Method for inhibiting commutation failure of inverter caused by failure of rectifier side ac system

Abstract

The application relates to an inverter commutation failure inhibition method caused by rectifier side AC system failure, which comprises the following steps: (1) improving the rectifier side trigger angle; (2) reducing the current margin; and (3) improving the rectifier side current instruction value. The application is scientific and reasonable in design. The commutation failure inhibition method based on trigger angle adjustment has rapid response, and the rectifier side DC voltage can be quickly changed. The method can reduce the rectifier side DC voltage and further promote the DC current reduction. The method of reducing the current margin and improving the rectifier side current instruction value is adopted to limit the recovery speed of the DC current, so that the system electrical quantity can be stably recovered and the commutation failure can be inhibited. The application can meet the requirements of system rapid response and stable recovery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power system stability analysis and control, and particularly relates to a method for inhibiting commutation failure of an inverter caused by fault of an AC system on a rectifier side. BACKGROUND

[0002] Under the goal of constructing a new power system mainly based on new energy, renewable energy such as wind power and photovoltaic power will be massively connected to the power system. Due to the reverse distribution of energy resources and load centers in China, the abundant wind and solar resources in the northwest region need to be transmitted to a remote place by high-voltage direct current (HVDC). The conventional HVDC adopts thyristors as the converter valve, and the thyristor is a semi-controlled device without self-turn-off capability and needs to rely on the receiving-end AC grid to complete commutation, which leads to commutation failure of the inverter becoming one of the most common faults of the HVDC system. The fault of the AC system on the inverter side will cause the commutation voltage to drop and the DC current to rise, thereby reducing the turn-off angle and triggering commutation failure. For the commutation failure problem under this scenario, the academia has carried out extensive research on its mechanism, discrimination criteria and inhibition methods, and has achieved relevant research results. However, in recent years, relevant research has found that the fault of the AC system on the rectifier side will also cause the commutation failure of the inverter.

[0003] During the fault of the AC system on the rectifier side, the DC transmission active power decreases, the DC voltage and the DC current decrease, the rectifier side may be switched from constant current control to minimum trigger angle control, the inverter side may be switched from constant turn-off angle control to constant current control, and at the same time, the turn-off angle on the inverter side will rise to a large value during this stage. When the fault is cleared, during the recovery of the AC system on the rectifier side, the DC transmission active power rises, the DC voltage and the DC current increase, and under the action of the constant current control on the rectifier side, the DC current gradually reaches the current command value on the rectifier side, which leads to the decrease of the output of the current deviation control, and further causes the further drop of the leading trigger angle output by the constant turn-off angle control on the inverter side, and finally causes the decrease of the turn-off angle. When the turn-off angle decreases below the critical turn-off angle, the inverter occurs commutation failure.

[0004] For the problem of commutation failure of the inverter caused by the fault of the AC system on the rectifier side, the application proposes to comprehensively coordinate and inhibit the commutation failure of the inverter from the aspects of improving the rectifier trigger angle, reducing the current margin and improving the current command value on the rectifier side. SUMMARY

[0005] The application aims to overcome the shortcomings of the prior art, improve the response of the control strategy on the rectifier side to the fault of the AC system on the rectifier side, and accordingly proposes a method for inhibiting commutation failure of the inverter caused by the fault of the AC system on the rectifier side.

[0006] The application solves the technical problem by the following technical scheme:

[0007] A method for preventing commutation failure of an inverter caused by a fault of an AC system on a rectifier side, characterized in that the method comprises the following steps:

[0008] (1) improving the firing angle on the rectifier side

[0009] After a fault of the AC system on the rectifier side, the voltage U Lr on the rectifier bus and the DC voltage U dr both decrease, and the relationship between them is shown in equation (1):

[0010]

[0011] In equation (1), α r is the firing angle on the rectifier side, I d is the DC line current, and R r is the equivalent commutation resistance on the rectifier side.

