CN107681698B  Doublyfed wind power rotor series resistance lowvoltage ridethrough control method based on power optimization  Google Patents
Doublyfed wind power rotor series resistance lowvoltage ridethrough control method based on power optimization Download PDFInfo
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 CN107681698B CN107681698B CN201711135475.1A CN201711135475A CN107681698B CN 107681698 B CN107681698 B CN 107681698B CN 201711135475 A CN201711135475 A CN 201711135475A CN 107681698 B CN107681698 B CN 107681698B
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Classifications

 H—ELECTRICITY
 H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
 H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
 H02J3/00—Circuit arrangements for ac mains or ac distribution networks
 H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
 H02J3/381—Dispersed generators
 H02J3/382—Dispersed generators the generators exploiting renewable energy
 H02J3/386—Wind energy

 H—ELECTRICITY
 H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
 H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
 H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
 H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]

 Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSSSECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSSREFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
 Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
 Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
 Y02E10/00—Energy generation through renewable energy sources
 Y02E10/70—Wind energy
 Y02E10/76—Power conversion electric or electronic aspects
Abstract
The invention discloses a doublyfed wind power rotor series resistance lowvoltage ridethrough control method based on power optimization, which changes the given value of active power of a doublyfed motor in real time according to the feedforward control of the grid voltage drop depth, so as to quickly and effectively control the power output of the doublyfed motor during the lowvoltage ridethrough period, realize the power balance of two sides of a rotor converter during the lowvoltage ridethrough period, inhibit the rising of the voltage of a direct current bus, and overshoot of the currents of a stator and a rotor, namely basically realize the safe, stable and zerovoltage ridethrough operation of a system without adding extra hardware equipment; by utilizing the controllability of the doublyfed motor during the lowvoltage ridethrough period of the rotor series resistor, under the condition of ensuring the stability of a system and not exceeding the limit of the current of the rotor converter, the reactive support as much as possible is provided for the power grid, and the fault voltage of the power grid is promoted to be quickly recovered. The invention also has the advantages of reducing the cost of the system and having certain engineering practicability.
Description
Technical Field
The invention relates to the technical field of electronics, in particular to a doublyfed wind power rotor series resistance lowvoltage ridethrough control method based on power optimization.
Background
With the increase of installed capacity and the structural characteristics of the variablespeed constantfrequency doublefed wind driven generator, modern power specifications require that the wind turbine generator still has uninterrupted gridconnected operation capability for a certain time when the terminal Voltage drops due to external grid faults, namely, the variablespeed constantfrequency doublefed wind driven generator has Low Voltage Ride Through (LVRT) capability. Currently, the LVRT control strategy of a Doublyfed induction generator (DFIG) can be classified into 3 types: hardware improvement measures, software improvement measures and hardware and software combined comprehensive improvement measures.
In terms of hardware improvement, an appropriate crowbar protection circuit is added on the rotor side of the doublyfed motor in an industrial field to suppress rotor converter overcurrent during LVRT. The method not only increases the cost, but also ensures that the DFIG is in an outofcontrol state during the LVRT period and cannot provide reactive current required by grid voltage drop recovery in time. In the document 'research on lowvoltage active ridethrough technology of doublyfed wind turbine generator with stator series impedance' (the Chinese Motor engineering report, 2015, 35 (12): 2943. The document 'doublyfed wind generator low voltage ride through based on rotor series resistance' (electric power automation equipment, 2015, 35 (12): 2833) proposes a rotor series resistance low voltage ride through power coordination control strategy, so that the doublyfed motor works in a reactive support mode while the rotor resistance value is optimized, and a certain inductive reactive power is preferentially output to a power grid, thereby being beneficial to the rapid recovery of the power grid voltage.
Software improvement measure aspect: in order to reduce rotor overcurrent, the existing literature proposes an LVRT scheme of virtual impedance according to the transfer function of a "current regulatorRSCDFIG model", but is only applicable to the case of relatively small grid voltage sag amplitude.
