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Report this Document. Description: asfsf. Flag for inappropriate content. Download now. Related titles. Carousel Previous Carousel Next. As Jump to Page. Search inside document. Dicky Ahmad Zaky. LiMe GrEen. Chandra Pal. The reactance of a large copper wire in this case is shunted by the surrounding earth, a path which may have slightly less reactance than the wire.
Therefore, a continuity test for buried wire would give indeterminate results if alternating current were used. The addition of copper connectors, however large, will not lower the reactance between the two ground grids.
The resistive component can be lowered by additional connectors, and this component is used to determine the integrity of the ground grid. One practical integrity test consists of passing about five amperes into the ground grid between two points to be checked. The voltage drop across these points is measured with a millivoltmeter or portable potentiometer and the effective resistance is calculated from the current and voltage readings. From these readings and the calculated resistance of copper it can be determined whether there is an adequate connection.
For those ground systems that have a direct voltage between points, the change of voltage caused by the test current is used to calculate the resistance. For the majority of large ground systems in service there will be a realtively large alternating voltage between the points to be measured compared with the direct millivolts to be detected. This capacitor should preferably have a liquid impregnated paper dielectric, but some modern electrolytic condensers have so little leakage that they can be used in this application.
These instruments are described in Section Earth Potential 9. The density of equipotential surfaces, having equal voltage differences between them, across a path in a given direction determines the step voltage which may be encountered. This gradient will be highest near the grounding electrode. The distance between equipotential surfaces, measured along the surface of the earth radially from the grounding connection, will vary with a number of factors.
These include variations in resistivity of the earth, the presence of buried pipes, conduit, railroad rails, steel fences, metallic cable sheaths, and the presence of overhead lines carrying ground current. As indicated in 8. Consequently it will be found that the ground potential under the transmission line carrying fault current will have a steeper gradient than in the adjoining earth.
This results in changing the pattern of the equipotential lines whenever a different transmission line terminating at the station is faulted. Therefore, equipotential lines cannot be established simply by measuring resistance from the grounding connection to various points around it. When once established, the voltage between the equipotential lines for a given fault condition can be expected to vary directly with ground-fault current magnitude. This assumes no change in the resistivity of the earth around the grounding system during the flow of fault current.
A potential contour map may then be drawn by connecting points of equal potential with continuous lines. If the contour lines have equal voltage differences between them, the closer the lines, the greater the hazard. Actual gradients due to ground-fault current are obtained by multiplying test current gradients by the ratio of the fault current to test current. The most accurate measurements of potential gradients are made with the voltmeter-ammeter method. A known current, between 50 A and A, held constant during test, is passed through the ground grid to a remote ground test electrode and returned through an insulated conductor.
A remotely located ground test electrode is necessary to prevent gradient distortion, caused by the mutual impedance of inadequately spaced ground electrodes. This distance may vary from m, for a small ground grid to a mile or more for larger installations. Measurements should be made with a very-high-impedance voltmeter on the surface of the earth along profile lines radial to the point of connection to the ground grid.
Unless suitable means are employed to mask out residual ground current, the test current must be of sufficient magnitude to do so.
At the same time care must be taken to prevent heating and drying of the soil in contact with the ground grid or test electrode to avoid variations in voltage gradients in a series of measurements.
Economics and the necessary detail required will determine the number of measurements to be made. When more than one overhead line or underground cable are connected to a substation, potential gradients in and around the substation may be quite different for faults on different lines or cables. Likewise, faults at different locations in large substations may result in differences in potential gradients in and around the substation.
It may, therefore, be advantageous to determine potential gradients in and around a large substation for two or more fault conditions.
Underground metallic structures, for example, neutral conductors, metallic cable sheaths, metallic water and gas lines, etc, metallic structures on the surface of the ground such as railroad rails and fences, and overhead ground wires in the vicinity of a substation, whether connected to the ground grid or not, will usually have a significant effect on potential gradients and should be considered when making potential gradient measurements.
When a potential gradient survey cannot be justified economically, potential gradients may be calculated from ground resistance or soil resistivity measurements. The accuracy of such calculations will be dependent upon the accuracy of the measurements, and the unknown abnormalities of the earth around and below the ground grid.
The adequacy of such calculations may be verified with relatively few potential gradient measurements. These values are proportional to the earth current and provided that the deep soil resistivity is constant to the top soil resistivity. NOTE — A variation of resistivity of the top soil in some cases increases the ground resistance. This in turn may cause a variation in the earth current. The changes in step and touch voltages should therefore be determined by taking into account simultaneously, top-soil resistivity and earth current variations.
