Transformer Bushing Ratings

[ivana24]

Transformer Bushing Ratings

I am checking Transformer Specification for one project where Transformer Bushing Rating Data are presented. That what I don’t understand there is a sizing of the Transformer Neutral Bushing .
Based on IEC 60137, for 500 kV transformer, following data are specified :

For Lightning impulse withstand voltage – (a) 1425kV crest is provided for line terminals, (b) 1550kV crest for line bushings , (c) 325kV crest for neutral terminal, (d) 325kV crest for neutral bushing

For Power frequency impulse withstand voltage – (a) 630kV rms for line terminals, (b) 680kV rms for line bushings, (c) 140kV rms for neutral terminal, (d) 140kV rms for neutral bushing.

Definitely, the neutral bushing has lower withstand voltages than line bushings what is OK. Based on IEC 60137, for 500 kV I agree with - 1425kV crest, 1550kV crest, 630kV rms, 680kV crest. I am trying to understand neutral insulation levels - 325kV crest and 140kV rms.

Based on IEC 60137 –Table 7
325kV relates to lightning impulse withstand for the voltage 72.5 kV
140kV relates to power frequency impulse withstand for the voltage 72.5 kV

So, line bushing s are specified for 500 (550) kV voltage, neutral bushing for 72.5 kV ( IEC 60137 –Table 7). Why ? What is going on if my highest voltage is 145kV instead 550kV, how to specify neutral bushings in this case.

 

[waross]

Will this transformer be solidly grounded or impedance grounded?

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

 

[ivana24]

solidly grounded

 

[cranky108]

Then it is custom to connect the neutral bushing to ground in the most direct route, and not to include the transformer steel case. The transformer steel case is grounded with a different ground connection.
The neutral bushing need to withstand the maximum voltage seen between the neutral and grounded case. Since both are connected to the ground grid, that voltage should be small, but will not be zero.

For the transformer manufacturer the neutral bushing rating becomes a guessing game. So it is typical to use a 15 kV bushing for a grounded neutral, where the transformer manufacturer would have no way of knowing the proper size.
The neutral bushing also must be able to carry the maximum momentary fault current that would be expected. Here the transformer manufacturer would not know this.

 

[prc]

The neutral end insulation level of a directly earthed winding is specified in 7.4.2 of IEC 60076-3 Power Transformers-Insulation levels and Dielectric Tests. For directly earthed neutral, the applied voltage test level shall be at least 38 kV (corresponding to Um ->17.5 kV)
When earthed through impedance, the voltage level may be calculated as per Annexure D " Neutral Insulation voltage level calculation" of the above IEC. But manufacturers have to provide more insulation to neutral since during the induced over voltage test, over voltage will appear at neutral end and the bushing has to withstand it. Some manufacturers use a temporary higher rated neutral bushing (72.5 or 145 kV) during tests, but supply transformer with a 17.5 kV or 36kV bushing.

The current rating of neutral bushing is indicated in Clause 9.2 of IEC 60076-1 Power Transformers-General. For K rated transformers, usually the neutral bushing will be provided with two times the line bushing current rating. To take care of transformer overloading requirements,bushing current rating shall be 1.2 times the rated current of transformer.