Pre-charge circuit for voltage converters having line interphase transformers
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-02-04
- Publication Date
- 2026-08-13
Smart Images
Figure IB2025051194_13082026_PF_FP_ABST
Abstract
Description
771982 (P240650W001 )1PRE-CHARGE CIRCUIT FOR VOLTAGE CONVERTERS HAVING LINE INTERPHASE TRANSFORMERSFIELD
[0001] The invention relates to voltage converters having line interphase transformers.BACKGROUND
[0002] Voltage converters are used to connect industrial devices to an alternating current (AC) grid. Voltage converters usually rely on DC-link capacitors to stabilize voltage, filter harmonics, and store energy for efficient power conversion. During startup of voltage converters, when the voltage converter is first connected to the alternating current (AC) grid, large inrush currents may be experienced due to the fully discharged DC-link capacitors. This sudden surge risks damaging the DC-link capacitors, creating overvoltage, harming semiconductor components, tripping grid protection systems, saturating magnetic devices, disturbing grid voltage, and causing mechanical stresses. To ensure a safe and controlled startup of the voltage converter, the DC-link capacitors must be gradually pre-charged to their operating voltage before the voltage converter is connected to the AC grid.
[0003] Typical pre-charging methods include using resistors in series with the grid to limit inrush currents, followed by a breaker to bypass the resistors once pre-charging is complete, or employing auxiliary voltage sources to charge the capacitors. These auxiliary sources could be high-voltage DC converters or low-power step-up transformers connected through a diode bridge. These typical pre-charging methods often require intensive resources and additional space, which is undesirable in applications requiring compact, high-power- density designs. This underscores the need for more efficient, resource-conscious solutions for safely starting up voltage converters.SUMMARY
[0004] A first aspect of the present disclosure provides a voltage converter. The voltage converter comprises: a line interphase transformer having a first end and a second end, the first end of the line interphase transformer being coupled to a multi-pulse rectifier, a plurality of direct current-link (DC-link) capacitors, each of the DC-link capacitors having a first end and a second end, wherein each of the plurality’ of DC-link capacitors is electrically’ coupled to a plurality of inverters at a first end and electrically coupled to the multi-pulse rectifier at a771982 (P240650W001 )2second end, and an auxiliary coil having a plurality of windings, wherein the auxiliary coil is electrically coupled to a low voltage AC network via low voltage resistors and a low voltage circuit breaker, one or more switching devices to electrically couple or decouple the voltage converter with a voltage grid. The voltage converter includes a controller configured to: provide to the one or more switching devices, a first instruction to electrically couple the aux i li ary coil to the low voltage AC network to begin charging of the plurality’ of DC-link capacitors; based on determining that the plurality of DC-link capacitors have reached a predetermined dc-link voltage, provide to the low' voltage circuit breaker, a second instruction to electrically decouple the auxiliary coil from the low voltage AC network; and provide to the one or more switching devices, a third instruction to electrically couple the voltage converter to the voltage grid.
[0005] According to an implementation of the first aspect, the controller is further configured to: initiate normal operation of the voltage converter, wherein the voltage converter controls operation of a multi-phase AC machine.
[0006] According to an implementation of the first aspect, the line interphase transformer comprises three single-phase cores, and wherein the plurality of windings of the auxiliary coil are disposed on any of the three single phase cores of the line interphase transformer.
[0007] According to an implementation of the first aspect, each single phase core of the line interphase transformer is shell-type or E-type.
[0008] According to an implementation of the first aspect, each single phase core of the line interphase transformer is core-type or U-type.
[0009] According to an implementation of the first aspect, the line interphase transformer comprises a five-limb core, and wherein the plurality of windings of the auxiliary' coil are disposed on any core limb of the five-limb core.
[0010] According to an implementation of the first aspect, the line interphase transformer comprises a four-limb core, and wherein the plurality of windings of the auxiliary' coil are disposed on any core limb of the four-limb core.
[0011] According to an implementation of the first aspect, the auxiliary coil with the plurality of windings is disposed on any limb of the shell-type or E-type single phase core.
[0012] According to an implementation of the first aspect, the auxiliary coil with the plurality of windings is disposed on any limb of the core-type or U-type single phase core.
[0013] According to an implementation of the firs t aspect, the multi-pulse rectifier is at least one of a 12-pulse rectifier, 18-pulse rectifier, and a 24-pulse rectifier.771982 (P240650W001 )3
[0014] According to an implementation of the first aspect, the plurality of inverters are arranged in a series connected topology and are configured to supply power to a multi-phase AC machine.
[0015] According to an implementation of the first aspect, the line interphase transformer is composed of at least one of ferrite, metal magnetic powders, amorphous-alloys, and electrical steel.
[0016] According to an implementation of the first aspect, the second end of the linear interphase transformer is connected to the one or more switching devices.
[0017] According to an implementation of the first aspect, the second end of the linear interphase transformer is connected to the one or more switching devices through a grid inductor.
[0018] A second aspect of the present disclosure provides a method to pre-charge a voltage converter, comprising: providing to one or more switching devices using a controller, a first instruction to electrically couple an auxiliary coil to a low voltage AC network to begin charging of a plurality of DC-link capacitors, wherein the auxiliary' coil comprises a plurality' of windings, wherein the auxiliary coil is electrically coupled to a low voltage AC network via low voltage resistors and a low voltage circuit breaker; based on determining that the plurality of DC-link capacitors have reached a predetermined DC-link voltage, providing to the low voltage circuit breaker, a second instruction to electrically decouple the auxiliary coil from the low voltage AC network; and providing to the one or more switching devices, a third instruction to electrically couple the voltage converter to the voltage grid.BRIEF DESCRIPTION OF DRAWINGS
[0019] Embodiments of the present disclosure will be described in even greater detail below based on the exemplary figures. The present disclosure is not limited to the exemplary' embodiments. All features described and / or illustrated herein can be used alone or combined in different combinations in embodiments of the present disclosure. The features and advantages of various embodiments of the present disclosure will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
[0020] FIG. 1 is a simplified block diagram depicting an exemplary pre-charge circuit for voltage converters having line interphase transformers, in accordance with one or more examples of the present disclosure.771982 (P240650W001 )4
[0021] FIGS. 2A-2D illustrate simplified circuit diagrams depicting the exemplary pre¬ charge circuit coupled to a single-phase core line interphase transformer, in accordance with one or more examples of the present disclosure.
