Real-Time Power System Simulation:

Transcription

1 OPAL-RT Technologies Real-Time Power System Simulation: EMT vs. Phasor White Paper OPAL-RT Technologies Inc. White Paper: opwp sa-reva Last update: 02 September 2016 By: Simon Abourida, Jean Bélanger, Vahid Jalili-Marandi OPAL-RT Technologies Inc., 2016

2 Table of Contents 1. Problem Statement Types of Transients Electromagnetic Transients (EMT) Electromechanical transients Simulation Methods Phasor Simulation EMT Simulation Real-Time Simulation ephasorsim emegasim and HYPERSIM Real-Time Simulation Step Size Hybrid Phasor-EMT Real-Time Simulation Typical EMT and Phasor-Type Simulator Performance An Ongoing Application Field References OPAL-RT Technologies Inc., of 15

3 ABOUT THIS DOCUMENT This white paper contains general technical and qualitative information aiming to improve the understanding about the technologies related to OPAL-RT Technologies Inc. solutions and technologies, and intended for general informative use by OPAL-RT customers and the general public. Note: While every effort has been made to ensure accuracy in this publication, no responsibility can be accepted for errors, omissions, data change. This publication is not intended to form a basis of a contract. OPAL-RT Technologies Inc., of 15

4 1. Problem Statement Simulation has been used for decades for power system analysis. And with the increased complexity of modern power grids, and the integration of more complex systems (renewable energy systems, switching power electronics, smart systems ), the industry is increasingly relying on simulation tools. In time-domain simulation of power systems, there are different studies, characterized by their different time frames that are directly tied to the frequency ranges of the phenomena and disturbances studied. The disturbances encountered in power systems can be broadly classified in two categories: electromagnetic transients and electromechanical transients (Figure 1). Figure 1: The two types of transients in power systems Of course, most simulation tools capable to simulate fast electromagnetic transients are normally capable to also simulate electromechanical transients if adequate dynamic models of machines and loads are used. The complete simulation would therefore be executed with time steps of 10 to 100 microseconds. The results would be like the detailed graph of Figure 1containing slow and fast phenomena combined together as in real-life. However such detailed simulation would take a long time to compute when the network includes thousands of busses. Specialized and expensive parallel simulators are therefore needed for such large grids. On the other hand, several power grid analysis and simulations require only evaluating the fundamental frequency transient (called electromechanical transients). Therefore, if EMT evaluation is not required, it is not any more required to simulate all state variables of the complete system at a very small time step. Over the years, phasor-type simulation tools (or RMS-value simulation tools) have been developed and widely used to obtain a good representation of low-frequency oscillation expected in power grids but using a time step of 10 to 20 milliseconds. Using such large time step enable to simulate large power grids with thousands of busses and generator at very high-speed using standard single-processor. The results presented by such tools are not any more the detailed waveforms but rather the variation of the amplitudes of voltages, current, P, Q and rotor angle as illustrated by the blue dashed curve of Figure 1. OPAL-RT Technologies Inc., of 15

5 Over the years, some specialists requested to have simulation tool with the capability to simulate both EMT and electromechanical transients but using the minimum amount of computing resources to decrease simulator cost. The main reason is the difficulty to develop approximate or average models of fast power electronic systems as required for phasor-type simulation. Furthermore, the development of such average model which would also be good for unbalanced fault conditions may be very difficult. One technology, discussed later, would be to develop hybrid simulation tools in which some parts of the grid is simulated in EMT mode and other part of the grid in phasor mode. Such technology would increase the simulation accuracy of sub-regions containing several fast power electronic models such as HVDC, FACTS and renewable energy systems. 2. Types of Transients 2.1. Electromagnetic Transients (EMT) Electromagnetic transients are very fast phenomena occurring in the microseconds to milliseconds range; they are triggered by sudden changes in the power grid configuration, that may be caused by closing or opening action of circuit breakers, or power electronic switches, by faults or equipment failures. The study of these EMTs requires accurate simulation of the network components such as transmission lines, transformers, protection devices, and power converters. On other hand, as the time constants of the power plant equipment (turbines, generators) is comparatively very long, EMT simulation often use simplified models of these equipment if the effect of slow fundamental-frequency is not relevant to the analysis. Some of the electromagnetic transient simulation software are: - Offline: EMTP, EMTP-RV, PSCAD - Real-Time: emegasim, HYPERSIM and others 2.2. Electromechanical transients Electromechanical transients are slower transients than EMTs occurring in the range of milliseconds to seconds. They are caused by a mismatch between power production and consumption, and therefore involve the oscillation of rotating machines. The analysis of this kind of transients is known as stability simulation. In stability simulation, the electromagnetic transients are filtered out and the mathematical models of electromechanical transients are therefore simplified or averaged from electromagnetic transient models. And this is achieved by using the quasi-steady-state approach, where the network is modeled using the conventional phasor technique while, contrary to this technique, the phasors are allowed to change in time, thus accounting for the dynamic response associated with the rotary and other mechanical equipment. [1] OPAL-RT Technologies Inc., of 15

