CN115085270B - Low-voltage ride through method and system for wind power converter - Google Patents

Low-voltage ride through method and system for wind power converter Download PDF

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CN115085270B
CN115085270B CN202210860610.3A CN202210860610A CN115085270B CN 115085270 B CN115085270 B CN 115085270B CN 202210860610 A CN202210860610 A CN 202210860610A CN 115085270 B CN115085270 B CN 115085270B
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power
current
voltage
converter
power grid
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CN115085270A (en
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张祯滨
李俊达
李�真
汪凤翔
时博
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Shandong University
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    • 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
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE 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

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  • Control Of Ac Motors In General (AREA)

Abstract

The invention belongs to the technical field of low-voltage ride through control of wind power generation systems, and provides a low-voltage ride through method and a system of a wind power converter. The method comprises the steps of obtaining voltage drop, and further determining a matched energy storage control strategy; the energy storage control strategy is as follows: when the voltage drop is smaller than a set threshold value, adopting a rotor energy storage strategy; when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on a power grid side, determining to predict power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor, distributing the power flowing through the unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; and for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side.

Description

Low-voltage ride through method and system for wind power converter
Technical Field
The invention belongs to the technical field of low-voltage ride through control of wind power generation systems, and particularly relates to a low-voltage ride through method and system of a wind power converter.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
The direct-drive permanent magnet synchronous wind power converter has the advantages of simple structure, low manufacturing cost, high power density, high efficiency, strong fault ride-through capability and the like of the converter, and is a main configuration type of a marine wind power generation system, as shown in fig. 1. The system comprises three control layers, namely an operation layer, a wind power system layer and a power converter control layer. The main control targets of the converter control layer are as follows: (1) And the motor side carries out maximum power tracking, so that the fan tracks and ensures the rotating speed of the maximum power, and a rotating speed outer ring is used. (2) The voltage of the direct current bus is ensured to be stable at the power grid side, so that the converter works normally, and a direct current bus voltage outer ring is used. And meanwhile, the neutral point voltage difference is controlled, so that the capacitance voltage balance is ensured.
However, when the grid voltage suddenly decreases, the grid voltage of the system decreases, and the grid-connected current of the converter cannot be increased too much, so that the output power of the fan converter decreases. Because the generated power of the fan cannot be changed in a short period, the generated power is larger than the power fed into the power grid, the energy in the converter can be increased, and the capacitor voltage is increased. In order not to damage the capacitor, only the fan can be cut off to cause further breakdown of the power grid. And a control means is required to be applied to carry out low-voltage ride through so that the fan keeps grid connection and outputs reactive power to support a power grid when the voltage drops. The traditional low-voltage ride through control method of the fan comprises two types of rotor energy storage and Chopper circuit unloading resistance unloading. However, these two types of conventional control methods have the following disadvantages: 1) The rotor stores limited energy, and it is difficult to cope with the national regulation grid voltage drop amplitude (drop to 80% of rated value for 0.625 seconds) by rotor energy storage alone. 2) The rotor energy storage can exchange control targets at the motor side and the power grid side, and the direct current bus voltage is controlled through the motor side, so that the rotating speed is uncontrolled, the rotating speed fluctuation is generated, and the direct current bus voltage is continuously fluctuated. 3) The Chopper circuit is led out of an unloading resistor loop through a direct current bus, energy is released through the unloading resistor, and a large amount of output power is converted into heat due to the fact that the fan power is large, so that the unloading resistor of the Chopper circuit is difficult to bear excessive power and cannot be unloaded for a long time. The super capacitor energy storage is used as a new low voltage ride through method, and has strong capacity of absorbing the output power of the fan; the duration of energy absorption is long; the energy absorbed can be released to the power grid after the crossing is finished, and the generated electric energy is not wasted. However, as voltage spikes exist in the capacitor energy storage and the voltage fluctuation of the bus can cause spike amplification, when the capacitor energy storage is matched with the rotor energy storage, the fluctuation of the rotating speed can cause the fluctuation of the voltage of the direct current bus, and the capacitor can amplify the fluctuation to generate severe direct current voltage vibration, so that the control is unstable.
