HV Neutral grounding 1

[mastergate]

Bonjour

We have an HV Neutral directly grounded using an insulated cable and a disconnecting switch (normally close).
The question is regarding the insulation level of this grounding material (cable+switch) per IEC standards?
should it be phase voltage 60kV (72,5) or 60kV/sqrt(3) or an LV cable is sufficient.

Thanks for your support

 

[waross]

Neutral to the switch may see phase to neutral voltage in the event of a grounded phase with the switch open.
From the switch to the grid will see the IR voltage rise in the cable in the event of a grounded phase with the switch closed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

If the switch is open ONLY during maintenance with dead-bus, the neutral might be rated phase-to-ground. Otherwise, the neutral insulation should be full rated for phase-to-phase.
If there is a surge arrester (SA)on the neutral phase, this could help to identify if the neutral should be rated for phase-to-ground or phase-to-phase insulation.

 

[RRaghunath]

Neutral potential can rise to phase-to-ground voltage during 1-phase to ground fault. Hence, it is appropriate to size the cable accordingly.
If the neutral is solidly connected to earth without the disconnecting switch, the voltage rating of cable could have been 80% of that.

 

[FPelec]

As waross said, the cable between the disconnector and the ground will only see the V = R x I voltage drop.
You can use normal LV (0.6/1 kV rated) cables, as we normally do for connecting the neutral to the ground in HV and EHV transformers and shunt reactors.

The connection between the neutral and the disconnector, as well as the disconnector itself, may see phase to neutral voltage in the event of a grounded phase with the switch open. Depending on the expected maximum duration of such overvoltage (hopefully, you are going to install a 59 N relay if you will use transformer without neutral grounding), you could use the same insulation level of the transformer phase bushing or a reduced insulation level.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[prc]

Normally in solidly grounded system (Um>72.5 kV) transformer neutral is provided with insulation of Um=17.5 kV class ( Applied voltage 38 kV BIL 125 kV) - clause 7.4.2.1 of IEC 60076-3 Insulation levels of Power Transformers. Transformer manufacturers usually run the neutral bus over the 17.5 kV or 33 kV bus support insulators from tank .

 

[RRaghunath]

17.5kV Um for neutral applies only for those transformers with graded insulation.

 

[prc]

Manufacturers no longer provide "graded insulation " in transformer windings ( almost4-5 decades now). The term used by IEC when the neutral is solidly grounded is non-uniform insulation, meaning reduced insulation to ground at neutral end. Up to 72.5 KV class line end, neutral shall be provided with same insulation level as line and above 72.5 kV class, applied voltage level for neutral is 38 kV ie 17.5 kV class for solidly grounded system. When neutral is grounded through resistor etc, neutral insulation level is decided as per clause 7.4.2.2 of IEC.

 

[RRaghunath]

Is non-uniform same as graded. Graded from 72.5kV at line end to 17.5kV at the neutral end. Am I making a mistake here!!
As per IEC, below 72.5kV, the transformer winding insulation shall be uniform.
This is because Grounding the neutral through resistance is only done in medium voltage systems as a practice internationally.

 

[NickParker]

I have a question. What is the purpose of disconnect switch here? Is is designed for system to operate as ungrounded system?