Showing papers on "Islanding published in 1999"

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

Abstract: 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.

Prevention of islanding in grid‐connected photovoltaic systems

Abstract: 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 © 1999 John Wiley & Sons, Ltd.

Systems and methods for preventing islanding of grid-connected electrical power systems

Abstract: A preferred embodiment of the electrical power system of the present invention includes a power conditioning unit which is configured to receive the DC electrical output signal to deliver an AC output signal to a grid-connected load. Preferably, the power conditioning unit includes a controller which is configured to monitor the AC output signal so that the power conditioning unit may cease delivering the AC output signal when a characteristic of the AC output signal satisfies an established criterion.

Islanding and load shedding schemes for captive power plants

Abstract: Summary form only given as follows. This paper outlines the various schemes adopted for islanding in industrial plants with captive generation in India. Considerations in the design of islanding and load shedding schemes and reliability aspects are described. Major benefits by interfacing with SCADA are briefly given.

Monitoring changes in system variables due to islanding condition and those due to disturbances at the utilities' network

Abstract: The continuous growth in embedded generation (EG) has emphasised the importance of being able to reliably detect loss of mains (LOM) especially when the EG is 'islanded' with part of the utility network. Current methods of detection have not proved completely dependable, particularly when the 'islanded' load's capacity matches that of the EG. This paper is the result of an investigation, using the Alternative Transients Program (ATP), into new methods of detecting LOM. To determine which system variables can be reliably used to detect LOM, it is necessary to accurately monitor all the relevant signals at the LOM relay location. This requires that the simulation realistically detects all the changes in system variables caused by disturbances on the network. In some cases, the results can vary markedly, depending on such factors as measurement technique and sampling rate. These problems can cause numerical instability leading to errors. The paper discusses these issues and the results obtained from ATP.

Analysis of a major load island outage on the Potomac Electric Power Company system

Abstract: This paper reviews the events of an incident, which occurred on June 20, 1997, that resulted in the islanding of a major load area in downtown Washington DC, USA, and the subsequent power outage to that load area. It discusses the investigation and analysis of the incident and lessons learned.

Intelligent load shedding schemes for industrial customers with cogeneration facilities

Abstract: In this paper a neural network-based methodology is proposed to develop load shedding schemes for industrial power systems. For a given post-disturbance scenario, an appropriate architecture of several neural networks suggests the nets necessary to provide the requested load-shedding action. It is assumed that, depending on the type of contingency and pre-disturbance network configuration, the post-disturbance scenario can involve the islanding of the entire power system or the decomposition into subsystems. Depending on the current generation-load mismatches of each subsystem, the exact amount of load to be shed is provided as output to a defined set of input signals. Since each net is trained using input-output patterns obtained from extended transient stability studies, the load shedding action is based on a 'dynamic' criterium, overcoming the limit of a load shedding action computed on the basis of the 'static' criterium of the anticipated overload. The effectiveness of the proposed procedure is tested on the power system of a petrochemical plant.

Comparison of digital simulation and field test results of islanding detection system for synchronous generators

Abstract: This paper presents two islanding detection methods for a dispersed synchronous generator and equivalent transformation techniques between the proposed two methods. The first method can be applied to a newly installed synchronous generator and the second method can be applied to an existing synchronous generator. The proposed two methods have the ability to detect all islanding cases, while the previous method has problems for detecting some islanding cases. The proposed methods and the equivalent transformation techniques were verified through a digital simulation and a field test. The islanding detection time of the simulation and the field test coincide perfectly. The results reveal the effectiveness of the proposed methods and the equivalent transformation techniques.

Design qualification and type approval of inverters for grid-connected operation of photovoltaic power generators: Dutch guidelines

Abstract: In the Netherlands and worldwide, the installation of grid-connectedPV-systems is rapidly gaining pace and volume. This makes the need for regulations of the requirements of grid- connected PV inverters urgent. This document lays down requirements and recommendations for the design qualification and type approval of inverters for grid-connected operation of photovoltaic power generators with a current up to and including 16 A per phase for single or three phase power conditioners (max. 11 kVA) feeding into the utility low-voltage mains. Inverters up to 5 kVA can be single-phase connected, and only a minimum level of protection is required. Islanding prevention is considered to be implemented with sufficient safety and at low costs by using a window for grid voltage and frequency, as well as the frequency shift method. The document addresses the design of grid connected PV-inverters with regard to performance, electromagnetic compatibility (EMC), safety, utility interface protection and reliability. lt describes the applicable standards for the design as well as for the experimental verification of a number of aspects related to the items mentioned above. This document can be used by manufacturers in the design process, by testing institutes for research and development and by certification bodies for type approval. 31 refs.