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UPS Nuisantly Transfers to Bypass
5

UPS Nuisantly Transfers to Bypass

UPS Nuisantly Transfers to Bypass

(OP)
I am presently working on a problem with one of the UPS systems in the plant. There are two Single UPS systems (480-208/120) feeding two UPS buses (A and B) separately. The two separate bus circuits feeds a couple of 208VAC-24Vdc rectifiers which in turn individually feed as redundant power supplies to downstream PLCs, SIS, etc.

Of the 24Vdc rectifiers, the 24Vdc, 400A rectifier is the largest. The UPS systems always shifts to bypass everytime the UPS output MCCB is closed or this Rectifier is energized. The UPS alarm shows "Output Overload". We suspect that it was the downstream in-rush which is causing the UPS to nuisantly transfer. An idea by one of the contractor was to put a reactor on the input of the rectifier. A 5% impedance reactor was connected in series but still the UPS keep to transfering to bypass.

It is observed that the UPS output transformer secondary is wye and the downstream rectifier isolation transformer primary is delta...is it possible that the problem is also caused by phase mismatch between the UPS and downstream charger?

is there a possibility that the UPS settings may be too sensitive?

I am gathering all the data so I can post them here soon. But any help or guidance will be very much appreciated. Thank you.

RE: UPS Nuisantly Transfers to Bypass

2
Wye feeding delta is not an issue.  Does the UPS come off bypass with the rectifier on line?  You will find that your UPS can be very finicky about what loads it serves.  What has the UPS vendor said about the situation?

RE: UPS Nuisantly Transfers to Bypass

(OP)
Well, the design/construction contractor tapped on another contractor and they bought these stuff from Mitsubishi while they also bought the rectifiers from Staticon. Coordination is really hard since each one of them tries to cover up their butts.

Well, actually everytime the staticon charger is energized, the UPS transfers to bypass and indicates an "output overload" and after the staticon charger is started, it shifts back to the inverter.

First, they tried to patch holes by using a reactor but did not do any good.

Now they the contractor is suggesting phase shifting isolation transformer between the UPS output and the downstream charger input.

The contractor later on suspected that the downstream charger is in resonance with the UPS meaning, the UPS sees a zero impedance and erroneously see as a fault.

So far, everything was just suggestion and everybody points one finger to another which makes the situation worse for the client.

Do you have or does anyone had experience with this problem. I appreciate any guidance I could get.

RE: UPS Nuisantly Transfers to Bypass

So, it is in bypass momentarily for what should be a rare event; is that actually a problem?  You may well find that any "cure" is worse than the "disease".  The best "cure" would be a bigger UPS.

RE: UPS Nuisantly Transfers to Bypass

(OP)
The concern that the plant is having is when there is a plant wide power failure and the UPS may not transfer the system to the batteries since it is sensing a system fault at it's output.

RE: UPS Nuisantly Transfers to Bypass

(OP)
Is there really an issue or disdvantage if a UPS output is wye and feeds a downstream rectifier with its isolation transformer in delta...The contractor is claiming that the phase shift mismatch is causing the problem..but I am doubting (unfortunately, I can't directly explain why it isn't so).

RE: UPS Nuisantly Transfers to Bypass

Nope, no issue at all between a wye source and a delta load; millions of installations around the world have a transformer wye winding feeding the delta winding of another transformer.

RE: UPS Nuisantly Transfers to Bypass

(OP)
We are planning to rent a high speed recorder and do the energization again hoping to measure the amount of in-rush. They showed me a Fluke measurement of 20A..but I think the fluke meter is not fast enough to catch a 0.1 second in-rush. The Contractor's consultant is insisting for an isolation transformer between the UPS output and rectifier input to match the phase shifts but we have held this up for further investigaton.

If the UPS is recording "output overload" this is definitely due to the rectifier in-rush during energization or because of swithing transients right? harmonics is not an issue due to the rectifier's isolation transformer. is there any other factors that I am not aware of?  

RE: UPS Nuisantly Transfers to Bypass

Have you tried a simulated power failure? If the UPS will handle a power failure you should be okay. I would follow Davids suggestions. If the UPS hangs and won't pick up the load during a power failure you must find a solution.
The thing to do now is to throw the main switch and see if the UPS can handle a power failure. That should be done as part of the acceptance testing.
Who's paying for the tries in the dark? Renting the proper test equipment may be cheaper and quicker than guessing what power equipment to buy next.
In the meantime and to keep the contractor diverted for a day or so, you may also try energizing the transformer with the rectifiers disconnected.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hmmm thanks waross and david, I'll arrange for that. I will keep you guys updated with this.

Any more suggestions will be appreciated.

RE: UPS Nuisantly Transfers to Bypass

Based on my UPS installation and maintenance experience, I think the inrush current to the charger is too high for the UPS.  High inrush pulls down the  output voltage and the UPS does what it is supposed to do - transfer to bypass to handle the inrush current (or the fault), then pop back to normal configuration when all is well.

The alternative, as mentioned, is a bigger UPS, or a ferro-resonant style which is a little more tolerant of inrush.  (But that design has other issues.)

