A commutation failure suppression method applicable to a UHV DC hierarchical access system

Abstract

The present invention proposes a commutation failure suppression method applicable to a UHV DC hierarchical access system, where the high and low end converter valves of the access system are connected in series on the DC side. The suppression method uses the trigger angle advance amount output by the commutation failure prevention control system CFPREV of the faulty layer as the electrical detection quantity, and by setting a coordination coefficient K, coordinates and cooperates it with the control systems of non-faulty layers to reduce the probability of commutation failure occurring in non-faulty layers; and reduces the DC current of the high and low end converters by triggering the VDCOL instruction in advance. Compared with the traditional commutation failure suppression method for hierarchical access systems, the present invention has stronger fault resistance and recovery effects.

Description

Technical Field

[0001] The present invention relates to the technical field of DC power transmission, and in particular to a commutation failure suppression method applicable to a UHVDC hierarchical access system. Background Art

[0002] In China, the power resources and load centers are distributed in a reverse manner geographically. To meet the strong demand for power resources in the load centers, it is necessary to conduct large-capacity and long-distance transmission of power resources. The Ultra High Voltage Direct Current (UHVDC) system has the advantages of large transmission capacity, long transmission distance, and flexible control mode, and plays an important role in realizing large-capacity and long-distance transmission of power resources. To meet the demand for power resources in the load centers, multiple UHVDC projects have been completed and put into operation in the load center areas, which not only meet the demand for power resources but also put forward higher requirements for the voltage support ability of the local power grid. To improve the voltage support ability of the receiving-end AC system, Chinese scholars have proposed the UHVDC hierarchical access mode. The hierarchical access mode has a more flexible power regulation mode, stronger power flow evacuation ability, lower project cost, smaller operation loss under the condition of transmitting the same power, and can effectively improve the economic efficiency of power grid operation. In actual projects, the Xizhou UHVDC, the Zhaqing UHVDC, and the Shangshan UHVDC, etc. have adopted the hierarchical access mode.

[0003] Commutation failure refers to the situation where when two valves of a converter conduct commutation, due to the commutation process not being completed or the originally scheduled turned-off valve not restoring the blocking ability during the reverse voltage action period, it re-conducts when it bears the forward voltage drop. Generally, when the extinction angle γ is less than 7°, it is considered that commutation failure occurs in the inverter. Since the receiving-end power grids of the UHVDC hierarchical access system are connected through impedances and there are complex electrical coupling relationships, its commutation characteristics are more complex. Due to the existence of the connection impedance, when a fault occurs in a certain pole, it will not only reduce the voltage of the commutation bus of this layer, but also cause the voltage of the commutation bus of the other pole to drop accordingly, but the drop degree is relatively low; in addition, the high- and low-end commutation valves are in a series relationship on the DC side, and the DC currents flowing through them will increase rapidly when a fault occurs. Since the detection thresholds of the high- and low-end CFPREV are the same, the CFPREV of the fault layer reaches the detection threshold first and starts, and then the CFPREV of the non-fault layer starts. There is a hysteresis effect between them. When the fault is relatively serious, the starting moment of the CFPREV of the non-fault layer may be later than the moment when commutation failure occurs, and it cannot play a suppression role, resulting in the risk of commutation failure in the non-fault layer converter. Since the time intervals between the high- and low-end converters having commutation failures are extremely short, it can be approximately considered that commutation failures occur simultaneously. The UHVDC system has a large capacity and a complex structure. Once simultaneous commutation failures occur in the receiving-end converters, it will bring a huge impact to the system. Summary of the Invention

[0004] The present invention proposes a commutation failure suppression method applicable to a UHV DC hierarchical access system, which has stronger fault resistance and recovery effects compared with the traditional commutation failure suppression methods for hierarchical access systems.

[0005] The present invention adopts the following technical solutions.

[0006] A commutation failure suppression method applicable to a UHV DC hierarchical access system, where the high and low end converter valves of the access system are connected in series on the DC side. This suppression method uses the trigger angle advance amount output by the commutation failure prevention control system CFPREV of the fault layer as the electrical detection quantity, and by setting a coordination coefficient K, coordinates and cooperates it with the control system of the non-fault layer to reduce the probability of commutation failure occurring in the non-fault layer; and reduces the DC current of the high and low end converters by triggering the VDCOL instruction in advance;

[0007] The suppression method includes the following steps;

[0008] Step S1, detect the DC current of the transmission line after the fault occurs, calculate the turn-off angle advance amount output by the CFPREV of the fault layer, and input both of them to the CFPREV of the non-fault layer;

[0009] Step S2, introduce the output quantity of the CFPREV of the fault layer, the DC current and the coordination coefficient K into the non-fault layer at the same time, and compare it with the original CFPREV output value of the non-fault layer;

[0010] Step S3, respectively send the output value to the valve group control and the trigger control, while increasing the maximum commutation area provided by the commutation voltage and reducing the commutation demand area.

