Insulation Resistance

<...are this difference acceptable...>

Carrying out a test without knowing where the acceptability criterion lays beforehand is abstruse.

The acceptability criterion needs to be in the test protocol before the test is carried out, and the protocol has been withheld from the forum. Without a defined acceptability, a test is not a test, merely a measurement.

<... what is the possible causes of this difference ...>

How about the quality of the insulation across <...R...S...[and]...T...>?

There is no reason the three phase insulation resistances should be the same. They are electrically isolated.

"Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. ... In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition"

https://www.instrumart.com/assets/Megger-Guide-to-Insulation-Testing.pdf

Before the generator was turned ON a spider spun a web from T to E. ZAP

An interesting possibility. GA

Resistances that high are usually measured using direct current. If you reverse the tester's leads the readings will change. Try heating the generator to near 100C to dry it, then allow it to cool. The moisture paths will go and higher resistances will be seen.

Zaf

These readings look acceptable to me. Is this an automobile generator? Please explain its usage.

No one will be able to answer your question unless until complete details of the Generator, Measuring instrument, conditions of testing, purpose of the testing are specified. So please come out with as much details as you have.

Are you a supplier of the generator or the operator /owner of the generator?

When we checked the insulation resistance for generator we show there are a difference

between reading:

R-E =320 M ohm ,S-E = 305 M ohm ,T-E = 240 M ohm

please what is the possible causes of this difference and are this difference

acceptable ,are there any standard value for difference in insulation between phases

You are missing the point. These numbers are not supposed to be equal, they are all supposed to be as high as possible.

There are many reasons for low insulation resistance. My experience is that the salty air in maritime environments is a prime cause. Salt deposits from the air draw moisture which provides a conductive layer.

I'm not even certain what the letters of R, S, T and E stand for. E might be chassis Earth but the other three might be Stator, Rotor and Terminal (aka Neutral, Common). On the other hand these sequential letters might be the three output phases in a star configuration with a floating common.

RST = three phases, E = earth or ground. I'm guessing that since there are different values of insulation resistance that the three phase stator windings are connected externally and disconnected (isolated) before meggaring.

https://www2.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=43105#:~:text=R%2DS%2DT%20was%20the%20general%20designation,Y%2C%20and%20W%20to%20Z.

Yes, some three phase generator manufacturers use these letters for the three phases. But some brushed single phase manufacturers have used these same letters to identify Rotor, Stator and Terminal. Without an identification by the OP to a model number, manufacturer, age or era of the generator or anything at all that describes the generator but what the OP doesn't understand then we are just as uninformed as the OP about what they have.

Rating of Generator in KVA?You can try standard methods to improve IR values and see if it improves .what is out Pput voltage?

Dear all

Thanks for all the responses.

generator rating 31 MVA ,5.5KV

R,S,T = Three phase

E=Earth

thanks

Kindly provide more information like, type of cooling (I hope that it is air cooled generator), type of insulation, like epoxy, bitumen etc, where the coolers are mounted, within the generator casing or out side? Voltage of the megger used for measuring the insulation resistance. Please also measure if not measured and provide the polarisation (PI) value and value of insulation resistance between the phases. This would help in advising on the subject.

Value is high enough to use it.Difference could be due to some defects developing in that phase insulation It could be due to various reasons.How old is the machine? Are the windings original or rewound or patch repaired any time? Try heating by circulating current using a low voltage source taking care .If still value continues to be low some defects are developing in the insulation of that phase

Dear all

Thanks for all the responses.

generator rating 31 MVA ,5.5KV

R,S,T = Three phase

E=Earth

thanks

Could the forum now please learn the outcome of the discussions that have taken place with the original equipment manufacturer?

Henry,

the value of the insulation resistance after 15 and 60 seconds of measurement and their ratio are important .If it's less than 1.3 it says a lot about his condition and maybe more than the maximum value you got .

On internet are good scripts on the subject of insulation resistance testing and I suggest you study one well before deciding and turn on the generator on .

Good luck and Happy New Year .

...a bit of damp/moisture/humidity somewhere....?

Another important piece of data is the history of insulation resistance tests in the past. From this you will see a trend which is important in assessing the condition of the machine.

--JMM

How do these figures compare with those taken when the equipment was originally commissioned?

What did the equipment manufacturer say during the telephone call(s) made on this topic?

https://www.ecmweb.com/maintenance-repair-operations/test-measurement/article/20898954/understanding-insulation-resistance-testing