CN107069682A - A kind of HVDC transmission line back-up protection method based on DC control system - Google Patents

Abstract

The invention relates to a backup protection method for a high-voltage direct current transmission line based on a direct current control system, and belongs to the technical field of electric power system relay protection. When faults occur in different areas of the high-voltage DC system, the DC control side will produce different responses. During the fault period, the converter firing angle α, the inverter shutdown angle γ, the differential current ΔI _re from the valve side to the DC outlet area at both ends, and The change characteristics of ΔI _inv can accurately distinguish faults inside and outside the zone, the trigger angle α response of the converter can identify the faults of the AC system on the rectifier side and the fault on the opposite pole, and the turn-off angle γ on the inverter side can identify the faults of the AC system on the inverter side , and the differential flow signals ΔI _re and ΔI _inv from the valve side to the DC outlet area are introduced to identify faults in the valve side to the outlet area. The invention has high action reliability, fast action speed, strong anti-transition resistance capability, and can satisfy remote faults. It has important reference significance for the configuration of the backup protection of the actual HVDC transmission line.

Description

A backup protection method for HVDC transmission line based on DC control system

technical field

The invention relates to a backup protection method for a high-voltage direct current transmission line based on a direct current control system, and belongs to the technical field of electric power system relay protection.

Background technique

At present, my country's power system has realized national networking, long-distance, large-capacity power transmission. It is estimated that by 2020, there will be more than 50 direct current transmission projects in my country, of which UHV direct current transmission projects account for about 60%. Due to the high voltage level of the DC transmission line and the complex geographical environment along the line, the probability of failure is also high, which seriously affects the safety and stability of the DC transmission system.

^At present, the main protection schemes for DC transmission lines include traveling wave protection, differential undervoltage protection, DC differential protection, etc. Undisclosed, China's DC transmission system is often in a passive situation in terms of protection configuration, setting, and protection device upgrades, which has a great impact on the safety and stability of China's AC-DC hybrid power system.

HVDC transmission systems generally use differential undervoltage protection and longitudinal differential protection as backup protection, but the operating characteristics of differential undervoltage protection undervoltage criteria are closely related to factors such as transition resistance and DC control system parameters; longitudinal differential protection It is greatly affected by the control system and in order to avoid out-of-area faults, its action delay is as long as 1.1s.

Contents of the invention

The technical problem to be solved by the present invention is to provide a backup protection method for HVDC transmission lines based on a DC control system. When faults occur in different areas of the HVDC system, the DC control side will produce different responses. , Inverter cut-off angle and the differential current change characteristics from the valve side of both ends to the DC outlet area can accurately distinguish the faults inside and outside the area, so that the faults in the area can be accurately and quickly acted on.

Technical scheme of the present invention is:

A backup protection method for a high-voltage direct current transmission line based on a direct current control system, the specific steps are:

(1) In the DC system, the area from the rectifier side DC outlet to the inverter side DC outlet is a high-voltage DC transmission line. The faults that occur in this area are internal faults, and the protection operates. The rest of the faults are external faults, and the protection does not operate. ;

(2) Monitor the differential current signals on the rectification side and the inverter side in real time. The differential current signal on the rectification side is the difference between the current at the rectifier valve and the current at the DC outlet of the rectification side, which is recorded as ΔI _re ; the differential current signal on the inverter side is The difference between the current at the inverter valve and the current at the DC outlet of the inverter side is recorded as ΔI _inv ; use these two differential current signals to detect the current free component caused by faults in the DC transmission system, when ΔI _re or ΔI _inv is greater than The action current threshold value I _set of the differential current protection indicates that there is a fault in the DC transmission system, the backup protection is started, and the trigger angle α criterion is started after the protection blocking is completed to further judge the area where the fault occurs;

(3) After the blocking time t _s is blocked, the α criterion is started, and the method of α criterion is as follows:

The sampling value of the trigger angle α on the rectification side is extracted from the extreme control measurement unit of the DC transmission system, and the sampling value is compared with the _set threshold value α _set . Reset; if established, start the γ criterion to further judge the area where the fault occurs;

