evaluation study on the effectiveness of the new proposed transmission tower earthing design under various soil conditions

Lightning is the primary cause trippings on overhead transmission lines in TNB. There have been incidents where faults occurred on transmission lines with tower footing resistance values below the TNB-specified limits (10ohm for 132kV and 275kV towers). Base from this problem, study and understand the dynamic behaviors and performance of transmission tower earthing system under the high frequency and transient conditions. This knowledge used in an effort to reduce faults on the transmission lines due to lighting. The design of tower earthing were modeled in and simulated using the Current Distribution, Electromagnetic, Grounding and Soil Analysis (CDEGS) software. The designs are to radial electrodes routed at different directions and electrodes arranged in a mesh configuration, with and without vertical electrodes.
For each design, the impedance remained constant for frequencies between 50H and up to 1kHz and 10kHz. Above these frequencies and up to 1MHz, the impedances increased at different rates depending on the design. At 50Hz and up to 50kHz, radial electrodes which cover larger lateral distance or area of the soil yielded the lower impedance value. Above these frequencies, the meshed electrodes concentrated and localized around the tower yielded lower impedance value. These radials electrodes coupled with vertical electrodes localized at the area immediate to the tower is the most effective design for both and low frequency, including the power frequency and lightning frequency. This design has fast current discharge and dissipation time an evident during the actual impulse testing at the study site.
For the tow-layer soil with the lower soil resistivity at the lower layer, the vertical electrodes are best to be driven to reach the lower layer. However, for soil with the lower resistivity at the upper layer, the vertical electrode depth to be kept to within the depth of the upper layer.
Key research elements are summarized below:
• Research on the effectiveness of present TNB tower earthing design in dissipating lightning direct into ground
• Research on the effectiveness on the new proposed tower earthing design in dissipating lightning direct into ground
• Research on the effect of different type of soil for lightning current dissipation
• Research on the best or mist appropriate tower earthing design to mitigate lightning induced faults on transmission lines

SOIL RESISTIVITY MEASUREMENT
Soil resistivity is a material with a low resistivity will behave as a “good conductor” and one with high resistivity will behave as a “bad conductor”. The resistance (R) of the conductor, can be derived from the resistivity as:
R=ρL/A
Where ρ Resistivity(Ω-m) of the conductor material
L length of the conductor (m)
A Cross sectional area (m2)

The importances of soil resistivity measurement are:
Such data re used to make sub-surface geophysical surveys as an aid in identifying ore location, depth to bedrock and other geological phenomena
Resistivity has direct impact on the degree of corrosion in underground pipelines.
Soil resistivity directly affects the design of a grounding system
Factors affecting soil resistivity
Type of earth
Stratification
Moisture content
Temperature
Chemical/salt composition
Presence of metal
Topography

Measurement of soil resistivity
A four-terminal earth tester is required, equipped with four short test rods and connecting leads. The leads should be mounted on reels for easy run-out and recovery. The calibration of the instrument should be checked before taking any readings. Before carrying out an testing, checks should be made both from cable records, and using above-ground detection equipment, for the presence of any buried cables, earth conductors or metal pipe work. These would adversely affect the accuracy of the readings taken, particularly if they are parallel to the measurement traverse.
The traverse locations chosen should be:
Away from existing cables, earth conductors or pipes
Not close and parallel to the overhead line route. The traverse should be ideally at 90° to the lie but no less than 45°. If they must be in parallel, then a separation of 20m or more from line is preferable to avoid induction affecting the results


Figure shows the general measurement arrangement. The four earth rods should be driven into the ground in a straight line, at a distance “a” meters apart and driven to a depth of “d” meters. The suggested dimensions are given in Table.
Spacing (m) Rod depth (m)
1.0 0.05
1.5 0.05
2.0 0.05
3.0 0.10
4.5 0.10
6.0 0.10
9.0 0.15
13.5 0.15
18.0 0.15

The four rods should be connected to the tester with outer rods connected to the C-1 and C-2 terminals, and the inner rods to the P-1 and P-2 terminals.
When the instrument is switched on, there will be an apparent resistance reading in the meter, which is “R” ohms. The meter should be left on to allow the built in filters to operate and the value after about 30 seconds should be fairly constants. The apparent soil resistivity (ρ) is then given by 2πaR Ωm.


Measurement interpretation
For application of power engineering – two layer equivalent model is accurate enough without being mathematically too involved. The measured apparent resistivity is plotted against the electrode spacing. The resulting curve indicates the soil structure. Software available fot simulation is CDEGS-RESAP modules.

EARTHING RESISTANCE MEASUREMENT
Earthing resistance is ohmic resistance between the earthing electrode and a remote earthing electrode of zero resistance. The importances of measurement are:
• To confirm design value – erathing system design
• To asses existing earthing system (substation/towers)
I. Measuring earthing resistance using Fall of Potential Method (FOP)
These measures involve measurement of voltages. R(R=V/I) as function of distance of voltage probe. Apparent R is R at the flat region of the curve. To obtain reliable results, the flat region must be clearly established, else other methods of interpretation shall be used. Required large electrode spacing (rule-of-thumb 5-10x (min.) the size of the grounding system)

II. Measurement interpretation
61.8% Rule
The apparent resistance is the resistance at the 61.8% of the distance between the earth under test and the current probe. Assumptions of FOP and 61.8% rule:
• Adequate probe distances
• Homogeneous soil (resistivity)
• Identical electrodes
Main advantages is P and I electrode can have substantially high R will not affect the accuracy.


Fault Lightning Analysis and Lightning Location System (FALLS)
The Fault Lightning Analysis and Lightning Location System (FALLS) software is a tool for studying the relationship between lightning events and the geographical reference asset. This program used to design maintenance, power quality and restoration, the ability to quickly and easily perform fault correlations on an event-by-event basis, obtain statistics concerning the number of lightning events or magnitudes for design, and evaluate lightning challenges to a line or area for input to maintenance prioritization.
Lightning Detection Lab (LDS) in TNB Research using this software to generate the lightning map for regional or exposure analysis to upgraded the performance of transmission because lightning is the primary cause of trappings on overhead transmission lines in TNB.