Disclosure of Invention
In order to solve the technical problems that the conventional low-voltage control of the direct-drive permanent magnet synchronous wind power generation converter is difficult to operate in a large power for a long time and direct-current bus voltage vibration can occur due to combined control of a super capacitor and a rotor energy storage, the invention provides a low-voltage ride-through method and a system of a wind power converter, which comprehensively utilize rotor energy storage, an unloading resistor and super capacitor energy storage, respectively use different control strategies by judging the voltage drop degree, and comprehensively control the direct-current bus voltage through model prediction control of a motor-side converter, a power grid-side converter and an unloading resistor-super capacitor Chopper circuit, so as to prevent direct-current bus voltage vibration caused by the cooperation of the super capacitor energy storage and the rotor energy storage.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
the first aspect of the invention provides a low-voltage ride through method of a wind power converter, which comprises the following steps:
acquiring the voltage drop, and further determining a matched energy storage control strategy;
wherein the energy storage control strategy is:
when the voltage drop is smaller than a set threshold value, determining to adopt a rotor energy storage strategy;
when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on the power grid side, predicting power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor of the Chopper circuit, distributing the power flowing through the unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; and for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side.
In one embodiment, a machine side converter model predictive control cost function penalty is set in the process of predicting stator current at the motor side, and a current penalty for rated speed recovery is added to the machine side converter model predictive control cost function penalty.
As one embodiment, the current penalty term for rating the rotational speed includes a current control penalty term and a rotational speedA fluctuation punishment term, wherein the current control punishment term is the sum of squares of differences between stator currents in dq coordinate system and corresponding reference values, and the rotation speed fluctuation punishment term is stator current i q And the stator current i recovered to a given rotational speed at the next time of the motor-side inverter qmc Is the sum of the squares of the differences of (a).
According to the technical scheme, based on a motor dynamics equation, a model of the converter at the improved machine side predicts a penalty term of a control cost function, and the fluctuation of the rotating speed is reduced by controlling current deviation.
As one embodiment, on the grid side, a grid side converter model predictive control cost function penalty term is set and a current out-of-limit penalty term is added to prevent busbar voltage oscillation.
As an implementation manner, the grid-side converter model predictive control cost function further comprises a current controller penalty term and a capacitor neutral point voltage controller penalty term.
According to the technical scheme, a current out-of-limit penalty term is used on the network side, so that the voltage oscillation amplitude of the direct-current bus is further reduced.
As an implementation mode, PI power control and predictive control are adopted for controlling the super capacitor, and the power difference at the next moment predicted on the motor side power grid side is used as reference power to charge and discharge.
As one embodiment, the control of the unloading resistor adopts a direct current bus voltage model and the next time power difference predicted on the motor side power grid side to predict the power to be absorbed by the unloading resistor at the next time, and the power is modulated.
A second aspect of the present invention provides a low voltage ride through controller for a wind power converter, comprising:
a voltage sag acquisition module for acquiring a voltage sag size;
the energy storage control strategy determining module is used for determining a matched energy storage control strategy according to the voltage drop;
wherein the energy storage control strategy is:
when the voltage drop is smaller than a set threshold value, determining to adopt a rotor energy storage strategy;
when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on the power grid side, predicting power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor of the Chopper circuit, distributing the power flowing through the unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; and for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side.
A third aspect of the present invention provides a wind power converter low voltage ride through system, which comprises a wind power converter low voltage ride through controller as described above.
A fourth aspect of the invention provides a computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, the processor implementing the steps in a wind power converter low voltage ride through method as described above when the program is executed.
Compared with the prior art, the invention has the beneficial effects that:
(1) The method for low-voltage ride through of the wind power converter adopts a method of combining the super capacitor, the unloading resistor circuit and the rotor energy storage, can solve the low-voltage problems of different grades with high effect, adopts an unloading resistor-super capacitor Chopper circuit controlled by improved model prediction, modulates the unloading circuit by predicting the power of the unloading resistor and the super capacitor which enable current to recover to a normal value at the next moment, and reduces direct-current bus shake.
(2) According to the invention, through improved model predictive control, the motor power equation is used for controlling the rotational speed to return stably at the machine side, and the predicted current change rate is limited at the network side, so that the direct current bus shake is further reduced.
(3) The invention is based on the Chopper circuit comprising the unloading resistor and the super capacitor, and the unloading resistor and the super capacitor share and absorb the redundant power emitted by the fan, so that the unloading resistor is prevented from bearing excessive power and is independently unloaded for too long, the capacity of the required super capacitor is reduced, and the cost is reduced.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.
FIG. 1 is a Chopper circuit topology including an unloading resistor and a super capacitor;
FIG. 2 is a schematic diagram of a model predictive control strategy motor side flow chart of an embodiment of the invention;
FIG. 3 is a grid-side flow chart of the proposed model predictive control strategy in an embodiment of the present invention;
FIG. 4 is a flow chart of a control strategy of a Chopper circuit with an unloading resistor and a super capacitor according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of motor side model predictive control in accordance with an embodiment of the present invention;
FIG. 6 is a schematic diagram of a prediction control of a grid-side model according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of the predictive control of a Chopper circuit including an unloading resistor and a supercapacitor according to an embodiment of the present invention.
