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Ac motor drives are becoming increasingly popular in the field of industrial processes and transportation electrification. Currently, many industrial applications are based on induction machines supplied by inverters and controlled with field-oriented control techniques. Such techniques require the knowledge of the machine parameters to ensure the correctness of the torque control both in dynamic and steady-state conditions. In particular, an accurate determination of the induction motor rotor time constant is crucial. Therefore, this article analyses in detail the physical phenomena involved during the flux-decay test used for the rotor time constant determination. The reported analysis has been performed on a 15 kW induction motor. The transient of the machine's back-electromotive force (back-emf) has been critically analyzed during its evolution, finding a link between its evolution in time and the magnetic phenomena that occur both in the stator and the rotor. In particular, the effects due to the lamination saturation, the stator and rotor leakage inductances, and the stator iron losses have been associated with the transient evolution of the machine's back-emf.

To make photovoltaic and wind power plants able to provide grid services (i.e., inertial behavior, grid support, and harmonic compensation), several control algorithms have been proposed in the literature during the last years. The most promising ones make the power electronic converters behave as conventional alternators, using the concept of virtual synchronous machine (VSM). Several VSM models are available in the literature, some of which can improve the voltage quality at the point of connection with the grid behaving as harmonic and unbalance sinks under nonideal grid voltage conditions. However, the literature lacks a general method to foresee the behavior of a generic VSM configuration in such conditions along with a well-established definition of the needed features to make VSMs able to work as harmonic or unbalance sinks. Therefore, this article proposes a simple and general method to foresee the behavior of different VSM configurations under nonideal grid voltage conditions before any experimental verification. The proposed method accurately foresees the VSMs behavior, as experimentally demonstrated on five VSM models available in the literature, working with fifth harmonic and inverse sequence voltage distortions. Moreover, the method identifies which VSM configuration can feature a beneficial harmonic and unbalance compensation.

The concept of Virtual Synchronous Machine (VSM) is a promising solution to integrate renewable energy sources into the grid. Thanks to this approach, renewable energy sources can provide grid services (e.g., inertial behavior), grid support during faults and island operation. Moreover, VSMs can enhance the voltage quality at the point of connection to the grid by behaving as harmonic and unbalance sinks under non–ideal conditions (i.e., grid harmonic distortion and unbalance). However, the inverter dead–time affects the harmonic and unbalance sink capability of grid–forming VSMs with no closed loop current control. Although the literature contains several papers concerning the dead–time issues, the negative influence of the switching dead–time on the harmonic and unbalance sink capability of VSMs has not been analyzed. Therefore, this article demonstrates through experiments that: (1) the inverter dead–time effect limits the harmonic and unbalance sink capability of grid–forming VSMs under non–ideal grid voltage conditions and (2) a dead–time compensation is mandatory to make grid–forming VSMs behave according to the theoretical analysis. A first experimental test shows that both the dead–time and its compensation do not influence the VSM response under normal operating conditions (i.e., symmetrical and sinusoidal grid voltage). Next, two experimental tests under a 5$\%$ grid voltage unbalance and a 10$\%$ grid voltage fifth harmonic distortion validate the negative influence of the dead–time and the beneficial effect of its compensation.

The Simplified Virtual Synchronous Compensator (S–VSC) is a Virtual Synchronous Machine (VSM) solution available in the literature, which operates as a virtual compensator. Previous works demonstrated that it can provide grid services (e.g., inertial behavior, grid support during faults) only in grid–following operation. However, the grid–forming capability of the S–VSC (and in general of a virtual compensator) has never been investigated in the literature. Therefore, this article proposes an S–VSC model able to operate both in grid–following and in grid–forming configuration. First, the article validates the small–signal stability of the control algorithm both in grid–mode and island operation through an eigenvalue–based stability analysis. Next, the control algorithm is experimentally validated on a 15 kVA inverter connected to a scaled microgrid. The experimental tests demonstrate that the S–VSC can operate both as grid–following and grid–forming. Moreover, it can seamlessly switch from the grid–connected to the island operation without requiring any communication system. Finally, the grid–forming capability of the S–VSC is experimentally validated even in case of a fault occurrence, thus representing a valid solution for the control of a microgrid.

The recent years have seen exponential growth in electric generation from renewable energy sources (RESs), such as wind and sun. This scenario represents an opportunity to decarbonize part of the energy sector and phase out fossil fuels. However, particular care must be taken to ensure the correct operation of the electric grid. With a large penetration of power electronics-based generation, special care must be taken in the case of grid faults, due to limited short-circuit current capabilities of static converters, supporting the grid in the case of faults. For this reason, static converters have to be transiently overloaded to inject fault currents larger than their nominal limit, mimicking the behavior of synchronous generators (SGs). Therefore, this article proposes the combined solution of a transiently overloadable virtual synchronous machine (VSM) converter equipped with real-time semiconductor junction temperature limitation. The VSM provides the necessary short-circuit current references and the inverter can be overloaded with an active thermal control strategy, avoiding the oversizing of the power semiconductors, by properly exploiting their thermal limits. This represents a possible path for further power electronics integration into the grid.