Outline Description of Program EPSIM

Overview

Program EPSIM has been developed by Power & Water Systems Consultants Ltd. (PWSC) for the rapid definition, evaluation, comparison and optimisation of power system expansion plans. EPSIM has been specifically designed to provide :

• modelling and economic evaluation of integrated generation and transmission system expansion plans, including explicit consideration of imports and exports;
• a graphical user-interface (GUI) to simplify and verify data inputs, and the screen display and printed output of detailed and consolidated results in both graphical and tabular form;
• the interactive definition and graphical display of alternative expansion plans;
• facilities for modelling Private Power Agreements (PPA's) and their associated impacts on system operation, as they may apply to power generation and transmission network usage;
• robust and computationally efficient routines for performing both deterministic and probabilistic load dispatch simulations, with the required level of detail being specified by the user;
• explicit consideration of demand management measures, as well as the costs of unserved energy, on a demand centre basis;
• full transparency of all analytical processes via permanent fully annotated (ASCII) input and output files, and their graphical interpretation.
• database storage of alternative expansion plan evaluation results, system component details and electricity demands;
• a Monte Carlo based optimisation facility for the automatic definition, screening, and evaluation of alternative expansion plans;
• data compatibility with the WASP and AS-PLAN generation system planning packages.

Development Philosophy top

EPSIM has been developed to provide a rapid and 'user friendly' facility for the definition, evaluation, comparison and optimisation of power system expansion plans. Development was prompted by limitations associated with widely used generation system 'optimisation' programs, such as WASP and AS-PLAN, such as their inability to model transmission systems, and a general lack of 'user friendliness' in terms of data input, execution, transparency and the presentation of results. It was also recognised that neither program can, in fact, be relied upon to 'optimise' the introduction of hydroelectric plants, and that difficulties may also be encountered with 'mutually exclusive' developments, and when generation modelling plants subject to complex PPA's.

In practice, the practical number of development alternatives is often quite limited, particularly in the case of hydroelectric development. In such cases the application of programs such as WASP and A/SPLAN, may not be capable of determining the 'optimal' development of the generation system let alone the integrated generation/transmission network. As a result, and it may well be preferable to utilise an inter-active simulation based approach.

EPSIM has been designed to enable the evaluation of defined system expansion plans in considerable detail, in terms of both economic analysis and the load dispatch simulation. A key attribute is that all such analyses are carried out with a monthly (rather than annual or 'seasonal') time step, and that the load dispatch simulations can be effected at various levels of detail as appropriate to the available data and specific study objectives.

In line with PWSC's overall modelling philosophy of fully integrating detailed simulation and mathematical programming techniques, EPSIM includes a Monte Carlo based module for the automatic definition, screening, evaluation, comparison and hence 'optimisation' of alternative power system expansion plans. While not employing a 'formal' optimisation technique, test results have demonstrated the effectiveness of the approach in terms of both computational efficiency and the identification of 'optimal' solutions. When combined with a detailed representation of the system and load dispatch, a fundamental attribute is that the user can have confidence that the resulting plans are 'practical' in terms of complying with system operation constraints.

Economic and Technical Data Inputs top

Input operating and investment cost data used by EPSIM to calculate the Net Present Worth (NPW) of a given expansion plan include :

• for hydroelectric plants - fixed operation and maintenance (O & M) costs ($/month), variable operating costs ($/MWh and $/MW), economic lifetime (months), (monthly) investment costs split into local, foreign and labour components (million$);
• for thermal plants - fixed O & M costs ($/month), variable operating costs at and above Minimum Stable Load ($/MWh), capacity costs ($/MW), economic lifetime (months), (monthly) investment costs split into local, foreign and labour components (million$);
• for transmission lines - fixed O & M costs ($/month), variable operating costs ($/MWh and $/MW), economic lifetime (months), (monthly) investment costs split into local, foreign and labour components (million$).

