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2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 1:14 AM

Hi everyone,

We have a 2000 KVA Dyn-11 Transformer at our site and it is earther from two earth pits one for neutral and for body. We have recently observed that there is a leakage of 24 A from transformer body. I just want to know whether it is normal or not?

Moreover, could it have any effect on Electronic Protection Relay installed on the upstream of transformer (HT Panel) as we are often finding our Main VCB of HT panel tripped showing IE>> during transformer excitation (start-up). We are using Siemens' 7SJ602 protection relay. Any suggestion/comment would be appreciated.

Regards

Farhan Javed

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#1

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 2:45 AM

The phase-to-ground voltage is probably at least 220 for this transformer. If so, 24A x 220V ≈ 5.3KW (and maybe more). That's a lot of energy leakage, and certainly not normal. If you are lucky, it may be something as simple as a loose wire strand, or insulation damage somewhere. In any event, prompt investigation and correction are essential. Good luck in tracking this down quickly and safely.

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#2

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 3:38 AM

Look for connections and crosses between the neutral conductor and the ground/earth conductor at all distribution boxes downstream, and eliminate them. This is an important safety issue, so do it before someone disconnects one of those conductors thinking all is well, and gets an armful of volts for the trouble. Do it today/now/immediately/without delay either.

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#3

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 9:28 AM

certainly it is more current. make independent neutral earthing with 50X6 mm GI flat, at the same time with Body also. transformer earthing should not connect with grid earthing, check tap operations, may be relay triping on instantaneous current. check settings, commonly megger values, you may found something if you open inspection top cover...like opened winding, etc, carbonised oil.

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#4

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 9:29 AM

24A earth leakage current is very dangerous. Indicates some insulation deterioration. Pl. investigate & rectify immediately.

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#5

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 9:41 AM

Are you saying that the grounding conductor that connects the metal tank/case of this transformer to your earth pit sees a current flow of 24 amps? If so, have you tried to measure the voltage across the circuit between the tank and the earth pit?

What happens to this current flow when you unload the transformer? Open its downstream breaker and see what the current flow is.

Does the transformer have powered cooling fans mounted on it? If so... check them very carefully.

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#6

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 11:04 AM

From transformer body to where...to neutral, to earth, phase (heaven forbid)?

What is the reference?

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#7

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 10:39 PM

1. PWSLACK has given some guidance on the 24A leakage current. However if the current persists when the LV side is disconnected, you may have a HV fault and this needs to be resolved quickly. With the transformer isolated from supply (HV and LV) check insulation resistance HV to Tank, LV to Tank and HV to LV. Check the arrangement of the HV system neutral earthing.

2. The tripping on energising is possibly due to inrush current. ((I>> is usually instantaneous and thus will operate if the inrush is greater than the setting) This can be up to 6 times full load current, and may not be balanced. Review the protection settings to ensure that instantaneous (I>>) settings take account of inrush current.

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#8

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/10/2013 11:38 PM

Let me put my cynical fly into the ointment, so to speak.

Is that measured current ohmic or capacitive?

What is the harmonic content, as measured with an oscilloscope?

I do not believe for a moment, that a 5 kWatts real leak can occur,

without loud and obvious signs.

On the other hand, say, a 100kHz capacitive coupling can do that

comfortably. And you will not be any wiser, until actually measuring

it. So, get on with it!

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#9

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 2:55 AM

Is it from the transformer or from your MV cables, (earth leakage and cirrcualting earth faults)? Did you disconnect the cables and measure the amps?

Also, it is time people defined their voltage ratrher than telling us HT. 11, 22, 33, 66kV, is NOT HT. If you place the correct information here, people can assist you better. i.e voltage, cable type, etc. Cheers.

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#10
In reply to #9

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 3:25 AM

I am surprised! What are you talking? If 11, 22, 33, 66kV are NOT HT, then what are they? Are they LT?

Reference is drawn to IEC 60038, wherein it is mentioned that LV is less than 1kV, HV Range 'A' is above 1kV but less than or equal to 245kV and HV Range 'B' is above 245kV.

And, just in case you don't know, LV is Low Voltage & HV is High Voltage.

Pl. do not post incorrect statements.

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#11
In reply to #10

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 3:36 AM

He never claimed he was High IQ. Just IQ.

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#12
In reply to #10

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 5:42 AM

As in real estate,location is everything.In power transmission, above 35Kv is considered High Voltage.

The IEC defines High Voltage as above 1000Volts for AC and above 1500 volts for DC.

The NEC defines it as any voltage above 600 volts AC,and British Standard has it's variance from these definitions also.

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#13
In reply to #10

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 5:44 AM

Rather than show ignorance here, IEC 60038 is for Standard Voltages. It does not state what is MV, HV or EHV. So dipstick, for your education it works as such.

1kv to 132kV Distribution voltage. which covers MV and HV. 11kV is MV, 132kV - 400kV is HV, 500kV and above is EHV and super EHV. 1MV is operational in China. Know you voltages.

