A robust power control strategy to enhance LVRT capability of grid‐connected DFIG‐based wind energy systems

This paper presents a new robust and effective control strategy to mitigate symmetrical voltage dips in a grid‐connected doubly fed induction generator (DFIG) wind energy conversion system without any additional hardware in the system. The aim is to control the power transmitted to the grid so as to keep the electrical and mechanical quantities above their threshold protection values during a voltage dip transient. To achieve this, the references of the powers are readjusted to adapt the wind energy conversion system to the fault conditions. Robust control strategies, combining the merits of sliding mode theory and fuzzy logic, are then proposed in this paper. These controllers are derived from the dynamic model of the DFIG considering the variations in the stator flux generated by the voltage drop. This approach is found to yield better performance than other control design methods which assume the flux in the stator to remain constant in amplitude. This control scheme is compliant with the fault‐ride‐through grid codes which require the wind turbine generator to remain connected during voltage dips. A series of simulation scenarios are carried out on a 3‐MW wind turbine system to demonstrate the effectiveness of the proposed control schemes under voltage dips and parameter uncertainty conditions.     

Role of flexible alternating current transmission systems devices in mitigating grid fault‐induced vibration of wind turbines

This paper focuses on the modeling of mechanical and structural vibrations in wind turbines due to the occurrence of electrical faults and an effective means of suppressing the vibrations with flexible alternating current transmission systems (FACTS) devices. A detailed model describing the dynamic interaction between the mechanical and the electrical subsystems of the turbine is presented. The model captures the effect of grid faults on the mechanical vibrations of drivetrain, flexible rotor blades and tower. Numerical investigation reveals that electrical disturbances have a significant impact on the mechanical/structural vibrations. In fact, the occurrence of severe vibrations due to voltage sags may compromise safe operation of the overall plant. The application of FACTS devices is then considered to suppress the effect of electrical fault‐induced vibrations. The performance comparison of static synchronous compensator and unified power quality conditioner devices in improving the mechanical/structural response has been carried out. A fault scenario compliant with Irish grid code has been simulated. Simulation results show that FACTS devices are successfully able to mitigate vibrations due to electrical faults, and they can be conveniently applied to stabilize the generator shaft speed, drivetrain oscillations, edgewise blade vibrations and tower responses. Further, superior performances of the unified power quality conditioner as compared with static synchronous compensator are also observed under certain conditions with increased fault duration. Copyright © 2013 John Wiley & Sons, Ltd.     

Modelling and control of synchronous generators for wide‐range variable‐speed wind turbines

The modelling and control of a wide‐range variable speed wind turbine based on a synchronous generator are presented. Two different methods to control the operation of the synchronous generator are investigated, i.e. load angle control and instantaneous vector control. The dynamic performance characteristics of these control strategies are evaluated and compared using three model representations of the generator: a non‐reduced order model including both stator and rotor transients, a reduced order model with stator transients neglected, and a steady‐state model that neglects generator electrical dynamics. Assessment on the performance of grid‐side controller is shown during network fault and frequency variation. A simplified wind turbine model representation is also developed and proposed for large‐scale power system studies. Simulation results in Matlab/Simulink are presented and discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Robust multi‐model control of an autonomous wind power system

This article presents a robust multi‐model control structure for a wind power system that uses a variable speed wind turbine (VSWT) driving a permanent magnet synchronous generator (PMSG) connected to a local grid. The control problem consists in maximizing the energy captured from the wind for varying wind speeds. The VSWT‐PMSG linearized model analysis reveals the resonant nature of its dynamic at points on the optimal regimes characteristic (ORC). The natural frequency of the system and the damping factor are strongly dependent on the operating point on the ORC. Under these circumstances a robust multi‐model control structure is designed. The simulation results prove the viability of the proposed control structure. Copyright © 2006 John Wiley & Sons, Ltd.     

