A deep-sea wind power DC transmission system based on a flexible thyristor converter and a control method thereof

Abstract

The application discloses a deep-sea wind power DC sending-out system based on a flexible thyristor converter and a control method thereof. The deep-sea wind power DC sending-out system based on the flexible thyristor converter comprises a plurality of offshore wind power DC sending-out systems, a DC large bus, an onshore converter station and an onshore AC power grid; the offshore wind power DC sending-out system is composed of a wind farm, a collection network, an offshore converter station and a DC submarine cable which are sequentially connected; and the onshore converter station is composed of an AC line, an onshore converter and a DC line which are sequentially connected. The deep-sea wind power DC sending-out system topology structure and the control method can realize centralized collection and large-capacity sending-out of a plurality of remote offshore wind farms, deeply utilize the complementarity of power fluctuation of the plurality of offshore wind farms, improve the construction economy of the deep-sea wind power grid-connected sending-out system and the annual utilization hours of the DC sending-out channel, and effectively enhance the flexibility and expansibility of the grid-connected system.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current power transmission, in particular to a deep-sea wind power direct current transmission system based on a flexible line commutated converter and a control method thereof. BACKGROUND

[0002] The typical converter topology of the existing offshore wind power end-to-end direct current transmission system is a modular multi-level converter (MMC). Due to the limitation of the current-carrying capacity of the switching device, the MMC usually adopts the modular expansion (such as increasing the number of parallel sub-modules) to improve the converter capacity, thereby bringing technical and economic challenges to the cost, volume and weight of the offshore converter station under the demand of large-capacity transmission of deep-sea wind power. In addition, the MMC with larger capacity has higher requirements for offshore platforms, further increasing the difficulty of engineering construction and operation and maintenance cost. The end-to-end structure can only realize the direct current transmission of a single wind farm, and cannot meet the requirements of flexible interconnection of multiple wind farms under the background of deep-sea wind power development, and cannot fully utilize the complementarity of power transmission of different regional offshore wind farms.

[0003] In terms of multi-terminal direct current transmission, due to the inherent defects of the series topology structure, the series multi-terminal system has problems in reliability and expandability. In order to ensure that the system can maintain normal operation when any port converter fails, the design of the control system is more complex, and the control accuracy and reliability are lower. In the face of the grid connection demand of large-scale newly added wind farms in the future, if the series structure needs to connect the newly added wind farms, the power transmission network must be reconstructed, which greatly increases the development cost. Due to the limitation of the current-carrying capacity of the switching device in the MMC, the grid connection capacity of a single onshore MMC converter station is limited, which cannot meet the large-capacity grid connection demand of deep-sea wind power. At present, the design of the parallel multi-terminal system usually adopts the scheme of constructing multiple MMC converter stations to realize the grid connection of large-capacity power, which brings economic problems to the construction of the transmission system.

[0004] Therefore, in order to solve the above key problems faced by deep-sea wind power, a deep-sea wind power direct current transmission system based on a flexible line commutated converter (FLCC) and a control method thereof are needed to realize the centralized collection and large-capacity transmission of multiple long-distance offshore wind farms. SUMMARY

[0005] The purpose of the present application is to provide a deep-sea wind power direct current transmission system based on a flexible line commutated converter and a control method thereof.

[0006] The application discloses a deep-sea wind power DC transmission system based on a flexible thyristor converter, which comprises a plurality of offshore wind power DC transmission systems, a DC bus, an onshore converter station and an onshore AC power grid; the offshore wind power DC transmission system is composed of a wind farm, a collection network, an offshore converter station and a DC submarine cable which are sequentially connected; the onshore converter station is composed of an AC line, an onshore converter and a DC line which are sequentially connected; wherein the DC submarine cable and the DC line are connected with the DC bus, and the AC line is connected with the onshore AC power grid.

[0007] Further, the wind farm is a grid-connected wind turbine, the collection network is an AC submarine cable, and the converter topology of the offshore converter station is a compact MMC.

