Patent Description:
Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modem wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and one or more rotor blades. The nacelle includes a rotor assembly coupled to the gearbox and to the generator. The rotor assembly and the gearbox are mounted on a bedplate support frame located within the nacelle. The one or more rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy and the electrical energy may be transmitted to a power conversion assembly and/or a transformer housed within the tower and subsequently deployed to a utility grid. Modern wind power generation systems typically take the form of a wind farm having multiple such wind turbine generators that are operable to supply power to a transmission system providing power to an electrical grid.

The power conversion assembly may include, for example, AC/AC voltage conversion, that is accomplished using a rotor side converter and a line side converter. The rotor side converter and the line side converter may be coupled via a DC link across which may include a DC link capacitor or capacitor bank. Thus, AC/AC conversion generally includes converting one AC supply to DC, then back to AC of a different voltage or frequency. The DC link between the two must remain stable for the second converter to successfully perform its function. Therefore, the capacitance of the DC link capacitance is used to provide a stabilizing force to maintain the DC link voltage within margins that the machine can function. The quantity of the capacitance scales with the power rating of the device, as more incoming current can shift the DC link more quickly, so more capacitance is required to maintain the DC link in a stable condition. Conventionally, a single type of capacitor has been used in DC link capacitor banks.

Thus, the art is continuously seeking new and improved systems and methods that decrease the cost and volume of the power conversion assembly, while also providing enough capacitance for the DC link to remain stable. As such, the present disclosure is directed to systems and methods that incorporate different types of capacitors in a configuration that highlights the primary benefits of each type, while also mitigating their respective weaknesses.

In one aspect, the present disclosure is directed to a power conversion assembly. The power conversion assembly includes a power converter having a plurality of switching devices, a power source electrically coupled to the power converter, and a direct current (DC) filter circuit bridging the power converter and the power source. The DC filter circuit includes a DC link having a positive rail, a negative rail, and a capacitor bank bridging the power converter and the power source. The capacitor bank is configured for maintaining a voltage of the DC link within a certain range. Further, the capacitor bank includes a plurality of capacitors coupled to the positive rail and the negative rail. The plurality of capacitors includes, at least, a first set of capacitors being a first type of capacitors and a second set of capacitors being a different, second type of capacitors. Moreover, each capacitor in the first set of capacitors is positioned closer to a respective switching device of the plurality of switching devices than a corresponding capacitor of the second set of capacitors to minimize impedance between each capacitor in the first set of capacitors and the respective switching device such that a majority of ripple current from the plurality of switching devices passes through the first set of capacitors.

In an embodiment, the power converter may be an AC-DC converter or a DC-AC converter. In another embodiment, the power converter may be a multi-level power converter. Further, in an embodiment, the plurality of switching devices may include one or more Insulated Gated Bipolar Transistors and/or one or more Silicon Carbide Metal Oxide Semiconductor Field Effect Transistors (Sic MOSFET).

In further embodiments, the first type of capacitors may be film capacitors, whereas the second type of capacitors may include electrolytic capacitors. Thus, in certain embodiments, the electrolytic capacitors have a higher capacitance than the film capacitors or vice versa.

In additional embodiments, the electrolytic capacitors are connected in parallel with the film capacitors. In particular embodiments, the electrolytic capacitors may be integral within the DC filter circuit. Alternatively, the electrolytic capacitors may be external to the DC filter circuit. For example, in an embodiment, the electrolytic capacitors may be placed in a separate box apart from the film capacitors or the electrolytic capacitors are more electrically removed from the DC filter circuit via physical positioning relative to the film capacitors.

In certain embodiments, the power source may be an converter, an inverter, or a DC energy source. For example, in an embodiment, the DC energy source may include a battery, a solar panel, or a fuel cell. In several embodiments, where the power source is the converter or the inverter, the power source may also be coupled to a generator and/or a motor.

In yet another embodiment, the power conversion assembly may be part of a wind turbine power system, a solar power system, a hydropower system, a tidal power system, a fuel cell power system, an energy storage power system, or a hybrid power system. For example, in an embodiment, the power conversion assembly may be part of the wind turbine power system. Thus, in such embodiments, the power converter may be a line side converter of the wind turbine power system and the power source may be a rotor side converter of the wind turbine power system.

