Patent Description:
Power generation systems can use power converters to convert power into a form of power suitable for an energy grid. In a typical power converter, a plurality of switching devices, such as insulated-gate bipolar transistors ("IGBTs") or metal-oxide-semiconductor field effect transistors ("MOSFETs") can be used in electronic circuits, such as half bridge or full-bridge circuits, to convert the power. Recent developments in switching device technology have allowed for the use of silicon carbide ("SiC") MOSFETs in power converters. Using SiC MOSFETs allows for operation of a power converter at a much higher switching frequency compared to conventional IGBTs. <CIT> describes system and method for optimizing wind turbine operation. <CIT> describes a converter device for wind generator system. <CIT> describes a power converter control system and method.

Aspects and advantages of embodiments of the present disclosure will be set forth in part in the following description, or may be learned from the description, or may be learned through practice of the embodiments. The invention is defined in the independent Claims.

One example aspect of the present disclosure is directed to a power generation system. The power generation system can include a power generator comprising a multiphase rotor. The power generator can be configured to generate multiphase alternating current power at a first voltage. The power generation system can also include a power converter comprising one or more silicon carbide MOSFETs and an isolation transformer. The power converter can be configured to convert the multiphase alternating current power from the power generator at the first voltage to multiphase alternating current power at a second voltage. The power generation system can further include a ground. The power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to the ground.

Another example aspect of the present disclosure is directed to a method of operating a power generation system. The method can include generating multiphase alternating current power at a first voltage with a power generator. The power generator can include a multiphase rotor and stator. The phases of the rotor can be configured in a delta or Wye configuration. Each phase of the rotor can include a high impedance resistor electrically connected to the phase and to a ground. The method can further include providing the multiphase alternating current power from the power generator to a power converter. The power converter can include one or more silicon carbide MOSFETs and an isolation transformer. The power converter can be configured to convert the multiphase alternating current power from the power generator at the first voltage to multiphase alternating current power at a second voltage. The method can further include sensing, by a control device, a voltage or current across the high impedance resistor of each phase of the rotor. The method can further include determining, by the control device, whether a voltage or current imbalance exists based at least upon the sensed voltage or current across the high impedance resistor of each phase of the rotor. When a voltage or current imbalance is determined to exist, the method can include shutting the power converter down by the control device to protect the power converter.

Another example aspect of the present disclosure is directed to power generation system. The power generation system can include a doubly fed induction generator comprising a multiphase rotor and a multiphase stator. The doubly fed induction generator can be configured to generate a multiphase low voltage power on the rotor side and a multiphase medium voltage power on the stator side. The power generation system can further include a low voltage bus electrically connected to the rotor. The power generation system can further include a medium voltage bus electrically connected to the stator. The power generation system can further include an AC to DC power converter electrically connected to the low voltage bus. The AC to DC power converter can be configured to receive low voltage power from the low voltage bus. The power generation system can further include a DC link electrically connected to the AC to DC power converter and configured to receive DC power from the AC to DC power converter. The power generation system can further include a DC to DC to AC power converter comprising at least one silicon carbide MOSFET and an isolation transformer. The DC to DC to AC power converter can be electrically connected to the DC link. The DC to DC to AC power converter can be configured to receive DC power from the DC link. The AC to DC power converter and the DC to DC to AC power converter can together be configured to convert the low voltage power from the multiphase rotor to medium voltage power. The power generation system can further include a ground. The power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer of the DC to DC to AC power converter to the ground.

Variations and modifications can be made to these example aspects of the present disclosure.

These and other features, aspects and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the related principles.

Example aspects of the present disclosure are directed to systems and methods for grounding a power system utilizing power converters with SiC MOSFETs and isolation transformers. For example, power generation systems, such as systems using doubly fed induction generators ("DFIGs"), can use one or more power converters to convert power into multiphase alternating current power suitable for a power grid. In some configurations, a power generation system can include a plurality of power converters, such as a first AC to DC converter and a second DC to DC to AC converter that includes an isolation transformer. The AC to DC and DC to DC to AC converters can convert the power from the power generator, such as a DFIG, from a low voltage to a medium voltage suitable for an electrical grid. This configuration can allow for the elimination of a three winding transformer at the grid interconnection, thereby reducing the cost of the power system.

However, in some applications, the primary- secondary capacitance and resistance associated with the isolation transformer can cause an associated leakage current to flow on the low voltage side of the power generation system. In such a case, the leakage current can cause faults to occur as a result of excessive voltage, and can damage the insulation system on the low voltage side of the power generation system if the system is not provided an adequately low level of impedance to earth. Further, any faults may disable or damage the power generator, and may require protection systems to trip the power generator offline, thereby reducing the availability of the power generation system.

Example aspects of the present disclosure are directed to systems and methods of grounding a low-voltage side of a power generator and/or power converter to shunt a leakage current associated with an isolation transformer of a power converter. For example, a power generation system can include a power generator, such as a doubly fed induction generator ("DFIG"), which can be configured to generate a multiphase (e.g., three phase) alternating current power at a first voltage and a second voltage. For example, a first voltage can be generated on the rotor side, which can be a low voltage. A second voltage power can be generated on the stator side, which can be a medium voltage power. In an embodiment, the medium voltage power from the stator can be provided to a medium voltage bus, and further can be provided to an electric grid, while the low voltage power can be provided to a low-voltage bus, and further to a power converter.

The power generation system can also include a power converter. The power converter can include an isolation transformer and one or more SiC MOSFETs. The power converter can be, for example, one or more power converters configured to convert the power from the first voltage, such as a low voltage, to the second voltage, such as a medium voltage. For example, a low-voltage ("LV") power can be a power less than or equal to about <NUM> kV, and a medium voltage ("MV") power can be a power greater than about <NUM> kV to less than about <NUM> kV. As use herein, the term "about" means within <NUM>% of the stated value.

