P. Buchanan is with Manito- ba HVDC Research Centre, Winnipeg, Canada. Power Electronics Spark. New Simulation Challenges. Om Nayak, Surya Santoso, ...
Power Electronics Spark New Simulation Challenges
© IMAGESTATE
Om Nayak, Surya Santoso, Paul Buchanan
he computer simulation of power systems has preprocessor-based controls and the associated advances in sented many challenges and opportunities over the power electronic devices over the past 10 years, the years. Fortunately, nature of modern power the general nature of systems has significantly power systems remained changed. This article disEnergy utilities are realizing that with relatively the same for a cusses some of the long period of time. This the appropriate tools they can train and changes that have taken allowed power system place in power systems sustain engineers who can maintain a engineers to improve and explores some of great insight into system dynamics modeling techniques the inherent requireprogressively and to ments for simulation apply computer hardware and software technology to technologies in order to keep up with this rapidly changdesign study tools that met the analysis requirements. ing environment. Industrial examples of how power system simulation has been applied by end-users to meet the The models were based on fundamental frequency responses. However, with the wide-spread use of microadvancing requirements is provided.
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O. Nayak is with Nayak Corp., Princeton, New Jersey, USA. S. Santoso is with Electrotek Concepts, Knoxville, Tennessee, USA. P. Buchanan is with Manitoba HVDC Research Centre, Winnipeg, Canada.
Changing Landscape In the early 1990s, the tools that previously had been appropriate for transmission system simulation and
ISSN 0895-0156/02/$17.00©2002 IEEE
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instantaneous system response over time. The instantaneous response, which is often referred to as the electromagnetic transient response, can provide detailed information about the system under study, such as the maximum over-voltage, harmonic distortions, and voltage sags. It can also accurately show the control system behavior. Electromagnetic transient simulation tools are not new. They have been in use for over 30 years. However, these tools were mainly used by utilities to study power surge phenomena or electromechanical system interaction. This was largely due to the highly numeric-intensive nature of the algorithms, as well as basic limitations in the simulation software. The end-user required a detailed understanding of the electrical system components and often required special engineering skills. Figure 1. Power system simulation workspace with interactive controls It is not surprising that these studies traand plots ditionally have been conducted only when analysis were simply no longer adequate for new fast-actthey are absolutely necessary. This is usually after a ing devices, such as static var compensators (SVC), statproblem has already occurred in the system rather than ic synchrounous compensators (STATCOM), via a proactive preengineering design study. In other high-voltage direct-current (HVdc) systems, unified words, it is the nature or cost of the problem that dicpower flow controllers (UPFC), and other modern power tates whether reactive action in the form of power syselectronic-based devices and their controls. Generation tem analysis is taken. This is not the ideal scenario under systems began to react much faster given this introducwhich to better understand the power system. tion of faster digital controls into the system. The impact Fortunately, things are changing. Energy utilities are of the extended spectrum of harmonics as a result of the realizing that with the appropriate tools, they can train fast reaction digital control systems was a phenomenon and sustain engineers who can maintain a great insight that suddenly could not be ignored. Analysis could no into system dynamics. These persons no longer have longer be restricted to fundamental frequency response. to be simulation study experts, but rather engineers Added to this, the emergence of unconventional distriband technologists with expertise in their area of uted generation systems, such as wind power, solar responsibility and who require modern software tools power, fuel-cells, and microturbines, presented new to facilitate their work. They use the simulation tools complexities to the behavior of power systems. as facilitators to accurate system modeling and system And, if this were not enough to complicate things, optimization. with this introduction of distributed generation systems Energy utilities will find it a necessity to perform tranand faster controls, we now face much more sensitive sient studies to understand events, such as the impact industrial loads (large computer data centers, high-precion the system of capacitor switching, arc furnace flicker, sion manufacturing systems), all of which require a high or to understand the impact of increased transmission degree of power quality. As energy utilities know all too capacity on existing transmission lines, coordinate conwell, the consumption patterns of these sensitive industrols, and set relays. Manufacturers of electrical equiptrial loads also are prone to introducing harmonics and ment will need to perform transient studies in order to voltage sags into the power grid. Thus, the modern enerdesign components, such as power electronic controls gy distribution environment is facing service and power and digital relays, optimally. quality challenges on many fronts. The bottom line is that power system simulation and analysis is becoming more important than ever before. A Why Analyze the System? better understanding of systems prior to their manufacPower systems are studied either to analyze their ture leads to optimally designed devices, and an analysis steady-state behavior or transient (time domain) behavof the impact of the introduction of new devices prior to ior. Transient analysis is conducted in order to analyze their installation into the electrical network results in system stability at the power frequency or to analyze optimized power systems and fewer surprises. 38 IEEE Computer Applications in Power
What Is Needed in Simulation Tools?
