Improvement and control method of power reverse transmission circuit of full-direct-current system

Abstract

The invention discloses an improvement and control method for a power reverse transmission circuit of a full-direct-current system. The method comprises the following steps: step 1, designing a low-cost power reverse transmission circuit of a direct-current wind turbine generator, wherein the power reverse transmission circuit comprises an input capacitor Ci, a diode rectifier and a transformer; and step 2, designing a collaborative starting scheme of the wind turbine generator and the output direct-current transformer. Compared with an existing full-direct-current system starting pre-charging scheme, the low-cost power back transmission circuit is provided, and the construction cost, the size and the weight of the full-direct-current system are reduced. According to the improvement and control method of the power reverse transmission circuit of the full-direct-current system, low-cost reverse power transmission of the wind turbine generator can be realized, and stable pre-charging when the full-direct-current system is started can be realized.

Description

Technical Field

[0001] The invention belongs to the field of power electronics and high voltage direct current transmission, and relates to an improved full direct current system power reverse transmission circuit and a control method thereof. Background Art

[0002] For large-capacity, long-distance deep-sea wind power bases, full DC technology with medium-voltage DC collection and high-voltage DC transmission has become a new generation of solutions. This solution replaces the DC / AC inverter and bulky power frequency transformer in the traditional AC wind turbine with a wind turbine DC transformer to form a DC wind turbine set. The medium-voltage DC is collected by the wind turbines in series or parallel, and a high-voltage and large-capacity DC transmission transformer is used on the offshore platform for secondary boosting to send the electric energy in the form of high-voltage DC. However, when both the wind turbine DC transformer and the DC transmission transformer adopt the topology of unidirectional power transmission, the onshore power grid cannot reversely transmit the starting power to the full DC offshore wind farm, nor can it obtain electric energy from the onshore power grid through an additional AC umbilical cable. At this time, an additional starting power supply is required to provide the energy required for the DC wind turbine set to start. For traditional AC collection systems, diesel generators or large-capacity battery energy storage devices are usually connected in parallel to the AC collection bus to provide starting power for the wind farm. However, it should be pointed out that for the DC collection solution, if the starting power supply is configured on the DC collection bus, all wind turbine DC transformers need to be designed as a bidirectional power transmission topology, which will greatly increase the construction cost, volume and weight of the full DC system. For scenarios where wind turbines do not have self-starting capabilities, how to achieve low-cost reverse power transmission through coordinated control of the full DC system is a key issue that needs to be solved urgently. Summary of the invention

[0003] In view of the above-mentioned deficiencies of the existing solutions, the present invention provides an improved power reverse transmission circuit of a full DC system and a control method thereof. In order to minimize the number of components, the present invention proposes a low-cost power reverse transmission circuit, and the proposed wind turbine and DC transformer coordinated starting scheme can achieve smooth pre-charging when the full DC system is started. The present invention has the characteristics of a small number of components, high efficiency and low cost.

[0004] The objective of the present invention is achieved through the following technical solutions:

[0005] An improved full DC system power reverse transmission circuit and a control method thereof, comprising the following steps:

[0006] Step 1: Design of low-cost power reverse transmission circuit for DC wind turbine:

[0007] The power feedback circuit includes an input capacitor C i , a diode rectifier and a transformer, where:

[0008] The diode rectifier circuit comprises a diode device D r1 、Diode device D r2 、Diode device D r3 、Diode device D r4 ;

[0009] The transformer comprises a transformer primary side and a transformer secondary side, the transformer primary side comprises a first terminal and a second terminal, the transformer secondary side comprises a first terminal, a second terminal and a center tap terminal, the first terminal of the transformer primary side and the first terminal of the secondary side are terminals of the same name, and the transformation ratio of the transformer is n:1;

[0010] The diode device D r1 The anode and diode device D r2 The cathode of the diode D is connected to form the first bridge arm of the output rectifier circuit; r3 The anode and diode device D r4 The cathode of the transformer is connected to form the second bridge arm of the output rectifier circuit; the first terminal of the primary side of the transformer is connected to the diode device D r1 With diode device D r2 The second terminal of the primary side of the transformer is connected to the diode device D r3 With diode device D r4 The intermediate node of