Detailed Description
The invention will be further described with reference to the drawings and examples.
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
Example 1
The embodiment provides a low-voltage ride through method of a wind power converter, which comprises the steps of obtaining voltage drop size and further determining a matched energy storage control strategy.
Wherein the energy storage control strategy is:
when the voltage drop is smaller than a set threshold value, determining to adopt a rotor energy storage strategy;
when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on the power grid side, predicting power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor of the Chopper circuit, distributing the power flowing through the unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; and for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side.
According to the method, rotor energy storage, unloading resistance and super capacitor energy storage are comprehensively utilized, and different control strategies are respectively used by judging the voltage drop degree. And when the voltage drop is smaller than a preset threshold value, the rotor is used for storing energy, and when the voltage drop is larger than or equal to the preset threshold value, three methods are comprehensively used. On the motor side, predicting stator current by using a motor electric model and a dynamic model, reducing voltage and rotation speed fluctuation of a direct current bus of the motor, and predicting power generation of the motor at the next moment; on a power grid side, predicting power grid current by using a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor, distributing the power flowing on the Chopper circuit unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; for super-capacitor, according to the predicted power sent by the machine side network side, the redundant power sent by most machine sides is born according to a certain proportion (such as 80%). The direct-current bus voltage is comprehensively controlled through model predictive control of the motor-side converter, the power grid-side converter and the unloading resistor-super capacitor Chopper circuit, and direct-current bus voltage oscillation caused by matching of super capacitor energy storage and rotor energy storage is prevented.
The main control objectives of the low voltage ride through method of the wind power converter of this embodiment can be summarized as follows: (a) The motor side is used for rapidly and accurately tracking a control instruction of the direct current bus voltage, the direct current bus voltage is controlled to be constant by using the motor side which is difficult to influence by low voltage ride through, and the multipurpose energy sent by the motor is converted into the kinetic energy of the fan rotor; (b) The control instructions of active power and reactive power are tracked rapidly and accurately on the power grid side, so that enough reactive power output is ensured to be used for maintaining the stability of the power grid during low voltage ride through, and meanwhile, the voltage balance of a neutral point is maintained; (c) And the bus unloading circuit is used for controlling the unloading resistors of the super capacitor and the Chopper circuit to absorb the multi-purpose energy transmitted from the machine side, ensuring the constant voltage of the direct current bus, ensuring the rotating speed not to exceed the rated rotating speed, and feeding the energy in the super capacitor back to the network side after the low voltage ride through is finished.
In the specific implementation process, the fan side adopts a PI direct current bus voltage outer ring and predicts the structure of the control inner ring. For predictive control of the inner ring, the control target includes current control, motor rotation speed fluctuation control. The power grid side adopts an active reactive power PI outer ring and predicts the structure of a control inner ring. For predictive control of the inner loop, control targets include current control, current out-of-limit control, and capacitor neutral voltage balancing. The handle comprises a Chopper circuit comprising an unloading resistor and a super capacitor, and adopts an overall control strategy combining hysteresis control and out-of-limit control, so that the rotating speed is not out-of-limit and the fluctuation is smaller under the condition of ensuring constant voltage of a direct current bus. And PI power control and predictive control are adopted for controlling the super capacitor, and the power difference at the next moment predicted by the power grid side at the motor side is used as reference power for charging and discharging. And the control of the Chopper circuit unloading resistor adopts a direct current bus voltage model and the next time power difference predicted by the motor side power grid side to predict the power which the unloading resistor should absorb at the next time, and the power is modulated.
For example: when the super capacitor is charged, the unloading resistor performs unloading sharing to send out redundant power by the fan, so that the capacity of the required capacitor is reduced, and the cost is reduced; the capacitor charging time is prolonged, the unloading power of the unloading resistor on the Chopper circuit and the independent unloading time of the unloading resistor when the super capacitor is fully charged are reduced, and the burning out of the unloading resistor is prevented. According to the model predictive control method, a current penalty item for recovering the rotating speed to a rated value is added on the motor side according to a motor power model; the current out-of-limit penalty term is added on the power grid side, so that bus voltage oscillation caused by overlarge current fluctuation is prevented; the super capacitor side predicts the power sent by the motor side and the power grid side at the next moment, and takes 80% of the predicted value of the power sent by the fan at the next moment as reference power; the unloading resistor predicts the absorption power of the unloading resistor at the next moment by taking the influence of the kinetic energy of the fan rotor and the fluctuation of the DC bus voltage into consideration while taking 20% of the predicted value of the multiple emitted power of the fan at the next moment as basic reference power through the DC bus voltage model.
