Improvements relating to providing grid formation control of power converters in power transfer network

Abstract

The name of the invention is' Improvements relating to providing grid formation control of power converters in a power transfer network '. There is provided a method of operating a controller (200) for providing grid formation control of a power converter (110) connected to an electrical grid (130), the method comprising determining, by a regulation module (210) of the controller (200), a regulation value (Xk) based on a measured power (Pmeas) and a feedforward signal (delta1), where the measured power (Pmeas) is indicative of an amount of power output from the power converter (110); determining, by a feed-forward module (220) of the controller (200), a feed-forward signal ([delta] 1) based on the power demand (Pdemand) and the adjustment value (Xk), wherein the power demand (Pdemand) indicates a demand for an amount of power to be output from the power converter (110); wherein the adjustment module (210) and the feed-forward module (220) form a first closed-loop control system to determine a feed-forward signal ([delta] 1) such that the feed-forward signal is automatically adjusted to compensate for a difference between the power demand (Pdemand) and the measured power (Pmeas) during a change in the power demand (Pdemand); and determining, by the controller (200), a phase angle command (delta) for the power converter (110) based on the feed-forward signal (delta 1).

Description

Technical Field

[0001] The subject matter herein generally relates to the field of power transmission networks, and more specifically, to a controller for providing grid-forming control of a power converter connected to a power grid and a method of operating the controller. Background Art

[0002] In a high voltage direct current (HVDC) power transmission network, alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines, submarine cables, and / or underground cables. This conversion eliminates the need to compensate for the AC reactive / capacitive load effects imposed by the power transmission medium (i.e., the transmission line or cable), and reduces the cost per kilometer of the line and / or cable, and thus becomes cost-effective when power needs to be transmitted over long distances. For example, DC power can also be directly transmitted from an offshore wind farm to an onshore AC power transmission network.

[0003] The conversion between DC power and AC power is used where it is necessary to interconnect DC and AC networks. In any such power transmission network, power conversion components, also known as converters (i.e., the power converters in the converter stations), are required at each interface between AC and DC power to achieve the desired conversion from AC to DC or from DC to AC.

[0004] For example, when converting DC power to AC power at the interface between a DC transmission line and an AC power grid, the power converter can operate in a grid-following mode (GFL) or a grid-forming mode (GFM).

[0005] In the GFL mode, the power converter uses a fast current regulation loop to control the active and reactive power exchanged with the AC power grid. The power converter uses the current reference from the AC power grid for the active component of the current to achieve the desired power output. Thus, the power converter operating in the GFL mode includes a function of managing voltage and reactive power in a way that generates a command for the reactive component of the current. Then, a wide-bandwidth current regulator generates a command for the voltage applied by the power converter to the AC power grid such that the actual current closely tracks the command. Thus, the power converter operating in the GFL mode provides a current source characteristic.

[0006] Alternatively, the power converter operating in the GFM mode provides a voltage source characteristic, where the phase angle and amplitude of the voltage are controlled to achieve the regulation function required by the power grid. With this configuration, the current will flow according to the demand of the power grid, and the converter helps to establish the voltage and frequency for the power grid. This characteristic can be compared to a traditional generator based on a turbine-driven synchronous machine.

[0007] For effectiveness, the resources forming the power grid should be able to maintain the internal voltage phasor such that when there are changes in the power grid conditions, such as sudden load increases / removals, opening or closing of the grid connection, which result in phase jumps and / or rapid frequency changes, the internal voltage phasor does not move rapidly. In other words, the power from the power converter should be able to change suddenly to stabilize the power grid, whereupon it is subsequently slowly reset to the power commanded by a higher-level control function. Additionally, the resources forming the power grid should be able to rapidly implement power limitations that exist due to constraints on the power handling part of the device, such as DC voltage / current in a battery, a solar array, and / or a wind power system. Such a response is preferred for mitigating severe disturbances on the power grid, such as faults, where the power limitations will be dynamically adjusted to coordinate with the power grid conditions for a safe recovery from the fault. Further, the power grid forming resources should be able to rapidly follow changes in commands from higher-level control, such as for damping mechanical vibrations in a wind turbine.

[0008] In some conventional systems, the foregoing problems can be solved by: determining a feedforward signal as a function of one or more power demands, one or more control signals, and one or more estimated electrical conditions of an inverter-based resource, and using the feedforward signal to position at least one control angle of the inverter-based resource to an expected value required to achieve one or more reference commands, thereby achieving a rapid response to one or more reference commands received from an external controller. Summary of the Invention

[0009] The present inventors have recognized that for greater effectiveness, the feedforward term should accurately track changes in the power demand, regardless of the electrical characteristics of the AC power grid (such as the impedance of the AC power grid). Thus, there is still room for improvement in providing grid-forming control of power converters in a power transmission network.

[0010] According to a first aspect, there is provided a method of operating a controller for providing grid-forming control of a power converter connected to a power grid, the method comprising determining an adjustment value by an adjustment module of the controller based on measured power and a feedforward signal, wherein the measured power indicates the amount of power output from the power converter; determining the feedforward signal by a feedforward module of the controller based on a power demand and the adjustment value, wherein the power demand indicates the demand for the amount of power to be output from the power converter; wherein the adjustment module and the feedforward module form a first closed-loop control system to determine the feedforward signal such that the feedforward signal is automatically adjusted to compensate for the difference between the power demand and the measured power during a change in the power demand; and determining a phase angle command of the power converter by the controller based on the feedforward signal.

[0011] Because the regulation module and the feedforward module form a closed-loop control system to determine the feedforward signal, the phase angle command provided to the power converter tends to improve the ability of the power converter output to closely and accurately track the power (measured power) that changes with the power demand. In other words, when operating in grid-forming mode, the power converter tends to be more effective in providing the desired changes in output power.

