Comparative study of magnetic energy storage control techniques for stabilizing wind farm integrated system under different conditions

Superconducting magnetic energy storage coupled static compensator for stability enhancement of the doubly fed induction generator integrated system

Doubly Fed Induction Generators (DFIGs) are widely used in variable speed wind turbine owing to its superior advantages that include ability to extract more energy from turbine, capability to control active and reactive power independently and the use of reduced converter rating that reduces its overall cost. On the other side fluctuating output power, weak fault ride through capability and high sensitivity to grid disturbances are the main issues that affect DFIG performance. In this paper, superconducting magnetic energy storage (SMES) unit is proposed to improve the fault ride through (FRT) capability of DFIG-based wind energy conversion system (WECS) during voltage sag and voltage swell events in the grid side. A new control approach for SMES unit using hysteresis current controller (HCC) along with proportional integral (PI) controller is introduced. Australian grid codes are used to examine the capability of the proposed controller to improve the FRT of the DFIG and hence maintaining the wind turbine connection to the grid during studied faults. Simulations results show the effectiveness of the SMES controller in maintaining system parameters within safety margins that comply with the Australian grid codes.

Technological developments such as Flexible AC Transmission Systems (FACTS) can improve the power quality of the power system and can respond to the ever-increasing demand for electrical energy.Superconducting magnetic energy storage (SMES) is a FACTS device that has been used for several years at utility and industrial sites throughout the United States, Japan, Europe, and South Africa to provide both transmission voltage support and power quality to customers who are vulnerable to fluctuating power quality [1]- [3].SMES systems are classified into two groups: voltage source inverter (VSI) and current source inverter (CSI) SMES.The VSI SMES has many advantages when compared with the CSI SMES; for example, in the VSI SMES, the power rating of power electronic devices that are used in the inverter is much less than that of the CSI SMES, resulting in fewer switching and power losses.Also, the VSI SMES can be used as a Static synchronous Compensator (STATCOM) when its chopper is out of service.Moreover, for the same condition, the VSI SMES can store more energy than the CSI SMES; these advantages encouraged the authors to study VSI SMES.This type of SMES is composed of a magnetic energy storage coil with various structures and power conditioning systems that are also composed of different parts, such as AC-DC filters, a multi-level chopper, a capacitor bank, and a multi-level converter (i.e. an inverter or rectifier).These power conditioning systems are also used in many different sustainable energy systems, such as bio fuels, solar power, wind power, wave power, geothermal power, and tidal power.The converter is an interface between the power network and the capacitor bank and controls the electrical energy exchange between the two.Likewise, the chopper is also an interface between the magnetic energy storage coil and the capacitor bank, and controls electrical energy exchange between them.To store the electrical energy in the capacitor bank and the magnetic energy storage coil in the range of mega joules, it is necessary to employ high power-rating converters; to overcome the limitations of the current and voltage range of the semiconductors, multi-level converters are used.The advantages of using such converters include reducing voltage on the switches, harmonic order correction, decreasing or eliminating lateral equipment, decreasing switching frequency, decreasing total harmonic distortion (THD), decreasing switching losses, and decreasing the output current ripple.

As the mainstream generation devices in the current wind power market, doubly fed induction generators (DFIG) based wind farms in China are always located in the northwest, northeast and north China and other remote areas where DFIG systems have to be connected with the weak grid. In that case, the resonance may occur in the system on account of the interaction between AC and DC system and lead to harmonic amplification. The subsynchronous resonance (SSR) lower than the fundamental frequency and the high-frequency resonance (HFR) are the two main resonance modes in the DFIG grid-connected system. However, researches on HFR characteristics of DFIG connected to grids have been drawn attention. This paper will fill this gap. The DFIG system model under the weak power grid with different kinds of compensations is set up firstly. Then, the grid impedance and the DFIG numbers are changed separately to discuss the influence of them on harmonic characteristics by Bode diagrams. Afterward, a detailed model for DFIG system under weak grid is developed in MATLAB/Simulink. The harmonic characteristics of the system are analyzed by fast Fourier transform (FFT). The study shows that HFR is more serious when the DFIG system is integrated into the parallel compensated weak grid than connected to the series compensated grid. HFR is more possibly to happen with the growth of the system capacity or the DFIG numbers.

