SYSTEM AND METHOD FOR OPERATING INVERTER-BASED RESOURCES USING INTERLEAVED PULSE PATTERNS - Patent application

Abstract

Interleaved pulse patterns are used to operate inverter-based resources. A method for operating a plurality of inverter-based resources (IBRs) connected to an electric power grid at a point of common coupling (PCC) includes providing pulse patterns for the at least two IBRs to respective local controllers of the at least two IBRs. The method also includes receiving one or more measured electrical signals from the electric power grid via the respective local controllers, and establishing a timing reference for interleaving the pulse patterns of the at least two IBRs based on the one or more measured electrical signals from the electric power grid via the respective local controllers. The method further includes operating the at least two IBRs in parallel on the electric power grid via the respective local controllers using the pulse patterns and the timing reference, such that the pulse patterns are interleaved with one another to reduce voltage distortion at the PCC.

Description

[Technical Field]

[0001] The present disclosure relates generally to inverter-based resources, and more particularly to interleaved pulse patterns for inverter-based resources such as wind turbines. [Background technology]

[0002] Modern wind turbines are commonly used to supply electricity to the electrical grid. This type of wind turbine typically includes a tower and a rotor disposed on the tower. The rotor typically includes a hub and multiple rotor blades that rotate under the influence of wind. This rotation generates torque, which is typically transmitted to a generator via a rotor shaft, either directly or via a gearbox. The generator generates electrical power, which can be supplied to the electrical grid. The hub may be rotatably coupled to the front of a nacelle. The hub is further coupled to a rotor shaft, which is rotatably mounted to the nacelle using one or more rotor shaft bearings disposed in a frame within the nacelle. The nacelle is a housing disposed at the top of the tower that houses and protects the gearbox (if present) and generator, as well as additional components such as a power converter and / or auxiliary systems, depending on the wind turbine.

[0003] Wind turbines are often grouped together as "wind farms," which generally refers to a common geographic location with multiple wind turbines. These wind turbines may be connected to a local (internal) power grid for the wind farm. The internal power grid for the wind farm may contain multiple strings, each of which may have many wind turbines connected to it. The wind farm's power grid may be connected to the main power grid at a point of common coupling (PCC).

[0004] A trend in the field of wind turbines is to install them in offshore wind farms, which may be connected to the mainland electrical grid via high-voltage power lines, for example, High Voltage Alternating Current (HVAC) or High Voltage Direct Current (HVDC).

[0005] Wind turbines can feed electricity into the power grid within a wind farm at voltages of, for example, 33 kilovolts (kV) or 66 kV. At the substation, the voltage is stepped up to several hundred kV by high-voltage transformers. The high-voltage power is then fed into high-voltage transmission lines that connect to the onshore power grid. Substations also contain circuit breakers, surge arresters, and capacitor banks.

[0006] Like most renewable energy sources, wind turbines typically include power-electronic conversion to regulate the power they inject into the power grid. Such power conversion systems include inverters, which generate energy at frequencies other than the nominal grid frequency (commonly called harmonics). This is a fundamental characteristic of all inverter-based resources (IBRs). As more IBRs are connected to the power grid, the combined effect of the individual harmonics can become significant. The impact is greatly amplified by unavoidable resonances within the power grid.

[0007] A well-known and highly effective way to mitigate the effects of IBR harmonics is to include damping filters in the power grid, but such filters can be costly or impractical in some cases, for example, for offshore wind farms.

[0008] Therefore, the conventional harmonic mitigation method is to improve the control algorithm of the IBR converter to minimize its impact on grid voltage distortion. However, due to the large amplification associated with grid resonance, minimizing the harmonics from individual IBRs may not be sufficient for effective mitigation.

[0009] To overcome this inherent limitation, methods have been proposed to interleave converter control of groups of IBRs within a facility, as taught, for example, in EP 2209200 B1 (entitled "Electrical System and Control Method"), filed August 12, 2009. However, such prior art methods rely on communication between the controllers of the various IBRs to enable the desired interleaving. This communication requirement can be problematic because a precise timing reference must be established to which all IBRs in the group refer to ensure effective interleaving.

[0010] Accordingly, the present disclosure is generally directed to a system and method for operating a pair of IBRs with interleaved pulse patterns without requiring communication between their respective individual controllers. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2018 / 184747 Summary of the Invention

[0012] Aspects and advantages of the present invention will be set forth in part in the description that follows, or will be obvious from the description, or may be learned through practice of the present invention.

