Jinia Roy

A high performance T-type single phase double grounded transformer-less photovoltaic inverter with active power decoupling

Transformer-less PV inverters are gaining widespread applications with lower cost, reduced footprint, and improved efficiency. This paper proposes a topology that can eliminate the common mode leakage current which is a major challenge in transformer-less PV inverters. In addition, an active power decoupling strategy is implemented in this topology instead of using large energy storage element for double line frequency power decoupling, thus achieving a smaller volume. A constant input voltage with negligible double line frequency ripple component ensuring high MPPT efficiency is achieved in this topology. Compared to the previously proposed topology, a T-type branch is added as an improvement to fully take advantage of the inherent three level structure resulting in much reduced switching loss and inductor current ripple. A 1 kW, 100 kHz single-phase prototype with 200 V DC input and 120 V/60 Hz AC output using SiC MOSFETs has been built to validate the theoretical analysis. The control strategy and modulation scheme are implemented in DSP TMS320F28335 resulting in 24% reduction in the total loss and 50% reduction in the inductor current ripple.

GaN based high gain non-isolated DC-DC stage of microinverter with extended-duty-ratio boost

Microinverter is attracting more attention due to its compact size, plug and play concept, easy installation, and higher power yield under partial shading condition. This paper explores a converter for the DC-DC stage of a non-isolated microinverter. The topology termed as extended-duty-ratio (EDR) boost, is a hybrid of the interleaved boost and switched capacitor concept which has the advantage of providing high gain but simultaneously maintaining reduced voltage and current stress on most of the switches. The inductor current is interleaved, reducing the equivalent ripple on the converter input current and the inductor power loss. The converter operation is identified to be divided into different zones depending on the duty ratio of the phases. The operating principles along with the details of the component design and loss analysis as a function of converter phases has been studied. A 250 W GaN based 3-level EDR prototype with input from 20 V–40 V and 225 V output and operating at switching frequency of 200 kHz has been developed to validate converters operation in hardware.

A GaN based doubly grounded, reduced capacitance transformer-less split phase photovoltaic inverter with active power decoupling

Transformer-less PV inverters are gaining widespread applications with lower cost, reduced footprint, and improved efficiency. This paper proposes a topology that can eliminate the common mode leakage current which is a major challenge in transformer-less PV inverters. In addition, an active power decoupling strategy is implemented in this topology instead of using large energy storage element for double line frequency power decoupling which is a common problem in single phase inverters / rectifiers, thus achieving a smaller volume and replacing electrolytic capacitors with film capacitors to increase reliability. A constant input voltage with negligible double line frequency ripple component less than 2% ensuring high MPPT efficiency is achieved in this topology through control strategy. Compared with previous topology, the 120 Hz inductor current ripple in the boost stage is reduced significantly resulting in the decrease of the RMS current value by 14% and the peak current value by 41%. Also the RMS current in the input capacitor is decreased by 91%. The dc link capacitor volume is also decreased with the increase of the voltage level. Finally, a 1 kW, 100 kHz single-phase prototype with 200 V DC nominal input and 120 V/60 Hz AC output in split phase configuration using GaN FETs has been built to validate the theoretical analysis. The control strategy and modulation scheme are implemented in DSP TMS320F28335.

A single phase transformerless string inverter with large voltage swing of half-bridge capacitors for active power decoupling

The transformerless single phase inverters are becoming common due to its advantages of reduced volume, lower cost, and higher efficiency but it has two implementation challenges-high frequency capacitive ground current and decoupling of double line frequency power. This paper proposes an optimized power decoupling topology for a single phase string inverter which addresses both the challenges as well as minimizes capacitance required to decouple the ripple power. Unlike conventional power decoupling techniques in full bridge converters, the proposed technique does not significantly increase the voltage stress on the devices. The combination of a half-bridge inverter and a buck boost converter ensure the complete elimination of high frequency capacitive coupled ground currents. The proposed technique requires only 40 μF/kW at 600 V for power decoupling. A closed loop controller design for the converter is detailed and the experimental results at 1 kW, 120 V, 60 Hz output for closed loop operation are provided.

GaN based transformer-less microinverter with extended-duty-ratio boost and doubly grounded voltage swing inverter

Microinverters are capturing the roof-top-PV market due to their plug and play feature, easy installation, and higher power yield under partial shading condition. This paper explores a converter for the transformer-less microinverter with extended-duty-ratio (EDR) boost as the DC-DC stage and doubly grounded inverter as the DC-AC stage. EDR boost is a hybrid of the interleaved boost and switched capacitor concept which has the advantage of providing high gain while simultaneously maintaining reduced voltage and current stress on most of the switches. The inductor currents are interleaved, reducing the equivalent ripple on the converter input current and the inductor power loss. The doubly grounded voltage swing inverter has the advantages of reduced capacitor requirement for 120 Hz power decoupling and zero capacitive-coupled common-mode ground currents, critical for transformer-less PV inverters. Also through dynamically variable DC-link with large voltage swing approach the decoupling capacitor is reduced allowing an all film capacitor implementation. A 300 W GaN based inverter prototype with 20 V–40 V DC input and 120 V, 60 Hz AC output and operating at switching frequency of 100 kHz has been developed to validate the converters operation in hardware.

