Synchronous grid-side harmonic filters and pre-charged battery capacitors in modular multilevel converters

Abstract

Synchronizing the grid-side harmonic filter and pre-charged battery capacitors in a modular multilevel converter can be achieved by: disconnecting a transmission circuit breaker on a first path between the grid and the converter system; closing a pre-charge contactor on a second path between the grid and the converter system, the pre-charge contactor comprising a set of pre-charge resistors; connecting the grid to the converter system and harmonic filter via the second path; selectively charging the battery capacitors in the converter system until a charge threshold is reached, wherein a smaller subset of the battery capacitors is charged at a given time compared to an earlier time, and each battery capacitor is charged to a higher battery voltage compared to an earlier time; and closing the transmission circuit breaker, and simultaneously connecting the generator to the converter system via a generator circuit breaker while closing the pre-charge contactor.

Description

Technical Field

[0001] The embodiments presented in this disclosure generally relate to modular multilevel converters (MMCs) and the capacitors contained therein. Specific embodiments described herein depict synchronization and pre-charging schemes for filters and capacitors. Background Technology

[0002] Wind turbine generators (WTGs) are an increasingly popular source of power generation, which can be deployed individually or comprised of several wind turbines, often referred to as a wind farm. In WTGs and other power generation or consumption systems connected to the grid or distribution lines, millimeter-scale converters (MMCs) can be used to electrically connect two powered systems operating with different voltage / current schemes. When initiating a connection between two powered systems, the MMC equalizes the voltage / current difference between the two systems to reduce power surges (e.g., inrush currents from the high-voltage side to the low-voltage side) and other anomalous effects. For example, in a WTG, the MMC can equalize the difference via a DC (direct current) link located between the machine-side converter (MSC) and the line-side converter (LSC), which is charged to a predefined level before contact via a series of charging components such as one or more transformers, diode bridges, current-limiting resistors, fuses, circuit breakers, switches, etc. Summary of the Invention

[0003] One embodiment of this disclosure is a method comprising: initializing a contactor to connect a power grid to a harmonic filter and a modular multilevel converter (MMC), wherein the MMC is disposed between the power grid and a generator, and the harmonic filter is disposed between the power grid and the MMC. The initializing contactor comprises: disconnecting a transmission circuit breaker disposed on a first path, wherein the transmission circuit breaker is disposed between the power grid and the MMC, and closing a pre-charging contactor disposed on a second path, wherein the pre-charging contactor is disposed between the power grid and the MMC, wherein the second path is parallel to the first path and includes a set of pre-charging resistors; connecting the power grid to the MMC and the harmonic filter via the second path; pre-charging a cell capacitor in the MMC in an un-driven stage; pre-charging a cell capacitor in a driven stage in response to the cell voltage of the cell capacitor satisfying a drive threshold; and pre-charging a generator connected to the MMC via a generator circuit breaker while the transmission circuit breaker and the pre-charging contactor are closed in response to the cell voltage satisfying a charge threshold, the drive threshold being based on the peak phase-to-phase voltage of the power grid divided by the number of cell capacitors disposed on a given phase of the MMC.

[0004] In one implementation, in conjunction with any of the methods described above or below, the drive threshold is based on the peak phase-to-phase voltage of the grid voltage divided by the number of battery capacitors set on a given phase of the MMC.

[0005] In one implementation, in conjunction with any of the methods described above or below, the driven phase employs a progressive driving scheme that iteratively increases by one relative to the number of battery capacitors that would be bypassed at a given time in the previous time period, in order to increase the charge level in the unbypassed battery capacitors.

[0006] In one implementation, in conjunction with any of the methods described above or below, the driven phase employs a halving drive scheme that exponentially increases the number of battery capacitors that would be bypassed at a given time relative to the previous time, in order to increase the charge level in the unbent battery capacitors.

[0007] In one implementation, in conjunction with any of the methods described above or below, the method further includes: disconnecting the pre-charge contactor after connecting the generator.

[0008] In one implementation, in conjunction with any of the methods described above or below, the resistance of the pre-charge resistor is selected based on the impedance of the harmonic filter at the grid frequency of the power grid.

[0009] In one implementation, in conjunction with any of the methods described above or below, the driven phase places a given dual battery in the MMC into an operating mode comprising: a natural blocking mode for charging a first battery capacitor and a second battery capacitor in the given dual battery; a first forced bypass mode for charging the first battery capacitor and bypassing the second battery capacitor; a second forced bypass mode for charging the second battery capacitor and bypassing the first battery capacitor; and a third forced bypass mode for bypassing the first battery capacitor and the second battery capacitor.

[0010] One embodiment of this disclosure is a power conversion and transmission system, comprising: a grid circuit breaker configured to selectively connect a grid to the power conversion and transmission system; a modular multilevel converter (MMC) including a plurality of batteries, each battery including a battery switch and a battery capacitor; a generator circuit breaker configured to selectively connect a generator to a machine-side converter of an MMC; and a harmonic filter connected to a transmission line connected to a line-side converter of the MMC, wherein the transmission line defines a first path when the transmission circuit breaker is closed and a second path when a pre-charge contactor is closed, wherein the first path connects the grid circuit breaker to the harmonic filter and the line-side converter, and wherein the second path connects the grid circuit breaker to the MMC. The harmonic filter and line-side converter are connected via a pre-charge resistor, wherein the first path is parallel to and bypasses the second path; and a controller configured to pre-charge the battery capacitors and synchronize the harmonic filter with the grid by: driving the battery switch into a forced bypass mode in response to the battery voltage in the battery capacitors meeting a drive threshold to raise the battery voltage to a charge threshold; and simultaneously closing the pre-charge contactor in response to the battery voltage in the battery capacitors meeting the charge threshold: closing the transmission circuit breaker and closing the generator circuit breaker, and the drive threshold is based on the peak phase-to-phase voltage of the grid voltage divided by the number of battery capacitors set on a given phase of the MMC.

[0011] In one implementation, in conjunction with any system described above or below, each of the plurality of batteries is a dual-battery system comprising two battery capacitors and four battery switches.

[0012] In one implementation, in conjunction with any of the systems described above or below, the controller is further configured to disconnect the pre-charge contactor after closing the generator circuit breaker.

[0013] In one implementation, in conjunction with any system described above or below, a forced bypass mode bypasses at least one battery capacitor included in the battery in order to apply a rectified voltage from the power grid across a subset of the battery capacitors.

[0014] In one implementation, in conjunction with any system described above or below, the controller is further configured to stop boosting the battery voltage in response to the battery voltage in the battery capacitor satisfying the charge, and to place the battery in an operating mode intended to invert and rectify the power supplied from the generator to provide it to the grid.

[0015] In one implementation, in conjunction with any system described above or below, the resistance of the pre-charge resistor is configured to be less than 10% of the impedance of the harmonic filter at the mains frequency.

[0016] In one implementation, in conjunction with any of the systems described above or below, the system is further configured for three-phase power transmission.

