Overcurrent protecting apparatus

In an overcurrent protecting apparatus for interrupting overcurrent in accordance with a coordination tripping curve characteristic, there are provided a microcomputer, a current/voltage converter for converting an overcurrent into a first voltage, D/A converting means for converting each output successively delivered from the microcomputer into a second voltage, and comparator means for comparing the first voltage with the second voltage. The delivering of the outputs from the microcomputer to the D/A converting means continues until an output is delivered from the comparator means to a first input of the microcomputer. Another comparator is also provided for delivering its output to a second input of the microcomputer when the first voltage is extremely high.

BACKGROUND OF THE INVENTION 
This invention relates to an overcurrent protection apparatus utilizing a 
microcomputer for providing overcurrent interrupting characteristics 
having a capability to coordinate with other protection apparatus 
characteristics concerned with high precision. 
In an ordinary overcurrent protection apparatus wherein an abnormal current 
flowing through a power line is detected for tripping a circuit breaker 
provided in the power line, it is essential that the overcurrent 
interrupting characteristics with coordinating capability have a 
performance which implements a specified coordination curve precisely. 
Heretofore, various attempts have been made for providing an overcurrent 
protection apparatus of the aforementioned type with the use of, for 
instance, an electronic circuit or a magnetically balancing type device. 
However, most of these apparatus require too many electronic components, 
or have insufficient capability in keeping the preset interruption 
characteristics immunized from the influence of ambient variations. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an overcurrent protecting 
apparatus wherein coordination tripping curve implemented overcurrent 
interrupting characteristics can be preset with high precision. 
Another object of the present invention is to provide an overcurrent 
protecting apparatus which is stable and reliable in operation having 
immunity from ambient variations. 
Still another object of the invention is to provide an overcurrent 
protecting apparatus inclusive of a coordination tripping curve setting 
device whereby the setting of any desired overcurrent interrupting 
characteristic can be made with ease and precision. 
According to the present invention, there is provided an overcurrent 
protecting apparatus for interrupting an overcurrent flowing through an 
electric power line in accordance with a current/time characteristic, 
comprising a microcomputer, a current/voltage converter for converting a 
current flowing through the power line into a voltage, D/A converting 
means connected to an output of the microcomputer for converting the 
output into a voltage, comparator means connected to compare the output of 
the current/voltage converter with the output of the D/A converting means, 
and to deliver an output to a first input of the microcomputer when the 
output of the current/voltage converter exceeds the output of the D/A 
converting means, and a separate comparator connected to compare the 
output of the current/voltage converter with a reference volage indicative 
of a current value to be interrupted instantaneously and to deliver an 
output to a second input of the microcomputer when the output of the 
current/voltage converter exceeds a voltage indicative of a current value 
to be interrupted instantaneously, the microcomputer including means for 
storing the current values and times corresponding to the current/time 
characteristic, means for delivering outputs indicative of current values 
sequentially, starting from the highest value to the lower values, to the 
input of the D/A converting means until an input is received at the first 
input, means for reading out a time value corresponding to the current 
value causing the reception of the first input, and means for delivering a 
tripping instruction after expiration of the time when the first input is 
received in the microcomputer, and for delivering the instruction 
immediately when the second input is received. 
In modifications of the present invention, the D/A converting means 
comprises a first D/A converter for delivering a basic current value P and 
another D/A converter for delivering additional current values 
.alpha..sub.i, while the comparator means comprises a first comparator for 
comparing the output of the first D/A converter with the output of the 
current/voltage converter, and a second comparator for comparing the 
output of the second D/A converter with the output of the first comparator 
and for delivering the output to the second input of the microcomputer. 
Preferably a current/time setting board is associated with the 
microcomputer for presetting the current values and times corresponding to 
the current-time characteristic used in interrupting overcurrent flowing 
through the electric power line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 2 showing a preferred embodiment of the present 
invention, there is illustrated a power line 9 interconnecting a group of 
power sources with a number of loads, or prices of equipment. 