The comprehensive improvement measure combined by the hardware and the software adopts a mode of connecting reactance in series in the stator to restrain the rotor current on one hand, on the other hand, the negative influence of prolonging the electromagnetic torque oscillation time caused by the series reactance in the stator is counteracted through an improved RSC control strategy, and a reactive compensation target in the fault period is added in the RSC control, so that the reactive output capability of the DFIG stator side is fully exerted, and a better LVRT effect is obtained.
Combining the theoretical research results, the common point of the theoretical research results is how to reduce the redundant energy generated by the LVRT, namely how to limit the rotor overvoltage and overcurrent during the LVRT within a safe range under the control of additional hardware circuits and software, which is a passive protection mode, and is not based on fundamentally reducing the imbalance of input and output energy brought by the LVRT to the DFIG.
Disclosure of Invention
In view of the above defects, the present invention provides a doublyfed wind power rotor series resistance low voltage ride through control method based on power optimization.
In order to achieve the above purpose, the technical scheme of the invention is as follows:
a doublyfed wind power rotor series resistance low voltage ride through control method based on power optimization comprises the following steps:
firstly, establishing a DFIG mathematical model in a threephase ABC coordinate system:
in the formula: u. of_{s}、u_{r}Respectively are stator voltage vectors and rotor voltage vectors; i.e. i_{s}、i_{r}Respectively are stator current vectors and rotor current vectors; psi_{s}、ψ_{r}Respectively are stator flux linkage vectors and rotor flux linkage vectors; l is_{s}，L_{r}The selfinductance of the stator and the rotor is achieved; l is_{m}Is mutual inductance; r_{s}、R_{r}Respectively a stator resistor and a rotor resistor; omega_{s}Is the synchronous angular velocity; omega_{r}Is the rotor angular velocity;
and secondly, simulating fault occurrence in a DFIG mathematical model, and obtaining a stator flux transient expression during fault according to the principle that the DFIG stator flux can not be mutated before and after the DFIG stator flux falls, wherein the transient expression comprises the following steps:
in the formula:
thirdly, analyzing according to a formula (3), on the basis of a rotor series resistance lowvoltage ridethrough structure, optimizing an active power set value controlled by the DFIG (doublyfed induction generator) by using the controllability of a doublyfed motor when the grid voltage has a drop fault, and increasing an optimized quantity P of the grid voltage drop depth in the active power_{1}And reducing the given value of active power so as to reduce the input energy of the DFIG during the LVRT period.
The fourth step is also included: optimizing transient reactive power, transmitting the reactive power to a power grid, and optimizing the transient power of a rotor string resistor structure to obtain a rotor reactive current instruction limit value during LVRT (linear variable resistor) period as follows:
wherein,  i_{drmax}I is a rotor reactive current instruction limit value during LVRT; i.e. i_{qr} ^{*}Setting a rotor active current value; i is_{rotormax}The current limiting upper limit value of the rotor converter; u shape_{0}Stator voltage amplitude for steady state operation, I_{N}And (4) taking the minimum value among the three for the rated current of the unit, wherein min { } represents the minimum value.
And in the third step, when the grid voltage has a drop fault, on the basis of a rotor series resistance lowvoltage ridethrough structure, the controllability of a doublyfed motor is utilized to optimize the given value of the active power controlled by the DFIG, and the optimized quantity P of the grid voltage drop depth is increased in the active power_{1}The method specifically comprises the following steps:
P_{1}＝P_{1max}P_{LVRT}(5)
wherein, P_{1max}For the calculated power value based on maximum wind energy capture, P_{LVRT}The dynamic power feedforward quantity is used for reflecting the voltage drop of the power grid.
