Meggar value for 4160 volt cable 4

[gsjhand]

Hello Everybody

Recently we had yearly shut down in the plant and as planned we got our substation checked. This was done by world wide reputed company and it involved checking of some underground 4160 volts cables. While checking they found one of the 4160 volt cables giving 2 Mega ohms meggar value. Cables were checked at 2.2 KV. When they went to 5 KV they found meggar value of same cable around 60 Mega ohms. Then they decided to check it on 7.5 KV and there was further improvement in the value.
When I questioned them for checking these cables at 2.2 KV they told me that this is their standard for high voltages cables.
Next no body was able to explain why meggar value improved on high voltage. These tests were done repeatedly with same results.
I would appreciate if somebody can explain to me that why meggar value improved by going to high voltage (I was expecting totally opposite results) &
Why H.V. cables are checked at 2.2 KV as I am under impression that cables should be checked at least at their operating voltage?

 

[rockman7892]

Davidbeach

I'm assuming that there is no capacitive current with DC voltage because of the fact that capacitors block DC current. When refering to this capacitive current, is this the current between the cable sheild and conductor, or leakage current between the sheild and ground? So with no capacitive leakage current then the only current present is that resistive current which is leaking through the insulation?

Is it recommended to use DC so as to not have this capacitive current interfere with results?

It was mentioned that leakage current will increase with voltage so therefore the resistance should stay the same. Is this why when plotting Hi-Pot results you look for the plot of leakage current vs voltage to smooth out?

 

[oldfieldguy]

Meggers (megohmmeters) are DC instruments. On long cables or on motor windings you can see a brief exhibit of the capacitive component of the insulation as the initial application of voltage shows the meter needle at the low end of the resistance scale. This part of the indication goes away after a fraction of a second or so.

After the voltage has reached its full value (charging the relatively small capacitance, then you see the resistance reading climbing. If you can accurately determine readings during this climb, you'd see an exponential (more or less) increase in the resistance reading as the dielectric absorbs the charge of the testing voltage. It is usually out of the picture after the first few seconds.

That leaves you with the REAL leakage current. In a good test, this current is ONLY through the insulation, but as I said in an earlier post, it CAN include paths over dirty terminations or stress cones. The real leakage current when using a Megger translates to megohms of resistance. In a good insulation system, the leakage current will drop off varying amounts over time, depending on the quality of the insulation.

We use this characteristic to quantify motor and generator windings by calculating a Polarization Index, or "P.I." which is the one minute megger reading divided into the ten-minute megger reading. We expect motor and generator windings to typically have PI's of 2 or greater. Less than 2 is cause of concern.

Cables, on the other hand, are not usually evaluated by PI, but it is typical to see a ratio of 1.2-1.5 between the one minute and ten-minute readings. In any case, the resistance reading should increase somewhat over time with a steady voltage applied. If the reading is low and steady, this usually indicates dirty or wet terminals or splices.

If the reading is getting lower as test voltage is maintained, terminate the test because something is headed towards breakdown.

Taking a megger to equipment is a good way to test things out, but to do a really good job of evaluation requires a bit of knowledge and thought. Unfortunately, too many "technicians" don't get far into the "knowledge and thought" and they just get the "easy".

I hope my brief explanation helps you.

old field guy

 

[Zogzog]

Or to put that post graphically....

 

[gsjhand]

It's really nice to know about effect of cable capacitance on it's megger value. There has been some discussion on cable length, so just want to mention that cable in question is around 700 ft long and is of 500 AWG. When I saw 2 mega ohms of megger values I was thinking about changing it. But then we asked service guys for other tests. After going through 7.5 KV test decided to put it back in service and it's running fine since then.

 

[flexoprinting]

Thanks David and oldfieldguy, I have a new perspective now!

gsjhand, thanks for your O.P.

 

[rockman7892]

I find that alot of times the techs we have working here at our plant dont have a complete understanding of how to use or interpret megger readings. A lot of times when a motor draws high current or has some other problem they will get out the megger and 90% of the time deam a motor as bad. I think a lot of time though like I said they jump to conclusinos or dont apply the test properly and therefore identify the motor as bad rather quickly.

It sounds like the polarization test is something that we should start implementing when testing motors. Besides this test is there any standards or information that talks about the steps and testing methods used when testing a motor? Something that would expalain a testing procedure, voltage levels etc...

I find that alot of times to they will connect the megger back at the starter rather than at the motor terminas itself. I have heard that a megger may not have enough power to go out thorugh the cables and at the same time will factor leakage current into the picture. It is for this reason that the megger should be applied at the motor leads.

 

[edison123]

rockman

IEEE std 43 gives the procedure and explanations for Motor IR testing. Testing the motor with cables is ok as long as the IR is good (which is not the case often). If bad Ir, you need to isolate them to see which is having the low IR, the cables (usually) or the motor.

 

[oldfieldguy]

Rockman--

If going out to see why the motor tripped, the "megger from the starter" check is a quick shot. If the megger reading is "good" with the cables and the motor connected, then insulation failure is NOT likely the problem. Meggers have enough "power" to test systems in this manner. I've meggered cables thousands of feet long. But if the reading is NOT good, then the tech is going to need to disconnect the motor and see whether the motor or the cable is the problem.

The full-blown ten- minute PI test is good, but a lot of my clients only performed that on larger motors, 100 HP and up, or medium voltage motors, not low voltage (480) motors. If you're not going to do a PI on the motor, then get your guys in the habit of doing a one-minute test for record-keeping. However, If when the megger is connected and the reading stays at zero, then there's no need to waste a minute. Ac ouple of seconds will show failed insulation. Bad is bad. There are, however, varying conditions of "good".

That little handbook from Megger (

http://www.biddlemegger.com/biddle/Stitch-new.pdf)

that somebody alluded to earlier is an excellent document to discuss with your electrical staff.

If you feel that you are not up to the discussion, you can call most companies in the electrical testing business and get somebody to come in and do a bit of training for you.

old field guy

 

[flexoprinting]

Oldfieldguy

Sigh...stupid tech here, I must admit I do believe I am a victim of manufacturing or standard production procedures in most facilities. Many people in Management really don't give a rats tail until something goes down. Basically what I have been doing is performing a go/no go test on motors. If I were to produce this data as described, they would look at me like I was talking about the boogie man! and possibly consider me a radical or something. so I bite my tongue and proceed, after all it's their money.

Anyhow I do appreciate the refresher on real preventive maintenance. Thanks again.

 

[oldfieldguy]

Flex--

I hear you, man! I've worked in facilities that had a "run it till it breaks" SOP. I made a lot of overtime, but in three years we NEVER had a scheduled unit outage. We had LOTS of unscheduled, 'hurry! the plant's down and we need it running NOW!' outages.



old field guy