US3745416A

US3745416A - A.c.power system couplings

Info

Publication number: US3745416A
Application number: US00159375A
Authority: US
Inventors: H Thanawala
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1969-10-21
Filing date: 1971-07-02
Publication date: 1973-07-10
Anticipated expiration: 1990-07-10
Also published as: DE2132292A1; GB1333380A

Abstract

In a short-circuit limiting coupling comprising a series combination of inductance and capacitance tuned to power-system frequency, at least a portion of the capacitance is connected in parallel with a bypass circuit which, at least in effect, comprises two parallel branches, one of the branches comprising a first saturable reactor in series with a first resistance shunted by a second saturable reactor, and the other branch comprising a second resistance in series with a rejector filter such as a parallel resonant circuit, also tuned to power-system frequency. The second reactor is designed to desaturate at a somewhat higher current level than the first; the first resistance will therefore assist in the recovery of the first reactor. The second resistance will damp harmonic and, more importantly, sub-harmonic oscillations.

Description

[111 3,745,416 July 10, 1973 A. C. POWER SYSTEM COUPLINGS llemesh Laxmidas Thanawala, Stafford, England Assignee: Associated Electrical Industries Limited, London, England Filed: July 2, 1971 Appl. No.2 159,375

Inventor:

[52] US. Cl 317/20, 317/50, 317/53,

323/77, 333/77, 333/79, 333/76 Int. Cl. H02h 9/00 Field of Search 317/20, 53, 50, 16;

Electrical World; Short Circuit Limiter Offers Flexibility; Jan. 5, 1970; pp. 50-51 Primary Examiner.l. D. Miller Assistant Examiner-Harvey Fendelman Attorney- Keith Misegades and George R. Douglas Jr.

[57] ABSTRACT In a short-circuit limiting coupling comprising a series combination of inductance and capacitance tuned to power-system frequency, at least a portion of the capacitance is connected in parallel with a bypass circuit which, at least in efi'ect, comprises two parallel branches, one of the branches comprising a first saturable reactor in series with a first resistance shunted by a second saturable reactor, and the other branch comprising a second resistance in series with a rejector filter such as a parallel resonant circuit, also tuned to power-system frequency. The second reactor is designed to desaturate at a somewhat higher current level than the first; the first resistance will therefore assist in the recovery of the first reactor. The second resistance will damp harmonic and, more importantly, subharmonic oscillations.

9 Claims, 12 Drawing Figures PATENIED JUL 101915 SHEEI 2 0F 2 A. C. POWER SYSTEM COUPLINGS This invention relates to electric circuit means for intercoupling two parts of an ac power system in such manner as to limit the flow of current from one part to the other under abnormal system conditions, such as a short-circuit in the latter part. Such means will be referred to herein as a short-circuit limiting coupling (or SLC).

The kind of SLC with which the invention is concerned comprises a series-connected combination of inductance and capacitance having such values that the combination is substantially series-resonant at the normal power-system frequency (say, 50 or 60 Hz) and means for effectively detuning the combination in response to over-current conditions. Thus the coupling offers negligible or permissibly low impedance under normal system-operating conditions, but increases its impedance and limits the magnitude of current flow under abnormal conditions involving current flow of greater than normal magnitude through the coupling.

It is known (for example, from our British Pat. specification No. 1,108,609) to provide an SLC of this kind in which the de-tuning means comprises a saturable re actor in parallel with at least a portion of the said capacitance. The saturable reactor is designed to be unsaturated and to allow very little current to flow through this by-pass branch of the coupling under normal system-operating conditions, but to saturate and allow a substantial current to flow through this branch under over-current conditions. Under the latter conditions, therefore, the effective value of the capacitance and its associated by-pass circuit is changed, and consequently the inductance-capacitance combination is detuned so that its impedance at the power-system frequency is increased.

It is also known (for example, from our'British Pat. specifications Nos. 1,108,608 and l,l46,676) to include resistance in such a by-pass branch to serve the dual purposes of damping out circuit transients and assisting the recovery of the saturable reactor to its unsaturated state within a short time after cessation of overcurrent conditions. The resistance (or part of the resistance) may itself be by-passed by a saturable reactor or by an acceptor filter such as a series resonant circuit tuned to the power-system frequency.

According to one aspect of the present invention, there is provided a short-circuit limiting coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of power supplied to the system, and circuit means connected across at least a portion of said capacitance means for detuning the resonant circuit in response to excess-current conditions, wherein said circuit means includes two parallel branches, a first branch comprising, in effect, first saturable reactance means, first resistance means for damping said first reactance means and second saturable reactance means for controlling the effectiveness of said first resistance means, and a second branch comprising, in effect, rejector filter means in series with second resistance means.

