Loss of phase or loss of three phase generator transfer

[stevenal]

An office building served three phase through a wye/wye service transformer has a standby generator and UPS system. When service is lost, the UPS keeps the critical loads up while the generator cranks and the transfer takes place. Over the weekend, a primary phase was lost. The UPS kept the critical loads up until the battery ran down, and the generator never started. The rumor is the generator control will only sense a full three phase outage. Obviously, the desired outcome would be for the generator control to be able to sense the loss of any single, two, or three phases, start the generator, and perform a three phase transfer. My question is: Are these controls generally configurable in this manner or would a replacement be required? I cannot be model specific at this point, so just looking for the "in general" sort of response. Thanks.

 

[dpc]

I recall that some smaller ATS units had only single-phase sensing. Most I'm familiar with look at voltage in all three phases, but if motors make up a sizeable percentage of the load, loss of a phase may not be detected, depending on setpoints and technology used. There's no standard that I'm aware of so I think you will need to get documentation on the specific generator start module that is on the ATS. They are generally an optional item and sometimes multiple types are available.

Dave

 

[davidbeach]

In general, yes that capability should exist. But I've also seen many silly set ups where the customer (or customer's engineer) didn't know what they were doing. Lots of places over the years that have responded to a trip and high speed reclose (total open time 20 cycles or so) by starting the generators and auto-transferring the load to the generator, and then auto-transferring back later. So instead of a brief outage (done with while the starter motor is still trying to get in the first full turn or two of the shaft) they instead have that brief outage followed 8-10 seconds later by a brief outage as the ATSs do their thing, followed 30 minutes or so later by yet another outage as the ATSs return to normal.

So you could have a situation where they had responded too many times to faults on other portions of the power system and then desensitized things too much.

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

 

[davidbeach]

Dave responded while I was typing. Another thing I've seen that fouls up an ATS is that if that wye-wye is actually a wye-delta-wye (real or phantom delta) you may get enough voltage on the missing phase to fool the ATS sensing if it's just sensing magnitude. Need to look at negative sequence voltage in that case.

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

 

[wayne440]

When/if you can state the make and model of your transfer switch, you may get more useful answers. Phase loss sensing is an option in many brands, your switch appears to lack that option.

 

[waross]

Respectfully, I've been there, David.
While you are correct, that information is seldom available at the ATS.
It is possible for a delta to provide the third phase, but with the result of a primary neutral current in the order of the primary phase currents.
The secondary effects, if any, are more subtle.
The third phase, supplied by the delta, may be considered as a single phase transformer fed from the open side of an open delta.
The voltage may be lower because of a lower effective impedance of the third phase.
In the event that the primary is fed by a long neutral, there may be phase shifts due to voltage drop on the neutral.

I have installed quite a few three phase ATS's on circuits that shared a wye-delta bank.
With three phase sensing, the systems worked well and did come on when one phase was lost.
When the third phase voltage was supplied by a delta feed-back, when the third phase voltage was high enough for customer use, the ATS would not trip.
When the feed-back voltage was too low, the ATS would trip.
This depended on the pick-up and drop out voltages set on the ATS.

I did have one ATS problem due to a transformer in a three phase bank on the wrong tap.
The voltage on one phase was chronically low.
When the power was lost, the ATS transferred to the generator, however, when power returned, the low voltage on one phase was not high enough to exceed the cut-in point of the ATS.
The set would hang on the generator and not transfer back to the grid.
I could have adjusted the settings on the ATS control board, but I did it right and had the tap set properly on the transformer.

Back to the problem at hand.
There are phase loss relays available starting at around 100$.
I suggest selecting a suitable phase loss relay and wiring the trip circuit in series with the existing single phase sensing circuit.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[JASGripen]

Most reputable three phase ATS control boards sense loss of phase, if the ATS did not switch then the voltage was in range based on the settings input by the technician.
I would check to see what the settings are on the UPS and why it transfered without activating the generator start on the ATS.
Could have been slighly lower or higher voltages on your grid which then transfered the UPS but not the ATS.
I have seen power providers change voltages on the grid which then created problems on the ATS's as was mentioned by waross, it does not take much of a change.
It would be smart to cooridnate the UPS and ATS settings.