Direct current side voltage stability control method and system of CLCC-HVDC system under power grid fault

Abstract

The invention discloses a direct current side voltage stability control method and system of a CLCC-HVDC system under a power grid fault, and aims to solve the problem of direct current side secondary ripples of the CLCC-HVDC system under a power grid imbalance working condition. According to the method, a frequency-doubled disturbance signal is actively injected into a trigger angle instruction of an inverter of a direct-current power transmission system, and unbalanced triggering of each valve arm of the converter is realized, so that a negative sequence component is introduced into a fundamental frequency switching function of the converter; the component coupling AC power grid positive sequence voltage can counteract a DC side double frequency component caused by a receiving end power grid voltage unbalance component. According to the method, the full-control characteristic of the controllable commutation converter is fully utilized, and the second harmonic generation disturbance signal is actively injected, so that the direct-current voltage and current secondary ripples of the direct-current power transmission system working under the unbalanced power grid working condition at the receiving end are effectively reduced, the power transmission stability of the direct-current power transmission system working under the unbalanced power grid working condition at the receiving end is improved, and the power transmission efficiency of the direct-current power transmission system is improved. The method has wide applicability and a good application prospect.

Description

Technical Field

[0001] The present invention relates to the field of high voltage direct current transmission, and in particular to a method and system for controlling the DC side voltage stability of a CLCC-HVDC system under a power grid fault. Background Art

[0002] The high-voltage direct current transmission system (LCC-HVDC) based on the grid-commutated converter has been widely used in the global power grid due to its large transmission capacity, long transmission distance, low operating loss and other advantages. Especially with the development of renewable energy, the long-distance power transmission through LCC-HVDC technology and the optimization of regional energy configuration have become important ways to absorb clean energy. However, since LCC-HVDC uses a semi-controlled commutation device with thyristor as the core, the transient operation characteristics of the LCC-HVDC system are poor when the AC power grid fails. In particular, when the AC system on the inverter side fails asymmetrically, the unbalanced component of the AC power grid will be mapped to the DC line through the converter, generating a secondary ripple voltage on the DC line. If the DC transmission line and its end equipment are in the second harmonic resonance range, dangerous resonant overvoltage will be generated. At the same time, due to the slow control response on the rectifier side, the DC secondary ripple voltage will lead to secondary ripple current, increasing the electrical stress of the converter. The corresponding current low-frequency oscillation will also affect the normal triggering of the converter valve through the LCC-HVDC controller, which is not conducive to the DC system fault recovery and may even cause the DC side protection to operate, thereby causing the DC system to lock. In addition, the low-frequency fluctuation of the DC line has a great impact on the system communication and the reliable access of the new energy at the sending end.

[0003] In order to reduce the impact of DC secondary ripple when the HVDC system works under unbalanced conditions of the AC grid on the inverter side, a series of studies have been carried out in academia and industry. At present, the most widely used secondary ripple suppression method in HVDC projects is to install a passive filter on the DC side. However, due to the low frequency of the secondary ripple, the required passive filter is large in size and high in cost, and will also reduce the response speed of the DC transmission system. Related studies have proposed that the flexible control characteristics of the HVDC system can be used to actively suppress DC secondary ripple fluctuations. This method does not require the installation of a passive filter and has great advantages in terms of economy and response speed. However, this method is only applicable to VSC-HVDC, and the control accuracy of conventional LCC-HVDC using semi-controlled devices is difficult to meet the requirements. Through the above research, it can be seen that the existing secondary ripple suppression methods are difficult to apply to LCC-HVDC systems. The fundamental reason is that the commutation element used in the LCC-HVDC system is a semi-controlled thyristor. The system control flexibility is low, especially considering that the receiving-end converter on the inverter side has the risk of commutation failure when the grid is unbalanced. During the fault, the system needs to reduce the trigger angle to obtain sufficient commutation margin, which makes it difficult to adjust the trigger angle in a wide range. Therefore, in actual projects, LCC-HVDC systems often use expensive passive filter solutions. However, with the development of large-capacity power electronic devices, a series of new hybrid converter solutions based on LCC-HVDC structure have been proposed, among which the most representative is the controllable commutation converter (CLCC). By constructing an auxiliary shutdown branch composed of IGBTs, active commutation can be achieved during grid faults. While retaining the operating advantages of LCC-HVDC, commutation failures are eliminated. This solution has been applied to the ±500kV project in Genan, China, and has repeatedly resisted commutation failures under the condition of receiving-end grid faults. At present, some research teams have also proposed an H-LCC solution that directly connects IGCTs in series with the thyristor valve arm. By introducing fully controlled devices, the active shutdown of the converter valve arm can also be achieved to eliminate commutation failures.

