With Today's Increases in System Voltages and Power Plant Ratings, more attention is being given to the safety of operational personnel in substations. When it comes to safety, the grounding system must be an important consideration in a substation's design. Following the installation of a new grounding system at a substation, testing is required to confirm the grounding resistance complies with the design value. In addition, the substation grounding system should be tested periodically to make certain no changes are required.

To ensure the safety of its operational field personnel, Israel Electric Corp. (IEC; Haifa, Israel) has taken steps to improve the integrity and accuracy of its substation grounding installation and test methods. Approximately 80% of IEC's substations are located in cities and suburban industrial zones, making it increasingly difficult or impossible for the utility to find available space in more than one direction to spread measuring wires so as to eliminate the coupling effect.

GROUNDING TEST METHOD

The fall-of-potential method is the most-widely adopted test used on the majority of grounding systems. By this method, the impedance measurements of a high-voltage substation grounding system are determined by passing an alternating current between the grounding grid and a remote current electrode. The potential electrode is placed at various positions between the current electrode and grounding system.

The ratio of voltage to current, known as the apparent resistance, is then plotted against the distance from the substation. The resultant curve, if the measurements are accurate, has a relatively flat segment in the middle section of this characteristic. The required value of the grounding system resistance is then determined from the characteristic at a position that is approximately 50% to 70% of the current wire length.

This measured value consists of two components:

• The actual voltage difference between grounding system under test and the auxiliary potential electrode
• The inducted potential due to alternating current flowing in the current test loop, known as the coupling effect.

At substations, the coupling effect between the current and potential leads can be considerable, giving rise to measurement discrepancies of 100% or more.

IMPEDANCE TO GROUND MEASUREMENT

The accuracy of measurements using the fall-of-potential method is dependent on several factors:

• Electromagnetic interference on the measuring wires

• Position of the potential and current wires with respect to the grounding system

• Metallic (conducting) objects between the electrodes and the substation.

The position of the potential electrode with respect to the current electrode when conducting ground measurement tests can vary widely. The potential electrode may be situated in line between the substation and the current electrode, either inserted at right angles to the current electrode or positioned on the opposite side of the substation from the current electrode.

Each configuration of the potential and current electrodes has advantages and disadvantages. Selecting a 90-degree or 180-degree configuration tends to eliminate the influence of the coupling effect. Positioning the potential electrode in line with the current electrode enables the recognition of services buried between ground level, such as water pipes and large metal bodies. A buried-object insert would deform the shape of the apparent resistance characteristic; therefore, to perform a test, the on-site technician would need to select alternative electrode positions, which, because of the coupling effect, would result in readings higher than the actual grounding resistance value.

Positioning the current and potential probes at opposite sides of the substation site does not eliminate the coupling effect, although the coupling effect will reduce the measured potential. Additionally, this method does not allow for identifying conducting objects in the ground. In practice, it is unlikely to have a substation site without buried communications and transmission cables; for this reason, the opposite electrodes method should not be used.

Another method often used by field testing staff is to place two electrode wires at 90 degrees, thereby eliminating the coupling effect. The disadvantage of this approach is the problem related to identifying the effects of buried conducting objects. Another possible source of error is the differences in ground soil resistivity in the areas used for the potential and current electrodes. It is also a technical fact that placing the potential electrode 90 degrees to the direction of the current electrode will always result in the measured apparent resistance being lower than the actual resistance.

THE COUPLING EFFECT

Today, substations are no longer located in open areas. Rather, they are often sited near load centers in cities or in suburban industrial zones. Therefore, it is difficult or impossible to find more than one direction to spread the measuring wires on a route that is free of transmission lines, buildings or underground communication cables. To overcome this problem, IEC has decided to adopt as standard the co-directional method of measurement.

Following mathematical computations, IEC undertook calculations for field tests that took into consideration variable distances between electrodes and differing values of soil resistivity. In addition, the utility also determined the influence of frequency on the coupling effect.

On-site field testing technicians are never able to position electrodes with 3000 m (9843 ft) apart. It is only necessary to identify the coupling effect at a single point — the point that refers to measuring resistance, which lies in the range of 50% to 70% of the total current wire length. Hence, the distance from that point to the substation will not be more than 2000 m (6562 ft).

