A back-to-back commutation system and a unilateral exit method of a commutator

Abstract

The embodiment of the present invention relates to a back-to-back commutation system and a method for unilateral exit of a converter, the method comprising: reducing the active power and reactive power of the commutation system to zero values ​​at preset rates; controlling the DC voltage of the converter on the opposite side of the converter to be exited to zero; locking the converter to be exited, disconnecting the grid-side incoming line switch on this side, so as to exit the converter to be exited; separating the DC bus isolation switches on each side of the double-sided converters; controlling the DC voltage of the converter on the opposite side of the converter to be exited to a rated value, and controlling it to operate in STATCOM mode. The technical solution of the present invention can quickly switch the flexible DC back-to-back system from the double-sided HVDC power transmission mode to the single-sided STATCOM operation mode when it is necessary to lock the single-sided converter, thereby improving the flexibility of the operation of the back-to-back commutation system and reducing the impact of restarting the system after locking on the main equipment.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of flexible direct current transmission engineering, and in particular to a back-to-back commutation system and a unilateral exit method of a converter. Background Art

[0002] Compared with traditional high-voltage direct current transmission, flexible direct current transmission has the characteristics of independent control of active power and reactive power, can more conveniently reverse the power flow, has no commutation failure problem, does not need to configure AC / DC filters, and has the advantages of fast adjustment during AC faults and reactive power support for AC voltage recovery. At present, the application scope of flexible direct current transmission technology is also increasing, including long-distance large-capacity transmission, DC back-to-back networking, and distributed new energy grid connection and other application scenarios.

[0003] In a flexible DC transmission back-to-back system, if the DC system is shut down due to a related fault in a converter on one side, the converter on the opposite side will usually be synchronously locked and shut down. At this time, the operator manually operates to separate the DC bus isolation switch. After the fault side is isolated, the converter without fault is charged and operated to the operating state in STATCOM operation mode. However, this method reduces the operational flexibility and reliability of the flexible DC transmission system and increases the impact on the recharging and starting of the main equipment such as the converter side transformer, charging resistor, and converter valve. Summary of the invention

[0004] Based on the above situation of the prior art, the purpose of the embodiments of the present invention is to provide a back-to-back commutation system and a single-sided exit method of the converter, which can switch the flexible DC back-to-back system from the double-sided HVDC power transmission mode to the single-sided STATCOM operation mode. When a single-sided converter fails and shuts down urgently, or needs to be repaired, the converter on the other side only needs to automatically reduce the voltage and current to zero by the control logic, and then automatically switch to the single-sided STATCOM mode to continue operation after isolating the faulty side, thereby improving the flexibility of the system and reducing the impact on the main equipment.

[0005] To achieve the above object, according to one aspect of the present invention, a method for withdrawing a converter on one side of a back-to-back commutation system is provided, wherein the back-to-back commutation system comprises a double-sided converter, each side of the converter is respectively connected to a different AC system, and the double-sided converters are connected to each other via a DC bus; the method comprises the steps of:

[0006] S1. reducing the active power and reactive power of the commutation system to zero values ​​at preset rates respectively;

[0007] S2. Control the DC voltage of the converter on the opposite side of the converter to be withdrawn to zero;

[0008] S3, locking the converter to be withdrawn, and disconnecting the grid-side incoming line switch on that side to withdraw the converter to be withdrawn;

[0009] S4, separate the DC bus isolation switches on both sides of the converter;

[0010] S5. Control the DC voltage of the converter on the opposite side of the converter to be withdrawn to a rated value, and control it to operate in STATCOM mode.

[0011] Further, when the method is executed due to a single-sided converter failure, steps S2, S4 and S5 are executed.

[0012] Furthermore, the back-to-back commutation system performs double-sided HVDC power transmission before the single-sided exit of the converter.

[0013] Furthermore, each side converter of the double-sided converter is a full-bridge topology structure or a full-bridge and half-bridge mixed topology structure.

[0014] Furthermore, a closed-loop controller is used to reduce the active power and reactive power to zero value at a preset rate.

[0015] Furthermore, a closed-loop controller is used to control the DC voltage to a zero value.

[0016] Furthermore, the STATCOM mode operation includes:

[0017] The DC voltage is controlled at the rated 1.0 psi, and the reactive power output is adjusted according to the reactive power target value or the AC voltage target value.

