Method and device for controlling rapid attenuation of fault current in overhead lines of flexible DC transmission systems

Abstract

The present invention belongs to the field of power system protection and control, and discloses a method and device for rapidly attenuating fault current in an overhead line of a flexible DC transmission system. The method comprises: upon receiving an overhead line fault signal and a sending-end converter arm output limiting signal, controlling the sending-end converter submodule to boost voltage, coordinating the power reference values ​​of the faulted pole and the non-faulted pole, and obtaining a sending-end converter AC voltage reference value; obtaining a target bridge arm three-phase AC reference voltage after third harmonic injection based on the sending-end converter AC voltage reference value; limiting the output voltage of the active damping control link based on the sending-end converter three-phase AC reference voltage and the DC active damping output voltage; and continuously executing these steps until the overhead line DC fault current decays to zero. The method of the present invention can reduce the constraints imposed by the bridge arm output voltage range on the fault current decay rate, thereby accelerating the decay rate of the overhead line fault current.

Description

Technical Field

[0001] The present application relates to the technical field of power system protection and control, and in particular to a method and device for rapidly attenuating fault current in an overhead line of a flexible direct current transmission system. Background Art

[0002] As a new generation of high-voltage direct current transmission, flexible direct current transmission technology has become the core technology for achieving ultra-long-distance and ultra-large-capacity power transmission with its advantages such as independent control of active / reactive power and no risk of commutation failure. It has shown great application potential in the fields of deep-sea wind power transmission and cross-regional power grid interconnection.

[0003] Ultra-long-distance direct current (DC) transmission typically uses overhead lines, which have a high probability of DC faults. When a DC short-circuit fault occurs on an overhead line in an ultra-long-distance flexible DC transmission system, the large amount of energy contained in the long-distance line leads to a long time for the DC fault current to clear. Introducing active DC damping control can accelerate the decay of the fault current. However, due to the physical constraints of the negative output range of the modular multilevel converter (MMC) bridge arm, the converter output is limited, resulting in slower decay of the DC fault current and a high risk of causing severe overvoltage in the submodule capacitors.

[0004] Therefore, it is necessary to develop a fault current rapid attenuation control method to improve the fault current attenuation speed while alleviating the sub-module voltage stress. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a method and device for controlling the rapid attenuation of overhead line fault current in a flexible direct current transmission system, which is used to reduce the restrictions on the rapid attenuation of fault current caused by the physical constraints of the negative level output range of the converter bridge arm during the process of riding through the overhead line fault in an ultra-long-distance flexible direct current transmission system, maximize the utilization of the bridge arm output voltage space, accelerate the attenuation speed of the overhead line fault current, and alleviate the voltage stress of the sub-module.

[0006] According to a first aspect of an embodiment of the present application, a method for rapidly attenuating fault current in an overhead line of a flexible direct current transmission system is provided, comprising:

[0007] S1: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the voltage reference value of the modulation link;

[0008] S2: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference value;

[0009] S3: According to the sending end converter d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected;

[0010] S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage;

[0011] S5: Continue to execute S1-S4 until the DC fault current of the overhead line decays to zero.

[0012] According to a second aspect of an embodiment of the present application, a device for rapidly attenuating fault current of an overhead line of a flexible direct current transmission system is provided, comprising:

[0013] The submodule boost control module is used to control the boost of the sending-end converter submodule when receiving the overhead line fault signal and the sending-end converter bridge arm output limit signal, and obtain the sending-end converter after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the voltage reference value of the modulation link;

[0014] The power coordination control module of the fault pole and the non-fault pole is used to coordinate the power reference values ​​of the fault pole and the non-fault pole according to the power reference values ​​of the fault pole and the non-fault pole when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, and obtain the sending-end converter after the power current double-loop PI control. q Shaft AC voltage reference value;

[0015] The third harmonic injection control module is used to control the output current of the sending end converter according to the d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected;

[0016] A DC active damping limiting control module is used to limit the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage;

[0017] The fault current attenuation control module is used to continuously execute the submodule boost control module-DC active damping limit control module until the overhead line DC fault current decays to zero.

[0018] According to a third aspect of the embodiments of the present application, there is provided an electronic device, including:

[0019] one or more processors;

[0020] a memory for storing one or more programs;

[0021] 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.