[0012] In order to ensure the transmission of active power, the firing angle on the rectifier side is reduced as much as possible during the fault until the minimum firing angle. In the case of a small firing angle, after the fault is cleared, the DC voltage U dr on the rectifier side will rapidly rise with the recovery of the voltage U Lr on the rectifier bus, thereby causing the rapid decrease of the extinction angle on the inverter side.

[0013] If the fault of the AC system on the rectifier side is serious, it may cause the extinction angle on the inverter side to decrease below the critical extinction angle, thereby causing the commutation failure. Therefore, the firing angle on the rectifier side needs to be improved according to the recovery state of the system, and the recovery speed of the firing angle is increased to prevent the rapid rise of the DC voltage.

[0014] The active power transmitted by the DC line contains voltage and current information, and can fully reflect the recovery state of the system. In order to establish the relationship between the active power and the firing angle on the rectifier side, the active power is written in the following form according to the expression of U dr in equation (1):

[0015]

[0016] In equation (2), γ r is the extinction angle on the rectifier side.

[0017] Suppose that the value of the extinction angle on the rectifier side before and after the fault is the same, i.e., γ r = 140°. According to equation (2), the improved firing angle α r ′ on the rectifier side can be calculated by obtaining the measured values of the active power and the DC current, and the calculation process is shown in the following equation:

[0018]

[0019] The improved rectifier-side trigger angle α r is calculated r is limited to (5°, 90°), and after the rectifier-side AC system fails, α r is compared with the trigger angle α dr_ord output by the rectifier-side constant current control in the CIGRE DC transmission standard model, and the larger one is taken as the final rectifier-side trigger angle, so as to reduce the rising speed of the DC voltage in the fault recovery process and further reduce the risk of commutation failure;

[0020] (2) Reduce the current margin

[0021] According to the current margin control principle of the DC transmission system, the rectifier-side current command I di_ord is always higher than the inverter-side current command I d by a value, which is the current margin ΔI d , satisfying ΔI dr_ord = I di_ord -I d ;

[0022] During the rectifier-side AC system failure, the DC current is mainly controlled by the inverter-side constant current control, that is, I di_ord = I CEC ; the greater the current margin is, the greater the difference between the DC current and the rectifier-side current command value is, and the greater the output γ d of the current deviation control is, so that I dr_ord tends to I CEC more and more during the fault recovery process, and γ CEC drops faster;

[0023] Therefore, in the case of rectifier-side AC system failure, by reducing the current margin from 0.1 pu to 0.05 pu, the difference between the rectifier-side and inverter-side current commands, that is, the difference between the rectifier-side current command and the actual DC current, can be reduced, and the output γ r of the current deviation control can be maintained at a smaller value, so as to prevent the rapid drop of the current deviation control output caused by the rapid recovery of the DC current, thereby preventing the occurrence of commutation failure;

[0024] (3) Improve the rectifier-side current command value

[0025] The faster the DC current rises, the more rapidly the inverter-side turn-off angle drops, and the greater the probability of commutation failure is. Since the DC current gradually rises following the rectifier-side current command value during the fault recovery process, the recovery speed of the DC current can be reduced by improving the rectifier-side current command value;

[0026] In the commutation time period (α r) inner, the commutation process of the rectifier side is integrated, and the expressions of the commutation area supply amount S pro , the demand amount S need and the deficiency amount S CF of the rectifier side are obtained as follows:

[0027]

[0028] Under normal operating conditions and fault steady states, the commutation area supply amount and the demand amount of the rectifier side are equal, that is, S pro =S need ; after the fault is cleared, the trigger angle of the rectifier side gradually rises, which leads to the decrease of the commutation area supply amount S pro ; and the DC current gradually recovers in this stage, which further increases the commutation area demand amount S need , and finally leads to S pro <S need , that is, the commutation area deficiency amount S CF <0, while the commutation failure has not yet occurred;

[0029] Therefore, before the commutation failure occurs, the rectifier side current command value is improved in time according to the size of the commutation area deficiency amount of the rectifier side, which can reduce the recovery speed of the DC current and avoid the commutation failure of the inverter side caused by the rapid rise of the DC current; the design coefficient k represents the rectifier side current command change amount corresponding to a unit commutation area deficiency amount (k>0), and since S CF <0 in the fault recovery process, the rectifier side current command change amount ΔI dr_ord =kS CF <0 in this stage;

