A Flexible Low-Frequency Power Transmission System Considering Frequency and Phase Angle Optimization

Abstract

The present invention discloses a flexible low-frequency power transmission system considering frequency and phase angle optimization. The flexible low-frequency power transmission system of the present invention includes: a first-frequency AC power grid operating at a first frequency; a second-frequency AC power grid operating at a second frequency, where the second frequency is lower than the first frequency; a modular multilevel matrix converter, one side of which is connected to the first-frequency AC power grid through a first-frequency transformer, and the other side is connected to the second-frequency AC power grid through a second-frequency transformer to achieve the conversion between the first frequency and the second frequency; a second-frequency optimization value and a first-frequency and second-frequency AC power grid side voltage initial phase angle difference optimization value, under which conditions, to achieve the same steady-state voltage on the second-frequency AC power grid side, the single-bridge-arm voltage requirement value of the modular multilevel matrix converter is minimized, and the number of modules required for each bridge arm is minimized, thereby reducing the cost of the converter in the low-frequency power transmission system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution in power systems, and more specifically, it is a flexible low-frequency power transmission system considering frequency and phase angle optimization. Background Art

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

[0003] In a flexible low-frequency power transmission system, the AC / AC converter is responsible for completing the frequency coupling and power transfer between the power-frequency power grid and the low-frequency power transmission system, and is the core equipment of the flexible low-frequency system. The modular multilevel matrix AC / AC converter adopts a 9-arm structure, and each arm is composed of several sub-modules, which is currently the most potential AC / AC converter for engineering applications. Since it is necessary to simultaneously meet the power-frequency and low-frequency voltage output requirements, the number of sub-modules in each arm is relatively large, resulting in a relatively high cost of the modular multilevel matrix AC / AC converter. Therefore, it is necessary to study the influence relationship between frequency and power-frequency / low-frequency voltage coupling on the number of sub-modules, and reduce the number of sub-modules required for a single arm through frequency optimization and coordinated control of power-frequency and low-frequency voltages, so as to reduce the cost of the modular multilevel matrix AC / 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 considering frequency and phase angle optimization, which, through optimizing the low-frequency frequency and the phase angle matching of power-frequency and low-frequency voltages, minimizes the number of series-connected sub-modules in the arm of the modular multilevel matrix converter under the condition of achieving the same low-frequency voltage amplitude, thereby reducing the cost of the AC / AC converter in the low-frequency power transmission system.

[0005] To this end, the present invention adopts the following technical solutions: A flexible low-frequency power transmission system considering frequency and phase angle optimization, comprising:

[0006] A first-frequency AC power grid operating at a first frequency;

[0007] A second-frequency AC power grid operating at a second frequency, where the second frequency is lower than the first frequency;

[0008] A modular multilevel matrix AC / AC converter, one side of which is connected to the first-frequency AC power grid through a first-frequency transformer, and the other side is connected to the second-frequency AC power grid through a second-frequency transformer to realize the conversion between the first frequency and the second frequency;

[0009] The preferred value of the second frequency and the preferred value of the phase angle difference between the first frequency and the voltage on the AC grid side of the second frequency. Under these conditions, to achieve the same steady-state voltage on the AC grid side of the second frequency, the required value of the voltage of a single arm of the modular multilevel matrix converter is minimized, and the minimum number of modules required for each arm is the least.

[0010] Furthermore, the modular multilevel matrix converter adopts a constant AC voltage amplitude and frequency control strategy on the AC grid side of the second frequency: according to the voltage amplitude command and voltage frequency command issued by the dispatching, through the voltage-current double-loop decoupling control, the voltage amplitude on the AC grid side of the second frequency of the converter is kept constant and the voltage frequency is kept constant.

[0011] Furthermore, for the modular multilevel matrix converter, the required value of the voltage of a single arm is:

[0012]

[0013] where U1 and U2 are respectively the rated voltages on the AC grid sides of the first frequency and the second frequency, f1 and f2 are respectively the first frequency and the second frequency, and are respectively the angular velocities of the AC grids of the first frequency and the second frequency, is the initial phase angle of the voltage on the AC grid side of the first frequency, is the initial phase angle of the voltage on the AC grid side of the second frequency, is the difference between the initial phase angle of the voltage on the AC grid side of the first frequency and the initial phase angle of the voltage on the AC grid side of the second frequency, and t is time.