[0004] In summary, with the development of new large-capacity power electronic devices, the controlled commutation converter type HVDC (CLCC-HVDC) system has no commutation failure risk. During the receiving grid fault, the inverter trigger angle can be adjusted in a wide range, which provides a new opportunity for the study of secondary ripple suppression in the DC transmission system under unbalanced grid conditions. However, how to use the operating characteristics of the controlled commutation converter to achieve active suppression of secondary ripples under unbalanced grid conditions at the receiving end, ensure the stability of the DC side voltage, and further improve the operating stability of the HVDC system under unbalanced grid conditions is still a heavy and difficult problem that needs to be solved. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a DC side voltage stability control method and system for a CLCC-HVDC system under a power grid fault in view of the deficiencies in the prior art, so as to solve the instability problem caused by secondary ripple interference on the DC side under unbalanced power grid conditions.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a DC side voltage stabilization control method of a CLCC-HVDC system under a power grid fault, the CLCC-HVDC system includes a controllable commutation converter, the controllable commutation converter is connected to the receiving end power grid through the power grid impedance; the controllable commutation converter includes three parallel bridge arms, each bridge arm includes two series-connected converter valve groups; each of the converter valve groups includes two parallel branches, one of which includes a thyristor and a first fully-controlled device connected in series with the thyristor, and the other branch includes a second fully-controlled device and a diode connected in series with the second fully-controlled device; the method includes the following steps:

[0007] Detect the three-phase AC voltage u of the receiving power grid a 、u b 、u c and CLCC-HVDC system DC voltage U D ;

[0008] The three-phase AC voltage u of the receiving power grid a 、u b 、u c Through the dual synchronous decoupling coordinate system phase-locked loop, the negative sequence crossover quantity is obtained The CLCC-HVDC system DC voltage U D Through the notch filter, extract the secondary ripple amplitude information U 2f0 ;

[0009] Using negative sequence crossover The phase information φ of the additional angle Δα to be injected into the secondary ripple disturbance is calculated by the inverter trigger angle command α in the CLCC-HVDC system Δα , the secondary ripple amplitude information U 2f0 The output result after the amplitude filter is input into the proportional integral controller to dynamically obtain the amplitude information A of the additional angle Δα that should be injected into the secondary ripple disturbance Δα ;

[0010] The phase information φ of the additional angle Δα of the secondary ripple disturbance should be injected Δα and the amplitude information A of the additional angle Δα of the secondary ripple disturbance to be injected Δα The signal of the additional angle Δα of the secondary ripple disturbance to be injected is calculated;

[0011] The obtained signal of the additional angle Δα to be injected into the secondary ripple disturbance is superimposed on the trigger angle command α to obtain the final output command α of the controllable commutation converterord ;

[0012] Using the final output instruction α ord Generate trigger signals for each valve arm to control the opening and closing of the thyristors in the CLCC-HVDC system.

[0013] The present invention realizes the unbalanced triggering of each valve arm of the converter by actively injecting a double frequency disturbance signal into the inverter trigger angle instruction of the DC transmission system, thereby introducing a negative sequence component in the converter base frequency switching function, which is coupled with the positive sequence voltage of the AC power grid to offset the double frequency component on the DC side caused by the unbalanced component of the receiving end power grid voltage. The present invention makes full use of the full control characteristics of the controllable commutation converter, and by actively injecting a double frequency disturbance signal, effectively reduces the DC voltage and current secondary ripples of the DC transmission system working under the unbalanced power grid condition at the receiving end, improves the power transmission stability of the DC transmission system working under the unbalanced power grid condition at the receiving end, and has wide applicability and good application prospects.

