The What, Where And Why Of Real-Time Simulation
37The What, Where and Whyof Real-Time SimulationJ. Bélanger, Member, IEEE, P. Venne, Student Member, IEEE,and J.-N. Paquin, Member, IEEE Abstract-- Simulation tools have been widely used for thedesign and improvement of electrical systems since the midtwentieth century. The evolution of simulation tools hasprogressed in step with the evolution of computing technologies.In recent years, computing technologies have improveddramatically in performance and become widely available at asteadily decreasing cost. Consequently, simulation tools have alsoseen dramatic performance gains and steady cost decreases.Researchers and engineers now have access to affordable, highperformance simulation tools that were previously too costprohibitive, except for the largest manufacturers and utilities.This paper introduces the role and advantages of using real-timesimulation by answering three fundamental questions: what isreal-time simulation; why is it needed and where does it best fit.The recent evolution of real-time simulators is summarized. Theimportance of model validation, mixed use of real-time andoffline modes of simulation and test coverage in complex systemsis discussed.Index Terms—accelerated simulation, hardware-in-the-loop(HIL), model-based design (MBD), power system simulation,rapid control prototyping (RCP), real-time simulation, softwarein-the-loop (SIL).the optimization of motor drives in transportation, simulationhas played a critical role in the successful development of alarge number of applications.For the last three decades, the evolution of simulation toolshas been driven by the rapid evolution of computingtechnologies. As computer technologies have decreased incost and increased in performance, the capability of simulationtools to solve increasingly complex problems in less time hasimproved. In addition, the cost of digital simulators has alsosteadily decreased, making them available to a larger numberof users for a wider variety of applications.The objective of this paper is to provide an introduction toreal-time digital simulators, with a focus on ElectromagneticTransients (EMT), power systems modeling & simulation, andcontrol prototyping techniques. First, real-time simulation isdefined. An overview of the evolution of real-time simulatorsis then presented. Two other essential questions are thenanswered. Why is real-time simulation needed? Where doesreal-time simulation fit best? Finally, this paper concludeswith discussion of the importance of model validation, themixed use of real-time & offline simulation and test coveragein complex systems.I. PWMRESRCPSILTNACommercial off-the-shelfDistributed GenerationDigital Signal ProcessorElectromagnetic TransientsFlexible AC Transmission SystemField-programmable Gate ArrayHardware-in-the-LoopHigh Voltage Direct CurrentInsulated-gate Bipolar TransistorInputs and OutputsModel-based DesignPulse Width ModulationRenewable Energy SourcesRapid Control PrototypingSoftware-in-the-LoopTransient Network AnalyzerII. INTRODUCTIONSIMULATORS have been used extensively in the planningand design of electrical systems for decades. From thelayout of transmission lines in large scale power systems toJ. Bélanger, P. Venne and J.-N. Paquin are with Opal-RT Technologies,1751 Richardson, suite 2525, Montréal, QC H3K 1G6 CANADA (e-mail:{jean.belanger, philippe.venne, jean-nicolas.paquin}@opal-rt.com)III. WHAT IS REAL-TIME SIMULATION?A. Time Runs Out and Real-Time PrevailsA simulation is a representation of the operation or featuresof a system through the use or operation of another [1]. For thetypes of digital simulation discussed in this paper, it isassumed a simulation with discrete-time and constant stepduration is performed. During discrete-time simulation, timemoves forward in steps of equal duration. This is commonlyknown as fixed time-step simulation [2]. It is important to notethat other solving techniques exist that use variable time-steps.Such techniques are used for solving high frequency dynamicsand non-linear systems, but are unsuitable for real-timesimulation [3]. Accordingly, they are not covered in thispaper.To solve mathematical functions and equations at a giventime-step, each variable or system state is solved successivelyas a function of variables and states at the end of thepreceeding time-step. During a discrete-time simulation, theamount of real time required to compute all equations andfunctions representing a system during a given time-step maybe shorter or longer than the duration of the simulation timestep. Figure 1 a) and Figure 1 b) represent these twopossibilities. In a), the computing time is shorter than a fixed
