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 modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.

During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. The associated electrical power can be transmitted to a main transformer that is typically connected to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.

In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Further, wind turbine power systems may include a variety of generator types, including but not limited to a doubly-fed induction generator (DFIG).

DFIG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the power converter can provide nearly instantaneous regulation of its output currents with respect to the grid frequency. Under steady operating conditions, the rotor-side converter controls the magnitude and phase of currents in the rotor circuit to achieve desired values of electromagnetic torque. Reactive power flow into the line-connected stator terminals of the generator can also be controlled.

Such DFIG wind turbines may or may not be equipped with a dynamic brake that includes parallel insulated-gate bipolar transistors (IGBTs) which feed power into a resistor. Minimum components for the dynamic brake typically include a switch (typically a semiconductor such as an IGBT) and a resistor and may also include one or more diode(s) in parallel with either the switch, the resistor, or both, as well as other components. Without dynamic braking, typical operation of a DFIG wind turbine is configured to regulate the positive sequence voltage with a closed-loop current regulation scheme which minimizes negative sequence current. As the length of the transmission line feeder to the DFIG wind turbine is increased, however, response to grid transients and grid disturbances causes oscillations of power into and out of the power converter, which can create disturbances on the DC bus voltage therein. As longer transmission line length is typically desired (and possibly coupled with larger grid voltage transients), the voltage overshoots on the DC bus voltage in the power converter may reach a level that damages the converter components. Thus, the dynamic brake may be used to control the peak voltage on the DC bus.

For conventional dynamic brakes, controls for the switch may be operated based solely on the level of the DC bus voltage in the power converter. As converter power levels continue to increase, additional IGBTs must be placed in parallel to conduct the current. Therefore, it is important to balance the loss in the parallel IGBTs because the loss directly impacts the junction temperature, and the IGBT with the highest junction temperature is the limit in the total energy that can be fed into the resistor.

<CIT> describes an optimized filter for battery energy storage on alternate energy systems; <NPL>; <CIT> describes a kind of transient state reconfiguration system for improving double-fed fan trouble ride-through capability and control method; <CIT> describes a circuit to be used in a wind power plant;<NPL>; <CIT> describes a system and method for optimization of dual bridge doubly fed induction generator (DFIG). Thus, the present disclosure is directed to an improved dynamic brake circuit assembly for a wind turbine that addresses the aforementioned issues.

In one aspect, the present subject matter is directed to a power converter assembly for an electrical power system connected to a power grid. The power converter assembly includes a rotor-side converter configured for coupling to a generator rotor of a generator of the electrical power system, a line-side converter electrically coupled to rotor-side converter via a DC link, at least one sensor configured to monitor at voltage parameter of the DC link, and a dynamic brake assembly electrically coupled to the DC link. The line-side converter is configured for coupling to the power grid. The dynamic brake assembly includes a plurality of switching devices connected in parallel and a plurality of inductors electrically coupled between the plurality of switching devices. Thus, when the voltage parameter is at or above a voltage threshold, the dynamic brake assembly is configured to turn on such that the plurality of inductors receives at least part of a load generated by the power converter assembly. The dynamic brake assembly includes at least one resistor electrically coupled to a node positioned between the plurality of inductors. In another embodiment, the resistor(s) may include a split resistor.

In further embodiments, the plurality of switching devices may be arranged in a plurality of pairs of switching devices connected in parallel. In such embodiments, each of the plurality of inductors may be connected to nodes between first and second switching devices of each of the plurality of pairs of switching devices.

additional embodiments, each of the plurality of inductors may be connected in parallel with the resistor(s). In alternative embodiments, the dynamic brake assembly may include a plurality of resistors. In such embodiments, each of the plurality of resistors may be connected in series with one of the plurality of inductors between the plurality of switching devices to form a plurality of dynamic brake circuits.

In several embodiments, the dynamic brake assembly may further include at least one snubber capacitor electrically coupled between the plurality of dynamic brake circuits. In such embodiments, the dynamic brake assembly may include at least one additional resistor connected in series with the snubber capacitor. In another embodiment, the dynamic brake assembly may include at least one additional resistor connected in parallel with the snubber capacitor.

In certain embodiments, the plurality of switching devices may be insulated-gate bipolar transistors (IGBTs). In additional embodiments, the electrical power system may be part of a wind turbine power system. In another embodiment, the generator may be a doubly-fed induction generator (DFIG).