[0017] One embodiment of this disclosure is a controller unit for a power conversion and transmission system, comprising: a processor; and a memory including pre-charge control logic, which, when executed by the processor, enables the controller unit to perform the following operations: initializing a circuit breaker to connect a power grid to a harmonic filter and a converter system, wherein initializing the circuit breaker includes: disconnecting a transmission circuit breaker disposed on a first path, wherein the transmission circuit breaker is disposed between the power grid and the converter system; and closing a pre-charge contactor disposed on a second path, wherein the pre-charge contactor is disposed between the power grid and the converter system, wherein the second path includes a... A set of pre-charge resistors, wherein a second path is parallel to a first path that bypasses the set of pre-charge resistors; the grid is connected to the converter system and harmonic filters via the second path; the battery capacitors in the converter system are selectively charged through a series of iterations until a charge threshold is reached, wherein each iteration in the series charges a smaller subset of the battery capacitors at a given time compared to an earlier iteration in the series, and each battery capacitor is charged to a higher battery voltage compared to an earlier iteration; and a transmission circuit breaker is closed, and the generator is connected to the converter system via a generator circuit breaker while the pre-charge contactor is closed. Attached Figure Description

[0018] To gain a more detailed understanding of the features of this disclosure, the disclosure briefly summarized above can be described in more detail with reference to embodiments, some of which are illustrated in the accompanying drawings. However, it should be noted that the accompanying drawings only show typical embodiments of this disclosure and should not be construed as limiting its scope, as other equally effective embodiments are permissible with respect to this disclosure.

[0019] Figure 1 A schematic diagram of a wind turbine according to an embodiment described in this disclosure is shown.

[0020] Figure 2 A schematic diagram showing the components inside the nacelle and tower of a wind turbine according to an embodiment described in this disclosure.

[0021] Figure 3 A schematic diagram of a power conversion and transmission system according to an embodiment of the present disclosure is shown.

[0022] Figure 4 This is a schematic diagram of a modular multilevel converter used as a converter system according to an embodiment of the present disclosure.

[0023] Figure 5This is a schematic diagram of a harmonic filter according to an embodiment of the present disclosure.

[0024] Figures 6A-6E These are a series of schematic diagrams of batteries in various operating modes according to embodiments of the present disclosure.

[0025] Figure 7A and 7B The operation of the line-side converter and the machine-side converter according to embodiments of the present disclosure is demonstrated.

[0026] Figure 8 This is a flowchart illustrating how a harmonic filter is synchronized while the battery in a modular multilevel converter is being precharged, according to an embodiment of this disclosure.

[0027] Figure 9 This is a block diagram of a controller unit that can be used as a controller according to one or more embodiments of the present disclosure.

[0028] For ease of understanding, the same reference numerals are used to denote common elements in the figures where possible. It is conceivable that elements disclosed in one embodiment may be usefully utilized in other embodiments without specific description. Detailed Implementation

[0029] This paper presents a system and method for a novel control scheme to precharge an MMC (Modular Multilevel Converter) while synchronizing a harmonic filter with the grid voltage. This control scheme allows inrush currents from the grid to be prevented from entering the harmonic filter, thereby protecting against overcurrent and increasing the lifespan of the filter components and other related system equipment. In the undriven phase, before the grid is directly connected to the MMC, the capacitors in the MMC and harmonic filter are precharged from the grid to a threshold level via precharge resistors. The undriven phase can continue until the sum of the battery voltages in the arms equals the rectified (peak) line-to-line voltage of the grid in the line-side converter of the MMC. After the undriven phase, the MMC enters a driven or controlled phase, during which the charge of the battery capacitors is increased to the rated operating voltage by progressively reducing the number of battery capacitors that are connected in series with the grid each time. In the driven phase, the MMC swaps or interchanges which capacitors are affected by the charging current until all capacitors reach the threshold voltage, at which point normal operation of the MMC can begin.

[0030] Example Implementation

[0031] Figure 1This diagram illustrates a horizontal axis wind turbine generator (WTG) 100. The WTG 100 typically includes a tower 102 and a wind turbine nacelle 104 positioned at the top of the tower 102. A wind turbine rotor 106 is connected to the nacelle 104 via a low-speed shaft extending outside the nacelle 104. The wind turbine rotor 106 includes three rotor blades 108 mounted on a common hub 110, rotating in the rotor plane, but may include any suitable number of blades, such as one, two, four, five, or more. Each blade 108 (or wing) typically has an aerodynamic shape, with a leading edge 112 facing the wind, a trailing edge 114 located at the opposite end of the chord of the blade 108, a tip 116, and a root 118 for connection to the hub 110 in any suitable manner.

[0032] In some implementations, blades 108 may be connected to hub 110 using pitch bearings 120, such that each blade 108 can rotate about its longitudinal axis to adjust the blade pitch. The pitch angle of blade 108 relative to the rotor plane can be controlled, for example, by a linear actuator, hydraulic actuator, or stepper motor connected between hub 110 and blade 108.

[0033] Figure 2 A schematic diagram showing typical components within the nacelle 104 and tower 102 of the WTG 100. When wind 200 drives the blades 108, the rotor 106 rotates, causing the low-speed shaft 202 to rotate. Gears in the gearbox 204 mechanically convert the low speed of the low-speed shaft 202 into the relatively high speed of the high-speed shaft 208, suitable for generating electricity using the generator 206.

[0034] The controller 210 can sense the rotational speed of one or both shafts 202, 208. If the controller determines that the shaft is rotating too fast, it can signal a braking system 212 to slow the rotation of the shaft, thereby slowing the rotation of the rotor 106—that is, reducing the revolutions per minute (RPM). The braking system 212 can prevent damage to the components of the WTG 100. The controller 210 can also receive input from an anemometer 214 (providing wind speed) and / or a wind vane 216 (providing wind direction). Based on the received information, the controller 210 can send control signals to one or more blades 108 to attempt to adjust the blade pitch 218. By adjusting the blade pitch 218 relative to the wind direction, the rotational speed of the rotor (and therefore the shafts 202, 208) can be increased or decreased. For example, based on wind direction, controller 210 can send control signals to components including yaw motor 220 and yaw drive 222 to rotate nacelle 104 relative to tower 102, thereby allowing rotor 106 to be positioned more (or in some cases less) upwind.

[0035] Figure 3 A schematic diagram of a power conversion and transmission system (PCTS) 300 according to an embodiment of the present disclosure is shown. As will be understood, the configuration shown is merely an example of a PCTS 300 schematic diagram and should not be construed as a limitation on the use of the disclosed embodiments. Those skilled in the art will understand that alternative arrangements of the proposed components, substitutions of the proposed components, and omissions of various components are all considered to be within the scope of this disclosure.

[0036] Figure 3 The PCTS 300 shown is a three-phase system that transmits power of three different phases on three separate transmission lines 301a-c (collectively referred to as transmission lines 301), although PCTS 300s with one, two, or more phases on corresponding numbers of transmission lines 301a-N may also employ the teachings of this disclosure. Unless otherwise stated or apparent from the context of this disclosure, each component of the PCTS 300 either comprises N instances, where each instance aN is associated with a phase aN, or is connected to each of the N phases.

