Frequency-based control of a micro-grid with multiple renewable energy sources

Abstract

Abstract-Stand-alone micro-grids need a proper management of the active power exchange. This work is focused on the parallel operation of multiple grid-connected converters in an islandgrid system. The proposed solution features a master inverter which emulates the grid and multiple grid-connected converters operating in parallel. The current sharing and overload protection is achieved by small frequency variations of master inverter's output, that are detected by the grid-connected converters. This mechanism exploits the behavior of the derating characteristics embedded in grid-connected inverters, that must reduce the output power if the grid frequency increases. In this case, standard grid-connected equipment can be used to realize microgrids without the need of digital communication between the power units. Two possible scenarios are analyzed: low-power microgrid with master/slave converters, and low voltage grid fed by a Smart Transformer (ST) which performs the frequency control.

Control and communication in the Smart Transformer-fed grid

“…It allows the ST to modify the LV side voltage waveform to intact with the DERs and loads with very limited impact on the MV grid. In [40], [29], two complementary control algorithms have been proposed to adapt the frequency and voltage to the power generation and consumption in order to achieve better power sharing and deal with some practical issue, such as overload and reverse power flow. Meanwhile, the load sensitivity should be identified to enable smooth operation during the LV frequency/voltage regulation.…”

Frequency adaptive control of a smart transformer-fed distribution grid

“…for voltage control). Of particular interest is the possibility to adapt the frequency to interact with the distributed generation [8] in order to change the power sharing between ST and distributed generation [9][10]. It is worth noticing that this solution has very limited impact on the main grid thanks to the electrical decoupling between the MV grid and LV grid.…”

Control strategies for multi-frequency power transfer in a smart-transformer-fed low-voltage grid

“…The ST is a solid-state transformer, already adopted in railway, harbor, and data center applications. In addition to implementing the step-down conversion, the ST can offer a series of ancillary services to the grid that are otherwise not possible [13], [14].…”

“…As a matter of fact, self-synchronizing solutions were presented in literature for single-frequency systems [14], and can be extended to this case. In this way, a method based on droop control between the transmitter and the ST is proposed, meeting the power demand of the receiver.…”