In another aspect, the present disclosure is directed to a power converter assembly for an electrical power system connected to a power grid. The power converter assembly includes a rotor-side converter configured for coupling to a generator rotor of a generator of the electrical power system, a line-side converter electrically coupled to rotor-side converter via a DC link, at least one sensor configured to monitor at voltage parameter of the DC link, and a dynamic brake assembly electrically coupled to the DC link. The line-side converter is configured for coupling to the power grid. The dynamic brake assembly includes a plurality of switching devices connected in parallel and at least one resistance-inductance component electrically coupled between the plurality of switching devices. Thus, when the voltage parameter is at or above a voltage threshold, the dynamic brake assembly is configured to turn on such that the resistance-inductance component receives at least part of a load generated by the power converter assembly. It should be understood that the power converter may further include any of the additional features as described herein.

In yet another aspect, the present disclosure is directed to a method for controlling peak voltage of a DC link of a power converter of an electrical power system connected to a power grid with minimal switching losses. The method includes electrically coupling a dynamic brake assembly to a DC link of the power converter. The dynamic brake includes a plurality of switching devices connected in parallel and a plurality of inductors electrically coupled between the plurality of switching devices. The method also includes receiving a voltage measurement of the DC link of the power converter. Further, the method includes comparing the voltage measurement of the DC link to a voltage threshold. When the voltage measurement is at or above the voltage threshold, the method includes turning on the dynamic brake assembly of the power converter such that the at least one inductor receives at least part of a load generated by the power converter.

In one embodiment, the method may include applying hysteresis to the voltage measurement. In another embodiment, the step of turning on the dynamic brake assembly of the power converter may include determining at least one gating command for each of the plurality of switching devices. More specifically, in certain embodiments, the step of determining the gating command(s) for each of the plurality of switching devices may include time-shifting on-delays and off-delays of the plurality of switching devices to optimize sharing of a load between the plurality of switching devices.

In further embodiments, as mentioned, the dynamic brake assembly may include a plurality of resistors and a plurality of inductors coupled between the plurality of switching devices, with each of the plurality of resistors connected in series with one of the plurality of inductors to form a plurality of dynamic brake circuits. In such embodiments, the dynamic brake assembly may also include at least one snubber capacitor coupled between the plurality of dynamic brake circuits. Thus, in particular embodiments, the step of determining the gating command(s) for each of the plurality of switching devices may include simultaneously turning on the plurality of switching devices to optimize sharing of a load between the plurality of switching devices and time-shifting turn-off commands for the plurality of switching devices such that each switching device turns off at a different time. It should be understood that the method may further include any of the additional steps and/or features as described herein.

Referring now to the drawings, <FIG> illustrates a perspective view of a portion of one embodiment of a wind turbine <NUM> according to the present disclosure that is configured to implement the method as described herein. As shown, the wind turbine <NUM> includes a nacelle <NUM> that typically houses a generator (not shown). The nacelle <NUM> is mounted on a tower <NUM> having any suitable height that facilitates operation of wind turbine <NUM> as described herein. The wind turbine <NUM> also includes a rotor <NUM> that includes three blades <NUM> attached to a rotating hub <NUM>. Alternatively, the wind turbine <NUM> may include any number of blades <NUM> that facilitates operation of the wind turbine <NUM> as described herein.

Referring to <FIG>, a schematic view of one embodiment of an electrical power system <NUM> that may be used with the wind turbine <NUM> is illustrated. It should be understood that <FIG> is provided as an example embodiment only and is not meant to be limiting. More specifically, as shown, the electrical power system corresponds to a doubly-fed induction generator (DFIG) power system. In alternative embodiments, however, the electrical power system <NUM> may correspond to a full power conversion system.