[0014] Even further, at the preferred value of the second frequency and the preferred value of the phase angle difference between the first frequency and the voltage on the AC grid side of the second frequency conditions, the required value of the voltage of a single arm of the modular multilevel matrix converter reaches the minimum, which is:

[0015] ,

[0016] In the formula, the required value of the voltage of a single arm reaches the minimum.

[0017] Even further, for the modular multilevel matrix converter, the minimum number of modules N required for each arm is:

[0018]

[0019] where U CNLet \(U_{sm}\) be the rated voltage of the sub-module of the modular multilevel matrix converter bridge arm, \(M\) be the rated modulation ratio of the converter, and \(k\) be the margin coefficient. \(M\) is equal to the sum of the amplitudes of the AC phase voltages on the first frequency side and the second frequency side of the converter divided by the sum of the capacitor voltages of the bridge arm sub-modules.

[0020] Furthermore, considering the voltage drop of the bridge arm reactor, \(k\) takes a value of 1.0 - 1.2.

[0021] Furthermore, the preferred value of the second frequency and the preferred value of the phase angle difference between the first frequency and the second frequency of the AC grid side voltage are obtained as follows:

[0022] Step 1: According to the design of the flexible low-frequency power transmission system, determine the second frequency range \([f_{2min}, f_{2max}]\) that meets the performance requirements of the flexible low-frequency power transmission system; 2min ,f 2max ;

[0023] Step 2: Under the condition that the initial value of the second frequency \(f_{20}\) is 0, \(t\) starts from 0 and gradually increases at a certain step size. In a cycle range, traverse the formula (2) to obtain the maximum value \(U_{max}\); 2min 、 is 0, \(t\) starts from 0 and gradually increases at a certain step size. In a cycle range, traverse the formula (2) to obtain the maximum value \(U_{max}\); brg maximum value \(U_{max}\) brg_max_0 ;

[0024] Step 3: Under the condition of the initial value of the second frequency \(f_{20}\), 2min condition, starts from 0 and gradually increases at a certain step size. In ∈[0, π] range, repeat Step 2 to obtain different conditions of \(U_{max}\), compare and obtain the minimum value among all \(U_{max}\), which is the minimum value of \(U_{max}\) under the condition of the second frequency \(f_{2}\), denoted as \(U_{min1}\); brg maximum value \(U_{max}\) brg_max_i Compare and obtain the minimum value among all \(U_{max}\), which is the minimum value of \(U_{max}\) under the condition of the second frequency \(f_{2}\), denoted as \(U_{min1}\); brg_max_i in the second frequency \(f_{2}\) 2min condition of \(U_{max}\) brg minimum value, denoted as \(U_{min1}\); brg_min_i ;

[0025] Step 4: Repeat Step 3, the second frequency \(f_{2}\) starts from \(f_{20}\) and gradually increases at a certain step size. In the range of \(f_{2}∈[f_{2min}, f_{2max}]\), repeat Step 2 and Step 3 to obtain the minimum value \(U_{min2}\) of \(U_{max}\) under different \(f_{2}\) conditions. Compare and obtain the minimum value among all \(U_{min2}\), which is the minimum value of the single-bridge arm voltage requirement of the modular multilevel matrix converter 2min starts from \(f_{20}\) and gradually increases at a certain step size. In the range of \(f_{2}∈[f_{2min}, f_{2max}]\), repeat Step 2 and Step 3 to obtain the minimum value \(U_{min2}\) of \(U_{max}\) under different \(f_{2}\) conditions. Compare and obtain the minimum value among all \(U_{min2}\), which is the minimum value of the single-bridge arm voltage requirement of the modular multilevel matrix converter 2min ,f 2max , denoted as \(U_{min}\), and the corresponding frequency is the preferred value of the second frequency brg minimum value \(U_{min2}\) brg_min_i Compare and obtain the minimum value among all \(U_{min2}\), which is the minimum value of the single-bridge arm voltage requirement of the modular multilevel matrix converter brg_min_i in the second frequency \(f_{2}\) minimum value, denoted as \(U_{min}\), and the corresponding frequency is the preferred value of the second frequency , the corresponding phase angle difference is the optimal value of the initial phase angle difference between the voltages of the first-frequency and second-frequency AC power grids .

