Offshore wind power DR-MMC DC grid loss analysis method and system

Abstract

The present invention relates to offshore wind power transmission control technology, and more particularly to a method and system for analyzing network losses of offshore wind power via a DR-MMC direct current (DC) grid, which is applicable to a control strategy for a DR-MMC multi-terminal hybrid DC transmission system for offshore wind power transmission. The method establishes an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system. By combining the external characteristics of DC line-to-line power flow controllers at different installation locations, control parameters are introduced into the DC grid power flow calculation to establish a sensitivity model for the operating network losses of each branch of the DC grid to the line-to-line power flow controller operating ratio M. The line-to-line power flow converter is selected in series according to a sensitivity formula for the network losses to the line-to-line power flow controller operating ratio, and the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding line-to-line power flow controller operating ratio are obtained. This method improves the power flow control capability of the DR-MMC multi-terminal DC transmission system and also improves the efficiency of offshore wind power transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of offshore wind power transmission control, and in particular relates to a method and system for analyzing network losses of an offshore wind power grid via a DR-MMC direct current (DC) network. Background Art

[0002] Offshore wind power resources are abundant, do not occupy land resources, and are close to coastal load centers. They are one of the key development directions of renewable energy in the future. Compared with AC transmission methods, flexible DC transmission technology based on modular multilevel converters (MMC) has the advantages of high long-distance transmission efficiency and flexible regulation. It is one of the main methods for long-distance offshore wind power transmission. However, the large size and weight of large-capacity MMC converter stations lead to high construction costs and difficult operation and maintenance of offshore platform projects. Therefore, DC transmission based on diode uncontrolled rectification (DR) is one of the important ways to solve the problem of low-cost and large-capacity transmission of offshore wind power.

[0003] DR-MMC transmission, a new, yet immature, transmission technology, differs from traditional flexible DC transmission in that the active power of DR transmission depends on the difference between DC and AC voltages. The DC voltage of an offshore transmission system composed of parallel DR-MMCs is naturally coupled to the offshore AC grid, making DC power flow control in the system more complex and cumbersome. Therefore, improving the power flow control capabilities of DR-MMC multi-terminal DC transmission can better ensure safe, economical, and reliable system operation. Currently, power flow control in multi-terminal DC grids generally falls into two approaches: one is to adjust the converter's active power output reference value at the DC transmission system level; the other is to install an auxiliary interlink DC power flow controller, known as an interlink DC power flow controller (IDCPFC).

[0004] In offshore wind power multi-terminal DC transmission scenarios, the onshore converter output active power must be kept stable as much as possible, making it unsuitable for system-level regulation of active power output. Therefore, a DC line-to-line power flow controller is needed to control the DC power flow and ensure optimal power distribution between the offshore MMC and DR converters. Furthermore, integrating an IDCPFC in series with the system not only increases the freedom of DC line power flow but also enhances the power flow control capabilities of the DR-MMC hybrid multi-terminal DC transmission system. IDCPFCs are typically small power converters that can support overall power flow control in multi-terminal DC grids. DC line-to-line power flow controllers are functionally classified into resistor-type and voltage-type. Variable resistor topology and control are relatively simple, but they increase line losses, leading to increased system losses, insufficient power flow freedom, and limited flexibility.

[0005] Currently, research on IDCPFC primarily focuses on improving PFC topologies, with control algorithms often focused on line power margin control. There is a lack of system-level power regulation design for IDCPFC, specifically designing the output reference value of IDCPFC in multi-terminal DC power grids with the goal of minimizing line losses. IDCPFC has the potential to minimize system losses while ensuring safe and reliable operation of DC transmission systems. Furthermore, limited research has explored integrating IDCPFC in series with offshore wind power DR-MMC multi-terminal hybrid DC transmission systems to improve the system's power regulation capabilities. DR-MMC hybrid DC power grids primarily focus on improving controller topologies, and power regulation for multi-terminal DC systems often relies on converter power regulation. However, this approach only achieves power regulation at the converter ports and cannot achieve power regulation for individual branches, resulting in limitations.

