Harmonic Elimination Pulse Width Modulation Research Articles

An optimal selective harmonic mitigation technique for high power converters

In medium-voltage medium- and high-power inverters, which are most likely to connect distributed generation systems to grids, it is of great importance to reach high quality waveforms using the least possible number of switching transitions. The power quality is also important to be kept as high as possible. Both inverter output harmonics and grid pre-existing distortions affect the power quality. The well-known selective harmonic elimination pulse width modulation (SHEPWM) method is able to totally eliminate some specific harmonics. But the recently introduced selective harmonic mitigation pulse width modulation (SHMPWM) method can mitigate a higher number of harmonics using the same switching frequency. In this paper, implementation of an optimal selective harmonic mitigation (OSHM) method using the least possible switching frequency to satisfy three grid codes EN 50160, CIGRE JWG C4.07 and IEC 61000-3-6 is presented. In the proposed method, also a technique is employed to minimize the individual harmonics as much as possible. Using the proposed method, there is no need to the bulky and heavy line side filters. Some simulations as well as experimental investigations are carried out to prove the claim and also compare the OSHM method with the conventional SHEPWM method using the same switching pattern to confirm its better performance over SHEPWM.

Particle swarm optimisation and its applications in power converter systems

Particle swarm optimisation PSO algorithm is known for its easy implementation and has been empirically shown to perform well in many optimisation problems. Thus, it is expected to mitigate the computational burden associated with the solutions of non-linear transcendental equations relevant to problems related to power converter systems. This paper starts with an overview of the general concept of PSO and its variants. It then continues with a review on the application of the PSO algorithm in power converter systems. Then, a sample application is described, focusing on the implementation of the harmonic elimination pulse width modulation HEPWM technique on a single-phase inverter circuit. The results of a simulation and experimental work on the inverter operation have revealed that the PSO method employed is capable of accurately calculating the relevant switching angles and generating the gate signals for the inverter power devices and improving its overall system performance.

Selective harmonic elimination pulse‐width modulation of modular multilevel converters

The modular multilevel converter (MMC) is the state-of-the-art for multilevel converter topologies. This study presents the operation of the MMC using the multilevel selective harmonic elimination pulse-width modulation (MSHE-PWM) technique. MSHE-PWM offers tight control of the low-order harmonics and the lowest switching frequency for the power semiconductors among all modulation techniques. A comprehensive analysis of the modulation methods for the MMC leads to two different modulation patterns for MSHE-PWM. A method for selecting the number of sub-modules in the phase-legs of the converter is also proposed in this study. Simulation results for both patterns are provided and verified through matching experimental results from a single phase 11-level laboratory prototype.

Selective Harmonic Elimination PWM Method Applied to Three-Level Photovoltaic NPC Inverter

This paper introduces a selective harmonic elimination (SHE) PWM method based on neutral-point potential balancing applied to a three-level Photovoltaic NPC Inverter. The method combines the SHE with neutral-point potential balancing， and introduces a method based on multi-objective ant colony algorithm to solve the SHE-PWM non-linear equations. SHE-PWM control method of multi-objective ant colony algorithm considering about neutral-point potential balancing, there is no need to solve the initial value of the equation when solving the SHEPWM equations. Experimental results demonstrate the validity of SHE-PWM method based on neutral-point potential balancing.

Improved selective harmonic elimination pulse-width modulation strategy in multilevel inverters

In the standard selective harmonic elimination (SHE)-pulse-width modulation (PWM) technique for multilevel converters, in order to eliminate N 2 1 non-triplen odd harmonics of the ac-side voltage and to regulate the output voltage amplitude, N switching angles need to be determined. Therefore a set of N non-linear transcendental equations must be solved to find the desired switching angles for any value of the modulation index. However, solutions to the non-linear equations are not feasible for the entire range of modulation region. This study proposes a modified SHE-PWM technique that extends the standard modulation region and generates a stepped voltage waveform within a wide range of modulation indices. As compared with the standard SHE-PWM technique, the constraint associated with elimination of the harmonic with the highest order is relaxed and the corresponding equation is disregarded. Consequently, a reduced number of transcendental equations are employed to find the N desired switching angles. Since the switching frequency remains constant, converter's efficiency does not change. However, in order to produce a pure sinusoidal waveform, a higher order filter is required. A genetic algorithm is used to solve the equations and to determine the switching angles. The proposed technique can generate stepped voltage waveforms with a wide range of modulation indices. Performance of the proposed SHE-PWM technique for a seven-level cascaded H-bridge converter, based on simulation studies, is evaluated and experimentally verified.

