Intermediate-frequency fan simulation model suitable for direct-current sending-out system control strategy

Abstract

The invention discloses an intermediate frequency fan simulation model suitable for a direct current sending-out system control strategy, which comprises a main circuit part, a controller part and a controller main circuit interface, and is characterized in that the main circuit part comprises a power grid side circuit, a fan side circuit and a direct current side circuit, and the power grid side circuit comprises a first equivalent voltage source, a high frequency filter and a boosting transformer; the direct current side circuit comprises four branches which are connected in parallel, namely a fan side equivalent injection current source branch, a power grid side equivalent injection current source branch, a direct current energy consumption device branch and an equivalent current source parallel equivalent resistance branch; the fan side circuit comprises a second equivalent voltage source, an LC filter and a wind driven generator; the controller main circuit interface comprises a first interface of the power grid side controller and the power grid side main circuit and a second interface of the fan side controller and the fan side main circuit. Simple circuit elements are adopted to replace a power electronic converter, the model is simplified, implementation is facilitated, and the simulation efficiency is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular to a medium frequency fan simulation model suitable for a control strategy of a direct current transmission system. Background Art

[0002] Among the existing offshore wind power grid-connected projects that have been put into operation, mature technologies include industrial frequency AC transmission and flexible DC transmission. For the industrial frequency AC transmission scheme, the investment in the transmission part mainly comes from AC submarine cables, offshore substations and onshore centralized control centers, which are suitable for small-capacity offshore wind farms to be connected to the grid. At present, it is believed that single-circuit cable transmission is suitable for offshore wind farms with a capacity of usually 400MW or less and a distance from the shore of less than 80km, which is suitable for offshore wind power transmission; when the transmission capacity increases, the number of transmission system loops needs to be increased, and the transmission system cost is approximately linearly related to the transmission capacity. For the flexible DC transmission scheme, the investment in the transmission part mainly comes from DC submarine cables, offshore converter stations and onshore inverter stations, which are suitable for large-capacity offshore wind farms to be connected to the grid. At present, the maximum capacity of the flexible DC scheme can reach 2000MW, and there is no theoretical limit on the transmission distance; with the increase in transmission capacity, the unit capacity cost of converter stations and DC submarine cables has decreased.

[0003] Under existing technical conditions, the flexible direct current transmission scheme is the standard technical route for the connection of one million kilowatt offshore wind power to the grid. According to the experience of domestic projects that have been put into operation, the equipment cost is an important limiting factor affecting the promotion and application of flexible direct current transmission technology in offshore wind grid-connected occasions. Taking a typical offshore wind farm with a capacity of 1000MW as an example, when a ±400kV flexible direct current transmission system based on MMC topology is used for grid connection, the equipment cost of the offshore converter station and the onshore converter station is about 2.5 billion, of which the converter valve cost is about 800 million, accounting for a large proportion of the overall equipment investment. In order to reduce the cost of offshore wind power transmission systems, the research on low-cost converters has received more and more attention from academia and industry in recent years.

[0004] Given that the current grid-type wind turbine technology is not mature, offshore wind farms that use flexible direct current transmission all use grid-type wind turbines. Considering the use of grid-type wind turbines, by increasing the operating frequency of the flexible direct current converter at the sending end to the medium frequency, the submodule capacitance and bridge arm reactance of the flexible direct current converter valve at the sending end are reduced, thereby reducing the volume, weight and cost of the flexible direct current converter valve. For flexible direct current transmission systems that use medium frequency operating frequency, it is necessary to model offshore wind farms (mainly medium frequency grid-type wind turbines) when simulating the dynamic characteristics of the system. Since there are a large number of wind turbines in offshore wind farms, and each wind turbine contains a large number of power electronic switches. In the simulation calculation, the power electronic switch increases the electrical nodes in the main circuit on the one hand, and on the other hand, the power electronic switch may involve the interpolation problem of numerical integration during the transient process, which will affect the efficiency of the simulation calculation.

[0005] In order to meet the requirements of the DC transmission system control strategy research in the project, the simulation efficiency of the entire offshore wind farm DC transmission system can be improved by using equivalent simplification for the medium-frequency grid-type wind turbines. Summary of the invention

[0006] In view of the defects of the above-mentioned prior art, the present invention provides a medium frequency fan simulation model suitable for the control strategy of the DC transmission system, which uses simple circuit elements to replace the power electronic converter, simplifies the model, and improves the simulation efficiency.