[0013] In one aspect, the disclosure is directed to a method for operating a plurality of inverter-based resources connected to an electric power grid at a common point of coupling. The method includes providing pulse patterns of at least two of the plurality of inverter-based resources to respective local controllers of at least two of the plurality of inverter-based resources. The method also includes receiving one or more measured electrical signals from the electric power grid via the respective local controllers. Further, the method includes establishing, via the respective local controllers, a timing reference for interleaving the pulse patterns of at least two of the plurality of inverter-based resources based on the one or more measured electrical signals from the electric power grid. Further, the method includes operating at least two of the plurality of inverter-based resources in parallel on the electric power grid via the respective local controllers such that the pulse patterns are interleaved with one another to reduce voltage distortion at the point of common coupling.

[0014] In another aspect, the present disclosure is directed to a wind farm connected to an electric power grid. The wind farm includes a plurality of wind turbines connected in parallel to the electric power grid at a point of common coupling and a plurality of local controllers. Each of the plurality of wind turbines is controlled by one of the plurality of local controllers. Each of the plurality of local controllers includes a pulse pattern programmed therein. The plurality of local controllers are configured to perform one or more operations. The operations include, but are not limited to, receiving one or more measured electrical signals from the electric power grid; establishing a timing reference for interleaving the pulse patterns of at least two of the wind turbines based on the one or more measured electrical signals from the electric power grid; and operating at least two of the wind turbines in parallel on the electric power grid using the pulse patterns and the timing reference such that the pulse patterns are interleaved with each other to reduce voltage distortion at the point of common coupling.

[0015] In yet another aspect, the present disclosure is directed to a method for operating a plurality of inverter-based resources connected to an electric power grid at a point of common coupling. The method includes determining a pulse pattern for each of the plurality of inverter-based resources. The method further includes receiving one or more measured voltage signals from the electric power grid via local controllers of each of the plurality of inverter-based resources. The method further includes establishing, via the respective local controllers, a timing reference for interleaving the pulse patterns of the plurality of inverter-based resources based on the one or more measured voltage signals. The method further includes operating, via the respective local controllers, the plurality of inverter-based resources in parallel on the electric power grid using the pulse patterns and the timing reference such that sets of pulse patterns of the plurality of inverter-based resources are interleaved together to reduce voltage distortion at the point of common coupling.

[0016] 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 explanation of the drawings]

[0017] 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 following specification, which makes reference to the accompanying drawings. [Figure 1] 1 is a perspective view of an embodiment of a wind turbine according to the present disclosure. [Figure 2] FIG. 2 is a schematic diagram illustrating one embodiment of a total power conversion system suitable for use with the wind turbine shown in FIG. 1. [Figure 3] FIG. 3 is a schematic diagram illustrating one embodiment of an individual inverter suitable for use with the overall power conversion system shown in FIG. 2. [Figure 4] FIG. 4 shows the nature of the voltage on the AC side of the inverter of the overall power conversion system of FIG. 3, as a result of the gate control used in that inverter. [Figure 5] FIG. 1 is a schematic diagram illustrating an example embodiment of an offshore wind farm according to the present disclosure. [Figure 6] FIG. 1 is a flow diagram of one embodiment of a method for operating a plurality of inverter-based resources connected to a power grid at a point of common coupling in accordance with the present disclosure. [Figure 7] FIG. 1 is a schematic diagram illustrating one embodiment of a string of inverter-based resources according to the present disclosure. [Figure 8] FIG. 10 is a flow diagram of another embodiment of a method for operating a plurality of inverter-based resources connected to a power grid at a point of common coupling in accordance with the present disclosure.

[0018] Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Reference will now be made in detail to embodiments of the present invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. Indeed, 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 example, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.

[0020] Terms such as "coupled," "fixed," and "attached" refer to both direct coupling, fixing, or attachment, as well as indirect coupling, fixing, or attachment through one or more intermediate components or features, unless otherwise specified herein.

[0021] Moreover, those skilled in the art will recognize the interchangeability of various features from different embodiments. Similarly, the various method steps and features described, and other known equivalents for each such method and feature, can be mixed and matched by those skilled in the art to construct additional systems and techniques in accordance with the principles of the present disclosure. Of course, it should be understood that not necessarily all such objects or advantages can be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein can be embodied or implemented in a way that achieves or optimizes one advantage or advantages as taught herein, without necessarily achieving other objects or advantages as may be taught or suggested herein.

