Fault risk assessment method and device after commutation failure of hierarchical access system

Abstract

The invention discloses a fault risk assessment method and device after commutation failure of a hierarchical access system, and belongs to the technical field of power system control, and the method comprises the steps: calculating the commutation voltage integral area needed by commutation of a high-end inverter and a low-end inverter after commutation failure of the hierarchical access system; if the commutation voltage integral area is lower than an area threshold value, obtaining a new energy low-power-on / off-network risk assessment index and a first direct current locking risk assessment index, and further calculating a new energy high-power-on / off-network risk assessment index; in consideration of an AC-DC-new energy reactive coupling relationship in a receiving-end power grid and a causal relationship between cascading failures, the cascading risk that a system may have new energy low-power-on / high-power-on off-network and DC blocking failures after a DC commutation failure occurs is evaluated, and theoretical guidance is provided for prevention and control of the cascading failures of the receiving-end power grid. Therefore, the operation safety of the power system is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system control, and more specifically, relates to a method and device for fault risk assessment after commutation failure in a hierarchical access system. Background Art

[0002] As a key technology for promoting large-scale development and long-distance transmission of clean energy, UHVDC transmission technology is of great significance for achieving the carbon neutrality goal. However, since the UHVDC project is still in its initial stage and with the continuous construction and development of multi-infeed DC systems, the AC-DC coupling characteristics are very complex. Among them, AC-DC coupling, AC-AC coupling, and DC-DC coupling may lead to the expansion of the fault range and cause a large impact on the receiving-end AC power grid. Therefore, the safe and stable operation control of UHVDC multi-infeed DC systems is a key support point for national major needs and strategies.

[0003] With the large-scale access of new energy to the receiving-end power grid of UHVDC, the fault characteristics of the original AC-DC system have changed: the fault characteristics of new energy are closely related to the AC-DC coupling and control system logic in the multi-infeed DC system. The combination of new energy grid connection and UHVDC hierarchical access system poses more complex challenges to the stability analysis and control of the power system, resulting in adaptability challenges for traditional fault calculation analysis methods and protection control strategies, and more accurate analysis of fault transient characteristics is required. Therefore, it has important research value to analyze the transient voltage characteristics of new energy units when they are large-scale connected to the receiving-end power grid.

[0004] To sum up, when a single AC fault is not properly handled, voltage instability may occur in the receiving-end power grid, usually resulting in DC commutation failure faults. Further, local new energy units may enter the high / low voltage ride-through stage or even trip due to fault shocks, while UHVDC may experience consecutive commutation failures or even blocking due to fault shocks, leading to more serious voltage instability and a wider range in the receiving-end power grid, and playing a key role in promoting the evolution of power system cascading faults. Mastering the fault characteristics of DC and new energy is an important basis for analyzing the evolution mechanism of cascading faults, and mastering the evolution path and mechanism of cascading faults is the basis for ensuring the safe and stable operation of the power system. There is an urgent need to find a method for risk assessment of cascading faults after commutation failure in a DC hierarchical access system to provide a theoretical basis for power system safety preventive control, thereby improving the operation safety of the power system. Summary of the Invention

[0005] Aiming at the above defects or improvement requirements of the prior art, the present invention provides a method and device for fault risk assessment after commutation failure in a hierarchical access system, aiming to solve the technical problem that the prior art lacks the assessment and prevention of the risk of cascading faults after commutation failure in a DC hierarchical access system, resulting in low operation safety.

[0006] To achieve the above object, according to one aspect of the present invention, a method for fault risk assessment after commutation failure of a hierarchical access system is provided, including:

[0007] S1: After the commutation failure of the hierarchical access system, calculate the commutation voltage integral area required for commutation of the high and low-end inverters; if the commutation voltage integral area is lower than the area threshold, obtain the new energy low-through network risk assessment index and the first DC blocking risk assessment index;

[0008] S2: If the first DC blocking risk assessment index is greater than the first number threshold, it is regarded as a DC system blocking, and it is determined that the system is severely unstable and the risk assessment is ended; otherwise, it is determined that the DC system has not blocked and the assessment continues; if the new energy low-through network risk assessment index is less than the first threshold, it is determined that a new energy low-through network has occurred and enter S4, otherwise it is determined that a new energy low-through network has not occurred and enter S3;

[0009] S3: After the short-circuit fault ends, collect the reactive power signal of the DC hierarchical access system to calculate the new energy high-through network risk assessment index; if the new energy high-through network risk assessment index is greater than the second threshold, it is determined that a new energy high-through network has occurred and enter S4, otherwise it is determined that a new energy high-through network has not occurred, and it is regarded that the system is not unstable and the risk assessment is ended;

[0010] S4: Collect the turn-off angle of the inverter station to obtain the second DC blocking risk assessment index; if the second DC blocking risk assessment index is greater than the second number threshold, it is regarded as a DC system blocking, and it is determined that the system is severely unstable and the risk assessment is ended; otherwise, it is determined that the DC system has not blocked and the risk assessment is ended.

[0011] Further, the calculation of the commutation voltage integral area required for commutation of the high and low-end inverters includes: using the expression Calculate the commutation voltage integral area required for commutation of the high and low-end inverters after the first-phase failure of the hierarchical access system; the expression for the corresponding area threshold is: L c I d1 (t2)+L c I d1 (t1); using the expression Calculate the commutation voltage integral area required for commutation of the high and low-end inverters after the first-phase failure of the hierarchical access system, and the expression for the corresponding area threshold is: L c I d2 (t2)+L c I d2 (t1); where L c is the commutation inductance, I d1 、I d2are the DC currents corresponding to the high-end inverter and the low-end inverter respectively, t1 and t2 are the start and end times of commutation respectively, and t max is the zero-crossing time of the commutation voltage, U b1 and U c1 are the voltages of phase b and phase c of the first-layer commutation bus in the hierarchical access system respectively, and U b2 and U c2 are the voltages of phase b and phase c of the second-layer commutation bus in the hierarchical access system respectively.

[0012] Furthermore, the obtaining of the new energy low-through network connection risk assessment index includes: using the expression to calculate the new energy low-through network connection risk assessment index corresponding to the first layer of the hierarchical access system; the expression of the corresponding first threshold is: k low (t)*U b1 ; using the expression to calculate the new energy low-through network connection risk assessment index corresponding to the second layer of the hierarchical access system; the expression of the corresponding first threshold is: k 1ow (t)*U b2 ; where, ΔQ ac1 and ΔQ ac2 are the reactive power change amounts sent from the corresponding AC systems of the first-layer and the second-layer commutation buses respectively, ΔQ new1 and ΔQ new2 are the reactive power change amounts output by the corresponding new energy power stations of the first-layer and the second-layer commutation buses respectively, ΔQ f1 and ΔQ f2 are the reactive power change amounts generated by the corresponding filters of the first-layer and the second-layer commutation buses respectively, ΔQ acac is the reactive power change amount sent to the first-layer commutation bus through the AC tie line, ΔQ * acac is the reactive power change amount flowing out of the second-layer commutation bus through the AC tie line, ΔQ d is the reactive power change amount flowing into the ground due to three-phase short-circuit grounding, ΔQ inv1 and ΔQ inv2 are the reactive power change amounts absorbed by the corresponding inverters of the first-layer and the second-layer commutation buses respectively, k low (t) is the new energy low-through network connection risk coefficient, and U b1 and U b2 are the reference voltage values of the first-layer and the second-layer commutation buses respectively.

