Grid Current Waveform Research Articles

Influence of photovoltaic microgeneration on the demand profile and its effects on the grid power quality

• Demand profile effects can cause a false impression of low power quality. • High PV inverter loading percentage conditions can improve the grid power quality. • PV inverter active current has a linear relationship with inverter loading. • PV inverter non-active current does not vary considerably with inverter loading. The connection of small-scale photovoltaic (PV) generation into the grid requires attention to the changes in load curve profile, the harmonic components in the current waveform, and the customer power factor (PF) variation measured by the grid operator. This paper evaluates aspects that can influence the power quality (PQ) at the point of common coupling (PCC) due to PV generation's insertion in the distribution grid, given the consumer demand profile. That evaluation considers different inverter loading percentage situations and the effects of the consumer's electric current considering its active and non-active components. This study used four grid-connected PV systems and a programmable electronic load to perform tests at different load demand situations. A data acquisition system collects the voltage waveforms at the PCC, and the grid's current waveforms, load, and PV inverter. Thus, it is possible to evaluate the effects of load profile (current intensity and waveform) on PQ at the PCC. At this point, there are different values of active and non-active currents, total harmonic distortion (THD), and PF when measured from the PV system, the grid, or the load sides. Depending on where the measure is, the PV system can cause a false impression of reducing the grid PQ.

LCL-filter design and analysis for PWM recuperating system used in DC traction power substation

Voltage source inverter (VSI) had been used in dc traction power substation to deliver the trains braking energy back to the utility grid. To mitigate low order harmonics components, pulse-width-modulation (PWM) technique is commonly used in VSI controlled. As a result, the ac voltage and current waveforms may contain high frequency ripples. This paper proposes to mitigate the high frequency harmonics using LCL-filter with series <em>R</em>-damper. This filter offers good attenuation on harmonics with smaller size compare to other passive filter topologies. Theoretical analysis and simulation verification are conducted in designing the proposed filter. System level simulation had also been carried out. Sinusoidal grid current and voltage waveforms are recorded (THD<2%). This paper also includes damping losses analysis in conjunction with resonance peak suppression in designing the <em>R</em>-damper.

In depth mathematical-analysis and experimentation of high-power SiC-FET based single-stage three-phase differential-based flyback inverter with practical design issues for grid-tied applications

This paper presents a detailed mathematical loss-modelling, design, hardware implementation issues, and experimental verification of high-power isolated single-stage three-phase differential-based flyback inverter (DBFI) for grid-tied photovoltaic applications. In addition, this paper studies the loss-modelling analysis of the DBFI components in terms of time and system parameters, which describes the power loss distribution over the circuit components for efficiency enhancement possibility. Also, it studies the practical design issues of DBFI; power stage design, HFT-based F3CC Nano-Crystalline C-Core design, passive elements selection effects on harmonics compensation of grid current waveforms, and the input DC current ripple for PV applications propriety. In addition, the input DC-voltage variation impact on the grid-current THD, system efficiency, and operational power factor are investigated. Also, continuous modulation scheme (CMS) is used to control the proposed DBFI for low-order harmonic components mitigation. The system is simulated by PSIM computer-aided simulator. Then, the proposed system is verified experimentally using a laboratory prototype controlled by TMS320C6713 DSP controller.

Suppression Strategy for Midpoint Potential Fluctuation of Three-Level Wind Power Converter

With the development of offshore wind power, three-level wind power converters have begun to be widely used in wind farms, but there has always been a problem of low-frequency fluctuations in bus voltage, which will seriously affect the power quality of wind farms. This paper studies a GA-BP neural network optimization control method, injecting zero-sequence components on the basis of the sine pulse width modulation strategy of carrier stacking, and then optimizing the neural network weights and thresholds through genetic algorithm, and the bus capacitor voltage Low-frequency fluctuations are tracked and compensated to improve the predictive ability of the control system. This combination of zero-sequence voltage injection and injection-compensated low-frequency fluctuation components solves the problem of bus voltage fluctuations. Through Matlab/simulink simulation analysis and 1140V wind power converter experimental verification, the verification results show that the bus voltage fluctuation is reduced by 50%, and the grid current waveform is improved. The proposed scheme can well solve some of the influences caused by the fluctuation of the bus voltage in the wind power converter, ensure the operation reliability of the wind power system and improve the operation efficiency of the wind power system.

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.

