Distorted Grid Conditions Research Articles

Dual Second Order Generalized Integrator – Phase Locked Loop technique applied to a distorted grid-connected solar energy based on a Z-Source Inverter

In this paper, a Dual Second Order Generalized Integrator_Phase Locked Loop (DSOGI_PLL) technique applied to the Photovoltaic (PV) system supplying a Z-Source five-level Inverter (ZSI) is presented. The ZSI assures the increasing voltage of the PV system and provides the desired output DC at the input of the five-level Neutral Point Clamped Inverter (5L_NPCI), without any control which reduces the complexity of the overall system, this is due to the impedance network in its structure. We use Z-Source in order to eliminate the controlled DC bus as well as the use of the DC-DC Boost converter as an intermediary. A DSOGI technique is employed to control and optimize the energy quality, especially in distorted grid conditions and allows one to obtain a very low value of Total Harmonic Distortion (THD) which means lower peak currents, and higher efficiency. Low THD is an essential feature in power systems that international standards such as IEC 61000-3-2, and IEEE-519 set limits on the harmonic currents of various classes of power equipment connected to a distorted three-phase grid. Compared to the classic structure of inverters, the NPC five-level inverter presents a very high performance.

Direct-Axis Current Orientation-Based Grid Voltage Sensorless Control Strategy for Vienna Rectifier Under Unbalanced and Distorted Grid Conditions

Direct-Axis Current Orientation-Based Grid Voltage Sensorless Control Strategy for Vienna Rectifier Under Unbalanced and Distorted Grid Conditions

Grid Synchronization in a 3-Phase Inverter using Double Integration Method

Distributed renewable energy sources (DRESs) have additional benefits compared with conventional fossil fuel-based energy sources. These sources are connected to the utility grid using a suitable synchronization method. It requires accurate information on grid voltage magnitude and phase angle. But the presence of harmonics, DC Offset, and voltage unbalances makes the synchronization process more challenging. Conventionally synchronous reference frame (SRF)- PLL is implemented for this purpose, however, it has a problem with 100Hz ripple during distorted grid conditions. A double integration method (DIM) is administered for the synchronization of a 3-phase inverter under non-ideal grid conditions. It is important for DIM implementation to remove the DC offset or integrating constants of pure integrators without involving any phase shift. This method is also compared with conventional SRF-PLL based on mathematical modelling and simulations using a MATLAB/Simulink toolbox. The results are verified based on ideal and non–ideal grid conditions and also tested on grid-connected 3-phase Inverter.

Neural Network Quadrature Signal Generator-Based Virtual Flux Estimation for Predictive Control of PWM Converters

In this paper, a frequency-adaptive neural network-based virtual flux (FANN-VF) estimator is developed for sensorless control of a pulse-width modulation converter under unbalanced and distorted grid conditions. This method exploits two parallel FANNs configured as quadrature signal generators. The VF fundamental positive and negative sequence components (PNSCs) are inherently separated in the estimator without employing any cascaded filters. A frequency-locked loop is introduced to accurately track the frequency variations. The estimated VF PNSCs are directly exploited to compute the power references in VF-based predictive direct power control scheme. The developed strategy ensures constant active power and sinusoidal current waveforms under nonideal grid conditions. Simulation and experimental tests are performed to verify the effectiveness of the proposed method.

Design and analysis of electric spring based on battery-less current-source inverter

PurposeElectric spring (ES) is a demand response method that can stabilize the voltage of critical loads and improve power quality, especially in a weak power grid with a high proportion of renewable energy sources. Most of existing ESs are implemented by voltage-source inverter (VSI), which has some shortcomings. For example, the DC-link capacitor limits the service life of ES, and the battery is costly and hard to recycle. Besides, conventional VSI cannot boost the voltage, which limits the application of ES in high-voltage occasions. This study aims to propose a novel scheme of ES to solve the above problems.Design/methodology/approachIn this work, an ES topology based on current-source inverter (CSI) without a battery is presented, and a direct current control strategy is proposed. The operating principles, voltage regulation range and parameter design of the proposed ES are discussed in detail.FindingsThe proposed ES is applicable to various voltage levels, and the harmonics are effectively suppressed, which have been validated via the experimental results in both ideal and distorted grid conditions.Originality/valueAn ES topology based on battery-less CSI is proposed for the first time, which reduces the cost and prolongs the service time of ES. A novel control strategy is proposed to realize the functions of voltage regulation and harmonic suppression.

