Earthing both sides of transmission neutrals

[Power0020]

I wonder about the practice at 66 kV levels with usually one substation (source) at 220/66 kV with the 66 kV side star neutral earthed, however, the (load) or distribution stations will have a delta or unearthed star 66/22 kV.

This is not the case with 220 kV or higher voltages where both ends substations have transformer sides with star and neutral point solidly earthed.

What is the purpose of keeping the network with only on side having an earth return? or both sides? I guess it doesn't have much to do with the overvoltages rather than the fault current detection/split.

Any clues?

 

[cuky2000]

The main reason the multi-grounded system is to maintain desirable reliability for lighting performance (see below). Also, a low ground with a metallic link (shield wire) help control the overvoltage and facilitate the protection system.

 

[Power0020]

My question is not focusing on the transmission structure, these will be earthed for safety and lightning performance.

I asked about the network configurations at source and distribution substations, with one side only usually having transformers star point earthed (directly or through a reactor/resistor) at source SS and the other side is left open or delta.

 

[RRaghunath]

In case of transmission substations, insulation costs determine that the HV be star connected with neutral solidly earthed. This allows grading of HV winding insulation making the transformer cheaper.
In distribution systems, it is the practice to have uniform insulated windings and the HV could be Delta or unearthed star. This also allows that the earth faults in 22kV system are not reflected on to the 66kV system and thus enabling 66kV earth fault protection to be set sensitive/fast.

 

[Mbrooke]

I have to admit, this is a very interesting thread.

In the US typically the end is not earthed mostly for the simple fact distribution is typically 4 wire while transmission is treated as 3 wire. You do not want neutral current on the T system. Its delta wye or wye ungrounded-delta-wye grounded when 230, 138, 115 and 69kv goes to 12.47kv or 115kv to 34.5kv sub-transmission. All else is typically auto transformer with a delta territary 90% of the time.

I know outside North America having a wye primary is very common when going from transmission to distribution.


Ideally you would not want the primary grounded. In Russia they actually have switches where the primary remains unearthed while operating unless the transformer is being switched on and off line.

You can get away with it most of the time because there is either a delta winding in the transformer or the secondary neutral is earthed through a very high impedance such that secondary faults do not present a significant zero sequence current.

Here is an example, watch the whole video to understand whats going on:

https://youtu.be/T1hqVIMRKdM?t=183

 

[Power0020]

@MBrooke,

The video is interesting as I couldn't locate any 110 KV CBs, and the DS operation leads to corona noise giving the inception that the DS is the last switching piece instead of a CB!.

I have noticed that in EHV levels (220kV+) both sides of a transmission circuit has neutral point solidly earthed (or through a DS as in Russia and some middle east countries).

I guess the neutral will be ensured connected in the source(say) substation and left open in remote-end substations, it must be earthed at one substation for overvoltage control as well as earth fault detection.