Simulation & Optimisation of Power System Short-Term Operation

Summary top

The SYRAP computer program has been developed by P.E. Robinson to perform detailed simulation, optimisation and production costing for generating plant on any interconnected power supply system, together with domestic power purchases, imports and exports. System demand is represented chronologically, employing a time step of either an hour or half an hour. The program is used daily in a number of system control centres around the world and also extensively in system operations studies and expansion planning. Thus applications include:

scheduling plant hourly operation to meet economic and environmental objectives;

determining volume and price bids into a central pool for a mixed portfolio of plant;

• assessing expected reliability of supply;
• production costing and determining marginal costs of supply;
• tariff setting; demand side management;
• evaluating alternative operating or purchase policies;
• fuel budgeting; maintenance planning; and
• calibrating programs based on load-duration-curve methods.

The program provides extensive facilities for load forecasting, based on characteristic daily load curves. All plant types, including combined cycle gas and steam components, together with associated operating restrictions can be represented. Hydroelectric representation can be in terms of power and energy only, or additionally in terms of water flow and storage. In the latter case iterative techniques with rapid convergence are employed to obtain water balances for each time step whilst satisfying all constraints. Security of supply is provided by: top

• modelling spinning reserve and its allocation within the system,
• providing for must-run plant, and
• allowing for transmission limitations.

Unit commitment top

Program input includes a list of generating plant units or power purchases in an initial commitment order. This order will normally be based on plant merit order, but will be adapted as necessary to allow for security considerations and any limitations on power and energy availabilities. For each daily load curve, plant starting and stopping is simulated to supply both the load and required spinning reserve, or to obtain savings against a tariff for imported supplies. At all times due account is taken of relevant operating considerations, such as: top

• temporary derating, transmission penalty factors or changes of fuel or plant availability,
• three shifting of nominated steam plant,
• ramping rates, minimum shut-down periods and time-varying start-up costs,
• output limitations, such as arising from fuel supply or emissions constraints,
• water availability for hydroelectric output and varying hydroelectric power capability,
• prescribed compensation flows or water demands,
• water flow limits and times of travel, and
• other hydroelectric cascade effects.

Economic dispatch top

With unit commitment thus determined and after any fixed outputs have been accommodated, such as from units when ramping or hydroelectric plant which is run-of-river or operated to meet water demands, an economic dispatch simulation allocates load to the plant synchronised in each time step. The dispatch is based on plant input/output characteristics, operating costs, minimum outputs and definable contributions to spinning reserve, and may thus optionally be constrained by security considerations. top

Water or energy values are used in the dispatch calculations to facilitate comparisons between alternative hydroelectric (or other free energy plant) and thermal plant outputs. For each hydroelectric plant, changes in output per unit of water discharge with hydraulic head can be accommodated by using input/output curves applicable over the operating range of upper water level. The water values are applied to such curves and the results directly compared with fuel costs and calorific values applied to thermal plant heat rate or input/output curves.

Program options top

The core of the program is the unit commitment and economic dispatch simulation as described above. In addition, various program options can be exercised in any combination to solve the diverse planning and operational problems arising with different power supply systems and changes in plant mix. As illustrated in Figure 1, the principal options are: top

• commitment order optimisation, designed to;
  • exploit trade-offs between off-load and running costs of different thermal plant types,
  • utilise capacities and energies of hydroelectric (or other energy limited plant) to minimise system total operating cost;
• energy or water valuation; values used in the core program are fixed as defined in data, but this option enables them to be optimised to match outputs to availabilities;
• fixed plant output creation; in the core program individual unit outputs can be fixed in data for periods from one time step to whole days, but this option enables parts of a solution to remain constant whilst the consequences of alternatives elsewhere in the system are evaluated; this facilitates retrospective analyses of actual operation or comparisons of alternative planning methods;
• probabilistic or partly probabilistic unit commitment and dispatch enabling effects of possible plant forced outages to be evaluated;
• full-colour graphical display of intermediate or final results;
• hydroelectric planning; hydroelectric representation in the core program and above options is in terms of power and energy; additional representation of water flow and storage, as illustrated in Figure 2, allows for varying i) tunnel and penstock losses with water flow, ii) turbine efficiency with hydraulic net head and flow, iii) generator efficiency with output, and iv) tail-water level with flow; this in turn enables;
  • hydroelectric input/output curve derivation for use in economic dispatch, and
  • hydroelectric availability calculation, based on expected water flows and demands and compliance with target storage levels;
• hydroelectric operation and spill utilisation; water flow and storage representation also enables simulation of actual hydroelectric operation together with hydro-thermal co-ordination, for example to utilise spill by modifying operation elsewhere in the system.

SYRAP can be used for a part or whole day, a week, a month, a year, or longer if desired, and results (e.g. energy allocations, running hours, numbers of unit starts, fuel consumption's and costs, together with any demands not supplied and details of water flows and hydroelectric operation) are accumulated and can be summarised at various levels of detail. Inherent in the whole process is the determination of short-run marginal costs of supply, and total cost is obtained as a summation of plant start-up and running fuel costs plus other operating and maintenance costs. The core program contains extensive facilities for load management, including automatic optimisation of the demand and load shape to be supplied (or, in a market-oriented power system, to be bid for) such that limited energy availabilities are respected. top

SYRAP is written in FORTRAN 77. When run on an IBM-compatible PC (either directly under DOS or within a Windows environment), input and output and option routines may be overlaid to keep memory requirements within the conventional 640 kB memory limit. Versions that access extended memory or run under UNIX are also available.

Data input top

Data entry to the standard program is via a variety of files, which are created and edited using a separate editor or word processor. This process is facilitated by the:

• detailed instructions for use of all files given in the program user manual [1],
• extensive provision made for the user to include within the files his own annotations and comments, perhaps in his own language,
• input of most data items being in free format, and the
• comprehensive data checking facilities built into the program and complemented by run-time error messages. top

Applications top

Use of SYRAP has generally identified improved methods of system operation leading to savings in system fuel costs of at least 1 per cent, and often far more, as compared to results simulated following previously established methods [2-4]. Program versions have been used to simulate and optimise operation of thermal, hydroelectric and mixed power supply systems in:

A demonstration diskette is available giving comparative examples utilising a variety of program option combinations for eighteen different supply systems. Sample results are also shown graphically as Figure 3. top

Figure 1 back

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