[0022] According to a fourth aspect of an embodiment of the present application, a computer-readable storage medium is provided, on which computer instructions are stored. When the instructions are executed by a processor, the steps of the method described in the first aspect are implemented.

[0023] The technical solutions provided by the embodiments of the present application may have the following beneficial effects:

[0024] It can be seen from the above embodiments that the present invention reduces the amplitude of the three-phase AC reference voltage by injecting the third harmonic when calculating the three-phase AC reference voltage during the overhead line DC fault ride-through, reduces the restriction of the bridge arm output voltage range on the negative voltage level of the bridge arm output, and thus accelerates the fault current decay speed; the present invention reduces the amplitude of the three-phase AC reference voltage by coordinating the power reference values ​​of the fault pole and the non-fault pole, and at the same time reduces the power transmission shortage of the sending-end converter during the DC fault, thereby avoiding the instability of the flexible DC transmission system; the present invention widens the output voltage range of the bridge arm of the sending-end converter by controlling the boost of the sending-end converter submodule and increasing the voltage reference value of the modulation link, so that the bridge arm outputs more negative levels, thereby accelerating the fault current decay speed; the present invention reduces the restriction of the bridge arm output voltage range by calculating and applying the optimal active damping control limit value, thereby accelerating the fault current decay speed and avoiding overvoltage of the submodule capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a flow chart of a method for rapidly controlling fault current attenuation of an overhead line in a flexible direct current transmission system provided by an embodiment of the present invention.

[0026] Figure 2 This is a block diagram of a method for rapidly controlling fault current decay of an overhead line in a flexible direct current transmission system provided by an embodiment of the present invention.

[0027] Figure 3 This is a flow chart of the submodule boost control strategy provided by an embodiment of the present invention.

[0028] Figure 4 This is a flow chart of the power coordination control strategy for faulty poles and non-faulty poles provided by an embodiment of the present invention.

[0029] Figure 5 This is a flow chart of the third harmonic injection strategy provided by an embodiment of the present invention.

[0030] Figure 6This is a waveform diagram of a DC fault current for rapid fault current decay control provided by an embodiment of the present invention.

[0031] Figure 7 The invention provides a fault current rapid decay control sending-end converter. a Phase AC voltage reference value waveform.

[0032] Figure 8 The invention provides a fault current rapid decay control sending-end converter. a Average switching function waveform of the phase bridge arm.

[0033] Figure 9 This is a block diagram of a device for rapidly attenuating fault current in an overhead line of a flexible direct current transmission system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0034] Here, exemplary embodiments will be described in detail.

[0035] Figure 1 FIG. 1 is a flow chart showing a method for rapidly controlling fault current attenuation of an overhead line in a flexible direct current transmission system according to an exemplary embodiment. Figure 1 As shown, the method may include the following steps:

[0036] S1: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the voltage reference value of the modulation link;

[0037] S2: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference value;

[0038] S3: According to the sending end converter d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected;

[0039] S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage;

[0040] S5: Continue to execute S1-S4 until the DC fault current of the overhead line decays to zero.

[0041] It can be seen from the above embodiments that the present invention reduces the amplitude of the three-phase AC reference voltage by injecting the third harmonic when calculating the three-phase AC reference voltage during the overhead line DC fault ride-through, reduces the restriction of the bridge arm output voltage range on the negative voltage level of the bridge arm output, and thus accelerates the fault current decay speed; the present invention reduces the amplitude of the three-phase AC reference voltage by coordinating the power reference values ​​of the fault pole and the non-fault pole, and at the same time reduces the power transmission shortage of the sending-end converter during the DC fault, thereby avoiding the instability of the flexible DC transmission system; the present invention widens the output voltage range of the bridge arm of the sending-end converter by controlling the boost of the sending-end converter submodule and increasing the voltage reference value of the modulation link, so that the bridge arm outputs more negative levels, thereby accelerating the fault current decay speed; the present invention reduces the restriction of the bridge arm output voltage range by calculating and applying the optimal active damping control limit value, thereby accelerating the fault current decay speed and avoiding overvoltage of the submodule capacitor.