When the falling amplitude does not exceed 10%, LVRT dynamic power feedforward quantity P_{LVRT}Is 0, the active power of the stator is given by P_{1} ^{*}Maintaining maximum wind energy capture value P before failure_{1max}The change is not changed; when the voltage drops to 0.9U_{0}When the following, the dynamic power feedforward quantity P_{LVRT}Greater than 0, stator active power given P_{1} ^{*}Corresponding to the condition of P_{1max}On the basis of reducing P_{LVRT}After PI regulation, the stator active current given value i_{qs} ^{*}Reducing the rotor active current given value i_{qr} ^{*}And reducing and balancing the DFIG input and output power.
The second step is to simulate the fault occurrence in a DFIG mathematical model, and obtain a stator flux transient expression during the fault according to the principle that the DFIG stator flux before and after the drop can not be suddenly changed, and the method specifically comprises the following steps:
2.1 setting the stator voltage amplitude value to u in steadystate operation_{0}Assuming that the drop depth of the power grid is P when t is equal to 0,the stator voltage before and after the sag is written as:
2.2, neglecting the stator resistance according to the relationship between flux linkage and voltage, the stator steady state flux linkage before and after the fault is according to the equation (1):
2.3, assuming open rotor, i.e. i_{r}When the value is 0, the stator flux linkage differential equation obtained from the equations (1) and (2) is:
according to the principle that the stator flux linkage can not change suddenly before and after the voltage of the power grid drops, the solution of the stator flux linkage differential equation can be decomposed into two parts, one part is a stator flux linkage component rotating at a synchronous speed, and the amplitude is determined by the amplitude of the voltage of the stator; the other part is a stator flux linkage directcurrent component caused by sudden drop of the stator voltage, the directcurrent component is kept still in space and is attenuated by a time constant, and the transient expression of the stator flux linkage during the fault is obtained by combining the formula (7):
order toSubstituting the stator flux linkage in the formula (2) into the rotor flux linkage to obtain the formula (10) as follows:
2.4 substituting the transient expressions (9) and (10) for the stator flux linkage during a fault into the equation for the rotor voltage in equation (1) to obtain a first order differential equation for the rotor voltage
And further obtaining a dynamic expression of sudden rise of the rotor current when the voltage of the gridconnected point drops as follows:
compared with the prior art, the invention has the beneficial effects that:
the invention provides a doublefed wind power rotor series resistance low voltage ride through control method based on power optimization, and provides a doublefed motor LVRT control strategy based on dynamic power optimization aiming at doublefed wind power generation rotor series resistance low voltage ride through control, namely, the given value of active power of DFIG during LVRT is dynamically reduced according to the feedforward of the grid voltage drop depth, the imbalance of input and output energy brought to the DFIG by the LVRT is reduced from the source, the power output of the doublefed motor during low voltage ride through is further rapidly and effectively controlled, the power balance of two sides of a rotor converter during low ride through is realized, the voltage rise of a direct current bus is restrained, and the overshoot of stator and rotor currents is basically realized, namely, the safe, stable and zero voltage ride through operation of a system is basically realized without adding extra hardware equipment; by utilizing the controllability of the doublyfed motor during the lowvoltage ridethrough period of the rotor series resistor, under the condition of ensuring the stability of a system and not exceeding the limit of the current of the rotor converter, the reactive support as much as possible is provided for the power grid, and the fault voltage of the power grid is promoted to be quickly recovered. The invention also has the advantages of reducing the cost of the system and having certain engineering practicability.
