Mohammad Hassan Hedayati

Common-Mode and Differential-Mode Active Damping for PWM Rectifiers

Modern pulse-width-modulated (PWM) rectifiers use LCL filters that can be applied in both the common mode and differential mode to obtain high-performance filtering. Interaction between the passive L and C components in the filter leads to resonance oscillations. These oscillations need to be damped either by the passive damping or active damping. The passive damping increases power loss and can reduce the effectiveness of the filter. Methods of active damping, using control strategy, are lossless while maintaining the effectiveness of the filters. In this paper, an active damping strategy is proposed to damp the oscillations in both line-to-line and line-to-ground. An approach based on pole placement by the state feedback is used to actively damp both the differential- and common-mode filter oscillations. Analytical expressions for the state-feedback controller gains are derived for both continuous and discrete-time model of the filter. Tradeoff in selection of the active damping gain on the lower order power converter harmonics is analyzed using a weighted admittance function. Experimental results on a 10-kVA laboratory prototype PWM rectifier are presented. The results validate the effectiveness of the active damping method, and the tradeoff in the settings of the damping gain.

Filter Configuration and PWM Method For Single-Phase Inverters With Reduced Conducted EMI Noise

Electromagnetic interference (EMI) noise is one of the major issues during design of grid-tied power converters. A novel LCL filter topology for a single-phase pulsewidth modulation (PWM) rectifier that makes use of bipolar PWM method is proposed for a single-phase to three-phase motor drive power converter. The proposed topology eliminates high dv/dt from the dc-bus common-mode (CM) voltage by making it sinusoidal. Hence, the high-frequency CM current injection to the ground and the motor-side CM current are minimized. The proposed filter configuration makes the system insensitive to circuit nonidealities such as mismatch in inductors values, unequal turn-on and turn-off delays, and dead-time mismatch between the inverter legs. Different variants of the filter topology are compared to establish the effectiveness of the proposed circuit. Experimental results based on the EMI measurement on the grid side and the CM current measurement on the motor side are presented for a 5-kW motor drive. It is shown that the proposed filter topology reduces the EMI noise level by about 35 dB.

Reliability of Profiled Blast Wall Structures

Stainless steel profiled walls have been used increasingly in the oil and gas industry to protect people and personnel against hydrocarbon explosions. Understanding the reliability of these blast walls greatly assists in improving the safety of offshore plant facilities. However, the presence of various uncertainties combined with a complex loading scenario makes the reliability assessment process very challenging. Therefore, a parametric model developed using ANSYS APDL is presented in this chapter. The significant uncertainties are combined with an advanced analysis model to investigate the influence of loading, material and geometric uncertainties on the response of these structures under realistic boundary conditions. To review and assess the effects of the dynamics and nonlinearities, four types of analyses including linear static, nonlinear static, linear transient dynamic, and nonlinear transient dynamic are carried out. The corresponding reliability of these structures is evaluated with a Monte Carlo simulation (MCS) method, implementing the Latin hypercube sampling (LHS) approach. The uncertainties related to dynamic blast loading, material properties, and geometry are represented in terms of probability distributions and the associated parameters. Dynamic, static, linear, and nonlinear responses of the structure are reviewed. Stochastic probabilistic analysis results are discussed in terms of the probability of occurrence, the cumulative distribution functions (CDFs), and the corresponding variable sensitivities. It is observed that using the approach taken in this study can help identify the important variables and parameters to optimize the design of profiled blast walls, to perform risk assessments, or to carry out performance-based design for these structures.