[0011] The input capacitance C i One end of the diode device D r1 and diode device D r3 The cathode of the input capacitor C i The other end of the diode device D r2 and diode device D r4 The anode in is connected;

[0012] The input capacitor C i Connected in parallel with the low-voltage DC busbar of the wind turbine, the first terminal and the second terminal of the secondary side of the transformer are respectively connected in series to the output end of the wind turbine DC transformer, and the center tap terminal is led out and grounded;

[0013] The fan DC transformer is composed of N resonant power modules with parallel input and series output;

[0014] Step 2: Design of coordinated start-up scheme for wind turbines and DC transformers:

[0015] Step 21: During the start-up phase of the wind turbine generator set, the full DC offshore wind farm is equivalent to the full DC offshore wind farm start-up phase equivalent circuit, wherein the equivalent circuit includes the wind turbine generator set equivalent circuit and the output DC transformer, wherein the wind turbine generator set equivalent circuit is the power reverse transmission circuit designed in step 1, and the first terminal and the second terminal of the secondary side of the transformer are respectively connected in series with the power module in the wind turbine DC transformer to output the equivalent capacitor C. eq One end is connected to the fan DC transformer, and the power module is connected in series with the output equivalent capacitor C eq The other end is connected to the bidirectional hybrid DC transformer, C eq =C o / 10, C o It is the output capacitor of the power module in the fan DC transformer;

[0016] Step 22: For wind turbines that do not have the self-starting capability, before the wind turbine is operated, the wind turbine and the output DC transformer submodule capacitors are not charged. It is necessary to charge the output DC transformer bridge arm submodule capacitors step by step from the onshore main grid, and then provide the wind turbine with the energy required for starting. In the charging stage of the output DC transformer submodule, it is first necessary to trigger the high-voltage side reverse thyristor valve group T A- ~T C- , charging the high-voltage side bridge arm submodule capacitor. After the high-voltage side bridge arm submodule capacitor is charged to the rated value, gradually increase the AC voltage applied to the transformer from 0 to the rated value, thereby charging the medium-voltage side bridge arm submodule capacitor. After the medium-voltage side bridge arm submodule capacitor is fully charged, the wind turbine start-up phase begins;

[0017] Step 2 and 3: Coordinated start-up of the wind turbine and the DC transformer: Active control of the medium voltage side bridge arm of the DC transformer is used to control the medium voltage side current i M Control the amplitude to I MF The sinusoidal current, after passing through the diode rectifier in the power reverse transmission circuit, provides reverse power for the wind turbine. After the sinusoidal current passes through the diode rectifier bridge, it will induce a square wave voltage on the transformer. The medium voltage DC bus voltage is at the rated DC voltage U M A value with amplitude U is superimposed on MF The medium frequency square wave AC voltage, of which the DC voltage U M The output capacitor C of the DC transformer of the fan eq The coordinated startup waveform design will not cause transformer saturation in the reverse transmission circuit.

[0018] 1) Medium voltage side AC frequency f MF :f MF Selected as 3 times the operating frequency f of the output DC transformer, that is, f MF=3f, and the thyristor valve group is triggered to turn on and off naturally when the sinusoidal AC current crosses the zero point, so there is no need to design the thyristor phase-to-phase commutation process;

[0019] 2) High-voltage side current continuity: The high-voltage side thyristor valve group current i TA ~i TC , high voltage side bridge arm current i PA ~i PC And the transformer current i a ~i c Designed with staggered trapezoidal waves;

[0020] 3) Medium frequency AC voltage amplitude U MF : Sinusoidal current i on the medium voltage side M The power P transmitted in the opposite direction with the same frequency and phase as the square wave AC voltage R It is expressed as:

[0021]

[0022] Limited by the current stress of the medium voltage side devices, the sinusoidal current i M The amplitude I MF The medium voltage DC current rating I during forward power transfer should not be exceeded M , then the medium frequency AC voltage amplitude U MF Need to meet:

[0023]

[0024] Where P is the rated power of the output DC transformer, that is, the rated power of the entire DC system.

