PROTECTION CIRCUIT FOR A POWER DISTRIBUTION SYSTEM

Abstract: A filtering and protection circuit (10), for use in a power distribution system including live, neutral and ground con­ ductors (L, N, G) and having a circuit-breaking device (12) in one of the conductors, includes transient protection devices (16) connected from a common circuit point (18) to the respective conductors (L, N, G) of the system. Failure (shorting) of any one of these transient protection devices can bring about actuation of the circuit-breaking device (12). The protection devices may comprise silicon diode transient voltage suppressors suitable for high voltage applications, clamping at a rated voltage in one direction and having a forward diode characteristic in the other direction. Each such suppressor may be paired with a parallel capacitor (22) protected by its diode from surges or transient voltages. Such pairing of suppressors and parallel capacitors can enable polarized capacitors to be used in AC circuitry.

Description: Protection Circuit For a Power Distribution System This invention relates generally to circuitry for protecting power distribution systems, for example protection and filtering circuits using protected capacitors. In the past transient voltage suppressors have been connected across multiple conductor alternatingcurrent power systems with each suppressor connected between each pair of conductors. As a result when a breaker such as a fuse is positioned in only one of those conductors, normally the live conductor, the failure of a suppressor not connected directly to the fused conductor does not activate the breaker. According to one aspect of the present invention there is provided protective circuitry for a power distribution system of the type having a live conductor, a neutral conductor, a ground conductor, and a circuitbreaking device connected in one of these conductors, characterised by three transient protection devices which are connected respectively between a common circuit point and said conductors of the system, each of said transient protection devices having a forward diode characteristic and the connections being such that the respective forward directions of those protection devices are similarly directed in relation to said common circuit point. Such circuitry can be arranged to provide fault protection using a single breaker for the live, neutral, and ground conductors that is activated in response to a fault between any of those conductors. A further problem encountered in power distribution systems is that line filters are sometimes unreliable because capacitors connected across the aforementioned suppressors are subjected to large transient voltages that can cause failure of the capacitors. In applications where AC capacitors are required, polarized capacitors have not been regarded as suitable because they are not designed to withstand substantial reverse voltage conditions. According to a second aspect of the present invention there is provided a capacitive device, for use in alternating current circuitry, comprising two polarized capacitors, connected in series between two terminals of the device, and clamping means connected with said polarized capacitors for preventing the positive peak of an alternating voltage,applied across said two terminals when the device is in use, from becoming destructively effective across one of said polarized capacitors and for preventing the negative peak of said alternating voltage from becoming destructively effective across the other of said polarized capacitors. Such a device can be used in a power distribution system protection and filtering circuit arrangement, since it can enable polarized capacitors to be used in high voltage alternating-current applications. The use of polarized capacitors in this manner, in AC circuits, can serve to reduce the likelihood of damage to filter capacitors from voltage transients and surges. Reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a circuit schematic showing protective circuitry embodying the said one aspect of the present invention; FIG, 2 is a circuit schematic of a capacitive device embodying the said second aspect of the present invention; and FIG. 3 is a circuit schematic showing another device embodying the said second aspect of the present invention. Referring to the drawings wherein like reference characters are used for like parts throughout, there is illustrated in FIG. 1 a three phase high voltage power distribution system or network with live conductors L1 through L3, neutral conductor N, and ground conductor G. Connected across these conductors is protective circuitry 10. Each of the live conductors L1 through L3 includes an over-current protection device such as a fuse or a conventional automatic breaker 12a, 12b or 12c, but the neutral and ground conductors do not include such a circuit-breaking device. In addition, choke input filters comprising inductors 14a, 14b and 14c are serially respectively located in the live conductors L1, L2 and L3, between the AC source (not shown) and the circuitry 10. A further inductor 14d is shown, in the neutral conductor N, but this may be omitted in some embodiments. The protective circuitry 10 includes a plurality of filtering and protection devices 16a through 16e enclosed in broken lines in FIG. 1. These transient protection devices 16 are connected in a wye (Y) arrangement between a common circuit point, or node, provided by line 18 in FIG. 1, and the live conductors L1 through L3, the neutral conductor N and the ground conductor G. With this arrangement even if a fault occurs in any of the devices 16a through 16e such that the device provides a short-circuit, a breaker 12 connected in one of the live conductors will be actuated since each of these devices 16 is connected by way of common node 18 to one of the live conductors including a breaker 12. Each device 16 includes a transient voltage suppressor 20 and, preferably, a parallel filter capacitor 22. The transient voltage suppressors 20 are preferably silicon p-n junction devices rated for high voltage and current applications. The current-voltage