Apparatus and method for flux management in an impedance isolated single conversion (ZISC) based UPS system

Abstract

Embodiments of the present disclosure relate to an apparatus and method for flux management in a ZISC-based UPS system. The apparatus includes a first determining unit configured to determine a first offset flux on a series reactor in the ZISC-based UPS system when a power quality event occurs in a power grid connected to the ZISC-based UPS system and the ZISC-based UPS system is operating in a grid-tied mode, and a first resetting unit configured to provide synthetic impedance damping to the series reactor to reset the first offset flux.

Description

Technical Field

[0001] Embodiments of the present disclosure relate generally to the field of power converter systems, and more particularly to apparatus and methods for flux management in a ZISC-based uninterruptible power supply (UPS) system. Background Art

[0002] Electrical or electronic equipment used in all types of industrial processes are susceptible to power quality events in the power grid. The power quality of the power grid is related to grid voltage variations in the form of momentary interruptions, voltage sags or swells, transients, harmonic distortion, electrical noise and flickering lights. In order to mitigate the impact of power quality events and reduce the overall cost of customers' continuous process applications to improve productivity, efficiency and safety, one of the most common solutions is the uninterruptible power supply (UPS). Different UPS systems have been developed for different customers. For example, the PCS100UPS for industrial customers is an offline single conversion UPS system that consists of a static switch, a power conversion system and an energy storage system. Recently, a new UPS system architecture has been developed, called a ZISC-based UPS system. The ZISC-based UPS system is a combination of components of an offline single conversion UPS and a rotary UPS system.

[0003] In steady-state grid-connected mode, when the grid is healthy, a ZISC-based UPS system regulates its output voltage by exchanging power with the grid to compensate for the voltage drop across the series reactor. In one scenario, when a voltage sag occurs while the ZISC-based UPS system is operating in grid-connected mode, the increased voltage across the series reactor can cause a flux offset. This flux offset across the series reactor can lead to magnetic saturation of the series reactor, resulting in very high currents in the power conversion system and significant distortion in the output voltage of the ZISC-based UPS system. In another scenario, when the ZISC-based UPS system is operating in grid-connected or islanded mode, if the network feeder with a downstream transformer is energized, this can cause high inrush currents to flow, potentially disrupting the flux of the coupling transformer. In both cases, this can lead to cross-inductance saturation between the coupling transformer and the downstream transformer, resulting in output voltage distortion. Consequently, the reliability of the ZISC-based UPS system in protecting customer loads can be adversely affected.

[0004] Therefore, in the above situation, an offset flux reset mechanism is needed to improve the reliability and robustness of the ZISC-based UPS system. Summary of the Invention

[0005] In view of the above problems, various exemplary embodiments of the present disclosure provide an apparatus and method for flux management in a ZISC-based UPS system to avoid magnetic saturation in the ZISC-based UPS system.

[0006] In a first aspect of the present disclosure, an exemplary embodiment of the present disclosure provides an apparatus for flux management in a ZISC-based UPS system. The apparatus includes a first determining unit configured to determine a first offset flux on a series reactor in the ZISC-based UPS system when a power quality event occurs in a power grid connected to the ZISC-based UPS system and the ZISC-based UPS system is operating in a grid-tied mode, and a first resetting unit configured to provide synthetic impedance damping to the series reactor to reset the first offset flux.

[0007] In some embodiments, the apparatus further includes a second determining unit and a second resetting unit, the second determining unit being configured to determine a second offset flux on a transformer in the ZISC-based UPS system when the ZISC-based UPS system operates in a grid-connected mode or an islanded mode, the second resetting unit being configured to reset the second offset flux in response to a magnitude of the second offset flux exceeding a flux magnitude limit.

[0008] In some embodiments, the transformer is a coupling transformer or a downstream transformer.

[0009] In some embodiments, the second determination unit includes: a voltage observer configured to determine a voltage across the transformer; and a flux estimator configured to estimate a second offset flux based on the determined voltage across the transformer and an inherent time constant term specific to the transformer.