[0025] Compared with the prior art, the present invention has the following advantages:

[0026] 1. Compared with the existing full DC system startup pre-charging solution, the present invention proposes a low-cost power reverse transmission circuit, which reduces the construction cost, volume and weight of the full DC system.

[0027] 2. The improved full DC system power reverse transmission circuit and its control method proposed in the present invention can realize low-cost reverse power transmission to the wind turbine and can achieve smooth pre-charging when the full DC system is started. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A circuit structure of a fan DC transformer with starting capability;

[0029] Figure 2 This is the equivalent circuit of the startup phase of a full DC offshore wind farm;

[0030] Figure 3 It is the operating principle waveform of the coordinated startup phase of the whole DC system;

[0031] Figure 4 A circuit structure of a DC transformer for transmitting power in reverse direction of the entire DC system;

[0032] Figure 5 This is a dynamic simulation diagram of the full DC system from startup charging, reverse power transmission to forward power transmission. DETAILED DESCRIPTION

[0033] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0034] The present invention provides an improved full DC system power reverse transmission circuit and a control method thereof, the method comprising the following steps:

[0035] Step 1: Design of low-cost power reverse transmission circuit for DC wind turbine:

[0036] Fan DC transformer circuit Figure 1 As shown, it is composed of N resonant power modules with parallel input and series output. Each power module has an input capacitor C i , a rectifier, a turns ratio of 1:n T A high-frequency transformer, an inverter, an output capacitor C o Since the power of the full DC system startup process mainly comes from the energy required by the auxiliary equipment of the DC wind turbine, including pitch and yaw drive, demisting and cooling system, control and protection system and communication system, the required power only accounts for 1% to 2% of the wind turbine capacity. In order to minimize the number of components, the present invention proposes a low-cost power reverse transmission circuit, the specific circuit is as follows Figure 1 As shown, it contains an input capacitor C i , a diode rectifier and a transformer. Among them: the DC side of the power reverse transmission circuit is connected in parallel with the low-voltage DC bus of the wind turbine, and the primary winding of the transformer is connected in series with the output end of the wind turbine DC transformer. In order to reduce the insulation requirements of the power reverse transmission circuit, the midpoint of the primary winding of the transformer is led out and grounded for total energy balance control. The diode rectifier circuit includes a diode device D r1 、Diode device D r2 、Diode device D r3 、Diode device D r4 The transformer includes a primary side and a secondary side of the transformer, wherein the primary side includes a first terminal and a second terminal, and the secondary side includes a first terminal, a second terminal and a center tap terminal. The first terminal of the primary side of the transformer and the first terminal of the secondary side are the same terminals, and the transformation ratio of the transformer is n:1. The diode device D r1The anode and diode device D r2 The cathode of the diode D is connected to form the first bridge arm of the output rectifier circuit; r3 The anode and diode device D r4 The cathode of the transformer is connected to form the second bridge arm of the output rectifier circuit; the first terminal of the primary side of the transformer is connected to the diode device D r1 With diode device D r2 The second terminal of the primary side of the transformer is connected to the diode device D r3 With diode device D r4 The middle node of the input capacitor C i One end of the diode device D r1 and diode device D r3 The cathode of the input capacitor C i The other end of the diode device D r2 and diode device D r4 connected to the anode in .

[0037] Step 2: Design of coordinated start-up scheme for wind turbines and DC transformers:

[0038] During the start-up phase of the wind turbine, the full DC offshore wind farm can be equivalent to Figure 2 The circuit shown in the figure shows the equivalent circuit of the wind turbine on the left (only one wind turbine is shown), and the circuit on the right shows the bidirectional hybrid DC transformer. eq The equivalent capacitance (C eq =C o / 10), R eq It is the equivalent load of the wind turbine startup process.

