Decentralized Secondary Control for Frequency Restoration of Microgrids with VF and PQ Droop Controlled Inverters

This paper presents a decentralized secondary control scheme for a Microgrid (MG) containing both VF and PQ controlled inverters. The proposed secondary controllers are based on identical PI controllers to generate the appropriate compensation signals for frequency deviation. The action of the secondary controller is used to modify reference of the primary controller, locally for each DG unit. This active droop positioning mechanism restores the system frequency back to its nominal value. The VF inverters modify their frequency reference while PQ ones modify their active power reference in proportion to the frequency error. Therefore, all inverters participate in the frequency restoration process while preserving the active power sharing ratio thus preventing overloading of any unit and providing more reliable and secure operation. Different simulation scenarios are conducted using PSCAD/EMTDC environment to evaluate the effectiveness and the feasibility of the proposed control strategy. The results show that the proposed system succeeds in fulfilling the two objectives of the proposed secondary controller, to restore system frequency and to keep the active power sharing ratio according to the DG capacities.