A First-Order PLL-Synchronized Voltage-Supporting Control Method for Parallel Inverters With Near-Zero Line Impedance

Synchronization and power-sharing control are essential for islanded parallel inverters. Traditionally, droop control and other grid-forming control are the typical methods with voltage-supporting abilities. However, those methods only apply to systems with a certain line impedance. For the application of uninterruptible power supply (UPS) systems with near-zero line impedance, droop-controlled inverters are prone to loss of stability. To address this issue, this letter introduces a new first-order phase-lock-loop (PLL) synchronized voltage-supporting control method for the islanded operation of parallel inverters in UPS systems. It is found that the first-order PLL is employed for frequency synchronization in islanded operation. Meanwhile, voltage support and power sharing can be guaranteed with modified current control. The experimental results validate the feasibility of the proposed method.