Electronic Protection Relay installed on the upstream of transformer (HT Panel) As quoted, HT panel? Rather state a voltage then folks know and can help.

God help you on a transmission line or sub station. But to answer the lads concern, it could be coming from the Tfx or more likely the cable and if this was oil filled cables, there is an earth issue on a cable. If this is XLPE cable it can be the termination, the cable screen, the cable core holed and the semi con leaking into the screen. It could even be a joint breaking down, if there is a joint in the cable. The far side earth protection should have tripped, if the cable is faulted to that side. If it is closer to the near side breaker the fault will not be seen by the relays. And this depends on earthing of the cables. So in your jest, you should perhaps keep quite as there are people who don't waste time with 17th edition as it has no value in 132kV systems. Wipe the shit from your eyes and learn for a change. And I guarantee this is a MV cable fault and very close the to the termination. It just makes it easier when one knows a voltage, rather than assumed terminology. But then if this is NOT your field, then tolerating fools and horses is hard work for us F'wits.

Read Note 3, page 10, IEC 60038 Ed 6.2, 2002-07.

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#14

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 7:35 AM

Thanks everyone for your suggestions and sarcasm!!

First of all Trandformer incoming voltage is 11 KV we conducted all the test of transformer like HV to LV Meggar, HV Meggar, LV Meggar, Cable Meggar, TTR, horn conditions and everything is fine.

We observed that the leakage is from our downstream network and is being observed at transfromer body as both LT and Transofrmer share a common earth.

We removed entire plant load and then tried to reset the VCB but it tripped again!!

Transformer vendor has suggested that our protection CTs "5P20" might be entering into saturation region and since we have set 0.0 sec delay on IE>> fault, VCB is being tripped instantly...

Interestingly after performing all transformer tests when we reset the VCB it did not tripped and since yesterday it is working fine!!!

Regards,

Farhan Javed

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#15
In reply to #2

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 7:39 AM

I agree with PW.

To answer the OP question (again). NO! It is not at all normal! Get working to fix it right now! Follow PW's guidance, as it is good.

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#16
In reply to #14

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 9:06 AM

Did the 24A leak go away at the same time, though?

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#17
In reply to #13

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 10:29 AM

Voltage levels have been defined and classified by various National and International Standards as also by certain electric power utility companies. The following are the definitions given in various Indian & International Standards for AC Voltages.

******

India: As per the Indian Electricity Rules, 1956, vide Rule 2(av), the following are the limits:

Low Voltage: Not exceeding 250V

Medium Voltage: Not exceeding 650V

High Voltage: Not exceeding 33000V

Extra High Voltage: Exceeding 33000V

******

But, in IS 13234 - 1992, which is the "Guide for short-circuit current calculation in Three-phase A.C. Systems" - it is mentioned vide Table I that:

Low Voltage: 100V to 1000V

Medium Voltage: > 1kV to 35kV

High Voltage: > 35kV to 230kV

As can be seen from the above, there is no harmony between the IS and the IE Rules in this issue.

******

France: Vide a publication dated 14th November 1998, the French Regulations define three voltage levels as below:

Low Voltage: < 1kV

High Voltage A (HTA): > 1kV but </= 50kV

High Voltage B (HTB): > 50kV

******

CENELEC (The European Electrotechnical Standardisation Committee): Vide a publication dated 27th July 1992, the CENELEC recognises the following voltage levels:

Low Voltage: < 1kV

Medium Voltage: > 1kV but </= 35kV

High Voltage: > 35kV

******

IEC (International Electrotechnical Commission): In IEC 60071, the following ranges are defined:

Low Voltage: < 1kV

High Voltage Range I: > 1kV but </= 245kV

High Voltage Range II: > 245kV

The acronyms EHV (Extra High Voltage) and UHV (Ultra High Voltage), though are being used elaborately in India, have never been officially defined in any of the above three standards.

USA:

i) Vide IEEE Std. 241-1990 (IEEE Recommended Practice for Electric Power Systems in Commercial Buildings), Clause 3.1.1.2,

Low Voltage: A class of nominal system voltages 1000V or less

Medium Voltage: A class of nominal system voltages greater than 1000V and less than 100000V

High Voltage: A class of nominal system voltages equal to or greater than 100000V or less than 230000V

******

ii) Vide ANSI C84.1-1989 (Voltage ratings for electric power systems and equipment),

Low Voltage: 120V to 600V

Medium Voltage: 2400V to 69000V

High Voltage: 115000V to 230000V

******

iii) Vide ANSI C92.2-1987 (Power Systems - Alternating Current Electrical Systems and Equipment Operating at Voltages above 230kV Nominal - Preferred Voltage Ratings)

Extra High Voltage: 345000V to 785000V

Ultra High Voltage: 1100000V

******

Note: IEEE - Institution of Electrical and Electronic Engineers

ANSI - American National Standards Institute

******

As can be seen from the above, there is no clear cut demarcation for classification of various voltage levels in AC Electric Power Transmission and Distribution. Also, there is no single standard across the globe, with various countries defining various limits for these voltages. And, as in the case of India and USA, even within one country, two standards do not match, when it comes to these limitations. It is wished that the electrical community across the globe meets and takes a decision soon to bring harmony across the countries of the world in standardizing the limits for various voltage classifications.

