Direct current transmission subsequent commutation failure prediction method and optimization method based on turn-off angle dynamic equation

Abstract

The invention discloses a DC power transmission subsequent commutation failure prediction method and optimization method based on a turn-off angle dynamic equation, and the method comprises the steps: obtaining a turn-off angle of an LCC-HVDC inverter side based on the output characteristics of the turn-off angle of the LCC-HVDC inverter side; based on constant direct current control, low-voltage current-limiting control, constant turn-off angle control and current deviation control of an inverter side, analyzing processes of first commutation failure and subsequent commutation failure of the LCC-HVDC system; in combination with control characteristics of low-voltage current-limiting control and fixed turn-off angle control, a transient nonlinear differential equation about a turn-off angle is obtained by analyzing dynamic characteristics of direct current and a trigger lead angle during a first commutation failure recovery period; and based on the transient nonlinear differential equation of the turn-off angle, drawing a phase diagram curve of the turn-off angle under different AC voltages and AC voltage change rates, and analyzing the phase diagram curve to obtain a criterion for predicting the occurrence of subsequent commutation failure. According to the invention, the stability and operation reliability of the system are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of transient stability assessment of high voltage direct current transmission, and in particular to a method and an optimization method for predicting subsequent commutation failure of direct current transmission based on a shutdown angle dynamic equation. Background Art

[0002] Line commutated converter base high voltage direct current (LCC-HVDC) has the advantages of long-distance large-capacity transmission, weak synchronous oscillation characteristics and low energy loss, and is widely used in long-distance power energy transmission. Since LCC-HVDC uses thyristors without self-shutoff capability, when the AC power grid at the receiving end fails, the converter valve lacks sufficient commutation support, and the system will experience commutation failure (CF), resulting in DC blocking at the receiving end and large fluctuations in transmission power. When a serious fault occurs in the AC system, if the control is improper, subsequent commutation failure (SCF) will occur after the first commutation failure (FCF), resulting in continuous fluctuations in DC power and continuous blocking of the converter station, which seriously affects the safe operation of the power system.

[0003] The problems caused by subsequent commutation failure are more significant, and its prediction and suppression strategies have received widespread attention. A large number of prediction methods for commutation failure are based on finding the critical value of the commutation voltage as the prediction criterion. The suppression strategies for subsequent commutation failure can be mainly divided into three categories: adding additional reactive compensation equipment, improving converter topology, and improving control strategies. The improved control strategy is highly economical and easy to implement, so it has received widespread attention. At present, the research on the improved control strategy lacks analysis of the dynamic characteristics of the system, and faces the problem of slow protection and control action due to the complex calculation process. Summary of the invention

[0004] In view of this, the present invention provides a method and an optimization method for predicting subsequent commutation failure of direct current transmission based on a dynamic equation of a turn-off angle.

[0005] The present invention discloses a method for predicting subsequent commutation failure of direct current transmission based on a shutdown angle dynamic equation, which comprises:

[0006] Based on the turn-off angle output characteristics of the LCC-HVDC inverter side, the turn-off angle of the LCC-HVDC inverter side is obtained; based on the constant DC current control, low-voltage current limiting control, fixed turn-off angle control and current deviation control on the inverter side, the process of the first commutation failure and subsequent commutation failure of the LCC-HVDC system is analyzed;

[0007] Combining the control characteristics of low-voltage current limiting control and fixed turn-off angle control, the transient nonlinear differential equation about the turn-off angle is obtained by analyzing the dynamic characteristics of DC current and trigger advance angle during the first commutation failure recovery period.

[0008] Based on the transient nonlinear differential equation of the turn-off angle, the phase diagram curves of the turn-off angle under different AC voltages and AC voltage change rates are plotted. By analyzing the phase diagram curves, the criterion for predicting the occurrence of subsequent commutation failure is obtained.

[0009] Furthermore, the calculation formula of the turn-off angle of the LCC-HVDC inverter side is:

[0010]

[0011] Among them, γ is the turn-off angle, β is the trigger advance angle, I dc with U L are the effective values ​​of the DC current and the line voltage of the AC busbar on the inverter side respectively; X c is the equivalent commutation reactance.

[0012] Furthermore, based on the constant DC current control link, the constant turn-off angle control link and the current deviation control link on the inverter side, the working process of the LCC-HVDC system includes:

[0013] The LCC-HVDC system includes a rectifier side and an inverter side. Both the rectifier side and the inverter side are composed of LCC converter valves. The converter valve AC busbars on the rectifier side and the inverter side are equipped with AC filters and reactive power compensators.

[0014] The rectifier side adopts a constant DC current control link, and its DC current reference signal is given by the inverter side; the inverter side is equipped with a constant DC current control link, a fixed turn-off angle control link and a current deviation control link; for the rectifier side, the controller compares the difference between the DC current command and the measured DC current, and generates the trigger lead angle command β of the rectifier side after the PI link. rec , and then generate a trigger angle signal α to control the operation of the converter valve; for the inverter side, the fixed DC current control and the fixed turn-off angle rec on the inverter side

[0015] The two control strategies respectively generate the trigger lead angle command β of the inverter side by passing the DC current error and the turn-off angle error through the PI link. inv_C With β inv_G, take the larger value of the two as the actual trigger advance angle of the inverter station, and then generate a trigger angle signal to control the operation of the converter valve.

