Direct-current transmission system direct-current line fault ride-through method and system

Abstract

The invention relates to a direct-current transmission system direct-current line fault ride-through method and system, and belongs to the field of power transmission and distribution of power systems. According to the method, after the alternating current energy consumption devices are put into use, when the direct current of an LCC converter of a rectification station is smaller than or equal to a current threshold value or the average voltage value of SVG sub-modules is smaller than or equal to a voltage threshold value, the put-into-group number of the alternating current energy consumption devices is determined again according to the average voltage value of the SVG sub-modules and a corresponding target value; the re-determined number of input groups can ensure that the average voltage value of the SVG sub-modules is close to the corresponding target value, so that the average voltage value of the SVG sub-modules is stabilized near the corresponding target value. Therefore, the problem that stable operation of the SVG sub-module is affected due to the fact that the group number of the alternating-current energy consumption devices input after forced phase shifting is fixed in the prior art is solved, and stable operation of the SVG sub-module is guaranteed.

Description

Technical Field

[0001] The invention relates to a method and system for riding through a DC line fault in a DC power transmission system, and belongs to the field of power transmission and distribution of electric power systems. Background Art

[0002] Line commutation converter based high voltage direct current (LCC-HVDC) is still the preferred route for building large-capacity, high-voltage (UHV) DC transmission systems due to the high withstand voltage, strong current-carrying capacity and engineering economy of its core device thyristor. However, the disadvantages of LCC-HVDC, such as its commutation process relying on a strong AC power grid and its lack of voltage / frequency construction capability, make it less adaptable in the scenario of large-scale isolated island transmission of new energy at the sending end.

[0003] Although voltage source converter based high voltage direct current (VSC-HVDC) has the advantages of flexibility, controllability and suitability for isolated island scenarios, it has the difficulty of DC fault ride-through, high operating losses and high manufacturing costs of core components. At present, the economic efficiency of VSC-HVDC in high voltage and large capacity applications is still poor.

[0004] Therefore, the introduction of VSC technology to improve LCC has been a major research hotspot at home and abroad in recent years. Among them, the multi-source adaptive phase-commutation converter type high voltage direct current (self-adaption statcom and line commutation converter based high voltage direct current, SLCC-HVDC) with static var generator (SVG) connected to the valve side of the LCC converter transformer is one of the typical solutions. SLCC-HVDC uses SVG in VSC technology to provide grid-side AC voltage / frequency support for LCC, so it can adapt to island application scenarios.

[0005] However, the SVG in the SLCC-HVDC used in the scenario of isolated island transmission of new energy is connected to the valve side of the LCC converter transformer. Its topological structure is special. After the LCC starts the forced phase shift strategy due to a DC line fault, the system DC current drops rapidly to 0, and the active power of the isolated new energy loses its transmission path. Therefore, it is necessary to configure an energy dissipation device at the sending-end isolated island rectifier station to dissipate the surplus power to avoid a large amount of surplus power accumulation in the SVG, resulting in overvoltage locking of its sub-modules, disconnection of the new energy station from the grid, and failure of the DC fault line crossing of the DC transmission system. Chain failure reaction.

[0006] At present, after the forced phase shift is started in the rectifier station and the inverter station, the number of AC energy-consuming devices put into operation is fixed. With the input of the AC energy-consuming devices, the voltage of the SVG submodule of the rectifier station will certainly fluctuate greatly, affecting the stable operation of the SVG submodule. Summary of the invention

[0007] The object of the present invention is to provide a DC line fault ride-through method and system for a DC power transmission system, so as to solve the problem in the prior art that the number of AC energy consuming devices put into operation after forced phase shifting remains unchanged, thus affecting the stable operation of SVG submodules.

