KR20110100692A - Control system of circulating current in a wind power system driven by 3-parallel back-to-back converters using a power theory - Google Patents

Abstract

The present invention is a cyclic current compensation method using the PQR power theory of a wind power generation system driven by a parallel back-to-back converter, circulating current by applying the PQR power theory to a wind power generation system driven by a parallel back-to-back converter Compensation is made, and through this, it is possible to control to reduce the circulating current generated by parallel operation, and as the capacity of wind power facilities is gradually increased with the growth of renewable energy, the problem of rating of the system in three parallel operation To reduce the rating of the back-to-back converter, the power semiconductor switching element, and to facilitate the design of the grid-side filter through the control of the circulating current. Will be reduced.

Description

Control system of Circulating Current in a Wind Power System driven by 3-parallel Back-to-Back Converters using a power theory

The present invention is a parallel operation system of a wind power generator driven by a parallel back-to-back converter using a permanent magnet synchronous generator, the PQR power theory to compensate for the circulating current, through which parallel operation The present invention relates to a circulating current compensation system using the PQR power theory of a wind power generation system driven by a parallel back-to-back converter that can reduce the circulating currents generated.

Wind power is a pollution-free energy source in the natural state and is the most economical energy source among alternative energy sources. The wind power generation system refers to a power generation system that converts wind energy into mechanical energy using various types of windmills, and generates electric power by driving a generator with this mechanical energy.

Since the wind power generation system uses the wind power of infinite clean energy as the power source, it is a pollution-free power generation method without problems such as heat pollution, air pollution, and radioactive leakage due to heat generation, unlike the existing power generation method using fossil fuel or uranium.

Accordingly, wind power is currently recognized as the most powerful alternative energy source, and the use of this wind power generation system is increasing worldwide because it is easy to operate and manage due to its simple structure or installation and easy operation and management. Is in.

The medium and large capacity wind power generation systems are the same as the power generation capacity of wind turbines and dual fed induction machines employing converters with capacities smaller than those of wind turbines. It is divided into Permanent Magnet Synchronous Generator (PMSG) with a capacity converter, and all of them adopt variable rotor speed type.

On the other hand, in the field of wind power generation, which is rapidly developing in recent years due to the seriousness of environmental problems, a parallel back-to-back converter using the permanent magnet synchronous generator, which is the variable speed driving method having high efficiency, high unit weight, torque per volume, and output power, is used. The demand for wind power systems driven by wind turbines is increasing rapidly.

However, in the wind power generation system driven by the parallel back-to-back converter using the permanent magnet synchronous generator, the design problem of the power converter has emerged as the capacity per unit gradually increases in MW. As in 1, the back-to-back converter 100 including the first converter 101 on the generator side and the first inverter 102 on the grid side as a power converter is directly connected between the generator and the grid 300. Therefore, it is necessary to select a switch corresponding to the rating of the system and a filter 301 to filter out harmonics.

Here, the first converter 101 and the first inverter 102 are each composed of a plurality of semiconductor switching elements, for example, an insulated gate bipolar transistor (IGBT).

That is, the power converter has a variable voltage and / or velocity due to a wind speed (or flow rate) characteristic having a variable characteristic when the mechanical energy recovered from the blade (or turbine, aberration) 200 is converted into electrical energy through the generator 201. It produces low-quality energy with variable frequency characteristics, which is refined into high-quality energy with constant voltage and constant frequency. For this purpose, AC-DC- converts AC to DC and then back to AC as shown in FIG. It has a structure of a back-to-back converter 100 having a built-in AC conversion function, the first converter 101 on the generator side and the first inverter on the grid side configured in the back-to-back converter 100 ( 102 is to operate in three parallel each to reduce the capacity each converter will bear 1/3.

