AC fault ride-through control method and device for hybrid flexible DC transmission system

Abstract

The present invention relates to an AC fault ride-through control method and control device for a hybrid flexible direct current transmission system. On the basis of the existing DC low-voltage current limiting function, an AC low-voltage current limiting function is added. The minimum value of the positive sequence voltage amplitude of the AC busbar of each converter on the inverter side of the direct current transmission system is selected for calculation to obtain an AC low-voltage current limiting value, and the smaller value of the AC low-voltage current limiting value and the DC low-voltage current limiting value is selected to control the output power of the converter on the rectifier side. The technical solution provided by the present invention adds an AC low-voltage current limiting function on the basis of the existing DC low-voltage current limiting function, thereby improving the system's response speed to AC faults and the system recovery speed. Different AC delay parameters are used when an AC fault occurs and when it is recovered, so that when a fault occurs, the voltage drops slowly to avoid causing greater interference to the system; when the fault ends, the voltage recovers as quickly as possible to minimize the impact of the fault on the system.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current (DC) transmission of power systems, and in particular to an AC fault ride-through control method and device for a hybrid flexible DC transmission system. Background Art

[0002] Conventional DC transmission systems face the risk of commutation failure when connected to weak AC grids. However, flexible DC transmission systems eliminate this risk and offer numerous advantages, such as increased flexibility. Therefore, they are widely used in a variety of scenarios, including renewable energy grid integration, island interconnection, and asynchronous power transmission. However, flexible DC transmission systems are expensive and have limited transmission capacity. Combining the advantages of both systems, hybrid DC transmission systems have become a hot research topic. In LCC-VSC hybrid DC transmission systems, when an AC system fault occurs in the receiving VSC, the valve group's power transmission capacity is reduced. The continued power output of the rectifier station exacerbates the power surplus on the DC side, causing a sharp increase in the VSC converter voltage. After the fault ends, system power recovery takes a long time, seriously impacting system operation and stability.

[0003] In the existing LCC-VSC hybrid DC transmission system, when an AC fault occurs at the receiving end, a forced phase shift is usually adopted at the sending end to reduce power transmission, thereby reducing the power received by the VSC converter at the receiving end and avoiding DC overvoltage. In other words, when an AC fault occurs in the VSC converter at the receiving end, the power transmission capacity is weakened, causing the DC voltage to rise. The low-voltage current limiting function configured in the LCC converter at the sending end cannot be used, and the output power of the sending end can only be limited by phase shifting. Since the LCC converter has a slow response speed, when the phase shift is completed, the valve group overvoltage has developed to a more serious level. At the same time, after the LCC converter phase shift is completed, it is necessary to wait for a period of de-ionization time before transmission can be resumed, which can easily cause the system recovery time to be longer, thereby affecting power transmission. Summary of the Invention

[0004] Based on the above-mentioned situation of the prior art, the purpose of the present invention is to provide an AC fault ride-through control method and device for a DC transmission system, which limits the output power of the sending end according to the AC voltage change at the receiving end of the DC transmission system to achieve more efficient AC fault ride-through.

[0005] To achieve the above object, according to one aspect of the present invention, there is provided an AC fault ride-through control method for a DC transmission system, comprising the steps of:

[0006] Get DC voltage U d , according to the DC voltage U d Get the DC low voltage current limit value I VDCL1 ;

[0007] Get the positive sequence voltage amplitude U of each converter AC busbar on the inverter side psfvsci , i=1,2,3……, and take the minimum value among them as U psfvscmin ;

[0008] According to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 ;

[0009] Using the rectifier side current reference value I dref The output power of the rectifier side converter is controlled. The rectifier side current reference value I dref The upper limit value is the DC low voltage current limit value I VDCL1 and AC low voltage current limit value I VDCL2 The smaller value.

[0010] Furthermore, the DC voltage U d Get the DC low voltage current limit value I VDCL1 ,include:

[0011] The DC voltage U d After first-order inertial filtering, linear amplification or reduction is performed to obtain the DC low-voltage current limit value I VDCL1 .

[0012] Furthermore, the first-order inertia filter coefficient is T is the DC delay parameter.

[0013] Furthermore, the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 ,include:

[0014] The minimum value U psfvscmin After first-order inertial filtering, linear amplification or reduction is performed to obtain the AC low-voltage current limit value I VDCL2 .

[0015] Furthermore, the first-order inertia filter coefficient is

[0016] When the AC fault begins, T = T2;

[0017] When the AC fault ends, T = T1;

[0018] Wherein, T1 and T2 are the first AC delay parameter and the second AC delay parameter, and T1>T2.