[0022] In general, the present disclosure is directed to a system and method for operating at least two inverter-based resources in parallel on a power grid. More specifically, for each inverter-based resource, the present disclosure utilizes timing information obtained from a locally sensed grid voltage to ensure that harmonic currents from the group are substantially canceled. Thus, in embodiments, converter pulse patterns may be designed such that selected harmonics have a combination of magnitude and phase relative to a fundamental frequency voltage such that the net effect of the selected harmonics is canceled when the group is operating on the power grid. In one embodiment, for example, a group of at least two inverter-based resources may have two different pulse patterns selected during commissioning. The pulse pattern characteristics may be, for example, that the selected harmonics have the same amplitude but are phase-angled 180 degrees apart. Because the grid voltage provides a means of maintaining the two inverter-based resources performing the desired interleaving function, the local controllers of each of the two inverter-based resources may operate independently of each other and utilize only one or more measured electrical signals to establish timing references for interleaving the pulse patterns without communicating with each other.

[0023] Referring now to the drawings, FIG. 1 is a perspective view of a portion of an inverter-based resource such as a wind turbine 100. The wind turbine 100 includes a nacelle 102 that houses a generator (not shown in FIG. 1). The nacelle 102 is mounted to a tower 104 (a portion of the tower 104 is shown in FIG. 1). The tower 104 may have any suitable height that facilitates operation of the wind turbine 100 as described herein. The wind turbine 100 also includes a rotor 106 that includes a plurality of rotor blades 108, e.g., three rotor blades 108, mounted to a rotating hub 110. Alternatively, the wind turbine 100 may include any number of rotor blades 108 that facilitate operation of the wind turbine 100 as described herein. In one embodiment, the wind turbine 100 may also include a gearbox (not shown in FIG. 1) operably coupled to the rotor 106 and the generator (not shown in FIG. 1).

[0024] 2, there is shown a schematic diagram of one embodiment of a full power conversion system 200 that may be used with wind turbine 100 in accordance with the present disclosure. Thus, as shown, in one embodiment, power conversion system 200 is configured to supply power to a power grid 212.

[0025] More specifically, as shown, power conversion system 200 includes a generator 118 coupled to a power conversion assembly 210. Power conversion assembly 210 includes a generator-side power converter 220 for AC-to-DC conversion electrically coupled to a line-side power converter 222 for DC-to-AC conversion. Further, as shown, generator-side power converter 220 is coupled to line-side power converter 222 via a single direct current (DC) link 226. Further, as shown, power conversion system 200 further includes a main transformer 234 electrically coupled between power conversion assembly 210 and power grid 212.

[0026] Thus, DC power is transmitted from DC link 226 to line-side power converter 222, which functions as an inverter configured to convert the DC power from DC link 226 into three-phase sinusoidal AC power having a predetermined voltage, current, and frequency. This conversion is monitored and controlled via converter controller 262. Power conversion assembly 210 compensates for or adjusts the frequency of the three-phase power from generator 118 for changes in wind speed at hub 110 and rotor blades 108, for example.

[0027] Additionally, in embodiments, as shown, power conversion system 200 may include a switchgear assembly 228 having one or more sensors coupled between main transformer 234 and power grid 212. Thus, in such embodiments, switchgear assembly 228 connects power conversion system 200 to a string of other wind turbine generators (e.g., WTGs). Furthermore, in such embodiments, the sensor(s) of switchgear assembly 228 are configured to detect voltage and / or current flow, which may be used by converter controller 262, as described herein.

[0028] 2 , power conversion system 200 may be coupled in electronic data communication with turbine controller 202 and / or converter controller 262 to control its operation. For example, in one embodiment, converter controller 262 is configured to receive control signals from turbine controller 202. The control signals are based on sensed conditions or operating characteristics of wind turbine 100 and power conversion system 200. The control signals are received by turbine controller 202 and used to control the operation of power conversion assembly 210. Feedback from one or more sensors may be used by power conversion system 200 to control power conversion assembly 210 via converter controller 262.

[0029] Turbine controller 202 and / or converter controller 262 include at least one processor and memory, at least one processor input channel, at least one processor output channel, and may include at least one computer. As used herein, the term computer is not limited to integrated circuits referred to in the art as computers but refers broadly to a processor, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. In an exemplary embodiment, memory may include a computer-readable medium such as, but not limited to, random access memory (RAM). Alternatively, one or more storage devices such as a floppy disk, a compact disc read only memory (CD-ROM), a magneto-optical disk (MOD), and / or a digital versatile disc (DVD) may be used. Also, in an exemplary embodiment, additional input channels (not shown in FIG. 2) may be, but are not limited to, computer peripherals associated with an operator interface, such as a mouse and keyboard. Further, in an exemplary embodiment, additional output channels may include, but are not limited to, an operator interface monitor (not shown in FIG. 2).

[0030] The processor of the turbine controller 202 and / or converter controller 262 processes information transmitted from multiple electrical and electronic devices, including, but not limited to, voltage and current converters. The RAM and / or storage device store and transfer information and instructions executed by the processor. The RAM and / or storage device may also be used to store and provide temporary variables, static (i.e., non-changing) information and instructions, or other intermediate information to the processor during execution of instructions by the processor. The instructions executed include, but are not limited to, resident conversion and / or comparator algorithms. Execution of instruction sequences is not limited to any specific combination of hardware circuitry and software instructions.