[0006] Through the above analysis, the problems and defects of the existing technology are as follows:

[0007] (1) The DC voltage of the offshore transmission system composed of parallel DR-MMCs is naturally coupled with the offshore AC power grid, and its power flow control capability is insufficient. In addition, the offshore MMC controller method is more complex and cumbersome, and cannot achieve frequent adjustments and actions.

[0008] (2) In the existing DC line-to-line power flow controller, the variable resistor topology and control are relatively simple, but it will increase line loss, increase the total network loss of the system, and has the characteristics of insufficient power flow freedom and poor flexibility.

[0009] (3) There is currently a lack of system-level power flow regulation design for DC transmission systems containing multiple IDCPFCs, that is, designing the output reference value of IDCPFC in a multi-terminal DC grid with the minimum line network loss as the control target. Summary of the Invention

[0010] In view of the problems existing in the background technology, the present invention provides a method and system for analyzing network losses of an offshore wind power DR-MMC direct current grid.

[0011] To solve the above technical problems, the present invention adopts the following technical solutions: a method for analyzing network losses of an offshore wind power DR-MMC DC grid, comprising the following steps: combining equivalent mathematical models of DR and MMC converters to establish an AC / DC power flow calculation model for a DR-MMC multi-terminal hybrid DC transmission system; combining the external characteristics of DC line power flow controllers at different installation locations, introducing control parameters into the DC grid power flow calculation, and establishing a sensitivity model for the operating network losses of each branch of the DC grid to the operating transformation ratio M of the line power flow controller; selecting the string insertion position of the line power flow converter based on the sensitivity formula of the network losses of the offshore wind power multi-terminal DC grid to the operating transformation ratio of the line power flow controller, and obtaining the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding operating transformation ratio of the line power flow controller.

[0012] In the above-mentioned offshore wind power DR-MMC DC grid loss analysis method, the specific steps of the method are as follows:

[0013] Step 1: Determine the control method of the converter of the offshore wind power DR-MMC multi-terminal hybrid DC transmission system;

[0014] Step 2: Combine the equivalent mathematical models of DR and MMC converters and the external characteristics of the DC line power flow controller to establish an AC and DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system;

[0015] Step 3: Based on the AC / DC power flow calculation model, a sensitivity model of the operating network loss of each branch of the DC power grid to the working ratio M of the line power flow controller is established;

[0016] Step 4. Select the nodes for installing the DC line power flow controller: Assume M = 1, and calculate the absolute value of the sensitivity of the network loss to the line power flow controller transformation ratio when the DC line power flow controller is connected in series to each line. Select the two groups of lines with the highest sensitivity and connect the two groups of DC line power flow controllers in series to the DC lines.

[0017] Step 5. Principle for adjusting the operating ratio M of the line power flow controller: given a minimum value ε, when S < 0 and |S| > ε, reduce the value of M; when S > 0 and |S| > threshold ε, increase the value of M and repeat steps 1 to 3.

[0018] Step 6: When |S| is less than the threshold ε, solve the line loss under the working ratio M of the line power flow controller, output the M value and line loss, and end the calculation; otherwise, repeat step 5.

[0019] An offshore wind power DR-MMC DC grid loss analysis system includes an AC / DC power flow calculation model building module for determining a control method for an offshore wind power DR-MMC multi-terminal hybrid DC transmission system. The system combines equivalent mathematical models of DR and MMC converters to establish an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system.

[0020] A sensitivity model building module is used to combine the external characteristics of DC line power flow controllers at different installation locations, introduce control parameters into the DC grid power flow calculation, and establish a sensitivity model of the operating network losses of each DC grid branch to the operating ratio M of the line power flow controller;

[0021] The system operation network loss analysis module is used to select the string insertion position of the line power flow controller based on the sensitivity formula of the network loss of the offshore wind power multi-terminal DC power grid to the working ratio of the line power flow controller, and to obtain the minimum operating network loss of the DR-MMC multi-terminal hybrid DC transmission system and the corresponding working ratio of the line power flow controller.

