Prediction method and device for continuous commutation failure of LCC-HVDC system

Abstract

The invention discloses a method and a device for predicting continuous commutation failure of an LCC-HVDC system, relates to the technical field of direct current transmission, and solves the problems existing in continuous commutation failure prediction of an existing LCC-HVDC system. The method comprises the following steps: calculating a minimum trigger angle after a corresponding commutation line voltage fault according to a trigger angle and a trigger angle lead amount; according to the commutation line voltage, calculating phase movement after the corresponding commutation line voltage fault; according to the commutation line voltage, the phase movement after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault and the minimum turn-off angle limit value of the inverter, the voltage time commutation area which can be provided by the current system is calculated; according to the minimum turn-off angle limit value of the inverter, calculating the minimum voltage time area required by commutation of the current inverter; and comparing the voltage time commutation area which can be provided by the current system with the minimum voltage time area required by commutation of the current inverter, and judging whether continuous commutation failure occurs or not.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission, and more specifically, to a method and device for predicting continuous commutation failure of an LCC-HVDC system. Background Art

[0002] High voltage direct current (HVDC) systems based on line converters (LCCs) are widely used to achieve long-distance and large-capacity power transmission. Commutation failure (CF), a frequent dynamic event in inverters of LCC-HVDC systems, is mainly caused by AC system faults, most of which are single-line ground faults (SLG). CF can be divided into three categories: first commutation failure (FCF), non-continuous commutation failure (NCCF), and continuous commutation failure (CCF). FCF refers to CF that lasts only one cycle after the fault occurs; NCCF refers to CF that occurs intermittently after FCF, especially during the recovery process; CCF refers to CF that occurs continuously after FCF and may continue until the protection system is activated to clear the fault. Compared with NCCF, CCF is of particular concern because it will bring about a continuous power interruption and may cause the inverter to block, thereby terminating the transmission of active power. This may further lead to: 1) tripping of the generator on the rectifier side and load shedding on the inverter side; 2) overloading of the parallel AC line and may cause cascading tripping.

[0003] In view of the above challenges, the present application provides a method and device for predicting continuous commutation failure of an LCC-HVDC system to solve the above problems. Summary of the invention

[0004] The purpose of this application is to provide a method and device for predicting continuous commutation failure in an LCC-HVDC system. This application calculates the phase shift (asymmetric lead angle difference AAAD) after the corresponding commutation line voltage fault, which is used to evaluate the impact of the commutation voltage phase angle offset and amplitude change on the commutation process, and uses AAAD to predict continuous commutation failure CCF under asymmetric faults.

[0005] The present application first provides a prediction method for continuous commutation failure of an LCC-HVDC system, comprising: S1, obtaining a commutation line voltage, a trigger angle, a trigger angle advance amount, and an inverter minimum turn-off angle limit; S2, calculating the minimum trigger angle after a corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance amount; S3, calculating the phase shift after a corresponding commutation line voltage fault according to the commutation line voltage; S4, calculating the voltage-time commutation area that can be provided by the current system according to the commutation line voltage, the phase shift after a corresponding commutation line voltage fault, the minimum trigger angle after a corresponding commutation line voltage fault, and the inverter minimum turn-off angle limit; S5, calculating the minimum voltage-time area required for the current inverter commutation according to the inverter minimum turn-off angle limit; S6, comparing the voltage-time commutation area that can be provided by the current system with the minimum voltage-time area required for the current inverter commutation, and determining whether a continuous commutation failure occurs in the LCC-HVDC system.

[0006] In a possible implementation, S2, according to the trigger angle and the trigger angle advance amount, calculate the minimum trigger angle corresponding to the commutation line voltage fault; the minimum trigger angle α corresponding to the commutation line voltage fault f.min , obtained by the following formula:

[0007] α f.min =α 0 -Δα max

[0008] Among them, α 0 is the initial trigger angle, Δα max is the maximum trigger angle advance; the trigger angle after the AB phase commutation line voltage fault is expressed as The trigger angle after the BC phase commutation line voltage fault is expressed as The trigger angle after the CA phase commutation line voltage fault is expressed as:

[0009] In a possible implementation, S3, according to the commutation line voltage, calculate the phase shift of the commutation voltage corresponding to the commutation process; the phase shift Δθ of the commutation voltage corresponding to the commutation process i , obtained by the following formula:

[0010]

[0011] in, and They are respectively the phase angle movement deviation of the CA phase commutation line voltage after the fault, the phase angle movement deviation of the AB phase commutation line voltage and the phase angle movement deviation of the BC phase commutation line voltage.

[0012] In a possible implementation, S4, according to the commutation line voltage, the phase angle movement deviation after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault and the minimum turn-off angle limit of the inverter, the voltage-time commutation area that the current system can provide is calculated; the voltage-time commutation area that the current system can provide It is obtained by the following formula:

[0013]

[0014] Among them, VTA if.max is the maximum voltage-time commutation area provided by the AC system to the inverter valve during the i-th commutation process, α f.min is the minimum trigger angle after the commutation line voltage fault, Δθ i is the phase shift of the commutation voltage corresponding to the commutation process, Δθ PLL is the phase shift of the phase-locked loop output, γ min is the minimum turn-off angle limit of the inverter, V if (t) is the commutation line voltage corresponding to the i-th commutation process.

[0015] In a possible implementation, S6, comparing the voltage-time commutation area that can be provided by the current system with the minimum voltage-time area required for the current inverter commutation, and determining whether the LCC-HVDC system has a continuous commutation failure; the determination is made by the following judgment formula:

[0016]

[0017] in, VTA is the voltage-time commutation area that the current system can provide. req.min It is the minimum voltage-time area required for the current inverter commutation. No CCF means that there is no continuous commutation failure in the current system, and CCF means that there is a continuous commutation failure in the current system.

[0018] The present application also provides a prediction device for continuous commutation failure of an LCC-HVDC system, comprising: a data acquisition unit, used to acquire a commutation line voltage, a trigger angle, a trigger angle advance amount, and a minimum turn-off angle limit of an inverter; a minimum trigger angle calculation unit, used to calculate the minimum trigger angle after a corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance amount; a phase shift calculation unit, used to calculate the phase shift after a corresponding commutation line voltage fault according to the commutation line voltage; a provided VTA calculation unit, used to calculate the voltage-time commutation area that can be provided by the current system according to the commutation line voltage, the phase shift after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault, and the minimum turn-off angle limit of the inverter; a required VTA calculation unit, used to calculate the minimum voltage-time area required for the current inverter commutation according to the minimum turn-off angle limit of the inverter; and a CCF judgment unit, used to compare the voltage-time commutation area that can be provided by the current system with the minimum voltage-time area required for the current inverter commutation, and to judge whether a continuous commutation failure occurs in the LCC-HVDC system.