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

[0027] The beneficial effects of the present invention are as follows: By optimizing the low-frequency frequency and the phase angle matching of the low-frequency and industrial-frequency voltages, under the condition of achieving the same low-frequency voltage amplitude, the number of series-connected sub-modules in the bridge arm of the modular multilevel matrix converter reaches the minimum, thereby reducing the cost of the converter in the low-frequency power transmission system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic diagram of the topology structure of the flexible low-frequency power transmission system of the present invention;

[0029] Figure 2 is a flowchart of the optimal frequency and phase angle difference of the flexible low-frequency power transmission system of the present invention;

[0030] Figure 3 is a flowchart of the total voltage output by the series-connected sub-modules at different t of the present invention;

[0031] Figure 4 is for the present invention different is a flowchart of the total voltage output by the series-connected sub-modules;

[0032] Figure 5 is a flowchart of the total voltage output by the series-connected sub-modules under different f2 conditions of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] In order to describe the present invention more specifically, the technical solutions of the present invention will be described in detail below in conjunction with the accompanying drawings of the specification and the specific embodiments.

[0034] This embodiment provides a flexible low-frequency power transmission system considering frequency and phase optimization.

[0035] Figure 1It is a schematic diagram of the topological structure of a flexible low-frequency power transmission system. The flexible low-frequency power transmission system includes a first-frequency AC power grid, a second-frequency AC power grid, and an AC frequency converter station. Among them, the first-frequency AC power grid operates at the first frequency, the second-frequency AC power grid operates at the second frequency, the first frequency is higher than the second frequency, and the first-frequency AC power grid and the second-frequency AC power grid are connected through the AC frequency converter station. The AC frequency converter station includes a modular multilevel matrix AC converter, a first-frequency transformer, and a second-frequency transformer. The modular multilevel matrix AC converter is connected to the first-frequency AC power grid through the first-frequency transformer; the modular multilevel matrix AC converter is connected to the second-frequency AC power grid through the second-frequency transformer. The modular multilevel matrix AC converter adopts a control strategy for determining the AC voltage amplitude and frequency on the second-frequency AC power grid side: according to the voltage amplitude command and voltage frequency command issued by the dispatching, through the voltage-current double-loop decoupling control, the voltage amplitude on the second-frequency AC power grid side of the AC converter is kept constant and the voltage frequency is kept constant.

[0036] When the modular multilevel matrix AC converter is operating, it outputs the voltage U on the first-frequency AC power grid side on the first-frequency AC power grid side f1 , and outputs the voltage U on the second-frequency AC power grid side on the second-frequency AC power grid side f2 :

[0037] (1)

[0038] Among them, U1 and U2 are the rated voltages on the first-frequency AC power grid side and the second-frequency AC power grid side respectively, and are the angular velocities of the first-frequency AC power grid and the second-frequency AC power grid respectively, is the initial phase angle of the voltage on the first-frequency AC power grid side, is the initial phase angle of the voltage on the second-frequency AC power grid side, and t is time.

[0039] Therefore, for each bridge arm, the total voltage output by its series sub-modules is:

[0040] (2)

[0041] Among them, 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, is the difference between the initial phase angle of the voltage on the second-frequency AC power grid side and the initial phase angle of the voltage on the second-frequency AC power grid side.

[0042] The present invention provides a preferred method for the second frequency f2 and the difference in the initial phase angles of the voltages on the first-frequency and second-frequency AC power grid sides The preferred method is that, at the preferred value, to achieve the same second-frequency steady-state voltage, the voltage requirement value of a single arm of the modular multilevel matrix converter is minimized, and the minimum number of modules required for each arm is the least. As Figure 2 shown, the specific preferred steps are as follows:

[0043] Step 1: According to the system design, determine the frequency range [f 2min , f 2max of the second frequency that meets the system performance requirements.

[0044] Step 2: Under the conditions that the initial value f 2min of the second frequency, the voltage of the first-frequency AC grid side, and the initial phase angle difference of the second-frequency AC grid side are 0, t gradually increases at a certain step length, and formula (2) is traversed within a cycle range to obtain the maximum value U brg U brg_max_0 . Here, the cycle starts from 0 and is the least common multiple of the first-frequency cycle and the second-frequency cycle.

[0045] Step 3: Under the condition of the initial value f 2min of the second frequency, starting from 0, gradually increases at a certain step length, and step 2 is repeated within ∈ [0, π] to obtain the maximum value U U brg U brg_max_i under different brg_max_i conditions. Compare and obtain the minimum value among all U brg_max_i , which is the minimum value U brg of U brg under the condition of the second frequency f min , denoted as U brg_min_i .