[0022] An electronic device, the computer device comprising a memory and a processor, the memory storing a computer program, and the computer program, when executed by the processor, causing the device to perform the steps of a method for analyzing network losses of an offshore wind power direct current grid via a DR-MMC.

[0023] A computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the processor executes the steps of a method for analyzing network losses of an offshore wind power grid via a DR-MMC direct current (DC) network.

[0024] An information data processing terminal is used to implement the offshore wind power DR-MMC DC grid loss analysis system.

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

[0026] The present invention provides a method for analyzing network losses of an offshore wind power DR-MMC DC power grid. First, a control strategy for a DR-MMC multi-terminal hybrid DC transmission system suitable for offshore wind power transmission is designed. Then, an AC / DC power flow calculation model of the DR-MMC multi-terminal hybrid DC transmission system is established by combining equivalent mathematical models of DR and MMC converters. In combination with the external characteristics of DC line power flow controllers at different installation positions, their control parameters are introduced into the DC power grid power flow calculation. A sensitivity model of the operating network losses of each branch of the DC power grid to the working transformation ratio M of the line power flow controller is established. The connection position of the line power flow converter is selected based on the sensitivity formula of the network loss to the working transformation ratio of the line power flow controller. Then, the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding working transformation ratio of the line power flow controller are obtained.

[0027] The present invention provides a method for analyzing network losses of offshore wind power via a DR-MMC DC grid. Combined with the mathematical model of AC and DC power flows of offshore wind power via a DR-MMC multi-terminal hybrid DC grid, the method solves for the optimal (minimizing network losses is the primary goal) DC line-to-line power flow controller installation location, sets the minimum network loss as the optimization control goal, and obtains the minimum network loss and DC line-to-line power flow controller ratio for offshore wind power via a DR-MMC multi-terminal hybrid DC grid. The present invention not only innovatively proposes a mathematical model of AC and DC power flows of a DR-MMC multi-terminal hybrid DC grid grid based on a DC line-to-line power flow controller, but also combines the optimal (minimizing network losses is the primary goal) DC line site selection and the numerical solution for DC network loss minimum control, thereby improving the economy and reliability of the offshore wind power hybrid DC transmission system.

[0028] This paper, combining the mathematical model characteristics of a DR-MMC hybrid DC grid, provides a method for analyzing network losses in offshore wind power transmission through a DR-MMC DC grid. This method improves the power flow control capability of the DR-MMC multi-terminal DC transmission system and reduces its transmission losses. The proposed method reduces the operating losses of the DR-MMC multi-terminal DC transmission system by rationally configuring line power flow controllers. This method not only controls the DC power flow of the lines but also reduces DC grid losses by rationally configuring its operating ratio.

[0029] The offshore wind power DR-MMC DC grid loss analysis method of the present invention significantly improves the transmission efficiency of the offshore wind power hybrid DC transmission system and reduces operating losses, improves the operating economy of offshore wind power, greatly reduces the cost of offshore wind power and accelerates the development of offshore wind power.

[0030] The offshore wind power DR-MMC DC grid loss analysis method of the present invention improves the transmission efficiency of the offshore wind power transmission system by 2 to 4 percentage points. Starting from the research on offshore wind power operation control, it breaks through technical barriers and develops a series of operation network loss analysis methods, thereby improving the line flow control capability of the offshore wind power hybrid DC transmission system.

