This letter presents an adaptive frequency locked loop with fixed-length transfer delay units for single-phase systems. By analyzing the relationship between the grid voltage and its transfer delay signals, a linear regression model of the grid voltage is established. Accordingly, a transfer delay-based adaptive frequency locked loop (TD-AFLL) is proposed. A mathematic proof indicates that the proposed TD-AFLL can reject both phase offset errors and double-frequency oscillatory errors. Thus, the grid voltage parameters can be estimated accurately, even when the frequency drifts away from its nominal value. Moreover, fast dynamics of the TD-AFLL are achieved due to the transfer delay structure. Experiments verify the effectiveness of the proposed method.

Distributed renewable energy systems play an increasing role in today’s energy paradigm. Thus, intensive research activities have been centered on improving the performance of renewable energy systems, including photovoltaic (PV) systems, which should be of multiple-functionality. That is, the PV systems should be more intelligent in the consideration of grid stability, reliability, and fault protection. Therefore, in this paper, the performance of single-phase grid-connected PV systems under an extreme grid fault (i.e., when the grid voltage dips to zero) is explored. It has been revealed that combining a fast and accurate synchronization mechanism with appropriate control strategies for the zero-voltage ride-through (ZVRT) operation is mandatory. Accordingly, the representative synchronization techniques (i.e., the phase-locked loop (PLL) methods) in the ZVRT operation are compared in terms of detection precision and dynamic response. It shows that the second-order generalized integrator (SOGI-PLL) is a promising solution for single-phase systems in the case of fault ride-through. A control strategy by modifying the SOGI-PLL scheme is then introduced to single-phase grid-connected PV systems for ZVRT operation. Simulations are performed to verify the discussions. The results have demonstrated that the proposed method can help single-phase PV systems to temporarily ride through zero-voltage faults with good dynamics.

The complex-coefficient filter-based phase-locked loop is widely used in state estimation for the constant frequency grid system. However, its performance under power systems with a variable frequency is generally unacceptable and related applications are consequently rare. This letter proposes an adaptive enhanced complex-coefficient filter-based phase-locked loop (AECCF-PLL) that accurately estimates the operational state of variable frequency grids. By deriving the relationship between model parameters and frequency, a frequency adaptive module is established to dynamically correct the model parameters in real time. Furthermore, a stability analysis of the system is provided to meet the requirements of stability and fast response for state estimation in more electric aircraft variable frequency grids and the computational workload is relatively small. Experimental tests show strong evidence for the effectiveness and validity of the proposed AECCF-PLL method.

Phase-locked loops (PLLs) are widely popular in applications of grid synchronization for single- and three-phase utility power systems. However, these PLL-based methods are usually designed for constant frequency grids and these schemes only have fast dynamics and good accuracy in a small local region (i.e., phase locked region). When applying these PLLs in the grids of more electric aircraft (MEA), in which the grid frequency varies in a wide range (360 Hz-800 Hz), both the dynamic response and the steady-state accuracy of PLLs reduce significantly. In light of this, this paper presents a global convergence method to estimate the frequency, positive- and negative-sequence components of grid voltage in MEA power systems. By analyzing the relationship between the DC-offset component, the grid voltage and their derivatives, a mathematic model with the unknown parameters, which are the DC-offset component and the grid frequency, is established. Accordingly, a global estimator is proposed to estimate the grid voltage parameters accurately. A Lyaponov-based argument ensures that the proposed estimator is global convergent and it has the capability to eliminate the DC offset effect completely. The experimental results are provided to verify the effectiveness of the proposed estimator.

Taking the advantage of easy implementation in the discrete digital control system, the finite‐control‐set model predictive control (FCS‐MPC) is widely used in multi‐phase or multi‐level power converters. As the smallest independent operation unit in photovoltaic (PV) distributed power generation systems, the PV inverter has the most direct influence on the power quality. However, the lack of switching states and output voltage levels restricts the development of FCS‐MPC in single‐phase PV inverters. In order to improve the quality of the PV inverter output current, a constant switching frequency FCS‐MPC (CFS‐FCS‐MPC) method is proposed for single‐phase grid‐connected PV inverter in this paper, which can thus reduce output current ripples and lower harmonics. Compared with the conventional FCS‐MPC method, the proposed CFS‐FCS‐MPC method synthesizes the switching state with a nonzero and a zero switching state in one sampling period. In addition, an optimum allocation of switching states and execution time for the synthesized switching state is presented. Experimental results have verified the effectiveness of the proposed CSF‐FCS‐MPC method for the output current, achieving lower current ripples and higher current quality with a constant switching frequency.