[0008] Further, the converter topology of the onshore converter station is an FLCC composed of a 12-pulse LCC and a FBMMC with a series connection of AC sides and a parallel connection of DC sides.

[0009] Further, the FBMMC is an MMC composed of full-bridge sub-modules.

[0010] A control method of a deep-sea wind power DC transmission system based on a flexible thyristor converter, wherein the FLCC adopts a U dc Q control mode, the deviation amount between the FLCC DC voltage reference value in the outer loop of the U dc Q control mode and the FLCC DC voltage measurement value is subjected to PI modulation to generate a DC port current reference value in the inner loop, and the deviation amount between the DC port current reference value and the DC port current measurement value is subjected to PI modulation and then added to 0.5 times of the FBMMC DC voltage measurement value to obtain a reference value of an intermediate variable for generating a bridge arm voltage.

[0011] Further, the control method further comprises that the offshore MMC converter station adopts a V / f control mode, and the LCC part adopts a fixed trigger angle control mode.

[0012] Further, the U dc Q control mode further comprises:

[0013] The voltage phase of the alternating three-phase voltage measurement value is obtained by using a phase-locked loop; the d, q component measurement values of the FBMMC grid-side current measurement value are obtained by abc / dq coordinate transformation according to the voltage phase; the d, q component measurement values of the FBMMC grid-side voltage measurement value are obtained by abc / dq coordinate transformation according to the voltage phase; the deviation amount between the reference value of the FBMMC bridge arm capacitor voltage sum average value of the outer loop and the measurement value of the FBMMC bridge arm capacitor voltage sum average value is subjected to PI modulation to generate the d-axis component reference value of the FBMMC grid-side current; the deviation amount between the FLCC reactive power reference value and the FLCC reactive power measurement value is subjected to PI modulation to generate the q-axis component reference value of the FBMMC grid-side current; the d, q component reference values of the valve-side voltage of the FBMMC are generated by using a conventional two-control variable inner loop control structure, and the two are subjected to dq / abc coordinate transformation to obtain the valve-side voltage reference value of the FBMMC, which is used to generate the bridge arm voltage.

[0014] Further, the intermediate variables are defined as follows:

[0015]

[0016] In the formula, E dcM is an intermediate variable, u a,b,cp , u a,b,cn are the upper and lower bridge arm voltages of the FBMMC respectively.

[0017] Further, the constant trigger angle in the fixed trigger angle control mode is in the range of 140°-160°.

[0018] The beneficial effects of the present application are:

[0019] 1, the present application contains U dc Q control mode, can realize the centralized collection of multiple remote offshore wind farm electric energy, the output of offshore wind farm in different geographical positions is complementary, smooth power output fluctuation, reduce the volatility of overall output power, facilitate the optimization of offshore wind farm operation in the receiving end;

[0020] 2, based on the FLCC, a single land converter station can realize large-capacity grid connection, avoid building multiple land MMC converter stations, and based on the compactness technology, several offshore MMC converter stations can realize the lightweight and compactness of the offshore platform, significantly improve the economy of offshore wind power grid connection system construction;

[0021] 3, based on the DC bus, the offshore grid connection system supports the access of future new wind farms, without reconfiguring the existing power transmission network, reducing long-term development cost, and enhancing the flexibility and expansibility of the grid connection system. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a topology diagram of the offshore wind power grid connection system based on the FLCC.

[0023] Figure 2 Control block diagram for the onshore FLCC converter station.

[0024] Figure 3 Control block diagram for the offshore MMC converter station. DETAILED DESCRIPTION

[0025] The application provides a deep-sea wind power DC transmission system based on a flexible thyristor converter and a control method thereof, which will be further described below in combination with the drawings and specific embodiments.

[0026] The deep-sea wind power DC transmission system based on a flexible thyristor converter (FLCC) specifically comprises a plurality of offshore wind power DC transmission systems, a DC bus, an onshore converter station and an onshore AC power grid. Figure 1 As shown in the figure.

