Implementation of Fault Ride-Through Techniques of Grid-Connected Inverter for Distributed Energy Resources With Adaptive Low-Pass Notch PLL

The amount of distributed energy resources (DERs) has constantly increased worldwide. As the power ratings of DERs have become considerably high, the grid code requirements are necessary to secure reliable power generation and transmission for the public electric network. In order to follow grid codes of the various countries and optimize the function of grid-connected inverters for DERs, a robust phase-locked loop (PLL) is essential for extracting the grid phase information accurately, when the grid voltage is polluted by harmonics and unbalanced. This paper proposes fault ride-through techniques based on a low-pass notch (LPN)-PLL. The LPN-PLL has not only fast and smooth transient responses to a sudden transition of the grid voltage but also has robustness to the distorted and unbalanced grid conditions. Therefore, the stable performance in the grid fault conditions is expected without the system trip. Furthermore, a universal voltage sag generator for the various grid codes with six parameters is proposed for the verification of low voltage ride-through (LVRT) performance. Experimental verifications are presented to show the LVRT performance based on the LPN-PLL with a three-phase grid-connected inverter (10 kV·A) and a prototype of the voltage sag generator (10 kV·A).