This article analyzes the global stability of synchronous reference frame phase-locked loops (SRF-PLLs) from a large signal viewpoint. First, a large-signal model of SRF-PLL is accurately established, without applying any linearization method. Then, according to the phase portrait and Lyapunov argument, the global performance of SRF-PLL is discussed in the nonlinear frame. Compared with the small-signal analysis methods, the proposed analysis, not relying on the small-signal model and linearization method, provides a global discussion of the SRF-PLL performance. The contributions of this article are as follows. First, it is found that SRF-PLL has infinite equilibrium points, including stable points and saddle points. Second, it provides a way to divide the global region of SRF-PLL into many small regions. In each small region, the SRF-PLL only has one stable equilibrium point. And for any initial states $(\tilde{\theta }(t_0),\tilde{\omega }(t_0))$ in a small region, all states $(\tilde{\theta }(t),\tilde{\omega }(t)),t>t_0$ will remain in this small region, and SRF-PLL will converge to the unique stable equilibrium point of this small region. Third, by dividing the global region of SRF-PLL into many small regions, it is found that when the frequency of grids varies largely, the SRF-PLL will converge to a new equilibrium point that is far away from the original equilibrium point. It is the reason why the frequency convergence of SRF-PLL has many oscillations and SRF-PLL has a rather slow dynamic, when the frequency changes largely. The experimental results are provided to verify the proposed global stability analysis of SRF-PLL.

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.

As an important ancillary service for smart grid, voltage support has the capability to improve the grid condition when grid voltage unbalanced fault occurs. Several solutions, taking into account not only the root mean square voltage, but also the shape of the voltage fault, have been proposed. In particular, the control scheme proposed in this study introduces two contributions: a novel reference generator for voltage support and a new current tracking controller. Taking into account of the shape of the grid voltage, this reference generator has the capacity to increase the positive sequence fundamental voltage magnitude and decrease the negative sequence fundamental voltage magnitude, which improves the grid condition when unbalanced fault and voltage sags occur. Compared with other works, phase locked loop is eliminated in this generator which simplifies the structure of the controller. On the basis of output regulation theory, a simple‐structure current tracking controller is proposed, which has a zero steady error under arbitrary order (or their combination) grid voltage harmonic disturbance. Simulations and experiments have verified the proposed control scheme.

To overcome the two shortcomings of the traditional phase-locked loop (PLL): incapability in realizing speed reversal and an unsatisfactory position tracking performance between dynamic and steady-state states, a new general PLL (GPLL) is proposed for permanent magnet synchronous motors sensorless control based on back electromotive force (BEMF) in both the stationary αβ-axis and the rotary dq -axis. Through constructing a new input error signal, the sign of the speed variable in position error signal can be eliminated. Nevertheless, this constructed error signal contains double-fundamental frequency position information, resulting in two sets of convergence points, i.e., 0ΰ or 180ΰ from the actual position. To avoid incorrect convergence, a polarity correction function, based on the relationship between estimated BEMF and estimated speed, is designed to cooperate with the GPLL. Then, the sensorless drive can converge into correct position both in positive and negative rotation conditions. Furthermore, an estimated speed feedforward compensation is integrated into the GPLL, then the position tracking error can be reduced in linear acceleration and deceleration conditions, which enhances the dynamic tracking ability of the sensorless drive. Finally, the GPLL are realized both in a sliding mode observer on αβ-axis and a Luenberger observer on dq -axis, and their effectiveness is verified in comparison with conventional PLL-based method and an active flux observer in terms of speed reversal ability, dynamic and steady-state performance, etc.

In aircraft grids, the voltage frequency is much higher than that in utility power systems and it challenges the dynamics of frequency estimations. In this letter, a fast and robust frequency estimation method is proposed for aircraft grids even when the grid voltage is distorted. In detail, a mathematical model is established based on the relationship among the fundamental grid voltage, harmonics and their time-delay signals. Accordingly, a robust parameter identification method is proposed to achieve grid frequency estimation and harmonic immunity. Compared with the frequency estimations for utility power systems, the proposed method, so-called robust frequency estimation (RFE), has faster dynamics and stronger harmonic suppression capability. Hence, it takes advantages in the applications of aircraft grids. Experimental results also indicate that the proposed RFE has good performance in terms of fast dynamics and strong robustness.

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.