Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Electrical Engineering


The protection of power distribution systems is essential to insure customer reliability of service, and minimize damage caused by man, nature, and material defects. Large currents flowing through devices and lines which are· not designed to carry these currents for long periods of time cause electrical damage. The distribution system must be protected from the product it delivers, electric energy, in magnitudes larger than the system was designed to carry. Distribution system protection is provided by devices which interrupt damaging currents. A distribution system fault study and a distribution system coordination study must be performed to provide distribution system protection. Data regarding the range of full load current and fault currents must be tabulated before protective devices can be applied to distribution circuits. Distribution system fault current data is obtained by representing the components of a power system in the positive-, negative-, and zero-sequence component networks, Power plants are represented as a voltage source with series impedance (Thevenin equivalent) in the positive-sequence network, and as an impedance in the negative- and zero-sequence networks. Subtransmission lines, subtransmission voltage to distribution voltage transformers, and distribution lines are all represented as impedances in the three networks. Conductor size, type, and configuration and transformer impedances vary from one distribution system fault study to another, but the basic goal, dividing driving point voltage by system impedance never changes. The first goal of the thesis was to develop a computer program to compile distribution system impedance data and calculate distribution system fault currents. This goal was realized for distribution systems supplied by 41.6 kV sub-transmission lines. The distribution system coordination study is designed to provide distribution system protection with the highest amount of customer reliability. Distribution system protective devices discussed have time-current operating characteristic curves. Equations were formulated to represent these curves. The second goal of the thesis was to develop a computer program which would select distribution system protective devices. This second goal was accomplished for distribution systems supplied by a symmetrical three-phase transformer or a single-phase transformer, utilizing a recloser-fuse scheme of distribution system protection. Distribution systems considered in this presentation are assumed to be radial, supplied by one source (subtransmission to distribution voltage transformer).

Library of Congress Subject Headings

Electric power distribution

South Dakota State University Theses



Number of Pages



South Dakota State University