Flexible DC power transmission system DC fault transient energy transfer time sequence control method and device

Abstract

The invention belongs to the field of power system protection and control, and discloses a flexible DC power transmission system DC fault transient energy transfer time sequence control method and device, and the method comprises the steps: increasing the d-axis current loop proportionality coefficient of a transmitting end converter when a DC fault signal of a flexible DC power transmission system is received, and obtaining a target d-axis voltage reference value; increasing the AC q-axis current reference value of the transmitting-end converter, limiting the AC q-axis output voltage, and obtaining a target q-axis voltage reference value; when the alternating-current d-axis current of the transmitting-end converter is attenuated to zero, adding direct-current active damping control; according to the target d-axis voltage reference value and the target q-axis voltage reference value, a direct-current active damping target amplitude limiting value is obtained through calculation; designing a direct-current active damping value according to a fault ride-through requirement; and the direct current control link is continued until the direct current fault current is attenuated to zero. By adopting the method, the energy accumulation in the transmitting-end converter can be reduced, the fault transient energy transfer speed is accelerated, and the capacitor voltage stress of the sub-module of the transmitting-end converter is inhibited.

Description

Technical Field

[0001] This application relates to the field of power system protection and control, and particularly to a transient energy transfer timing control method and device for a flexible DC transmission system during DC faults. Background Art

[0002] Under an overhead line fault, the DC side of an ultra-high-capacity and ultra-long-distance flexible DC transmission system contains huge transient energy. Limited by the physical constraints of the negative voltage output range of the modular multilevel converter (MMC) arm, the transfer process of DC fault transient energy is accompanied by multiple transient stresses such as DC current and sub-module capacitor voltage, which are difficult to suppress. Existing overhead line fault ride-through technologies are difficult to achieve rapid transfer of multiple fault transient energies in a short time, resulting in key equipment needing to continuously bear excessive stress impacts. The superposition of multiple transient stresses and excessive energy will significantly increase the damage risk of the flexible DC transmission system, threaten the safe and stable operation of the system, and become a key technical bottleneck restricting the construction of a new power system.

[0003] Therefore, it is necessary to develop a transient energy transfer timing control method for a flexible DC transmission system to improve the transfer speed of fault transient energy and avoid serious overvoltage of the converter sub-module at the same time. Summary of the Invention

[0004] In view of this, the present invention provides a transient energy transfer timing control method and device for a flexible DC transmission system, which are used to decouple the AC / DC loop control of the sending-end converter, reduce the energy accumulation in the sending-end converter, accelerate the transfer speed of fault transient energy, and suppress the voltage stress of the sub-module capacitor of the sending-end converter during the DC fault ride-through process of an ultra-long-distance flexible DC transmission system.

[0005] According to the first aspect of the embodiments of the present application, a transient energy transfer timing control method for a flexible DC transmission system is provided, including:

[0006] S1: When receiving a DC fault signal of the flexible DC transmission system, increase the proportional coefficient of the d-axis current loop PI controller of the sending-end converter to obtain a target d-axis voltage reference value;

[0007] S2: When receiving a DC fault signal of the flexible DC transmission system, increase the AC q-axis current reference value of the sending-end converter and limit the AC q-axis output voltage of the sending-end converter to obtain a target q-axis voltage reference value;

[0008] S3: Continuously execute S1 - S2 until the AC d-axis current of the sending-end converter decays to zero;

[0009] S4: When the AC d-axis current of the sending-end converter decays to zero, add DC active damping control to the control link of the sending-end converter;

[0010] S5: Calculate the target limiting value of the DC active damping control link according to the target d-axis voltage reference value and the target q-axis voltage reference value;

[0011] S6: Use the target limiting value as the target negative level of the sending converter bridge arm, design the target DC active damping value according to the fault ride-through requirement, and obtain the DC active damping output voltage;

[0012] S7: Continuously execute S4 - S6 until the DC fault current decays to zero.

[0013] According to the second aspect of the embodiments of the present application, there is provided a flexible DC transmission system DC fault transient energy transfer timing control device, including:

[0014] An AC d-axis control module, configured to increase the proportional coefficient of the d-axis current loop PI controller of the sending converter when receiving a DC fault signal of the flexible DC transmission system, and obtain a target d-axis voltage reference value;

[0015] An AC q-axis control module, configured to increase the AC q-axis current reference value of the sending converter and limit the AC q-axis output voltage of the sending converter when receiving a DC fault signal of the flexible DC transmission system, and obtain a target q-axis voltage reference value;

[0016] An AC transient energy transfer control module, configured to continuously execute the AC d-axis control module - AC q-axis control module until the AC d-axis current of the sending converter decays to zero;

[0017] A DC damping input control module, configured to add DC active damping control to the control link of the sending converter when the AC d-axis current of the sending converter decays to zero;

[0018] A DC output limiting control module, configured to calculate the target limiting value of the DC active damping control link according to the target d-axis voltage reference value and the target q-axis voltage reference value;

[0019] An active damping output control module, configured to use the target limiting value as the target negative level of the sending converter bridge arm, design the target DC active damping value according to the fault ride-through requirement, and obtain the DC active damping output voltage;

[0020] A DC transient energy transfer control module, configured to continuously execute the DC damping input control module - active damping output control module until the DC fault current decays to zero.

