Impedance-based stability analysis techniques are increasingly used to assess the stability of power converter-dominated power systems. Among these techniques, the Generalized Bode Criterion (GBC) has emerged as a robust method, combining the strengths of the Nyquist Stability Criterion (NSC) with the ease of application of the Bode criterion. However, the GBC requires knowledge of the phase derivative at 0 Hz, which can be challenging to obtain when the system transfer function is analytically unknown. This paper proposes and proves a mathematical relation to eliminate the need for the phase derivative in the GBC. Consequently, all necessary information required for the GBC can be directly obtained from the system Bode plot, enabling the application of the GBC to power systems represented by black-box models with no detailed internal information. The proposed approach is validated by analyzing the stability of a DC-DC converter connected to a DC grid and comparing the results with time-domain simulations.

The increased grid configuration variability and, among others, the massive integration of renewable energies, have created a challenging scenario for power converters. Due to this fact, active impedance estimation is an interesting characteristic that will allow power converter online control tuning and/or stability analysis. Pseudo Random Binary Sequence (PRBS) injection is one the most interesting and proposed methods in the literature to obtain this impedance estimation. Many papers use impedance estimation on dedicated power converters, or they often overlook the digital and power hardware constraints that power converters must meet. This is especially important in high-power converters, where low switching and control frequencies are used. This paper defines and proposes an implementation and design method for PRBS injection considering the abovementioned constraints. The proposed method is aimed to be implemented in an already operating converter, thus adding the impedance estimation as one additional functionality of the converter. Therefore, no dedicated power converter to estimate the grid frequency is required. Multi-harmonic and multi-sequence impedance estimation is also analyzed, thus demonstrating the ability of the proposed method to operate under highly variable and demanding grid conditions. The analysis is validated via simulation results in different case scenarios.

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.

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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.

Low switching frequency modulation techniques such as Selective Harmonic Elimination - Pulse Width Modulation (SHE-PWM) or Optimal Pulse Pattern (OPP) are commonly used in medium voltage - high power multilevel converters to improve their efficiency. These techniques require solving non-linear equations in order to calculate the firing angles. These equations can be solved by meta-heuristic optimization algorithms, which depend on a set of hyperparameters that must be properly adjusted to ensure their effectiveness. This paper presents a methodology to tune different meta-heuristic algorithms to fairly compare them and select the best algorithm to solve SHE-PWM/OPP formulation. The methodology is applied considering different meta-heuristic algorithms such as genetic algorithm, differential evolution, harmony search and simulated annealing. It has been validated in SHE-PWM and OPP techniques with different number of firing angles.

Medium-voltage high-power converters are usually modulated using low switching frequency techniques such as Selective Harmonic Elimination - Pulse Width Modulation (SHE-PWM) in order to improve their efficiency. To calculate the firing angles, SHE-PWM transcendental equations are usually solved by offline calculation methods, whose computational burden depends on the number of firing angles or eliminated harmonics and the required accuracy in the solutions. Consequently, with the aim of reducing this computational burden, this paper presents an analysis that estimates the maximum accuracy required in the calculated SHE-PWM solutions considering the main non-idealities of the converter. In this sense, a systematic methodology to evaluate the error in the harmonic amplitudes due to control and dead time effects has been developed, providing an optimal error tolerance for the calculation methods that solve the SHE-PWM problem.

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.

The Vienna rectifier is an attractive converter solution due to the three-level voltage generation and its simple structure. When the Vienna rectifier operates with nonunity power factor, the reference voltage and the input current have different signs during some intervals around the current zero crossings. This creates low-frequency distortion in the current waveforms. One of the preferable methods to reduce this distortion is the zero sequence injection which, however, risks the converter entering into overmodulation. This paper analyses the above distortion and introduces the operation of the Vienna rectifier in two modes, which includes injecting a proper zero sequence and reactive power compensation. This allows the converter to operate in a wide range of power factors without constraining the modulation index. The required reactive current is obtained analytically from the instantaneous values of the converter at any operating point.

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.

Selective harmonic elimination (SHE) - pulse with modulation (PWM), when applied to multilevel converters (MCs), can be optimized in case a half-wave (HW) symmetry is implemented. This kind of symmetry is able to provide a wide variety of solutions. This paper proposes a novel method, based on optimization algorithms, which provides a universal formulation of the SHE-PWM problem with HW symmetry, in such a way that different equation systems are not required to find a solution when different switching patterns are considered. In this way, the proposed technique is able to search simultaneously both the switching patterns and their associated firing angles that solve the SHE-PWM problem, without using predefined waveforms. Therefore, the search process is significantly simplified and the flexibility to find solutions with different waveforms, throughout the reference modulation index, ma, range, is increased. Different sets of solutions provided by this method have been presented and simulation results, obtained from a five-level modular multilevel converter (MMC), have proved the validity of the new proposed technique.