[0007] To this end, the present invention adopts the following technical solution: a medium frequency fan simulation model suitable for a DC transmission system control strategy, comprising a main circuit part, a controller part and a controller main circuit interface;

[0008] The main circuit part includes a grid side circuit, a fan side circuit and a DC side circuit; the grid side circuit includes a first equivalent voltage source, a high frequency filter and a step-up transformer connected in series in sequence; the DC side circuit includes four parallel branches, namely, a fan side equivalent injection current source branch, a grid side equivalent injection current source branch, a DC energy consumption device branch and an equivalent current source parallel equivalent resistance branch; the fan side circuit includes a second equivalent voltage source, an LC filter and a wind turbine generator connected in series in sequence;

[0009] The controller part includes a machine-side controller and a grid-side controller, both of which adopt the form of an outer-loop power controller + an inner-loop current controller. The outer-loop power controller outputs a dq-axis current command as the input of the inner-loop current controller. The inner-loop current controller outputs a dq-axis voltage signal, and the voltage modulation wave signal in the abc stationary coordinate system is obtained through coordinate transformation. The power outer loop of the machine-side controller adopts zero d-axis current control or unit current maximum torque control, and the power outer loop of the grid-side controller adopts constant DC voltage + constant reactive power control.

[0010] The controller main circuit interface includes a first interface between the grid-side controller and the grid-side circuit and a second interface between the machine-side controller and the wind turbine-side circuit.

[0011] Furthermore, when the power outer loop of the machine-side controller adopts zero d-axis current control, the wind turbine generator adopts a hidden-pole synchronous generator.

[0012] Furthermore, when the power outer loop of the machine-side controller adopts unit current maximum torque control, the wind turbine generator adopts a salient-pole synchronous generator.

[0013] Furthermore, in the grid-side circuit, the high-frequency filter is a capacitor, the first equivalent voltage source is connected in parallel with one end of the capacitor, and the other end of the capacitor is grounded; the parallel terminal is directly connected to the low-voltage side of the step-up transformer, and the high-voltage side of the step-up transformer is directly connected to the external grid; the first equivalent voltage source is replaced by a three-phase ideal controlled voltage source, and its voltage instantaneous value signal u gk Given by the first interface, k=a, b, c.

[0014] Furthermore, the second equivalent voltage source is first connected in series with one end of the inductor, and the other end of the inductor is connected in parallel with the capacitor, and the parallel terminal is directly connected to the wind turbine generator; the second equivalent voltage source is replaced by a three-phase ideal controlled voltage source, and its voltage instantaneous value signal u mk Given by the second interface, k=a, b, c.

[0015] Furthermore, at time t, the equivalent injection current source i dcm (t) is calculated using formula (1):

[0016] i dcg (t)=[u ga (t)*i ga (t-Δt)+u gb (t)i gb (t-Δt)+u gc (t)i gc (t-Δt)] / u dc (t-Δt)(1)

[0017] Where k = a, b, c, u gk (t) and i gk (t-Δt) represents the instantaneous value of the k-phase AC voltage of the first equivalent voltage source at time t and the instantaneous value of the k-phase AC current injected into the first equivalent voltage source at the last simulation step; u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step.

[0018] Furthermore, at time t, the grid-side equivalent injection current source i dcg (t) is calculated using formula (2):

[0019] i dcm (t)=[u ma (t)i ma (t-Δt)+u mb (t)i mb (t-Δt)+u mc (t)i mc (t-Δt)] / u dc (t-Δt)(2)

[0020] Where k = a, b, c, u mk (t) and i mk (t-Δt) represents the instantaneous value of the k-phase AC voltage of the second equivalent voltage source at time t and the instantaneous value of the k-phase AC current injected into the second equivalent voltage source at the last simulation step; u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step.

[0021] Furthermore, in the equivalent current source connected in parallel with the equivalent resistance branch, at time t, the equivalent current source i eq (t) and equivalent resistance R eq (t) is calculated according to formula (3) and formula (4) respectively:

[0022]

[0023] Among them, C dc represents the DC capacitance, i inj (t-Δt) represents the current injected into the equivalent current source parallel equivalent resistance branch at the last simulation step, u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step; Δt represents the time difference between two adjacent simulation steps.

