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© 2012 Eaton Corporation. All rights reserved. Transformerless UPS Neutral handling and grounding considerations Ed Spears May 2013

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Transformerless UPS - Neutral Handling and Grounding Consideration

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Page 1: Spears Grounding 2013

© 2012 Eaton Corporation. All rights reserved.

Transformerless UPSNeutral handling and grounding considerations

Ed Spears

May 2013

Page 2: Spears Grounding 2013

2© 2012 Eaton Corporation. All rights reserved.

What do I need to know about neutral handling in Eaton transformer-less UPS applications?

• Installation practices, especially in the U.S.• Considerations when designing systems with transformer-less UPS• How to answer engineers’, installers’, and inspectors’ questions

• What we recommend (and why), and what we will accept • How the transformerless UPS handles faults downstream of the UPS

A

C B

NTo the UPS

3W + Gnd Source

Page 3: Spears Grounding 2013

3© 2012 Eaton Corporation. All rights reserved.

Installation practices

• North American power systems at 480V are almost exclusively fed through a “3-wire + ground” wiring system from “Wye” (neutral grounded) facility transformers. This is commonly known as a “3-wire solidly grounded system.”

The neutral from the source transformer is not run to load-side equipment, AND it may not be necessary to run a neutral to the UPS.

• Why not? Because we can provide an internal center point reference, as long as we know that the source is 3W+G.

FUSE

FUSE

FUSE

A

BC

C

B

A

FUSE

FUSE

N

N

G

SAFETY

GND TO FRAME

PDU

GND

B

C

N

G

A

SSW

SOURCE

X FMR

Page 4: Spears Grounding 2013

4© 2012 Eaton Corporation. All rights reserved.

Installation practices

• 208V data center distribution establishes a neutral at the output of the distribution transformer (PDU), so in this case we do not need to provide a neutral from the UPS (PDU transformer primary is delta, and its wye secondary makes a new neutral).

• As systems get larger and more complex, design practices call for ground fault protection on feeders 1000A and above, on 480V and higher sources of supply.This can be an issue if the neutral is bonded to ground before the PDU transformer. It may cause nuisance tripping of the feeder due to ground current.

• So what do we do about this? If this is an issue, it can be solved by not bonding E12 bus in the UPS. Note that the UPS is NOT a separately derived system per the definition in the NEC, and therefore it is not required to be bonded.

FUSE

FUSE

FUSE

A

BC

C

B

A

FUSE

FUSE

E12

N

G

SAFETY

GND TO FRAME

PDU

GND

B

C

N

G

A

SSW

SOURCE

X FMR

Page 5: Spears Grounding 2013

5© 2012 Eaton Corporation. All rights reserved.

Neutral and Ground: What does the site require?

• Safety: protect personnel from hazardous voltages.• Performance: The datacenter equipment needs a

common reference point to work reliably and safely.• Fault Handling: When an electrical fault occurs, the

operation of circuit breakers and fuses function to “isolate” the fault; that is, to limit the potential for electrical shock, and avoid a fire. So their operation must be “predictable”. That includes the response of the UPS and power distribution network.

Page 6: Spears Grounding 2013

6© 2012 Eaton Corporation. All rights reserved.

Neutral and Ground: What does the UPS require?

• Safety: protect personnel from hazardous voltages.

• Performance: The UPS requires a “center point reference” in order to create a 3-phase output, and keep it aligned with the bypass source. That “CP reference” could become a neutral under certain circumstances, but that is not always the case. The UPS only requires that the CP reference be stationary, not “moving around.”

• Fault Handling: When an electrical fault occurs, the operation of circuit breakers and fuses function to limit the potential for electrical shock, and avoid a fire in the UPS. So their operation must be “predictable”.

A

C B

3-Phase UPS Output

Center

Point

Reference

Page 7: Spears Grounding 2013

7© 2012 Eaton Corporation. All rights reserved.

What if the CP Reference moves?

• The UPS center point reference should not “drift”! If it does drift, and we need to transfer to bypass, the phase voltages may not be in sync or have the same magnitude as shown at right…….

• How do we keep the reference stable? Three choices:• Tie it to ground (bond E12-G)• Use our internal center point

reference• Tie it back to source neutral

A

C B

UPS Output

Center

Point

References

A

C

B

Bypass

Page 8: Spears Grounding 2013

8© 2012 Eaton Corporation. All rights reserved.

So, what’s the Process when specifying the 9390 or 9395 UPS?

• Step One: Find out the configuration of the Utility source:• 3-wire plus ground, wye source, neutral solidly bonded: no

neutral available to UPS• 4-wire plus ground, solidly bonded: neutral is available to be

connected to the UPS.• Delta Source: no neutral available to be connected to the UPS

• Delta source is NOT supported for 9395 at this time• Corner grounded delta, or “open jaw” delta, or “high leg” sources are

never allowed. In these cases a transformer with a solidly grounded secondary must be placed ahead of the UPS.

• High Resistance Gnd Source (HRG): This is a wye source with a resistance connected between the secondary neutral terminal and ground, often seen in factory environments, but appearing in datacenter applications more often lately.

