Flexible low-frequency power transmission system considering frequency and phase angle optimization

Abstract

The invention discloses a flexible low-frequency power transmission system considering frequency and phase angle optimization. The flexible low-frequency power transmission system comprises a first-frequency alternating-current power grid operating at a first frequency; the second-frequency alternating-current power grid operates at a second frequency, and the second frequency is lower than the first frequency; one side of the modularized multi-level matrix type alternating-current converter is connected with a first frequency alternating-current power grid through a first frequency transformer, and the other side of the modularized multi-level matrix type alternating-current converter is connected with a second frequency alternating-current power grid through a second frequency transformer, so that conversion between the first frequency and the second frequency is realized; under the conditions of a first frequency optimal value, a second frequency optimal value and a first frequency and second frequency alternating-current power grid side voltage initial phase angle difference optimal value, the same second frequency alternating-current power grid side steady-state voltage is achieved, the single-bridge-arm voltage demand value of the modular multilevel matrix type alternating-current converter is minimum, the number of modules needed by each bridge arm is minimum, and the number of modules needed by each bridge arm is small. And the cost of the alternating current converter of the low-frequency power transmission system can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular to a flexible low-frequency power transmission system taking frequency and phase angle optimization into consideration. Background Art

[0002] Flexible low-frequency power transmission is a new type of AC power transmission technology, and its transmission frequency is between industrial frequency and DC. Due to the reduction of frequency, flexible low-frequency power transmission combines the technical characteristics of industrial frequency and DC power transmission, and has broad application prospects in the fields of urban power grid interconnection, new energy grid connection, and long-distance power supply. Especially in the application of COSCO offshore wind power transmission, flexible low-frequency power transmission technology provides a new means for the economical and efficient transmission of COSCO offshore wind power.

[0003] In the flexible low-frequency transmission system, the AC converter is responsible for completing the frequency coupling and power transfer between the industrial frequency power grid and the low-frequency transmission system, and is the core equipment of the flexible low-frequency system. The modular multi-level matrix AC converter adopts a 9-bridge arm structure, each bridge arm is composed of several sub-modules, and is currently the AC converter with the greatest engineering application potential. Due to the need to meet the industrial and low-frequency voltage output requirements at the same time, the number of sub-modules in each bridge arm is large, resulting in a high cost for the modular multi-level matrix AC converter. Therefore, it is necessary to study the influence of frequency and industrial and low-frequency voltage coupling on the number of sub-modules, and reduce the number of sub-modules required for a single bridge arm by optimizing the frequency and coordinating the industrial and low-frequency voltage, thereby achieving the effect of reducing the cost of the modular multi-level matrix AC converter. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned prior art and provide a flexible low-frequency power transmission system taking into account the optimization of frequency and phase angle, which minimizes the number of bridge arm sub-modules in series of the modular multi-level matrix converter while achieving the same low-frequency voltage amplitude by optimizing the low-frequency frequency and the phase angle coordination of the industrial low-frequency voltage, thereby reducing the AC converter cost of the low-frequency power transmission system.

[0005] To this end, the present invention adopts the following technical solution: a flexible low-frequency power transmission system taking into account the optimization of frequency and phase angle, comprising: a first frequency AC power grid, operating at a first frequency; a second frequency AC power grid operating at a second frequency, wherein the second frequency is lower than the first frequency; A modular multi-level matrix AC switch, one side of which is connected to a first-frequency AC grid via a first frequency transformer, and the other side of which is connected to a second-frequency AC grid via a second frequency transformer, to achieve conversion between the first frequency and the second frequency; The preferred value of the second frequency and the preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltages. Under this condition, the same second frequency AC grid side steady-state voltage is achieved, the voltage requirement value of a single bridge arm of the modular multi-level matrix AC converter is minimized, and the number of modules required for each bridge arm is minimized.

[0006] Furthermore, the second frequency AC grid side of the modular multi-level matrix AC converter adopts a constant AC voltage amplitude and frequency control strategy: according to the voltage amplitude instruction and voltage frequency instruction issued by the dispatcher, the voltage amplitude and voltage frequency on the second frequency AC grid side of the AC converter are kept constant through voltage and current dual-loop decoupling control.

