CET

3350 Scott Blvd., Bldg. 55. Unit 1
Santa Clara, CA 95054 USA
Voice: 408/988-1346 | Fax: 408/988-4869
URL: http://www.powercet.com
E-mail: consulting@powercet.com
Sample Consulting Report
ATS Transfer Testing with Chiller’s Operational
Data Center One
Background
The purpose of the testing is to determine if any undesirable transient conditions are injected into the
electrical distribution or loads during a fast synchronous transition operation of the Automatic Transfer
Switch (ATS) associated with the chiller equipment operating at the Data Center One facility.
Methodology / Technical Approach
Two Dranetz-BMI PowerGuide 4400s were installed as shown in Figure 1 below. Each of the PG4400’s
was connected to the load side of the ATS with the 4th channel of monitor #1 connected to the utility
supply on the line side of the ATS and the 4th channel of monitor #2 connected to the line side of the
emergency source for the ATS. This configuration provides information relative to the power being
supplied to the chillers under any source condition as well as documenting the phase relationship
between the sources at the time of transfer.
Figure 1 – Power monitor ATS test configuration.
Date: 7/19/2007
Filename: Sample_ATS_Testing_Report.doc
Page 1 of 10
Sample Report
PowerCET Corporation
Key Findings
Introduction
During the test window the power source for the chillers was transferred a total of four times—utility to
generator, generator to utility and then the sequence was repeated. The following sections document
each of the four transfers.
The transfer from utility power is generally not a problem unless there is a significant phasing issue as the
power source impedance of the generators is significantly higher than the utility service and limits the
surge currents. Also the transfer from utility to generator, under test conditions, is an artificial condition.
In the case of an actual utility power interruption the generators would not be on so most of the rotating
equipment would have had time to decay and the soft-starts would re-engage for the restart of the
equipment after the generators are brought on-line.
The transfer from generator power to utility is generally when operational problems are experienced. In
the case of a short duration transfer the phase synchronization of the generator to the utility is critical.
Any significant mismatch in the phase relationships can result in very high peak surge currents which the
utility service, unlike the generator, is capable of supplying.
The actual ATS transfer is basically a three-cycle event.
•
•
•
The initiation cycle is the point at which the synchronization (phase relationship) between the
utility and generator are within the ATS’s programmed window and the transfer is enabled.
The open cycle is the period that the ATS wiper is moving from Source 1 (S1) to Source 2 (S2)
and prior to closing on the new source.
The closing cycle is the point at which the new source is physical connected to the load.
During the testing the transfer open period varied between 30ms and 38ms which is about as fast as the
mechanical ATS can transfer.
Some ATS equipment has a neutral (intermediate) position for the wiper which allows for variable open
delays to be programmed. The ATS wiper is actually paused in the neutral position before closing on the
alternate source.
Date: 7/19/2007
Page 2 of 10
Sample Report
ATS Transfer Testing
Data Center One
Transfer #1: Utility to Generator
Figure 2 (below) shows the first transfer from utility-to-generator with a 2° phase difference between the
two power sources at the time transfer is initiated. The phase drift during the open transition can be seen
in figure 4 on the voltage waveforms (upper trace)…gray (Va-b generator voltage) and brown ATS load
Va-b. Total open period was approximately 31.2ms. (Note: The inrush current at the time the ATS
recloses on the new power source over-ranged the 3000A current probes.)
Event Details/Waveforms
BOA3_070429
750
500
C-AV
250
Volts
CI
0
-250
BI
DI
A-BV
-500
DV
-750
A -B Generator V
A-B V
B-C V
C-A V
525
AI
500
Volts
B-CV
475
450
Phase
B
C
425
A -B V Generator V rms (val)
A -B Vr ms (val)
B-C V rms (val)
C-A Vr ms (val)
5000
4000
3000
2000
Amps
A
D
358°
V
A 298° 179° 55° 307°
A- B B- C C- A
240° 120°
V 0°
Magnitude
A
B
C
D
487.8
V
A 863.5 898.6 882.2 17.15
A- B
B-C
C- A
V 476.5 477.6 474.6
1000
0
-1000
31.243 ms
-2000
-3000
-4000
-5000
AI
BI
CI
Ph A Generator I
3000
2500
Amps
2000
1500
1000
500
0
A Irms (val)
01:06:43.50
04/29/2007
Sunday
01:06:43.52
B Irms (val)
01:06:43.54
C Irms (val)
Ph A Gener ator Ir ms ( val)
01:06:43.56
01:06:43.58
01:06:43.59
Event #4 at 04/29/2007 01:06:43.508
Figure 2 - Transfer from utility to generator. Channel DV is connected between phases A-B of the
generator and channel DI’s current probe is connected to generator phase A.