Page 9: Spears Grounding 2013

9© 2012 Eaton Corporation. All rights reserved.

So, what’s the Process when specifying the 9390 or 9395 UPS?

• Step Two: Find out if the load that is directly connected to the UPS, will require a “current carrying neutral.” That is, are 277V or 120V loads connected to the UPS output terminals?• If yes, then a source neutral must ALWAYS be connected to the E12

terminal in the UPS, then that neutral wire will be passed on to the critical load distribution. Do not bond E12 to Gnd.

• If no, then we have some choices; the Installation Manual states our preferred choice, but other choices are safe and Code compliant.

• 1) Bond E12 to ground inside the UPS. The UPS is NOT a separately derived source, but NEC does not prohibit a N-G bond at this point.

• Some inspectors may disagree, and are unlikely to “change”• Ground noise may be a problem, when unbalanced utility exists

• 2) If #1 above is not desirable, connect our internal reference to E12, and do not bond E12 to ground in this case.

• 3) Pull in a minimal code rated neutral from the source. Do not bond in UPS.• Step Three: Communicate this info to the consultants, engineers,

and UPS installers. The next slides will help answer typical questions…….

Page 10: Spears Grounding 2013

10© 2012 Eaton Corporation. All rights reserved.

Lots of Questions…..

What if I have a delta source feeding the UPS?• NOT ALLOWED on 9395; get a delta-wye

transformer and place it ahead of the UPS• What if I want to feed a 9395 with a “corner-

grounded delta”? Use NRK? NRP? NNRK? ......NFG! (No). • This configuration is not allowed

• What if I want to feed a UPS with an “ungrounded wye” source? That is, where the center point of the wye source is connected to …….nothing.• This configuration is not allowed

Page 11: Spears Grounding 2013

11© 2012 Eaton Corporation. All rights reserved.

Lots of Questions…..

What if I have a High Resistance Ground (HRG) source? • OK for 9390 and 9395, for single source applications.

• No phase to neutral loads allowed with HRG, per NEC 250.186• Internal center point reference is required (CTO option in 9390)

FUSE

FUSE

FUSE

A

BCC

B

A

FUSEFUSE

N

N

G

SAFETY

GND TO FRAME

PDU

GND

B

CN

G

A

SSW

G

C

B

A

HRG

SOURCE

Page 12: Spears Grounding 2013

12© 2012 Eaton Corporation. All rights reserved.

Lots of Questions…..

• What if I have a 9390 208/208 UPS which supports phase to neutral loads?• Note there is no PDU or IDC cabinet. The UPS has phase to

neutral loads directly connected at its output.• Source neutral passes right thru the UPS, but connects to

E12, so the output neutral will be there when the UPS is on battery or on bypass.

FUSE

FUSE

FUSE

A

BC

C

B

FUSEFUSE

N

N

G

SAFETY

GND TO FRAME

GND

G

A

SSW208 OR 400V

LOAD PANEL

W/ P-N LOADS

G

N

C

B

A

N

SOURCE

X FMR

Page 13: Spears Grounding 2013

13© 2012 Eaton Corporation. All rights reserved.

Battery Operation with ungrounded output

What does it mean that my UPS output is an ungrounded

system? Is it safe? Will it work? What about faults?• The UPS output Wye is ungrounded when on battery without a ground referenced center point. • Transformer-less UPS pose no threat to the critical load during battery operation.• The same safety requirements apply to both grounded and ungrounded systems. The UPS

chassis still has a safety ground. The UPS frame will not rise above ground potential under any scenario

• Downstream distribution will locate the 3 phase conductors with respect to ground. This is the case with 480v to 208v PDUs.

Page 14: Spears Grounding 2013

14© 2012 Eaton Corporation. All rights reserved.

Battery Operation - continued

Is my load at risk?• The NEC describes grounded systems as generating fault current upon

the first phase-to-ground fault. Ungrounded systems generate current on the second fault.

• Two concerns with ungrounded systems include not detecting output faults, and ground fault detectors tripping “unexpectedly” upon the return of utility voltage.• Phase to ground faults while on battery are rare, but• We can set the UPS to shutdown on a detected fault while on battery• BTW, Load faults are ALWAYS “unexpected”.• Phase to phase faults will ALWAYS generate fault current.• Phase to ground faults downstream of step-down transformers (PDU) will

ALWAYS generate fault current upon the first output-ground fault.• Phase to ground faults before downstream distribution ALWAYS generate

fault current upon the second fault.• Bottom line? The UPS will operate safely in all modes, on utility or on

battery.

Page 15: Spears Grounding 2013

15© 2012 Eaton Corporation. All rights reserved.

What about faults?

Is the transformerless UPS safe? And does it operate predictably when a fault occurs?

• What if a fault occurs between the UPS and the PDU?• What is the “return path” for fault current in a transformerless UPS?• What if a fault occurs on the load side of the PDU?

FUSE

FUSE

FUSE

A

BC

C

B

A

FUSEFUSE

N

N

G

SAFETY

GND TO FRAME

PDU

GND

B

CN

G

A

SSW

SOURCE

X FMR

Phase to Gnd Fault

Fuse will not clear. UPS will enter current limit, and transfer to bypass. Bypass fuse will clear.

