Synchronizing grid side harmonic filter and pre-charging cell capacitors in modular multilevel converters

Synchronizing a grid side harmonic filter and pre-charging cell capacitors in modular multilevel converters by: opening a transmission circuit breaker disposed on a first path between a power grid and a converter system; closing a pre-charging contactor disposed on a second path between the power grid and the converter system that includes a set of pre-charge resistors; connecting the power grid to the converter system and the harmonic filter over the second path; selectively charging cell capacitors in the converter system until a charge threshold is reached, wherein a smaller subset of the cell capacitors is charged at a given time than in an earlier time, and each cell capacitor is charged to a higher cell voltage than in the earlier time; and closing the transmission circuit breaker and connecting a generator to the converter system via a generator circuit breaker while leaving the pre-charging contactor closed.

BACKGROUND

Field of the Invention

Embodiments presented in this disclosure generally relate to Modular Multilevel Converters (MMCs) and the capacitors included therein. Particular embodiments herein describe synchronizing and pre-charging schema for the filters and capacitors.

Description of the Related Art

Wind Turbine Generators (WTGs) are an increasing popular source for generating electricity and may be deployed singly or in groups of several wind turbines, often referred to as a wind farm. In WTGs and other power generating or consuming systems connected to a power grid or distribution line, MMCs can be used to electrically link two powered systems running different voltage/current schemas. When initiating a link between two powered systems, the MMC equalizes the differences in voltages/currents between the two systems to reduce power surges (e.g., inrush current from the higher voltage side to the lower voltage side) and other aberrant effects. For example, in a WTG, an MMC can equalize the difference via a DC (Direct Current) link located between the machine side converter (MSC) and line side converter (LSC) that is charged to a predefined level before contact is made via a series of charging components (e.g., one or more transformers, diode bridges, current limiter resistors, fuses, circuit breakers, switches, etc.)

SUMMARY

One embodiment of the present disclosure is a method comprising; initializing contactors to connect a power grid to a harmonic filter and a modular multilevel converter, 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, wherein initializing the contactors comprises: opening 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-charge resistors; connecting the power grid to the MMC and the harmonic filter over the second path; pre-charging cell capacitors in the MMC in an un-driven stage; in response to a cell voltage of the cell capacitors satisfying a driving threshold, pre-charging the cell capacitors in a driven stage; and in response to the cell voltage satisfying a charge threshold, completing pre-charging by connecting the generator to the MMC via a generator circuit breaker while the transmission circuit breaker and the pre-charging contactor are closed, the driving threshold is based on a peak phase-to-phase voltage of a grid voltage divided by a number of cell capacitors disposed on a given phase of the MMC.

In one embodiment, in combination with any method described above or below, the driving threshold is based on a peak phase-to-phase voltage of a grid voltage divided by a number of cell capacitors disposed on a given phase of the MMC.

In one embodiment, in combination with any method described above or below, the driven stage employs a progressive driving schema to iteratively increase a number of cell capacitors bypassed at a given time by one relative to a preceding time to boost a charge level in non-bypassed cell capacitors.

In one embodiment, in combination with any method described above or below, the driven stage employs a halving driving schema to exponentially increase a number of cell capacitors bypassed at a given time relative to a preceding time to boost a charge level in non-bypassed cell capacitors.

In one embodiment, in combination with any method described above or below, the method further comprises: after connecting the generator, opening the pre-charging contactor.

In one embodiment, in combination with any method described above or below, a resistance of the pre-charge resistors is selected based on an impedance of the harmonic filter at a grid frequency of the power grid.

In one embodiment, in combination with any method described above or below, the driven stage places a given dual-cell in the MMC into operational modes including: a natural blocking mode, for charging a first cell capacitor and a second cell capacitor in the given dual-cell; a first forced bypass mode, for charging the first cell capacitor and bypassing the second cell capacitor; a second forced bypass mode, for charging the second cell capacitor and bypassing the first cell capacitor; and a third forced bypass mode, for bypassing the first cell capacitor and the second cell capacitor.

One embodiment of the present disclosure is a Power Conversion and Transmission System, comprising: a grid circuit breaker, disposed to selectively connect a power grid with the Power Conversion and Transmission System; a Modular Multilevel Converter, including a plurality of cells each including cell switches and a cell capacitor; a generator circuit breaker, disposed to selectively connect a generator with a machine side converter of the MMC; 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 a transmission circuit breaker is closed and a second path when a pre-charging contactor is closed, wherein the first path connects the grid circuit breaker with the harmonic filter and the line side converter, wherein the second path connects the grid circuit breaker with the harmonic filter and the line side converter over a pre-charge resistor, and wherein the first path is parallel to the second path and bypasses the pre-charge resistors; and a controller configured to pre-charge the cell capacitors and synchronize the harmonic filter with the power grid by: in response to a cell voltage in the cell capacitors satisfying a driving threshold, driving the cell switches into forced bypass modes to boost the cell voltage to a charge threshold; and in response to the cell voltage in the cell capacitors satisfying the charge threshold, while the pre-charging contactor is closed: closing the transmission circuit breaker, and closing the generator circuit breaker, and the driving threshold is based on a peak phase-to-phase voltage of a grid voltage divided by a number of cell capacitors disposed on a given phase of the MMC.

In one embodiment, in combination with any system described above or below, each cell of the plurality of cells is a dual cell that includes two cell capacitors and four cell switches.

