A voltage-power coordinated control method and device for a flexible direct current transmission system

Abstract

The present invention relates to a voltage-power coordinated control method and device for a flexible direct current transmission system, which monitors the direct current voltage of a multi-terminal direct current transmission system. When the direct current voltage offset exceeds the upper limit due to a fault or interference, the power instruction of the receiving-end converter is increased proportionally according to the direct current voltage overvoltage situation, so that the non-fault converter absorbs more surplus power, thereby achieving the purpose of suppressing the overvoltage on the direct current side. The present invention makes full use of the advantages of parallel converters in a multi-terminal direct current transmission system. When the direct current voltage increases, the transmission power of the non-fault converter in the active power control mode is appropriately increased to reduce the residual power on the direct current side, thereby achieving the purpose of suppressing the overvoltage on the direct current side. When an alternating current fault is detected to occur on the converter of this station, the upper limit of the power instruction of the receiving-end converter is set to 0 to increase the risk of the expansion of the alternating current fault due to the increase in power output, thereby improving the reliability of system control.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transmission of electric power systems, and in particular to a voltage-power coordinated control method and device for a flexible direct current transmission system. Background Art

[0002] Flexible DC transmission system is widely used in various application scenarios such as new energy grid connection, island interconnection, asynchronous transmission, etc. due to its many advantages such as economy and flexibility. Multi-terminal flexible DC transmission system is one of the hot research directions of flexible DC. Multi-terminal flexible DC transmission system can connect multiple AC power grids with different transmission and absorption capabilities, realize multi-power supply and multi-point power reception, and save transmission line corridors at the same time, which is a more flexible DC transmission method.

[0003] For a multi-terminal flexible DC transmission system, when an AC system fault occurs in a certain MMC converter or an AC bus, the AC bus voltage will drop, causing the converter transmission capacity to weaken, the MMC submodule voltage to increase, and the system DC overvoltage to occur. At present, the fault handling solution usually adopts the solution of reducing the transmission power at the sending end and locking or not taking any action at the receiving end.

[0004] However, the above processing methods in the prior art all have disadvantages: for the scheme of reducing the transmission power at the sending end, due to the existence of inter-station communication delay, when the sending end detects the occurrence of AC fault, the overpressure of the valve group has developed to a more serious level. If the receiving end is locked or no operation is adopted, the system power will recover slowly after the fault ends, affecting power transmission. Summary of the invention

[0005] Based on the above situation of the prior art, the purpose of the present invention is to provide a voltage-power coordinated control method and device for a flexible direct current transmission system, which utilizes the characteristics of a multi-terminal flexible direct current transmission system. When an AC fault occurs in a valve group, the power transmission level of the non-faulty valve group is appropriately increased, thereby alleviating the overvoltage level of the faulty valve group and accelerating the system recovery speed.

[0006] To achieve the above object, according to one aspect of the present invention, a voltage-power coordinated control method for a flexible direct current transmission system is provided, comprising the steps of:

[0007] Collect the DC voltage value U of the DC transmission system d , calculate its difference with the DC voltage setting value U dref The difference ΔU d :ΔU d =U d -U dref ;

[0008] When ΔU d >X, the power command value Ud_H Increase proportionally according to the size of the difference; when ΔU d ≤X, the power command value U d_H =0; where X is the upper limit of DC overvoltage;

[0009] Using the power command value U d_H , Active power reference value P ref The current power value P generates a power reference value P_ref_P of the inner loop current controller of the DC transmission system, thereby adjusting the power delivered by the converter to the grid.

[0010] Furthermore, the power command value U d_H The difference is increased in proportion to the difference, including:

[0011] U d_H =ΔU d ·μ

[0012] Wherein, μ is the power command coefficient.

[0013] Furthermore, the power command value U d_H The upper limit of is P_H,

[0014]

[0015] Among them, U C is the AC bus positive sequence voltage amplitude, U CB is the rated value of the positive sequence voltage of the AC bus, P max is the maximum active power that the converter can transmit, P ref is the active power reference value.

[0016] Furthermore, when an AC failure occurs at this station, the upper limit value P_H=0.

[0017] Furthermore, the power command value U is used d_H , Active power reference value P ref And the current power value P generates a power reference value P_ref_P of the inner loop current controller of the direct current transmission system, comprising the steps of:

[0018] The power command value U d_H and active power reference value P ref Add them together to get the power value P add ;

[0019] The power and value P add The difference between the current power value P and the current power value P is then added to the power value P add Add them together to get the power reference value P_ref_P of the inner loop current controller of the DC transmission system.

