Abnormal Grid Conditions Research Articles

CC‐ROGI‐FLL based control for grid‐tied photovoltaic system at abnormal grid conditions

This study presents a single stage grid-interfaced solar photovoltaic (SPV) system using a multifunctional complex coefficient reduced ordered generalised integrator (CC-ROGI) based frequency-locked loop (FLL) control. The main contributions of this study, include (i) the extrication of fundamental active components from load currents using CC-ROGI-FLLbased control to generate the reference grid currents for voltage source converter (VSC) switching, (ii) VSC works as a distribution static compensator, which performs multi-functions like harmonics mitigation, reactive power compensation, unity power factor operation and grid currents balancing, (iii) this CC-ROGI-FLL also extricates the fundamental grid voltages component even under grid voltage harmonic distortion and grid voltages unbalancing. Moreover, SPV power feed-forward component is assimilated in this control to reduce the oscillations in grid currents under variable irradiance. Simulated and experimental results show satisfactory behaviour of the system at different conditions like load unbalancing, variable solar irradiance and abnormal grid conditions. The comparative performance of CC-ROGI-FLL control with other FLL and phaselocked loop controls are shown in this study. This CC-ROGI-FLL control has manifested a good steady state performance and dynamic response. The total harmonic distortions of grid currents are found well within the limits of the IEEE-519 standard, even under abnormal grid conditions.

Modified rotor-side converter control design for improving the LVRT capability of a DFIG-based WECS

The rapid integration of wind-power generation with existing power grids has caused reliability and stability concerns owing to the negative impact on the dynamic behaviors of power systems. During a fault, large electromotive force is induced in the rotor circuit of a doubly fed induction generator (DFIG) as the circuit is highly vulnerable to it. Such circumstances increase the importance of the low-voltage ride-through (LVRT) capability of a DFIG to ensure stability of the electric grid during transient conditions. Considering these factors, this study focuses on the mitigation of rotor overcurrents and DC-link voltage variations by modifying the control structure of the DFIG converter, thereby enhancing its LVRT capability. Additional voltage terms are injected into the rotor-voltage references to improve the transient behavior of the DFIG control system. In the proposed design, transient rotor currents and DC-link voltage variations are effectively suppressed. Because the voltage terms are introduced outside the current loops, there is no impact on their stability. Furthermore, electromagnetic torque oscillations during the faults are considerably suppressed. Finally, the validity of the proposed design during abnormal grid conditions is demonstrated via MATLAB/Simulink. The results confirm the feasibility and effectiveness of the improved converter control design to enhance the LVRT capability of a DFIG.

Utility-scale photovoltaic generators: a review on trends, grid code requirements and challenges

<p>This paper provides an overview of the global trends in utility-scale photovoltaic (PV) installed capacity. This paper also presents a comparison of grid-connection requirements of six countries in the continents of Europe, Asia, Africa, and Australia for utility-scale PV generators in normal and abnormal grid conditions. Many country-based grid codes and international standards (IEEE 1547) for interconnection of inverter-based renewable energy generators are demanding stricter grid-connection compliance from utility-scale PV generators to ensure power system safety and reliability as its penetration level increases. This paper then discusses the economic and technical impacts, and explores the preparedness of PV generation systems in meeting the grid code requirements.</p>

Self-Adjustable Step-Based Control Algorithm for Grid-Interactive Multifunctional Single-Phase PV-Battery System Under Abnormal Grid Conditions

This article deals with the single-phase grid interactive multifunctional solar PV (photovoltaic) system with seamless power transfer capability. This multifunctional PV-battery system is also helpful to enhance the quality of power in the utility grid. The system provides an uninterrupted power to nonlinear loads during the nonavailability of utility and renewable energy resource. This PV-battery system comprises two stages. First stage is a PV array-boost converter along with a battery energy storage-bidirectional converter that is integrated at dc-link and another is voltage source converter (VSC), which performs the task of harmonics elimination and injection of harvested energy to the utility along with nonactive power compensation. In standalone mode, the shape of load voltage is sustained sinusoidal by islanded technique. A proportional integral controller is used to sustain the dc-link voltage to a specific value. In order to improve the dynamic response of the PV-battery system, a feedforward control for solar PV participation is used. A self-adjustable step-based control is introduced for the VSC in this work. This estimates the real power reflecting portion of the load current in order to show the features of the PV-battery system. Simulation studies are carried out in MATLAB Simulink. The experimental performance of the developed PV-battery system is found satisfactory even under abnormal grid condition.

