DAR or PI is the same ? 5

[petronila]

Hello All,

Some paper that I have readed do not make difference between D.A.R and P.I, we know DAR is the ratio between IR measurements taken after 60 and 30 seconds or R 60/R30 and PI is ratio between IR measurements taken after ten minutes and one minute IR = R10/R1 but result interpretation are different in each test.

IMO the test should be called Dielectric Absorption Test or Time-Resistance Test , and depending of how was performed then re-name like DAR or PI, it this corect?

What do you think about this?

Thanks for your inputs

Regards

Petronila

 

[7anoter4]

In EPRI POWER PLANT ELECTRICAL REFERENCE SERIES VOL.6 MOTORS [EL-5036 ] see:

http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=EL-5036-V6

ch. 6.8 Insulation Testing.
D.A.R. is the total current measured all minute for 10 minutes at constant [d.c.] voltage.
PI it is the ratio between final IR and the value measured at first 1 minute.
In order to consider the insulation is in good situation this PI has to be more than 2.
If one cannot maintain the voltage 10 minutes then a shorter time PI may be considered:
R60sec/R30sec.This PI has to be more than 1.5 to be considered good.

 

[petronila]

Hello 7anoter4,

Thanks for the good information, excellent technical guide.Do you have more documents like this for generators and transformers?

Regarding my question and searching more about it, seems that DAR and PI are the same, but DAR test only evaluate the absorption current, like EPRI document sayds, this test is used mainly when one electronic powered megger is not available ( crank megger test) DAR = IR 30 sec/ IR 15 sec or IR 60 Sec / IR 30 Sec test. PI evaluate complete polarization process.

From:

http://www.kew-ltd.co.jp/en/support/mame_03.html

"DAR measurement is a diagnostic test similar to the Polarization Index (PI), but DAR takes the ratio of the Insulation Resistance usually measured at 30 sec and 1 min (other time settings are possible) instead of 1 min and 10 min typically of the PI.
DAR measurements are useful for instance when the PI is 2 or less even for new objects under test. In such cases, a min DAR value of 1.25 is required"


DAR 1.4 or more 1.25~1.0 1.0 or less
Criteria Best Good Bad

Thanks again

Carlos

carloshernandoramirez at gmail dot com

 

[7anoter4]

Very interesting,Carlos.Thanks.

 

[prc]

Please see IEEE Standard 43-2000(R2006) Testing of Insulation Resistance of Rotating Machinery for a complete tutorial on the subject. This standard is not giving any recommended values for DAR, but only suggest values for PI

 

[electricpete]

Do you have more documents like this for generators and transformers?

thread237-364123
More for motors - EPRI 5936, volume 16, volume 17
generators - EPRI 5936, volume 1
transformers - EPRI 5936, volume 2


IEEE 43 mentions in an appendix there are other variants of the ratios using different times, but the limits aren't standardized.

There are 3 main components of currents measured in insulation resistance testing:
1 - capacitive
2 - polarization
3- resistive (includes surface leakage and volume leakage).

Capacitive decays the fastest, mostly gone within 30-60 seconds
polarization lasts longer, mostly gone in 5-10 minutes (for form wound)
resistive is generally constant.

The rough idea of the P.I. is that you're using polarization current as a benchmark for evaluating resistive current. In simplified terms you might think 1 minute current is mostly polarization and 10 minute is resisitve. So it is something close to (not exactly) ratio of resistive current to polarization which (to me) is vaguely reminiscent of ratio resistive to capacitive current we look at in power factor measurements. In both bases we use something like a ratio to a reference current (capacitive for pf or polarization for PI) to account for geometry factors when we evaluate resistive current.

The motivation to use D.A. (ratio 1 minute/30 sec) is as mentioned in the Biddle document to save time. But it also comes into play more often in conjunction with random wound machines. While form wound machines have almost exclusively dieelctric insulating materials between conductor and ground, random wound are going to have slot liner materials and air (and varnish for that matter). So random wound tend to not have as much polarization components. For some random wound motors it is hard to pass PI even when the insulation is dry and healthy.

As a result PI is not often recommended for random wound motors. D.A. is sometimes recommended in its place and it makes a little sense that in the case of D.A. we are using an earlier time for the reference which may include the capacitive current. Also saving s in time may be more relevant for a small random wound motor than a big form wound.

Here are some references directly or indirectly implying limitations of applying PI to random wound
* IEEE 62.2 states that PI values will often be around 1.00 for small motors
• EPRI NP7502 – Motor predictive maintenance – recommends PI for form wound, not for random wound.
• EPRI PM Basis for Medium voltage motors 106857 Volume 9 recommends PI. EPRI PM basis for low voltage motors 106857V8 does not.
• EPRI 1008964 (Repair Spec for low voltage motors) states PI is required for form wound stators only.
• EPRI 5036, Volume 6, page 6-94 states "A PI of 2 or more is considered satisfactory for form-wound coil insulation. (The less uniform nature of random-wound insulation renders both IR and PI tests of little value; a PI of 1 is generally acceptable.)"
• Army Corps of Engineers UPS Guide - PI for >= 200HP
• NETA MTS-1: PI for motors > 200hp, DA for motors =<200hp
Note the last one - DA is substituted for PI for smaller motors which would likely be random wound.


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(2B)+(2B)' ?

 

[electricpete]

I'd like to summarize my last post:

P.I. relies on having significant polarization current present in the 1-minute reading. (that is used as a basis for evaluating the resistive current present in the 10 minute reading)

But some random-wound motors have very little polarization current at 1-minute due to the small amount of dielectric material. In this case, the D.A. can be useful. The 30 second primarily-capacitive current is used as basis for evaluating the primarily-resistive current at the 1-minute reading in these motors.

Review of industry recommendations confirms PI has less favor for random wound motors and DA finds occasional use but only in random wound motors.


=====================================
(2B)+(2B)' ?

 

[7anoter4]

Thank you, electricpete, for your very interesting information !

 

[petronila]

Hello Pete,

Very Good information, thank you.

Also The most relevant is the recommendation about DAR test is better for electric motor below 200 HP.

An star for you

Regards

Carlos