[0012] In some embodiments, the method further includes determining, by a control module in the controller, a first control signal based on the power demand, the measured power, and the measured frequency, where the measured frequency is a measure of the frequency component of the AC grid; and determining, by the controller, a phase angle command for the power converter based on the feedforward signal and the first control signal.

[0013] In some embodiments, the method further includes transforming, by a first transformation module of the controller, the feedforward signal and outputting the transformed feedforward signal; summing, by a summing component of the controller, the first control signal and the transformed feedforward signal and outputting a second control signal; and determining, by the controller, a phase angle command based on the second control signal such that the power converter can track the changes in power demand and the power changes in the AC grid.

[0014] By using the feedforward signal and the first control signal to determine the phase angle command, the power converter tends to be able to effectively respond to the changes in power demand and the changes in the AC grid.

[0015] In some embodiments, the method further includes determining, by a second transformation module of the controller, a phase angle command by transforming the second control signal and outputting the phase angle command to the power converter.

[0016] In some embodiments, the control module includes a second closed-loop control system with an integral term.

[0017] In some embodiments, the regulation module implements an equation that includes: multiplying the grid voltage by the sine of the power converter voltage and the feedforward signal to determine a first product, and then dividing the first product by the measured power; where the grid voltage indicates the voltage at the connection point between the power converter and the grid, and the power converter voltage indicates the output voltage of the power converter.

[0018] In some embodiments, the feedforward module implements an equation that includes summing the converter impedance and the regulation value to determine a third product; multiplying the third product by the power demand to determine a fourth product; multiplying the power converter voltage by the grid voltage (Vt) to determine a fifth product; dividing the fourth product by the fifth product to determine a sixth product; and determining the arcsine of the sixth product; where the grid voltage indicates the voltage at the connection point between the power converter and the grid, and the power converter voltage indicates the output voltage of the power converter.

[0019] In some embodiments, the method further includes comparing the adjustment value with a threshold value by a comparator in the controller.

[0020] In some embodiments, the method further includes enabling or disabling the adjustment module based on the comparison.

[0021] In some embodiments, enabling or disabling the adjustment module is done by the controller.

[0022] In some embodiments, enabling or disabling the adjustment module is done by a comparator in the controller.

[0023] In some embodiments, if the adjustment value is equal to or lower than a first threshold value, the comparator forwards the adjustment value determined by the adjustment module to the feed - forward module and thereby enables the adjustment module.

[0024] In some embodiments, if the adjustment value is higher than the first threshold value, the comparator updates the adjustment value to a reference value or to zero, and forwards the updated adjustment value to the feed - forward module and thereby disables the adjustment module.

[0025] In some embodiments, the enabling or disabling of the adjustment module is further determined based on hysteresis, such that if the adjustment value is higher than a second threshold value, the adjustment module is disabled, and if the adjustment value is equal to or lower than a third threshold value, the adjustment module is enabled.

[0026] According to a second aspect, there is provided a controller for providing grid - forming control of a power converter connected to a power grid, the controller including an adjustment module configured to determine an adjustment value based on a measured power and a feed - forward signal, wherein the measured power indicates the amount of power output from the power converter; and a feed - forward module configured to determine a feed - forward signal based on a power demand and the adjustment value, wherein the power demand indicates the demand for the amount of power to be output from the power converter; wherein the adjustment module and the feed - forward module form a first closed - loop control system configured to determine the feed - forward signal such that the feed - forward signal is automatically adjusted to compensate for the difference between the power demand and the measured power during a change in the power demand; and the controller is further configured to determine a phase - angle command of the power converter based on the feed - forward signal.

[0027] Generally, the controller is preferably configured to perform the methods described herein.

[0028] In some embodiments, the controller further includes a control module configured to determine a first control signal based on a power demand, a measured power, and a measured frequency, where the measured frequency is a measure of the frequency component of the AC grid; and a first transformation module configured to transform a feedforward signal into a transformed feedforward signal; wherein the controller is further configured to sum the first control signal with the transformed feedforward signal to output a second control signal and determine a phase angle command based on the second control signal such that the power converter can track changes in the power demand and power changes in the AC grid.

[0029] In some embodiments, the controller further includes a comparator configured to compare an adjustment value with a threshold value; wherein if the adjustment value is equal to or lower than the threshold value, the comparator is configured to forward the adjustment value determined by the adjustment module to the feedforward module and thereby enable the adjustment module; and wherein if the adjustment value is higher than the threshold value, the comparator is configured to update the adjustment value to a reference value or to zero and forward the updated adjustment value to the feedforward module and thereby disable the adjustment module.

[0030] According to a third aspect, there is provided a power converter including a DC side for connection to a DC source; an AC side for connection to an AC grid; and the controller of the second aspect.

[0031] In some embodiments, the AC grid is one or more of the following: a consumer network; an island power generation network; or an island load.

[0032] In some embodiments, the DC source is an HVDC transmission system.

[0033] According to a fourth aspect, there is provided a computer program including instructions which, when executed by a processor of a controller for controlling a power converter, cause the controller to perform the method of the first aspect.

[0034] According to a fifth aspect, there is provided a non-transitory computer-readable storage medium including the computer program of the fourth aspect.

[0035] It will be appreciated that the specific features of the different aspects of the present invention share the technical effects and benefits of the corresponding features of other aspects of the present invention. More specifically, the controller, the power converter, the computer program, and the non-transitory computer-readable medium share the technical effects and benefits of the method of the present invention.

[0036] It will also be appreciated that the use of terms such as "first" and "second" is merely intended to assist in differentiating between similar features and is not intended to indicate the relative importance of one feature over another, unless otherwise specified.