6 Some of the commercial software for electromechanical transient simulation are: - Offline: EUROSTAG, PSS/E, CYME, ETAP, etc. - Real-Time: ephasorsim 3. Simulation Methods To illustrate the difference between the two types of simulation (phasor and electromagnetic transient - EMT), the simple linear circuit [2] of Figure 2 is simulated with both methods and the current and voltage waveforms are shown (Figure 3). Figure 2: Linear circuit simulated with Phasor and EMT methods Figure 3: Phasor versus EMT waveforms OPAL-RT Technologies Inc., of 15

7 3.1. Phasor Simulation As its name implies, in phasor simulation the voltages and currents are computed as phasors. Phasors are complex numbers representing sinusoidal voltages and currents at a particular frequency. They can be expressed either in Cartesian coordinates (real and imaginary) or in polar coordinates (amplitude and phase). As the electrical states are ignored since each network impedance is represented as complex impedance R+jwL or a complex admittance G+jwC, the network solver need only to solve for YV=I, where the admittance matrix is linear, since all fast transients, caused by the grid states are eliminated. The simulation is therefore very fast to execute because a large time step of 10 to 20 milliseconds can be used to represent the dynamic of machine, loads, power electronic system and controller. However, this fast method gives the solution only at one particular frequency 1. Since phasor-type tools use simplified models allowing the use of large time step ranging from 10 to 20 milliseconds, they are capable of simulating very large power grids of several thousand nodes using conventional single-processor computers in a relatively short time. These tools are optimized for general system planning, protection and control studies to determine the best transmission and distribution system topology and station design to meet economical and reliability constraints. They are also used by system operators (TSOs) to determine power transfer stability limits under several operating conditions and contingencies expected during operation EMT Simulation On the other hand, instantaneous electromagnetic simulation tools (EMT) such as EMTP and PSCAD use very detailed models based on differential equation solvers to evaluate fast system transients required for insulation co-ordination and detailed control and protection system design. These software tools can not only simulate fundamental frequency phenomena but also harmonic and fast transients. The computational time taken to simulate a phenomenon lasting a few seconds is however rather long as compared to phasor tools, even for a small power grid of few hundred nodes, because of the level of detail. The main reason for this very long simulation time is that electrical systems contain a very large number of system states (i.e. very large number of differential equations) that must be simulated with very small time-step values ranging from 100 nanoseconds to 100 microseconds depending on the speed of the phenomena analyzed. A 10-second EMT simulation with a time step of 100 microseconds would require the calculation of 100,000 steps, while the same circuit simulated over the same 10-second time span at 10 millisecond will require computing 1,000 steps, which is 1000 times less than the 100,000 steps required by EMT simulation. 1 Note that a load flow (power-flow) simulation is a type of phasor simulation. It is a phasor simulation of a power system at nominal frequency (50Hz or 60Hz.) It assumes that the system is at sinusoidal steady state and that nothing is changing. OPAL-RT Technologies Inc., of 15

