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 rotor 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. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology. Standard power converters typically include a bridge circuit, a power filter, and an optional crowbar circuit. The bridge circuit typically includes a plurality of cells, for example, one or more power switching elements and/or one or more diodes.

<CIT> describes that the rotor speed of to a double-fed rotating machine (motor) is increased by increasing the converter frequency to a given value below the synchronization speed while the stator turns are short-circuited. The network voltage is then applied to the stator and the converter frequency is lowered to make the machine turn at a second speed. At this point, the transformation ratio of the machine is changed and the frequency reduced to the nominal speed of the machine, all three speed being defined by the load. The transformation ratio is the ratio of the primary voltage applied across the terminals of a stationary rotor to the secondary voltage appearing across the stator and is controlled by an adaptation device which switches from a lower to a higher value. Further, <CIT> describes an apparatus for the conversion of electric energy into mechanic energy and vice versa including a wound-rotor induction machine provided with a stator and a rotor both connected to a same three-phase line through respective connection lines, a motor connected to the rotor and capable to take it to a speed twice the synchronous speed, as well as a step-down transformer and a switch arranged on the connection lines in such a way as to allow to perform the parallel connection of the stator or of the rotor. As a consequence, the machine generates twice the power available for a given size, and weight of iron and copper, because the rotation speed of the machine is twice the synchronous speed and because has a double connection to the line. In this way the induction machine can operate as generator or as synchronous motor. Furthermore, <CIT> describes a method for operating a power generation system that supplies power for application to a load. The method includes receiving, at a power converter, an alternating current power generated by a generator operating at a speed that is substantially equal to its synchronous speed and converting, with the power converter, the alternating current power to an output power, wherein the power converter includes at least one switching element. In addition, the method includes receiving a control command to control a switching frequency of the at least one switching element and adjusting the switching frequency to an adjusted switching frequency that is substantially equal to a fundamental frequency of the load.

When the wind turbine is operating within a low rotor speed operating range, the Annualized Energy Production (AEP) is reduced due to the off-line/zero production time periods of the wind turbine. As such, it would be beneficial to increase the operating RPM (rotations per minute) range of the wind turbine so as to reduce such off-line/zero production time periods so as to increase the AEP and reduce the levelized cost of electricity with minimal risk and cost.

Thus, a system and method for operating the wind turbine power system that allows for more opportunities to harness the wind energy when operating at low RPM levels would be welcomed in the art. Accordingly, the present disclosure is directed to a system and method for operating the wind turbine power system to increase a rotor speed operating range thereof so as to address the aforementioned issues.

Various aspects and advantages of the invention will be set forth in part in the following description, or may be clear from the description, or may be learned through practice of the invention.

Various features, aspects and advantages of the present invention will become better understood with reference to the following description.

Generally, the present subject matter is directed to a system and method for operating an electrical power circuit e.g. a wind turbine power system, connected to a power grid so as to increase a rotor speed operating range thereof. Further, the electrical power circuit has a power converter electrically coupled to a generator having a rotor and a stator. Thus, the method includes operating rotor connections of the rotor of the generator in a wye configuration during a first rotor speed operating range. The method further includes monitoring a rotor speed of the rotor of the generator and transitioning the rotor connections of the rotor from the wye configuration to a delta configuration if the rotor speed changes to a second rotor speed operating range so as to increase the rotor speed operating range of the generator.

The present disclosure provides many advantages not present in the prior art. For example, increasing the operating range of the rotor speed of the generator can allow for an increased annualized energy production (AEP). Further, increasing the operating range of the rotor speed of the generator may also provide a reduction in the cost of electricity for renewable energy solutions as it can reduce the off-line/no production time periods and allow for more opportunities to harness the wind energy when operating at very low rotor speed levels.

Referring now to the drawings, <FIG> illustrates a perspective view of a portion of an exemplary wind turbine <NUM> according to the present disclosure that is configured to implement the method and apparatus as described herein. 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 an exemplary electrical and control system <NUM> that may be used with the wind turbine <NUM> is illustrated. During operation, wind impacts the 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, double-fed induction (asynchronous) generator (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 and control system <NUM> may include a wind turbine controller <NUM> configured to control any of the components of the wind turbine <NUM>. 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.

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 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> 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 a stator bus <NUM>. In one embodiment, to facilitate the DFIG configuration, the generator rotor <NUM> has rotor connections <NUM> that are electrically coupled to a bi-directional power conversion 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 and control system <NUM> as described herein. In a further embodiment, the stator synchronizing switch <NUM> may be electrically coupled to a main transformer circuit breaker <NUM> via a system bus <NUM>.

