Islanding in power system can be intentional or unintentional. In the past most of the methods were developed for the phenomenon of unintentional islanding. Recently as the distributed generator (DG) sources are able to control voltage and frequency in the islanded part, the phenomenon of unintentional islanded is not common. If the autonomous operation of sources or intentional islanding is desired, then a fast islanding detection technique is necessary . The main philosophy of detecting an islanding situation is to monitor the DG output parameters and system parameters to decide whether or not an islanding situation has occurred from change in the parameters. islanding detection techniques can be classified as remote techniques and local techniques, and local techniques can be divided into passive methods, active methods and hybrid methods. In this paper, several remote and local techniques (passive, active, and hybrid methods) for islanding detection are briefly described. The advantage and limitation of each technique are also introduced. Meanwhile, the comparison of these methods is also discussed.

This paper presents the VFTO calculation and analyzing results of XiMeng UHV pilot project in China. The result shows that the maximum voltage amplitude of VFTO in GIS is 1981kV, so that the pre-insert switching on off resistance is not necessary to installed on disconnector. In this case , it will reduce the probability of GIS failure operation and the potential supply current, and the safety and reliability of UHV GIS system will be improved. Meanwhile,the VFTO of a transformer in GIS substation is not high when the transformer is connected to GIS by overhead line.