[0013] Furthermore, the calculation of the new energy high-through network connection risk assessment index includes: using the expression Calculate the new - energy high - penetration connection / disconnection network risk assessment index corresponding to the first layer of the hierarchical access system. The expression of the corresponding second threshold is: k high (t)*U b1 ; Use the expression Calculate the new - energy high - penetration connection / disconnection network risk assessment index corresponding to the second layer of the hierarchical access system. The expression of the corresponding second threshold is: k high (t)*U b2 ; Among them, k high (t) is the new - energy high - penetration connection / disconnection network risk coefficient.

[0014] Further, the expression of the reactive power absorbed by the inverter is:

[0015] Among them, is the reactive power absorbed by the inverter, P d is the active power transmitted by DC, γ is the reference value of the turn - off angle obtained by the turn - off angle control of the inverter, Δγ cfprev is the turn - off angle correction amount considering the early trigger of the CFPREV control of the inverter, and μ is the commutation angle.

[0016] Further, the first DC blocking risk assessment index is the number of commutation failures when the integral area of the commutation voltage is lower than the area threshold and the turn - off angle of the inverter station is lower than the minimum turn - off angle required for commutation in the DC inverter station; the second DC blocking risk assessment index is the number of commutation failures when new - energy low - penetration / high - penetration connection / disconnection network occurs and the turn - off angle of the inverter station is lower than the minimum turn - off angle required for commutation in the DC inverter station.

[0017] Further, if the first DC blocking risk assessment index is greater than the first number threshold, it is considered that the DC system is blocked, including: if the first DC blocking risk assessment index N1 is greater than the first number threshold N0, it is considered that the DC system is blocked; if the second DC blocking risk assessment index is greater than the second number threshold, it is considered that the DC system is blocked, including: if the second DC blocking risk assessment index N2 is greater than the second number threshold (N0 - N1), it is considered that the DC system is blocked.

[0018] According to another aspect of the present invention, a fault risk assessment device after commutation failure of a hierarchical access system is provided, including: an index calculation module, a first determination module, a second determination module, and a third determination module;

[0019] The index calculation module is used to calculate the integral area of the commutation voltage required for commutation of the high - and low - end inverters after the commutation failure of the hierarchical access system; if the integral area of the commutation voltage is lower than the area threshold, obtain the new - energy low - penetration connection / disconnection network risk assessment index and the first DC blocking risk assessment index;

[0020] The first determination module is configured to, if the first DC blocking risk assessment index is greater than the first number threshold, consider that the DC system is blocked, determine that the system is severely unstable and end the risk assessment; otherwise, determine that the DC system is not blocked and continue the assessment; if the new energy low penetration and disconnection risk assessment index is less than the first threshold, determine that new energy low penetration and disconnection has occurred and enter the third determination module; otherwise, determine that new energy low penetration and disconnection has not occurred and enter the second determination module;

[0021] The second determination module is configured to collect the reactive power signal of the DC hierarchical access system after the short-circuit fault ends to calculate the new energy high penetration and disconnection risk assessment index; if the new energy high penetration and disconnection risk assessment index is greater than the second threshold, determine that new energy high penetration and disconnection has occurred and enter the third determination module; otherwise, determine that new energy high penetration and disconnection has not occurred, consider that the system is not unstable and end the risk assessment;

[0022] The third determination module is configured to collect the turn-off angle of the inverter station to obtain the second DC blocking risk assessment index; if the second DC blocking risk assessment index is greater than the second number threshold, consider that the DC system is blocked, determine that the system is severely unstable and end the risk assessment; otherwise, determine that the DC system is not blocked and end the risk assessment.

[0023] According to another aspect of the present invention, there is provided a power system, including a memory and a processor, where the memory stores a computer program, and when the processor executes the computer program, the steps of the above method are implemented.

[0024] According to another aspect of the present invention, there is provided a computer-readable storage medium, on which a computer program is stored, and when the computer program is executed by a processor, the steps of the above method are implemented.

[0025] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following beneficial effects can be achieved:

[0026] (1) The present invention provides a fault risk assessment method after commutation failure of a hierarchical access system, considering the reactive power coupling relationship and causal relationship between chain faults of AC-DC-new energy in the receiving-end power grid, and evaluating the chain risk of new energy low penetration / high transmission and disconnection and DC blocking faults that may occur after DC commutation failure, providing theoretical guidance for the prevention and control of chain faults in the receiving-end power grid, thereby improving the operation safety of the power system.

[0027] (2) In this solution, calculate the commutation voltage integral area required for commutation of high- and low-end inverters and make a determination. Compared with the prior art, the commutation conditions of high- and low-end converters in the DC hierarchical access system are considered simultaneously, and local commutation failure and simultaneous commutation failure can be identified in a timely manner.

[0028] (3) In this solution, Obtain and evaluate the risk assessment index for new energy low penetration and disconnection from the grid. Compared with the existing technology, it considers the reactive power characteristics of the DC hierarchical access system, can calculate the risk index of new energy low penetration and disconnection from the grid in real time according to the reactive power coupling characteristics of the system, and realizes the risk assessment of new energy low penetration and disconnection from the grid.

[0029] (4) In this solution, Obtain and evaluate the risk assessment index for new energy high penetration and disconnection from the grid. Compared with the existing technology, it considers the reactive power characteristics of the DC hierarchical access system, can calculate the risk index of new energy high penetration and disconnection from the grid in real time according to the reactive power coupling characteristics of the system, and realizes the risk assessment of new energy high penetration and disconnection from the grid.

[0030] (5) In this solution, Calculate the reactive power absorbed by the inverter. Compared with the existing technology, it considers the reactive power characteristics of the DC hierarchical access system, can calculate the risk index of new energy high penetration and disconnection from the grid in real time according to the reactive power coupling characteristics of the system, and realizes the risk assessment of new energy high penetration and disconnection from the grid.