[0010] In some embodiments, the voltage observer is further configured to determine the voltage across the transformer based on a voltage at terminals of a power conversion system in the ZISC-based UPS system or an output voltage of the ZISC-based UPS system.

[0011] In some embodiments, the second reset unit includes: a flux magnitude extractor configured to extract the magnitude of the second offset flux; a comparator configured to compare the magnitude of the second offset flux with a flux magnitude limit; a multiplier configured to multiply the second offset flux by a correction factor to provide a compensation voltage in response to the magnitude of the second offset flux exceeding the flux magnitude limit; and a subtractor configured to subtract the compensation voltage from the target voltage reference to obtain a corrected voltage reference.

[0012] In a second aspect of the present disclosure, an example embodiment of the present disclosure provides a method for flux management in a ZISC-based UPS system. The method includes: when a power quality event occurs in a grid connected to the ZISC-based UPS system and the ZISC-based UPS system is operating in a grid-tied mode, determining a first offset flux on a series reactor in the ZISC-based UPS system; and providing synthetic impedance damping to the series reactor to reset the first offset flux.

[0013] In some embodiments, the method further includes: determining a second offset flux on a transformer in the ZISC-based UPS system when the ZISC-based UPS system operates in a grid-connected mode or an islanded mode; and resetting the second offset flux in response to a magnitude of the second offset flux exceeding a flux magnitude limit.

[0014] In some embodiments, the transformer is a coupling transformer or a downstream transformer.

[0015] In some embodiments, determining the second offset flux includes: determining a voltage across the transformer; and estimating the second offset flux based on the determined voltage across the transformer and an intrinsic time constant term specific to the transformer.

[0016] In some embodiments, determining the voltage across the transformer includes determining the voltage across the transformer based on a voltage at terminals of a power conversion system in the ZISC-based UPS system or an output voltage of the ZISC-based UPS system.

[0017] In some embodiments, resetting the second offset flux in response to the magnitude of the second offset flux exceeding the flux magnitude limit includes: extracting the magnitude of the second offset flux; comparing the magnitude of the second offset flux to the flux magnitude limit; multiplying the second offset flux by a correction factor in response to the magnitude of the second offset flux exceeding the flux magnitude limit to provide a compensation voltage; and subtracting the compensation voltage from the target voltage reference to obtain a corrected voltage reference.

[0018] It should be understood that the "Summary of the Invention" section is not intended to identify the key or essential features of the embodiments of the present disclosure, nor is it intended to limit the scope of the present disclosure. Other features of the present disclosure will become readily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and advantages of the exemplary embodiments disclosed herein will become more readily understood through the following detailed description with reference to the accompanying drawings, in which several exemplary embodiments disclosed herein are illustrated by way of example and not limitation, in which:

[0020] Figure 1is a schematic diagram illustrating a ZISC-based UPS system operating in a grid-connected mode according to an embodiment of the present disclosure;

[0021] Figure 2 is a schematic diagram illustrating an apparatus for flux management in a ZISC-based UPS system according to an embodiment of the present disclosure;

[0022] Figure 3 is a schematic diagram illustrating a second determining unit and a second resetting unit according to an embodiment of the present disclosure;

[0023] Figure 4 is a flow chart illustrating a method for controlling a first offset flux on a series reactor in a ZISC-based UPS system according to an embodiment of the present disclosure; and

[0024] Figure 5 is a flow chart illustrating a method for controlling a second offset flux on a transformer in a ZISC-based UPS system according to an embodiment of the present disclosure.

[0025] Throughout the drawings, the same or similar reference numbers are used to refer to the same or similar elements. DETAILED DESCRIPTION

[0026] The principles of the present disclosure will now be described with reference to several example embodiments shown in the accompanying drawings. Although example embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the description of these embodiments is only for the purpose of facilitating a better understanding by those skilled in the art to implement the present disclosure, and is not intended to limit the scope of the present disclosure in any way.