In one embodiment, in combination with any system described above or below, the controller is further configured to open the pre-charging contactor after closing the generator circuit breaker.

In one embodiment, in combination with any system described above or below, the forced bypass modes bypass at least one cell capacitor included in the cells to apply a rectified voltage from the power grid across a subset of the cell capacitors.

In one embodiment, in combination with any system described above or below, the controller is further configured to cease boosting the cell voltage in response to the cell voltage in the cell capacitors satisfying the charge and place the cells in an operational mode in anticipation of inverting and rectifying power supplied from the generator for provision to the power grid.

In one embodiment, in combination with any system described above or below, a resistance of the pre-charge resistor is configured to be at less than 10% of an impedance of the harmonic filter at a grid frequency of the power grid.

In one embodiment, in combination with any system described above or below, the system is further configured for three-phase power transmission.

One embodiment of the present disclosure is controller unit a for a Power Conversion and Transmission System, comprising: a processor; and a memory, including pre-charging control logic that when executed by the processor, enable the controller unit to perform an operation comprising: initializing circuit breakers to connect a power grid to a harmonic filter and to a converter system, wherein initializing the circuit breakers includes: opening 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-charging contactor disposed on a second path, wherein the pre-charging contactor is disposed between the power grid and the converter system, wherein the second path includes a set of pre-charge resistors, wherein the second path is parallel to the first path which bypasses the set of pre-charge resistors; connecting the power grid to the converter system and the harmonic filter over the second path; selectively charging cell capacitors in the converter system over a series of iterations until a charge threshold is reached, wherein each iteration of the series of iterations charges a smaller subset of the cell capacitors at a given time than in an earlier iteration of the series of iterations, and each cell capacitor is charged to a higher cell voltage than in the earlier iteration; and closing the transmission circuit breaker and connecting a generator to the converter system via a generator circuit breaker while leaving the pre-charging contactor closed.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Systems and methods for a novel control schema for pre-charging an MMC (Modular Multilevel Converter) at the same time as synchronizing a harmonic filter to grid voltage is provided herein that allows for preventing inrush currents from the grid into the harmonic filter, thus protecting against over currents and increasing the lifetime of the filter components together with other related system equipment. In an un-driven stage, the capacitors in the MMC and the harmonic filter are pre-charged from the grid over the pre-charge resistors to threshold levels before connecting the grid directly to the MMC. The un-driven stage may last until the sum of cell voltages in an arm becomes equal to the rectified (peak) voltage of grid line-to-line voltage for a Line Side Converter in the MMC. After the un-driven stage, the MMC enters a driven or controlled stage, during which the charge of the cell capacitors is boosted to the rated operation voltage by progressively reducing the number of cell capacitors to which the grid applies a voltage in series to at a time. The MMC swaps or interchanges which capacitors are subject to the charging currents in the driven stage until all of the capacitors have reached a threshold voltage, at which point normal operations of the MMC may begin.

EXAMPLE EMBODIMENTS

FIG.1illustrates a diagrammatic view of a horizontal-axis wind turbine generator (WTG)100. The WTG100typically comprises a tower102and a wind turbine nacelle104located at the top of the tower102. A wind turbine rotor106may be connected with the nacelle104through a low speed shaft extending out of the nacelle104. The wind turbine rotor106comprises three rotor blades108mounted on a common hub110which rotate in a rotor plane, but may comprise any suitable number of blades, such as one, two, four, five, or more blades. The blades108(or airfoil) typically each have an aerodynamic shape with a leading edge112for facing into the wind, a trailing edge114at the opposite end of a chord for the blades108, a tip116, and a root118for attaching to the hub110in any suitable manner.

For some embodiments, the blades108may be connected to the hub110using pitch bearings120such that each blade108may be rotated around its longitudinal axis to adjust the blade's pitch. The pitch angle of a blade108relative to the rotor plane may be controlled by linear actuators, hydraulic actuators, or stepper motors, for example, connected between the hub110and the blades108.

FIG.2illustrates a diagrammatic view of typical components internal to the nacelle104and tower102of a WTG100. When the wind200pushes on the blades108, the rotor106spins and rotates a low-speed shaft202. Gears in a gearbox204mechanically convert the low rotational speed of the low-speed shaft202into a relatively high rotational speed of a high-speed shaft208suitable for generating electricity using a generator206.

A controller210may sense the rotational speed of one or both of the shafts202,208. If the controller decides that the shaft(s) are rotating too fast, the controller may signal a braking system212to slow the rotation of the shafts, which slows the rotation of the rotor106—i.e., reduces the revolutions per minute (RPM). The braking system212may prevent damage to the components of the WTG100. The controller210may also receive inputs from an anemometer214(providing wind speed) and/or a wind vane216(providing wind direction). Based on information received, the controller210may send a control signal to one or more of the blades108in an effort to adjust the pitch218of the blades. By adjusting the pitch218of the blades with respect to the wind direction, the rotational speed of the rotor (and therefore, the shafts202,208) may be increased or decreased. Based on the wind direction, for example, the controller210may send a control signal to an assembly comprising a yaw motor220and a yaw drive222to rotate the nacelle104with respect to the tower102, such that the rotor106may be positioned to face more (or, in certain circumstances, less) upwind.

