KR101116000B1 - Power conversion system and power conversion method using series connected current source ac/dc power converters - Google Patents

Abstract

The present invention relates to a power conversion system and a power conversion method that enables a high voltage direct current transmission with a simple structure. To this end, the power conversion system of the present invention includes a current type pulse width modulation converter connected to two or more AC power sources, respectively, characterized in that the DC terminals of the current type pulse width modulation converter are connected in series. With this configuration, it is possible to comply with the grid code (Grid Code) without the need for a separate boost transformer or a large DC-to-AC power converter, and at the same time DC power transmission of the generated power by the high-voltage power supply class.

Description

POWER CONVERSION SYSTEM AND POWER CONVERSION METHOD USING SERIES CONNECTED CURRENT SOURCE AC / DC POWER CONVERTERS}

The present invention relates to a power conversion system and a power conversion method.

Recently, the capacity of a single generator of a wind power generation system is increasing significantly, and a 10MW wind generator is also expected to be installed. In addition, the wind power generation system is developed in the form of a wind farm in which several wind generators are collectively installed. In the case of a large wind farm, the total capacity of the wind farm exceeds hundreds of MW, and the generated power is reliably transferred to the existing system. Connecting is a very important issue.

As wind power systems grow in size, countries in recent years have established grid codes that must be followed when connecting wind power systems to existing power systems, and that each wind power system must follow grid linkage regulations. Doing. In order for large-scale wind power generation systems to comply with these grid linking regulations, a full power conversion system is essential for linking with the grid.

Typically, a voltage pulse width modulated boost converter (PWM VSC) is used for grid linkage. In the voltage type pulse width modulated boost converter, the converter may be configured in parallel through a multi-winding boost transformer and connected to an AC system, or may be implemented as a multi-level converter and connected to an AC system. In addition, the AC output of the variable voltage variable frequency of the wind generator is connected to the DC terminal of the voltage pulse width modulated boost converter through a diode rectifier and a DC / DC converter, or the AC output of the wind generator is a separate voltage type pulse width modulated boost. After conversion to a DC voltage source of a constant voltage through the type converter can be connected to the DC terminal of the preceding voltage pulse width modulation step-up converter.

In the case of such wind power generation, the terminal voltage of the unit wind generator also adopts distribution level medium voltage in order to send generation power of several MW or more generated from individual generators to the grid. Transmission level high voltages are used to link the entire power plant to the grid. In order to transfer the power of a wind farm using a transmission class AC voltage, it is necessary to apply a large-capacity boosting transformer to the output of the voltage-type pulse width modulation converter individually or collectively to the wind generator. These transformers not only contribute to reduced efficiency, but also detrimental to the area and weight of the installation.

In addition, the direct current transmission system is known to be very advantageous when the distance between the wind farm and the existing AC system, such as offshore wind farms. However, in order to realize direct current transmission at high transmission voltage, a high voltage AC / DC converter for converting an AC voltage into a DC high voltage for a constant voltage in addition to the AC boosting transformer mentioned above is required. There is also a need for a DC / AC converter that converts a transmission-grade DC voltage back into an AC of the transmission voltage. This complicates the entire wind power system and increases the installation cost significantly.

The present invention proposes a power converter and a power conversion system that enables a high voltage direct current transmission with a simple structure.

To this end, the power conversion system according to an embodiment of the present invention includes a current type pulse width modulation converter connected to each of two or more AC power sources, characterized in that the DC terminal of the current type pulse width modulation converter is connected in series.

In this case, the two or more AC power sources may be different wind generators, or different power generators in one wind generator.

In addition, the power conversion system according to an embodiment of the present invention further includes a control unit for adjusting the modulation index of the current type pulse width modulation converter to obtain the output of the AC power source corresponding to the direct current of the current type pulse width modulation converter. can do.

In addition, the power conversion system according to an embodiment of the present invention may further include a transformer connecting the AC power supply and the current type pulse width modulation converter, the transformer may be a multi-winding transformer.

In addition, the power conversion system according to an embodiment of the present invention may further include a power absorption device consisting of a resistor and a magnetic contactor at the output terminal of the AC power source or the AC terminal of the current type pulse width modulation converter.

