TWI730649B - Power conversion system and method - Google Patents

Abstract

The present disclosure provides a power conversion system. The power conversion system includes at least one power conversion device and a controller. The power conversion device is connected to a DC grid. Each power conversion device includes a power generating unit, an inverting unit, an intermediate-frequency transformer and a rectifying device. The inverting unit is electrically connected to the corresponding power generating unit and is configured for converting an output voltage of the power generating unit into a first AC voltage. The intermediate-frequency transformer is electrically connected to the inverting unit for boosting the first AC voltage to a second AC voltage. The rectifying device is electrically connected to the intermediate-frequency transformer and the DC grid and is configured for rectifying the second AC voltage and outputting a DC voltage. The controller receives the high-level control command and controls the power conversion operation of the power conversion device accordingly.

Description

Power conversion system and method

The present disclosure relates to a power conversion system and method, and more particularly to a power conversion system and method coupled to a DC power grid.

With the rapid development of new energy power generation, various power generation and energy storage systems are gradually connected to the grid. Before being connected to the AC power grid, long-distance power transmission is required. In the prior art, AC cables are often used for power transmission. However, long-distance AC cables are less efficient and costly. In addition, long-distance AC cables are coupled with a large number of converters, which is prone to wide-band oscillations, which is not conducive to system stability.

Therefore, how to develop a power conversion system and method that can improve the above-mentioned prior art is actually an urgent need at present.

The purpose of the present disclosure is to provide a power conversion system and method, which can provide high-voltage DC power to the DC power grid through power conversion devices coupled in parallel to the DC power grid. In addition, the power conversion operation of the power conversion device can be coordinated according to upper-level control commands. Therefore, the power conversion system and method of the present disclosure adopts high-voltage direct current transmission, which can reduce the cost and reduce the loss in the power transmission process.

To achieve the above objective, the present disclosure provides a power conversion system coupled to a DC power grid. The power conversion system includes at least one power conversion device and a main controller. The power conversion device is coupled to the DC grid in parallel, and each power conversion device includes at least one power generation unit, at least one inverter unit, an intermediate frequency transformer, and a rectifier device. The inverter unit is electrically connected to the corresponding power generation unit, and is configured to convert the output voltage of the power generation unit into a first AC voltage. The intermediate frequency transformer is electrically connected to the inverter unit and boosts the first AC voltage to the second AC voltage. The rectifying device is electrically connected to the intermediate frequency transformer and the DC power grid to rectify the second AC voltage and output the DC voltage. The main controller is configured to receive an upper-level control command to coordinate the power conversion operation of the power conversion device based on the upper-level control command.

To achieve the above objective, the present disclosure further provides a power conversion method suitable for a power conversion system, wherein the power conversion system is coupled to a DC power grid and includes at least one power conversion device and a main controller, and the power conversion device is coupled in parallel to the DC power grid Each power conversion device includes at least one power generation unit, at least one inverter unit, an intermediate frequency transformer, and a rectifier device. The power conversion method includes the following steps: (a) Use an inverter unit to convert the output voltage of the corresponding power generation unit into a first AC voltage; (b) Use an intermediate frequency transformer to boost the first AC voltage to a second AC voltage; c) Utilizing the rectifying device to rectify the second AC voltage and generating a DC voltage; and (d) Utilizing the main controller to receive upper-level control commands, and coordinate the power conversion operation of the power conversion device based on the upper-level control commands.

Some typical embodiments embodying the features and advantages of the present disclosure will be described in detail in the following description. It should be understood that the present disclosure can have various changes in different implementations, which do not depart from the scope of the present disclosure, and the descriptions and illustrations therein are essentially for illustrative purposes, rather than being constructed to limit the present disclosure. public.

FIG. 1A is a schematic diagram of a circuit structure of a power conversion system according to a preferred embodiment of the present disclosure. As shown in FIG. 1A, the power conversion system 2 is coupled to the DC power grid 1 and includes at least one power conversion device 21 and a main controller 20. The power conversion devices 21 are coupled to the DC power grid 1 in parallel. The main controller 20 is configured to receive an upper-level control command and coordinate the power conversion operation of the power conversion device 21 based on the upper-level control command. The main controller 20 may be, for example, but not limited to, configured to control the output power of the power conversion device 21 as Fixed value. Further, the upper-level control command may be sent to the main controller by the upper-level DC grid dispatching center, such as a power command, but the present disclosure is not limited to this. Each power conversion device 21 includes at least one power generating unit 22, at least one inverter unit 23, an intermediate frequency transformer 24 and a rectifying device 25. Each inverter unit 23 is electrically connected to the corresponding power generation unit 22, and the inverter unit 23 is configured to convert the output voltage of the power generation unit 22 into a first AC voltage. The intermediate frequency transformer 24 is electrically connected to all the inverter units 23, and is configured to boost the first AC voltage output by each inverter unit into a second AC voltage. In some embodiments, the intermediate frequency transformer 24 includes a plurality of low-voltage windings and one high-voltage winding, wherein the low-voltage winding is electrically connected to the corresponding inverter unit 23. The rectifying device 25 is electrically connected to the intermediate frequency transformer 24 and the DC power grid 1, and the rectifying device 25 rectifies the second AC voltage and outputs high-voltage DC power to the DC power grid 1. In this way, the power conversion system 2 of the present disclosure is coupled to the DC power grid, and high-voltage DC power transmission can be adopted, which reduces the cost and the loss in the power transmission process.