[0027] The terms "comprises" or "includes" and variations thereof are to be understood as open terms meaning "including but not limited to". Unless the context clearly indicates otherwise, the term "or" should be understood as "and / or". The term "based on" should be understood as "based at least in part on". The term "operable to" means that a function, action, movement or state can be achieved through an operation induced by a user or an external mechanism. The terms "one embodiment" and "embodiment" should be understood as "at least one embodiment". The term "another embodiment" should be understood as "at least one other embodiment". The terms "first", "second", etc. may refer to different or the same objects. Additional definitions (explicit and implicit) may be included below. Unless the context clearly indicates otherwise, the definitions of terms are consistent throughout the description.

[0028] Figure 1is a schematic diagram illustrating a ZISC-based UPS system operating in a grid-connected mode according to an embodiment of the present disclosure. The ZISC-based UPS system is electrically coupled to the grid 102 via an optional delta-wye transformer 122. The ZISC-based UPS system includes an input coupled to the grid 102 or the delta-wye transformer 122 and an output coupled to a load 124. An input voltage Vin and an input current Iin can be provided to the UPS system from the grid 102. An output voltage Vout and an output current Iout can be provided from the UPS system to the load 124. Furthermore, the ZISC-based UPS system includes a network feeder with a downstream transformer 141.

[0029] like Figure 1 As shown, the ZISC-based UPS system includes an input switch 104 electrically coupled to an input of the ZISC-based UPS system, an output switch 106 electrically coupled to an output of the ZISC-based UPS system, and a bypass switch 108 coupled between the input and output of the ZISC-based UPS system.

[0030] The ZISC-based UPS system further includes a series inductor 120, a coupling transformer 140, a power conversion system 126, and an energy storage device 128. The series inductor 120 is electrically coupled between the input switch 104 and the output switch 106. The coupling transformer 140 is electrically coupled to a node between the series inductor 120 and the output switch 106. The power conversion system 126 is electrically coupled to the coupling transformer 140. The energy storage device 128 is electrically coupled to the power conversion system 126.

[0031] The ZISC-based UPS system also includes a system controller 160 coupled to the input switch 104, the output switch 106, and the bypass switch 108. The system controller 160 is configured to regulate the exchange of power between the UPS system and the grid 102 by operating the switches 104, 106, and 108. The ZISC-based UPS system can operate in a grid-connected mode or an islanded mode.

[0032] During grid-connected mode, input switch 104 and output switch 106 are closed and bypass switch 108 is open. The UPS system controls the output voltage magnitude and frequency by regulating the power exchange between the UPS system and grid 102. The steady-state output voltage is controlled to achieve a high-fidelity voltage source for connected load 124.

[0033] If the voltage sag is significant in magnitude and duration, the UPS system transitions to island mode. In island mode, the input switch 104 and the bypass switch 108 are open, and the output switch 106 is closed. Thus, the UPS system operates as a grid-forming converter system by controlling the output voltage Vout to have a given voltage magnitude and frequency reference.

[0034] During a voltage sag event, when the UPS system is operating in grid-tied mode, the incremental voltage across the series inductor 120 results in a first offset flux across the series inductor 120. The extent of the first offset flux is a function of the system's damping characteristics. During a voltage sag event, the peak flux across the series inductor 120 should not exceed a saturation limit. To reset the first offset flux across the series inductor 120, the system controller 160 may provide a synthetic impedance damping to reset the first offset flux across the series inductor 120 to zero.

[0035] Figure 2 FIG. 1 is a schematic diagram illustrating an apparatus for flux management in a ZISC-based UPS system according to an embodiment of the present disclosure. The apparatus may be embodied as the system controller 160 of the UPS system or another controller of the UPS system. Figure 2 As shown, system controller 160 includes a first determining unit 161 and a first resetting unit 162. When a power quality event occurs in grid 102 and the ZISC-based UPS system operates in grid-connected mode, first determining unit 161 determines a first offset flux across series reactor 120. Here, first determining unit 161 may only determine the occurrence of the first offset flux across series reactor 120, without determining the value of the first offset flux. First resetting unit 162 then provides synthetic impedance damping to series reactor 120 to reset the first offset flux.