[0018] According to another aspect of the present invention, there is provided a back-to-back commutation system, comprising:

[0019] A double-sided converter, each side of which is connected to a different AC system, and the double-sided converters are connected to each other through a DC bus; each side of the double-sided converter includes a transformer, a reactor and a power conversion module, one end of the transformer is connected to the AC system, and the other end is connected to the power conversion module through the reactor;

[0020] The power conversion module is a full-bridge topology structure or a full-bridge and half-bridge hybrid topology structure

[0021] A controller, which is connected to each converter and is used to output a modulation signal to the converter to control the output power and output voltage of each converter; the controller is also used to execute the converter unilateral exit method as described in the first aspect of the present invention.

[0022] Furthermore, it also includes:

[0023] A grid-side incoming line switch, through which the transformer is connected to the busbar of the AC system;

[0024] A DC bus isolating switch, through which the converter is connected to the DC bus.

[0025] Furthermore, when a single-side converter fails or the converter system needs to be repaired, the controller executes the above-mentioned converter single-side exit method.

[0026] In summary, an embodiment of the present invention provides a back-to-back commutation system and a single-sided exit method of a converter, the exit method comprising the steps of: reducing the active power and reactive power of the commutation system to zero values ​​at preset rates respectively; controlling the DC voltage of the converter on the opposite side of the converter to be exited to zero value; locking the converter to be exited, disconnecting the grid-side incoming line switch on that side, so as to exit the converter to be exited; separating the DC bus isolation switches on each side of the double-sided converters; controlling the DC voltage of the converter on the opposite side of the converter to be exited to the rated value, and controlling it to operate in STATCOM mode. The technical solution provided by the embodiment of the present invention can quickly switch the flexible DC back-to-back system from the double-sided HVDC power transmission mode to the single-sided STATCOM operation mode when a single-sided converter fails and stops operating urgently, or when maintenance operations need to be performed and the single-sided converter needs to be locked. The opposite-side converter to be exited only needs to automatically reduce the voltage and current to zero by the control logic, and then automatically switch to the single-sided STATCOM mode to continue operation after isolating the faulty side, thereby improving the flexibility of the back-to-back commutation system operation and reducing the impact of restarting the system on the main equipment after locking. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a main wiring diagram of a back-to-back commutation system according to an embodiment of the present invention;

[0028] Figure 2 is a circuit diagram of a power conversion module according to an embodiment of the present invention;

[0029] Figure 3 It is a flow chart of a method for unilaterally exiting a converter in a back-to-back commutation system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0031] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in one or more embodiments of the present invention should be understood by people with ordinary skills in the field to which the present disclosure belongs. The "first", "second" and similar words used in one or more embodiments of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0032] The technical solution of the present invention is described in detail below in conjunction with the accompanying drawings. An embodiment of the present invention provides a back-to-back commutation system. Figure 1 The main wiring diagram of the back-to-back commutation system of this embodiment of the present invention is shown in FIG. The back-to-back commutation system includes a double-side converter, Figure 1 The figure shows the connection of the converter on one side and the converter on the other side. Figure 1 The wiring of the converters shown in the figure is identical and symmetrical. Each side of the converter is connected to a different AC system, and the converters on both sides are connected to each other through a DC bus. Each side of the converter in the double-sided converter includes a transformer, a reactor and a power conversion module, one end of the transformer is connected to the AC system, and the other end is connected to the power conversion module through the reactor. The system also includes a grid-side incoming line switch ( Figure 1 The transformer is connected to the bus of the AC system through the grid-side incoming line switch; the DC bus isolation switch, for example Figure 1 Q1 and Q2 shown in the figure, the converter is connected to the DC bus through the DC bus isolation switch. The back-to-back commutation system usually operates under the double-sided HVDC power transmission mode. Under the double-sided HVDC power transmission mode, the active power is the concept of double-sided coupling synchronization. The power is transmitted by the rectifier side and received by the inverter side. Among them, the power conversion module is a full-bridge topology structure or a full-bridge and half-bridge hybrid topology structure. Figure 2 The circuit diagram of the power conversion module is shown in FIG. Figure 2As shown, each phase unit of the power conversion module is composed of two upper and lower bridge arm units, each bridge arm unit is composed of n sub-modules connected in series, each bridge arm is a mixture of half-bridge modules and full-bridge modules, and the ratio of full-bridge to half-bridge modules is not limited. The half-bridge sub-module includes two IGBT power devices T1 and T2, corresponding to two anti-parallel diodes D1 and D2 on T1 and T2 and a capacitor C connected in parallel. The full-bridge sub-module includes four IGBT power devices T1, T2, T3, and T4 connected in counterclockwise order, corresponding to four anti-parallel diodes D1, D2, D3, and D4 on T1, T2, T3, and T4 and a capacitor C, L is the bridge arm reactance, and Udc is the DC voltage. The positive and negative poles of the DC side of the converter are respectively equipped with isolation switches Q1 and Q2 to connect to the converter on the other side. The system may also include a controller connected to each converter and configured to output a modulation signal to the converter to control the output power and output voltage of each converter; the controller may be configured to execute a single-side converter exit method.

