Grounding using FCDIST module of CDEGS

[rezaa]

I am looking for the actual 12 kV (Star) ground fault current flowing into a grounding system using FCDIST module of CDEGS. What I have is SLG 6025 A at LV distribution side (12 kV) and 2382 A at HV side (66 kV Delta ). The distribution side has three phases and a single neutral cable. HV side is without OHSW. Having these information,how can I model model LV side using FCDIST?

 

[jghrist]

You need to know the substation grid resistance (normally provided by the MALT module), the configuration of the distribution lines and transmission lines (faulted phase and neutral/shield size, material, and location), the average distance between neutral/shield grounds, the average footing resistance, the length of the lines, the average soil resistivity, the grid resistance of the substation at the end of each line, and the source current of each line.

I usually model distribution lines as a typical length with the "end station" grid resistance equal to the average pole ground resistance. For 69 kV faults (the worst case for faults in the substation), the source current of distribution lines is zero.

 

[rezaa]

I am aware of all above data. The question is that do I need to start one tower away from the 69 kV station and assuming this tower as a central station and the 69 kV station as one of the terminal stations with the only source current 6025 A and the other distribution lines as terminals without any sources and calculate the ground fault current flowing into a grounding for this case? Do the same thing for the second tower away and calculate the ground current and so on to find which tower directs the maximum current into ground.

 

[jghrist]

The source of the 6025A at 12 kV is the transformer which is connected to the grid. For a fault in the station, none of this current flows through the earth. Since it doesn't flow through the earth, it doesn't contribute to GPR, step-voltage, or touch-voltage, and does not need to be considered.

I'm not sure what you are getting at by considering one 69 kV tower away from the station as a central station. The worst case fault will normally be a SLG fault on the 69 kV bus in the substation. The substation will be the central station. The substation at the end of the 69 kV line will be a terminal. The current of importance is the "Total Earth Current" at the central station (your substation). This is what causes GPR, step-voltage, and touch-voltage at the substation.

I don't know why you want to "find which tower directs the maximum current into ground." FCDIST does calculate the current into earth at each tower, but not all of this flows in the substation grid. At towers close to the central station, the earth current flows from the shield wire into earth and ultimately to the terminal substation. At towers close to the terminal station, the earth current flows from earth into the shield wire to get to the terminal station ground grid. Unless you are worried about potentials around the towers, tower earth currents do not need to be considered.

I suggest that you go into CDEGS Help and look at the section "FCDIST: An Overview".

 

[rezaa]

In the case of having a good grounding (a low foot tower resistance) outside of 12kV distribution side. The amount of current flowing via towers to soil can be in the order of the worst case fault(a SLG fault on the 69 kV bus in the substation). For this reason, I would like to compute the contribution of fault current 6025 A at towers near to near to distribution side.

 

[rezaa]

By the way, I mean one tower away from 12 kV side not 69 kV. It was my mistake.

 

[jghrist]

For a 12 kV fault on a distribution line, treat the faulted pole as the central station as you said. There will be 2 terminals, one being the 69/12 kV substation (source) and the other being the end of the faulted line (no source). You will find that the earth current and GPR for faulted poles near the station will be low because practically all of the fault current will return through the neutral. Another way to think about it is to consider that a fault one pole out will be practically the same as a fault in the station. A 12 kV fault in the station will produce no earth current and no GPR.

The worst case for GPR at distribution poles may be for a 69 kV fault at the station. In this case, FCDIST will give GPR and earth current at each pole. I recently did an analysis of a facility on a distribution line about a mile from the substation. The 12 kV fault level at the facility was 2919A, of which 46A returned through the earth, causing a 1460V GPR. For a 6700A high side fault at the substation, the earth current at the facility was 470A, with a 11070V GPR, almost as high as the GPR at the substation. This may have been an unusual case because of the relatively high (5.2 ohm) substation grid resistance and high earth resistivity (1220 ohm-m).

 

[rezaa]

The high GPR of 11070 V was due to transfer GPR of the station to the facility through the shield wire.

 

[jghrist]

The high GPR of 11070 V was due to transfer GPR of the station to the facility through the shield wire.

That is essentially the case, but the calculation was based on determining the amount of current in the neutral and earth along the distribution line. The GPR was a result of the calculated earth current at the facility (a remote terminal) and the ground resistance at the facility.

 

[rezaa]

The neutral conductor from 12kV to facility has small resistance compare to Grid resistance(5.2) and facility resistance than can be of order 15 ohm. So most of fault currents circulate within phases and only small fraction goes via soil. So, the GPR of the station due to fault at HV side can be transferred to facility location.

 

[jghrist]

rezaa,

I agree that the GPR due to fault at HV side can be transferred to the distribution facilities. In the case I investigated, this was a much worse case than 12 kV faults. That is why I cautioned that considering faults on the 12 kV line (which I think pertained to your original question) may not be sufficient.