Method for quantitatively evaluating subsequent commutation failure immune level of alternating-current and direct-current hybrid system

Abstract

The invention discloses a method for quantitatively evaluating the immune level of subsequent commutation failure of an AC-DC hybrid system, and the method comprises the steps: carrying out the analysis of the transient process of the system based on the transient reactive dynamic characteristics of the AC-DC hybrid system on the basis of achieving the cross-regional power transmission through a high-voltage DC power transmission technology; the essential relation among the electrical quantities in the transient process is obtained; and deducing an online evaluation index GCFII for reflecting the immune level of the subsequent commutation failure of the system in combination with known system parameters and electrical signals acquired by the PMU. According to the method, the influence of the direct current control mode is considered by obtaining the online evaluation index GCFII, and real-time updating can be carried out along with the change of system operation parameters, so that the requirements of real-time performance and accuracy can be met, a theoretical basis is provided for power grid construction and operation of operation personnel, and safe operation of a power grid is effectively ensured.

Description

Technical Field

[0001] The present invention relates to online monitoring and control of an electric power system, belongs to the field of electric power system operation analysis, and in particular to a method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system. Technical Background

[0002] Since the advent of the electrical age, the widespread use of electric energy has fundamentally changed people's lives. Now electric energy has penetrated into all aspects of social life and has become the most basic and main energy form in modern industrial and agricultural production, economic and social development, and people's daily lives. However, while my country's power industry is booming, it is also facing many problems. For example, the distribution of my country's energy resources and actual load demand are spatially inversely distributed. Primary energy is mainly concentrated in the western region, while load demand is mainly concentrated in the economically developed central and eastern regions. Therefore, in order to achieve a reasonable allocation of resources and coordinate the promotion of social development, my country has proposed a national networking pattern of "west-to-east power transmission" and "north-south mutual supply", which requires the use of high-voltage direct current transmission to achieve long-distance and large-scale transmission of electric energy.

[0003] In an AC / DC hybrid system, commutation failure caused by a fault in the receiving AC system is one of the most common fault forms. If the AC system fault that caused the first commutation failure is not cleared in time, the system may experience subsequent commutation failures. Usually, the first commutation failure is usually less harmful and can be recovered by itself, while subsequent commutation failures are prone to develop into large-scale cascading failures and cause voltage collapse in weak power grids. Therefore, by studying the dynamic characteristics of the system and proposing appropriate evaluation indicators to measure the immunity level of subsequent commutation failures in the AC / DC hybrid system, it can provide a theoretical basis for the design of system operation control strategies and ensure the voltage stability and safety of the system.

[0004] However, the existing evaluation method using the commutation failure immunity factor (CFII) is computationally intensive and time-consuming, and cannot achieve online evaluation of the system's commutation failure immunity capability. Therefore, it cannot adapt to changes in the system's operating mode, and it is difficult to meet the needs of flexibility in actual engineering, and cannot achieve ideal results in actual use.

[0005] In order to effectively solve the above problems, the present invention proposes a generalized commutation failure immunity factor (GCFII). GCFII can effectively evaluate the subsequent commutation failure immunity level of the AC / DC hybrid system online. Through quantitative comparison of the calculated values, the physical meaning is clear, and it can provide effective indicator basis for power grid operators, thereby effectively ensuring the stable operation and safety of the power grid. Summary of the invention

[0006] The purpose of the present invention is to provide a method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system, by deriving an online evaluation index GCFII used to reflect the subsequent commutation failure immunity level of the system. The index is derived taking into account the influence of the DC control mode and can be updated in real time with the changes in the system operating parameters. Therefore, it can meet the requirements of real-time and accuracy, provide a theoretical basis for the construction and operation of power grid operators, and effectively ensure the safe operation of the power grid.

[0007] The technical solution to achieve the above purpose is: a method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system. On the basis of using high-voltage direct current transmission technology to realize cross-regional power transmission, the transient process of the system is analyzed based on the transient reactive dynamic characteristics of the AC / DC hybrid system; and then the essential connection between various electrical quantities in the transient process is obtained; combined with the known system parameters and the electrical signals collected by the PMU, the online evaluation index GCFII used to reflect the subsequent commutation failure immunity level of the system is derived.

[0008] In the above-mentioned method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system, the derivation of the index GCFII takes into account changes in the DC control mode and can be updated in real time with changes in system operating parameters to meet the requirements of real-time and accuracy, providing data basis for the operation of power grid infrastructure and operators, thereby effectively ensuring the safe operation of the power grid.

