WO2017001483A1

WO2017001483A1 - Retrofit uninterruptable power supply

Info

Publication number: WO2017001483A1
Application number: PCT/EP2016/065154
Authority: WO
Inventors: Sridhar PULIKANTI; Simon James Walton; Robert Turner; Dustin MURDOCK; Nick Elliot
Original assignee: Abb Schweiz Ag
Priority date: 2015-06-29
Filing date: 2016-06-29
Publication date: 2017-01-05

Abstract

The present invention provides a method for operation of an offline uninterrupted power supply (100), in particular a medium-voltage uninterrupted power supply, for providing power to a load (102) in case of failure of a power source (106), the offline uninterrupted power supply (100) comprising at least one disconnect switch (112), which is arranged between the power source (106) and the load (102), at least one energy storage (114), at least one power converter (116), which is arranged between the at least one energy storage (114) and the load (102) at a load side of the disconnect switch (112), and a control device (12) for controlling the at least one power converter (116) to provide power from the at least one energy storage (114) to the load (102) in case of failure of the power source (106), comprising the steps of performing a compensation of impedance between the at least one power converter (116) and the load (102), and performing a compensation of impedance between the at least one power converter (116) and the disconnect switch (112). The present invention also provides an offline uninterrupted power supply to perform the above method.

Description

RETROFIT UNINTERRUPTABLE POWER SUPPLY

Technical Field

[0001] The present invention relates to the area of offline uninterruptable power supplies. In particular, the present invention refers to a method for operation of an offline uninterrupted power supply, in particular a medium- voltage uninterrupted power supply, for providing power to a load in case of failure of a power source.

Background Art

[0002] Offline uninterruptable power supplies (offline-UPS) are commonly used for power protection in industrial environments where efficiency and footprint are primary cost drivers.

[0003] In offline-UPS systems, a load, which is provided at a downstream side, is directly connected to a power source, which is provided at an upstream side, as incoming utility supply by a power bus, whereby a disconnect switch, also referred to as utility disconnect, is provided in the power bus. The power source is typically a grid supply. The offline-UPS comprise an energy storage, which is connected via a power converter to the power bus downstream of the disconnect switch.

[0004] When the offline-UPS detects a voltage disturbance in the power source, the disconnect switch disconnects the load from the power source and provides power from the storage via the power converter to the power bus to keep the load operational. The process of detecting a disturbance in the power source, disconnecting the power source by switching off the disconnect switch and transferring the load support to the power converter is known as a transfer.

[0005] Existing state of the art offline-UPSs typically have the energy storage and the power converter located physically close to the disconnect switch for the several reasons. In high-power applications around several megawatts, the power connections between the energy storage, power converter and disconnect switch become very large. At low voltage, cables or bus-bars may carry currents of thousands of amperes. Running cables or bus-bars carrying currents of thousands of amperes requires lots of space, cooling, structural supports and can result in detrimental voltage drops. It may also be important to minimize cable impedances between the disconnect switch and the power converter, as larger impedances can have a detrimental effect on the transfer performance. Finally, coordinating the switching of the disconnect switch and power converter during a transfer requires tight communication coordination, which is difficult to achieve. These constraints have resulted in current offline-UPS systems specifying maximum distances of no more than a couple of meters between the disconnect switch and the power converter.

[0006] Furthermore, in high power multi-megawatt applications the disconnect switch, power converter, energy storage as well as optional transformers are physically large, typically each having a footprint of at least several square meters. In the case of retrofit applications, this may cause problems due to space constraints. Furthermore, the shape of available space is often not configurable. This problem is typically further increased for retrofit applications, since in many retrofit applications the new equipment to be installed is of higher power and, consequently, larger than the space that was previously designed for the UPS. Hence, flexibility of locating components of new equipment separately can be a deciding factor when choosing new equipment.

