Method for controlling an uninterruptible power supply

Abstract

Embodiments of the present disclosure relate to a method for controlling an uninterruptible power supply, the uninterruptible power supply including: a grid connection member, a grid switch connected to the grid connection member, a rectifier and an inverter interconnected via a DC link, an energy storage device connected to the DC link, a load connection member to which the inverter is connected, and a bypass switch connected in parallel with the rectifier, the DC link, and the inverter between the grid switch and the load connection member; the grid switch is a mechanical switch, and the bypass switch is a semiconductor switch; the method includes: in a normal operation mode, operating the rectifier in a rectification mode, operating the inverter to convert DC current from the DC link into AC current supplied to the load; receiving a fault signal, sending a disconnection signal to the grid switch to disconnect the grid switch and disconnect the uninterruptible power supply from the grid; when receiving an acknowledgement signal from the grid switch, sending a closing signal to the bypass switch and sending a mode change signal to the rectifier to operate the rectifier in an inversion mode.

Description

Technical Field

[0001] The present invention relates to a method for controlling an uninterruptible power supply and a corresponding computer program. In particular, the present invention relates to a control device for controlling an uninterruptible power supply and an uninterruptible power supply having such a control device. Background Art

[0002] If a power failure occurs in the power grid connected to the uninterruptible power supply, the energy storage device and the inverter of the uninterruptible power supply are designed to supply power to the rated load for a predetermined period of time. If an electrical fault occurs in the load connected to the uninterruptible power supply, the energy storage device and the inverter may have to supply power to an overload greater than the rated load for a predetermined period of time. The electrical and electronic components of the uninterruptible power supply must be sized larger than those required for the rated load to withstand the thermal stress caused by the overload. Summary of the Invention

[0003] An object of the present invention is to provide an improved method for controlling an uninterruptible power supply. Another object of the present invention is to provide an economically reliable uninterruptible power supply.

[0004] These objects are achieved by the subject matter of the independent claims. Further exemplary embodiments will be apparent from the dependent claims and the following description.

[0005] The present invention relates to a method for controlling an uninterruptible power supply. For example, the uninterruptible power supply can be used to provide uninterrupted power to critical infrastructures such as communication networks or railway control centers. The uninterruptible power supply can be capable of providing power in the range of up to several megawatts.

[0006] According to an embodiment of the present invention, the uninterruptible power supply includes: a grid connection member, a grid switch connected to the grid connection, a rectifier and an inverter interconnected via a DC link, an energy storage device connected to the DC link, a load connection member to which the inverter is connected, and a bypass switch that is connected in parallel with the rectifier, the DC link, and the inverter between the grid switch and the load connection member.

[0007] The grid connection component can be an interface for connecting an uninterruptible power supply to the grid. The grid connection component can be a polyphase connection component for polyphase alternating current (AC). The grid switch can be an electrical switch for making and breaking. The grid switch can be a polyphase switch, which has a separate switching element for each wire connected to the grid. The rectifier can be configured to convert alternating current (AC) to direct current (DC). The rectifier can be an active component with active control electronic components. The electrical input of the rectifier for alternating current (AC) can be polyphase. The electrical output of the rectifier for direct current (DC) can be a single DC voltage. Thus, the DC link can be a single voltage link for direct current (DC). The electrical output of the rectifier for direct current (DC) can also have multiple DC voltage levels. Thus, the DC link can also have multiple DC voltage levels. The DC link can also include one or more capacitors. The electrical input of the inverter for direct current (DC) can be a single DC voltage or have multiple DC voltage levels. The inverter can be configured to convert direct current (DC) or multiple DC voltage levels to alternating current (AC). The alternating current (AC) can have the same voltage as the alternating current (AC) of the grid. The inverter can also be an active component with active control electronic components. The electrical output of the inverter for alternating current (AC) can be polyphase. The energy storage device can include at least a battery. The energy storage device can also include a capacitor. The load connection component can be an interface for connecting an uninterruptible power supply to an electrical load. The load connection component can be polyphase. The bypass switch can be an electrical switch for making and breaking. The bypass switch can be used for alternating current (AC) and can be polyphase.