In addition, the power conversion system according to an embodiment of the present invention may further include a current type pulse width modulation inverter connected to the DC terminal of the current type pulse width modulation converter.

In addition, the power conversion system according to an embodiment of the present invention may further include a link transformer connected to the AC terminal of the current type pulse width modulation inverter, the link transformer may be a multi-winding link transformer.

In the power conversion method according to an embodiment of the present invention, the output terminals of two or more AC power sources are respectively connected to a current pulse width modulation converter, and the DC terminals of the current pulse width modulation converter are connected to each other in series to generate the AC power. The converted AC power is characterized by converting to DC power.

In addition, the power conversion method according to an embodiment of the present invention further comprises adjusting the modulation index of the current type pulse width modulation converter to obtain the output of the AC power source corresponding to the direct current of the current type pulse width modulation converter. Can be.

In addition, the power conversion method according to an embodiment of the present invention may further include converting the DC power to AC power by connecting the current type pulse width modulation inverter to the DC terminal of the current type pulse width modulation converter.

By using the current type pulse width modulation converter as described above, by enabling a high voltage direct current transmission, it is possible to implement a high-efficiency wind power generation system that transmits a large amount of power at a long distance.

1 is a first embodiment of an AC-to-DC power converter of a power conversion system according to the present invention.
FIG. 2 is a diagram illustrating a DC equivalent model of the AC-DC power converter of FIG. 1.
3 is a second embodiment of the AC-DC power conversion unit of the power conversion system according to the present invention.
4 is a third embodiment of an AC-to-DC power conversion unit of the power conversion system according to the present invention.
5 is a first embodiment of the DC-to-AC power conversion unit of the power conversion system according to the present invention.
6 is a second embodiment of the DC-to-AC power conversion unit of the power conversion system according to the present invention.
7 is a view showing the overall configuration of a power conversion system according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. However, the accompanying drawings and the following description are only possible embodiments of the power conversion system according to the present invention, and the technical idea of the present invention is not limited by the above contents.

1 is a first embodiment of an AC-to-DC power converter of a power conversion system according to the present invention.

As shown in FIG. 1, the AC-to-DC power converter of the power conversion system according to the first embodiment includes a _first wind power generator 10, a _first current type pulse width modulation converter 12, and a a _1. DC current stage 14, a _second wind turbine 20, a _second current pulse width modulation converter 22, a _second DC current stage 24, and a _k wind turbine 30 ), A _ak current type pulse width modulation converter 32, a a _k direct current current stage 34, and a a _{k th} full direct current current stage 40.

That is, the current source pulse width modulation converters (Current Source Pulse Width Modulation Converter: 12, 22, 32) and the DC current link (DC Current Link: 14, 24, 34 are connected, and the output of each of the DC current stages 14, 24, 34 is connected in series with each other. The outputs of the series connected DC current stages 14, 24, and 34 form one _{ak th} full DC current stage 40.

Each of the k wind turbines 10, 20, 30 generates AC power of a three-phase variable voltage variable frequency (VVVF).

Each current source pulse width modulation converter 12, 22, 32 is composed of six switch elements made of GTO thyristor or IGCT, and three capacitors. Of course, there is a switch element.

Each DC current stage 14, 24, 34 includes a smoothing reactor that can attenuate the ripple of the current passing through the rectifying circuit composed of the switch element. The smoothing reactor consists of inductors at the input and output terminals of each of the DC current stages 14, 24 and 34, respectively, and the two inductors have mutual inductances. Such smoothing reactor may be installed collectively in the entire DC stage (40).

The current type pulse width modulation converters 12, 22, and 32 have an Insulated Gate Bipolar Transistor (IGBT) and a Field Effect (FET) in addition to the GTO Thyristor (GTO Turn-off Thyristor), the Integrated Gate-Cummutated Thyristor (IGCT), and the like, as shown in FIG. It can be implemented with various power semiconductor devices such as transistors.

