Electrical Power Transmission

Abstract

A connection-and-protection device having two capacitively connected conductors respectively connected in use to a supply cable at one end and a load cable at the other end is provided, the connection and protection device including a connection terminal for connection to either of the supply or load cables, a pair of cable terminals for the respective capacitively connected conductors, a direct connection within the device between the connection terminal and one of the pair of cable terminals, the other of the pair cable terminals not normally being connected to the connection terminal and means for connecting the other of the pair of cable terminals to the connection terminal to protect the capacitive connection of the two conductors, if the voltage between the pair of cable terminals exceeds a threshold and event detection means comprising means for detection the voltage between the pair of cable terminals exceeding a threshold.

Claims

1. A power-transmission cable protection-and-control arrangement for a capacitive power-transmission cable, the arrangement comprising: at a one end termination of the cable a load or supply terminal for a load or supply cable a pair of cable terminals for a pair of conductors of the capacitive power-transmission cable, the conductors being connected directly or via extension conductors to the terminals in use, one of the pair of cable terminals being connected by a rail or busbar to the load or supply terminal an electromagnetic or electronic switch between the rail or busbar and the other of the pair of cable terminals and at the one end or at another end termination of the cable: means for measuring the differential voltage between the pair of cable terminals, i.e. between the capacitive cable conductors in use, and means for controlling the switch to close in the event that the differential voltage exceeds a threshold below dielectric breakdown of the capacitive cable.

2. The power-transmission cable protection-and-control arrangement of claim 1, wherein the switch controlling means is adapted to control the switch in a second or third event or separate switch controlling means is provided therefor, the second event is that the current in the busbar or rail is below a certain threshold and the third event is that the current in the busbar or rail is above a certain threshold.

3. The power-transmission cable protection-and-control arrangement of claim 1, wherein a second switch controlling means is adapted to control a second switch between the rail or busbar and the other of the pair of cable terminals in a second or third event, the second event is that the current in the busbar or rail is below a certain threshold and the third event is that the current in the busbar or rail is above a certain threshold.

4. The power-transmission cable protection-and-control arrangement of claim 1, including a capacitor providing or augmenting the capacitance between the conductors of the capacitive cable.

5. The power-transmission cable protection-and-control arrangement of claim 1, arranged as a connection-and-protection device housed in an earthed or earthable conductive casing or cabinet, with the terminals insulated to a transmission voltage and the switch being rated for the differential breakdown voltage.

6. The connection-and-protection device of claim 5, wherein the measuring and controlling means are housed with the terminals and the switch in casing or cabinet.

7. The connection-and-protection device of claim 6, in combination with a second such device for an opposite end of the capacitive cable, the devices being adapted for wireless, wired or optic-fibre communication.

8. The connection-and-protection device of claim 5, including autonomous devices between the between the rail or busbar and the other of the pair of cable terminals adapted to conduct and lower the differential voltage faster than the switch can close.

9. The connection-and-protection device of claim 5, including both an electromagnetic switch and an electronic switch, the electronic switch being adapted to be closed faster than the electromagnetic switch.

10. The connection-and-protection device of claim 5, wherein the connection means is both rated for not more than 20% of a voltage to be transmitted by the capacitive cable and adapted to be operated by lower voltage control, event detection means comprising means for detecting the voltage between the pair of cables exceeding a threshold of more than 20% of the voltage to be transmitted voltage and the device comprises a low voltage power supply in the casing or cabinet and a low voltage controller in the housing for causing the connection means to effect the said connection in the event of detection of the said voltage exceeding the threshold.

11. The connection-and-protection device of claim 5, in combination externally of the device with one or more capacitor(s) connected between the pair of cable terminals.

12. The connection-and-protection device and cable combination of claim 11, wherein the cable comprises a pair of conductors in capacitive relationship along their length.

13. The connection-and-protection device of claim 5, in combination with a power-transmission cable having two capacitively connected conductors respectively connected in use to a supply cable at one end and a load cable at the other end.

14. The connection-and-protection device and cable combination of claim 13, wherein the cable comprises a pair of conventional elevated voltage power transmission cables, capacitively connected at either or both ends by one or more conventional capacitor, either housed as a circuit element of the connection-and-protection device or separately housed.

