Combined switching system for an electrically powered railway vehicle

Abstract

A switching system for an electrically powered railway vehicle, in which an electrical storage system provides power to a traction system, includes a first enclosure 206 provided externally of the veh

Description

02 05 25 COMBINED SWITCHING SYSTEM FOR AN ELECTRICALLY POWERED RAILWAY VEHICLE Field of the Invention The present invention relates to a combined switching system for an electrically powered railway vehicle. Background Conventional drive systems for railway vehicles include hybrid systems in which primary power can be selectably drawn from an overhead line or a generator powered by a diesel engine. In such drive systems it is known to include on-board energy storage systems, such as lithium ion batteries, or lead batteries, connection to the energy storage system being between the primary power source and the traction transmission system driving the wheels. Surplus energy from the primary power source, or energy derived from regenerative braking, can be stored in the energy storage system. During acceleration, this stored energy is directed to the transmission system, boosting that available from the primary power source. Figure 1 shows schematically a conventional railway vehicle 100 with a hybrid drive system comprising a main transformer 101 which steps down primary AC power supplied via a pantograph 106 from an overhead line, and supplies the stepped down AC power to a line converter 102. A DC link then joins the line converter 102 to a line inverter 103 which supplies three phase AC power to drive traction motor or motors 104. A battery-based energy storage system 105 connected to the DC link stores surplus power, and releases that power as needed. Less common are railway vehicles carrying traction batteries, where the batteries are the prime mover for the vehicle. Such trains are used for “gap-filling” operation, in which the trains are used on both electrified and non-electrified track sections, allowing their batteries to be recharged by overhead pickup when the trains are travelling on the electrified sections. However, with improvements in energy storage capability enabling longer distances to be travelled on non-electrified track sections, there are growing possibilities for using on-board energy storage systems as the primary power source for traction. Indeed, improvements in energy storage capability also allow larger batteries to be used in hybrid systems where the batteries are a primary power source on an equal footing with e.g. an overhead line, a generator powered by a diesel engine, or fuel cells. Safety is a key factor for acceptance of on-board energy storage. The traction systems of railway vehicles such as those discussed above require a high-power input to drive the vehicle. As such, the electrical storage system of the vehicle is typically a high voltage source. In normal operation, a control system of the railway vehicle (for example, a local controller of the traction system or a higher level Train Control and Management System - TCMS) can disconnect and reconnect 02 05 25 the electrical storage system from the rest of the vehicle via suitable load breakers. It is also important to manage the high voltage source safely in case of an accident. The present invention has been devised in light of the above considerations. 5 Summary of the Invention In a first aspect, the present disclosure provides a combined switching system for an electrically powered railway vehicle according to claim 1. The combined switching system of the first aspect advantageously provides externally accessible emergency actuators, manual operation of which enables emergency workers or railway crew to reduce 10 the danger that the high voltage electrical storage system poses to passengers in the event of a fault or railway accident by electrically isolating the electrical storage system. The provision of an emergency actuator on both sides of the railway vehicle advantageously enables emergency workers or railway vehicle crew to disconnect the electrical storage system from the traction system from whichever side of the railway vehicle is most readily accessible to them. This may reduce the 15 time taken for an emergency actuator to be actuated, and therefore improves safety. The first enclosure may contain electrical insulation between its internal electrical components and a metal exterior wall of the enclosure. In particular, such electrical insulation can be rated to withstand electrical arcing expected from the circuit breaker when interrupting a high current on the first power line under emergency conditions. The electrical insulation may be in the form of electrical insulation board 20 which lines the metal exterior wall. Typically, the electrical storage system provides power to the traction system under the control of a computer-based control system of the vehicle. This computer-based control system may be a local traction control system or a more general TCMS. Furthermore, each of the first and second circuit breakers may be configured to be reuseably openable under fault current conditions by the computer-25 based control system of the vehicle to isolate the electrical storage system from the traction system. Advantageously, this allows for certain railway vehicle failures, such as those detectable by an internal current sensor, to be responded to