[0036] During the process of resetting the first offset flux across series reactor 120 to zero, the offset flux of coupling transformer 140 migrates away from its center. As a result, when the peak flux of coupling transformer 140 exceeds its saturation level, it causes current limiting in power conversion system 126. Transformer saturation also occurs during other grid or load events, such as grid phase shift events, large load steps (e.g., motor startup), parallel operation, and collapse recovery during a fault.

[0037] One solution to minimize the saturation behavior of the coupling transformer 140 is to increase the flux margin. This solution will add additional iron in the coupling transformer 140, which increases the cost of the entire UPS system. In addition, increasing the flux margin of the coupling transformer 140 is not a solution because the downstream transformers 141 (distribution transformers) have the same behavior and their design is based on customer power system requirements (beyond the scope of the UPS manufacturer). A second solution is to minimize the UPS output voltage magnitude reference during upstream voltage sags to increase the flux margin of the coupling transformer 140. The reduction in UPS output voltage magnitude during upstream power quality events is driven by UPS system performance standards. To implement this solution, one of the key challenges is to determine the power quality event in a very short time. A third solution is to disconnect the input switch 104 faster to prevent magnetic saturation of the coupling transformer 140. However, the solutions available on the open market are very limited.

[0038] When magnetic saturation of the coupling transformer 140 occurs due to the offset flux of the coupling transformer 140 migrating from its center, the UPS output voltage Vout may be distorted and further cause a mutual inductive saturation phenomenon. Figure 2 As shown, to avoid the mutual inductive saturation phenomenon, the system controller 160 further includes a second determining unit 163 and a second resetting unit 164. When the ZISC-based UPS system operates in the grid-connected mode or the island mode, the second determining unit 163 determines the second offset flux on the coupling transformer 140. The second resetting unit 164 resets the second offset flux in response to the magnitude of the second offset flux exceeding the flux magnitude limit.

[0039] A cost-effective, robust, and reliable solution for avoiding magnetic saturation is obtained by controlling the core flux of the coupling transformer 140 based on the voltage across the coupling transformer 140 .

[0040] Figure 3 1 is a schematic diagram illustrating the second determining unit 163 and the second resetting unit 164 according to an embodiment of the present disclosure. Figure 3 As shown, the system controller 160 considers the measured or estimated voltage Vmeas across the coupling transformer 140 and estimates a second offset flux across the coupling transformer 140. The second determination unit 163 includes a voltage observer 1631 and a flux estimator 1632.

[0041] Voltage observer 1631 is adapted to determine the voltage across coupling transformer 140 based on a measured or estimated voltage Vmeas. Voltage Vmeas can be the terminal voltage of power conversion system 126 or the output voltage Vout of a ZISC-based UPS system, depending on the operating mode of the UPS system and transformer flux optimization. For example, during startup mode, voltage observer 1631 can select an internal voltage estimate when output switch 106 is open. Voltage observer 1631 estimates the terminal voltage of power conversion system 126 using the measured output voltage Vout of the UPS system and the estimated voltage drop across coupling transformer 140. The choice of the terminal voltage of power conversion system 126 or the UPS output voltage Vout depends on the flux model's better flux estimate.

[0042] Flux estimator 1632 is adapted to estimate a second offset flux based on the voltage determined across coupling transformer 140 and an intrinsic time constant term specific to transformer 140. For example, the resolver flux vector φ(t) can be estimated based on first principles, including time integration of the observer voltage assuming the flux starts at a known value (e.g., zero or other value). The resolver flux vector φ(t) serves as the second offset flux across coupling transformer 140.

