A hybrid DC power transmission start-up control system and method

Abstract

The present invention discloses a hybrid DC power transmission startup control system and method, the system includes a conventional DC side device, a flexible DC side device, a control protection device, a conventional DC valve control and a flexible DC valve control, the conventional DC side device and the flexible DC side device are connected through a transmission line, and the control protection device is connected to the conventional DC side device and the flexible DC side device through a control line; the present invention discloses a hybrid DC power transmission startup control method, which completes the charging of the conventional DC side device and the flexible DC device and completes the unlocking process of the conventional DC and the flexible DC through the control protection device and the conventional DC valve control and the flexible DC valve control. The hybrid DC power transmission startup control system and method of the present invention can realize the smooth and stable startup of the hybrid DC power transmission system, the implementation method is simple, and it is convenient to control the startup process.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current power transmission, and in particular to a hybrid direct current power transmission starting control system and method. Background Art

[0002] In recent years, with the development of power electronic devices and the application of renewable energy generation technologies such as wind power and solar energy, flexible DC transmission technology has been rapidly developed and applied at home and abroad. Compared with traditional high-voltage DC transmission technology, flexible DC transmission based on modular multi-level converters can independently control active and reactive power, and the control is more flexible. It is a new generation of DC transmission technology, which has been widely used in the fields of new energy grid connection, long-distance transmission of renewable energy, and flexible grid partition interconnection.

[0003] Conventional DC is mature in application and more economical. Hybrid DC transmission technology solutions with conventional DC at the sending end and flexible DC at the receiving end are gradually being applied to engineering. Previously, there were few studies on hybrid DC transmission technology solutions for this method. There is no literature reporting on the specific unlocking strategy after charging of the flexible DC converter valve and the control logic and timing of the conventional DC side startup in the hybrid DC transmission system. The hybrid DC transmission system startup control method proposed in the present invention and the receiving end flexible DC constant voltage control can quickly provide reactive power support to the power grid when the AC voltage of the receiving end large power grid drops due to an AC fault, thereby ensuring the stability of the AC voltage of the power grid and the safety and stability of the large power grid. Conventional DC is used at the sending end without losing economy.

[0004] The control method proposed in the present invention has been fully tested and verified on a three-terminal hybrid DC physical prototype platform, and can be used as a reference method for starting a hybrid DC transmission system in engineering or developing physical prototypes. Summary of the invention

[0005] In view of the above-mentioned defects of the prior art, the purpose of the embodiments of the present invention is to provide a hybrid DC transmission starting control system and method, which can achieve smooth and stable starting of the hybrid DC transmission system, and the implementation process is simple, which facilitates the control of the starting process.

[0006] To achieve the above object, the present invention provides a hybrid DC power transmission start-up control system, the system comprising a conventional DC side device, a flexible DC side device, a control and protection device, a conventional DC valve control and a flexible DC valve control;

[0007] The conventional DC side device includes a first AC circuit breaker, a filter, a first switch, a converter transformer, a thyristor converter valve and a smoothing reactor;

[0008] The flexible DC side device includes a second AC circuit breaker, a charging resistor, a second switch, a flexible DC transformer and a flexible DC converter valve;

[0009] The first terminal of the first AC circuit breaker is connected to the first power supply, the second terminal of the first AC circuit breaker is connected to the first terminal of the filter, the second terminal of the filter is connected to the first terminal of the converter transformer, in addition, the first switch is connected in parallel between the first terminal and the second terminal of the filter, the second terminal of the converter transformer is connected to the first terminal of the thyristor converter valve, and the second terminal of the thyristor converter valve is connected to the smoothing reactor;

[0010] The first terminal of the second AC circuit breaker is connected to the second power supply, the second terminal of the second AC circuit breaker is connected to the first terminal of the charging resistor, the second terminal of the charging resistor is connected to the first terminal of the flexible DC transformer, in addition, the second switch is connected in parallel between the first terminal and the second terminal of the charging resistor, and the second terminal of the flexible DC transformer is connected to the flexible DC converter valve;

[0011] The smoothing reactor of the conventional DC side device and the flexible DC converter valve of the flexible DC side device are connected via a transmission line;

[0012] The control protection device is connected to the conventional DC side device via a control line;

[0013] The control protection device is connected to the flexible DC side device through a control line.