[0021] According to the third aspect of the embodiments of the present application, there is provided an electronic device, including:

[0022] One or more processors;

[0023] A memory, configured to store one or more programs;

[0024] When the one or more programs are executed by the one or more processors, the one or more processors implement the method as described in the first aspect.

[0025] According to a fourth aspect of the embodiments of the present application, a computer-readable storage medium is provided, on which computer instructions are stored, and when the instructions are executed by a processor, the steps of the method as described in the first aspect are implemented.

[0026] The technical solutions provided by the embodiments of the present application may include the following beneficial effects:

[0027] As can be seen from the above embodiments, the present invention takes into account the physical constraints of the negative level output range of the sending converter bridge arm, separates the AC transient energy control and DC transient energy control processes of the sending converter in time sequence, so that good effects can be achieved in the AC-DC loop control of the sending converter; the present invention increases the proportional coefficient of the d-axis current loop of the sending converter, increases the reference value of the AC q-axis current, and limits the reference value of the AC q-axis voltage, so as to accelerate the decay rate of the AC d-axis current of the sending converter, and further accelerate the decay rate of the energy fed from the AC side to the sending converter; the present invention designs the DC active damping value and the limit value of the DC active damping control link to accelerate the transfer speed of the DC side fault transient energy. Overall, the present invention can alleviate the energy accumulation in the sending converter during the DC fault crossing period of the flexible DC transmission system and suppress the capacitor voltage stress of the sub-modules of the sending converter. Description of the Drawings

[0028] Figure 1 is a flowchart of a method for controlling the time sequence of DC fault transient energy transfer in a flexible DC transmission system provided by an embodiment of the present invention.

[0029] Figure 2 is a block diagram of a method for controlling the time sequence of DC fault transient energy transfer in a flexible DC transmission system provided by an embodiment of the present invention.

[0030] Figure 3 is a flowchart of the AC d-axis control in a method for controlling the time sequence of DC fault transient energy transfer in a flexible DC transmission system provided by an embodiment of the present invention.

[0031] Figure 4 is a flowchart of the AC q-axis control in a method for controlling the time sequence of DC fault transient energy transfer in a flexible DC transmission system provided by an embodiment of the present invention.

[0032] Figure 5 is a flowchart of the DC control in a method for controlling the time sequence of DC fault transient energy transfer in a flexible DC transmission system provided by an embodiment of the present invention.

[0033] Figure 6It is a waveform diagram of the energy change of the sending converter in the DC fault transient energy transfer timing control of the flexible DC transmission system provided by the embodiment of the present invention.

[0034] Figure 7 It is a waveform diagram of the DC fault current in the DC fault transient energy transfer timing control of the flexible DC transmission system provided by the embodiment of the present invention.

[0035] Figure 8 It is a waveform diagram of the capacitor voltage of the sub-module of the sending converter in the DC fault transient energy transfer timing control of the flexible DC transmission system provided by the embodiment of the present invention.

[0036] Figure 9 It is a flowchart of the device for the DC fault transient energy transfer timing control of the flexible DC transmission system provided by the embodiment of the present invention. Detailed implementation manners

[0037] Here, the exemplary embodiments will be described in detail.

[0038] Figure 1 It is a flowchart of the method for the DC fault transient energy transfer timing control of the flexible DC transmission system provided by the embodiment of the present invention. As Figure 1 shown, the method may include the following steps:

[0039] S1: When receiving the DC fault signal of the flexible DC transmission system, increase the proportional coefficient of the PI controller of the d-axis current loop of the sending converter to obtain the target d-axis voltage reference value;

[0040] S2: When receiving the DC fault signal of the flexible DC transmission system, increase the AC q-axis current reference value of the sending converter and limit the AC q-axis output voltage of the sending converter to obtain the target q-axis voltage reference value;

[0041] S3: Continuously execute S1 - S2 until the AC d-axis current of the sending converter decays to zero;

[0042] S4: When the AC d-axis current of the sending converter decays to zero, add DC active damping control to the control link of the sending converter;

[0043] S5: According to the target d-axis voltage reference value and the target q-axis voltage reference value, calculate the target limit value of the DC active damping control link;

[0044] S6: Use the target limit value as the target negative level of the bridge arm of the sending converter, and design the target DC active damping value according to the fault ride-through requirement to obtain the DC active damping output voltage;

[0045] S7: Continuously execute S4 - S6 until the DC fault current decays to zero.