[0030] In addition, since the current margin has been reduced to 0.05pu in the foregoing step, the rectifier side current command change amount corresponding to the commutation area deficiency amount should not exceed 0.05pu, that is, |ΔI dr_ord |≤0.05;

[0031] The rectifier side current command value I dr_ord of the CIGRE DC transmission standard model is the output of VDCOL, and the improved rectifier side current command value I dr_ord ′ is obtained by adding ΔI dr_ord to I dr_ord , and the calculation method is as follows:

[0032] I dr_ord ′=I dr_ord +ΔI dr_ord (5)

[0033] S CFThe smaller means that the DC current is recovered faster in the fault recovery process, and the commutation area demand is larger. pro And S need will not exceed its rated value S proN And S needN , considering the extreme case S pro = 0 and S need = S needN , at this time, the minimum value of the commutation area deficiency of the rectifier side S CFmin = -S needN can be considered; when S CF takes the minimum value, ΔI dr_ord also takes the minimum value, that is, ΔI dr_ord = -0.05pu, and thus the value of k is:

[0034] S needN = 2X c I d = 53.28 (6)

[0035]

[0036] The advantages and beneficial effects of the present application are:

[0037] 1. The commutation failure suppression method based on trigger angle adjustment of the present application has fast response, and the rectifier side DC voltage can be quickly changed. The method can further reduce the rectifier side DC voltage while further reducing the DC current.

[0038] 2. The rapid adjustment of the trigger angle of the present application may cause a large power fluctuation, so that the system bears a large electrical quantity fluctuation during recovery. In order to avoid this situation, the current margin is reduced and the rectifier side current command value is improved to limit the recovery speed of the DC current, so as to ensure the smooth recovery of the system electrical quantity while suppressing the occurrence of commutation failure.

[0039] 3. The present application can meet the needs of fast response and smooth recovery of the system, and has wide application range. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 is a DC control system response process diagram after the rectifier side AC system fault, wherein figure (a) is an electrical quantity response process diagram, and figure (b) is a control quantity response process diagram;

[0041] Figure 2 is a fault process flow diagram under the rectifier side AC system fault. DETAILED DESCRIPTION

[0042] The application will be further described in detail by specific examples. The following examples are only descriptive and not restrictive, and cannot limit the protection scope of the application.

[0043] A method for inhibiting commutation failure of an inverter caused by a fault of an AC system on a rectifier side, comprising the following steps:

[0044] (1) improving the firing angle on the rectifier side

[0045] After a fault occurs in the AC system on the rectifier side, the voltage U Lr and the DC voltage U dr on the rectifier side will both decrease, and the relationship between them is shown in equation (1):

[0046]

[0047] In the equation, αr is the firing angle on the rectifier side, Id is the DC line current, and Rr is the equivalent commutation resistance on the rectifier side.

[0048] To ensure the transmission of active power, the firing angle on the rectifier side will be reduced as much as possible during the fault until the minimum firing angle. In the case of a small firing angle, after the fault is cleared, the DC voltage Udr on the rectifier side will quickly rise with the recovery of the commutation bus voltage ULr, thereby causing the quick decrease of the extinction angle on the inverter side. If the fault of the AC system on the rectifier side is more serious, it may cause the extinction angle on the inverter side to decrease below the critical extinction angle, thereby causing the commutation failure phenomenon. Therefore, the firing angle on the rectifier side needs to be improved according to the recovery state of the system, and the recovery speed of the firing angle is increased to prevent the rapid rise of the DC voltage.

