A flexible low frequency power transmission system control method and apparatus

Abstract

The present application belongs to the technical field of flexible low-frequency power transmission, and particularly relates to a control method and device for a flexible low-frequency power transmission system. The method first calculates a power frequency component of a bridge arm reference voltage of a converter according to acquired power frequency three-phase grid-side voltage, power frequency three-phase grid-side current, bridge arm voltage of the converter and bridge arm current of the converter; then calculates a bridge arm circulating current component of the bridge arm reference voltage of the converter according to acquired bridge arm current of the converter; then calculates a low-frequency component of the bridge arm reference voltage of the converter according to acquired low-frequency three-phase grid-side voltage and low-frequency three-phase grid-side current; finally adds the power frequency component, the bridge arm circulating current component and the low-frequency component of the bridge arm reference voltage of the converter, and obtains the bridge arm reference voltage of the converter according to the addition result; and controls the converter by using the obtained bridge arm reference voltage of the converter. The present application provides a feasible scheme for reliable operation of the converter, and ensures the operation reliability of the flexible low-frequency power transmission system control.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flexible low-frequency power transmission, and particularly relates to a flexible low-frequency power transmission system control method and device. BACKGROUND

[0002] China has a vast sea area, and the offshore wind energy resources are relatively abundant. The conditions for developing offshore wind power are relatively superior, and the connection between offshore wind farms and land-based main networks can usually adopt two power transmission technical solutions: power frequency high-voltage alternating current power transmission and high-voltage direct current power transmission.

[0003] The design technology of the power frequency high-voltage alternating current power transmission system is mature, and the engineering operation experience is rich. At present, the offshore wind farms that have been put into operation mostly adopt the power frequency high-voltage alternating current power transmission to directly access the land-based main network. However, due to the influence of the cable charging current, the maximum power transmission capacity of the three-phase alternating current cable is significantly reduced with the increase of the power transmission distance. Corresponding to the 50Hz alternating current cable, the economic and reasonable power transmission distance is within 100 kilometers.

[0004] For medium and long distance wind farms, high-voltage direct current is generally used at present. The use of direct current power transmission technology can avoid the influence of the cable capacitance, increase the power transmission capacity and distance. However, the offshore wind power direct current power transmission system, especially the offshore bus platform and the converter station required by it, is expensive and difficult to maintain. In addition, although the line loss of the direct current power transmission is low, after the multiple conversion losses are added, the total loss exceeds the traditional HVAC mode in the medium and short distance, and the space charge accumulation effect will be generated in the direct current cable, which has an adverse effect on the insulation of the cable.

[0005] Flexible low-frequency power transmission provides a new choice for offshore wind power grid connection. The low-frequency alternating current power transmission technology can overcome the shortcomings of the power frequency alternating current power transmission and the high cost of the direct current power transmission. The transmission distance of the submarine cable power transmission is inversely proportional to the frequency adopted. If the power transmission frequency is reduced to about 15Hz, the economic and reasonable power transmission distance of the submarine cable can reach 300km, which can completely solve the problem of sending out the offshore wind power. At the same time, the alternating current cable does not have the space charge accumulation effect, and the capacitive current in the line is reduced by reducing the frequency, which is beneficial to the cable insulation. Moreover, the alternating current power transmission does not have the problem of no circuit breaker, and the offshore wind farm can be easily formed into an alternating current power grid. Compared with the offshore converter station required by the direct current power transmission, the land-based converter station has a greatly reduced manufacturing and maintenance cost, reduces the downtime, and thus increases the wind power supply.

[0006] Compared with the current MMC-based flexible HVDC transmission project and power frequency high-voltage AC transmission project, the flexible low-frequency transmission project learns from the frequency conversion idea of the flexible HVDC transmission, and takes advantage of the technical advantage of zero-point opening of AC transmission, in the frequency range of 0-50Hz, to improve the transmission capacity and distance of the project, and make up for the shortcomings of the flexible HVDC transmission project and the power frequency high-voltage AC transmission project. For the flexible low-frequency transmission project, the control strategy is essentially different from the flexible HVDC transmission project and the power frequency high-voltage AC transmission project, and the operation reliability is a crucial evaluation index, and the reliable control of the converter in the flexible low-frequency transmission project is the key to ensuring the operation reliability of the low-frequency project. The method in the prior art cannot realize the reliable control of the converter, so that the operation reliability of the flexible low-frequency transmission project is low. SUMMARY

[0007] The purpose of the present application is to provide a flexible low-frequency transmission system control method and device to solve the problem of low operation reliability of the flexible low-frequency transmission project caused by the inability to realize the reliable control of the converter in the prior art.

