Receiving end fault ride-through control method and system for new energy through flexible direct current island sending-out system

Abstract

The invention provides a receiving end fault ride-through control method and a receiving end fault ride-through control system for a new energy via flexible direct current islanding sending-out system, and the method comprises the steps: setting a voltage reference value of a sending end alternating current line through a converter station after detecting that a direct current voltage exceeds a safety threshold value, and injecting a negative sequence voltage harmonic component into the alternating current line; after a network side converter of the new energy power plant detects that the voltage is reduced, a low-voltage ride-through strategy is started, and negative-sequence voltage harmonic waves in a line are detected; and when the detected negative sequence voltage harmonic wave is detected, the new energy power plant is switched to an active load shedding strategy. Under the condition of actively reducing the AC voltage of the sending end, the grid-side converter actively reduces the active input according to the AC voltage detected locally and the negative sequence voltage harmonic component, the problem of matching of the load shedding power and the surplus power is solved, meanwhile, the reactive component is not supported, the reduction of the local input power is accelerated, and the power consumption is reduced. And the interference problem of reactive power support left and right on active voltage reduction control is optimized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of new energy power supply, in particular to a new energy through flexible direct current island sending-out system receiving end fault ride-through control method and system. BACKGROUND

[0002] With the growing demand for clean energy worldwide, the proportion of new energy power generation in the power system is increasing. Large-scale new energy through flexible direct current transmission system has become a typical scenario in new-type power system. Flexible direct current technology is particularly suitable for long-distance and large-capacity transmission of new energy such as offshore wind power due to its advantages of being able to be connected to passive network and reducing transmission loss.

[0003] However, there are many complex technical problems in the new energy through flexible direct current sending-out system. On the one hand, the AC transmission line between the new energy station and the flexible direct current converter is on both sides of the power electronic device. When the line fails, the short-circuit current is affected by different converter control strategies, resulting in problems such as waveform distortion, phase angle control, and amplitude limitation. On the other hand, new energy generation has randomness and volatility, which poses a serious challenge to the stability and reliability of the system.

[0004] At the same time, the new energy station needs to have a certain fault ride-through capability during the fault to ensure the stability of the system. At present, the new energy station usually detects the degree of positive sequence voltage drop at the grid-connected point, adjusts the output reactive current to support the grid voltage, and realizes low voltage ride-through. However, this method has poor effect in the receiving end fault scenario, and cannot effectively cooperate with the protection and control strategy of the flexible direct current system. When the receiving end of the flexible direct current system fails, a series of key problems need to be solved.

[0005] Specific problems are as follows:

[0006] 1. Receiving end fault will cause the DC voltage to rise sharply. The excessively high DC voltage not only causes irreversible damage to the power electronic devices in the flexible direct current system, but also may trigger the overvoltage protection action of the converter, thereby causing the system to be split, and seriously affecting the safe and stable operation of the power system;

[0007] 2. The unbalanced power generated by the system when the fault occurs and the wind turbine load shedding power are difficult to accurately match. If the wind turbine load shedding power is too large, it will cause waste of new energy. If the load shedding power is insufficient, the system's power imbalance problem cannot be effectively alleviated, resulting in intensified system frequency and voltage fluctuations;

[0008] 3. In the case of communication delay or failure, the load reduction signal is difficult to quickly and reliably transmit to the fan side, so that the fan cannot respond to the load reduction instruction in time, and the active load reduction of new energy and the coordination of the protection control strategy of the flexible DC system cannot be realized. Although some current research focuses on the control problem of new energy and flexible DC system, the active load reduction of new energy that does not depend on communication and takes into account the influence on differential protection under the fault of the receiving end still needs to be further studied and improved.