quickly and automatically by the control system. Each of the first and second circuit breakers may be a high-speed circuit breaker. Advantageously, highspeed circuit breakers provide near instantaneous activation. For example, such circuit breakers can 30 activate in 15 ms or less under peak fault current conditions. Due to their high-power rating, high-speed circuit breakers are openable under load, and are typically rated for operating during short circuit conditions of up to 30kA (and even higher peak currents of up to e.g. 42kA). The first enclosure may further contain a first load breaker arranged in series with the first circuit breaker on the first power line, wherein the first load breaker is configured to be openable and closable under 35 normal operating conditions of the traction system by the computer-based control system of the vehicle to 02 05 25 respectively isolate the electrical storage system from and reconnect the electrical storage system to the traction system. For example, the first load breaker may be operated by the computer-based control system to switch between: a self-powered mode in which the electrical storage system provides all the traction power; an externally powered mode in which an external supply (e.g. an overhead line or third 5 rail) provides all the traction power; and a hybrid mode in which the provision of traction power is shared between the external supply and the electrical storage system. It may also be operated to enable regenerative braking during any of these modes, or to charge the battery from an external power supply. The first load breaker is typically rated to be operable only under no load or normal load conditions, for example up to currents of 750A. 10 The first load breaker may comprise a first switch, and, in parallel with the first switch, a series arrangement of a second switch and a resistor, the first load breaker being configured such that, to isolate the electrical storage system from the traction system, both the first switch and the second switch are opened by the control system, with the first switch being opened before the second switch. This configuration can help to reduce current through the first load breaker in a staged manner when opening 15 the first load breaker, and thus reduce the possibility of arcing. Conveniently, the first enclosure may be configured to provide the first externally-accessible emergency actuator such that, when the first enclosure is installed on a side of the railway vehicle and the railway vehicle is halted, the first externally-accessible emergency actuator is manually actuatable by persons at track level or platform level. 20 Typically, the combined switching system has a driver’s emergency actuator which is configured to be installed in a driver’s cab and is manually actuatable in emergencies to open the first circuit breaker to thereby isolate the electrical storage system from the traction system. This actuator allows a driver of the railway vehicle to isolate the electrical storage system in an emergency as well as persons at track level or platform level. 25 The driver’s emergency actuator may be manually actuatable in emergencies to open both the first and the second circuit breakers to thereby isolate the electrical storage system from the traction system. The routine maintenance actuator may be lockable in at least the open position. Thus the routine maintenance actuator and disconnectors provide a convenient means for railway maintenance workers to guarantee disconnection of the electrical storage system from the traction system when they are 30 performing inspections or maintenance. The first and second disconnectors are typically rated only to be operable under no-load conditions. The first enclosure may typically further include a status indicator panel which visually indicates the open / close status of at least the first circuit breaker and the first disconnector. The status indicator panel may also indicate the open / close status of the first load breaker when the switching system has the first 35 load breaker. The panel advantageously allows maintenance workers to have immediate confirmation of the status of these components. 02 05 25 Conveniently, the status indicator panel may visually indicate the open / close status of the second circuit breaker and the second disconnector. The status indicator panel may also visually indicate the open / close status of the second load breaker when the switching system has the second load breaker. The second enclosure may further contain a second load breaker arranged in series with the second 5 circuit breaker on the second power line, wherein the second load breaker is configured to be openable and closable under normal operating conditions of the traction system by the computer-based control system to respectively isolate the electrical storage system from and reconnect the electrical storage system to the traction system. The second load breaker is typically rated to be operable only under no load or normal load conditions, for example upto currents of 750A. 10 Like the first load breaker, the second load breaker may comprise a first switch, and, in parallel with the first switch, a series arrangement of a second switch and a resistor, the load breaker being configured such that, to isolate the electrical storage system from the traction system, both the first switch and the second switch are opened by the control system, with the first switch being opened before the second switch. 