[0046] As can be seen from the above embodiments, the present invention takes into account the physical constraints of the negative voltage output range of the sending-end converter bridge arm, separates the AC transient energy control and DC transient energy control processes of the sending-end converter in time sequence, so that the AC and DC loop controls of the sending-end converter can achieve good effects; the present invention increases the proportional coefficient of the d-axis current loop of the sending-end converter, increases the reference value of the AC q-axis current, and limits the reference value of the AC q-axis voltage, so as to accelerate the decay rate of the AC d-axis current of the sending-end converter, and further accelerate the decay rate of the energy fed from the AC side to the sending-end converter; the present invention designs the DC active damping value and the limit value of the DC active damping control link to accelerate the transfer speed of the DC-side fault transient energy. Overall, the present invention can alleviate the energy accumulation in the sending-end converter during the DC fault crossing of the VSC-HVDC system and suppress the capacitor voltage stress of the sending-end converter sub-module.

[0047] Figure 2 It is a block diagram of a method for controlling the time sequence of DC fault transient energy transfer in a VSC-HVDC system provided by an embodiment of the present invention. Each step will be elaborated in detail below.

[0048] In the specific implementation of S1: when receiving the DC fault signal of the flexible DC transmission system, increase the proportional coefficient of the PI controller of the d-axis current loop of the sending-end converter to obtain the target d-axis voltage reference value; this step includes the following contents:

[0049] Specifically, Figure 3 It is a flowchart of the AC d-axis control in a method for controlling the time sequence of DC fault transient energy transfer in a VSC-HVDC system shown according to an exemplary embodiment. As Figure 3 shown, when receiving the DC fault signal of the flexible DC transmission system, increase the proportional coefficient of the PI controller of the d-axis current loop of the sending-end converter. Calculate the AC d-axis current error according to the AC d-axis current reference value and the actual value of the AC d-axis current of the sending-end converter. Take the AC d-axis current error as the input value of the PI controller of the d-axis current loop of the sending-end converter, and through the proportional-integral link of the d-axis current loop PI controller, obtain the AC d-axis output voltage of the sending-end converter.

[0050] Calculate the d-axis decoupling compensation term according to the actual value of the AC q-axis current of the sending-end converter, the AC angular frequency, and the equivalent inductance of the AC loop. Combine the AC d-axis output voltage of the sending-end converter and the d-axis decoupling compensation term to calculate the target d-axis voltage reference value of the sending-end converter. The above process can be expressed by the following formula:

[0051] ;

[0052] ;

[0053] ;

[0054] Wherein, I derror is the AC d-axis current error of the sending-end converter; I dref is the reference value of the AC d-axis current of the sending-end converter; I d is the actual value of the AC d-axis current of the sending-end converter; V dref0 is the output voltage of the AC d-axis of the sending-end converter of the sending-end converter; K p3 is the proportional coefficient of the PI controller of the d-axis current loop; K i3 is the integral coefficient of the PI controller of the d-axis current loop; p is the differential operator; V dref_targ is the reference value of the target d-axis voltage of the sending-end converter; ω is the angular frequency of the AC side of the sending-end converter; L is the equivalent inductance of the AC circuit of the sending-end converter; I q is the actual value of the AC q-axis current of the sending-end converter; V sd is the AC d-axis voltage of the sending-end converter.

[0055] In the specific implementation of S2: when receiving the DC fault signal of the flexible DC transmission system, increase the reference value of the AC q-axis current of the sending-end converter, limit the output voltage of the AC q-axis of the sending-end converter, and obtain the reference value of the target q-axis voltage.

[0056] Specifically, Figure 4 is the AC q-axis control flowchart in a DC fault transient energy transfer timing control method of a flexible DC transmission system shown according to an exemplary embodiment.

[0057] The expressions of the AC active power and the AC circuit equation of the sending-end converter in the flexible DC transmission system are as follows:

[0058] ;

[0059] ;

[0060] Wherein, P ac is the active power transmitted from the AC side to the sending-end converter; u sd is the AC d-axis voltage of the sending-end converter; i d is the AC d-axis current of the sending-end converter; e d is the internal potential of the AC d-axis of the sending-end converter; L t is the equivalent inductance of the transformer on the AC side of the sending-end converter; L0 is the arm inductance of the sending-end converter; R0 is the arm resistance of the sending-end converter; ω is the angular frequency of the AC side of the sending-end converter; i q is the AC q-axis current of the sending-end converter.

