Nasser Kutkut

A Review of Power Decoupling Techniques for Microinverters With Three Different Decoupling Capacitor Locations in PV Systems

The reliability of the microinverter is a very important feature that will determine the reliability of the ac-module photovoltaic (PV) system. Recently, many topologies and techniques have been proposed to improve its reliability. This paper presents a thorough study for different power decoupling techniques in single-phase microinverters for grid-tie PV applications. These power decoupling techniques are categorized into three groups in terms of the decoupling capacitor locations: 1) PV-side decoupling; 2) dc-link decoupling; and 3) ac-side decoupling. Various techniques and topologies are presented, compared, and scrutinized in scope of the size of decoupling capacitor, efficiency, and control complexity. Also, a systematic performance comparison is presented for potential power decoupling topologies and techniques.

Power decoupling techniques for micro-inverters in PV systems-a review

This paper reviews the power decoupling techniques of micro-inverters used in single-phase, grid-tied PV systems. The power decoupling techniques are categorized into three groups: (1) PV side decoupling; (2) DC link decoupling; and (3) AC side decoupling. Various topologies and techniques are presented, compared, and evaluated against the size of capacitance, efficiency and control complexity. Finally, potential topologies and technologies are pointed out as the best options for power decoupling implementation.

Design and Implementation of Three-Phase Two-Stage Grid-Connected Module Integrated Converter

Module integrated converters (MICs) in single phase have witnessed recent market success due to unique features such as improved energy harvest, improved system efficiency, lower installation costs, plug-and-play operation, and enhanced flexibility and modularity. The MIC sector has grown from a niche market to mainstream, especially in the United States. Assuming further expansion of the MIC market, this paper presents the microinverter concept incorporated in large size photovoltaic (PV) installations such as megawatts (MW)-class solar farms where a three-phase ac connection is employed. A high-efficiency three-phase MIC with two-stage zero voltage switching (ZVS) operation for the grid-tied PV system is proposed which will reduce cost per watt, improve reliability, and increase scalability of MW-class solar farms through the development of new solar farm system architectures. The first stage consists of a high-efficiency full-bridge LLC resonant dc-dc converter which interfaces to the PV panel and produces a dc-link voltage. A center points iteration algorithm developed specifically for LLC resonant topologies is used to track the maximum power point of the PV panel. The second stage is comprised of a three-phase dc-ac inverter circuit which employs a simple soft-switching scheme without adding auxiliary components. The modeling and control strategy of this three-phase dc-ac inverter is described. Because the dc-link capacitor plays such an important role for dual-stage MIC, the capacitance calculation is given under type D voltage dip conditions. A 400-W prototype was built and tested. The overall peak efficiency of the prototype was measured and found to be 96% with 98.2% in the first stage and 98.3% in the second stage.

A full bridge soft switched telecom power supply with a current doubler rectifier

This paper presents a full bridge soft switched PWM telecom power supply with a current doubler rectifier. The full bridge PWM DC-to-DC power converter offers high power handling capability, minimum VA ratings for the main devices, no additional auxiliary components and simple PWM-based control strategy. Phase shift control is employed to regulate the output voltage and achieve soft switching. The transformer leakage inductance is utilized effectively to achieve zero voltage turn-on for the main devices. The current doubler rectifier has only one diode conduction drop in addition to frequency doubling in the output capacitor. The transformer secondary winding current rating is one half the load current which improves the overall efficiency of the power converter.

A three-port Photovoltaic (PV) micro-inverter with power decoupling capability

This paper presents a new micro-inverter topology that is intended for single-phase grid-connected PV systems. The features of the proposed topology are: (1) eliminating the double-frequency power ripple using small film capacitor; (2) improving the maximumpower-point tracking (MPPT) performance; (3) using long life-time film capacitors, which will improve the reliability of the inverter; and (4) requiring no additional circuitry to manage the transformer leakage energy.

Efficiency Improvement of Grid-Tied Inverters at Low Input Power Using Pulse-Skipping Control Strategy

A pulse-skipping control strategy is proposed to improve efficiency of grid-tied inverters at light loads. To maximize the efficiency of pulse-skipping operation mode, three key parameters are identified and optimized based on a loss model, which is developed to find the maximal efficiency points using a 3-D search technique. A 200-W prototype inverter was setup to verify the proposed control strategy. The experimental results show that the proposed pulse-skipping control strategy greatly improves the inverters efficiency at light loads and match the simulation results fairly well, thus, validating the proposed optimization method for pulse-skipping operation.

Efficiency improvement of grid-tied inverters at low input power using pulse skipping control strategy

Pulse skipping control strategy is applied to improve efficiency of grid-tied inverter at light load. To maximize the efficiency of pulse skipping operation mode, three key parameters are identified and can be optimized based on a loss model, which is developed to find the maximal efficiency points using three-dimension searching technique. To reduce the potential of pulsating on the power grid, the synchronization of the power pulse to the grid is addressed by varying the DC bus voltage window, namely varying the upper and lower DC voltage limits. A 200W prototype is setup to verify the proposed strategy. The experimental results show that the proposed strategy greatly improves the efficiency at light load and match the simulation results fairly well, thus verifying the validity of the proposed optimization method for pulse skipping.

A Center Point Iteration MPPT Method With Application on the Frequency-Modulated LLC Microinverter

Maximum power point tracking (MPPT) is an essential technique to harvest PV power under varying environments. Perturb and observe (P& O) algorithms are the most broadly used MPPT due to their effectiveness and simplicity. However, it is difficult to balance the tracking speed and oscillation requirements in the conventional P& O with fixed perturb. Adaptive P& O techniques have been proposed as a solution to these problems. However, they are based on duty cycle modulation for conventional pulse width modulation converters. None of them deal with the variable frequency modulation for resonant converters. In this paper, a center point iteration MPPT is proposed, with its variable perturb in frequency. The proposed scheme overcomes the drawbacks of conventional P& O with a simple calculation. Moreover, it is suitable for various power curves, especially the LLC microinverter power curves, which may confuse conventional MPPT algorithms. The effectiveness of the proposed MPPT method was verified in theory. A 300 W prototype was constructed, and the experimental results verified the effectiveness of the proposed center point iteration MPPT. An advanced version was also introduced in order to accelerate the tracking speed.

Distributed Battery Micro-Storage Systems Design and Operation in a Deregulated Electricity Market

This paper covers some design and operation aspects of distributed battery micro-storage systems in a deregulated electricity market system. In this paper, the term “micro” refers to the size of the energy storage (ES) system compared to the grid generation, with a capacity from few kilowatt-hours and up. Generally, ES enhances the performance of renewable distributed generators (DGs) and increases the efficiency of the entire power system. Energy storage allows for leveling the load, shaving peak demands, and furthermore, transacting power with the utility grid. In this paper, different design aspects of distributed micro-storage systems are covered such as system architecture, system sizing, power stage design, battery management system (BMS), economic aspects and operation in a deregulated electricity market with and without renewable DGs.

High Efficiency Dual-Mode Current Modulation Method for Low-Power DC/AC Inverters

Boundary conduction mode (BCM) zero voltage switching (ZVS) current control is a promising soft switching method for microinverter applications. In this letter, different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching schemes is proposed to improve the efficiency of the microinverter. The experimental results show that by using this proposed current modulation scheme, higher efficiency of 0.5% can be achieved with no additional cost for a 400-W three-phase microinverter.