A coordinated control method of firing angles in multiple DC systems to suppress cascading commutation failures

Abstract

The present invention discloses a method for coordinated control of trigger angles of multiple DC systems for suppressing interlocking commutation failures, and belongs to the technical field of power generation, transformation or distribution. First, based on fast Fourier decomposition and nonlinear least squares fitting, the predicted value of the DC system turn-off angle considering the subsequent changes of harmonic voltage is calculated; then, based on the predicted value of the turn-off angle of each DC system, the adjustment amount of the trigger angle of each DC system that does not cause interlocking commutation failure is calculated. The present invention can perform online real-time control of the trigger angles of all DC systems, and does not rely on the switching of fixed gears, and has a better effect of suppressing multi-DC interlocking commutation failures.

Description

Technical Field

[0001] The invention relates to power system safety and stability analysis and control technology, and in particular discloses a multi-DC system trigger angle coordinated control method for suppressing chain commutation failure, belonging to the technical field of power generation, transformation or distribution. Background Art

[0002] The high-voltage direct current transmission system based on grid-commutated converters has great technical and economic advantages in large-capacity and long-distance power transmission, and is a key technical means to achieve cross-regional optimization of energy allocation. Therefore, the direct current transmission system has received great attention and has achieved rapid development. At present, State Grid Corporation of China and China Southern Power Grid Corporation have officially put into operation more than 30 direct current transmission projects. The large-capacity out-of-region power they transmit effectively alleviates the energy supply pressure in the load center area, greatly improving and enriching the power supply capacity and technical means of my country's power system.

[0003] However, the voltage drop caused by the AC grid fault at the receiving end often causes the commutation failure of the DC system inverter side. Since the DC systems fed into the same AC grid have strong mutual influence, the control strategy of the DC system after the AC fault will have an adverse effect on the commutation of the adjacent DC system, and even cause the commutation failure of multiple DC systems, which seriously affects the safe and stable operation of the power system in terms of power transmission, reactive voltage and transient power angle. Reducing the trigger angle of the DC system inverter side based on the commutation failure prediction control (CFPREV) mechanism after the fault is an effective technical means to suppress the commutation failure, but reducing the trigger angle will inevitably increase the reactive power consumption of the DC system. Due to the mutual influence of reactive voltage between multiple DC systems, in a multi-feed DC system with a close electrical distance, excessive adjustment of the trigger angle may have an adverse effect on the commutation of the adjacent DC system, and even cause the chain commutation failure of multiple DC systems.

[0004] Currently, only a few studies are devoted to the coordinated control method of the trigger angle of multiple DC systems. Some studies have developed a coordinated optimization method for the starting threshold of multiple DC CFPREVs considering the interaction of reactive power, but it is essentially an offline parameter design method, and the setting of the starting threshold depends on the priority index calculated offline. In order to achieve its online real-time control, some studies have adjusted the adjustment gear of the CFPREV gain coefficient in real time according to the shutdown angle margin of other DC systems, but it can only select one of the DC systems for control, and the adjustment depends on the pre-set fixed gear, which is not suitable for different fault scenarios. In addition, in order to suppress the failure of multi-DC chain commutation, it is necessary to reduce the trigger angle of the DC in this loop, which will weaken the ability to suppress the failure of the DC commutation in this loop in some cases. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and to provide a method for coordinated control of the trigger angles of multiple DC systems for suppressing chained commutation failures. The method can realize online real-time control of the trigger angles of all DC systems, is independent of a fixed adjustment gear, and takes into account the degree of active impact influence of each DC loop when determining the trigger angle adjustment value. The method can achieve a reasonable balance between the control target of the DC loop and the interactive influence of multiple DC loops, and is suitable for more and more complex fault conditions.

[0006] The present invention adopts the following technical solutions to achieve the above-mentioned invention object:

[0007] A method for coordinated control of firing angles of multiple DC systems for suppressing interlocking commutation failures comprises the following steps:

[0008] Step 1: From the time of fault occurrence to the initial stage of recovery, the predicted value of the turn-off angle of each DC system is calculated taking into account the subsequent changes of harmonic voltage;

[0009] Step 2: Based on the predicted value of the turn-off angle of each DC system, the trigger angle adjustment amount of each DC system that does not cause chain commutation failure is calculated.