[0031] (6) In this solution, Determine and evaluate the first and second DC blocking risk assessment indexes. Compared with the existing technology, it considers the DC blocking risk and the causal relationship between faults in the whole process, can evaluate the DC blocking risk after AC faults and new energy disconnection from the grid at the same time, and realizes the DC blocking risk assessment in cascading faults. Description of the Drawings

[0032] Figure 1 It is a flowchart of the risk assessment method for cascading faults after commutation failure in the DC hierarchical access system provided by the embodiment of the present invention;

[0033] Figure 2 It is a topology diagram of new energy incorporated into the UHV DC hierarchical access system in the PSCAD simulation scenario provided by the embodiment of the present invention;

[0034] Figure 3 It is a diagram of the reactive power coupling relationship and causal relationship of AC - DC - new energy after commutation failure occurs in the DC hierarchical access system;

[0035] Figures 4(a), 4(b), 4(c) and 4(d) are respectively the change curve of the inverter AC bus voltage, the DC current and voltage waveforms of the UHV DC transmission system, the change curve of the inverter extinction angle γ, and the dynamic process diagram of the reactive power exchange between the direct - drive wind farm and the AC - DC system during the commutation failure process of the DC system when a short - circuit fault occurs in the receiving - end power grid. Detailed Embodiments

[0036] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0037] Embodiment 1

[0038] As Figure 1 shown, this embodiment provides a method for fault risk assessment after commutation failure of a hierarchical access system, including S1-S4. S1: After the commutation failure of the hierarchical access system, calculate the commutation voltage integral area required for commutation of the high and low-end inverters; if the commutation voltage integral area is lower than the area threshold, obtain the new energy low voltage ride-through risk assessment index and the first DC blocking risk assessment index. S2: If the first DC blocking risk assessment index is greater than the first number threshold, it is regarded as a DC system blocking, and it is determined that the system is severely unstable and the risk assessment ends; otherwise, it is determined that the DC system blocking has not occurred and the assessment continues; if the new energy low voltage ride-through risk assessment index is less than the first threshold, it is determined that new energy low voltage ride-through has occurred and enter S4, otherwise it is determined that new energy low voltage ride-through has not occurred and enter S3. S3: After the short-circuit fault ends, collect the reactive power signal of the DC hierarchical access system to calculate the new energy high voltage ride-through risk assessment index; if the new energy high voltage ride-through risk assessment index is greater than the second threshold, it is determined that new energy high voltage ride-through has occurred and enter S4, otherwise it is determined that new energy high voltage ride-through has not occurred, and it is regarded that the system is not unstable and the risk assessment ends. S4: Collect the turn-off angle of the inverter station to obtain the second DC blocking risk assessment index; if the second DC blocking risk assessment index is greater than the second number threshold, it is regarded as a DC system blocking, and it is determined that the system is severely unstable and the risk assessment ends; otherwise, it is determined that the DC system blocking has not occurred and the risk assessment ends. It should be noted that in this embodiment, the first-layer access system corresponds to 500 kV, and the second-layer access system corresponds to 1000 kV.

[0039] Specifically, collect the DC current and commutation bus voltage signals of the hierarchical access system, substitute them into the AC-DC double-coupled commutation voltage integral area formula to determine the risk of simultaneous commutation failure, and enter the first low voltage risk stage; if the commutation voltage integral areas (Voltage-Time Area, VTA) required for commutation of the high and low-end inverters are both less than or equal to the VTA that the system can provide, it is regarded that the local commutation failure has caused simultaneous commutation failure through the current coupling on the DC side of the system and the voltage coupling on the AC side, and the range of commutation failure expands.

[0040] Collect the grounding fault, new energy low-voltage ride-through, inverter, AC tie line, AC power grid, reactive power signals of AC filter, and turn-off angle signal of inverter station. Calculate the risk assessment indexes of new energy low-voltage ride-through disconnection and the first DC blocking according to the reactive power coupling relationship of the hierarchical access system, and enter the second low-voltage risk stage. If the risk assessment index of new energy low-voltage ride-through disconnection is less than the threshold (this threshold varies with the fault time), it is regarded that the reactive power support ability of new energy low-voltage ride-through is insufficient, and new energy low-voltage ride-through failure and disconnection will occur, resulting in a shortage of new energy power in the system. If the first DC blocking risk assessment index is greater than the commutation failure times threshold, it is regarded that the DC system has a blocking, resulting in a large shortage of DC power in the system.

[0041] If new energy low-voltage ride-through disconnection and DC blocking do not occur, after the short-circuit fault ends, collect the reactive power signals of each part to calculate the risk assessment index of new energy high-voltage ride-through disconnection, and enter the third high-voltage risk stage. If the risk assessment index of new energy high-voltage ride-through disconnection is greater than the threshold (this threshold also varies with the fault time), it is regarded that the reactive power support ability of new energy high-voltage ride-through is insufficient, and new energy high-voltage ride-through failure and disconnection will occur, resulting in a shortage of new energy power in the system.

[0042] If new energy disconnection occurs, collect the turn-off angle signal of the inverter station to calculate the DC blocking risk, and enter the fourth voltage oscillation risk stage. If the second DC blocking risk assessment index is greater than the commutation failure times threshold, it is regarded that the DC system has a blocking, resulting in a large shortage of DC power in the system, and the shortage power is the steady-state transmission power of DC current.

[0043] Furthermore, calculate the commutation voltage integral area required for commutation of high- and low-end inverters, including: using the expression Calculate the commutation voltage integral area required for commutation of high- and low-end inverters after the first-phase failure of the hierarchical access system; the expression for its corresponding area threshold is: L c I d1 (t2)+L c I d1 (t1); use the expression Calculate the commutation voltage integral area required for commutation of high- and low-end inverters after the first-phase failure of the hierarchical access system, and the expression for its corresponding area threshold is: L c I d2 (t2)+L c I d2 (t1); where, L c is the commutation inductance, I d1 , I d2 are the DC currents corresponding to the high-end inverter and the low-end inverter respectively, t1 and t2 are the start and end times of commutation respectively, t max is the zero-crossing time of the commutation voltage, U b1 , U c1are the voltages of phase b and phase c of the first-layer commutation bus in the hierarchical access system, U b2 and U c2 are the voltages of phase b and phase c of the second-layer commutation bus in the hierarchical access system respectively.

[0044] Specifically, the simultaneous commutation failure occurrence risk is determined using the AC-DC double-coupled commutation voltage integral area formula, and the AC-DC double-coupled commutation voltage integral area formula can be represented by the following set of equations:

[0045]

[0046] where L c is the commutation inductance, I d1 and I d2 are the DC currents corresponding to the high-end inverter and the low-end inverter respectively, t1 and t2 are the start and end times of commutation respectively, t max is the zero-crossing time of the commutation voltage, U b1 and U c1 are the voltages of phase b and phase c of the 500 kV commutation bus in the hierarchical access system respectively, U b2 and U c2 are the voltages of phase b and phase c of the 1000 kV commutation bus in the hierarchical access system respectively.

[0047] For the DC hierarchical access system, its structure has a series structure of DC-side inverters and AC tie lines, resulting in current coupling on the DC side and voltage coupling on the AC side during the commutation process of the high and low-end inverters. The current coupling on the DC side can be expressed as follows: I d1 (t) = I d2 (t); after a local commutation failure occurs, it will cause an increase in the VTA required for the simultaneous commutation process, increasing the risk of commutation failure. In addition, the coupling of the AC tie line causes the voltage changes between the commutation bus voltages to be positively correlated. After a local commutation failure occurs, it will cause a decrease in the VTA provided by the system during the simultaneous commutation process, increasing the risk of commutation failure. Therefore, this AC-DC double-coupled commutation voltage integral area formula can better evaluate the risk of simultaneous commutation failure.