[0039] For wind turbines that do not have self-starting capabilities, before the wind turbine is running, the wind turbine and the DC transformer submodule capacitors are not charged. It is necessary to charge the DC transformer bridge arm submodule capacitors step by step from the onshore main grid, and then provide the wind turbine with the energy required for starting. In the charging stage of the DC transformer submodule, the high-voltage side reverse thyristor valve group T A- ~T C- , charging the high-voltage side bridge arm submodule capacitor. After the high-voltage side bridge arm capacitor is charged to the rated value, the AC voltage applied to the transformer is gradually increased from 0 to the rated value, thereby charging the medium-voltage side bridge arm submodule capacitor. After the medium-voltage side bridge arm is charged, the wind turbine start-up phase begins. For the power reverse start-up phase, a wind turbine and the output DC transformer coordinated start-up strategy is proposed. The corresponding operating principle waveform is as follows: Figure 3 The basic idea of ​​coordinated startup is to actively control the medium voltage side bridge arm of the bidirectional hybrid DC transformer toM Control Figure 3 The amplitude shown is I MF The sinusoidal current, after passing through the diode rectifier in the power reverse circuit, provides reverse power for the wind turbine. After the sinusoidal current passes through the diode rectifier bridge, it will induce a square wave voltage on the transformer, so the medium voltage DC bus voltage is at the rated DC voltage U M A value with amplitude U is superimposed on MF The medium frequency square wave AC voltage, of which the DC voltage U M The output capacitor C of the DC transformer of the fan eq The load will not cause transformer saturation in the reverse transmission circuit. Figure 3 The coordinated startup operation waveform shown has the following designs:

[0040] 1) Medium voltage side AC frequency f MF :

[0041] Since the three-phase circuit of the DC transformer is staggered, the thyristor valve groups on the medium voltage side are triggered and turned on in sequence, and the current flowing through each thyristor valve group needs to maintain the same waveform but staggered by 120° electrical angle. M is a sinusoidal current, so f MF It is better to select 3 times the operating frequency f of the output DC transformer, that is, f MF =3f. The thyristor valve group can be triggered to turn on and off naturally when the sinusoidal AC current passes through the zero point, and there is no need to design the thyristor phase-to-phase commutation process.

[0042] 2) High-voltage side current continuity: In order to ensure the high-voltage side current continuity during the reverse power transmission stage, the high-voltage side thyristor valve group current i TA ~i TC , high voltage side bridge arm current i PA ~i PC And the transformer current i a ~i c Designed as Figure 3 The staggered trapezoidal waves are shown.

[0043] 3) Medium frequency AC voltage amplitude U MF : Sinusoidal current i on the medium voltage side M The power P transmitted in the opposite direction with the same frequency and phase as the square wave AC voltage R It can be expressed as:

[0044]

[0045] Limited by the current stress of the medium voltage side devices, the sinusoidal current i M The amplitude I MF The medium voltage DC current rating I during forward power transfer should not be exceeded MTherefore, the medium frequency AC voltage amplitude U MF Need to meet

[0046]

[0047] P is the rated power of the DC transformer, that is, the rated power of the entire DC system. Since the reverse transmission power is less than 2% of the system rated power, according to formula (2), U MF Size is about 4% U M , and the number of bridge arm sub-modules will not increase.

[0048] In addition, the power reverse transmission circuit only works during the start-up process of the wind turbine. Although the primary winding of the transformer is connected in series with the medium voltage side of the wind turbine DC transformer, when the wind turbine is transmitting forward power, the diode rectifier D r1 ~D r4 In the blocking state, the transformer is equivalent to an inductor, and whether it is saturated or not will not affect the normal operation. During forward transmission, a knife switch can also be used to bypass the power reverse transmission circuit.

