Multilayer Control for Inverters in Parallel Operation without Intercommunications

In this paper, a multilayer control is proposed for inverters able to operate in parallel without intercommunications. The first control layer is an improved droop method that introduces power proportional terms into the conventional droop scheme, letting both active and reactive power to be shared among the inverters. The second layer is designed to compensate the voltage deviations caused by the mentioned droop control, thus improving the load-voltage regulation of the system. The third layer is a quasi-synchronization control aiming to roughly adjust the angle of the inverter to be close to the common ac bus. This layer ensures that the phase difference of each inverter inside a limited margin with the help of the phase signal sensed from the common ac bus. The principle of operation of the control scheme has been analyzed in detail. A small-signal model has been developed in order to study the system dynamics, which can be used for adjust the main control parameters. A prototype consisted of a two 35 KVA-inverter system has been build and tested in order to verify the feasibility of the proposed approach