[0019] Furthermore, when an inter-station communication failure occurs, the AC low voltage current limit value I VDCL2 =I max , I max It is the maximum limit of DC current.

[0020] Furthermore, the hybrid flexible direct current transmission system includes an LCC-VSC hybrid flexible direct current transmission system.

[0021] According to another aspect of the present invention, an AC fault ride-through control device for a hybrid flexible DC transmission system is provided, comprising a DC low-voltage current limiting value calculation module, an AC low-voltage current limiting value calculation module, a rectifier-side current reference value calculation module, and a rectifier-side output power control module, wherein the modules are connected in sequence; wherein,

[0022] The DC low voltage current limiting value calculation module obtains the DC voltage U d , according to the DC voltage U d Get the DC low voltage current limit value I VDCL1 ;

[0023] The AC low voltage current limit value calculation module obtains the AC bus positive sequence voltage amplitude U of each converter on the inverter side psfvsci , i=1,2,3……, and take the minimum value among them as U psfvscmin , according to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 ;

[0024] The rectifier side current reference value calculation module, the rectifier side current reference value I dref The upper limit value is the DC low voltage current limit value I VDCL1 and AC low voltage current limit value I VDCL2 The smaller value;

[0025] The rectifier side output power control module uses the rectifier side current reference value I dref Control the output power of the rectifier-side converter.

[0026] Furthermore, the AC low-voltage current limiting value calculation module includes a first-order inertial filter unit and a linear unit;

[0027] The first-order inertial filtering unit minimizes the value U psfvscmin Perform first-order inertial filtering;

[0028] The linear unit linearly amplifies or reduces the filtered signal to obtain the AC low-voltage current limiting value I VDCL2 .

[0029] Furthermore, the DC low-voltage current limiting value calculation module includes a first-order inertial filter unit and a linear unit;

[0030] The first-order inertial filter unit converts the DC voltage U d Perform first-order inertial filtering;

[0031] The limiting unit linearly amplifies or reduces the filtered signal to obtain the DC low-voltage current limiting value I VDCL1 .

[0032] In summary, the present invention provides an AC fault ride-through control method and control device for a hybrid flexible direct current transmission system. On the basis of the existing DC low-voltage current limiting function, an AC low-voltage current limiting function is added. The minimum value of the positive-sequence voltage amplitude of the AC bus of each converter on the inverter side of the direct current transmission system is selected for calculation to obtain the AC low-voltage current limiting value, and the smaller value of the AC low-voltage current limiting value and the DC low-voltage current limiting value is selected to control the output power of the rectifier-side converter.

[0033] The technical solution of the present invention has the following beneficial technical effects:

[0034] (1) Based on the existing DC low-voltage current limiting function, the AC low-voltage current limiting function is added, and the two are combined to control the output power of the sending end, thereby improving the system's response speed to AC faults and the system recovery speed. The control accuracy is high and will not affect the system's power transmission.

[0035] (2) Different AC delay parameters are used when an AC fault occurs and when it is restored, so that when a fault occurs, the voltage drops slowly to avoid causing major interference to the system; when the fault ends, the voltage recovers as quickly as possible to minimize the impact of the fault on the system.

[0036] (3) In the event of an inter-station communication failure, the maximum DC current limit is selected as the AC low-voltage current limit to shield the AC low-voltage current limit function, thereby avoiding malfunction caused by inter-station communication failure. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a flow chart of an AC fault ride-through control method for a hybrid flexible DC transmission system according to the present invention;

[0038] Figure 2 It is a circuit structure diagram of the LCC-VSC hybrid flexible DC transmission system;

[0039] Figure 3 This is a control strategy block diagram of the AC fault ride-through control method of the hybrid flexible HVDC system of the present invention;

[0040] Figure 4 It is a structural block diagram of the AC fault ride-through control device of the hybrid flexible DC transmission system of the present invention. DETAILED DESCRIPTION

[0041] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present invention.

[0042] The technical solution of the present invention is described in detail below with reference to the accompanying drawings. According to one embodiment of the present invention, a method for controlling AC fault ride-through of a hybrid flexible DC transmission system is provided. The flow chart of the method is shown in FIG. Figure 1 This control method is used to handle AC faults in a DC transmission system. Based on the existing DC low-voltage current limiting function, an AC low-voltage current limiting function is added, and the two are combined to control the output power of the sending end. The DC transmission system can be, for example, an LCC-VSC hybrid DC transmission system. The circuit structure diagram of the LCC-VSC hybrid DC transmission system is shown in FIG. Figure 2 As shown in the figure, the system adopts a series structure of high-end and low-end LCC valve groups at the sending end, and a series structure of a high-end LCC valve group and three low-end parallel VSC valve groups at the receiving end. Each VSC is a modular multilevel converter topology based on series sub-modules.

