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aunghlaingtun
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Join Date: Apr 2009
Location: singapore
Posts: 16

Zero Current Transformer

08/06/2010 2:13 AM

Hi all,

I would like to know how to usage for z ct . (zero current transformer) with elcb.

Because, I'm here working at one of the oil and gas terminals in happen using that case.

They are using this zct for protection at mccb. 400 v. 4 wire. So before we are using for this 3phase 4 wire ( r y b and natural) together pass through at zct. So that is our using is correct usage or because, now frequently happening trip for that mccb. So we have to found out leaking a lot at outgoing side circuit so we trace to found out is control circuit at leaking more than 500ma.so that make to trip for main mccb. So client engineer say to us, our design is wrong. Cuz, he take out one cable from this z ct and solve this problem tripping . Then he say that our design is wrong. So that is it he do the way is correct. He just take out natural cable from zct.

So I'm blar blar. I have done my study before is 4 wire( r y b and n ) all must pass through this zct to sensing and measure for unbalance of milli ampere .( ma ) . Sp i would like to know he do the way is correct usage or not. Anybody know, help me discuss and solve for me. Thank.

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kvsridhar
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#1

Re: Zero Current transformer

08/06/2010 2:22 AM

You are correct in making all 4 wires R,Y,B and N go through the CT. Tripping is due to some earth leakage in the system. That must be found and eliminated.

Please download and read this document : ect114

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aunghlaingtun
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#2
In reply to #1

Re: Zero Current transformer

08/06/2010 2:31 AM

thank for reply. i hv to prove that .

because now they are using like that wrong. then i say that engineer , is it correct or not .i just asking . but ......

we are handover 3 year ago .now just happen . so he dont know the system . but he never accept. i try to advise also dont want to accept. i just technician and he is engineer.

thank u for replying

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kvsridhar
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#3
In reply to #2

Re: Zero Current transformer

08/06/2010 3:42 AM

Don't get disheartened. i have worked with many outstanding technicians and also with incompetent engineers (who just got the degree because their parents were rich enough to pay their college fees)

Not to worry. That article from Schneider is a world-class reference document. Which MCCB (4-pole) are you using ? If it is a from a good company like Schneider, ABB, Siemens etc, you can request one of the technical people in their Singapore office to speak to the client engineer and convince him. Just a thought.

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aunghlaingtun
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#4
In reply to #3

Re: Zero Current transformer

08/06/2010 11:24 AM

yes, thank u very much

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SHAMITHGAMBHIR1
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India - Member - design engineer Engineering Fields - Electrical Engineering - New Member United Arab Emirates - Member - New Member

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#5

Re: Zero Current Transformer

08/07/2010 2:16 AM

The Zero CT protection responds to the residual current (also called neutral or zero-sequence current) shown below:i(neutral)=3I=i(r) + i(y) + I (b)The protection setting should take into consideration that the 3 CTs+ 1CT do not have identical characteristics and will perform differently for heavy phase-to-phase faults or for initial asymmetrical motor starting currents. This can produce false residual currents. The setting should also be above the line maximum unbalance current.

The above conditions must be satisfied to avoid nuisance tripping. In addition, the ground fault protection must be sensitive to minimum ground fault current at the end of the line. It is often difficult to determine a setting that satisfies both sensitivity and selectivity requirements.

there s also another method of ground fault protection ....ElR + CBCt

This method is based on primary current vector addition or flux summation. The conductor cables pass through the center hole of the core balance CT. Secondary current is the system zero-sequence current 3I_0

R u using elcb or elr? coz elcb has an inbuilt ct in it......u don need to provide an additional ct for tht...........

and yes if it s a 4 wire system thn neutral must be connected to CT or CBCT....

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aunghlaingtun
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#6
In reply to #5

Re: Zero Current Transformer

08/07/2010 3:52 AM

thanks U.

yes, we using ELR

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#7

Re: Zero Current Transformer

08/07/2010 6:10 AM

Hi Aunghlaigtun

Your problem could come from:

significant earth leakage in systems before your MCCB which can cause main neutral point on supply system getting elevated potential against real ground potential. in this case if your neutral is not properly insulated from the ground you will have current flowing from main neutral trough your neutral conductor into point in part of installation after your ELCB device which has connection to the ground. To prove it disconnect all actives R Y B from your ELCB but not neutral and if this is the case most probably ELCB will still trip (under condition that CTs are connected properly as they were originaly).