For example, a power converter can include a first AC to DC converter, which can convert the low voltage multiphase alternating current power into a DC power. The first AC to DC converter can be, for example, a two-level AC to DC power converter. The AC to DC converter can be coupled to a low voltage bus, which can receive the low voltage power from the rotor. The AC to DC converter can further be coupled to a DC link. A second converter, such as a DC to DC to AC converter, can further be coupled to the DC link. The second converter can be configured to receive the DC power from the DC link, and convert the DC power into an AC power at a second voltage, such as AC power at the medium voltage such that the converted power is at the same voltage as the electric grid, and/or the power provided by the stator. In an embodiment, the second converter can be a DC to DC to AC converter, and can include one or more SiC MOSFETs and an isolation transformer.

The power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to a ground. For example, at least one phase of a rotor can be electrically connected to a ground. The power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer to the ground through the at least one phase electrically connected to the ground. Another embodiment, one or more high impedance resistor(s) can be electrically connected to the at least one phase and the ground. The high impedance resistor(s) can be resistor(s) selected to carry a leakage current across all insulation barriers from all leakage current sources, including capacitance for high frequency AC sources, such as SiC MOSFETs. Further, the high impedance resistor(s) can be selected to ensure that any surge protectors, such as metal oxide varistors ("MOVs") connected to the power generation system do not carry the leakage current. The power generation system can be configured to shunt the leakage current associated with the isolation transformer to the ground through the high impedance resistor.

In an embodiment, each phase of the rotor can include a high impedance resistor electrically connected to the phase and a ground. In such a configuration, the power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer to the ground through one or all of the high impedance resistors. For example, the phases of a rotor could be configured in a delta configuration, wherein each phase is coupled to a high impedance resistor which is further coupled to a ground. In another configuration, the phases of the rotor can be in a Wye configuration, wherein each phase is coupled to a high impedance resistor which is further coupled to a ground. In either the Wye or delta configuration, a leakage current can be shunted to ground through the high impedance resistors. Further, the power generation system can include a control device, which can be configured to sense a voltage or current imbalance across the high impedance resistors. The control device can further be configured to shut down the power converter, open one or more switches (not shown), or perform other control actions to electrically isolate the faulted power generation unit when the control device senses a voltage or current imbalance across the high impedance resistors.

In an embodiment, the phases of the rotor can be in a Wye configuration which can include a midpoint. A neutral conductor can be connected to the midpoint and a ground. The power generation system can electrically shunt a leakage current associated with the isolation transformer to the ground through the neutral conductor. Further, a high impedance resistor can be connected to the neutral conductor and a ground. Such a configuration, the power generation system can shunt the leakage current to the ground through the high impedance resistor.

The power generation system can further be grounded by connecting at least one pole of the DC link to a ground. For example, a DC link can include a first pole at a first voltage and a second pole at a second voltage. Either the first pole or the second pole can be electrically connected to a ground. The power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer through the at least one pole of the DC link connected to the ground. In an embodiment, a high impedance resistor can be connected to the at least one pole of the DC link and the ground. The power generation system can be electrically grounded to shunt the leakage current associated with the isolation transformer to the ground through the high impedance resistor connected to the at least one pole of the DC link.

The power generation system can further include one or more filters. For example, each phase of a multiphase power from the rotor can be filtered to remove one or more frequencies from the AC power. The filter can include, for example, an inductor and a capacitor. For example, filter can include an inductor electrically coupled to the low voltage bus and a terminal of the AC to DC converter. The capacitor can be electrically coupled to the inductor and a ground. The power generation system can be electrically grounded to shunt a leakage current through the capacitor of a filter. In an embodiment, each phase can include a filter, wherein the power generation system can shunt the leakage current associated with the isolation transformer through the capacitor of each filter on each phase.

The power generation system can further include a multiphase crowbar circuit. The multiphase crowbar circuit can be used to prevent an overvoltage condition from damaging the power generation system.

In this way, the systems and methods according to example aspects of the present disclosure can have a technical effect of grounding the low-voltage side of a power system in order to shunt a leakage current associated with an isolation transformer to a ground. This can help to ensure that the insulation system of the power generation system is not damaged by high voltage as a result of leakage current flow. Further, this can allow for increased reliability and/or availability of the power generation system. Additionally, in configurations in which a high impedance resistor is used to ground the power generation system, the fault current can be limited, which can reduce the incident energy associated with an arc flash.

With reference now to the figures, example aspects of the present disclosure will be discussed in greater detail. <FIG> depicts a power generation system <NUM> according to example aspects of the present disclosure, which includes a DFIG <NUM>. The present disclosure will be discussed with reference to the example power generation system <NUM> of <FIG> for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, should understand that aspects of the present disclosure are also applicable in other systems, such as full power conversion wind turbine systems, solar power systems, energy storage systems, and other power systems.

In the example power generation system <NUM>, a rotor <NUM> includes a plurality of rotor blades <NUM> coupled to a rotating hub <NUM>. In the exemplary embodiment, the power generation unit includes a gear box <NUM>, which is, in turn, coupled to a generator <NUM>. In accordance with aspects of the present disclosure, the generator <NUM> is a doubly fed induction generator (DFIG) <NUM>.

DFIG <NUM> is typically coupled to a stator bus <NUM> and a power converter <NUM> via a rotor bus <NUM>. The stator bus provides an output multiphase power (e.g. three-phase power) from a stator of DFIG <NUM> and the rotor bus <NUM> provides an output multiphase power (e.g. three-phase power) of DFIG <NUM>. The power converter <NUM> can be a bidirectional power converter configured to provide output power to the electrical grid <NUM> and/or to receive power from the electrical grid <NUM>. As shown, DFIG <NUM> is coupled via the rotor bus <NUM> to a rotor side converter <NUM>. The rotor side converter <NUM> is coupled to a line side converter <NUM> which in turn is coupled to a line side bus <NUM>. An auxiliary power feed can be coupled to the line side bus <NUM> to provide power for components used in the wind turbine system, such as fans, pumps, motors, and other components of the wind turbine system.