[0024] Furthermore, the input of the first interface is the voltage modulation wave signal in the abc stationary coordinate system given by the grid-side controller, and then the instantaneous value u of the k-phase AC voltage of the first equivalent voltage source is obtained through two proportional links. gk , k = a, b, c, the proportional coefficients of the two proportional links are the rated phase voltage amplitude u of the grid-side converter gphbase and the DC voltage u across the DC side circuit dc .

[0025] Furthermore, the input of the second interface is the voltage modulation wave signal in the abc stationary coordinate system given by the machine-side controller, and then the instantaneous value u of the k-phase AC voltage of the second equivalent voltage source is obtained through two proportional links. mk , k = a, b, c, the proportional coefficients of the two proportional links are the rated phase voltage amplitude u of the machine-side converter mphbase and the DC voltage u across the DC side circuit dc .

[0026] The present invention has the following beneficial technical effects:

[0027] 1. The present invention adopts simple circuit elements such as controlled voltage source and current source to replace the power electronic converter, which simplifies the simulation model, is simple and convenient to implement, and improves the simulation efficiency.

[0028] 2. The present invention has strong versatility and is theoretically applicable not only to medium-frequency mesh fans, but also to mesh fans at other frequencies. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of the medium frequency fan simulation model of the present invention;

[0030] Figure 2 It is a structural schematic diagram of the grid side circuit of the present invention;

[0031] Figure 3 It is a structural schematic diagram of the fan side circuit of the present invention;

[0032] Figure 4 It is a structural schematic diagram of the DC side circuit of the present invention;

[0033] Figure 5 A schematic diagram of a first interface between a grid-side controller and a grid-side main circuit of the present invention;

[0034] Figure 6 A schematic diagram of a second interface between the grid-side controller and the grid-side main circuit of the present invention;

[0035] FIG. 7( a ) is a diagram showing the simulation results of active and reactive power on the high-voltage side of the step-up transformer of the present invention;

[0036] FIG7( b ) is a simulation result diagram of the instantaneous value of the AC voltage on the high-voltage side of the step-up transformer;

[0037] FIG7( c ) is a diagram showing the DC voltage simulation results of the DC side circuit. DETAILED DESCRIPTION

[0038] 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.

[0039] Assume that the main circuit parameters of the medium frequency fan are as shown in Table 1.

[0040] Table 1

[0041]

[0042] The structure of the medium frequency fan simulation model applicable to the DC transmission system control strategy of the present invention is as follows: Figure 1 As shown, it includes a main circuit part, a controller part, and a controller main circuit interface; the main circuit part includes a grid side circuit, a DC side circuit, and a fan side circuit; the controller main circuit interface includes a first interface between a grid side controller and a grid side main circuit, and a second interface between a fan side controller and a fan side main circuit, wherein:

[0043] The grid side circuit includes a first equivalent voltage source, a high frequency filter and a step-up transformer, such as Figure 2 The high-frequency filter is a capacitor, the first equivalent voltage source is connected in parallel with one end of the capacitor, and the other end of the capacitor is grounded; the parallel terminal is directly connected to the low-voltage side of the step-up transformer, and the high-voltage side of the step-up transformer is directly connected to the external power grid; the first equivalent voltage source is represented by a three-phase ideal controlled voltage source, and its voltage instantaneous value signal u gk (k=a, b, c) is given by the first interface.

[0044] The wind turbine side circuit includes a second equivalent voltage source, an LC filter and a wind turbine generator, such as Figure 3 The second equivalent voltage source is first connected in series with one end of the inductor, and the other end of the inductor is connected in parallel with the capacitor, and the parallel terminal is directly connected to the wind turbine generator; the second equivalent voltage source is represented by a three-phase ideal controlled voltage source, and its voltage instantaneous value signal u mk (k=a, b, c) is given by the second interface.

[0045] The controller part includes a generator-side controller and a grid-side controller, both of which adopt the form of an outer loop power controller + an inner loop current controller (see Wu B, Lang Y, Zargari N, et al. Power Conversion and Control of Wind Energy Systems [J]. IEEE Press, Wiley, 2011. for details).

[0046] The DC side circuit includes an equivalent injection current source, a DC energy consumption device, and an equivalent current source in parallel with an equivalent resistor, such as Figure 4 The DC side circuit is composed of four branches connected in parallel, including an equivalent injection current source on the wind turbine side, an equivalent injection current source on the grid side, a DC energy consumption device, and an equivalent current source parallel equivalent resistance branch.

