Uninterruptible power supply system with dynamic bypass power distribution

Abstract

A UPS system (1) comprising a plurality of UPS (2a to 2e) connected in parallel, each UPS comprising a bypass switch (10) and a controller (20), the controller (20) communicating with the controllers of the other UPS (2a to 2e) and comprising: a first unit (220) monitoring the current flowing in the bypass line 5 of the UPS of the first unit (220) and determining the effective value of the current intensity, a second unit (224) collecting the effective values ​​of all UPS with the bypass switch (10) closed and defining a reference effective value corresponding to the collected minimum effective value, a third unit (226) determining a delay for closing the bypass switch (10) of the UPS, if the effective value of the UPS of the controller (20) is equal to the reference effective value, the delay is zero, and a fourth unit (246), if the delay is zero, the fourth unit (246) keeping the bypass switch (10) closed, or opening the bypass switch (10) for a duration corresponding to the delay before closing the bypass switch (10).

Description

Technical Field

[0001] The present invention relates generally to uninterruptible power supplies (UPS) and, more particularly, to an automatic bypass system for distributing power in an installation including a plurality of uninterruptible power supplies electrically connected in parallel. Background Art

[0002] The present invention can be applied to all uninterruptible power supplies with static bypass switches that are independently controlled phase by phase (ie, using silicon controlled rectifier (SCR) technology).

[0003] An uninterruptible power supply (UPS) is an electrical device that is capable of supplying power to a load even if the quality and / or availability of energy provided by a utility varies.

[0004] A conventional online UPS utilizes a power factor correction (PFC) circuit to rectify the input power provided by the grid in order to power a direct current (DC) bus. When the mains is available, the rectified DC voltage is typically used to charge the battery and to power the DC bus. When there is no mains power supply, the battery powers the DC bus. The inverter generates an AC output voltage from the DC bus to the load. Since the DC bus is powered by the mains or the battery, the output power of the inverter remains uninterrupted when the mains fails and the battery is fully charged. Conventionally known online UPSs can also operate in a mode known as bypass mode, in which an unregulated power supply with basic protection is provided directly to the load from an alternating current (AC) power supply via a bypass line. The AC power supply used in bypass mode can be the same as the AC power supply used for the rectifier, or it can be a separate (auxiliary) power supply.

[0005] Therefore, an uninterruptible power supply system (UPS system) has a bypass circuit or bypass path, which is a power supply path that bypasses the UPS, that is, bypasses the rectifier, inverter and direct current (DC) power supply (for example, a battery that forms an uninterruptible power supply unit (UPS unit)). If the UPS system is overloaded or an internal fault occurs, the UPS unit can use an automatic bypass circuit to switch its load to the mains power supply. A manual bypass circuit, a maintenance bypass circuit or a service bypass circuit will enable the user to switch the load to the mains through the bypass circuit, thereby isolating, maintaining or removing the UPS unit without interrupting the power supply to the load.

[0006] Therefore, a bypass switch is a non-essential addition to a UPS system and, although it is not an integral part of uninterruptible power supply operation, it is obviously useful in the event of maintenance or repair. If a problem occurs or the unit needs to be removed for repair, the bypass switch ensures that power is maintained continuously in the event of a fault.

[0007] There are two main types of bypass switches that perform similar but different functions. These two bypass switches are the static bypass switch and the external maintenance bypass switch.

[0008] The static bypass switch is used as a fail-safe device for UPS. When a UPS fails, the load automatically drops to the main power input to ensure continuous power supply. This switch is suitable for almost any online UPS system as an additional safety measure to ensure continuous power supply.

[0009] The UPS system has an external maintenance bypass switch or external maintenance bypass (sometimes also called a "wrap-around bypass") connected externally. This bypass is used to provide continuous power while the UPS is isolated and easily removed for repair. The alternative is to shut down the entire network to remove the UPS, which can mean several hours of downtime and cost a lot of money for some companies. In addition, in the event of a failure of the UPS system itself, the switch will ensure that the load is safely transferred and enable the UPS unit to be removed and replaced more easily.

[0010] To improve scalability and / or redundancy, two or more UPS can be electrically connected to form a single UPS system in parallel with one output. In such a system, the combination of multiple UPS can improve the power supply capacity to the load connected to the parallel UPS system. In addition, if one of the parallel connected UPS fails, the other parallel connected UPS can serve as a backup for the failed UPS.

[0011] Typically, a parallel UPS system operating in bypass mode cannot actively control the current flowing through the individual bypass paths.

