Analysis of Internal Overvoltages in Transformer Windings during Transients in Electrical Networks

Jakub Furgał,Maciej Kuniewski,Piotr Pająk

doi:10.3390/en13102644

Abstract

Due to the increasing requirements for the reliability of electrical power supply and associated apparatus, it is necessary to provide a detailed analysis of the overvoltage risk of power transformer insulation systems and equipment connected to their terminals. Exposure of transformer windings to overvoltages is the result of the propagation condition of electromagnetic waves in electrical networks and transformer windings. An analysis of transformer winding responses to transients in power systems is of particular importance, especially when protection against surges by typical overvoltage protection systems is applied. The analysis of internal overvoltages in transformers during a typical transient related to switching operations and selected failures is of great importance, particularly to assess the overvoltage exposure of insulation systems in operating conditions. The random nature of overvoltage phenomena in electrical networks implies the usage of computer simulations for the analysis of overvoltage exposures of electrical devices in operation. This article presents the analysis of the impact of transient phenomena in a model of a medium-voltage electrical network during switching operations and ground faults on overvoltages in the internal insulation systems of transformer windings. The basis of the analysis is simulations of overvoltages in the windings, made in the Electromagnetic Transients Program/Alternative Transients Program (EMTP/ATP) using a model with lumped parameters of transformer windings. The analysis covers the impact of the cable line length and the ground fault resistance value on internal overvoltage distributions.

Highlights

An analysis of insulation system overvoltage exposure and overvoltage protection effectiveness is of high importance
Closing the circuit breaker VCB connected to the cable line Lk and the HF transformer winding model, the variable length of the cable was changed in a range of 10 m to 60 km (Figure 5a); analysis of the impact of a single-phase line to a ground fault with distance changing from 10 m to 60 km from the transformer terminals (Figure 5b); analysis of the impact of short circuit resistance during a ground fault on the level and mechanism of internal overvoltage distribution; the analysis covered low and high ohmic faults, Rf = {0.01 Ω, Ω, 2 Ω, 10 Ω, 100 Ω}
The simulation results of overvoltages inside transformer windings performed in the EMTP/ATP

Summary

Introduction

An analysis of insulation system overvoltage exposure and overvoltage protection effectiveness is of high importance. The common cases in which resonance can occur are the switching of a circuit breaker or ground faults at a feeding power line During these events, the transient voltage appearing in the network contains oscillating components whose frequencies depend on the propagation condition in power lines, the length of lines, and power transformer electrical parameters. This article presents the analysis of the internal overvoltages in power transformer windings caused by selected switching operations of a circuit breaker and failure states in electrical networks. The overvoltage analysis covered the simulation cases: energization of the distribution transformer connected with line Lk with variable length; single-phase earth fault in the cable line Lk with different distances from the transformer and different values of the short circuit resistance. For steep and high-frequency signals, the transformer can be treated as a linear time-invariant class object [7,22,28], ignoring the silicon steel magnetic core

Simulation Models of a Power Transformer and Power Apparatus

Equivalent scheme ofofa awinding lumpedRLC

Waveforms of overvoltages at selected of athe

Discussion

Summary