CN105429183A - Permanent magnetic direct-drive type offshore wind power plant grid-connected system topology structure and control method thereof - Google Patents

Abstract

The invention discloses a permanent magnetic direct-drive type offshore wind power plant grid-connected system topology structure and a control method thereof. A grid-connected system comprises a DC bus current collection type wind power plant, a wind field network-side converter station, a dual-stage boost transformer, an offshore rectification station, a seabed DC cable, a shore inversion station, a grid-connected side boost transformer and a land power grid which are successively connected, wherein the DC bus current collection type wind power plant employs a DC bus current collection topology structure and comprises multiple groups of wind turbines which are successively connected, a permanent magnetic synchronous generator, a machine-side rectifier and a DC bus for current collection. The control method is that the machine-side rectifier employs dual-closed-loop control of a rotating speed outer loop and a current inner loop, the wind field network-side converter station employs dual-closed-loop control of a voltage outer loop and the current inner loop, the offshore rectification station employs control of fixed AC voltages and fixed frequencies, and the shore inversion station employs dual-closed-loop control of fixed DC voltages and fixed reactive power. According to the invention, multi-machine parallel-connected operation can be realized, the electric energy conversion efficiency is improved, and the grid-connected AC harmonic quantity is reduced.

Description

Permanent magnet direct-drive type offshore grid-connected wind farm system topology and control method thereof

Technical field

The present invention relates to a kind of offshore grid-connected wind farm system, particularly relate to a kind of permanent magnet direct-drive type offshore grid-connected wind farm system topology and control method thereof, belong to wind-electricity integration field.

Background technology

The plurality of advantages such as wind energy on the sea has that wind speed is high, wind-force is stable and interference is few, therefore, greatly develop the new trend that offshore wind farm has become Wind Power Development.At present, because permanent magnet direct drive synthronous wind-mill generator has the many advantages such as non-gear box, good reliability and efficiency is high, the application in MW class offshore wind turbine is increasing.

For the marine wind electric field built by permanent magnet direct drive synthronous wind-mill generator, its grid connection topology not only comprises the inner current collection topological structure of wind energy turbine set, also comprises grid-connected power transmission mode.It is not only related to the stability of whole wind energy turbine set, economy and service efficiency, is also related to the reliability of wind energy turbine set access electrical network.

At present, the inner general employing ac bus current collection topological structure of wind energy turbine set, but its energy conversion efficiency is lower, and because every platform wind turbine generator has independently control system, the control system of whole wind energy turbine set is relatively more complicated; Offshore grid-connected wind farm mode generally adopts ac transmission (HVAC) grid-connected, and wherein ac transmission synchronizing mode electrical power transmission system structure is simple, and cost is lower, technology maturation, but transmission capacity and transmission range are restricted.Concrete visible, wind farm grid-connected system as depicted in figs. 1 and 2.

Figure 1 shows that the wind farm grid-connected topological structure of ac bus current collection type based on HVAC, the electric energy that in its wind energy turbine set, every permanent magnet synchronous motors sends obtains alternating current through total power converter, the ac bus place of AC energy in wind energy turbine set collects, then by after the marine primary substation of 35kV:220kV through subsea AC cable transfer to land electrical network.

Figure 2 shows that the wind farm grid-connected topological structure of DC bus current collection type based on HVAC, the electric energy that in its wind energy turbine set, every permanent magnet synchronous motors sends becomes direct current through rectifying conversion, direct current energy after DC bus place collects through current conversion station concentrate be reverse into alternating current, then after 35kV:220kV primary substation through subsea AC cable transfer to land electrical network.

So the inside current collection topological structure of wind energy turbine set and its synchronizing mode will be directly connected to stability and the reliability of wind energy turbine set long distance power transmission.

Summary of the invention

Main purpose of the present invention is, overcome deficiency of the prior art, a kind of permanent magnet direct-drive type offshore grid-connected wind farm system topology and control method thereof are provided, not only can realize multi-machine parallel connection to run, and the energy conversion efficiency of wind energy turbine set can be improved, reduce synchronization AC current harmonics amount, improve high efficiency and the stability of wind energy turbine set operation, there is the value in industry.

In order to achieve the above object, the technical solution adopted in the present invention is:

A kind of permanent magnet direct-drive type offshore grid-connected wind farm system topology, comprises DC bus collecting current type wind energy turbine set connected successively, wind field net side current conversion station, two-stage step-up transformer, marine converting plant, subsea DC cable, on the bank Inverter Station, grid-connected side boosting transformer and land electrical network.

Wherein, described DC bus collecting current type wind energy turbine set adopts DC bus current collection topological structure, comprises the some groups of wind energy conversion systems be connected successively, magneto alternator and pusher side rectifier, and the DC bus of current collection; Described wind energy conversion system converts the wind energy of catching to alternating current by magneto alternator, alternating current through pusher side rectifier rectification be transformed into direct current after DC bus place collects, through wind field net side current conversion station concentrate be reverse into wind field net side alternating current, wind field net side alternating current inputs marine converting plant after the boosting of two-stage step-up transformer, be transformed into voltage boosting dc electricity through marine converting plant again delivers to Inverter Station on the bank by subsea DC cable and is reverse into grid-connected side alternating current, and grid-connected side alternating current is incorporated to land electrical network finally by after grid-connected side boosting transformer boost.

Grid-connected system topological structure of the present invention is set to further: described two-stage step-up transformer comprises the 3KV:35KV step-up transformer and 35KV:115KV step-up transformer that are connected successively, the input of described 3KV:35KV step-up transformer is connected with the output of wind field net side current conversion station, and the output of described 35KV:115KV step-up transformer is connected with the input of marine converting plant.

Grid-connected system topological structure of the present invention is set to further: described subsea DC cable is ± 100KV direct current cables.

Grid-connected system topological structure of the present invention is set to further: described subsea DC cable is 100km.

Grid-connected system topological structure of the present invention is set to further: described marine converting plant is the voltage-source type current conversion station that topological structure is identical with Inverter Station on the bank; Described voltage-source type current conversion station is the converter based on full-controlled switch device, and concrete topological structure is three-phase two level-type converter, three-phase tri-level type converter, clamper type multi-level voltage source current converter, cascade multi-level converter, modular multi-electrical-level voltage source current converter or many pulse wave electric voltages source converter.

Grid-connected system topological structure of the present invention is set to further: described three-phase two level-type converter comprises the filter capacitor being parallel to DC side, the voltage-type Three-phase full-bridge inverter circuit in parallel with filter capacitor, and be series at AC and the converter reactor be connected respectively with the three-phase mid point that voltage-type Three-phase full-bridge inverter circuit exports and filter;

Wherein, described filter capacitor comprises the first direct current capacitor and second direct current capacitor of series connection, the connected node ground connection of described first direct current capacitor and the second direct current capacitor; Described converter reactor comprises the converting resistance and change of current inductance of connecting successively, and described converting resistance exports mid point with two-stage step-up transformer output with voltage-type Three-phase full-bridge inverter circuit respectively with the two ends after the series connection of change of current inductance and is connected; Described filter is high pass filter, comprises filter resistance, filter inductance and the filtering capacitor of connecting successively; One end of described filter resistance is connected between two-stage step-up transformer output and converting resistance, one end ground connection of described filtering capacitor.

