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More About EPSIM

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Program EPSIM : Outline Description

Outline Description of Program EPSIM


Program EPSIM has been developed by Power & Water Systems Consultants Ltd. (PWSC) to facilitate the rapid definition, evaluation, comparison and optimisation of power system expansion plans. It 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 impacts on system operation, as they may apply to both power generation and transmission network usage;
  • robust and computationally efficient routines for performing both deterministic and probabilistic load dispatch simulations;
  • 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 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 perceived limitations of generation system 'optimisation' programs, such as WASP and AS-PLAN, such as their inability to model transmission systems and limited 'user friendliness' in terms of data input, execution, transparency and the presentation of results. It was also recognised that neither WASP nor AS-PLAN can be relied upon to 'optimise' the introduction of hydroelectric plants, or cope with 'mutually exclusive' developments.

Due to the computational constraints associated with mathematical programming techniques such as Mixed Integer Linear and Dynamic Programming, formal optimisation models often need to make significant simplifications when estimating the operating costs associated with alternative system configurations and their associated supply reliabilities. As a result, they may not always be capable of determining the true 'optimal' development of a generation system, let alone an integrated generation/transmission network.

With EPSIM all economic analyses and load dispatch simulations 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.

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

Technical data considered during the load dispatch simulations include:

  • for hydroelectric and wind 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, thereby providing 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' ('firm') and 'average' hydrological conditions. Monthly thermal plant availabilities can be used to reflect planned maintenance schedules

It has long been known that the optimal role of individual hydro plants can vary with the development of an integrated electricity supply system, and EPSIM allows monthly hydro plant capabilities to be varied as a function of the expansion plan year. This facility can be particularly pertinent when optimising the development of systems which include non-hydro forms of generation, such as wind and solar, which have significant seasonal availability constraints. Such data can be provided directly by PWSC's AQUARIUS program

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),
  • the load in MW for each of up 8 blocks, and the duration of each block in hours.

In each case the load can be varied as a function of hydrological probability, thus enabling the modelling of preventive rationing or demand management. 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, transmission lines and electricity demand areas. Such plans can be manually defined (by editing the appropriate input file), constructed interactively by selecting from candidate system elements or generated automatically.

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 data specified earliest and latest commissioning dates and economic lifetimes, and mutual exclusivities, are also checked.

Plans can be generated automatically on the basis of $/MW installed, $/GWh (firm), $/GWh (average) and other heuristic ranking indices. On completion, the corresponding commissioning and retirement dates are automatically written to the Expansion Plan Definition (*.EPD) input file.

An example of the EPSIM screen provided for the display, construction and automatic generation of system expansion plans is shown as Figure 1.

Figure 1 : View, Construct and Generate System Expansion Plan

The installed capacity or energy availability associated with a given expansion plan, together with the annual additions and subtractions can be displayed graphically, as shown in Figure 2.

Figure 2 : Monthly Energy Demands, Availability and Annual Totals

The coverage of forecast peak load and energy demands can be plotted in the form shown in Figure 3 below.

Figure 3 : Monthly Peak Load and Energy Coverage

Automatic Definition, Screening and Evaluation of Alternative Expansion Planstop

EPSIM also includes an optional 'optimisation' module for the randomised 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.

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 performed 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 for systems with a relatively small number of development alternatives. Where there are a large number of potential generation projects, or a long expansion planning period is being investigated, the process can be used to refine plans for which a significant number of commissioning dates are fixed.

It can 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 of environmental aspects 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 demands for electricity and to estimate the operating costs, including those relating to any 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 by the peak and base loads (MW), and the total monthly energy requirement (MWh), with the ILDC being represented by a high order polynomial;
  • 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 rapid execution times, it is not possible to model transmission line constraints or 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 can be 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 block. 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 block are given; if modelling of transmission lines is specified, the tables also indicate for each block 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 block. An example of a load dispatch output plot is shown in Figure 4 below.

Figure 4 : Program EPSIM - Monthly Load Dispatch Plot (Linear Programming Method)

An example of a 'mimic' diagram representation, which can be used to show the status of system components in any month of the expansion planning period, is given in Figure 5.

Figure 5 : Program EPSIM - Mimic Diagram Representation of Load Dispatch Results

Graphs can also be produced which show 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 month by month. For each alternative expansion plan and hydrological condition, graphical screen and printer outputs can 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. Such estimates can be provided by PWSC's AQUARIUS computer program.

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 is also calculated, and the associated net cash flow can be plotted.

An example plot of total investment and operating costs is shown as Figure 6.

Figure 6 : Program EPSIM - Plot of Investment and Operating Cost Streams

Graphs showing the sensitivity of the PWV to the input ranges of the economic parameters can be displayed and printed, as illustrated in Figure 7 below

Figure 7 : Program EPSIM - Present Worth Value Sensitivity Plot

Program Details and Usage top

The EPSIM Graphical User Interface (GUI) is written in Microsoft Visual Basic© (Version 6) and runs under Microsoft Windows 95, 98, NT4, 2000, XP and 7 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 overall structure of this program is shown as Figure 8.

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 level of detail used for the load dispatch simulations will vary with the application and, within a planning study, this level may be increased as 'optimal' plans are 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.

The principal benefits of the program lie in 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.

Contact Details top

For further details on Program EPSIM and AQUARIUS, for optimising the operation of hydro-thermal and multi-purpose water resource systems, please contact .

Tim Wyatt, Managing Director
Power & Water Systems Consultants Ltd.

Telephone : +44 (0) 7980985170

e-mail :
Skype : pwsc

Figure 8 : Program EPSIM - Block Diagram of FORTRAN Program


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Water Resource & Hydro-Thermal Power Systems Modelling - Optimization of Large-Scale Hydropower System Operations - Power Generation Optimisation