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345kv vs 500kv

345kv vs 500kv

(OP)
For areas where 345kv is sufficient for both capacity and distance, is there any advantage to using a 500kv bulk power network? Such as having higher Kv equipment but at a lower current rating, ie lines, cables, 550kv 2000amp 40ka breakers vs 362kv 3000amp 63ka breakers.


I know that is very broad question- like asking what ocean life or bacteria might evolve into billions of years from now- but any standing specifics or general facts that come to mind such as cost difference? I'm all ears.

RE: 345kv vs 500kv

I would be looking at at least two factors.
1. The cost of the 345 kV equipment vs the cost of the lower rated 500 kV equipment.
2. The estimated losses of both systems: Both I2R losses and corona losses.
There may be other factors but that may be a good starting place.

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

RE: 345kv vs 500kv

(OP)
Good points.

Any known values between 345kv and 500kv equipment for those who have experience in pricing? I know little about the price of 500kv equipment.

RE: 345kv vs 500kv

Don't forget to add in consideration of the voltages already available. If the buses on both ends are already at 345kV, then any savings in downsizing the equipment in amperage may be swallowed up by the transformation costs to get to 500kV on both ends.

RE: 345kv vs 500kv

For what you're describing I can't imagine any scenario where the 500kV gear would be cheaper. I think it would be significantly more expensive. There are a tonne of variables involved here and not all of them are technical. Also, you are assuming fault currents go down with higher voltages, but that's not necessarily true so I doubt you'll be using lower rated breakers.

RE: 345kv vs 500kv

(OP)

Quote (Marks1080)

For what you're describing I can't imagine any scenario where the 500kV gear would be cheaper. I think it would be significantly more expensive. There are a tonne of variables involved here and not all of them are technical.


Any idea by how much?


Quote:

Also, you are assuming fault currents go down with higher voltages, but that's not necessarily true so I doubt you'll be using lower rated breakers.

Talk to me more about this. Perhaps we are thinking different scenarios.

RE: 345kv vs 500kv

No idea how much extra. There's probably as many variables within the RFP process than anything technical to influence price. But, generally speaking the cost of insulating for higher voltages doesn't go up proportionally, I think it goes up geometrically (inverse square law right?). Also, right of ways have to be increased, so how much for that real estate? How much for the extra steel in the towers? Are there labour issues working on larger towers that you were unaware of? You probably want to use larger transmission cable, because why the hell would you limit a 500kV system by undersizing the cable.

Also, the costs that go into a power system have to be recovered by the loads. If a 345kV system is the appropriate one to use for an area I dont think your customers will be thrilled to find out they're paying off a 500kV system instead.


Regarding the Fault Currents:
Fault currents depend on the available power to supply the fault. A typical bottleneck would be a transformer, for example (but any piece of equipment could be your bottleneck)... a 345kV unit will likely have a lower MVA rating than a 500kV unit, therefore the 500kV transformer will be able to deliver more power to a fault. This is all very general. I think for a larger system it would be fair to say (as a rule of thumb) that fault currents probably get higher at higher voltages - but this is not always true. It really is system dependent. On a small system I believe you would be correct - in general the 500kV faults would be of lower magnitude, but that's just due to the small system itself being the bottle next in terms of power available to deliver to a fault. Usually a the reason we have a HV system (500kV +) is to marshal out huge amounts of generation around an area, so USUALLY there will be a tonne of available energy to feed the fault. If your system is small enough a fault could take it out completely, vs. having a large system which could continue to feed a fault without major consequences. As far as I know this isn't possible or realistic for a 500kV system. I'm in North America - part of NERC and NPCC. From what I know about the entire north eastern power grid in north america and sustained 500kV fault has a high potential to take out the grid.


RE: 345kv vs 500kv

(OP)

Quote (marks1080)

because why the hell would you limit a 500kV system by undersizing the cable.


But why oversize the cable if load of that level is never anticipated?



Quote:

Regarding the Fault Currents:
Fault currents depend on the available power to supply the fault.


