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Optimizing
the Hydro System in First
published in Hydro Review, volume 18, no. 5, August, 1999. by Abstract FPL Energy - The FPLE
Hydroelectric System The FPLE hydro system has
31 conventional hydroelectric stations containing 92 units scattered
throughout its service territory. Nearly 57% of FPLE's
hydro capacity is located on the The FPLE stations range
in capacity from less than one MW to over 88 MW, and in head from less than
20 feet to over 150 feet. Unit types include horizontal and vertical Francis,
horizontal and vertical Kaplan, horizontal and vertical propeller, and
horizontal multiple runner Francis. The 31 stations have a total of capacity
of 373 MW. Harris and Wyman on the The Kennebec and Overview of
the Hydro Modeling System The Real-time Hydro
Operations Model (RHOM) is FPLE's implementation of
Charles Howard & Associates Ltd.'s HYDROPS
system. It was developed over a period of approximately three years starting
from existing HYDROPS software developed for other hydroelectric systems. The
modeling system envisioned in 1995 covered all aspects of water and hydro
operations management on time scales ranging from near real-time to annual.
The time steps used by the computer system range from minutes, to hours, to
weeks. Functions included in the operations support system include:
Hourly scheduling of the
generating units in the system has developed into the central part of the
RHOM project. With the advent of retail competition, there are clear
advantages to having an operations model that can quickly provide an optimum
dispatch schedule based on projections of market prices, future inflow
forecasts and the current watershed state. The following is a brief
description of the hardware and the software system including the components
and capabilities of each module.
The modules schedule the
operation of the generating units, the stations, the use of storage, and in
total effect they determine the operation of the river. They also pinpoint
the schedules for annual maintenance. There are nine main
software modules:
Satellite Down-link,
Hydrometric Data-viewer, & Editing Real-time hydrometric
data is picked up from the three river basin areas via direct reception of
satellite transmissions. Temperature and rainfall gages located at USGS river
gage sites store and transmit the meteorological data along with river stage
to the GOES-8 satellite. The rebroadcast from a commercial satellite is
received by a dish and decoder system at FPLE where a communication server
formats and stores the data in the central database. Other meteorological
data from the internet are automatically collected and loaded into the
database. The Satellite Data-viewer converts river stage to discharge and
provides convenient system-wide views of the river flows and the
meteorological data. These data are required by the hydrologic inflow
forecast system. Inflow
Short-Term Forecast Module Twice each day the
communication server automatically downloads a 7-day weather forecast from a
commercial weather forecasting service. The weather forecast along with the hydrometric
data obtained from the satellite and the Internet are loaded into the
forecast module. Based on the current watershed situation, this module
provides forecasts of future hourly inflows to the rivers and reservoirs over
the next 7 days. The module is operated at least once each day for each river
basin. The results are automatically loaded into the database for use by
other modules. The inflow-forecasting
module is a custom version of HFAM hydrologic model developed by Hydrocomp,
Inc. HFAM is a derivative of USEPA's HSPF and the
Stanford Model, which is the precursor of all physically based hydrologic
digital models. Selection of the proper
watershed model is an important consideration in the early stages of the
project. The inflow-forecasting module is a physically based model which
incorporates the physics of all hydrological processes. This type of model
was chosen because of the scarcity of gages in the river basins and the fact
that river gages are frozen and useless during much of the winter.
Calibration of this model was an expensive and time-consuming process.
Operation of the model is also time consuming.
Frequent adjustments are required to keep the model on track with the
observed hourly flows in the unregulated indicator streams. New interfaces to
expedite this process are currently being tested. Inflow
Stochastic Forecast Module This module provides
probabilistic forecasts of inflows to the major storage reservoirs for the
next 52 weeks. The inflow calculation is based on 50 years of historical
daily meteorological data. The calculations begin from the current initial
soil moisture and snow conditions determined by the short-term forecast
model. For each year in the
weather record the model determines the daily inflow for one year ahead from
the current date. This process is repeated until inflow forecasts have been
made for all 65 years of weather record. The result is 65 forecast sequences
of inflows for the coming year, all starting from the current date. This
ensemble of forecasts is used to construct probability distributions of
cumulative inflows to the reservoirs. These are used in optimizing the
reservoir operations and the maintenance schedule. The operator's level of
risk aversion is reflected by selecting 52 week inflow sequences from up to
seven probability levels. The entire process is repeated when initial
conditions of soil moisture or snow change. Dispatch
(Short-term) Decision Support System The Dispatch Decision
Support System (DDSS) module is the heart of the system. It optimizes the
schedules of hourly generation from hydro units for the next seven days (168
hours). The objective is to maximize revenue within the limits of the
available water resources, contract commitments, and the license constraints.
