Abdul Hamid Bhat

Capacitor Voltage Balancing of Three-Phase Neutral-Point-Clamped Rectifier Using Modified Reference Vector

This paper presents a control strategy using optimized switching sequences which results in capacitor voltage balancing of a three-phase neutral-point-clamped rectifier. In front end rectifiers, magnitude and lag angle of reference current vector (w.r.t supply voltage vector) varies depending upon amount of reactive power compensation and load current. Accordingly, the effectiveness of small and medium vectors (using SVPWM) for same switching state varies in each sector and its regions. This observation along with detailed analysis of current pattern for both the capacitors and their effect on neutral point voltage forms the basis of the proposed approach, which is to modify the reference vector for changing the number of sampling periods within each sector and its regions. The rectifier exhibits excellent performance in terms of other critical parameters like unity input power factor, low input current THD, minimum possible switching losses, reduced-rippled, and well-regulated dc-bus voltage, particularly, the capacitor voltage balancing. The proposed control algorithm is tested through exhaustive simulations and validated through experimental results obtained from a laboratory prototype of the rectifier.

An artificial-neural-network-based space vector PWM of a three-phase high power factor converter for power quality improvement

In this paper, an artificial neural network (ANN) based space vector pulse width modulation (SVPWM) for a three-phase, two-level high power factor converter is presented. A multilayer feedforward neural network with backpropagation is used. The ANN receives amplitude and angle of the reference vector to calculate the duty cycles of various space vectors in different sectors which can be used for the generation of PWM pulses for the control of converter. The ANN significantly reduces the computational efforts of the modulation technique and makes the implementation of space vector modulation algorithm very fast without losing precision compared to the conventional SVM algorithm implementation using look up table. The performance of the converter in terms of its power quality on the input and output side is investigated through computer simulations and the results have been found excellent. A mathematical model of the rectifier is also developed. A comparative analysis of the converter using space vector modulation without and with ANN implementation is also presented to validate the usefulness of the ANN implementation of space vector modulation. The principle can be extended to multilevel converters with certain modifications.

Improved Power Quality AC/DC Converters

In this chapter, various power quality problems created by the widespread use of conventional diode bridge rectifiers and line-commutated AC/DC converters have been discussed. The impact of these problems on the health of power systems and working of sensitive equipments has also been discussed. Need for addressing these burning power quality issues has been emphasized. Recent trends of addressing these issues have been briefly discussed. A new breed of improved power quality AC/DC converters has been discussed in details. Development of various topologies and state-of-art of this breed of converters has been discussed briefly. The state-of-the-art multilevel converters used as improved power quality converters have been covered in this chapter with emphasis on three-level neutral-point clamped converters. This converter has been investigated for improved power quality and various simulation results have been presented to prove their effectiveness in terms of excellent power quality like nearly unity input power factor and negligible harmonic distortion of source current. The simulation results have been obtained with sinusoidal PWM and sapce-vector PWM modualtion algorithms. Simulation restls have been validated through experimental results which are obtained on a three-level converter by real-time implementation of space-vector PWM technique using a real-time DSP board. The performance investigation of the converter proves the effectiveness of three-level converter in elegantly addressing the burning power quality issues. We know that power systems are designed to operate at frequencies of 50 or 60 Hz. However, certain types of loads produce harmonic currents in the power system. The power system harmonics are not a new phenomenon. Concern over harmonic distortions has ebbed and flowed during the history of electrical power systems. Traditionally the saturated iron in transformers and induction machines, electric arc furnaces, welding equipment, fluorescent lamps (with magnetic ballasts), etc. have been responsible for the generation of harmonics in electric power systems. Most of these equipments also cause the flow of reactive component of current in the system. In recent years, many power electronic converters utilizing switching devices are being widely used in domestic, commercial and industrial applications, ranging from few watts to MWs. However these converters suffer from the drawbacks of harmonic generation and reactive power flow from the source and offer highly non-linear characteristics. The generation of harmonics and reactive power flow in the power systems has given rise to the ‘Electric Power Quality’ problems. Any significant deviation in the magnitude of the voltage, current and frequency, or their waveform purity may result in a

Performance evaluation of a three phase shunt active power filter for power quality improvement

This paper deals with the performance evaluation of a three-phase, Shunt Active Power Filter for Power Quality Improvement. Power Quality problems can be addressed using Shunt Active Power Filters. A simulation model based on mathematical model of a three-phase, Shunt Active Power Filter has been developed. The results show an inherent power quality improvement in terms of reduced Total Harmonic Distortion (THD) of source current. It also exhibits Power Factor Correction (PFC) properties to a large extent as the source current and source voltage are in phase.

Optimising the performance of three-phase neutral-point clamped rectifier under disturbed AC mains

In this paper, a novel simplified control technique is proposed to control three-phase neutral-point clamped bidirectional rectifier using optimised switching sequences for disturbed AC mains. In space-vector modulation (SVM) technique using optimised switching sequences, duty ratio and sampling periods in each region are very important. In other words, reference vector trajectory shape and the time spent by the reference vector in each region are very important to obtain exact switching under any supply conditions. With the use of optimised switching sequences, even under ideal supply conditions, it is depicted that source-side and load-side parameters deviate from acceptable limits, and a DC-bus capacitor voltage unbalance occurs. Under the influence of disturbed AC supply, source-side and load-side parameters deviate more beyond acceptable limits which cause a very large unbalance in DC bus capacitor voltages. This non-ideal performance of the converter is responsible for the deterioration of quality of source currents and a large stress on power semiconductor devices. A new algorithm is proposed in this paper to make the converter performance ideal even under ideal and non-ideal supply conditions. According to supply conditions, the proposed technique varies the speed of the reference vector and, also at the same time, forms a required trajectory while passing through the most effective regions of SVM hexagon. The proposed technique maintains parameters within acceptable limits for source side and load side in terms of unity power factor, low input current total harmonic distortion (THD), minimum switching losses and reduced-rippled, well-regulated DC-bus voltage and also at the same time DC-bus capacitor voltage balance. The simulation results are presented to demonstrate the effectiveness of the proposed control technique.