[0022] FIGS. 3A-3C illustrate simplified circuit diagrams depicting the exemplary pre- charge circuit coupled to a five-limb core line interphase transformer, in accordance with one or more examples of the present disclosure.
[0023] FIGS. 4A-4B illustrate simplified circuit diagrams depicting a voltage converter system with a line interphase transformer having auxiliary' pre-charge winding, in accordance with one or more examples of the present disclosure.
[0024] FIG. 5 depicts exemplary graphs associated with the performance of the voltage converter system with a line interphase transformer having auxiliary pre-charge winding, according to one or more examples of the present disclosure.
[0025] FIGS. 6A-6C illustrate simplified circuit diagrams depicting the exemplary pre¬ charge circuit coupled to a four-limb core line interphase transformer, in accordance with one or more examples of the present disclosure;
[0026] FIG. 7 is a simplified block diagram of one or more devices or systems within the exemplary environment of FIG, 1; and
[0027] FIG. 8 illustrates an exemplary' process to pre-charge voltage converters having line interphase transformers using a pre-charge circuit, in accordance with one or more examples of the present application.DETAILED DESCRIPTION
[0028] Examples of the presented application will now be described more fully hereinafter with reference to the accompanying FIGs., in which some, but not all, examples of the application are shown. Indeed, the application may be exemplified in different forms and should not be construed as limited to the examples set forth herein; rather, these examples are provided so that the application will satisfy applicable legal requirements. Where possible, any terms expressed in the singular form herein are meant to also include the plural form and vice versa, unless explicitly stated otherwise. Also, as used herein, the term “a” and / or “an” shall mean “one or more” even though the phrase “one or more” is also used herein. Furthermore, when it is said herein that something is “based on” something else, it may be based on one or more other things as well. In other words, unless expressly indicated771982 (P240650W001 )5otherwise, as used herein "‘based on” means “based at least in part on” or “based at least partially on”.
[0029] DC-link capacitors are integral part of series-connected inverters (SCI) topology of voltage converters that are configured to connect multi-phase alternating current (AC) industrial machines to an alternating current (AC) grid. In some cases, the series-connected inverters (SCI) interface with the AC grid via a rectifier and a line interphase transformer to supply the multi-phase AC industrial device. DC-link capacitors of the SCI are used to stabilize voltage, filter harmonics, and store energy for efficient power conversion from the AC grid so that it may be supplied to the multi-phase AC machine. During startup of the voltage converters, huge inrush currents (e.g., 15 times the nominal value) may be detected at the voltage converter when the voltage converter is connected to the AC grid. The huge inrush currents may lead to charging of the DC-link capacitors to about 2 times the nominal values, which slowly drops depending on how much stand-by loading exists. The huge inrush currents may lead to overcurrent and overvoltage events damaging system components and leading to unstable operation of the voltage converters and the multi-phase industrial device.
[0030] In order to avoid the huge inrush currents in the voltage converters at the time of startup, the DC-link capacitors of the SCI may need to be charged to near their nominal operating voltage before the voltage converters are connected to the AC grid. Conventional techniques to charge DC-link capacitors before the voltage converter is connected to the AC grid, come with notable challenges. The resistor-based pre-charging approach, which limits inrush current using a series resistor and bypasses it with a breaker after pre-charging, requires additional components and reduce system efficiency. Auxiliary voltage sources, such as step-up DC-DC converters or step-up transformers with diode rectifiers, effectively charge the capacitors but add to the system resource utilization, complexity', and space requirements. Lastly, utilizing existing converter components — such as switches, diodes, and inductors in specific configurations — reduces the need for additional hardware but increases the complexity’ of control algorithms and may impose additional stress on those components. These limitations highlight the need for innovative pre-charging solutions that balance reliability, efficiency, and resource utilization.
[0031] Embodiments of the present disclosure provide a novel approach for pre-charging the DC-link capacitors of the voltage converters that have line interphase transformers (LIT) at a front-end side. In some embodiments, the material out of which the line interphase transformer may be made is ferrite (Mn-Zn based ferrite, Ni-Zn based ferrite, Ni-Zn-Cu771982 (P240650W001 )6based ferrite, Mn-Mg based ferrite, Ba based ferrite, Li based ferrite, or the like), metal magnetic powders (includes one or more of selected group consisting Fe, Si, Cr, Al, and Ni), amorphous-alloys or metallic glass (includes one or more of selected group consisting Fe, Si, B, Nb, Cu, Co, etc.), or electrical steel (e.g., silicon steel). In such embodiments, an existing core of the LIT is util ized to pre-charge the vol tage con verter by adding an auxiliary' pre¬ charge coil. The auxiliary’ coil may be connected to a low voltage AC network through a low voltage resistor and a low voltage breaker. Arranging an auxiliary coil as described herein may help in reducing total resources and space, since it avoids the use of a separate auxiliary step-up transformer.
[0032] The auxiliary coil added to an existing core of an LIT works as a single-phase transformer during a start-up (pre-charge) period. Since existing phase-shifting coils of the LIT would already be connected to the DC-link capacitors through a rectifier, this system will perform the pre-charge function, when the auxiliary' coil is connected to the low-voltage network.
[0033] FIG. 1 is a simplified block diagram depicting an exemplary pre-charge circuit for voltage converters having line interphase transformers, in accordance with one or more examples of the present application. System 100 of FIG. 1 includes an AC grid 102 that is connected to a multiphase AC machine 112 via voltage converter 120. The voltage converter 120 may include a line interphase transformer (LIT) 104, a rectifier 106, DC-link capacitors 122, and series connected inverters (SCI) 110.
[0034] In some embodiments, the voltage converter 120 may be connected to the AC grid 102 using circuit breakers 118. For example, grid connection breakers may be used as circuit breakers 118 to connect the LIT 104 to the AC grid 102. In some embodiments, the LIT 104 may be connected to the circuit breakers 118 via a grid inductor.[003S] In some embodiments, the system 100 is configured such that when the circuit breakers 118 are closed, power flows from the AC grid 102 to the line interphase transformer (LIT) 104 of the voltage converter 120. The LIT 104 conditions the alternating current (AC) power from the AC grid 102 and feeds it into the rectifier 106. The rectifier 106 converts the AC power received from the AC grid 102 to DC and the DC power is provided to the DC- link capacitors 122. The DC-link capacitors stabilize DC power received from the rectifier 106 before providing the DC power to the series connected inverters 110. The series connected inverters 110 may convert the stabilized DC power back to three-phase AC for the multi-phase AC machine 112.771982 (P240650W001 )7
[0036] According to certain embodiments, the DC-link capacitors 122 are key to maintaining the proper DC voltage between the rectifier 106 (which converts AC to DC) and the SCI 110 (which converts DC back to AC). Without a properly sized DC-link capacitor, the system would experience voltage instability, increased ripple, and poor power quality.