Long-Term
To address the challenges of the modern electric power transmission and genMid-Term Steady State eration environment, relevant Power Flow simulation technology is a must. From a general perspective, the two Stability Electromechanical most important aspects of any power system simulation toolset are the accuracy of Electromechanical Transients the simulation and the ease-of-use of the tools. More specific critical requirements Switching Transients of simulation tools are: Electromagnetics ■ Accuracy: accurate system solutions, Steep Front particularly where power electronic devices are involved 0.01 0.1 1 10 100 1000 10000 100000 Seconds ■ Intuitive user interface: a user interface Electromagnetic and Traveling Wave Analysis Fundamental Frequency Analysis and navigation system geared towards efficient power system analysis providing immediate visual feedback (Figure 1) Figure 2. Types of simulation tools versus analysis time duration ■ Size and duration: the ability to model Selection of Simulation Tools large systems and for a longer period of time than in the past A power system simulation tool must be carefully select■ Component library: a comprehensive library of ed to study a given problem. power system components, including all modern Figure 2 displays different types of simulation tools for power system devices and controls. analyzing power flow, stability, or system transients and It is the responsibility of power system simulation their applicability in terms of the various dynamics to be developers to provide tools that can add a valuable studied. What this chart indicates is that you must select extension to end-user capability. The tools must incorpothe appropriate tool for the task at hand. If you want to rate the ability to simulate the basic electrical elements analyze power electronic switching transients, for examof a power system, such as generators, transformers, ple, you will apply a toolset that supports full timetransmission lines, power electronics, diodes, thyristors, domain analysis. gate turn-off thyristors (GTO), as well as complex control Today, electromagnetic transient applications must and protection systems. The simulation tools must be be able to simulate larger systems than in the past and intuitive, they must be accurate, and they must be open, for longer time durations (minutes versus seconds). This providing support for existing end-user data formats. is now feasible given advances in computer technology and extensions to the software.
Simulating Power Electronics and Controls The simulation of power electronics and control systems State of the Art in Solution Techniques places very high demands on the precision of the simulaComputers are, of course, digital devices and must tion techniques. This is likely the most significant issue approximate a continuous response with a series of disthat simulation software developers have faced in terms of creating relevant Simulation tools must be intuitive, they must be tools for the modern electric accurate, and they must be open, providing support power system environment. For example, high-frequency for existing end-user data formats pulse width modulation (PWM) signals require great precision in the numerical calculation of the switching continuous calculation steps (often referred to as time instant. If the switching times are not determined accusteps). As a result, various numerical stability and mathrately, serious errors can be introduced into the simulaematical limitation problems can occur in simulation tion. But, in this search for accuracy, it is not generally depending on the solution techniques used. practical for simulation tools to enforce a small time Numerical accuracy is of particular importance in the increment (in the order of 1 µs) in order to achieve simulation of power electronic devices for which eleswitching accuracy. This just makes for an unacceptably ments constantly change state (turn on and off). The slow simulation. switching times of these devices most often will fall October 2002 39
three-phase insulated gate bipolar transistor (IGBT) based converter bridge feeding an RLC load. The results obtained from simulation when the interpolation algorithm is not applied show uncharacteristic harmonics that should not be present. The use of interpolation with PSCAD in determining the exact switching times allows the simulation to run at high speed and does not introduce inaccurate results.