The number of wind turbines connected to power grids has significantly increased during the last decade. This is mainly due to the convincing revolution in power electronic technology and the growing concern about greenhouse effect that is intensified due to the burning of fossil fuels. Variable-speed wind energy conversion systems (WECSs) such as doubly fed induction generators (DFIGs) are dominating the wind energy market due to their superior advantages over fixed-speed-based WECS which include more captured energy, less mechanical stress, and acoustical noise. DFIG is interfaced to the ac network through the grid-side voltage source converter (VSC) and rotor-side VSC to enable the variable-speed operation of the wind turbine and to provide reactive power support to the ac grid during disturbance events. Converter switching malfunction such as misfire and fire-through may influence the power dispatch capability of the DFIG. In this paper, a superconducting magnetic energy storage (SMES) unit is utilized to improve the power dispatch and dynamic performance of DFIG-based WECS during internal converter switching malfunctions such as misfire and fire-through faults. Simulation results without and with SMES connected to the system are presented, compared, and analyzed.

The wind farm based on Doubly Fed Induction Generator (DFIG) is built. Not only it can be as active power source to provide active power to network, but also as reactive power source to stabilize voltage or supply Var compensation (reactive power absorbed or generated). Therefore, Var coordinated control strategy is proposed, instead of expensive equipments, such as Static Var Compensator(SVC) or Static Synchronous Compensator (STATCOM), in order to provide sufficient reactive power to maintain system stability. DFIG comprehensive model is built based on correlation models in Dig SILENT/Power Factory. The eigen value analysis method is employed to verify its impact on low-frequency oscillation modes and small signal stability of power system integrated with wind farm. The results from eigen value analysis are elucidated the effectiveness of Var coordinated control in DFIG system.

Recently, a doubly fed induction generator (DFIG) wind turbine has been extensively paid attentions as a renewable energy source in the DC microgrid. However, the inevitable problems of DFIG wind turbine are low voltage ride through (LVRT) due to the occurrence of faults as well as the power fluctuation. To overcome both problems, a superconducting magnetic energy storage (SMES) unit which is able to provide fast control of active and reactive power, can be used. This paper applies a SMES unit to enhance LVRT performance and alleviate the power fluctuation of DFIG wind turbine in the DC microgrid. The SMES circuit is mainly composed of the DC chopper. The PI controllers of DC chopper are used to control the exchange energy between the superconducting coil and the system. Simulation study using MATLAB/SimPowerSystems ensures the SMES control effect on both enhancing LVRT performance and smoothing power fluctuation of DFIG wind turbine.

Variable speed doubly fed induction generators (DFIG) are preferred in wind generation systems because they offer several advantages in terms of control and aerodynamic efficiency. While it is generally a stable machine, its converter systems may not work properly under severe grid fault conditions. This article presents a strategy of riding through the low voltage conditions through control of superconducting magnetic energy storage (SMES) system. The reactive power supplied by the SMES helps the voltage recovery, while the real power improves the damping properties of the post disturbance system. Simulation studies show that the proposed SMES control strategy restores normal operation of the wind system in reasonably short period of time.

Enhancing stability via coordinated control of generators, wind farms, and energy storage: Optimizing system parameters

This paper proposes an advanced control strategy for DFIG based Wind Turbine. The designed method is based on a combination of Particle Swarm Optimization (PSO) and Artificial Neural Network (ANN). PSO combined ANN is suggested to track the available maximum power point (MPPT) at varying wind speeds. Thus, PSO is used to improve the dynamic performance of Doubly Fed Induction Generator (DFIG) by optimizing its Proportional Integral (PI) controller gains. This work highlights the performance of the PSO optimized PI controller against classical one. The proposed control strategy is verified via 2 MW DFIG-WT using the MATLAB/Simulink environment. The obtained results validate the proposed PSO-PI as an effective tool for improving the dynamic behavior of DFIGWT, it reveals that the overshoots are reduced by 50 % compared to the classical PI. In addition, a faster transient response is achieved using the proposed control strategy.