[0037] In order to ensure smooth connection of the AC grid 102 to the voltage converter 120 such that power is supplied smoothly from the AC grid 102 to the AC machine 112, the DC-link capacitors 122 may need to be pre-charged to their nominal value before the voltage converter 120 is connected to the AC grid 102. System 100 of FIG. 1 includes a controller 116 that ensures the proper pre-charging of the DC-link capacitors 122 before the connection of the LIT 104 to the AC grid 102 via circuit breakers 118. In some embodiments, in a pre¬ charge phase before the voltage converter 120 is connected to the AC grid 102, the controller 116 may instruct a pre-charge circuit 114 to charge the DC-link capacitors 122 to their nominal value by coupling an auxiliary coil of the pre-charge circuit 114 with a low voltage AC network. Once the DC-link capacitors 122 are properly pre-charged, the controller 116 may provide instructions to the circuit breakers 118 to close, so that the voltage converter 120 is connected to the AC grid 102. Because of the pre-charge of the DC-link capacitors 122 performed by the pre-charge circuit 114, the inrush current of the voltage converter 120 during the connection between the voltage converter 120 and AC grid 102 is significantly- reduced.
[0038] In some embodiments, the pre-charge circuit 114 includes an auxiliary coil that is coupled to a core of the LIT 104. The auxiliary coil of the pre-charge circuit 114 may be coupled to low voltage AC network through a low-voltage resistor and a low voltage breaker. At the same time as the voltage converter 120 is connected to the AC grid 102, the controller 116 instructs the auxiliary coil of the pre-charge circuit 114 to disconnect from the low-voltage alternating network. In such cases, the pre-charge circuit 114 works as a single-phase transformer during the pre-charge phase of the system 100. The arrangement of the various elements of the system 100 are depicted in more detail with respect to FIGs. 4A and 4B.
[0039] In some embodiments, the LIT 104 may be comprised of three single phase cores, one for each AC phase. In such embodiments, each of the three single phase cores may be a shell-type or E-type cores) or a core-type (or U-type cores). Based on depending on the types of construction of the LIT 104, the auxiliary coil of the pre-charge circuit 114 may be coupled to any limb or either of the cores of the LIT 104 in different ways. This is described in more details with respect to FIGS. 2A-2D and FIGS. 3A-3C.771982 (P240650W001 )8
[0040] In some embodiments, before the voltage converter 120 is connected to the voltage grid 102, the controller 116 initiates the pre-charge circuit, which causes current to flow in the pre-charge circuit 114. The current flowing in the pre-charge circuit 114 creates flux in the LIT 104, which leads to alternating current to flow' from the LIT 104 towards the rectifier 106. The rectifier 106 accepts the alternating current from the LIT 104 and converts it to DC-link current, which is used to charge the DC -link capacitors 122 that are coupled to the rectifier 106. Once the DC-link capacitors are charged to a predetermined pre-charge value, the controller 116 may instruct the controller 116 to disconnect the pre-charge circuit and close the breakers 118 to connect the voltage converter 120 to the voltage grid 102. Once the voltage converter 120 is connected to the voltage grid 102, the voltage converter is able to initiate normal operation of the AC machine 112, without causing high inrush current in the voltage converter 120, thereby preserving the elements of the system 100 during operation.
[0041] FIGS. 2A-2D illustrate simplified circuit diagrams depicting the exemplary pre-charge circuit as part of a single-phase core line interphase transformer, in accordance with one or more examples of the present application.
[0042] The LIT 104 depicted in FIGS. 2A-2D includes three cores, one for each phase of the AC grid 102. Each of the single-phase cores of the LIT 104 may be shell or E-type cores, or core or U-type cores.
[0043] FIGS. 2A-2D depict simplified diagrams of the different single-phase cores (shell or E-type and core or U-type type) of the LIT 104. The windings of tire auxiliary coil of the pre-charge circuit 114 may be placed on different cores or different limbs of the LIT 104. Position of the auxiliary winding of the auxiliary' coil of the pre-charge circuit 114 may depend on many factors such as size and available space, insulation requirements, magnetic saturation of the core, etc. For successful pre-charge of the DC-link capacitors of the SCI 110, the pre-charge circuit 114 may include a single auxiliary coil as part of the auxiliary circuit that may be coupled to a single-phase core. Such a design simplifies the voltage converter 120 and reduces the resources that may be required to create it.
[0044] FIGS. 2 A and 2B depict an LIT 104 with three cores that are shell or E-type.
[0045] In the di agram 200 of FIG. 2 A, the LIT 104 includes three shell-type single phase cores. The three shell-type single phase windings are installed on the central leg 224. For example, in the LIT 104 of FIG. 2A, a first winding wa, a second winding wb and a third winding Wab are placed on the central leg 224 of the LIT 104. In some embodiments, the different windings wa, wb, and waband their appropriate turn ratio create phase shift between771982 (P240650W001 )9currents of two 6-pulse rectifiers (e.g., 12-pulse rectifier) reducing the input current total harmonic distortion (THD). The first winding wa, a second winding wband a third winding wabmay be connected to the rectifier 106.
[0046] FIG. 2A depicts a pre-charge circuit 114 that is similar to the pre-charge circuit 114 as shown in system 100 of FIG. 1. An auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 is placed on a side-limb 222 of the LIT 104, as shown diagram 200 of FIG. 2A. The auxiliary coil 210 is connected to a low-voltage alternating current (LVAC) source 204 via a relay 208, and a resistor network 206. In some alternate embodiments, the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 may be placed on the other side-limb 218 of the LIT 104.