In an ordinary operation, a current equal to or less than the rated value 
flows through the power line 9. However, in the case of an abnormal 
condition, an excessive current will flow through the power line 9, which 
must be interrupted by a circuit breaker 12 connected in the power line 9 
in order to protect downstream equipment from damage. Assuming that the 
rated current of the power line is 100%, a relationship as shown in FIG. 1 
must be kept between the current, in percentage, and the time between 
detection and tripping of the circuit breaker 12. With this relation, the 
current which is assumed to be 100% will not be interrupted, while an 
overcurrent of about 200% will be interrupted upon expiration of an 
interval t.sub.1 from an instant of the detection of the overcurrent. The 
reason why the delay time is provided before the interruption of the 
current resides in the maintenance of coordination between operations of 
the circuit breaker 12 and other circuit breakers connected with the power 
line 9. For example, a circuit breaker located downstream of the circuit 
breaker 12 shall be tripped earlier than the circuit breaker 12. 
The characteristic of FIG. 1 further shows that a current of a magnitude 
exceeding a point Q must be interrupted instantaneously, and that a 
current lower than a point P must not be interrupted. In FIG. 1, the point 
P is selected to be equal to the rated current. However, in actual usage, 
the current value of the location of the points P and Q, and the shape of 
the coordination tripping curve must be changeable in a preset stage in a 
wide range. 
Turning now to FIG. 2 showing a preferred embodiment of the present 
invention wherein a microcomputer 10 is employed, an overcurrent flowing 
through the power line 9 is interrupted by the circuit breaker 12 upon 
expiration of a predetermined time defined by the coordination tripping 
curve shown in FIG. 1. More specifically, the coordination tripping curve 
is stored in a memory device of the microcomputer, the contents of which 
can be easily rewritten from the outside by utilizing a well known 
rewriting circuitry. The rewriting of the memory device is preferably 
carried out in a manner such that the value of point P is varied from 70% 
to 150% at 5% intervals, and the value of the point Q as well as the delay 
time for a current between the points P and Q are varied in accordance 
with the variation of the point P. It should be noted that, the value of 
K=(current corresponding to the point Q)/(current corresponding to the 
point P) is held substantially constant regardless of the variation of the 
point P. 
The current flowing through the power line 9 is detected by a current 
transformer 8 and a current/voltage converter 7 which, in this example, is 
so selected that an output of 7.6 V is obtained when a rated current flows 
through the power line 9. 
The apparatus of FIG. 2 further includes an operational amplifier 5 wherein 
the output of the converter 7 is compared with the output of a D/A 
converter 2. The difference voltage obtained from the operational 
amplifier 5 is applied to an input of a comparator 4. The output of the 
current/voltage converter 7 is further connected to an input of a 
comparator 6 while the output of the D/A converter 2 multiplied by a 
constant K in a multiplier 11 is connected to the other input of the 
comparator 6. The output of the comparator 6 is connected to an 
interruption input I of the microcomputer 10. The output of another D/A 
converter 3 is connected to the other input of the comparator 4, and the 
output of the comparator 4 is connected to a sense input S of the 
microcomputer 10. 
It is assumed that the current in percentage of the point P is also 
represented by P% (the rated current being assumed to be 100%), and that 
the current in percentage of the point Q is also represented by Q%. The 
values of P to be represented by the output of the D/A converter 2 are 
shown in Table 1 together with the outputs from the terminals 0, 1, 2, and 
3 of the microcomputer 10 depending on the value of P preset in the 
microcomputer 10. Also, outputs brought out from the output of the D/A 
converter 2 by inputting the above described microcomputer terminal 
voltages are shown in Table 1. 
TABLE 1 
______________________________________ 
Preset mag- 
Output of Outputs (V) of microcom- 
nitude of P 
D/A conver- puter 10 at terminals 
(%) ter 2 (V) 0 1 2 3 
______________________________________ 
70 7 0 0 0 0 
75 7.1 0 0 0 1.5 
80 7.2 0 0 1.5 0 
85 7.3 0 0 1.5 1.5 
90 7.4 0 1.5 0 0 
95 7.5 0 1.5 0 1.5 
100 7.6 0 1.5 1.5 0 
105 7.7 0 1.5 1.5 1.5 
110 7.8 1.5 0 0 0 
115 7.9 1.5 0 0 1.5 
120 8.0 1.5 0 1.5 0 
125 8.1 1.5 0 1.5 1.5 
130 8.2 1.5 1.5 0 0 
135 8.3 1.5 1.5 0 1.5 
140 8.4 1.5 1.5 1.5 0 
145 8.5 1.5 1.5 1.5 1.5 
______________________________________ 
The D/A converter 2 or 3 is of type well known in the art. 
