With global resource shortages and environmental degradation, the development of clean energy has become a key focus. The increasing integration of distributed renewable energy sources like solar and wind into the grid presents challenges due to their instability, affecting microgrid voltage and frequency. Frequency stability is crucial for the grid operation and user safety, with fluctuations potentially causing equipment failures. The Power Conversion System (PCS), as the core of energy storage systems, must provide traditional charge/discharge functions and off-grid power during outages for critical loads. Current PCS parallel control methods include interconnected and non-interconnected approaches, with the latter offering advantages such as module independence and reliability but prone to instability due to a lack of inertia. Introducing Virtual Synchronous Generators (VSG) can enhance stability against power disturbances and increase system capacity. However, improper control parameter settings in multi-unit VSG systems can lead to active power oscillations during grid voltage and frequency changes, affecting overall stability. Thus, effective solutions for multi-unit parallel stability in non-interconnected control are crucial. In response to the inadequacies of existing technologies in addressing stability issues in parallel control without interconnection lines, this paper proposes an improved VSG operation strategy. Aimed at finely controlling the VSG system by combining droop control and an improved second-order generalized integrator phase-locked loop, the strategy aims to enhance its stability and performance under parallel control without interconnection lines. The simulation results show that the control strategy can effectively solve the active power oscillation caused by the sudden change of voltage and frequency of the power grid, so as to ensure the stability of the whole system, improve the detection accuracy of the power grid, and reduce the THD of the multi-unit VSG system in the transient and steady state.