[0012] This is because in a parallel UPS system, successful (i.e., equal) load sharing between parallel-connected online UPSs can be achieved in online or battery mode by operating the inverter of each online UPS to properly regulate the power provided to each online UPS at a single output (connected to the load). However, it is much more difficult to achieve successful load sharing between parallel-connected online UPSs in bypass mode, in which unregulated power is provided by each UPS to a single output. More specifically, even if similar online UPSs are connected in parallel to a single output and each UPS supplies power to the output (i.e., the load) in bypass mode, manufacturing differences in the components of each UPS and differences in the cables connecting each UPS to the single output can result in uneven load distribution between each UPS connected in parallel.

[0013] For example, in the case where the bypass circuits of two UPS are electrically connected to each other in parallel, if the bypass circuit impedance of each UPS is the same, each UPS will have exactly 50% load. However, any impedance mismatch of the bypass circuit will result in different power distribution between the two UPS.

[0014] Also, if the loads connected to the devices utilize nearly 100% of the power the devices can provide (ie, nearly 100% load factor), the impedance mismatch may cause one UPS to operate overloaded while the other does not, resulting in potential failure of the entire system.

[0015] This is because if the load is unevenly distributed between the online UPSs connected in parallel and operating in bypass mode, one of the UPSs may be overloaded, which may cause damage to components of that UPS. Uneven load distribution between the online UPSs connected in parallel may also cause an upstream protection circuit (e.g., a circuit breaker) to trip when one of the UPSs trips, which may cause that UPS to transfer the load to the other UPSs connected in parallel. The additional load transferred to the other UPS may cause the upstream protection circuit in the other UPS to trip, and its load to be transferred to the other UPSs connected in parallel. As the circuit breaker tripping / load transfer process continues to the other UPSs, the remaining UPS may no longer be able to support the load, and the load may be abandoned.

[0016] Generally speaking, power mismatch is directly proportional to impedance mismatch and indirectly proportional to impedance. For this reason, and also taking into account the impedance characteristics of the cable, high-power UPS is more affected by bypass power distribution problems.

[0017] A common technique for dealing with uneven load distribution between parallel UPS's operating in bypass mode is to identify the actual portion of the load supported by each UPS connected in parallel (i.e., the load distribution portion), and adjust the impedance between each UPS and the load to attempt to evenly distribute the load across the UPS's. The impedance between each UPS and the load can be configured by adjusting the length of the cable connecting each UPS to the load and / or by adding an inductor (i.e., an inductor also referred to as a bypass distribution inductor) between the UPS's and the load.

[0018] However, such a solution can only alleviate the impedance imbalance and therefore cannot guarantee a high level of bypass power distribution. In fact, due to the unbalanced inductance distribution, perfect distribution is difficult to achieve and is usually ±10%.

[0019] In addition, this hardware solution has disadvantages such as increased total cost, reduced efficiency, reduced reliability, increased device congestion and increased weight.

[0020] For example, it is generally believed that a maximum of four online UPS or UPS units operating in bypass mode can be connected in parallel, despite adjusting the length of the cables connecting the parallel UPS to the load and / or adding buffers on said cables, because connecting more than four online UPS in parallel will result in a deviation of more than 10% in the load distribution portion between the UPS. Even when four parallel online UPS are operating in bypass mode, the load distribution portion of each UPS may deviate by up to 10%.

[0021] A parallel UPS system comprising a plurality of UPSs connected in parallel is known from document US 2017 / 163086, each UPS comprising:

[0022] - an input configured to be connected to a power supply and to receive an input power supply,

[0023] an output connected to a load and configured to provide output power to the load derived from at least the input power, and

[0024] a bypass line selectively connected between the input and the output via a bypass switch, the bypass switch or static switch being configured to: close in a first operating mode to connect the UPS input with the UPS output via the bypass line; and open in a second operating mode to disconnect the UPS input from the UPS output, and

[0025] - A common controller configured to measure the value and effective value or root mean square value of the current flowing through the bypass line and control the bypass switches of the individual UPSs.

[0026] The common system controller described in US 2017 / 0163086 obviously creates a critical single point of failure, which can be fatal to the system, as a controller failure will jeopardize the current distribution of the entire system and all bypasses may experience cascading failures.

[0027] In the UPS field, parallel systems are used to take advantage of their redundancy, but the existence of a single point failure in the system described in US2017 / 0163086 eliminates the possibility of such redundancy, making the parallel system inoperable and therefore undesirable. Summary of the invention

[0028] The present invention aims to provide a solution which eliminates the concept of adding physical impedances to eliminate the above mentioned drawbacks and enables to achieve zero steady state error and perfect power matching between the bypass circuits of the various uninterruptible power supplies of the installation.