[0027] The offshore wind power DC transmission system is composed of an offshore wind farm, a collection network, an offshore converter station and a DC submarine cable connected in sequence, wherein the offshore wind farm adopts grid-connected wind turbines, the collection network uses AC submarine cables, and the offshore converter station adopts compact MMC. Figure 1 The onshore converter station is composed of an AC line, an onshore converter and a DC line connected in sequence, wherein the onshore converter adopts FLCC, the FLCC is composed of a 12-pulse LCC and a FBMMC DC side series AC side parallel connection, the FBMMC is an MMC composed of full-bridge submodules (FBSM), dc U dcL and U dcM are DC voltages of the FLCC, LCC and FBMMC parts respectively, and I dc is a DC port current of the FLCC. The DC submarine cable and the DC line are connected with the DC bus, and the AC line of the FLCC AC side is connected with the onshore AC power grid.

[0028] The AC power generated by the offshore AC wind farm is converted into DC power by the MMC converter station through the collection network, and then transmitted by the DC submarine cable. The DC power transmitted by each offshore wind farm through the DC submarine cable has the same voltage level, and the DC sides of the plurality of offshore wind farms are connected in parallel to the DC bus, which is converted into AC power by the FLCC and then connected to the AC power grid. The offshore MMC converter station adopts compact technology, such as MMC with high ripple voltage (HR-MMC) operation mode and MMC operation mode based on improved control of circulating current suppression, to reduce the volume and weight of the offshore converter station and improve the construction economy of the offshore platform.

[0029] Since the wind farms are controlled in grid-following mode, the V / f control mode is adopted in each offshore MMC converter station to maintain the voltage stability of each offshore wind farm. The U dc / Q control mode is adopted in the onshore FLCC converter station to control the voltage stability of the onshore DC bus node U dc .

[0030] Figure 2 The control block diagram of the onshore FLCC converter station is shown in FIG. 2, where "*" and "m" represent the reference value and the measured value of a variable, respectively; "d" and "q" represent the equivalent values of a variable in the d-axis and the q-axis in the dq coordinate, respectively.v ga,b,c is the AC three-phase voltage of the FLCC PCC, θ is the phase angle of v ga,b,c , and ω g is the angular frequency of v ga,b,c .i' Ma,b,c is the grid-side current of the FBMMC, i' Md and i' Mq are the d, q components of i' Ma,b,c ;v Ma,b,c is the grid-side voltage of the FBMMC, V Md and V Mq are the d, q components of v Ma,b,c ;u Ma,b,c is the valve-side voltage of the FBMMC, u Md and u Mq are the d, q components of u Ma,b,c ;U dcM is the DC voltage of the FBMMC part; V capΣ is the average value of the FBMMC bridge arm capacitor voltage sum; Q s is the reactive power of the FLCC; L cal is the equivalent inductance of the FLCC; I dc is the DC port current of the FLCC; E dcM is an intermediate variable; u a,b,cp ~ u a,b,cn are the bridge arm voltages of the FBMMC, and the subscripts "p" and "n" represent the upper bridge arm and the lower bridge arm, respectively.

[0031] The dynamic of the AC side of the FBMMC can be expressed as:

[0032]

[0033] In the formula, V g and V g are the vector and the amplitude of v ga,b,c , respectively, and V gd,q is vga,b,c The equivalent values ​​of the d-axis and q-axis in the dq coordinate system are then represented by V. g Based on, i.e., V gd =V g And V gq =0, J1 is the coefficient matrix, which can be calculated as:

[0034]

[0035] In the formula, L cal and R cal Let represent the equivalent inductance and resistance of the FLCC, respectively, and calculate as follows:

[0036]

[0037] In the formula, L arm and R arm For the bridge arm inductance and resistance of the FBMMC; k M For FBMMC, the transformer turns ratio is L. eM and R eM For the converter transformer T required by FBMMC M The equivalent inductance and resistance.