[0016] Furthermore, when the inverter side is in the constant DC current control mode, the DC current is controlled by the controllers on the rectifier side and the inverter side at the same time. According to the current margin principle, the reference command in the constant DC current control on the inverter side is lower than that on the rectifier side. The function of the constant DC current control is to achieve a smooth transition between the two control strategies of the constant DC current control and the constant turn-off angle control on the inverter side when the DC current command is not equal to the actual current.

[0017] At the same time, the inverter side is equipped with a low-voltage current limiting control link. When a fault occurs in the AC system on the inverter side, the low-voltage current limiting control detects the voltage at the midpoint of the DC line after the DC voltage on the inverter side is superimposed on the compensation resistor voltage drop. When it drops to the threshold value of the low-voltage current limiting control, the low-voltage current limiting control will adjust the DC current command and thus reduce the DC current to ensure safe operation of the system.

[0018] Furthermore, the process of analyzing the first commutation failure and subsequent commutation failure of the LCC-HVDC system includes:

[0019] When a short circuit occurs in the inverter-side AC system, the commutation voltage and the turn-off angle decrease accordingly. When the turn-off angle decreases to the minimum turn-off angle, the first commutation failure will occur. At this time, the DC side is short-circuited, the DC voltage decreases rapidly, and the DC current increases instantaneously. Then the control system starts to act, and the low-voltage current limiting control cooperates with the rectifier side to reduce the DC current and reduce the inverter-side commutation valve trigger angle. The commutation valve resumes normal commutation and enters the first commutation failure recovery stage.

[0020] During the first commutation failure recovery phase, if the output DC current of the low-voltage current limiting control rises too fast or the fixed shutdown angle control on the inverter side cannot reduce the trigger angle at a speed that meets the reactive power demand of the converter valve under the fault, and the shutdown angle continues to decrease, subsequent commutation failures will occur.

[0021] Furthermore, if the following conditions occur, it is considered that the first commutation failure occurs:

[0022] During the commutation of the LCC converter valve, if the converter valve that has just exited the commutation process has not yet restored its blocking capacity and is subjected to a forward voltage drop again, or if the commutation process of the converter valve has not been completed while it is subjected to a reverse voltage drop, the converter valve will be turned on again without a trigger signal; the electrical angle corresponding to the time required for the converter valve group to restore its forward blocking capacity is defined as the limit shut-off angle, also known as the minimum shut-off angle γ min ; If the turn-off angle of the inverter side is less than its minimum turn-off angle, it is considered that commutation failure has occurred;

[0023] If the following conditions occur, subsequent commutation failure is considered to have occurred:

[0024] During the recovery period after the first commutation failure, if the control system causes the turn-off angle to be less than the minimum turn-off angle γ again min ; then it is considered that subsequent commutation failure has occurred.

[0025] Furthermore, the control characteristics of the combination of low-voltage current limiting control and fixed turn-off angle control are analyzed by analyzing the dynamic characteristics of the DC current and the trigger advance angle during the first commutation failure recovery period to obtain a transient nonlinear differential equation about the turn-off angle, including:

[0026] The mathematical model of DC current under low voltage current limiting control is:

[0027]

[0028] in, is the per unit value of the DC line midpoint voltage after compensation, I dh and I dl are the upper and lower limits of DC current, U dl and U dh is the lower and upper limits of the DC voltage, I dn is the rated value of DC current;

[0029] DC voltage U of LCC inverter dc It is expressed as:

[0030]

[0031] Among them, U L represents the effective value of the line voltage of the AC busbar on the inverter side; β and γ represent the trigger advance angle and the shutdown angle respectively; the DC line midpoint voltage after compensation is:

[0032]

[0033] in, is the DC line midpoint voltage after compensation, I dc is the average value of the sum of the upper and lower limits of the DC current, I dn is the rated value of DC current;

[0034] The dynamic characteristics of DC current are expressed as:

[0035]

[0036] The turn-off angle γ passes through the fixed turn-off angle control structure on the inverter side, and the mathematical expression of the trigger advance angle is obtained as follows:

[0037] β=(k p +k i / s)(γref -γ)

[0038] Among them, β is the trigger lead angle, γ ref and γ are the cut-off angle reference value and the cut-off angle respectively, k i and k p These are the control parameters of the fixed turn-off angle control on the inverter side;

[0039] The transient nonlinear differential equation about the cut-off angle is expressed as:

[0040]

[0041] Where γ is the cut-off angle, and They represent the derivative of the turn-off angle and the AC voltage respectively, and C3 and C4 are coefficients determined by the system parameters.

[0042] Furthermore, the expressions of C3 and C4 are:

[0043]

[0044] Furthermore, the criterion for predicting the occurrence of subsequent commutation failure includes:

[0045] Calculate C1, C2, C3, C4 according to system parameters, and give the effective value of AC voltage at current time k and previous time k-1 Calculate the rate of change of the AC voltage at the current moment Will and Bring in C SCF In the calculation expression of, the subsequent commutation failure prediction criterion C is calculated in real time SCF And determine its size; if C SCF is less than zero, it is considered that when the cut-off angle is reduced to γ min After that, it will still decrease, and subsequent commutation failure will occur; otherwise, it means that the turn-off angle will not decrease to γ min Below, no subsequent commutation failure fault will occur.

