An Enhancing Fault Current Limitation Hybrid Droop/V-f Control for Grid-Tied Four-Wire Inverters in AC Microgrids

Abstract

Microgrid integration and fault protection in complex network scenarios is a coming challenge to be faced with new strategies and solutions. In this context of increasing complexity, this paper describes two specific overload control strategies for four-wire inverters integrated in low voltage four-wire alternating current (AC) microgrids. The control of grid-tied microgrid inverters has been widely studied in the past and mainly focused on the use of droop control, which hugely constrains the time response during grid-disconnected operation. Taking into account the previous knowledge and experience about this subject, the main contribution of these two proposals regards providing fault current limitation in both operation modes, over-load capability skills in grid-connected operation and sinusoidal short-circuit proof in grid-disconnected operation. In the complex operation scenarios mentioned above, a hybrid combination of AC droop control based on dynamic phasors with varying virtual resistance, and voltage/frequency master voltage control for grid-(dis)connected operation modes are adopted as the mechanism to enhance time response. The two proposals described in the present document are validated by means of simulations using Matlab/Simulink and real experimental results obtained from CENER (The National Renewable Energy Centre) experimental ATENEA four-wire AC microgrid, obtaining time responses in the order of two-three grid cycles for all cases.

Special Issue on Microgrids

“…The authors from [1] propose overload control strategies for four-wire inverters in low voltage AC (Alternating Current) microgrids. The developed strategies provide a fast and appropriate fault current limitation in both operation modes, grid connected and grid isolated.…”

“…While [1][2][3] focus on a single microgrid, [4] proposes a hierarchical optimization method for the energy scheduling of multiple microgrids connected to the distribution grid with participation in the energy market. The optimization procedure is separated into two stages.…”

Impedance Measurement and Detection Frequency Bandwidth, a Valid Island Detection Proposal for Voltage Controlled Inverters

“…A droop control strategy is usually used in inverters (GSCI case) due to its ease of parallelization [4,5,9,10]. The droop control manages the active and reactive power, modifying the module and phase of the AC capacitor voltage U C [9,10] with respect to the PCC voltage U PCC .…”

“…The AI method suggested in this paper is based on a combination of some existing solutions [29,38,39], which have been selected and adapted to fit in an experimental platform with low availability of computational time. These high computational requirements are typical in the case of GSCI due to the extra control needs (more control loops) and extra secondary/tertiary loops to enhance droop control [4,5,9,10]. This situation makes it especially important not to overload the inverter source code with excessive computational burden to add an AIDM.…”

“…New control techniques, the use of information technologies, and the progress of power electronics have boosted alternative options for either grid-connected or grid-disconnected operation. Other behaviors like voltage-controlled (VC)-VSI inverters enhance grid flexibility services such as power sharing or non-zero voltage crossing between operation modes [2][3][4]. Currently, it is possible to distinguish between three main types of grid integration inverters [5]: -Grid supply inverters (GSI) are unidirectional CC-VSI when grid-connected, and their aim is to deliver the maximum power to the mains by means of maximum power point tracking (MPPT) algorithms [6].…”