KR101997089B1 - Wind turbine system and method for operating the same - Google Patents

Abstract

A wind power generation system and a method of operating the same are disclosed. A wind power generation system in which a plurality of wind turbines are connected in series to perform an ultra high voltage direct current transmission (HVDC) is connected to each of the wind turbines, and a plurality of wind turbines 1 tap change-over transformers, and the first tap-change transformers, converts the AC voltage output from the first tap-change transformer to DC, and outputs the AC voltage to the DC link of the modular multilevel converter (MMC) A second tap switching transformer connected between an output terminal of the MMC and the power system, and a plurality of wind turbine generators grouped into a plurality of groups, wherein the plurality of rectifiers are connected in series, Adjusts the tap of the first tap change-over transformer so that the magnitude remains constant, and the voltage output from the MMC changes to the grid voltage And a control unit for adjusting the tap of the second tap switching transformer so as to be turned on.

Description

Wind turbine system and method for operating same

The present invention relates to a wind power generation system and a method of operating the same. More particularly, when a plurality of wind power generators are connected in series to perform high voltage direct current transmission (HVDC), the power system side of a modular multilevel converter (MMC) To a wind power generation system having a variable DC link voltage by installing a transformer and a method of operating the same.

Large-scale offshore wind farms are being actively researched to overcome the geographical and geographical limitations of onshore wind power generation and to utilize excellent wind conditions. However, in case of offshore wind power plant, there are disadvantages in terms of cost, such as installation and maintenance, compared with onshore wind power generation plant. Therefore, a variety of grid connection technologies for reducing the installation and operation cost of offshore wind power generation plant are being studied. Among the grid interconnect technologies, there are high voltage alternative currnet transmission (HVAC) and high voltage direct current transmission system (HVDC). HVDC is considered to be more economical than HVAC in long-distance transmission of more than 52 km based on 400 MW offshore wind farm. Therefore, it is expected that HVDC will play an important role in future power grid connection plans of large offshore wind farms.

In the case of a DC-parallel structure in which a plurality of wind turbines are connected in a dc-parallel manner, a plurality of wind turbines are connected in parallel so that stability is excellent. Even when only a small number of wind turbines are started, It is necessary to install a DC / DC conversion platform for boosting at sea for driving the HVDC, and there is a technical and cost problem of the large capacity DC / DC converter at this time.

In the case of a DC-to-serial structure in which a plurality of wind turbines are connected in a dc-series manner, there is no need for an offshore platform, which is economically most advantageous among the offshore wind turbine grid connection schemes. However, depending on the wind speed of the wind turbine, The voltage may fluctuate, and when a small number of wind turbines are started, the DC link voltage may not be sufficiently increased so that the HVDC can not be operated.

In order to solve these problems, a DC-DC converter using a DC-DC converter has been proposed. However, when the operation of only one wind turbine is assumed, the output DC voltage of the wind turbine generator is changed by a modular multilevel converter (MMC) The DC voltage of the DC power source must be equal to the DC voltage of the DC power source. This is because it is difficult to realize practical operation due to the capacity of the switch semiconductor device for electric power and the insulation problem of the equipment. Even if it is possible, there is an additional cost problem in the case of the offshore wind power generation complex due to the increase of the insulation level.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a tap changer for a power system side of a modular multilevel converter (MMC) and a wind turbine generator when a plurality of wind turbines are connected in series to perform high voltage direct current transmission (HVDC) And a method of operating the wind power generation system.

According to an aspect of the present invention, there is provided a wind power generation system in which a plurality of wind turbines are connected in series to perform HVDC, the wind turbine generator is connected to each of the wind turbines, A plurality of first tap change-over transformers for converting a voltage of the power produced by the first tap-changing transformers and outputting the converted voltage, A plurality of rectifiers whose output terminals are connected in series to a DC link of a modular multilevel converter (MMC), a second tap switching transformer connected between the output stage of the MMC and the power system, and a plurality of wind turbines And the tabs of the first tap change-over transformer are arranged such that the magnitude of the voltage output to the DC link is maintained constant for each group Jeonghamyeo, the voltage output from the MMC may be provided with a control unit for adjusting the tap of the second transformer tap changer to be converted to the system voltage.