[0007] In this context WO 2004/054065 A1 describes a method, computer

program product, and apparatus and control system for providing substantially uninterrupted power to a load. The apparatus includes a control system coupled with an electrical power storage subsystem and an electric power generator. The control system is configured to provide a plurality of modes of operation including at least a static compensator (STATCOM) mode, an uninterruptible power supply (UPS) mode and a generator mode, and to control transitions between each of the plurality of modes. [0008] US 2014/0371929 A1 describes systems and methods for estimating a source impedance value. One embodiment includes an intelligent electronic device (I ED) configured to interface with an electric power distribution system. The IED includes a communications interface, a processor, and a non-transitory computer-readable storage medium. The computer-readable storage medium includes software instructions executable on the processor that enable the IED to identify a source impedance modeling event at a node in the power distribution system. The software instructions enable the IED to receive a plurality of

measurements representing an electrical condition at the node prior to the source impedance modeling event and subsequent to the source impedance modeling event. The IED calculates a source impedance value based on the first plurality of measurements at the node. Based on the source impedance value, a control action is generated.

Disclosure of Invention

[0009] It is an object of the present invention to provide an offline uninterruptable power supply suitable for retrofit applications as well as providing a method for operation of an offline uninterruptable power supply which facilitates the design of an offline uninterruptable power supply for use in retrofit applications.

[0010] This object is achieved by the independent claims. Advantageous

embodiments are given in the dependent claims.

[001 1] In particular, the present invention provides a method for operation of an offline uninterrupted power supply, in particular a medium-voltage uninterrupted power supply, for providing power to a load in case of failure of a power source, the offline uninterrupted power supply comprising at least one disconnect switch, which is arranged between the power source and the load, at least one energy storage, at least one power converter, which is arranged between the at least one energy storage and the load at a load side of the disconnect switch, and a control device for controlling the at least one power converter to provide power from the at least one energy storage to the load in case of failure of the power source, comprising the steps of performing a compensation of impedance between the at least one power converter and the load, and performing a compensation of impedance between the at least one power converter and the disconnect switch.

[0012] The present invention also provides an offline uninterrupted power supply, in particular a medium-voltage uninterrupted power supply, for providing power to a load in case of failure of a power source, the offline

uninterrupted power supply comprising at least one disconnect switch, which is arranged between the power source and the load, at least one energy storage, at least one power converter, which is arranged between the at least one energy storage and the load at a load side of the disconnect switch, and a control device for controlling the at least one power converter to provide power from the at least one energy storage to the load in case of failure of the power source, whereby the offline uninterrupted power supply is adapted to perform the above method.

[0013] The present invention further provides a computer program product

comprising computer executable instructions to perform the above method.

[0014] The present invention still further provides a software package for

upgrading an offline uninterruptable power supply, whereby the software package contains instructions for controlling the offline uninterruptable power supply to perform the above method.

[0015] The basic idea of the invention is to enable a method for operation of the offline uninterrupted power supply (offline-UPS) including performing a compensation of impedance between the at least one power converter, the disconnect switch, and the load. By performing a compensation of impedance, the disconnect switch, the energy storage, the power converter as well as other components of the offline-UPS can be located away from each other, so that they can be separated. Being able to separate the disconnect switch, the energy storage, and the power converter by more than a couple of meters already greatly facilitates retrofit applications of offline-UPS. The impedance compensation enables that the offline-UPS can tolerate increased impedances between the power converter and the connect switch. This enables in particular the use of transformers as well as long cables between the disconnect switch and the power converter, thus allowing the disconnect switch and the power converter to be separated.

[0016] Preferably, the step of performing impedance compensation with respect to the load comprises performing an impedance measurement between the power converter and the load.

[0017] Preferably, the step of performing impedance compensation comprises estimating the voltage drop across the offline uninterrupted power supply due to downstream load.

[0018] In systems comprising an offline-UPS, the load is provided at a

downstream side of the offline-UPS and directly connected to the power source, which is provided at an upstream side of the offline-UPS as incoming utility supply, typically a grid supply. The connection between the load and the power supply is achieved by a power bus, which may comprise cables or rails. Since the power bus typically refers to an existing system installation, it is typically not considered as part of the offline-UPS, although the offline-UPS also comprises cables and/or bus bars for providing electrical power connections.

