Control Strategy of Three-Phase Battery Energy Storage Systems for Frequency Support in Microgrids and with Uninterrupted Supply of Local Loads

 

ABSTRACT

Frequency control in autonomous microgrids (MG) with high penetration of renewable energy sources represents a great concern to ensure the system stability. In this regard, this paper presents an enhanced control method for battery energy storage systems (BESS) to support the frequency of MG and with the ability of disconnecting from the MG to supplying in the island mode a local consumer. A frequency controller, combining a conventional droop control with an inertia emulation function, governs the BESS active power transfer during the primary frequency control level. The BESS may also provide voltage support in the point of common coupling with the MG. Moreover, the proposed BESS may compensate, partially or totally, the power absorbed by the local loads in order to improve the MG frequency response. When the MG power quality worsens below a certain level, in terms of voltage and frequency, the BESS detaches from the MG and continues to operate islanded.

The reconnection is accomplished following a smoothly resynchronization of the local voltage with the MG, without disturbing the local loads supply. Additionally, this paper also discusses about the aspects related to the BESS management and its integration within the proposed system. The simulation and experimental results assess the feasibility of the proposed control solutions. Frequency control in autonomous microgrids (MG) with high penetration of renewable energy sources represents a great concern to ensure the system stability. In this regard, this paper presents an enhanced control method for battery energy storage systems (BESS) to support the frequency of MG and with the ability of disconnecting from the MG to supplying in the island mode a local consumer. A frequency controller, combining a conventional droop control with an inertia emulation function, governs the BESS active power transfer during the primary frequency control level. The BESS may also provide voltage support in the point of common coupling with the MG.

Moreover, the proposed BESS may compensate, partially or totally, the power absorbed by the local loads in order to improve the MG frequency response. When the MG power quality worsens below a certain level, in terms of voltage and frequency, the BESS detaches from the MG and continues to operate islanded. The reconnection is accomplished following a smoothly resynchronization of the local voltage with the MG, without disturbing the local loads supply. Additionally, this paper also discusses about the aspects related to the BESS management and its integration within the proposed system. The simulation and experimental results assess the feasibility of the proposed control solutions.

 

KEYWORDS

  1. Battery energy storage systems (BESS)
  2. Frequency control
  3. Inverter, microgrid (MG)
  4. Seamless transfer

 

SOFTWARE: MATLAB/SIMULINK

  

BLOCK DIAGRAM:

 

 

 

 

 

Fig. 1 BESS Structure

 

EXPECTED SIMULATION RESULTS:

 

 

 

 

 

Fig. 2. MG frequency (Top) and BESS active power (Bottom) for different operating conditions (simulation results).

 

 

 

 

 

Fig. 3 MG frequency (Top) and BESS active power (Bottom) for different levels of the local load compensation

 

CONCLUSION

This paper presented a Battery Energy Storage Systems BESS mainly designed to provide frequency support in MG, but having special control features. The BESS can operate both connected to the MG (G-mode) or in (I-mode), whereas the transition between the two states is seamlessly coordinated by an original control method. The BESS may serve local sensitive consumers connected on the local bus, by including special control functions to protect them in adverse MG operating conditions. The BESS management is also taking into discussion from the perspective of its influence upon the proposed controller performance. Simulations and experimental results were provided to validate the proposed BESS. An improved frequency controller, with conventional droop and virtual inertia was proposed and in the simulation results, it proved to be an efficient solution, resulting in faster damping of the MG frequency oscillations. Moreover, by partially or totally compensating the local loads, the MG is relieved by the corresponding power disturbance produced by their stochastic operation and thus the MG frequency deviation can be diminished.

By this approach, the BESS along with the local loads may be considered as a sort of smart load. The transition between G-mode to I-mode took place when the PCC power quality worsened and the experimental results showed a clean transfer without important voltage and frequency variations. The transition between I-mode to G-mode included a smoothly synchronization period of the local voltage with the MG voltage, after which the switching to G-mode did not disturb either the local loads or the MG. During I-mode, the local loads are supplied directly by the BESS and the presented experimental results including a comprehensive operating case, proved that the voltage control quality falls into the required standards. Future studies are intended to be carried out on the system availability to contribute to the MG power quality improvement.

 

REFERENCES

  • European Commission, Energy Roadmap 2050, 2011. [Online].Available: http://ec.europa.eu/energy/energy2020/roadmap/index_en.htm
  • Bevrani, A. Ghosh, and G. Ledwich, “Renewable energy sources and frequency regulation: Survey and new perspectives,” IET Renew. Power Gen., vol. 4, no. 5, pp. 438–457, Sep. 2010.
  • Tan, Q. Li, and H. Wang, “Advances and trends of energy storage technology in Microgrid,” Int. J. Elect. Power Energy Syst., vol. 44, no. 1, pp. 179–191, Jan. 2013.
  • Bottrell, M. Prodanovic, and T. C. Green, “Dynamic stability of a microgrid with an active load,” IEEE Trans. Power Electron., vol. 28, no. 11, pp. 5107–5119, Nov. 2013.
  • A. P. Lopes, F. J. Soares, and P. M. R. Almeida, “Integration of electric vehicles in the electric power system,” Proc. IEEE, vol. 99, no. 1, pp. 168–183, Jan. 2011.

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