Lightning Protection of Substations and the Effects of the Frequency-Dependent Surge Impedance of Transformers

The reliability of electrical power transmission and distribution depends upon the progress in the insulation coordination, which results both from the improvement of overvoltage protection methods and new constructions of electrical power devices, and from the development of the surge exposures identification, affecting the insulating system. Owing to the technical, exploitation, and economic nature, the overvoltage risk in high and extra high voltage electrical power systems has been rarely investigated, and therefore the theoretical methods of analysis are intensely developed. This especially applies to lightning overvoltages, which are analyzed using mathematical modeling and computer calculation techniques. The chapter is dedicated to the problems of voltage transients generated by lightning overvoltages in high and extra high voltage electrical power systems. Such models of electrical power lines and substations in the conditions of lightning overvoltages enable the analysis of surge risks, being a result of direct lightning strokes to the tower, ground, and phase conductors. Those models also account for the impulse electric strength of the external insulation. On the basis of mathematical models, the results of numerical simulation of overvoltage risk in selected electrical power systems have been presented. Those examples also cover optimization of the surge arresters location in electrical power substations.

Transformer Modelling for Impulse Voltage Distribution and Terminal Transient Analysis

Voltage surges arising from transient events, such as switching operations or lightning discharges, are one of the main causes of transformer winding failure. The voltage distribution along a transformer winding depends greatly on the waveshape of the voltage applied to the winding. This distribution is not uniform in the case of steep-fronted transients since a large portion of the applied voltage is usually concentrated on the first few turns of the winding. High frequency electromagnetic transients in transformers can be studied using internal models (i.e., models for analyzing the propagation and distribution of the incident impulse along the transformer windings), and black-box models (i.e., models for analyzing the response of the transformer from its terminals and for calculating voltage transfer). This chapter presents a summary of the most common models developed for analyzing the behaviour of transformers subjected to steep-fronted waves and a description of procedures for determining the parameters to be specified in those models. The main section details some test studies based on actual transformers in which models are validated by comparing simulation results to laboratory measurements.

Transmission Line Theories for the Analysis of Electromagnetic Transients in Coil Windings

The chapter contains the basic theory of a distributed-parameter circuit for a single overhead conductor and for a multi-conductor system, which corresponds to a three-phase transmission line and a transformer winding. Starting from a partial differential equation of a single conductor, solutions of a voltage and a current on the conductor are derived as a function of the distance from the sending end. The characteristics of the voltage and the current are explained, and the propagation constant (attenuation and propagation velocity) and the characteristic impedance are described. For a multi-conductor system, a modal theory is introduced, and it is shown that the multi-conductor system is handled as a combination of independent single conductors. Finally, a modeling method of a coil is explained by applying the theories described in the chapter.

Z-Transform Models for the Analysis of Electromagnetic Transients in Transformers and Rotating Machines Windings

High voltage power equipment with winding structures such as transformers, HV motors, and generators are important for the analysis of high frequency electromagnetic transients in electrical power systems. Conventional models of such equipment, for example the leakage inductance model, are only suitable for low frequency transients. A Z-transform model has been developed to simulate transformer, HV motor, and generator stator windings at higher frequencies. The new model covers a wide frequency range, which is more accurate and meaningful. It has many applications such as lightning protection and insulation coordination of substations and the circuit design of impulse voltage generator for transformer tests. The model can easily be implemented in EMTP programs.

Frequency Characteristics of Generator Stator Windings

A generator stator winding consists of a number of stator bars and overhang connections. Due to the complicated winding structure and the steel core, the attenuation and distortion of a pulse transmitted through the winding are complicated, and frequency-dependent. In this chapter, pulse propagation through stator windings is explained through the analysis of different winding models, and using experimental data from several generators. A low voltage impulse method and digital analysis techniques to determine the frequency characteristics of the winding are described. The frequency characteristics of generator stator windings are discussed in some detail. The concepts of the travelling wave mode and capacitive coupling mode propagations along stator winding, useful in insulation design, transient voltage analysis, and partial discharge location are also discussed. The analysis presented in this chapter could be applied to other rotating machines such as high voltage motors.