Failure modes and effect analysis of module level power electronics

The scope for module level power electronics (MLPE) is immense in modern day PV industry but it lacks in assessing the reliability. This paper presents the work performed by PREDICTS (Physics of Reliability: Evaluating Design Insights for Component Technologies in Solar) team lead by Sandia National Laboratories to develop a standard reliability assessment protocol for MLPE including microinverters and microconverters. This paper discusses the ground work performed to develop the reliability assessment protocol by conducting FMEA (Failure Mode Effect & Analysis) and identify the top five failure modes that drive to the development of accelerated test similar to the existing PV module and power electronics industries.

A single phase transformer-less string inverter with integrated magnetics and active power decoupling

This paper proposes a transformer-less single phase inverter for photovoltaic (PV) application with integrated magnetics and active power decoupling minimizing the capacitance requirement, converter volume, and cost. The topology is a combination of boost and half-bridge stages along with a buck-boost power decoupling stage. This converter ensure the complete elimination of high frequency capacitive coupled ground current by the combination of the half-bridge buck-boost stages. The inductors of these two stages are integrated in one single core to further reduce the core loss and converter volume and cost. Closed loop experimental results of the converter with integrated magnetics are provided to validate its operation.

A single phase doubly grounded, PV inverter using coupled inductor with integrated magnetics and active power decoupling technique

Transformer-less PV inverters are gaining widespread applications with lower cost, reduced footprint, and improved efficiency. This paper builds on a recently proposed topology with doubly grounded structure that can eliminate the common mode leakage current which is a major challenge in transformer-less PV inverters. An active power decoupling strategy is implemented in the above topology for double line frequency power decoupling, thus achieving high power density and improved reliability by use of film capacitors. A constant input voltage with negligible double line frequency ripple component ensuring high MPPT efficiency is achieved in this topology. Compared to the previously proposed topology, the dc-dc stage and dc-ac stage inductors are coupled with an integrated magnetics design to take advantage of the currents in the two stages to reduce the flux, core loss and the current ripple. A 500 W, 100 kHz single-phase prototype with 200 V DC input and 120 V/60 Hz AC output using SiC MOSFETs has been built to validate the theoretical analysis. The total inductor volume can be reduced by 31%, and the inductor current ripple in the dc-dc stage and dc-ac stage can be reduced by 35% and 50% respectively. The efficiency improves by 0.3% due to savings in the core loss and copper loss compared with topology using discrete inductors.

Optimal variable switching frequency scheme for grid connected full bridge inverters with bipolar modulation scheme

This paper proposes a scheme to improve the system overall efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency scheme in a full bridge inverter. Previous work has analyzed variable frequency scheme for unipolar PWM. This paper aims at minimizing the switching loss for inverters using bipolar PWM scheme by including characteristics of the loss and current ripple relative to instantaneous switching frequency in the optimization process. Compared with unipolar modulation, bipolar modulation is much more suitable to implement the variable switching frequency scheme due to its ripple distribution pattern. An additional motivation is that the common mode leakage current performance of bipolar PWM is superior to that of the unipolar PWM for grid connected converters. A 1 kW full bridge inverter using SiC MOSFETs and ferrite core has been built to validate the theoretical analysis. Both constant switching frequency scheme (200 kHz) and optimal variable frequency scheme with simple code modification have been implemented in the experimental prototype using DSP TMS320f28335. The optimal variable switching frequency scheme can increase the inverter efficiency at rated power (1 kW) from 95.5% to 96.8% and has a correspondingly significant 38.1% saving on the switching loss compared with constant switching frequency scheme.

Sliding mode control of a single phase transformer-less PV inverter with active power decoupling

This paper proposes a fixed frequency based sliding mode (SM) controller for transformer-less single phase inverter for photovoltaic (PV) application with active power decoupling minimizing the capacitance requirement. Unlike conventional power decoupling techniques in full bridge converters, the proposed technique incorporates a large sinusoidal swing of the half-bridge capacitor voltages and a limited ripple on the DC-link to address the power decoupling. This invokes coupling and complexity in the system making the control of the converter a challenging task. SM control technique is used to provide a robust and stable controller platform for this converter. Extensive simulation results are provided for verification of the SM controller performance and to support the mathematical formulation of the fixed frequency based non-linear sliding mode control for this converter in standalone operation mode.