[0011] The suppression method uses a VDCOL transformation method considering the combination of the fault layer and the non-fault layer, introduces the maximum value of the CFPREV outputs of the fault layer and the non-fault layer into the VDCOL, and realizes the early start of the VDCOL.

[0012] In step S1, the method for obtaining the parameters input from the fault layer to the non-fault layer is: according to the trigger angle advance amount output by the CFPREV of the fault layer after the fault occurs, multiply it by the DC current and the coordination coefficient K to obtain the parameters output from the fault layer to the non-fault layer. The formula is

[0013] Δα' = Δα F ·I d ·K Formula 1.

[0014] The method for early starting the VDCOL is: introduce the obtained output quantity of the fault layer into the VDCOL, and add it to the DC voltage value to realize the early start of the VDCOL.

[0015] The formula for determining the DC current output by VDCOL is

[0016]

[0017] Δα is the trigger angle advance.

[0018] The suppression method uses a coordinated control method considering the fault layer and the non-fault layer CFPREV. Specifically: compare the output parameters of the fault layer to be obtained with those of the non-fault layer CFPREV, and input the larger one to the trigger control of the non-fault layer to achieve the effect of suppressing commutation failure of the non-fault layer.

[0019] The process of the fault layer CFPREV determining the fault type is to detect the value of the commutation bus voltage after the fault, and judge the fault type that occurs in the system after zero-sequence component detection and Clarke transformation;

[0020] The setting process of the trigger angle advance output by CFPREV after commutation failure occurs in the fault layer is to compare the values after zero-sequence component detection and Clarke transformation, and perform arcsine transformation on the larger value to obtain the trigger angle advance Δα.

[0021] The coupling effect between systems after the fault reduces the commutation bus voltage of the non-fault layer. Steps S2 and S3 are the coordinated control methods for the high and low-end converters. Specifically:

[0022] First step: Multiply the trigger angle advance output by the fault layer CFPREV by the coordination coefficient K, and then multiply it by the DC current value that can reflect the severity of the fault. Compare the obtained result with the output value of the non-fault layer CFPREV, and determine the larger value as the compensation amount for the non-fault layer;

[0023] Second step: Input the determined compensation amount into the VDCOL of the non-fault layer, add it to the original input voltage value of the VDCOL, and trigger the VDCOL control in advance to reduce the DC current value and reduce the demand for the commutation time voltage integration area;

[0024] Third step: Input the compensation amount into the trigger control of the non-fault layer to improve the sensitivity of the non-fault layer CFPREV and increase the maximum commutation area that the commutation voltage can provide.

[0025] When the suppression method is implemented, if commutation failure is caused by a fault in a certain layer of the AC system at the receiving end of the UHVDC hierarchical access system, calculate the trigger angle advance output by the fault layer CFPREV, multiply it by the coordination coefficient K and the DC current I d and input the product into the non-fault layer; compare the input quantity of the fault layer with the trigger angle advance output by the non-fault layer CFPREV, and use the larger value as the compensation amount to input to the trigger control and pole control of the non-fault layer; judge whether commutation failure is suppressed. If it is successfully suppressed, exit. If it is not successfully suppressed, continue to compensate;

[0026] In the hierarchical access structure, the converters on the rectifier side are connected to the AC power grid of the same voltage level and are all equipped with constant current control and minimum firing angle control; the converters on the inverter side are respectively connected to the AC power grids of different voltage levels and are all equipped with constant extinction angle control, constant current control, current deviation control, and voltage-dependent current order limit control (VDCOL). Under normal conditions, the rectifier side operates with constant current control, and the inverter side operates with constant extinction angle control.

[0027] E n is the equivalent electromotive force of the AC system, and Z n is the equivalent reactance of the AC system, where n = 1, 2, 3; Z 23 is the equivalent connection impedance between the high and low-end converter buses on the inverter side; R d is the equivalent resistance value of the DC transmission line, and L d is the equivalent inductance value of the DC transmission line; I d is the DC current of the line; U dr and U di are the positive DC voltages on the rectifier side and the inverter side respectively.

