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DESSEM – Short Term Hydrothermal Dispatch Model

DESSEM project conducts research and develops computational models related to Daily Operation scheduling (PDO, by the Portuguese acronym) of electric power systems. DESSEM aims to implement techniques and tools that allow, from the theoretical and practical point of view, to model and solve the optimization problems related to the daily operation scheduling of hydrothermal systems, considering, as accurately as possible, both aspects related to the electrical network and aspects related to the operation of hydrothermal plants, thermoelectric plants and other components of the system.
Computational Models
In this project, the following software programs are developed:
• DECODESS, which is a data converter tool between the DECOMP and DESSEM models
• SIMHIDR – Hydropower simulator, which calculates the water balance for the hydropower plants in cascaded from hourly power generation targets established for each plant during a given scheduling horizon. The problem is solved by linear programming.
• DESSEM – Operation scheduling of hydrothermal systems, with a half-hourly time discretization accuracy and an horizon of up to two weeks. The electric network is represent by a DC model with losses with additional security constraints; complex thermal unit commitment constraints are considered, including the modeling of combined-cycle plants; finally a large set of constraints for the operation of hydro plants and reservoirs is taken into account. The problem is solved by mixed integer linear programming (MILP), with piecewise linear approximations of some nonlinear characteristics.
DESSEM program belongs to the chain of mathematical models developed by Cepel and used by the Independent System Operator (ONS) and the Market Operator (CCEE) for the operation planning and scheduling and prices setting of the Brazilian Interconnected System (SIN), with emphasis on cost minimization but taking into account some risk-aversion criteria. The main principle of this chain models is to coordinate the operation of the hydropower plants of the system, aiming to decrease the use of thermal energy and operate the system more efficiently, thus contributing to the reduction of operating costs and deficit risks. Nowadays, the NEWAVE and DECOMP models (for mid and short term planning, respectively), are officially used in a combined way to set the weekly dispatch and spot prices in Brazil, in three load blocks,
The DESSEM model is currently being validated by ONS and CCEE, in a task force comprising the main institutions and a large number of utilities of the Brazilian power system, in order to be also officially used to set hourly prices in Brazil, starting in January 2020.
Features of the model
 The main features of DESSEM model are:
• DC modeling of the electrical network, including line flow limits constraints, losses in the transmission lines and additional security constraints of the system. 
• Individualized representation of the reservoirs, modeling several complex aspects, such as the river routing in consecutive cascade plants, the backwater in the operation of reservoirs and operation of river channels.
• Modeling of hydro production function of the power plants, considering the variation of efficiency with the water head, by modeling the generation as a piecewise linear function of storage, turbine outflow and spillage in the reservoir.
• Modeling of several aspects o the unit commitment of thermal generating units, such as minimum generation (once the unit is turned on), minimum up/down times, ramp constraints for generation, startup/shutdown trajectories, operation of combined cycle units, as well as startup and non-linear generation costs.
• Constraints of minimum outflows for the reservoirs, and waiting volume for flood control.
• Representation of pumped-storage plants, operation of small hydro plants and intermittent generation (e.g., wind and solar), and power import/exports with neighboring subsystems.
• Modeling of river channels, such as the Pereira Barreto channel between Ilha Solteira and Três Irmãos reservoirs, as well as maximum hourly/daily river level variation constraints, as for example in Itaipu’s R-11 river gauge (Tripartite agreement).
• Coupling with DECOMP model through the future cost function or by establishment of weekly thermal generation targets and/or energy interchanges between system areas (subsystems).
Besides its main purpose, which is to determine the minimum cost hydrothermal operation scheduling of the system, the models developed within the DESSEM project can be applied in a series of studies, such as:
• calculation of the optimal dispatch, given pre-defined storage targets for the reservoirs at the end of the horizon;
• calculation of evolution of reservoir and/or the DC power flow dispatch of the system, from a pre-defined generation dispatch for the plants;
• sensitivity analyses regarding configuration changes or external factors, such as load increase or variations in the inflows to the reservoirs.
 Lines of Research
The current lines of research related to DESSEM project are:
• Consideration of hydro unit commitment constraints in the problem;
• Incorporation of the AC modeling of the electrical network.

• Explicit representation of uncertainties in uncontrollable generation sources such as wind and solar, which leads to a stochastic hydrothermal unit commitment problem.


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