[0031] Converter controller 262 is substantially similar to turbine controller 202 and is coupled in electronic data communication with turbine controller 202, as shown generally in Figure 2. Moreover, in the exemplary embodiment, converter controller 262 is physically integrated within power conversion assembly 210. Alternatively, converter controller 262 has any configuration that facilitates operation of power conversion system 200 as described herein.

[0032] Referring now to FIG. 3 , a schematic diagram of an embodiment of an individual inverter, such as line-side converter 222, suitable for use with the overall power conversion system shown in FIG. 2 is shown. Thus, as shown in FIG. 3 , wind turbine generators (e.g., WTGs), like other IBRs, connect to a three-phase AC load (such as power grid 212 in FIG. 2 ) through an inverter. Furthermore, as shown, generator 118 is represented as a DC power source. Thus, it should be understood that, in embodiments, AC-DC converters may take several forms, e.g., multilevel, and may also include several converters within an IBR, all operating according to commands from converter controller 262. In particular, as shown, line-side converter 222 is a three-phase assembly with multiple semiconductor power switches 224 (e.g., insulated gate bipolar transistors (IGBTs)). In other embodiments, semiconductor power switches 224 may be any other type of switching device now known in the art or later developed.

[0033] Thus, in one embodiment, the generator-side power converter 220 and the line-side power converter 222 may be in a three-phase, synchronous gating configuration including IGBT switching devices operating as known in the art. In another embodiment, the generator-side power converter 220 and the line-side power converter 222 may be in a three-phase, pulse width modulation (PWM) gating configuration including IGBT switching devices operating as known in the art. Alternatively, the generator-side power converter 220 and the line-side power converter 222 may have any configuration using any switching devices that facilitate operation of the power conversion system 200 described herein.

[0034] 4, the generator-side power converter 220 and / or the line-side converter 222 shown in FIGS. 2 and 3 have a voltage waveform 500 (also referred to herein as a pulse pattern). In such an embodiment, the voltage waveform 500 is the actual voltage at the converter AC terminals when the pulse pattern includes a fundamental frequency component exemplified by a sinusoidal waveform 502. The pulse pattern includes distortion components, as is well known in the art. The frequency, magnitude, and phase of the distortion components are functions of the switching time.

[0035] Additionally, in one embodiment, there are several means for creating the switching times of the voltage waveform 500, all designed to match the voltage of the generator-side power converter 220 and / or the line-side converter 222 to commands created by a higher level control function, such as a current regulator. In certain embodiments, the switching times are defined by gating logic, such as pulse width modulation (PWM) or synchronous gating.

[0036] In one embodiment, for example, PWM is used, which establishes a gating time at a fixed rate based on the magnitude of the instantaneous voltage desired within the next switching interval. In this example, the switching speed is much faster than the grid frequency, e.g., a switching speed of a few kHz is used, while the grid frequency is 60 Hz or 50 Hz.

[0037] Another approach is synchronous gating, where the switching time is based on an angular reference provided by a phase-locked loop synchronized to the fundamental frequency component of the positive voltage measured on the bus. In synchronous gating, the pulse pattern is predetermined by calculation and programmed into the gating control. The pulse pattern is determined in a way that minimizes selected harmonics. A benefit of synchronous gating versus PWM is to enable slower device switching rates than would be needed with a PWM approach.

[0038] Referring now to FIG. 5 , a schematic diagram of one embodiment of an offshore wind farm 300 according to the present disclosure is shown. As shown, the wind farm 300 includes a substation 302 at a point of common coupling 308. Further, as shown, in an embodiment, the wind farm 300 includes one or more wind farm strings 304 connected in parallel to the point of common coupling and including multiple wind turbines 306. The wind farm strings 304 may also be referred to as “lines” or “feeders.” One or more of the wind farm strings 304 may include a wind turbine, such as the wind turbine 100 described herein. In the illustrated embodiment, all of the strings 304 have the same length and the same number of wind turbines 306. However, it should be understood that in further embodiments, the strings 304 may have different lengths (and therefore different capacities) and different numbers of wind turbines 306.

[0039] In additional embodiments, while the strings 304 are depicted as being substantially parallel to one another, it should also be understood that the strings 304 may be arranged in various ways depending on the layout of the wind farm 300. For example, in one embodiment, the strings 304 may extend radially away from the common coupling point 308.

