Phase-locked loop (PLL) is commonly used for three-phase grid-connected inverters to obtain the information of grid synchronization, and PLL dynamics are the key factors for stable operation of the inverters. Under weak grid conditions, the coupling between PLL and grid impedance can result in harmonic resonance, or even instability in the system. In this paper, the effect of PLL dynamics and grid impedance on the stability of three-phase grid-connected inverters is studied with d-q impedance model. Besides, the variable transfer relationship is used to analyze the influence of PLL perturbations on output current under weak grid conditions. To suppress low order harmonics of the network current caused by PLL perturbations under weak grid conditions, a novel feed-forward control method is proposed to compensate PLL perturbations and revise the output impedance, where the operation of the inverter and PLL dynamics have been taken into account in the design process. By analyzing the impedance characteristic of the system, it can be demonstrated that the proposed method can enhance the stability of grid-connected inverters under weak grid conditions and reduce the impact of PLL perturbations on grid-connecting current. The experimental results indicate that the low order harmonics of the network current can be suppressed effectively, which verifies the analysis.

This paper proposes a new sensorless control scheme of interior permanent magnet synchronous motors (IPMSM) using high-frequency (HF) orthogonal square-wave voltage injection into stationary reference frame (SRF). This scheme is an improvement on the conventional HF rotating sinusoidal voltage injection into SRF. Firstly, discrete sequences of HF currents are applied to establish the whole signal process. Then, a position estimation method without using low-pass filters is proposed and the phase shift caused by high-pass filter is also reduced. Therefore, the proposed sensorless control method would not be adversely affected by digital filters, which can solve the common drawbacks of the current SRF-based injection methods. Furthermore, digital delay in drive systems is considered and its negative influence can be eliminated effectively as well. The sensorless control method is verified by experiments on a 2.2-kW IPMSM drive platform.

The shared energy storage service provided by independent energy storage operators (IESO) has a wide range of application prospects, but when faced with the interrelated and uncertain output of renewable energy on the supply side, how to size for energy storage capacity is a highly challenging problem. To this end, this paper firstly proposes a hybrid shared energy storage framework, in which the private energy storage of power suppliers and IESO jointly provide shared energy storage services for users. To generate low-dimensional scenarios that consider the correlation between the multiple uncertainties of wind and solar generation, combining the Kernel function estimation and the Gaussian mixture model forms the Bayesian estimation to fit the historical generation curve precisely. The uncertain output of same generation resources is coupled based on Kullback-Leibler divergence. The Frank-Copula function is used to couple the uncertain output between diverse generation resources. The energy storage planning problem is formulated as a Bayesian distributionally robust optimization model. The related algorithms are designed to solve the model. The effectiveness of the proposed uncertainties coupling and capacity sizing method is validated through a case study in the IEEE 118-bus system.

Although high-frequency circulating current exists in parallel-operated inverters, it has not attracted as much attention as low-frequency circulating current, and thus has not been analyzed in detail previously. In this paper, a mathematical model and analysis are presented, which reveal the spectrum components and their changing trends of the circulating currents caused by carrier phase differences. The impact of carrier phase difference on high-frequency circulating current is also analyzed and verified. A closed-loop control scheme is proposed to suppress the switching frequency circulating current caused by carrier phase difference. In addition, the deadbeat and closed-loop control are combined to suppress the low- and high-frequency circulating currents simultaneously, to improve the overall performance of circulating current suppression. The effectiveness of the proposed scheme is verified by the simulation and experimental results acquired from a laboratory platform.

Under weak grid condition, negative influence is brought in on system stability because the grid impedance and the grid-connected converter control system are coupled to each other. The key factor affecting the stability of the system is the perturbation of the voltage fluctuation at the point of common coupling (PCC) to the phase-locked loop (PLL) and the dc-link voltage loop. In this paper, the impedance model of three-phase grid-connected converter including the current loop, PLL and dc-link voltage loop in d-q frame is adopted, because it includes coupling information of the converter and reflects the transferring relationship among grid-connected system variables. Based on the transferring relationship, this paper analyzes the perturbation paths of PCC voltage and proposes the corresponding compensation methods, i.e. the PLL and dc-link voltage loop compensation terms, which are inserted into the q-axis and d-axis of the current loop controller respectively. The impedance analysis results show that the proposed method can effectively improve the system stability. In addition, the experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed method.

The asymmetrical rotating (dq-) frame control leads to frequency coupling dynamics, which tends to bring in harmonic instability. In this paper, based on the established complex signal impedance model, the influence of the grid impedance and the frequency coupling on system stability is analyzed. For the phase-locked loop (PLL) mainly affects the q-axis dynamics, it is proposed that the q-axis feedforward and the d-axis compensation control methods to decrease the asymmetric influence caused by the PLL. Similarly, for the dc-link voltage controller mainly affects the d-axis current reference, this paper proposes a d-axis feedforward and q-axis compensation method to improve the asymmetry dynamics introduced by the dc-link voltage controller. When the proposed methods are adopted, the components introduced by the PLL and the dc-link voltage controller in the coupling terms are eliminated, thereby achieving the purpose of suppressing the frequency coupling phenomenon and improving the system stability. The theoretical analysis and the experimental results show that the proposed methods are effective.