阅读说明：本技术 基于中-低-工频汇集的新型深远海风电输电系统 (Novel deep and distant sea wind power transmission system based on medium-low-power frequency collection ) 是由 刘闯 朱炳达 蔡国伟 葛维春 郭东波 裴忠晨 邵鑫铭 孙赫阳 杜天鹏 牟晓春 王 于 2021-07-19 设计创作，主要内容包括：本发明公开了本发明基于中-低-工频汇集的新型深远海风电输电系统,包含多个风电机组,多个汇流升压变压器,中频汇流母线,海上升压变频站,海底电缆,陆上变频并网站,陆上工频电网。本发明降低了输电线路的电抗,提升了线路输送容量,并且不含有建设和维护成本高昂的直流设备。此外,在海上采用体积和成本更小的中频变压器替代低频或工频变压器,具有显著的优势。本发明实现了各个环节的优化设计,有针对性的解决了传统方案的主要缺点,具有较高的实际应用价值和较好的未来发展前景。(The invention discloses a novel deep and distant sea wind power transmission system based on medium-low-power frequency convergence, which comprises a plurality of wind power generation sets, a plurality of convergence step-up transformers, an intermediate frequency convergence bus, a sea step-up frequency conversion station, a sea cable, a land frequency conversion grid and a land power frequency grid. The invention reduces the reactance of the transmission line, improves the transmission capacity of the line, and does not contain direct current equipment with high construction and maintenance cost. In addition, the medium-frequency transformer with smaller volume and cost is adopted to replace a low-frequency or power-frequency transformer at sea, so that the method has remarkable advantages. The invention realizes the optimization design of each link, pertinently solves the main defects of the traditional scheme, and has higher practical application value and better future development prospect.)

1. A novel deep and offshore wind power transmission system based on medium-low-power frequency convergence is characterized by comprising a plurality of wind power generation sets which emit a plurality of medium-frequency alternating currents of 100-500 Hz, wherein each energy transmission port of each wind power generation set is connected with the primary side of a convergence boosting transformer, the secondary sides of the convergence boosting transformers are connected in parallel and connected with a medium-frequency convergence bus, the medium-frequency convergence bus is connected with an offshore boosting frequency conversion station which converts medium-frequency electric energy into low-frequency alternating currents of 10-20 Hz, the offshore boosting frequency conversion station is connected with a submarine cable, the submarine cable is connected with a land frequency conversion station which converts the low-frequency electric energy into 50/60Hz power frequency electric energy, and the land frequency conversion station is connected with a land power frequency power grid.

2. The system according to claim 1, wherein the offshore boost converter station uses a direct ac-ac converter a, and the onshore variable frequency grid-connected station uses a direct ac-ac converter b.

3. The system according to claim 1, wherein the direct-to-AC converter a and the direct-to-AC converter B each comprise a three-phase core multi-winding transformer having three core legs, each core leg of the three-phase core multi-winding transformer has a set of windings connected thereto, each set of windings comprises a primary winding and a plurality of secondary windings, each primary winding is connected to one of a phase a, a phase B and a phase C of an input voltage, and six cascaded AC/AC converters are included, the six cascaded AC/AC converters are grouped into two by two, the input end of each set of cascaded AC/AC converters is connected to the plurality of secondary windings of a set of windings, and the three sets of cascaded AC/AC converters are set as the a set of cascaded AC/AC converters according to the voltages connected to each set of the windings, The cascade AC/AC converter comprises a group B of cascade AC/AC converters and a group C of cascade AC/AC converters, wherein the anode of a first cascade AC/AC converter in the group A is connected with the cathode of a first cascade AC/AC converter in the group B through an LC filter a, the cathode of the first cascade AC/AC converter in the group A is connected with the anode of a first cascade AC/AC converter in the group B, the anode of a second cascade AC/AC converter in the group B is connected with the cathode of a first cascade AC/AC converter in the group C through an LC filter B, the cathode of the second cascade AC/AC converter in the group B is connected with the anode of a first cascade AC/AC converter in the group C, the anode of the second cascade AC/AC converter in the group C is connected with the cathode of a second cascade AC/AC converter in the group A through an LC filter C, and the cathode of the second cascade AC/AC converter in the group C is connected with the anode of the second cascade AC/AC converter in the group A The cathode of the second cascade AC/AC converter in the group A is respectively connected with the cathode of the first cascade AC/AC converter in the group B and the cathode of the first cascade AC/AC converter in the group C, the LC filter a is connected with the anode of the phase A output port, the LC filter B is connected with the anode of the phase B output port, the LC filter C is connected with the anode of the phase C output port, the primary winding of the direct AC-AC converter a is connected with a medium-frequency bus bar, the anode of the phase A output port, the anode of the phase B output port and the anode of the phase C output port of the direct AC-AC converter a are all connected with a submarine cable, the primary winding of the direct type AC-AC converter B is connected with a land power frequency power grid, and the anode of the phase A output port, the anode of the phase B output port and the anode of the phase C output port of the direct type AC-AC converter B are connected with the output end of the submarine cable.

