Synchronization of Inverters in Grid Forming Mode

Abstract

This article compares two strategies for seamless (re)connection of grid-forming inverters to a microgrid powered by droop-controlled inverters. While an incoming inverter must be synced to the microgrid, seamless syncing and power-sharing are technical challenges for grid-forming inverters. In the first strategy, called the output-sync method, an incoming inverter is synced to the microgrid, and then the circuit breaker is closed for power-sharing. In the second strategy, called the controller-sync method, the inverter initially contributes at zero power following the microgrid frequency, and then the controller is transferred to the mode of power-sharing. Remarkably, the circuit breaker can be kept closed during the syncing process using the controller-sync method as the inverter starts from zero power contribution. In this novel strategy, two controller sets run in parallel, i.e., two parallel control paths are in place for obtaining the magnitude and the phase angle of the PWM reference signal. These parallel paths are synced for seamless transitions when only one of them is engaged to generate the PWM reference signal. The efficacy of these control strategies has been tested in a hardware setup of a microgrid fed by two 5kVA 208V droopcontrolled inverters, and the results are presented in this article. This article is also accompanied by a video clip demonstrating the performance of the output-sync and controller-sync methods in the laboratory.INDEX TERMS Autonomous synchronization, power-sharing, grid-forming inverter.

Design, Modeling, and Validation of Grid-Forming Inverters for Frequency Synchronization and Restoration

“…A seamless connection to the MG/grid is essential for both GFM and grid-following inverters. A synchronization method for inverters in GFM, known as the controller-sync, is proposed in [27]. In this approach, the inverter initially aligns with the MG frequency without contributing to power sharing.…”

Frequency, Phase, and Magnitude Difference Locked-Loop Based Linear Synchronization Scheme for Islanded Inverters and Microgrids

“…There are many studies in the literature on these topics. However, in the literature, the synchronization methods for Grid-forming inverters (GFIs) in the islanded microgrid are limited [10], [33], [34]. Also, the synchronization methods of islanded microgrids each other has yet to be sufficiently studied in the literature as well [35].…”

“…GFIs are generally used in Microgrids, wind, solar power plants, and hybrid distribution systems with different energy sources [36], [37], [38], [39]. As grid-forming inverters become more widespread, the need for advanced control strategies and algorithms to synchronize these inverters to the islanded microgrid or other GFIs becomes increasingly important [33], [34], [40], [41]. GFIs are an advanced type of inverter used in power systems to provide stable and reliable power to the grid [42], [43].…”

“…The parallel operation with synchronization methods for GFIs in IGs improves energy efficiency and increases voltage and frequency stability [45], [46], [47], [48], [49], [50]. However, many technical and operational challenges still need to be investigated because of limited studies for synchronization for GFIs in IG [5], [33]. This study investigates the nonlinearity caused by the initial condition dependency and the cross-coupling effect.…”

“…The cross-coupling occurs due to the simultaneous operation of PSL and VSL [89], [90]. In this case, it occurs voltage collapse and frequency oscillations [31], [33], [69], [71], [73]. The initial condition dependence and cross-coupling cause the conventional synchronization control system to be highly nonlinear [19], [20], [21], [22], [23], [57].…”