During operation, wind impacts the rotor blades <NUM> and the blades <NUM> transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft <NUM> via the hub <NUM>. The low-speed shaft <NUM> is configured to drive a gearbox <NUM> that subsequently steps up the low rotational speed of the low-speed shaft <NUM> to drive a high-speed shaft <NUM> at an increased rotational speed. The high-speed shaft <NUM> is generally rotatably coupled to a generator <NUM> so as to rotatably drive a generator rotor <NUM>. In one embodiment, the generator <NUM> may be a wound rotor, three-phase, DFIG that includes a generator stator <NUM> magnetically coupled to a generator rotor <NUM>. As such, a rotating magnetic field may be induced by the generator rotor <NUM> and a voltage may be induced within a generator stator <NUM> that is magnetically coupled to the generator rotor <NUM>. In one embodiment, the generator <NUM> is configured to convert the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in the generator stator <NUM>. The associated electrical power can be transmitted to a main transformer <NUM> via a stator bus <NUM>, a stator synchronizing switch <NUM>, a system bus <NUM>, a main transformer circuit breaker <NUM>, and a generator-side bus <NUM>. The main transformer <NUM> steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to a grid via a grid circuit breaker <NUM>, a breaker-side bus <NUM>, and a grid bus <NUM>.

In addition, the electrical power system <NUM> may include a wind turbine controller <NUM> configured to control any of the components of the wind turbine <NUM> and/or implement the method steps as described herein. For example, as shown particularly in <FIG>, the controller <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> may also include a communications module <NUM> to facilitate communications between the controller <NUM> and the various components of the wind turbine <NUM>, e.g. any of the components of <FIG>. 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 one or more sensors to be converted into signals that can be understood and processed by the processors <NUM>. It should be appreciated that the sensors (e.g. sensors <NUM>, <NUM>, <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>, <NUM>, <NUM> may be coupled to the sensor interface <NUM> via a wired connection. However, in other embodiments, the sensors <NUM>, <NUM>, <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. As such, the processor <NUM> may be configured to receive one or more signals from the sensors <NUM>, <NUM>, <NUM>, <NUM>.

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. The processor <NUM> is also configured to compute advanced control algorithms and communicate to a variety of Ethernet or serial-based protocols (Modbus, OPC, CAN, etc.). 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 magnetooptical 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> to perform the various functions as described herein.

Referring back to <FIG>, the generator stator <NUM> may be electrically coupled to a stator synchronizing switch <NUM> via the stator bus <NUM>. In one embodiment, the generator rotor <NUM> may be electrically coupled to a bi-directional power converter assembly <NUM> or power converter via a rotor bus <NUM>. Alternatively, the generator rotor <NUM> may be electrically coupled to the rotor bus <NUM> via any other device that facilitates operation of electrical power system <NUM> as described herein. In a further embodiment, the stator synchronizing switch <NUM> may be electrically coupled to the main transformer circuit breaker <NUM> via the system bus <NUM>.

In addition, as shown, the power converter assembly <NUM> (also referred to herein as a power converter) may include a rotor-side power converter <NUM> electrically coupled to a line-side power converter <NUM> via a single direct current (DC) link <NUM>. Alternatively, the rotor-side power converter <NUM> and the line-side power converter <NUM> may be electrically coupled via individual and separate DC links. In addition, as shown, the DC link <NUM> may include a positive rail <NUM>, a negative rail <NUM>, and at least one capacitor <NUM> coupled therebetween.

In addition, the line-side power converter <NUM> may be electrically coupled to a line bus <NUM> that includes a line contactor <NUM>. In addition, the line contactor <NUM> may be electrically coupled to a conversion circuit breaker <NUM> via a conversion circuit breaker bus <NUM>. In addition, the conversion circuit breaker <NUM> may be electrically coupled to the main transformer circuit breaker <NUM> via system bus <NUM> and a connection bus <NUM>. The main transformer circuit breaker <NUM> may be electrically coupled to an electric power main transformer <NUM> via a generator-side bus <NUM>. The main transformer <NUM> may be electrically coupled to a grid circuit breaker <NUM> via the breaker-side bus <NUM>. In addition, as shown, the grid circuit breaker <NUM> may be connected to the electric power transmission and distribution grid via the grid bus <NUM>.

In operation, alternating current (AC) power generated at the generator stator <NUM> by rotation of the rotor <NUM> is provided via a dual path to the grid bus <NUM>. The dual paths are defined by the stator bus <NUM> and the rotor bus <NUM>. On the rotor bus side <NUM>, sinusoidal multi-phase (e.g. three-phase) AC power is provided to the power converter assembly <NUM>. The rotor-side power converter <NUM> converts the AC power provided from the rotor bus <NUM> into DC power and provides the DC power to the DC link <NUM>. Switching elements (e.g. IGBTs) used in bridge circuits of the rotor-side power converter <NUM> can be modulated to convert the AC power provided from the rotor bus <NUM> into DC power suitable for the DC link <NUM>. In addition, as shown, the power converter assembly <NUM> may also include a dynamic brake assembly <NUM> electrically coupled between the rotor-side converter <NUM> and the line-side converter <NUM>, which will be discussed in more detail below in reference to <FIG>.

