Current sensor responsive to symmetrical and asymmetrical currents and current limiting protector utilizing same

A current limiting protector for use in electric power transmission lines includes a conductive strip of material and a chemical charge for physically interrupting the conductive strip in response to a fault current. A current sensor triggers the chemical charge when the line current is symmetrical and the RMS value of the line current exceeds a first value, and the current sensor triggers the chemical charge when the line current is asymmetrical and the RMS value of line current exceeds a second value.

Referring now to the drawings, FIG. 1 is an electrical schematic of an 
electrical power transmission system including a source 10 and a load 12 
with a current limiting protector 14 and a circuit breaker 16 serially 
connected in the transmission line between the source and the load. As 
described above, the current limiting protector 14 contains a parallel 
current limiting fuse element 18 whereby the main conductor of the current 
limiting protector carries the normal line currents and responds to an 
excessive or fault current due to a fault 20 by directing the line current 
through the fusible element 18. Further, the current limiting protector 14 
is connected in series with the breaker 16 whereby smaller fault currents 
will be interrupted by the breaker 16 without triggering the current 
limiting protector 14. Since the current limiting protector is a one shot 
device, i.e. the current limiting protector must be replaced after each 
trigger thereof, longer use of a current limiting protector and less down 
time of the transmission line are experienced by utilizing the breaker 16 
to interrupt excess currents of a lower magnitude due to fault 20. 
As above described, current limiting fuses cannot handle continuous 
currents above approximately 100 amperes and still provide low let through 
current characteristics. On the other hand, the current limiting protector 
can handle high continuous currents typically as high as 1,000 or 2,000 
amperes and yet limit the let through currents under short circuit 
conditions to less than 15,000 amperes. When a fault occurs, a current 
level sensor triggers strategically located chemical charges that 
pyrotechnically cut gaps in the copper conductor of the current limiting 
protector thus diverting the fault current to the fuse where it is 
interrupted in the normal manner. 
Heretofore, the current is sensed independently and the current limiting 
protector has been triggered in response to the instantaneous load current 
exceeding a preselected level. 
FIG. 2 is a perspective view of a portion of the current limiting protector 
14 and shows the metal strip 30, typically copper, which is mounted on a 
nonconducting support member 32 by means of metallic plates 34 which are 
bolted to support member 32. The chemical charges 36 are provided in 
recessed portions of support member 32 under the strip 30 near a support 
member 34. The conductive layer 30 may be notched as shown at 40 to 
facilitate a rupture of the metal strip. As above described, in response 
to a sensed fault current, an electrical signal is applied to trigger the 
chemical charges 36 and disrupt the metal layer 30 as shown in FIG. 3. 
Thus, the explosive chemical charges instantaneously disrupt the current 
path through the conductive strip 30 thereby diverting current through the 
fuse 18 in FIG. 1. 
In accordance with one embodiment of the present invention a fault current 
sensor is provided which responds to the root mean square (RMS) value of 
the fault current and can be responsive to different current values 
depending on whether the fault current is symmetrical or asymmetrical. In 
determining whether the fault current is asymmetrical or symmetrical, the 
integral of the current between two time periods is compared with the 
average of the current or the trapezoid area formed by the corresponding 
currents at the two time periods, as illustrated in FIGS. 4 and 5. In FIG. 
4, a symmetrical current 40 is illustrated and the current curve is convex 
between the two time periods t.sub.1 and t.sub.2. Thus, the integral of 
the actual current line 40 between t.sub.1 and t.sub.2 will be greater 
than the trapezoidal area defined by the current i.sub.1 at t.sub.1 and 
the current i.sub.2 at t.sub.2. Conversely, as shown in FIG. 5 an 
asymmetrical current line 42 is concave between the time periods t.sub.1 
and t.sub.2. Thus, the integral of the actual current line 42 between 
t.sub.1 and t.sub.2 will be less than the trapezoidal area defined by the 
points i.sub.1 and i.sub.2 at times t.sub.1 and t.sub.2. 