[0009] In view of the above problems, in the prior art, the control mode of the flexible DC transmission network mainly relies on another communication line, and when the communication fails, the reliability of the scheme is low; the imbalance power is mostly dissipated by the DC energy dissipation device, and the economy is poor; when the imbalance power matching algorithm is complex, it is not conducive to the quick and reliable action of load shedding; the load reduction strategy of the flexible DC transmission network does not depend on communication, has stepped load reduction, cannot accurately match power, and the reliability is not perfect.

[0010] In view of the above, there is still a need for a receiving end fault ride-through control strategy under the scene of new energy through flexible DC island transmission, which can adaptively control the AC voltage to more flexibly and accurately control the new energy load reduction, and does not depend on communication between stations; at the same time, the DC energy dissipation device is put into operation, and the economy of the system is improved. SUMMARY

[0011] The present application provides a new energy through flexible DC island transmission system receiving end fault ride-through control method and control system based on active voltage reduction load reduction control to solve the problems in the prior art.

[0012] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0013] On the one hand, the present application provides a new energy through flexible DC island transmission system receiving end fault ride-through control method, wherein in the normal operation of the transmission system, the converter adopts a negative sequence voltage suppression strategy; when the transmission system detects a fault at the receiving end through DC voltage, a coordinated ride-through strategy is started;

[0014] The coordinated ride-through strategy comprises the following steps:

[0015] After detecting that the DC voltage exceeds a safety threshold, the sending end AC line voltage reference value of the converter station is set, and a negative sequence voltage harmonic component is injected into the AC line;

[0016] After the grid-side converter of the new energy power plant detects that the sending end AC line voltage is reduced, a low-voltage ride-through strategy is started, and a negative sequence voltage harmonic in the line is detected;

[0017] When the detected negative sequence voltage harmonic satisfies a preset judgment condition of the negative sequence voltage harmonic component, the new energy power plant switches to an active load reduction strategy.

[0018] After the DC voltage returns to normal, the sending system exits the coordinated ride-through strategy and continues to detect the DC voltage.

[0019] Optionally, setting the sending-end AC line voltage reference value comprises the following steps:

[0020] solving the voltage deviation between the real-time voltage and the safe voltage;

[0021] the voltage deviation is obtained by a coefficient weighting method to obtain a corrected voltage;

[0022] based on the rated voltage and the corrected voltage, the sending-end AC line voltage reference value is solved.

[0023] Optionally, in the low-voltage ride-through strategy, the q-axis current is increased and the d-axis current is reduced through current loop control.

[0024] Optionally, in the low-voltage ride-through strategy, the q-axis current reference value is calculated as follows:

[0025]

[0026] In the formula, V wf is the detected AC voltage value, and I NG is the rated current value of the line in normal operation;

[0027] the d-axis current reference value is calculated as follows:

[0028]

[0029] In the formula, I MAX is the maximum current value of the AC line.

[0030] Optionally, in the active load shedding strategy, the d-axis current reference value is set to a preset parameter through current loop control, and the q-axis current reference value is set to zero.

[0031] Optionally, in the active load shedding strategy, the d-axis current reference value is calculated as follows:

[0032]

[0033] In the formula, I dN is the d-axis rated current value of the line in normal operation, V wfN is the rated value of the AC voltage on the new energy unit side in normal operation, and V wf is the detected AC voltage value on the new energy unit side.

[0034] Optionally, when the direct current voltage exceeds the safety threshold, the method further comprises the following steps:

[0035] calculating a power difference on both sides of the receiving converter;

[0036] solving the energy consumption power based on the power difference;

[0037] corresponding to the energy consumption device according to the energy consumption power.

[0038] Optionally, the frequency of the negative sequence voltage harmonic component is 300 Hz.

[0039] The condition for starting the low-voltage ride-through strategy is that the grid-side converter of the new energy power plant detects that the sending-end AC line voltage decreases, and the grid-side converter detects that the AC voltage standard value enters a preset range.

[0040] In another aspect, the application also provides a new energy through a flexible island sending system receiving end fault ride-through control system, which is used to implement the new energy through a flexible island sending system receiving end fault ride-through control method.