[0014] The transfer function of the notch filter is G notch (s) is expressed as:

[0015]

[0016] where ω 0 is the grid voltage fundamental angular frequency, η is the quality factor, and s is the complex frequency variable in the Laplace transform.

[0017] The amplitude information A of the additional angle Δα of the secondary ripple disturbance should be injected Δα The specific acquisition process includes:

[0018] The voltage secondary ripple information U output by the notch filter 2f0 After the filter, the maximum value U of the half cycle is obtained. 2f ;

[0019] Subtract the maximum value U of the filter output half cycle from 0 2f After that, the control error is obtained, and the control error is input into the PI controller. The upper and lower limit values ​​of the PI controller are π / 6 and -π / 6 respectively. Then, the output value of the PI controller with the upper and lower limit values ​​of π / 6 and -π / 6 respectively is the amplitude information A of the additional angle Δα of the secondary ripple disturbance that should be injected. Δα ;

[0020] in, U 2f_0 (n) represents the nth voltage secondary ripple information U within half a control cycle 2f0 The sampling value of N represents the number of sampling values ​​within a control cycle, that is, U 2f_0 (1) indicates U 2f0 Current sample value, Indicates the sample value half the control cycle length ago.

[0021] The phase information φ of the additional angle Δα of the secondary ripple disturbance should be injected Δα The expression is:

[0022]

[0023] in, and They respectively represent the negative-sequence q-axis double frequency component and the negative-sequence d-axis double frequency component obtained by the dual synchronous decoupled coordinate system phase-locked loop DDSRF-SPLL.

[0024] The expression of the additional angle Δα signal that should be injected into the secondary ripple disturbance is:

[0025]

[0026] in, and It represents the negative-sequence q-axis voltage amplitude and d-axis voltage amplitude obtained by DDSRF-SPLL.

[0027] The final output command of the controllable commutation converter is α ord The expression is:

[0028]

[0029] As an inventive concept, the present invention also provides a DC side voltage stability control system of a CLCC-HVDC system under a power grid fault, comprising a memory, a processor and a computer program stored in the memory; the processor executes the computer program to implement the steps of the above method.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention makes full use of the wide range trigger angle adjustment characteristics of the controllable commutation converter, proposes an asymmetric triggering method for the converter valve with trigger angle secondary ripple injection, realizes active suppression of secondary ripple under unbalanced grid conditions at the receiving end, further improves the operating stability of the DC transmission system under unbalanced grid conditions, and reduces the investment in DC filters. The proposed method is also applicable to other low-frequency phase-controlled converters with flexible trigger angle adjustment capabilities, and has wider applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is an inverter topology diagram of a DC power transmission system based on a controllable commutation converter according to an embodiment of the present invention;

[0032] Figure 2 This is the implementation process of the DC side voltage stabilization control method of the CLCC-HVDC system under power grid fault in an embodiment of the present invention;

[0033] Figure 3 is a block diagram of a DC power transmission control system according to an embodiment of the present invention;

[0034] Figure 4 This is the real-time simulation experiment result of the embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] Figure 1 is an inverter topology diagram of a DC power transmission system based on a controllable phase-commutated converter according to an embodiment of the present invention. Figure 1 As shown, the controllable commutation converter transforms the commutation valve group in the original grid commutation converter based on thyristor elements, introduces fully controlled devices and utilizes the fully controlled characteristics of the fully controlled devices to achieve controllable shutdown of the commutation current without the risk of commutation failure. During the receiving-end grid fault, the inverter trigger angle can be adjusted over a wide range, providing a basis for secondary ripple suppression in the DC transmission system operating under unbalanced grid conditions.

[0038] Figure 2 This is the implementation process of the DC side voltage stability control method of the CLCC-HVDC system under power grid fault in the embodiment of the present invention. Figure 2 Describe the method. Figure 2 As shown, the method comprises the following steps:

[0039] Step 1: Detect the three-phase AC voltage u of the receiving power grid abc , detect the DC voltage U of the DC transmission system D ;

[0040] Figure 3 FIG. 1 is a block diagram of a DC power transmission control system according to an embodiment of the present invention. Figure 3 As shown, the dual synchronous decoupling coordinate system phase-locked loop can provide negative sequence crossing quantity information, and the original DC control system can provide initial inverter trigger angle information. The implementation modules required by the proposed method are introduced to easily implement the application.

