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According to an accident analysis report, with regard to sub-synchronous oscillation (SSO) in Xinjiang, China, dynamic reactive compensation devices have great influence on the existence of sub-synchronous harmonics. This paper based on the practical phenomenon for further study, focuses on the interaction between direct-drive permanent synchronous generators (D-PMSGs)-based wind farm and static var compensator (SVC). Detailed electro-magnetic transient simulation model, such as D-PMSGs-based wind farm compensated by SVC, is established in PSCAD/EMTDC. Time-domain simulation results demonstrate that there is interaction between D-PMSGs-based wind farm and SVC, which is closely related with the control parameters of SVC and D-PMSGs-based wind farm. This type of control interaction will lead to sub-synchronous oscillation. Therefore, the influence of above control parameters on the oscillation characteristics is further studied here. The practical significance of this paper is indicating the necessity of coordinating and optimising control parameters in large-scale wind farm. [ABSTRACT FROM AUTHOR]

This paper proposes a WECS based on a back to back diode‐clamped multilevel inverter systems (DCMLI) fired comparatively by sinusoidal pulse width modulation (SPWM) and third harmonic injection pulse width modulation (THIPWM) techniques. DFIG performance is compared using these technologies for different wind speed under normal operation condition. The proposed approach shows that the DCMLI systems generate a nearly sinusoidal voltage with reduced total harmonic distortion (THD) thus upgrading the power quality of that produced by DFIG. As SPWM and THIPWM MLI are most common usage techniques in research, we applied them in the system to discuss a comparison of using those techniques to determine the number of levels that achieve the IEEE 519 criteria of THD which is less than 5% for both techniques (5th level for SPWM and 4th level for THIPWM) in all operation wind speed. Compared with simple PWM, and THIPWM have many advantages as lower THD, minimal number of switching to decrease switching losses and the output fundamental voltage is increased. Lastly, the paper investigates the variation of the frequency of induced rotor voltage and the active power flow due to the wind speed change when the rotor speed changes from super‐synchronous to sub‐synchronous speeds. [ABSTRACT FROM AUTHOR]

Inertial and droop control are common methods for improving grid frequency control. Using the first‐order derivative of the frequency in the inertial control loop only investigates the rate of frequency variation, while other useful information is not taken into consideration. Accordingly, in this study, the application of a fractional‐order derivative (between 1 and 2) in the inertial control loop is proposed to have more useful frequency‐based information. To achieve this goal, firstly continued fraction approximations and secondly, the discretization is adopted by using the Tustin Method with Prewarping. Inertia loop and droop loop gain have a significant impact on the operation of the wind turbines and the power system frequency control. Therefore, in this paper, a novel method for the adaptive adjusting of droop and inertia control loops gains in DFIG is proposed. To utilize more frequency error information, fractional‐order adaptive law is proposed to update the loop gains and the inertia control loop. In order to have a robust system, which is permanently stable for every fractional order (FO), is used parameter space method. Simulation results demonstrate the proper performance of the FO‐based adaptive approach to increase the frequency nadir (FN) and decrease the frequency variations. [ABSTRACT FROM AUTHOR]

DC grid has a prospect of application in collecting and transmitting large-scale renewable energy over long distance. However, the randomness and fluctuation of the renewable power may hamper the stable operation of DC grid. Energy storage device is able to actively absorb or supplement active power, which can be used to smooth the power fluctuation. The paper investigates a DC grid topology with energy storage and proposes a coordinated control strategy. The proposed strategy enables coordinated control of the renewable energy output power, pumped storage power and the grid consuming power. During the steady state, the control strategy enables the system DC voltage stable and power flow of the grid under zero-error control. During the faulted time, the coordinate control strategy enables the converter fast response and maintains the system DC voltage within safety margin. The effectiveness of the coordinated control strategy is verified by simulation in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Large-scale wind farms integrated with multi-terminal direct current (MTDC) system decouples the frequency between the wind farms and the receiving end, resulting in weakened frequency regulation ability of the onshore AC grid. Recently a new topology is proposed, called diode rectifier based HVDC, which recouples the wind farm and onshore AC system due to its characteristics, where offshore AC voltage changes with the DC voltage of HVDC system. A coordinated power control strategy for frequency regulation participation is proposed in this paper for offshore wind farms connected with diode-rectifier based high voltage direct current (HVDC) system. Considering the primary frequency and inertia response control, variable power supplementary control is implemented in wind turbines, and a telecommunication-less method of frequency event detection via detection of offshore AC voltage is proposed, avoiding communication delay and communication failure. Simulations using EMTDC/PSCAD show that frequency dip can be accurately detected by the AC voltage of offshore wind farm, and wind farm can quickly compensate active power. [ABSTRACT FROM AUTHOR]

The Kriegers Flak combined grid solution (KF CGS) is the first hybrid‐asset project utilising an offshore high voltage alternating current/high voltage direct current (HVAC/HVDC) interconnector for large offshore wind power plants between Eastern Denmark and Germany in the Baltic Sea. On the Danish side, above 200 km 220 kV cables are arranged into a meshed offshore transmission grid (MOG). In order to maintain good voltages in the MOG and wind power plants, on the Danish side, the control is realised by an automatic voltage regulation and reactive power control (AVR/RPC) of several switchable and variable reactors and transformers with tap‐position controllers, and the wind power plant controllers (WPPC); on the German side, the voltage control is achieved by the HVDC back‐to‐back converter, WPPC, and part of the optimal power flow by the master controller for interconnector operation. This paper presents the proposed coordinated voltage and RPC of the HVAC MOG on the Danish side of the KF CGS interconnector. The efficiency and constraints of the proposed coordinated control scheme of the KF CGS at fluctuating voltage and reactive power on both ends of the MOG are presented by simulations using the entire set up of the 220 kV AC MOG. [ABSTRACT FROM AUTHOR]