[0031] Targeted at the distributed grid connection and DC transmission network construction of deep-sea wind farm clusters, this paper utilizes an additional DC power flow controller to achieve power regulation across multiple ports on the DC side of a converter. The paper analyzes the DC power flow controller's adjustable capabilities within a complex, multi-terminal network structure comprised of multiple converter types, and its enhanced regulation and improved transmission efficiency of offshore wind power transmission systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a flow chart of a method for analyzing network losses of an offshore wind power grid via a DR-MMC DC power grid provided by an embodiment of the present invention;

[0033] Figure 2 This is a flow chart of a method for analyzing network losses in offshore wind power transmission via DR-MMC multi-terminal DC transmission based on a DC line flow controller provided by an embodiment of the present invention;

[0034] Figure 3 This is a schematic diagram of a five-terminal DC grid for offshore wind power via DR-MMC including a DC line-to-line flow controller provided by an embodiment of the present invention;

[0035] Figure 4 This is a topological diagram of a DC line power flow controller provided by an embodiment of the present invention;

[0036] Figure 5 This is a schematic diagram of network loss changes in an offshore wind power DR-MMC multi-terminal hybrid DC transmission system containing multiple IDCPFC groups provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0038] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0039] The present invention will be further described below with reference to specific examples, but they are not intended to limit the present invention.

[0040] This embodiment provides a method for analyzing network losses in an offshore wind power grid using a DR-MMC DC power grid. First, a control strategy for a DR-MMC multi-terminal hybrid DC power transmission system suitable for offshore wind power transmission is designed. Then, an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC power transmission system is established by combining equivalent mathematical models of DR and MMC converters. The control parameters of DC line power flow controllers at different installation locations are introduced into the DC power grid power flow calculation, and a sensitivity model for the operating network losses of each branch of the DC grid to the line power flow controller operating ratio M is established. The line power flow converter insertion position is selected based on the sensitivity formula for the line operating network losses to the line power flow controller operating ratio, thereby determining the minimum operating network loss of the DR-MMC multi-terminal DC power transmission system and the corresponding line power flow controller operating ratio. This method improves the power flow control capability of the DR-MMC multi-terminal DC power transmission system while also improving the efficiency of offshore wind power transmission.

[0041] This embodiment is implemented through the following technical solutions: a method for analyzing network losses of offshore wind power through a DR-MMC DC grid, applicable to the control strategy of a DR-MMC multi-terminal hybrid DC transmission system for offshore wind power transmission; an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system is established by combining equivalent mathematical models of DR and MMC converters; control parameters are introduced into the DC grid power flow calculation based on the external characteristics of DC line power flow controllers at different installation locations, and a sensitivity model of the operating network losses of each branch of the DC grid to the operating ratio M of the line power flow controller is established; the string insertion position of the line power flow converter is selected based on the sensitivity formula of the network loss to the operating ratio of the line power flow controller, and the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding operating ratio of the line power flow controller are obtained.

[0042] Furthermore, the offshore wind power DR-MMC DC grid loss analysis method includes the following specific steps:

[0043] S1. Determine the converter control strategy based on the converter characteristics of the offshore wind power DR-MMC multi-terminal hybrid DC transmission system; including:

[0044] The offshore MMC converter uses active power-AC voltage droop control to provide a stable frequency and amplitude for the wind farm AC bus voltage, absorbing the power transmitted by the wind farm, thereby ensuring the stable operation of the wind farm. At the same time, the offshore AC voltage amplitude is adjusted in real time to ensure the reasonable distribution of DR and MMC active power. The relationship between the offshore wind power AC bus voltage and the offshore MMC converter output power is as follows:

[0045] U m_off =U m0_off +k r P MMC_off ;

[0046] Where U m0_off is the initial reference value of AC voltage; U m_off is the AC voltage at the outlet of offshore MMC; P MMC_off is the active power transmitted by the offshore MMC; k r is the droop coefficient.