[0028] The receiving-end equivalent structure of the UHV DC hierarchical access system consists of a high-voltage AC system and a UHV AC system, which are connected by an impedance. Assuming that when a fault occurs in the high-voltage AC system, the bus line voltage drop value is ΔU H , and the bus line voltage drop value of the UHV AC system is ΔU L , then the relationship between the two is as follows:

[0029] The receiving-end converters of the hierarchical access structure are connected in series on the DC side, so that the DC currents of the two are the same, which is expressed by the formula:

[0030]

[0031] In the UHV DC hierarchical access system, when a fault occurs in a certain layer of the system, resulting in changes in electrical quantities, due to the coupling effect between systems, the converter bus voltage of the non-fault layer will also decrease and the DC current will increase, causing a risk of commutation failure in the non-fault layer; at this time, CFPREV detects the three-phase instantaneous values of the converter bus to judge the type of fault that occurs in the system, and obtains the trigger angle advance amount after inverse cosine transformation;

[0032] Among them, U a , U b , U c are the three-phase voltage instantaneous values of the converter bus; D_LEVEL and A_LEVEL are the detection thresholds for asymmetric faults and symmetric faults, taking 0.14 and 0.15 respectively; Δα is the trigger angle advance amount output by CFPREV, which is related to the magnitude of the converter bus voltage drop;

[0033] The analysis method of the commutation process is the commutation voltage time integral area theory, and its formula is as follows:

[0034]

[0035] In the formula, S demand is the demand for the commutation area, and Smax is the maximum commutation area that the commutation voltage can provide; L c is the commutation reactance; ΔU is the commutation voltage; t 1 and t 2max are the commutation start time and the moment when the commutation voltage area is the largest respectively; I d (t 1 ) and I d (t 2max ) are the DC current values at the corresponding moments respectively;

[0036] When a fault occurs in the AC system on the inverter side, due to the DC voltage drop and the sudden increase in the DC current, it is reflected in the commutation time voltage integral area as S demand increases and Smax decreases. When the two cannot meet the constraints of Formula 5, it will cause commutation failure of the inverter-side converter valve; the commutation failure suppression method adopts an improved coordinated control strategy to suppress the commutation failure of the non-fault layer from the following two aspects:

[0037] Smax increase link: This link utilizes the characteristic that the fault layer CFPREV can respond to faults more quickly, and sends the trigger angle advance amount output by the fault layer to the non-fault layer to increase the sensitivity of the non-fault layer CFPREV; Δα H and Δα L are the output values of CFPREV of the high-end valve group and the low-end valve group respectively. After multiplying these values by the per-unit value of the DC current and the coordinated control coefficient respectively, the maximum value among them is selected and input to VDCOL in the pole control level and the valve group control level; among them, the coordinated control coefficients K 1 and K 2 are selected with the goal of effectively suppressing commutation failure and minimizing the impact on the transmission efficiency; the coordinated control coefficients K 1 and K 2 are taken as 0.7 and 0.75 respectively; S demand decrease link: This link modifies VDCOL, increases U H and U L , and sends the trigger angle advance amount coordinated by the high and low end valve groups to VDCOL, and adds it to the DC voltage to realize the rapid startup of VDCOL and reduce the demand for the commutation voltage time integral area; the VDCOL characteristic curve in this link is expressed by the formula as

[0038]

[0039] The present invention uses the trigger angle advance amount output by the fault layer CFPREV as an electrical detection quantity. By setting a reasonable coordination coefficient K, it coordinates with the control system of the non-fault layer, effectively reducing the probability of commutation failure in the non-fault layer. At the same time, since the high and low-end converter valves are connected in series on the DC side, this method can effectively reduce the DC current of the high and low-end converters by triggering the VDCOL instruction in advance. Compared with the traditional method for suppressing commutation failure in a hierarchical access system, it has a stronger fault resistance ability and recovery effect.

[0040] In summary, the beneficial effects of the present invention are as follows:

[0041] 1. The trigger angle advance amount output by the fault layer CFPREV of the present invention is used as a compensation amount, which coordinates with the control system of the non-fault layer, improves the sensitivity of the non-fault layer CFPREV to faults, and at the same time reduces the DC current of the system during faults. It has the advantages of quickly triggering the non-fault layer CFPREV and reducing the DC current during faults.