[0037] Within the scope of the present application, it is clearly intended that the various aspects, embodiments, examples, and alternatives set forth in the foregoing paragraphs and claims and / or the following description and drawings, and in particular their various features, can be adopted independently or in any combination. That is, all embodiments and all features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The present invention provides a set of technical solutions as follows. Technical solution 1. A method of operating a controller (200) for providing grid-forming control of a power converter (110) connected to a power grid (130), the method comprising: Determining an adjustment value (X meas ) by an adjustment module (210) of the controller based on a measured power (P k ) and a feedforward signal (δ1), wherein the measured power (P meas ) indicates the amount of power output from the power converter (110); Determining the feedforward signal (δ1) by a feedforward module (220) of the controller based on a power demand (P demand ) and the adjustment value (X k ), wherein the power demand (P demand ) indicates the demand for the amount of power to be output from the power converter; wherein the adjustment module (210) and the feedforward module (220) form a first closed-loop control system to determine the feedforward signal (δ1) such that the feedforward signal is automatically adjusted to compensate for the difference between the power demand and the measured power during a change in the power demand; and Determining a phase angle command (δ) of the power converter (110) by the controller based on the feedforward signal (δ1). Technical solution 2. The method according to technical solution 1, further comprising: Determining a first control signal (240) by a control module (230) in the controller based on the power demand, the measured power, and a measured frequency (f g ), wherein the measured frequency is a measure of the frequency component of an AC power grid; and Determining the phase angle command (δ) of the power converter by the controller based on the feedforward signal (δ1) and the first control signal (240). Technical solution 3. The method according to technical solution 2, further comprising: Transforming the feedforward signal (δ1) by a first transformation module (250) of the controller and outputting a transformed feedforward signal (255); The summing component (260) of the controller sums the first control signal (240) and the transformed feedforward signal (255), and outputs a second control signal (Δω); and The controller determines the phase angle command (δ) based on the second control signal (Δω), such that the power converter can track changes in the power demand and power changes in the AC grid. Technical solution 4. The method according to technical solution 3 further includes: The second transformation module (270) of the controller determines the phase angle command (δ) by transforming the second control signal (Δω), and outputs the phase angle command (δ) to the power converter. Technical solution 5. The method according to technical solutions 2, 3, or 4, wherein the control module (230) includes a second closed-loop control system (237) containing an integral term. Technical solution 6. The method according to any of the foregoing technical solutions, wherein the adjustment module implements an equation, and the equation includes: Multiply the grid voltage (V t ) by the power converter voltage (V cnv ) and the sine of the feedforward signal (δ1) to determine a first product, and then divide the first product by the measured power; Wherein the grid voltage indicates the voltage at the connection point between the power converter and the grid, and the power converter voltage indicates the output voltage of the power converter. Technical solution 7. The method according to any of the foregoing technical solutions, wherein the feedforward module implements an equation, and the equation includes: Sum the converter impedance (X cnv ) and the adjustment value (X k ) to determine a third product; Multiply the third product by the power demand to determine a fourth product; Multiply the power converter voltage (V cnv ) by the grid voltage (V t ) to determine a fifth product; Divide the fourth product by the fifth product to determine a sixth product; and Determine the arcsine of the sixth product; Wherein the grid voltage indicates the voltage at the connection point between the power converter and the grid, and the power converter voltage indicates the output voltage of the power converter. Technical solution 8. The method according to any of the foregoing technical solutions further includes: The comparator (280) in the controller compares the adjustment value with a first threshold; and The adjustment module (210) is enabled or disabled based on the comparison. Technical solution 9. The method according to technical solution 8, wherein If the adjustment value is equal to or lower than the first threshold, the comparator forwards the adjustment value determined by the adjustment module to the feed - forward module, and thereby enables the adjustment module; and If the adjustment value is higher than the first threshold, the comparator updates the adjustment value to a reference value or to zero, and forwards the updated adjustment value to the feed - forward module, and thereby disables the adjustment module. Technical solution 10. The method according to technical solution 8 or 9, wherein the enabling or disabling of the adjustment module (210) is further determined based on hysteresis, such that if the adjustment value (X k ) is higher than a second threshold, the adjustment module is disabled, and if the adjustment value (X k ) is equal to or lower than a third threshold, the adjustment module is enabled. Technical solution 11. A controller (200) for providing grid - forming control of a power converter (110) connected to a power grid (130), the controller comprising: An adjustment module (210) configured to determine an adjustment value (X meas ) based on a measured power (P k ) and a feed - forward signal (δ1), wherein the measured power (P meas ) indicates the amount of power output from the power converter; and A feed - forward module (220) configured to determine the feed - forward signal (δ1) based on a power demand (P demand ) and the adjustment value (X k ), wherein the power demand indicates the demand for the amount of power to be output from the power converter; wherein the adjustment module (210) and the feed - forward module (220) form a first closed - loop control system configured to determine the feed - forward signal (δ1) such that the feed - forward signal is automatically adjusted to compensate for the difference between the power demand and the measured power during a change in the power demand; and The controller is further configured to determine a phase - angle command (δ) of the power converter based on the feed - forward signal (δ1). Technical solution 12. The controller according to technical solution 11, further comprising: A control module (230), the control module (230) being configured to determine a first control signal (240) based on the power demand, the measured power, and the measured frequency (f g ), where the measured frequency is a measure of the frequency component of the AC grid; and A first transformation module (250), the first transformation module (250) being configured to transform the feedforward signal into a transformed feedforward signal (255); wherein the controller is further configured to sum the first control signal (240) with the transformed feedforward signal (255) to output a second control signal (Δω), and determine the phase angle command (δ) based on the second control signal (Δω), such that the power converter can track changes in the power demand and power changes in the AC grid. Technical solution 13. The controller according to technical solution 11 or 12, further comprising: A comparator configured to compare the regulated value with a threshold; wherein if the regulated value is equal to or lower than the threshold, the comparator is configured to forward the regulated value determined by the regulation module to the feedforward module, and thereby enable the regulation module; and wherein if the regulated value is higher than the threshold, the comparator is configured to update the regulated value to a reference value or to zero, and forward the updated regulated value to the feedforward module, and thereby disable the regulation module. Technical solution 14. A power converter (110), comprising: A DC side (110a) for connection to a DC source (120); An AC side (110b) for connection to an AC grid (130); and A controller (200) according to any one of technical solutions 11 to 13. Technical solution 15. A computer program comprising instructions that, when executed by a processor of a controller for controlling a power converter, cause the controller to perform the method according to any one of technical solutions 1 to 10. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Embodiments of the present invention will now be described only by way of example and with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram of an example of a power converter connected between a DC source and an AC grid; Figure 2 is a schematic diagram of a controller; Figure 3is a graph showing the power demand and power output of a converter operating with and without a feedforward term; Figure 4 is a graph showing the power demand and converter power output of a strong AC system and a weak AC system; Figure 5 is a graph showing the power demand of a power converter connected to a weak AC grid and the power output from the power converter when operating according to a conventional method and the power output from the power converter when operating according to the method disclosed herein. DETAILED DESCRIPTION