A fast hybrid PLL with an adaptive all-pass filter under abnormal grid conditions

Recently, various phase locked loop (PLL) methods have been improved for a reliable and robust synchronization under non-ideal grid conditions. One of the PLLs is a hybrid PLL (HPLL) which is an improved structure of the quasi-type-1 PLL (QT1-PLL) method. In this paper, an enhanced HPLL that offers both fast transient response and increased disturbance rejection capability for dominant harmonics is proposed. The proposed PLL includes adaptive all-pass-filters (APFs) in pre-filtering stage to eliminate fundamental frequency negative sequence (FFNS) component. Thus, window-width of the moving average filter (MAF) is reduced by three times compared with the HPLL by means of removing the FFNS component. So, the proposed PLL, which is called fast-hybrid PLL (FH-PLL), considerably ensures a fast transient response. Also, in case of grid faults (for example, type-B, type-C, type-D, and etc.), at the same time frequency change occurs, the FH-PLL completely blocks the FFNS component by means of using adaptive DAPF. In this study, it is experimentally implemented HPLL, QT1-PLL, and the proposed FH-PLL methods. The effectiveness of the FH-PLL is verified owing to experimental results and comparison with the QT1-PLL and HPLL.

Control of Grid Fed PV Generation Using Infinite Impulse Response Peak Filter in Distribution Network

An upsurge in the integration of renewable sources to the utility grid has resulted into escalating concerns regarding power quality (PQ) improvement. In order to provide efficient operation under weak grid conditions, the necessity of a competent control technique is dominant. Consequently, the implementation of an infinite impulse response (IIR) peak filter is used here for harmonics mitigation, power factor correction, and alleviating other PQ issues by providing distribution static compensator (DSTATCOM) capabilities. In addition, this control technique also works during conditions when solar photovoltaic (PV) power is not procurable. During day, the requirement of load is fed from the solar PV array and the stored grid power. On the other hand, at night when the power from solar PV array is unavailable, the requirements of load are fulfilled from the grid and DSTATCOM operation ensures power quality improvement. In order to validate this system, it is maintained according to the IEEE-519 standard. The considered weak grid conditions include load unbalancing, intermittent solar insolation, voltage sag, voltage swell, voltage distortion, and voltage unbalance conditions. The benefits of IIR peak filter include adaptive computation during filtering, thereby alleviating the computational complexity. Moreover, less memory space is required during execution guaranteeing fast performance during weak grid conditions. Simulated results are utilized to depict the behavior of the system during abnormal grid conditions. Moreover, its comparative performance with the existing control techniques, is also presented. For further substantiation and in order to observe reliable operation, a laboratory prototype is developed and tested during weak grid conditions such as voltage distortion, voltage unbalance, voltage sag, voltage swell, and load unbalancing.

Utility Grid Interfaced Solar WPS Using PMSM Drive With Improved Power Quality Performance for Operation Under Abnormal Grid Conditions

Owing to the intermittency issue associated with renewable energy sources, a solar photovoltaic (PV) array fed grid interfaced encoder-less permanent magnet synchronous motor (PMSM) based solar water pumping (SWP) system is presented in this paper. The grid integration enables a continuous water flow regardless of available solar insolation. However, it has been observed that the grids in developing and underdeveloped countries are mostly weak. Moreover, as SWP systems are typically established at remote locations, which are generally the grid radial ends, therefore, prone to various grid abnormalities. Such abnormalities affect the SWP system performance. Therefore, a hybrid multi-resonant generalized integrator-frequency locked loop (HMRGI-FLL) control structure is presented in this paper to deal with the abnormalities in grid voltage and makes the SWP system operational even under weak grid condition. The proposed structure not only rejects the dc-offset but it also eliminates the dominant lower order harmonic with its selective harmonic elimination capability. A boost converter is used on the grid side to facilitate the power transfer from the grid. In condition of grid failure, the output water flow is regulated in accordance with available solar insolation. The speed of the PMSM is regulated using sensor-less vector control. Performance of the proposed system is experimentally validated using a laboratory prototype under varying solar insolation, during grid failure, during voltage sag, voltage swell, and distorted grid voltage conditions.

