Deep and far sea wind power direct current delivery system based on flexible thyristor converter and control method

Abstract

The invention discloses a deep and far sea wind power direct current delivery system based on a flexible thyristor converter and a control method. The deep and far sea wind power direct current delivery system based on the flexible thyristor converter comprises a plurality of offshore wind power direct current delivery systems, a direct current large bus, an onshore converter station and an onshore alternating current power grid. The offshore wind power direct current sending-out system is composed of a wind power plant, a collection network, an offshore converter station and a direct current submarine cable which are connected in sequence. The land converter station is composed of an alternating current line, a land converter and a direct current line which are connected in sequence. The invention discloses a deep and far sea wind power direct current delivery system topological structure and a control method, which can realize concentrated collection and high-capacity delivery of receiving ends of a plurality of long-distance offshore wind fields, deeply utilize complementarity of power fluctuations of the plurality of offshore wind fields, improve construction economy of a deep and far sea wind power grid-connected delivery system and annual utilization hours of a direct current delivery channel, and reduce the construction cost of the deep and far sea wind power grid-connected delivery system. And the flexibility and expansibility of the grid-connected system are effectively enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current (DC) transmission, and in particular to a deep-sea wind power DC transmission system and a control method based on a flexible thyristor converter. Background Art

[0002] The typical converter topology for existing end-to-end DC transmission systems for offshore wind power is the Modular Multi-level Converter (MMC). Limited by the current-carrying capacity of switching devices, MMCs typically increase converter capacity through modular expansion (e.g., increasing the number of parallel submodules). This poses technical and economic challenges in terms of cost, size, and weight of offshore converter stations, especially given the demand for large-scale offshore wind power transmission. Furthermore, larger MMCs place higher demands on offshore platforms, further increasing construction complexity and operational costs. This end-to-end structure can only achieve DC transmission from a single wind farm, failing to meet the flexible interconnection requirements for multiple wind farms in the context of deep-sea wind power development. Furthermore, it fails to fully utilize the complementary power transmission capabilities of offshore wind farms in different regions.

[0003] In terms of multi-terminal DC transmission, due to the inherent flaws of the series topology, series multi-terminal systems face reliability and scalability issues. To ensure the system can maintain normal operation when a converter failure occurs at any port, the control system design becomes more complex, and the control accuracy and reliability decrease. In the face of the future demand for large-scale grid connection of new wind farms, the series topology must restructure the transmission network to connect new wind farms, significantly increasing development costs. Due to the current-carrying capacity limitations of the switching devices in the MMC, the grid-connected power capacity that can be achieved by a single onshore MMC converter station is limited, unable to meet the large-scale grid connection requirements of offshore wind power. Current parallel multi-terminal system designs typically adopt the solution of building multiple MMC converter stations to achieve large-capacity grid connection, which brings economic problems to the construction of the transmission system.

[0004] Therefore, in order to solve the above-mentioned key problems faced by deep-sea wind power, a deep-sea wind power DC transmission system and control method based on flexible thyristor converter (Flexible Line Commutated Converter, FLCC) is needed to achieve centralized collection and large-capacity transmission of multiple remote offshore wind farms. Summary of the Invention

[0005] The purpose of the present invention is to provide a deep-sea wind power DC transmission system and a control method based on a flexible thyristor converter.

[0006] A deep-sea wind power DC transmission system based on flexible thyristor converters includes multiple offshore wind power DC transmission systems, a DC busbar, an onshore converter station, and an onshore AC power grid. 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. The DC submarine cable and the DC line are both connected to the DC busbar, and the AC line is connected to the onshore AC power grid.

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

[0008] Furthermore, 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.

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

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

[0011] Furthermore, the control method also includes: the offshore MMC converter station adopts a V / f control mode, and the LCC part adopts a fixed trigger angle control mode.

[0012] Furthermore, U dc Q control mode also includes:

[0013] A phase-locked loop is used to obtain the voltage phase of the AC three-phase voltage measurement value; the FBMMC grid-side current measurement value is transformed into the d and q component measurement values according to the voltage phase through the abc / dq coordinate transformation; the FBMMC grid-side voltage measurement value is transformed into the d and q component measurement values according to the voltage phase through the abc / dq coordinate transformation; the deviation between the reference value of the average value of the sum of the FBMMC bridge arm capacitor voltages in the outer loop and the measured value of the average value of the sum of the FBMMC bridge arm capacitor voltages is subjected to PI modulation to generate the d-axis component reference value of the FBMMC grid-side current, and the deviation 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; 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, and the two are transformed into the dq / abc coordinate transformation to obtain the FBMMC valve-side voltage reference value, which is used to generate the bridge arm voltage.

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

[0015]

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

[0017] Furthermore, the value range of the constant trigger angle in the fixed trigger angle control mode is 140° to 160°.

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

[0019] 1. The present invention comprises U dc The Q control mode enables centralized collection of electricity from multiple remote offshore wind farms, complementing the output of offshore wind farms in different geographical locations, smoothing power output fluctuations, reducing overall output power volatility, and facilitating optimized offshore wind farm operation at the receiving end.