[0010] As a further optimization scheme for the multi-DC system firing angle coordinated control method to suppress chain commutation failure, step one is specifically as follows:

[0011] Step 1: Using the two cycles before the valve is triggered as a sliding window for measuring the commutation voltage of each DC system, and constructing the Fourier series of the commutation voltage of each DC system after the fault by considering the nth harmonic according to the measured commutation voltage of each DC system;

[0012] Step 2, determining the Fourier transform coefficients of the Fourier series of the commutation voltage of each DC system after the fault, and obtaining the predicted waveform of the commutation voltage of each DC system after the fault and the zero-crossing offset angle of the commutation voltage;

[0013] Step 3: Calculate the commutation angle of each DC system after the fault, and obtain the predicted value of the shutdown angle of each DC system considering the change of harmonic voltage.

[0014] As a further optimization scheme for the coordinated control method of firing angles of multiple DC systems to suppress chain commutation failures, in step 1, the Fourier series of the commutation voltages of each DC system after the fault constructed by the nth harmonic is: in, The Fourier series of the commutation voltage of the DC system from the k-1th cycle to the k+1th cycle when the nth harmonic is taken into account is: is the commutation voltage waveform of the DC system from the k-1th cycle to the k+1th cycle, k is an integer greater than 1, T is the cycle, ω d is the angular frequency, and It is the time domain steady-state and transient response of the commutation voltage of the DC system.

[0015] As a further optimization scheme for the coordinated control method of trigger angles of multiple DC systems to suppress chained commutation failures, the specific method for determining the Fourier transform coefficients of the commutation voltage Fourier series of each DC system after the fault in step 2 is: performing fast Fourier transform and nonlinear least squares fitting on the commutation voltage Fourier series of each DC system after the fault.

[0016] As a further optimization scheme for the multi-DC system trigger angle coordinated control method to suppress chain commutation failure, the specific method of step 2 to obtain the predicted waveform of the commutation voltage of each DC system after the fault and the commutation voltage zero-crossing offset angle is: perform inverse Fourier transform and series summation according to the obtained Fourier transform coefficients to obtain the predicted waveform of the DC system commutation voltage in the (k+1)th cycle, and then Calculate the zero-crossing angle of the DC system commutation voltage in the k+1th cycle and are the amplitude and phase angle of the jth harmonic voltage in the k+1th cycle, is the amplitude of the fundamental voltage in the k+1th cycle.

[0017] As a further optimization scheme for the coordinated control method of trigger angles of multiple DC systems to suppress cascading commutation failures, step 3 calculates the commutation angles of each DC system after the fault according to the commutation voltage-time area theory. The commutation angle μ′ of the DC system after the fault is expressed as Solve, the predicted value of the DC system turn-off angle γ′ considering the change of harmonic voltage is: in, is the zero-crossing offset angle of the DC system commutation voltage when considering the jth harmonic voltage in the k+1th cycle, I d (α) and I d (α+μ′) are the DC currents at the start and end of the DC system commutation, α is the trigger angle of the DC system, L c is the commutation inductance on the inverter side of the DC system.

[0018] As a further optimization scheme for the coordinated control method of trigger angles of multiple DC systems to suppress chained commutation failures, step two is specifically as follows: calculate the minimum trigger angle adjustment value that avoids commutation failure of the DC system in question and the maximum trigger angle adjustment value of the DC system in question that does not cause chained commutation failure in the adjacent DC system, and calculate the trigger angle adjustment amount of each DC system in combination with the commutation failure situation of the adjacent DC system.

[0019] As a further optimization scheme for the coordinated control method of firing angles of multiple DC systems to suppress cascading commutation failures, the expression for calculating the minimum firing angle adjustment value to avoid commutation failure in this DC system is: Δα a =γ min -γ′ a , where Δα a To avoid the minimum trigger angle adjustment value of the DC system a commutation failure, γ min is the inherent limit turn-off angle of the DC system, γ′ a is the predicted value of the turn-off angle of this DC system a.