[0014] Preferably, the conventional DC side device further includes a closing resistor and a third switch;

[0015] The closing resistor is connected in parallel with the third switch, a first end of the closing resistor is connected to a second end of the filter, and a second end of the closing resistor is connected to a first end of the converter transformer.

[0016] Preferably, the flexible DC side device further includes a bridge arm reactor;

[0017] The first end of the bridge arm reactor is connected to the second end of the flexible DC transformer, and the second end of the bridge arm reactor is connected to the first end of the flexible DC converter valve.

[0018] Furthermore, the control protection device is connected to the conventional DC side device through a control line, specifically:

[0019] The control protection device is connected to the conventional DC valve control via a control line, and the conventional DC valve control is connected to the thyristor commutation valve via a control line;

[0020] The control protection device is connected to the first AC circuit breaker via a control line;

[0021] The control protection device is connected to the first switch via a control line;

[0022] The control protection device is connected to the third switch via a control circuit;

[0023] The control protection device is connected to the converter transformer via a control line.

[0024] Furthermore, the control protection device is connected to the flexible DC side device through a control line specifically as follows:

[0025] The control protection device is connected to the flexible DC valve control via a control line, and the flexible DC valve control is connected to the flexible DC converter valve via a control line;

[0026] The control protection device is connected to the second AC circuit breaker via a control line;

[0027] The control protection device is connected to the second switch via a control line;

[0028] The control protection device is connected to the flexible DC transformer via a control line.

[0029] The control and protection device also includes a control workbench, which is connected to an internal computing board of the device via a data line, and the internal computing board is connected to the conventional DC valve control and the flexible DC valve control via a control line.

[0030] The present invention proposes a hybrid DC power transmission start-up control system, in which the control protection device, conventional DC valve control and flexible DC valve control of the system can control the charging process of the conventional DC side device and the flexible DC side, can control the unlocking of the flexible DC and conventional DC, and can complete the control of the hybrid DC power transmission start-up. In addition, when the conventional DC side device is charged, the closing resistor and smoothing reactor of the system can reduce the commutation transformer excitation surge current and AC pulse component at the conventional DC end; when the flexible DC side device is charged, the current limiting effect of the charging resistor of the system and the filtering effect of the bridge arm reactor can maintain the stability of the circuit. When the flexible DC and conventional DC are unlocked, the starting current values ​​of the conventional DC and the flexible DC can be set by controlling the protection device, so that the current value of the starting process increases according to the slope, and the control device can adjust the instruction of the DC current setting value so that the system reaches the rated working state. This hybrid DC power transmission start-up control system has a simple structure, and can control the startup process by controlling the protection device, conventional DC valve control and flexible DC valve control, so that the hybrid DC power transmission system can achieve smooth and stable startup.

[0031] The present invention also provides a hybrid DC power transmission startup control method, using the hybrid DC power transmission startup control system, the method is:

[0032] The control protection device sends a first charging instruction to the conventional DC side device, so that the converter transformer of the conventional DC side device is charged, and the thyristor converter valve of the conventional DC side device is charged;

[0033] The control protection device sends a waiting instruction to the conventional DC side device, and the conventional DC side device waits for the charging and unlocking process of the flexible DC side device after receiving the instruction;

[0034] The control protection device sends a second charging instruction to the flexible DC side device, so that the flexible DC converter valve of the flexible DC side device performs uncontrolled charging and controlled charging;

[0035] The flexible DC valve control determines that the uncontrolled charging is completed, and sends a signal to the control protection device. The control protection device sends a first unlocking instruction to the flexible DC valve control. The flexible valve control controls the flexible DC converter valve to make the flexible DC voltage rise to the rated DC voltage according to the set slope.

[0036] The control protection device controls the disconnection of the first switch and puts the filter into the circuit. The control protection device sends a second unlocking instruction to the conventional DC valve control. The conventional DC valve control controls the thyristor commutator valve to cancel the phase shift and make the conventional DC run according to the DC current set value.