The output of the D/A converter 2 corresponding to the value in percentage 
of P is compared in the operational amplifier 5 with the output of the 
current/voltage converter 7, and the output of the operational amplifier 5 
is applied to an input of the comparator 4. 
In the comparator 4, the amount of the overcurrent in excess of the 
percentage of P, that is the value of .alpha..sub.i, is determined. More 
specifically, the computer 10 can present any one of the voltages shown in 
Table 2 below at the output of the D/A converter 3, and this voltage is 
compared in the comparator 4 with the output of the operational amplifier 
5. In the Table 2, there are also shown those voltages to be delivered 
from the terminals 4, 5, 6, and 7 of the microcomputer 10 for obtaining 
voltages indicative of the values of .alpha..sub.i at the output of the 
D/A converter 3. 
TABLE 2 
______________________________________ 
Value of .alpha..sub.i 
in percentage 
to be compar- 
Output of Outputs (V) of microcom- 
ed in compa- 
D/A con- puter 10 at terminals 
rator 4 (%) 
verter 3 4 5 6 7 
______________________________________ 
0 0 0 0 0 0 
50 1 0 0 0 15 
100 2 0 0 15 0 
150 3 0 0 15 15 
200 4 0 15 0 0 
250 5 0 15 0 15 
300 6 0 15 15 0 
350 7 0 15 15 15 
400 8 15 0 0 0 
450 9 15 0 0 15 
500 10 15 0 15 0 
550 11 15 0 15 15 
600 12 15 15 0 0 
650 13 15 15 0 15 
700 14 15 15 15 0 
750 15 15 15 15 15 
______________________________________ 
The output from the D/A converter 2 corresponding to the value of P preset 
in the microcomputer 10 is multiplied by a constant K in a multiplier 11, 
and is compared in the comparator 6 with the output of the current/voltage 
converter 7. The object of this comparison is to determine whether the 
current flowing through the power line 9 reaches the current value 
represented by the point Q or not. While the output of the comparator 4 is 
connected to the sense input S of the microcomputer 10, the output of the 
comparator 6 is connected to the interrupting input I of the same computer 
10. 
The operation of the overcurrent protecting apparatus shown in FIG. 2 will 
now be described. 
It is assumed that the apparatus operates based on the interruption 
characteristics as shown in FIG. 3, wherein the preset values of the 
current in percentage P+.alpha..sub.i are interrupted after the following 
times in seconds: 
______________________________________ 
P + 0 not interrupted 
P + 100 t.sub.1 sec. 
P + 150 t.sub.2 sec. 
P + 200 t.sub.3 sec. 
P + 250 t.sub.4 sec. 
P + 300 t.sub.5 sec. 
P + 400 t.sub.6 sec. 
P + 500 t.sub.7 sec. 
P + 600 t.sub.8 sec. 
______________________________________ 
The current values P+.alpha..sub.i preset in the microcomputer are not 
necessarily limited to the above described 8 values, but can be selected 
arbitrarily from 15 values (determined by the number of the inputs to the 
D/A converter 3) shown in Table 2, and the value of P may be preset to any 
one of those determined by the number of inputs of the D/A converter 2 as 
shown in Table 1. 
The currents in percentage P+.alpha..sub.i (i.ltoreq.15) and the delay 
times t.sub.i as listed above are stored in the microcomputer 10, and are 
used as data required in the execution of a program and routine as shown 
in FIGS. 4 and 5. 
In the microcomputer 10, the program illustrated in FIG. 4 is executed. 
When the program is entered, the A/D converters 2 and 3 are set to the 
value so as to output a prescribed value at the steps 41 and 42, and the 
presence of the sense input is searched for in the following step 43. The 
step 43 is repeated until a sense input is received at the input S of the 
microcomputer 10. When the sense input is received, a current interrupting 
routine as shown in FIG. 5 is entered. 
The presence of the sense input indicates that the current flowing through 
the power line 9 exceeds a current in percentage of P+.alpha..sub.i. For 
this reason, the steps 43 searching the sense input are carried out 
starting from a current value chosen in considerating unnecessary delay. 
Thus, in the routine shown in FIG. 5, i=i.sub.max is set at the step 51, 
and the presence of the sense input is searched successively at the step 
53 while the value of i in each time is subtracted by one (at step 54). In 
this way, the lowest limit of the current value producing the sense input 
is found. 
