Title of Invention

AN APPARATUS FOR PROTECTING AGAINST SLOW CLOUSER OF A CONNECTION CIRCUIT BREAKER DURING SYNCHRONIZATION OF AN ELECTRIC POWER GENERATOR AND AN ASSOCIATED ELECTRIC POWER SYSTEM

Abstract The apparatus comprises a circuit for automatically and continuously determining the difference voltage between the voltage at the input side of a connecting circuit breaker and the voltage at an output side thereof between an electric generator and an associated power system, following imitation of closure of the connecting circuit breaker. The difference voltage is representative of the voltage angle of the generator relative to the electrical system. The difference voltage is compared against a reference value, which is selected such that a voltage angle value greater than the reference value will result in damage to the generator and/or the electrical system. An output signal is developed which can then be used to trip circuit breakers serving the electrical system if the reference value is exceeded by the difference voltage.
Full Text Description

Technical Field
This invention relates generally to the
synchronization of an electric power generator and an
associated electrical system to which the power generator is
to be connected, and more specifically concerns a circuit for
preventing damage to the generator and/or the electrical
system when the connecting circuit breaker is too slow in
closing.
Background of the Invention
In connecting an electric generator, transformer or
similar equipment to an associated electrical system, a
circuit breaker connecting the generator and the electrical
system is closed after selected operating conditions of the
generator and the electrical system are satisfied. The
connection must occur when the two systems are synchronous in
their respective operation; the systems can be damaged when
the breaker closes when the systems are not synchronous. The
general arrangement of two connecting systems is shown in
Figure 1, which includes a generator 10, an electrical system
12, a transformer 14 and a connecting circuit breaker 16.
Circuit breakers 18, 20 and 22 are associated with the
electrical system 12.
Synchronous power generators are started when they
are electrically isolated from the electrical system.
Connecting such a generator to a power system is dynamic and
requires the coordinated operation of electrical and
mechanical functions as well as human action. The generator
must be connected to the power system with minimal power
surges or swings. This is accomplished by closing the

connecting breaker (16 in Figure 1) when the generator
substantially matches the power system in voltage magnitude,
phase angle and frequency. Typically, there will not be an
exact match and some power will flow into or our or tne
generator to force it into synchronization with the power
system. If that power is excessive because of a Door match,
severe damage to the qenerator or the power system can result.
The generator 10, once started, will begin to
increase in speed prior to connection with the electrical
system 12, and once the speed is relatively near (typically
slightly greater than) the synchronous speed appropriate for
the electrical system, closure of the system circuit breaker
16 will be initiated to connect the generator to the
electrical power system. Various ways have been used to
determine the best point in time for initiating closure of the
circuit breaker, including monitoring of the generator
operating angle, the speed (frequency) of the generator and
the voltage magnitudes of the generator and the power system.
Typically, the generator slip frequency and the closing time
for the circuit breaker are important variables in determining
the time at which to initiate the closing of the system
circuit breaker. The slip frequency is the difference in
frequency between the generator voltage and the system
voltage.
Once circuit breaker closure is initiated, however,
there is no way to stop the circuit breaker from closing, even
though closing may have been initiated at the wrong time
and/or the closing may be too late. Attempting to reverse a
partially closed breaker could result in extreme failure of
the breaker.
When the circuit breaker begins to close, it will
either close within the expected and acceptable time limits,
or it will close slower than expected but still within an
acceptable time for the power system and the generator, or
lastly, the circuit breaker will not close at all, or at a
slower rate than is acceptable. Delay in closing can be due
to a variety of causes, including electrical problems or
mechanical problems, such as corrosion, degraded lubrication,
etc.
There is thus a range of anales between the infinite
and the generator voltage which is acceptable at the time of
actual closure of the circuit breaker. Generally, for a
particular angle, as long as the circuit breaker is able to
physically close without causing severe damage to the
generator or electrical system, that particular difference in
angle is acceptable. Existing circuit breaker controls
typically take into account the estimated circuit breaker
closing time and initiate closing of the breaker at such a
time that when the circuit breaker actually closes, the
generator and system voltage angles will be within a small
difference angle. If the circuit breaker operates slower than
expected, but not too slowly, it still may actually close
within an acceptable angle such that damage does not result.
However, if a circuit breaker operates so slowly that the
generator and the system voltage angles are no longer within
an acceptable range of difference, damage can result to the
system or the generator. As indicated above, however, even if
it is known that the breaker closing is going to take too
long, the breaker could not itself be stopped from closing.
Figure 2 is a diagram illustrating the voltage angle
relationship between the generator and the system. The solid
line 26 represents for illustration a system angle (at 12
o'clock), while solid line 28 represents a generator voltage
angle which is rotating clockwise, due to a slightly higher
frequency. The position of line 28 is for illustration only.
Area 30 shown between the dotted lines 31-31 represents a
"desired" voltage angle range, i.e. ±3 degrees. Area 32
between the solid lines 33-33 represents an angle difference
which is beyond the desired angle difference but which is
acceptable for proper operation. This could be as much as ±7 0
degrees. A more typical value, depending upon the particular
application, will be ±10-15 degrees. If the circuit breaker,
however, is even slower, it may actually close beyond lines
36-36. In this situation, the voltage angles are so different
that severe system damage will result.