[0047] FIG. 2B is similar to FIG. 2A, except that in the diagram 220 of FIG. 2B, the auxiliary winding 210 of the auxiliary' coil of the pre-charge circuit 114 is pl aced on the central leg 224 of the LIT 104 instead of the side-legs 218 or 222.
[0048] FIGS. 2C and 2D depict an LIT 104 with three cores that are core or U-type.
[0049] In the diagram 240 of FIG. 2C, the LIT 104 includes a first leg 228 and a second leg 226. In the LIT 104 of FIG. 2C, the first winding Wa, and the third winding Wab are placed on the first leg 228. The second winding wb is placed on the second leg 226 of the LIT 104. The first winding wa, a second winding wband a third winding wabare connected to the rectifier 106.
[0050] FIG. 2C depicts a pre-charge circuit 114 that is similar to the pre-charge circuit 114 as shown in system 100 of FIG. 1. The auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 is placed on the first leg 228 of the LIT 104 along with the first winding waand the third winding wab, as shown diagram 240 of FIG. 2C. The auxiliary coil 210 is connected to a low-voltage alternating current (LVAC) source 204 via a relay 208, and a resistor network 206. In some alternate embodiments, the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 may be placed on the other side-limb 218 of the LIT 104.
[0051] FIG. 2D is similar to FIG. 2C, except that in FIG. 2D, the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 is placed on the second leg 226 of the LIT 104 along with the second winding wb.
[0052] In some embodiments, the number of windings in the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 may be based on the desired DC-link voltage to771982 (P240650W001 )10which the DC-link capacitors may need to be charged. In some embodiments, the number of win dings of the auxiliary' winding 210 may be based on the following formula:^^LVAC.rms ^'aux zs,,. V 2 ^LVAC,nns-■ - > Naux= (Nwat)+ Nwa) - - -VDCwabVDCwhere VLVAc,rms is the rms voltage of the low-voltage alternating current (LVAC) source 204, V DC is the DC-link voltage of the MV converter 120, Naux is the number of auxiliary windings of the auxiliary coil of the pre-charge circuit 114, Nwabis number of windings for winding wab, and Nwais number of windings for winding wa.
[0053] In some embodiments, the number of auxiliary windings 210 may be decreased to increase transformer flux, so the desired DC-link voltage may be reached. In such a way, higher voltage is induced in the windings connected to the rectifier 106, leading to higher charge on the DC-link capacitors 122. As the resistor network 206 and the auxiliary relay 208 are placed in connection with a low voltage circuit, they do not need to be rated for high voltage. In some embodiments, even during the normal operation of the LIT 104, induced voltage on the auxiliary winding 210 of the auxiliary' coils of the pre-charge circuit 114 is low, which significantly reduces cost and space required from components of the pre-charge circuit 114.
[0054] FIGS. 3A-3C illustrate simplified circuit diagrams depicting the exemplary pre¬ charge circuit as part of a five-limb core line interphase transformer, in accordance with one or more examples of the present application.
[0055] FIGS. 3A-3C depict a five-limb LIT 104. In some embodiments, the five-limb LIT 104 may be configured for three phase systems. For example, diagram 300 of FIG. 3 A depicts the five-limb LIT 104 with side-limbs 320 and 328, outer main limbs 322 and 326, and a central limb 324. Each of the outer main limbs 322 and 326, and the central limb 324 include the first winding wa, the second winding wb and the third winding w?.b. For example, the outer main limb 326 includes a first winding WAB316, a second winding WbB318, and a third winding WABb314. Similarly, the outer main limb 320 includes a first winding WAc304, a second winding WBc306, and a third winding WABc302. The central limb 324 includes a first winding WaA 310, a second winding wbA 312, and a third winding WabA 308.
[0056] Each of the first winding wa, a second winding wband a third winding wabfrom each of the limbs 322, 324, and 326 are connected to the rectifier 106.
[0057] FIG. 3 A depicts a pre-charge circuit 114 that is similar to the pre-charge circuit 114 as shown in system 100 of FIG. 1. An auxiliary'' winding 210 of the auxiliary' coil of the771982 (P240650W001 )11pre-charge circuit 114 is placed on a side-limb 328 of the LIT 104, as shown diagram 300 of FIG. 3A. The auxiliary coil 210 is connected to a low-voltage alternating current (LVAC) source 204 via a relay 208, and a resistor network 206. In some alternate embodiments, the auxiliary' winding 210 of the auxiliary coil of the pre-charge circuit 114 may be placed on the other side-limb 320 of the five-limb LIT 104.
[0058] FIG. 3B is similar to FIG. 3A, except that in the diagram 340 of FIG. 3B, the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 is placed on the outer main limb 326 of the LIT 104. In some alternate embodiments, the auxiliary' winding 210 of the auxiliary' coil of the pre-charge circuit 114 may be placed on the other outer main limb 322 of the LIT 104.
[0059] FIG. 3C is similar to FIG. 3 A, except that in the diagram 360 of FIG. 3C, the auxiliary' winding 210 of the auxiliary' coil of the pre-charge circuit 114 is pl aced on the central limb 324 leg 224 of the LIT 104.
[0060] As shown with respect to FIGS. 3A-3C, the pre-charge circuit 114 may include a single auxiliary' coil as part of the auxiliary circuit 210 that may be coupled to a single-phase core for successful recharging of the DC-link capacitors of the SCI 110. As discussed previously, the position of the auxiliary winding 210 of the auxiliary' coil of the pre-charge circuit 114 may depend on many factors such as size and available space, insulation requirements, magneti c saturation of the core, etc.
[0061] In some embodiments, with the LIT 104 as shown in FIGS. 3A-3C constructed with five-limbs in a single core, waand w windings from all three phases (central and side main legs) during pre-charge are connected to the DC-link through a rectifier and participate in charging.
[0062] FIGS. 4A-4B illustrate simplified circuit diagrams depicting a voltage converter system with a line interphase transformer having a pre-charge circuit, in accordance with one or more examples of the present disclosure.
