The performance of modular multilevel converters (MMCs) in medium-voltage applications, where the number of required submodules is not high, can be improved utilizing low switching frequency ...modulations such as (2N+1) selective harmonic elimination-pulse width modulation (SHE-PWM), which provides tight control of lower order harmonics and low switching losses. This paper proposes a calculation method, which is based on a novel formulation, to solve the SHE-PWM problem. In particular, MMCs with (2N+1) phase output voltage levels are considered, obtaining a (2N+1) SHE-PWM waveform. This method utilizes a unique system of equations that is valid for any possible waveform. Therefore, it is able to calculate simultaneously, without predefined waveforms, both the switching patterns and the associated firing angles that solve the (2N+1) SHE-PWM problem. Consequently, the search process is simplified and optimized. Furthermore, this paper also proposes a circulating current control technique, which can be applied along with (2N+1) SHE-PWM without disturbing the phase output voltage. Simulation results and experimental tests obtained with a single-phase laboratory prototype prove the validity of the novel (2N+1) SHE-PWM implementation method and the proposed circulating current control technique.
Selective-harmonic-eliminated pulsewidth modulation (SHE-PWM) can be utilized to improve the efficiency of multilevel voltage-source converters due to its ability to provide low switching frequency ...and tight control of low-order harmonics. In addition, SHE-PWM with half-wave (HW) symmetry provides a higher number of solutions than quarter-wave (QW) symmetry, and therefore, the waveform design can be improved. This work proposes a universal formulation, which can be utilized with HW symmetry, that provides a unique system of equations valid for any possible multilevel waveform. Thereby, without using predefined waveforms, this formulation provides the ability to search simultaneously both the firing angles and the switching patterns, simplifying significantly the search process and providing a high number of solutions. With the aim of selecting the optimum sets of firing angles, the solutions provided by HW and QW symmetries are compared, based on several metrics of harmonic performance, for particular test cases. Experimental results also validate the universal formulation with HW symmetry.
This study introduces a novel adaptive technique to accelerate the process of reclosing in a Battery Energy Storage System (BESS)-based microgrid system to provide an uninterrupted power supply ...(UPS). Two different methodologies, Fault Current Contribution Ratio (FCCR) and Variational Mode Decomposition (VMD) are used to implement the proposed technique. First, the FCCR between the healthy and faulty phases is estimated in the relay after the occurrence of the transient. In the next stage, exact fault occurrences and clearance instances are detected using the VMD technique. The exact detection of fault clearance time will help reduce the conventional outage time. This will reduce the unwanted burden on the BESS as it can be used adaptively during the fault only. The comparative assessment is done to show the efficacy of the proposed reclosing method. The proposed technique will also help distinguish faults from switching operations. The performance of the proposed method is validated through a modified IEEE 13-bus BESS-based microgrid architecture. The EMTDC/PSCAD software is used for simulation. The algorithms are developed on the MATLAB platform. Real-time test results are also provided for the signals obtained from the Smart Grid Technology Laboratory (SGTL) lab setup. The results prove the efficacy of the proposed technique.
Selective harmonic elimination-pulse width modulation (SHE-PWM) is a widely used low switching frequency modulation technique for medium-voltage high-power converters. This approach is able to adjust ...the converter fundamental component while eliminating low-order harmonics. However, some applications such as active power filters (APFs) require regulating simultaneously, both the fundamental and low-order harmonics in amplitude and phase. This article presents a novel selective harmonic control-PWM (SHC-PWM) modulator, valid for APFs, based on artificial neural networks (ANNs) and sequential quadratic programming (SQP). A new offline search methodology, based on a hybrid metaheuristic-numerical algorithm, is defined to calculate the solution space when both the fundamental and a low-order harmonic are controlled in phase and amplitude. The solutions obtained are used to train the ANNs offline. Afterwards, the ANN + SQP calculation method is used to solve the SHC-PWM problem in real-time (RT). Experimental results are provided for a three-level converter to verify the effectiveness of the proposed RT control method.