[0008] In one embodiment, the grid switch is a mechanical switch and the bypass switch is a semiconductor switch. The mechanical switch can have at least one movable switching element per phase. The mechanical switch can be mechanically actuated. For example, the mechanical switch can be spring-loaded. The mechanical switch can also be electrically actuated. For example, the mechanical switch can have an electromagnetic opener and / or an electromagnetic closer. The mechanical switch can be robust. The mechanical switch may take some time to open or close. All phases of the mechanical switch can open or close simultaneously. The grid switch can have a control device to signal the state of the grid switch. For example, the control device can be a circuit that closes when the switching element of the grid switch reaches the open position. The semiconductor switch can have electronically switchable electronic components. The semiconductor switch can have no movable parts. The semiconductor switch can be fast. The semiconductor switch can open or close almost instantaneously. The semiconductor switch can have at least one semiconductor switching element per phase.

[0009] According to one embodiment, in normal operating mode, the bypass switch is open, the grid switch is closed, and the uninterruptible power supply is connected to the grid via the grid connection and to the load via the load connection. In normal operating mode, the rectifier operates in rectification mode, in which the AC current from the grid is converted into a DC current supplied to the DC link. In normal operating mode, the inverter operates in inversion mode to convert the DC current from the DC link into an AC current supplied to the load. In normal operating mode, the energy storage device can be charged and / or used to buffer load variations to provide additional energy in the case of a large overload. For example, the rectifier can be sized for 120% of the rated load. When an overload above 120% of the rated load occurs, energy is taken from the battery to maintain the load.

[0010] According to one embodiment, upon receiving a fault signal, the method includes sending a disconnection signal to the grid switch to disconnect the grid switch and disconnecting the uninterruptible power supply from the grid, the fault signal signaling an electrical fault related to the load or the grid. A fault related to the grid can be, for example, undervoltage, power outage, or surge of at least one phase. The fault can also be a frequency deviation. If the fault is related to the grid, bypass may not be available. For example, a fault related to the load can be a short circuit. Since the bypass switch is a semiconductor switch and the grid switch is a mechanical switch, the closing of the bypass switch is faster than the opening of the grid switch.

[0011] When the load is higher than the rated load, the inverter output is higher than the rated load. The temperature of the inverter may thus rise above the rated temperature. Since the rectifier supports the inverter after the confirmation signal is received, the absolute temperature rise during the exclusive use of the inverter will be within the specified range.

[0012] According to one embodiment, upon receiving a confirmation signal from the grid switch confirming disconnection from the grid, the method includes sending a closing signal to the bypass switch and sending a mode change signal to the rectifier to operate the rectifier in inversion mode, in which the DC current from the energy storage device is converted into an AC current so that the load is powered by the inverter and the rectifier. The rectifier can operate in reverse because it includes actively controlled electronic components. The AC current generated by the rectifier flows to the load connection through the closed bypass switch. During the closing duration of the grid switch, the closing signal for the bypass switch can be sent interleaved with the disconnection signal of the grid switch.

[0013] According to one embodiment, upon receiving a fault signal, the method includes sending a termination signal to the rectifier to terminate operation in rectification mode. The termination signal can be sent before the mode change signal. The rectifier can become idle upon receiving the termination signal. The rectifier can be restarted in inversion mode after being idle.

[0014] According to one embodiment, the termination signal is staggered with a predetermined delay after receiving the fault signal. As long as the power grid remains connected to the inlet of the rectifier, the rectifier can operate in a normal operating mode. The disconnection duration of the power grid switch is known, such that the rectifier can still continue rectifying as long as there is an electrical contact in the power grid switch. The termination signal can be sent synchronously with the alternating current. In particular, the termination signal can be staggered at the zero crossing of the phase.

[0015] According to one embodiment, upon receiving the confirmation signal, the method includes sending a balance signal to the rectifier and the inverter to balance the load between the rectifier and the inverter. For example, the rectifier and the inverter can each supply power to half of the load. In the case of load balancing, the rectifier and the inverter can actually cool down from the rated operating temperature.

[0016] According to one embodiment, upon receiving the balance signal, the rectifier and the inverter operate at a lower load compared to the rated load. For example, the rectifier and the inverter can each operate at 75% of the rated load. Through the cooperation between the rectifier and the inverter, both components can operate below the rated load and can still provide an overload of 150% of the rated load to the load.

[0017] According to one embodiment, upon receiving a short - circuit signal indicating a short - circuit in the load, the method includes sending a lag signal to the inverter to operate the inverter in a lag mode, wherein until the lag signal is received, the inverter operates in a linear mode to provide a sinusoidal output voltage. The shape of the current depends on the load. For example, for a non - linear load, the current is not sinusoidal. In the lag mode, the inverter provides a square - wave output current. In the lag mode, the inverter can limit the output current, where the output voltage may be close to zero due to the short - circuit. The resulting output voltage may be due to the impedance of the load. In the lag mode, the frequency of the square - wave current can be equal to the frequency of the sinusoidal voltage.