In practice, the soil resistivity will not exceed 10,000 Ω/m, and based on experience, the most convenient distance between the current and potential wires for field measurements is 1 m (3.28 ft). The measurements should be taken at a frequency preferably within ±2% of the power frequency.

Based on a power frequency of 50 Hz and soil resistivity values in the range of ρ = 25 Ω/m and ρ = 3000 Ω/m, the calculations confirmed that for the same soil resistivity, the curves of the coupling effect versus the potential wire lengths were practically identical. Thus, for every given soil resistivity value, only a single characteristic is required as the length of the potential wire does not influence the coupling effect.

Based on these results, IEC has developed a family of coupling effect curves to be determined for different soil resistivities, for any current wire lengths up to 3000 m (9843 ft) and potential wires up to 2000 m.

FIELD TESTS

IEC conducted grounding tests at four 170/24-kV substations located in rural areas to verify its theoretical design calculations. The sites were checked to ensure there were no underground communication cables that could influence the measurements in close proximity to the sites. The measurements have been performed using two methods: with the potential and current electrodes positioned as co-directional, in parallel and laid with a 1-m (3.3-ft) gap between them, and with the electrodes positioned at 90 degrees.

It is assumed the 90-degree measurement gives a true result, so it is reasonable to assume the values obtained from 0 degrees should be higher because of the coupling impedance value. Differences in measurement results are acceptable, provided they are within the range of 5% to 10%. These differences are due to the earth resistance value that can vary with changes in climate or temperature.

Currently, IEC determines the grounding resistance by inserting the current electrode at a point some four to five substation diameters from the substation center. Substations diameters vary from 40 m (131 ft) for mobile substations to 700 m (2289 ft) for the large high-voltage switching stations. The potential electrode is placed at various positions between the current electrode and grounding system.

The distance between the current and potential wires is about 1 m. The resultant curve — the ratio of voltage to current — is plotted against the distance from the substation. The constant value section of this characteristic occurs in the region of 50% to 70% of the current wire length that determines a substation resistance.

Applying the basic standard (i.e., the Wenner method), by using the soil resistivity of the area where the grounding mat is installed and knowing the lengths of potential wire and soil resistivity curve, the coupling effect can be easily found. By subtracting the coupling effect value from the measured ground resistance, the substation grounding resistance can be determined.

TRUSTED MEASUREMENTS

The coupling effect significantly influences grounding-resistance measurement results. The determination of this effect is not easy because of its dependence on current wire length and soil resistivity. Proper test method selection is important to obtain accurate results and to identify buried conducting objects as well as other objects that can influence measurement results.

The method of substation resistance measurement and calculation that IEC uses has several advantages:

• Easy to perform
• Eliminates complicated computations
• Provides results with greater accuracy
• Allows for the detection of underground conducting objects, thus obtaining more reliable results.

Equally important are the financial benefits that accrue when the coupling effect is considered. Savings stem from reduced measured ground resistance as well as reduced earthing conductor and deep-driven rods required for the substation earthing system.

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Alexander Farber (ferber_a@iec.co.il) received BSEE and MSEE degrees from Leningrad Polytechnic Institute in Russia. In 1986, he joined Electroapparat, where he worked on the development and design of high-voltage test equipment for gas-insulated substations. He joined Israel Electric in 1990, working as a senior section manager in the Central Electrical Laboratory in the field of substation equipment testing.

Boris Katz (katz_b@iec.co.il) received BSEE and MSEE degrees from Ural Polytechnic Institute in Sverdlovsk, Russia. In 1984, he joined TechEnergo, where he worked as a test engineer on power stations and industrial sites. He joined Israel Electric in 1990, working as a senior section manager in the Central Electrical Laboratory in the fields of relay protection testing and test equipment calibration.

Summary of Measurement Results

No.	Substation diagonal (m)	Current wire (m)	ρ(Ω/m)	Measured value (Ω)	Coupling effect (Ω)	Final result 0 degrees (Ω)	Final result 90 degrees (Ω)	Results difference (%)
1	100	550	200	0.252	0.112	0.140	0.151	7.3
2	35	250	30	0.232	0.035	0.197	0.185	-6.5
3	85	400	450	0.225	0.075	0.150	0.164	8.5
4	40	200	85	0.320	0.035	0.285	0.270	-5.6