Transformer Neutral Earthing 4

[magpie2000]

This 132/33kV YNynD1 transformer has been operated for years with the HV neutral floating even though the HV primary winding has graded insulation (270kV line and 90kV neutral end) and the HV neutral is brought out. The LV star netral is earthed through NER. Delta tertiary has one tip connected to earth.

What could be the reason for not earthing the HV neutral?

Implications (protection?) if it is earthed now?

 

[RalphChristie]

Hi

Check

http://www.beckwithelectric.com/powerlines/powerlines-37.html

for some pros and cons for the different connections. (Interconnection Transformer Winding Arrangement Implications on IPP Protection)

I would think the biggest reason for not earthing your trsf on the 132kV side is for earthfaults on the 132kV line. If this neutral was grounded, some of the fault-current will flow up in the neutral (during earthfault on line), causing an unnessecary earth-fault trip on the trsf.

For an earthed neutral, use a directional earthfault-unit.

Regards
Ralph

 

[magpie2000]

Thanks Ralph

the question remains. Should the xmer neutral be earthed or not given that it has graded insulation.

Your feedbak will be most appreciated.

Regards

 

[RalphChristie]

From the J&P Transformer Book: (Twelfth Edition - Martin Heathcote)

The subject of neutral earthing is a complex one and, whenever discussed by electrical engineers, views are varied and discussion lenghthy. Practices vary in different countries, and even within different utilities in the same country.

Guiding principles in relations to earthing in the UK are determined by statue, in the form of the Electricity Supply Regulations of 1988. Part II of the regulations says that:
•Every electrical system rated at greater than 50V shall be connected to earth.
•How that earth connection is to be made differs between high-voltage and low-voltage systems.

In the UK, non-uniform insulation is used for system voltages of 132kV and above.
A stable earth fault on one line of a system having an isolated neutral raises the voltage of the two sound lines to full line voltage above earth, which is maintained so long as the fault persist. The isulation of all equipment connected to the sound lines is subjected to this higher voltage, and although it may be able to withstand some overvoltage, it will eventually fail. In extra-high-voltage systems, because of capacitance evects, the voltage of the two sound lines may, initially, reach a value approaching twice the normal line voltage by the same phenomenon as that of voltage doubling which takes place when switching a pure capacitance into circuit, and the insulation of the system will be correspondingly overstressed.

Hope above info/quote is helpful

Regards
Ralph

 

[RRaghunath]

Magpie2000,

I think it is better to leave the 132kV neutral unearthed. This ensures that the earth faults in 33kV system do not get reflected on to 132kV side with possible disturbances in the 132kV system.

With uneartehd 132kV neutral the transformer behaves like a DY connected transformer. 132kV system, any way, would have been earthed at source end upstream nd thus there is no possiblity of over voltages and such related problems.

The above presumes that there is no generation on 33kV side of the transformer.

Not withstanding all that said above, if you wish to earth the transformer primary neutral, I think it can still be done considering that the 33kV side is resistance earthed. You need to take care in relay setting calculations that the protection on the 132kV side doesn't respond to the fault on 33kV side / has enough margin in the current setting.

Trust the above helps.

 

[rbulsara]

I agree with rraghunath....

 

[magpie2000]

Thanks rargunath

The 132kV source is indeed solidly earthed.

The 33kV side neutral is earthed through a NER.

33kV side has some big induction motors. The largest being 20MW.

The magintued of the rise on helthy phases for a sigle phase to ground fault will be determined by the system's effective X/R ratio and the stray capaciatnce to ground.

The transformer has failed!! Preliminary investigations suggest red phase winding fault close to neutral (where we have the graded insulation). No short circuit protection operated. Bucholz gas was the only protection that operated. Suggesting a fault close to neutral and most probably an inter turn fault. Obviously HV REF was not there.

Referering to the J&P transformer book I am very tempted to say that we must solidly earth the HV neutral because the transformer was initialy designed with this intention and take care of the protection settings as you suggest. I suspect transformer manufacturer may void gurantees if HV netrl was not solidly earthed.

Regards
magpie2000

 

[RRaghunath]

I can understand your concern with failed transformer in your hands and pressures from different corners.

But I beg to differ on the point that the isolated neutral is responsible for the failure of the transformer.

1) The transformer neutral is isolated but the source neutral is earthed solidly and this makes the 132kV system solidly earthed removing possibilities of over voltages (in healthy phases) during system single phase-to-ground faults.

2) The fault as indicated by your preliminary investigations is inter-turn fault and not phase-to-ground fault. If the graded insulation coupled with isolated neutral had been responsible for the failure, I would expect the fault to be phase-to-ground type (afterall, the insulation grading is between and phase and ground (not interturn).

3) Lastly, neutral isolated operation as in your case is not uncommon in grid connected transformers. I understand it is a stipulation by grid authorities in many cases.

Considering the above I don't see any case for transformer manuafacturer for not honouring his warranties.

 

[magpie2000]

Hi rraghunath

I guess I should explore further whether the solidly earthed source is in fact "effectively" earthed with X0/X1 < 3.0 and R0/X1 <1.0.

If the above is established, I think you are correct that the graded floating neutral will not be subjected to the excessive overvoltages due to neutral shift.

Regards

 

[RRaghunath]

I guess the upstream 132kV system is effectively earthed. This will be true in general unless long OHL separates the upstream transformer and your plant transformer primary. Consider the values below:

The Xo = X1 for transformers with negligible R0 and
Xo ¬ 2.5 times X1 (approximately) and R0 ia about 5 to 7.5 times X1 (Hope I am right?) for OHLs.

Hence, unless really long OHL separates your plant from the source, the source can be taken as effectively earthed.

 

[cuky2000]

Maggie2000,

Just as a curiosity, what is the BIL rating of the primary winding of the transformer?

Here is a summary of additional pro and cons for different grounding method as published by Schweitzer Eng Lab. In the enclose reference.

http://cuky2000.250free.com/GND-Methods_MV.jpg

Enclose is supplementary information from IEEE Std C62.2 if you are interested to determine the Coefficient of Grounding to verify how well the system is grounded.

http://cuky2000.250free.com/Coefficient_of_Gnd.JPG

I hope this help you with your final grounding recommendation for the transformer.

 

[GamezBeCJ]

Magpie2000:

I would differ with some of the opinions stated here. I worked for a transformer manufacturer and it is strongly recommended to ground neutral bushings of a graded insulation winding. The main reason is that in this type of insulation technology, is a well known fact that the voltage distribution in the winding during an impulse (atmospheric or switching) is very different when grounded or not grounded. When not grounded, the voltage oscillaitons at the neutral end, tend to be much more severe than when the neutral is grounded.
Finally, I doubt that a design engineer would recomend the use of a graded insulation design on an ungrounded neutral application.

I hope this helps.

Regards.

Carlos

 

[magpie2000]

cuky2000

Insulation levels are as follows:

HV : 650/275kV (Lightning inpulse/Power frequency)
HVN : -/91.7kV (Lightning inpulse/Power frequency)

According to text in the effectively grounded system the voltage on the healthy phase (in the event of a L-G fault on one of the phases) will rise no more than 80% of L-L voltage.

That means for 132kV sytem the healthy phase L-G voltage could rise from 76kV (132kV/sqrt3) to 105.6kV (0.8 x 132.

The transformer neutral insulation is 91.7kV. I am not sure if this is power frequency test volatge or the actual dielectric strength of neutral insulation.

Regards
magpie2000