变速恒频风力发电机组动态模型及并网研究

分析了风力发电系统的特点,着重介绍了变速恒频（Variable Speed Constant Frequency,VSCF）风力发电机组运行的基本原理．在此基础上,探讨了风力发电机组各部分的仿真建模,并分析了目前适用于不同条件下的双馈感应式异步发电机（Double—Fed Induction Generator,DFIG）变速恒频（AC—Exited Variable Speed Constant Frequency,AEVSCF）风力发电机的数学模型。对含变速恒频风机电网系统的故障扰动过程进行仿真分析．结果验证了建模的正确性。在故障扰动的最后对未来风电机组建模的研究重点提出了一些建议。     

PSCAD/EMTDC-Based Modeling and Analysis of a Gearless Variable Speed Wind Turbine

This paper presents dynamic modeling and simulation of a grid connected variable speed wind turbine (VSWT) using PSCAD/EMTDC, a widely used power system transient analysis tool. The variable speed wind system with a direct-drive generator and power electronics interface is modeled for dynamic analysis. Component models and equations are addressed and their implementations into PSCAD/EMTDC are described. Controllable power inverter strategy is intended for capturing the maximum energy from varying wind speed and maintaining reactive power generation at a pre-determined level for constant power factor or voltage regulation. The component models and entire control scheme are constructed by user-defined function provided in the program. Simulation studies provide control performance and dynamic behavior of a gearless VSWT under varying wind speeds. In addition, the system responses to network fault conditions have been simulated. This modeling study can be employed to evaluate the control scheme, output performance and impacts of a VSWT on power grid at planning or designing stage.     

Faulty phase active power characteristics‐based adaptive single‐phase reclosing schemes for shunt reactors‐compensated wind power outgoing line

This paper proposes two novel adaptive single‐phase auto‐reclosing (ASPAR) schemes for shunt reactors‐compensated wind power outgoing (WPO) line. After extinction of a fault on a single phase‐tripped WPO line, due to the existence of shunt reactors and distributed capacitors, a free frequency component will emerge in the tripped faulty phase, which will make faulty phase active power calculated with instantaneous power algorithm have a bigger magnitude and more frequency components. According to these characteristics, transient fault identification (TFI) methods based on active power root mean square (RMS) value and twisty beat frequency were put forward. Subsequently, in view of abovementioned TFI methods, ASPAR schemes for a shunt reactors‐compensated WPO line based on faulty phase active power characteristics were established. Simulation experiments, which were conducted using the software PSCAD/EMTDC, verified that the proposed ASPAR schemes can distinguish transient faults from permanent ones rapidly and reliably. The experiments also proved that the proposed adaptive single‐phase reclosing schemes can minimize the influence of changes in fault location and transition resistance and, thus, are suitable for the inconsistent output power of a wind farm and the complex working condition of an outgoing line.     

Short‐circuit analysis of a doubly fed induction generator wind turbine with direct current chopper protection

Modern doubly fed induction generator (DFIG) wind turbines can ride through a symmetrical fault in the network by using a chopper protection on the direct current (DC) link without triggering a crowbar protection. A novel method to model the DC link system of such wind turbines as an equivalent resistance during symmetrical faults is presented in this paper. The method allows looking at the DFIG with chopper protection as to one with an equivalent crowbar protection and, hence, to apply to the former type of DFIG short‐circuit calculation methods developed for a DFIG with crowbar protection. This may be a valid help in short‐circuit calculations, for example, for protection settings. It also allows simulating for short‐circuit studies a DFIG with chopper protection, often not available in a standard power system simulation software, by using an equivalent DFIG with crowbar protection, which is a standard model in power system simulation software. The results for the short‐circuit current obtained through the proposed method are compared with simulations of a detailed model of a DFIG with chopper protection under different conditions, which showed good agreement. It is also shown that the DFIG with chopper protection delivers lower short‐circuit current than a DFIG with standard crowbar protection, especially for low initial loading. Copyright © 2011 John Wiley & Sons, Ltd.     

Optimising residential electric vehicle charging under renewable energy: Multi‐agent learning in software simulation and hardware‐in‐the‐loop evaluation

The integration of intermittent renewable energy sources coupled with the increasing demand of electric vehicles (EVs) poses new challenges to the electrical grid. To address this, many solutions based on demand response have been presented. These solutions are typically tested only in software‐based simulations. In this paper, we present the application in hardware‐in‐the‐loop (HIL) of a recently proposed algorithm for decentralised EV charging, prediction‐based multi‐agent reinforcement learning (P‐MARL), to the problem of optimal EV residential charging under intermittent wind power and variable household baseload demands. P‐MARL is an approach that can address EV charging objectives in a demand response aware manner, to avoid peak power usage while maximising the exploitation of renewable energy sources. We first train and test our algorithm in a residential neighbourhood scenario using GridLAB‐D, a software power network simulator. Once agents learn optimal behaviour for EV charging while avoiding peak power demand in the software simulator, we port our solution to HIL while emulating the same scenario, in order to decrease the effects of agent learning on power networks. Experimental results carried out in a laboratory microgrid show that our approach makes full use of the available wind power, and smooths grid demand while charging EVs for their next day's trip, achieving a peak‐to‐average ration of 1.67, down from 2.24 in the baseline case. We also provide an analysis of the additional demand response effects observed in HIL, such as voltage drops and transients, which can impact the grid and are not observable in the GridLAB‐D software simulation.     