[0020] As a further optimization scheme for the coordinated control method of firing angles of multiple DC systems to suppress chained commutation failures, the expression for calculating the maximum firing angle adjustment value of the DC system that does not cause chained commutation failures in adjacent DC systems is: Δα a max =min(Δα a1 ,Δα a2 ,Δα ab ,...,Δα am ), where Δα a max The maximum trigger angle adjustment value of the DC system a that does not cause a cascading commutation failure in the adjacent DC system, Δα ab It is the maximum trigger angle adjustment value of the DC system a that does not cause the chain commutation failure of the adjacent DC system b, b=1,2,…,m, m is the total number of the DC systems adjacent to the DC system a.

[0021] As a further optimization scheme for the coordinated control method of firing angles of multiple DC systems to suppress chain commutation failures, the specific method for calculating the firing angle adjustment of each DC system in combination with the commutation failure of the adjacent DC system is as follows:

[0022] Δα a ≤Δα a max When the trigger angle adjustment of this DC system is Δα a ;

[0023] Δα a >Δα a max When only one adjacent DC system b fails to commutate, quantify the impact of the active power impact of the DC system a on the receiving grid λ a , the impact of the active power impact of the adjacent DC system b on the receiving grid λ b ,λ a ≥λ b The trigger angle adjustment of this DC system is Δα a ,λ a <λ b The trigger angle adjustment of this DC system is Δαa max , ΔE a is the magnitude of active power impact in the transient process of the DC system after the fault, S ac_a is the short-circuit capacity at the converter busbar a of this DC system;

[0024] Δα a >Δα a max When there are two adjacent DC systems with commutation failure, the impact of the active power impact of the DC system a on the receiving grid and the impact of the active power impact of each adjacent DC system with commutation failure on the receiving grid are quantified. Determine the final trigger angle adjustment value Δα of the DC system a, g and h are the adjacent DC systems whose commutation failures are caused by the commutation failure of the DC system a, λ g is the impact degree of active power rate formed by the adjacent DC system g on the receiving grid, λ h is the impact degree of active power rate impact caused by the adjacent DC system h on the receiving grid, Δα ah The maximum trigger angle adjustment value of the DC system a that does not cause the adjacent DC system h to fail, Δα ag It is the maximum trigger angle adjustment value of the DC system a in this loop so as not to cause the interlocking commutation failure of the adjacent DC system g.

[0025] The present invention adopts the above technical solution and has the following beneficial effects:

[0026] (1) The method proposed in the present invention first considers the change of harmonic voltage after the fault to predict the shutdown angle of each DC system, and then calculates the trigger angle adjustment value of each DC system based on the predicted value of the shutdown angle and the commutation failure of the adjacent DC system. It can control the trigger angle of all DC systems online in real time and does not rely on the switching of fixed gears, which has a better effect of suppressing multi-DC chain commutation failure.

[0027] (2) The method proposed in the present invention accurately calculates the zero-crossing offset angle based on the predicted commutation voltage waveform, providing an accurate data source for subsequent shutdown angle prediction, thereby improving the accuracy of the trigger angle adjustment value of each DC system.

[0028] (3) The method proposed in the present invention uses the short-circuit capacity at the converter bus to quantify the impact of the active power of each DC loop when determining the trigger angle adjustment value. The physical meaning is clear. At the same time, the impact of the active power of each DC loop is taken into account, and a reasonable balance is achieved between the control target of the DC loop and the interactive impact of multiple DC loops. The method is suitable for more and more complex fault conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1It is a simulation model built in PSCAD / EMTDC based on the CIGRE DC standard model in an embodiment of the present invention.

[0030] Figure 2 It is a comparison diagram of the shutdown angle simulation curve of the DC system 2 under the coordinated control strategy proposed by the present invention and the traditional control strategy when a three-phase fault is set at the commutation bus in an embodiment of the present invention.

[0031] Figure 3 It is a comparison diagram of the simulation curves of the DC system 3 shut-off angles under the coordinated control strategy proposed in the present invention and the traditional control strategy when a three-phase fault is set at the commutation bus in an embodiment of the present invention.

