System and method for reducing grid current imbalance in an electrical power system

Abstract

A method for controlling an electrical power system connected to a power grid includes receiving, via a controller, one or more electrical feedback signals from one or more windings of a transformer of the electrical power system. The method also includes separating, via the controller, one or more negative sequence components from the one or more electrical feedback signals from the one or more windings of the transformer. Further, the method includes determining, via the controller, an error signal based on the one or more negative sequence components. Moreover, the method includes determining, via the controller, a negative sequence output based on the error signal. In addition, the method includes synthesizing, via the controller, gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

Description

SYSTEM AND METHOD FOR REDUCING GRID CURRENT IMBALANCE IN AN ELECTRICAL POWER SYSTEMFIELD

[0001] The present disclosure relates generally to wind turbines and. more particularly, to a system and method for controlling a negative sequence current in a generator of a wind turbine.BACKGROUND

[0002] Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modem wind turbine typically includes a tower, 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.

[0003] 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.

[0004] In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Further, wind turbine power systems may include a variety of generator types, including but not limited to a doubly fed induction generator (DFIG).

[0005] DFIG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the power converter can provide nearly instantaneous regulation of its output currents with respect to the grid frequency. Under steady operating conditions, the rotor-side converter controls the magnitude and phase of currents in the rotor circuit to achieve desired values of electromagnetic torque. Reactive power flow into the line- connected stator terminals of the generator can also be controlled.

[0006] A simplified, schematic diagram of an embodiment of a main circuit 10 of a DFIG is illustrated in FIG. 1. As shown, the main circuit 10 includes a generator 12 connected to a power converter 14 (as well as any required power electronics) and a transformer 16 that is connected to a power grid 18. More specifically, as shown, the connection to the power grid 18 is at the high side of the transformer 16, where the voltage and current are indicated as VG and IG, respectively. Further, the powder grid 18 is illustrated conceptually as a Thevenin (Th) equivalent. As used herein, the Thevenin equivalent generally refers to an equivalent voltage source in series connection with an impedance that is an approximation of a complex, non-linear grid that constantly changes based on the number of wind turbines, external grid status, etc. The Thevenin impedance ZTh varies with the number of wind turbines in operation and the status of the transmission system beyond the wind park.

[0007] When an imbalance occurs in the power grid 18. a negative-sequence component of voltage appears in the Thevenin voltage, which is represented in FIG. 1 as VTh. Such a negative sequence component of voltage can have a negative impact on the power grid 18 as well as the wind turbine power system.

[0008] Accordingly, the present disclosure is directed to systems and methods for controlling a negative sequence current in a generator of a wind turbine.BRIEF DESCRIPTION

[0009] Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may be learned through practice of the invention.

[0010] In an aspect, the present disclosure is directed to a method for controlling an electrical power system connected to a power grid. The electrical power system has a power conversion assembly electrically coupled to a generator. The method includes receiving, via a controller, one or more electrical feedback signals from one or more windings of a transformer of the electrical power system. Further, the method includes separating, via the controller, one or more negative sequence components from the one or more electrical feedback signals from the one or more windings of the transformer. Moreover, the method includes determining, via the controller, an error signal based on the one or more negative sequence components. In addition, the method includes determining, via the controller, a negative sequence output based on the error signal. Further, the method includes synthesizing, via the controller, gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

[0011] In another aspect, the present disclosure is directed to an electrical power system having a transformer connected to a power grid. The transformer includes a primary' winding and secondary' windings. The electrical power system also includes a generator electrically coupled to the transformer. The generator has a rotor and a stator and a power conversion assembly electrically coupled to the transformer and the generator. Further, the electrical power system includes a controller configured to control the electrical power system. The controller has a processor configured to perform a plurality of operations, including but not limited to receiving one or more current feedback signals from the secondary windings of the transformer; separating one or more negative sequence components from the one or more current feedback signals from the secondary' windings of the transformer; determining an error signal based on the one or more negative sequence components; determining a negative sequence output based on the error signal; and synthesizing gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

[0012] These and other features, aspects and advantages of the present invention 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 invention and, together with the description, serve to explain the principles of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

[0014] FIG. 1 illustrates a simplified schematic diagram of an embodiment of a main circuit according to the present disclosure;

