Voltage requirement for Tan-Delta Test

[power2020]

We are planning to carry tan-delta test for 16.5kV,260MVA generators & 400/16.5kV generator-transformers. Experts pl. discuss that how much voltage needs to be applied as per standards?
Advanced thanks...

 

[Skogsgurra]

No expert. But been there and seen how it is done.

Tan(delta) is normally done up to rated voltage. I know that Siemens does it up to 1.4 times rated voltage on new machines before letting them out of factory.

Divide rated voltage in five equal parts. Start at 20 % and then 40 % and so on. There is a limit to how much tan(d) may change between steps, I remember something like 0.01/step - or was it 0.005? Normal values are around 0.01 at 20 % but up to 0.03 can be seen without too much worries.



Gunnar Englund

http://www.gke.org

 

[power2020]

Thank you Mr.Skogsgurra,
The rated voltage is rated Phase voltage or rated line voltage if we need to test between phase to earth (R-Y+B+E)
And pl. give if any standard available,

 

[chris8410]

Power2020,
I have conducted tan delta measurements on large generators and GSU transformers rated up to 600MVA for over 20 years. In theory, tan delta is independent of voltage. For oil-filled transformers, this is true in practice. However, on solid insulation systems, such as a generator, tan delta can be effected by corona activity in the stator winding. The change in power factor as the voltage is increased is refered to as "tip-up."

I usually perform tan delta measurements on transformes at 10kV maximum, but never exceed rated LN voltage. I have performed tan delta measurements on the same transformer using a small battery power test set (9V) and have acheived nearly the same value (ie: 0.5%). In the USA, tan delta is very commonly performed at 10kV.

I use tan delta during the dryout process of large power transformers, using the 9V test set. Tan delta is very sensitive to the degree of moisture in the windings, thus is an excellant indicator of the dryout progress. Below 1%, "percent tan delta = percent moisture" is a good rule of thumb. Thus, we continue our dryout until the numbers are flat and below our limits.


Chris

 

[power2020]

I gone through last year readings for 5.1mW motors, tan delta readings are 6% at line voltage applied to LN. Is this reading o.k ? What is the limitation and what is the significance of tip-up value?
Thanks a lot..

 

[Skogsgurra]

Six percent is rather high. What is your L-N voltage?

"Tip up" means that the curve you get when you do a 20, 40, ... 80, 100 % tan(d) test isn't straight, but bends upward at the end. That indicates the onset of PD.

Gunnar Englund

http://www.gke.org

 

[power2020]

Thank you Mr.skogsgurra
L-N Voltage: 6351V but as per records, 10kV test voltage applied to L-N of the motor windings. This year again we are planning to carry preventive maintenance and tan-delta test. If the reading is high as 6%, shall we need to go for drying the winding?
What is the max Tip-up allowed? If Tip-up is more due PD what precauionary measures can we take to avoid PD in dry type insulation. We are planning to check PD as well at the same time of tan-Delta test.

 

[Skogsgurra]

There must be standards for that. I do not have the numbers. It is possible that the high tan(d) is because you overstressed the winding with 10 kV to ground. The tip up gets more pronounced the higher you go. There are other guys here that know those standards by heart. You better wait for them to come in.

Gunnar Englund

http://www.gke.org

 

[power2020]

To reduce Tan-Delta test kit size, we are using Resonance inductor. Is there any deviation of tan-delta value by using the kit with out inductor and with inductor? Which is advisable?