[0047] Figure 4 At time t, the equivalent injection current source i on the fan side is dcm (t) and the grid-side equivalent injection current source i dcg (t) is calculated using formula (1) and formula (2):

[0048] i dcg (t)=[u ga (t)*i ga (t-Δt)+u gb (t)i gb (t-Δt)+u gc (t)i gc (t-Δt)] / u dc (t-Δt)(1)

[0049] i dcm (t)=[u ma (t)ima (t-Δt)+u mb (t)i mb (t-Δt)+u mc (t)i mc (t-Δt)] / u dc (t-Δt)(2)

[0050] Among them, u gk (t) and i gk (t-Δt)(k=a, b, c) respectively represent the instantaneous value of the k-phase AC voltage of the first equivalent voltage source at time t and the instantaneous value of the k-phase AC current injected into the first equivalent voltage source at the last simulation step; u mk (t) and i mk (t-Δt)(k=a, b, c) respectively represent the instantaneous value of the k-phase AC voltage of the second equivalent voltage source at time t and the instantaneous value of the k-phase AC current injected into the second equivalent voltage source at the last simulation step; u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step.

[0051] Figure 4 In the equivalent current source parallel equivalent resistance branch, the equivalent current source i at time t eq (t) and equivalent resistance R eq (t) is calculated according to formula (3) and formula (4) respectively:

[0052] i eq (t) = i inj (t-Δt)+4900u dc (t-Δt) (3)

[0053]

[0054] Among them, i inj (t-Δt) represents the current injected into the equivalent current source parallel equivalent resistance branch at the last simulation step.

[0055] The input of the first interface is the voltage modulation wave signal in the abc stationary coordinate system given by the grid-side controller, denoted as u gkref (k = a, b, c), and then through two proportional links, the instantaneous value u of the k-phase AC voltage of the first equivalent voltage source is obtained gk ,like Figure 5 As shown in the figure, the proportional coefficients of the two proportional links are the rated phase voltage amplitude u of the grid-side converter and gphbase and the DC voltage u across the DC side circuit dc .

[0056] The input of the second interface is the voltage modulation wave signal in the abc stationary coordinate system given by the machine-side controller, denoted as u mkref (k = a, b, c), and then through two proportional links, the instantaneous value u of the k-phase AC voltage of the second equivalent voltage source is obtained mk ,like Figure 6 As shown in the figure, the proportional coefficients of the two proportional links are respectively the rated phase voltage amplitude u of the machine-side converter mphbase and the DC voltage u across the DC side circuit dc .

[0057] FIG7 shows the simulation waveform of a three-phase metallic fault occurring on the high voltage side of the medium frequency fan step-up transformer, which proves the effectiveness of the present invention.

[0058] Specific implementation methods are given above, but the present invention is not limited to the described implementation methods. The basic idea of ​​the present invention lies in the above basic scheme. For ordinary technicians in this field, it does not take creative work to design various deformed models, formulas, and parameters according to the teachings of the present invention. Changes, modifications, substitutions, and deformations of the implementation methods without departing from the principles and spirit of the present invention still fall within the scope of protection of the present invention.

Claims

1. A medium frequency fan simulation model suitable for DC transmission system control strategy, including a main circuit part, a controller part and a controller main circuit interface; The main circuit part includes a grid side circuit, a fan side circuit and a DC side circuit; the grid side circuit includes a first equivalent voltage source, a high frequency filter and a step-up transformer connected in series in sequence; the DC side circuit includes four parallel branches, namely, a fan side equivalent injection current source branch, a grid side equivalent injection current source branch, a DC energy consumption device branch and an equivalent current source parallel equivalent resistance branch; the fan side circuit includes a second equivalent voltage source, an LC filter and a wind turbine generator connected in series in sequence; The controller part includes a machine-side controller and a grid-side controller, both of which adopt the form of an outer-loop power controller + an inner-loop current controller. The outer-loop power controller outputs a dq-axis current command as the input of the inner-loop current controller. The inner-loop current controller outputs a dq-axis voltage signal, and the voltage modulation wave signal in the abc stationary coordinate system is obtained through coordinate transformation. The power outer loop of the machine-side controller adopts zero d-axis current control or unit current maximum torque control, and the power outer loop of the grid-side controller adopts constant DC voltage + constant reactive power control. The controller main circuit interface includes a first interface between the grid-side controller and the grid-side circuit and a second interface between the machine-side controller and the wind turbine-side circuit.