I can't see where the phase difference has anything to do with yoru problem. The downstream equipment has no way of telling what phase it is getting.  There is a remote, but unlikely, possibility that the parallel rectifiers have insufficient filtering on the output causing a "dip" or gap in the DC waveform that coincides with a peak of the subject rectifier/s output waveform.  So the rectifier supplies more than its share of the current.  Usually, the DC power supply has a "walk-in" circuit to ramp up to full voltage output which would make this theory invalid.  Also, the rectifier would be overloaded all of the time.

What is the size of your UPS, its current limit setting and the rating of the DC power supply that is causing the problem?

Another thought - if the UPS is transferring on inrush now, it will probably transfer when it tries to energize the interposing transformer also.  Putting a transformer between the UPS and the charger may just change the location of the inrush and not solve the problem.
 

RE: UPS Nuisantly Transfers to Bypass

(OP)
Thank you rcwilson. I am also going to check if the UPS was also properly sized. Normally, the UPS size should be calculated to consider both normal loading and during transient peak load conditions right?

I am in the process of preparing some field request to obtain all information needed to evaluate the system. I will verify if the UPS was functionally tested on mains supply outage. I will also going to get the UPS settings, operations, manual and vendor drawings to see what kind of design we are dealing here.

I will keep the forum updated with this.

For the meantime I appreciate any technical guidance I could get. Thank you for all the support.  

RE: UPS Nuisantly Transfers to Bypass

(OP)
I also would like to request any helpful links about related to our subject matter to have a quick grasp of the technical aspect of our subject matter. My deepest thanks for the assistance. I will keep everybody updated.

RE: UPS Nuisantly Transfers to Bypass

(OP)
is the in-rush possibly mitigated using series reactors? I know that series reactors are used to bring the short circuit current, but will it help for this application?

As I know, the series reactor will again be seen as an additional load by the UPS during starting and the starting voltage drop Ist*Z will even pull down the voltage at the UPS output and it will also consume power in form of a loss Ist^2 *Z

I am thinking that this will aggravate the problem rather thah mitigating it.

RE: UPS Nuisantly Transfers to Bypass

The series reactor increases the impedance of the circuit. The increased impedance reduces the current drawn. Additionally, it acts in phase with the magnetizing current and has the most effect reducing the magnetizing component of the inrush. The reactance and the reactive voltage drop is at aphase angle of 90 deg to the real current and may not have much effect on load inrush. Hence the suggestion to try to energize the rectifier transformer with the rectifiers disconnected. If you can energize without the rectifiers connected then you have a load inrush rather than (or in addition to) transformer inrush. First identify the problem, then look for a solution.
If you are able to perform this test and get back to us, we can help with a solution.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
Thank you waross. I will definitely do that.

RE: UPS Nuisantly Transfers to Bypass

nightfox, a few questions

Do both of the UPSs have the same problem?

What is the size of the UPSs and who is the manufacturer and model number? This would help as we could then look up the specs on the unit.

As to the rectifiers, do you have a single line of the units. Knowing the topology of the units will help out.

To me, the suggestion of a phase shifting transformer just seems ridiculous. Someone is definitely grasping at straws.

I'd be getting a reasonably fast oscilloscope with a decent 'roll mode' and examain the level of the inrush current.

 

RE: UPS Nuisantly Transfers to Bypass

(OP)
Yes, its odd to look at the phase shifts as a culprit. The phase shift matching is significant if you are paralleling two transformers.

I will be able to get the data soon and keep this forum posted. I have requested all vendor info and site info as well. Thank you for all the support.

RE: UPS Nuisantly Transfers to Bypass

Yes,there is a phase shift of 30 degrees between wye and delta on the same system. But, this is an issue when paralleling transformers. How would the invertor or the rectifiers sense a phase shift? The phase shift will be inherent in a rectifier designed to be delta connected to a wye network. Not an issue.  
Feeding a delta load from a wye inverter  may be a problem if the inverter has a problem with line to line loads but apparently the load is not an issue, just getting energized is the issue.
Another issue that we haven't considered is the original reactor. We have assumed that because a reactor was installed that it was properly sized. Possibly a reactor is needed but the original reactor may be wildly inappropriate.
It happens when the "Hope, Faith, and Charity" method of engineering is used.
(I Hope that this will work, but whatever, I have Faith that through the Charity of the owner I will be paid.)
Nonetheless, please continue testing. If you can get the specs on the reactor it may be helpful.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

Actually, there may not be a problem at all.

If the rectifier does have a substansial transformer at its input then you wold expect a decent amount of inrush current when it was energised. If the size of the rectifier is a decent percentage of the size of the UPS then I would expect the UPS to transfer to bypass (reserve) when the rectifier is turned on. Of course, once the rectifier is up and running and the UPS has transferred back to inverter, then any power interruptons shouldn't cause any problems.

I have seen many UPS instalations where downstream transformers are used, for harmonic cancellation etc, and in the vast majority of cases the UPS will transfer to bypass when these transformers are turned on.

I suspect you may be chasing something that really doesn't need fixing.

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hello Marke. I will be suggesting the UPS be tested further at site to determine what is causing it to sense an "output overload".

1. Energize the the 24Vdc rectifier with the converter isolated and only the input transformer is energized. If UPS do not transfer,
    proceed to item 2 below.

2. Energize the 24Vdc rectifier with both input transformer and inverter connected but the 24Vdc loads and batteries isolated. If UPS
   do not transfer, proceed to item 3 below.