FIG.3illustrates a schematic of a power conversion and transmission system (PCTS)300, according to embodiments of the present disclosure. As will be appreciated, the configuration shown is but one example of a schematic of a PCTS300, and should not be considered as limiting use of the disclosed embodiments. One of ordinary skill in the art will appreciate alternative arrangements of the presented components, substitutions for the presented components, and the omission of various components are contemplated as being within the scope of the present disclosure.

The PCTS300illustrated inFIG.3is a three-phase system that carries power of three different phases on three separate transmission lines301a-c(generally, transmission line301), although PCTS300of one, two, or more phases with a corresponding number of transmission lines301a-N can also employ the teachings of the present disclosure. Unless stated otherwise or when clear from the context of the present disclosure, each component of the PCTS300either includes N instances, where each instance a-N is associated with one phase a-N, or is connected to each of the N phases.

In the PCTS300, a converter system310receives alternating current (AC) power from a generator320(e.g., a WTG100) on a number of generator lines302a-c(generally, generator line302) corresponding to a number of phases of power generated by the generator320. When the generator320produces power to supply to the grid330, the converter system310rectifies the AC power to a direct current (DC) power, and inverts the DC power to another AC power that is suitable to be supplied to a power grid330via the transmission lines301. In various embodiments, the frequency that the power grid330receives power can be different than the frequency at which the generator320provides power. The components of a three-phase converter system310are discussed in more detail in regard toFIG.4. A power grid transformer340can adjust the voltage of the AC power output by the converter system310to an appropriate voltage for the power grid330(e.g., a grid voltage), and a harmonic filter350connected to the transmission lines301between the converter system310and the grid transformer340(or grid330) can be used to condition and adjust the AC power provided to the grid330. The components of a three-phase harmonic filter350are discussed in greater detail in regard toFIG.5.

Various additional components are included in the PCTS300to control the transfer of power to/from the converter system310and the grid330, such as during start-up and connection or shutdown and disconnect procedures. A controller900(not illustrated inFIG.3) in communication with various components and current or voltage sensors371a,371b(generally, sensors371) disposed throughout the PCTS300may control the operation of the generator320, auxiliary power source (APS)370, auxiliary transformer380, and/or various switches to affect whether and how power is provided to/from the grid330or components within the PCTS300. The controller900may be fed power from the APS370and is in communication with the various components of the PCTS300via wireless channels or wired connections (not illustrated). The components of a controller900are discussed in greater detail in regard toFIG.9.

Several circuit breakers and/or contactors360a-f(generally, circuit breaker or contactor360) are disposed at various points in the PCTS300to direct or block the transmission of power to/from particular portions of the PCTS300. For example, a grid circuit breaker360adisposed between the grid330and the grid transformer340can act as a Point of Common Coupling (PCC) between the PCTS300and the grid330to connect or disconnect the PCTS300to/from the grid330. In another example, an auxiliary transformer circuit breaker360band an APS circuit breaker360ccan connect or disconnect an auxiliary transformer380to/from the converter system310and the APS370respectively. In some embodiments, the auxiliary transformer circuit breaker360bis a medium voltage circuit breaker disposed between the grid330and the auxiliary transformer380to isolate the auxiliary transformer380from faults or during service/inspection on the primary side. In some embodiments, the APS circuit breaker360cis a low voltage circuit breaker disposed between the auxiliary transformer380and the APS370to protect against faults on the APS side (or secondary side) of the auxiliary transformer380or during de-energization of the auxiliary equipment powered via the APS370.

Similarly, a generator circuit breaker360ddisposed between the generator320and the converter system310can connect or disconnect the generator320to/from the converter system310.

A transmission circuit breaker360eis disposed on a first path between the converter system310(and the harmonic filter350) and the grid transformer340. The transmission circuit breaker360ecan include a first set of switches361eto make or break electrical contact on the electrical transmission lines301between the converter system310and the grid330, and a second set of switches362eto make or break electrical contact on an electrical pathway between ground and the transmission lines301.

A pre-charging contactor360fis disposed on a second path in the PCTS300. The second path may alternatively be referred to as a pre-charge path, as the second path provides an alternative pathway around the transmission circuit breaker360e(i.e., connects to the transmission lines301both a first point upstream and a second point downstream of the first set of switches361eof the transmission circuit breaker360e) and is used when pre-charging the converter system310. The pre-charging contactor360fcan include a first set of switches361fto make or break electrical contact on the electrical transmission lines301between the converter system310and the power grid330, and a second set of switches362fto make or break electrical contact on an electrical pathway between ground and the transmission lines301. The pre-charging contactor360falso includes (or can be associated with) a set of fuses363fon the transmission lines301and a set of pre-charge resistors390, which ensure that a pre-charging current provided from the power grid330does not exceed the designed capabilities of the converter system310. The pre-charge resistors390are provided with a predefined resistance, which is selected based on the impedance of the harmonic filter350.

By opening and closing the transmission circuit breaker360e, the controller900blocks or allows (respectively) current from the grid330to flow over the first path through the transmission circuit breaker360eto the converter system310. Similarly, by opening and closing the pre-charging contactor360f, the controller900blocks or allows (respectively) current from the grid330to flow over the second path through the pre-charging contactor360fto the converter system310. Due to the presence of the pre-charge resistors390on the second path and lack of similar resistors on the first path, current prefers to flow over the first path when both paths are available. Therefore, to force current to flow through the pre-charge resistors390, the controller900blocks the first path (i.e., opens the transmission circuit breaker360e) and allows the second path (i.e., closes the pre-charging contactor360f) when pre-charging the converter system310.

