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Meggar value for 4160 volt cable
4

Meggar value for 4160 volt cable

Meggar value for 4160 volt cable

(OP)
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?
 

RE: Meggar value for 4160 volt cable

Hello gsjhand

Those readings indicate good insulation, your megger is calculating the resistance based on higher voltage increments I think these reading would vary on different units even of the same model. It is when the ohms reading lowers you should be concerned, this would indicate a problem.

The Fluke web site has information on using a meg ohmmeter, also years back Avo had a good down loadable PDF on this subject.

I am no expert on this someone else will most likely follow with a better explanation.

 

RE: Meggar value for 4160 volt cable

2
There is a high voltage megohmmeter that requires the use of multipliers to calculate readings.  I had issues with technicians who couldn't grasp the concept of having to multiply a meter reading by two different multipliers based on voltage level and scale.  Questionable results often appeared on test reports due to this "complexity".

A reading of two megohms on a 4160-volt cable would cause me to seriously question its serviceability.  The other two readings are more in line with what one would expect.  The whole set, though, causes me to question the ability of the technician performing the test.

As for the "world-wide reputed company", I have some doubts.  I was formerly a senior field engineer for one of those companies, and I can tell you that I lost sleep worrying about the capabilities of some of the "technicians" we had on our roster.

I recently had to have several commissioning tests redone because the quality of the data on the test reports by a nationally-known testing company could not be interpreted to indicate that my brand-new equipment was fit for service.

As for the low reading followed by two satisfactory subsequent tests, I'd likely suspect poor preparation of the test specimen before the first test.

old field guy

RE: Meggar value for 4160 volt cable

2 megaohms sounds low to me also for typical cable run lengths.  Of course, the longer the cable is, the lower the insulation resistance.  This might be normal for some very very very long run of cable (several thousand yards?)

Also of course, sometimes it is not the cable but some connected equipment that is the culprit.

NETA Maintenance Testing Standard identifies a single test voltage 2500 vdc.  There is no 2-voltage P.I. test for cable like there is for motors.  Personally, I wouldn't expect the resistance to go down as voltage increases, but I wouldn't worry about it either. I would worry about the relatively low reading.

=====================================
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RE: Meggar value for 4160 volt cable

A MEgger test aloone wont tell you much about the dielectric strength of the cable and any "World wide reputed company" should know this and have also performer a VLF, Tan Delta or PD test on your cable also.

OFG made a great point, dont rely on the companies reputation, I also have worked as a SFE for one of these world wide companies and some of the techs were pretty weak. Power system testing is a small world and most of the best techs prefer to work for the independants, better pay, better benifits, more freedom, so most of the big guys get the bottom of the barrel, dont get me wrong they have some great techs too but you cant count on it. Also the big guys use random job generation, in other words the next job goes to the next tech, regardless of his area of expertise, you might get a relay guy assigned to your cable testing job, the smaller companies send thier cable guys to every cable job, relay guy to every relay job, etc....

What specs was he testing to? Obviously not NETA.  

RE: Meggar value for 4160 volt cable

Zog--

You and I, we need to sit down and do a lunch sometime...

old field guy

RE: Meggar value for 4160 volt cable

Yep, what part of the country are you in? Will you be at Powertest (NETA) in San Antonio in March?

RE: Meggar value for 4160 volt cable

I'm down here in southwest Louisiana.  I may make that NETA conference.

old field guy

RE: Meggar value for 4160 volt cable


Why does the insulation resistance decrease with long cable runs as mentioned in an earlier post.  Does it have something to do with capacitive coupling?

 

RE: Meggar value for 4160 volt cable

Think about having the insulation of a foot of cable in parallel with the insulation of the next foot of cable in parallel with the insulation of the next foot of cable and so forth and so on.

If (totally made up numbers) you have 1μA of leakage current at a test voltage of 1000V, for a 100 foot chunk of cable, you have 1000MΩ of resistance.  Add another 100 feet of cable, which will add another 1μA of leakage current and now your resistance is 500MΩ.  take it up to 1000 feet of cable and now you have 10μA of leakage current and your resistance is 100MΩ.

RE: Meggar value for 4160 volt cable

Hello Davidbeach

"If (totally made up numbers) you have 1μA of leakage current at a test voltage of 1000V"

What if there is no leakage current, does the resistance reading decrease with length of cable?

 

RE: Meggar value for 4160 volt cable

I wish there was an edit button!

The original question by the O.P. was
"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."

These are the readings I get with a Fluke 1587 under good conditions, or so I thought until now!

