Isochronous Architecture-Based Voltage-Active Power Droop for Multi-Inverter Systems

Abstract

Advanced microgrids consisting of distributed energy resources interfaced with multi-inverter systems are becoming more common. Consequently, the effectiveness of voltage and frequency regulation in microgrids using conventional droopbased methodologies is challenged by uncertainty in the size and schedule of loads. This article proposes an isochronous architecture of parallel inverters with only voltage-active power droop (VP-D) control for improving active power sharing as well as plug-and-play of multi-inverter based distributed energy resources (DERs). In spite of not employing explicit control for frequency regulation, this architecture allows even sharing of reactive power while maintaining reduced circulating currents between inverters. The performance is achieved even when there are mismatches between commanded reference and power demanded from the actual load in the network. The isochronous architecture is implemented by employing a global positioning system (GPS) to disseminate the clock timing signals that enable the microgrid to maintain nominal system frequency in the entire network. This enables direct control of active power through voltage source inverter (VSI) output voltage regulation, even in the presence of system disturbances. A small signal eigenvalue analysis of a multi-inverter system near the steady-state operating point is presented to evaluate the stability of the multi-inverter system with the proposed VP-D control. Simulation studies and hardware experiments on an 1.2 kVA prototype are conducted. The effectiveness of the proposed architecture towards active and reactive power sharing between inverters with load scenarios are demonstrated. Results of the hardware experiments corroborate the viability of the proposed VP-D control architecture.Index Terms-Common-clock/GPS, low-voltage multi-inverter microgrid, virtual impedance, voltage active power droop, voltage source inverter I. INTRODUCTIONM ODERNIZATION of rapidly dispatchable DERs to provide demand response and ancillary services are transforming emergent microgrids into advanced microgrids. Advanced microgrids enable additional flexibility, resilience and reliability for local resources as well as support of the large scale grid when connected [1]. These microgrids can be operated in both grid-connected as well as islanded modes. In the islanded mode, DERs support local loads in the microgrid through either centralized, distributed or de-centralized control architectures. Controllers employed either locally at individual DERs or at a microgrid level are responsible for stabilizing S. Patel, S. Chakraborty, M. Salapaka are with the