15. The connection-and-protection device and cable combination of claim 13, wherein the cable comprises a pair of conductors in capacitive relationship along their length and an additional capacitor is included as a circuit element in the device.

16. A connection-and-protection device for a power-transmission cable having two capacitively connected conductors respectively connected in use to a supply cable at one end and a load cable at the other end, the connection-and-protection device comprising: an earthable conductive housing; three connection terminals, each being rated to provide for termination at elevated voltage insulation from the housing and an internal connection point within the housing: one of the connection terminals being for insulated termination and internal connection of either of the supply or load cables, the others of cable terminals being ones of a pair for insulated termination and internal connection of the respective capacitively connected conductors, or interconnecting pieces of cable from the terminals to the respective conductors, a direct connection within the housing between the one of the connection terminals and one of the pair of cable terminals, the other of the pair of cable terminals not normally being connected to the connection terminal and means in the housing for connecting the other of the pair of cable terminals to the connection terminal to protect the capacitive connection of the two conductors, in the event that the voltage between the pair of cable terminals exceeds a threshold.

17. The connection-and-protection device of claim 16, including event detecting means comprising means for detection the voltage between the pair of cables exceeding a threshold.

18. A connection-and-protection device in combination with a power-transmission cable having two capacitively connected conductors respectively connected in use to a supply cable at one end and a load cable at the other end, the connection-and-protection device comprising: an earthable conductive housing; three connection terminals, each being rated to provide for termination at elevated voltage insulation from the housing and an internal connection point within the housing: one of the connection terminals being for insulated termination and internal connection of either of the supply or load cables, the others of cable terminals being ones of a pair for insulated termination and internal connection of the respective capacitively connected conductors, or interconnecting pieces of cable from the terminals to the respective conductors, a direct connection within the housing between the one of the connection terminal and one of the pair of cable terminals, the other of the pair of cable terminals not normally being connected to the connection terminal and means in the housing for connecting the other of the pair of cable terminals to the connection terminal to protect the capacitive connection of the two conductors, in the event that the voltage between the pair of cable terminals exceeds a threshold: the connection means being rated for not more than 20% of the elevated voltage and the connection means being adapted to be operated by low voltage control, event detection means comprising means for detecting the voltage between the pair of cables exceeding a threshold of more than 20% of the elevated voltage a low voltage power supply in the housing and a low voltage controller in the housing for causing the connection means to effect the said connection in the event of detection of the said voltage exceeding the threshold.

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28. A power-transmission cable protection-and-control arrangement for a capacitive power-transmission cable, the arrangement comprising: at a one end termination of the cable a load or supply terminal for a load or supply cable a pair of cable terminals for a pair of conductors of the capacitive power-transmission cable, the conductors being connected directly or via extension conductors to the terminals in use, one of the pair of cable terminals being connected by a rail or busbar to the load or supply terminal an electromagnetic or electronic switch between the rail or busbar and the other of the pair of cable terminals and at the one end or at another end termination of the cable: means for measuring the differential voltage between the pair of cable terminals, i.e. between the capacitive cable conductors in use, and means for controlling the switch to close in the event that the differential voltage exceeds a threshold below dielectric breakdown of the capacitive cable, wherein the switch controlling means is adapted to control the switch in a second or third event or separate switch controlling means is provided therefor, the second event is that the current in the busbar or rail is below a certain threshold and the third event is that the current in the busbar or rail is above a certain threshold.

29. The power-transmission cable protection-and-control arrangement of claim 28, arranged as a connection-and-protection device housed in an earthed or earthable conductive casing or cabinet, with the terminals insulated to a transmission voltage and the switch being rated for the differential breakdown voltage.

Description

[0135] To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

[0136] FIG. 1 is a diagram of a basic connection-and-protection device of the invention;

[0137] FIG. 2 is a three phase supply including connection-and-protection devices of the invention;

[0138] FIG. 3 is a block diagram of grouping of the components of a protection-and-control device of the invention; and

[0139] FIG. 4 is a circuit diagram of an exemplary connection-and-protection device of the invention.