In this case, as shown in FIG. 1, power for driving the first converter 101 on the generator side and the first inverter 102 on the grid side, respectively, included in the back-to-back converter 100 in three parallel operations. The converter is a back-to-back converter (100) for the grid-side current controller to send all the power transmitted to the DC terminal through the first converter 101 of the generator side to the grid 300 side through the three-phase reactance (301) The power factor is controlled so that the reactive power outputted to the system 300 is 0 while controlling the DC terminal voltage of the power supply unit 300 at a constant value. The efficiency of the overall power system is reduced by using a high efficiency low capacity converter instead of a high capacity converter. This system has advantages in terms of accident occurrence, maintenance, and maintenance while sharing power by improving the dynamic characteristics and load response characteristics of the system. It is a parallel external control method that measures and controls parallel external current to minimize the number of fish.

However, the parallel external control method for measuring the parallel external current as described above and performing the control operation is generally performed in the parallel connected power converter as shown in FIG. Since there is no potential difference between the parallels, no circulating current (ic) occurs.However, when the potential difference occurs due to external conditions such as impedance difference and switching difference between parallel branches (part A of FIG. 2), the current unbalance as well as the image Due to the current, a circulating current ic circulating in the parallel interior of the grid-side first inverter 102 is generated, and this current unbalance and circulating current ic are the amount of circulating current in addition to the rated current generated when the system is operated near the rated voltage. The first converter 101 and the first inverter 102 of the parallel back-to-back converter 100, which is a designed power switching device, are There is a problem that adversely affects the wind power generation system.

In addition, the parallel external control has a problem in that it is impossible to control the current unbalance and the circulating current ic that can occur in parallel because the parallel external control measures each parallel switch equally.

Accordingly, the present invention is to improve the conventional problems as described above, the parallel back-to-back converter is used a parallel internal control method that can independently control each parallel converter and inverter by measuring the parallel internal current By applying the PQR power theory to the wind power system driven by the wind turbine, the circulating current compensation is made, and the wind power driven by the parallel back-to-back converter enables the control to reduce the circulating current generated by the parallel operation. The purpose is to provide a circulating current compensation system using the system's PQR power theory.

The circulating current compensation system using the PQR power theory of the wind power generation system driven by the parallel back-to-back converter for achieving the above object, the generator generates three-phase AC power of variable frequency from the rotation of the blade, The generated three-phase AC power is converted into DC power by the converter of the back-to-back converter, and the converted power of DC is converted into three-phase AC power of fixed frequency by the inverter of the back-to-back converter. In the wind power generation system for supplying to the system through a three-phase reactance, and connecting the capacitor between the converter and the inverter to perform the role of a filter for storing and transmitting the DC power converted through the converter and removing harmonics, At the output of the inverter, a phase angle detector, a current controller, a plurality of coordinate converters for converting axes of the stationary coordinate system and the synchronous coordinate system, and PWM Connect an inverter control unit including a living unit, and calculates a compensation component of the circulating current generated by the inverter using the three-phase current and voltage output from the inverter control unit between the output terminal of the inverter and the inverter control unit, The circulating current control unit is configured to provide the calculated compensation component to the current controller to control the circulating current.

The cyclic current controller may include a pqr-coordinate conversion unit configured to calculate current values in a p-q-r coordinate axis by converting current and voltage of each phase into currents and voltages in a p-q-r coordinate axis; A filter unit extracting an ac component from a current value in the p-q-r coordinate axis calculated from the pqr-coordinate transformation unit; A compensation current calculator configured to control the current of the r-axis to zero from the current of the ac component extracted by the filter unit, and control the current of the extracted ac component to the current of the dc component from which noise is removed; And a current control unit converting the compensation current calculated from the compensation current calculating unit into a current of the d-q-n axis, which is a synchronous coordinate axis of the current controller, so as to control the circulating current in the inverter, and adding the current to the command value of the current controller. It includes.

In addition, the circulating current compensation method implemented by the circulating current compensation system using the PQR power theory of the wind power generation system driven by the parallel converter, three-phase AC power to DC power in the back-to-back converter, and A first step of converting three-phase current and voltage of the electric power into current and voltage of the pqr coordinate axis when a cyclic current due to a potential difference occurs in a multi-level generator-side converter or grid-side inverter that converts DC power into three-phase AC power; A second step of extracting ac components after calculating the three-phase current and voltage of the p-q-r coordinate axis in the first step; A third step of detecting whether an image current due to an unbalanced component is included in the p-q-r coordinate axis current and voltage of the ac component extracted from the second step; A fourth step of calculating a compensation component for removing the image split current when the detection result of the third step includes the image split current; A fifth step of converting the compensation component calculated from the fourth step into a compensation current of an o-α-β stationary coordinate system through an inverse conversion process; The converted compensation current is converted into the current of the dqn synchronous coordinate system of the current controller and then added to the command value of the current controller. A sixth step of controlling and attenuating circulating currents circulated in the multilevel inverter; It includes.