[0046] Furthermore, C SCF The calculation expression is:

[0047]

[0048] The present invention also discloses an optimization method for suppressing the occurrence of subsequent commutation failure, which suppresses the subsequent commutation failure on the basis of the prediction of the subsequent commutation failure by the above-mentioned DC power transmission subsequent commutation failure prediction method based on the shutdown angle dynamic equation, and comprises:

[0049] If the occurrence of subsequent commutation failure is predicted, the compensation amount Δγ of the turn-off angle γ is superimposed on the turn-off angle reference value of the fixed turn-off angle control on the inverter side, changing the dynamic characteristics of the turn-off angle during the first commutation failure recovery period and suppressing the occurrence of subsequent commutation failure faults.

[0050] Further, according to the criterion of subsequent commutation failure, a setting value n is obtained, and according to the setting value n, it is judged whether the LCC-HVDC system is likely to have subsequent commutation failure or whether it will cause the DC voltage on the inverter side to be too low and reduce the transmission efficiency.

[0051] Furthermore, the expression of the setting value n is:

[0052]

[0053] Among them, γ ref is the cut-off angle, k i and k p is the control parameter of the fixed turn-off angle control on the inverter side, γ ref are the cut-off angle reference value, γ min is the minimum cut-off angle, C3 and C4 are coefficients determined by system parameters, It is the effective value of the line voltage of the AC bus on the inverter side of the system in steady state.

[0054] Furthermore, the expression of the compensation amount Δγ is:

[0055]

[0056] Among them, U L is the AC voltage, is the rate of change of AC voltage.

[0057] Due to the adoption of the above technical solution, the present invention has the following advantages:

[0058] 1. The present invention proposes a prediction method for subsequent commutation failure of a conventional DC transmission receiving-end system under an AC power grid fault, which fills the gap in the prediction of subsequent commutation failure of the system under an AC power grid fault. The established prediction method based on the phase diagram analysis method can accurately predict whether a subsequent commutation failure will occur before it occurs, and has a certain speed, which is beneficial to the emergency protection action of the power system.

[0059] 2. The optimization control strategy for suppressing the occurrence of subsequent commutation failures in the conventional DC transmission receiving-end system proposed in the present invention is intended to ensure that the LCC converter valve operates safely under AC power grid faults without continuous commutation failures. The proposed optimization control strategy for subsequent commutation failures dynamically compensates for the reference value of the fixed shut-off angle control on the inverter side, thereby changing the dynamic characteristics of the shut-off angle during the recovery period of the first commutation failure. This strategy can effectively suppress the occurrence of subsequent commutation failures under different ground fault conditions, significantly improving the stability and operational reliability of the system.

[0060] 3. The present invention first analyzes the dynamic characteristics of DC current and turn-off angle during the recovery period of subsequent commutation failure in the LCC-HVDC system, so as to clarify the mechanism of subsequent commutation failure, and then proposes a prediction method for whether subsequent commutation failure will occur and an optimization control strategy for suppressing subsequent commutation failure, which is of great significance to maintaining safe operation of high-voltage direct current transmission systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments recorded in the embodiments of the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0062] Figure 1 A conventional high-voltage direct current transmission topology diagram provided in an embodiment of the present application;

[0063] Figure 2 A high voltage direct current transmission control structure diagram provided in an embodiment of the present application;

[0064] Figure 3 A commutation failure recovery process and a controller action sequence diagram during a system failure process provided by an embodiment of the present application;

[0065] Figure 4 The control structure and control characteristic diagram of the low-voltage current limiting control provided in the embodiment of the present application;

[0066] Figure 5 A phase diagram curve diagram of the shutdown angle at different times during a fault process provided by an embodiment of the present application;

[0067] Figure 6 A subsequent commutation failure prediction flow chart provided in an embodiment of the present application;

[0068] Figure 7 A test result diagram of the proposed prediction method under a three-phase grounding fault provided in an embodiment of the present application;

[0069] Figure 8A test result diagram of the prediction method proposed under a single-phase grounding fault provided in an embodiment of the present application;

[0070] Fig. 9 An optimized control strategy diagram for suppressing subsequent commutation failure provided in an embodiment of the present application;

[0071] Fig.10 A graph showing the test results (transition resistance 20Ω) of the proposed optimization control strategy under a three-phase grounding fault provided in an embodiment of the present application;

[0072] Fig.11 The test results of the proposed optimization control strategy under the three-phase grounding fault provided in the embodiment of the present application (transition resistance 5Ω);

[0073] Fig.12 The test results of the proposed optimization control strategy under the single-phase grounding fault provided in the embodiment of the present application (transition resistance 50Ω);

[0074] Fig.13 This is the test result of the proposed optimization control strategy under a single-phase grounding fault provided in an embodiment of the present application (transition resistance 5Ω). DETAILED DESCRIPTION

[0075] The present invention is further described in conjunction with the accompanying drawings and embodiments, and the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field should fall within the scope of protection of the embodiments of the present invention.