Electricpete stated above "2 megaohms sounds low to me also for typical cable run lengths.  Of course, the longer the cable is, the lower the insulation resistance.  This might be normal for some very very very long run of cable (several thousand yards?)"

I am a motor guy and have no experience with the big wire, however find this statement confusing.

Rockman7892 questioned it in relation to capacitive coupling, this condition also would indicate a leakage or improper procedure I believe.

Will post later, gotta go!

 

RE: Meggar value for 4160 volt cable

gsjhand

The disparity in cable IR values with different test voltage levels could be due to

1. Different meggers were used and there were instrumental errors.

2. The IR values were noted at different times during these tests. Since cable insulation currents are mainly capacitive (i.e. they tend to decrease rapidly initially and then slowly over a period of time), you need to measure the IR values at the same time (e.g. 1 min)for all the test voltage levels.

3. Also, you need to discharge the cables to ground for at least 30 minutes before you reapply the test (even if it is at the same test voltage).

RE: Meggar value for 4160 volt cable

Edison 123 has it nailed, I think 2 ro 3 is your culprit.  

RE: Meggar value for 4160 volt cable


"If (totally made up numbers) you have 1μA of leakage current at a test voltage of 1000V, for a 100 foot chunk of cable, you have 1000MΩ of resistance.  Add another 100 feet of cable, which will add another 1μA of leakage current and now your resistance is 500MΩ.  take it up to 1000 feet of cable and now you have 10μA of leakage current and your resistance is 100MΩ."

I like davidbeach's example above!

Following this example it would show that indeed the resistance values would increase with using a higher voltage.  If 5000V was used in the example above with 1uA of leakage the resistance in the first 100ft would be 5000MΩ.  I believe this would help answwer the question in the OP.

The one question that I have about this example however is the resistances being in parallel. As mentioned by others this leakage current is represented by a capacitance, and capacitors in parallel are added directly not inversely?  With that said I thought 1000MΩ  + 1000MΩ would equal 2000MΩ?  

RE: Meggar value for 4160 volt cable

I'm going to add one more caveat to the disparity in the readings:  Cable preparation.

A typical scenario:  Client tells the test technician that the cable is ready for testing.  The tech,assuming (there's that word) that the client has properly cleaned the stress cones, etc., conducts a test.  The readings don't look good.  A second test is conducted.  This time, before starting the test, the stress cones are carefully wiped off and dried.  The readings are an order of magnitude better.

I'm just saying that a good test technician would have made sure of the validity of his test results before scaring the client.  

old field guy

RE: Meggar value for 4160 volt cable

rockman

You have turned around the ohm's law. The cable leakage current would increase proportionately with the increasing voltage so that R (i.e. IR) is constant. Generally, all the good insulations tend to have a constant IR (at a particular temperature) meaning that applied voltage level (within reason) does not change it. A bad insulation will tend to have more than proportionate increase in current with increasing voltages leading to lower IR values.

In OP's case, he says that the IR increases with increasing voltage. Something wrong with the procedure or the instruments.

RE: Meggar value for 4160 volt cable

It's not just capacitance.  Even if it were just capacitance you'd find that the longer the cable the more charging current which is all driven by capacitance.

It might help to think of the cable insulation as providing a certain admittance (inverse of impedance) per unit length.  In this cable the admittance will be mostly conductance (inverse of resistance) and not much susceptance (inverse of reactance).  Capacitive susceptance is a function of voltage and the voltages in question are much too low for this to be a major factor.  More cable means more admittance; more admittance means less impedance.

You'll find that using DC as the test voltage you get leakage current through the insulation.  But with DC there is no capacitive current, only resistance or conductance.

In my made up example the leakage current would be expected to increase with increasing voltage.  With a test voltage of 2000V, I'd expect something much closer to 2μA of leakage current (it isn't a straight proportionality) rather than the leakage remaining at 1μA.

RE: Meggar value for 4160 volt cable

OFG, did you catch Don Genuitis article in the Summer NETA World on corona supression? Good stuff.

RE: Meggar value for 4160 volt cable


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?

RE: Meggar value for 4160 volt cable

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

RE: Meggar value for 4160 volt cable

(OP)
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.

RE: Meggar value for 4160 volt cable


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

gsjhand, thanks for your O.P.

RE: Meggar value for 4160 volt cable


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.  

RE: Meggar value for 4160 volt cable

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.

RE: Meggar value for 4160 volt cable

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

RE: Meggar value for 4160 volt cable

Oldfieldguy

blushSigh...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.

RE: Meggar value for 4160 volt cable

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

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