Flexible direct current starting loop and operation method thereof

Abstract

The invention discloses a flexible direct-current starting loop and an operation method thereof, and belongs to the field of primary main wiring design of flexible direct-current power transmission power equipment, during operation, a starting resistor only bears voltage and does not bear current during operation, the starting resistor only needs to be configured with over-current protection, and protection is simple; according to the method, a valve side circuit breaker is additionally arranged on a connection transformer valve side, when equipment such as a converter valve breaks down, protection action consequences can act on the valve side circuit breaker and a network side circuit breaker at the same time, converter valve short-circuit current can be cut off only through opening of one circuit breaker, and the other circuit breaker can serve as near backup protection and failure action consequences.

Description

Technical Field

[0001] The invention belongs to the field of primary main connection design of flexible direct current power transmission power equipment, and in particular relates to a flexible direct current starting circuit and an operation method thereof. Background Art

[0002] With the rapid development of my country's economy, a large number of ultra-high voltage and ultra-high voltage direct current transmission projects have been put into operation, and the flexible direct current transmission system has been more and more widely used in the power grid. The flexible direct current transmission system has the characteristics of low harmonic level, power supply for passive systems, small footprint, and convenient flow reversal. However, at present, the starting circuit of the flexible direct current transmission project adopts the method of connecting the starting resistor and the bypass circuit breaker in parallel between the connecting transformer and the bridge arm reactor. The existing wiring method has the disadvantages of complex wiring of the starting resistor protection, low sensitivity of the starting resistor protection, no isolation point during maintenance on the converter transformer valve side, and failure of the circuit breaker after the DC system protection is activated when used for the grid-type flexible direct current system. There is no backup circuit breaker nearby. Once the circuit breaker on the converter transformer grid side refuses to operate in the existing wiring method, the scope of power outage will be expanded, and the converter valve IGBT equipment will flow through the short-circuit current for a long time, resulting in equipment damage. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a flexible DC starting circuit and an operating method thereof. In this method, the starting resistor only bears voltage but not current during operation. The starting resistor only needs to be configured with overcurrent protection, and the protection is simple. This method adds a valve side circuit breaker on the connecting transformer side. When a fault occurs in the converter valve and other equipment, the protection action can act on the valve side circuit breaker and the grid side circuit breaker at the same time. Only one circuit breaker is opened to cut off the short-circuit current of the converter valve, and the other circuit breaker can be used as a close backup protection and failure action.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: A flexible DC starting circuit includes a starting resistor, one side of which is connected in series with a photocurrent transformer, the other side of which is connected in series with a series switch, and then connected in parallel with a bypass switch; One side of the photocurrent transformer is connected to the circuit breaker on the connecting transformer valve side, and the circuit breaker on the connecting transformer valve side is connected to the connecting transformer; One side of the series knife switch is connected to the converter valve bridge arm reactor and the converter valve.

[0005] A method for operating a flexible DC starting circuit, characterized by the following steps: During the converter valve charging process, the starting resistor is first connected to the converter valve through a series knife switch, and then the grid-side circuit breaker of the connecting transformer is closed in turn to charge the connecting transformer. After the valve-side voltage of the connecting transformer reaches the rated voltage value, the valve-side circuit breaker is closed; Since the converter valve contains a large number of capacitors, direct charging will cause a huge impact on the power grid, which is equivalent to a three-phase short-circuit process. At the same time, fast charging will also cause the capacitor to be broken down. Therefore, the charging current needs to flow through the connecting transformer and then through the charging resistor to charge the converter valve. In this way, the charging current of the converter valve is limited by the charging resistor; when the charging voltage of the converter valve is close to the valve-side voltage of the connecting transformer, the current flowing through the starting resistor measured by the photocurrent transformer will become smaller. When the current measured by the photocurrent transformer is less than the opening capacity of the series knife switch, the starting resistor is bypassed by closing the parallel knife switch, so that the current flowing through the starting resistor is close to 0; Then open the series switch so that the starting resistor only bears voltage but no current during the normal operation of the flexible DC system, ensuring that the starting resistor will not be burned out due to poor contact with other equipment, causing equipment accidents.

[0006] Preferably, the setting condition of the photocurrent transformer is: the current value flowing through the photocurrent transformer is less than the set value as the closing condition of the parallel knife switch and the opening condition of the series knife switch.

[0007] A controller, characterized in that it comprises: one or more processors; The processor is used to store one or more programs; When the one or more programs are executed by the one or more processors, the operating method of the flexible DC starting loop is implemented.

[0008] A controller-readable storage medium stores a control program, and when the control program is executed, an operation method of a flexible DC starting circuit as described above is implemented.

[0009] The present invention can achieve the following beneficial effects: (1) Compared with the traditional starting circuit wiring method, the flexible DC starting circuit wiring method of the present invention can play the role of connecting the transformer and the converter valve to be isolated separately, thereby reducing the scope of fault isolation equipment; (2) During operation, the starting resistor is isolated from the circuit by a series knife switch. The starting resistor only bears the voltage on the connected transformer side. Even if the bypass knife switch has poor contact or high contact resistance, current will not flow through it and the starting resistor will not burn out. This can greatly improve the service life of the starting resistor and the reliability of flexible DC operation. In the traditional starting circuit wiring method, when the starting resistor is connected in parallel with the circuit breaker and the closing is not in place or the contact resistance is too high, the starting resistor will share a part of the load current, which may easily cause the starting resistor to burn out.

