Michael Ropp

Analysis and performance assessment of the active frequency drift method of islanding prevention

Islanding of photovoltaic (PV) systems can cause a variety of problems and must be prevented. However, if the real and reactive powers of the load and PV system are closely matched, islanding detection by passive methods becomes difficult. Also, most active methods lose effectiveness when there are several PV systems feeding the same island. The active frequency drift method (AFD), also called the frequency bias method, enables islanding detection by forcing the frequency of the voltage in the island to drift up or down. In this paper, AFD is studied analytically, using the describing function analysis technique, and by simulation, using MATLAB. It is shown that AFD has a nondetection zone (NDZ) in which it fails to detect islanding, and that this NDZ includes a range of unity-power-factor loads. Finally, the paper describes a novel method using positive feedback which significantly shrinks the size of the AFD NDZ.

Determining the relative effectiveness of islanding detection methods using phase criteria and nondetection zones

Islanding of a utility-interactive photovoltaic (UIPV) system occurs if the UIPV system continues to power a section of the utility system after that section has been disconnected from the utility source. Since islanding creates hazards for personnel and equipment, UIPV systems are required to detect and prevent it. It is desirable to have a simplified method of determining which islanding detection methods (IDMs) are most effective. In this paper, a previously described method for finding the nondetection zones (NDZs) of IDMs is experimentally verified. This method is used to determine the NDZs of several common IDMs. These results indicate that, of the IDMs discussed in this paper: (1) Sandia Frequency Shift (SFS) is most effective; and (2) the worst-case loads are low-power loads that are near resonance at the line frequency and have a large capacitance and small inductance (a high value of the quality factor Q).

Development of a MATLAB/Simulink Model of a Single-Phase Grid-Connected Photovoltaic System

Because of their deployment in dispersed locations on the lowest voltage portions of the grid, photovoltaic (PV) systems pose unique challenges to power system engineers. Computer models that accurately simulate the relevant behavior of PV systems would thus be of high value. However, most of todays models either do not accurately model the dynamics of the maximum power point trackers (MPPTs) or anti-islanding algorithms, or they involve excessive computational overhead for this application. To address this need, a Matlab/Simulink model of a single-phase grid-connected PV inverter has been developed and experimentally tested. The development of the PV array model, the integration of the MPPT with an averaged model of the power electronics, and the Simulink implementation are described. It is experimentally demonstrated that the model works well in predicting the general behaviors of single-phase grid-connected PV systems. This paper concludes with a discussion of the need for a full gradient-based MPPT model, as opposed to a commonly used simplified MPPT model.

Prevention of islanding in grid-connected photovoltaic systems

Recently there has been a resurgence of concern about islanding of grid-connected photovoltaic (PV) systems. This condition occurs when the PV system continues to energize a section of the grid after that section has been isolated from the main utility voltage source. Generally, islanding is undesirable because it poses a safety hazard to utility service personnel, and also because it can lead to asynchronous reclosure which can damage equipment. It is therefore important that PV systems incorporate methods to prevent islanding. The purpose of this paper is threefold: (1) to critically review the literature on islanding prevention methods for PV systems and discuss their strengths and shortcomings; (2) to review and analyze the islanding behavior of four converters which are prominent in the literature in order to demonstrate the implementation and effectiveness of some islanding prevention methods; and (3) to introduce a new islanding prevention scheme, active frequency drift with positive feedback, which overcomes many of the shortcomings of existing schemes. It is concluded that no ‘perfect’ islanding prevention method yet exists, but also that many existing methods or combinations thereof work very well in practical situations. Finally, it is noted than an investigation of what constitutes ‘sufficient’ islanding prevention is needed. Copyright

Organic photovoltaic cells made from sandwich-type rare earth phthalocyaninato double and triple deckers

This work presents organic-inorganic hybrid solar cells, which possess the capability for broad band photon harvesting from an ultraviolet-visible to a near infrared range. These solar cells are bulk heterojunction devices, which have been fabricated by free base phthalocyanine and rare earth phthalocyaninato double or triple deckers (electron donors) with a perylenediimide derivative (electron acceptor). Two-type cell structures with or without nanostructured TiO2 layers have been presented. A characterization of the structures, steady state spectroscopy, fluorescence dynamics, and photoelectrical property of these cells and the active materials has been carried out. A cell structure of In2O3:SnO2∕TiO2-active material-TiO2∕Al has shown a significant improvement in conversion efficiency.