[0076] Reference Figure 1 and Figure 2 The present method provides an embodiment of a method for predicting subsequent commutation failure of direct current transmission based on a dynamic equation of a shutdown angle, which comprises the following steps:

[0077] S1. Based on the output characteristics of the cut-off angle of the LCC-HVDC inverter side, the specific calculation formula of the cut-off angle of the LCC-HVDC inverter side was constructed; based on the constant extinction angle control (CEAC), low-voltage current limiting control (VDCOL), current error control (CEA) and constant DC current control (CCC) on the inverter side, the process of the first commutation failure and subsequent commutation failure of the LCC-HVDC system was analyzed. Among them, the constant cut-off angle control is a control strategy to maintain the cut-off angle of the inverter side at a constant value by adjusting the trigger angle; the low-voltage current limiting control is a control strategy to adjust the output DC current command according to the DC voltage drop on the inverter side; the current deviation control is a control strategy to adjust the output command to help the system achieve smooth switching of the control strategy when the DC current command is not equal to the actual DC current; the constant DC current control is a control strategy to maintain the DC current on the inverter side at a constant value by adjusting the trigger angle.

[0078] S2. Based on the first commutation failure recovery process analyzed in step S1, it is clarified that the dominant control strategies that determine whether the system will experience subsequent commutation failures during the first commutation failure recovery process are fixed turn-off angle control and low-voltage current limiting control; combined with the control characteristics of low-voltage current limiting control and fixed turn-off angle control, the transient nonlinear differential equation about the turn-off angle is obtained by analyzing the dynamic characteristics of the DC current and the trigger advance angle during the first commutation failure recovery period.

[0079] S3. Based on the transient nonlinear differential equation of the turn-off angle established in step S2, the phase diagram curves of the turn-off angle under different AC voltages and AC voltage change rates are plotted, and the criterion for predicting whether subsequent commutation failure will occur is obtained by analyzing the phase diagram curves.

[0080] As an example of this embodiment, refer to Figure 1 The rectifier and inverter sides of the HVDC transmission system are both 12-pulse LCC converter valves consisting of two six-pulse LCC converter valves connected in series, and the two are connected via a DC line. The AC busbars of the converter valves on the rectifier and inverter sides are equipped with AC filters and reactive power compensators.

[0081] As another example of this embodiment, refer to Figure 2As shown in the figure, the rectifier side adopts constant DC current control (CCC), and its DC current reference signal is given by the inverter side. The inverter side is equipped with constant DC current control (CCC), constant cut-off angle control (CEAC) and current deviation control (CEA). For the rectifier side, the DC current I dc_rec After passing through the first-order filter, the difference is made with the DC current command, and after passing through the PI (proportional-integral) link, the trigger lead angle command β of the rectifier side is generated. rec , and then generate a trigger angle signal α to control the operation of the converter valve, where G1 and T1 are the first-order rec in the constant DC current control

[0082] For the inverter side, the constant DC current control is based on the DC current I dc_inv The error between the first-order filter and the DC current command is converted into a trigger lead angle command β after PI (proportional-integral) inv_C ; Among them, G3 and T3 are the parameters of the first-order filter in the constant DC current control on the inverter side. At the same time, when the inverter side is under constant DC current control, the DC current is controlled by the controllers on both the rectifier side and the inverter side. According to the current margin principle, the reference command in the constant DC control on the inverter side is lower than that on the rectifier side (here 0.1pu). The fixed turn-off angle control compares the minimum turn-off angle measured in the previous cycle with the turn-off angle reference value γ ref The error between them is generated by PI (proportional-integral) to trigger the lead angle command β inv_G ; The error has a certain upper threshold, which is -0.544 here. inv_C and β inv_G The maximum value among them is taken as the actual trigger advance angle signal β inv , and then generate the trigger angle signal α inv_G Control the operation of the converter valve. The function of current deviation control is to achieve a smooth transition between the two control strategies of fixed DC control and fixed cut-off angle control when the DC current command is not equal to the actual current.

[0083] At the same time, the inverter side is equipped with a low voltage current limiting control, namely VDCOL. When a fault occurs in the inverter side AC system, VDCOL detects the DC voltage U dc_inv , and after first-order filtering, the compensation resistor voltage drop is superimposed as the voltage at the midpoint of the DC line; where G2 and T2 are the parameters of the first-order filter in the low-voltage current limiting control. When the voltage drops to the threshold value of VDCOL, VDCOL will adjust the DC current command and reduce the DC current to ensure the safe operation of the system. Compare this current command with the DC current command I under normal operation des The smaller value is used as the actual DC current command transmitted to the rectifier side.

[0084] As another embodiment of the present invention, this embodiment further supplements and elaborates on the technical solution of the present invention on the basis of the above embodiment. In this embodiment, the turn-off angle of the LCC-HVDC inverter side constructed in step S1 is related to the DC current, AC voltage and trigger advance angle, and the specific calculation formula is:

[0085]

[0086] Among them, γ is the turn-off angle, β is the trigger advance angle, I dc with U L are the effective values ​​of the DC current and the line voltage of the AC busbar on the inverter side respectively; X c is the equivalent commutation reactance.

[0087] In step S1, during the commutation of the LCC converter valve, if the valve that has just exited the commutation process has not yet restored its blocking capability and is subjected to a forward voltage drop again, or if the commutation process has not been completed while the converter valve is subjected to a reverse voltage drop, the converter valve will be turned on again without a trigger signal. This phenomenon is called commutation failure. The electrical angle corresponding to the time required for the converter valve group to restore its forward blocking capability is defined as the limit shutoff angle, also known as the minimum shutoff angle γ min (Generally 7.2°). If the turn-off angle on the inverter side is less than its minimum turn-off angle, it is considered that a commutation failure has occurred.

