Active filter for single-phase charging using switch-mode converter

Abstract

A converter for electrically connecting to a three-phase grid and a single-phase grid is provided. The converter includes three DAB modules, each module for converting a corresponding alternating current of the three-phase grid. When connected to the single-phase grid, the third DAB module is bidirectional so that it is operable to filter the power output of the first and second DAB modules. The converter further includes a filter capacitor in electrical communication with the third DAB module through a relay, wherein the relay is responsive to a controller for coupling the third DAB module to the filter capacitor when a single-phase grid is detected, and for coupling the third DAB module to a grid rectifier when a three-phase grid is detected.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 754,641, filed on November 2, 2018, the disclosure of which is incorporated by reference in its entirety. Technical Field

[0003] The present invention relates to an active filter using a bidirectional switch mode converter for an on-board charger of an electric vehicle. Background Art

[0004] Electric vehicles include batteries that power the electric drive system. Plug-in electric vehicles are a type of electric vehicle that has a rechargeable battery pack that can be recharged from the power grid. Because most power grids provide AC voltage, and because most rechargeable battery packs require DC voltage, when recharging an electric vehicle, the power from the grid must first be converted to a suitable DC voltage.

[0005] There are various circuits for converting AC voltage from the grid to a DC voltage for a rechargeable battery. One known circuit includes a dual active bridge (DAB) converter. The DAB converter receives a DC voltage from a front-end rectifier and outputs it to a DC link capacitor. In operation, its DC input is inverted by the primary side bridge and rectified by the secondary side bridge. The DAB converter is typically operated by a controller to adjust the amplitude of the DC output according to the battery charging requirements.

[0006] The electrical grid exists as either a three-phase grid or a single-phase grid, often changing between the two. While high power charging is more ideally suited to a three-phase grid than a single-phase grid, the latter may be the only available option. Therefore, it is desirable to have an on-board charger that can support both grid types. In particular, it is desirable to have an on-board charger that removes second-order harmonics during single-phase charging to provide a pure DC output from the single-phase grid. Summary of the invention

[0007] An on-board charger is provided having a dual-purpose switch mode converter for electrically connecting to a three-phase grid and a single-phase grid. When connected to a three-phase grid, three switch mode converters (e.g., three DAB converters) convert rectified voltages to produce direct current at the output. When connected to a single-phase grid, the third switch mode converter is bidirectional so that it is operable to filter the power output of the first and second switch mode converters and thereby maintain direct current at the output. The on-board charger includes a filter capacitor in electrical communication with the third switch mode converter via a relay. The relay is responsive to a controller to couple the third switch mode converter to the filter capacitor when a single-phase grid is detected, and to couple the third switch mode converter to the third phase of the grid when a three-phase grid is detected.

[0008] In one embodiment, the on-board charger comprises a first DAB converter, a second DAB converter and a third DAB converter, each of which is operable to convert a rectified voltage into a DC component when coupled to a three-phase grid. However, when coupled to a single-phase grid, the input stage of the third DAB converter is coupled to a filter capacitor as an active filter to remove harmonics from the DC output of one or both of the first and second DAB converters. In particular, if the DC output is greater than a power reference, the filter capacitor draws power from the DC output through the third DAB converter, and if the DC output is less than the power reference, the filter capacitor supplies power to the DC output through the third DAB converter. The input stage of each DAB converter is electrically coupled to a rectifier, optionally to a full-bridge rectifier. Depending on whether a single-phase grid or a three-phase grid is detected, the relay can alternatively couple the input stage of the third DAB converter to a rectifier or a filter capacitor.

[0009] In another embodiment, an operating method is provided. The method includes determining the presence of a single-phase grid or a three-phase grid at the input of the on-board charger. If a three-phase grid is detected, a controller operates a first rectifier and a first DAB converter to convert a first phase of the AC input into a first DC component, operates a second rectifier and a second DAB converter to convert a second phase of the AC input into a second DC component, and operates a third rectifier and a third DAB converter to convert the third phase into a second DC component. If a single-phase grid is detected, the controller decouples the third DAB converter from the third rectifier and couples it to a filter capacitor. The controller further causes the third DAB converter to operate as an active filter of the DC outputs of the first and second DAB converters, such that if the DC output is greater than a power reference, the filter capacitor draws power from the DC output through the third DAB converter, and if the DC output is less than the power reference, the DC output is supplied with power through the third DAB converter.