[0020] According to another aspect of the present invention, a voltage-power coordinated control device for a flexible DC power transmission system is provided, comprising a DC voltage difference calculation module, a power command value generation module, and a power reference value generation module; wherein:

[0021] The DC voltage difference calculation module uses the collected DC voltage value U d , calculate its difference with the DC voltage setting value U dref The difference ΔU d :ΔU d =U d -U dref ;

[0022] The power command value generating module is used to generate a power command value: when ΔU d >X, the power command value U d_H Increase proportionally according to the size of the difference; when ΔU d ≤X, the power command value U d_H =0; where X is the upper limit of DC overvoltage;

[0023] The power reference value generating module uses the power command value U d_H , Active power reference value P ref The current power value P generates a power reference value P_ref_P of the inner loop current controller of the DC transmission system, thereby adjusting the power delivered by the converter to the grid.

[0024] Furthermore, the power command value generating module is used to generate a power command value. d >X,

[0025] U d_H =ΔU d ·μ

[0026] Wherein, μ is the power command coefficient.

[0027] Furthermore, the power command value U d_H The upper limit of is P_H,

[0028]

[0029] Among them, U C is the AC bus positive sequence voltage amplitude, U CB is the rated value of the positive sequence voltage of the AC bus, P max is the maximum active power that the converter can transmit, P ref is the active power reference value.

[0030] Furthermore, when an AC failure occurs at this station, the upper limit value P_H=0.

[0031] Furthermore, the power reference value generation module uses the power command value U d_H , Active power reference value P ref The current power value P generates a power reference value P_ref_P of the inner loop current controller of the DC transmission system, including:

[0032] The power command value U d_H and active power reference value P ref Add them together to get the power value P add ;

[0033] The power and value P add The difference between the current power value P and the current power value P is then added to the power value P add Add them together to get the power reference value P_ref_P of the inner loop current controller of the DC transmission system.

[0034] In summary, the present invention provides a voltage-power coordinated control method and control device for a flexible direct current transmission system, which monitors the direct current voltage of a multi-terminal direct current transmission system. When the direct current voltage offset exceeds the upper limit due to a fault or interference, the power command of the receiving-end converter is increased proportionally according to the direct current overvoltage situation, so that the reference value of the inner-loop current controller increases accordingly, so that the non-fault converter absorbs more surplus power, thereby achieving the purpose of suppressing the overvoltage on the direct current side.

[0035] The technical solution provided by the present invention has the following beneficial technical effects:

[0036] (1) Make full use of the advantages of parallel converters in multi-terminal DC transmission systems. When the DC voltage increases, appropriately increase the transmission power of non-fault converters in active power control mode to reduce the residual power on the DC side. This can effectively stabilize the DC voltage to a certain extent, thereby achieving the purpose of suppressing overvoltage on the DC side.

[0037] (2) When an AC fault is detected on the converter of this station, the upper limit of the power command of the receiving-end converter is set to 0, and the voltage-power coordinated control function is exited to avoid the risk of the AC fault expanding due to the increase of power output by the control function, thereby improving the reliability of system control.

[0038] (3) Based on power control and combined with DC voltage control, intervention is implemented at the early stage of fault occurrence, which improves system reliability and speeds up the effectiveness of fault handling. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a flow chart of a voltage-power coordinated control method of a flexible direct current transmission system of the present invention;

[0040] Figure 2 It is a circuit structure diagram of a multi-terminal flexible DC transmission system;

[0041] Figure 3 It is the active outer loop control block diagram of the flexible DC transmission system;

[0042] Figure 4 It is the block diagram of the current inner loop control of the flexible DC transmission system;

[0043] Figure 5 It is a control strategy block diagram of the voltage-power coordinated control method of the flexible direct current transmission system of the present invention;

[0044] Figure 6 It is a structural block diagram of the voltage-power coordinated control device of the flexible direct current transmission system of the present invention. DETAILED DESCRIPTION

[0045] 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.

[0046] The technical solution of the present invention is described in detail below with reference to the accompanying drawings. According to one embodiment of the present invention, a voltage-power coordinated control method for a flexible DC transmission system is provided. The flowchart of the method is shown in FIG. Figure 1 As shown. The multi-terminal flexible DC transmission system can connect multiple AC power grids with different transmission and absorption capabilities, and can realize multi-power supply and multi-point power reception. The circuit structure diagram of a common multi-terminal flexible DC transmission system is shown in Figure 2 As shown, the system sending end adopts a high-end and low-end LCC valve group series structure, and the receiving end adopts a high-end LCC valve group and a low-end three parallel VSC valve groups series structure. Each VSC is a modular multilevel converter topology based on sub-module series connection.