[0140] Referring to FIG. 1 of the drawings, a first embodiment is described in which there are two active controlled components, one component 1 is normally closed and controlled to open when a current threshold is reached and the other 2 is controlled to close in the event of excessive voltage. The device has a conductive housing 3, with three terminals 4,5,6 insulatedly provided in the wall 7 of the housing. Within the housing a busbar 8 extends between the terminals 4,5. A further busbar extends from the terminal 6 to an insulated receptacle 9 on the other side of the housing.

[0141] An inductive current sensing coil 11 is provided around the busbar 8 in series with a coil 12 of a normally closed (NC) relay 14 having contacts connected to the respective busbars. On start-up when no current is flowing through the device, via the busbar 8, the terminals 5,6 are connected together by the NC relay. When the current reaches a current threshold, sufficient current flows in the coils 11,12 to open the relay and allow capacitive operation of a capacitive cable having its conductors connected to the terminals 5,6.

[0142] A voltage sensing line 21, with a high series resistor 22 (sized to cause sufficient current to flow only for the following purpose), is connected in series with the coil 23 of a normally open (NO) relay 24. Its contacts are connected respectively to the busbars. Normally, on start-up and normal operation, the voltage between the busbars and the conductors is within the capabilities of dielectric between the conductors and the relay 24 remains open. Should the voltage between the busbars 6,8/the terminals 5,6/the cable conductors (not shown in FIG. 1) exceed a threshold at which the dielectric is in potential danger, the sufficient current flows in the line 21, resistor 22 & coil 23 for the NO relay to close, connecting the conductors together as at start-up, but by the relay 24 as opposed to the relay 14.

[0143] As described, this embodiment is expected to require special tuning of the relays for the currents in their coils to operate at the desired current and voltage thresholds. For this reason, the relays are preferably controlled by a control circuit and indeed a single relay is preferably employed as now described in a second embodiment.

[0144] It should however be noted that the simplest embodiment of the invention requires only one NO relay, or other electronic switch, arranged to close on the inter-conductor voltage exceeding the threshold.

[0145] Referring to FIGS. 2 to 4 of the drawings, a second embodiment is described. An elevated, transmission voltage, typically 33 kV, distribution network 101 is provided between a supply 102 and a load 103. These and other features of the network are shown diagrammatically in FIG. 1. It is a three phase network, with the phases indicated as R,G,B, as such some of the features are triplicated. However, insofar as the features of this invention are provided in each phase, i.e. the three of them as respective features, they have common reference numerals.

[0146] The main cables of the network are capacitive cables 104, each having a supply-connected plate 105 and a load-connected plate 106 with dielectric 107 between them. The plates and dielectric are shown diagrammatically. One of the plates is connected at one end and the other at the other end. The non-connected ends are normally simply disconnected.

[0147] Connection-and-protection devices 108 are provided in each phase between supplies cable 109 and load cables 110, with the inter-position of short lengths 111 of conventional cable from the devices 108 and the capacitive cables 104. Connection between the cables 104 and 111 are by connectors 112, shown diagrammatically. They can be as described in our WO 2019/234449, in particular FIGS. 6 to 10 thereof.

[0148] The connection-and-protection devices 108 are all identical and a representative one only is described with reference to FIG. 4. These are elevated voltage devices utilising lower voltage components, some of which are operated by much lower, electronic voltage control circuitry. By this is intended that the devices have insulation to earth appropriate for an electricity distribution network. For example, this elevated voltage is typically 33 kV. As explained above, the instantaneous voltage between the capacitor plates is an order of magnitude lower than the elevated voltage. This has the consequence that the capacitive plates of the cables, in practice made up of multiple strands and which are the capacitive conductors, can be separated by dielectric rated much lower. In other words, the breakdown voltage of dielectric can be of the order of one third of the cable's rated voltage. In turn, the connection-and-protection devices can be rated to react to unusual voltage of 15% to 20% of the rated voltage to protect the dielectric. This is the lower voltage. The associated circuitry for operating at least some on the lower voltage components can operated at electronic circuit voltage, typically 12 volts. This relationship of voltages in shown in FIG. 3.