[0010] In single-phase operation, the third DAB converter acts as the switch-mode and inductive element of the output active filter to eliminate second-order harmonics in the single-phase system. By using one DAB converter as an active filter, the hardware utilization of the entire system is maximized, while the amount of additional hardware and software requirements is minimized. The inductive component of the active filter is provided by the transformer of the DAB converter, and the output sensors that control the reactive power are already in place for three-phase operation. Further, because the control algorithm of the DAB converter can be bidirectionally modulated with zero power, only the new power reference needs to be applied to the same open-loop algorithm, which means that the control loop that maintains the capacitor voltage can be designed using the same transfer function as the other two modules.

[0011] One advantage of locating the relay between the grid rectifier and the converter module is that the relay allows the grid rectifier to short its unused phase to the neutral line and add an additional conduction path to the neutral line. When switching from three-phase operation to single-phase operation, if one converter is used as an active filter and the other two converters are working in parallel to supply power to the output, the neutral connection must handle the return current of both phase connections. By shorting the neutral line to the third, unused phase via the unused rectifier, the current distribution between the neutral line and the phase connection is restored to balance.

[0012] These and other features and advantages of the present invention will become apparent from the following description of the invention when viewed in light of the accompanying drawings and appended claims.

[0013] Before explaining the embodiments of the present invention in detail, it should be understood that the present invention is not limited to the operational details or construction details and component arrangements set forth in the following description or illustrated in the accompanying drawings. The present invention can be implemented in various other embodiments and can be practiced or implemented in alternative ways that are not explicitly disclosed herein. In addition, it should be understood that the wording and terminology used herein are for the purpose of description and should not be considered restrictive. The use of "include" and "comprise" and their variations means covering the items listed thereafter and their equivalents and additional items and their equivalents. Further, enumeration can be used in the description of various embodiments. Unless otherwise expressly stated, the use of enumeration should not be interpreted as limiting the present invention to any particular order or number of components. The use of enumeration should not be interpreted as excluding from the scope of the present invention any additional steps or components that may be combined with or combined to the steps or components of the enumeration. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a circuit diagram of a three-phase / single-phase, single-stage on-board charger for a vehicle battery;

[0015] Figure 2 yes Figure 1 A circuit diagram of a first alternative embodiment of a vehicle charger;

[0016] Figure 3 yes Figure 1 A circuit diagram of a second alternative embodiment of a vehicle charger;

[0017] Figure 4 Includes three embodiments of relays for disconnecting the grid and connecting filter capacitors;

[0018] Figure 5 Six embodiments including relay placement for switching the electrical connection of the DAB module between the grid and the filter capacitor;

[0019] Figure 6is a circuit diagram of an on-board charger, the on-board charger comprising a full-bridge rectifier and a filter capacitor between the primary of the full-bridge DAB;

[0020] Figure 7 yes Figure 6 A circuit diagram comprising a third and a fourth relay coupled between the filter capacitor and the output;

[0021] Figure 8 is a graph of instantaneous grid voltage, current and power provided by a single phase of the grid under unity power factor operation;

[0022] Fig. 9 is a graph of the idealized operation of a three-phase / single-phase, single-stage on-board charger using one phase as an output active filter. DETAILED DESCRIPTION

[0023] Reference now Figure 1 , a three-phase / single-phase on-board charger (OBC) is illustrated and is generally designated 10. As discussed below, one DAB module of the OBC 10 optionally functions as a switch mode and inductive element of an output active filter and employs a single relay on the primary high-side DC link to dynamically switch from a three-phase configuration when only a single-phase connection is detected.