In addition, the first step may include a seventh step of converting three-phase current and voltage of the a-b-c coordinate system into three-phase current and voltage of the o-α-β static coordinate system; And an eighth step of converting the three-phase current and voltage of the o-α-β stationary coordinate system into three-phase current and voltage of the p-q-r coordinate system. It includes.

In addition, the fourth step, when the three-phase current is in an equilibrium state, the r-axis current component does not exist, the image component is present in the system due to the unbalanced component, the image component, A ninth step of controlling the current on the r-axis to zero by calculating an additional compensation component for removing the current; And a tenth step of calculating a compensation component by controlling a current of the p-axis and the q-axis to a dc component from which an ac component has been removed to obtain an output current as a sinusoidal waveform having three phases balanced. It includes.

Further, in the sixth step, the three-phase command pole voltage includes a command voltage for controlling the zero component current to zero to the PWM command voltage.

As described above, the present invention is configured to perform circulating current compensation by applying PQR power theory to a wind power generation system driven by a parallel back-to-back converter, so that the control to reduce the circulating current generated by parallel operation is possible. As the renewable energy grows and the capacity of wind turbines increases, the parallel problem of the system is solved through three parallel operations. The circulating current control reduces the back-to-back converter, the power semiconductor switching element. In addition to improving the design of the system-side filter, it is possible to expect the effect of reducing the construction cost of the direct wind power generation system by solving the rating problem.

1 is a configuration diagram of a wind power generation system driven by a back-to-back converter.
2 is an enlarged view of a grid-side inverter made up of multiple levels.
3 is a block diagram of circulating current compensation using PQR power theory of a wind power generation system driven by a parallel converter according to an embodiment of the present invention.
4 is a configuration diagram of a circulating current controller according to an embodiment of the present invention.
5 is a graph illustrating a relationship between an o-α-β coordinate system and a pqr coordinate system according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

1 to 5, a circulating current compensation system using PQR power theory of a wind power generation system driven by a parallel converter according to an embodiment of the present invention is characterized in that the generator 200 is rotated from the rotation of the blade 201. Generates three-phase AC power of variable frequency, the three-phase AC power is converted into DC power by the converter 101 of the back-to-back converter 100, the power converted to DC is a back-to-back converter The inverter 102 of the (100) is converted into a three-phase AC power of a fixed frequency and then supplied to the system 300 via the three-phase reactance 301, between the converter 101 and the inverter 102 In the wind power generation system that connects the capacitor 400 that serves as a filter for removing harmonics while storing and transmitting the DC power converted through the converter 101, the inverter control unit on the output side of the grid-side inverter 102 Connecting the 20 and the circulating current control unit 30 One will.

The inverter controller 20 senses the voltage on the grid side and detects the phase angle on the side of the grid 300. The phase angle detector (PLL 21) is configured to transfer power transmitted from the generator-side converter to the grid 300. A DC stage voltage controller 22 is configured to maintain a constant DC voltage by sensing the DC stage voltage of the -to-back converter 100 and to control the power factor so that the reactive power of the system 300 becomes zero.

At this time, the DC stage voltage controller 22 receives an actual value and a command value, and generates an axis of a current controller (PI) 23, a stationary coordinate system (o-α-β), and a synchronous coordinate system (ndq). A plurality of coordinate conversion unit 24 for conversion, and PWM generation unit 25 for outputting a command voltage (V _as-ref ) (V _bs-ref ) (V _cs-ref ) using a power semiconductor device. It is configured by.