This study firstly derives the relationship between the grid voltage and the rotor current of doubly-fed induction generator (DFIG) during the grid voltage swell. Then, an effective control strategy aiming to restrain rotor over-current is proposed and the control strategy of the grid-side converter is also improved. The proposed rotor current suppression and grid voltage suppression method, cooperating with the DC chopper, can not only solve the problem of short circuit of the rotor side converter resulting from the frequent switching of crowbar device, but also maximally improve the capability of wind turbine in dynamic reactive power support without adding any other power electronic devices. With the help of this control strategy, the wind turbine could maintain its normal operation even during the fault and the high voltage ride through of wind turbine can be achieved. As a result, the stability of the wind power integration is significantly improved. Finally, the simulations and analysis in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) verify the effectiveness and feasibility of the proposed control scheme. [ABSTRACT FROM AUTHOR]

With the increasing proportion of wind power installed capacity, the influences of transient characteristics of wind turbines generator on safety and stability of power grid is becoming more and more important. The rotor side of doubly fed induction generator (DFIG) is connected to the grid through a back-to-back inverter, and vector control based on phase-locking synchronisation is used generally. Phase-locked loop (PLL) drive internal potential phase changes to achieve synchronous operation with the grid. Therefore, PLL has great influence on transient characteristics of DFIG. A dynamic model of PLL is built in this study. Phase-locking performance of PLL under voltage phase jump is studied by changing its parameters, and its impact mechanism on transient characteristics of DFIG is proposed. Simulation models are built with DIgSILENT/PowerFactory. The influence of parameters of PLL on phase-locking performance of DFIG under voltage phase jump is simulated. [ABSTRACT FROM AUTHOR]

The complex torque coefficients (CTC) method has been widely adopted to investigate the conventional sub-synchronous oscillation (SSO) issues. However, the application of the CTC method is limited when large-scale wind power generations are integrated into power systems. Therefore, in order to overcome the disadvantages of the CTC method on the aspects of frequency scan and stability determination, this paper proposes an improved CTC method based on the shaft system modal decoupling and small-signal self-excitation theory. The improved method is demonstrated and validated through the IEEE first benchmark power system (FBMPS). By establishing the small-signal model of the series-compensated DFIG-based transmission system, the proposed method is applied to examine the SSO in a grid-connected DFIG system. Moreover, the impacts of different parameters on the electrical damping characteristics of the system are deeply analysed. In addition, an effective VSC control strategy for suppression of the SSO in series-compensated DFIG-based transmission system is proposed. [ABSTRACT FROM AUTHOR]

Marked with flexible interconnection and control, the high-voltage direct current (HVDC) gird has captured much attention of industries and academics. Hybrid dual-infeed or multi-infeed HVDC composed of line-commutated-converter HVDC (LCC-HVDC) and voltage source converter HVDC (VSC-HVDC) will form the main pattern in a further power grid; meanwhile, the new gird pattern will bring new opportunities and challenges to security and stability control in the power system. First, research works on the control strategies and operation performances of LCC-HVDC and VSC-HVDC are stated in this paper; then, a model of wind power integration into a dual-infeed DC transmission system is established in PowerFactory, and case studies are conducted in both steady and transient states. On this basis, a new control strategy for variable-speed constant-frequency wind power generators to promote voltage characteristics of the DC network is designed in this paper, and two additional active power control segments are designed in the traditional control system; thus, DC voltage stability can be improved by fast regulation of active power output due to quick power adjustment of wind power generators; simulations are implemented and the results will lay a foundation for safe and stable operation in the DC transmission system with renewable energy integration. [ABSTRACT FROM AUTHOR]

Zhangbei demonstration project will be the world's highest voltage level and largest capacity VSC-HVDC power grid, by which millions of kilowatts of renewable energy and pumped storage power will be supplied to Beijing. Due to the uncertainty of wind power and solar power, and the operating range restriction of pumped storage unit, it is urgent to find the solution to the problem of plan making of pumped storage generation and convertor substation after the Zhangbei demonstration project completes. This paper analyses the fluctuation characteristics of renewable energy generation in converter substation, based on the t-location scale, then proposes a multi-objective optimal scheduling strategy for wind power, PV, and pumped storage plant in VSC-HVDC grid, with maximum energy consumption and minimum power fluctuation at the load side and vicinity of the adjusting side as the target, considering the constraints from day-ahead forecasts, VSC-HVDC capacity, unit start-up–shutdown, and state conversion. Simulation results show that authors' strategy can make use of pumped storage power plant to inhibit the fluctuation of renewable energy power and make the maximum renewable energy consumption within the allowed power fluctuation at the sides, attaining a good balance between energy consumption and grid security. [ABSTRACT FROM AUTHOR]

Converting the surplus power into rotor kinetic energy by changing the control functions of machine side converter (MSC) and grid side converter (GSC) is considered to be an efficient way for permanent magnet synchronous generator (PMSG)‐based wind turbine generator system (WTGS) to accomplish low‐voltage ride through (LVRT) without additional hardware components. However, because of the time delay of grid fault detection and the control system inertia, the dc‐link capacitor may still be subjected to overvoltage in the early tens of milliseconds after a grid fault happens. This study proposes a compositive control method of LVRT for PMSG‐based WTGS, in which the surplus power during LVRT is converted into rotor kinetic energy by controlling the electrical power of the PMSG according to the grid‐connected power of the GSC, and a crowbar circuit is reserved to consume the surplus power before MSC reacts to the grid fault. The proposed method does not change the control functions of MSC and GSC, and the related control parameters resetting can be avoid. Besides, it can also provide reactive power support to assist the grid voltage recovery. Simulations and experiments proved the proposed method to be feasible and effective. [ABSTRACT FROM AUTHOR]

