDC).

Power Smoothing of a Variable-Speed Wind Turbine Generator Based on a Two-Valued Control Gain

This article proposes a power-smoothing scheme for a variable-speed wind turbine generator (WTG) that can maintain the frequency within a narrow range under normal operations. The proposed scheme uses an additional control loop based on the frequency deviation that operates in conjunction with maximum power point tracking (MPPT). To improve the energy-absorbing capability in the overfrequency section (OFS) and energy-releasing capability in the underfrequency section (UFS), the scheme suggests separate control gains, which are determined based on the ratio of the output of the additional loop to that of the MPPT loop. The ratio in the OFS is set to decrease with respect to the rotor speed whereas the ratio in the UFS is set to increase with respect to the rotor speed. The simulation results demonstrate that the proposed scheme can significantly lessen the WTG output power fluctuations by adjusting the control gains of the UFS and the OFS even though the WTG output power is severely fluctuating, thereby mitigating the frequency fluctuations. The scheme effectively uses the heavy rotating masses of a WTG as an energy storage system for enhancing the power grid flexibility in terms of frequency, particularly in high penetration levels of variable renewable energy.

Power Smoothing of a Variable-speed Wind Turbine Generator

This paper presents a power-smoothing scheme of a variable-speed wind turbine generator (VSWTG) that employs separate control gains for the over-frequency section (OFS) and under-frequency section (UFS). In the proposed scheme, an additional proportional control loop based on the system frequency deviation operating in conjunction with maximum power point tracking operation is used. In the OFS, to improve the energy-storing capability, the scheme suggests the gain of the frequency deviation control loop, which is set to be monotonously decreasing with the rotor speed while being significantly larger than that in the UFS. In the UFS, to improve the energy-releasing capability while preventing over-deceleration, the gain of the frequency deviation control loop is set to be a linear function of the rotor speed. The simulation results under continuously varying wind speeds with different wind patterns and wind speeds clearly demonstrate that the proposed scheme significantly mitigates the output power fluctuations of a VSWTG. The proposed scheme keeps the frequency within a narrow range, thereby reducing the required primary frequency control reserve for regulating the frequency under normal operations.

Fast Frequency Support From Wind Turbine Generators With Auxiliary Dynamic Demand Control

Wind penetration is expected to reach substantially higher levels of penetration in the near future. A variable speed wind turbine generator, such as, the doubly-fed induction generator (DFIG) or the permanent magnet synchronous generator, does not naturally provide rotating inertia due to the converter interface, which leads to a reduction of the overall system inertial response. The replacement of the conventional generators which have governors also reduces primary frequency control capability. Multiple control strategies have been proposed to enable a DFIG to provide emulated inertial response through reducing rotor speed that can release some kinetic energy. However, this has no effect on primary settling frequency and may even bring a secondary frequency dip to the system due to rotor speed recovery. Moreover, the rotor speed is not guaranteed to stay in an allowable range during the reduction period. This paper proposes a new control strategy for DFIG to provide adequate inertial response with guaranteed rotor security. To mitigate the potential of the secondary frequency dip and improve the performance of primary frequency control, a dynamic demand control strategy is proposed to coordinate with DFIG control. The coordination strategy can provide fast frequency support from the time-scale of inertial response through primary frequency control. The proposed control can be realized in both small-scale and large-scale power systems.

Advanced non-linear backstepping control design for variable speed wind turbine power maximization based on tip-speed-ratio approach during partial load operation

This study suggests an effective non-linear control scheme to improve wind energy production for getting maximum output power during partial load operation from a variable speed wind turbine generator. The proposed control strategy is based on an integral backstepping control using Tip-Speed-Ratio approach (TSR) for Wind Turbine Generators applications to optimize the wind energy captured by the system operating under rating wind speed. The proposed method has a fast and robust tracking capability. In addition, it is not necessary to know the parameters of the generator so as to get the control signal. This control method is known to scale back the mechanical stress on the generator and turbine shafts. Moreover, the Integral Backstepping Control (IBSC) policy is relatively simple, that considerably reduces the online computational cost and time. The time-domain simulation results prove the efficient operation of the proposed TSR-IBSC with fast system response compared to standard TSR approach.

Impact of Short-Circuit Impedance Model for Variable-Speed Wind Turbine Generators on Operation of Overcurrent Relays

Impact of Short-Circuit Impedance Model for Variable-Speed Wind Turbine Generators on Operation of Overcurrent Relays

Modeling and Control of Variable Speed Wind Turbine Generators for Frequency Regulation

Wind turbine generators (WTG) can participate in system frequency regulation via virtual inertial controllers (VIC). In the presence of frequency disturbances, VIC temporarily regulates the WTG power output forcing it to release/absorb kinetic energy into/from the grid. With increasing penetration of WTGs in power systems, grid operators require these generators to provide frequency regulation services; however, kinetic energy release/absorption can destabilize WTGs. Hence, to address these issues, a new large-perturbation nonlinear WTG model is proposed in this paper, based on the WTG internal response that is used to tune typical VICs. Novel worst case and optimal VIC tuning approaches are also proposed and discussed based on the developed WTG nonlinear model. Several simulations are presented to test and validate the proposed model and VIC tuning techniques, demonstrating their adequate performance and advantages.