38Computationf(t)Sim. Clockf(tn)f(tn 1)tn-1f(tn 2)tnTimetn 1Time(a) Offline simulation: Faster then real-timef(tn)Computationf(t)(a) Thyristor Converter CircuitTsf(tn 1)TimeTimetn-1tntn 1TimeJitterSim. Clocktn-10.8f(tn 1)tnTimetn 110.8AmplitudeVariation0.60.60.40.40.20.2Time(c) Real-time simulation: SynchronizedFigure 1: Real-Time Simulation Requisites and OtherSimulation Techniquesvoltage (V)Computationf(t)Time1(b) Offline simulation: Slower then real-timef(tn)Jitter00- 0.2- 0.2Source Voltage- 0.4- 0.4Thyristor Curent(Real Circuit)- 0.6time-step (also referred to as accelerated simulation) while inb), the computing time is longer. These two situations arereferred to as offline simulation. In both cases, the moment atwhich a result becomes available is irrelevent. Typically,when performing offline simulation, the objective is to obtainresults as fast as possible. The system solving speed dependson available computation power and the system’smathematical model complexity.Conversely, during real-time simulation, the accuracy ofcomputations not only depends upon precise dynamicrepresentation of the system, but also on the length of timeused to produce results [4]. Figure 1 c) illustrates thechronological principle of real-time simulation. For a real-timesimulation to be valid, the real-time simulator used mustaccurately produce the internal variables and outputs of thesimulation within the same length of time that its physicalcounterpart would. In fact, the time required to compute thesolution at a given time-step must be shorter than the wallclock duration of the time-step. This permits the real-timesimulator to perform all operations necessary to make a realtime simulation relevant, including driving inputs and outputs(I/O) to and from externally connected devices (furtherdiscussed in section III. D. and E. ). For a given time-step,any idle-time preceding or following simulator operations islost; as opposed to accelerated simulation, where idle time isused to compute the equations at the next time-step. In such acase, the simulator waits until the clock ticks to the next timestep. However, if simulator operations are not all achievedwithin the required fixed time-step, the real-time simulation isconsidered erroneous. This is commonly known as an“overrun”.Based on these basic definitions, it can be concluded that areal-time simulator is performing as expected if the equations- 0.6Thyristor Curent(uncompensatedFixed-Step Simulation)- 0.8current (mA)Sim. Clock- 0.8-1-10246810time (us)(b) Thyristor Converter Voltages12Figure 2: Timing Problem in a Thyristor Converterand states of the simulated system are solved accurately, withan acceptable resemblance to its physical counterpart, withoutthe occurence of overruns.B. Timing and ConstraintsAs previously discussed, real-time digital simulation isbased on discrete time-steps where the simulator solves modelequations successively. Proper time-step duration must bedetermined to accurately represent system frequency responseup to the fastest transient of interest. Simulation results can bevalidated when the simulator achieves real-time withoutoverruns.For each time-step, the simulator executes the same seriesof tasks: 1) read inputs and generate outputs 2) solve modelequations 3) exchange results with other simulation nodes 4)wait for the start of the next step. A simplified explanation ofthis routine suggests that the state(s) of any externallyconnected device is/are sampled once at the beginning of eachsimulation time-step. Consequently, the state(s) of thesimulated system is/are communicated to external devicesonly once per time-step. As introduced in section III. A. , ifnot all real-time simulation timing conditions are met,overruns occur and discrepancies between the simulatorresults and its physical counterpart’s responses are observed.The required use of a discrete-time-step solver is an
39Slow and Fast Dynamics &TransientsSlowDynamicsVeryLargeDyn.Sim.Computing Powerinherent constraint of today’s real-time simulators, and can bea major limitation when simulating non-linear systems, suchas HVDC, FACTS, active filters or drives. Because of thenature of discrete-time-step solvers, the occurence of nonlinear events in a real-time simulation, such as transistorswitching, can cause numerical instability. Solving methods toprevent this problem have been proposed in [5] and [6], butthey cannot be used during real-time simulation. Achievingreal-time is one thing, but achieving it synchronously isanother. With non-linear systems, such as the simple rectifiercircuit illustrated in Figure 2, there is no guarantee thatswitching events will occur (or should be simulated) at adiscrete time instance. Furthermore, multiple events can occurduring a single time-step, and without proper handling thesimulator may only be aware of the last one. Recently, realtime simulator manufacturers have proposed solutions totiming and stability problems. Proposed solutions generallyknown as discrete-time compensation techniques usuallyinvolve time-stamping and interpolation algorithms. State-ofthe-art real-time simulators take advantage of advanced I/Ocards running at sampling rates considerably faster than fixedstep simulation [7], [8]. The I/O card acquires data faster thanthe simulation, and can read state changes in betweensimulation steps. Then, at the beginning of the next time-step,the I/O card not only