Waveform control method for mitigating harmonics of inverter systems with nonlinear load

Comparison of the input filter effect to PV panel by SEPIC MPPT converter

In modern power systems, renewable energy, such as solar and wind, has been increasingly exploited to replace traditional fossil fuels in hope of resolving the issue of energy crisis. Normally, the renewable generators are integrated into the power grid through power converters. A common issue with the photovoltaic (PV) powered single-phase inverter systems is that the performance and lifespan of the dc source might be undermined by the second-order current ripple (SCR) due to the double line frequency instantaneous power oscillating at the ac side. This issue tends to exacerbate when the inverter interfaces more complicated nonlinear loads, where the input current is adulterated with multiple low-frequency ripple components besides SCR. To solve this issue, this paper presents a repetitive controller based dual-mode control method for PV powered single-phase buck-boost inverter under nonlinear load scenario. Without introducing any additional circuit or passive electrical components, the proposed method manages to significantly mitigate ripple components in the input current with the reduced dc-link capacitance. Finally, simulated results are demonstrated to verify its effectiveness in current ripple reduction.

Lately, on the power rectifier circuit with semiconductor switching device, PWM control is generally employed for the purposes of reducing the harmonic currents on the DC side and improving the power factor of fundamental wave. In this case, the analysis of the current waveform becomes fairly complex. However, we express the PWM controlled voltage waveform by the step function, and can easily analyze the current waveform. We reported formerly some results of analysis about the waveform and harmonics of the current. In this paper, on the power rectifier circuit to which the nonequi-PWM and equi-PWM control are applied, we exactly calculate the harmonic currents on the DC and AC side, and the power factor by the use of the above analytical results. Thecharacteristics of both control methods are compared on condition that each controlled factor of the load voltage is equal. These results are shown in the calculated charts. The following items can be seen from these charts on the power rectifier circuit:(1) The nonequi-PWM control can let the harmonic currents on the DC side decrease more than the equi-PWM control. But on the contrary, the harmonic currents on the AC side increase. Consequently, when the nonequi-PWM control is applied, the total power factor decreases by the increase of distortion factor of the current on the AC side.(2) The difference between the harmonic currents on the DC and AC side to both of the nonequi-PWM and equi-PWM control increases with the increase of controlled factor of the load voltage.

Non-linear loads have become more prevalent due to the increasing use of power electronics for power conditioning and regulation. The current harmonics generated from non-linear loads can cause long-term effects on generator sets, such as the degradation of their lifetime due to higher operating temperatures and component failures due to high current transients. A testbed was developed to run a 3-kW military tactical generator set under non-linear loads. The 3-kW generator set includes a permanent magnet alternator (PMA) and a power electronic converter to deliver a 120VAC/240VAC output at 60 Hz. A range of non-linear loads was developed to create several load profiles for the generator set. The generator set was tested in four-hour and two-hour intervals for several weeks under different load profiles to stress its alternator and internal power electronics. In addition, several measurements were taken during testing, such as the output power, output voltage and current, and stator winding temperature. The collected data and testing suggest that the power electronics converter is more susceptible to the current harmonics than the PMA. These issues can result in premature failures and overall degradation of the lifetime of the generator set.

With the wide use of power conversion devices - 'nonlinear loads' - many harmonic currents are being injected into the power grid. Shunt Active Power Filters SAPF are the power electronic equipment to compensate the harmonic currents caused by nonlinear loads. As the foundation of the harmonics recognition and compensation, harmonic extraction is the key technology in SAPF. Artificial Neural Networks ANN method has the features of parallel computation and satisfactory results for distorted source voltages over traditional extraction methods. This paper proposes a new harmonic extraction method based on ANN. To test the feasibility of different types of neural networks in this application, this paper compares the performances of three types of ANN: Echo State Networks ESN, Recurrent Neural Networks RNN and Multilayer Perceptron Networks MLP.

