Hybrid low-frequency power transmission system and starting method and device thereof

Abstract

The invention belongs to the technical field of low-frequency alternating-current power transmission, and particularly relates to a hybrid low-frequency power transmission system and a starting method and device thereof. The method comprises the following steps: firstly, pre-charging an MMC and an M3C in the hybrid frequency converter by using an onshore power frequency alternating current power grid, unlocking the MMC after the pre-charging is completed, and controlling the MMC to enable the direct current side voltage to reach a rated value; secondly, locking the M3C so as to enable the hybrid frequency converter to be communicated with the offshore low-frequency wind turbine generator set, unlocking the M3C after the hybrid frequency converter is communicated with the offshore low-frequency wind turbine generator set, and controlling the M3C so as to enable the amplitude of the low-frequency side alternating voltage to rise to an initial instruction value of the amplitude of the alternating voltage; then the offshore low-frequency wind turbine generator set is unlocked so that the offshore wind turbine generator set can output power, and starting of the hybrid low-frequency power transmission system is completed; the problem that an existing starting method of the power transmission system is not suitable for the hybrid low-frequency power transmission system is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-frequency alternating current power transmission, and in particular relates to a hybrid low-frequency power transmission system and a starting method and device thereof. Background Art

[0002] Clean energy represented by photovoltaic and wind power is one of the important technologies to reduce carbon emissions in the energy production process. my country has abundant offshore wind power resources, which have developed rapidly in recent years. At present, for offshore wind farms that can be connected to the AC power grid nearby, AC transmission can be selected, while for the development of offshore wind power resources in COSCO, which are far away from the AC power grid, more flexible DC transmission systems are used. The use of flexible DC transmission technology can avoid the influence of cable capacitance and increase the capacity and distance of power transmission. However, the cost and maintenance cost of offshore DC transmission systems, especially the offshore convergence platforms and converter stations required by them, are expensive; in addition, although the line loss of DC transmission is low, after adding the loss caused by multiple converters, its total loss exceeds that of traditional HVAC methods in medium and short distances. Low-frequency AC transmission (LFAC) provides a new option for medium and short-distance offshore wind power grid connection. The capacitive current in the line is reduced by frequency reduction, and the flexible low-frequency transmission system only needs to build a converter station on land, which greatly reduces the manufacturing and maintenance costs compared to the offshore converter station required by the flexible DC transmission system.

[0003] At present, the low-frequency transmission projects that have been put into operation in China all use modular multilevel matrix converters (M3C) as the core inverter. Since M3C contains 9 bridge arms composed of full-bridge sub-modules connected in series, the cost of a single M3C frequency conversion valve is relatively high. There are also AC-DC-AC inverters based on modular multilevel converters (MMC) used in low-frequency transmission systems, but this inverter has the disadvantages of high cost and large power loss.

[0004] In order to reduce the cost of inverters in flexible low-frequency power transmission systems, the research team of Zhejiang University proposed to use a diode uncontrolled rectifier unit (DRU) to replace the low-frequency side converter in the AC-DC-AC inverter to form a DRU-MMC AC-DC-AC inverter, which can effectively reduce the investment cost and operating loss of low-frequency power transmission projects. Since DRU does not have active control capabilities, the Zhejiang University team proposed to use grid-type wind turbines with DRU-MMC inverters. However, when large-scale grid-type wind turbines are currently operated in parallel, there are problems such as synchronization, coordinated control, stability and oscillation suppression between converters.

[0005] Studies have found that connecting the M3C inverter and the DRU-MMC inverter in parallel to form a hybrid inverter can not only solve the problem that the DRU cannot control the AC voltage, but also improve the economy of the low-frequency transmission system. However, there is a lack of research on this low-frequency transmission system containing a hybrid inverter. The system's startup control method is the basis for the stable operation of the entire system, and the existing startup method is not suitable for this system. Therefore, research on the startup control method of the hybrid low-frequency transmission system is a necessary and key link in promoting the development of low-frequency transmission technology. Summary of the invention

[0006] The object of the present invention is to provide a hybrid low-frequency power transmission system and a starting method and device thereof, so as to solve the problem that the starting method of the existing power transmission system is not suitable for the hybrid low-frequency power transmission system.