(4) The discriminant method of γ criterion is:

The sampling value of the turn-off angle γ of the inverter side is extracted from the extreme control measurement unit, and the sampling value is compared with the set threshold value γ _set . If the sampling value γ>γ _set is not established, it is judged to be an out-of-area fault and the protection is reset. ; If true, start the differential flow criterion to further judge the area where the fault occurs;

(5) The discriminant method of differential flow criterion is:

Sampling the differential current ΔI _re and ΔI _inv to judge whether the differential current ΔI _re >I _set and ΔI _inv >I _set is true. If any formula is established, it can be judged that the fault is a fault in the corresponding area, which is an external fault, and the protection is reset; if two If none of the formulas is established, it is judged as an internal fault.

The blocking time t _s described in step (3) is specifically determined as:

In order to avoid the interference of the attenuation free component produced by the nonlinear element on various physical quantities after the fault, and cooperate with the main protection of the system at the same time, set the blocking time t _s after the protection starts, and take the time taken for the free component to decay to zero and the time used for avoiding the main protection The larger value among the times is taken as the blocking time t _s .

The setting scheme of the action setting value α _set of the α criterion described in step (3) is:

Build a simulation model based on the actual parameters of the DC transmission system, set various extreme faults on the rectification side AC line and the opposite pole line of the simulation system, and obtain the maximum value of the firing angle on the rectification side of the system under various extreme fault conditions. α _max , set α _set = k ₁ α _max , k ₁ is the reliability coefficient.

The setting scheme of the action setting value γ _set of the described γ criterion of step (4) is:

Faults under various extreme conditions are set on the AC line of the inverter side of the simulation system, and the maximum value γ _max of the turn-off angle of the inverter side under various extreme fault conditions is obtained by simulation, and the setting γ _set = k ₂ γ _max , k ₂ is the reliability factor.

The setting scheme of the differential flow criterion described in step (5) is specifically:

After the blocking time t _s ends, the free component released by the nonlinear element basically decays to 0, and the interference effect of the free component disappears. If the differential current signal ΔI _re or ΔI _inv is still greater than the set value at this time, it means that the fault occurs in the differential current signal protection. In the area, that is, the area from the rectifier valve or inverter valve to the DC outlet, this area is outside the protection area, so the protection is reset; if the differential current signal is less than the set value, it is determined that the fault occurred in the high-voltage DC transmission line, and the protection operates.

Principle of the present invention is:

1. Scope of protection

The scope of protection of the present invention is the entire length of the DC transmission line, and the area outside the line is an out-of-area fault that needs to be identified. It is mainly divided into: faults on opposite poles, faults on the AC system on the rectifier side, faults on the AC system on the inverter side, and outgoing lines on the rectifier side. There are 5 types of faults and inverter side outgoing faults.

2. Protection start

When the DC system fails, the backup protection should start quickly and accurately regardless of the fault distance, transition resistance and fault type. Since all faults will lead to a sudden change in the electrical quantity of the system, it will enter the electromagnetic transient process and cause the free components of each nonlinear element to be released, and the signals of the ΔI _re and ΔI _inv differential current criteria come from the current at both ends of the smoothing reactor, and the electromagnetic transient Repeated charging and discharging of the smoothing reactor during the period will bring obvious attenuating and oscillating differential current signals, and the differential current signals are not easily affected by fault distance and transition resistance for system faults, so this scheme uses differential current signals as the start-up of backup protection model. Judging whether the differential current on both sides ΔI _re >I _set or ΔI _inv >I _set is true, if it is true, start the backup protection, and start the trigger angle α criterion after the protection is blocked; if it is not true, continue to detect the electrical quantity.

In the formula, ΔI _re and ΔI _inv are the differential current signals from the rectifier valve side to the DC outgoing line at the sending end and from the inverter valve side to the DC outgoing line at the receiving end. I _set is the operating current threshold value of the differential current protection, I _set = k ₁ I ₀ , k ₁ is the reliability coefficient, and I ₀ is the larger value of the differential current signals on both sides after the fault blocking in the zone ends.