[0061] Therefore, the rapid decay of AC active power can be controlled by quickly reducing the AC d-axis current of the sending-end converter. According to the AC circuit equation of the sending-end converter and the physical constraints of the arm output voltage range, it is difficult to optimize the transient energy transfer process through the AC internal potential and the AC d-axis voltage. The value of the arm resistance is small, and its influence on the AC d-axis current can be ignored. Therefore, it is considered to increase the value of the AC q-axis current, so as to accelerate the reduction speed of the AC d-axis current of the sending-end converter.

[0062] Step S2 includes the following sub-steps:

[0063] S21: When receiving the DC fault signal of the flexible DC transmission system, increase the reference value of the AC q-axis current of the sending-end converter, use the difference between the reference value of the AC q-axis current of the sending-end converter and the actual value of the AC q-axis current as the input value of the q-axis current loop PI controller, and through the q-axis current loop PI controller, calculate the output voltage of the AC q-axis of the sending-end converter.

[0064] Specifically, increase the reference value of the AC q-axis current of the sending-end converter, calculate the AC q-axis current error according to the reference value of the AC q-axis current of the sending-end converter and the actual value of the AC q-axis current. Use the AC q-axis current error as the input value of the q-axis current loop PI controller of the sending-end converter, and through the proportional-integral link of the q-axis current loop PI controller, obtain the output voltage of the AC q-axis of the sending-end converter. The above process can be expressed by the following formula:

[0065] ;

[0066] ;

[0067] In the formula, I qerror is the AC q-axis current error of the sending-end converter; I qref is the reference value of the AC q-axis current of the sending-end converter; I q is the actual value of the AC q-axis current of the sending-end converter; V qref0 is the output voltage of the AC q-axis of the sending-end converter; K p4 is the proportional coefficient of the q-axis current loop PI controller; K i4 is the integral coefficient of the q-axis current loop PI controller; p is the differential operator.

[0068] It should be noted that according to the relationship between the synchronous rotating coordinate system and the three-phase static coordinate system in the present invention, the AC q-axis current of the negative pole of the sending-end converter is negative during normal operation. The reference value of the AC q-axis current of the sending-end converter described above only represents the numerical value and does not represent the positive or negative direction, and its actual value is reduced. At the same time, the reference value of the AC q-axis current of the sending-end converter has an upper limit, which is determined by the reactive power capacity transmitted by the sending-end converter.

[0069] The comparison relationship between the AC q-axis current reference values of the sending-end converter before and after the fault ride-through control in this embodiment is as follows:

[0070] ;

[0071] In the formula, I qref0 is the AC q-axis current reference value of the sending-end converter before the fault ride-through; I qref1 is the AC q-axis current reference value of the sending-end converter during the fault ride-through in this embodiment.

[0072] S22: Limit the AC q-axis output voltage of the sending-end converter, control the rapid decay of the AC d-axis current of the sending-end converter, and perform amplitude limiting on the q-axis output voltage according to the target d-axis voltage reference value and the AC q-axis output voltage of the sending-end converter to obtain the target q-axis voltage reference value of the sending-end converter.

[0073] Specifically, perform amplitude limiting on the AC q-axis output voltage of the sending-end converter so that the AC q-axis output voltage occupies less space of the bridge arm output voltage. Calculate the amplitude of the AC reference voltage of the sending-end converter according to the target d-axis voltage reference value and the AC q-axis output voltage of the sending-end converter, determine the q-axis voltage amplitude limit value, and perform amplitude limiting on the AC q-axis output voltage. The calculation formula for the amplitude of the AC reference voltage of the sending-end converter is expressed as follows:

[0074] ;

[0075] In the formula, V mref is the amplitude of the AC reference voltage of the sending-end converter; V dref_targ is the target d-axis voltage reference value of the sending-end converter; V qref0 is the AC q-axis output voltage of the sending-end converter.

[0076] The amplitude limiting process of the q-axis voltage reference value is expressed by the following formula:

[0077] ;

[0078] ;

[0079] In the formula, V qlim is the q-axis voltage amplitude limit value of the sending-end converter; V dcn is the rated DC voltage of the flexible DC transmission system; V dmref is the amplitude of the target d-axis reference voltage of the sending-end converter; V qref0 ' is the q-axis voltage reference value of the sending-end converter after considering amplitude limiting; V qref0 is the AC q-axis output voltage of the sending-end converter; V mref is the amplitude of the AC reference voltage of the sending-end converter.