[0008] To solve the above technical problems, the present application provides a flexible low-frequency transmission system control method, comprising the following steps:

[0009] 1) According to the obtained power frequency three-phase grid side voltage u abc , power frequency three-phase grid side current i abc , converter bridge arm voltage and converter bridge arm current , the power frequency component of the converter bridge arm reference voltage is calculated

[0010] 2) According to the obtained converter bridge arm current , the bridge arm circulating current component of the converter bridge arm reference voltage is calculated

[0011] 3) According to the obtained low-frequency three-phase grid side voltage u uvw and low-frequency three-phase grid side current i uvw , the low-frequency component of the converter bridge arm reference voltage is calculated

[0012] 4) The power frequency component of the converter bridge arm reference voltage , the bridge arm circulating current component and the low-frequency component are added, and the converter bridge arm reference voltage is obtained according to the addition result The obtained converter bridge arm reference voltage is used to control the converter.

[0013] The beneficial effect is that the converter bridge arm reference voltage is calculated by considering various factors, including a power frequency side factor, a converter bridge arm factor and a low frequency side factor, so that three components, including a power frequency component, a bridge arm circulating current component and a low frequency component of the converter bridge arm reference voltage, are calculated correspondingly, the three components are added and processed, the final converter bridge arm reference voltage is obtained according to the result after the addition and processing, a feasible scheme is provided for reliable operation of the converter, and the operation reliability of the flexible low frequency power transmission system is ensured.

[0014] Further, the power frequency component of the converter bridge arm reference voltage calculated in step 1) is obtained by the following means:

[0015] The obtained power frequency three-phase network side voltage u abc is subjected to three-phase phase-locked loop control to obtain the power frequency side phase θ s .

[0016] According to the obtained power frequency three-phase network side voltage u abc and the power frequency three-phase network side current i abc , the power frequency reactive power Q s is calculated.

[0017] The obtained power frequency reactive power Q s is subjected to power frequency reactive power outer loop closed loop control to obtain the reactive current component reference value i sqref of the power frequency current inner loop closed loop control.

[0018] The total capacitor voltage of the sub-module is subjected to closed loop control to obtain the active current component reference value i spref of the power frequency current inner loop closed loop control.

[0019] According to the obtained reactive current component reference value i sqref and the active current component reference value i spref of the power frequency current inner loop closed loop control, power frequency reactive current inner loop closed loop control and power frequency active current inner loop closed loop control are respectively performed, and the power frequency side phase θ s calculated is combined to calculate the power frequency component of the converter bridge arm reference voltage.

[0020] The beneficial effect is that the active current component reference value of the power frequency current inner loop closed loop control is obtained by the total capacitor voltage closed loop control of the sub-module, so that the total capacitor voltage of the sub-module can reach the required value.

[0021] Further, the bridge arm circulating current component of the converter bridge arm reference voltage calculated in step 2) is obtained by the following means:

[0022] ​​The bridge arm circulating current reference value of the bridge arm circulating current closed-loop control is calculated through the differential regulation control of the capacitor voltage of the converter sub-module

[0023] According to the obtained converter bridge arm current and the calculated bridge arm circulating current reference value of the converter bridge arm circulating current closed-loop control The converter bridge arm circulating current control is performed to obtain the converter bridge arm circulating current component of the converter bridge arm reference voltage

[0024] The beneficial effects are that the converter bridge arm circulating current component of the converter bridge arm reference voltage is obtained by taking the sub-module capacitor voltage balancing control into account, and the sub-module capacitor voltage balancing is ensured.

