UNINTERRUPTIBLE POWER SUPPLY SYSTEM WITH OPTIMIZED

AUTONOMY

Technical Field

[0001] The present invention relates to the area of uninterruptible power supply systems comprising a main power line interconnecting a power supply side and a load side, a utility disconnect switch provided in the main power line, and multiple power delivery units, which are connected to the main power line at the load side of the utility disconnect switch, thereby providing an off-line UPS configuration. The present invention in particular relates to a control method for such an uninterruptible power supply system.

Background Art

[0002] Offline uninterruptable power supply (UPS) systems, also referred to

as single conversion UPS, are commonly used for power protection in case of power quality events. Such power quality events comprise voltage sags (short-term under-voltages), voltage swells (short-term over-voltages), voltage short-term interruptions, voltage long-term interruptions, voltage drops and failures of AC power supply. In

particular, typical power quality events comprise shallow sags of at least one phase of the AC power supply, failures of at least one

phase of the AC power supply, i.e. a one-phase failure or a multiple phase failure, an AC supply outage or deep sag, e.g. a failure of all phases of the AC power supply. Offline-UPS systems are commonly used for power protection in industrial environments where efficiency and footprint are primary cost drivers.

[0003] In a typical offline-UPS system 10, which can be seen in Fig. 1 , the

offline UPS system 10 comprises a main power line 12, which

directly connects a power supply 14, also denoted as grid in Fig. 1 , at a power supply side 16 and a load 18 at a load side 20 of a given installation. The power supply 14 and the load 18 are typically an AC power supply and an AC load, respectively, in particular when considering industrial applications, more typically a three-phase AC power supply and/or a three-phase AC load. The installation can be provided with three or four wires depending on different

requirements, including national installation practices or standards.

[0004] The main power line 12 comprises a utility disconnect switch 22, which is provided in the main power line 12 and disconnects the power supply side 16 as required in case of a power quality event. The utility disconnect switch 22 is typically a semiconductor switching device.

[0005] The offline UPS system 10 comprises one power delivery unit 24

(PDU), which is connected to the main power line 12 at the load side 20 of the utility disconnect switch 22. The PDU 24 of the shown embodiment comprises an optional transformer 26, multiple power electronic building blocks 28 (PEBBs) and an energy storage device 30. The energy storage device 30 typically provides power to the load through multiple PEBBs 28. In an alternative embodiment, the offline UPS system 10 comprises multiple energy storage devices 30 connected in parallel.

[0006] The PDU 24 further has a control unit 32 for controlling the operation of the PDU 24, which is labeled master controller in Fig. 1. The utility disconnect switch 22 is operated by the control unit 32 of the PDU. In an alternative embodiment, the offline UPS system 10 comprises multiple PDUs, which are all connected to the main power line 12. In this alternative embodiment, the UPS system 10 further comprises a control device for controlling the operation of the PDUs 24 and the utility disconnect switch 22.

[0007] Each of the PEBBs 28 typically comprises an DC/AC converter, also referred to as inverter, for converting electrical power from the energy storage device 30, i.e. typically DC power, into AC power, as required for powering the load 18. The PEBBs 28 are electrically connected to the transformer 26, which transforms the AC power provided by the PEBBs 28 to provide a required load voltage. Each DC/AC converter unit typically comprises an individual controller for controlling its operation within the offline-UPS system 10.

[0008] When the offline-UPS system 10 detects a power quality event, the utility disconnect switch 22 disconnects the load 18 from the grid, i.e. the AC power supply 14, and uses the PEBBs 28 with the DC/AC converter units and the energy storage device 30 to keep the load 18 running.

[0009] The process of detecting a power quality event, turning off the utility

disconnect switch 22 and transferring the support of the load to the PDUs 24 is known as a transfer. In order to increase power capacity of offline UPS system 10, there are two possibilities. The first possibility is to increase power capacity of a PDU 24, i.e. power capacity of all of its components, and the second possibility is to provide additional PDUs, 24 which are connected in parallel.

[0010] However, when increasing power capacity of a PDU, there are several technical challenges. First, common control units of the PDUs with existing controller processor technology are typically limited to a given number of PEBBs. Hence, there is a limitation in increasing the number of PEBBs within the PDUs. Second, when increasing energy storage capacity of a PDU, also an increase in short-circuit current occurs. Hence, high rating direct current (DC) circuit-breakers are required, which are expensive compared to alternating current (AC) circuit breakers. Third, adding additional hardware components increases footprint size of the offline UPS system, which makes it complicated to implement retrofit applications.