As indicated above, the circuit breaker takes a
finite time to physically close after closure is initiated.
In order to accommodate this finite time, the operator (or the
automatic control system) who is initiating the closure must
actually initiate closure of the breaker prior to its reaching
the desired angular difference region. The actual value, of
generator slip and the circuit breaker close time determines
the point at which the circuit breaker closure is initiated.
There are various known methods to accomplish the
synchronizing process which takes into account various closing
issues. The closing procedures, however, do rely on the
breaker closing in a specified time. To protect against a
slow breaker, a timer has been used. If the breaker does not
actually close within that predetermined time, referring again
to Figure 1, breakers 16, 18 and 20 will all be tripped, thus
clearing the system bus serviced by the generator. This
clearing of the bus is an extreme situation and should be
avoided if possible, although it is better than permitting the
system to be damaged. Further, the timer is typically set
very conservatively, i.e. a relatively short time, to ensure
against system damage. It is thus possible that the bus
circuit breakers could be tripped in a particular situation
even though the system circuit breaker would have in fact
closed in time.
Hence, it is desirable to provide a control circuit
which monitors the actual operating characteristics of the
generator and the power system such that the bus circuit
breakers will trip substantially only when the system circuit
breaker is in fact going to close late. Such a system would
thus be both secure yet efficient for generator/power system
synchronization.
disclosure of the Invention
Accordingly, the present invention is an apparatus
for protecting against slow closure of a connecting circuit
breaker during synchronization of an electric power generator
and an associated electric power system, comprising: means
for automatically determining a value representative of the
voltage angle of the generator relative to that of the
electrical system to which it is to be connected, following
the start of closure of the connecting circuit breaker; means
for comparing the representative value with a reference value;
and means for developing an output signal which is used to
trip circuit breakers serving the electric power system if the
reference value is exceeded by the representative value.
accompanying
Brief Description of the/Drawings
Figure 1 is a simplified schematic drawing showing
the connection of the circuit of the present invention
relative to generator and power system synchronization.
Figure 2 is a vector representation of the angle
difference between the generator and the system, illustrating
one example of selected safe regions for generator-power
system synchronization.
Figure 3 is a block diagram showing a portion of the
overall circuit of the present invention.
Figure 4 is a block diagram showing a system for
determining initiation of closure of a circuit breaker for
synchronization of a generator and an associated power system.
Figure 5 is a block diagram showing the system for
enabling and resetting the control circuit portion of the
present invention.
Figure 6 is a block diagram showing the control
circuit of the present invention.
Best Mode for Carrying Out the Invention
Referring again to Figure 1, the circuit 40 of the
present invention is shown in dotted lines as part of a
synchronization system comprising a generator 10, an electric
power system 12, a transformer 14, and system circuit breaker
16... In Figure 1, control circuit 40 obtains two sets of
voltages from the connecting power line 41, V* from before the
circuit breaker 16, through transformer 43, and Vy from after
the circuit breaker 16, through transformer 45. Further,
current I on the power line 41 is obtained. All of these
values are used in the control circuit 40, as discussed below.
Referring now to Figure 3, the currents for all
three phases (I*, IB and Ic) are compared against a threshold
setting in comparators 42, 44 and 46. These comparators
provide one determination as to whether the breaker 16 is in
fact actually closed because of the presence of current
flowing in the power line 41. If there is current, then the
control circuit of the present invention is not enabled, as
discussed further hereinafter.
Voltages for all three phases (Vax, VBX and Vcx) from
the power line before circuit breaker 16 are compared with a
first threshold setting by comparators 50, 52 and 54. This
threshold comparison determines whether or not there is a
voltage present for all three phases before the breaker. The
outputs of comparators 50, 52 and 54 are applied to an AND
gate 56. The same determination is made for all three phases
of the voltage following the circuit breaker 16 by comparators
58, 60 and 62. The outputs of comparators 58, 60 and 62 are
applied to an AND gate 64. The output of AND gates 56 and 64
are applied to AND gates 66 and 68, the outputs of which are
then applied to an OR gate 70. AND gates 66 and 68 and OR
gate 70 determine whether one (or both) sides of the system
is a~ feixable indication that a fuse has blown (BPF) , the
control circuit of the present invention is disabled, as
discussed more in detain KeTrein&FEeF.
In addition, the voltage *of one phase (phase A) of
Vx is compared against a second threshold setting, in
comparator 72 to provide an overvoltage indication. If the
value of phase A of Vx exceeds the threshold, indicating an
overvoltage condition, the signal from comparator 72 will be
used to disable the control circuit.
Lastly in Figure 3, each phase of voltages Vx and Vy
are applied, respectively, to difference determination
circuits 76, 78 and 80 to determine the difference voltage.
The difference voltage is important in the present invention,
since it is known _ to fee. repxesentative of the voltage angle
difference. The resulting difference values are then compared
against a first, "high" setting in comparators 84, 85 and 86