[0040] 5 , as previously mentioned, the wind farm 300 may be an offshore wind farm. Furthermore, the substation 302 may include a high-voltage transformer and may be connected to a high-voltage transmission line 310, such as an HVAC line or an HVDC line. Such a high-voltage transmission line may be several kilometers long, for example, 20 kilometers, 70 kilometers, or more. The high-voltage transmission line 310 may be connected to a point of common coupling (PPC) 312 with an onshore power grid 314. Reference numeral 316 denotes a coastline.

[0041] Thus, in an embodiment, an objective of the present disclosure is to have adjacent generators 254 of a string 304 have different switching times so that selected harmonics of the voltage are 180 degrees out of phase. This allows currents at these harmonics to enter the string 304 at one connection and be extracted at the connection point of the adjacent generator 254, thereby not entering the substation with the resulting distortion of the voltage at the substation. This is commonly referred to as "interleaving." As used herein, interleaving is a well-known approach for canceling net harmonics from multiple converters. For example, interleaving is widely used in converter-based systems when multiple converters are included, as described in U.S. Patent No. 7,944,068, entitled "Optimizing Converter Protection for Wind Turbine Generators," filed June 30, 2008.

[0042] However, in the present disclosure, the difference in gate timing must be enforced in some manner. Prior art systems utilize communication between controllers to achieve this function. However, the present disclosure achieves such interleaving without turbine-to-turbine communication. In particular, the systems and methods of the present disclosure utilize the voltage measured at the inverter-based resources to set the timing of the gating logic. This voltage is typically very similar between adjacent inverter-based resources on a string, such as string 304. This allows the voltage to be used to determine the reference time for the gating logic to achieve effective interleaving.

[0043] In one embodiment, referring now to FIG. 6 , a flow diagram of an exemplary method 400 for operating multiple inverter-based resources connected to a power grid at a common coupling point is shown, in accordance with some embodiments of the present disclosure. The flow diagrams and methods described herein do not imply a fixed order to the steps, and embodiments of the present invention may be performed in any order possible. Note that any of the methods described herein may be performed by hardware, software, or any combination of these approaches. For example, a non-transitory computer-readable storage medium may store thereon instructions that, when executed by a machine, result in performance according to any of the embodiments described herein.

[0044] As shown at (402), method 400 includes providing pulse patterns for at least two inverter-based resources of the plurality of inverter-based resources to a local controller for each of the at least two inverter-based resources of the plurality of inverter-based resources. In one embodiment, for example, the pulse patterns are determined offline, such as during commissioning of the inverter-based resources, and provided to local turbine controller 202 at the time of installation and any time during operation of wind farm 300. In certain embodiments, for example, the pulse patterns for the inverter-based resources are determined based on one or more selected harmonics. The selected one or more harmonics may include, for example, a combination of a magnitude and a phase angle with respect to a fundamental-frequency voltage.

[0045] Thus, in certain embodiments, the method 400 may further include storing pulse patterns of the at least two inverter-based resources of the plurality of inverter-based resources in a local controller (e.g., the turbine controller 202) for each of the at least two inverter-based resources of the plurality of inverter-based resources.

[0046] As shown at (404), method 400 includes receiving, via the respective local controllers, one or more measured electrical signals from the power grid. The electrical signals may include, for example, voltage, current, or combinations or functions thereof. Accordingly, as shown at (406), method 400 includes, via the respective local controllers, establishing a timing reference for interleaving pulse patterns of at least two of the plurality of inverter-based resources based on the one or more measured electrical signals from the power grid.

[0047] Further, as shown in (408), method 400 includes operating at least two of the plurality of inverter-based resources in parallel on the power grid via respective local controllers utilizing pulse patterns and timing references such that the pulse patterns are interleaved with one another to reduce voltage distortion at the point of common coupling. In such embodiments, the inverter-based resources may be operated using synchronous gating or pulse width modulation. Thus, in such embodiments, when the two inverter-based resources are operating on the power grid with interleaved pulse patterns, the net effects of selected harmonics cancel each other out.

[0048] Thus, for the methods of the present disclosure, the grid voltage provides a means for maintaining individual inverter-based resources performing the desired interleaving function such that communication between the respective controllers is not necessary or required. In particular, in one embodiment, each local controller operates independently of one another to establish timing references for interleaving pulse patterns, utilizing only one or more measured electrical signals without communicating with one another.

[0049] Referring now to FIG. 7 , a schematic diagram of one embodiment of a string 250 of inverter-based resources 252 according to the present disclosure is shown. As shown, each inverter-based resource 252 includes a generator 254 and a buck-buck power conversion system 256 coupled to the high-voltage bus 260 of the string 250 via a transformer 258. However, FIG. 8 is provided to illustrate how a desired timing reference can be achieved even when there is some impedance (e.g., Z12) between the connection points. In such an embodiment, establishing the desired timing reference requires measuring the current flow (e.g., I12) on the portion of the string 250 between adjacent generators 254. This current flow can be sensed within the switchgear assemblies of the individual inverter-based resources, so no communication between controllers is required.

