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A cascaded NPC/H-bridge inverter with simplifiied control strategy and minimum component count

A comprehensive study of power system harmonics

  • Authors: Kasemuana Matototo, Seraphin
  • Date: 2014-04-16
  • Subjects: Electric power systems , Harmonics (Electric waves) , Electric power distribution , Electric networks
  • Type: Thesis
  • Identifier: uj:10802 , http://hdl.handle.net/10210/10309
  • Description: M.Tech. (Electrical and Electronic Engineering) , With the increasing use of nonlinear loads in power systems, harmonic pollution becomes more frequent with disastrous consequences on the electrical network and other loads. Harmonic currents cause problems such as system resonance, protective relays and switchgear malfunction, communication interference, incorrect meter, equipment and process failure, over heating of transmission and distribution systems, insulation degradation and power transformer failure. It appears therefore absolutely essential to determine the current profile of the loads connected to the same Point of Common Coupling (PCC). This study aims to assess a distorted power system in order to determine the characteristics components of distorted waveforms (DC components, fundamental and harmonics components), and to quantify the respective distortion contributions in the power network. The numerical method is applied to analyse the current waveforms captured with the aid of the digital scope meter (DSO) 1200 series at different sections of the network. The power network under study is designed, constructed and analysed with the MatLab/Simulink 7.0 package. In both methods, the Individual Harmonic Distortion (IHD) and the current total harmonic distortion (ITHD) for the line, the non-linear and linear loads are computed for the harmonic components 5th, 7th, 11th and 13th. The results showed that for the analysis of the power system, numerical analysis based on the resultant harmonic curve method and the MatLab/Simulink as applied to the network all converged to highlight that harmonic components appear both on the line, the PCC and the linear load. This is due to the fact that the converter characteristic harmonics (5th, 7th, 11th, 13th…) are found in the network components (line, PCC, linear load…). The resultant harmonic curve method indicated a randomly distributed percentage IHD in different sections of the network for harmonic orders higher than 7th, whereas the Matlab/Simulink results normally show a non-uniform decay pattern of the IHD as the harmonic order increases within the respective section of the network.
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An increase of a down-hole nuclear magnetic resonance tool’s reliability and accuracy by the cancellation of a multi-module DC/AC converter's output’s higher harmonics

An investigation of the effects of voltage and current harmonics on an electrical distribution island network

  • Authors: Modipane, Kabelo Clifford
  • Date: 2009-02-26T12:20:58Z
  • Subjects: Electric networks , Harmonics (Electric waves)
  • Type: Thesis
  • Identifier: uj:8167 , http://hdl.handle.net/10210/2172
  • Description: M.Ing. , With the advances in technology and the increase in industrial facilities, the harmonic content of an electrical network has always been a major concern among power system engineers. This is especially in an industrial environment, where the usage of power is very high as there are many large-scale types of equipment being used. Thus, it would be useful to know the sources, distortion level, impact on the power system and the equipment of harmonic currents on the harmonically rich electrical network. The presence of harmonics on electrical networks poses many problems to power system engineers and inconvenience or loss to the industries. Their effects on power system apparatus include resonance, reduced operating life of rotating machines and error in power calculations. For this study, the author would like to find out the implications of this issues, following these harmonic problems by analyzing, with some amount of simulations, practical measurements and assessments. In this way, a better understanding could be gained about these harmonic problems and harmonic contents. Steps could be taken to protect the power system equipment that could be affected by high harmonic currents and raise the quality of power supply.
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Assessing the contribution of harmonics at the point of common coupling in networks

  • Authors: Kasemuana Matototo, Séraphin
  • Date: 2020
  • Subjects: Harmonics (Electric waves)
  • Language: English
  • Type: Doctoral (Thesis)
  • Identifier: http://hdl.handle.net/10210/444400 , uj:38839
  • Description: Abstract: The presence of harmonics in voltage and current waveforms is a result of an increase in use of nonlinear loads in power systems. Utility and end users are in disagreement over who is responsible of polluting the Point of Common Coupling (PCC) and therefore poor power quality. Hence, there is a need for dedicated techniques of analysis to determine the contributions of harmonics between utility and customer... , Ph.D. (Electrical and Electronic Engineering Science)
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Harmonic impedance estimation of a power system implementing frequency domain measurement techniques

  • Authors: Venter, Fredrik Hendrik
  • Date: 2012-08-13
  • Subjects: Harmonics (Electric waves) , Electric power systems
  • Type: Thesis
  • Identifier: uj:9129 , http://hdl.handle.net/10210/5585
  • Description: M.Ing. , The increased used of non-linear loads within the power system during recent years have resulted in deviations from the perfect sinusoidal voltage and current waveform. This deviation from the perfect sinusoidal waveform can be expressed according to Fourier analysis by a set of co-sinusoidal waveforms having frequencies, which are a multiple of the fundamental frequency, referred to as harmonics. Non-linear loads are generally characterised by harmonic currents. It is therefore often preferred to express the harmonic emission limits in terms of harmonic currents rather than harmonic voltages. In order to translate the harmonic currents into harmonic voltages the harmonic impedance of the power system must be known. The power system's harmonic impedance can be assessed via computational, simulation and measurement techniques. The study will concentrate on the estimation of the harmonic impedance of a 33kV power system by implementing two online frequency domain measurement techniques. Both techniques are applied at the point of delivery of a dominant nonlinear load during normal and energisation load conditions with its power factor correction capacitor bank in and out off service. The estimated harmonic impedance is then compared with the calculated and simulated harmonic impedance.
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Impact of the converter operating modes on line current harmonic generation

