A commutation failure prediction control method for hybrid cascade UHVDC transmission system

Abstract

In the field of DC transmission system control and protection technology, the present invention discloses a method for predicting and controlling commutation failure in a hybrid cascaded ultra-high voltage DC transmission system, comprising: S1, obtaining power data of the hybrid cascaded ultra-high voltage DC transmission system; S2, calculating the comprehensive voltage drop amplitude and comprehensive zero-sequence component of the hybrid cascaded ultra-high voltage DC transmission system based on the power data; and S3, determining whether it is necessary to trigger the converter in advance to achieve early triggering of the LCC based on the comprehensive voltage drop amplitude and comprehensive zero-sequence component. The present invention uses the voltage drop or distortion of the AC bus connected to the MMC as one of the starting conditions for the commutation failure predictive control. Since the AC bus connected to the MMC is not equipped with an AC filter, its voltage changes are more sensitive than those of the AC bus connected to the LCC. Therefore, the commutation failure predictive control provided by the present invention has a faster response speed and better commutation failure suppression effect.

Description

Technical Field

[0001] The present invention relates to the technical field of control and protection of direct current (D.C.) transmission systems, and in particular to a commutation failure prediction and control method for a hybrid cascaded ultra-high voltage (UHV) direct current (D.C.) transmission system. Background Art

[0002] Ultra-high voltage direct current (UHVDC) transmission systems offer advantages such as large transmission capacity, long transmission distances, and low transmission losses. They are a crucial support for my country's "West-to-East Power Transmission" initiative and cross-regional energy allocation. The basic principle of UHVDC transmission is that converters at the transmitting end of the system rectify and convert three-phase AC power into DC power. This power is then transmitted through the UHVDC transmission lines. At the receiving end of the system, converters invert the DC power into three-phase AC power, which is then fed into the receiving AC system.

[0003] Current DC transmission technologies fall into two main categories: conventional DC transmission using line commutated converters (LCCs); and flexible DC transmission using modular multilevel converters (MMCs). Furthermore, to leverage the technical advantages of both LCCs and MMCs, existing domestic projects have employed both technologies simultaneously, creating hybrid DC transmission systems. A typical hybrid DC transmission system topology is a hybrid cascade configuration, where an LCC is still used at the sending end, an LCC is used at the receiving end's high-voltage side, and three MMCs are connected in parallel at the low-voltage side, with 50% of the power distributed between the high and low voltage sides. Furthermore, the receiving end's LCC and three MMCs are distributed across different AC systems to achieve power transmission.

[0004] In conventional DC transmission technology using LCC converters, if the valves that are supposed to shut down fail to completely shut down within the reverse voltage time during the LCC converter's rectification and inversion processes, they will reopen after the voltage changes from negative to positive. This is known as commutation failure. Commutation failure is one of the most common fault types in conventional DC transmission systems, easily causing the DC system to operate at reduced power or even shut down, threatening the safety and stability of the larger power grid. Hybrid cascaded UHVDC transmission systems use MMC converters on the low-voltage side, which does not pose a commutation failure issue. However, the high-voltage side still uses LCC converters, making commutation failure a significant concern.

[0005] Existing commutation failure prediction and control technologies are mostly targeted at conventional UHVDC transmission systems. There are no commutation failure prediction and control technologies that specifically consider the control and operating characteristics of hybrid cascaded HVDC transmission systems. The commutation failure suppression effect of hybrid cascaded HVDC transmission systems still needs to be improved. Summary of the Invention

[0006] The purpose of the present invention is to provide a commutation failure prediction and control method for a hybrid cascaded ultra-high voltage direct current (UHVDC) transmission system. This method comprehensively considers the voltage conditions of multiple AC systems connected to the inverter side of the system, and can more quickly detect fault conditions that may cause commutation failure, thereby achieving early prevention of commutation failure, which is beneficial to ensuring the safe and stable operation of the ultra-high voltage direct current (UHVDC) transmission system and the large power grid.