The circulating current controller 30 calculates a compensation component of the circulating current ic generated in the grid-side inverter 102 using the three-phase current and voltage output from the inverter controller 20, and calculates the calculated components. It is configured to control the circulating current (ic) by providing a compensation component to the current controller 23, the pqr-coordinate transformation unit (PQR Transformation) 31, the filter unit (High Pass Filter) 32, Compensation current calculation unit (Calculate Compensation Currents) 33, and Reference Current Control (34).

The pqr-coordinate conversion unit 31 converts the currents i _a , i _b , i _c and the voltages e _a , e _b , e _c into the phases of the pqr coordinate axes (i _p , i _q , i _r). ) to give the coordinate transformation into a voltage (e _{p, q} e, _r e) is configured to calculate the current value of the coordinate axis pqr _(p i, _q i, _r i).

The filter part 32 is configured to extract an ac component from the current value i _p , i _q , i _r in the pqr coordinate axis calculated from the pqr-coordinate conversion part 31.

The compensation current computing unit 33 are controlled by the currents (i _pac, i _qac, i _rac) zero (zero) the r-axis current (i _rac) from the ac component extracted by the filter unit 32, the Extraction The current of the ac component i _pac , i _qac is configured to calculate the compensation current i _pcomp , i _qcomp , i _rcomp by controlling the current of the dc component without noise.

The current controller 34 is a compensation current calculated from the compensating current computing unit (33) (i _pcomp, i _qcomp, i _rcomp) to the dqn axis current synchronous coordinate axes of the current controller _{(22) (i dcomp, i} qcomp, i _ncomp ), and the neutral-side voltage (V _sn ) is added to the command voltage (V _{as -ref} ) (V _{bs -ref} ) (V _{cs -ref} ) of the current controller 22 to convert the grid-side inverter 102. It is configured to generate a three-phase command pole voltage (V _an-ref ) (V _bn-ref ) (V _cn-ref ) to control and attenuate the circulating current (ic) at.

That is, in the case of the wind power generation system using the permanent magnet synchronous generator as shown in FIG. 1, when the generator 201 generates three-phase AC power of variable frequency from rotation of the blade 200, the three Phase AC power is converted into DC power by the generator side converter 101 of the back-to-back converter 100.

The power converted into DC by the generator-side converter 101 is fixed by the grid-side inverter 102 of the back-to-back converter 100 after the harmonics are removed by the filter 400. After being converted to three-phase AC power is supplied to the system 300 via the three-phase reactance (301).

At this time, the generator-side converter 101 performs a torque control through the generator rotor speed to produce the maximum output by using the control method of Maximum Power Point Tracking (MPPT), the grid-side inverter 102 is Since all power transmitted to the DC terminal through the generator-side converter 101 must be sent to the system 300, the inverter controller 20 including the phase angle detector 21 and the DC terminal voltage controller 22 is connected to the Back-. While controlling the DC voltage of the to-Back converter 100 at a constant time, the power factor is controlled so that the reactive power output to the system becomes zero.

That is, when the inverter controller 20 controls the back-to-back converter 100 connected in multiple levels (3-parallel) as shown in FIG. 1, as shown in Equations 1 and 2 below. After calculating the generator side q side command current for generator torque control from the torque command value and dividing it into three parts, it is outputted to the current controller 23, which is the front end of each PWM generator 25, as shown in FIG. The grid-side q-axis command current for controlling the DC-stage voltage is calculated from the DC-stage voltage and divided into three parts and outputted to the current controller 23 located in front of the PWM generator 25 of the grid-side inverter 102. .

[Equation 1]

here,

[Equation 2]

Here, T ^* _e is the torque reference value of the generator, K _t is the torque constant of the permanent magnet synchronous generator, P is the pole number of the rotor, φ _f is the magnetic field flux.

In this case, in the power converters connected in parallel as multiple levels as described above, when each parallel branch is ideally controlled to have the same voltage output, since the potential difference between the parallels does not exist, the circulating current ic does not occur, but in parallel When the potential difference occurs due to external conditions such as impedance difference and switching difference between branches, as shown in FIG. 2, a circulating current ic is generated inside the grid-side inverter 102. Since the amount of circulating current is added in addition to the rated current generated when the wind power generation system is operated near the rated, it adversely affects the PWM generating unit 25, which is a designed power device, and thus must be attenuated through control.

