HVDC commutation failure risk prediction method considering optimal CFPREV

Abstract

The invention relates to an HVDC commutation failure risk prediction method considering an optimal CFPREV, belongs to the technical field of commutation failure defense of a high-voltage direct-current transmission system, and aims to predict the commutation failure risk of an LCC-HVDC system under the control of the optimal CFPREV and judge whether the system can suppress the commutation failure only by changing the size of a trigger angle after a fault occurs. And a support is provided for the investment of an off-station commutation failure defense device. In the implementation process, an inverter station trigger angle under the CFPREV limit effect is calculated through real-time measurement data in the operation process of the high-voltage direct-current power transmission system, then the commutation failure risk under the optimal CFPREV control is predicted based on the commutation area theory, and the change of direct current under the CFPREV effect is considered in the calculation process, so that the prediction accuracy is improved. And the reliability of prediction results is ensured.

Description

Technical Field

[0001] The present invention relates to a HVDC commutation failure risk prediction method taking into account optimal commutation failure prediction control (CFPREV), belonging to the technical field of commutation failure prevention of high voltage direct current transmission systems. Background Art

[0002] Line-commutated Converter HVDC (LCC-HVDC) based on grid commutation has effectively solved the problem of uneven distribution of energy centers and load centers in my country with its large transmission capacity, low loss and low cost, and ensured the reliable supply of electric energy. Commutation failure (CF) is a common fault in LCC-HVDC, which refers to the failure of thyristors to turn off or turn on smoothly during the commutation process. Commutation failure will cause abnormal fluctuations in the AC and DC systems, and may even affect the stability and reliability of the entire power system. If continuous commutation failure occurs, it may cause more serious system failures or large-scale shutdowns. Therefore, commutation failure defense technology has become a current research hotspot. In commutation failure defense, the prediction of commutation failure risk is one of the most important links. It is an important basis for the action of commutation failure control and protection system. By predicting the risk of commutation failure in advance, the system operation state can be adjusted before the fault occurs to increase the commutation margin, such as optimizing the trigger angle in the station and switching the reactive compensation device outside the station, so as to avoid the occurrence of commutation failure and improve the safety and stability of system operation. In summary, it is crucial to quickly and accurately predict the risk of commutation failure.

[0003] For the above problems, current domestic and foreign research mostly analyzes the commutation process of the converter based on the commutation voltage-time integral area theory, and predicts commutation failure by comparing the defined commutation demand area and the commutation provided area, as recorded in the following literature: Wang Zengping, Liu Xiyang, Zheng Bowen, et al. Rapid prediction and suppression measures of commutation failure based on voltage waveform fitting [J]. Transactions of the Chinese Society of Electrotechnical Engineering, 2020, 35(07): 1454-1463. DOI: 10.19595 / j.cnki.1000-6753.tces.190759.

[0004] The existing commutation failure prediction method based on the commutation voltage-time integral area theory has the following basic principles: When the converter is working, the process of current transfer from the bridge arm is called commutation. Taking the commutation from valve 1 (VT1) to valve 3 (VT3) as an example, the commutation equivalent circuit is as follows: Figure 1 As shown in the figure, VT1-VT3 are valves 1, 2 and 3 involved in the commutation process. ua , u b , u c is the phase voltage of valve 1, valve 2 and valve 3 on the AC side of the converter, that is, the three-phase voltage of the AC grid after being transformed by the converter transformer, i 1. i 2. i 3 are the currents flowing through valve 1, valve 2, and valve 3 respectively, L d is the inductance of the DC side smoothing reactor, i d is the DC side current of the converter, L c is the equal commutation inductance of each phase.