4. The system according to claim 3, wherein the LC filter a, the LC filter b and the LC filter c each comprise an inductor with one end connected to a positive electrode of the cascaded AC/AC converter, the other end of the inductor is connected to a positive electrode plate of the capacitor and a positive electrode of the output port, and the negative electrode plate of the capacitor is connected to a negative electrode of the cascaded AC/AC converter.

5. The novel deep and offshore wind power transmission system based on medium-low-power frequency convergence according to claim 3, wherein each cascaded AC/AC converter comprises one or more bipolar direct AC/AC converters, the bipolar direct AC/AC converters are sequentially connected in a manner that an output negative electrode is connected with an output positive electrode of an adjacent bipolar direct AC/AC converter, a positive electrode of a first-end bipolar direct AC/AC converter is a positive electrode of the cascaded AC/AC converter, a negative electrode of a tail-end bipolar direct AC/AC converter is a negative electrode of the cascaded AC/AC converter, and an input end of each bipolar direct AC/AC converter is connected with a secondary winding.

6. The new type deep sea wind power transmission system based on mid-low-power frequency convergence according to claim 4, wherein the first cascaded AC/AC converter of the A-group cascaded AC/AC converters, the second cascaded AC/AC converter of the B-group cascaded AC/AC converters, and the second cascaded AC/AC converter of the C-group cascaded AC/AC converters all adopt modulation waves as shown in formula (1):

D₁＝V_D1cos[(ω₂-ω₁)t+30°+α] (1)

the second cascaded AC/AC converter of the A-group cascaded AC/AC converter, the first cascaded AC/AC converter of the B-group cascaded AC/AC converter and the first cascaded AC/AC converter of the C-group cascaded AC/AC converter all adopt modulation waves shown in a formula (2):

D₂＝V_D2cos[(ω₂-ω₁)t-90°+α] (2)

wherein ω is₁Is the angular frequency, omega, of the input voltage₂To the angular frequency of the output voltage, V_D1And V_D2The amplitudes of the two modulated waves; v_D1Value range of 0 to 1, V_D2Value of V_D1The carrier wave adopts a triangular wave with the amplitude of-1 to 1, and alpha is a phase deviation angle and also is an initial phase angle of the output voltage.

Technical Field

The invention belongs to the technical field of wind power transmission, and particularly relates to a novel deep and distant sea wind power transmission system based on medium-low-power frequency convergence.

Background

In recent years, the problems of energy depletion and environmental pollution are increasingly severe, and new energy power generation has the characteristics of no pollution, sustainability and the like, so that the application prospect is extremely wide. Wind power generation is one of the most mature and developed scale conditions in new energy power generation technology. With the development and application of wind power generation technology, the scale of wind power plants is becoming larger. Due to the limitation of land resources, wind energy resources and the like, the large wind power plant is located in a remote area or on the sea far away from the power node. At present, the onshore wind power development of some countries tends to be saturated, and the offshore wind power is not utilized yet, so that the offshore wind power has huge development potential. For example, germany has clearly focused the development of future renewable energy on offshore wind power and made an ambitious offshore wind power plan. Furthermore, the largest offshore wind power plant in the world is the "london array project" in the uk, with a total installed capacity of 630 MW.

China has abundant offshore wind resources, and the development of offshore wind resources has important practical significance. Therefore, large-capacity and long-distance offshore wind power is a trend of future development of wind power in China, and how to realize long-distance transmission of large-capacity offshore wind power is a very realistic and urgent subject. At present, the offshore wind power transmission modes mainly include 3 types: power frequency high voltage alternating current transmission (HVAC) technology, high voltage direct current transmission (HVDC) technology, and Fractional Frequency Transmission Systems (FFTS).