characteristic of each of the suppressors 20 is preferably similar to that of a zener diode, clamping at a given rated voltage in one direction and having a forward diode characteristic in the other direction. Suitable transient voltage suppressors are distributed by TRW, 301 West O Street, Ogallala, Nebraska with identifying numbers TVP1500 through TVP1534. Since the clamping time from zero volts to the breakdown voltage is a nanosecond the response of these devices is effectively instantaneous. These suppressors are available with clamping voltages higher than normal line voltages. Each suppressor 20 is arranged to permit forward current flow from the common node 18 into the corresponding one of the conductors L1, L2, L3, N and G, whilst allowing reverse current flow only when a clamping voltage is exceeded. Each suppressor 20 can also be combined with a parallel surge suppressor (not shown). For example, a shunt diode with a high forward surge rating increases the forward surge rating of the combination by shunting high node-to-line currents across the suppressor 20. The other shunt diode characteristics are not critical, provided that the clamping voltage of the suppressor is less than the reverse breakdown voltage of the shunt diode. Similarly a bi-directional clamp or protection device, such as a gas discharge tube, a metal oxide varistor, a bi-directional transient voltage suppressor or a combination of such devices can be shunted across the suppressor 20 for surge protection. For example, a bi-directional transient voltage suppressor made up of back-to-back p-n junction silicon devices, such as those described above, can be utilized. The capacitors 22 are protected by the ability of the suppressors 20 to quickly conduct to shunt the capacitor 22 in response to a surge or transient. As regards other possible embodiments, it should be realized that the wye arrangement of the protection devices 16 is suitable for single phase operation as well as other conductor arrangements of a power distribution network. In one embodiment of the Figure 1 circuitry, the capacitors 22 are polarized capacitors such as electrolytics. Each polarized capacitor has its positive terminal connected to the cathode of the associated suppressor 20. In this way, the polarized capacitors are capable of safe operation in an AC circuit without experiencing reverse voltages that could result in failure of those capacitors 22. FIG. 2 shows a more generally applicable AC capacitive device, or filter network, 23 which may be regarded as equivalent to the parallel arrangement of capacitors 22 and suppressors 20 in Figure 1. The capacitive network 23 of FIG. 2 includes a pair of diodes 26a and 26b connected in series, anode to anode. A polarized capacitor 24a is connected across diode 26a with the positive terminal of the capacitor 24a connected to the cathode of the diode 26a. Similarly, a polarized capactor 24b is connected across the diode 26b with the positive terminal of the capacitor 24b connected to the cathode of the diode 26b. The capacitor 24a on one side of the network and the diode 26b on the other side of the network form one diode damper, and the diode 26a together with the capacitor 24b form another diode damper. The two diode dampers are connected together at common node 28 with each diode 26 shunting current flow from common node 28 around their parallel capacitors 24 and preventing either capacitor 24 from discharging. The steady state voltage across the diode 26 of each damper is therefore the difference between the voltage across the filter network 23 and the voltage across clamper's capacitor, Thus the voltage across each diode 26 is a non-zero crossing sinusoid, one positive and one negative, one clamping the positive peak of the applied voltage at zero and the other clamping the negative peak at zero. Although the magnitude of the waveform is dependent on the values of capacitors 24a and 24b, when the capacitors are of equal value, the waveform representing the voltage across each diode 26 has a frequency equal to that of the applied voltage with half the amplitude. Thus each polarized electrolytic capacitor 24 is capable of acting in a high energy alternating current environment without experiencing a destructive reverse voltage. The filter network 23 has an overall effective AC capacitance equal to the product of the capacitances of each capacitor 24 divided by their sum. Such a network 23 could be used in place of each capacitor 22 in Figure 1. Conveniently a very large resistance (not shown) can be connected across the filter network 23 so that the charge trapped on the capacitors 24 will eventually bleed off after a long period of non-use. Such a resistance draws negligible current and does not alter the characteristics of the circuit. The diodes 26a and 26b can be any suitably rated p-n junction devices including conventional diodes or zener diodes. However, in order to avoid overvoltage of the capacitors, silicon transient voltage suppressors, such as the silicon transient suppressor discussed above, commercially distributed by TRW, are preferred. The forward diode current of the diodes 26 is effectively zero and the reverse breakdown voltage or the clamp voltage in the case of transient suppressors should be greater than the D.C. working voltage rating of the capacitors 24 which should at least equal the peak applied voltage. In an alternative embodiment (not shown) the polarity of the capacitors 24 and diodes 26 can be reversed to form a pair of peak rectifiers one including the capacitor 24a and diode 26b and the other including capacitor 24b and diode 26a. The circuit operation is otherwise identical to that shown in FIG. 2 and described above. Filter networks 23 as shown in FIG. 2 (or alternative versions as just described, using a peak rectifier arrangement) can be stacked as shown in FIG. 3 to increase the voltage capability of the overall device or to reduce the required ripple current rating of each unit. The filter networks 23 can be stacked to form a ladder network of any desired length with the polarity of the capacitors 24 and the diodes 26 alternating.