[0010] (3) The flexible DC starting circuit wiring method of the present invention only needs to configure overcurrent protection for the starting resistor to meet the protection requirements of fault conditions such as reduced starting resistor resistance and internal short circuit. At the same time, the starting circuit differential protection can quickly respond to the starting resistor grounding fault, without the need to separately configure differential protection and more complex other protections.

[0011] (4) For the back-to-back flexible DC system, the starting circuit wiring method of the present invention can isolate the converter transformer and the converter valve. The converter valve can continue to be connected to the AC power grid through the converter transformer on the other side of the back-to-back flexible DC system to realize STACOM mode operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will be further described below in conjunction with the accompanying drawings and embodiments: Figure 1 This is a system structure diagram of the present invention.

[0013] In the figure: In the figure, 1—connecting transformer grid side circuit breaker, 2—connecting transformer, 3—connecting transformer valve side circuit breaker, 4—starting resistor photocurrent transformer, 5—starting resistor, 6—first grounding switch, 7—series switch, 8—second grounding switch, 9—connecting transformer valve side grounding resistor, 10—third grounding switch, 11—parallel switch, 12—bridge arm reactor, 13—flexible DC converter valve. DETAILED DESCRIPTION

[0014] The preferred solution is Figure 1 As shown, a flexible DC starting circuit includes a valve-side circuit breaker, a starting resistor, a bypass knife switch and a series knife switch. The transformer-grid side is connected to the grid-side circuit breaker, the valve side is connected to the valve-side circuit breaker, and the valve-side circuit breaker is connected to the starting resistor photocurrent transformer, the starting resistor and the series knife switch in sequence. After the photocurrent transformer, the starting resistor and the series knife switch are connected in series, a parallel knife switch is connected together, and the parallel knife switch and the series knife switch are connected to the upper and lower bridge arm reactors and the converter valve.

[0015] The flexible DC starting circuit wiring method of the present invention has a working principle of first closing the starting circuit series switch during operation, then closing the connection transformer grid side circuit breaker to charge the converter transformer, and then closing the valve side circuit breaker in sequence. The grid AC flows through the connection transformer, the starting resistor, and the series switch in sequence to charge the converter valve capacitor. As the converter valve capacitor voltage increases, the charging current flowing through the charging resistor decreases. When the charging current is very small, the parallel switch is closed, and then the series switch is opened to complete the converter valve charging start process. The charging resistor plays a role in limiting the converter valve charging current in the circuit. After charging is completed, the parallel switch is closed and the series switch is opened, so that the starting resistor is isolated in the circuit. When the converter valve is in normal operating state, the starting resistor only bears the connection transformer valve side voltage. Because the series switch is opened, no current flows through the starting resistor. Therefore, during the startup and operation process, the starting resistor only needs to be configured with overcurrent protection to reflect the operating status of the starting resistor. When the starting resistor fails, the overcurrent protection can reflect the fault conditions such as reduced resistance and internal short circuit of the starting resistor. At the same time, the starting circuit differential protection can quickly respond to the grounding fault of the starting resistor, and there is no need to separately configure differential protection and more complex other protections.

[0016] Embodiment 1: Since the traditional flexible DC starting circuit uses the method of directly connecting the starting resistor and the bypass circuit breaker in parallel, when the bypass circuit breaker is not closed properly, the starting resistor will heat up and burn. The patent of the present invention adopts the method of adding a connecting transformer valve side circuit breaker 3 connected to the connecting transformer 2. When the DC equipment such as the converter valve 13 fails and trips, the connecting transformer grid side circuit breaker 1 and the valve side circuit breaker 3 can be tripped at the same time. The traditional flexible DC starting circuit wiring method does not have a valve side circuit breaker. When the traditional flexible DC grid side circuit breaker refuses to operate, due to the poor overcurrent capacity of the converter valve IGBT, the backup protection action time is long, which will cause the converter valve IGBT to bear the short-circuit current for a long time and be damaged by overcurrent.