and a second, 'low' setting in comparators 87, 88 and 89. The
high setting is a voltage which represents the largest voltage
angle permissible (in volts) between the generator and the
power system, for example 69 volts in the embodiment shown,
while the low setting is small enough such that when the
difference voltage drops below this setting for a selected
period of time, the circuit breaker is considered to be
closed. The low setting should therefore be quite small, such
as, for example, 3,5 volts.
Hence, the outputs of Figure 3 provide both
threshold circuit conditions necessary for operation of the
control circuit, as well as values of difference voltage which
are an indication of voltage angle, which can be analyzed to
reliably determine when the voltage angle difference is in or
approaching the danger area so that the bus circuit breakers
should be tripped to avoid damage to the system.
While the above-described circuit uses all three
phases of the current and three phases of the two voltages, it
should be understood that all three phases are not necessary
for the present invention. One phase could be used. The
three phases provide additional security and reliability.
Figure 4 shows a circuit which .analyzes the
frequency slip and the voltage angle between the two systems
to determine when closure of the circuit breaker should be
at a particular point in time io necessary, tne circuit shown
the breaker to close. It should be understood that while the
determination of whether or not the breaker should be closed
at a particular point in time io necessary, tne circuit shown
in Figure 4 is only one example of several different circuits
and other approaches which could be used to make such a
determination. A high output from timer 126 is the indication
that it is an appropriate time for initiation of the closure
of the circuit breaker.
The circuit of Figure 4 has four enable conditions
which are combined by an AND gate 98. A first enable
condition which is applied to a NOT input 100 of AND gate 98
is the overvoltage indication from comparator 72 in Figure 3.
If an overvoltage is present, then the output of AND gate 98