Correct- but imagine a system where the entire bulk power system is either 345kv or 500kv interconnecting many 1,200MW generating stations and load clusters miles apart. Won't the 345kv fault current always be higher near generation (and usually most everywhere) as apposed to the 500kv? Same generation- just one case those same MVA GSUs pump out more current under any condition.



Quote:

If your system is small enough a fault could take it out completely, vs. having a large system which could continue to feed a fault without major consequences. As far as I know this isn't possible or realistic for a 500kV system. I'm in North America - part of NERC and NPCC. From what I know about the entire north eastern power grid in north america and sustained 500kV fault has a high potential to take out the grid.

I don't think thats entirely a fair analysis IMHO- couldn't a sustained fault on a 345kv line do the same if the bulk of generation is connected at that voltage level? My understanding is that in the upper eastern portion of North America such as NY state and New England the entire system is 345kv as apposed to 500kv. Those areas have very limited support from 500Kv.

RE: 345kv vs 500kv

I agree with your counter points. It's very much system specifically dependant, which makes general conversation difficult.

I do believe that most of the time (95% +) what you're suggesting would not be cost effective.

RE: 345kv vs 500kv

(OP)
But Mark, I think you gave a perfect real world example we can work with. Picture if most- or rather all of the 345kv in NY and New England was 500kv.

RE: 345kv vs 500kv

My assumption, without any real data, would be that if the entire NY/NE 500kV system was magically converted to a 345kV system the grid would collapse because of way too many variables to list.... You'd be affecting system impedance in such a way that I doubt the existing 'normal' load flows would stay constant, or within an acceptable range. Start messing with the major load flows in the north east and your system will go down - see 2003 blackout.

Honestly, the most cost effective thing we could do today to lower the cost of power to the end user would be to eliminate the market structure that's been created. This entire layer of the industry really doesn't do anything except dramatically increase the cost of providing power. That's a tough talk to have with the MBA decision makers who run todays system, all who depend on this unnecessary layer of 'Corporate Vars*' to get a paycheck.



* - I wish I could claim credit for the term 'corporate vars.' I stole it from a colleague :)


RE: 345kv vs 500kv

If my recollection is correct, NY has their 345 kV and 765 kV as well, so it is not all 345 kV transmission.

RE: 345kv vs 500kv

(OP)
@Magoo2: correct, but 765kv is limited, mostly as an interconnection to Canada from my understanding. 345kv is most of the bulk backbone, even for the NYC area.


@Marks1080: Perhaps, without taking for or adjusting for any other variables beforehand. Maybe I am wrong, However I would theorize that 500kv is more likely to survive a 2003 type disturbance vs a 345kv back bone. In fact I would argue that is why PJM faired better with their 500kv system- and why 765kv took off in the Ohio area latter.

Second load flows on the 115kv, 138kv and 230kv systems would decrease due to the lower impedance of the bulk backbone. Although fault current may go up at this level- however for the sake of this discussion I think we can ignore that.

But from your real world experience, you can say that 500kv equipment in of itself (circuit breakers, isolaters, CTs, VTs) will always cost more than 345kv?


Quote (Marks1080)

Honestly, the most cost effective thing we could do today to lower the cost of power to the end user would be to eliminate the market structure that's been created. This entire layer of the industry really doesn't do anything except dramatically increase the cost of providing power. That's a tough talk to have with the MBA decision makers who run todays system, all who depend on this unnecessary layer of 'Corporate Vars*' to get a paycheck.



* - I wish I could claim credit for the term 'corporate vars.' I stole it from a colleague :)


I think we can both agree here. And maybe get rid of NERC requirements/ government over-site. However I will leave my opinions at that for now.

"corporate vars"- I will have to use that someday :)

RE: 345kv vs 500kv

"But from your real world experience, you can say that 500kv equipment in of itself (circuit breakers, isolaters, CTs, VTs) will always cost more than 345kv? "


Yes.