[0037] Furthermore, the control protection device sends a first charging instruction to the conventional DC side device to charge the converter transformer of the conventional DC side device and to charge the thyristor converter valve of the conventional DC side device specifically as follows:

[0038] The control and protection device sends a first charging instruction to control the closing of the first circuit breaker on the conventional DC side, closes the first switch in parallel with the filter, and opens the third switch in parallel with the closing resistor. The current charges the converter transformer through the closing resistor. The conventional DC valve control receives the signal from the control and protection device to charge the thyristor converter valve, and is filtered through the smoothing reactor to complete the charging of the conventional DC side device.

[0039] Furthermore, the control and protection device sends a second charging instruction to the flexible DC side device, so that the flexible DC converter valve of the flexible DC side device performs uncontrolled charging and controllable charging specifically as follows:

[0040] The second charging instruction sent by the control and protection device controls the closing of the second AC circuit breaker of the flexible DC side device and disconnects the second switch. After the current passes through the charging resistor, the flexible DC transformer, and the bridge arm reactor, the flexible DC converter valve is charged uncontrolled. After the flexible DC valve control determines that the uncontrolled charging is completed, it sends a signal to the control and protection device. The control and protection device sends a controllable charging instruction to the flexible DC side device, closes the second switch, cuts off the charging switch, and continues to controllably charge the flexible DC converter valve.

[0041] The present invention proposes a hybrid DC power transmission startup control method, which charges a conventional DC side device and a flexible DC side device through a control protection device, then unlocks the flexible DC side device and the conventional DC side device, and during the charging process of the conventional DC side device, reduces the commutation transformer excitation surge current and AC pulse component of the conventional DC side device through a closing resistor and a smoothing reactor; during the charging process of the flexible DC side device, the circuit stability can be maintained through the current limiting effect of the charging resistor and the filtering effect of the bridge arm reactor; during the unlocking process of the flexible DC and conventional DC, the DC voltage can be ramped up according to a set slope through the control of the control protection device, the conventional DC valve control and the flexible DC valve control, and the system can reach the rated working state by adjusting the set slope and the DC current setting value instruction. This hybrid DC power transmission startup control method can enable the hybrid DC power transmission system to achieve smooth and stable startup, and the process can be easily controlled through the control protection device. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a structural diagram of an embodiment of a hybrid direct current transmission starting control system provided by the present invention.

[0043] Figure 2 It is a structural diagram of another embodiment of a hybrid direct current transmission starting control system provided by the present invention.

[0044] Figure 3 It is a flow chart of an embodiment of a hybrid direct current transmission starting control method provided by the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] See also Figure 1 , is a structural diagram of an embodiment of a hybrid DC transmission starting control system provided by the present invention.

[0047] like Figure 1 As shown, a hybrid DC power transmission start-up control system includes a conventional DC side device 207, a flexible DC side device 212, a control protection device 214, a conventional DC valve control 215 and a flexible DC valve control 216;

[0048] The conventional DC side device 207 includes a first AC circuit breaker 202, a filter 203, a first switch 204, a converter transformer 205, a thyristor converter valve 206 and a smoothing reactor 219;

[0049] During the implementation process, the circuit connection of the conventional DC side device 207 is specifically as follows: the first end of the first AC circuit breaker 202 is connected to the first power supply 201, the second end of the first AC circuit breaker 202 is connected to the first end of the filter 203, the second end of the filter 203 is connected to the first end of the converter transformer 205, and in addition, a first switch 204 is connected in parallel between the first end and the second end of the filter 203, the second end of the converter transformer 205 is connected to the first end of the thyristor converter valve 206, and the second end of the thyristor converter valve 206 is connected to the smoothing reactor 219.