[0048] Furthermore, obtaining the new energy low-voltage ride-through and islanding risk assessment indicators includes: using the expression to calculate the new energy low-voltage ride-through and islanding risk assessment indicator corresponding to the first layer of the hierarchical access system; the expression for the corresponding first threshold is: k low (t) * U b1 ; using the expression to calculate the new energy low-voltage ride-through and islanding risk assessment indicator corresponding to the second layer of the hierarchical access system; the expression for the corresponding first threshold is: k 1ow (t) * U b2 ; where, ΔQac1 and ΔQ ac2 are the reactive power change amounts sent from the AC systems corresponding to the first - layer and second - layer commutation buses respectively. ΔQ new1 and ΔQ new2 are the reactive power change amounts output from the new - energy power stations corresponding to the first - layer and second - layer commutation buses respectively. ΔQ f1 and ΔQ f2 are the reactive power change amounts emitted by the filters corresponding to the first - layer and second - layer commutation buses respectively. ΔQ acac is the reactive power change amount sent to the first - layer commutation bus through the AC tie line. ΔQ * acac is the reactive power change amount flowing out of the second - layer commutation bus through the AC tie line. ΔQ d is the reactive power change amount flowing into the ground due to three - phase short - circuit grounding. ΔQ inv1 and ΔQ inv2 are the reactive power change amounts absorbed by the inverters corresponding to the first - layer and second - layer commutation buses respectively. k low (t) is the risk coefficient of new - energy low - voltage ride - through and disconnection from the grid. U b1 and U b2 are the reference voltage values of the first - layer and second - layer commutation buses respectively.

[0049] Specifically, based on the analysis of the reactive - power coupling relationship of the DC hierarchical access system, the reactive - power coupling relationship of the DC hierarchical access system can be expressed by the following set of equations:

[0050]

[0051] Among them, Q ac1 and Q ac2 are the reactive powers sent from the AC systems corresponding to the 500 - kV and 1000 - kV commutation buses respectively. Q new1 and Q new2 are the reactive powers output from the new - energy power stations corresponding to the 500 - kV and 1000 - kV commutation buses respectively. Q f1 and Q f2 are the reactive powers emitted by the filters corresponding to the 500 - kV and 1000 - kV commutation buses respectively. Q acac is the reactive power sent to the 500 - kV commutation bus through the AC tie line. Q * acac is the reactive power flowing out of the 1000 - kV commutation bus through the AC tie line. Q d is the reactive power flowing into the ground due to three - phase short - circuit grounding. Q inv1 and Q inv2 are the reactive powers absorbed by the inverters corresponding to the 500 - kV and 1000 - kV commutation buses respectively.

[0052] Specifically, according to the reactive power coupling relationship of the DC hierarchical access system, the new energy low penetration and disconnection risk assessment index is calculated. The new energy low penetration and disconnection risk assessment index of the DC hierarchical access system can be expressed by the following set of equations:

[0053]

[0054] Among them, ΔQ ac1 and ΔQ ac2 are the changes in reactive power sent out by the AC systems corresponding to the 500 kV and 1000 kV converter buses respectively. ΔQ new1 and ΔQ new2 are the changes in reactive power output by the new energy power stations corresponding to the 500 kV and 1000 kV converter buses respectively. ΔQ f1 and ΔQ f2 are the changes in reactive power emitted by the filters corresponding to the 500 kV and 1000 kV converter buses respectively. ΔQ acac is the change in reactive power sent to the 500 kV converter bus through the AC tie line. ΔQ * acac is the change in reactive power flowing out of the 1000 kV converter bus through the AC tie line. ΔQ d is the change in reactive power flowing into the ground during a three-phase short circuit to ground. ΔQ inv1 and ΔQ inv2 are the changes in reactive power absorbed by the inverters corresponding to the 500 kV and 1000 kV converter buses respectively. k low (t) is the new energy low penetration and disconnection risk coefficient, which is determined according to the new energy fault voltage crossing curve and increases with the passage of the fault time. Generally, -0.8 is taken for short-time faults. U b1 and U b2 are the reference voltage values of the 500 kV and 1000 kV converter buses respectively.

[0055] Once the new energy low penetration and disconnection risk assessment index on the left side of the above formula is less than or equal to the threshold value, it is considered that the new energy low penetration and disconnection risk of the DC hierarchical access system is extremely high, which will cause a shortage of new energy power in the power system, and power system stability control strategies must be taken.

[0056] Furthermore, the new energy high penetration and disconnection risk assessment index is calculated, including: using the expression to calculate the new energy high penetration and disconnection risk assessment index corresponding to the first layer of the hierarchical access system. The expression for the corresponding second threshold is: k high (t)*U b1 ; using the expression to calculate the new energy high penetration and disconnection risk assessment index corresponding to the second layer of the hierarchical access system. The expression for the corresponding second threshold is: k high (t)*Ub2 ; where k high (t) is the risk coefficient of new energy high penetration and disconnection from the grid.

[0057] Specifically, the reactive power signals of each part are collected to calculate the risk assessment index of new energy high penetration and disconnection from the grid. It is characterized in that after the fault is cleared, the reactive power surplus of new energy during low penetration and the hysteresis of the reactive power compensation device will cause transient overvoltage in the receiving-end power grid, and new energy may enter high-voltage penetration. The risk assessment index of new energy high penetration and disconnection from the grid in a DC hierarchical access system can be expressed by the following set of equations:

[0058]

[0059] where ΔQ ac1 and ΔQ ac2 are the changes in reactive power sent from the AC systems corresponding to the 500 kV and 1000 kV converter buses respectively, ΔQ new1 and ΔQ new2 are the changes in reactive power output from the new energy stations corresponding to the 500 kV and 1000 kV converter buses respectively, ΔQ f1 and ΔQ f2 are the changes in reactive power emitted by the filters corresponding to the 500 kV and 1000 kV converter buses respectively, ΔQ acac is the change in reactive power sent to the 500 kV converter bus through the AC tie line, ΔQ * acac is the change in reactive power flowing out of the 1000 kV converter bus through the AC tie line, ΔQ inv1 and ΔQ inv2 are the changes in reactive power absorbed by the inverters corresponding to the 500 kV and 1000 kV converter buses respectively, k high (t) is the risk coefficient of new energy high penetration and disconnection from the grid, which is determined according to the new energy fault voltage penetration curve and decreases with the passage of the fault time. For commutation failure faults, it is generally taken as 1.2, U b1 and U b2 are the reference voltage values of the 500 kV and 1000 kV converter buses respectively.