[0015] FIG. 2 illustrates a perspective view of a portion of an embodiment of a wind turbine according to the present disclosure;

[0016] FIG. 3 illustrates a schematic view of an embodiment of an electrical power system suitable for use with the wind turbine shown in FIG. 2;

[0017] FIG. 4 illustrates a block diagram of an embodiment of a controller suitable for use with the wind turbine shown in FIG. 2;

[0018] FIG. 5 illustrates a flow- diagram of an embodiment of a method for controlling a negative sequence current in a generator of a wind turbine according to the present disclosure;

[0019] FIG. 6 illustrates a schematic diagram of an embodiment of negative sequence current flow7for a wind turbine power sy stem according to the present disclosure; and

[0020] FIGS. 7A-7J illustrate various graphs of an embodiment of vanous parameters and outputs of a negative sequence regulator of a controller according to the present disclosure.DETAILED DESCRIPTION OF THE INVENTION

[0021] Reference now will be made in detail to embodiments of the invention, one or more examples of w hich are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. Infact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of an embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0022] Generally, the present disclosure is directed to a systems and methods for controlling a negative sequence current in a generator of a wind turbine power system. In an embodiment, for example, systems and methods of the present disclosure use a line-side converter of the wind turbine power system to provide negative-sequence current, such that net grid negative-sequence current sensed in a high voltage winding of the transformer is near zero, during steady-state operation and grid conditions deemed acceptable. In this scenario, small negative-sequence current content is expected to be caused by asymmetries in electrical components and / or imbalanced grid voltage. The small negative-sequence current becomes more prominent with respect to symmetrical current, when wind turbine operates at low power levels. The ratio of asymmetrical to symmetrical current is sometimes required to be below a certain threshold over the entire turbine operating power range, in order for wind turbines to be allowed grid connection. Accordingly, systems and methods of the present disclosure are configured to reduce such compliance risk.

[0023] In particular embodiments, systems and methods of the present disclosure are applicable to any number of three-phase current feedback sets, wherein regulation can be referenced to zero or non-zero negative-sequence current targets and are intended to activate during turbine normal operation. Moreover, in an embodiment, only the line-side converter is used, thereby avoiding interferences with generator control the rotor-side converter, when the turbine is operating under normal conditions and with low grid power flow. More specifically, in an embodiment, systems and methods of the present disclosure are directed to a net grid negativesequence regulator that momentarily replaces a conventional line bridge resonator, which has always targeted elimination of bridge only negative-sequence current.Once the grid power increases above a hysteresis band defined by preset parameters, the line current resonator resumes operation and net grid negative-sequence regulationstops. Accordingly, systems and methods of the present disclosure are primarily intended to reduce a ratio of negative-sequence over positive-sequence grid current, also known as current imbalance or current asymmetry, which in some instances, is required to be below 1 .5% per certain grid codes.

[0024] In certain embodiments, systems and methods of the present disclosure includes separating all transformer low voltage side current feedbacks into positively and negatively rotating components, e.g., by transforming A-B-C static-frame signals into corresponding rotating frames (one associated with +w and another with -w). Each rotating frame includes two orthogonal axes, typically labeled as X and Y. Accordingly, systems and methods of the present disclosure may include summing the negative-sequence currents in the X- and Y-axes and the using the summations as error inputs to PI regulators acting on X and Y. The regulator outputs are then transformed from the negatively rotating frame back to the positively rotating frame, where all existing current regulating loops can be manipulated consistently, and eventually translated into unified voltage command signals for the line bridge. Thus, in an embodiment, the regulator effectively controls the line bridge to compensate for any existing negative-sequence current, and also maintains any negative-sequence current isolated from the transformer high-voltage side.

[0025] Referring now to the drawings, FIG. 2 illustrates a perspective view of a portion of an exemplary wind turbine 100 according to the present disclosure that is configured to implement the method as described herein. The wind turbine 100 includes a nacelle 102 that ty pically houses a generator (not shown). The nacelle 102 is mounted on a tower 104 having any suitable height that facilitates operation of wind turbine 100 as described herein. The wind turbine 100 also includes a rotor 106 that includes three blades 108 attached to a rotating hub 110. Alternatively, the wind turbine 100 may include any number of blades 108 that facilitates operation of the wind turbine 100 as described herein.

