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Claims  |
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We claim:
1. A system for timing the opening and closing of a switching arrangement
used in high power electrical transmission systems which transmit at least
one phase of a power signal having a sinusoidal variation, comprising:
switch means for providing an OPEN/CLOSE initiating signal for initiating
the opening/closing of said switch arrangement;
phase angle detector means for detecting a phase of said power signal and
for providing a phase indication signal;
temperature sensing means for sensing the temperature of said switching
arrangement and producing a temperature signal; and control means
connected to said switch means, said phase detector means, and said
temperature sensing means for opening and closing said switching
arrangement in response to said initiating signal timed as a function of
said temperature signal and said phase indication signal.
2. A system as defined in claim 1 wherein said switching arrangement
includes two electrodes and a coil;
said electrodes, when in contact with each other, being separated upon
application of an opening signal to said coil;
said electrodes, when separated from each other, being moved towards each
other to contact each other upon application of a closing signal to said
coil.
3. A system as defined in claim 2 wherein said control means carry out
steps including, when said electrodes are in contact with each other:
after a waiting time t.sub.y after a zero crossing of said power signal
applying said opening signal to said coil;
said electrodes being separated from each other after a period of time
t.sub.mo2 ;
the termination of said period t.sub.mo2 occurring a period t.sub.arc
before the next zero crossing of said power signal;
said control means including means for calculating t.sub.mo2 for different
temperatures according to the formula:
t.sub.mo2 =t.sub.mo1 -a.sub.o (T.sub.2 -T.sub.1)
where
a.sub.o is a value which is indicative of the sensitivity of the breaker to
temperature and is given by the breaker manufacturer
T.sub.2 is equal to the temperature of interest
T.sub.1 is equal to the standard temperature is equal to, in a particular
embodiment, 20.degree. C.
t.sub.mo1 is equal to the switch opening time at 20.degree. C.
t.sub.mo2 is equal to the switch opening time at temperature T.sub.2.
4. A system as defined in claim 3 wherein said control means includes means
for calculating a waiting time t.sub.y from the formula:
t.sub.o =t.sub.y +t.sub.mo2 +t.sub.arc
where
t.sub.o =a predetermined integral number of periods of said power signal
t.sub.y =waiting time
t.sub.arc =arcing time.
5. A system as defined in claim 2 wherein said control means carry out
steps, including when said electrodes are separated from each other:
after a waiting period t.sub.x after a zero crossing of said power signal
applying said closing signal to said coil;
said coil being closed after a period t.sub.mc2 ;
said control means including means for calculating t.sub.mc for different
temperatures according to the formula:
t.sub.mc2 =t.sub.mc1 -a.sub.c (T.sub.2 -T.sub.1)
where
a.sub.c =a value which is indicative of the sensitivity of the breaker to
temperature and is given by the breaker manufacturer
T.sub.2 =temperature of interest
T.sub.1 =standard temperature is equal to, in a particular embodiment,
20.degree. C.
t.sub.mc1 =switch closing time at 20.degree. C.
t.sub.mc2 =switch closing time at temperature T.sub.2.
6. A system as defined in claim 5 wherein said control means includes means
for calculating t.sub.x from the formula:
t.sub.c =t.sub.x +t.sub.mc2 +8.33 msec-t.sub.del
where
t.sub.c =a predetermined integral number of periods of said power signal
t.sub.x =waiting time
t.sub.del =a time delay period.
7. A system as defined in any one of claims 1, 2, 3, 4, 5 or 6 wherein said
electrical transmission system transmits three phases of said power signal
comprising a first phase, a second phase and a third phase;
said first phase being separated from said second phase by a phase angle
P.sub.a ;
said second phase being separated from said third phase by a phase angle
P.sub.b ;
said control means including means for initiating opening and closing a
second phase portion of said switching arrangement at a time P.sub.a /360
(t.sub.cycle) after said control means has initiated opening and closing
for said first phase; and
said control means including means for initiating opening and closing a
third phase portion of said switching arrangement at a time P.sub.b /360
(t.sub.cycle) after said control means has initiated opening and closing
for said first phase.