[0007] Furthermore, the voltage requirement value of a single bridge arm of the modular multi-level matrix AC switch is for:

[0008] Among them, U1 and U2 are the rated voltage of the first frequency AC grid side and the rated voltage of the second frequency AC grid side, f1 and f2 are the first frequency and the second frequency, respectively. and are the angular velocity of the first frequency AC grid and the angular velocity of the second frequency AC grid, is the initial phase angle of the voltage on the AC grid side at the first frequency, is the initial phase angle of the voltage on the second frequency AC grid side, is the difference between the initial phase angle of the voltage on the AC grid side at the first frequency and the initial phase angle of the voltage on the AC grid side at the second frequency, and t is the time.

[0009] Furthermore, at the second frequency preferred value The preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltage Under these conditions, the voltage requirement of a single bridge arm of the modular multi-level matrix AC converter reaches the minimum value, which is: , In the formula, The voltage requirement of a single bridge arm is minimized.

[0010] Furthermore, in the modular multi-level matrix AC switch, the minimum number of modules N required for each bridge arm is:

[0011] Among them, U CNis the rated voltage of the bridge arm submodule of the modular multi-level matrix AC converter, M is the rated modulation of the AC converter, and k is the margin coefficient. M is equal to the sum of the AC phase voltage amplitude on the first frequency side of the AC converter and the AC phase voltage amplitude on the second frequency side divided by the sum of the capacitor voltages of the bridge arm submodules.

[0012] Furthermore, considering the voltage drop of the bridge arm reactor, the k is set to 1.0-1.2.

[0013] Furthermore, the preferred value of the second frequency and the preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltage are obtained in the following steps: Step 1: According to the design of the flexible low-frequency power transmission system, determine the second frequency range [f 2min , f 2max ]; Step 2, at the second frequency initial value f 2min , Under the condition that t is 0, t starts from 0 and gradually increases according to a certain step size. Formula (2) is traversed within a period range to obtain U brg Maximum value U brg_max_0 ; Step 3, at the second frequency initial value f 2min conditions, Starting from 0, it gradually increases in steps. Repeat step 2 in the range of ∈[0,π] to obtain different Under the condition U brg Maximum value U brg_max_i , compare and get all U brg_max_i The minimum value in the second frequency f 2min U under the condition brg The minimum value, denoted as U brg_min_i ; Step 4, repeat step 3, the second frequency f2 is f 2min Start to increase gradually according to a certain step size, and then 2min , f 2max ], repeat steps 2 and 3 to obtain U under different f2 conditions. brg Minimum value U brg_min_i , compare and get all U brg_min_i The minimum value of the voltage requirement of a single bridge arm of the modular multi-level matrix AC converter is , the corresponding frequency is the second frequency preferred value , the corresponding phase angle difference is the preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltage .

[0014] Furthermore, when the modular multi-level matrix AC switch is running, it always runs at the second frequency preferred value. The optimal value of the initial phase angle difference At this time, the frequency of the AC grid side voltage of the second frequency AC converter adopts a fixed value , the angle value adopts an open-loop control method, the first frequency AC grid side voltage phase angle calculation is realized by a phase-locked loop, the first frequency AC grid side voltage value of the AC converter is calculated by the power flow transmitted between the AC converter and the first frequency AC grid and the initial phase angle of the first frequency AC grid side, and the initial phase angle of the second frequency AC grid side voltage of the AC converter differs from the initial phase angle of the first frequency AC grid side voltage .

[0015] The beneficial effects of the present invention are as follows: by optimizing the low-frequency frequency and the phase angle coordination of the industrial low-frequency voltage, the number of bridge arm submodules connected in series in the modular multi-level matrix converter is minimized under the condition of achieving the same low-frequency voltage amplitude, thereby reducing the AC converter cost of the low-frequency power transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the topological structure of the flexible low-frequency power transmission system of the present invention; Figure 2 A flow chart showing the optimal frequency and phase angle difference of the flexible low-frequency power transmission system of the present invention; Figure 3 It is a total voltage flow chart of the output of the series-connected submodules under different t of the present invention; Figure 4 The present invention is different The total voltage flow chart of the series submodule output under; Figure 5 This is a flow chart of the total voltage output by the series-connected submodules under different f2 conditions of the present invention. DETAILED DESCRIPTION

[0017] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0018] This embodiment provides a flexible low-frequency power transmission system taking into account frequency and phase angle optimization.