[0047] The present invention utilizes the characteristics of a multi-terminal flexible direct current transmission system. When an AC fault occurs in a valve group, the power transmission level of the non-fault valve group is appropriately increased, thereby alleviating the overvoltage level of the faulty valve group and accelerating system recovery. For a multi-terminal direct current transmission system, when the AC bus voltage of a certain converter at the receiving end (for example, a VSC converter) drops, since each station is connected to a different AC power grid, the remaining converters still have a stronger power transmission capacity than the faulty converter. In order to fully utilize the advantages of parallel converters, in the technical solution provided by this embodiment, when the DC voltage increases, the transmission power of the non-fault converter in the active power control mode is appropriately increased to reduce the residual power on the DC side and stabilize the DC voltage to a certain extent. The voltage-power coordinated control method provided by this embodiment mainly acts on the outer loop of the power control of the multi-terminal flexible direct current transmission system. Figure 3 The active outer loop control block diagram of the flexible DC transmission system is shown. Figure 4 The control block diagram of the current inner loop of the flexible DC transmission system is shown, and the control methods commonly used in the field are all used, which will not be repeated here.

[0048] Figure 5 The control strategy block diagram of the voltage-power coordinated control method of the flexible DC transmission system of the present invention is shown in FIG. Figure 1 and Figure 5 , the control method provided in this embodiment includes the following steps:

[0049] Collect the DC voltage value U of the DC transmission system d , calculate its difference with the DC voltage setting value U dref The difference ΔU d :ΔU d =U d -U dref .

[0050] The difference between the DC voltage and the upper limit value X is judged. When the difference between the DC voltage and the set value does not exceed the upper limit value X, the power command value U sent to the regulator is d_H =0, the active power controller operates in normal mode; when the DC voltage offset (i.e., the difference) exceeds the upper limit value X due to a fault or interference, the power command value U of the converter will be increased proportionally according to the DC overvoltage situation. d_H ,like Figure 5 As shown, the reference value of the inner loop current controller finally outputs P_ref_P, which increases accordingly, so that the non-fault converter absorbs more surplus power, thereby achieving the purpose of suppressing the overvoltage on the DC side. The control can be carried out in the following steps:

[0051] When ΔU d >X, the power command value U d_HThe power command value U can be calculated by the following formula: d_H :

[0052] U d_H =ΔU d ·μ

[0053] Among them, μ is the power command coefficient, which can be determined according to actual engineering needs, and the value range is usually 0.1-1.

[0054] When ΔU d ≤X, the power command value U d_H =0.

[0055] The above steps of increasing the power command value are for the case where the AC fault does not occur in the station. At this time, the power command value U d_H The upper limit value is P_H, which can be set according to the following formula:

[0056]

[0057] Among them, U C is the AC bus positive sequence voltage amplitude, U CB is the rated value of the positive sequence voltage of the AC bus, P max is the maximum active power that the converter can transmit, P ref is the active power reference value.

[0058] However, when the AC fault happens to occur at this station, in order to avoid the risk of AC fault expansion caused by increasing the power output control, the above method of increasing the power command value cannot be used for control. When an AC fault occurs at this station, the upper limit value P_H = 0. That is, at this time, the voltage-power coordinated control function is launched to avoid the above risks.

[0059] Using the power command value U d_H , Active power reference value P ref The current power value P generates the power reference value P_ref_P of the inner loop current controller of the DC transmission system, thereby adjusting the power delivered by the converter to the grid. The following steps can be used:

[0060] The power command value U d_H and active power reference value P ref Add them together to get the power value P add ;

[0061] The power and value P add The difference between the current power value P and the current power value P is then added to the power value P addAdd them together to get the power reference value P_ref_P of the inner loop current controller of the DC transmission system.

[0062] According to another embodiment of the present invention, a voltage-power coordinated control device for a DC power transmission system is provided. The structural block diagram of the device is as follows: Figure 6 As shown, it includes a DC voltage difference calculation module, a power command value generation module, and a power reference value generation module; wherein,

[0063] The DC voltage difference calculation module uses the collected DC voltage value U d , calculate its difference with the DC voltage setting value U dref The difference ΔU d :ΔU d =U d -U dref .

[0064] The power command value generation module is used to generate the power command value: when ΔU d >X, the power command value U d_H The difference is increased proportionally, for example, it can be calculated according to the following formula:

[0065] U d_H =ΔU d ·μ

[0066] Wherein, μ is the power command coefficient.

[0067] When ΔU d ≤X, the power command value U d_H =0; where X is the upper limit of DC overvoltage.

[0068] When the AC fault does not occur at this station, the power command value U d_H The upper limit of is P_H,

[0069]

[0070] Among them, U C is the AC bus positive sequence voltage amplitude, U CB is the rated value of the positive sequence voltage of the AC bus, P max is the maximum active power that the converter can transmit, P ref is the active power reference value.

[0071] When an AC failure occurs at this station, the upper limit value P_H=0.