[0009] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, the following steps are included:

[0010] Step S1: Transient characteristics analysis of AC / DC hybrid system:

[0011] Step S2: Analytical calculation of electrical quantity connections based on transient characteristics;

[0012] Step S3: deriving a generalized commutation failure critical impedance based on electrical quantity connections;

[0013] Step S4: deriving a generalized commutation failure immunity factor based on the generalized commutation failure critical impedance.

[0014] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, the step S1, i.e., the transient characteristic analysis method of the AC / DC hybrid system is:

[0015] When a short circuit occurs at the commutation bus, critical commutation failure occurs on the inverter side, that is, the shutdown angle γ = γ min When , the fault severity can be expressed by the short-circuit impedance.

[0016] At this time, the reactive power balance at the commutation bus can be expressed as:

[0017] Q d =Q ac +Q f +Q fault (1)

[0018] In the formula, Q d , Q ac , Q f and Q fault The expressions of are shown in formula (2) to formula (5) respectively.

[0019]

[0020] Where E is the amplitude of the equivalent voltage source of the receiving AC system, U ac is the amplitude of the inverter-side commutation bus voltage, Z is the amplitude and phase angle of the equivalent impedance of the receiving-end system, P d With Q d are respectively the active and reactive power fed into the commutation bus by the inverter, P ac With Q ac are the active and reactive power fed into the receiving-end AC system by the commutation bus, I d is the DC current, B f is the equivalent susceptance of the filter, Q f is the reactive power fed into the commutation bus by the filter, X fault is the critical fault inductance when critical commutation failure occurs, Q fault is the reactive power fed into the commutation bus by the critical fault inductor, γ is the arc extinction angle of the inverter, β is the leading trigger angle of the inverter, k is the transformer ratio, N p is the number of pole pairs of the transformer, X T is the impedance of the transformer, U cr It is the critical voltage of the commutation bus when critical commutation failure occurs.

[0021] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, the step S2, i.e., the electrical quantity connection analytical calculation method based on transient characteristics, is:

[0022] Substituting the analytical expressions of reactive power in equations (2) to (5) into equation (1) for calculation, we can obtain the functional relationship of various electrical quantities, as shown in the following equation:

[0023]

[0024] The expressions of coefficients a, b, c, d, and e are:

[0025]

[0026] The critical arc extinction angle is the actual value, that is, γmin =7.2°.

[0027] And through the known system operating parameters and the real-time acquisition of electrical quantity signals by PMU, this functional relationship is further simplified to the DC current I d , commutation bus critical voltage U cr With critical fault inductance X fault The function is:

[0028] I d =f(U cr ,X fault ) (8)

[0029] Its specific expression is:

[0030]

[0031] Among them, the expressions of E, F, X, and Y are:

[0032]

[0033] In the above formula, the expressions of G, p, and q are:

[0034]

[0035] p=-(12G+F 2 ) / 3 (15)

[0036] q=(27(4FG-E 2 G)-36FG-2F 3 ) / 27 (16)

[0037] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, the step S3, i.e., the method for deriving the generalized commutation failure critical impedance based on the electrical quantity connection, is:

[0038] According to the analytical expression of the arc extinction angle, the minimum arc extinction angle when critical commutation failure occurs can also be expressed as:

[0039]

[0040] After the first commutation failure occurs in the system, the control system has been fully functional. Therefore, when the system ends the first commutation failure, the turn-off angle γ returns to a normal level, and the corresponding advance trigger angle β returns to near the rated value.

[0041] 7. A method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 6, characterized in that: when the subsequent commutation failure critical occurs, the arc extinction angle γ suddenly decreases to a critical value. At this time, due to the instantaneous change of the turn-off angle, the control system has not yet taken effect, so the leading trigger angle is still near the rated value, that is, β in formula (17) can be regarded as the rated value.

[0042] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, a low-voltage limiting link is used to regulate the DC current.

[0043] The mathematical relationship between its input and output is:

[0044]

[0045] Among them, I order is the output DC current command value, I max is the maximum output value, I min is the minimum output value, U dc is the collected commutation bus voltage, U H The maximum voltage that makes VDCOL operate, U L The minimum voltage for VDCOL to operate is:

[0046] Combining equations (8), (17) and (18), the critical fault impedance X when critical commutation failure occurs can be obtained: fault .