In example configurations, the rotor side converter <NUM> and/or the line side converter <NUM> are configured for normal operating mode in a three-phase, pulse width modulation (PWM) arrangement using SiC MOSFETs and/or IGBTs as switching devices. SiC MOSFETs can switch at a very high frequency as compared to conventional IGBTs. For example, SiC MOSFETs can be switched at a frequency from approximately. <NUM> to <NUM>, with a typical switching frequency of <NUM> to <NUM>, whereas IGBTs can be switched at a frequency from approximately. <NUM> to <NUM>, with a typical switching frequency of <NUM> to <NUM>. Additionally, SiC MOSFETs can provide advantages over ordinary MOSFETs when operated in some voltage ranges. For example, in power converters operating at 1200V-1700V on the LV side, SiC MOSFETs have lower switching and conduction losses than ordinary MOSFETs.

In some implementations, the rotor side converter <NUM> and/or the line side converter <NUM> can include a plurality of conversion modules, each associated with a an output phase of the multiphase power, as will be discussed in more detail with respect to <FIG>. The rotor side converter <NUM> and the line side converter <NUM> can be coupled via a DC link <NUM> across which can be a DC link capacitor <NUM>.

In some embodiments, the DC link <NUM> can include a dynamic brake (not shown). The dynamic brake can include a switching element (e.g., an IGBT) coupled in series with a dissipative element (e.g., a resistor). The switching element can be controlled using pulse width modulation techniques via one or more control devices (e.g., controller <NUM> or control system <NUM>) to control the voltage on the DC link <NUM>.

In some embodiments, the DC link <NUM> can include a plurality of resistors (e.g., two resistors) coupled in series between the positive and negative bus. A ground can be coupled at a midpoint between the resistors.

The power converter <NUM> can be coupled to a control device <NUM> to control the operation of the rotor side converter <NUM> and the line side converter <NUM>. It should be noted that the control device <NUM>, in typical embodiments, is configured as an interface between the power converter <NUM> and a control system <NUM>.

In operation, power generated at DFIG <NUM> by rotating the rotor <NUM> is provided via a dual path to electrical grid <NUM>. The dual paths are defined by the stator bus <NUM> and the rotor bus <NUM>. On the stator bus side <NUM>, sinusoidal multiphase (e.g. three-phase) is provided to the electrical grid. In particular, the AC power provided via the stator bus <NUM> can be a medium voltage ("MV") AC power. As used herein, MV AC power can be an alternating current power greater than about <NUM> kilovolts and less than about <NUM> kilovolts. As used herein, the term "about" can mean within <NUM>% of the stated value. On the rotor bus side <NUM>, sinusoidal multiphase (e.g. three-phase) AC power is provided to the power converter <NUM>. In particular, the AC power provided to the power converter <NUM> via the rotor bus <NUM> can be a low voltage ("LV") AC power. As used herein, LV AC power can be an alternating current power less than or equal to about <NUM> kilovolts. The rotor side power converter <NUM> converts the LV AC power provided from the rotor bus <NUM> into DC power and provides the DC power to the DC link <NUM>. Switching devices (e.g. SiC MOSFETs and/or IGBTs) used in parallel 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>. Such DC power can be a LV DC power.

In a power generation system <NUM>, the power converter <NUM> can be configured to convert the LV power to MV AC power. For example, the line side converter <NUM> converts the LV DC power on the DC link <NUM> into a MV AC power suitable for the electrical grid <NUM>. In particular, SiC MOSFETs 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>. SiC MOSFETs can be operated at a higher switching frequency than conventional IGBTs. In addition, one or more isolation transformers coupled to one or more of the bridge circuits can be configured to step the voltage up or down as needed. Additionally, a plurality of inverter blocks can be connected in series on the MV side to collectively step up the voltage of the power on the DC link <NUM> to a MV AC power. The MV AC power from the power converter <NUM> can be combined with the MV power from the stator of DFIG <NUM> to provide multiphase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the electrical grid <NUM> (e.g. <NUM>/<NUM>). In this manner, the MV line side bus <NUM> can be coupled to the MV stator bus <NUM> to provide such multiphase power.

Various circuit breakers and switches, such as grid breaker <NUM>, stator sync switch <NUM>, etc. can be included in the power generation system <NUM> for isolating the various components as necessary for normal operation of DFIG <NUM> during connection to and disconnection from the electrical grid <NUM>. In this manner, such components can be configured to connect or disconnect corresponding buses, for example, when current flow is excessive and can damage components of the power generation system <NUM> or for other operational considerations. Additional protection components can also be included in the power generation system <NUM>. For example, as depicted in <FIG>, a multiphase crowbar circuit <NUM> can be included to protect against an overvoltage condition damaging circuits of the power generation system <NUM>.

The power converter <NUM> can receive control signals from, for instance, the control system <NUM> via the control device <NUM>. The control signals can be based, among other things, on sensed conditions or operating characteristics of the power generation system <NUM>. Typically, the control signals provide for control of the operation of the power converter <NUM>. For example, feedback in the form of sensed speed of the DFIG <NUM> can be used to control the conversion of the output power from the rotor bus <NUM> to maintain a proper and balanced multiphase (e.g. three-phase) power supply. Other feedback from other sensors can also be used by the control device <NUM> to control the power converter <NUM>, including, for example, stator and rotor bus voltages and current feedbacks. Using the various forms of feedback information, switching control signals (e.g. gate timing commands for switching devices), stator synchronizing control signals, and circuit breaker signals can be generated. In an embodiment, the control device <NUM> can be configured to sense a voltage or current imbalance across one or more high impedance resistors in a power generation system <NUM>. For example, as will be discussed in greater detail below, each phase of a multiphase power from a rotor of a DFIG <NUM> can include a high impedance resistor electrically connected between the phase and a ground. In an embodiment, the control device <NUM> can be configured to determine whether a voltage or current imbalance exists across the high impedance resistors, and further can be configured to shut down the power converter <NUM> when the control device <NUM> determines a voltage or current imbalance across the high impedance resistors exists.

