Abstract-The International Electrotechnical Commission (IEC) has recently released a technical report (IEC/TR 61000-3-13, Ed. 1.0, 2008) in relation to the assessment of voltage unbalance emission by installations connected to MV, HV and EHV power systems. As in the cases of harmonics and flicker, this requires a quantitative measure of propagation of voltage unbalance from upstream (higher voltage) to downstream (lower voltage) systems in terms of transfer coefficients. Naturally, these transfer coefficients depend on the downstream load composition. The existing method for determining MV to LV voltage unbalance transfer coefficient suggests a value of unity in relation to passive loads in general. However, this paper reports that MV to LV transfer coefficient of unity is conservative in the presence of commonly prevailing constant power loads. Further, the paper reports on an improved methodology for estimating MV to LV voltage unbalance transfer coefficient taking system and load characteristics into account.
Abstract-The International Electrotechnical Commission (IEC) has recently released the Technical Report IEC/TR 61000-3-13 for the assessment of voltage unbalance emission limits to individual customer installations connected to medium-voltage (MV), high-voltage, and extra-high-voltage power systems. As in the counterpart IEC technical reports 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 customers in proportion to their agreed apparent power. However, noting that voltage unbalance at a busbar can arise due to load and system (essentially lines) asymmetries, IEC/TR 61000-3-13 applies a scaling factor to the apportioned allowance. Tis factor represents the fraction of the global emission allowance that can be allocated to installations. Conversely, the factor = 1 accounts for the emission arising as a result of system inherent asymmetries. This paper addresses the global voltage unbalance emission in MV power systems arising as a result of line asymmetries on which a systematic evaluation method is not given in IEC/TR 61000-3-13. First, a detailed study with the view to investigate the influence of line asymmetries on the global emission and its dependency on various load types is carried out by employing a simple radial network. Second, a matrix-based methodology, covering radial and interconnected networks, for the assessment of this emission at the nodal level taking line, system and load characteristics, system operating conditions, and downstream load composition into account are proposed. The methodology is verified by using unbalance load-flow analysis.Index Terms-Line asymmetries, medium-voltage (MV) power systems, radial and interconnected networks, voltage unbalance, voltage unbalance allocation.
Abstract-The levels of voltage unbalance which exist in some interconnected sub-transmission systems in Australia have been observed to exceed the stipulated levels. The causes of the problem are not well known to system operators, ie. whether the problem mainly arises as a result of system asymmetry or loads or both. Hence, system operators have found difficulties in identifying potential mitigation approaches which provide the optimum level of correction at various busbars. This paper reports, employing a study network, a suitable theoretical basis that can be used to identify the level of contribution made by the asymmetrical transmission lines and the unbalanced loads in relation to the problem of voltage unbalance and a systematic approach for identifying the optimum mitigation options.
The levels of voltage unbalance that exist in some interconnected sub-transmission networks in Australia has been observed to be above the code requirements (e.g. 1% in Victoria). There is a perception that these high levels arise as a result of asymmetrical loads. However, it is identified that the network asymmetry in relation to transmission lines also play a significant role in the lead up to this situation, employing an interconnected 66kV sub-transmission system as the study case. The individual transmission lines of the study network are analysed in order to observe their possible influence arising as a result of line impedance characteristics and line loading levels. A methodology is developed to quantify the contribution of individual transmission lines to the role played by the network at nodal level. The leading contributors to the problem in the study network are explored employing the proposed approach. Disciplines Physical Sciences and Mathematics Publication DetailsThis conference paper was originally published as Paranavithana, P, Perera, S, and Sutanto, D Abstract -The levels of voltage unbalance that exist in some interconnected sub-transmission networks in Australia has been observed to be above the code requirements (e.g. 1% in Victoria). There is a perception that these high levels arise as a result of asymmetrical loads. However, it is identified that the network asymmetry in relation to transmission lines also play a significant role in the lead up to this situation, employing an interconnected 66kV sub-transmission system as the study case. The individual transmission lines of the study network are analysed in order to observe their possible influence arising as a result of line impedance characteristics and line loading levels. A methodology is developed to quantify the contribution of individual transmission lines to the role played by the network at nodal level. The leading contributors to the problem in the study network are explored employing the proposed approach.
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