Fault Current Limiter

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.

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.

FIG. 1 is a block diagram showing the fault current limiter included in an AC power system.

FIG. 2 is an electrical schematic diagram showing one embodiment of the fault current limiter.

FIG. 3 is an electrical schematic diagram showing another embodiment of the fault current limiter.

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.