In the microcomputer, the delay time t.sub.i for the value of i providing 
the sense input is calculated, and the circuit breaker 12 is tripped after 
t.sub.i seconds by an output delivered through the line 1. 
According to the above described procedure, the overcurrent protective 
characteristic is approximated by a stepped line as shown in FIG. 6. 
However, if it is desired, the program may be so arranged that the 
protective characteristic is approximated by a broken line obtained by 
connecting the points P.sub.1, P.sub.2, . . . P.sub.8. 
In a case where a current K.times.P % flows through the power line 9, the 
current must be interrupted instantaneously. When a current greater than 
K.times.P % flows through the line, an output is produced from the 
comparator 6, and this output is applied to the interrupting input I of 
the microcomputer 10. The execution of the program shown in FIG. 4 is thus 
interrupted, and an output for tripping the circuit breaker 12 is 
immediately issued through the line 1. 
The microcomputer 10 is ordinarily formed in one chip containing a RAM 
(random access memory) and a ROM (read only memory). The microcomputer 10 
has input/output terminals 0, 1, . . . , 8, a sense input S, and an 
interruption input terminal I. Although it is not shown, a series of clock 
pulses and a power source voltage are supplied from the outside to the 
microcomputer 10. 
The microcomputer 10 used in this invention may preferably be of type TMS 
9940 made by Texas Instrument Inc. or type TLCS-43 made by Tokyo Shibaura 
Denki Kabushikigaisha in Japan. 
Each of the input/output terminals is constituted by a well known circuit 
as shown in FIG. 7. In the case where the input/output terminal is used 
only for an output terminal, 0 V (earth potential) or +5 V is delivered 
from the output terminal 75 when a logic signal is applied to a line 71 in 
accordance with a program and another line 72 is made into WRITE MODE. 
In FIG. 8, there is illustrated an example (in the form of a ladder 
resistor circuit) of a well known converter which is used as the D/A 
converter 2 or 3 shown in FIG. 2. In this example, each amplifier 81 
converts +5 V of the output from the microcomputer 10 to +1.5 V in the 
case of the D/A converter 2, and to +15 V in the case of the D/A converter 
3. A bias device 83 is used in the D/A converter 2 for the purpose of 
applying a bias voltage of 7 V to the output of the converter. 
The comparators 4 and 6 shown in FIG. 2 are of well known types, whereas 
the comparator 5 is of a type delivering +5 V when the voltage applied to 
the positive terminal is greater than the voltage applied to the negative 
terminal, and the difference exceeds 0.1 V. The terminals S and I of the 
microcomputer 10 can discriminate the presence of the sense input and the 
interrupting input, respectively, when these inputs are equal to or 
greater than +5 V. 
A modification of the present invention is illustrated in FIG. 9 wherein 
elements corresponding to those shown in FIG. 2 are designated by the same 
reference numerals. 
In this modification, the D/A converter 2 having the biasing device is 
omitted, and the value of P is set by a sliding resistor 13. In this way, 
a function of presetting P can be omitted from the computer 10. 
Another modification of the invention is illustrated in FIG. 10 wherein 
also like members and parts are designated by the same reference numerals. 
In this modification, although the manner of setting the value of P, by 
utilizing a sliding resistor 15, is different from the modification shown 
in FIG. 9, the feature of eliminating the function of presetting the value 
of P from that of the microcomputer is similar to the first modification. 
In this modification, the setting of a reference voltage for the 
comparator 6 is also achieved by another sliding resistor 14 interlocked 
with the sliding resistor 15. 
The values of the current in percentage P+.alpha..sub.i and the delay times 
t.sub.i as shown in FIG. 3 may be preset in the microcomputer 10 by the 
use of a current-time setting board 14 as shown in FIG. 12 wherein also 
like members and parts as in FIG. 2 are designated by the same reference 
numerals. 
