Achieving frequency stability through controlled active power injection using energy storage systems

Abstract

As solar Photovoltaic solar generation increases in the power system the conventional generators are being replaced or lose their dominance. Therefore, the resultant inertia of the system reduces. Due to this reason, in case of a large machine tripping in the system, or a line being tripped, the frequency deviation in a unit time increases compared to the fequency deviation in a system without high solar pentration. By using a fast active power injector to the system, such frequency deviation could be mitigated in a high solar power pentrated system. An energy storage system of such magnitude is not economically possible in the Sri Lankan context. During a fault in the system, Under Frequency Load Shedding (UFLS) occurs in the feeders and 33 kV feeders are diconected from the grid. A control mechanism has been developed to be used as an Uniterrupted Power Supply for the feeder. so that the feeder’s voltage and frequency are kept intact until the system is restored. By incorporating the said method, the selected feeder will not undergo any power interruptions during UFLS, as the fast active power injector which has been developed will cater to supplying the required power to the feeder. The fast active power injection method has been developed by combining both the virtual synchronous generator concept and the indirect current control pulse width modulation technique. Further, the research also addresses the selection criteria of the Battery Energy Storage System (BESS) to be implemented when developing the fast active power injector. The study has shown that a unity power factor 50 MW BESS can stop UFLS occurring in case of the tripping of a medium scale power plant or line tripping. and other common system faults. As such a BESS would not be economical, it also shows that a 13.7 MW, 3.88 MWh of BESS would be sufficient to prevent UFLS operating 90% of the time at night peak and day peak, with the UFLS operating probability during the off peak being less than 0.1%. Thus, the study has also shown that depleting the energy storage during the day to account for solar fluctuations would be to great advantage, with building up the battery energy reserves before night time.