Compressor In-rush During ATS Transfer

[ron]

ASCO 940 Automatic Transfer Switch with in-phase monitoring.

Hospital tests ATS with generators running to reduce outage time to loads. If the in-phase monitoring is functioning properly, the transfer happens only when the sources are within 10 degrees of sync.

With the belief that the sources are sync'd, the ATS has an open transition condition (break before make) for about 20-30 electrical cycles.

Would you expect significant inrush on a typical air compressor to blow fuses sized at 2 x FLA?

It is a new install and it happens randomly on occasion from utility to gen and from gen to utility.

 

[retirede]

What type of compressor?
Does it try try to restart immediately? I would expect the compressor control system to prevent restart until it is completely unloaded. If this were true, the restart after ATS shift should be no different than a normal startup.

 

[ron]

What type of compressor?
Does it try try to restart immediately? I would expect the compressor control system to prevent restart until it is completely unloaded. If this were true, the restart after ATS shift should be no different than a normal startup.

It is running when this happens, so it continues to try and run.

It normally has a reduced voltage starter, but the break before make of the ATS occurs too fast for it to recognize it as an outage, so it tries to ride through, and in the process, it experiences an inrush which blows the fuses.

 

[texie]

It is running when this happens, so it continues to try and run.

It normally has a reduced voltage starter, but the break before make of the ATS occurs too fast for it to recognize it as an outage, so it tries to ride through, and in the process, it experiences an inrush which blows the fuses.

Assuming the in phase transfer is actually working correctly, is it possible that the compressor motor is is sized so close to the load that during the transfer it heads toward stalling and drawing enough current to blow the fuses?
I guess the most elegant fix would be to use a true closed transition ATS, but that is probably not very appealing. Seems like a compressor load could tolerate being off line for the period of transfer. Maybe set up some controls to shed the compressor during transfer and then it will restart unloaded and thru the reduced voltage starter.

 

[Ingenieur]

is it on a contactor or solid state starter?
you mentioned soft start but does it xfer to a contactor

when interrupted the motor becomes a generator
it is decelerating
so it has a torque slowing opposite the nml direction
when power is applied it has to counter this torque
could be significant 'starting torque'

 

[Jraef]

It is running when this happens, so it continues to try and run.

It normally has a reduced voltage starter, but the break before make of the ATS occurs too fast for it to recognize it as an outage, so it tries to ride through, and in the process, it experiences an inrush which blows the fuses.

Not surprising. As mentioned, the motor is likely still still holding onto flux in the core, so when re-connected the motor acts like a generator but the regen power in the motor is out of sync with the line power, so it's like connecting two generators out of sync and boom!


I suggest you either:
1) shut down the compressor when the ATS is engaged, then turn it back on again after the ATS is done, by putting in a Restart Delay Timer in the compressor control circuit (you should have one anyway)
or
2) make the transition time longer on the ATS. It likely takes a few seconds for the motor field to fully collapse in the compressor, so set the transition delay for 3 seconds, unless that causes other problems, in which case revert to option 1.

 

[Ingenieur]

Not surprising. As mentioned, the motor is likely still still holding onto flux in the core, so when re-connected the motor acts like a generator but the regen power in the motor is out of sync with the line power, so it's like connecting two generators out of sync and boom!


I suggest you either:
1) shut down the compressor when the ATS is engaged, then turn it back on again after the ATS is done, by putting in a Restart Delay Timer in the compressor control circuit (you should have one anyway)
or
2) make the transition time longer on the ATS. It likely takes a few seconds for the motor field to fully collapse in the compressor, so set the transition delay for 3 seconds, unless that causes other problems, in which case revert to option 1.

I like 1

the comp likely cycles to fill a tank
on the occasion it is running during xfer it blows the fuses
other times tank full, not running, no blown fuses
if it has a tank a small restart delay should be ok

 

[texie]

Not surprising. As mentioned, the motor is likely still still holding onto flux in the core, so when re-connected the motor acts like a generator but the regen power in the motor is out of sync with the line power, so it's like connecting two generators out of sync and boom!