In some embodiments, in any power conversion device 21, the inverter unit 23 converts the output voltage of the corresponding power generating unit 22 into a first AC voltage according to the operating frequency ω of the intermediate frequency transformer 24, wherein The frequency is equal to the operating frequency ω of the intermediate frequency transformer 24.

It should be noted that, for the power conversion device in the power conversion system 2, the power conversion device may be, for example, but not limited to, wind power generation or photovoltaic power generation, as shown in FIG. 2C and FIG. 2D, for example. Of course, if the power conversion system 2 includes multiple power conversion devices, different power generation sources may be used respectively. For example, as shown in FIG. 2A, some of the multiple power conversion devices may use wind power generation, and some of the power conversion devices may use photovoltaic power generation. The following examples illustrate the circuit structures of power conversion devices (21a, 21b) that use wind power generation and photovoltaic power generation.

3A is a schematic diagram of a circuit structure of a power conversion device using wind power generation according to a preferred embodiment of the present disclosure. As shown in FIG. 3A, the power conversion device 21a uses wind power generation, the power generation unit of the power conversion device 21a is a wind power generation unit 22a, and the wind power generation unit 22a includes a wind turbine 28 and a machine-side converter 26. The inverter unit of the power conversion device 21a is a grid-side converter 23a, which is electrically connected between the machine-side converter 26 and the intermediate frequency transformer 24, and is constructed to stabilize the DC bus voltage and control the output voltage frequency. The machine-side converter 26 converts the alternating current with varying frequency and amplitude generated by the fan 28 into direct current, and the grid-side converter 23a converts the direct current into intermediate frequency three-phase alternating current (ie, the first alternating voltage). The intermediate frequency three-phase alternating current is boosted by the intermediate frequency transformer 24 with a high step-up ratio to obtain the second alternating current, and finally the second alternating current is converted into direct current by the uncontrolled rectifier device 25 and output to the high voltage direct current transmission grid (ie, direct current grid 1) . Among them, the intermediate frequency transformer 24 has a high step-up ratio to convert low-voltage three-phase alternating current (for example, 690V) to high-voltage three-phase alternating current (for example, 26KV). The use of an intermediate frequency transformer can reduce the volume of the transformer and reduce the cost. Further, the high-voltage three-phase alternating current is converted into direct current through a high withstand voltage uncontrolled rectifier device 25, and the rectifier device 25 of the present invention can also adopt a semi-controlled device. The bus voltage of the DC power grid 1 is 35KV, but the present invention is not limited to this.

In some embodiments, the output voltage of the wind power generation unit 22a is a low voltage less than 1KV or a high voltage greater than 1KV, and the inverter unit corresponds to a low voltage inverter unit or a high voltage inverter unit. In some embodiments, the power conversion device 21a further includes a filter 27. The filter 27 is electrically connected between the grid-side converter 23a and the intermediate frequency transformer 24. The filter 27 is constructed to output current from the grid-side converter 23a. Perform filtering.

The rectifier device 25 includes a plurality of bridge arms. In this embodiment, the rectifier device includes 3 bridge arms. Each bridge arm has a node. The node is electrically connected to the intermediate frequency transformer 24 and the bridge arm is divided into two branches. Route multiple rectifier elements connected in series. In this way, the requirements for the withstand voltage of the rectifier element can be reduced. The rectifying element can be, for example, but not limited to, a diode or a semi-control device. For the voltage level of DC grid 1 (for example, 35KV), a diode or semi-control device with high withstand voltage needs to be selected. Each half of the bridge arm is composed of two or more diodes or semi-control devices in series, which can reduce The withstand voltage level of a rectifier element reduces the cost.

In some embodiments, the fan 28 is a high-pressure fan (for example, 10KV), and the corresponding machine-side converter 26 and the grid-side converter 23a are high-voltage converters. As shown in FIG. 3B, the machine-side converter 26 and the grid-side converter 23a are all multi-level converters, but not limited to this, so that the turns ratio of the intermediate frequency transformer 24 can be reduced. For example, using 10KV grade wind turbines and converters (including machine-side converters and grid-side converters), compared with the conventional 690V wind power generation system, the turns ratio of the selected intermediate frequency transformer 24 can be reduced by more than ten times For the 35KV DC transmission system, the 24 turns ratio of the intermediate frequency transformer is reduced from 26KV/690V to 26KV/10KV.

In addition, the number of wind power generation units 22a and inverter units (ie, grid-side converters 23a) is not limited to one. For example, as shown in FIG. 3C, the power conversion device 21a may include two wind power generation units 22a and two grid-side converters. In the converter 23a, the two grid-side converters 23a are electrically connected to the two wind power generation units 22a, respectively, and the intermediate frequency transformer 24 is electrically connected to the two grid-side converters 23a. In this embodiment, the intermediate frequency transformer 24 is a dual-winding step-up transformer, which can effectively save costs compared to a solution in which two independent transformers are respectively connected to two grid-side converters 23a. Of course, the wind turbine 28 can also be a wind turbine with multiple output windings. As shown in FIG. 3D, the wind turbine 28 includes two sets of output windings, and each set of output windings is electrically connected to the corresponding machine-side converter 26 to form a wind power generation unit 22a. .