HVAC adopts traditional power frequency transmission mode, need not to convert the electric energy that the fan sent into low frequency or direct current electric energy, and this kind of transmission mode simple structure, the cost is lower, and has many years's abundant operation and practical experience, has great advantage in the aspect of closely small capacity wind-powered electricity generation transport and being incorporated into the power networks. Because wind power is transmitted through the submarine cable, compared with a common overhead power transmission line, the submarine cable has the advantages that reactance is reduced, capacitance is increased, capacitive charging current in the cable is increased rapidly along with the increase of power transmission distance and power transmission capacity, line loss is increased, and the effective utilization rate of line capacity is greatly reduced. HVAC applications to long-distance and high-capacity wind power delivery and grid integration are difficult.

For medium and long distance wind farms, High Voltage Direct Current (HVDC) solutions are currently commonly used. The direct current transmission technology can avoid the influence of cable capacitance and increase the electric energy transmission capacity and distance. However, offshore dc transmission systems, particularly the required offshore convergence platforms and converter stations, are expensive to build and maintain. Two power conversions from ac to dc and then to ac must be carried out during the power transmission, so both onshore and offshore converter stations must be constructed. The construction of the offshore converter station is far higher than that of the onshore converter station in technical difficulty and investment cost, the operation and maintenance cost is high, and the economy of the HVDC scheme is reduced to a great extent. Although the line loss of direct current transmission is low, the total loss exceeds the traditional HVAC mode within a medium-short distance after the loss caused by multi-stage commutation is added. In addition, the technical problems of a direct current breaker and the like are not effectively solved, and the wind power direct current grid connection in a short period can only be carried out between an offshore converter station and a land converter station in a point-to-point mode, so that a series of problems of low reliability, high failure rate and the like are brought.

For the transportation of deep and offshore wind power, the FFTS has unique advantages. The FFTS scheme reduces the transmission frequency and reduces the impedance of the whole transmission system, so that the transmission capacity of a line can be improved in multiples, the voltage fluctuation of the line is stabilized, and the wind power absorption capacity of a power grid is improved. Capacitive reactive power of the FFTS wind power system is reduced compared with 50Hz alternating current transmission, and the service life of the submarine cable can be prolonged. Compared with the HVDC scheme, the FFTS does not need to establish an offshore converter station, so that the construction investment, the operation maintenance cost and the cost are greatly reduced, the downtime can be reduced, and the wind power supply time is increased. However, the size and cost of the low frequency side transformer of FFTS increases, and its weight is at least twice that of the industrial frequency transformer, and it has been found that the cost of the transformer increases by about 70%. In addition, the frequency converter is used as a key component in the FFTS system, and has the application problems and disadvantages in both a frequency converter based on a thyristor and a frequency converter based on a full-control device.

Disclosure of Invention

The invention aims to provide a novel deep and distant sea wind power transmission system based on medium-low-power frequency collection, wherein medium-frequency collection is adopted at sea, so that medium-frequency transformers are adopted, and the size and the installation difficulty of the transformers are reduced; through low frequency transmission, the reactance of a transmission line is reduced, and the transmission capacity of the line is improved.

The technical scheme includes that the novel deep and remote sea wind power transmission system based on medium-low-power frequency convergence comprises a plurality of wind power units which emit 100-500 Hz medium-frequency alternating current, an energy transmission port of each wind power unit is connected with a primary side of a convergence boosting transformer, secondary sides of the convergence boosting transformers are connected in parallel and connected with a medium-frequency convergence bus, the medium-frequency convergence bus is connected with an offshore boosting frequency conversion station which converts medium-frequency electric energy into 10-20 Hz low-frequency alternating current, the offshore boosting frequency conversion station is connected with a submarine cable, the submarine cable is connected with a land frequency conversion station which converts the low-frequency electric energy into 50/60Hz power-frequency electric energy, and the land frequency conversion station is connected with a land power-frequency power grid.

The invention is also characterized in that:

the offshore boosting frequency conversion station adopts a direct type AC-AC converter a, and the onshore frequency conversion grid-connected station adopts a direct type AC-AC converter b.

