Islanding Detection Principle And Implementation For Distributed Synchronous Generators

With the extensive application of renewable energy generation, distributed generation become a new focus in power system research. Distributed resources are usually connected into the public grid from low voltage system, so distributed generation brought many impacts on operation and control of distribution networks, and islanding is one of them. Because unintentional islanding would adversely affect utility, users and distributed generator itself, islanding must be detected as soon as possible, and appropriate response measures should be taken.Islanding detection is key issue on island related research. As the diversity of the types of distributed generators, islanding detection method also appears to diversity. This paper focuses on theory study and device developing for islanding detection of distributed synchronous generators. The main contents are as follows:Basing on widely survey of literatures, the characteristics of existing islanding detection methods to distributed synchronous generators are analyzed in detail. The advantages, shortingcomings and applicability are summarized. As passive detection method has no influence on the connected distribution networks and is easy to implement, it becomes effective way on islanding detection. However, the sensitivity and reliability issues need to be addressed.The passive detection method based on vector shift is simulated comprehensively using PSCAD/EMTDC. Factors affecting sensitivity and reliability are analyzed in detail. Comparing the curve which is fitted with exponential function based on digital simulation results and the curve which reflects relationship between detection time and unbalanced power, they are well matched. Using the second curve, the factors affecting phase shift are comprehensively analyzed and evaluated.Basing on the study to traditional phase shift algorithms, two new algorithms calculating phase shift are proposed and analyzed. The first one is called fast method based on Discrete Fourier Transform, and the second one is called integration method. Comparing with the traditional zero-cross algorithm, the first method is able to reduce detecting time, and the second one has a higher sensitivity.To improve both sensitivity and reliability of traditional islanding detecting method based on phase shift, a new method, combining phase shift with frequency variation, is developed. The frequency variation is measured and combined with phase shift to form a complex criterion for islanding detection when power imbalance is small. Both digital simulation and dynamic test have been carried out to evaluate the new detection method. The simulation and test results show that the new method can significantly reduce the non-detection zone traditional method while keeping same reliability.On the basis of theoretical study, a new islanding detection device is developed using similar hardware platform as micro-processor based relay. The proposed new detection method and the phase shift computing algorithm are taken in the device. Both static and dynamic tests results show that the technical specifications of the device can meet the designing requirements. The permitted power imbalance level is able to reach 5%, and so the non-detection zone is significantly reduced. Meanwhile, the nuisance tripping caused by the load fluctuation is greatly reduced. The developed device will be suitable for the islanding detection of distributed synchronious generators.