When pre-charging and synchronizing using power from the grid330, the first set of switches361fin the pre-charging contactor360fand the switches in the grid circuit breaker360aare closed. Meanwhile, the first set of switches361e, in the transmission circuit breaker360e, the second set of switches362ein the transmission circuit breaker360e, the second set of switches362fin the pre-charging contactor360f, the switches in the generator circuit breaker360dare open, and the switches in the auxiliary transformer circuit breaker360bare closed. Thus, power from the grid330flows on the transmission lines301into the PCTS300and over the pre-charge resistors390to the harmonic filter350and the converter system310.

FIG.4is a schematic of an MMC400for use as a converter system310according to embodiments of the present disclosure.FIG.4may be understood in conjunction withFIG.3. The MMC400operates by converting the AC power supplied by the generator320to DC power via a Machine Side Converter (MSC)410, and from DC power back to AC power for supply to the grid330via a Line Side Converter (LSC)420. The MSC410and the LSC420are coupled together via a DC link430. Various switches in the MSC410and LSC420control how the MMC400is charged or discharged and how power is rectified or inverted, which may be controlled by the controller900or another subordinate or independent control device. Each of the MSC410and the LSC420include several cells, which can be modularly added or removed from the converters in series with one or more other cells to adjust the capabilities of the converters in handling different voltage inputs and outputs when converting power from AC to DC or from DC to AC. The cells include various switches and capacitors, which are discussed in greater detail in regard toFIGS.6A-6E.

The DC link430carries DC power between the MSC410and the LSC420on a first rail401aand a second rail401b(generally, rail401), and includes DC-link capacitors440a-b(generally, DC-link capacitor440) disposed between the rails401. In various embodiments, the DC-link capacitors440regulate a DC voltage between the MSC410and the LSC420. In some embodiments, the DC-link capacitors440are connected to a neutral (common) voltage node connected to earth ground through a transmission resistor450and the non-common terminals of the DC-link capacitors440are connected to opposing DC-link voltage rails401(e.g., to one of a positive or a negative rail401). Although shown inFIG.4as having two DC-link capacitors440, the DC-link may include more or fewer DC-link capacitors440(including none) in other embodiments.

An operator pre-charges the DC-link capacitors440and the capacitors in the MSC410and LSC420before connecting the generator320to the grid330(via the converter system310and associated circuit breakers360) to reduce the amount of inrush current. Generally, when pre-charging the capacitors, the controller900manages the switches in the MMC400to gradually build power across the rails401to charge the capacitors to store a threshold voltage before connecting the converter system310to the generator320. The generator320is soft started via the MSC410and are at a standstill until the MMC400has completed pre-charge. As described herein, the pre-charging of the capacitors is performed in parallel (i.e., at substantially the same time) with the synchronization of the harmonic filter350to the grid power.

FIG.5is a schematic of a harmonic filter350according to embodiments of the present disclosure.FIG.5may be understood in conjunction withFIG.3. The harmonic filter350is shunt connected with the transmission lines301between the converter system310and the grid330. The harmonic filter350mitigates the risks and effects of different current frequencies between the grid330and the converter system310causing harmonic currents in the PCTS300using a set of resonant circuits connected to each transmission line301. Although illustrated as a single-tuned filter, the harmonic filter350may be a double-tuned filter, high-pass filter, etc., in other embodiments.

As illustrated, the harmonic filter350includes, connected on a first side to each corresponding transmission line301, filter fuses510a-c(generally, filter fuse510), inductors520a-c(generally, filter inductors520), and filter capacitors530a-c(generally, filter capacitors530) that are connected on a second side to a shared node540. The filter fuses510protect the filter inductors520and filter capacitors530on the same line from over currents from the transmission line301associated with a difference between the grid voltage and the stored voltage in the harmonic filter350.

The filter inductors520are selected to have a filter inductance of Lf, and the filter capacitors530are selected to have a filter capacitance of Cfthat result in a filter impedance at grid frequency (generally, 50 Hz or 60 Hz±10%) that is significantly higher than the resistance Rpreof the pre-charge resistors390(i.e., |Zfilter|>>Rpre). In various embodiments, the resistance Rpreis less than 10% of the filter impedance Zfilter(i.e., 0.1*|Zfilter|>Rpre). When the resistance Rpreof the pre-charge resistors390is negligible compared to the impedance of the harmonic filter350, the voltage drop over the pre-charge resistors390is also negligible, and the voltage Vfof the harmonic filter350and the grid voltage Vgare substantially equal in magnitude (e.g., |Vg|−5%≤|Vf|≤|Vg|). Accordingly, for an angular speed of ω, based on the grid frequency f (e.g., ω=2πf), the values for the pre-charge resistors390, filter inductors520, and filter capacitors530are selected according to Formula 1.

FIG.6A-6Eare a series of schematics of dual-cells600in various modes of operation, according to embodiments of the present disclosure. Each of the dual-cells600inFIGS.6A-6Eallow for rectification or inversion of current by the controlled switching and discharge of the components therein. For ease of reference, a series of nodes610a-e(generally, node610or cell node) are provided in each of the dual-cells600. A first node610aand a fifth node610eare contacts for the dual-cell600by which the dual-cell600can be connected to another dual-cell600, a rail402, arm reactor710(discussed in relation toFIGS.7A and7B), or other component external to the dual-cell600.