[0049] The active power transmitted by the DC line contains voltage and current information, and can fully reflect the recovery state of the system. To establish the relationship between the active power and the firing angle on the rectifier side, the active power is written in the following form by combining the Udr expression in equation (1):

[0050]

[0051] In the equation, γr is the extinction angle on the rectifier side. It is assumed that the value of the extinction angle on the rectifier side before and after the fault is the same, i.e., γr = 140°. According to equation (2), the improved firing angle αr' on the rectifier side can be calculated by obtaining the measured values of the active power and the DC current, and the calculation process is shown in the following equation:

[0052]

[0053] The improved rectifier side trigger angle αr' calculated is subjected to amplitude limiting processing, and the amplitude limiting value is (5°, 90°). After the rectifier side AC system fails, αr' is compared with the trigger angle αr output by the rectifier side constant current control under the CIGRE DC power transmission standard model, and the larger value is taken as the final rectifier side trigger angle, so as to reduce the rising speed of the DC voltage in the fault recovery process, and further reduce the commutation failure risk.

[0054] (2) Reduce the current margin:

[0055] According to the current margin control principle of the DC power transmission system, the rectifier side current command I dr_ord is always higher than the inverter side current command I di_ord by a value, which is the current margin ΔI d , satisfying ΔI d = I dr_ord -I di_ord .

[0056] During the failure of the rectifier side AC system, the DC current is mainly controlled by the inverter side constant current control, that is, I d = I di_ord . When the current margin is larger, the difference between the DC current and the rectifier side current command value is larger, the output γ CEC of the current deviation control is larger, and in the fault recovery process, I d tends to I dr_ord more and more, and γ CEC drops faster. Therefore, in the case of failure of the rectifier side AC system, by reducing the current margin from 0.1pu to 0.05pu, the difference between the rectifier side and inverter side current commands, that is, the difference between the rectifier side current command and the actual DC current, can be reduced, and further the output γ CEC of the current deviation control can be maintained at a smaller value, so as to prevent the rapid recovery of the DC current from causing the rapid drop of the output of the current deviation control, thereby preventing the occurrence of commutation failure.

[0057] (3) Improve the rectifier side current command value

[0058] The faster the DC current rises, the more rapidly the inverter side turn-off angle drops, and the greater the probability of commutation failure. Since the DC current gradually rises following the rectifier side current command value in the fault recovery process, the recovery speed of the DC current can be reduced by improving the rectifier side current command value. In the commutation time period (α r , π-γ r ), the rectifier side commutation process is integrated to obtain the expressions of the rectifier side commutation area supply amount S pro , demand amount S need and deficiency amount S CF , as shown in the following formula:

[0059]

[0060] Under normal operating conditions and fault steady-state conditions, the supply and demand of commutation area on the rectifier side are equal, i.e., S pro =S need After the fault is cleared, the rectifier-side firing angle will gradually increase, which will cause the commutation area supply S to increase. pro The current decreases; and the DC current gradually recovers during this stage, thereby reducing the commutation area requirement S. need Increase, ultimately leading to S pro need That is, the commutation area lacks a certain amount S. CF <0, and commutation failure has not yet occurred. Therefore, before commutation failure occurs, timely adjustments to the rectifier-side current command value based on the magnitude of the commutation area deficiency on the rectifier side can reduce the DC current recovery speed and prevent commutation failure on the inverter side due to a rapid increase in DC current. The design factor k represents the change in rectifier-side current command corresponding to a unit commutation area deficiency (k>0). Due to the fault recovery process, S CF <0, therefore the change in rectifier-side current command ΔI during this stage dr_ord =kS CF <0. Furthermore, since the aforementioned steps have already reduced the current margin to 0.05 pu, the change in rectifier-side current command corresponding to the commutation area deficiency should not exceed 0.05 pu, i.e., |ΔI dr_ord |≤0.05.

[0061] The rectifier-side current command value I under the CIGRE DC transmission standard model dr_ord For the output of VDCOL, through I dr_ord Add ΔI to the basis dr_ord The improved rectifier-side current command value I is obtained. dr_ord ′, its calculation method is shown in the following formula:

[0062] I dr_ord ′=I dr_ord +ΔI dr_ord (5)

[0063] S CF A smaller value means a faster DC current recovery during fault recovery, but a larger commutation area requirement. This is due to the S... pro and S need None will exceed its rated value S proN and S needN Considering the extreme case S pro =0 and S need =S needN At this point, the minimum value S of the commutation area deficiency on the rectifier side can be considered.​CFmin = -S needN . When S CF takes the minimum value, ΔI dr_ord also takes the minimum value, i.e. ΔI dr_ord = -0.05 pu. Thus the value of k is:

[0064] S needN = 2X c I d = 53.28 (6)

[0065]

[0066] The technical solutions of the application will be described in detail below with reference to the examples shown in the accompanying drawings. Figure 1 、 2 The technical solutions of the application will be described in detail below with reference to the examples shown in the accompanying drawings.