[0008] To achieve the above object, the solution of the present invention includes:

[0009] A method for riding through a DC line fault in a DC power transmission system of the present invention comprises:

[0010] 1) When a fault is detected in a DC line in a DC transmission system, the rectifier station and the inverter station in the DC transmission system are controlled to perform forced phase shifting respectively;

[0011] 2) After forced phase shift, control the N in the rectifier station R1 AC energy consumption device input, N R1 It is determined based on the current active power of the isolated island renewable energy power plant and the rated active power of each group of AC energy consumption devices;

[0012] 3) In the rectifier station N R1 After the group of AC energy consuming devices is put into operation, when the DC current of the LCC converter of the rectifier station is less than or equal to the current threshold or the average voltage of the SVG submodule is less than or equal to the voltage threshold, the number of AC energy consuming devices put into operation is redetermined according to the average voltage of the SVG submodule and the corresponding target value. The redetermined number of AC energy consuming devices put into operation makes the difference between the average voltage of the SVG submodule and the corresponding target value less than the set threshold; the corresponding number of AC energy consuming devices are controlled to be put into operation according to the redetermined number of put into operation.

[0013] Furthermore, the corresponding target value range is (1.1~1.2)U sm_rated , where Usm_rated is the rated submodule voltage of SVG.

[0014] Furthermore, the re-determined number of groups of AC energy consuming devices is obtained by performing closed-loop control on the difference between the average voltage of the SVG submodule and the corresponding target value.

[0015] Furthermore, the controller adopted by the closed-loop control is a proportional controller or a proportional-integral controller.

[0016] Furthermore, the re-determined number of AC energy consuming devices is obtained by limiting the number of groups obtained according to the closed-loop control and rounding it to the nearest integer. The upper limit amplitude limit value used in the limiting is the total number of AC energy consuming devices, and the lower limit amplitude limit value is 0.

[0017] Furthermore, the number of AC energy consumption devices put into operation is N R1 The calculation formula is as follows:

[0018]

[0019] Among them, P W is the current active power of the isolated island renewable energy power plant; P R0 is the rated active power of each group of AC energy consuming devices; celing(·) is rounded up.

[0020] Furthermore, the average voltage of the SVG submodule is U sm_ave The calculation formula is as follows:

[0021]

[0022] Among them, N SVG N is the number of SVGs running in the system; sm The total number of submodules in a single SVG; u sm,ij is the voltage of the jth submodule of the i-th SVG.

[0023] Furthermore, the current threshold value range is (0.05~0.1)I dc_rated , where I dc_rated Rated DC current of the DC transmission system.

[0024] Furthermore, the voltage threshold value range is (0.8~0.9)U sm_rated , where U sm_rated is the rated submodule voltage of SVG.

[0025] The present invention also provides a DC line fault ride-through system for a DC power transmission system, comprising a processor, wherein the processor is used to execute a computer program to implement the steps of any of the above DC line fault ride-through methods for a DC power transmission system.

[0026] The beneficial effects of the present invention are as follows: as an improved invention, after the AC energy consuming device is put into operation, when the DC current of the LCC converter of the rectifier station is less than or equal to the current threshold or the average voltage of the SVG submodule is less than or equal to the voltage threshold, it indicates that the rectifier station needs to redetermine the number of AC energy consuming device groups put into operation. At this time, the number of AC energy consuming device groups put into operation is redetermined according to the average voltage of the SVG submodule and the corresponding target value. The redetermined number of groups put into operation can ensure that the average voltage of the SVG submodule is close to the corresponding target value, that is, stable near the target value. In this way, the corresponding number of AC energy consuming devices are put into operation according to the redetermined number of groups put into operation, which can stabilize the average voltage of the SVG submodule near the corresponding target value. Therefore, the present invention solves the problem in the prior art that the number of groups of AC energy consuming devices put into operation after forced phase shifting remains unchanged, affecting the stable operation of the SVG submodule, and ensures the stable operation of the SVG submodule. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the topological structure of the direct current transmission system targeted by the present invention;

[0028] Figure 2 It is a flow chart of a DC line fault ride-through method of a DC power transmission system of the present invention;

[0029] Figure 3 It is a closed-loop control block diagram of the average voltage of the SVG submodule in the closed-loop control stage of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail in a clear and complete manner in conjunction with the accompanying drawings and embodiments.