[0047] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, the step S4, i.e., the method for deriving a generalized commutation failure immunity factor based on the generalized commutation failure critical impedance, is as follows: the generalized commutation failure immunity factor is selected as the ease of occurrence of commutation failure, and its expression is:

[0048]

[0049] Where U ac is the rated value of the line voltage, P ac is the DC rated power.

[0050] In the above-mentioned method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system, it can be seen from formula (19) that GCFII is related to the rated line voltage, the rated power of DC transmission and the critical impedance of generalized commutation failure. The larger the GCFII, the stronger the system's ability to be immune to subsequent commutation failure.

[0051] The beneficial effect of the present invention is that, in the context of using high-voltage direct current transmission technology to achieve cross-regional power transmission, the transient process of the system is analyzed based on the transient reactive dynamic characteristics of the AC / DC hybrid system. Then, the essential connection between the electrical quantities in the transient process is obtained, and the online evaluation index GCFII used to reflect the subsequent commutation failure immunity level of the system is derived by combining the known system parameters and the electrical signals collected by the PMU. The derivation of this index takes into account the influence of the DC control mode, and can be updated in real time with the changes in the system operating parameters, so it can meet the requirements of real-time and accuracy, provide a theoretical basis for the construction of the power grid and the operation of the operation personnel, and effectively ensure the safe operation of the power grid.

[0052] The present invention adopts the above technical solution. Compared with the prior art, the present invention has the following advantages:

[0053] 1. The present invention obtains a system equivalent model under the most serious fault state by analyzing the transient process of the AC / DC hybrid system. On the basis of this model, the essential mathematical connection between various electrical quantities is obtained by combining the reactive power balance relationship and the analytical expressions of various reactive powers. This essential mathematical connection can more intuitively reflect the factors affecting the possibility of subsequent commutation failure of the system and their influencing effects, and has clear physical meaning, which can provide a model and theoretical basis for the proposal of commutation failure evaluation methods and the design of corresponding control strategies.

[0054] 2. The calculation process of the subsequent commutation failure immunity evaluation index GCFII proposed in the present invention does not rely on simulation. It can be calculated through known system operating parameters and electrical quantity signals collected in real time by PMU. Therefore, online calculation can be realized during system operation, which meets the real-time performance required for system operation monitoring and provides a theoretical basis for the operation of power grid operators.

[0055] 3. The calculation process of the subsequent commutation failure immunity evaluation index GCFII proposed by the present invention takes into account the influence of different DC current control modes, so the calculation results under different system operation modes are accurate, so it can be widely applied to various power grids. And when the system operation mode changes, the proposed GCFII can still be applied, meeting the power grid's requirements for the flexibility of evaluation indicators. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is the system equivalent model when the three-phase inductive grounding fault of the commutation bus leads to critical commutation failure;

[0057] Figure 2 It is the VDCOL static characteristic diagram;

[0058] Figure 3 This is a schematic diagram of the Cigre HVDC standard model. DETAILED DESCRIPTION

[0059] The present invention will be further described below in conjunction with the accompanying drawings.

[0060] The present invention mainly includes four aspects:

[0061] S1: Transient characteristics analysis of AC / DC hybrid system

[0062] When a short circuit occurs at the commutation bus, critical commutation failure occurs on the inverter side, that is, the shutdown angle γ = γ min When the fault severity is , the short-circuit impedance can be used to represent it. According to simulation verification, in the case of a three-phase inductive grounding fault, the critical impedance is the largest when a critical commutation failure occurs, that is, the system is most likely to have a commutation failure. Therefore, the transient characteristics of the system under a three-phase inductive grounding fault are analyzed, that is, the most serious system fault, so that the analysis is most universal.

[0063] When a three-phase inductive grounding fault occurs at the commutation busbar of the AC / DC hybrid system, resulting in a critical commutation failure, its equivalent model is as follows: Figure 1 shown.

[0064] Figure 1 Where E∠φ is the amplitude and phase angle of the equivalent voltage source of the receiving AC system, U ac ∠δ is the amplitude and phase angle of the inverter-side commutation bus voltage, Z∠θ is the amplitude and phase angle of the equivalent impedance of the receiving-end system, P d With Q d are respectively the active and reactive power fed into the commutation bus by the inverter, P ac With Q ac are the active and reactive power fed into the receiving-end AC system by the commutation bus, I d is the DC current, B f is the equivalent susceptance of the filter, Q f is the reactive power fed into the commutation bus by the filter, X fault is the critical fault inductance when critical commutation failure occurs, Q fault is the reactive power fed into the commutation bus by the critical fault inductor, γ is the arc extinction angle of the inverter, β is the leading trigger angle of the inverter, k is the transformer ratio, N p is the number of pole pairs of the transformer, X T is the impedance of the transformer.