Transient Impedance It has been shown [B4], [B15] that the impedance of a simple grounding electrode depends on the amplitude of the impulse current and also varies with time, depending on the impulse form. The nonlinearity of the grounding impedance is caused by local discharges in soil in the area where the electric field gradient exceeds 2. Since the field gradient attains the highest value at the ground electrode the discharges partly short circuit the layer of soil adjacent to the electrode.
Consequently the transient impedance of the grounding system for high-current impulses is lower than the value measured with the conventional steady-state methods, or with an impulse of lower amplitude which does not produce the discharges in soil. An opposite effect has been observed in the case of extended ground electrodes, wires or strips more than m ft long, when tested with steep front impulses. The voltage drop across the grounding impedance shows then a large inductive component.
The instantaneous impedance is normally determined as a quotient of the applied transient voltage and current recorded at the same instant. The additional voltage component which appears across the grounding inductance at the steep impulse front or at an abrupt collapse of the impulse current is then interpreted as an increase of the grounding impedance.
To perform such measurements a testing circuit is required which contains a high-voltage impulse current generator of adequate energy, as well as a precise voltage divider, current measuring shunt, and double beam impulse oscillograph. Considering these typical requirements a mobile impulse generator which is normally used by power utilities for testing of insulation coordination in high-voltage substations can be suitable for measurements of the transient grounding impedance. Another possible solution consists of installing a prototype ground system in the soil near a high-voltage laboratory and connecting the laboratory generator, as well as the measuring apparatus, to the ground system under test.
The simultaneous oscilloscope recording of the voltage drop across the grounding impedance, and of the applied impulse current, requires a reference grounding point. The reference ground can be conveniently located at the impulse generator base, provided that there is sufficient distance to the examined ground. The transient impedance of ground is derived from the voltage and current oscillograms as a quotient of these two transients, calculated point by point for consecutive time intervals.
Since the variation of the grounding impedance depends on the impulse current amplitude and form, as well as on the electrode geometry and the type of soil, several measurements have to be taken to permit a more general interpretation of results and for a definite conclusion.
Attention should also be drawn to possible common mode interference which may appear in the measuring circuit if the grounding points of the voltage divider and shunt are shifted from the reference ground potential. The high-voltage and high-current impulse is generated by discharge of a large capacitor into an impulse forming network.
Although such a circuit can be improvized on the test site, in most practical cases a mobile impulse generator is used. Apart from the ground to be measured the test circuit has to have another auxiliary ground which carries the return current from the impulse generator.
This ground is preferably of the distributed type, such as a substation or a laboratory grounding mesh, and its impedance must be significantly lower than that of the measured ground. The impulse generator is connected to this ground through a high-current shunt. Voltage drop across the resistance of the measured ground is measured by a voltage divider preferably of the resistive type and designed for the expected voltage range.
It is essential to keep the shunt and the divider grounding points directly connected to the auxiliary ground by short, low-inductance leads. The simultaneous recording of the voltage and current impulses is normally performed with a double beam oscilloscope. The two coaxial cables connecting the divider and the shunt to the oscilloscope have to be of the same length to avoid time lags between the recorded transients.
Taking into account the nonlinear character of the transient grounding impedance, the measurements should be performed at different impulse current shapes and amplitudes. Each set of recorder oscillograms permits plotting a family of the grounding impedance curves, which will characterize the performance of the grounding at the high- and low-impulse currents. Model Tests A 20 to I scale is often satisfactory.
The top layer should preferably be water with some quantity of common salt or copper sulfate to achieve the desired resistivity. The second layer could be simulated by a concrete block of appropriate dimensions. Use of a frequency in the range of Hz to Hz aids in eliminating electrolytic polarization which causes potential distortions. Figure 13—Electrolytic Tank Measure the voltage V between P and A.
Instrumentation Current from the hand-cranked direct-current generator is reversed periodically by the current reverser and exists in the earth between ground X under test and current electrode C. The fall-of-potential between X and the potential electrode P is rectified by the potential reverser, which is on the same shaft, and therefore, operates in synchronism with the current reverser.
The coils operate in a field provided by a permanent magnet. The Current coil tends to turn the pointer toward zero, while the potential coil tends to turn the pointer toward a higher ohm reading. The operating current through these coils is furnished respectively by the current through and the voltage drop across the ground under test, therefore, the scale of the instrument can be calibrated in ohms. A suitable range switch provides a divider to the scale values.