[0063] FIG. 4A depicts a simplified circuit diagram 400 of a voltage converter that connects the AC machine 112 to the AC grid 102. As shown in FIG. 4A, the simplified circuit diagram 400 includes a LIT 104, a rectifier 106, DC-link capacitors 122, and an SCI 110. As discussed with respect to FIG. 1, the LIT 104, the rectifier 106, the DC-link capacitors 122 and the SCI 110 together form a voltage converter 120. The voltage converter 120 may be connected to the AC grid 102 via circuit breakers 118 on one end and an AC machine 112 on another end.771982 (P240650W001 )12
[0064] In some embodiments, the LIT 104 of the voltage converter 120 may be coupled to circuit breakers 118 on one end and the rectifier 106 on another end. In some cases, the rectifier 106 may be a multi-pulse rectifier. For example, the rectifier 106 may be a 12 pule rectifier, 18 pulse rectifier, or a 24 pule rectifier. The LIT 104 shown in FIG. 4A includes a collection of single-phase cores 402, 404, and 406. Each of the single-phase cores of the LIT 104 of FIG. 4A may be associated with a corresponding phase of the AC grid 102. The LIT 104 may be connected to the AC grid 102 via circuit breakers 118.
[0065] The rectifier 106 may be coupled to DC-link capacitors 122. The DC-link capacitors may be coupled to the rectifier 106 on one end and SCI 110 on another end. The SCI may be coupled to the AC machine 112 on an opposite end. In some embodiments, the rectifier 106 may be a 12-pulse rectifier that includes two 6-pulse rectifiers. Each 6-pulse rectifier comprises 6 diodes connected in a manner shown in FIGs. 4A and 4B.
[0066] In order to ensure smooth connection of the AC grid 102 to the voltage converter 120 such that power is supplied smoothly from the AC grid 102 to the AC machine 112, the DC-link capacitors 122 may need to be pre-charged to their nominal value before the voltage converter 120 is connected to the AC grid 102. System 100 of FIG. 1 includes a controller 116 that ensures the proper pre-charging of the DC-link capacitors 122 before the connection of the LIT 104 to the AC grid 102 via circuit breakers 118. In some embodiments, in a pre¬ charge phase before the voltage converter 120 is connected to the AC grid 102, the controller 116 may instruct a pre-charge circuit 114 to charge the DC-link capacitors 122 to their nominal value. In some embodiments, the controller 116 instructs the pre-charge circuit 114 to couple an auxiliary winding 210 of the auxiliary coil to a low-voltage alternating current (LVAC) source 204 via a relay 208, and a resistor network 206.
[0067] Once the DC-link capacitors 122 are properly pre-charged, the controller 116 may provide instructions to the circuit breakers 118 to close, so that the voltage converter 120 is connected to the AC grid 102. Simultaneously, the controller 116 may also instruct the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 to disconnect from the LVAC source 204. Because of the pre-charge of the DC-link capacitors 122 performed by the pre-charge circuit 114, the inrush current felt in the voltage converter 120 during the connection between the voltage converter 120 and AC grid 102 is significantly reduced.
[0068] A pre-charge circuit 114 may be coupled to core 406 of the LIT 104. In some alternate embodiments, the pre-charge circuit 114 may be coupled to any of the other cores 402 or 404 of the LIT 104,771982 (P240650W001 )13
[0069] FIG. 4B depicts a simplified circuit diagram 450 of a voltage converter that connects the AC machine 112 to the AC grid 102. FIG. 4B is similar to FIG. 4A except that the LIT 104 shown in FIG. 4B is a five-limb LIT instead of a collection of single-phase LITs shown in FIG. 4A. As discussed with respect to FIGS, 3A-3C, the five-limb LIT 104 includes two side limbs 320 and 328, two main outer limbs 322 and 326, and a central limb 324. FIG.4B depicts the pre-charge circuit 114 coupled to a side limb 328 of the five-limb LIT 104. Alternatively, the pre-charge circuit 114 may be placed on the other side limb 320, the two outer main limbs 322 and 326, or the central limb 324 of the LIT 104.
[0070] FIG. 5 depicts exemplary graphs associated with the performance of the voltage converter system with a line interphase transformer having auxiliary pre-charge winding, according to one or more examples of the present disclosure.
[0071] The graphs 502-548 are associated with the circuit layout 400 as shown in FIG.4A with an LIT 104 having a collection of single-phase cores 402-406. Because the operation of the LIT 104 as shown in FIG. 4A, with a collection of single-phase cores 402-406 is similar to the operation of the LIT 104 having a five-limb core as shown in circuit layout 450 of FIG. 4B, the same graphs may be associated with the circuit layout 450 as shown in FIG.4B.
[0072] The graph 502 of FIG. 5 depicts a curve that plots the three phase grid voltage (Vgrid,abc) of the voltage grid 102 on the y-axis against time on the x-axis during the pre¬ charge and startup of the voltage converter 120. In some embodiments, the grid voltage of the voltage grid 102 is measured at the voltage grid 102 using voltage sensors. The graph 502 is divided into four portions, a pre-charge period 550, a grid connection period 552, a motor-startup period 554, and a nominal operation 556. Graph 504 depicts a magnified version of the pre-charge period portion 550 of the voltage grid graph 502. Graph 506 depicts a magnified version of the grid connection period portion 552 of the voltage grid graph 502. Graph 508 depicts a magnified version of the nominal operation portion 556 of the voltage grid graph 502.
[0073] The graph 510 of FIG. 5 depicts a curve that plots the three phase grid current (Igrid.abc) on the y-axis against time on the x-axis during the charge and startup of the voltage converter 120. In some embodiments, the grid current is measured at the circuit breakers 118 that connect the voltage converter 118 to the voltage grid 102 using current sensors. The graph 510 is divided into four portions, a pre-charge period 550, a grid connection period 552, a motor-startup period 554, and a nominal operati on 556. Graph 512 depicts a magnified771982 (P240650W001 )14version of the pre-charge period portion 550 of the grid currents graph 510. Graph 514 depicts a magnified version of the grid connection period portion 552 of the grid currents graph 510. Graph 516 depicts a magnified version of the nominal operation portion 556 of the grid currents graph 510.
[0074] The graph 518 of FIG. 5 depi cts a curve that plots DC-link current (Ide) on the y- axis against time on the x-axis during the charge and startup of the voltage converter 120. In some embodiments, the DC-link current is measured using current sensors after the rectifier 106 and before the DC-link capacitors 122 that connect the rectifier 106 to the SCI 110. The graph 518 is divided into four portions, a pre-charge period 550, a grid connection period 552, a motor-startup period 554, and a nominal operation 556. Graph 520 depicts a magnified version of the pre-charge period portion 550 of the DC-link current graph 518. Graph 522 depicts a magnified version of the grid connection period portion 552 of the DC-link current grid graph 518. Graph 524 depicts a magnified version of the nominal operation portion 556 of the DC-link current graph 518.