Referring now to <FIG>, a topology of a component in a DC to DC to AC converter is depicted. <FIG> depicts an example DC to DC to AC inverter block <NUM>, which can be included in a conversion module <NUM> of a line side converter <NUM>, as depicted in <FIG>. Each inverter block <NUM> can include a plurality of conversion entities. For instance, inverter block <NUM> can include conversion entity <NUM>, conversion entity <NUM>, and conversion entity <NUM>. Each conversion entity <NUM>-<NUM> can include a plurality of bridge circuits coupled in parallel. For instance, conversion entity <NUM> includes bridge circuit <NUM> and bridge circuit <NUM>. As indicated, each bridge circuit can include a plurality of switching devices coupled in series. For instance, bridge circuit <NUM> includes an upper switching device <NUM> and a lower switching device <NUM>. The switching devices can be SiC MOSFETs, which can be operated at higher switching frequencies than conventional IGBTs. As shown, inverter block <NUM> further includes an isolation transformer <NUM>. The isolation transformer <NUM> can be coupled to conversion entity <NUM> and conversion entity <NUM>. As shown, the inverter block <NUM> can further include capacitors <NUM> and <NUM>.

First conversion entity <NUM>, isolation transformer <NUM>, and second conversion entity <NUM> can together define an inner converter <NUM>. Inner converter <NUM> can be operated to convert a LV DC power from the DC link <NUM> to a second LV DC power bus at the outer converter. In an embodiment, inner converter <NUM> can be a high-frequency resonant converter. In a resonant converter configuration, a resonant capacitor <NUM> can be included in inner converter <NUM>. In various embodiments, a resonant capacitor <NUM> can be included on a LV side of the isolation transformer <NUM> as depicted in <FIG>, on an MV side of the isolation transformer <NUM> (not depicted), or on both the LV and MV sides of the isolation transformer <NUM> (not depicted). In another embodiment, inner converter <NUM> can be a hard-switched converter by removing the resonant capacitor <NUM>. Third conversion entity <NUM> can also be referred to as an outer converter <NUM>. Outer converter <NUM> can convert a LV DC power from the inner converter to a LV AC power suitable for use on an energy grid <NUM>. In a typical application, outer converter <NUM> can be a hard-switched converter, and therefore not include a resonant capacitor.

Additionally, as depicted in <FIG>, in an embodiment, a plurality of resistors (e.g., resistors <NUM> and <NUM>) can be coupled across the DC link <NUM>, and together can define a midpoint ("M"). In an embodiment, the midpoint "M" can be can be connected to a ground <NUM>. The resistors <NUM> and <NUM> can be, for example, high impedance resistors selected to carry a leakage current associated with the isolation transformer. Further, in an embodiment, a leakage current associated with the isolation transformer of the power converter can be shunted to the ground through at least one of the plurality of resistors.

<FIG> depicts an example line side converter <NUM> according to example embodiments of the present disclosure. As shown, the line side converter <NUM> includes conversion module <NUM>, conversion module <NUM>, and conversion module <NUM>. The conversion modules <NUM>-<NUM> can be configured to receive a LV DC power from the rotor side converter <NUM>, and to convert the LV DC power to a MV AC power for feeding to the electrical grid <NUM>. Each conversion module <NUM>-<NUM> is associated with a single phase of three-phase output AC power. In particular, conversion module <NUM> is associated with the phase A output of the three-phase output power, conversion module <NUM> is associated with the phase B output of the three-phase output power, and conversion module <NUM> is associated with the phase C output of the three-phase output power.

Each conversion module <NUM>-<NUM> includes a plurality of inverter blocks <NUM>-<NUM>. For instance, as shown, conversion module <NUM> includes inverter blocks <NUM>, inverter block <NUM>, and inverter block <NUM>. In an embodiment, each conversion module <NUM>-<NUM> can include any number of inverter blocks <NUM>-<NUM>. The line side converter <NUM> can be a bidirectional power converter. The line side converter <NUM> can be configured to convert a LV DC power to a MV AC power and vice versa. For instance, when providing power to the electrical grid <NUM>, the line side converter <NUM> can be configured to receive a LV DC power from the DC link <NUM> on a LV side of the line side converter <NUM>, and to output a MV AC power on a MV side of the line side converter <NUM>. The inverter blocks <NUM>-<NUM> can be coupled together in parallel on the LV side and can be coupled together in series on the MV side.

In one particular example implementation, when providing power to the electrical grid <NUM>, the conversion entity <NUM> can be configured to convert the LV DC on the DC link <NUM> to a LV AC power. The isolation transformer <NUM> can be configured to provide isolation. The conversion entity <NUM> can be configured to convert the LV AC power to a LV DC power. The conversion entity <NUM> can be configured to convert the LV DC power to a LV AC power suitable for provision to the electric grid <NUM>. A plurality of inverter blocks can be connected in series to build a MV AC voltage suitable for use on a MV AC energy grid.

The inverter blocks <NUM>-<NUM> can be configured to contribute to the overall MV AC power provided by the conversion module <NUM>. In this manner, any suitable number of inverter blocks can be included within the inverter blocks <NUM>-<NUM>. As indicated, each conversion module <NUM>-<NUM> is associated with a single phase of output power. In this manner, the switching devices of the conversion modules <NUM>-<NUM> can be controlled using suitable gate timing commands (e.g. provided by one or more suitable driver circuits) to generate the appropriate phase of output power to be provided to the electrical grid. For example, the control device <NUM> can provide suitable gate timing commands to the gates of the switching devices of the bridge circuits. The gate timing commands can control the pulse width modulation of the SiC MOSFETs and/or IGBTs to provide a desired output.