Selective harmonic elimination-pulsewidth modulation (SHE-PWM) is a modulation technique widely used to improve the efficiency and harmonic content in medium-voltage high-power converters. Among ...these converters, the three-level neutral-point-clamped (3L-NPC) converter is one of the most utilized topologies. However, the 3L-NPC converter requires a proper balancing of dc-link capacitor voltages. This article presents a control technique, based on pulse shifting method, to regulate the neutral point voltage when SHE-PWM is utilized. The proposed technique performs well with any power factor and it can also be directly applied to T-type converters and easily extrapolated to other multilevel topologies. The proposed method is experimentally validated in a scaled-down power converter and a full-scale medium-voltage 3L-NPC converter.
Modular multilevel converters (MMCs) require control of the circulating current, <inline-formula><tex-math notation="LaTeX">i_{\text{circ}}</tex-math></inline-formula>, to improve their operation and ...efficiency. This is particularly important when low switching frequency modulation techniques, such as selective harmonic elimination pulsewidth modulation (SHE-PWM) are applied. This article provides a novel method to control the circulating current along with (N+1) SHE-PWM. Unlike the case of (2N+1) SHE-PWM, explicit redundant levels are not available and, therefore, different modulation indexes, <inline-formula><tex-math notation="LaTeX">m_{1}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">m_{2}</tex-math></inline-formula>, are employed in the upper and lower arms to obtain the desired modulation index <inline-formula><tex-math notation="LaTeX">m_{a}</tex-math></inline-formula>. Unlike previous (N+1) circulating current methods, the distances between <inline-formula><tex-math notation="LaTeX">m_{a}</tex-math></inline-formula>, <inline-formula><tex-math notation="LaTeX">m_{1}</tex-math></inline-formula>, and <inline-formula><tex-math notation="LaTeX">m_{2}</tex-math></inline-formula> remain constant to not disturb the phase output voltage, with an interchange of <inline-formula><tex-math notation="LaTeX">m_{1}</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">m_{2}</tex-math></inline-formula> between the arms used to follow the desired <inline-formula><tex-math notation="LaTeX">i_{\text{circ}}</tex-math></inline-formula>. The control adjusts the dc component of the circulating current and the energy stored in the SMs to their references, while maintaining the energy balance between the upper and lower arms. Simulation tests and experimental results, obtained from a single-phase laboratory prototype MMC, validate the proposed control technique.
Different modulation techniques need to be combined in medium-voltage high-power variable-speed drives as the fundamental output frequency changes. Due to this fact, transitions between different ...modulation techniques are required in a real-time converter operation. However, these transitions may generate phase overcurrent due to changes in the switching patterns. This article presents a method that analyzes and models the modulation transitions. It also identifies the optimal fundamental period angle interval where the transition should take place to minimize or eliminate any overcurrent. The proposed method is generic and can be extrapolated to any converter topology and modulation technique. It is demonstrated for transitions between selective harmonic elimination pulsewidth modulation and space vector modulation on a three-level neutral-point-clamped converter through simulations and experimentally validated in a 45-kW scaled-down converter and 6.5-MW full-scale drive.
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.
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.
The increase of non-linear loads connected to the grid has led to the use of active power filters (APF) to reduce the low-order current harmonics injected by this kind of loads. In case of high-power ...medium-voltage applications, low switching frequency optimized modulators are preferred over standard carrier based-PWM. Therefore, in this work the selective harmonic control - PWM (SHC-PWM) modulation technique is used. SHC-PWM allows to synthesize low-order harmonics in amplitude and phase with low switching frequency. A closed-loop control system for APFs is proposed in order to address the control dynamic challenges related to low switching frequency modulation techniques. The closed-loop control uses a Kalman filter to estimate the harmonic currents in the grid side, a proportional-integral controller and SHC-PWM modulator based on artificial neural network. Simulation results are provided for a 3-level NPC converter to verify the effectiveness of the proposed control.
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