ANALYSIS AND CONTROL OF POWER SYSTEM HARMONICS

Abstract

The conventional power supply systems are designed to operate with sinusoidal waveforms, with the aim to maintain the voltage level at equipment terminal within certain limits. Ideally electrical energy must be supplied at a single constant frequency and specified voltage levels of constant magnitudes. However, this situation is difficult to achieve in practice. The undesirable deviation from a perfect sinusoidal waveform (variations in the voltage magnitude and /or the frequency) is generally expressed in terms of power quality. Power quality (PQ) is the combination of voltage quality and current quality. Voltage quality is concerned with deviations of the voltage from the ideal. The ideal voltage is a single frequency sine wave of constant amplitude and frequency. Current quality is the complementary term to voltage quality with the additional requirement that the current sine wave is in phase with the voltage sine wave. Any compromise with either attribute is a power quality concern. Globally, increasing trend of liberalization of the energy market and privatization of the power supply industry has made the quality of electrical power an important feature of consumer goods on the market. Deficient quality of electrical power supply results in performance degradation of equipment. In general, the foremost power quality issues can be identified as waveform distortion due to harmonics, voltage unbalance, voltage dips, voltage swells /over-voltages, transients, flicker, and frequency variations etc. Among these, voltage unbalance and harmonics distortion are probably the most degenerative conditions to power quality because of being a steady state condition. Voltage unbalance is a condition in which the three phase voltages differ in amplitude or are displaced from their normal 120 degree phase relationship or both. Thenature of the unbalance includes unequal voltage magnitudes at the fundamental system frequency, under-voltage or over-voltage, fundamental phase angle deviation, and unequal levels of harmonic distortion between the phases. The voltage unbalance is inherent to electrical system due to unequal phase impedances and unbalanced load distribution and it can exist anywhere in a power distribution system. Power system operation under unbalanced conditions putting worries for the power system engineers. Rectifiers are widely used in power conversion industries for converting AC power to DC power. However, these rectifiers generate a large amount of current harmonic components back to the input utility side, due to their nonlinear nature. In case ofunbalanced voltage supply, these rectifiers produce non-characteristic harmonics and changes in characteristic harmonic pattern. In general, it can contribute for high DC distortion, overvoltages, highly unbalanced currents, protection relay malfunction etc. The harmonic distortion, and the means of keeping it under control, is a growing concern. This is primarily due to the increase in the number and application of nonlinear power-electronic equipments used in the control of power apparatus and the presence of sensitive electronic equipment. In electrical power systems, the harmful effects of harmonics are very real and they cause various problems in power systems. The control or mitigation of the harmonic distortion may be realized through the use of harmonic filters. Traditionally, passive LC filters have been used to eliminate line current harmonics, to increase the power factor, and thus improving the quality of power system. However, these are tuned to specific loads and are unsuitable for use in power systems with widely varying load currents. Furthermore, their operation depends on the electricity network impedance and the characteristics of the nonlinear load. An undesirable parallel resonance could occur between the source impedance and the shunt filter, at a specific frequency. Accordingly, active power filters (APFs) have been researched, developed and gradually recognized as a feasible solution to the problems caused by nonlinear loads. APFs are used to eliminate the undesirable harmonics and reactive power components ofload currents by injecting equal but opposite compensation currents. One ofthe most popular active power filters is the shunt active power filter. It is a current controlled device, connected in parallel with the nonlinear load. Classically, a shunt APF is controlled in such a way as to inject harmonic and reactive compensation currents based on calculated reference currents. The injected currents are meant to 'cancel' the harmonic and reactive currents drawn by the non-linear loads. An increasingly attractive alternative is to use so-called intelligent control schemes involving tools such as expert systems, neural networks, or fuzzy logic. Artificial intelligence (AI) is one of the major fields developed since past four decades, and is popular due to its ability to handle complex problems at difficult situations. The AI techniques like fuzzy logic have already been utilized for harmonic minimization and powerquality improvement. The design of APF under 'fixed load' conditions is the most important observation from the work reported by various authors, though the load is continuously and randomly varying, in practical situations. Therefore, it is considered to design an APF for variable load conditions. In view of these, there is strong motivation to undertake a thorough and systematic study on effects of power system voltage unbalance on the harmonics injected by an AC-DC rectifier load (nonlinear load), and its impact on AC-DC rectifier performance, and to develop a simple, robust fuzzy logic based active power filter to control the harmonics under variable load conditions. The first part of this thesis reviews two typical power quality attributes viz. supply voltage unbalance and power system harmonics in terms of their causes and effects. Experimental survey of harmonics injected by various home appliances, equipments and devices is carried out to quantify and compare their individual and combined harmonic distortion. Ill In second part, a comprehensive study aboutthe impact of source voltage unbalance on the performance of a three-phase AC-DC rectifier has been presented. The study includes the effect of source voltage unbalance on harmonic distortion of different input and output characteristics of the rectifier. Also, the harmonics injected by the rectifier load are investigated in terms of variation in magnitude of different characteristic and non-characteristic harmonics with variable degree of unbalance. In final part, a simple fuzzy logic based robust active power filter to minimize the harmonics for wide range of variations of load current under stochastic conditions, is proposed, which is found very simple and also capable of maintaining the compensated line currents balanced, irrespective of the unbalance in the load currents. The proposed methodology is extensively tested for wide range of variable loads under stochastic conditions and results are found to be quite satisfactory to mitigate harmonics and reactive power components from the utility current.