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.

Postfault oscillations in active power and voltage responses of grid-connected voltage source inverters (VSIs) have been reported in the literature. They are caused by nonideally tuned controllers and the implemented current limitations used for protecting the VSIs from overcurrents. These oscillations become even more significant when the VSI is connected to a weak grid, deteriorating the recovery process of the VSI. This article presents a method for damping the postfault oscillations by using an adaptive virtual resistor (VR). Even though these oscillations are observed with both grid-following and grid-forming inverters, this article focuses on droop-based grid-forming inverters. The proposed method dynamically integrates a VR into the VSI control and removes it in the normal operation mode of the VSI. This method is implemented in the synchronous reference frame, which is commonly used due to its decoupled active and reactive power control. The amount of virtual resistance used for oscillation damping is adaptive to the recovery rate of the VSI. Hence, the proposed method is robust against changes in grid strength. Finally, the performance of the method is evaluated in PSCAD/EMTDC and also experimentally validated.

The majority of inverter-based resources (IBRs) currently operate as grid-following inverters (GFLIs). However, these inverters exhibit certain stability issues when integrated in low-strength areas of the grid. To enhance the grid strength in a GFLI-dominant area, virtual synchronous generators (VSGs) can be installed. If the VSG and the GFLI are investigated as a paralleled system, despite the benefits brought by the VSG, in some cases, the transient angle instability process, caused by a voltage sag, is accelerated, making the whole system more prone to instability. Therefore, this paper aims to study the transient angle stability of a paralleled VSG-GFLI system via investigating the voltage at a common bus between the two IBRs. Subsequently, a stable region of the voltage angles of the two IBRs is determined. Based on this stable region, the stability margin of the paralleled system can be determined and used to quantitatively evaluate the system stability. Moreover, an additional control loop is proposed to improve the transient stability of the paralleled system in this paper. The proposed controller can adaptively reduce the power set-point of the VSG to avoid a complete failure of the VSG even when no stable equilibrium point exists during a voltage sag. The stability investigation and performance evaluation of the proposed method are conducted in PSCAD/EMTDC and experimentally validated.

A current limiter (CL), which protects virtual synchronous generators (VSGs) from overcurrents, may significantly alter the operation of the VSGs when activated. In this letter, a transient stability analysis of VSGs equipped with quadrature (q-)prioritized CL is performed. This study shows that in addition to the instability caused by the positive feedback of the primary controller, a VSG could become unstable after large disturbances due to the failure of the inner voltage controller. In this study, a stability criterion is provided for the voltage control loop.

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 nonislanding situations; however, to better analyze the $dv\!/\!dt$ changes, a 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 $\text{FFT}$ $(dv\!/\!dt)$ 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 also eliminates the nondetection zone entirely in comparison with other available methods.

This study suggests a model to include a solar power system or photovoltaic system (PV) in the control of frequency by taking into account a percentage of the PV power production for back-up reserve. This is done by investigating two scenarios: PV contribution in (1) initial primary frequency control and (2) entire primary frequency control. As explained in section three, 10% power of the PV modules which receive more than 400 w/m² irradiation is allocated for the power reserve. The power generation of photovoltaic systems depends largely on weather conditions which makes their output power associated with some degree of uncertainty. For this reason, in this paper, a PV system is considered along with conventional hydro and thermal units and they are modeled in MATLAB/Simulink (version 9.3, MathWorks, Natick, MA, USA) with the purpose of exploring the behavior of the intended method. In the next phase, for further studies, this system is extended to multi-area power systems including gas turbines. The results of the simulation demonstrated that the photovoltaic involvement in the control of frequency can successfully amend the frequency of the overall network. Not only it can decrease the overshoot and undershoot of the frequency response, it has the ability to improve the settling time as well, which helps the system reach the steady state easily and in shorter time. Specifically, the overshoot has reached nearly zero in both one area and two area systems and undershoot has declined up to 60% and 50% in the one area and two-area system, respectively. Considering settling time, while it had a negligible improvement in the one area system, it showed a remarkable enhancement in the two-area system, which improved from about 25 s to 6 s by using the proposed method.