[0047] S2. Combine the equivalent mathematical models of DR and MMC converters and the external characteristics of the DC line power flow controller to establish an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system; including:

[0048] (1) Input the grid parameters and calculate the Jacobian matrix J ac , set the initial value of each node voltage and the wind farm transmission power;

[0049] (2) The active and reactive power of each group DRi are obtained by solving the following formula:

[0050]

[0051] Where, P DR and Q DR are the active power and reactive power of DR respectively; I DR is the DC side current of DR; n DR is the ratio of the converter transformer of DR; N1 is the number of 6-pulse rectifier bridges of DR at the sending end; U DR0 and U DR are the DR no-load DC side voltage and actual DC voltage respectively; V m is the AC bus voltage amplitude; Y f is the admittance of reactive power compensation and filter on the DR AC side.

[0052] (3) The power transmission equation of each wind turbine and the network matrix is obtained:

[0053]

[0054] Where V i , V j is the AC node voltage amplitude, j∈i represents all nodes connected to node i; θ ij , G ij , B ij are the phase angle difference, conductance and susceptance between nodes i and j respectively; P ac_i , Q ac_i Active power and reactive power injected into the AC node; ΔP ac_i , ΔQ ac_i The deviation of active power and reactive power injected into the AC node.

[0055] Obtain the imbalance of the modified equation:

[0056]

[0057] Where ΔP ac_i , ΔQ ac_i Active power and reactive power deviation injected into the AC node; Δθ ij , ΔV i are the phase angle deviation and voltage deviation of node i respectively; J ac is the Jacobian matrix, and the partial derivatives of each deviation with respect to each variable are obtained.

[0058] (4) Calculate the new value of the AC bus voltage. If the iterative accuracy calculation formula is satisfied, output the voltage of the DR node and the active power of MMC3. Otherwise, take the new voltage value and start the next iteration from step (3).

[0059]

[0060] Where ε1 is a pre-given threshold; ΔP (n) ac_i , ΔQ (n) ac_i is the difference between the nth and n-1th active and reactive powers injected into the AC node.

[0061] (5) Combined with the above AC iteration results, the AC node voltage U mi and offshore MMC active power P MMC_off , solve for the DR DC side current I DRi and offshore MMC active power I MMC_off .

[0062] The DC power flow of the DC grid is determined by the voltage at each port and the DC line structure; if the line l between converter i and converter j is ij If a DC line power flow controller is connected in series, the line current and voltage expressions are:

[0063] I ij =G ij (MU i -U j );

[0064] Where G ij G in the system ij is the mutual conductance between node i and node j; I ij is the line current between DC node i and node j; M is the transformation ratio of the line power flow controller; U i is the voltage at node i; U j is the voltage at node j.

[0065] If there is no series line power flow controller, the line current expression is as follows:

[0066] I ij =G ij (U i -U j );

[0067] Where, I i is the current injected into node i; U i is the voltage at node i; U j is the voltage at node j; G ij is the node conductance matrix of the DC network.

[0068] The DC power flow equation is solved by Gaussian elimination method to calculate the DC voltage and DC current at each node.

[0069] (6) Calculate the new value of the DC voltage. If the convergence criterion is satisfied, the DC voltage matrix and current matrix and other variables are calculated; otherwise, start the next iteration from step (5);

[0070]

[0071] Where ε2 and ε3 are pre-given thresholds; ΔI MMCi , ΔI DRi are the DC current differences between the i-th MMC and DR nodes respectively.

[0072] S3. Based on the AC / DC power flow calculation model, establish a sensitivity model for the operating network losses of each branch of the DC grid to the line power flow controller ratio M; including:

[0073] The converter transmission power and line transmission expressions are as follows:

[0074]

[0075] Where, the total power of converter port i; P ij For line l ij Transmission power; n is the total number of nodes; I i is the current injected into node i; U i is the voltage at node i; U j is the voltage at node j.

[0076] The derivative of the network loss of the multi-terminal DC grid for offshore wind power with respect to the working ratio M of the line power flow controller is obtained:

[0077]

[0078] Where U i and U j is the partial derivative of the DC node voltage amplitude; M is the partial derivative of the power flow controller ratio.