[0039] Figure 1 is a schematic diagram of an example DC transmission system node 100, the node including a power converter 110 connected between a DC source 120 and an AC grid 130. This illustration is not intended to be limited to representing a specific interconnection, but is also provided as a general example to illustrate the operating principles of the interconnection useful for understanding the present invention. Thus, while specific features in the illustration are shown as being connected to each other using a specific number of connections, it will be understood that this is not intended to be restrictive either, but also illustrates the general connections between features / components. Relatedly, the relative sizes or distances between components perceived in the illustration are not intended to be restrictive either. Thus, it will be understood that the principles and features in the DC transmission system node 100 and discussed herein can be applied to an interconnection including Figure 2 the controller 200 shown in, or a power converter or network operated using the controller 200.

[0040] The DC transmission system node 100 includes a first inverter-based resource 110 (also referred to as the power converter 110). The power converter 110 is configured to convert DC power into AC power (substantially acting as an inverter), or convert AC power into DC power (acting as a rectifier). In other words, the power converter 110 can perform bidirectional power conversion. The power converter 110 can include a single converter in the case of a monopolar system, or two converters in the case of a bipolar system. The power converter 110 can represent multiple converter stations arranged as a multi-terminal power transmission system. In this example, the power converter 110 includes a DC side 110a and an AC side 110b.

[0041] The power converter 110 is connected to the DC source 120. The DC source 120 is connected to the DC side 110a of the power converter 110.

[0042] The power converter 110 is connected to the AC grid 130. The AC grid 130 is connected to the AC side 110b of the power converter 110.

[0043] The DC source 120 and / or the AC power grid 130 can be an electric power transmission system, including a power generation device, a transmission device, a power distribution device, and an electrical load. The DC source 120 can be part of an HVDC transmission system. The AC power grid 130 can be a consumer network. As a non-limiting example, for instance, the DC source 120 is part of an HVDC transmission system, where the AC power grid 130 is a consumer network.

[0044] The AC power grid 130 can be of any size and, due to operating factors, can have varying electrical characteristics. In particular, the AC power grid 130 can have an "strength" characteristic, which, from the perspective of impedance, can be defined as in CIGRE TB 68 "Guide for Planning DC Links Terminating at AC Systems Locations having Low Short-Circuit Capabilities; Part 1: ACDC Interaction Phenomena", June 1992. The "strength" of an AC system can be determined by the ratio of the AC system short-circuit capacity to the DC link power (also known as the "short-circuit ratio", SCR). A "weaker" system may have a lower ratio of AC system short-circuit capacity to DC link power, such as an SCR value less than 2. A "stronger" system may have a higher ratio of AC system short-circuit capacity to DC link power, such as an SCR value greater than 3. In other words, a strong AC system may have a low impedance amount between the DC source and the AC system, while a weak AC system may have a high impedance amount between the DC source and the AC system. When referring to weak and strong AC systems, these definitions as known to those skilled in the art are used herein.

[0045] In Figure 1 the examples disclosed, the AC power grid 130 can be a weak AC system or a strong AC system, or can change from a weak system to a strong system, or vice versa, during the operation of the AC power grid 130. The change in the strength of the AC power grid 130 may encounter the result of a change in the AC power grid 130, such as the disconnection or connection of a high-impedance load. Since there is no limit to the size of the AC power grid 130, the strength of the AC power grid 130 can and will actually change during the operation of the AC power grid 130. Therefore, it is desirable for the power converter 110 to operate in a manner that supports the AC power grid 130, regardless of whether the AC power grid 130 is a strong AC system or a weak AC system.

[0046] The operation of the DC transmission system node 100 can generally be described as follows. The DC power source 120 either supplies DC power to the power converter 110 at the DC side 110a or consumes DC power from the power converter 110 at the DC side 110a. The power converter 110 converts the received DC power into AC power for the AC grid 130, thus acting as an inverter. Alternatively, the power converter 110 converts the AC power received from the AC grid 130 into DC power to be absorbed by the DC power source 120, thus acting as a rectifier. For example, following the example where the DC power source 120 supplies power to the power converter 110, the AC power output from the power converter 110 is transmitted from the AC side 110b to the AC grid 130 for consumption.

[0047] It will be appreciated that in the exemplary DC transmission system node 100, various other electrical components can be located at any specific location or positioned with any specific feature / component. These can include switches, transformers, resistors, reactors, surge arresters, harmonic filters, and other components known in the art.