[0042] 2. The present invention does not require adding complex control links. It is all based on the controllers equipped in the existing hierarchical system for improvement, with good economy and does not affect the operation of the system under normal conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The following further details the present invention in conjunction with the drawings and specific embodiments:

[0044] Figure 1 is the flow schematic diagram of the present invention;

[0045] Figure 2 is the structural schematic diagram of the hierarchical access system built in the embodiment of the present invention;

[0046] Figure 3 is the schematic diagram of the U-I characteristic curve of VDCOL in the hierarchical access system built in the embodiment of the present invention;

[0047] Figure 4 is the schematic diagram of the equivalent structure at the receiving end of the hierarchical access system built in the embodiment of the present invention;

[0048] Figure 5 is the schematic diagram of the basic structure of CFPREV in the hierarchical access system built in the embodiment of the present invention;

[0049] Figure 6 is the schematic diagram for showing the time integral area of the commutation voltage in the embodiment of the present invention;

[0050] Figure 7 is the schematic diagram of the improved control system in the embodiment of the present invention;

[0051] Figure 8Schematic diagram of the U-I characteristic curve of the improved VDCOL in the embodiments of the present invention;

[0052] Figure 9 Schematic diagram for showing the variation of each electrical quantity parameter when adopting the commutation failure suppression strategy of the hierarchical system proposed by the present invention, only adopting the coordinated control strategy, and adopting the standard control strategy in Embodiment 1 of the present invention;

[0053] Figure 10 Schematic diagram for showing the variation of each electrical quantity parameter when adopting the commutation failure suppression strategy of the hierarchical system proposed by the present invention, only adopting the coordinated control strategy, and adopting the standard control strategy in Embodiment 2 of the present invention. Specific implementation manners

[0054] As shown in the figure, a commutation failure suppression method applicable to a UHV DC hierarchical access system, where the high and low end converter valves of the access system are connected in series on the DC side. This suppression method uses the trigger angle advance amount output by the commutation failure prevention control system CFPREV of the fault layer as the electrical detection quantity, and coordinates with the control system of the non-fault layer by setting a coordination coefficient K to reduce the probability of commutation failure in the non-fault layer; and reduces the DC current of the high and low end converters by triggering the VDCOL instruction in advance;

[0055] The suppression method includes the following steps;

[0056] Step S1: Detect the DC current of the transmission line after a fault occurs, calculate the turn-off angle advance amount output by the CFPREV of the fault layer, and input both of them to the CFPREV of the non-fault layer;

[0057] Step S2: Introduce the output quantity of the CFPREV of the fault layer, the DC current, and the coordination coefficient K into the non-fault layer at the same time, and compare it with the original output value of the CFPREV of the non-fault layer;

[0058] Step S3: Send the output values to the valve group control and trigger control respectively, while increasing the maximum commutation area provided by the commutation voltage and reducing the commutation demand area.

[0059] The suppression method uses a VDCOL transformation method considering the combination of the fault layer and the non-fault layer, and introduces the maximum value of the output of the CFPREV of the fault layer and the non-fault layer into the VDCOL to realize the early start of the VDCOL.

[0060] In Step S1, the method for obtaining the parameters input from the fault layer to the non-fault layer is: based on the trigger angle advance amount output by the CFPREV of the fault layer after a fault occurs, multiply it by the DC current and the coordination coefficient K to obtain the parameters output from the fault layer to the non-fault layer. The formula is

[0061] Δα' = Δα F ·Id · K formula one.

[0062] The method for early start of VDCOL is as follows: introducing the obtained output quantity of the fault layer into VDCOL and adding it to the DC voltage value to achieve the early start of VDCOL.

[0063] The formula for determining the DC current output by VDCOL is

[0064]

[0065] Δα is the trigger angle advance.

[0066] The suppression method uses a coordinated control method considering the fault layer and the non-fault layer CFPREV. Specifically: comparing the obtained output parameters of the fault layer with those of the non-fault layer CFPREV, and inputting the larger one to the trigger control of the non-fault layer to achieve the effect of suppressing commutation failure of the non-fault layer.

[0067] The process for the fault layer CFPREV to determine the fault type is to detect the commutation bus voltage value after the fault, and judge the fault type that occurs in the system after zero-sequence component detection and Clarke transformation;

[0068] The setting process of the trigger angle advance output by CFPREV after commutation failure occurs in the fault layer. Compare the values after zero-sequence component detection and Clarke transformation, and perform the arcsine transformation on the larger value to obtain the trigger angle advance Δα.

