Lightning caused overvoltages on power transformers recorded by on-line transient overvoltage monitoring system

Abstract

Transient overvoltages generated by lightning strikes or switching operations represent a significant risk to bushings and windings of power transformers. They cause stress on the insulation system and can, over time, cause dielectric failure and damage to power transformers. Many transformer failures are reported as dielectric failures and they are not necessarily linked to any particular event when they occur but may be the result of prior damage from transient overvoltage events. Lightning and switching overvoltage waveforms appearing at transformer terminals in real operating conditions may significantly differ from standard impulse voltage waveforms used during laboratory testing. The number and amplitudes of overvoltages which stress the insulation depend on various parameters such as the lightning strike density in the considered area, since it determines how often the transformer is stressed by lightning overvoltages. Since the overvoltage amplitudes at transformer terminals are usually unknown, an on-line overvoltage transient recorder can be used with the ability to sample, analyse and store transients in real-time. In this paper, an on-line transient overvoltage monitoring system (TOMS) for power transformers is presented that is capable to continuously record in real-time various kinds of transient overvoltages such as lightning or switching overvoltages. Special attention is paid to lightning caused transient overvoltages recorded at the terminals of 150 MVA power transformer. Recorded waveforms originating from lightning strikes to overhead lines are correlated with data from the lightning location system (LLS) and supervisory control and data acquisition (SCADA) system. Collected data about overvoltage stresses can be used as the basis for the assessment of the transformer insulation condition, estimation of health index and for analysis of various kinds of events such as faults or equipment failures.

Calculation of Fast Front Overvoltages in Vicinity of a Power Transformer

“…In order to obtain admittance by measuring the voltage ratios, it is necessary to use the internal resistance of the response measurement terminal as a shunt resistor to measure the current. In this case, the diagonal coefficient of the admittance matrix can be calculated as: 4 The connection between the measuring device and the connection box is made using short coaxial cab while the connection between the connection box and the top of the transformer bushing is made usin meter long coaxial cables, which are part of the standard SFRA equipment.…”

“…In order to obtain admittance by measuring the vol ratios, it is necessary to use the internal resistance of the response measurement terminal as a sh resistor to measure the current. In this case, the diagonal coefficient of the admittance matrix can calculated as: (4) From figure 2 it can be seen that the measurement circuit is grounded before the measu device so the current that closes through the transformer tank towards the measuring device contribute the voltage drop on the internal resistance of the response measurement terminal. By doing this, all sheaths of the coaxial cables must be short-circuited and left at the floating potential, so that the vol drop across the internal resistance of the response measurement terminal is not equal to 0 V.…”

“…atrix. In order to obtain admittance by measuring the voltage l resistance of the response measurement terminal as a shunt case, the diagonal coefficient of the admittance matrix can be (4) at the measurement circuit is grounded before the measuring he transformer tank towards the measuring device contributes to e of the response measurement terminal. By doing this, all the rt-circuited and left at the floating potential, so that the voltage esponse measurement terminal is not equal to 0 V.…”

“…Due to the significant share of high frequencies in such overvoltages, re phenomena may occur within the windings of the power transformer, which can endanger its di insultation. For this reason, on-line monitoring systems for measurement of overvoltages can be in and usually they are connected over measuring tap of transformer bushing [4], [5]. The electrom behaviour of the transformer at its terminals is a function of these resonant phenomena.…”

A Low Cost Real-Time Transient Recorder for High Voltage Systems

“…The proposed use of optical sensors would require replacing conventional current and voltage measurement transformers and would not be applicable for recording transients on transmission line towers or surge arresters. In [19], the authors describe a dedicated system with the option to record transients on a transformer by extending the existing transformer monitoring with the necessary components and a recorder for very fast transient events. In [13], the authors provide results of the application of a transient monitoring system on a transformer and a shunt resistor described in [19] in the context of the power system in Croatia.…”