Still referring to <FIG>, the line-side converter <NUM> converts the DC power on the DC link <NUM> into AC output power suitable for the electrical grid bus <NUM>. In particular, switching elements (e.g. IGBTs) used in bridge circuits of the line-side power converter <NUM> can be modulated to convert the DC power on the DC link <NUM> into AC power on the line-side bus <NUM>. The AC power from the power converter assembly <NUM> can be combined with the power from the generator stator <NUM> to provide multi-phase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the electrical grid bus <NUM> (e.g. <NUM>/<NUM>). It should be understood that the rotor-side power converter <NUM> and the line-side power converter <NUM> may have any configuration using any switching devices that facilitate operation of electrical power system <NUM> as described herein.

Further, the power converter assembly <NUM> may be coupled in electronic data communication with the turbine controller <NUM> and/or a separate or integral converter controller <NUM> to control the operation of the rotor-side power converter <NUM> and the line-side power converter <NUM>. For example, during operation, one or more of the controllers <NUM>, <NUM> may be configured to receive one or more measurement signals from the sensors <NUM>, <NUM>, <NUM>, <NUM>. Thus, the controllers <NUM>, <NUM> may be configured to monitor and control at least some of the operational variables associated with the wind turbine <NUM> via the sensors <NUM>, <NUM>, <NUM>, <NUM>. In the illustrated embodiment, each of the sensors <NUM>, <NUM>, <NUM>, <NUM> may be electrically coupled to each one of the three phases of the power grid bus <NUM>. Alternatively, the sensors <NUM>, <NUM>, <NUM>, <NUM> may be electrically coupled to any portion of electrical power system <NUM>, such as the DC link <NUM>, that facilitates operation of electrical power system <NUM> as described herein.

It should also be understood that any number or type of sensors may be employed within the wind turbine <NUM> and at any location. For example, the sensors <NUM>, <NUM>, <NUM>, <NUM> may be current or voltage transformers, shunt sensors, rogowski coils, Hall Effect current or voltage sensors, Micro Inertial Measurement Units (MIMUs), and/or any other suitable voltage or electric current sensors now known or later developed in the art. Thus, the converter controller <NUM> is configured to receive one or more feedback signals from the sensors <NUM>, <NUM>, <NUM>, <NUM>. In addition, the converter controller <NUM> may be configured with any of the features described herein in regards to the main controller <NUM>. Further, the converter controller <NUM> may be separate from or integral with the main controller <NUM>. As such, the converter controller <NUM> is configured to implement the various method steps as described herein and may be configured similar to the turbine controller <NUM>.

Referring now to <FIG>, schematic diagrams of various embodiments of the dynamic brake assembly <NUM> according to the present disclosure are illustrated. More specifically, as shown, the dynamic brake assembly <NUM> is electrically coupled to the DC link <NUM> between the positive and negative rails <NUM>, <NUM>. Though the figures generally illustrate the dynamic brake assembly <NUM> connected to the positive rail <NUM>, it should be understood that the dynamic brake assembly <NUM> may also be coupled to the negative rail <NUM>. Further, as shown, the dynamic brake assembly <NUM> includes a plurality of switching devices <NUM> connected in parallel. For example, as shown in the illustrated embodiments, the switching devices <NUM> are insulated-gate bipolar transistors (IGBTs). In additional embodiments, the switching devices <NUM> may also include one or more diodes. In addition, as shown, the dynamic brake assembly <NUM> includes a plurality of inductors <NUM> electrically coupled between the switching devices <NUM>. In addition, as shown in <FIG>, the dynamic brake assembly <NUM> may include at least one resistor <NUM> electrically coupled to a node <NUM> positioned between the inductors <NUM>.

