Grid-Forming Type Converter Decoupling and Resynchronization for an AC Microgrid

The Microgrid (MG) operating modes are Grid-Tied Mode (GTM) and Islanded Mode (ISM). However, the transitions from GTM to ISM (GTM-ISM) and from ISM to GTM (ISM-GTM) are currently studied as operation modes. To correctly perform GTM-ISM and ISM-GTM, in this paper an Islanding Detection Algorithm (IDA) and Grid-Synchronization Algorithm (GSA) are developed. The development of IDA and GSA is based on the DQ transformation and the IEEE 1547 Standard. Both IDA and GSA operate together as a secondary control level to provide synchronizing signals to the primary control level. The results show a correct IDA and GSA performance, which allowed the Grid-Forming Type Converter (GFTC) to be capable of carrying out both GTM-ISM and ISM-GTM, correctly. This is possible because of the cascade control schemes outstandingly perform tasks such as: to extract mains power, to charge the battery, to regulate the DC bus voltage, and to synchronize and to regulate the GFTC output voltage.