Control of DSTATCOM with Parallel Tangent Method Based Adaptive Control Algorithm under Distorted Grid Conditions

This paper presents a new adaptive control algorithm for a cascaded multilevel inverter (CHB-MLI) used as a Distribution Static compensator (DSTATCOM) unit to mitigate various power quality (PQ) issues. A Parallel Tangent (PARTAN) Least Mean Square (LMS) adaptive control algorithm controls the compensator. The shunt CHB-MLI exports reactive power needed by the nonlinear load and improves the power factor to unity on the supply side. The system’s closed-loop performance is also studied under a distorted grid employing an additional Second-order generalized integrator (SOGI) block. Modeling of a five-level inverter along with the proposed PARTAN-LMS controller has been done in Matlab/Simulink. Experimental verification of the same system is tested in the laboratory. Simulation and experimental results demonstrate satisfactory performance of the proposed system under dynamic load variation and distorted grid conditions. Further, the performance of the proposed algorithm is compared with conventional and new adaptive algorithms to ascertain its advantages and effectiveness. The THD obtained in supply current and voltages are within stipulated IEEE-519 limits.

Delay-Tracking Loop for Frequency Estimation of Grid-Interfaced Inverter Under Distorted Conditions

Fast and accurate frequency estimation is difficult for grid-interfaced inverter under distorted grid conditions with off-nominal frequency. A popular solution considers the multiple second-order generalized integrator-based frequency-locked loop yet can hardly achieve a fast dynamic response under significant phase jump and voltage sag. Another solution uses an adaptive sliding window filter with an additional frequency estimator. However, it is challenging to handle numerical instability due to the adjusting marginally-stable resonator and the accumulation error. This letter proposes a delay-tracking loop to overcome these problems. This loop allows the delay length to track the frequency with an inherent harmonics rejection at off-nominal frequency. At the same time, the fast response speed is maintained, and the marginally stable resonator is avoided. Comparative experimental results validate the above claims.

A Frequency/Phase/Amplitude Estimator for Three-Phase Applications Operating at a Low Sampling Rate

A frequency/phase/amplitude estimator is arguably one of the most crucial components in the control and synchronization of grid-connected equipment. Such an estimator may also be useful for monitoring and protection purposes in power systems. In this paper, an open-loop (and therefore unconditionally stable) estimator for the accurate and rapid extraction of the grid voltage phase, frequency, and amplitude is presented. In designing the proposed technique, special focus is given to applications operating at a low sampling rate. Examples of such applications include high-power converters, where both switching and sampling frequencies are very low. The study concludes with a comprehensive evaluation of the proposed estimator, demonstrating its effectiveness in accurately and swiftly estimating the fundamental parameters of grid voltage under low sampling rates. It highlights the estimator’s enhanced performance in scenarios of distorted grid conditions and its superiority in filtering capabilities compared to traditional methods. These findings underline the estimator’s potential for broad applicability in power system monitoring, protection, and control.

Robust and efficient control strategy for single-phase two-stage PV grid-tied system with improved power quality

In the last couple of decades, the integration of photovoltaic (PV) systems with the utility grid has gained significant attention due to its potential to harness renewable energy and contribute to sustainable power generation. However, the integration of the PV system into the grid leads to issues related to power quality and system stability under varying solar irradiation, dc offset, and distorted grid conditions. Therefore, this paper focus on the robust and efficient control of a single-phase, two-stage PV grid-tied system to improve power quality and stable system operation under abnormal grid conditions. These objectives can be achieved using the synchronization method, dc link voltage control, and grid current control. The hybrid PLL is proposed in this paper for synchronization and to mitigate the effect of dc offset and lower-order harmonics in the estimated phase and frequency. Further, the sliding mode voltage controller and integral sliding mode current controller are proposed for dc link voltage regulation and grid current tracking. The stability of voltage and current control loops is analyzed using the Lyapunov stability theory. Furthermore, the proposed schemes are validated using MATLAB simulation and experimental analysis. The proposed approach outperforms conventional methods and exhibits better disturbance rejection capability under both dynamic and steady-state conditions, making it a robust and efficient control strategy for grid-tied systems.