8. A method for timing the opening and closing of a switching arrangement
used in high power electrical transmission systems which transmit at least
one phase of a power signal having a sinusoidal variation, comprising:
providing an OPEN/CLOSE initiating signal to initiate the opening/closing
of said switching arrangement;
detecting a phase angle of said power signal and providing a phase
indication signal to said processor;
sensing a temperature of said switching arrangement and producing a
temperature signal;
controlling opening and closing of said switching arrangement in response
to said initiating signal timed as a function of said temperature signal
and said phase indication signal.
9. A method as defined in claim 8 wherein said switching arrangement
includes a coil and two electrodes and wherein said step of controlling
includes, when said electrodes are in contact with each other:
after a waiting time t.sub.y applying an opening signal to said coil;
said electrodes being separated from each other after a period of time
t.sub.mo ;
the termination of said period t.sub.mo occurring a period t.sub.arc before
the next zero crossing of said power signal;
said processor calculating t.sub.mo for different temperatures according to
the formula:
t.sub.mo2 =t.sub.mo1 -a.sub.o (T.sub.2 -T.sub.1)
where
a.sub.o is a value which is indicative of the sensitivity of the breaker to
temperature and is given by the breaker manufacturer
T.sub.2 is equal to the temperature of interest
T.sub.1 is equal to the standard temperature is equal to, in a particular
embodiment, 20.degree. C.
t.sub.mo1 is equal to the switch opening time at 20.degree. C.
t.sub.mo2 is equal to the switch opening time at T.sub.2.
10. A method as defined in claim 9 wherein t.sub.y is calculated from the
formula:
t.sub.o =t.sub.y +t.sub.mo2 +t.sub.arc
where
t.sub.o =a predetermined integral number of periods of said power signal
t.sub.y =waiting time
t.sub.arc =arcing time.
11. A method as defined in claim 8 wherein said step of controlling, when
said electrodes are separated from each other, comprises:
after a waiting period t.sub.x applying a closing signal to said coil;
said electrodes being closed after a period t.sub.mc ;
calculating t.sub.mc for different temperatures according to the formula:
t.sub.mc2 =t.sub.mc1 -a.sub.c (T.sub.2 -T.sub.1)
where
a.sub.c =a value which is indicative of the sensitivity of the breaker to
temperature and is given by the breaker manufacturer
T.sub.2 =temperature of interest
T.sub.1 =standard temperature is equal to, in a particular embodiment,
20.degree. C.
t.sub.mc1 =switch closing time at 20.degree. C.
t.sub.mc2 =switch closing time at temperature T.sub.2.
12. A method as defined in claim 11 wherein t.sub.x is calculated from the
formula:
t.sub.c =t.sub.x +t.sub.mc2 +8.33 msec-t.sub.del
where
t.sub.c =a predetermined integral number of periods of said power signal
t.sub.x =waiting time at temperature T.sub.2
t.sub.del =a time delay.
13. A method as defined in any one of claims 8, 9, 10, 11 or 12 wherein
said electrical transmission system transmits three phases of said power
signal comprising a first phase, a second phase and a third phase;
said first phase being separated from said second phase by a phase angle
P.sub.a ;
said second phase being separated from said third phase by a phase angle
P.sub.b ;
said step of controlling further comprising steps of controlling a portion
of said switching arrangement for said second phase and for said third
phases, wherein
opening and closing of said portions for said second phase is initiated at
a time P.sub.a /360 (t.sub.cycle) after initiation for said first phase;
opening and closing of said portions for said third phase is initiated at a
time P.sub.b /720 (t.sub.cycle) after initiation for said first phase. |
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Claims |
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Description |
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BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates to a system for timing the opening and closing of
switching arrangements used in high power electrical transmission systems.