I suggest you either:
1) shut down the compressor when the ATS is engaged, then turn it back on again after the ATS is done, by putting in a Restart Delay Timer in the compressor control circuit (you should have one anyway)
or
2) make the transition time longer on the ATS. It likely takes a few seconds for the motor field to fully collapse in the compressor, so set the transition delay for 3 seconds, unless that causes other problems, in which case revert to option 1.

I don't think these are options for the install ron is describing. This is an issue with in phase monitoring type switches designed to limit total outage while testing and both sources are live. 1) A resart timer won't work because the voltage is not fully removed to trigger it. 2) Making the transition longer defeats the purpose of having the in phase transition.

 

[Ingenieur]

something like this
operates < 1 cycle of 20% drop
http://m.littelfuse.com/~/media/pro...s/littelfuse_timedelayrelays_ta_datasheet.pdf

they make adjustable versions
quite common

another, 5 sec off, 6 cycle loss, adj 50-480 vac
http://www.ia.omron.com/data_pdf/cat/k8ak-vw_n182-e1_2_3_csm1020912.pdf

 

[don_resqcapt19]

Install a closed transition ATS.

 

[ron]

It is an existing ATS and a new compressor.

I was hoping that 30 electrical cycles without power would be fast enough that there wouldn't be Boom, as described.

This is a single actuator ATS, so it is either in one position or the other, no off position (Delayed transition would have been my preference if we could have specify this from new).

I suspect I need to find a fast response 3 phase voltage protection relay that will take the compressor off line for several seconds so the contactor based reduced voltage wye-delta starter can recognize the outage. Maybe Allan Bradley Bulletin 813S Voltage Monitoring Relay
https://ab.rockwellautomation.com/C...r-Protectors/Monitoring-Relays/Phase-Monitors
I wonder if it is sensitive enough to pickup a quick transient like 15 cycles without power.

 

[Ingenieur]

It is an existing ATS and a new compressor.

I was hoping that 30 electrical cycles without power would be fast enough that there wouldn't be Boom, as described.

This is a single actuator ATS, so it is either in one position or the other, no off position (Delayed transition would have been my preference if we could have specify this from new).

I suspect I need to find a fast response 3 phase voltage protection relay that will take the compressor off line for several seconds so the contactor based reduced voltage wye-delta starter can recognize the outage. Maybe Allan Bradley Bulletin 813S Voltage Monitoring Relay
https://ab.rockwellautomation.com/C...r-Protectors/Monitoring-Relays/Phase-Monitors
I wonder if it is sensitive enough to pickup a quick transient like 15 cycles without power.

funny, does not give a response time
the 2 I posted do

 

[ron]

funny, does not give a response time
the 2 I posted do

Yeah,
I don't know why.

 

[ATSman]

It is an existing ATS and a new compressor.

I was hoping that 30 electrical cycles without power would be fast enough that there wouldn't be Boom, as described.

This is a single actuator ATS, so it is either in one position or the other, no off position (Delayed transition would have been my preference if we could have specify this from new).

I suspect I need to find a fast response 3 phase voltage protection relay that will take the compressor off line for several seconds so the contactor based reduced voltage wye-delta starter can recognize the outage. Maybe Allan Bradley Bulletin 813S Voltage Monitoring Relay
https://ab.rockwellautomation.com/C...r-Protectors/Monitoring-Relays/Phase-Monitors
I wonder if it is sensitive enough to pickup a quick transient like 15 cycles without power.