Therefore, as a method of controlling and attenuating the circulating current ic, first, in the pqr-coordinate conversion unit 31 included in the circulating current controller 30, the three-phase current i _a , i _b , i _c of electric power. And convert the voltage (v _a , v _b , v _c ) into the current (i _p , i _q , i _r ) and voltage (v _p , v _q , v _r ) in the pqr coordinate axis.

That is, from the three-phase current (i _a , i _b , i _c ) and the voltage (v _a , v _b , v _c ) of the abc coordinate system as shown in Equation 3 below, the three-phase current of the o-α-β stationary coordinate system ( i _o , i _α , i _β ) and the voltages v _o , v _α , v _β .

&Quot; (3) "

Next, as shown in Figure 5 the equation 4 and 5 below, as well as attached, the o-α-β stop the three-phase current of the coordinate system (i _o, i _α, i _β) and a voltage (v _o, v _α , v _β ) is converted into a three-phase current (i _p , i _q , i _r ) and a voltage (v _p , v _q , v _r ) of the pqr coordinate system.

&Quot; (4) "

here,

[Equation 5]

In this case, as shown in FIG. 5, since the voltage Vp exists only on the p-axis in the pqr coordinate system, the instantaneous real and imaginary power is derived from the current and voltage generated from the coordinate transformation in the pqr coordinate system as described above. It is obtained as in Equation 6 below.

&Quot; (6) "

In this case, since the instantaneous powers (p, q _q and q _r ) are each linearly independent, three types of instantaneous powers can be independently controlled through the currents of the pqr coordinate system, and accordingly, the three-phase currents of the pqr coordinate axes ( i _p , i _q , i _r ) and the voltage v _p , v _q , v _r .

Then, as shown in Equation 7 below, the three-phase current (i _p , i _q , i _r ) of the pqr coordinate axis includes an ac component and a dc component, respectively.

[Equation 7]

i _p = i _pac + i _pdc

_q i = i + i _{qac qdc}

i _r = i _rac + i _rdc

Herein, the ac component is related to the value of the non-linear state such as harmonics. Accordingly, in the state where the three-phase current is parallel, no current on the r-axis exists.

However, if an image current is present in the wind power generation system due to an unbalanced component, the current of the r-axis (i _r ) is present. The compensation component for the current (ir) of the axis is controlled by the equation of i _r = -i _p tanθ (tanθ = v _α / v _αβ ), whereby the current i _{r of} the r axis can be controlled to zero. It can be.

At this time, in order to obtain the output current of the wind power generation system as a sinusoidal wave having a three-phase equilibrium, the current i _p and i _{q of} the p-axis and the q-axis are ac components through the filter unit 32 included in the circulating current controller 30. Only is extracted, and the compensation current calculation unit 33 controls the compensation component (i _pcomp ) (i _qcomp ) (i _rcomp ) from the extracted ac component to the dc component from which the ac component has been removed, as shown in Equation 8 below. Will be calculated.

[Equation 8]

i _pcomp = i _pac

i _qcomp = i _qac

i _rcomp = i _r + (v _o / v _αβ ) i _p

Then, the current control unit 34 included in the circulating current control unit 30 converts the calculated compensation component i _pcomp (i _qcomp ) (i _rcomp ) through an inverse conversion process as shown in Equation 9 below. -β thereby converted to a compensation current _{(i dcomp) (i qcomp)} (i ncomp) of a stationary coordinate.

[Equation 9]

At this time, the current control unit 34 calculates the compensation current (i _dcomp ) (i _qcomp ) (i _ncomp ) of the o-α-β stationary coordinate system as shown in Equation 10 below, i _n , i _d , i _q ).

[Equation 10]

Here, the n-axis is called a neutral axis (or image axis) like the r-axis in the pqr axis, a component orthogonal to the dq axis does not contribute to the formation of the rotor magnetic field, and the neutral point of the motor is connected to the circuit If there is no and each phase is in equilibrium, it is ignored.

