mbrooke
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True, though I would think that a 500kv underground right of way would get more attention in mapping.
Technical Feasibility of Underground AC Transmission Cable Grid
- Thread starter mbrooke
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True, though I would think that a 500kv underground right of way would get more attention in mapping.
petersonra
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- Northern illinois
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- engineer
there used to be a MV line on poles that ran down the middle of the street I lived on. when they final got around to rebuilding the street they buried the MV lines in plastic conduit under the sidewalk. No idea what voltage it is.
I will attempt to answer your question from a technical perspective and disregard the cost aspect as you requested. Cost data for constructing various voltages of overhead and underground transmission lines is readily available online and doesn’t need to be rehashed here.
You raised the first issue to work through, which is the use of shunt reactors to compensate for charging current due to the high shunt capacitance of the cables. Reactors are used to supply charging current to the line capacitance so the power system doesn’t have to supply the charging current - essentially a way to increase the efficiency of the transmission circuit by minimizing the losses that have to be supplied by generation.
The next limitation is that the cable itself has to carry the charging current in addition to the power we are trying to move through the circuit. Each cable design has a continuous rating that is calculated from heat transfer equations based on the thermal conductivity of the surrounding soil and other things that may heat the soil, like other nearby cables. For a particular cable design with some calculated continuous current rating, that current rating will be taken up partially with cable charging current and partially with load current. So the longer the cable is, the more charging current it will have to carry at the ends, then a smaller portion of the ampacity rating is available to carry load current. In the extreme case, if we make the circuit length long enough the charging current will be equal to the circuit ampacity, leaving no copper available to carry load current. From a practical perspective an economic study can be done to determine the relationship between maximum load current and length for each cable design. If we consider a particular cable and find that it can’t carry the amount of load current want for the required distance we can try selecting a larger cable.
At some point we will still run into some limits where we cannot move the desired amount of power the distance it needs to go. The next option is selecting cables with less charging current - DC! Of course this requires converter stations at both ends of the line, which are large, expensive, and require maintenance. With DC links right now there are essentially no power and distance limits - reliability and redundancy concerns tend to crop up and cause us to select multiple smaller lines before DC link technology constraints limit the capacity of the circuit.
As to the viability of an all cable transmission system, some folks are headed in that direction. There are some countries and areas where it is virtually impossible to get approval for an overhead transmission line so underground has essentially become the standard. Offshore wind generation plants use extensive networks of 66 kV collector circuits and transmission level export cables. Some of the export cables have lengths and power requirements in excess of what any AC cable can provide so offshore DC converter stations are being installed and a DC subsea export cable is used. We have made some conceptual studies of more extensive all cable transmission networks and they appear to be technically feasible.
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You raised the first issue to work through, which is the use of shunt reactors to compensate for charging current due to the high shunt capacitance of the cables. Reactors are used to supply charging current to the line capacitance so the power system doesn’t have to supply the charging current - essentially a way to increase the efficiency of the transmission circuit by minimizing the losses that have to be supplied by generation.
The next limitation is that the cable itself has to carry the charging current in addition to the power we are trying to move through the circuit. Each cable design has a continuous rating that is calculated from heat transfer equations based on the thermal conductivity of the surrounding soil and other things that may heat the soil, like other nearby cables. For a particular cable design with some calculated continuous current rating, that current rating will be taken up partially with cable charging current and partially with load current. So the longer the cable is, the more charging current it will have to carry at the ends, then a smaller portion of the ampacity rating is available to carry load current. In the extreme case, if we make the circuit length long enough the charging current will be equal to the circuit ampacity, leaving no copper available to carry load current. From a practical perspective an economic study can be done to determine the relationship between maximum load current and length for each cable design. If we consider a particular cable and find that it can’t carry the amount of load current want for the required distance we can try selecting a larger cable.
