Method for generating circuit-breaking signals

A method for generating a circuit-breaking signal in dependence upon the magnitude and duration of an excess current, where a measured quantity, derived from the excess current, is sampled, is converted into digital values, and these values are weighted in accordance with a predetermined function and are added in order to obtain a circuit-breaking signal when the sum exceeds a predetermined comparison value. In order to improve upon a method of this kind in such a manner that, in the case of a diminishing excess current, a false interruption of the circuit is safely prevented, an auxiliary quantity (.DELTA.i) is formed which corresponds to the difference quotient or differential quotient of the measured quantity (M) and when the absolute amount of the auxiliary quantity (.DELTA.i) is already below a predetermined limit value and the sampled value of the excess current is below a predetermined threshold (S), the addition is interrupted.

BACKGROUND OF THE INVENTION 
The present invention relates to methods for generating circuit-breaking 
signals, in dependence upon the magnitude and the duration of an excess 
current, where a rectified measured quantity is derived from the excess 
current and sampled, the samples measured values are converted into 
corresponding digital values which are then individually weighted in 
accordance with a predetermined function, and added, and if the sum total 
exceeds a predetermined comparison value, a circuit-breaking signal is 
formed, whereas addition is interrupted prior to the formation of the 
circuit-breaking signal if the excess current falls below a given 
threshold for a period of time. 
A known monitoring device described in German Patent Specification No. 29 
50 031 (See U.S. Pat. Nos. 4,219,860 and 4,219,858) operates in accordance 
with a method of this kind. In this known monitoring device, a measured 
quantity is obtained from the current which is to be monitored by 
rectification, sampled by a sampling circuit, and the sampled values 
obtained in this way are converted into corresponding digital values in an 
A/D-converter. The digital values are weighted in a function generator, 
and subsequent to weighting, are added in an adder circuit. When the count 
of the adder circuit reaches a given comparison value, a circuit-breaking 
signal is produced. In the known monitoring device, the A/D-converter is 
connected to a circuit assembly which continuously checks whether the 
current to be monitored comprises excess current values. When this occurs, 
a clock-controlled counter connected to the output of the circuit assembly 
is continuously reset. If the reset signal for the circuit assembly fails 
to appear, the clock-controlled counter counts up to a predetermined count 
and then resets the adder circuit. To prevent this from taking place when, 
on the basis of the curve of the rectified masured quantity, the 
instantaneous values thereof fall below a value which characterizes an 
excess current value, in the known monitoring device, the predetermined 
count must be set to be such that this count can be reached only following 
a length of time which exceeds the duration of a half-cycle of the current 
which is to be monitored. In certain situations, this can lead to the 
circuit being broken even when such is unnecessary. In any case, the known 
monitoring device which operates in accordance with the described method 
for currents of 50 Hz or 60 Hz, requires counters which are differently 
set in respect of the critical count. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a method for generating a 
circuit-breaking signal in dependence upon the magnitude and duration of 
an excess current, where the time at which the addition is interrupted, in 
the event of a diminishing excess current, is dependent upon the curve of 
the measured quantity, and thus, independently of the frequency of the 
current which is to be monitored in respect of excess current values, 
occurs approximately after one half-cycle of the current which is to be 
monitored. 
In accordance with the present invention, a method is provided for 
generating a circuit-breaking signal in dependence upon the magnitude and 
duration of an excess current, where the time at which the addition is 
interrupted, in the event of a diminishing excess current, is dependent 
upon the curve of the measured quantity, and thus, independently of the 
frequency of the current which is to be monitored in respect of excess 
current values, occurs approximately after one half-cycle of the current 
which is to be monitored. 