Claims: CLAIMS: 1. Protective circuitry for a power distribution system of the type having a live conductor (L1), a neutral conductor (N), a ground conductor (G),and a circuit-breaking device (12a) connected in one of these conductors, characterised by three transient protection devices (16a, 16d, 16e) which are connected respectively between a common circuit point (18) and said conductors (L1, N,G) of the system, each of said transient protection devices having a forward diode characteristic and the connections being such that the respective forward directions of those protection devices are similarly directed in relation to said common circuit point. 2. Protective circuitry as claimed in claim 1, including two choke input filters (14a, 14d) connected respectively in said live and neutral conductors (L,N). 3. Protective circuitry as claimed in claim 1 or 2, wherein each of said transient protection devices comprises a semiconductor transient voltage suppressor (20). 4. Protective circuitry as claimed in claim 3, wherein a capacitor (22) is connected in parallel across each of the transient voltage suppressors (20). 5. Protective circuitry as claimed in claim 4, wherein the capacitors (22) are polarized capacitors. 6. Protective circuitry as claimed in claim 3, wherein a polarized capacitor is connected in parallel across each of the transient voltage suppressors so that a positive terminal of the capacitor is connected to a cathode terminal of the transient voltage suppressor. 7. Protective circuitry as claimed in any one of claims 3 to 6, wherein each of the transient voltage suppressors comprises a varistor in parallel with a diode. 8. Protective circuitry as claimed in any one of claims 3 to 6, including three surge suppressors shunted respectively across the transient voltage suppressors. 9. Protective circuitry as claimed in claim 8, wherein each of said surge suppressors is a bi-directional clamp. 10. Protective circuitry as claimed in claim 8, wherein each of said surge suppressors is a gas discharge tube. 11. Protective circuitry as claimed in claim 8, wherein each of said surge suppressors is a varistor. 12. Protective circuitry as claimed in claim 8, wherein each of said surge suppressors is a bi-directional transient voltage suppressor. 13. Protective circuitry as claimed in any one of claims 3 to 6, wherein there is shunted across each said transient voltage suppressor a diode having a forward surge rating higher than that of the transient voltage suppressor and having a reverse breakdown voltage greater than a clamping voltage of the transient voltage suppressor. 14. A capacitive device, for use in alternating current circuitry, comprising two polarized capacitors (24a, 24b), connected in series between two terminals of the device, and clamping means (26a, 26b) connected with said polarized capacitors for preventing the positive peak of an alternating voltage applied across said two terminals when the device is in use, from becoming destructively effective across one of said polarized capacitors and for preventing the negative peak of such alternating voltage from becoming destructively effective across the other of said polarized capacitors. 15. A device as claimed in claim 14, including a pair of diodes (26a, 26b), one said capacitor (24a) and one diode (26b) forming a first diode damper, and the other said capacitor (24b) and the other diode (26a) forming a second diode damper, the diode of each clamper being shunted across the capacitor of the other damper. 16. A device as claimed in claim 14, including a pair of diodes (26a, 26b), one said capacitor (24a) and one diode (26b) forming a first peak rectifier, and the other said capacitor (24b) and the other diode (26a) forming a second peak rectifier, the diode of each peak rectifier being shunted across the capacitor of the other peak rectifier. 17. A device as claimed in claim 15, wherein said diodes are high voltage silicon p-n junction transient voltage suppressors. 18. A device as claimed in claim 15, wherein said reverse breakdown voltage of each of said diodes is greater than the voltage rating of each of said capacitors. 19. A device as claimed in claim 14, wherein said capacitors are of equal capacitance. 20. A device as claimed in claim 15 or 16, wherein each of said polarized capacitors includes a positive terminal and each of said diodes includes a cathode; said positive terminal of each capacitor being connected to said cathode of said respective diode. 21. A device as claimed in claim 15, wherein each of said polarized capacitors includes a negative terminal, said negative terminals of said capacitors being interconnected, and each of said diodes includes an anode, the said anodes being connected to said negative terminals of said capacitors. 22. A device as claimed in claim 16, wherein each of said polarized capacitors includes a positive terminal, said positive terminals of said capacitors being interconnected, and each of said diodes includes a cathode, the said cathodes being connected to said positive terminals of said capacitors.

Inventors: MCCARTNEY, THOMAS
Applicants: ONEAC CORPORATION
Application year: 1981
Main classification: H02H9
IPC codes: H02H9