PSS™MUST uses a linearized
DC load flow to calculate the incremental flow changes on monitored
elements due to changes in generation, load, or topology. These
incremental flow changes are added to the existing initial flows
to obtain the final flows. Flow changes due to branch outages
also require the initial flow on the line to be outaged. Therefore,
initial flows are required for monitored branches and for branches
to be outaged.
Initial flows can be obtained from
another source other than the currently processed load flow case.
PSS™MUST allows several options for obtaining initial flows:
Flows computed using DC load flow.
Flows computed using AC load flow.
Flows provided to PSS™MUST from an external database.
Most likely, these flows will be obtained from the SCADA/EMS;
hence, it is called the Energy Management System (EMS) Flows
Method.
Traditionally, the first two options
have been used in power system planning applications and are available
in PSS™E. The EMS Flows Method provides a simple and efficient
means to apply planning methods for ATC calculations in an operation
or an operations-support environment. This approach is based on
using a planner’s load flow case topology for incremental
flow updates, and updated branch flow and branch status from the
EMS environment.
Combining planning and on-line data
brings many benefits for both planning and on-line applications.
The main benefits of using a planner’s load flow case for
EMS applications are:
A planner’s load flow case will most likely have a better
representation of external subsystems than in EMS cases. This
allows, for example, to model the impact of through- system
transactions on the study systems.
The more sophisticated and flexible methods available in planning
applications can be applied to EMS applications.
If a utility does not have complete implementation of advanced
EMS applications and cannot build on-line network topology,
then the planning case can be used for incremental flow updates.
If the SCADA/EMS cannot run the State Estimator (for example,
due to observability problems), then directly measured flows
can still be used as an intermediate solution.
Updating a planner’s load flow
case to closely match real-time flows is a labor-intensive process.
It can be accomplished only approximately as it is almost impossible
to reconstruct external transactions, load levels, and generation
dispatch in the external systems. Updating the planning model
with real-time on-line flows brings the following benefits:
Obtain much better estimates of initial flows.
The EMS Flows Method can be efficiently combined with the
PSS™MUST transaction scheduling system.
With additional postprocessing, one can identify the source
of differences between planning and on-line cases. Future developments
are required in this area.
The use of EMS flows provides a
simple alternative to an extremely complicated and time-consuming
process of creating solvable on-line cases for OASIS TTC computations.
The EMS Flows Method can be implemented within several weeks without
solving complicated external systems observability problems.
The main drawback of using EMS flows
is that it requires the manual update of the planner’s load
flow case to accommodate topology changes. To apply the EMS Flows
Method, the user has to provide the EMS flows for all monitored
elements and all contingency branches to be processed.