Technical data considered during the load dispatch simulations include :

• for hydroelectric plants - installed (maximum) capacity (MW), 'firm' and average annual energy production (GWh) or calendar monthly plant availabilities in terms of MW and GWh (by hydrological condition), forced outage rate (%);
• for thermal plants - installed (maximum) capacity (MW), Minimum Stable Load (MW), maximum and minimum annual energy production (GWh) or calendar monthly availabilities in terms of MW and GWh, forced outage rate (%);
• for transmission lines - maximum carrying capacity (MW), forced outage rate (%), and loss factor (%).

For hydro plants, up to five 'hydrological conditions' can be considered, with different calendar monthly power and energy availabilities being assigned to each condition. Their probabilities of occurrence. may, for example, be associated with 'very dry', 'dry', 'average', 'wet' and 'very wet' conditions, and this feature provides consistency with data inputs to the WASP and A/SPLAN programs. If only annual values for 'firm' and average energies are input, these are automatically pro-rated within EPSIM to give the requisite monthly values for the 'very dry' and 'average' hydrological conditions. Monthly thermal plant availabilities can be used to reflect planned maintenance schedules.

The demands associated with each demand centre in the system can be given in terms of :

peak and base loads (MW), and the total monthly energy requirement (GWh),
or
the load in MW for each of up 8 blocks, and duration of each block.

In each case the load can be varied as a function of hydrological probability, thus enabling the modelling of preventive rationing. For example, the demands to be met can be reduced by a specified amount in the event of 'very dry' hydrological conditions being experienced. Differential unit prices for supplied energy and costs of unserved energy can also be associated with each demand centre.

Expansion Plan Definition top

Alternative expansion plans define the commissioning and retirement dates (year and month) of system elements, namely hydroelectric plants, thermal generation plants and transmission lines.

With EPSIM, such plans can be manually defined (by editing the appropriate input file), or constructed interactively by selecting from of candidate system elements. The user is provided with guidance regarding the ranking of candidate plants, in terms of $/MW installed, $/GWh (firm), $/GWh (average) etc., the monthly peak load (MW) and energy (GWh) demands to be met, and construction time constraints. Compliance with earliest and latest commissioning dates and economic lifetimes, as specified by input data, are also checked. On completion, the corresponding commissioning and retirement dates are automatically written to the Expansion Plan Definition (*.EPD) input file.

A given expansion plan can be displayed graphically and as a 'mimic' diagram indicating the status of system components in each month of the expansion planning period.

Automatic Definition, Screening and Evaluation of Alternative Expansion Plans top

EPSIM can include an optional 'optimisation' module for the automatic definition, screening, and evaluation of alternative power system expansion plans.

The overall procedure it is follows :

Step 1 : Commissioning and retirement dates (in terms of year and month of the planning period) for all candidate system elements are selected using a Monte Carlo sampling approach, taking into account input data on earliest and latest permissible commissioning dates, mutual exclusivity and dependencies, construction periods, economic lifetimes, and associated transmission lines.

Step 2 : Checks are made to ensure that, for each month of the planning period, the planned system configuration can meet the total electricity demand in terms of both peak load and 'firm' energy. If a failure is registered, return to Step 1.

Step 3 : Load dispatch simulations are performed for each month of the planning period to ensure compliance with all system operating constraints e.g. including minimum plant outputs and transmission line capacities, and to calculate system operating costs. If any such constraints are violated, return to Step 1.

Step 4 : Calculate the NPW of the expansion plan and automatically store all run details and evaluation results in the integrated data base If the user defined maximum number of iterations has not been exceeded, return to Step 1.

In practice it has been found that only approximately 1 in 100,000 of the plans selected in Step 1 reach Step 4. While such rates will obviously vary depending on the number and tightness of the constraints, it can be noted that Steps 1 and 2 are effected extremely rapidly, with the main computational load being associated with Step 3.