Date: 7/19/2007
Page 3 of 10
Sample Report
ATS Transfer Testing
Data Center One
Transfer #2: Generator to Utility
Figure 3 (below) shows the first transfer from generator-to-utility with a 2° phase difference between the
two power sources at the time transfer is initiated. The phase drift during the open transition can be seen
in figure 3 on the voltage waveforms (upper trace)…gray (Va-b utility voltage) and brown ATS load Va-b.
Total open period was approximately 31.0ms. (Note: The inrush current at the time the ATS recloses on
the new power source over-ranged the 3000A current probes.)
Event Details/Waveforms
BOA1_070429
750
C-AV
500
250
Volts
CI
0
-250
DI
BI
-500
DV
-750
A-BV
DV
A -B V
B-C V
C-A V
490
480
AI
470
460
Volt s
B-CV
450
440
V
A 297° 176° 49°
A- B B- C C- A
238° 119°
V 0°
Magnitude
A
B
C
430
420
D
2°
297°
D
486.0
V
A 748.8 876.8 804.1 15.38
A-B
B- C
C- A
V 477.9 478.5 462.9
410
D V rms (val)
A -B Vrms (val)
B-C V rms (val)
C-A Vr ms (val)
5000
4000
3000
2000
1000
Amps
A
Phase
B
C
0
-1000
31.025 ms
-2000
-3000
-4000
-5000
AI
BI
CI
DI
3000
2500
Amps
2000
1500
1000
500
0
A Irms (val)
01:14:36.35
04/29/2007
Sunday
B Irms (val)
C Irms (val)
D Irms (val)
01:14:36.40
01:14:36.45
Event #11 at 04/29/2007 01:14:36.357
Figure 3- Transfer from generator to utility. Channel DV is connected between phases A-B of the
utility supply and channel DI’s current probe is connected to utility supply phase A.
Date: 7/19/2007
Page 4 of 10
Sample Report
ATS Transfer Testing
Data Center One
Transfer #3: Utility to Generator
Figure 4 (below) shows the second transfer from utility-to-generator with a 1° phase difference between
the two power sources at the time transfer is initiated. The phase drift during the open transition can be
seen in figure 4 on the voltage waveforms (upper trace)…gray (Va-b generator voltage) and brown ATS
load Va-b. Total open period was approximately 37.9ms which was about 7ms longer than any of the
other open intervals measured during the testing. (Note: The inrush current at the time the ATS recloses
on the new power source over-ranged the 3000A current probes.)
Event Details/Waveforms
BOA3_070429-2
500
C-AV
Volts
250
CI
0
-250
DI
-500
DV
BI
DV
A-BV
A -B V
B-C V
C-A V
500
490
AI
480
470
Volts
460
B-CV
450
440
430
420
V
A 304° 190° 45°
A- B B- C C- A
238° 119°
V 0°
Magnitude
A
B
C
410
400
D V rms (val)
D
1°
304°
A -B Vrms (val)
B-C V rms (val)
C-A Vr ms (val)
5000
4000
3000
2000
Amps
A
Phase
B
C
D
488.2
V
A 251.9 424.3 389.8 15.34
A-B
B- C
C- A
V 442.8 446.0 429.9
1000
0
-1000
37.929 ms
-2000
-3000
-4000
-5000
AI
BI
CI
DI
3500
3000
Amps
2500
2000
1500
1000
500
0
A Irms (val)
02:07:44.06
04/29/2007
Sunday
02:07:44.08
B Irms (val)
02:07:44.10
C Irms (val)
D Irms (val)
02:07:44.12
02:07:44.14
02:07:44.16
Event #3 at 04/29/2007 02:07:44.083
Figure 4 – Transfer from utility to generator. Channel DV is connected between phases A-B of the
generator and channel DI’s current probe is connected to generator phase A.