[0037] In PCTS 300, converter system 310 receives alternating current (AC) from generator 320 (e.g., WTG 100) on a number of generator lines 302a-c (collectively referred to as generator lines 302) corresponding to the number of phases of the power generated by generator 320. When generator 320 produces power to supply the grid 330, converter system 310 rectifies the AC to direct current (DC) and inverts the DC to another type of AC suitable for supplying the grid 330 via transmission line 301. In various embodiments, the frequency at which the grid 330 receives power may differ from the frequency at which generator 320 provides power. Figure 4 The components of the three-phase converter system 310 are discussed in more detail. The grid transformer 340 regulates the voltage of the AC power output from the converter system 310 to a voltage suitable for the grid 330 (e.g., the grid voltage). A harmonic filter 350 connected to the transmission line 301 between the converter system 310 and the grid transformer 340 (or the grid 330) can be used to adjust and regulate the AC power supplied to the grid 330. In conjunction with... Figure 5 The components of the three-phase harmonic filter 350 will be discussed in more detail.

[0038] Various additional components are included in the PCTS 300 to control the power transfer to / from the converter system 310 and the power grid 330, such as during startup and connection or shutdown and disconnection procedures. A controller 900 communicates with various components and current or voltage sensors 371a, 371b (collectively referred to as sensors 371) located throughout the PCTS 300. Figure 3(Not shown) The controller 900 can control the operation of generator 320, auxiliary power supply (APS) 370, auxiliary transformer 380, and / or various switches to influence whether and how power is supplied to / from the grid 330 or components within the PCTS 300. The controller 900 can be fed power from the APS 370 and communicates with various components of the PCTS 300 via a wireless channel or a wired connection (not shown). Figure 9 The components of controller 900 will be discussed in more detail.

[0039] Several circuit breakers and / or contactors 360a-f (collectively referred to as circuit breakers or contactors 360) are located at different points in the PCTS 300 to direct or block the transmission of power to / from specific parts of the PCTS 300. For example, a grid circuit breaker 360a located between the grid 330 and the grid transformer 340 can serve as a common coupling point (PCC) between the PCTS 300 and the grid 330 to connect or disconnect the PCTS 300 to / from the grid 330. In another example, an auxiliary transformer circuit breaker 360b and an APS circuit breaker 360c can connect or disconnect the auxiliary transformer 380 to / from the converter system 310 and the APS 370, respectively. In some embodiments, the auxiliary transformer circuit breaker 360b is a medium-voltage circuit breaker located between the grid 330 and the auxiliary transformer 380 to isolate the auxiliary transformer 380 from faults or during maintenance / inspection on the main side. In some implementations, the APS circuit breaker 360c is a low-voltage circuit breaker located between the auxiliary transformer 380 and the APS 370 to resist faults on the APS side (or secondary side) of the auxiliary transformer 380 or to provide protection during power outages of auxiliary equipment powered via the APS 370.

[0040] Similarly, a generator circuit breaker 360d located between the generator 320 and the converter system 310 can connect or disconnect the generator 320 to / from the converter system 310.

[0041] The transmission circuit breaker 360e is disposed on a first path between the converter system 310 (and harmonic filter 350) and the grid transformer 340. The transmission circuit breaker 360e may include a first set of switches 361e for establishing or disconnecting electrical contact on the power transmission line 301 between the converter system 310 and the grid 330, and a second set of switches 362e for establishing or disconnecting electrical contact on the electrical path between the ground and the transmission line 301.

[0042] A pre-charge contactor 360f is disposed on the second path in the PCTS 300. This second path can be alternatively referred to as the pre-charge path because it provides an alternative path around the transmission circuit breaker 360e (i.e., connected to the transmission line 301 at both a first point upstream and a second point downstream of the first set of switches 361e of the transmission circuit breaker 360e) and is used during pre-charging of the converter system 310. The pre-charge contactor 360f may include a first set of switches 361f to establish or disconnect electrical contact on the transmission line 301 between the converter system 310 and the grid 330, and a second set of switches 362f to establish or disconnect electrical contact on the electrical path between ground and the transmission line 301. The pre-charge contactor 360f also includes a set of fuses 363f and a set of pre-charge resistors 390 (or may be associated with them) on the transmission line 301, which ensure that the pre-charge current supplied from the grid 330 does not exceed the design capacity of the converter system 310. The precharge resistor 390 is configured with a predefined resistor, which is selected based on the impedance of the harmonic filter 350.

[0043] By opening and closing transmission circuit breaker 360e, controller 900 (respectively) blocks or allows current from grid 330 to flow through transmission circuit breaker 360e to converter system 310 via the first path. Similarly, by opening and closing pre-charge contactor 360f, controller 900 (respectively) blocks or allows current from grid 330 to flow through pre-charge contactor 360f to converter system 310 via the second path. Because a pre-charge resistor 390 is present in the second path, while a similar resistor is lacking in the first path, current tends to flow through the first path when both paths are available. Therefore, to force current through pre-charge resistor 390, when pre-charging converter system 310, controller 900 blocks the first path (i.e., opens transmission circuit breaker 360e) and allows the second path (i.e., closes pre-charge contactor 360f).

[0044] When using power from the grid 330 for pre-charging and synchronization, the first set of switches 361f in the pre-charging contactor 360f and the switch in the grid circuit breaker 360a are closed. Simultaneously, the first set of switches 361e in the transmission circuit breaker 360e, the second set of switches 362e in the transmission circuit breaker 360e, the second set of switches 362f in the pre-charging contactor 360f, and the switch in the generator circuit breaker 360d are opened, while the switch in the auxiliary transformer circuit breaker 360b is closed. Therefore, power from the grid 330 flows into the PCTS 300 on transmission line 301 and through the pre-charging resistor 390 to the harmonic filter 350 and the converter system 310.

[0045] Figure 4This is a schematic diagram of an MMC 400 used as a converter system 310 according to an embodiment of the present disclosure. Figure 4 Can be with Figure 3 To understand this in context, the MMC 400 operates by converting AC power supplied by generator 320 to DC power via machine-side converter (MSC) 410, and then converting the DC power back to AC power via line-side converter (LSC) 420 to supply the grid 330. MSC 410 and LSC 420 are coupled together via DC link 430. Various switches in MSC 410 and LSC 420 control how the MMC 400 charges or discharges, and how the power is rectified or inverted, which can be controlled by controller 900 or another subordinate or independent control unit. Each of MSC 410 and LSC 420 includes multiple batteries, which can be added to or removed from the converter in a modular fashion, in series with one or more other batteries, to adjust the converter's ability to handle different voltage inputs and outputs when converting power from AC to DC or from DC to AC. These batteries include various switches and capacitors, combined with... Figures 6A-6E A more detailed discussion.

[0046] DC link 430 transmits DC power between MSC 410 and LSC 420 on first rail 401a and second rail 401b (collectively referred to as rail 401) and includes DC link capacitors 440a-b (collectively referred to as DC link capacitors 440) disposed between rails 401. In various embodiments, DC link capacitors 440 adjust the DC voltage between MSC 410 and LSC 420. In some embodiments, DC link capacitors 440 are connected to a neutral (common) voltage node connected to ground via a transmission resistor 450, and the non-common terminal of DC link capacitors 440 is connected to the opposite DC link voltage rail 401 (e.g., to one of the positive or negative rails 401). Although in Figure 4 The diagram shows two DC link capacitors 440, but in other embodiments, the DC link may include more or fewer DC link capacitors 440 (including none).

