[Substations] Effect of tying-in overhead shield wires to design a safe ground grid?

[il102]

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!

 

[magoo2]

I look at the tying in of the shield wire to the ground grid as a measure intended to improve the performance of the shield wire. I don't see it as the other way around. The impedance of the shield wire grounds may be an order of magnitude greater than that of the ground grid so the ground grid improvement would hardly be noticeable in my opinion.

 

[jghrist]

The overall resistance of remote earth is lower with the shield wire connected because the shield wire and the transmission tower grounds are a parallel path for ground fault return current. It's similar to the ground grid and the shield wire being two parallel resistances. Similar, but not the same because there are a lot of mutual impedances to be considered.

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.

 

[cuky2000]

See the information on the link below.

http://www.icee-con.org/papers/2004/521.pdf

 

[Marmite]

Major substations contain expensive equipment that if damaged can take a significant time to replace/repair (eg transformers, switchgear and cables). Such equipment is at risk from lightning overvoltages. Circuits connected to the substation serve to collect lightning. Overhead lines are the dominant source. They have a greater collection area per unit length many times that of the substation site itself.Lightning related overvoltages are usually due to strikes on the line which travel to the substation along the wire. When a tower earth wire takes a direct strike, the voltage created on connected tower cross-arms can cause a flashover onto the phase conductor(s), known as a ‘back-flashover’. A very steep-fronted overvoltage wave is created when this occurs. This is particularly onerous if it occurs near the substation as attenuation is only small before the overvoltage reaches the substation. The protective zone of surge arresters at the substation is smallest when overvoltage is steepest. The likelihood of back-flashover is reduced by keeping tower footing resistance low and by connecting the terminal towers to the substation earthing system. The provision of a low impedance tower footing and good earth wire connectivity to the substation earth mat are therefore key to preventing overvoltages appearing in the first place. A high resistance can increase the risk of back flashover when lightning strikes the earth wire. Tower footing resistance should aim to be <10 Ohm and the last circuit tower should be connected to the substation earth mat which should have an impedance an order of magnitude lower.

Regards
Marmite