[0032] Figure 4 It is a comparison diagram of the reactive power simulation curve absorbed by the DC system 2 under the cooperative control strategy proposed by the present invention and the traditional control strategy when a three-phase fault is set at the commutation bus in an embodiment of the present invention.

[0033] Figure 5 It is a comparison diagram of the voltage simulation curve of the commutation bus of the DC system 3 under the coordinated control strategy proposed by the present invention and the traditional control strategy when a three-phase fault is set at the commutation bus in an embodiment of the present invention.

[0034] Figure 6 The present invention is a flowchart of the method. DETAILED DESCRIPTION

[0035] The technical solution provided by the present invention will be described in detail below in conjunction with specific embodiments. It should be understood that the following specific implementation methods are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0036] like Figure 6 As shown, the method for coordinated control of trigger angles of multiple DC systems for suppressing chained commutation failures proposed in the present invention includes two parts: the first part, for the trigger angle control from the occurrence of a fault to the initial stage of recovery, calculates the predicted value of the turn-off angle of each DC system considering the subsequent change of the harmonic voltage, which is specifically implemented through steps 1 to 3; the second part, based on the predicted value of the turn-off angle of each DC system, calculates the trigger angle adjustment amount of each DC system that does not cause chained commutation failure, which is specifically implemented through step 4.

[0037] Step 1: Consider the harmonic voltage to construct the Fourier series of the commutation voltage of each DC system after the fault

[0038] The two cycles before the valve is triggered are set as the sliding window for measurement, and the subsequent commutation voltage is predicted by the curve fitting method based on the measured AC voltage. The following describes the prediction process of the commutation voltage of each DC system after the fault by combining equations (1) to (4). It should be noted that the commutation voltage involved in equations (1) to (4) refers to the commutation voltage of the same DC system. The transient voltage of the system in the time domain after the fault can be characterized by steady-state response and transient response. represents the commutation voltage waveform from the k-1th cycle to the k+1th cycle. The Fourier series of the commutation voltage considering the nth harmonic is:

[0039]

[0040] In formula (1): is the Fourier series of the commutation voltage from the k-1th period to the k+1th period when the nth harmonic is taken into account, k is an integer greater than 1, T is the period, ω d is the angular frequency, and It is the time domain steady-state and transient response of the commutation voltage.

[0041] Step 2: Perform fast Fourier transform and nonlinear least squares fitting on the commutation voltage Fourier series of each DC system after the fault to determine the Fourier transform coefficient, and obtain the predicted waveform of the commutation voltage of each DC system after the fault and the zero-crossing offset angle of the commutation voltage

[0042] After calculation and simplification, The frequency domain form is expressed in the form of real and imaginary parts, thereby obtaining the frequency domain mathematical form of the Fourier series of the commutation voltage when considering the nth harmonic in the k-1th to k+1th period as shown in equation (2). The coefficients are determined by nonlinear least squares fitting. For example, the coefficients of the real part can be solved by equation (3):

[0043]

[0044] In formula (2) and formula (3): A kn , B kn , C kn and D kn is the fitting coefficient of the Fourier series frequency domain form of the commutation voltage when considering the nth harmonic from the k-1th cycle to the k+1th cycle, lsq is the nonlinear least squares fitting function, and real represents the real part of the complex number. This is the Fourier series of the commutation voltage from the i-1th period to the i+1th period when the nth harmonic is taken into account, and τ represents the damping ratio.

[0045] Similarly, the fitting coefficient C of the imaginary part kn and D knIt can also be determined by nonlinear least squares fitting method. When an asymmetric fault occurs, the frequency domain voltage is decomposed into positive sequence, negative sequence and zero sequence components by symmetrical component method, and then the real coefficient and imaginary coefficient of positive sequence, negative sequence and zero sequence components are determined by nonlinear least squares fitting respectively. Thus, the predicted waveform of the commutation voltage in the k+1th cycle can be obtained after inverse Fourier transform and series summation, and the commutation voltage zero-crossing offset angle can be calculated by formula (4):

[0046]

[0047] In formula (4): and It represents the amplitude and phase angle of the jth harmonic voltage in the k+1th cycle.