In this case, when an electric potential difference between parallel branches occurs due to an external requirement such as an impedance difference of each phase or a difference in switching, and an image current that causes a circulating current ic occurs, control must be made to protect the power device. The control section 34 adds the compensation current i _n , i _d , i _q of the ndq synchronous coordinate system in which the coordinate transformation is performed to the command value of the current controller 23.

That is, the neutral point voltage (V) from the current of the dqn synchronous coordinate system to the command voltage (V _{as -ref} ) (V _{bs -ref} ) (V _cs -ref) of the PWM generator 25 as shown in Equations 11 and 12 below. _sn is applied to the three-phase command pole voltage (V _an-ref ) (V _bn-ref ) (V _cn-ref ) is produced, the three-phase command pole voltage (V _an-ref ) (V _bn-ref ) ( V _cn -ref) to control and attenuate the circulating current ic circulated in the multi-level grid side inverter 102.

At this time, the three-phase command pole voltage (V _an-ref ) (V _{bn -ref} ) (V _cn-ref ), the command voltage (V _{as -ref} ) (V _{bs -ref} ) (V) of the PWM generator 25. _cs-ref ) to include a command voltage (V _n -ref) for controlling the zero component current to zero, which is implemented as shown in Equations 11 and 12 below.

[Equation 11]

[Equation 12]

V _an-ref = V _as-ref + V _sn + V _n-ref

V _bn-ref = V _bs-ref + V _sn + V _n-ref

V _cn-ref = V _cs-ref + V _sn + V _n-ref

Therefore, if the cyclic current (ic) is controlled and attenuated as described above, the rating problem in the large-capacity wind power generation system can be solved, and the effect of reducing the cost for the construction of the large-capacity wind power generation system can be expected. Will be.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention as defined by the appended claims. It is to be understood that such changes and modifications are within the scope of the claims.

20; Inverter control unit 21; Phase angle detector
22; DC stage voltage control unit 23; Current controller
24; Coordinate transformation unit 25; PWM generator
30; Circulating current controller 31; pqr-coordinate transformation
32; Filter section 33; Compensation current calculator
34; Current control unit 100; back-to-back converter
101; Generator-side converter 102; Grid-side converter
200; Generator 201; blade
300; System 400; filter

Claims (13)

The generator generates three-phase AC power of variable frequency from the rotation of the blade, and the generated three-phase AC power is converted into DC power by the generator-side converter of the back-to-back converter, and the converted power into DC It is converted into fixed-phase three-phase AC power by the grid-side inverter of the back-to-back converter and then supplied to the grid via a three-phase reactance, and through the generator-side converter between the generator-side converter and the grid-side inverter. In the wind power generation system that connects a capacitor that performs the filter role of storing and transmitting the converted DC power and remove harmonics,
An inverter controller including a phase angle detector, a current controller, a plurality of coordinate converters for converting axes of the stationary coordinate system (o-α-β) and a synchronous coordinate system (ndq), and a PWM generator are connected to an output terminal of the grid-side inverter. But
The compensation component of the circulating current generated in the grid-side inverter is calculated between the output terminal of the grid-side inverter and the inverter controller by using a three-phase current and a voltage output from the inverter controller. A circulating current controller configured to provide a current controller to control the circulating current; Cyclic current compensation system using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that the connection configuration. The method of claim 1, wherein the circulating current control unit,
A pqr-coordinate transformation unit configured to calculate a current value in the pqr coordinate axis by converting the current and voltage in each phase into a current and voltage in the pqr coordinate axis;
A filter unit for extracting an ac component from a current value in a pqr coordinate axis calculated from the pqr-coordinate transformation unit;
Compensation current calculation unit for controlling the current of the r-axis from the current of the ac component extracted by the filter unit to zero, and the current of the extracted ac component is controlled by the current of the dc component from which the noise is removed ; And,
A current control unit for converting the compensation current calculated from the compensation current calculating unit into a current of the dqn axis, which is a synchronous coordinate axis of the current controller, to control the circulating current in the grid-side inverter, and adding the current to the command voltage of the current controller; Cyclic current compensation system using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that the configuration including a. In the back-to-back converter, when a circulating current occurs due to a potential difference in a multi-level generator-side converter or grid-side inverter that converts three-phase AC power into DC power and DC power into three-phase AC power,
A first step of converting three-phase current and voltage of electric power into current and voltage of a pqr coordinate axis;
A second step of extracting an ac component after calculating the three-phase current and voltage of the pqr coordinate axis in the first step;
A third step of detecting whether an image current due to an unbalanced component is included in the pqr coordinate axis current and voltage of the ac component extracted from the second step;
A fourth step of calculating a compensation component for removing the image split current when the detection result of the third step includes the image split current;
A fifth step of converting the compensation component calculated from the fourth step into a compensation current of an o-α-β stationary coordinate system through an inverse conversion process;
The converted compensation current is converted into the current of the dqn synchronous coordinate system of the current controller and then added to the command value of the current controller, and the three-phase command pole voltage is applied to the command voltage of PWM from the current of the dqn synchronous coordinate system. A sixth step of controlling and attenuating circulating currents circulated in the multilevel inverter; Cyclic current compensation method using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that proceeding. The method of claim 3, wherein the first step,
a seventh step of converting three-phase current and voltage of the abc coordinate system into three-phase current and voltage of the o-α-β static coordinate system; And,
An eighth step of converting the three-phase current and voltage of the o-α-β static coordinate system into three-phase current and voltage of a pqr coordinate system; Cyclic current compensation method using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that further comprising. The method of claim 4, wherein
The coordinate transformation of the seventh step is a cyclic current compensation method using the PQR power theory of a wind power generation system driven by a parallel back-to-back converter, characterized in that the following equation.