[0042] It should be noted that, in the specific implementation, the DC short circuit fault signal of the overhead line is given by the fault detection link, and the output limiting signal of the bridge arm of the sending end converter is obtained by real-time detection of the bridge arm reference voltage. V dcn / 2 or less than - V dcn / 2 is considered that the output of the bridge arm of the sending-end converter is limited, where V dcn is the rated DC voltage of the sending-end converter. At this time, the output limit signal of the bridge arm of the sending-end converter is given, and it is necessary to start the rapid decay control of the overhead line fault current of the flexible DC transmission system.

[0043] Figure 2 This is a block diagram of a method for rapidly attenuating fault current in overhead lines of a flexible DC transmission system, provided by an embodiment of the present invention. Each step is described in detail below.

[0044] In the specific implementation of S1: when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The shaft AC voltage reference value is used to increase the modulation link voltage reference value; this step includes the following sub-steps:

[0045] S11: Control the boost voltage of the sending-end converter submodule to increase the reference value of the average capacitor voltage of the sending-end converter submodule. After the voltage and current double-loop PI control, the sending-end converter is obtained. d Shaft AC voltage reference value;

[0046] Specifically, Figure 3 FIG. 1 is a flow chart showing a submodule boost control in a fast decay control strategy for an overhead line fault current in a flexible DC transmission system according to an exemplary embodiment. Figure 3 As shown, when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the reference value of the average capacitor voltage of the sending-end converter submodule is switched to V cref1 , improves the reference value of the average capacitor voltage of the submodule. The error between the reference value of the average capacitor voltage of the sending-end converter submodule and the actual value of the average capacitor voltage of the sending-end converter submodule is used as the input value of the voltage loop PI controller. According to the output of the voltage loop PI controller, the AC voltage of the sending-end converter d The shaft voltage and decoupling compensation are used to obtain the AC current of the sending-end converter. d The shaft voltage reference value. The above process can be expressed by the following formula:

[0047] ;

[0048] ;

[0049] ;

[0050] Where, V cref1 is the reference value of the capacitor voltage of the sending-end converter submodule during fault ride-through; V cref0 is the initial reference value of the capacitor voltage of the sending-end converter submodule; k c is the submodule capacitor boost coefficient, k c >1; I dref For the sending end converter d Shaft AC current reference value; K p1 、 K i1 are the proportional coefficient and integral coefficient of the submodule voltage loop PI controller; p is the differential operator; V c1 is the average voltage of the capacitor of the sending-end converter submodule; V dref1 For the sending end converter d Shaft AC voltage reference value; K p3 、 K i3 for d Proportional coefficient and integral coefficient of the axis current loop PI controller; I d For the sending end converter d Shaft AC current; ω is the AC side angular frequency of the sending-end converter; L is the equivalent inductance of the AC circuit of the sending-end converter;I q For the sending end converter q Shaft AC current; V sd AC for the sending-end converter d Shaft voltage.

[0051] In the boost control of the submodule of the sending-end converter, the submodule boost coefficient k c >1, the upper limit is determined by the multiple of the submodule's short-term overvoltage operation. k c =1.3.

[0052] S12: Calculating the maximum bridge arm output voltage based on the reference value of the average capacitor voltage of the sending-end converter submodule, and performing capacitor modulation using the maximum bridge arm output voltage as a voltage reference;

[0053] Specifically, the sending-end converter of the ultra-long-distance flexible direct current transmission system is a modular multilevel converter, and the modulation process generally adopts direct modulation. When the boost control of the sending-end converter submodule is started, the maximum output voltage of the sending-end converter bridge arm is calculated based on the average voltage reference value of the sending-end converter submodule capacitor. The submodule modulation signal generation link uses the maximum output voltage of the bridge arm as the voltage reference to generate the average switching function of the sending-end converter bridge arm. The above process can be expressed by the following formula:

[0054]

[0055]

[0056] Where, V N is the calculated maximum output voltage of the bridge arm of the sending-end converter; N is the number of bridge arm submodules of the sending-end converter; V cref1 is the reference value of the capacitor voltage of the sending-end converter submodule during fault ride-through; S jp1 、 S jn1 is the average switching function of the upper and lower bridge arms of the sending-end converter during the fault ride-through period; V jref1 For the sending end converter abc Three-phase AC voltage reference value, where j represent abc three-phase; V cirj Output voltage of circulating current suppression link of sending-end converter; V vir It is the output voltage of the DC active damping control link of the sending-end converter.