[0049] As a preferred embodiment, the circuit of the DC transformer is as follows: Figure 4 As shown, both the medium voltage side and the high voltage side are composed of three thyristor valve groups and three submodule bridge arms. The medium voltage side and the high voltage side are connected through a medium frequency transformer, where:

[0050] The thyristor valve group is composed of two groups of series thyristors connected in reverse parallel. j+ and T J+ They are the forward thyristor valve groups on the medium voltage side and the high voltage side, T j- and T J- They are reverse thyristor valve groups on the medium voltage side and high voltage side, j = a, b, c, J = A, B, C;

[0051] The bridge arm of the medium voltage side submodule and the bridge arm of the high voltage side submodule are respectively composed of N M and N H Half-bridge sub-modules, where L M and L H are the bridge arm inductances of the submodules on the medium voltage side and the high voltage side, respectively, U M and I M is the DC voltage and current on the medium voltage side, U H and I H is the DC voltage and current on the high voltage side, u Pj and i Pj They represent the bridge arm voltage and current of the medium voltage side submodule, u PJ and i PJ Respectively represent the voltage and current of the high-voltage side submodule bridge arm, i Tj and i TJ are the thyristor valve group currents on the medium voltage side and high voltage side, ij is the primary current of the transformer;

[0052] Dynamic simulation of the full DC system from startup charging, reverse power transmission to forward power transmission Figure 5 As the onshore converter station gradually establishes the high-voltage side voltage U H , send out the DC transformer high voltage side submodule capacitor U CHA ~U CHC It is also charged (during this period, the high-voltage side reverse thyristor valve group T is triggered A- ~T C- ). Wait for U CHA ~U CHC After charging to the rated value, the high-voltage side bridge arm gradually increases the internal AC voltage component, thereby charging the medium-voltage side bridge arm submodule capacitor U through the transformer. CMa ~U CMa Charging, while gradually building up the medium voltage DC bus voltage U M . Wait for U CMa ~U CMa After the capacitor is charged to the rated value, the system enters the wind turbine startup phase. The DC transformer injects a square wave voltage with an amplitude of into the medium voltage DC bus. Therefore, in this phase, U M According to the coordinated startup control strategy, the DC transformer actively converts the medium voltage DC bus current i M The control is a sinusoidal current, which transmits the power required for starting the wind turbine. The low-voltage DC bus voltage of the wind turbine is U L After the wind turbine has obtained the energy required for startup, it starts to generate electricity normally, and the full DC system begins to enter the forward power transmission stage.

