IEEE Power & Energy Magazine - January/February 2016 - 53

higher voltages, and the lower layers will represent lower voltage parts of the grid; but the ciM only provides the partitioning mechanism-each group of collaborating model sources
may tailor this layering to their own purposes.
tsos will tend to be a logical choice at some layer of
transmission partitioning, but it often makes sense to partition
within a tso based on criteria that separate the bulk power
model (which interconnection studies would be interested in)
from local detail (which is of interest only in the tso's own
studies). the local-interest building block is used only in certain studies, so it is convenient to separate it to avoid having to
extract it or equivalence it over and over again for interconnection usage. in a large interconnection, there may also be a layer
above the tsos, representing reliability authorities, planning
authorities, or markets that have model responsibility.
partitioning down the voltage levels may be continued into
the distribution system and even into customer premises, providing a means to support completely seamless transmissionto-distribution modeling. this is an important feature for the
future, where the clear trend is that transmission and distribution are less separable from one another. seamless modeling would enable, for example, a straightforward method of
processing distribution models to get an accurate, real-time
picture of what part of a transmission "load" was wind or solar
or under a demand constraint and so forth.
one final wrinkle deserves mention. engineers currently
often depend on "equivalents" to create appropriate sized
models for a given analysis. equivalents may imply anything
from manual simplification to mathematically-derived models. equivalents are also troublesome, as a rule. they take
time to build and test, and they always pose an accuracy risk
compared to models with greater fidelity. the first ciM idea
is that, through creating the right framework of parts, models
can be built up with minimal waste by selecting the model
parts that should be in view, rendering most equivalents
unnecessary. the second ciM idea is that when equivalents
are necessary, they are created as the output of equivalent
analysis in the form of ciM model parts that fit into the same
overall framework.

Partitioning Data by Types of Model Parts

Invariant Types

invariant types include the data that describe the qualities
of the grid that are inherent in its construction and will not
typically change except as a result of new construction activity. (the term "invariant" here emphasizes the goal that this
information about a given element should be the same in
every study representing that element.)
as noted earlier, the foundation on which any model
assembly rests is the set of eQ model parts that are used
in defining the framework. Model parts of type eQ declare
which grid components are part of the model. in other words,
if a line, transformer, switch, busbar, generator, or other element is to be represented in a case, then a corresponding data
object must be supplied by an eQ model part that is part of
the case. these eQ data objects also describe how the grid
elements are connected together electrically and provide
their basic steady-state characteristics.
other invariant model data are defined in model parts
that reference eQ model parts, which means they have
dangling associations that reference objects defined in eQ
model parts. these include the following:
✔ diagram layout (dl) types define how grid parts may
be organized into schematic diagrams.
✔ short-circuit (sc) types define additional data
required for short-circuit analysis.
✔ dynamic (dY) types define additional data required
for dynamic analysis.
✔ geographical layout (gl) types define geographical data.
in recommended practice, invariant model parts are
maintained within business processes that reflect the result
of new construction activity or the development of future
planned changes. their maintenance is typically independent of study case preparation. thus, an engineer setting
up a study case might select which invariant modeling is
required for the study but would not normally make any
modifications to invariant model parts.
Variant Types

variant types are those that the engineer will set up differently for different kinds of studies. For example, a capacityplanning study might look at a heavy-load scenario, while
a real-time state estimator studies current load conditions.

Model parts are also modularized by the kind of data they contain. (there is a standard set of ciM
model part types defined in the iec
61970-4xx series of documents.)
SC:
DY:
DL:
SV:
these different types are defined
Short Circuit
Dynamics
Diagram Layout
State Variables
primarily on the basis that the information each type contains typically
TP:
Topology
comes from a different source or a
EQ:
different stage in data-development
Equipment
SSH:
processes. broadly, there are two
Steady-State Hypothesis
major categories of types, correInvariant Model Part Types
Variant Model Part Types
sponding to variant and invariant
data identified in requirement three.
these are illustrated in Figure 3.
figure 3. The partitioning of network data by types of model parts.
january/february 2016

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - January/February 2016