[0057] It should be noted that by controlling the voltage boost of the sending-end converter submodule and increasing the maximum bridge arm output voltage in the submodule modulation signal generation stage, the amplitude of the bridge arm average switching function is reduced, reducing the limitation of the sending-end converter bridge arm output voltage range on the fault current decay rate. A comparison of the sending-end converter bridge arm average switching function in conventional fault ride-through control and the fault ride-through control of this embodiment can be expressed by the following formula:

[0058]

[0059] Where, S jk0 is the average switching function of the bridge arm of the sending-end converter during the traditional fault ride-through period; S jk1 is the average switching function of the bridge arm of the sending-end converter during the fault ride-through period of this embodiment; V jref1 For the sending end converter abc Three-phase AC voltage reference value, where j represent abc three-phase; V cirj Output voltage of circulating current suppression link of sending-end converter; V vir Output voltage of the DC active damping control link of the sending-end converter; V cref0 is the reference value of the capacitor voltage of the sending-end converter submodule during the traditional fault ride-through period; V cref1 It is the reference value of the capacitor voltage of the sending-end converter submodule during the fault ride-through period of this embodiment.

[0060] This embodiment expands the voltage range of the bridge arm output by increasing the average voltage of the sub-module capacitor of the sending-end converter during fault ride-through. The sending-end converter bridge arm can output more negative levels, thereby reducing the physical constraints of the bridge arm output voltage range, thereby achieving the goal of accelerating the attenuation speed of the overhead line DC fault current.

[0061] In the specific implementation of S2: when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power current double-loop PI control. q Shaft AC voltage reference value; reference Figure 4 , which includes the following sub-steps:

[0062] S21: according to the active power reference values ​​and reactive power reference values ​​of the faulty pole and the non-faulty pole, within the power limit range of the converter, reducing the reactive power reference value of the faulty pole, reducing the reactive power reference value of the non-faulty pole, and increasing the active power reference value of the non-faulty pole;

[0063] Specifically, when an overhead line fault signal and a sending-end converter arm output limiting signal are received, the power reference values ​​of the fault pole and the non-fault pole are coordinated based on the power reference values ​​of the fault pole and the non-fault pole, and the active and reactive power transmitted between the fault pole and the non-fault pole is actively regulated. The reactive power reference value of the converter non-fault pole is reduced, the active power reference value of the converter non-fault pole is increased, and the reactive power reference value of the converter fault pole is increased. The comparison of the non-fault pole reactive power, non-fault pole active power, and fault pole reactive power before fault ride-through and after the fault current rapid decay control strategy is activated can be expressed by the following formula:

[0064]

[0065]

[0066]

[0067] Where, Q acpref0 、 Q acpref1 are the non-fault pole reactive power reference values ​​before fault ride-through and after the fault current rapid decay control strategy is initiated; P acpref0 、 P acpref1 are the reference values ​​of the active power of the non-fault pole before fault ride-through and after the fault current rapid decay control strategy is started; Q acnref0 、 Q acnref1 They are the fault pole reactive power reference values ​​before fault ride-through and after the fault current rapid decay control strategy is started, respectively.

[0068] It should be noted that the reference value of the positive reactive power of the sending-end converter is generally positive, the negative reactive power is generally negative, and the positive and negative active power are positive. Q acpref0 、 Q acpref1 、 P acpref0 、 P acpref1 、 Q acnref0 、 Q acnref1 All are numerical values ​​and do not include positive or negative signs.

[0069] S22: After the power and current dual-loop PI control, the sending-end converter is obtained q Shaft AC voltage reference value;

[0070] Specifically, after the power and current double-loop PI control, the sending-end converter is obtained q Axis AC voltage reference value; at this time, the faulty AC pole of the sending end converter q The shaft current model and the comparison of the relationship before and after the fault are expressed as follows:

[0071]

[0072] Where, I qn0 、I qn1 The power coordination control strategy for the fault pole and the non-fault pole in this embodiment starts the AC of the front and rear sending end converters. q Shaft current; Q acn1 After the fault current fast decay control strategy is started, the reactive power of the fault pole of the sending-end converter is calculated; V sd AC for the sending-end converter d Shaft voltage.

[0073] It should be noted that the negative reactive power of the sending-end converter is generally negative, the AC q-axis current of the sending-end converter is negative, and the AC current of the sending-end converter is negative after the fault current fast decay control strategy is started. q The shaft current decreases.

