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.
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
RE: UPS Nuisantly Transfers to Bypass
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
RE: UPS Nuisantly Transfers to Bypass
RE: UPS Nuisantly Transfers to Bypass
RE: UPS Nuisantly Transfers to Bypass
RE: UPS Nuisantly Transfers to Bypass
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
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
Any more suggestions will be appreciated.
RE: UPS Nuisantly Transfers to Bypass
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
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
RE: UPS Nuisantly Transfers to Bypass
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
[IMG]http:
RE: UPS Nuisantly Transfers to Bypass
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
RE: UPS Nuisantly Transfers to Bypass
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
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
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
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
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
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
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:/
RE: UPS Nuisantly Transfers to Bypass
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:
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I appreciate any comments and guidance that would help us resolve the problem. Thank you.
RE: UPS Nuisantly Transfers to Bypass
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
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 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
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
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
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
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
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
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
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
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
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
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
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
RE: UPS Nuisantly Transfers to Bypass
*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
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
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
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
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://i53
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://i53
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://i53
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
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
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
RE: UPS Nuisantly Transfers to Bypass
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
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
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
RE: UPS Nuisantly Transfers to Bypass
I am curious though what made almost all of them think that it what they think it is...
RE: UPS Nuisantly Transfers to Bypass
RE: UPS Nuisantly Transfers to Bypass
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
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
RE: UPS Nuisantly Transfers to Bypass
RE: UPS Nuisantly Transfers to Bypass
RE: UPS Nuisantly Transfers to Bypass
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
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
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.