The present invention also provides a kind of control method of permanent magnet direct-drive type offshore grid-connected wind farm system topology, and the rectify control of described marine converting plant, based on the intermittence of Power Output for Wind Power Field and uncontrollability, adopts and determines alternating voltage and determine FREQUENCY CONTROL; Specifically comprise the following steps,

1) wind energy turbine set output AC voltage effective value U is obtained _wF, wind energy turbine set output AC voltage effective value reference value U _wFref, wind energy turbine set output the fundamental frequency f of alternating voltage _wF, wind energy turbine set output the reference value f of fundamental frequency of alternating voltage _wFref;

2) by wind energy turbine set output AC voltage effective value U _wFwith the reference value U of wind energy turbine set output AC voltage effective value _wFrefdeviation export as alternating voltage amplitude M, by the fundamental frequency f of the alternating voltage of wind energy turbine set output through pi regulator and [0,1] amplitude limit _wFwith the reference value f of the fundamental frequency of the alternating voltage of wind energy turbine set output _wFrefdeviation through pi regulator and [-arcsinX ^*, arcsinX ^*] amplitude limit exports as alternating voltage phase δ, passes through formula calculate the perunit value X of change of current reactance X ^*, wherein, S _nfor current conversion station rated capacity, U _nfor current conversion station rated voltage.

Control method of the present invention is set to further: the inversion control of described Inverter Station on the bank adopts the double-closed-loop control determined direct voltage and determine reactive power, maintains stable DC side voltage, equalizing system active power; Specifically comprise the following steps,

1) VSC-HDC system inverter side direct voltage V is obtained _dc4, VSC-HVDC system inverter side direct voltage reference value V _dcref4, the reactive power Q that is connected to the grid _s4, grid-connected idle merit value and power reference Q _sref4;

2) by VSC-HDC system inverter side direct voltage V _dc4with VSC-HVDC system inverter side direct voltage reference value V _dcref4deviation export d axle reference current i through pi regulator _sdref4, by the reactive power Q be connected to the grid _s4with grid-connected idle merit value and power reference Q _sref4deviation export q axle reference current i through pi regulator _sqref4;

3) by VSC-HDC system inverter side alternating current d axle component i _sd4with VSC-HVDC system inverter side alternating current d axle reference current i _sdref4deviation export converter d axle modulation voltage, by VSC-HDC system inverter side alternating current q axle component i through pi regulator and q axle coupling terms, d axis AC combinations of voltages _sq4with VSC-HVDC system inverter side alternating current q axle reference current i _sqref4deviation export converter q axle modulation voltage through pi regulator and d axle coupling terms, q axis AC combinations of voltages.

Control method of the present invention is set to further: the rectify control strategy of the pusher side rectifier of described DC bus collecting current type wind energy turbine set controls based on zero rotor field-oriented d shaft current, adopt the double-closed-loop control of rotating speed outer shroud and current inner loop, its medium speed outer shroud reference value is provided by maximal power tracing algorithm, catches maximal wind-energy; Specifically comprise the following steps,

1) with the center line of rotor permanent magnet for d axle, be q axle along the advanced d axle of rotor direction of rotation 90 ° of electrical degree directions, under dq0 rotating coordinate system, setting magneto alternator stator voltage equation is formula (1),

$\left\{\begin{array}{c}{u}_{sd1}=\frac{{\mathrm{d\ψ}}_{sd}}{dt}+R_{s1}{i}_{sd1}-{\mathrm{\ω}}_e{L}_{sq1}{i}_{sq1}\\ u_{sq1}=\frac{{\mathrm{d\ψ}}_{sq}}{dt}+R_{s1}{i}_{sq1}+{\mathrm{\ω}}_e{L}_{sd1}{i}_{sd1}+{\mathrm{\ω}}_e{\mathrm{\ψ}}_f\end{array}\right.---\left(1\right)$

In formula (1), u _sd1, u _sq1for d, q axle component of generator unit stator voltage, ψ _sd, ψ _sqfor d, q axle component of stator magnetic linkage, i _sd1, i _sq1for d, q axle component of stator current; ω _efor rotor electric angle frequency, ω _e=p ω _g, wherein, p is motor number of pole-pairs, ω _g=ω _mfor the rotating speed of generator; L _sd1, L _sq1for stator d, q axle synchronous inductance;

2) to d, q axle component i of stator current in formula (1) _sd1, i _sq1carry out uneoupled control, introduce feedforward compensation item u according to formula (2) _d', u _q', compensate the voltage drop on equivalent reactance device;

$\left\{\begin{array}{c}{u}_d^{\′}=\frac{{\mathrm{d\ψ}}_{sd1}}{dt}+R_{s1}{i}_{sd1}\\ u_q^{\′}=\frac{{\mathrm{d\ψ}}_{sq1}}{dt}+R_{s1}{i}_{sq1}\end{array}\right.---\left(2\right)$

Formula (2) is converted an accepted way of doing sth (2-1) by passing ratio integral element,

$\left\{\begin{array}{c}{u}_d^{\′}=K_{p1}(i_{sdref1}-i_{sd1})+K_{i1}\∫(i_{sdref1}-i_{sd1})dt\\ u_q^{\′}=K_{p1}(i_{sqref1}-i_{sq1})+K_{i1}\∫(i_{sqref1}-i_{sq1})dt\end{array}\right.---(2-1)$

In formula (2-1), K _p1, K _i1be respectively current inner loop ratio and integral coefficient; i _sdref1, i _sqref1be respectively current i _sd1, i _sq1reference value, i _sdref1=0, i _sqref1by speed reference ω _mrefwith rotary speed actual value ω _mdeviation through pi regulator regulate obtain;

3) according to formula (2-1), formula (1) is converted an accepted way of doing sth (3),

$\left\{\begin{array}{c}{u}_{sd1}=K_{p1}(i_{sdref1}-i_{sd1})+K_{i1}\∫(i_{sdref1}-i_{sd1})dt-{\mathrm{\ω}}_e{L}_{sq1}{i}_{sq1}\\ u_{sq1}=K_{p1}(i_{sqref1}-i_{sq1})+K_{i1}\∫(i_{sqref1}-i_{sq1})dt+{\mathrm{\ω}}_e{L}_{sd1}{i}_{sd1}+{\mathrm{\ω}}_e{\mathrm{\ψ}}_f\end{array}\right.---\left(3\right)$

By regulating the current inner loop ratio K of magneto alternator _p1, integral coefficient K _i1with rotary speed actual value ω _m, and the speed reference ω of wind energy conversion system _mref, the output of magneto alternator stator voltage is calculated according to formula (3).