8 3.3. Real-Time Simulation ephasorsim ephasorsim is a phasor-mode, real-time simulator of large-scale transmission and distribution systems performing real-time transient stability simulation for transmission and distribution grids with thousands of buses. Its phasor solver with a typical time-step of few millisecond allows to compute the RMS and angle values of voltages and currents, as well as active and reactive powers, machine frequencies, etc. of the power system. Being a real-time simulator (with contrast to offline desktop simulation software), it is suited for interactive simulation and operation studies. Typical applications of ephasorsim are: Test wide area control and protection systems implemented on control center using SCADA systems by connecting actual communication system and control equipment in the loop with the simulator (HIL simulation) Testing of interaction between FACTS and HVDC transmission systems on system stability of interconnected systems using fundamental-frequency simulation before making detailed EMT simulations System studies with massive number of renewable penetration Wide area control/protection/state estimation algorithms PMU streams and PDC applications Advanced metering and information network Impact of load profiles in distribution networks Test and optimization of machine controls on system stability (voltage regulators, speed regulators, power system stabilizers) Capabilities: Offline and real-time transient stability simulation For power systems without phase-shifts (symmetric) up to nodes on one CPU core with a guaranty time step of 10 milliseconds without beaker switching or tap changer position changes, and up to nodes with breaker or tap changer operations For power systems with phase-shifts, up to 20,000 nodes on one CPU core with a guaranty time step of 10 milliseconds with no breaker operation, and up to nodes with beaker switching or tap changer position changes; Up to 30,000 nodes using 10 processors. Up to 4000 generator and loads with full dynamic models Positive sequence simulator or detailed phase-by-phase unbalanced systems, Import network data from third party tools such as PSSe and CYME Interface to Simulink for local and wide area control development Compatible with FMU s created by FMI V1 for model exchange mode Interface to real protection and SCADA using IEC61850, C37.118, DNP3 and OPC OPAL-RT Technologies Inc., of 15

9 Interface to real PMU and protection with analog converters emegasim and HYPERSIM emegasim and HYPERSIM are electromagnetic transient (EMT), real-time simulators of power systems and power electronics. The EMT solver used by these real-time simulators with a typical time step of microsecond using standard Intel processors (or less than 1 microsecond for fast switching power converters and other systems with very fast transients using dedicated FPGA processor) allows to compute the instantaneous continuous waveforms of voltage and current and other electrical quantities. Such performance is essential for hardware-in-the-loop (HIL) testing where real control and protection systems are interfaced with the simulator as they would be interconnected with the real power system. Typical applications of EMT RT simulators (emegasim and HYPERSIM) are: Design and testing of power electronics controllers in Hardware-In-the-Loop setup Design and testing of protection devices (including IEC protection relays) Design and testing of micro grids (PV, wind turbine, generator, fuel cells, capacitor banks) and Smart grids HVDC and FACTS controller development and testing Large scale power systems EMT simulation and integration testing to analyze interactions between an important power electronic systems (HVDC, FACTS), fast local protection and wide area special protection and control systems. Several EMT real-time simulators have been developed over the years by universities and R&D center since the basic solver method are well known. However, very few RTS are commercially available due to the complexity of advanced solver development optimized for parallel processing. Achieving time step between 20 to 50 microseconds to simulate very large power grids with 20 to 100 processors or more is still a challenge. Furthermore, simulating large distribution power grid poses an additional challenge due to the very short transmission lines with propagation delays smaller than the simulation time step. OPAL-RT has anticipated these challenges 10 years ago by developing new mathematical solvers enabling to solve distribution systems of about 1200 state at 100 microseconds using 4 processors Real-Time Simulation Step Size In simulating dynamic systems in real-time, a real-time simulator (RTS) uses only a fixed-step solver; no iterations or variable step solvers are allowed. A key parameter in RTS is therefore the selection of the time-step (also called step size): the simulation step size should be small enough to give accurate results, but not too small so that the real-time processor can solve all the model equations describing the system within the time step. Since that RTS time step cannot exceed a specified value, the only way to maintain simulation speed is at the end, to add more processors, adding complexity and cost. The step size should therefore be an optimal selection between these three criteria: accuracy, speed and simulator cost. OPAL-RT Technologies Inc., of 15

