[Substations] Effect of tying-in overhead shield wires to design a safe ground grid?
[Substations] Effect of tying-in overhead shield wires to design a safe ground grid?
(OP)
There has been some confusion among my peers regarding the following circumstances. Often during the design of a ground grid, an engineer will specify that overhead shield wires from incoming lines must be tied into the ground grid prior to energization for a safe system. I've been told by a few people that tying in these shield wires reduces the ground grid impedance. While this might true in a certain abstract way, I disagree with that notion.
I've even been told by people testing in the field, for example, that the ground grid's impedance was measured at 4 ohms before tying in a shield wire and 2 ohms after tying in a shield wire. I suspect there are more variables at play here. Given the way a ground grid test works (injecting current over a distance in the sub and measuring the voltage drop) I see no physical explanation that would account for this value changing as a result of connecting a random shield wire. The test current won't do anything differently as a result.
It's my understanding that connecting a shield wire ensures that less ground fault current flows into the substation through the earth (rather, a substantial portion comes in through the shield wire). This reduces the safety hazards associated with large amounts of fault current flowing through the earth and instead there is a more efficient, lower resistance path directly through the shield wire - ground grid connection.
But at the end of the day, you can't prove this with a simple measurement in a substation. Is my understanding correct?
Thanks!
I've even been told by people testing in the field, for example, that the ground grid's impedance was measured at 4 ohms before tying in a shield wire and 2 ohms after tying in a shield wire. I suspect there are more variables at play here. Given the way a ground grid test works (injecting current over a distance in the sub and measuring the voltage drop) I see no physical explanation that would account for this value changing as a result of connecting a random shield wire. The test current won't do anything differently as a result.
It's my understanding that connecting a shield wire ensures that less ground fault current flows into the substation through the earth (rather, a substantial portion comes in through the shield wire). This reduces the safety hazards associated with large amounts of fault current flowing through the earth and instead there is a more efficient, lower resistance path directly through the shield wire - ground grid connection.
But at the end of the day, you can't prove this with a simple measurement in a substation. Is my understanding correct?
Thanks!






RE: [Substations] Effect of tying-in overhead shield wires to design a safe ground grid?
RE: [Substations] Effect of tying-in overhead shield wires to design a safe ground grid?
Usually an acceptance test for a new ground grid is to measure the resistance of the grid alone, not connected to the shield wire. If the ground grid was designed with consideration of the current split between the ground grid and the shield wires, then operation of the substation would be unsafe with the shield wires disconnected.
Magoo: While I'm sure that the ground grid improves the performance of the shield wire, I think the main benefit is improving substation safety. When you calculate the zero-sequence impedance of a transmission line, you don't consider the station grid resistance. When you calculate the fault current at the station, the system model only includes sequence impedances of the transmission lines, not the ground grid resistance.
RE: [Substations] Effect of tying-in overhead shield wires to design a safe ground grid?
http://www.icee-con.org/papers/2004/521.pdf
RE: [Substations] Effect of tying-in overhead shield wires to design a safe ground grid?
Regards
Marmite