Control method of the present invention is set to further: the concentrated inversion control of described wind field net side current conversion station is based on the vector control of grid voltage orientation, adopt the double-closed-loop control of outer voltage and current inner loop, pusher side current transformer rectification DC inverter is out become and line voltage, alternating current that amplitude is the same, keep the constant of DC voltage simultaneously; Specifically comprise the following steps,

1) under d-q rotating coordinate system, determine that the Controlling model of wind field net side current conversion station is formula (4),

$\left\{\begin{array}{c}{u}_{cd2}=u_{sd2}+L\frac{{\mathrm{di}}_{sd2}}{dt}+R_2{i}_{sd2}-{\mathrm{\ω}}_s{L}_2{i}_{sq2}\\ u_{cq2}=u_{sq2}+L\frac{{\mathrm{di}}_{sq2}}{dt}+R_2{i}_{sq2}+{\mathrm{\ω}}_s{L}_2{i}_{sd2}\end{array}\right.---\left(4\right)$

In formula, u _cd2, u _cq2for AC voltage d, q axle component; R ₂, L ₂be respectively the equivalent resistance of connection transformer and commutating reactor, reactance; i _sd2, i _sq2for inverter ac side electric current d, q axle component; ω _sfor wind field line voltage angular frequency; u _sd2, u _sq2for wind field line voltage d, q axle component;

And the active power of wind field net side current conversion station and reactive power are represented for formula (5),

$\left\{\begin{array}{c}{P}_s=\frac{3}{2}(u_{sd2}{i}_{d2}+u_{sq2}{i}_{q2})\\ Q_s=-\frac{3}{2}(u_{sd2}{i}_{q2}-u_{sq2}{i}_{d2})\end{array}\right.---\left(5\right)$

2) space vector control method based on grid voltage orientation is adopted, under the condition of three-phase power grid voltage balance, power taking net space vector of voltage direction be d direction of principal axis, the advanced d axle of q axle 90 ° of electrical degrees, then u _sd2=U _s, u _sq2=0, U wherein _sfor the modulus value of line voltage space vector;

Formula (5) is rewritten as formula (6),

$\left\{\begin{array}{c}{P}_s=\frac{3}{2}{U}_s{i}_{d2}\\ Q_s=-\frac{3}{2}{U}_s{i}_{q2}\end{array}\right.---\left(6\right)$

Learn according to formula (6), the electric current by changing d, q axle controls active power and the reactive power of the current conversion station output of wind field net side;

3) in the current conversion station of wind field net side, introduce feedforward compensation item in ring controller, and be achieved by a proportional integral link, the Controlling model of wind field net side current conversion station is transformed to formula (7),

$\left\{\begin{array}{c}{u}_{cd2}=u_{sd2}+K_{p2}(i_{sdref2}-i_{sd2})+K_{i2}\∫(i_{sdref2}-i_{sd2})dt-{\mathrm{\ω}}_s{L}_2{i}_{sq2}\\ u_{cq2}=u_{sq2}+K_{p2}(i_{sqref2}-i_{sq2})+K_{i2}\∫(i_{sqref2}-i_{sq2})dt+{\mathrm{\ω}}_s{L}_2{i}_{sd2}\end{array}\right.---\left(7\right)$

In formula (7), K _p2, K _i2be respectively current inner loop ratio and integral coefficient; i _sdref2, i _sqref2be respectively meritorious and reactive current reference value; Wherein, i _sdref2for net side converter direct voltage set point V _dcref2with actual value V _dc2deviation transform and obtain after pi regulator regulates; i _sqref2for net side converter reactive power reference qref Q _ref2with actual value Q ₂deviation transform and obtain after pi regulator regulates.

Compared with prior art, the beneficial effect that the present invention has is:

By adopting the DC bus collecting current type wind energy turbine set of DC bus current collection topological structure, the electric energy of production is carried out concentrated inversion through wind field net side current conversion station after DC bus place collects, and by marine converting plant, the setting of subsea DC cable and on the bank Inverter Station, alternating current is transformed into direct current through marine converting plant deliver to Inverter Station inversion on the bank by subsea DC cable and return alternating current, finally send into grid-connected land electrical network, whole grid-connected system topological structure not only can realize multi-machine parallel connection and run, and the energy conversion efficiency of wind energy turbine set can be improved, reduce synchronization AC current harmonics amount, improve high efficiency and the stability of wind energy turbine set operation.Meanwhile, control method provided by the invention, can realize the uneoupled control of active power and reactive power flexibly, and effectively isolation electric network fault is on the impact of wind-powered electricity generation, has stronger antijamming capability.

Foregoing is only the general introduction of technical solution of the present invention, and in order to clearer understanding technological means of the present invention, below in conjunction with accompanying drawing, the invention will be further described.

Accompanying drawing explanation

Fig. 1 is the wind farm grid-connected topological structure of ac bus current collection type based on HVAC of prior art;

Fig. 2 is the wind farm grid-connected topological structure of DC bus current collection type based on HVAC of prior art;

Fig. 3 is the wind farm grid-connected topological structure of DC bus current collection type based on VSC-HVDC of the present invention;

Fig. 4 is the vector control block diagram of magneto alternator pusher side rectifier in Fig. 3;

Fig. 5 is the concentrated contravariant vector control block diagram of Fig. 3 Wind Field net side current conversion station;

Fig. 6 is the three-phase two level topological structure of marine converting plant or Inverter Station on the bank in Fig. 3;

Fig. 7 be in Fig. 3 marine converting plant determine alternating voltage control block diagram;

Fig. 8 is the control block diagram of Inverter Station on the bank in Fig. 3;

Fig. 9 is wind farm wind velocity situation of change;

The wind energy turbine set active power of output of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 10 is wind speed change;

The wind energy turbine set output reactive power of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 11 is wind speed change;

The grid-connected grid side active power of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 12 is wind speed change;

The grid-connected grid side reactive power of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 13 is wind speed change;

The wind energy turbine set output alternating current of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 14 is wind speed change;

The grid-connected grid side alternating current of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 15 is wind speed change;

The direct voltage of wind farm side and grid-connected grid side in Fig. 3 when Figure 16 is wind speed change;

The grid-connected grid side alternating voltage of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 17 is electric network fault;

The grid-connected grid side active power of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 18 is electric network fault;

The grid-connected grid side reactive power of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 19 is electric network fault;

The wind energy turbine set active power of output of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 20 is electric network fault;

The wind energy turbine set output reactive power of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 21 is electric network fault;

The wind energy turbine set output alternating voltage effective value of Fig. 1, Fig. 2 and Fig. 3 tri-kinds of topological structures when Figure 22 is electric network fault;

The direct voltage of VSC-HVDC system in Fig. 3 when Figure 23 is electric network fault.

Embodiment

Below in conjunction with Figure of description, the present invention is further illustrated.