In the current-time setting board shown in FIG. 12, sixteen current values 
from P to P+750% are presented along X axis, while the delay times of 
substantially similar number are presented along Y axis of a rectangular 
coordinate system. Assuming that the number of the delay times is also 16, 
there are a total of 16.times.16=256 points having coordinates selected 
from the current values and delay times. At these points, coordination 
tripping curve setting switches, each made up of a hole 122 and a plug 121 
to be inserted therein, are provided in a matrix-like pattern. The switch 
is opened when the plug 121 is pulled out of the hole 122. On the 
coordination tripping curve setting board 14, there are further provided 
digital switches 123 used for setting the value of P%, and a power switch 
124 corresponding to a power switch 132 in FIG. 13 showing a circuit 
associated with the coordination tripping curve setting board 14. In this 
circuit, the voltage of a power source 131 is applied across power lines 
138 and 139 through the power switch 132. Upon closing a switch 133 which 
corresponds to one of the switches arranged in a matrix form in the 
coordination tripping curve setting board, flip-flops 96 and 97 connected 
thereto are set, and when the switch 133 is opened, the two flip-flops are 
reset. The flip-flops 136 are arranged in a row in a register 134 so as to 
correspond to the currents in percentage P+.alpha..sub.i. That is, the 
leftmost flip-flop 136 corresponds to a current in percentage P+50%, while 
the following flip-flops 136 correspond to currents in percentage of 
P+100%, P+150%, and so on. 
On the other hand, the flip-flops 137 are arranged in a row in a register 
135 so as to correspond to delay times t.sub.i. That is, the uppermost 
flip-flop 137 corresponds to a longest delay time, and the following 
flip-flops 137 correspond to succeeding delay times of reduced values. 
The registers 134 and 135 are connected to the microcomputer 10 through a 
multiplexer 13 shown in FIG. 11. In order to utilize four input/output 
terminals of the microcomputer 10, the multiplexer 13 selectively connects 
four bits out of the registers 134 and 135 alternately starting from the 
leftmost end of the registers, so that these bits are read out to be 
stored in the microcomputer 10. In this manner, the values shown in Tables 
1 and 2, and a cross-reference between these values and the bits in the 
register 134 are all stored in the memory device of the microcomputer 10. 
In operation, an operation of the current-time setting board 14 firstly 
operates the digital switches 123 for setting the value of P, and then 
operates the switches such as 133 in FIG. 13 for setting the current 
values and the delay times in accordance with an overcurrent interruption 
characteristic of a coordinating nature as shown in FIG. 3. For instance, 
the plugs 121 in FIG. 12 are inserted in the holes 122 at predetermined 
positions, thereby providing further advantageous features of permitting 
the operator to recognize the overcurrent protection characteristic at a 
glance. 
In relation to the operating step 41 of the flow chart shown in FIG. 4, the 
microcomputer 10 reads out the setting of the digital switches 123 shown 
in FIG. 12, thereby knowing the value of P, and produces at the output 
terminals 0, 1, 2, 3 these voltages adapted to produce a voltage 
corresponding to the value of P at the output of the A/D converter 2 as 
described hereinbefore. Furthermore, for the purpose of knowing 
.alpha..sub.i and t.sub.i at the time of executing the operating steps 52 
and 56 in the flow chart shown in FIG. 5, the microcomputer 10 surveys the 
registers 134 and 135 in FIG. 13 from right to left and from the bottom to 
upward, respectively. Upon finding a pair of flip-flops set to "1" on the 
respective registers, the microcomputer calculates .alpha..sub.i and 
t.sub.i corresponding to the positions of the flip-flops while referring 
to Tables 1 and 2 stored in the microcomputer 10. 
For the execution of the next following program loop, the microcomputer 
starts surveying the flip-flops in the registers 134 and 135, leftwardly 
and upwardly, starting from the bit positions of the above described pair 
of flip-flops, and finds out the next pair of flip-flops which have been 
set to "1". 
The above described operation is repeated for required times for finding 
out the values of .alpha..sub.i and t.sub.i to be used in the execution of 
the subsequent program loops. 
When it is desired to reduce the time required for the microcomputer in 
finding out the values of .alpha..sub.i and t.sub.i successively, the 
program of the microcomputer may be so arranged that the setting of the 
digital switches 123 and the registers 134 and 135 of the coordination 
tripping curve setting board 14 are read out immediately after the 
entrance of the program, and the values of P+.alpha..sub.i and t.sub.i 
(i=1, 2, 3, . . . ) thus read out are stored in the memory device of the 
microcomputer. According to this method, it is not necessary to read the 
settings of the current-time setting board for the execution of subsequent 
program steps, but merely the contents of the memory device are referred 
to. For this reason, if desired, the coordination tripping curve setting 
board may be eliminated from the circuit after the settings on the board 
are stored in the microcomputer 10.