[0029] In a first object of the present invention, there is provided an uninterruptible power supply system (UPS system), comprising a power supply of a plurality of uninterruptible power supplies (UPS) connected in parallel between a main input configured to be connected to a power supply and receive input power, and a main output configured to be connected to a load and provide output power derived from at least the input power to the load, each UPS comprising:

[0030] - input, which is connected to the main input,

[0031] - output, which is connected to the main output, and

[0032] a bypass line connected between the input and the output via a bypass switch, the bypass switch being configured to close in a first operating mode, thereby connecting the input to the output via the bypass line, and to open in a second operating mode, thereby decoupling the input from the output.

[0033] According to the general features of the present invention, each UPS of the UPS system comprises a control unit, which communicates with the control units of all other UPS of the UPS system. The control unit of each UPS comprises:

[0034] - a first block, comprising a first unit, a second unit and a third unit, the first unit being configured to monitor a current signal flowing in a bypass line of a UPS of the first unit and determine a root mean square value (RMS), i.e., an effective value, of a current intensity of the current signal, the second unit being configured to collect RMS values ​​of the UPS of the second unit and other UPSs in a UPS system whose bypass switches are closed and set an RMS reference value corresponding to a minimum RMS value collected, the third unit being configured to determine an activation delay for closing a bypass switch of a UPS of a control unit, if the RMS value of the UPS of the third unit is equal to the RMS reference value, the activation delay is zero, otherwise, the activation delay is not zero, and

[0035] - A second block, comprising a fourth unit, the fourth unit being configured to control a bypass switch of the UPS of the fourth unit according to the activation delay sent by the third unit, if the activation delay is zero, the bypass switch remains closed, otherwise the bypass switch is opened within a specific time corresponding to the activation delay and then closed.

[0036] There is always at least one closed bypass switch to ensure a direct path between the main input and main output of the UPS system.

[0037] Therefore, the UPS system according to the present invention comprises an electrical structure including a plurality of control units distributed in each UPS of the UPS system. As described above, this electrical structure is produced without a master-slave relationship between the control units of different UPS, which is different from the systems described in the prior art, for example, the system described in document US 2017 / 0163086, which still has at least one master controller or one master controller managing different UPS or UPS controllers.

[0038] The structure of the present invention using control units distributed in each UPS eliminates the severity of single point failure, thereby creating a current distribution structure suitable for practical parallel systems. Using this distributed structure, the failure control unit of the UPS can be eliminated while making limited compromises on bypass power balance.

[0039] This solution eliminates the concept of increased physical impedance used in the prior art and, therefore, all the drawbacks associated therewith concerning cost, efficiency, reliability, crowding and weight.

[0040] Utilizing activation delay to control the opening and closing of bypass switches of bypass lines of different UPS provides a solution for a UPS system, which can provide a continuous output current signal to a load connected to the UPS system while balancing the current distribution flowing in the bypass lines of different UPS.

[0041] The UPS system according to the present invention is capable of having a high level of bypass power distribution to eliminate physical inductance, reduce the overall cost, overall volume and overall weight of the system, and simplify the parallel installation procedure.

[0042] The AC supply used in bypass mode can be the same as the AC supply used for the rectifier, or it can be a separate (auxiliary) supply.

[0043] In a first aspect of the UPS system, the first block can be configured to operate at a first frequency and the second block can be configured to operate at a second frequency higher than the first frequency, the ratio of the second frequency to the first frequency being proportional to the required control accuracy.

[0044] Therefore, the system is divided into two blocks operating at two different frequencies. The first block operates at a lower frequency and is responsible for managing the logic and calculating the delay angle, and the second block operates at a higher frequency to be able to control the bypass switch according to the delay angle.

[0045] In the second aspect of the UPS system, the first unit of each control unit may include an AC-DC converter for sampling the monitored current signal and an RMS calculation module for determining an RMS value of the current sample.

[0046] In the third aspect of the UPS system, the second unit of each control unit may include a real-time communication module.

[0047] In the fourth aspect of the UPS system, the third unit of each control unit may include a comparator and a delay unit, wherein the comparator calculates the difference between the RMS value of the current signal of the UPS of the comparator and the RMS reference value; the delay unit is configured to calculate the activation delay, after which the bypass switch of the UPS can be closed.

[0048] In a fifth aspect of the UPS system, the delay unit may include a proportional-integral corrector to have a zero steady-state error.

[0049] In a sixth aspect of the UPS system, the activation delay determined by the third unit may be a delay angle relative to a phase angle, and the third unit may further include a saturation module configured to limit the delay angle of the activation delay to between a minimum delay angle of 0° and a maximum delay angle according to the number of UPSs in the UPS system.