3. Energize the 24Vdc rectifier with both input transformer, inverter and batteries connected but the 24Vdc loads isolated. If UPS
   do not transfer, proceed to item 4 below.

4. Energize the 24Vdc rectifier with both input transformer, inverter, batteries and 24Vdc loads connected.

I will proposing these operational tests to determine which current in-rush is making the UPS transfer to bypass (as waross suggested) and then from there propose some mitigation.

By the way, I heard from Marke about putting a DC choke coil on rectifier's converter output as an option..(in case its a load in-rush as a culprit) to mitigate peak currents. Does anyone also have further explanation on this?

Thanks for all the support.
 

RE: UPS Nuisantly Transfers to Bypass

nightfox, I take it from the choke suggestion that the rectifier is a standard six pulse style thyristor controlled rectifier. Placing a choke on the output of the rectifier is equivalent to placing a choke at the three phase input.

Draw it out. In a six pulse rectifier, two of the thyristors are turned on during each switching cycle. Now you can either put a choke at the input of the rectifier and therefore have a choke followed by a thyristor to the DC bus; or you can put the choke on the output and then have a thyristor followed by a choke.

Electrically equivalent :)

 

RE: UPS Nuisantly Transfers to Bypass

(OP)
To update everybody, we will still go to site tomorrow to get all the infro we need on these equipment and will gladly post the diagrams here.

I have said before that the UPS output transformer is wye and the downstream rectifier input transformer is delta.

Is possible to mitigate the downstream in-rush by placing a specially manufactured wye-delta transformer so designed to have a wye primary winding (in consideration to the downstream in-rush)which will draw a lesser in-rush below the overload capability of the transformer?

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/Alternativeoption.jpg[/IMG]

  

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hello gentlemen,

The UPS in question is a 100kVA, 208VAC, 288.3A, 0.98 P.F. lagging, 103.8kVA, 3Ph, 3W input and 208VAC, 277.6A, 100kVA, 0.8P.F. lagging, 3Ph, 4W ouput. The UPS rectifier component is rated for 360Vdc to charge the UPS batteries and feed the UPS inverter. The Inverter has an output overload capability of 105%~125% for 10 mins. and 126%~150% for 10seconds with voltage regulation maintained. The converter and inverters are insulated gate bipolar transistor (IGBTs). The UPS is a Mitsubishi model 9700 series UPS.

The UPS feeds two Staticon rectifier units:

1- 208VAC, 60A input and 24Vdc, 500A output Staticon
   rectifier unit, delta connected primary input X'former
   with series saturable reactor to control the x'former
   input.

1- 208VAC, 7A input and 24Vdc, 50A output Staticon
   rectifier unit, delta connected primary input X'former
   with series saturable reactor to control the x'former
   input.

It was observed that everytime the 500A staticon charger is energized, the UPS transfers to bypass. We are looking at it as an in-rush problem.

The solutions being considered is to adjust the overload setting of the UPS. The present setting was verified to be at 100%, hence measuring the 500A rectifier in-rush and adding it to the other 208VAC continuous loads will determine the %overload to overrride (if the setting allows it). My question, will this endanger the electronic circuitry and protection of the UPS?

The other solution is to consider an isolation transformer, delta-delta between the UPS output and the rectifier input. The isolation transformer may be designed with an impedance and magnetic property that will absorb or damp the in-rush from the rectifier input through it secondary winding and still maintain an acceptable value of primary in-rush at its primary winding. The purpose is also to provide electrical isolation between the UPS output circuit and the rectifier input  circuit. Will this probably work?

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/UPSVendorDiagram.png[/IMG]

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/Staticonrectifierdiagram.png[/IMG]

I appreciate any comments and guidance that would help us resolve the problem. Thank you.
  

RE: UPS Nuisantly Transfers to Bypass

Settings. I would not like to exceed the 150% 10 second rating. Have you checked the actual magnitude of the inrush?
If the series reactor didn't work, don't let the same person calculate the transformer specs.
More on the transformer;
Are you able to check the action of the saturable reactor? Is it providing any current limiting at startup or is it allowing full conduction?
Adding a second transformer to avoid the inrush of the first transformer may be counter productive. The added transformer will probably have a similar input unless it is wound with a high resistance primary. That will cost you a lot of heat losses and give poor regulation.
I'll through this out for comments from the other regulars.
It may be cheaper and easier to follow another course of action.
How about using resistors to limit the surge and then shorting out the resistors. Either an electro-mechanical contactor, back to back SCRs, IGBTs or some other electronic device.
It may be time to rent the equipment to record the current profile during turn on. Remember the difference between peak and RMS.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hi Waross,

The saturable reactors connected in series to the staticon rectifier input transformer are part of the rectifier package. These reactor is a series synchronous switch (analog to 2 anti-parallel SCR's). The power control is derived from the concept of equal ampere-turns. These reactors are used as to regulate the DC output of the rectifier.

Do you have a typical model of measuring instrument (preferably portable) that can measure the in-rush within say less than 6 cycles?

We will conduct an in-rush measurement next week. If you have a contact of a third party test group within the edmonton or within alberta, we will be happy to get in touch with them.

Did you say resistors as an option? Is this gonna be connected temporarily in series with the line? If you won't mind sharing the concept and a contact person or link where I can start with...

Thank you.

