Èñòî÷íèê:International Conference on Power Systems Transients (IPST2009) in Kyoto, Japan June 3–6, 2009
This paper describes a versatile, multi–domain,
real–time digital simulator of large power grids. Its capability to
conduct multiple tests for protection coordination studies is
described. A large grid model built using the EMTP–RV software
and simulated in real–time using the eMEGAsim real–time digital
simulator and EMTP–RT software tool is described.
Comparisons between off-line and real–time simulations with
different solvers are made using superimposed steady–state and
fault condition waveforms. A multiple random tests application
for protection coordination studies using eMEGAsim simulator’s
built-in software TestDrive GUI and Python API scripting tool is
described. The paper concludes with a discussion on the off-line,
real-time and acceleration modes of simulation of the PC-based
eMEGAsim simulator and its advantages for studies of modern
power systems.
Index Terms real–time simulation, accelerated simulation,
off–line simulation, electrical network, wind energy, detailed
modeling, doubly–fed induction generator, electromagnetic
transients, hardware–in–the–loop, multi–core processors
Imulators have been extensively used in the planning
and design of transmission systems for decades. Simulator
technology has evolved from physical/analogue simulators
(HVDC simulators, TNA’s) for electromagnetic transients and
protection and control studies, to hybrid
TNA/Analogue/Digital simulators with the capability of
studying electro-mechanical transient behaviour [1], to fully
digital real–time simulators.
Physical simulators served their purpose well, however they
were very large, expensive and required highly skilled
technical teams for the tedious job of setting up networks and
maintaining the extensive inventory of complex equipment.
With the development of microprocessors and floating–point
DSPs, physical simulators have been replaced with fully digital real–time simulators.
DSP–based real–time simulators for use in (HIL) hardwarein–
the–loop studies became available first [2]. However, the
limitations of using proprietary hardware were soon
recognized and commercial supercomputer–based simulators
such as HYPERSIM from Hydro–Quebec [3] were developed.
Hydro-Quebec has since ceased commercializing their
Hypersim product. Attempts have been made by a number of
universities and research organisations to develop fully digital
real–time simulators using low-cost standard PC technology,
in an effort to eliminate the high costs associated with the use
of high–end supercomputers [4]. Such development was very
difficult due to the lack of fast, low-cost inter–computer
communication links. However, the advent of low-cost, easily
obtainable multi–core processors [5] (INTEL or AMD) and
related Commercial–off–the-Shelf (COTS) computer
components has directly addressed this issue, clearing the
way for the development of much lower cost and easily
scalable real–time simulators. In fact, today’s low-cost
computer boards with 8 processor cores provide greater
performance than 24–CPU supercomputers that were available
only 10 years ago. The availability of this low-cost, high
performance processor technology has also reduced the need
to cluster multiple PCs to conduct complex parallel
simulation, thereby reducing dependence on sometimes costly
fast inter–computer communication technology.
COTS-based high-end real-time simulators using INTEL or
AMD multi–core processor computers have been used in
aerospace, robotics, automotive and power electronic system
design and testing for a number of years [6]. Recent
advancements in multi–core processor technology means that
such simulators are now available for the simulation of
electromagnetic transients expected in large-scale power grids,
micro-grids, wind farms and power systems installed in large
electrical ships and aircraft. These simulators, operating under
Windows, LINUX and standard real–time operating systems,
are potentially compatible with all power system analysis
software such as PSS/E, EMTP-RV and PSCAD, as well as
multi-domain software tools such as SIMULINK and
DYMOLA. The integration of multi-domain simulation tools
with electrical simulators enables the analysis of interactions
between electrical, power electronic, mechanical and fluid
dynamic systems.
This paper discusses the simulation challenges involved and
solutions implemented in the digital real–time simulation of
large-scale power systems with integrated power electronic
devices and control systems. Off–line and real–time simulation
results obtained through the use of the PC-based eMEGAsim
simulator [6], equipped with the latest INTEL quad-core
processors, will be presented and compared with results
obtained with the famous EMTP–RV off–line simulation
tool[1].
A. Application Challenges
The secure operation of power systems has become more
and more dependent on complex control systems and power
electronic devices. Furthermore, the proliferation of
distributed generation plants, often based on the use of
renewable energy resources, presents significant challenges to
the design and stable operation of today’s power systems.
Examples include the integration of wind farms, photovoltaic
cells or other power electronic based distributed energy
generation systems, domestic loads and future plug-in electric
vehicles into the existing power grid.