in Figure 4 will be low. The output of comparator 50 in
Figure 3, applied to input 102 of AND gate 96, indicates the
presence of voltage before circuit breaker 16, while the
output of comparator 58 in Figure 3, applied to input 104 of
AND gate 98, indicates the presence of voltage following
circuit breaker 16. Both of these voltages must be present to
enable the circuit of Figure 4- Lastly, a SYNCEN (synchronous
enable) signal from Figure 5 is applied to output 106 of AND
gate 98. If all the enable conditions are present (including
a lack of overvoltage condition), the output of AND gate 98
will be high, which initiates a timer 108.
The output of timer 108 will be high after the
output of AND gate 98 has been high for 15 cycles, and is
applied to both an angle calculation circuit 110 and a
frequency calculation circuit 1T2~. vector (not instantaneous)
voltages V« angle. The result of frequency calculation block 112 is the
frequency slip of the generator. This is a conventional
calculation and the result is applied to comparators 114 and
116. Comparator 114 compares the resulting frequency slip
value from calculator 112 against a first, high threshold,
while comparator 116 compares the calculated frequency slip
against a second, low threshold. The first (high) and second
(low) thresholds provide a window of acceptable slip value.
In one example, the high threshold could be 0.26, while the
low threshold could be 0.008. The output of comparator 114 is
applied as one input to an AND gate 118, while the output of
comparator 116 is applied as one input to AND gate 120.
The Vax and VAY voltages are also used to make the
angle calculation at 110. The angle calculation takes into
account the slip frequency and the nominal circuit breaker
close time to provide an indication of when closure of the
breaker should be initiated. The circuit breaker nominal
operating time is a setting established within the relay or
other protective device, while the slip frequency is
calculated as discussed above. The angle calculation is then
compared in comparators 122 and 124 against first and second
settings. The first setting is typically a desired setting,
while the second setting could be a different value, although
in this application it can be the same as well.
The angle comparison in comparators 122 and 124
determines that the actual voltage angle of the generator is
within the desired angle range for initiation of closure of
the breaker. The angle and frequency comparisons from
comparators 122 and 114 are applied to AND gate 118, while the
angle and frequency comparisons from comparators 124 and 116
are applied to AND gate 120. a high output from AND gate 118
indicates that the circuit is ready to close, i.e. the
generator is actually not at the desired angle calculation but
will be when the time-to-close of the breaker and the
frequency slip are taken into account.
The output of AND gate 118 is applied to a 4.5 cycle
timer 126, while the output of AND gate 120 is applied to a
4.5 cycle timer 128. Since there is a "window" of frequency
slip established, the output of timer 128 should be low
(indicating that the slip is above the minimum threshold,
while the output of timer 126 should be high, indicating that
the slip is below the maximum threshold. The outputs of
timers 126 and 128 are applied to the control circuit shown in
Figure 6.
Figure 5 shows a portion of the circuit 40 which
provides the SYNCEN (sync enable) signal and system reset
signal RCLS, which are also applied to the circuit of Figure
6. The circuit 40 of the present invention has three possible
operator-selected conditions: (1) an OFF condition, in which
the system is not operational; (2) a TEST condition, in which
the system is operated under normal conditions, but the
circuit breaker 16 is electrically isolated so that when it
closes it does not electrically connect the generator with the
system; and (3) an ON condition, in which the circuit breaker
16 is electrically connected between the generator and the
electrical system.
In Figure 5, if the system is neither in the TEST or
ON condition, the output of AND gate 134 will be high to input
135 of OR gate 136, as will the output of OR gate 136
accordingly. A high output from OR gate 136 will result in a

high reset signal (RCLS) and, because of inverter 133, a low
SYNCEN output. A low SYNCEN signal will effectively disable
the circuit of Figure 4, so that closure of circuit breaker 16
cannot be initiated.
The second input 137 to OR gate 136 reflects the
condition of an auxiliary contact, which is in a closed
condition when current can flow in the power line 41 through
breaker 16. In this case as well, the output of OR gate 136
is high, providing a high reset signal RCLS and a low SYNCEN
signal. Still further, if the three current outputs
(comparators 42, 44 and 46) from Figure 3 are all high, as
determined by AND gate 140, indicating that current is
actually flowing in power line 41/ there is no need to
initiate a breaker closure procedure. Lastly, the low
threshold difference voltage values from comparators 87, 88
and 89 in Figure 3 are applied to three not inputs of AND gate
142. If the difference voltages are low (below the low
threshold), a high signal is applied to a timer 144, which has
a long enough pick up (delay) time that an output of timer 144
indicates that the circuit breaker is in fact closed. The
output of timer 144 is applied to input 139 of OR gate 136.
As an example, the pickup time of timer 144 in the embodiment
shown is 167 cycles, based on a selected frequency slip and a
desired close angle. If any of the above conditions are
true, then the circuit will be reset by the RCLS signal and
SYNCEN will be low.
However, if none of the above conditions is true,
the output of OR gate 136 is low, so that there is no reset
(RCLS) and a SYNCEN signal is present. The SYNCEN signal is
applied as an enable signal back to Figure 4.
Figure 6 shows the key part of the control circuit
40 of the present invention. When the circuit 40 is in the
TEST mode or the ON mode as selected by the operator, a high
output from OR gate 146 is applied as one input to a first
output AND date 148.
The two outputs from Figure 4 (timers 126 and 128)
are applied as inputs to AND gate 154, the high threshold
signal from timer 126 being applied to one input 156, and the