[0007] The present invention is achieved through the following technical solutions:

[0008] A commutation failure prediction control method for a hybrid cascade ultra-high voltage direct current transmission system, comprising:

[0009] S1. Obtaining power data of a hybrid cascaded UHVDC transmission system;

[0010] S2. Calculating a comprehensive voltage drop amplitude and a comprehensive zero-sequence component of the hybrid cascaded ultra-high voltage direct current transmission system based on the power data;

[0011] S3. Determine whether it is necessary to trigger the converter in advance to achieve early triggering of the LCC according to the comprehensive voltage drop amplitude and the comprehensive zero-sequence component.

[0012] As an optimization, the power data includes the equivalent impedance Zn (n=1, 2, 3) of the AC bus connected to the inverter side MMCn and the AC bus connected to the LCC of the hybrid cascaded ultra-high voltage direct current transmission system, and the instantaneous value u of the AC three-phase voltage of the AC bus connected to the inverter side LCC. A 、u B 、u C , the instantaneous value u of the three-phase AC voltage of the AC bus connected to the inverter side MMCn An 、u Bn 、u Cn (n=1, 2, 3).

[0013] As an optimization, in S2, the comprehensive voltage drop amplitude of the hybrid cascaded UHVDC transmission system is calculated by specifically calculating the voltage drop amplitude ΔU of the AC bus connected to the inverter-side LCC. LCC The voltage drop amplitude ΔU of the AC bus connected to the inverter side MMCn MMCn , ΔU LCC The value of is as follows:

[0014]

[0015] Among them, u αβ_LCC_N is u αβ_LCC The value under normal operating conditions of the hybrid cascade UHVDC transmission system;

[0016] ΔU MMCn The value of is as follows:

[0017]

[0018] Among them, u αβ_MMCn_N is u αβ_MMCn Values under normal operating conditions of the hybrid cascade UHVDC transmission system (n=1, 2, 3).

[0019] As an optimization, u αβ_LCC The real-time calculation is as follows:

[0020]

[0021] Among them, u A 、u B 、u C They are respectively the instantaneous values of the three-phase AC voltages of the AC busbar connected to the inverter-side LCC.

[0022] As an optimization, u αβ_MMCn The real-time calculation is as follows:

[0023]

[0024] Among them, u An 、u Bn 、u Cn It is the instantaneous value of the AC three-phase voltage of the AC bus connected to the inverter-side MMCn.

[0025] As an optimization, in S2, the specific formula for finally obtaining the comprehensive voltage drop amplitude ΔU is:

[0026]

[0027] Among them, Z1, Z2, and Z3 are the equivalent impedances of the AC bus connected to the inverter-side MMCn and the AC bus connected to the LCC, respectively.

[0028] As an optimization, in S2, the comprehensive zero-sequence component of the hybrid cascaded ultra-high voltage direct current transmission system is calculated based on the power data, specifically by calculating the voltage zero-sequence component u of the AC bus connected to the inverter side LCC. 0_LCC , and u 0_LCC for u' 0_LCC The maximum value within ams, u' 0_LCC The real-time calculation is as follows:

[0029] u' 0_LCC =u A +u B +u C ;

[0030] Among them, u A 、u B 、u Care the instantaneous values of the three-phase AC voltages of the AC busbar to which the inverter-side LCC is connected;

[0031] Calculate the voltage zero sequence component u of the AC bus connected to the inverter side MMCn 0_MMCn (n=1,2,3),u 0_MMCn Take u' 0_MMCn The maximum value within ams, u' 0_MMCn The real-time calculation is as follows:

[0032] u' 0_MMCn =u An +u Bn +u Cn (n=1,2,3);

[0033] Among them, u An 、u Bn 、u Cn It is the instantaneous value of the AC three-phase voltage of the AC bus connected to the inverter-side MMCn.

[0034] As an optimization, in S2, the specific formula for obtaining the comprehensive zero-sequence component u0 is as follows:

[0035]

[0036] Among them, Z1, Z2, and Z3 are the equivalent impedances of the AC bus connected to the inverter side MMCn and the AC bus connected to the LCC, respectively. N is the rated voltage of the AC bus on the inverter side.

