400V Dual Source UPS 60hz... Neutral Question

[upsguy]

Looking to install a large three phase UPS 3 phase 400V 60hz that has no internal transformers. The UPS is designed to be feed by two separate utility sources, in this case 400v 60hz. The only transformers in the entire lineup will be the Medium Voltage/400V at the building entrance. There are two 3000kva transformers being feed from two separate utility sources.

So the primary load Data Center servers, will be feed 230V L-N voltage. The UPS requires 3w + G on the Mains 1 Rectifier input and 4w + G on the bypass.

My question is:

Primary operation is Mains 1 through the UPS to the load. The only Neutral in the system is derived on the secondary of the Mains 2 transformer. How can the loads operate 230 L-N when the N being used is from a different source as the primary transformer?

 

[Smart $]

Is the drawing something you made up, or was it furnished by UPS company? Regardless, it appears to be more of a representive illustration than a true schematic or wiring diagram.

Detailed information on entire system would help. Both transformers in illustration are shown as delta-wye configured. The neutral of both transfomers need to be connected to the system. Is bypass switch integral with UPS? Got any true schematics of the UPS you can post or link?

 

[upsguy]

Its just something I drew up based off a conversation of a concept. Bypass switch is integral to UPS.

Normally for Dual Feed UPS applications it isn't a big issue where the two sources come from because the Neutral is derived downstream at the PDU(480/208) transformer.

This newer application for the US is common overseas and I wondered how they overcome this issue. The wiring diagram calls out specifically 3w + G on Input and 4w + G on bypass.

I cannot provide any schematics of the UPS, but its a normal Double Conversion with an IGBT Rectifier and IGBT Inverter. The Static switch is in parallel with the Rec/Inv power path and is a SCR type device. The neutral is simply passed through the system.

I searched NEC and can't find a section about tying together two different neutrals together, but I assume this would cause a loop?

I've used dual Source Remote Power Panels in the past that had switching Neutral Contactors and thought that might be neccessary for this application.

Thanks for any help with this High Level question. As I stated its all in the concept stage to remove all but primary step down transformers from the site. We could just source the UPS from a single feed and Dual source the path using 2 breakers and two sets of conduits, but maximum reliability requires true Dual Sources as well.

 

[shmojoe]

Have not seen dual fed UPS without PDU derived neutral downstream. Would think an output iso-transformer on UPS would be needed. Would also wonder how ATS and generator would play into both input and bypass wiring. Out of curiosity, why the focus on removing transformers completely downstream? Reliability? Space? Transformers are normally the last thing that keep my up at night worrying.

Would like to hear more, interesting setup.

 

[upsguy]

Highest efficiency possible is the main driver. A possible side gain is equipment CAPEX and install costs.

 

[weressl]

Looking to install a large three phase UPS 3 phase 400V 60hz that has no internal transformers. The UPS is designed to be feed by two separate utility sources, in this case 400v 60hz. The only transformers in the entire lineup will be the Medium Voltage/400V at the building entrance. There are two 3000kva transformers being feed from two separate utility sources.

So the primary load Data Center servers, will be feed 230V L-N voltage. The UPS requires 3w + G on the Mains 1 Rectifier input and 4w + G on the bypass.

My question is:

Primary operation is Mains 1 through the UPS to the load. The only Neutral in the system is derived on the secondary of the Mains 2 transformer. How can the loads operate 230 L-N when the N being used is from a different source as the primary transformer?

Hmmmm.

Is the UPS output 3phase or 3phase+neutral?
UPS output IS a separately derived system, therefore if there is a neutral, it needs to be separately grounded and the neutrals switched as the 4th pole. (Same rule as thefor the ATS between an emergency generator and normal power source.)
There MAY be an integral transformer in the UPS, that you're not aware.
There is a static switch to provide transfer from Normal to Bypass source and there may be isolating contactors motor operated breakers to open after transfer to prevent backfeed onto failed electronics. (Output is always synchronized to bypass frequency/voltage.)