Detection of Transformer Faults Using Frequency Response Analysis with Case Studies

Power transformers encounter mechanical deformations and displacements that can originate from mechanical forces generated by electrical short-circuit faults, lapse during transportation or installation and material aging accompanied by weakened clamping force. These types of mechanical faults are usually hard to detect by other diagnostic methods. Frequency response analysis, better known as FRA, came about in 1960s (Lech & Tyminski 1966) as a byproduct of low voltage (LV) impulse test, and since then has thrived as an advanced non-destructive test for detecting mechanical faults of transformer windings by comparing two frequency responses one of which serves as the reference from the same transformer or a similar design. This chapter provides a background to the FRA, a brief description about frequency response measuring methods, the art of diagnosing mechanical faults by FRA, and some case studies showing typical faults that can be detected.

Ferroresonance in Power and Instrument Transformers

This chapter describes the fundamental concepts of ferroresonance phenomenon and analyzes its symptoms and the consequences in transformers and power systems. Due to its nonlinear nature, the ferroresonance phenomenon can result in multiple oscillating modes which can be characterized based on the concepts of the nonlinear dynamic systems, e.g., Poincare map. Among numerous system configurations which can experience the phenomena, a few typical systems scenarios, which cover the majority of the observed ferroresonance incidents in power systems, are introduced. This chapter also classifies the ferroresonance study methods into the analytical and the time-domain simulation approaches. A set of analytical approaches are presented, and the corresponding fundamentals, assumptions, and limitations are discussed. Furthermore, key parameters for accurate digital time-domain simulation of the ferroresonance phenomenon are introduced, and the impact of transformer models and the iron core representations on the ferroresonance behavior of transformers is investigated. The chapter also presents some of the ferroresonance mitigation approaches in power and instrument transformers.

Frequency Characteristics of Transformer Windings

Transformers are subjected to voltages and currents of various waveforms while in service or during insulation tests. They could be system voltages, ferroresonance, and harmonics at low frequencies, lightning or switching impulses at high frequencies, and corona/partial discharges at ultra-high frequencies (a brief explanation is given at the end of the chapter). It is of great importance to understand the frequency characteristics of transformer windings, so that technical problems such as impulse distribution, resonance, and partial discharge attenuation can be more readily solved. The frequency characteristics of a transformer winding depend on its layout, core structure, and insulation materials.

Partial Discharge Detection and Location in Transformers Using UHF Techniques

Power transformers can exhibit partial discharge (PD) activity due to incipient weaknesses in the insulation system. A certain level of PD may be tolerated because corrective maintenance requires the transformer to be removed from service. However, PD cannot simply be ignored because it can provide advance warning of potentially serious faults, which in the worst cases might lead to complete failure of the transformer. Conventional monitoring based on dissolved gas analysis does not provide information on the defect location that is necessary for a complete assessment of severity. This chapter describes the use of ultra-high frequency (UHF) sensors to detect and locate sources of PD in transformers. The UHF technique was developed for gas-insulated substations in the 1990s and its application has been extended to power transformers, where time difference of arrival methods can be used to locate PD sources. This chapter outlines the basis for UHF detection of PD, describes various UHF sensors and their installation, and provides examples of successful PD location in power transformers.

Transformer Insulation Design Based on the Analysis of Impulse Voltage Distribution

In this chapter, the calculation of transient voltages over and between winding parts of a large power transformer, and the influence on the design of the insulation is treated. The insulation is grouped into two types; minor insulation, which means the insulation within the windings, and major insulation, which means the insulation build-up between the windings and from the windings to grounded surfaces. For illustration purposes, the core form transformer type with circular windings around a quasi-circular core is assumed. The insulation system is assumed to be comprised of mineral insulating oil, oil-impregnated paper and pressboard. Other insulation media have different transient voltage withstand capabilities. The results of impulse voltage distribution calculations along and between the winding parts have to be checked against the withstand capabilities of the physical structure of the windings in a winding phase assembly. Attention is paid to major transformer components outside the winding set, like active part leads and cleats and various types of tap changers.