Year

2009

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering - Faculty of Informatics

Abstract

Although voltage unbalance is a well understood concept, its presence as a power quality problem in electricity transmission and distribution networks has continued to be an issue of concerns primarily due to difficulties found by some network service providers in maintaining acceptable levels. This emphasises the lack of recommendations on engineering practices governing voltage unbalance that would facilitate the provision of adequate supply quality to connected customers.

The International Electrotechnical Commission (IEC) has recently released the Technical Report IEC/TR 61000-3-13 which provides guiding principles for coordinating voltage unbalance between various voltage levels of a power system through the allocation of emission limits to installations. Although the IEC report is based on widely accepted basic concepts and principles, it requires refinements and original developments in relation to some of the key aspects. This thesis primarily focuses on making contributions for further improvements to the IEC report so as to present a more comprehensive voltage unbalance allocation procedure.

Similar to the counterpart IEC guidelines for harmonics (IEC 61000-3-6) and flicker (IEC 61000-3-7) allocation, IEC/TR 61000-3-13 also apportions the global emission allowance to an installation in proportion to the ratio between the agreed apparent power, and the total available apparent power of the system seen at the busbar where it is connected. However, noting that voltage unbalance at a busbar can arise as a result of both load and system (essentially lines) asymmetries, IEC/TR 61000-3-13 applies an additional factor which is referred to as ‘Kue’ to the apportioned allowance. This factor Kue represents the fraction of the global emission allowance that can be allocated to customers, whereas the factor K0ue (= 1 − Kue) accounts for voltage unbalance which arises as a result of line asymmetries. Although IEC/TR 61000-3-13 recommends system operators to assess the factors Kue and K0ue for prevailing system conditions, a systematic method for its evaluation is not provided other than a rudimentary direction. This thesis initially examines, employing radial systems, the influence of line asymmetries on the global emission levels in medium voltage (MV) and high voltage (HV) power systems in the presence of various load types/bases including three-phase induction motors. It is shown that the factor K0ue is seen to be dependant not only on line parameters as evident from IEC/TR 61000-3-13, but also on the downstream load composition. In essence, the global emission levels in HV power systems is seen to arise as a result of both the local HV lines and the downstream MV lines in the presence of considerable proportions of induction motor loads. Eventually, generalised methodologies, covering both radial and interconnected networks, for the assessment of the global emission in MV and HV power systems which arises due to line asymmetries are proposed.

In allocating voltage unbalance based on the IEC/TR 61000-3-13 recommendations, quantitative measures of its propagation from higher voltage to lower voltage levels in terms of transfer coefficients, and from one busbar to other neighbouring busbar of a sub-system in terms of influence coefficients are required. IEC/TR 61000-3-13 gives a method for evaluating the MV to LV transfer coefficient suggesting a value less than unity for industrial load bases containing large proportions of mains connected three-phase induction motors, and a value of unity for passive loads in general. Upon detailed examination, it is noted that a transfer coefficient > 1 can arise in the presence of commonly prevailing constant power loads. Incorporating these different influences exhibited by various load types under unbalanced supply conditions on the propagation, comprehensive methods for assessing the MV to LV and HV to MV transfer coefficients are proposed. A systematic approach for estimating influence coefficients for interconnected network environments taking their dependency on the downstream load composition into account is developed.

The IEC allocation policy with regard to harmonics and flicker has been found not to guarantee that the emission limits allocated to customers ensure non-exceedance of the set planning levels. This thesis reports that the above is an issue with voltage unbalance as well. Overcoming this problem, an alternative allocation technique referred to as ‘constraint bus voltage’ (CBV) method which closely aligns with the IEC approach has been suggested for harmonics and flicker. The work presented in this thesis extends the suggested CBV method to voltage unbalance allocation adding appropriate revisions to address the additional aspect of the emission which arises as a result of line asymmetries.

In the application of the IEC/TR 61000-3-13 principles to better manage existing networks already experiencing excessive voltage unbalance levels, the initial development of insights into the influences made by various sources of unbalance is required. Employing an existing 66kV interconnected sub-transmission system as the study case, deterministic studies are carried out in a systematic manner considering each of the asymmetrical elements. Approaches for studying the voltage unbalance behaviour exhibited by various sources which exist in interconnected network environments are established. These are employed to identify the most favourable line transposition options for the study system. Further, this knowledge that facilitates the identification of contributions made by individual unbalanced sources forms a platform for developing techniques to assess the compliance with emission limits, which is another subject of relevance to future editions of IEC/TR 61000-3-13.

As an essential tool for carrying out the studies, an unbalanced load flow program based on the phase coordinate reference frame incorporating the component level load flow constraints and the three-phase modelling of system components is developed.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.