[0031] The idea of ​​the present invention is that after the AC energy consuming device is put into operation, when the DC current of the LCC converter of the rectifier station is less than or equal to the current threshold or the average voltage of the SVG submodule is less than or equal to the voltage threshold, the number of put-in groups of the AC energy consuming device is re-determined according to the average voltage of the SVG submodule and the corresponding target value. The re-determined number of put-in groups can ensure that the average voltage of the SVG submodule is close to the corresponding target value, that is, stabilized near the target value. In this way, the corresponding number of AC energy consuming devices are put into operation according to the re-determined number of put-in groups, so that the average voltage of the SVG submodule is stabilized near the corresponding target value.

[0032] Method Example:

[0033] The topological structure of the DC power transmission system of the present invention is as follows: Figure 1As shown, the DC transmission system is mainly composed of a wind farm, a rectifier station, and an inverter station. The rectifier station is connected to the wind farm through an AC line, and the rectifier station is connected to the inverter station through a DC line. There is no AC grid at the sending end of the DC transmission system, and it is in an island operation state. It is mainly composed of a wind farm and a rectifier station. One side of the AC bus of the rectifier station is connected to a grouped AC energy-consuming device, and the other side is connected to SVG1 and LCC1, SVG2 and LCC2 through a transformer. SVG1 is connected in series with LCC1, and SVG2 is connected in series with LCC2; the inverter station at the receiving end of the DC transmission system is a conventional LCC converter, which is connected to the AC bus through a transformer and connected to the AC grid. As another embodiment, the wind farm can also be other power plants, such as a hydroelectric power plant.

[0034] In this embodiment, it is assumed that the rated active power of the DC transmission system is P W 2000MW, rated voltage on DC side 400kV, rated current on DC side I dc_rated The total number of SVG submodules is 216. The rated voltage of SVG submodule is U sm_rated The voltage is 2.4kV, the total number of grouped AC energy consuming devices is 4, and the rated active power of each group of AC energy consuming devices is P R0 It is 525MW.

[0035] The flow chart of the DC line fault ride-through method of the DC power transmission system of the present invention is as follows: Figure 2 As shown, after the DC transmission system enters the DC fault ride-through process, the following steps are performed:

[0036] (1) The LCC converters of the rectifier station and inverter station start forced phase shifting respectively.

[0037] Detect whether the DC line between the rectifier station and the inverter station is faulty. When a fault occurs, control the LCC converters of the rectifier station and the inverter station to start forced phase shifting respectively. For example, 2ms and 5ms after a ground fault occurs in the DC line, the rectifier station and the inverter station start forced phase shifting respectively.

[0038] (2) Control N in the rectifier station R1 AC energy consumption device input, N R1 It is determined based on the current active power of the isolated island renewable energy power plant and the rated active power of each group of AC energy consumption devices.

[0039] After the rectifier station and the inverter station are forced to shift phase, this embodiment calculates the number of groups N of grouped AC energy consuming devices according to the current active power of the wind farm and the rated active power of each group of AC energy consuming devices. R1 , input group number N R1 The calculation formula is as follows:

[0040]

[0041] Among them, P W is the current active power of the wind farm; P R0 is the rated active power of each group of AC energy consuming devices; celing(·) is rounded up.

[0042] In this embodiment, P W =2000MW, P R0 =525MW, calculated to get N R1 =4.

[0043] (3) According to the DC current I of the LCC converter of the rectifier station dc Or the average voltage of SVG submodule U sm_ave Determine whether it is necessary to readjust the number of groups of AC energy consuming devices.