Drawings
FIG. 1 is a block diagram of a rotor series resistance structure of a doublyfed wind turbine generator system adopted by the transient power optimization method;
FIG. 2 is a block diagram of the transient power optimization control of the present invention;
FIG. 3a is a waveform diagram of a DC bus voltage experiment when the grid voltage drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 3b is a waveform diagram of an active power experiment of a stator when the voltage of a power grid drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 3c is a waveform diagram of a reactive power test of the stator when the voltage of the power grid drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 3d is a waveform diagram of a stator current experiment when the grid voltage drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 3e is a waveform diagram of an experimental rotor current when the grid voltage drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 4a is a waveform diagram of a DC bus voltage experiment when the grid voltage drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 4b is a waveform diagram of an active power experiment of a stator when the voltage of a power grid drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 4c is a waveform diagram of a reactive power test of the stator when the voltage of the power grid drops to 50% after the transient power optimization method of the present invention is adopted;
FIG. 4d is a waveform diagram of a stator current experiment when the grid voltage drops to 50% after the transient power optimization method of the present invention is adopted;
fig. 4e is a waveform diagram of an experimental rotor current when the grid voltage drops to 50% after the transient power optimization method of the present invention is adopted.
Detailed Description
The present invention will now be described in detail with reference to the drawings, wherein the described embodiments are only some, but not all embodiments of the invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, belong to the scope of the present invention.
The structure of the transient power optimizationbased doublyfed wind power rotor series resistance low voltage ride through control system provided by the invention is shown in figure 1, wherein L is_{s}Is a network side filter inductor, L_{r}Is a filter inductor at the rotor side, C is a DC bus capacitor, V_{dc}Is straightCurrent bus voltage, R_{ch}For direct current side dump resistance, R_{rs}Is a bypass current limiting resistor connected in series with the rotor. The converter directly connected to the rotor is called a rotor side converter (RSC for short), and regulates the active power and the reactive power output by the stator end of the DFIG by controlling the terminal voltage of the rotor fed into the DFIG. The other converter is called a grid side converter (GSC for short), and is connected to the rotor side converter through a dc bus, and the ac side of the converter is connected to a threephase grid, so as to maintain the dc bus voltage constant and provide a certain reactive power support to the grid.
The invention provides a doublyfed wind power rotor series resistance low voltage ride through control method based on power optimization, which comprises the following steps:
firstly, establishing a DFIG mathematical model in a threephase ABC coordinate system:
in the formula: u. of_{s}、u_{r}Respectively are stator voltage vectors and rotor voltage vectors; i.e. i_{s}、i_{r}Respectively are stator current vectors and rotor current vectors; psi_{s}、ψ_{r}Respectively are stator flux linkage vectors and rotor flux linkage vectors; l is_{s}，L_{r}The selfinductance of the stator and the rotor is achieved; l is_{m}Is mutual inductance; r_{s}、R_{r}Respectively a stator resistor and a rotor resistor; omega_{s}Is the synchronous angular velocity; omega_{r}Is the rotor angular velocity;
secondly, theoretically analyzing a rotor current mutation mechanism during a fault period:
simulating fault occurrence in a DFIG mathematical model, and obtaining a stator flux transient expression during fault according to the principle that the stator flux of the DFIG before and after falling can not be suddenly changed:
in the formula:
the method specifically comprises the following steps:
2.1 setting the stator voltage amplitude value to u in steadystate operation_{0}If the grid has a symmetrical fault with a drop depth of P when t is equal to 0, the stator voltage before and after the drop is written as follows:
2.2, neglecting the stator resistance according to the relationship between flux linkage and voltage, the stator steady state flux linkage before and after the fault is according to the equation (1):
2.3, assuming open rotor, i.e. i_{r}When the value is 0, the stator flux linkage differential equation obtained from the equations (1) and (2) is:
according to the principle that the stator flux linkage can not change suddenly before and after the voltage of the power grid drops, the solution of the stator flux linkage differential equation can be decomposed into two parts, one part is a stator flux linkage component rotating at a synchronous speed, and the amplitude is determined by the amplitude of the voltage of the stator; the other part is a stator flux linkage directcurrent component caused by sudden drop of the stator voltage, the directcurrent component is kept still in space and is attenuated by a time constant, and the transient expression of the stator flux linkage during the fault is obtained by combining the formula (7):
order toStator in the formula (2)The flux linkage is substituted into the rotor flux linkage to obtain the following formula (10):
2.4, substituting the transient expression (9) and the expression (10) of the stator flux linkage during the fault into the rotor voltage equation in the formula (1) to obtain a first order differential equation of the rotor voltage
And further obtaining a dynamic expression of sudden rise of the rotor current when the voltage of the gridconnected point drops as follows:
as can be seen from equation (3), the rate of change of the rotor current ramp during LVRT is related to the rotor voltage, the rotor angular velocity, and the magnitude of the rotor resistance. As shown in FIG. 1, the doublyfed wind turbine generator structure adopting the rotor series resistance is shown in FIG. 1, wherein R is_{rs}The currentlimiting resistor is a currentlimiting resistor in series with the rotor and bypasses the currentlimiting resistor. Before the grid voltage drops, the thyristor bypass switch is switched on, R_{rs}Is bypassed; after the fault occurs, the thyristor is turned off, R_{rs}Limiting the sudden rotor current in series on the rotor side.
