Bonding earthgrids together
Bonding earthgrids together
(OP)
I have two earthgrids A and B. A is a power station with an 11kV 30MW generator and a 11/22kV step-up trfr. There is a 22kV cable from the trfr to a swbd on grid B 40metres away. Grid B is fed either from the power station or utility via a 132/22kV transformer situated on Grid B. Earthgrid B has been designed for a 132kV fault from the utility.
1) What fault level should Grid A be designed to?
2) Should grids A and B be joined or left seperate.
Thanks in advance.
1) What fault level should Grid A be designed to?
2) Should grids A and B be joined or left seperate.
Thanks in advance.






RE: Bonding earthgrids together
Regards
Marmite
RE: Bonding earthgrids together
Thanks for some very useful insights. What fault current would you use to size the power station earthgrid? The way I see it any EF on the power station will not lead to an EPR as the EF current flows in the actual grid. So what do you use?
Thanks.
RE: Bonding earthgrids together
However, in your case the 132kV fault will more than likely be worst case from an EPR perspective.
Regards
Marmite
RE: Bonding earthgrids together
Regards
Marmite
RE: Bonding earthgrids together
It makes sense to me that the conductor sizing can only be due to a 132kV earthfault (btw the 22kV system is EF limited to 25A) BUT a 132kV fault cannot possibly give rise to an EPR since the 132kV winding of the trfr is delta connected. Though a 132kV EF close to grid B could have some EF current flowing to the grid an into the OHEW of the transmission line? But how to quantify?
Sizing of Grid A would be due to an EF on the 11kV side as the 22kV is limited to 25A.
RE: Bonding earthgrids together
RE: Bonding earthgrids together
In terms of EPR the transformer winding doesn't matter. What if you get a Tx bushing fault, or a cable box fault if it's cable connected? Are there 132kv arresters, station post insulators etc? Any of them could flashover/conduct to earth raising the earth potential.
I would first calculate your EPR without factoring in current returning via the transmission line earth wire. If it is within limits then you need go no further. If it's out of limits then bring in the earth wire to the calculation, and any other fortuitous earth return paths such as the sheaths of distribution cables connected to the same sub.
Regards
Marmite
RE: Bonding earthgrids together
I was not going to size the earthgrids based on 25A as the 11kV fault current with a generator EF is in the order of kA (have not done the detailed calcs yet). Also, this is a remote unmanned site so I cannot foresee many issues with transfer potentials when tying the grids together. As you say there is the benefit of reduced grid resistance.
The utility has advised that a 50:50 split be used for 132kV EF current to ground and via the OHEW. This means about 13kA going to ground.
I can see the problem of transfer potentials between the two grids if they're not bonded together. There are services, control and power cables running between them and so I agree it's best to tie the earthgrids together.
Thanks again for the insights marmite.
Regards.
RE: Bonding earthgrids together
- does the local supply authority allow the two grids to be connected together;
- what is the soil resistivity in the area? If it is high then joining the two grids together may not be best.
- are the earth grids existing? If so you can test to understand the current configuration and how the currents split, identify touch hazards, etc.
Then there is the question of what standard are you designing it too? From your previous posts I believe you are in Australia, so are you designing to AS2067, or Ena eg0 (risk based approach), Ena eg1, local authority requirements/design requirements.
I don't think you can simply ask the question of if I should bond the two grids together, get an answer and run with it. You really need to model it in some software to understand what the ramifications of each decision are. There have been several earthing related fatalities in recent years so this is an area that needs thorough investigation.
RE: Bonding earthgrids together
Thanks for the pointers.
Yes, it's and Australian design to AS 2067 and ENA EG1-2006 and drawing upon IEEE 80:2000. This is a greenfields project so no existing earthmats. Am using a two layer soil resistivity model. Soil tests have revealed resistivity of 10ohm.meters up to 50m and 8.49 ohm.meters further down.
PTW Groundmat is being used to do the modelling. So far no problem with step and touch. The power station is a temporary addition as the utlity supply will not be ready in time.
I am not responsible for the detailed design but have a keen interest in earthing and am assisting.