In order to control the conversion of electric energy in the power conversion device 21a, in this embodiment, the power conversion device 21a is a wind power generation system, the machine-side converter 26 controls the output power of the wind turbine 28, and the grid-side converter 23a stabilizes the DC bus 29 At the same time, the frequency of the output first AC voltage is controlled by the bus voltage of, and the amplitude of the first AC voltage is clamped by the DC power grid 1 and the rectifier 25. As shown in FIG. 4, the inverter unit of the power conversion device 21a includes a first control circuit 200a. The first control circuit 200a generates a switching signal according to the operating frequency ω of the intermediate frequency transformer 24 and a plurality of electrical signals of the inverter unit. And through the switch signal to control the conversion operation of the inverter unit. The first control circuit 200a includes a calculator 201, a coordinate converter 202, a voltage regulator 203, a current regulator 204, and a PWM (Pulse Width Modulation) modulator 205. The calculator 201 generates an angle signal θ according to the operating frequency ω of the intermediate frequency transformer 24, that is, integrates the frequency ω to generate an angle signal. The intermediate frequency frequency is conventionally taken as 400 Hz, which is the rated operating frequency of the intermediate frequency transformer 24, but the present invention is not limited to this. The coordinate converter 202 generates an active current I _d and a reactive current I _{q according to the three-phase current I abc} and the angle signal θ at the AC end of the inverter unit (ie, the grid-side converter 23 a ). The voltage regulator 203 samples the DC bus voltage U _{dc of the} DC bus 29, and generates an active current command I _dr according to the DC bus voltage U _dc and its command value U _dcr . The current regulator 204 generates a three-phase control potential E _abc according to the active current command I _dr , the reactive current command I _qr , the active current I _d , the reactive current I _q and the angle signal θ. The PWM modulator 205 generates a switching signal according to the three-phase control potential E _abc , wherein the first control circuit 200a controls the switching operation of the grid-side converter 23a through the switching signal. The PWM modulator 205 may be, for example, but not limited to, generating a switching signal through SPWM (Sinusoidal PWM) or SVPWM (Space Vector PWM) technology. Among them, the reactive current command is obtained by closed-loop adjustment of the reactive power.

FIG. 5A is a schematic diagram of a circuit structure of a power conversion device using photovoltaic power generation according to a preferred embodiment of the present disclosure. As shown in Figure 5A, the power conversion device 21b uses photovoltaic power generation, the power generation unit of the power conversion device 21b is a photovoltaic power generation unit 22b, and the inverter unit 23b converts the output voltage of the photovoltaic power generation unit 22b into intermediate frequency three-phase AC power (ie, the first AC voltage). The intermediate frequency three-phase alternating current is boosted by the intermediate frequency transformer 24 with a high step-up ratio to obtain the second alternating current, and finally the second alternating current is converted into direct current by the uncontrolled rectifier device 25 and output to the high voltage direct current transmission grid (ie, direct current grid 1) . The circuit elements similar to those in FIG. 3A are denoted by the same reference numerals, and the similar circuit elements have similar functions. For details, please refer to the above description, which will not be repeated here. In addition, the number of photovoltaic power generation units 22b and inverter units 23b is not limited to one. For example, as shown in FIG. 5B, the power conversion device 21b may include two photovoltaic power generation units 22b and two inverter units 23b. The transformation unit 23b is electrically connected to the two photovoltaic power generation units 22b, and the intermediate frequency transformer 24 is electrically connected to the two inverter units 23b.

In order to control the conversion of electric energy in the power conversion device 21b, in this embodiment, the power conversion device 21b is a photovoltaic power generation system, and the inverter unit 23b controls the frequency of the output first AC voltage, and the amplitude of the first AC voltage It is clamped by the DC grid 1 and the rectifier 25. As shown in FIG. 6, the inverter unit 23b of the power conversion device 21b includes a second control circuit 200b. The second control circuit 200b generates a switch according to the operating frequency ω of the intermediate frequency transformer 24 and a plurality of electrical signals from the inverter unit 23b. Signal, and the switching operation of the inverter unit 23b is controlled by the switch signal. The second control circuit 200b includes a calculator 201, a coordinate converter 202, an MPPT (Maximum power point tracking) controller 206, a voltage regulator 203, a current regulator 204, and a PWM modulator 205. The calculator 201 generates an angle signal θ according to the operating frequency ω of the intermediate frequency transformer 24, that is, integrates the frequency ω to generate an angle signal. The intermediate frequency frequency is conventionally taken as 400 Hz, which is the rated operating frequency of the intermediate frequency transformer 24, but the present invention is not limited to this. The coordinate converter 202 generates an active current I _d and a reactive current I _{q according to the three-phase current I abc} and the angle signal θ at the AC end of the inverter unit 23 b. The MPPT controller 206 obtains the output voltage U _o and the output current I _{o of the} photovoltaic power generation unit 22, and calculates the DC power according to the output voltage U _o and the output current I _o to obtain the output voltage command U _or according to the MPPT algorithm. The voltage regulator 203 generates an active current command I _dr according to the output voltage U _o and the output voltage command U _{or of the} photovoltaic power generation unit 22 b. The current regulator 204 generates a three-phase control potential E _abc according to the active current command I _dr , the reactive current command I _qr , the active current I _d , the reactive current I _q and the angle signal θ. The PWM modulator 205 generates a switching signal according to the three-phase control potential E _abc , wherein the second control circuit 200b controls the switching operation of the inverter unit 23b through the switching signal. The PWM modulator 205 can be, for example, but not limited to, modulating and generating a switching signal through SPWM (Sinusoidal PWM) or SVPWM (Space Vector PWM) technology. Among them, the reactive current command is obtained by closed-loop adjustment of the reactive power.