[0018] According to one embodiment, upon receiving the short - circuit signal and upon having received the confirmation signal, the method includes sending a lag signal to the rectifier to operate the rectifier in a lag mode, wherein in the lag mode, the rectifier converts the DC current from the energy storage device into a square - wave output current. The rectifier and the inverter can operate synchronously while providing the square - wave output current. In particular, the zero crossing of the square - wave output current can be synchronized between the rectifier and the inverter.

[0019] According to one embodiment, upon receiving the lag signal, the inverter changes to the lag mode, thereby providing a square - wave output current at a higher load compared to the rated load. For example, the inverter can provide 110% of the rated load. The output current can be limited to 110% of the rated output current.

[0020] According to one embodiment, the inverter operates in a linear mode until a confirmation signal is received. In the linear mode, the inverter provides a sinusoidal output voltage. When a mode change signal and a balance signal are received, the inverter and the rectifier operate in the linear mode and balance the load between the rectifier and the inverter. When a hysteresis signal is received, the inverter and the rectifier operate in a hysteresis mode to provide a square wave output current, where the inverter provides a higher load compared to the rated load.

[0021] The present invention also relates to a computer program which, when executed by a processor, is adapted to perform the method of one of the foregoing embodiments.

[0022] The present invention also relates to a computer-readable medium storing a computer program according to an embodiment. The computer-readable medium may be a floppy disk, a hard disk, a USB (Universal Serial Bus) storage device, a RAM (Random Access Memory), a ROM (Read-Only Memory), an EPROM (Erasable Programmable Read-Only Memory), or a flash memory. The computer-readable medium may also be a data communication network, such as the Internet, which allows downloading of program code. Generally, the computer-readable medium may be a non-transitory or transitory medium.

[0023] The present invention also relates to a control device for an uninterruptible power supply, the control device being configured to perform the method of one of the foregoing embodiments.

[0024] The present invention also relates to an uninterruptible power supply device, comprising: a grid connection; a grid switch connected to the grid connection, where the grid switch is a mechanical switch; a rectifier and an inverter interconnected via a DC link; an energy storage device connected to the DC link; a load connection to which the inverter is connected; and a bypass switch connected in parallel with the grid switch, the rectifier, the DC link, and the inverter between the grid switch and the load connection, where the bypass switch is a semiconductor switch and includes a control device according to an embodiment.

[0025] According to one embodiment, the bypass switch includes two antiparallel thyristors. A thyristor can conduct current in one direction and not in the opposite direction. Therefore, in order to conduct an AC current with a changing current direction, two thyristors with opposite directions can be connected in parallel with each other. The thyristor can be switched to the conducting state by a closing signal and remain conducting without maintaining the closing signal. Similarly, the thyristor can be switched to the non-conducting state by an opening signal and remain non-conducting without maintaining the opening signal.

[0026] According to one embodiment, the energy storage device includes a battery. The battery can be an electrochemical energy storage device. When the battery is charged, electrical energy is converted into chemical energy stored in the battery. When the battery is discharged, the chemical energy in the battery is converted into electrical energy. Other energy storage devices can be used instead of the battery. For example, energy can be stored in a flywheel or a fuel cell.

[0027] With reference to the embodiments described below, these and other aspects of the present invention will become apparent and be elucidated. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The subject matter of the present invention will be explained in more detail hereinafter with reference to the exemplary embodiments shown in the drawings.

[0029] Figure 1 An uninterruptible power supply according to an embodiment is schematically shown; and

[0030] Figures 2 to 3 A control sequence for an uninterruptible power supply according to an embodiment is schematically shown.

[0031] The reference numerals used in the drawings and their meanings are listed in a summary form in the list of reference numerals. In principle, the same components have the same reference numerals in the figures. DETAILED DESCRIPTION

[0032] Figure 1 An uninterruptible power supply 100 according to an embodiment is schematically shown. The grid connection 102 of the uninterruptible power supply 100 is connected to the grid switch 104 of the uninterruptible power supply 100. The grid switch 104 is a mechanical switch. The rectifier 106 of the uninterruptible power supply 100 and the inverter 108 of the uninterruptible power supply 100 are connected to each other via the DC link 110. The energy storage device 112 of the uninterruptible power supply 100 is connected to the DC link 110. The energy storage device 112 can include a battery converter. The load connection 114 of the uninterruptible power supply 100 is connected to the inverter 108. The bypass switch 116 of the uninterruptible power supply 100 is connected in parallel with the rectifier 106, the DC link 110, and the inverter 108 between the grid switch 104 and the load connection 114. The bypass switch 116 is a semiconductor switch. The grid connection 102, the bypass switch 116, and the load connection 114 can have multiple phases. For clarity, only one phase is shown.