[0019] In a possible implementation manner, the minimum trigger angle calculation unit is specifically used to execute the following formula to obtain the minimum trigger angle α corresponding to the commutation line voltage fault. f.min :

[0020] α f.min =α 0 -Δα max

[0021] Among them, α 0 is the initial trigger angle, Δα max is the maximum trigger angle advance; the trigger angle after the AB phase commutation line voltage fault is expressed as The trigger angle after the BC phase commutation line voltage fault is expressed as The trigger angle after the CA phase commutation line voltage fault is expressed as:

[0022] In a possible implementation manner, the phase shift calculation unit is specifically configured to execute the following formula to obtain the phase shift Δθ of the commutation voltage corresponding to the commutation process: i :

[0023]

[0024] in, and They are respectively the phase angle movement deviation of the CA phase commutation line voltage after the fault, the phase angle movement deviation of the AB phase commutation line voltage and the phase angle movement deviation of the BC phase commutation line voltage.

[0025] In a possible implementation, a VTA calculation unit is provided, which is specifically used to execute the following formula to obtain the voltage-time commutation area that the current system can provide:

[0026]

[0027] Among them, VTA if.max is the maximum voltage-time commutation area provided by the AC system to the inverter valve during the i-th commutation process, α f.min is the minimum trigger angle after the commutation line voltage fault, Δθ i is the phase shift of the commutation voltage corresponding to the commutation process, Δθ PLL is the phase shift of the phase-locked loop output, γ min is the minimum turn-off angle limit of the inverter, V if (t) is the commutation line voltage corresponding to the i-th commutation process.

[0028] In a possible implementation manner, the CCF determination unit is specifically configured to execute the following determination formula to determine whether a continuous commutation failure occurs in the LCC-HVDC system.

[0029]

[0030] in, VTA is the voltage-time commutation area that the current system can provide. req.min It is the minimum voltage-time area required for the current inverter commutation. No CCF means that there is no continuous commutation failure in the current system, and CCF means that there is a continuous commutation failure in the current system.

[0031] Compared with the prior art, the present application has the following beneficial effects: an indicator of phase shift (asymmetric lead angle difference AAAD) corresponding to a commutation line voltage fault is proposed to evaluate the influence of commutation voltage phase angle offset and amplitude change on the commutation process, and the indicator is used to predict continuous commutation failure (CCF). Compared with the research on general commutation failure prediction, the present application emphasizes the prediction of CCF under asymmetric faults, and its importance lies in that CCF has a serious impact on the LCC-HVDC system that cannot be ignored. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0033] Figure 1 A flow chart of a method for predicting continuous commutation failure of an LCC-HVDC system provided in an embodiment of the present application;

[0034] Figure 2A structural diagram of a device for predicting continuous commutation failure of an LCC-HVDC system provided in an embodiment of the present application;

[0035] Figure 3 Schematic diagram of the CIGRE LCC-HVDC benchmark system.

[0036] Figure 4 is the equivalent single-line diagram of the fault phase;

[0037] Figure 5 for and E 1f The phasor diagram of

[0038] Figure 6 for and E 2f The phasor diagram of

[0039] Figure 7 For E CV The phasor diagram of

[0040] Figure 8 is the phasor diagram of the phase commutation voltage and the line commutation voltage;

[0041] Fig. 9 Schematic diagram of line commutation voltage during an asymmetric fault and PLL output during an asymmetric fault;

[0042] Fig.10 is a schematic diagram of a control system of LCC-HVDC;

[0043] Fig.11 It is a stage diagram of a zero-impedance single-phase-to-ground (SLG) fault occurring on the system bus 4;

[0044] Fig.12 It is a schematic diagram of the arc extinction angle after the system restores FCF;

[0045] Fig.13 It is the commutation process under asymmetric fault;

[0046] Fig.14 This is a schematic diagram of a 6-machine 2-area (6M2A) system;

[0047] Fig.15 is the commutation voltage on inverter bus 1 when T2 transmission line fails Schematic diagram of calculation and simulation results;

[0048] Fig.16 Schematic diagram of the phase angle change of the phase voltage at node 1B;

[0049] Fig.17 It is a schematic diagram of AAAD with different commutation processes when a single-phase grounding fault occurs;

[0050] Fig.18 Schematic diagram of CCF prediction. DETAILED DESCRIPTION

[0051] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with examples and drawings. The illustrative implementation scheme of the present application and its description are only used to explain the present application and are not intended to limit the present application.

[0052] First, some of the terms and related technologies involved in this application are explained to facilitate understanding by those skilled in the art.

[0053] 1. Trigger angle (α): The trigger angle refers to the phase angle at which the thyristor starts to conduct.

[0054] 2. Lead trigger angle (β): The lead trigger angle is a specific parameter on the inverter side, which refers to the advance angle of the inverter thyristor relative to the negative zero crossing point of the ideal sinusoidal commutation voltage.

[0055] 3. Trigger angle advance (Δα): The trigger angle advance refers to the adjustment amount of the control system on the inverter side trigger angle under specific circumstances, such as system failure or other times that require adjustment.

[0056] 4. Overlap angle (μ): The overlap angle refers to the overlapping time period of two adjacent thyristors on the inverter side.

[0057] 5. Arc extinction angle (γ): The arc extinction angle refers to the angle before the AC voltage crosses the zero point when the thyristor changes from the on state to the off state on the inverter side.

[0058] 6. Three-phase commutation voltage E A 、E B 、E C :The three-phase commutation voltage refers to the commutation voltage of each phase in the three-phase system. The phase shift of the three-phase commutation voltage is Δθ A , Δθ B and Δθ C ;

[0059] 7. Three-phase line-to-line (LL) commutation voltage E AB 、E BC 、E CA :Line-to-line commutation voltage refers to the voltage difference between any two phases in a three-phase system. The phase shift of the three-phase line-to-line commutation voltage is Δθ AB , Δθ BC and Δθ CA .

[0060] 8. FCF (First Commutation Failure): FCF refers to the first commutation failure event caused by AC side fault or other disturbance in the LCC-HVDC system.

[0061] 9. CCF (Continuous Commutation Failure): CCF refers to the commutation failure that occurs again in one or more subsequent grid cycles due to improper system control or other factors after the first commutation failure (FCF).

