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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to reclosing relays which automatically reclose
circuit breakers in electric power transmission and distribution systems
after they have been opened by other protective relays. More specifically,
it relates to a microcomputer based reclosing relay and particularly to
aspects of such a relay which provide continuity of operation despite
interruption in power to the relay microcomputer.
2. Background Information
Reclosing relays are commonly used to automatically reclose circuit
breakers in electric power systems which have been opened by overcurrent
or other protective relay action. Typically, the reclosing relay provides
several reclosures at predetermined intervals in case the fault which
initiated tripping of the circuit breaker takes time to clear and the
circuit breaker does not remain closed in response to the first closure.
The first reclosure is usually made without a delay since operating
experience has shown that the majority of faults are of a temporary nature
such as lighting flashovers, and will not be reestablished after an
interruption of the fault current.
If the circuit breaker does not remain closed after the first reclosure,
the relay makes additional reclosures at suitably graded intervals. It is
common practice for the reclosing relay to make two additional reclosures.
If the circuit breaker does not remain closed after the third reclosure,
the reclosing relay goes to a lockout state, and manual action is required
to reset the circuit breaker and the reclosing relay.
Conventionally, reclosing relays have been electromechanical devices While
these devices are very reliable, they are not very flexible.
It is a primary object of the invention to provide a reclosing relay which
can be more easily adapted to varying conditions.
It is also an object of the invention to provide such a reclosing relay
which maintains continuity of operation despite interruption in power to
the relay.
SUMMARY OF THE INVENTION
These and other objects are realized by the invention which is directed to
a microcomputer based reclosing relay which includes a digital processor
operated to perform a plurality of sequences, identified by sequence
flags, which generate a preselected number of reclosure signals for a
circuit breaker at timed intervals. The sequences include implementation
of counters and timers. The relay includes volatile memory means in which
the sequence flags, counts in the counters, and running times of the
timers are maintained during use by the digital processor. The digital
processor copies updated values of the flags, counts and running times
into a non-volatile memory such as an EEPROM. Upon restoration of power
following an interruption of power to the volatile memory which results in
loss of the contents of the volatile memory, the digital processor
recopies the flags, counts and running times from the non-volatile memory
back into the volatile memory. The digital processor then resumes
generating the succession of reclosing signals at the point where it left
off when power was lost using the recopied flags, counts, and running
times recopied from the non-volatile memory. Thus, in accordance with the
invention, various working variables used by the digital processor in
generating the succession of timed reclosing signals and which are
maintained in volatile memory for use by the digital processor are
periodically copied to a non-volatile memory so that upon restoration of
power following a loss of power to the non-volatile memory, the digital
processor can recopy the latest values of the working variables back into
the volatile memory and can resume generating its succession of reclosing
signals from the point where it was when power was lost.
The sequences performed by the digital processor include a lockout sequence
in which no reclosure signals are generated. The lockout sequence is
entered if the permitted number of reclosures have been attempted and the
circuit breaker does not remain closed If a power-up to lockout option is
selected, the relay goes to the lockout sequence upon restoration of power
rather to the point where it left off in generating the succession of
reclosure signals. However, if the circuit breaker remains closed for a
selected interval after restoration of power, the digital processor
transfers to a home sequence from which a complete new succession of
reclosing signals can be generated if the circuit breaker again trips.
As another aspect of the invention, the relay includes a follow breaker
feature in which closure of the breaker through other means, while the
reclosing relay is timing a delay for generating a reclosure signal, is
counted as a reclosure by the relay. This is an antipumping feature which
prevents continued operation of the breaker on its own. By counting the
self-closing of the breaker as a reclosure by the relay, the relay is
advanced toward the lockout sequence which terminates further attempts at
reclosure by the reclosing relay.
When an EEPROM is used as the non-volatile memory, the flags and counts in
the counters are only copied to the EEPROM when they change state, and the
running times in the timers are only entered into the EEPROM at spaced
intervals of time while timing This is done to extend the life of the
EEPROM which can only perform a given number of operations. In the
exemplary system, the running times in the timers are copied to the EEPROM
once each second.