The direct type AC/AC converter a and the direct type AC/AC converter B respectively comprise three-phase core type multi-winding transformers with three iron core columns, each iron core column of each three-phase core type multi-winding transformer is connected with a group of windings, each group of windings comprises a primary winding and a plurality of secondary windings, each primary winding is respectively connected with one of the phases A, B and C of input voltage, the direct type AC/AC converter a and the direct type AC/AC converter B further comprise six cascaded AC/AC converters, the six cascaded AC/AC converters are divided into a group in a pairwise manner, the input end of each group of cascaded AC/AC converters is connected with the plurality of secondary windings of the group of windings, the three groups of cascaded AC/AC converters are correspondingly set as an A group cascaded AC/AC converter, a B group cascaded AC/AC converter and a C group cascaded AC/AC converter according to the voltage accessed by each group of windings, the positive pole of the first cascaded AC/AC converter in the A group is connected with the first cascaded AC/AC converter in the B group through an LC filter a The negative pole of the first cascade AC/AC converter in the group A is connected with the positive pole of the first cascade AC/AC converter in the group B, the positive pole of the second cascade AC/AC converter in the group B is connected with the negative pole of the first cascade AC/AC converter in the group C through an LC filter B, the negative pole of the second cascade AC/AC converter in the group B is connected with the positive pole of the first cascade AC/AC converter in the group C, the positive pole of the second cascade AC/AC converter in the group C is connected with the negative pole of the second cascade AC/AC converter in the group A through an LC filter C, the negative pole of the second cascade AC/AC converter in the group C is connected with the positive pole of the second cascade AC/AC converter in the group A, and the negative pole of the second cascade AC/AC converter in the group A is respectively connected with the negative pole of the first cascade AC/AC converter in the group B, The first cascade AC/AC converter cathode in the group C, the LC filter a is connected with the anode of the phase A output port, the LC filter B is connected with the anode of the phase B output port, the LC filter C is connected with the anode of the phase C output port, the primary winding of the direct type AC-AC converter a is connected with the intermediate frequency bus bar, the anode of the phase A output port, the anode of the phase B output port and the anode of the phase C output port of the direct type AC-AC converter a are all connected with the submarine cable, the primary winding of the direct type AC-AC converter B is connected with the onshore power frequency power grid, and the anode of the phase A output port, the anode of the phase B output port and the anode of the phase C output port of the direct type AC-AC converter B are all connected with the output end of the submarine cable.

The LC filter a, the LC filter b and the LC filter c respectively comprise an inductor with one end connected with the anode of the cascade AC/AC converter, the other end of the inductor is connected with the positive electrode of the capacitor and the anode of the output port, and the negative electrode of the capacitor is connected with the cathode of the other cascade AC/AC converter.

Each cascade AC/AC converter comprises one or more bipolar direct AC/AC converters, the bipolar direct AC/AC converters are sequentially connected in a mode that an output negative electrode is connected with an output positive electrode of an adjacent bipolar direct AC/AC converter, the positive electrode of the first-end bipolar direct AC/AC converter is the positive electrode of the cascade AC/AC converter, the negative electrode of the tail-end bipolar direct AC/AC converter is the negative electrode of the cascade AC/AC converter, and the input end of each bipolar direct AC/AC converter is connected with a secondary winding.

A first cascade AC/AC converter of the A group of cascade AC/AC converters, a second cascade AC/AC converter of the B group of cascade AC/AC converters and a second cascade AC/AC converter of the C group of cascade AC/AC converters all adopt modulation waves shown in a formula (1):

D₁＝V_D1cos[(ω₂-ω₁)t+30°+α] (1)

the second cascaded AC/AC converter of the A group of cascaded AC/AC converters, the first cascaded AC/AC converter of the B group of cascaded AC/AC converters and the first cascaded AC/AC converter of the C group of cascaded AC/AC converters all adopt modulation waves shown in the formula (2):

D₂＝V_D2cos[(ω₂-ω₁)t-90°+α] (2)

wherein ω is₁Is the angular frequency, omega, of the input voltage₂To the angular frequency of the output voltage, V_D1And V_D2The amplitudes of the two modulated waves; v_D1Value range of 0 to 1, V_D2Value of V_D1The carrier wave adopts a triangular wave with the amplitude of-1 to 1, alpha is a phase offset angle and is also outputInitial phase angle of voltage.