[0026] Referring to FIG. 3, a schematic view of an embodiment of an electrical power system 200 that may be used with the wind turbine 100 is illustrated. During operation, wind impacts the blades 108 and the blades 108 transform wind energy into a mechanical rotational torque that rotatably drives a low -speed shaft 112 via the hub 110. The low -speed shaft 112 is configured to drive a gearbox 114 that subsequentlysteps up the low rotational speed of the low-speed shaft 112 to drive a high-speed shaft 116 at an increased rotational speed. The high-speed shaft 116 is generally rotatably coupled to a generator 118 so as to rotatably drive a generator rotor 122. In an embodiment, the generator 118 may be a wound rotor, three-phase, doubly -fed induction (asynchronous) generator (DFAG) that includes a generator stator 120 magnetically coupled to a generator rotor 122. As such, a rotating magnetic field may be induced by the generator rotor 122 and a voltage may be induced within a generator stator 120 that is magnetically coupled to the generator rotor 122. In an embodiment, the generator 118 is configured to convert the rotational mechanical energy to a sinusoidal, three-phase alternating current (AC) electrical energy signal in the generator stator 120. The associated electrical power can be transmitted to a main transformer 234 via a stator bus 208, a stator synchronizing switch 206, a system bus 216, a main transformer circuit breaker 214, and a generator-side bus 236. The main transformer 234 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 238, a breaker-side bus 240. and a grid bus 242.

[0027] In addition, the electrical power system 200 may include a wind turbine controller 202 configured to control any of the components of the wind turbine 100 and / or implement the method steps as described herein. For example, as shown particularly in FIG. 4, the controller 202 may include one or more processor(s) 204 and associated memory device(s) 207 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 202 may also include a communications module 209 to facilitate communications between the controller 202 and the various components of the wind turbine 100. e.g. any of the components of FIG. 3. Further, the communications module 209 may include a sensor interface 211 (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 204. It should be appreciated that the sensors (e g. sensors 252, 254, 256, 258) may be communicatively coupled to the communications module 209 using any suitable means. For example, as shown in FIG. 4, the sensors 252, 254, 256, 258 may be coupled to the sensor interface 211 viaa wired connection. However, in other embodiments, the sensors 252, 254, 256, 258 may be coupled to the sensor interface 211 via a wireless connection, such as by using any suitable wireless communications protocol known in the art. As such, the processor 204 may be configured to receive one or more signals from the sensors.

[0028] 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 204 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) 207 may generally comprise memory element(s) including, but not limited to, computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory). a floppy disk, a compact disc-read only memory (CD-ROM), a magnetooptical disk (MOD), a digital versatile disc (DVD) and / or other suitable memory elements. Such memory device(s) 207 may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s) 204, configure the controller 202 to perform the various functions as described herein.

[0029] Referring back to FIG. 3, the generator stator 120 may be electrically coupled to a stator synchronizing switch 206 via a stator bus 208. In an embodiment, the generator rotor 122 may be electrically coupled to a bi-directional power conversion assembly 210 or power converter via a rotor bus 212. Alternatively, the generator rotor 122 may be electrically coupled to the rotor bus 212 via any other device that facilitates operation of electrical power system 200 as described herein. In a further embodiment, the stator synchronizing switch 206 may be electrically coupled to a main transformer circuit breaker 214 via a system bus 216.

[0030] The power conversion assembly 210 may include a rotor filter 218 that is electrically coupled to the generator rotor 122 via the rotor bus 212. In addition, the rotor filter 218 may include a rotor-side reactor. A rotor filter bus 219 electrically couples the rotor filter 218 to a rotor-side converter 220. Further, the rotor-side converter 220 may be electrically coupled to a line-side converter 222 via a single direct current (DC) link 244. Alternatively, the rotor-side converter 220 and the line-side converter 222 may be electrically coupled via individual and separate DC links. In addition, as shown, the DC link 244 may include a positive rail 246, a negative rail 248, and at least one capacitor 250 coupled therebetween.