15 The second enclosure may be configured to provide the second externally-accessible emergency actuator such that, when the second enclosure is installed on a side of the railway vehicle and the railway vehicle is halted, the second externally-accessible emergency actuator is manually actuatable by persons at track level or platform level. The second disconnector is typically rated only to be operable under no-load conditions. 20 Ina second aspect, the present disclosure provides an electrically powered railway vehicle according to claim 8. As mentioned above, typically the vehicle further has a computer-based control system and the electrical storage system provides power to the traction system under the control of this computer-based control system. 25 Conveniently, the first and second enclosures may be installed to the underside of the vehicle. This position is consistent with access to the enclosures by persons at track level or platform level. The electrical storage system may also be installed to the underside of the vehicle, with a bogie of the vehicle separating the first and second enclosures from the electrical storage system. In the event of an electrical fire in the electrical storage system, this separation of the electrical storage system from the 30 enclosures, with the bogie interposing between, can help to enable continued access to the enclosures so that the ability to electrically isolate the electrical storage system is not compromised. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. 02 05 25 Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows schematically a conventional railway vehicle; 5 Figure 2 shows a circuit diagram of an electric drive system of a railway vehicle; Figure 3 shows a circuit diagram of a part of the electric drive system; Figure 4 shows a safety mechanism circuit diagram; Figure 5 shows schematically a perspective view of a first enclosure; Figure 6 shows schematically a perspective view of a second enclosure; and 10 Figure 7 shows schematically a perspective view of an underframe at one end of the railway vehicle of Figure 2. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the 15 accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Figure 2 shows a circuit diagram of an electric drive system of an electric railway vehicle. An overhead line 207, supplies a main transformer 201 with AC power via a pantograph 205. The main transformer steps down the AC power and supplies the stepped down power to a line converter 202 which converts 20 the power into DC. A DC link having positive and negative rails and an optional ground rail joins the line converter 202 to a line inverter 203 and also provides branches to respectively a positive line 212, a negative line 216 and an optional ground line 214. The line inverter converts the DC power to three phase AC power to drive traction motors 204. The lines 212, 214, 216 extend from the DC link to connect to an energy storage system 210 via a plurality of disconnectors 218, 220, 222, circuit breakers 224, 226 and 25 load breakers 228, 230 (discussed in more detail below) contained in a first enclosure 206 and a second enclosure 208. The energy storage system typically comprises a battery formed from a plurality of lithium ion cells. The positive line connects to a positive pole of the energy storage system, and the negative line connects to a negative pole of the energy storage system. Under the instruction of a computer-based control system (not shown) of the vehicle, the energy storage system can store electrical power obtained 30 either from regenerative braking via the traction motors 204 and the line inverter 203 or from the overhead line 207 via the line converter 202, and can also supply power to drive the traction motors. The control system may be a local traction control system or a more general TCMS. The flow of electrical energy into and out of the energy storage system will depend on the instantaneous operating conditions, and also the particular driving mode selected via the control system. Possible driving modes include a 35 self-powered mode in which the electrical storage system provides all the power to the traction motors; an 02 05 25 externally powered mode in which the overhead line provides all the power to the traction motors; and a hybrid mode in which the provision of power to the traction motors is shared between the overhead line and the electrical storage system. The first enclosure 206 and second enclosure 208 lie between the energy storage system 210 and the 5 DC link of the line converter 202 and the line inverter 203. The positive line 212 from the DC link passes through the first enclosure, and the negative line 216 from the DC link passes through both the first and second enclosures. The ground line 214 is optional, but may be provided depending on the voltage of the energy storage system. In Figure 2, the ground line 214 is shown following the path of the positive line through the first enclosure, but alternatively it could follow the path of the negative line through both 10 enclosures. The first enclosure contains three disconnectors: a first disconnector 218 on the positive line, a second disconnector 222 on the negative line, and a third disconnector 220 on the ground line. The disconnectors are configured to be opened or closed only under no-load conditions to respectively electrically isolate or reconnect the energy storage system from the other parts of the electric drive system. 