[0048] It will be appreciated that the converter or power conversion component can include a variety of different technologies, such as a voltage source converter (e.g., using insulated gate bipolar transistor (IGBT) valves). Such converters can generally be considered to use "power electronics". For example, a power electronic converter can include a multilevel voltage source converter.

[0049] It will be appreciated that the cables used as power transmission media can include the following non-limiting examples of cross-linked polyethylene (XLPE) and / or mass-impregnated (MI) insulated cables. Such cables can include a conductor (such as copper or aluminum) surrounded by an insulating layer. The dimensions of the cable and its associated layers can vary according to the specific application (and particularly, the operating voltage requirements). In applications such as subsea installations, the cable may also include reinforcement or "armouring". The cable can also include a sheath / shield grounded at one or more locations.

[0050] Furthermore, it will be understood that the DC transmission system node 100 can be used with a three-phase power system. In a three-phase power system, three conductors supply first, second, and third phase AC power to consumers respectively.

[0051] Figure 2 An embodiment of the controller 200 is shown as being available for implementing the methods described herein for controlling Figure 1 the power converter 110 shown in the grid-forming mode of operation.

[0052] As Figure 2As shown in FIG. 0, the controller 200 includes an adjustment module 210, a feedforward module 220, a control module 230, a first transformation module 250, a summing component 260, and a second transformation module 270. In addition, the controller 200 receives a power demand P demand , a measured power P meas , and a measured frequency f g , each of which is further discussed below.

[0053] The power demand P demand is the demand for the amount of power to be output from the power converter 110. The measured power P meas is the amount of power output from the power converter 110. The measured frequency f g is a measure of the frequency component of the AC grid 130. The power demand P demand , the measured power P meas , and the measured frequency f g are provided to the control module 230. The control module 230 performs processing and outputs a first control signal 240 to the summing component 260.

[0054] The power demand P demand is provided to the feedforward module 220. The feedforward module 220 receives the power demand P demand and an adjustment value X k , performs processing, and outputs a feedforward signal δ1.

[0055] The feedforward module 220 outputs the feedforward signal δ1 to the adjustment module 210. The measured power P meas is also provided to the adjustment module 210. The adjustment module 210 receives the measured power P meas and the feedforward signal δ1, performs processing, and outputs an adjustment value X k . The adjustment value X k is provided to the feedforward module 220. In this way, the feedforward module 220 and the adjustment module 210 form a first closed-loop control system 225.

[0056] The feedforward module 220 also outputs the feedforward signal δ1 to the first transformation module 250. The first transformation module 250 receives the feedforward signal δ1, performs processing, and outputs a transformed feedforward signal 255 to the summing component 260.

[0057] The summing component 260 sums the transformed feedforward signal 255 and the first control signal 240, and outputs the result as a second control signal Δω to the second transformation module 270.

[0058] The second transformation module 270 receives the second control signal Δω, performs processing, and outputs a phase angle command δ for providing grid-forming control to the power converter 110.

[0059] Although Figure 2 not shown in Figure 2 , the controller 200 may include a memory and at least one processor. The memory may include computer-readable instructions that, when executed by the at least one processor, cause the controller 200 to perform the method(s) described herein.

[0060] In addition, although Figure 2 not shown in Figure 2 , the controller 200 may also include a transceiver device. The transceiver device may include separate transmitter and receiver. The transceiver device can be used to operate communicatively with other components described herein directly or via another interface such as a network interface, using either wired or wireless components. The transceiver device may, for example, send and receive control signals using the transmitter and receiver. The control signals may contain or define electrical control parameters such as a reference current or a reference voltage.

[0061] The at least one processor may be capable of executing computer-readable instructions and / or performing logical operations. The at least one processor may be a microcontroller, a microprocessor, a central processing unit (CPU), a field-programmable gate array (FPGA), or a similar programmable controller. The controller 200 may also include a user input device and / or an output device. The processor may be communicatively coupled to the memory and to the transceiver.

[0062] The memory may be a computer-readable storage medium. For example, the memory may include a non-volatile computer storage medium. For example, the memory may include a hard disk drive, a flash memory, etc.

[0063] In addition, although Figure 2 not shown in Figure 2 , the controller 200 may additionally include a user input device interface and / or a user output device interface, which may allow visual, auditory, or tactile input / output. Examples of such user input / output devices include, but are not limited to, interfaces for touchscreens, keyboards, mice, speakers, microphones, and electronic displays.

[0064] As discussed above, in the disclosed example, the power converter 110 operates in the GFM mode. As will now be discussed, aspects of the present disclosure provide improvements when the power converter 110 operates in the GFM mode.

[0065] To implement grid-forming control, the control module 230 of the controller 200 implements a second closed-loop control system 237 that includes an integral function 235. During steady-state conditions, the power demand P demand is provided to the control module 230, and the measured power P meas closely matches or is the same as the power demand P demand

[0066] The power demand P demand ​is a demand for the amount of power to be output from the power converter 110. The measured power P meas is the amount of power output from the power converter 110.

[0067] If the voltage vector of the AC grid 130 changes due to, for example, an AC grid 130 load switching event or a generator switch, the control module 230 will detect the change in the measured power P meas and the measured frequency f g The measured frequency f g is a measure of the frequency component of the AC grid 130.

[0068] The second closed-loop control system 237 in the control module 230 determines a first control signal 240 that will maintain the internal voltage vector of the power converter 110. Thus, due to the angle difference between the power converter 110 and the AC grid 130, the active power flow between the power converter 110 and the AC grid 130 will change rapidly. The new power flow is only intended to be temporary, and thus the (synchronization grid formation) controller 200 will modify its internal voltage vector based on the new AC grid 130 frequency to restore the pre-disturbance conditions or the modified steady state, if the controller 200 incorporates a frequency droop function. The time period within which such a modification will be implemented will be a parameter defined by the integral function 235.