[0048] Step 3: Calculate the commutation angle of each DC system after the fault according to the commutation voltage-time area theory, and obtain the predicted value of the shutdown angle of each DC system considering the harmonic voltage change.

[0049] According to the commutation voltage-time area theory, to complete commutation, the voltage-time area that can be provided should be equal to the required voltage-time area. The required voltage-time area depends on the DC current at the start and end of commutation. Therefore, the commutation angle μ′ after the fault is:

[0050]

[0051] In formula (5): is the zero-crossing offset angle of the commutation voltage when considering the jth harmonic voltage in the k+1th cycle, I d (α) and I d (α+μ′) represents the DC current at the start and end of commutation, α represents the trigger angle, L c Represents the commutation inductance on the inverter side of the DC system.

[0052] The predicted commutation angle μ′ obtained by solving is compared with the zero-crossing offset angle Substituting into equation (6), the predicted value of the DC system turn-off angle γ′ considering the harmonic voltage change is calculated.

[0053]

[0054] It should be noted that the commutation voltages involved in equations (5) and (6) refer to the commutation voltages of the same DC system.

[0055] Step 4: Calculate the minimum trigger angle adjustment value to avoid commutation failure of the DC system and the maximum trigger angle adjustment value of the DC system that does not cause chain commutation failure in the adjacent DC system, and calculate the trigger angle adjustment value of each DC system in combination with the commutation failure of the adjacent DC system.

[0056] Step 4.1, based on equations (7) and (8), calculate the minimum trigger angle adjustment value Δα required to avoid commutation failure of the DC system a: a , and the maximum trigger angle adjustment value Δα of the DC system that does not cause chain commutation failure in the adjacent DC system a max ,

[0057] Δα a =γ min -γ′ a (7)

[0058] Δα a max =min(Δα a1 ,Δα a2 ,Δα ab ,...,Δα am ) (8)

[0059] In formula (7) and formula (8): γ min represents the inherent limit turn-off angle of the DC system, γ′ a represents the predicted value of the cut-off angle of the DC system a, Δα ab It represents the maximum trigger angle adjustment value of the DC system a that will not cause the interlocking commutation failure of the adjacent DC system b, b=1,2,…,m, where m is the total number of the DC systems adjacent to the DC system a.

[0060] Step 4.2, if Δα a ≤Δα a max , then let the trigger angle adjustment of this DC system be Δα a .

[0061] Step 4.3, if Δα a >Δα a max , then considering the chain reaction effect of commutation failure of this DC system, calculate the trigger angle adjustment of each DC system:

[0062] Step 4.3.1, if Δα a >Δα a max If only one adjacent DC system b fails in commutation, the final trigger angle adjustment value is determined by further comprehensively considering the active power impact degree caused by the commutation failure and the critical value of the trigger angle.

[0063] According to the active power characteristics of the DC system in the transient process after the fault, the active power impact size ΔE is approximately quantified using formula (9):

[0064]

[0065] In formula (9): η represents the active power P d The active power P transmitted when the system returns to steady state operation d0 time,

[0066] According to formula (10), the influence degree of active power impact formed by the DC system a on the receiving grid and the influence degree of active power impact formed by the adjacent DC system b on the receiving grid are quantified. The larger the value, the higher the influence degree of active power rate impact formed by the DC system a on the receiving grid. The influence degree of active power rate impact formed by the DC system a on the receiving grid is λ a for:

[0067]

[0068] In formula (10): ΔE a is the magnitude of active power impact in the transient process of the DC system after the fault, S ac_a It represents the short-circuit capacity at the converter busbar a of this DC system.