An Improved Passivity-Based Direct Power Control Strategy for AC/DC Converter Under Unbalanced and Distorted Grid Conditions

An Improved Passivity-Based Direct Power Control Strategy for AC/DC Converter Under Unbalanced and Distorted Grid Conditions

Control for Power Quality Improvement of Solar Photovoltaic-Distributed Static Synchronous Compensator Interfaced with Weak Grid Using Multi-Variable Filter Dual Second-Order Generalized Integrator Phase-Locked Loop

This study proposes a control method for transferring power generated by a solar photovoltaic system. The control algorithm, called MVF-DSOGI-PLL or Multi-Variable Filter Dual Second-Order Generalized Integrator Phase-Locked Loop), accurately estimates symmetrical components, phase angle, and positive sequence voltages of the Common Point of Interface (CPI) in distorted grid conditions. The approach creates a compensatory current to reduce PQ difficulties by extracting the size of the basic current load undergoing operation employing the SOGI method. These systems can operate in either modes of PV-DSTATCOM or DSTATCOM, depending on solar insolation’s availability. The simulation models of the grid-interfaced SPV systems are developed in Matlab/Simulink to evaluate the performances of the suggested control method. The results of simulation demonstrate that the suggested control systems efficiently transfers the power generated by PV arrays to a grid while performing tasks like load balancing, power factor correction (PFC), and harmonics reduction. Furthermore, the MVF-DSOGI-PLL-based control algorithm successfully mitigates PQ issues in distribution networks with weak grids, ensuring that PQ levels remain between a limited set by the IEEE-519 standard. A laboratory prototype of the proposed system is implemented to assess its steady states and dynamic efficient under fluctuating solar insolation, and load imbalance distortions.

Space vector-based model predictive current controller for grid-connected converter under unbalanced and distorted grid without a phase-locked loop

This paper presents an improved current control strategy for a three-phase voltage source converter connected to distorted grid. The proposed controller can successfully compensate the induced harmonic currents, while suppressing possible active power ripple. To manage various objectives and deal with tough restrictions, model predictive control can offer a promising solution. However, a modified extended complex Kalman filter is introduced to estimate the positive and negative fundamental components of the grid voltages which are used with the real and reactive power references to generate pure sinusoidal reference currents without the need for a phase-locked loop (PLL), avoiding its challenges under abnormal grid conditions. An improved space vector-based model predictive control (SVM-MPC) is proposed to regulate the real and reactive power components. The main target of this controller is to maintain harmonic free grid currents, even in unbalanced and distorted grid conditions, with enhanced performance in terms of fast dynamic response and good steady-state behavior. The proposed SVM MPC uses Pythagoras theorem to determine duty factors, avoiding trigonometric functions, which improves the computational burden and implementation complexity. In addition, fixed switching frequency can be ensured. Simulation and experimental results are carried out to validate the effectiveness and the practical feasibility of the proposed controller under distorted grid conditions.

A Cost Optimized-Reliable-Resilient-Realtime- Rule-Based Energy Management Scheme for a SPV-BES-Based Microgrid for Smart Building Applications

This paper presents a cost-optimized, reliable, resilient, real-time, rule-based energy management scheme (EMS) for a solar photovoltaic (SPV) array and battery energy storage (BES) interfaced, grid-integrated microgrid (MG) for smart building applications. To solve several economic issues, the EMS is designed to provide power leveling, emergency backup and optimized power consumption from the grid. The introduced EMS controls the power utilized from the BES to regulate the power consumption from the grid to maximize the net profit earned and minimize the energy cost incurred based on the real-time system states. Moreover, this paper presents a variable gain enhanced second-order generalized integrator frequency locked loop (VGESOGI-FLL) based control algorithm for MG. It solves the power quality (PQ) problems in smart building-based MG due to the distorted grid conditions. Moreover, the control also improves the grid side PQ with the local loads at the demand side by eliminating dominant harmonics, providing reactive power support, and operating the grid at the unity power factor, further improving the system efficiency. The system performance is analyzed using simulation results from a designed Simulink model along with hardware results during extreme conditions.