Also you shoul trip ELCB and disconnect neutral from incoming neutral, then measure resitance of your neutral to the ground and also measure votage of incomming neutral to the good ground. If it is problem which I'm suggesting then you should measure low resitance to the ground and most probable a few volts between icoming neutral and ground. This few volts can cause significant currents flowing through neutral even when you do not have any load connected. You have find thing which has bad insulation/short circuit between your neutral and ground and clear it. I do not believe that is design problem but may be bad workmanship which developped along the time or damage to some cable or device connected to the system.

To work it out you will may be have to use megger to measure insulation resistance between each phase and ground and neutral and ground. Be wigilant to disconnect first devices which can be damaged by megger's high voltage.

Good luck

Djuro
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LAA_Lucke
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#8

Re: Zero Current Transformer

08/07/2010 6:20 PM

Sorry but you english is very poor and could not understand all what you really meant!

Points to clear: You said that he removed the neutral from the zct output and now it is not tripping any more(?). If this is correct, then the wiring is either not well clarified to us OR it is definitely Wrong downstream!

Explanation: Using this zct and passing the 4 wires through it, means that all the currents (AC) will balance to zero: 3 phases + Neutral, for it not to trip. Now if some current is leaking to ground instead of returning via the neutral line, then it will trip. If the circuits downstream the zct are using the neutral line (means your installation has load using a phase and neutral and not only 3 phases without the neutral), then definitely, removing the neutral from the zct output, you will need to connect it directly for the circuits to work... then it should trip!! FOR THIS REASON I SAY THAT THE CIRCUIT IS EITHER WRONG OR WE COULD NOT UNDERSTAND YOUR ENGLISH! (I am not shouting ...just highlighting).
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aunghlaingtun
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#9
In reply to #8

Re: Zero Current Transformer

08/08/2010 2:47 PM

thanks u alot.

yes ,i accept. whtever u say.

let me know how to design . my meaning is 4 wires( RYB & N ) go through ZCT is correct ,or without N is correct. Bcuz, he just take out N is now working. that mean which one is correct .if correct, why ?

ckt is

ELR wth Zct for protect Earth Leakage.

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LAA_Lucke
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#10
In reply to #9

Re: Zero Current Transformer

08/09/2010 5:03 AM

From What I can gather following your input:

1- If the circuitry after the zct is suppluing a 3 phase load without Neutral: i.e. comprising 3phase motors etc and no phase to neutral loads --> then I can understand that the zct is not tripping { the current going through the 3 wires of the 3 phases will always balance to zero residual current. --> If the zct trips when you connect the neutral through the zct (in this context) then it means that somewhere along the N wires, there is a leak to ground: i.e. the insulation of the N wire is damaged at a point and it is touching the ground, OR it is connected to ground by mistake.

WHY?: Somehow, the N wire is bringing current from another network passing through the zct and going to ground. This will create an unbalance in the total current going through the zct...TRIP.

2- If the circuit network after the zct comprises loads that are using the N wire (single phase loads) as well as 3 phase loads, the total current resulting from the 4 wires through the zct will also cancel and will have a zero residual so as not to trip. Since it is tripping, it will mean that somehwere some current is not returning back through the zct. If you remove the N wire and leave it OFF in the air, and you start the equipment (put the power ON), the culprit circuit will still have a leak and will trip!! BUT if the tripping is not happening only when nothing is powered and when you energise the circuits it trips, then the same explanation as above stands.

trouble shoot: Follow the N wire after the zct to what is called a Neutral BAR (where all the neutrals are connected from it to the different output circuits). then Isolate (remove) the outgoing Neutrals (all of the outgoing) and reconnect them one by one until it trips. You will find the line that is leaking to ground (Neutral leaking to ground or connected by mistake to ground).
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aunghlaingtun
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Location: singapore
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#11
In reply to #10

Re: Zero Current Transformer

08/09/2010 6:01 AM

thank for that.

i want to know ,thus he do the way is correct or not. just want to know only. bcuz we know this N at leaking that why he take out from Zct .that mean he bypass N wire from Zct.and until now . so my meaning is he do the way is correct. jst want to know. if correct whY? Cuz, Earth leakage system we have to 4 wire (RYB and N) must go through Zct . is it .?

cuz, he say can be done by without N .i say , that not truth. 4 wire must go through Zct. now he do like this, without N , is it safety for this system .

anyway , thanks u for reply.

i have take note of that.

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