[0037] As an optimization, the specific steps of S3 are:

[0038] S3.1. Compare the comprehensive voltage drop amplitude ΔU with the voltage drop threshold ΔU set If ΔU ≥ ΔU set , the voltage drop triggers the logic output F1=1 in advance, if ΔU<ΔU set , F1=0;

[0039] S3.2. Compare the comprehensive zero-sequence component u0 with the zero-sequence component threshold u 0set The size of u0≥ΔU set , the zero sequence component triggers the logic output F2=1 in advance, if u0<ΔU set , F2=0.

[0040] S3.3, if F1=1 or F2=1, set the advance trigger total logic output F=1 and jump to S3.4, otherwise jump to S1;

[0041] S3.4. Calculate the converter advance triggering amount α of the inverter-side LCC based on the comprehensive voltage drop amplitude ΔU and the comprehensive zero-sequence component u0. pre , and α pre Input to the UHVDC control and protection system to trigger the LCC in advance, α pre The calculation is as follows:

[0042] α pre =max[arccos(1-kΔU),arccos(1-ku0)]

[0043] Wherein, k is the calculation coefficient of the advance trigger amount.

[0044] As an optimization, the voltage drop threshold ΔU set Take 0.15, the zero sequence component threshold u 0set The value is 0.15, and the advance trigger amount calculation proportional coefficient k is 0.075.

[0045] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0046] 1. The present invention utilizes the characteristic of a hybrid cascaded UHVDC transmission system with multiple AC systems connected to the inverter side to synchronously monitor the AC bus voltages of multiple stations connected to different AC systems. This technology also integrates the severity of faults at multiple locations based on the electrical distances between each AC system and the AC bus connected to the LCC, enabling faster and more accurate detection of regional grid faults that may cause commutation failures.

[0047] 2. The present invention uses the voltage drop or distortion of the AC bus connected to the MMC as one of the starting conditions for the commutation failure predictive control. Since no AC filter is configured on the AC bus connected to the MMC, its voltage change is more sensitive than that of the AC bus connected to the LCC. Therefore, the commutation failure predictive control provided by the present invention has a faster response speed and a better commutation failure suppression effect.

[0048] Third, the present invention features simple calculations and clear physical implications. It requires no changes to the UHVDC system's structure or hardware; simple addition, subtraction, multiplication, and division operations based on the system's existing electrical and control parameters are sufficient to implement the present invention. This approach requires minimal hardware and software requirements, is fast, and is suitable for engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the examples. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be considered as limiting the scope. A person of ordinary skill in the art can also derive other relevant drawings based on these drawings without inventive effort. In the drawings:

[0050] Figure 1 Simulation experiment results using conventional commutation failure prediction control;

[0051] Figure 2 A diagram showing the simulation results using the commutation failure prediction control provided by the present invention. DETAILED DESCRIPTION

[0052] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with examples and drawings. The exemplary embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0053] Before introducing the specific embodiments, it should be noted that all the names in this application are defined based on the transmitting antenna, but in fact, this application serves as a receiving antenna, the arrow is the transmission direction of the received signal, the starting end of the receiving antenna in this application is the end of the transmitting antenna, and the end of the receiving antenna is the starting end of the transmitting antenna. Therefore, the transmission direction of the Gaussian beam in this application is from the end of a certain segment to the beginning of the segment.

[0054] Step 1: Obtain calculation parameters

[0055] Based on actual grid operating parameters, calculate the equivalent impedance Zn (n = 1, 2, 3) of the AC busbar connected to the inverter-side MMCn and the AC busbar connected to the LCC in the hybrid cascaded UHVDC transmission system. These equivalent impedances are provided in the complete design documentation for UHVDC transmission projects. This is prior art and will not be further elaborated here.

[0056] Step 2: Real-time data collection and processing

[0057] The control and protection device of the hybrid cascade UHVDC transmission system collects in real time: the instantaneous value of the three-phase AC voltage of the AC bus connected to the LCC on the inverter side u A 、u B 、u C ; Instantaneous value u of the AC three-phase voltage of the AC bus connected to the inverter side MMCn An 、u Bn 、u Cn (n=1, 2, 3).

