Mohammed S. Agamy

A comparative study of central and distributed MPPT architectures for megawatt utility and large scale commercial photovoltaic plants

In this paper different distributed PV architectures are studied from an energy yield perspective. These distributed architectures are applied to massively paralleled thin film plants employing high voltage PV modules, mc-Si plants with long series strings of low voltage modules and plants with medium voltage thin film modules in order to evaluate the effectiveness of the distributed architecture in each case. The effects of partial shading, module mismatch and cable losses are quantified in order to obtain the energy yield for each of the architectures under study. The results of this trade-off study are used to quantify the benefits of a distributed architecture as well as determine the optimal location of the dc/dc converters that perform the MPPT function.

A Three-Level Resonant Single-Stage Power Factor Correction Converter: Analysis, Design, and Implementation

This paper presents a new single-stage power factor correction AC/DC converter based on a three-level half-bridge resonant converter topology. The proposed circuit integrates the operation of the boost power factor preregulator and the three-level resonant DC/DC converter. A variable-frequency asymmetrical pulsewidth modulation controller is proposed for this converter. This control technique is based on two integrated control loops: the output voltage is regulated by controlling the switching frequency of the resonant converter, whereas the DC-bus voltage and input current are regulated by means of duty cycle control of the boost part of the converter. This provides a regulated output voltage and a nearly constant DC-bus voltage regardless of the loading condition; this, in turn, allows using smaller switches and consequently having a lower on resistance helping to reduce conduction losses. Zero-voltage switching is also achieved for a wide range of loading and input voltage. The resulting circuit, therefore, has high conversion efficiency making it suitable for high-power wide-input-voltage-range applications. The effectiveness of this method is verified on a 2.3-kW 48-V converter with input voltage (90-265 Vrms).

A Variable Frequency Phase-Shift Modulated Three-Level Resonant Single-Stage Power Factor Correction Converter

This paper presents a new single-stage three-level resonant power factor correction ac-dc converter suitable for high power applications (in the order of multiple kilowatts) with a universal input voltage range (90-265 Vrms). The proposed topology integrates the boost input power factor preregulator with a half-bridge three-level resonant dc-dc converter. The converter operation is controlled by means of a combination of phase-shift and variable frequency control. The phase-shift between the switch gate pulses is used to provide the required input current shaping and to regulate the dc-bus voltage to a set reference value for all loading conditions, whereas, variable frequency control is used to tightly regulate the output voltage. An auxiliary circuit is used in order to balance the voltage across the two dc-bus capacitors. Zero voltage switching (ZVS) is also achieved for a wide range of loading and input voltage by having a lagging resonant current in addition to the flowing of the boost inductor current through the body diodes of the upper pair of switches in the free wheeling mode. The resulting circuit, therefore, has high conversion efficiency and lower component stresses making it suitable for high power, wide input voltage range applications. The effectiveness of the proposed converter is verified by analysis, simulation, and experimental results.

Dc-dc converter topology assessment for large scale distributed photovoltaic plant architectures

Distributed photovoltaic (PV) plant architectures are emerging as a replacement for the classical central inverter based systems. However, power converters of smaller ratings may have a negative impact on system efficiency, reliability and cost. Therefore, it is necessary to design converters with very high efficiency and simpler topologies in order not to offset the benefits gained by using distributed PV systems. In this paper an evaluation of the selection criteria for dc-dc converters for distributed PV systems is performed; this evaluation includes efficiency, simplicity of design, reliability and cost. Based on this evaluation, recommendations can be made as to which class of converters is best fit for this application.

Adaptive fuzzy variable structure control of induction motors

In variable structure sliding mode control schemes, one has to adjust the controller gains in order to obtain an acceptable response. The controller gains are required to be readjusted under variation of the disturbance load torque and/or of the parameters of the induction motor. To compensate automatically for the uncertainties experienced by the system, the controller gains are adjusted by a fuzzy inference mechanism. Furthermore, an adaptive fuzzy sliding mode controller is proposed. It combines the merits of sliding mode control, fuzzy inference mechanism and the adaptive algorithm. First, a sliding mode controller is designed, and then a fuzzy inference mechanism is used to compensate for the uncertainties experienced by the system by adjusting the reaching rates of the sliding mode controller. Finally an adaptation algorithm is used to adjust the centers of the fuzzy sets in order to reduce the control effort and chattering. Simulation results verify the effectiveness of the proposed algorithm.

