Distribution Operations - Multi-Level Feeder Reconfiguration Function
Contents
This application recommends an optimal selection
of feeder(s) connectivity for different objectives. It supports three
modes of operation:
1. Closed-loop mode, in
which the application is initiated by the Fault Location, Isolation
and Service Restoration application, unable to restore service by
simple (one-level) load transfer, to determine a switching order for
the remotely-controlled switching devices to restore service to the
non-faulted sections by using multi-level load transfers. .
2. Advisory mode, in which
the application is initiated by SCADA alarms triggered by overloads of
substation transformer, segments of distribution circuits, or by DOMA
detecting an overload, or by operator who would indicate the objective
and the reconfiguration area. In this mode, the application recommends
a switching order to the operator.
3. Study mode, in which the
application is initiated and the conditions are defined by the user.
The application performs a multi-level feeder
reconfiguration to meet one of the following objectives:
a.
Optimally restore service to customers utilizing multiple
alternative sources. The application meets this objective by operating
as part of Fault Location, Isolation and Service Restoration.
b.
Optimally unload an overloaded segment. This objective is
pursued if the application is triggered by the overload alarm from
SCADA, or from the Distribution Operation Modeling and Analysis, or
from Contingency analysis. These alarms are generated by overloads of
substation transformer or segments of distribution circuits, or by
operator demand.
c.
Minimize losses
d.
Minimize exposure to faults
e.
Equalize voltages
The last three objectives are selected by
engineer/planner.
The feeder reconfiguration process is a
multi-objective function with a very large number of variables.
Theoretically, the number of possible combinations to consider during
the search of the best solution is equal 2n, where n is the
number of switching devices in the interconnected circuits. With DERs
connected to the distribution circuits, the circuit configuration
solution may be different. Just by adding the two states of DER: ON
or OFF, the number of combinations for searching the configuration
solution increases to 2n+m, where m is the number of DERs
connected to the subject circuit. Different modes of operation and
different load schedules of DER can also change the solution. Hence,
the number of combination may increase even more. The nominal optimal
configuration is typically selected for a long-term interval (season,
year). If the solutions are different with DERs ON from those with
DERs OFF, then the application should take into account the probable
schedules of the DERs during the entire time interval for which the
configuration solution is sought and find a configuration which is the
best for this time interval. The application should be capable of
recommending smaller time intervals with different circuit boundaries.
When the reconfiguration solution implies the
participation of DER devices, and the circuits can be overloaded if
the DER devices are disconnected, then an additional condition that
the DER devices are always available when needed by the Area EPS
should be present. In this case the maintenance of DER devices will
lead to temporary change of configuration, and an outage of DER at
heavy-load times is a contingency. Hence, DERs introduce additional
opportunities and additional constraints to the configuration and to
the migration from one configuration to another.
When the distribution
circuits are reconfigured, a DER may move from being electrically
connected to one substation bus to being connected to another bus, and
this may be unacceptable by the DER owner due to, e.g., a change of
the DER capability to provide ancillary services, or due to an
increase in exposure to faults. (The DER owner may have a reliability
guarantee or similar agreement with DISCO, limiting the exposure to
faults.) The input data describing such kind of conditions should be
made available to the reconfiguration application.
Other constraints can be imposed by the
transient process during switching operations while implementing the
new configuration, which consist of paralleling and breaking. The
change in voltage magnitude and phase angle during the switching
operations will present some kind of disturbance for the DER and in
some instances may be unacceptable. Such parameters as acceptable
rate of voltage and angle changes may be needed for deciding whether
the configuration with a DER connected to the circuit can be
accomplished. In some cases, a temporary intentional islanding during
the transition from one configuration to another can be included in
the reconfiguration solution.
The specific of multi-level feeder
reconfiguration process is that the more connectivity alternatives are
available, the greater are the optimization benefits. As was mentioned
above, the number of alternatives increases with the increase of the
number of sectionalizing and transfer switching devices. In order to
consider the maximum number of alternatives all interconnected feeders
with all switching devices available for control should be included in
the optimization process. The presence of DER devices in the
distribution system increases the number off alternatives due to the
various combinations of kW and kvar injections, possibilities of
islanding, and protection schemes, and contractual agreements. The
run-times of the multi-level feeder reconfiguration program ranges
from seconds to hours depending on the reconfiguration objective.
The feeder reconfiguration solution will be
used for different timeframes, such as:
1.
For several hours after clearing a fault for service
restoration to healthy sections. The solution should be found in the
matter of seconds.
2.
For several hours or days for voltage equalization, when there
is an urgent need in load reduction via Volt, Var, and Watt control.
The solution should be found in the matter of minutes.
3.
For several days or weeks for load balancing during maintenance
of distribution facilities. The solution should be found in the matter
of seconds. The solution should be found in the matter of tens of
minutes.
4.
For a season or a year for minimization of customer exposure to
interruptions, normal load balancing, and loss minimization. The
solution should be sought during several hours.
The input data for this computing application shall include the
following DER object model attributes for different reconfiguration
objectives and different timeframes:
a.
For minimizing exposure of the distribution system to customer
interruptions, the following attributes of the DER object model (or
other sources of information) will be used:
·
Rated characteristics
·
Long-term operation and maintenance schedules
·
Contractual constraints
·
Conditions for automatic islanding
b.
For balancing loads of substation transformers and distribution
feeders, the following attributes may be needed:
·
Rated characteristics, including the capability curves
·
Short-term operation and maintenance schedules
·
Contractual constraints
c.
For equalizing voltages between feeders and substations, the
following attributes should be considered:
·
Rated characteristics, including the capability curves
·
Short-term operation and maintenance schedules
·
Contractual constraints
·
P-Q-V modes of operations
·
Availability to start DER devices in Area EPS demand
d.
For minimizing energy losses, in addition to the attributes
mentioned above, the cost of DER operations will be needed.