More specifically, the invention relates to such a system which takes into
account conditions of temperature surrounding the switching arrangements
as well as the mechanical displacement time of the electrical contacts of
the switching arrangements.
2. Description of Prior Art
Switching arrangements, for example, circuit breakers, are used in
electrical transmission lines or distribution lines to redirect power, or
are used to connect the lines to reactive elements to correct power
factor. Such breakers, because of the large amounts of power they must
handle, are very large (approximately the size of a small house on each
phase) and are very costly.
Associated with such breakers are resistive elements, which are connected
in parallel to the breakers just before the opening and closing of the
breakers, to absorb the "overvoltages" which accompany the opening and
closing of the breakers to thereby protect the switching elements of the
breakers as well as the reactive elements. The resistive elements are also
large and expensive.
It is a well known fact in the art that the temperature surrounding the
breaker has an effect on the speed of operation of the breakers. Generally
speaking, the lower the temperature, the greater amount of time needed to
open or close the breakers and vice-versa.
SUMMARY OF INVENTION
It is an object of the invention to provide a system for timing the opening
and closing of switching arrangements which obviates the needs for
resistive elements.
It is a more specific object of the invention to provide such a timing
system which will open and close the breakers at such a time in the cycle
of the transmitted signal whereby to minimize the overvoltage due to the
opening and closing of the breaker.
In accordance with a particular embodiment of the invention there is
provided a system for timing the opening and closing of a switching
arrangement used in high power electrical transmission systems which
transmit at least one phase of a power signal having a sinusoidal
variation, comprising:
switch means for providing an OPEN/CLOSE initiating signal for initiating
the opening/closing of said switch arrangement;
zero crossing detector means for detecting zero crossings of said power
signal and for providing a zero crossing signal upon detection of a zero
crossing;
processor means;
controller means;
analog-to-digital converter means;
temperature sensing means for sensing the temperature of said switching
arrangement;
first conductor means connecting said power signal to a first input of said
analog-to-digital converter means when said switching arrangement is open;
second conductor means connecting said power signal to a second input of
said analog-to-digital converter means when said switching arrangement is
closed;
third conductor means connecting said power signal to a first input of said
zero crossing detector when said switching arrangement is open;
fourth conductor means connecting said power signal to a second input of
said zero crossing detector means when said switching arrangement is
closed;
fifth conductor means connecting said temperature sensing means to a third
input of said analog-to-digital converter means;
said analog-to-digital converter means being connected to a first input of
said processor means;
said zero crossing detector being connected to a second input of said
processor means;
said switch means being connected to a third input of said processor;
said processor means being connected to an input of said controller means;
whereby, upon detection of an initiating signal, said processor, after
receiving a zero crossing signal, causes said controller to carry out a
series of predetermined steps to open/close said switching arrangement.
From a different aspect and in accordance with a particular embodiment of
the invention there is provided a method for timing the opening and
closing of a switching arrangement used in high power electrical
transmission systems which transmit at least one phase of a power signal
having a sinusoidal variation, comprising:
providing an OPEN/CLOSE initiating signal to a processor to initiate the
opening/closing of said switching arrangement;
detecting a zero crossing of said power signal and providing a zero
crossing signal to said processor upon detection of said zero crossing;
said processor, upon detection of a first zero crossing signal after an
opening/closing signal, causing a controller to carry out a series of
predetermined steps.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be better understood by an examination of the following
description, together with the accompanying drawings, in which:
FIG. 1 is a block diagram of the system;
FIG. 2A illustrates the number of integral cycles in which the complete
opening procedure is performed in the preferred embodiment;
FIG. 2B illustrates the first phase power signal;
FIG. 2C illustrates the initiating signal;
FIG. 2D illustrates the phase indication signal according to the preferred
embodiment;
FIG. 2E illustrates the breaker activation signal according to the
preferred embodiment;
FIG. 2F illustrates the state of the breaker; and
FIGS. 3A through 3F correspond to FIGS. 2A through 2F for the equivalent
sequence of events during closing of the breaker according to the
preferred embodiment.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIG. 1, a circuit breaker, illustrated schematically at 1, and
having coil means represented schematically at 1A and electrode means
represented schematically at 1B and 1C, is connected between the three
phases, A, B and C, of transmitted power, and a reactive element
illustrated schematically at 3. When the breaker is opened, the measured
tension of one of the phases, in the illustrated embodiment phase A, is
connected to an analog-to-digital (A/D) converter 5 by conductor D. The
magnitude, frequency and other characteristics of the phase A signal are
translated from an analog value to a digital value in A/D converter 5, and
the digital signal is then fed to a microprocessor 7. In addition, the
phase A signal is fed to zero detector 9 wherein the zero crossings of the
phase A signal are detected. When a phase A zero crossing is detected, a
pulse or other indication is fed to the microprocessor 7. As will be
apparent, the zero crossings of phase A are used for synchronization
purposes.
