Undervoltage protection on service entrance switchgear

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mshields

mshields

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Boston, MA
I've got a 3200A double ended switchgear serving a hospital. I'm wondering what to spec for the mains and the feeders for undervoltage protection.

My assumptions are these that a) it should only be required on the mains and that b) should trip the breaker on a time delay - never instantaneous.

Do you agree with that and if so, what would you set at both in terms of pick up and delay.

Is there any value in adding to the feeder breakers?

Thanks,

Mike
 
mshields said:
I've got a 3200A double ended switchgear serving a hospital. I'm wondering what to spec for the mains and the feeders for undervoltage protection.

My assumptions are these that a) it should only be required on the mains and that b) should trip the breaker on a time delay - never instantaneous.

Do you agree with that and if so, what would you set at both in terms of pick up and delay.

Is there any value in adding to the feeder breakers?

Thanks,

Mike
Click to expand...

I wWould talk to the Utility to find out what kind of reclosers they employ upstream, 3 sec. and 70% are fairly common recommende values. I can't thimk of any benefit on the feeders.
 
I have concerns with undervoltage protection on mains, especially for a hospital which probably has an emergency generator and maybe several automatic transfer switches (ATS).

Imagine power going out for five seconds, the mains trip open and the diesel starts and the ATS's transfer within 10 seconds. 30 minutes later the power comes back but the transfer switches that get normal power from the Main-Tie-Main switchgear can't sense normal power available, because the Mains are tripped. The generator is forced to run until the hospital maintenance staff notices the neighbors have power and the utility is back on. Then they have to manually close the 3200A main(s) to get the ATS's to transfer back to normal power and let the generator shut down.

Without 27 undervoltage protection on the mains, the return to normal operation is automatic, the normal power returns when the utility comes back. No breaker operations are needed.

There are good reasons for using 27 protection. But look at the complete system, the level of expertise of the facility, and how the system will operate. A comprehensive automatic emergency control system may be beneficial or it may be best just to keep it very simple. Maybe trip if under voltage is in the 20%-70% range, and inhibit tripping if all three phases are <20%, indicating a utility outage.
Just some things to consider.
 
What I typically see at the Medium Voltage level is that the PTs for the MAIN Breakers undervoltage protection are located on the line side. When bus PTs are used for the 27s, they tend not to allow the MAINS to close on good voltage. I have seen many creative solutions to avoid re-cabling the PTs.

Usually, in my case the switchgear is the service entrance equipment & the utility is heavily involved. There is almost always a TIE Breaker & some type of automatic transfer with good line seeking. The trend in the last few years is a manual transfer back to normal.
 
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Excellent input

Excellent input

Thanks very much. At the hospital I'm looking at, they want to control when they go back to the normal source. Still, I'll discuss this with them.

Thanks,

Mike
 
Hospital Electrical Distribution System Design

Hospital Electrical Distribution System Design

I agree with Bob. Don't try to re-invent the wheel. Look at typical hospital 1-line drawings. All of hospital loads are usually divided up into different categories base on the nature of the loads: Critical, Life-Safety, Non-critical, etc. Most of this is explained in the NFPA 99 documents and tons of other literature out there on the subject (just Google NFPA hospital standards.) By nature the Transfer Switches (ATS) are designed to sense the undervoltage condition, start the gen(s) and transfer the load to the good source. This is done selectively if the LV condition is internal to the facility and only affect that part of the system. If the condition is system wide (utility related) then all the ATSs will sense the LV and make the decision to transfer the load based on their drop out and pickup settings (typicaly: 80% DO and 90% PU of nominal.) With the newer type ATSs, they can be programmed to be automatic to genset, then manual on return to utility power. With the older units you can always cheat and set the return timer to 100 hrs (typical is 30 min) which would give the operator plenty of time to control the re-transfer back to utility.
Another issue to consider is load shedding: the decision to dump certain non-critical loads (HVAC, lighting, etc) should the genset become overloaded or with the case with mutiple gensets; one gen runs out of gas and shuts down. I think I have some good articles/case studies on the subject that I can list if you are interested :)
 
UV or not to UV

UV or not to UV

I'm sold. Doesn't make sense in a hospital and for that matter probably any system with a Generator and ATS's.
 
I once did a project where the spec called for undervoltage trip on EVERY motor starter.

There must have been hundreds or thousands of motors in the plant.

I can only imagine the nightmare of trying to reset them after they all trip.

I think we eventually convinced them to put an UV trip only on the feeder to our equipment.
 
mshields said:
I'm sold. Doesn't make sense in a hospital and for that matter probably any system with a Generator and ATS's.
Click to expand...

There are times when you want the normal power turned off, with a manual restart. I know of a data center that wishes they had employed a 27 function.
As Laszlo mentioned, if the utility is using reclosers it can cause some problems due to their repeated opening then closing. You could experience equipment crashes based on trying to shut down then immediately trying to restart as well as power quality issues which may be present until the fault is cleared.
 
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jim dungar said:
There are times when you want the normal power turned off, with a manual restart. I know of a data center that wishes they had employed a 27 function.
As Laszlo mentioned, if the utility is using reclosers it can cause some problems due to their repeated opening then closing. You could experience equipment crashes based on trying to shut down then immediately trying to restart as well as power quality issues which may be present until the fault is cleared.
Click to expand...

Normally a critical load such as a UPS, VFD or motor has UV protection built into the device or MCC starter.
 
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