The invention has the beneficial effects that:

(1) the invention adopts lower electric energy transmission frequency, reduces the reactance of a transmission line and improves the transmission capacity of the line.

(2) The invention does not need high direct current equipment construction and maintenance cost, does not adopt immature direct current breakers, and is beneficial to the continuous transmission of wind power plant energy and the fault ride-through capability.

(3) The invention adopts the intermediate frequency transformer on the sea, and the volume and the weight of the low frequency transformer are multiplied compared with the intermediate frequency transformer of the invention, and simultaneously, the invention is not beneficial to the actual installation and operation of the wind power plant in deep and far sea.

(4) The novel direct AC-AC converter and the modulation method adopted by the invention can continuously carry out frequency conversion, voltage transformation and phase shifting on the three-phase input voltage of the converter, have higher controllability and more flexible operation of the device, and can meet various application requirements. And meanwhile, the energy storage capacitor is not contained, the cost and the volume are lower, the problems of voltage sharing and starting of the capacitor do not exist, and the complexity of a control system is reduced. In addition, the input sides of the bipolar direct AC/AC converters in the converters are coupled through a transformer magnetic circuit, so that current harmonics with the same frequency as the output voltage can be eliminated, and the current harmonics are prevented from being introduced into the primary side of the transformer.

(5) According to the invention, through medium-frequency collection, low-frequency transmission and power frequency grid connection, the optimal design of each link is realized, the main defects of the traditional scheme are pertinently solved, and the method has high practical application value and good future development prospect.

Drawings

FIG. 1 is a diagram of a medium-low-power frequency-based novel deep ocean wind power transmission system;

FIG. 2 is a diagram of the topology of the direct AC/AC converter according to the present invention;

FIG. 3 is a diagram of a cascaded AC/AC converter topology;

FIG. 4 is a graph of three phase voltage waveforms at nodes B and C of FIG. 1;

fig. 5 is a waveform diagram of three-phase voltages at nodes D and E of fig. 1.

Detailed Description

The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

The invention relates to a novel deep and distant sea wind power transmission system based on medium-low-power frequency convergence, which is shown in figure 1, wherein a windmill pattern represents a deep and distant sea wind power plant and mainly comprises a plurality of wind turbine generators and corollary equipment thereof; the novel direct AC-AC converter is a direct AC-AC converter based on a three-phase core type multi-winding transformer; the left side of the dotted curve is the sea and the right side is the land; and the right side of the node G is a land power frequency alternating current power grid. The overall process of the present invention will be described in detail with reference to fig. 1.

The invention relates to a novel deep and distant sea wind power transmission system based on medium-low-power frequency convergence, wherein a plurality of wind generation sets of a deep and distant sea wind power plant generate a plurality of medium-frequency alternating currents of 100-500 Hz, and the medium-frequency alternating currents are electric energy of a node A. And then all the intermediate-frequency alternating currents are converged at an intermediate-frequency convergence bus through a plurality of convergence step-up transformers, the node B is intermediate-frequency alternating current, the voltage grade of the converged intermediate-frequency alternating current is improved through a direct alternating-current and alternating-current converter a in the offshore step-up frequency conversion station, electric energy frequency conversion is carried out at the same time, low-frequency alternating current of 10-20 Hz is output, and the node C is low-frequency alternating current. The low-frequency alternating current is transmitted to the land direction through the submarine cable, and the node D is the low-frequency alternating current. After the submarine cable sends the electric energy to land, the submarine cable is connected to a direct AC-AC converter b in a land frequency conversion and website to carry out electric energy frequency conversion, power frequency alternating current of 50/60Hz is output, finally the power frequency alternating current is merged into a land power frequency AC power grid through the direct AC-AC converter b, and a node E is the power frequency alternating current.

For ac long-distance transmission, the transmission capacity is mainly expressed in the transmission power limit, and the transmission power limit of the ac transmission system can be estimated by the following formula:

P_max＝V²/X (Ⅰ)

in the formula (I), P_maxFor the transmission power limit, V is the transmission line rated voltage and X is the reactance of the transmission line.