3. Protection lock

The free component of the electromagnetic transient process makes the electrical quantity violently oscillate, and the oscillating process lasts for about 300ms. This oscillation brings great interference to the setting of the backup protection. Therefore, the backup protection scheme should set a relevant blocking signal to avoid the free component. Considering that after commutation failure occurs in the DC system, the commutation failure diagnosis should be performed by the DC system and the relevant protection actions should be taken. Therefore, the backup protection scheme should set a phase lock signal to cooperate with the commutation failure protection. Due to the need to cooperate with the commutation failure protection, it will lead to a delay in the action of the backup protection. The delay time is determined by the commutation failure diagnosis time, generally about 200ms. The blocking time of backup protection should take the larger one of the two, so the blocking time of this backup protection scheme is taken as ts=300ms.

4. Trigger angle α criterion

When a symmetrical fault occurs in the AC system on the rectifier side, the rectifier side rapidly reduces the α angle under the action of constant current control to suppress the DC current drop, and the rectifier side finally operates under the constant α _min control; the fault characteristics of the AC system on the rectifier side when a symmetrical short-circuit fault occurs It is: α=α _min =5°.

When an asymmetrical fault occurs in the AC system on the rectifier side, the DC current at the outlet of the rectifier will continue to fluctuate, resulting in the continuous switching of the control mode of the DC side between constant current control and constant α _min control, resulting in constant fluctuations in the α angle, and its minimum value is α=α _min = 5°, the maximum value α _max is determined by the fault type, and the fault characteristic when an asymmetric short-circuit fault occurs in the AC system on the rectification side is: the α angle fluctuates continuously between α = α _min = 5° and α _max .

At the instant of a short-circuit fault on the DC line, the DC current on the rectifier side increases and the DC current on the inverter side decreases. The rectifier side rapidly increases the α angle under the action of constant current control to suppress the DC current. When the influence of transition resistance is not considered, the low-voltage current-limiting link starts to work, the DC current setting value decreases with the DC voltage, and the rectifier side finally operates under constant current (minimum current limit) control. The fault characteristics in the steady state of the DC line short-circuit fault are : α>α _N .

Due to the line coupling effect, the opposite pole line fault, the change trend of α angle is similar to that of the local pole line fault, but the amplitude is quite different.

In this method, the trigger angle α is used to identify the fault of the AC system on the rectification side and the fault of the opposite pole line. The sampling value of the trigger angle α on the rectification side is extracted from the extreme control measurement unit, and it is judged whether the sampling value α<α _set is established. If it is established, the fault can be judged to be a fault of the AC system of the rectification side or a fault of the opposite pole DC line, which is an out-of-area fault. The protection is reset; if not established, the γ criterion is started.

In the formula, α _set is the action setting value of α criterion, α _set =k ₂ α _max , k ₂ is the reliability coefficient, α _max is the maximum value of the firing angle α in each fault of the AC system on the rectification side and the fault of the opposite pole system.

5. Criterion of turn-off angle γ

When a symmetrical fault occurs in the AC system of the inverter side, the inverter side is always controlled by a fixed γ angle, and the γ angle decreases due to the decrease of the AC bus voltage on the inverter side. The broken angle controller has been unable to eliminate the error between the γ angle setting value and the actual value, resulting in the γ angle being smaller than its rated value. When a symmetrical short-circuit fault occurs in the AC system on the inverter side, the fault characteristics are: γ<γ _N .

When an asymmetrical fault occurs in the AC system on the inverter side, the control mode of the inverter side and the change trend of the actual value of the γ angle are the same as those in the case of a symmetrical fault on the inverter side. The fault characteristics of the asymmetrical short-circuit fault on the AC bus on the inverter side are: γ<γ _N .