The power conversion assembly <NUM> may include a rotor filter <NUM> that is electrically coupled to the generator rotor <NUM> via the rotor bus <NUM>. In addition, the rotor filter <NUM> may include a rotor-side reactor. A rotor filter bus <NUM> electrically couples the rotor filter <NUM> to a rotor-side power converter <NUM>. Further, the rotor-side power converter <NUM> may be 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, a line-side power converter bus <NUM> may electrically couple the line-side power converter <NUM> to a line filter <NUM>. Also, a line bus <NUM> may electrically couple the line filter <NUM> to a line contactor <NUM>. In addition, the line filter <NUM> may include a line-side reactor. Moreover, 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 a breaker-side bus <NUM>. The grid circuit breaker <NUM> may be connected to the electric power transmission and distribution grid via a 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 conversion 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>.

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 conversion assembly <NUM> can be combined with the power from the 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 and control system <NUM> as described herein.

Further, the power conversion 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, the controller <NUM> may be configured to receive one or more voltage and/or electric current measurement signals from the first set of voltage and electric current sensors <NUM>. Thus, the controller <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>. In the illustrated embodiment, each of the sensors <NUM> may be electrically coupled to each one of the three phases of the power grid bus <NUM>. Alternatively, the sensors <NUM> may be electrically coupled to any portion of electrical and control system <NUM> that facilitates operation of electrical and control system <NUM> as described herein. In addition to the sensors described above, the sensors may also include a second set of voltage and electric current sensors <NUM>, a third set of voltage and electric current sensors <NUM>, a fourth set of voltage and electric current sensors <NUM> (all shown in <FIG>), and/or any other suitable sensors.

It should also be understood that any number or type of voltage and/or electric current sensors may be employed within the wind turbine <NUM> and at any location. For example, the sensors may be current transformers, shunt sensors, rogowski coils, Hall Effect current sensors, Micro Inertial Measurement Units (MIMUs), or similar, 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 voltage and/or electric current feedback signals from the sensors <NUM>, <NUM>, <NUM>, <NUM>. More specifically, in certain embodiments, the current or voltage feedback signals may include at least one of line current feedback signals, line-side converter feedback signals, rotor-side converter feedback signals, stator current feedback signals, line voltage feedback signals, or stator voltage feedback signals. For example, as shown in the illustrated embodiment, the converter controller <NUM> receives voltage and electric current measurement signals from the second set of voltage and electric current sensors <NUM> coupled in electronic data communication with stator bus <NUM>. The converter controller <NUM> may also receive the third and fourth set of voltage and electric current measurement signals from the third and fourth set of voltage and electric current sensors <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>.

The maximum allowable instantaneous operating magnitude of the DC link <NUM> is determined by the design of the line-side and rotor-side converters <NUM>, <NUM>, including but not limited to selection of the power switching device types and ratings, selection of the DC link capacitance type and ratings, parasitic elements such as stray inductance and the operation of the gate drivers that govern the switching of the power devices and consequently the transient overshoot voltage seen by the power switching devices. Further, the steady state DC link operating voltage set point impacts a number of items including but not limited to the maximum magnitude of fundamental AC voltage available at the rotor and line side converters <NUM>, <NUM>, the semiconductor losses and the failure rate of the switching devices.

The maximum available fundamental output voltage at the rotor <NUM> and the line-side converter <NUM> is a function of the boost converter modulator design, the magnitude of the DC link voltage and the modulation index. Thus, if the power converter <NUM> uses a typical Space Vector Modulation (SVM), the voltage gain VG from the DC link <NUM> to either line to line output (line or rotor) without over-modulating is represented by Equation (<NUM>) below: <MAT>.

The rotor-side voltage of the rotor-side converter <NUM> may be calculated as a function of the stator voltage and frequency, a number of poles, rotor speed, machine impedance and turns ratio, and stator and rotor current. In order to maximize the operating voltage range of the generator <NUM>, the steady state DC link operating voltage set point of the power converter <NUM> should be set as high as possible. Conversely, to obtain robustness in regards to grid voltage capability (e.g. high-voltage right through (HVRT) and islanding capability), a margin is required between the steady state DC link operating voltage set point and the maximum allowable instantaneous DC link voltage to avoid component failure, thereby necessitating a lower steady state DC link voltage set point. Minimizing the reliability effects of Single Event Burnout (SEB) and/or terrestrial cosmic radiation also calls for a minimization of the steady state DC link voltage set point.

