Fault current limiter

A current limiting network is connected in a power line between an AC power supply and a load, and has a pair of series connected resonant branch circuits, each of which include a capacitance and an inductance tuned to the power supply frequency. Under normal operation virtually no impedance is offered by the network. Each branch circuit has a circuit node between the capacitor and inductor contained therein. A switch, having a resistance in series therewith, is connected between the circuit nodes in the branch circuits. The branch circuits are connected in parallel, and a resistance is connected in parallel with the branch circuits. The switch is responsive to a fault current in the power system and when actuated, detunes the series resonant branch circuits to thereby present a high impedance to the power line. The parallel resistance reduces the level of any transient surges and the time of any oscillation appearing on the capacitors when the switch is actuated by a fault current. Another embodiment selectively suppresses transients and reduces oscillation on the branch circuit capacitors.

BACKGROUND OF THE INVENTION 
This invention relates to current limiting circuits, and more particularly 
to such circuits for use in AC power lines. 
Circuits are known for use in AC power systems which are tuned to series 
resonance at the AC power frequency, and which are detuned in response to 
a fault current on the AC power line to afford a high impedance in the 
line and subsequent current limiting. A number of such circuits are shown 
in U.S. Pat. No. 3,418,532, issued to Becker. In FIG. 2 of the Becker 
patent a pair of series resonant inductive-capacitive circuit legs are 
shown connected in parallel, having a cross leg between the circuit nodes 
connecting the inductors and capacitors in each of the parallel legs. The 
cross leg contains a saturable choke X.sub.S in series with a resistor R. 
In normal operation the saturable choke presents a high impedance between 
the nodes, and the series resonant legs present substantially no 
resistance to the flow of current. When a fault current creates a voltage 
across the saturable choke, the choke is driven into saturation rendering 
it highly conductive and detuning the series inductive capacitive legs. 
Subsequent current flow through the circuit, when detuned, is limited by 
the resistance R to a safe value. Oscillation prevention is also provided 
by resistance R. However, the Becker disclosure, as well as all other 
known references, allows the voltage across the capacitors to increase 
momentarily to a very high level when the saturable choke assumes a 
conductive condition upon the occurrence of a fault. Building capacitors 
to withstand this momentary high voltage is quite costly, and renders any 
resulting circuit module very bulky. A short circuit limiting network 
having small volume is desirable which provides fault current protection 
in an AC power system, and at the same time provides protection for the 
network itself. 
SUMMARY AND OBJECTS OF THE INVENTION 
A current limiting circuit is disclosed for insertion in a power line 
between an AC power source and the load which includes first and second 
series resonant branch circuits each having a capacitor and an inductor 
therein, and each being tuned to the AC power frequency. Connection 
between the capacitor and the inductor in each branch circuit is made at a 
common circuit node therebetween. Consequently, substantially no impedance 
is offered to the power line by the branch circuits at resonance. 
Switching means is connected between the common circuit nodes in the 
branch circuits. The switching means is responsive to a fault current in 
the power line and operates, when actuated, to detune the branch circuits, 
and thereby offer a high impedance and subsequent limiting of the fault 
current in the power line. Resistance connected in parallel with the 
capacitors operates to reduce switching transients and voltage 
oscillations thereacross. 
In general, it is an object of the present invention to provide an improved 
fault current limiter which reduces the level of switching transients and 
voltage oscillations in the limiting network. 
Another object of the present invention is to provide an improved fault 
current limiter which specifically protects, and therefore reduces the 
size and required capability of capacitors contained in the limiting 
circuit. 
Another object of the present invention is to provide an improved fault 
current limiter which provides damping and reduction in electrical 
oscillation subsequent to switching. 
Additional objects and features of the invention will appear from the 
following description in which the preferred embodiment has been set forth 
in detail in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A rudimentary AC power system is shown in FIG. 1 including an AC power 
supply 11 coupled to a power line 12 in which is contained a fault current 
limiter 13. Power line 12 is coupled to a load 14 to be energized by power 
supply 11. 
Turning now to FIG. 2, a pair of branch circuits are shown, the first of 
which includes the series combination of capacitor C1 and inductor L1, and 
the second of which includes the series combination of inductor L2 and 
capacitor C2. The inductor and capacitor elements are chosen to provide 
series resonance at the AC power supply frequency, and consequent minimal 
impedance between terminals 16 and 17 shown thereon when coupled to power 
line 12. A circuit node is shown between capacitor C1 and inductor L1 and 
another circuit node is shown between inductor L2 and capacitor C2. A 
resistor R2, in series with a switch S1, is shown connected between the 
aforementioned nodes in the two branch circuits. A resistor R1 is 
connected in parallel with both of the two branch circuits. 