[0072] The power reference value generating module uses the power command value U d_H , Active power reference value P refThe current power value P generates a power reference value P_ref_P of the inner loop current controller of the DC transmission system, including:

[0073] The power command value U d_H and active power reference value P ref Add them together to get the power value P add ;

[0074] The power and value P add The difference between the current power value P and the current power value P is then added to the power value P add Add them together to get the power reference value P_ref_P of the inner loop current controller of the DC transmission system.

[0075] In summary, the present invention relates to a voltage-power coordinated control method and control device for a flexible direct current transmission system, which monitors the direct current voltage of a multi-terminal direct current transmission system. When the direct current voltage offset exceeds the upper limit due to a fault or interference, the power instruction of the receiving-end converter is increased proportionally according to the direct current voltage overvoltage situation, so that the reference value of the inner loop current controller increases accordingly, so that the non-fault converter absorbs more surplus power, thereby achieving the purpose of suppressing the overvoltage on the direct current side. The technical solution provided by the present invention makes full use of the advantages of parallel converters in a multi-terminal direct current transmission system. When the direct current voltage increases, the transmission power of the non-fault converter in the active power control mode is appropriately increased to reduce the residual power on the direct current side, and the direct current voltage can be effectively stabilized to a certain extent, thereby achieving the purpose of suppressing the overvoltage on the direct current side. When it is detected that an alternating current fault occurs on the converter of this station, the upper limit of the power instruction of the receiving-end converter is set to 0, and the voltage-power coordinated control function is exited to avoid the risk of the control function increasing the power output and causing the expansion of the alternating current fault, thereby improving the reliability of system control. The present invention combines direct current voltage control with power control to implement intervention at the early stage of a fault, thereby improving the reliability of the system and accelerating the effectiveness of fault handling.

[0076] 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 voltage-power coordinated control method for a flexible direct current transmission system, characterized in that: Includes steps: Collect the DC voltage value U of the DC transmission system d , calculate its difference with the DC voltage setting value U dref The difference ΔU d :ΔU d =U d -U dref ; When ΔU d >X, the power command value U d_H Increase proportionally according to the size of the difference; when ΔU d ≤X, the power command value U d_H =0; where X is the upper limit of DC overvoltage; Using the power command value U d_H , Active power reference value P ref The method comprises the following steps: The power command value U d_H and active power reference value P ref Add them together to get the power value P add ; The power and value P add The difference between the current power value P and the current power value P is then added to the power value P add Add them together to obtain the power reference value P_ref_P of the inner loop current controller of the DC transmission system; This adjusts the amount of power the converter delivers to the grid.

2. The control method according to claim 1, characterized in that: The power command value U d_H The difference is increased in proportion to the difference, including: U d_H =ΔU d ·m Wherein, μ is the power command coefficient.

3. The control method according to claim 1, characterized in that: The power command value U d_H The upper limit of is P_H, Among them, U C is the AC bus positive sequence voltage amplitude, U CB is the rated value of the positive sequence voltage of the AC bus, P max is the maximum active power that the converter can transmit, P ref is the active power reference value.

4. The control method according to claim 3, characterized in that: When an AC failure occurs at this station, the upper limit value P_H=0.

5. A voltage-power coordinated control device for a flexible direct current transmission system, characterized in that: It includes a DC voltage difference calculation module, a power command value generation module, and a power reference value generation module; wherein, The DC voltage difference calculation module uses the collected DC voltage value U d , calculate its difference with the DC voltage setting value U dref The difference ΔU d :ΔU d =U d -U dref ; The power command value generating module is used to generate a power command value: when ΔU d >X, the power command value U d_H Increase proportionally according to the size of the difference; when ΔU d ≤X, the power command value U d_H =0; where X is the upper limit of DC overvoltage; The power reference value generating module uses the power command value U d_H , Active power reference value P ref The current power value P generates a power reference value P_ref_P of the inner loop current controller of the DC power transmission system, including: converting the power command value U d_H and active power reference value P ref Add them together to get the power value P add ; The power and value P add The difference between the current power value P and the current power value P is then added to the power value P add The power reference value P_ref_P of the inner loop current controller of the DC transmission system is obtained by adding them together, thereby adjusting the power transmitted by the converter to the power grid.

6. The control device according to claim 5, characterized in that: The power command value generating module is used to generate a power command value. d >X, U d_H =ΔU d ·m Wherein, μ is the power command coefficient.

7. The control device according to claim 6, characterized in that: The power command value U d_H The upper limit of is P_H, Among them, U C is the AC bus positive sequence voltage amplitude, U CB is the rated value of the positive sequence voltage of the AC bus, P max is the maximum active power that the converter can transmit, P ref is the active power reference value.

8. The control device according to claim 7, characterized in that: When an AC failure occurs at this station, the upper limit value P_H=0.

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