X＝2πfL (Ⅱ)

In the formula (II), f is the frequency of the transmitted alternating current, and L is the equivalent inductance. The formula (II) shows that the line reactance is in direct proportion to the frequency of the transmitted alternating current, and the formula (I) shows that the transmission power limit is in inverse proportion to the frequency of the transmitted alternating current.

Induced electromotive force E of transformer_BCan be represented by formula (III):

in the formula (III), f is the operating frequency of the transformer, N is the number of turns of the coil,at maximum magnetic flux, B_mS is the effective cross-sectional area of the core for maximum magnetic flux density. When the transformer is designed, the rated electromotive force E is kept unchanged, when f is low frequency or intermediate frequency, the numerical value of f is different by dozens of times, and the effective sectional area S of the corresponding iron core is also different by dozens of times. Therefore, compared with a low-frequency transformer, the cross section and the length of the iron core of the medium-frequency transformer are greatly reduced, the size and the cost are also greatly reduced, and the medium-frequency transformer has obvious advantages in an offshore wind farm.

As shown in fig. 2, each of the direct-type AC-AC converters a and the direct-type AC-AC converters B includes a three-phase core-type multi-winding transformer having three core legs, each core leg of the three-phase core-type multi-winding transformer is connected with a set of winding, each set of winding includes a primary winding and a plurality of secondary windings, each primary winding is connected with one of the phases a, B and C of the input voltage, and further includes six cascaded AC/AC converters, the six cascaded AC/AC converters are divided into a set in pairs, the input end of each set of cascaded AC/AC converter is connected with the plurality of secondary windings of a set of winding, the three sets of cascaded AC/AC converters are set as an a-set cascaded AC/AC converter, a B-set cascaded AC/AC converter and a C-set cascaded AC/AC converter according to the voltage accessed by each set of winding, the positive pole of the first cascaded AC/AC converter in the a-set is connected with the first cascaded AC/AC converter in the B-set through the LC filter a The negative poles of the first cascade AC/AC converter in the group A are connected with the positive pole of the first cascade AC/AC converter in the group B, the positive pole of the second cascade AC/AC converter in the group B is connected with the negative pole of the first cascade AC/AC converter in the group C through an LC filter B, the negative pole of the second cascade AC/AC converter in the group B is connected with the positive pole of the first cascade AC/AC converter in the group C, the positive pole of the second cascade AC/AC converter in the group C is connected with the negative pole of the second cascade AC/AC converter in the group A through an LC filter C, the negative pole of the second cascade AC/AC converter in the group C is connected with the positive pole of the second cascade AC/AC converter in the group A, and the negative poles of the second cascade AC/AC converter in the group A are respectively connected with the negative pole of the first cascade AC/AC converter in the group B, The first cascade AC/AC converter cathode in the group C, the LC filter a is connected with the anode of the phase A output port, the LC filter B is connected with the anode of the phase B output port, the LC filter C is connected with the anode of the phase C output port, the primary winding of the direct type AC-AC converter a is connected with the intermediate frequency bus bar, the anode of the phase A output port, the anode of the phase B output port and the anode of the phase C output port of the direct type AC-AC converter a are all connected with the submarine cable, the primary winding of the direct type AC-AC converter B is connected with the onshore power frequency power grid, and the anode of the phase A output port, the anode of the phase B output port and the anode of the phase C output port of the direct type AC-AC converter B are all connected with the output end of the submarine cable.

The LC filter a, the LC filter b and the LC filter c respectively comprise an inductor with one end connected with the anode of the cascade AC/AC converter, the other end of the inductor is connected with the positive electrode of the capacitor and the anode of the output port, and the negative electrode of the capacitor is connected with the cathode of the other cascade AC/AC converter. U in FIG. 2_IA，U_IB，U_ICIs a three-phase input interface, U_OA，U_OB，U_OCIs a three-phase output interface.

As shown in fig. 3, the internal structure of the cascaded AC/AC converter in fig. 2 is shown, each cascaded AC/AC converter includes one or more bipolar direct AC/AC converters, the plurality of bipolar direct AC/AC converters are connected in a manner that an output negative electrode is connected to an output positive electrode of an adjacent bipolar direct AC/AC converter in sequence, a positive electrode of a first-end bipolar direct AC/AC converter is a positive electrode of the cascaded AC/AC converter, a negative electrode of a last-end bipolar direct AC/AC converter is a negative electrode of the cascaded AC/AC converter, and an input end of each bipolar direct AC/AC converter is connected to a secondary winding. Each secondary winding of the three-phase core transformer supplies power to a direct AC/AC converter, and the topology of the direct AC/AC converter is not limited.