10 EMT simulation tools compute the instantaneous values of the voltages and currents. Therefore EMT solvers use a very small simulation step to simulate these transients, depending on the highest frequency present in those transients. The rule of thumb is to select the time step value between 5 to 10 times the period of the highest oscillation frequency expected following a disturbance. For real-time simulators, EMTs must be simulated in real-time using a fixed step solver. In discrete fixedstep simulation, as those used in digital Real-Time Simulators (RTS), the resolution of the waveforms is tightly related to the discrete step size used in the simulation, the smaller yielding a better resolution. As digital processors are not of infinite power, and despite the large computational power achieved by multi-processing (as used in OPAL-RT s Simulators), the step size cannot be decreased in digital RTS lower than a certain value (depending on the size of the power system simulated versus the processing power and communication overhead of the simulator); therefore, with RTS, step size is selected as a compromise between the need for a good resolution and the ability of the processor to solve the power system equations in real-time using an affordable simulator. In EMT simulation, the step size is traditionally in the range of us (microsecond). And with the advent of more power electronics, especially to connect the traditional power system to renewable energy sources and active loads, and with the switching frequency of these electronic converters being pushed ever higher, and with it the harmonic frequencies, the step size needed could be as low as 1 us or below, usually achieved only by dedicate chips (FPGAs). Phasor domain simulation does not compute the instantaneous waveform but solely the RMS values of the electric signals; for that reason, and in order to accelerate the simulation of very large power grids, Phasor simulation uses simplified models of the transmission line and power electronic devices valid for fundamental frequency only. Simulation time step of 5 to 20 milliseconds are typically used. This type of simulation is also called Fundamental-Frequency (FF) approximation, quasi-sinusoidal, or Transient Stability (TS) approximation. 4. Hybrid Phasor-EMT Real-Time Simulation The integration of several types of energy resources has transformed the nature of distribution systems from passive to active. Currently, a large number of wind turbines, micro-turbine generators, and photovoltaic panels are connected to distribution networks. The massive penetration of small but geographically distributed generators (DG) has changed the operation of distribution networks during the steady and dynamic states. From the dynamic state viewpoint, the disturbance in modern distribution systems cannot be considered anymore an insignificant local event. First, each type of DG unit presents a different behavior during transients in the system. Second, a disturbance in the transmission system can easily lead to the tripping of the DG units in the distribution system, and in OPAL-RT Technologies Inc., of 15

11 return the disconnection of DG units increases the chance of load-shedding due to the lack of generation. Therefore, the overall stability of power systems is affected from both the transmission and the distribution networks. This poses a great challenge to traditional simulation tool using classical phasor-type solver with simplified models of power electronic systems. OPAL-RT offers the unique feature of combining traditional Phasor-type Transient-Stability (TS) solver (computing amplitude / RMS values and angles, P and Q at each 10 milliseconds) with EMT solver computing fast transients at each 50 microsecond or less. Optional FPGA-based power electronic models with less than one microsecond time step can also be supply to analyze new power electronic systems if required. Such hybrid simulation enables the simulation of network subsystems with fundamental-frequency simulation at 10 ms connected to network subsystems using the detailed EMT simulation of fast power electronic systems used in Photo-Voltaic cells, wind turbines, battery storage, Fuel cell systems, HVDC and FACTS systems. This feature can also be used to develop and validate simplified models to be used with the phasor simulator. Of course, this mixed-mode hybrid simulation may not be needed for simple cases but offers nevertheless a big advantage to analyze the behavior of distribution systems integrated with distributed energy systems and large transmission systems. The TS simulator (i.e. ephasorsim) is based on the fundamental frequency, phasor component, while the EMT simulator (emegasim) is based on the three-phase instantaneous waveform, which includes several frequency components. Thus, to connect these two types of solutions two converter blocks are needed: phasor-to-waveform and waveform-to-phasor as depicted on Figure 4. Figure 4: Interfacing TS and EMT simulators One of the main challenges in this hybrid simulation is how to establish an interface between two different types of solution methodologies which are running in different time-steps. Several types of serial and parallel protocols are proposed in literature to coordinate the data exchange and update the equivalent circuits in TS and EMT domains. OPAL-RT offers hybrid simulation between ephasorsim and emegasim as detailed in this paper [3], integrating TS and EMT domain solutions in one working model. OPAL-RT Technologies Inc., of 15