This paper describes an intelligent sensor fault detection and compensation (FDC) scheme for a hybrid grid involving renewable energy (RE) sources with power electronic interfaces. To evaluate the sensor FDC scheme, a wound rotor induction generator (WRIG)‐based wind energy system connected to the hybrid grid is examined. The system employs a WRIG with excitation from DC RE grid via a single, rotor side converter. To analyse the dynamic performance, a decoupled voltage vector control is employed. The sensors involved in the control algorithm may not be ideal and if fault occurs might lead to system collapse. To overcome electrical sensor failure, a robust, intelligent sensor fault control scheme is proposed with ANFIS. A hardware‐free solution with PLL block is proposed for mechanical sensor fault. The ANFIS‐based FDC scheme involves a bank of observers that computes residuals, detects and isolates faults. When fault occurs, the observer output is utilised for the execution of control algorithm thereby avoiding system collapse. The focused system is simulated in MATLAB and experimentally tested with a dSPACE controller for various fault scenarios. The results show the effectiveness of the FDC scheme. [ABSTRACT FROM AUTHOR]

Low-voltage ride-through (LVRT) compliance of wind energy conversion systems is one of the significant concerns for both the wind industry and the system operator. In this study, power electronic-assisted on load tap changer (OLTC) is proposed as a pragmatic solution to enhance LVRT performance of wind turbine generator (WTG). Furthermore, a coordinated control strategy between OLTC and WTG controller is proposed to mitigate post-fault over-voltage issues that are encountered due to the relatively slow dynamics of hybrid OLTC. The results suggest that employing hybrid OLTC does not only improve LVRT compliance of WTG but also significantly improve voltage dip-induced delayed active power recovery of WTG. [ABSTRACT FROM AUTHOR]

This study presents a dq impedance‐decoupled network modelling method of offshore wind power plant (OWPP) for stability analysis. DQ impedance frequency responses of grid‐connected inverters (GCIs) and long transmission cables (LTCs) are first measured by frequency scanning method, which are fitted as transfer function matrices using matrix fitting algorithm. Then, the GCIs are modelled as Norton equivalent circuits. In addition, per‐unit‐length electrical parameters of the LTCs are extracted from the measured dq impedance frequency responses of a specific LTC, based on which dq impedance‐decoupled two port network models of the LTCs are established. DQ impedance‐decoupled network model of the whole OWPP is then established based on connection relationships of these GCIs and LTCs. Finally, generalised Nyquist criterion (GNC) is performed in all the dq impedance‐decoupled subsystems, and the subsystems where the GNC is not satisfied are identified as instability sources. Compared with conventional impedance‐based stability analysis methods of OWPP, the proposed dq impedance‐decoupled modelling method is able to facilitate the application of GNC and further instability source identification based on partitioning the whole OWPP into several decoupled subsystems. The effectiveness of the network modelling method is validated based on time‐domain simulation results in Matlab/Simulink and real‐time verification results in OPAL‐RT. [ABSTRACT FROM AUTHOR]

When a hybrid wind farm based on doubly fed induction generator (DFIG) and fixed speed induction generator (FSIG) is connected to an unbalanced network, most of the negative sequence current caused by the unbalanced grid will be centralised at the FSIG for its inherent small negative sequence output impedance (NSOI). The centralised negative sequence current can cause significant torque pulsation in the drive train of FSIG, which may go beyond the pressure endurance of the drive train and cause some damages. Meanwhile, losses and risks of overcurrent and grid code violation can also be increased. This study presents a control strategy for DFIG in hybrid wind farm during network unbalance, which can disperse the negative sequence current and the consequent pressure among the three parallel power sources without communication, i.e. among the DFIG stator, the grid side converter (GSC), and the FSIG, rather than centralising the pressure at any of them. In order to achieve this target, a novel hybrid virtual impedance method is proposed to flexibly control the NSOI of the DFIG stator and the GSC from zero to infinity. Theoretical analysis and simulation results are provided to verify the operation performance of the proposed strategy. [ABSTRACT FROM AUTHOR]

Small‐signal sequence impedance models have been developed for different types of wind turbines and used in industry to study wind farm and high‐voltage direct current (HVDC) system harmonics and resonances. Compared to other small‐signal methods, a major advantage of system modelling and analysis based on sequence impedance is its scalability: an equivalent impedance model can be easily built for any complex system by aggregating the impedance of individual turbines and the network. A recent development in the small‐signal sequence impedance theory is the modelling of frequency coupling to improve the accuracy of system analysis and explain a common characteristic of harmonics created by system resonance. In light of this new development, this study presents and compares different aggregation methods to build farm‐level impedance models in the presence of coupling in individual turbine responses. The objective is to identify practical methods that can meet the needs of different system analyses and are easily to use. Experiences of China State Grid and TenneT with the application of sequence impedance models in renewable energy and HVDC system resonance and harmonic analysis are also discussed, along with an overview of their ongoing efforts to develop new grid codes and system analysis tools based on sequence impedances. [ABSTRACT FROM AUTHOR]

This study presents an analysis method of harmonic interaction characteristics between direct‐drive wind turbines (DDWTs) based on αβ coordinates and the relative gain array (RGA). First, considering the digital delay of inverters, the capacitive current feedback active damping is applied to suppress the resonance of LCL filter, and the output current of DDWT is regulated by quasi‐proportional resonant controller in the αβ coordinates and then combined with the topology of the direct‐drive wind farm (DDWF), the transfer function matrix of the DDWF is established. Second, based on the RGA principle, the relationship between parameters and harmonic interaction characteristics is quantitatively analysed. The results show that with the proportional coefficient increasing, the degree of negative interaction between DDWTs is strengthened, which increases the harmonic content in the negative interaction frequency band. However, the increase of resonance coefficient and number of DDWTs will weaken the degree of negative interaction. In addition, with the increase of capacitive current feedback coefficient and grid inductance, the interaction frequency gradually decreases. Finally, the DDWF model is built both on Matlab/Simulink and StarSim + PXI hardware‐in‐the‐loop platform to verify the analytical results. The harmonic interaction characteristics can be used to guide the parameter design of DDWTs, so as to optimise power quality of the wind farm. [ABSTRACT FROM AUTHOR]