[0062] 10. CCFs (Multiple Continuous Commutation Failures): CCFs refers to the situation where continuous commutation failures occur multiple times in the power system.

[0063] See also Figure 1 As shown, Figure 1 A flow chart of a method for predicting continuous commutation failure of an LCC-HVDC system provided in an embodiment of the present application. The method includes: S1, obtaining the commutation line voltage, trigger angle, trigger angle advance amount and inverter minimum turn-off angle limit; S2, calculating the minimum trigger angle after the corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance amount; S3, calculating the phase shift after the corresponding commutation line voltage fault according to the commutation line voltage; S4, calculating the voltage-time commutation area that the current system can provide according to the commutation line voltage, the phase shift after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault and the inverter minimum turn-off angle limit; S5, calculating the minimum voltage-time area required for the current inverter commutation according to the inverter minimum turn-off angle limit; S6, comparing the voltage-time commutation area that the current system can provide with the minimum voltage-time area required for the current inverter commutation, and determining whether the LCC-HVDC system has a continuous commutation failure.

[0064] Specifically, Figure 1 In step S1, the commutation line voltage (LL Commutation Voltages) is obtained. and and They are the AB phase commutation line voltage, the BC phase commutation line voltage and the CA phase commutation line voltage respectively; obtain the trigger angle α, the trigger angle advance Δα and the inverter minimum turn-off angle limit γ min In step S2, the minimum trigger angle after the corresponding commutation line voltage fault is calculated according to the trigger angle α and the trigger angle advance Δα. and and They correspond to the trigger angle after the commutation line voltage fault. In step S3, according to the commutation line voltage and Calculate the phase shift of the corresponding commutation line voltage after a fault In step S4, according to the commutation line voltage and Phase shift after the corresponding commutation line voltage fault Minimum trigger angle after corresponding commutation line voltage fault and Inverter minimum turn-off angle limit γ min , calculate the voltage-time commutation area that the current system can provide In step S5, according to the inverter minimum turn-off angle limit γ min , calculate the minimum voltage time area VTA required for the current inverter commutation req.min In step S6, compare the voltage-time switching area that the current system can provide and the minimum voltage time area VTA required for the current inverter commutation req.min , determine whether continuous commutation failures CCFs will occur in the LCC-HVDC system in the future.

[0065] In a possible implementation, S2, according to the trigger angle and the trigger angle advance amount, calculate the minimum trigger angle corresponding to the commutation line voltage fault; the minimum trigger angle α corresponding to the commutation line voltage fault f.min , obtained by the following formula:

[0066] α f.min =α 0 -Δα max

[0067] Among them, α 0 is the initial trigger angle, Δα max is the maximum trigger angle advance; the trigger angle after the AB phase commutation line voltage fault is expressed as The trigger angle after the BC phase commutation line voltage fault is expressed as The trigger angle after the CA phase commutation line voltage fault is expressed as:

[0068] In a possible implementation, S3, according to the commutation line voltage, calculate the phase shift of the commutation voltage corresponding to the commutation process; the phase shift Δθ of the commutation voltage corresponding to the commutation process i , obtained by the following formula:

[0069]

[0070] in, and They are respectively the phase angle movement deviation of the CA phase commutation line voltage after the fault, the phase angle movement deviation of the AB phase commutation line voltage and the phase angle movement deviation of the BC phase commutation line voltage.

[0071] In a possible implementation, S4, according to the commutation line voltage, the phase angle movement deviation after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault and the minimum turn-off angle limit of the inverter, the voltage-time commutation area that the current system can provide is calculated; the voltage-time commutation area that the current system can provide It is obtained by the following formula:

[0072]

[0073] Among them, VTA if.max is the maximum voltage-time commutation area provided by the AC system to the inverter valve during the i-th commutation process, α f.min is the minimum trigger angle after the commutation line voltage fault, Δθ i is the phase shift of the commutation voltage corresponding to the commutation process, Δθ PLL is the phase shift of the phase-locked loop output, γ min is the minimum turn-off angle limit of the inverter, V if (t) is the commutation line voltage corresponding to the i-th commutation process.

[0074] In a possible implementation, S6, comparing the voltage-time commutation area that can be provided by the current system with the minimum voltage-time area required for the current inverter commutation, and determining whether the LCC-HVDC system has a continuous commutation failure; the determination is made by the following judgment formula:

[0075]

[0076] in, VTA is the voltage-time commutation area that the current system can provide. req.min It is the minimum voltage-time area required for the current inverter commutation. No CCF means that there is no continuous commutation failure in the current system, and CCF means that there is a continuous commutation failure in the current system.

[0077] See also Figure 2 As shown, Figure 2A structural diagram of a prediction device for continuous commutation failure of an LCC-HVDC system provided in an embodiment of the present application. The prediction device includes: a data acquisition unit for acquiring a commutation line voltage, a trigger angle, a trigger angle advance amount, and an inverter minimum turn-off angle limit; a minimum trigger angle calculation unit for calculating the minimum trigger angle after a corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance amount; a phase shift calculation unit for calculating the phase shift after a corresponding commutation line voltage fault according to the commutation line voltage; a provision VTA calculation unit for calculating the voltage-time commutation area that the current system can provide according to the commutation line voltage, the phase shift after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault, and the inverter minimum turn-off angle limit; a demand VTA calculation unit for calculating the minimum voltage-time area required for the current inverter commutation according to the inverter minimum turn-off angle limit; and a CCF judgment unit for comparing the voltage-time commutation area that the current system can provide with the minimum voltage-time area required for the current inverter commutation, and judging whether a continuous commutation failure occurs in the LCC-HVDC system.

[0078] In a possible implementation manner, the minimum trigger angle calculation unit is specifically used to execute the following formula to obtain the minimum trigger angle α corresponding to the commutation line voltage fault. f.min :

[0079] α f.min =α 0 -Δα max

[0080] Among them, α 0 is the initial trigger angle, Δα max is the maximum trigger angle advance; the trigger angle after the AB phase commutation line voltage fault is expressed as The trigger angle after the BC phase commutation line voltage fault is expressed as The trigger angle after the CA phase commutation line voltage fault is expressed as:

[0081] In a possible implementation manner, the phase shift calculation unit is specifically configured to execute the following formula to obtain the phase shift Δθ of the commutation voltage corresponding to the commutation process: i :

[0082]

[0083] in, and They are respectively the phase angle movement deviation of the CA phase commutation line voltage after the fault, the phase angle movement deviation of the AB phase commutation line voltage and the phase angle movement deviation of the BC phase commutation line voltage.

