Prasad N. Enjeti

Programmed PWM techniques to eliminate harmonics - A critical evaluation

The authors establish the superiority of programmed PWM (pulsewidth modulation) techniques over the conventional carrier-modulated PWMs and provide a critical evaluation of the programmed PWM schemes applied to single and three-phase inverters, thereby providing the framework and guidelines for the selection of the appropriate technique for each application area. Evaluation criteria include harmonic loss factor and total harmonic distortion factors defined at the input and output of the inverter terminals. A simple low-cost solution for obtaining the required PWM switching points is proposed. Selected results are verified experimentally.<<ETX>>

High-Performance Adaptive Perturb and Observe MPPT Technique for Photovoltaic-Based Microgrids

Solar photovoltaic (PV) energy has witnessed double-digit growth in the past decade. The penetration of PV systems as distributed generators in low-voltage grids has also seen significant attention. In addition, the need for higher overall grid efficiency and reliability has boosted the interest in the microgrid concept. High-efficiency PV-based microgrids require maximum power point tracking (MPPT) controllers to maximize the harvested energy due to the nonlinearity in PV module characteristics. Perturb and observe (P&O) techniques, although thoroughly investigated in previous research, still suffer from several disadvantages, such as sustained oscillation around the MPP, fast tracking versus oscillation tradeoffs, and user predefined constants. In this paper, a modified P&O MPPT technique, applicable for PV systems, is presented. The proposed technique achieves: first, adaptive tracking; second, no steady-state oscillations around the MPP; and lastly, no need for predefined system-dependent constants, hence provides a generic design core. A design example is presented by experimental implementation of the proposed technique. Practical results for the implemented setup at different irradiance levels are illustrated to validate the proposed technique.

A new single phase to three phase converter with active input current shaping for low cost AC motor drives

A single-phase-to-three-phase converter for a low-cost AC motor drive is proposed. The converter employs only six switches and incorporates a front-end half-bridge active rectifier structure that provides the DC link with an active input current shaping feature, which results in sinusoidal input current at close to unity power factor. The front-end rectifier in the converter permits bidirectional power flow and provides for excellent regulation against fluctuations in source voltage, facilitating regenerative braking of the AC motor drive. A control strategy that maintains a near-unity power factor over the full operating range and is easy to implement is described. Suitable design guides for the selection of filter components are presented. Simulation and experimental results that verify the developed theoretical models are also presented.<<ETX>>

Sharing of nonlinear load in parallel connected three-phase converters

In this paper, a new control method is presented which enables equal sharing of linear and nonlinear loads in three-phase power converters connected in parallel, without communication between the converters. The paper focuses on solving the problem that arises when two converters with harmonic compensation are connected in parallel. Without the new solution, they are normally not able to distinguish the harmonic currents that flow to the load and harmonic currents that circulate between the converters. Analysis and experimental results on two 90-kVA 400-Hz converters in parallel are presented. The results show that both linear and nonlinear loads can be shared equally by the proposed concept.

Design of a Wide Input Range DC–DC Converter With a Robust Power Control Scheme Suitable for Fuel Cell Power Conversion

This paper analysis and design of a wide input range DC-DC converter is proposed along with a robust power control scheme. The proposed converter and its control is designed to be compatible to a fuel cell power source, which exhibits 2:1 voltage variation as well as a slow transient response. The proposed approach consists of two stages: a conventional boost converter stage cascaded with a current-fed two-inductor boost converter topology, which has a higher voltage gain and isolation from the input source. The function of the first boost converter stage is to maintain a constant voltage at the input of the cascaded DC-DC converter to ensure optimal performance characteristics with high efficiency. At the output of the first boost converter a battery or ultracapacitor energy storage is connected to take care of the fuel cell slow transient response (200 watts/min). The robust features of the proposed control system ensure a constant output DC voltage for a variety of load fluctuations, thus limiting the power being delivered by the fuel cell during a load transient. Moreover, the proposed configuration simplifies the power control management and can interact with the fuel cell controller. The simulation results and the preliminary experimental results confirm the feasibility of the proposed system.

