Evaluation of 34.5kV vs 13.8kV Distribution Voltages

[rockman7892]

We are working on developing a standard distribution design from customer owned HV substation to facility distribution. Currently we are evaluating us of 34.5kV vs 13.8kV for this distribution. I wanted to see if there were any rules of thumbs or thresholds based on system capacity and incoming HV utility feed that are typically used for evaluation.

This secondary distribution is fed from secondary of a transformer with a 72MVA base rating and HV incoming of either 138kV or 230kV.

Is there a particular limiting factor with HV incoming that dictates needs for secondary voltage. For example step from HV to 15kV transformer is much less common and unavailable then stepping to 34.5kV?

In terms of capacity customer is evaluating going from (5) smaller rated 30MVA transformers at 15kV to (3) 80MVA transformers at 34.5kV. Obviously a cost savings with less transformers (I believe) as well as Equpment sizing but wanted to see if there were any obvious things I'm overlooking that make 34.5kV an unattractive option? This is a new build.

 

[jghrist]

Is this a single industrial system? Utility distribution to multiple customers? Residential, commercial, industrial, or combination? Urban? Suburban? Rural?

15 kV class equipment is more common and available than 34.5 kV.

 

[rockman7892]

jghirst

This is for a data center site. Customer owns HV Substation (fed from nearby utility owned substation) with secondary feeding into data center facility distribution.

 

[Paulsey88]

rockman
A big deciding factor will be location and number of points of delivery/utilization. How many locations on this site will the distribution voltage need to be stepped down again to a utilization voltage? 34.5kV to utilization voltage (480, 120, etc.) transformers are much less common and more expensive than 15kV equipment, as mentioned by jghrist.

 

[davidbeach]

Depends on where you are and what capacity the data center is looking for. That (3) 80MVA looks like a 150MVA load with the capability of feeding the whole load on two. Becoming quite common.

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

 

[cranky108]

Distribution 34 or 13kV are quite common, and are mass produced. But with current delivery times an intermediate 34 system may or may not be possible in the time frame you may be looking for.
Actually a 34 line in general can transmit at least 30 MVA, depending on the conductor, so it is possible to get more with larger conductor. But if you are using switchgear the limit maybe 3000 amps for the bus rating. Yes you can go higher, but so does the cost.
Best is look at different systems, and then price, and find delivery times.
That's why you are there.