With the second saturable reactance means designed to desaturate at somewhat higher current levels than the first saturable reactance means, the first resistance means will serve to assist the recovery of the first saturable reactance means. With the rejector filter tuned to said frequency, the second resistance means will serve to damp harmonic and sub-harmonic oscillations appearing across the circuit means. The damping of subharmonics is most important because rotating electrical machines in the power system are otherwise likely to produce sustained self-generated sub-harmonic oscillation.

The separation of the two damping functions (reactor damping and capacitor damping) in this way offers the possibility of making the SLC cheaper and/or technically better for stability in particular. However, some of the components of the two effective branches can be combined with certain advantages, as will become apparent from the following description of various embodiments of the invention, by way of example, with reference to the accompanying drawings (FIGS. 1 10).

Each of the Figures is a circuit diagram of a shortcircuit limiting coupling comprising, between terminals 1 and 2, a main inductance 3 of inductive reactance X1 and a main capacitance 4 of capacitive reactance Xc, constituting a series resonant circuit tuned to the normal frequency (say, 50 or 60 B2) of a power system in which the coupling is to be used to interconnect two parts of the system: the capacitance 4 may of course be constituted by more than one capacitor. In parallel with a portion or preferably, as shown, the whole of the capacitance 4 there is a by-pass circuit B which is different in the several embodiments.

In the SLC shown in FIG. 1 the by-pass circuit B consists of two parallel branches B1 and B2. Branch Bl consists of a first saturable reactor 5 of reactance X2u when unsaturated, in series with a first resistance 6 of ohmic value R3 shunted by a second saturable reactor 7 of reactance X4u when unsaturated. Branch B2 consists of a second resistance 8 of ohmic value Rd in series with a rejector filter constituted by a parallel resonant circuit comprising an inductance 9 of inductive reactance Xld and a capacitance 10 of capacitive reactance Xcd.

The values of Xld and Xcd are so chosen that the filter F is tuned to the normal frequency of the power system. Branch B2 therefore has a negligible efi'ect on the operation of the coupling under normal conditions. However, the filter F offers low impedance to circuit transients and therefore brings the second resistor 8 into play to damp the transients appearing across the main capacitor 4, including those which arise from the other branch B1. The value of Rd is chosen to achieve such capacitor damping in an optimum manner.

The first saturable reactor 5 is designed so that its reactance X2u allows very little current to flow through branch Bl under normal system-operating conditions and so that it will saturate when the current flowing through the coupling (and therefore the voltage, appearing across the main capacitance 4) exceeds a predetermined value. Thus this reactor functions as a magnetic switch. The second saturable reactor 7 is designed so that it will saturate at a slightly higher level of current in the coupling than the first but that, during reduction of the current in branch B1 on recovery of the system, it will desaturate at a somewhat higher current level than the first. Thus the resistance 6 will be brought into play to assist the recovery of the first saturable reactor 5. The value of R3 ischosen to achieve such reactor damping in an optimum manner.

In the SLC shown in FIG. 2 the FIG. 1 arrangement is modified in that the reactor damping resistance 6 is replaced by a resistance 20 of ohmic value R311 which.

is suitable for both damping functions. The separate damping resistance 8 is dispensed with and the rejector filter F is connected across the first saturable reactor 5. Since the optimum values of Rd and R3 in the FIG. 1 arrangement are different (Rd being usually larger than R3) the values of Xld and Xcd are smaller in the FIG. 2 arrangement to achieve similar sub-harmonic performance. For the smaller value of Xcd the filter capacitance has to be constituted by a larger number of individual capacitors in parallel, but this disadvantage is offset by the saving in dispensing with the separate resistor 8. Moreover the whole or a part of the filter F could now be placed, for convenience and economy, in the tank of an oil-immersed reactor constituting the first saturable reactor 5.

Although the two resistors have been combined into one in the FIG. 2 arrangement, it will be understood that, functionally or effectively, the by-pass circuit B can still be regarded as comprising two parallel branches one (as branch Bl) being the first reactor 5 in series with the resistance shunted by the second reactor 7, and the other (as branch B2) being the filter F in series with the resistance 20.

In the SLC shown in FIG. 3 the FIG. 1 arrangement is modified in that the second saturable reactor 7 is provided with a secondary winding 30 and the reactordamping resistance 6 is connected across this winding. However this resistance is still, in effect, in series with the first saturable reactor 5 and shunted by the second 7. With this arrangement the level of voltage across the resistance can be chosen to permit the use of a resistor of lower ohmic value than would be required if it were directly connected: thus, for example, this arrangement may permit the use of a steel resistor instead of a less reliable carbon resistor. Moreover, the use of low voltage winding with one end earthed (as shown) is particularly convenient in SLCs for higher system voltages, e.g. 132 kV or 275 kV.

