dear sirs,

we have a alternator 15000 KVA 6000 Volt 3 Phase Ac, the insulation resistence as folow:

U-Earth 14 G-ohm
V-Earht 5.5 G-ohm
W-earth 22 G-ohm
U-V 24 G-ohm
U-W 60 G-ohm
V-W 18 G-ohm
tester volt 5000 VDC for 2 mints.

are the above insulation results are normal??
please advvise me

 

Hussain, please provide addditonal detail. The six measurements numbers inicate, somehow, that you were able to isolate the three windings, that is, with respect to earth and to each other.

Regards, Phil Corso([email protected])

 

we isolate the 3 windings and megger the each with other and as well earth

regards
hussain

 

Hussain, thank you for your reply. Regrding the variation in values:

1) How old is the generator?

2) What is the insulation material?

3) What is the environment in which it is operating?

4) Why was the test done?

5) Did you pre-heat the windings by applying low voltage to their terminals, before you isolated the windings

6) Do you have a set of values taken in the past?

7) You mentioned using a 5kV DC source for testing. Did you plot a micro-ampere curve vs time?

Regards, Phil Corso ([email protected])

 

Dear,

thank u reply,

Genrator Make Siemens Germany. 6000 V 3 Phase, 1500 rpm. 50 Hz.
answer of u qustion as below,
1. Manfucture in 1979.firstly operated in Norway upto 2008. now we purchase and insattal in Pakistan.

2. Insullation material not mention.Insullation class F.

3. Envirment Dry weather, Ambent Temp. 40-48 deg C

4. cause of test we newely installed.

5. we preheat the winding by short the UVW terminal and by appling excition up to stator 70 % nominal Current.
1. Insulation resistance (G-Ohm)
U-V U-W V-W U-E V-E W-E Tester Volt
24.0 60.0 18.0 14.0 5.5 22.0 5000V DC
8.5 4.4 4.4 4.1 1.2 4.4 1000V DC
uper reading is after heating and below is before heating.

6. past data not available

7. curent vs time not available.

regards,
Hussain

 

Hussain,

The readings are not "normal" which is obvious from the wide variation. Exacerbating the problem is that the so-called "spot-test" you carried out, without prior tests as a guide, can't really tell you how "good" the insulation is!

The positive fact: the measurements are well above the usual Rule-of-Thumb, that is, one-megohm for each 1,000 volts of operating voltage.

The negative fact: the inulation is 30+ years old!

However, to further allay your concerns, and if you do not have access to an "expert" from Siemens or a respected rewind shop, I suggest you perform a "Dielectric Absorption Test" modified per Robert Scott's suggestion in:

Control.com/Thread/1026224486 "Insulation Resistance"

The Dielectric Abdsorption Test results in a time-resistance plot which yields a more qualitative indication of insulation "health!"

This test can be carried out with the windings connected, but the supply cables should be diconnected if they are lengthy.

Caution: Do not exceed 5,000V for test voltage. Why you ask? See:

Control.com/Thread/1001589379 "Di-Electric Strength Tests"

Regards, Phil Corso ([email protected])

 

dear,

u called thumbe rule is 1 Gohm per 1000 volt. but what are the tester voltage on 6000 volt generator?

i cant search the thread 1001589379 "di-electric strength tests"

please reply me

regards,
Hussain

 

Hussain,

1) Your 5,000 V meter is adequate. The point is to use the Tester to determine Dielectric-Absorption-Rate. Knowing just Insulation Resistance magnitude is not enough to reveal how "good" the insulation is!

2) Sorry, the thread number id # 1001509379

Phil Corso

 

dear sir,

tomorow i clean the altornator stator winding with air. after that we check the results are as below,

U-Earth V-Earth W-earth
1 mint reading (G-ohm) 14 14 22
2 mint reading (G-ohm) 23 22 30

please send me method of dielcetric absorption test.

regards.
hussain

 

Using the string '+dielectric +strength +test' (without the quotes) in the Search function of control.com returns several threads:

http://www.control.com/thread/1001509379

http://www.control.com/thread/1026224486

The two above are the threads Mr. Phil Corso, PE, is referring to.

The Search function of control.com searches the archived threads and responses. While not entirely user-friendly it is very fast and it does have a 'Help' function which can be referred to if having problems. It's best to precede each word of a multi-word search term or string with a plus sign (+).

It's very likely that a question has been asked previously on control.com, so before posting a question it's a good idea to search the archives using several differently worded search terms to see if you can find any relevant threads and responses, and if you don't find what you're looking for then you can post a new thread and let us know that you have searched for a relevant thread/response and haven't found what you're looking for.

 

Hussain,

1) Within the last month, there has been a proliferation of articles questioning the use of the Insulation Resistance and (IR)Polarization Index (PI) for determining insulation integrity, on rotating equipment having Class F or H insulation.

2) In my view the single IR measurement you are contemplating is useless. Your best course-of-action is to plot current over a period of time. The polarization index can provide some additional information to help with your decision. However, it wont provide much help in your situation where the IR magnituse is in Gig-ohms in magnitude.

3) How long has it been since the alternator was last energized? If in storage, then what were the storage conditions, that is, in a location exposed to large temperature swings, or in wet-dry location?

Regards, Phil Corso ([email protected])