RE: 345kv vs 500kv

(OP)
Making note- thanks :)

RE: 345kv vs 500kv

Seems like voltage class of the lower voltages also matters since typical voltage classes are separated by a ratio of 1.732 to 3. For example 115 kV, 230 kV, and 500 kV or 69kV, 138 kV, 345 kV, and 765 kV are common system designs. In a system with only 138 kV and 500 kV options, there would be a very large jump in both capacity and cost when deciding which voltage to utilize.

RE: 345kv vs 500kv

(OP)
@Bacon4Life: Excellent point and worth mentioning. Typically 500kv is stepped down to no less than 161kv, often 230kv too. Similarly in Europe 400kv is usually stepped down to no less than 132kv. Reason being is that auto transformers become more costly (basically they become a standard 'isolation' unit in size/cost) when the step up/down is more than 3x.

RE: 345kv vs 500kv

There's a wealth of transmission unit costs to be found in Link

Table 2-1 shows line budget costs per mile for each voltage.
345kV = $1.34M/mile
500kV = $1.92M/mile

Tables 3.1, 3.3, 4.4 show similar higher costs for 500kV substations vs. 345kV substations


RE: 345kv vs 500kv

(OP)
^^^ That paper is gold, much thanks :)


The cost for a 115/500kv vs 230/500kv auto has me surprised. Can someone explain?

RE: 345kv vs 500kv

Mbrooke: I'm assuming you're surprised because the costs are so similar?

It makes sense for an Auto... as there is only one winding. So for a 500/230 or a 500/115 unit still has the same 500kV main winding. The biggest cost in the unit is the core, and I believe either of these auto's would have the same core. The minor price differences probably come down to secondary equipment and insulation ratings.

RE: 345kv vs 500kv

(OP)
Yup- similar costs. I was taught that when you step down from more than 3:1, that the cost of an auto transformer starts to increase. Maybe this is wrong however- but its what I have always held in my limited knowledge base.

RE: 345kv vs 500kv

Marks1080- Although an auto has one electrically continuous winding, they may have more then one physical winding that are then electrically interconnected. The 230/115 kV autotransformer we recently purchased actually had 4 physical windings: Tertiary, Common, Tap/LTC, and HV. Each of the 4 windings was separately created using different kinds of wire. During assembly, the four windings were all placed concentrically around each other with space between each winding for paper insulation and oil ducts.

In distribution substation transformer purchases, we have recently found that a 40 MVA transformer is a small adder rather than twice the cost of a 20 MVA transformer. Thus I suspect there is a lot of nuance detail that couldn't be captured in the B&V study.

RE: 345kv vs 500kv

(OP)
Could this be because 40MVA units are so common?

RE: 345kv vs 500kv

We purchase custom built to order transformers of varying secondary voltages and sizes, not just 40 MVA units.

RE: 345kv vs 500kv

(OP)
I know- but 40MVA is very common in the POCO world. 40-60MVA makes the bulk of all modern transmission to distribution transformers.

RE: 345kv vs 500kv

Quote (Considering a line at 550kv 2000amp (~1,732 MVA) vs 362kv 3000amp (~1,792 MVA))

.

At 345 kV a double circuit is required vs. at 500 kV a single circuit can handle the desired MVA.

See the line capability with few scenarios with single vs. double circuit with bundle conductors.

RE: 345kv vs 500kv

bacon4life: Thanks for that information. I haven't had as many opportunities as I would like to see the insides of transformers. Early on when I was training I got to climb inside some smaller units, but never a big Auto.

I've always considered the main winding to be one giant winding. So a 500/230 unit would have the same size primary winding as a 500/115 unit.

I remember hearing conversations a few years back now about the global steel supply affecting transformer costs. I know we have had to take a second look at how we deploy 2nd harmonic blocking because we were having nuisance trips that came back to the core quality of the units. Do you have any insight on that conversation? I think it was pretty much a cost thing if i remember correctly (newer transformers cheaping out with lower quality iron)....