Date: 7/19/2007
Page 5 of 10
Sample Report
ATS Transfer Testing
Data Center One
Transfer #4: Generator to Utility
Figure 5 (below) shows the first transfer from generator-to-utility with a 2° phase difference between the
two power sources at the time transfer is initiated. The phase drift during the open transition can be seen
in figure 5 on the voltage waveforms (upper trace)…gray (Va-b utility voltage) and brown ATS load Va-b.
Total open period was approximately 31.4ms. Prior to this test the 3000A current probes were
exchanged for 6000A probes which allowed the peak inrush currents to be fully displayed.
The table in figure 5 summarizes the MIN/MAX readings for each of the traces. Please note that values
for waveform plots reflect peak values and the time plot represents RMS values for the above waveform
in each of the graphs.
Event Details/Waveforms
BOA1_070429-2
C-AV
500
DI
DV
BI
Volts
CI
250
0
-250
-500
A-BV
AI
DV
D
485.0
V
A 699.4 821.1 825.0 14.38
A- B
B- C
C- A
V 472.7 473.5 462.2
M in
M ax
-686.4 687.9
DV
-681.1 681.0
A-BV
-681.2 681.1
B-CV
-681.0 681.5
C-AV
-1758 6432
AI
-5371 2822
BI
-4955 2628
CI
-1730 6338
DI
466.3 485.3
DVrms
A-BVrms 415.0 483.2
B-CVrms 426.8 483.7
C-AVrms 422.4 483.1
3.926 3807
AIrms
8.164 3169
BIrms
13.03 2829
CIrms
7.530 3765
DIrms
Volts
V
A 297° 182° 52°
A- B B- C C- A
239° 120°
V 0°
Magnitude
A
B
C
B-C V
C-A V
500
D
3°
298°
475
450
425
D Vrms (val)
A-B Vrms (val)
B-C Vrms (val)
C-A Vrms (val)
5000
Amps
Phase
B
C
2500
0
-2500
31.420 ms
-5000
AI
BI
CI
DI
3500
Amps
A
A-B V
525
B-CV
2500
1500
500
0
A Irms (val)
02:16:37.30
04/29/2007
Sunday
02:16:37.32
B Irms (val)
02:16:37.34
C Irms (val)
02:16:37.36
D Irms (val)
02:16:37.38
02:16:37.40
Event #300 at 04/29/2007 02:16:37.324
Figure 5 - Transfer from generator to utility. Channel DV is connected between phases A-B of the
utility supply and channel DI’s current probe is connected to utility supply phase A.
Date: 7/19/2007
Page 6 of 10
Sample Report
ATS Transfer Testing
Data Center One
Additional Discussions
Figure 6 is a waveform capture documenting the phase drift between the two different power sources.
Once the ATS receives the signal to transfer the synchronizing circuitry monitors the phase relationships
between the respective power sources and does not initiate (enable) transfer until the phase difference is
within a specified window. In some of the more sophisticated synchronous transfer equipment there is a
feedback signal to the generator. The slew rate for the generator/ATS configuration tested appears to be
about 60-cycles long (6°/cycle).
Event Details/Waveforms
DV
500
A-BV
Phase
D
V 119°
A-B
V 0°
Magnitude
D
V 485.3
A- B
V 481.0
Volts
250
0
-250
-500
02:08:05.9
04/29/2007
Sunday
02:08:06.0
DV
02:08:06.1
02:08:06.2
A-B V
Event #150 at 04/29/2007 02:08:06.008
Figure 6 – Waveform capture showing phase drift between the utility source A-BV and the
generator source DV.
