Lightning Arresters On Pole Mounted Transformers

[alingstone]

The problem,

Pole Mounted Transformer (Delta-Star), 22000/415V.
Proposal is to install lightning arresters on 22kV side, mounted to tank.
Tank earthed to pole earth, LV cable also connected to same earth.
If lightning arrester discharges then possibility that HV voltage transferred into LV mains through earth system thus customer earth rises above remote earth.
Is this a good practice?
What is done elsewhere? Comments?
If lightning discharges the arrester will the inductance of the earth wire be enough to stop EPR being transferred into customer LV network?

Thanks

 

[cranky108]

In the US we do use LA's and connections like you described. There is also a requirement that the customer use a single ground point (usually at the meter point).

Yes a lightning strike can rase the local ground level potential, but at the same time the secondary voltage will rase at the same level. Note: the phone normally uses the same ground.

Because the pole ground is tied to the customer ground, and the grounds on every pole in the distribution system, as well as customer grounds, the effect is to reduce the potential rise across the distribution system.

The key here is the customer should have only one ground point, so there is no current path for the local potential rise to flow to.

 

[pwrtran]

Where can you dissipate the lightning current other than through the earth connection? You have no other choice.

"will the inductance of the earth wire be enough to stop EPR being transferred into customer LV network? " - no, V=L*di/dt, the higher the "L", the greater the potential raise!

 

[cuky2000]

It is common practice in many utilities to use the same ground conductor for the surge arrester and the LV wye secondary ground conductor.

The emphasis with this practice is to save material and labour recognizing that two separate ground conductors if grounded (earthed) at the bottom of the pole will create virtually the same effect from the GPR (EPR) prospective.

 

[rtronics]

I live in country NSW, Australia, and I checked out my pole transformer yesterday. There are two earths coming down the pole, they are spaced 180 degrees, and they go into the ground straight out from the pole, forming an underground 'T'.
At the top of the pole, one earth connects to the casing, to which the LA's are mounted, and the other earth connects to the centre tap of the 460 volt secondary, which is our neutral conductor. So we have 230 volts * 2 at the house and the multiple earthed neutral system.
Primary is 22kV, 2 conductors of the 3-phase distribution.
Ray.