 

[upsguy]

There are no transformers in the system I know this for a fact. Also since there are no transformers this UPS output is not considered a separately derived source since the grounded conductor originates in another system.

The output is 4w + G or 3 + N + G since the loads are all 230V L-N

 

[weressl]

There are no transformers in the system I know this for a fact. Also since there are no transformers this UPS output is not considered a separately derived source since the grounded conductor originates in another system.

The output is 4w + G or 3 + N + G since the loads are all 230V L-N

You don't need to have a transformer for it to be a separately derived system. The input and output are galvanically isolated from each other, (re. previous post of what the output is synchronized by) and the 4th wire or the neutral is referenced to this separately derived system. All residual currents resulting from the unbalanced single phase loads will 'return' through this wire to complete the circuit flow. They will not flow back through the bypass neutral to the bypass source. Both the bypass source and the UPS output 4th wire should be independently grounded at their origin and switched.

 

[upsguy]

You don't need to have a transformer for it to be a separately derived system. The input and output are galvanically isolated from each other, (re. previous post of what the output is synchronized by) and the 4th wire or the neutral is referenced to this separately derived system. All residual currents resulting from the unbalanced single phase loads will 'return' through this wire to complete the circuit flow. They will not flow back through the bypass neutral to the bypass source. Both the bypass source and the UPS output 4th wire should be independently grounded at their origin and switched.

Please excuse my ignorance but I don't see how the Output is Galvanically isolated from the input or bypass when the Neutral is passed through the system.

I believe I am seeing that without switching the Neutral upstream of the UPS their isn't a way the single phase loads can have a return to complete circuit flow.

Also in a "typical" ups design using an Output Neutral we would only be grounded in the UPS if the UPS had an output transformer, otherwise we rely on the source ground bond when their is no transformer in the system(in my case)

 

[Smart $]

Please excuse my ignorance but I don't see how the Output is Galvanically isolated from the input or bypass when the Neutral is passed through the system.

I believe I am seeing that without switching the Neutral upstream of the UPS their isn't a way the single phase loads can have a return to complete circuit flow.

Also in a "typical" ups design using an Output Neutral we would only be grounded in the UPS if the UPS had an output transformer, otherwise we rely on the source ground bond when their is no transformer in the system(in my case)

As I see it, with the neutral carried through the UPS, it is not a separately derived system.

Also you said...

There are no transformers in the system I know this for a fact. Also since there are no transformers this UPS output is not considered a separately derived source since the grounded conductor originates in another system.

The output is 4w + G or 3 + N + G since the loads are all 230V L-N

The highlighted portion needs corrected to say, "originates in the same system." You only have one system regarding the UPS and its output (for the sake of discussion, I am not considering any other equipment connected directly to either service).

What I don't understand is why MAIN 2 bypasses the UPS function??? I think a system which syncs and switches between sources upstream from the UPS function would be much more preferable. What if MAIN 2 goes down while switched over? For servicing the UPS, you'd provide completely external bypass around the UPS unit to isolate it.

 

[weressl]

Please excuse my ignorance but I don't see how the Output is Galvanically isolated from the input or bypass when the Neutral is passed through the system.

I believe I am seeing that without switching the Neutral upstream of the UPS their isn't a way the single phase loads can have a return to complete circuit flow.

Also in a "typical" ups design using an Output Neutral we would only be grounded in the UPS if the UPS had an output transformer, otherwise we rely on the source ground bond when their is no transformer in the system(in my case)

I need to have a model or a catalog number to further comment on this, because in my lengthy expereince with UPS's I have not seen any double conversion units that would be configured the way you're describing it.


Galvanic isolation comes from the fact that the incoming AC is converted to DC and that is reconverted back to a synthetized AC. In other words there is no continuous metallic conductive path between the incoming and outgoing power.

 

[shmojoe]

What I don't understand is why MAIN 2 bypasses the UPS function??? I think a system which syncs and switches between sources upstream from the UPS function would be much more preferable. What if MAIN 2 goes down while switched over? For servicing the UPS, you'd provide completely external bypass around the UPS unit to isolate it.