Increasing penetration of wind power has a negative effect on system frequency deviation due to the fast variability and uncertainty characteristics of wind. In this study, a wind power variation prediction‐based automatic generation control (AGC) feedforward control method is applied within the conventional AGC framework to minimise system frequency and tie‐line deviations. Firstly, the variogram function is introduced for analysing the characteristics of wind power variations. A relationship between the variogram of wind power and the trend component of wind power is found and then a three‐parameter power‐law model is established. Then, the generation rate constrains of AGC generators is considered as the main constraint factor that influences wind power smoothing. By predicting the wind power variations in AGC control time‐scale, a coordination AGC feedforward control strategy is proposed. The control strategy makes different types of AGC generators act in advance to improve the ability of the power system, corresponding to the forecasted ramp variations in wind power generation. Finally, the performance of the proposed coordination control strategy is verified and tested using the actual data from China. Simulation results show that with this new strategy, frequency deviation under wind power variations can be effectively decreased. [ABSTRACT FROM AUTHOR]

With the proliferation of intermittent renewable energy sources, power systems need to withstand an increasing number of disturbances that may affect system frequency. In this paper, we focus on the effects of high penetration level of wind turbine‐generators (WTG) on the power system operational planning from the frequency point of view. Specifically, an exact formulation for the minimum frequency calculation is presented to increase accuracy and speed of analyses. Then, the effect of WTG's frequency response on the power system frequency stability is investigated to compute the maximum acceptable generation outage as a function of penetration level. Finally, as an application to power system operational planning, the obtained frequency dynamics of the power system and the wind turbine frequency response are integrated into the unit commitment problem. Simulations are performed on the IEEE 39‐bus test system to study the effects of the WTGs on the frequency stability of the system. Results illustrate a more secure and reliable operation of the power system when the wind turbines participate in the frequency regulation task. Moreover, including the frequency response of the wind turbines in the unit commitment problem can reduce the ancillary cost, meanwhile increase the frequency stability of the system. [ABSTRACT FROM AUTHOR]

Converters are the key for increasing the development of renewable energy generation but their dynamic interaction with the grid has an important impact on stability. Oscillatory instabilities in different grid‐connected converter systems at several frequency ranges are reported. In particular, sub and supersynchronous oscillatory instabilities in AC power systems with type‐3 and type‐4 wind turbine generators (WTGs) were recently registered at several wind farm areas. A number of works based on eigenvalue analysis and frequency domain approaches are carried out to analyse this new stability issue and more research is going on to analyse in detail the electric and control parameters that may affect this phenomenon. This study contributes analysing in detail the influence of system parameters on the subsynchronous oscillations (SSOs) in AC power systems with type‐4 WTGs. This study is based on a new approach for subsynchronous stability assessment which could also be used for analysing other types of SSOs (e.g. subsynchronous control interactions between doubly‐fed induction generators and series‐compensated networks) as well as supersynchronous oscillations. A representative example of weak AC grids with type‐4 WTGs is used to illustrate the contributions of the study. Results are validated with PSCAD/EMTDC time‐domain simulations. [ABSTRACT FROM AUTHOR]

The sub‐synchronous control interaction (SSCI) has frequently occurred in the direct‐drive permanent magnetic synchronous generators (PMSGs) based wind farm integrated to the weak AC network. The system strength, i.e. short‐circuit ratio (SCR) has been believed as the critical factor so far. However, the impedance–frequency characteristics of the practical network exhibit a strong resonance that might have impacts on the SSCI as well. In this study, considering the network resonance, the eigenvalue‐ and impedance‐based analyses are conducted to study the SSCI phenomenon in the grid‐connected PMSG system. The impacts on SSCI from system strength, network resonance and network‐resonant frequency are investigated firstly. Then, the parameter limits of PMSG control for the stable region are calculated to discuss the effects of network resonance on the parameter design. The results show that the network resonance plays a critical role in the SSO mode. When the network resonance is considered in the PMSG system, the SSO would arise more easily, and the stable region of the control parameter will be reduced. Meanwhile, the frequency of the SSO mode matches well with the network resonance frequency in dq frame. The analytical results are all verified by simulation. [ABSTRACT FROM AUTHOR]

The offshore all‐DC wind farm with increasing capacity will bring problems such as the weakening of grid frequency stability and the increase of equivalent grid impedance. To overcome this, a coordinated control strategy for the offshore all‐DC wind farm is proposed here with two salient features: better performance under weak grid condition and real‐time frequency support from the wind farm. The control strategy consists of three parts: the inertia synchronising control of the receiving‐end converter, the constant ratio control of the DC transformer and the frequency response of the wind farm. With the proposed strategy, the all‐DC wind farm operates like a synchronous generator to the onshore grid, which provides fast frequency support when the onshore grid frequency changes. The effectiveness of the proposed method is validated in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) using a typical IEEE 9 bus system. [ABSTRACT FROM AUTHOR]

With higher penetration of converter‐connected renewable energy sources (RES) into power systems, the successful operation of the system is challenged by significant reductions in system inertia. Presently, given the dominant share of the conventional synchronous power plant, RES power plants are not demanded to provide ancillary services. However, as RES connections increase, RES power plants will play a major role in power system operation, contributing to frequency control. This study demonstrates that photovoltaic power plants (PVPPs) can provide effectively different types of frequency support based on a power reserve and an offline maximum power point tracking (MPPT) technique. An innovative method to de‐load the PVPP without significantly increasing the MPPT complexity is proposed. Results from different PVPP frequency support methods, under varying levels of photovoltaic penetration, are presented which demonstrate their capability to provide inertia support comparable to that of synchronous generators. A new variable droop control method, which releases maximum power during the inertial response and returns to fixed droop gain value after a specified time is also presented. The results from using the variable droop show that the frequency nadir and the rate‐of‐change‐of‐frequency can be significantly reduced and some power reserve still maintained after a frequency event. [ABSTRACT FROM AUTHOR]

The rapidly increasing penetration of renewable energies has introduced severe challenges to power system frequency controls due to the highly intermittent and uncertain power output of renewable energies. This study proposes a proactive frequency control method to control traditional synchronous generators in advance in anticipation of sudden power fluctuation. Therefore, the frequency deviations can be well mitigated by such early‐acted control signal. In order to obtain this new control reference, an ensemble‐forecasting model based on the extreme learning machine algorithm is designed to predict ultra‐short‐term power fluctuations, which serves as an extra signal for automatic generation control. The proposed method was verified on an equivalent model of the Singapore power system with various types of generations and loads. The simulation results clearly demonstrate the accuracy of the forecasting model and the advantages of the proposed control method. The proposed method can also reduce the frequency charging/discharging of energy storage systems, which can effectively extend their lifetime. [ABSTRACT FROM AUTHOR]