[0031] In addition, a line-side converter bus 223 may electrically couple the lineside converter 222 to a line filter 224. Also, a line bus 225 may electrically couple the line filter 224 to a line contactor 226. In addition, the line filter 224 may include a line-side reactor. Moreover, the line contactor 226 may be electrically coupled to a conversion circuit breaker 228 via a conversion circuit breaker bus 230. In addition, the conversion circuit breaker 228 may be electrically coupled to the main transformer circuit breaker 214 via system bus 216 and a connection bus 232. The main transformer circuit breaker 214 may be electrically coupled to an electric power main transformer 234 via a generator-side bus 236. The main transformer 234 may be electrically coupled to a grid circuit breaker 238 via a breaker-side bus 240. The grid circuit breaker 238 may be connected to the electric power transmission and distribution grid via a grid bus 242.

[0032] In operation, alternating current (AC) power generated at the generator stator 120 by rotation of the rotor 106 is provided via a dual path to the grid bus 242. The dual paths are defined by the stator bus 208 and the rotor bus 212. On the rotor bus side 212, sinusoidal multi-phase (e.g. three-phase) AC power is provided to the power conversion assembly 210. The rotor-side converter 220 converts the AC power provided from the rotor bus 212 into DC power and provides the DC power to the DC link 244. Switching elements (e.g. IGBTs) used in bridge circuits of the rotor side power converter 220 can be modulated to convert the AC power provided from the rotor bus 212 into DC power suitable for the DC link 244.

[0033] The line side converter 222 converts the DC power on the DC link 244 into AC output power suitable for the electrical grid bus 242. In particular, switching elements (e.g. IGBTs) used in bridge circuits of the line side power converter 222 can be modulated to convert the DC power on the DC link 244 into AC power on the line side bus 225. The AC power from the power conversion assembly 210 can be combined with the power from the stator 120 to provide multi-phase power (e.g. three-phase power) having a frequency maintained substantially at the frequency of the electrical grid bus 242 (e g. 50 Hz / 60 Hz). It should be understood that the rotor-side converter 220 and the line-side converter 222 may have any configuration using any switching devices that facilitate operation of electrical power system 200 as described herein.

[0034] Further, the power conversion assembly 210 may be coupled in electronic data communication with the turbine controller 202 and / or a separate or integral converter controller 262 to control the operation of the rotor-side converter 220 and the line-side converter 222. For example, during operation, the controller 202 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 252. Thus, the controller 202 may be configured to monitor and control at least some of the operational variables associated with the wind turbine 100 via the sensors 252. In the illustrated embodiment, each of the sensors 252 may be electrically coupled to each one of the three phases of the power grid bus 242. Alternatively, the sensors 252 may be electrically coupled to any portion of electrical power system 200 that facilitates operation of electrical power system 200 as described herein. In addition to the sensors described above, the sensors may also include a second set of voltage and electric current sensors 254, a third set of voltage and electric current sensors 256, a fourth set of voltage and electric current sensors 258 (all show n in FIG. 2), and / or any other suitable sensors.

[0035] It should also be understood that any number or type of voltage and / or electric current sensors may be employed within the wind turbine 100 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.

[0036] Thus, the converter controller 262 is configured to receive one or more voltage and / or electric current feedback signals from the sensors 252, 254, 256, 258. More specifically, in certain embodiments, the current or voltage feedback signals may include at least one of line feedback signals, line-side converter feedback signals, rotor-side converter feedback signals, or stator feedback signals. For example, as shown in the illustrated embodiment, the converter controller 262 receives voltage and electric current measurement signals from the second set of voltage and electriccurrent sensors 254 coupled in electronic data communication with stator bus 208. The converter controller 262 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 256, 258. In addition, the converter controller 262 may be configured with any of the features described herein in regards to the main controller 202. Further, the converter controller 262 may be separate from or integral with the main controller 202. As such, the converter controller 262 is configured to implement the various method steps as described herein and may be configured similar to the turbine controller 202.

[0037] Referring now to FIGS. 5-7J, various illustrations are provided to further describe the systems and methods of the present disclosure. For example, FIG. 5 illustrates a flow diagram of an embodiment of a method 300 for controlling an electrical power system connected to a power grid according to the present disclosure; FIG. 6 illustrates a schematic diagram of an embodiment of negative sequence current flow for a wind turbine power system 400 according to the present disclosure; and FIGS. 7A-7J illustrate various graphs of an embodiment of various parameters and outputs of a negative sequence regulator of a controller according to the present disclosure.