In addition, although not shown in Figure 1, the power conversion system according to the present invention may further include a control unit for adjusting the modulation index of the current type pulse width modulation converter (12, 22, 32). The controller controls the modulation index to obtain the output of each of the wind turbines 10, 20, 30 corresponding to the direct current of the current type pulse width modulation converter 12, 22, 32.

That is, the current type pulse width modulation converters 12, 22, and 32 convert the input AC power into direct current power. Since the current type pulse width modulation converters 12, 22, and 32 are connected in series, the direct current The currents of the stages are the same, but the voltages of the respective DC current stages 14, 24, and 34 may all be different. At this time, the controller (not shown) adjusts the modulation indexes of the current type pulse width modulation converters 12, 22, and 32 so as to satisfy this condition. More detailed operation will be described later with reference to FIG. 2.

In the conventional AC power transmission system, a large capacity booster transformer has to be installed in order to supply power by using a power supply AC voltage. However, as shown in FIG. 1, when the current type pulse width modulation converters 11, 22, and 32 are connected in series, power can be directly supplied by using a high voltage DC voltage without installing a separate boosting transformer. . In addition, such a DC power transmission system is highly efficient because power loss can be minimized when an AC system to be powered such as an offshore wind farm is very far apart.

In FIG. 1, when the voltage of each of the DC current stages 14, 24, and 34 is several kV to several tens of kV, the voltage of the a _{k th} full DC current stage 40 may be a voltage of several tens of kV to several hundred kV. The a- _k total DC current stage 40 serves as a DC transmission line of an AC-to-DC power converter consisting of k current-type pulse width modulation converters 10, 20, and 30. In the case of power generation systems, this is a long transmission line connected to a remote alternating current system.

FIG. 2 is a diagram illustrating an equivalent model seen from a DC terminal of the AC-DC power converter of FIG. 1.

In the AC-DC power converter of the power conversion system of FIG. 1, the power output of each wind turbine 10, 20, 30 is dependent on the wind speed applied to the wind turbine and the pitch of the blades. Accordingly, the current type pulse width modulated converters 12, 22, and 32 connected to each of the wind turbines 10, 20, and 30 have a modulation index of PWM in order to obtain a corresponding output of the generator corresponding to the input current of the direct current current source. Index). If the pitches of the wind turbines 10, 20, 30 are the same, the output power of the wind turbines 10, 20, 30 is usually proportional to the square of the wind speed n. In this case, each wind turbine 10, 20 , The modulation index of 30) is proportional to the square of the wind speed. Therefore, if the modulation index of the wind power generator with the largest output is set to a value less than or equal to 1 and the modulation index of the current pulse width modulation converter connected to another generator is adjusted in proportion to the quadratic of the wind speed, each wind generator (10, 20, 30) ), Even if the output of the power is different from each other, the continuous operation is possible at the maximum output point of each wind turbine (10, 20, 30).

Applying the above control technique, each current type pulse width modulation converter (10, 20, 30) can be equivalent to a variable voltage source proportional to the square of the wind speed as shown in FIG. If the wind power generators 10, 20, 30 are driven by an electric motor, the polarity of the equivalent voltage source is changed to absorb energy from the current source. When the wind speed applied to the entire wind farm is low, if the DC current is kept constant, as shown in Fig. 2, the voltage of the DC power transmission stage naturally decreases. At this time, in order to reduce the transmission loss, the modulation index of the current type pulse width modulation inverter (not shown) of the AC system power supply may be adjusted to reduce the magnitude of the DC stage current and maintain the DC stage transmission voltage constant.

3 is a second embodiment of the AC-DC power conversion unit of the power conversion system according to the present invention.

As shown in FIG. 3, the AC-DC DC conversion unit of the power conversion system according to the second embodiment includes the b wind turbine 50, the b _first current type pulse width modulation converter 51, and the b ₁ direct current. and the current stage (52), a b ₂ a current-type pulse-width modulation converter 53 and, first b ₂ a direct current stage (54), a b ₃ a current type pulse-width-modulation converter (55), the b ₃ DC And a current stage 34 and a b- _{k th} full DC current stage 40.

3 is similar to the AC-to-DC power converter shown in FIG. 1, but does not connect the output power of a plurality of wind generators in series to a current type pulse width modulated converter. Shows a structure in which the output power in the circuit is connected in series to the current type pulse width modulation converters 51, 53, and 55.

