Double frequency harmonic suppression method, device and equipment for energy storage participating flexible direct current grid-connected system

Abstract

The invention discloses a second harmonic generation harmonic suppression method, device and equipment for an energy storage participation flexible direct current grid-connected system, and the method comprises the steps: obtaining a grid-connected end electric parameter of an energy storage inverter and a combined output current of a grid-connected point in real time when an asymmetric fault occurs in a wind power flexible direct current grid-connected system; measuring a vector phase corresponding to the grid-connected end electric parameter, and performing coordinate transformation on the grid-connected end electric parameter and the combined output current according to the vector phase to obtain a dq-axis electric parameter and a negative sequence current q-axis component; generating a negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value; generating a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electric parameter; according to the negative-sequence current reference value and the positive-sequence current reference value, current inner loop control is carried out on the dq-axis electric parameters, a pulse control signal is generated and output to an energy storage inverter, so that frequency doubling harmonic waves generated by the wind power flexible direct current grid-connected system are restrained, and the control stability of the wind power flexible direct current grid-connected system is effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of harmonic suppression, and particularly to a method, device and equipment for suppressing second-harmonic in a flexible DC grid-connected system with energy storage participation. Background Art

[0002] Offshore wind power is clean, efficient and does not occupy land resources. The voltage source converter based high voltage direct current transmission (VSC-HVDC) technology can meet the voltage support and reactive power compensation requirements for long-distance transmission of offshore wind power, and has become the preferred solution for offshore wind power grid connection.

[0003] However, when the flexible DC transmission technology experiences an asymmetric fault in the receiving-end power grid, the negative-sequence component causes DC voltage fluctuations and power oscillations through the converter station. When an asymmetric fault occurs in the AC-DC system, a second-harmonic will be generated in the DC voltage, which will affect the AC system of the converter station and generate a third-harmonic AC voltage. Conversely, the third-harmonic in the AC system will generate a fourth-harmonic in the DC-side voltage, affecting the system stability.

[0004] In response to this, in the existing flexible DC transmission system, the negative-sequence component suppression strategies for asymmetric faults in the receiving-end power grid of the flexible DC transmission system are mainly divided into three categories: positive-negative sequence decoupling strategies based on double-loop control, power compensation control strategies based on second-harmonic power suppression, and voltage compensation type suppression strategies. However, the above suppression strategies mainly suppress the negative-sequence current by changing the flexible DC transmission system, and do not block the propagation path of the negative-sequence harmonic current from the source, resulting in low control stability. Summary of the Invention

[0005] The present invention provides a method, device and equipment for suppressing second-harmonic in a flexible DC grid-connected system with energy storage participation, which solves the technical problem that the existing negative-sequence component suppression strategies mainly suppress the negative-sequence current by changing the flexible DC transmission system, and do not block the propagation path of the negative-sequence harmonic current from the source, resulting in low control stability.

[0006] A method for suppressing second-harmonic in a flexible DC grid-connected system with energy storage participation provided by the first aspect of the present invention involves a wind power flexible DC grid-connected system and an energy storage inverter connected at the grid connection point. The method includes:

[0007] When an asymmetric fault occurs in the wind power flexible DC grid-connected system, the electrical parameters at the grid connection end of the energy storage inverter and the combined output current at the grid connection point are acquired in real time;

[0008] Measure the vector phase corresponding to the grid-connected terminal electrical parameters, and perform coordinate transformation on the grid-connected terminal electrical parameters and the combined output current according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current;

[0009] Generate a negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value;

[0010] Generate a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters;

[0011] Perform current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generate a pulse control signal to be output to the energy storage inverter to suppress the second-harmonic generated by the flexible HVDC grid-connected wind power system.

[0012] Optionally, the grid-connected terminal electrical parameters include the grid-connected terminal voltage and the grid-connected terminal current, and the dq-axis electrical parameters include the dq-axis voltage and the dq-axis current; the measurement of the vector phase corresponding to the grid-connected terminal electrical parameters, and the coordinate transformation of the grid-connected terminal electrical parameters and the combined output current according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current includes:

[0013] Measure the vector phase corresponding to the grid-connected terminal voltage;

[0014] Convert the grid-connected terminal voltage from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis voltage;

[0015] Convert the grid-connected terminal current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis current;

[0016] Convert the combined output current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the combined output dq-axis current;

[0017] Separate the positive and negative sequences of the combined output q-axis current in the combined output dq-axis current, and extract the q-axis component of the negative-sequence current.

[0018] Optionally, the generating of the negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value includes:

[0019] Calculate the difference current between the q-axis negative-sequence reference value and the q-axis component of the negative-sequence current;

[0020] Perform proportional operation and integral operation on the difference current through a proportional-integral controller to generate the q-axis reference component of the negative-sequence current;

[0021] Use the negative - sequence current q - axis reference component and the preset negative - sequence current d - axis reference component as the negative - sequence current reference value.

[0022] Optionally, the positive - sequence power reference value includes a positive - sequence active - power reference value and a positive - sequence reactive - power reference value, and the positive - sequence current reference value includes a positive - sequence current d - axis reference component and a positive - sequence current q - axis reference component; generating the positive - sequence current reference value according to the positive - sequence power reference value and the dq - axis electrical parameters includes:

[0023] Use the dq - axis voltage and the dq - axis current to calculate the actual active power and the actual reactive power;

[0024] Calculate the active - power deviation between the positive - sequence active - power reference value and the actual active power;

[0025] Perform proportional - integral regulation on the active - power deviation according to the active - power loop to obtain the active - power regulation voltage;

[0026] Determine the positive - sequence current d - axis reference component according to the d - axis voltage among the positive - sequence active - power reference value, the active - power regulation voltage, and the dq - axis voltage;

[0027] Calculate the reactive - power deviation between the positive - sequence reactive - power reference value and the actual reactive power;

[0028] Perform proportional - integral regulation on the reactive - power deviation according to the reactive - power loop to obtain the reactive - power regulation voltage;

[0029] Determine the positive - sequence current q - axis reference component according to the q - axis voltage among the positive - sequence reactive - power reference value, the reactive - power regulation voltage, and the dq - axis voltage.

[0030] Optionally, the current inner - loop control of the dq - axis electrical parameters according to the negative - sequence current reference value and the positive - sequence current reference value to generate a pulse control signal and output it to the energy - storage inverter includes:

[0031] Separate the positive and negative sequences of the dq - axis current to obtain the positive - sequence dq - axis actual current and the negative - sequence dq - axis actual current;

[0032] Perform current inner - loop control on the negative - sequence dq - axis actual current according to the negative - sequence current reference value to determine the negative - sequence modulation reference voltage;

[0033] Perform current inner - loop control on the positive - sequence dq - axis actual current according to the positive - sequence current reference value to determine the positive - sequence modulation reference voltage;

[0034] Combine the negative - sequence modulation reference voltage and the positive - sequence modulation reference voltage according to the d - axis and q - axis respectively to obtain the d - axis modulation reference voltage and the q - axis modulation reference voltage;

[0035] Perform pulse-width modulation using the d-axis modulation reference voltage and the q-axis modulation reference voltage, and generate a pulse control signal to be output to the energy storage inverter.