  

RE: UPS Nuisantly Transfers to Bypass

The 500Amp staticon charger has an input iso-tx that is about 20kVA right?
The inrush from an iso-tx is from 10 to 16 times nominal (so about 500-600A).
The UPS practically has a short circuit on the output during transformer energisation.
The Static switch SCRs will fire on a quarter cycle output undervotage detection.

I've had 500kVA 600V output UPS systems transfer to bypass upon energising a 75kVA distrib ution iso-tx.

If your UPS has a seperate bypass feed to it, try opening the bypass feed breaker. Now the UPS will not see the bypass availibilty and will leave the inverter to current limit the inrush rather than let utility take the hit.

You will probably see an output UV alarm but load will not be dropped.

The IGBT inverter is super fast and will react to the inrush, but if the bypass is available it will transfer the load to and from utility.



 

RE: UPS Nuisantly Transfers to Bypass

Quote:

Do you have a typical model of measuring instrument (preferably portable) that can measure the in-rush within say less than 6 cycles?
I'll let some others answer that. There are a lot of people here with more knowledge of test equipment than I.

Quote:

We will conduct an in-rush measurement next week. If you have a contact of a third party test group within the edmonton or within alberta, we will be happy to get in touch with them.

Quote:

Did you say resistors as an option? Is this gonna be connected temporarily in series with the line? If you won't mind sharing the concept and a contact person or link where I can start with...
The idea is to limit the inrush by connecting resistance in line. A short time later the resistance may be shorted out. The transformer will now be energized with two smaller inrushes instead of one large one.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

You can use the resistor in-line method to limit the inrush current, with a few caveats.

Normally with this type of circuit the in-line resistors are shorted out by a contactor or motorised circuit breaker shortly after energisation of the transformer.

With the rectifier you would need to be sure that the rectifier doesn't turn on and start drawing substansial current before the resistors are shorted out. Otherwise you'll quickly find that your resistors are now charred stumps which have released a considerable amount of smoke.

I will reiterate a previous statement.

I still think you are chasing and trying to fix a problem which 'really' doesn't cause any issues with the site reliability. Once the transformer has been energised the UPS should then operate correctly during all grid outages etc. There are countless sites around the world where it is accepted that the UPS will transfer to bypass as it first energises a load and then operate 'correctly'. Of course you still need to test that the UPS operates as expected, but that is just standard commisioning practice.

RE: UPS Nuisantly Transfers to Bypass

Come on guys, this is way too much effort to solve a non-problem.  The UPS is doing exactly what it is intended to do.  UPS systems have a self supplied fault current capability below 2 per unit and go to bypass for everything above that level.  On the assumption that this inrush condition only occurs occasionally, why saddle the system with components, and associated losses, that are only needed during inrush and provide no benefit at any other time.

When the rectifier is started, just let the UPS go to bypass for the few cycles/seconds the inrush lasts.  Months/years later when the rectifier is started the next time let the UPS go to bypass again.

Just because a "problem" can be solved doesn't mean that it needs to be solved.

RE: UPS Nuisantly Transfers to Bypass

(OP)
I understand davidbeach's and sibeen's logic on this issue. However, I also explained this to the client but it seems they are still insistent on trying to mitigate it. It may sound absurd on our part as electrical practitioners but this is not how the operations people look at it. As I agree that the bypass transfer dur to in-rush is part of the UPS designed operation, it still gives off a nuisant alarm that makes the operation folks at the control room jumping out on their seats and yeah they sure do not like the fact that this happens and are somehow concern on the fact that it may create a wrong impression aside from causing some panic to them everytime an alarm happens.

While I also agree that placing some mitigation also causes additional maintenance and "point of failure", I do intend to present them the disadvantages of making external mitigation and what I gather here are also essential to help or guide me the way (or a better way to explain to them).

The specs read as "The Inverter has an output overload capability of 105%~125% for 10 mins. and 126%~150% for 10seconds with voltage regulation maintained."

If I set the UPS overload setting to 150%, does this mean that the UPS algorithm will pickup automatically in 10seconds? Is the time to pickup already inherent to the UPS and all its protection components (fuse or breakers) already coordinated to trip within 10 seconds should a continuous overload of 150% persists (more than 10 seconds). I would like to have an understanding on this since the vendor supplied specs does not explain this? Anyone's idea is appreciated. Thank you.

RE: UPS Nuisantly Transfers to Bypass

What it means is that the UPS won't exceed 150%, and after 10 seconds it will back off to 125% and after 10 minutes it will current limit at 100%.  The inverters just don't have much overload/fault capability.

If this is really in inrush condition, lasting less than a second, why not just delay response to the alarm?  Make the control system wait until it has had the bypass alarm for more than 1 second (or what ever is necessary) before annunciating that alarm point.

RE: UPS Nuisantly Transfers to Bypass

(OP)
By the way Waross, is the sizing of the resistor (or reactor) for in-rush the same as if sizing a limiting reactor for short circuit limitation?

Taking the system P.U. impedances and then determining the P.U. R (or X) with given specified maximum allowable in-rush? What company does this type of design? Postglover supply this type of resistors?

Sibeen, in your statement for the in-line resistors, do you mean that the converter should be fitted with an input circuit breaker so that the converter be isolated when the rectifier input transformer is energized.