Ron,
Unfortunately you are caught between a rock and a hard place. The solution we use in this situation is to employ a motor disconnect circuit that ASCO calls Selective Load Disconnect Control, accessory 31F. See page 5-6 of the ASCO Operator Manual 1D4980R9 for more info. The ASCO 940 is an older model that was sold in the '80 up to around the mid '90's. At that time this accessory was provided by a separate add-on panel wired to the Group 7 controller board which is no longer sold. This type of load disconnect is also used to signal elevators (elev pre-signal) in high rises prior to and after transfer to emergency (see attached.) This feature is only active on a Hot Bus - Hot Bus transfer: during a test panel test or on re-transfer to normal after a power failure.
Since ASCO's pat answer was to replace the ATS with the newer controller that had the feature built in, we designed a circuit panel (simple control relay and timer) and modified the controller wiring to provide this feature.
We also used a control panel sold by Kohler (see pic.)
Depending on the size (amperage) of the ATS it was usually cheaper to modify it than to replace it.
So your options are to call in a specialty controls contractor to modify the existing ATS or replace the ATS. All newer designs have this feature that is accessed on the display panel of the controller.
As others mentioned, a closed transition ATS will also work but they are pricey. You will have to decide the cheaper route.
What is the amperage of the ATS?

 

[ron]

What is the amperage of the ATS?

1200A, 480V with a Group 5 controller

 

[ATSman]

1200A, 480V with a Group 5 controller

Ok, that changes the whole picture. Sorry I did not ask earlier but the info I gave still applies to the old group 7 controllers. Looks like yours was upgraded to the 7000 series group 5 controller. If so then you already have the motor disc/ elev presignal option available in the new controller. All is needed is to activate it in the program and connect the motor starter run contact to the to the TB provided with the group 5 controller.

 

[ron]

Looks like yours was upgraded to the 7000 series group 5 controller. If so then you already have the motor disc/ elev presignal option available in the new controller. All is needed is to activate it in the program and connect the motor starter run contact to the to the TB provided with the group 5 controller.

It was upgraded a few years ago.

I also was withholding information, in that the operations team will refuse to modify anything in the ATS even to allow a TB to be extended and to have the presignal option turned on. There are lots of politics going on and the other HVAC type loads fed by this ATS seem to be able to survive this testing sequence for many years and it is only these 3 new compressors that are having this problem.

That's why I was trying to understand if the inrush with the loss of 15-30 cycles of power is reasonable with in-sync sources during the transfer, and since it sounds like it is, now it is a matter of finding good 27 protection (undervoltage protection) that will drop voltage long enough for the wye delta starter to recognize an outage to start in the typical sequence.

 

[rlundsrud]

Since this condition only occurs during testing when both utility and Generator power is on, why can't you just shut down the compressors. During a normal utility outage this wouldn't be an issue. I don't see how using a closed transition would be effective unless a pending power loss was anticipated and the generators were spun up in advance.

Just out of curiosity, what kind of compressor Motors are we talking about? These aren't maglev Motors or anything like that are they?

 

[retirede]

Since this condition only occurs during testing when both utility and Generator power is on, why can't you just shut down the compressors. During a normal utility outage this wouldn't be an issue. I don't see how using a closed transition would be effective unless a pending power loss was anticipated and the generators were spun up in advance.

I like it!

 

[ATSman]

It was upgraded a few years ago.

I also was withholding information, in that the operations team will refuse to modify anything in the ATS even to allow a TB to be extended and to have the presignal option turned on. There are lots of politics going on and the other HVAC type loads fed by this ATS seem to be able to survive this testing sequence for many years and it is only these 3 new compressors that are having this problem.

That's why I was trying to understand if the inrush with the loss of 15-30 cycles of power is reasonable with in-sync sources during the transfer, and since it sounds like it is, now it is a matter of finding good 27 protection (undervoltage protection) that will drop voltage long enough for the wye delta starter to recognize an outage to start in the typical sequence.

Ok, but you are not "modifying" anything in the ATS. You are simply using the the available presignal/ motor disconnect feature of the controller that is normally applied to this application to solve a specific problem.
Sounds like you have a "mindset education" problem rather than a technical problem.