[0038] Referring particularly to FIG. 5, a flow diagram of an embodiment of a method 300 for controlling an electrical power system connected to a power grid is illustrated in accordance with aspects of the present disclosure. As used herein the electrical power system may include the wind turbine power system 100, a solar power system, an energy storage power system, or a hybrid power system containing combinations thereof. In general, the method 300 will be described herein as being implemented using a controller of a wind turbine system, such as the turbine controller 202 or the converter controller 262 of the wind turbine power system 100 described above with reference to FIGS. 1-4. However, it should be appreciated that the disclosed method 300 may be implemented using any other suitable power generation system. In addition, although FIG. 5 depicts steps performed in a particular order for purposes of illustration and discussion, the methods described herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of themethods can be omitted, rearranged, performed simultaneously, combined and / or adapted in various ways. Additional steps not disclosed herein may be performed without departing from the scope or spirit of the present disclosure.

[0039] As shown at (302), the method 300 includes determining, via the controller, whether a grid fault is detected in the power grid. When the grid fault is detected, as shown at (304), the method 300 includes providing negative sequence current injection during a duration of the fault. In contrast, as shown at (306), when the grid fault is not detected or the grid fault has cleared, the method 300 includes determining whether a net power output of the electrical power system is below a predefined threshold.

[0040] As shown at (308), when the net power output is above the predefined threshold, the method 300 further includes providing negative sequence current regulation based, e.g., solely on line bridge current feedbacks and rotor bridge current feedbacks.

[0041] Still referring to FIG. 5, when the net power output is below the predefined threshold, the method 300 includes implementing a negative sequence current regulator sequence 310 that provides net grid current to the power grid. More specifically, as shown at (312), the negative sequence current regulator sequence 310 includes disabling, via the controller, negative sequence regulation of the line-side converter 222 of the power conversion assembly 210 based on one or more bridge current feedback signals. As shown at (314), the negative sequence current regulator sequence 310 includes receiving one or more electrical feedback signals, such as three-phase current feedback signals, from one or more windings of the transformer 234 of the electrical power system 200.

[0042] Further, as shown at (316). the negative sequence current regulator sequence 310 includes separating, via the controller, one or more negative sequence components from the three-phase current feedback signals from a plurality of secondary' windings of the transformer 234 into a properly oriented rotating frame.

[0043] Moreover, as shown at (318), the negative sequence current regulator sequence 310 includes determining, via the controller, an error signal based on the one or more negative sequence components. For example, in an embodiment, determining the error signal based on the negative sequence component(s) mayinclude determining the error signal based on a difference between a summation of the plurality of negative sequence components and predefined thresholds.

[0044] As shown at (320). the negative sequence current regulator sequence 310 includes feeding the error signal to a proportional integral (PI) regulator and determining, via the PI regulator, a negative sequence output based on the error signal. As shown at (322), the negative sequence current regulator sequence 310 includes transforming the rotating frame to ensure the negative sequence output is compatible with remaining current regulation signals of the power conversion assembly 210.

[0045] As shown at (324), the negative sequence current regulator sequence 310 includes synthesizing gating commands for the power conversion assembly 210 based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance. For example, in an embodiment, synthesizing the gating commands for the pow er conversion assembly 210 may include determining the gating commands for the power conversion assembly 210 based on the negative sequence output and the remaining current regulation signals.

[0046] Referring particularly to FIG. 6, a schematic diagram of an embodiment of negative sequence current flow- 402 for a wind turbine power system 400, such as the electrical power system 200. according to the present disclosure is illustrated. In particular, as shown, the wind turbine pow er system 400 includes a negative sequence regulator 404 applied to the pow er conversion assembly 210. During normal operation, there are small negative-sequence currents created by asymmetries in the wind turbine power system 400 and / or imbalanced grid voltage. As such, the lineside converter 222 (e.g., the line bridge) supplies and / or consumes negative-sequence current as needed to maintain balance on the high voltage side of the transformer 234. Thus, in an embodiment, there is a net zero negative-sequence current on the high voltage side of the transformer 234.