[0084] In a possible implementation, a VTA calculation unit is provided, which is specifically used to execute the following formula to obtain the voltage-time commutation area that the current system can provide:

[0085]

[0086] Among them, VTA if.max is the maximum voltage-time commutation area provided by the AC system to the inverter valve during the i-th commutation process, α f.min is the minimum trigger angle after the commutation line voltage fault, Δθ i is the phase shift of the commutation voltage corresponding to the commutation process, Δθ PLL is the phase shift of the phase-locked loop output, γ min is the minimum turn-off angle limit of the inverter, V if (t) is the commutation line voltage corresponding to the i-th commutation process.

[0087] In a possible implementation manner, the CCF determination unit is specifically configured to execute the following determination formula to determine whether a continuous commutation failure occurs in the LCC-HVDC system.

[0088]

[0089] in, VTA is the voltage-time commutation area that the current system can provide. req.min It is the minimum voltage-time area required for the current inverter commutation. No CCF means that there is no continuous commutation failure in the current system, and CCF means that there is a continuous commutation failure in the current system.

[0090] It should be noted that the device for predicting continuous commutation failure of the LCC-HVDC system is used to implement the method for predicting continuous commutation failure of the LCC-HVDC system, corresponds to the method one by one, and has corresponding effects, so it is not elaborated herein.

[0091] See also Figure 3-14 The present application also studies the influence of phase shift and amplitude change on commutation under asymmetric faults and the influence of control system on commutation under asymmetric faults. The formation process of the prediction method for continuous commutation failure of LCC-HVDC system provided by the present application is described.

[0092] In order to develop a commutation failure (CCF) prediction method under asymmetric faults, two aspects need to be considered: first, the impact of the phase shift and amplitude change of the commutation voltage on the commutation process under asymmetric faults; second, the impact of the control system on the commutation process under asymmetric faults. Based on the first two points, a CCF prediction method for symmetric faults is finally formed.

[0093] First, the influence of the phase shift and amplitude change of the commutation voltage on the commutation process under asymmetric fault.

[0094] About the phase and amplitude changes of the commutation voltage. In order to illustrate the phase and amplitude changes of the commutation voltage, the CIGRE LCC-HVDC reference system is used, such as Figure 3 As shown. To simplify the analysis, the inverter is roughly regarded as a current source. This assumption has little effect on the accuracy of the commutation failure (CCF) prediction. According to the superposition theorem, the equivalent single-line diagram of the fault phase can be drawn, as shown in Figure 4 shown.

[0095] Depend on Figure 4 It can be seen that the commutation voltage E CV You can do this by Figure 2 (a) and Figure 2 The corresponding voltages in (b) are added together to calculate.

[0096] E CV =E 1f +E 2f (1)

[0097] 1) By Figure 4 (a) It can be seen that for a circuit with a voltage source, the commutation voltage E 1f It can be calculated as follows:

[0098]

[0099] in, is from the voltage source through Z 0 The current flowing to the fault point; It's Z 0 and Z 1f The series impedance of 1f It's Z f (fault impedance) and Z 1 The parallel impedance of the converter transformer is assumed to be resistive. f The impedance varies from 0.1 to 10 ohms based on formula (4) Figure 5 Shown in.

[0100] exist Figure 5 middle, express The pre-fault value (i.e. Z f =+∞); is its phase angle. 1f and The endpoint trajectory of is a part of a circle, which can be derived from formula (4). Figure 5 (a) It can be seen that when a fault occurs, Z 1fThe amplitude of the fault continues to decrease, and its phase angle continues to lag behind the state before the fault (see Figure 5 (dashed line trajectory in (a)).

[0101] Assumption E s is the reference voltage, different Z f The voltage phasor diagram is as follows Figure 5 (b) as shown. Is the fault period through Z 0 The current from Figure 5 (b) It can be observed that as the fault worsens, the voltage E 1f Continuous decrease and hysteresis compared to the pre-fault value.

[0102] 2) By Figure 4 (b) It can be seen that for a circuit with a current source, the commutation voltage E 2f It can be calculated as:

[0103]

[0104] Assuming the reference phase angle of the current source is zero, With Z f The relationship can be as follows Figure 6 (a) shows that the commutation voltage E 2f The phasor diagram of Figure 6 (b) in.

[0105] Figure 6 It shows that as the fault impedance decreases, the voltage E on the inverter bus 2f By superimposing the above two circuits according to formula (1), the final commutation voltage E CV The phasor diagram can be drawn as Figure 7 .

[0106] The influence of phase shift and amplitude change on the commutation process under asymmetric fault conditions. Under asymmetric fault conditions, the phasor diagram of the three-phase commutation voltage is shown in Figure 8 (a). Figure 8 (a) shows that: 1) under asymmetric faults, the amplitudes of all three-phase voltages decrease and the phase angles lag; 2) the changes and phase shifts of the three-phase commutation voltages under asymmetric faults are different from each other.

[0107] The asymmetry of the phase shift and amplitude change of the three phases will lead to an imbalance between the commutation voltages. Figure 8 (a), the phasor diagram of the three-phase line-to-line (LL) commutation voltage is drawn on Figure 8 (b) From Figure 8(b) It can be seen that the phase shift and amplitude change of the three-phase line-to-line commutation voltages are different from each other, which means that the phase angle difference between any two-phase line-to-line commutation voltages is no longer equal to 120 electrical degrees.

[0108] This change in line-to-line commutation voltage has a direct impact on the commutation process. The commutation voltage before and after an asymmetric fault is shown in Fig. 9 middle.

[0109] Fig. 9 The black solid line and dotted line in the figure represent the three-phase line-to-line commutation voltage waveform and phase-locked loop (PLL) output before the fault, respectively, while the red solid line represents the situation during an asymmetric fault. It can be seen that:

[0110] In the first cycle after the fault occurs, a) the amplitude of the commutation voltage decreases immediately, which has an adverse effect on the commutation process; while the phase lag is beneficial to the commutation process. The DC current increases to a relatively high level, and if the DC current is not successfully commutated, a commutation failure (CF) will occur.

[0111] In the next few cycles, the PLL tracks the phase angle of the commutation voltage and generates a gradually lagging sawtooth reference waveform. The asymmetrically distributed zero crossings of the three-phase line-to-line commutation voltage have a significant impact on the overlap and arc extinction angles, thereby affecting the commutation process. In summary, a phase angle between two adjacent zero crossings of the three-phase line-to-line commutation voltage less than 120 electrical degrees may lead to insufficient arc extinction angles; while a phase angle greater than 120 electrical degrees will be beneficial to the commutation process; once the arc extinction angle is less than the minimum value required by the thyristor, commutation failure (CF) will occur.