[0065] At this time, the reactive power balance at the commutation bus can be expressed as:

[0066] Q d =Q ac +Q f +Q fault (1)

[0067] In the formula, Q d , Q ac , Q f and Q fault The expressions of are shown in formula (2) to formula (5):

[0068]

[0069] Where U cr It is the critical voltage of the commutation bus when critical commutation failure occurs.

[0070] S2: Analytical calculation of electrical quantity connections based on transient characteristics

[0071] Substituting the analytical expressions of reactive power in equations (2) to (5) into equation (1) for calculation, we can obtain the functional relationship of various electrical quantities, as shown in the following equation:

[0072]

[0073] The expressions of coefficients a, b, c, d, and e are:

[0074]

[0075] The critical arc extinction angle is the actual value, that is, γ min =7.2°.

[0076] And through the known system operating parameters and the real-time acquisition of electrical quantity signals by PMU, this functional relationship is further simplified to the DC current I d , commutation bus critical voltage U cr With critical fault inductance X fault The function is:

[0077] I d =f(U cr ,X fault ) (8)

[0078] Its specific expression is:

[0079]

[0080] Among them, the expressions of E, F, X, and Y are:

[0081]

[0082] In the above formula, the expressions of G, p, and q are:

[0083]

[0084] p=-(12G+F 2) / 3 (15)

[0085] q=(27(4FG-E 2 G)-36FG-2F 3 ) / 27 (16)

[0086] S3: Generalized commutation failure critical impedance based on electrical quantity connection

[0087] According to the analytical expression of the arc extinction angle, the minimum arc extinction angle when critical commutation failure occurs can also be expressed as:

[0088]

[0089] After the first commutation failure occurs in the system, the control system has fully functioned. Therefore, when the system ends the first commutation failure, the turn-off angle γ returns to the normal level, and the corresponding lead trigger angle β returns to the rated value. When the subsequent commutation failure occurs, the arc extinction angle γ suddenly decreases to the critical value. At this time, due to the instantaneous change of the turn-off angle, the control system has not yet functioned. Therefore, the lead trigger angle is still near the rated value, that is, β in formula (17) can be regarded as the rated value.

[0090] In actual engineering, a low voltage limiting link (voltage dependent current order limiter, VDCOL) is usually used to regulate the DC current. Its static characteristics are as follows: Figure 2 As shown:

[0091] Among them, I order is the output DC current command value, I max is the maximum output value, I min is the minimum output value, U dc is the collected commutation bus voltage, U H The maximum voltage that makes VDCOL operate, U L It is the minimum voltage for VDCOL to operate.

[0092] The mathematical relationship between its input and output is:

[0093]

[0094] Combining equations (8), (17) and (18), the critical fault impedance X when critical commutation failure occurs can be obtained: fault .

[0095] S4: Generalized commutation failure immunity factor derived based on generalized commutation failure critical impedance

[0096] Since the use of generalized commutation failure critical impedance as the criterion for commutation failure is not intuitive enough, it is further sorted out and a generalized commutation failure immunity factor is proposed as the ease of commutation failure. Its expression is:

[0097]

[0098] Where U ac is the rated value of the line voltage, P ac is the DC rated power.

[0099] It can be seen from formula (19) that GCFII is related to the rated line voltage, DC transmission rated power and generalized commutation failure critical impedance. The larger the GCFII, the stronger the system's ability to be immune to subsequent commutation failures.

[0100] by Figure 3 The Cigre HVDC standard model shown is a specific embodiment, in which the DC part voltage level is 500KV, the rated power is 1000MW, the rectifier side AC system voltage is 382.87KV, the sending end AC system impedance is 47.6∠84°Ω, the rectifier side transformer rated capacity is 603.7MVA, the impedance per unit value is 0.18pu, the inverter side AC system voltage is 215.05KV, the receiving end AC system impedance is 21.2∠75°, the inverter side reactive power compensation is 626Mvar, the inverter side transformer rated capacity is 591.8MVA. The impedance per unit value is 0.18pu.