In accordance with the present invention, a method of producing a 
circuit-breaking signal in dependence upon the magnitude and the duration 
of an excess current, includes the steps of: 
(a) sampling a rectified measured quantity derived from the excess current, 
and converting the sampled measured values into corresponding digital 
values; 
(b) weighting the digital values in accordance with a predetermined 
function and then adding them, and if the sum value exceeds a 
predetermined comparison value, generating the circuit-breaking signal; 
(c) interrupting the addition prior to generation of the circuit-breaking 
signal if the excess current falls below a predetermined threshold for a 
period of time; 
(d) the method further including the formation of an auxiliary quantity 
which corresponds to at least one of a difference quotient or differential 
quotient of the rectified measured quantity; 
(e) determining the sign of the auxiliary quantity and in the event of a 
change in its sign from positive to negative, storing the sampled maximum 
value occurring at that time, and continuing the addition in 
clock-controlled fashion until the sign again changes from positive to 
negative; 
(f) comparing the absolute amount of the auxiliary quantity with a 
predetermined limit value and each sampled value with the predetermined 
threshold of the excess current; and 
(g) interrupting the addition if the absolute amount of the auxiliary 
quantity is below the predetermined limit value and the sampled value of 
the excess current is below the predetermined threshold. 
Thus, a device operating in accordance with this method can be designed 
independently of the frequency of a current which is to be monitored in 
respect of excess current values, since an interruption of the addition 
process in the case of a diminishing excess current is actuated not by a 
counter which counts to a given count, but on the basis of a comparison of 
the absolute amount of the auxiliary quantity with a predetermined limit 
value and a comparison of the sampled value of the excess current with a 
predetermined threshold. Interruption of the addition process always takes 
place in dependence upon the actual curve of the measured quantity. 
The auxiliary quantity can be obtained in different ways in the method in 
accordance with the invention. For example, the auxiliary quantity can be 
obtained by differentiation of the rectified measured quantity. However, 
in order to execute the method in accordance with the invention in a 
simple fashion, it would appear more advantageous to compare consecutive 
sampled values with one another and to determine the sign and the absolute 
amount of the auxiliary quantity on the basis of the comparison. Thus the 
auxiliary quantity can be obtained by simple difference formation. 
In order to execute the method in accordance with the invention in a manner 
which is both simple and reliable, it has proved advantageous to produce 
an energizing signal on the first occasion on which the predetermined 
threshold is exceeded by the current which is to be monitored, thus in the 
case of an excess current, the signal is produced, in response to which 
the sampled value is stored and addition is commenced, starting from zero. 
In this way, it is not only possible precisely to define the starting 
conditions for the addition process, but it is also ensured that from the 
first instant at which an excess current occurs, the values sampled at 
that time can be included in the addition. As a result, the accuracy of 
the method in accordance with the invention is extremely high. 
In order to adapt the method in accordance with the invention to the 
fulfillment of a frequently imposed requirement, namely, that of not 
falling below a minimum circuit-breaking time, the energizing signal 
advantageously starts a fixed time counter, on the completion of the count 
of which, if the predetermined comparison value is reached during the 
addition process, a circuit-breaking signal is produced. It has also 
proved advantageous, if, in the method in accordance with the invention, 
in response to the energizing signal, a predetermined number of 
consecutive sampled values is stored and an investigation carried out, by 
forming the difference in pairs of the last three stored, sampled values, 
to establish when there is occurrence of a maximum value of the excess 
current, and when a maximum value is determined, this is stored. In the 
practical implementation of the method in accordance with the invention, 
the paired difference formation of the three last-stored, sampled values 
allows it to be established, without any noticeable outlay, as to whether 
one difference formation results in a positive sign and the other 
difference formation results in a negative sign; if this is so, an 
intermediate value can only represent a maximum value, which can then be 
stored and further processed in a simple manner. 
In the method in accordance with the invention, it has also proved 
advantageous to withdraw the energizing signal when a maximum value occurs 
which is below the given threshold of the excess current and the absolute 
amount of the auxiliary quantity is below the given limit value. When the 
energizing signal is withdrawn, for example, the weighting and the 
addition can be interrupted. Thus, this also prevents the formation of a 
circuit-breaking signal. 
In order also to interrupt the addition process in those relatively rare 
situations in which the excess current continuously converges towards 
zero, in accordance with a further development of the method of the 
invention, the predetermined number of stored, sampled values results in a 
value below a predetermined minimum value. As a result of the withdrawal 
of the energizing signal, the addition is interrupted and thus the 
formation of a circuit-breaking signal is prevented.