[0020] 2. A single onshore converter station based on FLCC can achieve large-capacity grid connection, eliminating the need to build multiple onshore MMC converter stations. Several offshore MMC converter stations based on compact technology can achieve lightweight and compact offshore platforms, significantly improving the economic efficiency of offshore wind power grid connection system construction.

[0021] 3. The offshore grid-connected system based on the DC busbar supports the connection of new wind farms in the future without the need to restructure the existing transmission network, reducing long-term development costs and enhancing the flexibility and scalability of the grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the topology diagram of the offshore wind power grid-connected system based on FLCC.

[0023] Figure 2 This is the control block diagram of the onshore FLCC converter station.

[0024] Figure 3 This is the control block diagram of the offshore MMC converter station. DETAILED DESCRIPTION

[0025] The present invention provides a deep-sea wind power DC transmission system and control method based on a flexible thyristor converter. The present invention is further described below with reference to the accompanying drawings and specific embodiments.

[0026] The deep-sea wind power DC transmission system based on flexible line commutated converter (FLCC) specifically includes multiple offshore wind power DC transmission systems, DC busbars, onshore converter stations and onshore AC power grids. Its topology is as follows: Figure 1 shown.

[0027] The offshore wind power DC transmission system consists of a sequentially connected offshore wind farm, a collection network, an offshore converter station, and a DC submarine cable. The offshore wind farms all use grid-following wind turbines, the collection network uses an AC submarine cable, and the offshore converter station converter topology all adopts a compact MMC. The onshore converter station consists of a sequentially connected AC line, an onshore converter, and a DC line. The onshore converter station converter topology adopts an FLCC, which consists of a 12-pulse LCC connected in series with an FBMMC on the DC side and in parallel with the AC side. The FBMMC is an MMC composed of full-bridge submodules (FBSMs). Figure 1 Middle U dc 、U dcL and U dcM are the DC voltages of the FLCC, LCC, and FBMMC parts, respectively. dc is the DC port current of the FLCC. The DC submarine cable and the DC line are both connected to the DC busbar, and the AC line on the FLCC side is connected to the onshore AC grid.

[0028] AC power generated by offshore AC wind farms passes through a collection network, where it is converted to DC by MMC converter stations. The DC power transmitted by each offshore wind farm via the DC cable has the same voltage level. The DC sides of multiple offshore wind farms are connected in parallel to a large DC busbar, where it is inverted by FLCCs to AC power before being connected to the AC grid. The offshore MMC converter stations utilize compact technologies, such as high-ripple MMC (HR-MMC) operation and MMC operation with improved circulating current suppression control, to reduce the size and weight of the offshore converter stations and improve the economic efficiency of offshore platform construction.

[0029] Since the wind farm adopts grid-following control, all offshore MMC converter stations adopt V / f control mode to maintain the voltage stability of each offshore wind farm; the onshore FLCC converter station adopts U dc Q control mode to control the shore DC bus node voltage U dc stability.

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

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

[0032]

[0033] Where V g and V g v ga,b,c The vector sum magnitude, V gd,q It is vga,b,c The equivalent values of the d-axis and q-axis in the dq coordinate are expressed as V g As the benchmark, that is, V gd =V g And V gq =0, J1 is the coefficient matrix, which can be calculated as:

[0034]

[0035] Where, L cal and R cal Represent the equivalent inductance and resistance of FLCC respectively, and are calculated as follows:

[0036]

[0037] Where, L arm and R arm is the bridge arm inductance and resistance of FBMMC; k M is the transformer ratio of FBMMC; L eM and R eM Converter transformer T required for FBMMC M The equivalent inductance and resistance.

[0038] Since the adjustment variable of the FBMMC part is u a,b,cn and u a,b,cp , in order to decouple the two, define an intermediate variable E dcM :

[0039]

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

[0041]

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

[0043]

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

[0045]

[0046] Where i gd,q is the AC port current i of FLCC ga,b,c The d,q components, i Ld,q is the LCC grid-side current i La,b,c The d,q components, V Ld,q is the LCC grid-side voltage v La,b,cThe d,q components, L eL is the equivalent inductance of the converter transformer required by the LCC, C e is the equivalent capacitance value of the AC reactive power compensation capacitor and filter bank, J2 and J3 are coefficient matrices, which can be calculated as:

[0047]

[0048] and

[0049]

[0050] Where α* and k L is the trigger angle of the LCC part and the converter transformer ratio.

