Distorted Grid Voltage Research Articles

Accuracy assessment of frequency-domain models for harmonic analysis of residential type photovoltaic-distributed generation units

Since Photovoltaic-Based Distributed Generation Units (PV-DGUs) have power electronic interfaces for their grid integration, they can cause harmonic pollution in the system. Thus, the harmonic analysis is required to determine the system’s harmonic constrained PV-DGU hosting capacity and proper planning of the harmonic mitigation devices and PV-DGUs. The accuracy of the harmonic analysis highly depends on the validity of the system equipment models. Accessible literature shows that there is not a report on accuracy assessment of the PV-DGU frequency-domain harmonic models. In this paper, firstly, for numerous distorted grid voltage conditions, the current harmonic distortion nature of the single-phase transformerless two-level H-bridge residential type on-grid PV inverter is examined in an experimental test system. Secondly, for current harmonic analysis of the examined PV inverter, the accuracy of the widely known frequency-domain models is statistically analyzed under numerous distorted grid voltages. For the accuracy assessment of the models, the measurements obtained from an experimental test system are considered as reference values. Finally, to investigate the effect of the models on the voltage harmonic distortion results of the harmonic analysis, the harmonic penetration analysis algorithm is integrated with the models, and it is simulated for IEEE 33 bus test system.

Three-Level T-Type Quasi-Z Source PV Grid-Tied Inverter With Active Power Filter Functionality Under Distorted Grid Voltage

Distributed energy resources (DER) such as solar photovoltaic (PV) interfaces with the utility grid by high-efficiency power electronic converters. This equipment is sometimes underutilized in terms of its power capacity; thus, the remaining capacity can be used to provide support to the distribution system. In this sense, this work assesses the performance of the three-level T-type quasi-impedance source inverter (3L-T-type qZSI) injecting not only active power to the grid, but also providing support to it by injecting reactive power and acting as an active power filter (APF) simultaneously. The global control strategy of the grid-connected inverter is derived from the instantaneous <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$i_{d}$ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$i_{q}$ </tex-math></inline-formula> power theory. The combination of an improved deadbeat current controller and a level-shifted carrier-based pulse-width-modulation (PWM) technique, using both the upper shoot-through (UST) and the lower shoot-through (LST) alternating states, allows a successful tracking of the current references. Both the dc-link voltage control and the neutral-point balancing are properly achieved by taking the advantage of the shoot-through (ST) states and the redundant states simultaneously. These control actions are implemented by simple proportional-integral (PI) controllers. The simulation and experimental results demonstrate the above-mentioned functionalities and verify the stability and good dynamic response of the grid-connected 3L-T-type qZSI.

Repetitive Predictive Control for Current Control of Grid-Connected Inverter Under Distorted Voltage Conditions

A repetitive predictive control for grid-connected inverter current control scheme is presented in this paper under voltage harmonic distortion in the stationary reference frame. Predictive control is an approach that uses a receding horizon to achieve the optimal track for the reference. In this approach, the repetitive controller behavior is added to the predictive control to increase its performance under distorted voltage conditions. In addition, to apply the controller in the generation system, controller is designed from the perspectives of the power system. The controller is designed considering the state-space model in the stationary reference frame. The controller performance was verified using a laboratory experimental setup under distorted grid voltage condition. Experimental results confirm that the proposed controller can effectively suppress harmonics under both normal operation and distorted grid voltage conditions, and also satisfy the requirements stipulated in IEEE Std. 1547.2-2008.