It is clear that under normal operating conditions with switch S1 in an 
open condition, line current may pass between terminals 16 and 17 
essentially unimpeded when the inductive reactance and capacitive 
reactance are equivalent in each branch circuit. This is a condition of 
series resonance. 
Switch S1 is sensitive to fault current and may be any one of a number of 
appropriate devices, such as a saturable reactor, a silicon controlled 
rectifier, a relay, spark gap or the like. Switch S1 is shown in FIG. 2 to 
be actuated by relay 18. Under fault conditions switch S1 is closed. 
Ordinarily when switch S1 is closed the voltage across capacitors C1 and 
C2 would increase momentarily to an extremely high level. However, 
resistor R1 reduces the voltage which may appear across the entire network 
terminals 16 and 17 during the occurrence of a fault in the AC system. 
Further, oscillations induced by the closing and opening of switch S1 are 
damped by both resistors R1 and R2. 
Inductors L1 and L2 may be air core devices, which inherently have a high 
over-voltage capability without increased cost of manufacture. As a 
consequence, the embodiment of FIG. 3 is feasible, wherein the switching 
transients and oscillations on capacitors C3 and C4 are lowered 
selectively. As in FIG. 2 above, a pair of parallel connected branch 
circuits including capacitor C3 and inductor L3 in one branch, and 
inductor L4 and capacitor C4 in another branch, are tuned to the AC power 
supply frequency to provide series resonance and minimal impedance between 
terminals 16 and 17 during normal operation. Capacitor C3 and inductor L3 
have a circuit node therebetween. A voltage divider including resistors 
R3, R4 and R5, is connected in parallel with the two resonant branch 
circuits containing capacitor-inductor combinations C3/L3 and L4/C4. A 
normally open switch S2 is connected between the circuit node which is 
common to capacitor C3 and inductor L3 and the junction between resistors 
R3 and R4 in the voltage divider. Another normally open switch S3 is 
connected between the circuit node located between inductor L4 and 
capacitor C4 and the junction between resistors R4 and R5 in the voltage 
divider. 
Switches S2 and S3 are both sensitive to fault currents in AC power line 
12, and are shown in FIG. 3 to be actuated by relays 19 and 20 
respectively, which are energized when a fault current is present in the 
circuit. Terminals 16 and 17 are coupled to AC power line 12 and switches 
S2 and S3 are positioned to a closed position in response to such fault 
currents. Resistors R3 and R5 in the voltage divider or multi-element 
resistor containing R3, R4 and R5 do not carry much, if any, current 
during normal operating conditions with switches S2 and S3 open, because 
the two branch circuits containing C3/L3 and L4/C4 present essentially 
zero impedance at the power system frequency. There is, therefore, no 
voltage drop across the multi-element resistor. In the event there is some 
small amount of current through resistors R3, R4 and R5 during normal 
operations, a spark gap (not shown) may be placed in series therewith. 
A fault current will cause switches S2 and S3 to close, thereby detuning 
the series resonant branch circuits containing C3/L3 and L4/C4. The 
impedance between terminals 16 and 17 rises to a high value as the series 
resonant branch circuits are detuned. The voltage divider effect at the 
junction between resistors R3 and R4 will reduce the steady state voltage 
across capacitors C3. Voltage division at the junction between resistors 
R4 and R5 will reduce the steady state voltage across capacitor C4. 
Further, the value of resistors R3 and R5 should be chosen so that the 
time constants of the combinations R3/C3 and R5/C4 is much less than 
one-half the period of one power cycle. This will provide network damping 
which will prevent short time voltage buildup across capacitors C3 and C4 
due to network signal oscillation. 
The resistors in the embodiments of FIG. 2 and FIG. 3 may be linear or 
non-linear. Non-linear, or voltage dependent resistors, such as those made 
of silicon carbide or zinc oxide, may more efficiently protect the 
capacitors in the networks of FIGS. 2 and 3. 
An improved fault current limiter providing controlled impedance to an AC 
power line has been disclosed, which provides protection for capacitor 
elements in the limiter circuit. The capacitors may therefore be less 
costly, and occupy less volume due to the lower voltage rating required.