Download Available Information on Voltage Sags and Interruptions

TR41.7.4-03-02-001
STANDARDS PROJECT
PN-3-3283-RV2, Environmental Considerations (Revision of TIA/EIA571-A)
TITLE
Proposal for AC Power Interruptions (Clause 4.3.1.3)
SOURCE
VTech Communications
2 Shannon Ct
Howell, NJ 07731
CONTACT
Stephen R Whitesell
Phone 732 751 1079
Fax
732 751 0095
Email [email protected]
DATE
January 29, 2003
DISTRIBUTION
TR-41.7.4
Abstract
The criteria in Clause 4.3.1.3 of TIA/EIA-571-A require products to span a complete power interruption of
125 ms in duration, with the ability to span a 2000 ms duration identified as desirable. These criteria have
little basis in reality since complete power outages lasting between 20 and 2000 ms are extremely rare.
On the other hand, voltage sags with time durations in this range are quite common. New criteria are
proposed that require the ability to withstand voltage sags to specified levels for given durations. Longer
durations are required for less severe sags. These criteria are based on information resulting from a
study of distribution system power quality conducted in the mid 1990s and on data gathered by the
semiconductor industry.
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Disclaimer:
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2
Current Criteria
Clause 4.3.1.3, Power Interruptions, of TIA/EIA-571-A reads as follows:
Equipment shall maintain the connections of established calls for all ac power interruptions lasting
125 ms or less. It is desirable that the equipment maintain connections over interruptions up to 2
seconds in duration. It is also desirable that other equipment functions (digit reception,
supervision, attendant operation, etc.) be maintained during interruptions lasting 125 ms or less.
Background on Power Distribution System Faults
Power distribution systems have a tree-like structure. Higher voltage main distribution feeders give way
to lower voltage local feeders and ultimately to the drop wires to individual homes and businesses. These
feeders are usually equipped with both fusible links and “reclosers”. A recloser is a circuit breaker with a
means of automatically re-closing the circuit after a predetermined time-out interval. A fault in one part of
the system may or may not have an effect on the power delivered by other parts of the system. An open
at some point in the system obviously affects everything downstream from it. A short may affect parts of
the system downstream from it; it may also affect other nearby branches in the system structure. A good
overview of voltage sags and interruptions is provided in Chapter 3 of Electrical Power Systems Quality
by Dugan, et al.1 The following discussion is based on information from that chapter.
It is first helpful to distinguish voltage sags as short duration events (typically 0.5 to 30 cycles) in which
the rms voltage level is between 10% and 90% of its nominal value. An interruption occurs when the rms
voltage drops below 10% of nominal. Momentary interruptions can be as short as 20 cycles, but are
typically on the order of 2 to 5 s. In rare instances, transients on the power line can result in interruptions
lasting for 1 cycle or less. Voltage sags and momentary interruptions are generally caused by short
duration or self-clearing faults on the power distribution system. Sustained interruptions of a minute or
more are usually caused by permanent faults that require corrective action by a repair crew.
Consider the customer who is supplied by a feeder on which a fault occurs due, for example, to a falling
tree limb. The customer will experience a voltage sag during the fault condition followed by an
interruption when the circuit breaker opens (typically in 3 to 6 cycles). The breaker will typically remain
open for 2 to 5 s and then re-close. If the fault condition has cleared (e.g., the tree branch has burned
due to arcing and fallen off the feeder), power is restored to its nominal value and life goes on. If the fault
is still present, the breaker will again open in a few cycles and the process will repeat. After the second
or third attempt, the breaker may go into a slow trip mode in which it takes perhaps 30 cycles to open. If
the fault still persists, the breaker may remain open. It is also possible that a high fault current during the
slow trip mode will cause the fusible link to open. In either case, the customer experiences a permanent
interruption until a repair crew can be dispatched to rectify the fault condition.
