Sigifredo Gonzalez

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.

Development and Testing of an Approach to Anti-Islanding in Utility-Interconnected Photovoltaic Systems

This report documents the development of an inverter control method that detects and avoids islanding in utility-interconnected photovoltaic installations. This method is applicable to single and multiple inverters connected to a single utility distribution line. The anti-islanding approach is described and its performance is demonstrated on both a theoretical basis and with results from tests conducted at Sandia National Laboratories and Ascension Technology, a division of Applied Power Corporation. It has been demonstrated that this approach is effective for single and multiple photovoltaic inverter installations for the special case where the inverter contains a version of anti-islanding software compatible with IEEE Std 929-2000.The report also describes the anti-islanding technique so that it can be incorporated into photovoltaic systems lacking this feature. A test procedure that ascertains whether an anti-islanding capability exists in a PV inverter is also presented. This page intentionally left blank

Grid-Tied PV system energy smoothing

Grid-tied PV energy smoothing was implemented by using a valve regulated lead-acid (VRLA) battery as a temporary energy storage device to both charge and discharge as required to smooth the inverter energy output from the PV array. Inverter output was controlled by the average solar irradiance over the previous 1h time interval. On a clear day the solar irradiance power curve is offset by about 1h, while on a variable cloudy day the inverter output power curve will be smoothed based on the average solar irradiance. Test results demonstrate that this smoothing algorithm works very well. Battery state of charge was more difficult to manage because of the variable system inefficiencies. Testing continued for 30-days and established consistent operational performance for extended periods of time under a wide variety of resource conditions. Both battery technologies from Exide (Absolyte) and East Penn (Advanced Valve Regulated Lead-Acid) proved to cycle well at a partial state of charge over the time interval tested.

Differentiating series and parallel photovoltaic arc-faults

The 2011 National Electrical Code® requires PV DC series arc-fault protection but does not require parallel arc-fault protection. As a result, manufacturers are creating arc-fault circuit interrupters (AFCIs) which only safely de-energize the arcing circuit when a series arc-fault occurs. Since AFCI devices often use the broadband AC noise on the DC side of the PV system for detection and series and parallel arc-faults create similar frequency content, it is likely an AFCI device will open in the event of either arc-fault type. In the case of parallel arc-faults, opening the AFCI will not extinguish the arc and may make the arc worse, potentially creating a fire. Due to the fire risk from parallel arc-faults, Tigo Energy and Sandia National Laboratories studied series and parallel arc-faults and confirmed the noise signatures from the two arc-faults types are nearly identical. As a result, three alternative methods for differentiating parallel and series arc-faults are presented along with suggestions for arc-fault mitigation of each arc-fault type.

Array Performance Characterization and Modeling for Real-Time Performance Analysis of Photovoltaic Systems

Improvements in the methods used for photovoltaic (PV) system design, performance rating, and long-term monitoring are needed by the rapidly growing industry, as well as by the U.S. Department of Energy in evaluating progress by solar technology development initiatives. This paper describes an improved model for rating and monitoring PV array performance, discusses initial results from an outdoor laboratory designed to assist industry in optimizing system components and integration, and provides a brief discussion of the system performance metrics currently being used by the PV community

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.

Design considerations for three-phase grid connected photovoltaic inverters

This paper discusses the design considerations for three-phase inverters for grid connected photovoltaic systems. Specific performance considerations are addressed including topology trade-offs, efficiency trade-offs, and maximum power point tracking. Additionally, utility interconnect issues are examined including power quality and anti-islanding. Testing results are provided for the Xantrex PV series of inverter products. These systems operate with a variety of PV panels and are designed to meet the UL-1741 utility interconnection standard. Power levels are from 5 kW to 300 kW, and the output is a nominal 208 VAC. Experimental results showing successful operation are given for the 10 kW system. Testing was performed at the Sandia National Laboratories Photovoltaic Systems Evaluation Lab in Albuquerque, NM.

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

PV array simulator development and validation

The ability to harvest all available energy from a photovoltaic (PV) array is essential if new system developments are to meet levelized cost of energy targets and achieve grid parity with conventional centralized utility power. Therefore, exercising maximum power point tracking (MPPT) algorithms, dynamic irradiance condition operation and startup and shutdown routines and evaluating inverter performance with various PV module fill-factor characteristics must be performed with a repeatable, reliable PV source. Sandia National Laboratories is collaborating with Ametek Programmable Power to develop and demonstrate a multi-port TerraSAS PV array simulator.