[001 1] Some of these disadvantages can be overcome by increasing the number of PDUs connected in parallel to the main power line. Further advantages comprise provided modularity and scalability. Accordingly, the offline-UPS system can be re-scaled based on load requirements and PDUs connect in parallel. Furthermore, offline UPS systems with parallel PDUs have a redundant setup, which increases system reliability, as any PDU can be isolated in the event of a fault without affecting remaining PDUs. Also flexibility of moving equipment in retrofit applications is provided. Still further, a separate, low DC short-circuit capacity is required within the PDUs compared to monolithic offline UPS system. Hence, each PDU can be provided with a low DC short-circuit capacity. Furthermore,

maintenance is facilitated since individual PDUs can be separated from the main power line while remaining PDUs are operative and keep the offline UPS system operational. Hence, maintenance and service can be performed without interrupting the offline UPS system.

[0012] However, the offline UPS system with parallel PDUs also has some

inherent disadvantages. First, a more complex control architecture is required with a system controller for controlling the operation of the individual PDUs. Second, the operation of parallel PDUs with various energy delivery elements having different time constants for delivering energy may constrain the overall autonomy of the offline UPS system. Third, the operation of parallel PDUs with different energy storage levels of energy storage device in PDUs are common due to differently aged energy storage devices, due to different state of charge of the energy storage devices, and/or due to different internal configurations of the PDUs. E.g. the energy storage devices of the different PDUs can have different numbers of parallel energy storage strings, which reduces overall autonomy of the offline UPS system. Fourth, if the number of available PEBBs is less in a PDU than configured, and all PDUs share load equally, overloading of PEBBs, in particular of the DC/AC converter, in that PDU may occur.

[0013] In this context, document WO 2013/058763 A1 refers to a method of

operating a UPS system having a first UPS and a second UPS. The method comprises powering on the first UPS, receiving power from a first input power source coupled to an input of the first UPS, providing power to a load coupled to an output of the first UPS, adjusting the power provided to the load by the first UPS in response to power characteristics of the first UPS and power characteristics of the second UPS. Furthermore, document US 2009/0230772 A1 refers to a load sharing, multi-module power supply system for supplying power to a load. The system involves: a first power supply module having a controller, and having a first per unit capacity (pu-c); a second power supply module having a controller, and having a second per unit capacity (pu-c); the controller of the first power supply module adapted to implement a reduction in an output power of the first power supply module upon the detection of an operating event, where a portion of the load being handled by the first power supply module is shed by a percentage, and such that the first power supply module remains operating during the operating event but at a reduced power output level; and upon the occurrence of the operating event the controller of the second power supply module is adapted to increase a power output of the second power supply module sufficient to accommodate the portion ofthe load that has been shed by the first power supply module.

[0014] US 2007/0007825 A1 refers to an uninterruptible power supply (UPS) including a plurality of UPS modules. Each of the UPS modules has a battery that provides power to a protected load in the event of a utility power failure. A plurality of controllers control how much power each ofthe batteries deliver to the protected load, and a communication bus allows the controllers to exchange information about the battery voltages. One of the controllers calculates the average battery voltage of the plurality of batteries and adjusts the amount of energy provided by an individual battery such that the battery voltage is about equal to the average battery voltage.

[0015] Document WO 2014/209377 A1 is directed toward a UPS system

including an input configured to receive input power from an input power source, an output configured to provide output power to a load, and a plurality of units coupled to the input and the output, each of the plurality of units configured to provide an output contributing to the output power, each of the plurality of units comprising at least one temperature sensor. The UPS also includes a main controller coupled to the plurality of units, the main controller configured to receive, from the temperature sensors, information relating to temperatures of each of the plurality of units, calculate at least one average temperature based on the temperatures of each of the plurality of units, and provide the at least one average temperature to each of the plurality of units. [0016] Still further, document EP 2 071 699 A2 provides an uninterruptible power supply (UPS) system including a plurality of UPSs connected in parallel at a load bus and configured to provide powerthereto from respective batteries of a plurality of batteries is operated such that a difference between a variable, for example, battery voltage, indicative of battery capacity for a battery associated with the subject UPS and an average value of the variable for the plurality of batteries is determined and a power flow between the subject UPS and the load bus is controlled responsive to the determined difference. Controlling a power flow between the subject UPS and the load bus responsive to the determined difference may include, for example, controlling a phase of an inverter of the subject UPS responsive to the determined difference.

