PMU Frequency Data Processing for A Planned Islanding Operation in Bornholm

Abstract: Because of the predictable higher penetration of distributed wind power and Combined Heat and Power (CHP) plants in Denmark, it is considered feasible to utilize those Distributed Generations (DGs) for planned islanding operation of distribution networks due to either a disturbance or fault in the upstream grid or an interconnection maintenance. However, at present, islanding operation of a system with many Wind Turbines (WTs) like the Danish island Bornholm has been found difficult in reality. In order to obtain adequate experience upon islanding operation in the system with high integration of wind power, and to study it in detail afterwards, the Centre for Electric Technology (CET) at Technical University of Denmark (DTU) was involved in a planned islanding operation of a full-scale distribution system. This islanding operation was conducted by the local Distribution System Operator (DSO) Østkraft from September 11th., 2007 to 14th., 2007 in the Danish island Bornholm, which has already integrated a considerable amount of Wind Turbines (WTs). However, they were completely shut down during the islanding transition moment and only three WTs were reconnected later under the islanding mode, due to the fluctuation of wind power and the system stability issue, which were not friendly to both WT owners and DSO. To avoid the disconnection of WTs and even utilize them to assist islanding operation in the future, it is necessary to analyze the data collected by Phasor Measurement Units (PMU) system in Bornholm during the planned islanding operation first, as it has high time resolution and it is synchronized to Global Position System (GPS). However, there were some problems with the PMU system during the islanding period, which made the data not as complete as expected. This paper describes the problems and provides proposals for a data processing procedure to ensure that the improved frequency data are acceptable for statistics analysis. The proposed data processing procedure is described and evaluated. Furthermore, this paper provides a reference for future PMU data acquisition and processing. Individually, section I in the paper generally describes the background for islanding operation and the Bornholm system. Section II introduces the installed PMU measurement system, followed by section III, which describes the PMU original frequency data, the problems during the acquisition processing and the solution to calculating those missing frequency from phase angle. In section IV, the authors explain how the data can be improved by a moving average filter, and the final frequencies, from both Nordic system and Bornholm system during the islanding operation, are analyzed. In the end, section V draws several conclusions related to data acquisition in power system.