[0050] The first AC circuit breaker 202 is used to disconnect the AC power supply from the conventional DC side device and provide circuit fault protection;

[0051] The converter transformer 205 is used to convert the AC system voltage into the commutation voltage required by the thyristor converter valve;

[0052] The filter 203 can reduce the harmonics in the AC voltage from entering the converter transformer and the thyristor converter valve;

[0053] The thyristor commutation valve 206 is used to convert the AC voltage of the power supply into a DC voltage;

[0054] The control protection device 214 can control the closing of the first AC circuit breaker 202 of the conventional DC side device 207, charge the converter transformer 205 from the first power supply 201, and control the conventional DC valve control 215 to charge the thyristor converter valve 206;

[0055] The control protection device 214 can control the second AC circuit breaker 209 to charge the flexible DC transformer 210, and complete the uncontrolled charging and controllable charging of the flexible DC converter valve 211 by controlling the flexible DC valve control 216;

[0056] After receiving the controllable charging completion signal, the control protection device 214 sends an unlocking flexible DC command to the flexible DC valve control 216 to control the flexible DC converter valve 211 so that the DC voltage is ramped up to the rated DC voltage according to the set slope;

[0057] After the flexible DC is unlocked, the control protection device 214 can control the first switch 204 to be disconnected, the filter 203 to be put into operation, and the conventional DC valve control 215 to cancel the phase shift of the thyristor commutation valve 206, so that the conventional DC runs according to the DC current setting value.

[0058] The flexible DC side device 212 includes a second AC circuit breaker 209, a charging resistor 217, a second switch 218, a flexible DC transformer 210 and a flexible DC converter valve 211;

[0059] The circuit connection of the flexible DC side device 212 is specifically as follows: the first end of the second AC circuit breaker 209 is connected to the second power source 208, the second end of the second AC circuit breaker 209 is connected to the first end of the charging resistor 217, the second end of the charging resistor 217 is connected to the first end of the flexible DC transformer 210, wherein a second switch 218 is connected in parallel between the first end and the second end of the charging resistor 217, and the second end of the flexible DC transformer 210 is connected to the flexible DC converter valve 211;

[0060] The smoothing reactor 219 of the conventional DC side device 207 and the flexible DC converter valve 211 of the flexible DC side device 212 are connected via a transmission line 213;

[0061] The control protection device 214 is connected to the conventional DC valve control 215 and the flexible DC valve control 216 through a control line;

[0062] The conventional DC valve control 215 is connected to the thyristor commutator valve 206 of the conventional DC side device 207 , and the flexible DC valve control 216 is connected to the flexible DC commutator valve 211 of the flexible DC side device 212 .

[0063] The charging resistor 217 can limit the size of the charging current for the flexible DC converter valve to prevent the large charging current generated by the short circuit at the moment of power-on from damaging the device;

[0064] The smoothing reactor 219 can suppress the AC pulsation component of the current after the thyristor commutation valve commutates, so that the output DC is close to the ideal;

[0065] The control protection device 214 is also directly connected to the conventional DC side device 207 and the flexible DC side device 212 through the control line;

[0066] Specifically, the control protection device 214 is connected to the first AC circuit breaker 202, the first switch 204 and the converter transformer 205 of the conventional DC side device 207 through a control line;

[0067] The control protection device 214 is connected to the second AC circuit breaker 209 of the flexible DC side device 212 and the flexible DC transformer 210 through a control line;

[0068] The control protection device 214 is connected to the conventional DC valve control 215 and the flexible DC valve control 216 through a control line;

[0069] In this embodiment, the control and protection device controls the charging of the conventional DC side and the flexible DC side devices, completes the unlocking and starting of the conventional DC through the conventional DC valve control, and completes the unlocking and starting of the flexible DC through the flexible DC valve control, thereby realizing the smooth startup of the hybrid DC transmission system and facilitating process control.

[0070] Another embodiment of a hybrid DC transmission starting control system proposed by the present invention is shown in Figure 2 , like Figure 2 As shown, compared with the previous embodiment, the difference is that the system of this embodiment further includes a closing resistor 301 , a third switch 302 and a bridge arm reactor 303 .

[0071] The first end and the second end of the closing resistor 301 are connected in parallel with a third switch 302, the first end of the closing resistor 301 is connected to the filter 203, and the second end of the closing resistor is connected to the first end of the converter transformer 205;

[0072] The first end of the bridge arm reactor 303 is connected to the second end of the flexible DC transformer 210 , and the second end of the bridge arm reactor 303 is connected to the first end of the flexible DC converter valve 211 .