50µs (Interpolated) 50µs (Not Interpolated)
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Figure 3. Typical harmonic spectrum of current from three-phase IGBT-based converter bridge feeding an RLC load
between time steps, and, due to the discrete nature of computation, the actual switching times will not be represented accurately. If not corrected, this phenomenon can lead to the inclusion of (among many other things) uncharacteristic harmonics and error-compounding and, if series inductance is present, spurious voltage spikes in the simulation results. A good way to measure the power of any particular simulation software package is to gauge how well these numerical issues are addressed, alongside the requirement for simulation accuracy and speed. Interpolation One method to remove the effects of inter-time-step switching is to decrease the time step itself. This, however, is not that practical, as it can greatly reduce simulation speed. It is possible to apply an interpolation algorithm to determine the exact instant of the switching event. This has proven to be much faster and more accurate than the alternative of reducing the time step. The use of the interpolation algorithm allows simulators, such as the PSCAD power system simulation toolset, to simulate any switching event accurately, while still allowing the use of a larger/faster time step. Figure 3 shows the current harmonic spectrum of a V L Natural Turn-Off
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Chatter Removal Chatter is a time-step to time-step symmetrical oscillation phenomenon inherent in the trapezoidal integration method used in the Dommel algorithm for transient simulation of electrical networks (H. Dommel from his classic April 1969 IEEE paper published in IEEE Transactions on Power Apparatus and Systems). This is a common base algorithm used in transient simulation. Chatter is generally introduced in simulation by the closing of a switch in a branch containing inductors. It does not matter, in this case, if the switching occurs between time steps or at a natural current zero. Figure 4 illustrates the presence of voltage chatter due to a natural turn-off of a series thyristor/inductor branch circuit. Chatter does not represent any true electrical network behavior and leads to inaccurate results. A modern simulation algorithm must recognize and correct this phenomenon. When chatter is detected or when a switching event takes place, a chatter removal algorithm is invoked. The user generally has the option to enable or disable the chatter algorithms; however, it is best to keep them enabled for all circuits, as there is an undetectable performance impact. Zero-Impedance Elements Zero-impedance circuit elements have traditionally caused problems in simulation algorithms. Examples of zero-impedance branches are infinite voltage sources, ideal short circuits, and an ideal switch that is in a closed state. Standard electromagnetic transient solution algorithms using what is termed a nodal admittance matrix require every branch to possess finite impedance. A zero-impedance branch would yield an infinite admittance and could lead to severe numerical problems.
Confidence in the Solution
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Chatter
Figure 4. Voltage chatter 40 IEEE Computer Applications in Power
3
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The complexities introduced by power electronics switching elements require advanced simulation algorithms. A great amount of research and development has continued over the past few years to maintain simulation accuracy and to optimize transient simulation software as it applies to modern power electronics based systems. For utilities to apply simulation tools to develop a better understanding of the nature of their power systems, the tools must be practical and easy to learn. At the same time, there must
VLs
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be confidence that the simulation tool developers have provided tools that do not need to be second-guessed. With an accurate foundation, endusers then need the flexibility to create and customize their own models as well as to access libraries of existing models and components. And, especially for the analysis of power systems involving control systems, the simulation toolset must provide the flexibility to integrate controls, including controls modeled with other software packages.
VGs 2 G2
4 G4
Figure 5. PQ-VR inverter bridge consists of four IGBTs and corresponding harmonic filters
Industrial Examples To provide an accurate picture of how simulation software can be applied successfully, one must look to industrial examples.