[0080] Calculate the q-axis decoupling compensation term according to the actual value of the AC d-axis current of the sending-end converter, the AC angular frequency, and the equivalent inductance of the AC loop. Combine the reference value of the q-axis voltage of the sending-end converter after amplitude limiting and the q-axis decoupling compensation term to obtain the target reference value of the q-axis voltage of the sending-end converter. The above process can be expressed by the following formula:

[0081] ;

[0082] In the formula, V qref0 ’ is the reference value of the q-axis voltage of the sending-end converter considering amplitude limiting; V qref_targ is the target reference value of the q-axis voltage of the sending-end converter; ω is the angular frequency of the AC side of the sending-end converter; L is the equivalent inductance of the AC loop of the sending-end converter; I d is the actual value of the AC d-axis current of the sending-end converter; V sq is the AC q-axis voltage of the sending-end converter.

[0083] In the specific implementation of S4: When the AC d-axis current of the sending-end converter decays to zero, DC active damping control is added to the control link of the sending-end converter.

[0084] Specifically, Figure 5 is the DC control flowchart in a transient energy transfer timing control method for a flexible DC transmission system shown according to an exemplary embodiment. As Figure 5 shown, the control link of the sending-end converter detects the AC current in real time. When it detects that the AC d-axis current of the sending-end converter decays to zero, the DC active damping control is put into the control link of the sending-end converter. The enabling input time of the DC active damping control added to the control link of the sending-end converter is selected according to the following model:

[0085] ;

[0086] In the formula, V vir_enable is the enabling signal of the DC active damping control. When V vir_enable = 0, the DC active damping control is removed. When V vir_enable = 1, the DC active damping control is put into operation; i d is the AC d-axis current of the sending-end converter.

[0087] The DC active damping control will be added when the AC d-axis current of the sending-end converter decays to zero after the fault occurs. At this time, the enabling signal of the DC active damping control changes to 1. During the stage of adding the DC active damping, the control bridge arm will output an additional negative level to accelerate the decay speed of the DC fault current. When the DC fault current decays to zero, that is, after the DC fault is cleared, the DC active damping control will be removed.

[0088] In the specific implementation of S5: According to the target d-axis voltage reference value and the target q-axis voltage reference value, the target limit value of the DC active damping control link is calculated. This step includes the following sub-steps:

[0089] S51: According to the target d-axis voltage reference value and the target q-axis voltage reference value, calculate the amplitude of the abc three-phase AC voltage reference value of the sending converter.

[0090] Specifically, in the synchronous rotating coordinate system, the AC target voltage reference value of the sending converter is composed of the target d-axis voltage reference value and the target q-axis voltage reference value. Calculate the amplitude of the target dq-axis voltage reference value of the sending converter through mathematical relations; According to the amplitude of the target dq-axis voltage reference value of the sending converter, calculate the amplitude of the abc three-phase voltage reference value of the sending converter through the conversion relationship between the synchronous rotating coordinate system and the three-phase stationary coordinate system. This process can be expressed by the formula:

[0091] ;

[0092] ;

[0093] In the formula, V dqm is the amplitude of the AC dq-axis voltage reference value of the sending converter; V dref1 is the target d-axis voltage reference value of the sending converter; V qref1 is the target q-axis voltage reference value of the sending converter; V abcm is the amplitude of the AC abc three-phase voltage reference value of the sending converter.

[0094] S52: According to the amplitude and the bridge arm output voltage range, calculate the target limit value of the DC active damping control link.

[0095] Specifically, the bridge arm output voltage range of the sending converter is -V dcn / 2 ≤ V jk ≤ +V dcn / 2, where V dcn is the rated DC voltage of the flexible DC transmission system. According to the amplitude of the abc three-phase AC voltage reference value of the sending converter and the bridge arm output voltage range, the voltage margin considering the bridge arm output voltage limit and the AC reference voltage can be calculated. According to the voltage margin, determine the target limit value of the DC active damping control link. The above process can be expressed by the following formula:

[0096] ;

[0097] ;

[0098] ;

[0099] Wherein, V jkmin is the minimum value of the output voltage of the sending converter bridge arm; V dcn is the rated DC voltage of the flexible DC transmission system; V margin is the voltage margin considering the limit of the bridge arm output voltage and the AC reference voltage; V abcm is the amplitude of the reference value of the AC abc three-phase voltage of the sending converter; V vir_lim is the target limit value of the DC active damping control link.

[0100] In the specific implementation of S6: taking the target limit value as the target negative level of the sending converter bridge arm, designing the target DC active damping value according to the fault ride-through requirement, and obtaining the DC active damping output voltage. This step includes the following sub-steps:

[0101] S61: taking the target limit value as the target negative level of the sending converter bridge arm, and designing the target DC active damping value according to the fault ride-through requirement.