In a distribution system, to compensate for the effect of voltage sag/swell and mitigate harmonic currents, a unified power quality conditioner (UPQC) is used. Conventionally, the power rating of UPQC is determined based on the expected sag/swell, linear and nonlinear loads. However, with the shift towards usage of electronic loads and electrical drives, the composition of nonlinear loads is increasing. Hence, the UPQC deployed earlier may not be capable of handling increased harmonic currents due to its limited current rating. While trying to compensate for both harmonic currents and voltage sag/swell, the shunt converter (of UPQC) may reach its current limit and would result in undesirable behaviour. To address this issue, a technique to prioritise sag/swell compensation over active filtering is proposed in this study. The remaining power capacity of UPQC (after sag/swell compensation) is utilised for harmonic filtering. Adaptive determination of harmonic compensation current also allows prioritising the various harmonics to be compensated. This helps in case passive tuned filters are installed for some harmonic frequencies for which UPQC need not respond. Detailed simulation studies are performed on MATLAB/SIMULINK platform and results are included in this study. A laboratory prototype of single phase UPQC is developed to validate the performance of the proposed scheme.

Harmonic distortion in electric distribution system is increasingly growing due to the widespread use of nonlinear loads. Large concentrations of these loads have the potential to raise harmonic voltages and currents in an electrical distribution system to unacceptable high levels that can adversely affect the system. Hence there is need to study the impact of nonlinear loads in the system. This paper presents a method based on bispectrum analysis to characterize typical single phase nonlinear loads by analyzing the harmonic currents produced by these loads. Measurements are conducted using a power quality monitor to record the harmonic current waveforms, individual harmonic distortion levels and total harmonic distortion. To investigate the harmonic characteristics of these loads, the fast Fourier transform technique and the advance signal processing technique such as the bispectrum analysis are used. The results of harmonic characteristics obtained from these loads are compared so as to investigate the effectiveness of the bispectrum analysis in characterizing the various nonlinear loads.

41 - Active Filters

In this paper, a novel control system structure is introduced for generating a sinusoidal wave on the differential boost inverter terminals using inverse model approach. To address the high-frequency resonance issues, an active damping technique is proposed. Proportional-resonant controllers are designed for tracking and harmonic rejection. Pulling direct and constant current from photovoltaic panels as the DC source is essential in maximum power point tracking. Therefore, a decoupling controller is utilised to suppress current oscillations. An innovative combination of tracking controller, harmonic rejector, and decoupling controller is proposed in order to provide a sinusoidal voltage output for linear and nonlinear loads with low total harmonic distortion. The proposed method reduces the number of controllers by half compare to conventional methods. In addition to simulation, experimental results show the effectiveness of the proposed method.

The main objectives of a load sharing control in dc power systems are proper power sharing among dc sources with an acceptable voltage regulation through the grid. Virtual resistor based droop methods also called voltage droop methods have low performance and accuracy. Hence, supervisory controllers reinforced by a communication system are required to improve the effectiveness of the voltage droop methods. Furthermore, frequency droop approaches have been presented for accurate load sharing control in dc power systems without utilizing a communication system. However, the ripples are superimposed on voltage and current waveforms affecting overall system efficiency and power quality. This paper proposes an Enhanced Frequency Droop Method (EFDM) to overcome the main drawbacks of the frequency droop approaches by stopping voltage superimposing at steady state. Preliminary simulation results show the effectiveness of the proposed approach.

This paper will discuss the monitoring and data analysis of nonlinear load power quality by analyzing electrical railway, which is a typical nonlinear load in Beijing. Firstly, this paper will introduce a method of designing the software and hardware of a kind of highly accurate and large-capacity energy quality data recorder, which is capable of accurately recording voltage, electric current wave, etc., data on site for a long standing and can restore the original data with upper computer in lab. Secondly, this paper will analyze the data collected by the energy quality data recorder from Beijing Xiazhuang transformer substation, which is responsible for supplying electricity to Daqin railway and is a typical nonlinear electrical railway load. On the one hand this paper presents the complete data of fundamental current and harmonic current in a typical working day (as shown in graphs) and analyzes the correlation between fundamental current and main harmonic current. On the other hand, this paper also analyzes the amplitude of main harmonic current distribution with statistical methods and establishes an autoregressive (AR) model to illustrate the distribution of main harmonic current. From the above analysis we can get the key data for evaluating the impact generated by this transformer substation on the grid's electrical energy quality.