[0047] Before connecting generator 320 to grid 330 (via converter system 310 and associated circuit breaker 360), the operator precharges the DC link capacitor 440 and the capacitors in MSC 410 and LSC 420 to reduce the amount of inrush current. Generally, while precharging the capacitors, controller 900 manages the switches in MMC 400 to gradually build up power across rail 401, charging the capacitors to store threshold voltages before connecting converter system 310 to generator 320. Generator 320 is soft-started via MSC 410 and remains stationary until MMC 400 completes precharging. As described herein, the precharging of the capacitors is performed in parallel with (i.e., substantially simultaneously with) the harmonic filter 350 synchronizing to grid power.

[0048] Figure 5 This is a schematic diagram of a harmonic filter 350 according to an embodiment of the present disclosure. Figure 5 Can be with Figure 3 To understand this in context, the harmonic filter 350 is shunt to the transmission line 301 between the converter system 310 and the power grid 330. The harmonic filter 350 uses a set of resonant circuits connected to each transmission line 301 to mitigate the risk and impact of harmonic currents in the PCTS 300 caused by the different current frequencies between the power grid 330 and the converter system 310. Although illustrated as a single-tuned filter, in other embodiments, the harmonic filter 350 can be a double-tuned filter, a high-pass filter, etc.

[0049] As shown in the figure, the harmonic filter 350 includes filter fuses 510a-c (collectively referred to as filter fuses 510) and inductors 520a-c (collectively referred to as filter inductors 520) connected to each corresponding transmission line 301 on a first side, and filter capacitors 530a-c (collectively referred to as filter capacitors 530) connected to a shared node 540 on a second side. The filter fuses 510 protect the filter inductors 520 and filter capacitors 530 on the same line from overcurrents originating from the transmission line 301, which are associated with the difference between the mains voltage and the stored voltage in the harmonic filter 350.

[0050] The filter inductor 520 was selected to have L f The filter inductor, and the filter capacitor 530 is selected to have C f The filter capacitor results in a significantly higher filter impedance at the mains frequency (typically 50Hz or 60Hz ± 10%) than the pre-charge resistor R390. pre (that is, |Z) filter |>>R pre In various implementations, the resistor R preLess than the filter impedance Z filter 10% of (i.e., 0.1*|Z) filter |>R pre When the resistance R of the pre-charge resistor 390... pre When the impedance of the harmonic filter 350 is negligible, the voltage drop through the pre-charge resistor 390 is also negligible, and the voltage V of the harmonic filter 350 is also negligible. f and grid voltage V g Their sizes are basically equal (e.g., |V g |-5%≤|V f |≤|V g Therefore, for the angular velocity ω, the values ​​of the pre-charge resistor 390, the filter inductor 520, and the filter capacitor 530 are selected according to Formula 1 based on the grid frequency f (e.g., ω = 2πf).

[0051]

[0052] Figures 6A-6E These are a series of schematic diagrams of the dual battery 600 in various operating modes according to embodiments of the present disclosure. Figures 6A-6E Each dual battery 600 allows for the rectification or inversion of current through controlled switching and discharging of its components. For ease of reference, a series of nodes 610a-e (collectively referred to as nodes 610 or battery nodes) are provided in each dual battery 600. The first node 610a and the fifth node 610e are the contacts of the dual battery 600 through which the dual battery 600 can be connected to another dual battery 600, rail 402, arm reactor 710 (in conjunction with...). Figure 7A and 7B (Discussion) or other external components of the dual-battery 600.

[0053] The illustrated dual-cell battery 600 includes a first battery capacitor 620a (collectively referred to as battery capacitor 620) disposed between a second node 610b and a third node 610c, and a second battery capacitor 620b disposed between the third node 610c and a fourth node 610d. Although illustrated as a four-switch, two-capacitor battery, other batteries with more or fewer switches and capacitors may be used in other embodiments. For example, in cases where a similar component-to-node arrangement is shown, two two-switch, one-capacitor batteries can be considered as a single four-switch, two-capacitor battery.

[0054] Switches 630a-d (collectively referred to as switches 630 or battery switches) and corresponding diodes 640a-d (collectively referred to as diodes 640) are arranged in parallel and driven to open or close to define various operating modes in the dual battery 600. A first switch 630a and a first diode 640a are disposed between a first node 610a and a second node 610b, wherein the first diode 640a is biased to block current flowing from the second node 610b to the first node 610a. A second switch 630b and a second diode 640b are disposed between the first node 610a and a third node 610c, wherein the second diode 640b is biased to block current flowing from the first node 610a to the third node 610c. A third switch 630c and a third diode 640c are disposed between the third node 610c and a fifth node 610e, wherein the third diode 640c is biased to block current flowing from the third node 610c to the fifth node 610e. A fourth switch 630d and a fourth diode 640d are disposed between the fifth node 610e and the fourth node 610d, wherein the fourth diode 640d is biased to block the current flowing from the fifth node 610e to the fourth node 610d.

[0055] When driven to the closed state by controller 900, switch 630 provides a path bypassing the associated diode 640, thereby allowing current to flow against the bias of the associated diode 640, thus bypassing one or more battery capacitors 620 in the dual battery 600. Although shown as an insulated-gate bipolar transistor (IGBT), switch 630 may include other power semiconductor devices (e.g., power metal-oxide-semiconductor field-effect transistors (MOSFETs) or bipolar junction transistors (BJTs)). Controller 900 (not shown) can thus control whether a given switch 630 is open or closed by controlling the gate of the associated power semiconductor device. The paired switches 630 and diodes 640 may be included in a single package or integrated component, or may be provided as discrete circuit components.

[0056] Although this disclosure primarily discusses the dual-cell 600, in other embodiments, a single cell (or a pair of single cells that functionally form the dual-cell 600) may freely replace the dual-cell 600. In contrast to the dual-cell 600, the single cell comprises a battery capacitor 620 and two switches 630 (with associated diodes 640); effectively half of the dual-cell 600. Operating modes of the single cell include a natural bypass mode (where switch 630 is not conducting), a natural blocking mode (where current flows through battery capacitor 620), and a forced blocking mode (where switch 630 is conducting to prevent charging of battery capacitor 620).

[0057] Figure 6AThe diagram illustrates the natural bypass operating mode of the dual-cell battery 600, where a first current flow 601 flows from the fifth node 610e, through the third diode 640c to the third node 610c, and through the second diode 640b to the first node 610a. In the natural bypass mode, switch 630 is not turned on. The natural bypass mode is achieved without driving switch 630 (i.e., all switches 630 can remain open); when a higher voltage is applied to the fifth node 610e than to the first node 610a, current naturally flows from the fifth node 610e to the first node 610a.