[0074] Faulty pole of the sending-end converter d The model of the shaft current loop equation and the comparison of the relationship before and after the fault are expressed as follows:

[0075]

[0076] Where, V dref0 、 V dref1 The fault current rapid decay control strategy in this embodiment starts the front and rear sending end converters d Shaft AC voltage reference value; K p3 、 K i3 for d Proportional coefficient and integral coefficient of the axis current loop PI controller; p is the differential operator; I d Sending-end converter AC d Shaft current; I dref0 、 I dref1The fault current rapid decay control strategy in this embodiment starts the front and rear sending end converters d Shaft AC current reference value; ω is the AC side angular frequency of the sending-end converter; L is the equivalent inductance of the AC circuit of the sending-end converter; I qn0 、 I qn1 The fault current rapid decay control strategy in this embodiment starts the front and rear sending end converters q Shaft AC current; V sd AC for the sending-end converter d Shaft voltage.

[0077] It should be noted that, in this embodiment, the coordination of active power and reactive power between the faulty pole and the non-faulty pole of the sending-end converter complies with the following requirements: the reactive power reference value of the faulty pole increases, but is less than the reactive power transmission limit of the faulty pole; the reactive power reference value of the non-faulty pole decreases, and at the maximum limit the reactive power reference value of the non-faulty pole is zero; the active power reference value of the non-faulty pole increases, and it is necessary to satisfy that the total transmission power of the non-faulty pole is less than the transmission capacity.

[0078] This embodiment coordinates the power reference values ​​of the fault pole and the non-fault pole of the sending-end converter during the fault ride-through period, so that the AC power of the fault pole of the sending-end converter is q The reduction in shaft current reduces the overall amplitude of the three-phase AC voltage reference, allowing the sending-end converter to increase the output of negative DC voltage, allowing the sending-end converter bridge arm to output more negative voltage, thereby accelerating the decay of DC fault current in the overhead line. Simultaneously, the active power transmitted by the non-fault poles of the sending-end converter increases, contributing to the stability of the flexible DC transmission system and significantly reducing the risk of system instability.

[0079] In the specific implementation of S3: according to the sending end converter d Shaft AC voltage reference and q The reference value of the shaft AC voltage is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and to determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected; Figure 5 , which includes the following sub-steps:

[0080] S31: According to the sending end converter d Shaft AC voltage reference and q The axis AC voltage reference value is used to obtain the amplitude and phase of the three-phase AC voltage through trigonometric function relationships;

[0081] Specifically, according to the AC converter of the sending end obtained in the above steps dq The axis reference voltage is obtained by Park inverse transformation abcAC reference voltage in a three-phase stationary coordinate system. The amplitude and phase of the three-phase AC reference voltage are obtained through trigonometric functions. The formula for the above process is as follows:

[0082]

[0083] Where, V m1 is the amplitude of the AC reference voltage of the sending-end converter; θ j1 is the phase of the AC reference voltage; V aref1 、 V bref1 、V cref1 for abc Three-phase AC reference voltage; f Represents the calculation process of trigonometric function relationships.

[0084] f The formula for calculating the trigonometric function relationship is as follows:

[0085]

[0086] Where, V m1 is the amplitude of the AC reference voltage of the sending-end converter; V iref1 for abc Three-phase AC reference voltage; θ j1 is the phase of the AC reference voltage; V jref1 for abc Three-phase AC reference voltage.

[0087] S32: Calculating the optimal amplitude and phase of the target third harmonic according to the amplitude and phase of the three-phase AC voltage;

[0088] Specifically, based on the calculated three-phase AC reference voltage amplitude and phase, the optimal amplitude and phase of the target third harmonic to be injected are calculated, and the formula is expressed as follows:

[0089]

[0090]

[0091] Where, V m3 is the amplitude of the target third harmonic voltage to be injected; V m1 is the amplitude of the AC reference voltage of the sending-end converter; θj3 is the phase of the target third harmonic voltage to be injected; θ j1 is the phase of the AC reference voltage of the sending-end converter; V jref3 is the target third harmonic voltage that needs to be injected.

