SUISHI – Detailed Simulation Model of Power Plant Operation for Interconnected Hydrothermal Systems
SUISHI is a monthly simulation model with an individual representation of power plants that make part of interconnected hydrothermal systems. The SUISHI model has the following characteristics:
• simulates the energy operation for several interconnected hydrothermal subsystems, taking into account energy interchange capacities among system areas (subsystems);
• can be coupled to the strategic decision model NEWAVE (Long and Medium Term Operation Planning Model of Interconnected Hydrothermal Systems), which provides an operation policy (given by an expected cost-to-go function) for each simulation stage (time step);
• considers local operational constraints due to multiple uses of water: maximum streamflow for flood control, minimum streamflow for sanitation or navigation, and river streamflow deviations for irrigation, as well as operating special basins such as the Paraíba do Sul and Tietê river basins;
• simulates multiple hydrological series in parallel, which improve the obtainment of probabilistic indexes for the system's performance in each simulation stage, such as the annual risk of deficit and the average operation marginal cost of the system ;
• considers four simulation modes: hydrothermal, assured energy calculation, firm energy calculation within a pre-established critical period and firm energy calculation with the determination of the critical period.
Interconnected Hydrothermal Generation Subsystems
Two types of hydropower plants can be represented in the SUISHI model: run-of-the-river, which has no water storage or a very limited storage capacity; and hydro plants with reservoir, which have a significant storage capacity. All other power plants are called thermoelectric plants (nuclear, coal, gas, oil, diesel, biomass, etc.). These plants are represented by a minimum and a maximum generation capacity, and also by a constant unit generation cost. The cost of deficit function may be represented in different levels. The generation of renewables, like wind and solar PV, are also considered.
The transmission system is represented taking into account large energy interchange capacity between subsystems. The electricity consumer market of each subsystem is represented by a load duration curve with up to five levels.
Hydrological Series: Historical, Synthetic and Alternative Uses
SUISHI model recognizes three types of hydrological sequences, as defined below.
· - The historical inflow sequence to a hydropower plant corresponds to the monthly average values of natural inflow. -- Synthetic sequences of inflows are generated by a Stochastic Inflow Model, such as the GEVAZP model (Energy and Periodic Streamflow Synthetic Series Generation).
· - Sequences of inflows for alternative uses, with monthly values, must be added to or subtracted from its corresponding series of incremental inflows in order to represent different water uses at a given site.
Simulation Modes: Hydrothermal, Firm Energy Calculation and Assured Energy Calculation
The main goal of the hydrothermal simulation mode is to determine, for each month of the planning horizon, generation targets for each hydro and thermal power plant, as well as targets for the energy interchanges among subsystems, that minimize the total operation cost over the whole planning horizon. In this case the SUISHI model makes use of the cost-to-go function provided by the NEWAVE model, which is built also taking into account a nested Conditional Value-at-Risk criterion.
The objective of the firm energy calculation mode is to determine the largest energy market that a purely hydroelectric system can meet without the occurrence of energy deficit, assuming the occurrence of the historical inflow sequence. This is called the firm energy of the system. In Brazil this concept is used in order to determine the assured energy of each hydro plant, which represents the maximum amount of energy that can traded in the long-run.
The objective of the assured energy calculation mode is to determine the largest energy market that a hydrothermal system can meet under a given supply adequacy criterion. In Brazil such criterion is satisfied when the equality between the operation marginal cost and the expansion marginal cost is observed, with a deficit risk lower or equal to 5%.
Hydrothermal Balance Optimization among Subsystems and Simulation of Hydro Power Plants Operation
The solution process of the SUISHI model, at each period of the planning horizon, is divided into two steps that are solved iteratively: the first step optimizes the hydrothermal balance between subsystems according the cost-to-go function provided by NEWAVE model, and the second step simulates the operation with an individual representation of the hydro plants.
In the first step, the objective of SUISHI model is to define generation targets for each thermal plant, for each Energy Equivalent Reservoir (EER) and also the energy interchanges among subsystems by solving an optimization problem which aims to minimize the sum of the present and future operation costs, subject to certain constraints, such as water balance, demand supply, maximum storage, maximum hydro generation and taking into account the cost-to-go function provided by the NEWAVE model.
In the second step, a heuristic approach, considering the nonlinearities embedded in the energy production function of hydro power plants, looks for to allocate the hydro generation targets of each EER (defined in the first step) to the respective hydro power plants. Such procedure is called operation simulation.
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