The method of claim 4, wherein
The coordinate transformation of the eighth step is a circulating current compensation method using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that the following equation.

The method of claim 3, wherein the fourth step,
Assuming that there is no r-axis current component in the state where the three-phase current is balanced, and that an image component exists in the system due to an unbalanced component, the r-axis current exists.
A ninth step of controlling the current on the r-axis to zero by calculating an additional compensation component for removing the image split current; And,
A tenth step of calculating a compensation component by controlling a current of the p-axis and the q-axis to a dc component from which an ac component is removed to obtain an output current as a sinusoidal waveform having three phases balanced; Cyclic current compensation method using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that further comprising. The method of claim 7, wherein
The r-axis current according to the ninth step is controlled by i _r = -i _p tanθ (tanθ = v _α / v _αβ )) PQR of the wind power generation system driven by a parallel back-to-back converter Cyclic current compensation method using power theory. The method of claim 7, wherein
The compensation component of the tenth step is a cyclic current compensation method using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that obtained by the following equation.
i _pcomp = i _pac
i _qcomp = i _qac
i _rcomp = i _r + (v _o / v _αβ ) i _p The method of claim 3, wherein
The reverse compensation current of the compensation component of the fifth step is a cyclic current compensation method using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, characterized in that the following equation.

The method of claim 3, wherein
Coordinate transformation of the inversely transformed compensation current in the fifth step into the ndq synchronous coordinate system is performed by the following equation, circulating current using the PQR power theory of the wind power generation system driven by the parallel back-to-back converter. Compensation Method.

The method of claim 3, wherein
In the sixth step, the three-phase command pole voltage (V _an-ref ) (V _{bn -ref} ) (V _{cn -ref} ) includes the command voltage of PWM (V _{as -ref} ) (V _{bs -ref} ) (V _{cs -ref} ). Circulating using the PQR power theory of the wind power generation system driven by a parallel back-to-back converter, further comprising a command voltage (V _n -ref) for controlling the zero component current to zero). Current compensation method. The method of claim 12,
Command voltage in the sixth step _{(as V-ref) (bs-V ref)} (V _cs-ref) to the neutral voltage (V _sn) a three-phase command voltage pole (V _ref-an) (V _bn- added _ref ) (V _cn-ref ) is a cyclic current compensation method using the PQR power theory of a wind power generation system driven by a parallel back-to-back converter, characterized in that obtained by the following equation.

V _an-ref = V _as-ref + V _sn + V _n-ref
V _bn-ref = V _bs-ref + V _sn + V _n-ref
V _cn-ref = V _cs-ref + V _sn + V _n-ref

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