[0036] Optionally, before performing the step of performing current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generating a pulse control signal to be output to the energy storage inverter, the method further includes:

[0037] Calculate the Euclidean norm between the negative-sequence current q-axis reference component, the positive-sequence current q-axis reference component, and the positive-sequence current d-axis reference component;

[0038] Determine whether the Euclidean norm is less than or equal to the current amplitude upper limit value;

[0039] If so, retain the negative-sequence current q-axis reference component, the positive-sequence current q-axis reference component, and the positive-sequence current d-axis reference component at the current moment;

[0040] If not, calculate a new positive-sequence current d-axis reference component using the current amplitude upper limit value, the negative-sequence current q-axis reference component, and the positive-sequence current q-axis reference component;

[0041] If the new positive-sequence current d-axis reference component is zero, calculate a new positive-sequence current q-axis reference component using the current amplitude upper limit value and the negative-sequence current q-axis reference component.

[0042] Optionally, the method further includes:

[0043] When the energy storage inverter has idle capacity, update the positive-sequence power reference value respectively according to a preset power update formula to obtain a new positive-sequence power reference value;

[0044] The power update formula includes:

[0045]

[0046] Wherein, is the positive-sequence reactive power reference value, is the positive-sequence active power reference value, is the total rated power during an asymmetrical fault, is the d-axis component of the negative-sequence voltage output by the energy storage inverter, is the negative-sequence current q-axis reference component.

[0047] Optionally, the method further includes:

[0048] If the grid-connected terminal voltage is greater than or equal to the over-limit voltage threshold, the positive-sequence current d-axis reference component and the positive-sequence current q-axis reference component are sequentially reduced to zero.

[0049] The second aspect of the present invention provides a device for suppressing second-harmonic in a flexible DC grid-connected system with energy storage participation, which relates to a flexible DC grid-connected system of wind power and an energy storage inverter connected at the grid connection point. The device includes:

[0050] An electrical parameter acquisition module, configured to, when an asymmetric fault occurs in the flexible DC grid-connected system of wind power, acquire the grid-connected terminal electrical parameters of the energy storage inverter and the combined output current of the grid connection point in real time;

[0051] An electrical parameter conversion module, configured to measure the vector phase corresponding to the grid-connected terminal electrical parameters, and perform coordinate conversion on the grid-connected terminal electrical parameters and the combined output current respectively according to the vector phase to obtain dq-axis electrical parameters and the negative-sequence current q-axis component;

[0052] A negative-sequence current reference value determination module, configured to generate a negative-sequence current reference value according to the negative-sequence current q-axis component and the q-axis negative-sequence reference value;

[0053] A positive-sequence current reference value determination module, configured to generate a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters;

[0054] A control signal generation module, configured to perform current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generate a pulse control signal to be output to the energy storage inverter to suppress the second-harmonic generated by the flexible DC grid-connected system of wind power.

[0055] The third aspect of the present invention provides an electronic device, including a memory and a processor. When the computer program stored in the memory is executed by the processor, the processor is caused to execute the steps of the method for suppressing second-harmonic in a flexible DC grid-connected system with energy storage participation according to any one of the first aspect of the present invention.

[0056] From the above technical solutions, it can be seen that the present invention has the following advantages:

[0057] When an asymmetrical fault occurs in the flexible HVDC grid-connected wind power system, the electrical parameters at the grid-connected end of the energy storage inverter and the combined output current at the grid connection point are obtained in real time; the vector phase corresponding to the electrical parameters at the grid-connected end is measured, and the electrical parameters at the grid-connected end and the combined output current are respectively subjected to coordinate transformation according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current; according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value, a negative-sequence current reference value is generated; according to the positive-sequence power reference value and the dq-axis electrical parameters, a positive-sequence current reference value is generated; according to the negative-sequence current reference value and the positive-sequence current reference value, current inner-loop control is performed on the dq-axis electrical parameters, and a pulse control signal is generated and output to the energy storage inverter to suppress the double-frequency harmonics generated by the flexible HVDC grid-connected wind power system, thereby blocking the propagation path of the negative-sequence harmonic current from the source and effectively improving the control stability of the flexible HVDC grid-connected wind power system. Description of the Drawings

[0058] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0059] Figure 1 It is a flowchart of the steps of a method for suppressing double-frequency harmonics in a flexible HVDC grid-connected system with energy storage participation provided by an embodiment of the present invention;

[0060] Figure 2 It is a schematic diagram of the topology and control structure of a flexible HVDC grid-connected wind power system provided by an embodiment of the present invention;

[0061] Figure 3 It is a schematic diagram of a detailed model of a VSC side converter station provided by an embodiment of the present invention;

[0062] Figure 4 It is a schematic diagram of the control flow of a method for suppressing double-frequency harmonics in a flexible HVDC grid-connected system with energy storage participation provided in an embodiment of the present invention;

[0063] Figure 5 It is a schematic diagram of the change in the positive and negative sequence voltage amplitudes of the receiving-end AC bus provided by an embodiment of the present invention;

[0064] Figure 6 It is a schematic diagram of the DC bus current and the DC bus voltage harmonics in the case of using the method of this embodiment during a fault provided by an embodiment of the present invention;

[0065] Figure 7 It is a schematic diagram of the DC bus current and the DC bus voltage harmonics in the case of not using the method of this embodiment during a fault provided by an embodiment of the present invention;

[0066] Figure 8 This is the structural block diagram of a second-harmonic suppression device for a flexible DC grid-connected system with energy storage participation provided by an embodiment of the present invention. Detailed implementation manners

[0067] When an asymmetric fault occurs in the receiving-end power grid, negative-sequence current will generate DC voltage fluctuations and power oscillations through the converter station, thereby inducing the stability of the AC-DC system. Given that there are commonalities between the receiving-end power grid faults and the corresponding faults in the flexible DC system when offshore wind power is transmitted through the VSC-HVDC system, they can learn from each other. Existing negative-sequence component suppression strategies for asymmetric faults in the receiving-end power grid of flexible DC transmission systems are mainly divided into the following three categories: (1) Positive and negative sequence decoupling strategy based on double-loop control; (2) Power compensation control strategy based on second-harmonic power suppression; (3) Voltage compensation type suppression strategy. The first type of method realizes the basic control function by constructing positive and negative sequence independent control loops, but has the defect of insufficient harmonic suppression ability. Especially when a fault occurs on the inverter side, the second-harmonic generated on the DC side will affect the operation of the AC system in the reverse direction. If the harmonic suppression ability is weak, the normal operation of the system will be affected. The second type of method realizes harmonic suppression through power balance control, but requires the configuration of notch filters, resulting in an increase in the complexity of the control system. The third type of method can effectively limit the negative-sequence current, but its complex vector phase calculation affects the control real-time performance. The above research mainly suppresses the negative-sequence current by changing the flexible DC transmission system, but fails to block the propagation path of negative-sequence harmonic current to the flexible DC system from the source.

