Control and Operation of Islanded Distribution System

A yearly demand growth of less than 3%, concern about the environment, and various benefits of onsite generation have all resulted in a significant increase in penetration of dispersed and distributed generation (DG) in many distribution systems. This has also resulted in some power system operational challenges. But, on the other hand, it has also opened up some opportunities. One opportunity/challenge is an islanded operation of a distribution system with DG unit(s). Islanding is a situation in which a distribution system becomes electrically isolated from the remainder of the power system and yet continues to be energized by DG unit(s) connected to it. Currently, it is seen as a challenge and so far all DG units need to shut down when a distribution system is islanded. However, with the DG penetration expected to increase sharply, islanding is an opportunity to improve the reliability of power supply provided that various issues with islanding are properly addressed. Some of the issues with islanding are state (islanded or grid connected) detection, control of voltage and frequency, load control and protection.

In this dissertation, some of the major technical issues with islanding are addressed. A hybrid islanding detection technique, based on the average rate of voltage change and real power shift, is developed to overcome the problems with most of the existing islanding detection techniques. It uses a passive technique (average rate of voltage change) and an active technique (real power shift). However, the active technique is used only when the passive technique cannot clearly discriminate between islanded and grid connected conditions. DG units perform the best if they are operated with droop control and power factor control when they are operating parallel to the grid and if they are operated with isochronous control and voltage control when the distribution system is islanded. It is proposed that the DG units are controlled differently when the distribution system changes state from grid connected condition to islanded condition. However, isochronous controllers cannot be used with more than one generator connected to the same system. An isochronous controller with feedback has been developed in this research study. It performs relatively well in both islanded and grid connected conditions. Hence, if there is more than one DG unit in the distribution system, employing isochronous control in one DG unit and employing isochronous control with feedback in other DG units, during islanding, results in better frequency profile of the islanded distribution system. The DG control strategy needs to be changed again when the islanded distribution system is reconnected back to the transmission grid. Hence, grid reconnection detection algorithms have been proposed to detect when an islanded distribution system is reconnected back to the transmission grid. One of the grid reconnection detection
algorithms is based on rate of change of speed over power. Another one is based on frequency deviation and real power shift. When a distribution system, with all its generators operating at maximum power, is islanded, the frequency will go down if the total load is more than the total generation. An under-frequency load shedding procedure for islanded distribution systems with DG unit(s) based on frequency information, rate of change of frequency, customers’ willingness to pay and loads’ histories is proposed in this research. It sheds an optimal number of loads and stabilizes the frequency of the islanded distribution system. Short circuit power of a distribution system changes when it changes states. Short circuit power also changes when some of the generators in the distribution system are disconnected. This may result in elongation of fault clearing time and hence disconnection of equipments (including generators) in the distribution system or unnecessary operation of protective devices. An adaptive protection has been proposed in the research study to overcome the problem with change in short circuit power.

The algorithms, models and methodologies developed during the course of this research study have been tested in a distribution system with gas turbine and wind turbine generators. Simulation results show that they are able to correctly identify the states of distribution systems, maintain the voltage and frequency when the distribution system is islanded, maintain the power and power factor when the distribution system is connected to the grid, maintain the voltage and frequency after load and stochastic generation changes, shed an optimal number of loads to stabilize the frequency when the total demand is more than the generation in an islanded distribution system, and protect the distribution system against the short circuits even with the changing short circuit power.