[0101] In order to verify that the GCFII proposed in the present invention can accurately reflect the subsequent commutation failure immunity level of the system, the calculation results under different receiving end system strengths, that is, different short circuit ratios (SCR), are selected for comparison with the existing evaluation method (CFII). At the same time, in order to verify that the GCFII proposed in the present invention has universal applicability to different DC current control methods, the system is selected to use VDCOL and adaptive DC current control methods for simulation verification, and the calculation results are compared.

[0102] (1) DC current adopts VDCOL control method:

[0103] Table 1 Comparison of calculation results of CFII and GCFII at different short circuit ratios under VDCOL control

[0104] SCR CFII / % GCFII / % error / % 2 6.01 5.82 3.16 2.5 14.03 13.86 1.21 3 30.6 29.79 2.65 3.5 49.37 48.46 1.84 4 72.83 71.85 1.35

[0105] As can be seen from Table 1, when the DC current adopts the VDCOL control mode, under different short-circuit ratios, the GCFII calculation results proposed in the present invention are similar to the CFII obtained by simulation calculation, which verifies the effectiveness of the GCFII proposed in the present invention. Compared with the CFII based on simulation and requiring a lot of calculation time, the proposed GCFII can be calculated online in real time, reflecting the timeliness of the invented indicator, so it can better adapt to the actual operation and monitoring of the power system, and ensure the safe and stable operation of the power system.

[0106] (2) The DC current adopts adaptive DC current control method:

[0107] Table 2 Comparison of calculation results of CFII and GCFII at different short-circuit ratios under adaptive DC current control

[0108] SCR CFII / % GCFII / % error / % 2 112.26 109.39 2.56 2.5 153.07 148.82 2.78 3 336.77 327.81 2.66 3.5 541.95 529.38 2.32 4 803.41 784.05 2.41

[0109] Similarly, it can be seen from Table 2 that when the DC current adopts an adaptive DC current control method, under different short-circuit ratios, the GCFII calculation results proposed in the present invention are also similar to the CFII obtained by simulation calculation, which verifies that the GCFII proposed in the present invention can be effectively applied to systems under different DC current control. Therefore, when the actual system operation mode changes, the GCFII proposed in the present invention is also applicable.

[0110] The above embodiments prove that the corresponding method is effective and reasonable.

[0111] In summary, the method for quantitatively evaluating the subsequent commutation failure immunity level of the AC / DC hybrid system of the present invention, by analyzing the transient process of the AC / DC hybrid system, obtains the system equivalent model under the most serious fault state. On the basis of this model, combined with the reactive power balance relationship and the analytical expression of each reactive power, the essential mathematical connection between each electrical quantity is obtained. Secondly, the calculation process of the proposed GCFII index does not rely on simulation, and can be calculated by the known system operation parameters and the electrical quantity signal collected by the PMU in real time. Therefore, online calculation can be realized during the system operation process, which meets the real-time performance required for system operation monitoring and provides a theoretical basis for the operation of power grid operators. In addition, the influence of different DC current control modes is considered in the calculation process of the proposed GCFII index, so the calculation results under different system operation modes are accurate, so it can be widely applied to various power grids. And when the system operation mode changes, the proposed GCFII can still be applied, which meets the requirements of the power grid for the flexibility of evaluation indicators.

[0112] The present invention is described in detail above in conjunction with the embodiments of the accompanying drawings. A person skilled in the art can make various variations of the present invention according to the above description. Therefore, certain details in the embodiments should not be construed as limiting the present invention, and the scope of protection of the present invention shall be the scope defined by the attached claims.

Claims

1. A method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system. On the basis of using high-voltage DC transmission technology to achieve cross-regional power transmission, the transient process of the system is analyzed based on the transient reactive dynamic characteristics of the AC / DC hybrid system; the essential connection between various electrical quantities in the transient process is then obtained; combined with the known system parameters and the electrical signals collected by the PMU, the online evaluation index GCFII used to reflect the subsequent commutation failure immunity level of the system is derived.

2. A method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 1, characterized in that: The derivation of the index GCFII takes into account the changes in the DC control mode and can be updated in real time with the changes in the system operating parameters to meet the requirements of real-time and accuracy, providing a data basis for the operation of the power grid foundation and operators, thereby effectively ensuring the safe operation of the power grid.

3. A method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 1 or 2, comprising the following steps: Step S1: Transient characteristics analysis of AC / DC hybrid system: Step S2: Analytical calculation of electrical quantity connections based on transient characteristics; Step S3: deriving a generalized commutation failure critical impedance based on electrical quantity connections; Step S4: deriving a generalized commutation failure immunity factor based on the generalized commutation failure critical impedance.