[0024] More specifically, the OBC 10 is coupled to a three-phase grid or a single-phase grid 12. The OBC 10 includes a controller 14 for providing control signals to each of three DAB modules 16, 18, 20, each DAB module having eight controllable switches. The controllable switches of the DAB modules 16, 18, 20 are operated by the controller 14 to provide a DC voltage to a battery 2 (e.g., a vehicle battery). The controller 14 is also operable to detect single-phase and three-phase connections, and is operable to control the operation of one or more relays 24 coupled to a filter capacitor 32. Three grid synchronous rectifiers 26, 28, 30 provide a varying DC voltage to each DAB module 16, 18, 20.

[0025] During three-phase operation, each DAB module 16, 18, 20 is responsible for power factor correction of a single alternating current. The resulting DC output is achieved by combining all three DAB outputs. When applied to a three-phase grid, the isolated AC / DC system can take advantage of a 120° offset between each phase of the grid voltage. When the phase shift is 120° and the amplitude between each voltage sinusoid is equal, the grid is said to be balanced. This means that, given a unity power factor, at any time t, the sum of all three phase voltages will be equal to zero, and by extension, the phase current and all its harmonics will also be equal to zero.

[0026] During single phase operation, and to eliminate second order harmonics in the single phase system, one DAB module 20 from the three phase system is dynamically reused as the switch mode and inductive element of the output active filter. Once connected to the filter capacitor 32, optionally to a capacitor bank, the individual DAB module is operated bidirectionally, approximately 180° out of phase with the other two DAB modules 16, 18, to achieve active filtering. While in Figure 1 Until Figure 3 Three possible embodiments are shown in FIG. Figure 1 Embodiments of are most advantageous for their lower conduction losses and additional separation from the grid, and are therefore discussed in further detail below.

[0027] By using one module 20 as an active filter, the hardware utilization of the entire system is maximized, while the amount of additional hardware and software requirements is minimized. In addition to the existing hardware for three-phase operation, only the relay mechanism 24, the filter capacitor 32 and the voltage sensor 34 across the filter capacitor 32 are required. The main controller does not require additional PWM channels, only one additional ADC channel, and potentially as little as one additional GPIO output. The inductive component of the active filter is provided by the transformer, and the output sensor that controls the reactive power is already in place for three-phase operation. Further, because the control algorithm can be bidirectionally modulated with zero power, only the new power reference needs to be applied to the same open-loop modulation algorithm, which means that the control loop that maintains the capacitor voltage can be designed using the same transfer function as the other two modules.

[0028] Figure 2 FIG. 1 shows a vehicle charger 10 according to a second embodiment. Figure 1 The embodiment of the invention differs in that the primary-side full bridge of each DAB 16, 28, 20 now comprises a matrix converter having a plurality of bidirectional semiconductor switches for each phase of the grid 12. In addition, a filter capacitor 32 is connected to the input side of the matrix converter for the third DAB, or as Figure 2 The "matrix DAB 20" depicted in FIG. Figure 3 As shown in FIG. 8 , a filter capacitor 32 is connected to the input side of the third grid synchronous rectifier 30 . Figure 2 and Figure 3 The embodiments are also functionally similar to Figure 1 The embodiments are similar, since when connected to a three-phase grid each matrix DAB or each DAB module is responsible for power factor correction of a single alternating current, when connected to a single-phase grid one matrix DAB or DAB module is reused as the switch mode and inductive element of the active filter.

[0029] The relay mechanism 24 can be any device that disconnects the grid connection and simultaneously connects the filter capacitors. This can be accomplished using a single device such as a single-pole double-throw type mechanical relay, or a combination of devices such as a complementary switch, solid-state or mechanical relay. Figure 4 Three device-independent embodiments are shown, where the relay consists of one or more devices. Although one or both of the grid, the filter capacitor, and the DAB module 20 may not be connected, the simultaneous connection of both to the DAB will be considered a fault condition. For this reason, Figure 4 (a) and Figure 4 A relay of form (c) may be preferred.