In another aspect, the present disclosure is directed to a wind turbine power system. The wind turbine power system includes a rotor having a rotatable hub with at least one rotor blade mounted thereto, a generator coupled to the rotor, and a power conversion assembly. The power conversion assembly includes a line side converter, a rotor side converter, and a direct current (DC) filter circuit bridging the line side converter and the rotor side converter. The line side converter includes a plurality of switching devices. The DC filter circuit includes a DC link having a positive rail, a negative rail, and a capacitor bank bridging the line side converter and a rotor side converter. The capacitor bank is configured for maintaining a voltage of the DC link within a certain range. The capacitor bank includes a plurality of capacitors coupled to the positive rail and the negative rail. The plurality of capacitors includes, at least, a first set of capacitors being a first type of capacitors connected in parallel with a second set of capacitors being a different, second type of capacitors. Further, each capacitor in the first set of capacitors is positioned closer to a respective switching device of the plurality of switching devices than a corresponding capacitor of the second set of capacitors to minimize impedance between each capacitor in the first set of capacitors and the respective switching device such that a majority of ripple current from the plurality of switching devices passes through the first set of capacitors. The wind turbine power system may also include any of the additional features described herein.

In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention which is defined by the appended claims.

Generally, the present disclosure is directed to a power conversion assembly that utilizes multiple types of capacitors arranged in a physical configuration that highlights each of their benefits. More specifically, a first type of capacitors, such as film capacitors, may be placed as close to the switching devices of a power converter of the power conversion assembly as possible so that there is very low impedance between these capacitors and the devices. This positioning will ensure that most of the high frequency ripple current caused by the semiconductor switching events will pass through these capacitors. As an aside, these low impedance capacitors also have a benefit of reducing stress on the semiconductors, while also contributing at least some capacitance. Some distance away (e.g. either in a separate container or box or combined with the first type of capacitors), the power conversion assembly includes a different, second type of capacitors, such as electrolytic capacitors. Thus, the impedance between the bank of electrolytic capacitors and the switching devices reduces the high frequency ripple that these capacitors experience, so the electrolytic capacitors are not stressed, which would reduce their lifetime. The electrolytic capacitors are also configured to provide the bulk capacitance to ensure control stability and contribute some ripple currents capability.

Accordingly, the present disclosure has many advantages not present in the prior art. For example, the hybrid capacitor bank reduces the total volume of capacitors required. This is especially advantageous is a three- or higher-level power converter because the difference between the minimum capacitors for control stability is much lower than the minimum electrolytic capacitors to carry ripple current. Moreover, the capacitor bank of the present disclosure provides cost savings by incorporating film capacitors because the cost of a hybrid capacitor bank is lower than the cost of a purely electrolytic bank with required capacitance value and ripple current capability. For example, in an embodiment, the film capacitors perform the function of a low impedance snubber capacitor on the switching devices to reduce voltage stress and thus improve lifetime expectations of the semiconductor devices without having to purchase another dedicated capacitor.

Referring now to the drawings, <FIG> illustrates a perspective view of one embodiment of a wind turbine <NUM> according to the present disclosure. As shown, the wind turbine <NUM> generally includes a tower <NUM> extending from a support surface <NUM>, a nacelle <NUM>, mounted on the tower <NUM>, and a rotor <NUM> coupled to the nacelle <NUM>. The rotor <NUM> includes a rotatable hub <NUM> and at least one rotor blade <NUM> coupled to and extending outwardly from the hub <NUM>. For example, in the illustrated embodiment, the rotor <NUM> includes three rotor blades <NUM>. However, in an alternative embodiment, the rotor <NUM> may include more or less than three rotor blades <NUM>. Each rotor blade <NUM> may be spaced about the hub <NUM> to facilitate rotating the rotor <NUM> to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub <NUM> may be rotatably coupled to an electric generator <NUM> (<FIG>) of an electrical system <NUM> positioned within the nacelle <NUM> to permit electrical energy to be produced.

The wind turbine <NUM> may also include a controller <NUM> centralized within the nacelle <NUM>. However, in other embodiments, the controller <NUM> may be located within any other component of the wind turbine <NUM> or at a location outside the wind turbine. Further, the controller <NUM> may be communicatively coupled to any number of the components of the wind turbine <NUM> in order to control the components. As such, the controller <NUM> may include a computer or other suitable processing unit. Thus, in several embodiments, the controller <NUM> may include suitable computer-readable instructions that, when implemented, configure the controller <NUM> to perform various different functions, such as receiving, transmitting and/or executing wind turbine control signals.

