Stray Voltage in MCC

[raj151857]

Dear Sir,
Am facing a problem in MCC Motor feeder not getting stoped from LCS. It was detected that the multicore control cables coming from LCS is having stray voltage in the range of 150V-270V with some mA.The distance of LCS from substation is in the range of 1-2 KMs. This stray voltage is not allowing contactor to drop as contactor VA may be less. I had grounded all the spare cores but problem does not solved. By accident I had grounded one end of snubber circuit (capacitor only)to ground & problem is solved. Now I want to asked experts how to calculate the capacitor value wrt to distance of cables & other parameters . I request please help me by sending link or formula from which I can calculate the value of capacitors.
Please find attach scheme.

 

[dpc]

What is the control voltage?

For long ac control circuits, this problem (contactor not dropping out) is generally caused by the capacitance in the long control circuit.

It is not stray voltage, but capacitive coupling. The standard solution is to convert the control circuit to dc voltage using interposing relays.

 

[rockman7892]

raj

like dpc mentioned, what you are seeing is likely a result of coupled capacitance on this long cable run. Aside from swiching the circuit to dc as he mentioned you can use a shunt impedence like you mentioned.

You mentioned measuring 150-270V. Does this vary when measuring each time? What voltage do you see when when the wire in question is connected to the contactor coil? What voltage do you see on the wire when this wire is lifted off of the contactor coil?

Basically you have to determine the impedence of your contactor coil, and then back calculate an estimation for the impedence of the capacitance coupling between wires in the circuit. From these two impedences, the situation basically becomes a voltage divider, and you can calclate a shunt impedence to put in parallel with the coil in order to lower the coil input impedence. Lowering the coil input impedence will cause more of the voltage to drop across the capacitance coupled impedence and lower the voltage measured on the coil

Start with trying to measure the different voltages I mentioned, and find the total impedence of you coil.

 

[raj151857]

Sir,
Control Voltage is 240 V 50 Hz Ac. You are correct It is not stray voltage, but capacitive coupling. I want to know how coaxial cable capacitance can be measured,Am interested in formula & calculations. There are many site which can calculate Cable capacitance, but am interested to calculate myself.
Dear Mr. Rockman,
Volatage vary in different feeders that why i had mention 150V-270V. The voltage i had measured by digital multimeter was 253V on diconnect from Contactor.
Sir. Can any one send link where i can get formulas & methods to calculate cable capacitance.

 

[dpc]

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680079349

Try this link.

 

[raj151857]

Dear Mr.dpc, Thanks for sending link, but it does not solve my problem.Sir, am looking for formulas & calculations to find cable capacitance myself. Please help .

 

[davidbeach]

You'll have better luck measuring the capacitance than calculating it, but even then it probably isn't going to be a value that remains constant over time. Just accept that you need to use a dc control voltage.

 

[dpc]

Calculating capacitance of a long run of conductors randomly situated in a conduit with varying amount of moisture in the dielectric is not a simple calculation.

You can make an approximation using capacitance calcs in any handbook.

As David Beach indicates, measurement would be more accurate. But knowing the capacitance is not going to help solve your actual problem.

 

[waross]

Measure the current. From that you may make an assumption of the X/R ratio of the insulation system and reverse engineer the capacitance value.
BUT, once you have the capacitance you will use that plus an assumption to calculate the leakage current and select the appropriate correction measures.
SO, why waste time and add two assumptions when you are going to base your correction on the current you have just measured.
And don't forget once you have determined the value of the capacitance, it may change by a factor of more than 2:1 in a few days.
Just measure the induced current and allow a generous safety factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[raj151857]

Dear Sirs,
I do agree what u said. Still am requesting the formulas for academic purpose for cable capicatance calculations . Can any body provide me.

 

[waross]

If you can't find the basic formula for capacitance in your textbook maybe you shouldn't be here. This forum is for professionals.
Google is your friend.
The Capacitance in Farads equals the facing Area of the conductors in square meters times the Dialectric constant, divided by the separation between the conductors in meters.

C=A*d/D

C=Farads
A=Area in square meters
d=dialectric constant
D=distance between the conductor surfaces

good luck.

Bill
--------------------
"Why not the best?"
Jimmy Carter