Waross, what is the disadvantage of setting the overload to say 150% to override the in-rush (if possible)? Trying to understand what the specs read out.

The UPS technical guy was indicating that placing an isolation transformer in between will provide mitigation and circuit isolation...however, it will be special design as I perceive it. I do hope you can expand this more.

Bottom line, if such suggested mitigation are having a lot of counter hickups that may out weigh the advantages, I will appreciate much for any further comments and guidance that would help my justify to live up with the situation and provide a written procedure for the rectifier start-up.
 

RE: UPS Nuisantly Transfers to Bypass

For a proof of concept, I would buy some replacement element coils for a residential clothes drier and cut and parallel them to get the resistance I needed.
As for settings, make sure that the UPS settings are the maximum allowable.
If you go with the resistors, I would assume a value for the energization surge (200%?) and use enough resistance to set the voltage drop to 50% at that current value. If the UPS is limiting the current to 150%, then the effective load impedance is the parameter of interest.
That way you will have two surges, one on energization and the second when you switch out the resistors, but both will be 50% of the present surge.
I'm concerned about a special design transformer. If it works I expect that it will waste a lot of energy and reduce the useful output of the UPS.
BUT we still should know the cause of the inrush. If it is a reflected load from the DC bus, and is real current, you want resistors limiting it. If it is magnetizing inrush it may be highly reactive and respond to reactors.
As it did not respont to the installation of the reactors, it may be a real current inrush or the reactors may have been improperly sized. We should know the quadrant of the inrush current before spending any money on transformers reactors or resistors.
I would prefer to solve a problem of this type with an adjustment of the present parameters. Transformers, reactors, resistors contactors. This seems kludgy if the system can be adjusted to make the customer happy as it is.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hi david,

we could have loved to delay the alarm response. The problem is that the UPS vendor do not want to change their UPS software algorithm..according to them, the alarm response is fixed as per their manufacturing standard. For the overload seeting, if I set the UPS overload say, 150% and then a continuous overload happens at 150%, does this mean that the UPS will start to pick up and then after 10seconds, it will resort to bypass? I may have mis-interpreted the term current limit...how does it actually work. (I apologize but, the manual we received does not explain this term further).

Hi waross,

What do you mean adjustment of the present parameters..do you mean the "overload" adjustment or maybe alarm time delay? Somebody even mentioned decreasing the UPS sensor gain (will this work also)?

Going back to the series resistors. Did they call these "buffer" resistors before? Is the concept actually putting the resistors in series while energizing the transformer so as to create a voltage drop on the line, consequently reducing the voltage input at the transformer primary and avoid saturating the transformer primary to decrease the starting flux generated in the core based from formula:

Flux (max) = 2Vmax / (2*pi*f*Np)

Is my understanding valid?

If ever an isolation transformer is provided to isolate UPS output circuit from the rectifier input circuit, then the isolation transformer should have a designed primary resistance (+ reactance) so as to develop a lower in-rush. The set-back of this is that, an increase primary resistance will increase copper loss on the transformer and may also create poor voltage regulation on the transformer.

is my understanding on your previous post correct? If there is a downstream in-rush on the transformer secondary, will this secondary load in-rush add up (via transformation ratio) to the primary magnetization in-rush?
 

RE: UPS Nuisantly Transfers to Bypass

(OP)
In addition, we will conduct in-rush measurements tomorrow and simulate the following scenarios:

With the UPS output breaker on:

1. Energize the downtream 500A, 24Vdc rectifier with its
   associated 24Vdc loads and batteries disconnected.

2. Energize the downtream 500A, 24Vdc rectifier with its
   associated 24Vdc loads disconnected and batteries
   connected.

3. Energize the downtream 500A, 24Vdc rectifier with its
   associated 24Vdc loads and batteries connected.

Check UPS alarm and pickup level. Keep everybody posted about this. Thank you.
   
  

RE: UPS Nuisantly Transfers to Bypass

Hi;
First, we should know the source of the inrush. Is it reactive or real? Different possible solutions.

Parameters, I meant any adjustments that may be made to the UPS. This would include anything that may be done with the rectifier unit to lessen the inrush.

Is the concept actually putting the resistors in series while energizing the transformer so as to create a voltage drop on the line, consequently reducing the voltage input at the transformer primary and avoid saturating the transformer primary to decrease the starting flux generated in the core based from formula:
Yes.   I'm not sure about the formula and I am away from my library and can't check.

If ever an isolation transformer is provided to isolate UPS output circuit from the rectifier input circuit, then the isolation transformer should have a designed primary resistance (+ reactance) so as to develop a lower in-rush. The set-back of this is that, an increase primary resistance will increase copper loss on the transformer and may also create poor voltage regulation on the transformer.
Yes. You are combining a reactance and a resistor in one unit. The losses will be permanent and the voltage regulation will be poor.
The disadvantages of the transformer are:
The extra losses and the poor voltage regulation.
It cannot be bypassed like the resistors or the reactor.
The losses will subtract from the available capacity of the UPS.
The advantages of the transformer are:.
No switching.

The UPS technical guy was indicating that placing an isolation transformer in between will provide mitigation and circuit isolation...however, it will be special design as I perceive it. I do hope you can expand this more.
The UPS guy is really the one to answer this question.
BUT, if he will gaurantee that the transformer will work that is a plus.
A heads up here. It is easy to lose sight of the overall picture when you are wrestling with a problem like this. If the next step may be buying a larger UPS to handle the losses of the transformer, consider going all the way to a UPS large enough to handle the inrush.