passes state information on to thesimulator, but also timing information as to when the statechange occurred. The simulator can then compensate for thetiming error.Figure 2 illustrates a classical case of simulation errorcaused by the late firing of a thyristor in a converter circuit. Inthis example, a thyristor is triggered at a 90-degree angle withrespect to the AC voltage source positive zero-crossing. Assoon as the thyristor is triggered, current begins to flowthrough it. The resulting load current obtained throughuncompensated real-time simulation (dotted line) isrepresented with a degree of error in comparison to the currentflowing through the real circuit (plain black line). This isbecause the event at 90 electrical degrees does not occursynchronously to the simulator fixed-time-step. Thus, thethyristor gate signal is only taken into account at the beginningof the next time-step. This phenomenon is commonly knownas “jitter”. When jitter occurs in a discrete-time simulation,sub-synchronous or uncharacteristic harmonics (amplitudevariations) may be visible in resulting waveforms. In this case,variations are evident in the thyristor current.Finally, the use of multiple simulation tools and differenttime-step durations during real-time simulation can causeproblems. When multiple tools are integrated in the samesimulation environment, a method known as co-simulation,data transfer between tools can present challenges sincesynchronization and data validity must be maintained [9].Furthermore, in multi-rate simulations, where parts of a modelare simulated at different rates (with different time-stepdurations), result accuracy and simulation stability are alsoissues [10]. For example, multi-rate simulation may be used tosimulate a thermal system with slow dynamics alongside anelectrical system with fast dynamics [11]. Multi-rateUltra FastTransientsMulti-areaPower SystemMultiUAVs sFuel CellsBatteries1 kHz1000 usVery Fast TransientsLargePower SystemMedium-areaPower SystemSmall equivalentPower SystemsController TestingFACTSActive FiltersMulti-ConvertersHigh-Power Drives(1-10 MW)Wind FarmsHigh-Power Drives (1-5 MW)1 - 3 kHz PWMTrains, Off Highway Electric Vehicles10 kHz100 us20 KHz50 us40 kHz25 usInterconnectedMid-PowerDrives (100 kW)10 kHz PWMVery Low PowerDrives ( 10 kW)Low PowerDrives (100 kW) 10 kHz PWMIGBT Detailed10 kHz PWMmodelsHybrid Vehicles100 kHz10 us250 kHz5 us1 MHz1usSimulation SpeedFigure 3: Simulation Time-step by Applicationsimulation and co-simulation environments, where multipletolls are used side by side, is an active research topic.C. Choosing the Right Simulator for the Right Time-stepThe first challenge faced by simulation specialists is toselect a real-time simulator that will meet their needs.Simulator capabilities, size and cost are determined by anumber of criteria, including 1) the frequency of the highesttransients to be simulated, which in turn dictates minimumtime-step, and 2) the complexity or the size of the system tosimulate, which along with the time-step duration, dictates thecomputing power required. The number of I/O channelsrequired to interface the simulator with physical controllers orother hardware is also critically important, affecting the totalperformance and cost of the simulator.Figure 3 outlines typical time-step and computing powerrequirements for a variety of applications. The left side of thechart illustrates mechanical systems with slow dynamics thatgenerally require a simulation time-step between 1 and 10milliseconds, according to the rule of thumb that thesimulation step should be smaller than 5% to 10 % of thesmallest time constant of the system. A smaller time-step maybe required to maintain numerical stability in stiff systems.When friction phenomena are present, simulation time-stepsas low as 100 microseconds to 500 microseconds may berequired.It is a common practice with EMT simulators to use asimulation time-step of 30 to 50 microseconds to provideacceptable results for transients up to 2 kHz. Because greaterprecision can be achieved with smaller time-steps, simulationof EMT phenomena with frequency content up to 10 kHztypically require a simulation time-step of approximately 10microseconds.Accurately simulating fast-switching power electronicdevices requires the use of very small time-steps to solvesystem equations [12]. Offline simulation is widely used, butis time consuming if no precision compromise is made onmodels (i.e. the use of average models). Power