The program user is able to specify whether the commissioning month as well as year is to be selected; if not, the month is automatically set to 1. In this way it is possible to successively refine the 'optimised' expansion plan. Similarly, by setting the earliest and latest commissioning dates of a given component to the same values it is possible to 'fix' its installation. There is also an option to 'suspend' and 'resume' the process at any point so that previously considered plans are not repeated.

While the methodology employed cannot 'guarantee' identification of the 'optimal' expansion plan, test results indicate that it is capable of producing plans with a lower PWV than those defined inter-actively, and with acceptable execution times.

It can also be noted that the method produces a collection of plans rather than a single 'optimum' plan, enabling their input to further studies taking into account such aspects as environmental consideration and financing requirements.

Load Dispatch Simulations top

The technical and economic characteristics of the available generation plant are used to perform load dispatch simulations for each month of the specified expansion planning period, taking into account the system composition as defined in the *.EPD file.

These simulations are used to establish the ability of the defined system configuration to meet the forecast demand for electricity, and to calculate the associated operating costs including those associated with unserved energy. Two alternative methods can be used to perform the load dispatch simulations: the Integrated Load Duration Curve (ILDC) method due to Jacoby, where the demand is specified only by the peak and base loads (MW), and the total monthly energy requirement (MWh), with the ILDC being represented by a high order polynomial; or a Linear Programming (LP) formulation where the demand is specified in blocks representing either the chronological load or the load duration curve.

While the ILDC method has a very rapid execution time, it is not possible to model transmission line constraints (and hence multiple demand areas), or always identify the exact dispatch of each generating plant.

The LP method can optionally consider multiple demand areas and transmission constraints/losses, and hence explicitly imports and exports, as well as minimum and maximum monthly power and energy outputs specified for both hydro and thermal plant. The position of each generating plant in each load block ('tranche') is identified and, if a load duration curve is used, an optional constraint can be imposed so that the output from each plant must be at least that dispatched in a lower load tranche. With large numbers of generating plant and transmission lines, execution times with this method are significantly greater than with the ILDC method.

Both the ILDC and LP load dispatch simulations can be performed in either deterministic or probabilistic mode. In deterministic mode the forced outage rate associated with all system components is assumed to be zero. In probabilistic mode multiple load dispatch simulations are performed, with the capacities of individual and combinations of system components being systematically set to zero. The user is able to set a loss of load probability limit, computed as the product of the individual component forced outage rates, below which the associated load dispatch simulations are suppressed. For each simulation are recorded the loss of load (MW) and the corresponding probability. With the ILDC method the user is able to specify whether hydro and/or thermal plant forced outages are to be considered; if the LP method is employed any combination of hydro plant, thermal plant and transmission line outages can be specified.

The results of all load dispatch simulations performed when evaluating a given expansion run are stored in fully annotated output files, one for each hydrological condition specified. With the ILDC method the monthly tables show the power and energy provided by each generation plant and the associated production costs. With the LP method the output of each generation plant and associated operating costs in each load dispatch tranche are given; if modelling of transmission lines is specified, the tables also indicate for each tranche the load flow through each line (MW), the energy carried (GWh), losses incurred and the associated transfer costs (if any).

The contents of these output files can be displayed in tabular, graphic or 'mimic' diagram form to show the production of each plant in each month and, if the LP method is used, in each load dispatch tranche. Graphs can also be displayed and printed showing the loss of load probability and either the loss of load (MW) or the loss of energy (GWh) in each month of the planning period. In both cases the facility is provided to 'page through' the planning period by month.

An example of the graphical load dispatch details that can be output when using the LP method is shown below. It can be noted that, in this example, the first load block is used to ensure coverage of the maximum 'instantaneous' (15 minute) load and that the dispatch corresponds to Hydro Condition 1.