[0005] In the commutation process from VT1 to VT3, the three thyristors VT1, VT2 and VT3 are turned on at the same time, and the DC current is distributed to the two parallel branches according to a certain rule. The Kirchhoff voltage equation is written for the parallel circuit: (1); In the formula, i 1 and i 3 are the currents flowing through VT1 and VT3 respectively. When in the commutation process, the current relationship satisfies i d = i 3 + i 1, and then we can get: (2); in u ba = u a -u b , which is the commutation voltage of the commutation process, t 1 and t 2 are the starting time and ending time of commutation respectively. According to the commutation theory, i 3( t 1)=0, i 3( t 2)= i d ( t 2), converting equation (2) into the form of electrical angle, we can get the conditions that each electrical quantity needs to meet during the commutation process: (3); In the formula, ω 0 is the system angular velocity, α It is the command value of the trigger angle at the beginning of VT1-VT3 commutation. γ is the arc extinction angle. From formula (3), we can see that the right part of the formula is the integral of the commutation voltage over time during the commutation period, and the area between the commutation voltage and the time axis is reflected in the commutation voltage waveform. The left part of formula (3) is defined as the commutation demand area S need It reflects that the converter needs a certain commutation area to complete its commutation process. This area value is affected by the DC current and the commutation reactance. γ min Substitute it into the right side of formula (3) and define it as the commutation area S pro , which reflects the maximum commutation area that the system can provide under the current commutation voltage and trigger angle, that is, the maximum commutation area. S pro Greater than or equal to the required commutation area S need , it is considered that the commutation process can be completed normally and commutation failure will not occur; on the contrary, when the maximum commutation area S pro Smaller than the area required for commutation S need , it is considered that commutation failure occurs. It can be seen that the traditional commutation area prediction method only predicts whether commutation failure occurs or not, and does not evaluate the risk of commutation failure based on the current state of the converter station.

[0006] Moreover, a commutation failure prediction control module CFPREV is usually provided in the current HVDC transmission system. CFPREV adjusts the trigger angle by detecting the fault degree of the AC system, thereby reducing the probability of commutation failure. However, the existing commutation failure prediction method fails to fully consider the adjustment effect of CFPREV on the trigger angle, which is prone to misjudgment of commutation failure, thereby causing unnecessary action of other protection devices in the system and affecting the safety and stability of the system. This problem has become the key to limiting the accuracy of commutation failure prediction in the field of HVDC transmission commutation failure defense. Summary of the invention

[0007] In order to solve the above problems, the present invention provides a HVDC commutation failure risk prediction method taking into account the optimal CFPREV, which can predict the risk of commutation failure under the consideration of the CFPREV limit effect.

[0008] The technical solution of the present invention is as follows: A method for predicting HVDC commutation failure risk taking into account optimal CFPREV comprises the following steps: S1: During the operation of LCC-HVDC, real-time acquisition of the effective value of the inverter station commutation voltage U com, CFPREV action time t f And the arc extinction angle when the system is operating normally γ 0; S2: Since the DC voltage will decrease when the firing angle decreases, and the DC voltage cannot be too low when the DC system is running, it is necessary to limit the minimum firing angle under the action of CFPREV. It is necessary to calculate the minimum firing angle under the DC voltage limit. α 1; During the commutation period, the commutation voltage waveform will suddenly change and produce a dent. When the trigger angle command suddenly decreases, it may cause the next commutation process to be excessively advanced, encroaching on the closing process of the valve of the previous commutation process, resulting in commutation failure. Therefore, in order to avoid the interval between two commutation processes being too small, it is necessary to limit the minimum trigger angle under the action of CFPREV after the fault for a short time, and calculate the minimum trigger angle under the encroachment limit of the commutation process after the fault. α 2; Select α 1 and α The larger of the two is used as the firing angle under the optimal CFPREV control α 3; S3: According to the firing angle under the optimal CFPREV control α 3. Effective value of commutation voltage of inverter station U com , calculate the maximum commutation area S pro.cfp ; S4: Consider the trigger angle under the action of CFPREV α 3. A sudden decrease will cause the DC current to increase in a short period of time, introducing a risk factor k 1. k 2. k 3. Assume DC current I d The sizes are k 1 times, k 2 times, k 3 times the rated DC current, k 1 to k 3 Increase successively, and then calculate the DC current I d for k 1 times, k 2 times, k Commutation area required at 3 times rated DC current S need.cfp(1) , S need.cfp(2) and S need.cfp(3) ; S5: Compare the maximum commutation areaS pro.cfp and commutation required area S need.cfp(1) , S need.cfp(2) and S need.cfp(3) The size of the commutation margin is determined to judge the risk of commutation failure in the system.