The illustrated dual-cells600include a first cell capacitor620a(generally, cell capacitor620) disposed between a second node610band a third node610c, and a second cell capacitor620bdisposed between the third node610cand a fourth node610d. Although illustrated as four-switch, two-capacitor cells, other cells with more or fewer switches and capacitors may be used in other embodiments. For example, two two-switch, one-capacitor cells may be treated as one four-switch, two-capacitor cell if arranged to exhibit similar component-to-node arrangements.

The switches630a-d(generally, switch630or cell switch) are arranged in parallel with corresponding diodes640a-d(generally, diode640), and are driven open or closed to define various operating modes in the dual-cell600. A first switch630aand first diode640aare disposed between the first node610aand the second node610b, wherein the first diode640ais biased to block current flowing from the second node610bto the first node610a. A second switch630band second diode640bare disposed between the first node610aand the third node610c, wherein the second diode640bis biased to block current flowing from the first node610ato the third node610c. A third switch630cand third diode640care disposed between the third node610cand the fifth node610e, wherein the third diode640cis biased to block current flowing from the third node610cto the fifth node610e. A fourth switch630dand fourth diode640dare disposed between the fifth node610eand the fourth node610d, wherein the fourth diode640dis biased to block current flowing from the fifth node610eto the fourth node610d.

When driven to a closed state by the controller900, the switches630provide a pathway that bypasses the associated diode640, thus allowing current to flow counter to the bias of the associated diode640and thereby bypass one or more cell capacitors620in the dual-cell600. Although illustrated as Insulated-Gate Bipolar Transistors (IGBT), the switches630may include other power semiconductor devices (e.g., a power Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or Bipolar Junction Transistor (BJT)). The controller900(not illustrated) can thereby control whether a given switch630is open or closed by controlling the gate of the associated power semiconductor device. The paired switches630and diodes640may be included in a single package or integrated component, or may be provided as discrete circuit components.

Although the present disclosure primarily discusses dual-cells600, single-cells (or paired single-cells functionally forming a dual-cell600) may be freely substituted for dual-cells600in other embodiments. A single-cell, in contrast to a dual-cell600, includes one cell capacitor620and two switches630(with associated diodes640); effectively half of a dual-cell600. The modes of operation of a single-cell include a natural bypass mode (in which the switches630are not conducting), a natural blocked mode (in which current flows over the cell capacitor620), and a forced blocked mode (in which the switches630are conducting to avoid charging the cell capacitor620).

FIG.6Aillustrates a natural bypass mode of operation of a dual-cell600, where a first current flow601runs from the fifth node610e, through the third diode640cto the third node610c, and through the second diode640bto the first node610a. In the natural bypass mode, the switches630are not conducting. The natural bypass mode is achieved without driving the switches630(i.e., all of the switches630may remain open); current naturally flows from the fifth node610eto the first node610awhen a higher voltage is applied to the fifth node610ethan the first node610a.

FIG.6Billustrates a natural blocked mode of operation of a dual-cell600, where a second current flow602runs from the first node610a, through the first diode640a, first cell capacitor620a, and second cell capacitor620bto the fourth node610d, and through the fourth diode640dto the fifth node610e. In the natural blocked mode, the switches630are open, and the second current flow602flowing over the cell capacitors620charges the cell capacitors620. The natural blocked mode is achieved without driving the switches630(i.e., all of the switches630may remain open); current naturally flows from the first node610ato the fifth node610eover the cell capacitors620when a higher voltage is applied to the first node610athan the fifth node610e.

FIG.6Cillustrates a first forced bypass mode of operation of a dual-cell600, where a third current flow603runs from the first node610a, through the first diode640aand first cell capacitor620ato the third node610c, and through the closed third switch630cto the fifth node610e. In the first forced bypass mode, the first, second, and fourth switches630a,630b,630dare open, and the third switch630cis closed, thus charging the first cell capacitor620a, and not charging the second cell capacitor620b.

FIG.6Dillustrates a second forced bypass mode of operation of a dual-cell600, where a fourth current flow604runs from the first node610a, through the closed second switch630bto the third node610c, and second cell capacitor620bto the fourth node610d, and through the fourth diode640dto the fifth node610e. In the second forced bypass mode, the first, third, and fourth switches630a,630c,630dare open, and the second switch630bis closed, thus charging the second cell capacitor620b, and not charging the first cell capacitor620a.

FIG.6Eillustrates a third forced bypass mode of operation of a dual-cell600, where a fifth current flow605runs from the first node610a, through the closed second switch630bto the third node610c, and through the closed third switch630cto the fifth node610e. In the third forced bypass mode, the first and fourth switches630a,630dare open, and the second and third switches630b,630care closed, thus bypassing and not charging the cell capacitors620.