[0038] Since the regulating variable of the FBMMC part is u a,b,cn and u a,b,cp To decouple the two, an intermediate variable E is defined. dcM :

[0039]

[0040] Bridge arm voltage u a,b,cn and u a,b,cp It can be calculated as follows:

[0041]

[0042] Ignoring circulating current, the DC dynamics of the FBMMC section can be expressed as:

[0043]

[0044] Finally, consider the LCC portion of FLCC:

[0045]

[0046] In the formula, i gd,q It is the AC port current i of the FLCC. ga,b,c The d and q components, i Ld,q For LCC network-side current i La,b,c The d and q components, V Ld,q For LCC grid-side voltage v La,b,cThe d and q components, L eL It is the equivalent inductance of the converter transformer required for LCC, C e These are the equivalent capacitance values ​​of the AC reactive power compensation capacitors and filter banks. J2 and J3 are both coefficient matrices, which can be calculated as follows:

[0047]

[0048] and

[0049]

[0050] In the formula, α* and k L It refers to the firing angle and converter transformer ratio of the LCC section.

[0051] The LCC section uses fixed trigger angle control, with the fixed trigger angle α being a constant value. * And since FLCC is a receiving-end inverter, α * It should be greater than 90°, but in practical applications it is usually taken as 140° to 160°, determined by the modulation system based on α. * Generate trigger pulse signals for all thyristors in the LCC.

[0052] FLCC uses U dc Q control mode, given reference value FLCC DC voltage U * dc The average value of the sum of the voltages across the bridge arm capacitors, V * capΣ and reactive power Q * s A phase-locked loop (PLL) is used to measure the AC three-phase voltage (V) of the external PCC of the converter. m ga,b,c Obtain the voltage phase at this moment; i' m Ma,b,c According to θ m i' is obtained after abc / dq coordinate transformation m Ma,b,c The d,q components i' m Md and i' m Mq ;v m Ma,b,c According to θ m v is obtained after abc / dq coordinate transformation m Ma,b,c The d,q components V m Md and V m Mq Outer ring U * dc with Um dc the deviation of I * dc , the inner loop I * dc and I m dc the deviation of I m dcM is added to 0.5 times U * dcM , used to generate the bridge arm voltage u * a,b,cp ~u * a,b,cn . The outer loop V * capΣ and V m capΣ the deviation of I * Md , Q * s and Q m s the deviation of I * Mq ; the inner loop uses the conventional two control variable inner loop control structure to generate u * Md and u * Mq , both of which are subjected to dq / abc coordinate transformation to obtain u * Ma,b,c , used to generate the bridge arm voltage u * a,b,cp ~u * a,b,cn . The modulation system generates the trigger pulse signals of all switching devices in the MMC according to u * a,b,cp ~u * a,b,cn .

[0053] Figure 3 is the control block diagram of the offshore MMC converter station, and the V / f control of the offshore MMC converter station is the conventional double closed loop control. Wherein, "*" and "m" represent the reference value and measured value of the variable respectively; "d" and "q" represent the equivalent values of the variable on the d-axis and q-axis in the dq coordinate respectively; L is the equivalent inductance of the MMC; v a,b,c is the MMC grid side voltage, ω is the angular frequency of v a,b,c , V d and V q are the d, q components of v a,b,c ; i a,b,c is the AC side current of the MMC, i d and iq Vd,q are the d, q components of i a,b,c * abc and θ * 1 are the given reference amplitude and phase angle, respectively, Vd,q are the d, q components of v * 1 after abc / dq coordinate transformation according to θ * 1 d and Vd,q are the d, q components of i * q ; v m Ma,b,c Vd,q are the d, q components of v * 1 after abc / dq coordinate transformation according to θ m 1 a,b,c and Vd,q are the d, q components of i m d 1 after abc / dq coordinate transformation according to θ m 1 q ; i m a,b,c Vd,q are the d, q components of i * 1 after abc / dq coordinate transformation according to θ m 1 a,b,c and i m d 1 after abc / dq coordinate transformation according to θ m 1 q .