Referring now to <FIG>, a simplified, internal view of one embodiment of the nacelle <NUM> according to the present disclosure is illustrated. As shown, the generator <NUM> may be coupled to the rotor <NUM> for producing electrical power from the rotational energy generated by the rotor <NUM>. For example, as shown in the illustrated embodiment, the rotor <NUM> may include a rotor shaft <NUM> coupled to the hub <NUM> for rotation therewith. The rotor shaft <NUM> may be rotatably supported by a main bearing <NUM>. The rotor shaft <NUM> may, in turn, be rotatably coupled to a high-speed shaft <NUM> of the generator <NUM> through an optional gearbox <NUM> connected to a bedplate support frame <NUM> by one or more torque arms <NUM>. As is generally understood, the rotor shaft <NUM> may provide a low-speed, high-torque input to the gearbox <NUM> in response to rotation of the rotor blades <NUM> and the hub <NUM>. The gearbox <NUM> may then be configured with a plurality of gears <NUM> to convert the low-speed, high-torque input to a high-speed, low-torque output to drive the high-speed shaft <NUM> and, thus, the generator <NUM>. In an embodiment, the gearbox <NUM> may be configured with multiple gear ratios so as to produce varying rotational speeds of the high-speed shaft for a given low-speed input, or vice versa.

Each rotor blade <NUM> may also include a pitch control mechanism <NUM> configured to rotate the rotor blade <NUM> about its pitch axis <NUM>. Each pitch control mechanism <NUM> may include a pitch drive motor <NUM> (e.g., any suitable electric, hydraulic, or pneumatic motor), a pitch drive gearbox <NUM>, and a pitch drive pinion <NUM>. In such embodiments, the pitch drive motor <NUM> may be coupled to the pitch drive gearbox <NUM> so that the pitch drive motor <NUM> imparts mechanical force to the pitch drive gearbox <NUM>. Similarly, the pitch drive gearbox <NUM> may be coupled to the pitch drive pinion <NUM> for rotation therewith. The pitch drive pinion <NUM> may, in turn, be in rotational engagement with a pitch bearing <NUM> coupled between the hub <NUM> and a corresponding rotor blade <NUM> such that rotation of the pitch drive pinion <NUM> causes rotation of the pitch bearing <NUM>. Thus, in such embodiments, rotation of the pitch drive motor <NUM> drives the pitch drive gearbox <NUM> and the pitch drive pinion <NUM>, thereby rotating the pitch bearing <NUM> and the rotor blade(s) <NUM> about the pitch axis <NUM>. Similarly, the wind turbine <NUM> may include one or more yaw drive mechanisms <NUM> communicatively coupled to the controller <NUM>, with each yaw drive mechanism(s) <NUM> being configured to change the angle of the nacelle <NUM> relative to the wind (e.g., by engaging a yaw bearing <NUM> of the wind turbine <NUM>).

Referring now to <FIG>, a schematic diagram of one embodiment of an electrical system <NUM> according to the present disclosure is illustrated. For example, as shown, the generator <NUM> may be a doubly-fed induction generator (DFIG). The generator <NUM> may be coupled to a stator bus <NUM> and a power conversion assembly <NUM> via a rotor bus <NUM>. In such a configuration, the stator bus <NUM> may provide an output multiphase power (e.g. three-phase power) from a stator of the generator <NUM>, and the rotor bus <NUM> may provide an output multiphase power (e.g. three-phase power) of the rotor of the generator <NUM>. More specifically, the power conversion assembly <NUM> may include a rotor side converter <NUM> and a line side converter <NUM>. Thus, as shown, the generator <NUM> may be coupled to the rotor side converter <NUM> via the rotor bus <NUM>. Furthermore, as shown, the rotor side converter <NUM> may be coupled to the line side converter <NUM> via a DC link <NUM> across which may be a capacitance bank <NUM>, which is described in more detail with respect to <FIG>. The line side converter, may, in turn, be coupled to a line side bus <NUM>.

