Research On Active Islanding Detection Based On System Impedance Measurement

Nowadays, environmental and energy concerns related to power generation have been drawing much attention worldwide. As a consequence, renewable or low emission energy sources associated with distributed generation technologies have gained a lot of momentum in the power industry. Because of more and more distributed generators (DGs) connected in power distribution systems, the impact to main grid has been becoming a hot topic. But when the breaker which connects the main grid and the distributed generating system opens, the distributed generator and its local load are isolated to be an island. If DGs run into the islanding state, due to the mismatch between DGs and their local loads, it may cause damages to the user’s power devices and the distributed generators. Meanwhile the power quality of power supply in the island may be impacted and the safety of the operators may be threatened. Thus, it is important for safe and stable distributed generations to detect islanding effectively.Depending on different detection positions, islanding detection methods can be roughly divided into two categories:remote and local method. Remote methods which usually have nothing to do with the type of DGs are efficient to detect islanding; however they involve high costs and only can be applied to some distributed generation with simple structure. Local methods can be divided into passive and active method. Passive methods are simple and have low influence on the grid, they tend to have large non-detection zones (NDZ); Active methods are more efficient than passive ones and have small NDZs, but if the disturbance injected is large, they may cause power quality problems and the interference between the signals is a defect which has not been overcame yet.Focused on the problem of islanding, this paper proposes a new and effective islanding detection scheme:a thyristor is placed between any two phases just by the port of the distributed generator (DG). An artificial short time phase-to-phase short circuit is formed by triggering the thyristor periodically, so the transient disturbance current and voltage signal can be extracted out. Because of this, the frequency response of system can be acquired according to the theory of harmonic impedance, so that the impedance measured from DG port can be obtained. Whether the DG is islanding can be determined quickly by monitoring the changes of this impedance. This method is proved to be correct and effective by theoretical analysis, simulation, experimental test and field experiment.The first chapter introduces the significance of the study and the research status of islanding detection; the second chapter contains the detailed theoretical analysis about principle for the active islanding detection based on the harmonic impedance measurement technology and the derivation of the transient disturbance current and voltage signal, at the same time, the effective islanding detection criterion has been built; in Chapter Ⅲ, the simulation model according to the theoretical analysis is built and the effectiveness of islanding detection criterion is verified by the sensitivity analysis, in addition, simulation and analysis of the impact on power quality have been illustrated; the interaction among signals of various anti-islanding active schemes is an important concern in multi-DG systems, therefore this issue is discussed in ChapterⅣ; in the fifth chapter, an experimental test is carried out and the ZK-1islanding detection device made based on this scheme has been tested in a wind farm belonging to Laizhou Luneng Wind Power Corporation for field experiment, moreover, this method is proved again.