Power quality issues involve all aspects of power system operation, from steady-state voltage regulation to the control of high-frequency transients caused by lightning or switching operations. Investigating power quality problems requires an equally broad range of monitoring equipment and analytical tools. Today, thanks to recent advancements, the tools for simulating power system performance during transient conditions are better than ever. Because many PQ problems involve complex interactions between power system elements, control systems, and end-use equipment, these tools are quite valuable for determining root causes and evaluating solutions.
Simulations play a critical role in power quality investigations (see Fig. 1). Typically, these investigations start with monitoring. Monitoring helps engineers determine if a problem relates to steady-state conditions (such as harmonic distortion, voltage regulation, and unbalance) or transient conditions associated with momentary disturbances. Once engineers understand the nature of a problem, they can use system models and simulation tools to track the causes and determine the effects of various system conditions and parameters. For instance, you could perform a simulation to find out if adding low-voltage capacitor banks to a customer's system would magnify a transient voltage. For a list of other power quality concerns where simulations can play an important role, see the table on page 12.
Simulations also enable engineers to identify technically viable solutions and evaluate the economics associated with each alternative. These solutions may include simple response characteristics or complex control-system issues involving interactions with power systems and customer facilities.
What They Do
Transient simulation tools work by performing a two-step process. They develop a set of differential equations that describe a power system's behavior and then solve those equations using digital integration techniques.
Simulation tools also evaluate voltages, currents, and control system parameters during disturbance intervals. Each interval consists of individual time steps with lengths that depend on the system being modeled and the particular phenomenon. For example, switching transients have rise times on the order of 10 μs. Therefore, time steps on the order of 1 μs or less are required to simulate the propagation of these transients around a system.
Historically, the most widely used program for transient analysis is the Electromagnetic Transients Program (EMTP). The EMTP is a general-purpose computer program for simulating high-speed transient effects on electrical power systems. The program features a wide variety of modeling capabilities encompassing electromagnetic and electromechanical oscillations ranging in duration from microseconds to seconds.
The EMTP method was originally developed in the late 1960s by Hermann Dommel at Bonneville Power Administration (BPA). Dr. Dommel's work demonstrated that a power system can be represented by a network of resistors (R), inductors (L), and capacitors (C). Inductors and capacitors can be represented by a resistor and a series time-varying current source. Since then, there have been significant developments by groups all over the world. In the early 1990s, Electrotek Concepts developed a version of the program for Microsoft® Windows that substantially increased its availability for power quality investigations.
Despite these developments, understanding the complexities of system modeling for transient studies required a significant investment. User groups have been formed to help with training requirements and share information about model development. Currently, EPRI and the EMTP's Development Coordination Group (DCG) coordinate the development and support of EMTP. Companies that are not members of EPRI or the DCG can obtain a separately supported version of the program known as the Alternative Transients Program (ATP). Information is available through the Canadian/American EMTP User Group by calling (360) 418-8640 or sending an e-mail to [email protected].
Recent technological advancements have solved some of the problems that have prevented the widespread use of simulation tools for power quality studies in the past. These advancements include:
Control system modeling
Transient simulation tools must often take into account the response and interaction of control systems, which must react quickly to improve power quality. An important simulation objective is to evaluate the performance of these devices under a wide variety of conditions to make sure they do not cause unexpected problems. Modern tools permit convenient and flexible representation of the control systems and the specific electrical elements under control.
Graphical user interface
This addition has greatly enhanced the value of simulation tools. Combined with model libraries for different components and types of studies, the user interface reduces the effort required to develop and modify system models for a wide variety of investigations.
Interactive viewing of simulation results
The traditional method of performing simulations involves three steps: setting up a case model, running the simulation, and viewing an output file with a separate program, such as TOP, The Output Processor® (www.pqsoft.com/top). New simulation tools allow users to view simulation results as they are progressing and interrupt simulations at any time to change the models or the status of switches.
Manufacturers can now develop models of their devices, complete with controls and proprietary characteristics. Prospective clients use these models to evaluate the performances of the devices for specific applications. These evaluations are the basis for model libraries. As they increase, so does the range of problems eligible for study.
Simulating power electronics circuits
Transient simulation tools have always been able to model the switching elements in power electronics circuits and the combinations of these elements in different topologies. However, the selected time-step size was based on the switching speeds of the power electronics equipment being modeled. Therefore, the extent of the model was limited and simulation times were long.
New tools like Manitoba's PSCAD® solve these problems. This tool includes interpolation between the solution time steps to improve accuracy during switching events. The network matrix is automatically reduced in order to reduce the size of the system under study and increase algorithm solution speed. The electrical network is then separated into subsystems that again decrease the overall simulation solution time. In addition, chatter removal eliminates unwanted numerical oscillations. As a result, the newer tools simulate more detailed models more quickly, even on a desktop computer.
PSCAD® is an example of a transient simulation tool that includes all of the above advancements. It is available for computers that use Microsoft® Windows or UNIX operating systems.
Transient simulation tools are ideal for evaluating capacitor-switching transients. These types of transients can significantly impact customer loads, oftentimes making it necessary to implement switching controls for large banks. Simulation tools can help optimize switching control technology (e.g., synchronous closing control, preinsertion resistors, etc.) and evaluate the effects on customer facilities. Important concerns include magnification at lower voltage capacitor banks and transient impacts on electronic loads, such as variable-speed drives.
Fig. 2, on page 10, is an example of a system model used to evaluate the impact on customer facilities during the energizing of a substation capacitor bank. Fig. 3 was developed by simulating the switching of the substation capacitor bank with different sized power factor correction capacitor banks at the customer facility. The figure shows the effect of the low-voltage capacitor on the magnified transient voltage. This magnified transient could be controlled by reducing the transient with control at the switched capacitor or by changing the characteristics of the low-voltage power factor correction (e.g., adding a tuning reactor).
Transient simulation tools are also beneficial in the area of arc furnace flicker control. When steel melts inside an electric arc furnace (EAF), it creates dynamic conditions that can cause voltage fluctuations (flicker) on power systems.
You can control the flicker somewhat by controlling the melting process itself. However, external compensation is often necessary to make sure the flicker does not impact customers in the area of the arc furnace facility.
Traditional compensation has been in the form of static var systems consisting of fixed capacitors/filters and thyristor-controlled reactors. More recently, static converters (STATCOMs) with faster response and better control characteristics have been applied for flicker control.
Transient simulation tools will model the nonlinear and dynamic characteristics of an arc furnace load and evaluate different options for controlling flicker. At the same time, these tools can evaluate harmonic control and switching concerns. Fig. 4, on page 14, illustrates the effectiveness of a STATCOM application. The STATCOM is operating from 0 sec to 2.5 sec and is not operating between 2.5 sec and 4 sec.
In the beginning, using transient simulation tools can be a daunting task, even with more user-friendly tools available. One way to solicit help and stay on top of the latest developments is to access various user groups and training classes. Here are a few online resources to get you started:
- Canadian/American EMTP Users Group; www.ee.mtu.edu/atp
- DCG/EPRI EMTP Users Group; www.emtp96.com
- Electrotek PQSoft Support; www.pqsoft.com
- PSCAD/EMTDC Web site (course information and link to users group); www.pscad.com
Once you become familiar with transient simulation tools, you'll add another method to your repertoire for finding and resolving costly power quality problems.
Mark McGranaghan directs power quality projects and product development at Electrotek Concepts in Knoxville, Tenn. You can reach him at [email protected].
Randy Wachal is the research project manager at the Manitoba HVDC Research Centre in Winnipeg, Manitoba, Canada. You can reach him at [email protected].