In an embodiment, the rotor side converter <NUM> and the line side converter <NUM> may be configured for normal operating mode in a three-phase, pulse width modulation (PWM) arrangement using insulated gate bipolar transistors (IGBTs) as switching devices. Other suitable switching devices may be used, such as insulated gate commuted thyristors, MOSFETs, bipolar transistors, silicone controlled rectifier's, and/or other suitable switching devices.

In an embodiment, the power conversion assembly <NUM> may be controlled via a converter controller <NUM>. For example, the converter controller <NUM> may send control commands to the rotor side converter <NUM> and the line side converter <NUM> to control the modulation of switching elements used in the power conversion assembly <NUM> to establish a desired generator torque setpoint and/or power output. The converter controller <NUM> may also be communicatively coupled to the turbine controller <NUM>.

As further depicted in <FIG>, the electrical system <NUM> may, in an embodiment, include a transformer <NUM> coupling the wind turbine <NUM> to an electrical grid <NUM>. The transformer <NUM> may, in an embodiment, be a three-winding transformer which includes a high voltage (e.g. greater than <NUM> KVAC) primary winding <NUM>. The high voltage primary winding <NUM> may be coupled to the electrical grid <NUM>. The transformer <NUM> may also include a medium voltage (e.g. <NUM> KVAC) secondary winding <NUM> coupled to the stator bus <NUM> and a low voltage (e.g. <NUM> VAC, <NUM> VAC, etc.) auxiliary winding <NUM> coupled to the line bus <NUM>. It should be appreciated that the transformer <NUM> can be a three-winding transformer as depicted, or alternatively, may be a two-winding transformer having only a primary winding <NUM> and a secondary winding <NUM>; may be a four-winding transformer having a primary winding <NUM>, a secondary winding <NUM>, and auxiliary winding <NUM>, and an additional auxiliary winding; or may have any other suitable number of windings.

In an additional embodiment, the electrical system <NUM> may include an auxiliary power feed <NUM> coupled to the output of the power conversion assembly <NUM>. The auxiliary power feed <NUM> may act as a power source for various components of the wind turbine system <NUM>. For example, the auxiliary power feed <NUM> may power fans, pumps, motors, and other suitable components of the wind turbine system <NUM>.

In an embodiment, the electrical system <NUM> may also include various circuit breakers, fuses, contactors, and other devices to control and/or protect the various components of the electrical system <NUM>. For example, the electrical system <NUM> may, in an embodiment, include a grid circuit breaker <NUM>, a stator bus circuit breaker <NUM>, and/or a line bus circuit breaker <NUM>. The circuit breaker(s) <NUM>, <NUM>, <NUM> of the electrical system <NUM> may connect or disconnect corresponding components of the electrical system <NUM> when a condition of the electrical system <NUM> approaches an operational threshold of the electrical system <NUM>.

Referring now to <FIG>, a block diagram of one embodiment of suitable components that may be included within the controller (e.g. the turbine controller <NUM> or the converter controller <NUM>) according to the present disclosure is illustrated. For example, as shown, the controller <NUM>, <NUM> may include one or more processor(s) <NUM> and associated memory device(s) <NUM> configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and the like and storing relevant data as disclosed herein). Additionally, the controller <NUM>, <NUM>, may also include a communications module <NUM> to facilitate communications between the controller <NUM>, <NUM>, and the various components of the wind turbine <NUM>. Further, the communications module <NUM> may include a sensor interface <NUM> (e.g., one or more analog-to-digital converters) to permit signals transmitted from various sensor(s) <NUM>, <NUM> to be converted into signals that can be understood and processed by the processors <NUM>. It should be appreciated that the sensor(s) <NUM>, <NUM> may be communicatively coupled to the communications module <NUM> using any suitable means. For example, as shown in <FIG>, the sensors <NUM>, <NUM> are coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <NUM>, <NUM> may be coupled to the sensor interface <NUM> via a wireless connection, such as by using any suitable wireless communications protocol known in the art. Additionally, the communications module <NUM> may also be operably coupled to an operating state control module <NUM> configured to change at least one wind turbine operating state.

As used herein, the term "processor" refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits. Additionally, the memory device(s) <NUM> may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s) <NUM> may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s) <NUM>, configure the controller <NUM>, <NUM> to perform various functions including, but not limited to, detecting an anonymous operational event and initiating an enhanced braking mode for the wind turbine <NUM> as described herein, as well as various other suitable computer-implemented functions.