The above applications take full advantage of several very
fast and distributed power electronic systems which, in many
cases, are of innovative design and consequently have never
been integrated together or with a power grid. Furthermore, in
most cases, these distributed systems are designed,
manufactured and commercialized as individual off-the-shelf
products, with no consideration given to total system
performance. Validated models suitable for electromagnetic
transients, as well as dynamic stability analysis under normal
and abnormal conditions, are usually not available. This poses
a new and significant challenge to utility and system engineers
who must guarantee total system performance and security.
B. Simulation Challenge 1: Simultaneous Simulation of Fast
and Long Phenomena
Simultaneous simulation of fast and long phenomena
pushes simulation tools that are required in the planning and
operation of power systems to their limits. Indeed, such
challenges are multi–disciplinary. Examples include
mechanical stresses on large generators due to potential subsynchronous
resonance and sudden loss of loads; rotation of
wind turbine palms in front of the mast, creating pulses on
mechanical and electrical torques of generators which must be
compensated for by special control loops; electrical systems
installed on large electrical ships involve the simulation of
several interconnected generators and propulsion plant,
together with the complex behaviour of the water and the
propeller.
The transient response of an interconnected power system
ranges from fast (microseconds) electromagnetic transients,
through electro–mechanical power swings (milliseconds), to
slower modes influenced by the prime mover boiler and fuel
feed systems (seconds to minutes). For the modeling of
electromechanical transients (EMT) caused by large
disturbances such as network faults and/or plant outages,
system states must be evaluated at intervals in the order of
milliseconds over time scales of seconds. For small-signal and
voltage stability assessment, the time scale needs to be
extended to minutes and for voltage security tens of minutes
to hours. During this period, accurate representation of power
electronic devices require relatively small time steps, typical
of electromagnetic transients (EMT) simulators, but
impractical for phasor-type electromechanical dynamic
simulation tools.
C. Simulation Challenge 2: Small Time Step
It is a common practice with EMT simulators to use a
simulation time step of 30 to 50 μs to provide acceptable
results for transients up to 2 kHz. Better precision can be
achieved with smaller time steps. Simulation of transient
phenomena with frequency content up to 10 kHz typically
requires a simulation time step of approximately 10 μs. Power
electronic converters with higher PWM carrier frequency in
the range of 10 kHz, such as those used in low-power
converters, require smaller time steps of less than 250
nanoseconds without interpolation, or 10 μs with an
interpolation technique. AC circuits with higher resonance
frequency and very short lines, as expected in a low-voltage
distribution circuit and railway power feeding system may
require time steps below 20 μs. Tests must be done with
practical system configurations and parameters to determine
minimum time step and the number of processors required to
reach the minimum time step.
Modern PC-based simulators such as eMEGAsim can
exhibit jitter and overhead of less than 1μs which enable time
step values as low as 10 μs with plenty of processing resource
per processor core available for computation of the model.
Simulation time steps can therefore be reduced to a very low
value when necessary to increase precision or to prevent
numerical instability.
D. Simulation Challenge 3: Multi–domain Simulation with
Heterogeneous Tools
While EMT simulation software such as EMTP–RV and
PSCAD represent the most accurate simulation tools available
for detailed representation of power electronic devices, such
tools are not practical for simulation of the dynamics of very
large systems. The EMT simulation of a system with
thousands of busses and many power electronic devices would
require an excessive amount of time to simulate long
transients at very small time step when using only one
processor. Conversely, fundamental–frequency transient
stability (TS) simulation software enables very fast simulation,
but such tools use relatively long integration steps in the order
of 10 to 20 milliseconds; consequently, highly non–linear
elements common in HVDC and FACTS can only be
represented as modified steady–state models. Since switching
devices and control systems are not represented in detail, the
overall accuracy of conventional transient stability programs
suffers, and contingencies involving mal-operation of FACTS
and AC–DC converters devices cannot be adequately
represented.
Consequently, all these simulation tasks are currently
performed using separate simulation tools, and significant
compromises are required to deal with the respective
shortcomings of the different simulations. The requirement to
simultaneously simulate all mechanical, electrical and power
electronic subsystems using heterogeneous tools provided by
several software houses is becoming essential for many
applications. Consequently real–time digital parallel processor
simulators with the capability to integrate all necessary
simulation tools in off–line or real-time co-simulation mode
[7] are certainly an advantage over real–time digital simulators
using closed computer systems that cannot execute third-party software.