[0041] Step 2: The detected receiving-end grid AC voltage information u abc Obtaining negative sequence crossover via dual synchronous decoupled frame phase-locked loop (DDSRF-SPLL) The detected DC voltage information U D Extract the secondary ripple amplitude information U through the notch filter 2f0 .

[0042] The positive sequence synchronous decoupling transfer matrix and the negative sequence synchronous decoupling transfer matrix are expressed as:

[0043]

[0044] in, is the positive sequence synchronous decoupling conversion matrix, is the negative sequence synchronous decoupling conversion matrix. After transformation, the d-axis and q-axis double frequency components under the negative sequence synchronous decoupling coordinates are obtained as follows:

[0045]

[0046] in, and They represent the d-axis and q-axis double frequency components in the negative sequence synchronous decoupling coordinates. n Indicates the negative sequence amplitude of the phase voltage, is the negative sequence voltage phase under this reference. Since the zero sequence voltage of the commutation bus has no effect on the zero sequence voltage output by the converter, the zero sequence voltage is ignored.

[0047] Step 3: Using negative sequence crossover The phase information φ of the additional angle Δα to be injected into the secondary ripple disturbance is calculated by the inverter trigger angle command α in the DC control system Δα , the secondary ripple amplitude information U 2f0 After passing through the amplitude filter, the proportional integral controller (PI) is input to dynamically obtain the amplitude information A of the additional angle Δα that should be injected into the secondary ripple disturbance Δα ;

[0048] pass The phase information φ of Δα is calculated with α Δα , whose expression is:

[0049]

[0050] in, and It represents the negative-sequence q-axis double frequency component and the negative-sequence d-axis double frequency component obtained by DDSRF-SPLL, and α represents the trigger angle command output by the DC control system.

[0051] The detected DC voltage information U D Extract the secondary ripple amplitude information U through the notch filter 2f0 , the notch filter transfer function is as follows:

[0052]

[0053] Among them, ω 0 is the grid voltage fundamental angular frequency, η is the quality factor;

[0054] The voltage secondary ripple information U of the notch filter output 2f0 After filtering, the half-period maximum value U is obtained 2f , the filter expression is:

[0055]

[0056] Among them, U 2f0 (n) represents the nth voltage secondary ripple information U within half a control cycle 2f0 The sampling value of N represents the number of sampling values ​​within a control cycle, that is, U 2f0 (1) indicates U 2f0 Current sampling value, U 2f0 (N / 2) represents the sampling value before half the control cycle;

[0057] Subtract the maximum value U of the filter output half cycle from 0 2f After that, the control error is obtained and input into the PI controller. Considering that the trigger angle of the new hybrid converter needs to be less than π, and the conventional trigger angle of the DC control system is about 130°~140°, the amplitude information A of the secondary ripple disturbance additional angle Δα should be injected. Δα It needs to be less than π / 6, so the upper and lower limits of the PI controller are π / 6 and -π / 6 respectively;

[0058] Get the output value of the PI controller with upper and lower limits of π / 6 and -π / 6 respectively as the amplitude information A of the additional angle Δα to be injected into the secondary ripple disturbance Δα .

[0059] Step 4: The phase information φ of the additional angle Δα to be injected into the secondary ripple disturbance obtained in step 3 Δα After normalization, the amplitude information A of the additional angle Δα of the secondary ripple disturbance should be injected Δ Multiply them together and calculate the additional angle Δα signal that should be injected into the secondary ripple disturbance. Its expression is:

[0060]

[0061] in, and It represents the negative sequence q-axis voltage amplitude and d-axis voltage amplitude obtained by DDSRF-SPLL, and the minimum value of the secondary ripple disturbance additional angle Δα signal to be injected is -π / 6, and the maximum value is π / 6;

[0062] Step 5: Superimpose the obtained secondary ripple disturbance additional angle Δα signal to be injected into the inverter side controller output trigger angle command α to obtain the final output command α of the converter ord Its expression is:

[0063]

[0064] The final output instruction α ord The trigger pulse generation module of the DC control system generates trigger signals for each valve arm to control the switching of the thyristors of the DC transmission system and suppress the secondary ripple on the DC side. It should be pointed out that the injection of the secondary ripple disturbance additional angle Δα will increase the trigger angle, so the converter needs to work in the self-shutdown mode to start the control, otherwise there will be a risk of commutation failure. According to the above requirements, the control can be started at the same time as the self-shutdown mode, and it can be completed in conjunction with the conventional DC system fault detection module without the need for an additional detection module.

