Accurate Calculation of VAR Demand for a fleet of Power Transformers

It has been recognized by the Industry that using accurate values of winding resistances of transformers on a Power System is paramount to the accuracy of the GIC Power System model and consequently on the accuracy of the calculation of the GIC flow in the Power System. Also, it has been recognized by the authors of this paper that the method, commonly used by the Industry for the calculation of the Additional VAR demand caused by GIC, is not sufficiently accurate for some transformer types, and in fact, incorrect for other transformer types. This paper presents results of a comprehensive study performed by the authors to demonstrate the significant impact that accurate values of transformer winding resistances, correct core types, and correct calculation of the VAR Demand on the results of the evaluation of the vulnerability of the power system to GIC. The most significant finding from this study is that the actual magnitudes of the Total VAR Demand on the system is a fraction of what was calculated using original values of winding resistances, core types, and calculation methods commonly used in the Industry for the calculation of VAR Demand. This is of a significant impact on the System Vulnerability to GMDs.