[0041] The control system comprises a direct current voltage detection module, a converter station V / F control module, a new energy grid-side converter ride-through control module, and a direct current energy consumption control module; the direct current voltage detection module is connected with the converter station V / F control module; the new energy grid-side converter ride-through control module is also connected with a new energy power plant; the direct current energy consumption control module is connected with the direct current voltage detection module and / or the converter station V / F control module.

[0042] The direct current voltage detection module is used to collect direct current voltage data.

[0043] The converter station V / F control module is used to generate and send the control instructions of the converter station, and control the inverter in the station to inject negative sequence voltage harmonic components into the line;

[0044] The new energy grid-side converter ride-through control module is used to generate and send ride-through control instructions, and detect negative sequence voltage harmonics.

[0045] The direct current energy consumption control module is used to control the energy consumption action of the direct current energy, and is connected with a direct current energy consumption device.

[0046] Optionally, the new energy grid-side converter ride-through control module comprises a low-voltage ride-through module and an active load shedding ride-through module.

[0047] The low-voltage ride-through module is used to start a low-voltage ride-through strategy.

[0048] The active load shedding ride-through module is used to start an active load shedding strategy.

[0049] Compared with the prior art, the application has the following beneficial effects:

[0050] The application solves the problem of matching the load reduction power and the surplus power by actively reducing the active input of the grid-side converter according to the locally detected AC voltage and negative sequence voltage harmonic component under the condition of actively reducing the sending end AC voltage, without supporting the reactive component, accelerating the reduction of the local input power, and optimizing the interference problem of the active voltage reduction control caused by the reactive support on both sides; further, in the case of serious fault, the part of surplus power is consumed, and the economic efficiency of the system is improved. BRIEF DESCRIPTION OF DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the drawings needed to be used in the embodiments will be briefly introduced below. Obviously, the drawings in the following description only constitute some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor under the premise of the drawings.

[0052] Figure 1 is the topology structure diagram of new energy transmission through the flexible direct current system in the specific embodiment of the application;

[0053] Figure 2 is the topology diagram of wind power AC collection line in the specific embodiment of the application;

[0054] Figure 3 is the method flow chart in the specific embodiment of the application;

[0055] Figure 4 is the active voltage reduction control strategy circuit diagram in the specific embodiment of the application;

[0056] Figure 5 is the sending end converter negative sequence voltage harmonic control strategy circuit diagram in the specific embodiment of the application;

[0057] Figure 6 is the DC overvoltage degree simulation verification under different crossing strategies in the specific embodiment of the application;

[0058] Figure 7 is the simulation verification of the cooperative strategy under different transition resistances in the specific embodiment of the application.

[0059] In the drawings: 1, point of common coupling, 2, sending end MMC, 3, receiving end MMC, 4, wind farm grid-side converter, 5, converter station. DETAILED DESCRIPTION

[0060] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0061] It should be noted that similar reference numerals and letters refer to similar items throughout the accompanying drawings, and thus, once an item is defined in one drawing, it is not necessary to further define and explain it in subsequent drawings.

[0062] It should be noted that the methods used in the present application are conventional methods unless otherwise specified, and the raw materials and devices used are conventional commercially available products unless otherwise specified, and their sources are not limited.

[0063] It should be further noted that, in order to facilitate understanding, the steps of the methods are described in a certain order in the embodiments of the present application, but a person of ordinary skill in the art can change the order of the steps according to actual needs, and therefore, this should not be regarded as a limitation; it should be further noted that in the description of the following specific embodiments, the upper and lower subscripts of each parameter should be understood as a distinction mark of similar identification unless otherwise specified, representing the parameter of the related or corresponding device of the subscript, and should not be understood as a specific model or special mark.