[0065] Step 6: Output the final instruction α ord The trigger pulse generation module of the DC control system generates trigger signals for each valve arm, which can control the switching of the thyristors in the DC transmission system and suppress the secondary ripple on the DC side.

[0066] Figure 4 The figure is a real-time simulation experiment result of an embodiment of the present invention.

[0067] Figure 4 The invention shows that the DC power transmission system with a controllable commutation converter can effectively suppress secondary ripples and ensure DC voltage stability under grid fault conditions. When the DC power transmission system based on a controllable commutation converter is unbalanced at the receiving end, the method exhibits the following advantages:

[0068] (1) With reference to the AC / DC voltage mapping equation of the phase-controlled converter and the self-shutdown characteristics of the controllable commutated converter, the unbalanced triggering of each valve arm is achieved by injecting a double-frequency disturbance signal into the system trigger angle, which effectively reduces the double-frequency fluctuation of the DC side voltage under unbalanced conditions of the HVDC system;

[0069] (2) The suppression of the double frequency fluctuation of the DC side voltage effectively reduces the voltage and current stress of the converter valve arm.

[0070] (3) Wide applicability. The simulation results under single-phase inductive grounding and metallic grounding faults show that the proposed control strategy effectively suppresses the secondary ripple voltage under unbalanced grid conditions and improves the stability of transient power transmission in HVDC system faults.

[0071] Figure 4The feasibility and practical value of this method have been verified, which can effectively improve the transient operation performance of the new hybrid converter, and is conducive to the further development of the new hybrid converter in actual engineering.

[0072] Example 2

[0073] Embodiment 2 of the present invention provides a DC side voltage stabilization control system of a CLCC-HVDC system under a power grid fault corresponding to the above-mentioned embodiment 1, including a memory, a processor and a computer program stored in the memory; the processor executes the computer program in the memory to implement the steps of the method in the above-mentioned embodiment 1.

[0074] In some implementations, the memory may be a high-speed random access memory (RAM), and may also include a non-volatile memory, such as at least one disk memory.

[0075] In some other implementations, the processor may be a central processing unit (CPU), a digital signal processor (DSP), or other general-purpose processors of various types, which are not limited herein.

[0076] Example 3

[0077] Embodiment 3 of the present invention provides a computer-readable storage medium corresponding to the above embodiment 1, on which a computer program / instruction is stored. When the computer program / instruction is executed by a processor, the steps of the method of the above embodiment 1 are implemented.

[0078] Computer readable storage media can be tangible devices that hold and store instructions used by instruction execution devices. Computer readable storage media can be, for example, but not limited to, electronic storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any combination thereof.

[0079] Those skilled in the art will appreciate that the embodiments of the present application can be provided as methods, systems, or computer program products. Therefore, the present application can adopt the form of complete hardware embodiments, complete software embodiments, or embodiments in combination with software and hardware. Moreover, the present application can adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code. The scheme in the embodiments of the present application can be implemented in various computer languages, for example, object-oriented programming language Java and literal scripting language JavaScript, etc.

[0080] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0081] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0082] Although the preferred embodiments of the present application have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0083] Obviously, those skilled in the art can make various changes and modifications to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.