[0051] The LCC part adopts fixed trigger angle control, and the fixed trigger angle α is a constant value α * , and since FLCC is the receiving-end inverter, α * Should be greater than 90°, usually 140° to 160° in practical applications, the modulation system is based on α * Generates 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 bridge arm capacitor voltages V * capΣ and reactive power Q * s ; Use Phase-Locked Loop (PLL) according to the external PCC AC three-phase voltage v m ga,b,c Get the voltage phase at this time; i' m Ma,b,c According to θ m After abc / dq coordinate transformation, we get i' m Ma,b,c The d,q components i' m Md and i' m Mq ;v m Ma,b,c According to θ m After abc / dq coordinate transformation, we get v m Ma,b,c The d,q components V m Md and V m Mq Outer Ring U * dc with Um dc The deviation is modulated by PI to generate I * dc , inner ring I * dc with I m dc The deviation is modulated by PI and 0.5 times U m dcM Add them together to get E * dcM , used to generate the bridge arm voltage u * a,b,cp ~u * a,b,cn Outer ring V * capΣ With V m capΣ The deviation is modulated by PI to generate i' * Md , Q * s With Q m s The deviation is modulated by PI to generate i' * Mq ; The inner loop uses the conventional two-control variable inner loop control structure to generate u * Md and u * Mq , the two are transformed by dq / abc coordinates to get u * Ma,b,c , used to generate the bridge arm voltage u * a,b,cp ~u * a,b,cn . The modulation system is based on u * a,b,cp ~u * a,b,cn Generate trigger pulse signals for all switching devices in the MMC.

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

[0054] The above-mentioned FLCC-based DC transmission system can realize the collection of electricity from multiple long-distance offshore wind farms, and the output of offshore wind farms in different geographical locations can complement each other and smooth out power output fluctuations. For example, the difference in wind speeds in offshore wind farms in different geographical locations can balance power generation fluctuations (such as when there is no wind in one area, there may be sufficient wind in another area), thereby reducing the volatility of the overall output power. Compared with the existing series multi-terminal system, the proposed FLCC-based DC transmission system design supports the access of new wind farms in the future without the need to reconstruct the existing transmission network, thereby reducing long-term development costs. Compared with the existing parallel multi-terminal system, since the current-carrying capacity of the switching device thyristor used in LCC is much greater than the switching device IGBT used in MMC, the DC transmission system of the present invention uses FLCC as an onshore converter station, and the power capacity that can be transmitted is much greater than that of the traditional MMC converter station, avoiding the construction of multiple MMC converter stations to achieve large-capacity wind power grid connection, and has higher economic efficiency.

Claims

1. A deep-sea wind power DC transmission system based on flexible thyristor converters, 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 is composed of a wind farm, a collection network, an offshore converter station and a DC submarine cable connected in sequence; the onshore converter station is composed of an AC line, an onshore converter and a DC line connected in sequence; Among them, the DC submarine cable and DC line are connected to the DC busbar, and the AC line is connected to the onshore AC power grid.

2. The deep-sea wind power DC transmission system based on flexible thyristor converter according to claim 1 is characterized in that: The wind farm is a grid-following type wind turbine, the collection network is an AC submarine cable, and the converter topology of the offshore converter station is a compact MMC.

3. The deep-sea wind power DC transmission system based on flexible thyristor converter according to claim 1 or 2, characterized in that: 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.

4. The deep-sea wind power DC transmission system based on flexible thyristor converter according to claim 3 is characterized in that: FBMMC is an MMC composed of full-bridge sub-modules.

5. A control method for a deep-sea wind power DC transmission system based on a flexible thyristor converter according to claim 4, characterized in that: FLCC uses U dc Q control mode, the U dc In the Q control mode, the deviation between the FLCC DC voltage reference value of the outer loop and the FLCC DC voltage measurement value is PI modulated to generate the DC port current reference value of the inner loop. The deviation between the DC port current reference value and the DC port current measurement value is PI modulated and added to 0.5 times the FBMMC DC voltage measurement value to obtain the reference value of the intermediate variable for generating the bridge arm voltage.

6. The control method of the deep-sea wind power DC transmission system based on the flexible thyristor converter according to claim 5 is characterized in that: The control method further includes: the offshore MMC converter station adopts a V / f control mode, and the LCC part adopts a fixed trigger angle control mode.

7. The control method of the deep-sea wind power DC transmission system based on the flexible thyristor converter according to claim 6 is characterized in that: The U dc Q control mode also includes: A phase-locked loop is used to obtain the voltage phase of the AC three-phase voltage measurement value; the FBMMC grid-side current measurement value is transformed into the d and q component measurement values according to the voltage phase through the abc / dq coordinate transformation; the FBMMC grid-side voltage measurement value is transformed into the d and q component measurement values according to the voltage phase through the abc / dq coordinate transformation; the deviation between the reference value of the average value of the sum of the FBMMC bridge arm capacitor voltages in the outer loop and the measured value of the average value of the sum of the FBMMC bridge arm capacitor voltages is subjected to PI modulation to generate the d-axis component reference value of the FBMMC grid-side current, and the deviation 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; 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, and the two are transformed into the dq / abc coordinate transformation to obtain the FBMMC valve-side voltage reference value, which is used to generate the bridge arm voltage.

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

9. The control method of the deep-sea wind power DC transmission system based on the flexible thyristor converter according to claim 6 is characterized in that: The value range of the constant trigger angle in the fixed trigger angle control mode is 140° to 160°.

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