In the process of predicting the stator current at the motor side, a current penalty for recovering the rotational speed to a rated value is increased.
The current penalty term for recovering the rotating speed to the rated value comprises a current control penalty term and a rotating speed fluctuation penalty term, wherein the current control penalty term is the sum of squares of differences between stator currents in dq coordinate system and corresponding reference values, and the rotating speed fluctuation penalty term is stator current i q And the stator current i recovered to a given rotational speed at the next time of the motor-side inverter qmc Is squared with the difference of (2). According to the technical scheme, based on a motor dynamics equation, a side penalty term is improved, and the rotation speed fluctuation is reduced by controlling current deviation.
The motor side low voltage ride through specific control steps are shown in fig. 2 and 5:
step 1: sensor sample motor side current I phm Motor stator flux linkage angle θ, motor rotational speed ω m DC bus voltage U dc 。
Step 2: the motor-side current is converted into a stator current dq-axis component by park transformation. Comparing the DC bus voltage with a reference and sending the comparison result to a PI controller to obtain q-axis current i q Reference is made to the following.
Step 3: the prediction controller predicts according to the current and the voltage at the k moment to obtain a dq axis component i of the current at the motor side d 、i q A value at time k+1.
Step 4: and calculating various penalty parameters of the next control period converter in different switching vector states.
Step 5: and performing cost function calculation, wherein the penalty items of the control target comprise:
(1) The current control penalty is the stator current i in the dq coordinate system d And i q 。i q Is obtained from the torque error by controlling i q The torque and thus the rotational speed can be controlled to reach the reference rotational speed. i.e d Then the control is set to 0 according to the maximum torque current. The two control targets are combined into one, the priority is highest, and the cost function is as follows:
Figure BDA0003758314260000091
wherein i is d * And i q * Is a reference value for the stator current in the dq coordinate system.
(2) A rotational speed fluctuation punishment term is set as a stator current i q And the stator current i recovered to a given rotational speed at the next time of the motor-side inverter qmc Is a difference in (c). During low voltage ride through, the rotor energy storage will be difficult to control the rotational speed, and unstable rotational speed results in stator current i d And i q Fluctuation occurs, resulting in dc bus voltage oscillation. Thus, the mechanical torque T can be calculated according to the dynamics model (formula (3)) t In the known case, the next time the rotational speed is brought back to the nominal rotational speed is deduced
Figure BDA0003758314260000101
Required electromagnetic fieldTorque T e The magnitude of the stator current i required for the electromagnetic torque at time (k+1) can be predicted according to equation (6) qmc Is of a size of (a) and (b). By adding the stator current i to the cost function q Stator current i recovered to a given rotational speed with a motor-side inverter qmc The penalty term for the difference of (2), then the cost function is:
J im ＝(i q -i qmc ) 2 , (2)
wherein, the rotation speed is restored to the rated rotation speed at the next moment
Figure BDA0003758314260000102
Required electromagnetic torque T e The method can be obtained by a motor dynamics model:
Figure BDA0003758314260000103
wherein J is motor rotational inertia, and the initial values of the rotational speed and the electromagnetic torque are the rotational speed and the electromagnetic torque when the motor is just in a low-pass state, so that the motor rotational inertia is obtained by performing backward Euler discretization:
Figure BDA0003758314260000104
wherein T is s For the control period, the mechanical torque is given, the suffix (k+1) represents the predicted value of the variable at the next moment, and the suffix (k) represents the value of the variable at the current moment. The model for obtaining the current reference by electromagnetic torque is:
T e (t)＝N p ψ pm i q (5)
wherein N is p Is the pole pair number of the motor, psi pm Is the magnetic flux of the permanent magnet. Then it is possible to obtain:
Figure BDA0003758314260000105
the total cost function is:
Figure BDA0003758314260000106
where α is a weight coefficient for controlling the importance of two control targets. Traversing all the switch vectors, and selecting the switch vector with the smallest total cost function in the selected switch vectors as the optimal switch vector.
Step 6: and (3) the selected optimal switching vector is driven out in the next control period, and the motor side converter is controlled. Outputting the active power predicted value P sent by the fan at the next moment under the action of the current selected optimal switching vector mc 。
In a specific implementation process, a current out-of-limit penalty term is added to the power grid model so as to prevent busbar voltage oscillation. The current out-of-limit penalty term comprises a current controller penalty term, a capacitor neutral point voltage controller penalty term and a current oscillation amplitude of the power grid side converter. According to the technical scheme, the current overrun penalty term is used on the network side, so that the voltage oscillation amplitude of the direct-current bus is further reduced.