[0007] In order to solve the above technical problems, the present invention provides a hybrid low-frequency power transmission system starting method, the method comprising:

[0008] First, the MMC and M3C in the hybrid inverter are pre-charged using the onshore industrial frequency AC power grid. After the pre-charging is completed, the MMC is unlocked first, and the MMC is controlled to make the DC side voltage reach the rated value; secondly, the M3C is locked, and then the hybrid inverter is connected to the offshore low-frequency wind turbine group. After the connection, the M3C is unlocked again, and the M3C is controlled to increase the AC voltage amplitude on the low-frequency side to the initial command value of the AC voltage amplitude; then the offshore low-frequency wind turbine group is unlocked to enable the offshore wind turbine group to output power, completing the startup of the hybrid low-frequency transmission system; wherein, the hybrid inverter includes M3C and DRU-MMC arranged in parallel.

[0009] Furthermore, after the offshore low-frequency wind turbine outputs power, the dq decoupling control strategy of the voltage outer loop and the current inner loop is adopted for M3C, wherein the d-axis component U of the actual command value of the AC voltage amplitude in the d-axis voltage outer loop control is d_ref is the deviation command value ΔU of the AC voltage amplitude d The d-axis component U of the initial command value of the AC voltage amplitude dref_ori The sum of ΔU d The calculation method is:

[0010] Calculate the difference between the actual active power input to M3C and the active power command value of M3C and adjust the difference. After adjustment and control, ΔU is obtained through limiting processing. d ; The limiting processing includes: when the input is less than 0, the output is 0.

[0011] Furthermore, the control strategy adopted for controlling M3C is a dq decoupling control strategy of a voltage outer loop and a current inner loop, wherein the d-axis component of the actual command value of the AC voltage amplitude in the d-axis voltage outer loop control is gradually increased to the initial command value of the AC voltage amplitude.

[0012] Furthermore, the initial command value of the AC voltage amplitude is lower than the AC voltage amplitude when the M3C is in rated operation.

[0013] Furthermore, the amplitude limiting process also includes: when the input is greater than a set amplitude threshold, the output is the set amplitude threshold.

[0014] Furthermore, the q-axis component of the actual command value of the AC voltage amplitude in the q-axis voltage outer loop control is zero.

[0015] Furthermore, the strategy adopted for controlling the MMC is constant DC voltage control.

[0016] Further, when the voltages of the submodules of the MMC and the submodules of the M3C inverter rise to corresponding set values, it is determined that the pre-charging is completed.

[0017] To solve the above technical problems, the present invention further provides a hybrid low-frequency power transmission system starting device, comprising a processor, wherein the processor executes a computer program to implement the steps of the hybrid low-frequency power transmission system starting method as described above.

[0018] In order to solve the above technical problems, the present invention also provides a hybrid low-frequency power transmission system, including a hybrid inverter controller and a hybrid inverter, the two ends of the hybrid inverter are respectively used to connect the offshore low-frequency wind turbine and the onshore industrial frequency AC power grid, the hybrid inverter controller includes a processor, and the processor executes a computer program to implement the steps of the hybrid low-frequency power transmission system startup method as mentioned above.

[0019] The beneficial effects are as follows: the present invention is an improved invention creation. The present invention considers that the high cost of the inverter in the existing low-frequency power transmission system can be well solved by adopting the M3C inverter and the DRU-MMC inverter to be set in parallel and integrated into a hybrid inverter, but there is no good starting method to make the system run smoothly; therefore, the present invention first uses the onshore industrial frequency AC power grid to pre-charge the MMC and M3C in the hybrid inverter, unlocks the MMC after the pre-charging is completed, and controls the MMC so that the DC side voltage is the rated value; secondly, the M3C is locked, and then the hybrid inverter is connected to the offshore low-frequency wind turbine set. The M3C is locked first and then the circuit is connected, which effectively avoids a large electrical impact on other equipment on the low-frequency side when the line is connected; after the connection, the M3C is unlocked, and the M3C is controlled to increase the AC voltage amplitude on the low-frequency side to the initial command value of the AC voltage amplitude, and then the offshore low-frequency wind turbine set is unlocked; so that the startup process of the entire offshore wind power hybrid low-frequency power transmission system does not have serious overvoltage and overcurrent phenomena, thereby improving the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the structure of an offshore wind power hybrid low-frequency power transmission system according to an embodiment of the method of the present invention;

[0021] Figure 2 Schematic diagram of the topological structure of the M3C frequency converter according to the method embodiment of the present invention;

[0022] Figure 3 is a start-up flow chart of an offshore wind power hybrid low-frequency power transmission system according to an embodiment of the method of the present invention;

[0023] Figure 4 It is a block diagram of the AC voltage control on the low frequency side of the M3C inverter according to the method embodiment of the present invention. DETAILED DESCRIPTION