low threshold signal from timer 128 being applied to a NOT
input 158. The output of AND gate 154 will be high when the
slip frequency is within the specified range and the angle
based on slip frequency and breaker close time is acceptable.
The output of AND gate 154 is applied to one input of an AND
gate 160.
An OR gate 162 has two inputs, one input being a
breaker close initiation signal (ACLOSE) by an auxiliary
device, such as an autosynchronizer, while the other is a
manual (operator-initiated) initiation signal (MCLOSE), from a
switch.
When one of the "close" inputs is high, timer 164 is
started. The output of timer 164 is applied as another input
to AND gate 160. The close initiation signal sets the timer
164 running for a specified {dropout) time. If synchronous
conditions are met as indicated by a high signal from AND gate
154, within the dropout time of timer 164, the output of AND
gate 160 goes high, which sets a latch circuit 168. If the
synchronous conditions do not occur within the dropout time of
timer 164, latch 168 is reset. When latch 168 is set, timer
170 is initiated. The output of timer l?0 is applied as
another input to AND gate 148.
The output of latch 168 is also applied as one input
to AND gate 172, the other input which is the SYNCEH signal
from Figure 5. Timer 174 is set to a preselected pickup time
which allows sufficient time for the breaker to close. In the
present case, as one example, the time is 5 cycles, which is
based on the time the breaker should take to close. The
output of timer 174 is applied to a NOT input of output AND
gate 148. With a combined output of OR gate 146, timer 170
and a lack of a high signal from timer 174, the resulting
output of AND gate 148 will be high, which signal is sent to
the circuit breaker coil to initiate closure of breaker 16.
The output of OR gate 146 is applied as one input to
a second output AND gate 176. The other input to AND gate 176
is from timer 174. When timer 174 times out, which means that
the breaker should have closed but did not, a high output from
AND gate 176 results, which results in an alarm, which could

take various forms. In the embodiment shown, the alarm will
at a particular point in time io necessary, tne circuit shown
it will do nothing to any of the elements in the electrical
power system. Its primary use is for the TEST mode, to
indicate that the breaker is not closing in time, although, as
indicated above, it will also sound when the device is in the
ON mode as well.
The output of a third output AND gate 178 is high
when the device is in the ON mode, when timer 174 has timed
out, and when there is an input from AND gate 180. AND gate
180 receives one input from OR gate 182. The three inputs to
OR gate 182 are difference voltage indications for phases A, B
and C from Figure 3 (comparators 84, 85, and 86) . If the
difference in voltages between the X and Y voltages exceed the
threshold, for each phase, this is a reliable indication that
at that point in time the permitted voltage angle of the
generator has been exceeded. Although the described
embodiment uses a voltage difference determination to
determine whether the permitted voltage angle has been
exceeded, other methods to determine voltage angle could be
used, such as an angle calculation like that shown in Figure
4. However, regardless of the output of OR gate 182, if the
BPF signal (from Figure 3) is present, the output of AND gate
180 will be low, and there will be no output from AND gate
178.
A high output from AND gate 180, however, when a BPF
signal is not present, will result in a high output from AND
gate 178, when the other inputs to AND gate 178 are high.
This output will be directed to breakers 18, 20 and 22 (Figure
1), which will basically clear the bus in advance of damage
occurring to the system.
Although Figure 6 shows that the output of AND gate
178 requires that timer 174 must time out, it is possible to
have an arrangement which does not require a timer 174 output.
In such a case, the inputs to AND gate 178 will be an *ON"
condition signal for the apparatus and the signal from AND
gate 180. Also, it is possible to utilize only one phase of
difference voltage input to OR gate 182. The additional
phases provide additional security and reliability for the
control system.
Hence, an apparatus has been described which
provides secure and reliable synchronization between a power
generator and an associated electrical system. The apparatus
is also efficient, since instead of the protection being bassrt
solely on a specific time after initiation of breaker closure,
it monitors the actual voltage angle of the generator by
ascertaining difference voltages between the voltages present
on the line before and after the breaker. The system in the
embodiment shown provides an alarm during both test and on
at a particular point in time io necessary, tne circuit shown
breaker.
Although a preferred embodiment of the invention has
been disclosed herein for illustration, it should be
understood that various changes, modifications and
substitutions may be incorporated in such embodiment without
departing from the spirit of the invention, which is defined
by the claims as follows:

WE CLAIM:
1. An apparatus for protecting against slow closure of a
connecting circuit breaker during synchronization of an electric
power generator and an associated electric power system,
comprisingi
means (110,178,148,180) for automatically determining a
value representative of the voltage angle of the generator
relative to that of the electrical system following initiation of
closure of the connecting circuit breakers
means (122,124|146,176) for comparing said representative
value with a reference value} and
means (172,174) for developing a signal which in turn can be
used to trip circuit breakers serving the electric system if the
reference value is exceeded by the representative value.
2. An apparatus as claimed in claim 1, wherein the reference
value is substantially the value of voltage angle beyond which
damage to at least one of the generator and the electric power
system would occur.
3. An apparatus as claimed in claim 2, wherein the
representative value is determined substantially continuously.
4. An apparatus as claimed in claim 1, wherein the
representative value is the difference voltage between a voltage
at an input side of the connecting circuit breaker and a voltage
present at an output side of the connecting circuit breaker.
9. An apparatus as claimed in claim 4, wherein the voltages at
the input and output of the circuit breaker are three phase
voltages and wherein the difference voltage comprises more than
one phase of the three-phase voltage signal.
6. An apparatus as claimed in claim 1, comprising means for
disabling the apparatus when current is determined to be flowing
through the connecting circuit breaker.
7. An apparatus as claimed in claim 1, comprising means for
initiating closure of the connecting circuit breaker.
8. An apparatus as claimed in claim 1, comprising means for
disabling the apparatus if a blow fuse is detected from either
the input or output voltages.
9. An apparatus as claimed in claim 1, comprising means for
connecting said apparatus in a test mode in which the connecting
circuit breaker is electrically isolated so that it does not
actually connect the generator to the electrical system, and
wherein the apparatus comprises means for providing an alarm
signal if the breaker does not close within a specified period of
time following initiation of closure when the apparatus is in the
test mode.
10. An apparatus as claimed in claim 1, comprising means for
developing the trip signal only if the reference value is
exceeded by the representative value and the breaker does not
close within a specified period of time following initiation of
closure of the breaker



The apparatus comprises a circuit for automatically and
continuously determining the difference voltage between the
voltage at the input side of a connecting circuit breaker and the
voltage at an output side thereof between an electric generator
and an associated power system, following imitation of closure of
the connecting circuit breaker. The difference voltage is
representative of the voltage angle of the generator relative to
the electrical system. The difference voltage is compared against
a reference value, which is selected such that a voltage angle
value greater than the reference value will result in damage to
the generator and/or the electrical system. An output signal is
developed which can then be used to trip circuit breakers serving
the electrical system if the reference value is exceeded by the
difference voltage.

Documents:

1810-cal-1998-abstract.pdf

1810-cal-1998-claims.pdf

1810-cal-1998-correspondence.pdf

1810-cal-1998-description (complete).pdf

1810-cal-1998-drawings.pdf

1810-cal-1998-examination report.pdf

1810-cal-1998-form 1.pdf

1810-cal-1998-form 2.pdf

1810-cal-1998-form 3.pdf

1810-cal-1998-form 5.pdf

1810-cal-1998-pa.pdf

1810-cal-1998-reply to examination report.pdf

1810-cal-1998-specification.pdf

1810-cal-1998-translated copy of priority document.pdf


Patent Number 238881
Indian Patent Application Number 1810/CAL/1998
PG Journal Number 09/2010
Publication Date 26-Feb-2010
Grant Date 24-Feb-2010
Date of Filing 12-Oct-1998
Name of Patentee SCHWEITZER ENGINEERING LABORATORIES INC
Applicant Address 2350 NE HOPKINS COURT, PULLMAN, WA 99163
Inventors:
# Inventor's Name Inventor's Address
1 LARRY C GROSS 620 SE MEADOWVALE DR. PULLMAN WA 99163
2 LUTHER S ANDERSON 745 SE HIGH STREET, PULLMAN, WA 99163
PCT International Classification Number H02H 7/06
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 08/950,970 1997-10-15 U.S.A.