[0017] The patent of the present invention adopts the method of adding a valve-side circuit breaker to avoid such consequences. One side of the starting resistor 5 is connected in series with the photocurrent transformer 4, and the other side is connected in series with the series knife switch 7, and then connected in parallel with the bypass knife switch 11. One side of the photocurrent transformer 4 is connected to the connecting transformer valve-side circuit breaker 3, and one side of the series knife switch 7 is connected to the converter valve bridge arm reactor 12 and the converter valve 13. In this wiring method, during the charging process of the converter valve, the starting resistor is first connected to the converter valve 13 through the series knife switch 7, and then the connecting transformer grid-side circuit breaker 1 is closed in turn to charge the connecting transformer 2. After the valve-side voltage of the connecting transformer 2 reaches the rated voltage value, the valve-side circuit breaker 3 is closed; since the converter valve contains a large number of capacitors, if it is charged directly, it will cause a huge impact on the power grid, which is equivalent to a three-phase short-circuit process. At the same time, rapid charging will also cause the capacitor to be broken down, so the charging current needs to pass through the connecting transformer 2 and then flow through the charging resistor 5 to charge the converter valve 13, so that the charging resistor 5 can be used to limit the impact on the converter valve The charging current of valve 13. When the charging voltage of converter valve 13 is close to the valve side voltage of connecting transformer 2, photocurrent transformer 4 can measure that the current flowing through starting resistor 5 will become smaller. When the current measured by photocurrent transformer 4 is less than the opening capacity of series knife switch 7, the starting resistor can be bypassed by closing parallel knife switch 11, so that the current flowing through the starting resistor is close to 0, and then the series knife switch 7 is opened, so that the starting resistor only bears voltage but not any current during the normal operation of the flexible DC system, thereby ensuring that the starting resistor will not be burned out due to poor contact of other equipment.

[0018] The above operation sequence can be completed by program control, in which the opening and closing sequence of the knife switch and the circuit breaker is set as the interlocking condition, and the current value flowing through the photocurrent transformer 4 is set to be less than the set value as the closing condition of the parallel knife switch 11 and the opening condition of the series knife switch 7. The wiring method of the present invention can close the first grounding knife switch, the second grounding knife switch, and the third grounding knife switch during the startup circuit maintenance process to ensure the risk of sudden power supply and the incident of induced electric injury, thereby ensuring the safety of people and equipment.

[0019] The above embodiments are only preferred technical solutions of the present invention and should not be regarded as limiting the present invention. The protection scope of the present invention shall be the technical solutions recorded in the claims, including equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A flexible DC starting circuit, characterized in that: It includes a starting resistor, one side of which is connected in series with a photocurrent transformer, and the other side of which is connected in series with a series switch, and then connected in parallel with a bypass switch; One side of the photocurrent transformer is connected to the circuit breaker on the connecting transformer valve side, and the circuit breaker on the connecting transformer valve side is connected to the connecting transformer; One side of the series knife switch is connected to the converter valve bridge arm reactor and the converter valve.

2. The method for operating a flexible DC starting circuit according to claim 1, It is characterized by the following steps: S1 connects the starting resistor and charges the connection transformer until its valve side voltage reaches the rated value, prepares for subsequent operations and limits the initial charging current to protect the system and equipment; S2 closes the valve-side circuit breaker to complete the main circuit connection and ensures safe charging by monitoring and adjusting the current; When the current drops to a safe level, the startup resistor is bypassed to reduce the current load on it; S3 disconnects the series switch, so that the starting resistor only bears voltage but does not pass current, ensuring that the system enters a stable operating state. At the same time, it prevents the starting resistor from being damaged by current, ensuring long-term reliable operation of the system.

3. The method for operating a flexible DC starting circuit according to claim 2, characterized in that: S1 is specifically: S1.1 Connect the starting resistor: connect the starting resistor to the circuit and connect it to the converter valve by closing the series switch; S1.2 Charging the connecting transformer: close the grid-side circuit breaker of the connecting transformer and start charging the connecting transformer; S1.3 Reaching rated voltage: Monitor the valve-side voltage of the connected transformer until it rises to the rated value.

4. The operating method of a flexible DC starting circuit according to claim 2, characterized in that: S2 is specifically: S2.1 Close the valve side circuit breaker: When the valve side voltage of the connected transformer reaches the rated value, close the valve side circuit breaker to complete the main circuit connection; S2.2 Monitor current changes: Use a photocurrent transformer to continuously monitor the current flowing through the starting resistor. As the converter valve voltage gradually approaches the valve side voltage of the connection transformer, pay attention to the current change trend; S2.3 Bypass starting resistor: When the detected current drops to a safe range, close the parallel knife switch to bypass the starting resistor so that the current flowing through it is close to zero to protect the starting resistor.

5. The operating method of a flexible DC starting circuit according to claim 2, characterized in that: S3 is specifically: S3.1 Open the series switch: The last step is to open the series switch to ensure that the starting resistor only bears voltage but not current during normal operation of the DC system; S3.2 Confirm system status: Confirm that all equipment has entered the working state correctly, the converter valve is ready to perform its intended function, and check the stability of the entire system.

6. The operating method of a flexible DC starting circuit according to claim 4, characterized in that: The setting condition of the photocurrent transformer in S2.2 is: the current value flowing through the photocurrent transformer is less than the set value as the closing condition of the parallel knife switch and the opening condition of the series knife switch.

7. A controller, characterized in that: include: one or more processors; The processor is used to store one or more programs; When the one or more programs are executed by the one or more processors, an operating method of a flexible DC starting circuit as claimed in any one of claims 2 to 6 is implemented.

8. A controller-readable storage medium, characterized in that: A control program is stored thereon, and when the control program is executed, an operating method of a flexible DC starting circuit as claimed in any one of claims 2 to 6 is implemented.

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