[0024] The present invention firstly utilizes the onshore industrial frequency AC power grid to pre-charge the MMC and M3C in the hybrid inverter, and after the pre-charging is completed, the MMC is first unlocked, and the MMC is controlled so that the DC side voltage reaches the rated value; secondly, the M3C is locked, and then the hybrid inverter is connected with the offshore low-frequency wind turbine group, and after the connection, the M3C is unlocked again, and the M3C is controlled to increase the AC voltage amplitude on the low-frequency side to the initial command value of the AC voltage amplitude; then the offshore low-frequency wind turbine group is unlocked to enable the offshore wind turbine group to output power, and the startup of the hybrid low-frequency power transmission system is completed, thereby providing a stable startup strategy for the hybrid low-frequency power transmission system.

[0025] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0026] System Example:

[0027] A hybrid low-frequency power transmission system of this embodiment, such as Figure 1 As shown, the hybrid low-frequency power transmission system includes: an offshore low-frequency wind turbine, a low-frequency collection system, a low-frequency side AC switch, a low-frequency side transformer, an onshore hybrid inverter (i.e., a hybrid inverter), an industrial frequency side transformer, an industrial frequency side starting resistor, an industrial frequency side AC switch, and an onshore industrial frequency AC power grid.

[0028] Among them, the onshore hybrid inverter includes an AC-AC inverter M3C and an AC-DC-AC inverter DRU-MMC composed of a diode uncontrolled rectifier unit DRU and a modular multilevel converter MMC. The low-frequency sides of these two inverters are connected in series with the low-frequency side transformer and then connected in parallel. After the low-frequency side AC switch and the low-frequency collection system are connected in sequence, they are connected to the offshore low-frequency wind turbines; the power frequency side is connected in sequence to the power frequency side transformer, the power frequency side starting resistor and the power frequency side AC switch, and then connected to different onshore power frequency AC power grids. In the DRU-MMC inverter, the AC side of the DRU is connected to the low-frequency AC side, the AC side of the MMC is connected to the power frequency AC side, and the DC side of the DRU and the MMC are connected back to back. Among them, the onshore power frequency AC power grid charges the MMC and M3C through the power frequency side starting resistor, which plays a role in voltage stabilization.

[0029] like Figure 2 As shown, the M3C inverter contains three sub-converters, corresponding to Figure 2 Each of the sub-converters 1, 2, and 3 is composed of three bridge arms, and each bridge arm is composed of N full-bridge sub-modules and bridge arm reactances connected in series. M3C has two AC output terminals, which are connected to the industrial frequency AC line and the low-frequency AC line respectively. The electrical quantities of the two frequencies are interactively transformed in the three sub-converters. The low-frequency side of M3C controls the AC voltage. In order to realize the power distribution control between the M3C inverter and the DRU-MMC inverter, a power distribution controller is added to the d-axis voltage outer loop control of M3C. This additional controller is enabled after the offshore low-frequency wind turbine is unlocked and outputs power.

[0030] The hybrid low-frequency power transmission system of this embodiment also includes a hybrid inverter controller, which includes a processor. The processor executes a computer program to implement the startup method described below to complete the startup. The specific steps of the startup method are as follows: Figure 3 As shown, including the following:

[0031] Step 1: Close the power frequency side AC switch of the DRU-MMC inverter, and pre-charge the MMC submodule in the inverter from the onshore power frequency AC grid through the power frequency side starting resistor.

[0032] Step 2: After the pre-charging of the submodules of the MMC is completed, the MMC is unlocked, and a given DC voltage control strategy is adopted for the MMC to control the DC voltage to a rated value.

[0033] Step 3: Close the AC switch on the power frequency side of the M3C inverter, and pre-charge the M3C submodule from the onshore power frequency side AC grid through the power frequency side starting resistor.

[0034] Step 4: After the pre-charging of the M3C submodule is completed, first lock the M3C inverter and then close the low-frequency side AC switch.

[0035] Among them, in step 2 and step 3 of this embodiment, when the voltage of the submodule of the MMC and the submodule of the M3C inverter rises to the corresponding set value, it is determined that the pre-charging is completed. As other implementations, they can be set according to actual needs, for example, when the pre-charging time reaches the set pre-charging time, it is determined that the pre-charging is completed. In addition, the pre-charging of the submodule of the MMC and the submodule of the M3C in step 1 and step 3 can be performed simultaneously.

[0036] Step 5: M3C is unlocked and ramps up the low-frequency side AC voltage, wherein the initial command value of the AC voltage amplitude of M3C should be lower than the AC voltage amplitude of M3C during rated operation.

