Islanding of a full power converter grid interface

Abstract

A part of smart grid ideology is a new approach to islanding of power system. Smart grids and distributed generation can e.g. enable power flow upstream from distribution network and operation of distribution branch as an individual power system. These scenarios introduce new requirements for protection and control of the power system such as detection of islanding and the ability of distributed generation to maintain voltage and frequency of the power system. Full power converter is the most used way of connecting new distributed generation to power system and it enables implementation of protection functions and grid controlling abilities into distributed generation units.

This thesis studies islanding of power systems and related phenomena such as safety aspects and possible benefits of islanding. Islanding detection methods presented in literature and their benefits and downsides are eviewed. Islanding and an anti-islanding method based on frequency diverging by reactive current injection are studied also from practical perspective by a miniature demonstration. Grid codes and standardization related to islanding are utilized in testing and results are studied on the grounds of stand-ardization.

Results show the frequency and voltage behavior of the islanded system with different reactive – and active power balances and behavior of the grid interface inverter with and without islanding prevention functionality. On basis of these results the functionality and applicability of this islanding detection method into different environments and compatibility with different grid codes can be evaluated. Functionality of the grid inverter without anti-islanding functionality is verified and as for currently existing products the possibility and probability of unintentional islanding can be assessed.