Performance Comparison of Single-Stage Three-Level Resonant AC/DC Converter Topologies

In this paper, the performance of different three-level resonant converters is studied for single-stage power factor correction operation. These converters are suitable for power ranges higher than that in the currently available single-stage converters, due to their high efficiency and reduced component stresses. All the converters presented here are characterized by their ability to regulate the output voltage as well as the dc bus voltage. This leads to lower voltage stresses, wider input voltage range, higher output power applications, and improved efficiencies compared to existing single-stage topologies. Due to the availability of more degrees of freedom in the presented converters, two types of control strategies can be used for this purpose: variable frequency asymmetrical pulsewidth modulation control and variable frequency phase-shift modulation control. Three resonant converters will be studied in this paper and their performances as well as the applicability of the aforementioned control methods for each converter are compared. A 2.3-kW, 48-V converter with input voltage range of 90-265 Vrms is used to study the performance of each case.

A new single stage power factor corrected three level resonant AC/DC converter with and without active current control

A new single-stage power factor correction converter based on a three-level LCC resonant topology that is suitable for high power applications and universal input voltage (90-265 Vrms) is proposed. It integrates the pulse width modulated (PWM) boost pre-regulators and variable frequency resonant circuits giving a variable frequency asymmetrical PWM (VFAPWM) resonant circuit operation. This regulates the output voltage, obtains a sinusoidal input current at near unity power factor and regulates the DC bus voltage to a controlled level leading to reduced switch ratings, therefore, improving the efficiency. Zero-voltage-switching is achieved for wide ranges of input voltage and loading. The circuit is studied in both continuous and discontinuous conduction modes. Simulation results for a 48 V, 2.3 kW, three-level half -bridge resonant converter operating at an input voltage of 90-265 Vrms and overall resonant frequency of 770 kHz, verify the effectiveness of the proposed method.

An Adaptive Energy Storage Technique for Efficiency Improvement of Single-Stage Three-Level Resonant AC/DC Converters

The use of single-stage power-factor-corrected (SSPFC) three-level resonant ac/dc converters solves many problems that present SSPFC converters face today, namely, high component stresses, high circulating currents, and low efficiency. This makes single-stage three-level resonant ac/dc converters a good candidate for high-power applications. These converters provide the flexibility of simultaneously using two control variables. They can operate with a combined variable-frequency and asymmetrical pulsewidth modulation or with a combined variable-frequency phase-shift modulation. This provides good regulation of the output voltage, dc-bus voltage, and input current. The drawback of these methods is that the efficiency of the converter drops as the load is reduced because the converter starts to drift away from its resonance frequency, thus leading to more circulating currents and conduction losses. Therefore, a load-adaptive energy storage technique is proposed in this paper to guarantee the converter operation near its maximum efficiency point for a wide range of loading. This leads to almost constant converter efficiency from full load to 40% load. The use of interleaved converters is also proposed to extend the constant efficiency range of operation to lighter loads (15%-20% of full load). Analytical simulation and experimental results are presented to verify the proposed methods.

A new zero voltage switching single stage power factor corrected three level resonant AC/DC converter

This paper presents a new single stage power factor correction (SSPFC) AC/DC converter based on a three level LCC resonant converter topology. The proposed circuit integrates the operation of the boost power factor pre-regulator and the three level resonant DC/DC converter. The proposed control technique for this circuit is based on two control loops: the output voltage is regulated by controlling the switching frequency of the resonant converter, whereas, the DC bus voltage is regulated by means of duty cycle control of the boost part of the converter. This provides a regulated output voltage and a nearly constant DC bus voltage regardless of the loading condition; this in turn allows using smaller switches and consequently having a lower on resistance helping to reduce conduction losses. Zero voltage switching (ZVS) is also achieved for a wide range of loading and input voltage by having a lagging resonant current in addition to the flowing of the boost inductor current through the body diodes of the upper pair of switches in the free wheeling mode. The effectiveness of this method is verified on a 2.3 kW, 48 V converter, input voltage (90-265 Vrms). The converter has a resonant frequency of 770 kHz, and is operated above resonant frequency.

An Adaptive Energy Storage Technique for Efficiency Improvement of Single Stage Three-Level Resonant AC/DC Converters

This paper presents an efficiency improvement method for single-stage PFC converters operating with variable frequency asymmetrical pulse width modulation. A load adaptive energy storage technique is proposed to guarantee the converter operation near its maximum efficiency point for a wide range of loading. This leads to almost constant converter efficiency from full load to 40% load. The use of interleaved converters is also proposed to extend the constant efficiency range of operation to lighter loads. Analytical, simulation and experimental results are presented to verify the proposed methods.