A Coordinated Frequency Regulation Framework Based on Hybrid Battery-Ultracapacitor Energy Storage Technologies

The replacement of conventional electricity generators by wind turbines and solar photovoltaic panels results in reduced system inertia, which jeopardizes the electric power system frequency. Frequency variation is critical as it may cause equipment damage and blackouts. Frequency regulation (FR) plays a crucial role in sustaining the stability of electric power grids by minimizing the instantaneous mismatches between electric power generation and load demand. Regulation service (RS) providers dynamically inject/absorb electric power to/from the grid, in response to regulation signals provided by independent system operators (ISOs), in order to keep the frequency within the permissible limits. The regulation signals are highly transient and hence require quick responding resources in order to provide FR effectively. This paper proposes innovative design and operation frameworks for state-of-the-art battery-energy storage system (BESS) and ultracapacitor (UC)-based hybrid energy storage system (HESS) employed for FR in electricity market. The proposed system design framework is based upon the initial investment cost, replacement cost, maintenance cost, and financial penalty imposed by ISO on RS provider for not supplying the required RS. The proposed system operation framework allocates power to both BESS and UC based upon their maximum power ratings while fulfilling their constraints at the same time. The frequent partial charge-discharge transitions, which are detrimental for BESS, are reduced by using two battery banks instead of one large battery bank. The charging and discharging of two battery banks are controlled innovatively to reduce the transitions between the partial charge-discharge transitions. Moreover, a comparison based upon cost per unit between two cases, that is: 1) HESS employed for FR and 2) BESS employed for FR, is presented, which shows that the HESS is more economical.