This is illustrated as follows: 
##EQU1## 
Thus in the two cases we find for the difference of the respective areas: 
##EQU2## 
As shown in FIG. 6 the area difference for the asymmetrical case is 
positive while the area difference for the symmetrical case is negative. 
FIG. 7 is a plot of the symmetrical integral and the asymmetrical integral 
and illustrates that the symmetrical integral is correspondingly larger. 
That is, for any given value of current the integral of current is higher 
for the symmetrical current than for the asymmetrical current. Thus, it is 
seen that if a firing level is set on the basis of RMS symmetrical 
current, the current limiting protector will not be triggered on an 
asymmetrical current having a symmetrical component of the same magnitude. 
FIG. 8 is a functional block diagram of a fault current sensor in 
accordance with the invention which distinguishes between symmetrical and 
asymmetrical currents. An input signal at 60 is derived from the 
instantaneous line current by means of a current transformer connected to 
the power line and the input signal is compared with a preselected current 
level in comparator 62. Only when the input signal exceeds the preselected 
value in comparator 62 is the current sensor operation initiated. The 
preselected value is set at a level such that the normal operating current 
will not be reached. 
When the input signal exceeds the preselected value, as determined by 
comparator 62, the switches 64 and 66 are opened to initiate the timing 
and integration functions. The opening of switches 64 and 66 starts 
integration of the input signal at 68. A timing signal is provided by the 
integration of a constant at 70, and this time period is applied to a time 
reference comparator 72 which defines the time interval for the 
integration in the output logic 74. The trapezoidal area computation is 
provided at 76, and the output from the computation of 76 is compared with 
the current integration in the comparator 78. The output of comparator 78, 
either positive or negative, gives an indication of an asymmetrical or 
symmetrical current input signal. Assuming an asymmetrical signal, the 
integrated current wave is compared with a reference level for the 
asymmetrical current in comparator 80. 
Assuming that the current is symmetrical, the current integration is 
compared with a symmetrical reference level in comparator 82. Thus, the 
decision logic 84 responds to a positive or negative input from 78 
indicative of symmetry or asymmetry of the current wave, and then responds 
to either the asymmetrical comparator 80 or the symmetrical comparator 82 
at the end of the selected time interval, established by the time 
reference 72, whereby the output logic 74 causes a trigger signal to be 
applied to the current limiting protector when the selected reference 
level is exceeded. 
FIG. 9 is a detailed schematic of the fault current sensor of FIG. 8, and 
like elements have the same reference numerals. 
The outputs from comparators 78, 80, and 82 are used as select lines to the 
multiplexer 84. The multiplexer is used as a function generator and 
produces an output which is the input to flip-flop 74. The flip-flop is 
triggered when clocked if the current limiting protector is to be 
operated. In one embodiment the following components were used: 
______________________________________ 
Component Part Number 
______________________________________ 
Integrators LM 148J 
Comparators 72, 78, 80, 82 
LM 139J 
Comparators 62, 68, 70, 76 
One LMlllJ 
Multiplexers DM 74151 
Flip-flop DM 7474 
______________________________________ 
The current limiting protector in accordance with the present invention 
provides more selective response to fault currents whereby maintenance is 
reduced and down time of the electric power transmission lines is 
minimized. The current sensor is fast acting in determining current 
symmetry or asymmetry and determining trigger levels. While the 
illustrated embodiment determines symmetry by integration of current and 
comparison with a trapezoidal area, symmetry can be determined by other 
means such as determining and comparing the differential quotient at two 
points along the current wave. Similarly, the simple time integration of 
current can be replaced by the integral of the square of the current, 
thereby giving a true RMS value. 
Thus, while the invention has been described with reference to a specific 
embodiment, the description is illustrative of the invention and is not to 
be construed as limiting the invention. Various modifications and 
applications may occur to those skilled in the art without departing from 
the true spirit and scope of the invention as defined by the appended 
claims.