Stray capacitance study for HVDC Classic : The use of PSCAD and Mathcad to calculate the equivalent stray capacitance

Abstract: The reactors placed in the thyristor valve designed by Hitachi Energy have reactor losses which are dependent on the stray capacitance. To find out the stray capacitance, and further get to know the reactor losses, a PSCAD simulation is performed. This is time consuming and the process would benefit if Hitachi Energy had a faster method of finding the stray capacitance. Before this thesis, previous tests were performed to prove that the losses of the reactor and the oscillation across the reactor is dependent on the stray capacitance by using different sizes of the stray capacitance. Tests in this thesis have been done as well when talking about the reactor. Every project is using different number of reactors in the thyristor valve as well as the inductance in the reactor. The tests were done to show if the inductance or the number of reactors would affect the stray capacitance or not.   This master thesis goes through the previously calculated equivalent stray capacitance from a project Hitachi has been working with. It goes through the PSCAD schemes, the definitions of the components and the equations in the Mathcad file. The Mathcad file includes capacitances from the transformer, the 12-pulse thyristor bridge and extra capacitances in the valve hall but outside of the thyristor valve. To validate if this Mathcad file could be the solution for Hitachi to speed up the process, two other projects were looked into. The values of the capacitances in these projects were put into the Mathcad file to see how big the equivalent stray capacitance is.    To know from which thyristor the stray capacitance should be calculated from, an analysis of all the thyristors was done. The calculation of the stray capacitance in PSCAD was done by analysing the period of the oscillation in the thyristor current. The results showed a difference between the Mathcad calculation and the PSCAD simulation, but both of the calculations showed a smaller equivalent stray capacitance than the value Hitachi Energy are using for the projects right now. This is a promising result and the conclusion is that Mathcad is a viable tool for calculation of the equivalent stray capacitance, and therefore it is possible for Hitachi Energy to save both time and money in the future.