Performance evaluation and benchmarking of PLL algorithms for grid‐connected RES applications

The phase-locked loop (PLL) is the main controller element for the fast and accurate synchronisation and operation of grid-connected renewable energy systems (RESs). It is used to extract the grid voltage information such as the phase angle, the frequency and amplitude. Subsequently, this information is used in the control system of the grid-side converter of the RES. The performance of the PLL is critical under abnormal grid conditions such as in the event of balanced and/or unbalanced faults, frequency and phase variations, the presence of harmonics, interharmonics and DC offset. This study sets out with a benchmarking study of the four latest state-of-the-art PLLs. The PLLs compared are the decoupled dual synchronous reference frame PLL, the decoupling network designed in αβ-frame PLL, the enhanced pre-filtering moving average filter type-2 PLL and the harmonic–interharmonic DC-offset PLL. The PLLs are analysed and compared based on their performance, their dynamic response and their computational complexity. The benchmarking concludes with a PLL selection guide depending on the application and other system constraints. Experiments and simulation results are presented to compare and analyse the performance of the selected PLLs.

Single phase NTD PLL for fast dynamic response and operational robustness under abnormal grid condition

Non-frequency-dependent transfer delay (NTD) approach is one of the simplest and the fastest solution for single-phase AC systems to achieve phase-locked loops (PLLs). It also exhibits immunity to error in estimated quantities in grid frequency variations. However it is susceptible to grid voltage disturbances, such as DC offset and distortions. This paper proposed an improved NTD scheme to achieve both fast dynamic response and operational robustness against abnormal grid conditions. The solution is based on a digital phase lead compensator (PLC), which is cascaded with the MAF inside the PLL control loop to enhance dynamic response and mitigate disturbance, DC offset, and harmonics. The filtering structure is also designed to be adaptive to grid frequency variations. The effectiveness of the proposed solution is verified via simulation and experimental results.

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.

Grid Integration of Three-Phase Single-Stage PV System Using Adaptive Laguerre Filter Based Control Algorithm Under Nonideal Distribution System

The increasing integration of utility with renewable sources of energy has resulted into serious issues of power quality (PQ) deterioration primarily in a scenario of weak distribution grid. In order to improve the PQ, efficient implementation of a control algorithm for the solar energy conversion system interfaced to the grid is paramount. This article illustrates the usage of an adaptive Laguerre filter based control technique for optimal operation providing distribution static compensator capabilities along with load balancing, power factor correction and harmonics mitigation. Moreover, it provides the active power transfer to the grid together with feeding the nonlinear and linear loads. In addition, the system performance according to an IEEE-519 standard has been verified, hence it is proficient in maintaining the PQ. The Laguerre filter offers the benefits of reduced computational time in addition with the alleviation in model complexity predominant during abnormal grid conditions. By utilizing maximum power point tracking, the efficient utilization of solar PV array is accomplished with a perturb and observe method. For validating performance of the proposed system, it is examined through simulation results. Furthermore, a prototype is developed for validation and test results corroborate reliable operation under nonideal grid conditions comprising of wide range of load variations, voltage sag, swell, distortion, and unbalance conditions.

Real-time simulator based hybrid controller of DFIG-WES during grid faults design and analysis

This paper presents the significant contribution of the combination of both vector control and direct torque control (HC) strategies for DFIG based WECS. The DTC is applied to rotor side converter (RSC) to control the rotor flux and generator speed. It also controls both active and reactive power components indirectly. Whereas, VC is applied to grid side converter (GSC) to control the DC-link voltage and also to improve the quality of power supplied into the utility grid. The greatest advantage of the proposed control is that the DTC and VC are de-coupling in nature, so that the rotor side control is independent of stator parameter variations. Especially, HC successfully tackles the problems related to voltage dip, LVRT and FRT. The simulations for the proposed VC-DTC strategy are carried out in MATLAB/SIMULINK environment and the results indicate that the proposed VC-DTC technique provides excellent dynamic response in terms of DC voltage and current transients, and smoothing of generator speed during grid disturbance. Further, the torque, flux and current ripples under steady state conditions are also minimized. The proposed HC doesn’t require any auxiliary protection circuit, which indicates that the complexity of the control scheme is reduced during abnormal grid conditions. Finally, the real time simulation of the proposed HC strategy is carried out on OPAL-RT (OP5700) real time simulator. The simulation and RTS results demonstrate the superiority of the proposed SMC based HC when compared to the conventional PI controller based DFIG WECS.