This paper presents a new six-level flying capacitor based (FC-based) multilevel converter with one dc source and the capability of operating in all power factors and modulation indexes. Multilevel converters with one dc voltage source are attractive in many applications as they do not need rather expensive and bulky multiwinding input transformer connection at the dc side. On the other hand, not all classic multilevel converters with one dc source can produce any desirable number of output voltage levels at all power factors and/or modulation indexes. In this paper, a hybrid structure is proposed in which six voltage levels can be realized at the ac terminals. The modulation technique and the control strategy for the FC voltage balancing are presented. To show the advantages of the proposed converter, different performance criteria, such as switch count and rating, the size of capacitors, switching frequency, and power losses, are compared with other existing six-level topologies. The results indicate that the proposed structure is superior to other six-level converters from different standpoints. Simulation results are used to further evaluate the performance of the proposed converter. A laboratory-type experimental setup is used to validate the theoretical results.

Introduction. To improve the power quality of a supply system, the total harmonic distortion (THD) is the most important parameter in the quantification of harmonics caused by nonlinear loads. In practice, it must be less than 5 %. The novelty of the proposed work consists in the use of a cascaded five level active filter, when the converter consisting of six H-bridge pairs, each one includes four transistors. Purpose. To increase the efficiency of this filter, two techniques for quantification of harmonic currents are proposed, first the PQ-theory which is simple but can only be used in case of a balanced grid, and second the synchronous reference frame theory (SFR-theory), which is capable of creating harmonic current not only in an unbalanced grid, but also in an unbalanced and distorted beam. Methods. Using the control techniques, the harmonic current is extracted from load current and considered as a reference. The constructed current should follow this reference. Results. The estimation of the active and reactive powers is based on the measurement of the currents crossing the load and the network voltages, these powers are used to determine the shape of the harmonic (reference) current. Using the PI regulator, the output current of the five-level inverter follows the reference current perfectly. The inverters output current is injected into the grid to eliminate harmonic currents. Practical value. In practice, the harmonic distortion rate THD is the most widely used criterion for criticizing the waveform of the currents and judging the quality of the energy involved. For currents on the source side, the THD is considered acceptable if it is less than 5 %, in our proposal the THD is 0.85 % with the PQ-theory and 2.34 % with SFR-theory, so it is optimal.

In this paper, average modeling is derived for both AC and DC sides of grid-tied cascaded H-bridge converter (CHB). The proposed technique uses the AC current harmonics to control and reduce the current harmonic ripples on the DC sides of the CHB converter. By using the proposed modeling technique, the DC current harmonics of all cells of the CHB can be balanced including the zero- and the second-order current harmonics. This balancing technique has several advantages. First, due to balancing and regulating the even order current harmonics of all cells, the DC side current harmonics of batteries of electric vehicle are equal. So, the lifetimes of all batteries are kept the same due to dealing with the similar current harmonic spectrums. Next, as will be derived in the equations, controlling the odd-order AC side current harmonics of the grid-tied converter can reduce the even-order harmonics of the DC sides of the converter. This can increase the lifespans of batteries. Finally, the low-frequency selective harmonic current mitigation-pulse width modulation (SHCM-PWM) technique is used in this paper which meets the requirements of IEEE-519 for AC side harmonics. The simulation and experimental results in this paper validate the advantages of the proposed current harmonic balancing technique.

