Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1994-07-05
2001-07-24
Nguyen, Vinh P. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S347000, C324S357000, C324S713000, C324S715000, C324S718000
Reexamination Certificate
active
06265881
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method and apparatus for measuring the impedance of the ground of an electric power grounding system for evaluating the performance of the grounding system.
BACKGROUND OF THE INVENTION
Traditionally, the performance of electric power grounding systems has been evaluated by measuring the impedance of the grounding system using the so-called “fall of potential” method. In the fall of potential method, a reference electrode is placed in the ground at a location far removed from the grounding system. A current is applied to the grounding system and the voltage potential between the grounding system and the reference electrode is measured. This voltage potential is referred to in the art as the “ground potential rise” and the method assumes that this ground potential rise or “GPR” is a measurement of the voltage potential between the grounding system and the most remote earth. The GPR simply is divided by the current applied, according to Ohm's Law (V=IR), to determine the resistance of the grounding system. The resistance is assumed to be equal to the impedance, thus providing an estimation of the impedance.
This fall of potential method suffers badly from several deficiencies. For example, very long wires are needed to place the voltage probe as far from the grounding system as possible in an attempt to get clear of the zone of influence or electrical interference in the vicinity of the grounding system. Typically, these wires are 2-3 miles long and some are much longer. However, in order to completely avoid the interference from the grounding system itself, one would have to place the voltage probe at an infinite distance from the grounding system. Obviously, this is not possible. Also, as the length of the wire is increased, the reliability of the signal suffers due to voltage interference induced in the long wire by electric sources, such as power distribution lines, adjacent the wire. Furthermore, where grounding systems are placed in the vicinity of populated areas, the use of long wires is generally impractical owing to problems in securing a needed right-of-way.
By assuming that the measured voltage potential between the grounding system and the probe is equal to the voltage potential between the grounding system and the most remote earth, the fall of potential method introduces a significant error in the estimation of impedance. This is compounded by the simplistic approach used to estimate impedance without considering the reactance component of impedance.
It has been common in the practice of the fall of potential method to use a steady-state, alternating current source for providing the electric current injected into the ground through the grounding system. Unfortunately, such a current source produces currents very much like currents applied to the grounding system by the electric power system, making it difficult to distinguish a useful voltage signal from common noise. Finally, the above-described shortcomings of the fall of potential method render the method generally useless for very large grounding systems having very low impedances, because in extending the probe and the wire to beyond the zone of influence of the grounding system, the probe and the wire typically would end up either in the zone of influence of another grounding system or the interference from power circuits will corrupt the useful signal, or both, and because in this case the useful signal is relatively low, the method is virtually prevented from making an accurate measurement of low impedance values.
Accordingly, it can be seen that a need yet remains for a method and apparatus for measuring ground impedance which does not require long wires, which provides an accurate measurement, and which is effective for evaluating large grounding systems with low impedances. It is to the provision of such a method and apparatus that the present invention is primarily directed.
SUMMARY OF THE INVENTION
Briefly described, in a preferred form the invention comprises an apparatus for measuring ground impedance to evaluate the performance of an electric power grounding system. The apparatus comprises a current electrode for placement in the ground and means for causing an electric current to flow from the current electrode through the grounding system. At least two voltage probes are provided for placement in the ground at first and second locations spaced from the grounding system for detecting ground potential differences resulting from the flow of current from the current electrode through the grounding system. The voltage probes are placed rather close to the grounding system, on the order of hundreds of feet. Electronic means are provided for evaluating the ground potential differences as detected by the voltage probes at these first and second close locations to estimate a ground potential rise at remote earth. The estimated ground potential rise is used to calculate the impedance of the grounding system.
Preferably, the electronic means are adapted for controlling the electric current and are linked with a computer for processing data, while filtering noise. Also, the current is injected as a random, transient current.
In operation, the transient electric current is caused to flow from the current electrode to the grounding system and the resulting transient ground potential differences at the various voltage probes at several points on or below the earth surface at small distances from the grounding system are measured and recorded. After the signals have been filtered of noise, a parameter estimation procedure is used which takes into consideration the distance of the voltage probes from the grounding system. Specifically, the ground potential rise with respect to remote earth is estimated by comparing the measured data with grounding system mathematical models.
In another form, the present invention comprises a method for measuring ground impedance to evaluate the performance of a grounding system, the method comprising the steps of placing a current electrode in the ground and placing at least one voltage probe in the ground at a position spaced a short distance from the grounding system. The method also includes the steps of causing current to flow from the current electrode to the grounding system, detecting a ground potential difference with the voltage probe, and evaluating the ground potential difference in relation to the distance of the voltage probe from the grounding system to determine a ground potential rise with respect to remote earth, and using the ground potential rise of remote earth to calculate the impedance of the grounding system.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for measuring ground impedance without requiring lengthy cables.
It is another object of the invention to provide a method and apparatus for measuring ground impedance which is suitable for use with a large grounding system with low impedance.
It is another object of the invention to provide a method and apparatus for accurately measuring ground impedance.
It is another object of the invention to provide a method and apparatus for measuring ground impedance and which is effective at filtering out extraneous noise, such as from adjacent power transmission lines.
Other objects, features and advantages of the invention will become apparent upon reading the following specification in conjunction with the accompanying drawing figures.
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Zwicker, Messung des Erdungswidertandes, Dec. 1980, pp. 18-24.
Cokkinides George J.
Meliopoulos Athanasios P.
Deb Anjan K.
Deveau, Esq. Todd
Georgia Tech Research Corporation
Nguyen Vinh P.
Schneider, Esq. Ryan A.
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