# Simulating and Analyzing Transients in Power Systems

Electric utility engineers designing new transmission and distribution (T&D) systems and evaluating existing systems have relied on sophisticated software programs that can simulate transients. Now design engineers and electrical contractors have begun to use these same simulation software programs to evaluate the performance of end-user distribution systems. Simulating electromagnetic, electromechanical,

Electric utility engineers designing new transmission and distribution (T&D) systems and evaluating existing systems have relied on sophisticated software programs that can simulate transients. Now design engineers and electrical contractors have begun to use these same simulation software programs to evaluate the performance of end-user distribution systems.

Simulating electromagnetic, electromechanical, and control system transients, these software tools allow you to insert various components that are a part of the power system under evaluation, and then test the entire power system to a range of transient conditions. Nearly all those programs designed for power systems solve problems in the time domain, re-creating the transient-effected waveform point by point. A few programs, however, solve in the frequency domain, using the Fourier transform method to convert to time domain. You would need a time-domain solution to model nonlinear elements such as surge arresters and transformer magnetizing characteristics.

According to the book “Electrical Power Systems Quality,” by Roger Duncan, Mark McGranaghan, Surya Santoso, and H. Wayne Beaty, McGraw-Hill (ISBN 0-07-138622-X), there are many quality software programs on the market for simulating transients on a specific power system. Let's take a look at what's available, in generic terms.

### EMTP/ATP

The most widely used computer program is Electromagnetic Transients Program (EMTP) and its derivatives such as Alternative Transients Program (ATP). A sample power circuit is shown in the Figure above.

System components you can insert in conducting an EMTP/ATP study include:

• Uncoupled and coupled linear elements and lumped resistance, inductance, and capacitance (RLC) elements

• Transmission lines and cables with distributed and frequency-dependent parameters

• Nonlinear resistances and inductances, hysteretic inductor, time-varying resistances, and controlled resistances

• Components with nonlinearities, such as transformers (including saturation and hysteresis), surge arresters (gapless and with gap), and arcs

• Ordinary switches, time-dependent and voltage-dependent switches, and statistical switching

• Valves (diodes, thyristors, triacs) and controlled switches

• Analytical sources such as step, ramp, sinusoidal and exponential surge functions

• Rotating machines such as 3-phase synchronous machines and universal machine models

In addition to transient studies, you can use EMTP/ATP to study:

• Statistical and systematic overvoltages

• Very fast transients in groundings

• Machine modeling

• Transient stability and motor startup

• Shaft torsional oscillations

• Transformer and shunt reactor/capacitor switching

• Ferroresonance

• Power electronic applications

• Circuit breaker duty (electric arc) and current chopping

• Harmonic analysis and network resonances

• Protective device testing

You can use an Intel 486/Pentium-based PC with MS-Windows 3.x/95/98/NT. A standard PC configuration with minimum 16MB RAM, 20MB of free hard disk space, and VGA graphics is sufficient to execute most commercially available EMTP/ATP programs under MS-DOS/MS-Windows. ATP itself is available for other computers and operating systems, too.

For more detailed information, visit http://www.emtp.org.

### PSCAD/EMTDC

You can also perform some transient simulations with a commercial analysis tool known as PSCAD/EMTDC (Power System Computer Aided Design/Electromagnetic Transient including DC), which was developed by the Manitoba-HVDC Research Centre. The EMTDC portion of this analysis tool is a numerical engine that generates simulation results for transient studies. The program features a very sophisticated graphical user interface and can be used to simulate the effects of transients on T&D networks, industrial distribution systems, rotating machines, power converters, embedded systems such as those on electric ships, aircraft, and electric railway systems, and de-centralized generation systems.

This program also allows you to use a variety of components, such as:

• Fixed and variable resistors (R), inductors (L), and capacitors (C)

• Mutually coupled windings, such as transformers and saturable inductors

• Frequency dependent transmission lines and cables

• Current and voltage sources

• Switches and breakers

• Diodes, thyristors, GTOs, IGBTs, and other power electronic switching devices

• Analog and digital control functions, Laplace functions, and signal generators

• AC and DC machines, exciters, governors, stabilizers, and inertial models

• Meters and measuring functions

• Generic DC and AC controls

• Power electronic converters and control circuits

You can combine mixed signal functionality to provide a complete transient solution or view simulated signals in real-time with a variety of meters, dials, and oscilloscope windows.

For more detailed information on this program, do a Google search of “PSCAD/EMTDC.”

### SPICE

Some power system analysts use computer programs such as Simulation Program for Integrated Circuits Emphasis (SPICE), which was developed more for the analysis of electronic circuits.

SPICE can do several types of circuit analyses, including:

• Non-linear DC analysis (calculates the DC transfer curve)

• Non-linear transient analysis (calculates the voltage and current as a function of time when a large signal is applied)

• Linear AC analysis (calculates the output as a function of frequency

• Noise analysis

• Sensitivity analysis

• Distortion analysis

• Fourier analysis (calculates and plots the frequency spectrum)

The circuit under evaluation can contain the following components:

• Independent and dependent voltage and current sources

• Resistors, inductors, and capacitors

• Mutual inductors

• Transmission lines

• Operational amplifiers

• Switches

• Diodes

• Bipolar transistors

• Metal oxide semiconductors

• Junction field effect transistors

• Metal oxide semiconductor field-effect transistors

• Digital gates

To specify the circuit topology and do your analysis, you construct a SPICE input file. This file contains data statements, which describe all of the components in your power circuit and their interconnections, control statements, which tell SPICE what type of analysis you wish to perform on the circuit, and output statements, which specify what outputs are to be printed or plotted.

You can get an overview of how this program works at http://bwrc.eecs.berkeley.edu/Classes/IcBook/SPICE.

### Benefits of automation

Although programs such as EMTP/ATP, PSCAD/EMTDC, and SPICE can provide remarkable simulations, you may need considerably more modeling expertise to do electromagnetic transient studies than you would for more common power system analyses such as power flow or short circuit studies. Nevertheless, these programs can help you design new distribution systems or evaluate existing ones.

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