[0088] The commutation failure process described in step S1 is as shown in the attached Figure 3 When a short circuit occurs in the inverter side AC system, the commutation voltage and the cut-off angle decrease accordingly. When the cut-off angle decreases to the minimum cut-off angle, FCF will occur, corresponding to Figure 3 At this time, the DC side is short-circuited, the DC voltage drops rapidly, and the DC current increases instantaneously. Then the control system starts to work, VDCOL (typical time of input is 10ms) cooperates with the rectifier side to reduce the DC current, and CEAC acts to reduce the trigger angle of the inverter valve. Under the joint action of the two, the converter valve resumes normal commutation, corresponding to Figure 3 At the moment t2 in the figure, the turn-off angle will instantly rise to its maximum value γ max Enter the first commutation failure recovery phase.

[0089] In the first commutation failure recovery phase, if the controller cooperates, the turn-off angle will change from γ max Gradually decrease to the reference value γ of the fixed cut-off angle controller ref , the system will return to stability; however, if the controller is not properly coordinated, such as the VDCOL output DC current rises too fast or the speed at which the CEAC reduces the trigger angle cannot adapt to the reactive power demand of the converter valve under fault conditions, the shutdown angle will continue to decrease until it is lower than γ again. min ,correspond Figure 3 At the t3 moment in the figure, SCF (subsequent commutation failure) will occur, corresponding to Figure 3 The t4 moment in.

[0090] In the process from the first commutation failure recovery to the subsequent commutation failure, control links such as VDCOL and CEAC have begun to act. These control links are key factors in predicting and suppressing the subsequent commutation failure. In this process, CCC is used on the rectifier side. If CCC is also used on the inverter side, the magnitude of the DC current is controlled by both the rectifier and inverter stations. The ambiguity of the control command will cause DC current fluctuations, increasing the risk of subsequent commutation failures. It is generally not recommended that the inverter side use CCC during the first commutation recovery period. Therefore, the analysis of subsequent commutation failures in the present invention mainly considers the influence of VDCOL and CEAC.

[0091] In step S2, the control structure and control characteristics of VDCOL are as follows: Figure 4 As shown in (a) and (b), the DC current mathematical model based on VDCOL is:

[0092]

[0093] in, is the per unit value of the DC line midpoint voltage after compensation. dh and I dl are the upper and lower limits of DC current respectively; U dl and U dh is the lower and upper limits of the DC voltage, I dn is the rated value of DC current.

[0094] In step S2, for the twelve-pulse LCC inverter under study, its DC voltage can be expressed as:

[0095]

[0096] Among them, U L represents the effective value of the line voltage of the AC busbar on the inverter side; β and γ represent the trigger advance angle and the cut-off angle respectively. The DC line midpoint voltage after compensation is:

[0097]

[0098] Among them, I dc is the average value of the sum of the upper and lower limits of the DC current, I dn is the rated value of DC current.

[0099] In step S2, the converter valve has restored normal commutation from the first commutation failure to the subsequent commutation failure, the commutation voltage remains basically unchanged at a lower value, the output of VDCOL remains basically unchanged, and the change of DC current is very small. Therefore, the dynamic characteristics of DC current can be expressed as:

[0100]

[0101] In step S2, the mathematical expression of the trigger advance angle obtained by the turn-off angle γ through the CEAC control structure is:

[0102] β=(k p +k i / s)(γ ref -γ)

[0103] Among them, β is the trigger lead angle, γ ref and γ are the cut-off angle reference value and the cut-off angle respectively, k i and k p is the control parameter of CEAC.

[0104] The transient nonlinear differential equation about the turn-off angle γ mentioned in step S2 specifically refers to the transient nonlinear differential equation about the turn-off angle γ obtained by combining the dynamic characteristics of the DC current and the trigger advance angle during the first commutation failure recovery period and the influence of the trigger advance angle β on the turn-off angle γ, and considering the influence of the line impedance and the AC voltage on the transient characteristics, which is expressed as:

[0105]

[0106] in, and Represent the cut-off angle and the derivative of the AC voltage respectively. C3 and C4 are coefficients determined by system parameters, expressed as:

[0107]

[0108] It can be seen that when the cut-off angle changes to a certain value, its rate of change It is determined by the system control parameters, AC voltage and the derivative of AC voltage. If after the first commutation failure is restored, the rate of change If the turn-off angle is always less than zero, the turn-off angle will continue to decrease until it is less than the minimum turn-off angle γ required for the carrier recombination of the converter valve. min , and subsequent commutation failure occurs.

[0109] As another embodiment of the present invention, this embodiment is based on the above embodiment, and further supplements and explains the technical solution of the present invention in detail. In this embodiment, the drawing of the phase diagram curve about the shutdown angle specifically refers to drawing the phase diagram curve under different AC voltages and AC voltage change rates in combination with the transient nonlinear differential equation of the shutdown angle γ in step S2. The specific process is as follows Figure 5 shown.