That is, when the winding of the b wind turbine 50 is composed of several three-phase windings having independent neutral points, the AC stages of the current type pulse width modulation converters 51, 53, and 55 are connected to the respective Samsung windings, and each current type pulse is used. By connecting the DC stages of the width modulation converters 51, 53, and 55 in series, the power conversion system of the unit generator 50 can be configured.

3 is an application example for a wind power generator having three three-phase windings, and the number of Samsung windings may be optimally determined according to the capacity of the power device used in the generator output and the converter. The role and operation principle of each component are similar to those described with reference to FIG.

4 is a third embodiment of an AC-to-DC power conversion unit of the power conversion system according to the present invention.

As shown in FIG. 4, the AC-DC DC conversion unit of the power conversion system according to the second embodiment includes a rotating unit 60, a c wind turbine 61, a c multi-winding transformer 62, and a c _{The first} current type pulse width modulation converter 63, the c _first DC current stage 64, the c _second current type pulse width modulation converter 65, the c _second DC current stage 66, and the c _{And a third} current pulse width modulation converter 67, a c _third DC current stage 68, and a c _k all DC current stage 69.

FIG. 4 is almost similar to the AC-to-DC power converter shown in FIG. 3, but in the case where the c wind turbine 61 is a three-phase single winding of distribution-high voltage, the AC output of the c wind turbine 61 is equal to the AC output. The low voltage current type pulse width modulation converters 63, 65, and 67 are connected in series using the winding transformer 62. FIG.

That is, instead of connecting the output power of the plurality of wind generators in series with the current type pulse width modulation converter, the output power in the unit B wind turbine 50 which is one unit generator is converted into the current type pulse width modulation converter 51, 53, 55. ) Shows the structure connected in series. The role and operation principle of each component are similar to those described with reference to FIG.

5 is a first embodiment of the DC-to-AC power conversion unit of the power conversion system according to the present invention.

As shown in FIG. 5, the DC-to-AC power conversion unit of the power conversion system according to the first embodiment includes a d-DC current stage 70, a d-current pulse width modulation inverter 72, and a d-th transformer. It is configured to include.

The d-th DC current stage 70 serves to attenuate the ripple of the DC current as in the DC current stages 14, 24, and 34 of FIG. 1, and the d-current current pulse width modulation inverter 72 receives the DC power. It converts into AC power. In this case, the term 'inverter' is used in terms of converting DC power into AC power. However, when power is supplied to the wind generator side, such as when the wind generator side is driven by an electric motor, that is, the flow of power On the contrary, it is possible to convert the AC power into DC power.

The d th connection transformer adjusts the voltage of the AC power converted through the d current type pulse width modulation inverter 72 according to the voltage level of the AC system, and serves as electrical insulation required in connection with the system. To perform.

Although FIG. 5 shows six switch elements of the d current type pulse width modulation inverter 72, a plurality of switch elements may be formed to correspond to the magnitude of generated power. In addition, individual switches, denoted as GTO thyristors or IGCTs, can be thought of as multiple GTO thyristors or IGCTs connected in series and on-off at the same time as a single switch in order to operate at a transmission-class DC voltage.

Thus, the electric power generated in the remote power generation such as in the offshore wind farm is transmitted to the high voltage DC power through the AC to DC power converter of Figure 1, which includes the current type pulse width modulation inverter 72 of FIG. By passing through the DC-to-AC power converter, AC power can be supplied to the AC system in compliance with the Grid Code.

6 is a second embodiment of the DC-to-AC power conversion unit of the power conversion system according to the present invention.

DC versus AC power conversion of the electric power conversion system according to the second embodiment, as shown in Fig unit comprises: e a direct current stage (80), a e ₁ direct-current stage (81), a e ₁ current pulses Width modulated converter 82, e- _second DC current stage 83, e- _second current pulse width modulation converter 84, e- _k DC current stage 85, and e- _k current pulse And a width modulated converter 86 and an e- _{k th} transformer 87.