In each of the forced bypass modes illustrated inFIGS.6C-6E, the selective closing of one or more switches630alters the pathway that current flows from the first node610ato the fifth node610e, thus altering how the cell capacitors620charge compared to the natural blocked mode ofFIG.6B. When the same voltage is applied across the dual-cell600, the voltage may be divided across the two cell capacitors620(as per the natural block mode), directed solely across one cell capacitor620(as per the first and second forced bypass modes), or none of the cell capacitors620(as per the third forced bypass mode). When in the natural block mode, the voltage Vcellin the N cell capacitors620of a given phase arm may be given as per Formula 2, where Vpeakis the peak phase-to-phase voltage of the grid330or power source used in pre-charging.
Vcell=Vpeak÷N(2)

In contrast, when in the first or second forced bypass mode, one or more cell capacitors620are bypassed; dividing the voltage over fewer cell capacitors620and allowing for a higher charge in the non-bypassed cell capacitors620, which may be expressed according to Formula 3, where K is the number of cell capacitors620bypassed (i.e., over which current does not flow).
Vcell=Vpeak÷(N−K)  (3)

The controller900selects which dual-cells600to operate in a given mode of operation to selectively charge the sum of voltages on the cell capacitors620up to the peak voltage Vpeak(less any voltage drops across the diodes640and resistive losses) or another pre-charging threshold set by an operator. Various sensors371(e.g., voltage probes371a-bacross the cell capacitors620a-b) allow the controller900to measure how the cell capacitors620are charging, and for the controller900to balance how quickly individual or a subset of the cell capacitors620are charging by controlling the switches630to charge or not charge the associated cell capacitors620at a given time when pre-charging the MMC400.

FIGS.7A and7Billustrate the LSC420and MSC410respectively in association with the charging paths for a phase AB positive current (i.e., Vab>0), according to embodiments of the present disclosure. Charging currents for the other phases (i.e., BC, AC), and negative currents thereof, have been omitted fromFIGS.7A and7Bfor clarity, but are contemplated by the present disclosure. The LSC layout700ainFIG.7Amay be understood to be linked with the MSC layout700binFIG.7Bby node A on the first rail401a(as a positive rail401in the present example), and node B on the second rail401b(as a negative rail401in the present example).

The LSC layout700afor a three-phase MMC400includes three legs connected between the transmission lines301and the rails401, each connected to a respective one of the three transmission lines301a-c. Each leg includes a plurality of dual-cells600, of which half are disposed between the corresponding transmission lines301a-cand the first rail401a, and may be referred to as a positive arm730a-c(generally, positive arm730or positive LSC arm) based on the corresponding transmission line301a-c. The other half of the dual-cells600are disposed between the corresponding transmission line301and the second voltage rail401b, and may be referred to as a negative arm740a-c(generally, negative arm740or negative LSC arm) based on the corresponding transmission line301a-c. Each half of the legs is also associated with a leg inductance, represented by a arm inductor710.

The MSC layout700bfor a three-phase MMC400includes three legs connected between the generator lines302and the rails401, each connected to a respective one of the three generator lines302a-c. Each leg includes a plurality of dual-cells600, of which half are disposed between the corresponding generator lines302a-cand the first voltage rail401a, and may be referred to as a positive arm750a-c(generally, positive arm750or positive MSC arm) based on the corresponding generator line302a-c. The other half of the dual-cells600is disposed between the corresponding generator line302a-cand the second voltage rail401b, and may be referred to as a negative arm760a-c(generally, negative arm760or negative MSC arm) based on the corresponding generator line302a-c. Each half of the legs is also associated with a leg inductance, represented by arm inductor710.

Three current loops770a-c(generally, current loop770) are shown inFIGS.7A and7Bflowing inward from the first transmission line301aand outward to the second transmission line301b.

The first current loop770acarries current from the first transmission line301athrough the first positive arm730aonto the first rail401a, and from the first rail401athrough the second positive arm730bto the second transmission line301b.

The second current loop770bcarries current from the first transmission line301athrough the first positive arm730aonto the first rail401a, from the first rail401athrough the positive and negative MSC arms to the second rail401b, and from the second rail401bthrough the second negative arms740bto the second transmission line301b.

The third current loop770ccarries current from the first transmission line301athrough the first negative arm740aonto the second rail401b, and from the second rail401bthrough the second negative arm740bto the second transmission line301b.

The dual-cells600in the first positive arm730aand the second negative arm740bare operated in the natural bypass mode (as perFIG.6A), such that the first, second, and third current loops770a-cdo not charge the cell capacitors620in those dual-cells600(e.g., passing through the respective second and third diodes640b-c).

A controller900may drive the cell switches to force the dual-cells600into a forced bypass mode (perFIGS.6C-6E), or may leave the cell switches un-driven to leave the dual-cells600to conduct current in a natural mode of operation (perFIGS.6A-6B).

When the controller900allows the dual-cells600in the LSC420and the MSC410to charge in an un-driven mode of operation (i.e., the switches630remain open for current to flow through the diodes640and the capacitors620), the peak voltage Vpeakapplied is across twice as many cell capacitors620in the MSC410relative to the LSC420. Accordingly, the voltage in the cell capacitors620of an LSC420Vcell-LSCand the voltage in the cell capacitors620of an MSC410Vcell-MSCmay equalize according to Formula 4, if the controller900does not drive the switches630to boost the voltages therein.
Vcell-LSC=2*Vcell-MSC(4)

Once the controller900has completed the un-driven mode of operation, the controller900can then selectively drive one or more cells (e.g., turn on one or two bypass switches in a dual-cell600) to boost the charge level in the cell capacitors620. During the driven stage, the controller900drives the switches630in one or more dual-cells600such that selected dual-cells600operate any one of the natural modes of operations and the forced bypass modes of operation. For example, in the driven stage, the dual-cells600in the second positive arm730b, the first negative arm740a, and the MSC arms are operated in any one of the natural blocked mode, first forced bypass mode, second forced bypass mode, and third forced bypass mode (as perFIGS.6B-6Ereceptively) to control how the cell capacitors620therein are charged. Details of cell charging are discussed in greater detail in regard toFIG.8. The controller900may force a bypass mode in a given dual-cell600to boost the charging of the cell capacitors620in an arm in total close to Vpeak, to account for manufacturing tolerances between cell capacitors620(e.g., to equalize the stored charge), etc.