Kind of what I was thinking. In terms of maximum reliability, put an upstream Static ATS. However, very expensive when you start getting into that area - probably more expensive that what a single UPS module could justify in terms of critical load.

 

[jdsmith]

What is the kVA or kW rating of this UPS? "Large" means different things to different people and it will affect the answers you're getting here.

The system configuration you have shown on your sketch is not normal for data centers or industrial plants - those are the two classes of facilities that have fairly well-developed families of standard system configurations that have been optimized over the years for:

1) size of UPS's in absolute terms and relative to other loads in the facility
2) Relative criticality of loads ($$ and life safety), cost of downtime ($$), mean time to repair/recover
3) Likelihood and nature of power disruptions and disturbances

In your second post you mentioned "The neutral is simply passed through the system." I assume you mean the neutral from the bypass source is passed through and not switched since you stated the rectifier input is 3W + ground.

The UPS does constitute a separately derived system because of the DC link in the middle - it decouples the AC systems, the DC link floats relative to ground, and you need to derive a new N-G bond after you go through a DC link to stabilize the fabricated AC voltage from the inverter. The output stage must be ground referenced (bonded) in some fashion so the 3 phase voltages don't float relative to ground. The hazard if they float too much is that the phase-ground voltage can become higher than the dielectric rating of the IGBTs. Therefore regardless of whether you are using 3W or 4W loads the neutral of the IGBT inverter must be bonded somewhere with a single N-G bond. (Read that again - this is an important issue even if you are using PDUs with step-down transformers!)

It is acceptable to use a 3 pole static transfer switch and bond the neutral of the inverter output to the neutral of the bypass source - this achieves the goal of exactly one neutral to ground bond - in this case the N-G bond at the MV service entrance transformer. The UPS input power is being fed from service entrance transformer #1 and the N-G bond for the UPS loads is being provided by service entrance transformer #2. Hokey, yes. Prone to problems, yes. But it would work if you're cheap and OK with weird power quality problems in the future from a very strange, roundabout N-G bonding scheme.

This is why industrial UPS's use output transformers and why UPS bypass sources are usually run through standard transformers, isolation/shielded transformers, or active power conditioning transformers. The output transformer and bypass source transformer make the N-G bonding more straightforward by confining the N-G bond to only UPS loads to eliminate power quality or bonding problems elsewhere in the building from affecting the UPS loads. Data centers do things differently for reasons I'm not going to get into - if you are using a true datacenter-class UPS I will elaborate.

It sounds like your client is more interested in efficiency than in a reliable UPS power system. If you can provide us a little more background on the nature of the facility and what other UPS systems or local generation are present we may be able to help justify a more sophisticated system.

 

[upsguy]

Kind of what I was thinking. In terms of maximum reliability, put an upstream Static ATS. However, very expensive when you start getting into that area - probably more expensive that what a single UPS module could justify in terms of critical load.

Didn't really want to get into to much of the upstream to muddle things further. However I'll provide you with a few reasons to not use an ATS if you have 2 true mains voltages.

An ATS is a single point of failure. A UPS with two true input sources has a higher reliabilty factor if their isn't and ATS(switching between two utility sources). They only reason this installation would have an ATS on Mains 1(and it will) is to switch between Mains 1 and Generator during a failure.

Also a Common configuration like this will be used in is for 2N source to the Load(Dual Corded Servers). Since we have an A/B configuration

UPS A is feed Rectifier(Utility A) Bypass(Utility B)
UPS B is feed Rectifier(Utility B) Bypass(Utility A)

In the event Utility B Fails, UPS A Stays online Normal Operation, bypass transfer locked out. UPS B will either run on battery(if no gen) and then once the battery is depleted run on Utility A on the Bypass. This also allows us to shut down the A side and perform maintenance and still keep the B side on inverter and the A side powered by the bypass.

Just wanted to provide a common example of why UPS have two different sources that are not switched with an ATS.