[0102] Specifically, as Figure 5 shown, calculating the target DC active damping value according to the fault ride-through requirement. For the DC fault ride-through process of the flexible DC transmission system, when the fault current decays to a certain value, it can be interrupted to achieve fault clearing. Taking the target limit value of the DC active damping control link as the target negative level of the sending converter bridge arm, the bridge arm is required to output the corresponding negative level during the fault ride-through. According to the fault ride-through requirement, using the numerical calculation method, the target DC active damping value of the converter DC active damping control of the flexible DC transmission system is calculated, and its model is as follows:

[0103] ;

[0104] ;

[0105] Wherein, V jkdc is the target negative level of the sending converter bridge arm; V vir_lim is the target limit value of the DC active damping control link; R vir is the target DC active damping value of the DC active damping control; I dc1 is the initial value of the DC current at the start of the fault ride-through; μ is a coefficient, indicating the ratio of the fault current magnitude when the active damping control exits the saturation limit to the peak value of the fault current during the ride-through.

[0106] It should be noted that the value of the coefficient μ affects the magnitude of the fault current when the active damping control exits the saturation limit. At the moment when the active damping control exits the saturation limit, the DC fault current does not actually decay to zero, but μI dc1, when the coefficient μ approaches zero, it can be approximated as zero. After the active damping control exits the saturation limit, the DC fault current will continue to decay to zero under the control of the sending-end converter. At this time, since the coefficient μ is very small, the DC fault current can be ignored because it is small enough.

[0107] S62: Use the actual value of the DC fault current, the reference value of the DC fault current, and the target DC active damping value as the inputs of the DC active damping link to obtain the DC active damping output voltage.

[0108] Specifically, use the actual value of the DC fault current, the reference value of the DC fault current, and the target DC active damping value as the inputs of the DC active damping link to calculate the DC active damping voltage. Limit the DC active damping voltage using the target limit value of the DC active damping control link to finally obtain the DC active damping control output voltage. Its model is as follows:

[0109] ;

[0110] ;

[0111] V vir is the DC active damping voltage of the sending-end converter; I dcref is the reference value of the DC current in the fault ride-through control, usually 0; I dc is the actual value of the DC current in the fault ride-through control; R vir is the target DC active damping value of the DC active damping control; V vir ’ is the DC active damping control output voltage of the sending-end converter; V vir_lim is the target limit value of the DC active damping control link.

[0112] In this embodiment, by separating the AC transient energy control and DC transient energy control processes of the sending-end converter in time sequence, increasing the proportional coefficient of the d-axis current loop of the sending-end converter, increasing the reference value of the AC q-axis current, and limiting the reference value of the AC q-axis voltage, designing the DC active damping value and limit value, on the one hand, accelerating the attenuation speed of the energy fed from the AC side to the sending-end converter, and on the other hand, accelerating the transfer speed of the DC-side fault transient energy, overall alleviating the energy accumulation in the sending-end converter during the DC fault ride-through of the VSC-HVDC system and suppressing the capacitor voltage stress of the sending-end converter sub-module.

[0113] The parameters of the ultra-long-distance VSC-HVDC system are shown in Table 1.

[0114] Table 1:

[0115] Parameter variable Value DC voltage 800 kV HVDC converter capacity 2800 MVA Rated voltage of sub-module capacitor 2 kV Number of full-bridge sub-modules in bridge arm 200 Number of half-bridge sub-modules in bridge arm 200 HVDC transmission distance 2700 km Unit resistance of line 0.00282 Ω / km Unit reactance of line <![CDATA[0.177×10 -7 Ω / km]]> <![CDATA[D-axis current loop K before fault p3 parameter]]> 50 <![CDATA[d-axis current loop K after fault p3 parameter]]> 500 Reference value of q-axis current before fault 0.778 kA Reference value of q-axis current after fault 2.333 kA Coefficient μ 0.02

[0116] The effectiveness of the DC fault transient energy transfer timing control method for the flexible DC transmission system is verified in the following aspects: the effectiveness of suppressing the energy accumulation in the sending converter during fault crossing, suppressing the capacitor voltage stress of the converter sub-module, and accelerating the decay rate of the DC fault current.

[0117] As the AC / DC loop interface, the MMC converter acts as a transient energy transfer medium. During the DC fault detection stage, a large amount of energy in the converter is transferred into the DC transmission line; after the fault crossing is initiated, the AC energy is fed into the converter in a rapidly decaying state under the control of the AC loop; under the control of the DC loop, a large amount of energy on the DC line is transferred into the converter. The separation of the AC / DC control timing makes the accumulation of AC / DC energy non-overlapping, effectively alleviating the capacitor voltage stress of the sub-module; in the later stage of fault crossing, the energy on the DC line is transferred from the converter to the AC side, and the sub-module voltage gradually stabilizes.

