Abstract
A self-adaptive controller is proposed to operate inverter in both grid-connected (GC) state and stand-alone (SA) state and ensure seamless transfer between them without reconfiguring the control structure. The controller is mainly composed of phase-locked-loop (PLL) and two cascaded control loops. The role of PLL is not only to estimate the frequency but also to generate the reference of voltage loop. When the grid is normal, the reference and the feedback value of voltage loop are always equal and the proposed controller accurately regulates the grid current of inverter. When islanding occurs, the inverter can automatically convert from current control mode to voltage control mode without critical islanding detection. The self-adaptive controller has several advantages. First, compared with direct current control, it can operate in both strong and weak grid conditions. Second, its dynamic performance in the GC state is much better than that of indirect current control. Third, when the grid returns to normal, the pre-synchronization of inverter's output voltage and grid voltage can be achieved automatically without any extra control algorithm. Finally, simulation and experimental results are provided to verify the effectiveness of proposed self-adaptive controller.
Highlights
With the substantial increased demand of energy and ever-worsening environmental pollution, distributed energy resources (DERs) have attracted more and more attention
Under the droop control method, the inverter in DER is controlled as a voltage source in GC and SA states; the load voltage quality is guaranteed during the transition from the GC state to SA state
A novel self-adaptive controller for inverter is proposed in this paper to operate inverter in both grid-connected (GC) state and stand-alone (SA) state and ensure seamless transfer between them without reconfiguring the control structure
Summary
With the substantial increased demand of energy and ever-worsening environmental pollution, distributed energy resources (DERs) have attracted more and more attention. The duration of this state depends on the time of the islanding detection employed, varying from 20 milliseconds to a few hundreds of milliseconds, no matter which kind of islanding detection scheme is adopted: remote, local passive and local active [19], [20]; and the quality of load voltage heavily depends on it To overcome this drawback, the self-adaptive control methods have been proposed. In the GC state, the voltage references produced by the PLL always equal the actual voltage feedback values; the capacitor voltage loop does not work and the inverter is controlled as a current source. The grid current reference can be designed freely and it doesn’t influence the dynamic characteristic of inverters It can overcome the limitation of direct current control.
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