A centralized vehicle‐to‐grid scheme with distributed computing capacity engaging internet of smart charging points: Case study

In order to accommodate additional plug‐in electric vehicle (PEV) charging loads for existing distribution power grids, the vehicle‐to‐grid (V2G) technology has been regarded as a cost‐effective solution. Nevertheless, it can hardly scale up to large PEVs fleet coordination due to the computational complexity issue. In this paper, a centralized V2G scheme with distributed computing capability engaging internet of smart charging points (ISCP) is proposed. Within ISCP, each smart charging point equips a computing unit and does not upload PEV sensitive information to the energy coordinator, to protect PEV users’ privacy. Particularly, the computational complexity can be decreased dramatically by employing distributed computing, viz., by decomposing the overall scheduling problem into many manageable sub‐problems. Moreover, six typical V2G scenarios are analyzed deliberately, and based on that, a load peak‐shaving and valley‐filling scheduling algorithm is built up. The proposed algorithm can be conducted in real‐time to mitigate the uncertainties in arrival time, departure time, and energy demand. Finally, the proposed scheme and its algorithm are verified under the distribution grid of the SUSTech campus (China). Compared with uncoordinated charging, the proposed scheme realizes load peak‐shaving and valley‐filling by 11.98% and 12.68%, respectively. The voltage values are ensured within the limitation range by engaging power flow calculation, in which the minimum voltage values are increasing and the maximum voltage values are decreasing with the expansion of PEV penetration. What is more, the computational complexity of peak‐shaving and valley‐filling strategy is near‐linear, which verifies the proposed scheme can be carried out very efficiently.     

Impact of fault ride‐through requirements on fixed‐speed wind turbine structural loads

The emphasis in this article is on the impact of fault ride‐through requirements on wind turbines structural loads. Nowadays, this aspect is a matter of high priority as wind turbines are required more and more to act as active components in the grid, i.e. to support the grid even during grid faults. This article proposes a computer approach for the quantification of the wind turbines structural loads caused by the fault ride‐through grid requirements. This approach, exemplified for the case of a 2MW active stall wind turbine, relies on the combination of knowledge from complimentary simulation tools, which have expertise in different specialized wind turbines design areas. Two complimentary simulation tools are considered i.e. the detailed power system simulation tool PowerFactory from DIgSILENT and the advanced aeroelastic computer code HAWC2, in order to assess of the dynamic response of wind turbines to grid faults. These two tools are coupled sequently in an offline approach, in order to achieve a thorough insight both into the structural as well as the electrical wind turbine response during grid faults. The impact of grid requirements on wind turbines structural loads is quantified by performing a rainflow and a statistical analysis for fatigue and ultimate structural loads, respectively. Two cases are compared i.e. one where the turbine is immediately disconnected from the grid when a grid fault occurs and one where the turbine is equipped with a fault ride‐through controller and therefore it is able to remain connected to the grid during the grid fault. Copyright copy; 2010 John Wiley & Sons, Ltd.     

A control scheme to enhance low voltage ride‐through of brushless doubly‐fed induction generators

The use of brushless doubly‐fed induction generator has been recently proposed for wind turbines because of its variable speed operation with fractional size converter without the need to brush and slip ring. This paper introduces a control scheme to improve low voltage ride‐through capability of doubly‐fed induction generator considering grid code requirements. The proposed control strategy is based on analysis of flux linkages and back electromotive forces and intends to retain the control‐winding current below the safety limit (typically 2 pu) during severe voltage dips. The time‐domain simulations validate effectiveness of the proposed scheme to protect the converter against failure as well as support reactive power required by German grid code. Copyright © 2015 John Wiley & Sons, Ltd.     