RE: 345kv vs 500kv

(OP)
^^ x2

RE: 345kv vs 500kv

Mbrooke: I just caught your comment above regarding a 500kV system riding through a fault better than a 345kV system. I see where you're coming from here: A larger system has more 'inertia' therefore can ride through a fault easier. However, the opposite is actually true on a system level. From the perspective of the system as a whole the 500kV fault is much more damaging for stability.

I've heard of an instance where, after maintenance, a set of three phase grounds were accidentally left on a 500kV line in Ontario. When operators closed the line in they closed it into a directly bolted, three phase fault. The worst kind you can get. The protections managed to clear everything with no major damage but during the 50 or so miliseconds the fault was in there were measured frequency deviations through the entire system - from northern Ontario to Florida. The 500kV system is beefier, but its analogous to cutting a major artery vs a capillary. the consequences are also much worse. I think how a system rides through high voltage faults has more to do with the inertia of it's lower voltage generators (usually around the 13.8kV level). A system with a few very heavy machines should withstand a fault better than a system with many lighter machines.

Do we have any system stability experts here that can weigh in?

RE: 345kv vs 500kv

In general, a short transmission line up to 50 mi with a transfer capability rating of ~1,700 MW in single circuit operated at 500 kV is more cost effective than a double circuit line rated for 345 kV. See below for additional information.

RE: 345kv vs 500kv

(OP)
Cuky2000, Excellent perspective and to be honest this what I was looking for. Not to say the other comparisons aren't just as good.


Quote (Marks1080)

Mbrooke: I just caught your comment above regarding a 500kV system riding through a fault better than a 345kV system. I see where you're coming from here: A larger system has more 'inertia' therefore can ride through a fault easier. However, the opposite is actually true on a system level. From the perspective of the system as a whole the 500kV fault is much more damaging for stability.

That makes perfect sense- I was thinking of the inertia from a larger system type perspective.


Question though- if more, yet smaller circuits are run (more impedance for any conductor) will this help increase system stability in that any fault will draw less current or such a practice has little effect?






Quote:

I've heard of an instance where, after maintenance, a set of three phase grounds were accidentally left on a 500kV line in Ontario. When operators closed the line in they closed it into a directly bolted, three phase fault. The worst kind you can get. The protections managed to clear everything with no major damage but during the 50 or so miliseconds the fault was in there were measured frequency deviations through the entire system - from northern Ontario to Florida. The 500kV system is beefier, but its analogous to cutting a major artery vs a capillary. the consequences are also much worse. I think how a system rides through high voltage faults has more to do with the inertia of it's lower voltage generators (usually around the 13.8kV level). A system with a few very heavy machines should withstand a fault better than a system with many lighter machines.



But don't smaller machines around 250MVA tend to have more inertia than large machines (2,000 MVA)? I only ask this because of stuff I have read in documents regarding critical clearing time. For example in Florida a nuclear generating plant had generators of such size that they had to modify the breaker failure design because the CCT was so short.

RE: 345kv vs 500kv

Mbrooke: It's really difficult to say without having an actual system to model. I think in general using more 'smaller' circuits will give you better system diversity, but higher overall system impedance. It's all kind of relative too depending on how many voltage levels are in any particular system. Basically, whatever the highest voltage level a system has, that part of the system will be most critical in terms of overall system stability - not always true, but generally it is. So if the highest voltage you have in your system is 115kV for example, than the 115kV part of your system is most critical. For this discussion ignore any HV DC stuff. Those circuits are pretty well isolated from the rest of the AC system from the valves, however sometimes just losing a large line can cause stability issues, not just faulting the line.

Everything I know about generation (which isn't much) says that large units are large in both physical size, MVA and electrical/mechanical inertia. A unit's inertia directly related to its mass as far as I know. But there are others here that can give a better answer. But I think it's safe to say that a system full of large, heavy nuclear units is a more stable system than one made of of many more, smaller gas turbines. Not to say there isn't an advantage to the gas turbines, but we are just talking about one thing

Cuky2000: Your chart is true, but misleading. Load carrying capability is only one variable. You also need diversity, which usually means a double circuit regardless of voltage level.