In some embodiments, the power conversion device 21 includes a first power conversion device and a second power conversion device, and the first power conversion device and the second power conversion device are coupled to the DC power grid 1 in parallel. The first power conversion device is a wind power generation system, and the second power conversion device is a photovoltaic power generation system.

In some embodiments, in addition to the power conversion device 21, the power conversion system 2 further includes an energy storage device 3. As shown in FIG. 1B, the energy storage device 3 and all the power conversion devices 21 are coupled to the DC grid 1 in parallel. Furthermore, as shown in FIG. 2B, the power conversion system 2 includes a power conversion device 21a that uses wind power generation, a power conversion device 21b that uses photovoltaic power generation, and an energy storage device 3, forming a large-scale wind-solar hybrid power generation system. Of course, the present invention is not limited to this. As shown in FIG. 2C, the power system 2 includes a power conversion device 21a that uses wind power generation and an energy storage device 3 to form a wind storage power generation system; as shown in FIG. 2D, the power system 2 includes The photovoltaic power conversion device 21b and the energy storage device 3 form a light storage power generation system.

Based on the foregoing embodiments and their variants, the present disclosure proposes a high-voltage DC coupling system, in which at least one power conversion device (such as a wind power generation device, a photovoltaic power generation device) and/or an energy storage device are independently coupled in parallel to the DC power grid, Convenient expansion; each power generation unit converts the generated electric energy into intermediate frequency alternating current through its own inverter unit, and converts it into high-voltage direct current through its respective step-up transformer and rectifier device to connect to the high-voltage direct current transmission grid, and the energy storage device through the DC converter Convert low-voltage direct current to high-voltage direct current and connect it to the high-voltage direct current transmission grid, and use direct current transmission technology to transmit the electric energy to the remote grid or user end. Under the same voltage level, the transmission capacity is stronger and the loss is smaller; it is controlled by the main controller. , To achieve power generation, grid regulation and dispatch functions.

2B schematically shows a high-voltage direct current coupling system, which includes a power conversion device 21a that uses wind power generation, a power conversion device 21b that uses photovoltaic power generation, an energy storage device 3, and a main controller 20. The power conversion device 21a, the power conversion device 21b, and the energy storage device 3 are respectively connected in parallel to the DC grid 1 as independent devices to realize high-voltage DC transmission of wind turbines or photovoltaic power generation, and each device can be integrated into the DC grid 1 or connected at any time. The DC grid 1 is disconnected to facilitate expansion. The main controller 20 detects the output power of the power conversion device 21a and the power conversion device 21b, and calculates the power command of the energy storage device 3 according to the grid dispatch command to control the charging and discharging operation of the energy storage device 3, so that the power conversion system 2 is fixed Power output; or the main controller 20 calculates the power command of the energy storage device 3 according to the upper-level control command to control the charging and discharging operation of the energy storage device 3, and achieve goals such as peak shaving and valley filling and smoothing of new energy. It should be noted that the above description is only used for schematic description, and does not constitute a limitation to the present disclosure. Of course, the system of this embodiment may include multiple wind power generation devices, multiple photovoltaic power generation devices, and multiple energy storage devices at the same time.

FIG. 2C schematically shows another high-voltage direct current coupling system, which includes a power conversion device 21a that uses wind power generation, an energy storage device 3, and a main controller 20. The power conversion device 21a and the energy storage device 3 are separately coupled to the DC grid 1 in parallel as independent devices to realize high-voltage DC transmission of wind power generation, and each device can be integrated into the DC grid 1 or disconnected from the DC grid 1 at any time, which is convenient Extension. The main controller 20 detects the output power of the power conversion device 21a, and calculates the power command of the energy storage device 3 according to the grid dispatch command to control the charging and discharging operation of the energy storage device 3, so that the power conversion system 2 achieves a fixed power output; or The controller 20 calculates the power command of the energy storage device 3 according to the superior control command, so as to control the charging and discharging operation of the energy storage device 3, so as to achieve goals such as peak shaving and valley filling, and smoothing of new energy. It should be noted that the above description is only used for schematic description, and does not constitute a limitation to the present disclosure. Of course, the system of this embodiment may include multiple wind power generation devices, that is, corresponding to one wind power plant, and the wind power generation device may be onshore wind power or offshore wind power.

FIG. 2D schematically shows another high-voltage direct current coupling system, which includes a power conversion device 21b that uses photovoltaic power generation, an energy storage device 3, and a main controller 20. The power conversion device 21b and the energy storage device 3 are separately coupled to the DC grid 1 in parallel as independent devices to realize high-voltage DC transmission of photovoltaic power generation, and each device can be integrated into the DC grid 1 or disconnected from the DC grid 1 at any time, which is convenient Extension. The main controller 20 detects the output power of the photovoltaic power generation device 21b, and calculates the power command of the energy storage device 3 according to the grid dispatch command to control the charging and discharging operation of the energy storage device 3, and the power conversion system 2 realizes a fixed power output; or the main control The device 20 calculates the power command of the energy storage device 3 according to the superior control command, so as to control the charging and discharging operation of the energy storage device 3, so as to achieve goals such as peak shaving and valley filling, and smoothing of new energy. It should be noted that the above description is only used for schematic description, and does not constitute a limitation to the present disclosure. Of course, the system of this embodiment may include multiple photovoltaic power generation devices, that is, one photovoltaic power plant corresponds to one.