DC-Link Voltage Ripple Compensation for Multilevel Active-Neutral-Point-Clamped Converters Operated With SHE-PWM

This paper presents a dc-link voltage ripple compensation method for flying-capacitor (FC)-based active neutral-point-clamped multilevel converters operating under selective harmonic elimination pulsewidth modulation. The method is based on feedforward modification of the modulation index according to the ripple on the dc-link voltage, effectively altering the switching control functions. The low-order harmonics in the output due to the presence of the dc-link ripple are eliminated. In addition, a control strategy that actively regulates the flying capacitor voltages of each phase to the reference value and controls the neutral point voltage deviation is implemented. The performance of the dc ripple harmonic compensation method and regulation strategies are evaluated through simulation and experimental results from a three-phase, five-level laboratory prototype.

Selective harmonic elimination pulse-width modulation seven-level cascaded H-bridge converter with optimised DC voltage levels

This study presents a novel formulation of multilevel selective harmonic elimination pulse width modulation (MSHE-PWM) technique with optimised DC voltage levels suitable for high-power voltage source converter-based applications. The study reformulates the problem in a way in which the levels of the DC voltage sources are made variables within certain constraints. Therefore the degrees of freedom for specifying the cost function are increased when compared to the existing family of selective harmonic elimination-based multilevel control for the same physical structure. Moreover, with the proposed approach, the solution of the switching angles can be sought for the entire range of the modulation index without affecting the number of harmonics being eliminated or the number of the output voltage levels. The effectiveness of the proposed method is investigated with various waveforms. The theoretical and simulation findings are validated through experimental results.

Application of the Bee Algorithm for Selective Harmonic Elimination Strategy in Multilevel Inverters

This paper presents the Bee optimization method for harmonic elimination in a cascaded multilevel inverter. The main objective in selective harmonic elimination pulsewidth modulation strategy is eliminating low-order harmonics by solving nonlinear equations, while the fundamental component is satisfied. In this paper, the Bee algorithm (BA) is applied to a 7-level inverter for solving the equations. The algorithm is based on the food foraging behavior of a swarm of a honeybees and it performs a neighborhood search combined with a random search. This method has higher precision and probability of convergence than the genetic algorithm (GA). MATLAB software is used for optimization and comparison of GA and BA. Simulation results show superiority of BA over GA in attaining accurate global minima and higher convergence rate. Also, its performance in 10 times run is the same as in 1 time run. Finally, for verifying purposes, an experimental study is performed.

Hybrid Selective Harmonic Elimination PWM for Common-Mode Voltage Reduction in Three-Level Neutral-Point-Clamped Inverters for Variable Speed Induction Drives

This paper proposes a hybrid selective harmonic elimination pulsewidth modulation (SHEPWM) scheme for common-mode voltage reduction in three-level neutral-point-clamped inverter-based induction motor drives. The scheme uses the conventional SHEPWM (C-SHEPWM) to control the inverter at high frequency (≥ 0.9 motor rated frequency) and uses the modified SHEPWM (M-SHEPWM) to control the inverter at low frequency. It also uses a scheme to ensure the smooth transition between the two SHEPWM schemes. As a result, at high frequency, the C-SHEPWM provides the required high modulation index for the motor, while at low frequency, when a passive filter is less effective for common-mode voltage reduction, the M-SHEPWM is used to suppress the common-mode voltage. Experimental results show that the proposed hybrid SHEPWM scheme could meet the modulation index need of the motor and reduce the common-mode voltage in the drive, and the two SHEPWM schemes could transition smoothly.

Low cost and high efficiency of single phase photovoltaic system based on microcontroller

This paper presents a theoretical and practical study of a single phase photovoltaic conversion system. It consists of a step down converter to charge a battery with the maximum power available from photovoltaic generator (PVG) and a single phase voltage source inverter (VSI) to produce a stable AC voltage (220V/50Hz) with lower total harmonic distortion (THD). A new perturb and observe algorithm is designed and implemented in a cheaper microcontroller PIC 16F876 where the duty cycle perturbation and the sampling period are selected to insure the stability of the PV system around the maximum power. The control strategy adopted for the inverter is the Selective Harmonic Eliminated Pulse Width Modulation (SHE PWM). The pulses are calculated and transferred on the PIC 16F876 memory. With this technique, inverter losses are decreased and the output voltage is easily filtered with a simple low pass filter producing a perfectly sine wave form voltage. The battery is sized to supply loads in non-sunny times.With optimization of its various components, the conventional single phase PV system has a low cost, high efficiency but also good power quality which represents a good opportunity to use it in many stand alone photovoltaic applications such as houses lighting. An experimental system has been made to demonstrate the efficiency of the photovoltaic system and to validate simulations done by Matlab–Simulink environment.

