Questions about design of grounding for overhead transmission lines in conditions of the extreme North often arise, because it is not easy at all to achieve the required resistance of the grounding device in permafrost soils.
Requirements to grounding for overhead transmission lines
Any engineer starts the design from the existing regulatory documents. Requirements to the grounding resistance is regulated by the EIC "section 2.4" for overhead transmission lines with the voltage of up to 1 kV and "section 2.5" for overhead lines with the voltage of more than 1 kV. Let's see the example of overhead power transmission lines of 110-500 kV, because the majority of questions are connected with them.
Metal and reinforce concrete bases on overhead transmission lines of 110-500 kV must be grounded. The resistance of the grounding devices is regulated in chart2.5.19 EIC 7th ed.
Chart2.5.19. The maximm resistance of the grounding devices of overhead transmission line bases
|Specific equivalent ground resistance, Ohm*m||The maximum grounding device resistance, Ohm|
|Up to 100||10|
|More than 100 to 500||15|
|More than 500 to 1000||20|
|Over 1000 to 5000||30|
It is necessary to remember that the foundation reinforcement must be considered and used as a natural ground electrode. The case when the ground resistivity exceeds 1000 Ohm*m is an exception. In such grounds the reinforced concrete structures should not be considered at the calculation of the grounding device resistance.
Grounding of overhead power tranismission lines in perfmafrost ground
In the conditions of extreme North the grounds are perfmafrost and their ground resistivity exceeds 1000 Ohm*m, that is why we can't use natural ground electrides for overhead line bases and it is necessary to calculate artificial ground electrodes for overhead power transmission lines.
The problem is that it is not easy to achieve the ground resistance of, for instance, 30 Ohm in the conditions of permafrost grounds using conventional vertical and horizontal ground electrode systems. The areas for the grounding of one base turn out to be enormous, that correspondingly means that the expenditures for ground electrodes and their installation will be large. The customers usually don't like that. So the designers think of what to do?
There is a solution for such issues. It is reasonable to use special ground electrodes which were developed specially for permafrost, hi-ohmic grounds in this case. These are - electrolytic ground electrodes which provide resistance up to 12 times weaker in comparison to ordinary electrodes.
Example of calculation of a grounding device for overhead power tranmission lines
We will study an example of what structure of a grounding device will be for one overhead tranmission line base of 220 kV with a ground resistivity of 3000 Ohm*m.
In accordance with item 2.5.129 chart2.5.19, EIC for overhead lines' bases of 220 kV in grounds with the resistivity of more than 1000 and up to 5000 Ohm, ground electrode device resistance should not exceed 30 Ohm.
To achieve the required resistance we will use 2 kits of electrolytic grounding ZZ-100-102-6MB which are united with the help of a copper-bonded steel tape with the cross-section of 30x4 mm. The burial depth is 0.5 – 0.7 meters.
Calculation of the grounding device:
You can see the algorithm here.
The design resistance of the grounding device is 27,94 Ohms, which is less than the required value of 30 Ohms.
Figure 1 shows the arrangement of equipment.
Figure 1 - Grounding device of overhead transmission line base of 220 kV.
Комплект электролитического заземления – Electrolytic grounding kit
Полоса омедненная – copper-bonded tape 30 x 4mm
The list of required material is given in Table 1.
Table 1 - Materials requirement list
|№||Figure||Product item||Name||Quantity, pcs.|
|1.||ZZ-100-102-6МВ||ZANDZ Electrolytic grounding kit (vertical; 6 meters)||2|
|2.||GL-11075-10||GALMAR Copper-bonded tape (30 * 4 mm / S 120 mm²; coil of 10 meters)||3|
|3.||ZZ-005-064||ZANDZ Clamp for connecting conductors (up to 40 mm)||7|
|4.||ZZ-007-030||ZANDZ Waterproof tape||2|
So, in order to achieve the necessary requirements to grounding resistance of overhead transmission line base of 220 kV we needed only 2 vertical electrolytic ground electrodes 6 m long. And it is necessary to know, that for overhead lines protected by cables, the ground resistance of the grounding devices made according to the lightning protection conditions, must be provided with a detached cable, and for the rest conditions - with a detached cable.
Do you have questions connected with the design of grounding of overhead transmission lines in the conditions of the Extreme North? Send your request to email@example.com.