It will be appreciated, that although <FIG> depicts only the line side converter <NUM>, the rotor side converter <NUM> depicted in <FIG> can include the same or similar topology. In particular, the rotor side converter <NUM> can include a plurality of conversion modules having one or more conversion entities as described with reference to the line side converter <NUM>. Further, it will be appreciated that the line side converter <NUM> and the rotor side converter <NUM> can include SiC MOSFETs, IGBT switching devices, and/or other suitable switching devices. In implementations wherein the rotor side converter <NUM> is implemented using SiC MOSFETs, the rotor side converter <NUM> can be coupled to a crowbar circuit (e.g. multiphase crowbar circuit <NUM>) to protect the SiC MOSFETs from high rotor current during certain fault conditions.

Referring generally to <FIG>, the LV side, or rotor side, of a power generation system <NUM> may need a method of voltage control to ensure the insulation system on the LV side is not damaged by a high voltage as a result of leakage current flow. For example, the isolation transformer <NUM> in a power converter <NUM> configured to convert a LV power to a MV power contains parasitic coupling capacitances between the two windings and between the windings and an earth reference which produce leakage currents. These capacitance effects can cause excessive transient voltages within the power converter <NUM>, which, if ungrounded, can cause faults to occur. During faults, the insulation system on a power converter or power generator may be compromised, thereby disabling the power generation system or requiring it to be taken off-line. According to example aspects of the present disclosure, a power generation system <NUM> can be electrically grounded to shunt a leakage current associated with the isolation transformer <NUM> of the power converter <NUM> to a ground.

Referring now to <FIG>, an example power generation system <NUM> according to example aspects of the present disclosure is depicted. Elements that are the same or similar to those in <FIG> are referred to with the same reference numeral. As shown in <FIG>, a DFIG <NUM> includes a rotor side and a stator side. The stator side is connected to a stator side bus <NUM>. The rotor side is connected to a rotor side bus <NUM>. Power provided by the rotor of the DFIG <NUM> is provided to the rotor side bus <NUM>, and to a rotor side converter <NUM>, which can be an AC to DC converter. Each phase of the three-phase power from the rotor can be filtered by a filter <NUM>. For example, as shown in <FIG>, a first filter 410A filters first phase A, a second filter 410B filters a second phase B, and a third filter 410C filters a third phase C. The rotor side converter <NUM> can convert the multiphase AC power into DC power and provide the DC power to a DC link <NUM>. A DC bus capacitor <NUM> can be connected to the DC link <NUM>. A line side converter <NUM> can convert the DC power from the DC link <NUM> into multiphase AC power and provide the multiphase AC power to a line side bus <NUM>. The power provided by the line side converter <NUM> can be suitable for application to a grid <NUM>. For example a line side converter <NUM> can be a DC to DC to AC converter configured to convert LV power to a MV power. In an embodiment, the line side converter <NUM> can include one or more SiC MOSFETs and an isolation transformer.

In an embodiment, at least one phase of the rotor side of the DFIG <NUM> can be electrically connected to a ground <NUM>. For example, as shown in <FIG>, each phase of the rotor side bus is electrically connected to a ground <NUM>. In additional embodiments, any number of phases can be electrically connected to a ground <NUM>. The power generation system can be electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to the ground <NUM> through the at least one phase of the rotor electrically connected to the ground <NUM>. For example, an isolation transformer <NUM> in a line side converter <NUM> may cause capacitance effect, particularly in instances in which power converter utilizes high frequency switching SiC MOSFETs. The capacitance effect may induce a leakage current within the power converter, which can be shunted to ground through the at least one phase connected to a ground <NUM>. In an embodiment, a high impedance resistor <NUM> can be electrically connected to the at least one phase and the ground <NUM>. For example, as shown in <FIG>, each phase includes a high impedance resistor <NUM> electrically connected between the phase and the ground <NUM>, a first high impedance resistor 420A for phase A, a second high impedance resistor 420B for phase B, and a third high impedance resistor 420C for phase C. In additional embodiments, any number of phases can include a high impedance resistor <NUM> electrically connected between the phase and the ground <NUM>.

The high impedance resistor can be selected to carry a leakage current associated with the isolation transformer of the power converter. For example, the high impedance resistor <NUM> can be a resistor selected to carry the leakage current across insulation barriers from all leakage current sources, including leakage current caused by high-frequency AC sources, such as SiC MOSFETs, through stray capacitances in the insulation system. Additionally, the high impedance resistor(s) <NUM> can be selected based on the ratings and capabilities of surge protection devices, such as MOVs (not shown). For example, the high impedance resistor <NUM> can be selected to ensure that MOVs do not end up carrying the leakage current.

In configurations in which at least one phase of the rotor is electrically connected to a high impedance resistor <NUM> which is connected to the ground <NUM>, the power generation system <NUM> can be electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter <NUM> to the ground <NUM> through the high impedance resistor <NUM>. By including a high impedance resistor <NUM> between the at least one phase and the ground <NUM>, the fault current to ground can be limited, which can reduce the risk of the incident energy associated with an arc flash event.

Referring now to <FIG>, an example configuration of a power system <NUM> according to example aspects of the present disclosure is depicted. The power system <NUM> can correspond to the power system <NUM> depicted in <FIG>. Elements that are the same or similar to those in previous FIGS. are labeled with the same reference numerals. As shown, power system <NUM> can include a multiphase DFIG <NUM> connected in a Wye configuration. For example, as shown in <FIG>, DFIG <NUM> includes three rotor windings for each of the three rotor phases, rotor winding 120A configured to generate power for phase A, rotor winding 120B configured to generate power for phase B, and rotor winding 120C configured to generate power for phase C. A high impedance resistor <NUM> can be electrically connected to one or more phases of the DFIG <NUM>. For example, as shown a first high impedance resistor 420A is electrically connected between phase A and the ground <NUM>, a second high impedance resistor 420B is electrically connected between phase B and the ground <NUM>, and a third high impedance resistor 420C is electrically connected between phase C and the ground <NUM>. The leakage current from a power converter <NUM> can be shunted to ground <NUM> through these high impedance resistors <NUM>.