BRIEF DESCRIPTION OF THE DRAWINGS
A full understanding of the invention can be gained from the following
description of the preferred embodiment when read in conjunction with the
accompanying drawings in which:
FIG. 1 is a schematic diagram of a portion of an electric power system
incorporating the reclosing relay of the invention.
FIG. 2 is a schematic diagram in block form of the microcompute which forms
part of the reclosing relay shown in FIG. 1.
FIGS. 3 through 6 are flowcharts for a program suitable for implementing
the invention in the microcomputer shown in FIG. 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a three-phase segment of an electric power system 1 is
protected by a circuit breaker 3. Tripping of the circuit breaker 3 is
controlled by a protective relay, such as for example, the overcurrent
relay 5. The overcurrent relay 5 monitors the current in each phase of the
power system 1 through current monitors 7. If the current in the protected
segment of the electric power system 1 exceeds limits set by the
overcurrent relay 5, a trip signal is sent to the circuit breaker 3 over
the lead 9. Other types of protective relays can be used to trip the
circuit breaker 3.
Reclosure of the circuit breaker 3 is controlled by the reclosing relay 11
of the invention. The reclosing relay 11 includes: a power supply 13, an
opto-isolator 15, a microcomputer 17 with a display 19 and a non-volatile
memory in the form of an electronically erasible programmable read only
memory (EEPROM) 21; input push button switches 23, 25, 27, 29, 31 and 33;
output relays 35, 37 and 9; and LEDs 41, 43 and 45.
The power supply 13 generates regulated +5 volt dc power for the
microcomputer 17 and LEDs 41, 43 and 45 and +24 volt dc power for the
relays 35, 37 and 39, from either an ac or dc external source. The
external source also provides power through a shielded twisted pair cable
47 to a normally closed switch 49 which generates a signal representative
of the open/closed state of the circuit breaker 3. The opto-isolator 15
converts this circuit breaker state signal to an isolated +5 volt logic
signal for input into the microcomputer 17. The switch 49 is closed when
the circuit breaker 3 is open.
As shown in FIG. 2, the microcomputer 17 includes a central processing unit
(CPU) 51 controlled by a clock 53, a non-volatile read only memory (ROM)
55, a random access memory (RAM) 57, and an input/output (I/O) unit 59
interconnected by a communications bus 61. The ROM 55 is a non-volatile
memory in which the program for the microprocessor is stored. The RAM 57
is a volatile working memory in which working variables and data used and
generated by the central processing unit are stored. By volatile, it is
meant that the information stored in RAM 57 is lost when power to the RAM
is interrupted. Ordinarily this would necessitate that the microcomputer
start from some set initial condition when power is restored; however, in
accordance with the invention, the microcomputer 17 can resume operation
from where it left off when power was lost. The input/output unit 59
allows the microcomputer 17 to communicate with the other components of
the system. This includes the EEPROM 21 which, as will be seen, stores
settings used by the microcomputer 17 and also variables whose values are
affected by operation of the microcomputer. As previously mentioned, the
EEPROM is non-volatile memory, meaning that it retains stored information
indefinitely with or without power. The stored information can be readily
changed, however, and in fact, variables used by the microcomputer are
updated in the EEPROM 21 on a repetitive basis.
In addition to the open/closed input signal from the circuit breaker, the
microcomputer 17 receives inputs through the push button switches. The
switch 23 is a test switch which permits the operator to initiate a
computer test. The "select/run" button 25 permits the operator to select a
function and when pushed again to enter the "run" mode. The "lower" and
"raise" buttons 27 and 29 allow the operator to scroll through functions
or to decrease and increase, respectively, the values of parameters. With
the "set/enter" push button 33, the operator can designate a parameter to
have its value changed by the "raise" and "lower" buttons, and by
subsequent actuation to enter the selected value into the microcomputer.
With the "failed reclose reset" button 33, the operator can reset the
failed to reclose LED 45.