[0033] The uninterruptible power supply 100 has a control device 118, and the control device 118 is connected to the grid switch 104, the rectifier 106, the inverter 108, and the bypass switch 116. The control device 118 is also connected to a monitoring device 120, and the monitoring device 120 monitors the grid 122 connected to the grid connection 102 and the load 124 connected to the load connection 114. The monitoring device 120 can also be incorporated into the control device 118.

[0034] During normal operation of the uninterruptible power supply 100, the grid switch 104 is closed and the bypass switch 116 is open. During normal operation, the rectifier 106 is in rectification mode, in which the rectifier 106 draws alternating current (AC) from the grid 122 and feeds rectified direct current (DC) to the DC link 110. The rectifier 106 controls the voltage of the DC link 110 to the required voltage by drawing appropriate current from the grid. These currents can be sinusoidal and in phase with the corresponding grid voltage to obtain a unity power factor. The bidirectional dc / dc battery converter included in the energy storage device 112 transfers power from the DC link 110 to the battery until the battery is fully charged. The rectifier 106 is controlled to keep the voltage of the DC link 110 within a tolerance range. During normal operation, the inverter 108 draws direct current (DC) from the DC link 110 and feeds inverted alternating current (AC) to the load 124. Thus, the direct current (DC) drawn by the inverter 108 can be supplied by the rectifier 106 and / or the energy storage device 112.

[0035] If the monitoring device 120 senses an electrical fault in the grid 122 or the load 124, the monitoring device 120 sends a fault signal 126 to the control device 118. Upon receiving the fault signal 126, the control device 118 sends a disconnection signal 130 to the grid switch 104 to disconnect the grid 122 from the uninterruptible power supply 100. The inverter 108 continues to draw direct current (DC) from the DC link 110.

[0036] The mechanical grid switch 104 has a moving part 132 that must be moved by applying a physical force 134. The force 134 on the moving part 132 is provided by the actuator 136 of the grid switch 104. The actuator 136 is activated by the disconnection signal 130. For example, the actuator 136 can be a spring and / or an electromagnet. The moving part 132 requires a short duration to move to the open position. Once the moving part 132 is in the open position, the contacts of the grid switch 104 are far enough apart so that arcing can be prevented. The uninterruptible power supply 100 is thus electrically isolated from the grid 122. Once the moving part 132 has reached the open position and the uninterruptible power supply 100 is disconnected from the grid 122, the confirmation device 138 of the grid switch 104 sends a confirmation signal 140 to the control device 118. The direct current (DC) of the DC link 110 is now supplied by the energy storage device 112.

[0037] When the control device 118 receives the confirmation signal 140 that the power grid switch 104 is disconnected, the control device 118 sends a closing signal 128 to the bypass switch 116. Once the closing signal 128 is received, the bypass switch 116 is electronically and almost instantaneously switched. Once the bypass switch 116 is closed, the rectifier 106 is connected in parallel between the energy storage device 112 and the load connection 114 to the inverter 108.

[0038] When the control device 118 receives the confirmation signal 140, the control device 118 sends a mode change signal 142 to the rectifier 106. When the mode change signal 142 is received, the rectifier 106 operates in an inversion mode and draws direct current (DC) from the DC link 110, and feeds inverted alternating current (AC) to the load 124 via the closed bypass switch 116, so that the load 124 is powered by both the inverter 108 and the rectifier 106.

[0039] In one embodiment, when the control device 118 receives the fault signal 126, the control device 118 sends a termination signal 144 to the rectifier 106. When the termination signal 144 is received, the rectifier 106 stops drawing alternating current (AC) from the power grid 122. Since the power grid switch 104 takes some time to disconnect, the termination signal 144 can also be sent with a time delay of the predetermined disconnection time of the power grid switch 104. In this way, the rectifier 106 continuously draws alternating current (AC) from the power grid 122 until the uninterruptible power supply 100 is truly disconnected from the power grid 122.

[0040] If the fault signal 126 signals that the load connection 114 is overloaded, the control device 118 sends a disconnection signal 130 to the power grid switch 104 to disconnect the power grid 122 from the uninterruptible power supply 100. When the control device 118 receives the confirmation signal 140 about the disconnection of the power grid switch, the control device 118 sends a closing signal 128 to the bypass switch 116.

