Multi-voltage power system design 2

[Kuratong007]

My prospect is asking me to find the most efficient and cost effective way to supply electricity for various loads with different input voltages.

Primary voltage from utility: 34.5 kV
Load 1 : 1,140 V 3phase
Load 2 : 440 V 3phase
Load 3 : 380 V 3phase
Load 4 : 230 V 3phase
Load 5 : 230 or 220 V 1phase

My plan is to recommend 4 step down transformers:
34.5 kV - 1,140 V
34.5 kV - 440 V
440 V - 380 V, Y
440 V - 230 V, Y

End to end length of the project, which is a tunnel, spans 550 meters long so part of my recommendation is to relocate this whole system every 175 meters so that wires needed will not be too long.

Addition to the engineering challenge is that the prospect is more interested with pole-mounted transformers than pad-mounted possibly due to their price difference. I still believe bias over the other is not yet final specially if I can properly recommend an efficient and cost-effective system.

I know the scheme I imagined can solve the variation of voltage but I'm not quite sure if it is the most efficient one. Also, I feel like there are more economical suggestions than providing 4 transformers. Unfortunately, my existing knowledge and experience can't tell yet.

I am asking for your experience and wisdom. What did you do during the time you experienced same or similar multi voltage requirements?

 

[TugboatEng]

You should be able to feed your 380, 230 3p and 230 1p all from the same transformer.

Transformers do catch fire from time to time. Is it good practice to put them inside the tunnel? I think a bank on each end would be safer.

 

[bacon4life]

Placing 4 sets of 4 transformers sounds pretty strange. How much power is needed at each voltage level? It is often more economical purchase equipment with voltage rates that match the available voltages.

Some folks don't open random hyperlinks, so you get may more responses by editing your post to show your attachment as an image.

Pole mounted transformers use air as the isolating medium. Although have seen them mounted inside of vaults, they take up a huge amount of room due the need to keep people away from the bushings. I had assumed you would need fire resistant transformers in a tunnel, which are typically vault style rather than overhead style.

 

[davidbeach]

I don’t notice load sizes, but I’d expect dry type transformers rather than oil. But there should be exactly one voltage other than the 34.5kV.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Kuratong007]

You should be able to feed your 380, 230 3p and 230 1p all from the same transformer.

Is it possible to have a single transformer that is capable to provide all of the output mentioned? I don't think so. I would like to ask for a reference/brochure/books/articles that may provide information regarding this specific reply.

Transformers do catch fire from time to time. Is it good practice to put them inside the tunnel? I think a bank on each end would be safer.

I am also thinking to do the same but I'm restricted with the cost. Imagine the cost of having 4 banks on one end and 4 on the other end. Having said that transformers do catch fire, may I know what are other factors that will contribute to this risk except overloading? To minimize risk of catching fire, I already recommended that transformers must be utilized at no more than 80% of its capacity only.

It is often more economical purchase equipment with voltage rates that match the available voltages.

There are equipment that was already been used from a different project and will now be brought to use again for this project. For the 1,140V equipment, they said that this is the only voltage rating available from manufacturer.

Although have seen them mounted inside of vaults, they take up a huge amount of room due the need to keep people away from the bushings. I had assumed you would need fire resistant transformers in a tunnel, which are typically vault style rather than overhead style.

This is a valuable input! Thanks for this. I now know that the tunnel will be located approx 200m below ground. Installing overhead style underground vs vault/pad-mounted style underground is one of important decision I need to make and there lot of things to consider such as this that I failed to notice. May I ask your more about your experience? What's the advantage of one over the other with respect to this situation?

I don’t notice load sizes, but I’d expect dry type transformers rather than oil. But there should be exactly one voltage other than the 34.5kV.

My initial recommendation is pad-mounted, liquid-filled transformers but got immediately turned down because of cost. They are more willing to install-and-relocate the whole system of poles and pole-mounted transformer than to purchase pad-mounted transformers. If I'm going to recommend dry-type transformers, "mobility" will be my reason and most likely be rejected as well due to same reason stated above. Is there any reason to justify this recommendation?

 

[bacon4life]

Where is this located? This proposed installation sounds quite strange compared to my local practices. Advice from those of us in the USA might not match your local practices.

What sort of tunnel is this? I am having trouble picturing what sort of tunnel as room for 10 meter tall poles, plus additional height for adequate clearance between energize parts and the ceiling.

In the USA, worker safety rules prohibit non-electrically qualified workers from approaching within about 3 meters of an air-insulated power line. Typically a worker with a tools in their hands can reach about 3 meters wide. This means pole mounted equipment needs to be at least 6 meters from any wall/ceiling so that that the is room for future maintenance (i.e. painting, window washing) of the surface. By contrast, cables and padmounted transformers have no clearance requirements.

You would also need separate breakers/fuses for each transformer. Although not hugely expensive, it does add cost, complexity and space requirements. If you do end up having to use multiple transformers, using an autotransformer instead of a regular two winding transformer can be more cost effective for turns ratios of less than 3:1.

 

[JezNZ]

Some thought from recent tunnel projects:

- It is typical practice to provide all major equipment at the ends of the tunnel. Think about access in emergency situation, it is not easy to get to the middle of a tunnel!

- it is typical practice to provide multiple voltage levels, usually a lower voltage for light & small power services and a higher voltage for mechanical services (jet fans/pumps etc)

- I have never seen so many voltages provided. It is a good idea to look at the loads and the rationale for requesting so many voltage levels.

- Regarding pole-mount vs pad-mount. I suspect whoever is requesting this is not familiar with 'true costs'. Space constraints are at a premium in a tunnel environment, including at the services buildings at either end. The cost of the ancillary requirements will be much more.