[0009] Preferably, in step S2, the minimum trigger angle under the DC voltage limit is calculated using the following formula: α 1: (4); In the formula, U dmin The minimum DC voltage for the system to maintain normal operation. N is the number of pulsating converters in inverter station 6, k d It is the compensation coefficient, which can be defined according to actual conditions. The recommended value is 1.1.

[0010] Preferably, in step S2, the following formula is used to calculate the minimum trigger angle under the post-fault commutation process encroachment limit: α 2: (5); In the formula, γ min is the minimum arc extinction angle required for valve shutoff, T is the reference frequency of the AC system on the inverter side of LCC-HVDC, k α It is a conservative coefficient, which can be defined according to actual conditions, and the recommended value is 1.05; Firing angle under optimal CFPREV control α 3 is: (6); Preferably, in step S3, the maximum commutation area is calculated using the following formula: S pro.cfp : (7); In the formula, ω 0 is the system angular frequency, which is 2π / T ; t represents the integration variable.

[0011] Preferably, in step S4, the commutation required area is calculated using the following formula: (8); In the formula, L crepresents the equivalent commutation inductance of each phase, I dN Indicates the rated DC current, which is determined by the actual system.

[0012] Preferably, in step S4, the risk factor k 1. k 2. k The values ​​of 3 are k 1=1.0, k 2=1.5, k 3=2.0.

[0013] Preferably, in step S5, if S pro.cfp ≥ S need.cfp(3) , it is considered that the system has no risk of commutation failure under the optimal CFPREV control; if S need.cfp(2) ≤ S pro.cfp < S need.cfp(3) , it is believed that the risk of commutation failure is low under the optimal CFPREV control; if S need.cfp(1) ≤ S pro.cfp < S need.cfp(2) , it is believed that the risk of commutation failure is high under the optimal CFPREV control; if S pro.cfp < S need.cfp(1) , it is believed that the risk of commutation failure is extremely high under the optimal CFPREV control.

[0014] For any details not provided in the present invention, please refer to the prior art.

[0015] The beneficial effects of the present invention are: The HVDC commutation failure risk prediction method taking into account the optimal CFPREV of the present invention can predict the commutation failure risk of the LCC-HVDC system under the control of the optimal CFPREV, and determine whether the system can suppress the occurrence of commutation failure by only changing the trigger angle after the fault, and provide support for the investment of the off-station commutation failure defense device. In the implementation process, the method calculates the inverter station trigger angle of the CFPREV limit effect through the real-time measurement data during the operation of the high-voltage direct current transmission system, and then predicts the commutation failure risk under the control of the optimal CFPREV based on the commutation area theory, and considers the change of DC current under the action of CFPREV in the calculation process to ensure the reliability of the prediction result. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0017] Figure 1 It is the valve 1-valve 3 commutation equivalent circuit; Figure 2 Flow chart of the HVDC commutation failure risk prediction method considering the optimal CFPREV; Figure 3 The valve current waveforms of the 6-pulse converter corresponding to the Y-connected winding during the fault period, where (a) is the current through valve 1, (b) is the current through valve 2, (c) is the current through valve 3, (d) is the current through valve 4, (e) is the current through valve 5, and (f) is the current through valve 6; Figure 4 The valve current waveforms of the 6-pulse converter corresponding to the D-type connected winding during the fault period, where (a) is the current through valve 1, (b) is the current through valve 2, (c) is the current through valve 3, (d) is the current through valve 4, (e) is the current through valve 5, and (f) is the current through valve 6; Figure 5 This is the prediction result of the traditional commutation area method; Figure 6 Predict outcomes for the methods of the present invention. DETAILED DESCRIPTION