  • Authors: Bokoro, Ntambu Pitshou
  • Date: 2012-06-04
  • Subjects: Electric power systems , Harmonics (Electric waves) , Electric networks
  • Type: Thesis
  • Identifier: uj:2333 , http://hdl.handle.net/10210/4790
  • Description: M.Ing. , Line commutated thyristor converters are proven to be natural line harmonic currentgenerating sources regardless of their operating modes. The quality of harmonic current components induced back to the lines is commonly described to be similar under both states of operation. This however does not seem to be quite obvious as far as the aggregate harmonic current contribution to the phase inputs per operating region of thyristor converters. It becomes thus important to examine the degree of association between thyristor converter operating states and the magnitude of current harmonic pollution induced back to the input lines to subsequently establish the most current harmonic polluting region of operation. For the purpose of this study, the Nyquist-Shannon theorem, best known as the sampling theorem, whereby the converter output signal or waveform is sampled by the switching function to reconstruct the input waveform, is numerically applied in conjunction with the MatLab/Simulink 7.0 which enables the simulation of the two-quadrant operation of direct current variable speed drive (DC VSD) and that of high voltage direct current link (HVDC) converter station operation, as well as physical measurements on the twoquadrant operation of the DC VSD (Saftronics DC2L) with the aid of the digital scope meter (DSO)1200 series. However, numerical analysis based on the sampling theorem, practical measurements obtained and the MatLab/Simulink simulations indicate that the converter operating states cause a randomly distributed harmonic current generation trend in the input lines as the firing angle is increased and the most harmonic current contribution cannot be attributed to a specific region of the controlled converter firing angle.
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Non linear load identification

  • Authors: Du Toit, Jacobus Petrus Verster
  • Date: 2015-03-02
  • Subjects: Harmonics (Electric waves) , Electric power systems , Wavelets (Mathematics)
  • Type: Thesis
  • Identifier: uj:13376 , http://hdl.handle.net/10210/13388
  • Description: M.Ing. , With the increased use of nonlinear loads such as variable speed motor drives and rectifiers, the voltages and currents on the power system grid are no longer sinusoidal. These non-sinusoidal waveforms cannot be analyzed by conventional power theories and the usual recourse is to decompose the nonlinear waveform into a set of harmonics. Harmonic voltage and current components are detrimental to the power system and may cause additional losses, or premature failure of equipment, and as such they have a definite influence on the quality of supply. This thesis shows the limitations and potential pitfalls of harmonic decomposition and other power theories, and examines various methods used for identifying, quantifying and modelling nonlinear loads. The aim of the thesis is to evaluate methods for attaching a specific disturbance or non-linearity on the voltage waveform to a specific load connected at the point of common coupling. The power theories examined include the total complex power, the IEEE working group definitions of apparent power, true power factor and harmonic adjusted power factor. Some new techniques for estimating the degree to which a load is disturbing the voltage at the point of common coupling is introduced, including the calculation of correlation indices, and the the use of wavelets.
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The impact of harmonic distortion on power transformers operating near the thermal limit

  • Authors: Geduldt, Owen Christopher
  • Date: 2009-02-26T12:19:56Z
  • Subjects: Electric distortion , Harmonics (Electric waves) , Eddy currents (Electric) , Electric transformers
  • Type: Thesis
  • Identifier: uj:8160 , http://hdl.handle.net/10210/2166
  • Description: M.Ing. , The study looks into the impact of harmonic distortion on power-plant equipment in general, and then focuses on the impact it has on power transformers operating near the thermal limit. The feasibility of the study is firstly evaluated and then the theory on harmonics and transformer losses is analysed. The study had been narrowed down to power transformers due to the high numbers of failures nationally and internationally attributed to unknown causes. A transformer model is then developed through theoretical considerations. Finally, a case study is done on the capability of a fully loaded transformer under harmonics conditions evaluated through transformer capability calculations and the proposed transformer model. Thereafter the transformer model developed is verified with measured results. The main impact of harmonic current distortion on power transformers is an increase in the rated power losses that results in a temperature rise inside the power transformer. The heat build-up can lead to degradation of insulation, which can shorten the transformer’s life and lead to eventual breakdown. The harmonic current distortion impacts transformer losses – namely, ohmic losses, the winding eddy current losses and other stray losses. All of these harmonic effects on transformer losses are verified theoretically, mathematically and practically. The harmonic impact on the transformer capability is then evaluated through a numerical example of a transformer feeding a harmonic load. The transformer capability is determined via two methods – namely, harmonic capability calculations in the standard “IEEE Recommended Practice for Establishing Transformer Capability when Supplying Nonsinusoidal Load Currents”, [11] and a proposed transformer model derived from theoretical and mathematical analysis. The results show that an increase in the winding eddy current losses can decrease the maximum permissible nonsinusoidal load current substantially. If the load current of the transformer is derated accordingly it translates into a loss of the output power capacity of the power transformer. The standard recommended capability calculations for winding eddy current losses are conservative and not satisfactorily accurate. This results in a large loss of power capacity. The proposed transformer model includes a parameter that estimates the winding eddy current loss in the transformer that results in a smaller loss in power capacity. Furthermore, it was shown that the harmonic current distortion levels could exceed the permissible levels although the harmonic voltage distortion levels are within acceptable levels. The proposed transformer equivalent model is thereafter practically verified with experimental results of papers published by M.A.S. Masoum, E.F. Fuchs and D.J. Roesler, [19], [20] and [29].
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