Dynamic behaviour of unstiffened stainless steel profiled barrier blast walls

ABSTRACT Performing optimum design, reliable assessment or suitable verification for stainless steel profiled barrier blast wall structures requires dealing with various challenges, stemming from the associated uncertainties in material properties, fabrication, installation, and more importantly variations in the blast load characteristics. In the analysis, assessment, and design of these blast walls, one of the key areas to be appreciated and understood is the dynamic response of these structures. This paper presents a methodology developed for identifying the predominant structural behaviour and characteristics of profiled barrier blast wall structures, using a probabilistic approach. Twenty parametric base models are developed using Ansys and by implementing a Latin hypercube sampling (LHS) approach, the section properties of the models are represented in terms of probability distributions. A number of models are generated stochastically and modal analyses performed to identify the dynamic sensitivity of these models. The corresponding response classification of these structures is evaluated from the load duration and natural periods of the structures. The results of the study confirm that structural response, for the wide range of profiled blast walls analysed, is mainly quasi-static or static, as opposed to dynamic. In fact, dynamic effects are negligible for unstiffened profiled barrier blast walls and structural responses in most cases can be estimated on a static or quasi-static basis. This conclusion would help a competent design engineer to consider a proper dynamic load factor at an early stage of the design, without involving complex advanced nonlinear dynamic analyses.

Reducing Peak Current and Energy Dissipation in Hybrid HVDC CBs Using Disconnector Voltage Control

Peak fault current and energy dissipation in high-voltage direct current (HVdc) circuit breakers (CBs) are very important parameters that impact dc grid protection development. This paper analyzes a hybrid DCCB (HCB) control that reduces peak current and energy dissipation by regulating the voltage across contacts of the ultrafast disconnector (UFD). This is achieved by manipulating the number of inserted surge arresters while contacts of the UFD are moving apart. The controller is seamlessly integrated with the current controller of HCBs. An analytical model for current and energy calculation is presented, verified, and employed for parametric studies. A PSCAD simulation with 320-kV, 16-kA test circuit confirms that the proposed voltage control reduces the peak current and energy dissipation by around 20%–30%. A 900-V, 500-A HCB laboratory hardware is described and the experimental results are shown to corroborate simulation conclusions.

High Performance Parallel Single-Phase Converter Reconfiguration for Enhanced Availability

Paralleling power converters is a common practice in industries to enhance total power rating, reliability, and availability of the system. In case of fault occurring in systems with parallel converters, the faulty power converter can be isolated and the system can still be operated at reduced power level. In this paper, a grid-connected power converter consisting of two parallel H-bridge converter, with low ground leakage current, is considered. Two contingency configurations, that are also of low ground leakage current, are proposed to enhance the availability of the system. This is done by reconfiguring the power circuit to a single H-bridge, in the case of failure of the other bridge. The power converter is experimentally tested with the proposed configurations for experimental validation. The results show that, the second configuration has better performance in terms of power loss and current THD.

High performance parallel single-phase converter reconfiguration for enhanced availability

Paralleling power converters is a common practice in industries to enhance total power rating, reliability, and availability of the system. In case of fault occurring in systems with parallel converters, the faulty power converter can be isolated and the system can still be operated at reduced power level. In this paper, a grid-connected power converter consisting of two parallel H-bridge converters with low ground leakage current is considered. Two contingency configurations, that are also of low ground leakage current, are proposed to enhance the availability of the system. This is done by reconfiguring the power circuit to a single H-bridge in the case of failure in one of the bridges. The power converter is experimentally tested with the proposed configurations for experimental validation. The results show that the second configuration has better performance in terms of power loss and current total harmonic distortion when operating at lower power level.

Ground leakage current reduction in single-phase grid-connected power converter

One of the major concerns during the design of grid-connected converters is meeting the electromagnetic interference (EMI) standards. Ground leakage current is another aspect which has a stringent limit due to safety concerns. Presence of high frequency pulses, with high dv/dt due to switching actions, in common-mode (CM) voltage excite parasitic capacitances of the power converters. This results in injection of peaky current to ground and causes high EMI noise level. In this paper a new method is proposed to make the CM voltage, of a single-phase grid-connected converter, sinusoidal and free of high frequency pulses. The proposed method consists of two parallel connected H-bridge power converters, which uses unipolar PWM with carrier interleaving angle of 180°. The proposed method reduces the ground leakage current by more than an order of magnitude. A modification to the LCL filter is adopted which further reduces the leakage current by 48%. It is shown that these modifications make the overall system insensitive to circuit nonidealities. Experimental results based on ground leakage current and EMI measurements on a 7.5kW parallel single-phase PWM-rectifier are presented. These results validate the effectiveness of the proposed method.