345kv vs 500kv 5

[Mbrooke]

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.

 

[Mbrooke]

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.

 

[bacon4life]

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.

 

[Mbrooke]

Could this be because 40MVA units are so common?

 

[bacon4life]

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

 

[Mbrooke]

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

 

[cuky2000]

.

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.

 

[marks1080]

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)....

 

[Mbrooke]

^^ x2

 

[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.

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?

 

[cuky2000]

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.

 

[Mbrooke]

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

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?

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.

 

[marks1080]

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.

 

[bacon4life]

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.

 

[Mbrooke]

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.

 

[marks1080]

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

 

[davidbeach]

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.

 

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

 

[Mbrooke]

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.

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.

 

[prc]

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.

 

[waross]

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
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"Why not the best?"
Jimmy Carter