[0110] Figure 5 The solid line in the middle ① indicates that the system is in a steady state. The intersection point (point a) of the phase diagram curve and the horizontal axis is the stable point, and the corresponding shutdown angle is γ ref At this time, the inverter side maintains γ=γ under the action of CEAC ref After the AC system fails, normal commutation is restored after a short first commutation failure. At different times after the first commutation failure is restored, the system operates at Figure 5 On the solid line of ②, ③ or ④.

[0111] Taking the solid line ② as an example, the stable point (point b) of the system is at γ min The right side of the cut-off angle will not continue to decrease to γ min , so no subsequent commutation failure will occur. When the system runs on the solid line ④, its stable point (point d) is at γ min The left side shows that in this case, γ will continue to decrease until it is less than γ during the recovery process of the first commutation failure. min , and then the subsequent commutation failure occurs. From the dynamic change of the turn-off angle, if γ = γ min There is always It shows that when the turn-off angle γ continues to decrease to its minimum turn-off angle, it will continue to decrease and subsequent commutation failure will occur.

[0112] The criteria for predicting whether a subsequent commutation failure will occur are further defined as follows:

[0113]

[0114] γ = γ determined by the current AC voltage and its rate of change and the interaction between the controller (VDCOL and CEAC) min The change trend of the inverter turn-off angle at the moment. SCF If it is less than 0, the subsequent commutation fails. The specific prediction process is as follows: Figure 6 shown.

[0115] First, C1, C2, C3, and C4 are calculated based on the system parameters. Then the measurement link gives the effective value of the AC voltage at the current time k and the previous time k-1. Calculate the rate of change of the AC voltage at the current moment k Where Δt is the sampling interval. The AC voltage and its rate of change at the current time k are and Bring in C SCF In the calculation expression of, the subsequent commutation failure prediction criterion C is calculated in real time SCF And determine its size. If the calculated C SCF is less than zero, it is considered that when the cut-off angle is reduced to γ min After that, it will still decrease, and subsequent commutation failure will occur; on the contrary, if C SCF If it is zero, it means that the turn-off angle will not decrease to γ min Therefore, no subsequent commutation failure fault will occur.

[0116] As another embodiment of the present invention, this embodiment is based on the above embodiment, and further supplements and elaborates on the technical solution of the present invention. This embodiment provides the verification results of the proposed method under different transition resistances and different fault times when a three-phase grounding fault or a single-phase grounding fault occurs in the AC measurement. The AC measurement fault is set to occur at 1s, 1.004s, 1.008s, 1.012s, 1.016s, and 1.20s, and the fault duration is 0.5s. The verification results are as follows: Figure 7 and Figure 8 shown.

[0117] Figure 7 and Figure 8 In the figure, the square with time indicates that the subsequent commutation failure is successfully predicted. The time is the time margin between the prediction of the subsequent commutation failure by the proposed method and the actual occurrence of the subsequent commutation failure in the system. The square without time and with a single slash indicates that only the first commutation failure is successfully predicted. The square without time and with two crossed slashes indicates that the prediction result is inconsistent with the actual result;

[0118] It can be seen that under different transition resistances and different fault time tests within a power frequency cycle, the proposed prediction method has relatively good prediction performance for both single-phase grounding faults and three-phase grounding faults, and can accurately predict whether subsequent commutation failure will occur. The prediction accuracy can reach more than 90%.

[0119] See also Fig. 9 The present invention also provides an embodiment of an optimization method for suppressing the occurrence of subsequent commutation failure, which includes: firstly, the measured AC voltage U L After processing, the change rate is obtained From the above mentioned CSCF The expression is used to calculate the subsequent commutation failure prediction criterion C SCF If the subsequent commutation failure is predicted, the optimization control strategy is started. The calculated compensation value Δγ of the turn-off angle γ is added to the turn-off angle reference value of the fixed turn-off angle control on the inverter side to change the dynamic characteristics of the turn-off angle during the first commutation failure recovery period and suppress the occurrence of subsequent commutation failure faults.

[0120] If n is too small, the improvement effect is not significant, and the system may still fail in subsequent commutation. If n is too large, the DC voltage on the inverter side will be too low, reducing the transmission efficiency. Considering the safety margin and energy loss, this paper takes n as C SCF 1.5 times of is taken as the setting value, that is:

[0121]

[0122] in, It is the effective value of the line voltage of the AC bus on the inverter side of the system in steady state.

[0123] The correctness of the above embodiments is verified by using specific examples below.

[0124] A three-phase resistive grounding fault is set to occur at the AC bus M at 1s, lasting for 0.5s, and the transition resistance is 20Ω. The shutdown angle γ and DC voltage U of the system without starting the optimization control and starting the optimization control are obtained. dc The results are as follows Fig.10 (a), (b), (c), and (d) are shown in the figure; the turn-off angle γ and DC voltage U of the system without starting the optimization control and starting the optimization control obtained by the transition resistance of 5Ω dc The results are as follows Fig.11 As shown in (a), (b), (c) and (d).

[0125] The test results show that when the original system has a three-phase grounding fault with a transition resistance of 20Ω and 5Ω on the AC bus, subsequent commutation failures will occur after the first commutation failure, causing the shutdown angle to decrease to 0, a DC short circuit, and a rapid drop in the DC voltage to zero; after starting the proposed optimization control, the system can resume stable operation after the first commutation failure in both test scenarios, and no subsequent commutation failures will occur. The system resumes stable operation, verifying the effectiveness of the proposed optimization control strategy under symmetrical faults.