[0047] Referring now to FIGS. 7A-7J. various time-series graphs of various parameters and outputs of the negative sequence regulator (FIG. 6) according to the present disclosure are illustrated. In particular, FIG. 7A illustrates a time-series graph of an output of the negative sequence regulator, which is active when the signal isfalse, according to the present disclosure. FIG. 7B illustrates a time-series graph of a ratio of asymmetrical to symmetrical current as calculated by the power conversion assembly 210 according to the present disclosure. FIG. 7C illustrates atime-series graph of a grid positive-sequence voltage magnitude according to the present disclosure. FIG. 7D illustrates a time-series graph of a grid negative-sequence voltage magnitude according to the present disclosure. FIG. 7E illustrates a time-series graph of a grid net current magnitude according to the present disclosure. FIG. 7F illustrates a time-series graph of an output of the negative sequence regulator in the x-axis according to the present disclosure. FIG. 7G illustrates a time-series graph of an output of the negative sequence regulator in the x-axis according to the present disclosure. FIG. 7H illustrates a time-series graph of negative sequence reactive current in the 690V winding according to the present disclosure. FIG. 71 illustrates a time-series graph of negative sequence reactive current in the 6kV winding according to the present disclosure. FIG. 7J illustrates a time-series graph of only the line bridge running when the signal is true according to the present disclosure.

[0048] Further aspects of the invention are provided by the subject matter of the following clauses:

[0049] A method for controlling an electrical power system connected to a power grid, the electrical power system having a power conversion assembly electrically coupled to a generator, the method comprising: receiving, via a controller, one or more electrical feedback signals from one or more windings of a transformer of the electrical power system; separating, via the controller, one or more negative sequence components from the one or more electrical feedback signals from the one or more windings of the transformer; determining, via the controller, an error signal based on the one or more negative sequence components; determining, via the controller, a negative sequence output based on the error signal; and synthesizing, via the controller, gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

[0050] The method of any preceding clause, further comprising determining, via the controller, whether a grid fault is detected in the power grid before receiving theone or more electrical feedback signals from the one or more windings of the transformer of the electrical power system.

[0051] The method of any preceding clause, wherein, when the grid fault is detected, providing negative sequence current injection during a duration of the fault.

[0052] The method of any preceding clause, wherein, when the grid fault is not detected or the grid fault has cleared, the method further comprises determining whether a net power output of the electrical power system is below a predefined threshold.

[0053] The method of any preceding clause, wherein, when the net power output is above the predefined threshold, the method further comprises providing negative sequence regulation based on line bridge current feedbacks and rotor bridge current feedbacks.

[0054] The method of any preceding clause, wherein, when the net power output is below the predefined threshold, the method further comprises disabling, via the controller, negative sequence regulation of a line-side converter of the power conversion assembly based on one or more bridge current feedback signals before receiving the one or more electrical feedback signals from the one or more windings of the transformer of the electrical power system.

[0055] The method of any preceding clause, wherein the one or more electrical feedback signals comprises three-phase electrical feedback signals, and wherein separating the one or more negative sequence components from the one or more electrical feedback signals from the one or more windings of the transformer further comprises: separating a plurality of negative sequence components from the three- phase electrical feedback signals from a plurality of secondary windings of the transformer into a rotating frame.

[0056] The method of any preceding clause, wherein determining the error signal based on the one or more negative sequence components further comprises determining the error signal based on a difference between a summation of the plurality of negative sequence components and predefined thresholds.

[0057] The method of any preceding clause, further comprising transforming the rotating frame after determining the negative sequence output to ensure the negativesequence output is compatible with remaining current regulation signals of the power conversion assembly.

[0058] The method of any preceding clause, wherein synthesizing the gating commands for the power conversion assembly based, at least in part, on the negative sequence output further comprises: after the transforming, determining the gating commands for the power conversion assembly based on the negative sequence output and the remaining current regulation signals.

[0059] The method of any preceding clause, wherein the controller comprises a proportional integral controller, the method further comprising determining the negative sequence output based on the error signal via the proportional integral controller.

[0060] The method of any preceding clause, wherein the one or more electrical feedback signals comprises current feedback signals.