FIG. 7 is a schematic diagram of a circuit structure of an energy storage device according to a preferred embodiment of the present disclosure. As shown in FIG. 7, the energy storage device 3 includes a plurality of energy storage units 30, and each energy storage unit 30 includes a connected energy storage element 31 and a DC converter 32, wherein the plurality of DC converters 32 are connected in series to form a DC The output terminal is coupled to the DC power grid 1. In this embodiment, the main controller 20 may be, for example, but not limited to, configured to control the sum of the output power of the plurality of power conversion devices 21 and the output power of the energy storage device 3 to a fixed value. In some embodiments, the main controller 20 receives the state of charge of the plurality of energy storage elements 31, and receives an upper-level control command from the DC grid 1, and the main controller 20 generates the plurality of DC converters 32 based on the upper-level control command and the state of charge. According to the actual demand, the output power of each energy storage unit 30 can be adjusted.

In order to control the electric energy conversion of the energy storage device 3, in some embodiments, as shown in FIG. 8, the energy storage unit 30 further includes a third control circuit 300, and the third control circuit 300 includes a calculator 301 and a power regulator 302. , Current regulator 303 and PWM modulator 304. The calculator 301 calculates the power P of the energy storage element according to the current I and the voltage U of the energy storage element. _{The power regulator 302 receives the power command P r} of the energy storage unit 30 provided by the main controller 20, and generates a current command I _{r of the} energy storage element according to the power P and the power command P _r of the energy storage element. The main controller 20 allocates the total power command of the energy storage device according to the state of charge of each energy storage element to obtain the power command P _r of each energy storage unit, and each energy storage unit automatically allocates the high-voltage side DC voltage according to the power command. The current regulator 303 generates a direct current potential E according to the current I of the energy storage element and the current command _Ir. The PWM modulator 304 generates a switching signal according to the DC potential E, and the third control circuit 300 controls the switching operation of the DC converter 32 through the switching signal.

By providing the energy storage device 3, the main controller 20 can flexibly adjust the distribution of power output in the power conversion system 2 by controlling the energy storage device 3. For example, the main controller 20 detects the generated power of the power conversion device 21, filters and smoothes the generated power to obtain the target power, and then subtracts the generated power from the target power, and adjusts the energy storage device 3 according to the difference. The power command of each energy storage unit controls the charging and discharging operation of the energy storage unit to smooth power fluctuations, thereby making the actual output power of the power conversion system 2 consistent with the target power. In addition, the main controller 20 can perform energy dispatch based on the load demand of the DC grid 1 and the power generation situation of the power conversion device 21. For example, when the power conversion device 21 has a large amount of power generation and the DC grid 1 has a small load demand, it can Excess energy is stored in the energy storage element 31 of the energy storage device 3. When the power conversion device 21 generates less power and the load demand of the DC grid 1 is large, the energy stored in the energy storage element 31 can be released. Output and supply to the DC grid 1.

The main controller 20 communicates with each power conversion device 21 and can implement different functions according to the actual needs of the DC power grid 1. When the DC grid 1 requires the power conversion system 2 to output fixed power, if the power conversion system 2 does not include an energy storage device, the main controller 20 detects the output power of each power conversion device 21 and limits the power of at least one power conversion device 21 Run to output target power to the DC grid 1 to meet the actual needs of the DC grid 1. If the power conversion system 2 includes an energy storage device 3, the main controller 20 detects the output power of each power conversion device 21, such as photovoltaic power generation devices and / Or the output power of the wind power generation device, the target power of the fixed power output minus the output power of the power conversion device 21 to obtain the power command of the energy storage device 3, and the power command of the energy storage device 3 according to the state of charge of the energy storage element 31 Allocate to each energy storage unit 30, and achieve a fixed power output by controlling the charging and discharging operation of the energy storage device 3.

In some embodiments, the main controller 20 detects the power generated by the wind power generation device and/or the photovoltaic power generation device, and performs filtering and smoothing processing on the power through the energy storage device 3. Specifically, the smoothed target power minus the actual generated power, Obtain a fluctuating power command that needs to be smoothed by the energy storage device 3. In some embodiments, the main controller 20 controls the energy storage device 3 to implement energy time shift according to the grid dispatch and the power generation conditions of each power generation device. For example, when the power generation device generates more electric energy and the demand is small, the energy is stored in the energy storage device. In the element 31; when the power generation device generates less electric energy and the demand is greater, the energy in the energy storage element 31 is output to meet the actual demand.

Fig. 9 is a schematic diagram of steps of a power conversion method according to a preferred embodiment of the present disclosure. The power conversion method can be applied to the power conversion system 2 shown in FIGS. 1A and 1B. As shown in FIG. 9, the power conversion method includes the following steps S1, S2, S3, and S4.

In step S1, the inverter unit 23 is used to convert the output voltage of the corresponding power generation unit 22 into a first AC voltage. In some embodiments, the inverter unit 23 converts the output voltage of the corresponding power generating unit 22 into the first AC voltage according to the operating frequency ω of the intermediate frequency transformer 24, wherein the frequency of the first AC voltage is equal to the operating frequency ω.