[0017] It is known from WO 2013/102782 A1 that an uninterruptible power supply (UPS) system comprises a plurality of UPS units connected in parallel. Each UPS unit comprises a power converter for supplying a share of a total load current (Uot). The total load is shared automatically between UPS units of power ratings, in a proportionate manner. A controller of each converter is arranged to establish real-time feedback control of a current supplied by the power converter. An exchange current for each converter represents an imbalance between an output current of the converter in question and output currents of the parallel converters. Exchange current sensing circuits of the parallel-connected UPS units are connected together. The controller steers the exchange current of each converter toward a value (i_ exc_c) that is a non-zero proportion of a current (Lrnut) sensed within the converter. Said non-zero proportion is calculated such that the exchange current will be steered towards a positive value in the case of a converter with higher than average nominal power rating and toward a negative value in the case of a converter with lower than average power rating.

[0018] It is further known from WO 2004/054065 A1 to provide a method, a

computer program product, and an apparatus and a control system and method 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.

Disclosure of Invention

[0019] It is an object of the present invention to provide an uninterruptible power supply system comprising a main power line interconnecting a power supply side and a load side, a utility disconnect switch provided in the main power line, and multiple power delivery units, which are connected to the main power line at the load side of the utility disconnect switch, thereby providing an off-line UPS configuration, and a control method for such an uninterruptible power supply system, which overcome at least some of the above problems. In particular, it is an object of the present invention to provide such an uninterruptible power supply system a control method, which enable an improved reliability of power supply to a load, and which have an enhanced autonomy of power supply to a load.

[0020] This object is achieved by the independent claims. Advantageous

embodiments are given in the dependent claims.

[0021] In particular, the present invention provides a control method for an

uninterruptible power supply system, the uninterruptible power supply system comprising a main power line interconnecting a power supply side and a load side, a utility disconnect switch provided in the main power line, and multiple power delivery units, which are connected to the main power line at the load side of the utility disconnect switch, thereby providing an offline UPS configuration, whereby each power delivery unit comprises components including multiple power electronic building blocks and an energy storage device, each of the components having a time constant for delivering energy, whereby the time constant refers to an indication of possible support of a load in operation, the method comprising the steps of performing an optimization of autonomy of the uninterruptible power supply system, comprising the step of managing power between the power delivery units such that each power delivery unit achieves longest autonomy determined by the component with the fastest time constant, which defines the shortest operation time, for delivering energy.

[0022] The present invention also provides an uninterruptible power supply

system comprising a main power line interconnecting a power supply side and a load side, a utility disconnect switch provided in the main power line, and multiple power delivery units, which are connected to the main power line at the load side of the utility disconnect switch, thereby providing an off-line UPS configuration, whereby each power delivery unit comprises components including multiple power electronic building blocks and an energy storage device, each of the components having a time constant for delivering energy, and the uninterruptible power supply system comprises a control device adapted to performing the above method.

[0023] The basic idea of the invention is to improve control of the offline

uninterruptible power supply system (UPS) system, so that autonomy of the uninterruptible power supply system is optimized by managing power between the power delivery units (PDUs). Hence, individual time constants for delivering energy of the components of the PDUs are considered to perform an overall control of the offline UPS system. Accordingly, autonomy of the single power delivery unit can be increased based on the fastest time constant.

[0024] In this context, the term time constant refers to an indication of possible support of the load in operation. The time constant can depend on operational states of the PDUs, e.g. a load support of DC/AC converter units in overload condition, or others. The time constant for the energy storage device depends primary on a storage capacity of the energy storage device. However, other time constants can be considered, e.g. a maximum support time of a given current. The fastest or shortest time constant is the time constant out of the time constants of all components of the PDUs, which defines the shortest operation time.

[0025] The PDUs may comprise additional, optional components having a time constant for delivering energy. The optional components comprise in particular a transformer for transforming power provided from the multiple PEBBs.

[0026] The control device performs a coordination and control of the multiple

PDUs. Hence, the control device monitors voltage of the AC power supply to detect power quality events, synchronizes with the voltage of the AC power supply when the power quality event is cleared. The control device is responsible for operation of the utility disconnect switch in order to separate the load side from the AC power supply in case of a power quality event and to connect the load side to the AC power supply in a synchronized way when the power quality event is cleared. The control device is connected to control units of the PDUs via a control bus. The control device controls the control units of the PDUs and propagates necessary information to the control units of the PDUs. The control device also performs a collection of status information of all PDUs e.g. operational mode of PDUs, estimated state of charge of energy storage devices and others.

[0027] The above control method of the offline UPS system can be applied to

offline UPS systems and the PDUs thereof independently from the kind of energy storage device employed in the PDUs. Hence, the control method can be employed independently from the parallel energy storage devices being provided e.g. as batteries, super-capacitors or others and the inherent different capacities thereof, as long as each PDU is provided with the same kind of energy storage device.