[0092] It should be noted that the amplitude of the target third harmonic voltage to be injected is V m3 Obtained through mathematical calculation, it meets the following requirements: After the third harmonic voltage is superimposed on the original AC reference voltage, the minimum value of the overall waveform is maximized, and the amplitude is obtained at this time V m3 .

[0093] S33: Calculating a target AC reference voltage of the sending-end converter based on the AC voltage reference value and the superposition of the third harmonic according to the optimal amplitude and phase of the target third harmonic.

[0094] Specifically, determine the amplitude and phase of the target three-phase AC reference voltage of the sending-end converter. abc The AC reference voltage in the three-phase stationary coordinate system is superimposed on the target third harmonic voltage to be injected, and the target three-phase AC reference voltage of the sending-end converter after the third harmonic injection is obtained and input. The formula is expressed as follows:

[0095]

[0096] Where, V jref is the target three-phase AC reference voltage after third harmonic injection; V m3 is the amplitude of the target third harmonic voltage to be injected; V m1 is the amplitude of the AC reference voltage of the sending-end converter; θ j3 is the phase of the target third harmonic voltage to be injected; θ j1 is the phase of the AC reference voltage of the sending-end converter.

[0097] During fault ride-through, this embodiment generates the target three-phase AC reference voltage for the sending-end converter by injecting the third harmonic. This reduces the overall amplitude of the sending-end converter's three-phase AC reference voltage, thereby providing the sending-end converter with more voltage headroom for negative output. This increases the negative output of the sending-end converter, thereby accelerating the decay of the overhead line DC fault current.

[0098] In the specific implementation of S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; this step includes the following sub-steps:

[0099] S41: Calculating an optimal active damping control limit value according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage;

[0100] The optimal limit value of the DC active damping control link is calculated based on the output voltage of the DC active damping control link and the target three-phase AC reference voltage of the sending-end converter obtained from the aforementioned links.

[0101] During fault ride-through, the arm output voltage is limited by the physical constraints of the arm output range. To simultaneously control the AC and DC circuits of the sending-end converter, the arm output voltage range must meet the output requirements of both the target AC reference voltage and the DC active damping output voltage. By coordinating the target AC reference voltage with DC active damping control, the arm output voltage range can be maximized, minimizing the impact of the arm output limit on the control performance of the sending-end converter. The formula for the output limit design of the active damping control link is expressed as follows:

[0102]

[0103]

[0104] Where, V jkref ’ The reference voltage of the target bridge arm output after adding DC active damping control; V jkref is the target three-phase AC reference voltage of the sending-end converter; V cirj It is the voltage output by the circulating current suppression link; V vir It is the DC active damping control output voltage; V virlim is the DC active damping control output limit value; V virmin The DC active damping control output voltage minimum value can be designed as -1.1 here. V m1 , V m1 is the amplitude of the AC reference voltage of the sending-end converter; V dcn It is the rated DC voltage of the DC terminal of the sending-end converter.

[0105] S42: limiting the output of the active damping control link according to the optimal active damping control limiting value.

[0106] This embodiment coordinates the target AC reference voltage and DC active damping control during fault ride-through, so that the output reference voltage of the bridge arm of the sending-end converter is reduced to within the output range of the bridge arm, so that the output voltage range of the bridge arm is maximized, which can improve the control effect of the AC and DC circuits of the sending-end converter, accelerate the attenuation speed of the overhead line DC fault current, and do not cause serious overvoltage of the sub-module capacitor.

[0107] The parameters of the ultra-long-distance flexible DC transmission system are shown in Table 1.

[0108] Table 1:

[0109]

[0110] The rapid attenuation control method for overhead line fault current of the flexible DC transmission system is verified to be effective in the following aspects: reducing the AC reference voltage amplitude, improving the utilization of the bridge arm output voltage, and accelerating the attenuation speed of the overhead line fault current.