[0050] Therefore, the saturation module is able to limit the maximum peak value of the current signal that will pass through the UPS once the delay angle is applied.

[0051] The maximum delay angle is used to limit the interval during which a single bypass switch is closed to prevent current sensor saturation or protection device tripping.

[0052] However, the value of the maximum delay angle may limit the distribution capacity and therefore the possibility of obtaining zero steady-state error, a limit necessary to avoid saturation of any current sensor or tripping of any protection device.

[0053] In the seventh aspect of the UPS system, the second block may further include an extraction unit, a phase-locked loop (PLL), a comparator, and a controller, wherein the extraction unit is configured to extract a fundamental harmonic from the current signal, the phase-locked loop (PLL) is configured to determine a phase angle of the current signal and detect a zero crossing of the current signal, the comparator compares the phase angle of the current signal detected by the phase-locked loop with the activation delay determined by the third unit of the first block, and the fourth unit of the second block controls the bypass switch according to the comparison.

[0054] In order to correctly apply the delay, the zero crossing of the current signal must be detected. Combining a standard PLL with a fundamental harmonic extraction unit allows this detection with a signal that has multiple harmonic components in the current due to loading and shaping.

[0055] By continuously comparing the phase angle of the current signal sent by the PLL with the activation delay, the activation time of the bypass switch can be detected. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The invention will be better understood on reading the following description given by way of illustration and not limitation with reference to the accompanying drawings.

[0057] Figure 1 is a schematic diagram showing an uninterruptible power supply system according to one embodiment of the present invention.

[0058] Figure 2 Schematically shows Figure 1 The control unit of one of the uninterruptible power supplies of the power supply system.

[0059] Figure 3 A flow chart showing a method of operating the parallel UPS system 1 in the "bypass" mode according to one embodiment. DETAILED DESCRIPTION

[0060] The present invention will be described in conjunction with specific embodiments and with reference to specific drawings, but the invention is not limited thereto and is limited only by the claims. The drawings described are only schematic and non-restrictive. In the drawings, for the purpose of illustration, the size of some elements may be exaggerated and not drawn to scale. When the term "comprising" is used in this specification and claims, other elements or steps are not excluded. When an undefined or defined article is used to represent a singular name, such as "a", "the", unless otherwise specified, the plural form of the name may be included.

[0061] The term "comprising" used in the claims should not be interpreted as being limited to the means listed below; the term does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to a device consisting only of components A and B. This means that, for the purposes of the present invention, the only relevant components of the device are A and B.

[0062] In addition, the terms first, second, third, etc. in the specification and claims are used to distinguish similar elements and are not necessarily used to describe a sequential or chronological order. It should be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the invention described herein are capable of operating in sequences other than those described or illustrated herein.

[0063] The present invention relates to a system for equal load distribution between UPS (i.e., uninterruptible power supplies) connected in parallel and operating in bypass mode. The system does not require adjusting cable length or adding expensive inductors, and can provide equal load distribution between more than four UPS connected in parallel.

[0064] Figure 1 is a block diagram of a parallel UPS system 1 according to aspects described herein.

[0065] The parallel UPS system 1 includes five UPS (UPS 2a, UPS 2b, UPS 2c, UPS 2d and UPS 2e). Each of UPS 2a to UPS 2e includes an input 3 connected to a main input 4, a bypass line 5, an AC / DC converter 6, a DC bus 7, a DC / AC inverter 8, a bypass switch 10, and an output 11 connected to a main output 12.

[0066] The main input 4 is connected to an AC power supply 13 (e.g., a mains power supply). For each of the UPS 2a to 2e, the input 3 of each of the UPS 2a to 2e is also connected to the input of the AC / DC converter 6, and the DC bus 7 is connected between the output of the AC / DC converter 6 and the input of the DC / AC inverter 8. The UPS 2a to 2e are connected together in parallel so that the output 11 of each of the UPS 2a to 2e is connected to the load 14 through the main output 12.

[0067] In each of the UPS 2a to 2e, a bypass switch 10 is connected between the input 3 and the output 11 via a bypass line 5. The bypass switch 10 may include two silicon controlled rectifiers (eg, thyristors) or other types of suitable switches (eg, transistors).

[0068] like Figure 1 As shown, in this embodiment, the DC bus 7 is connected to a battery 9 via a DC-DC converter.

[0069] Each of the UPS 2a to UPS 2e further includes a general controller (not shown) and a bypass controller 20, both of which are configured to monitor the input power of its own UPS 2a to UPS 2e provided by the input power supply 13 in different situations, and the monitoring of the input power is achieved by monitoring the input current. Based on the presence and quality of the input power provided by the input power supply 13 to each of the UPS 2a to UPS 2e, the bypass controller 20 is configured to operate its UPS 2a to UPS 2e in different operating modes.