[0041] In one embodiment and depending on the amount of overload, the inverter 108 outputs up to 150% of the rated load within the time required for the power grid switch 104 to disconnect, which can typically be up to 120 milliseconds for a mechanical contactor.

[0042] After the rectifier 106 has started to invert direct current (DC) from the energy storage device 112 into alternating current (AC) when receiving the mode change signal 142, the load 124 is shared between the inverter 108 and the rectifier 106, so that the inverter 108 can reduce its load.

[0043] In one embodiment, when the control device 118 receives the confirmation signal 140, it sends a balancing signal 148 to the inverter 108 and the rectifier 106 to share an equal portion of the load 124. In one embodiment, both the rectifier 106 and the inverter 108 supply power to up to 75% of the rated load when they receive the balancing signal 148. The overload capacity can also be, for example, 200%. Then the inverter 108 alone withstands a full overload of 200% for the time required for the grid switch 104 to open. After that, the loads of the rectifier 106 and the inverter 108 are only 100%.

[0044] If the fault signal 126 signals a short circuit or flashover of the load 124, the control device 118 sends a lag signal 150 to the inverter 108 when it receives the fault signal 126. When receiving the lag signal 150, the inverter goes into the lag mode and provides a square wave current 152 to the load 124. The square wave current 152 is different from the sinusoidal voltage 154 provided by the inverter 108 when operating in the linear mode. When operating in the lag mode, the inverter can output more than the rated load. In one embodiment, when operating in the lag mode, the inverter 108 provides 110% of the rated RMS current.

[0045] Figure 2 is schematically shown Figure 1 The control sequence of the uninterruptible power supply 100. The control sequence starts with detecting that the bypass is unavailable (i.e., the grid is out of tolerance). At the same time, the grid switch 104 is commanded to open by sending a disconnection signal 130. The rectifier 106 continues to operate in the current control mode and draws a sinusoidal current from the grid. Typically, about one to two milliseconds after receiving the fault signal 126, a termination signal 144 is sent to the rectifier 106 and the rectifier 106 stops operating. About 60 to 120 milliseconds after receiving the disconnection signal 130, the grid switch 104 opens and a confirmation signal 140 is generated. When receiving the confirmation signal 140, a closing signal 128 is sent to the bypass switch. When receiving the closing signal 128, the rectifier 106 is connected in parallel with the inverter 108 between the energy storage device and the load connection. While sending the closing signal 128, a mode change signal 142 is sent to the rectifier 106. When receiving the mode change signal 142, the rectifier 106 starts operating in the inversion mode and draws DC current from the energy storage device and inverses it into a sinusoidal AC current. The thus-converted AC current flows to the load through the closed bypass switch. In addition, while sending the mode change signal 142, a balancing signal 148 is sent to the rectifier 106. While the inverter 108 maintains operation in the voltage control mode to apply the load voltage, the rectifier 106 operates in the current control mode when receiving the balancing signal 148 to share the load evenly with the inverter.

[0046] In one embodiment, sometimes after receiving a fault signal 126 signaling that the bypass is unavailable, another fault signal 126 signals a short circuit 200 in the load. When the fault signal 126 is received, a hysteresis signal 150 is sent to the inverter 108 and the rectifier 106. When the hysteresis signal 150 is received, both converters operate in a hysteresis current control mode to provide a square wave current 152 to the fault. For example, when in the hysteresis current control mode, the inverter 108 outputs 110% of the rated load.

[0047] In the case of an overload, the grid switch is commanded to open. Once opened, the bypass closes and the rectifier is used to share the load with the inverter.

[0048] A short circuit is more complex and different strategies can be used. In the first strategy, if the grid is available, the grid is used to provide fault current by activating the bypass. If the grid is unavailable, the grid switch can be opened in advance, for example once the grid is detected to be out of tolerance. Thus, as long as a short circuit occurs without the grid, the UPS is already ready to deliver fault current to the load using both the inverter and the rectifier. Once the grid switch is opened, the rectifier and the inverter share the applied load. Each converter takes half of the load (L / 2). In this case, if a short circuit occurs, both converters will switch to the hysteresis current control mode to provide the desired amount of fault current (e.g., 110% each to reach a total of 220%). When a short circuit occurs before the grid switch is opened, the inverter alone provides the fault current until the grid switch is opened. At this time, the rectifier is used to increase the amount of fault current.