Thirdly, according to the analysis of the formula (3), a rotor series resistance structure is added on the rotor side, and the rotor series resistance structure comprises: thyristor and currentlimiting resistor R connected in series with rotor_{rs}Before the grid voltage drops, the thyristor bypass switch is conducted, R_{rs}Is bypassed; after the fault occurs, the thyristor is turned off, R_{rs}Limiting the sudden rise of the rotor current in series on the rotor side;
fourthly, optimizing a low voltage ride through control strategy by using the transient power:
when the grid voltage has a drop fault, the controllability of the doublyfed motor is utilized to carry out DFIG control on the given value of the active power based on the rotor series resistance lowvoltage ridethrough structureOptimizing, increasing the optimization quantity P of the grid voltage drop depth in the active power_{1}And reducing the given value of active power so as to reduce the input energy of the DFIG during the LVRT period.
The above theoretical analysis shows that during LVRT, the rotor resistance R is removed_{rs}In addition, the rate of change of the sudden rise of the rotor current also corresponds to the rotor voltage u_{r}And rotor angular velocity omega_{r}It is related. On one hand, the rotating speed of the DFIG depends on the difference between the input power of the wind turbine and the output power of the DFIG, and as for the rotor series resistance LVRT, the doublyfed converter and the DFIG are in the normal working state in steadystate operation, so that stator flux linkage directional vector control, namely the control mode of a power outer ring and a current inner ring, is mostly adopted for DFIG control, and in order to utilize wind energy to the maximum extent, the given value P of active power is set_{1} ^{*}Typically at the point of maximum wind energy capture power. When the voltage of the power grid drops, the input active power of the wind turbine and the DFIG is kept unchanged, but the output power of the DFIG, namely the active power transmitted to the power grid, is reduced, and the rotating speed omega of the DFIG is caused by the unbalanced power difference between the two_{r}Increase, according to the formula (3), ω_{r}The increase in rotor current ramp rate increases. Meanwhile, the voltage of the intermediate direct current bus is increased due to energy imbalance between the doublefed converters. On the other hand, according to analysis of a DFIG stator flux linkage directional vector control algorithm, after active power and reactive power are decoupled, active components and reactive components of rotor current are determined by active power and reactive power of the DFIG respectively. This clearly increases the rotor current during a fault if the active power setpoint remains at the maximum wind energy capture point during rotor string resistance LVRT.
In conclusion, during the rotor series resistance LVRT, the active power set value controlled by the DFIG is optimized, so that the generation of LVRT excess power is reduced from the source, and thus, the rotor overcurrent and the dc bus overvoltage are suppressed.
The main idea of the transient power optimization control algorithm is as follows: when the grid voltage is in a drop fault, in order to maintain the power balance as much as possible, the DFIG active power given value can not keep the previous maximum value unchanged, an optimization quantity related to the grid voltage drop depth is added on the basis of the given value, the given value of the active power is actively reduced, and the DFIG input energy in the LVRT period is further reduced. The rotor string resistance transient power optimization control strategy is shown in fig. 2.
