A ferroresonance analysis related to HV Inductive Voltage Transformers (IVT) and power Voltage Transformers or Station Service Transformers (PVT/SST) was addressed to develop models based on EMT formulation, enabling detailed ferroresonance risk analyses in specific HV station conditions and helping in the equipment technical specifications (including anti-ferroresonance remedial provisions).
The research relied on a comparison between simulation results and measurements obtained during two sets of specific HV laboratory full-voltage ferroresonance tests performed on 220 kV IVT and PVT/SST. Preliminary EMT time domain simulation models, drawn starting from standard type test results and manufacturing information of IVT and PVT, were applied with different external network conditions, at first, analysing the influence of the equivalent fundamental capacitive and inductive parameters on the ferroresonance inception and evolution, and then, designing suitable ferroresonance tests to be made on the sample devices in the HV laboratory, by switching HV Circuit Breakers with different grading capacitance values. The first set of test showed results not corresponding with those coming from simulations and no stable ferroresonance in the case of SST was measured. Deeper analysis led to the conclusion that the saturation characteristic and the equivalent self-stray capacitance of IVT/PVT are key factors for the model quality; both of them can be easily evaluated using non-standard tests made in the manufacturer’s factory. Models were adapted accordingly and a new set of test was performed, in which moreover higher values of grading capacitances were applied. The second set of lab tests showed that, with such “series” grading capacitance circuital condition, both IVT and PVT can be forced in a state of “stable” ferroresonance. Depending on different test conditions, two types of ferroresonance were experienced: a fundamental frequency type, with relatively high voltage peaks, and a sub-harmonic 3rd order type, with lower voltage peaks. Tests also showed that, in case of a stable ferroresonance state, the oscillation can be quickly damped by inserting a low-value resistor on the IVT or PVT secondary; furthermore, a ferroresonance oscillation can rise again if such secondary resistance is switched off just after the damping action. Procedures, through a parametric approach, with reference to the circuital main parameters of the system in which the device will be installed, enable assessing the ferroresonance risk. Finally, a parametric approach, based on EMT simulations (by ATP and Matlab®) as per the models above, was applied to evaluate anti-ferroresonance solutions to be associated with IVT/PVT. In fact, additional losses by forced dissipation in a circuit connected to the LV secondary terminals of the device after the ferroresonance condition has started proves to be an effective solution .