[0050] Referring now to FIG. 8 , a flow diagram of another exemplary method 700 for operating a plurality of inverter-based resources connected to an electric power grid at a point of common coupling is shown, in accordance with some embodiments of the present disclosure. As shown at (702), method 700 includes determining a pulse pattern for each of the plurality of inverter-based resources. As shown at (704), method 700 includes receiving one or more measured voltage signals from the electric power grid via a local controller of each of the plurality of inverter-based resources. As shown at (706), method 700 includes establishing, via the respective local controllers, a timing reference for interleaving the pulse patterns of the plurality of inverter-based resources based on the one or more measured voltage signals. Accordingly, as shown at (708), method 700 includes operating the plurality of inverter-based resources in parallel on the electric power grid via the respective local controllers using the pulse patterns and timing reference such that sets of pulse patterns of the plurality of inverter-based resources are interleaved together to reduce voltage distortion at the point of common coupling.

[0051] In this disclosure, the wind turbines and wind farms are generally illustrated and described as offshore wind turbines and wind farms, although it will be apparent that the same or similar arrangements may be used on land.

[0052] Further aspects of the invention are provided by the subject matter of the following clauses. [Embodiment 1] A method for operating a plurality of inverter-based resources connected to an electrical grid at a point of common coupling, comprising: providing pulse patterns for at least two of the plurality of inverter-based resources to respective local controllers of the at least two of the plurality of inverter-based resources; receiving, via the respective local controllers, one or more measured electrical signals from the electrical grid; establishing, via the respective local controllers, a timing reference for interleaving the pulse patterns for the at least two of the plurality of inverter-based resources based on the one or more measured electrical signals from the electrical grid; operating the at least two of the plurality of inverter-based resources in parallel on the electrical grid via the respective local controllers utilizing the pulse patterns and the timing reference such that the pulse patterns are interleaved with each other to reduce a voltage distortion at the point of common coupling.

[0019] The step of providing the pulse pattern to the at least two of the plurality of inverter-based resources further comprises: 2. The method of embodiment 1, comprising determining pulse patterns for the at least two of the plurality of inverter-based resources based on one or more selected harmonics comprising a combination of a magnitude and a phase angle with respect to a fundamental-frequency voltage such that a net effect of the one or more selected harmonics cancel each other when the at least two of the plurality of inverter-based resources are operating on the electrical grid. [Embodiment 3] The method described in embodiment 2, wherein the pulse patterns comprise a first pulse pattern having a first amplitude of a selected harmonic and a second pulse pattern having a second amplitude of the selected harmonic, and the first and second amplitudes of the selected harmonic are equal. [Embodiment 4] The method described in embodiment 3, wherein the first pulse pattern has a first phase angle and the second pulse pattern has a second phase angle, the first phase angle being shifted from the second phase angle by 180 degrees. [Embodiment 5] A method as described in any of the preceding embodiments, wherein the step of operating the at least two of the plurality of inverter-based resources in parallel on the electrical grid further comprises utilizing synchronous gating. [Embodiment 6] A method as described in any of the preceding embodiments, wherein the step of operating the at least two of the plurality of inverter-based resources in parallel on the electrical grid further comprises utilizing pulse width modulation. [Embodiment 7] A step of predetermining the pulse patterns for the at least two of the plurality of inverter-based resources offline; Storing the pulse patterns for the at least two inverter-based resources in the respective local controllers of the at least two inverter-based resources. [Embodiment 8] A method described in any of the preceding embodiments, wherein the respective local controllers operate independently of each other to establish the timing reference for interleaving the pulse patterns and utilize only the one or more measured electrical signals without communicating to each other. [Embodiment 9] A method described in any of the preceding embodiments, wherein one or more of the plurality of inverter-based resources are wind turbines. [Embodiment 10] A wind farm connected to an electrical grid, a plurality of wind turbines connected in parallel to the electrical grid at a point of common coupling; a plurality of local controllers; each of the plurality of wind turbines being controlled by one of the plurality of local controllers; each of the plurality of local controllers comprising a pulse pattern programmed therein; the plurality of local controllers configured to perform one or more operations; The one or more actions include: receiving one or more measured electrical signals from the electrical grid; establishing a timing reference for interleaving the pulse patterns for the at least two of the wind turbines based on the one or more measured electrical signals from the electrical grid; operating the at least two of the wind turbines in parallel on the electrical grid utilizing the pulse patterns and the timing reference such that the pulse patterns are interleaved with each other to reduce a voltage distortion at the point of common coupling. [Embodiment 11] A wind farm as described in embodiment 10, wherein the pulse pattern for each of the plurality of wind turbines is predetermined based on one or more selected harmonics comprising a combination of a magnitude and a phase angle with respect to a fundamental-frequency voltage such that a net effect of the one or more selected harmonics cancel each other when the at least two of the plurality of wind turbines are operating on the electrical grid. [Embodiment 12] A method for operating a plurality of inverter-based resources connected to an electrical grid at a point of common coupling, comprising: determining a pulse pattern for each of the plurality of inverter-based resources; receiving, via respective local controllers of the plurality of inverter-based resources, one or more measured voltage signals from the electrical grid; establishing, via the respective local controllers, a timing reference for interleaving the pulse patterns for the plurality of inverter-based resources based on the one or more measured voltage signals; and operating, via the respective local controllers, the plurality of inverter-based resources in parallel on the electrical grid utilizing the pulse patterns and the timing reference such that pairs of the pulse patterns of the plurality of inverter-based resources are interleaved together to reduce a voltage distortion at the point of common coupling.