As shown in Figure 3, the invention provides a kind of permanent magnet direct-drive type offshore grid-connected wind farm system topology, it is the wind farm grid-connected topological structure of DC bus current collection type based on VSC-HVDC, comprises DC bus collecting current type wind energy turbine set 1 connected successively, wind field net side current conversion station 2, two-stage step-up transformer 3, marine converting plant 4, subsea DC cable 5, on the bank Inverter Station 6, grid-connected side boosting transformer 7 and land electrical network 8; Described DC bus collecting current type wind energy turbine set 1 adopts DC bus current collection topological structure, comprises the some groups of wind energy conversion systems be connected successively 11, magneto alternator 12 and pusher side rectifier 13, and the DC bus 14 of current collection.

Marine converting plant 4 of the present invention, subsea DC cable 5 and on the bank Inverter Station 6 form VSC-HVDC system 10.Wind energy conversion system 11 in DC bus collecting current type wind energy turbine set 1 converts the wind energy of catching to alternating current by magneto alternator 12, alternating current becomes direct current after DC bus 14 place collects through pusher side rectifier 13 rectifying conversion, concentrate through wind field net side current conversion station 2 and be reverse into wind field net side alternating current, wind field net side alternating current inputs marine converting plant 4 after two-stage step-up transformer 3 boosts, be transformed into voltage boosting dc electricity through marine converting plant 4 again to deliver to Inverter Station 6 on the bank by subsea DC cable 5 and be reverse into grid-connected side alternating current, grid-connected side alternating current is incorporated to land electrical network 8 after boosting finally by grid-connected side boosting transformer 7.

Wherein, two-stage step-up transformer 3 comprises the 3KV:35KV step-up transformer 21 and 35KV:115KV step-up transformer 32 that are connected successively; End direct current cables be 100km ± 100KV direct current cables, grid-connected side boosting transformer 7 is 115kV:220kV step-up transformer.

As shown in Figure 4, the essence that in DC bus collecting current type wind energy turbine set, pusher side rectifier controls converts the alternating current that magneto alternator exports to direct current, and ensureing the sinusoidal galvanic current that magneto alternator output of a generator factor is higher, it is relevant to the running status of whole wind-driven generator unit rectifying part.

With the center line of rotor permanent magnet for d axle, be q axle along the advanced d axle of rotor direction of rotation 90 ° of electrical degree directions.Under dq0 rotating coordinate system, magneto alternator stator voltage equation is:

$\left\{\begin{array}{c}{u}_{sd1}=\frac{{\mathrm{d\ψ}}_{sd}}{dt}+R_{s1}{i}_{sd1}-{\mathrm{\ω}}_e{L}_{sq1}{i}_{sq1}\\ u_{sq1}=\frac{{\mathrm{d\ψ}}_{sq}}{dt}+R_{s1}{i}_{sq1}+{\mathrm{\ω}}_e{L}_{sd1}{i}_{sd1}+{\mathrm{\ω}}_e{\mathrm{\ψ}}_f\end{array}\right.---\left(1\right)$

In formula, u _sd1, u _sq1for d, q axle component of generator unit stator voltage; ψ _sd, ψ _sqfor d, q axle component of stator magnetic linkage; i _sd1, i _sq1for d, q axle component of stator current; ω _efor rotor electric angle frequency, ω _e=p ω _g, wherein, p is motor number of pole-pairs, ω _g=ω _mfor the rotating speed of generator; L _sd1, L _sq1for stator d, q axle synchronous inductance.

From formula (1), stator d, q shaft current component i _sd1, i _sq1except controlled voltage u _sd1, u _sq1impact outside, also by compensating for coupling item ω _el _sq1i _sq1and ω _el _sd1i _sd1impact.Therefore, in order to realize current i _sd1, i _sq1uneoupled control, introduce feedforward compensation item u _d', u _q', thus compensate the voltage drop on equivalent reactance device.

$\left\{\begin{array}{c}{u}_d^{\′}=\frac{{\mathrm{d\ψ}}_{sd1}}{dt}+R_{s1}{i}_{sd1}\\ u_q^{\′}=\frac{{\mathrm{d\ψ}}_{sq1}}{dt}+R_{s1}{i}_{sq1}\end{array}\right.---\left(2\right)$

As can be seen from formula (2), u _d', u _q' be respectively and i _sd1, i _sq1there is the component of voltage of first differential relation,

Formula (2) passing ratio integral element can be converted an accepted way of doing sth (2-1) by it.

$\left\{\begin{array}{c}{u}_d^{\′}=K_{p1}(i_{sdref1}-i_{sd1})+K_{i1}\∫(i_{sdref1}-i_{sd1})dt\\ u_q^{\′}=K_{p1}(i_{sqref1}-i_{sq1})+K_{i1}\∫(i_{sqref1}-i_{sq1})dt\end{array}\right.---(2-1)$

In formula (2-1), K _p1, K _i1be respectively current inner loop ratio and integral coefficient; i _sdref1, i _sqref1be respectively current i _sd1, i _sq1reference value, i _sdref1=0, i _sqref1by speed reference ω _mrefwith rotary speed actual value ω _mdeviation through pi regulator regulate obtain.

Then according to formula (2-1), formula (1) is converted an accepted way of doing sth (3),

$\left\{\begin{array}{c}{u}_{sd1}=K_{p1}(i_{sdref1}-i_{sd1})+K_{i1}\∫(i_{sdref1}-i_{sd1})dt-{\mathrm{\ω}}_e{L}_{sq1}{i}_{sq1}\\ u_{sq1}=K_{p1}(i_{sqref1}-i_{sq1})+K_{i1}\∫(i_{sqref1}-i_{sq1})dt+{\mathrm{\ω}}_e{L}_{sd1}{i}_{sd1}+{\mathrm{\ω}}_e{\mathrm{\ψ}}_f\end{array}\right.---\left(3\right)$

By regulating the current inner loop ratio K of magneto alternator _p1, integral coefficient K _i1with rotary speed actual value ω _m, and the speed reference ω of wind energy conversion system _mref, the output of magneto alternator stator voltage is calculated according to formula (3).

As shown in Figure 5, the Main Function of the concentrated inversion control of wind field net side current conversion station is maintain DC voltage constant, and needs according to electrical network the adjustment carrying out reactive power.

In the Mathematical Modeling of the off line side converter of d-q rotating coordinate system be:

$\left\{\begin{array}{c}{u}_{cd2}=u_{sd2}+L\frac{{\mathrm{di}}_{sd2}}{dt}+R_2{i}_{sd2}-{\mathrm{\ω}}_s{L}_2{i}_{sq2}\\ u_{cq2}=u_{sq2}+L\frac{{\mathrm{di}}_{sq2}}{dt}+R_2{i}_{sq2}+{\mathrm{\ω}}_s{L}_2{i}_{sd2}\end{array}\right.---\left(4\right)$

In formula, u _cd2, u _cq2for AC side of converter voltage d, q axle component; R ₂, L ₂be respectively the equivalent resistance of connection transformer and commutating reactor, reactance; i _sd2, i _sq2for inverter ac side electric current d, q axle component; ω _sfor line voltage angular frequency; u _sd2, u _sq2for line voltage d, q axle component.

