Weak Grid System Research Articles

A Self-Regulating Virtual Synchronous Generator Control of Doubly Fed Induction Generator-Wind Farms

The inverter-driven renewable energy sources (RESs), such as wind energy conversion systems (WECS), pose major threats toward system stability due to lack of inertia. Hence, virtual inertia concepts have gained popularity, for control and improvisation of the dynamic behavior of RESs, by simulating the kinetic inertia of the synchronous generator. This article focuses on developing an improved self-regulating virtual synchronous generator (VSG) control for grid-tied doubly fed induction generator (DFIG)-wind farms (WFs). The proposed scheme provides frequency support to the system while ensuring the low-voltage ride through (LVRT) capability at transient conditions, as per grid code requirements (GCRs). This has been achieved by introducing an additional control at grid side converter (GSC). This auxiliary control consists of a combined approach of VSG control and a current limiting approach. The VSG loop that alters the inertia of the system improves the frequency of the system and the current limiting loop provides the required inductance to limit fault current. This overall loop uses a self-regulating approach, and the developed concept helps to suppress the transients in stator current. The study obtained on a multimachine system and also for a weak grid system confirms the effectiveness and viability of the modified converter control structure.

Decentralized Grid-Forming Control Strategy and Dynamic Characteristics Analysis of High-Penetration Wind Power Microgrids

As wind power generation transits from centralized development mode to decentralized on-site consumption mode, microgrid (MG) can provide an efficient solution for wind power integration into the distribution network. However, the high-penetration wind power MG is the typical weak power grid system. The traditional wind turbine generator (WTG) participates in system frequency regulation through grid-following current source, which relies on the phase-locked loop for voltage phase synchronization and is unable to provide strong frequency support in weak power grid conditions. To fill this gap, this paper presents a decentralized grid-forming control strategy of high-penetration wind power MG. A wind power adaptive dynamic droop mechanism considering wind energy characteristics and rotor speed dynamic is proposed, cooperating with the implementation of wind maximum power point tracking (MPPT) for economical operation. A detailed small-signal model for voltage source wind power-based system considering electromechanical transients and adaptive droop mechanism is developed. The dominant modes are figured out and the critical control parameters are established and optimized. The proposed wind power adaptive droop mechanism can effectively provide frequency regulation and robust control performance. Theoretical analysis, time-domain simulation results and hardware-in-the-loop experiments under various scenarios verify the feasibility and effectiveness of the proposed strategy.

An Inductance Online Identification Method for Model Predictive Control of V2G Inverter With Enhanced Robustness to Grid Frequency Deviation

In this article, an inductance online identification method is proposed for model predictive control (MPC) of vehicle-to-grid (V2G) inverter with an enhanced robustness against grid frequency deviation. First, a full-order sliding mode observer is established to observe the virtual flux of the power grid. Second, a new inductance online identification method based on the dot product of the grid voltage vector and the observed virtual-flux vector is creatively proposed. A detailed theoretical analysis is carried out, which shows that the robustness of the proposed inductance identification method against frequency deviation can be enhanced by properly selecting sliding mode gains. Finally, by substituting the identified inductance into the MPC algorithm, the current control accuracy of the V2G inverter is improved accordingly. Detailed comparative experimental results verify the effectiveness and feasibility of the proposed strategy. When there is a frequency deviation of 5 Hz, which may occur in a weak grid system, the inductance identification errors of the conventional gradient correction method, the forgetting-factor least-squares method, and the direct calculation method are 2, 2.3, and 3 mH, respectively, while it is reduced to 1.75 mH for the proposed method.

Performance Enhancement of Grid Integrated Photovoltaic System Using Luo Converter

