Abstract
With the rapid development of the micro-grid associated with new and clean energies, the smooth switching between grid-connected and islanded operation modes of the micro-grid is a key issue that needs to be addressed urgently. In traditional solutions, V/f (Voltage/frequency) control is adopted for the master micro sources when the micro-grid works in islanded mode, while PQ (real and reactive power) control is adopted when in grid-connected mode. However, when the two controllers switch when mode transfer occurs, transient oscillations usually occur and thereafter the dynamic response will be degraded. This paper considers an archetypical micro-grid with Wind-PV-ES (Wind, Photovoltaic and Energy Storage) hybrid system, which forms the basis of our case study. The underlying reason for such transient oscillation is analyzed in this paper. Thereafter a modified control strategy for seamless mode transfer is designed and implemented. An improved PQ control method is designed by which the output of the PQ controller always synchronously tracks the output of the V/f controller for micro-grid switches from islanded mode to grid-connected; furthermore, a dq rotating coordinate synchronization based V/f control method is proposed for transition from grid-connected mode to islanded mode. Finally, experiments and analysis are undertaken on some basic and important operating cases; the results in our case study indicate that the modified control strategy is effective in dominating the micro-grid during mode transfer and thus yielding significantly better performances.
Highlights
In recent years, micro-grids, as the low-voltage distribution systems, have captured considerable attention due to the many advantages they afford including enhanced reliability, scalability, and flexible control when compared with large centralized power systems
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In order to ameliorate the effect of power quality during mode transfer, this paper establishes the underlyinsssg cause of the transient oscillation associated with mode transfer via a case study focusing on an archetypical micro-grid with the Wind-PV-ES hybrid system
Summary
Micro-grids, as the low-voltage distribution systems, have captured considerable attention due to the many advantages they afford including enhanced reliability, scalability, and flexible control when compared with large centralized power systems. To limit the switching transient and enables a smooth mode transfer from islanded to grid-connected mode, a modified voltage based droop control method was proposed in [11]. Motivated by the aforementioned issues, the aim of this paper is to bring to light the underlying reason of the transient oscillations associated with a case study, and to develop a modified control strategy for seamless mode transfer in micro-grids. It can be seen that there are great output voltage saltations both in d and q axis coordinates This is because the V/f control method is switched to PQ strategy when the micro-grid switches from islanded mode to grid-connected mode, while the outputs of the two controllers are different.
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