[0060] Once the risk assessment index of new energy high penetration and disconnection from the grid on the left side of the above formula is greater than or equal to the threshold value, it is considered that the risk of new energy high penetration and disconnection from the grid in the DC hierarchical access system is extremely high, which will cause a shortage of new energy power in the power system, and power system stabilizer control strategies must be adopted.

[0061] Furthermore, the expression for the reactive power absorbed by the inverter is:

[0062]

[0063] where is the reactive power absorbed by the inverter, P d is the active power transmitted by DC, γ is the reference value of the turn-off angle obtained by the turn-off angle control of the inverter, Δγ cfprev is the turn-off angle correction amount considering the early trigger of the CFPREV control of the inverter, and μ is the commutation angle.

[0064] Specifically, for the reactive power characteristics of the DC inverter station, during the short-circuit fault, the reactive power absorbed by the inverter is mainly affected by the active power transmitted by the DC system and the extinction angle (denoted by γ):

[0065]

[0066] Among them, Q inv is the reactive power absorbed by the inverter, P d is the active power transmitted by DC, is the power factor angle, γ is the turn-off angle of the inverter, μ is the commutation angle, u d 、i d are the DC voltage and DC current respectively; u d0 is the ideal no-load DC voltage of the inverter; U ac is the high-voltage bus voltage of the inverter station; X c is the commutation reactance; T is the turn ratio of the converter transformer.

[0067] Considering that there is also Commutation Failure Prevention (CFPREV) in the turn-off angle control, one part of the CFPREV control logic is the zero-sequence component detection module, and the other part is the U αβ component detection module. The former is used to detect asymmetric ground faults, and the amplitude of the latter is calculated from the positive and negative sequence components.

[0068] This control will trigger in advance to directly increase the commutation margin, thereby reducing the risk of commutation failure. At the same time, it will also result in a larger turn-off angle and an increase in the reactive power absorbed by the inverter station. Therefore, the reactive power correction formula for the DC inverter station can be expressed as:

[0069]

[0070] Among them, is the corrected value of the reactive power absorbed by the inverter, P d is the active power transmitted by DC, γ is the reference value of the turn-off angle obtained by the turn-off angle control of the inverter, Δγ cfprev is the turn-off angle correction amount considering the early trigger of the CFPREV control of the inverter, and μ is the commutation angle.

[0071] Furthermore, the first DC blocking risk assessment index is the number of commutation failures that occur when the commutation voltage integral area is lower than the area threshold and the extinction angle of the inverter station is lower than the minimum extinction angle required for commutation in the DC inverter station; the second DC blocking risk assessment index is the number of commutation failures that occur when new energy experiences low / high penetration network disconnection and the extinction angle of the inverter station is lower than the minimum extinction angle required for commutation in the DC inverter station.

[0072] Specifically, the DC blocking risk assessment index is calculated based on the extinction angle signal of the inverter station. It is characterized in that the DC blocking risk assessment index of the DC hierarchical access system can be expressed by the following set of equations:

[0073]

[0074] where γ is the commutation extinction angle of the DC inverter station, γ min is the minimum extinction angle required for commutation in the DC inverter station. N1 and N2 are the number of commutation failures that occur in the DC inverter station during the second low-voltage risk stage and the fourth voltage oscillation stage respectively. N0 is the threshold for the number of consecutive commutation failures that the DC inverter station can tolerate within a short period, and generally takes a value of 2 or 3.

[0075] For the series structure of inverters in the DC hierarchical access system, the blocking of any inverter station will cause the adjacent-side inverter stations to be blocked simultaneously. Therefore, in the above formula, γ can be the commutation extinction angle of any DC inverter station

[0076] Once the DC blocking risk assessment index on the left side of the above formula is greater than or equal to the threshold for the number of consecutive commutation failures, it can be regarded that the DC hierarchical access system has a very high DC blocking risk, which will cause a large deficit in the DC power of the power system, and power system stabilizer control strategies must be adopted.

[0077] Furthermore, if the first DC blocking risk assessment index is greater than the first number threshold, it is regarded as a DC system block, including: if the first DC blocking risk assessment index N1 is greater than the first number threshold N0, it is regarded as a DC system block; if the second DC blocking risk assessment index is greater than the second number threshold, it is regarded as a DC system block, including: if the second DC blocking risk assessment index N2 is greater than the second number threshold (N0 - N1), it is regarded as a DC system block.

[0078] Specifically, if the risk assessment index of new energy low penetration and disconnection from the grid is less than the threshold (this threshold varies with the fault time), it is considered that the reactive power support ability of new energy low voltage ride-through is insufficient, and new energy low voltage ride-through failure and disconnection from the grid will occur, resulting in a shortage of new energy power in the system; if the DC blocking risk assessment index N1 is greater than the commutation failure times threshold N0, it is considered that the DC system has a block, resulting in a large shortage of DC power in the system. If new energy is disconnected from the grid, the DC blocking risk is calculated by collecting the turn-off angle signal of the inverter station, and the fourth voltage oscillation risk stage is entered. If the DC blocking risk assessment index N2 is greater than the commutation failure times threshold N0 - N1, it is considered that the DC system has a block, resulting in a large shortage of DC power in the system, and the shortage power is the steady-state transmission power of the DC current.

[0079] The following is a detailed example of the risk assessment method for cascading faults after commutation failure in a DC hierarchical access system.

[0080] First, collect the DC current and converter bus voltage signals of the hierarchical access system, substitute them into the AC-DC double-coupled commutation voltage integral area formula to determine the risk of simultaneous commutation failure, and enter the first low voltage risk stage; if the commutation voltage integral area (VTA) required for commutation of the high and low-end inverters is less than or equal to the VTA provided by the system, it is considered that local commutation failure has caused simultaneous commutation failure through the current coupling on the DC side and voltage coupling on the AC side of the system, and the scope of commutation failure has expanded. Due to the series converter structure of the DC hierarchical access system, combined with Figure 2 the shown system topology diagram and Figure 3 the shown simulation result diagram, analyze the mechanism process of local commutation failure (LCF) causing simultaneous commutation failure (CCF) at the remote converter station.

[0081] Combined with Figure 2 the simulation topology diagram, the series converter structure of the DC hierarchical access system results in the same DC current in the high and low-end converter buses, and the AC-DC double-coupled commutation voltage integral area theory can be applied to the analysis of the commutation process of the DC hierarchical access system:

[0082]

[0083] Among them, L c is the commutation inductance, I d1 and I d2 are the DC currents corresponding to the high-end inverter and the low-end inverter respectively, t1 and t2 are the start and end times of commutation respectively, t max is the commutation voltage zero-crossing time, U b1 and U c1 are the voltages of phase b and phase c of the 500 kV converter bus in the hierarchical access system respectively, U b2 and U c2They are the voltages of phase b and phase c of the 1000 kV commutation bus in the hierarchical access system respectively. Among them, from the DC current of the UHVDC transmission system in Figure 4(b) and the voltage waveform change of the commutation bus shown in Figure 4(a) under the condition of single commutation failure, it can be seen that the DC current I d rises, the amplitude of the commutation voltage drops, and the insufficient VTA provided by the system causes the integral area of the commutation voltage not to be satisfied, thus triggering simultaneous commutation failure.