Regarding PMSG-based wind turbine generation system, this study proposes a super-capacitor energy storage unit (SCECU) connected parallel to the DC-link of the back-to-back converter to enhance its high voltage ride through performance. The analysis of the operation and control for the grid-side converter and SCECU are conducted. Based on real-time digital simulators (RTDS), a model of PMSG-based wind turbine with SCECU is developed, and the simulation results show that the SCECU absorbs the imbalanced energy and the grid-side converter absorbs inductive reactive power during the period of voltage swell. The simulation results based on RTDS verify the correctness and feasibility of the high voltage ride through control strategy. [ABSTRACT FROM AUTHOR]

This article presents a harmonic compensation capability‐based coordinated control strategy for a hybrid wind farm including doubly fed induction generator (DFIG)‐based and direct‐driven permanent‐magnet synchronous generator (PMSG)‐based wind turbines operation under distorted grid voltage conditions. First, the maximum harmonic current compensation capabilities of the DFIG and PMSG systems are analysed in detail when the grid voltage is harmonically distorted. Combining the maximum harmonic current compensation capabilities with the harmonic current requirements of different control targets, the controllable operating regions of each control target of the DFIG and PMSG systems during network voltage distortions are presented. Furthermore, the effects of the voltage total harmonic distortion (THD) and the system operating conditions on the controllable operating regions are analysed. Based on the analysis, the coordination strategy is proposed for a hybrid wind farm without communication equipment between the different wind power generators. The proposed strategy can improve the interoperability and the output power quality of the entire hybrid wind farm during grid‐voltage harmonic distortions. Finally, the effectiveness of the proposed coordination strategy for the hybrid wind farm is validated by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Series capacitor compensation technology has been widely used in long‐distance wind power transmission, which can improve the power transmission capacity and enhance the static stability of the power system. However, adding a series capacitor in the line may present a potential risk of subsynchronous resonance (SSR) to the system. This study focuses on the SSR caused by induction generator effect of the doubly‐fed induction generator‐based wind farms. The SSR phenomenon of wind power system is researched by the impedance analysis method. A mitigation strategy of adding a virtual inductance based on integral control is proposed for enhancing the system stability and alleviating the SSR. It has clear and intuitive physical concept, which is beneficial to system integration and provides a simpler and more economic method for mitigating SSR. Moreover, the principle and process of controller design are elaborated. Moreover, the time‐domain simulation is carried out to prove the effectiveness of the proposed control strategy under different operating conditions considering various compensation levels and wind speeds. Finally, the suppression effects of the virtual inductance in rotor‐side converter and grid‐side converter are compared. [ABSTRACT FROM AUTHOR]

In future power systems with little system inertia, grid operators will require the provision of synthetic inertia (SI) from renewable energy sources. Unlike today, grid operators may require continuous provision of SI. This can lead to an unwanted disconnection of wind turbine generators (WTGs) from the grid, and has the potential to cause a significant decrease of the energy yield and financial losses for the turbine operator. In order to avoid such situations a controller is proposed, which interprets the grid codes to the benefit of all parties involved. This can be achieved by a variable inertia constant, which changes with the operating point of the WTG. In this study, the behaviour of the variable inertia constant controller is described, assessed and verified with time domain simulations. [ABSTRACT FROM AUTHOR]

Flexible AC transmission systems have shown effective functionalities in promoting the system operation security and service reliability. Facing with series-compensated lines, subsynchronous resonance (SSR) may jeopardise the stability and mechanical facilities. The control strategy and simulation model of a unified power flow controller (UPFC) based on the dq decoupling control are deduced and constructed. Considering the impacts of grid-connected wind farms, SSR characteristics of the series-compensated system with UPFC are studied by the complex torque coefficient method and time-domain simulation. The results verify the capability of UPFC in attenuating SSR in wind farm integrations by utilising subsynchronous damping controllers coordinated with UPFC's main controllers. It also suggests that the shunt transformer−converter has a slight effect on SSR, whereas the series transformer−converter affects the system resonance characteristics significantly. Simulations are carried out on a typical power system model integrated with wind turbines. [ABSTRACT FROM AUTHOR]

The doubly fed induction generator (DFIG) has insufficient capacity to respond to the frequency inertia of the power grid due to the isolation between the rotor kinetic energy and the power grid by the generator-side converter. At the same time, the 'source' of the DFIG cannot be as stable as the thermal generator, so sudden changes in the wind speed on the ultra-short time scale can affect the system inertia. In this study, the inertia of a doubly fed wind generator with a super capacitor energy storage system (SCESS-DFIG) is defined from the standpoint of a single machine. Based on fuzzy logic controller, an inertia response control strategy of multi energy coordinated control is proposed. Then the SCESS-DFIG inertia response control strategy under variable wind speed is proposed at the wind field level. Based on the establishment of DFIG, DC bidirectional buck–boost converter, and supercapacitor energy storage unit model, a wind farm with three SCESS-DFIGs is simulated. When the grid frequency is dropped, the effectiveness of the proposed control strategy is verified by simulating their operation under the same rated parameters, different wind speed curves, and different initial voltages of the supercapacitors. [ABSTRACT FROM AUTHOR]