A thermometer, illustrated schematically at 10, measures the temperature
surrounding the circuit breaker. An electrical analog of the temperature
is then fed to the A/D (analog to digital) converter 5, and the digital
conversion of the temperature is also fed to the microprocessor 7.
When the breaker is closed, phase A, B and C signals are fed along
conductors X, Y and Z, and the phases A, B and C measured currents are fed
to the A/D converter 5 as shown in FIG. 1. Once again, the analog signals
are converted to digital signals and the digital signals are fed to the
microprocessor 7. The signal of the phase A is also fed to the zero
detector 9, and, once again, a pulse or other indication is fed to the
processor 7 when a zero crossing is detected.
The currents on phases A, B and C are monitored in order to detect any
restrike that might occur when the circuit breaker opens or high inrush
current when the circuit breaker closes.
Alarm signals are generated when a restrike or a high inrush current occurs
on any of the three phases.
The opening or closing of the breaker is initiated by ON/OFF switch 11. The
signal from the ON/OFF switch is, once again, fed to the microprocessor 7.
The output of the microprocessor 7 is fed to a controller 13 which will
either open or close the breakers, associated with the A, B or C phases
under the control of the microprocessor 7, by carrying out a series of
predetermined, timed, steps as described below. If the system cannot
operate to open or close the breaker under the control of the controller
13, an emergency override 15 is provided to open or close the breakers,
once again, under control of the microprocessor 7.
A keyboard 17 is provided for the purpose of programming the microprocessor
7, as is well known in the art, and a display unit 19 is provided for
examining various parameters and alarm signals, once again, as is well
known in the art.
To understand the operation of the system, reference is had to FIG. 2, for
an understanding of the opening operation, and to FIG. 3 for an
understanding of the closing operation. Generally, the system is either in
a waiting mode, that is, when an opening or closing has not been
commanded, or an active mode in which the breaker is either being opened
or closed. In the waiting mode, temperature readings are taken at
predetermined intervals by the thermometer 10, and an electrical analog of
the temperature is provided to the A/D converter 5. The digital
representation of the temperature is then provided to the processor 7.
At the same time, during the waiting mode, the functionality of the system
is verified by means well known in the art. Parameters are also calculated
taking into account the changing temperature.
Turning now to FIG. 2, in accordance with the invention, the complete
opening procedure, t.sub.o, is performed during an integral number of
cycles, i.e. in a time n (t.sub.cycle), where t.sub.cycle =period of a
cycle and n=a predetermined integer. As illustrated in FIG. 2A, the number
of integral cycles in which the complete opening procedure is performed in
one particular embodiment is 3. As illustrated in FIG. 2B, the transmitted
signal is a sinusoid. In North America, the frequency of the transmitted
signal is, of course, 60 Hz so that t.sub.cycle =16.67 msec..