Tolerance and component aging can cause significant differences in the capacitance values of the submodules (SMs) in a modular multilevel converter (MMC). Depending on the modulation technique, capacitance mismatches may produce uneven switching transitions of the SMs, hence imbalances in the power losses that can lead to reliability problems. In this paper, a new algorithm that helps to achieve evenly distributed switching and conduction losses within the converter SMs is presented. The proposed algorithm is based on a modification of the common voltage balancing algorithms, balancing a weighted function of voltage and losses. Even distribution of power losses is achieved at the cost of slightly increasing the capacitor voltage ripples. The effectiveness of the strategy has been demonstrated by simulation results of a high-power grid-connected MMC.

Research on dc-fault tolerant multilevel converters has gained noticeable attention over recent years. The alternate arm converter (AAC) is one of such emerging multilevel converter topologies, and a hybrid topology of the two-level converter and the modular multilevel converter (MMC). Bipolar sub-modules (SMs) that can produce both positive and negative voltages are the building blocks of the AAC. This paper analyses the operation of an AAC with the full-bridge SM (FB-SM) and the cross-connected SM (CC-SM). The conduction and switching losses of the two SM configurations are evaluated and compared, in order to identify the suitability of CC-SM for AACs and its performance compared to the FB-SM. The CC-SM with identical semiconductor devices has reduced losses compared to the CC-SM with higher rated devices in the cross-connected path. It is concluded that the CC-SM does not offer advantages in the losses, construction, and application to the AAC, compared to FB-SM.

Control of the circulating current in the modular multilevel converter (MMC) is one of the major tasks for the proper operation of the converter topology. This paper develops a controller for the circulating current of the MMC that utilises the redundant voltage levels in 2N+1 modulated MMCs in order to regulate the current to its reference. The main advantages of the approach are the elimination of control loops that generate the voltages for the control of the circulating current, simple implementation and very fast dynamic performance. The application of the control is also independent of the circulating current reference. The simplicity and effectiveness of the proposed controller is illustrated through simulations and experimental results.

In this paper, a new approach to the discontinuous modulation technique for the operation of the modular multilevel converter (MMC) is presented. Discontinuous modulation is based on adding a zero-sequence to the original modulation signals so that each MMC arm is clamped to the upper or lower terminals of the dc-link bus during some intervals. In combination with a circulating current control, the original discontinuous modulation can reduce the capacitor voltage ripple amplitudes and the switching power losses. However, additional submodules (SMs) are required to control the circulating current. This new approach presents a clamping algorithm that eliminates the requirement of additional SMs. As a result, the conduction losses are reduced while the capacitor voltage ripples are maintained low. Simulation and experimental results on a silicon-carbide-based MMC are reported and compared against the original discontinuous modulation and a conventional carrier-based pulse-width modulation.

This paper proposes a PWM based modulation strategy particularly useful for three-level multi-phase Neutral-Point-Clamped Converters. It achieves similar results to those obtained with the space vector modulation but allowing a very fast and simple implementation in a digital controller. The proposed algorithm is able to maintain the neutral-point voltage under control by the addition of a zero-sequence voltage component to the modulation signals. A simple and efficient methodology to calculate the most appropriate zero-sequence component is described. In addition the modulation strategy is completely general, regardless the number of phases of the converter, and therefore it is easy extendible to n-phase converters.

The voltage ripple of the capacitors of the modular multilevel converter (MMC) increases when the converter output frequency decreases. This occurs in motor drive applications at low motor speeds, where the output frequency is reduced proportionally to the converter modulation index. In this paper, a discontinuous modulation technique for reducing the amplitude of the capacitor voltage ripples and the switching losses, especially at low modulation indices, is presented. The technique is based on adding a zero-sequence to the original modulation signals, clamping the MMC arms to the upper or lower rails of the fictitious dc-link bus. A current control strategy suitable for this modulation strategy is also proposed. Simulation results under various operating points are reported along with evaluation and comparison results against the conventional carrier-based pulse-width space-vector modulation.

The modular multilevel converter (MMC) is one of the most potential converter topologies for medium/high power/voltage applications. One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents within the legs. Circulating currents, if not properly controlled, increase the amplitude of capacitor voltage variations, rating values of the converter components and converter losses. This paper proposes a closed-loop circulating current control strategy for an MMC to specifically minimize the amplitude of capacitor voltage variations. The proposed strategy is based on adding an offset signal to the modulating signal of each arm. To minimize the amplitude of the capacitor voltage oscillations, an optimal circulating current component is determined and used as a reference signal for the current control of each MMC leg. Performance of the proposed control strategy is evaluated based on simulation studies in the MATLAB/Simulink environment. The reported study results demonstrate effectiveness of the proposed strategy to reduce the amplitude of the capacitor voltage oscillations.