Robust Control of GTIs under wide grid impedance ranges: An approach combining metaheuristics and LMIs

This paper provides a new design procedure for robust state feedback current controllers suitable for grid-tied inverters under wide grid impedance ranges. The proposed procedure is based on the combination of particle swarm optimization and linear matrix inequalities. The control gains, designed offline, are obtained through the minimization of a novel objective function that takes into account the grid current tracking error in the stationary reference frame, limits for the control signal, and robust stability. Linear matrix inequalities are used in the design stage to theoretically certify the closed-loop robust stability systematically and efficiently, based on a polytopic model of the grid-tied inverter, which avoids the need for exhaustive and time-consuming a posteriori tests. Real-time simulations and experimental results are presented, attesting that the closed-loop system with control gains provided by the proposed procedure is robustly stable for wide ranges in the grid inductances. Moreover, the grid currents exhibit an optimized transient performance and steady-state responses in compliance with harmonic distortion limits from IEEE 1547 Std., even encompassing operation from stiff to weak grid conditions and in the presence of distorted grid voltages. To provide a comparison, results based on other well-established state feedback design techniques are also presented, highlighting the superior performance obtained with the proposed procedure.

Grid integrated solar energy transfer system with a two‐layer complex coefficient filter‐based control

In this paper, a novel control strategy for grid-integrated solar energy transfer system (SETS) is proposed which enhances the grid power quality along with active power injection. Some of the conventional grid synchronization controls face problems due to imperfect estimation of fundamental active load current during both grid-side and load-side disturbances. The proposed two-layer multiple-complex coefficient filter control improves the performance of SETS by accurately estimating the fundamental active load current at adverse operating conditions. One layer extracts the fundamental positive sequence of the sensed three-phase grid voltages and derives the unit in-phase voltage templates for grid currents reference generation to achieve unity power factor operation. The second layer evaluates the average active fundamental component of the load currents, and finally derives the reference amplitude of three-phase grid currents. The two-stage SETS consists of a quadratic boost DC-DC converter to extract the maximum solar array power forming an interface between PV array and inverter. It also provides high voltage gain and decreases the length of the PV array string while maintaining a low DC-link voltage ripple, extending the service life of the SETS. The effect of a nonlinear unbalanced load is nullified and therefore the grid current's waveform quality is maintained in compliance with the IEEE-519 standard during the dynamic operating conditions. The SETS provides sinusoidal, balanced, and steady grid currents at unity power factor withstanding adverse operating conditions such as distorted grid voltages, varying irradiance and varying unbalanced nonlinear load currents. Due to its flexibility to operate in such adverse conditions, the proposed control based SETS is suitable for grid-connected applications. Simulations are carried out in MATLAB/Simulink and results validate the performance of the SETS. The hardware prototype of the proposed SETS is developed and rigorously tested under various operating conditions to validate the proposed claims.

Adaptive finite control set model predictive control for three-phase inverters connected to distorted grid with fewer voltage sensors

This paper proposes an adaptive finite control set model predictive control (FCS-MPC) scheme for three-phase inverters connected to distorted grid with fewer voltage sensors. For the distorted grid voltage construction, a composite observer is introduced. The observer is composed of two parts, a phase detection module and time-varying observers. The phase information is obtained by the measured voltage of one phase. Then the other two-phase grid voltage is identified by time-varying observers. The convergence of time-varying observers is guaranteed by a Lyapunov function and the Lasalle invariance principle. Moreover, the outputs of the observer are the DC component and the amplitude of each harmonic, which is helpful to obtain positive components without complex post-processing. Finally, the FCS-MPC scheme with simplified cost function is employed for current tracking in natural (abc) coordinate rather than αβ coordinate. Simulation on Matlab/Simulink and experiments on FPGA platform for the three-phase inverter under distorted grid conditions prove that the proposed control approach can perform well with fewer voltage sensors.