Grid-side converter control steps, as shown in fig. 3 and 6:
step 1: the sensor adopts the voltage V at the power grid side ph And current I ph DC bus voltage V of back-to-back converter dc And the voltage difference V of the two capacitors dc12 And calculating the active power P and the reactive power Q sent by the power grid side.
Step 2: and converting the current phase voltage at the power grid side into an alpha beta axis coordinate system through clark conversion. The active power P, the reactive power Q and the reference active and reactive power are compared and sent to a PI controller to obtain a reference dq axis current reference i dref And i qref 。
Step 3: current prediction is carried out according to the alpha beta axis component of the existing current and voltage, and the alpha beta axis component i of the current at the power grid side is obtained α 、i β At the time of k+1, predicting the alpha beta axis component V of the power grid side voltage α 、V β At the time k+1, the voltage difference between the two capacitors of the DC bus is predicted at the time k+1.
Step 4: and predicting various penalty parameters of the next control period converter in different switching vector states.
Step 5: and performing cost function calculation, wherein the penalty items of the control target comprise:
(1) The penalty of the current controller is the grid side current i in dq coordinate system d And i q 。i d The reference is obtained by controlling the outer ring by the DC bus voltage by controlling i d Control of the dc bus voltage can be achieved. i.e q The reference value is set to 0, and the power factor at the power grid side is ensured to be 1. The two control targets are combined into one, the priority is highest, and the cost function is as follows:
Figure BDA0003758314260000111
wherein i is d * And i q * Is a reference value for grid-side current in dq coordinate system.
(2) The capacitive neutral point voltage controller penalty is the capacitive neutral point voltage imbalance of the back-to-back converter. For neutral point clamping type three-level converter, under normal working condition, the upper voltage-equalizing capacitor C and the lower voltage-equalizing capacitor C are used 1 、C 2 The voltages of the upper bridge arm and the lower bridge arm should be equal, and the maximum voltage born by the switching tube of the upper bridge arm and the lower bridge arm is the voltage V of the direct current bus dc Half of (a) is provided. However, under certain working conditions, the neutral point voltage can deviate, so that the waveform of the output voltage is distorted, and when the deviation is serious, the switching tube breaks down. It is therefore necessary to ensure the capacitor voltage balance by means of a control algorithm. The cost function is as follows:
J V ＝(V c1 -V c2 ) 2 , (9)
wherein V is c1 And V c2 Is a DC bus upper and lower voltage-equalizing capacitor C 1 、C 2 Is set in the above-described voltage range.
(3) Current oscillation amplitude of the grid-side converter. To ensure that the voltage fluctuation of the direct current bus does not affect the output power of the network side, the current i of the dq axis of the network side is added into the cost function d And i q Penalty term for fluctuation, if network sideIf the dq axis current exceeds the maximum and minimum limit values, a penalty term needs to be set to enable the current fluctuation to be within a certain range, and for the priority control target, the cost function is as follows:
Figure BDA0003758314260000121
wherein i is qmax 、i dmax Is i d And i q Upper limit of oscillating current, i qmin 、i dmin Is i d And i q Is lower than the oscillation current. If i q Greater than i qmax Or less than i qmin ，i d Greater than i dmax Or less than i dmin Then the penalty term is taken as the portion thereof that exceeds the specified upper and lower limits.
The total cost function is:
Figure BDA0003758314260000122
where α and β are weight coefficients for controlling the overall extent of the two control targets. Traversing all the switch vectors, and selecting the switch vector with the smallest total cost function in the selected switch vectors as the optimal switch vector.
Step 6: and (3) the selected optimal switching vector is driven out in the next control period, and the motor side converter is controlled. Outputting the current selected optimal switching vector, and inputting the active power predicted value P of the power grid at the next moment gc 。
The topological structure diagram of the Chopper circuit including the unloading resistor and the super capacitor in the embodiment is shown in fig. 1, and the control flow diagrams are shown in fig. 4 and fig. 7.
The IGBT tube label of the unloading circuit is shown in figure 1, wherein R2 is a Chopper circuit resistor, C1 is a super capacitor, R1 is the internal resistance of the super capacitor, and L1 is a flat wave inductor. The switching tubes 1 and 2 control the super capacitor loop, the switching tube 1 is turned on, the switching tube 2 is turned off, the super capacitor loop works in the buck converter mode, and the super capacitor is charged. The switching tube 2 is turned on, the switching tube 1 is turned off, the super capacitor works in a boost converter mode, and the super capacitor discharges. The switching tube 3 controls the switching of the resistor loop, if the switching tube 3 is switched on, R2 is connected into the loop, and the resistor loop works; the switching tube 3 is opened and the resistor loop is not operated.
The control method thereof is described below.