device is commonly known as a sag protector, dynamic voltage restorer, or power quality voltage restorer. Electrotek Concepts recently applied electromagnetic transient simulation software to model accurately the operation of a 3.2 MVA SMES power quality voltage restorer (PQ-VR) developed by American Superconductor Corporation, prior to its installation in a semiconductor manufacturing facility (IBM’s Bromont, Quebec, Canada, PowerPC manufacturing facility). The semiconductor manufacturing facility is supplied from a 49 kV system via two 49/13.8 kV transformers. The facility houses both sensitive and conventional loads. The downtime and loss of production associated with voltage sag disturbances are huge and, thus, the primary reason to install a sag protector device. For economic reasons, the PQ-VR is intended to protect only the most sensitive and critical equipment. A key component of the system is an inverter bridge consisting of four IGBTs, which turn on and off according to commands generated by a PWM controller. A simplified inverter bridge is shown in Figure 5. The switching scheme is performed by comparing the grid voltage with its expected voltage on an instantaneous waveform basis. Simulation plays a crucial role in the successful commercial deployment of this type of device. The performance of the PQ-VR in mitigating voltage sags can be
Voltage (kV)
Power Quality Power quality (PQ) problems generally appear in the form of voltage sags, transients, and harmonics. Among these problems, voltage sags are the most common PQ disturbance. They are typically caused by a fault on the power system. The voltage sag occurs over a significant area while the fault is actually on the system. As soon as the fault is cleared by a protective device, voltage returns to normal on most parts of the system, except the specific line or section that is actually faulted. The typical duration for a transmission system fault is about six cycles. Distribution system faults can have significantly longer durations, depending on the protection philosophy employed. The voltage magnitude during the fault will depend on the distance from the fault, the type of fault, and the system characteristics. Over the years, many solutions have been proposed to mitigate voltage sags. In principle, these solutions can be divided into two approaches: ■ Redesign the sensitive equipment such that it does not trip out during a voltage sag event ■ Maintain the supply voltage at a Voltage at Grid and Load Sides nominal level, i.e., VT1A VT2A +12 one per-unit rms at all times. +8 In this power sys+4 tem simulation exam+0 ple, we focus on the second approach: a -4 device-based super-8 conducting magnetic energy storage -12 0.1 0.1333 0.1667 0.2 0.2333 0.2667 0.3 (SMES) technology Time (s) designed to maintain load voltage at one Figure 6. PQ-VR in carryover mode; the load voltage is maintained at 1.0 per unit when the grid per-unit rms. This voltage experiences voltage sags (VT1A is the source supply; VT2A is the output of the PQ-VR)
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accurately verified through simulation prior to its industrial installation. As a sample test case, a voltage sag can be simulated with software by applying a transmission fault that causes a voltage sag of 60% at the 13.8 kV level. The sag duration is set at six cycles. The voltages at the grid and load sides are shown in Figure 6. It can be seen in Figure 6 that the PQ-VR can boost the load voltage back up to its nominal level; thus, the deployed dynamic voltage restorer would successfully handle this scenario. In this situation, the software toolset made it possible to accurately model the operation of the dynamic voltage restoration device and resulted in an optimal implementation. Specifically, the power simulation software was used to evaluate the PQ-VR design parameters, control algorithms, performance in mitigating voltage sags, and to characterize its harmonic distortion during the idling and carryover mode. The actual switching schemes and controls were realistically and accurately represented. The protection system and the interaction between the PQ devices and the distribution systems, including the protected loads, were also accurately modeled. With the new software tools and a basic personal computer, such complex systems can be modeled, and we can be confident in the results due to the accuracy of the solution algorithm. The power electronic switching (such as the IGBTs in this example) is performed at the exact time instant; thus, there is no delay in performing a switching action, the results are accurate, and the simulation is fast. Electric Propulsion PSCAD and other transient simulation applications have historically been used by energy utilities for system planning and power quality applications. However, there are many other emerging applications for transient simulation. With the trend toward the use of electric and hybrid-electric vehicles, electric ships, and the expansion of rapid transit systems based on electric power, power system simulation programs will need to adapt to specifically address these mini power systems. The electric power grid of a ship can be regarded as a small-scale, autonomous, industrial type compact power system. A ship’s electric energy system consists of a generator set and a distribution cabling system serving the loads. In the case of an ac conventional ship power system, there are at least two main ac generators driven by diesel engines, gas turbines, or the main propulsion engine. Electric power is often supplied via substations to all electric loads consisting of: ■ Conventional auxiliary systems (lighting, motors driving rotating machinery), similar to an industrial plant ■ Main and auxiliary ship propulsion systems. Typical shipboard electrical networks can operate at 50/60 Hz, while warship applications often require 400 Hz systems. Increased electric power demands in ships have led to the introduction of medium (high in ship installa42 IEEE Computer Applications in Power