An Advanced Proportional Multiresonant Controller for Enhanced Harmonic Compensation With Power Ripple Mitigation of Grid-Integrated PV Systems Under Distorted Grid Voltage Conditions

This article proposes an advanced integrated control strategy for enhanced grid current harmonic compensation with power ripple mitigation in a grid-integrated photovoltaic systems (GIPVS) under distorted grid voltage conditions. The proposed control strategy consists of an advanced proportional multiresonant (APMR) current controller and a compensator for grid voltage sag (CGVS), which is implemented with a phase-locked loop block. The APMR is designed in the α-β reference frame and minimizes the harmonics of the grid injected current. The CGVS, along with the APMR, eliminates the active and reactive power ripples under unbalanced voltage sags conditions. Further, an adaptive step size incremental conductance (ASINC) maximum power point tracking controller in integration with the proposed control strategy is implemented. Along with current harmonic reduction by APMR, the ASINC method ensures effective and faster tracking of maximum power point and negligible oscillations in the steady-state under different environmental conditions. By utilizing the proposed APMR control strategy, the frequency adaptability of the GIPVS is improved compared to the conventional proportional multiresonant (PMR) control strategy under grid frequency deviations. It successfully keeps the grid current total harmonics distortion below the acceptable limits for the grid voltage polluted by several odd harmonics, up to the nineteenth harmonics. The proposed control strategy of APMR with CGVS almost completely eliminates the power ripples in even severe unbalanced voltage sag conditions. Experimental studies are carried out using a real-time digital simulator to verify the effectiveness of the proposed control strategy in comparison with the conventional PMR controller.

An Improved Droop Control Strategy for Grid-Connected Inverter Applied in Grid Voltage Inter-Harmonics and Fundamental Frequency Fluctuation

This paper presents a current suppression method based on a droop control strategy under distorted grid voltage with inter-harmonics and fundamental frequency fluctuation. In this proposed strategy, the current incomplete derivation controller is employed to decrease the negative impact caused by harmonic and inter-harmonic grid voltage. This method provides a good dynamic response and has low complexity against the inter-harmonics with unfixed fundamental frequency. Based on a mathematical model of the grid-connected inverter, we designed novel instantaneous frequency detection and feed-forward methods to suppress the grid fundamental frequency fluctuation impacts. Then the main parameters were analyzed. The simulation and experimental results verified the feasibility and effectiveness of the proposed method.

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;

Inverter Control Strategy Based on Improved Deadbeat Control and QPR Compound

In the process of photovoltaic power generation, it is particularly important to choose a suitable inverter control strategy. Aiming at problems such as the poor dynamic response of the traditional controller in the three-phase LCL grid-connected inverter, the current harmonic disturbance caused by the grid voltage distortion and the inability to achieve the optimal tracking of the AC quantity, this paper presented a method to control voltage by fuzzy PI, control current by discrete QPR and improved DBC. In the voltage outer loop, the fuzzy control principle was used to adjust the P and I parameters in real time, and its output was used as the reference value for the current inner loop. In the current inner loop, the repeated control improved deadbeat and discretized form of QPR were used for compound control. The simulation results show that the control strategy greatly improves the distortion of the grid-connected current, which can optimally track the AC volume, and makes the entire system have good stability and dynamic response capabilities.

Power quality improvement using three‐phase transformerless grid interfaced SECS enabling leakage current suppression

This paper presents a harmonic compensation strategy for a 3-ϕ grid-tied solar energy conversion system with leakage current attenuation feature. As parasitic capacitance exists between the solar photovoltaic panels with the ground, the deviation in common-mode voltage compels the probable safety hazards such as electromagnetic interference and harmonics injection in the grid currents. Additionally, the power quality indices of the grid, are exacerbated with the coupling of the non-linear load. Owing to these issues, a generalized integrator based approach is introduced to ensure the harmonics suppression, power factor correction, leakage current alleviation. Additionally, the grid current distortion limits are ensured within limits in order to comply with the IEEE std. 519. The presented algorithm ensures a robust operation with respect to various anomalies in the grid voltages. Simulation results show the satisfactory response of the harmonics controller even under abnormal operating scenarios including the symmetrical/unsymmetrical faults in the grid side network. In comparison with the traditional algorithms, the harmonic controller suppresses the leakage current and harmonics within restricted limits despite the abnormal conditions. Real-time simulator results demonstrate the performance of the controller under distinct operating conditions such as load unbalancing, distorted grid voltages, etc.