Bernoulli polynomial‐based control technique for PV‐integrated grid system under distorted supply

SummaryThis paper discusses the performance of the Bernoulli Polynomial‐based artificial neural network (BePANN) for the control of a single‐phase grid‐integrated solar photovoltaic (PV) system. A single‐phase, single‐stage topology of a grid‐integrated PV system is utilized to feed nonlinear loads at the point of common coupling. The fundamental load component is extracted from distorted load current using a single‐layer neural network. There are several control techniques mentioned in the literature for load compensation. However, many techniques show slower convergence response, higher oscillations, and a large computational burden. The designed BePANN controller attenuates harmonic components from nonlinear current and improves the power quality (PQ) under normal and distorted grid conditions. The single‐layer BePANN control has a simplified structure based on several polynomial terms; this reduces the computational burden and complexity of the controller. The objective of the designed controller is to fulfill the load's active power demand from the generated solar PV power and feed the excess power back to the grid when surplus. When solar PV is not integrated with the grid, the voltage source converter acts as a distribution static compensator, improving the system's utilization factor. The proposed control technique is simulated in MATLAB Simulink, and results are experimentally verified through laboratory prototype under normal and abnormal grid conditions. The application of higher order polynomials for PV‐integrated systems with multifunctional PQ capabilities has been implemented for the first time in this article. The main outcomes of the proposed control technique are solving current‐related PQ issues and reactive power compensation and providing improved power factor under both normal and distorted polluted grids.

Enhancing the Filtering Capability and the Dynamic Performance of a Third-Order Phase-Locked Loop under Distorted Grid Conditions

This work proposes a structural enhancement and a new technique to design the loop filter (LF) of a third-order phase-locked loop (PLL) to enhance the PLL dynamic performance under abnormal grid conditions. The proposed PLL combines a moving average filter (MAF) and an arbitrarily delayed signal cancelation (ADSC) for structural enhancement to achieve DC-offset rejection and harmonics elimination. The window length of the MAF is selected to be one-sixth of the fundamental grid period to remove non-triple odd harmonics and speed up the PLL dynamic response. The triple harmonics are eliminated, adopting the line-to-line voltage concept, while the ADSC operator rejects the DC offset. The LF design is based on a modified third-order polynomial tuned using stochastic optimization to minimize the settling time of the frequency deviation, offering better dynamic performance over the symmetrical optimum method (SOM) and achieving synchronization within one grid cycle. The PLL mathematical model, small-signal model, and LF design based on the modified polynomial are discussed. Finally, the proposed PLL performance is verified numerically and experimentally with comparisons with other PLLs to demonstrate the effectiveness of the proposed work.

Hybrid-Modulation Hysteresis Scheme Based Decoupled Power Control of Grid-Connected Inverter

Most existing power control methods for grid-connected inverter are implemented in synchronous reference frame. In order to achieve soft-switching operation, those methods usually need additional design considerations, which could introduce complex computation. This article presents a control strategy for single-phase grid-connected inverter system with LCL filter that can be used for grid-connected battery/photovoltaic (PV) system, with the target to implement decoupled active and reactive power control capability in stationary frame and achieve soft-switching behavior for higher efficiency. In order to improve system efficiency, a new hybrid-modulation-based hysteresis current scheme is used to achieve soft-switching while improving the inherent zero-crossing distortion issue existing in traditional unipolar hysteresis current control. Multiproportional and resonant compensators are utilized to mitigate output low-frequency distortions further, together with a capacitor current feedback-based active damping design embedded in an inner loop. The proposed controller also demonstrates good robustness and immunity to distorted grid conditions. Simulation and experimental results are presented in this article with essential theoretical analysis to verify the validity of the proposed control strategy.

Enhanced PLL-less grid synchronization algorithm amidst unbalanced and distorted three-phase grid conditions

Fast and precise tracking of grid voltage attributes is essential for the smooth and reliable control and operation of grid-side converters. The synchronous reference-frame phase-locked loop (SRF-PLL) approach is commonly used for this purpose, although it has issues related to the stability limit and sluggish transient response. Consequently, PLL-less grid synchronization approaches are becoming increasingly prevalent. In this context, in the present paper, a discrete Fourier transform-based band pass filter (DFT-BPF) is designed in the prefiltering stage for the purpose of extracting the fundamental frequency positive sequence component (FFPS) from unbalanced and distorted grid conditions. Despite being widely used, the SRF-PLL is struggling with stability issues and demands tedious tuning effort. For this objective, a simple frequency estimator has been developed, which is capable of accurately estimating the fundamental frequency. Additionally, a curve fitting based error correction technique is employed to adaptively estimate the amplitude and phase-angle of the fundamental component without using frequency feedback loop. The experimental results validate the efficacy of the proposed scheme.