[0019] The control device for controlling the at least one power converter can be provided integral with the at least one power converter or as a separate device, depending on the system design. The control device performs a control of the entire offline UPS. Independently, the different components of the offline UPS, in particular the at least one power converter, may comprise individual control units for performing an internal control of the respective components.

[0020] The failure of the power source comprises any event, that might endanger the operation of the load, in particular voltage drops and failures of the power source, as well as voltage disturbances in the power source, which might endanger operation of the load. It is therefore not required, that the power source is completely out of operation for the occurrence of a failure of the power source. [0021] When the offline-UPS, in particular the control device, detects a failure of the power source, the disconnect switch disconnects the load from the power source and provides power from the energy storage via the power converter to the power bus to keep the load operational. The process of detecting a failure of the power source, disconnecting the power source by switching off the disconnect switch and transferring the load support to the power converter is known as a transfer.

[0022] According to a modified embodiment of the invention the step of

performing a compensation of impedance between the at least one power converter and the disconnect switch comprises employing a current- sourcing control method during transfer to negate the effect of the impedances between the at least one power converter and the disconnect switch to assist in commutating the static switch current to zero during transfer of the power supply from the power source to the offline uninterrupted power supply. This enables the at least one power converter of the offline-UPS to assist in commutating a static switch current to zero during transfer, whereby the effect of cable or transformer impedances between the at least one power converter and the disconnect switch is negated. Voltage drops due to these impedances can be automatically compensated for by the power converter. Hence, the current source control enables to overcome in particular the influence of different cable lengths or bus bar lengths to reliably provide power from the at least one energy storage to the load. Also in general, the impedances from the power converter to the load or the power bus can be compensated so that the power converter can be located essentially at any place independently from impedances.

[0023] According to a modified embodiment of the invention the step of

performing a compensation of impedance between the at least one power converter and the load comprises providing the same voltage as the power supply prior to the transfer based on the load current subsequently to the transfer of the power supply from the power source to the offline uninterrupted power supply. Hence, the power converter can be put in operation prior to taking over the load current from the power source, whereby output voltage is already adjusted to the power source.

Accordingly, a smooth transfer can be achieved. Effects of increased impedances due to a placement of components of the offline-UPS at a great distance as well as effects of different impedances when using multiple components in parallel, in particular multiple power converters, can be overcome to enable smooth operation. Also impedances of optional transformers can be compensated based on the load current.

[0024] According to a modified embodiment of the invention the offline

uninterrupted power supply comprises a communication link, in particular a high speed , long distance communication link, which interconnects the at least one disconnect switch, and/or the at least one energy storage, and/or the at least one power converter, and/or the control device. The

communication link is preferably an industrially robust, low-latency and/or long-distance communication link. The communication link can be provided as a communication bus, where all components of the offline- UPS are connected to, or with individual connections from the control device to the components of the offline-UPS or a combination thereof. The communication link is used for commonly controlling all components of the offline-UPS, in particular the disconnect switch and the at least one power converter. The communication link is particularly important for

synchronization of the disconnect switch and power converter. Preferably, the communication link provides a physical link based on fiber-optic or a twisted-pair connection, which are industrially-robust and can be used at distances of up to one hundred meters or even more. To achieve low- latency of the communication link, either a custom communication protocol can be used, or an existing industry communication protocol may be employed.

[0025] According to a modified embodiment of the invention the offline

uninterrupted power supply comprises multiple disconnect switches, which are arranged in parallel between the power source and the load, whereby the multiple disconnect switches are individually controlled by the control device. Depending on the design of the entire system, in particular in the case of retrofit applications, where a offline-UPS is to be substituted or modified, multiple disconnect switches can be provided. They are all controlled by the control device to perform an efficient and reliable transfer in the case of failure of the power source. Accordingly, delays in switching the disconnect switches can be easily implemented as required.