[0054] The above FLCC-based DC transmission system can realize power collection of multiple long-distance offshore wind farms, complementary output of offshore wind farms in different geographical locations, smooth power output fluctuation, such as balancing power generation fluctuation due to wind speed difference of offshore wind farms in different geographical locations (for example, when there is no wind in a certain area, another area may have sufficient wind power), and reducing the volatility of overall output power. Compared with the existing series multi-terminal system, the FLCC-based DC transmission system design supports the access of future new wind farms, without the need to reconfigure the existing power transmission network, thereby reducing long-term development costs. Compared with the existing parallel multi-terminal system, since the current-carrying capacity of the thyristor switch used by the LCC is much larger than that of the IGBT switch used by the MMC, the DC transmission system using the FLCC as the onshore converter station can transmit much larger power capacity than the traditional MMC converter station, avoiding the construction of multiple MMC converter stations to realize large-capacity wind power grid connection, and having higher economic efficiency.

Claims

1. A deep-sea wind power DC transmission system based on a flexible thyristor converter, characterized in that, It includes multiple offshore wind power DC transmission systems, DC busbars, onshore converter stations, and onshore AC power grids; The offshore wind power DC transmission system consists of a wind farm, a collection network, an offshore converter station, and a DC submarine cable connected in sequence; the onshore converter station consists of an AC line, an onshore converter, and a DC line connected in sequence. Among them, the DC submarine cable and DC line are both connected to the DC bus, and the AC line is connected to the onshore AC power grid; The wind farm is a grid-connected wind turbine, the collection network is an AC submarine cable, and the converter topology of the offshore converter station is a compact MMC. The converter topology of the onshore converter station is an FLCC consisting of a 12-pulse LCC and an FBMMC connected in series on the DC side and in parallel on the AC side; the FBMMC is an MMC composed of full-bridge submodules.

2. A control method for a deep-sea wind power DC transmission system based on a flexible thyristor converter as described in claim 1, characterized in that, FLCC uses U dc Q control mode, the U dc In Q control mode, the deviation between the outer loop FLCC DC voltage reference value and the FLCC DC voltage measurement value is modulated by PI to generate the inner loop DC port current reference value. The deviation between the DC port current reference value and the DC port current measurement value is modulated by PI and added to 0.5 times the FBMMC DC voltage measurement value to obtain the intermediate variable reference value used to generate the bridge arm voltage.

3. The control method for a deep-sea wind power DC transmission system based on a flexible thyristor converter according to claim 2, characterized in that, The control method also includes: the offshore MMC converter station adopts V / f control mode, and the LCC section adopts fixed trigger angle control mode.

4. The control method for a deep-sea wind power DC transmission system based on a flexible thyristor converter according to claim 3, characterized in that, The U dc Q control mode also includes: A phase-locked loop (PLL) is used to obtain the voltage phase of the AC three-phase voltage measurement. The FBMMC grid-side current measurement is transformed into d and q component values ​​based on the voltage phase using an abc / dq coordinate transformation. The deviation between the reference value of the sum of the FBMMC arm capacitor voltages in the outer loop and the measured value of the sum of the FBMMC arm capacitor voltages is modulated by a PI to generate the d-axis component reference value of the FBMMC grid-side current. The deviation between the FLCC reactive power reference value and the measured value of the FLCC reactive power is modulated by a PI to generate the q-axis component reference value of the FBMMC grid-side current. A conventional two-control-variable inner-loop control structure is used to generate the d and q component reference values ​​of the FBMMC valve-side voltage. The two are transformed into dq / abc coordinates to obtain the FBMMC valve-side voltage reference value, which is used to generate the arm voltage.

5. The control method for a deep-sea wind power DC transmission system based on a flexible thyristor converter according to claim 2, characterized in that, The intermediate variable is defined as follows: In the formula, E dcM u is an intermediate variable. a,b,cp u a,b,cn These are the upper and lower arm voltages of the FBMMC, respectively.

6. The control method for a deep-sea wind power DC transmission system based on a flexible thyristor converter according to claim 3, characterized in that, In the constant trigger angle control mode, the constant trigger angle ranges from 140° to 160°.

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