The control unit adjusts the tabs of the first tap change-over transformer so that the magnitude of the voltage per group output to the DC link is kept constant when at least one wind turbine generator is started regardless of the number of wind turbines operating in each group .

Wherein the control unit controls the first group wind turbines so that the voltage outputted to the DC link is the first voltage when at least one of the wind turbines included in the first group of wind turbines is started When at least one of the wind turbines included in the second group is started when the voltage output from the DC link is the first voltage, the taps of the first tap change- The taps of the first tap switching transformers connected to the wind turbines of the second group can be adjusted while maintaining the taps of the first tap switching transformers connected to the first group of wind turbines so that the output voltage is the second voltage have.

The wind turbine generator may include a permanent magnet synchronous generator (PMSG).

The wind power generation system may further include a back-to-back converter connected to an output terminal of each of the wind power generators, for converting an AC voltage output from the wind power generator into a DC voltage, And the first tap switching transformer is connected to the output terminal of the converter to convert the DC voltage output from the converter to AC and output the AC voltage.

The first tap change-over transformer is connected to each of the plurality of wind turbines according to the technical idea of the present invention, and the voltage output from the first tap-change transformers is converted into a direct current (MMC) And a second tap-changing transformer is connected between the output terminal of the MMC and the power system to perform HVDC, the method comprising the steps of: Adjusting the tap of the first tap switching transformer so that the magnitude of the voltage output to the DC link is kept constant for each group, And adjusting the tabs of the two-tap change-over transformer.

The step of adjusting the taps of the first tap-changing transformer may include adjusting the taps of the first and second tap-changing transformers so that when the at least one wind turbine generator is activated independently of the number of the wind turbines operating in each group, And adjusting the tab of the first tap change-over transformer.

Wherein the step of adjusting the taps of the first tap-changing transformer comprises the steps of: when at least one of the wind turbines included in the first group of the groups is started, Adjusting the taps of the first tap switching transformers connected to the first group of wind turbines so that at least one of the wind turbines included in the second group in the state where the voltage output from the DC link is at the first voltage When the wind turbine is started, the taps of the first tap switching transformers connected to the wind turbines of the first group are connected to the wind turbines of the second group so that the voltage output to the DC link is the second voltage And adjusting the taps of the first tap change-over transformers.

The wind turbine system according to an embodiment of the present invention and the method of operating the same according to an embodiment of the present invention can control variable DC link voltage well even when the turbine starts sequentially from only one wind turbine to the rated output of the wind turbine, There is an advantage that it can be transmitted. Even when the wind turbine generator of the present invention has different wind speeds, the DC link voltage of the MMC can be stably maintained and output power can be stably transmitted. In addition, when the present invention is applied, it is possible to save the installation cost and operational cost of the marine platform by installing the three-phase rectifier of the wind turbine generator and the tap change-over transformer even though the marine MMC platform is not available. . Therefore, when the present invention is applied to the operation of a large-scale offshore wind farm, stable power transmission is possible despite the removal of the offshore platform, and the insulation level of each wind turbine generator can be lowered by variable operation of the DC link, It is possible to contribute to securing economical efficiency in the introduction of the offshore wind farm because it can reduce the distance in the facility.

BRIEF DESCRIPTION OF THE DRAWINGS A brief description of each drawing is provided to more fully understand the drawings recited in the description of the invention.
1 is a schematic view of a wind turbine system according to an embodiment of the present invention.
FIG. 2 is a view schematically showing a structure between a DC link in a wind turbine generator of the wind turbine generator of FIG. 1. FIG.
3 is a flowchart showing a method of operating the wind power generation system of FIG.
4 is an algorithm representation of the method of operating the wind power generation system of FIG.
FIG. 4 and FIG. 5 are circuit diagrams illustrating the converter of the wind power generator among the converters of the back-to-back system of FIG.
FIG. 6 is a circuit diagram showing a rectifier-side converter of the back-to-back system converter of FIG.
Fig. 7 is a circuit diagram showing an embodiment of the first tap change-over transformer or the second tap-change transformer of Fig. 1;
8 is a diagram illustrating an embodiment of the MMC of FIG.
FIG. 9 is a view showing the operation of the sub-module of the MMC of FIG.