[0026] According to a modified embodiment of the invention the offline

uninterrupted power supply comprises multiple energy storages, which are arranged in parallel, whereby the multiple energy storages are individually controlled by the control device. With the multiple energy storages, space requirements can be easily met for retrofit applications, as well as energy requirements for the offline-UPS in case of failure of the power source. Also different kinds of energy storages can be combined. By splitting the energy storage into small units, they can be located in different places. Additionally the individual cabling or bus-bar size to each energy storage can be reduced.

[0027] According to a modified embodiment of the invention the offline

uninterrupted power supply comprises multiple power converters, which are arranged in parallel, whereby the multiple power converters are individually controlled by the control device. Splitting the power converter into smaller units allows them to be located in different places, which is beneficial in particular in retrofit applications, where a offline-UPS of an existing system has to be replaced, typically by a more powerful and therefore a bigger offline-UPS. Hence, space constraints can be easily resolved by using available space as far as possible and providing remaining power converters in different places. Additionally, the individual connections to the power bus, e.g. via cables or bus-bars, can be simplified, since each connection can be realized with small components, which are easy to handle and easily available. The same refers to the connection of the power converters to the at least on energy storage. Common control of the multiple power converters by the control device enables reliable operation of the offline UPS in case of voltage

disturbances, whereby the power converters commonly provide power from the at least one energy storage to the load. [0028] According to a modified embodiment of the invention the offline

uninterrupted power supply is connectable to multiple individual load units, which are arranged in parallel forming together the load. Also this connection facilitates retrofit application of the offline-UPS to the needs of an existing system. The previously mentioned advantages in respect to cabling and/or bus bar connections also apply.

[0029] According to a modified embodiment of the invention the offline

uninterrupted power supply comprises at least one coupling transformer, which is arranged between the at least one power converter and the load at a load side of the disconnect switch. The coupling transformer can perform a voltage adaptation of the provided power. The coupling transformer can be provided on the power bus, either upstream or downstream of the at least one disconnect switch, upstream of the at least one load, or in the connection of the at least one power converter to the power bus.

Brief Description of Drawings

[0030] These and other aspects of the invention will be apparent from and

elucidated with reference to the embodiments described hereinafter.

[0031] In the drawings:

[0032] Fig. 1 shows a schematic drawing of an offline uninterruptable power supply together with a load and a power source according to a first embodiment, and

[0033] Fig. 2 shows a schematic drawing of an offline uninterruptable power supply having multiple energy storages and multiple power converters together with a load and a power source according to a second embodiment.

Detailed Description of the Invention

[0034] Fig. 1 shows an electrical system comprising an offline uninterruptable power supply 100 (offline-UPS) in accordance with a first, preferred embodiment of the present invention. [0035] The electrical system further comprises a load 102, which is provided at a downstream side 104 of the offline-UPS 100 and a power source 106, which is provided at an upstream side 108 of the offline-UPS 100. The power source 106 in this embodiment is a grid supply. The load 102 and the power supply 106 are connected by a power bus 1 10, which comprise cables or bus bars.

[0036] The offline-UPS 100 according to the first embodiment is a medium- voltage uninterrupted power supply. The offline-UPS 1 10 comprises a disconnect switch 1 12, which is arranged between the power source 106 and the load 102, an energy storage 1 14, a power converter 1 16, which is arranged between the energy storage 1 14 and the load 102 at a load side of the disconnect switch 1 12, which corresponds to the downstream side 104, a coupling transformer 1 18, which is arranged between the power converter 1 16 and the load 102, and a control device 120. The control device 120 performs a control of the entire offline UPS 100, and the coupling transformer 1 18 performs a voltage adaptation of the provided power from the power converter 1 16.