In step S2, the intermediate frequency transformer 24 is used to boost the first AC voltage to the second AC voltage.

In step S3, the rectifying device 25 is used to rectify the second AC voltage and generate a DC voltage.

In step S4, the main controller 20 is used to receive the upper-level control command, and the power conversion operation of the plurality of power conversion devices 21 is coordinated based on the upper-level control command. In some embodiments, the main controller 20 is used to control the output power of the plurality of power conversion devices 21 to a fixed value. In other embodiments, the main controller 20 is used to control the sum of the output power of the multiple power conversion devices 21 and the output power of the energy storage device 3 to a fixed value.

In some embodiments, in order to control the power conversion device 21a using wind power generation, the power conversion method further includes the following steps. First, the angle signal θ is generated according to the operating frequency ω of the intermediate frequency transformer 24. _{Next, the active current I d} and the reactive current I _{q are generated according to the three-phase current I abc} and the angle signal θ of the AC end of the inverter unit (ie, the grid-side converter 23 a) corresponding to the wind power generation unit 22 a. Then, the bus voltage U _{dc of the} DC bus 29 is sampled, and the active current command I _{dr is} generated according to the DC bus voltage U _dc and its command value U _dcr . Then, a three-phase control potential E _{abc is} generated according to the active current command I _dr , the reactive current command I _qr , the active current I _d , the reactive current I _q and the angle signal θ. Finally, a switching signal is generated according to the three-phase control potential E _abc , and the operation of the inverter unit (ie, the grid-side converter 23a) corresponding to the wind power generation unit 22a is controlled by the switching signal.

In some embodiments, in order to control the power conversion device 21b using photovoltaic power generation, the power conversion method further includes the following steps. First, the angle signal θ is generated according to the operating frequency ω of the intermediate frequency transformer 24. _{Then, the active current I d} and the reactive current I _{q are generated according to the three-phase current I abc} and the angle signal θ at the AC end of the inverter unit 23 b corresponding to the photovoltaic power generation unit 22 b. _{Then, the output voltage U o} and the output current I _{o of the} photovoltaic power generation unit 22 are obtained, the DC power is calculated according to the output voltage U _o and the output current I _o , and the output voltage command U _{or is} obtained according to the MPPT algorithm. Then, the active current command I _{dr is} generated according to the output voltage U _o and the output voltage command U _{or of the} photovoltaic power generation unit 22 b. Then, a three-phase control potential E _{abc is} generated according to the active current command I _dr , the reactive current command I _qr , the active current I _d , the reactive current I _q and the angle signal θ. Finally, a switching signal is generated according to the three-phase control potential E _abc , and the operation of the inverter unit 23b corresponding to the photovoltaic power generation unit 22b is controlled by the switching signal.

In some embodiments, in order to control the energy storage device 3, the power conversion method further includes the following steps. First, calculate the power P of the energy storage element according to the current I and the voltage U of the energy storage element. _{Then, the power command P r} of the energy storage unit 30 provided by the main controller 20 is received _{, and the current command I r of the} energy storage element is generated according to the power P and the power command P _r of the energy storage element. Then, a direct current potential E is generated according to the current I of the energy storage element and the current command _Ir. Finally, a switching signal is generated according to the potential E, and the operation of the DC converter 32 is controlled by the switching signal.

In summary, the present disclosure provides a power conversion system and method, which can provide high-voltage DC power to the DC power grid through at least one power conversion device coupled in parallel to the DC power grid. In addition, the power conversion operations of multiple power conversion devices can be coordinated according to the upper-level control commands of the DC grid. Therefore, the power conversion system and method of the present disclosure adopts high-voltage direct current transmission, which can reduce the cost and reduce the loss in the power transmission process. Furthermore, by setting the energy storage device, the main controller can flexibly adjust the distribution of power output in the power conversion system by controlling the energy storage device to achieve the control target. What's more, the main controller can perform energy dispatch according to the supply load of the DC grid and the power generation situation of the power conversion device. For example, when the power conversion device has a large amount of power generation and the supply load of the DC grid is small, the excess energy can be It is stored in the energy storage element of the energy storage device, and when the power conversion device generates less power and the supply load of the DC grid is large, the energy stored in the energy storage element can be released and supplied to the DC grid.

It should be noted that the foregoing are only preferred embodiments proposed for explaining the present disclosure, and the present disclosure is not limited to the described embodiments, and the scope of the present disclosure is determined by the scope of the patent application. Moreover, the present disclosure can be modified in many ways by those who are familiar with this technology, but it does not deviate from those who wish to protect the scope of the patent application.

1: DC grid 2: Power conversion system 20: Main controller 21, 21a, 21b: Power conversion device 22: Power generation unit 22a: Wind power generation unit 22b: Photovoltaic power generation unit 23, 23b: Inverter unit 23a: Grid-side converter 24: Intermediate frequency transformer 25: Rectifier 26: Machine-side converter 27: Filter 28: Fan 29: DC bus 200a: First control circuit 200b: Second control circuit 201: Calculator 202: Coordinate converter 203: Voltage Regulator 204: Current regulator 205: PWM modulator 206: MPPT controller 3: Energy storage device 30: Energy storage unit 31: Energy storage element 32: DC converter 300: Third control circuit 301: Calculator 302: Power Regulator 303: current regulator 304: PWM modulator ω: operating frequency θ: angle signal I _abc : three-phase current I _d : active current I _dr : active current command I _q : reactive current I _qr : reactive current Command U _o : output voltage U _dc : DC bus voltage U _dcr : command value I _o : output current I: current U: voltage P: power P _r : power command E _abc , E: electric potential

FIG. 1A is a schematic diagram of a circuit structure of a power conversion system according to a preferred embodiment of the present disclosure.