[0037] like Figure 4 As shown, the power distribution controller is not enabled at this time, and the M3C low-frequency side control strategy is the dq decoupling control of the voltage outer loop and the current inner loop. Specifically:

[0038] d-axis: deviation command value ΔU of AC voltage amplitude d is 0, the d-axis component U of the initial command value of the AC voltage amplitude dref_ori It increases in a ramp, and at this time the d-axis component U of the actual command value of the AC voltage amplitude d_ref Equal to U dref_ori ;Change U d_ref Subtract the d-axis component U of the actual feedback value of the AC voltage amplitude d_fdb The obtained value is used for PI regulation control.

[0039] q-axis: The value obtained by subtracting the q-axis component of the actual feedback value of the AC voltage amplitude from the q-axis component of the actual command value of the AC voltage amplitude is subjected to PI regulation control; wherein, the q-axis component of the actual command value of the AC voltage amplitude is 0.

[0040] Step 6: The offshore low-frequency wind turbine is unlocked and outputs power. The M3C in the onshore hybrid inverter controls the low-frequency side AC voltage amplitude to increase steadily in a closed-loop manner according to the output power of the wind turbine. The wind turbine power is output through the M3C inverter and the DRU-MMC inverter respectively according to the system allocation instructions, and the wind turbine power is transmitted to the onshore industrial frequency AC power grid.

[0041] like Figure 4 As shown, at this time, the power distribution controller starts to be enabled. In the d-axis outer loop control of M3C, the d-axis component of the actual command value of the AC voltage amplitude in the d-axis voltage outer loop control is calculated using the output result of the power distribution controller, and the control of the q-axis is consistent with step five; the d-axis voltage outer loop control is specifically:

[0042] The active power P actually input to M3C real_M3C The active power command value P of M3C ref_M3C The difference is then processed by PI regulation and control, and the result of the regulation and control output is processed by limiting to obtain the deviation command value ΔU of the AC voltage amplitude in M3C. d , add the deviation command value to the d-axis component U of the initial command value of the AC voltage amplitude dref_ori Get the d-axis component U of the actual command value of the AC voltage amplitude d_ref , subtract the d-axis component U of the actual feedback value of the AC voltage amplitude from the d-axis component of the actual command value of the AC voltage amplitude d_fdb The obtained value is used for PI regulation control.

[0043] The limiting process includes: when the input is less than 0, the output is 0, that is, ΔU d is 0; when the input is greater than or equal to 0 and less than or equal to the set amplitude threshold, the output is equal to the input, that is, ΔU d is the value of the PI regulation control output; when the input is greater than the set amplitude threshold, the output is the set amplitude threshold, that is, ΔU d To set the amplitude threshold.

[0044] The regulation control in this embodiment adopts PI regulation. Other implementation methods can be selected according to actual needs, for example: PID regulation.

[0045] The distribution process of the AC voltage on the low-frequency side is as follows: after the offshore low-frequency wind turbine is unlocked and outputs power, the power distribution controller attached to the AC voltage control of M3C is enabled; when the active power P actually input to M3C is real_M3C Greater than the active power command value P of M3C ref_M3C When the actual command value of the AC voltage amplitude of M3C is U d_ref Increase, under the action of the power distribution controller, the low-frequency side AC voltage amplitude of the entire system increases; combined with the characteristics of DRU, when its AC side voltage amplitude increases and the DC side voltage amplitude remains basically unchanged, the active power input to DRU will increase, and the actual active power input to M3C will decrease, thus forming a closed-loop control. Ultimately, the active power input to M3C by the wind turbine is controlled to be close to the active power command value P ref_M3CThe remaining active power output by the wind turbine is output through the DRU-MMC inverter.

[0046] A hybrid low-frequency power transmission system of the present invention can realize the smooth startup of the M3C inverter, the DRU-MMC inverter and the wind farm in the hybrid low-frequency power transmission system. After the pre-charging of the M3C and the MMC is completed, the MMC is first unlocked, and a fixed DC voltage control is adopted for the MMC. Then, the M3C is first locked, and then the hybrid inverter is connected with the offshore low-frequency wind turbine group to avoid a large electrical impact on other equipment on the low-frequency side when the line is connected. After the connection is completed, the M3C is unlocked again, and power distribution is performed through the power distribution controller in the d-axis voltage outer loop control of the M3C, so that the startup process of the entire offshore wind power hybrid low-frequency power transmission system does not have a serious overvoltage and overcurrent phenomenon, thereby protecting the safety of each equipment in the system and improving the safety, stability and reliability of the entire system.