[0028] In the same way, also PDUs with differently aged or degraded energy

storage devices can be connected in parallel. This also applies to control of PDUs energy storage devices with different state of charge or energy storage level. Hence, energy storage devices with normal variations in energy storage levels can be connected in parallel. Based on the above control method, autonomy of the offline UPS system can be increased and the offline UPS system can withstand different energy storage levels due to relatively aged energy storage devices, different state of charge of energy storage devices, and due to differences in internal structures of the energy storage devices, or others. [0029] With the described offline UPS system, a modular control approach for offline UPS with parallel power delivery units can be implemented.

Preferably, also the PDUs have a modular structure with the PEBBs being provided as modules. The offline UPS system can be easily scaled to provide essentially any load capacity. Merely further parallel PDUs have to be added to the offline UPS system to achieve an extension of load capacity. The modular setup of the offline UPS system enables flexibility for retrofit applications. Also reliability of the offline UPS system is satisfactory since the parallel PDUs can be replaced independently and during operation of the offline UPS system.

[0030] According to a modified embodiment of the invention the uninterruptible power supply system is provided as a short autonomy system, whereby the energy storage device has the shortest time constant out of the components of the power delivery units, and the step of managing power between the power delivery units comprises performing a fine utilization of energy available between power delivery units to support the load with optimum autonomy. Depending on the design of the components, in particular the multiple PEBBs and the energy storage device, different components of the PDUs out of the listed components can have the lowest time constant. Hence, in a short autonomy system, which typically provides an autonomy of e.g. up to several seconds, the energy storage device will likely be the component having the shortest time constant. Energy storage devices for these short autonomy system may comprise e.g. super capacitors or a flywheel. After the transfer, i.e. when the PDUs have started to power the load, the offline UPS system is subsequently running out of PDUs depending on the energy stored in the PDUs.

Consequently, at a certain time, the offline UPS system cannot support the load anymore, since available PDUs cannot provide sufficient power to the load. Hence, to achieve increased autonomy of the overall UPS system, the support of the load is shared between the different PDUs in order to avoid that individual PDUs ran out of energy and the offline UPS system can power the load for an extended time. [0031] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises energy balancing for optimizing or sub-optimizing overall energy storage devices available in the uninterruptible power supply system for extension of autonomy. E.g. if each PDU of the offline UPS system is provided with an energy storage device comprising super capacitors, and the energy storage devices have different energy levels, the autonomy of the PDU with the lowest energy level determines the autonomy of overall offline UPS system. By enabling energy balancing of energy storage, the energy can be utilized in optimum manner while power overloading the PEBBs. In this case, the time constant for delivering energy of PEBBs is higher than time constant of energy storage device, so that the PEBBs can withstand high power loading throughout the autonomy. Energy balancing between PDUs of the offline UPS system optimizes or sub-optimizes overall energy storage available in the system for maximum autonomy. Optimization and sub- optimization comprises that the operation of the offline UPS system can be optimized under consideration of different aspects, whereby also other aspects than maximum autonomy can be considered.

[0032] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises balancing the demands of the components, each of which having a time constant for delivering energy, depending on the component having the shortest time constant. When considering the component having the shortest time constant, a proper control of the parallel PDUs can increase the overall autonomy of the offline UPS system. Accordingly, support of the load between the PDUs can be adapted. This includes to operate the PEBBs, in particular the DC/AC converters above the designed power ratings. However, since the DC/AC converters have an overload capability, such an operation of the DC/AC converters can be performed without harming the DC/AC converters. Typical overload capabilities are up to several seconds.

[0033] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises fine utilization of power delivery units with components having different time constants for delivering energy. This refers to utilization of the PDUs of the offline UPS system under consideration of their detailed capability for supporting the load under current operation conditions. Hence, variations of the time constants can be considered for optimizing the overall autonomy.

Accordingly, also PDUs with components having different time constants amongst the PDUs, e.g. based on amount of energy stored in the energy storage device, temperature, aging, degradation, or others, can be commonly controlled to achieve a control for optimization of autonomy of the offline UPS system.

[0034] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises performing utilization of components in the power delivery units which optimizes or sub-optimizes overall energy storage available in the uninterruptible power supply system for maximization of overall autonomy. Maximization of the overall autonomy refers to an optimization which in particular increases the overall autonomy. Hence, other aspects of the overall control of the offline UPS system are considered of less importance. Accordingly, upon

requirements, also power ratings of e.g. the DC/AC converters can be overridden in order to achieve the maximization.