[0018] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0019] Example 1 A HVDC commutation failure risk prediction method considering the optimal CFPREV is proposed. Figure 2 As shown, the following steps are included: S1: During the operation of LCC-HVDC, real-time acquisition of the effective value of the inverter station commutation voltage U com , CFPREV action time t f And the arc extinction angle when the system is operating normally γ 0; S2: Since the DC voltage will decrease when the firing angle decreases, and the DC voltage cannot be too low when the DC system is running, it is necessary to limit the minimum firing angle under the action of CFPREV. It is necessary to calculate the minimum firing angle under the DC voltage limit. α 1; During the commutation period, the commutation voltage waveform will suddenly change and produce a dent. When the trigger angle command suddenly decreases, it may cause the next commutation process to be excessively advanced, encroaching on the closing process of the valve of the previous commutation process, resulting in commutation failure. Therefore, in order to avoid the interval between two commutation processes being too small, it is necessary to limit the minimum trigger angle under the action of CFPREV after the fault for a short time, and calculate the minimum trigger angle under the encroachment limit of the commutation process after the fault. α 2; Select α 1 and α The larger of the two is used as the firing angle under the optimal CFPREV control α 3; S3: According to the firing angle under the optimal CFPREV control α 3. Effective value of commutation voltage of inverter station U com , calculate the maximum commutation area S pro.cfp ; S4: Consider the trigger angle under the action of CFPREV α 3. A sudden decrease will cause the DC current to increase in a short period of time, introducing a risk factor k 1. k 2. k 3. Assume DC current I d The sizes are k 1 times, k 2 times, k 3 times the rated DC current, k 1 to k 3 Increase successively, and then calculate the DC current I d for k 1 times, k 2 times, k Commutation area required at 3 times rated DC current S need.cfp(1) , S need.cfp(2) and S need.cfp(3) ; S5: Compare the maximum commutation area S pro.cfp and commutation required area S need.cfp(1) , S need.cfp(2) and S need.cfp(3) The size of the phase change failure can be used to determine the risk of phase change failure in the system.

[0020] Commutation failure is one of the most common faults in the operation process of LCC-HVDC, which threatens the safe and stable operation of the high-voltage direct current transmission system. Therefore, the prediction of commutation failure risk is crucial to the subsequent suppression of commutation failure and maintaining the safety and stability of the system. In addition, the actual high-voltage direct current transmission system is usually equipped with a commutation failure prediction control module CFPREV, which has the function of reducing the inverter trigger angle and increasing the commutation margin after the fault. However, most of the existing commutation failure prediction methods do not consider the impact of CFPREV on commutation failure, and only perform commutation failure prediction, and do not evaluate the commutation failure risk based on the current state of the converter station. It is easy to misjudge commutation failure, which leads to the malfunction of other protection measures and causes unnecessary impact on the system. Therefore, this embodiment proposes a HVDC commutation failure risk prediction method taking into account the optimal CFPREV. The method can calculate the trigger angle under the limit action of CFPREV according to the electrical quantity after the system fault without adding other collected data, and then predict the risk of commutation failure under the control of the optimal CFPREV, thereby improving the accuracy and reliability of commutation failure risk prediction. During the implementation process, this method deduced the trigger angle under the optimal CFPREV control from the two aspects of DC voltage limitation and commutation process encroachment. In the commutation failure risk prediction link, this method considered the impact of CFPREV on DC current and determined the risk of commutation failure according to different degrees of DC current change, ensuring the reliability of the final prediction result.

[0021] Example 2 A method for predicting HVDC commutation failure risk taking into account the optimal CFPREV is as described in Example 1, except that in step S2, the minimum trigger angle under the DC voltage limit is calculated using the following formula: α 1: (4); In the formula, U dmin The minimum DC voltage for the system to maintain normal operation. N is the number of pulsating converters in inverter station 6, k d is the compensation coefficient, and its value is 1.1.

