Transient Analysis of Power Systems

Solution Techniques, Tools and Applications

Juan A. Martinez-Velasco, Juan A. Martinez-Velasco

• Publisher: Wiley-IEEE Press
• Year: 2014
• ISBN:
  9781118694176
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

The simulation of electromagnetic transients is a mature field that plays an important role in the design of modern power systems. Since the first steps in this field to date, a significant effort has been dedicated to the development of new techniques and more powerful software tools. Sophisticated models, complex solution techniques and powerful simulation tools have been developed to perform studies that are of supreme importance in the design of modern power systems. The first developments of transients tools were mostly aimed at calculating over-voltages. Presently, these tools are applied to a myriad of studies (e.g. FACTS and Custom Power applications, protective relay performance, simulation of smart grids) for which detailed models and fast solution methods can be of paramount importance.

This book provides a basic understanding of the main aspects to be considered when performing electromagnetic transients studies, detailing the main applications of present electromagnetic transients (EMT) tools, and discusses new developments for enhanced simulation capability.

Key features:

• Provides up-to-date information on solution techniques and software capabilities for simulation of electromagnetic transients.
• Covers key aspects that can expand the capabilities of a transient software tool (e.g. interfacing techniques) or speed up transients simulation (e.g. dynamic model averaging).
• Applies EMT-type tools to a wide spectrum of studies that range from fast electromagnetic transients to slow electromechanical transients, including power electronic applications, distributed energy resources and protection systems.
• Illustrates the application of EMT tools to the analysis and simulation of smart grids.

1
Introduction to Electromagnetic Transient Analysis of Power Systems

Juan A. Martinez-Velasco

1.1 Overview

Electrical power systems are among the most complex, extensive and efficient systems designed to date. The goal of a power system is to generate, transport and distribute the electrical energy demanded by consumers in a safe and reliable way.

Power systems play a crucial role in modern society, and their operation is based on some specific principles. Since electricity cannot be stored in large quantities, the operation of the power system must achieve a permanent balance between its production in power stations and its consumption by loads in order to maintain frequency within narrow limits and ensure a reliable service.

Even when the power system is running under normal operation, loads are continually connected and disconnected, and some control actions are required to maintain voltage and frequency within limits. This means that the power system is never operating in a steady state. In addition, unscheduled disturbances can alter the normal operation of the power system, force a change in its configuration, cause failure of some power equipment or cause an interruption of service that can affect a significant percentage of the system demand, such as a blackout.

The analysis and simulation of electromagnetic transients has become a fundamental methodology for understanding the performance of power systems, determining power component ratings, explaining equipment failures or testing protection devices. The study of transients is a mature field that can be used in the design of modern power systems. Since the first steps in this field, a significant effort has been dedicated to the development of new techniques and more powerful software tools. Sophisticated models, complex solution techniques and powerful simulation tools have been developed to perform studies that are of paramount importance in the design of modern power systems. The first developments of transients tools were mostly aimed at calculating overvoltages. Presently, these tools are applied in a myriad of studies (e.g. FACTS and custom power applications, protective relay performance, power quality studies) for which detailed models and accurate solutions can be extremely important.

Transient phenomena in power systems are associated with disturbances caused by faults, switching operations, lightning strikes or load variations. These phenomena can stress and damage power equipment. The paramount importance of their study relates to the effects they can have on system performance or the failures they can cause to power equipment.

Two types of stress can be caused by transient phenomena in power systems: (1) overcurrents, which can damage power equipment due to excessive heat dissipation, and (2) overvoltages, which can cause insulation breakdown (failure through solid or liquid insulation) or flashovers (insulation failure through air). Protection against these stresses is therefore necessary. This protection can be provided by specialized equipment whose operation is aimed at either isolating the power system section where the disturbance has occurred (e.g. a power component failure that causes short-circuit) or limiting the stress across power equipment terminals (e.g. by installing a surge arrester that will mitigate voltage stresses). In addition, a better ability to handle stresses caused by transient phenomena can be also achieved through good design of power equipment (e.g. by shielding overhead transmission lines to limit flashovers caused by direct lightning strikes). That is, although the power system operates most of the time under normal operating conditions, its design must enable it to cope with the consequences of transient phenomena.