[0084] Then, collect the reactive power signals of the grounding fault, new energy low-voltage ride-through, inverter, AC tie line, AC power grid, AC filter, and the turn-off angle signal of the inverter station, calculate the risk assessment indexes of new energy low-voltage ride-through and DC blocking according to the reactive power coupling relationship of the hierarchical access system, and enter the second low-voltage risk stage; if the risk assessment index of new energy low-voltage ride-through is less than the threshold (this threshold changes with the fault time), it is considered that the reactive power support ability of new energy low-voltage ride-through is insufficient, and new energy low-voltage ride-through failure and disconnection will occur, resulting in a shortage of new energy power in the system; if the DC blocking risk assessment index N1 is greater than the commutation failure times threshold N0, it is considered that the DC system is blocked, resulting in a large shortage of DC power in the system.

[0085] In the case of a single commutation failure fault, due to the high severity of the three-phase short-circuit fault, it directly causes the commutation failure of the DC inverter, making the extinction angle drop to zero. The DC power drops significantly, and the extinction angle drops to zero, which leads to a decreasing trend in the reactive power absorbed by the inverter in the initial stage of the fault. However, within a short time after the fault occurs, the fixed extinction angle control strategy of the inverter intervenes, making the extinction angle increase rapidly, which briefly increases the reactive power absorbed by the inverter. During this period, the reactive power provided by the filter is not enough to meet the requirements of the inverter, resulting in the DC system having to obtain additional reactive power from the direct-drive wind farm and the AC power grid. During the fault, the commutation bus voltage drops, causing the direct-drive wind farm to enter low-voltage ride-through, with the output active power decreasing and the reactive power increasing. The active power output during the fault decreases, and at the same time, the commutation failure causes the DC active power transmission to decrease, resulting in heavy load on the commutation bus and instability of the system frequency. The receiving-end power grid needs to shed loads and generators to maintain frequency stability; at the same time, the reactive power output supports the voltage of the receiving-end power grid. In addition, in the hierarchical access system, the AC tie line and the filter also provide reactive power support.

[0086] Combined with the reactive power signals of the grounding fault, new energy low-voltage ride-through, inverter, AC tie line, AC power grid, and AC filter shown in Figure 4(d) of the simulation results, according to the reactive power coupling relationship equations of the DC hierarchical access system:

[0087]

[0088] Among them, Q ac1 、Q ac2are the reactive powers sent out by the AC systems corresponding to the 500 kV and 1000 kV commutation buses respectively, Q new1 、Q new2 are the reactive powers output by the new energy power stations corresponding to the 500 kV and 1000 kV commutation buses respectively, Q f1 、Q f2 are the reactive powers generated by the filters corresponding to the 500 kV and 1000 kV commutation buses respectively, Q acac is the reactive power sent to the 500 kV commutation bus through the AC tie line, Q * acac is the reactive power flowing out of the 1000 kV commutation bus through the AC tie line, Q d is the reactive power flowing into the ground during three-phase short-circuit grounding, Q inv1 、Q inv2 are the reactive powers absorbed by the inverters corresponding to the 500 kV and 1000 kV commutation buses respectively.

[0089] Substitute the reactive power signals into the new energy low-voltage ride-through and islanding risk assessment index equations of the DC hierarchical access system and calculate and judge:

[0090]

[0091] Among them, ΔQ ac1 、ΔQ ac2 are the changes in the reactive powers sent out by the AC systems corresponding to the 500 kV and 1000 kV commutation buses respectively, ΔQ new1 、ΔQ new2 are the changes in the reactive powers output by the new energy power stations corresponding to the 500 kV and 1000 kV commutation buses respectively, ΔQ f1 、ΔQ f2 are the changes in the reactive powers generated by the filters corresponding to the 500 kV and 1000 kV commutation buses respectively, ΔQ acac is the change in the reactive power sent to the 500 kV commutation bus through the AC tie line, ΔQ * acac is the change in the reactive power flowing out of the 1000 kV commutation bus through the AC tie line, ΔQ d is the change in the reactive power flowing into the ground during three-phase short-circuit grounding, ΔQ inv1 、ΔQ inv2 are the changes in the reactive powers absorbed by the inverters corresponding to the 500 kV and 1000 kV commutation buses respectively, k low (t) is the new energy low-voltage ride-through and islanding risk coefficient, which is determined according to the new energy fault voltage ride-through curve and increases with the passage of the fault time. Generally, -0.8 is taken for short-time faults, U b1 、U b2They are the reference voltage values of the 500 kV and 1000 kV converter busbars respectively. The evaluation results obtained by using per-unit values are -0.46 and -0.42, which are greater than the new energy low-voltage ride-through disconnection risk coefficient -0.8 within 0.1 s after the fault occurs. It can be considered that there is no new energy low-voltage ride-through disconnection risk in the second low-voltage risk stage, and the simulation results and the evaluation results confirm each other.

[0092] Combined with the waveform of the change in the extinction angle γ of the inverter depicted in Figure 4(c) of the simulation results diagram, substitute the turn-off angle signal into the DC latching risk assessment index equation set of the DC hierarchical access system and calculate and judge:

[0093]

[0094] Among them, γ is the commutation turn-off angle of the DC inverter station, γ min is the minimum turn-off angle required for commutation at the DC inverter station. N1 and N2 are the number of commutation failures at the DC inverter station in the second low-voltage risk stage and the fourth voltage oscillation stage respectively. N0 is the threshold value of the number of consecutive commutation failures that the DC inverter station can accept within a short time, and the general value is 2 or 3. It can be seen that the number of DC commutation failures in the second low-voltage risk stage only occurs once, which is less than the threshold value of the number of consecutive commutation failures that the DC inverter station can accept within a short time. It can be considered that there is no DC latching risk, and the simulation results and the evaluation results confirm each other.

[0095] Then, if there is no new energy low-voltage ride-through disconnection and DC latching, after the short-circuit fault ends, collect the reactive power signals of each part to calculate the new energy high-voltage ride-through risk assessment index and enter the third high-voltage risk stage; if the new energy high-voltage ride-through risk assessment index is greater than the threshold value (this threshold value also changes with the fault time), it is considered that the reactive power support ability for the new energy high-voltage ride-through is insufficient, and there will be a new energy high-voltage ride-through failure and disconnection, resulting in a new energy power shortage in the system.