[0069] If a ≥λ b , then the final trigger angle adjustment of this DC system a is Δα a ,

[0070] If a <λ b , then the final trigger angle adjustment of this DC system a is Δα a max ;

[0071] Step 4.3.2, if Δα a >Δα a max And it causes commutation failure of multiple adjacent DC systems, then the final trigger angle adjustment value Δα of the DC system a is determined according to formula (11):

[0072]

[0073] In formula (11), g and h are the adjacent DC systems that are caused by the commutation failure of the DC system a, and λ g is the impact degree of active power rate formed by the adjacent DC system g on the receiving grid, λ h is the impact degree of active power rate impact caused by the adjacent DC system h on the receiving grid, λ g , h According to formula (10), Δα ahThe maximum trigger angle adjustment value of the DC system a that does not cause the adjacent DC system h to fail, Δα ag It is the maximum trigger angle adjustment value of the DC system a in this loop so as not to cause the interlocking commutation failure of the adjacent DC system g.

[0074] Based on the CIGRE DC standard model, the following Figure 1 In the simulation system shown, the DC transmission power is 1000MW, and the impedance parameters of the simulation system are shown in Table 1.

[0075] Table 1 Impedance parameters of the simulation system

[0076] impedance <![CDATA[Z 1 ]]> <![CDATA[Z 2 ]]> <![CDATA[Z 3 ]]> <![CDATA[Z 12 ]]> <![CDATA[Z 13 ]]> <![CDATA[Z 23 ]]> Numeric 2.75+j10.23 2.75+j10.23 5.50+j20.46 25+j314 25+j314 12.5+j157

[0077] A three-phase fault with a grounding inductance of 0.21H is set at the commutation bus of DC system 3, which occurs at 1.0s and lasts for 0.05s. The simulation curves of the shutdown angles of DC system 2 and DC system 3 are shown in Figure 2. Figure 2 and Figure 3 As shown, it can be seen that the commutation failure in the two DC systems is effectively suppressed. The reason is that, with the method of the present invention, when the turn-off angle of DC system 3 decreases, the trigger angle adjustment value of DC system 2 is reduced, thereby reducing the reactive power absorbed by it. Figure 4 The reduction of reactive power consumption increases the commutation bus voltage of the adjacent DC system 3, as shown in Figure 5 As shown in the figure, the commutation failure of the DC system is effectively suppressed. As the turn-off angle curve in the figure shows, the proposed coordinated control method effectively suppresses the chain commutation failure of multiple DC systems.

[0078] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above specific embodiments. The above specific embodiments and the description in the specification are only for further illustrating the principles and preparation effects of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. A method for coordinated control of firing angles of multiple DC systems to suppress chain commutation failures, characterized in that: The steps include: Step 1: From the time of fault occurrence to the initial stage of recovery, the predicted value of the turn-off angle of each DC system is calculated taking into account the subsequent changes of harmonic voltage; Step 2: Based on the predicted value of the turn-off angle of each DC system, the trigger angle adjustment of each DC system that does not cause chain commutation failure is calculated, specifically: Calculate the minimum trigger angle adjustment value to avoid commutation failure in this DC system, Δα a =γ min -γ a ′, where Δα a To avoid the minimum trigger angle adjustment value of the DC system a commutation failure, γ min is the inherent limit turn-off angle of the DC system, γ a ′ is the predicted value of the cut-off angle of the DC system a. Calculate the maximum trigger angle adjustment value of the DC system that does not cause chain commutation failure in the adjacent DC system, Δα amax =min(Δα a1 ,Δα a2 ,Δα ab ,...,Δα am ), where Δα amax The maximum trigger angle adjustment value of the DC system a that does not cause a cascading commutation failure in the adjacent DC system, Δα ab is the maximum trigger angle adjustment value of the DC system a that does not cause the adjacent DC system b to fail, b = 1, 2, ..., m, m is the total number of the adjacent DC systems of the DC system a, Combined with the commutation failure of the adjacent DC system, the trigger angle adjustment of each DC system is calculated: Δα a ≤Δα amax When the trigger angle adjustment of this DC system is Δα a , Δα a >Δα amax When only one adjacent DC system b fails to commutate, quantify the impact of the active power impact of the DC system a on the receiving grid λ a , the impact of the active power impact of the adjacent DC system b on the receiving grid λ b ,λ a ≥λ b The trigger angle adjustment of this DC system is Δα a ,λ a <λ b The trigger angle adjustment of this DC system is Δα amax , ΔE a is the magnitude of active power impact in the transient process of the DC system after the fault, S ac_a is the short-circuit capacity of the converter busbar a of this DC system, Δα a >Δα amax When there are two adjacent DC systems with commutation failure, the impact of the active power impact of the DC system a on the receiving grid and the impact of the active power impact of each adjacent DC system with commutation failure on the receiving grid are quantified. Determine the final trigger angle adjustment value Δα of the DC system a, g and h are the adjacent DC systems whose commutation failures are caused by the commutation failure of the DC system a, λ g is the impact degree of active power impact caused by the adjacent DC system g on the receiving grid, λ h is the impact degree of active power impact caused by the adjacent DC system h on the receiving grid, Δα ah The maximum trigger angle adjustment value of the DC system a that does not cause the adjacent DC system h to fail, Δα ag It is the maximum trigger angle adjustment value of the DC system a in this loop so as not to cause the interlocking commutation failure of the adjacent DC system g.