[0073] The closing resistor 301 can reduce the commutation transformer excitation inrush current caused by the closing moment of the first AC circuit breaker during the charging process of the conventional DC side device, and can limit the charging current during the charging process of the thyristor commutation valve;

[0074] The bridge arm reactor 303 can suppress the short-circuit current, play a certain filtering role, and suppress the harmonic components of the output voltage of the flexible DC converter valve.

[0075] The closing resistor and the bridge arm reactor of this embodiment can play a role in maintaining the stability of the hybrid DC power transmission system during the startup process.

[0076] It should be noted that in the hybrid DC transmission start-up control system provided in the above embodiment, the control workbench of the control protection device can set a DC current unlocking instruction. After receiving the instruction, the control protection device outputs the instruction to the conventional DC valve control and the flexible DC valve control through the calculation of the internal board of the device, thereby controlling the converter valves at both ends and controlling the DC current to run according to the set value;

[0077] The control workbench of the control protection device will display the current operating parameters of the system, such as DC voltage, DC current, power, etc. By observing the display screen, it can be determined whether it is running stably under the set working conditions.

[0078] The present invention also proposes a hybrid direct current power transmission starting control method, which applies a hybrid direct current power transmission starting control system.

[0079] See also Figure 3 , Figure 3 FIG. 1 is a flow chart of an embodiment of a hybrid DC transmission startup control method provided by the present invention. Figure 3 As shown, the process of implementing this method is:

[0080] S401, controlling the protection device to send a first charging instruction to the conventional DC side device, so that the converter transformer of the conventional DC side device is charged, and the thyristor converter valve of the conventional DC side device is charged;

[0081] S402, the control protection device sends a waiting instruction to the conventional DC side device, and the conventional DC side device waits for the charging and unlocking process of the flexible DC side device after receiving the instruction;

[0082] S403, the control protection device sends a second charging instruction to the flexible DC side device, so that the flexible DC converter valve of the flexible DC side device performs uncontrolled charging and controlled charging;

[0083] S404, the flexible DC valve control determines that the uncontrolled charging is completed, and sends a signal to the control protection device, and the control protection device sends a first unlocking instruction to the flexible DC valve control, and the flexible valve control controls the flexible DC converter valve to increase the flexible DC voltage to the rated DC voltage according to the set slope;

[0084] S405, the control protection device controls to disconnect the first switch, puts the filter into the circuit, and sends a second unlocking instruction to the conventional DC valve control. The conventional DC valve control controls the thyristor commutator valve to cancel the phase shift and makes the conventional DC run according to the DC current set value.

[0085] In the specific implementation of this embodiment, the first charging instruction sent by the control protection device controls the first AC circuit breaker of the conventional DC side device to close, so that the first power supply is connected to the conventional DC side device, the control protection device controls the first switch to close, the third switch to open, the filter is cut out of the circuit, the closing resistor is put into the circuit, the first power supply charges the converter transformer through the closing resistor, and the thyristor converter valve is charged through the converter transformer. The thyristor converter valve is also connected to a smoothing reactor to filter the circuit after commutation, so that the current output by the thyristor converter valve is closer to the ideal DC. After the thyristor converter valve is charged, the current is transmitted to the smoothing reactor, and the charging process of the conventional DC side device is completed;

[0086] The control protection device sends a waiting instruction to the conventional DC side device, and the conventional DC side device waits for the charging and unlocking process of the flexible DC side device after receiving the instruction;

[0087] The second charging instruction sent by the control and protection device controls the closing of the second AC circuit breaker and the disconnection of the second switch to cut the charging resistor into the circuit. The second power supply charges the flexible DC transformer through the charging resistor, and performs uncontrolled charging on the flexible DC converter valve through filtering of the bridge arm inductor. The flexible DC valve control determines that the uncontrolled charging is completed and sends a signal to the control and protection device. The control and protection device controls the closing of the second switch and cuts out the charging resistor. The control and protection device sends a controllable charging instruction to the flexible DC valve control to control the controllable charging of the flexible DC converter valve.