A much more common condition for an individual customer is for a fault to occur on one of the many other
feeders supplied by the same substation or even on the transmission line providing power to the
substation. In this case, the customer will experience a voltage sag during the time the fault is actually on
the system, but will have normal power restored as soon as the breakers open on feeder experiencing the
fault. There is a possibility of a repeated sag in 2 to 5 s due to the re-closing of the breaker as described
above. Given the large number of feeders supplied by a single substation, the probability that a given
customer will experience one or two voltage sags due to a fault event is much greater than the probability
of experiencing an actual interruption.
Available Information on Voltage Sags and Interruptions
The Computer Business and Equipment Manufacturers Association (CBEMA) first issued a curve in the
late 1970s describing the percentage of nominal voltage vs. time that computing equipment should be
able to tolerate without malfunctioning. This curve addressed voltage swells as well as sags. The
Information Technology Industry (ITI) Council (successor to CBEMA) revised and republished the CBEMA
curve in 1996 and again in 2000. Details of the revised curve are shown in Figure 1. 2
3
0.01
500
Cycles of 60 Hz Power
1.0
10
0.1
100
1000
Percent of Nominal Value
400
300
Prohibited Region
200
100
No Interruption in Function Region
No Damage Region
0
0.001 0.003
0.020
0.5
10.0
Duration in Seconds
Figure 1: ITI (CBEMA) Curve
The Electric Power Research Institute (EPRI) conducted a distribution system power quality study in the
mid 1990s that collected data at 277 sites across the U.S. for a period of 27 months. 3 The results of the
study, including the complete measurement database, have been published in three volumes. 4,5,6 The
average number of sags per year identified in the EPRI study are shown in graphical form in Figure B.4,
Annex B, of IEEE Standard 1346-1998.7 This graph is reproduced in Figure 2. Contours for 5, 10, etc.
sags per year are shown. These contour lines represent events having sag voltages at least as low as
the percentage shown and durations of at least as long as the time shown. For example, there were on
average five sag events to 65% of nominal voltage or less lasting for 200 ms or longer.
Comparing Figures 1 and 2 suggests there may be several sag events each year not covered by the
CBEMA curve. For example, the EPRI data shows an average of 10 sags per year to 70% of nominal
voltage or less lasting for more than 100 ms and 5 sags to 45% of nominal voltage or less lasting for at
least 100 ms. Equipment compliant with the CBEMA curve could be susceptible to these sag events.
4
Figure 2: Average Results from EPRI Distribution System Power Quality Study
Concerns about sag events not covered by the CBEMA curve led the Semiconductor Equipment and
Materials International (SEMI) organization to conduct their own study of the power quality delivered to 15
semiconductor manufacturing facilities. Data was collected on over 1000 voltage sag events, with 166 of
the events (15.4%) found to be below the CBEMA curve. On average, each site experienced 5.4 sags
per year that were below the CBEMA curve. The results of the study, the CBEMA curve and a
modification to the curve adopted by the semiconductor industry in their own SEMI F47 standard are
shown in Figure 3.8,9 This figure clearly shows a large number of sag events lasting for 4, 5, or 6 cycles
below the CBEMA curve.
CBEMA Curve
SEMI F47 Modification
Figure 3: Results of Semiconductor Industry Voltage Sag Study
5
Discussion
Based on the available information, the existing criteria in clause 4.3.1.3 of TIA/EIA-571-A has little basis
in reality. Momentary interruptions, when they occur, last for a minimum of 20 cycles (333 ms) and more
typically for a period of 2 to 5 seconds. On the other hand, short duration voltage sags lasting for 4, 5, or
6 cycles (6 cycles = 100 ms) do frequently occur. Thus, applying the 125 ms criteria in clause 4.3.1.3 to
voltage sags instead of to interruptions would give it validity.
Requiring equipment to span sags to 50% of nominal voltage for durations up to 125 ms (7.5 cycles)
would pick up the high-density events from the SEMI study falling below the CBEMA curve as shown in
Figure 4. Continuing in a step-wise fashion by requiring products to span sags to 70% of nominal for 500
ms (30 cycles) and 80% of nominal for 1 s (60 cycles) picks up most of the longer duration high-density
events in the SEMI study results. It is also consistent with the slope of the EPRI study shown in Figure 2.