The signal for opening the breaker (separating the electrodes of the
breakers from each other: the signal is initiated by pressing the ON
button in the switch 11 in FIG. 1) is given at the beginning of a period
t.sub.co. The signal t.sub.co is illustrated in FIG. 2C and is the time
duration during which the opening signal remains high. As can be seen in
FIG. 2C, t.sub.co remains high during the entire opening procedure and
stays open until a closing signal is initiated.
The high level at the onset of t.sub.co is fed to the microprocessor 7 and
the microprocessor 7 then seeks a zero of the sinusoid at the first zero
crossing after the initiation of t.sub.co. As seen in FIGS. 2B and 2D,
this occurs at the beginning of the period t.sub.y in FIG. 2D.
It is only after the waiting period t.sub.y, that is, at the beginning of
the period t.sub.mo, (see FIG. 2D) that power is applied to the coil of
the circuit breaker to initiate the movement for the physical separation
of the electrodes of the breaker as shown in FIG. 2E.
As seen in FIGS. 2F and 2D, the contacts separate at the conclusion of the
period t.sub.mo, that is, at a period t.sub.arc before the next zero
crossing.
When the electrodes of the breakers are physically separated, an arc is
formed between the electrodes. The arc is extinguished when the current
reaches the zero level, that is, at the conclusion of the period
t.sub.arc.
To prevent restrikes inside the breaker after the current goes to zero, the
duration of the arc, identified as t.sub.arc in FIG. 2D, should be greater
than 3 milliseconds. If it is less than this, then the current will pass
through zero and increase (in either a positive or negative direction)
while the arc is still strong enough to restrike. Accordingly, t.sub.arc
should be a minimum of 3 milliseconds.
In addition, to guard against the uncontrollable variation in the amount of
time that it takes for the physical separation of the electrodes to occur
(t.sub.mo), which variation could be of the order of 2 milliseconds, it is
preferable that the period t.sub.arc should be of the order of 5
milliseconds.
The actual magnitude t.sub.arc is entered into microprocessor 7 by keyboard
17. The period t.sub.mo is determined by a calibration procedure at a
standard temperature, for example, 20.degree. C.
It will then be observed that
t.sub.o =t.sub.y +t.sub.mo +t.sub.arc (1)
As t.sub.o is known (in the present example, t.sub.o =3 cycles. In the
North American case, each cycle is equal to 16.6 msec so that t.sub.o =50
msec) and t.sub.arc is selected to be of the order of 5 milliseconds. The
value of t.sub.mo is determined, at the standard temperature, by
calibration, and the value of t.sub.y is calculated by the microprocessor
7.
In order to determine the values of the above periods at temperatures other
than 20.degree. C., the opening time t.sub.mo2 at temperature T.sub.2 is
calculated using the relationship
t.sub.mo2 =t.sub.mo1 -a.sub.o (T.sub.2 -T.sub.1) (2)
where
a.sub.o is a value which is indicative of the sensitivity of the breaker to
temperature and is given by the breaker manufacturer
T.sub.2 is equal to the temperature of interest
T.sub.1 is equal to the standard temperature is equal to, in a particular
embodiment, 20.degree. C.
t.sub.mo1 is equal to the switch opening time at 20.degree. C.
t.sub.mo2 is equal to the switch opening time at T.sub.2.
The value of t.sub.mo2 is calculated with equation (2), and the value of
t.sub.y is calculated using the programmed value of t.sub.arc and the
calculated value of t.sub.mo2 applied in equation (1) above.
With the above calculation, the parameters for opening the breaker are
determined. The processor 7 sends out signals to the controller 13 which
initiates appropriate action (e.g. applying an opening signal to the coil
of the breaker) to affect the opening in accordance with the calculated
timing.
As seen from FIG. 1, the zero crossing is determined only for phase A.