Referring now to <FIG>, schematic diagrams of various embodiments of a power conversion assembly <NUM> having a capacitor bank <NUM> according to the present disclosure are illustrated. As an example, and as referenced with respect to <FIG>, the power conversion assembly <NUM> may be part of a wind turbine power system <NUM>. Further, <FIG> illustrates a power conversion assembly <NUM> similar to the power conversion assembly <NUM> of the wind turbine power system of <FIG>. However, as further illustrated in <FIG>, it should be understood that the capacitor bank <NUM> of the present disclosure may be used with any three-phase variable or fixed AC supply or power grid, represented as element <NUM> in <FIG>. Accordingly, the power conversion assembly <NUM> described herein may be utilized with a solar power system, a hydropower system, a tidal power system, a fuel cell power system, an energy storage power system, or a hybrid power system.

Thus, as shown in <FIG>, the capacitor bank <NUM> can be utilized with any power converter/inverter <NUM> electrically coupled with any suitable power source <NUM>. For example, in certain embodiments, the power converter <NUM> may be an AC-DC converter or a DC-AC converter, as well as any two-level or any higher multi-level power converter with neutral-point-clamped (NPC), active neutral-point-clamped (ANPC), T-type NPC or Cascaded H-bridges or flying capacitor configurations).

Moreover, in certain embodiments, the power source <NUM> may be an converter, an inverter, or a DC energy source. For example, as shown in <FIG>, the power source <NUM> corresponds to a converter/inverter <NUM>. More specifically, in particular embodiments, in which the power conversion assembly <NUM> is part of the wind turbine power system <NUM>, the power converter <NUM> may be the line side converter <NUM>, whereas the power source <NUM> may be the rotor side converter <NUM>.

Alternatively, as shown in <FIG>, the power source <NUM> may correspond to a DC energy source <NUM>. In such embodiments, the DC energy source <NUM>, as an example, may include a battery/battery-powered source, a solar panel, a fuel cell, a wind power source, or combinations thereof. Moreover, as shown in <FIG>, in several embodiments, where the power source <NUM> is the converter or the inverter, the power source <NUM> may also be coupled to a generator and/or a motor, as shown at <NUM>. Thus, the generator/motor <NUM> may be a wind source, a tidal/hydro source, a fuel cell, and/or any other power conversion application/load, such as marine, healthcare, automation, and/or avionics applications.

Furthermore, as shown in <FIG>, the power converter <NUM> described herein may include any suitable switching devices <NUM>, such as semiconductor switches. In particular, the switching elements <NUM> may include one or more Insulated Gated Bipolar Transistors and/or one or more Silicon Carbide Metal Oxide Semiconductor Field Effect Transistors (Sic MOSFET). In addition, as shown in <FIG>, the power conversion assembly <NUM> may also include a direct current (DC) filter circuit <NUM> bridging the power converter <NUM> and the power source <NUM>.

Further, as shown, the DC filter circuit <NUM> includes a DC link <NUM> having a positive rail <NUM> and a negative rail <NUM>, such that the capacitor bank <NUM> bridges the power converter <NUM> and the power source <NUM>. Thus, the capacitor bank <NUM> is configured for maintaining a voltage of the DC link <NUM> within a certain range. Further, the capacitor bank <NUM> includes a plurality of capacitors <NUM> coupled to the positive rail <NUM> and the negative rail <NUM>. More specifically, as shown, the plurality of capacitors <NUM> includes, at least, a first set <NUM> of capacitors being a first type of capacitors and a second set <NUM> of capacitors being a different, second type of capacitors. For example, in an embodiment, the first type of capacitors <NUM> may be film capacitors, whereas the second type of capacitors <NUM> may include electrolytic capacitors. Thus, in certain embodiments, the electrolytic capacitors <NUM> may have a higher capacitance than the film capacitors <NUM>. In additional embodiments, as shown generally in <FIG>, the electrolytic capacitors <NUM> may be connected in parallel with the film capacitors <NUM>.

In particular embodiments, as shown in <FIG> and <FIG>, the electrolytic capacitors <NUM> may be integral within the DC filter circuit <NUM>. Alternatively, as shown in <FIG> and <FIG>, the electrolytic capacitors <NUM> may be external to the DC filter circuit <NUM>. For example, as shown, in particular embodiments, the electrolytic capacitors <NUM> may be placed in a separate box <NUM> apart from the film capacitors <NUM>. In yet another embodiment, the electrolytic capacitors <NUM> may be more electrically removed from the DC filter circuit <NUM>, e.g. via any suitable physical positioning relative to the film capacitors <NUM>.