A second curve is proposed in Figure 4 as a desirable objective. It picks up most of the lower-density
data points from the SEMI study by using thresholds of 40% of nominal voltage for 125 ms, 50% of
nominal for 500 ms, 70% of nominal for 1 s, and 80% of nominal for 2 s. In all cases equipment is
expected to operate continuously at 90% of nominal voltage.
CBEMA Curve
Proposed Requirement
Proposed Objective
Figure 4: Proposed Voltage Sag Requirement and Objective
Proposal
Change the title of clause 4.3.1.3 to “Short Duration Power Interruptions and Voltage Sags”. Replace the
text of clause 4.3.1.3 with the following text and figure:
Commercial ac power may experience transient voltage interruptions lasting for less than one
cycle, short duration voltage sags lasting for several cycles, and momentary interruptions lasting
on the order of two to five seconds. Figure N shows the transient voltage interruption and sag
criteria for which equipment using commercial ac power is expected to operate. The criteria are
expressed in terms of the percentage of nominal voltage as a function of time and equivalent
number of cycles of ac power. A nominal utilization voltage of 115 VRMS shall be assumed when
checking for compliance with these criteria.
6
Equipment in any normal operating state shall continue to operate for any voltage sag or
interruption in the region of Figure N identified as Operation Required. It is desirable that
equipment continue to operate for any voltage sag in the region of Figure N identified as
Operation Desirable. Continue to operate in this case means the equipment shall not change
state or lose any information during the voltage sag or interruption and shall continue to function
normally afterwards.
Note: Two or three voltage sags could occur in succession. Such sags would normally be
separated by intervals of 2 to 5 seconds or longer. It is recommend that 5-second intervals be
used when applying multiple sags for compliance testing.
20
Percent of Nominal Voltage Value
100
125
Time in ms
500
1000
2000
Operation
Required
80
Operation
Desirable
60
40
Operation
Not Required
20
0
1
1.2
7.5
30
10
Cycles of 60 Hz Power
60
120
100
Figure N: Transient Voltage Interruption and Sag Criteria
Equipment using commercial ac power is not required to operate during short duration voltage
interruptions lasting for several seconds. However, such equipment shall not lose any stored
information during the voltage interruption and shall continue to function normally afterwards.
7
References
1
Roger C. Dugan, Mark F. McGranaghan, and H. Wayne Beaty. Electrical Power Systems Quality. New
York: McGraw-Hill. 1996.
2
Technical Committee 3 of the Information Technology Industry Council. ITI (CBEMA) Curve Application
Note. www.itic.org/technical/iticurv.pdf. 2000.
3
Dan Sabin. Overview of the EPRI Distribution System Power Quality Project.
www.electrotek.com/PROJECTS/DPQ/Dpq.htm. May 2, 2001.
4
Thomas E. Grebe, D. Daniel Sabin, and Mark F. McGranaghan. An Assessment of Distribution System
Power Quality, Volume 1: Executive Summary. EPRI Report TR-106294-V1. Palo Alto, California. May
1996.
5
D. Daniel Sabin. An Assessment of Distribution System Power Quality, Volume 2: Statistical Summary
Report. EPRI Report TR-106294-V2. Palo Alto, California. May 1996.
6
Daniel L. Brooks and D. Daniel Sabin. An Assessment of Distribution System Power Quality, Volume 3:
The Library of Distribution System Power Quality Monitoring Case Studies. EPRI Report TR-106294-V3.
Palo Alto, California. May 1996.
7
IEEE 1346-1998, IEEE Recommended Practice for Evaluating Electric Power System Compatibility with
Electronic Process Equipment. July 21, 1998.
Mark Stephens, John Soward, Dennis Johnson, and Jim Ammenheuser. “Power Quality Solutions for
Semiconductor Tools, Part 2: Voltage Sag Immunity Standard and Testing Methodology,” Power Quality
Assurance. www.f47testing.com/semi_documents/Semi_part_2_PQ_Magazine_Article.pdf. May/June
2000.
8
9
SEMI F47-0200, Specification for Semiconductor Processing Equipment Voltage Sag Immunity.
December 15, 1999.