Control of three wire DSTATCOM using cascaded delayed signal cancellation effect

ABSTRACT In this paper demonstration of three-wire distributed static compensator (DSTATCOM) for compensating disturbed ac main currents with a Frequency Locked Loop (FLL). An improvement with delayed signal cancellation (DSC) technique to a standard FLL is used. An improvement has been observed in the quality of grid current waveform in terms of distortion to 2.2% from load current distortion of 23.4% by applying DSC in cascaded mode. The standard FLL capability to extract fundamental from disturbed load current has increased with cascaded DSC operators. In order to provide fast, accurate, and precise fundamental extraction with a satisfactory dynamic response without any stability struggle, delay factors of 4 and 24 are cascaded and unified with the FLL in the recognized cascaded αβ delayed signal cancellation (Cαβ-DSCFLL).The dynamic response of the Cαβ-DSCFLL is taking three fundamental cycles to settle the system and it shows satisfactory harmonic rejection capability. A small signal model is provided for the FLL structure through which gain estimation and stability analysis is made. An optimization algorithm named JAYA algorithm which is an algorithm designed to perform a specific task, parameterless optimization framework is used for estimating the PI controller gains in modes for correcting of power factor and regulating zero voltage. An optimization problem is formulated with errors by DC as well as AC bus voltages. JAYA optimization algorithm is used for estimating gains. The disturbance removal capability of the recognized Cαβ-DSCFLL is better when compared with the other conventional ones. The efficacy of the recognized FLL has been demonstrated with experimental results on the laboratory proto type. Reactive power mitigation and removal of grid current harmonics in both modes of DSTATCOM with nonlinear load are achieved with recognized FLL and optimized gains. In realizing the recognized control algorithm, the dSPACE-1104 controller is used to generate gate pulses for DSTATCOM.

Research on harmonic current suppression of three-phase four-wire power conversion system

The harmonic current of the three-phase four-wire gridconnected power conversion system (PCS) will be increased due to gridside harmonics and neutral loop. To solve these problems, the three-phase four-wire I-type three-level converter is applied as the topology in this paper to study the harmonic current suppression of the grid-connected PCS. A compound control strategy with the proportional integral (PI) and repetitive controller in parallel is presented and the interaction between the parameters of the compound controller is analyzed. The comparative research is carried out from the theoretical perspective of model analysis, and a controller optimization scheme is proposed. Finally, under the grid and neutral current waveform quality of simulation and experiment, the correctness and effectiveness of the proposed scheme is verified.

Artificial neural network based unity power factor corrector for single phase DC-DC converters

Due to the negative effects of the non-linear semiconductor devices and the passive electrical components (inductor and capacitor) in the converter circuits, and that are deteriorating the power factor (PF) and total harmonics distortion (THD) of grid current, this study proposes a novel unity PF correction controller based on a new algorithm of neural network to improve the performance of a single phase boost DC-DC converter with respect to the mentioned concerns. The controller guarantees stable load voltage. The PF corrector, firstly measures the phase shift between grid voltage and grid current waveforms, then through a new artificial neural network (ANN) algorithm, a suitable duty cycle is predicted to guide and control the converter to reduce the phase shift between grid voltage and grid current as possible to have maximum PF which is unity PF, and to improve the THD level of grid current. The proposed system is simulated and evaluated via Simulink of MATLAB, the simulation results are collected at constant duty cycle and at controlled duty cycle through the proposed PF controller using different loads. The presented PF controller guarantees the unity power factor, and enhances the grid alternating current THD.

Methods of Modulation for Current-Source Single-Phase Isolated Matrix Converter in a Grid-Connected Battery Application

This paper discusses three methods of modulation for a single-phase isolated matrix converter. The matrix converter is combined with a transformer integration to perform power decoupling control in order to reduce the number of component and capacitor volumes. Due to the reason of (i) Alternating current (AC/AC) direct conversion and (ii) transformer integration, obtaining a clean sinusoidal grid current waveform in the modulation of matrix converter (MC) is important. Three methods of modulation are compared in terms of control complexity, quality waveform, and inductive-capacitive-inductive (LCL) filter sizing. The principal control of each method is described. Finally, a prototype was tested to verify the validity and the effectiveness of grid current control and power decoupling in the spoken circuit structure.

Effective and Simple Control of Grid-Connected Three-Phase Converter Operating at a Strongly Distorted Voltage

Three-phase power electronics converters (PECs) are commonly used as an interconnection of DC loads/sources with the electrical AC power system. Their important goal is to ensure three-phase symmetrical and pure sinusoidal grid current waveforms. Unfortunately, frequent adverse of grid voltage conditions like the occurrence of higher order harmonics and dips/sags leads to problems in maintaining high-quality parameters of current and stable operation of PEC. Therefore, a more complex control algorithms are usually used to solve those problems. In this paper, effective and simple control is proposed to mitigate grid current distortions caused by grid-voltage harmonics. This improvement of current quality can be easily obtained in control of grid-connected PEC using only band notch filtered grid voltage as a feedforward without any additional control loops parallel connected to linear current controllers (tuned to fundamental frequency). Theoretical analysis with simulation and experimental investigations are presented, which prove very good properties and advantages of the proposed control idea.

