A developed control strategy for mitigating wind power generation transients using superconducting magnetic energy storage with reactive power support

The fast variations of wind speed during extreme wind gusts result in fluctuations in both generated power and the voltage of power systems connected to wind energy conversion system (WECS). This paper presents a control strategy which has been tested out using two scenarios of wind gusts. The strategy is based on active and reactive powers controls of superconducting magnetic energy storage (SMES). The WECS includes squirrel cage induction generator (SCIG) with shunt connected capacitor bank to improve the power factor. The SMES system consists of step down transformer, power conditioning unit, DC–DC chopper, and large inductance superconducting coil. The WECS and SMES are connected at the point of common coupling (PCC). Fuzzy logic controller (FLC) is used with the DC–DC chopper to control the power transfer between the grid and SMES coil. The FLC is designed so that the SMES can absorb/deliver active power from/to the power system. Moreover, reactive power is controlled to regulate the voltage profile of PCC. Two inputs are applied to the FLC; the wind speed and SMES current to control the amount active and reactive power generated by SMES. The proposed strategy is simulated in MATLAB/Simulink®. The proposed control strategy of SMES is robust, as it successfully controlled the PCC voltage, active and reactive powers during normal wind speeds and for different scenarios of wind gusts. The PCC voltage was regulated at 1.0 pu for the two studied scenarios of wind gusts. The fluctuation ranges of real power delivered to the grid were decreased by 53.1% for Scenario #1 and 56.53% for Scenario #2. The average reactive power supplied by the grid to the wind farm were decreased by 27.45% for Scenario #1 and 31.13% for Scenario #2.     

A new control strategy of WFSG-based wind turbine to enhance the LVRT capability

This paper proposes a competent and effective scheme to enhance the low voltage ride through (LVRT) capability of wound field synchronous generator (WFSG) based wind turbines (WTs) under unbalanced voltage dip conditions. A technique for grid synchronization against voltage excursions, i.e., a PLL using positive grid voltages with a high selectivity filters (HSFs) is utilized to extract a robust grid voltage synchronization signal irrespective of the mains condition to enhance the overall system performance. Besides, a new controller (adaptive fuzzy RST) for both the stator side converter (SSC) and grid side converter (GSC) current regulation are employed to further improve dynamic performance. Also, a reactive power support scheme to manage the WFSG reactive power during contingencies and fulfill the grid codes obligations is presented. Moreover, an additional device such as braking chopper (BC) circuit is used in the DC-link circuit for stable operation of the wind energy conversion system (WECS) under-line fault. Effectiveness of the proposed control strategy is verified by the numerical simulations.     

Performance of wound rotor induction generators with the combination of input voltage and slip power control

Input voltage control had been used for the improvement of power factor in grid-connected wind power based induction generators. However, efficiency in this case is very poor and the control range is also limited. The conventional rotor resistance control had also been used for wind power generation but in this case too, the input power factor becomes very poor. In this paper, a new control method is being proposed where both input voltage and slip power control are combined to achieve better performance of wound rotor induction generators (WRIG) for wind power generation. For each operating point, an optimum input voltage is set and the slip power is controlled by rotor impedance such that the maximum efficiency is achieved. Both power factor and efficiency improve for a wide range of speed variation. Moreover, the reactive power demand remains nearly constant which is compensated by a fixed value capacitor whose optimum value is found through the simulation. Moreover, the capacitors being placed at the rotor side, the efficiency of the proposed controller improves further. This scheme is useful for low power wind energy conversion system (WECS) where the wind speed varies over wide range. The performance of the proposed system under various wind conditions is experimentally verified.     