2. A method for coordinated control of firing angles of multiple DC systems for suppressing chained commutation failures according to claim 1, characterized in that: The step 1 is specifically as follows: Step 1: Using the two cycles before the valve is triggered as a sliding window for measuring the commutation voltage of each DC system, and constructing the Fourier series of the commutation voltage of each DC system after the fault by considering the nth harmonic according to the measured commutation voltage of each DC system; Step 2, determining the Fourier transform coefficients of the Fourier series of the commutation voltage of each DC system after the fault, and obtaining the predicted waveform of the commutation voltage of each DC system after the fault and the zero-crossing offset angle of the commutation voltage; Step 3: Calculate the commutation angle of each DC system after the fault, and obtain the predicted value of the shutdown angle of each DC system considering the change of harmonic voltage.

3. A method for coordinated control of firing angles of multiple DC systems for suppressing chained commutation failures according to claim 2, characterized in that: In step 1, the Fourier series of the commutation voltage of each DC system after the fault constructed by considering the nth harmonic is: in, The Fourier series of the commutation voltage of the DC system from the k-1th cycle to the k+1th cycle when the nth harmonic is taken into account is: is the commutation voltage waveform of the DC system from the k-1th cycle to the k+1th cycle, k is an integer greater than 1, T is the period, ω d is the angular frequency, and It is the time domain steady-state and transient response of the commutation voltage of the DC system.

4. A method for coordinated control of firing angles of multiple DC systems for suppressing chained commutation failures according to claim 3, characterized in that: The specific method of determining the Fourier transform coefficients of the Fourier series of the commutation voltages of each DC system after the fault in step 2 is: performing fast Fourier transform and nonlinear least squares fitting on the Fourier series of the commutation voltages of each DC system after the fault.

5. A method for coordinated control of firing angles of multiple DC systems for suppressing chained commutation failures according to claim 4, characterized in that: The specific method of obtaining the predicted waveform of the commutation voltage of each DC system after the fault and the zero-crossing offset angle of the commutation voltage in step 2 is: performing inverse Fourier transform and series summation according to the obtained Fourier transform coefficients to obtain the predicted waveform of the commutation voltage of the DC system in the k+1th cycle, and Calculate the zero-crossing angle of the DC system commutation voltage in the k+1th cycle and are the amplitude and phase angle of the jth harmonic voltage in the k+1th cycle, is the amplitude of the fundamental voltage in the k+1th cycle.

6. A method for coordinated control of firing angles of multiple DC systems for suppressing chained commutation failures according to claim 5, characterized in that: The step 3 calculates the commutation angle of each DC system after the fault according to the commutation voltage-time area theory. The commutation angle μ′ of the DC system after the fault is expressed as Solve, the predicted value of the DC system turn-off angle γ′ considering the change of harmonic voltage is: in, is the zero-crossing offset angle of the DC system commutation voltage when considering the jth harmonic voltage in the k+1th cycle, I d (α) and I d (α+μ′) are the DC currents at the start and end of the DC system commutation, α is the trigger angle of the DC system, L c is the commutation inductance on the inverter side of the DC system.

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