The microcomputer 17 also receives settings from the EEPROM 21, and as will
be seen periodically, transmits values for storage in the EEPROM 21. The
microcomputer 17 has two types of outputs, the coils 35, 37, and 39 of
output relays K.sub.1, K.sub.2 and K.sub.3, respectively, and the LEDs 41,
43 and 45. The relay K.sub.1 is the close relay. The coil 35 of this relay
is energized when the microcomputer 17 generates a reclose signal for the
circuit breaker. Energization of the coil 35 results in closure of the
normally open contacts 63 in the control circuit of circuit breaker 3 to
initiate closure of the circuit breaker. The K.sub.2 relay is an
instantaneous trip relay. When the instantaneous trip function is selected
by the microprocessor 17, the coil 37 of this relay is energized to close
the normally open contacts 65. These contacts 65 are in parallel with time
delay contacts within the protective relay 5 (not shown). Thus, when these
contacts are closed, the circuit breaker is tripped instantaneously
without the delay provided by the overcurrent relay 5.
The K.sub.3 relay is an alarm relay. The coil 39 of this relay is normally
energized, but is deenergized when microcomputer 17 detects an alarm
condition. The K.sub.3 relay has one set of normally closed contacts 67
and one set of normally open contacts 69. Either of these contacts can be
selected by a jumper 71 to activate an alarm 73. The position of jumper 71
is selected dependent upon whether the alarm 73 is to be energized or
deenergized for the alarm condition. The display 19 is a two-line,
16-character per line, liquid crystal display used for displaying the
sequence of relay operation, including timing and failure modes, as well
as parameter settings.
The reclosing relay 11 has several operating states or sequences it
performs. Most of the time, the relay is in an inactive, rest condition,
called, HOME. The reclosing relay also has a RECLOSING state in which
reclosings are timed, a state in which the relay is waiting for the
circuit breaker to respond to a RECLOSURE signal, and in which it detects
failure of the circuit breaker to close within a selected interval, hence
called the "FAILED RECLOSURE" state, and a RESET state in which the
reclosing relay determines if the circuit breaker is going to remain
closed. The reclosure relay also has a LOCKOUT state in which automatic
reclosure is prevented.
The reclosing relay 11 can be programmed to attempt up to four reclosures
of the circuit breaker, each at a separately settable predetermined time
interval. If the circuit breaker fails to remain closed after the last
reclosure, the relay goes to the LOCKOUT state, and any further attempts
at reclosure must be made by manual operation.
Typically, the protective relay 5 may be provided with a delay in tripping
the circuit breaker. The reclosing relay 11 can be operated, if desired,
to provide instantaneous tripping of the circuit breaker by energization
of the relay K.sub.2 which closes the contact 65 to bypass the protective
relay delayed trip. The reclosing relay 11 can be operated to provide
instantaneous trips when in LOCKOUT, and can also operate the contact 65
to permit a selected number of the trips between reclosures to be
instantaneous.
The reclosing relay 11 includes two counters and several timers. The first
counter keeps track of the reclosures and the second counter counts the
number of instantaneous trips. The timers include four reclose, or time
delay, timers which time the delays for the four reclosures. They start
timing when the contacts 49 on the circuit breaker close indicating that
the circuit breaker has been tripped. When one of these timers times out,
the K.sub.1 relay is energized to initiate reclosure of the circuit
breaker. Another timer is the RECLOSE FAILURE timer which, if selected,
times the interval required for the circuit breaker to reclose after the
reclose signal is generated. If the circuit breaker fails to reclose
within the selected time interval, the reclosing relay goes to LOCKOUT.
An additional third timer of the reclosing relay is the reset timer which
begins timing when the circuit breaker closes. If the circuit breaker
remains reclosed until the reset timer times out, the circuit breaker goes
to the HOME state. If the circuit breaker reopens before the reset timer
times out, another reclosure is attempted. Timing out of the reset timer
is an indication that the event which caused the circuit breaker to trip
initially has terminated. This could occur after any one of the
reclosures. In returning to the HOME state the counters and timers are
reset for the next event. The reclosing relay also includes a maximum
cycle timer which begins timing when the circuit breaker first trips. This
timer is set for an interval which exceeds the maximum time that would be
required for the circuit breaker to go through a full succession of time
delayed reclosures. Thus, timing out of this timer is an indication of a
malfunction and the relay goes to LOCKOUT.