[0058] Figure 6B The diagram illustrates the natural blocking operation mode of the dual-cell battery 600, where a second current flow 602 flows from the first node 610a, through the first diode 640a, the first battery capacitor 620a, and the second battery capacitor 620b to the fourth node 610d, and then through the fourth diode 640d to the fifth node 610e. In the natural blocking mode, switch 630 is open, and the second current flow 602 through the battery capacitor 620 charges the battery capacitor 620. The natural blocking mode is achieved without driving switch 630 (i.e., all switches 630 can remain open); when a higher voltage is applied to the first node 610a than to the fifth node 610e, current naturally flows from the first node 610a through the battery capacitor 620 to the fifth node 610e.

[0059] Figure 6C The first forced bypass operation mode of the dual battery 600 is demonstrated, in which a third current flow 603 flows from the first node 610a, through the first diode 640a and the first battery capacitor 620a to the third node 610c, and then through the closed third switch 630c to the fifth node 610e. In the first forced bypass mode, the first, second, and fourth switches 630a, 630b, and 630d are open, while the third switch 630c is closed, thus charging the first battery capacitor 620a but not the second battery capacitor 620b.

[0060] Figure 6D The second forced bypass operation mode of the dual battery 600 is demonstrated, in which a fourth current flow 604 flows from the first node 610a, through the closed second switch 630b to the third node 610c, through the second battery capacitor 620b to the fourth node 610d, and through the fourth diode 640d to the fifth node 610e. In the second forced bypass mode, the first, third, and fourth switches 630a, 630c, and 630d are open, and the second switch 630b is closed, thereby charging the second battery capacitor 620b without charging the first battery capacitor 620a.

[0061] Figure 6EThe third forced bypass operating mode of the dual battery 600 is demonstrated, in which a fifth current flow 605 flows from the first node 610a, through the closed second switch 630b to the third node 610c, and through the closed third switch 630c to the fifth node 610e. In the third forced bypass mode, the first and fourth switches 630a and 630d are open, and the second and third switches 630b and 630c are closed, thereby bypassing the battery capacitor 620 and preventing it from being charged.

[0062] exist Figure 6C-6E The diagram illustrates that in each forced bypass mode, the selective closing of one or more switches 630 alters the path of current flow from the first node 610a to the fifth node 610e, thereby affecting... Figure 6B Compared to the natural blocking mode, the charging method of battery capacitor 620 is changed. When the same voltage is applied across the dual batteries 600, this voltage can be divided across both battery capacitors 620 (according to the natural blocking mode), guided across only one battery capacitor 620 (according to the first and second forced bypass modes), or without any battery capacitors 620 (according to the third forced bypass mode). When in the natural blocking mode, the voltage V among the N battery capacitors 620 of a given phase arm is... cell It can be given according to Formula 2, where V peak It is the peak phase-to-phase voltage of the grid 330 or the power supply used for pre-charging.

[0063] V cell =V peak ÷N (2)

[0064] Conversely, when in the first or second forced bypass mode, one or more battery capacitors 620 are bypassed; the voltage is divided across fewer battery capacitors 620 and higher charge is allowed in the un-bypassed battery capacitors 620, which can be expressed as Equation 3, where K is the number of battery capacitors 620 that are bypassed (i.e., no current flows through them).

[0065] V cell =V peak ÷(NK) (3)

[0066] The controller 900 selects which dual batteries 600 operate in a given operating mode to selectively charge the sum of the voltages on the battery capacitors 620 to the peak voltage V. peak(Subtract any voltage drop and resistive loss across diode 640) or another precharge threshold set by the operator. Various sensors 371 (e.g., voltage probes 371a-b across battery capacitors 620a-b) allow controller 900 to measure the charging status of battery capacitors 620, and controller 900 controls switch 630 to make the charging rate of a single or subset of battery capacitors 620 equal, so that during precharging of MMC 400, the associated battery capacitors 620 are charged or not charged at a given time.

[0067] Figure 7A and 7B The embodiments of this disclosure are shown respectively, with the positive current (i.e., V) of phase AB. ab The charging path associated with >0) is LSC 420 and MSC 410. For clarity, Figure 7A and 7B The charging currents and negative currents of other phases (i.e., BC, AC) are omitted, but this disclosure also takes this into account. Figure 7A The LSC layout 700a in the text can be understood as being similar to... Figure 7B The MSC layout 700b is connected via node A on the first track 401a (positive polar track 401 in this example) and node B on the second track 401b (negative polar track 401 in this example).

[0068] The LSC layout 700a of the three-phase MMC 400 includes three legs connected between transmission lines 301 and rails 401, each leg connected to one of the three transmission lines 301a-c. Each leg includes multiple dual batteries 600, half of which is disposed between the corresponding transmission line 301a-c and the first rail 401a, and can be referred to as the positive arm 730a-c (collectively referred to as the positive arm 730 or positive LSC arm) based on the corresponding transmission line 301a-c. The other half of the dual battery 600 is disposed between the corresponding transmission line 301 and the second voltage rail 401b, and can be referred to as the negative arm 740a-c (collectively referred to as the negative arm 740 or negative LSC arm) based on the corresponding transmission line 301a-c. Each half of the leg is also associated with a leg inductor, represented by an arm inductor 710.

[0069] The MSC layout 700b of the three-phase MMC 400 includes three legs connecting the generator line 302 to the rail 401, each leg connecting to one of the three generator lines 302a-c. Each leg includes multiple dual batteries 600, half of which is disposed between the corresponding generator line 302a-c and the first voltage rail 401a, and can be referred to as the positive arm 750a-c (collectively referred to as the positive arm 750 or positive MSC arm) based on the corresponding generator line 302a-c. The other half of the dual battery 600 is disposed between the corresponding generator line 302a-c and the second voltage rail 401b, and can be referred to as the negative arm 760a-c (collectively referred to as the negative arm 760 or negative MSC arm) based on the corresponding generator line 302a-c. Each half of the leg is also associated with a leg inductor, indicated by an arm inductor 710.

[0070] Figure 7A and 7B The diagram shows three current loops 770a-c (collectively referred to as current loops 770) that flow inward from the first transmission line 301a and outward to the second transmission line 301b.

[0071] The first current loop 770a carries current from the first transmission line 301a through the first positive arm 730a to the first track 401a, and from the first track 401a through the second positive arm 730b to the second transmission line 301b.

[0072] The second current loop 770b carries current from the first transmission line 301a through the first positive arm 730a to the first track 401a, from the first track 401a through the positive and negative MSC arms to the second track 401b, and from the second track 401b through the second negative arm 740b to the second transmission line 301b.

[0073] The third current loop 770c carries current from the first transmission line 301a through the first negative arm 740a to the second track 401b, and from the second track 401b through the second negative arm 740b to the second transmission line 301b.

[0074] The dual battery 600 in the first positive arm 730a and the second negative arm 740b operates in a natural bypass mode (according to...). Figure 6A Thus, the first, second, and third current loops 770a-c do not charge the battery capacitors 620 in these dual batteries 600 (e.g., via the corresponding second and third diodes 640b-c).