When a short-circuit fault occurs on the DC line, the inverter side is converted from constant γ-angle control to constant current control, and the current setting value is reduced under the action of the low-voltage current limiting link, and finally the inverter side operates at the minimum current limit. At this time , the setting value of the constant current controller on the inverter side decreases, the output β angle increases compared with normal operation, the decrease of the inverter side DC current and the increase of β angle will cause the cut-off angle γ increased. The fault characteristics of the asymmetrical short-circuit fault in the DC line are: γ>γ _N .

This method uses the cut-off angle γ to distinguish the faults of the AC system on the inverter side. The sampling value of the turn-off angle γ on the inverter side is extracted from the extreme control measurement unit, and it is judged whether the sampling value γ<γ _set is established. If it is established, the fault can be judged to be an AC system fault on the inverter side, which is an out-of-area fault, and the protection is reset; If not, start the differential flow criterion.

In the formula, γ _set is the action setting value of γ criterion, γ _set =k ₃ γ _max , k ₃ is the reliability coefficient, and γ _max is the maximum value of turn-off angle γ in each fault of the AC system on the inverter side.

6. Differential flow criterion

After the blocking time is over, the free component released by the nonlinear element basically decays to 0, and judge whether the differential current signal ΔI _re < I _set and ΔI _inv < I _set are true. If both are not true, the fault occurs in the outgoing line area on both sides. If both are established, it will happen in the zone, and the protection action will be taken.

The beneficial effects of the present invention are:

1. This method uses the turn-off angle γ of the inverter side to identify the fault of the AC system on the inverter side, and does not use the transient electrical quantity, which avoids the problem that the transient electrical quantity is affected by the control system after the fault and makes the action unreliable;

2. This protection should be able to operate reliably in about 300ms. DC differential protection is mainly used to detect remote high-resistance ground faults, but its action delay is up to 1.1s. Therefore, compared with the DC longitudinal difference protection, the action time of this protection is greatly improved;

3. The action trends of the rectification side trigger angle α and cut-off angle γ are hardly affected by the fault distance; this protection uses the rectifier side trigger angle α and cut-off angle γ as criteria to reduce the impact of fault distance on protection reliability.

Description of drawings

Figure 1 is the Yunguang ± 800kV UHV transmission system model and fault classification diagram in the embodiment of the present invention; f1 to f6 in the figure respectively represent faults in the area, faults on opposite poles, faults on rectifier side outlets, inverter side outlet faults, and rectification AC system failure on the inverter side and AC system failure on the inverter side;

Fig. 2 is the α criterion action figure when different transition resistances of the present invention;

Fig. 3 is the action situation figure of gamma criterion when different transition resistances of the present invention;

Fig. 4 is the operation situation diagram of the rectification side differential current criterion ΔI _re when different transition resistances of the present invention;

Fig. 5 is a diagram of the action of the inverter side differential current criterion ΔI _inv when different transition resistances are used in the present invention;

Fig. 6 is a situation diagram of the α criterion action during different fault distances of the present invention;

Fig. 7 is a diagram of the gamma criterion action situation when the present invention has different fault distances;

Fig. 8 is an action diagram of the differential current criterion ΔI _re on the rectification side at different fault distances in the present invention;

Fig. 9 is a diagram of the action of the differential current criterion ΔI _inv on the inverter side at different fault distances in the present invention.

detailed description

The present invention will be further described below in combination with the accompanying drawings and specific embodiments.

Embodiment 1: A backup protection method for HVDC transmission lines based on a DC control system, the specific steps are:

(1) In the DC system, the area from the rectifier side DC outlet to the inverter side DC outlet is a high-voltage DC transmission line. The faults that occur in this area are internal faults, and the protection operates. The rest of the faults are external faults, and the protection does not operate. ;

(2) Monitor the differential current signals on the rectification side and the inverter side in real time. The differential current signal on the rectification side is the difference between the current at the rectifier valve and the current at the DC outlet of the rectification side, which is recorded as ΔI _re ; the differential current signal on the inverter side is The difference between the current at the inverter valve and the current at the DC outlet of the inverter side is recorded as ΔI _inv ; use these two differential current signals to detect the current free component caused by faults in the DC transmission system, when ΔI _re or ΔI _inv is greater than The action current threshold value I _set of the differential current protection indicates that there is a fault in the DC transmission system, the backup protection is started, and the trigger angle α criterion is started after the protection blocking is completed to further judge the area where the fault occurs;