Claims

1. An improved full DC system power reverse transmission circuit and its control method, characterized in that The method comprises the following steps: Step 1: Design of low-cost power reverse transmission circuit for DC wind turbine: The power feedback circuit includes an input capacitor C i , a diode rectifier and a transformer, where: The diode rectifier circuit comprises a diode device D r1 、Diode device D r2 、Diode device D r3 、Diode device D r4 ; The transformer comprises a transformer primary side and a transformer secondary side, the transformer primary side comprises a first terminal and a second terminal, the transformer secondary side comprises a first terminal, a second terminal and a center tap terminal, the first terminal of the transformer primary side and the first terminal of the secondary side are terminals of the same name, and the transformation ratio of the transformer is n:1; The diode device D r1 The anode and diode device D r2 The cathode of the diode D is connected to form the first bridge arm of the output rectifier circuit; r3 The anode and diode device D r4 The cathode of the transformer is connected to form the second bridge arm of the output rectifier circuit; the first terminal of the primary side of the transformer is connected to the diode device D r1 With diode device D r2 The second terminal of the primary side of the transformer is connected to the diode device D r3 With diode device D r4 The intermediate node of The input capacitance C i One end of the diode device D r1 and diode device D r3 The cathode of the input capacitor C i The other end of the diode device D r2 and diode device D r4 The anode in is connected; The input capacitance C i Connected in parallel with the low-voltage DC busbar of the wind turbine, the first terminal and the second terminal of the secondary side of the transformer are respectively connected in series to the output end of the wind turbine DC transformer, and the center tap terminal is led out and grounded; The fan DC transformer is composed of N resonant power modules with parallel input and series output; Step 2: Design of coordinated start-up scheme for wind turbines and DC transformers: Step 21: During the start-up phase of the wind turbine generator set, the full DC offshore wind farm is equivalent to the full DC offshore wind farm start-up phase equivalent circuit, wherein the equivalent circuit includes the wind turbine generator set equivalent circuit and the output DC transformer, wherein the wind turbine generator set equivalent circuit is the power reverse transmission circuit designed in step 1, and the first terminal and the second terminal of the secondary side of the transformer are respectively connected in series with the power module in the wind turbine DC transformer to output the equivalent capacitor C. eq One end is connected to the fan DC transformer, and the power module is connected in series with the output equivalent capacitor C eq The other end is connected to the bidirectional hybrid DC transformer, C eq =C o / 10, C o It is the output capacitor of the power module in the fan DC transformer; Step 22: For wind turbines that do not have the self-starting capability, before the wind turbine is operated, the wind turbine and the output DC transformer submodule capacitors are not charged. It is necessary to charge the output DC transformer bridge arm submodule capacitors step by step from the onshore main grid, and then provide the wind turbine with the energy required for starting. In the charging stage of the output DC transformer submodule, it is first necessary to trigger the high-voltage side reverse thyristor valve group T A- ~T C- , charging the high-voltage side bridge arm submodule capacitor. After the high-voltage side bridge arm submodule capacitor is charged to the rated value, gradually increase the AC voltage applied to the transformer from 0 to the rated value, thereby charging the medium-voltage side bridge arm submodule capacitor. After the medium-voltage side bridge arm submodule capacitor is fully charged, the wind turbine start-up phase begins; Step 2 and 3: Coordinated start-up of the wind turbine and the DC transformer: Active control of the medium voltage side bridge arm of the DC transformer is used to control the medium voltage side current i M Control the amplitude to I MF The sinusoidal current, after passing through the diode rectifier in the power reverse transmission circuit, provides reverse power for the wind turbine. After the sinusoidal current passes through the diode rectifier bridge, it will induce a square wave voltage on the transformer. The medium voltage DC bus voltage is at the rated DC voltage U M A value with amplitude U is superimposed on MF The medium frequency square wave AC voltage, of which the DC voltage U M The output capacitor C of the DC transformer of the fan eq It will not cause transformer saturation in the feedback circuit.

2. The improved full DC system power reverse transmission circuit and control method thereof according to claim 1 is characterized in that In the steps 2 and 3, the coordinated startup operation waveform is designed as follows: 1) Medium voltage side AC frequency f MF :f MF Selected as 3 times the operating frequency f of the output DC transformer, that is, f MF =3f, and the thyristor valve group is triggered to turn on and off naturally when the sinusoidal AC current crosses the zero point, so there is no need to design the thyristor phase-to-phase commutation process; 2) High-voltage side current continuity: The high-voltage side thyristor valve group current i TA ~i TC , high voltage side bridge arm current i PA ~i PC And the transformer current i a ~i c Designed with staggered trapezoidal waves; 3) Medium frequency AC voltage amplitude U MF : Sinusoidal current i on the medium voltage side M The power P transmitted in the opposite direction with the same frequency and phase as the square wave AC voltage R It is expressed as: Limited by the current stress of the medium voltage side devices, the sinusoidal current i M The amplitude I MF The medium voltage DC current rating I during forward power transfer should not be exceeded M , then the medium frequency AC voltage amplitude U MF Need to meet: Where P is the rated power of the output DC transformer, that is, the rated power of the entire DC system.

3. The improved full DC system power reverse transmission circuit and control method thereof according to claim 1 is characterized in that The medium voltage side and the high voltage side of the output DC transformer are both composed of three thyristor valve groups and three submodule bridge arms, and the medium voltage side and the high voltage side are connected through a medium frequency transformer.

4. The improved full DC system power reverse transmission circuit and control method thereof according to claim 3 is characterized in that The thyristor valve group is composed of two groups of series thyristors connected in reverse parallel; the bridge arm of the medium voltage side submodule and the bridge arm of the high voltage side submodule are respectively composed of N M and N H It is composed of half-bridge sub-modules.

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