Mitigating Ferroresonance in HV inductive transformers W. Piasecki, M. Stosur, M. Florkowski, M. Fulczyk, B. Lewandowski Abstract--Ferroresonant oscillations involving voltage transformers (VTs) may be initiated by transient events such as switching operations or intermittent faults. In the present article the problem of ferroresonance involving HV VTs is demonstrated with a particular stress on inductive voltage transformers. ATP/EMTP simulations based on a realistic VT model demonstrated that stable ferroresonant oscillations characterized by significant overcurrent values may be initiated by opening a circuit beaker. A new approach towards damping the ferroresonance by means of a compact active damping device is shown. Experimental verification of the device performance proved the ability of effectively damping the ferroresonant oscillations without re-triggering after the load rejection. 1 Keywords: Voltage transformers, ferroresonance damping. I. INTRODUCTION Voltage transformers are characterized by a special construction and their rated power is typically very low due to their metrological, rather than power supply function. Nominal primary currents in the voltage transformer (VT) winding are typically of the order of single milliamps at primary voltage ranging from several up to tens of kilovolts in MV networks, and hundreds of kV in HV networks. In ungrounded networks typically the ferroresonance involves the phase-to-ground connected VTs and the phase to ground capacitances of the lines [2], [4], [7]. In grounded networks the ferroresonant oscillations may involve a single voltage transformer and a series capacitance e.g. a grading capacitance of a circuit breaker [3], [6]. Special construction of voltage transformers characterized by their relatively low power ratings makes them very sensitive to the ferroresonance problem since large overcurrent in the primary windings may lead to the overheating and, in consequence, to the permanent equipment damage. In MV networks the ferroresonance damping is typically achieved by introducing a damping burden into the open-delta connected auxiliary windings of the VTs [5]. In HV instrument transformers however, ferroresonance free performance is required for each individual device. W. Piasecki, M. Stosur, M. Florkowski, M. Fulczyk are with ABB Corporate Research Center in Krakow, Starowislna 13A 31-038, Poland (e-mails: wojciech.piasecki@pl.abb.com, mariusz.stosur@pl.abb.com, marek.florkowski@pl.abb.com, marek.fulczyk@pl.abb.com). B. Lewandowski is with ABB Ltd, Aleksandrowska 67/93 Lodz 91-205, Poland (e-mail: bogusz.lewandowski@pl.abb.com). Presented at the International Conference on Power Systems Transients (IPST’09) in Kyoto, Japan on June 3-6, 2009 II. FERRORESONANCE SIMULATIONS The ATP/EMTP simulations demonstrating the ferroresonance problem in inductive HV Voltage Transformers were performed on the basis of a realistic model of a VT. For the VT model built, the dependence of the ability to initiate the stable ferroresonant oscillations by circuit breaker switching on the grading capacitance value was studied [1],[7]. In order to create an electrical simulation model of the VT, a magnetizing characteristic was measured for the real core of the HV instrument transformer using test windings. Instantaneous current and voltage values were measured using a digital oscilloscope. Characteristic U=f(I) 0 20 40 60 80 100 120 140 160 0 5000 10000 15000 20000 25000 I [mA] U [V] Fig.1. Measured U-I characteristic for the voltage transformer core Then the voltage values were re-calculated for real numbers of turns present in the HV device (rated voltage 123kV/ 3 ) . The U-I characteristic for the HV winding can be than directly obtained by re-calculating the characteristic obtained from the measurement. The complete model however, requires realistic values of the leakage inductance as well as of the winding capacitance value and the winding resistance. The U-I characteristic obtained for the test winding (60 turns) is shown in Fig. 1. Due to the negligible influence of the leakage inductance for the very low number of turns, this characteristic was used to determine the B-H curve for the core (see Fig. 2). The HV U-I characteristic was obtained by: – re-calculating the current-fluxlinked (I-Fluxlinked) characteristic for the required number of turns, – adding a series resistor representing the winding resistance, – adding a series linear inductance representing the leakage inductance of the winding, – adding a parallel capacitor representing the equivalent capacitance of the winding.