[0037] The offline-UPS 100 further comprises a communication link 122, which interconnects the disconnect switch 1 12, the energy storage 1 14, the power converter 1 16, and the control device 120. The communication link 122 is an industrially robust, low-latency and/or long-distance

communication link. The communication link 122 is provide as a communication bus, to which all components of the offline-UPS 100 are connected to. In an alternative embodiment, all components of the offline- UPS 100 are individually connected to the control device 120. The communication link 122 comprises a physical link based on fiber-optic. Alternatively, the physical link is based on a twisted-pair connection. The communication link 122 enables a high speed, long distance

communication link 122 with low-latency. In this embodiment, a custom communication protocol is implemented. In an alternative embodiment, an existing industry communication protocol is employed.

[0038] Subsequently, the operation of the offline-UPS 100 of fig. 1 will be

described. [0039] The basic operation of the offline-UPS 100 is providing power from the energy storage 1 14 to the load 102 in case of failure of the power source 106. The failure of the power source 106 comprises any event, that might endanger the operation of the load 102, in particular voltage drops, complete failures of the power source 106, or voltage disturbances in the power source 106.

[0040] The control device 120 continuously monitors the power source 106 for failures. In case the control device 120 detects a failure of the power source 106, the control device 120 performs a transfer of power supply from the power source 106 to the power converter 1 16. During transfer, the control device 120 disconnects the power source 106 by switching off the disconnect switch 1 12 and transferring the load support to the power converter 1 16.

[0041] The operation of the offline-UPS comprises performing a compensation of impedance between the power converter 1 16 and the load 102 as well as performing a compensation of impedance between the power converter 1 16 and the disconnect switch 1 12. In particular, this comprises two components.

[0042] First, the same voltage as provided from the power supply 106 prior to the transfer based on the load current is provided subsequently to the transfer from the offline-UPS 100. Hence, the power converter 1 16 is operated by the control device 120 prior to taking over the load current from the power source 106 to provide the output voltage as provided prior to the failure from the power source 106.

[0043] Second, a current-sourcing control method is employed by the control device 120 during transfer to negate the effect of the impedances between the power converter 1 16 and the disconnect switch 1 12 to assist in commutating the static switch current to zero during transfer of the power supply from the power source 102 to the offline uninterrupted power supply 100. Accordingly, the power converter 1 16 assists in commutating a static switch current to zero during transfer, whereby the effect of cable or transformer impedances is negated. Voltage drops due to the these impedances are automatically compensated for by the power converter

1 16.

[0044] Summarizing, the effects of impedances between the power converter 1 16 and the load 102 are compensated to provide operating conditions at the load 102 identical to operating conditions prior to the failure of the power supply 106. The compensation includes compensation of impedances in the electrical connections as well as impedances of e.g. coupling transformers.

[0045] Fig. 2 refer to an offline-UPS 100 according to a second embodiment. The offline-UPS 100 of the second embodiment is in many components and the general operation identical to that of the first embodiment. Hence, details not explicitly described with reference to the second embodiment are implemented as discussed in respect to the first embodiment.

[0046] The offline-UPS 100 of the second embodiment comprises multiple energy storages 1 14 and multiple power converters 1 16. The energy storages 1 14 and the power converters 1 16 are arranged in parallel and individually controlled by the control device 120.

[0047] During operation, the power converters 1 16 are individually controlled by the control device 120 to provide commonly the power to the load 102.

[0048] Apart from this, setup and operation of the offline-UPS of the second

embodiment is as described above with respect to the first embodiment.

[0049] In a further modified embodiment, the offline-UPS 100 comprises multiple disconnect switches 1 12, which are arranged in parallel between the power source 106 and the load 102, and which are individually controlled by the control device 120 during transfer.

[0050] In a further modified embodiment, the offline-UPS 100 is connectable to multiple individual load units, which are arranged in parallel forming together the load 102.

[0051] While the invention has been illustrated and described in detail in the

drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.