15 The positive and negative lines also each have a circuit breaker, the positive line circuit breaker 224 being contained within the first enclosure and the negative line circuit breaker 226 being contained within the second enclosure. These circuit breakers may be reuseably opened or closed under fault conditions to respectively electrically disconnect or reconnect the energy storage system from the other systems of the railway vehicle. The circuit breakers can be high-speed circuit breakers which are able to open under 20 high load, such as that experienced in a power surge. Such circuit breakers can open near instantaneously, allowing the electric storage system to be disconnected promptly and safely in the event of a fault or accident. Moreover, the positive and negative lines each have a load breaker, the positive line load breaker 228 being contained within the first enclosure and the negative line load breaker 230 being contained within 25 the second enclosure. Each load breaker comprises a first switch, and, in parallel with the first switch, a series arrangement of a second switch and a resistor. Each load breaker is configured so that the first switch is opened before the second switch such as to reduce current through the load breaker in a staged manner, and thus reduce the possibility of arcing. When the load breaker is closed, the switches are closed in the opposite order to increase the current in a staged manner, i.e. the second switch is closed 30 before the first switch. The load breakers may be opened or closed by the control system to respectively electrically isolate the energy storage system from or reconnect it to other parts of the drive system during normal use depending on instantaneous operating conditions, and the driving mode selected via the control system. For example, the load breakers may be open when the externally powered mode has been selected. Conversely, the load breakers must be closed when there is a demand for traction power 35 and the self-powered mode has been selected. The positioning and ordering of the circuit breaker 224, 226 and the load breakers 228, 230 on the lines 212,214 (with the circuit breaker 224, 226 electrically closer to the energy storage system 210 than the load breakers 228, 230) can ensure that the energy storage system is disconnected from the vehicle and 02 05 25 its electric drive system as close to the energy storage system as possible. Any fault current inrush from the energy storage system which would activate the circuit breakers can therefore be isolated from other vehicle system components by the circuit breakers’ opening. This is particularly important should the fault be caused by a system component short to vehicle earth, which may lead to a thermal event if power 5 from the energy storage system is not removed. Figure 3 shows a circuit diagram of the part of the drive system within the first 206 and second 208 enclosures. Also shown are: a routine maintenance actuator 236 for the first 218, second 222 and third 220 disconnectors; a first emergency actuator 232 for the positive line 224 and negative line 226 circuit breakers; and a second emergency actuator 234 for the positive line 224 and negative line 226 circuit 10 breakers. The first emergency actuator 232 is provided externally on the first enclosure, and the second emergency actuator 234 is provided externally on the second enclosure. Actuation of either of the emergency actuators opens both the positive line and negative line circuit breakers. Although not shown in Figure 3, a driver’s emergency actuator may also be provided and located in a driver’s cab of the vehicle to perform the same function of opening the positive line and negative line circuit breakers. The 15 routine maintenance actuator 236 is provided within the first enclosure. This actuator may be used by maintenance workers to open or close the disconnectors under no-load conditions and thereby guarantee disconnection of the electrical storage system during inspections or maintenance. Figure 4 shows a circuit diagram providing detail of a safety mechanism by which any of the first 232 and second 234 emergency actuators and the driver’s emergency actuator 238 can operate the positive line 20 224 and negative line 226 circuit breakers. More particularly, each of the emergency actuators 232, 234, 236 are operable to open a first trigger line 237 and a second trigger line 239. A first coil 247 on the first trigger line located adjacent the positive line circuit breaker 224 is de-energised by the opening of the first trigger line and trips the positive line circuit breaker open. Similarly, a second coil 249 on the second trigger line located adjacent the negative line circuit breaker 226 is de-energised by the opening of the 25 second trigger line and trips the negative line circuit breaker open. A first indicator lamp 248 and a second indicator lamp 250 on respectively the first trigger line and the second trigger line are extinguished when the coils de-energise, thereby indicating the changed status of the circuit breakers. Conveniently, these lamps can be located on a status indicator panel discussed below in relation to Figure 5. The emergency actuators may be spring-loaded so that after such operation they automatically re-close the triggers lines. 