[0069] The coupling effect between systems after the fault causes the commutation bus voltage of the non-fault layer to decrease. Steps S2 and S3 are the coordinated control methods for the high and low end converters. Specifically:

[0070] First step: Multiply the trigger angle advance output by the fault layer CFPREV by the coordination coefficient K, and then multiply the result by the DC current value that can reflect the severity of the fault. Compare the obtained result with the output value of the non-fault layer CFPREV, and determine the larger value as the compensation amount for the non-fault layer;

[0071] Second step: Input the determined compensation amount into the VDCOL of the non-fault layer, add it to the original input voltage value of VDCOL, and trigger the VDCOL control in advance to reduce the DC current value and reduce the demand for the commutation time voltage integration area;

[0072] Third step: Input the compensation amount into the trigger control of the non-fault layer to improve the sensitivity of the non-fault layer CFPREV and increase the maximum commutation area that the commutation voltage can provide.

[0073] Such as Figure 1As shown, when implementing the suppression method, if commutation failure is caused by a fault in a certain layer of the AC system at the receiving end of the UHV DC hierarchical access system, calculate the trigger angle advance amount output by CFPREV of the faulty layer, multiply it by the coordination coefficient K and the DC current I d and input the result to the non-faulty layer; compare the input quantity of the faulty layer with the trigger angle advance amount output by CFPREV of the non-faulty layer, and use the larger value as the compensation quantity to input to the trigger control and pole control of the non-faulty layer; determine whether commutation failure is suppressed. If it is successfully suppressed, exit. If it is not successfully suppressed, continue to compensate;

[0074] As Figure 2 shown, in the hierarchical access structure, the rectifier side converters are connected to the AC power grid of the same voltage level and are all configured with constant current control and minimum trigger angle control; the inverter side converters are respectively connected to the AC power grids of different voltage levels and are all configured with constant extinction angle control, constant current control, current deviation control and voltage-dependent current order limiting control VDCOL; under normal conditions, the rectifier side operates with constant current control and the inverter side operates with constant extinction angle control;

[0075] In this example, the U-I characteristic curve of VDCOL is as Figure 3 shown.

[0076] E n is the equivalent electromotive force of the AC system, Z n is the equivalent reactance of the AC system, where n = 1, 2, 3; Z 23 is the equivalent connection impedance between the high and low end converter buses on the inverter side; R d is the equivalent resistance value of the DC transmission line, L d is the equivalent inductance value of the DC transmission line; I d is the DC current of the line; U dr and U di are the positive DC voltages of the rectifier side and the inverter side respectively;

[0077] As Figure 4 shown, the equivalent structure at the receiving end of the UHV DC hierarchical access system consists of a high-voltage AC system and a UHV AC system, which are connected by an impedance. Assuming that when a fault occurs in the high-voltage AC system, the bus line voltage drop value is ΔU H , and the bus line voltage drop value of the UHV AC system is ΔU L , then the relationship between the two is as follows:

[0078]

[0079] The converters at the receiving end of the hierarchical access structure are connected in series on the DC side to make the DC currents of the two the same, which is expressed by the formula:

[0080]

[0081] In the UHV DC hierarchical access system, when a fault occurs in a certain layer of the system, resulting in changes in electrical quantities, due to the coupling effect between systems, the commutation bus voltage of the non-fault layer will also decrease, and the DC current will increase, causing a risk of commutation failure in the non-fault layer; such as Figure 5 shown, at this time, CFPREV detects the three-phase instantaneous values of the commutation bus to judge the type of fault that occurs in the system, and obtains the trigger angle advance amount after inverse cosine transformation;

[0082] Among them, U a 、U b 、U c are the three-phase voltage instantaneous values of the commutation bus; D_LEVEL and A_LEVEL are the detection thresholds for asymmetric faults and symmetric faults, taking 0.14 and 0.15 respectively; Δα is the trigger angle advance amount output by CFPREV, which is related to the magnitude of the commutation bus voltage drop;

[0083] such as Figure 6 shown, the analysis method of the commutation process is the commutation voltage time integral area theory, and its formula is as follows:

[0084]

[0085] In the formula, S demand is the required commutation area, and Smax is the maximum commutation area that the commutation voltage can provide; L c is the commutation reactance; ΔU is the commutation voltage; t 1 and t 2max are the commutation start time and the moment when the commutation voltage area is the largest respectively; I d (t 1 ) and I d (t 2max ) are the DC current values at the corresponding moments respectively;

[0086] When a fault occurs in the AC system on the inverter side, due to the DC voltage drop and the surge of the DC current, it is reflected in the commutation time voltage integral area as S demand increases and Smax decreases. When the two cannot meet the constraints of Formula 5, the commutation valves on the inverter side will experience commutation failure; the commutation failure suppression method adopts an improved coordinated control strategy to suppress the commutation failure of the non-fault layer from the following two aspects:

[0087] such as Figure 7 、 Figure 8 shown, the improved coordinated control strategy suppresses the commutation failure of the non-fault layer from two aspects: the Smax increase link: this link uses the characteristic that the CFPREV of the fault layer can respond to the fault more quickly, and sends the trigger angle advance amount output by the fault layer to the non-fault layer to increase the sensitivity of the CFPREV of the non-fault layer; Δα Hand Δα L are the output values of the high - end valve group and the low - end valve group CFPREV respectively. After multiplying these values by the per - unit value of the DC current and the coordination control coefficient respectively, the maximum value is selected and input into VDCOL in the pole control stage and the valve group control stage. Among them, the coordination control coefficients K 1 and K 2 are selected with the goal of effectively suppressing commutation failure and minimizing the impact on transmission efficiency at the same time; the coordination control coefficients K 1 and K 2 are taken as 0.7 and 0.75 respectively; S demand Reduction link: This link modifies VDCOL, increases U H and U L , and conveys the trigger - angle advance amount coordinated by the high - and low - end valve groups to VDCOL, adding it to the DC voltage to achieve the rapid start of VDCOL and reduce the demand for the commutation voltage time - integral area. The VDCOL characteristic curve in this link is expressed by the formula

[0088]

[0089] Example 1:

[0090] Referring to Figure 4 , the receiving - end equivalent structure of the UHVDC hierarchical access system consists of a 500 kV AC system and a 1000 kV AC system, which are connected by an impedance. Assuming that a fault occurs in the 500 kV AC system, the bus line - voltage drop value is ΔU H , and the bus line - voltage drop value of the 1000 kV AC system is ΔU L , then the relationship between the two is as follows:

[0091]

[0092] Since the receiving - end converters of the hierarchical access structure are connected in series on the DC side, their DC currents are the same:

[0093]

[0094] When an electrical quantity changes due to a fault in a certain layer of the system, due to the coupling effect between systems, the converter bus voltage of the non - fault layer will also decrease and the DC current will increase. Therefore, there is also a risk of commutation failure in the non - fault layer.

[0095] Referring to Figure 5 , CFPREV detects the three - phase instantaneous values of the converter bus to judge the type of fault that occurs in the system, and obtains the trigger - angle advance amount after inverse - cosine transformation. Among them, U a , U b , U c$u_a$, $u_b$, $u_c$ are the instantaneous values of the three-phase voltages of the commutation bus; D_LEVEL and A_LEVEL are the detection thresholds for asymmetrical faults and symmetrical faults respectively, generally taken as 0.14 and 0.15; $\Delta\alpha$ is the trigger angle advance output by CFPREV, which is related to the magnitude of the voltage dip of the commutation bus.

[0096] Example 2:

[0097] This example is based on Example 1, referring to Figure 9 ;

[0098] A single-phase grounding fault is set on the commutation bus on the 1000 kV side, with a fault inductance of 0.2 H, a fault occurrence time of 1 s, and a duration of 0.1 s. Among them, Control Method 1: Both the high-end and low-end converters adopt the CIGRE HVDC standard test control strategy; Control Method 2: Only the coordinated control strategy is carried out for the high-end and low-end converters; Control Method 3: The control strategy proposed in this paper is adopted for the high-end and low-end converters.

[0099] It can be seen from Figure 9 that commutation failures will occur at the fault level when all three control strategies are adopted. However, when Control Method 3 is adopted, the fluctuation of the extinction angle is smaller. The three control strategies cause two, one, and zero commutation failures at the non-fault level respectively, and the degree of extinction angle fluctuation decreases in turn; when Control Strategy 3 is adopted, the extinction angle at the non-fault level tends to be stable at about 1.38 s, which is 0.12 s earlier than Control Strategies 1 and 2. The minimum values of the DC voltage on the inverter side of Control Methods 1, 2, and 3 are 0 pu, 0.12 pu, and 0.25 pu respectively; the minimum values of the DC power are 0.28 pu, 0.37 pu, and 0.50 pu respectively. Therefore, when a single-phase grounding fault occurs, the control strategy proposed in this paper can significantly improve the system's ability to resist commutation failures while improving the system's transmission capacity and voltage stability, which is more conducive to the safe and stable operation of the system.

[0100] Example 3:

[0101] This example is based on Example 1, referring to Figure 10 ;

[0102] A three-phase grounding fault is set on the commutation bus on the 500 kV side, with a fault inductance of 0.15 H, a fault occurrence time of 1 s, and a duration of 0.1 s.