The control strategy of the unloading circuit topology is divided into two parts, namely an operation control part and an out-of-limit control part. The operation control part ensures that the unloading circuit operates efficiently, and the out-of-limit control part ensures that the motor rotation speed, the DC bus voltage and the capacitance current are not out-of-limit. Specific control steps of the operation control section and the out-of-limit control section are described, respectively, and a control flow chart is shown in fig. 5.
At the initial moment, the switching tubes 1, 2, 3 are all turned off.
An operation control section:
1) And if the power grid voltage is in a low voltage ride through state and is smaller than 90% of the rated voltage but larger than 80% of the rated voltage, the unloading circuit does not work.
2) If the voltage of the direct current bus exceeds 5% in an out-of-limit mode and is in a severe low-voltage ride through state (the voltage of a power grid is reduced to below 80% of a rated value), the super capacitor does not reach the highest voltage, the super capacitor is charged, and the unloading resistor is blocked. The switching tube 1 can be opened, the switching tube 2 is closed, and the switching tube 3 is closed.
3) If the voltage of the direct current bus is reduced to be less than 1% in an out-of-limit mode, the system exits from a severe low-voltage ride through state (the power grid voltage is greater than 80% of the rated value), the super capacitor is not reduced to the lowest voltage, the super capacitor discharges, and the unloading resistor is blocked. The switching tube 1 is turned off, the switching tube 2 can be turned on, and the switching tube 3 is turned off.
4) If the super capacitor reaches the highest voltage (full charge) and the voltage of the direct current bus exceeds 5%, the unloading resistor of the chopper circuit independently absorbs all the redundant power emitted by the fan. The switching tubes 1, 2 are turned off, and the switching tube 3 can be turned on.
An out-of-limit control section:
1) And if the voltage of the super capacitor is lower than the minimum voltage, charging the super capacitor. The switching tube 1 can be opened, the switching tube 2 is closed, and the switching tube 3 is closed.
2) If the rotating speed of the permanent magnet synchronous generator exceeds the rated rotating speed, and if the super capacitor reaches the highest voltage, the unloading resistor of the chopper circuit independently absorbs the redundant power generated by the motor side, and the rotating speed of the motor is reduced. The switching tubes 1, 2 are turned off, and the switching tube 3 can be turned on.
3) If the rotating speed of the permanent magnet synchronous generator exceeds the rated rotating speed, and if the super capacitor does not reach the highest voltage, the super capacitor is charged, and most of redundant power generated by the motor side is absorbed; the dump resistor of the Chopper circuit absorbs a small portion of the excess power emitted from the motor side. In this time, the super capacitor and the unloading resistor share the redundant power sent by the fan, 80% of the redundant power is absorbed by the super capacitor, and the unloading resistor is set on the basis of absorbing 20% of the redundant power and considering the basis of recovering the rotating speed to the reference value. The switching tube 1 can be opened, the switching tube 2 is closed, and the switching tube 3 can be opened.
4) If the voltage of the direct current bus exceeds 6%, emergency unloading is carried out, the super capacitor is blocked, and the super capacitor is prevented from being damaged by overvoltage, and the unloading resistor independently absorbs all the redundant power emitted by the fan to stabilize the voltage peak of the capacitor. The switching tubes 1, 2 are turned off, and the switching tube 3 can be turned on.
5) And if the discharge current of the super capacitor exceeds the rated value, locking the super capacitor, enabling the super capacitor to be communicated with the unloading resistor to temporarily freewheel, and stabilizing the current peak. The switching tubes 1, 2 are turned off, and the switching tube 3 can be turned on.
6) If the voltage drop of the direct current bus exceeds 6%, the direct current bus is in a motor rotation speed rising period, the super capacitor is locked when a large amount of direct current bus energy flows out, the super capacitor is prevented from being absorbed by the energy, and the switching tubes 1, 2, 3, 4 and 5 are all turned off.
The specific control steps of the charging and discharging of the super capacitor and the specific control steps of the unloading resistor are described below.
Super capacitor charge control step, as shown in fig. 7:
step 1: sensor sampling super capacitor charging power P cc The active power predicted value P sent by the fan at the next moment and transmitted by the converter controller at the motor side is received mc And grid-side converter controlThe active power predicted value P of the power grid is input at the next moment transmitted by the device gc 。
Step 2: calculating the reference charging power of the super capacitor through a formula (12)
Figure BDA0003758314260000151
Charging super capacitor with power P cc And comparing the duty ratio with a reference, sending the duty ratio to a PI controller to obtain the duty ratio of the PWM modulator, and inputting the duty ratio to the PWM modulator to control the switching tube 1 to be switched on and off.
