High Voltage Pylon Earth Measurements

Speaker: Gavin van Rooy

Authors: Frank Barnes and Gavin van Rooy

Tycom (Pty) Ltd

PO Box 3546, Randburg, 2125, South Africa

E-mail: frank@tycom.co.za

Phone: 011 787 8508 Fax: 011 787 8518

Abstract

The earth connection of high voltage electrical power line pylons is obviously very important. The different design of pylons can change the earth bond task drastically. This paper will give feedback from an actual experiment conducted with a new design Pylon in South Africa and discusses the difficulty of getting repeatable measurements and knowing that the Pylon is properly earthed.

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centres. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as

electric power distribution. Transmission lines, when interconnected with each other, become transmission networks.

Figure 1.Network of Power

1. Earthing

The simple definition of an earth is to connect the electric circuit or equipment to the earth’s conductive surface. The reason for this is personal safety and protection of equipment in the case of a lighting strike. Any power quality expert will relate that poor grounding is second only to improper wiring as the leading cause of equipment malfunction. There are a myriad of methods and systems to do this, starting with a simple stake driven into the ground to multiple ground rods connected, mesh or grid networks. One of these systems is then chosen depending on the requirements of the site. Considerations would include the type of installation. For example a substation, electrical pylon, residential residence or cell phone tower and anything in between. Another consideration would be the soil resistivity which is dependant many factors i.e. sandy desert and rocky areas which would be very much less conductive (1000 ohms per meter) than loamy and clay soil (100 ohms per meter). These factors are not always constant and can be affected by climate, rainfall, frost and temperature. Changes to the site can result in changes of the soil resistivity, for example, if the area were to be paved and developed after the soil resistivity was measured, this would result in less moisture reaching the earth and the soil resistivity would increase.

2. Soil Resistivity

Soil resistivity measurements have a threefold purpose. First, the data is used to make subsurface geophysical surveys as an aid in identifying ore locations, depth to bedrock and other geological phenomena. Second resistivity has a direct impact on the degree of corrosion on underground pipelines. A decrease in resistivity relates to an increase in corrosion activity. Third the resistivity directly affects the design of the grounding system.

Before we measure the soil resistivity we must consider a few factors which will have an effect on the measurement. The first of these is the physical dimensions and the depth of a typical earth electrode assuming all connections to the electrode are in perfect condition and present a very low resistance. Because the earth is made up of various layers and these layers do not have a constant resistivity it follows that the deeper the electrode is driven into the earth the better the resistance to earth will be. If the length of the electrode is doubled the resistance level can be reduced by 40%. The diameter of the electrode does not have much influence on the resistance levels.

The second is the “sphere of influence”. When using multiple earth electrodes is does not necessarily follow that the more electrodes used the lower the earth resistance levels will be.

The definition of a ground electrode is a conductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the soil. This definition does not refer to an actual ohm resistance value of the electrode. The resistance value is determined by the resistivity of soil which these electrodes are in contact. When an object is grounded, it is then forced to assume the same zero potential as the earth. If the potential of the grounded

object is higher or lower, current will pass through the grounding connection until the potential of the object and the earth are the same. The earth electrode is that connection path from the equipment to the earth. The effect of the concentric shell is that it takes a finite amount of earth for a ground rod to fully realise its resistance value. This finite amount of earth is known as ground rod’s sphere of influence. The sphere of influence of a ground rod is commonly thought to be a radius around the ground rod equal to its length; the ground rod achieves approximately 94% of its resistance value at this radius. (100% is achieved at approximately 2.5 times the rod length). The resistance of the electrode, measured in ohms, determines how quickly and at what potential energy is equalised. Hence, grounding is necessary to maintain an object’s potential equal to that of the earth’s.

The volume formula is: V = (5 x PI x L cubed) / 3 which can be simplified by rounding PI down to 3 to cancel the 3 below the line. This will leave us with V=5 L cubed. Where V is the volume of soil in the sphere of influence and L is the depth of the electrode.

Using this formula we can calculate that a single 10 foot driven rod would utilize 5000 cubic feet. An 8 foot rod would utilize only 2560 cubic feet.

For additional electrodes to be effective they must be spaced so that their spheres of influence do not intersect. The minimum distance is roughly the depth of the electrode for the second electrode to be effective.