[0022] The following formula is used to calculate the minimum firing angle under the encroachment limit of the commutation process after a fault: α 2: (5); In the formula, γ min is the minimum arc extinction angle required for valve shutoff, T is the reference frequency of the AC system on the inverter side of LCC-HVDC, k αis the conservative coefficient, with a value of 1.05; Firing angle under optimal CFPREV control α 3 is: (6).

[0023] Example 3 A method for predicting HVDC commutation failure risk taking into account the optimal CFPREV is as described in Example 2, except that in step S3, the maximum commutation area provided is calculated using the following formula: S pro.cfp : (7); In the formula, ω 0 is the system angular frequency, which is 2π / T ; t represents the integration variable.

[0024] Example 4 A method for predicting HVDC commutation failure risk taking into account an optimal CFPREV is as described in Example 3, except that in step S4, the commutation requirement area is calculated using the following formula: (8); In the formula, L c represents the equivalent commutation inductance of each phase, I dN Indicates the rated DC current, which is determined by the actual system.

[0025] Risk Factors k 1. k 2. k The values ​​of 3 are k 1=1.0, k 2=1.5, k 3=2.0.

[0026] Example 5 A method for predicting HVDC commutation failure risk taking into account the optimal CFPREV is as described in Example 4, except that in step S5, if S pro.cfp ≥ S need.cfp(3) , it is considered that the system has no risk of commutation failure under the optimal CFPREV control; if S need.cfp(2) ≤ S pro.cfp < S need.cfp(3) , it is believed that the risk of commutation failure is low under the optimal CFPREV control; if Sneed.cfp(1) ≤ S pro.cfp < S need.cfp(2) , it is believed that the risk of commutation failure is high under the optimal CFPREV control; if S pro.cfp < S need.cfp(1) , it is believed that the risk of commutation failure of the system under the optimal CFPREV control is extremely high, as shown in Table 1.

[0027] Table 1 S pro.cfp , S need.cfp(i) Corresponding relationship between size and commutation failure risk

[0028] The verification was carried out in the LCC-HVDC standard model using the parameters in Table 2, where both the LCC-HVDC rectifier and inverter side converter stations use 12-pulse converters (composed of two 6-pulse converters connected in series), the rectifier side uses a constant current control method, and the inverter side uses a control method that combines constant voltage and fixed arc extinction angle; the converter transformer uses a three-phase three-winding transformer, the high-voltage side uses a Yg type connection, and the two low-voltage side windings use a Y type connection and a D1 type connection, and provide commutation voltages for the two 6-pulse converters respectively. During the normal operation of the LCC-HVDC, a three-phase grounding fault is set at the inverter side AC bus at 0.75s, and the risk of commutation failure after the fault is predicted based on the traditional commutation area method and the proposed method, respectively, to verify the effectiveness of the method proposed in the present invention.

[0029] Table 2 Simulation parameters of LCC-HVDC standard model

[0030] Set the fault grounding reactance to 0.1H and the fault duration to 100ms. Figure 3 The figure shows the valve current waveforms of the 12 converter valves during the fault period. Figure 3 The figure shows the valve current of the 6-pulse converter corresponding to the Y-connected winding during the fault period. VT1-VT6 represent valve 1-valve 6 respectively. Figure 3 (a) to (f) are the currents passing through valves 1 to 6 respectively. Figure 4 is the valve current of the 6-pulse converter corresponding to the D-type connected winding, Figure 4 (a) to (f) are the currents through valves 1 to 6, respectively. From the definition of commutation failure, we can see that commutation failure means that the valve is not completely closed and then reopened. Because there are 12 valves in the converter station in the LCC-HVDC system, Figure 3 , Figure 4The middle is the current flowing through each valve during the fault period. By observing the valve current, it can be determined whether the actual commutation failure occurs. When a fault occurs in the high-voltage DC transmission system, the valve current increases or decreases under the influence of the DC current. By observing the valve current waveform, it can be seen that no valve current suddenly rises again during the decline process, or suddenly rises again after just dropping to zero, that is, during the fault period, each valve can be smoothly turned on or off in a predetermined order, and commutation failure does not occur.