[0064] On the one hand, the embodiments of the present application provide a fault ride-through control method for a receiving end of a new energy through flexible island sending-out system. In the embodiments, the new energy through flexible island sending-out system takes an offshore wind farm transmission scenario as an example, and combines the Figure 1 and Figure 2 As shown in the drawings, the alternating current generated by the wind turbine generator set in the wind farm is adjusted through the wind farm set side converter and the wind farm grid side converter 4, and then collected at the point of common coupling 1 (PCC) to the alternating current bus, and then sent to the converter station 5; the converter station adopts a modular multilevel converter (MMC), which is used as a sending end MMC 2 to convert the alternating current into high-voltage direct current, and remotely transmit it to the receiving end MMC 3, and then convert it into alternating current required by the power grid through inversion.

[0065] Further, in the normal operation of the above sending-out system, the wind farm grid side converter adopts a negative sequence voltage suppression strategy.

[0066] When the receiving end of the sending-out system fails to detect a direct current voltage, the voltage of the receiving end of the flexible direct current system falls, the power sending is limited, if the direct current communication signal and the alternating current communication signal appear time delay or failure at this time, the wind power is not reduced in time, the power is gathered in the capacitor of the converter sub-module, and the direct current voltage rises. Therefore, the system needs to start the coordinated ride-through strategy, and the coordinated ride-through strategy includes the following steps as shown in Figure 3 After the direct current voltage is detected to exceed the safety threshold, the sending end alternating current line voltage reference value is set by the converter station; optionally, setting the sending end alternating current line voltage reference value includes the following steps:

[0067] After the direct current voltage is detected to exceed the safety threshold, the sending end alternating current line voltage reference value is set by the converter station; optionally, setting the sending end alternating current line voltage reference value includes the following steps:

[0068] The voltage deviation between the real-time voltage and the safety voltage is solved, and the formula is:

[0069] U dc0 -U dcset ;

[0070] In the formula, U dcset is the safety voltage, and U dc0 is the real-time voltage.

[0071] The voltage deviation is obtained by the coefficient weighting method to obtain the correction voltage K(U dc0 -U dcset ), wherein K is the voltage reduction coefficient;

[0072] The sending end alternating current line voltage reference value U WFset is solved based on the rated voltage and the correction voltage, and the formula is:

[0073] U WFset =U WFN -K(U dc0 -U dcset );

[0074] In the formula, U WFN is the rated value of the sending end alternating current line in normal operation.

[0075] The setting formula introduces the overvoltage degree of the direct current voltage into the reference value setting, and then controls the sending end converter to inject negative sequence voltage harmonic components into the alternating current line. The converter station injects negative sequence voltage harmonic components into the alternating current line, and the negative sequence voltage harmonic components are single negative sequence voltage 300Hz harmonic components. Because the sending end MMC converter adopts the control strategy of negative sequence voltage suppression of 0 in the normal operation process of the system, the negative sequence voltage fundamental and harmonic content is low, and the 50Hz fundamental component content is the highest, and the 300Hz high frequency harmonic component is low, so the embodiment considers actively injecting the 300Hz harmonic component as a characteristic signal to realize the transmission of the communication signal through the power cable.

[0076] At the same time, when the direct current voltage is detected to exceed the safety threshold, the following steps are further included:

[0077] The power difference on both sides of the receiving end converter is calculated, and the current required energy consumption power is solved based on the power difference to dissipate part of the surplus power; and the energy consumption device is put into operation according to the energy consumption power, including but not limited to starting the corresponding number of energy consumption devices according to the energy consumption power, or adjusting the power of the energy consumption device.

[0078] When the sending end AC line voltage detected by the wind farm grid-side converter is reduced, the low-voltage ride-through strategy is started, and the line negative sequence voltage harmonic is detected. The condition for starting the low-voltage ride-through strategy is that the wind farm grid-side converter detects that the sending end AC line voltage is reduced, and the grid-side converter detects that the AC voltage reference value enters the preset range; specifically, when the wind farm grid-side converter detects that the AC voltage reference value satisfies 0.9>V * wf >0.2, the conventional low-voltage ride-through strategy is prepared to be started; V * wf represents the wind turbine side of the system detects the local AC voltage reference value.