Optimization quantity P for increasing grid voltage drop depth in grid system_{1}The method specifically comprises the following steps:
P_{1}＝P_{1max}P_{LVRT}(5)
wherein, P_{1max}For the calculated power value based on maximum wind energy capture, P_{LVRT}The dynamic power feedforward quantity is used for reflecting the voltage drop of the power grid.
After the DFIG adopts a stator flux linkage directional vector control algorithm, stator active current i in d and q coordinate systems_{qs}Reactive current i_{ds}Decoupling control is achieved. The variation of the power in the LVRT period is reflected to the voltage drop amplitude, and the voltage drop value of the power grid and the voltage amplitude U in normal operation_{0}In comparison, when the falling amplitude does not exceed 10%, the LVRT dynamic power feedforward quantity P_{LVRT}Is 0, the active power of the stator is given by P_{1} ^{*}Maintaining maximum wind energy capture value P before failure_{1max}The change is not changed; when the voltage drops to 0.9U_{0}When the following, the dynamic power feedforward quantity P_{LVRT}Greater than 0, stator active power given P_{1} ^{*}Corresponding to the condition of P_{1max}On the basis of reducing P_{LVRT}After PI regulation, the stator active current given value i_{qs} ^{*}Reducing the rotor active current given value i_{qr} ^{*}And the input and output power of the DFIG is reduced, and the rotor current amplitude and the DC bus voltage increase during the LVRT period are reduced.
And step five, a transient reactive power optimization control strategy:
optimizing transient reactive power, transmitting the reactive power to a power grid, and optimizing the transient power of a rotor string resistor structure to obtain a rotor reactive current instruction limit value during LVRT (linear variable resistor) period as follows:
wherein,  i_{drmax}I is a rotor reactive current instruction limit value during LVRT; i.e. i_{qr} ^{*}Setting a rotor active current value; i is_{rotormax}The current limiting upper limit value of the rotor converter; u shape_{0}Stator voltage amplitude for steady state operation, I_{N}And (4) taking the minimum value among the three for the rated current of the unit, wherein min { } represents the minimum value.
The specific method comprises the following steps:
according to the optimization scheme, the upper limit value of the current limit of the rotor converter is set as I_{rotormax}Then the optimized rotor reactive current given value i_{dr} ^{*}The following conditions should be satisfied:
the technical specification requirements of accessing a largescale wind power plant to a power grid are as follows: during LVRT the DFIG should deliver some reactive power to the grid to facilitate rapid recovery of the faulty grid. According to requirements, the given value i of the reactive current of the DFIG stator during the fault period_{ds} ^{*}The following formula should be satisfied:
according to the daxis current relationship of the stator and the rotorTo obtain
The magnitude of the transmitted reactive power should also take into account the transient stability constraints of the DFIG. Based on the Lyapunov stability criterion, the necessary condition for keeping the transient stability of DFIG is
By combining (12), (14) and (15), the reactive current instruction limit value of the rotor during the transient power optimization LVRT based on the rotor series resistance can be obtained
Wherein,  i_{drmax}I is a rotor reactive current instruction limit value during LVRT; i.e. i_{qr} ^{*}Setting a rotor active current value; i is_{rotormax}The current limiting upper limit value of the rotor converter; u shape_{0}Stator voltage amplitude for steady state operation, I_{N}And (4) taking the minimum value among the three for the rated current of the unit, wherein min { } represents the minimum value.
In order to verify the method, the transient power optimizationbased LVRT control method for the rotor series resistance of the doublyfed wind power generation system is experimentally verified, and the method specifically comprises the following steps:
on the basis of a doublyfed wind power generation simulation platform and an impedance type voltage sag generator, transient power optimizationbased experimental research is carried out.