The UPS will be sized around 460-500kVA.

Your question of where the Neutral bonds goes back to my question. In a normal system that shares a common source transformer even if Dual input the Bypass Neutral is Bonded at the Upstream source. If no Neutral is pulled then a N-G bond is installed in the UPS.

I agree that this is an ODD N-G bonding scheme but my real question is since these loads will be primarily 230L-N, can the Bypass Transformer Neutral support current flow when the Power is actually being drawn from the Main Input transformer.

Thanks for your support.

 

[jdsmith]

Didn't really want to get into to much of the upstream to muddle things further. However I'll provide you with a few reasons to not use an ATS if you have 2 true mains voltages.

An ATS is a single point of failure. A UPS with two true input sources has a higher reliabilty factor if their isn't and ATS(switching between two utility sources). They only reason this installation would have an ATS on Mains 1(and it will) is to switch between Mains 1 and Generator during a failure.

Also a Common configuration like this will be used in is for 2N source to the Load(Dual Corded Servers). Since we have an A/B configuration

UPS A is feed Rectifier(Utility A) Bypass(Utility B)
UPS B is feed Rectifier(Utility B) Bypass(Utility A)

In the event Utility B Fails, UPS A Stays online Normal Operation, bypass transfer locked out. UPS B will either run on battery(if no gen) and then once the battery is depleted run on Utility A on the Bypass. This also allows us to shut down the A side and perform maintenance and still keep the B side on inverter and the A side powered by the bypass.

Just wanted to provide a common example of why UPS have two different sources that are not switched with an ATS.

The UPS will be sized around 460-500kVA.

Your question of where the Neutral bonds goes back to my question. In a normal system that shares a common source transformer even if Dual input the Bypass Neutral is Bonded at the Upstream source. If no Neutral is pulled then a N-G bond is installed in the UPS.

I agree that this is an ODD N-G bonding scheme but my real question is since these loads will be primarily 230L-N, can the Bypass Transformer Neutral support current flow when the Power is actually being drawn from the Main Input transformer.

Thanks for your support.

The system you described is not a dual input UPS - it is a single input UPS that feeds a static switch on the output (aka solid state auto transfer switch) where the other source into the static switch is from a second power source. The problem with this design is you still have a single point of failure in that one static switch.

The short answer to your question is yes, if it is sized and installed properly. The neutral to ground bond in a solidly grounded system is always sized to handle unbalanced currents. If you're worried about both main transformers being fully loaded and the UPS loads are running fully unbalanced, you could increase the size of the N-G bond on the second main transformer to account for the additional unbalance from the UPS loads.

This whole arrangement boggles my mind a bit - on one hand you're concerned about single points of failure, then you put a static switch on the output of the UPS where all UPS load power flows through that one static switch. Do you have a wraparound manual maintenance bypass switch? With a wraparound manual bypass you can't maintain that static switch under load - now you have something worse - a single point of failure that can't be tested as part of a preventative maintenance program.

Wraparound bypass switches are not typical with 460-500 kVA UPS systems - in this class of system the norm is for the system designer to implement a manual bypass that basically "shorts out" the UPS, and furnish disconnecting means so the UPS can be disconnected and repaired/maintained while the owner-designed bypass is powering the load.

 

[ron]

Transformerless UPS's are the new rage. Eaton, GE and soon MGE have (or will) have them.

It will likely be a while until the IEEE 1100-2005 (Emerald Book) specifically covers the newer UPS modules, but the intent is covered in Section 8.5.2 System grounding.

I think the key is that the N-G is bonded in the bypass. That is the reference.

The imbalanced current wants to return to its neutral, so when in rectifier/inverter mode, the neutral is in the UPS module. During a ground fault downstream, the current has a long way to go, but it can still get back to its neutral source, keeping in mind the UPS module will quickly do its job and transfer to bypass and use the regularly expected neutral from the bypass source.

.

 

[weressl]

The system you described is not a dual input UPS - it is a single input UPS that feeds a static switch on the output (aka solid state auto transfer switch) where the other source into the static switch is from a second power source. The problem with this design is you still have a single point of failure in that one static switch.