Single-stage AC–AC power conversion for WECS

This paper investigates a novel single stage AC–AC power conversion, as an alternative to multistage AC–DC–AC power conversion topology, for interfacing the wind energy conversion system (WECS) to a grid as a distributed load system. A comprehensive dynamic model of proposed AC–AC converter is developed to satisfy all the functions of the converter. A new time switching pattern and a control mechanism are described to convert a variable frequency input power proportion to the wind power to a constant frequency output power for a distributed load system in a single unit. The converter control functions are adapted to control active and reactive powers injected into the distributed load system. Based on time-domain simulations in the MATLAB environment, a comparative study has been made of the dynamic behavior of wind turbine generation system with the proposed AC to AC converter and conventional AC–DC–AC converter. The study concludes that an AC–AC converter is technically a viable option to interface a wind turbine to a distributed load system or utility grid application. A prototype of the proposed converter is developed in the lab taking variable frequency input voltage and then converting it to a constant output frequency voltage. The performance of the converter has been found satisfactory.     

一种新颖的模糊自耦合PI在风力机MPPT控制中的应用

针对风速随机性给风能转换系统(wind energy conversion system, WECS)带来的非线性和参数不确定性,提出了一种模糊自耦合PI控制方案用于低风速的最大功率点跟踪。自耦合PI被用来完成基本的转速跟踪,以实现对风力机尖速比的最优化控制。而模糊控制器则被用来获取自耦合PI在不同工作点下的控制参数,以提高系统对风速变化的适应能力。为了验证所提方案的可行性,在Matlab/Simulink搭建的2 MW风能转换系统仿真模型中开展了与传统方法的对比实验。仿真结果表明,相比于传统PI、模糊PI以及GA-PI,所提出方法拥有更佳的转速跟踪性能、更平滑的响应曲线以及更多的电能输出。     

Double input Z-source inverter applicable in dual-star PMSG based wind turbine

This paper proposes a new double-input Z-network for application in wind energy conversion system (WECS) which is composed of two same DC voltage sources as input sources, two inductors and one capacitor. As a result, the presented structure requires less capacitor number compared to traditional Z-network and it will be able to deliverer energy of both DC sources to local load or grid. The proposed inverter is applicable in dual-star PMSG based WECS, since it requires two DC voltage sources in same value. Besides, dynamic modeling of dual-star PMSG is presented to analyze proposed WECS connected to grid which employs dual-star PMSG and double-input Z-source inverter. The proposed dual-input Z-source inverter controls maximum power point tracking (MPPT) and delivering power to the grid. Therefore, other DC–DC chopper is not required to control two sets of rectified output voltage of generator in view of MPPT. As a result, the proposed topology requires less power electronic switches and the suggested system is more reliable against short circuit. The ability of proposed WECS with dual-star PMSG and double-input Z-source inverter is validated with simulation results and experimental tests using PCI-1716 data acquisition system.     

Reliability assessment of bulk electric systems containing large wind farms

Wind power is an intermittent energy source that behaves quite differently than conventional energy sources. Bulk electric system reliability analysis associated with wind energy conversion systems (WECS) provides an opportunity to investigate the reliability benefits when large-scale wind power is injected at specified locations in a bulk electric system. Connecting the WECS to different locations in a bulk system can have different impacts on the overall system reliability depending on the system topology and conditions. Connecting a large-scale WECS to an area which has weak transmission could create system operating constraints and provide less system benefit than connecting it to an area with stronger transmission. This paper investigates bulk electric system transmission constraints associated with large-scale wind farms. The analyses presented in this paper can be used to determine the maximum WECS installed capacity that can be injected at specified locations in a bulk electric system, and assist system planners to create potential transmission reinforcement schemes to facilitate large-scale WECS additions to the bulk system. A sequential Monte Carlo simulation approach is used as this methodology can facilitate a time series modeling of wind speeds, and also provides accurate frequency and duration assessments. An auto-regressive moving average (ARMA) time series model is used to simulate hourly wind speeds.     