In order to fully understand the present invention, operational advantages of the present invention, and objects achieved by the practice of the present invention, reference should be made to the accompanying drawings and the accompanying drawings which illustrate preferred embodiments of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the accompanying drawings. Like reference symbols in the drawings denote like elements.

FIG. 1 is a schematic view of a wind power generation system 100 according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating a structure between a wind power generator and a DC link in the wind power generation system 100 of FIG. Fig.

Referring to FIGS. 1 and 2, a wind turbine system 100 according to an embodiment of the present invention includes a plurality of wind turbines 210 connected in series to a high voltage direct current transmission system (HVDC) A plurality of rectifiers 240, a second tap-change transformer 120, and a control unit (not shown) for controlling the operation of the first tap-change transformer 230, The wind power generator 210 may be a wind power generator composed of a permanent magnet synchronous generator (PMSG).

Each of the plurality of first tap change-over transformers 230 is connected to each of the wind power generators 210 and can convert and output the voltage of the power generated by the wind power generator 201. Each of the plurality of rectifiers 240 is connected to the output terminal of each of the first tap-change transformers 230 to convert the AC voltage output from the first tap-change transformer 230 into DC, a multilevel converter 120 may be connected in series to the DC link. The rectifier 240 may be a diode rectifier. The second tap change-over transformer 120 may be connected between the output terminal of the MMC 110 and the power system.

The control unit groups the plurality of wind turbines 230 into a plurality of groups and controls the tap of the first tap switching transformer 230 so that the magnitude of the voltage output to the DC link Can be adjusted. That is, when the at least one wind turbine generator is started, regardless of the number of the wind turbine generators 230 activated for each group, the control unit controls the first The tabs of the tap-changing transformer 230 can be adjusted. For example, when five wind turbines are grouped into one group, whether the wind turbine included in the group is activated by one or five generators is activated or the magnitude of the voltage output to the DC link is The taps of the first tap change-over transformer 230 can be adjusted so as to be kept constant.

Wherein the control unit controls the first and second wind turbines so that a voltage output from the DC link is a first voltage when at least one of the wind turbines 210 of the wind turbine generators 210 included in the first group is activated, The taps of the first tap switching transformers 230 connected to the wind turbines of the group are adjusted and the wind turbine 210 included in the second group with the voltage output from the DC link being the first voltage, The taps of the first tap switching transformers connected to the wind turbines of the first group are maintained such that the voltage output to the DC link is the second voltage when at least one of the wind turbines is activated, The taps of the first tap change-over transformers connected to the wind generators of the second group can be adjusted.

For example, it is assumed that the first group includes the first wind turbine generator to the fifth wind turbine generator, and the second group includes the sixth wind turbine to the tenth wind turbine generator. As described above, when at least one of the wind turbines of the first to fifth wind turbines of the first group is activated, the first tap switching transformer 230 connected to the wind turbines of the first group adjusts the taps So that the voltage output to the DC link can always be the first voltage. If all of the wind turbines of the first group are operating and the sixth wind turbine belonging to the second group is started in a state where the first voltage is being output by the DC link, Adjusts the taps of the first tap switching transformer connected to the sixth wind turbine of the second group while maintaining the taps of the first tap switching transformers connected to the wind turbines belonging to the first group, May be the second voltage. In this example, the control unit selects the uppermost tab of the first tap switching transformer connected to the sixth wind power generator while keeping the taps of the first tap switching transformers connected to the wind turbines belonging to the first group, So as to boost the voltage of the DC link from the first voltage to the second voltage.

The controller may adjust the tap of the second tap switching transformer 120 so that the voltage output from the MMC 110 is converted into a constant grid voltage. Since the system voltage must be maintained constantly, the control unit can adjust the tap of the second tap-changing transformer 120 so that the output voltage of the second tap-changing transformer 120 can be maintained at a constant system voltage have.

The wind power generation system 100 is connected to an output terminal of each of the wind power generators 210 to convert the alternating voltage output from the wind power generator into a direct current And may further include converters 220_1 and 220_2. The first tap change-over transformer 230 is connected to the output terminals of the converters 220_1 and 220_2 to convert the DC voltage output from the converters 220_1 and 220_2 into AC and output the DC voltage.