[0043] Figure 3 The control strategy block diagram of the AC fault ride-through control method of the hybrid flexible DC transmission system of the present invention is shown in FIG. Figure 1 and Figure 3 , the control method provided in this embodiment includes the following steps:

[0044] Measure the DC voltage and obtain the DC voltage U d , according to the DC voltage U d Get the DC low voltage current limit value I VDCL1 The following steps can be used: d After first-order inertial filtering, linear amplification or reduction is performed to obtain the DC low-voltage current limit value I VDCL1 The first-order inertial filter coefficient can be T is the DC delay parameter, and s is the differential operator of Laplace transform.

[0045] Get the positive sequence voltage amplitude U of each converter AC busbar on the inverter side psfvsci , for example, the AC bus positive sequence voltage amplitude of each VSC converter, where i = 1, 2, 3, ..., represents the serial number of the converter, and the minimum value is taken as U psfvscmin .

[0046] According to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2The following steps can be used: psfvscmin After first-order inertial filtering, linear amplification or reduction is performed to obtain the AC low-voltage current limit value I VDCL2 The first-order inertial filter coefficient can be The value of T varies according to the different stages of the AC fault:

[0047] When an AC fault begins, T = T2; when the AC fault ends, T = T1. T1 and T2 are the first and second AC delay parameters. By setting different AC delay parameters at the beginning and end of an AC fault, the rate at which the current reference value is limited and restored during the fault period is adjusted, avoiding significant system disruption and minimizing the impact of the fault on the system.

[0048] Using the rectifier side current reference value I dref The output power of the rectifier side converter is controlled. The rectifier side current reference value I dref The upper limit value is the DC low voltage current limit value I VDCL1 and AC low voltage current limit value I VDCL2 The smaller value. Rectifier side current reference value I dref The lower limit value is LL, which can be set according to actual needs and is usually 0.1.

[0049] The steps may also be included: when an inter-station communication failure occurs, the AC low voltage current limit value I VDCL2 =I max , I max The maximum DC current limit is selected as the AC low-voltage current limit in the event of an inter-station communication failure, thereby shielding the AC low-voltage current limit function.

[0050] According to another embodiment of the present invention, a control device for an AC fault ride-through control method of a DC power transmission system is provided. The structural block diagram of the device is shown in FIG. Figure 4 As shown, it includes: a DC low-voltage current limit value calculation module, an AC low-voltage current limit value calculation module, a rectifier side current reference value calculation module, and a rectifier side output power control module, and the modules are connected in sequence; wherein,

[0051] The DC low voltage current limiting value calculation module obtains the DC voltage U d , according to the DC voltage U d Get the DC low voltage current limit value I VDCL1 The DC low voltage current limit value calculation module may further include a first order inertia filter unit and a linear unit; the first order inertia filter unit converts the DC voltage U dPerform first-order inertial filtering; the limiting unit linearly amplifies or reduces the filtered signal to obtain a DC low-voltage current limiting value I VDCL1 .

[0052] The AC low voltage current limit value calculation module obtains the AC bus positive sequence voltage amplitude U of each converter on the inverter side psfvsci , i=1,2,3……, and take the minimum value among them as U psfvscmin , according to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 The AC low-voltage current limit value calculation module may further include a first-order inertial filter unit and a linear unit; the first-order inertial filter unit converts the minimum value U psfvscmin Perform first-order inertial filtering; the linear unit linearly amplifies or reduces the filtered signal to obtain the AC low-voltage current limiting value I VDCL2

[0053] The rectifier side current reference value calculation module, the rectifier side current reference value I dref The upper limit value is the DC low voltage current limit value I VDCL1 and AC low voltage current limit value I VDCL2 The smaller value;

[0054] The rectifier side output power control module uses the rectifier side current reference value I dref Control the output power of the rectifier-side converter.