Wind farms integrated to an AC power system through a voltage source converter-based high-voltage direct current system (VSC-HVDC) are viewed as islanded wind farms. The wind farm side converter of VSC-HVDC maintains a constant frequency of the wind farm collection network and also controls its voltage. However, the transient responding time of VSC-HVDC is very short, and its fault responding feature is at the millisecond (ms) level. As a result, under grid fault conditions, the VSC station could be overloaded and even operation could be quitted because of blocking if islanded wind farms fail to limit their power generation in time. This study proposes a fault ride through control method for VSC-HVDC connected with islanded wind farms based on the forced short circuit of the AC-side bus of the VSC station. When there is a fault in the VSC-HVDC and the protection equipment is not able to cut off the excessive power at the ms level, the short-circuit resistance installed on the AC side of the VSC station would be switched to make a forced short circuit. That would activate the low-voltage ride through mode of islanded wind farms, and therefore reduce the power flowing into the VSC quickly. The HVDC fault and short circuit on the AC side would be cleared at the same time. The VSC-HVDC system accomplishes fault ride through. [ABSTRACT FROM AUTHOR]

The dynamic performance of voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) grid for large-scale renewable energy integration is becoming a concern. This study proposes a system-level dynamic response model of wind farm (WF)-MTDC-main AC systems, and then analyses the dynamic performance based on the singular value decomposition (SVD) technique. In the modelling, an improved virtual synchronous machine control is developed, and the interaction between AC frequency and DC voltage can be readily described. Using the SVD technique, parameters of the controllers are tuned, thereby making the AC frequency and DC voltage deviations within the limitations. Some oscillation modes of the system are observed and virtual power system stabiliser is proposed to suppress the oscillations. Additionally, the oscillation can be mitigated by emulating capacitance-based inertia response. The efficiency of the proposed model and analysis is verified through the frequency-domain and time-domain results. [ABSTRACT FROM AUTHOR]

Aiming at the voltage stability problem of wind power grid‐connected systems, a cooperative control scheme of Thyristor Controlled Series Compensator (TCSC) and Static Synchronous Compensator (STATCOM) is proposed in this study. The proposed scheme is applied in the wind power grid‐connected system, a combined control strategy based on STATCOM double closed‐loop feedback control and TCSC device is used to ensure that the system has enough reactive power to maintain its normal work during operation. The corresponding simulation model of wind power grid‐connected system is built in MATLAB/Simulink. By analysing and comparing the waveforms of the operating examples with only TCSC, only STATCOM and TCSC‐STATCOM, it is shown that the cooperative control of TCSC and STATCOM can quickly and effectively recover the bus voltage at the point of grid‐connected wind farms after fault, which has a good performance on improving the voltage stability of wind power grid‐connected system and increasing the low‐voltage ride‐through capability, and the cooperative control effect of TCSC and STATCOM is better than only using the control of TCSC or STATCOM. [ABSTRACT FROM AUTHOR]

High‐voltage direct‐current (HVDC) grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms and synthetic inertia contribution from offshore wind farms. However, when all converters within the HVDC grid are fitted with these supplementary controllers, undesirable power flows and reduced power transfers may occur during a power imbalance. This is due to simultaneous frequency oscillations on the different ac systems connected to the HVDC grid arising during the support operation. To prevent this adverse effect, an auxiliary dead‐band controller (ADC) is proposed in this study. The ADC modifies the dead‐band set‐point of the fast frequency controllers using measurements of the rate of change of frequency and frequency deviation. A four‐terminal HVDC integrated with an offshore wind farm is modelled to analyse and study the effectiveness of three different supplementary fast frequency control algorithms. Results show that the proposed ADC scheme improves the performance of fast frequency control algorithms. For completeness, a small‐signal stability analysis is carried out to confirm that a stable system operation is maintained. [ABSTRACT FROM AUTHOR]

Sub‐synchronous oscillation (SSO) caused by direct‐drive permanent magnet synchronous generators (PMSGs) occurs frequently in Xinjiang Uygur Autonomous Region, China. As a new type of SSO, the mechanism of power oscillation triggered by PMSGs has not been clarified yet. In this study, a small‐signal analysis method was used to investigate the response process of the grid‐side converter (GSC) controller and a phase‐locked loop (PLL) to harmonics. Components with the same frequency of input disturbance signal and an output signal which generated by response process of controller superpose at the outlet of PMSGs could be amplified by positive feedback. Amplified harmonics will cause sustained power oscillation in sub‐synchronous frequency. A criterion for judging dangerous frequencies of SSO was deduced based on phase relation between input and output signals, and the form of output signal was predicted under the condition of certain parameters setting. The theoretical analysis is then validated by the simulation results and the on‐site data measured by the phasor measurement unit. Furthermore, the impacts of GSC and PLL parameters on SSO were discussed. This study provides the theoretical basis for SSO mechanism and judgement method in practical engineering. [ABSTRACT FROM AUTHOR]

This study presents a novel application of a hybrid adaptive neuro‐fuzzy inference system (ANFIS)‐genetic algorithm (GA)‐based control scheme to enhance the performance of a variable‐speed wind energy conversion system. The variable‐speed wind turbine drives a permanent‐magnet synchronous generator, which is connected to the power grid through a frequency converter. A cascaded ANFIS‐GA controller is introduced to control both of the generator‐side converter and the grid‐side inverter. ANFIS is a non‐linear, adaptive, and robustness controller, which integrates the merits of the artificial neural network and the FIS. A GA‐based learning design procedure is proposed to identify the ANFIS parameters. Detailed modelling of the system under investigation and its control strategies are demonstrated. For achieving realistic responses, real wind speed data extracted from Zaafarana wind farm, Egypt, are considered in the analyses. The effectiveness of the ANFIS‐GA controller is compared with that obtained using optimised proportional–integral controllers by the novel grey wolf optimiser algorithm taking into consideration severe grid disturbances. The validity of the ANFIS‐GA control scheme is verified by the extensive simulation analyses, which are performed using MATLAB/Simulink environment. With the ANFIS‐GA controller, the dynamic and transient stability of grid‐connected wind generator systems can be further enhanced. [ABSTRACT FROM AUTHOR]