Active power and reactive power can be expressed as:

$\left\{\begin{array}{c}{P}_s=\frac{3}{2}(u_{sd2}{i}_{d2}+u_{sq2}{i}_{q2})\\ Q_s=-\frac{3}{2}(u_{sd2}{i}_{q2}-u_{sq2}{i}_{d2})\end{array}\right.---\left(5\right)$

When adopting the space vector control method based on grid voltage orientation, under the condition of three-phase power grid voltage balance, power taking net space vector of voltage direction be d direction of principal axis, the advanced d axle of q axle 90 ° of electrical degrees, then u _sd2=U _s(modulus value of line voltage space vector), u _sq2=0.

Then (5) can be rewritten as:

$\left\{\begin{array}{c}{P}_s=\frac{3}{2}{U}_s{i}_{d2}\\ Q_s=-\frac{3}{2}{U}_s{i}_{q2}\end{array}\right.---\left(6\right)$

As can be seen from formula (6), the active power of AC and the electric current of d axle are directly proportional, and AC reactive power is only directly proportional to q shaft current.Therefore, the electric current by changing d, q axle controls active power and the reactive power of current conversion station output.

Similar with the inner ring Controller gain variations of pusher side rectifier in DC bus collecting current type wind energy turbine set, in the current conversion station of wind field net side ring controller design process in have also been introduced feedforward compensation item, and be achieved by a proportional integral link, its final Systematical control equation is:

$\left\{\begin{array}{c}{u}_{cd2}=u_{sd2}+K_{p2}(i_{sdref2}-i_{sd2})+K_{i2}\∫(i_{sdref2}-i_{sd2})dt-{\mathrm{\ω}}_s{L}_2{i}_{sq2}\\ u_{cq2}=u_{sq2}+K_{p2}(i_{sqref2}-i_{sq2})+K_{i2}\∫(i_{sqref2}-i_{sq2})dt+{\mathrm{\ω}}_s{L}_2{i}_{sd2}\end{array}\right.---\left(7\right)$

In formula, K _p2, K _i2be respectively current inner loop ratio and integral coefficient; i _sdref2, i _sqref2be respectively meritorious and reactive current reference value.Wherein i _sdref2for net side converter direct voltage set point V _dcref2with actual value V _dc2deviation transform and obtain after PI regulates; i _sqref2for net side converter reactive power reference qref Q _ref2with actual value Q ₂deviation transform and obtain after pi regulator regulates.

Because VSC-HVDC system is positioned at the marine converting plant WFVSC at two ends with Inverter Station GSVSC structure is identical on the bank, existing for one end converter VSC.Converter VSC generally has the various topological structures such as two level, three level and many level, herein the typical three-phase two level VSC topological structure of main research, as shown in Figure 6, with Inverter Station on the bank for three-phase two level-type converter is described in detail.

Described three-phase two level-type converter comprises the filter capacitor 61 being parallel to DC side, the voltage-type Three-phase full-bridge inverter circuit 62 in parallel with filter capacitor 61, and be series at AC and the converter reactor 63 be connected respectively with the three-phase mid point that voltage-type Three-phase full-bridge inverter circuit 62 exports and filter 64 (as shown in Figure 3); Wherein, described filter capacitor 61 comprises the first direct current capacitor 2C1 and the second direct current capacitor 2C2 of series connection, the connected node ground connection of described first direct current capacitor 2C1 and the second direct current capacitor 2C2; Described converter reactor 63 comprises the converting resistance R and change of current inductance L that connect successively, and described converting resistance R exports mid point with two-stage step-up transformer 3 output with voltage-type Three-phase full-bridge inverter circuit 62 respectively with the two ends after the series connection of change of current inductance L and is connected; Described filter 64 is high pass filter, comprises filter resistance, filter inductance and the filtering capacitor (not shown) of connecting successively; One end of described filter resistance is connected between two-stage step-up transformer output and converting resistance, one end ground connection of described filtering capacitor.

For three-phase two level-type converter, the three-phase VSC voltage circuit equation can setting up AC according to Kirchhoff's second law is:

$\left\{\begin{array}{c}{u}_{cA}=L\frac{{\mathrm{di}}_A}{dt}+{\mathrm{Ri}}_A+u_{sA}\\ u_{cB}=L\frac{{\mathrm{di}}_B}{dt}+{\mathrm{Ri}}_B+u_{sB}\\ u_{cC}=L\frac{{\mathrm{di}}_C}{dt}+{\mathrm{Ri}}_C+u_{sC}\end{array}\right.---\left(8\right)$

In formula, u _sA, u _sB, u _sCgrid side or wind farm side three-phase alternating voltage, u _cA, u _cB, u _cCfor the PWM voltage that converter exports, L is the inductance of converter reactor, and R is the resistance of converter reactor.

The dynamical equation that can be obtained DC side by Kirchhoff's current law (KCL) is:

$I_{dl}-I_{dc}=C\frac{{\mathrm{dV}}_{dc}}{dt}---\left(9\right)$

In formula, I _dlfor direct current, I _dcfor being injected into the direct current of converter, C is DC bus capacitor, V _dcfor VSC DC voltage.

Park conversion is carried out to formula (8), obtains the Mathematical Modeling of VSC under dq0 coordinate system:

$\left\{\begin{array}{c}L\frac{{\mathrm{di}}_d}{dt}=u_{cd}-{\mathrm{Ri}}_d-u_{sd}+{\mathrm{\ωLi}}_q\\ L\frac{{\mathrm{di}}_q}{dt}=u_{cq}-{\mathrm{Ri}}_q-u_{sq}-{\mathrm{\ωLi}}_d\end{array}\right.---\left(10\right)$

According to instantaneous reactive power theory, the active-power P that under dq0 coordinate system, VSC and AC system exchange _s1and Q _s1can be expressed as:

$\left\{\begin{array}{c}{P}_{s1}=\frac{3}{2}(u_{sd}{i}_d+u_{sq}{i}_q)\\ Q_{s1}=-\frac{3}{2}(u_{sd}{i}_d+u_{sq}{i}_d)\end{array}\right.---\left(11\right)$

Under stable situation, supposing the system three-phase symmetrical runs, and therefore, does not have zero-sequence component.When d axle and ac bus fundamental voltage same-phase, u _sq=0, then formula (11) changes to formula (12):

$\left\{\begin{array}{c}{P}_{s1}=\frac{3}{2}{u}_{sd}{i}_d\\ Q_{s1}=-\frac{3}{2}{u}_{sd}{i}_q\end{array}\right.---\left(12\right)$

During for VSC stable operation, suppose that AC system is enough powerful, therefore, u _sdfor steady state value, from formula (12), the electric current that is meritorious and d axle of AC is directly proportional, and AC reactive power is only directly proportional to q shaft current.Therefore, alternating current can be analyzed to two independently component i _dand i _q.From formula (12), by changing the electric current of d, q axle, active power and the reactive power of current conversion station output can be changed, the decoupling zero of active power and reactive power of this model realization, according to the VSC-HVDC controller under the dq0 coordinate system Design of Mathematical Model steady state operating conditions of three-phase VSC.