[0035] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises determining an individual estimated state of charge of the multiple power delivery units, determining a mean estimated state of charge of the multiple power delivery units, and adapting the output current of the multiple power delivery units based on the individual estimated state of charge of the multiple power delivery units and the mean estimated state of charge of the multiple power delivery units. Determining an individual estimated state of charge of the multiple power delivery units comprises determining the state of charge in particular of the energy storage devices, which are supposed to provide a major part of the energy stored in the PDUs. By adapting the output current of the multiple power delivery units, load support is provided by the PDUs to maintain an increased number of PDUs actively supporting the load. Accordingly, load support from all or almost all PDUs will continue essentially for the same time, so that autonomy of the offline UPS system can be increased.

[0036] According to a modified embodiment of the invention the uninterruptible power supply system is provided as a long autonomy system, whereby the energy storage device has the longest time constant out of the

components of the power delivery unit, and the step of managing power between the power delivery units comprises performing optimal power sharing between power delivery units to support the load with optimum autonomy. Based on the design of the offline UPS system, the lowest time constant component may change depending on the designed autonomy time. For systems with longer autonomy, the thermal rating of

components, in particular the PEBB, may have the shortest time constant, whereas the energy storage device typically has a long time constant. Such an energy storage device can be provided e.g. as a battery. Hence, in offline UPS systems, components with relative short time constant become the dominant constraint. Hence, the control method is performed based on time constants of components for delivery energy in PDUs.

[0037] Based on the above the control method enables a decrease of power loading of DC/AC converter units in PDUs comprising less DC/AC converter units. The reduced number of DC/AC converter units can be either by design or because one or more DC/AC converter units are not available, e.g. because of failure or maintenance. Due to the reduced power loading, passive components in PDU do not reach their maximum operation limits. The passive components in PDUs may comprise filter components of the DC/AC converter units, the coupling transformer, switching devices and others. Furthermore, the DC/AC converter units can be driven by the control unit of the PDU based on a control command from the control device of the offline UPS system to provide a bigger or smaller share to support of the load. Hence, the offline UPS system can withstand a loss of DC/AC converter units in single PDUs. Following the transfer, the load is running off PDUs. To achieve decreased power loading of PEBBs in PDUs with less PEBBs, the PDU output current can be varied based on number of DC/AC converter units available, and an average current provided by all PDUs.

[0038] By way of example, if the PDUs of the offline UPS are provided with

energy storage devices with batteries, whereby the batteries have different energy levels, by enabling energy balancing, some of the PEBBs are power overloaded. Since the time constant of the battery is bigger than the time constant of other components of the PDUs, in particular the DC/AC converter unit, overloading the other components of the PDUs, in particular the DC/AC converter unit, can exceed their maximum

operational limitation which are defined by time constants of the

components of PDUs, which could lead to tripping overall offline UPS system. Hence, power balancing can be performed to avoid failure of components of the PDUs based on the time constants for delivering energy, so that the individual components of the PDUs do not suffer from operation at maximum operation limitation and sustain the power overloading. Hence, the offline UPS system can be kept running for an extended time, thereby achieving an optimization of overall autonomy of the offline UPS system. Hence, power sharing can be performed depending on time constants for delivering energy of the components in the PDUs. This way, utilization of energy available between PDUs to support the load is optimized or sub-optimized so that autonomy of the offline UPS can be extended.

[0039] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises the step of fine power balancing between power delivery units under power delivery units with different energy storage levels or loss of power electronic building blocks in power delivery units. Hence, a detailed evaluation of energy storage levels of the energy storage devices and available PEBBs is performed by the control device for calculating optimum power to perform optimum power sharing. The optimized or sub-optimized autonomy of a PDUs from constrained energy delivery elements is achieved by balancing the demands of the fastest time constant within the PDU. [0040] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises reducing power loading in the power delivery units with a reduced number of power electronic building blocks which have reduced capability. Hence, power loading in the off-line UPS system can be reduced even in case of a reduced number of DC/AC converter units of a PDU or in case of the offline UPS system having a reduced number of PDUs. Reduction of number of PDUs and/or PEBBs can occur based on failure and on maintenance of the respective component.

[0041] According to a modified embodiment of the invention the step of power sharing between the multiple power delivery units comprises determining a number of power electronic building blocks available in each power delivery unit, and adapting an output current of the multiple power delivery units based on the number of power electronic building blocks available in each power delivery unit. With this deterministic approach, a detailed configuration of an offline UPS can be achieved as basis for active power sharing.

[0042] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises power balancing to reduce power loading with a reduced number of power delivery units or power electronic building blocks. The power balancing can be performed to avoid a failure of a PEBB or a PDU in order to sufficiently power the load.