[0070] In response to determining that the AC power provided by the input power supply 13 is below or above a desired level (eg, in a drooping or ramping state), the universal controller of each of the UPS 2a to 2e operates its UPS 2a to 2e into an "online" operating mode.

[0071] In the "online" operating mode, for each of the UPS 2a to UPS 2e, the universal controller controls the closing of the reset relay (relais de rappel) in its UPS 2a to UPS 2e (thereby connecting the input power supply 13 to the input 3 of the UPS of the universal controller), and disconnects the bypass switch 10 of each of the UPS 2a to UPS 2e. The AC / DC converter 6 of each of the UPS 2a to UPS 2e receives AC power from the input power supply 13, and is operated by the universal controller to convert the received AC power into DC power and provide the DC power to the DC / AC inverter 8 via the DC bus 7. The DC / AC inverter 8 of each of the UPS 2a to UPS 2e is controlled by the universal controller to convert the received DC power into the required AC power and provide the required AC power to the output 11.

[0072] Still in the "online" operating mode, the DC-DC converter connected to the DC bus 7 can be supplied with the DC power on the DC bus 7 of each of the UPS 2a to UPS 2e. The DC-DC converter converts the DC power received from the DC bus into the DC power of the desired load level, and provides the DC power of the desired load level to the corresponding battery 9 to charge the battery 9.

[0073] In response to a failure of the mains power provided by the input power supply 13 (eg in the event of a voltage drop or power failure), the common controller of each of the UPSs 2a to 2e controls its UPS to switch to an operating mode referred to as a "battery" operating mode.

[0074] In the "battery" operating mode, the universal controller of each of the UPS 2a to UPS 2e controls the reset relay in its UPS to open (thereby isolating the power supply 13 from the input 3) and closes the bypass switch 10 of its UPS. The DC power of the battery 9 connected to the DC bus 7 is provided to the DC / AC inverter 8, and the universal controller controls the DC / AC inverter 8 to convert the received DC power into the desired AC power provided to the output 11.

[0075] In response to determining that the AC power provided by the input power supply 13 is at a desired level, the bypass controller 20 of each of the UPS 2a to 2e controls its UPS to switch to an operating mode referred to as a "bypass" operating mode.

[0076] In the "bypass" operation mode, the bypass controller 20 of each of the UPS 2a to UPS 2e controls the closing of the reset relay (thereby connecting the input power supply 13 to the input 3), and closes the bypass switch 10 of its UPS. Therefore, in the "bypass" operation mode, the input line 3 (connected to the input power supply 13) of each of the UPS 2a to UPS 2e is directly connected to the output line 11 of the corresponding UPS 2a to UPS 2e via the bypass line 5. In the "bypass" operation mode, the AC power on the input line 3 of each of the UPS 2a to UPS 2e received from the input power supply 13 is directly provided to the output 11.

[0077] Figure 2 Schematically shows Figure 1 A bypass controller 20 of one of the UPS 2a to 2e of the UPS system 1.

[0078] The bypass controller 20 includes a first block 22, a second block 24, and a current sensor 26 for determining a current signal flowing in the UPS 2a to 2e.

[0079] The first block 22 operates at a first frequency of about 50 Hz and is responsible for managing logic and calculating the delay angle. The second block 24 operates at a second frequency of about 5 kHz and is responsible for controlling the bypass switch based on the delay angle calculated by the first block 22 .

[0080] The first block 22 includes a first unit 220, a second unit 224, and a third unit 226. Therefore, with the UPS in the "bypass" mode, the first unit 220 monitors the current signal flowing in the bypass line 5 of its UPS and determines the effective value of the current intensity. In this example, the second unit 224 collects the RMS value of its UPS (e.g., UPS2a) and other UPS (i.e., UPS 2b to UPS 2e) from the UPS system 1 with the bypass switch 10 closed. Then, the second unit 224 sets the RMS reference value corresponding to the minimum RMS value collected. The third unit 226 is configured to determine the activation delay of the bypass switch 10 for closing its UPS (UPS 2a in this example).

[0081] The first unit 220 of the bypass controller 20 includes an AC / DC current converter 221 for sampling the monitored current signal through the bypass resistor 26. The first unit 220 also includes an RMS calculation module 222 for determining the RMS value of the current sample delivered by the AC / DC current converter 221.

