What size conductors should I use? And, is there a simple table I can use for this HV high voltage earthing design project. These are great questions and often asked. This post answers these questions together with the choice of conductor materials and jointing method within a high voltage earthing design project.
Assessing conductor size is entirely dependent on the electrical configuration and the load that the conductor must take. For example, an above-ground bonding conductor serves to transfer current with minimal voltage drop, from A to B. A directly buried conductor has an additional purpose, that of leaking the fault current or voltage into the local geology (as part of an electrode).
When considering Lightning, the same conductor above might also see a high-frequency component and which will impose yet another requirement.
High Voltage Earthing Design Considerations
In IEEE-std80 Guide for Safety in AC Substation Grounding, section 11 – states the basic requirements are:
Each element of the grounding system, including grid conductors, connections, connecting leads, and all primary electrodes, should be so designed that for the expected design life of the installation, the element will :
- a) Have sufficient conductivity, so that it will not contribute substantially to local voltage differences.
- b) Resist fusing and mechanical deterioration under the most adverse combination of a fault magnitude and duration.
- c) Be mechanically reliable and rugged to a high degree.
- d) Be able to maintain its function even when exposed to corrosion or physical abuse.
High Voltage Earthing Materials
Copper has traditionally been the go-to material for years in high voltage earthing because it is not only highly conductive but also resistant to most sources of in-ground corrosion.
Similarly, aluminium has sufficient conductivity but suffers from in-ground corrosion, and the oxidation that forms around its surface is not conductive, therefore, compromises the conductor’s ability to leak current when buried, which is why buried aluminium conductors is a no-no.
Stainless Steel or mild steel (when appropriately coated – galvanised or copper bonded), on the other hand, may not have the conductivity of copper or aluminium but it is sufficiently conductive to be utilised as a buried conductor.
Dealing with the Heat From HV Current Flow
So now we’ve covered the materials widely used for HV earthing. Point ‘b’ from IEEE-std 80 above calls for the conductor to retain its mechanical strength when exposed to a fault.
Conductors get hot when current flows through them. And they will potentially their hottest when subjected to a fault from the electrical system. This fault means the conductor needs to maintain its physical integrity as temperature increases and not transform into a shower of hot molten metal!
Up to this point, we’ve discussed conductor conductivities, materials and thermal-mechanical characteristics.
Another consideration for high voltage earthing is the method of the joint between the conductors.
High Voltage Conductor Joints
Joints in the conductors are; as critical to current-flow as the conductors themselves. The jointing method must not allow the joint to introduce excessive resistance. Therefore, the selection of the jointing method will impact the HV earthing system’s thermal resilience massively. This thermal resilience is why welding joints are the go-to choice for the high voltage earthing system.
Welded joints achieve the nearest physical match to the native conductor itself and are the gold-standard in conductor jointing because they accomplish a molecular similarity to the native material’s conductivity, as well as similar mechanical robustness.
Here’s an example of a safe method of igniting an exothermic weld.
A Word of Caution
Bolted joints and clamps made up of multiple parts which can creep and loosen over time. Loosening adds resistance across the conductor’s joints. Which as previously mentioned… is not a good thing (thermal runaway is a topic for another day).
Bolted joints fall into the category of mechanical joint methods and have a derating factor to keep the conductors operating at lower temperatures. When designing a high voltage earthing system, this means the seemingly unrelated selection of jointing method will also impact the conductor size calculation.
IEEE adiabatic equation.
So there you have it. Without going into great detail, the following factors influence conductor sizing in high voltage earthing design:
- The magnitude and duration of fault current (i.e. the heat source),
- The method used for the joint
- The conductor material
- The conductor’s role in the high voltage earthing design
These factors may vary project by project, so, using a particular size of the conductor on a previous project does not mean that the same will apply to the next project, which involves calculating conductor sizes (adiabatic equation) to fit the particular requirements of the high voltage earthing design project is usually very necessary.