The main aim of this article is to implement a novel active power decoupling-based Luo converter for the weak grid system interfacing photovoltaic (PV) system. The modified Perturb and Observe (P&#x0026;O) algorithm has been used for maximum power extraction from the PV source. The proposed system is capable of operating in the standalone mode and the grid-connected mode. The Luo converter is promising with the viewpoint of low inrush current, high voltage gain, small ripples along with high power density, high efficiency, and simple structure. Hence, the Luo converter has been used for maximum power extraction from the PV source, which is appropriate for both grid-tied and islanding modes of operation. The modified P&#x0026;O algorithm has been used for maximum power extraction. The proposed MPPT technique reduces the drift problem, which occurs in the conventional MPPT methods by including the data of change in current (<inline-formula> <tex-math notation="LaTeX">$\Delta I$</tex-math> </inline-formula>) in addition to the data used in the conventional P&#x0026;O algorithm. This extraction has achieved by using a novel decoupling method, which, in turn, reduces the size of the filter. The converter along with the decoupling method also ensures reduced harmonics at the grid output. The operational principle and the stability analysis of the proposed converter have been analyzed in different modes of operation, such as grid-connected mode and standalone. The simulation has been carried out in MATLAB/Simulink environment. The implementation of the converter is carried out using a microcontroller-based prototype model to validate the performance. The efficiency of the proposed converter has been calculated to be 87.53% when operating under the grid-connected mode, whereas the efficiency is 92.46% when operating under the standalone mode.

A study of a single phase grid connected pv inverter performance under a weak grid conditions and distorted grid voltage for Cambodia

&lt;span lang="EN-US"&gt;A single-phase grid-connected PV inverter performance under a weak grid is a model designed to penetrate PV energy with a weak grid. Usually, this model gets complex and unstable in power system control such as THD growth, harmonic effects, voltage surges, inverter performance. Experimental results would present the impacts on the system which would lead to instability in the grid system. This study was based on inverter performance control, a weak grid control, the grid distortion examination, and harmonic effects. To optimize this grid system, hence, the proposed methods of oscilloscope and power meter were proportionally used to control the grid impacts and stabilities. The results showed that current THD in distorted grid voltage system surged to 8.88%. V&lt;sub&gt;PCC&lt;/sub&gt; growth was 238.11V, equivalent to 8.23% in a weak grid (X&lt;sub&gt;L&lt;/sub&gt; and R). Such a huge increase could prevent the grid system from transferring power for later operation. During the experiment, the inverter, S power and Q power performances stood for good operating processes without impacts on the grid performance. This method should be applied with a weak grid system because it provided information about grid stability control.&lt;/span&gt;

Designing and Analysis of Single Stage and Two Stage PV Inverter Connected to Weak Grid System

In this research paper design, analysis and comparison of single stage and two stages Photovoltaic inverter connected to weak grid system is executed in terms of their maximum power point tracking, DC link voltage regulation, power factor and overall efficiency. Majority of the commercial and industrial loads are inductive in nature and result in a very low lagging power factor. However renewable energy sources have no reactive power generation and lagging power factor results in a weak grid system. For this purpose control mechanism comprises of three objectives is proposed in this research paper. These objectives are to obtain highest amount of power from photovoltaic array, the power must be deliver from photovoltaic array into the utility grid at unity power factor and to maintain desired voltage at the input of the inverter. In order to achieve these objectives nonlinear control mechanism of Photovoltaic inverter connected to weak grid system is established and implemented based on accurate mathematical modeling and by using Backstepping technique and Lyapunov Stability analysis. PI controller is used for the purpose to maintain desired voltage at input of the inverter according to the requirement of inverter. Both single stage and two stage models are developed and simulated in Simulink/Matlab environment.

Performance of three phase AI controller based UPQC to enhance power quality of hybrid RES

Day-by-day to reduce the greenhouse effects and increase the power generating capacity the dependency of renewable energy systems are increasing. Due to its uncertainty and continuously changing characteristics introducing power quality issues into the power system. The challenging task for electrical engineers is to produce quality of electrical power with less carbon emissions, especially in a weak grid systems. In this paper, built a new featured hybrid technological concept of renewable energy generating resources to reduce the greenhouse carbon emissions. For this, developed mathematical models of two renewable energy sources, proton exchange membrane fuel cell and wind energy system with a squirrel cage induction generator. To enhance the quality of power built a most promising custom power devise, an Artificial Neural Network controller based Unified Power Quality Conditioner. Further, presented performance of an Articial Neural Network Controller based unied power quality conditioner: to handle the voltage variations due to wind fluctuations, to enhance quality of power due to sags and swells.