Reference signs list

100 offline uninterruptable power supply, offline-UPS

102 load

104 downstream side

106 power source

108 upstream side

1 10 power bus

1 12 disconnect switch

1 14 energy storage

1 16 power converter

1 18 coupling transformer

120 control device

122 communication link

Claims

1 . Method for operation of an offline uninterrupted power supply (100), in

particular a medium-voltage uninterrupted power supply, for providing power to a load (102) in case of failure of a power source (106), the offline uninterrupted power supply (100) comprising

at least one disconnect switch (1 12), which is arranged between the power source (106) and the load (102),

at least one energy storage (1 14),

at least one power converter (1 6), which is arranged between the at least one energy storage (1 14) and the load (102) at a load side of the disconnect switch (1 12),

and a control device (12) for controlling the at least one power converter (1 16) to provide power from the at least one energy storage (1 14) to the load (102) in case of failure of the power source (106),

comprising the steps of

performing a compensation of impedance between the at least one power converter (1 16) and the load (102), and

performing a compensation of impedance between the at least one power converter (1 16) and the disconnect switch (1 12).

2. Method according to claim 1 ,

characterized in that

the step of performing a compensation of impedance between the at least one power converter (1 16) and the disconnect switch (1 12) comprises employing a current-sourcing control method during transfer to negate the effect of the impedances between the at least one power converter (1 16) and the disconnect switch (1 12) to assist in commutating the static switch current to zero during transfer of the supply from the power source (106) to the offline uninterrupted power supply (100).

3. Method according to preceding claims 1 or 2,

characterized in that the step of performing a compensation of impedance between the at least one power converter (1 16) and the load (102) comprises providing the same voltage as the power supply (106) prior to the transfer based on the load current subsequently to the transfer of the supply from the power source (106) to the offline uninterrupted power supply (100).

4. Offline uninterrupted power supply (100), in particular a medium-voltage

uninterrupted power supply, for providing power to a load (102) in case of failure of a power source (106), the offline uninterrupted power supply (100) comprising

at least one energy storage (1 14),

at least one power converter (1 16), which is arranged between the at least one energy storage (1 14) and the load (102) at a load side of the disconnect switch (1 12),

and a control device (120) for controlling the at least one power converter (1 16) to provide power from the at least one energy storage (1 14) to the load (102) in case of failure of the power source (106),

whereby the offline uninterrupted power supply (100) is adapted to perform the method according to any of the above method claims.

5. Offline uninterrupted power supply (100) according to claim 4,

characterized in that

the offline uninterrupted power supply (100) comprises a communication link (122), in particular a high speed , long distance communication link, which interconnects the at least one disconnect switch (1 12), and/or the at least one energy storage (1 14), and/or the at least one power converter (1 16), and/or the control device (120).

6. Offline uninterrupted power supply (100) according to preceding claims 4 or 5, characterized in that the offline uninterrupted power supply (100) comprises multiple disconnect switches (1 12), which are arranged in parallel between the power source (106) and the load (102), whereby

the multiple disconnect switches (1 12) are individually controlled by the control device (120).

7. Offline uninterrupted power supply (100) according to any of preceding claims 4 to 6,

characterized in that

the offline uninterrupted power supply (100) comprises multiple energy storages (1 14), which are arranged in parallel, whereby

the multiple energy storages (1 14) are individually controlled by the control device (120).

8. Offline uninterrupted power supply (100) according to any of preceding claims 4 to 7,

characterized in that

the offline uninterrupted power supply (100) comprises multiple power converters (1 16), which are arranged in parallel, whereby

the multiple power converters (1 16) are individually controlled by the control device (120).

9. Offline uninterrupted power supply (100) according to any of preceding claims 4 to 8,

characterized in that

the offline uninterrupted power supply (100)is connectable to multiple individual load units, which are arranged in parallel forming together the load

(102).

10. Offline uninterrupted power supply (100) according to any of preceding claims 4 to 8,

characterized in that the offline uninterrupted power supply (100) comprises at least one coupling transformer (1 18), which is arranged between the at least one power converter (1 16) and the load (102) at a load side of the disconnect switch (1 12).

1 1. Computer program product comprising computer executable instructions to perform the method steps according to any of above method claims 1 to 3.

12. Software package for upgrading an offline uninterruptable power supply (100), whereby the software package contains instructions for controlling the offline uninterruptable power supply (100) to perform the method according to any of above method claims 1 to 3.