[0118] When the DC fault transient energy transfer timing control method for the flexible DC transmission system starts to respond, the energy change ΔE in the sending converter of the flexible DC transmission system MMC has a waveform as Figure 6 shown. Under the DC fault transient energy transfer timing control method, the peak value of the energy absorbed by the converter is 3,058,150 J, which is 47.3% less than the peak value of the energy absorbed by the converter under the traditional fault crossing control, which is 6,463,570 J. The fault transient energy transfer timing control method can significantly reduce the total energy absorbed by the converter from the AC / DC sides in the initial stage of fault crossing, effectively alleviating the accumulation of transient energy in the converter.

[0119] The waveforms of the DC fault current and the capacitor voltage of the sub-module under the DC fault transient energy transfer timing control method for the flexible DC transmission system are as Figure 7 and Figure 8 shown. The fault transient energy transfer timing control can be effectively realized for both by separating the control of the AC loop and the DC loop in terms of timing. The DC fault current clearing time is 211 ms, which is 5.8% faster than the DC fault current clearing time of 224 ms under the traditional fault crossing control. The peak value of the capacitor voltage stress of the sub-module is 1.24 p.u., which is 6.1% less than the peak value of the capacitor voltage stress of the sub-module of 1.32 p.u. under the traditional fault crossing control.

[0120] Corresponding to the embodiment of the DC fault transient energy transfer timing control method for the flexible DC transmission system described above, the present application also provides an embodiment of a DC fault transient energy transfer timing control device for the flexible DC transmission system.

[0121] Figure 9 is a block diagram of a DC fault transient energy transfer timing control device for a flexible DC transmission system shown according to an exemplary embodiment. Referring to Figure 9 , the device includes:

[0122] The AC d-axis control module 1 is used to increase the proportional coefficient of the d-axis current loop PI controller of the sending-end converter when receiving the DC fault signal of the flexible DC transmission system, so as to obtain the target d-axis voltage reference value;

[0123] The AC q-axis control module 2 is used to increase the AC q-axis current reference value of the sending-end converter and limit the AC q-axis output voltage of the sending-end converter when receiving the DC fault signal of the flexible DC transmission system, so as to obtain the target q-axis voltage reference value;

[0124] The AC transient energy transfer control module 3 is used to continuously execute the AC d-axis control module - AC q-axis control module until the AC d-axis current of the sending-end converter decays to zero;

[0125] The DC damping input control module 4 is used to add DC active damping control to the control link of the sending-end converter when the AC d-axis current of the sending-end converter decays to zero;

[0126] The DC output limit control module 5 is used to calculate the target limit value of the DC active damping control link according to the target d-axis voltage reference value and the target q-axis voltage reference value;

[0127] The active damping output control module 6 is used to use the target limit value as the target negative level of the sending-end converter bridge arm, design the target DC active damping value according to the fault ride-through requirement, and obtain the DC active damping output voltage;

[0128] The DC transient energy transfer control module 7 is used to continuously execute the DC damping input control module - active damping output control module until the DC fault current decays to zero.

[0129] Regarding the device in the above embodiments, the specific ways for each module to execute operations have been described in detail in the embodiments related to the method, and will not be elaborated here.

[0130] For the device embodiments, since they basically correspond to the method embodiments, the relevant parts can be referred to the partial descriptions of the method embodiments. The device embodiments described above are only illustrative. The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the solution of the present application. Those of ordinary skill in the art can understand and implement it without creative efforts.

[0131] Correspondingly, the present application further provides an electronic device, including: one or more processors; a memory for storing one or more programs; when the one or more programs are executed by the one or more processors, the one or more processors implement the DC fault transient energy transfer timing control method of the flexible DC transmission system as described above.

[0132] Correspondingly, the present application further provides a computer-readable storage medium, on which computer instructions are stored, and when the instructions are executed by a processor, the DC fault transient energy transfer timing control method of the flexible DC transmission system as described above is implemented.

[0133] It should be understood that the present application is not limited to the exact structure already described and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present application is only limited by the appended claims.

Claims

1. A method for controlling the timing of transient energy transfer in a DC fault in a flexible DC power transmission system, characterized in that: include: S1: when receiving a DC fault signal of the flexible DC transmission system, increasing the proportional coefficient of the PI controller of the d-axis current loop of the sending-end converter to obtain a target d-axis voltage reference value; S2: when receiving a DC fault signal of the flexible DC transmission system, increasing the AC q-axis current reference value of the sending-end converter, limiting the AC q-axis output voltage of the sending-end converter, and obtaining a target q-axis voltage reference value; S3: Continue to execute S1-S2 until the AC d-axis current of the sending-end converter decays to zero; S4: When the AC d-axis current of the sending-end converter decays to zero, a DC active damping control is added to the control link of the sending-end converter; S5: Calculate a target amplitude limit value of a DC active damping control link according to the target d-axis voltage reference value and the target q-axis voltage reference value; S6: taking the target amplitude limit value as the target negative level of the bridge arm of the sending-end converter, designing the target DC active damping value according to the fault ride-through requirement, and obtaining the DC active damping output voltage; S7: Continue to execute S4-S6 until the DC fault current decays to zero.