[0088] The flexible DC valve control determines that the uncontrolled charging is completed, and sends a signal to the control protection device. The control protection device sends a first unlocking instruction to the flexible DC valve control. The flexible valve control controls the flexible DC converter valve to make the flexible DC voltage rise to the rated DC voltage according to the set slope.

[0089] The control protection device controls the disconnection of the first switch and puts the filter into the circuit. The control protection device sends a second unlocking instruction to the conventional DC valve control. The conventional DC valve control controls the thyristor commutator valve to cancel the phase shift and make the conventional DC run according to the DC current set value.

[0090] The slope and the DC current setting value are set on the control workbench of the control protection device. The control protection device receives the set instructions, and through the calculation of the internal board of the device, the control protection device gives control instructions to the conventional DC valve control and the flexible DC valve control, respectively controlling the thyristor commutator valve and the flexible DC commutator valve, so that the DC current runs according to the set value.

[0091] The present invention proposes a hybrid DC power transmission startup control system and method, which is composed of a conventional DC side device and a flexible DC side device connected through a transmission line. The conventional DC side device includes an AC circuit breaker, a filter, a first switch, a closing resistor, a third switch, a converter transformer, a thyristor converter valve and a smoothing reactor; the flexible DC end includes an AC circuit breaker, a charging resistor, a second switch, a flexible DC transformer, a bridge arm reactor and a flexible DC converter valve; the conventional DC side device and the flexible DC side device are controlled by conventional DC valve control, flexible DC valve control and control protection devices. The circuit is filtered and limited by the closing resistor, the smoothing reactor, the charging resistor and the bridge arm reactor, which can maintain the stability of the circuit. In the process of charging and unlocking the flexible DC and conventional DC, the setting and control of the control protection device can enable the hybrid DC power transmission system to achieve smooth and stable startup, and the implementation process is simple, which is convenient for controlling the startup process.

[0092] It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A hybrid DC transmission starting control system, characterized in that: The system includes a conventional DC side device, a flexible DC side device, a control and protection device, a conventional DC valve control and a flexible DC valve control; The conventional DC side device includes a first AC circuit breaker, a filter, a first switch, a converter transformer, a thyristor converter valve and a smoothing reactor; The flexible DC side device includes a second AC circuit breaker, a charging resistor, a second switch, a flexible DC transformer and a flexible DC converter valve; The first terminal of the first AC circuit breaker is connected to the first power supply, the second terminal of the first AC circuit breaker is connected to the first terminal of the filter, the second terminal of the filter is connected to the first terminal of the converter transformer, in addition, the first switch is connected in parallel between the first terminal and the second terminal of the filter, the second terminal of the converter transformer is connected to the first terminal of the thyristor converter valve, and the second terminal of the thyristor converter valve is connected to the smoothing reactor; The first terminal of the second AC circuit breaker is connected to the second power supply, the second terminal of the second AC circuit breaker is connected to the first terminal of the charging resistor, the second terminal of the charging resistor is connected to the first terminal of the flexible DC transformer, in addition, the second switch is connected in parallel between the first terminal and the second terminal of the charging resistor, and the second terminal of the flexible DC transformer is connected to the flexible DC converter valve; The smoothing reactor of the conventional DC side device and the flexible DC converter valve of the flexible DC side device are connected via a transmission line; The control protection device is connected to the conventional DC side device via a control line; The control protection device is connected to the flexible DC side device via a control line; The conventional DC side device also includes a closing resistor and a third switch; The closing resistor is connected in parallel with the third switch, the first end of the closing resistor is connected to the second end of the filter, and the second end of the closing resistor is connected to the first end of the converter transformer; The flexible DC side device also includes a bridge arm reactor; The first end of the bridge arm reactor is connected to the second end of the flexible DC transformer, and the second end of the bridge arm reactor is connected to the first end of the flexible DC converter valve.

