Haiyu Zhang

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.

Control strategy for seamless transfer between island and grid-connected operation for a dual-mode photovoltaic inverter

The dual-mode photovoltaic inverter is capable of operating either in grid-connected mode or island mode, acting as a current source for the ac grid in the former and a voltage source for the load in the latter. Transitioning from one mode to the next is non-trivial and can cause large deviations in voltage, current, and frequency because a mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. This paper presents a model predictive control (MPC) for a single phase, grid-connected voltage source inverter (VSI) to support dual-mode operation and seamless transfer between the two. This technique uses decoupled power control in grid-connected mode and voltage control in island mode respectively. Seamless transfer is implemented through synchronization algorithm and smoothly changing weighting factor for the cost function of the MPC. Compared with PI based controller, the MPC controller is simpler to implement since there is only one parameter to tune, the weighting factor. Combined with second order generalized integrator (SOGI), the synchronization algorithm is used to achieve fast phase detection and improved disturbance rejection. The theoretical analysis, simulation and experimental results validate the effectiveness of the proposed control strategy.

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.

Loss analysis during dead time and thermal study of gallium nitride devices

By virtue of the advantages in breakdown field strength and saturated electron speed, gallium nitride field-effect transistors (GaN FETs) have been attracting attentions as next generation power devices in recent years. Compared with silicon-based metal-oxidesemiconductor field-effect transistors (MOSFETs), GaN FETs have no intrinsic anti-parallel body diodes and exhibit poor reverse conduction characteristics. Thus it is necessary to optimize the dead time to not only avoid shoot-through current during the transient period in the single phase leg structure, but also to minimize reverse conduction power losses. Another characteristic of GaN FETs is their relatively low thermal conductivity. To satisfy the demands of higher power density, the thermal performance and cooling requirements of GaN FETs should be evaluated. In this paper, loss analysis during dead time and the thermal study of GaN FETs are presented. As a case study, a 200W GaN FETs based resonant converter is considered. The simulation and experimental results verify the validity of the theoretical analysis.

Photovoltaic hybrid power harvesting system for emergency applications

A photovoltaic (PV) based hybrid power harvesting system is proposed for emergency applications. It can compensate for the sudden loss of grid power and is suitable for developing countries and rural areas with unreliable power systems. The PV energy harvesting module uses a reconfigurable PV harvester which combines an isolated SEPIC DC-DC converter with Maximum Power Point Tracking (MPPT) to reduce complexity and increase efficiency. Source-selection control has been implemented to decide which source, PV or ac grid, to use for delivering power under different operating conditions. A synchronous-rectified flyback converter with non-complimentary active clamping and peak current mode control is applied to verify the feasibility of the PV hybrid power harvesting system. The simulation results with a Light-Emitting Diode (LED) load have shown that the PV hybrid power harvesting system can be an optional solution to rural lighting problems.

Model Predictive Control of a Voltage-Source Inverter With Seamless Transition Between Islanded and Grid-Connected Operations

Inverter-based distributed generation (DG) system is becoming an attractive solution for high penetration of renewable energy sources to the main grid. DG system should be able to supply power to the local loads whenever necessary even in case of utility power outage. Thus, the inverters in DG systems are expected to operate in both grid-connected and islanded mode, where they are acting as a current source for the ac grid and a voltage-source for the load, respectively. Transition between modes of operation is nontrivial and can cause deviations in voltage and current, because of mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. Thus, it is necessary to have seamless transition between grid-connected and islanded mode. This paper presents a new control strategy with seamless transfer characteristics for a grid-connected voltage-source inverter using model predictive control (MPC) framework. The main objectives of the proposed predictive controller are: 1) decoupled power control in grid-connected mode, which enables the proposed power electronics interface to provide ancillary services such as reactive power compensation; 2) load voltage control in islanded mode; and 3) seamless transition between modes of operation through proposed synchronization and phase adjustment algorithm. The proposed controller features simplicity to implement since only one cost function should be minimized for all modes of operation, and hence no ambiguity in the control algorithm that could cause mode transition problems. An autotuning strategy for weight factors in MPC cost function is proposed to simplify the weight factor tuning strategy. The stability analysis of the proposed controller is provided. Simulation and experimental results validate the expected performance and effectiveness of the proposed control strategy.