[0019] Figure 1The figure is a schematic diagram of the topological structure of the flexible low-frequency power transmission system. The flexible low-frequency power transmission system includes a first-frequency AC grid, a second-frequency AC grid and an AC-to-AC switching station. Among them, the first-frequency AC grid operates at a first frequency, the second-frequency AC grid operates at a second frequency, the first frequency is higher than the second frequency, and the first-frequency AC grid and the second-frequency AC grid are connected through an AC-to-AC switching station. The AC-to-AC switching station includes a modular multi-level matrix AC converter, a first frequency transformer and a second frequency transformer. The modular multi-level matrix AC converter is connected to the first-frequency AC grid through the first frequency transformer; the modular multi-level matrix AC converter is connected to the second-frequency AC grid through the second frequency transformer. The modular multi-level matrix AC converter adopts a fixed AC voltage amplitude and frequency control strategy on the second-frequency AC grid side: according to the voltage amplitude command and voltage frequency command issued by the dispatcher, the voltage amplitude and voltage frequency on the second-frequency AC grid side of the AC converter are kept constant through voltage and current dual-loop decoupling control.

[0020] When the modular multi-level matrix AC converter is running, the first frequency AC grid side voltage U is output at the first frequency AC grid side. f1 , outputs a second frequency AC grid side voltage U at the second frequency AC grid side f2 : (1) Among them, U1 and U2 are the rated voltage of the first frequency AC grid side and the rated voltage of the second frequency AC grid side, respectively. and are the angular velocity of the first frequency AC grid and the angular velocity of the second frequency AC grid, is the initial phase angle of the voltage on the AC grid side at the first frequency, is the initial phase angle of the voltage on the second frequency AC grid side, and t is the time.

[0021] Therefore, for each bridge arm, the total voltage output by its series-connected submodules is: (2) Wherein, f1 and f2 are the first frequency (i.e., the frequency of the first-frequency AC power grid) and the second frequency (i.e., the frequency of the second-frequency AC power grid), respectively. It is the difference between the initial phase angle of the voltage on the second frequency AC grid side and the initial phase angle of the voltage on the second frequency AC grid side.

[0022] The present invention provides a method for optimizing the second frequency f2 and an initial phase angle difference between the first frequency and the second frequency AC grid side voltage. The optimization method, under the optimal value, achieves the same second frequency steady-state voltage, the modular multi-level matrix AC switch single bridge arm voltage demand value is the smallest, and the number of modules required for each bridge arm is the least. Figure 2 As shown, the specific preferred steps are as follows: Step 1: According to the system design, determine the frequency range of the second frequency [f 2min , f 2max ].

[0023] Step 2, at the second frequency initial value f 2min , the initial phase angle difference between the voltage on the first frequency AC grid side and the voltage on the second frequency AC grid side When t is 0, t increases gradually according to a certain step length, and formula (2) is traversed within a period range to obtain U brg Maximum value U brg_max_0 , starting from 0, the period here is the least common multiple of the first frequency period and the second frequency period.

[0024] Step 3, at the second frequency initial value f 2min conditions, Starting from 0, it gradually increases in steps. Repeat step 2 in the range of ∈[0,π] to obtain different Under the condition U brg Maximum value U brg_max_i , compare and get all U brg_max_i The minimum value in the second frequency f min U under the condition brg The minimum value, denoted as U brg_min_i .

[0025] Step 4, repeat step 3, the second frequency f2 is f 2min Start to increase gradually according to a certain step size, and then 2min , f 2max ] Repeat steps 2 and 3 to obtain U under different f2 conditions. brg Minimum value U brg_min_i , compare and get all U brg_min_i The minimum value of is the minimum value of the voltage requirement of a single bridge arm of the modular multi-level matrix AC converter U brg_min , the corresponding frequency is , the corresponding phase angle difference is .at this time, (3) Correspondingly, the minimum number of modules N required for each bridge arm is: (4) Among them, U CNis the rated voltage of the bridge arm submodule of the modular multi-level matrix AC converter, M is the rated modulation index, and k is the margin coefficient, which can be 1.0-1.2. M is equal to the sum of the AC phase voltage amplitude on the first frequency side of the AC converter and the AC phase voltage amplitude on the second frequency side divided by the sum of the capacitor voltages of the bridge arm submodules.