[0069] A limitation of the control module 230 is that it may have poor ability to track changes in the power demand P demand This problem is solved by the traditional controller by bypassing the slow inertial response of the control module 230 (which occurs as a result of the integral function 235) to directly modify the phase angle command δ of the power converter 110 as a function of the power demand P demand applied to the power converter 110. The basic concept is based on the transfer equation shown below as Equation 1.

[0070] Where: P demand is the demand for the amount of power to be output from the power converter 110; V cnv is the output voltage of the power converter 110; V thev is the Thévenin equivalent voltage of the AC grid 130; θ cnv is the angle of the output voltage of the power converter 110; θ thev is the angle of the Thévenin equivalent AC grid 130 voltage; X cnv is the impedance of the power converter 130; and X thev is the Thévenin equivalent impedance of the AC grid 130.

[0071] However, in practice, the Thévenin equivalent of the AC grid 130 may be unknown. To overcome this, an assumption can be made that the value of X thev can be neglected (as discussed further below). This essentially allows Equation 1 to be simplified to the equation shown in Equation 2.

[0072] where: V t is the voltage at the connection point of the power converter 110 to the AC grid 130; and θ t is the voltage angle at the connection point of the power converter to the AC grid 130.

[0073] Equation 2 tends to be more practical because the quantities V t and θ t can be directly measured at the power converter 110. If it is assumed that both V cnv and V t are equal to 1.0 pu under steady-state conditions, further simplification can be made.

[0074] As Figure 2 shown, the feed-forward module 220 and the resulting feed-forward signal δ1 are thus implementations based on the feed-forward terms of the rearranged Equation 2. Additionally, further simplification can be made by noting that typically the AC grid 130 voltage will be close to 1.0 pu. Thus, if desired, for example to improve the calculation time, it can be assumed that V t and V cnv are equal to 1.0.

[0075] Figure 3 shows a first curve 301 of the power demand P demand of the power converter 110, a second curve 302 of the measured power P meas output from the power converter 110 with the feed-forward module 220 active, and a third curve 303 of the measured power P meas output from the power converter 110 with the feed-forward module 220 inactive. In Figure 3 , the x-axis 320 is time in seconds and the y-axis 310 is power in per unit. As can be seen, the power converter 110 controlled with the feed-forward module 220 tends to more actively track changes in the power demand P demand than the power converter 110 controlled with the feed-forward module 220 inactive.

[0076] Furthermore, the feed-forward module 220 and the resulting feed-forward signal δ1 can also be used after a fault clearance on the AC grid 130 (i.e., to resume the power demand P demand(The time taken) accelerates the recovery of the power converter 110.

[0077] The feed - forward module 220 outputs a feed - forward signal δ1 to the first transformation module 250. The first transformation module 250 applies a transformation, such as a Laplace transform, to the feed - forward signal δ1. This transformation converts the feed - forward signal δ1 from an angle to a rotational speed, and the output is the transformed feed - forward signal 255. The first transformation module 250 provides the transformed feed - forward signal 255 to the summing component 260.

[0078] The summing component 260 sums the first control signal 240 and the transformed feed - forward signal 255. By summing the first control signal 240 and the transformed feed - forward signal 255, in this way, a signal with a larger magnitude can and will actually dominate the phase - angle command δ. This tends to allow the controller 200 to track changes in power demand and power changes in the power grid.

[0079] In a "strong" AC system (as discussed above), the value of X in Equation 1 thev is small, and thus the assumption introduced by Equation 2 (ignoring X thev ) is generally correct. As such, in a strong AC system, ignoring X thev does not tend to significantly change or affect the resulting ability of the power converter 110 to track the ramp of the power demand P demand when using the feed - forward module 220. However, in applications where the AC system strength is low or weak (as discussed above), the value of X thev is not negligible. In this case, the feed - forward module 220 and the resulting feed - forward signal δ1 may not be able to effectively track the ramp of the power demand P demand . This is schematically shown in Figure 4 , Figure 4 showing a fourth curve 404 of the power demand P demand provided to the power converter 110, a fifth curve 405 of the measured power P meas output from the power converter 110 (with the feed - forward module 220 active) for a weak AC system, and a sixth curve 406 of the measured power P meas output from the power converter 110 (with the feed - forward module 220 active) for a strong AC system. In Figure 4 , the x - axis 420 is time in seconds, and the y - axis 410 is power in per - unit. As can be seen, compared to a strong AC system, there are more overshoots and resonances in a weak AC system. Thus, for a weak AC system, the feed - forward module 220 and the resulting feed - forward signal δ1 cannot effectively track the ramp or changes in the power demand P demand .

[0080] The object of the present disclosure is to overcome the aforementioned problems, or in other words, to provide improvements in ramping the power demand P demand in a ramp.

[0081] Referring again to Figure 2 , the controller 200 includes an adjustment module 210. The general operating principle of the adjustment module 210 is that the adjustment module 210 corrects the error between the expected power demand P demand and the measured power P meas to improve the active power ramping tracking.

[0082] To achieve this, the adjustment module 210 in the controller 200 uses the measured power P meas , and thereby, by operating Equation 2, the adjustment value X k can be derived as shown in Equation 3.

[0083] The adjustment module 210 thus implements Equation 3 rearranged to solve for the adjustment value X k . However, as can be seen, in order to fully implement Equation 3, the value of the feed-forward signal δ1 must be determined. The adjustment module 210 thus receives the value of the feed-forward signal δ1 from the feed-forward module 220.

