This work proposes the minimum dc-link voltage control for efficiency and reliability improvement of two-stage grid-connected photovoltaic (PV) inverters. The main goal is to compute in real-time the minimum dc-link voltage required for power transfer to reduce the stresses on capacitors and semiconductor devices. Analytical expressions of current and voltage stresses in the converter components are derived to support the viability of the technique. The feasibility of this control strategy is demonstrated by experimental results. The efficiency and system-level reliability are computed based on a case study of a 3-kW commercial PV inverter. The results indicate that the proposed solution leads to suitable dynamic response, reduced power losses and increased system-level reliability. Since no additional hardware is required to implement the proposal, this approach can improve the reliability of current PV inverter technology through simple modifications in the firmware of installed systems.

This paper proposes a coordinated multi-stage centralized/decentralized control used to mitigate voltage fluctuation caused by planned direct on-line heavy induction motor start in oil and gas offshore platform. The proposed control remotely coordinates the active front-end variable frequency drives (AFE-VFDs) using a low-bandwidth communication link. It is split into three stages, in which stage 1 accurately regulates the power factor at the main feeder, stage 2 is a transitioning operating mode and stage 3 comprises the autonomous volt-var function to mitigate voltage fluctuation. The proposed control shows a centralized architecture for stages 1 and 2, and a decentralized architecture for stage 3. The proposed control is evaluated by means of simulation results considering a real case test bench of a Brazilian offshore platform, being comparatively confronted with two other literature solutions. Compared to other solutions, results show 33% and 29% of improvement in sag and swell voltage regulation at the main feeder, respectively; relief the generator field windings up to 8%, in steady state; and increase the power factor at the generator terminals from 0.92 to 0.97, using two AFE-VFDs.

Voltage regulation in low-voltage (LV) distribution grids is essential to increase the penetration of photovoltaic (PV) rooftop units. Hence, PV inverters are commonly embedded with volt-var (VV) and volt-watt (VW) control functions to support the grid voltage regulation within the grid codes. Such an approach usually increases the LV grid hosting capacity (HC) by 1.5 to 2 times at the cost of energy curtail. Thus, the use of decentralized battery energy storage systems (BESS) near to nodes sensible to overvoltage become appealing, as it stores energy without creating current circulation within the grid. However, there are still gaps in sizing of decentralized BESSs and in quantifying their improvement in terms of HC. Thus, this paper contributes with a comprehensive study that evaluates the HC of a generic LV grid comprised of PV units with embedded VV and VW control functions, and proposes a methodology to guide the sizing of small decentralized BESS used to increase the grid HC. Using OpenDSS software, the VV and VW control functions are evaluated and compared with a base scenario where PV units operate at a unity power factor. The active power curtailment is used to size decentralized BESS to absorb excess energy, avoiding PV active power derating. Results show that BESS units, sized at about 5-10% of the PV power capacity, can reduce revenue impacts and increase HC by over 30% in LV grids.

The high penetration of distributed energy resources (DERs) and non-linear loads in low-voltage (LV) grids may cause overvoltage and voltage harmonic distortion. This paper proposes an automatic control strategy to flexibly steer dispatchable DERs in AC microgrids (MGs) while simultaneously mitigating harmonic voltage distortion and over- and undervoltage at the MG nodes. Voltage support is performed through integrated local and coordinated volt-var and volt-watt functionalities, with a well-defined coordinated sequence of actions to avoid active power curtailment. A voltage detection-based decentralized selective harmonic current compensation (HCC) is enabled when the voltage total harmonic distortion exceeds the established threshold at the MG point of common coupling (PCC). The proposed scheme consists of a model-free control that provides power flow controllability at the MG PCC. It does not require computationally intensive algorithms, does not require extra grid-side and/or load-side current measurements, and effectively regulates the internal MG node voltages regardless of the line impedance X/R feature. The strategy is evaluated based on multiple simulations and experimental results, considering a LV MG model with several DERs as well as unbalanced, non-linear loads and non-dispatchable photovoltaic inverters. The simulations and experimental results showed an improved overall voltage profile throughout the MG nodes in terms of magnitude and distortion, which certifies the feasibility of the proposed scheme in real-field applications.

The expansion of grid-connected photovoltaic (PV) systems over the years has been followed by concerns about the PV inverter reliability. It is well known that single-phase grid-connected PV inverter has a double-line frequency ripple in the dc-link voltage, which increases the wear-out failure probability of the dc-link capacitor and affects the overall inverter reliability. This work uses a third-harmonic current injection to attenuate the double-line frequency ripple in the dc-link capacitor voltage and to improve the PV system reliability. An analytical model capable to describe the effects of the third-harmonic current injection method in the dc-link capacitor is derived. The proposed strategy is validated by simulation and experimental results. The lifetime evaluation is developed considering a 3 kVA PV inverter and one-year mission profiles. The PV inverter failure probability considering an operation period equal to 20 years decreased in 27.17% for the adopted case study. Moreover, this strategy provides a straightforward retrofit of the current inverter technology, requiring no hardwarechanges.