medium voltage

[New EE]

Are there any advantages by having electrical distribution at medium voltage vs 480 volts in a medical facility?
A medical center is trying to decide either to bring in medium voltage or 480 volts so I was curious to what
would be the advantages and disavantages of doing that vs bringing in 480 volts instead. Thanks.

 

[broadgage]

It depends on how large the facility is and on how great the load is.
In the case of many buildings over a large site, then MV is likely to be viable. Use of 480 volts in such cases would require long runs of very large cable, at substantial expense, or if allowed a number of utility services to different parts of the complex.

In the case of very large loads, even over a small area, then MV can be worthwhile, especialy if any large loads such as central chiller plant can use the MV supply directly. This saves both the cost of the transformer and the losses therein.

Presumably some form of backup power will be required ?
For a large facility, emergency generation at MV can be attractive. 4 generators each of say 2MW at MV are likely a lot cheaper than the same capacity provided by numerous small sets scattered around an extensive site.

MV is likely to be cheaper per KWH.

Drawbacks of MV include the requirement for more highly skilled operating/maintenance staff, and the risks of transformer failures.
Modern transformers are very reliable, but they can and do fail. Duplication adds substantial expense.
Transformers and MV switchgear take up probably valuable space.
The customers pays for the losses in the transformers.

 

[mayanees]

IE Reduction in MV Systems

IE Reduction in MV Systems

As broadgage wrote, the higher capacity systems typically drive the application of MV systems.
But one big advantage that isn't intuitively obvious is that MV systems are inherently safer from an arc-flash standpoint. I've attached an example of two 3750 kVA transformers, one with a 480 Volt secondary, and the other at 13.8 kV. The systems are set up for a worst-case incident energy accumualtion, using the IEEE 1584 2-second timeout value. Note that the 480 Volt system is HRC Dangerous in a huge way with 214 cal/cm^2, whereas the 13.8 kV system is HRC 2 with 4.6 cal/cm^2.

John M

 

[masterinbama]

Drawbacks of MV include the requirement for more highly skilled operating/maintenance staff, and the risks of transformer failures.
Modern transformers are very reliable, but they can and do fail. Duplication adds substantial expense.
Transformers and MV switchgear take up probably valuable space.
The customers pays for the losses in the transformers.

With modern M-T-M setups transformer failures can be worked around with ease

 

[ron]

But one big advantage that isn't intuitively obvious is that MV systems are inherently safer from an arc-flash standpoint. I've attached an example of two 3750 kVA transformers, one with a 480 Volt secondary, and the other at 13.8 kV. The systems are set up for a worst-case incident energy accumulation, using the IEEE 1584 2-second timeout value. Note that the 480 Volt system is HRC Dangerous in a huge way with 214 cal/cm^2, whereas the 13.8 kV system is HRC 2 with 4.6 cal/cm^2.

There is a bit of contention in the industry as to the use of the 2 second limit by default. It is not in the main body of the IEEE calculation standard, and many feel that is for a reason. Annex B is identified as "informative", not a standard or guide as the body is identified. When there is only 5' from the MV SWGR to the wall (working clearance), and someone is blasted backward and slams the wall it is unlikely that they will be able to get away in 2 seconds. A quote from IEEE 1584 "A person in a bucket truck or a person who has crawled into equipment will need more time to move away."

Some study specs say: Arc Flash calculations shall be based on actual overcurrent protective device clearing time. Maximum clearing time will be capped at 2
seconds based on IEEE 1584-2002 section B.1.2. Where it is not physically possible to move outside of the flash protection boundary in less than 2 seconds during an arc flash event, a maximum clearing time based on the specific location shall be utilized.

Also consider that NFPA 70E's method doesn't include the 2 second limit that I'm familiar with.

Sorry to highjack the thread with this, but I wanted to share.

 

[mayanees]

... agreed Ron.
I used that 2 second limit for comparison purposes to illustrate the difference between like systems.
I've attached another pdf that shows the resultant incident energy levels after the secondary overcurrent protective device. The ocpd's were all set similarly, relative to the ANSI damage curves, and represent typical adjustments. This further illustrates the point that MV systems have inherently lower incident energy levels.
John M

 

[ron]

... agreed Ron.
I used that 2 second limit for comparison purposes to illustrate the difference between like systems.
I've attached another pdf that shows the resultant incident energy levels after the secondary overcurrent protective device. The ocpd's were all set similarly, relative to the ANSI damage curves, and represent typical adjustments. This further illustrates the point that MV systems have inherently lower incident energy levels.
John M

What are the results on the secondary of the transformer (line side of the secondary overcurrent protective device) without the 2 second limit?

 

[mayanees]

That's a loaded question Ron!
In order for the primary device to coordinate with the secondary main, I can't get lower than the 2 second delay. Of course that's totally dependent on the adjustments to the secondary main that I've selected.
I based those adjustments on allowance of the breaker to tolerate an inrush from about 1800 kVA of small dry-type K-13 transformers that get picked up upon energization, which was from an actual power study I did recently.
That particular job had two 3750 kVA transformers with 480 Volt secondaries. One was loaded up with the 1800 kVA of dry-type transformers, and the other had two large VFD-driven motors, at around 1500 HP of load. The secondary of the one with dry-type xf loads did the 2-second timeout with the primary device, resulting from breaker adjustments to allow the inrush, with a resultant HRC Dangerous. The VFD/motors xf secondary had very little inrush, and the ocpd's were adjusted to minimize the incident energy level, resulting in a Hazard Risk Category 2.

The arc-flash advantage that MV systems have over LV systems is a direct result of the magnitude of the arcing fault current. LV systems have af currents on the order of 55% of the bolted fault current level, whereas the MV systems acring fault current is closer to 95% of the bolted fault current, resulting in much faster operation of the protective device.

John M

 

[New EE]

Thank you everyone. This is very helpful information.