[0079] Since the power of the constant active power converter station does not change with the change of the IDCPFC transformation ratio M, and the voltage of the converter station operating with constant DC voltage does not change with the change of the IDCPFC transformation ratio M, then:

[0080]

[0081] Where, P i_C is the converter node power with known active power; U i_C is the node voltage of the converter using constant DC voltage control; M is the partial derivative of the power flow controller ratio.

[0082] According to the iterative calculation results of AC and DC power flows, the power flow of the DC transmission system is known, and M, I ij , U i , Uj Substitute into the formula to get and

[0083] According to the DC node voltage partial derivative and Substitute the value into the following formula:

[0084]

[0085] Where, is the partial derivative of the line current between DC nodes i and j.

[0086] Therefore, the partial derivative of the line loss of the multi-terminal DC grid with respect to M is expressed as follows:

[0087]

[0088] Where, P loss is the line loss power of the DC system; R ij is the resistance of the line node ij.

[0089] S4. Selection of DC power flow control nodes: When M = 1, calculate the absolute value of the sensitivity of the network loss to the transformation ratio of the line power flow controller when the line power flow controller is connected in series to each line. Select the two groups of lines with the highest sensitivity and connect the two groups of DC line power flow controllers in series to the DC lines.

[0090] Options for installing DC power flow control nodes in S4 include:

[0091] Offshore wind power is transmitted through the DR-MMC multi-terminal hybrid DC grid. The DC grid contains multiple DC lines. The different installation locations of the DC line flow controllers result in different DC flow control capabilities.

[0092] When the line power flow controller ratio M = 1, the network loss is the same regardless of which line the DC line power flow controller is connected in series. Therefore, when M = 1, the absolute value of the sensitivity of the network loss to the line power flow controller ratio |S| is calculated for each DC line.

[0093] When the line power flow controllers are connected in series in different DC lines, the higher the sensitivity of the grid loss to the DC line power flow controller ratio M, the lower the minimum value of the DC grid loss, and thus the optimal line power flow controller connection position with the minimum loss is obtained;

[0094] Select the two groups of lines with the highest sensitivity, connect the two groups of DC line power flow controllers in series to the DC lines, and adjust the DC power flow equation expression in step 2.

[0095] S5. Principle for adjusting the value of M: Given a minimum value ε, when S < 0 and |S| > ε, reduce the value of M; when S > 0 and |S| > threshold ε, increase the value of M and repeat S1 to S3.

[0096] The principles for adjusting the M value in S5 are:

[0097] The network loss value of the DC grid will change with the change of M. When S<0 and |S|>ε, the value of M is reduced, otherwise the value of M is increased.

[0098] Therefore, the minimum value ε is set. When |S|>ε, it means that the value of M fails to minimize the system loss. In the process of adjusting the value of M, there must be a point where M minimizes the loss. When the sensitivity S=0, the grid loss is minimized.

[0099] S6. When |S| is less than the threshold ε, solve the line loss under the line power flow controller ratio M, output the M value and line loss, and end the calculation; otherwise, repeat S5.

[0100] The minimum line loss under the ratio M of the line power flow controller in S6 is:

[0101]

[0102] Where, P loss is the line loss power of the DC system; R ij is the resistance of the line node ij.

[0103] Furthermore, this embodiment also provides a system for analyzing network losses of an offshore wind power DR-MMC DC grid, including:

[0104] AC / DC power flow calculation model building module, used to design control strategies for DR-MMC multi-terminal hybrid DC transmission systems suitable for offshore wind power transmission; combining the equivalent mathematical models of DR and MMC converters to establish the AC / DC power flow calculation model of the DR-MMC multi-terminal hybrid DC transmission system;

[0105] A sensitivity model building module is used to combine the external characteristics of DC line power flow controllers at different installation locations, introduce control parameters into the DC grid power flow calculation, and establish a sensitivity model of the operating network losses of each DC grid branch to the operating ratio M of the line power flow controller;

[0106] The system operation network loss analysis module is used to select the string insertion position of the line power flow converter based on the sensitivity formula of the network loss to the operating ratio of the line power flow controller, and then obtain the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding line power flow controller operating ratio.