12 The paper shows how emegasim is interfaced with ephasorsim transient stability simulation. The ephasorsim phasor-mode simulation module is interfaced with Simulink and can be executed simultaneously with SimPowerSytems /ARTEMiS electromagnetic transient simulation Typical EMT and Phasor-Type Simulator Performance EMT simulators update the simulation at each 50 or even 10 microseconds while a PHASOR type simulator can update the simulation at each 10 or 20 milliseconds to simulate electromechanical oscillations. It is therefore obvious that for the same power grid size, the Phasor-type simulator will require less processing power. Or one can say that for the same processing power, the size of the simulated systems in Phasor mode would be much larger than the size of simulated systems using EMT mode. Since that power system dynamic is affected by all generators, transmission line and loads located over a very wide area, while fast transients do not propagate very far, Phasor-type simulation is normally used to analyze system dynamics. The size of the system that can be simulated in real time can reach about 250 three-phase busses (750 single-phase node) with emegasim EMT simulator using a single high-end Simulator 2 equipped with 12 Intel processors. emegasim can simulate unbalanced distribution and transmission systems on a phaseby-phase basis including neutral. Standard PC server with 32 processors could simulate a system of about 600 three-phase busses (1,800 single-phase nodes) using emegasim or HYPERSIM. A power grid with about 2000 three-phase buses (6,000 single-phase nodes) could be simulated with HYPERSIM EMT simulator with a time step of 50 microseconds and using about 100 to 150 processors. This performance is estimated and would require an SGI super computer. Of course, such system would be very expensive and should be used only for cases requiring detailed EMT simulation of very large power systems equipped with several HVDC and FACTS systems. A large power grid with about 20,000 nodes can be simulated in real-time with the phasor method available now with ephasorsim. This benchmark was achieved with the actual ephasorsim simulator with only one processor. The simulated network can be a balanced transmission systems using positive sequence only or an unbalanced transmission or distribution systems simulated on a phase-by-phase basis. A system of about 10,000 nodes can be simulated with the real-time Phasor method available now with line and load switching and tab changer modification. This benchmark was made for a balanced transmission system. 2 This benchmark was achieved with 11 processor cores using a 12-Core Intel computer in which one core is used for the operating system functions (Ethernet, local hard disk ). OPAL-RT Technologies Inc., of 15

13 5. An Ongoing Application Field Less than a decade ago, not many companies including our customers were using RTS for distribution systems (DS). So using RTS for DS is a recent subject. However, we now have several customers (Utilities, R&D centers and universities) using RTS for DS applications. The main reason as has been explained above, is that modern distribution systems are not any more simple loads which damp transmission systems transients. New distribution systems now include local unpredictable renewable energy generation, active power electronic loads, energy storage systems and fast local protection and control systems as well as wide area control and protection. The integration of modern distribution and transmission systems is therefore becoming very complex since that both distribution and transmission relies on controls to achieve maximum performance and security. Consequently distribution systems engineers must now use the same dynamic simulation tools traditionally used only by TS engineers. Such tools include also real-time simulators (RTS), which are standard tools to design and test protection and control systems installed on transmission systems. But in many cases, such tools are new to distribution specialists and furthermore, RTS designed for transmission systems are not always well adapted for distribution systems. This is therefore the duty of universities, utility R&D center and national laboratories to analyze how to use RTS for DS and to dictate RTS requirement to RTS vendors. Let us consider that this is a new age for distribution specialists who must defined new design and testing method for modern smart distribution systems and micro grids. A decade ago, OPAL-RT has predicted that distribution specialists will need sophisticated RTS specifically designed to meet distribution systems challenges. These challenges are more demanding than traditional transmission RTS due to the short distances between each electrical nodes and the very large number of nodes. These new RTS technologies will be discussed later, but the following application examples will explain the diversity of RTS applications for DS. RTS must be considered as complementary tool to standard phasor and EMT modeling and simulation tools. Phasor, EMT simulation tools (off-line and real-time) have each their own advantages, features and limitations (refer to Table 1 below). Power system engineers must therefore use the best available and efficient tool based on the specific study objectives. OPAL-RT Technologies Inc., of 15

14 This table illustrates typical uses for each type of tool: Table 1: Comparison of Phasor and EMT capabilities OPAL-RT Technologies Inc., of 15

15 6. References [1] S. Henschel, Analysis of Electromagnetic and Electromechanica Power System Transients with Dynamic Phasors, Ph.D. Thesis, The University of British Columbia, [2] T. MathWorks, "Introducing the Phasor Simulation Method," The MathWorks, [Online]. Available: [Accessed ]. [3] V. Jalili-Marandi, F. J. Ayres, C. Dufour and J. Bélanger, "Real-time Electromagnetic and Transient Stability Simulations for Active Distribution Networks," in IPST, OPAL-RT Technologies Inc., of 15