The synchronous reversing switch combination current and potential reverser used in this instrument makes it relatively insensitive to stray voltages in the potential circuit. In most cases a cranking speed, which eliminates the effect of relatively large stray voltages, can be used.
Some difficulty may be experienced in obtaining a reading in an extreme case of a ground of less than 0. In this method current from the alternating-current source exists in two parallel circuits. The lower circuit includes fixed resistance A, electrode X under test, and auxiliary current electrode C.
The upper circuit includes fixed resistance B and an adjustable slide rheostat on which two sliders, Sa and Sb, make contact. With the detector switch closed to the left, slider Sa is adjusted until the detector shows a balance. The currents in the two branch circuits are then inversely proportional to resistances A and B. The switch then is closed to the right, and slider Sb is adjusted until the detector again shows a balance.
The tone of the buzzer usually can be recognized and balanced out even in the presence of considerable background noise caused by stray alternating currents. Resistance at P merely reduces the sensitivity of the detector. Excessive resistance at C may limit the range of resistance that can be measured.
The locations of electrodes P and C are determined by the same considerations as in the fall-of-potential method, given in 8. In this instrument a battery is used to drive a vibrator that has two sets of contacts. The first set of contacts reverses the direction of primary current to a transformer that provides test current between the current electrode and the ground under test.
The second set of contacts gives sense direction to the balancing galvanometer, which then can indicate whether the dial setting is low or high. When the slider of the potentiometer is adjusted until there is no potential between the slider and auxiliary electrode P, as shown by a galvanometer null, the portion of rheostat R, bears a definite relationship to the resistance of the ground under test.
Therefore the potentiometer can be calibrated in ohms with appropriate multipliers provided by taps on the ratio transformer as selected by the range switch.
Since a negligible current exists in the potential electrode circuit at balance, the resistance of the potential electrode does not affect the accuracy but does have an effect on the sensitivity of the galvanometer. The instrument is relatively insensitive to stray voltages and only in an extreme case will difficulty be experienced see NOTE — The above three instruments are often equipped with a fifth terminal called the guard terminal.
If the test electrode ground resistance is high, currents within an instrument may produce a small deviation of the sensitive galvanometer and so cause erroneous readings.
The guard terminal eliminates this error by bypassing these leakage currents to earth. The voltmeter requirements, when there is no stray voltage, are simply that the impedance of the voltmeter be high in relation to the resistance of the potential electrode and the test leads. The impedance of the potential electrode must be considered when measuring the voltage caused by current into the measured ground. It is obvious that less error is introduced when using a high-impedance voltmeter, and this error will become negligible when an electronic type of voltmeter is used.
When there is a stray current in the ground to be measured and it produces a voltage which is large compared with the voltage caused by the test current, this stray voltage must be balanced out, both in magnitude and phase, before test current is applied. The voltmeter in this case should be frequency selective, because only one frequency can be balanced out. Usually the only case where such a selective frequency voltmeter is required is in the measurement of a large grounding system with an impedance of less than 0.
A simplified schematic diagram of the test connections for a selective-frequency voltmeter-ammeter circuit is given as Fig The test current is measured by taking the voltage drop across a 0. This arrangement provides a form of ratio measurement and thus limits the errors to scale errors of the instrument and ratio errors of the shunts and multipliers.
It is a highly sensitive apparatus which is well suited for earth resistivity and resistance measurements. The instrument is a four-terminal type with, however, a different measuring circuitry and power source. The instrument is composed of two units, the receiver and the transmitter as shown in Fig Figure 18—Induced Polarization Units Thus no direct cable connection is required between the receiver and transmitter.
The transmitter passes a strong direct current into the ground through two electrodes and then abruptly interrupts this current. Usually the main design features of the receiver console include: 1 Automatic self potential compensation 2 Remote ground triggering special filters for ac noise suppression 3 Curve shape discrimination and automatic integral summations for random noise suppression. Due to the inherent noise suppression capability of this system, surveys can be conducted much closer to sources of spurious electrical noise such as power lines, and deeper effective penetration can be obtained without increasing power requirements.
Also the coupling between leads can be completely eliminated. Finally, the light weight and low-power requirements allow for the maximum field mobility and versatility of operation.