[0075] The graph 526 of FIG. 5 depicts a curve that plots DC-link voltage (Vdc) on the y- axis against time on the x-axis during the charge and startup of the voltage converter 120. In some embodiments, the DC-link voltage is measured at the DC-link capacitors 122 that connect the rectifier 106 to the SCI 110 using voltage sensors. The graph 526 is divided into four portions, a pre-charge period 550, a grid connection period 552, a motor-startup period 554, and a nominal operation 556. Graph 520 depicts a magnified version of the pre-charge period portion 550 of the DC-link voltage graph 526. Graph 522 depicts a magnified version of the grid connection period portion 552 of the DC-link voltage graph 526. Graph 524 depicts a magnified version of the nominal operation portion 556 of the DC-link voltage graph 526.
[0076] The graph 534 of FIG. 5 depicts a curve that plots auxiliary voltage measured in the winding 210 of the auxiliary coil of the pre-charge circuit 114 (Vaux) on the y-axis against time on the x-axis during the charge and startup of the voltage converter 120. In some embodiments, the auxiliary voltage is measured at the windings 210 of the auxiliary coil of the pre-charge circuit 114 using voltage sensors. The graph 534 is divided into four portions, a pre-charge period 550, a grid connection period 552, a motor-startup period 554, and a nominal operation 556. Graph 536 depicts a magnified version of the pre-charge period portion 550 of the auxiliary voltage graph 534. Graph 538 depicts a magnified version of the771982 (P240650W001 )15grid connection period portion 552 of the auxiliary voltage graph 534. Graph 540 depicts a magnified version of the nominal operation portion 556 of the auxiliary voltage graph 534.
[0077] The graph 542 of FIG. 5 depicts a curve that plots auxiliary current measured in the winding 210 of the auxiliary coil of the pre-charge circuit 114 (Lux) on the y-axis against time on the x-axis 'during the charge and startup of the voltage converter 120. In some embodiments, the auxiliary current is measured at the windings 210 of the auxiliary coil of the pre-charge circuit 114 using current sensors. The graph 542 is divided into four portions, a pre-charge period 550, a grid connection period 552, a motor-startup period 554, and a nominal operation 556. Graph 544 depicts a magnified version of the pre-charge period portion 550 of the auxiliary current graph 542. Graph 546 depicts a magnified version of the grid connection period portion 552 of the auxiliary current graph 542. Graph 548 depicts a magnified version of the nominal operation portion 556 of the auxiliary current graph 542.
[0078] In some embodiments, in the pre-charge period, the controller 116 may instruct the auxiliary relay 208 of the pre-charge circuit 114 to close so as to connect the LVAC 204 to the auxiliary winding 210 of the auxiliary' coil via the resistor network 206 of the pre¬ charge circuit 114.
[0079] Once the auxiliary relays 208 is closed, current starts flowing into the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114. The flow' of auxiliary current in the auxiliary winding of the auxiliary' coil is shown in graph 544 of FIG. 5. The flow' of current in the auxiliary winding 210 of the auxiliary' coil of the pre-charge circuit 114 creates magnetic flux through a core of the LIT 104. In some embodiments, the phase shifting windings of the LIT 104 operate as secondary windings of a transformer. The creation of magnetic flux in the core of the LIT 104 generates alternating current in the windings of the core of the LIT 104. Since the phase shifting windings of the LIT 104 are connected to the DC-link capacitors 122 via the rectifier 106, the alternating current from the LIT 104 is rectified so that only the positive current values are provided as DC-link current to the DC- link capacitors 122. The flow of DC-link current in the pre-charge period is shown in graph 520. The flow of the DC-link current to the DC-link capacitors 122 charges the DC-link capacitors 122 in the pre-charge period, as shown in graph 528.
[0080] In some embodiments, to limit the inrush current in the auxiliary' winding 210 'hen the auxiliary relay’ 208 is closed, a relatively large resistance is used as part of the resistance network 206. The resistance value of the resistor network 206 is chosen that the DC-link capacitors 122 are charged to the desired values. In some embodiments, in order to771982 (P240650W001 )16balance the resistance of the resistor network 206, one of the resistors of the resistor network 206 may be bypassed during the pre-charge period with another low voltage relay.
[0081] Once the DC-link capacitors are charged to the desired voltage (e.g., Vdc ≈ 7200 volts (V)), the controller 116 may instruct the auxiliary relay 208 to open so as to disconnect the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114. Once the pre¬ charge circuit 114 is disconnected, the charging of the DC-link capacitors 122 is stopped, and the current in the auxiliary winding 210 and the DC-link current go to zero, as shown in graphs 522 and 546.
[0082] As the controller 116 instructs the auxiliary' relay 208 to create a break in the precharge circuit 114, the controller 116 may simultaneously instruct the circuit breakers 118 to close, so as to connect the voltage grid 102 to the voltage converter 120 via the LIT 104.
[0083] As shown in graph 522, once the circuit breakers 118 are closed and the voltage converter 120 is connected to the voltage grid 102, a spike in the DC-link current is observed. In some embodiments, the spike the DC-link current may be observed because the DC-link voltage measured at the DC-link capacitors 122 is lower than the rectified grid voltage during stand-by condition. This current spike as shown in graph 522 raises the voltage of the DC-link to the values around Vdc ≈ 7700 V which is higher than nominal DC-link voltage for this type of rectifier, since the loading of the DC-link capacitors at the moment is low as the AC machine 112 is in stand-by during the pre-charge period. This is captured in the formula below:37T Y7 >Npulse line > 12 line ~ 74 Q / ’ 1 / n x ^dc,nom iso0rx. iso0~!*33 v \ 172 sm--- - v -3 2 sm - v opulse12where Vdc,nom is a nominal dc-link voltage for the line interphase transformer; Viine is AC grid line-to-line rms voltage, and Npulseis a rectifier pulse number.
