PLL-Less Nonlinear Current-Limiting Controller for Single-Phase Grid-Tied Inverters: Design, Stability Analysis, and Operation Under Grid Faults

Abstract

Abstract-A nonlinear controller for single-phase grid-tied inverters, that can operate under both a normal and a faulty grid with guaranteed closed-loop stability, is proposed. The proposed controller acts independently from the system parameters, does not require a phase-locked loop (PLL) and can achieve the desired real power regulation and unity power factor operation. Based on nonlinear input-to-state stability theory, it is analytically proven that the inverter current always remains below a given value, even during transients, independently from grid variations or faults (short circuit or voltage sag). The desired performance and stability of the closed-loop system are rigorously proven since the controller has a structure that does not require any switches, additional limiters or monitoring devices for its implementation. Therefore, nonlinear stability of a grid-tied inverter with a given current-limiting property is proven for both normal and faulty grid conditions. The effectiveness of the proposed approach is experimentally verified under different operating conditions of the grid.

Current-Limiting Droop Control of Grid-Connected Inverters

“…This allows a simple change of the operating modes of the inverter from the droop mode to the set mode at any time. In fact, this is one of the main differences of the proposed controller compared to the one in [38]. The control structure (6), together with the control dynamics (7)- (10), allow the regulation of both the real and the reactive power as well as the implementation of droop functions, but the approach in [38] can only achieve unity power factor.…”

“…Additionally, external limiters and saturation units are often added into the current or voltage loop control loops to achieve the desired current-limiting property, but these approaches can lead to undesired oscillations and instability [35]- [37]. A controller for grid-connected inverters that partially overcomes these limitations has been recently reported in [38], based on a bounded control structure [39], but only unity power factor can be accomplished. Hence, the reactive power could not be controlled and remains always close to zero.…”

“…It is also shown that the current-limiting property is guaranteed independently from grid voltage and frequency variations, extending the application of the controller to gridfault cases. With comparison to [38], a generic control structure is proposed to achieve both current-limiting and droop control. Moreover, the proposed approach can control both the real and the reactive power of the inverter, providing the opportunity to switch between two operating modes for gridconnected inverters: i) real and reactive power regulation to reference values and ii) droop control.…”

A Novel Parameter-independent Fictive-axis Approach for the Voltage Oriented Control of Single-phase Inverters

“…To cope with these requirements, multilevel inverters, modified topologies, high order passive filters and different grid current control strategies have been explored [9]- [16]. Recently, different current controllers for single-phase controllers have been published, such as: hysteresis control, dead beat control, Proportional Integral (PI), proportional resonant current regulators, PLL compensators, fuzzy controllers, non-linear controllers and Voltage Oriented Control (VOC) [2], [16]- [25].…”

“…However, reactive power regulation and compatibility with advanced filters have not been covered in recent work [22], [25].…”