Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.

Grid-connected photovoltaic power systems: Technical and potential problems—A review

This report describes the various methods and circuits that have been developed to detect an islanding condition for photovoltaic applications and presents three methods that have been developed to test those methods and circuits. Passive methods for detecting an islanding condition basically monitor parameters such as voltage and frequency and/or their characteristics and cause the inverter to cease converting power when there is sufficient transition from normal specified conditions. Active methods for detecting the island introduce deliberate changes or disturbances to the connected circuit and then monitor the response to determine if the utility grid with its stable frequency, voltage and impedance is still connected. If the small perturbation is able to affect the parameters of the load connection within prescribed requirements, the active circuit causes the inverter to cease power conversion and delivery of power to the loads. The methods not resident in the inverter are generally controlled by the utility or have communications between the inverter and the utility to affect an inverter shut down when necessary. This report also describes several test methods that may be used for determining whether the anti-islanding method is effective. The test circuits and methodologies used in the U.S. have been chosenmore » to limit the number of tests by measuring the reaction of a single or small number of inverters under a set of consensus-based worst-case conditions.« less

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Evaluation of Islanding Detection Methods for Utility-Interactive Inverters in Photovoltaic Systems

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.

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

For pt.I see ibid., p.420-5 (2006).This paper offers an overview of the active methods used for islanding detection. The inverter control algorithm is based on the use of harmonic compensators in order to create ideal working conditions (i.e. the PV-system produces a sinusoidal current and reject grid harmonics). This condition is the indispensable starting point to implement active islanding detection methods. In fact these methods are based on the perturbation of the ideal conditions (e.g. the injection of active/reactive power, the injection of current harmonics, or the insertion of an external capacitor). All the reported algorithms are carefully analysed in their advantage and drawbacks and the considerations are validated by tests results obtained in accordance with the IEEE Std. 929-2000 procedure

Overview of Anti-Islanding Algorithms for PV Systems. Part I: Passive Methods

Increasing numbers of photovoltaic arrays are being connected to the power utility through power electronic inverters. This has raised potential problems of network protection. If, due to the action of the inverter or inverters, the local network voltage and frequency remain within regulatory limits when the utility is disconnected, then islanding is said to occur. Many methods to prevent this have been proposed and a SPICE-based model and analysis of the system are presented. Specifically aimed at predicting the effectiveness of two of these. These are: active frequency drift, which causes the inverter current to be generated slightly lower or higher in frequency than the frequency of the terminal voltage; and slip mode frequency shift, which controls the phase angle of the inverter current as a function of the frequency deviation of the terminal voltage from that of the normal network frequency. Experimental verification of the model is provided. Active frequency drift using frequency speed-up rather than speed-down is shown to be preferable for inductive loads, but conditions that resulted in islanding could be predicted. Slip mode frequency shift proved to be a more robust anti-islanding protection technique.

Predicting islanding operation of grid connected PV inverters

Residential, utility-interactive, photovoltaic systems consist of a photovoltaic (PV) array, and a Power Conditioning System (PCS), or inverter, which converts the dc array energy to an ac voltage or current at the proper frequency for connection to an electric utility distribution feeder. One of the concerns wth such an interconnection is the potential for "islanding." This is a phenomenon in which one or more PCSs continue to operate, and keep the system energized even after the utility supply is disconnected. The loss of utility may be related to a scheduled outage, or the operation of circuit breakers, sectionalizers, or fuses in response to a system disturbance. Since the electric utility company cannot exert direct control on residential photovoltaic systems, islanding is generally considered to be undesirable. Concern is primarily related to the safety of personnel attempting to restore service in a system energized by islanded PCSs. Additionally, there is cause for concern as to the effect of transients and off-nominal frequency and/or voltage on connected load and on the PCS itself. Therefore, an understanding of the islanding problem is extremely important to the commercialization of utility-interactive PV systems. The objective of the work described in this paper and its companion [1] is to develop simple analytical methods to determine if islanding is possible in a system consisting of one or more PCSs connected to a single-phase distribution lateral. Attention is focused on three commercially available PCSs, the 'Gemini' manufactured by Windworks, Inc., the 'Teslaco' manufactured by Teslaco, Inc.

A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part I-Models and Analytical Methods

Islanding is the continued operation of a group of utility-connected photovoltaic systems even after loss of the utility source. Such operation can be hazardous to both utility personnel as well as to components in the system. Models and methods for the analysis of this phenomenon are presented in this paper. The methods are used in a companion paper to explore the possibility of islanding in practical systems. Although all such systems have a natural propensity to island, it is shown that the controls used in many commercially available units make islanding a rather unlikely phenomenon.

A study of islanding in utility-connected residential photovoltaic systems

The phenomenon wherein a group of residential, utility-connected, photovoltaic inverters continue to operate even after loss of the utility source is known as 'islanding.' Islanded operation is undesirable since it can jeopardize the safety of utility personnel as well as cause damage to connected equipment. Analytical methods for the study of islanding have been presented in the companion paper [1]. It has been shown therein that a power-flow analysis type of method can be effectively used to identify the electrical frequency at which a power balance can be achieved in an islanded system. If such a frequency exists then islanding is possible. At this predicted frequency, largesignal Describing Function analysis and small-signal stability analysis can be used to determine if islanding will actually occur. In the present paper the methods are extended to the analysis of systems containing multiple power conditioning systems (PCS). Beginning with a general state-variable format, models are defined for power-flow analysis, describing functions analysis, and small-signal stability analysis. The methods above are used to study islanding in several test systems. Three types of PCSs are studied-the Gemini manufactured by Windworks, Inc., the Teslaco manufactured by Teslaco, Inc., and

Steve Omick

Papers

A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part I-Models and Analytical Methods

A study of islanding in utility-connected residential photovoltaic systems

A Study of Islanding in Utility-Connected Residential Photovoltaic Systems Part II--case Studies

Islanding in dispersed, utility-interactive photovoltaic systems