Inertia Emulation-Oriented Evaluation Method of Sustaining Power Boundary for Lithium-Ion Battery Energy Storage System

In the low-inertia power system, the lithium-ion (Li-ion) battery energy storage system (BESS) is expected to provide virtual inertia support to the power system. However, the state-of-the-art output power boundary evaluation standards have not considered the time-varying feature of inertia emulation profile, based on which the inertia emulation capability of BESS cannot be accurately evaluated. Therefore, this paper firstly proposes the concepts of sustaining power (SP) and SP boundary (SPB) for evaluating the output boundary of BESS in inertia emulation condition. The sustaining current (SC) profile in inertia emulation condition that can opportunely make the response voltage reach the cut-off voltage is identified as the SC boundary (SCB), and the SPB can then be calculated. Then, an offline experimental SPB test method is proposed with improved efficiency, which can provide a predefined reference to the BESS operator and the grid before decision-making regarding frequency control. Applying the SC profiles on the BESS, the response voltage states are classified into three cases. In different cases, the SC profile is adjusted to approach the critical cut-off condition according to the proposed adjustment method. All the cases can achieve the critical cut-off condition with minimum attempts. The proposed SPB test method is validated by experimental tests on the 18650 Li-ion battery at different temperatures and with different state of charge (SOC). The experimental results indicate that the proposed SPB concept significantly increases the output boundary of battery, which can be captured offline with a limited number of trials under the proposed test method. The obtained SPB helps in quantifying the maximum virtual inertia that the BESS can deliver to the grid.