[0023] [Embodiment 13] Determining a pulse pattern for each of the plurality of inverter-based resources includes: 13. The method of embodiment 12, further comprising determining the pulse pattern for each of the plurality of inverter-based resources based on one or more selected harmonics comprising a combination of a magnitude and a phase angle with respect to a fundamental-frequency voltage such that a net effect of the one or more selected harmonics cancel each other when the at least two of the plurality of inverter-based resources are operating on the electrical grid. [Embodiment 14] The method described in embodiment 13, wherein the amplitudes of the pairs of the pulse patterns are equal. [Embodiment 15] The method described in embodiment 14, wherein the phase angles of the pairs of the pulse patterns shifted from each other as a function of the number of the plurality of inverter-based resources.

[0023] [Embodiment 16] Determining the pulse pattern for each of the plurality of inverter-based resources includes: A method as described in any one of embodiments 12 to 15, comprising a step of predetermining the pulse patterns for each of the plurality of inverter-based resources offline. [Embodiment 17] A method as described in embodiments 12 to 16, wherein operating at least two of the multiple inverter-based resources in parallel on the power grid further includes utilizing synchronous gating. [Embodiment 18] A method as described in embodiments 12 to 17, wherein operating at least two of the plurality of inverter-based resources in parallel on the power grid further includes utilizing pulse width modulation. [Embodiment 19] A method as described in embodiments 12 to 18, wherein the respective local controllers operate independently of each other to establish the timing reference for interleaving the pulse patterns and utilize only the one or more measured electrical signals without communicating to each other. [Embodiment 20] A method of embodiments 12 to 19, wherein one or more of the multiple inverter-based resources is a wind turbine.

[0053] This specification 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 the 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 that do not differ substantially from the literal language of the claims. [Explanation of symbols]

[0054] 100: Wind turbine 102: Nacelle 104: Tower 106: Rotor 108: Rotor blade 110: Rotating hub 118, 254: Generator 200: Total power conversion system / power conversion system 202: Turbine controller 210: Power conversion assembly 212: Power grid 220: Generator side power converter / Three-phase inverter circuit 222: Line side power converter 224: Solid state power switch 226: DC link 228: Switchgear assembly 234: Main transformer 250: String 252: Inverter-based resource 256: Back-back power conversion system 258: Transformer 260: High voltage bus 262: Converter controller 300: Wind farm 302: Substation 304: Wind farm string / string 306: Wind turbine 308: Common coupling point 310: High voltage transmission line 312: Common coupling point 314: Onshore power grid 316: Coastline 500: Voltage waveform / pulse pattern 502: Sine wave waveform / fundamental frequency component

Claims

1. 1. A method for operating a plurality of inverter-based resources connected to a power grid at a point of common coupling, comprising: providing at least two pulse patterns of the plurality of inverter-based resources to at least two respective local controllers of the plurality of inverter-based resources; receiving, via each local controller, one or more measured electrical signals from the power grid; establishing, via the respective local controllers, a timing reference for interleaving pulse patterns of at least two of the plurality of inverter-based resources based on one or more measured electrical signals from the power grid; and operating at least two of the plurality of inverter-based resources in parallel on the power grid via respective local controllers utilizing pulse patterns and timing references such that the pulse patterns are interleaved with one another to reduce voltage distortion at a point of common coupling.