Page 16: Spears Grounding 2013

16© 2012 Eaton Corporation. All rights reserved.

What about faults?

What if I have a fault between the UPS and the PDU while on battery?? • There is NOT a return path for fault current! (so no current will flow)• What happens to the load? (hint: nothing)• What happens when Utility returns or Generator comes on line?• Note: the UPS can be set to shutdown instantly on this condition (ref

moves away from gnd.)

FUSE

FUSE

FUSE

A

BC

C

B

FUSEFUSE

N

N

G

SAFETY

GND TO FRAME

PDU

GND

A

G

A

SP5W

B

C

SOURCE

X FMR

Phase to Gnd Fault

Return current can NOT flow; IGBTs are off when utility has failed

Page 17: Spears Grounding 2013

17© 2012 Eaton Corporation. All rights reserved.

What about faults?

OK, wise guy, what happens if I have a fault while on battery in a 9390 208V UPS with no PDU?

• In this case, there IS a return path, so fault currents will cause UPS to enter 300% current limit for 20 cycles, then trip off.

FUSE

FUSE

FUSE

A

BC

C

B

FUSEFUSE

N

N

G

SAFETY

GND TO FRAME

GND

G

A

ON BATTERY 208/120V OR 415/240V

SSW208 OR 400V

LOAD PANEL

W/ P-N LOADS

G

N

C

B

A

N

SOURCE

X FMR

Phase to Gnd Fault

Return current can NOT flow; IGBTs are off when utility has failed

Fuse will not clear. UPS will enter current limit at 300% nominal for 20 cycles

Page 18: Spears Grounding 2013

18© 2012 Eaton Corporation. All rights reserved.

What about faults?

• What if I have an HRG source?

FUSE

FUSE

FUSE

A

BCC

B

A

FUSEFUSE

N

N

G

SAFETY

GND TO FRAME

PDU

GND

B

CN

G

A

SSW

G

C

B

A

HRG

SOURCE

Phase to Gnd Fault

Resistance limits return current, and triggers gnd fault alarm

Page 19: Spears Grounding 2013

19© 2012 Eaton Corporation. All rights reserved.

What if I have a parallel system?

• If there are phase to neutral loads connected at the output of the tie cabinet or SBM, the same rule applies: bring in a source neutral to each unit’s E12 terminal, and pass that neutral on to the SBM or tie cabinet. Do not bond E12-G in the UPS or at the tie point.

Page 20: Spears Grounding 2013

20© 2012 Eaton Corporation. All rights reserved.

Parallel Systems—Distributed Bypass

If there are NO phase to neutral loads (more typical), we still need a full-rated 4th wire from each UPS E12 to a common tie point in the tie cabinet.

• This may confuse inspectors/installers, but the reason is that we like to have a SINGLE center point reference for the whole parallel system. The impedance from each UPS to its common CP reference needs to be the same. That’s why we require a “neutral” wire from each UPS.

• The Neutral bar in the tie cabinet can be bonded to ground. If the inspector objects, we can use NRK kits in each UPS, and remove the N-G bond in the tie cabinet.

DC/ACAC/DC

Bypass9395

DC/ACAC/DC

Bypass9395∆

Y

Y

33

DC/ACAC/DC

Bypass9395

DC/ACAC/DC

Bypass9395

33

33

3333

33

44

44

Tie

Ca

bin

et

Pull an E12 wire from each UPS

Page 21: Spears Grounding 2013

21© 2012 Eaton Corporation. All rights reserved.

Parallel Systems—with SBM

If there are no phase to neutral loads, a full-rated 4th wire should be connected from E12 in each UPS to a common tie point in the SBM.

• This may confuse inspectors/installers, but the reason is that we like to have a SINGLE center point reference for the whole parallel system. The impedance from each UPS to its CP reference needs to be the same. That’s why we require an E12 wire from each UPS.

• The E12 bar in the tie cabinet “can” be bonded to ground. If the inspector objects, we can use NRK kits in each UPS, and remove the E12-G bond in the tie cabinet.

STSW

CBS

CBP

FBP

SSBM

Y

Y

AC/DC DC/AC

9395

33 33

33

33

44

Pull an E12 wire from each UPS

Page 22: Spears Grounding 2013

22© 2012 Eaton Corporation. All rights reserved.

Key Take-Aways

• The Installation manual is your best (first) reference for our grounding recommendations.• If the engineer/inspector balks, then we do have options.• There are many ways to handle grounding, that are safe and code-

compliant…..it’s not just ONE answer. We do have our “preferences”, for best operation of the UPS, and that’s what shows in the Manual.

• If there are phase to neutral loads, we MUST ALWAYS bring in a source neutral, so no delta or HRG sources allowed in that case.

• The absence of transformers in the UPS does not impair the safety or code-compliant nature of the UPS, but it can cause confusion…..for the real grounding gurus, refer them to the white paper. It should help answer the really deep questions.

• Refer to the “Q&A” section of the paper to answer most installers and engineers’ concerns.

Page 23: Spears Grounding 2013

23© 2012 Eaton Corporation. All rights reserved.