[0047] Method Example:

[0048] A hybrid low-frequency power transmission system starting method in this embodiment is used to realize the starting of a hybrid low-frequency power transmission system as described in the above system embodiment. The hybrid low-frequency power transmission system starting method is the same as the starting method in the above system embodiment. The principle, implementation process, and achievable effects of the method have been fully introduced in the method embodiment and will not be repeated here.

[0049] Device Example:

[0050] A hybrid low-frequency power transmission system startup device of the present invention includes a memory, a processor and an internal bus, and the processor and the memory communicate and exchange data with each other through the internal bus. The memory includes at least one software function module stored in the memory, and the processor executes various functional applications and data processing by running the computer program and module stored in the memory to implement a hybrid low-frequency power transmission system startup method introduced in the method embodiment of the present invention. The principle, implementation process, and achievable effects of the method have been fully introduced in the method embodiment and will not be repeated here.

Claims

1. A method for starting a hybrid low-frequency power transmission system, characterized in that: The method includes: First, the MMC and M3C in the hybrid inverter are pre-charged using the onshore industrial frequency AC power grid. After the pre-charging is completed, the MMC is unlocked first, and the MMC is controlled to make the DC side voltage reach the rated value; secondly, the M3C is locked, and then the hybrid inverter is connected to the offshore low-frequency wind turbine group. After the connection, the M3C is unlocked again, and the M3C is controlled to increase the AC voltage amplitude on the low-frequency side to the initial command value of the AC voltage amplitude; then the offshore low-frequency wind turbine group is unlocked to enable the offshore wind turbine group to output power, completing the startup of the hybrid low-frequency transmission system; wherein, the hybrid inverter includes M3C and DRU-MMC arranged in parallel.

2. The hybrid low-frequency power transmission system startup method according to claim 1, characterized in that: After the offshore low-frequency wind turbine outputs power, the dq decoupling control strategy of voltage outer loop and current inner loop is adopted for M3C, among which the d-axis component U of the actual command value of the AC voltage amplitude in the d-axis voltage outer loop control is d_ref is the deviation command value ΔU of the AC voltage amplitude d The d-axis component U of the initial command value of the AC voltage amplitude dref_ori The sum of ΔU d The calculation method is: Calculate the difference between the actual active power input to M3C and the active power command value of M3C and adjust the difference. After adjustment and control, ΔU is obtained through limiting processing. d ; The limiting processing includes: when the input is less than 0, the output is 0.

3. The hybrid low-frequency power transmission system startup method according to claim 1, characterized in that: The control strategy adopted for controlling M3C is a dq decoupling control strategy of a voltage outer loop and a current inner loop, wherein the d-axis component of the actual command value of the AC voltage amplitude in the d-axis voltage outer loop control is gradually increased to the initial command value of the AC voltage amplitude.

4. The hybrid low-frequency power transmission system starting method according to any one of claims 1 to 3, characterized in that: The initial command value of the AC voltage amplitude is lower than the AC voltage amplitude when the M3C is in rated operation.

5. The hybrid low-frequency power transmission system startup method according to claim 2, characterized in that: The amplitude limiting process further includes: when the input is greater than a set amplitude threshold, the output is the set amplitude threshold.

6. The hybrid low-frequency power transmission system startup method according to claim 2 or 3, characterized in that: The q-axis component of the actual command value of the AC voltage amplitude in the q-axis voltage outer loop control is 0.

7. The hybrid low-frequency power transmission system startup method according to claim 1, characterized in that: The strategy adopted for controlling the MMC is constant DC voltage control.

8. The hybrid low-frequency power transmission system startup method according to claim 1, characterized in that: When the voltage of the submodule of the MMC and the submodule of the M3C inverter rises to the corresponding set value, it is determined that the pre-charging is completed.

9. A hybrid low-frequency power transmission system starting device, comprising a processor, characterized in that: The processor executes the computer program to implement the steps of the hybrid low-frequency power transmission system startup method according to any one of claims 1 to 8.

10. A hybrid low-frequency power transmission system, comprising a hybrid inverter controller and a hybrid inverter, wherein two ends of the hybrid inverter are respectively used to connect an offshore low-frequency wind turbine and an onshore industrial frequency AC power grid, and the hybrid inverter controller comprises a processor, characterized in that: The processor executes the computer program to implement the steps of the hybrid low-frequency power transmission system startup method according to any one of claims 1 to 8.

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