[0067] A three-phase 20 Ω fault resistor is connected to the rectifier bus, the fault time is 1.0 s, and the fault duration is 0.5 s. After the fault of the rectifier-side AC system, the voltage and current and other electrical quantities fluctuate dramatically, and the rectifier-side and inverter-side DC control systems adjust the trigger angle to maintain system stability, and the response process of the DC control system is shown in FIG. 2. In the figure, the subscripts r and i are used to distinguish the rectifier-side and inverter-side variable symbols. According to the variation characteristics of electrical quantities and control quantities, the rectifier-side AC system fault period and fault recovery process are divided into three stages. Figure 1

[0068] Stage I is the fault stage: the fault of the rectifier-side AC system causes the active power P d transmitted by the DC to decrease, the rectifier-side DC voltage U dr drops rapidly, the DC current I d decreases, and the inverter-side turn-off angle γ i rises significantly. Influenced by the rise of the turn-off angle, the β CEA output by the inverter-side constant turn-off angle control decreases. To ensure the transmission of active power, the rectifier-side and inverter-side constant current controls timely adjust the trigger angle to raise the DC current, and the specific adjustment process is: the rectifier-side constant current control decreases the trigger angle until it reaches the minimum value, at which time the rectifier-side switches from the constant current control to the minimum trigger angle control; the inverter-side constant current control increases the advance trigger angle β CC , and since β CC is greater than β CEA , the inverter-side switches from the constant turn-off angle control to the constant current control, and finally the system reaches the fault steady state.

[0069] Stage II is the fault recovery stage: after the fault is cleared, the active power P d transmitted by the DC begins to recover, the rectifier-side DC voltage U dr rises, and the DC current I​d Increase the inverter-side turn-off angle γ i The β value of the inverter-side fixed-turn-off angle control output decreases as the turn-off angle decreases. CEA Increase. At the moment the fault is cleared, the rectifier-side firing angle experiences a brief increase, and the rectifier side switches back to constant current control. As the DC current recovers, the β of the inverter-side constant current control output increases. CC It gradually decreases, but due to the β level during this stage CC Still greater than β CEA Therefore, both the rectifier and inverter sides operate in constant current control mode.

[0070] Stage III is the commutation failure stage: the continuous decrease in the inverter-side turn-off angle leads to β CEA Greater than β CC At this point, the inverter side switches from constant current control back to constant turn-off angle control. During this stage, the DC current is controlled only by the rectifier side, and under the constant current control of the rectifier side, I... d Gradually reaching the rectifier-side current command value I dr_ord This results in the output γ of the current deviation control. CEC The decrease is due to the input of the PI element in the inverter-side fixed-off angle control satisfying Δγ = γ. re f-γ i +γ CEC γ CEC Decreasing it will cause a decrease in Δγ, which in turn will decrease its output β. CEA Further drops eventually lead to a decrease in the turn-off angle. When the turn-off angle falls below the critical turn-off angle, the inverter experiences commutation failure.

[0071] In summary, the inverter will not experience commutation failure during a fault in the rectifier-side AC system or immediately after the fault is cleared. After the fault in the rectifier-side AC system is cleared, the turn-off angle on the inverter side gradually decreases as electrical quantities gradually recover. If the fault in the rectifier-side AC system is severe, the rapid recovery of electrical quantities in Phase II will cause the turn-off angle on the inverter side to drop below the near-turn-off angle, thereby triggering inverter commutation failure. This invention addresses the issue of suppressing rapid increases in DC voltage and DC current by comprehensively and coordinately suppressing inverter commutation failure through three aspects: improving the firing angle on the rectifier side, reducing the current margin, and improving the current command value on the rectifier side.