[0084] Although in this example, the adjustment module 210 implements the rearranged Equation 3, other formulas can also be used in other embodiments. For example, the option of the formula can be selected based on a specific type of application.

[0085] To configure the power converter 110 with the feed-forward signal δ1 to effectively track the ramp of the power demand P demand , the feed-forward module 220 must also be updated to take into account the adjustment value X k .

[0086] Then, the adaptation of Equation 2 can be used to incorporate the result from Equation 3, as shown in Equation 4.

[0087] The feed-forward module 220 of the present disclosure implements Equation 4 rearranged to solve for the feed-forward signal δ1. And as can be seen, the adjustment value X k is required to fully solve this equation. The adjustment value X k is provided by the adjustment module 210 to the feed-forward module 220. Thus, in this way, the adjustment module 210 and the feed-forward module 220 form a first closed-loop control system 225. The feed-forward signal δ1 is thus automatically adjusted to compensate for the difference between the power demand and the measured power during the change of the power demand.

[0088] Under startup conditions, the adjustment value X can be used k and the reference value of the feedforward signal δ1.

[0089] Although in this example, the feedforward module 220 implements the rearranged equation 4, other formulas can also be used in other embodiments. For example, the options of the formula can be selected based on a specific type of application.

[0090] Figure 5 The seventh curve 507 of the power demand P provided to the power converter 110 connected to the weak grid is shown demand , the measured power P in the case where the regulation module 210 is enabled to provide the adjustment value X k , the eighth curve 508 of the measured power P meas and the ninth curve 509 of the measured power P in the case where the regulation module 210 is disabled. In meas , the x-axis 520 is time in seconds, and the y-axis 510 is power in per unit. As can be seen, when the regulation module 210 is enabled, the measured power P Figure 5 tracks the power demand P significantly better compared to when the regulation module 210 is disabled meas . demand

[0091] Because the regulation module 210 and the feedforward module 220 form a closed-loop control system to determine the feedforward signal δ1, the phase angle command δ provided to the power converter 110 tends to improve the ability of the power converter 110 to output power closely and accurately track the power demand P demand (the measured power P meas ). In other words, when operating in the grid-forming mode, the power converter 110 is more effective in providing the desired output power.

[0092] This also tends to improve the ability of the power converter 110 and the controller 200 to recover from faults.

[0093] However, if the AC system is too weak and the power converter 110 recovers active power too quickly, the regulation module 210 may cause further problems. Therefore, in an embodiment of the present disclosure, if the adjustment value X k will cause the power converter 110 to operate in a manner that will have an undesirable impact on the AC grid 130, the regulation module 210 can be automatically disabled.

[0094] To achieve this, a comparator 280 can be included in the controller 200. The comparator 280 is connected to the output of the regulation module 210 at a point between the regulation module 210 and the feedforward module 220. As a result, the comparator 280 is capable of modifying the adjustment value X k if necessary.​

[0095] Comparator 280 receives the adjustment value X k , and compares the adjustment value X k with a threshold value and outputs the result to the feedforward module 220. If the adjustment value is equal to or lower than the threshold value, comparator 280 forwards the adjustment value X k determined by the adjustment module 210 to the feedforward module 220 and thereby enables the adjustment module 210. However, if the adjustment value X k is higher than the threshold value, comparator 280 updates the adjustment value X k to a reference value or updates it to zero, and forwards the updated adjustment value to the feedforward module 220 and thereby disables the adjustment module 210. In this way, the adjustment module 210 can be enabled or disabled.

[0096] In another embodiment, the enabling or disabling of the adjustment module 210 is also determined based on hysteresis, such that if the adjustment value X k is higher than a second threshold value, the adjustment module 210 is disabled, while if the adjustment value X k is equal to or lower than a third threshold value, the adjustment module 210 is enabled.

[0097] Although the embodiments described herein disclose the DC source 120 and the AC power grid 130, it is to be understood that these are merely examples of the interconnection between a DC system and an AC system.

[0098] Throughout this specification, references to examples of a particular method or apparatus or similar language mean that the particular features, structures, or characteristics described in connection with that example are included in at least one implementation of the methods and apparatuses described herein. The terms "comprises," "comprising," "has," and their variants mean "including but not limited to" unless expressly specified otherwise. A list of enumerated items does not imply that any or all of the items are mutually exclusive unless expressly specified otherwise. The terms "a," "an," and "the" also refer to "one or more" unless expressly specified otherwise.

[0099] As used herein, a list with the conjunction "and / or" includes any single item in the list or a combination of items in the list. For example, the list of A, B, and / or C includes only A, only B, only C, the combination of A and B, the combination of B and C, the combination of A and C, or the combination of A, B, and C. As used herein, a list using the term "one or more of..." includes any single item in the list or a combination of items in the list. For example, one or more of A, B, and C includes only A, only B, only C, the combination of A and B, the combination of B and C, the combination of A and C, or the combination of A, B, and C. As used herein, a list using the term "one of..." includes one and only one of any single item in the list. For example, "one of A, B, and C" includes only A, only B, or only C, and does not include the combination of A, B, and C. As used herein, "a member selected from the group consisting of A, B, and C" includes one and only one of A, B, or C, and does not include the combination of A, B, and C. As used herein, "a member selected from the group consisting of A, B, and C and combinations thereof" includes only A, only B, only C, the combination of A and B, the combination of B and C, the combination of A and C, or the combination of A, B, and C.

[0100] Aspects of the disclosed methods and devices are described with reference to schematic flowcharts and / or schematic block diagrams of methods, devices, systems, and program products. It will be understood that each block of the schematic flowcharts and / or schematic block diagrams, and combinations of blocks in the schematic flowcharts and / or schematic block diagrams, can be implemented by code. This code can be provided to a processor of a general purpose computer, a special purpose computer, or other programmable data processing device to produce a machine such that the instructions executed by the processor of the computer or other programmable data processing device create means for implementing the functions / actions specified in the schematic flowcharts and / or schematic block diagrams.

