Dynamic analysis and field verification of an innovative anti-islanding protection scheme based on directional reactive power detection

Based on current utility practice, anti-islanding protection is one of the main protection requirements for interconnection of a distribution generation (DG) to the medium and low voltage grids. For connecting a small synchronous generator to the utility grid, DG interconnection guidelines require the use of a transfer-trip scheme when the minimum load of a connecting feeder is less than twice the rated capacity of the total DG units. Some standards (e.g. IEEE Std. 1547) suggest a more aggressive generation to load ratio of one-third. Implementation of a fast communication based transfer-trip scheme with a detection time of less than a second is very expensive and not economically feasible for small DG projects. The fast islanding detection is mainly required to comply with the feeder protection coordination, especially the first reclosing time of the feeder automatic reclosure (in this case 1.5 seconds). This paper presents the computer modelling, simulation, and field verification of a proposed passive, local anti-islanding protection scheme based on directional reactive power measurement. The protection scheme was tested on a farm-based biogas DG to demonstrate compliance with the utility requirements. The detailed simulation studies and field tests consistently yielded detection times of less than 0.25 seconds. Based on these results the solution and the DG interconnection were approved by the utility. The proposed protection scheme can effectively be utilized for anti-islanding protection of synchronous generator based DG units on feeders with inductive load.