An Enhanced LVRT Scheme for DFIG-based WECSs under Both Balanced and Unbalanced Grid Voltage Sags

Due to the latest grid codes, wind energy conversion systems (WECSs) are required to remain connected to grid under grid voltage sags and supply reactive power into the grid. So, this paper proposes an enhanced scheme to improve low-voltage ride through (LVRT) capability of doubly fed induction generator (DFIG)-based WECSs under both balanced and unbalanced grid voltage sags. The proposed scheme is composed of active and passive LVRT compensators. The active compensator is performed by controlling the rotor- and grid-side converters of the DFIG to decrease the stator flux oscillations and inject reactive power into the grid. The passive compensator is based on a three-phase stator damping resistor (SDR) located in series with the stator windings. The proposed scheme decreases the negative effects of grid voltage sags in the DFIG system including the rotor over-currents, electromagnetic torque oscillations, and DC-link over-voltage and also injects reactive power into grid to support the grid voltage. So, the LVRT capability of DFIG is enhanced and new grid code requirements are addressed. Simulation results on a 1.5-MW DFIG-based WECS using MATLAB/Simulink demonstrate the effectiveness of the proposed LVRT scheme under both balanced and unbalanced grid voltage sags.     

Stability and nonlinear controller analysis of wind energy conversion system with random wind speed

In the recent years, globally wind energy has played a vital role in renewable sources in order to minimize the environmental consequence on power generation, As a result of this, computer models of wind turbines for power system stability studies have been developed and supplied to the consumer. Therefore, the development of such models is of particular consequence for stability of power system, which has been studied and can be structured and integrated into network simulation software are needed. However, in this contribution a nonlinear control design modeling is required to stabilize and analyze wind energy conversion system (WECS) by regulating the electrical frequency and stator voltage amplitude of the squirrel-cage induction generator (SCIG) at random wind speed approach is presented. A design scheme consists of dynamic wind turbine system and 3-phase SCIG unit. In this research study, we employ a unique technique based on feedback linearization technique through field oriented control concepts. The controllers were designed in simulated software Matlab in order to regulate the SCIG constraints. The validation of the developing system models will be appropriate to provide for large system stability and control.     

Improving power quality of wind energy conversion system with unconventional power electronic interface

The increasing interest to utilize wind energy as a power source prompted more researches to be dedicated to the unconventional integration of this power source into the current grid. In this paper, one avenue to achieve this efficient utilization, through the use of integrated wind energy conversion system (WECS) using doubly fed induction generator (DFIG) is presented. Wind grid integration brings the problems of voltage fluctuation and harmonic distortion. This paper presents an Unconventional Power Electronic Interface (UPEI) to reduce the total harmonic distortion (THD) and enhance power quality during disturbances. The models used in the paper includes a pitch-angled controlled wind turbine model, a DFIG model, power system model and an UPEI having controlled converters. A phase to phase fault is simulated on 132 kV bus and the measured results obtained from grid connection of the wind generation system are presented. The results have demonstrated the ability of UPEI to regulate pitch angle, VAR and to reduce THD. The proposed system increases the effectiveness of the utilization of wind energy.     

Power generation maximization of distributed photovoltaic systems using dynamic topology reconfiguration

The ‘mismatch losses’ problem is commonly encountered in distributed photovoltaic (PV) power generation systems. It can directly reduce power generation. Hence, PV array reconfiguration techniques have become highly popular to minimize the mismatch losses. In this paper, a dynamical array reconfiguration method for Total-Cross-Ties (TCT) and Series–Parallel (SP) interconnected PV arrays is proposed. The method aims to improve the maximum power output generation of a distributed PV array in different mismatch conditions through a set of inverters and a switching matrix that is controlled by a dynamic and scalable reconfiguration optimization algorithm. The structures of the switching matrix for both TCT-based and SP-based PV arrays are designed to enable flexible alteration of the electrical connections between PV strings and inverters. Also, the proposed reconfiguration solution is scalable, because the size of the switching matrix deployed in the proposed solution is only determined by the numbers of the PV strings and the inverters, and is not related to the number of PV modules in a string. The performance of the proposed method is assessed for PV arrays with both TCT and SP interconnections in different mismatch conditions, including different partial shading and random PV module failure. The average optimization time for TCT and SP interconnected PV arrays is 0.02 and 3 s, respectively. The effectiveness of the proposed dynamical reconfiguration is confirmed, with the average maximum power generation improved by 8.56% for the TCT-based PV array and 6.43% for the SP-based PV array compared to a fixed topology scheme.     