[0043] According to a modified embodiment of the invention the step of power sharing between the multiple power delivery units comprises determining an individual current delivered from each of the multiple power delivery units to the load side, determining a mean current delivered from the multiple power delivery units to the load side, and the step of adapting an output current of the multiple power delivery units based on the number of power electronic building blocks available in each power delivery unit comprises adapting the output current of the multiple power delivery units based on the mean current delivered from the multiple power delivery units to the load side. In other words, time constants are provided individually for the PDUs of the offline UPS system and are individually assessed in order to enable power balancing between the different PDUs.

[0044] According to a modified embodiment of the invention the step of managing power between the power delivery units comprises adapting the output current of the multiple power delivery units depending on the time constants of the power delivery units.

[0045] According to a modified embodiment of the invention the uninterruptible power supply system comprises a long distance communication link interconnecting the multiple power delivery units and the control device. The long distance communication link is preferably a high speed

communication link. The long distance communication link is preferably designed as an industrially-robust, low-latency communication link. The long distance communication link is used for common control of the PDUs in order to perform optimization of autonomy of the uninterruptible power supply system and manage power between the power delivery units. The long distance communication link is preferably based on a fiber-optic or twisted pair copper physical link, which are industrially robust and can be used at distances of up to one hundred meters or more. Further preferred, the long distance communication link is provided with a low latency. To achieve the low-latency requirement, either a custom communication protocol can be developed, or an existing industry communication protocols may be employed depending on a use case.

[0046] According to a modified embodiment of the invention the utility disconnect switch is connected via the long distance communication link to the control device. Hence, the control device can also control the utility disconnect switch to perform a separation of the power supply side from the AC power supply in case of a power quality event, which is detected by the control device. Furthermore, also a control of the utility disconnect switch to perform a synchronization of the voltages of the AC power supply and the power provided by the PDUs upon end of the power quality event can be performed.

Brief Description of Drawings [0047] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0048] In the drawings:

[0049] Fig. 1 shows an offline UPS system known in the Art with a single power delivery unit as a schematic drawing, and

[0050] Fig. 2 shows an offline UPS system according to a first, preferred embodiment comprising multiple parallel power delivery units as a schematic drawing.

Detailed Description of the Invention

[0051] Fig. 2 shows an offline-UPS system 100 in accordance with a first,

preferred embodiment of the present invention. The offline UPS system 100 comprises a main power line 1 12, which directly connects a power supply 1 14 at a power supply side 1 16 and a load 1 18 at a load side 120 of a given electrical installation. The power supply 1 14 and the load 1 18 are typically an AC power supply and an AC load, respectively, in particular when considering industrial applications, more typically a three- phase AC power supply and/or a three-phase AC load. The installation can be provided with three or four wires depending on different requirements, including national installation practices or standards.

[0052] The main power line 1 12 comprises a utility disconnect switch 122,

which is provided in the main power line 1 12 and disconnects the power supply side 1 16 as required in case of a power quality event.

The utility disconnect switch 122 is typically a semiconductor

switching device.

[0053] The offline UPS system 100 comprises by way of example three

power delivery units 124 (PDUs), which are connected to the main power line 1 12 at the load side 120 of the utility disconnect switch

122. In an alternative embodiment, the offline UPS system 100

comprises a different number of PDUs 124. Each PDU 24 comprises a transformer 126, multiple power electronic building blocks 128

(PEBBs) and an energy storage device 130. In an alternative embodiment, the offline UPS system 100 is provided without

transformer 126.

[0054] The energy storage device 130 provides DC power to the multiple

PEBBs 128. In an alternative embodiment, the offline UPS system

100 comprises multiple energy storage devices 130 connected in parallel. The PDUs 124 have a modular structure with the PEBBs

128 being provided as modules.

[0055] The PDU 124 further has a control unit 132 for controlling the

operation of the PDU 124, which is labeled PDU controller in Fig. 2.

The utility disconnect switch 122 is operated by the control unit 132 of the PDU 124. In an alternative embodiment, the offline UPS

system 100 comprises multiple PDUs 124, which are all connected to the main power line 1 12.

[0056] The UPS system 100 further comprises a control device 134 for controlling the operation of the PDUs 124 and the utility disconnect switch 122 and a long distance communication link 136. Hence, the control device 134 is connected via the long distance communication link 136 to the control units 132 of the PDUs 124. Also the utility disconnect switch 122 is connected via the long distance communication link 136 to the control device 134. Hence, the control device 134 controls the utility disconnect switch 122 to perform a separation of the power supply side 1 16 from the AC power supply 1 14 in case of a power quality event, which is detected by the control device 134 and to perform a synchronization of the voltages of the power supply 1 14 and the power provided by the PDUs 124 upon clearing the power quality event.

