Communication-Based Adaptive Overcurrent Protection for Distribution Systems with DistributedGenerators

Abstract

Traditional distribution systems are designed with a radial structure. As a result, their protection systems are designed and set based on one utility source feeding the whole system. Nowadays, Distributed Generators (DG) are increasingly connected to distribution systems to meet the load demand and increase the reliability of the system. With the additionally connected sources, the system is no longer radial. Moreover, during a fault condition, the fault is fed from all the sources connected to the power system. Therefore, the fault current level is different compared with the radial system. The DGs affect the operation of the protection relays in distribution feeders as they reduce the reach of the relays. This is due to the fact that DGs increase the equivalent impedance of the feeder which decreases the fault current. Furthermore, protective relays on the main feeder must see fault currents in forward or reverse directions, and they have to detect the fault direction. Another important problem is that DGs can get disconnected from the grid due to disturbances or for maintenance. Consequently, a new configuration for the system results and, if a fault occurs, a different fault current level flows. Therefore, one setting for the protective relays cannot respond to the continuously changing system configuration. Thus, relays have to be adaptively coordinated for each new system configuration to achieve correct fault clearance operation. In this thesis, a communication-based adaptive protection system is proposed. The proposed system operates during islanding and normal connection operation modes. Linear optimization is used for overcurrent relay setting. DNP3 communication protocol is chosen to facilitate the communication between the various devices in the adaptive protection system. Centralized and decentralized adaptive protection schemes are investigated. In each scheme, two simultaneous algorithms are executed. The first algorithmworks when the system configuration is changed due to the disconnection or reconnection of a DG. While the other algorithm operates when faults are detected to speed up fault clearance. A backup scheme is developed to operate in case of communicationfailure between the various devices in the adaptive protection system. The proposed schemes are tested for different types of faults, different system configurations and during communication failure. The obtained results show that the proposed adaptive overcurrent protection system is able to respond to system changes accordinglyto ensure reliable protection operation and fast response during fault occurrences.