FIG. 1B is a schematic diagram of a circuit structure of a power conversion system according to another preferred embodiment of the present disclosure.

FIG. 2A is a schematic diagram of a circuit structure of a variation of the power conversion system of FIG. 1A.

FIG. 2B, FIG. 2C, and FIG. 2D are schematic diagrams of the circuit structure of a modification of the power conversion system of FIG.

3A is a schematic diagram of a circuit structure of a power conversion device using wind power generation according to a preferred embodiment of the present disclosure.

3B, 3C, and 3D are schematic diagrams of the circuit structure of a modified example of the power conversion device of FIG. 3A.

4 is a schematic diagram of the circuit structure of the power conversion device and its control circuit of FIG. 3A.

FIG. 5A is a schematic diagram of a circuit structure of a power conversion device using photovoltaic power generation according to a preferred embodiment of the present disclosure.

FIG. 5B is a schematic diagram of a circuit structure of a modification of the power conversion device of FIG. 5A.

6 is a schematic diagram of the circuit structure of the power conversion device and its control circuit of FIG. 5A.

FIG. 7 is a schematic diagram of a circuit structure of an energy storage device according to a preferred embodiment of the present disclosure.

FIG. 8 is a schematic diagram of the circuit structure of the energy storage device and its control circuit of FIG. 7.

Fig. 9 is a schematic diagram of steps of a power conversion method according to a preferred embodiment of the present disclosure.

1: DC grid

2: Power conversion system

20: main controller

21: Power conversion device

22: power generation unit

23: Inverter unit

24: Intermediate frequency transformer

25: Rectifier

Claims (22)

