Sequence impedances (positive – Z1 and zero – Z0) of overhead lines and underground cables are important characteristics of electric power systems since the accuracy and performance of protective relay systems, load-flow applications and energy management systems very much depend on the accuracy of these characteristics. Many engineers and systems use sequence impedance values that have been computed from software models of overhead and underground lines. There are several commercial software applications that offer this feature. However, in most cases software models cannot accurately model the zero sequence component due to the irregular composition of the earth in various locations. It is not uncommon to have the soil model (i.e. quantity of soil layers of various depths and apparent resistivity) change several times throughout the length of an overhead transmission line. There is also the inherent obstacles of buried conductive objects that can also impact (gas and water lines, railroads, distribution and communications lines, etc.) the zero sequence component. As a result, the zero sequence component can be a moving target and an impracticality to accurately characterize with simplistic software tools.
The ramifications of not characterizing the zero sequence component accurately is manifested in the performance of distance relay schemes. Since a majority of faults on electric power systems are faults to ground/earth, it is important to obtain a zero sequence value that closely represents the circuit or line in order to avoid over and under-reaching operations of the relay scheme.
A simplistic, accurate and economically desirable alternative to modeling overhead and underground lines is direct measurement of the sequence impedances. While the equipment used to capture these values is sophisticated, the actual test is quick and benign to other parts of the power system. An outage on the line or cable system for approximately 1 hour is all that is required. OrionMagnetics would be pleased to render a quotation or provide consultation for your project or maintenance requirements.
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