[0107] Moreover, this embodiment also provides a computer device, which includes a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the offshore wind power DR-MMC DC grid loss analysis method.

[0108] Moreover, this embodiment further provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, the processor executes the steps of the offshore wind power DR-MMC DC grid loss analysis method.

[0109] Moreover, this embodiment further provides an information data processing terminal, which is used to implement the offshore wind power DR-MMC DC grid loss analysis system.

[0110] Example 1

[0111] like Figure 1 As shown in the figure, the DR-MMC DC grid loss analysis method for offshore wind power includes the following steps:

[0112] S101: Design a control strategy for a DR-MMC multi-terminal hybrid DC transmission system suitable for offshore wind power transmission; establish an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system by combining equivalent mathematical models of DR and MMC converters;

[0113] S102, combining the external characteristics of DC line power flow controllers at different installation locations, introducing control parameters into the DC grid power flow calculation, and establishing a sensitivity model of the operating network loss of each branch of the DC grid to the operating transformation ratio M of the line power flow controller;

[0114] S103, according to the sensitivity formula of the network loss to the working ratio of the line power flow controller, the string insertion position of the line power flow converter is selected, and then the minimum network loss of the DR-MMC multi-terminal DC transmission system and the corresponding working ratio of the line power flow controller are obtained.

[0115] like Figure 2 As shown, the offshore wind power DR-MMC DC grid loss analysis method provided in this embodiment includes the following specific steps:

[0116] Step 01: Based on the converter characteristics of the offshore wind power DR-MMC multi-terminal hybrid DC transmission system, the control strategy of each converter is determined;

[0117] Step 02: Combine the equivalent mathematical models of DR and MMC converters and the external characteristics of the DC line power flow controller to establish an AC and DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system;

[0118] Step 03: Based on the AC / DC power flow calculation model, a mathematical model of the sensitivity of the offshore wind power multi-terminal DC power grid loss to the line power flow controller ratio M is established;

[0119] Step 4, installation of DC power flow control nodes: Assuming M = 1, calculate the absolute value of the sensitivity of the network loss to the transformation ratio of the line power flow controller when the line power flow controller is connected in series to each line. Select the two groups of lines with the highest sensitivity and connect the two groups of DC line power flow controllers in series to the DC lines.

[0120] Step 05, principle of adjusting the M value: given a minimum value ε, if S<0 and |S|>ε, then reduce the value of M; if S>0 and |S|>threshold ε, then increase the value of M and repeat steps 01 to 03;

[0121] Step 06: When |S| is less than the threshold ε, solve the line loss under the line power flow controller ratio M, output the M value and line loss, and end the calculation; otherwise, repeat step 05.

[0122] The control strategy of the converter of the offshore wind power DR-MMC multi-terminal hybrid DC transmission system in step 01 provided in this embodiment includes:

[0123] The offshore MMC converter uses active power-AC voltage droop control to provide a stable frequency and amplitude for the wind farm AC bus voltage, absorbing the power transmitted by the wind farm, thereby ensuring the stable operation of the wind farm. At the same time, the offshore AC voltage amplitude is adjusted in real time to ensure the reasonable distribution of DR and MMC active power. The relationship between the offshore wind power AC bus voltage and the offshore MMC converter output power is as follows:

[0124] U m_off =U m0_off +k r P MMC_off (1)

[0125] Where U m0_off is the initial reference value of AC voltage; U m_off is the AC voltage at the outlet of offshore MMC; P MMC_off is the active power transmitted by the offshore MMC; k r is the droop coefficient.