A New Near Optimal Harmonics Elimination PWM Algorithm for AC Traction Drives

For AC traction system, it is necessary to develop voltage source inverters (VSI) which is compatible with existing types of signaling systems adopted by various railway operators. The varying harmonics incidents of conventional sinusoidal–type PWM VSI is not particularly suitable as it produces unwanted harmonics within the signaling frequency band. One well–known solution to ensure that the unwanted harmonics do not appear on the spectra is by using the harmonics elimination PWM (HEPWM) switching method. However, the application of HEPWM has been somewhat limited by the fact that the switching angles cannot be calculated online by a microprocessor–based waveform generator. This is due to the fact that the equations involved are non–linear and transcendental in nature. The paper presents an algorithm to calculate near optimal switching angles, which will permit a fast and efficient realization using a microprocessor. The method is based on quadratic approximation approach which is derived from the computed trajectories of angles. Key words: power electronics; inverter; pulse-width modulation (PWM); railway signalling; harmonics

Zero-Sequence Circulating Current Reduction Method for Parallel HEPWM Inverters Between AC Bus and DC Bus

In this paper, a real-time circulating current reduction method for parallel harmonic-elimination pulsewidth modulation (HEPWM) inverters is proposed. HEPWM techniques are often used in high-capacity inverters. For instance, in a hybrid microgrid, the inverters are employed to transfer power between the dc and ac buses. If the inverters are in parallel operation, the zero-sequence path can be established, and the zero-sequence circulating current will circulate among the inverters. The proposed method installs a cascade null-vector control system behind the conventional three-phase HEPWM modulator. The proposed null-vector control system can be disabled to save switching losses when the zero-sequence circulating current is small, whereas it can be enabled when the zero-sequence circulating current becomes large. The proposed method does not affect the line-to-line voltage waveforms of HEPWM inverters, and it can easily enable/disable the null-vector control system to provide bumpless transfer. Compared with the conventional HEPWM with zero-sequence harmonics elimination, the proposed method can provide an extra 15% modulation index range. Results that were obtained from both simulation and experiments confirmed the performance and effectiveness of the proposed method.

Solution trajectories for selective harmonic elimination pulse-width modulation for seven-level waveforms: analysis and implementation

A detailed analysis of a seven-level selective harmonic elimination pulse-width modulation method suitable for multilevel DC–AC converters is presented in this study. Solution trajectories that cover the normal range of modulation indices typical for the operation of multilevel inverters are documented. The properties and characteristics of these solutions with regard to the distribution of switching instants (angles) to the levels of the waveform are also reported. Selected simulation and experimental results are included for both steady state operation and under dynamic changes of the modulation index and the output frequency. The results confirm the effectiveness of the method.

Design of an Optimized Modulation for AC-DC Harmonic Immunity in VSC HVDC Transmission

Control methods based on selective harmonic elimination pulse-width modulation (SHE-PWM) techniques offer the lowest possible number of switching transitions. This feature also results in the lowest possible level of converter switching losses. For this reason, they are very attractive techniques for the voltage- source-converter-(VSC) based high-voltage dc (HVDC) power transmission systems. The paper discusses optimized modulation patterns which offer controlled harmonic immunity between the ac and dc side. The application focuses on the conventional two-level converter when its dc-link voltage contains a mix of low- frequency harmonic components. Simulation and experimental results are presented to confirm the validity of the proposed switching patterns.

Research on a cascaded multilevel inverter by employing three-phase transformers

Cascaded multilevel H-Bridge inverter is a promising topology and is an alternative for converters that are used for grid-connected photovoltaic/wind-power generators, flexible alternating current systems and motor drive applications. The cascade multilevel inverter (CMI) can flexibly expand the output power capability and favourable to develop because the converter provides modularity in topology, control structure and modulation. A recent version of CMI topology employs a single dc source and low-frequency three-phase transformers. Compared with conventional topologies, this CMI with three-phase transformers facilitates high-quality output waveforms with reduced number of components. Additionally, structure has high degree of freedom for specifying the cost function in terms of filter size, losses and total harmonic distortion (THD). In the present study, the authors proposed three major control techniques for this CMI namely (i) fundamental frequency, (ii) selective harmonic elimination PWM (SHEPWM) and (iii) sinusoidal PWM (SPWM) methods. To demonstrate presented CMI effectively, THD comparison is carried out with conventional seven-level CMI. Selected experimental results are reported to verify and validate the theoretical findings.