Modelling analysis of transient stability simulation with high penetration of grid‐connected wind farms of DFIG type

This paper shows a model of a doubly fed induction generator (DFIG) including a simplified model of a wind turbine for the purpose of transient stability analysis of large‐scale power systems with great wind farms penetration. The wind turbine model and the DFIG model are systematically deducted in this paper. Specially, the improved model of rotor‐side converter and the simplified grid‐side converter model are considered in our work. The corresponding machine–network interface solution based on the synchronously rotating common x‐y reference frame is elaborately issued in this paper. Furthermore, a method is proposed to calculate the DFIG initial conditions as well. A simplified model of the turbine is used excluding among other components the pitch control. Copyright © 2007 John Wiley & Sons, Ltd.     

Distributed energy storage system‐based control strategy for hybrid DC/AC microgrids in grid‐connected mode

This paper concentrates on the issues with the aim of providing a constant dc‐link voltage and desired power sharing for a distributed energy storage system (DESS)‐based hybrid microgrid under load variations. The hybrid microgrid which is consisted of PV system, lithium battery‐based storage system and a grid‐connected dc/ac converter are controlled by designing a controller based on the zero dynamics‐based mathematical equations of all used converters. Two buck and bidirectional buck‐boost dc/dc converters employed in PV and DESS systems, respectively, are responsible for damping the dc‐link voltage fluctuations, and also the grid‐connected converter is set to enhance the grid power quality and supply continuously the grid‐connected loads. The main contributions of the proposed control technique are simplicity and providing the simultaneous stable performance for both DC and AC sides under both DC and grid‐connected loads variations. Moreover, another contribution of the proposed control technique is providing accurate coordination in both steady‐state and dynamic conditions. To analyze the proposed controller, the dynamic operations of the converters in various operating conditions are evaluated. In this evaluation, several curves based on their zero dynamics are achieved, and their desired operations are completely investigated in different operating conditions. Simulation results in MATLAB/SIMULINK verify the proposed controller ability at reaching the desired zero dynamics and the stable performance of the proposed hybrid microgrid.     

Modeling and simulation of multi‐scale transients for PMSG‐based wind power systems

In a wind energy conversion system (WECS), multiple‐time‐scale transients that cover a wide frequency range from low‐frequency transient stability up to high‐frequency switching events are observed. This paper presents a methodology of modeling diverse transients for a permanent magnet synchronous generator (PMSG)‐based WECS within the same study. Multiple physical areas of the PMSG‐based WECS are given depending on the appearance of carriers contained in the considered waveforms. In order to eliminate different carrier frequencies, the PMSG and generator‐side voltage source converter (VSC) are modeled in the dq0‐reference frame. On the other hand, the grid‐side VSC and utility grid are dealt with in the multi‐scale model of the network in which the shift frequency is available. The switching‐function and average‐value models of the VSC are selected depending on the carrier shifted. In addition, interface between the control and electrical subsystems is redesigned to offset the computation error caused by one time‐step delay. Two test cases are performed to study the wind power fluctuations and faults ride‐through. The results show that the proposed multi‐scale model is able to simulate slow‐changing dynamic responses up to high‐frequency transients accurately while decreasing the simulation burden. In comparison with the results obtained from the EMTP (electromagnetic transients program) type simulators, the effectiveness and accuracy of the multi‐scale model are verified. Copyright © 2017 The Authors Wind Energy Published by John Wiley & Sons Ltd.     

New design of robust PID controller based on meta‐heuristic algorithms for wind energy conversion system

This paper proposes a new robust control method for a wind energy conversion system. The suggested method can damp the deviations in the generator speed because of the penetration of wind speed and load demand fluctuations in the electrical grid. Furthermore, it can overcome the uncertainties of the plant parameters because of load demand fluctuations and the errors of the implementation. The new method has been built based on new simple frequency‐domain conditions and the whale optimization algorithm (WOA). This method is utilized to design a robust proportional‐integral‐derivative (PID) controller based on the WOA in order to enhance the damping characteristics of the wind energy conversion system. Simulation results confirm the superiority and robustness of the proposed technique against the wind speed fluctuations and the plant parameters uncertainties compared with other meta‐heuristic algorithms.     