DETAILED DESCRIPTION 
With reference now to the drawings, FIG. 1 represents, by way of example, a 
measured quantity M plotted against time t, where the measured value M is 
formed by rectification from a single-phase alternating current which is 
to be monitored, together with a train of sampling pulses A, and waveforms 
DY, SWU and ANG, to be described with reference to FIG. 3. 
As shown in FIG. 2, the measured value M obtained in this way is 
investigated in a first interrogation 50 to establish whether it comprises 
instantaneous values above a predetermined threshold S. In respect of the 
curve, represented in FIG. 1, the measured quantity M, this is so in the 
case of the second sampling signal A2. Therefore, the first interrogation 
of the flow diagram shown in FIG. 2 is responded to at this instant in 
time by "yes", and the sampling of further values of the measured quantity 
M, as shown at 52, commences, as can be seen from FIG. 1. The sampled 
values are then checked in a second interrogation 54 to establish whether 
the sampled value i(t+.DELTA.t)&gt;i(t), and thus whether the later-sampled 
value is greater than the previous. If this is so, then the last-sampled 
instantaneous value i(t+.DELTA.t) of the measured quantity M is stored and 
the absolute amount of the difference between the last-sampled 
instantaneous value and the previous instantaneous value--i.e., the 
absolute amount of the auxiliary quantity .DELTA.i--is formed, as shown at 
56. 
In a third interrogation 58, the sampled instantaneous value of the 
measured quantity M is checked to determine whether it is greater than the 
predetermined threshold S. If so, the sampling is continued. If the 
sampled instantaneous value is below the predetermined threshold S, then a 
further (fourth) interrogation 60 is carried out in which the absolute 
amount of the auxiliary quantity .DELTA.i obtained by difference formation 
is checked to determine if it is greater than a predetermined limit value 
.DELTA.I. If this is so, the sampling is continued. If this is not so, the 
sampling is interrupted. 
If the result of the second interrogation 54 is that a last-sampled 
instantaneous value of the measured quantity M is smaller than the 
previously-sampled instantaneous value, then the preceding instantaneous 
value i(t) is stored, is weighted in accordance with a function f=i.sup.2 
t and the weighted digital values are added. See 62. In a further (fifth) 
interrogation 64, the addition is checked to determine if it has reached a 
predetermined comparison value K. If this is so, a circuit-breaking signal 
is produced as shown at 66. Othrwise, the addition is continued or is 
interrupted at 70 without the formation of a circuit-breaking signal via 
flow line 68. 
When the circuit has been broken, the addition is returned to zero as shown 
at 70, which generally takes place at the start of a new incidence of 
excess current. 
The method in accordance with the invention can advantageously be carried 
out using a micro-computer; therefore the block circuit diagram shown in 
FIG. 3 is merely a simplified representation by way of explanation of the 
sequence of the method proposed in accordance with the invention. 
The alternating current I which is to be monitored is rectified in a 
rectifier circuit 1 and sampled by a sampling device represented 
schematically by a contact 2. If currents are to be monitored in a 
multi-phase system, then, in a manner known per se, in place of the 
rectifier 1, an envelope curve former is used which is acted upon at its 
input by quantities obtained from the currents in all phases. The sampled 
values of the measured quantity M, occurring at the output of the 
rectifier 1, are then fed to means for A/D-conversion, not shown for the 
sake of simplicity in the drawing, and then to a module 3 which sets a 
circuit-breaking threshold which corresponds to the predetermined 
threshold S in FIG. 1. The output of the module 3 is connected to an 
excitation marker 4 which, in response to an output signal from the module 
3, undergoes a change in potential at its output Q. This change in 
potential at the output Q of the excitation marker 4 also causes an enable 
signal to be fed to a fixed-time counter 7. Previously, the fixed-time 
counter 7 has been reset by the output signal of the monostable circuit 5. 
An additional contact 8 is also closed as a result of the change in 
potential at the output Q of the excitation marker 4. 