[0149] As shown in FIGS. 2 & 4, in broad outline the connection-and-protection devices have a straight-through connection rail 121, from a supply/load cable 109,1010 to one of the plates 105,106 of a capacitive cable for, and an isolated rail 122 for the other plate 106,105. The cables 104 being capacitive, at the supply end, the plate 105 is directly connected straight through by the rail 121 with the plate 106 connected to the isolated rail 122; whilst at the load end of the capacitive cable, the plate 106 is connected straight through to the load by the connection rail 121 whilst the plate 105 is connected to the isolated rail 122.

[0150] That said, the exemplary connection-and-protection device will now be described as a standalone device. It has a steel outer case 123, which is provided with an earth connection appropriate for its distribution cable connection. The structural arrangement of the case or cabinet is determinable by the skilled person in accordance with the electrical features now described.

[0151] The two rails 121,122 are arranged across the case 123. The straight-through/direct-connection rail 121 has two cable terminals in the form of connectors 125,126, one at either end. They can be socket parts of Pfisterer MV-Connex cable connectors, as described at https://www.pfisterer.com/fileadmin/pfisterer/downloads_en/CableSystemMV-CT-EN.pdf. The connected cables are provided with plug parts, connecting the core of the cables to the rail and the cable sheath to the outer case 121. The isolated rail is insulatingly mounted at one end and provided with a cable terminal/connector 127 at the other end. Various circuit elements are connected to the rails and between them.

[0152] As shown in FIG. 3 the circuitry now described is arranged in three groups: [0153] High voltage components in group I which are connected directly between the busbar rails 121,122 for current flow from one to the other, in the event that the voltage between the rails exceeds the voltage threshold; [0154] Low voltage control circuitry components in group III which are receive power from the rails, monitor the current they are carrying and their relative voltage; [0155] A threshold voltage rated switch 128 in group II for conducting current between the rails as required under control from the group III components.

[0156] The components are all fully insulated via high voltage insulation from the conductive, earthed in use casing 123, to guard against failure of insulation from one group to the next. In other words, despite the switch 128 and the Group I components being connected by their terminals to the rails, it and they experiences only inter-rail voltage and can be rated for the inter-rail voltage. The internal insulation of the switch insulates the rail voltage from the low voltage on its control wiring, which is connected to the low voltage control circuitry.

[0157] The components can be mounted all on a single board, with the board being insulated with line voltage rated insulation from the casing. However, the Group I components can be mechanically and electrically connected between the rails. The contacts of the switch 128 can be also.

[0158] The low voltage components are mounted on a printed circuit board, fully insulated to transmission line voltage from the casing.

[0159] An element between the rails is the normally open switch 128, with a driver circuit 129 for the switch. As shown in FIG. 3, the switch is adapted to receive wireless control signals. The purpose of the switch is to short circuit the two rails together so that no potential can exist between them in the event of the inter-rail voltage exceeding a threshold. It should be particularly noted that although the cables, the connectors and the rails are rated for 33 kV, the switch operates at 4 kV. The possibility for this apparent dichotomy is that under normal operating conditions, the capacitive connection of the two plates 105,106 of the capacitive network cable 104 causes the voltage differential between the load plate and the supply plate to be dependent on the cable current acting on the capacitance between the plate elements of the capacitor. Therefore, the instantaneous voltage between the plates and the rails is smaller, in fact substantially an order of magnitude smaller, than the nominal peak voltage of 33 kV, or other voltage concerned. The dielectric in cables such as our WO 2019/234449 is rated for the instantaneous voltage between the plates. This is why circuit components in the connection-and-protection device can be rated at the lower voltage.

[0160] In another mode switch 128 is normally closed, with the above-described operation adapted mutatis mutandis. When normally closed this state allows the cable to “fail safe” to conventional mode by shorting red and green in the event of system losing external DC power to the control circuits. Thus upon instruction to enter capacitive mode the relay switch is held open electromechanically and the trigger removes the power to the relay thus allowing it to revert to the closed position.

[0161] Referring to FIG. 4, the voltage between the rails is measured by a measuring circuit 130. If the voltage exceeds a threshold, typically 4 kV, the circuit passes a signal to a driver 129 for the switch 128. The driver closes the switch and the two rails 121, 122 are connected together by the switch, removing the voltage difference between them. Because the switch is an electromagnetic relay switch, it has a takes a discernible time to close in the case of events such as lightning strike, in other words because it is too slow, a thyristor switch 228 is provided in parallel with it and operated by the same driver 129. Other even faster components are provided as described below.