FIG.8is a flowchart of a method800for synchronizing a harmonic filter350while pre-charging the dual cells600in an MMC400, according to embodiments of the present disclosure.

Method800begins with block810, where the controller900initializes the open/closed state of various circuit breakers360. During pre-charging, a generator circuit breaker360dremains open to disconnect the generator320from the converter system310being pre-charged. A first set of switches361fin a pre-charging contactor360fare closed and a first set of switches361ein a transmission circuit breaker360eare opened to direct power from the grid transformer340over a set of pre-charge resistors390before reaching the harmonic filter350and the converter system310. The second sets of switches362e,362fin the transmission circuit breaker360eand the pre-charging contactor360frespectively remain open, unless the power from grid330needs to be shunted to ground (e.g. in cases where inspection or service is needed on the system).

Once the circuit breakers360are initialized at block810, method800proceeds to block820, where the controller900ensures that the grid circuit breaker360ais closed to thereby connect the grid330to the harmonic filter350and the converter system310over the pre-charge resistors390and to provide power from the grid330(over the auxiliary transformer380) to the APS370and controller900.

At block830, the MMC400enters into an un-driven stage of pre-charging, where the controller900monitors the status of the dual-cells600and the voltages of the cell capacitors620, but does not drive the switches630in the dual-cells600. The voltage in the harmonic filter350equalizes (less any voltage drops over the pre-charge resistors390) with the grid voltage Vg, and the voltage Vcell-LSCacross the cell capacitors620in the LSC420equalizes to the peak phase-to-phase voltage of the grid330Vpeak, divided by the number of cell capacitors620N in each arm of the LSC, and the voltage Vcell-MSCacross the cell capacitors620in the MSC410equalizes to the peak phase-to-phase voltage of the grid330Vpeak, divided by the number of cell capacitors620N in each leg of the MSC (typically Vcell-LSC=2*Vcell-MSC). During block830, the controller900leaves the switches630in all of the dual-cells600open; allowing current to flow through the dual-cells600according to the natural bypass mode or natural blocking mode (perFIGS.6A and6B, respectively).

Method800may remain at block830until a measured voltage satisfies a driving threshold. In some embodiments, the measured voltage may be a voltage Vfstored in the filter capacitors530compared against a driving threshold based on the grid voltage Vg(e.g., x % of Vg). In some embodiments, the measured voltage may be a voltage Vcell-LSCin the one of the cell capacitors620(or an average, highest, or lowest Vcell-LSCmeasured across several cell capacitors620) compared against a driving threshold based on the peak phase-to-phase voltage Vpeak(e.g., x % of Vpeak÷N). In some embodiments, the measured voltage may be a voltage Vcell-MSCin the one of the cell capacitors620(or an average, highest, or lowest Vcell-MSCmeasured across several cell capacitors620) compared against a driving threshold based on the peak phase-to-phase voltage Vpeak(e.g., x % of Vpeak÷N). Once the measured voltage satisfies the driving threshold, method800proceeds to block840.

At block840, the MMC400enters into a driven stage of pre-charging, where the controller900continues to monitor the status of the dual-cells600and the voltage of the cell capacitors620, but may actively drive the switches630in the dual-cells600to boost the charge stored in the cell capacitors620. The status of the dual-cells600can include the health of the various electronics included in the dual-cell600, including the cell switches630, communication and temperature sensors, pressure sensors, etc. In various embodiments, the controller900drives the switches630in the MSC410and the LSC420to decrease the number of cell capacitors620over which the voltage from the grid330is applied using the different forced bypass modes to gradually increase the voltage applied across the cell capacitors620; applying the rectified or peak grid phase-to-phase voltage VPeakacross a subset of the cell capacitors620over the diodes640.

In a progressive driving schema to decrement one cell at a time how many dual-cells600are being simultaneously charged, for example for LSC420, the controller900begins by placing one dual-cell600in each arm into the first or the second forced bypass mode and the rest of the dual-cells600in the natural blocking mode so that one cell capacitor620at a time is not charged, and the other cells capacitors620are charged to a voltage of Vcell=Vpeak÷(N−K), where K=1 due to one cell capacitor620being bypassed. In some embodiments, the controller900sweeps through which one cell capacitor620the current bypasses until all of the cell capacitors reach a voltage Vcell=Vpeak÷(N−1). The controller900can then proceed to increase the number of cell capacitors620bypassed at any given time by driving one or more dual-cells600into the first, second, or third forced bypass modes or leaving one or more dual-cells600in the natural blocked mode to that K increases and Vcell=Vpeak÷(N−K) increases accordingly. The controller900can thus cycle through which cell capacitors620are bypassed in an arm, and which are charged until a charge threshold is reached. Stated differently, when the controller900is ready to boost the cell voltage Vcell, the progressive driving schema employs iterative increases in the number of cell capacitors620bypassed at a given time relative to a preceding time (e.g., K1=K0+1) to thereby increase the voltage applied per non-bypassed cell capacitors620.