2. The step of providing a pulse pattern to at least two resources of the plurality of inverter-based resources includes:

10. The method of claim 1, comprising determining pulse patterns for at least two of the plurality of inverter-based resources based on one or more selected harmonics comprising a combination of magnitude and phase angle relative to a fundamental frequency voltage such that a net effect of the one or more selected harmonics cancels each other when at least two of the plurality of inverter-based resources are operating on the power grid.

3. 3. The method of claim 2, wherein the pulse patterns include a first pulse pattern having a first amplitude of the selected harmonic and a second pulse pattern having a second amplitude of the selected harmonic, the first and second amplitudes of the selected harmonic being equal.

4. 4. The method of claim 3, wherein the first pulse pattern has a first phase angle and the second pulse pattern has a second phase angle, the first phase angle being shifted 180 degrees from the second phase angle.

5. The method of claim 1 , wherein operating at least two of the plurality of inverter-based resources in parallel on the power grid further comprises utilizing synchronous gating.

6. The method of claim 1 , wherein operating at least two of the plurality of inverter-based resources in parallel on the power grid further comprises utilizing pulse width modulation.

7. offline predetermining pulse patterns for at least two of the plurality of inverter-based resources; 10. The method of claim 1, further comprising: storing pulse patterns for at least two inverter-based resources of the plurality of inverter-based resources in respective local controllers of the at least two inverter-based resources of the plurality of inverter-based resources.

8. 10. The method of claim 1, wherein each local controller operates independently of one another to establish a timing reference for interleaving the pulse patterns, without communicating with one another, and utilizing only one or more measured electrical signals.

9. The method of claim 1 , wherein one or more of the plurality of inverter-based resources is a wind turbine.

10. 1. A wind farm connected to an electric power grid, comprising: a plurality of wind turbines connected in parallel to an electric power grid at a common coupling point; a plurality of local controllers; each of the plurality of wind turbines is controlled by one of a plurality of local controllers; each of the plurality of local controllers having a pulse pattern programmed therein; the plurality of local controllers are configured to perform one or more operations; The one or more actions may include: receiving one or more measured electrical signals from an electrical power grid; establishing a timing reference for interleaving pulse patterns of at least two wind turbines based on one or more measured electrical signals from the power grid; and operating at least two wind turbines in parallel on an electric power grid using pulse patterns and timing references such that the pulse patterns are interleaved with one another to reduce voltage distortion at a common coupling point.

11. 11. The wind farm of claim 10, wherein the pulse pattern for each of the plurality of wind turbines is predetermined based on the one or more selected harmonics including a combination of magnitude and phase angle relative to the fundamental frequency voltage such that a net effect of the one or more selected harmonics cancels each other when at least two of the plurality of wind turbines are operating on the power grid.

12. 1. A method of operating a plurality of inverter-based resources connected to an electric power grid at a point of common coupling, comprising: determining a pulse pattern for each of a plurality of inverter-based resources; receiving one or more measured voltage signals from a power grid via a local controller of each of a plurality of inverter-based resources; establishing, via respective local controllers, a timing reference for interleaving pulse patterns of the plurality of inverter-based resources based on the one or more measured voltage signals; and operating, via respective local controllers, a plurality of inverter-based resources in parallel on the power grid using pulse patterns and timing references, and interleaving pairs of pulse patterns of the plurality of inverter-based resources to reduce voltage distortion at a point of common coupling.

13. Determining a pulse pattern for each of the plurality of inverter-based resources includes:

13. The method of claim 12, further comprising determining a pulse pattern for each of the plurality of inverter-based resources based on the one or more selected harmonics comprising a combination of magnitude and phase angle relative to a fundamental frequency voltage such that a net effect of the one or more selected harmonics cancels each other when at least two of the plurality of inverter-based resources are operating on the power grid.

14. The method of claim 13 , wherein the amplitudes of the pairs of pulse patterns are equal.

15. 15. The method of claim 14, wherein phase angles of pairs of pulse patterns shift relative to one another as a function of the number of inverter-based resources in the plurality.

16. Determining a pulse pattern for each of the plurality of inverter-based resources further includes: The method of claim 12 , comprising predetermining a pulse pattern offline for each of a plurality of inverter-based resources.

17. The method of claim 12 , wherein operating at least two of the plurality of inverter-based resources in parallel on the power grid further comprises utilizing synchronous gating.

18. The method of claim 12 , wherein operating at least two of the plurality of inverter-based resources in parallel on the power grid further comprises utilizing pulse width modulation.

19. 13. The method of claim 12, wherein each local controller operates independently of one another to establish a timing reference for interleaving the pulse patterns, without communicating with one another, and utilizing only one or more measured electrical signals.

20. The method of claim 12 , wherein one or more of the plurality of inverter-based resources is a wind turbine.

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