Moreover, as shown particularly in <FIG>, each capacitor <NUM> in the first set <NUM> of capacitors is positioned closer to a respective switching device <NUM> of the plurality of switching devices than a corresponding capacitor <NUM> of the second set <NUM> of capacitors to minimize impedance between each capacitor <NUM> in the first set of capacitors and the respective switching device <NUM>. This positioning/arrangement ensures that the majority of the high frequency ripple current caused by the semiconductor switching events <NUM> passes through the film capacitors <NUM>. In addition, in certain embodiments, these low impedance capacitors <NUM> also reduce stress on the switching device <NUM>, while contributing some overall capacitance.

Still referring to <FIG>, some distance away, e.g. further down the busbar <NUM>, the electrolytic capacitors <NUM> are coupled thereto. Accordingly, the impedance provided by the film capacitors <NUM> (and also between the electrolytic capacitors <NUM> and the switching devices <NUM> is configured to reduce the high frequency ripple current that the electrolytic capacitors <NUM> experience, such that the electrolytic capacitors <NUM> are not unduly stressed, thereby increasing their lifetime. Moreover, the electrolytic capacitors <NUM> are configured to provide the bulk capacitance to ensure control stability and contribute some ripple currents capability.

Referring now to <FIG>, a flow diagram of an embodiment of a system <NUM> for operating a power conversion assembly according to the present disclosure is illustrated. For example, as shown, the system <NUM> may include a DC link circuit design optimization function <NUM> that determines an electrolytic capacitor dominant hybrid configuration <NUM> and/or a film capacitor dominant hybrid configuration <NUM>. More specifically, as shown, the DC link filter circuit design optimization function <NUM> receives one or more parameters (such as cost, power loss, the number of components, the loading margins of the capacitors, expected life, and/or the power density) and determines the electrolytic and film capacitor dominant hybrid configurations <NUM>, <NUM>, respectively, by adhering to one or more design constraints (e.g. voltage, ripple current capability, and/or capacitance).

Claim 1:
A power conversion assembly (<NUM>), comprising:
a power converter (<NUM>, <NUM>) comprising a plurality of switching devices (<NUM>);
a power source (<NUM>, <NUM>) electrically coupled to the power converter (<NUM>, <NUM>); and
a circuit bridging the power converter (<NUM>, <NUM>) and the power source (<NUM>, <NUM>), the circuit comprising a direct current (DC) link (<NUM>, <NUM>) comprising a positive rail (<NUM>), a negative rail (<NUM>), and a capacitor bank (<NUM>) bridging the power converter (<NUM>, <NUM>) and the power source (<NUM>, <NUM>), the capacitor bank (<NUM>) configured for maintaining a voltage of the DC link (<NUM>, <NUM>) within a certain range, the capacitor bank (<NUM>) comprising a plurality of capacitors (<NUM>) coupled to the positive rail (<NUM>) and the negative rail (<NUM>), the plurality of capacitors (<NUM>) comprising a first set (<NUM>) of capacitors being a first type of capacitors and a second set (<NUM>) of capacitors being a different, second type of capacitors,
wherein each capacitor in the first set (<NUM>) of capacitors is positioned closer to a respective switching device (<NUM>) of the plurality of switching devices than a corresponding capacitor of the second set (<NUM>) of capacitors to minimize impedance between each capacitor in the first set (<NUM>) of capacitors and the respective switching device (<NUM>)
wherein the first type of capacitors (<NUM>) are film capacitors and the second type of capacitors (<NUM>) are electrolytic capacitors,
characterized in that the first set (<NUM>) of capacitors and the respective switching devices (<NUM>) are located along one side of a planar busbar (<NUM>), and the corresponding capacitors of the second set (<NUM>) of capacitors are located along the opposite side of the planar busbar (<NUM>), wherein each respective switching device (<NUM>) and each capacitor of the first set (<NUM>) has a planar configuration perpendicular to the busbar (<NUM>), and wherein each capacitor of the first set (<NUM>) is parallel and adjacent to its respective switching device (<NUM>).