In the case of extremely large fault currents induced into the ground by a phase to earth fault, or by lightning. The ground can not immediately reduce the potential to zero. This condition is called “Ground Potential Rise”. In these conditions not only will the grounding system rise in electrical potential but will radiate into the soil from the centre of the grounding point.

The formula for earth potential

Earth Potential = Soil Resistivity x current / (2 x PI x Distance)

Where Earth potential is in volts, Soil resistivity is in ohm-meters, Current is in Amps, PI is 3.14 and Distance is in meters.

This phenomenon is demonstrated by cows in a field after a lighting strike. Some of the cows are killed but others close by are unaffected. This is because the potential difference radiates from the point of the strike outwards, those cows facing the strike are more likely to be killed as the potential difference between the front and back legs is high and across the animal’s heart. Those standing side on to the strike have a much higher chance of survival as there is a much lower potential difference between one front foot and the other.

Step potential, touch potential and mesh potential

Step potential is the voltage between the feet of a person standing near an energized grounded object. It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the electrode. A person could be at risk of injury during a fault simply by standing near the grounding point.

Touch potential is the voltage between the energized object and the feet of a person in contact with the object. It is equal to the difference in voltage between the object and a point some distance away. The touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. For example, a crane that was grounded to the system neutral and that contacted an energized line would expose any person in contact with the crane or its uninsulated load line to a touch potential nearly equal to the full fault voltage.

Mesh potential is a factor calculated when a grid of grounding conductors is installed. Mesh potential is the difference between the metallic object connected to the grid, and the potential of the soil within the grid. It is significant because a person may be standing inside the grid at a point with a large potential relative to the grid itself.

3. Measuring Earth Resistance

Earth resistance is measured in ohms per meter or ohms per centimetre and represents the resistance of a cubic meter of earth. There are a number of methods of measuring soil resistivity of which the Wenner is most popular.

Figure 2. Frank Wenner 4-Point Test

Wenner simplified formula: Rho = 2 x PI x A x R

Where Rho is the average soil resistivity at depth A

PI is 3.141

A is the distance between the electrodes in cm

R is the measured resistance value in ohms.

Using the Wenner method we need to place four electrodes in a straight line at equal distances apart. The minimum distance apart is three times the depth of the electrode in the ground to avoid the effects of the “sphere of influence”. The depth of these electrodes is typically six inches. The distance that the electrodes are apart is the depth at which the resistivity is calculated. The length of a standard, “off the shelf electrode”, is a good start. It is good practice to take a number of readings in a pattern and calculate the average resistivity.

As results are sometimes distorted by geological anomalies and spurious induced currents, it is recommended that readings be taken at various depths and at 90 degrees to the original axis. This data can then be plotted to form a profile which will be useful in choosing an appropriate grounding system.

The above is all well and good but in today’s terms there are a number of instruments available which will measure the ground resistance at the touch of a button and using automatic frequency control to compensate for ground currents and harmonics. Similarly these instruments implement the four stake method.

4. Transmission Line Towers

One of the most important aspects of transmission line towers is the earthing for the reasons described previously. The two pylons we tested are the 400Kv and 765Kv which is a single mast guyed pylon. The earth connection is the cast in the concrete foundation system of the tower with a connection between the rebar and the tower stub steel. These connections are within the concrete. The importance of this is that there is no earth strap to disconnect in order to measure the earth resistance of one foot alone. To further complicate matters further each pylon is connected to the next by an overhead earth cable. This cable runs along the top of the pylon above the three phases and would be dangerous and difficult to isolate from the pylon. In fact by using the fall of potential method of measuring the earth resistance of a single pylon foot, all the earthing points together including the adjacent pylons will be measured. In cases where an acceptable connection to earth cannot be achieved, a counterpoise earth is used. This method has other advantages in the case of lightning protection. Counterpoise grounding consists of conductors buried below the surface of the earth that are connected to a power-system ground point. In the case of a transmission tower, the connection point could be the tower footing or the grounded side of a lightning arrestor. This counterpoise provides a relatively high capacitance and therefore a relatively low impedance path to earth. The counterpoise is sometimes used in medium- and low-frequency applications where it would be difficult to provide an effective ground connection. The function of the counterpoise is to lower the transmission in areas where the impedance needs to be lowered. The reduction of impedance will reduce the insulator flashover due to lightning strikes. Since ground rods and mats have been found to be ineffective in high-resistivity soils, some utilities have implemented counterpoise grounding. It was found, for example, that in rural areas counterpoise provided some lightning protection, depending of course on soil resistivity. Since performance is directly dependant on field conditions, such as soil resistivity and homogeneity, the use of field measurements is required to ensure the effectiveness of counterpoise grounding. Counterpoise is an effective means of reducing the impedance to ground presented to a lightning strike in areas where high soil resistivity and rocky ground prevent conventional grounding. Fundamentally, counterpoise, which should be considered an alternative to other