[0061] The method of any preceding clause, wherein the generator comprises a doubly fed induction generator (DFIG).

[0062] The method of any preceding clause, wherein the electrical power system is part of a wind turbine power system.

[0063] An electrical power system, comprising: a transformer connected to a power grid, the transformer comprising a primary winding and secondary windings; a generator electrically coupled to the transformer, the generator comprising a rotor and a stator; a power conversion assembly electrically coupled to the transformer and the generator; and a controller configured to control the electrical power system, the controller comprising a processor configured to perform a plurality of operations, the plurality of operations comprising: receiving one or more current feedback signals from the secondary windings of the transformer; separating one or more negative sequence components from the one or more current feedback signals from the secondary' windings of the transformer; determining an error signal based on the one or more negative sequence components; determining a negative sequence output based on the error signal; and synthesizing gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

[0064] The electrical power system of any preceding clause, wherein the plurality of operations further comprise determining whether a grid fault is detected in the power grid before receiving the one or more current feedback signals from the one or more windings of the transformer of the electrical power system, wherein, when the grid fault is detected, the plurality of operations further comprise providing negative sequence current injection during a duration of the fault, wherein, when the grid fault is not detected or the grid fault has cleared, the plurality of operations further comprise determining whether a net power output of the electrical power system is below a predefined threshold.

[0065] The electrical power system of any preceding clause, wherein, when the net power output is above the predefined threshold, the plurality of operations further comprise providing negative sequence regulation based on line bridge current feedbacks and rotor bridge current feedbacks, and wherein, when the net power output is below the predefined threshold, the plurality of operations further comprise disabling, via the controller, negative sequence regulation of a line-side converter of the power conversion assembly based on one or more bridge current feedback signals before receiving the one or more current feedback signals from the one or more windings of the transformer of the electrical power system.

[0066] The electrical power system of any preceding clause, wherein the one or more current feedback signals comprises three-phase current feedback signals, wherein separating the one or more negative sequence components from the one or more current feedback signals from the one or more windings of the transformer further comprises: separating a plurality of negative sequence components from the three-phase current feedback signals from a plurality of secondary’ windings of the transformer into a rotating frame.

[0067] The electrical power system of any preceding clause, wherein determining the error signal based on the one or more negative sequence components further comprises determining the error signal based on a difference between a summation of the plurality of negative sequence components and predefined thresholds.

[0068] The electrical power system of any preceding clause, wherein the plurality of operations further comprise transforming the rotating frame after determining the negative sequence output to ensure the negative sequence output is compatible withremaining current regulation signals of the power conversion assembly, and wherein synthesizing the gating commands for the power conversion assembly based, at least in part, on the negative sequence output further comprises: after the transforming, determining the gating commands for the power conversion assembly based on the negative sequence output and the remaining current regulation signals.

[0069] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

WHAT IS CLAIMED IS:

1. A method for controlling an electrical power system connected to a power grid, the electrical power system having a power conversion assembly electrically coupled to a generator, the method comprising: receiving, via a controller, one or more electrical feedback signals from one or more windings of a transformer of the electrical power system; separating, via the controller, one or more negative sequence components from the one or more electrical feedback signals from the one or more windings of the transformer; determining, via the controller, an error signal based on the one or more negative sequence components; determining, via the controller, a negative sequence output based on the error signal; and synthesizing, via the controller, gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

2. The method of claim 1, further comprising determining, via the controller, whether a grid fault is detected in the power grid before receiving the one or more electrical feedback signals from the one or more windings of the transformer of the electrical power system.

3. The method of claim 2, wherein, when the grid fault is detected, providing negative sequence current injection during a duration of the fault.

4. The method of claim 2. wherein, when the grid fault is not detected or the grid fault has cleared, the method further comprises determining whether a net power output of the electrical power system is below a predefined threshold.

5. The method of claim 4, wherein, when the net power output is above the predefined threshold, the method further comprises providing negative sequence regulation based on line bridge current feedbacks and rotor bridge current feedbacks.

6. The method of claim 4, wherein, when the net power output is below the predefined threshold, the method further comprises disabling, via the controller, negative sequence regulation of a line-side converter of the power conversionassembly based on one or more bridge current feedback signals before receiving the one or more electrical feedback signals from the one or more windings of the transformer of the electrical power system.

