Grounding resistance in mine area 2

[HamidEle]

We are trying to limit Ground potetial rise in mine area to 100V or less. The system ground fault is 500A in 69kv system. The only way of achieving it is to limit Mine Substation resistance to 0.2 Ohms. We have only 2 substaions in mine area, fed from 69KV Substation via overhead lines. We could ground some poles or drill deep ground wells, or add a isolated ground bed. Want to know the pro and cons of each option. Thanks in advance.

 

[mgtrp]

We have two mines in our area that are trying to achieve the same as you (I believe that it is a requirement in some of the mining standards), and these are the two approaches that they are using:
1. Put the step-down transformer outside of the mine area, and install a resistor in the neutral to minimise the fault current. Ensure that there are no connections between the mine earth grid and the step-down transformer substation.
2. Ignore the utility contribution to ground potential rise and adopt a "head in the sand" approach.

Neither has tried to lower the earthing resistance to such a low level.

 

[HamidEle]

Yes, according to mine code, the GPR can be transfered to shovels under the fault conditions. Puting an isolation substation ground bed is a reliable solution?

 

[podobing]

Is this voltage limitation for a fault on the primary or the secondary side of the substation distribution system?

 

[HamidEle]

On the both sides. The GPR on the primary side can be transferred to the secondary as well.

 

[Kiribanda]

HamidEle,
Just a thought.Can't we have a high resistance grounded system?

 

[mgtrp]

The issue is generally that the incoming line from the supply utility probably won't be high-resistance earthed (especially for a 69 kV system), and so can contribute to the GPR regardless of whether the LV side of the step-down transformer is resistance earthed. This is why a prospective mine in our area is considering installing the step-down transformer outside of the mine area, and ensuring no connections between the earth grids.

 

[podobing]

In an underground coal mine in the US, all three phase power that enters the mine must be resistance grounded. The grounded side of the neutral grounding resistor along with the ground wires of the cable that feeds the power to the underground mine is connected to a "safety ground bed". This safety ground bed must be at least 25 feet away from the substation ground bed. I always thought that this was to keep lightning discharge(s) from being coupled directly into the mine. I could also see where this could help with your particular problem.

I cannot explain the 25 feet requirement other than it is a law. However, if the three phase power from the station stays on the surface (portal building, ventilation fan, water pumps, etc.), and does not enter the underground area of the coal mine, there is no requirement to establish this 25 foot separation distance, whether it is resistance grounded or solidly grounded.

I am not sure what type of mining you are involved with, but the laws that govern coal mining in the US can be found in 30CFR (Code of Federal Regulations) - Part 75 (Underground) and Part 77 (Surface). The website is

http://www.msha.gov.

MSHA is the Mining Safety and Health Administration.

Dave