[0046] Step 4: Repeat step 3. The second frequency f2 gradually increases at a certain step length starting from f 2min , and steps 2 and 3 are repeated within f2 ∈ [f 2min , f 2max to obtain the minimum value U brg U brg_min_i of U brg under different f2 conditions. Compare and obtain the minimum value among all U brg_min_i , which is the minimum value U brg_min of the voltage requirement value of a single arm of the modular multilevel matrix converter. The corresponding frequency is , and the corresponding phase angle difference is . At this time,

[0047] (3)

[0048] Correspondingly, the minimum number of modules N required for each arm is:

[0049] (4)

[0050] Among them, U CN is the rated voltage of the sub-module of the modular multilevel matrix converter bridge arm, M is the rated modulation degree, k is the margin coefficient, and can take 1.0 - 1.2. M is equal to the sum of the amplitudes of the AC phase voltages on the first frequency side of the converter divided by the sum of the capacitor voltages of the sub-modules of the bridge arm.

[0051] When the modular multilevel matrix converter is operating, it always operates at the second frequency optimal value and the optimal value of the initial phase angle difference . At this time, the frequency of the voltage on the second frequency AC grid side of the converter adopts a fixed value , the angle value adopts an open-loop control method, the calculation of the phase angle of the voltage on the first frequency AC grid side is realized by a phase-locked loop, the value of the voltage on the first frequency AC grid side of the converter is jointly calculated by the power flow transmitted between the 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 voltage on the second frequency AC grid side of the converter differs from the initial phase angle of the voltage on the first frequency AC grid side by .

[0052] The following is an application example:

[0053] The frequency of the first frequency AC grid is 50 Hz, and the initial phase angle of the voltage on the power frequency side of the converter is 0 degrees. According to the system design, the frequency range of the second frequency that meets the system performance requirements is 10 Hz - 30 Hz. Assume that the rated voltages on both the first frequency AC grid side and the second frequency AC grid side are 64 kV. Then:

[0054] The initial value of the second frequency is 10 Hz, and the initial value of the initial phase angle difference of the voltage on the second frequency AC grid side is 0 degrees. The formula (2) is traversed within a period of 0.1 s, and the step size of the increase of t is selected as 0.0001 s, and U brg The maximum value U brg_max_0 is 107.8 kV, and the total voltage output by the series sub-modules at different t is as Figure 3 shown.

[0055] Second, gradually increases from 0 in a certain step size, and the above steps are repeated within the range of [0, π], The step size of the increase of is selected as 1 degree, and the minimum value U min of U brg is obtained under the condition that the initial value f brg_min_i of the second frequency is 10 Hz. The total voltage output by the series sub-modules at different is asFigure 4 as shown

[0056] Third, the initial value f2 of the second frequency starts from 10 Hz and gradually increases in a certain step. The first two steps are repeated within the range of f2 ∈ [10, 30]. The increasing step of f2 is selected as 0.01 Hz, and the U brg minimum value U brg_min is 58.8 kV. The corresponding frequency is 25 Hz. The preferred value of the phase angle difference is 135 degrees. The total voltage output by the series sub-modules under different f2 conditions is as Figure 5 shown

[0057] It can be seen that under the conditions of the preferred value of the second frequency of 25 Hz and the preferred value of the phase angle difference between the first frequency and the second frequency of 135 degrees, the total voltage requirement for the series sub-modules is 58.8 kV. Compared with the arm voltage of 107.8 kV required when the second frequency is 10 Hz and the phase angle difference between the first frequency and the second frequency is 0 degrees, the number of sub-modules is saved by about 45.5%, and the cost of the AC / DC converter will be significantly reduced.

[0058] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all 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 which 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; 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 at the first frequency AC grid side and the initial phase angle of the voltage at the second frequency AC grid side, and t is the time; 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, is the minimum value of the voltage requirement of a single bridge arm; 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 length. 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 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 .

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 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.

5. The flexible low-frequency power transmission system according to claim 4, 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.

6. The flexible low-frequency power transmission system according to claim 4, characterized in that: Considering the voltage drop of the bridge arm reactor, the k is set to 1.0-1.

2.

7. The flexible low-frequency power transmission system according to claim 1, 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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