Claims

1. A DC side voltage stabilization control method for a CLCC-HVDC system under a power grid fault, wherein the CLCC-HVDC system comprises a controllable commutation converter, wherein the controllable commutation converter is connected to a receiving-end power grid through a power grid impedance; the controllable commutation converter comprises three parallel bridge arms, each bridge arm comprises two series-connected converter valve groups; each of the converter valve groups comprises two parallel branches, wherein one branch comprises a thyristor and a first fully-controlled device connected in series with the thyristor, and the other branch comprises a second fully-controlled device and a diode connected in series with the second fully-controlled device; wherein: The method comprises the following steps: Detect the three-phase AC voltage u of the receiving end power grid a 、u b 、u c and CLCC-HVDC system DC voltage U D ; The three-phase AC voltage u of the receiving end power grid a 、u b 、u c Through the dual synchronous decoupling coordinate system phase-locked loop, the negative sequence crossover quantity is obtained The CLCC-HVDC system DC voltage U D Through the notch filter, extract the secondary ripple amplitude information U 2f0 ; Using negative sequence crossover The phase information φ of the additional angle Δα to be injected into the secondary ripple disturbance is calculated by the inverter trigger angle command α in the CLCC-HVDC system Δα , the secondary ripple amplitude information U 2f0 The output result after the amplitude filter is input into the proportional integral controller to dynamically obtain the amplitude information A of the additional angle Δα that should be injected into the secondary ripple disturbance Δα ; The phase information φ of the additional angle Δα of the secondary ripple disturbance should be injected Δα and the amplitude information A of the additional angle Δα of the secondary ripple disturbance to be injected Δα The signal of the additional angle Δα of the secondary ripple disturbance to be injected is calculated; The obtained signal of the additional angle Δα to be injected into the secondary ripple disturbance is superimposed on the trigger angle command α to obtain the final output command α of the controllable commutation converter ord ; Using the final output instruction α ord Generate trigger signals for each valve arm to control the opening and closing of the thyristors in the CLCC-HVDC system.

2. The DC side voltage stabilization control method of the CLCC-HVDC system under power grid fault according to claim 1, characterized in that: The transfer function of the notch filter is G notch (s) is expressed as: Where ω0 is the fundamental angular frequency of the grid voltage, η is the quality factor, and s is the complex frequency variable in the Laplace transform.

3. The DC side voltage stabilization control method of the CLCC-HVDC system under power grid fault according to claim 1, characterized in that: The amplitude information A of the additional angle Δα of the secondary ripple disturbance should be injected Δα The specific acquisition process includes: The voltage secondary ripple information U output by the notch filter 2f0 After the filter, the maximum value U of the half cycle is obtained. 2f ; Subtract the maximum value U of the filter output half cycle from 0 2f After that, the control error is obtained, and the control error is input into the PI controller. The upper and lower limit values ​​of the PI controller are π / 6 and -π / 6 respectively. Then, the output value of the PI controller with the upper and lower limit values ​​of π / 6 and -π / 6 respectively is the amplitude information A of the additional angle Δα of the secondary ripple disturbance that should be injected. Δα ; in, U 2f_0 (n) represents the nth voltage secondary ripple information U within half a control cycle 2f0 The sampling value of N represents the number of sampling values ​​within a control cycle, that is, U 2f_0 (1) indicates U 2f0 Current sample value, Indicates the sample value half the control cycle length ago.

4. The DC side voltage stabilization control method of the CLCC-HVDC system under power grid fault according to claim 1, characterized in that: The phase information φ of the additional angle Δα of the secondary ripple disturbance should be injected Δα The expression is: in, and They respectively represent the negative-sequence q-axis double frequency component and the negative-sequence d-axis double frequency component obtained by the dual synchronous decoupled coordinate system phase-locked loop DDSRF-SPLL.

5. The DC side voltage stabilization control method of the CLCC-HVDC system under power grid fault according to claim 1, characterized in that: The expression of the additional angle Δα signal that should be injected into the secondary ripple disturbance is: in, and represents the negative sequence q-axis voltage amplitude and d-axis voltage amplitude obtained by DDSRF-SPLL, and They respectively represent the negative-sequence q-axis double frequency component and the negative-sequence d-axis double frequency component obtained by the dual synchronous decoupled coordinate system phase-locked loop DDSRF-SPLL.

6. The DC side voltage stabilization control method of the CLCC-HVDC system under power grid fault according to claim 1, characterized in that: The final output command of the controllable commutation converter is α ord The expression is:

7. A DC side voltage stability control system of a CLCC-HVDC system under a power grid fault, comprising a memory, a processor and a computer program stored in the memory; characterized in that: The processor executes the computer program to implement the steps of the method according to any one of claims 1 to 6.

8. A computer-readable storage medium having a computer program / instruction stored thereon; characterized in that: When the computer program / instructions are executed by a processor, the steps of the method according to any one of claims 1 to 6 are implemented.

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