[0025] Further, the low-frequency component of the converter bridge arm reference voltage is calculated in step 3) The means are:

[0026] The obtained low-frequency three-phase voltage u uvw is subjected to phase-locked loop control to obtain the low-frequency side phase θ m ;

[0027] According to the obtained low-frequency three-phase grid-side voltage u uvw and the low-frequency three-phase grid-side current i uvw , the low-frequency active power P m and the low-frequency reactive power Q m are calculated.

[0028] According to the obtained low-frequency active power P m and the low-frequency reactive power Q m , the low-frequency active power outer loop closed-loop control and the low-frequency reactive power outer loop closed-loop control are respectively performed, and the active current component reference value i mpref and the reactive current component reference value i mqref of the low-frequency current inner loop closed-loop control are respectively obtained.

[0029] According to the obtained active current component reference value i mpref and the reactive current component reference value i mqref of the low-frequency current inner loop closed-loop control, the low-frequency active current inner loop closed-loop control and the low-frequency reactive current inner loop closed-loop control are respectively performed, and the low-frequency side phase θ m is combined to calculate the low-frequency component of the converter bridge arm reference voltage.

[0030] Further, the obtained converter bridge arm reference voltage is sent to the valve control system through an optical fiber for the valve control system to control the converter.

[0031] Its beneficial effect is: transmission rate can be improved by using optical fiber transmission, and the timeliness of the action of the converter is ensured.

[0032] Further, the converter is an M3C converter.

[0033] To solve the above technical problems, the application also provides a flexible low-frequency power transmission system control device, comprising a memory and a processor, the processor is used for executing computer program instructions stored in the memory to realize the flexible low-frequency power transmission system control method introduced above, and achieve the same beneficial effects as the method. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is the flexible low-frequency power transmission system circuit topology structure of the application;

[0035] Figure 2 It is the control block diagram of the flexible low-frequency power transmission system control method of the application;

[0036] Figure 3 It is the structure diagram of the flexible low-frequency power transmission system control device of the application. DETAILED DESCRIPTION

[0037] The converter in the flexible low-frequency power transmission system of the application, the converter bridge arm reference voltage Three factors are considered, which are power frequency side, low frequency side and bridge arm itself, so that the converter bridge arm reference voltage Composed of three components, which are the power frequency component Bridge arm circulating component And low frequency component Complete the regulation and control of the flexible low-frequency power transmission system.

[0038] In order to make the purpose, technical scheme and advantages of the application more clear and obvious, the application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the application and are not used to limit the application, that is, the described examples are only a part of the examples of the application, not all examples. Therefore, the following detailed description of the embodiments of the application provided in the drawings is not intended to limit the scope of the claimed application, but only represents selected embodiments of the application. Based on the embodiments of the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0039] Method embodiment:

[0040] An embodiment of the flexible low-frequency power transmission system control method of the application, the circuit topology structure is as Figure 1The control block diagram is shown as Figure 2 The whole process is as follows:

[0041] Step one, the following control calculation is performed to obtain the power frequency component of the converter bridge arm reference voltage

[0042] 1) The power frequency three-phase network side voltage and current signals are collected by using the power frequency electrical quantity collection module, and the power frequency three-phase network side voltage u abc and the power frequency three-phase network side current i abc are obtained by completing A / D conversion.

[0043] 2) The phase-locked loop control is performed by using the power frequency three-phase network side voltage u abc , the power frequency side phase θ s is obtained, and the power frequency active power P s and the power frequency reactive power Q s are obtained by using the power frequency three-phase network side voltage u abc and the power frequency three-phase network side current i abc , so as to complete the power frequency electrical quantity operation.

[0044] 3) According to the obtained power frequency reactive power Q s , the power frequency reactive power outer loop closed-loop control is performed by using the power frequency reactive power PI controller, the reactive current component reference value i sqref of the power frequency current inner loop closed-loop control is calculated, and the power frequency power outer loop closed-loop control is realized.

[0045] 4) The closed-loop control is performed by using the sub-module total capacitor voltage PI controller, the active current component reference value i spref of the power frequency current inner loop closed-loop control is calculated, and the sub-module total capacitor voltage control is realized.