[0068] Therefore, the embodiment of the present invention combines the four-quadrant operation characteristics of the energy storage device, can effectively control the amplitude of its output current and the vector angle with the power grid, and can effectively suppress negative-sequence current under faults, and provides a second-harmonic suppression method, device and equipment for a flexible DC grid-connected system with energy storage participation, which is used to solve the technical problem that the existing negative-sequence component suppression strategy mainly suppresses the negative-sequence current by changing the flexible DC transmission system, does not block the propagation path of negative-sequence harmonic current from the source, and has low control stability.

[0069] The wind power flexible DC grid-connected system refers to a system that includes a wind power generation device and a flexible DC transmission device and is used to integrate the electric energy generated by wind power into the power grid in a flexible DC manner. Its topology and control structure are as Figure 2 shown.

[0070] The offshore wind farm consists of multiple permanent magnetic synchronous generators (PMSG), which generate 50Hz electricity that is first connected to the wind farm side VSC (WFVSC), and then connected to the onshore power grid through the grid side VSC (GSVSC) in the point-to-point flexible direct current system. The VF control strategy is adopted at the sending end converter station WFVSC.

[0071] like Figure 3 As shown, a detailed model of the converter station on the VSC side is given. When the system is in steady state, the mathematical model of the topology shown is:

[0072]

[0073] Where: and are the DC current and DC voltage of GSVSC respectively; It is the component of the three-phase bridge arm current fed into the DC side; , , is the three-phase current flowing from the AC grid to the GSVSC; and are the converter station exit resistance and inductance respectively; is the voltage at the commutation bus; , , They are the three-phase voltages output by GSVSC; , , They are the switching functions of the three-phase bridge arms of GSVSC.

[0074] Normally, the transformer connected to the GSVSC is of YNd type, so the zero-sequence current will not pass through the VSC converter station, and only the positive and negative sequence components of the fault current need to be considered. At this time, the three-phase short-circuit current can be expressed as:

[0075]

[0076] Where: and are the amplitudes of positive sequence current and negative sequence current respectively; and are the phase angles of the positive sequence current and the negative sequence current respectively.

[0077] Assuming that at the moment when the AC system of the VSC converter station is short-circuited, the switching functions of the three-phase bridge arms in the converter station remain symmetrical, the following relationship exists:

[0078]

[0079] Wherein: is the amplitude of the three-phase switching function, is the phase angle of the switching function.

[0080] At this time, the component of the current flowing into the DC side is:

[0081]

[0082] It can be seen from the above formula that when an asymmetric fault occurs in the AC system, a second-harmonic negative-sequence component will be generated in the DC current of the three-phase bridge arm, and then the DC side voltage will also contain second-harmonic components. At this time, the DC side voltage after the fault can be defined as:

[0083]

[0084] Wherein: is the amplitude of the second-harmonic voltage; is the phase angle of the second-harmonic voltage.

[0085] There is the following relationship between the output voltage of the VSC converter station and the DC side voltage, ( ). Taking phase A as an example, the voltage of phase A output by the VSC converter station after the fault is:

[0086]

[0087] It can be seen from the above formula that the second-harmonic component of the DC voltage will generate a third-harmonic AC voltage harmonic in the AC system of the converter station. Conversely, the third-harmonic component of the AC system will generate a fourth-harmonic component in the DC side voltage. Therefore, it is particularly important to propose corresponding control strategies to suppress the negative-sequence component.

[0088] In order to make the object, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0089] Please refer to Figure 1 , Figure 1 which is the step flow chart of a method for suppressing second-harmonic components of a flexible DC grid-connected system with energy storage participation provided by an embodiment of the present invention.

[0090] A method for suppressing second - harmonic in a flexible DC grid - connected system with energy storage involved provided by the present invention relates to a wind power flexible DC grid - connected system and an energy storage inverter connected at the grid connection point. The method includes:

[0091] Step 101, when an asymmetric fault occurs in the wind power flexible DC grid - connected system, obtain in real - time the grid - connected terminal electrical parameters of the energy storage inverter and the combined output current at the grid connection point;

[0092] An asymmetric fault refers to a fault that causes asymmetry in the three - phase parameters in the power system, such as single - phase ground short - circuit, two - phase short - circuit, etc., which will lead to unbalance of the three - phase current and voltage in the system.

[0093] An energy storage inverter refers to a power electronic device that connects an energy storage device to the power grid. It converts the direct current of the energy storage device into alternating current and injects it into the power grid at the grid connection point, and can also achieve the reverse conversion.

[0094] The grid - connected terminal electrical parameters refer to electrical parameters such as the grid - connected terminal current and grid - connected terminal voltage that are output by the energy storage inverter and flow into the grid connection point.

[0095] The combined output current refers to the three - phase current flowing from the grid connection point into the power grid, including amplitude and phase information.

[0096] In an embodiment of the present invention, voltage and current sensors can be installed at the grid - connected terminal of the energy storage inverter to collect the grid - connected terminal electrical parameters in real - time. At the same time, a current sensor capable of measuring the combined output current is installed at the grid connection point. In addition, by continuously monitoring the operating state of the wind power flexible DC grid - connected system, by comparing the amplitude and phase relationships of the three - phase voltage or current, it is judged whether an asymmetric fault occurs. For example, when the amplitude of a certain phase voltage is significantly lower than the other two phases and the phase difference does not conform to the normal operating range, it is determined that an asymmetric fault has occurred.

[0097] When it is detected that an asymmetric fault occurs in the wind power flexible DC grid - connected system, at this time, the grid - connected terminal electrical parameters of the energy storage inverter are obtained, including the grid - connected terminal voltage and grid - connected terminal current in three - phase form, and the combined output current at the grid connection point, so as to obtain the data basis for subsequent second - harmonic suppression.

[0098] Step 102, measure the vector phase corresponding to the grid - connected terminal electrical parameters, and perform coordinate transformation on the grid - connected terminal electrical parameters and the combined output current respectively according to the vector phase to obtain the dq - axis electrical parameters and the q - axis component of the negative - sequence current;

[0099] In this embodiment, the phase-locked loop technology is used to measure the vector phase corresponding to the grid-connected end electrical parameters. The phase-locked loop outputs a signal with the same frequency and phase as the input signal by tracking the phase of the input signal, so as to obtain an accurate vector phase. Using this vector phase, coordinate transformation is performed on the grid-connected end electrical parameters and the combined output current, so as to transform the grid-connected end electrical parameters and the combined output current from the three-phase stationary coordinate system to the synchronous rotating coordinate system through the Park transformation, obtain the dq-axis electrical parameters, and extract the negative sequence current q-axis component from the q-axis current obtained by coordinate transformation of the combined output current.