Dynamic Voltage Restorer (DVR) with a novel robust control strategy

With the advancement of technology, electric equipment and loads have become more sensitive to problems related to power quality, such as voltage sag, swell, imbalances, and harmonics. To detect faults and to protect sensitive loads from these voltage distortions, a Dynamic Voltage Restorer (DVR) series compensator is among the best available cost-effective solutions. One of the main goals of the DVR is to achieve a control structure that is robust, stable, and can handle properly the disturbances (e.g., grid voltage issues, load current, and fluctuations at the DC link voltage) and model uncertainties (e.g., inverters and filter parameters). In this work, a novel framework control strategy based on Uncertainty and Disturbance Estimator (UDE) is proposed to improve the response of the DVR to properly compensate the load voltage under a variety of power quality issues, particularly the ones associated with the grid voltage disturbances. Additionally, the stability of the proposed control system is analyzed and validated using the Lyapunov stability theory. The advantages of the new control system are robustness, simplified design, good harmonic rejection, low tracking error, fast response, and sinusoidal reference tracking without the need for voltage transformations or specific frequency tuning (e.g., abc-dq0 and Proportional-Resonant). This research uses the MATLAB/Simulink software to validate the effectiveness of the proposed scheme under a diverse set of conditions with no control limitations. Moreover, the designed controller is tested under real conditions using Hardware-In-the-Loop (HIL) validation with OPAL-RT real-time simulator coupled with a TI Launchpad microcontroller. The results demonstrate a good performance of the proposed control strategy for a quick transient response and a great harmonic rejection when subject to grid voltage distortions.

Power Quality Enhancement of Grid-Connected Solar Photovoltaic System Using ANN based Filter

Grid-connected photovoltaic (PV) systems are increasingly attracting the attention of industry and academia as a means of providing an alternative to conventional fossil-fuel generation and pollution-free power. This project aims to improve the power quality level of a grid-tied PV distribution system using shunt active power filter (APF) along with adaptive current control technique. In this work Fuzzy Logic controller used to destroy the voltage and current harmonics in a grid-tied PV system. A reference current generation strategy is implemented to mitigate the current harmonics by extracting the fundamental constituents (FCs) from the nonlinear load currents. MCCF is employed to separate the FC from the distorted grid voltages and eliminates the voltage harmonics during extremely polluted grid voltage condition. The comparative analysis is analyzed to check the effectiveness of the proposed hybrid control scheme with existing and adaptive control techniques in respect of power quality, better dc offset rejection, better FC and frequency extraction, and grid synchronization.

Power Quality Enhancement of Grid-Connected Solar Photovoltaic System using ANN based Filter

Grid-connected photovoltaic (PV) systems are increasingly attracting the attention of industry and academia as a means of providing an alternative to conventional fossil-fuel generation and pollution-free power. This project aims to improve the power quality level of a grid-tied PV distribution system using shunt active power filter (APF) along with adaptive current control technique. In this work Artificial Neural Network controller used to destroy the voltage and current harmonics in a grid-tied PV system. A reference current generation strategy is implemented to mitigate the current harmonics by extracting the fundamental constituents (FCs) from the nonlinear load currents. MCCF is employed to separate the FC from the distorted grid voltages and eliminates the voltage harmonics during extremely polluted grid voltage condition. The comparative analysis is analyzed to check the effectiveness of the proposed hybrid control scheme with existing and adaptive control techniques in respect of power quality, better dc offset rejection, better FC and frequency extraction, and grid synchronization.