The number of reclosures, the timing intervals, and various options are
fully selectable as indicated in the following Table 1.
TABLE 1
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FUNCTION VALUE
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Reclosure to LOCKOUT
0 to 4
Instantaneous Trip Enable
0 to 4
First Reclose Timer, sec
0 to 250
Second Reclose Timer, sec
0 to 250
Third Reclose Timer, sec
0 to 250
Fourth Reclose Timer, sec
0 to 250
Follow Breaker Yes or No
Reset Timer, sec 0 to 250
Instantaneous Trip Yes or No
from LOCKOUT
Fail Reclose Enable
Yes or No
Reclose Fail Timer, sec
0 to 250
Max Cycle Enable Yes or No
Maximum Cycle Timer sec
0 to 999
______________________________________
These various functions can be called up on the display 19 one at a time
and the values set by the operator. Table 1 shows the range of values that
can be selected. For instance, up to four reclosures are available, and if
desired each can be an instantaneous trip. If less than the number of
reclosures are selected for instantaneous trip, the instantaneous trips
are selected for the earliest reclosures. As can be seen, the delay for
each reclosure can be independently selected by setting the appropriate
reclose timer. Various options can also be selected, such as "follow
breaker", which is explained below, and "instantaneous trip from lockout."
If "fail reclose enable" is selected, the relay goes to lockout if the
circuit breaker does not close within the interval set by the "reclose
fail timer." As can also be seen, the maximum cycle timer is an optional
feature.
The microcomputer 17 of the reclosing of relay 11 maintains the contents of
the counters and timers in the RAM 57. It also maintains in the RAM,
software flags for the different stages or sequences of operation. That
is, a flag for HOME, a flag for RECLOSURE, a flag for RESET, a flag for
FAILED TO RECLOSE, and a flag for each output contact. Since the RAM 57 is
a volatile memory, this information is lost if power to the microcomputer
17 is interrupted. However, in accordance with the invention, loss of the
information in the RAM 57 is precluded by operating the microcomputer 17
to repetitively write the count in the counters, the running times in the
timers and the flags to the EEPROM 21. In the exemplary system, the flags
and counts in the counters are only copied to the EEPROM when they change
state while the running times in the timers are only copied once each
second during timing operations. While the cycle time of the microcomputer
17 is much higher and thus the timers are being incremented at a much
higher rate, timing information is transferred to the EEPROM 21 only every
second to prolong the service life of the EEPROM. When power to the relay
is lost, the point in the reclosing sequence is remembered in the EEPROM
by saving appropriate flag, counter count, timer content and contact
output state. Then upon restoration of power, the relay will resume
operation at the point where the power was lost and re-establish the relay
status as if there was no power loss.
FIGS. 3 through 6 illustrate flow charts for a suitable computer program
for operating the microcomputer 17 of the reclosing relay 11 in accordance
with the invention. As can be seen from FIG. 3, on power up at 75, the
relays K.sub.1 through K.sub.3 are reset at 77 and the liquid crystal
display 19 is cleared at 79. If a RAM check is passed at 81, the LEDs 41,
43, and 45 are turned off at 83 and a ROM is check is made at 85. If
either the RAM check of ROM check are failed, the coil 39 of the alarm
relay K.sub.3 is deenergized to actuate the alarm circuit 73, as shown at
87. Operation of the microcomputer 17 then stops until an operator
intervenes.
If all of the checks are passed, the microcomputer ports are initialized at
89 and the settings are downloaded from the EEPROM 21 as indicated at 91.
The settings downloaded include the selected parameters from Table 1
above, and the stored values for the flags and timers.