[0075] The controller 900 can drive the battery switch to force the dual batteries 600 into a forced bypass mode (according to...). Figure 6C-6EAlternatively, the battery switch can be left undone, allowing the dual-battery 600 to conduct current in natural operating mode (according to...). Figures 6A-6B ).

[0076] When controller 900 allows the dual batteries 600 in LSC 420 and MSC 410 to charge in an undriven operating mode (i.e., switch 630 remains open to allow current to flow through diode 640 and capacitor 620), the applied peak voltage V relative to LSC 420 is... peak This is twice the voltage across the battery capacitor 620 in MSC 410. Therefore, if the controller 900 does not drive the switch 630 to boost the voltage therein, the voltage V in the battery capacitor 620 of LSC 420 can be adjusted according to Formula 4. cell-LSC The voltage V in the battery capacitor 620 of MSC 410 cell-MSC equal.

[0077] V cell-LSC =2*V cell-MSC (4)

[0078] Once the controller 900 has completed the undriven operation mode, it can selectively drive one or more batteries (e.g., open one or both bypass switches in the dual battery 600) to increase the charge level in the battery capacitor 620. During the driven phase, the controller 900 drives one or more switches 630 in the dual battery 600, thereby causing the selected dual battery 600 to operate in either a natural operation mode or a forced bypass operation mode. For example, in the driven phase, the dual battery 600 in the second positive arm 730b, the first negative arm 740a, and the MSC arm operates in a natural blocking mode, a first forced bypass mode, a second forced bypass mode, and a third forced bypass mode (according to...). Figure 6B-6E Any one of the operations in the () to control how the battery capacitor 620 is charged. Combined Figure 8 The details of battery charging will be discussed in more detail. The controller 900 can force a bypass mode in a given dual-battery configuration 600 to boost the overall charging of the battery capacitor 620 in the arm to near V. peak To account for manufacturing tolerances between the battery capacitors 620 (e.g., to make the stored charge equal), etc.

[0079] Figure 8 This is a flowchart of a method 800 for synchronizing a harmonic filter 350 while pre-charging a dual battery 600 in an MMC 400, according to an embodiment of the present disclosure.

[0080] Method 800 begins at block 810, where controller 900 initializes the open / closed states of various circuit breakers 360. During pre-charging, generator circuit breaker 360d remains open to disconnect generator 320 from converter system 310, which is being pre-charged. The first set of switches 361f in pre-charging contactor 360f is closed, and the first set of switches 361e in transmission circuit breaker 360e is open to guide power from grid transformer 340 through a set of pre-charging resistors 390 before reaching harmonic filter 350 and converter system 310. The second sets of switches 362e, 362f in transmission circuit breaker 360e and pre-charging contactor 360f remain open, unless power from grid 330 needs to be diverted to ground (e.g., in the event of system inspection or maintenance).

[0081] Once circuit breaker 360 is initialized at block 810, method 800 proceeds to block 820, where controller 900 ensures that grid circuit breaker 360a is closed, thereby connecting grid 330 to harmonic filter 350 and converter system 310 via pre-charge resistor 390, and supplying power from grid 330 (via auxiliary transformer 380) to APS 370 and controller 900.

[0082] In block 830, the MMC 400 enters a pre-charge, undriven phase, where the controller 900 monitors the state of the dual battery 600 and the voltage of the battery capacitor 620, but does not drive the switch 630 in the dual battery 600. The voltage in the harmonic filter 350 is related to the grid voltage V. g Equal to (minus any voltage drop across the pre-charge resistor 390), and the voltage V across the battery capacitor 620 in the LSC 420. cell-LSC Equal to the peak phase-to-phase voltage V of the power grid 330 peak Divide by the number N of battery capacitors 620 in each arm of the LSC, and the voltage V across the battery capacitors 620 in the MSC 410. cell-MSC Equal to the peak phase-to-phase voltage V of the power grid 330 peak Divide by the number N (typically V) of the 620 battery capacitors in each leg of the MSC. cell-LSC =2*V cell-MSC During block 830, controller 900 disconnects all switches 630 in the dual batteries 600; allowing current to flow through the dual batteries 600 according to either the natural bypass mode or the natural blocking mode (as per the respective conditions). Figure 6A and 6B ).

[0083] Method 800 can remain at block 830 until the measured voltage meets the drive threshold. In some embodiments, the measured voltage may be the voltage V stored in the filter capacitor 530. f , and based on grid voltage V g (For example, V) g The measured voltage is compared to the driving threshold of (x%). In some embodiments, the measured voltage may be the voltage V in one of the battery capacitors 620. cell-LSC (or the average, highest, or lowest V measured across multiple battery capacitors 620) cell-LSC ), and based on the peak phase-to-phase voltage V peak (For example, V) peak The driving threshold is compared with (x%) ÷ N. In some embodiments, the measured voltage may be the voltage V in one of the battery capacitors 620. cell-MSC (or the average, highest, or lowest V measured across multiple battery capacitors 620) cell-MSC ), and based on the peak phase-to-phase voltage V peak (For example, V) peak The driving threshold (x%) is compared with N. Once the measured voltage meets the driving threshold, method 800 proceeds to block 840.

[0084] At block 840, MMC 400 enters the pre-charge driven phase, where controller 900 continues to monitor the state of dual battery 600 and the voltage of battery capacitor 620, but can actively drive switch 630 in dual battery 600 to increase the charge stored in battery capacitor 620. The state of dual battery 600 can include the health status of various electronic devices included in dual battery 600, including battery switch 630, communication and temperature sensors, pressure sensors, etc. In various embodiments, controller 900 drives switch 630 in MSC 410 and LSC 420 to reduce the number of battery capacitors 620 through which voltage from grid 330 is applied using different forced bypass modes to gradually increase the voltage applied across battery capacitors 620; rectified or peak grid phase-to-phase voltage V is applied across a subset of battery capacitors 620 via diode 640. Peak .

[0085] In a progressive drive scheme where the number of dual batteries 600 being charged simultaneously decreases by one at a time, for example for LSC 420, controller 900 first places one dual battery 600 in each arm into a first or second forced bypass mode, while the remaining dual batteries 600 are placed into a natural blocking mode, so that one battery capacitor 620 is not charged at a time, while the other battery capacitors 620 are charged to voltage V. cell =Vpeak ÷(NK), where K = 1 because one battery capacitor 620 is bypassed. In some embodiments, the controller 900 sweeps the current through the bypassed battery capacitor 620 until all battery capacitors reach voltage V. cell =V peak ÷(N-1). Then, the controller 900 can increase the number of battery capacitors 620 bypassed at any given time by driving one or more dual batteries 600 into a first, second, or third forced bypass mode, or by placing one or more dual batteries 600 in a natural blocking mode, so that K increases and V... cell =V peak ÷(NK) increases accordingly. Therefore, controller 900 can cycle through which battery capacitors 620 are bypassed and which are charged in one arm until a charge threshold is reached. In other words, when controller 900 is ready to boost the battery voltage V... cell In this case, the progressive drive scheme employs an iterative increase (e.g., K1 = K0 + 1) relative to the number of battery capacitors 620 that will be bypassed at a given time in the previous time, thereby increasing the voltage applied to each unbobble battery capacitor 620.