[0100] In an example of the present invention, the grid-connected end electrical parameters include the grid-connected end voltage and the grid-connected end current, and the dq-axis electrical parameters include the dq-axis voltage and the dq-axis current; step 102 may include the following sub-steps:

[0101] Measure the vector phase corresponding to the grid-connected end voltage;

[0102] Convert the grid-connected end voltage from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis voltage;

[0103] Convert the grid-connected end current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis current;

[0104] Convert the combined output current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the combined output dq-axis current;

[0105] Separate the positive and negative sequences of the combined output q-axis current in the combined output dq-axis current, and extract the negative sequence current q-axis component.

[0106] In this embodiment, after obtaining the grid-connected end voltage and the grid-connected end current, the vector phase of the grid-connected end voltage is detected by a phase-locked loop PLL for energy storage control, and the transformation of each control quantity from the three-phase stationary coordinate system to the synchronous rotating coordinate system is realized with this vector phase as a reference.

[0107] Specifically, after measuring the vector phase through the phase-locked loop then, the voltage components in the three-phase stationary coordinate system are transformed to the synchronous rotating coordinate system by using the Park transformation to obtain the dq-axis voltage including the d-axis voltage and the q-axis voltage:

[0108]

[0109] Similarly, the grid-connected end current is transformed from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase by using the Park transformation to obtain the dq-axis current including the d-axis current and the q-axis current:

[0110]

[0111] Similarly, the combined output current is transformed from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase by Park transformation. Further, the positive and negative sequence separation of the combined output dq-axis current is performed by using the symmetrical component transformation matrix, and the combined output q-axis current is extracted therefrom. :

[0112]

[0113] Wherein, is the component after specific phase shift processing through vector phase, component, is the combined output q-axis current in the combined output dq-axis current.

[0114] Step 103: Generate a negative sequence current reference value according to the q-axis component of the negative sequence current and the q-axis negative sequence reference value;

[0115] After obtaining the q-axis component of the negative sequence current, the negative sequence current reference value is generated by combining the q-axis negative sequence reference value.

[0116] Wherein, the q-axis negative sequence reference value can be taken as 0 in this embodiment.

[0117] In an example of the present invention, step 103 may include the following sub-steps:

[0118] Calculate the difference current between the q-axis negative sequence reference value and the q-axis component of the negative sequence current;

[0119] Perform proportional operation and integral operation on the difference current through a proportional integral controller to generate a q-axis reference component of the negative sequence current;

[0120] Use the q-axis reference component of the negative sequence current and the preset d-axis reference component of the negative sequence current as the negative sequence current reference value.

[0121] In the grid connection control of the power system, the negative sequence current control under the voltage unbalance condition is a key link to improve the system stability. In this embodiment, the q-axis component of the negative sequence current is used as the input of the energy storage negative sequence control for feedback control. By comparing the q-axis negative sequence reference value and the q-axis component of the negative sequence current, the difference current is determined as the input of the proportional integral controller (i.e., PI controller). The proportional integral controller adjusts according to the difference current to generate a q-axis reference component of the negative sequence current :

[0122]

[0123] Wherein, is the proportional gain, is the integral gain, is 0.

[0124] In addition, the PI controller can be discretized to calculate the q-axis reference component of the negative-sequence current, which will not be elaborated in this embodiment.

[0125] Meanwhile, the d-axis reference component of the negative-sequence current is set to 0 and, together with the q-axis reference component of the negative-sequence current is used as the negative-sequence current reference value.

[0126] Step 104: Generate a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters;

[0127] After obtaining the dq-axis electrical parameters, the active power control and the reactive power control are respectively performed in combination with the positive-sequence power reference value of the energy storage inverter at the current moment to generate a positive-sequence current reference value.

[0128] In an example of the present invention, the positive-sequence power reference value includes a positive-sequence active power reference value and a positive-sequence reactive power reference value, and the positive-sequence current reference value includes a positive-sequence current d-axis reference component and a positive-sequence current q-axis reference component; Step 104 may include the following sub-steps:

[0129] Use the dq-axis voltage and the dq-axis current to calculate the actual active power and the actual reactive power;

[0130] Calculate the active power deviation between the positive-sequence active power reference value and the actual active power;

[0131] Perform proportional-integral regulation on the active power deviation according to the active power loop to obtain the active regulation voltage;

[0132] Determine the positive-sequence current d-axis reference component according to the d-axis voltage in the positive-sequence active power reference value, the active regulation voltage, and the dq-axis voltage;

[0133] Calculate the reactive power deviation between the positive-sequence reactive power reference value and the actual reactive power;

[0134] Perform proportional-integral regulation on the reactive power deviation according to the reactive power loop to obtain the reactive regulation voltage;

[0135] Determine the positive-sequence current q-axis reference component according to the q-axis voltage in the positive-sequence reactive power reference value, the reactive regulation voltage, and the dq-axis voltage.

[0136] In this embodiment, after obtaining the dq-axis electrical parameters, they are adjusted according to the positive-sequence power reference value to determine the positive-sequence current d-axis reference component and the positive-sequence current q-axis reference component . Taking the generation process of the positive-sequence current d-axis reference component as an example for illustration:

[0137] First, the actual active power is calculated using the d-axis voltage and the q-axis current and the actual reactive power :

[0138]

[0139] wherein is the d-axis voltage, is the q-axis voltage, is the d-axis current, is the q-axis current

[0140] The positive-sequence active power reference value is calculated and the active power deviation between the positive-sequence active power reference value and the actual active power :

[0141]

[0142] The process of performing proportional-integral regulation on the active power deviation according to the active power loop to obtain the active regulation voltage is similar to the process of generating the q-axis reference component of the negative-sequence current in step 103 above

[0143] After obtaining the active regulation voltage based on the active power balance, the d-axis reference component of the positive-sequence current is calculated by combining feed-forward control

[0144]

[0145] The generation process of the q-axis reference component of the positive-sequence current is similar to the above process and is calculated using the positive-sequence reactive power reference value for calculation

[0146] In another example of the present invention, the method may further include the following steps

[0147] When there is idle capacity in the energy storage inverter, the positive-sequence power reference value is updated respectively according to a preset power update formula to obtain a new positive-sequence power reference value

[0148] The power update formula includes

[0149]

[0150] wherein is the positive-sequence reactive power reference value, is the positive-sequence active power reference value, is the total rated power during an asymmetrical fault, is the d-axis component of the negative-sequence voltage output by the energy storage inverter is the q - axis reference component of the negative - sequence current.