In order to provide adequate protection against both types of stresses, it is fundamental to know their origin, calculate their main characteristics and estimate the most adverse operating conditions. A rigorous and accurate analysis of transients in power systems is difficult due to the size of the system, the complexity of the interaction between power devices and the physical phenomena that need to be analysed. Presently, the study and simulation of transients in actual power systems is carried out with the aid of a computer.

Aspects that contribute to this complexity are the variety of causes, the nature of the physical phenomena and the timescale of the power system transients.

Disturbances can be external (lightning strikes) or internal (faults, switching operations, load variations).

Power system transients can be electromagnetic, when it is necessary to analyse the interaction between the (electric) energy stored in capacitors and the (magnetic) energy stored in inductors, or electromechanical, when the analysis involves the interaction between the electric energy stored in circuit elements and the mechanical energy stored in rotating machines.

Physical phenomena associated with transients make it necessary to examine the power system over a time interval as short as a few nanoseconds or as long as several minutes.

This latter aspect is a challenge for the analysis and simulation of power system transients, since the behaviour of power equipment is very dependent on the transient phenomena: it depends on the range of frequencies associated to transients. An accurate mathematical representation of any power device over the whole frequency range of transients is very difficult, and for most components is not practically possible.

Despite the powerful numerical techniques, simulation tools and graphical user interfaces currently available, those involved in electromagnetic transients studies, sooner or later, face the limitations of models available in transients packages, the lack of reliable data and conversion procedures for parameter estimation or insufficient studies for validating models.

Figure 1.1 presents a typical procedure when simulating electromagnetic transients in power systems. The entire procedure implies four steps, that are summarized as follows:

1. The selection of the study zone and the most adequate representation of each component involved in the transient
   The system zone is selected, taking into account the frequency range of the transients to be simulated: the higher the frequencies, the smaller the zone modelled. In general, it is advisable to minimize the study zone, because a larger number of components does not necessarily increase accuracy; instead it will increase the simulation time, and there will be a higher probability of insufficient or incorrect modelling. Although many works have been dedicated to providing guidelines on these aspects [1–3], some expertise is usually needed to choose the study zone and the models.
2. The estimation of parameters to be specified in the mathematical models
   Once the mathematical model has been selected, it is necessary to collect the information that could be useful for obtaining the values of parameters to be specified. For some components, these values can be derived from the geometry; for other components these values are not readily available and they must be deduced by testing the component in the laboratory or carrying out field measurements. In such case, a data conversion procedure will be required to derive the final parameter values. Details of parameter determination for some power components were presented in [4].
   Interestingly, an idealized/simplified representation of some components may be considered when the system to be simulated is too complex. This representation will enable the data file to be edited and the analysis of the simulation results to be simplified.
   A sensitivity study should be carried out if one or several parameters cannot be accurately determined. Results derived from such a study will show what parameters are of concern.
3. The application of a simulation tool
   The steadily increasing capabilities of hardware and software tools have led to the development of powerful simulation tools that can cope with large and complex power systems. Modern software for transient analysis incorporates powerful and friendly graphical user interfaces that can be very useful when creating the input file of the test system model.
4. The analysis of simulation results
   Simulation of electromagnetic transients can be used, among other things, for determining component ratings (e.g. insulation levels or energy absorption capabilities), testing control and protection systems, validating power component representations or understanding equipment failures. A deep analysis of simulation results is an important aspect of the entire procedure, since each of these studies may involve an iterative procedure in which models and parameters values must be adjusted.

Figure 1.1 Simulation of electromagnetic transients in power systems.

Pioneering work in this field was presented in [2, 5, 6]; see also [7]. Readers interested in electromagnetic transient analysis can consult other specialized literature [8–15].

1.2 Scope of the Book

This book provides a basic background to the main solution techniques presently applied to the calculation of electromagnetic transients, gives details of the main applications of the most popular transient tools (insulation coordination, power electronics applications, protection) and discusses new developments (e.g. dynamic average models, interfacing techniques) mostly aimed at overcoming some limitations of the present software tools.

The main topics to be covered by this book are as follows:

Solution Methods and Simulation Tools: The analysis of electromagne...