Voltage Source Converters Connected to Very Weak Grids: Accurate Dynamic Modeling, Small-Signal Analysis, and Stability Improvement

The interfacing of the vector-controlled voltage-source converters (VSCs) into weak grid (WG) systems can induce severe instabilities. This is attributed to interactions between the vector control of the VSCs and the high impedance of the WG. The weak connection limits the amount of active power that can be injected by the VSC to the WG. In this work, the small-signal analysis is used to derive; first, a transfer function that helps to design the controller of the VSC output voltage based on the VSC-WG accurate dynamics; second, the full-order state-space model of the VSC-WG system. A modal analysis is then conducted to develop the participation factors to characterize the influencing states on the dominant modes of the system, followed by a sensitivity analysis to evaluate the influence of the vector control gains and other parameters on the dominant modes. The results of the modal analysis confirm that instabilities at the nominal power are inevitable for VSCs connected to very weak grids (VWGs). Inspired by this shortcoming, a novel compensation method is proposed to mitigate the dynamic instability of VSCs connected to VWGs. Finally, several offline time-domain simulations and hardware-in-the-loop (HIL) real-time experiments are conducted to verify the validity of small-signal analysis, validate the effectiveness of the proposed compensation method; and investigate the performance of the system under challenging scenarios such as sudden variations in the phase angle of the grid.

Framework of Gradient Descent Least Squares Regression-Based NN Structure for Power Quality Improvement in PV-Integrated Low-Voltage Weak Grid System

This paper proposes a novel gradient descent least squares regression (GDLSR)-based neural network (NN) structure for the control of grid-integrated solar photovoltaic (PV) system with improved power quality. Here, a single-layer neuron structure is used for the extraction of fundamental component (FC) from the load current. During FC extraction, GDLSR-based NN structure attenuates harmonic components, noise, dc offsets, bias, notches and distortions from the nonlinear current, which improves the power quality under normal as well as under abnormal grid conditions. This single-layer GDLSR-based NN structure has a very simple architecture, which decreases the computational burden and algorithm complexity. Therefore, it is easy in implementation. In this work, the GDLSR-based control technique is tested on a single-phase single-stage grid-integrated PV topology with the nonlinear loads. The prime objective of the GDLSR-based NN structure is to provide reactive power compensation, power factor correction, harmonics filtering and mitigation of other power quality issues. Moreover, when solar irradiation is zero, the dc-link capacitor and voltage-source converter act as a distribution static compensator, which enhances the utilization factor of the system. The proposed system is modeled, and its performances are verified experimentally on a developed prototype in different grid disturbances as well as solar insolation variation conditions, which performances have satisfied the motive of the proposed technique as well as the IEEE-519 standard.

Integration of Solar PV With Low-Voltage Weak Grid System: Using Normalized Laplacian Kernel Adaptive Kalman Filter and Learning Based InC Algorithm

This paper proposes a novel normalized Laplacian kernel adaptive Kalman filter (NLKAKF) based control technique and learning based incremental conductance (LIC) maximum power point tracking (MPPT) algorithm, for low-voltage weak grid-integrated solar photovoltaic (PV) system. Here, a two-stage topology of three-phase grid integrated solar PV system is implemented, where the loads are connected at the point of common coupling. Proposed LIC is the improved form of incremental conductance (InC) algorithm, where inherent problems of traditional InC technique, such as steady-state oscillation, slow dynamic responses, and fixed step size issues, are successfully mitigated. The prime objective of proposed NLKAKF control is to meet the active power requirement of the loads from generated solar PV power, and after feeding load, excess power is fed to the grid. However, when generated PV power is less than the required load power, then NLKAKF control meets the load by taking extra required power from the grid. During this process, power quality is improved at the grid. The controller action provides reactive power compensation, power factor correction, and harmonics filtering and mitigation of other power quality issues. Moreover, when the solar irradiation is zero than voltage source converter acts as a distribution static compensator (DSTATCOM), which enhances the utilization factor of the system. The proposed techniques are modeled and their performances are verified experimentally on a developed prototype, in solar insolation variation conditions, unbalanced loading, as well as in different grid disturbances such as overvoltage, undervoltage, phase imbalance, harmonics distortion in the grid voltage, etc.