[0031] The effectiveness of the method proposed by the present invention is verified by comparing the prediction results of the HVDC commutation failure risk prediction method taking into account the optimal CFPREV and the existing commutation area method. The relevant parameters are set as follows: The minimum DC voltage for the system to maintain normal operation U dmin =250kV, compensation coefficient k d =1.1, conservative coefficient k α =1.05, risk factor k 1=1.0, k 2=1.5, k 3=2.0. The minimum DC voltage for the system to maintain normal operation U dmin Determined by the system structure and actual needs, it can be seen from formula (4) that: U dmin The smaller it is, the more the system can accept a smaller trigger angle, and the stronger the ability to resist commutation failure under the optimal CFPREV control. k d The function is to reduce the minimum firing angle under DC voltage limitation. α 1 calculation error, which is mainly caused by the product of the commutation reactance and the DC current. k d The larger the value, the better. α 1 has a greater impact. Conservative coefficient k α The role is to set the minimum firing angle under the phase change process encroachment limit α 2 Keep a certain margin to prevent α 2 is too close to the limit value of the commutation process encroachment. From formula (5), we can know that k α The larger the value, the smaller the minimum firing angle under the phase change process encroachment limit. α 2 The greater the margin reserved. Risk Factor k 1. k 2. k 3 represents the assumed increase factor of DC current under CFPREV, k 1. k 2.k The larger the value of 3 is, the higher the requirement of the proposed method for the risk level of commutation failure is, that is, the easier it is to judge as high risk. U dmin , k d , k α , k 1. k 2. k 3 and other parameters directly affect the accuracy and reliability of the prediction, so it is necessary to reasonably set these parameters according to the actual situation of the HVDC transmission system before prediction.

[0032] Figure 5 The commutation failure prediction results of the traditional commutation area method under the above fault conditions are shown. The horizontal axis in the figure is the time near the fault process, and the vertical axis is the maximum commutation area provided S pro Reduce the required area for commutation S need From Figure 5 It can be seen that when the LCC-HVDC is operating normally before 0.75s, the maximum commutation area is always greater than the commutation area required, and the commutation process can be completed smoothly. S pro - S need The value of decreases rapidly and decreases to zero at 0.755s. At this time, the prediction method based on the traditional commutation area theory determines that commutation failure occurs. Figure 4 , Figure 5 From the waveform of the valve current, it can be seen that there is no commutation failure during the system fault in practice. Therefore, the prediction results of the traditional area method under this condition are consistent with Figure 3 and Figure 4 The simulation results are inconsistent and a misjudgment of commutation failure occurs.

[0033] Figure 6 The prediction results of the commutation failure risk of the method proposed in the present invention are shown. The horizontal axis in the figure is the time near the fault process, and the vertical axis is the maximum commutation area provided by the optimal CFPREV control. S pro.cfp Reduce the required area for commutation S need.cfp(i) From Figure 6 It can be seen that when LCC-HVDC is operating normally before 0.75s, the maximum commutation area S pro.cfp Always larger than the commutation area required S need.cfp(i) , the commutation process can be completed smoothly. When the fault occurs 0.75s later,S pro.cfp - S need.cfp(i) The value of decreases rapidly, S pro.cfp - S need.cfp(1) and S pro.cfp - S need.cfp(2) The value of is always greater than 0 during a fault. S pro.cfp - S need.cfp(3) It decreases to less than 0 at 0.761s. Therefore, the risk of commutation failure predicted by the proposed method is lower. Compared with the misjudgment of the traditional prediction method, the prediction result of the proposed method is more in line with reality and has higher accuracy and reliability.