30 However, as the coils remain de-energised, the circuit breakers remain open. To return the first 247 and second 249 coils to an energised state in which they hold the positive line 224 and negative line 226 circuit breakers closed, the control system first recognises a demand for traction power and issues a temporary close command to operate mechanical actuators (not shown) which close the positive line and negative line circuit breakers. A respective auxiliary contact (also not shown) on 35 each circuit breaker then creates a holding circuit by which electricity for its respective coil is supplied. The control system then stops issuing the temporary close command to the mechanical actuators and the circuit breakers are again held in their closed position by their coils, whereby the electrical storage system 210 is reconnected to other parts of the drive system. The first 248 and second 250 indicator lamps are 02 05 25 also relit when the coils are re-energised. This mechanism advantageously ensures that the coils cannot be energised and the circuit breakers closed without an explicit command for traction power from the control system. Figure 5 shows schematically a perspective view of a first enclosure 206. The first enclosure has a metal 5 external wall for protection against ballast strikes. It is lined with electrical insulation board, and is attachable to an underframe of the railway vehicle via mounting brackets 254. The first emergency actuator 232 is located externally on the first enclosure in a convenient location where railway staff and emergency workers at track side or on a platform can operate the first emergency actuator to isolate the energy storage system 210 from the other systems of the vehicle in the event of a fault or accident. A 10 lockable access door (not shown) covers an instrument area 256 in which are located the routine maintenance actuator 236 and a status indicator panel 257. The access door may be unlocked with a standard “T” rail key, so as to suitably restrict access to the maintenance switch. Furthermore, the maintenance actuator is lockable in each of the open and closed positions to prevent accidental movement to the other position. The status indicator panel 257 comprises lamps indicating the open or 15 closed status of the circuit breakers 224, 226 and load breakers 228, 230. Removable access panels (not shown) cover side maintenance windows 270, 271 allowing access from the outside of the vehicle to the electrical components within the enclosure. These components are preferably distributed along the length direction of the vehicle so that each component is readily accessible through one of the windows without obstruction from another component 20 Figure 6 shows schematically a perspective view of a second enclosure 208. The second enclosure also has a metal external wall lined with electrical insulation board and is similarly attachable to the underframe of the vehicle via mounting brackets 260. The second emergency actuator 234 is located externally on the second enclosure in a convenient location for access by railway staff and emergency workers. Again, removable access panels (not shown) cover side maintenance windows 272, 273 25 allowing access from the outside of the vehicle to the electrical components within the enclosure. These components are preferably distributed along the length direction of the vehicle so that each component is readily accessible through one of the windows without obstruction from another component. Figure 7 shows schematically a perspective view of the underframe 256 at an end of the railway vehicle. The mounting brackets 254, 260 are located in T slots of the underframe so that the first enclosure 206 30 and the second enclosure 208 are on opposite lateral sides of the vehicle with their respective emergency actuators 232, 234 facing outwardly and conveniently accessible from track side or at platform level. A bogie 258 of the vehicle separates the first and second enclosures from the electrical storage system (not shown in Figure 7), which is also located beneath the underframe but at a more central position of the vehicle. As such, continued access to the emergency actuators 232, 234 is facilitated even in the event of 35 an electrical fire in the electrical storage system. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, 02 05 25 or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this 5 disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of 10 these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be 15 understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range 20 is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A switching system for an electrically powered railway vehicle in which an electrical storage system (210) provides power to a traction system (204), the switching system having a first enclosure (206) which includes a first externally-accessible emergency actuator (232) and contains a first circuit breaker (224) on a first power line (212) which is part of a circuit connecting a positive or negative pole of the electrical storage system to the traction system, wherein the first externally-accessible emergency actuator is manually actuatable in emergencies to open the first circuit breaker and thereby isolate the electrical storage system from the traction system.