Assessment of Sequence Extraction Methods Applied to MMC-SDBC STATCOM Under Distorted Grid Conditions

Under grid distortions, Modular Multilevel Converters (MMC) must adopt proper control strategies to fulfill the power system requirements and ensure a stable operation. An inappropriate control under such conditions may lead to energy unbalances between the MMC legs, inaccurate current injection, and failure in the synchronization process. In this context, sequence extraction methods play a critical role in enhancing the performance of the control, firstly, by aiding the Phase-Locked Loop (PLL) to maintain the synchronization with the AC grid by following the positive sequence fundamental component of the voltage, secondly, by allowing accurate active and reactive currents injection via the decoupled Voltage Oriented Control (dVOC), thirdly, by properly managing the internal energy of the MMC through the circulating current control. In prior researches, some sequence extraction methods have been used for MMC STATCOM. However, the sequence extraction was not the core of the performed studies and their impact on the system behavior has not been highlighted or tested in several grid conditions. This work fills this gap by first assessing the performance of a Single Delta Bridge Cell MMC (SDBC-MMC) STATCOM with four well-known sequence extraction methods (i.e., Decoupled Double Synchronous Reference Frame (DDSRF), Dual Second Order Generalized Integrator (DSOGI), Improved DSOGI, and Fortescue matrix-based (FMB) filter under normal and abnormal grid conditions, then, finding the most suitable one in terms of stability, dynamics, and functionalities.

Solar PV integrated dynamic voltage restorer for enhancing the power quality under distorted grid conditions

• PV integrated DVR for voltage compensation is presented. • An enhanced INC controller is used for MPPT. • An optimum tuned PI controller using ANFIS controller is presented. • The load voltages with and without DVR along with the compensated load voltages of both sag and swell for existing PI-tuned controllers with the proposed PI-ANFIS controller are compared. Renewable energy sources (RES) play an essential role in the generation of electrical power, particularly in grid-connected systems. As a result of RES interaction with the grid, power quality issues arise. The grid suffers from voltage sag, swell, and harmonics due to power quality Problems, which have an impact on consumers. The most cost-effective alternative for overcoming the aforementioned issues is a series compensator, such as the Dynamic Voltage Restorer (DVR). This article presents a solar PV integrated DVR using a rotating dq reference frame controller to improve power quality issues. The application of an enhanced INC MPPT approach and the design of an efficient PI controller for DVR to improve power quality is the primary objective of this paper. The optimal tuning of the PI controller is achieved by using the Adaptive Neuro-Fuzzy Inference System (ANFIS). Furthermore, the proposed solar PV, boost converter, and rotating dq reference frame control are mathematically analyzed. In the presence of a balanced load, the system dynamic performance is tested. The suggested system performance is verified in real time with MATLAB/SIMULINK software, and Opal-RT closely resembles real-world hardware supported by FPGA processors. The results show that the design is effective at limiting voltage sag, swell, and THD% levels at the load side under the IEEE standard limit.

A current sensorless control method for multi‐level active front‐end rectifiers with LCL filter

AbstractThis paper presents a current sensorless control strategy for a three‐phase multi‐level active front‐end rectifier with an LCL filter. The proposed control strategy is based on the mathematical model of the system. Opposed to the existing control methods that measure three‐phase currents as well as three‐phase voltages, the proposed control eliminates the current measurement requirement which leads to cost reduction. Moreover, unlike the existing current control methods that use two proportional‐integral (PI) controllers, the proposed current control does not use any PI controller in the current loop. The elimination of two PI controllers simplifies the controller design. The resonance damping requirement arising due to the LCL filter is tackled by active damping that is realized by using filter capacitor voltage feedback. The steady‐state and dynamic performances of the proposed control strategy are investigated experimentally under balanced, unbalanced and distorted grid conditions.