FIG. 3 is a flowchart illustrating an operation method of the wind power generation system 100 of FIG. 1, and FIG. 4 is an algorithm representation of the method of operating the wind power generation system of FIG.

은 MMC(110) 초기 직류 링크 전압,

은 그룹에 포함되는 풍력발전기 수,

는 그룹 내 풍력발전기 개수

,

는 전체 풍력발전기 수

,

은 제 2 탭 절환 변압기(120)탭 변환 레벨,

는 제 1 탭 절환 변압기(230) 2차측 출력 교류 전압,

는 그룹 내 바이패스 스위치가 온(on) 되어진 풍력발전기 수,

은 MMC(110) 출력 교류 전압,

는 MMC(110) 연계 계통 전압, m은 MMC(110)의 변조비,

는 j번 풍력발전기의 직류 출력 전압을 나타낸다.Referring to FIGS. 1 to 3, the control unit groups the plurality of wind turbines into a plurality of groups (S310), and controls the first tap switching The tap of the transformer can be adjusted (S320). The controller may adjust the tap of the second tap switching transformer 120 to convert the voltage output from the MMC 110 to a constant grid voltage at step S330. Since steps S310 to S330 have been described in detail above, duplicate descriptions will be omitted. 4,

The MMC 110 initial DC link voltage,

The number of wind power generators included in the group,

The number of wind turbines in the group

,

Total Wind Turbine Number

,

The second tap change-over transformer 120 has a tap conversion level,

A second tap output transformer 230, a secondary output AC voltage,

The number of wind generators in which the in-group bypass switch is turned on,

The output AC voltage of the MMC 110,

M is the MMC 110 coupled system voltage, m is the modulation ratio of the MMC 110,

Represents the DC output voltage of the j-th wind turbine generator.

이 5일 경우, 1대부터 5대의 풍력발전기가 기동할 때 제 1 탭 절환 변압기(230)의 탭 조정이 이루어져, 직류 링크 전압을 일정하게 유지한다. 이때 1대의 풍력발전기가 추가로 기동할 경우에는 기존 5대의 풍력발전기의 탭은 그대로 유지하되 1대의 풍력발전기는 가장 상위 탭을 선택하여 직류 링크 전압을 승압하고, 이와 동시에 제 2 탭 절환 변압기(120)의 탭을 조정하여 출력 전압을 승압하게 된다.E.g,

In this case, when the first to fifth wind turbines are started, the taps of the first tapped switching transformer 230 are adjusted to keep the dc link voltage constant. In this case, when one wind turbine generator is further activated, the taps of the existing five wind turbines are maintained while one turbine generator selects the uppermost tap to boost the DC link voltage. At the same time, the second tap switching transformer 120 ) To adjust the output voltage.

FIGS. 4 and 5 are circuit diagrams illustrating the wind power generator side converter 220_1 of the back-to-back type converters 220_1 and 220_2 of FIG.

1 to 5, control of a PMSG wind turbine generator connected to a grid and using a general control method and a PMSG wind turbine generator for DC-to-serial coupling is controlled by a voltage having a constant magnitude and frequency in a rectifier- It is necessary to control the DC link voltage at the wind power generator side converter 220_1 to contribute to the stabilization of the output voltage of the rectifier side converter 220_2.

는 도 5의 직류 링크 전압제어기와 연결되며,

는 0으로 제어된다. 직류 링크 전압 제어기는 직류 링크 전압에 맞춰 PMSG 풍력발전기의 출력을 제어할 수 있도록 도 5와 같이 제어기를 설계하였으며, 도 5에서

는 직류 링크 전압 지령값,

는 직류 링크 전압,

는 PMSG 풍력발전기의 지령값 그리고

는 PMSG 풍력발전기의 유효전력을 의미한다.In this case, the voltage equation in the dq synchronous coordinate system of the PMSG wind power generator can be expressed by the following equations (1) and (2), which can be expressed by a current controller as shown in FIG. In Figure 4

Is connected to the DC link voltage controller of Figure 5,

Is controlled to be zero. 5, the DC link voltage controller is designed to control the output of the PMSG wind power generator according to the DC link voltage,

DC link voltage command value,

DC link voltage,

Is the command value of the PMSG wind turbine generator and

Means the effective power of the PMSG wind power generator.

