Acceptance Criteria for Dissipation Factor of a Coil/bar

[Insulectric]

Hi,

IEEE has a standard (recommended practice) for measurement of power-factor tip-up of electric machinery stator coil insulation (IEEE 286-2000). Test methods are described there but no acceptance criteria is recommended. I was wonder if anyone can provide me an acceptance criteria for dissipation factor(power-factor) for new coils (at low voltages(0.1-.2Un))?

Thanks

 

[TheBlacksmith]

Generally accepted (I think it's mentioned in the IEEE standard) for an epoxy/mica system without gradient tapes on the end turns is less than 1%. If you have gradient tapes, all bets are off as the gradient tape by design has increasing conductivity with increased voltage, mimicing the dissipation factor. You can test a sample coil with guard bands applied per the standard for a representative number of the supplier's insulation system.

 

[Insulectric]

TheBlacksmith,

Thank for reply. I also heard about 1% criteria for epoxy/mica coils but I don`t have any reference for that. I could not find it (or any criteria) at IEEE286. Could you please let me know any reference or standard for this value?

The another question is: we have a coil with dissipation factor at low voltages (slightly)higher than 1%, but with good tip-up and PD results. Is it Ok or it should be rejected?

 

[electricpete]

IEEE 286-2000 gives the definitive response: "It depends"

"The expected power factor and power factor tip-up vary with the type and age of the insulation system. The type of insulation system shall be identified, i.e., asphalt-mica, epoxy-mica, or polyester-mica. The power factor is also affected by the temperature of the insulation, and accordingly, periodic tests should be performed at similar temperatures. The power factor tip-up is also affected by the atmosphere during the test, e.g., air versus sealed with rated hydrogen pressure."

http://www.usbr.gov/power/data/fist/fist3_1/vol3-1.pdf

It depends:
"The power-factor-voltage characteristic (power- factor tip-up) is used primarily as a quality-control criterion in manufacturing. It is sometimes used as an acceptance test on individual coils. Power-factor tip-up has been used as a maintenance test because a change in the tip-up value over a period of time is an indication of change of condition of the coil insulation. "


Doble has a database of test results by machine voltage, insulation type, cooling etc. They provide periodic summaries of that database. Example "UPDATE OF THE POWER FACTOR DATABASE FOR GENERATOR STATOR INSULATION (A REVIEW), dated 2004, by Eileen M. Duarte, Doble Engineering Company"
Doble information is available only to Doble clients.

Many user's have adopted limits such as 0.5% power factor or 1% tip-up in their specifications for either new coils or assembled winding. It is then up to the supplier to justify deviations from that specification.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[TheBlacksmith]

You're right, maybe 1% is an urban legend that everyone has heard. Regardless, I have personal experience with two different suppliers of MV generators that both demonstrated tip-ups less than 1% as calculated from the 0.2 and 1.0 line to ground readings. These were custom designs for a specific application but I think with a modern VPI epoxy mica system 1% should be achievable. However, if other QA readings are on the money, slightly over 1% shouldn't be an outright rejection.