[0058] Step 3: Calculate the comprehensive voltage drop

[0059] Calculate the voltage drop ΔU of the AC bus connected to the inverter-side LCC LCC , ΔU LCC The value of is as follows:

[0060]

[0061] Among them, u αβ_LCC_N is u αβ_LCC The value under normal system operation conditions, u αβ_LCC The real-time calculation is as follows:

[0062]

[0063] Calculate the voltage drop ΔU of the AC bus connected to the inverter side MMCn MMCn (n=1,2,3), ΔU MMCn The value of is as follows:

[0064]

[0065] Among them, u αβ_MMCn_N is u αβ_MMCn The value under normal system operation (n=1, 2, 3), u αβ_MMCn The real-time calculation is as follows:

[0066]

[0067] The ΔU calculated above LCC and ΔU MMCn Substitute (n=1, 2, 3) into the following formula to obtain the comprehensive voltage drop amplitude ΔU:

[0068]

[0069] Step 4: Comprehensive zero-sequence component calculation

[0070] Calculate the voltage zero-sequence component u of the AC bus connected to the inverter-side LCC 0_LCC ,u 0_LCC Take u' 0_LCC The maximum value within 20ms, u' 0_LCC The real-time calculation is as follows:

[0071] u' 0_LCC =u A +u B +u C

[0072] Calculate the voltage zero sequence component u of the AC bus connected to the inverter side MMCn 0_MMCn(n=1,2,3),u 0_MMCn Take u' 0_MMCn The maximum value within 20ms, u' 0_MMCn The real-time calculation is as follows:

[0073] u' 0_MMCn =u An +u Bn +u Cn (n=1,2,3)

[0074] The u calculated above is 0_LCC and u 0_MMCn Substitute (n=1, 2, 3) into the following formula to obtain the comprehensive zero-sequence component u0:

[0075]

[0076] Among them, U N It is the rated AC voltage of the inverter side commutation bus.

[0077] Step 5: Trigger logical judgment in advance

[0078] Compare the comprehensive voltage drop amplitude ΔU calculated in step 3 with the voltage drop threshold ΔU set If ΔU ≥ ΔU set , the voltage drop triggers the logic output F1=1 in advance, if ΔU<ΔU set , F1=0.

[0079] Compare the comprehensive zero-sequence component u0 calculated in step 4 with the zero-sequence component threshold u 0set The size of u0≥ΔU set , the zero sequence component triggers the logic output F2=1 in advance, if u0<ΔU set , F2=0.

[0080] Once F1=1 or F2=1 is established, the overall logic output F=1 is triggered in advance.

[0081] Step 6: Trigger the inverter early

[0082] When the advance trigger total logic output F=1 in step 5 is established, the converter advance trigger amount α of the inverter side LCC is calculated based on the comprehensive voltage drop amplitude ΔU and comprehensive zero sequence component u0 calculated in steps 3 and 4. pre , and α pre Input to the UHVDC control and protection system to trigger the LCC in advance. pre The calculation is as follows:

[0083] α pre=max[arccos(1-kΔU),arccos(1-ku0)]

[0084] Wherein, k is the calculation coefficient of the advance trigger amount.

[0085] Principle of the invention:

[0086] Assume that the instantaneous value of the three-phase voltage of the AC bus connected to the LCC on the inverter side of the hybrid cascade UHVDC transmission system is u A 、u B 、u C The three-phase instantaneous voltage can be converted into an amplitude value by the following transformation. Under normal operating conditions, u αβ_LCC Fixed, once a symmetrical fault occurs in the AC system, u αβ_LCC will fall, the extent of the fall being related to the severity of the fault.

[0087]

[0088] Similarly, using u αβ_MMCn (n=1, 2, 3) can represent the symmetrical fault condition of the AC system to which MMCn (n=1, 2, 3) is connected.

[0089] Then we can base on u αβ_LCC 、u αβ_MMCn The difference between the measured value and the rated value of (n=1, 2, 3) is used to obtain the normalized voltage drop amplitude:

[0090]

[0091]

[0092] Based on the above, the comprehensive voltage drop amplitude ΔU considering both the LCC AC bus and the MMC AC bus is obtained using the following formula:

[0093]

[0094] Because the MMC AC bus's voltage response is more sensitive, calculations are performed separately for the MMC AC bus, ultimately taking the maximum value of the MMC and LCC AC buses to accelerate the response to ΔU at the initial stage of a fault. When calculating the three MMC AC buses, coefficients are allocated based on the difference in equivalent impedance between the MMC and LCC buses. The smaller the equivalent impedance, the more significant its impact on LCC commutation failure, and the higher the coefficient contribution.

