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This article presents a stability analysis of photovoltaic (PV) clusters forming a dc microgrid with a distributed power reserve capability. The PV clusters feature flexible PV power generation under fluctuating ambient conditions via the distributed power reserve control. The PV set-points are adjusted based on grid operator command that is accomplished in a decentralized manner. Stability analysis of the PV clusters has been carried out to determine the safe operation regions of the PV units with respect to operation set-point adjustments. The proposed stability analysis approach considers all the characteristics of the PV subsystems. The stability analysis demonstrates that the PV cluster architecture is stable as far as the dc bus stray resistance has a finite value. Also, the grid-interfacing converter must operate in input current control mode (CCM) instead of input voltage control mode (VCM) for better stability margin. Several case studies are emulated on an IEEE 5-Bus system using real-time controller hardware in the loop (C-HIL) to verify the stability and the theoretical expectations of the proposed system.

This paper proposes a hybrid constant power generation - power ramp rate control (CPG-PRRC) scheme to regulate the ramp-rate of the power injected into the grid by a cluster of photovoltaic (PV) sources. The ramp-rate regulation of the injected PV active power is achieved dynamically in real-time under highly fluctuating PV ambient conditions and without considering PV forecasting data. The proposed hybrid CPG-PRRC control scheme integrates the concept of constant power generation with power ramp rate control to ensure mitigation of the potential voltage/ power fluctuations seen by the grid and protects it from source side instabilities. Moreover, the proposed scheme is capable of regulating the PV power ramp-rate within the set limit during power ramp-up as well as ramp-down scenarios and does so without the use of energy storage devices. The ramp-rate is regulated by maintaining the PV operation set– point on the left side of the maximum power point (MPP) and dynamically increasing or decreasing the power reference at a rate which is within the ramp limit. Furthermore, this paper addresses potential stability concerns for the PV architecture in hand with the CPG-PRRC. Several test scenarios are provided to verify the theoretical expectations.

This paper presents an effective resonance suppression control when grid interactive inverter with LCL filter experience weak grid conditions. The resonance suppression mechanism is based on finite-set model predictive control (FS- MPC) with adaptive cost function to alter the controller objectives seamlessly. The large parasitic impedance and low short circuit ratio (SCR) of weak grid challenges the operation of grid- connected inverters. Specifically, the LCL filter resonance may get excited, resulting in collapse of the inverter operation. To address this issue, an effective active damping approach is implemented in the proposed control. In the proposed active damping scheme for resonance suppression, the feedback currents are switched based on the operating conditions of inverter. During the stiff grid conditions, grid current serves as feedback to the controller. While in the weak grid conditions, the inverter current is selected as feedback signal. The toggling action between these two feedback currents is determined by comparing moving RMS of grid current with threshold current as constraint in the cost function of the proposed MPC scheme. The threshold current for toggling action is based on the reference active and reactive power setpoints plus an acceptable band of variation. The theoretical analysis is verified by several case studies.

This paper provides a review of the control schemes implemented for microgrids (MGs) or grid clusters to enable seamless transition between grid-connected (GC) and islanded (IS) modes of operation. Grid of MGs is a potential solution towards a resilient power grid with high penetration of renewable energy resources (RES). The deployed MGs must be capable of operating in both GC as well as IS mode of operation in the occurrence of anomalies or natural disasters. Such required ability of the MG will ensure uninterrupted energy services to critical loads and infrastructures. This mandates the implementation of control strategies to enable smooth transition of the MG between GC and IS modes and avoid deviations in voltage/current due to misalignment in phase and frequency during the transition process. The main purpose of this paper is to provide an overview of the challenges and existing techniques in literature to mitigate voltage and frequency $(\pmb{V}/\pmb{f})$ fluctuations at the MG's point of common coupling (PCC) and the power grid during the transition process; and to motivate the development of advanced control schemes in this area of MG-related researches.