Protection of the Inter-Tie between an Industrial Cogenerator and a Utility

Abstract

The work presented in this thesis is concerned with the development of a microprocessor based technique to detect and locate symmetrical and unsymmetrical faults on the interfacing network of a utility interconnected industrial cogenerator system; and to identify the type of the detected fault. In order to provide operational flexibility and supply security to such an interconnected industrial cogenerator plant, fault location is regarded as being of prime importance. Current protection practices using conventional technology for this sort of interconnected cogeneration system were reviewed. A relay coordination study was carried out on a simulated utility interconnected cogeneration system. This resulted in identification of the problems and limitations of the application of standard IDMT relays for overcurrent protection of such a system. A review of digital protection, and its various algorithms, was undertaken. This provided the basis for a rational choice of algorithm for the proposed new technique. The sequence of functions of the protection scheme's algorithm, based on the proposed technique, can be presented in brief as follows: First, phase currents at selected positions on the system are sampled. The positive phase sequence (pps) currents at those positions are then obtained in digitised form from the sampled phase currents. Then, these pps currents, containing fundamentals along with other harmonics and dc offsets, are used in a Fourier filtering algorithm to extract only fundamental components in rectangular forms. Signs of imaginary parts of these components are then used to determine the directions of pps currents at the selected positions. The industrial bus voltage is used as the polarising reference. Finally these directions are based in a fault detection aridlocat1on subroutine to determine the location of any fault under such conditions. After detecting a fault and locating its position, the type of fault is determined from off-line analysis of the postfault phase currents. The analysis is based on identifying the phases carrying the maximum fault current and checking the presence of zero sequence current. The proposed protection scheme has been tested on a computer simulated system which consists of a utility interconnected to an industrial plant with cogeneration. The performance has been found to be satisfactory for all kinds of solid and low are resistance faults anywhere on the interfacing network between the systems.