[0101] The schematic flowcharts and / or schematic block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of devices, systems, methods, and program products. In this regard, each block in the schematic flowcharts and / or schematic block diagrams can represent a portion, segment, or module of code that includes one or more executable instructions for implementing the specified logical function(s).

[0102] It will be appreciated that the numerical values recited herein are merely intended to assist in illustrating the operation of the invention and can vary depending on the requirements of a given power transmission network, its components, or power transmission application.

[0103] The listing or discussion in this specification of a document or information that is clearly prior art or clearly prior published information should not necessarily be taken as an admission that the document or information is part of the prior art or common general knowledge.

[0104] Unless the context otherwise indicates, preferences and options for a given aspect, feature, or parameter of the present invention should be considered to have been disclosed in combination with any and all preferences and options for all other aspects, features, and parameters of the present invention.

[0105] The disclosure herein also provides a converter controller operating in a synchronous grid-forming mode, that is, the controller instantaneously controls the AC voltage vector of the converter in terms of phase and magnitude, independent of the phase and magnitude of the AC system as measured at the connection point of the converter to the AC system. The converter automatically adjusts its voltage phase and magnitude to compensate for the difference between the user demand value (either active power or DC voltage) and the actual measured value, thereby giving an inertial response defined in a way that provides attenuation of demand value violations, and wherein the attenuation time is a user-defined parameter.

[0106] This converter also includes a feedforward term that modifies the inertial response of the converter to better track changes in the actual demand rather than sudden network events.

[0107] In addition, the feedforward term can automatically adjust the changes it makes to the converter voltage phase to compensate for the difference between the expected and measured values at any moment during a change in the demand value.

[0108] In a further enhancement, the correction factor can be automatically disabled based on the magnitude of the correction factor.

[0109] In a further enhancement, the correction factor can be automatically enabled based on the magnitude of the correction factor.

Claims

1. A method of operating a controller (200) for providing grid forming control of a power converter (110) connected to a grid (130), the method comprising: The regulation module (210) of the controller is based on the measured power (P meas ) and the feedforward signal (δ1) determine the adjustment value (X k ), where the measured power (P meas ) indicates the amount of power output from the power converter (110); The controller's feedforward module (220) generates a power demand (P demand ) and the adjustment value (X k ) determines the feedforward signal (δ1), wherein the power demand (P demand ) indicating a demand for an amount of power to be output from said power converter; wherein the adjustment module (210) and the feedforward module (220) form a first closed-loop control system to determine the feedforward signal (δ1), so that the feedforward signal is automatically adjusted to compensate for the difference between the power demand and the measured power during the power demand change; as well as A phase angle command (δ) of the power converter (110) is determined by the controller based on the feedforward signal (δ1).

2. The method according to claim 1, further comprising: The control module (230) in the controller determines the power demand, the measured power and the measured frequency (f g ) to determine a first control signal (240), wherein the measured frequency is a measure of a frequency component of the AC power grid; as well as The phase angle command (δ) of the power converter is determined by the controller based on the feedforward signal (δ1) and the first control signal (240).

3. The method according to claim 2, further comprising: The first transformation module (250) of the controller transforms the feedforward signal (δ1) and outputs the transformed feedforward signal (255); The controller's summing component (260) sums the first control signal (240) and the transformed feedforward signal (255) and outputs a second control signal (Δω); as well as The phase angle command (δ) is determined by the controller based on the second control signal (Δω) so that the power converter can track changes in the power demand and power changes in the AC grid.

4. The method according to claim 3, further comprising: The second transformation module (270) of the controller determines the phase angle command (δ) by transforming the second control signal (Δω), and outputs the phase angle command (δ) to the power converter.

5. The method according to claim 2, 3 or 4, wherein: The control module (230) includes a second closed-loop control system (237) including an integral term.

6. A method according to any preceding claim, wherein: The adjustment module implements an equation, which includes: The grid voltage (V t ) multiplied by the power converter voltage (V cnv ) and the sine of the feedforward signal (δ1) to determine a first product, and then dividing the first product by the measured power; The grid voltage indicates a voltage at a connection point between the power converter and the grid, and the power converter voltage indicates an output voltage of the power converter.

7. A method according to any preceding claim, wherein: The feedforward module implements an equation, which includes: The converter impedance (X cnv ) and the adjustment value (X k ) are summed to determine the third product; multiplying the third product by the power demand to determine a fourth product; The power converter voltage (V cnv ) multiplied by the grid voltage (V t ) to determine the fifth product; dividing the fourth product by the fifth product to determine a sixth product; and determining the arc sine of the sixth product; The grid voltage indicates a voltage at a connection point between the power converter and the grid, and the power converter voltage indicates an output voltage of the power converter.

8. The method according to any preceding claim, further comprising: The comparator (280) in the controller compares the adjustment value with a first threshold value; as well as The regulation module (210) is enabled or disabled based on the comparison.

9. The method according to claim 8, wherein If the regulation value is equal to or below the first threshold, the comparator forwards the regulation value determined by the regulation module to the feedforward module and thereby enables the regulation module; and If the regulation value is above the first threshold value, the comparator updates the regulation value to a reference value or to zero and forwards the updated regulation value to the feedforward module and thereby disables the regulation module.

10. The method according to claim 8 or 9, wherein: The enabling or disabling of the adjustment module (210) is further determined based on hysteresis, such that if the adjustment value (X k ) is higher than a second threshold, the regulation module is disabled, and if the regulation value (X k ) is equal to or lower than a third threshold, the adjustment module is enabled.

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