[0096] After the fault is cleared, due to the high severity of the fault, the DC power drops significantly, which causes the AC bus voltage of the inverter station to drop at a faster rate. Therefore, the constant current control with a low-voltage current limiting link on the rectifier side takes a longer time to respond, making the recovery process of the DC current relatively slow. After a period of time after the fault is cleared, when the AC bus voltage of the inverter station recovers to its rated value, the reactive power absorbed by the inverter decreases simultaneously with the decrease of the DC power, which results in a reduction in the reactive power absorbed by the DC system from the direct-drive wind farm and the receiving-end AC grid. This characteristic is beneficial for promoting the voltage recovery of the receiving-end AC system under severe fault conditions. In addition, at the moment of fault clearance, due to the surplus reactive power generated by the direct-drive wind turbines during low-voltage ride-through, it helps to mitigate the oscillatory drop of the AC bus voltage of the inverter station at the moment of fault clearance. This phenomenon leads to a significantly lower increase in the extinction angle γ compared to the system without parallel wind turbines. Similarly, this also makes the growth rate of the reactive power absorbed by the inverter slower, resulting in the earlier occurrence and larger amplitude of the transient overvoltage phenomenon.

[0097] Combined with the reactive power signals of the grounding fault, new energy low-voltage ride-through, inverter, AC tie line, AC grid, and AC filter shown in the simulation result diagram in Figure 4(d), substitute the reactive power signals into the new energy high-penetration network connection and disconnection risk assessment index equation set for calculation and discrimination:

[0098]

[0099] Among them, ΔQ ac1 and ΔQ ac2 are the changes in reactive power sent from the AC systems corresponding to the 500 kV and 1000 kV converter buses respectively, ΔQ new1 and ΔQ new2 are the changes in reactive power output from the new energy stations corresponding to the 500 kV and 1000 kV converter buses respectively, ΔQ f1 and ΔQ f2 are the changes in reactive power generated by the filters corresponding to the 500 kV and 1000 kV converter buses respectively, ΔQ acac is the change in reactive power sent to the 500 kV converter bus through the AC tie line, ΔQ * acac is the change in reactive power flowing out of the 1000 kV converter bus through the AC tie line, ΔQ inv1 and ΔQ inv2 are the changes in reactive power absorbed by the inverters corresponding to the 500 kV and 1000 kV converter buses respectively, k high (t) is the new energy high-penetration network connection and disconnection risk coefficient, which is determined according to the new energy fault voltage ride-through curve and decreases with the passage of the fault time. For commutation failure faults, it is generally taken as 0.2, Ub1 , U b2 are the reference voltage values of the 500 kV and 1000 kV commutation busbars respectively.

[0100] The results of calculating the risk assessment indicators using per-unit values are 0.02 and 0.04 respectively, which are less than the new energy high-through and tripping risk coefficient threshold of 0.2. It can be considered that there is no new energy high-through and tripping risk in the third high-voltage risk stage, and the simulation results are consistent with the assessment results. Finally, if new energy tripping occurs, the commutation failure angle signal of the inverter station is collected to calculate the DC blocking risk, and the system enters the fourth voltage oscillation risk stage. If the DC blocking risk assessment indicator N2 is greater than the commutation failure times threshold N0 - N1, it is considered that the DC system has a block, resulting in a large deficit in the DC power of the system, and the deficit power is the steady-state transmission power of the DC current. According to the new energy tripping assessment, when a single commutation failure occurs, there is basically no new energy high / low through and tripping risk in the DC hierarchical access system, and it can be considered that the system has not entered the fourth voltage oscillation risk stage and there is no DC blocking risk, and the simulation results are consistent with the assessment results.

[0101] Based on the reactive power coupling relationship and the causal relationship between cascading faults among AC-DC-new energy in the receiving-end power grid, this example divides the cascading fault period into four risk periods, evaluates the risks of new energy tripping and DC blocking faults that may occur after commutation failure in the DC system, and provides theoretical guidance for the prevention and control of cascading faults in the receiving-end power grid. The innovation of this method lies in the comprehensive analysis and evaluation of the cascading fault paths in the receiving-end power grid with DC hierarchical access and large-scale new energy access, and research on the field of transient stability of the receiving-end power grid, which has been less studied.

[0102] Embodiment 2

[0103] This embodiment provides a fault risk assessment device after commutation failure in a hierarchical access system, including: an index calculation module, a first determination module, a second determination module, and a third determination module.

[0104] The index calculation module is used to calculate the commutation voltage integral area required for commutation of the high and low-end inverters after commutation failure in the hierarchical access system; if the commutation voltage integral area is lower than the area threshold, obtain the new energy low-through and tripping risk assessment index and the first DC blocking risk assessment index.

[0105] The first determination module is used to consider that the DC system has a block if the first DC blocking risk assessment index is greater than the first number threshold, and determine that the system is severely unstable and end the risk assessment; otherwise, determine that the DC system has not had a block and continue the assessment; if the new energy low-through and tripping risk assessment index is less than the first threshold, determine that new energy low-through and tripping has occurred and enter the third determination module, otherwise determine that new energy low-through and tripping has not occurred and enter the second determination module.

[0106] The second determination module is used to collect the reactive power signal of the DC hierarchical access system after the short-circuit fault ends, so as to calculate the risk assessment index of new energy high-throughput network disconnection; if the risk assessment index of new energy high-throughput network disconnection is greater than the second threshold, it is determined that new energy high-throughput network disconnection has occurred and enters the third determination module, otherwise it is determined that new energy high-throughput network disconnection has not occurred, and it is regarded that the system is not unstable and the risk assessment ends.

[0107] The third determination module is used to collect the turn-off angle of the inverter station to obtain the second DC blocking risk assessment index; if the second DC blocking risk assessment index is greater than the second frequency threshold, it is regarded that the DC system is blocked, and it is determined that the system is severely unstable and the risk assessment ends; otherwise, it is determined that the DC system is not blocked and the risk assessment ends.

[0108] Embodiment 3

[0109] This embodiment provides a power system, including a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, the steps of the above method are implemented.

[0110] Embodiment 4

[0111] This embodiment provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the above method are implemented.

[0112] Those skilled in the art can easily understand that the above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for evaluating the fault risk after commutation failure in a hierarchical access system, characterized in that, Including: S1: After the commutation failure of the hierarchical access system, calculate the commutation voltage integral area required for commutation of the high and low-end inverters; if the commutation voltage integral area is lower than the area threshold, obtain the new energy low-through network risk assessment index and the first DC blocking risk assessment index; S2: If the first DC blocking risk assessment index is greater than the first number threshold, it is regarded as a DC system block, and it is determined that the system is seriously unstable and the risk assessment ends; otherwise, it is determined that no DC system block occurs and the assessment continues; if the new energy low-through network risk assessment index is less than the first threshold, it is determined that a new energy low-through network occurs and enters S4, otherwise it is determined that no new energy low-through network occurs and enters S3; S3: After the short-circuit fault ends, collect the reactive power signal of the DC hierarchical access system to calculate the new energy high-through network risk assessment index; if the new energy high-through network risk assessment index is greater than the second threshold, it is determined that a new energy high-through network occurs and enters S4, otherwise it is determined that no new energy high-through network occurs, and it is regarded that the system is not unstable and the risk assessment ends; S4: Collect the commutation angle of the inverter station to obtain the second DC blocking risk assessment index; if the second DC blocking risk assessment index is greater than the second number threshold, it is regarded as a DC system block, and it is determined that the system is seriously unstable and the risk assessment ends; otherwise, it is determined that no DC system block occurs and the risk assessment ends.