[0046] 5) According to the reactive current component reference value i sqref and the active current component reference value i spref of the power frequency current inner loop closed-loop control, the power frequency reactive current inner loop closed-loop control and the power frequency active current inner loop closed-loop control are respectively performed by using the power frequency reactive current PI controller and the power frequency active current PI controller, and the power frequency side phase θ s is combined, the power frequency component of the converter bridge arm reference voltage is calculated

[0047] Step two, the following control calculation is performed to obtain the bridge arm circulating current component of the converter bridge arm reference voltage

[0048] 1) The M3C converter 9 bridge arm voltage and current signals

[0049] 2) The collected 9 bridge arm voltage and current signals are respectively subjected to double αβ0 transformation to obtain and to complete the converter electrical quantity operation.

[0050] 3) The sub-module capacitor voltage equalization control module calculates the bridge arm circulating current reference value of the bridge arm circulating current closed-loop control through the differential regulation controller of the sub-module capacitor voltage

[0051] 4) According to the bridge arm circulating current reference value of the bridge arm circulating current closed-loop control of the converter , the bridge arm circulating current PI controller is used to control the bridge arm circulating current of the converter, and the bridge arm circulating current component of the converter bridge arm reference voltage is calculated

[0052] Step three, the low-frequency component of the converter bridge arm reference voltage is calculated through the following control calculation

[0053] 1) The low-frequency three-phase grid-side voltage and current signals are collected by using the low-frequency electrical quantity collection module, and A / D conversion is completed to obtain the low-frequency three-phase grid-side voltage u uvw and the low-frequency three-phase grid-side current i uvw .

[0054] 2) The low-frequency three-phase voltage u uvw is used for phase-locked loop control to obtain the low-frequency initial phase θ m ; and the low-frequency three-phase grid-side voltage u uvw and the low-frequency three-phase grid-side current i uvw are used to obtain the low-frequency active power P m and the low-frequency reactive power Q m to complete the low-frequency electrical quantity operation.

[0055] 3) According to the obtained low-frequency active power P m and the low-frequency reactive power Q m , and using the low-frequency active power PI controller and the low-frequency reactive power PI controller, the low-frequency active power outer loop closed-loop control and the low-frequency reactive power outer loop closed-loop control are respectively performed, and the active current component reference value i mpref and the reactive current component reference value i mqref of the low-frequency current inner loop closed-loop control are respectively calculated.

[0056] 4) According to the obtained low-frequency current inner loop closed-loop control active current component reference value i mpref and the reactive current component reference value i mqrefLow-frequency active current PI controller and low-frequency reactive current PI controller are used to perform low-frequency active current inner loop closed-loop control and low-frequency reactive current inner loop closed-loop control respectively, combined with the low-frequency side phase θ m The low-frequency component of the converter arm reference voltage was calculated.

[0057] Step 4: Utilize the power frequency component of the converter arm reference voltage. Low frequency components and converter bridge arm circulating component Calculate the double αβ0 components of the converter arm reference voltage. Performing a double αβ0 inverse transform on it yields the converter arm reference voltage. The signal is transmitted via optical fiber to the valve control system, which then sends commands to the M3C converter to control it.

[0058] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on several computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0059] Device Example:

[0060] An embodiment of the flexible low-frequency power transmission system control device of the present invention, such as... Figure 3 As shown, the system includes a memory, a processor, and an internal bus. The processor and memory communicate and exchange data with each other via the internal bus. The memory includes at least one software function module stored in the memory. The processor executes various functional applications and data processing by running the software programs and modules stored in the memory, thereby implementing the flexible low-frequency power transmission system control method described in the embodiments of the present invention.

[0061] The processor can be a microprocessor (MCU), a programmable logic device (FPGA), or other processing devices. The memory can be any type of memory that stores information using electrical energy, such as RAM and ROM; it can also be any type of memory that stores information using magnetic energy, such as hard disks, floppy disks, magnetic tapes, magnetic core memory, bubble memory, and USB flash drives; it can also be any type of memory that stores information using optical methods, such as CDs and DVDs; and of course, it can also be other types of memory, such as quantum memory and graphene memory.