(3) After the blocking time t _s is blocked, the α criterion is started, and the method of α criterion is as follows:

The sampling value of the trigger angle α on the rectification side is extracted from the extreme control measurement unit of the DC transmission system, and the sampling value is compared with the _set threshold value α _set . Reset; if established, start the γ criterion to further judge the area where the fault occurs;

(4) The discriminant method of γ criterion is:

The sampling value of the turn-off angle γ of the inverter side is extracted from the extreme control measurement unit, and the sampling value is compared with the set threshold value γ _set . If the sampling value γ>γ _set is not established, it is judged to be an out-of-area fault and the protection is reset. ; If true, start the differential flow criterion to further judge the area where the fault occurs;

(5) The discriminant method of differential flow criterion is:

Sampling the differential current ΔI _re and ΔI _inv to judge whether the differential current ΔI _re >I _set and ΔI _inv >I _set is true. If any formula is established, it can be judged that the fault is a fault in the corresponding area, which is an external fault, and the protection is reset; if two If none of the formulas is established, it is judged as an internal fault.

The blocking time t _s described in step (3) is specifically determined as:

In order to avoid the interference of the attenuation free component produced by the nonlinear element on various physical quantities after the fault, and cooperate with the main protection of the system at the same time, set the blocking time t _s after the protection starts, and take the time taken for the free component to decay to zero and the time used for avoiding the main protection The larger value among the times is taken as the blocking time t _s .

The setting scheme of the action setting value α _set of the α criterion described in step (3) is:

Build a simulation model based on the actual parameters of the DC transmission system, set various extreme faults on the rectification side AC line and the opposite pole line of the simulation system, and obtain the maximum value of the firing angle on the rectification side of the system under various extreme fault conditions. α _max , set α _set = k ₁ α _max , k ₁ is the reliability coefficient.

The setting scheme of the action setting value γ _set of the described γ criterion of step (4) is:

Faults under various extreme conditions are set on the AC line of the inverter side of the simulation system, and the maximum value γ _max of the turn-off angle of the inverter side under various extreme fault conditions is obtained by simulation, and the setting γ _set = k ₂ γ _max , k ₂ is the reliability factor.

The setting scheme of the differential flow criterion described in step (5) is specifically:

After the blocking time t _s ends, the free component released by the nonlinear element basically decays to 0, and the interference effect of the free component disappears. If the differential current signal ΔI _re or ΔI _inv is still greater than the set value at this time, it means that the fault occurs in the differential current signal protection. In the area, that is, the area from the rectifier valve or inverter valve to the DC outlet, this area is outside the protection area, so the protection is reset; if the differential current signal is less than the set value, it is determined that the fault occurred in the high-voltage DC transmission line, and the protection operates.

Embodiment 2: As shown in Figure 1, a ±800kV UHV DC transmission model. In this model, the rated power of the DC transmission is 5000MW, the rated current is 3125A, the rectification side: the voltage of the AC system is 525kV, the short-circuit ratio SCR=2.5/840, the reactive power compensation capacity is 3000Mvar, and the DC filter is 12/24/ 36 triple-tuned filters, each pole commutation unit is composed of two 12-pulse converters connected in series, and the voltage in series is distributed according to ±(400+400)kV. The reactive power compensation capacity of the inverter side is 3040Mvar, and other parameters are consistent with the rectification side. The total length of the direct current transmission line is 1500km (the actual project is 1418km), and the control system is established according to the test system of the direct current transmission standard of the International Conference on Large Power Grids (CIGRE).

As shown in Figure 1, after various faults occurred in the f1~f6 area, statistics were made on each criterion, and the results are shown in Table 1-Table 4.