[0111] When the fault current of the overhead line of the flexible direct current transmission system is rapidly attenuated, the fault current and the sending end converter are controlled. a Phase AC voltage reference value, sending end converter a The average switching function waveform of the phase bridge arm is as follows: Figure 6 、 Figure 7 、 Figure 8 The fault current decay time for conventional fault ride-through control is 4.88s, while the fault current decay time for the flexible DC transmission system's overhead line rapid fault current decay control is 132ms, a 37-fold increase in fault current decay speed. The negative amplitude of the reference voltage of the sending-end converter bridge arm for conventional fault ride-through control is 329kV, while the negative amplitude of the reference voltage of the sending-end converter bridge arm for the flexible DC transmission system's overhead line rapid fault current decay control is 292kV, a reduction of 11.3% in the reference voltage amplitude. The voltage modulation index of the sending-end converter bridge arm for conventional fault ride-through control is 0.823, while the voltage modulation index of the sending-end converter bridge arm for the flexible DC transmission system's overhead line rapid fault current decay control is 0.664, a reduction of 19.3% in the voltage modulation index. Under the control strategy of the present invention, the average switching function of the sending-end converter bridge arm is controlled within the output voltage limit of the sending-end converter bridge arm. The sending-end converter bridge arm reference voltage is reduced by the output voltage limit constraint, allowing the sending-end converter to output more negative voltage levels, thereby accelerating the decay of DC fault current. The rapid decay control of overhead line fault current in the flexible DC transmission system significantly improves the utilization of the bridge arm output voltage and significantly accelerates the decay of overhead line fault current.

[0112] Corresponding to the aforementioned embodiment of the method for rapidly attenuating fault current of an overhead line of a flexible DC power transmission system, the present application also provides an embodiment of a device for rapidly attenuating fault current of an overhead line of a flexible DC power transmission system.

[0113] Figure 9 This is a block diagram of a device for controlling the rapid decay of fault current in an overhead line of a flexible DC power transmission system according to an exemplary embodiment. Figure 9 , the device comprises:

[0114] Submodule boost control module 1 is used to control the boost of the sending-end converter submodule when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, and obtain the sending-end converter after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the voltage reference value of the modulation link;

[0115] The power coordination control module 2 for the fault pole and the non-fault pole is used to coordinate the power reference values ​​of the fault pole and the non-fault pole according to the power reference values ​​of the fault pole and the non-fault pole when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, and obtain the sending-end converter after the power current double-loop PI control. q Shaft AC voltage reference value;

[0116] The third harmonic injection control module 3 is used to control the third harmonic injection control module 3 according to the sending end converter. d Shaft AC voltage reference and q The shaft AC voltage reference value is used to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and determine the target three-phase AC reference voltage of the sending-end converter after the third harmonic is injected;

[0117] A DC active damping limiting control module 4 is configured to limit the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage;

[0118] The fault current attenuation control module 5 is used to continuously execute the submodule boost control module-DC active damping limit control module until the DC fault current of the overhead line decays to zero.

[0119] Regarding the apparatus in the above embodiment, the specific manner in which each module performs operations has been described in detail in the embodiment of the method, and will not be elaborated here.

[0120] For the device embodiments, since they basically correspond to the method embodiments, the relevant parts can be referred to the partial description of the method embodiments. The device embodiments described above are merely schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules may be selected according to actual needs to achieve the purpose of the present application scheme. A person of ordinary skill in the art can understand and implement it without paying any creative work.

[0121] Correspondingly, the present application also provides an electronic device, comprising: 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 above-mentioned method for rapid attenuation control of overhead line fault current in a flexible direct current transmission system.

[0122] Accordingly, the present application also provides a computer-readable storage medium having computer instructions stored thereon, which, when executed by a processor, implements the above-mentioned method for controlling the rapid attenuation of the overhead line fault current of the flexible direct current transmission system.

[0123] Those skilled in the art will readily appreciate other embodiments of the present application after considering the specification and practicing the contents disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present application that follow the general principles of the present application and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered merely as exemplary, and the true scope and spirit of the present application are indicated by the claims.

[0124] It should be understood that the present application is not limited to the exact structures described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present application is limited only by the appended claims.

Claims

1. A method for rapidly attenuating fault current in overhead lines of a flexible DC transmission system, characterized in that: include: S1: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the sending-end converter submodule is controlled to boost the voltage, and the sending-end converter is obtained after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the voltage reference value of the modulation link; S2: When receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, the power reference values ​​of the fault pole and the non-fault pole are coordinated according to the power reference values ​​of the fault pole and the non-fault pole, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference value; S3: According to the sending end converter d Shaft AC voltage reference and q The axis AC voltage reference value is obtained to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and the target three-phase AC reference voltage of the sending-end converter after the third harmonic injection is determined; the expressions of the amplitude and phase are as follows: ； Where, V m3 is the amplitude of the target third harmonic voltage to be injected; V m1 is the amplitude of the AC reference voltage of the sending-end converter; θ j3 is the phase of the target third harmonic voltage to be injected; θ j1 is the phase of the AC reference voltage of the sending-end converter; S4: limiting the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; S5: Continue to execute S1-S4 until the DC fault current of the overhead line decays to zero.