The grid-connected Doubly Fed Induction Generator (DFIG) system is exposed to severe grid faults. The DFIG is sensitive to grid disturbances, which lead to consideration in the controller design. This paper proposed the Modified Super Twisting (MST) algorithm for the Low Voltage Ride Through (LVRT) enhancement under voltage sag condition. This proposed algorithm is implemented using the 2nd Order Sliding Mode (SOSM) to control the DFIG based wind generator. The higher-order switching functions are introduced in the SOSM for sliding surface control. Moreover, the Lyapunov analysis for the MST algorithm brings down the chattering amplitude. The advantages of the proposed algorithm are that it reduces the system uncertainties, chattering effect and improves the settling period of the system parameters. The performance of the proposed algorithm is compared with existing algorithms in the literature with the help of MATLAB/SIMULINK. The Hardware-In-Loop (HIL) is used to validate the simulation results, which have been performed on the OPAL-RT setup. In addition, the proposed algorithm is also tested on an equivalent model of the practical Wind Farm (WF). Based on the studies, it is found that the proposed algorithm enhanced the LVRT performance of the single Wind Turbine (WT)-DFIG system as well as the practical WF under transient conditions.

Nowadays, the use of superconductive magnetic energy storage (SMES) devices in grid integration are highly escalated with the technological amelioration of magnetic components. The power quality and energy conversion efficiency of the power conditioning system of the SMES and renewable energy systems depend on the control algorithms of the voltage source inverter (VSI). However, the traditional control schemes cause inferior response and conversion efficiency. In this paper, an advanced control technique named PI+LLC controller is proposed, which is based on the proportional integral (PI) controller and lead-lag compensator (LLC). The proposed control technique offers significant reduction in the total harmonic distortion (THD), superior dynamic response, smooth response against fault, excellent reference tracking capability of grid current and improved power quality performance at both inverter side and grid side for a 5-level neutral point clamped inverter based grid-tied photovoltaic system. It is expected that, the proposed control scheme can also be used to mitigate the excessive heat of the VSI based SMES system by improving the performance of the power conditioning system. The performance of the proposed control technique is evaluated in MATLAB/Simulink environment to validate the excellent features of the proposed control scheme.

This paper proposes an adaptive output power control of superconducting magnetic energy storage (SMES) to solve the output power fluctuation problem of the grid connected hybrid permanent magnet synchronous generators (PMSG) wind and photovoltaic generations (HWPV). By the power control of SMES, the adaptive output power reference is employed to achieve the desired output power of the HWPV while the variation of SMES coil current is regulated between the minimum and maximum limits. Study result shows that the proposed adaptive power control of SMES is able to alleviate the HWPV output power fluctuation, and maintain the SMES coil current in the acceptable range effectively.

Since 2004, Doubly Fed Induction Generator (DFIG) has become the most admired wind turbine generator type that is widely installed worldwide. The popularity of DFIG is due to its capability to extract more energy than fixed speed wind turbine generator type. Moreover, a DFIG requires only one-third of the full converter size used by type-4 wind turbine generator which in turn reduces the total cost of DFIG installation. The main weakness of DFIG is its vulnerability to grid faults such as voltage dip and swell. Grid faults may have adverse impacts on the overall performance of the DFIG including the voltage at the point of common coupling that, at certain fault levels, may violate the permissible margins of the grid codes established by the transmission line operators. This paper investigates the application of Superconducting Magnetic Energy Storage (SMES) unit to improve the fault ride-through capability of a DFIG during grid faults. Results show that the connection of a SMES unit at the point of common coupling between the DFIG and the grid can enhance the overall performance of the DFIG during such faults events.

This paper presents the modeling and simulation of wind power systems based on two different induction generators; the squirrel-cage induction generator (SCIG) and the doubly-fed induction generator (DFIG). The techniques of direct grid integration of SCIG system and independent power control of DFIG system are discussed. Particularly, to solve the droop in distribution line voltage in SCIG system, a reactive power static compensator (STACOM) is used and a comparison of distribution line voltage is conducted between the SCIG and DFIG systems. Besides, in DFIG system, a fitting curve for optimal power versus speed is proposed for the turbine model and a cross-coupling relation between three phase choke and stator-side converter is also emphasized. Both wind power systems and their grid integration techniques are modeled and simulated in Matlab/Simulink. The results demonstrated the characteristics of both systems under varying wind speeds. Compared to the conventional constant speed SCIG system, variable speed DFIG reveals its superiority in terms of optimal power capture as well as constant distribution line voltage.