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This paper surveys current literature on modeling methods, control techniques, protection schemes, applications, and real-world implementations pertaining to grid forming inverters (GFMIs). Electric power systems are increasingly being augmented with inverter-based resources (IBRs). While having a growing share of IBRs, conventional synchronous generator-based voltage and frequency control mechanisms are still prevalent in the power industry. Therefore, IBRs are experiencing a growing demand for mimicking the behavior of synchronous generators, which is not possible with conventional grid following inverters (GFLIs). As a solution, the concept of GFMIs is currently emerging, which is drawing increased attention from academia and the industry. This paper presents a comprehensive review of GFMIs covering recent advancements in control technologies, fault ride-through capabilities, stability enhancement measures, and practical implementations. Moreover, the challenges in adding GFMIs into existing power systems, including a seamless transition from grid-connected mode to the standalone mode and vice versa, are also discussed in detail. Recently commissioned projects in Australia, the UK, and the US are taken as examples to highlight the trend in the power industry in adding GFMIs to address issues related to weak grid scenarios. Research directions in terms of voltage control, frequency control, system strength improvement, and regulatory framework are also discussed. This paper serves as a resource for researchers and power system engineers exploring solutions to the emerging problems with high penetration of IBRs, focusing on GFMIs.INDEX TERMS Current control, fault ride-through, grid forming inverters, power synchronization control, small-signal and transient stability, virtual inertia.

This paper presents a new function-based modulation control technique for modular multilevel converters (MMC). The main contribution of the study is the formulation of two new modulation functions for the required switching signals of the MMC's upper and lower submodules respectively. The output and circulating current equations of the converter are employed to attain the arm's currents which are utilized for the proposed modulation functions, which have two important features: i) it is much less complex compared to the existing control methods of MMC; and ii) the proposed controller can be regulated properly to deal with parameter variations in a bid to ensure stable and accurate performance. In this controller, the MMC output current magnitude and phase angle required for special active and reactive power sharing can be easily applied to the modulation functions. Also, the equivalent capacitors of upper and lower sub-modules are discussed based on the proposed modulation functions. Finally, simulations are performed in Matlab/Simulink environment to evaluate the performance of the proposed control technique in both the dynamic conditions of load as well as varying parameters.

This paper deals with a new islanding detection method based on parallel inductive impedance (PII) switching at distributed generation (DG) connection point, and monitoring the rate of change of voltage (dv/dt) at DG output. In the proposed approach, switching the PII causes variation in the dv/dt. This variation is very small in the case that DG operates in parallel with the main grid, whereas the changes will be very large during the occurrence of islanding. Therefore, the dv/dt is employed to identify both the islanding and non-islanding situations; however, to better analyze the dv/dt changes, the Fast Fourier Transform (FFT) is employed to process the variation of the dv/dt. To select the type of inserted impedance, the effect of switching various impedance types on the FFT(dv/dt) is thoroughly analyzed. It is demonstrated that the purely inductive impedance has better performance than other impedance types. The performance of the proposed method is examined through comprehensive simulation studies in MATLAB. The simulation results indicate that the proposed method retains its efficiency for both inverters-and synchronous-based DGs. Moreover, the proposed method not only has a substantial influence on the reduction of disturbances due to the small amount of PII, but it also eliminates the non-detection zone entirely in comparison with other available methods.Index Terms--Islanding, parallel inductive impedance switching, fast Fourier transform, the rate of change of voltage.

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Abstract:In this paper, a novel modulation function-based method including analyses of the modulation index and phase is proposed for operation of modular multilevel converters (MMCs) in high voltage direct current (HVDC) transmission systems. The proposed modulation function-based control technique is developed based on thorough and precise analyses of all MMC voltages and currents in the a-b-c reference frame in which the alternating current (AC)-side voltage is the first target to be obtained. Using the AC-side voltage, the combination of the MMC upper and lower arm voltages is achieved as the main structure of the proposed modulation function. The main contribution of this paper is to obtain two very simple new modulation functions to control MMC performance in different operating conditions. The features of the modulation function-based control technique are as follows: (1) this control technique is very simple and can be easily achieved in a-b-c reference frame without the need of using Park transformation; and (2) in addition, the inherent properties of the MMC model are considered in the proposed control technique. Considering these properties leads to constructing a control technique that is robust against MMC parameters changes and also is a very good tracking method for the components of MMC input currents. These features lead to improving the operation of MMC significantly, which can act as a rectifier in the HVDC structure. The simulation studies are conducted through MATLAB/SIMULINK software, and the results obtained verify the effectiveness of the proposed modulation function-based control technique.