[0112] Second, the influence of the control system on the commutation process under asymmetric faults. Fig.14 Taking the LCC-HVDC control system in the 6-machine 2-zone (6M2A) system as an example, the LCC-HVDC control system is as follows: Fig.10 As shown. The DC current is controlled by constant current (CC) on the rectifier side, and the DC voltage is controlled by constant voltage (CV) on the inverter side. The inverter is equipped with minimum arc extinction angle (CEA) control and CC control. CEC and VDCOL represent current error control and current-dependent voltage limiter, respectively. α_(inv_order) is the firing angle command on the inverter side, i.e., the Alpha order (AOI) of the inverter.

[0113] About the influence of control system on commutation. Fig.11The valve current and firing angle command of the system are shown when a zero-impedance single-phase-to-ground (SLG) fault occurs on bus 4 for 140 milliseconds, and the situation when CCF occurs is shown in the figure. It can be seen that the inverter is under CV control at the beginning, and then under CEA and CC control at the same time in the first two cycles after the fault. After t=0.15 seconds, it is mainly controlled by CEA, and the influence of CC control is small. In order to explain this process in detail, the fault situation is divided into the following stages:

[0114] Phase 1 (from t = 0.07 seconds to t = 0.08 seconds: FCF occurs): In this phase, the increase in DC current and the decrease in commutation voltage will prolong the commutation process, thereby compressing the arc extinction angle. In addition, CV control will increase AOI to maintain the DC voltage level, which further reduces the margin of the arc extinction angle, resulting in the occurrence of FCF.

[0115] Phase 2 (from t = 0.08 seconds to t = 0.11 seconds: recovery from FCF): After FCF occurs, the output of CV control continues to increase, while the output of CEA control continues to decrease. As a result, the final AOI will be determined by CEA control, such as Fig.11 As shown at t = 0.08 seconds. However, due to parameter settings, CEA control changes the output slowly, so the commutation process cannot recover from FCF before the AOI of CC control drops to the minimum value. At this stage, the AOI of the inverter valve group is determined by CEA control and CC control, as shown in Fig.11 As shown from t = 0.08 seconds to t = 0.11 seconds.

[0116] Phase 3 (from t=0.11 sec to t=0.21 sec: CCF continues until fault is removed): Starting from t=0.11 sec, CEA control controls AOI for the next five cycles. Under this fault condition, the commutation voltage cannot provide enough VTA for commutation after FCF, so CF occurs again at t=0.11 sec. Once CF occurs, CEA control continues to control AOI and reduce its output in an attempt to maintain the arc extinction angle. Due to insufficient commutation voltage, CF continues to occur in each subsequent cycle, resulting in CCF. Therefore, in this phase, the system continues to operate in CCF until the fault is removed.

[0117] Fig.12 It shows that after the system recovers FCF at t = 0.09 seconds, the measured minimum arc extinction angle reaches zero again at t = 0.102 seconds until the fault ends. This shows that CCF cannot be alleviated by adjusting the control system at this stage.

[0118] It has been noted that Fig.12As shown, the measured value of the arc extinction angle is the minimum value. Under balanced fault, the arc extinction angles of the valve groups are almost the same due to the three-phase balance. However, under asymmetrical fault, the minimum value of the arc extinction angle will be affected by the imbalance of the commutation voltage.

[0119] Phase 4 (from t = 0.21 seconds to t = 0.23 seconds: recovery from CCF after fault removal): After the fault is removed, CCF cannot be eliminated immediately. The reason is that after the fault is removed, the phase angle of the commutation voltage will be relative to the conduction during the fault (see the analysis in Section II.B). Therefore, the AOI controlled by CEA continues to decrease until the CCF disappears, as shown in Figure 2. Fig.11 As shown from t = 0.21 seconds to t = 0.23 seconds. Once the CCF disappears, the AOI controlled by the CEA will increase to keep the arc extinction angle close to its reference value.

[0120] However, increasing the firing angle in order to increase the DC voltage will also reduce the DC current due to the reduction in the voltage difference between the two converters. Fig.11 The reason why the CC controlled AOI drops sharply at t = 0.23 seconds is that the reduction of DC current leads to a reduction in the CC controlled AOI output, the purpose of which is to keep the DC current at a preset level, such as Fig.11 As shown at t = 0.23 seconds. Therefore, the lower AOI of CC control will further increase the voltage difference between the two converters and then maintain the DC current at its reference level.

[0121] Phase 5 (after t=0.23 seconds: recovery from unbalanced state to balanced state): The recovery process is mainly affected by the parameters of the AC system and the control system.

[0122] Summarizing the above stages 1 to 5, we can see that stage 3 has the most serious negative impact on the system. First, due to the abnormally long conduction time during CCF, there is a risk of overheating of the thyristors. Second, CCF may cause the HVDC link to be blocked. Finally, the blocking of the HVDC link may cause the collapse of the adjacent transmission link, which in turn leads to a blackout in the power system.

[0123] Derivation of the maximum value of the control system's influence on commutation - the maximum lead angle. CEA control dominates the AOI during a fault, and its purpose is to ensure a sufficient arc extinction angle to successfully complete the commutation. Therefore, it is very important to theoretically analyze the influence of the control system on the commutation process. This section will quantify the maximum influence of the control system on commutation.

[0124] In order to consider the influence of the control system, it is necessary to consider the conduction angle Δα of the control system to the AOI during the fault. As the fault worsens, the conduction angle Δα will increase. In an asymmetric fault, the maximum conduction angle Δα max It can be obtained under the most extreme fault condition, i.e., a zero-impedance single-phase-to-ground fault on the inverter bus.

[0125] CEA control dominates AOI in the third stage, from Fig.11 The CEA output trend line shown can be approximated as a straight line with a constant slope. Therefore, Δα max It can be obtained by multiplying the slope and the control duration. According to the topology of CEA control in the commonly used LCC-HVDC, the slope k_(trendline_CCF) can be derived as:

[0126] k_(trendline_CCF)=γ_min·(P_CEA / t+1 / T_I)(7)

[0127] Where P_CEA and T_I are the proportional coefficient and the time constant of the integral part of the PI controller in CEA control. Therefore, the maximum conduction trigger angle Δα can be obtained max :

[0128] Δα max =k_(trendline_CCF)γt_max(8)

[0129] Where t_max is the maximum time duration during which the control system adjusts the firing angle sequence during a fault, which can be regarded as the maximum fault duration before the circuit breaker is activated. In summary, the maximum influence of the control system on the commutation, i.e. the maximum conduction firing angle Δα max , can be determined by using the slope of the AOI trend line under the CCF fault condition and the maximum fault duration through formula (8). max Will be used to predict CCF.