Power system transient stability assessment based on the multiple paralleled convolutional neural network and gated recurrent unit

In order to accurately evaluate power system stability in a timely manner after faults, and further improve the feature extraction ability of the model, this paper presents an improved transient stability assessment (TSA) method of CNN?+?GRU. This comprises a convolutional neural network (CNN) and gated recurrent unit (GRU). CNN has the feature extraction capability for a micro short-term time sequence, while GRU can extract characteristics contained in a macro long-term time sequence. The two are integrated to comprehensively extract the high-order features that are contained in a transient process. To overcome the difficulty of sample misclassification, a multiple parallel (MP) CNN?+?GRU, with multiple CNN?+?GRU connected in parallel, is created. Additionally, an improved focal loss (FL) function which can implement self-adaptive adjustment according to the neural network training is introduced to guide model training. Finally, the proposed methods are verified on the IEEE 39 and 145-bus systems. The simulation results indicate that the proposed methods have better TSA performance than other existing methods.     

单相级联H桥整流电路非线性控制策略

针对电力电子牵引变压器输入级单相级联H桥(CHB)整流电路的非线性及扰动工况,提出一种基于静止坐标系的非线性优化控制策略。根据系统仿射非线性模型及微分几何理论,提出基于部分反馈线性化的零动态设计方案,对其线性部分采用二次型最优方法以确定反馈增益,并引入谐振环节以实现零相差跟踪;对于零动态则采用基于扩张状态观测器的自抗扰控制策略,以提高系统在负载大范围扰动时的控制品质。实验结果表明,该控制策略能提高CHB系统在电网电压及负载扰动时的动态响应速度,保证网侧电流及直流电压快速稳定调节,同时使网侧在单位功率因数下运行,各模块直流电压均衡。     

Current Source Converter Based Wind Energy Conversion Systems

The increase in the installed capacity of wind energy conversion systems(WECS) has triggered the devel-opment of more demanding grid codes and additional requirements on performance.In order to meet these require-ments the industry trend has shifted to full-scale power converter interfaces in modern multi-megawatt WECS.As con-sequence,a wide variety of new power converter topologies and WECS configurations have been introduced in recent years.Among them,current source converter(CSC) based configurations have attracted attention due to a series of advantages like:simple structure,grid friendly waveforms,controllable power factor,and reliable grid short-circuit pro-tection.This paper presents the latest developments in CSC interfaces for WECS and related technologies such as modulation methods,control schemes and grid code compatibility.     

Wind energy conversion system regulation via LMI fuzzy pole cluster approach

This paper addresses the design of fuzzy state feedback controller that has not only the ability to stabilize the fuzzy model/system but also to control the transient behavior and closed loop poles location for wind energy conversion system (WECS) that presents interesting control demands and exhibits intrinsic non-linear characteristics. The proposed fuzzy controller is employed to regulate indirectly the power flow in the grid connected WECS by regulating the DC current flows in the interconnected DC link. First, a Takagi–Sugeno fuzzy model is employed to represent the non-linear WECS. Then a model-based fuzzy controller design utilizing the concept of parallel-distributed compensation is developed. Satisfactory time response and closed loop damping over wide operating range are achieved by forcing the closed loop poles into a suitable sub-region of the complex frequency plane. Sufficient stability conditions are expressed in terms of linear matrix inequalities (LMI’s) which can be solved very efficiently using convex optimization techniques. The design procedures are applied to a dynamic model of a typical wind energy conversion system to illustrate the feasibility and the effectiveness of the proposed control techniques via simulation example.     

Double-fed induction generator control for variable speed wind power generation

Vector control of a doubly fed induction generator drive for variable speed wind power generation is described. A wound rotor induction machine with back-to-back three phase power converter bridges between its rotor and the grid forms the electrical system. The control scheme uses stator flux-oriented control for the rotor side converter bridge control and grid voltage vector control for the grid side converter bridge. A complete simulation model is developed for the control of the active and reactive powers of the doubly fed generator under variable speed operation. Several studies are performed to test its operation under different wind conditions. A laboratory test setup consisting of a wound rotor induction machine driven by a variable speed dc motor is used to validate the software simulations.     