[0043] like Figure 3 As shown, the second reset unit 164 includes a flux magnitude extractor 1641 adapted to extract the magnitude of the second offset flux from the vector components of the second offset flux. The transformer flux magnitude limit is defined based on a specified or nominal rated flux value of the coupling transformer 140, where the nominal flux vector magnitude generally corresponds to a nominal or peak operating voltage. In a flux magnitude comparator 1643, the magnitude of the second offset flux is compared to the specified flux magnitude limit, and if the magnitude of the second offset flux exceeds the flux magnitude limit, a correction factor (e.g., gain K, referenced as 1644) based on the excess flux magnitude is provided to a multiplier 1645. In the multiplier 1645, in response to the magnitude of the second offset flux exceeding the flux magnitude limit, the second offset flux is multiplied by the correction factor to provide a compensation voltage. The compensation voltage is then subtracted from the target voltage reference Vref' in a subtractor 1646 to obtain a corrected voltage reference Vref. The corrected reference voltage Vref is used to generate a corresponding pulse width modulated (PWM) signal.

[0044] It should be understood that magnetic saturation of the downstream transformer 141 can be eliminated in a similar manner. The proposed concept also improves system output voltage performance during energization of the downstream transformer 141. The proposed concept effectively manages saturation, and better results can be achieved by increasing the flux margin of the UPS transformer (by increasing the transformer flux margin or minimizing the UPS output voltage magnitude reference, as described above) because it sees more voltage during upstream voltage sags.

[0045] For a three-wire, three-phase system, the voltage compensation and flux level terms can be expressed in a stationary or rotating reference frame, or individually in the time domain using phase quantities or other non-orthogonal axes.

[0046] It should be understood that the proposed ideas are applicable to other flux management supporting parallel grid voltage source systems.

[0047] The exemplary embodiment of the present disclosure also provides a method for managing flux in a UPS system based on ZISC. The method can be referred to above. Figure 1-Figure 3 The described device is implemented.

[0048] Figure 4 FIG. 1 is a flow chart illustrating a method for controlling a first offset flux on a series inductor according to an embodiment of the present disclosure. Figure 4 As shown, the method 400 includes: at 402, when a power quality event occurs in a grid connected to a ZISC-based UPS system and the ZISC-based UPS system operates in a grid-tied mode, determining a first offset flux on a series reactor in the ZISC-based UPS system; and at 404, providing synthetic impedance damping to the series reactor to reset the first offset flux.

[0049] Figure 5 FIG. 1 is a flow chart illustrating a method for controlling a second offset flux on a transformer in a ZISC-based UPS system according to an embodiment of the present disclosure. Figure 5 As shown, the method 500 further includes: at 502, when the ZISC-based UPS system operates in a grid-connected mode or an islanded mode, determining a second offset flux on a transformer in the ZISC-based UPS system; and at 504, resetting the second offset flux in response to a magnitude of the second offset flux exceeding a flux magnitude limit.

[0050] In some embodiments, the transformer is a coupling transformer or a downstream transformer.

[0051] In some embodiments, determining the second offset flux includes: determining a voltage across the transformer; and estimating the second offset flux based on the determined voltage across the transformer and an intrinsic time constant term specific to the transformer.

[0052] In some embodiments, determining the voltage across the transformer includes determining the voltage across the transformer based on a voltage at terminals of a power conversion system in the ZISC-based UPS system or an output voltage of the ZISC-based UPS system.

[0053] In some embodiments, resetting the second offset flux in response to the magnitude of the second offset flux exceeding the flux magnitude limit includes: extracting the magnitude of the second offset flux; comparing the magnitude of the second offset flux to the flux magnitude limit; multiplying the second offset flux by a correction factor in response to the magnitude of the second offset flux exceeding the flux magnitude limit to provide a compensation voltage; and subtracting the compensation voltage from the target voltage reference to obtain a corrected voltage reference.

[0054] Although several inventive embodiments have been described and illustrated herein, a person of ordinary skill in the art will readily envision various other devices and / or structures for performing the functions and / or obtaining the results and / or one or more of the advantages described herein, and each of these variations and / or modifications is considered to be within the scope of the embodiments of the present invention described herein. More generally, those skilled in the art will readily understand that all parameters, dimensions, materials, and configurations described herein are exemplary, and that actual parameters, dimensions, materials, and / or configurations will depend on one or more specific applications in which the present invention is used. Those skilled in the art will recognize or be able to determine many equivalents to the specific inventive embodiments described herein using only routine experimentation. Therefore, it should be understood that the foregoing embodiments are presented only as examples, and within the scope of the appended claims and their equivalents, embodiments of the present invention may be practiced in a manner different from that specifically described and claimed. The inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and / or methods (if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent) is included within the inventive scope of the present disclosure.