In certain embodiments, it should be noted that operation of the DC Link voltage at the level required to meet the extreme ends of the extended operating rotor speed range is not required, nor is it desired. For example, when operating at speeds near the synchronous speed of the generator <NUM>, the DC Link voltage set point can be reduced to a level dictated solely by peak value of the line-side voltage, plus some additional margin to allow for the forcing of current through the line-side or rotor-side inductors. By reducing the DC Link voltage set point when permitted by the operating RPM, the present disclosure reduces converter losses, provides an additional DC link voltage margin for HVRT events, and increases power switching device reliability.

Referring now to <FIG>, a flow diagram of one embodiment of a method <NUM> for operating an electrical power system (e.g. the wind turbine power system of <FIG>) is illustrated. As shown at <NUM>, the method <NUM> includes operating the rotor connections <NUM> of the rotor <NUM> of the generator <NUM> in a wye configuration (<FIG>) during a first rotor speed operating range. For example, as shown in the graph <NUM> of <FIG>, the first rotor speed operating range may include rotor speeds equal to and above synchronous speed <NUM> of the wind turbine <NUM>. More specifically, in certain embodiments, for a <NUM>-Hertz power converter, the first rotor speed operating range may include rotor speeds from about <NUM> rotations per minute (RPM) to about <NUM> RPM. Further, <FIG> illustrates the limitations of the operating RPM based on the power converter <NUM> only being about to output a limited rotor voltage.

Referring still to <FIG>, as shown at <NUM>, the method <NUM> includes monitoring a rotor speed of the rotor <NUM> of the generator <NUM> during operation thereof, e.g. via one or more sensors <NUM>, <NUM>, <NUM>, <NUM>. For example, in particular embodiments, the sensor(s) <NUM>, <NUM>, <NUM>, <NUM> may include electric current or voltage sensors configured to generate one or more current or voltage feedback signals of the electrical power circuit <NUM> as well as an RPM sensor in the drivetrain.

As shown at <NUM>, the method <NUM> also includes transitioning the rotor connections of the rotor <NUM> from the wye configuration to a delta configuration if the rotor speed changes to a second rotor speed operating range. For example, as shown in the graph <NUM> of <FIG>, the second rotor speed operating range may include rotor speeds below a synchronous speed <NUM> of the wind turbine <NUM>. More specifically, in one embodiment, for a <NUM>-Hertz power converter, the first rotor speed operating range may include from about <NUM> rotations per minute (RPM) to about <NUM> RPM, such as from about <NUM> RPM to about <NUM> RPM, as shown in <FIG>. It should be further understood that the second rotor speed operating range may also depend on various other factors, such as turns ratio of the generator <NUM> and/or a gear ratio of the gearbox <NUM>, which must occur at an RPM/converter rotor frequency at or below a magnitude that allows the current of the power converter <NUM> to remain at or below its rating once the machine is connected into the delta configuration.

Referring now to <FIG>, a schematic diagram of one embodiment of a wye-delta contactor configuration of the rotor connections <NUM> (<FIG>) is illustrated according to the present disclosure. It should be understood that <FIG> illustrates the use of a conventional wye-delta motor starter inserted between the generator rotor <NUM> and the power converter <NUM>, where one of the contactors (e.g. contactor <NUM>) for each phase is always closed. In alternative embodiments, as shown in <FIG>, each phase of the wye-delta configuration may include a single contactor (i.e. contactor <NUM>) according to the present disclosure. More specifically, the top diagrams (<FIG>) illustrate various embodiments of a wye-delta contactor configuration of the electrical power circuit <NUM>, whereas <FIG> illustrate the wye and delta configurations, respectively. Thus, as shown in the illustrated embodiment, to operate the rotor connections <NUM> of the rotor <NUM> of the generator <NUM> in a wye configuration (<FIG>), the <NUM> and S contactors are closed for the first rotor speed operating range. In contrast, to operate the rotor connections <NUM> of the rotor <NUM> of the generator <NUM> in a delta configuration (<FIG>), the <NUM> and <NUM> contactors are closed and the S contactors are open, e.g. during the second rotor speed operating range. As such, by transitioning the rotor connections of the rotor <NUM> from the wye configuration to the delta configuration at the bottom end of the RPM range (i.e. from about <NUM> RPM to about <NUM> RPM, more preferably from about <NUM> RPM to about <NUM> RPM as shown in <FIG>), an overall rotor speed operating range is increased for the electrical power circuit <NUM>. For example, in certain embodiments, the increased overall rotor speed operating range may be larger than the initial operating range by about <NUM>% to about <NUM>%. Further, the transition from wye to delta is configured to increase the current seen by the power converter <NUM> by the square-root of <NUM>; however, the power converter <NUM> is able to operate at this level of load current. In addition, as shown in <FIG>, the transition from the wye configuration to the delta configuration and vice versa can include RPM hysteresis in the controller <NUM> to avoid excessive transitioning, i.e. the transition should occur quickly.

