Souhib Harb

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.

A Single-Stage Microinverter Without Using Eletrolytic Capacitors

This paper presents a new microinverter topology that is intended for single-phase grid-connected PV systems. The proposed microinverter topology is based on a flyback converter, where an extra switch is added to separate the decoupling capacitor from the PV Module, which allows for a high voltage and voltage ripples across its terminals. This results in reducing the power decoupling required capacitance. In this manner, long life-time low power density film capacitors can be used instead of life-time limited high power density electrolytic capacitors, resulting in remarkable increase of microinverters lifespan. The main advantages of the proposed topology are summarized as: 1) eliminating the double-frequency power ripple using a small film capacitor; 2) using long lifetime film capacitors, which will improve the reliability of the inverter; and 3) requiring no additional circuitry to manage the transformer leakage energy. A 100-W microinverter prototype was built to verify the proposed topology. Experimental results show that the proposed topology and its control scheme can realize the power decoupling, while maintaining very good conversion efficiency numbers.

Reliability of Candidate Photovoltaic Module-Integrated-Inverter (PV-MII) Topologies—A Usage Model Approach

This paper proposes a new methodology for calculating the mean time between failure (MTBF) of a photovoltaic module-integrated inverter (PV-MII). Based on a stress-factor reliability methodology, the proposed technique applies a usage model for the inverter to determine the statistical distribution of thermal and electrical stresses for the electrical components. The salient feature of the proposed methodology is taking into account the operating environment volatility of the module-integrated electronics to calculate the MTBF of the MII. This leads to more realistic assessment of reliability than if a single worst case or typical operating point was used. Measured data (module temperature and insolation level) are used to experimentally verify the efficacy of the methodology. The proposed methodology is used to examine the reliability of six different candidate inverter topologies for a PV-MII. This study shows the impact of each component on the inverter reliability, in particular, the power decoupling capacitors. The results confirm that the electrolytic capacitor is the most vulnerable component with the lowest MTBF, but more importantly provide a quantified assessment of realistic MTBF under expected operating conditions rather than a single worst case operating point, which may have a low probability of occurrence.

A Three-port Flyback for PV Microinverter Applications With Power Pulsation Decoupling Capability

A novel single-stage photovoltaic (PV) microinverter with power decoupling capability is proposed in this paper. The proposed topology is based on three-port flyback with one port dedicated to power decoupling function so as to reduce the decoupling capacitance, thus allowing for long lifetime film capacitor to be used. Operation principle is analyzed in details. Key design considerations, including key parameter selections, predictive control strategy, and the dc voltage balance control across the power decoupling capacitor, are given in this paper. A 100-W microinverter prototype is built to verify the proposed topology. Experimental results show the proposed topology can achieve power decoupling, while maintaining good efficiency.

Microinverter and string inverter grid-connected photovoltaic system — A comprehensive study

This paper present a comparison between a string inverter based photovoltaic (PV) energy system and a microinverter based system. Reliability, environmental factors, inverter failure, and electrical safety of a test case 6kW residential PV system are thoroughly evaluated and compared using the two different approaches. The impact of all these features on the cost of the PV system is estimated. The results showed that when the levelized cost of energy (LCOE) is considered the break-even cost can be reached by the microinverter more quickly than with a string inverter operating in the same environment Moreover, considering the replacement costs associated with the expected string inverter failure, the microinverter configuration is the more cost effective.

Ripple-Port Module-Integrated Inverter for Grid-Connected PV Applications

The single-phase inverter has inherent double-frequency power ripple, which if not mitigated internally appears at the input port and deteriorates the MPPT performance. Conventional dc link inverter topologies filter this significant double-frequency ripple by means of the bus capacitance, usually in the form of electrolytic capacitors which have well-known lifetime challenges. This paper presents a double-frequency ripple cancelation concept and experimental proof of concept. The proposed module-integrated inverter is based on the commonly used two-stage inverter. However, a third port is added for ripple cancelation purposes. Hence, a very small capacitance is needed, and, as a result, high reliability film capacitor can be used instead of the bulky, low reliability electrolytic ones.

Single-phase PWM rectifier with power decoupling ripple-port for double-line-frequency ripple cancellation

This paper proposes a new technique for double-frequency ripple-power decoupling in a single-phase PWM rectifier that does not require an electrolytic capacitor, which improves the reliability of the system for long-life applications such as LED lighting. A ripple-port is added in parallel with the output of the PWM rectifier to filter the double-line-frequency ripple using the minimum capacitance necessary for power buffering. Hence, a very small, highly reliable film capacitor is used, which will improve the reliability dramatically compared to the bulky electrolytic capacitor option, which increased the power density of the system. The proposed topology doubles the MTBF increases the lifetime by one order of magnitude. Moreover, the design and control of the ripple-port is independent from the PWM rectifier circuit, so it can be dropped in as an auxiliary circuit to an existing design.

Reliability of candidate photovoltaic module-integrated-inverter topologies

This paper examines the reliability of candidate topologies for a PV module-integrated inverter (MII). A new approach to calculate the mean time between failure (MTBF) using the MIL-HDBK-217 stress factor method is proposed. The new approach takes in consideration the usage model of the inverter, the statistical distribution of expected operating temperature and power processed, rather than a single (typically worst-case) operating point. The technique is applied to the systematic reliability comparative study for six different inverter topologies. This study shows the impact of each component on the inverters reliability, in particular, the power decoupling capacitors.

AC-link, single-phase, photovoltaic Module Integrated Inverter

This paper discusses a new PV-Module Integrated Inverter (PVMII) topology that is ideally suited for use in single-phase grid-connected PV application. The proposed MII is based on bipolar, AC-link principle, so there are no electrolytic capacitors, and a new power decoupling approach, which can use the minimum value of decoupling capacitance. Hence, a very small, highly reliable film capacitor is cost-effectively used, which also improves dramatically the reliability compared to dc-link topologies with electrolytic capacitors. Further, the proposed MII allows bidirectional power flow; a beneficial feature for advanced grid-support features such as local energy storage and reactive power compensation.

Ripple-port module-integrated inverter for grid-connected PV applications

The single-phase inverter has inherent double-frequency power ripple, which if not internally mitigated, appears at the input port and deteriorates the maximum power point tracker performance. Conventional dc link inverter topologies filter this significant double-frequency ripple by means of the bus capacitance, usually in the form of electrolytic capacitors, which have well known lifetime challenges. This paper presents a double-frequency ripple cancellation concept and experimental proof of concept. The proposed module-integrated inverter is based on the commonly used two-stage inverter. However, a third port is added for ripple cancellation purposes. Hence, a very small capacitance is needed, and, as a result, a high-reliability film capacitor can be used instead of the bulky low-reliability electrolytic ones.