At some point we will still run into some limits where we cannot move the desired amount of power the distance it needs to go. The next option is selecting cables with less charging current - DC! Of course this requires converter stations at both ends of the line, which are large, expensive, and require maintenance. With DC links right now there are essentially no power and distance limits - reliability and redundancy concerns tend to crop up and cause us to select multiple smaller lines before DC link technology constraints limit the capacity of the circuit.
As to the viability of an all cable transmission system, some folks are headed in that direction. There are some countries and areas where it is virtually impossible to get approval for an overhead transmission line so underground has essentially become the standard. Offshore wind generation plants use extensive networks of 66 kV collector circuits and transmission level export cables. Some of the export cables have lengths and power requirements in excess of what any AC cable can provide so offshore DC converter stations are being installed and a DC subsea export cable is used. We have made some conceptual studies of more extensive all cable transmission networks and they appear to be technically feasible.
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mbrooke
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Do you have any idea at what length the charging current becomes impractical for various voltages and wire sizes?
.
Ic = V/Xc = V×2πfC.
example.
132 kV line with a cable current capacity of 1020 amps, .18 microfarads/km, 136.6 km
Ferranti effect also has to be considered with long cables
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mbrooke
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Does this hold true for all cable sizes of the same voltage?
same size cable core will have lower capacitance with higher voltage.
cables with a larger core, (higher ampacity) will have higher capacitance at the same voltage
The ferranti effect is going to limit your lengths before charging current limits IMO.
mbrooke
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Thanks
Do you have a list of various capacitances for various sizes and voltages?
Can ferranti be mitigated?
Shunt reacotors, but then resonance could become a problem.
Increase loads on the cable is the simplest way
mbrooke
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Can you go into more detail on resonance?
Looks like operators might have to switch out cables light load periods or conditions.... but that is less likely, since a storm would not remove 80% of the distro.
here’s some reading. Too much typing…
mbrooke
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Anything about CCVT ferro? Would simple shunt resistors on the secondary voltage circuits be required or enough?
Open Neutral
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Don't the coax's need dielectric liquid cooling pumped through them. I ran into a crew at a PEPCO sub & chatted with their underground engineer. They were upgrading the pumping/cooling system for their 69KV feeders.
Literally, he was so specialized he didn't even know of another sub with only overhead feeds that I mentioned.
Literally, he was so specialized he didn't even know of another sub with only overhead feeds that I mentioned.
mbrooke
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Some do, other I know are high pressure fluid filled cables that rely on the oil being at a high pressure to maintain dielectric strength.
To me solid dielectric looks best, but then again I'm saying that with next to no underground knowledge. Everything I know about is overhead.
drcampbell
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- The Motor City, Michigan USA
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- Registered Professional Engineer
Everything's a tradeoff.
Solid dielectric has the greatest capacitance.
Gas or liquid filling is self-healing. When a solid dielectric gets a pinhole or other small defect, it progresses and becomes a large defect.
Solid dielectric has the greatest capacitance.
Gas or liquid filling is self-healing. When a solid dielectric gets a pinhole or other small defect, it progresses and becomes a large defect.
mbrooke you were looking for a job on the east coast right?
You should move out west and get a job at one of the POCO's out here;
the bean counters at the POCO here are comparing the cost of rebuilding an entire town that burns in a wild fire (caused by overhead lines) and not to mention legal bills for manslaughter charges and investor relations
to
the perceived 'high' cost of underground lines.
That increases the 'cost' of doing things the old way.
You should move out west and get a job at one of the POCO's out here;
the bean counters at the POCO here are comparing the cost of rebuilding an entire town that burns in a wild fire (caused by overhead lines) and not to mention legal bills for manslaughter charges and investor relations
to
the perceived 'high' cost of underground lines.
That increases the 'cost' of doing things the old way.
mbrooke
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Indeed, the old way all to often crosses the line into life and property:
I think it will only be a matter of time before the east coast finds themselves in the same shoes as the west coast
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