The DDSS module produces the optimum hourly unit operations schedule based on
the following information:
To initialize to current
conditions in the river the DDSS uses current pond levels and current river
flows collected by the SCADA system. These are automatically fed into the
database by a communications server. Other information used by the DDSS
includes:
This information provides
hard constraints in the optimization problem formulation. Annual
Storage (Long-term) Module Inflow forecasts produced
by the Stochastic Forecast module are used by the Annual Storage Module,
which incorporates the Maintenance Scheduling routine. This module is
operated at least once each week to re-optimize the storage operation
schedules for the Kennebec and
The output is the
generation flow schedule from each reservoir that will maximize the expected
value of power and meet all of the license constraints. The operator can
select up to seven levels of inflow probability to analyze simultaneously.
The module will produce the optimum release schedule for the next week,
considering all of the selected levels of probability for future inflows. The Annual Storage Module
includes customized spreadsheet-type worksheets that are used to fine tune
the storage releases during the week and to allow for special conditions such
as rafting releases. Maintenance
Module The Annual Storage module
contains a sub-module called the Maintenance Module which provides an
optimized maintenance schedule. This module optimizes the schedule for unit
outages throughout the year with the objective of minimizing lost revenue.
Maintenance is incorporated into the optimization routine of the Annual
Storage module because outages directly impact the operation of storage. The
optimization is constrained within an operator
specified window and outage duration for each unit. Outages can also be
entered with fixed schedules. The maintenance routine determines the
scheduled outages which are posted to the database. From there they are used
by the Dispatch Decision Support System and the Annual Storage module. The appropriate river
engineer runs the Maintenance Module. Outage schedules must be coordinated
with the Independent System Operator (ISO/Maine Satellite). Station
Optimization (Near Real Time) Module The current output of
each unit and water level data is provided by a SCADA system to the Energy
Management System (EMS) at FPLE. The
The station operators at
the River Control Centers use the Station Optimization module. Through the
wide area network, managers can monitor the current operating status and
efficiency of all stations in the system. Off-line, the Station Optimization
module provides a tool to study station operations under various conditions. Engineering
Module The Engineering module
provides a convenient interface for editing portions of the database that
deal with physical characteristics of the projects. These data include time
dependent licensing restrictions; maximum and minimum pond levels, minimum
flows, and bypass flows. Station level and unit level engineering data that
are controlled through this module include; turbine maximum and minimum
limits, generator limits, turbine rough zones, unit efficiencies,
stage-storage curves, and tailwater curves. The
module records the dates of time dependent licensing restrictions, which are
used by the Dispatch Decision Support Module to alert the operator to license
violations that might be inherent in the problem setup. Re-licensing
Module The Re-licensing module
provides the Annual Storage and Dispatch Decision Support modules with a
database of historical flows and energy prices. With this information a study
can accurately determine the effect on operations and revenues of changes in
licensing or other operating restrictions. The Re-licensing module will also
be a valuable engineering tool to study changes or additions to generating units.
In the past, re-licensing
and engineering studies have used either HEC-5, which is a simulation model,
or been based on manual calculations with flow duration curves. Limitations
of simulation models for studies include having to fix operating parameters
ahead of time and the use of fixed rule curves. Simulation models do not
automatically redevelop the operating rules for the changes being examined in
the studies. The Re-licensing module uses the automatic optimization routines
in the Dispatch Decision Support System and the Annual Storage Model. Re-licensing studies
typically examine operating conditions for many combinations of flow
conditions by simulating the operating environment over many years. The
Re-licensing module accurately simulates the operating environment by
including all of the details of actual hourly operations. For a study
encompassing a year or more, this can require several hours of computer time. Operating a
Hydro System in the New NEPOOL Beginning in May 1999 the
New England Power Pool began operation as a "residual wholesale
electricity market". This means that electric generation companies may
sell electricity into a wholesale market, subject to the rigid bidding and
operating rules of the Pool. The rules are described on the ISO web page at www.iso-ne.com
under the section on Market Rules and Procedures. In brief, the rules require
that by noon of the current day hydro generation resources must be scheduled
for each hour of the next day. Depending on the
resource's characteristics, there are three methods of bidding a hydro
generation resource:
A self-schedule is a
listing of the MW hours and associated bid price for each hour for the
following day for each ISO hydro unit. To further complicate the
issue 'ISO Hydro Unit' under the ISO definition may consist of a single hydro
unit, multiple units in a single station, or multiple stations closely
connected in parallel or series and operated in combination. Each ISO Hydro
Unit must be bid separately and operated to the hourly schedule to within
tolerance of +- 1 MW per hour. Within the bid structure,
scheduling is particularly problematic for run-of river hydro stations. For
these stations, the range of allowable operations is very narrow because
little or no pond is available to re-regulate flows from upstream. If the
operator finds it impossible to meet the schedule which was bid the previous
day, the ISO must be notified through a process know as redeclaration.