[0151] In this embodiment, when a three - phase unbalance fault occurs and there is still idle capacity in the energy storage inverter, the reference value of positive - sequence reactive power and / or the reference value of positive - sequence active power can be updated through the above - mentioned power update formula, so as to control the q - axis reference component of the positive - sequence current to raise the positive - sequence voltage, and control the d - axis reference component of the positive - sequence current to output for improving the system stability.

[0152] Furthermore, the method may further include the following steps:

[0153] If the grid - connected terminal voltage is greater than or equal to the over - limit voltage threshold, the d - axis reference component and the q - axis reference component of the positive - sequence current are sequentially reduced to zero.

[0154] During the operation of the power system, the voltage needs to be maintained within a certain range. Too high or too low voltage will affect the normal operation of equipment or even damage the equipment. In this embodiment, when the grid - connected terminal voltage is greater than or equal to the over - limit voltage threshold, that is , at this time, the positive - sequence d - axis current control reference value and the positive - sequence q - axis current control reference value of the energy storage system are set to 0. In this way, the power output of the energy storage system is adjusted, the effect on the power grid is reduced, and further voltage rise is avoided, so that the voltage returns to a reasonable range, ensuring the safe and stable operation of the system.

[0155] Furthermore, before executing step 105, the method may further include the following steps:

[0156] Calculate the Euclidean norm among the q - axis reference component of the negative - sequence current, the q - axis reference component of the positive - sequence current, and the d - axis reference component of the positive - sequence current;

[0157] Judge whether the Euclidean norm is less than or equal to the current amplitude upper limit value;

[0158] If so, retain the q - axis reference component of the negative - sequence current, the q - axis reference component of the positive - sequence current, and the d - axis reference component of the positive - sequence current at the current moment;

[0159] If not, use the current amplitude upper limit value, the q - axis reference component of the negative - sequence current, and the q - axis reference component of the positive - sequence current to calculate the new d - axis reference component of the positive - sequence current;

[0160] If the new d - axis reference component of the positive - sequence current is zero, use the current amplitude upper limit value and the q - axis reference component of the negative - sequence current to calculate the new q - axis reference component of the positive - sequence current.

[0161] In this embodiment, since the output current amplitude is an important factor affecting the safe operation of the energy storage converter, it is necessary to simultaneously consider the constraints of the phase current amplitude and voltage, as follows:

[0162]

[0163] Among them, is the Euclidean norm, is the upper limit value of the current amplitude.

[0164] If the above constraints are met, it indicates that no adjustment is required at present, and the negative-sequence current q-axis reference component, positive-sequence current q-axis reference component, and positive-sequence current d-axis reference component at the current moment are retained. If the above constraints are not met, with the positive-sequence current d-axis reference component as the first constraint priority, the upper limit value of the current amplitude, negative-sequence current q-axis reference component, and positive-sequence current q-axis reference component are used to calculate the new positive-sequence current d-axis reference component, and the positive-sequence current d-axis reference component is re-determined as until it is reduced to 0.

[0165] If the positive-sequence current d-axis reference component is 0, with the positive-sequence current q-axis reference component as the second constraint priority, and there is still , then the upper limit value of the current amplitude and the negative-sequence current q-axis reference component are used to calculate the new positive-sequence current q-axis reference component until it is reduced to 0.

[0166] In addition, it can also as the third constraint priority. When is reduced to 0, the constraint .

[0167] Step 105: According to the negative-sequence current reference value and positive-sequence current reference value, perform current inner-loop control on the dq-axis electrical parameters to generate a pulse control signal and output it to the energy storage inverter to suppress the second-harmonic generated by the wind power VSC-HVDC grid-connected system.

[0168] In this embodiment, the current inner-loop control can be implemented by a current inner-loop controller. By obtaining the negative-sequence current reference value and positive-sequence current reference value, the PI control algorithm is used to adjust the dq-axis current in the dq-axis electrical parameters to obtain the modulation reference voltage. By performing pulse width modulation on the modulation reference voltage, a pulse control signal is generated and output to the energy storage inverter. By controlling the on-off of the power switching devices in the energy storage inverter through the duty cycle, the grid-connected terminal current output of the energy storage inverter is adjusted to suppress the second-harmonic generated by the wind power VSC-HVDC grid-connected system.

[0169] In an example of the present invention, the negative-sequence current reference value includes a negative-sequence current q-axis reference component and a negative-sequence current d-axis reference component; Step 105 may include the following sub-steps S11-S12:

[0170] Separate the positive and negative sequence components of the dq-axis current to obtain the positive-sequence actual dq-axis current and the negative-sequence actual dq-axis current;

[0171] Perform current inner-loop control on the negative-sequence actual dq-axis current according to the negative-sequence current reference value to determine the negative-sequence modulation reference voltage;

[0172] Perform current inner-loop control on the positive-sequence actual dq-axis current according to the positive-sequence current reference value to determine the positive-sequence modulation reference voltage;

[0173] Combine the negative-sequence modulation reference voltage and the positive-sequence modulation reference voltage separately for the d-axis and q-axis to obtain the d-axis modulation reference voltage and the q-axis modulation reference voltage;

[0174] Use the d-axis modulation reference voltage and the q-axis modulation reference voltage to perform pulse-width modulation to generate a pulse control signal and output it to the energy storage inverter.

[0175] In the embodiment of the present invention, after obtaining the negative-sequence current reference value and the positive-sequence current reference value, use the current inner-loop controller to perform current inner-loop control on the dq-axis current in the dq-axis electrical parameters based on this to determine the modulation reference voltage corresponding to the energy storage inverter. First, separate the positive and negative sequence components of the dq-axis current to obtain the positive-sequence actual dq-axis current and the negative-sequence actual dq-axis current, and then perform current inner-loop control on the corresponding dq-axis current using the negative-sequence current reference value and the positive-sequence current reference value respectively to determine the modulation reference voltage. Taking the current inner-loop control process corresponding to the negative-sequence modulation reference voltage as an example, the process is as follows:

[0176] First, calculate the negative-sequence current deviation between the negative-sequence current reference value and the negative-sequence actual dq-axis current and :

[0177]

[0178] Then, use this negative-sequence current deviation as the input of the PI controller to determine the dq-axis negative-sequence voltage output by the PI control. Further considering the cross-coupling relationship between the d-axis and q-axis, add the corresponding cross-coupling term after the output of the PI controller to obtain the negative-sequence modulation reference voltage:

[0179]

[0180] Similarly, calculate the positive-sequence modulation reference voltage corresponding to the positive-sequence dq-axis and .

[0181] Then, combine the corresponding positive and negative sequence components for the d-axis and q-axis to obtain the d-axis modulation reference voltage and the q-axis modulation reference voltage Pulse width modulation is performed using the d-axis modulation reference voltage and the q-axis modulation reference voltage to generate a pulse control signal and output it to the energy storage inverter.

[0182] As Figure 4 shown, Figure 4 this is a schematic diagram of the control flow of a method for suppressing second-harmonic in a flexible DC grid-connected system with energy storage participation provided in an embodiment of the present invention.