[0033] An embodiment of the present invention further provides a method for withdrawing a converter on one side of the back-to-back commutation system described in the above embodiment. Figure 3 The flowchart of the method is shown in FIG. 1 . For example, when carrying out maintenance work such as planned single-side converter withdrawal, the method can be implemented to achieve single-side converter withdrawal, including the following steps:

[0034] First, the operator closes the converter DC bus isolation switches Q1 and Q2 of the flexible DC back-to-back system, and operates it to the operating state in the HVDC power transmission mode. Usually, the rectifier side of the flexible DC system controls the DC voltage, and the inverter side controls the DC power. The reactive power control mode can flexibly select "fixed reactive power control" and "AC voltage control". Flexible DC power control consists of outer loop control and inner loop control. The outer loop generates a reference current command, and the inner loop current control generates a three-phase reference voltage output by the converter through a series of processing based on the vector control principle, modulates it into a six-way bridge arm voltage reference value, and sends it to the valve control unit for execution.

[0035] S1. Reduce the active power and reactive power of the commutation system to zero value at a preset rate. In this step, the active power can be reduced to zero value at a preset rate according to the single-sided converter exit instruction issued by the operator, and the reactive power of the double-sided converter can also reduce the reactive output to zero value at a preset rate. The preset rate can be set according to actual needs and is not specifically limited here, but if the impact on the AC system is reduced, the preset rate can be set to a lower rate. The active power and reactive power can be reduced to zero value through a closed-loop controller, and a power slope generator can be used to generate a power reference value in real time according to the set change rate, and the difference between the real-time reference value and the actual power value is used as the input of the closed-loop controller, and the corresponding modulation wave is output to the valve control system, thereby adjusting the power output value in real time.

[0036] S2. Control the DC voltage of the converter on the opposite side of the converter to be withdrawn to zero. The DC voltage of the converter on the opposite side can be controlled to zero by a closed-loop controller. The difference between the voltage target value and the actual voltage value is used as the input of the closed-loop controller, and the corresponding modulation wave is output to the valve control system to adjust the actual value of the DC voltage to zero in real time.

[0037] S3, lock the converter to be withdrawn, and disconnect the grid-side incoming line switch on this side to withdraw the converter to be withdrawn. The lock can be achieved by the converter valve control system no longer sending modulation wave instructions to the converter valve power module, and the disconnection switch can be achieved by the control system separating the incoming line switch through the operating mechanism.

[0038] S4, separate the DC bus isolation switches on both sides of the converter;

[0039] S5. Control the DC voltage of the converter on the opposite side of the converter to be withdrawn to the rated value, and control it to operate in STATCOM mode. Here, operating in STATCOM mode means that the flexible DC controller controls the DC voltage at the rated 1.0 s / u, and adjusts the reactive power output according to the reactive power target value or AC voltage target value issued by the operator. Controlling the DC voltage to the rated value is achieved through a closed-loop controller. The voltage target value is the difference between the rated value 1.0 s / u and the actual value as the input of the closed-loop controller, and the corresponding modulation wave is output to the valve control system to adjust the actual DC voltage value to the rated target value in real time.

[0040] According to certain embodiments, when a single-side converter fails and needs to exit online, the single-side converter exit can also be achieved by executing this method. In this case, only S2, S4 and S5 in the above steps need to be executed. When a fault occurs in the transformer, converter valve, bridge arm reactor and other equipment on either side of the converter, resulting in a protection action trip, the active power and reactive power will quickly drop to zero, that is, the result of the protection action trip caused by the fault is to lock the converter and trip the grid-side incoming line switch, so that the active power and reactive power automatically drop to zero. At this time, the converter valve control system stops sending modulation wave instructions to the converter valve power module, and the IGBT power device of the converter valve is no longer periodically opened, so that the converter on the fault side is locked and the grid-side incoming line switch is tripped. In this embodiment, the specific process of executing steps S2, S4 and S5 is the same as that described in the above embodiment, and will not be repeated here.