[0049] Figure 3 is schematically shown for Figure 1 another control sequence of the uninterruptible power supply 100. As Figure 2 shown, the control sequence starts with the detection of "bypass unavailable". A fault signal 126 signaling "bypass unavailable" is generated. While sending the "bypass unavailable" signal 126, the grid switch 104 is commanded to open by sending a disconnect signal 130. The rectifier 106 continues to operate in a current control mode and draws a sinusoidal current from the grid. Approximately one to two milliseconds after receiving the fault signal 126, a termination signal 144 is sent to the rectifier 106 and the rectifier 106 stops operating.

[0050] While the grid switch 104 is still in the process of opening, another fault signal 126 signals a short circuit 200 in the load. Upon receiving the another fault signal 126, a lag signal 150 is sent separately to the inverter 108. Upon receiving the lag signal 150, the inverter operates in a lagging current control mode to supply a square wave current 152 to the fault. For example, when in the lagging current control mode, the inverter 108 outputs 110% of the rated load.

[0051] Approximately 60 to 120 milliseconds after receiving the disconnect signal 130, the grid switch 104 has completed opening and an acknowledgement signal 140 is generated. Upon receiving the acknowledgement signal 140, a close signal 128 is sent to the bypass switch. Upon receiving the close signal 128, the rectifier 106 is connected in parallel with the inverter 108 between the energy storage device and the load connection. While sending the close signal 128, a mode change signal 142 and another lag signal 150 are sent to the rectifier 106. Upon receiving the mode change signal 142 and the lag signal 150, the rectifier 106 operates in a lagging current control mode to also supply the square wave current 152 to the fault.

[0052] In the following paragraphs, the proposed idea is described in different words. The idea is to increase the power supply capacity of an uninterruptible power supply (UPS).

[0053] UPS converters are typically designed to provide specific overload and fault capabilities, typically 150% overload for 30 seconds and 220% short circuit for 100 ms. These limitations severely affect the size, thermal design, and cost of the UPS inverter semiconductors. This is because, under overload and fault conditions, the inverter must supply all of the specified power alone.

[0054] During overload operation, when the bypass switch is open, the inverter typically supports 150% for 30 s. The inverter operates linearly and controls the voltage on the critical load. The bypass supports higher overloads, typically 150% for 60 s.

[0055] In current UPS designs, various overload strategies are used, which also depend on the amount of overload. For example, in the first strategy when 125% overload is detected, the load can remain on the inverter until the inverter capacity is exhausted (e.g., 60 s). Then the load will be transferred to the bypass. In the second strategy, when 120% overload is detected, the load can be transferred to the bypass until the bypass overload capacity is exhausted. Then the load will be transferred back to the inverter. Obviously, if the bypass is not available, the inverter must supply the overload alone.

[0056] In a short - circuit operation, when the bypass switch is open, the inverter typically provides 220% of the short - circuit current for 100 ms. The bypass provides more than 220% of the short - circuit current for more than 100 ms, typically 1000% for half a cycle.

[0057] In a short - circuit situation, the inverter is typically controlled in a lagging - current control mode to supply a square - wave current to the fault. The voltage is very close to zero, which depends on the impedance of the short - circuit. When a short - circuit is detected at the UPS output, the normal sinusoidal voltage collapses to zero due to the short - circuit. Then the inverter - bridge current is controlled to provide a square - wave current at the output after the LC filter.

[0058] The idea here is to supply fault current from the bypass when the grid is available and from both the rectifier and the inverter when the grid is not available. Also, by using the rectifier to support the inverter, the stress during overload operation can be reduced.

[0059] The proposed idea helps with the economic feasibility of UPSs with SiC devices, making it possible to significantly increase the fault - current capacity of traditional UPSs, for example, from 220% to 440%, or it can reduce the cost of a UPS with the same performance.

[0060] In an overload operation with a new design, when the bypass switch is open, the inverter typically supports 150% for 5 s. The inverter operates in a linear manner and controls the voltage on the critical load. The bypass supports a higher overload than the inverter, typically 150% for 60 s.

[0061] In a new UPS design, various overload strategies can be used, which typically depend on the amount of overload. From the perspective of overload capacity, the bypass can be designed in the same way as the traditional design. The main difference lies in the overload of the inverter. To improve the overload operation in the new design, a contactor is added at the UPS input.

[0062] The overload of the inverter is initiated by commanding the input switch to open and the rectifier to turn off. During this period, the inverter alone bears the overload (e.g., 150%). Once the input switch is open (for a contactor, typically after 60 to 120 ms, depending on the size), the bypass is switched on and the rectifier is used in an inverted mode to share the load with the inverter. From this point on, both the inverter and the rectifier share the overload (e.g., 75% each).