[0079] In the low-voltage ride-through strategy, the q-axis current is increased and the d-axis current is reduced through current loop control. The formula of the wind turbine q-axis current reference value is as follows:

[0080]

[0081] In the formula, V wf is the AC voltage value detected by the wind turbine, and I NG is the line rated current value when the wind turbine is normally running.

[0082] The formula of the wind turbine d-axis current reference value is as follows:

[0083]

[0084] In the formula, I MAX is the maximum AC current value of the wind turbine.

[0085] When the wind farm grid-side converter starts the low-voltage ride-through strategy, the 300Hz harmonic component V nf300 of the AC line negative sequence voltage is detected immediately within T0 nfset when the threshold V nf300 is reached (i.e. V nfset >V nfsetThe 300Hz harmonic critical value represents the ability to distinguish the sending end and the receiving end fault. Further, in the active load shedding strategy, the d-axis current reference value is set to a preset parameter by the current loop control, and the q-axis current reference value is set to zero.

[0086] The formula of the d-axis current reference value is:

[0087]

[0088] In the formula, I dN is the d-axis rated current value of the line when the fan is running normally, V wfN is the rated value of the AC voltage on the fan unit side when the system is running normally, V wf is the detected AC voltage value on the fan unit side; at the same time, the q-axis reference value is controlled to to avoid reactive power support.

[0089] In one example operation, the line is set to fail at 0ms, and the sampling frequency is set to 5kHz, i.e. the sampling interval is 0.2ms. After the receiving end AC fault occurs, the modular multilevel converter detects the rise in DC voltage, reaches the threshold value of 510kV, and switches to the active voltage reduction control mode, limiting the modular multilevel converter outlet AC voltage to the vicinity of the reference value, K is 2.2, and the control strategy circuit is as shown in Figure 4 , u s is the preset reference voltage, f is the preset reference frequency, i s is the preset reference current, and the subscripts ref, max, min, d, q represent the reference value, maximum, minimum, d-axis, q-axis, respectively, and abc is the three-phase; at the same time, a single negative sequence voltage 300Hz harmonic component is delivered to the fan side, and the control strategy circuit of the negative sequence voltage harmonic is as shown in Figure 5 .

[0090] Based on the above control method, the fault period T N is experienced, and after the DC voltage returns to normal, the sending system exits the cooperative ride-through strategy and continues to detect the DC voltage.

[0091] This embodiment does not rely on communication equipment, but only relies on the electrical parameters of the line, i.e. the active load shedding cooperative ride-through can be realized. First, the receiving end fault degree and the surplus power signal are transmitted, and the overvoltage amount of the DC voltage is transmitted to the sending end MMC converter, then the converter actively reduces the voltage and injects a negative sequence voltage high frequency component, which transmits the fault signal and the surplus power signal to the wind farm grid side converter through the sending end AC line, finally, the wind farm controls the wind turbine to accurately shed load according to the signal, avoiding the influence of communication delay or fault on the transmission of the load shedding signal, and realizing the accurate matching of the power surplus. As shown in Figures 6-7 ​The simulation results show that the method can realize active and accurate load reduction of the fan under the premise of not relying on communication, by setting the voltage reduction reference value and inputting the characteristic harmonic signal, the highest DC voltage is 525 kV, the overvoltage degree is 1.04 pu, compared with other control strategies, the overvoltage degree is lower and the effect is better; and under different faults, the overvoltage degree is within the allowable range, and the scheme has high adaptability.

[0092] In another aspect, the application also provides a new energy through flexible island sending-out system receiving end fault ride-through control system, which is used to implement the control method.

[0093] Firstly, the converter station of the embodiment specifically adopts a V / F control method (a motor speed regulation technology of maintaining a constant voltage-to-frequency ratio, V / f=C).

