3 phase 4 wire system

[imranahmed]

Please let me know about issue given below,

There is 3 phase 4 wire system, when load connected through Circuit Breaker MCCB 250 Amps 4 pole to 3 phase with equal value i.e, 200W.

If its main neutral wire is broken ,then MCCB should be trip automatically, but problem is that during test when I disconnect its main neutral

wire ,the MCCB could not trip by its shunt trip relay. MCCB trips when I connect unequal load 100W,200W and 60W at every phase.

I want to know that when equal load is connected then MCCB should trip after neutral disconnection.

 

[milind.a.kulkarni]

Ideally, Neutral do not carry the current when there is balanced load .....In other words neutral carries negligible current when there is balance load .....that is what it can be even removed from load ......which is your situation no 1 ( 200 watts load case)....... Now when you have other case of unbalance load neutral is expected to carry current .....but as you said you are removed the neutral from load then phase carries unequal distribution of currents which is you case 2(100w,200w,600w) case...... In this situation the current driven by the phase connected to 600w load will be more than other two...and now the question is how much.......of course it depends on the topology that you used to connect the loads I mean is it star connection or delta connection....both have different formula.... Now as your current in the respective load is higher than over load current capacity of MCB ......it is tripping.... Refer this example given by link-
http://www.electrical-engineering-assignment.com/12-unbalanced-delta-connected-load

Good luck

 

[FvM]

If its main neutral wire is broken ,then MCCB should be trip automatically

Why and how? That's not the purpose of a circuit breaker.

 

[imranahmed]

Dear FvM,
MCCB consists of internal trip relay.

 

[FvM]

You mean, it has an undervoltage trip function? If so, it still can't reliably detect neutral discontinuity.

 

[milind.a.kulkarni]

Hi FvM,

I think he means that it has over-current protection ( that what he is calling as trip) .... I don't think that the circuit breaker that he is using have under voltage trip.....

with regards,

Milind

 

[Venkadesh_M]

You cant use trip function for neutral discontinuity it is only for single phasing protection(It cuts out if any one phase absent)....

 

[FvM]

I think he means that it has over-current protection ( that what he is calling as trip) .... I don't think that the circuit breaker that he is the under voltage trip.....

Post #1 suggests that the device has an unvervoltage trip. It's an option available with some MCCB devices.

 

[milind.a.kulkarni]

Hi FvM,

Just to make myself clear.....Can it is possible in 3 phase system of under voltage trip situation due to unbalance load condition?

with regards,

Milind

 

[Venkadesh_M]

It is not going to happen If the neutral presents as prescribed...

 

[imranahmed]

Dear all,

I don't need to trip MCCB on under or over voltage trip but I need only that the MCCB should trip when neutral wire is disconnect on 3-phase system both with balanced and unbalanced load.

Thank you for replies.

 

[milind.a.kulkarni]

This is very tricy.....I mean with unbalance load this case is till ok as neutral carries certain amount of current.... but in case of balance load conditions it carries very less current mostly leakage kind of current...thus if you use current sense method for neutral discontinuity using some CT..... this will not work in one of the case and also lower threshold will be problem....what you should do is you need to use some kind of modulated signal feed at the point you want to check to neutral and detect it at point you want to test the continuity of neutral......

Good Luck

 

[imranahmed]

Dear milind.a.kulkarni,

You are understand my point of view very nicely but (check & detect point) is same is mains neutral broken point, during balanced condition the amount

of current is negligible , can I sense current by Hall Sensor?. I checked during balanced condition the current through neutral wire is 0.8 apms approx.,

If I program uC interface with Hall Sensor it detects current if current pass through neutral wire is greater than 1.5 Amps ,uC gives signal.

Is this right approach?

 

[Venkadesh_M]

Hi sensing the current is only option but there may not any current in the balanced situation.....

so if you found a 0 current then add a small load on a single phase and check once again if it changed then neutral is fine......

but you cant do it by only sensing the current.........

may be there is some current in all time but if the load goes balanced you will get a warning.....

 

[milind.a.kulkarni]

Hi IMRAN,

I think 'Venkadesh_M' had bought very nice point if there is no load..... you will not have even current flowing (except very little may be leakage current) through the Neutral.... I am not sure you will able to use Hall sensor in such a variable range .... I mean If there is load and unbalance condition neutral may carry current based on load ( may be in amperes) and if no load then you will have only leakage current or even no current case..... My suggestion here is go with some kind of wire continuity detection approach rather than believing that there is a current flow through the neutral.....

Good Luck

 

[imranahmed]

Thank you very much for suggestions,

 

[ftsolutions]

I think that a reliable way to be able to detect and respond appropriately to broken neutral line would need multiple Hall effect sensors, so that one could tell if there was no load, or balanced load, or imbalanced load, as well as monitoring the neutral line. This could require up to 4 sensors, and either PLC or programmable device to perform the logic

 

[Venkadesh_M]

I think that a reliable way to be able to detect and respond appropriately to broken neutral line would need multiple Hall effect sensors, so that one could tell if there was no load, or balanced load, or imbalanced load, as well as monitoring the neutral line. This could require up to 4 sensors, and either PLC or programmable device to perform the logic

Hi still its difficult to find the neutral continuity in balanced or no load situation because you will have same current in three phases and zero current in neutral, how will you come to a decision?

Its about a continuity of a voltage line, did you ever thought about how a fuse blown indicator works !! that may be the solution...

 

[milind.a.kulkarni]

Yes,

I even agree with Venkadesh_M as the basic of current sensing assumes that there is some flow of current let it be hall effect current sensor or Current Transformer(CT).....however in the sensing method one of difficulty is identify the proper value of threshold against the noise level.... However....the problem is continuity detection of wire......

with regards,

milind

 

[ftsolutions]

If the 3 phase loads are PERFECTLY balanced, there would be 0 current in the neutral line, and, if you had current sensors on all lines, then your logic could detect that and know of the perfectly balanced situation and expect to see '0' current on neutral line. If the loads are NOT perfectly balanced, there would be some current in the neutral line, which the logic would check to verify it is there. IF it is '0' then there is an open circuit. But, in the case of a perfectly balanced load with '0' neutral line current, I agree that just monitoring the power current in the 4 lines wouldn't detect an open circuit on the neutral line as long as the load was perfectly balanced. This would be a fairly rare condition in many systems, but one which could certainly happen, and especially with the possibility of high noise to filter against as well.

If you modulate a distinct frequency signal onto the line(s) it might be done, but again there may be significant noise to sift through.