[0034] It is worth noting that the present invention does not directly determine the occurrence of commutation failure, but rather evaluates the risk of commutation failure at a certain moment. Neither the proposed method nor the traditional method can determine the specific time when commutation failure will occur, but only predicts it within a period of time before the commutation failure occurs.

[0035] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for predicting HVDC commutation failure risk taking into account optimal CFPREV, characterized in that: The steps include: S1: During the operation of LCC-HVDC, real-time acquisition of the effective value of the inverter station commutation voltage U com , CFPREV action time t f And the arc extinction angle when the system is operating normally γ 0; S2: Calculate the minimum firing angle under DC voltage limit α 1, and the minimum firing angle under the post-fault commutation process encroachment limit α 2, and take α 1 and α The larger of the two is used as the firing angle under the optimal CFPREV control α 3; S3: According to the firing angle under the optimal CFPREV control α 3. Effective value of commutation voltage of inverter station U com , calculate the maximum commutation area S pro.cfp ; S4: Consider the trigger angle under the action of CFPREV α 3. A sudden decrease will cause the DC current to increase in a short period of time, introducing a risk factor k 1. k 2. k 3. Assume DC current I d The sizes are k 1 times, k 2 times, k 3 times the rated DC current, k 1 to k 3 Increase successively, and then calculate the DC current I d for k 1 times, k 2 times, k Commutation area required at 3 times rated DC current S need.cfp(1) , S need.cfp(2) and S need.cfp(3) ; S5: Compare the maximum commutation area S pro.cfp and commutation required area S need.cfp(1) , S need.cfp(2) and S need.cfp(3) The size of the phase change failure can be used to determine the risk of phase change failure in the system.

2. The HVDC commutation failure risk prediction method taking into account the optimal CFPREV according to claim 1, characterized in that: In step S2, the minimum trigger angle under the DC voltage limit is calculated using the following formula: α 1: ； In the formula, U dmin The minimum DC voltage for the system to maintain normal operation. N is the number of pulsating converters in inverter station 6, k d is the compensation coefficient, and its value is 1.

1.

3. The HVDC commutation failure risk prediction method taking into account the optimal CFPREV according to claim 2, characterized in that: In step S2, the following formula is used to calculate the minimum trigger angle under the post-fault commutation process encroachment limit: α 2: ； In the formula, γ min is the minimum arc extinction angle required for valve shutoff, T is the reference frequency of the AC system on the inverter side of LCC-HVDC, k α is the conservative coefficient, with a value of 1.05; Firing angle under optimal CFPREV control α 3 is: 。 4. The HVDC commutation failure risk prediction method taking into account the optimal CFPREV according to claim 3, characterized in that: In step S3, the maximum commutation area is calculated using the following formula: S pro.cfp : ； In the formula, ω 0 is the system angular frequency, which is 2π / T ; t represents the integration variable.

5. The HVDC commutation failure risk prediction method taking into account the optimal CFPREV according to claim 4, characterized in that: In step S4, the commutation required area is calculated using the following formula: ； In the formula, L c represents the equivalent commutation inductance of each phase, I dN Indicates the rated DC current.

6. The HVDC commutation failure risk prediction method taking into account the optimal CFPREV according to claim 5, characterized in that: In step S4, the risk factor k 1. k 2. k The values ​​of 3 are k 1=1.0, k 2=1.5, k 3=2.

0.

7. The HVDC commutation failure risk prediction method taking into account the optimal CFPREV according to claim 6, characterized in that: In step S5, if S pro.cfp ≥ S need.cfp(3) , it is considered that the system has no risk of commutation failure under the optimal CFPREV control; if S need.cfp(2) ≤ S pro.cfp < S need.cfp(3) , it is believed that the risk of commutation failure of the system is low under the optimal CFPREV control; like S need.cfp(1) ≤ S pro.cfp < S need.cfp(2) , it is believed that the risk of commutation failure is high under the optimal CFPREV control; if S pro.cfp < S need.cfp(1) , it is believed that the risk of commutation failure is extremely high under the optimal CFPREV control.

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