[0072] Improved rectifier-side firing angle: The improved rectifier-side firing angle α is calculated using equation (3). r ′, and the calculated α r Amplitude limiting is applied, with a limiting value of (5°, 90°). After a fault occurs in the rectifier-side AC system, α... r The firing angle α of the rectifier-side constant current control output under the CIGRE DC transmission standard model. rThe comparison is made, and the larger value is taken as the final rectifier-side trigger angle to reduce the rising speed of the DC voltage during the fault recovery process, and further reduce the commutation failure risk of the inverter.

[0073] Reducing current margin: in the case of a fault in the rectifier-side AC system, when the inverter-side control system detects β CC >β CEA After the fixed off-angle control switching to fixed current control, the current margin is reduced from 0.1 pu to 0.05 pu, which can reduce the difference between the actual DC current and the rectifier-side current command value, avoid the rapid drop of the current deviation control output caused by the rapid recovery of the DC current, and prevent the occurrence of commutation failure.

[0074] Improving the rectifier-side current command value: the rectifier-side current command value I dr_ord is the output of VDCOL, which is obtained by adding the rectifier-side current command change amount ΔI dr_ord to I dr_ord , to obtain the improved rectifier-side current command value I dr_ord ′, and the calculation method is I dr_ord ′=I dr_ord +ΔI dr_ord . Wherein, ΔI dr_ord =kS CF , k is the rectifier-side current command change amount corresponding to the unit commutation area deficiency (k>0), and S CF is the rectifier-side commutation area deficiency. According to formula (4), the rectifier-side commutation area deficiency is calculated, and then the rectifier-side current command change amount ΔI dr_ord is obtained, and further according to formula (5), the improved rectifier-side current command value I dr_ord ′ is calculated.

[0075] Although the embodiments and drawings of the present application are disclosed for the purpose of illustration, those skilled in the art can understand that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present application and the appended claims, therefore, the scope of the present application is not limited to the disclosed content of the embodiments and drawings.