[0183] In this embodiment, the energy storage control is divided into positive-sequence control and negative-sequence control. The energy storage control uses conventional dq decoupling control, and the reference dq control coordinate system of the control is provided by a phase-locked loop (PLL) measurement. The PLL of the energy storage control measures the grid-connected terminal voltage of the energy storage by realizing the measurement of the phase of the grid-connected terminal voltage vector and realizing the conversion of each control quantity from the rotating coordinate system abc to the stationary coordinate system dq with the phase of the grid-connected terminal voltage vector as a reference.

[0184] Among them, the positive-sequence control of the energy storage is divided into energy storage active power control and energy storage reactive power control. The reference value of the active power control and the reference value of the reactive power control are determined by the conventional active and reactive power control of the energy storage station.

[0185] After passing through the energy storage active power control and the energy storage reactive power control, the reference value of the active and reactive power control will obtain the reference value of the positive-sequence d-axis current control of the energy storage and the reference value of the positive-sequence q-axis current control of the energy storage .

[0186] In the control, by sampling the current jointly sent out by the flexible DC and the energy storage and separating the positive and negative sequences, the positive-sequence component and the negative-sequence component of the jointly sent out current are obtained. By performing coordinate transformation on the negative-sequence component in the rotating coordinate system with the phase of the grid-connected terminal voltage vector measured by the PLL, the d-axis component and the q-axis component of the negative-sequence current in the stationary coordinate system are obtained, and the q-axis component of the negative-sequence current is used as the input of the energy storage negative-sequence control.

[0187] When a single-phase ground fault occurs, the energy storage mainly adopts a negative-sequence suppression strategy. In the energy storage negative-sequence control, is used as the input for feedback control, and the reference value of the q-axis component of the system output negative-sequence current is set to 0. Through the PI control link, the reference value of the energy storage negative-sequence q-axis current control is finally obtained., and set the reference value of the positive-sequence d-axis current of the energy storage to 0.

[0188] Finally, the energy storage inverter passes the input , , , , and finally outputs the control reference voltage of the inverter through the current inner-loop control , . After pulse modulation, the control signal is output to the energy storage inverter.

[0189] In another example of the present invention, taking the flexible HVDC grid-connected wind power system shown in Figure 2 as an example, this embodiment is described. The rated active power of a single permanent magnet synchronous generator (PMSG) wind turbine is set to 5 MW, and there are 60 wind turbines in total, delivering 300 MW of power. The DC bus voltage of the flexible HVDC transmission is set to 300 kV, the DC side capacitor C is set to 0.002 F, and the impedance parameters of the receiving-end AC system are RL = 75×10−3 Ω and LL = 25×10−3 H. , , .

[0190] After incorporating the energy storage inverter, the energy storage power station to which the energy storage inverter belongs is configured with a rated capacity of 50 MVA. To verify the advantages in harmonic suppression, set t = 1 s to occur an A-phase grounding fault on the receiving-end AC bus of the offshore wind power. The fault lasts for 0.5 s, and the energy storage power station outputs a rated power of 0.6 S during the fault N . The following is from the perspectives of the magnitudes of the positive and negative sequence voltages, DC voltage, and harmonic content of the DC current at the receiving end during the fault, and the influence of the scheme summarized in this embodiment on the system frequency stability.

[0191] As Figure 5 shown, , are the amplitudes of the positive and negative sequence voltages of the receiving-end AC bus of the offshore wind power without using the method of this embodiment, , are the amplitudes of the positive and negative sequence voltages of the receiving-end AC bus of the offshore wind power when using the method of this embodiment. In contrast, the method of this embodiment can reduce the negative sequence voltage from 18.4 kV to 9.4 kV, effectively suppressing the growth of the negative sequence current under the fault. At the same time, due to setting the power, voltage, and current upper limits, almost all the capacity is used for the output of the negative sequence q-axis current at this time, so the positive sequence voltage rise is not obvious.

[0192] As Figure 6 and Figure 7 shown, Figure 6 is the schematic diagram of the DC bus current and the harmonic of the DC bus voltage when using the method of this embodiment during the fault.Figure 7 Schematic diagram of DC bus current and DC bus voltage harmonics when the method of this embodiment is not adopted during a fault.

[0193] It can be seen from Figure 6 that under the condition of single-phase grounding fault, the DC current harmonic of the offshore wind power grid-connected system is 91.74%, and the DC voltage harmonic is 177.20%. However, when an asymmetric fault occurs without adopting a harmonic control strategy, a second-harmonic will be generated in the DC voltage, which will affect the AC system of the converter station at the same time, generating a third-harmonic AC voltage. Conversely, the third-harmonic of the AC system will generate a fourth-harmonic in the DC side voltage. At this time, the DC current harmonic is 242.03%, and the DC voltage harmonic is 322.24%. By comparing the simulation results, it can be seen that the proposed collaborative strategy involving energy storage can reduce the DC current harmonic by 0.62 times and the DC current harmonic by 0.45 times, effectively suppressing the harmonics of electrical quantities under the condition of single-phase grounding fault and improving the frequency stability of the offshore wind power grid-connected system under the receiving-end asymmetric fault.

[0194] In the embodiment of the present invention, when an asymmetric fault occurs in the wind power VSC-HVDC grid-connected system, the electrical parameters at the grid-connected end of the energy storage inverter and the combined output current at the grid connection point are obtained in real time; the vector phase corresponding to the electrical parameters at the grid-connected end is measured, and the electrical parameters at the grid-connected end and the combined output current are respectively subjected to coordinate transformation according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current; according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value, a negative-sequence current reference value is generated; according to the positive-sequence power reference value and the dq-axis electrical parameters, a positive-sequence current reference value is generated; according to the negative-sequence current reference value and the positive-sequence current reference value, current inner-loop control is performed on the dq-axis electrical parameters, and a pulse control signal is generated and output to the energy storage inverter to suppress the second-harmonic generated by the wind power VSC-HVDC grid-connected system, thereby blocking the propagation path of the negative-sequence harmonic current from the source and effectively improving the control stability of the wind power VSC-HVDC grid-connected system.

[0195] Please refer to Figure 8 , Figure 8 which shows the structural block diagram of a device for suppressing the second-harmonic in a VSC-HVDC grid-connected system with energy storage participation in the embodiment of the present invention.