However, as phases B and C have a known phase relationship to phase A
(e.g. phase B is separated from phase A by angle P.sub.a and phase C is
separated from phase B by angle P.sub.b), timing for these phases is
determined in a straightforward manner. Specifically, the zero crossing
occurs at P.sub.a /360 (t.sub.cycle) msec after the zero crossing for
phase A. In a like manner, the zero crossing for phase C occurs at p.sub.b
/360 (t.sub.cycle) after the zero crossing for phase A.
In practice, temperature readings are taken at predetermined intervals and
the value for t.sub.mo is calculated whenever a temperature reading is
taken. When an actuating signal is received, the value of the last
calculated t.sub.mo is used.
In addition, the t.sub.mo of phase A may not be identical with the t.sub.mo
of phase B or of phase C. Accordingly, separate calculations have to be
made at each temperature for the value t.sub.mo of each phase. Further,
the value a.sub.o may also be different from each phase. The values for
a.sub.o for each phase are stored in the processor 7 and are identified as
such to perform appropriate calculations.
As is also well known, it is not possible to continuously convert the
analog signal to a digital value. Instead, samples have to be taken. In
accordance with a particular embodiment of the invention, 32 samples are
taken during each cycle of the voltage/current.
The parameters for determining the closing times for the breakers are
illustrated in FIG. 3. As seen in FIG. 3A, the total closing time t.sub.c
is once again equal to an integral number of cycles. Once again, the
number of cycles illustrated in FIG. 3 is 3.
The closing signal is, as seen in FIG. 3C, initiated at the beginning of
the time period t.sub.cc. Once again, the computer monitors for the first
zero crossing, illustrated in FIGS. 3B and 3D as appearing at the
beginning of the time period t.sub.x. t.sub.x is a waiting period and a
closing signal is applied to the coil of the breaker at the expiration of
the period t.sub.x. As seen in FIGS. 3D and 3E, this occurs at the
beginning of the period t.sub.mc. The period t.sub.mc, that is, the time
that it takes the contacts to move from an open to a closed position, is
once again a function of the particular breaker and is once again
calibrated at a standard temperature, for example, 20.degree. C. In order
to determine the period t.sub.mc2 for a temperature T.sub.2, different
from 20.degree. C., use is made of the relationship
t.sub.mc2 =t.sub.mc1 -a.sub.c (T.sub.2 -T.sub.1) (3)
where
a.sub.c is once again given by the manufacture of the breakers.
It can also be seen from FIG. 3 that
t.sub.c =t.sub.x +t.sub.mc +8.33 msec-t.sub.del (4)
As t.sub.c and t.sub.mc are already known, and as t.sub.del is selected to
enable the exact point of initiation (the onset of the period t.sub.mc) to
be fixed with exactness, the period t.sub.del is also known, and the
period t.sub.x can be determined from equation (4).
By definition, t.sub.del is the time delay between the last zero crossing
of the phase voltage before the mechanical closure of the circuit breaker
contacts and the actual contact closure. When the circuit breaker is used
with an inductance or with a transformer, t.sub.del should be set around 2
ms in order to avoid the high inrush currents which can cause high
electrodynamic stresses on the windings. High inrush currents occur when
the breaker contacts close near zero phase voltage i.e. when t.sub.del is
close to zero. Conversely, when the circuit breaker is used with a
capacitor bank, t.sub.del should be close to zero in order to prevent high
inrush currents which would stress the capacitors and damage the contacts
of the circuit breaker.
As seen in FIG. 3F, the contacts move from an open to a closed position
upon termination of the period t.sub.mc. Once again, the timing of phases
B and C are determined knowing the relationship between the signals on
phases A, B and C. In addition, the value t.sub.mc2 must be separately
calculated for each phase A, B or C taking into account the value of
a.sub.c and of T.sub.2.
Although a particular embodiment has been described, this was for the
purpose of illustrating, but not limiting, the invention. Various
modifications, which will come readily to the mind of one skilled in the
art, are within the scope of the invention as defined in the appended
claims.
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