2. The medium frequency fan simulation model according to claim 1 is characterized in that: When the power outer loop of the machine-side controller adopts zero d-axis current control, the wind turbine generator adopts a hidden-pole synchronous generator.

3. The medium frequency fan simulation model according to claim 1 is characterized in that: When the power outer loop of the machine-side controller adopts unit current maximum torque control, the wind turbine generator adopts a salient-pole synchronous generator.

4. The medium frequency fan simulation model according to claim 1 is characterized in that: In the grid-side circuit, the high-frequency filter is a capacitor, the first equivalent voltage source is connected in parallel with one end of the capacitor, and the other end of the capacitor is grounded; the parallel terminal is directly connected to the low-voltage side of the step-up transformer, and the high-voltage side of the step-up transformer is directly connected to the external grid; the first equivalent voltage source is replaced by a three-phase ideal controlled voltage source, and its voltage instantaneous value signal u gk Given by the first interface, k=a, b, c.

5. The medium frequency fan simulation model according to claim 1 is characterized in that: The second equivalent voltage source is first connected in series with one end of the inductor, and the other end of the inductor is connected in parallel with the capacitor. The parallel terminal is directly connected to the wind turbine generator. The second equivalent voltage source is replaced by a three-phase ideal controlled voltage source, and its voltage instantaneous value signal u mk Given by the second interface, k=a, b, c.

6. The medium frequency fan simulation model according to claim 1, characterized in that: At time t, the equivalent injection current source i on the fan side dcm (t) is calculated using formula (1): i dcg (t)=[u ga (t)*i ga (t-Δt)+u gb (t)i gb (t-Δt)+u gc (t)i gc (t-Δt)] / u dc (t-Δt) (1) Where k = a, b, c, u gk (t) and i gk (t-Δt) represents the instantaneous value of the k-phase AC voltage of the first equivalent voltage source at time t and the instantaneous value of the k-phase AC current injected into the first equivalent voltage source at the last simulation step; u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step.

7. The medium frequency fan simulation model according to claim 1, characterized in that: At time t, the grid-side equivalent injection current source i dcg (t) is calculated using formula (2): i dcm (t)=[u ma (t)i ma (t-Δt)+u mb (t)i mb (t-Δt)+u mc (t)i mc (t-Δt)] / u dc (t-Δt)(2) Where k = a, b, c, u mk (t) and i mk (t-Δt) represents the instantaneous value of the k-phase AC voltage of the second equivalent voltage source at time t and the instantaneous value of the k-phase AC current injected into the second equivalent voltage source at the last simulation step; u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step.

8. The medium frequency fan simulation model according to claim 1, characterized in that: In the equivalent current source parallel equivalent resistance branch, the equivalent current source i at time t eq (t) and equivalent resistance R eq (t) is calculated according to formula (3) and formula (4) respectively: Among them, C dc represents the DC capacitance, i inj (t-Δt) represents the current injected into the equivalent current source parallel equivalent resistance branch at the last simulation step, u dc (t-Δt) represents the DC voltage across the DC circuit at the last simulation step; Δt represents the time difference between two adjacent simulation steps.

9. The medium frequency fan simulation model according to claim 1, characterized in that: The input of the first interface is the voltage modulation wave signal in the abc stationary coordinate system given by the grid-side controller, and then the instantaneous value u of the k-phase AC voltage of the first equivalent voltage source is obtained through two proportional links. gk , k = a, b, c, the proportional coefficients of the two proportional links are the rated phase voltage amplitude u of the grid-side converter gphbase and the DC voltage u across the DC side circuit dc .

10. The medium frequency fan simulation model according to claim 1, characterized in that: The input of the second interface is the voltage modulation wave signal in the abc stationary coordinate system given by the machine side controller, and then the instantaneous value u of the k-phase AC voltage of the second equivalent voltage source is obtained through two proportional links. mk , k = a, b, c, the proportional coefficients of the two proportional links are the rated phase voltage amplitude u of the machine-side converter mphbase and the DC voltage u across the DC side circuit dc .

Images