tions) voltage operating levels of 3.3 and 6 kV. The ship grounding system is usually of the unearthed type. Recent developments in power semiconductor and power electronics technology and their application in drive and control systems have enabled integrated full electric propulsion (IFEP) of ships. The term electric ship now generally refers to a ship with a full electric propulsion system. The advent of ac electric propulsion in ships has introduced a whole new set of complexities on shipboard: ■ The main propulsion system comprises at least one huge power motor driving the ship propeller. New electric motor types (permanent magnet synchronous motors, advanced induction motors, multiphase motors of radial, axial, or transverse flux motors) have been constructed in an attempt to achieve higher efficiency and reliability indices. The propulsion motors are fed by power converters, which are responsible for the ship speed control in conjunction with their associated controllers. Speed control is achieved mainly either by vector flux control or direct torque control. ■ The auxiliary propulsion system (also called dynamic positioning system, DPS) consisting of side propellers, bow and stern thrusters, assisting the ship maneuvers. The DPS is usually driven by ac induction motors supplied via PWM controllers. ■ According to the electric ship concept, every major or minor system onboard the ship, propulsion being the predominant one, will eventually be electrified and served by a rather large-scale electric system (of several tens of MW) with multiple redundancies in components and circuits. The emergence of electric propulsion in ships has made accurate simulation and modeling a must. Transient simulation analysis is used to perform studies involving the modeling of electromagnetic and other switching transients on shipboard electric energy systems comprising both ac and dc subsystems. The systems that require modeling are the same as in any modern power system. The systems contain a high degree of power electronic switching components and require an understanding of the impact of all system modules on the operation of the shipboard electric system. The broad areas of study include propulsion, switching transients, and power quality investigations. ■ Propulsion: Main or auxiliary propulsion motor startup and steady-state operation in conjunction with their power electronic converters (cyclo-converters, synchro-converters, and PWM inverters) ■ Switching transients: Overvoltages and overcurrents due to making or breaking circuit breakers (e.g., transformer electrification, cable disconnection, fault clearances, load shedding etc.) ■ Power quality: Investigation of power quality problems introduced by the extensive use of power electronics in
With the new software tools and a basic PC, complex systems can be modeled, and we can be confident in the results due to the accuracy of the solution algorithm
propulsion drives, pulsed loads (e.g., weaponry in warships), and navigation instruments (e.g., radar). The harmonic distortion in both current and voltage waveforms provokes increased reactive losses, but also electromagnetic interference (EMI) problems. The requirement for power system simulation is a necessity on such a critical self-contained entity as a ship. The same tools that have played a role in the optimization of our electric power utility grids will play a significant role in research and deployment of advanced shipboard electric power and propulsion systems.
transmission or a flexible ac transmission system (FACTS) solution must be found. The initial indication of dynamic performance can be evaluated with a stability study. If this study demonstrated a problem, a more detailed analysis is needed to find the best solution for dynamic voltage control. The detail required in the simulation must include: ■ Wind turbine characteristics, particularly its speed control and protection system ■ Detailed generator, such as an induction or synchronous generator including the power electronics that might be applied, such as for a doubly wound induction generator ■ Strategy for ac voltage control. The study must demonstrate that ac voltages are retained within desirable limits and that transient overvoltages are properly coordinated to protect the equipment. If further investment is needed to achieve acceptable operation, its form should be determined. That would include: ■ Wind turbine and generator type ■ Additional transmission facilities required ■ Power electronic solutions (FACTS or dc transmission) ■ Impact of wind turbine protection ■ Control strategy and controls. Electromagnetic simulation with modern tools is criti-
AES2000
Dynamic Study of Wind Turbines Another area that is garnering much attention and promise is wind energy. As a renewable energy source, wind turbines are gaining popularity for the reasons that, as a generator of green power, they are relatively low in capital cost and can be brought into service relatively quickly. It is not surprising then that investors are looking at large wind farms with power ratings of the order of hundreds of MW, and, in some cases, studies are investigating capacities of thousands of MW. When large wind farms are being considered, they would be physically located where the normal wind velocities are high enough to produce the energy needed for profitability. However, these windy locations are not usually near any sizable load center where the energy is in demand. Thus, any existing grid system transmission in the location of the wind farm is not likely to be adequate for the full load power level generated. The first step in the study is to show that, under steady-state conditions with adequate switched shunt capacitor compensation, full load power generated by the wind farm can be connected satisfactorily to the grid transmission. But that is not enough. It must be determined that, during dynamic conditions following a fault, the ac transmission voltage is retained within acceptable limits. If not, either additional Figure 7. All-Electric Ship (AES) research
October 2002 43
D.A. WOODFORD, ELECTRANIX CORP.