A power conversion system, coupled to a DC power grid, and including: At least one power conversion device is coupled to the DC grid in parallel, and each of the power conversion devices includes: At least one power generation unit; At least one inverter unit, each of the inverter units is electrically connected to the corresponding power generation unit, and an output voltage of the power generation unit is converted into a first AC voltage; An intermediate frequency transformer, electrically connected to the at least one inverter unit, and boosting the first AC voltage to a second AC voltage; and A rectifier device, electrically connected to the intermediate frequency transformer and the DC grid, to rectify the second AC voltage and output a DC voltage; and A main controller is configured to receive an upper-level control command to coordinate the power conversion operation of the at least one power conversion device based on the upper-level control command. The power conversion system according to claim 1, wherein the rectifying device includes a plurality of bridge arms, each of the bridge arms has a node, the node is electrically connected to the intermediate frequency transformer and the bridge arm is divided into two branches Each branch is composed of multiple rectifier elements connected in series. The power conversion system according to claim 1 or 2, wherein the intermediate frequency transformer includes a plurality of low-voltage windings and a high-voltage winding, and each of the low-voltage windings is electrically connected to the corresponding inverter unit. The power conversion system according to claim 1, wherein in any of the power conversion devices, the inverter unit converts the output voltage of the corresponding power generation unit into the first AC according to an operating frequency of the intermediate frequency transformer Voltage, and the frequency of the first AC voltage is equal to the operating frequency. The power conversion system according to claim 4, wherein in at least one of the power conversion devices, the power generation unit is a wind power generation unit; each of the inverter units includes a first control circuit, wherein the first control circuit is based on the The operating frequency and the multiple electrical signals of the inverter unit generate a switching signal, and the switching operation of the inverter unit is controlled by the switching signal. The power conversion system according to claim 5, wherein the first control circuit includes: A calculator to generate an angle signal according to the operating frequency; A standard converter generates an active current and a reactive current according to a three-phase current of the AC terminal of the inverter unit and the angle signal; A voltage regulator, sampling the DC bus voltage of the DC bus, and generating an active current command based on the DC bus voltage and its command value; A current regulator that generates an electric potential according to the active current command, a reactive current command, the active current, the reactive current, and the angle signal; and A PWM modulator generates the switching signal according to the electric potential, wherein the first control circuit controls the operation of the inverter unit through the switching signal. The power conversion system according to claim 5, wherein the output voltage of the wind power generation unit is a low voltage less than 1KV or a high voltage greater than 1KV, and the inverter unit corresponds to a low voltage inverter unit or a high voltage inverter unit. The power conversion system according to claim 4, wherein in at least one of the power conversion devices, the power generation unit is a photovoltaic power generation unit, and each of the inverter units includes a second control circuit, wherein the second control circuit is based on the The operating frequency and the multiple electrical signals of the inverter unit generate a switching signal, and the switching operation of the inverter unit is controlled by the switching signal. The power conversion system according to claim 8, wherein the second control circuit includes: A calculator to generate an angle signal according to the operating frequency; A standard converter generates an active current and a reactive current according to a three-phase current and angle signal at the AC terminal of the inverter unit; An MPPT controller, which obtains an output voltage command according to the output voltage and an output current of the photovoltaic power generation unit; A voltage regulator, which generates an active current command according to the output voltage of the photovoltaic power generation unit and the output voltage command; A current regulator that generates an electric potential according to the active current command, a reactive current command, the active current, the reactive current, and the angle signal; and A PWM modulator generates the switching signal according to the electric potential, wherein the second control circuit controls the operation of the inverter unit through the switching signal. The power conversion system according to claim 4, wherein at least one of the power conversion devices includes a first power conversion device and a second power conversion device, coupled to the DC grid in parallel; in the first power conversion device, The power generation unit is a wind power generation unit; in the second power conversion device, the power generation unit is a photovoltaic power generation unit. The power conversion system according to any one of 8 and 10, further comprising an energy storage device, wherein the energy storage device and the power conversion device are coupled to the DC grid in parallel, and the energy storage device includes a plurality of energy storage units, Each of the energy storage units includes an energy storage element and a DC converter connected to each other, and a plurality of the DC converters are connected in series to form a DC output terminal coupled to the DC power grid. The power conversion system according to claim 11, wherein the main controller receives the state of charge of the plurality of energy storage elements, and generates power commands of the plurality of DC converters based on the upper-level control command and the state of charge. The power conversion system according to claim 12, wherein the energy storage unit further includes a third control circuit, wherein the third control circuit includes: A calculator for calculating and obtaining a power of the energy storage element according to a current and a voltage of the energy storage element; A power regulator, which generates a current command according to the power and a power command; A current regulator that generates an electric potential according to the current command and the current; and A PWM modulator generates a switching signal according to the electric potential, wherein the third control circuit controls the operation of the DC converter through the switching signal. The power conversion system according to claim 11, wherein the main controller is configured to control the output power of the at least one power conversion device to a fixed value; or the main controller is configured to control the output power of the at least one power conversion device The sum of the output power and the output power of the energy storage device is the fixed value. A power conversion method is suitable for a power conversion system, wherein the power conversion system is coupled to a DC power grid and includes at least one power conversion device and a main controller, the at least one power conversion device is coupled in parallel to the DC power grid, Each power conversion device includes at least one power generation unit, at least one inverter unit, an intermediate frequency transformer, and a rectifier device, and the power conversion method includes: (a) Using the inverter unit to convert an output voltage of the corresponding power generation unit into a first AC voltage; (b) Using the intermediate frequency transformer to boost the first AC voltage into a second AC voltage; (c) Using the rectifying device to rectify the second AC voltage and generate a DC voltage; and (d) Using the main controller to receive an upper-level control command, and coordinate the power conversion operation of the at least one power conversion device based on the upper-level control command. The power conversion method according to claim 15, wherein in the step (a), the inverter unit converts the output voltage of the corresponding power generating unit into the first AC according to an operating frequency of the intermediate frequency transformer Voltage, wherein the frequency of the first AC voltage is equal to the operating frequency. The power conversion method according to claim 16, wherein the power generation unit of at least one of the power conversion devices is a wind power generation unit, and the power conversion method further includes the steps: (e1) Generate an angle signal according to the operating frequency; (e2) Generate an active current and a reactive current according to a three-phase current and the angle signal of the AC terminal of the inverter unit corresponding to the wind power unit; (e3) Sampling the DC bus voltage of the DC bus, and generate an active current command based on the DC bus voltage and its command value; (e4) Generate an electric potential based on the active current command, a reactive current command, the active current, the reactive current, and the angle signal; and (e5) A switch signal is generated according to the electric potential, and the operation of the inverter unit corresponding to the wind power unit is controlled by the switch signal. The power conversion method according to claim 16, wherein the power generation unit of at least one of the power conversion devices is a photovoltaic power generation unit, and the power conversion method further includes the steps: (f1) Generate an angle signal according to the operating frequency; (f2) Calculate an active current and a reactive current according to a three-phase current and the angle signal of the AC terminal of the inverter unit corresponding to the photovoltaic power generation unit; (f3) Obtain an output voltage command according to the output voltage and an output current of the photovoltaic power generation unit; (f4) Generate an active current command based on the output voltage of the photovoltaic power generation unit and the output voltage command (f5) Generate an electric potential based on the active current command, a reactive current command, the active current, the reactive current, and the angle signal; and (f6) A switch signal is generated according to the electric potential, and the operation of the inverter unit corresponding to the photovoltaic power generation unit is controlled by the switch signal. The power conversion method according to any one of claim 15-18, wherein the power conversion system further includes an energy storage device, the energy storage device and the at least one power conversion device are coupled to the DC grid in parallel, and the storage The energy device includes a plurality of energy storage units, each of the energy storage units includes an energy storage element and a DC converter connected in series, and a plurality of the DC converters are connected in series to form a DC output terminal to be coupled to the DC power grid. The power conversion method according to claim 19, further comprising the step of: receiving the state of charge of the plurality of energy storage elements by the main controller, and generating the power of the plurality of DC converters based on the superior control command and the state of charge instruction. The power conversion method according to claim 19, further comprising the steps: (g1) Calculate and obtain a power of the energy storage element based on a current and a voltage of the energy storage element; (g2) Generate a current command based on the power and a power command; (g3) Generate an electric potential based on the current command and the current; and (g4) A switching signal is generated according to the electric potential, and the operation of the DC converter is controlled by the switching signal. The power conversion method according to claim 19, further comprising the step of: using a main controller to control the output power of the at least one power conversion device to a fixed value, or using the main controller to control the output power of the at least one power conversion device The sum of the output power of the energy storage device is the fixed value.

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