The microcomputer 17 is then ready for operation. The first item that is
checked is whether the feature "power up to lockout" has been selected as
determined at 93. If it has, the LOCKOUT LED 41 is turned on at 95. If the
instantaneous trip from LOCKOUT feature has been selected as determined at
97, the K.sub.2 relay is energized at 99; otherwise, it is deenergized at
101. The computer then monitors the contact 49 at 103 to determine if the
circuit breaker is opened or closed. If the contact 49 is closed
indicating that the circuit breaker is tripped, the program loops back to
95. However, if the circuit breaker is closed as indicated by open
contacts 49, the reset timer is started at 105. If the circuit breaker
remains closed as determined at 107, until the reset timer times out, the
LOCKOUT light is turned off at 109 and the program goes to HOME. However,
if the circuit breaker 3 opens, as determined at 107, before the reset
timer times out, the program remains in the LOCKOUT routine.
If power-up to LOCKOUT was not selected, as determined at 93, the power-up
to last state routine 111 is selected. If the HOME flag is set at 113, the
program goes to HOME. If the system was not at HOME at the time of the
power failure, the reclose flag is checked at 115 to determine if the
relay was going through a reclose sequence at the time of power failure.
If not, and since it was also not in HOME, it had to be in LOCKOUT, and
the program returns to the LOCKOUT sequence.
If the relay was going through a reclose sequence at the time of the power
failure, the number of reclosures performed before the power loss is
downloaded from the EEPROM 21 at 117. Next, it must be determined what
point of the reclose cycle the system was in at the time of the power
failure. If the failed to reclose flag is present at 119 indicating that
the relay had sent a reclose signal to the circuit breaker and was waiting
for the circuit breaker to respond, the contents of the failed to reclose
timer are downloaded at 121 and the program goes to the failed to reclose
sequence If instead, the circuit breaker had closed and the relay was
timing the interval that the circuit breaker had remained reset, as
determined by the presence of the reset flag at 123, the contents of the
reset timer are downloaded at 125, and the program goes to the reset
sequence. If neither the failed to reclose flag nor the reset flag are
set, then the relay was timing the interval for reclosure and the contents
of the timed delay reclose timer are downloaded at 127, and the program
transfers to the reclose sequence.
The program enters the HOME routine at 129 in FIG. 4 and the HOME flag is
saved in the EEPROM at 131 and the HOME LED 43 is turned on at 133. The
input switches are then scanned at 135. If the "select/run" button 25 has
been actuated to call up the "select" function, as determined at 137, the
settings for the relay may be changed by the operator at 139. To perform
this function, the operator can use the "lower" and "raise" buttons 27 and
29 respectively to scroll through on the display 19 the functions listed
in Table 1. When the function to be changed is displayed, the "set/enter"
button 31 is depressed and the operator may then use the "raise" and
"lower" buttons to change the value of the function. The desired value is
then entered by again pressing the "select/enter" button 31 and the new
value is stored in the EEPROM. When all the desired changes have been
made, the updated values are entered in the RAM 141. If the button 25 is
again depressed to select the run function, the program enters the run
subroutine at 143. The number of instantaneous trips allowed is checked at
145, and if there are still instantaneous trips left, the K.sub.2 relay is
energized at 147; otherwise, it is deenergized at 149. The status of the
circuit breaker is then monitored by checking the contact 49 at 151. If
the contact 49 is open indicating that the circuit breaker is closed, the
timed delay reclose timer is reset at 152 and the program remains in the
HOME routine. Otherwise, it goes to the reclosing sequence.