As a result of the closure of the contact 6, the first sampled value (in 
the case of the sampling signal A2 in FIG. 1), which occurs following the 
occurrence of an excess current value, can be switched to a hold element 9 
and therefore is stored in this hold element 9. Via a contact 10, which is 
also closed as a result of the change in potential at the output Q of the 
excitation marker 4, the stored value is weighted in a function generator 
11 in accordance with a predetermined function, in particular, an i.sup.2 
t-function, and the weighted value is fed into an integrator 13 via a 
contact 12 controlled by a clock signal T. This integrator 13 is followed 
by a threshold value module 14 in which a predetermined comparison value 
is stored. The output of the threshold value module 14 is connected to one 
input of an AND gate 15, whose other input is connected to the output of 
the fixed time counter 7. 
Via the contact 8, the sampled instantaneous values of the measured 
quantity M act upon a sampled value store 16 in which a predetermined 
number of consecutive, sampled values can be stored. In the exemplary 
embodiment, four consecutive, sampled values can be stored in the sampled 
value store 16. The output of the first storage position 17 is connected 
to a plus-input or an adder 18 whose minus-input is connected to the 
output of a further storage position 19. The output of this storage 
position 19 is connected to a plus-input of a further adder 20 whose 
minus-input is connected to an output of a further storage position 21. By 
means of the adders 18 and 20, the consecutive, sampled values are 
compared in pairs by subtraction and when a predetermined difference 
exists, signals are fed via threshold value modules 22 and 23 to a further 
AND-gate 24 whose upper input is an inverting input. This AND-gate 24 is 
followed by a monostable circuit 25 whose output is connected both to an 
input of an additional AND-gate 26 and to a control input of a contact 27. 
The storage position 19 of the sampled value store 16 is connected via a 
connection line 28 to the contact 27 via which a further connection can be 
established to the hold element 9 when the contact 27 is actuated by an 
appropriate output quantity of the monostable circuit 25. The connection 
line 28 also leads to the input of a threshold value module 29 which is 
set at the predetermined threshold S for the circuit-breaking current. If 
this threshold S is exceeded, then a signal occurs at the output of the 
module 29 and is inverted at the input of the additional AND-gate 26. 
The additional AND-gate 26 is connected at its output to an OR-gate 30 
whose other input is an inverting input connected to the output of an 
additional threshold value module 31. This additional threshold value 
module 31 is connected to the output of an adder 32 which has respective 
inputs connected to the storage positions 17, 19 and 21, and to a fourth 
storage position 33 of the sampled value store 16. The output of the 
OR-gate 30 is connected to a reset-input R of the excitation marker 4. 
The sequence of the method in accordance with the invention will now be 
explained, making reference to the circuit arrangement illustrated in FIG. 
3. 
An alternating current I, which is to be monitored, is rectified in the 
rectifier 1 and produces a measured quantity M as plotted in FIG. 1 in 
dependence upon the time. The rectified measured quantity M, obtained in 
this way, is sampled pulse-wise in a form represented in FIG. 1 by the 
sampling signals A. If the sampling results in a sampled value which is 
above the predetermined threshold value S in FIG. 1, then the threshold 
value module 3 is FIG. 3 responds at the time t1 and a change in potential 
occurs at the output of the excitation marker 4, which, in certain 
circumstances, leads to the activation of the fixed-time counter 7 and to 
the closure of the contact 8. For a predetermined length of time, the 
contact 6 is closed by the output signal of the monostable circuit 5 and a 
reset signal is fed to the fixed-time counter 7 and to the integrator 13. 
As a result, via contact 6, the first value which is sampled by the 
sampling signal A2 and which is above the predetermined threshold S can be 
stored in the hold element 9. Since the contact 10 is also closed in the 
event of a change in potential at the output Q of the excitation marker 4, 
this value is weighted and is added in a predetermined clock signal in the 
integrator. 