In additional embodiments, during the transition from the wye configuration to the delta configuration, any one or more of the following conditions may occur: the generator stator <NUM> may remain connected, the torque may be ramped to zero, the power converter rotor IGBTs may stop gating, the contactor S may be dropped out, the contactor <NUM> may be pulled in, the rotor converter output voltage may be synched to the rotor voltage, the rotor IGBTs may be enabled, and/or the torque may be ramped up. Further, during the time this process is occurring, the rotor blade speed increases as the energy from the wind has no energy sink, so the time allotted to make the transition must be short enough to avoid an excessive level of blade RPM increase. Thus, as shown in <FIG>, the minimum level of RPM hysteresis is determined by the time required to carry out the above process and the related analysis of the speed increase that can be expected during this time period.

In further embodiments, during the transition from the delta configuration to the wye configuration, any one or more of the following conditions may occur: the generator stator <NUM> may remain connected, the torque may be ramped to zero, the converter rotor IGBTs may stop gating, the contactor <NUM> may be dropped out, the contactor S may be pulled in, the rotor converter output voltage may be synched to the rotor voltage, the rotor IGBTs may be enabled, and/or the torque may be ramped up.

More specifically, in particular embodiments, for a <NUM>-Hertz power converter, the overall rotor speed operating range, before transitioning from the wye configurations to the delta configuration, may include rotor speeds from about <NUM> rotations per minute (RPM) to about <NUM> RPM. As such, in certain embodiments, the switchover may increase the overall rotor speed operating range from about <NUM> RPM-<NUM> RPM to about <NUM> RPM-<NUM> RPM, more preferably from about <NUM> RPM-<NUM> RPM, and still more preferably from about <NUM> RPM-<NUM> RPM. More specifically, where the DC Link voltage of the power converter <NUM> is limited to approximately 1051V, the overall rotor speed operating range may be increased from <NUM> RPM-<NUM> RPM to about <NUM> RPM-<NUM> RPM, depending on the system design, machine turns ratio, gearbox ratio, etc.). In addition, where the DC link <NUM> is permitted to increase to 1300V at both ends of the RPM range (i.e. the upper and lower range), the overall rotor speed operating range may be increased from <NUM> RPM-<NUM> RPM to about <NUM> RPM-<NUM> RPM, again, depending on the system design, machine turns ratio, gearbox ratio, etc..

For certain embodiments, transitioning from the wye configuration to the delta configuration can be accomplished in a short time frame, e.g. from about <NUM> seconds to about <NUM> second, such as about <NUM> seconds, without the concern for over-speed issues. More specifically, in certain embodiments, to prevent over-speed conditions, the system <NUM> remains on-line with power generation being interrupted until the switchover is complete by providing for switchover to occur at the low end of the operating RPM range of the wind turbine, turning off the power converter <NUM>, maintaining the stator <NUM> in an energized state, and/or sensing the rotor back electromotive force (EMF) voltage for line-side converter synchronization once the switchover from the wye configuration to the delta configuration (or vice versa) is accomplished.

Claim 1:
A method (<NUM>) for operating an electrical power circuit (<NUM>) connected to a power grid (<NUM>), the electrical power circuit having a power converter (<NUM>) comprising a DC link (<NUM>), the power converter (<NUM>) electrically coupled to a three-phase, double-fed induction generator, DFIG, (<NUM>) having a rotor (<NUM>) and a stator (<NUM>), the DFIG (<NUM>) comprising a synchronous speed, the electrical power circuit (<NUM>) comprising a first rotor speed operating range of rotor speeds equal to and above the synchronous speed of the DFIG (<NUM>) and a second rotor speed operating range of rotor speeds below the synchronous speed of the DFIG (<NUM>), the method comprising:
monitoring (<NUM>) a rotor speed of the rotor (<NUM>) of the DFIG (<NUM>);
operating (<NUM>) rotor connections (<NUM>) of the rotor (<NUM>) of the DFIG (<NUM>) in a wye configuration during the first rotor speed operating range; and
transitioning the rotor connections (<NUM>) of the rotor (<NUM>) from the wye configuration to a delta configuration if the rotor speed changes to the second rotor speed operating range to increase an overall rotor speed operating range of the electrical power circuit (<NUM>).