Optimal scheduling of a renewable micro-grid in an isolated load area using mixed-integer linear programming

https://doi.org/10.1016/j.renene.2009年02月03日1 Get rights and content

Abstract

In the energy management of the isolated operation of small power system , the economic scheduling of the generation units is a crucial problem. Applying right timing can maximize the performance of the supply. The optimal operation of a wind turbine, a solar unit, a fuel cell and a storage battery is searched by a mixed-integer linear programming implemented in General Algebraic Modeling Systems (GAMS). A Virtual Power Producer (VPP) can optimal operate the generation units, assured the good functioning of equipment, including the maintenance, operation cost and the generation measurement and control. A central control at system allows a VPP to manage the optimal generation and their load control. The application of methodology to a real case study in Budapest Tech, demonstrates the effectiveness of this method to solve the optimal isolated dispatch of the DC micro-grid renewable energy park. The problem has been converged in 0.09 s and 30 iterations.

Introduction

Keeping the balance between load and generation is the basic rule of all power systems. In the autonomous power supply systems, as spacecrafts, airplanes or small islands, the critical amount of fuel and specific program of the generation capability give the scheduling problem special importance.
In the power system the daily load curve can be well forecasted. The forecast is based on statistical, analytical or technological models.
Characteristics of the system elements:
  • •
    Load – The power system contains controlled and uncontrolled loads. The time curve can be well forecasted.
  • •
    Generator – The generators have many constraints that are minimal/maximal capacity, fuel amount (total generated energy) and speedup ratio.
  • •
    Storage – A large amount of electricity can't be stored economically. Only small systems use super condensers and batteries, the greater systems contain pumped water storage, pressurized air, hydrogen generator, etc. The storage units have double characteristics: these are loads with limited capabilities, and later they may turn into generators. Due to the losses of transformation the storage has never 100% efficiency.
The scheduling problem exists in all power systems [1]. In the autonomous isolated renewable power systems and in the isolated micro-grids for the limited power source, the dynamic portfolio management is really important.
Nowadays there is an upward tendency for using small isolated power systems, against central power producing system when regarding rural and distant places [2].
In this type of system the most important producers are the renewable sources of energy (e.g, photovoltaic panels (PV), fuel cells, wind turbine, etc.) in combination with diesel generators. These small power producing networks need a distributed and autonomous power generation control [3], [4].
Interest in small isolated power systems is also attractive for power utility companies, since they can help in improving the power quality and power supply flexibility. Also, they can provide spinning reserve and reduce the transmission and distribution costs, and can be used to feed the customers in the event of an outage in the primary substation [5].
Although the advantages of using small power systems are considerable, the systems that subsist only from a unique renewable energy source, have their differences because if the source considered is wind power it has more availability than one using photovoltaic panels. Adding storage capability increases the availability more for solar-based systems [6].

Section snippets

VPP operation

The aggregation of DG plants gives place to a new concept: the Virtual Power Producer (VPP). VPPs are multi-technology and multi-site heterogeneous entities. In the scope of a VPP, producers can make sure their generators are optimally operated. At the same time, VPPs will be able to commit to a more robust generation profile, raising the value of non-dispatched generation technologies [7].
The VPP can also operate in isolated networks. In this type of installation the VPP can detain the system

Budapest Tech equipment

In the Budapest Tech exists a small renewable system that has a wind turbine, photovoltaic panels and a fuel cell. The test system is implemented at the roof [10] (Fig. 1).
The equipment of generation has the following characteristics:
  • •
    Photovoltaic panel(s) – type: DS 40; Nominal power: 40 W; Expected production of 4 panels 150 W peak
  • •
    Wind turbine – type: Air-X 401; Nominal power: 400 W at 11.5 m/s; Daily production (day and night) approx. 800 Wh
  • •
    Fuel cell – type: Flexiva; Nominal power: 80 W
  • •
    Load –

Problem formulation

For the formulation of the problem we take a real case of the Budapest Tech system and develop a VPP operation in the isolated grid as following structure. Fig. 4 presents the structure of real case study system.
For the optimal operation system, the VPP same information needs, to define the amount of energy generated by wind energy, photovoltaic energy, fuel cell and the storage battery charging and discharging taking into account the following considerations:
  • •
    The wind power generation strongly

Case study

For the application of the methodology we developed a digital program GAMS [13], where has been applied to real case corresponding to Budapest Tech Renewable Equipment in Hungary. The considered prices are the following: Wind energy 0.4 €/kWh; photovoltaic 0.4 €/kWh; fuel cell 0.9 €/kWh; storage discharging 0.6 €/kWh; storage charging 0.4 €/kWh; undelivered power is 1.5 €/kWh and the excess energy is 0 €/kWh.
To illustrate the generality and the effectiveness of a proposed methodology we implemented a

Conclusion

In this paper we presented the optimal operation of an isolated system by a VPP. The main goal is to decide the best VPP management strategy to minimize the generation costs and optimize storage charging and discharging time subjected to all the operation technical constraints.
The dispatch has been formulated as a mixed-integer linear programming problem and solved by the developed program elaborated in GAMS Platform using CPLEX.
The VPP must control of all system to balance the generation and

Acknowledgments

The authors would like to acknowledge FCT, FEDER, POCTI, POSI, POCI and POSC for their support to R&D Projects and GECAD Unit.

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