[0196] The embodiment of the present invention provides a device for suppressing the second-harmonic in a VSC-HVDC grid-connected system with energy storage participation, which relates to a wind power VSC-HVDC grid-connected system and an energy storage inverter connected at the grid connection point. The device includes:

[0197] An electrical parameter acquisition module 801, configured to, when an asymmetric fault occurs in the wind power VSC-HVDC grid-connected system, acquire the electrical parameters at the grid-connected end of the energy storage inverter and the combined output current at the grid connection point in real time;

[0198] The electrical parameter conversion module 802 is configured to measure the vector phase corresponding to the grid-connected end electrical parameters, and perform coordinate conversion on the grid-connected end electrical parameters and the combined output current according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current;

[0199] The negative-sequence current reference value determination module 803 is configured to generate a negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value;

[0200] The positive-sequence current reference value determination module 804 is configured to generate a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters;

[0201] The control signal generation module 805 is configured to perform current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generate a pulse control signal for output to the energy storage inverter to suppress the second-harmonic generated by the wind power flexible DC grid-connected system.

[0202] Optionally, the grid-connected end electrical parameters include the grid-connected end voltage and the grid-connected end current, and the dq-axis electrical parameters include the dq-axis voltage and the dq-axis current; specifically, the electrical parameter conversion module 802 is configured to:

[0203] Measure the vector phase corresponding to the grid-connected end voltage;

[0204] Convert the grid-connected end voltage from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis voltage;

[0205] Convert the grid-connected end current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis current;

[0206] Convert the combined output current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the combined output dq-axis current;

[0207] Separate the positive and negative sequences of the combined output q-axis current in the combined output dq-axis current, and extract the q-axis component of the negative-sequence current.

[0208] Optionally, the negative-sequence current reference value determination module 803 is specifically configured to:

[0209] Calculate the difference current between the q-axis negative-sequence reference value and the q-axis component of the negative-sequence current;

[0210] Perform proportional operation and integral operation on the difference current through a proportional-integral controller to generate the q-axis reference component of the negative-sequence current;

[0211] Use the q-axis reference component of the negative-sequence current and the preset d-axis reference component of the negative-sequence current as the negative-sequence current reference value.

[0212] Optionally, the positive-sequence power reference value includes a positive-sequence active power reference value and a positive-sequence reactive power reference value, and the positive-sequence current reference value includes a positive-sequence current d-axis reference component and a positive-sequence current q-axis reference component; specifically, the positive-sequence current reference value determination module 804 is configured to:

[0213] Use the dq-axis voltage and the dq-axis current to calculate the actual active power and the actual reactive power;

[0214] Calculate the active power deviation between the positive-sequence active power reference value and the actual active power;

[0215] Perform proportional-integral regulation on the active power deviation according to the active power loop to obtain the active regulation voltage;

[0216] Determine the positive-sequence current d-axis reference component according to the positive-sequence active power reference value, the active regulation voltage, and the d-axis voltage in the dq-axis voltage;

[0217] Calculate the reactive power deviation between the positive-sequence reactive power reference value and the actual reactive power;

[0218] Perform proportional-integral regulation on the reactive power deviation according to the reactive power loop to obtain the reactive regulation voltage;

[0219] Determine the positive-sequence current q-axis reference component according to the positive-sequence reactive power reference value, the reactive regulation voltage, and the q-axis voltage in the dq-axis voltage.

[0220] Optionally, the control signal generation module 805 is specifically configured to:

[0221] Separate the positive and negative sequences of the dq-axis current to obtain the positive-sequence dq-axis actual current and the negative-sequence dq-axis actual current;

[0222] Perform current inner-loop control on the negative-sequence dq-axis actual current according to the negative-sequence current reference value to determine the negative-sequence modulation reference voltage;

[0223] Perform current inner-loop control on the positive-sequence dq-axis actual current according to the positive-sequence current reference value to determine the positive-sequence modulation reference voltage;

[0224] Combine the negative-sequence modulation reference voltage and the positive-sequence modulation reference voltage respectively according to the d-axis and the q-axis to obtain the d-axis modulation reference voltage and the q-axis modulation reference voltage;

[0225] Use the d-axis modulation reference voltage and the q-axis modulation reference voltage to perform pulse-width modulation to generate a pulse control signal and output it to the energy storage inverter.

[0226] Optionally, the device further includes an update module, which is specifically configured to:

[0227] Calculate the Euclidean norm between the negative-sequence current q-axis reference component, the positive-sequence current q-axis reference component, and the positive-sequence current d-axis reference component;

[0228] Determine whether the Euclidean norm is less than or equal to the upper limit value of the current amplitude;

[0229] If so, retain the negative-sequence current q-axis reference component, positive-sequence current q-axis reference component, and positive-sequence current d-axis reference component at the current moment;

[0230] If not, use the upper limit value of the current amplitude, negative-sequence current q-axis reference component, and positive-sequence current q-axis reference component to calculate a new positive-sequence current d-axis reference component;

[0231] If the new positive-sequence current d-axis reference component is zero, use the upper limit value of the current amplitude and the negative-sequence current q-axis reference component to calculate a new positive-sequence current q-axis reference component.

[0232] Optionally, the device further includes a power update module, specifically used for:

[0233] When there is idle capacity in the energy storage inverter, update the positive-sequence power reference value respectively according to the preset power update formula to obtain a new positive-sequence power reference value;

[0234] The power update formula includes:

[0235]

[0236] Wherein, is the positive-sequence reactive power reference value, is the positive-sequence active power reference value, is the total rated power during an asymmetric fault, is the d-axis component of the negative-sequence voltage output by the energy storage inverter, is the negative-sequence current q-axis reference component.

[0237] Optionally, the device further includes a voltage limiting module, specifically used for:

[0238] If the grid-connected terminal voltage is greater than or equal to the over-limit voltage threshold, reduce the positive-sequence current d-axis reference component and positive-sequence current q-axis reference component to zero in sequence.

[0239] An embodiment of the present invention provides an electronic device, including a memory and a processor. When the computer program stored in the memory is executed by the processor, the processor executes the steps of the method for suppressing the second-harmonic in the energy storage participating in the flexible DC grid-connected system as described in any embodiment of the present invention.

[0240] Those skilled in the art can clearly understand that for the convenience and simplicity of description, the specific working processes of the above-described device and module can refer to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0241] In several embodiments provided by the present invention, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the modules is only a logical function division. In actual implementation, there may be other division methods. For example, multiple modules or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed coupling, direct coupling, or communication connection between each other can be through some interfaces. The indirect coupling or communication connection of devices or modules can be in electrical, mechanical, or other forms.

[0242] The modules described as separate components may or may not be physically separated. The components displayed as modules may or may not be physical modules, that is, they can be located in one place or distributed to multiple network modules. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0243] In addition, in each embodiment of the present invention, the functional modules can be integrated in a processing module, or each module can exist physically alone, or two or more modules can be integrated in one module. The above integrated modules can be implemented in the form of hardware or in the form of software functional modules.

[0244] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of each embodiment of the present invention.