Claims

1. A device for flux management in an uninterruptible power supply (UPS) system based on an impedance-isolated single-conversion (ZISC), the device comprising: a first determining unit configured to determine a first offset flux on a series reactor in the ZISC-based UPS system when a power quality event occurs in a power grid connected to the ZISC-based UPS system and the ZISC-based UPS system operates in a grid-connected mode; as well as A first resetting unit is configured to provide synthetic impedance damping to the series inductor to reset the first offset flux.

2. The apparatus according to claim 1, further comprising: a second determining unit configured to determine a second offset flux on a transformer in the ZISC-based UPS system when the ZISC-based UPS system operates in the grid-connected mode or the islanded mode; as well as The second resetting unit is configured to reset the second offset flux in response to a magnitude of the second offset flux exceeding a flux magnitude limit. The apparatus according to claim 2 , wherein the transformer is a coupling transformer or a downstream transformer.

4. The apparatus according to claim 2 or 3, wherein the second determining unit comprises: a voltage observer configured to determine a voltage across the transformer; as well as A flux estimator is configured to estimate the second offset flux based on the determined voltage across the transformer and an intrinsic time constant term specific to the transformer.

5. The apparatus according to claim 4 , wherein the voltage observer is further configured to: The voltage across the transformer is determined based on a voltage at terminals of a power conversion system in the ZISC-based UPS system or an output voltage of the ZISC-based UPS system.

6. The apparatus according to claim 4, wherein the second resetting unit comprises: a flux magnitude extractor configured to extract the magnitude of the second offset flux; a comparator configured to compare the magnitude of the second offset flux with the flux magnitude limit; a multiplier configured to multiply the second offset flux by a correction factor to provide a compensation voltage in response to the magnitude of the second offset flux exceeding the flux magnitude limit; and The subtractor is configured to subtract the compensation voltage from the target voltage reference to obtain a corrected voltage reference.

7. A method for flux management in an uninterruptible power supply (UPS) system based on an impedance-isolated single-conversion (ZISC), the method comprising: determining a first offset flux across a series reactor in the ZISC-based UPS system when a power quality event occurs in a grid connected to the ZISC-based UPS system and the ZISC-based UPS system operates in a grid-tied mode; as well as A synthetic impedance damping is provided to the series reactor to reset the first offset flux.

8. The method according to claim 7, further comprising: determining a second offset flux on a transformer in the ZISC-based UPS system when the ZISC-based UPS system operates in the grid-connected mode or the islanded mode; as well as The second offset flux is reset in response to the magnitude of the second offset flux exceeding a flux magnitude limit.

9. The method of claim 8, wherein the transformer is a coupling transformer or a downstream transformer.

10. The method according to claim 8 or 9, wherein determining the second offset flux comprises: determining a voltage across the transformer; as well as The second offset flux is estimated based on the determined voltage across the transformer and an intrinsic time constant term specific to the transformer.

11. The method of claim 10, wherein determining the voltage across the transformer comprises: The voltage across the transformer is determined based on a voltage at terminals of a power conversion system in the ZISC-based UPS system or an output voltage of the ZISC-based UPS system.

12. The method of claim 10, wherein resetting the second offset flux in response to the magnitude of the second offset flux exceeding the flux magnitude limit comprises: extracting the magnitude of the second offset flux; comparing the magnitude of the second offset flux to the flux magnitude limit; multiplying the second offset flux by a correction factor to provide a compensation voltage in response to the magnitude of the second offset flux exceeding the flux magnitude limit; and The compensation voltage is subtracted from the target voltage reference to obtain a corrected voltage reference.

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