In a halving driving schema to successively halve how many dual-cells600are being simultaneously charged, for example for MSC410, the controller900begins by alternating between the first forced bypass mode and the second forced bypass mode of operation for each dual-cell600in a given arm to charge half of the cell capacitors620at any given time, until each cell capacitor620is charged to 2*Vpeak÷N (e.g., where K=N/2, Vpeak÷(N−K)=2*Vpeak÷N). After reaching 2*Vpeak÷N, the controller900can then cycle between placing pairs of dual-cells600through the third forced bypass mode and the first and second forced bypass modes to bypass three out of four cell capacitors620to charge one cell capacitor620of the four at a time to 4*Vpeak÷N (e.g., where K=N*3/4, Vpeak÷(N−K)=4*Vpeak÷N). The controller900may thus cycle through bypassing greater percentages of the cell capacitors620to double the voltage applied across the cell capacitors620in each arm until a charge threshold is reached. Stated differently, when the controller900is ready to boost the cell voltage Vcell, the halving driving schema employs exponential increases (by a power of two) in the number of cell capacitors620bypassed at a given time relative to a preceding time (e.g., K2=2*K1=4*K0) to thereby increase the cell voltage Vcellapplied across the non-bypassed cell capacitors620.

As will be appreciated, other driving schema and variations of the above schema are also contemplated, such as, for example, by identifying the cell capacitors620with the lowest charges to balance the charges in a given arm, such as by a Sorting and Selecting (SoS) algorithm used during normal (i.e., inverting/rectifying) operation of the MMC. The controller900may thus be configured for pre-charging the cell capacitors620to account for inherent differences in the circuitry of the dual-cells600in a series of iterations where each successive iteration includes a smaller subset of cell capacitors620charged at a given time to a correspondingly higher cell voltage Vcell.

In some embodiments, the controller900drives the switches in the MSC410while leaving the switches in the LSC420un-driven until Vcell-MSC=Vcell-LSC; equalizing the charge in the MSC410and the LSC420before boosting all of the cell capacitors620beyond Vpeak÷N. In some embodiments, the controller900drives each of the arms equally, until the LSC arms reach the charge threshold, and then continues to drive the MSC arms until Vcell-LSC=Vcell-MSC.

Method800may remain at block840until a measured voltage satisfies a charge threshold. In some embodiments, the charge threshold is reached when K=(N−1) and the voltage Vcellin each cell capacitor620has been charged to Vpeak/N (or y % of Vpeak/N to account for manufacturing tolerances in the cell capacitors620, voltage drops across the diodes640, and resistive losses). In other embodiments, where an operator may be more tolerant of inrush currents, the charge threshold may be reached before K=(N−1), and where nominal/rated/operating Vcellis a predefined value less than Vpeak/N (or y % of Vpeak/N). Once the measured voltage of Vcellsatisfies the charge threshold (nominal/rated/operating Vcell), method800proceeds to block850.

At block850, pre-charging is complete, and the controller900closes the first set of switches361ein the transmission circuit breaker, closes the generator circuit breaker360d, and begins switching the converter system310to rectify and invert the power produced by the generator320for supply to the power grid330rather than to boost the charge levels in the cell capacitors620. To avoid causing (or reduce the amplitude of) transient currents in the harmonic filter350, the first set of switches361fin the pre-charging contactor360fremain closed until after the first set of switches361ein the transmission circuit breaker360eare closed. The difference in resistance on the two paths over the transmission lines (i.e., through the transmission circuit breaker360eand through the pre-charging contactor360f) cause the current to bypass the pre-charge resistors390while maintaining the synchronization to the grid330in the harmonic filter350established during the pre-charging of the dual-cells600from the grid330.

Method800may then conclude as the PCTS300enters normal operations, where the generator320provides power to the grid330via the converter system310, which rectifies and inverts the power for consumption by the grid330. The controller900may continue to control the switches630in the cells to affect rectification and inversion, and continue to control the circuit breakers360; optionally opening the pre-charging contactor360fafter normal operations have begun.

FIG.9is a block diagram of a controller unit900, according to one or more embodiments. The controller unit900includes one or more computer processors910and a memory920(e.g., a memory storage device). The one or more processors910represent any number of processing elements that each can include any number of processing cores. The memory920can include volatile memory elements (such as random access memory), non-volatile memory elements (such as solid-state, magnetic, optical, or Flash-based storage), and combinations thereof. Moreover, the memory920can be distributed across different mediums (e.g., network storage or external hard drives).

As shown, the one or more processors910are communicatively coupled with a communication system930to send/receive communication via fiber optic cables, electrical wires, and/or radio signals with various sensors371, circuit breakers/contactors360, switches630and other controller units900associated with the WTG100, APS370, and auxiliary transformer380, etc.

The memory920may include a plurality of “modules” for performing various functions described herein. In one embodiment, each module includes program code that is executable by one or more of the processors910. However, other embodiments may include modules that are partially or fully implemented in hardware (i.e., circuitry) or firmware. The memory920includes a pre-charging control logic940that enables the controller unit900to charge the cell capacitors620while synchronizing the harmonic filter350to the grid330as described herein. In some embodiments, the pre-charging control logic940is preloaded with setpoints and thresholds for various control schemes, such as are described in relation toFIG.8by way of example.

The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer-readable storage medium (or media) (e.g., a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.