Claims

1. A method for suppressing the second harmonic in a flexible DC grid-connected system with energy storage participation, characterized in that, It involves a flexible HVDC wind power grid-connected system and an energy storage inverter connected at the point of common coupling. The method includes: When an asymmetrical fault occurs in the flexible HVDC wind power grid-connected system, the electrical parameters at the grid-connected end of the energy storage inverter and the combined output current at the point of common coupling are acquired in real time. Measure the vector phase corresponding to the electrical parameters at the grid-connected end, and perform coordinate transformation on the electrical parameters at the grid-connected end and the combined output current respectively according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current. Generate a negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value. Generate a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters. Perform current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generate a pulse control signal to be output to the energy storage inverter to suppress the second-harmonic generated by the flexible HVDC wind power grid-connected system.

2. The method according to claim 1, characterized in that, The electrical parameters at the grid-connected end include the grid-connected end voltage and the grid-connected end current. The dq-axis electrical parameters include the dq-axis voltage and the dq-axis current. The measurement of the vector phase corresponding to the electrical parameters at the grid-connected end, and the coordinate transformation of the electrical parameters at the grid-connected end and the combined output current respectively according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current includes: Measure the vector phase corresponding to the grid-connected end voltage. Convert the grid-connected end voltage from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis voltage. Convert the grid-connected end current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the dq-axis current. Convert the combined output current from the three-phase stationary coordinate system to the synchronous rotating coordinate system according to the vector phase to obtain the combined output dq-axis current. Separate the positive and negative sequences of the combined output q-axis current in the combined output dq-axis current, and extract the q-axis component of the negative-sequence current.

3. The method according to claim 2, wherein The generation of the negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value includes: Calculate the difference current between the q-axis negative-sequence reference value and the q-axis component of the negative-sequence current. Perform proportional operation and integral operation on the difference current through a proportional-integral controller to generate the q-axis reference component of the negative-sequence current. Use the q-axis reference component of the negative-sequence current and the preset d-axis reference component of the negative-sequence current as the negative-sequence current reference value.

4. The method according to claim 3, wherein The positive-sequence power reference value includes the positive-sequence active power reference value and the positive-sequence reactive power reference value. The positive-sequence current reference value includes the d-axis reference component of the positive-sequence current and the q-axis reference component of the positive-sequence current. The generation of the positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters includes: Use the dq-axis voltage and the dq-axis current to calculate the actual active power and the actual reactive power. Calculate the active power deviation between the positive-sequence active power reference value and the actual active power. Perform proportional-integral regulation on the active power deviation according to the active power loop to obtain the active power regulation voltage. Determine the d-axis reference component of the positive-sequence current according to the positive-sequence active power reference value, the active power regulation voltage, and the d-axis voltage in the dq-axis voltage. Calculate the reactive power deviation between the positive-sequence reactive power reference value and the actual reactive power; Perform proportional-integral regulation on the reactive power deviation according to the reactive power loop to obtain the reactive power regulation voltage; Determine the q-axis reference component of the positive-sequence current according to the positive-sequence reactive power reference value, the reactive power regulation voltage, and the q-axis voltage in the dq-axis voltage.

5. The method according to claim 2, characterized in that The step of performing current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generating a pulse control signal to be output to the energy storage inverter includes: Separate the positive and negative sequences of the dq-axis current to obtain the positive-sequence dq-axis actual current and the negative-sequence dq-axis actual current; Perform current inner-loop control on the negative-sequence dq-axis actual current according to the negative-sequence current reference value to determine the negative-sequence modulation reference voltage; Perform current inner-loop control on the positive-sequence dq-axis actual current according to the positive-sequence current reference value to determine the positive-sequence modulation reference voltage; Combine the negative-sequence modulation reference voltage and the positive-sequence modulation reference voltage according to the d-axis and q-axis respectively to obtain the d-axis modulation reference voltage and the q-axis modulation reference voltage; Use the d-axis modulation reference voltage and the q-axis modulation reference voltage to perform pulse-width modulation, and generate a pulse control signal to be output to the energy storage inverter.

6. The method according to claim 4, characterized in that Before performing the step of performing current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generating a pulse control signal to be output to the energy storage inverter, the method further includes: Calculate the Euclidean norm between the negative-sequence current q-axis reference component, the positive-sequence current q-axis reference component, and the positive-sequence current d-axis reference component; Judge whether the Euclidean norm is less than or equal to the current amplitude upper limit value; If so, retain the negative-sequence current q-axis reference component, the positive-sequence current q-axis reference component, and the positive-sequence current d-axis reference component at the current moment; If not, use the current amplitude upper limit value, the negative-sequence current q-axis reference component, and the positive-sequence current q-axis reference component to calculate a new positive-sequence current d-axis reference component; If the new positive-sequence current d-axis reference component is zero, use the current amplitude upper limit value and the negative-sequence current q-axis reference component to calculate a new positive-sequence current q-axis reference component.

7. The method according to claim 1, characterized in that, The method further includes: When there is idle capacity in the energy storage inverter, update the positive-sequence power reference value respectively according to a preset power update formula to obtain a new positive-sequence power reference value; The power update formula includes: Among them, is the positive-sequence reactive power reference value, is the positive-sequence active power reference value, is the total rated power during the occurrence of an asymmetric fault, is the d-axis component of the negative-sequence voltage output by the energy storage inverter, is the q-axis reference component of the negative-sequence current.

8. The method according to claim 4, characterized in that The method further includes: If the grid-connected terminal voltage is greater than or equal to the over-limit voltage threshold, reduce the positive-sequence current d-axis reference component and the positive-sequence current q-axis reference component to zero in sequence.

9. A device for suppressing the second harmonic in a flexible DC grid-connected system with energy storage participation, characterized in that, Relates to a wind power flexible DC grid-connected system and an energy storage inverter connected at the grid connection point, the device includes: An electrical parameter acquisition module, configured to, when an asymmetric fault occurs in the wind power flexible DC grid-connected system, acquire the grid-connected terminal electrical parameters of the energy storage inverter and the combined output current of the grid connection point in real time; An electrical parameter conversion module is used to measure the vector phase corresponding to the grid-connected end electrical parameters, and perform coordinate conversion on the grid-connected end electrical parameters and the combined output current respectively according to the vector phase to obtain the dq-axis electrical parameters and the q-axis component of the negative-sequence current; A negative-sequence current reference value determination module is used to generate a negative-sequence current reference value according to the q-axis component of the negative-sequence current and the q-axis negative-sequence reference value; A positive-sequence current reference value determination module is used to generate a positive-sequence current reference value according to the positive-sequence power reference value and the dq-axis electrical parameters; A control signal generation module is used to perform current inner-loop control on the dq-axis electrical parameters according to the negative-sequence current reference value and the positive-sequence current reference value, and generate a pulse control signal to be output to the energy storage inverter to suppress the second-harmonic generated by the wind power HVDC grid-connected system.

10. An electronic device, characterized in that, It includes a memory and a processor. When the computer program stored in the memory is executed by the processor, the processor executes the steps of the method for suppressing the second-harmonic of the energy storage participating in the HVDC grid-connected system according to any one of claims 1-8.

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