[0030] The realization of the relay can have various embodiments, since, contrary to any physical connection, only the electrical circuit must be opened to disconnect the grid or the filter capacitors. Figure 5 Six embodiments of relay placement are shown which will adequately disconnect the electrical connection from the DAB module to the grid or filter capacitors. Figure 5 a, 5c and 5e are possible but less desirable because the rectifier will add heat losses during the active filter configuration. Figure 5 b, 5d and 5f can avoid the above situation. Among them, from the perspective of cost and space optimization, Figure 5 d and Figure 5 f is the most desirable, and from the perspective of isolation or EMI, Figure 5 b may be preferred. Figure 1 The embodiment shows Figure 5 d in the relay placement, while Figure 6 Shows Figure 5 b. Relay placement. Figure 7 A further embodiment is shown in which a filter capacitor 32 is coupled to the output mains voltage via a third and fourth relay 25. In this embodiment, a total of four relays are used, which allows the filter capacitor 32 to be switched between the output mains voltage in three-phase mode and the input DC link during single-phase mode. Thus, the filter capacitor 32 is used for both three-phase operation and single-phase operation. In three-phase operation, switches Sr7 and Sr8 are closed so that the filter capacitor 32 reduces the output current ripple of the battery 22 and generally increases the ability to absorb transients on the grid so that they are not seen by the battery. In single-phase operation, switches Sr7 and Sr8 are open. Thus, Figure 7 The configuration in allows maximum utilization of the filter capacitor 32 regardless of the grid configuration. Connecting the filter capacitor 32 to the output rail voltage can amplify the output capacitance by five to six times, depending on the specific system, while also greatly increasing the ability to supply transient demands to the output bus from other systems connected to the battery.

[0031] An additional advantage of placing a relay on the DC link between the unrolled rectifier and the DAB primary full bridge is that it allows the rectifier to short its unused phase to neutral and add an additional conduction path to the neutral. When switching from three-phase and single-phase operation, if one DAB module is used as an active filter and the other two are working in parallel to supply the output, the neutral connection must handle the return current of both phase connections. By shorting the neutral to the third, unused phase through the unused rectifier, the balance between the neutral and phase connections is restored.

[0032] The operating principles of the OBC 10 will now be described. The control algorithm used to modulate the power of the DAB modules of each phase allows the instantaneous power through each phase to be controlled to any reference waveform. Therefore, the power reference supplied to the third DAB module 20 used as an active filter only needs to be formulated to destructively interfere with the undesirable output harmonics produced by the power modulation of the other DAB modules. To illustrate this concept, Figure 8 shows the power factor of 277 V RMS The power command required for a single-phase grid to draw 14 kW average power. Therefore, this will be applied to Figure 1 The power reference of the first two DAB modules (16) and (18) in the figure. Assuming an ideal system, the output power P 输出 will be equal to the input power P 电网 , is a 120Hz sine wave with an amplitude of 14kW and a DC offset of 14kW. The desired output power is Fig. 9 The average power output plotted in P avg Therefore, to eliminate the output harmonics, the power that must be supplied to the output is P act = P avg – P 输出 .if P 输出 It is represented by the expression 14k*sin(120πt)+14kW, and if P avg Expressed by the expression 14kW, then P act is equal to -14k*sin(120πt), which has The RMS value and net contribution of 0W. Fig. 9 During the negative portion of , the filter DAB module 20 is drawing power from the output and storing this energy in the filter capacitor 32. This is the area where the PFC DAB is supplying more power than the desired average.P act During the positive region, the filter DAB module 20 is pulling stored energy from the filter capacitor 32 and delivering it to the output. This is the region where the PFC DAB modules 16, 18 are delivering less power than desired. In this way, the load only sees the average power as the desired DC output.

[0033] The above description is a current embodiment of the present invention. Various changes and modifications may be made without departing from the spirit and broader aspects of the present invention. The present disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the present invention, or as limiting the scope of the claims to the specific elements illustrated or described in conjunction with these embodiments. For example, any reference to an element in the singular form using the article "one", "the", "the" or "said" should not be interpreted as limiting the element to the singular.