Table 1 Statistics of differential current signals at the time of fault occurrence under various faults in areas f1 to f6

Fault type f1 f3 f4 I _re1,2 /pu 0.004 0.1 0.003 I _inv1,2 /pu 0.004 0.001 0.1

Table 2 Statistics of differential current signals after the end of blocking under faults in f1, f3, and f4 areas

Fault type f1 f2 f5 three-phase short circuit f5 single phase short circuit F5 two-phase short circuit F5 two-phase short circuit to ground α _max /° 90 36 5 47 45 25

Table 3 Statistics of the maximum value of α after the end of the lockout under various faults in areas f1, f2, and f5

Fault type f1 f6 three-phase short circuit f6 single-phase short circuit F6 two-phase short circuit F6 two-phase short circuit to ground γ _max /° 90 18 36 31 27

Table 4 Statistics of the maximum value of γ after the end of the lockout under various faults in areas f1 and f6

When setting the criterion, it must be ensured that this set of protection schemes will not malfunction if any fault occurs outside the zone. Therefore, the most difficult to identify fault outside the area should be used as the benchmark when setting. For the starting criterion, it should be guaranteed that all faults can be started; for the α criterion, it should be guaranteed to identify all faults in the f2 and f5 regions; for the γ criterion, it should be guaranteed to be able to identify all the faults in the f6 region; for the differential flow criterion , it should ensure that all faults in f3 and f4 areas can be identified; in summary, in the setting of α criterion, α _max should take the maximum value of α angle that the control system may output in all faults in f2 and f5 areas; the setting of γ criterion The value should be set according to the maximum value of γ angle γ _max that may occur in all faults in the f6 area; the calculation of the setting value of the differential current criterion should be based on the initial blocking time of the fault, and the maximum differential current that may occur during a short-circuit fault of f1 value is adjusted. The setting results are shown in Table 5.

Table 5 The setting values of each criterion

Taking short-circuit faults of 20Ω, 100Ω and 200Ω at the midpoint of the positive line at 0.1s as an example, the simulation results are shown in Figure 7.

It can be seen from Figure 2-5 that the protection can operate correctly when the transition resistance is 20Ω, 100Ω and 200Ω. The action degree of the trigger angle α and cut-off angle γ on the rectifier side shows a downward trend with the increase of the transition resistance, and the time to reach the set value increases, but after the end of the blocking, they are both greater than the set value and remain stable; the transition The resistance limits the free component of the short-circuit current to a certain extent, so the amplitude and decay time of the differential current on both sides will decrease with the increase of the transition resistance. . In addition, through a large number of simulations, this paper has obtained the "critical" transition resistance of the correct action of each criterion of the backup protection when the DC line is faulty, all of which are greater than 300Ω.

Taking the metallic grounding faults of 100km, 600km, and 1400km positive lines from the M terminal at 0.1s as an example, the simulation results are shown in Figure 8.

It can be seen from Figure 6-9 that this protection can operate correctly when there is a metal short-circuit fault on the positive line 100km, 600km, and 1400km away from the M terminal. However, with the increase of the fault distance, the duration of the 2nd harmonic component of the DC voltage at the initial stage of the fault gradually increases, but after the initial blockage of the fault, it decreases to around 0; The influence of fault distance; the farther the fault distance is, the longer it takes for the rectifier side differential current criterion to drop to near 0, while the time required for the inverter side differential current criterion to fall to near 0 increases with the increase of fault distance. Decrease after increasing.

Table 4 shows the protection actions of f2-f6 in Figure 1 under different transition resistance and fault location conditions. It can be seen from Table 4 that the DC line backup protection proposed in this paper can still operate reliably when the remote high-impedance grounding fault occurs. At the same time, considering the protection blocking time, data transmission time and protection criterion calculation time, the protection should be able to operate reliably in about 300ms . DC differential protection is mainly used to detect remote high-resistance ground faults, but its action delay is up to 1.1s. Therefore, compared with DC differential protection, the protection action time is greatly improved.

Table 6 Protection actions under different transition resistances

The specific embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings, but the present invention is not limited to the above embodiments. Variations.