Fig. 7 is wind farm side current conversion station in VSC-HVDC system, namely marine converting plant determine alternating voltage control block diagram.Because wind energy turbine set has intermittence and uncontrollability, the active power that wind energy turbine set sends is also along with wind speed constantly changes.Therefore, during wind energy turbine set employing VSC-HVDC system grid connection, marine converting plant is difficult to the control realizing determining active power.The power delivery sent to make wind energy turbine set is as far as possible to VSC-HVDC system and send into electrical network, and the present invention at sea wind farm side of converting plant adopts and determine alternating voltage control and determines FREQUENCY CONTROL.In Fig. 7, U _wFfor wind energy turbine set output AC voltage effective value, U _wFreffor the reference value of wind energy turbine set output AC voltage effective value; f _wFfor the fundamental frequency of the alternating voltage of wind energy turbine set output, f _wFreffor the reference value of the fundamental frequency of the alternating voltage of wind energy turbine set output; By wind energy turbine set output AC voltage effective value U _wFwith the reference value U of wind energy turbine set output AC voltage effective value _wFrefdeviation through pi regulator and [0,1] amplitude limit output AC voltage amplitude M, by the fundamental frequency f of the alternating voltage of wind energy turbine set output _wFwith the reference value f of the fundamental frequency of the alternating voltage of wind energy turbine set output _wFrefdeviation through pi regulator and [-arcsinX ^*, arcsinX ^*] amplitude limit output AC voltage phase place δ, pass through for calculating the perunit value X of change of current reactance X ^*, wherein, S _nfor current conversion station rated capacity, U _nfor current conversion station rated voltage.

Net side current conversion station in VSC-HVDC system, namely the control of inverter on the bank mainly maintains the stable of DC voltage, to maintain the balance of active power.The concentrated inversion control of its control principle and wind field net side current conversion station is basically identical, and its control block diagram as shown in Figure 8.In Fig. 8, V _dc4for VSC-HDC system inverter side direct voltage, V _dcref4for VSC-HVDC system inverter side direct voltage reference value, Q _s4for the reactive power be connected to the grid, Q _sref4for grid-connected idle merit value and power reference.By VSC-HDC system inverter side direct voltage V _dc4with VSC-HVDC system inverter side direct voltage reference value V _dcref4deviation export d axle reference current i through pi regulator _sdref4, by the reactive power Q be connected to the grid _s4with grid-connected idle merit value and power reference Q _sref4deviation export q axle reference current i through pi regulator _sqref4; By VSC-HDC system inverter side alternating current d axle component i _sd4with VSC-HVDC system inverter side alternating current d axle reference current i _sdref4deviation through pi regulator and q axle coupling terms ω _sl ₄i _sq4, d axis AC voltage u _sd4array output converter d axle modulation voltage u _cd4, by VSC-HDC system inverter side alternating current q axle component i _sq4with VSC-HVDC system inverter side alternating current q axle reference current i _sqref4deviation through pi regulator and d axle coupling terms ω _sl ₄i _sd4, q axis AC voltage u _sq4array output converter q axle modulation voltage u _cq4.

The present invention utilizes PSCAD/EMTDC software to carry out simulation analysis to designed permanent magnet direct-drive type offshore grid-connected wind farm system topology, by emulating wind speed change and grid side single phase ground fault two kinds of situations, validity of the present invention and superiority can be verified.

Fig. 9 is wind speed change curve.When the wind speed of wind energy turbine set changes, Figure 10-11 is active power and the reactive power curve of output of wind energy turbine set, and as we can see from the figure, the active power that wind energy turbine set exports all increases along with the increase of wind speed, and reactive power does not change with the change of wind speed.

Figure 12-13 is that wind farm grid-connected active power and reactive power export, and can find out, the active power that wind energy turbine set exports has the reduction of certain amplitude after the AC transmission line road or direct current transmission circuit of 100km; For DC bus current collection type wind energy turbine set, the active power delivering to electrical network through VSC-HVDC transmission system is less slightly.Although this is because direct current cables loss ratio ac cable is little, in VSC-HVDC system, two ends current conversion station exists switching loss, but along with the increase of transmission distance, the loss that direct current cables is saved will be obvious gradually; The grid-connected grid side reactive power of VSC-HVDC transmission system is stabilized near set point 0Mvar always, and the grid-connected grid side reactive power of HVAC transmission system can change along with the change of the active power of conveying, and this will be unfavorable for wind farm grid-connected stable operation.

Figure 14 and 15 is respectively the alternating current of wind energy turbine set output and wind farm grid-connected alternating current, as we can see from the figure, during the employing ac bus current collection of wind energy turbine set inside, the alternating current harmonic content that wind energy turbine set exports is maximum, and wind farm grid-connected alternating current harmonic content is also maximum.

Based on VSC-HVDC system grid connection, inverter side direct voltage is stabilized in set point 200kV always, and the lifting to some extent along with the increase of the active power of conveying of rectification side direct voltage, as shown in figure 16.

Grid side generation single phase ground fault, causes falling of system alternating voltage, and Voltage Drop situation as shown in figure 17.The active power of grid-connected grid side and reactive power are as depicted in figs. 18-19.What wind energy turbine set exported attacks power and reactive power as shown in figures 20-21.The alternating voltage that wind energy turbine set exports as shown in figure 22.As we can see from the figure, when wind energy turbine set adopts HVAC grid-connected, cannot effectively isolated fault, grid side active power and reactive power all receive obvious impact, and the active power that wind energy turbine set exports, reactive power and alternating voltage also all receive obvious impact, have occurred certain fluctuation and decline.

The direct voltage of wind farm side and grid-connected grid side when Figure 23 is wind energy turbine set employing VSC-HVDC system grid connection, as we can see from the figure, there is slight fluctuations in the direct voltage of VSC-HVDC system inverter side, but VSC-HVDC system rectification side direct voltage does not change substantially.

So can see, wind energy turbine set adopts DC bus current collection topological structure to contribute to improving energy conversion efficiency, reduces the harmonic content of wind energy turbine set output AC electric current; And fault ride-through of power grid can be realized smoothly; Adopt VSC-HVDC system power transmission mode then effectively can isolate the impact of electric network fault on wind energy turbine set, there is good runnability.

To sum up analyze, to the wind farm grid-connected topological structure of DC bus current collection type based on VSC-HVDC provided by the invention, carry out simulation analysis by wind speed change and grid side single phase ground fault two kinds of situations, fully can show that the present invention possesses feasibility and superiority.