The short answer to your question is yes, if it is sized and installed properly. The neutral to ground bond in a solidly grounded system is always sized to handle unbalanced currents. If you're worried about both main transformers being fully loaded and the UPS loads are running fully unbalanced, you could increase the size of the N-G bond on the second main transformer to account for the additional unbalance from the UPS loads.

This whole arrangement boggles my mind a bit - on one hand you're concerned about single points of failure, then you put a static switch on the output of the UPS where all UPS load power flows through that one static switch. Do you have a wraparound manual maintenance bypass switch? With a wraparound manual bypass you can't maintain that static switch under load - now you have something worse - a single point of failure that can't be tested as part of a preventative maintenance program.

Wraparound bypass switches are not typical with 460-500 kVA UPS systems - in this class of system the norm is for the system designer to implement a manual bypass that basically "shorts out" the UPS, and furnish disconnecting means so the UPS can be disconnected and repaired/maintained while the owner-designed bypass is powering the load.

Actually there are manufacturers that do make such bypass available as an intergaqted product, such as the EATON 9395 SBM. That's a 225-1100kVA class system.

It doesn't really matter how many bypasses, static switches one provides; a single user will always have a common point of failure. Worth keeing in mind that the simpler the device is at the common failure point, the more reliable it is, the least likely is it to fail.

 

[weressl]

Transformerless UPS's are the new rage. Eaton, GE and soon MGE have (or will) have them.

It will likely be a while until the IEEE 1100-2005 (Emerald Book) specifically covers the newer UPS modules, but the intent is covered in Section 8.5.2 System grounding.

I think the key is that the N-G is bonded in the bypass. That is the reference.

The imbalanced current wants to return to its neutral, so when in rectifier/inverter mode, the neutral is in the UPS module. During a ground fault downstream, the current has a long way to go, but it can still get back to its neutral source, keeping in mind the UPS module will quickly do its job and transfer to bypass and use the regularly expected neutral from the bypass source.

.

Regardless if it with or without transformer the circuit is always completed WITHIN the source. If you have parallelled trasnformers, you will find circulating current between the phase and even the grounded wire. BUT if your transformers do not have at least TWO wires paralleled there will be no current flow from one to the another - not counting the small amounts of capcacitive differential coupled voltage difference - all balanced and unbalanced ccurrent will flow back to its respective transformer. Should you choose a 'transformerless' design, the same principle will still apply. Usually there would be no reason to supply a neutral as an input to a UPS, but even if one chooses to do so, I fail to see how the neutral would carry back ANY current to the input source from the UPS output block to the input transformer(way upstream). I don't see the functionality and logic of the so-called 'pass through neutral' wire.

 

[upsguy]

The system you described is not a dual input UPS - it is a single input UPS that feeds a static switch on the output (aka solid state auto transfer switch) where the other source into the static switch is from a second power source. The problem with this design is you still have a single point of failure in that one static switch.

.

Thank you for all of your advice. Just a couple of comments.

I would say the opposite... It is very rare to see a 500kVA withougt Maintenance Bypass and that will be included in this design but wasn't relevant.

I'm not sure how you think the UPS I've described isn't a Dual input but anytime you have one Feed powering the rectifier and the other feed powering the Bypass Static switch it is dual input. There is an Output static switch on the inverter side however I didn't mention that because it was also irrelevant. The Bypass and Output static switches operate only to transfer the load from inverter to bypass.


The design I've described is a single Module Double Conversion with Static Bypass, however yes I am aware of all the major manufactures that make distributed static switch parallel systems as well as centralized static switch systems.

Thanks again.

 

[weressl]

Highest efficiency possible is the main driver. A possible side gain is equipment CAPEX and install costs.

When it comes to UPS highest efficiency should not be the main driver. That is just plain silly. The main driver is RELIABILITY and AVAILABILITY, otherwise you won't or shouldn't be bothering with UPS in the first place.