Medium power distributed energy resources (DERs) are commonly connected to medium voltage distribution systems via voltage source converters (VSCs). Several guidelines and standards have been developed to establish the needed criteria and requirements for DERs interconnections. In this respect, it is preferred to reinforce the VSC fault ride through (FRT) capability, which considerably minimizes the DG outage period and reconnection time and results in a resilient system against short circuits. Considering the significant number of asymmetrical faults in distribution systems, the VSC response in such conditions must be investigated, and consequently, its FRT capability must be reinforced. In this paper firstly a comprehensive review on existing FRT methods has been presented and discussed. Accordingly, an adaptive virtual impedance-based voltage reference generation method is proposed, which enhances the VSC behavior under short circuits and increases the VSC FRT capability. Also, a fast sinusoidal current reference limiter is proposed to improve the performance. To evaluate the performance of the proposed scheme, state space analysis is presented, and a complete set of simulations is performed in PSCAD/EMTDC environment. Also, a comparison with the conventional method is presented.

This paper proposes a new control scheme to eliminate the $3^{\text{rd}}$ harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical $3^{\text{rd}}$ harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a $3^{\text{rd}}$ harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup.

This paper proposes a fault detection and protection strategy for islanded inverter-based microgrids (IBMGs). Reliable and accurate protection is one of the main challenges in the proliferation of modern microgrids (MGs). Considering the limited fault current of the voltage-frequency controlled inverter-based distributed energy resources (VF-IBDERs), the protection is more challenging in islanded IBMGs. In this regard, the control scheme of VF-IBDER with a current limiting strategy plays an important role. Due to the limited fault currents close to the converter nominal current, the conventional fault detection methods do not work properly. In addition, bi-directional fault currents worsen protection coordination. In this paper, first, an analytical sequence network modeling for VF-IBDERs is derived to specify their behavior under fault conditions. Then, a voltage-restrained negative-sequence resistance-based fault detection approach is proposed, which is based on the derived sequence networks. This quantity inherently detects the fault and its direction and is independent from the fault current magnitude. The proposed feature can be employed in both conventional and communication-assisted coordination strategies. Also, a protection coordination strategy based on definite-time grading approach is employed. Finally, the performance of the proposed scheme is demonstrated by applying different faults in a test MG in PSCAD/EMTDC environment.

Microgrid (MG) protection is one of the main challenges in proliferation of microgrids. Due to limited fault current feeding of inverter-based distributed generations (DGs), in islanded operation of MG, protection problems become more complicated; and, therefore, conventional protection strategies cannot be applied. Hence, new protection methods that are applicable in islanded and grid-connected modes of operation are necessary. In this paper, a comprehensive digital-relay based protection is introduced for the protection of MGs. The proposed method includes protection of lines, distributed generations, and the point of common coupling. The structure and graphical schematic of the proposed digital relays are also presented. The proposed method is independent of the MG operation mode and benefits from single-phase tripping. In this method, due to fault isolation from both sides of a line, downstream loads and DGs can operate after fault clearance. To demonstrate the effectiveness of the proposed protection method, numerous simulations are carried out on an MG test system in the PSCAD/EMTDC environment.