2. The method according to claim 1, characterized in that Control the boost voltage of the sending-end converter submodule, and obtain the sending-end converter after voltage and current dual-loop PI control d The shaft AC voltage reference value increases the voltage reference value of the modulation link, including: Control the boost voltage of the sending-end converter submodule to increase the reference value of the average capacitor voltage of the sending-end converter submodule, and obtain the sending-end converter after voltage and current dual-loop PI control. d Shaft AC voltage reference value; The maximum value of the bridge arm output voltage is calculated according to the reference value of the average voltage of the capacitor of the sending-end converter submodule, and the capacitance modulation is performed with the maximum value of the bridge arm output voltage as the voltage reference.

3. The method according to claim 1, characterized in that According to the power reference values ​​of the fault pole and the non-fault pole, the power reference values ​​of the fault pole and the non-fault pole are coordinated, and the sending-end converter is obtained after the power and current double-loop PI control. q Shaft AC voltage reference values, including: According to the active power reference values ​​and reactive power reference values ​​of the faulty pole and the non-faulty pole, within the power limit range of the converter, the reactive power reference value of the faulty pole is reduced, the reactive power reference value of the non-faulty pole is reduced, and the active power reference value of the non-faulty pole is increased; After the power and current double loop PI control, the sending end converter is obtained q Shaft AC voltage reference value.

4. The method according to claim 1, wherein According to the sending-end converter d Shaft AC voltage reference and q The shaft AC voltage reference value is obtained to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and the target three-phase AC reference voltage of the sending-end converter after the third harmonic injection is determined, including: According to the sending-end converter d Shaft AC voltage reference and q The axis AC voltage reference value is used to obtain the amplitude and phase of the three-phase AC voltage through trigonometric function relationships; Calculate the optimal amplitude and phase of the target third harmonic according to the amplitude and phase of the three-phase AC voltage; According to the optimal amplitude and phase of the third harmonic, the target three-phase AC reference voltage of the sending-end converter based on the AC voltage reference value and the superposition of the third harmonic is calculated.

5. The method according to claim 1, characterized in that According to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage, the output voltage of the active damping control link is limited, including: Calculating an optimal active damping control limit value according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; According to the optimal active damping control limit value, the output of the active damping control link is limited.

6. A device for controlling the rapid attenuation of fault current in overhead lines of a flexible DC power transmission system, characterized in that: include: The submodule boost control module is used to control the boost of the sending-end converter submodule when receiving the overhead line fault signal and the sending-end converter bridge arm output limit signal, and obtain the sending-end converter after the voltage and current double-loop PI control. d The shaft AC voltage reference value increases the voltage reference value of the modulation link; The power coordination control module of the fault pole and the non-fault pole is used to coordinate the power reference values ​​of the fault pole and the non-fault pole according to the power reference values ​​of the fault pole and the non-fault pole when receiving the overhead line fault signal and the output limit signal of the sending-end converter bridge arm, and obtain the sending-end converter after the power current double-loop PI control. q Shaft AC voltage reference value; The third harmonic injection control module is used to control the sending end converter according to the d Shaft AC voltage reference and q The axis AC voltage reference value is obtained to obtain the amplitude and phase of the AC voltage reference value of the sending-end converter, and the target three-phase AC reference voltage of the sending-end converter after the third harmonic injection is determined; the expressions of the amplitude and phase are as follows: ； Where, V m3 is the amplitude of the target third harmonic voltage to be injected; V m1 is the amplitude of the AC reference voltage of the sending-end converter; θ j3 is the phase of the target third harmonic voltage to be injected; θ j1 is the phase of the AC reference voltage of the sending-end converter; A DC active damping limiting control module is used to limit the output voltage of the active damping control link according to the target three-phase AC reference voltage of the sending-end converter and the DC active damping output voltage; The fault current attenuation control module is used to continuously execute the submodule boost control module-DC active damping limit control module until the overhead line 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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