Integration of Solar PV With Low-Voltage Weak Grid System: Using Maximize-M Kalman Filter and Self-Tuned P&amp;O Algorithm

This paper introduces a novel self-tuned perturb and observe (SPO) algorithm for quick maximum power point tracking (MPPT) and a novel maximize-M Kalman filter (MMKF)-based control technique for optimal operation of grid-integrated solar photovoltaic (PV) energy conversion system, where linear/nonlinear loads are attached at point of common coupling (PCC). The proposed SPO is the improved form of perturb and observe (P&O) algorithm, where inherent problems of traditional P&O such as steady-state oscillation, slow dynamic responses, and fixed step size issues, are successfully mitigated. Therefore, SPO tracks maximum power peak (MPP) very rapidly, and it very accurately extracts maximum power from the PV array. The extracted power is used to meet the active power requirement of loads, and after meeting the load demand, the excess power is supplied to the grid. During power feeding, the power quality and power management are maintained by the MMKF-based control technique. In control strategy, the MMKF is used for fundamental harmonic component extraction from the grid voltage and load current, even when the grid voltage is characterized by adverse situations, such as sag, swell, harmonic distortion, dc offset, etc. Here, the SPO MPPT algorithm and MMKF-based control techniques are tested on a developed prototype. The efficient and reliable performances of SPO MPPT algorithm and MMKF-based control algorithm, in dynamic as well as in steady-state condition, are demonstrated under insolation variation conditions, nonlinear loading, as well as in different grid disturbances such as overvoltage, undervoltage, phase imbalance, harmonics distortion in the grid voltage, etc.

Feedback linearization based control for weak grid connected PV system under normal and abnormal conditions

This paper proposes a control strategy for interface of distributed energy sources into the weak grid system with a focus on the energy and ancillary services. A novel controller has been designed and implemented to tackle the challenges of coupling terms in the LCL filter, the transient behavior under sudden changes, and the voltage support under fault condition using the feedback linearization technique. The controller proposed has been implemented on the PV system connected with the weak grid using the LCL filter and the performance of the controller has been verified using Matlab/Simulink through simulation under different conditions. The results of the controller proposed have been compared with the conventional PI dual loop controller. The simulation results obtained demonstrate the effectiveness and simplicity of the controller design strategy.

약한 계통에 연계된 풍력발전기의 과도안정도 해석

The margin of transient stability of a weak grid system is very low because of the small short-circuit ratio and large impedance. If the fault of the weak grid is cleared by the protection system, one of the major lines is disconnected after the fault is cleared. This further reduces the system strength. Therefore, it is necessary that a new generation system be added to the weak grid to enhance the transient stability margin. A conventional synchronous generator and wind turbine generator were added to a base grid system. The results of transient stability analysis with additional generators using PSSE were compared. The simulations showed that wind turbine generators provide good damping performance and enhance the transient stability margin based on CCT up to 5 times.

Control of variable pitch and variable speed direct‐drive wind turbines in weak grid systems with active power balance

In the case of operating in a weak grid system, when wind power becomes a significant portion of the power system or even the sole energy source, the wind power generators and converters are expected to help maintain the grid voltage. The grid-side converter needs to work as a voltage source to help regulate the terminal (grid) voltage amplitude and frequency by adjusting the reactive power and active power flow, respectively. For a direct-drive permanent magnet synchronous generator with a full power converter, the active power must be provided by the captured wind power. The active power flow between the source (captured wind power) and the grid (load) must be balanced by actively controlling the generator speed and wind turbine pitch angle. In the study, the coordinated control of generator speed and blade pitch angle is proposed together with a dc-link voltage controller. A model of the grid-side converter operating as a voltage source has been built and the strategy regarding voltage and frequency regulation is presented. Simulation is carried out with different wind and load profile. The results show the wind energy can help support the weak grid and power the local grid in stand-alone mode as well.