More than show and describe general principle of the present invention, principal character and advantage.The technical staff of the industry should understand; the present invention is not restricted to the described embodiments; what describe in above-described embodiment and specification just illustrates principle of the present invention; without departing from the spirit and scope of the present invention; the present invention also has various changes and modifications, and these changes and improvements all fall in the claimed scope of the invention.Application claims protection range is defined by appending claims and equivalent thereof.

Claims (10)

1. a permanent magnet direct-drive type offshore grid-connected wind farm system topology, is characterized in that: comprise DC bus collecting current type wind energy turbine set connected successively, wind field net side current conversion station, two-stage step-up transformer, marine converting plant, subsea DC cable, on the bank Inverter Station, grid-connected side boosting transformer and land electrical network;

Described DC bus collecting current type wind energy turbine set adopts DC bus current collection topological structure, comprises the some groups of wind energy conversion systems be connected successively, magneto alternator and pusher side rectifier, and the DC bus of current collection;

Described wind energy conversion system converts the wind energy of catching to alternating current by magneto alternator, alternating current through pusher side rectifier rectification be transformed into direct current after DC bus place collects, through wind field net side current conversion station concentrate be reverse into wind field net side alternating current, wind field net side alternating current inputs marine converting plant after the boosting of two-stage step-up transformer, be transformed into voltage boosting dc electricity through marine converting plant again delivers to Inverter Station on the bank by subsea DC cable and is reverse into grid-connected side alternating current, and grid-connected side alternating current is incorporated to land electrical network finally by after grid-connected side boosting transformer boost.

2. permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 1, it is characterized in that: described two-stage step-up transformer comprises the 3KV:35KV step-up transformer and 35KV:115KV step-up transformer that are connected successively, the input of described 3KV:35KV step-up transformer is connected with the output of wind field net side current conversion station, and the output of described 35KV:115KV step-up transformer is connected with the input of marine converting plant.

3. permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 1, is characterized in that: described subsea DC cable is ± 100KV direct current cables.

4. the permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 1 or 3, is characterized in that: described subsea DC cable is 100km.

5. permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 1, is characterized in that: described marine converting plant is the voltage-source type current conversion station that topological structure is identical with Inverter Station on the bank; Described voltage-source type current conversion station is the converter based on full-controlled switch device, and concrete topological structure is three-phase two level-type converter, three-phase tri-level type converter, clamper type multi-level voltage source current converter, cascade multi-level converter, modular multi-electrical-level voltage source current converter or many pulse wave electric voltages source converter.

6. permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 5, it is characterized in that: described three-phase two level-type converter comprises the filter capacitor being parallel to DC side, the voltage-type Three-phase full-bridge inverter circuit in parallel with filter capacitor, and be series at AC and the converter reactor be connected respectively with the three-phase mid point that voltage-type Three-phase full-bridge inverter circuit exports and filter;

Described filter capacitor comprises the first direct current capacitor and second direct current capacitor of series connection, the connected node ground connection of described first direct current capacitor and the second direct current capacitor;

Described converter reactor comprises the converting resistance and change of current inductance of connecting successively, and described converting resistance exports mid point with two-stage step-up transformer output with voltage-type Three-phase full-bridge inverter circuit respectively with the two ends after the series connection of change of current inductance and is connected;

Described filter is high pass filter, comprises filter resistance, filter inductance and the filtering capacitor of connecting successively; One end of described filter resistance is connected between two-stage step-up transformer output and converting resistance, one end ground connection of described filtering capacitor.

7. the control method of the permanent magnet direct-drive type offshore grid-connected wind farm system topology according to any one of claim 1 ~ 6, it is characterized in that: the rectify control of described marine converting plant, based on the intermittence of Power Output for Wind Power Field and uncontrollability, adopts and determines alternating voltage and determine FREQUENCY CONTROL; Specifically comprise the following steps,

1) wind energy turbine set output AC voltage effective value U is obtained _wF, wind energy turbine set output AC voltage effective value reference value U _wFref, wind energy turbine set output the fundamental frequency f of alternating voltage _wF, wind energy turbine set output the reference value f of fundamental frequency of alternating voltage _wFref;

2) by wind energy turbine set output AC voltage effective value U _wFwith the reference value U of wind energy turbine set output AC voltage effective value _wFrefdeviation export as alternating voltage amplitude M, by the fundamental frequency f of the alternating voltage of wind energy turbine set output through pi regulator and [0,1] amplitude limit _wFwith the reference value f of the fundamental frequency of the alternating voltage of wind energy turbine set output _wFrefdeviation through pi regulator and [-arcsinX ^*, arcsinX ^*] amplitude limit exports as alternating voltage phase δ, passes through formula calculate the perunit value X of change of current reactance X ^*, wherein, S _nfor current conversion station rated capacity, U _nfor current conversion station rated voltage.

8. the control method of permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 7, it is characterized in that: the inversion control of described Inverter Station on the bank adopts the double-closed-loop control determined direct voltage and determine reactive power, maintain stable DC side voltage, equalizing system active power; Specifically comprise the following steps,

1) VSC-HDC system inverter side direct voltage V is obtained _dc4, VSC-HVDC system inverter side direct voltage reference value V _dcref4, the reactive power Q that is connected to the grid _s4, grid-connected idle merit value and power reference Q _sref4;

2) by VSC-HDC system inverter side direct voltage V _dc4with VSC-HVDC system inverter side direct voltage reference value V _dcref4deviation export d axle reference current i through pi regulator _sdref4, by the reactive power Q be connected to the grid _s4with grid-connected idle merit value and power reference Q _sref4deviation export q axle reference current i through pi regulator _sqref4;

3) by VSC-HDC system inverter side alternating current d axle component i _sd4with VSC-HVDC system inverter side alternating current d axle reference current i _sdref4deviation export converter d axle modulation voltage, by VSC-HDC system inverter side alternating current q axle component i through pi regulator and q axle coupling terms, d axis AC combinations of voltages _sq4with VSC-HVDC system inverter side alternating current q axle reference current i _sqref4deviation export converter q axle modulation voltage through pi regulator and d axle coupling terms, q axis AC combinations of voltages.