[0086] In a halving drive scheme where the number of dual batteries 600 being charged simultaneously is continuously halved, for example for the MSC410, the controller 900 first causes each dual battery 600 in a given arm to alternate between a first forced bypass operation mode and a second forced bypass operation mode to charge half of the battery capacitor 620 at any given time, until each battery capacitor 620 is charged to 2*V. peak ÷N (for example, where K = N / 2, V peak ÷(NK)=2*V peak ÷N). Reaching 2*V peak After ÷N, the controller 900 can cycle between placing the paired dual batteries 600 in a third forced bypass mode and the first and second forced bypass modes to bypass three of the four battery capacitors 620, so that one of the four battery capacitors 620 is charged to 4*V at a time. peak ÷N (for example, where K = N * 3 / 4, V peak ÷(NK)=4*V peak (÷N). Therefore, the controller 900 can cyclically bypass larger percentages of the battery capacitor 620 to double the voltage applied across the battery capacitor 620 in each arm until a charge threshold is reached. In other words, when the controller 900 is ready to boost the battery voltage V cellIn this case, the halving drive scheme increases the number of battery capacitors 620 that will be bypassed at a given time exponentially (in powers of 2) relative to the previous time (e.g., K2 = 2 * K1 = 4 * K0), thereby increasing the battery voltage V applied across the unbypassed battery capacitors 620. cell .

[0087] As will be understood, other driving schemes and variations thereof are also envisioned, such as balancing the charge in a given arm by identifying the battery capacitor 620 with the lowest charge, such as through a sorting and selection (SoS) algorithm used during normal (i.e., inverter / rectifier) ​​operation of the MMC. Therefore, the controller 900 can be configured to precharge the battery capacitors 620 to account for the inherent differences in the circuitry of the dual-cell 600 over a series of iterations, where each successive iteration includes charging a smaller subset of the battery capacitors 620 to the corresponding higher battery voltage V at a given time. cell .

[0088] In some implementations, controller 900 drives the switch in MSC410 while deactivating the switch in LSC 420, until V cell-MSC =V cell-LSC ; Increasing all battery capacitors by 620V or more peak Before dividing by N, the charges in MSC410 and LSC 420 are made equal. In some implementations, controller 900 drives each arm evenly until the LSC arm reaches the charge threshold, and then continues to drive the MSC arms until V. cell-LSC =V cell-MSC .

[0089] Method 800 can remain at block 840 until the measured voltage meets the charge threshold. In some implementations, when K = (N-1) and the voltage V in each battery capacitor 620 is... cell Charged to V peak / N (or V) peak / N y%, to account for manufacturing tolerances of battery capacitor 620, voltage drop across diode 640, and resistive losses), reaches the charge threshold. In other embodiments where the operator may have greater tolerance for surge current, the charge threshold may be reached before K = (N-1), and the nominal / rated / operating V cell It is less than V peak / N (or V) peak The predefined value of / N (y%). Once the V is measured. cell The voltage meets the charge threshold (nominal / rated / operating V). cell Method 800 will then proceed to block 850.

[0090] At block 850, pre-charging is complete. Controller 900 closes the first set of switches 361e in the transmission circuit breaker, closes the generator circuit breaker 360d, and begins switching the converter system 310 to rectify and invert the power generated by generator 320 to supply the grid 330, rather than boosting the charge level in battery capacitor 620. To avoid inducing transient currents (or reducing their magnitude) in harmonic filter 350, the first set of switches 361f in pre-charging contactor 360f remains closed until the first set of switches 361e in transmission circuit breaker 360e is closed. The resistance difference across the two paths of the transmission line (i.e., through transmission circuit breaker 360e and through pre-charging contactor 360f) causes current to bypass pre-charging resistor 390 while maintaining synchronization with grid 330 established in harmonic filter 350 during pre-charging of dual batteries 600 from grid 330.

[0091] Then, method 800 can end when PCTS 300 enters normal operation, wherein generator 320 supplies power to grid 330 via converter system 310, which rectifies and inverts the power to supply power to grid 330 for consumption. Controller 900 can continue to control switch 630 in the battery to affect rectification and inversion, and continue to control circuit breaker 360; optionally, the precharge contactor 360f can be disconnected after normal operation begins.

[0092] Figure 9 This is a block diagram of a controller unit 900 according to one or more embodiments. The controller unit 900 includes one or more computer processors 910 and a memory 920 (e.g., a memory storage device). The one or more processors 910 represent any number of processing elements, each of which may include any number of processing cores. The memory 920 may include volatile memory elements (such as random access memory), non-volatile memory elements (such as solid-state, magnetic, optical, or flash-based memory), and combinations thereof. Furthermore, the memory 920 may be distributed across different media (such as network storage or external hard drives).

[0093] As shown in the figure, the one or more processors 910 are communicatively coupled to the communication system 930 to send / receive communications with various sensors 371, circuit breakers / contactors 360, switches 630 and other controller units 900 associated with WTG 100, APS 370 and auxiliary transformer 380 via optical fiber, wires and / or radio signals.

[0094] The memory 920 may include multiple "modules" for performing the various functions described herein. In one embodiment, each module includes program code executable by one or more processors 910. However, other embodiments may include modules implemented partially or entirely in hardware (i.e., circuitry) or firmware. The memory 920 includes precharge control logic 940, which enables the controller unit 900 to charge the battery capacitor 620 while synchronizing the harmonic filter 350 with the power grid 330, as described herein. In some embodiments, the precharge control logic 940 is pre-loaded with setpoints and thresholds for various control schemes, such as those described by way of example. Figure 8 Described.

[0095] The embodiments proposed in this disclosure have been mentioned above. However, the scope of this disclosure is not limited to the specifically described embodiments. Rather, any combination of the features and elements provided above, whether or not related to different embodiments, is contemplated for implementing and carrying out the contemplated embodiments. Furthermore, although the embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether a particular advantage is achieved by a given embodiment does not limit the scope of this disclosure. Therefore, the aspects, features, embodiments, and advantages described herein are merely illustrative and should not be considered as elements or limitations of the appended claims unless expressly mentioned in the claims.

[0096] As those skilled in the art will understand, the embodiments disclosed herein can be embodied as systems, methods, or computer program articles. Therefore, aspects can take the form of entirely hardware implementations, entirely software implementations (including firmware, resident software, microcode, etc.), or implementations combining software and hardware aspects, all of which can be referred to herein as “circuit,” “module,” or “system.” Furthermore, aspects can take the form of computer program articles embodied in one or more computer-readable media having computer-readable program code thereon.

[0097] This invention can be a system, a method, and / or a computer program article. The computer program article may include a computer-readable storage medium (or medium) having computer-readable program instructions thereon (e.g., a portable computer floppy disk, hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable optical disc read-only memory (CD-ROM), optical storage device, magnetic storage device, or any suitable combination thereof) for causing a processor to execute various aspects of the invention.

[0098] Various aspects of this disclosure are described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program articles according to embodiments presented herein. It will be understood that each block in the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing apparatus to create a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions / behaviors specified in one or more blocks of the flowchart illustrations and / or block diagrams.