electric vehicles, and other upcal, particularly when dynamic and-coming areas. voltage control and the performance of the wind farm is an In order to optimize solutions, important aspect. PSCAD has more efficient methods of deterrecently been used in cooperamining optimal system design tion with the Danish energy utiliand operating conditions must ty, ELTRA, where the Horns Rev be mathematically integrated 160 MW off shore wind farm is into simulation tools. These now under construction. Of conmethods could apply optimizacern was the performance of the tion algorithms to move more new wind turbine design with a quickly to an optimal condition. double wound induction generaTo make efficient use of the tor and how it performs dynamivast amount of data that is availcally in a system setting. A able, simulation systems must detailed model of the turbine and support open data formats, such generator with its power elecas relational databases. Simulatronics, controls, and protection tion toolsets are modernizing, was developed in software. Studand the learning curve to use ies were undertaken to confirm these tools is lessening. The the insulation coordination for requirement to model complex the undersea cable, and to coorelectrical systems is only going dinate its protection strategy, to increase. such as preventing self-excitation of the induction generator. TakAcknowledgments The authors acknowledge the insight and ing this further, the simulation contributions of the following individuals: model became the basis to build Dennis Woodford, president, Electranix and test the equivalent model for Corp., for his contribution on wind energy Figure 8. Large wind turbine in Denmark inclusion in a stability program. modeling (http://www.electranix.com); The future Laeso Syd offshore John M. Prousalidis, Marine Engineering Laboratory, Naval Architecture and Marine Engineering Department, wind farm in northern Denmark is now being modeled to National Technical University of Athens, Greece (http://www. determine whether the transmission to ELTRA’s main naval.ntua.gr); Charles Neumeyer, Princeton University Plasma Physics grid should be accomplished with ac or dc transmission. Laboratory, for his insight into ultra-high-voltage dc simulation require-
Keep Pace with Change One type of simulation technology is not a fit for all analysis. Engineers need to recognize what tools fit the requirements of their studies. Modern power system simulation packages will need to consist of various simulation modules all sharing the same graphical design and visualization environment. These power system simulation modules must include algorithms for the analysis of power flow, fault analysis, harmonic analysis, and stability analysis, as well as transient analysis. If simulation tools are going to be applicable to the modeling of new systems, such as power components on ships, for example, mathematical models for new types of propulsion motors and their associated power electronic converters will have to be developed. In some cases, the controllers will require new flexible programming modules to be integrated into the component libraries. Continuing with ships as an example, there will be nonlinear loads (pulsed loads and navigation systems) operating at various voltage and frequency levels that could be added to the simulation model libraries. The same holds for the requirements of wind energy systems (wind turbines), 44 IEEE Computer Applications in Power
ments; Bharat Bhargava, Southern California Edison, for his helpful information from the energy utility perspective; John Nordstrom and Randy Wachal, researchers with the Manitoba HVDC Research Centre, Winnipeg, Canada; Takashi Aihara of Hitachi, Power & Industrial Systems, Japan, for his information on industrial requirements for simulation technologies.
For Further Reading K. Søbrink, P.L. Sorensen, E. Joncquel, and D. Woodford, “Feasibility study regarding integration of the læsø syd 160 mw wind farm using VSC transmission.” Available http://www.electranix.com/publications.html. A.M. Gole, S.A. Woodford, J.E. Nordstrom, and G.D. Irwin, “A fully interpolated controls library for electromagnetic transients simulation of power electronic systems,” in Proc. 2001 Int. Conf. Power Syst. Transients, Rio de Janeiro, Jun. 2001, p. 681. A.M. Gole, G.D. Irwin, and D.A. Woodford, “Precision simulation of PWM controllers,” Manitoba HVDC Research Centre, 1999. Available http://www.hvdc.ca/download.html).
Biographies Om Nayak received a Ph.D. degree and is president of Nayak Corporation, Princeton, New Jersey, USA. Surya Santoso received a Ph.D. degree and is a senior power quality engineer at Electrotek Concepts, Knoxville, Tennessee, USA. Paul Buchanan is a power engineer at Manitoba HVDC Research Centre, Winnipeg, Canada, and is involved in the development and application of PSCAD power system simulation software.