I once had an issue with some small control transformers. The transformers would blow the factory installed fuses if the load was connected when the transformers were energized.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
To all, can anybody explain how the "current limiting" function of the UPS works...for my benefit of understanding.

What if we leave the system as it is, and then for some reason, the bypass input supply was out of service. can we still be able to energize the staticon charger through the converter + inverter UPS path? Will it withstand the in-rush and then normalize and sends off an alarm during the event of transient overloading? I just want to check out that the inverter will still assume the transient in-rush load.

Overload capability of 150% at 10 seconds, is the 10 seconds count inherent to the UPS even if the UPS is set at 100% overload?

RE: UPS Nuisantly Transfers to Bypass

(OP)
Waross, is sizing the series reactor to mitigate the transformer in-rush similar as if you are sizing a reactor for short circuit limitation?

RE: UPS Nuisantly Transfers to Bypass

Going back to the series resistors. Did they call these "buffer" resistors before? Is the concept actually putting the resistors in series while energizing the transformer so as to create a voltage drop on the line, consequently reducing the voltage input at the transformer primary and avoid saturating the transformer primary to decrease the starting flux generated in the core based from formula:

*caveat* Lots of hand waving explanations and not much maths.

Nightfox, have a look at the small single line I have included. The resistors are placed in-line to the transformer and are shorted out by the contactor K1 after energisation of the transformer. What you need to do is size the resistors to such a size that the current into the transformer is going to be smaller than the current that will force the UPS to bypass. If, for example, we make the three resistors one ohm each, then the current into the transformer will be limited to approximately 100 amps.

By doing this the length of the inrush will be extended and may last as long as a second or so before the transformer reaches its 'steady state' magnetisation level. For a 20 kVA transformer I'd guess that this would be about 5 amps. So, the wattage of the resistor needs to be sized to be able to handle a quite considerable amount of power for a short duration and then a reasonably lower level of power (25 watts?).

The problem with this is that the rectifier must not be allowed to turn on before the contactor has closed, otherwise you will then be trying to draw maximum load through the resistors (10000 watts), this will produce smoke in large quantities. So the timing of the contactor closure must be long enough to ensure that the transformer has been substantially magnetised and short enough to ensure that it closes before the rectifier has a chance to start.


To all, can anybody explain how the "current limiting" function of the UPS works...for my benefit of understanding.

There is a difference between "current limiting" and overload operation in a UPS. The current limiting function is normally only used when the Bypass of the UPS is not available. In this case, if there is an overcurrent on the output of the UPS, the inverter of the UPS will then ramp down its voltage so that a maximum current is provided at the output. If there was a short on the output, for instance, the output voltage may go as low as a few volts to limit the output current.

Overload capability of 150% at 10 seconds, is the 10 seconds count inherent to the UPS even if the UPS is set at 100% overload?

I'm not sure what you're asking here. Whilst I've never seen a Mitsubishi UPS, this is my take on it. If the output load is >110% and <125% and the bypass supply is available, then a timer will start in the UPS and transfer the load to bypass 10 minutes later.

If the load is between 125% and 150% the same timer will transfer the UPS to bypass in 10 seconds. With a load >150% the UPS will transfer to bypass immediately.
 

RE: UPS Nuisantly Transfers to Bypass

(OP)
We finally was able to complete the site measurements at site using a high speed analyzer. We made tests with the following scenarios:

1. Rectifier is energized with DC output breaker on

2. Rectifier is energized with DC output breaker OFF (DC
   loads and batteries disconnected).

3. Rectifier is energized with DC output breaker reclosed
   after 20 minutes (to partially discharge the batteries)

Among the 3 scenarios, item 3 gave us the worse current up to 430A peak to peak. The rectifer AC input is rated for a full load of 48A at 208VAC. This results to approximately 8.9X or say 9X (9 P.U.) as inrush.

The upstream UPS in this location was having an output rated for 75kVA at 208VAC. The overload capability for 150% is 112.5kVA for 10 seconds. The inrush equates to 154.9kVA pk-pk which is 206% (2.06P.U.)! and this is not even including the other 208VAC continuous loads from then other panelboards which amounts to 56.2kVA. Adding them up to the inrush kVA will result to 154.9+56.2 = 211.1kVA, a whooping 281% which totally exceed even the 200% rating at 1 sec. This is why the UPS instantaneously transfer itself to bypass.

In sizing the appropriate mitigation, am I going to use the in-rush peak-to-peak or the equivalent in-rush RMS?

Thanks

RE: UPS Nuisantly Transfers to Bypass

I am not sure but may I make a suggestion.
The UPS rating is probably RMS. The peak will probably not be a sign wave. How about converting the UPS's RMS rating to peak and then comparing to the peak current. Use peak to zero rather than peak to peak. If this is a transformer inrush the current will be asymmetrical. Peak to peak will ignore the DC component which should not be ignored.

At this point, now that you have hard numbers, can you contact the manufacturer to determine if this is acceptable operation for this UPS? Ask in writing and for a written response.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hi Waross,

The official measurement results will be provided by the Third Party testing company tommorow. I will definitely send it to Mitsubishi including a copy of the wave form. I will scan the wave form and post it here as well.