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#18

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 12:10 PM

I find this thread an exercise in human obtuseness.

There is a 24Amps leak somewhere, supposedly.

There is no measurement presented, but somebody's say so.

Well, I do not believe so.

An approximately 5kWatts leakage is loud, cannot be hidden.

I asked for some measurements with independent instrumentation, like oscilloscope. The fellow comes back with Meggering parts. By the way, it is Megger, not meggar. Who cares?

A relay trips for a reason, that is for sure. That is not sure at all, what the reason is. The relay not tripping for a day for unknown reasons is not an achievement in my book.

Actual measurements, anyone. As in verifying, if the 24Amps is real in the first place, and its nature, if it exist at all?!?

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#19
In reply to #14

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/11/2013 4:15 PM

Ok Farhan, everyone here has express their best opinion but far away from site, now take your notebook and choice the better of each one because you are touching the problem.

Reference to the voltage designation, please check standard ANSI-84 1 -2006 despite voltage designation now is not your top priority.

Good luck

Bye

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#20
In reply to #14

Re: 2000 KVA Transformer - Body Leakage of 24 A

04/12/2013 2:14 AM

If LV and HT earths are common, and there is no problem on HV side, the current is probably LV neutral current which is finding parallel paths through neutral to earth connections. Check if the LV neutral is connected to earth at the transformer and another place.

As for the HV VCB tripping. What is the CT ratio? Since the transformer is 2000kVA (primary current 104amps), the CT primary is at least 100A. 5p20 will take 2000A before saturating. This is not likely to be the reason for the VCB tripping on Ie>>.

Inrush current could be as high as 600A. However this level of current will not always happen as it depends on the point in the cycle that the closing occurs. Sometimes the VCB will not trip, and other times it will.

What is the current setting for the Ie>> function? You could put a short time delay (e.g. 0.2 seconds) or increase the current setting if it is too low. The Ie> function with appropriate time setting should pick up low level earth faults

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#21
In reply to #18

Re: 2000 KVA Transformer - Body Leakage of 24 A

08/11/2014 8:54 AM

Hi,

Leveles thank you for pointing out the tests. I have taken over maintenance of this particular transformer after my predeccesor Farhan. I conducted the tests. Below are the results;

1) Unloaded LT, leakage current on earth : 16A

2) Unloaded LT, voltage on transformer body to earth busbar : 0V

3) Unloaded LT, voltage on neutral to earth : 0V

4) Unloaded LT, leakage current frequency : 150 Hz

5) Unloaded LT, leakage current THD : 250%

6) Unloaded LT, power quadrant 1, net power 0W, PF 1.00, tan 0.00

With LT on load the above values remain same except the frequency changes to 50Hz (our system freq).

We do not have an oscilloscope on site, the THD values were noted from a power analyser we use (PEL 100). An interesting observation here is that while the transformer neutral is earthed seperately, the transformer body earth is common with the plant distribution ( body earth connection made to a bus bar where multiple equipment and feeders have been earthed including a generator neutral connection). I do not believe that the leakage is resulting from damage to transformer or some neutral to phase cross over. I do not believe the current is ohmic (though the 1.0 PF seemed disturbing). What are your thoughts?

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#22
In reply to #21

Re: 2000 KVA Transformer - Body Leakage of 24 A

08/11/2014 10:39 AM

A few questions need answering to satisfy my curiosity,and eliminate some variables that may be obvious to some, but ambiguous to other:

Re: The generator you mentioned:Is it running while conducting the tests you mentioned?

Do you have a lot of Switch-mode power supplies in the plant;computers,UPS's etc?

The fact that the current is a 3rd harmonic indicates such devices on premises that are supplied by another source.

Even though the individual currents may be small, due to the nature of 3rd harmonics,the currents are additive.

I suspect this may be the source of the current.

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#23
In reply to #22

Re: 2000 KVA Transformer - Body Leakage of 24 A

08/11/2014 11:56 PM

HiTek the answers to your queries are given below;

1) Is the generator running while test was conducted : Yes, Transformer primary was also available, LT was unloaded. Load was on generator.

2) Yes there are significant SMPs installed at our plant. UPSes, computers and DC power supplies.

3) Why would you say that the current is 3rd harmonic dominanat. Beacuse of the frequency of 150Hz?

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#24
In reply to #23

Re: 2000 KVA Transformer - Body Leakage of 24 A

08/12/2014 1:08 AM

Yes,the 150hz frequency is the reason,presuming your system is 50hz.

Instead of going into possible remedies here, I can direct you to a good source to help resolve your issues.

Link:http://www.hammondpowersolutions.com/files/HPS_article_Harmonics_PhaseShifting.pdf

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