For each alternative expansion plan analysed, and each hydrological condition, graphical screen and printer outputs can also be produced showing:

• the monthly peak load and capacity coverage, split between hydro and thermal plants, and with any deficits highlighted;
• the monthly energy demand and energy availability, split between hydro and thermal plants, and with any deficits highlighted;
• the monthly peak load and dispatched capacity, split between hydro and thermal plants, and with any deficits highlighted;
• the monthly energy demand and dispatched energy, split between hydro and thermal plants, and with any deficits highlighted.

If the expected operating costs for given demand/system configurations are available, these can be input directly to EPSIM via the System Operating Cost (*.SOC) input file and the load dispatch simulations are suppressed.

Calculation of Present Worth Costs and Sensitivity Analysis top

The Present Worth Cost associated with a given expansion plan is computed as the total discounted value of all investment and operating costs, including the cost of supply deficits ($/MWh) and any 'salvage' credits associated with components with an economic lifetime extending beyond the planning period. The user may also specify an 'evaluation' period longer than the planning period, during which the demand to be met and operating costs are assumed to be the same as for the last year of the planning period.

All expenditures and benefits are accounted for on a monthly basis. In the event of more than one ('average') hydrological condition is analysed, the monthly operating and unserved energy costs are taken to be those associated with each load dispatch simulation weighted by the assigned hydrological probability. The Present Worth Costs for each expansion plan are calculated for each combination of input ranges of economic parameters, namely:

• discount rates;
• differential ('real') operating cost escalation rates;
• shadow prices on local, foreign and labour investment costs, and on local and foreign operating costs.

The 'base case' present worth cost associated with each alternative expansion plan is automatically stored within an integrated data base, together with the names of the input data and output files used and supply reliability indicators i.e. the number of months when demand could not be met and, if probabilistic load dispatch is employed, the maximum Loss of Load Probability in any month. All plan results stored in the (Microsoft ACCESS) database can be tabulated in increasing order of present worth cost or run code.

Graphical screen and hard copy outputs can be produced showing the monthly cash flow of investment, operating costs and, optionally, revenues and deficit costs. Total investment costs can be shown or disaggregated into local, foreign and labour components.

The required (constant) price of generated electricity necessary to meet total expenditures over the planning period can also be calculated, and the associated net cash flow plotted.

Three dimensional graphs showing the sensitivity of the PWV to the input ranges of the economic parameters can be displayed and printed.

Program Details and Usage top

The EPSIM Graphical User Interface (GUI) is written in Microsoft Visual Basic (Version 3) and runs under Microsoft Windows 95, 98, NT4, 2000 & XP operating systems. For rapid execution, the load dispatch simulation and economic analyses are performed by a FORTRAN program which the user calls directly from the GUI. The structure of the FORTRAN program is shown as Figure 1.

The execution times required to evaluate a given expansion plan will depend upon :

• the number of generation plants and transmission lines modelled;
• whether the ILDC or LP load dispatch method is used;
• the number of 'hydrological conditions' to be analysed;
• whether deterministic or probabilistic load dispatch simulations are performed.

In practice the appropriate level of detail for load dispatch simulations will vary with the application and can be increased as the 'optimal' plan is successively refined.

Applications and Availability top

EPSIM has been designed to be used either as a 'stand alone' expansion planning tool, or to refine, and conduct sensitivity analyses on, 'optimised' generation plans produced by programs such as WASP or AS-PLAN. In addition, it facilitates the combined economic analysis of separately derived generation and transmission expansion plans.

EPSIM's principal benefits are its ability to model integrated generation and transmission systems, a high level of 'user friendliness', integrated graphical output and database facilities, rapid execution times, and the facilities to model Private Power Agreements as they may apply to hydroelectric installations, thermal power plants and transmission lines. Thus, in addition to its application within expansion planning studies, it is also intended for use in comparing and selecting from alternative proposals submitted by potential Independent Power Producers (IPP's).

EPSIM is available for outright purchase by individual utilities, subject to standard software protection measures being installed. Usage by consultants and international agencies on a project-by-project basis is by negotiation.