In an embodiment, the control device, such as a control device <NUM>, can be configured to sense a voltage or current across each high impedance resistor <NUM> connected to the phases of a DFIG <NUM>. For example, each phase can include a high impedance resistor <NUM> connected between the phase and the ground <NUM>. A voltage or current sensor can be configured to sense a voltage across and/or current through the high impedance resistors <NUM>. Further, the sensors can be configured to provide the sensed values to a control device, such as a control device <NUM>. The control device can be configured to determine whether a voltage or current imbalance exists based on the sensed values, and further can be configured to shut down the power converter <NUM>, open one or more switches (not shown), or perform other control actions to electrically isolate the faulted power generation unit when the control device <NUM> determines a voltage or current imbalance across the high impedance resistors <NUM> exists.

Referring now to <FIG>, an example configuration of a power system <NUM> according to example aspects of the present disclosure is depicted. The power system <NUM> can correspond to the power system <NUM> depicted in <FIG>. Elements that are the same or similar to those in previous FIGS. are labeled with the same reference numerals. As shown, power system <NUM> can include a multiphase DFIG <NUM> connected in a Wye configuration. For example, as shown in <FIG>, DFIG <NUM> includes three rotor windings for each of the three phases, rotor winding 120A configured to generate power for phase A, rotor winding 120B configured to generate power for phase B, and rotor winding 120C configured to generate power for phase C. Further, as depicted in <FIG>, the three phases of the DFIG <NUM> connected in a Wye configuration can together define a midpoint M. A neutral conductor <NUM> can be electrically connected to the midpoint M and the ground <NUM>.

For example, as shown in <FIG>, a neutral conductor <NUM> is electrically connected between the midpoint M and the ground <NUM>. Further, a high impedance resistor <NUM> is electrically connected to the neutral conductor <NUM> between the midpoint M and the ground <NUM>. In an embodiment, the neutral conductor <NUM> can be electrically connected between the midpoint M and the ground <NUM> without a high impedance resistor <NUM>. The power generation system <NUM> can be electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter <NUM> to the ground <NUM> through the neutral conductor <NUM>. Further, in applications in which a high impedance resistor <NUM> is electrically connected to the neutral conductor, the power generation system <NUM> can be electrically grounded to shunt a leakage current associated with the isolation transformer and other components of the power converter <NUM> to the ground <NUM> through the high impedance resistor <NUM>.

Referring now to <FIG>, an example configuration of a power system <NUM> according to example aspects of the present disclosure is depicted. The power system <NUM> can correspond to the power system <NUM> depicted in <FIG>. Elements that are the same or similar to those in previous FIGS. are labeled with the same reference numerals. As shown, power system <NUM> can include a multiphase DFIG <NUM> connected in a delta configuration. For example, as shown in <FIG>, DFIG <NUM> includes a rotor winding with three phases, windings 120A and 120B configured to generate power for phase A, windings 120B and 120C configured to generate power for phase B, and windings 120C and 120A configured to generate power for phase C. A high impedance resistor <NUM> can be electrically connected to one or more phases of the DFIG <NUM>. For example, as shown a first high impedance resistor 420A is electrically connected between phase A and the ground <NUM>, a second high impedance resistor 420B is electrically connected between phase B and the ground <NUM>, and a third high impedance resistor 420C is electrically connected between phase C and the ground <NUM>. The leakage current from a power converter <NUM> can be shunted to ground <NUM> through these high impedance resistors <NUM>.

Further, a control device, such as a control device <NUM>, can be configured to determine whether a voltage or current imbalance exists across each high impedance resistor <NUM> connected to the phases of a DFIG <NUM>. For example, each phase can include a high impedance resistor <NUM> connected between the phase and the ground <NUM>. A voltage or current sensor can be configured to sense a voltage across and/or current through the high impedance resistors <NUM>. Further, the sensors can be configured to provide the sensed values to the control device, such as a control device <NUM>. The control device can be configured to determine whether a voltage or current imbalance exists based on the sensed values, and further can be configured to shut down the power converter <NUM> when the control device <NUM> determines a voltage and/or current imbalance exists across the high impedance resistors <NUM>.

Referring now to <FIG>, a power generation system <NUM> according to example aspects of the present disclosure is depicted. Elements that are the same or similar to those as depicted in previous FIGS. are referred to with the same reference numerals. As shown, a DFIG <NUM> includes a rotor connected to a rotor side bus <NUM> and a stator connected to a stator side bus <NUM>. In an embodiment, at least one phase of a multiphase power provided by a rotor of a DFIG <NUM> can be connected to a filter <NUM>. The filter <NUM> can be, for example, and inductor <NUM> and a capacitor <NUM>, wherein the capacitor <NUM> is electrically connected to a ground <NUM>. The at least one filter <NUM> can be electrically connected to the at least one phase of the rotor. Additionally, each phase of the rotor can be electrically connected to a filter <NUM>. Each filter <NUM> can include an inductor <NUM> and a capacitor <NUM>.

For example, as shown in <FIG>, phase A of the three-phase power from the rotor includes a first filter 410A, which includes an inductor 411A electrically connected between the rotor side converter <NUM> and the rotor side bus <NUM>. Additionally, the first filter 410A includes a capacitor 412A electrically connected between the phase A of the rotor and the ground <NUM>. Similarly, phases B and C of the three-phase power from the rotor include a second filter 410B for phase B, and a third filter 410C for phase C, respectively. The power generation system <NUM> can be electrically grounded to shunt a leakage current associated with an isolation transformer of a power converter <NUM>, such as an isolation transformer <NUM> of a line side converter <NUM>, to the ground <NUM> through the capacitor <NUM> of each filter. Further, in applications in which only one phase includes a filter <NUM> electrically connected to the at least one phase, the power generation system <NUM> can be electrically grounded to shunt a leakage current associated with an isolation transformer of a power converter <NUM> to a ground <NUM> through the capacitor <NUM> of the at least one filter <NUM>.