Date: 7/19/2007
Page 7 of 10
Sample Report
ATS Transfer Testing
Data Center One
Sample Report
ATS Transfer Testing
Data Center One
Prior to disconnecting the monitoring equipment the thresholds were adjusted to capture the current
waveform envelop, figure 7, as the chiller spins-down. The white boxes document the time for the
respective segment in the current envelope.
Event Details/Waveforms
1000
56.161 Amps
00:00:10.143651
500
Amps
00:00:23.706510
0
00:00:33.878633
-500
-1000
CI
02:45:20
04/29/2007
Sunday
02:45:30
02:45:40
Event #441 at 04/29/2007 02:46:19.547
Timed
Figure 7 – Chiller spin-down current waveform envelop.
Date: 7/19/2007
Page 8 of 10
Summary / Recommendations
The ATS testing did not reveal any significant transients being generated as a result of switching between
power sources. The open period experienced during the transfer operation was between 30ms and 38ms
which is about as fast and the mechanical ATS can switch between sources.
The phase synchronization at the time transfer was initiated was between 1° and 3° degrees with a slew
rate of about 6° per cycle.
At the closing cycle of the ATS operation, when the load is connected to the new source, an inrush
current is generated with peak values between 6000Apeak and 7000Apeak which corresponds to RMS
values of between 3000Arms and 4000Arms maximum. These peak values may not be all that different
from levels that would be experienced during start of the chillers. (Note: These values are for combined
operation of two chillers that were in operation during the testing.)
While the testing indicates no significant problems with this type of operation there is always a risk
associated with switching large loads between sources. The Data Center One facility appears to have an
ATS/Generator configuration with a narrow phase synchronization window and in the four instances
measured initiated transfer with a very small phase angle difference. If the equipment can maintain this
level of operation then the worse case peak surge currents should be in the range measured in the
testing.
Some ATS equipment has the ability to send a turn-off signal to selected equipment prior to initiating the
transfer that allows rotating equipment to shutdown, allowing the soft-starts to be re-engaged. This
method of operation would be preferred as the equipment can be restarted (sequenced-ON) after the
successful transfer to the new source. The advantage of this type of operation is that there is little, if any,
danger of equipment being connected to an out-of-phase source and, secondly, the loads can be
sequenced so that multiple loads are not competing for start-up energy at the same time from the new
source.
The sequencing of loads on to a new source is common practice in many applications. Generally benign
loads are connected first—lighting being a good example—and followed by more critical load—elevators,
HVAC, UPS, etc.
RECOMMENDATION: Investigate the feasibility of implementing an automatic shutdown of the chillers
(and other critical equipment) prior to the initiation of a transfer followed by a sequenced restart to avoid
any possibility of out-of-phase transfers and to limit current rush on the new power source.
Data Center One conducts preemptive transfer to emergency generator power when certain weather
related conditions are present—i.e., thunderstorm activity in the immediate area. The preemptive transfer
practice is not all that uncommon for critical installations…DHS, NASA (Johnson Space Center) and
Charles Schwab are examples of organizations that practice preemptive transfers to emergency
(generator) power.
The preemptive transfer practice significantly increases the number of operations for all the electrical
switching equipment involved. Conventional ATS equipment was never intended for this frequency of
usage and accelerated equipment aging and degradation can be expected. The surge currents, even for
a well synchronized break-before-make transfer can be substantial as demonstrated by the 6000 to 7000
peak amp currents recorded during the testing at the Data Center One facility.
If the intention is to implement a regular pre-emptive transfer to emergency (generator) power during
weather related events then consideration should be given to upgrading the facility to a synchronous
Date: 7/19/2007
Page 9 of 10
Sample Report
ATS Transfer Testing
Data Center One
(paralleling) make-before-break type of transfer which will result in substantially reduced surge currents
and decreased over stressing of the electrical equipment supporting the Data Center One facility.
RECOMMENDATION: Investigate the feasibility of implementing a synchronous, paralleling, makebefore-break type of system for the Data Center One facility.
Prepared by:
Bruce Lonie
President, PowerCET Corporation
Date: 7/19/2007
Page 10 of 10
Sample Report
ATS Transfer Testing
Data Center One