Danger will be avoided as work shall not be done near energized conductors. For operating principle see Fig The high-frequency meter is fully transistorized. A Ni-Cd battery is used as the power source. The generator is a self- excited power oscillator at 25 kHz. The loop current i flows through the current electrode H and the tower's ground M.
The high-frequency receiver compares the measured voltage with a reference internal voltage. Therefore, adequate spacing between the test electrodes must be used in order to obtain reliable results.
Figure 19—High-Frequency Meter Practical Aspects of Measurements Performing resistivity and resistance tests can be physically exhausting especially if poor equipment is used during measurements. High-quality measuring instruments should be selected in order to obtain reliable data.
Also, in many cases, special auxiliary equipment may be necessary to drive rods, to measure distances, and wind-up test leads. Steel ground rods are preferred to lightweight aluminum rods since aluminum rods may be damaged if a hammer is used to drive them in hard soil.
Screw type rods should not be used. The screw type rod fluffs up the soil and creates air in the area of the rod above the screw which results in high contact resistances.
The driven rod compacts the soil giving minimum contact resistance. The current electrode resistance is in series with the power source and is, therefore, one of the factors governing the testing current. If this current is low, it may be necessary to obtain a lower current electrode resistance by driving additional ground rods. In rocky soil it is a good practice to drive rods at an angle with respect to the vertical. Inclined rods will slide over the top of a rock.
The device used to measure the potential difference should have an internal resistance which is large compared with the potential electrode resistance. If this is not the case, additional ground rods may be required to lower the potential- electrode resistance. The temperature at the site must also be considered to determine the adequate test lead. The lead insulation should not freeze or crack because of low temperatures. The test lead impedance should be low especially when testing low impedance ground systems.
The driving force should be axial to the rod in order to avoid undue whipping. A practical type of hammer useful for the prevention of whipping consists of a chuck and sliding hammer Fig This device has the advantage that the work may be at a level convenient to the individual making the test without using an auxiliary platform. Also the blow is delivered to the rod at a point not far from the ground line. When normal hand driving is not possible hard or frozen soils, etc it may be necessary to use mechanically operated hammers.
These can be operated by either electric, pneumatic, or gasoline engines. When the distances are larger, the use of an odometer may be more practical and less time consuming.
Extremely long distances may be read from appropriately scaled charts or maps of the area. The mobile trolley should be light and compact for ease of handling. Fig 21 shows a possible design for a convenient container equipped with four lead reels which could be spring cranked to wind up the test leads.
The testing instruments are located on the upper shelf. The dc battery if required , hammers, clips, and other handy tools may be stored in the lower shelf. Figure 21—Test Table The conduction through the soil is electrolytic in nature, and back voltages can develop at the auxiliary electrodes. An easy way to eliminate electrolytic effects is to use alternating test currents.
If the current is of power frequency, electrolysis is not completely eliminated and stray alternating current at power frequencies may influence the results. At higher frequencies electrolysis is negligible but the self and mutual impedance of the leads are increased and errors may be introduced. Also if an impedance test is performed, the reactance component will be different from the 60 Hz value.
Usually a compromise using frequencies in the order of 80 Hz is considered adequate. If direct current is used, the effects of inductance and mutual impedance are eliminated, but electrolysis can be very troublesome. This problem can be solved by reversing the direct current periodically. The effects of inductance and mutual impedance are then evident only as transients which will be negligible, if the time constants of the various circuits are sufficiently low.
However, it is not adequate for impedance measurements. The preceding discussion points to the necessity of caution when handling the test leads, and under no circumstancesshould the two hands or other parts of the body be allowed to complete the circuit between points of possible high-potential difference.
It is true that the chances are remote that a station-ground fault will occur while test leads arebeing handled, but this possibility should not be discounted and therefore the use of insulating shoes, gloves, blankets,and other protection devices are recommended whenever measurements are carried out at an energized power station. In all cases, safety procedures and practices adopted by the particular organization involved shall be followed. These grounds fall in a special category because of the extremely high short-duration lightning currents carried bysurge-arrester grounds.
These currents may be in excess of 50 A for surge currents,with a possibility of fault-system currents in the case of a defective surge arrester. An isolated surge arrester ground should never bedisconnected to be measured, since the base of the arrester can be elevated to the line potential. A surge-arrester groundcan be tested as long as precautions axe taken to minimize arrester discharge. Another precaution concerns possible high-potential gradients around the current electrode.
If current is passed into aremotely located electrode, as in the fall-of-potential method, it is worthwhile to ensure against a curious person beingallowed near the current electrode while tests are in progress.
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