Referring now to <FIG>, a power generation system <NUM> according to example aspects of the present disclosure is depicted. Elements that are the same or similar to those as depicted in previous FIGS. are referred to with the same reference numerals. As shown, a DFIG <NUM> includes a rotor connected to a rotor side bus <NUM> and a stator connected to a stator side bus <NUM>. Each phase of three-phase power output from the rotor is filtered by a filter <NUM>, such as a first filter <NUM> A for phase A, a second filter <NUM> B for phase B, and a third filter <NUM> C for phase C. The three-phase power from the rotor side bus <NUM> is provided to the rotor side converter <NUM>. Further, at least one phase of the three-phase rotor side bus <NUM> can be electrically connected to a ground <NUM>. For example, as shown in <FIG>, phase C is electrically connected to a ground <NUM>. Additionally, any number of phases can be electrically connected to the ground <NUM>.

Referring now to <FIG>, an example power generation system <NUM> according to example aspects of the present disclosure is depicted. Elements that are the same or similar to those depicted in previous FIGS. are referred to with the same reference numerals. As shown, a DFIG <NUM> includes a rotor connected to the rotor side bus <NUM> and a stator connected to a stator side bus <NUM>. The rotor side bus <NUM> can be, for example, a LV bus. A stator side bus <NUM> can be, for example, a MV bus. The DFIG <NUM> can be configured to generate a LV AC power and provide the LV AC power to the rotor side bus <NUM>. The DFIG <NUM> can be configured to generate a MV AC power and provide the MV AC power to the stator side bus <NUM>. The LV AC power can be provided to the power converter <NUM>, such as a rotor side converter <NUM> by the rotor side bus <NUM>.

A filter <NUM> can be included to filter each phase of the three-phase power from the rotor before being provided to a rotor side converter <NUM>. Additionally, a filter <NUM> can be included to filter each phase of the three-phase power between the line side converter <NUM> and the electrical grid <NUM>. A multi-phase (e.g., three phase) crowbar protection circuit <NUM> can also be included to protect against overvoltage. A rotor side converter <NUM> can be coupled to a rotor side bus <NUM> and configured to convert three-phase LV AC power into a LV DC power. Rotor side converter <NUM> can be, for example, a two level AC to DC power converter configured to convert the three-phase alternating current power to a LV DC power, and can provide the DC power to a DC link <NUM>. A DC capacitor <NUM> can be electrically connected to the DC link <NUM>. A line side converter <NUM> can be coupled to the DC link <NUM>, and configured to receive DC power from the DC link and convert it to a multiphase MV AC power suitable for a grid <NUM>. The line side converter <NUM> can be, for example, a DC to DC to AC converter comprising one or more SiC MOSFETs and an isolation transformer, as depicted in <FIG>.

In an embodiment, at least one pole of the DC link <NUM> can be connected to a ground <NUM>. For example, the DC link can be connected directly to a ground <NUM>, and the power generation system <NUM> can be electrically grounded to shunt a leakage current to the ground <NUM> through the at least one pole of the DC link <NUM> connected to the ground <NUM>.

Additionally, in an embodiment, a tap to provide power for auxiliary loads can be coupled to the DC link <NUM>. For example, an auxiliary inverter can be coupled to the DC link <NUM> to provide power to one or more auxiliary loads. In an embodiment, a filter can be included at the output of the auxiliary inverter. For example, a filter can include an inductor and a filter capacitor. In such a configuration, the neutral of the filter capacitor can be electrically connected to a ground, which can allow for a leakage current to be shunted to ground. Additionally, in a power generation system <NUM> which includes a DC power source, such as one or more solar panels or batteries/energy storage devices, the DC power source can be connected to the DC link <NUM>. In such a system, one of the poles of the DC power source, such as the negative or positive pole of a solar panel or battery, can be electrically connected to a ground. A leakage current can be shunted to ground through the pole of the DC power source connected to the ground.

In an embodiment not depicted in <FIG>, a high impedance resistor, such as a high impedance resistor <NUM>, can be electrically connected between the at least one pole of the DC link <NUM> and the ground <NUM>. In such a power generation system <NUM>, the power generation system can be electrically grounded to shunt a leakage current from a power converter to the ground <NUM> through the high impedance resistor connected to the at least one pole of the DC link <NUM>.

Referring now to <FIG>, a method (<NUM>) for providing power according to example aspects of the present disclosure is depicted. At (<NUM>), the method (<NUM>) can include generating three-phase alternating current power at a first voltage with a power generator. The power generator can include a multiphase rotor and stator. The phases of the rotor can be configured in a delta or Wye configuration. Each phase of the rotor can include a high impedance resistor electrically connected to the phase and a ground. For example, each phase of a rotor can include a high impedance resistor <NUM> connected between the phase and a ground <NUM>.

At (<NUM>), the method (<NUM>) can include providing the three-phase alternating current power from the power generated to a power converter. The power converter can include one or more SiC MOSFETs and an isolation transformer. The power converter can be configured to convert the three-phase alternating current power from the power generator at the first voltage to three-phase alternating current power at a second voltage. For example, a power converter <NUM> can include a rotor side converter <NUM> and a line side converter <NUM>. The rotor side converter <NUM> can be, for example, a two-level AC to DC power converter. The AC to DC power converter can be coupled to a DC link <NUM>, which can further be coupled to a line side converter <NUM>, which can be a DC to DC to AC power converter. The DC to DC to AC power converter can include one or more SiC MOSFETs and an isolation transformer. The power converter can be configured to convert the three-phase alternating current power at the first voltage to three-phase alternating current power at a second voltage. For example, the power converter can be a power converter <NUM> configured to convert a LV AC power to a MV AC power suitable for a grid <NUM>.