: PMSG 풍력발전기 고정자 dq 등가 전압here,

: PMSG wind power generator stator dq equivalent voltage

: PMSG 풍력발전기 고정자 dq 등가 전류

: PMSG wind power generator stator dq equivalent current

: 고정자 인덕턴스

: Stator inductance

: 고정자 저항

: Stator resistance

: PMSG 풍력발전기 회전 각속도

: PMSG wind turbine rotation angular velocity

FIG. 6 is a circuit diagram showing the rectifier-side converter 220_2 of the back-to-back converter 220_1 and 220_2 of FIG.

Referring to FIGS. 1 to 6, a rectifier-side converter 220_2 of a dc-to-dc connected wind turbine generator must generate a voltage having a predetermined size, phase and frequency, unlike a control structure of a general wind turbine generator. Therefore, the voltage for generating the PWM signal is expressed by Equation (3) to Equation (5), which can be expressed as shown in FIG.

: 정류기 측 컨버터(220_2)의 a,b,c 상전압 지령치 here,

: The a, b, and c phase voltage command values of the rectifier side converter 220_2

: 출력 전압 지령값의 크기

: Size of output voltage command value

: 출력 주파스

: Output frequency

: 위상각 지령값

: Phase angle command value

FIG. 7 is a circuit diagram showing one embodiment of the first tap change-over transformer 230 or the second tap change-over transformer 120 of FIG.

이며, 턴오프(turn-off) 상태에서

과 같다.Referring to Figs. 1 to 7, Fig. 7 shows a case where the first tap change-over transformer 230 or the second tap change-over transformer 120 is a tap-changing transformer using a triac. In this case, the tap-changing transformer may generate a variable output voltage using a constant input voltage by operation of the triac, or may obtain an output voltage of a fixed size by using an input voltage varying with a constant width. The secondary voltage of the tap-changing transformer using the triac in the structure as shown in FIG. 7 can be expressed by Equation (6) below. When the triac is in a turn-on state

, And in a turn-off state

Respectively.

FIG. 8 is a diagram illustrating an embodiment of the MMC 110 of FIG.

가 연결되어 있다.Referring to FIGS. 1 to 8, the MMC 110 may be a 3-phase N + 1 level MMC as shown in FIG. 8, in which sub-modules of a half bridge structure are connected in series, Arm Inductors for Fault Current Suppression

Respectively.

At this time, the circulating current flowing through each arm can be calculated as shown in Equation (7) below, and the arm inductor value for suppressing the circulating current can be calculated by Equation (8).

: a상 순환 전류 [A]here,

: a Phase Cycle current [A]

: 서브모듈 수

: Number of submodules

: a상 전류 최대값 [A]

: a phase current maximum value [A]

: 변조비

: Modulation ratio

: 역률

: Power factor

: 직류 전류 [A]

: DC current [A]

: 무효율

: No Efficiency

: 위상각속도 [rad/s]

: Phase angular velocity [rad / s]

: 서브모듈 커패시턴스 [F]

: Sub-module capacitance [F]

: 암 인덕턴스 [H]

: Arm Inductance [H]

The sub-module of the half-bridge structure is composed of a capacitor, a switching device and a diode connected in an anti-parallel structure to the sub-module. The voltage of the sub-module is expressed by Equation (9) .

: 서브모듈 전압here,

: Submodule Voltage

: 직류 링크 전압 [V]

: DC link voltage [V]

: 피상전력 [VA]

: Apparent power [VA]

: 커패시터 전압 리플율 [%]

: Capacitor voltage ripple rate [%]

: 커패시터 평균 전압 [V]

: Capacitor average voltage [V]

FIG. 9 is a diagram illustrating the operation of the sub-module of the MMC 110 of FIG.

