| The most common method of detecting the islanding in a machine-based distributed generator (DG) is to establish an under/over frequency and under/over voltage window within which a synchronous DG is allowed to operate. When such a DG is islanded from the utility system, the frequency or voltage will quickly move outside the operating window if there is a sufficient difference between the local load and DG power generation level. However, there is a possibility that the system voltage and frequency will be maintained within the specified limits following loss of the grid in cases where the load and generator are matched. Therefore, special means are required to detect the loss of the grid in this particular situation.;This thesis investigates effective ways to detect islanding of machine-based DG unit when there is a match between generation and demand in the islanded section of the distribution system. A combination of both passive and active techniques will be considered in the analysis. Typical passive techniques are based on the rate of change of frequency, output power, voltage, power factor, voltage unbalance, or voltage total harmonic distortion. On the other hand, active schemes detect the islanding by directly interacting with the utility system. Three active methods that were recently proposed include the following: (A) Reactive error export detection (which controls the embedded generator excitation current so that it generates a known value of reactive current, which cannot be supported unless the generator is connected to the grid). (B) Fault level monitoring (which makes detection possible in half a cycle by using point-on-wave thyristor switching triggered near the voltage zero point). (C) Positive feedback (that results in unstable frequency or voltage once the DG is islanded). The analysis will be illustrated through computer simulations. |
