Input Power Factor Problem and Correction for Industrial Drives

Osunde, D.O (2010-03)

A Thesis Submitted to the School of Postgraduate Studies, University

Thesis

In an ideal power system, the voltage supplied to customer equipment and the resulting input currents should be sinusoidal waves. In practice, however, these waveforms can be quite distorted. This deviation from perfect sinusoids is usually expressed in terms of harmonic content of the voltage and current waveforms. Most equipment connected to an electricity distribution network usually may need controlled power conversion equipment which produces a non - sinusoidal line current due to the nonlinear load. With such loads as RLC, the switching action of the devices makes the system non- linear. Also, with the steadily increasing use of such equipment, line current harmonics have become a significant problem. Their adverse effects on the power system are well recognized. Harmonics are unwanted frequency components, which arise from the use of semi-conductor controllers. Modern industries and applications which include the steel plants, traction systems, industrial drives, furnaces etc generate voltage and current harmonics which have adverse effects on the supply lines and equipment connected to such lines. The harmonics generated according to Okoro (1982, 1986) and Redl (1994) result in distortion of line voltages, degradation of power factor of electrical equipment thereby increasing the reactive power consumption and also overall running cost of equipment. The overall effects are reduced efficiency, increased heating effect and lead to Poor Power Factor on the AC inputs of the industrial drives. Also, voltage distortion produces such effects as motor prematurely burning out due to overheating, increased losses and lower efficiency. There are many problems associated with harmonics within an industrial plant (Agu 1997) and there have been many efforts made without results in the past aimed at collecting data on harmonics from industrial companies operating in Nigeria.(Agu 1997). The up - coming Ajaokuta Steel Company and subsequent industrialization from subsidiary companies are expected to increase the harmonic currents in the National Grid. This study investigates the impact of these harmonics on the AC power supply inputs to these industries. In steel plants, most equipment for moving raw materials and finished products are fed from controlled single – phase AC – DC bridge converters which produce the worst case of harmonic distortion. There is therefore the need to mitigate harmonics at the point where the offending equipment is connected to the power system. Power system harmonic distortion is not a new phenomenon. Effort to limit it to acceptable proportions has been a concern to power engineers from the early days of utility systems Okoro (1982). At that time, the distortion was typically caused by the magnetic saturation of transformers by certain industrial loads such as arc furnaces or arc welders. The major concerns were the effects of harmonics on synchronous and induction machines, telephone interference and power capacitor failures (Agu 1997). In the past, harmonic problems could often be tolerated because equipment was of conservative design and grounded WYE- Delta transformer connection was used judiciously. Also, star connections of three – phase windings in rotating machines eliminate the 3rd - order harmonics. S.M. Bashi et. al (2005) proposed a harmonic injection technique, which reduces the line frequency harmonics of the single switch three-phase boost rectifier. In this method, a periodic voltage is injected in the control circuit to vary the duty cycle of the rectifier switch within a line cycle so that the fifth-order harmonic of the input current is reduced to meet the total harmonic distortion (THD) requirement. Ying-Tung Hsiao (2001) presents a method capable of designing power filters to reduce harmonic distortion and correct the power factor in an Industrial distribution network. The proposed method minimizes the designed filters’ total investment cost such that the harmonic distortion is within an acceptable range. The optimization process considers the discrete nature of the size of the element of the filter. It is to be noted that the presence of harmonics in the supply waveforms has other wide-ranging effects on the supply system. These include: • Communication system interference. • Degradation of equipment performance and effective life • Sudden equipment failure • Protective system mal-operation • Increased power transmission losses • Overheating in transformer, shunt capacitor, power cables, AC machines and switchgear leading to pre-mature ageing Harmonics result in distortion of line voltages and currents, degradation of power factor of electrical equipment thereby increasing the reactive power consumption and also overall running cost of equipment. The poor Power Factor problems on the AC input of Industrial drives is expected to increase with increased industrialisation where large numbers of such drives are connected to the National Power (PHCN) Network. In order to completely understand the effects of harmonic distortions and poor input power factor on controlled drives, the single – phase asymmetrical bridge Drive was chosen for the study. The choice of this Drive is influenced by the fact that • it presents a high level of harmonic content • it has a wide range of applications in traction and industrial motor control systems • it is increasingly being applied to main-line rail propulsion systems • it is widely used in low power motor control systems • it is simple and inexpensive

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