An Uninterruptable PV Array-Battery Based System Operating in Different Power Modes With Enhanced Power Quality

This article aims to develop a solar- battery energy storage (BES) based system, which ensures an uninterruptable supply to loads irrespective of availability of the grid. This system comprises of a solar photovoltaic (PV) array, a BES, the grid and local residential loads. A new control is implemented such that the active power demand of residential loads, is fed from the PV array, a BES unit and the utility grid. In this system, the power control operates in different power modes, which delivers the benefits to the end users with an integration of BES and an excess of PV array power, which is sold back to the grid. For this, an effective control logic is developed for the grid tied voltage source converter. Moreover, this system deals with the issue of an integrating power quality enhancement along with the power generation from the solar PV source. The cascaded delayed signal cancellation based phase locked loop is implemented for grid synchronization during the grid voltage distortion. The developed control is easily implemented in a real time controller (dSPACE-1202). Test results validate the performance of the implemented control in different operating conditions such as varying solar power generation, load variations and unavailability of the grid.

Power Quality Enhancement of Grid-Connected Solar Photovoltaic System using ANN based Filter

Grid-connected photovoltaic (PV) systems are increasingly attracting the attention of industry and academia as a means of providing an alternative to conventional fossil-fuel generation and pollution-free power. This project aims to improve the power quality level of a grid-tied PV distribution system using shunt active power filter (APF) along with adaptive current control technique. In this work Artificial Neural Network controller used to destroy the voltage and current harmonics in a grid-tied PV system. A reference current generation strategy is implemented to mitigate the current harmonics by extracting the fundamental constituents (FCs) from the nonlinear load currents. MCCF is employed to separate the FC from the distorted grid voltages and eliminates the voltage harmonics during extremely polluted grid voltage condition. The comparative analysis is analyzed to check the effectiveness of the proposed hybrid control scheme with existing and adaptive control techniques in respect of power quality, better dc offset rejection, better FC and frequency extraction, and grid synchronization.

Modelling and prediction of current harmonics generated by power converters in distribution networks

Predicting current harmonics in distribution networks is essential to find harmonic losses and power quality indices. In order to predict the current harmonics, the effect of grid impedance and voltage distortions on grid-connected power converters need to be considered. The effects of these parameters on current harmonics can be nonlinear, which complicates the system analysis. Consequently, this paper investigates the impacts of background voltage harmonics, grid impedance, and multi-parallel grid-connected converters on current harmonics at a point of common coupling. Moreover, a novel method called combined least squares and kriging (CLSK) is proposed to predict the current harmonics after evaluating the level of system nonlinearity. In the proposed CLSK method, the prediction model is developed by measured voltage and current harmonics at the point of common coupling to identify and predict the current harmonics. This analysis shows that using the proposed CLSK method, the accuracy of the current harmonic prediction is increased compared to the existing methods. The accuracy and the efficiency of the proposed CLSK method are verified by lab experimental results and field measurement data.

Direct active damping control for grid‐connected AC/DC converter with LCL filter using augmented look‐up table

This paper presents a novel approach to adapt the conventional direct power control (DPC) methods that are applied to ac/dc converters with an inductive capacitive inductive (LCL) filter. These new solutions improved the conventional DPC with an active damping algorithm via multi-variable constraint adjustment, which is based on an augmented look-up table and multi-variable comparators. The novel look-up table proposed for the active damping algorithm of the LCL filter allows an optimal voltage vector assignment to be chosen using differences in converter-side current and capacitor voltage. This method is also compared with the DPC method with active damping via the addition of a virtual damping resistor and a harmonic rejection loop. The steady-state (total harmonic distortion of the grid current) and dynamic performances (response time on step changes of the set current), as well as the immunity under distorted grid voltage conditions of the proposed system, are verified through simulation results and experimental measurements.

Modeling and control of unbalanced and distorted grid voltage of grid‐connected DFIG wind turbine

Modeling and control of unbalanced and distorted grid voltage of <scp>grid‐connected DFIG</scp> wind turbine

High quality repetitive control system for a grid-tied converter under distorted grid voltage conditions – design and implementation

High quality repetitive control system for a grid-tied converter under distorted grid voltage conditions – design and implementation