As shown particularly in <FIG>, the plurality of switching devices <NUM> may be arranged in a plurality of pairs <NUM> of switching devices <NUM> connected in parallel. In such embodiments, as shown, each of the plurality of inductors <NUM> may be connected to nodes <NUM> between first and second switching devices <NUM> of each of the plurality of pairs of switching devices <NUM>. Alternatively, as shown in <FIG>, each of the plurality of inductors <NUM> may be connected to nodes <NUM> associated with separate switching devices <NUM> that are connected in parallel. In addition, as shown in <FIG>, the dynamic brake assembly <NUM> may also include a freewheel diode <NUM> connected in series with each of the separate switching devices <NUM>. Further, each of the plurality of inductors <NUM> may be connected in parallel with the resistor(s) <NUM>.

Referring now to <FIG>, the dynamic brake assembly <NUM> may include a plurality of resistors <NUM>. In such embodiments, as shown particularly in <FIG>, each of the plurality of resistors <NUM> may be connected in series with one of the plurality of inductors <NUM> between the switching devices <NUM> to form a plurality of dynamic brake circuits <NUM>. Thus, as shown in <FIG>, the dynamic brake assembly <NUM> may further include at least one snubber capacitor <NUM> electrically coupled between the plurality of dynamic brake circuits <NUM>. In such embodiments, the addition of the snubber capacitor <NUM> between the two dynamic brake circuits is configured to change the operation of the circuit by allowing significant reduction of the turn-off switching loss. For example, in the illustrated circuit, the turn-on of the switching devices <NUM> can be simultaneous, whereas turn-off can be staggered, such that one switching device turns off before the other.

In addition, as shown in <FIG>, the dynamic brake assembly <NUM> may also include at least one additional resistor <NUM> connected in series with the snubber capacitor <NUM>. More specifically, as shown, the dynamic brake assembly <NUM> includes two additional resistors <NUM> connected in series on opposing sides of the snubber capacitor <NUM>. In another embodiment, the dynamic brake assembly <NUM> may include at least one additional resistor <NUM> connected in parallel with the snubber capacitor <NUM>. More specifically, as shown in <FIG>, the dynamic brake assembly <NUM> includes two additional resistors <NUM> connected in parallel with the snubber capacitor <NUM>.

Referring now to <FIG> and <FIG>, the resistor(s) <NUM> of the dynamic brake assembly <NUM> may include a split resistor <NUM>, i.e. one that divides voltage between multiple routes. For example, as shown particularly in <FIG>, at least one inductor <NUM> is connected to each connection end of the split resistor <NUM> to the dynamic brake phase module. In addition, as shown in <FIG>, such inductors <NUM> are separate components in the circuit. In an alternative embodiment, as shown in <FIG>, the dynamic brake assembly <NUM> includes a plurality of switching devices <NUM> connected in parallel with at least one resistance-inductance component <NUM> electrically coupled between the plurality of switching devices <NUM>. In such embodiments, the resistance-inductance component <NUM> is a single component having both resistance <NUM> and inductance <NUM> capabilities, i.e. in separate sections of the component.

Referring now to <FIG>, a schematic diagram of yet another embodiment of the dynamic brake assembly <NUM> according to an example useful to understand the invention. As shown, the dynamic brake assembly <NUM> includes a plurality of switching devices <NUM> connected in parallel with an inductance component <NUM> electrically coupled between the switching devices <NUM>. In such embodiments, the inductance component <NUM> is a single component having inductance capabilities that also has specified and controlled resistance. In such embodiments, the inductance and resistance are distributed in the composition of the component.

Referring now to <FIG>, a flow diagram of one embodiment of a method <NUM> for controlling peak voltage of a DC link of a power converter of an electrical power system connected to a power grid with minimal switching losses is illustrated. In general, the method <NUM> will be described herein with reference to the wind turbine <NUM> and dynamic brake assembly <NUM> shown in <FIG>. However, it should be appreciated that the disclosed method <NUM> may be implemented with wind turbines having any other suitable configurations. In addition, although <FIG> depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways without deviating from the scope of the present disclosure.

As shown at (<NUM>), the method <NUM> includes electrically coupling the dynamic brake assembly <NUM> (such as any of the embodiments illustrated in <FIG>, to the DC link <NUM> of the power converter <NUM>. As shown at (<NUM>), the method <NUM> includes receiving a voltage measurement of the DC link <NUM> of the power converter <NUM>. For example, as shown in <FIG>, a schematic diagram of one embodiment of a control scheme that may be implemented by one of the controllers described herein <NUM>, <NUM> is illustrated. As shown, the controller <NUM>, <NUM> receives the voltage measurement <NUM> via comparator <NUM>. Referring back to <FIG>, as shown at (<NUM>), the method <NUM> may also include applying hysteresis to the voltage measurement <NUM>.