2. A hybrid DC power transmission starting control system according to claim 1, characterized in that: The control protection device is connected to the conventional DC side device through a control line, specifically: The control protection device is connected to the conventional DC valve control via a control line, and the conventional DC valve control is connected to the thyristor commutation valve via a control line; The control protection device is connected to the first AC circuit breaker via a control line; The control protection device is connected to the first switch via a control line; The control protection device is connected to the third switch via a control circuit; The control protection device is connected to the converter transformer via a control line.

3. A hybrid DC power transmission starting control system according to claim 1, characterized in that: The control protection device is connected to the flexible DC side device through a control line specifically as follows: The control protection device is connected to the flexible DC valve control via a control line, and the flexible DC valve control is connected to the flexible DC converter valve via a control line; The control protection device is connected to the second AC circuit breaker via a control line; The control protection device is connected to the second switch via a control line; The control protection device is connected to the flexible DC transformer via a control line.

4. A hybrid DC power transmission starting control system according to claim 1, characterized in that: The control and protection device also includes a control workbench, which is connected to an internal computing board of the device via a data line, and the internal computing board is connected to the conventional DC valve control and the flexible DC valve control via a control line.

5. A hybrid DC transmission startup control method, characterized in that: A hybrid DC power transmission starting control system according to any one of claims 1 to 4 is adopted, wherein the process is as follows: The control protection device sends a first charging instruction to the conventional DC side device, so that the converter transformer of the conventional DC side device is charged, and the thyristor converter valve of the conventional DC side device is charged; The control protection device sends a waiting instruction to the conventional DC side device, and the conventional DC side device waits for the charging and unlocking process of the flexible DC side device after receiving the instruction; The control protection device sends a second charging instruction to the flexible DC side device, so that the flexible DC converter valve of the flexible DC side device performs uncontrolled charging and controlled charging; The flexible DC valve control determines that the uncontrolled charging is completed, and sends a signal to the control protection device. The control protection device sends a first unlocking instruction to the flexible DC valve control. The flexible valve control controls the flexible DC converter valve to make the flexible DC voltage rise to the rated DC voltage according to the set slope. The control protection device controls the disconnection of the first switch and puts the filter into the circuit. The control protection device sends a second unlocking instruction to the conventional DC valve control. The conventional DC valve control controls the thyristor commutator valve to cancel the phase shift and make the conventional DC run according to the DC current set value.

6. A hybrid DC transmission startup control method according to claim 5, characterized in that: The control protection device sends a first charging instruction to the conventional DC side device to charge the converter transformer of the conventional DC side device and to charge the thyristor converter valve of the conventional DC side device as follows: The control and protection device sends a first charging instruction to control the closing of the first circuit breaker on the conventional DC side, closes the first switch in parallel with the filter, and opens the third switch in parallel with the closing resistor. The current charges the converter transformer through the closing resistor. The conventional DC valve control receives the signal from the control and protection device to charge the thyristor converter valve, and is filtered through the smoothing reactor to complete the charging of the conventional DC side device.

7. A hybrid DC transmission startup control method according to claim 6, characterized in that: The control and protection device sends a second charging instruction to the flexible DC side device, so that the flexible DC converter valve of the flexible DC side device performs uncontrolled charging and controllable charging. Specifically: The second charging instruction sent by the control and protection device controls the closing of the second AC circuit breaker of the flexible DC side device and disconnects the second switch. After the current passes through the charging resistor, the flexible DC transformer, and the bridge arm reactor, the flexible DC converter valve is charged uncontrolled. After the flexible DC valve control determines that the uncontrolled charging is completed, it sends a signal to the control and protection device. The control and protection device sends a controllable charging instruction to the flexible DC side device, closes the second switch, cuts off the charging switch, and continues to controllably charge the flexible DC converter valve.

8. A hybrid DC transmission startup control method according to claim 5, characterized in that: The set slope and the DC current set value are set on the control workbench of the control protection device. The control protection device receives the set instructions, and through the calculation of the internal board of the device, the control protection device gives conventional DC valve control and flexible DC valve control control instructions to control the thyristor commutator valve and the flexible DC commutator valve respectively, so that the DC current runs according to the set value.

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