A Multiobjective GI-Based Control for Effective Operation of PV Pumping System Under Abnormal Grid Conditions

The increasing renewable energy penetration has encouraged solar water pumping (SWP) as an effective solution to escalating water demand in remote areas. Despite receiving wide-spread popularity, such systems exhibit an unreliable operation due to intermittent weather conditions. As the utility grid integration enables the bidirectional power flow, it has emerged as a viable solution for uninterrupted pumping. However, since most of the SWP systems are remotely installed at radial ends of the distribution network, they encounter problems associated with abnormal grid conditions, which degrade their performance. A generalized integrator (GI) is used to deal with these issues. However, convention GI structures are incapable of effectively mitigating them. Therefore, a multiresonant cascaded second-order generalized integrator (MC-SOGI) based control approach is developed in this article to deal with these issues. The proposed MC-SOGI structure provides effective dc-offset rejection along with filtering of harmonics as well as subharmonics. Therefore, the proposed control approach is capable of effectively extracting the balanced fundamental positive sequence components even under unbalanced distorted grid voltages and maintaining unity displacement power factor under all conditions. An enhanced system efficiency is achieved by employing a permanent magnet synchronous motor (PMSM) for driving the pump. The well-known vector control technique and an incremental conductance algorithm are used for controlling the speed of PMSM and optimum power harvesting, respectively. The estimation of rotor speed and position is carried out by using voltage and current templates for eliminating the encoder thereby enhancing the system reliability and reducing the cost. The system performance is investigated experimentally and the acquired results validate the system feasibility.

Seamless Control of Solar PV Grid Interfaced System With Islanding Operation

This paper presents a robust control strategy for a solar photovoltaic (PV)-based distributed generation system (DGS) with seamless transition capabilities from islanded to the grid-connected mode and vice versa. The proposed DGS consists of a solar PV array, a dc–dc boost converter, voltage source converter (VSC), and local nonlinear loads. In grid-connected mode, VSC regulates the dc-link voltage and the boost converter operates the solar PV array at the maximum power point. Moreover, the load reactive power compensation and harmonics elimination with unity power factor operation are achieved using the advanced robust shrinkage normalized sign (ARSNS)-based control algorithm. Therefore, the grid current distortion is maintained within the IEEE-519 standard and the IEEE-1547 standard. Under the grid fault condition, the proposed DGS operates in an islanded mode without any storage unit. The grid synchronization and resynchronization operations are executed through the intelligent synchronization control (SYC) algorithm with fast Fourier transform phase-locked loop (FFT-PLL). Test results demonstrate the system capabilities under the abnormal grid and unbalanced nonlinear load conditions.

Delta-Bar-Delta Neural-Network-Based Control Approach for Power Quality Improvement of Solar-PV-Interfaced Distribution System

A serious concern regarding deterioration in power quality has emerged with the increasing integration of solar photovoltaic (PV) energy sources to the utility primarily in the scenario of a weak distribution grid. Therefore, power quality improvement of the grid-tied solar energy conversion system is paramount by implementation of a robust control technique. This paper deals with a delta-bar-delta neural network (NN) control for operating optimally by feeding active power to the loads and remaining power to the grid as a function of distribution static compensator capabilities, such as mitigating harmonics, balancing of load, and improving power factor. The control algorithm provides the ability to adjust weights adaptively in an independent manner, and hence, it offers alleviation in model complexity predominant during abnormal grid conditions along with reduction in computational time. Moreover, the NN-based control technique offers enhanced accuracy due to the combinational neural structure in the estimation process. In addition, the system performance according to the IEEE-519 standard has been verified; hence, it is proficient in maintaining the power quality. The solar-PV-array-efficient utilization is accomplished through an incremental-conductance-based maximum power point tracking technique. For validating the behavior of the proposed system, its performance is studied using simulation results. Moreover, a prototype is developed for validation, and experimental results corroborate reliable operation under nonideal grid conditions comprising of a wide range of load variations, voltage sag, and varying solar insolation conditions.

Voltage Support under Grid Faults with Inherent Current Limitation for Three-Phase Droop-Controlled Inverters

A novel nonlinear current-limiting controller for three-phase grid-tied droop-controlled inverters that is capable of offering voltage support during balanced and unbalanced grid voltage drops is proposed in this paper. The proposed controller introduces a unified structure under both normal and abnormal grid conditions operating as a droop controller or following the recent fault-ride-through requirement to provide voltage support. In the case of unbalanced faults, the inverter can further inject or absorb the required negative sequence real and reactive power to eliminate the negative sequence voltage at the point of common coupling (PCC) whilst ensuring at all times boundedness for the grid current. To accomplish this task, a novel and easily implementable method for dividing the available current into the two sequences (positive and negative) is proposed, suitably adapting the proposed controller parameters. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the total grid current remains limited below its given maximum value under both normal and abnormal grid conditions. Asymptotic stability for any equilibrium point of the closed-loop system in the bounded operating range is also analytically proven for first time using interconnected-systems stability analysis irrespective of the system parameters. The proposed control concept is verified using an OPAL-RT real-time digital simulation system for a three-phase inverter connected to the grid.