40selecting a suitable fixed step-size for models with increasingcomplexity is a time-domain comparison of waveforms forrepeated runs with different step-sizes.D. Rapid Control PrototypingReal-time simulators are typically used in three differentapplication categories, as illustrated in Figure 4. In RCPapplications (Figure 4 (a)), a plant controller is implementedusing a real-time simulator and is connected to a physicalplant. RCP offers many advantages over implementing anactual controller prototype. A controller prototype developedusing a real-time simulator is more flexible, faster toimplement and easier to debug. The controller prototype canbe tuned on the fly or completely modified with just a fewmouse clicks. In addition, since every internal controller stateis available, an RCP can be debugged faster without having totake its cover off.E. Hardware-in-the-LoopFor HIL applications, a physical controller is connected to avirtual plant executed on a real-time simulator, instead of to aphysical plant. Figure 4 (b) illustrates a small variation to HIL;an implementation of a controller using RCP is connected to avirtual plant via HIL. In addition to the advantages of RCP,HIL allows for early testing of controllers when physical testbenches are not available. Virtual plants also usually cost lessand are more constant. This allows for more repeatable resultsand provides for testing conditions that are unavailable on realhardware, such as extreme events testing.Figure 4: Applications Categorieselectronic converters with a higher PWM carrier frequency inthe range of 10 kHz, such as those used in low-powerconverters, require time-steps of less than 250 nanosecondswithout interpolation, or 10 microseconds with aninterpolation technique. AC circuits with higher resonancefrequency and very short lines, as expected in low-voltagedistribution circuits and electric rail power feeding systems,may require time-steps below 20 microseconds. Tests that usepractical system configurations and parameters are necessaryto determine minimum time-step size and computing powerrequired to achieve the desired time-step.State-of-the-art digital real-time simulators can exhibit jitterand overhead of less than 1microsecond, thereby enablingtime-step values as low as 10 microseconds, leaving plenty ofprocessing resources available for computation of the model.This means that simulation time-steps can be reduced to aconsiderably low value, as necessary, to increase precision orto prevent numerical instability.Regardless of the simulator used, both numerical solverperformance and the bandwidth of interest are considerationswhen selecting the right time-step. The standard approach forF. Software in the loopSIL represents the third logical step beyond thecombination of RCP and HIL. With a powerful enoughsimulator, both controller and plant can be simulated in realtime in the same simulator. SIL has the advantage over RCPand HIL that no inputs and outputs are used, therebypreserving signal integrity. In addition, since both thecontroller and plant models run on the same simulator, timingwith the outside world is no longer critical; it can be slower orfaster than real-time with no impact on the validity of results,making SIL ideal for a class of simulation called acceleratedsimulation. In accelerated mode, a simulation runs faster thanreal-time, allowing for a large number of tests to be performedin a short period. For this reason, SIL is well suited forstatistical testing such as Monte-Carlo simulations. SIL canalso run slower than real-time. In this case, if the real-timesimulator lacks computing power to reach real-time, asimulation can still be run at a fraction of real-time, usuallyfaster than on a desktop computer.IV. EVOLUTION OF REAL-TIME SIMULATORSSimulator technology has evolved from physical/analoguesimulators (HVDC simulators &TNAs) for EMT andprotection & control studies, to hybrid TNA/Analogue/Digitalsimulators capable of studying EMT behavior [13], to fullydigital real-time simulators, as illustrated in Figure 5.Physical simulators served their purpose well. However,they were very large, expensive and required highly skilled
41CostReal-Time SimulationAnalog SimilatorsRequirementsHybrid (Analog/Digital) SimulatorsTestOffline SimulationCustom Digital SimulatorsArchitectureVerificationDigital SupercomputerSimulatorsDesignIntegrationDigital COTSSimulatorsFPGASimulationOn DemonstrationManufactureIn-service2010 TimeFigure 5: Evolution of Real-Time Simulation TechnologiesFigure 6: Model-based Design Workflowtechnical teams to handle the tedious jobs of setting upnetworks and maintaining extensive inventories of complexequipment. With the development of microprocessor andfloating-point DSP technologies, physical simulators havebeen gradually replaced with fully digital real-time simulators.DSP-based real-time simulators developed usingproprietary technology, and used primarily for HIL studies,were the first of the new breed of digital simulator to becomecommercially available [14]. However, the limitations of usingproprietary hardware were recognized quickly, leading to thedevelopment of commercial supercomputer-based simulators,such as HYPERSIM from Hydro-Quebec [15], which is nolong
Transients (EMT), power systems modeling & simulation, and . First, real-time simulation is defined. An overview of the evolution of real-time simulators is then presented. Two other essential questions are then answered. Why is real-time simulation needed? Where does real-time simulation fit best? Finally, this paper concludes
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