20-08-2014, 12:17 PM
Power Semiconductors Seminar Report
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INTRODUCTION
Power converters are used in many applications in power
systems, both in the power delivery system and as part of the end
use applications. Power delivery applications include HVdc
transmission, flexible ac transmission system (FACTS) devices at
the transmission level and Custom Power devices at the distribution
level. Many distributed generation and storage devices also
incorporate power electronic interfaces. Load-based applications
include motor drives, and reactive compensators.
Models of the power converters and their control systems are
needed with the appropriate degree of detail for performing transient
simulations. Power electronic devices introduce non-linearities to the
computer simulation. Representing this non-linear behavior in
sufficient detail to produce results with the desired degree of
accuracy represents a significant challenge. In many cases,
approximations in the converter modeling can be made without
impacting accuracy too significantly. In other cases, more exact detail
is needed. Also in some situations, data sufficient to model the
power semiconductor devices and related components is lacking.
This could result from using a converter built and installed by
another party who prefers you to view it as a “black box.”
Different models for power converters and the power electronic
devices will be presented along with guidelines for when different
levels of modeling should be used will be presented. A description
of detailed device models will be presented. This will be followed
by a discussion on where to get appropriate device data for different
levels of modeling and how to convert it for use in the device models
CONVERTER MODELS
Averaged Models
For many studies, averaged or steady state models of the
converters can be used. The power semiconductor devices are not
modeled in these studies, although the internal device characteristicshe converter based on terminal characteristics is developed. The
converter is often represented as either a dependent current source or
a dependent voltage source. These models are typically used for
steady-state operation points and to study the response of slower
converter control schemes.
can be incorporated into the model. Instead an averaged behavior for
Switching Models
The degree of detail in the converter model often depends on the
relationship between frequency of interest in the simulation results
and the switching speeds in the converter. If the dynamic or
transient response of the converter is required, a converter model
that represents the switching of the power electronic devices is used.
The control model now must include gating circuits and the
sychronization scheme. In some cases a model of the snubber
circuits is also appropriate, in other cases the snubber circuit can be
ignored. In some cases the converter can be reduced to a simpler
equivalent. For example, a HVdc converter could have 50 or 60
thyristors connected in series, but a model including one thyristor
may be equivalent
SWITCH MODELING
Ideal Device Models
Converter terminal characteristics are often sufficient for many
simulations involving power converters. In such cases, it could be
appropriate to model series and parallel connected power electronic
devices as one or two equivalent devices. Or a 48-pulse voltage
source converter could be represented with a simpler, lower pulse
order model if response is sufficient.
In addition, if the converter is connected to a system where the
time scales of the dynamic response of interest are very long
compared to the device turn-on and turn-off times, ideal switch
models can be used. In this case, the power electronic device is
assumed to open or close in one time-step as the simulation
progresses (or essentially instantly as far as the external system is
concerned)
GATHERING MODEL DATA
The first step is to determine the level of detail needed for the
simulation studies to be performed. For studies of the impact of the
power converter on real or reactive power flows, voltage magnitude
control, or motor control ideal device models are often sufficient. In
such studies, choosing the appropriate converter topology and
control scheme to model is the main concern. Emtp-like programs
have built-in switch models adequate to represent diodes, thyristors,
and self-commutating devices such as IGBTs, GTOs, and GCTs.
If studies requiring detailed converter and device models are
required, obtaining device data can be more difficult. Simulations
conducted by the parties designing and building the converter can be
conducted with knowledge of the specific devices used, and values of
resistance, inductance and capacitance for the passive components in
the circuit. The main source of data will be the manufacturers
specification sheets for the devices. Tabular data will be available
describing turn-on and turn-off behavior and on-state conduction
behavior. The turn-on and turn-off characteristics are generally
approximated with piecewise linear approximations of the curves or
with equation models. It will also be necessary to include the
external snubber circuits, and any circuits to control voltage sharing
in series connected devices and current sharing in parallel connected
devices.