Comparing-the harmonic characteristics of typical single-phase nonlinear loads

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.

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.

This paper discusses the design of a passive filter system for Energy-Saving Lamps (LHE) and Light Emitting Diodes (LED) using the MATLAB Simulink software. This type of lamp is a type of non-linear load that produces harmonics of current and voltage. However, this harmonic problem can be reduced using passive filters. To determine the size of the passive filter components, research was carried out in the form of measurements of power, power factor, voltage, current, THDi and THDv produced by the LHE and LED. The results of these measurements were simulated using MATLAB Simulink to determine the passive filter design that reduces the THD value on the LHE and LED. To reduce the level of current harmonics, a single tuned LC passive filter was designed. The filter, designed to work at a frequency of 50 Hz and is expected to reduce the level of harmonics in the 3rd, 5th, 7th, 9th, 11th harmonic orders so that the THD produced by LHE and LED meets the IEEE 519-2014 standards. The simulation results of single tuned LC passive filter design can reduce THDi by 46.78% from the initial THD of 84.55% so that it becomes 37.77%.

The connection of nonlinear loads such as thyristor drive systems to the power system creates waveform distortion due to the harmonic currents which are injected. The propagation of the harmonic currents throughout the power system is governed by a number of factors including resistance, inductance and capacitance of circuit elements and the system load. The harmonic current at the nonlinear load may be substantially balanced, but due to asymmetry in overhead line and transformer configurations, in particular, residual harmonic currents are produced, which circulate through line capacitance-to-ground and transformer grounded star points. These currents, being of zero sequence order, may cause noise interference in neighbouring telephone circuits. Since the harmonic currents in the power system may become amplified as they progress to the power frequency source, the interference could be considerably increased. The paper addresses the basic theory involved and studies particular systems with a view to using simplified manual calculations. From a knowledge of the residual current magnitude produced by a nonlinear load, noise interference could be determined.

As modern three-phase distribution systems continue to add more nonlinear and fast-changing loads, the need to maintain steady voltage and good power quality has become much more critical. A commonly used solution for these issues is the Unified Power Quality Conditioner (UPQC). It is a custom device that helps the system handle voltage sags, voltage swells, harmonic distortion and unbalanced conditions. The actual performance of a UPQC depends a lot on the control method used to operate it. In this study, two control techniques are compared, fuzzy logic and artificial neural networks (ANN), to understand how well they support voltage regulation and compensation during varying load conditions. The analysis first looks at how the system behaves under induction motor loads and nonlinear loads when no UPQC is present. After that, a UPQC with a basic PI controller is introduced so that it can serve as a reference point. Then the UPQC models controlled by fuzzy logic and by ANN are developed and tested using MATLAB and Simulink. The work focuses on the waveforms of voltage and current, the total harmonic distortion, and how effectively each method maintains system stability when the load changes. The results show that both fuzzy logic and ANN improve the dynamic performance of the system and help bring down the harmonic levels when compared with the traditional PI controller. Among the two methods, the ANN controlled UPQC shows better adaptability and handles sudden load variations more smoothly.

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.

In this modern generation almost every power supplies are non-linear loads which contains electronic devices that do not conduct current over the full cycle of the applied voltage, and thus they introduce harmonics into the power network. Electronic front end of non linear load makes them more efficient than linear loads and these non-linear front ends are one of the source of harmonic. Harmonic is nothing but a signal or a wave having frequency is an integral (whole-number) multiple of the frequency of some reference signal or wave or fundamental frequency. Harmonic causes losses in the power distribution network and can lead to increased cost due to maintenance, failures or device de-rating and thus decreases the power quality of the power distribution network. Power quality may be defined as a set of electrical characteristics that allows a piece of equipment to function in its desired manner without any interruption and significant loss of performance or life expectancy. Total harmonic distortion (THD) is an important figure of merit used to quantify the level of harmonics in voltage or current waveforms. In this paper Distribution Static Compensator (DSTATCOM) with Synchronous Reference Frame based control which taking into account Three-Level Cascaded multi level Inverter for minimization of harmonics in Power System. This Three level Cascaded multilevel inverter supply low harmonics distortion, reduced number of switches, suppression of switching losses and take out the Total Harmonics Distortion (THD).

This paper proposes the implementation of a single-phase low cost grid connected axial flux permanent magnet (AFPM) generator system with harmonic mitigation functionality for various types of nonlinear loads. The design, construction and testing of the AFPM generator are given. The method of current reference calculation derived from a combination between power transfer and harmonic compensation for nonlinear loads is described. The synchronization with the grid does not require phase locked loop (PLL). It employs a simple technique based on a sinusoidal look-up table The whole converter system is implemented on a low cost dsPIC30F4011 microcontroller. The simulation and experimental results confirm the validity of the proposed system under various conditions.