[0026] When the modular multi-level matrix AC switch is running, it always runs at the second frequency preferred value. The optimal value of the initial phase angle difference At this time, the frequency of the AC grid side voltage of the second frequency AC converter adopts a fixed value , the angle value adopts an open-loop control method, the first frequency AC grid side voltage phase angle calculation is realized by a phase-locked loop, the first frequency AC grid side voltage value of the AC converter is calculated by the power flow transmitted between the AC converter and the first frequency AC grid and the initial phase angle of the first frequency AC grid side, and the initial phase angle of the second frequency AC grid side voltage of the AC converter differs from the initial phase angle of the first frequency AC grid side voltage .

[0027] The following are application examples: The frequency of the first frequency AC grid is 50Hz, the initial phase angle of the voltage on the power frequency side of the AC converter is 0 degrees, and according to the system design, the frequency range of the second frequency that meets the system performance requirements is 10Hz-30Hz. Assuming that the rated voltage on the first frequency AC grid side and the rated voltage on the second frequency AC grid side are both 64kV. Then: The initial value of the second frequency is 10 Hz, and the initial value of the initial phase angle difference of the second frequency AC grid side voltage is 0 degrees. Formula (2) is traversed within a cycle of 0.1 s, and the increase step of t is selected as 0.0001 s. brg Maximum U brg_max_0 is 107.8kV. The total voltage output by the series submodules at different t is as follows: Figure 3 shown.

[0028] second, Starting from 0, the step size is gradually increased, and the above steps are repeated in the range of [0, π]. The increase step size is selected as 1 degree, and the second frequency initial value f min is U at 10Hz brg Minimum value U brg_min_i is 97.8kV, different The total voltage output by the series submodules is as follows: Figure 4 shown.

[0029] Third, the initial value of the second frequency f2 starts from 10Hz and gradually increases according to a certain step size. The first two steps are repeated in the range of f2∈[10,30], and the increase step size of f2 is selected as 0.01Hz. The U under different f2 conditions is obtained. brg Minimum value U brg_min is 58.8kV, and the corresponding frequency is The corresponding phase angle difference is 25Hz. is 135 degrees, the total voltage output by the series submodules under different f2 conditions is as follows Figure 5 shown.

[0030] It can be seen that under the conditions of the preferred value of the second frequency being 25 Hz and the preferred value of the first frequency / second frequency phase angle difference being 135 degrees, the total voltage requirement for the output of the series sub-modules is 58.8 kV, which is 107.8 kV compared to the required bridge arm voltage of 107.8 kV when the second frequency is 10 Hz and the first frequency / second frequency phase angle difference is 0 degrees. The number of sub-modules is saved by about 45.5%, and the cost of the AC converter will be significantly reduced.

[0031] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A flexible low-frequency power transmission system taking into account frequency and phase angle optimization, characterized in that: include: a first frequency AC power grid, operating at a first frequency; a second frequency AC power grid operating at a second frequency, wherein the second frequency is lower than the first frequency; A modular multi-level matrix AC switch, one side of which is connected to a first-frequency AC grid via a first frequency transformer, and the other side of which is connected to a second-frequency AC grid via a second frequency transformer, to achieve conversion between the first frequency and the second frequency; The preferred value of the second frequency and the preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltages. Under this condition, the same second frequency AC grid side steady-state voltage is achieved, the voltage requirement value of a single bridge arm of the modular multi-level matrix AC converter is minimized, and the number of modules required for each bridge arm is minimized.

2. The flexible low-frequency power transmission system according to claim 1, characterized in that: The second frequency AC grid side of the modular multi-level matrix AC switch adopts a fixed AC voltage amplitude and frequency control strategy.