4. According to the method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system according to claim 3, the step S1, i.e., the transient characteristic analysis method of the AC / DC hybrid system is: When a short circuit occurs at the commutation bus, critical commutation failure occurs on the inverter side, that is, the shutdown angle γ = γ min When , the fault severity can be expressed by the short-circuit impedance. At this time, the reactive power balance at the commutation bus can be expressed as: Q d =Q ac +Q f +Q fault (1) In the formula, Q d , Q ac , Q f and Q fault The expressions of are shown in formula (2) to formula (5) respectively. Where E is the amplitude of the equivalent voltage source of the receiving AC system, U ac is the amplitude of the inverter-side commutation bus voltage, Z is the amplitude and phase angle of the equivalent impedance of the receiving-end system, P d With Q d are respectively the active and reactive power fed into the commutation bus by the inverter, P ac With Q ac are the active and reactive power fed into the receiving-end AC system by the commutation bus, I d is the DC current, B f is the equivalent susceptance of the filter, Q f is the reactive power fed into the commutation bus by the filter, X fault is the critical fault inductance when critical commutation failure occurs, Q fault is the reactive power fed into the commutation bus by the critical fault inductor, γ is the arc extinction angle of the inverter, β is the leading trigger angle of the inverter, k is the transformer ratio, N p is the number of pole pairs of the transformer, X T is the impedance of the transformer, U cr It is the critical voltage of the commutation bus when critical commutation failure occurs.

5. According to the method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 3, the step S2, i.e., the electrical quantity connection analytical calculation method based on transient characteristics, is: Substituting the analytical expressions of reactive power in equations (2) to (5) into equation (1) for calculation, we can obtain the functional relationship of various electrical quantities, as shown in the following equation: The expressions of coefficients a, b, c, d, and e are: The critical arc extinction angle is the actual value, that is, γ min =7.2°. And through the known system operating parameters and the real-time acquisition of electrical quantity signals by PMU, this functional relationship is further simplified to the DC current I d , commutation bus critical voltage U cr With critical fault inductance X fault The function is: I d =f(U cr ,X fault ) (8) Its specific expression is: in, The expressions of E, F, X, and Y are: In the above formula, the expressions of G, p, and q are: p=-(12G+F 2 ) / 3 (15) q=(27(4FG-E 2 G)-36FG-2F 3 ) / 27 (16)。 6. According to the method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system in claim 3, the step S3, i.e., the method for deriving the generalized commutation failure critical impedance based on the electrical quantity connection, is: According to the analytical expression of the arc extinction angle, the minimum arc extinction angle when critical commutation failure occurs can also be expressed as: After the first commutation failure occurs in the system, the control system has been fully functional. Therefore, when the system ends the first commutation failure, the turn-off angle γ returns to a normal level, and the corresponding advance trigger angle β returns to near the rated value.

7. A method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 6, characterized in that: When the subsequent commutation failure occurs, the arc extinction angle γ suddenly drops to the critical value. At this time, due to the instantaneous change of the turn-off angle, the control system has not yet taken effect, so the leading trigger angle is still near the rated value, that is, β in formula (17) can be regarded as the rated value.

8. The method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 7, characterized in that: The DC current is regulated by using a low voltage limiting link. The mathematical relationship between its input and output is: Among them, I order is the output DC current command value, I max is the maximum output value, I min is the minimum output value, U dc is the collected commutation bus voltage, U H The maximum voltage that makes VDCOL operate, U L The minimum voltage for VDCOL to operate is: Combining equations (8), (17) and (18), the critical fault impedance X when critical commutation failure occurs can be obtained: fault .

9. According to the method for quantitatively evaluating the subsequent commutation failure immunity level of an AC / DC hybrid system in claim 3, the step S4, i.e., the method for deriving the generalized commutation failure immunity factor based on the generalized commutation failure critical impedance, is: selecting the generalized commutation failure immunity factor as the ease of occurrence of commutation failure, and its expression is: Where U ac is the rated value of the line voltage, P ac is the DC rated power.

10. The method for quantitatively evaluating the immunity level of subsequent commutation failure of an AC / DC hybrid system according to claim 9, characterized in that: It can be seen from formula (19) that GCFII is related to the rated line voltage, DC transmission rated power and generalized commutation failure critical impedance. The larger the GCFII is, the stronger the system's ability to be immune to subsequent commutation failures is.

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