[0099] The flowcharts and block diagrams in the figures illustrate the possible structures, functions, and operations of systems, methods, and computer program articles according to various embodiments. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of code, comprising one or more executable instructions for implementing a specified logical function. It should also be noted that in some alternative implementations, the functions indicated in the blocks may not appear in the order shown in the figures. For example, two blocks shown consecutively may actually be executed substantially simultaneously, or these blocks may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagrams and / or flowchart descriptions, and combinations of blocks in the block diagrams and / or flowchart descriptions, may be implemented by a special-purpose hardware system to perform the specified function or behavior, or a combination of special-purpose hardware and computer instructions.

[0100] In view of the foregoing, the scope of this disclosure is defined by the following claims.

Claims

1. A method of synchronizing a grid-side harmonic filter and a pre-charge battery capacitor in a modular multilevel converter, comprising: initializing a contactor (360) to connect a grid (330) to the harmonic filter (350) and the modular multilevel converter (400), wherein the modular multilevel converter is disposed between the grid and a generator (320), and the harmonic filter is disposed between the grid and the modular multilevel converter, wherein initializing the contactor comprises: opening a transfer breaker (360e) disposed on a first path, wherein the transfer breaker is disposed between the grid and the modular multilevel converter, and closing a pre-charge contactor (360f) disposed on a second path, wherein the pre-charge contactor is disposed between the grid and the modular multilevel converter, wherein the second path is parallel to the first path and includes a set of pre-charge resistors (390), wherein the pre-charge resistors are selected to have a resistance lower than an impedance of the harmonic filter at a grid frequency of the grid; connecting the grid to the modular multilevel converter and the harmonic filter through the second path to simultaneously pre-charge battery capacitors (620) in the modular multilevel converter and synchronize the harmonic filter with the grid; pre-charging the battery capacitors (620) in the modular multilevel converter in an un-driven phase; in response to a battery voltage of the battery capacitors satisfying a drive threshold, pre-charging the battery capacitors in a driven phase; and in response to the battery voltage satisfying a charge threshold, completing the pre-charging by: closing the transfer breaker (360e) to establish a first path parallel to the second path, and connecting the generator to the modular multilevel converter via a generator breaker (360d) while the transfer breaker and the pre-charge contactor are closed, wherein the drive threshold is based on a peak phase-to-phase voltage of a grid voltage divided by a number of the battery capacitors disposed on a given phase of the modular multilevel converter.

2. The method of claim 1, wherein the driven phase employs a progressive drive scheme that iteratively increases the number of battery capacitors bypassed at a given time by one relative to a previous time to boost a charge level in battery capacitors not bypassed.

3. The method of claim 1, wherein the driven phase employs a halving drive scheme that exponentially increases the number of battery capacitors bypassed at a given time relative to a previous time to boost a charge level in battery capacitors not bypassed.

4. The method of any of claims 1-3, further comprising: after connecting the generator, opening the pre-charge contactor.

5. The method of any of claims 1-3, wherein the driven phase places a given double battery (600) in the modular multilevel converter in an operating mode comprising: a natural block mode for charging a first battery capacitor (620a) and a second battery capacitor (620b) in the given double battery; a first forced bypass mode for charging the first battery capacitor and bypassing the second battery capacitor; a second forced bypass mode for charging the second battery capacitor and bypassing the first battery capacitor; and a third forced bypass mode for bypassing the first battery capacitor and the second battery capacitor.

6. A power conversion and transmission system (300) comprising: a grid breaker (330a) arranged to selectively connect a grid (330) with the power conversion and transmission system; a modular multilevel converter (400) comprising a plurality of cells, each cell comprising a cell switch (630) and a battery capacitor (620); a generator breaker (360d) arranged to selectively connect a generator (320) with a machine side converter (410) of the modular multilevel converter; a harmonic filter (350) connected to a transmission line (301) connected with a line side converter (420) of the modular multilevel converter, wherein the transmission line defines a first path when a transmission breaker (360e) is closed and a second path when a pre-charge contactor (360f) is closed, wherein the first path connects the grid breaker with the harmonic filter and the line side converter, wherein the second path connects the grid breaker with the harmonic filter and the line side converter through a set of pre-charge resistors (390), and wherein the first path is parallel to the second path and bypasses the pre-charge resistors, and wherein a resistance of the pre-charge resistors is configured to be less than 10% of an impedance of the harmonic filter at a grid frequency of the grid; and a controller (900) configured to simultaneously pre-charge the battery capacitors and synchronize the harmonic filter with the grid by: initializing the pre-charge through the second path when the first path is open; in response to a cell voltage in the battery capacitor satisfying a drive threshold, driving the cell switch into a forced bypass mode to boost the cell voltage to a charge threshold; and in response to the cell voltage in the battery capacitor satisfying the charge threshold, while the pre-charge contactor is closed: closing the transmission breaker, and closing the generator breaker, wherein the drive threshold is based on a peak phase-to-phase voltage of the grid voltage divided by a number of battery capacitors arranged on a given phase of the modular multilevel converter.

7. The system of claim 6, wherein each cell of the plurality of cells is a double cell (600) comprising two battery capacitors and four cell switches.

8. The system of claim 6 or 7, wherein the controller is further configured to open the pre-charge contactor after closing the generator breaker.

9. The system of claim 6 or 7, wherein the forced bypass mode bypasses at least one battery capacitor included in the cell to apply a rectified voltage from the grid across a subset of the battery capacitors.

10. The system of claim 6 or 7, wherein the controller is further configured to stop boosting the cell voltage in response to the cell voltage in the battery capacitor satisfying a charge and place the cell in an operational mode that is expected to invert and rectify power supplied from the generator for providing to the grid.

11. The system of claim 6 or 7, further configured for three-phase power transmission.

12. A controller unit (900) for a power conversion and transmission system, comprising: a processor (910); and a memory (920) comprising pre-charge control logic (940) that, when executed by the processor, enables the controller unit to perform operations comprising: initializing a circuit breaker (360) to connect a power grid (330) to a harmonic filter (350) and a converter system (310), wherein initializing the circuit breaker comprises: opening a transfer circuit breaker (360e) disposed on a first path, wherein the transfer circuit breaker is disposed between the power grid and the converter system; and closing a pre-charge contactor (360f) disposed on a second path, wherein the pre-charge contactor is disposed between the power grid and the converter system, wherein the second path includes a set of pre-charge resistors (390), wherein the second path is parallel to the first path that bypasses the set of pre-charge resistors; connecting the power grid to the converter system and the harmonic filter through the second path to simultaneously pre-charge battery capacitors (620) in the converter system (310) and synchronize the harmonic filter with the power grid; selectively charging the battery capacitors (620) in the converter system through a series of iterations until a charge threshold is reached, wherein each iteration in the series of iterations charges a smaller subset of the battery capacitors at a given time compared to earlier iterations in the series of iterations, while each battery capacitor is charged to a higher battery voltage compared to the earlier iterations; and closing the transfer circuit breaker and connecting a generator (320) to the converter system via a generator circuit breaker (360d) while the pre-charge contactor is closed.

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