Figure BDA0003758314260000152
The super capacitor discharge control step is as shown in fig. 7:
step 1: sensor sampling super capacitor discharge power P ccf And comparing the duty ratio with the fixed reference power, sending the duty ratio into a PI controller to obtain the duty ratio of the PWM modulator, and inputting the duty ratio into the PWM modulator to control the switching tube 2 to be switched on and off.
The step of controlling the unloading resistance of the Chopper circuit is shown in fig. 7:
step 1: sensor sampling chopper circuit charging power P r The active power predicted value P sent by the fan at the next moment and transmitted by the converter controller at the motor side is received mc And the active power predicted value P of the power grid is input at the next moment transmitted by the power grid side converter controller gc 。
Step 2: calculating the reference unloading power of the unloading resistor through a formula (15)
Figure BDA0003758314260000153
Referencing the unloading resistor to the unloading power
Figure BDA0003758314260000161
And the active power P absorbed by the unloading resistor under the voltage of the direct current bus at the next moment rm Comparing (P) rm The calculation formula of (a) is shown as formula (16)), the duty ratio d of the PWM modulator is obtained, and the input PWM modulator controls the switching tube 3 to be switched on and off.
The calculation formula of the DC bus voltage is as follows:
Figure BDA0003758314260000162
wherein U is dc Is the DC bus voltage. The initial value was set to 0.25 times using the backward Euler method
Figure BDA0003758314260000163
Discretizing can be carried out to obtain:
Figure BDA0003758314260000164
wherein T is s For controlling the period, C is the capacitance of the super capacitor, the suffix (k+1) represents the variable as the predicted value at the next moment, and the suffix (k) represents the variable as the value at the current moment. Wherein P is mc 、P gc 、
Figure BDA0003758314260000165
Has calculated U dc (k+1) is set as the target DC bus voltage value U dc * ，U dc (k) For the current DC bus voltage value U dc . Reference unload power of unload resistor>
Figure BDA0003758314260000166
The method comprises the following steps:
Figure BDA0003758314260000167
active power P absorbed by unloading resistor under direct current bus voltage at next moment rm The method comprises the following steps:
Figure BDA0003758314260000168
the duty cycle d of the PWM modulator is:
Figure BDA0003758314260000169
aiming at the direct-drive permanent magnet synchronous wind power converter system, the embodiment provides a low-voltage ride through comprehensive control method based on the Chopper circuit topology comprising an unloading resistor and a super capacitor and improved model predictive control, and the defects of the method when the method is used independently can be reduced by integrating the rotating speed control, the super capacitor and the Chopper circuit resistance unloading, and the problem of direct-current bus voltage oscillation when the rotating speed control and the super capacitor are used in a combined mode is avoided by using the improved model predictive control.
The benefits produced by this embodiment are significant, as detailed below:
the method comprehensively uses three low-voltage ride-through methods of rotating speed control, super capacitor and unloading resistor, and can use different methods when low-voltage ride-through with different amplitudes, and the advantages of the three methods are considered.
The method is based on a Chopper circuit comprising an unloading resistor and a super capacitor, the unloading resistor and the super capacitor share and absorb the redundant power emitted by a fan, the unloading resistor is prevented from bearing excessive power and is independently unloaded for too long, the capacity of the required super capacitor is reduced, and the cost is reduced.
The improved model predictive control method is characterized in that the rotating speed fluctuation is represented by current at the machine side by improving a punishment item, the rotating speed fluctuation is incorporated into the punishment item, and the rotating speed fluctuation is reduced; and taking the current overrun condition into a punishment term at the network side, and reducing output power fluctuation.
The improved model prediction control method is characterized in that by means of prediction control, the power setting of the unloading resistor is divided into a part of multiple power of the fan at the next moment, the energy required by predicting the voltage regression rated value of the direct current bus at the next moment is considered, the absorption power of the unloading resistor is comprehensively considered, and the fluctuation of the direct current bus voltage can be reduced besides the absorption of the multiple power of the fan.
Example two
The embodiment provides a wind power converter low voltage ride through controller, which comprises:
a voltage sag acquisition module for acquiring a voltage sag size;
the energy storage control strategy determining module is used for determining a matched energy storage control strategy according to the voltage drop;
wherein the energy storage control strategy is:
when the voltage drop is smaller than a set threshold value, determining to adopt a rotor energy storage strategy;
when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on the power grid side, predicting power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor of the Chopper circuit, distributing the power flowing through the unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; and for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side.
It should be noted that, each module in the embodiment corresponds to each step in the first embodiment one to one, and the implementation process is the same, which is not described here.
Example III
The embodiment provides a low voltage ride through system of a wind power converter, which comprises the low voltage ride through controller of the wind power converter in the second embodiment.
