11kV vs 22kV distribution voltage

[asela115]

Hi,

what are the possible reasons for selecting 11kV underground distribution system vs 22kV underground distribution system for urban area.
I know about 22kV can carry twice the load compared to 22kV.

How about impacts of pricing, fault level, stepdown from 132 kv to 22/kv etc.

Thanks

 

[PlantBoy]

...22kV can carry twice the load of 22kV.

O rly? =p

For two reasons for choosing 11kV over 22kV, one is that protection systems would be cheaper and smaller. The other is power correction of an 11kV system would require smaller devices as well.

May not seem like much on a small network but on a larger distribution scale, the savings of using a smaller voltage could be significant.

What do others think?

 

[ScottyUK]

What loads are you serving?

An 11kV system can economically carry sufficent current to handle up to about 10MVA at a reasonable distance from the primary substation. Not many urban distribution substations are in the > 10MVA range, so unless you need to supply fairly large loads and/or your loads are a long way out from the primary substation then the economics probably favour the lower voltage where everything is cheaper and smaller. As with almost all engineering questions though, the real answer is "It depends".

 

[jghrist]

There is a reliability trade-off. If you carry twice the load on a feeder, you will interrupt twice the load for each feeder fault.

 

[Mbrooke]

Ferroresonance is a greater risk at 22kv for single phase switching and protection.


As for 11kv vs 22kv its a balance between cost, load, distance and reliability.

 

[davidbeach]

Not sure where in the world you are, but distribution voltages took a very long time to cross the 15kV threshold. So, if 11kV is a reasonable voltage in your area, it has a lot more installed base than 22kV would have. From my perspective, 22 seems like an odd duck anyway; if you have to go from 15kV gear to 35kV gear, why not get a whole lot closer to 35kV? When we finally had need to go beyond our standard 13.2kV, we considered 26.4 for a bit, but went forth with 34.5kV. But that's only for a few specific customers. As jghrist points out, more, smaller, feeders means fewer customers out for any feeder fault.

 

[Mbrooke]

I thought you could get 25kv gear? 23kv has been used by some US utilities very extensively, for some since the early 60s. This was due to 22kv delta sub transmission being converted to MGN distribution allowing existing equipment to be re-used while bypassing the intermediate 22-2.4 or 4.8kv substation.

 

[Mbrooke]

Question for the OP, in your 11kv base, is the system fed via 33kv sub transmission or directly from the 132kv system? I think I know what you are trying to do...

 

[FPelec]

For a given active power, conductor cross section is significantly reduced at 22 kV, as well as line losses.
No significant differences in stepdown transformer construction and cost, both 132/22 kV and 22/ 0.4 kV.
This could lead to lower life cycle costs if compared to 11 kV.
Here 20 kV (Um=24 kV equipment) is the standard voltage for distribution; older networks at 8.4 or 11 kV are slowly being upgraded.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Mbrooke]

@FPelec:

Is the trend to bypass the 33/11kv substation and feed the 22kv system directly from the 132kv system?

 

[FPelec]

@Mbrooke,
yes, the 20 kV is fed directly from the 132kV, 150 kV or 230 kV network.



Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[Mbrooke]

Same for the 11kv?

 

[asela115]

thank you all for your valuable feedback (I should have shared following details beforehand)
this is a special project with following details. it has residential, hotels, government offices, cultural & educational centers etc. compacted in small area (a luxury compacted city build in small area).
[ul]
[li]land area - 275 hectare[/li]
[li]population -300,000[/li]
[li]total expected maximum demand - 250 WM[/li]
[li]total demand- 500 MW[/li]
[/ul]
we are to justify a suitable distribution voltage for this special project. based on the feedback and details observed so far, i have summerised below details,
advantage of 22kv
1)Reduce by 50%
[ul]
[li]space requirement for laying cables[/li]
[li]No of feeder bays[/li]
[/ul]
2)For double the capacity
[ul]
[li]Increase only by around 10% to 15% cable cost[/li]
[li]Small price difference between 11kV and 22kV panel -design the panels for 22kV and fix 12kV equipment[/li]
[/ul]
3)Low energy loss in MV network
4)Low energy loss in HV equipment- 220kV can be directly stepped down to 22kV
5)No much difference in operation & maintenance of 11kV & 22kV cables & equipment
why 33kv is not best for this kind of project
1)33kV is a sub transmission voltage not a distribution voltage.
2)Cables and equipment very expensive.
3)RMUs are very expensive and very rare.
4)Cable systems are difficult to handle, operate & maintain.
5)Need more space for switch gear and cables
6)High fault level.
7)Cable capacitance would be high and need to have reactors at GSS

are there any technical matters we should consider beyond this. Especially I'm missing with accurate pricing details of cables and RMU/Switchgears (pricing comparison of 11kV, 22kV and 33kV).
Further in some posts I saw that having higher fault levels is good when considering starting of large motors etc. where neighboring consumers won't feel much (So, is 22kV better than 11kV)
Are there any examples where this kind of projects were carried out using 22kV (instead of 11kV)

 

[Parchie]

There are lots of empirical formulas that tried to relate the demand load and the transmission line length to choose an economical transmission voltage level. Read one here.
FWIW, I tried to calculate the economical transmission voltage from your given data, assuming your source is on the periphery of the community and your substation is at the center:
area = 275 has.;
line length = sqrt(2.75 sq. km) = 1.66 km
Power demand = 500MW
Voltage = 5.5 x (1.66/1.6 + 3(500)/100)^(0.5) = 22 kV!
Coincidence?

 

[FPelec]

@Mbrooke
Yes, it is the same for the 11 kV.
@asela115,
the 22 kV would be more suitable for such an high power demand density (100 MW/ km2).




Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[asela115]

I'm so glad and thankful to you all for taking time on assisting me on this.
@Mbrooke
the 22kV or 11kV sub is to be directly fed via 132kV (or may be by 220kV).
@Parchie
I'm going through the material suggested

 

[Mbrooke]

How many customers or peak loading per feeder? I know that when I reach for 22kv distribution, I find myself using recloser loop schemes (more probable) due to the increased amount of customers per feeder. Generally for me 11kv means roughly about 1,500 average customers per feeder, but at 22kv due to the increased MW capacity per feeder and increased length (fewer substations for the same load) that often comes to about 2,500-3000 customers per feeder. As a result, good practice is to have each feeder divided into two segments with the second segment being linked into another circuit via a normally open tie recloser. That way if a fault occurs in any part of a circuit, only half the customers are left without power.


In a networked system this generally is not a concern since the intent of most networks is to have the ability to run under an N+1 or N+2 feeder outage.