For the mean time, do you have any canadian based transformer, current limiting reactor/resistor manufacturer for me to contact?

Thank you for your patience and assistance.

RE: UPS Nuisantly Transfers to Bypass

(OP)
Hello everybody,

We have received the measurement results and waveform for the in-rush measurements for the downtream rectifier. This rectifier is connected to a 100kVA, 208VAC, 277.6A, 0.8P.F. UPS inverter at its output. The UPS transfers to bypass everytime the large downstream rectifier is energized.

The other UPS loads in addition to this large recitifier is a 2.521kVA rectifier and a 208VAC UPS panelboard with connected load of 56.206kVA.

The downstream large rectifier is having an input of 208VAC, 60Hz, 3Ph, 60A and an output of 24Vdc, 500A and connected to a 12cells, 755Ah storage batteries.

The In-rush currents along with their respective scenarios are as follows:

1. The Recitifer is energized with its 24vdc Loads +
   batteries connected. Batteries are fully charged

   Time: 11:36:21
   Phase A: +420Arms, +594Apk
   Phase B: -560Arms, -792Apk
   Phase C: +340Arms, +481Apk

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/Inrushwaveform_Page_1.jpg[/IMG]

2. The Recitifer is energized with its 24vdc Loads +
   batteries DISABLED (Converter DC output MCCB OPEN)

   Time: 11:41:42
   Phase A: -460Arms, -650Apk
   Phase B: -620Arms, -877Apk
   Phase C: +500Arms, +707Apk

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/Inrushwaveform_Page_2.jpg[/IMG]

3. The Recitifer is energized with its 24vdc Loads +
   batteries CONNECTED (Batteries partially discharged for
   10 minutes).

   Time: 11:45:18
   Phase A: +80Arms, +113Apk
   Phase B: +160Arms, +226Apk
   Phase C: -140Arms, -198Apk

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/Inrushwaveform_Page_3.jpg[/IMG]

How can we possibly distinguish and determine how much component of the in-rush is real and how much is reaactive? I understand that  if the in-rush has a significant load (real) component, a resistor mitigation is effective, if the magnitizing in-rush is significant, then a reactor will be effective. I appreciate any further comments.

RE: UPS Nuisantly Transfers to Bypass

nightfox, I'm not sure why you think that the resistor wont work if the current is reactive.

Ohms law still applies. If you have 208 volts and 2 ohms of resistance you going to have a maximum current of 104 amps.

RE: UPS Nuisantly Transfers to Bypass

(OP)
I guess I misinterpreted something here. My bad. You're right Sibeen, the total equivalent impedance is the vector sum of the resistance and reactance, so it does not matter whether its purely resistance or reactance.

Looking into waross statement:

"BUT we still should know the cause of the inrush. If it is a reflected load from the DC bus, and is real current, you want resistors limiting it. If it is magnetizing inrush it may be highly reactive and respond to reactors."

Sibeen, do you have any comment on this so I could be further enlightened? Is this statement has something to do with the current displacement?


 

RE: UPS Nuisantly Transfers to Bypass

(OP)
Waross, if we were to size a series reactor to limit the in-rush, which measured in-rush values will be used, the RMS values or the Peak Values?  

RE: UPS Nuisantly Transfers to Bypass

I think that I should defer to one of our protection experts.
I think that there are others here with experience with current limiting reactors. I have a good idea, but I don't want to lead you astray.
For a symmetrical inrush I would use the RMS, Or 0.707 of the peak value.
For an asymmetrical inrush I would spend several hours that I can't spare just now doing some study and a lot of cut and try calculations on the DC offset issues. I would be looking at the phase angle of the AC component and trying to combine the resistive voltage drops of both the AC and DC components and the the reactive voltage drop of the AC component.
I don't have the time just now but I am sure that we have some posters here who work with current limiting reactors and will be able to supply a better answer quicker than I on the sizing of reactors on asynchronous surges.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: UPS Nuisantly Transfers to Bypass

(OP)
Thank you very much for all the guidance. I am actually, looking into several options to mitigate the issue. Although I fully support the idea that placing a proper start up procedure for the downstream rectifier and appropriate switching (since redudancy of 24Vdc supplies are in place)will tend to make the present set-up operational.

However, this is not the kind of perception that the operations are having and further convincing has to be done. I am also considering other options like placing current limiting reactors and a low in-rush isolation transformers. I will coordinate with the manufacturers of these equipment with regards to properly sizing them with the available In-rush data and continuous load data and as well as the operating limits of both UPS and rectifier. All advantages and disadvantages will be part of the study so the client will understand the operating limitations and possible hickups in choosing each option.

Waross and Sibeen, thanks for all the technical comments, it helped me understand the situation. Waross, thanks for reminding me about the UPS manufacturer with regards of getting their side with this actual in-rush data. I may post further separately on the deal of selecting and sizing reactors and hope I will also get opinions on that subject matter from the associated experts of this forum.  

RE: UPS Nuisantly Transfers to Bypass

(OP)
By the way, it is stated here that there is no issue of a UPS output transformer whose secondary is wye and feeding a rectifier whose input transformer is having a Delta primary winding.