RE: 345kv vs 500kv

The 500 kV winding would have 50% more turns for a 115/500 kV transformer than for a 230 kV/500 kV transformer, though I suppose it might use the same type of conductor for the HV winding. The 500 kV voltage class would set the requirements for insulation levels, factory cleanliness and assembly tolerances. There are also fewer factories capable of designing and manufacturing 500 kV equipment as compared to 230 kV equipment. I don't have an idea of how the number of manufacturers compares for 345 kV versus 500 kV equipment.

The higher levels of second harmonics are actually due to better quality core steel. Shifting to higher quality steel has dramatically reduced core losses, but does have the drawback of higher ratios of harmonics to fundamental. I don't know whether the actual amount of harmonic current has gone up, or whether the fundamental current went down resulting in a higher ratio of harmonics.

In addition to size of unit, the type of generator has a big influence on inertia and transient performance. Small hydro units have more per unit inertia than gas turbines. Gas turbine output is highly dependent on RPM, so they have tend not to be much help stabilizing the system during a frequency disturbance. However, unit inertia is a different issue from what happens during fault for 345 kV vs 500 kV system. Two factors push for high voltages having short CCTs. First, the per unit impedance of lines decreases as voltage increases, so a larger geographic area is impacted by a fault. Second, typically there are fewer redundant lines at high voltage, so a larger portion of the post fault transfer capability disappears by isolating the fault.

RE: 345kv vs 500kv

(OP)

Quote:

Cuky2000: Your chart is true, but misleading. Load carrying capability is only one variable. You also need diversity, which usually means a double circuit regardless of voltage level.


But under NERC, double circuit is basically treated as a single circuit. I also know of many cases where lightning (and even trees) simultaneously took out both circuits.

RE: 345kv vs 500kv

So does that mean you think double circuits are a bad idea?

RE: 345kv vs 500kv

Double circuit construction is a great way of getting multiple lines through a constricted right-of-way corridor, but you have to plan on both being out at the same time. If they're parallel lines that may be much harder to deal with than if they're two unrelated line that just happen to be going in the same general direction. Like a 230kV line on one side of the tower and a succession of different 115kV lines on the other side of the tower.

Two singles, each capable of carrying the total load, on different paths will always be more reliable (and more expensive) than one double circuit line with half the total necessary capacity on each side.

RE: 345kv vs 500kv

Mbrooke, we are moving out off the original question, to explore any advantage of using 500 kV considering that a 345 kV is sufficient for both capacity and distance. The intention of the loadability is to show that for the given ampacity and voltage rating the 345 kV require double circuit and single circuit for a 500 kV. So the graph was useful to show that 500 kV could be lower cost in many instance.

For a new T. Line in a network, many other factors need to be considered such as system reliability and contingency. For example, if a sudden fault in one circuit how this will impact the system stability a readjustment of the power flow. I do not see any data in the post to make any comment.

Still the curve provide other inside info for the postulated cases:
  • The 345 kV double circuit operate closer to the thermal rating of the OH conductor
  • The 500 kV line operate in the voltage drop region
  • For fault in one circuit, the 345 kV could use larger bundle conductor and the 500 kV will require another redundant circuit
  • Line series compensation for short line do not provide significant advantage for 345 kV or 500 kV
  • To prevent or mitigate overload in parallel lines, FACTS or other systems are options to increase reliability but add capital cost


  • RE: 345kv vs 500kv

    (OP)

    Quote (Cuky2000)

    Mbrooke, we are moving out off the original question, to explore any advantage of using 500 kV considering that a 345 kV is sufficient for both capacity and distance

    I would disagree- double vs single circuit (as well as CCT) is very relevant to this conversation. If contingency planning and system operators must view a double circuit line as a single circuit, then the 500kv single circuit option becomes a lot more attractive, even cheaper as presented in another post.


    Thank you again for the graph- this helps a ton and its exactly what I had in mind.