[0063] If the bypass is available, it can deliver more than 220% of the short - circuit current for more than 100 ms (usually 1000% for half a cycle). Usually, even when connected to a common power grid for the bypass and the rectifier input, the conditions of "bypass available" and "rectifier power available" are different and depend on different conditions related to the grid amplitude and phase in relation to other internal variables. Additionally, usually, the condition for stating "rectifier power available" is more difficult to determine than the condition for determining "available bypass".

[0064] To improve short - circuit operation in the new design, a switch is added at the UPS input. This switch (e.g., a contactor) is typically controlled by a command signal that commands the switch to open or close an auxiliary contact, actually measures whether the contact is open or closed, and supplies power to the voltage terminals to feed the contactor coil.

[0065] If the bypass is not available, the input switch opens, the rectifier shuts down, and the inverter continues to control the voltage of the critical load to draw energy from the battery. In the event of a short - circuit, the bypass switch closes and the short - circuit current is provided by both the inverter and the rectifier operating in the anti - phase mode. Usually, both the inverter and the rectifier provide 110% of the short - circuit current for 100 ms.

[0066] A variant of the above strategy is to share the load between the inverter and the rectifier immediately after detecting that the input contactor is open by reading the auxiliary contact of the contactor. The advantage of this is that the inverter and the rectifier are already operating in a cooperative mode before a potential short - circuit occurs. Various control techniques can be used for the coordinated operation of the inverter and the rectifier in the anti - phase mode.

[0067] If a short - circuit occurs before the input contactor opens, the inverter will operate alone in the lagging - current control mode. Once the contactor opens, the rectifier will also operate in the lagging - current control mode to assist the inverter. If this happens and to compensate for the loss of short - circuit current, the short - circuit operation can be extended from the normal 100 ms to (100 ms + 3T / 4), where T is the time in [ms], during which only the inverter feeds the short - circuit. This can maintain the same I2t.

[0068] The probability of a short - circuit occurring before the input contactor opens is extremely low, and in such a case, the above strategy is adopted. To further reduce this possibility or even completely avoid this situation, a faster disconnector can be used instead of a mechanical contactor. For example, when using IGBT or IGCT, the disconnection time is almost zero, while when using SCR (thyristor), the disconnection time may be about 2 to 3 ms, using inverter - biasing techniques to force the turn - off.

[0069] Although the present invention has been described in detail in the drawings and the foregoing description, such description and illustration are to be considered illustrative or exemplary and not restrictive; the present invention is not limited to the disclosed embodiments. By studying the drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement other variations of the disclosed embodiments by practicing the claimed invention. 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. A single processor or controller or other unit may implement the functions of several items recited in the claims. The 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 the scope.