Investigation of two anti-islanding methods in the multi-inverter case

As photovoltaic systems proliferate on grids, larger numbers of inverters will be interconnected with each other through their host utility. This becomes especially true if string inverters (SIs) or AC module ldquomicroinvertersrdquo are used. It will thus become increasingly important to ensure that the islanding detection methods used are effective in this so-called ldquomulti-inverter caserdquo. In this paper, two methods, one that uses classical controls concepts to obtain positive feedback (the dasiaclassical linear instability methodpsila, or CLIM), and a more recent method called the Sandia frequency shift (SFS) that applies positive feedback directly to the inverterpsilas output frequency, are investigated in multi-inverter use via MATLAB/Simulink simulation. The results indicate that the effectiveness of these two anti-islanding methods can depend on the number of inverters on the system, depending on the system configuration.

Simulation and Experimental Study of the Impedance Detection Anti-Islanding Method in the Single-Inverter Case

The impedance detection method of islanding prevention is often touted as one that possesses no nondetection zone in the single-inverter case. However, simulation results and experiments indicate that this is not necessarily true; the effectiveness of impedance detection is highly dependent on the specific implementation of the method. This paper discusses a simulation and experimental study to examine the origins of this nondetection zone, and a simple modification that appears to significantly reduce the size of the nondetection zone

Design Considerations for Large Roof-integrated Photovoltaic Arrays

This paper describes calculations and modeling used in the design of the photovoltaic (PV) array built on the roof of the Georgia Tech Aquatic Center, the aquatic sports venue for the 1996 Olympic and Paralympic Games. The software package PVFORM (version 3.3) was extensively utilized; because of its importance to this work, it is thoroughly reviewed here. Procedures required to adapt PVFORM to this particular installation are described. The expected behavior and performance of the system, including maximum power output, annual energy output and maximum expected temperature, are then presented, and the use of this information in making informed design decisions is described. Finally, since the orientation of the PV array is not optimal, the effect of the unoptimized array orientation on the systems performance is quantified.

Power line carrier permissive as a simple and safe method of enabling inverter ride-through operation of distributed grid-tied photovoltaic systems

Conventional anti-islanding techniques used in grid-tied photovoltaic (PV) systems pose many disadvantages at high levels of PV deployment. One such issue is the inability of these systems to ride-through grid disturbances. In this paper, the use of a Power Line Carrier Communications (PLCC) Permissive anti-islanding scheme is investigated as a means of safely enabling ride-through operation of grid-tied photovoltaic systems. Here potential fault scenarios are considered, along with performance, cost, and design considerations for the PLCC Permissive components, as well as potential system configurations and methods of implementation. While PV systems are the largest (and growing) form of distributed generation (DG) generating in parallel with utility feeders, it is important to note that this technique is effective for any DG technology, whether inverter-based or rotating, including wind, hydro and fossil fueled bio-gas machines.

New results for power line carrier-based islanding detection and an updated strengths and weaknesses discussion

As PV deployment levels increase, loss of mains detection, or islanding detection, has again arisen as a primary concern among the utility community. This is true especially in multi-inverter cases, cases with a mix of distributed resources, and on difficult feeders on which false tripping may be a disproportionately significant problem. Power line carrier communications can be effective in solving this problem for all types of distributed generation. This paper provides an update on laboratory and field testing of this technique; discusses some of its unique but lesser-known advantages; and examines some of its weaknesses.