41 - Active Filters

The extensive use of power electronics devices in power systems, including rectifiers and inverters, presents considerable issues regarding power quality. One problem is the creation of current and voltage harmonics, which result in load waveform distortion, voltage fluctuation, voltage drop, and equipment overheating. Harmonic current sources are prevalent in commercial, institutional, and medical establishments, encompassing computer power supplies, fluorescent lights with electronic ballasts, elevator motors, and electrical devices using switch-mode power supplies. Active power filters (APF) represent the most effective method for harmonic reduction and may be used across many applications. This study employs a digital active power filter to eliminate harmonics from the source current in a grid system characterized by non-linear demand. Nonlinear loads are essential components of every power system. The advent of power electronics has resulted in switching components constituting a substantial portion of electrical demand. These features create discontinuities in lines, exacerbating line losses and diminishing power quality and sine wave integrity. Passive filters effectively eliminate harmonics at specified frequencies within the design. Active filters, conversely, operate in accordance with the prevailing harmonics. This work develops a shunt active power filter that mitigates current harmonics generated by non-linear loads. The active filter includes a voltage source converter (VSC) that functions in conjunction with a DC coupling capacitor. The VCS delivers power at the point of common coupling (PCC). The reference current is obtained by the instantaneous power theory. The instantaneous power theory, or PQ theory, has been enhanced with the FIS control system to optimize DC voltage generation over the DC link capacitor. The fuzzy system substituting the PI controller has a 49-rule base structure organized in a 7x7 configuration for the generation of error signals. A comparative study of DC voltage and total harmonic distortions (THDs) is conducted using MATLAB software and the FFT analysis tool..

Concentric neutral cables are commonly used in medium-voltage power distribution systems. Presently, the cable ampacity calculation methods can take into account only the fundamental frequency current and the harmonic currents in the phase conductors. In reality, the cable neutral can carry fundamental frequency and harmonic currents (induced and unbalanced), leading to a further increase of cable temperature. In view of the increased harmonics and load imbalance in power distribution systems and the need to determine their impact on concentric neutral cables, this paper presents a method to estimate the ampacity of concentric neutral cables, especially for multigrounded neutral systems. The proposed cable derating factor can be used to determine the increase in cable loading caused by unbalanced load and harmonics. A chart is also presented as a simplified method for cable ampacity and cable loading estimation, for which only the total harmonic distortion and zero-sequence current ratio of the phase current are needed.

This paper proposes new and simple control method for a stand-alone doubly fed induction generator (DFIG) connected to unbalanced and nonlinear loads. This type of load causes harmonic distortion in the stator current and voltage. To solve this problem, a novel control method is proposed in the load side converter (LSC). In this method, instantaneous active and reactive power theory in stationary [α-β] frame is used. Unlike other control strategies, which needs to extract and compensate each harmonic or unbalanced component individually, in this paper, all harmonics and unbalanced components are injected by LSC. Finally, stator voltage becomes sinusoidal with very fast dynamic response. The performance of the DFIG feeding nonlinear and unbalanced loads is evaluated for steady state and dynamic conditions, using MATLAB/Simulink.

Three-phase four-wire (3P4W) electrical distribution systems are extensively utilized in commercial and industrial sectors. These electrical distribution systems are facing severe power quality issues such as current harmonics, reactive power, and load unbalancing, excessive neutral current. This paper presents the analysis, design and digital simulation of a hysteresis current controller (HCC) based active power filter (APF) to improve the power quality in a 3P4W electrical distribution system. In the present work APF is implemented with a 4-leg voltage source inverter (VSI) which is connected in shunt to the 3P4W electrical distribution system. The VSI switching signals are generated through a hysteresis current controller (HCC), which has good current tracking accuracy, unconditioned stability and easy implementation. The proposed HCC based APF compensates current harmonics, reactive power, and load unbalancing and neutral current in 3P4W electric distribution system feeding nonlinear and unbalanced loads. The performance of the HCC based APF is demonstrated using MATLAB© simulation.

Harmonic analysis currently plays an important role for the power quality analysis because the non-linear loads, as source of the harmonic distortion, have significantly increased. The harmonic distortion influences power quality in power systems such as overheating of electrical equipment and aging of electrical equipment. Therefore, non-linear loads should be monitored in real time. This research proposes the design and implementation of the prototype current harmonic analysis that can be applied in real time based on S-transform Method. The equipment of this prototype can be explained as follows: CT-235, TZ2L9 and CT for KWH meters as current sensor, microcontroller AT mega 8535 as data acquisition, and Visual Basic software as displaying current waveform, current spectrum, rms current value, and total harmonic distortion (THD) for current. S-transform method is used to obtain the THD for current. The experimental result is validated by FLUKE 43B and the THD for current is accurately measured.

This paper deals with the harmonic mitigation and reactive power compensation using Shunt Active filter. The performance of the Shunt active filter depends on the design and current control technique for generating a reference current. The design of the filter involves the iterative procedure for choosing the filter components so that it compensates the harmonics and reactive power for maximum load condition. The current controller method adopted for indirect current control technique where the reference signal is similar to the supply current signal. The Adaptive filter used in current controller mitigates harmonics and reactive power and voltage control loop of SAF balancing the active power between the load and supply unit. The learning rate is fixed by repeated simulation. The SAF active filter is tested in four load condition ie., nonlinear load(37% Total Harmonic Distortion(THD)),highly nonlinear load(>100% THD), linear load(<8% THD, power factor(pf) <0.5) and both nonlinear and linear load(37% THD and 0.7 pf). The study is carried out in MATLAB Simulink and the results are presented.