Influence of capacitive no-load current component in large power transformers

Abstract

No load current of large power transformers consists of active and reactive component. Active component covers hysteresis and eddy current losses in the core. Reactive component (usually called magnetizing current) creates magnetic field / magnetic flux and lags the voltage by 90°. In most models these two components are enough to describe no-load current in large power transformer at power grid frequency (50 or 60 Hz). Due to the low frequency modeling the winding capacitance are usually neglected. For higher frequencies and/or transients, ground capacitive reactances and capacitive reactances between winding turns and windings are significant and they need to be modeled and, therefore, capacitive current component occurs [1]. If capacitances can be neglected, reactive component of no load current consists only of magnetizing part and depends only on magnetic steel properties, following its B-H curve.In this paper the presence of capacitive component of large power transformer no-load current is presented when powered by 50 Hz sinusoidal voltage. Capacitive component exist mainly as sum of the capacitive component of supplied windings and transferred currents from other windings [2], [3] Its influence on the no-load characteristic and an approach for determination of the inductive component in the no-load current based on the no-load current measurement is explained. Equivalent capacitance is compared with capacitance measurements in test field.