[0057] The long distance communication link 136 is a high speed

communication link, which is designed as an industrially-robust, low- latency communication link. The long distance communication link

136 in this embodiment is based on a fiber-optic or twisted pair

copper physical link, which is provided with a low latency. To achieve the low-latency requirement, either a custom communication protocol can be developed, or an existing industry communication protocols is employed depending on a use case. [0058] Each of the PEBBs 128 comprises a DC/AC converter unit, also referred to as inverter, for converting electrical power from the energy storage device 130, which provides DC power, into AC power, as required for powering the load 1 18. The PEBBs 128 are electrically connected to the transformer 126, which transforms the AC power provided by the PEBBs 128 to provide a required load voltage. Each DC/AC converter unit typically comprises an individual controller for controlling its operation within the offline-UPS system 100. The

DC/AC converter units are driven by the control unit 132 of the

respective PDU 124 based on a control command from the control device 134 of the offline UPS system 100.

[0059] Each of the transformer 126, the multiple PEBBs 128 and the energy storage device 130 have a time constant for delivering energy, i.e.

when the PDU powers the load 1 18. The term time constant refers to an indication of possible support of the load 1 18 in operation. The time constant can depend on operational states of the PDUs 124, e.g. a load support in overload condition, or others. The time

constant for the energy storage device 130 depends first on a

storage capacity of the energy storage device 130. However, other time constants can be considered, e.g. a maximum time for support of a given current. The fastest or shortest time constant is the time constant out of the time constants of all components of the PDUs

124, which defines the shortest operation time of the PDU 124.

[0060] When the offline-UPS system 100 detects a power quality event, the

control device 134 controls the utility disconnect switch 122 to disconnect the load 1 18 from the power supply side 1 16, i.e. the AC power supply 1 14, and uses the PEBBs 128 with the DC/AC converter units and the energy storage device 130 to keep the load 1 18 running. Hence, the control device 134 monitors voltage of the power supply 1 14 to detect power quality events. Furthermore, the control device 134 synchronizes with the voltage of the power supply 1 14 when the power quality event is cleared. The control device 134 also performs a collection of status information of the three PDUs 124. The status information comprises operational mode of PDUs 124, estimated state of charge of energy storage devices 130 and others.

[0061] A control of the offline UPS system 100 in accordance with the described embodiment comprises the steps of performing an optimization of autonomy of the offline UPS system 100 comprising the step of managing power between the PDUs 124 such that the single PDU 124 achieves longest autonomy determined by the component with the fastest time constant for delivering energy, e.g. in this embodiment determined by the fastest time constant for delivering energy out of the transformer 126, the PEBBs 128, and the energy storage device 130.

[0062] According to a second embodiment the offline UPS system 100 is

provided as a short autonomy system. However, in general, the offline UPS system 100 of the third embodiment is implemented based on the UPS system 100 of the first embodiment. Hence, a repeated description of the details of the offline UPS system 100 is omitted.

[0063] In the offline UPS system 100 of the second embodiment, the energy

storage device 130 is provided with super capacitors or a flywheel, which typically provide an autonomy of up to several seconds. Hence, the energy storage device 130 has the shortest time constant out of the transformer 126, the PEBBs 128 and the energy storage device 130. According to the second embodiment, the step of managing power between the PDUs 124 comprises performing a fine utilization of energy available between PDUs 124 to support the load 1 18 with optimum autonomy. Hence, after a transfer, i.e. when the PDUs 124 have started to power the load 1 18, the offline UPS system 100 controls the PDUs 124 to share support of the load 1 18 between the different PDUs 124 in order to avoid that individual PDUs 124 ran out of energy.

[0064] According to the second embodiment, the step of managing power

between the PDUs 124 comprises energy balancing for optimizing or sub- optimizing overall energy storage available in the offline UPS system 100 for extension of autonomy. Hence, when the energy storage devices 130 have different energy levels, the control is performed to utilize energy in optimum manner while power overloading the PEBBs 128. Based on the time constant for delivering energy of the PEBBs 128 being higher than the time constant of the energy storage device 130, the PEBBs 128 withstand high power loading throughout the time constant of the energy storage device 130.

[0065] According to the second embodiment, the step of managing power

between the PDUs 124 comprises balancing the demands of the transformer 126, the PEBBs 128 and the energy storage device 130, each of which having a time constant for delivering energy depending on the device having the shortest time constant. Accordingly, support of the load 1 18 amongst the PDUs 124 is adapted to operate the PEBBs 128, in particular the DC/AC converter units, above the designed power ratings in accordance with an overload capability of the DC/AC converter units.

[0066] According to the second embodiment, the step of managing power

between the PDUs 124 comprises fine utilization of PDUs 124 with components having different time constant for delivering energy. Hence, components of the PDUs 124 with different time constants for delivering energy from the energy storage device 130 to the load 1 18, i.e. the transformer 126, the PEBBs 128 and the energy storage device 130, are be commonly controlled to achieve optimization of autonomy of the offline UPS system 100.