A first cascade AC/AC converter of the A group of cascade AC/AC converters, a second cascade AC/AC converter of the B group of cascade AC/AC converters and a second cascade AC/AC converter of the C group of cascade AC/AC converters all adopt modulation waves shown in a formula (1):

D₁＝V_D1cos[(ω₂-ω₁)t+30°+α] (1)

the second cascaded AC/AC converter of the A group of cascaded AC/AC converters, the first cascaded AC/AC converter of the B group of cascaded AC/AC converters and the first cascaded AC/AC converter of the C group of cascaded AC/AC converters all adopt modulation waves shown in the formula (2):

D₂＝V_D2cos[(ω₂-ω₁)t-90°+α] (2)

wherein ω is₁Is the angular frequency, omega, of the input voltage₂To the angular frequency of the output voltage, V_D1And V_D2The amplitudes of the two modulated waves; v_D1Value range of 0 to 1, V_D2Value of V_D1The carrier wave adopts a triangular wave with the amplitude of-1 to 1, and alpha is a phase deviation angle and also is an initial phase angle of the output voltage. Based on the proposed modulation wave, the converter can output three-phase voltages with controllable frequency, amplitude and initial phase, and current harmonics with the same frequency as the output voltage can be eliminated, thereby avoiding introducing the three-phase voltages to the primary side of the transformer.

The turn ratio of each winding of each phase is 1:2:2, and the output voltage U is_OACan be represented by formula (3).

U_OA＝2D₁U_IA+2D₂U_IB (3)

By substituting the formulae (1) to (2) for the formula (3)

Wherein V_mIs the magnitude of the input voltage. As can be seen from the formula (4), the three-phase voltage output device can output three-phase voltages with expected frequency, amplitude and initial phase.

Taking the phase-B core limb of the transformer as an example, the harmonic current is analyzed. The coupling current of the B-phase primary winding of the three-phase core type multi-winding transformer can be represented by equation (5).

Wherein I_mThe amplitude of the three-phase current is output for the invention. From the equation (5), the frequency of the coupling current is ω₁And the current has the same frequency as the input voltage current, so that the current harmonic wave with the same frequency as the output voltage of the invention is eliminated by coupling, and the current harmonic wave is prevented from being introduced into the primary side of the transformer.

Based on the direct ac-ac converter and the modulation method of the present invention, fig. 4 shows voltage waveform diagrams of the node B and the node C. U in the first grid_BA，U_BBAnd U_BCThe three-phase voltages of the node B are respectively, and the frequency of the three-phase voltages is 500 Hz. U in the second grid_CA，U_CBAnd U_CCThe three-phase voltage of the node C is respectively, and the frequency of the three-phase voltage is 15 Hz.

Based on the direct ac-ac converter and the modulation method of the present invention, fig. 5 shows voltage waveform diagrams of the node D and the node E. U in the first grid_DA，U_DBAnd U_DCThe three-phase voltage of the node D is respectively, and the frequency of the three-phase voltage is 15 Hz. U in the second grid_EA，U_EBAnd U_ECThe three-phase voltage of the node E is respectively, and the frequency of the three-phase voltage is 50 Hz. Fig. 4 and 5 verify the correctness and feasibility of the adopted direct-mode ac-ac converter and the modulation method.

Through the mode, the novel deep and distant sea wind power transmission system based on medium-low-power frequency convergence comprises a plurality of wind power generation sets, a plurality of convergence step-up transformers, an intermediate frequency convergence bus, a sea step-up frequency conversion station, a sea cable, a land frequency conversion and website and a land power frequency power grid. The invention reduces the reactance of the transmission line, improves the transmission capacity of the line, and does not contain direct current equipment with high construction and maintenance cost. In addition, the medium-frequency transformer with smaller volume and cost is adopted to replace a low-frequency or power-frequency transformer at sea, so that the method has remarkable advantages. The invention realizes the optimization design of each link, pertinently solves the main defects of the traditional scheme, and has higher practical application value and better future development prospect.