[0094] Therefore, the control system of the embodiment at least includes a DC voltage detection module, a converter station V / F control module, a new energy grid-side converter ride-through control module and a DC energy consumption control module.

[0095] The DC voltage detection module is connected with the converter station V / F control module; the new energy grid-side converter ride-through control module is also connected with a new energy power plant; and the DC energy consumption control module is connected with the DC voltage detection module.

[0096] Further, the DC voltage detection module is used to collect DC voltage data to provide a basis for fault judgment. Specifically, the DC voltage detection module detects the DC system voltage level in real time during system operation, and transmits the difference value between the DC voltage and the rated DC voltage to the setting calculation module of the sending-end converter station.

[0097] The converter station V / F control module is used to generate and send the control instruction of the converter station, and control the converter in the station to inject negative sequence voltage harmonic components into the line; specifically, the converter needs to provide stable voltage support for new energy grid connection, and outputs a constant AC voltage reference value during normal operation; after the sending-end MMC converter detects that the DC line voltage of the flexible DC system reaches a threshold value, the AC voltage reference value is actively controlled after the fault occurs; the control system frequency is 50 Hz during normal operation; after the fault occurs, a single negative sequence voltage 300 Hz harmonic component is injected into the AC line.

[0098] The new energy grid-side converter ride-through control module is used to generate and send the ride-through control instruction, and detect the negative sequence voltage harmonic; further, the new energy grid-side converter ride-through control module includes a low-voltage ride-through module and an active load reduction ride-through module.

[0099] The low-voltage ride-through module is used to start a low-voltage ride-through strategy; under the AC line fault of the sending end, the AC voltage of the sending end drops, and after the fan detects that the voltage drop is between 0.2-0.9 (unit value), the conventional low-voltage ride-through strategy is started, the q-axis current is increased, and the d-axis current is reduced.

[0100] The active load shedding ride-through module is used to start an active load shedding strategy; under the AC line fault of the receiving end, the AC voltage of the receiving end drops, the d-axis reference value is directly controlled, and the q-axis reference value is controlled to avoid reactive power support.

[0101] The DC energy consumption control module is used to control the energy consumption action of the DC energy and is connected with the DC energy consumption device. The DC energy consumption device can be composed of multiple energy consumption plates, and the energy consumption plates are provided with energy consumption elements, which are specifically pure resistance elements that irreversibly convert electric energy into other forms of energy such as heat. The DC energy consumption control module is arranged on the DC line close to the outlet of the receiving end converter, and detects the power difference on both sides of the receiving end converter in real time. When the DC voltage reaches the threshold value, the number of energy consumption plates required to be put in is calculated, and the energy consumption plates are accurately put in. Without affecting the fault characteristics required by the strategy, part of the surplus power is dissipated, and the loss of the converter caused by the serious overvoltage of the DC system is avoided.

[0102] Optionally, the system further comprises a sending end AC voltage detection module integrated in the grid-side converter of the wind farm to detect the voltage level of the AC line in real time, and transmit the voltage value to the calculation control board in the fan converter for the setting calculation of the fan low ride-through or load shedding. Further, a fault recovery module is arranged in the system monitoring unit, and after the ride-through strategy is put in for a certain time, such as a fault cycle T N , the DC line voltage of the system is detected, and if the voltage returns to the normal level, the strategy is exited and the normal operation mode is restored.

[0103] Finally, it should be noted that the above content is only used to illustrate the technical solutions of the present application, and is not a limitation on the protection scope of the present application. Simple modifications or equivalent replacements of the technical solutions of the present application made by those skilled in the art do not deviate from the essence and scope of the technical solutions of the present application.