[0103] It can be seen from Figure 10It can be seen that commutation failures will occur at the fault layer when all three control strategies are adopted. However, when control method 3 is used, the fluctuation value of the extinction angle is smaller. At the non-fault layer, commutation failures occur twice, once, and zero times respectively for the three control methods, and the fluctuation of the extinction angle decreases successively. The minimum value of the extinction angle is 7.4° when control method 3 is used. Compared with control methods 1 and 2, control method 3 has smaller drop values of the DC voltage at the inverter side and the active power transmitted by the line during a fault, which is more conducive to improving the operation ability of the system after a fault and enhancing the immunity of the system to faults.

Claims

1. A commutation failure suppression method applicable to a UHV DC hierarchical access system, where the high- and low-end converter valves of the access system are connected in series on the DC side. Characterized in that: This suppression method uses the trigger angle advance amount output by the commutation failure prevention control system CFPREV of the faulty layer as an electrical detection quantity. By setting a coordination coefficient K, it coordinates with the control systems of non-faulty layers to reduce the probability of commutation failure occurring in non-faulty layers. And the DC current of the high- and low-end converters is reduced by triggering the VDCOL instruction in advance. The suppression method includes the following steps: Step S1: Detect the DC current of the transmission line after a fault occurs, calculate the turn-off angle advance amount output by the CFPREV of the faulty layer, and input both to the CFPREV of the non-faulty layer. Step S2: Introduce the output quantity of the CFPREV of the faulty layer, the DC current, and the coordination coefficient K into the non-faulty layer at the same time, and compare it with the original CFPREV output value of the non-faulty layer. Step S3: Send the output values to the valve group control and trigger control respectively, while increasing the maximum commutation area provided by the commutation voltage and reducing the commutation demand area. After a fault, the coupling effect between systems reduces the commutation bus voltage of the non-faulty layer. Steps S2 and S3 are specifically as follows: The first step: Multiply the trigger angle advance amount output by the CFPREV of the faulty layer by the coordination coefficient K, and then multiply it by the DC current value that can reflect the severity of the fault. The obtained result is compared with the CFPREV output value of the non-faulty layer, and the larger value is determined as the compensation amount of the non-faulty layer. The second step: Input the determined compensation amount into the VDCOL of the non-faulty layer, add it to the original input voltage value of the VDCOL, and trigger the VDCOL control in advance to reduce the DC current value and reduce the demand for the commutation time voltage integral area. The third step: Input the compensation amount into the trigger control of the non-faulty layer, improve the sensitivity of the CFPREV of the non-faulty layer, and increase the maximum commutation area that the commutation voltage can provide. When the suppression method is implemented, if commutation failure is caused by a fault in a certain layer of the AC system at the receiving end of the UHVDC hierarchical access system, calculate the trigger angle advance amount output by CFPREV of the fault layer, multiply it by the coordination coefficient K and the DC current I d and input the result to the non-fault layer; compare the input quantity of the fault layer with the trigger angle advance amount output by CFPREV of the non-fault layer, and use the larger value as the compensation quantity to input to the trigger control and pole control of the non-fault layer; determine whether commutation failure is suppressed. If it is successfully suppressed, exit. If it is not successfully suppressed, continue to compensate; In the hierarchical access structure, the rectifier-side converters are connected to the AC power grid of the same voltage level and are all equipped with constant current control and minimum trigger angle control; the inverter-side converters are respectively connected to the AC power grids of different voltage levels and are all equipped with constant turn-off angle control, constant current control, current deviation control, and low-voltage current limiting control VDCOL; under normal conditions, the rectifier side operates with constant current control, and the inverter side operates with constant turn-off angle control. E n is the equivalent electromotive force of the AC system, and Z n is the equivalent reactance of the AC system, where n = 1, 2, 3; Z 23 is the equivalent connection impedance between the high and low voltage commutation buses on the inverter side; R d is the equivalent resistance value of the DC transmission line, and L d is the equivalent inductance value of the DC transmission line; I d is the DC current of the line; U dr and U di are the positive DC voltages on the rectifier side and the inverter side respectively; The receiving-end equivalent structure of the UHV DC hierarchical access system consists of a high-voltage AC system and a UHV AC system, which are connected by an impedance. Assuming that when a fault occurs in the high-voltage AC system, the bus line voltage drop value is ΔU H , and the bus line voltage drop value of the UHV AC system is ΔU L , then the relationship between the two is as follows: The receiving-end converters in the hierarchical access structure are connected in series on the DC side to make their DC currents the same, which is expressed by the formula:

2. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 1. Characterized in that: This suppression method uses a VDCOL transformation method considering the combination of the faulty layer and non-faulty layers, and introduces the maximum value of the CFPREV outputs of the faulty layer and non-faulty layers into the VDCOL to achieve the early start of the VDCOL.

3. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 2. Characterized in that: In step S1, the method for obtaining the parameters input from the faulty layer to the non-faulty layer is as follows: Based on the trigger angle advance amount output by the CFPREV of the faulty layer after the fault occurs, multiply it by the DC current and the coordination coefficient K to obtain the parameters sent from the faulty layer to the non-faulty layer. The formula is Δα' = Δα F ·I d ·K Formula 1.

4. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 2. Characterized in that: The method for early startup of VDCOL is as follows: Introduce the obtained output quantity of the fault layer into VDCOL and add it to the DC voltage value to achieve the early startup of VDCOL.

5. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 2, characterized in that: The formula for determining the DC current output by VDCOL is Δα is the trigger angle advance.

6. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 1, characterized in that: The suppression method uses a coordinated control method considering the fault layer and the non-fault layer CFPREV. Specifically: Compare the obtained output parameters of the fault layer with those of the non-fault layer CFPREV, and input the larger one to the trigger control of the non-fault layer to achieve the effect of suppressing commutation failure of the non-fault layer.

7. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 1, characterized in that: The process for the fault layer CFPREV to determine the fault type is to detect the voltage value of the commutation bus after the fault, and judge the fault type that occurs in the system after zero-sequence component detection and Clarke transformation; The setting process of the trigger angle advance output by CFPREV after commutation failure occurs in the fault layer. Compare the values after zero-sequence component detection and Clarke transformation, and perform an arcsine transformation on the larger value to obtain the trigger angle advance Δα.

8. A commutation failure suppression method applicable to a UHV DC hierarchical access system according to claim 1, characterized in that: In a UHV DC hierarchical access system, when a fault occurs in a certain layer of the system, resulting in changes in electrical quantities, due to the coupling effect between systems, the commutation bus voltage of the non-fault layer will also decrease and the DC current will increase, causing a risk of commutation failure in the non-fault layer; At this time, CFPREV detects the three-phase instantaneous values of the commutation bus to judge the type of fault that occurs in the system, and obtains the trigger angle advance after an arccosine transformation; Among them, U a , U b , U c are the instantaneous values of the three-phase voltages of the commutation bus; D_LEVEL and A_LEVEL are the detection thresholds for asymmetric faults and symmetric faults, taking 0.14 and 0.15 respectively; Δα is the trigger angle advance output by CFPREV, which is related to the magnitude of the commutation bus voltage dip; The analysis method of the commutation process is the commutation voltage time integral area theory, and its formula is as follows: Wherein, S demand is the demand for commutation area, and Smax is the maximum commutation area that the commutation voltage can provide; L c is the commutation reactance; ΔU is the commutation voltage; t 1 and t 2max are the commutation start time and the moment when the commutation voltage area is the largest, respectively; I d (t 1 ) and I d (t 2max ) are the DC current values at the corresponding moments, respectively. After a fault occurs in the AC system on the inverter side, due to the DC voltage drop and the sharp increase in DC current, it is reflected in the voltage integration area of the commutation time as S demand increases, Smax decreases. When the two cannot meet the constraints of Formula Five, it will cause commutation failure of the converter valve on the inverter side; the commutation failure suppression method adopts an improved coordinated control strategy to suppress commutation failure of the non-fault layer from the following two aspects: Smax increase section: This section utilizes the characteristic that the fault layer CFPREV can respond to faults more quickly, and conveys the trigger angle advance amount output by the fault layer to the non-fault layer to increase the sensitivity of the non-fault layer CFPREV; Δα H and Δα L are the output values of CFPREV of the high-end valve group and the low-end valve group respectively. After multiplying these values by the per-unit value of the DC current and the coordination control coefficient respectively, the maximum value among them is selected and input into VDCOL in the pole control stage and the valve group control stage; among them, the coordination control coefficients K 1 and K 2 are selected with the goal of effectively suppressing commutation failures while minimizing the impact on transmission efficiency; the coordination control coefficients K 1 and K 2 are taken as 0.7 and 0.75 respectively; S demand Decrease section: This section modifies VDCOL, increases U H and U L , conveys the trigger angle advance amount coordinated by the high and low-end valve groups to VDCOL, adds it to the DC voltage, and realizes the rapid startup of VDCOL, reducing the demand for the commutation voltage time integral area; the VDCOL characteristic curve in this section is expressed by the formula

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