In addition, as shown at (<NUM>), the method <NUM> further includes comparing the voltage measurement <NUM> of the DC link <NUM> to a voltage threshold <NUM>. For example, as shown in <FIG>, the comparator <NUM> is configured to compare the voltage measurement <NUM> and the voltage reference <NUM> or threshold. Referring back to <FIG>, as shown at (<NUM>), the controller <NUM>, <NUM> is configured to determine whether the voltage measurement <NUM> is equal to or exceeds the voltage threshold <NUM>. If so, as shown at (<NUM>), the method <NUM> includes turning on the dynamic brake assembly <NUM> of the power converter <NUM> such that the inductor(s) <NUM> receives at least part of a load generated by the power converter <NUM>. More specifically, as shown in the illustrated embodiment of <FIG>, the controller <NUM> may send an "ON" signal <NUM> to the converter controller <NUM> such that the converter controller <NUM> can send appropriate gating commands to the switching devices <NUM> (i.e. IGBT <NUM> and IGBT <NUM>). More specifically, in certain embodiments, the controller <NUM>, <NUM> may alternate on-delays and off-delays of the switching devices <NUM> to optimize sharing of a load between the switching devices <NUM>.

In further embodiments, such as those that include the snubber capacitor <NUM>, the controller <NUM>, <NUM> may simultaneously turn on the switching devices <NUM> to optimize sharing of a load between the switching devices <NUM>. As such, there are minimal to no switching losses at turn on. Being a primarily resistive load, the load current is zero when the switching devices <NUM> turn on, allowing a zero-current turn-on with no switching loss, as long as both switching devices <NUM> can be previously turned off long enough to allow the load current to drop to zero.

In addition, the controller <NUM>, <NUM> may stagger turn-off commands for the switching devices <NUM> such that each switching device turns off at a different time. As such, the first switching device <NUM> that turns off will have zero (or much reduced) switching loss because the load current will shift into the capacitor <NUM> during the switching event. Hence, the capacitor <NUM> becomes a turn-off snubber circuit. The second switching device <NUM> that turns off will have reduced switching loss (nearly zero) because the capacitor <NUM> will again act as a turn-off snubber. It may be advantageous, but not necessary, to operate the turn-off of the switching devices <NUM> in a way to alternate the sequence for every other turn-off event, (one switching device <NUM> turns off first, then the other), to better balance their switching, conduction and diode losses.

Claim 1:
A power converter assembly (<NUM>) for an electrical power system (<NUM>) connected to a power grid (<NUM>), the power converter assembly (<NUM>) comprising:
a rotor-side converter (<NUM>) coupled to a generator rotor (<NUM>) of a generator (<NUM>) of the electrical power system (<NUM>);
a line-side converter (<NUM>) electrically coupled to rotor-side converter (<NUM>) via a DC link (<NUM>), the line-side converter (<NUM>) coupled to the power grid (<NUM>);
at least one sensor configured to monitor a voltage parameter of the DC link (<NUM>); and,
a dynamic brake assembly (<NUM>) electrically coupled to the DC link (<NUM>), the dynamic brake assembly (<NUM>) comprising a plurality of switching devices (<NUM>) connected in parallel and a plurality of inductors (<NUM>) electrically coupled between the plurality of switching devices (<NUM>),
wherein, when the voltage parameter is at or above a voltage threshold, the dynamic brake assembly (<NUM>) is configured to turn on such that the plurality of inductors (<NUM>) receives at least part of a load generated by the power converter assembly (<NUM>);
wherein the plurality of switching devices (<NUM>) is arranged in a plurality of pairs (<NUM>) of switching devices (<NUM>) connected in parallel and each of the plurality of inductors (<NUM>) are connected to nodes (<NUM>) between first and second switching devices (<NUM>) of each of the plurality of pairs of switching devices (<NUM>).;
and wherein the dynamic brake assembly (<NUM>) further comprises a resistor (<NUM>) electrically coupled to a node (<NUM>) positioned between the plurality of inductors (<NUM>).