    Quote:

    For fault in one circuit, the 345 kV could use larger bundle conductor and the 500 kV will require another redundant circuit

    To prevent or mitigate overload in parallel lines, FACTS or other systems are options to increase reliability but add capital cost




    Assume a meshed system with plenty of generation re-dispatch. Any N-1 is covered, even N-1-1 and most N-2s without any thermal or voltage violations.

    RE: 345kv vs 500kv

    There were some discussions on transformer prices. Let me respond to them.
    1)Core size of a transformer depends on MVA rating and not on kV.In case of auto-transformers,core is not the costliest item,but copper in the windings.(approx30%) But with generator transformers,core will form maximum percentage.
    2)With modern steels, second harmonic content in excitation inrush current has come down .Earlier days it was as high as 20-30%. But today it is only 5-10% causing occasionally mal-operation of differential relay during switching on the transformer.
    3)Cost of transformers vary as (MVA)raised to 0.75.ie Cost of a 200 MVA will be only 1.7 times the cost of 100 MVA transformer.With voltage variation,but for same MVA ,it varies as kV raised to 0.3-0.5 ie a 200MVA 500 kV unit may cost 1.45 times that of 220KV.But in reality this may vary because of the cost of OLTC.
    4)When we compare a 100 MVA 200/132 kV two winding and auto-transformers, the auto-transformer losses and core-winding size will be only that of a 40MVA two winding unit (100x 220-132/220) This is because the power transferred through core is only 40MVA and balance 60 MVA will jump in to secondary through the galvanic connection between HV and LV windings.But the cost of a 100 MVA auto in reality will be more than 40 MVA two winding unit, as it requires 3 poles of OLTC,tertiary winding etc.So the price difference between auto and two winding comes down when co-ratio (HV-LV/HV)goes up.This why you will not see auto-transformers with a voltage ratio more than 3:1.
    5)Generally, a 500kV will be better than 345 kv line,esp if we expect increased load flow in future.One reason is the BIL (basic impulse level) has drastically came down thanks to zinc oxide lightning arresters.BIL of first 400kV line in Sweden was 1950kV,today it is 1300kV or less.In India, we thought 1200 kV lines will be required in near future.But distributed generation changed the whole situation and it may never be necessary.

    RE: 345kv vs 500kv

    An example of a small lighting transformer capacity when used as an auto-transformer at different voltage ratios may be helpful.
    Consider a 10 KVA transformer rated 240:12/24 Volts used as an auto-transformer.
    The current rating of the secondary windings is 10000 VA / 24 V = 417 Amps
    This is two 12 Volt windings so the current rating at 12 Volts is 833 Amps.
    When the secondary windings are in parallel the KVA rating is 240 V x 833 A = 200,000 VA or 200 KVA
    When the secondary windings are in series the KVA rating is 240 V x 417 A = 100,000 VA or 100 KVA
    As the secondary voltage drops in relation to the applied voltage, the capacity of a given size of transformer tends to drop also.

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

    RE: 345kv vs 500kv

    Parallel circuits don't have to run on the same towers. For smaller systems this will be common, but for large systems you can often run your parallel (double circuits) on different towers.

    PRC - Thanks for the transformer information. Very interesting!

    RE: 345kv vs 500kv

    PRC-Thanks for the correction on second harmonics.

    Quote (But under NERC, double circuit is basically treated as a single circuit. I also know of many cases where lightning (and even trees) simultaneously took out both circuits.)


    While NERC TPL-001-4 does have an additional performance Category of P7 for double circuit towers, Category P6 requires utilities to consider the loss of any two transmission lines. The difference between P6 and P7 is that P6 allows manual system adjustments between the two outages and P7 assumes the outages happen simultaneously.

    Additionally, TPL-001-4 requires utility's to simulate extreme events including loss of all transmission lines on the a common right of way and then to evaluate possible actions to mitigate the consequences. There have certainly been instances within WECC of fires taking out multiple circuits on the same right-of-way.

    On some tubular steel double circuit towers, it is challenging to work on one circuit while the other circuit is energized. On older lattice towers, there seems to be much more circuit to circuit clearance.

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