[0162] Also included in the device is a current transformer 132 for detecting excess current in the rail 121 and passing a signal to the driver 129 for closure of the switch and sharing of the excess current by both capacitive conductors. The driver is also adapted to maintain the switch closed after start-up and at other times, when the current in the rail 121 is below anther, lower threshold.

[0163] In other words, the current transformer performs a further purpose in detecting when the current is below a low threshold, belong which it advantageous for the capacitive conductors to operate as a conventional conductor. The switch is arranged as a normally closed relay switch, whereby it is closed when the capacitive cable is first switched into operation—by conventional network switches not shown.

[0164] Other circuit components in the device are: [0165] a low voltage power supply 133, including a rectifier and a battery, powering other components, including the switch driver 131, [0166] a voltage divider 134 between the rails for supplying AC at electronic circuitry voltage to the power supply, [0167] a metal oxide surge arrester 135 for limiting transitory voltage peaks between the rails, such as caused by impulses. This is via the semiconductor nature of the surge arrester allowing current to flow between the rails when the transitory voltage rises to the break down voltage of the arrester. Preferably the arrester is specified to break down at substantially half the voltage at which the switch is controlled to operate, i.e. 2 kV in this embodiment. The ensures that whilst the transitory voltage is limited by the surge arrester to 2 kV, the switch will not be operated. If the power of the transitory voltage, i.e. the power of the impulse is too high, this may not be possible, particularly if the surge arrester is burned out, a spark gap 136 for passing steep sided impulses between the rails. In the same way that the surge arrester can be expected to act faster than the switch and its driver, the spark gap is provided to conduct before a steep sided spike causes the surge arrester to start conducting. Conveniently the conduction voltage of the spark gap is the same as that of the surge arrester. A further device in parallel with the surge arrester and the spark gap is a varistor 236 to provide further assurance that the dielectric of the capacitive cable is protected before the switch, in its electromagnetic and thyristor forms, are able to react.

[0168] It should be noted that surge arrestor, spark gap, varistor and indeed the capacitor 139 below fall into group I as regards insulation in that they are fully exposed to line voltage vis-à-vis the earthed conductive casing.

[0169] The power supply and the monitoring circuitry fall in group III, operating at low voltage and only being exposed the line voltage at in a manner to protect this circuitry from experiencing the full line voltage or indeed the inter-conductor voltage, in the manner of the group I components.

[0170] The switch is referred to as being a group II component, because the voltage between its contacts to the rails and its control contacts should never exceed an order of magnitude or so below the line voltage to earth.

[0171] The switch when closed due to voltage threshold or current threshold being exceeded, remains closed until the disturbance causing the excess voltage or current has dissipated. Only when the current as measured by the current measurement device 132 falls below the high current threshold, does the switch driver open the switch again and normal operation resumes. The monitoring and switching is managed by a controller 137 driven by the power supply and passing “close” or “open” signals to the switch driver as appropriate.

[0172] Each switch operation can be expected to give rise to capacitive discharge currents in the switch. To protect this, a current damper 38 is in series with the switch 128 as part of the connection means.

[0173] Also included within the device is an optional capacitor 139. One or more of these may be mounted in the case 123 or externally. Its function is to increase the capacitance between the capacitively connected conductors, particularly when these are conventional cables capacitively connected only by such capacitors 139.

[0174] The invention is not intended to be restricted to the details of the above-described embodiments. For instance, the connection-and-distribution devices can be mounted in an earthed cabinet within a substation. In this case the outer earthed case 123 can be dispensed with. Again, where the connection-and-distribution devices are installed in such a substation cabinet, they can be powered from the substation's own power supply. The switch driver 129 can be provided with a further input from a wired or wireless input port, whereby the switch 128 can be closed as network conditions indicate to be desirable. Indeed, measurements of network current and/or inter-conductor voltage can be provided from without the earthed case &/or indeed the earther cabinet and control signal &/or power for closure of the relay can be provided into the case/cabinet for closure of the inter-conductor switch in accordance with the measurements and/or external control. This is particularly the case where measurements at one end of a capacitive cable cause closure at the one end and it is desirable to effect closure at the other end as well.