[0084] The current spike depicted in graph 522 may be below the rated values of the DC-link capacitors 122. According to some embodiments, the DC-link voltage stays roughly around values of 7700 V until the AC machine 112 is engaged. Additionally, and / or alternatively, it is seen in the graphs 502-548, that even after voltage converter 120 is connected to the voltage grid 102, voltage over the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 remains to be low voltage. This hugely simplifies the design of the auxiliary' winding 210 of the auxiliary coil, and in turn, the pre-charge circuit 114.771982 (P240650W001 )17
[0085] In some embodiments, after the voltage converter 120 is connected to the voltage grid 102, normal operation of the AC machine 112 is initiated. In such embodiments, the AC machine 112 may be a motor and a corresponding motor stall sequence may be initiated in which the motor ramps up speed to nominal values. Graphs 508, 516, 524, 532, 540, and 548 depict nominal operation waveforms (from front end side perspective). In some embodiments, the grid current shown in graphs 514 and 516 appear as a 12-pulse form, as is to be expected. In such cases, the DC-link capacitors are charged to a value of approximately 7430 V as shown in graphs 528 and 530. At the same time, the DC-link current is approximately 210 A, with a minor 12 pule ripple, as shown in graphs 522 and 524.
[0086] As the pre-charge circuit 114 is disconnected from the LIT 104 during nominal operation, the voltage over the auxiliary winding remains to be low voltage as shown in graphs 538 and 540, and auxiliary current is approximately 0 A as shown in graphs 546 and 548.
[0087] According to embodiments of the present disclosure, the pre-charge circuit 114 utilizes the existing line interphase transformer 104 core to help pre-charge the DC-link capacitors 122. By utilizing the core of the existing LIT 104 the resources and complexity required to implement the pre-charge circuit 114 is considerably reduced.
[0088] FIGS. 6A-6C illustrate simplified circuit diagrams depicting the exemplary pre¬ charge circuit as part of a four-limb core line interphase transformer, in accordance with one or more examples of the present disclosure.
[0089] FIGS. 6A-6C depict a four-limb LIT 104. In some embodiments, the four-limb LIT 104 may be configured for three phase systems. For example, system 600 of FIG. 6A depicts the four-limb LIT 104 with main limbs 602, 604, 606, and common (or central) limb 608. This type of core is equivalent to three single phase U-cores in contact with each other. Each of the main limbs 602, 604, and 606 include the first winding wa, the second winding wt and the third winding Wab. For example, the first main limb 604 includes a first winding WaB 316, a second winding WI, B 318, and a third winding WabB 314. Similarly, the second main limb 602 includes a first winding wac 304, a second winding wbc 306, and a third winding Ware 302. The third main limb 606 includes a first winding WaA 310, a second winding wbA 312, and a third winding wa A 308.
[0090] Each of the first winding wa, a second winding wb and a third winding Wab from each of the limbs 602, 604, and 606 are connected to the rectifier 106.771982 (P240650W001 )18
[0091] FIG. 6A depicts a pre-charge circuit 114 that is similar to the pre-charge circuit 114 as shown in system 100 of FIG. 1. An auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 is placed on the first main limb 604 of the LIT 104, as shown diagram 600 of FIG. 6A. The auxiliary coil 210 is connected to a low-voltage alternating current (LVAC) source 204 via a relay 208, and a resistor network 206.
[0092] FIG. 6B is similar to FIG. 6A, except that in the diagram 625 of FIG. 6B, the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 is placed on the third main limb 606 of the LIT 104. In some alternate embodiments, the auxiliary winding 210 of the auxiliary' coil of the pre-charge circuit 114 may be placed on the second main limb 602 of the LIT 104.
[0093] FIG. 6C is similar to FIG. 6A, except that in the diagram 650 of FIG. 6C, the auxiliary winding 210 of the auxiliary' coil of the pre-charge circuit 114 is placed on the central limb 608 of the LIT 104.
[0094] As shown with respect to FIGS. 6A-6C, the pre-charge circuit 114 may include a single auxiliary coil as part of the auxiliary circuit 210 that may be coupled to a four-limb core for successful charging of the DC~link capacitors of the SCI 110. As discussed previously, the position of the auxiliary winding 210 of the auxiliary coil of the pre-charge circuit 114 may depend on many factors such as size and available space, insulation requirements, magnetic saturation of the core, etc.
[0095] In some embodiments, with the LIT 104 as shown in FIGS. 6A-6C constructed with four-limbs in a single core, waand wbwindings from all three phases (located on main legs) during pre-charge are connected to the DC-link through a rectifier and participate in charging.
[0096] FIG. 7 is a block diagram of an exemplary system or device 700 within the environment 100 such as the controller 720. The system 600 includes a. processor 704, such as a central processing unit (CPU), and / or logic, which executes computer executable instructions for performing the functions, processes, and / or methods described herein. In some examples, the computer executable instructions are locally stored and accessed from a non-transitory computer readable medium, such as storage 710, which may be a hard drive or flash drive. Read Only Memory- (ROM) 706 includes computer executable instructions for initializing the processor 704, while the random-access memory' (RAM) 708 is the main memory for loading and processing instructions executed by the processor 704. The network interface 712 may connect to a wired network or cellular network and to a local area network771982 (P240650W001 )19or wide area network. The system 700 may also include a bus 702 that connects the processor 704, ROM 706, RAM 708, storage 710, and / or the network interface 712. The components within the system 700 may use the bus 702 to communicate with each other. The components within the system 700 are merely exemplary and might not be inclusive of every component within the controller 116. Additionally, and / or alternatively, the system 700 may further include components that might not be included within every’ entity of environment 700. For instance, in some examples, the controller 116 might not include a network interface 712.
[0097] FIG. 8 illustrates an exemplary process to pre-charge voltage converters having line interphase transformers using a pre-charge circuit, in accordance with one or more examples of the present application. In some embodiments, the process 800 may’ be performed by the environment 100 of FIG, 1. However, it will be recognized that any of the following blocks may be performed in any suitable order and that the process 800 may be performed in any’ environment and by any suitable computing device and / or controller. For instance, the process 800 may also be performed by the controller 116 shown in FIG. 1.