[0044] 1) Set the DC current threshold I dc_set and SVG submodule voltage average value threshold U sm_set0 ;

[0045] I dc_set The recommended value range is (0.05~0.1)I dc_rated , I dc_rated The rated DC current of the DC transmission system is 5kA; U sm_set0 The recommended value range is (0.8~0.9)U sm_rated , U sm_rated The rated submodule voltage of SVG is 2.4kV.

[0046] In this embodiment, Take U sm_set0 =0.8U sm_rated =1.92kV.

[0047] 2) Obtain the DC current I of the LCC converter of the rectifier station dc and the average voltage of the SVG submodule U sm_ave .

[0048] The DC current I dc It can be directly detected that the average voltage of the SVG submodule is U sm_ave It can be calculated based on the voltage of each SVG submodule, and the average voltage of the SVG submodule U sm_ave The calculation formula used is:

[0049]

[0050] Among them, N SVG N is the number of SVGs running in the system; sm The total number of submodules in a single SVG; u sm,ijis the voltage of the jth submodule of the i-th SVG.

[0051] In this embodiment, N SVG =2, N sm =216, we can get

[0052] 3) Determine the DC current I of the LCC converter of the rectifier station dc Or the average voltage of SVG submodule U sm_ave Whether the requirements are met.

[0053] When I dc ≤I dc_set or U sm_ave ≤U sm_set0 When the number of groups of grouped AC energy consuming devices enters the closed-loop control stage, the number of groups of AC energy consuming devices is readjusted.

[0054] (4) Re-determining the number of AC energy consuming devices put into operation according to the average voltage value of the SVG submodules and the corresponding target value; and controlling the corresponding number of AC energy consuming devices to be put into operation according to the re-determined number of groups put into operation.

[0055] 1) Set the closed-loop control target U of the SVG submodule voltage average value sm_set1 :

[0056] U sm_set1 The recommended value range is (1.1~1.2)U sm_rated ;

[0057] In this embodiment, take U sm_set1 =1.12U sm_rated =2.688kV.

[0058] 2) In the closed-loop control stage, the average voltage U of the SVG submodule is sm_ave and the closed-loop control target U of the SVG submodule voltage average value sm_set1 After the difference is closed-loop controlled, the new number of grouped AC energy consumption devices N is obtained. R2 .

[0059] The controller of closed-loop control can be a proportional controller or a proportional-integral controller. When the controller is a proportional controller, the controller parameter is only the proportional coefficient k. At this time, N R2 For U sm_set1 and U sm_ave The result of multiplying the difference of and the proportional coefficient k; when the controller is a proportional-integral controller, the controller parameters include the proportional coefficient k and the integral time constant T. Accordingly, at this time, N R2 It is the sum of the proportional and integral results. The proportional part is U sm_set1 and U sm_aveThe integral part has a continuous accumulation effect, which is the integral part value at the end of the previous cycle and the current cycle U sm_set1 and U sm_ave The result of multiplying the difference between and 1 / T*dt, where dt is the control cycle time. Through closed-loop control, U sm_ave and U sm_set1 The difference is smaller than a set threshold value to make the number of energy-consuming device input groups more accurate. The threshold value is a very small value close to 0, for example, 0.01.

[0060] In this embodiment, the closed-loop control controller is a proportional-integral controller with a proportional coefficient of 4.17 and an integral time constant of 0.012.

[0061] 3) For N R2 After amplitude limitation and rounding to the nearest integer, the final number of groups N of grouped AC energy-consuming devices in the closed-loop control stage is R3 .

[0062] Considering the closed-loop control, N R2 Usually it is not an integer, but the number of energy-consuming devices put into operation is N. R3 is an integer, so we need to R2 Perform rounding operation to get N R3 , so that the number of energy-consuming devices put into operation is as close as possible to the closed-loop control result; for example, if the closed-loop control result N R2 =2.85, then round up to get the final number of groups N R3 =3.