[0055] In summary, the present invention relates to an AC fault ride-through control method and control device for a hybrid flexible direct current transmission system. Based on the existing DC low-voltage current limiting function, an AC low-voltage current limiting function is added. The minimum value of the positive-sequence voltage amplitude of the AC busbars of each converter on the inverter side of the hybrid flexible direct current transmission system is selected for calculation to obtain an AC low-voltage current limiting value. The smaller of the AC low-voltage current limiting value and the DC low-voltage current limiting value is selected to control the output power of the rectifier-side converter. The technical solution provided by the present invention, based on the existing DC low-voltage current limiting function, adds an AC low-voltage current limiting function. The two functions are combined to control the output power of the sending end, thereby improving the system's response speed to AC faults and the system's recovery speed, achieving high control accuracy and not affecting the system's power delivery. By using different AC delay parameters when an AC fault occurs and when it recovers, the voltage slowly drops when the fault occurs, avoiding significant interference to the system. When the fault ends, the voltage recovers as quickly as possible, minimizing the impact of the fault on the system. And in the case of inter-station communication failure, the maximum limit of DC current is selected as the AC low-voltage current limiting limit value to achieve the effect of shielding the AC low-voltage current limiting function.

[0056] It should be understood that the above-described specific embodiments of the present invention are merely illustrative or illustrative of the principles of the present invention and do not constitute limitations of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included within the scope of protection of the present invention. In addition, the appended claims are intended to cover all variations and modifications that fall within the scope and metes and bounds of the appended claims, or equivalents thereof.

Claims

1. A method for controlling AC fault ride-through of a hybrid flexible DC transmission system, characterized in that: Including steps: Get the DC voltage U d , according to the DC voltage U d Get the DC low voltage current limit value I VDCL1 ; Get the positive sequence voltage amplitude U of each converter AC busbar on the inverter side psfvsci , i=1,2,3……, and take the minimum value among them as U psfvscmin ; According to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 ; Using the rectifier side current reference value I dref The output power of the rectifier side converter is controlled. The rectifier side current reference value I dref The upper limit value is the DC low voltage current limit value I VDCL1 and AC low voltage current limit value I VDCL2 The smaller value.

2. The control method according to claim 1, characterized in that: The DC voltage U d Get the DC low voltage current limit value I VDCL1 ,include: The DC voltage U d After first-order inertial filtering, linear amplification or reduction is performed to obtain the DC low-voltage current limit value I VDCL1 .

3. The control method according to claim 2, characterized in that: The first-order inertial filter coefficient is T is the DC delay parameter.

4. The control method according to claim 1, wherein: According to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 ,include: The minimum value U psfvscmin After first-order inertial filtering, linear amplification or reduction is performed to obtain the AC low-voltage current limit value I VDCL2 .

5. The control method according to claim 4, characterized in that: The first-order inertial filter coefficient is T is the DC delay parameter; When the AC fault begins, T = T2; When the AC fault ends, T = T1; Wherein, T1 and T2 are the first AC delay parameter and the second AC delay parameter.

6. The control method according to claim 1, characterized in that: When an inter-station communication failure occurs, the AC low voltage current limit value I VDCL2 =I max , I max It is the maximum limit of DC current.

7. The control method according to claim 1, characterized in that: The hybrid flexible direct current transmission system includes an LCC-VSC hybrid flexible direct current transmission system.

8. An AC fault ride-through control device for a hybrid flexible DC transmission system, characterized in that: It includes a DC low-voltage current limiting value calculation module, an AC low-voltage current limiting value calculation module, a rectifier side current reference value calculation module, and a rectifier side output power control module, and the modules are connected in sequence; wherein, The DC low voltage current limiting value calculation module obtains the DC voltage U d , according to the DC voltage U d Get the DC low voltage current limit value I VDCL1 ; The AC low voltage current limit value calculation module obtains the AC bus positive sequence voltage amplitude U of each converter on the inverter side psfvsci , i=1,2,3……, and take the minimum value among them as U psfvscmin , according to the minimum value U psfvscmin Get the AC low voltage current limit value I VDCL2 ; The rectifier side current reference value calculation module, the rectifier side current reference value I dref The upper limit value is the DC low voltage current limit value I VDCL1 and AC low voltage current limit value I VDCL2 The smaller value; The rectifier side output power control module uses the rectifier side current reference value I dref Control the output power of the rectifier-side converter.

9. The control device according to claim 8, characterized in that The AC low-voltage current limiting value calculation module includes a first-order inertial filter unit and a linear unit; The first-order inertial filtering unit minimizes the value U psfvscmin Perform first-order inertial filtering; The linear unit linearly amplifies or reduces the filtered signal to obtain the AC low-voltage current limiting value I VDCL2 .

10. The control device according to claim 8, characterized in that The DC low-voltage current limiting value calculation module includes a first-order inertial filter unit and a linear unit; The first-order inertial filter unit converts the DC voltage U d Perform first-order inertial filtering; The linear unit linearly amplifies or reduces the filtered signal to obtain the DC low-voltage current limiting value I VDCL1 .

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