Multimodule HVDC System Using SHE-PWM With DC Capacitor Voltage Equalization

Multimodule converter-based high-voltage dc (HVDC) power transmission incorporates numerous voltage-source converters (VSCs) connected in series/parallel to provide increased voltage/current and, hence, power ratings. When a selective harmonic elimination pulsewidth-modulation (SHE-PWM) technique is employed, the switching losses of the VSC modules are reduced but the voltages of the series-connected dc capacitors are not naturally balanced. In this paper, a voltage-balancing technique is reported. The technique relies on monitoring the voltage of each dc capacitor and when such voltage exceeds its preset band, the realization of the multilevel waveform reallocates the task to another VSC module to ensure that voltage balance between all capacitors is achieved. Three-phase bipolar VSC modules are considered in this paper and each one is modulated by a precalculated multilevel SHE-PWM waveform. The time-domain waveforms and their line-to-line voltage spectra are verified by simulation using PSCAD/EMTDC. The selected results are presented to confirm the viability of the proposed method.

Study on MPPT Control of the Single-Stage Grid-Connected Photovoltaic System

The fuzzy control can react to the environment change quickly and keep the photovoltaic system work on its maximum power point all the time. However, the limitation in the fuzzy control itself and the characteristic of self-optimization lead to serious concussion of the system around the maximum power point. Whereas the PID control method is effective in the elimination of concussion and is easy to be realized. To tackle the problem, the fuzzy parameter self-tuning PID control method is adopted in the paper to achieve the MPPT control of PV system. The method can effectively eliminate the concussion around the maximum power point and improve the stability of the system; and achieve grid-connected operation with a high power factor in combination with the harmonic elimination PWM control of inverter. Simulation results show that the scheme has merits of simple structure, high power factor and reliable operation, etc. and is worthwhile to be popularized.

Multiple Switching Patterns for SHEPWM Inverters Using Differential Evolution Algorithms

An investigation on multiple patterns for both odd and even number of switching angles in a three-phase selective harmonic elimination pulse width modulation (SHEPWM) inverter is presented. The switching patterns are eand classified on the basis of the harmonic performance of the waveforms output from the inverter. Differen tial evolution algorithms are employed to calculate the optimum switching angl es. Selected cases are verified experimentally by a digital signal processorbased hardware implementation. The experimental res ults show that all types of the patterns offer low ha rmonic distortions on the inverter output after filtering. However, one tgives better harmonic performance is identified .

Hybrid Flying-Capacitor-Based Active-Neutral-Point-Clamped Five-Level Converter Operated With SHE-PWM

A five-level flying-capacitor (FC)-based active-neutral-point-clamped (ANPC) converter is an arrangement of a three-level ANPC converter and a two-level cell. In this paper, a control strategy is proposed to regulate the voltage across the FCs at their respective reference voltage levels by swapping the switching patterns of the switches based on the polarity of the output current, the polarity of the FC voltage, and the polarity of the fundamental line-to-neutral voltage under selective harmonic elimination pulsewidth modulation. The voltage across the FCs and the dc-link capacitors are simultaneously controlled at their reference voltage levels. The proposed control strategy is applied using power system computer aided design/electromagnetic transients including dc on a static synchronous compensator operating under a power-factor-correction mode to verify its performance. Experimental results are also presented for low and high number of angles per quarter period using a low-power laboratory prototype.

A Hybrid PWM Applied to High-Power Three-Level Inverter-Fed Induction-Motor Drives

A hybrid pulsewidth modulation (PWM), combining the merits of both space-vector PWM (SVPWM) and selective harmonic elimination (SHE) PWM (SHEPWM), is proposed for three-level neutral-point-clamped (NPC) inverter-fed high-power adjustable-speed drives, which uses asynchronous SVPWM at low frequency and SHEPWM at high frequency. For SHEPWM, a novel formula is proposed to obtain the initial values of switching angles, leading to another valid solution that is different from the known ones in the literature. Furthermore, it is shown that, by eliminating the quarter-wave symmetry, unlimited groups of solutions to three-level SHEPWM can be obtained. The characteristics of the multiple solutions in terms of harmonic distribution, pulsewidth, and total harmonic distortion are investigated for the application of SHE in practical drives. Switching between SVPWM and SHEPWM is problematic if no appropriate measure is taken, particularly when the influence of the minimum pulsewidth (MPW) is considerable. A simple but effective method, taking into account the MPW, is proposed in this paper to address this problem. The multiple groups of solutions to SHE are simulated and experimentally verified on a low-voltage three-level NPC inverter prototype. Experimental results obtained from a low-voltage prototype and an industrial 6-kV/1250-kW three-level drive are presented to validate the smooth switching between SVPWM and SHEPWM.