Claims

1. A fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding, characterized in that: During normal operation of the power transmission system, the converter adopts a negative sequence voltage suppression strategy; when a fault occurs at the DC voltage detection receiving end of the power transmission system, a cooperative ride-through strategy is activated. The cooperative traversal strategy includes the following steps: After detecting that the DC voltage exceeds the safety threshold, the converter station adjusts the reference value of the AC line voltage at the sending end and injects negative sequence voltage harmonic components into the AC line. After the grid-side converter of the new energy power plant detects a drop in the voltage of the sending-end AC line, it initiates a low-voltage ride-through strategy and detects negative sequence voltage harmonics in the line. When the detected negative sequence voltage harmonics meet the preset judgment conditions of the negative sequence voltage harmonic components, the new energy power plant switches to an active load reduction strategy. After the DC voltage returns to normal, the sending system exits the cooperative crossover strategy and continues to monitor the DC voltage.

2. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 1, characterized in that: Setting the reference value for the AC line voltage at the sending end includes the following steps: Solve for the voltage deviation between the real-time voltage and the safe voltage; The voltage deviation is calculated using a coefficient weighting method to obtain the corrected voltage. The reference value of the AC line voltage at the sending end is determined based on the rated voltage and the corrected voltage.

3. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 1, characterized in that: In the low-voltage ride-through strategy, the q-axis current is increased and the d-axis current is decreased by current loop control.

4. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 3, characterized in that: In the low-voltage ride-through strategy, the q-axis current reference value The formula is: In the formula, V wf To detect the AC voltage value, I NG This is the rated current value of the line during normal operation; d-axis current reference value The formula is: In the formula, I MAX This represents the maximum current value of the AC line.

5. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 1, characterized in that: In the active load reduction strategy, the d-axis current reference value is set to a preset parameter through current loop control, and the q-axis current reference value is set to zero.

6. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 5, characterized in that: In the active load reduction strategy, the d-axis current reference value The formula is: In the formula, I dN V represents the rated d-axis current of the line during normal operation. wfN V represents the rated value of the AC voltage on the side of the new energy unit during normal operation. wf This refers to the AC voltage value detected on the side of the new energy unit.

7. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 1, characterized in that: When the DC voltage is detected to exceed the safety threshold, the following steps are also included: Calculate the power difference between the two sides of the receiving-end converter; The energy consumption power is calculated based on the power difference. The energy-consuming device is put into operation according to the energy-consuming power.

8. The fault ride-through control method for the receiving end of a new energy transmission system via flexible DC islanding as described in claim 1, characterized in that: The frequency of the negative sequence voltage harmonic component is 300Hz; The conditions for activating the low-voltage ride-through strategy are: the grid-side converter of the new energy power plant detects a decrease in the voltage of the sending-end AC line, and the per-unit value of the AC voltage detected by the grid-side converter enters a preset range.

9. A fault ride-through control system for the receiving end of a new energy transmission system via a flexible DC islanding system, characterized in that: Used to implement the new energy transmission system via flexible direct current islanding fault ride-through control method as described in any one of claims 1-8; The control system includes: a DC voltage detection module, a converter station V / F control module, a new energy grid-side converter ride-through control module, and a DC energy consumption control module; the DC voltage detection module is connected to the converter station V / F control module; the new energy grid-side converter ride-through control module is also connected to the new energy power plant; the DC energy consumption control module is connected to the DC voltage detection module and / or the converter station V / F control module; The DC voltage detection module is used to collect DC voltage data; The converter station V / F control module is used to generate and send control commands for the converter station, and to control the converters in the station to inject negative sequence voltage harmonic components into the line. The new energy grid-side converter ride-through control module is used to generate and send ride-through control commands and detect negative sequence voltage harmonics. The DC energy consumption control module is used to control the energy consumption of DC power and is connected to the DC energy consumption device.

10. The fault ride-through control system for the receiving end of the new energy transmission system via flexible DC islanding as described in claim 9, characterized in that: The new energy grid-side converter ride-through control module includes a low-voltage ride-through module and an active load shedding ride-through module; The low-voltage ride-through module is used to initiate the low-voltage ride-through strategy. The active load reduction crossing module is used to initiate the active load reduction strategy.

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