[0041] In summary, an embodiment of the present invention relates to a back-to-back commutation system and a single-sided exit method of a converter, the exit method comprising the steps of: reducing the active power and reactive power of the commutation system to zero values ​​at preset rates respectively; controlling the DC voltage of the converter on the opposite side of the converter to be exited to zero value; locking the converter to be exited, disconnecting the grid-side incoming line switch on that side, so as to exit the converter to be exited; separating the DC bus isolation switches on each side of the double-sided converters; controlling the DC voltage of the converter on the opposite side of the converter to be exited to the rated value, and controlling it to operate in STATCOM mode. The technical solution provided by the embodiment of the present invention can quickly switch the flexible DC back-to-back system from the double-sided HVDC power transmission mode to the single-sided STATCOM operation mode when a single-sided converter fails and stops operating urgently, or when a maintenance operation needs to be carried out and the single-sided converter needs to be locked. The opposite-side converter to be exited only needs to automatically reduce the voltage and current to zero by the control logic, and then automatically switch to the single-sided STATCOM mode to continue operation after isolating the faulty side, thereby improving the flexibility of the back-to-back commutation system operation and reducing the impact of restarting the system on the main equipment after locking.

[0042] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, and do not constitute a limitation of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalent forms of such scope and boundaries.

Claims

1. A method for withdrawing a converter from a back-to-back commutation system on one side, characterized in that: The back-to-back commutation system comprises a double-sided converter, each side of the converter is connected to a different AC system, and the double-sided converters are connected to each other through a DC bus; the method comprises the steps of: S1, reducing the active power and reactive power of the commutation system to zero values ​​at preset rates respectively; reducing the active power and reactive power to zero values ​​through a closed-loop controller, generating a power reference value in real time according to a set change rate through a power slope generator, and using the difference between the real-time power reference value and the actual power value as an input of a closed-loop controller, outputting a corresponding modulation wave to control the active power and reactive power; S2. Control the DC voltage of the converter on the opposite side of the converter to be withdrawn to zero; The DC voltage of the opposite converter is controlled to zero by a closed-loop controller, the difference between the voltage target value and the actual voltage value is used as the input of the closed-loop controller, and a corresponding modulation wave is output to control the DC voltage; S3, locking the converter to be withdrawn, and disconnecting the grid-side incoming line switch on that side to withdraw the converter to be withdrawn; S4, separate the DC bus isolation switches on both sides of the converter; S5. Control the DC voltage of the converter on the opposite side of the converter to be withdrawn to the rated value, and control it to operate in STATCOM mode. The STATCOM mode includes: controlling the DC voltage at the rated 1.0 s / u, and adjusting the reactive power output according to the reactive power target value or the AC voltage target value.

2. The method according to claim 1, characterized in that When the method is executed due to a single-sided converter failure, steps S2, S4 and S5 are performed.

3. The method according to claim 1 or 2, characterized in that: The back-to-back commutation system performs double-sided HVDC power transmission before the single-sided exit of the converter.

4. The method according to claim 3, characterized in that: Each side converter of the double-sided converter is a full-bridge topology structure or a full-bridge and half-bridge mixed topology structure.

5. A back-to-back commutation system, characterized in that: include: A double-sided converter, each side of which is connected to a different AC system, and the double-sided converters are connected to each other through a DC bus; each side of the double-sided converter includes a transformer, a reactor and a power conversion module, one end of the transformer is connected to the AC system, and the other end is connected to the power conversion module through the reactor; The power conversion module is a full-bridge topology or a full-bridge and half-bridge mixed topology controller, and the controller is connected to each converter and is used to output a modulation signal to the converter to control the output power and output voltage of each converter; the controller is also used to execute the converter single-sided exit method as described in any one of claims 1-4.

6. The system according to claim 5, characterized in that Also includes: A grid-side incoming line switch, through which the transformer is connected to the busbar of the AC system; A DC bus isolating switch, through which the converter is connected to the DC bus.

7. The system according to claim 6, characterized in that When a single-side converter fails, or the converter system needs to be repaired, the controller executes the above-mentioned converter single-side exit method.

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