Claims (5)

1. A backup protection method for high-voltage direct current transmission lines based on a direct current control system, characterized in that: the specific steps of the method are: (1) In the DC system, the area from the rectifier side DC outlet to the inverter side DC outlet is a high-voltage DC transmission line. The faults that occur in this area are internal faults, and the protection operates. The rest of the faults are external faults, and the protection does not operate. ; (2) Monitor the differential current signals on the rectification side and the inverter side in real time. The differential current signal on the rectification side is the difference between the current at the rectifier valve and the current at the DC outlet of the rectification side, which is recorded as ΔI _re ; the differential current signal on the inverter side is The difference between the current at the inverter valve and the current at the DC outlet of the inverter side is recorded as ΔI _inv ; use these two differential current signals to detect the current free component caused by faults in the DC transmission system, when ΔI _re or ΔI _inv is greater than The action current threshold value I _set of the differential current protection indicates that there is a fault in the DC transmission system, the backup protection is started, and the trigger angle α criterion is started after the protection blocking is completed to further judge the area where the fault occurs; (3) After the blocking time t _s is blocked, the α criterion is started, and the method of α criterion is as follows: The sampling value of the trigger angle α on the rectification side is extracted from the extreme control measurement unit of the DC transmission system, and the sampling value is compared with the _set threshold value α _set . Reset; if established, start the γ criterion to further judge the area where the fault occurs; (4) The discriminant method of γ criterion is: The sampling value of the turn-off angle γ of the inverter side is extracted from the extreme control measurement unit, and the sampling value is compared with the set threshold value γ _set . If the sampling value γ>γ _set is not established, it is judged to be an out-of-area fault and the protection is reset. ; If true, start the differential flow criterion to further judge the area where the fault occurs; (5) The discriminant method of differential flow criterion is: Sampling the differential current ΔI _re and ΔI _inv to judge whether the differential current ΔI _re >I _set and ΔI _inv >I _set is true. If any formula is established, it can be judged that the fault is a fault in the corresponding area, which is an external fault, and the protection is reset; if two If none of the formulas is established, it is judged as an internal fault. 2. The backup protection method for HVDC transmission line based on DC control system according to claim 1, characterized in that: The blocking time t _s described in step (3) is specifically determined as: In order to avoid the interference of the attenuation free component produced by the nonlinear element on various physical quantities after the fault, and cooperate with the main protection of the system at the same time, set the blocking time t _s after the protection starts, and take the time taken for the free component to decay to zero and the time used for avoiding the main protection The larger value among the times is taken as the blocking time t _s . 3. The backup protection method for HVDC transmission line based on DC control system according to claim 1, characterized in that: The setting scheme of the action setting value α _set of the α criterion described in step (3) is: Build a simulation model based on the actual parameters of the DC transmission system, set various extreme faults on the rectification side AC line and the opposite pole line of the simulation system, and obtain the maximum value of the firing angle on the rectification side of the system under various extreme fault conditions. α _max , set α _set = k ₁ α _max , k ₁ is the reliability coefficient. 4. The backup protection method for HVDC transmission line based on DC control system according to claim 1, characterized in that: The setting scheme of the action setting value γ _set of the described γ criterion of step (4) is: Faults under various extreme conditions are set on the AC line of the inverter side of the simulation system, and the maximum value γ _max of the turn-off angle of the inverter side under various extreme fault conditions is obtained by simulation, and the setting γ _set = k ₂ γ _max , k ₂ is the reliability factor. 5. The backup protection method for HVDC transmission line based on DC control system according to claim 1, characterized in that: The setting scheme of the differential flow criterion described in step (5) is specifically: After the blocking time t _s ends, the free component released by the nonlinear element basically decays to 0, and the interference effect of the free component disappears. If the differential current signal ΔI _re or ΔI _inv is still greater than the set value at this time, it means that the fault occurs in the differential current signal protection. In the area, that is, the area from the rectifier valve or inverter valve to the DC outlet, this area is outside the protection area, so the protection is reset; if the differential current signal is less than the set value, it is determined that the fault occurred in the high-voltage DC transmission line, and the protection operates.