Referring to FIGS. 1 to 9, the sub-module of the MMC 110 can be broadly divided into two states according to the on-off operation of the switches T1 and T2 as shown in FIG. 9, (a) and (c), and the off state is divided into (b) and (d) in FIG. In case of positive current, the capacitor of the submodule is charged through D1 diode in the case of positive current, and when the negative current flows, the capacitor of submodule is discharged due to the ON operation of T1 switch. Respectively. When the positive current flows in the OFF state, the submodule is bypassed by the ON operation of the T2 switch. When negative current flows, the voltage of the submodule in the OFF state bypassed through the D2 diode is equal to zero.

As described above, an optimal embodiment has been disclosed in the drawings and specification. Although specific terms have been employed herein, they are used for purposes of illustration only and are not intended to limit the scope of the invention as defined in the claims or the claims. Therefore, those skilled in the art will appreciate that various modifications and equivalent embodiments are possible without departing from the scope of the present invention. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

Claims (8)

1. A wind power generation system in which a plurality of wind turbines are connected in series to perform an ultra high voltage direct current transmission (HVDC)
A plurality of first tap switching transformers connected to each of the wind turbines and converting and outputting a voltage of the power generated by the wind turbine generator;
A plurality of first switching transistors connected in series to a DC link of an MMC (Modular Multilevel Converter), each of the plurality of first switching transistors being connected to an output terminal of each of the first tapped switching transformers, Rectifiers;
A second tap switching transformer connected between the output terminal of the MMC and the power system; And
Wherein the plurality of wind turbines are grouped into a plurality of groups and a tap of the first tap switching transformer is adjusted so that the magnitude of the voltage output to the DC link is maintained constant for each group, And a control unit for adjusting a tap of the second tap switching transformer so as to be converted into a voltage. The apparatus of claim 1,
The taps of the first tap switching transformers are adjusted so that the magnitude of the voltage of each group output to the DC link is kept constant when only at least one wind turbine generator is activated regardless of the number of wind generators activated for each group. Wind power generation system. The apparatus of claim 1,
Connected to the first group of wind turbines so that the voltage output to the DC link is at a first voltage when at least one of the wind turbines included in the first group of wind turbines is activated, Adjust the taps of the switching transformers,
Wherein when a voltage outputted from the DC link is a first voltage and at least one of the wind turbines included in the second group is started, a voltage output from the DC link is a second voltage, Wherein the taps of the first tap-change transformers connected to the wind turbines of the second group are adjusted while maintaining the taps of the first tap-change transformers connected to the wind turbines of the group. The wind power generator according to claim 1,
And a permanent magnet synchronous generator (PMSG). The wind power generation system according to claim 1,
Further comprising a back-to-back converter connected to an output terminal of each of the wind turbines to convert the alternating voltage output from the wind turbine generator to a direct current,
Wherein the first tap change-
And converts the DC voltage output from the converter into AC to be connected to the output terminal of the converter. A first tap change-over transformer is connected to each of the plurality of wind power generators, and the voltage output from the first tap-change transformers is converted into DC to be connected in series to a DC link of a modular multilevel converter (MMC) A method for operating a wind power generation system in which a second tapped switching transformer is connected between an output terminal and a power system to perform ultra high voltage direct current transmission (HVDC)
Grouping the plurality of wind power generators into a plurality of groups;
Adjusting a tap of the first tap switching transformer so that the magnitude of the voltage output to the DC link is maintained constant for each group; And
And adjusting a tap of the second tap switching transformer so that a voltage output from the MMC is converted into a grid voltage. 7. The method of claim 6, wherein adjusting the taps of the first tap-
And adjusting the tap of the first tap switching transformer so that the magnitude of the voltage of each group output to the DC link is kept constant when at least one wind turbine generator is started regardless of the number of wind turbines operating in each group A method of operating a wind power system. 7. The method of claim 6, wherein adjusting the taps of the first tap-
Connected to the first group of wind turbines so that the voltage output to the DC link is at a first voltage when at least one of the wind turbines included in the first group of wind turbines is activated, Adjusting the taps of the switching transformers; And
Wherein when a voltage outputted from the DC link is a first voltage and at least one of the wind turbines included in the second group is started, a voltage output from the DC link is a second voltage, Adjusting the taps of the first tap-changing transformers connected to the wind turbines of the second group while maintaining the taps of the first tap-changing transformers connected to the wind turbines of the group Way.

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