Comparison of GaN and SiC power devices in application to MW-scale quasi-Z-source cascaded multilevel inverters

Wide bandgap (WBG) semiconductors including gallium nitride (GaN) and silicon carbide (SiC) offer significant performance improvement compared with conventional silicon power devices. The quasi-Z-source cascaded multilevel inverter (qZS-CMI) provides many advantages over the conventional CMI while applied in photovoltaic (PV) systems. In this paper, two solutions are proposed and compared to the design goal of a high efficiency and low-cost qZS-CMI based 1 MW/11 kV PV system. The first solution is based on 650 V GaN enhancement mode high-electron-mobility transistors (E-HEMT) and 650 V SiC Schottky diodes. The second solution uses 1200 V SiC power modules and 1200 V SiC Schottky diodes. The power losses and costs of the two candidate designs are compared in details. It is concluded that the first solution shows lower power losses and costs per quasi-Z-source inverter (qZSI) module. However, due to the low voltage rating of GaN E-HEMTs, more qZSI modules are needed to achieve the overall 11 kV inverter rating. Therefore, the second solution shows lower total power loss and cost in the medium-voltage, MW-scale qZS-CMI PV system.

Dual buck based power decoupling circuit for single phase inverter/rectifier

Single phase rectifiers and inverters are inherently subject to double-line frequency ripple power, at both the ac and dc sides, which has adverse effects on the overall system performance. Normally a bulky capacitor is placed at the dc side of the circuit to prevent this ripple power ripple from reaching the dc source. This approach results in low power density and reliability issues. As a result, active power decoupling methods have been proposed such that the total capacitance required can be orders of magnitude smaller. This paper proposed a new power decoupling circuit, which is composed of two separate buck converters operating in each half cycle and two split dc-link capacitors. The dc link capacitors can be used to store ripple power while supporting transient power to the main output. The capacitance needed is reduced largely by allowing high voltage fluctuation on capacitors, while the dc link voltage can be controlled with small fluctuation. The dc link capacitors can be fully charged and discharged with full energy utilization. The added power decoupling module does not need dead time, and totally eliminates the shoot through concerns, which could enhance the system reliability. Another advantage of the proposed power decoupling method is that its control is independent with that of the main power stage. Modulation and control strategy are proposed for the power decoupling circuit. The operating principles together with parameters design are discussed in detail. Both simulation and experimental results prove the effectiveness of this method.

Experimental verification of energy harvest from non-planar photovoltaic surfaces

Non-planar photovoltaic (PV) surfaces have the potential to increase the total energy harvested from the sun compared to a planar surface of the same two-dimensional footprint. This offers opportunities to increase the electricity generated from a given roof surface. This paper presents a modeling approach to determine the view factor of an arbitrary geometry so that the total incident solar energy can be computed. Experimental measurements confirm the increase in generation and provide insights into the requirements for the balance of system to make most effective use of the non-planar geometry.

Capacitance, dc Voltage Utilizaton, and Current Stress: Comparison of Double-Line Frequency Ripple Power Decoupling for Single-Phase Systems

Double-line frequency ripple power is inherent in single-phase rectifiers and inverters, and, if not managed properly, it can be adverse to system performance at both the ac and dc sides. Therefore, numerous active power-decoupling techniques have been introduced to decouple the double-line frequency ripple power. However, no comprehensive comparisons are available on the permitted minimum capacitance for power decoupling, dc voltage utilization, current stress, modulation complexity, and even application evaluations, except for power rating and component counts. All of these aspects are critical when choosing appropriate power-decoupling techniques for different applications. In this article, the minimum capacitance to decouple the ripple power and the current stress of power devices in the main circuit are derived in light of different voltages across energy storage capacitors. By considering the ripple power paths, we investigate the dc voltage utilization of both the main circuit and power-decoupling circuit. Combined with other features, including component counts and modulation complexity, the overall characteristics of different power-decoupling techniques are compared and summarized to effectively evaluate their performance in different applications.