At (<NUM>), the method (<NUM>) can include a sensing, by a control device, a voltage or current across the high impedance resistor of each phase of the rotor. For example, a control device <NUM> can be in communication with one or more voltage or current sensors, which can be configured to sense a voltage or current across a high impedance resistor <NUM> connected to each phase of a rotor. The control device <NUM> can be configured to receive the sensed measurements from the one or more voltage or current sensors.

At (<NUM>), the method (<NUM>) can include determining, by the control device, whether a voltage or current imbalance exists based at least upon the sensed voltage or current across the high impedance resistor of each phase of the rotor. For example, a control device <NUM> can be configured to compare a voltage across at least two high impedance resistors <NUM>, such as by comparing the voltage across each high impedance resistor <NUM> of each phase of the rotor. If the sensed voltage across the high impedance resistors <NUM> is imbalanced, such as, for example, when the voltage levels of two or more high impedance resistors <NUM> vary from each other by an amount that exceeds a threshold, the control device <NUM> can determine that a voltage imbalance exists. Similarly, the control device can be configured to determine whether a current in balance exists based on one or more current measurements by comparing the current through at least two high impedance resistors <NUM>, such as by comparing the current through each high impedance resistor <NUM> of each phase of the rotor. If the sensed current is imbalanced, such as, for example, when the current levels of two or more high impedance resistors <NUM> vary from each other by an amount that exceeds a threshold, the control device <NUM> can determine that a current imbalance exists.

When a voltage or current imbalance exists, at (<NUM>), the method (<NUM>) can include shutting the power converter down, open one or more switches (not shown), or perform other control actions to electrically isolate the faulted power generation unit by the control device to protect the power converter. For example, a control device <NUM> can disconnect a power converter <NUM> from a power generation system <NUM> in order to protect the power converter <NUM> from a fault current.

In this way, the systems and methods according to example embodiments of the present disclosure can have a technical effect of shunting a leakage current associated with an isolation transformer and other components in a power converter to a ground, and further, in applications in which one or more high impedance resistors are used, can allow for voltage and current imbalance detection and system protection schemes to be implemented.

<FIG> depicts an example computing system <NUM> according to example embodiments of the present disclosure. The computing system <NUM> can be used, for example, as a control device <NUM> in a power generation system. The computing system <NUM> can include one or more computing device(s) <NUM>. The computing device(s) <NUM> can include one or more processor(s) 1210A and one or more memory device(s) 1210B. The one or more processor(s) 1210A can include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, logic device, and/or other suitable processing device. The one or more memory device(s) 1210B can include one or more computer-readable media, including, but not limited to, non-transitory computer-readable media, RAM, ROM, hard drives, flash drives, and/or other memory devices.

The one or more memory device(s) 1210B can store information accessible by the one or more processor(s) 1210A, including computer-readable instructions 1210C that can be executed by the one or more processor(s) 1210A. The instructions 1210C can be any set of instructions that when executed by the one or more processor(s) 1210A, cause the one or more processor(s) 1210A to perform operations. In some embodiments, the instructions 1210C can be executed by the one or more processor(s) 1210A to cause the one or more processor(s) 1210A to perform operations, such as any of the operations and functions for which the computing system <NUM> and/or the computing device(s) <NUM> are configured, the operations for operating a power generation system (e.g., method <NUM>), as described herein, and/or any other operations or functions of the one or more computing device(s) <NUM>. The instructions 1210C can be software written in any suitable programming language or can be implemented in hardware. Additionally, and/or alternatively, the instructions 1210C can be executed in logically and/or virtually separate threads on processor(s) 1210A. The memory device(s) 1210B can further store data 1210D that can be accessed by the processor(s) 1210A. For example, the data 1210D can include data indicative of power flows, current flows, actual voltages, ground fault currents, nominal voltages, and/or any other data and/or information described herein.

The computing device(s) <NUM> can also include a network interface 1210E used to communicate, for example, with the other components of system <NUM> (e.g., via a network). The network interface 1210E can include any suitable components for interfacing with one or more network(s), including for example, transmitters, receivers, ports, controllers, antennas, and/or other suitable components. For example, the network interface 1210E can be configured to communicate with one or more sensors in a power generation system.

Claim 1:
A power generation system (<NUM>), comprising:
a ground, a power generator comprising a multiphase rotor (120A, 120B, 120C) and stator, the phases of the rotor configured in a delta or Wye configuration, the power generator configured to generate multiphase alternating current power at a first voltage, characterized by
a power converter (<NUM>) comprising one or more silicon carbide MOSFETs and an isolation transformer (<NUM>), the power converter configured to convert the multiphase alternating current power from the power generator at the first voltage to multiphase alternating current power at a second voltage;
wherein the power generation system is electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to the ground; wherein at least one phase of the rotor is electrically connected to the ground, and
wherein the power generation system is electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to the ground through the at least one phase of the rotor electrically connected to the ground; and wherein the at least one phase of the rotor electrically connected to the ground further comprises a high impedance resistor electrically connected to the at least one phase and the ground; and
wherein the power generation system is electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to the ground through the high impedance resistor; and wherein each phase of the rotor comprises a high impedance resistor (420A, 420B, 420C) electrically connected to the phase of the rotor and the ground; and
wherein the power generation system is electrically grounded to shunt a leakage current associated with the isolation transformer of the power converter to the ground through the high impedance resistors; and further comprising:
a control device configured to determine whether a voltage or current imbalance exists across the high impedance resistors; and
wherein the control device is further configured to shut down the power converter when the control device determines a voltage or current imbalance exists across the high impedance resistors.