Capacitor voltage balancing in neutral-point clamped rectifier using modified modulation index technique

Abstract In this paper, a simplified control technique is proposed to control capacitor voltages of three-phase Neutral-Point Clamped bi-directional rectifier based on the modified modulation index technique. The modified modulation index technique generates new position of the reference vector in the space vector hexagon keeping both capacitor voltages constant. The modified reference vector is made either large or small enough so that the effects of redundant switching patterns become prominent to balance capacitor voltages. Changing commutation state for the adoption of redundant switching patterns is dictated by reference vector. The modulation index is modified in proportion to the mean difference in the capacitor voltages plus calculated modulation index. The calculated modulation index is due to change in duty cycle of redundant switching states of the three nearest reference vectors in the sectors in which reference vector lies. The proposed algorithm maintains minimum neural point potential variations and also keeps source side and load side parameters within acceptable limits.

Performance investigation of nine-level cascaded H-bridge inverter-based STATCOM for mitigation of various power quality problems

This paper deals with the design and implementation of cascaded H-bridge multilevel voltage source converter-based static synchronous compensator (STATCOM) for simultaneous active and reactive power compensation, voltage stability and damping of power oscillations. The main objective is to provide active power support and maintain the voltage stability by compensating the reactive power in the power system. In this paper, the multi pulse inverter-based STATCOM has been replaced by multilevel inverter-based STATCOM to overcome the drawbacks of the former. Cascaded H-bridge (CHB) MLI for implementation of STATCOM has been used as CHB MLI is considered the best for STATCOM applications due to its modular structure and requirement of lesser number of components. MLI-based STATCOM is used to provide simultaneous reactive power and active power compensation established by using super-capacitor. The super-capacitors are connected with each DC link capacitor of CHB MLI by using the DC-DC buck boost converter. The STATCOM is also used to provide voltage stability during the power quality problems such as voltage sag and voltage swell, it is established by injecting or absorbing reactive power. The STATCOM is also used for damping the power oscillations by controlled injection of reactive power hence enhancing transient stability. Nine-level cascaded H-bridge MLI-based STATCOM is designed for simultaneous reactive power and active power compensation, voltage stability and damping the power oscillations. Exhaustive simulations have been carried out using MATLAB/Simulink and SimPowerSystems software. The simulation results are presented to prove the effectiveness of MLI-based STATCOM for simultaneous active and reactive power compensation, voltage stability and damping of power oscillation.

Fuzzy logic controlled dynamic voltage restorer for mitigation of various power quality problems

Dynamic voltage restorer (DVR) is used to protect sensitive loads from harmonic distortion, voltage sag/voltage swell, and unbalance in supply voltage economically. In this paper, fuzzy logic-based control scheme has been developed for the production of compensating voltages for controlling DVR. Linear as well as nonlinear loads can be controlled by this scheme. A three-phase programmable voltage source is taken for producing voltage sag/swell and voltage harmonic distortion and this voltage is compared with a reference voltage of same magnitude. At the time of any disturbance, an error signal is generated which is processed by controller based on fuzzy logic theory. The controller generates the gate pulses for PWM inverter energised by a DC source and inverter injects the missing voltage into the distribution line and tightly regulates the voltage at the load terminals. By extensive simulation studies in MATLAB/Simulink and SimPowerSystems environment, the above control strategy is tested. The simultion results for voltage harmonic distortion, voltage sag/swell, and voltage unbalanced mitigation using the proposed control algorithm prove the effectiveness of DVR.

Fuzzy logic controlled hybrid filter for power quality improvement

This paper presents the performance evaluation of a hybrid filter for current harmonic elimination and improved input power factor. Hybrid filter consists of a shunt active power filter and passive filters. In the proposed scheme, shunt active power filter operates at a very low switching frequency, thus giving higher conversion efficiency. The combined operation of shunt active power filter and passive filter ensures THD of source current within the permissible limits set by various regulatory agencies and near unity supply power factor. The effectiveness of the hybrid filter has been verified by the extensive simulation results carried out in MATLAB/Simulink and SimPowerSystem software.

Performance evaluation of D-STATCOM for mitigation of power quality problems at different source and load side fault conditions

One of the major issues in the present electrical power system is power quality. The power quality problems may be of many types whether at source side or load side. Distributed Static Synchronous Compensator (D-STATCOM) is the most convenient device among all FACTS devices for all these power quality problems. This paper deals with performance evaluation with and without D-STATCOM for voltage swell and sag at source side in one model and different types of faults conditions like LG, LLG, LLLG in other model. With the help of MATLAB/Simulink & SimPowerSystems software, the performance analysis of D-STATCOM has been done with conventional PI controller using Hysteresis Current Control (HCC) for the mitigation of these fault conditions.