methods of grounding, is a leaky transmission line that is intentionally connected to the earth with large amounts of conductance. At the instant of a lightning strike, the counterpoise acts as surge impedance mutually coupled with both the ground wires and phase conductors of the transmission line. The energy from the lightning strike travels down the counterpoise and is reflected at the terminal end. The counterpoise will act as a series resistance with a distributed leakage to ground. A counterpoise would be in excess of a grounding electrode system, not in lieu of.

5. Fall of Potential Method

Two stakes are placed in the earth in a direct line away from the foot of the pylon. These auxiliary stakes must be placed at least 20m away from the earth under test in order to avoid the sphere of influence and associated unreliable readings.

A known current is generated between the outer stake and the earth under test. The volt drop across the middle stake and the earth under test is measured. From ohms law V=IR we can calculate R which in the case of a pylon will be Re i.e. the total resistance of all the earth resistances collectively. To test the reliability of the readings, vary the direction of the stakes relative to the foot of the tower and make 3 or 4 more readings. Compare these reading to ensure that they are within 20% of each other.

This method requires that the earth under test must be disconnected from the system and any other earthing points. In the case of a 765Kv pylon, if it were possible, this would be a dangerous exercise as substantial currents could exist in the earth line. One would prefer not to test this theory using one’s body as a substitute earth conductor.

6. Selective Method

The fall of potential method and many variations thereof was, until the 1990’s, the established method of measuring earth resistance. A whole new type of instrument was developed with the advent of clamp on testing. The major significance of this with respect to pylon testing is that tests can now be carried out on each individual foot separately without disconnecting the system or other earths.

Just as with the fall-of-potential method, two earth stakes are placed in the soil in a direct line, away from the earth stake. Normally, spacing of 20 meters is sufficient. The tester is then connected to the earth stake of interest; a special clamp is placed around the earth stake, which eliminates the effect of parallel resistances in a ground system, so that only the earth stake of interest is measured.

Just as before, a known current is generated between the outer stake and the earth stake, while the drop in voltage potential is measured between the inner earth stake. Only the current flowing through the earth stake of interest is measured using the clamp. The generated current will also flow through other parallel resistances, but only the current through the clamp (i.e. the current through the earth stake of interest because the value of these resistances is negligible) is used to calculate resistance.

R=V/I=Sum of all the other parallel earth resistances

In the case of transmission pylons which the pylons and their earth paths are all inter connected via the top earth cable, this assumption more than valid.

This means that we can now measure the earth resistance of any foot of a pylon without disconnecting any other earth paths.

By using the 4 wire selective method we can vastly (10x) improve on the accuracy of the three pole method and null out the resistance of the test leads. This method is used to measure very low resistances and is therefore particularly suited to measuring the earth resistance of transmission pylons.

At Beaufort West we measured two pylons, one operational with earth connections intact, and the other with the overhead earth not connected and with the rebar earthing system. The operational pylon was a 400kv four footed pylon. The other was a765kV guyed pylon which required five measurements (each of the four guys and the centre of the tower). The two pylons are fairly close to each other and run parallel to each other. The pylons were situated in semi desert with sandy soil. The distance to the potential (inner) electrode was 60 m and to the current (outer) electrode100m. The resistance of both pylons was found to be similar and very low, in the order of 3.4 Ohms. Multiple measurements were done in different directions, on both sites, and all the results were similar.

References

Fluke: Earth Ground Resistance

LEM: Earth Grounding Principals

AEMC: Understanding Soil Resistivity Testing

Lightning Engineers: Electrical Ground Systems – Testing and Importance

Chauvin Arnoux: Earth/ground measurement guide

Mike Holt’s Forum: BPH Gravity

The Internet: About Electrical Grounding

Wikipedia