[0130] Third, predicting CCF under asymmetric faults. Based on the analysis in the first and second parts above, this part proposes a prediction method for continuous commutation failure (CCF) of LCC-HVDC system under asymmetric faults based on VTA theory. Next, the detailed commutation process when CCF occurs is described, and then the CCF prediction method is described.

[0131] Regarding the detailed commutation process when CCF occurs. In order to predict CCF, the following two factors need to be considered when using VTA theory:

[0132] The actual overlap angle of the commutation process during a fault. To do this, the actual trigger instant and the negative to positive zero crossing of the commutation voltage need to be estimated. The actual trigger instant is the intersection of the AOI and the PLL output.

[0133] The actual commutation voltage during the fault. To determine these values, as described in the first section, every two adjacent commutation voltages are considered as a pair of commutation voltages, which will affect the corresponding commutation process. Due to the phase shift Δθ of the PLL output PLLThe maximum and minimum phase shifts Δθ at the commutation voltage CA \ and Δθ AB As described in the first part, the reference of the trigger time is earlier than the zero crossing point of the most relatively lagging commutation voltage, but later than the zero crossing point of the most relatively conducting commutation voltage, such as Fig.13 shown. Fig.13 (a) shows the I before and during the fault under an asymmetric fault. T2 →I T4 and I T4 →I T6 Comparison of the commutation process. It can be seen that:

[0134] Under asymmetrical fault conditions, the commutation voltages have different amplitudes and phase shifts. and The phase shifts are marked as Δθ CA and Δθ AB In order to analyze without loss of generality, we assume And Δθ CA is the largest, and Δθ AB is the smallest (this assumption will not affect the final conclusion);

[0135] I T4 in I T2 →I T4 The trigger moment in the process is advanced, which will increase I T2 The arc extinction angle margin is beneficial to the commutation process;

[0136] I T4 →I T6 The arc extinction angle γ of the process f4-6 Will be I T2 →I T4 The arc extinction angle γ of the process f2-4 4 is much smaller. If the amplitude changes are the same and the overlap angle is the same, the difference between them is |Δθ CA -Δθ AB ||As expressed in formula (9):

[0137] γ f2-4 -γ f4-6 =Δθ CA -Δθ AB (9)

[0138] But due to |E CAf ∣>∣E ABf ∣, according to VTA theory, affected by E ABf The overlap angle μ of the IT4→IT6 process affected f4-6 will be more affected by E CAf The overlap angle μ of the IT2→IT4 process affected f2-4Therefore, γ f2-4 and γ f4-6 The difference between them will be greater than |ΔθCA-ΔθAB|, as shown in formula (10):

[0139] γ f2-4 -γ f4-6 >Δθ CA -Δθ AB (10)

[0140] In order to consider both formulas (9) and (10), the leading firing angle β is used, which is equal to the sum of the commutation overlap angle μ and the arc extinction angle γ. Then, the difference between the leading firing angles of the above two commutation processes can be expressed by formula (11):

[0141] Δβ 2-4 / 4-6 =β f2-4 -β f4-6 =(μ f2-4 +γ f2-4 )-(μ f4-6 +γ f4-6 )(11)

[0142] =Δθ CA -Δθ AB

[0143] Δβ 2-4 / 4-6 represents the difference between the phase shifts of the two corresponding commutation voltages under an asymmetrical fault. This can have a significant adverse effect, causing CF to occur due to the mismatch between the firing angle reference instant and the commutation voltage zero crossing; once the minimum extinction angle γ f4-6 Less than the required minimum extinction angle γ min , CF will occur. Fig.13 (b) shows the six commutation processes in the inverter under an asymmetric fault. It can be seen that the insufficient extinction angle will affect the corresponding commutation processes in the upper thyristor arm and the lower thyristor arm, i.e., I T1 →I T3 and I T4 →I T6 The CCF will persist until the fault is removed.

[0144] In summary, the relatively lagging LL commutation voltage and the next relatively conducting LL commutation voltage will lead to a phase angle mismatch between the firing angle reference instant and the actual zero crossing point of the commutation voltage under an asymmetric fault. This mismatch has not been considered in existing studies, but will significantly reduce the upcoming commutation margin and lead to the occurrence of CCF.

[0145] Regarding the prediction method of CCF: Based on the above analysis of the detailed commutation process when CCF occurs, it is necessary to estimate the commutation voltage and overlap angle before predicting CCF.

[0146] according to Fig.13 And formula (11), the leading trigger angle of each commutation process can be derived:

[0147] β f2-4 =μ f2-4 +γ f2-4 =180°+Δθ CA -Δθ PLL -α f (12)

[0148] β f4-6 =μ f4-6 +γ f4-6 (13)

[0149] α f.min =α 0 -Δα max (14)

[0150] where α f.min It is the minimum firing angle before the fault is removed, which enables the AC system to provide the maximum VTA value to the inverter valve. 0 is the initial firing angle. Formulas (11)-(13) are used to calculate AAADij in Table 1 below.

[0151]

[0152] Where i = 1, 2, ..., 6 is the sequence of six commutation processes in one cycle; V if is the LL commutation voltage corresponding to the i-th commutation process, Δθi is its phase shift; VTA if.max It represents the maximum VTA value provided by the AC system at the minimum AOI of the i-th commutation process under fault conditions. It should be noted that the starting point of the integration is not α f.min The reason is that α f.min It does not start from the actual zero crossing of the corresponding commutation voltage. f.min The starting moment of the commutation voltage is Δθi-ΔθPLL ahead of the actual commutation voltage zero crossing point, which is referred to as "asymmetric conduction angle (AAA)" in this application. Fig.13 shown.

[0153] By comparing AAA with formula (11), Δβ 2-4 / 4-6 is the AAA difference between the i-th and j-th commutation processes (AAAD ij ).

[0154] AAAD ij =AAA i -AAA j =(Δθ i -Δθ j )(16)

[0155] according to Fig.13 From the above analysis, it can be concluded that the six commutation processes are affected by three AAADs: Δθ CA -Δθ AB , Δθ BC -Δθ CA , and ΔθA B -Δθ BC , as shown in Table 1.

