AM/FM/GIS Modeling to support Power System Planning
A Basic Appreciation of Circuits
It sounds obvious, but connect facilities in the GIS as you need them to be
connected for the electrical analysis you plan to perform. In a node-edge model,
series devices need to be modeled with two nodes (e.g. a conductor, switch, or
fuse) and shunt devices (e.g. a shunt capacitor) can be modeled with a single
node. Typically, source and spot load objects are generated from other GIS
facilities. Source objects are used to model where power is being injected into
the network and spot loads indicate where power is being absorbed from the
network. For example, a substation transformer may be used to model a source,
while a secondary meter or distribution transformer may be used to model a spot
load. If a series device needs to break the electrical flow, then it should always
be modeled with two nodes. In other situations, the decision may not be quite so
evident.
Consider a bus for a moment. A bus acts a connection point, so you may
consider modeling it as a single node facility, which implies that the electrical
properties of the bus are inconsequential. However, if you consider electrical
properties of the bus significant, you may deem it appropriate to model the bus
with two nodes and treat it similar to a conductor for analysis. By most
estimates, buses are relatively short and have minimal impedance, so it is
acceptable to model them as single node facilities in the GIS and neglect them in
your analysis.
Secondly, consider a distribution transformer. You could model the transformer
and all of the devices on the secondary side of the distribution transformer right down to the secondary meters (which you could model as load objects), but it
greatly increases the size of the network that you are to analyze. Another
approach would be to model the transformer and associate a load object to the
secondary side of the distribution transformer to account for the load absorbed by
all of the devices downstream of the distribution transformer. An even simpler
approach would be to simply model a distribution transformer as a load. In either
of these last two scenarios, you will need to create a relationship between the
distribution transformers and the secondary meters they feed if you plan to derive
load information from these secondary meters.
Your short-term needs should be your highest priority, but you need to consider
what analysis you may want to pursue in the future. There may be some simple
things that you can do in your GIS model now that will avoid a lot of re-work and
data collection later. For example, though it may not be necessary for analysis of
your primary network, you may want to go ahead and maintain connectivity
between your secondary devices if you plan to analyze your secondary network
in the future.
All things considered, try to keep your model as simple as possible.
Knowledge of the Vendor Specific Analysis Package
Fundamentally, we are trying to model a physical situation using mathematical
techniques. It is not always practical to collect/maintain the data necessary to
support the more complex/sophisticated algorithms, so we have to strike a
delicate balance between the complexity and accuracy of the analysis. For
example, let’s examine the phase rotation of conductors. The impedance of the
circuit can be more accurately determined knowing the phase rotation of the
conductors; however, most utilities find it impractical to gather and maintain this
information and therefore choose to ignore this affect.
All electrical analysis packages need the same basic information, and generally
large portions of the electrical properties of the connected network are
determined using catalog tables. To support analysis using your GIS data, you
model the conductors and devices with some basic attribution, but you retrieve
many of the electrical properties of the items from catalog tables. This avoids
complexity and redundancy in your GIS data storage.
Leveraging Enterprise Data
Further leveraging the wealth of information in the enterprise, switching status
from the outage system and metered data from SCADA/MV90 can be used to
analyze summer and winter heavy and light-loading situations. Transformer load
statistical analysis derived from customer billing information can be used to
derive load data for the electrical analysis.
Typically, only the design status or planned configuration of switching devices is
represented in the GIS. This is fine for some studies, but it may not be OK for
others. If you have an outage management system (OMS), you can get the
operations configuration of switching devices at defined points in time. However,
be aware this may not represent the “real” network configuration if cuts, jumpers,
or other devices that change the connectivity are inserted in the OMS that are not
represented in the GIS. Also, consider that the GIS may have updates that affect
the network configuration that may not yet be reflected in OMS. In the end, you
will have to weigh the inaccuracies that may be introduced by this approach
against the possible benefits.
Another enterprise data source that may be leveraged is the voltages and
currents being measured by SCADA/MV90. You may have to do some work to
tie the SCADA/MV90 data to facilities in the GIS, and you need to consider what
the actual network configuration was when the readings were taken.
You may be able to derive satisfactory load data from your customer billing
information. The monthly reading can be used as a weighting factor. Another
weighting factor can be generated based on the customer category and the time
of day. Lastly, the data can be scaled using SCADA/MV90 voltage/current
values where you have meters. This will not give you a precise reading at a point
in time for a device, but it may be satisfactory.