Advanced PWM Switching Technique for Accurate Unity Power Factor of Bidirectional Three-Phase Grid-Tied DC–AC Converters

Matrix-based high-frequency linked (MBHFL) bidirectional dc–ac converters are getting much interest in grid-tied applications due to their high efficiency and the compacted footprint. This paper proposes a new PWM switching technique of the MBHFL dc–ac converters based on phase shift angle considering compensation of the LC filter current at the grid side in order to realize accurate unity power factor and to reduce the total harmonic distortion (THD) of the grid current waveform. The proposed technique estimates the phase shift angle error between the grid voltage and current waveforms due to the capacitor injected current that reduces the power factor at the grid side. It considers two operational modes based on the patterns of three-phase grid voltage and current waveforms. Therefore, it provides the matrix converter switches with different duties according to the pattern of the detected grid voltage and the reference grid current to compensate the phase shift angle error. Also, grid LC filter design and the system controllable limit of the matrix converter output voltage are presented. In addition, the PWM switching technique of the H-bridge circuit is modified to realize safe operation of the converter during step changes at the grid voltage. The effectiveness of the proposed PWM switching technique and its related mathematical model have been investigated experimentally using laboratory prototype based 200 V, 2 kW.

Comparative study of two potential recuperating converters in DC railway electrification system for harmonic mitigation

<span>The regenerative braking energy produced by Light-Rail-Transit (LRT) train is commonly transferred back to power grid via a conventional three-phase inverter (recuperating converter). Although this is a cost saving solution but the ac grid current and voltage waveforms were distorted. Hence passive filters are integrated to mitigate the harmonics. This paper proposed to replace the conventional inverter system with a multilevel converter. Cascaded H-Bridge (CHB) converter and Modular Multilevel Converter (MMC) are selected to be evaluated in this paper due to their modularity structures. The aim of this study is to determine the most potential multilevel converter to be implemented without additional passive filters. Nine-level CHB and nine-level MMC converters are modeled with MATLAB/Simulink simulation tool. Both converters are modulated with Level-Shifted Pulse Width Modulation technique. The output voltage and current waveforms generated by CHB and MMC are presented with full analysis. It is concluded that MMC converter is more suitable to be used as a recuperating converter. It produces a clean voltage and current waveforms. The voltage and current Total Harmonic Distortion (THD) indexes are found approximate to 8% and 3%.</span>

Direct Power Control of Matrix Converter-Fed DFIG with Fixed Switching Frequency

In this paper, a direct power control (DPC) technique is proposed for matrix converter-fed grid-connected doubly fed induction generators (DFIGs). In contrast to what has been investigated in the past for direct torque control (DTC) or DPC of matrix converter-fed DFIGs, the active and reactive powers are regulated in a fixed switching frequency using indirect space vector modulation (ISVM) technique. Hence, designing input filters for matrix converters (MCs) becomes convenient. In addition, the reactive component of input side of MC is controlled which leads to reduction of distortion in grid current waveform. Also, an extensive discussion is addressed for nonlinear voltage errors of MC that may cause inaccurate power control. Simulation results done in MATLAB/Simulink show the effectiveness of the proposed method.

Power and Voltage Control for Single-Phase Cascaded H-Bridge Multilevel Converters under Unbalanced Loads

The conventional control method for a single-phase cascaded H-bridge (CHB) multilevel converter is vector (dq) control; however, dq control requires complicated calculations and additional time delays. This paper presents a novel power control strategy for the CHB multilevel converter. A power-based dc-link voltage balance control is also proposed for unbalanced load conditions. The new control method is designed in a virtual αβ stationary reference frame without coordinate transformation or phase-locked loop (PLL) to avoid the potential issues related to computational complexity. Because only imaginary voltage construction is needed in the proposed control method, the time delay from conventional imaginary current construction can be eliminated. The proposed method can obtain a sinusoidal grid current waveform with unity power factor. Compared with the conventional dq control method, the power control strategy possesses the advantage of a fast dynamic response. The stability of the closed-loop system with the dc-link voltage balance controller is evaluated. Simulation and experimental results are presented to verify the accuracy of the proposed power and voltage control method.

Power System Compensation Using a Power-Electronics Integrated Transformer

This paper presents a new transformer, that is, the custom power active transformer (CPAT)—which integrates shunt and series equivalent circuits within the transformer's magnetic structure. Thus, it provides power system services using a single transformer. The CPAT equipped with a power converter can be utilized in distribution systems to control grid current and load-voltage waveforms while operating as a step-up or step-down transformer between the grid and load. Moreover, it can provide other services that any typical shunt-series compensation arrangement provides. The design and analysis of a single-phase CPAT are presented, showing the effect of coupling between windings and transformer parameters affecting CPAT operation. In this paper, control of the CPAT in a unified power-quality controller application is investigated to attenuate grid-current and load-voltage harmonics as well as compensate for reactive power requirements and mitigate grid inrush current. Through simulation and experimental implementation, the merits and performance of the CPAT were validated.