In the SLC shown in FIG. 4, the FIG. 1 arrangement is modified in both the ways of the FIG. 2 and FIG. 3 arrangements i.e., a common damping resistance 20 of ohmic value R3d is connected across a secondary winding 30 of the second saturable reactor, and the filter F is connected across the first saturable reactor 5. Alternatively, as indicated by the dotted lines 40 and 43, the filter F may be connected physically in series with the resistance 20, the connections indicated by the full lines 41 and 44 being omitted: then, as indicated by the dotted line 42, the resistance 20, which acts directly as the capacitor damping resistance (Rd), is reflected to act as the reactor damping resistance (R3). This alternative permits the use of components having the same values of Xld, Xcd and Rd as in the FIG. 1 arrangement. Another alternative would have connections 40 and 44, and omit 41 and 43, the component values Xld and Xcd then being different.

Further alternatives to the FIG. 4 arrangement are shown in FIG. 4A and FIG. 4B. In the FIG. 4A arrangement the common damping resistance 20 is connected across the primary winding of the second reactor 7 while the secondary winding 30 is connected in series with the filter F. In the FIG. 4B arrangement the second reactor 7 is provided with a third winding 45 which is connected in series with the filter F.

In the SLC shown in FIG. 5 the FIG. 4 arrangement is modified in that the filter F is connected across a secondary winding 50 on the main reactor 5. As the voltage provided by this winding can be suitably chosen and one end can be earthed (as shown) it is easier to design the filter economically.

The SLC shown in FIG. 6 is similar to that of FIG. 5 except in that only the filter reactor 9 is connected across the secondary 50, while the filter capacitance I0 is connected across the primary 5 of the main reactor. The filter capacitance can therefore be formed of capacitors of the same design as those of the main bank constituting the main capacitance 4.

The SLCs shown in the remaining Figures have a common feature which differentiates them from those shown in the preceding Figures namely a parallel equivalent branch BIA in the by-pass circuit B. The second saturable reactor 7 shunts the first saturable reactor 5 as well as the resistance 6 in series with the latter. However, it has the same effect as if it were shunting the resistance 6 only. Of course it now becomes es sential to ensure that the second reactor 7 saturates at a higher current level of the coupling than the first 5, as well as desaturating at a higher current level than the first.

In the SLC's shown in FIGS. 7 and 8 the second reactor 7 has a single winding. In those shown in FIGS. 9 and 10 the second reactor 7 has a secondary winding across which the series combination of first reactor 5 and resistance 6 is connected; in other words, the second reactor is constituted by a saturating transformer with thesame knee-point voltage as the reactor 7 of FIGS. 7 and 8. These arrangements are functionally or effectively equivalent but the provision of the secondary winding enables the voltage across the latter to be chosen to make it easier to design ecoomically the components connected thereto: with one end of the secondary winding 90 earthed, the FIG. 10 arrangement is a particular advantageous form of SLC for high voltage systems (e.g. 132 kV or 275 kV or higher voltages).

Each of the SLCs of FIGS. 7 10 includes, at least in effect, a capacitor damping branch as in the SLCs of FIGS. 1 6, consisting of a rejector filter F in series with a resistance. In FIGS. 7 and 9 this resistance 8 (of ohmic value Rd) is separate from the reactor damping resistance 6 (of ohmic value R3). In FIGS. Sand 10 there is a common resistance 20 (of ohmic value R311) as in FIG. 2, and the values of Xld and Xcd are different from those of FIG. 7 as is the case between FIG. 2 and FIG. 1.

It will be understood that there are various other possibilities for deriving different combinations from those which have been described, on the basis of the principles which have been mentioned.

I claim:

1. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response -to excesscurrent conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the a first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected in series with the first saturable reactance means and the rejector filter means, and resistance means for damping the first saturable reactance means connected across a secondary winding of the saturable transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

2. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excesscurrent conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little curent to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected in series with the first saturable reactance means, and a secondary winding connected in series with the rejector filter means across at least part of the first saturable reactance means, and resistance means for damping the first saturable reactance means connected across the primary winding of the saturable transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

3. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excesscurrent conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected in series with the first saturable reactance means, a first secondary winding connected in series with the rejector filter means across at least part of the first capacitance means, and resistance means for damping the first saturable reactance means connected across a further secondary winding of the transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

4. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excesscurrent conditions and comprising a first saturable reactance means in the form of a saturable transformer having its primary winding shunting the first capacitance means and arranged to be unsaturated to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, a rejector filter means at least part of which is connected across a secondary winding of the transformer so that it effectively shunts the first capacitance means and which is tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a further saturable transformer having its primary winding connected in series with the primary winding of the first saturable reactance means across the first capacitance means, and resistance means for damping the first saturable reactance means connected across a secondary winding of the further saturable transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

5. A system as claimed in claim 4 wherein a capacitance forming part of the rejector filter means is connected across the primary winding of the saturable transformer forming the first saturable reactor means.

6. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excesscurrent conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in parallel with the first saturable reactance means and with the rejector filter means, and resistance means comprising a first resistance connected in series with the first saturable reactance means for damping the latter and a second resistance connected in series with the rejector filter means for damping harmonic and sub-harmonic oscillations.

7. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excesscurrent conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means tuned to the frequency of the power supplied to the system and connected across the first saturable reactor means, a second saturable reactor means connected in parallel with the first saturable reactor means, and resistance means in series with the first saturable reactor means and the rejector filter for damping harmonic and sub-harmonic oscillations.

8. An ac power system coupling including first'capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excesscurrent conditions and comprising a first saturable reactance means. operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected across at least part of the first capacitance, the first saturable reactance means and the rejector filter means being connected in parallel across a secondary winding of the transformer, and resistance means comprising a first resistance connected in series with the first saturable reactance means for damping the latter and a second resistance connected in series with the rejector filter means for damping harmonic and sub-harmonic oslillations.

9. An ac power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for detuning the resonant circuit in response to excess current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected across at least part of the first capacitance, the first saturable reactance monic and sub-harmonic oscillations.

Claims

1. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connEcted in series with the first saturable reactance means and the rejector filter means, and resistance means for damping the first saturable reactance means connected across a secondary winding of the saturable transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

2. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little curent to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected in series with the first saturable reactance means, and a secondary winding connected in series with the rejector filter means across at least part of the first saturable reactance means, and resistance means for damping the first saturable reactance means connected across the primary winding of the saturable transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

3. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected in series with the first saturable reactance means, a first secondary winding connected in series with the rejector filter means across at least part of the first capacitance means, and resistance means for damping the first saturable reactance means connected across a further secondary winding of the transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

4. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means in the form of a saturable transformer having its primary winding shunting the first capacitance means and arranged to be unsaturated to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, a rejector filter means at least part of which is connected across a secondary winding of the transformer so that it effectively shunts the first capacitance means and which is tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a further saturable transformer having its primary winding connectEd in series with the primary winding of the first saturable reactance means across the first capacitance means, and resistance means for damping the first saturable reactance means connected across a secondary winding of the further saturable transformer so as also to provide a resistance effectively in series with the rejector filter means to damp harmonic and sub-harmonic oscillations.

6. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in parallel with the first saturable reactance means and with the rejector filter means, and resistance means comprising a first resistance connected in series with the first saturable reactance means for damping the latter and a second resistance connected in series with the rejector filter means for damping harmonic and sub-harmonic oscillations.

7. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means tuned to the frequency of the power supplied to the system and connected across the first saturable reactor means, a second saturable reactor means connected in parallel with the first saturable reactor means, and resistance means in series with the first saturable reactor means and the rejector filter for damping harmonic and sub-harmonic oscillations.

8. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected across at least part of the first capacitance, the first saturable reactance means and the rejector filter means being connected in parallel across a secondary winding of the transformer, and resistance means comprising a first resistance connected in series with the first saturable reactance means for damping the latter and a second resistance connected in series witH the rejector filter means for damping harmonic and sub-harmonic oscillations.

9. An a.c. power system coupling including first capacitance means and first inductance means together forming a series resonant circuit at the frequency of the power supplied to the system, circuit means for de-tuning the resonant circuit in response to excess-current conditions and comprising a first saturable reactance means operatively associated with a current path shunting at least part of the first capacitance means so as to be unsaturated and to allow very little current to flow therethrough under normal operating conditions, but to allow a substantial current to flow therethrough under excess-current conditions, rejector filter means connected so as to shunt at least part of the first capacitance means and tuned to the frequency of the power supplied to the system, a second saturable reactance means in the form of a saturable transformer having a primary winding connected across at least part of the first capacitance, the first saturable reactance means and the rejector filter means being connected in parallel across a secondary winding of the transformer, and resistance means in series with the first saturable reactor means and the rejector filter for damping harmonic and sub-harmonic oscillations.