[0095] The above principle applies to symmetrical faults in AC systems. For asymmetrical faults, the fault severity is characterized by the voltage zero-sequence component instead of the voltage drop amplitude:

[0096] u' 0_LCC =u A +u B +u C

[0097] u' 0_MMCn =u An +u Bn +u Cn (n=1,2,3)

[0098] The comprehensive method for responding to LCC AC bus and MMC AC bus faults is similar to that for symmetrical faults and will not be described in detail.

[0099] Since the fault type cannot be determined in advance under the actual operation state of the system, the changes of the comprehensive voltage drop amplitude ΔU and the comprehensive zero sequence component u0 are monitored in real time and synchronously. set , the voltage drop triggers the logic output F1=1 in advance, otherwise F1=0; if u0≥ΔU set , the zero-sequence component triggers the logic output F2 = 1 in advance, otherwise F2 = 0. Once either F1 = 1 or F2 = 1 is met, it is necessary to trigger the LCC converter in advance to ensure sufficient turn-off angle to prevent possible commutation failure. At this time, according to the comprehensive voltage drop amplitude ΔU and the comprehensive zero-sequence component u0, the converter trigger amount α of the inverter-side LCC is calculated. pre , and α pre Input to the UHVDC control and protection system to trigger the LCC in advance. pre The calculation is as follows: the larger the value of ΔU or u0, the more serious the fault is, and the larger the advance triggering amount is.

[0100] α pre =max[arccos(1-kΔU),arccos(1-ku0)]

[0101] Simulation experiment

[0102] To verify the effectiveness of the proposed equivalent method, simulation experiments were conducted using a hybrid cascaded UHVDC system model developed on the PSCAD / EMTDC simulation platform. A three-phase ground fault with a fault inductance of 0.2 Hz and a fault time of 1.706 s was set on the AC busbar connected to MMC2 on the inverter side of the hybrid cascaded UHVDC system. Fault simulations were conducted using both conventional commutation failure predictive control and the commutation failure predictive control provided by the present invention. The effectiveness of the present invention was verified by comparing the response of the inverter-side LCC shutdown angle.

[0103] The simulation results are as follows Figure 1 、 Figure 2As shown in the results, it can be seen that: under the conditions of conventional commutation failure prediction control, after a fault occurs in the inverter-side AC system, the LCC shutdown angle drops to 0°, i.e., commutation failure occurs; under the conditions of the commutation failure prediction control provided by the present invention, due to the more sensitive fault detection, a faster converter early triggering effect can be achieved, and the shutdown angle only drops to 10.5°, i.e., commutation failure is avoided. In summary, compared with conventional commutation failure prediction control, the present invention helps to achieve better commutation failure suppression effect in hybrid cascaded UHVDC transmission systems.

[0104] 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 and controlling commutation failure in a hybrid cascaded ultra-high voltage direct current transmission system, characterized in that: include S1. Obtaining power data of a hybrid cascaded UHVDC transmission system; S2. Calculating a comprehensive voltage drop amplitude and a comprehensive zero-sequence component of the hybrid cascaded ultra-high voltage direct current transmission system based on the power data; S3. Determine whether it is necessary to trigger the converter in advance to achieve early triggering of the LCC according to the comprehensive voltage drop amplitude and the comprehensive zero-sequence component; Specific steps for S3: S3.

1. Compare the comprehensive voltage drop amplitude ΔU with the voltage drop threshold ΔU set If ΔU ≥ ΔU set , the voltage drop triggers the logic output F1=1 in advance, if ΔU<ΔU set , F1=0; S3.

2. Compare the comprehensive zero-sequence component u0 with the zero-sequence component threshold u 0set The size of u0≥ΔU set , the zero sequence component triggers the logic output F2=1 in advance, if u0<ΔU set , F2=0; S3.3, if F1=1 or F2=1, set the advance trigger total logic output F=1 and jump to S3.4, otherwise jump to S1; S3.