Reactive Power Control for Single-Phase Grid-Tie Inverters Using Quasi-Sinusoidal Waveform

This paper presents a reactive power control technique for single-phase photovoltaic (PV) inverters, especially unfolding inverters. The proposed system retains the benefit of the unfolding inverters having low material cost and semiconductor losses, and tackles the drawback of the standard unfolding inverter not having capability of reactive power injection. It is important to note that reactive power delivery is mandatory for PV inverters according to the recent announced regulations. The concept is based on changing the shape of the grid current waveform but keeping the same zero crossing points as in the unity power factor condition. The current waveform is governed by real power and reactive power, at the price of an acceptable deformation. The operating principles of the proposed technique and mathematical derivations of the grid current function are provided in this paper. Experimental results in a grid-tie inverter prototype have shown a good agreement with the derived theory, and they confirm the feasibility of using the proposed technique in grid-tie inverters.

A Novel Multi-Parallel Power Frequency Communication Technology

Power line frequency communication technology using the small distortion about the grid voltage and current waveform to carry information, and cross the transformer remote communication, suitable for line structure and load characteristics in distribution network. It has the advantages of low cost, easy installation, free communication and so on. But the power line frequency communication there is a low rate of defects, and the traditional modulation and demodulation are based on the local voltage zero, because the transceiver system voltage phase difference caused by receiving time domain deviation, which seriously affect the communication performance. This paper established a multi-channel signal transmission model based on the technical mechanism of frequency communication. We use single-phase multi-channel, three-phase transmission, multi-feeder parallel transmission and other means to improve the communication rate, At the same time, this paper based on time-frequency analysis to determine the frequency domain communication signal modulation time domain, combined with a variety of anti-jamming measures, which can adapt to the distribution network voltage difference between nodes, thereby enhancing communication performance.

High‐performance control system for grid‐tied ESSs

In this study, a high-performance controller is proposed for single-phase grid-tied energy storage systems (ESSs). To control power factor and current harmonics and manage time-shifting of energy, the ESS is required to have low steady-state error and fast transient response. It is well known that fast controllers often lack the required steady-state accuracy and trade-off is inevitable. A hybrid control system is therefore presented that combines a simple yet fast proportional derivative controller with a repetitive controller which is a type of learning controller with small steady-state error, suitable for applications with periodic grid current harmonic waveforms. This results in an improved system with distortion-free, high power factor grid current. The proposed controller model is developed and design parameters are presented. The stability analysis for the proposed system is provided and the theoretical analysis is verified through stability, transient and steady-state simulations.

Mathematical Modeling of Electrical Grid Current and Voltage Waveforms for Protective Relay Tests Under Actual Faulty Grid Conditions

Protective relays need to be tested under actual faulty grid conditions. On the other hand, their subsystems, as the signal conditioners, A/D converters, synchronization devices and digital filters, often need to be tested with current and voltage waveforms containing known DC, fundamental and harmonic components, since the relay operation principles are usually based on these components. Available on the market, there are many EMTP to generate high-fidelity faulty waveforms and many test sets to playback COMTRADE files capable of supporting most test cases for relays. However, these devices do not have the ability to formulate a physically meaningful grid current and voltage mathematical model. That model can be useful to improve protective relays performance tests by comparing the relay operation with its mathematical model. Besides, the waveforms model grants flexibility and accuracy to resample an actual fault register. Further, the model can be useful in relay coordination studies, performed with EMTP. In this work, we propose a fitting scheme to build a physically meaningful current and voltage waveforms model from EMTP or actual grid fault registers. Five hundred fifty current or voltage waveforms were fitted by our scheme and suitable models were achieved, based on the coefficients of determination, which is the selected measure of goodness of fit.

Fuzzy logic controller vs. PI controller for MPPT of three-phase grid-connected PV system considering different irradiation conditions

In this paper, a comparison between fuzzy logic controller (FLC) and conventional proportional-integral controller (PIC) is presented for a connected PV array system to utility grid via a three-phase line-commutated inverter. The main objective of the controller is providing maximum power for utility grid in different solar radiation conditions. The simulink model of the studied schemes is built using MATLAB/PSB for generating firing pulses of the thyristors in the inverter by applying PIC and FLC methods separately. Three different radiation conditions are examined on both controller which are constant solar radiation, step solar radiation and a combination of step and ramp scheme. According to simulations, FLC dominates PIC for all radiation types and in many important aspects. Some of these aspects could be named as, extracting power from PV array, providing active power for grid, shape of output current of inverter, shape of grid current waveform and total harmonic distortion (THD) of these currents.