[0156] Table 1

[0157]

[0158] Max AAAD max It can be expressed as:

[0159] AAAD max =(Δθ max -Δθ min )(17)

[0160] Where Δθmax and Δθmin represent the maximum and minimum phase shift values ​​of the corresponding LL commutation voltage in the six commutation processes, respectively. In addition, in a symmetrical fault, AAA will be equal to zero because the three commutation voltages are balanced.

[0161] CCF will first occur in the phase with the minimum VTA. Therefore, the minimum value of the six maximum VTAs in the six commutation processes is used to predict CCF, as expressed in formula (18):

[0162]

[0163] It can be seen that in order to predict CCF based on the VTA theory, the amplitude and phase angle of the commutation voltage and the maximum conduction trigger angle are required.

[0164] Δα obtained from formula (8) max , the CCF can be predicted by comparing the minimum VTA under asymmetric fault and the minimum VTA required by the thyristor valve, as shown in formula (19):

[0165]

[0166] Finally, the flowchart of the proposed CCF prediction method is shown in Figure 1 As shown in the figure, the prediction method only requires local measurement and has a low computational load (no complex calculations are required), which makes the proposed method both reliable and practical. Its input is the three-phase commutation voltage on the inverter side, the trigger angle on the inverter side, the trigger angle advance amount and the minimum turn-off angle limit. According to the input, the voltage-time commutation area that the current system can provide is calculated respectively and the minimum voltage time area VTA required for the current inverter commutation req.min , by comparison and VTA req.min The size of the phase change failure can be determined by the LCC-HVDC system.

[0167] It is important to note that the proposed method is applicable to both single-infeed and multi-infeed systems since it only requires the steady-state system parameters (i.e., the current rating of the LCC-HVDC, the leakage inductance of the inverter transformer, the impedance of the filter bank and reactive power compensation connected to the inverter bus, and the Thevenin equivalent parameters of the AC system) to obtain the commutation voltages.

[0168] See also Figure 15-18 As shown, the embodiment of the present application also provides experimental simulation data to confirm the accuracy of the method of the present application. Fig.14 The modified 6M2A model is shown. The synchronous generator on the inverter side of the original 6M2A model is replaced by the Thevenin equivalent circuit. Fig.14 Faults on transmission lines T2 and T3 are highlighted in Figure .

[0169] About AAAD max and Δα max Calculation. In order to predict CCFs, it is necessary to calculate Δα max , commutation voltage and AAAD max The following is a brief introduction to Δα max and the calculation method of commutation voltage, and then focus on AAAD max Derivation of .

[0170] First, for Δα max , in the above model, γ min , P CEA and T I The settings for are 7 degrees, 0.75 and 0.0544 seconds respectively. The maximum time duration t max is set to 140 milliseconds. Therefore, by applying formula (8), the maximum conduction angle Δα max can be calculated as 23 degrees, which is Fig.11 Compared with the simulation results in max is about 20 degrees, a difference of 3 degrees. This difference has very little effect on the accuracy of the proposed CCF prediction method, which is demonstrated in the sequel.

[0171] Secondly, the commutation voltage and its change under fault conditions are calculated. The calculation results are very accurate compared with the simulation results. For example, when a fault occurs on the T2 transmission line, the commutation voltage on the inverter bus 1 is The calculation and simulation results change as Fig.15 shown. Fig.15The percentage shown in indicates the fault location. For example, 50% T2 means that the fault is located on T2, and the distance between the fault point and the HVDC side bus (bus 3) is 50% of the length of T2.

[0172] from Fig.15 It can be concluded that: 1) When the fault impedance is lower, that is, the fault is more severe, the voltage amplitude changes more and the phase shift continues to decrease, which means that the phase continues to lag; 2) When the fault location is closer to the inverter, the voltage amplitude changes more and the phase shift becomes smaller (that is, more lagging);

[0173] It is worth noting that once FCF occurs, Fig.15 As shown, when Z f When the resistance is less than 60 ohms, the phase angle of the LL commutation voltage lags by as much as 20 degrees. This has a significant impact on the commutation process.

[0174] It is also important to note that the instant when the fault occurs, i.e., the point of the waveform (POW), has little impact on the above calculations, as these calculations are based on quasi-steady-state estimation of the system under asymmetric faults. Fig.16 The conduction angle caused by the control system and the phase angle change of the B phase voltage on bus 1 under faults of different POW are shown.

[0175] Fig.16 (a) shows that the phase angle shift of the B-phase voltage on bus 1 is kept between 10 and 20 degrees of lag, and the oscillation of the phase angle shift does not exceed 3 degrees in each case. Fig.16 (b) shows that the conduction angle of the trigger signal caused by the control system varies by no more than 3 degrees under different POW faults on T2 or T3, and there is no obvious relationship between the variation and POW. Fig.16 It shows that POW has little effect on the parameters and factors used to predict CCF.

[0176] Finally, the calculated commutation voltage is used to derive AAAD according to equations (16) and (17). ij and AAAD max . Fig.17 The three AAADs for inverters 1 and 2 when a single-phase ground fault occurs at T2 0% (on bus 3) and T3 0% (on bus 1) are shown: Δθ CA -Δθ AB ,Δθ BC -Δθ CA ,andΔθ AB -Δθ BC , where AAAD max It is the largest curve among the three AAAD curves.

[0177] Fig.17It shows that: 1) the calculated results match the simulation results, with the maximum error less than 1%; 2) when a single-phase ground fault occurs, the AAAD of different commutation processes changes from -15° to 23°; 3) when the fault impedance is lower, that is, the fault is more serious, the absolute value of AAAD increases; 4) AAAD max The curve with the largest value among the three AAADs can reach 23 degrees when the fault impedance drops to 10 ohms; 5) when AAAD max When the commutation margin continues to increase and compress, CCF may occur according to formula (19).

[0178] It is worth noting that AAAD has not been considered in existing studies. max The above simulation verification shows that this significantly reduced commutation margin has a high risk of causing CCF.

[0179] About CCF prediction. Minimum VTA required for successful commutation of the inverter valve group req.min According to the minimum extinction angle γ min Then, according to Figure 1 The flowchart shown is compared and VTA req.min . According to the flow chart, calculate the inverter 1 and 2 under different fault impedance and position and VTA req.min .

[0180] from Fig.18 It can be observed that: 1) When the fault impedance is low, that is, the fault is more serious, the VTA difference becomes larger, which means that the margin of the commutation process continues to be compressed; 2) When the fault location is closer to the inverter, the VTA difference becomes larger, which means that the margin of the commutation process is also decreasing; 3) When the VTA difference is equal to zero, that is Equal to VTA req.min , CCF is about to occur. Therefore, the fault impedance at the intersection of the curved surface and the zero vertical position of the horizontal plane indicates the critical fault impedance of CCF.

