Calculating voltage dips in power systems using probability distributions of dip durations and implementation of the Moving Fault Node method

Examensarbete för masterexamen
Master Thesis
Wämundsson, Mikael
This report is based on a master thesis work aimed at improving the Simpow Dips program, by implementing new features. Simpow Dips, in its existing version, simulates faults in all nodes in a given network and creates a result-file containing the during-fault voltages at each node for every fault. Improvements have been made in three aspects: the ability to also calculate the dip duration, an improved method for calculating the magnitude of the dip and a graphical presentation of the calculation results. A method for defining statistical distributions of dip durations for subsets of the network has been implemented in Simpow Dips. The method is able to produce calculation results with good accuracy under an important condition: that the data used as basis for the distributions is of sufficient accuracy. This data can be collected in different ways, but monitoring the network (or a network with similar characteristics) togetherwith knowledge of the protection relay settings can give data of desired accuracy. In the existing version of Simpow Dips the dip magnitudes are calculated with faults only occurring at the nodes. The Moving Fault Node method implemented in the new version of Simpow Dips will result in more accurate calculations of the magnitude. The calculation according to this method requires the bus impedance matrix, the impedance for the faulted line, the fault position on the line and pre-fault voltages at three locations in the system: the two terminals of the faulted line and the node where the customer is connected. A uniformdistribution of the fault position over the line length has been assumed. The calculation of the dip magnitude for a fault at a line is not yet implemented in Simpow Dips itself, but is done in an analysis program, written under Matlab. The result-file produced by Simpow Dips gives pre-fault voltages and sequence impedances necessary for the calculation. The above-mentioned analysis program also produces graphical presentations of the results: scatter plots, voltage dip coordination charts and voltage dip density charts are produced. The voltage dip coordination chart is helpful when estimating the severity of the dip situation for a certain customer by introducing the voltage-tolerance curve. By using the voltage dip density chart and cumulative chart an impression of the power quality is given for the site. The different plots also simplify the analysis of the effect of changes in the power system. This is illustrated for a number of different changes to a small network.
Elkraftteknik , Electric power engineering
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