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.

Cascaded H-bridge (CHB) static compensator (Stat-Com) enhances stability of the electricity power grid, which is becoming ever increasingly vulnerable as a consequence of replacing fossil fuel-based synchronous generators by intermitten renewable sources. A CHB StatCom needs large dc capacitors in the submodules to operate, which is a major drawback that hinders a widespread use of this technology. In order to overcome this drawback, the low-capacitance StatCom (LC-StatCom) technology was developed. Other than reducing the capacitors size, improvement in power quality and reduced losses were listed as further advantages of the LC-StatCom. On the other hand, control challenges and reduced stable operating range are the main challenges of this technology. This study aims to provide a better understanding of the prospects and challenges of the LC-StatCom technology as compared to the conventional$C$HB StatComs.

Intermittent changes in irradiance caused by passing clouds inevitably lead to fluctuations in photovoltaic (PV) power generation. To ensure the stability of a power grid with integrated solar PV generation, a battery energy storage system (BESS) is an intrinsic solution to effectively process the PV power before transmitting it into the grid. Alternatively, a PV software-based ramp rate (RR) control can be applied to mitigate the PV power fluctuations without any BESS, which is known to be cost-effective. The contribution of this paper is to compare various BESS-based and cost-effective software-based RR control strategies operating with real-time measured information, and analyze their effectiveness in regulating the PV power fluctuations. Furthermore, this paper studies the BESS utilization behavior among hardware-based control strategies, and also provides insights into which strategy is more appropriate for specific applications. The investigations are carried out through simulation case studies with a real-field one-day fluctuated irradiance profile.

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.

This paper presents a novel zero-sequence voltage injection method for discontinuous modulation (DM) in star-connected cascaded H-bridge (CHB) static compensators (Stat-Coms). Conventional DM methods in CHB StatComs suffer from control interactions during unbalanced grid conditions. This paper proposes a DM that eliminates this control interaction by inserting a clamping transition within a third of the fundamental period. The transition instant is decided by comparing a duty cycle, which is calculated to mitigate the fundamental-frequency in the zero-sequence voltage for discontinuous operation, with a triangular carrier signal with a frequency of three times the fundamental. The clamping intervals under the proposed DM are affected by the phase-shift of this carrier. Specifically, the paper analyses the effects on the harmonic content of the proposed zero-sequence voltage, switching loss, twice-fundamental-frequency capacitor voltage ripple, and current total harmonic distortion, for different phase-shift angles of the DM carrier signal. Besides, the feasibility of the proposed DM in dealing with a grid voltage sag is also discussed.

In this paper, the effect of min-max zero-sequence voltage (ZSV) injection on the maximum average neutral-point (NP) current control limits of a neutral-point-clamped (NPC) converter is analyzed. These current limits determine the stable operating range of the NPC converter in applications that impose a nonzero average NP current, like- NPC converters feeding multiterminal dc loads, back-to-back-connected NPC converter-based wind and high voltage dc transmission systems, etc. Previously, a dual-mode modulation technique was implemented in the literature which provided the maximum average NP current limits without considering the ZSV effect. As the ZSV injection affects the NP duty cycles, and in turn the NP current, it is necessary to analyze and quantify its effect. In this paper, the effect of ZSV on the current limits is analytically calculated. Simulation results are obtained in MATLAB/Simulink environment to validate the derived analytical equations.

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.

In a modular multilevel converter (MMC) based battery energy storage system (BESS), a fault tolerant design ensures uninterrupted operation of the MMC when a given number of submodules (SMs) have faulty batteries or no batteries. This paper quantitatively investigates the fault tolerance improvement in MMC-based BESSs with different numbers of redundant SMs, which is commonly practiced to improve the fault tolerance in modular converter topologies. Simulation results are obtained to verify the proposed analysis. The presented analysis provides guidelines for designers to choose an appropriate number of redundant SMs to achieve a desired fault tolerance.

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 a horizon-one constrained model predictive control (MPC) with integral action for dc-dc H-bridge converters. An integral-action MPC law is analytically derived. The proposed controller takes into account the bilinear dynamics of the converter both in the control law derivation and constraints modeling. In addition, the proposed cost function does not presume the knowledge of the equilibrium point. In this paper, a three-level H-bridge converter for regulation purposes is considered to illustrate the proposed approach, though it is applicable to other dc-dc converter topologies as well. The paper compares the performance of the proposed analytical MPC law and the solution provided by an iterative optimisation algorithm. Simulation results are provided to verify the theoretical predictions, that is, the control law rejects constant disturbances while satisfying the designed constraints.

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.

In this paper, application of thyristor switched reactors as a filtering inductor for the cascaded H-bridge low-capacitance static compensator (LC-StatCom) is demonstrated. Replacing the fixed-value filter inductor with thyristor switched reactors reduces the overall energy storage capacity and voltage rating of the LC-StatCom. The proposed concept takes advantage of LC-StatCom's ability to synthesize high quality ac voltage to reduce the overall energy storage requirement of the filter inductance. Using modular filter inductor concept adds an additional degree of freedom to control the maximum voltage drop on the filter inductor by changing the inductance value. This additional capability decreases the overall voltage rating of the converter. The effects of adding the modular inductor unit on the I-V characteristic of the LC-StatCom are evaluated. The effectiveness of the proposed solution is demonstrated by simulation results on an 11-level 3 MVA LC-StatCom.