This work aims to carry out a comprehensive analytical evaluation of a quasi‐Z‐source matrix converter (qZSMC), which is comprised of a three‐phase quasi‐Z‐source network (TPQZSN) followed by an MC, as the grid interface of a permanent magnet synchonous generator (PMSG)‐based wind energy conversion system (PMSG‐WECS). First, the small‐signal model of the TPQZSN in space domain is obtained by the state‐space averaging method, while the parasitic elements are also considered. The TPQZSN's transfer functions are further calculated and analysed. The results together with a frequency‐domain analysis are used to render a comprehensive guide for the selection of proper passive components of the TPQZSN. The qZSMC is then employed as the grid interface of a PMSG‐WECS. A control system is designed for the grid‐connected operation. The small‐signal model of the whole system is developed. The qZSMC is compared with conventional MC (CMC) from stability point of view, indicating that, unlike the CMC, the qZSMC has no trouble working stable for all ranges of output power. Furthermore, it is shown that the proposed WECS can operate stably without using digital filter. The system performance is compared with CMC‐based WECS. [ABSTRACT FROM AUTHOR]

Requirements for inertial response (IR) from wind turbines (WTs) have been implemented or drafted by power systems (PSs) operators worldwide. This is a response to the replacement of conventional power plants using synchronous generators by non‐synchronous, power electronics‐based generation and the resulting effect on frequency dynamics in the case of contingency events. The additional active power provided during operation in IR mode must be drawn from the rotating masses. A re‐acceleration accompanied by reduced active power output follows the activation phase. The allowed depth and duration of the post‐inertial recovery will be regulated in future versions of grid codes, e.g. in Québec and Ireland. This study describes an improved version of an IR control system that enables a more PSs‐friendly provision of such short‐term frequency support. The new controls allow adjusting the duration of the recovery period. Potential negative effects of IR from WTs on the PS in the form of a second frequency nadir during the recovery phase can be minimised. The outcome of simulations and of field testing will be presented. All results shown in this study include the initial and the future inertia emulation performance which allows easy comparison of the two controls. [ABSTRACT FROM AUTHOR]

An intelligent wind power smoothing control using recurrent fuzzy neural network (RFNN) is proposed in this study. First, the modeling of wind power generator and the designed battery energy storage system (BESS) are introduced. The BESS is consisted of a bidirectional interleaved DC/DC converter and a 3‐arm 3‐level inverter. Then, the network structure of the RFNN and its online learning algorithms are described in detail. Moreover, actual wind data is adopted as the input to the designed wind power generator model. Furthermore, the three‐phase output currents of the wind power generator are converted to dq‐axis current components. The resulted q‐axis current is the input of the RFNN power smoothing control and the output is a gentle wind power curve to achieve the effect of wind power smoothing. The difference of the actual wind power and smoothed power is supplied by the BESS. The minimum energy capacity of the BESS with a small fluctuation of the grid power can be achieved by the RFNN power smoothing control. A digital signal processor (DSP) based BESS is built using two TMS320F28335. From the experimental results of various wind variation sceneries, the effectiveness of the proposed intelligent wind power smoothing control is verified. [ABSTRACT FROM AUTHOR]

This study presents two enhancements at the Electric Reliability Council of Texas (ERCOT) for coping with the uncertainties and variations brought by high penetration of wind generation resources (GRs). The first enhancement is the incorporation of a novel short‐term wind forecast algorithm into the look‐ahead dispatch process to improve the calculation of locational marginal prices and generation schedules. The second enhancement is the development of a multi‐time‐scale method for quantifying regulation service requirements considering the combined impact of wind variations on governor response, automatic generation control, and economic dispatch results. A simulation model that is capable of simulating all relevant scheduling and operation processes from a few seconds to a few hours is developed to verify the performance of the proposed method. The ERCOT operational model and historical data are used to demonstrate that the derived regulation requirements can meet the ERCOT reliability needs by considering additional generation variations and uncertainties caused by wind GRs. The proposed methods are readily applicable to other regional power grids with high penetration of variable GRs. [ABSTRACT FROM AUTHOR]

Offshore wind farms connected to the mainland through high‐voltage DC links based on voltage source converters (VSC‐HVDC) are subject to grid code requirements, such as fault ride‐through (FRT) capability and dynamic voltage support. To address the challenge of FRT capability, sophisticated control strategies are required, capable of handling the power imbalance between the two interconnection ends during onshore grid faults. Τhis study proposes an FRT method which combines a de‐loading control strategy for the offshore wind turbines, utilising the communication infrastructure of the VSC‐HVDC system, with a DC chopper to dissipate the power surplus that cannot be effectively curtailed via the communication link. The impact of communication system latency on the expected FRT response and the required rating of the DC chopper is investigated using a linearised small‐signal model introduced in this study, whose results are validated against time‐domain simulations using detailed electromagnetic transient‐type models for the entire system. It is concluded that the required rating of chopper resistors can be substantially reduced when existing communication capabilities are exploited, even in the presence of relatively high communication delays. [ABSTRACT FROM AUTHOR]

Following from smaller‐scale investigations of grid‐forming converter control applied to wind turbines in 2017–2018, this study describes a much larger trial involving an entire wind farm, owned and operated by ScottishPower Renewables. To the authors' knowledge, this was the first UK converter‐connected wind farm to operate in grid‐forming mode, and the largest in the world to date. The 23‐turbine, 69 MW farm ran in the grid‐forming mode for 6 weeks, exploring inertia contributions between H = 0.2 s and H = 8 s. A number of unscheduled frequency disturbances occurred due to interconnector, combined cycle gas turbine (CCGT) and other trips, to which un‐curtailed turbines were able to respond. In addition, several deliberate tests were carried out. The turbines were able to provide a stable and appropriate response at relatively high inertia levels to the frequency events commonly occurring today. The captured responses stimulated a debate as to whether external damping power might be required in a grid‐forming converter, or whether internal damping is sufficient to allow stable and robust power‐sharing with parallel devices in all grid event scenarios. Analysis in this study suggests that, practically, internal damping is probably appropriate, and that any deficiency in external damping power can be more than mitigated by reactance and/or droop‐slope response‐time management in the grid‐forming converters. [ABSTRACT FROM AUTHOR]