[0067] According to the second embodiment, the step of managing power

between the PDUs 124 comprises performing utilization of components in the PDUs 124 which optimizes or sub-optimizes overall energy storage available in the offline UPS system 100 for maximization of overall autonomy.

[0068] According to the second embodiment, the step of managing power

between the PDUs 124 comprises determining an individual estimated state of charge of the PDUs 124, determining a mean estimated state of charge of the PDUs 124, and adapting the output current of the PDUs 124 based on the individual estimated state of charge of the PDUs 124 and the mean estimated state of charge of the PDUs 124.

[0069] According to a third embodiment the offline UPS system 100 is provided as a long autonomy system. However, in general, the offline UPS system 100 of the third embodiment is implemented based on the UPS system 100 of the first embodiment. Hence, a repeated description of the details of the offline UPS system 100 is omitted.

[0070] In the third embodiment, the energy storage device 130 is a battery,

which enables a typical load support by the offline UPS system 100 of e.g. several minutes. Hence, the energy storage device 130 has the longest time constant out of the transformer 126, the PEBBs 128 and the energy storage device 130. The time constant of the PEBBs 128 is defined by their maximum operation limitations. Hence, the step of managing power between the PDUs 124 comprises

performing optimal power sharing between PDUs 124 to support the load 1 18 with optimum autonomy. Hence, the DC/AC converter units are driven to provide a bigger or smaller share to support of the load 1 18. In particular, the PDU output current is varied based on number of DC/AC converter units available, and an average current provided by all PDUs 124.

[0071] Accordingly, when the energy storage devices 130 have different energy levels, by enabling energy balancing, some of the PEBBs 128 are power overloaded. In this case power balancing is performed to avoid failure of DC/AC converter units by sharing power amongst available DC/AC converter units, so that a single DC/AC converter unit does not suffer from operation at maximum operation limitation and the PEBBs 128 sustain the power overloading.

[0072] According to the third embodiment, the step of managing power between the PDUs 124 further comprises the step of fine power balancing between PDUs under PDUs with different energy storage levels or loss of PEBBs 128 in PDUs 124. Hence, a detailed evaluation of energy storage levels of the power storage devices 130 and available PEBBs 128 is performed by the control device 134, which calculates optimum power for each PDU 124 to perform optimum power sharing by balancing the demands of the fastest time constant within the PDU 124.

[0073] According to the third embodiment, the step of managing power between the PDUs 124 further comprises reducing power loading in the PDUs 124 with a reduced number of PEBBs 128 which have reduced capability. Hence, power loading in the off-line UPS system 100 is reduced in case of a reduced number of DC/AC converter units of a PDU 124 or in case of the offline UPS system 100 having a reduced number of PDUs 124.

[0074] Furthermore, the step of power sharing between the PDUs 124 comprises determining a number of PEBBs 128 available in each PDU 124, and adapting an output current of the PDUs 124 based on the number of PEBBs 128 available in each PDU 124.

[0075] According to the third embodiment, the step of managing power between the PDUs 124 comprises power balancing to reduce power loading with a reduced number of PDUs 124 or PEBBs 128.

[0076] According to the third embodiment, the step of power sharing between the PDUs 124 comprises determining an individual current delivered from each of the PDUs 124 to the load side 120, determining a mean current delivered from the PDUs 124 to the load side 120, and the step of adapting an output current of the multiple PDUs 124 based on the number of PEBBs 128 available in each PDU 124 comprises adapting the output current of the multiple PDUs 124 based on the mean current delivered from the multiple PDUs 124 to the load side 120.

[0077] According to the third embodiment, the step of managing power between the PDUs 124 comprises adapting the output current of the PDUs 124 depending on the time constants of the PDUs 124.

[0078] 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

10 offline uninterruptable power supply system, offline UPS system (state of the art)

12 main power line (state of the art)

14 power supply (state of the art)

16 power supply side (state of the art)

18 load (state of the art)

20 load side (state of the art)

22 utility disconnect switch (state of the art)

24 power delivery unit, PDU (state of the art)

26 transformer (state of the art)

28 power electronic building block, PEBB (state of the art)

30 energy storage device (state of the art)

32 control unit (state of the art)

100 offline uninterruptable power supply system, offline UPS system

1 12 main power line

1 14 power supply

1 16 power supply side

1 18 load

120 load side

122 utility disconnect switch

124 power delivery unit, PDU

126 transformer

128 power electronic building block, PEBB

130 energy storage device

132 control unit

134 control device

136 long distance communication link

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