4. Calculate the converter advance triggering amount α of the inverter-side LCC based on the comprehensive voltage drop amplitude ΔU and the comprehensive zero-sequence component u0. pre , and α pre Input to the UHVDC control and protection system to trigger the LCC in advance, α pre The calculation is as follows: a pre =max[arccos(1-kΔU),arccos(1-ku0)]; Wherein, k is the calculation coefficient of the advance trigger amount.

2. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 1, characterized in that: The power data includes the equivalent impedance Zn (n=1, 2, 3) of the AC bus connected to the inverter side MMCn and the AC bus connected to the LCC of the hybrid cascade ultra-high voltage direct current transmission system, and the instantaneous value u of the AC three-phase voltage of the AC bus connected to the inverter side LCC. A 、u B 、u C , the instantaneous value u of the three-phase AC voltage of the AC bus connected to the inverter side MMCn An 、u Bn 、u Cn (n=1, 2, 3).

3. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 1, characterized in that: In S2, the comprehensive voltage drop amplitude of the hybrid cascaded UHVDC transmission system is calculated by calculating the voltage drop amplitude ΔU of the AC bus connected to the inverter-side LCC. LCC The voltage drop amplitude ΔU of the AC bus connected to the inverter side MMCn MMCn , ΔU LCC The value of is as follows: Among them, u αβ_LCC_N is u αβ_LCC The value under normal operating conditions of the hybrid cascade UHVDC transmission system; ΔU MMCn The value of is as follows: Among them, u αβ_MMCn_N is u αβ_MMCn Values under normal operating conditions of the hybrid cascade UHVDC transmission system (n=1, 2, 3).

4. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 3, characterized in that: u αβ_LCC The real-time calculation is as follows: Among them, u A 、u B 、u C are the instantaneous values of the three-phase AC voltages of the AC busbar connected to the inverter-side LCC.

5. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 4, characterized in that: u αβ_MMCn The real-time calculation is as follows: Among them, u An 、u Bn 、u Cn It is the instantaneous value of the AC three-phase voltage of the AC bus connected to the inverter-side MMCn.

6. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 5, characterized in that: In S2, the specific formula for finally obtaining the comprehensive voltage drop amplitude ΔU is: Among them, Z1, Z2, and Z3 are the equivalent impedances of the AC bus connected to the inverter-side MMCn and the AC bus connected to the LCC, respectively.

7. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 6, characterized in that: In S2, the comprehensive zero-sequence component of the hybrid cascaded ultra-high voltage direct current transmission system is calculated based on the power data, specifically the voltage zero-sequence component u of the AC bus connected to the inverter side LCC is calculated. 0_LCC , and u 0_LCC for u' 0_LCC The maximum value within ams, u' 0LCC The real-time calculation is as follows: in' 0_LCC =in A +in B +in C ; Among them, u A 、u B 、u C are the instantaneous values of the three-phase AC voltages of the AC busbar to which the inverter-side LCC is connected; Calculate the voltage zero sequence component u of the AC bus connected to the inverter side MMCn 0_MMCn (n=1,2,3),u 0_MMCn Take u'0_ MMC The maximum value of n in ams, u'0_ MMC The real-time calculation of n is as follows: u' 0_MMCn =u An +u Bn +u Cn (n=1,2,3); Among them, u An 、u Bn 、u Cn It is the instantaneous value of the AC three-phase voltage of the AC bus connected to the inverter-side MMCn.

8. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 7, characterized in that: In S2, the specific formula for obtaining the comprehensive zero-sequence component u0 is as follows: Among them, Z1, Z2, and Z3 are the equivalent impedances of the AC bus connected to the inverter side MMCn and the AC bus connected to the LCC, respectively. N is the rated voltage of the AC bus on the inverter side.

9. The method for predicting and controlling commutation failure of a hybrid cascaded ultra-high voltage direct current transmission system according to claim 1, characterized in that: The voltage drop threshold ΔU set Take 0.15, the zero sequence component threshold u 0set The value is 0.15, and the advance trigger amount calculation proportional coefficient k is 0.075.

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