What if the UPS output transformer whose secondary is wye is feeding a 3Ph, 4 wire panelboard and then the delta connected rectifier is fed from this panelboard via 3 wire and the panelboard is feeding other line-to-neutral loads?

Is this going to increase the inrush drawn by the rectifier input transformer?

RE: UPS Nuisantly Transfers to Bypass

Nope, not a problem.

RE: UPS Nuisantly Transfers to Bypass

(OP)
Thanks david. There are rectifier or UPS manufacturer who strongly believe that phase mismatch is a cause for the high inrush. However, they can't present anything when I asked them to justify this in terms of physics and calculations. Thye just based it from their "experiences". Unfortunately I don't want to place this phase matching isolation transformers by trial and error.

I am curious though what made almost all of them think that it what they think it is...

RE: UPS Nuisantly Transfers to Bypass

What is the frequency tolerance setting on the UPS?

RE: UPS Nuisantly Transfers to Bypass

nightfox,
I don't understand this "phase mismatch". Are they saying that having a wye connected transformer serving a 3-wire load somehow creates high inrush?   

RE: UPS Nuisantly Transfers to Bypass

By the way, it is stated here that there is no issue of a UPS output transformer whose secondary is wye and feeding a rectifier whose input transformer is having a Delta primary winding.

Surely all, or at least the vast, vast majority of three phase, on-line, UPSs have an output transformer which has a wye secondary?

There are thousands of computer centres around the world where a 'standard' UPS with a wye output transformer is feeding an isolation trandsformer at a PDU, or similar, which has a delta primary.

In a lot of these cases the UPS will transfer to bypass when energising the transformer. This is becauses of the inrush current to the transformer and has nothing to do with some percieved phase miss-match.
 

RE: UPS Nuisantly Transfers to Bypass

(OP)
norman63, to answer you question, the UPS free running output frequency (asynchronous or on battery) is 60Hz, +/-0.01%

RE: UPS Nuisantly Transfers to Bypass

Check the frequency pot setting.  I've had problems in the past with machines when trying to sync to generator power, batteries and/or outside power sources/draws.  You should be able to set the tolerance to plus or minus 5%.  1% is a very tight setting considering your set up and load.  Try backing it off 1% at a time until the units stop(s) transfering into bypass.  Are both of the UPS units Mitsubishi?  Do they have internal batteries, external batteries or both?  What size are the batteries?  Are the rectifiers your feeding part of a D/C plant (batteries included or are you relying on the UPS for battery power?) Are all UPS, Rectifier and/or D/C plants new installations and are the batteries new?  What is the float voltage set at for the UPS units?  Are both UPS untis 100KVA?  I will dig into this a little deeper at my end.  I have excellent tech support available and they should be able to nail the problem down quickly.  Try what I suggested first and send me the info on the machines involved.

RE: UPS Nuisantly Transfers to Bypass

As a application engineer for a UPS manufacturer (not Mitsubishi), it appearts to me the issue is UPS sizing.  There is nothing in the thread that indicated there is a problem with the Mitsubishi UPS.  The UPS is a 100KVA box @ .8 PF - meaning the 100% full load is 80 KW.  At roughly 105% load (84 KW) the unit will transfer to bypass if the bypass is avaiable.  The best solution is to determine the maximun load (including inrush) and size the UPS according. Industry standard is for the design load to be approximately 80% of the full load rating of the UPS - so for the 100KVA UPS about 80 KVA/64KW.

 

RE: UPS Nuisantly Transfers to Bypass

I agree with Handlin. The UPS is not faulty at all and is doing exactly what it should be doing. The problem is in the sizing of the UPS.
That does not cure yor problem though. May I suggest that you look at installing a soft start unit between the output of the UPS and the input to the rectifier. This will control the inrush to the system by gradually increasing the output voltage available to the rectifier input therefore limiting the inrush current to the transformer. This will prevent the UPS tripping to bypass, and also is a far better and safer way to limit inrush that using a high wattage resistor and contactor circuit.
Vist this site for an idea: http://www.softstartuk.com/
I am a power electronics engineer working for the following company: http://www.powerups.co.uk

RE: UPS Nuisantly Transfers to Bypass

Nightfox, are you still having the UPS problem with transfer to Bypass when energizing the rectifier? Can you check the manual on the Static switch to determine what the threshold limts are for transfer to Bypass? These can be a low inverter output voltage threshold (Undervoltage), high Inverter output voltage (Overvoltage). There may also be Inverter output frequency limits and Inverter output current limits, but I am not sure of the last two parameters (Frequecy & Current). You can energize the rectifier, as suggested earlier in this thread, with the Bypass supply isolated, and determine Inverter output Current peaks (as already done), Voltage and Frequency excursion limits. You can then compare these with the transfer thresholds of the Static Switch to see which threshold (Voltage, Current or Frequency) is being violated.
You can also test the static switch to ensure that it transfers accurately at its set threshold limits. If the static switch is operating correctly at its preset threshold limits, then we are back to limiting inrush current on energising the rectifier.

RE: UPS Nuisantly Transfers to Bypass

(OP)
For update on this topic.

Staticon decided to internally modify the existing rectifiers by changing the transformers, line reactors and providing DC choke and some kind of an internal circuitry which will somehow soft-start the input transformer.

The resulting transformer in-rush was even less than the transformer FLA. It was good news but eventually we paid a lot in extra because of it.

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