Claims

1. A method for suppressing inverter commutation failure caused by a fault in an ac system on the rectifier side, characterized by: The method comprises the following steps: (1) improving the trigger angle of the rectifier side After the failure of the rectifier-side AC system, the rectifier-side bus voltage U Lr and the DC voltage U dr both decrease, and the relationship between them is shown in equation (1): In the formula: α r is the rectifier-side trigger angle, I d is the DC line current, R r is the rectifier-side equivalent commutation resistance; To ensure the transmission of active power, the rectifier side will reduce its firing angle as small as possible until the minimum firing angle during the fault. In the case of small firing angle, the rectifier side DC voltage U dr will rise rapidly with the recovery of the commutation bus voltage U Lr , thereby causing the rapid decrease of the inverter side turn-off angle. If the fault of the AC system on the rectifier side is more serious, the turn-off angle on the inverter side can be reduced below the critical turn-off angle, causing the commutation failure phenomenon. Therefore, the trigger angle of the rectifier side needs to be improved according to the system recovery state, and the recovery speed of the trigger angle is increased to prevent the DC voltage from rising too fast; The active power transmitted by DC line contains voltage and current information, which can fully reflect the recovery state of the system. To establish the relationship between the active power and the trigger angle on the rectifier side, the active power is written in the following form by combining the expression of U dr in equation (1) In the formula, γ r is the off angle of the rectifier side; Assuming that the off angle of the rectifier side is the same before and after the fault, i.e. γ r = 140°, according to formula (2), by obtaining the active power and DC current measurement values, the improved rectifier side trigger angle α r ′ can be calculated, and the calculation process is as follows: The improved rectifier-side trigger angle α r is subjected to amplitude limiting processing with an amplitude limiting value of (5°, 90°), and after a failure of the rectifier-side AC system, α r is compared with a trigger angle α of a constant current control output of the rectifier-side in a CIGRE DC power transmission standard model r The larger one is taken as the final rectifier-side trigger angle to reduce the rising speed of the DC voltage in the fault recovery process, thereby reducing the risk of commutation failure. (2) reducing the current margin According to the current margin control principle of the DC power transmission system, the rectifier-side current command I dr_ord is always higher than the inverter-side current command I di_ord by a value, which is the current margin ΔI d , satisfying ΔI d = I dr_ord -I di_ord ; During AC system faults on the rectifier side, the DC current is mainly controlled by the inverter side constant current, i.e., I d =I di_ord When the current margin is larger, the difference between the DC current and the rectifier-side current command value is greater, and the output γ of the current deviation control is also greater. CEC The larger the value, the more the fault recovery process will progress with I. d Increasingly trending towards I dr_ord γ CEC The faster the descent; Therefore, in the case of failure of the rectifier-side AC system, by reducing the current margin from 0.1 pu to 0.05 pu, the difference between the current command of the rectifier side and the inverter side, i.e., the difference between the current command of the rectifier side and the actual DC current, can be reduced, and thus the output γ of the current deviation control is maintained at a smaller value, avoiding the rapid decrease of the output of the current deviation control caused by the rapid recovery of the DC current, thereby preventing the occurrence of commutation failure. CEC Therefore, in the case of failure of the rectifier-side AC system, by reducing the current margin from 0.1 pu to 0.05 pu, the difference between the current command of the rectifier side and the inverter side, i.e., the difference between the current command of the rectifier side and the actual DC current, can be reduced, and thus the output γ of the current deviation control is maintained at a smaller value, avoiding the rapid decrease of the output of the current deviation control caused by the rapid recovery of the DC current, thereby preventing the occurrence of commutation failure. (3) improving the current command value of the rectifier side The faster the DC current rises, the more rapidly the turn-off angle on the inverter side decreases, and the greater the probability of commutation failure. Since the DC current gradually rises following the current command value of the rectifier side during the fault recovery process, the recovery speed of the DC current can be reduced by improving the current command value of the rectifier side; In the commutation time period (α r , π-γ r ), the commutation process of the rectifier side is integrated to obtain expressions of the rectifier side commutation area providing quantity S pro , demand quantity S need and deficiency quantity S CF , as shown in the following formula: Under normal operation and fault steady state, the commutation area provided quantity and demand quantity are equal, i.e. S pro = S need ; after the fault is cleared, the commutation side trigger angle gradually rises, which will lead to the commutation area provided quantity S pro decrease; and the DC current gradually recovers in this stage, thereby making the commutation area demand quantity S need increase, eventually leading to S pro < S need , i.e. the commutation area deficiency quantity S CF < 0, while the commutation failure has not yet occurred; Therefore, before the commutation failure occurs, the recovery speed of the DC current can be reduced by improving the rectifier-side current command value in time according to the size of the rectifier-side commutation area deficiency, and the commutation failure caused by the rapid rise of the DC current at the inverter side can be avoided; the design coefficient k represents the rectifier-side current command change corresponding to a unit commutation area deficiency, and since S CF <0, the rectifier-side current command change ΔI dr_ord =kS CF <0 in this stage. In addition, since the aforementioned step has reduced the current margin to 0.05 pu, the commutation area lack amount corresponding to the rectifier side current command change amount should not exceed 0.05 pu, i.e., |ΔI dr_ord ≤ 0.05; Rectifier side current command value I under CIGRE DC transmission standard model dr_ord is output as VDCOL by adding ΔI dr_ord to I dr_ord , and the improved rectifier side current command value I dr_ord ′ is obtained, and its calculation method is shown in the following formula: I dr_ord ′=I dr_ord +ΔI dr_ord (5) S CF The smaller means the faster the direct current recovers in the fault recovery process and the larger the commutation area demand, since S pro and S need will not exceed their rated values S proN and S needN , considering the extreme case of S pro = 0 and S need = S needN , at this time the minimum value of the commutation area deficiency of the rectifier side S CFmin = -S needN can be considered; when S CF takes the minimum value, ΔI dr_ord also takes the minimum value, namely ΔI dr_ord = -0.05pu, and thus the value of k is obtained: S needN = 2X c I d = 53.28 (6)

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