[0126] A single-phase resistive ground fault is set to occur at the AC bus M at 1s, lasting 0.5s, with a transition resistance of 50Ω. The shutdown angle γ and DC voltage U obtained by the test are as follows: dc The results are as follows Fig.12The transition resistance is 5Ω, and the system shutoff angle γ and DC voltage U obtained by the test are not started and started with the optimization control. dc The results are as follows Fig.13 As shown in (a), (b), (c) and (d).

[0127] The test results show that when a single-phase grounding fault with a transition resistance of 50Ω occurs in the original system, multiple subsequent commutation failures will occur after the first commutation failure, causing the shutdown angle to drop to 0 multiple times and the DC voltage to fluctuate greatly multiple times; after starting the proposed optimization control, no subsequent commutation failures will occur, the shutdown angle will always fluctuate above the minimum shutdown angle, and the DC voltage will not drop to zero.

[0128] When the original system had a single-phase grounding fault with a transition resistance of 5Ω on the AC bus, commutation failures occurred multiple times in a short time after the fault, causing the turn-off angle to drop to zero and the DC voltage to continue to fluctuate. After the proposed optimization control was started, although the DC voltage still fluctuated, the turn-off angle would no longer drop to zero after the first commutation failure. This shows that even if an asymmetric fault occurs on the AC side, the proposed optimization control strategy still has a certain degree of reliability, which can reduce the risk of subsequent commutation failures and improve the stability of the system.

[0129] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for predicting subsequent commutation failure of direct current transmission based on the dynamic equation of the turn-off angle, characterized in that: include: Based on the cut-off angle output characteristics of the LCC-HVDC inverter side, the cut-off angle of the LCC-HVDC inverter side is obtained; Based on the constant DC current control, low voltage current limiting control, fixed turn-off angle control and current deviation control on the inverter side, the process of the first commutation failure and subsequent commutation failure of the LCC-HVDC system is analyzed; Combining the control characteristics of low-voltage current limiting control and fixed turn-off angle control, the transient nonlinear differential equation about the turn-off angle is obtained by analyzing the dynamic characteristics of DC current and trigger advance angle during the first commutation failure recovery period. Based on the transient nonlinear differential equation of the turn-off angle, the phase diagram curves of the turn-off angle under different AC voltages and AC voltage change rates are plotted. By analyzing the phase diagram curves, the criterion for predicting the occurrence of subsequent commutation failure is obtained.

2. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 1 is characterized in that: The calculation formula of the turn-off angle of the LCC-HVDC inverter side is: Among them, γ is the turn-off angle, β is the trigger advance angle, I dc with U L are the effective values ​​of the DC current and the line voltage of the AC busbar on the inverter side respectively; X c is the equivalent commutation reactance.

3. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 1 is characterized in that: Based on the constant DC current control, constant turn-off angle control and current deviation control on the inverter side, the working process of the LCC-HVDC system includes: The LCC-HVDC system includes a rectifier side and an inverter side. Both the rectifier side and the inverter side are composed of LCC converter valves. The converter valve AC busbars on the rectifier side and the inverter side are equipped with AC filters and reactive power compensators. The rectifier side adopts constant DC current control, and its DC current reference signal is given by the inverter side; the inverter side is equipped with constant DC current control, fixed turn-off angle control and current deviation control; for the rectifier side, the controller compares the difference between the DC current command and the measured DC current, and generates the trigger lead angle command β of the rectifier side after the PI link. rec , and then generate the trigger angle signal α rec Control the action of the converter valve; for the inverter side, the two control strategies of fixed DC current control and fixed shutdown angle on the inverter side respectively generate the trigger lead angle command β on the inverter side after the DC current error and the shutdown angle error pass through the PI link inv_C With β inv_G , take the larger value of the two as the actual trigger advance angle of the inverter station, and then generate a trigger angle signal to control the operation of the converter valve.

4. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 3 is characterized in that: When the inverter side is in the constant DC current control mode, the DC current is controlled by the controllers on both the rectifier side and the inverter side. According to the current margin principle, the reference command in the constant DC current control on the inverter side is lower than that on the rectifier side. The function of the constant DC current control is to achieve a smooth transition between the constant DC current control and the constant turn-off angle control on the inverter side when the DC current command is not equal to the actual current. At the same time, the inverter side is equipped with a low-voltage current limiting control. When a fault occurs in the AC system on the inverter side, the low-voltage current limiting control detects the voltage at the midpoint of the DC line after the DC voltage on the inverter side is superimposed on the compensation resistor voltage drop. When it drops to the threshold value of the low-voltage current limiting control, the low-voltage current limiting control will adjust the DC current command and thus reduce the DC current to ensure safe operation of the system.

5. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 1 is characterized in that: The process of analyzing the first commutation failure and subsequent commutation failure of the LCC-HVDC system includes: When a short circuit occurs in the inverter-side AC system, the commutation voltage and the turn-off angle decrease accordingly. When the turn-off angle decreases to the minimum turn-off angle, the first commutation failure will occur. At this time, the DC side is short-circuited, the DC voltage decreases rapidly, and the DC current increases instantaneously. Then the control system starts to act, and the low-voltage current limiting control cooperates with the rectifier side to reduce the DC current and reduce the inverter-side commutation valve trigger angle. The commutation valve resumes normal commutation and enters the first commutation failure recovery stage. During the first commutation failure recovery phase, if the output DC current of the low-voltage current limiting control rises too fast or the speed of reducing the trigger angle by the fixed shutdown angle control cannot adapt to the reactive power demand of the converter valve under the fault, the shutdown angle continues to decrease, which will lead to subsequent commutation failures.

6. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 1, characterized in that: The first commutation failure is considered to occur if the following conditions occur: During the commutation of the LCC converter valve, if the converter valve that has just exited the commutation process has not yet restored its blocking capacity and is subjected to a forward voltage drop again, or if the commutation process of the converter valve has not been completed while it is subjected to a reverse voltage drop, the converter valve will be turned on again without a trigger signal; the electrical angle corresponding to the time required for the converter valve group to restore its forward blocking capacity is defined as the limit shut-off angle, also known as the minimum shut-off angle γ min ; If the turn-off angle of the inverter side is less than its minimum turn-off angle, it is considered that commutation failure has occurred; If the following conditions occur, subsequent commutation failure is considered to have occurred: During the recovery period after the first commutation failure, if the control system causes the turn-off angle to be less than the minimum turn-off angle γ again min ; then it is considered that subsequent commutation failure has occurred.

7. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 1, characterized in that: The control characteristics of the low-voltage current limiting control and the fixed turn-off angle control are combined, and the transient nonlinear differential equation about the turn-off angle is obtained by analyzing the dynamic characteristics of the DC current and the trigger advance angle during the first commutation failure recovery period, including: The DC current mathematical model of VDCOL is: in, is the per unit value of the DC line midpoint voltage after compensation, I dh and I dl are the upper and lower limits of DC current, U dl and U dh is the lower and upper limits of the DC voltage, I dn is the rated value of DC current; DC voltage U of LCC inverter dc It is expressed as: Among them, U L represents the effective value of the line voltage of the AC busbar on the inverter side; β and γ represent the trigger advance angle and the shutdown angle respectively; the DC line midpoint voltage after compensation is: in, is the DC line midpoint voltage after compensation, I dc is the average value of the sum of the upper and lower limits of the DC current, I dn is the rated value of DC current; The dynamic characteristics of DC current are expressed as: The turn-off angle γ passes through the fixed turn-off angle control structure on the inverter side, and the mathematical expression of the trigger advance angle is obtained as follows: β=(k p +k i / s)(c ref -c) Among them, β is the trigger lead angle, γ ref and γ are the cut-off angle reference value and the cut-off angle respectively, k i and k p These are the control parameters of the fixed turn-off angle control on the inverter side; The transient nonlinear differential equation about the cut-off angle is expressed as: Where γ is the cut-off angle, and They represent the derivative of the turn-off angle and the AC voltage respectively, and C3 and C4 are coefficients determined by the system parameters.

8. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 7 is characterized in that: The expressions of C3 and C4 are:

9. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 7, characterized in that: The criterion for predicting the occurrence of subsequent commutation failure includes: Calculate C1, C2, C3, C4 according to system parameters, and give the effective value of AC voltage at current time k and previous time k-1 Calculate the rate of change of the AC voltage at the current moment k Will and Bring in C SCF In the calculation expression of, the subsequent commutation failure prediction criterion C is calculated in real time SCF And determine its size; if C SCF is less than zero, it is considered that when the cut-off angle is reduced to γ min After that, it will still decrease, and subsequent commutation failure will occur; otherwise, it means that the turn-off angle will not decrease to γ min Below, no subsequent commutation failure fault will occur.

10. The method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claim 9, characterized in that: C SCF The calculation expression is:

11. An optimization method for suppressing the occurrence of subsequent commutation failure, based on the prediction of subsequent commutation failure by the method for predicting subsequent commutation failure of direct current transmission based on the shutdown angle dynamic equation according to claims 1-10, suppressing subsequent commutation failure, characterized in that: include: If the occurrence of subsequent commutation failure is predicted, the compensation amount Δγ of the turn-off angle γ is superimposed on the turn-off angle reference value of the fixed turn-off angle control on the inverter side, changing the dynamic characteristics of the turn-off angle during the first commutation failure recovery period and suppressing the occurrence of subsequent commutation failure faults.

12. The optimization method for suppressing subsequent commutation failure according to claim 11, characterized in that: According to the criterion of subsequent commutation failure, the setting value n is obtained. According to the setting value n, it is judged whether the LCC-HVDC system may have subsequent commutation failure or whether it will cause the DC voltage on the inverter side to be too low and reduce the transmission efficiency.

13. The optimization method for suppressing subsequent commutation failure according to claim 12, characterized in that: The expression of the setting value n is: Among them, γ ref is the cut-off angle, k i and k p is the control parameter of the fixed turn-off angle control on the inverter side, γ ref are the cut-off angle reference value, γ min is the minimum cut-off angle, C3 and C4 are coefficients determined by system parameters, It is the effective value of the line voltage of the AC bus on the inverter side of the system in steady state.

14. The optimization method for suppressing subsequent commutation failure according to claim 13, characterized in that: The expression of compensation amount Δγ is: Among them, U L is the AC voltage, is the rate of change of AC voltage.

Images