The uncertain demeanour from wind generators and loads adversely affect the grid operational stability. Various control approaches have been explored to remedy the system uncertainties while maintaining generation and load demand balance. This study proposes a fuzzy‐based proportional–fractional integral–derivative with filter controller to sustain frequency stability in wind integrated power systems having different configurations. The controller parameters have been tuned using a recently developed coyote optimisation algorithm (COA). The proposed control approach is executed and validated on three distinct configurations of two‐area power systems. All test models are integrated with a doubly fed induction generator (DFIG) type wind turbine units (WTUs). Different case scenarios have been considered to analyse the efficacy of the proposed control strategy in the presence of WTU. Furthermore, the impact of inertial support delivered by the DFIG‐WTU and higher penetration of wind energy in the power system has been studied. The analysis reveals that the control scheme in coordination with WTU support reduces the stress on a wind turbine during the inertial control scheme and maintains the grid frequency stability under unexpected load disturbances. Stability and robustness analysis are also conducted to verify the validity of the introduced control approach. [ABSTRACT FROM AUTHOR]

The transmission lines between the sending and receiving ends of an interconnected power grid may be tripped under severe disturbance or equipment failure. The tripping will result in a large power imbalance in the sending grid, thereby causing frequency instability and even collapse. The effect of existing emergency control methods based on synchronous generators, such as generator tripping control, is limited due to the lack of available tripped generators, high cost and long recovery time. The high penetration and flexible controllability provide wind farms with the potential to participate in system emergency control. To evacuate excess power from the tripping of transmission lines and improve frequency stability, an emergency power balance control method based on the doubly‐fed induction generator (DFIG)‐based wind turbine (DFWT) is proposed in this study. A novel idea of the motor operation of DFWT is presented and applied to the proposed method. To realise steady and adjustable power consumption of DFWT under motor operation, a rotor virtual resistor control and supplementary pitch control are introduced under safety constraints. The validity of the proposed method is verified by case studies on single DFWT and DFWT‐based wind farms. [ABSTRACT FROM AUTHOR]

Doubly-fed induction generators (DFIGs) have drawn prominent interest in the field of wind power generation, but they are vulnerable to grid faults. Grid codes mandate DFIGs to employ a sort of fault ride-through (FRT) technique during faults. Fault current limiters (FCLs) always help to augment the FRT capability of DFIGs and a non-linear controller boosts their performances. In this study, a non-linear auto-regressive moving average-L2 (NARMA-L2) controller-based bridge-type flux coupling non-superconducting FCL (BFC-NSFCL) is proposed to enhance the FRT capability of the wind farm. The authors analysed the performance of the proposed NARMA-L2-based BFC-NSFCL (NL2-BFC-NSFCL) against that of the conventionally used series dynamic braking resistor (SDBR), bridge-type FCL (BFCL), and proportional–integral (PI) controller-based BFC-NSFCL (PI-BFC-NSFCL). They tested the performance of the NL2-BFC-NSFCL through multiple temporary and permanent fault scenarios and carried out the mathematical and graphical analysis in MATLAB/Simulink platform. They found that the proposed NL2-BFC-NSFCL's performance surpasses the performances of the SDBR, the BFCL, and the PI-BFC-NSFCL. Moreover, the NL2-BFC-NSFCL has faster system recovery capability after the occurrence of any fault than other competitors. [ABSTRACT FROM AUTHOR]

As a result of the increasing penetration of renewables in power systems, wind farms (WFs) have to satisfy the low-voltage ride-through (LVRT) requirement to stay connected to the power grid under fault conditions. This study proposes a novel method that uses the static synchronous compensator (STATCOM) to improve the LVRT capability of WFs during grid faults. The proposed method is based on the Taguchi method which involves orthogonal experiments. The allocation (locations and sizes) and fuzzy control gains of STATCOM are determined by the analysis of mean (ANOM) technique as part of the Taguchi method to achieve a robust design that is insensitive to variations of operating conditions and faults. The optimal STATCOM that is obtained by the proposed method is validated in both a single WF-infinite bus power system and the IEEE 39-bus power system. The simulation results show that the proposed method can enhance the LVRT capability of WFs. [ABSTRACT FROM AUTHOR]

Matching of the colossal energy demand with carbon‐free resources originates a global challenge to produce and innovate advanced energy technologies that generate clean and friendly energy supplies without threatening our environment. Nevertheless, with the increase in the penetration from renewable energy resources like solar and wind farms many technical issues are presented typically; power stability, power quality, and frequency fluctuations. The aforementioned problems stem from the intermittent nature of renewable resources which makes the generated power unpredictable and requires controlling and smoothing before dispatching. Conventionally low pass filter (LPF) is used with a controlled energy storage system to smoothen the fluctuated power due to its simplicity, but a delay problem increases with higher values of the filtering the time constant. This study suggests a smoothing controller‐based fuzzy logic that determines the value of the filtering time constant based on the present solar power ramp rate to avoid the battery overworking, which results in the predetermined configuration of the LPF. The presented simulation results prove the smoothing performance of the proposed controller and compare its performance to the LPF during clear, normal, and cloudy irradiance profiles. [ABSTRACT FROM AUTHOR]

In this study, a novel methodology is proposed for sensitivity‐based tuning and analysis of derivative‐based fast active power injection (FAPI) controllers in type‐4 wind turbine units integrated into a low‐inertia power system. The FAPI controller is attached to a power electronic interfaced generation (PEIG) represented by a generic model of wind turbines type 4. It consists of a combination of droop and derivative controllers, which is dependent on the measurement of the frequency. The tuning methodology performs parametric sensitivity to search for the most suitable set of parameters of the attached FAPI that minimises the maximum frequency deviation in the containment period. The FAPI is adjusted to safeguard system stability when increasing the share of PEIG. Since the input signal of the FAPI is the measured frequency, the impact of different values and parameter settings of the phase‐locked loop used for the FAPI controller is also investigated. Detailed validation with a full‐scaled wind power converter is also provided with a real‐time digital simulator testbed. Obtained simulation results using a three‐area test system, identify the maximum achievable degree of increase in the share of wind power when a proper combination of wind park locations considering their suggested settings for inertia emulation. [ABSTRACT FROM AUTHOR]