Abstract

Application of micro-grids (MGs) as a solution for future energy systems are significantly increasing in recent years. On the other hand, more stringent requirements are added to grid codes (GCs) of low-voltage distribution networks. Accordingly, this paper proposes a low-voltage ride-through (LVRT) control scheme for four-wire multi-source MGs. The novel strategy is composed of two layers for controlling each phase of the grid-connected sources independently. The primary layer contains a reverse-droop function for each phase and fractional-order sliding-mode-control (FOSMC). The secondary layer determines the total requested reactive power for each phase during symmetrical and asymmetrical voltage drops. Then, the mentioned power is shared among the phases of each inverter based on the reactive power-sharing strategy and GC requirements. Based on the proposed grid-following controller, the voltage of faulty phases is compensated by reactive power injection. In addition, power quality indexes are kept in acceptable ranges during abnormalities, and the FOSMC performs better than the conventional methods. The effectiveness of the proposed scheme is verified through offline simulations in MATLAB/Simulink as well as validation by real-time results.

Introduction

۱٫۱٫ Background
The utilization of small-scale distributed energy resources (DERs) in low and medium-voltage networks is increasing, and as a result, their sudden and collective disconnection from the power system might cause voltage collapse on the grid during severe voltage perturbations. Therefore, the severe impact of the DERs cannot be ignored since their contribution to the power supply is considerable. Thus, the incremental usage of DER systems has become leverage for setting new requirements for power systems. These regulations are called grid codes (GCs), which vary from country to country and cover some grid-connected power sources such as wind turbines and photovoltaic (PV) systems [1]. The mentioned regulations force the sources to stay connected to the main grid during temporary voltage sags and support it by injecting reactive current. An essential requirement of the GC, which has gained much attraction in recent studies, is the low-voltage ride-through (LVRT) capability of the distributed generators (DGs). LVRT curves of the German and Danish GCs are highlighted in Fig. 1a [2]. Besides, in Fig. 1b amount of the reactive power injection per voltage drop is depicted for different scenarios [3]. Most studies cover symmetrical voltage disturbances that only include a minor percentage of the faults such as [4,5], etc. However, asymmetrical faults such as single-phase short-circuits are more probable with an 85 % occurrence rate [6]. In most cases, the conventional three-phase control methods for compensating voltage drops are not applicable for asymmetrical faults since they might affect the operation of safe phases by deteriorating voltage imbalance. Also, reactive power-sharing and reference power tracking in multi-source MGs are important operation factors, which has not been addressed sufficiently in previous studies.

Conclusion

MGs integration into power systems has boosted the system’s reliability and caused operational challenges for the network operators. Hence, some countries have considered a set of requirements to improve the functionality of their power systems. For this purpose, in this paper, an LVRT scheme is proposed for four-wire MGs, which can control each phase of the DGs independently through a grid-following method. The local controller of the DGs, known as the primary layer, utilizes reverse-droop control for voltage and frequency regulation and a nonlinear FOSMC. On the other hand, the secondary layer fulfills the modern GC requirements by calculating the required reactive power for the faulty phases. Next, this amount is shared between the corresponding phases of the DGs based on their free capacity. The effectiveness of the presented controller is proved by analyzing the results obtained from implementing 30 % single-phase, two-phase, and three-phase voltage sags. Accordingly, each DG phase injects reactive power during voltage sags according to the determined amount. Also, comparing the injected reactive power with the amounts of German GC demonstrates that the reactive power injection is done with acceptable accuracy. The results point out that during the external sags, the voltage of PCC, inverter terminals, and load terminals are restored to their nominal values, 30 % compensation, as if there is no perturbation. Consequently, the presented controller can realize the GC requirements and ride through during severe voltage conditions. Moreover, this control method keeps the power quality indices like THD and voltage imbalance factors in acceptable ranges during the sags. Also, the superiority of designed FOSMC over conventional current controllers such as PI and SMC are demonstrated. This outperformance is highlighted with several aspects such as MPAE index, THD values, voltage compensation, and reactive power tracking during the imposed voltage sags. It is shown that the best value for the alpha is 0.76, by which the response of the controller is fast and accurate. Finally, the simulations are validated through real-time results. Our future work is focused on the LVRT capability of the MGs including both grid-forming and grid-following inverters. In addition, the effect of grid strength and R/X ratio of the power lines can be discussed in the future investigations, which has not been addressed sufficiently in this literature.