2. The switching system according to claim 1, wherein the first enclosure (206) is configured to provide the first externally-accessible emergency actuator (232) such that, when the first enclosure is installed on a side of the railway vehicle and the railway vehicle is halted, the first externally-accessible emergency actuator is manually actuatable by persons at track level or platform level.

3. The switching system according to claim 1 or 2, which further has a driver’s emergency actuator (236) which is configured to be installed in a driver’s cab and is manually actuatable in emergencies to open the first circuit breaker (224) to thereby isolate the electrical storage system (210) from the traction system (204).

4. The switching system according to claim any one of the previous claims, wherein the first enclosure (206) further contains a routine maintenance actuator (236) and a first disconnector (218) which is arranged in series with the first circuit breaker (224) on the first power line (212), wherein the routine maintenance actuator is manually actuatable to open or close the first disconnector under no-load conditions to respectively isolate the electrical storage system (210) from and reconnect the electrical storage system to the traction system (204).

5. The switching system according to claim 4, wherein the first enclosure (206) further includes a status indicator panel (257) which visually indicates the open / close status of at least the first circuit breaker (224) and the first disconnector (218).

6. A combined switching system including a first switching system which is the switching system according to any one of claims 1 to 5, and a second switching system for the electrically powered railway vehicle, the second switching system having a second enclosure (208) which includes a second externally-accessible emergency actuator (234) and contains a second circuit breaker (226) on a second power line (216) which is part of the circuit connecting the other of the positive and negative pole of the electrical storage system (210) to the traction system (204), wherein the second externally-accessible emergency actuator is manually actuatable in emergencies to open the second circuit breaker and thereby isolate the electrical storage system from the traction system, and wherein the first (206) and the second (208) enclosures are configured for installation on opposite lateral sides of the railway vehicle.

7. The combined switching system according to claim 6, wherein the second enclosure (208) is configured to provide the second externally-accessible emergency actuator (234) such that, when the second enclosure is installed on a side of the railway vehicle and the railway vehicle is halted, the second externally-accessible emergency actuator is manually actuatable by persons at track level or platform level.

8. The combined switching system according to claim 6 or 7, wherein each of the first (232) and second (234) externally-accessible emergency actuators is manually actuatable in emergencies to open both the first circuit breaker (224) and the second circuit breaker (226).

9. The combined switching system according to any one of claims 6 to 8, as dependent on claim 3, wherein the driver’s emergency actuator is manually actuatable in emergencies to open both the first (224) and the second (226) circuit breakers to thereby isolate the electrical storage system (210) from the traction system (204).

10. The combined switching system according to any one of claim 6 to 9 as dependent on claim 4, wherein the first enclosure (206) further contains a second disconnector (222) which is arranged in series with the second circuit breaker (226) on the second power line (216), wherein the routine maintenance actuator (236) is manually actuatable to open or close the first (212) and the second (216) disconnectors under no-load conditions to respectively isolate the electrical storage system (210) from and reconnect the electrical storage system to the traction system (204).

11. The combined switching system according to claim 10, as dependent on claim 5, wherein the status indicator panel (257) also visually indicates the open / close status of the second circuit breaker (226) and the second disconnector (222).

12. An electrically powered railway vehicle having an electrical storage system (210), and a traction system (204) which is provided with power by the electrical storage system, wherein the vehicle further has:the switching system according to any of claims 1 to 5, orthe combined switching system according to any of claims 6 to 11, the first (206) and second (208) enclosures being installed on opposite lateral sides of the vehicle.

13. The vehicle according to claim 12 having the combined switching system according to any of claims 6 to 11, the first (206) and second (208) enclosures being installed on opposite lateral sides of the vehicle,wherein the first and second enclosures are installed to the underside of the vehicle.

14. The vehicle according to claim 13, wherein the electrical storage system (210) is also installed to the underside of the vehicle, and a bogie of the vehicle separates the first (206) and second (208) enclosures from the electrical storage system.

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