A high-performance single-phase rectifier with input power factor correction

In this paper, a high-performance single-phase AC-to-DC rectifier with input power factor correction is proposed. The proposed approach has many advantages, including fewer semiconductor components, simplified control, and high-performance features, and satisfies IEC 555 harmonic current standards. Simulation and experimental results obtained on a laboratory prototype are discussed. A hybrid power module of the proposed approach is also shown.

An improved inverter output filter configuration reduces common and differential modes dv/dt at the motor terminals in PWM drive systems

In this paper, an improved inverter output filter is proposed for pulsewidth-modulated (PWM) drive systems. The proposed filter is shown to effectively reduce both the differential and common modes dv/dt at the motor terminals, even in the presence of long motor leads. Reducing differential mode dv/dt reduces overvoltages at the motor terminals and lowers the stress on the motor insulation. Lowering common mode dv/dt is shown to significantly reduce high-frequency leakage currents to ground and induced shaft voltage in the motor. An important advantage of the approach is that the filter can be installed within the inverter enclosure to achieve both the differential and common modes dv/dt reductions at the motor terminals. Thus, the use of the filter can contribute to enhanced bearing life and improve reliability of PWM drive systems. Analysis, design equations, and experimental results on a 480-V 20-hp PWM drive system are presented. The filter configuration is an excellent candidate for many new and retrofit PWM 480-V/575-V drive systems.

Design considerations for an inverter output filter to mitigate the effects of long motor leads in ASD applications

Design considerations for an inverter output filter to mitigate the effects of long motor leads in adjustable-speed drive (ASD) applications are presented. It is shown by analysis that, for a given length of cable, reducing the dv/dt of the pulsewidth modulated (PWM) inverter output voltage applied to the cable below a critical value will eliminate overvoltages due to voltage reflections. Design issues for a low-pass filter at the inverter output terminals to reduce the dv/dt of the inverter output pulse are examined in detail. The filter operation is verified for the entire variable frequency range of the inverter. The performance of the filter is evaluated through simulations and experimentally on a 460 V commercially available AC motor drive (PWM insulated gate bipolar transistor (IGBT)). The proposed inverter output filter is then compared with a motor terminal shunt filter also designed to reduce overvoltages and ringing at the motor terminals.

A new approach to improve power factor and reduce harmonics in a three phase diode rectifier type utility interface

A novel approach to improving power factor and reducing harmonics generated by a three-phase diode-rectifier-type utility interface is proposed. This approach is passive and consists of a novel interconnection of a star/delta power transformer between the AC and DC sides of the diode rectifier topology. This interconnection, in combination with the 120 degrees conduction intervals of each diode, is shown to generate a circulating third-harmonic current between the AC and DC side of the rectifier bridge. This current is shown to drastically improve the performance of the diode-rectifier-type interface. The resulting input current is near-sinusoidal, with significant reduction in line current harmonics. The proposed system can be viewed as a cost-effective retrofit to the existing diode rectifier utility interface applications. A design example illustrates the sizing of the necessary passive components. Selected results were verified experimentally on a laboratory prototype system.<<ETX>>

Development of an equivalent circuit model of a fuel cell to evaluate the effects of inverter ripple current

In this paper an impedance model of the proton exchange membrane fuel cell stack (PEMFCS) is proposed. The proposed study employs an equivalent circuit of the PEMFCS derived by frequency response analysis (FRA) technique. An equivalent circuit for the fuel cell stack is developed to evaluate the effects of ripple currents generated by the power-conditioning unit. The calculated results are then verified by means of experiments on three commercially available fuel cells: Avista Labs SR-12 (500 W), Ballard Nexa (1.2 kW) and BCS-Tech (300 W) PEMFC system. The relationship between ripple current and fuel cell performance: such as power loss and fuel consumption is investigated. Experimental results show that the ripple current can contribute up to 10% reduction in the available output power.