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Capacitive Charging On Ungrounded Systems

02/15/2017 5:27 AM

In order to find the fault current in ungrounded systems, if many cables are used in series it is normal that the distributed capacitance is considered as single lumped capacitance to find the charging current. But one of my collegue as says the fault current flows as the below picture. Any comments.

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#1

Re: Capacitive charging on Ungrounded systems

02/15/2017 6:55 AM

You should ask your friend why he thinks that the positive sequence currents seek a different route than the negative sequence currents; then look at a good book on Symmetrical Component Theory.

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#2

Re: Capacitive Charging On Ungrounded Systems

02/15/2017 10:47 AM

If this is a 3 phase AC circuit, I would think you would have 3 capacitors as an equivalent circuit for the parasitic capacitance, delta connected between the three phases.

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#3

Re: Capacitive Charging On Ungrounded Systems

02/15/2017 12:18 PM

Whatever you hope, this looks like a bad idea about to happen.

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#4

Re: Capacitive Charging On Ungrounded Systems

02/15/2017 2:08 PM

As a personal exercise why don't you calculate an example phase to phase or phase to ground fault current and compare it to the miniscule capacitive component current obtained from the lumped cable capacitive element (which will be lower in real life because the cable capacitance is distributed over the length of the line).

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#5
In reply to #4

Re: Capacitive Charging On Ungrounded Systems

02/15/2017 2:42 PM

I would say many orders of magnitude difference in current.

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#6
In reply to #5

Re: Capacitive Charging On Ungrounded Systems

02/15/2017 4:23 PM

Well......yes, but I wanted the original poster to come to that conclusion on their own.

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#7

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 1:18 AM

I understand during single phase to ground faults, the fault current will be very low and for phase to phase faults the current will be high.

The discussion arose because of capacitive charge currents. To find the capacitive charging current theoretically, the capacitance of entire cable length from source to load should be considered, but he says the capacitance of the cable from the source till the ZCT should be considered. I fail to understand why?

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#10
In reply to #7

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 9:29 AM

Your colleague is looking at the quantity from the viewpoint of the protective equipment connected to the CT, the unmeasured offset.

The amount of heating provided by the capacitive coupling current is the total distributed current, heating that you would experience in your broken delta ground sensing equipment resistor, for instance, if you provide a circuit.

On (HV) shielded cables, the charging current is high enough to be sensed by typical zero sequence CTs, that is why you bring the shield drain wires back through the CT at the source end.

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#8

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 4:24 AM

Er, there is no fault current illustrated.

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#9

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 8:44 AM

Voltage. Cable type, Cable run length. 3 Cored or 1 Cores, Sheath type, collective screening, individual screening or both, are the 'many cables used in series' all the same cable type and configuration.

Might be of value to know.

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#11
In reply to #9

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 11:48 AM

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#12
In reply to #11

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 12:24 PM

Is this for an underground ungrounded system? Don't you think that is confusing? I think it should all be above board.

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#13
In reply to #9

Re: Capacitive Charging On Ungrounded Systems

02/16/2017 4:08 PM

For a single core screen cable .

C = e r / [18 * log e ( D / d )] ( m F/km)

where er = relative permittivity of insulation = dielectric constant

XLPE _ 2.5 er

F= Farads

D- Diameter over insulation

d = diameter of conductor

You really don't have much to worry about on these short lengths and if the screens are properly earthed.

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