Example IV
The embodiment provides a computer device, which comprises a memory, a processor and a computer program stored in the memory and capable of running on the processor, wherein the processor executes the program to realize the steps in the low voltage ride through method of the wind power converter according to the embodiment.
The present invention is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each flow and/or block of the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. The low-voltage ride through method for the wind power converter is characterized by comprising the following steps of:
acquiring the voltage drop, and further determining a matched energy storage control strategy;
wherein the energy storage control strategy is:
when the voltage drop is smaller than a set threshold value, determining to adopt a rotor energy storage strategy;
when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on the power grid side, predicting power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the Chopper circuit, distributing the power flowing through an unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side;
for the super capacitor, the distribution power is as follows: sensor sampling super capacitor charging power P cc Receiving the next-time wind transmitted from the motor-side converter controllerThe predicted value of the active power sent by the machine is P mc And the active power predicted value of the input power grid at the next moment transmitted by the power grid side converter controller is P gc :
Figure FDA0004230395760000011
Calculating the reference charging power of the super capacitor through a formula (12)
Figure FDA0004230395760000012
Charging super capacitor with power P cc And reference charging power->
Figure FDA0004230395760000014
Comparing and sending the signals to a PI controller to obtain the duty ratio of a PWM modulator, and inputting the duty ratio into the PWM modulator to control the switching tube 1 to be switched on and off;
for the Chopper circuit, the distributed power of the unloading resistor is as follows: sensor sampling chopper circuit charging power P r The active power predicted value P sent by the fan at the next moment and transmitted by the converter controller at the motor side is received mc And the active power predicted value P of the power grid is input at the next moment transmitted by the power grid side converter controller gc ；
Figure FDA0004230395760000013
Calculating the reference unloading power of the unloading resistor through a formula (15)
Figure FDA0004230395760000015
Reference the unload resistance to unload power>
Figure FDA0004230395760000021
And the active power P absorbed by the unloading resistor under the voltage of the direct current bus at the next moment rm Comparing to obtain duty ratio d of PWM modulator, and inputting PWM modulator control switchThe pipe 3 is broken;
u in dc Is the voltage of a direct current bus, T s For controlling period, C is the capacitance of the super capacitor, U dc * For the target value of the dc bus voltage,
Figure FDA0004230395760000022
and (5) charging power for the reference of the super capacitor.
2. The low voltage ride through method of a wind power converter according to claim 1, wherein a machine side converter model predictive control cost function penalty is set in the process of predicting stator current at a motor side, and a current penalty for rated speed recovery is added to the machine side converter model predictive control cost function penalty.
3. The method for low-voltage ride through of a wind power converter according to claim 2, wherein the current penalty term for rating the rotational speed comprises a current control penalty term and a rotational speed fluctuation penalty term, the current control penalty term is the sum of squares of differences between stator currents in dq coordinate system and corresponding reference values, and the rotational speed fluctuation penalty term is stator current i q And the stator current i recovered to a given rotational speed at the next time of the motor-side inverter qmc Is the sum of the squares of the differences of (a).
4. The low voltage ride through method of a wind power converter according to claim 1, wherein a grid-side converter model predictive control cost function penalty term is set on a grid side and a current out-of-limit penalty term is added to prevent bus voltage oscillation.
5. The method for low voltage ride through of a wind power converter of claim 4, wherein the grid-side converter model predictive control cost function further comprises a current controller penalty term and a capacitive neutral point voltage controller penalty term.
6. A wind power converter low voltage ride through controller configured to perform a wind power converter low voltage ride through method as recited in any one of claims 1-5, comprising:
a voltage sag acquisition module for acquiring a voltage sag size;
the energy storage control strategy determining module is used for determining a matched energy storage control strategy according to the voltage drop;
wherein the energy storage control strategy is:
when the voltage drop is smaller than a set threshold value, determining to adopt a rotor energy storage strategy;
when the voltage drop is greater than or equal to a set threshold value, predicting stator current by adopting a motor electric model and a dynamic model at the motor side, and predicting power emitted by the motor at the next moment; on the power grid side, predicting power grid current by adopting a power grid model, controlling the power grid current change not to exceed a current limit value, and predicting power emitted by the power grid side at the next moment; for the unloading resistor of the Chopper circuit, distributing the power flowing through the unloading resistor according to the direct-current bus voltage model and the predicted power sent by the machine side network side; and for the super capacitor, redundant power sent by the machine side is born according to a set proportion according to the predicted power sent by the machine side network side.
7. A wind power converter low voltage ride through system comprising the wind power converter low voltage ride through controller of claim 6.
8. A computer device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, wherein the processor performs the steps in the wind power converter low voltage ride through method of any of claims 1-5 when the program is executed.
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