[0098] At 802, the controller 116 provides to one or more switching devices, a first instruction to electrically couple the auxiliary coil to the low voltage AC network to begin charging of a plurality of DC -link capacitors. For example, in order to ensure smooth connection of the AC grid 102 to the voltage converter 120 such that power is supplied smoothly from the AC grid 102 to the AC machine 112, the DC-link capacitors 122 may need to be pre-charged to their nominal value before the voltage converter 120 is connected to the AC grid 102, System 100 of FIG. 1 includes a controller 116 that ensures the proper pre¬ charging of the DC-link capacitors 122 before the connection of the LIT 104 to the AC grid 102 via circuit breakers 118. In some embodiments, in a pre-charge phase before the voltage converter 120 is connected to the AC grid 102, the controller 116 may’ instruct a pre-charge circuit 114 to charge the DC-link capacitors 122 to their nominal value. In some embodiments, the pre-charge circuit 114 includes an auxiliary coil that is coupled to a core of the LIT 104. The auxiliary- coil of the pre-charge circuit 114 may be coupled to low voltage AC network through a low-voltage resistor and a low voltage breaker.
[0099] At 804, the controller 116 provides to a low-voltage circuit breaker, a second instruction to electrically decouple the auxiliary coil from the low voltage AC network, based on determining that the plurality'’ of DC-link capacitors have reached a predetermined DC-link voltage. For example, the controller 116 may instruct the pre-charge circuit 114 to disconnect from LVAC.771982 (P240650W001 )20
[0100] At 806, the controller 116 provides to the one or more switching devices, a third instruction to electrically couple the voltage converter to the voltage grid. For example, once the DC-link capacitors 122 are properly pre-charged, the controller 116 may provide instructions to the circuit breakers 118 to close, so that the voltage converter 120 is connected to the AC grid 102.
[0101] While embodiments of the invention have been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. For example, the various embodiments of the kinematic, control, electrical, mounting, and user interface subsystems can be used interchangeably without departing from the scope of the invention. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.
[0102] The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or ‘"the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and / or C” or “at least one of A, B or C” should be interpreted as including any singular entity' from the listed elements, e.g., A, any' subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
Claims
771982 (P240650W001 )21CLAIMS1. A voltage converter comprising:a line interphase transformer having a first end and a second end, the first end of the line interphase transformer being coupled to a multi-pulse rectifier;a plurality of direct current-link (DC-link) capacitors, each of the DC-link capacitors having a first end and a second end, wherein each of the plurality' of DC-link capacitors is electrically coupled to a plurality of inverters at a first end and electrically coupled to the multi-pulse rectifier at a second end;an auxiliary coil having a plurality of windings, wherein the auxiliary coil is electrically coupled to a low voltage AC network via low voltage resistors and a low voltage circuit breaker;one or more switching devices to electrically couple or decouple the voltage converter with a voltage grid; anda controller configured to:provide to the one or more switching devices, a first instruction to electrically couple the auxiliary coil to the low voltage AC network to begin charging of the plurality' of DC-link capacitors;based on determining that the plurality of DC-link capacitors have reached a predetermined dc-link voltage, provide to the low voltage circuit breaker, a second instruction to electrically decouple the auxiliary' coil from the low' voltage AC network; andprovide to the one or more switching devices, a third instruction to electrically couple the voltage converter to the voltage grid.
2. The voltage converter of claim 1, wherein the controller is further configured to:initiate normal operation of the voltage converter, wherein the voltage converter controls operation of a multi-phase AC machine.
3. The voltage converter of claim 1, wherein the line interphase transformer comprises three single-phase cores, and wherein the plurality of windings of the auxiliary' coil are disposed on any' of the three single phase cores of the line interphase transformer.771982 (P240650W001 )224. The voltage converter of claim 3, wherein each single phase core of the line interphase transformer is shell-type or E-type.
5. The voltage converter of claim 3, wherein each single phase core of the line interphase transformer is core-type or U-type.
6. The voltage converter of claim 1, wherein the line interphase transformer comprises a five-limb core, and wherein the plurality of windings of the auxiliary' coil are disposed on any' core limb of the five-limb core.
7. The voltage converter of claim 1, wherein the line interphase transformer comprises a four-limb core, and wherein the plurality of windings of the auxiliary' coil are disposed on any core limb of the four-limb core.
8. The voltage converter of claim 4, wherein the auxiliary' coil with the plurality of windings is disposed on any limb of the shell-type or E-type single phase core.
9. The voltage converter of claim 5, wherein the auxiliary coil with the plurality' of windings is disposed on any limb of the core-type or U-type single phase core.
10. The voltage converter of claim 1, wherein the multi-pulse rectifier is at least one of a 12-pulse rectifier, 18-pulse rectifier, and a 24-pulse rectifier.
11. The voltage-converter of claim 1, wherein the plurality of inverters are arranged in a series connected topology' and are configured to supply power to a multi-phase AC machine.
12. The voltage converter of claim 1, w'herein the line interphase transformer is composed of at least one of ferrite, metal magnetic powders, amorphous-alloys, and electrical steel.
13. The voltage converter of claim 1, w'herein the second end of the linear interphase transformer is connected to the one or more switching devices.
14. The voltage converter of claim 1, w'herein the second end of the linear interphase transformer is connected to the one or more switching devices through a grid inductor.771982 (P240650W001 )2315. A method to pre-charge a voltage converter, comprising:providing to one or more switching devices using a controller, a first instruction to electrically couple an auxiliary coil to a low voltage AC network to begin charging of a plurality of DC-link capacitors, wherein the auxiliary coil comprises a plurality of windings, wherein the auxiliary coil is electrically coupled to a low voltage AC network via low voltage resistors and a low voltage circuit breaker;based on determining that the plurality of DC-link capacitors have reached a predetermined DC-link voltage, providing to the low voltage circuit breaker, a second instruction to electrically decouple the auxiliary coil from the low voltage AC network; and providing to the one or more switching devices, a third instruction to electrically couple the voltage converter to the voltage grid.
16. The method of claim 15, wherein the line interphase transformer comprises three single-phase cores, and wherein the plurality' of windings of the auxiliary' coil are disposed on any of the three single phase cores of the line interphase transformer.
17. The method of claim 17, wherein each single phase core of the line interphase transformer is shell-type or E-type.
18. The method of claim 17, wherein each single phase core of the line interphase transformer is core-type or U-type.
19. The method of claim 15, wherein the line interphase transformer comprises a five-limb core, and wherein the plurality' of windings of the auxiliary’ coil are disposed on any core limb of the five-limb core.
20. The method of claim 15, wherein the line interphase transformer comprises a four-limb core, and wherein the plurality of windings of the auxiliary coil are disposed on any core limb of the four-limb core.