2. The method according to claim 1, characterized in that: When a DC fault signal of the flexible DC transmission system is received, the proportional coefficient of the PI controller of the d-axis current loop of the sending-end converter is increased to obtain a target d-axis voltage reference value, including: When a DC fault signal of the flexible DC transmission system is received, the proportional coefficient of the d-axis current loop PI controller of the sending-end converter is increased, and the difference between the AC d-axis current reference value of the sending-end converter and the actual value of the AC d-axis current is used as the input value of the d-axis current loop PI controller. The target d-axis voltage reference value of the sending-end converter is calculated through the d-axis current loop PI controller.

3. The method according to claim 1, characterized in that When a DC fault signal of the flexible DC transmission system is received, the AC q-axis current reference value of the sending-end converter is increased, the AC q-axis output voltage of the sending-end converter is limited, and a target q-axis voltage reference value is obtained, including: When a DC fault signal of the flexible DC transmission system is received, the reference value of the AC q-axis current of the sending-end converter is increased, and the difference between the reference value of the AC q-axis current of the sending-end converter and the actual value of the AC q-axis current is used as the input value of the q-axis current loop PI controller, and the AC q-axis output voltage of the sending-end converter is calculated through the q-axis current loop PI controller; The AC q-axis output voltage of the sending-end converter is limited, and the AC d-axis current of the sending-end converter is controlled to decay rapidly. According to the target d-axis voltage reference value of the sending-end converter and the AC q-axis output voltage, the q-axis output voltage is limited to obtain the target q-axis voltage reference value of the sending-end converter.

4. The method according to claim 1, characterized in that: According to the target d-axis voltage reference value and the target q-axis voltage reference value, a target amplitude limit value of a DC active damping control link is calculated, including: According to the target d-axis voltage reference value and the target q-axis voltage reference value, the amplitude of the three-phase AC voltage reference value of the sending-end converter abc is calculated; According to the amplitude and the bridge arm output voltage range, the target amplitude limit value of the DC active damping control link is calculated.

5. The method according to claim 1, characterized in that The target amplitude limit value is used as the target negative level of the bridge arm of the sending-end converter, and the target DC active damping value is designed according to the fault ride-through requirement to obtain the DC active damping output voltage, including: Taking the target amplitude limit value as the target negative level of the bridge arm of the sending-end converter, the target DC active damping value is designed according to the fault ride-through requirement; The actual value of the DC fault current, the reference value of the DC fault current and the target DC active damping value are used as inputs of the DC active damping link to obtain a DC active damping output voltage.

6. A timing control device for transient energy transfer of DC fault in flexible DC transmission system, characterized in that: include: The AC d-axis control module is used to increase the proportional coefficient of the PI controller of the d-axis current loop of the sending-end converter to obtain a target d-axis voltage reference value when receiving a DC fault signal of the flexible DC transmission system; The AC q-axis control module is used to increase the AC q-axis current reference value of the sending-end converter and limit the AC q-axis output voltage of the sending-end converter to obtain a target q-axis voltage reference value when receiving a DC fault signal of the flexible DC transmission system; The AC transient energy transfer control module is used to continuously execute the AC d-axis control module-AC q-axis control module until the AC d-axis current of the sending-end converter decays to zero; A DC damping input control module is used to add DC active damping control to the control link of the sending-end converter when the AC d-axis current of the sending-end converter decays to zero; A DC output limiting control module, used for calculating a target limiting value of a DC active damping control link according to the target d-axis voltage reference value and the target q-axis voltage reference value; An active damping output control module is used to design a target DC active damping value according to the fault ride-through requirement with the target amplitude limit value as the target negative level of the bridge arm of the sending-end converter, and obtain a DC active damping output voltage; The DC transient energy transfer control module is used to continuously execute the DC damping input control module-active damping output control module until the DC fault current decays to zero.

7. An electronic device, characterized in that: include: one or more processors; A memory for storing one or more programs; When the one or more programs are executed by the one or more processors, the one or more processors implement the method according to any one of claims 1 to 5.

8. A computer-readable storage medium having computer instructions stored thereon, characterized in that: When the instruction is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

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