Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes Detection of ferroresonance occurrence in inductive voltage transformers through vibration analysis A. Arroyo a, ⁎ , R. Martinez a , M. Manana a , A. Pigazo a , R. Minguez b a University of Cantabria, Department of Electrical and Energy Engineering, Av. Los Castros s/n, 39005 Santander, Spain b Viesgo, C/ Isabel Torres 25, (PCTCAN), 39011 Santander, Spain ARTICLEINFO Keywords: Ferroresonance Inductive voltage transformer Power transformer ABSTRACT Steady state ferroresonances in isolated neutral electrical systems result in sustained abnormal system oscilla- tions, which can damage inductive voltage transformers (IVTs) and other system elements. During this phe- nomenon, the IVT becomes saturated; consequently, the dimensions of the ferromagnetic material of the IVT change owing to magnetostriction. Because of this inherently nonlinear behaviour, the natural vibration modes of the IVT are excited. The aim of this study is to take advantage of these anomalous vibration modes to reveal the occurrence of ferroresonance in IVTs. The methodology for ferroresonance detection in IVTs through vi- bration analysis is described and demonstrated experimentally. 1. Introduction Ferroresonance is a nonlinear phenomenon characterised by over- voltages and oscillations in waveforms. This phenomenon is produced by a change of state due to certain power system operation conditions, for example, transients, lightning, switching operations, and faults, which stimulate system non-linearities and, due to the association of capacitances and inductances, results in an abnormal oscillation state. Ferroresonance is common in power systems owing to the large number of saturable inductances [e.g., in power transformers (PTs), inductive voltage transformers (IVTs), and shunt reactors] and capacitances (e.g., in underground and overhead lines, coupling capacitor voltage trans- formers (CCVTs), and capacitor banks). Generally, IVTs are used in medium and high voltage power systems and CCVTs in high voltage power systems [1,2]. This paper will be focused on medium voltage power systems and consequently on IVTs. Fig. 1 shows the main elements of a power system: the voltage source (VS), current transformer (CT), IVT, switch stray capacity (SSC), PT, line capacity (LC), and line. The physical phenomenon of ferroresonance is explained using the equivalent electrical circuit in Fig. 2. In this section, a series RLC system is analysed [3,4]. The system voltage V in Fig. 2 is calculated as = + + V V V V R L C (1) where V V , R L and V C are the resistive, inductive, and capacitive voltages [V], respectively. Eq. (1) can be also expressed as = V V V V L R C (2) and, according to Ohm’s Law, = + V V RI jX I · · L C (3) where I is the system current [A], R is the line equivalent resistance [ ], and X C is the line capacitive reactance [ ]. = V jX I · L L (4) where X L is the nonlinear inductive reactance of the IVT [ ]. The possible operation points of the system can be obtained by combining Eqs. (3) and (4) (Fig. 3). Point B represents a normal state, whereas points A and C represent saturated states, wherein voltages and currents are very high. Operation points A and C lead the system to ferroresonance, and they depend strongly on the initial conditions. The consequences of ferroresonance will depend on the voltage and current levels at these operation points. In a real power system, the con- sequences of ferroresonance are: core saturation, waveform distortion, overvoltages and overcurrents, increase in transformer temperature, noise and vibration and primary winding destruction. Ferroresonance is so widespread and harmful that various systems have been developed to mitigate it [5,6]. In the traditional method of mitigating ferroresonance, a low resistance is installed in the open-delta protection winding of the IVT. In this type of connection, the resistance works when there is an imbalance in the transformer (e.g., a fault or ferroresonance). Another system consists of a burden at the neutral https://doi.org/10.1016/j.ijepes.2018.10.011 Received 26 February 2018; Received in revised form 14 May 2018; Accepted 7 October 2018 ⁎ Corresponding author. E-mail address: arroyoa@unican.es (A. Arroyo). Electrical Power and Energy Systems 106 (2019) 294–300 0142-0615/ © 2018 Elsevier Ltd. All rights reserved. T