Advanced reserve power point tracking (RPPT) algorithms are becoming popular and are expected to replace the conventional maximum power point tracking (MPPT) and flexible power point tracking (FPPT) algorithms. RPPT enables power reserve availability while injecting flexible power into the grid. Hence, it combines the MPPT and FPPT functionalities simultaneously. However, due to inherent continuous sweeping across the PV curve, RPPT suffers from high dc-link voltage fluctuations. For this reason, this paper extends the RPPT concept from the single-string photovoltaic (PV) system to the multi-string PV system. Specifically, a two-string PV system is proposed, where two PV strings that are interleaved at the dc-link. This approach offers advantages over using a single PV string of the same capacity, notably by reducing the ripple in the dc-link voltage caused by RPPT. The effectiveness of this proposed algorithm in the interleaved PV system is demonstrated through simulation results under varying grid frequency and irradiance conditions.

High penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.

Photovoltaic (PV) grid support functionality (frequency support) can be accomplished with the aid of flexible power point tracking (FPPT) technology. However, the convergence rate of FPPT algorithms needs to be fast, to provide grid support under transients. To fulfil this purpose, a Newton’s method-based (NM-B) FPPT algorithm is proposed in this paper that offers quadratic convergence, which is a higher convergence rate compared to the existing methods. Moreover, the search algorithm requires only a single initialization point, thus eliminating the difficulty of setting up the search boundary as in the case of the binary search-based and secant method-based (SM-B) FPPT algorithms. Simulation validation is performed for the proposed NM-B FPPT algorithm and the results are compared with the SM-B FPPT algorithm.

Frequency support from photovoltaic (PV) power plants is required with new grid codes. To provide frequency support, the PV system needs to regulate the PV power in such a way that a predefined amount power reserve is kept in the PV system. Accordingly, a dynamic flexible power point tracking algorithm (FPPT) for grid-connected PV power plants is introduced in this paper. The proposed algorithm regulates the average PV power by continuously sweeping some portion of the PV curve that includes the given power reference point. The proposed strategy is different from traditional static FPPT techniques as it ensures a predefined amount of power reserve in the PV system. This power reserve provides the flexibility of increasing the power injected into the grid that can be withdrawn with a fast dynamic. Compared to the traditional solutions, in the proposed algorithm, the PV injects a constant power into the grid without any fluctuations. Effectiveness of the proposed technique in providing frequency support in a two-stage grid-connected PV plan is demonstrated by simulation results.

The neutral-point-clamped (NPC) converter employed in applications like bipolar dc-bus, back-to-back converters, power compensator, etc., requires a nonzero average neutral-point (NP) current injection under certain operating conditions. In order to better implement the average NP current injection, this paper studies the maximum and minimum average NP current that can be injected with an NPC converter. For this study, a three-level modulation technique is employed in which one of the three phases is selected to switch between the positive dc-rail and the negative dc-rail while the other two phases switch in two consecutive voltage levels. The maximum and minimum average NP current injection limits are analyzed under different loading conditions. The presented study also evaluates the effects of modulation index and operating power factor on the limits.

This paper proposes sectional theory and binary search-based maximum power point tracking (MPPT) algorithms for solar photovoltaic (PV) systems. The maximum power of the solar PV under various environmental conditions are obtained by employing these algorithms. The corresponding voltage of the maximum power is computed in every step with the proposed logic, which moves the operation point closer to the maximum power point. The proposed methods are validated using the MATLAB/SIMULINK software tool. The results are bench-marked against the conventional adaptive step-based perturb and observe method. The proposed algorithms improve the performance in terms of convergence rate and reduced steady state oscillations. The results demonstrate that the sectional theorem-based algorithms are either less accurate or has slower convergence rate while the binary search-based MPPT technique is the most efficient among the algorithms tested.

The frequency response support has been added to the new standards and grid codes for the grid-connected photovoltaic (PV) systems, to retain the reliability and quality of the power system under the high penetration of renewable energy resources. This paper proposes an algorithm for the control of cascaded H-bridge (CHB)-based PV systems to achieve frequency response for them. The required power reference, based on the grid operation condition, is distributed among the sub-modules (SMs) of the CHB converter considering the available power from each SM and operational constraints of the system. The details of the proposed control algorithm are provided, while its effectiveness is evaluated on a 1.5-MW PV simulation setup, connected directly to a 6.6-kV grid.

In this paper, a cascaded H-bridge low capacitance StatCom with modular capacitors is proposed. The low capacitance StatCom benefits from having large capacitor voltage ripple to reduce losses and improve harmonic performance. The proposed LC-StatCom can operate with large capacitor voltage ripple even in light loading conditions by keeping redundant capacitor modules in standby mode when the current is low. Effectiveness of the proposed system is confirmed using a simulated 8.5-MVA StatCom system.

Flexible power point tracking (FPPT) is control of active power generated by grid-connected photovoltaic power plants (GCPVPPs) to provide various grid-support functionalities, such as frequency response and voltage support. Fast transient response is required during grid voltage and frequency variations. An algorithm, based on dynamic voltage reference design and model predictive control for the dc-dc converter in GCPVPPs is introduced in this paper. The main novelty of the proposed algorithm is fast dynamic response based on the maximum capacity of the system. This feature enables fast FPPT operation in GCPVPPs. The proposed control algorithm results in several features for the FPPT operation, including: higher bandwidth, low power oscillations during steady-state, flexibility for adding constraints based on the operation conditions, and ease of implementation without requiring any complex control parameters tuning and design procedure. The transient performance of the proposed algorithm is evaluated under conditions of voltage sag and frequency deviation. Experimental results are also provided to demonstrate the effectiveness of the proposed algorithm when subjected to rapid changes in PV reference power.

This paper proposes a multi-input linear time-variant control law based on the incremental passivity theory for cascaded H-bridge (CHB) low-capacitance static compensators (LC-StatComs). The proposed control law guarantees global asymptotic stability of the system and controls the capacitor voltages and inductor current without assuming fast dynamics in the current and slow dynamics in voltages. This is particularly important for LC-StatCom applications as the control of the inductor current and capacitor voltages are inherently coupled due to the reduced capacitance. After introducing the incremental model of the converter, a passivity-based control that ensures the global stability is derived. Simulation results on a seven-level 1-kVA CHB LC-StatCom are shown to corroborate its excellent performance in steady-state and during transients.

Due to the high penetration of the installed distributed generation units in the power system, the injection of reactive power is required for the medium-scale and large-scale grid-connected photovoltaic power plants (PVPPs). Because of the current limitation of the grid-connected inverter, the injected active power should be reduced during voltage sags. In order to obtain a constant dc-link voltage in a multi-string PVPP, the extracted power from PV strings should be equal to the injected power to the grid in all operating conditions (excluding power losses). Therefore, the extracted power of PV strings should be reduced during voltage sags. In this paper, an algorithm is proposed for determining the reference voltage of the PV string which results in a reduction of the output power to a certain amount. The proposed algorithm calculates the reference voltage for the dc/dc converter controller, based on the characteristics of the power-voltage curve of the PV string and therefore, no modification is required in the the controller of the dc/dc converter. Simulation results on a 50-kW PV string verified the effectiveness of the proposed algorithm in reducing the power from PV strings under different irradiation and reference power values.