Rated power P of the doublyfed generator is 10 KW; number of pole pairs n_{p}3; frequency f is 50 Hz; stator connection mode Y connection, resistance R_{s}＝0.7Ω；L_{s}2.1 mH; the rotor is connected in a Yconnection mode, and after being converted to the stator side, the resistor R_{r}＝0.59Ω；L_{r}4.1 m; mutual inductance L_{m}72.6mH, DC bus voltage U_{dc}690V. A selfmade impedance type voltage drop generator is adopted to simulate the voltage drop fault of a power grid, the rotating speed of the DFIG before the fault is 917r/min, and the active power output by the stator side is 8.5 KW. The parameter of the PILVRT in the voltage drop of the power grid is K_{pL}＝0.251，K_{iL}1.524. The system control is realized by adopting a DSP TMS320F28335 chip of Ti company, and the experimental waveform is captured by a DPO 3054 oscilloscope of Tek company.
Fig. 3 and 4 show experimental waveforms of dc bus voltage, active power of stator and rotor, reactive power of stator and rotor, respectively, when the grid voltage drops to 50% and 30% before the fault, and the fault duration is 625 ms. It can be seen that during the optimization of the transient power of the rotor series resistance LVRT, the active power input by the DFIG is dynamically reduced, so that the voltage of the direct current bus only fluctuates when the voltage drops and recovers, and the maximum fluctuation amplitude value is not more than 7% of that in steadystate operation. Fig. 3(b), (c) and fig. 4(b), (c) show experimental active and reactive power waveforms generated by the DFIG stator. It can be seen that the active power is respectively reduced to about 5.5KW and 4.5KW from the maximum wind energy capture value of 8.5KW before the fault, the reactive power is respectively increased to about 2Kvar and 3.5Kvar from 0Kvar before the fault, and no secondary droop occurs in the fault voltage recovery process, which shows that the rotor series resistance transient power control strategy powerfully provides the reactive support required by the grid during the LVRT period, and promotes the rapid recovery of the grid fault voltage. And 3(d), (e) and 4((d), (e) show experimental waveforms of the stator current and the rotor current during LVRT, it can be seen that the active power set value of the DFIG is dynamically reduced along with the increase of the voltage drop depth, so that the current impact of the stator and the rotor does not exceed 1.1 times of the rated limit value, and the safety of the converter is ensured.
It will be appreciated by those skilled in the art that the foregoing embodiments are merely preferred embodiments of the invention, and thus, modifications, variations and other changes which may be made in the details of the abovedescribed embodiments by those skilled in the art may be made without departing from the spirit and scope of the invention.
Claims (3)
1. A doublyfed wind power rotor series resistance low voltage ride through control method based on power optimization is characterized by comprising the following steps:
firstly, establishing a DFIG mathematical model in a threephase ABC coordinate system:
in the formula: u. of_{s}、u_{r}Respectively are stator voltage vectors and rotor voltage vectors; i.e. i_{s}、i_{r}Respectively are stator current vectors and rotor current vectors;ψ_{s}、ψ_{r}respectively are stator flux linkage vectors and rotor flux linkage vectors; l is_{s}，L_{r}The selfinductance of the stator and the rotor is achieved; l is_{m}Is mutual inductance; r_{s}、R_{r}Respectively a stator resistor and a rotor resistor; omega_{s}Is the synchronous angular velocity; omega_{r}Is the rotor angular velocity;
and secondly, simulating fault occurrence in a DFIG mathematical model, and obtaining a stator flux transient expression during fault according to the principle that the DFIG stator flux can not be mutated before and after the DFIG stator flux falls, wherein the transient expression comprises the following steps:
in the formula:
thirdly, according to the analysis of the formula (3), when the grid voltage has a drop fault, on the basis of a rotor series resistance lowvoltage ridethrough structure, the controllability of a doublyfed motor is utilized to optimize the given value of the active power controlled by the DFIG, and the optimized quantity P of the grid voltage drop depth is increased in the active power_{1}Reducing the given value of active power, so that the input energy of the DFIG during the LVRT period is reduced;
the fourth step: optimizing transient reactive power, transmitting the reactive power to a power grid, and optimizing the transient power of a rotor string resistor structure to obtain a rotor reactive current instruction limit value during LVRT (linear variable resistor) period as follows:
wherein,  i_{drmax}I is a rotor reactive current instruction limit value during LVRT; i.e. i_{qr} ^{*}Setting a rotor active current value; i is_{rotormax}The current limiting upper limit value of the rotor converter; u shape_{0}Stator voltage amplitude for steady state operation, I_{N}And (4) taking the minimum value among the three for the rated current of the unit, wherein min { } represents the minimum value.