NEPOOL is currently reviewing bidding rules hydro stations due to the
difficult many run-of-river stations have in meeting NEPOOL's
bidding criteria. Frozen
Schedule The software described
above can reschedule the entire hydro system quickly to take advantage of
changes in hydrology or market conditions. But the software was designed
before the new system of bidding had been developed. A number of software
changes were required to work within the complex bidding requirements of the
new NEPOOL. Now, when updating the schedule for the coming week the schedule
already committed to NEPOOL for the remainder of the current day can be
"frozen". The system operator now may select the fixed schedule
from the previous day's optimization run, edit it and then use it to
"freeze" the output schedule for each ISO hydro unit for the
remainder of the current day. Thus, the current day's operating schedule,
which was produced the previous day, is not modified by the program. Each morning, with
today's schedule frozen, the Dispatch Decision Support System module
generates the schedule for the next day. All continuity and licensing
requirements must apply to the frozen schedule, but since the schedule was
determined, conditions such as the hydrologic forecast may have changed.
Under the old NEPOOL system this was not a problem because the software would
automatically re-optimize the schedule. Under the new system, the program
will determine whether the schedule remains hydrologically
feasible for the remainder of the current day. If the schedule is no longer
feasible it must be re-declared to the ISO. Available
Reserve Analysis The new bidding system
initiated a new requirement for a schedule of High Operating Limit. This is
required for bidding reserve generating capability. In order to qualify for
reserve capability, an ISO hydro unit must have a reserve of 1 MW or more
available and have the pondage capability to
sustain the High Operating Limit for one hour. Reserve bids must be submitted
for specific units for each hour of the bid period. The difference between
the specific output self schedule (SOSS) and the High Operating Limit for a
unit in a given hour is the available reserve. A routine was added to the
software to deal with reserves by determining the additional energy that can
be generated for one hour from each unit with the available water. Wholesale
Market Operation The NEPOOL wholesale
electricity market began on May 1, 1999. The modeling system was tested in a
"mock" market operation conducted by NEPOOOL in November of 1998
and performed well. Since the market's opening day the RHOM model has
functioned as the scheduling tool for the river system and the interface
between energy marketing and station operations. The process of coordination
with river operations personnel and the marketing group continues to evolve
and personnel training continues, but it is already
apparent the model functions as intended and its use
will continue to evolve and grow. Conclusions FPL Energy -
The RHOMS system operates
from a relational database controlled by a dedicated computer which acts as a
communication server. Convenient user interfaces and data set version
tracking minimize manual data entries and opportunities for errors in the
input data. The on-line unit-loading
module has been in use for over one year. The operations scheduling software
has successfully been successfully operated in the The NT computer
environment is adequate for this application. The fast pace of Windows
software upgrades has created problems due version incompatibilities of
components. Management and tracking of software and system configuration
changes has been an important function during the development. The CPLEX optimization
system is reliable, fast, and flexible. Its speed for large problems has been
key to making these detailed operating models viable
for near real time operations planning. The development of this
software was a complex interwoven process of thoughtful design, re-direction,
engineering judgment and re-judgment, and previous related software
development experience. There were low points of frustration and high points
of satisfaction. During the development period there were rapid major
advances in proprietary software and new hardware that enhanced RHOM/HYDROPS
system. These advances are likely to continue. Acknowledgments The software was
developed by a closely coordinated team led by Alan Livingstone of FPL Energy
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