[0082] The second unit 224 of the bypass controller 20 includes a real-time communication module, which receives the RMS value determined by the RMS calculation module 222 of the same first block 22 (i.e., UPS2a in this example) including the second unit 224 and the RMS value determined by the RMS calculation module of the first block of other UPS (UPS2b to UPS 2e) in this example.

[0083] The second unit 224 determines the minimum RMS value among all different RMS values ​​of the UPS whose previous delay angle is equal to zero in the previous step, and sets the minimum value as the RMS reference value whose delay angle is set to zero.

[0084] The RMS calculation module 222 also transmits the calculated RMS value to the controllers of all other UPSs of the UPS system 1 .

[0085] The third unit 226 of the bypass controller 20 includes a comparator 227 and a delay module 228, wherein the comparator 227 receives the RMS value of the current signal of its UPS determined by the RMS calculation unit 222 of the monitoring unit 220 and the RMS reference value determined by the second unit 224, and the delay module 228 is connected to the output of the comparator 227 and includes an integral proportional corrector to have a zero steady-state error.

[0086] The comparator 227 calculates the difference between the effective value of the current signal of the UPS and the reference effective value, and then transmits the result to the delay module 228 .

[0087] If the difference calculated by the comparator 227 is zero, the delay module 228 sets the activation delay to zero, which then corresponds to a command to keep the bypass switch 10 closed.

[0088] If the difference calculated by the comparator 227 is not zero, the delay module 228 calculates an activation delay corresponding to the delay time for the bypass switch 10 of its UPS to be closed, and the bypass switch 10 is controlled to be opened between the moment when the current signal crosses zero and the end of the activation delay.

[0089] In this embodiment, the activation delay determined by the delay module 228 of the third unit 226 is a delay angle relative to the phase angle, and the third unit 226 is a saturation module 229. The saturation module 229 limits the delay angle of the activation delay sent by the delay module 228 to between a minimum delay angle of 0° and a maximum delay angle according to the number of UPS in the UPS system 1, so as to limit the maximum peak value of the current signal that will flow through the UPS once the delay angle is applied.

[0090] The second block 24 comprises an extraction unit 240 which receives the current sample delivered by the AC / DC current converter 221 of the first unit 220 of the first block 22 and a phase-locked loop (PLL) unit 242 which is connected to the output of the extraction unit.

[0091] The extraction unit 240 extracts fundamental harmonics from the current signal and transmits the fundamental harmonics to the PLL unit 242 , which determines the phase angle of the current signal and detects zero crossing of the current signal.

[0092] The second block 24 further includes a comparator 244 and a control unit 246 .

[0093] The comparator 244 receives the delay angle determined by the third unit 226 of the first block 22 and the phase angle of the current signal determined by the PLL 242 of the second block 24. The comparator 244 compares the phase angle of the current signal detected by the PLL 242 with the activation delay determined by the third unit 226, and sends the result to the control unit 246 of the second block 24.

[0094] The control unit 246 controls the UPS bypass switch 10 based on the comparison. If the activation delay is zero, the bypass switch 10 remains closed and opens from the moment the UPS current signal crosses zero until the end of the period corresponding to the activation delay when the bypass switch 10 is closed again.

[0095] Therefore, there is always at least one closed bypass switch to ensure a direct path between the main input and the main output of the UPS system.

[0096] Combine the following Figure 2 and Figure 3 Switching the UPS to "Bypass" mode is described in more detail.

[0097] Figure 3 is a flow chart illustrating a process of an embodiment of operation of the parallel UPS system 1 in "bypass" mode.

[0098] In step 302, the bypass controller 20 indicates that the UPS to which the bypass controller 20 belongs should operate in the "bypass" operating mode.

[0099] In step 304, in response to determining that the bypass controller 20 operates in the "bypass" operation mode, the bypass controller 20 controls the bypass switch 10 of its UPS to close, thereby connecting the input 3 of its UPS to its output 10. After closing the bypass switch 10 of each UPS, each of the UPS 2a to UPS 2e provides unregulated output power (directly provided from the input 3) to the load 14 via its bypass line 5. More specifically, once the bypass switch 10 of each of the UPS 2a to UPS 2e is closed, each of the UPS 2a to UPS 2e provides an output current having a current waveform to the load 14.

[0100] In step 306, the first unit 220 of the first block 22 of the bypass controller 20 of each of the UPS 2a to 2e monitors the output current of its UPS.

[0101] Then, in step 308, the second unit 224 of the first block 22 of each bypass controller 20 determines the minimum output current (ie, the minimum value of the root mean square (RMS) value) of all UPS 2a to 2e and defines it as an RMS reference value.

[0102] In step 310 , the comparator 227 of the third unit 226 of the first block 22 of each bypass controller 20 compares its RMS value with the RMS reference value.