2. The doublyfed wind power rotor series resistance lowvoltage ridethrough control method based on power optimization according to claim 1, characterized in that in the third step, when grid voltage has a drop fault, the active power set value controlled by the DFIG is optimized by using the controllability of the doublyfed motor on the basis of a rotor series resistance lowvoltage ridethrough structure, and the optimization quantity P of grid voltage drop depth is increased in the active power_{1}The method specifically comprises the following steps:
P_{1}＝P_{1max}P_{LVRT}(5)
wherein, P_{1max}For the calculated power value based on maximum wind energy capture, P_{LVRT}A dynamic power feedforward quantity for reflecting the voltage drop of the power grid;
when the falling amplitude does not exceed 10%, LVRT dynamic power feedforward quantity P_{LVRT}Is 0, the active power of the stator is given by P_{1} ^{*}Maintaining maximum wind energy capture value P before failure_{1max}The change is not changed; when the voltage drops to 0.9U_{0}When the following, the dynamic power feedforward quantity P_{LVRT}Greater than 0, stator active power given P_{1} ^{*}Corresponding to the condition of P_{1max}On the basis of reducing P_{LVRT}After PI regulation, the stator active current given value i_{qs} ^{*}Reducing the rotor active current given value i_{qr} ^{*}And reducing and balancing the DFIG input and output power.
3. The doublyfed wind power rotor series resistance low voltage ride through control method based on power optimization according to claim 1, wherein the second step simulates fault occurrence in a DFIG mathematical model, and the obtaining of the transient expression of the stator flux linkage during fault according to the principle that the stator flux linkage of the DFIG before and after the DFIG falls can not be suddenly changed specifically comprises:
2.1 setting the stator voltage amplitude value to u in steadystate operation_{0}If the grid has a symmetrical fault with a drop depth of P when t is equal to 0, the stator voltage before and after the drop is written as follows:
2.2, neglecting the stator resistance according to the relationship between flux linkage and voltage, the stator steady state flux linkage before and after the fault is according to the equation (1):
2.3, assuming open rotor, i.e. i_{r}When the value is 0, the stator flux linkage differential equation obtained from the equations (1) and (2) is:
according to the principle that the stator flux linkage can not change suddenly before and after the voltage of the power grid drops, the solution of the stator flux linkage differential equation can be decomposed into two parts, one part is a stator flux linkage component rotating at a synchronous speed, and the amplitude is determined by the amplitude of the voltage of the stator; the other part is a stator flux linkage directcurrent component caused by sudden drop of the stator voltage, the directcurrent component is kept still in space and is attenuated by a time constant, and the transient expression of the stator flux linkage during the fault is obtained by combining the formula (7):
order toSubstituting the stator flux linkage in the formula (2) into the rotor flux linkage to obtain the formula (10) as follows:
2.4, substituting the transient expression (9) and the expression (10) of the stator flux linkage during the fault into the rotor voltage equation in the formula (1) to obtain a first order differential equation of the rotor voltage
And further obtaining a dynamic expression of sudden rise of the rotor current when the voltage of the gridconnected point drops as follows:
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