ESTIMATION OF ROTOR VOLTAGE VECTOR ON THE DOUBLE EXCITED INDUCTION MACHINE USED IN WECS

ABSTRACT

The aim of this work is to estimate and control the rotor voltage of a double excited induction machine DEIM that is necessary to operate it as a generator in wind energy conversion systems (WECS), with speed range extending from the sub synchronous range to double the synchronous speeds. For efficient power generation, the DEIM is to be operated at its rated electric torque which is realised by having both rotor and stator currents magnitudes constant throughout the whole operating range. Also the angle between the rotor current and stator current is kept constant irrespective of the rotor speed.     

Reliability-Based Transmission Reinforcement Planning Associated With Large-Scale Wind Farms

The reliability impacts of a highly variable energy source such as wind power is an important aspect that needs to be assessed as wind power penetration becomes increasingly significant. Bulk electric system (BES) reliability analysis associated with wind energy conversion systems (WECS) provides an opportunity to investigate the reliability benefits at potential BES connection points in close proximity to the wind power development area. A bulk electric system can encounter transmission capacity limitation problems when a large-scale WECS is connected to a weak transmission area. In this case, transmission reinforcement may be required in order to increase the system capability to absorb more wind power at specified locations. BES reliability analysis associated with large-scale wind farms as demonstrated in this paper can assist system planners to create potential transmission reinforcement schemes to facilitate large-scale WECS additions to a bulk system. Reliability cost/worth analysis is also incorporated in the examination of reinforcement alternatives. A sequential Monte Carlo simulation approach is used as this methodology can facilitate a time series modeling of wind speeds and also provides accurate frequency and duration assessments. An auto-regressive moving average (ARMA) time series model is used to simulate hourly wind speeds     

Considering wind speed correlation of WECS in reliability evaluation using the time-shifting technique

Wind power is a very useful renewable energy source that is attracting consideration around the world due to its non-exhaustive nature and its environmental and social benefits. The correlation between multiple wind sites has a significant impact on the reliability of power systems containing wind energy conversion systems (WECS). Conventional methods cannot be directly applied to evaluate the reliability of WECS in the absence of comprehensive modeling techniques that recognize the correlation of the wind speeds at different wind farm locations. This paper proposes a model for power system reliability assessment that can consider the wind speed correlation and preserve the statistical characteristics of wind speeds, such as the mean and deviation of the wind speed time series (WSTS). A time-shifting technique is used to produce a new WSTS for a given correlation between two wind sites. The optimal shifted time at the two sites is determined using a linear interpolation technique. The probability distributions of the generated power and the system reliability indices with different degrees of correlation between the two sites are compared using two reliability test systems, i.e. the RBTS and the IEEE-RTS. The results show that the proposed method is useful in evaluating the reliability of WECS with correlation between two wind sites.     

Sliding mode control of a dual-stator induction generator for wind energy conversion systems

This paper presents a sliding mode control (SMC) associated to the field oriented control (FOC) of a dual-stator induction generator (DSIG) based wind energy conversion systems (WECSs). The DSIG has two sets of stator three-phase windings spatially shifted by 30 electrical degrees. The study of operation of the wind turbine leads us to two essential cases: optimization of the power for wind speeds lower than the nominal speed of the turbine and limitation of the power for higher speeds. Conventional electrical grid connected WECS present interesting control demands, due to the intrinsic nonlinear characteristic of wind mills and electric generators. The SMC is a robust nonlinear algorithm which uses discontinuous control to force the system states trajectories to join some specified sliding surface, it has been widely used for its robustness to model parameter uncertainties and external disturbances, is studied. In order to verify the validity of the proposed method, a dynamic model of the proposed system has been simulated, to demonstrate its performance.