3. The flexible low-frequency power transmission system according to claim 2, characterized in that: The constant AC voltage amplitude and frequency control strategy is: according to the voltage amplitude instruction and voltage frequency instruction issued by the dispatcher, the voltage amplitude and voltage frequency on the second frequency AC grid side of the AC converter are kept constant through voltage and current dual-loop decoupling control.

4. The flexible low-frequency power transmission system according to claim 1, characterized in that: The modular multi-level matrix AC switch has a single bridge arm voltage requirement value of for: (2) Among them, U1 and U2 are the rated voltage of the first frequency AC grid side and the rated voltage of the second frequency AC grid side, f1 and f2 are the first frequency and the second frequency, respectively. and are the angular velocity of the first frequency AC grid and the angular velocity of the second frequency AC grid, is the initial phase angle of the voltage on the AC grid side at the first frequency, is the initial phase angle of the voltage on the second frequency AC grid side, is the difference between the initial phase angle of the voltage on the AC grid side at the first frequency and the initial phase angle of the voltage on the AC grid side at the second frequency, and t is the time.

5. The flexible low-frequency power transmission system according to claim 4, characterized in that: At the second frequency preferred value The preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltage Under these conditions, the voltage requirement of a single bridge arm of the modular multi-level matrix AC converter reaches the minimum value, which is: ， In the formula, It is the minimum value of the voltage requirement of a single bridge arm.

6. The flexible low-frequency power transmission system according to claim 5, characterized in that: The minimum number of modules N required for each bridge arm of the modular multi-level matrix AC switch is: ， Among them, U CN is the rated voltage of the bridge arm submodule of the modular multi-level matrix AC converter, M is the rated modulation index of the AC converter, and k is the margin coefficient.

7. The flexible low-frequency power transmission system according to claim 6, characterized in that: The rated modulation index M of the AC converter is equal to the sum of the AC phase voltage amplitude at the first frequency side and the AC phase voltage amplitude at the second frequency side of the AC converter divided by the sum of the bridge arm submodule capacitor voltages.

8. The flexible low-frequency power transmission system according to claim 6, characterized in that: Considering the voltage drop of the bridge arm reactor, the k is set to 1.0-1.

2.

9. The flexible low-frequency power transmission system according to claim 5, characterized in that: The steps of obtaining the preferred value of the second frequency and the preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltage are as follows: Step 1: According to the design of the flexible low-frequency power transmission system, determine the second frequency range [f 2min , f 2max ]; Step 2, at the second frequency initial value f 2min , Under the condition that t is 0, t starts from 0 and gradually increases according to a certain step size. Formula (2) is traversed within a period range to obtain U brg Maximum U brg_max_0 ; Step 3, at the second frequency initial value f 2min conditions, Starting from 0, it gradually increases in steps. Repeat step 2 in the range of ∈[0,π] to obtain different Under the condition U brg Maximum U brg_max_i , compare and get all U brg_max_i The minimum value in the second frequency f 2min U under the condition brg The minimum value, denoted as U brg_min_i ; Step 4, repeat step 3, the second frequency f2 is f 2min Start to increase gradually according to a certain step size, and then 2min , f 2max Repeat steps 2 and 3 within the range of ] to obtain U under different f2 conditions. brg Minimum value U brg_min_i , compare and get all U brg_min_i The minimum value of the voltage requirement of a single bridge arm of the modular multi-level matrix AC converter is , the corresponding frequency is the second frequency preferred value , the corresponding phase angle difference is the preferred value of the initial phase angle difference between the first frequency and the second frequency AC grid side voltage .

10. The flexible low-frequency power transmission system according to claim 5, characterized in that: When the modular multi-level matrix AC switch is running, it always runs at the second frequency preferred value. The optimal value of the initial phase angle difference At this time, the frequency of the AC grid side voltage of the second frequency AC converter adopts a fixed value , the angle value adopts an open-loop control method, the first frequency AC grid side voltage phase angle calculation is realized by a phase-locked loop, the first frequency AC grid side voltage value of the AC converter is calculated by the power flow transmitted between the AC converter and the first frequency AC grid and the initial phase angle of the first frequency AC grid side, and the initial phase angle of the second frequency AC grid side voltage of the AC converter differs from the initial phase angle of the first frequency AC grid side voltage .

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