[0103] If the RMS value of the UPS of the bypass controller 20 of the UPS is equal to the RMS reference value, then in step 312, the second unit 226 of the first block 22 of the bypass controller 20 of the UPS sets the activation delay to zero and keeps the bypass switch 10 of the UPS closed even at the zero-crossing moment of the output current waveform. In this case, the current flowing through the bypass switch 10 of the UPS is a complete sinusoidal waveform, and the load 14 connected to the output 11 of the UPS continuously receives the complete sinusoidal waveform of the UPS.

[0104] If the RMS value of its UPS is greater than the RMS reference value, then in step 314, the bypass controller 20 of the UPS opens the bypass switch 10 of its UPS and performs a regulation process on the second unit 226 of the first block 22 of its controller to determine the activation delay at which the bypass switch 10 will close.

[0105] In step 316, the bypass controller 20 of the UPS with the bypass switch opened eventually closes the bypass switch 10 at the end of the activation delay.

[0106] For example, according to at least one embodiment, the bypass controller 20 of a UPS having an effective value greater than an RMS reference value controls the bypass switch 10 of the UPS to be opened within a delay period when its output current waveform passes through zero.

[0107] In other words, the bypass controller 20 of each UPS (i.e., the UPS without the minimum output current) having an RMS value greater than the RMS reference value operates the bypass switch 10 of its UPS so that the bypass switch 10 is disconnected within the delay time of the output current waveform of the UPS crossing zero. When the bypass switch 10 is disconnected at the zero crossing, the current cannot flow through the bypass switch 10. After the delay period, the bypass controller 20 closes the bypass switch 10, the current flows through the bypass switch 10, and the output current waveform is provided from the corresponding UPS to the load 14.

[0108] When the output current of each UPS passes through zero, at least one UPS (the UPS with the smallest RMS value) supplies power to the load 14. The load 14 is gradually transferred from one UPS to another UPS (i.e., after a corresponding delay has passed after the output current waveform of each other UPS passes through zero). Therefore, the load 14 is gradually distributed among the different UPS. Each bypass controller 20 controls this "switching" of the bypass line 5 of its UPS (which is connected in parallel with the bypass line 5 of the UPS that remains closed (after each zero crossing)) so that the energy provided to the load 14 is evenly distributed among UPS2a to UPS2e.

[0109] More specifically, each bypass controller 20 configures the activation delay of its UPS 2a to UPS 2e so that the root mean square (RMS) of the output current of its UPS is substantially the same as the RMS reference value. Although there may be differences in the components and wiring of each of the UPS 2a to UPS 2e, by actively adjusting the activation delay of each UPS whose RMS value is different from the RMS reference value, the root mean square (RMS) of the output current of each of the UPS2a to UPS2e can be configured in almost the same manner. When the effective value of the output current of each of the UPS 2a to UPS 2e (i.e., the load sharing portion of each UPS) is substantially the same, the difference between the load sharing portions of each of the UPS 2a to UPS2e is relatively small (e.g., approximately 0.1%), and more than 8 UPSs can be successfully connected in parallel (i.e., equal load sharing).

[0110] By providing a complete output current waveform to the load 14 via the UPS with the smallest RMS value, and actively introducing a delay period into the output current waveform of each UPS to control the effective value of the output current of each of the UPS 2a to UPS 2e to be substantially the same, an equal distribution of loads can be provided between different UPSs of UPS 2a to UPS 2e, and overloading of UPS 2a to UPS 2e can be prevented. In addition, the power flowing through the UPSs having an RMS value greater than the RMS reference value can be reduced because each of these UPSs provides an output current waveform of less than a complete cycle to the load 14. The reduction in the power flowing through these UPSs can result in a reduction in the amount of heat generated in each of these UPSs, and can also reduce the impact on the circuit breakers in each of these UPSs.

[0111] According to one embodiment, the bypass controller 20 is configured to analyze and update the load distribution between different UPS 2a to 2e each time the UPS system 1 switches to the “bypass” mode.

[0112] As described above, the systems and methods enabling equal load sharing are used with parallel UPS systems (including online UPSs). However, in other embodiments, the systems and methods described herein can be used with other types of parallel-connected UPSs or power systems to provide equal load distribution with unregulated power output.