9. the control method of permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 7, it is characterized in that: the rectify control strategy of the pusher side rectifier of described DC bus collecting current type wind energy turbine set controls based on zero rotor field-oriented d shaft current, adopt the double-closed-loop control of rotating speed outer shroud and current inner loop, its medium speed outer shroud reference value is provided by maximal power tracing algorithm, catches maximal wind-energy; Specifically comprise the following steps,

1) with the center line of rotor permanent magnet for d axle, be q axle along the advanced d axle of rotor direction of rotation 90 ° of electrical degree directions, under dq0 rotating coordinate system, setting magneto alternator stator voltage equation is formula (1),

$\left\{\begin{array}{c}{u}_{sd1}=\frac{{\mathrm{d\ψ}}_{sd}}{dt}+R_{s1}{i}_{sd1}-{\mathrm{\ω}}_e{L}_{sq1}{i}_{sq1}\\ u_{sq1}=\frac{{\mathrm{d\ψ}}_{sq}}{dt}+R_{s1}{i}_{sq1}+{\mathrm{\ω}}_e{L}_{sd1}{i}_{sd1}+{\mathrm{\ω}}_e{\mathrm{\ψ}}_f\end{array}\right.---\left(1\right)$

In formula (1), u _sd1, u _sq1for d, q axle component of generator unit stator voltage, ψ _sd, ψ _sqfor d, q axle component of stator magnetic linkage, i _sd1, i _sq1for d, q axle component of stator current; ω _efor rotor electric angle frequency, ω _e=p ω _g, wherein, p is motor number of pole-pairs, ω _g=ω _mfor the rotating speed of generator; L _sd1, L _sq1for stator d, q axle synchronous inductance;

2) to d, q axle component i of stator current in formula (1) _sd1, i _sq1carry out uneoupled control, introduce feedforward compensation item u according to formula (2) _d', u _q', compensate the voltage drop on equivalent reactance device;

$\left\{\begin{array}{c}{u}_d^{\′}=\frac{{\mathrm{d\ψ}}_{sd1}}{dt}+R_{s1}{i}_{sd1}\\ u_q^{\′}=\frac{{\mathrm{d\ψ}}_{sq1}}{dt}+R_{s1}{i}_{sq1}\end{array}\right.---\left(2\right)$

Formula (2) is converted an accepted way of doing sth (2-1) by passing ratio integral element,

$\left\{\begin{array}{c}{u}_d^{\′}=K_{p1}(i_{sdref1}-i_{sd1})+K_{i1}\∫(i_{sdref1}-i_{sd1})dt\\ u_q^{\′}=K_{p1}(i_{sqref1}-i_{sq1})+K_{i1}\∫(i_{sqref1}-i_{sq1})dt\end{array}\right.---(2-1)$

In formula (2-1), K _p1, K _i1be respectively current inner loop ratio and integral coefficient; i _sdref1, i _sqref1be respectively current i _sd1, i _sq1reference value, i _sdref1=0, i _sqref1by speed reference ω _mrefwith rotary speed actual value ω _mdeviation through pi regulator regulate obtain;

3) according to formula (2-1), formula (1) is converted an accepted way of doing sth (3),

$\left\{\begin{array}{c}{u}_{sd1}=K_{p1}(i_{sdref1}-i_{sd1})+K_{i1}\∫(i_{sdref1}-i_{sd1})dt-{\mathrm{\ω}}_e{L}_{sq1}{i}_{sq1}\\ u_{sq1}=K_{p1}(i_{sqref1}-i_{sq1})+K_{i1}\∫(i_{sqref1}-i_{sq1})dt+{\mathrm{\ω}}_e{L}_{sd1}{i}_{sd1}+{\mathrm{\ω}}_e{\mathrm{\ψ}}_f\end{array}\right.---\left(3\right)$

By regulating the current inner loop ratio K of magneto alternator _p1, integral coefficient K _i1with rotary speed actual value ω _m, and the speed reference ω of wind energy conversion system _mref, the output of magneto alternator stator voltage is calculated according to formula (3).

10. the control method of permanent magnet direct-drive type offshore grid-connected wind farm system topology according to claim 9, it is characterized in that: the concentrated inversion control of described wind field net side current conversion station is based on the vector control of grid voltage orientation, adopt the double-closed-loop control of outer voltage and current inner loop, pusher side current transformer rectification DC inverter is out become and line voltage, alternating current that amplitude is the same, keep the constant of DC voltage simultaneously; Specifically comprise the following steps,

1) under d-q rotating coordinate system, determine that the Controlling model of wind field net side current conversion station is formula (4),

$\left\{\begin{array}{c}{u}_{cd2}=u_{sd2}+L\frac{{\mathrm{di}}_{sd2}}{dt}+R_2{i}_{sd2}-{\mathrm{\ω}}_s{L}_2{i}_{sq2}\\ u_{cq2}=u_{sq2}+L\frac{{\mathrm{di}}_{sq2}}{dt}+R_2{i}_{sq2}+{\mathrm{\ω}}_s{L}_2{i}_{sd2}\end{array}\right.---\left(4\right)$

In formula, u _cd2, u _cq2for AC voltage d, q axle component; R ₂, L ₂be respectively the equivalent resistance of connection transformer and commutating reactor, reactance; i _sd2, i _sq2for inverter ac side electric current d, q axle component; ω _sfor wind field line voltage angular frequency; u _sd2, u _sq2for wind field line voltage d, q axle component;

And the active power of wind field net side current conversion station and reactive power are represented for formula (5),

$\left\{\begin{array}{c}{P}_s=\frac{3}{2}(u_{sd2}{i}_{d2}+u_{sq2}{i}_{q2})\\ Q_s=-\frac{3}{2}(u_{sd2}{i}_{q2}-u_{sq2}{i}_{d2})\end{array}\right.---\left(5\right)$

2) space vector control method based on grid voltage orientation is adopted, under the condition of three-phase power grid voltage balance, power taking net space vector of voltage direction be d direction of principal axis, the advanced d axle of q axle 90 ° of electrical degrees, then u _sd2=U _s, u _sq2=0, U wherein _sfor the modulus value of line voltage space vector;

Formula (5) is rewritten as formula (6),

$\left\{\begin{array}{c}{P}_s=\frac{3}{2}{U}_s{i}_{d2}\\ Q_s=-\frac{3}{2}{U}_s{i}_{q2}\end{array}\right.---\left(6\right)$

Learn according to formula (6), the electric current by changing d, q axle controls active power and the reactive power of the current conversion station output of wind field net side;

3) in the current conversion station of wind field net side, introduce feedforward compensation item in ring controller, and be achieved by a proportional integral link, the Controlling model of wind field net side current conversion station is transformed to formula (7),

$\left\{\begin{array}{c}{u}_{cd2}=u_{sd2}+K_{p2}(i_{sdref2}-i_{sd2})+K_{i2}\∫(i_{sdref2}-i_{sd2})dt-{\mathrm{\ω}}_s{L}_2{i}_{sq2}\\ u_{cq2}=u_{sq2}+K_{p2}(i_{sqref2}-i_{sq2})+K_{i2}\∫(i_{sqref2}-i_{sq2})dt+{\mathrm{\ω}}_s{L}_2{i}_{sd2}\end{array}\right.---\left(7\right)$

In formula (7), K _p2, K _i2be respectively current inner loop ratio and integral coefficient; i _sdref2, i _sqref2be respectively meritorious and reactive current reference value; Wherein, i _sdref2for net side converter direct voltage set point V _dcref2with actual value V _dc2deviation transform and obtain after pi regulator regulates; i _sqref2for net side converter reactive power reference qref Q _ref2with actual value Q ₂deviation transform and obtain after pi regulator regulates.