[0070] List of Reference Signs

[0071] 100: Uninterruptible power supply

[0072] 102: Grid connection member

[0073] 104: Grid switch

[0074] 106: Rectifier

[0075] 108: Inverter

[0076] 110: DC link

[0077] 112: Energy storage device

[0078] 114: Load connection member

[0079] 116: Bypass switch

[0080] 118: Control device

[0081] 120: Monitoring device

[0082] 122: Power grid

[0083] 124: Load

[0084] 126: Fault signal

[0085] 128: Closing signal

[0086] 130: Disconnection signal

[0087] 132: Moving part

[0088] 134: Force

[0089] 136: Opener

[0090] 138: Confirmation device

[0091] 140: Confirmation signal

[0092] 142: Mode change signal

[0093] 144: Termination signal

[0094] 148: Balance signal

[0095] 150: Hysteresis signal

[0096] 152: Square wave current

[0097] 154: Sinusoidal voltage

[0098] 200: Short circuit

Claims

1. A method for controlling an uninterruptible power supply (100), wherein the uninterruptible power supply (100) includes: a grid connection member (102), a grid switch (104) connected to the grid connection member (102), a rectifier (106) and an inverter (108) interconnected via a DC link (110), an energy storage device (112) connected to the DC link (110), a load connection member (114) to which the inverter (108) is connected, and a bypass switch (116), the bypass switch (116) being connected in parallel with the rectifier (106), the DC link (110) and the inverter (108) between the grid switch (104) and the load connection member (114); wherein the grid switch (104) is a mechanical switch and the bypass switch (116) is a semiconductor switch; wherein the grid switch (104) includes a confirmation device (138), and in response to the uninterruptible power supply (100) being disconnected from the grid connection member (102), the confirmation device (138) generates a confirmation signal (140), the method comprising: in a normal operation mode, operating the rectifier (106) in a rectification mode and operating the inverter (108) to convert a DC current from the DC link (110) into an AC current supplied to a load (124), in the normal operation mode, the bypass switch (116) is open, the grid switch (104) is closed, the uninterruptible power supply (100) is connected to a grid (122) via the grid connection member (102) and is connected to the load (124) via the load connection member (114), and in the rectification mode, an AC current from the grid (122) is converted into a DC current supplied to the DC link (110); upon receiving a fault signal (126), sending a disconnection signal (130) to the grid switch (104) to disconnect the grid switch (104) and disconnecting the uninterruptible power supply (100) from the grid (122), the fault signal (126) signaling an electrical fault related to at least one of the load (124) and the grid (122); upon receiving the confirmation signal (140) from the grid switch (104) confirming the disconnection from the grid (122), sending a closing signal (128) to the bypass switch (116) and sending a mode change signal (142) to the rectifier (106) to operate the rectifier (106) in an inversion mode, in the inversion mode, a DC current from the energy storage device (112) is converted into an AC current such that the load (124) is powered by the inverter (108) and the rectifier (106), If the fault signal (126) signals a short circuit (200) in the load (124), a lag signal (150) is sent to the inverter (108) to operate the inverter (108) in a lag mode, wherein the inverter (108) operates in a linear mode to provide a sinusoidal output voltage (154) until the lag signal (150) is received, and wherein in the lag mode, the inverter (108) provides a square wave output current (152).

2. The method according to claim 1, further comprising: Upon receiving the fault signal (126), sending a termination signal (144) to the rectifier (106) to terminate operation in the rectification mode.

3. The method according to claim 2, wherein the termination signal (144) is sent staggered with a predetermined delay after receiving the fault signal (126).

4. The method according to any one of the preceding claims, further comprising: Upon receiving the confirmation signal (140), sending a balance signal (148) to the rectifier (106) and the inverter (108) to balance the load (124) between the rectifier (106) and the inverter (108).

5. The method according to claim 4, wherein upon receiving the balance signal (148), the rectifier (106) and the inverter (108) operate at a lower load compared to the rated load.

6. The method according to claim 1, further comprising: Upon receiving the fault signal (126) signaling the short circuit (200) and after the confirmation signal (140) has been received, sending the lag signal (150) to the rectifier (106) to operate the rectifier (106) in the lag mode, wherein in the lag mode, the rectifier (106) converts the DC current from the energy storage device (112) into a square wave output current (152).

7. The method according to any one of claims 1 to 3 and 5 to 6, wherein upon receiving the lag signal (150), the inverter (108) enters the lag mode and thus provides a square wave output current (152) at a higher load compared to the rated load.

8. The method according to claim 4, Among them, The inverter (108) operates in a linear mode until the confirmation signal (140) is received. In the linear mode, the inverter (108) provides a sinusoidal output voltage (154). When the mode change signal (142) and the balance signal (148) are received, the inverter (108) and the rectifier (106) operate in the linear mode, and balance the load (124) between the rectifier (106) and the inverter (108). When the hysteresis signal (150) is received, the inverter (108) and the rectifier (106) operate in the hysteresis mode to provide a square wave output current (152), where the inverter (108) provides a higher load compared to the rated load.

9. A computer program which, when executed by a processor, is adapted to perform the method according to any one of the preceding claims.

10. A computer-readable medium, in which the computer program according to claim 9 is stored.

11. A control device (118) for an uninterruptible power supply (100), configured to perform the method according to any one of claims 1 to 8.

12. An uninterruptible power supply (100), comprising: a grid connection (102); a grid switch (104) connected to the grid connection (102), where the grid switch (104) is a mechanical switch, and where the grid switch (104) includes a confirmation device (138) which generates a confirmation signal (140) in response to the uninterruptible power supply (100) being disconnected from the grid connection (102); a rectifier (106) and an inverter (108) interconnected via a DC link (110); an energy storage device (112) connected to the DC link (110); a load connection (114) to which the inverter (108) is connected; a bypass switch (116) connected in parallel with the rectifier (106), the DC link (110) and the inverter (108) between the grid switch (104) and the load connection (114), where the bypass switch (116) is a semiconductor switch; and a control device (118) according to claim 11.

13. The uninterruptible power supply (100) according to claim 12, where the bypass switch (116) includes two antiparallel thyristors.

14. The uninterruptible power supply (100) according to claim 12 or 13, where the energy storage device (112) includes a battery.

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