[0113] As described above, the present invention provides a system for providing equal load sharing between UPSs connected in parallel and operating in bypass mode. The system does not require adjusting the length of the cable or adding expensive inductors, and can include more than four UPSs connected in parallel. Although there may be differences in the components and wiring of each UPS, by providing a complete output current waveform to the load through a UPS that continuously operates in bypass mode, and actively introducing a delay period into the output current waveform of each of the other UPSs connected in parallel, so that each UPS has a relatively equal RMS output current value, it is possible to ensure equal load sharing between the UPSs. Since the output current of each UPS (i.e., the load sharing portion of each UPS) is substantially the same, the difference between the load sharing portions of each UPS is relatively small (e.g., approximately 0.1%), and more than four UPSs can be successfully connected in parallel, while reducing many risks associated with load sharing between UPSs connected in parallel and operating in bypass mode.

[0114] Thus, having described several aspects of at least one embodiment of the present invention, it will be appreciated that various modifications, changes and improvements will be apparent to those skilled in the art. Such substitutions, modifications and improvements are intended to form a part of this specification and are intended to fall within the spirit and scope of the present invention. Therefore, the above description and accompanying drawings are given as examples only.

Claims

1. An uninterruptible power supply system comprising a plurality of uninterruptible power supplies connected in parallel between a main input and a main output, the main input being configured to be connected to a power supply and receive input power, the main output being configured to be connected to a load and provide output power derived from at least the input power to the load, each uninterruptible power supply comprising: - input, which is connected to the main input, - output, which is connected to the main output, and a bypass line connected between the input and the output via a bypass switch, the bypass switch being configured to be closed in a first operating mode, thereby connecting the input to the output via the bypass line, and to be opened in a second operating mode, thereby disconnecting the input from the output, Characterized in that each uninterruptible power supply of the uninterruptible power supply system includes a control unit, each of the control units communicates with the control units of all other uninterruptible power supplies of the uninterruptible power supply system, and the control unit of each uninterruptible power supply includes: - a first block, comprising a first unit, a second unit and a third unit, the first unit being configured to monitor a current signal flowing in a bypass line of an uninterruptible power supply of the first unit and determine a root mean square value (RMS), i.e., an effective value, of a current intensity of the current signal, the second unit being configured to collect the RMS value of the uninterruptible power supply of the second unit and the RMS values ​​of other uninterruptible power supplies in an uninterruptible power supply system whose bypass switches are closed, and to set an RMS reference value corresponding to a minimum RMS value collected, the third unit being configured to determine an activation delay for closing the bypass switch of the uninterruptible power supply of the third unit, if the RMS value of the uninterruptible power supply of the third unit is equal to the RMS reference value, the activation delay is zero, otherwise, the activation delay is not zero, and - A second block, comprising a fourth unit, the fourth unit being configured to control a bypass switch of the uninterruptible power supply of the fourth unit according to the activation delay sent by the third unit, if the activation delay is zero, the bypass switch remains closed, otherwise, the bypass switch is opened for a specific time corresponding to the activation delay and then closed.

2. The uninterruptible power supply system according to claim 1, wherein: The first block can be configured to operate at a first frequency, and the second block can be configured to operate at a second frequency higher than the first frequency, the ratio of the second frequency to the first frequency being proportional to the accuracy of the control required.

3. The uninterruptible power supply system according to claim 1, wherein: The first unit of each control unit includes an AC-DC converter and an RMS calculation module, wherein the AC-DC converter is used to sample the monitored current signal, and the RMS calculation module is used to determine the RMS value of the current sample.

4. The uninterruptible power supply system according to claim 1, wherein: The second unit of each control unit comprises a real-time communication module.

5. The uninterruptible power supply system according to claim 1, wherein: The third unit of each control unit comprises a comparator and a delay unit, wherein the comparator calculates the difference between the RMS value of the current signal of the uninterruptible power supply of the comparator and the RMS reference value, and the delay unit is configured to calculate the activation delay, after which the bypass switch of the uninterruptible power supply can be closed.

6. The uninterruptible power supply system according to claim 5, wherein: The delay unit includes a proportional-integral corrector to have a zero steady-state error.

7. The uninterruptible power supply system according to claim 1, wherein: The activation delay determined by the third unit is a delay angle relative to the phase angle, and the third unit further includes a saturation module, which is configured to limit the delay angle of the activation delay to between a minimum delay angle of 0° and a maximum delay angle according to the number of uninterruptible power supplies of the uninterruptible power supply system.

8. The uninterruptible power supply system according to claim 1, wherein: The second block further includes an extraction unit, a phase-locked loop (PLL), and a comparator, wherein the extraction unit is configured to extract a fundamental harmonic from the current signal, the phase-locked loop (PLL) is configured to determine a phase angle of the current signal and detect a zero crossing of the current signal, the comparator compares the phase angle of the current signal detected by the phase-locked loop with the activation delay determined by the third unit of the first block, and the fourth unit of the second block controls the bypass switch according to the comparison.

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