Zero non‐detection zone assessment for anti‐islanding protection in rotating machines based distributed generation system

The challenges for a reliable operation of electrical power system have increased due to the presence of multi‐distributed generation units (DGs) in the distribution systems in order to meet the increase of the load demand. Detection of unintentional islanding situation is very important as non‐detection of islanding situation could result in a cascaded failure of the system. If the islanding situation remains undetected, the instability in the islanded part can lead to a complete failure of the electrical power system. This paper introduces a new passive scheme for islanding detection, which is suitable for multi‐distributed generation units based on rotating machines. The proposed method is based on the measurements of the system voltage and frequency to compute two indices called the islanding index and harmonics index. The islanding index is the main index used to discriminate and identify the islanding situation. However, the harmonics index in conjunction with a strategy called speed reduction strategy assists the islanding index to discriminate between islanding situation in case of a close power match and system disturbances. The simulation studies were conducted in MATLAB/SIMULINK environment, and various cases have been considered, such as normal operation, islanding operation, sudden load change, DG tripping, separation of some DG units, faults, etc. The novelty of the proposed strategy is that it provides fast detection and has zero nondetection zone compared with the existing detection methods. Moreover, the proposed strategy has no effect on the power quality, and the maximum detection time is almost 350 ms at a close power match. The results indicate that the proposed scheme is successful in discrimination of the islanding conditions from other grid disturbances, revealing its great potential to be able to detect islanding events. Finally, the proposed method is applied only for rotating machine based DGs, such as wind turbines. Wind farms' power generation system based on doubly‐fed induction generators is introduced in this paper as an example of DGs units.     

Fault tolerant control strategy for modular PWM current source inverter

In this paper, a fault-tolerant control method for an input-series output-parallel modular grid-tied pulse-width modulation (PWM) current source inverter is proposed to address the most commonly seen single symmetrical gate-commutated thyristor (SGCT) open-circuit fault problems. This method actively offsets the neutral point of the current space vector to ensure a sinusoidal output of the grid current, and it can achieve the upper limit power of the inverter under the condition of a single SGCT open-circuit fault. In addition, an active damping control method based on grid harmonic current feedback is proposed after analyzing the influence of the transformer ferromagnetic resonance caused by the neutral point offset on the power quality of the grid current. It has been demonstrated that the proposed method effectively suppresses the resonance caused by the transformer and the modified modulation, improving the grid current’s power quality.     

Validation of a double fed induction generator wind turbine model and wind farm verification following the Spanish grid code

Wind turbine manufacturers are required by transmission system operators for fault ride‐through capability as the penetration of wind energy in the electrical systems grows. For this reason, testing and modeling of wind turbines and wind farms are required by the national grid codes to verify the fulfillment of this capability. Therefore, wind turbine models are required to simulate the evolution of voltage, current, reactive and active power during faults. The simulation results obtained from these wind turbine models are used for verification, validation and certification against the real wind turbines measurement results, although evolution of electrical variables during the fault and its clearance is not easy to fulfill. The purpose of this paper is to show the different stages involved in the fulfillment of the procedure of operation for fault ride‐through capability of the Spanish national grid code (PO 12.3) and the ‘procedure for verification, validation and certification of the requirements of the PO 12.3 on the response of wind farms in the event of voltage dips’. The process has been applied to a wind farm composed of Gamesa G52 wind turbines, and the results obtained are presented. Copyright © 2011 John Wiley & Sons, Ltd.     

A review of fault ride through of PV and wind renewable energies in grid codes

This paper presents a comprehensive review of fault ride through (FRT) in the grid code of 38 selected countries with an emphasis on renewable energy (REN) sources–related rules. Grid codes are the rules legislated usually by the transmission system operators (TSOs) to determine the grid integration requirements of electrical power generators. Each country establishes its grid code for satisfying the minimum required technical criteria and revises it frequently to cope with new modifications of the utility. Growing the penetration of REN sources have influenced many operational aspects of the power system such as protection, power quality, reliability, and stability. Thereupon, regulations must ensure the power system's secure and controllable operation of REN sources. FRT is one of the main parts of the grid code, and its characteristics affect the performance and rating of power system apparatus. FRT defines the performance of electric power generators during and in postfault conditions. FRT of solar photovoltaic (PV) and wind turbines (WTs) as the main REN sources of energy has great importance in the grid codes. In this paper, a comparison of FRTs in the grid code of 38 countries, including low‐voltage ride through (LVRT), zero‐voltage ride through (ZVRT), and high‐voltage ride through (HVRT) are provided and surveyed.