6 is similar to the structure of the wind farm shown in FIG. 4, in which small current type pulse width modulation converters 82, 84, and 86 are connected in series and an e- _k multi-winding transformer 87 is used on an AC grid side. To deliver power. Since the operation principle is similar to that described in FIG. 4, the description thereof is omitted.

7 is a view showing the overall configuration of a power conversion system according to an embodiment of the present invention.

As shown in FIG. 7, the power conversion system according to the embodiment of the present invention includes a _first wind power generator 100, a _first power absorber 102, and a _first current pulse width modulated converter. 104, f _1st switch element 106, f ₁ DC current stage 108, f _2nd wind power generator 200, f _2nd power absorber 202, f _2nd current and type pulse-width-modulation converter (204), the f ₂ switch elements (206), a f _2, and a direct current stage (208), a f _k wind turbine 300, the f _k power absorber 302 F _k current type pulse width modulation converter 304, f _k switch element 306, f _k DC current stage 308, f _k all DC current stage 400, The f current type pulse width modulation inverter 500 and the f th transformer are comprised.

 The power conversion system of FIG. 7 has a structure similar to that of connecting the AC-DC power converter of FIG. 1 to the DC-to-AC power converter of FIG. 5. That is, the output terminals of the k wind turbines 100, 200, and 300 are connected to the k current pulse width modulation converters 104, 204, and 304, and each of the current pulse width modulation converters 104, 204, and 304 is connected. A DC line is connected to each other in series to form a transmission line, and then connected to a current type pulse width modulation inverter 500 having a transmission class capacity, thereby supplying AC power that meets the grid linkage rule to the AC system.

However, in the power conversion system of FIG. 7, power absorbing devices 102, 202, and 302 and switch elements 106, 206, and 306 are added to the configurations shown in FIGS. 1 and 5, and current type pulse width modulation is performed. Instead of six switch elements including GTO thyristors or IGCTs of the inverter 500, a plurality of switch elements have a structure connected in series.

First, the role of the power absorbing devices 102, 202, and 302 will be described. In order to absorb the effective power of each wind generator (100, 200, 300) in the event of an AC system of the wind power generation system, a resistor is installed in series at the output terminal of each wind generator (100, 200, 300), and then In operation, short-circuit the resistance by using a magnetic contactor, and in case of an accident, by opening the magnetic contactor terminal and inserting a resistor into a circuit, power generation can be absorbed to prevent over speed of the generator. In case of such an accident, the effective power absorber may be similarly installed in the AC system, and of course, may be installed in a DC power transmission terminal.

The reason for adding the switch elements 106, 206, 306 is as follows. If there is a problem in each unit wind generator and the current type pulse width modulation converter connected to the wind turbine, there is a possibility that the entire power generation system may be stopped due to the circuit structure characteristics of the power conversion system of the present invention having a series of DC current stages connected in series. . To prevent this, in case of failure of an individual generator or corresponding current pulse width modulation converter, it is possible to short-circuit the DC terminal of each converter by completely conducting the semiconductor switch of each current pulse width modulation converter. This allows the faulty wind turbine to be separated from the grid, and the entire power generation system can continue to be powered by the remaining generators.

In addition, in preparation for an accident such as the inability to control semiconductor switches of the current type pulse width modulation converters 104, 204, and 304, the current type pulse width modulation converters 104, 204, 304, and the like as shown in FIG. Power semiconductor switches, such as thyristors, can be installed in parallel to keep the current flowing through the semiconductor switch when the current pulse-width modulated converter is out of control, enabling continuous generation using the remaining generators. have.

As described with reference to FIG. 5, the number of switch elements including GTO thyristors or IGCTs of the current type pulse width modulation inverter 500 connected in series instead of six corresponds to power generated in the wind turbine. This is to have a capacity to.

In addition, although FIG. 7 shows that inductors for maintaining a constant current source are provided in the individual current type pulse width modulation converters 104, 204, and 304, respectively, the current type pulse width modulation converters 104, 204 are shown. Of course, the DC terminal of the 304 may be collectively configured in the f _k all DC current terminal 400 connected in series.