TLBO-Optimized FOPI Controller for Three-Phase Active Rectifier Using ZDPC Technique

Traditional direct power control (DPC) method is a simple and proficient way to control the three-phase active rectifiers, but its response is unacceptable when the power converter is subjected to abnormal grid conditions. This paper proposes the zero direct power control (ZDPC) methodology with teaching–learning-based optimal fractional order proportional integral (FOPI) controller to improve the performance of DPC method under unbalanced and harmonically polluted grid voltage conditions. In the proposed approach, a better performance is obtained by directly controlling the instantaneous active and reactive powers of the harmonic component of currents considering a predefined lookup table. In the proposed technique to achieve the complete elimination of the impact of the grid disturbance, the reference of both active and reactive powers is explicitly provided from outside of the controller and is set to zero. The teaching–learning-based optimized (TLBO) FOPI controller is utilized for better regulation of dc bus voltage. Additionally, the proposed approach does not require any prior familiarity with disturbance behavior, sequence components of voltages and currents. Also, it does not need harmonic component extractions. In the proposed control technique, a simple phase-locked loop (PLL) is utilized to extract the fundamental component of currents and to obtain the angular position of the grid voltage space vector in the stationary $$\upalpha $$ - $$\upbeta $$ reference frame. The simulation results have confirmed the supremacy of the proposed ZDPC with optimal FOPI under unbalanced and harmonically contaminated grid conditions as compared to conventional DPC.

A modernistic PLL based on feed forward frequency estimator with selective harmonic pre filter for grid imperfection

This paper describes a synchronisation technique to track phase and frequency of input signal under abnormal grid condition like frequency variation. Proposed control topology consist of a conventional synchronous reference frame phase locked loop (SRF PLL) type-2 along with a feed forward frequency estimator loop with selective harmonic filtering technique. It eliminates phase and frequency error under wide frequency deviation. Phase error is fed into proportional integral regulator which reduces it to zero. It is observed that faster and better dynamic performance is achieved by frequency estimator with higher stability margin. The stability is evaluated by bode plot. An estimated frequency is smooth and close to fundamental frequency as it uses the information from all three phase to estimate a single value for frequency. The feed forward action is implemented with pre filtering stage to improve dynamic performance of system. Simulation results are presented to validate effectiveness of proposed technique.

GI-Based Control Scheme for Single-Stage Grid Interfaced SECS for Power Quality Improvement

This paper presents an improved generalized integrator (GI)-based control with a frequency locked loop for multifunctional three-phase single-stage grid interfaced solar energy conversion system for power quality enhancement of the distribution network under abnormal grid conditions. The perturb and observe-based maximum power point tracking technique is utilized to obtain peak power from solar photovoltaic array under varying atmospheric conditions. This control scheme provides unity power factor operation, load balancing, harmonics mitigation, and reactive power compensation. The improved GI control algorithm has an advantage of better dc offset and harmonics rejection capabilities as compared to a conventional second-order GI algorithm. To substantiate the control scheme, tests are performed on a paradigm in the research laboratory for manifold operating conditions. Test results show the satisfactory behavior under steady state and dynamic operating scenario such as unbalanced load, solar irradiations variation, voltage sag, and swell and distorted voltage grid. The total harmonic distortions of the grid voltages–currents are achieved within constraints of grid code compliance of an IEEE 519 and 1564 standards.

Analysis of Smart Loads in Nanogrids

Low-inertia power generation units make nanogrids vulnerable to the voltage and power fluctuations caused by pulse loads and abnormal grid conditions. The conversion of critical loads to smart loads is a potential solution for improving the stability and power quality in nanogrids. This paper investigates the effects of utilizing smart loads on the performance of nanogrids. A smart load can compensate for sudden deviations between supply and demand and therefore can mitigate voltage and power oscillations in low-inertia nanogrids. The conversion of critical loads to smart loads can reduce the stress on the energy storage units and minimize the required battery banks in nanogrids. In this paper, several case studies are considered to verify the stability and power quality improvement of nanogrids when some loads are converted to smart loads.