Doctor of Philosophy
School of Electrical, Computer and Telecommunications Engineering
Zhao, Kun, Performance of LV loads in the presence of voltage fluctuations: preliminary investigations based on critical component analyses, Doctor of Philosophy thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 2013. http://ro.uow.edu.au/theses/3991
Voltage fluctuations in distribution networks are a power quality disturbance that can produce a number of undesirable eects, particularly if the level of fluctuation becomes excessive. Utilities and customers have a responsibility of ensuring that levels remain below values that are regarded as acceptable. In order to determine what an acceptable level might be, measurement techniques have been devised that are able to establish a quantitative measure for voltage fluctuations. The basis of these methods is the effect that voltage fluctuations have on the luminous output of the incandescent lamp. This phenomena is referred to as lamp flicker, or simply flicker.
The IEC Technical Reports IEC/TR 61000-2-2 and 61000-2-12 define flicker levels that arise in response to voltage fluctuations by using short term and long term flicker severity indices, Pst and Plt, on the basis of human visual sensation which is generated by the traditional incandescent lamp types. The output from new lamp types such as compact flourescent lights are less sensitive to voltage fluctuations as their flicker characteristics are considerably different compared to those of the traditional incandescent lamp. These differences could support the moderation of the present voltage fluctuation and flicker standards and hence the associated limits. However, the severity indices used to quantify flicker do not account for effects that may be observed in the terminal characteristics of electrical equipment connected to the public supply network. Therefore, the potential detrimental effects on electrical equipment which may be caused by relaxation of these flicker limits should be investigated before any changes to the present standards take place.
The related investigation presented in this thesis is motivated by this issue in addition to developing a greater insight into the impact of voltage fluctuations on common items of electrical equipment.
The impact of voltage fluctuations on a single-phase, full-bridge rectifier with a capacitor filteris investigated. Such a circuit is found in many different types of equipment. By comparing the DC-link capacitor ripple current characteristic between non-fluctuating and regular fluctuating AC power supplies using mathematical analyses, simulation, and experimental verification, the investigation reveals that the DC-link capacitor will accumulate and dissipate increased amounts of charge when the rectifier is subjected to AC source voltage fluctuations. The consequence of this phenomena is that the RMS value of the capacitor current will increase and the magnitude of this increase is related to the modulation frequency and magnitude of the fluctuating voltage. Therefore, an AC supply with a voltage fluctuation component will cause the capacitor filter of the rectifier circuit to sustain increased stress. This stress may accelerate the capacitor ageing process, possibly resulting in premature equipment failure.
Using a simple thermal model of an aluminium electrolytic capacitor coupled with the newly determined relationship between supply voltage fluctuations and capacitor RMS current, a lifetime model is developed. This model is used to estimate the reduction in the useful lifetime of the capacitor as a function of the severity of the voltage fluctuations applied to the rectifier and capacitor-filter combination. As a consequence of the method used for establishing the short term flicker index, Pst, the lifetime model demonstrates that Pst is an ineffective metric.
The performance of widely used mains-connected three-phase induction motors heavily depends on power supply quality. Thus, the impact of voltage fluctuations on mains connected induction motors is another effect worthy of investigation. A per- phase equivalent circuit and a dynamic model has been used to analyse the impact of regular voltage fluctuations on the stator and rotor current characteristics of a squirrel cage induction motor. The outcomes of the simulation work and experimental verification indicate that the stator and rotor RMS current magnitudes of are affected significanlty by voltage fluctuations. This effect worsens as the modulation frequency and the modulation depth of the supply voltage increase. Such an increase in the RMS current will increase copper losses in the motor, resulting in winding temperature rise and subsequently accelerating the ageing process.
Although the squirrel cage induction motor is a complex device, simple thermal models have been developed by a range of researchers. By applying such a thermal model, coupled with the known relationship between supply voltage fluctuations and stator RMS current, a lifetime model for the induction motor is developed. This model is used to estimate the reduction in the lifetime of the induction motor as a function of the severity of the source voltage fluctuations The short term flicker index, Pst ,is shown to be an ineective metric with regard to this phenomenon.
The impact of regular voltage fluctuations on an adjustable speed drive (ASD) is also investigated in this thesis. The outcomes of simulation work and experimental verification of an open-loop ASD structure connected to an induction motor coupled to various loading levels indicate that a fluctuating supply voltage will lead to an increase in the three-phase rectifier DC-link capacitor RMS current of the ASD. The magnitude of this increase is related to the modulation frequency and magnitude of the fluctuating voltage. The RMS current is observed to continue to increase as the voltage change and modulation frequency increases. The supply voltage fluctuations are also observed to pass through the rectifier circuit to the inverter and cause induction motor stator-current distortion.
This thesis presents a series of impact investigations on common electrical loads and circuits found in electrical equipment. The results reveal that the standard methods of flicker measurement, Pst and Plt, cannot represent the risk to equipment degradation caused by voltage fluctuations. The evidence collected in this thesis will provide useful information to equipment manufacturers, electricity utilities, end- user customers and those involved in the development of relevant electromagnetic compatibility (EMC) standards.