DFIG 풍력발전기가 연계된 배전선로의 고조파 공진 특성에 관한 연구

There were telecommunication noise and malfunctions of the electronic devices occurred over a wide area due to the high harmonic voltage and/or current levels of the Back-to-back converter in the DFIG wind power system even though the magnitude of all harmonics is within the international standards. The triangular carrier signals of the PWM used in the power converter system is related to the telecommunication noise because they are in the range of audible frequencies and amplified by a variety of the standing waves that were excited by harmonic voltage sources in the weak grid system such as a long distance distribution transmission lines. This paper describes the characteristics of the harmonics in the wind turbine-generator, numerical analysis and simulation of the harmonics resonance phenomena in the distribution lines as well as measuring induced voltage of the telecommunication lines in parallel with power lines in order to verify the root cause of the telecommunication noise. These noise problems can occur in a wind turbine power system with a non-linear converter at any time, as well as photovoltaic power system. So, the preliminary review of suitable filter devices and switching frequencies of the PWM have to be required by considering the stability of the controller at the design stage but as part of the measures the effect of the telecommunication cable shields was analyzed by comparing the measured data between multi-conductor with/without shields so as to attenuate the sources of the harmonics voltage induced into the telecommunication lines and to apply the most cost-effective measures in the field.

Stability Analysis of Grid-Connected Inverter in Weak Grid Systems

Distributed Generation may be located far away from load centers due to their use of new energy resource distribution characteristics. In this case, Long distance transmission lines will introduce an equivalent impedance to the grid, so that the grid showing the characteristics of a weak grid. In this paper, the interaction between grid-connected inverter and weak grid is studied by means of impedance analysis in the frequency domain. The experimental results are used to evaluate the stability of grid-connected inverter under different grid impedance condition. In a weak grid system, the increasing grid inductance results in a smaller stability margin of impedances ration. To improve the stability of the grid-connected inverter, the paper introduces a kind of active damping method with a virtual resistor in parallel with the capacitor. The new active damping control scheme without differential factor has a simple algorithm. The experimental results are depicted to demonstrate the correctness of theoretical analysis and the feasibility of the active damping algorithm.

Flicker Suppression Scheme for Variable-Speed Wind Turbine Systems

This paper proposes a strategy of flicker mitigation for doubly-fed induction generator (DFIG) wind turbine systems. In the weak grid system where the grid impedance ratio is low, the reactive power compensation only cannot suppress the flicker sufficiently due to the limited power capacity of the converters or the DFIG. For the full suppression of flickers, the active power smoothening using the energy storage system (ESS) needs to be utilized together with the reactive power compensation. The effectiveness of the proposed method is verified by PSCAD/EMTDC simulation results for a 2[MW] DFIG wind turbine system and by experimental results for a 3[kW] wind turbine simulator.

Fuzzy Logic Control of DFIG to Eliminate Unbalances in Weak Grid System

Fuzzy Logic Control of DFIG to Eliminate Unbalances in Weak Grid System

Studies on the Grid-Connected Characteristics about Large-Scale Photovoltaic Power Station in Weak Grid Systems

This paper made some researches of modeling on the systems of the large-scale photovoltaic（PV） power station, and established the PV model, the rectifier and inverter circuits and so on. Using MATLAB and PSASP, simulation and analysis of grid-connected characteristics about the large-scale PV power station in weak grid systems were carried out, focusing on the transient stability in the fault condition of the line-side and the power quality. These two simulation results had good consistency, which demonstrated the accuracy of the large-scale PV power station model in this paper and the analysis of its grid-connected characteristics. The conclusion of this paper will provide some theoretical basis of improving the stability of grid-connected and reducing the harmonic current in weak grid systems, and will also provide some theoretical basis and the guide of grid-connected for the new energy strategy of our country in the future.

Five-phase induction motor drive for weak and remote grid system

Multi-phase (more than three-phase) motor drive systems have attracted much attention in recent years due to some inherent advantages which they offer when compared to the three-phase counterpart. Presently the grid power available is only limited to three-phase so the supply to multi-phase motors is invariably given from power electronic converters. Thus the paper focuses on the inverter controlled five-phase induction motor drive system for variable speed applications. The paper presents inverter control method for five-phase variable speed induction motor drives. The proposed solution may be employed in the applications not requiring very precise position and speed control such as water pumping especially in weak grid system with poor power quality. The inverter is operated in different operating modes with conduction angle varying from 180° to 108° conduction modes and the performance is evaluated in terms of the harmonic contents in the output phase voltages . It is shown that optimum performance is achieved by operating inverter at 144° conduction mode. Experimental and analytical results are included in the paper. Keywords: Voltage source inverter, ten-step operation, conduction modes, power grid, Induction motor drive