[0181] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for predicting continuous commutation failure in an LCC-HVDC system, characterized in that: include: S1, obtaining the commutation line voltage, trigger angle, trigger angle advance value and inverter minimum turn-off angle limit; S2. Calculate the minimum trigger angle after the corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance amount; S3, calculating the phase shift of the corresponding commutation line voltage after the fault according to the commutation line voltage; S4, calculating the voltage-time commutation area that the current system can provide based on the commutation line voltage, the phase shift after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault, and the minimum turn-off angle limit of the inverter; S5. Calculate the minimum voltage-time area required for the current inverter commutation according to the minimum turn-off angle limit of the inverter; S6. Compare the voltage-time commutation area that can be provided by the current system with the minimum voltage-time area required for the current inverter commutation to determine whether the LCC-HVDC system has a continuous commutation failure.

2. The method for predicting continuous commutation failure of an LCC-HVDC system according to claim 1, characterized in that: S2. Calculate the minimum trigger angle after the corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance; the minimum trigger angle after the corresponding commutation line voltage fault α f.min , obtained by the following formula: a f.min =α0-Δα max Among them, α0 is the initial trigger angle, Δα max is the maximum trigger angle advance; the trigger angle after the AB phase commutation line voltage fault is expressed as The trigger angle after the BC phase commutation line voltage fault is expressed as The trigger angle after the CA phase commutation line voltage fault is expressed as:

3. The method for predicting continuous commutation failure of an LCC-HVDC system according to claim 1, characterized in that: S3. Calculate the phase shift of the commutation voltage corresponding to the commutation process according to the commutation line voltage; the phase shift Δθ of the commutation voltage corresponding to the commutation process i , obtained by the following formula: in, and They are respectively the phase angle movement deviation of the CA phase commutation line voltage after the fault, the phase angle movement deviation of the AB phase commutation line voltage and the phase angle movement deviation of the BC phase commutation line voltage.

4. The method for predicting continuous commutation failure of an LCC-HVDC system according to claim 1, characterized in that: S4, calculating the voltage-time commutation area that the current system can provide based on the commutation line voltage, the phase angle shift deviation after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault, and the minimum turn-off angle limit of the inverter; The voltage-time commutation area that the current system can provide It is obtained by the following formula: Among them, VTA if.max is the maximum voltage-time commutation area provided by the AC system to the inverter valve during the i-th commutation process, α f.min is the minimum trigger angle after the commutation line voltage fault, Δθ i is the phase shift of the commutation voltage corresponding to the commutation process, Δθ PLL is the phase shift of the phase-locked loop output, γ min is the minimum turn-off angle limit of the inverter, V if (t) is the commutation line voltage corresponding to the i-th commutation process.

5. The method for predicting continuous commutation failure of an LCC-HVDC system according to claim 1, characterized in that: S6. Compare the voltage-time commutation area that the current system can provide and the minimum voltage-time area required for the current inverter commutation to determine whether the LCC-HVDC system has a continuous commutation failure; determine by the following judgment formula: in, VTA is the voltage-time commutation area that the current system can provide. req.min It is the minimum voltage-time area required for the current inverter commutation. No CCF means that there is no continuous commutation failure in the current system, and CCF means that there is a continuous commutation failure in the current system.

6. A device for predicting continuous commutation failure in an LCC-HVDC system, characterized in that: include: A data acquisition unit, used to acquire a commutation line voltage, a trigger angle, a trigger angle advance amount and a minimum turn-off angle limit of an inverter; A minimum trigger angle calculation unit, used to calculate the minimum trigger angle after the corresponding commutation line voltage fault according to the trigger angle and the trigger angle advance amount; A phase shift calculation unit, used for calculating the phase shift after the corresponding commutation line voltage fails according to the commutation line voltage; A VTA calculation unit is provided, which is used to calculate the voltage-time commutation area that can be provided by the current system according to the commutation line voltage, the phase shift after the corresponding commutation line voltage fault, the minimum trigger angle after the corresponding commutation line voltage fault and the inverter minimum turn-off angle limit; The demand VTA calculation unit is used to calculate the minimum voltage time area required for the current inverter commutation according to the inverter minimum turn-off angle limit; The CCF judgment unit is used to compare the voltage-time commutation area that can be provided by the current system with the minimum voltage-time area required for the current inverter commutation, and judge whether the LCC-HVDC system has a continuous commutation failure.

7. The device for predicting continuous commutation failure of an LCC-HVDC system according to claim 6, characterized in that: The minimum trigger angle calculation unit is specifically used to execute the following formula to obtain the minimum trigger angle α after the corresponding commutation line voltage fault f.min : a f.min =α0-Δα max Among them, α0 is the initial trigger angle, Δα max is the maximum trigger angle advance; the trigger angle after the AB phase commutation line voltage fault is expressed as The trigger angle after the BC phase commutation line voltage fault is expressed as The trigger angle after the CA phase commutation line voltage fault is expressed as:

8. The device for predicting continuous commutation failure of an LCC-HVDC system according to claim 6, characterized in that: The phase shift calculation unit is specifically used to execute the following formula to obtain the phase shift Δθ of the commutation voltage corresponding to the commutation process: i : in, and They are respectively the phase angle movement deviation of the CA phase commutation line voltage after the fault, the phase angle movement deviation of the AB phase commutation line voltage and the phase angle movement deviation of the BC phase commutation line voltage.

9. The device for predicting continuous commutation failure of an LCC-HVDC system according to claim 6, characterized in that: The VTA calculation unit is specifically used to execute the following formula to obtain the voltage-time commutation area that the current system can provide: Among them, VTA if.max is the maximum voltage-time commutation area provided by the AC system to the inverter valve during the i-th commutation process, α f.min is the minimum trigger angle after the commutation line voltage fault, Δθ i is the phase shift of the commutation voltage corresponding to the commutation process, Δθ PLL is the phase shift of the phase-locked loop output, γ min is the minimum turn-off angle limit of the inverter, V if (t) is the commutation line voltage corresponding to the i-th commutation process.

10. The device for predicting continuous commutation failure of an LCC-HVDC system according to claim 6, characterized in that: The CCF judgment unit is specifically used to execute the following judgment formula to judge whether a continuous commutation failure occurs in the LCC-HVDC system. in, VTA is the voltage-time commutation area that the current system can provide. req.min It is the minimum voltage-time area required for the current inverter commutation. No CCF means that there is no continuous commutation failure in the current system, and CCF means that there is a continuous commutation failure in the current system.

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