[0062] The above gives specific embodiments, but the present application is not limited to the described embodiments. The basic idea of the present application is in the above basic scheme, and according to the teaching of the present application, designing various transformed models, formulas, and parameters does not require creative labor for those skilled in the art. Changes, modifications, replacements, and variations of the embodiments without departing from the principles and spirits of the present application still fall within the protection scope of the present application.

Claims

1. A method of controlling a flexible low frequency power transmission system, characterized by, The method comprises the following steps: 1) from the acquired line frequency three-phase network-side voltage u abc , line frequency three-phase network-side current i abc , converter bridge arm voltage and converter bridge arm current , the line frequency component of the converter bridge arm reference voltage is calculated ; 2) according to the obtained , the bridge arm circulating current component of the converter bridge arm reference voltage is calculated in the following way : the bridge arm circulating current reference value of the converter bridge arm circulating current closed-loop control is calculated through the differential regulation control of the converter sub-module capacitor voltage ; the converter bridge arm circulating current control is performed according to and , and is obtained; 3) a low-frequency component of the converter bridge arm reference voltage is calculated from the acquired low-frequency three-phase network-side voltage u uvw and the low-frequency three-phase network-side current i uvw ;​ 4) adding , and to obtain the inverter bridge arm reference voltage from the addition result; Utilizing The inverter is controlled.

2. The flexible low-frequency power transmission system control method according to claim 1, wherein The fundamental component of the converter bridge arm reference voltage calculated in step 1) The means are: The acquired power frequency three-phase network side voltage u abc The three-phase phase-locked loop control is performed to obtain the power frequency side phase θ s ; According to the acquired power frequency three-phase network side voltage u abc and the power frequency three-phase network side current i abc , the power frequency reactive power Q s is calculated According to the obtained power frequency reactive power Q s The power frequency reactive power outer loop control is performed to obtain a reactive current component reference value of the power frequency current inner loop closed-loop control i sqref ; The active current component reference value of the power frequency current inner loop closed loop control is obtained by using the total capacitor voltage closed loop control of the sub-module i spref ; According to the obtained reactive current component reference value of the power frequency current inner loop closed-loop control i sqref And the active current component reference value i spref , respectively, the power frequency reactive current inner loop closed-loop control and the power frequency active current inner loop closed-loop control are carried out, and the calculated power frequency side phase θ s , the power frequency component of the converter bridge arm reference voltage is calculated.

3. The control method of the flexible low-frequency power transmission system according to claim 1, wherein The low frequency component of the converter bridge arm reference voltage calculated in step 3) The means are: The acquired low-frequency three-phase voltage u uvw The phase on the low-frequency side is obtained by phase-locked loop control θ m ; According to the acquired low-frequency three-phase network-side voltage u uvw and the low-frequency three-phase network-side current i uvw , the low-frequency active power P m and the low-frequency reactive power Q m are calculated. According to the obtained low-frequency active power P m and low-frequency reactive power Q m Low-frequency active power outer loop closed-loop control and low-frequency reactive power outer loop closed-loop control are respectively performed, and low-frequency current inner loop closed-loop control active current component reference value i mpref and reactive current component reference value i mqref are respectively obtained. According to the obtained low-frequency current inner loop closed-loop control active current component reference value i mpref And the reactive current component reference value i mqref Low-frequency active current inner loop closed-loop control and low-frequency reactive current inner loop closed-loop control are respectively carried out, and the obtained low-frequency side phase θ m The low-frequency component of the converter bridge arm reference voltage is calculated.

4. The method of claim 1-3, wherein The obtained converter bridge arm reference voltage The obtained converter bridge arm reference voltage is transmitted via an optical fiber to a valve control system for controlling the converter.

5. The method of claim 1-3, wherein The converter is an M3C converter.

6. A flexible low frequency power transmission system control device, characterized by, The method comprises a memory and a processor, the processor being used to execute computer program instructions stored in the memory to realize the flexible low-frequency power transmission system control method as claimed in any one of claims 1-5.

Images