2. The fault risk assessment method after commutation failure of the hierarchical access system according to claim 1, wherein The calculation of the commutation voltage integral area required for commutation of the high and low-end inverters includes: Using the expression calculate the commutation voltage integral area required for the commutation of the high and low end inverters after the phase failure of the first layer of the hierarchical access system; the expression for the corresponding area threshold is: L c I d1 (t2)+L c I d1 (t1); Using the expression calculate the commutation voltage integration area required for the commutation of the high and low-end inverters after the phase failure of the first layer of the hierarchical access system. The expression for the corresponding area threshold is: L c I d2 (t2)+L c I d2 (t1); Among them, L c is the commutation inductor, I d1 , I d2 are the DC currents corresponding to the high - end inverter and the low - end inverter respectively. t1 and t2 are the start and end times of commutation respectively, and t max is the zero - crossing time of the commutation voltage. U b1 , U c1 are the voltages of phase b and phase c of the first - layer commutation bus in the hierarchical access system respectively. U b2 , U c2 are the voltages of phase b and phase c of the second - layer commutation bus in the hierarchical access system respectively.

3. The fault risk assessment method after commutation failure of the hierarchical access system according to claim 1, wherein The obtaining of the new energy low-through network risk assessment index includes: Using the expression Calculate the new energy low voltage ride-through network risk assessment index corresponding to the first layer of the hierarchical access system; the expression of the corresponding first threshold is: k low (t)*U b1 ; Using the expression calculate the new energy low penetration and disconnection risk assessment index corresponding to the second layer of the hierarchical access system; the expression for the corresponding first threshold is: k 1ow (t)*U b2 ; Among them, ΔQ ac1 and ΔQ ac2 are respectively the reactive power change amounts sent out by the AC systems corresponding to the first-layer and second-layer commutation buses, ΔQ new1 and ΔQ new2 are respectively the reactive power change amounts output by the new energy power stations corresponding to the first-layer and second-layer commutation buses, ΔQ f1 and ΔQ f2 are respectively the reactive power change amounts emitted by the filters corresponding to the first-layer and second-layer commutation buses, ΔQ acac is the reactive power change amount sent to the first-layer commutation bus through the AC tie line, ΔQ * acac is the reactive power change amount flowing out of the second-layer commutation bus through the AC tie line, ΔQ d is the reactive power change amount flowing into the ground due to three-phase short-circuit grounding, ΔQ inv1 and ΔQ inv2 are respectively the reactive power change amounts absorbed by the inverters corresponding to the first-layer and second-layer commutation buses, k low (t) is the new energy low-voltage ride-through and disconnection risk coefficient, U b1 and U b2 are respectively the reference voltage values of the first-layer and second-layer commutation buses.

4. The fault risk assessment method after commutation failure of the hierarchical access system according to claim 3, wherein The calculation of the new energy high-through network risk assessment index includes: Using the expression Calculate the new energy high penetration network connection and disconnection risk assessment index corresponding to the first layer of the hierarchical access system. The expression for the corresponding second threshold is: k high (t)*U b1 ; Using the expression Calculate the new energy high penetration network connection and disconnection risk assessment index corresponding to the second layer of the hierarchical access system. The expression for the corresponding second threshold is: k high (t)*U b2 ; Among them, k high (t) is the risk coefficient of new energy for high penetration and disconnection from the grid.

5. The fault risk assessment method after commutation failure of the hierarchical access system according to claim 3 or 4, characterized in that, The expression of the reactive power absorbed by the inverter is: wherein, is the reactive power absorbed by the inverter, P d is the active power transmitted by DC, γ is the reference value of the turn-off angle obtained by the turn-off angle control of the inverter, Δγ cfprev is the turn-off angle correction amount considering the early trigger of the CFPREV control of the inverter, and μ is the commutation angle.

6. The fault risk assessment method after the commutation failure of the hierarchical access system according to claim 1, characterized in that The first DC blocking risk assessment index is the number of commutation failures when the commutation voltage integral area is lower than the area threshold and the commutation angle of the inverter station is lower than the minimum commutation angle required for commutation of the DC inverter station; The second DC blocking risk assessment index is the number of commutation failures when new energy low / high-through network occurs and the commutation angle of the inverter station is lower than the minimum commutation angle required for commutation of the DC inverter station.

7. The fault risk assessment method after the commutation failure of the hierarchical access system according to claim 6, characterized in that The case where if the first DC blocking risk assessment index is greater than the first number threshold, it is regarded as a DC system block includes: if the first DC blocking risk assessment index N1 is greater than the first number threshold N0, it is regarded as a DC system block; The case where if the second DC blocking risk assessment index is greater than the second number threshold, it is regarded as a DC system block includes: if the second DC blocking risk assessment index N2 is greater than the second number threshold (N0 - N1), it is regarded as a DC system block.

8. A fault risk assessment device after commutation failure of a hierarchical access system, characterized in that, Including: An index calculation module, a first determination module, a second determination module, and a third determination module; The index calculation module is configured to calculate the commutation voltage integration area required for commutation of the high- and low-end inverters after commutation failure of the hierarchical access system; if the commutation voltage integration area is lower than the area threshold, obtain the new energy low-through network risk assessment index and the first DC blocking risk assessment index; The first determination module is configured to, if the first DC blocking risk assessment index is greater than the first number threshold, consider that a DC system blocking has occurred, determine that the system is severely unstable and end the risk assessment; otherwise, determine that no DC system blocking has occurred and continue the assessment; if the new energy low-through network risk assessment index is less than the first threshold, determine that a new energy low-through network has occurred and enter the third determination module, otherwise determine that no new energy low-through network has occurred and enter the second determination module; The second determination module is configured to collect the reactive power signal of the DC hierarchical access system after the short-circuit fault ends to calculate the new energy high-through network risk assessment index; If the new energy high-through network risk assessment index is greater than the second threshold, determine that a new energy high-through network has occurred and enter the third determination module, otherwise determine that no new energy high-through network has occurred, consider that the system is not unstable and end the risk assessment; The third determination module is configured to collect the turn-off angle of the inverter station to obtain the second DC blocking risk assessment index; if the second DC blocking risk assessment index is greater than the second number threshold, consider that a DC system blocking has occurred, determine that the system is severely unstable and end the risk assessment; otherwise, determine that no DC system blocking has occurred and end the risk assessment.

9. A power system, comprising a memory and a processor, the memory storing a computer program, characterized in that, When the processor executes the computer program, the steps of the method according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, the steps of the method according to any one of claims 1 to 7 are implemented.

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