7. The method of claim 1, wherein the one or more electrical feedback signals comprises three-phase electrical feedback signals, and wherein separating the one or more negative sequence components from the one or more electrical feedback signals from the one or more windings of the transformer further comprises: separating a plurality of negative sequence components from the three-phase electrical feedback signals from a plurality of secondary' windings of the transformer into a rotating frame.

8. The method of claim 7. wherein determining the error signal based on the one or more negative sequence components further comprises determining the error signal based on a difference between a summation of the plurality of negative sequence components and predefined thresholds.

9. The method of claim 7, further comprising transforming the rotating frame after determining the negative sequence output to ensure the negative sequence output is compatible with remaining current regulation signals of the power conversion assembly.

10. The method of claim 9, wherein synthesizing the gating commands for the power conversion assembly based, at least in part, on the negative sequence output further comprises: after the transforming, determining the gating commands for the powder conversion assembly based on the negative sequence output and the remaining current regulation signals.

11. The method of claim 1. wherein the controller comprises a proportional integral controller, the method further comprising determining the negative sequence output based on the error signal via the proportional integral controller.

12. The method of claim 1. wherein the one or more electrical feedback signals comprises current feedback signals.

13. The method of claim 1, wh erein the generator comprises a doubly fed induction generator (DFIG).

14. The method of claim 1, wherein the electrical power system is part of a wind turbine power system.

15. An electrical power system, comprising: a transformer connected to a power grid, the transformer comprising a primary winding and secondary' windings; a generator electrically coupled to the transformer, the generator comprising a rotor and a stator; a power conversion assembly electrically coupled to the transformer and the generator; and a controller configured to control the electrical power system, the controller comprising a processor configured to perform a plurality of operations, the plurality of operations comprising: receiving one or more current feedback signals from the secondary windings of the transformer; separating one or more negative sequence components from the one or more cunent feedback signals from the secondary windings of the transformer; determining an error signal based on the one or more negative sequence components; determining a negative sequence output based on the error signal; and synthesizing gating commands for the power conversion assembly based, at least in part, on the negative sequence output to provide a net grid negative sequence current equal to or near zero, thereby reducing grid current imbalance.

16. The electrical power system of claim 15. wherein the plurality of operations further comprise determining whether a grid fault is detected in the power grid before receiving the one or more current feedback signals from the one or more windings of the transformer of the electrical power system, wherein, when the grid fault is detected, the plurality of operations further comprise providing negative sequence current injection during a duration of the fault, and wherein, when the grid fault is not detected or the grid fault has cleared, theplurality of operations further comprise determining whether a net power output of the electrical power system is below a predefined threshold.

17. The electrical power system of claim 16. wherein, when the net power output is above the predefined threshold, the plurality of operations further comprise providing negative sequence regulation based on line bridge current feedbacks and rotor bridge current feedbacks, and wherein, when the net power output is below the predefined threshold, the plurality of operations further comprise disabling, via the controller, negative sequence regulation of a line-side converter of the power conversion assembly based on one or more bridge current feedback signals before receiving the one or more current feedback signals from the one or more windings of the transformer of the electrical power system.

18. The electrical power system of claim 15, wherein the one or more current feedback signals comprises three-phase current feedback signals, wherein separating the one or more negative sequence components from the one or more current feedback signals from the one or more windings of the transformer further comprises: separating a plurality7of negative sequence components from the three-phase current feedback signals from a plurality of secondary windings of the transformer into a rotating frame.

19. The electrical power system of claim 18, wherein determining the error signal based on the one or more negative sequence components further comprises determining the error signal based on a difference between a summation of the plurality of negative sequence components and predefined thresholds.

20. The electrical power system of claim 18. wherein the plurality of operations further comprise transforming the rotating frame after determining the negative sequence output to ensure the negative sequence output is compatible with remaining current regulation signals of the power conversion assembly, and wherein synthesizing the gating commands for the power conversion assembly based, at least in part, on the negative sequence output further comprises: after the transforming, determining the gating commands for the power conversion assembly based on the negative sequence output and the remaining currentregulation signals.

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