Transformerless UPS
Neutral handling and grounding considerations
Ed Spears
May 2013
© 2012 Eaton Corporation. All rights reserved.
What do I need to know about neutral handling in Eaton transformerless 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
3W + Gnd Source
N
To the UPS
C
B
© 2012 Eaton Corporation. All rights reserved.
2
Installation practices
FUSE
SSW
SOURCE
X FMR
PDU
FUSE
FUSE
FUSE
A
N
B
A
B
N
C
N
B
A
C
C
FUSE
G
GND
G
SAFETY
GND TO
FRAME
• 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.
© 2012 Eaton Corporation. All rights reserved.
3
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
SSW
SOURCE
X FMR
PDU
FUSE
FUSE
FUSE
A
N
B
A
B
N
E12
C
B
A
C
C
FUSE
G
GND
G
SAFETY
GND TO
FRAME
© 2012 Eaton Corporation. All rights reserved.
4
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.
© 2012 Eaton Corporation. All rights reserved.
5
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.”
C
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”.
© 2012 Eaton Corporation. All rights reserved.
3-Phase UPS Output
A
B
Center
Point
Reference
6
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
UPS Output
Bypass
C
B
Center
Point
References
© 2012 Eaton Corporation. All rights reserved.
7
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.
© 2012 Eaton Corporation. All rights reserved.
8
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…….
© 2012 Eaton Corporation. All rights reserved.
9
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 “cornergrounded 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
© 2012 Eaton Corporation. All rights reserved.
10
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
SSW
HRG
SOURCE
A
B
N
C
G
PDU
FUSE
FUSE
A
FUSE
B
A
B
N
C
N
B
A
C
C
FUSE
GND
G
G
SAFETY
GND TO
FRAME
© 2012 Eaton Corporation. All rights reserved.
11
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.
N
FUSE
208 OR 400V
SSW
LOAD PANEL
SOURCE
W/ P-N LOADS
X FMR
FUSE
FUSE
A
N
N
FUSE
A
B
N
C
A
B
B
C
FUSE
C
G
G
GND
G
SAFETY
GND TO
FRAME
© 2012 Eaton Corporation. All rights reserved.
12
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.
© 2012 Eaton Corporation. All rights reserved.
13
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.
© 2012 Eaton Corporation. All rights reserved.
14
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
SSW
SOURCE
X FMR
PDU
FUSE
FUSE
A
N
FUSE
B
A
B
N
C
N
B
A
C
C
FUSE
G
GND
G
SAFETY
GND TO
FRAME
Fuse will not clear.
UPS will enter
current limit, and
transfer to bypass.
Bypass fuse will
clear.
© 2012 Eaton Corporation. All rights reserved.
Phase
to Gnd
Fault
15
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
SP5W
SOURCE
X FMR
PDU
FUSE
FUSE
A
N
FUSE
A
A
B
N
C
B
B
C
C
FUSE
G
GND
G
SAFETY
Return current can
NOT flow; IGBTs
are off when utility
has failed
GND TO
FRAME
© 2012 Eaton Corporation. All rights reserved.
Phase to
Gnd
Fault
16
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.
ON BATTERY 208/120V OR 415/240V
N
FUSE
208 OR 400V
SSW
LOAD PANEL
SOURCE
W/ P-N LOADS
X FMR
FUSE
FUSE
A
N
N
FUSE
A
B
N
C
A
B
B
C
FUSE
C
G
G
GND
G
SAFETY
Return current can
NOT flow; IGBTs
are off when utility
has failed
GND TO
FRAME
Fuse will not
clear. UPS will
enter current limit
at 300% nominal
for 20 cycles
© 2012 Eaton Corporation. All rights reserved.
Phase to
Gnd
Fault
17
What about faults?
• What if I have an HRG source?
FUSE
SSW
HRG
SOURCE
A
B
N
PDU
FUSE
C
G
FUSE
A
FUSE
B
A
B
N
C
N
B
A
C
C
FUSE
GND
G
G
SAFETY
Resistance limits
return current, and
triggers gnd fault alarm
GND TO
FRAME
© 2012 Eaton Corporation. All rights reserved.
Phase to
Gnd
Fault
18
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.
© 2012 Eaton Corporation. All rights reserved.
19
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.
9395
∆
Y
3
3
3
Pull an E12
wire from
each UPS
Bypass
AC/DC
DC/AC
4
Tie Cabinet
9395
3
3
Bypass
AC/DC
DC/AC
© 2012 Eaton Corporation. All rights reserved.
3
4
20
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.
SSBM
STSW
FBP
∆
Y
3
CBP
3
3
Pull an E12
wire from
each UPS
CBS
9395
3
AC/DC
DC/AC
4
© 2012 Eaton Corporation. All rights reserved.
21
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 codecompliant…..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.
© 2012 Eaton Corporation. All rights reserved.
22
© 2012 Eaton Corporation. All rights reserved.
23