[0063] On the other hand, considering that the calculation results of closed-loop control may exceed the total number of energy-consuming devices, it is necessary to limit the calculation results, such as Figure 3 As shown, the amplitude limit includes an upper amplitude limit and a lower amplitude limit, where the upper amplitude limit value is the total number of grouped AC energy consuming devices N. RMAX , the lower limit amplitude limit value is 0. In this embodiment, N RMAX =4, if the closed-loop control result N R2 =5, since the upper limit is 4, the final number of groups is N R3 =4.

[0064] System Example:

[0065] A DC line fault ride-through system for a DC power transmission system of the present invention includes a processor, and the processor is used to execute computer program instructions to implement a DC line fault ride-through method for a DC power transmission system introduced in a method embodiment. The method has been introduced clearly enough in the method embodiment and will not be repeated here.

Claims

1. A DC line fault ride-through method for a DC power transmission system, characterized in that: The method includes: 1) When a fault is detected in a DC line in a DC transmission system, the rectifier station and the inverter station in the DC transmission system are controlled to perform forced phase shifting respectively; 2) After forced phase shift, control the N in the rectifier station R1 AC energy consumption device input, N R1 It is determined based on the current active power of the isolated island renewable energy power plant and the rated active power of each group of AC energy consumption devices; 3) In the rectifier station N R1 After the group of AC energy consuming devices is put into operation, when the DC current of the LCC converter of the rectifier station is less than or equal to the current threshold or the average voltage of the SVG submodule is less than or equal to the voltage threshold, the number of AC energy consuming devices put into operation is redetermined according to the average voltage of the SVG submodule and the corresponding target value. The redetermined number of AC energy consuming devices put into operation makes the difference between the average voltage of the SVG submodule and the corresponding target value less than the set threshold; the corresponding number of AC energy consuming devices are controlled to be put into operation according to the redetermined number of put into operation.

2. The DC line fault ride-through method of a DC power transmission system according to claim 1, characterized in that: The corresponding target value range is (1.1~1.2)U sm_rted , where U sm_rted is the rated submodule voltage of SVG.

3. The DC line fault ride-through method of a DC power transmission system according to claim 1, characterized in that: The re-determined number of groups of AC energy consuming devices is obtained by performing closed-loop control on the difference between the average voltage value of the SVG submodule and the corresponding target value.

4. The DC line fault ride-through method of a DC power transmission system according to claim 3, characterized in that: The controller used in closed-loop control is a proportional controller or a proportional-integral controller.

5. The DC line fault ride-through method of a DC power transmission system according to claim 3, characterized in that: The re-determined number of AC energy consuming devices is obtained by limiting the number of groups obtained according to the closed-loop control and rounding it to the nearest integer. The upper limit amplitude limit value used in the limiting is the total number of AC energy consuming devices, and the lower limit amplitude limit value is 0.

6. The DC line fault ride-through method of a DC power transmission system according to claim 1, characterized in that: The number of groups N of AC energy consuming devices put into operation R1 The calculation formula is as follows: Among them, P W is the current active power of the isolated island renewable energy power plant; P R0 is the rated active power of each group of AC energy consuming devices; celing(·) is rounded up.

7. The DC line fault ride-through method of a DC power transmission system according to claim 1, characterized in that: The SVG submodule voltage average value U sm_ave The calculation formula is as follows: Among them, N SVG N is the number of SVGs running in the system; sm The total number of submodules in a single SVG; u sm,ij is the voltage of the jth submodule of the i-th SVG.

8. The DC line fault ride-through method of a DC power transmission system according to claim 1, characterized in that: The current threshold value range is (0.05~0.1)I dc_rated , where I dc_rated Rated DC current of the DC transmission system.

9. The DC line fault ride-through method of a DC power transmission system according to claim 1, characterized in that: The voltage threshold value range is (0.8~0.9)U sm_rated , where U sm_rated is the rated submodule voltage of SVG.

10. A DC line fault ride-through system for a DC power transmission system, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of a DC line fault ride-through method for a DC power transmission system as claimed in any one of claims 1 to 9.

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