Chronic hepatitis C virus (HCV) viremia has been known to provoke a plethora of autoimmune syndromes referred to as extrahepatic manifestations of chronic HCV infection. Aim of the current study was to assess the prevalence of rheumatologic manifestations among Egyptians with hepatitis C infection and its' association with cryoglobulin profile. The current research represents a cross-sectional study where patients with chronic HCV infection attending the outpatient clinic of the National Hepatology and Tropical Medicine Research Institute over a period of 1 year were interviewed. Patients with decompensated liver disease, on interferon therapy, having end-stage renal disease or coexisting viral infection like hepatitis B surface antibody positive patients were all excluded from the research. Laboratory investigations as well as serological assay including cryoglobulin profile, rheumatoid factor, antinuclear antibody, HCV-PCR were performed. Three hundred and six patients having chronic HCV infection were interviewed in this research. The overall estimated prevalence of rheumatologic manifestations in the current research was 16.39%, chronic fatigue syndrome 9.5%, sicca symptoms 8.8%, arthralgia 6.5%, fibromyalgia 1.9%, myalgia 1.3%, arthritis 0.7%, cryoglobulinemic vasculitis 0.7%, autoimmune hemolytic anemia 0.7%, thrombocytopenia 0.7%. Xerophthalmia was significantly present in male population (p = 0.04), whereas fibromyalgia, cryoglobulinemic vasculitis, arthritis, and autoimmune hemolytic anemia were significantly present in female population under study (p < 0.05). In chronic HCV genotype 4 infection, the prevalence of rheumatologic manifestations was 16.3% with chronic fatigue syndrome and sicca symptoms being the most common with no significant correlation to the degree of elevation of liver disease or viral load.

An increasing intake of grid-connected inverters could change the characteristics of low voltage networks including the equivalent grid impedance seen by each inverter at its point of common coupling (PCC). This can impact the overall performance of the inverters, and thus it becomes necessary for grid-connected inverters to estimate the grid impedance online. However, there are some challenges when it comes to grid impedance estimation using grid-connected inverters. These include the estimation accuracy and the associated power quality issues that are mainly related to the amplitude, frequency and time period of the injected disturbance into the grid. To address these challenges, a novel online event-based grid impedance estimation technique for grid-connected inverters is proposed in this paper. This technique combines an active grid impedance estimation technique, based on variations of the inverter's output power (active (P) and reactive (Q) power variations), and the continuous monitoring of the positive-sequence amplitude of the PCC voltage. The main advantage is that the estimation technique only performs a deliberate perturbation of P and Q if there is a certain change in voltage magnitude related to grid impedance change. This will significantly reduce the occurrence of required PQ variations, thus minimizing the power losses and the impact on power quality. Simulation and experimental results of a grid-connected inverter system validate the performance of the proposed technique.

In islanded AC microgrids, the mismatched impedances of the interfacing feeders between the inverters and the load bus cause poor reactive power sharing when the conventional frequency and voltage droop control technique is employed. Such operation endangers the whole microgrid reliability as it may lead to overloading certain inverters and, consequently, triggering protection relays and causing cascaded failure. Thus, this paper proposes an accurate reactive power sharing strategy that considers the mismatched feeder impedances in islanded AC microgrids. It is based on the optimal tuning of the virtual complex impedance for each inverter. The proposed strategy has several advantages. First, it has a physical meaning as it establishes an explicit relationship between the mismatched values of the actual resistive-inductive feeders and the assigned values for the proposed virtual complex impedance to each inverter. Hence, further degradations in the microgrid voltages are prevented. Second, there is no need for prior knowledge about the actual feeder impedances in the design stage as they are estimated online from the available measurements. Lastly, the proposed control is reliable and fault-tolerant as it copes with unexpected failures such as failure of (or sudden switching off)some inverters and communication disruptions/delays. It ensures accurate power sharing even under the primary controllers after losing communication links with the secondary controller. The simulation and experimental verification results are presented to validate the performance of the proposed technique.

Whereas power-electronics-based power systems are expected to enable more integration of renewable energy sources, they could pose crucial challenges including stability issues due to the Thévenin impedance seen by inverters. Such problems could be mitigated by enabling the inverters to estimate the grid impedance by including a grid impedance estimation technique into their control loop. However, one aspect which has been overlooked thus far is that the accuracy of such grid impedance estimation techniques is significantly affected by various grid conditions. For instance, the unbalance in three-phase power systems causes unwanted oscillations at double the fundamental frequency in the inverters control loops. Therefore, this paper proposes a simple and reliable online estimation of the grid impedance under unbalanced conditions. The technique is based on wide-band impedance estimation incorporated into the control loop of the positive sequence of a three-phase grid-connected inverter equipped with a positive-and negative-sequence control (PNSC) strategy. Additionally, complex curve fitting is utilized to obtain parametric models of the grid impedance. To demonstrate the efficacy of the proposed grid impedance estimation technique, extensive case studies are performed. These include: (1) unbalanced operations of both resistive-inductive (RL) and resistive-inductive-capacitive (RLC) models of the grid, (2) background harmonics, and (3) asymmetrical impedances of the network.

Interactions between grid-connected inverters bring major problems such as increased harmonic distortion and instability. Furthermore, as existing literature on inverter-based online grid impedance estimation mainly focuses on one inverter unit, the effects of the interactions on the estimation accuracy have been neglected. This paper initially investigates how the interactions among the grid-connected inverters impact the accuracy of the estimated fundamental and wideband grid impedances. Afterwards, a communication-free approach for multi-inverter systems is proposed to avoid the possible interactions between the inverters while estimating the equivalent grid impedance. Hence, accurate estimation results are ensured. The presented method is implemented in the local controller of each inverter, relying only on local measurements. Thus, it is more practical as coordinating different communication protocols between inverters is challenging. Furthermore, it is a generic approach as it can be used for single/three-phase inverters and for implementing fundamental/wideband impedance estimation algorithms. The proposed strategy also minimizes the power quality issues as it is a trigger-based approach. The simulation and experimental results validate the effectiveness of the proposed communication-free strategy to accurately coordinate impedance estimation in multi grid-connected inverter systems.