Overvoltage Impact on Internal Insulation Systems of Transformers in Electrical Networks with Vacuum Circuit Breakers

Marek Florkowski,Jakub Furgał,Maciej Kuniewski,Piotr Pająk

doi:10.3390/en13236380

Abstract

Vacuum circuit breakers are increasingly used as switching apparatus in electric power systems. The vacuum circuit breakers (VCBs) have very good operating properties. VCBs are characterized by specific physical phenomena that affect overvoltage exposure of the insulation systems of other devices. The most important phenomena are the ability to chop the current before the natural zero crossing, the ability to switch off high-frequency currents, and the rapid increase in dielectric strength recovery. One of the devices connected directly to vacuum circuit breakers is the distribution transformer. Overvoltages generated in electrical systems during switching off the transformers are a source of internal overvoltages in the windings. The analysis of the exposure of transformers operating in electrical networks equipped with vacuum circuit breakers is of great importance because of the impact on the insulation systems of switching overvoltages (SO). These types of overvoltages can be characterized by high maximum values and atypical waveforms, depending on the phenomena in the circuit breaker chambers, system configuration, parameters of electrical devices, and overvoltage protection. Overvoltages that stress the internal insulation systems are the result of the windings response to overvoltages at transformer terminals. This article presents an analysis of overvoltages that stress the transformer insulation systems, which occur while switching off transformers in systems with vacuum circuit breakers. The analysis was based on the results of laboratory measurements of switching overvoltages at transformer terminals and inside the winding, in a model medium-voltage electrical network with a vacuum circuit breaker.

Highlights

The basic task of distribution companies is to ensure the reliability of electrical power delivery to the customers
Due to the use of many devices in power systems with parameters different from those used so far, insulation systems for power devices are exposed to the effects of surges not occurring
The aim of the research was a comparative analysis of various configurations and an analysis of the influence of surge arresters on the shapes of switching switching overvoltages occurring during switching off transformers in systems with various overvoltages in systems vacuum circuit breakers

Summary

Introduction

The basic task of distribution companies is to ensure the reliability of electrical power delivery to the customers. The analysis is based on the results of laboratory tests of switching overvoltages in transformers, protected by metal oxide surge arresters, operating in systems with vacuum circuit breakers, modeling part of a typical medium-voltage network. With a slow rate of increase in the strength of the gap between the UWp contacts and a rapid increase in the recovery voltage (u0 ≈ uC ), restrikes of the arc may occur This is accompanied by the flow of high-frequency current in the Cz -W-L0 -C0 circuit (Figure 1) and a very quick recharge of the capacitance C0 to a higher voltage uC (j), if the high-frequency current is switched off at the first zero crossing. As a consequence of this, the maximum values of the voltage expected at the terminals of the disconnected circuit increase

Characteristics of the Laboratory Stand

Tests of Switching

Transient overvoltage waveformsat at transformer transformer terminals

10. Overvoltage waveforms insidethe the L3