Year

2011

Degree Name

Master of Engineering - Research

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

Modern domestic loads are considerably different to those found in households 10 to 15 years ago. In addition to traditional heating, cooking and refrigeration loads, modern loads are likely to include a variety of sophisticated devices containing power electronic front ends. Such front ends can lead to a range of power quality (PQ) impacts on the electricity network, primarily harmonic distortion. Indications are that penetration levels of non-linear residential load are likely to increase. The decision by the Australian Federal Government to ban the sale of traditional incandescent light globes by 2010 is one driver of the increase in non-linear load. At present, the only viable alternative to the incandescent light globe is the compact fluorescent lamp (CFL), a well-known non-linear load.

The evolution of the residential load in terms of magnitude and characteristics into a significant distorting load and as such a potential source of significant power quality disturbances is worthy of detailed study. Distribution utilities are required by law to maintain acceptable power quality levels on their distribution networks to ensure safe and reliable operation of the network as well as equipment connected thereto. This is only possible if detailed understanding of the behaviour of equipment connected to the distribution network has been achieved so that proper planning schemes can be put in place.

In order to understand the electrical behaviour of modern loads the electrical performance of a range of appliances, including a range of CFLs, was examined through laboratory investigations. Examination was made of performance under a range of input voltage conditions (magnitude and waveform distortion). A realistic low voltage (LV) electricity distribution network simulation model was developed to assess their impact on distribution system PQ levels using the appliance load characteristics determined through laboratory testing. This flexible distribution system model allowed a range of feeder lengths and loading scenarios to be examined.

Field monitoring conducted in conjunction with theoretical work is used to substantiate the simulation studies. Broadly, the majority of the results obtained from the modelling work were found to be indicative of credible harmonic magnitudes and profiles. Results most reflective of recent field observations were obtained using the simulation models where the injected current levels were based on appliance behaviour when such appliances were supplied with distorted input voltages rather than sinusoidal input voltages. There are a number of reasons that make direct comparison of model outputs with field data very difficult. Notwithstanding these difficulties, good correlation between field measurements and model outputs was achieved for 3rd and 5th harmonic voltage levels. Correlation between model outputs and field data for the higher order harmonics which were examined (7th, 15th and 21st) was not as strong.

On the whole, results obtained from the simulation models indicate that a very high penetration of modern domestic appliances (i.e. large numbers in each residence) will result in harmonic voltage levels that will exceed the Australian harmonic voltage planning levels on LV feeders. The modelling also indicated that the profile of the harmonic voltages due to modern domestic appliances may be different to the traditional profile where low order harmonics dominate. There is evidence to show that if high penetration of modern appliances occurs, the dominant voltage harmonics due to interaction of appliance load currents with network impedance may be the 7th or 15th harmonic. Special emphasis has been placed on the impact of CFLs on harmonic levels. Results of modelling clearly show that a large number of CFLs can have a significant impact on voltage harmonic levels.

It was clear that some of the simulation models which were developed produced values which were unlikely to be representative of those seen in practice and this is an avenue for future research. Methods of harmonic modelling are complex, especially for higher order harmonics, and if very accurate models are required, some of the assumptions used may need closer examination. Further, as appliances develop over time, it may be necessary to periodically perform studies such as the one presented in this thesis in order to maintain a complete understanding of the electrical behaviour both of modern loads and the LV distribution network.

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