With the following sampling signal, the instantaneous value of the measured 
quantity M virtually reaches the peak value; this sampled value is stored 
via the contact 8 in the storage position 17 of the sampled value store 
16. It is not passed immediately to the hold element 9 because the contact 
6 has since been reopened. Additional sample values are input 
consecutively into the sampled value store 16. The difference formation of 
consecutively-sampled sample values by the adders serve to produce the 
auxiliary quantity .DELTA.i, and in particular the sign thereof is 
determined, as shown by the curve DY in FIG. 1. A signal SWU is also 
produced by means of the threshold value module 29, as also shown in FIG. 
1. This signal SWU characterizes the occasions when the predetermined 
threshold S is undershot. If the first three storage positions 17, 19 and 
21 are filled, the adders 18 and 20 establish whether a maximum value is 
stored. If such a maximum value is contained in the storage position 19 
(this is so when the adder 18 produces a quantity having a positive value 
and the adder 20 produces a quantity having a negative sign), it is fed 
via the contact 27, which is then closed, to hold element 9, where it is 
stored. This stored value is now weighted and the weighted values are 
added in the predetermined clock signal T in the integrator 13. The 
addition on the base of the determined maximum value continues until the 
adders 18 and 20 again produce output quantities with the above-described 
sign ratio. This is so in the case of the sampling signal A9. In this 
case, the contact 27 is then temporarily closed again via the monostable 
circuit 25 and the maximum value which is then contained in the storage 
position 19 is fed to the hold element 9 where it is stored. This (lower) 
maximum value is then weighted again and a clock-controlled addition in 
the integrator 13 takes place. This is carried out with further maximum 
values until the time t2 (see FIG. 1), because at this time a sampled 
value is established which is smaller than the predetermined threshold S 
and furthermore the adders 18 and 20 indicate that the difference between 
adjacent sampled values is relatively small, and thus the differential 
quotient is relatively small. In this case, the contact 27 is in fact no 
longer actuated so that a maximum value below the predetermined threshold 
S is no longer input into the hold element 9. 
The additional AND-gate 26 is now acted upon by the outputs of the 
threshold value module 29 and the additional monostable circuit 25 in such 
manner that an output signal is emitted from the AND-gate 26, as a result 
of which the excitation marker 4 is acted upon at its reset input 4 by the 
OR-gate 30. Then, as a result of a signal change at the output Q of the 
excitation marker 4, the contact 8 is reopened so that no further sampled 
values can be stored. Furthermore, the contact 10 is opened and thus the 
integrator 13 is cut off from the hold element 9 so that the addition is 
interrupted. Consequently, the integrator 13 does not reach the 
predetermined comparison value which is given by the threshold value 
module 14 and no circuit-breaking signal is produced at the output of the 
AND-gate 15. 
Different conditions prevail, on the other hand, if, although this is not 
represented in FIG. 1, the measured quantity M comprises values above the 
predetermined threshold S for a long period of time. In this case, the 
excitation is not canceled via the output signal OR-gate 30 before the 
integrator 13 has reached the predetermined comparison value, but the 
integrator 13 can in fact add up to this predetermined comparison value 
whereupon the threshold value module 14 then responds and from its output 
emits a signal to the following AND-gate 15. If the fixed-time counter 7 
has then already completed its count, which is generally the case after 
300 ms, a circuit-breaking signal occurs at the output of the AND-gate 15. 
It should also be noted that under certain circumstances, the curve of the 
measured quantity M can be such that it continuously converges towards 
zero, and thus does not exhibit marked maximum values. In order to be able 
to prevent a circuit-breaking signal in this case, in accordance with the 
block-circuit diagram in FIG. 3, the stored values of all the storage 
positions 17, 19, 21 and 33 are added in the adder 32 and when a 
predetermined value is exceeded, the threshold value module 31 emits a 
signal which, having been inverted, produces an output signal from the 
OR-gate 30, whereupon the excitation marker 4 is reset. Thus, in this case 
also, a false interruption of the circuit is prevented. 
In the foregoing specification, the invention has been described with 
reference to a specific exemplary embodiment thereof. It will, however, be 
evident that various modifications and changes may be made thereunto 
without departing from the broader spirit and scope of the invention as 
set forth in the appended claims. The specification and drawings are, 
accordingly, to be regarded in an illustrative rather than in a 
restrictive sense.