In FIG. 7, the current type pulse width modulation converter 104, 204, 304 also enables the wind turbines 100, 200, 300 to be driven by an electric motor, thereby facilitating startup of the wind turbines 100, 200, 300. In the installation and repair of the wind turbines 100, 200 and 300, it is also possible to move the wind turbine blades to a specific position as needed.

In addition, through the phase control of the AC output current of the current type pulse width modulation converter 104, 204, 304 with respect to the terminal voltage of the wind power generator (100, 200, 300), the generator can operate to generate the maximum power at the minimum current. .

In addition, when the current type pulse width modulation converters 104, 204, and 304 are connected in series, a carrier wave for pulse width modulation of each unit converter appears in the DC stage by phase shifting with a carrier of an adjacent converter. The frequency of the pulsating voltage can be made significantly higher than the PWM switching frequency and can be used to greatly reduce the inductance of the DC stage.

In order to meet the power factor control regulations of the Grid Code, the current in the DC stage controls the phase of the alternator output voltage of the alternating current of each power conversion section to a certain level even if the output of the wind farm is very low. In this case, by controlling the phase of the alternating current of the current type pulse width modulation inverter 500 connected to the alternating current system, the phase power is absorbed by the capacitor connected to the alternating current system and viewed from the alternating current system. Power factor can be controlled extensively from truth to ground. This control can also be used to provide continuous reactive power in the event of an AC power system accident as required by the Grid Code.

With the above configuration, the power conversion system according to an embodiment of the present invention complies with the Grid Code without a separate booster transformer or a large DC-to-AC power conversion device, and at the same time, the wind power generated by the high voltage of the transmission class. To enable direct current transmission. In addition, the installation area and the weight of the power conversion system of the entire wind farm are minimized to simplify the structure and improve the power generation and transmission efficiency.

100: f ₁ wind power generator 102: f ₁ power absorber
104: f ₁ current type pulse width modulation converter 106: f ₁ switch element
108: f ₁ DC current stage 200: f ₂ wind power generator
202: f _2nd power absorption device 204: f ₂ current type pulse width modulation converter
206: f _2nd switch element 208: f ₂ DC current stage
300: f _k wind power generator 302: f _k power absorber
304: The switch element f _k: f _k the current type converter 306, a pulse width modulation
308: The f _k direct current stage 400: The total DC current stage f
500: f current type pulse width modulation inverter

Claims (14)

A plurality of current type pulse width modulation converters each connected to two or more AC power sources to convert AC into DC,
And a DC terminal of the plurality of current type pulse width modulation converters is connected in series with each other. The method of claim 1,
The two or more AC power is a power conversion system, characterized in that each different wind generator. The method of claim 1,
And the two or more AC power sources are different power generators in one wind generator. The method of claim 1,
And a control unit for adjusting a modulation index of the current type pulse width modulation converter to obtain an output of the AC power source corresponding to the DC terminal current of the current type pulse width modulation converter. The method of claim 1,
And a transformer for connecting the AC power supply and the current type pulse width modulation converter. The method of claim 5, wherein
And the transformer is a multi-winding transformer. The method of claim 1,
And a power absorbing device including a resistor and a magnetic contactor at an output terminal of the AC power source or an AC terminal of the current type pulse width modulation converter. The method of claim 1,
And a current type pulse width modulation inverter connected to the DC terminal of the current type pulse width modulation converter. The method of claim 8,
And a switch element at an AC terminal of the current type pulse width modulation inverter. The method of claim 8,
And a link transformer connected to an AC terminal of the current pulse width modulation inverter. The method of claim 10,
The associated transformer is a power conversion system, characterized in that the multi-winding coupled transformer. Connect the output terminals of two or more AC power supplies to the AC terminals of a plurality of current pulse width modulation converters,
And converting AC power generated by the AC power source into DC power by connecting the DC terminals of the plurality of current type pulse width modulation converters in series. The method of claim 12,
And adjusting the modulation index of the current type pulse width modulation converter to obtain an output of the AC power source corresponding to the direct current of the current type pulse width modulation converter. The method of claim 12,
And converting the DC power into AC power by connecting a current pulse width modulation inverter to a DC terminal of the current pulse width modulation converter.

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