The flow chart for the reclosing sequence is shown in FIG. 5. First the RAM
values are checked at 153. The microcomputer 17 copies the values of the
parameters from the EEPROM and stores them in three separate locations in
the RAM. The integrity of the data is confirmed before the reclosing
sequence is initiated by checking to see that at least two out of three of
the values stored in the RA1 for each parameter are the same, as
determined at 155. If not, the alarm is set at 157 by deenergizing the
K.sub.3 relay and the system goes to LOCKOUT as indicated by the tags A in
FIGS. 3 and 5. Upon entering LOCKOUT, the reclosure count is reset at 158
(see FIG. 3). If two out of three of the RAM values check, the third value
is conformed at 159. If the max cycle timer has timed out, the system goes
to LOCKOUT. If the relay is performing normally, the max cycle timer will
not be timed out at 161 and the number of instantaneous trips remaining
will be checked at 163. If instantaneous trips are remaining, the
instantaneous trip counter is decremented at 165. If the preselected
number of instantaneous trips have occurred, then the K.sub.2 relay is
deenergized at 167. In either case, the reclose flag is set and saved in
the EEPROM at 169 and the count of the number of reclosures is saved in
the EEPROM at 171. In addition, the reclose timer content is saved in the
EEPROM at 173. The time delay reclose timer then begins to count down and
the contents of the timer are checked at 175. While the reclose timer is
timing out, the status of the circuit breaker is monitored at 177, and
every second as determined at 179, the time remaining in the reclose timer
is saved to the EEPROM at 173. Even if the circuit breaker opens, if the
follow breaker function has not been selected as determined at 181, the
reclose timer is permitted to time out and the time in the timer is saved
to the EEPROM every second. If the follow breaker function has been
selected, and the breaker closes on its own, the program advances to the
reset sequence as indicated by the tag B. This is an antipumping feature
which counts the self closures of the circuit breaker as one of the timed
counts which will eventually cause the reclosing relay to go into lockout
and prevent further closures of the circuit breaker if the circuit breaker
does not remain closed.
When the timed delay reclose timer times out at 175, the close relay
K.sub.1 is energized at 183 and the failed to reclose flag is set and
saved in the EEPROM at 185. The reclose sequence continues in FIG. 6, as
shown by the tags C, by storing of the contents of the failed to reclose
timer in the EEPROM at 187. While the failed to reclose timer is timing
out at 189, the status of the circuit breaker is checked at 191. Every
second, as determined at 193, the time remaining in the timer is saved to
the EEPROM at 187. If the circuit breaker closes before the failed to
reclose timer times out, the failed to reclose timer and flag are reset at
195 and the program enters the reset sequence. If the circuit breaker
fails to reset reclose the interval determined by the failed to reclose
timer, as determined at 189, the program goes to LOCKOUT.
When the circuit breaker closes either through operation of the reclosing
relay, or on its own as detected by the follow breaker routine, the reset
flag is set and saved in the EEPROM at 197 and the reset timer content is
saved to the EEPROM at 199. While the reset timer is timing out at 201,
the status of the circuit breaker is monitored at 203. The time remaining
in the reset timer while the circuit breaker remains closed is saved in
the EEPROM every second as determined at 205. If the circuit breaker
remains closed for the interval set by the reset timer, the program goes
to HOME as indicated by the tags D.
If the circuit breaker opens, as determined at 203, before the reset timer
times out, the number of reclosures attempted is checked at 207. If the
maximum number of reclosures have been attempted, the program goes to
LOCKOUT as indicated by the tags C. If additional reclosures are
available, the reclose counter is decremented at 209 and the program goes
to the running routine, as indicated by the tags E, for the next reclosing
attempt.
As can be seen from the above, the reclosing relay periodically stores the
flags and timer and counter contents in the non-volatile EEPROM 21 so that
if power to the microcomputer is interrupted and information in the
non-volatile RAM 57 is lost, the reclosing relay can, upon restoration of
power, resume operation at the point where it left off. The information is
retained in the EEPROM indefinitely and thus is not dependent upon the
life of a battery or the amount of charge that can be stored in the
capacitor. Accordingly, the reclosing relay of the invention provides
reliable operation even with prolonged power interruptions.
While specific embodiments of the invention have been described in detail,
it will be appreciated by those skilled in the art that various
modifications and alternatives to those details could be developed in
light of the overall teachings of the disclosure. Accordingly, the
particular arrangements disclosed are meant to be illustrative only and
not limiting as to the scope of the invention which is to be given the
full breadth of the appended claims and any and all equivalents thereof.
* * * * *
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