As a continuation of the work done by the QSE (Electrical Supply Quality) research group at the UPC (Polytechnic University of Catalonia) on non-linear load modelling to determine harmonic distortion in electrical power systems, this thesis consists of the carrying out of a series of studies of the most usual non-linear loads in the said systems (single-phase and three-phase rectifiers with capacitive filter, three-phase 6-pulse rectifiers and discharge lamps), basically aiming to characterize their behaviour and assess their influence on electrical power systems. The thesis was prepared in cooperation with the GNOM (Group of Numerical Optimization and Modelling) research group at the UPC in order to be able to analyse the numerical problem raised by the system of non-linear equations which characterizes each of the non-linear loads, as well as being able to approach optimization issues with some certainty of success. In summary, the aims of this thesis are as follows: -To analyse modelling of the most usual four non-linear loads in electrical power systems in order to incorporate the invariant concept in a justified way and establish ranges of values for the said invariants depending on their usual operating conditions. -To exploit the possibilities offered by invariants and, on the basis of the ranges of values obtained for each of them, to proceed to characterize the deterministic behaviour of the non-linear loads which are the object of the study and also to analyse the phenomena of harmonic cancellation and interaction. -To describe and study experimentally, also on the basis of invariants, the fundamental and harmonic currents (magnitude and angle) consumed by the non-linear loads considered in this thesis, whether by obtaining experimental models (databases) validated both experimentally and numerically, by validating the studied analytical models experimentally or by fitting the parameters of any of the previous models (parameter estimation) on the basis of experimental acquisitions. -To study the neutral conductor current in three-phase systems to which single-phase non-linear loads are connected (in particular, single-phase rectifiers with capacitive filter), by using invariants and validating the results of this experimentally. Como continuación de los trabajos realizados por el grupo de investigación QSE (Calidad del Suministro Eléctrico) de la UPC (Universidad Politécnica de Cataluña) sobre modelización de cargas no lineales para la determinación de la distorsión armónica en los sistemas eléctricos de potencia, se ha realizado como tesis una serie de estudios sobre las cargas no lineales más habituales en dichos sistemas (rectificadores monofásicos y trifásicos con filtro capacitivo, rectificadores trifásicos de 6 pulsos y lámparas de descarga) encaminados básicamente a la caracterización de su comportamiento y al análisis de su influencia en los sistemas eléctricos de potencia. La tesis se ha desarrollado en colaboración con el grupo de investigación GNOM (Grupo de Optimización Numérica y Modelización) de la UPC para poder analizar el problema numérico planteado por el sistema de ecuaciones no lineales que caracteriza cada una de las cargas no lineales, así como poder abordar con garantías problemas de optimización. Resumidamente, los objetivos de esta tesis son: -Analizar la modelización de las cuatro cargas no lineales más habituales en los sistemas eléctricos de potencia con el objetivo de introducir de forma justificada el concepto de invariante y establecer rangos de valores para dichos invariantes de acuerdo a sus condiciones habituales de funcionamiento. -Aprovechar las posibilidades que ofrecen los invariantes y, a partir de los rangos de valores obtenidos para cada uno de ellos, proceder a caracterizar el comportamiento determinístico de las cargas no lineales objeto de estudio y también a analizar los fenómenos de la cancelación y la interacción armónica. -Caracterizar y estudiar experimentalmente, también a partir de los invariantes, las intensidades fundamental y armónicas (módulo y ángulo) que consumen las cargas no lineales consideradas en esta tesis, ya sea mediante la obtención de modelos experimentales (bases de datos) validados tanto experimental como numéricamente, la validación experimental de los modelos analíticos estudiados o bien el ajuste de los parámetros de cualquiera de los anteriores modelos (estimación de parámetros) a partir de adquisiciones experimentales. -Estudiar la intensidad del neutro en sistemas trifásicos en los que hay conectadas cargas no lineales monofásicas (en particular, rectificadores monofásicos con filtro capacitivo), utilizando los invariantes y validando experimentalmente los resultados que se obtengan.