Claims

1. A vehicle charger for a vehicle, comprising: a first switch-mode converter operable to convert the first rectified voltage into a first DC component; a second switch-mode converter operable to convert the second rectified voltage into a second DC component; a third switch-mode converter operable to convert the third rectified voltage into a third DC component, the third switch-mode converter being bidirectional such that the third switch-mode converter is operable as an active filter to remove harmonics from the DC outputs of the first and second switch-mode converters when coupled to a single-phase grid, wherein the third switch-mode converter is a dual active bridge converter including a primary-side full bridge, an isolation transformer, and a secondary-side full bridge; a first rectifier, a second rectifier, and a third rectifier electrically coupled in series with the first switch-mode converter, the second switch-mode converter, and the third switch-mode converter, respectively; as well as A filter capacitor is selectively coupled between the primary side full bridge of the third switch mode converter and the third rectifier via a relay, wherein the relay is configured to alternately couple the primary side full bridge of the third switch mode converter to either one of the third rectifier and the filter capacitor, and wherein if the DC output is greater than a power reference, the filter capacitor draws power from the DC output through the third switch mode converter, and if the DC output is less than the power reference, supplies power to the DC output through the third switch mode converter. 2 . The on-board charger of claim 1 , wherein each of the first and second switch-mode converters comprises a dual active bridge topology. 3 . The on-board charger of claim 1 , wherein the first rectifier, the second rectifier, and the third rectifier each comprise a full-bridge topology. 4 . The vehicle charger according to claim 1 , further comprising a controller to detect the single-phase AC input voltage and the three-phase AC input voltage and control the operation of the relay. 5 . The on-board charger of claim 4 , wherein each of the first, second, and third switch-mode converters provides a unity power factor when coupled to a three-phase AC input voltage. 6 . The on-board charger of claim 4 , wherein when the controller detects a single-phase AC input voltage, the third switch-mode converter operates 180° out of phase with respect to the first switch-mode converter and the second switch-mode converter. 7 . The on-board charger of claim 4 , further comprising a voltage sensor across the filter capacitor, the voltage sensor coupled to the controller.

8. The on-board charger of claim 1, wherein the power reference is selected such that the third switch-mode converter destructively interferes with harmonics of the DC output.

9. A method for converting an AC input voltage into a DC output voltage, comprising: Providing an on-board charger, the on-board charger comprising a first rectifier, a second rectifier, and a third rectifier electrically coupled in series with a first switch-mode converter, a second switch-mode converter, and a third switch-mode converter, respectively, wherein the third switch-mode converter is a dual active bridge converter including a primary-side full bridge, an isolation transformer, and a secondary-side full bridge; In response to detecting a three-phase AC input at the on-board charger, operating a first rectifier and a first switch-mode converter to convert a first phase of the AC input into a first DC component, operating a second rectifier and a second switch-mode converter to convert a second phase of the AC input into a second DC component, and operating a third rectifier and a third switch-mode converter to convert a third phase of the AC input into a third DC component; as well as In response to detecting a single-phase AC input at the on-board charger, filtering a DC output of at least one of the first and second switch-mode converters by a third switch-mode converter to reduce harmonics in the DC output, wherein filtering the DC output includes decoupling the third switch-mode converter from a third rectifier and coupling an input stage of the third switch-mode converter to a filter capacitor via a relay, the filter capacitor being coupled between the third rectifier and a primary-side full-bridge of the third switch-mode converter, wherein the relay is configured to alternately couple the primary-side full-bridge of the third switch-mode converter to either the third rectifier and the filter capacitor, such that if the DC output is greater than a power reference, the filter capacitor draws power from the DC output, and if the DC output is less than the power reference, the filter capacitor supplies power to the DC output.

10. The method of claim 9, wherein the first, second, and third rectifiers each comprise a full-bridge topology.

11. The method of claim 9, wherein the first switch-mode converter and the second switch-mode converter each comprise a dual active bridge topology.

12. The method of claim 9, wherein the filter capacitor comprises a capacitor bank.

13. The method of claim 9, further comprising coupling a DC output of the on-board charger to a vehicle battery.

14. The method of claim 9, further comprising coupling a filter capacitor to the DC output of the on-board charger through the first and second switches to reduce current ripple in the DC output of the on-board charger when a three-phase AC input is detected. 15 . The method of claim 14 , further comprising decoupling the filter capacitor from the DC output of the on-board charger via the first and second switches when a single-phase AC input is detected.

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