Improvement of admittance measurements for modeling of an ungrounded reactor/transformer winding

In this paper,a model of an ungrounded reactor winding is developed for transient voltage studies.The developed model is a black box model where the state-space model is developed using the directly measured admittance matrix of the reactor winding.Furthermore,an improved model is developed,where the accuracy of the admittance matrix measurement set in the low frequency band is obtained indirectly using an additional set of voltage ratio measurements.When the measured admittances are low,the accuracy of the diagonal admittance vectors in the low frequency band is low as well.However,the accuracy at low frequencies obtained using the indirect measurements is improved at nodes with the lowest admittances.In this paper,different approaches for indirect measurements are studied and the most accurate method is identified considering the physical structure of windings.