[0126] The steps for establishing the AC / DC power flow calculation model of the DR-MMC five-terminal hybrid DC transmission system in step 02 provided in this embodiment are as follows:

[0127] ① Input the grid parameters and calculate the Jacobian matrix J ac , set the initial value of each node voltage and the wind farm transmission power;

[0128] The specific parameters of the DR-MMC offshore wind power multi-terminal hybrid DC transmission system topology are shown in Table 1.

[0129] Table 1 Main electrical parameters of hybrid DC grid system

[0130]

[0131] ② The reactive power expression of each DR group can be solved by the following formula:

[0132] Electrical quantities on the AC side of offshore converter DR1:

[0133]

[0134] Where U m1 、U m2 is the node voltage of the offshore AC system; X 12 is the reactance between the offshore AC voltage nodes 12; N1 is the number of 6-pulse rectifier bridges at the sending end DR; Q DR1 is the reactive power of converter DR1; X r1 is the equivalent leakage reactance of DR transformer; U DR1 and I DR1 are the current and voltage of converter DR1 respectively; Y f is the admittance of reactive power compensation and filter on the DR AC side.

[0135] Offshore converter DR2 AC side equation relationship:

[0136]

[0137] Where U m2 、U m3 is the node voltage of the offshore AC system; X 23 is the reactance between the offshore AC voltage nodes 2 and 3; N1 is the number of 6-pulse rectifier bridges at the sending end DR; Q DR2 is the reactive power of converter DR2; X r1 is the equivalent leakage reactance of DR transformer; U DR2 and I DR2 are the current and voltage of converter DR2 respectively; Y f is the admittance of reactive power compensation and filter on the DR AC side.

[0138] ③ The power flow calculation equation on the offshore AC side is obtained by considering the power of each wind turbine and the network matrix of the offshore AC system:

[0139]

[0140] Where V i , V jis the AC node voltage amplitude, j∈i represents all nodes connected to node i; θ ij , G ij , B ij are the phase angle difference, conductance and susceptance between nodes i and j respectively; P ac_i , Q ac_i Active power and reactive power injected into the AC node; ΔP ac_i , ΔQ ac_i The deviation of active power and reactive power injected into the AC node.

[0141] Through the above equation, the imbalance of the correction equation is obtained:

[0142]

[0143] Where ΔP ac_i , ΔQ ac_i Active power and reactive power deviation injected into the AC node; Δθ ij , ΔV i are the phase angle deviation and voltage deviation of node i respectively; J ac is the Jacobian matrix, and the partial derivatives of each deviation with respect to each variable are obtained.

[0144] ④ Calculate the new value of the AC bus voltage. If the following iterative accuracy calculation formula is satisfied, output the voltage of the DRi node and the active power of MMC3. Otherwise, take the new voltage value and start the next iteration from step ③.

[0145]

[0146] Where ε1 is a pre-given threshold; ΔP (n) ac_i , ΔQ (n) ac_i is the difference between the nth and n-1th active and reactive powers injected into the AC node.

[0147] ⑤ Combined with the AC iteration results, the AC node voltage U mi and offshore MMC active power P MMC_off , solve for DR DC side current I DRi and offshore MMC active power I MMC_off .

[0148] The DC power flow of the DC grid is determined by the voltage at each port and the DC line structure; if the line l between converter i and converter j is ij The DC line power flow controller is connected in series, and the DC line power flow controller adopts two-terminal control. Figure 1 The current equation and voltage equation of the DC line network are as follows:

[0149] Current equations:

[0150]

[0151] Where, I 12 , I 13 , I 23 , I 24 , I 35 , I 45 are the DC line current; I DR1 , I DR2 , I MMC are offshore DR and MMC currents respectively; I ON1 , I ON2 is the current of two onshore converters.

[0152] Node voltage equations:

[0153]

[0154] Where R 12 、R 13 、R 23 、R 24 、R 35 and R 45 is the line resistance; I 12 , I 13 , I 23 , I 24 , I 35 and I 45 are the DC line current; U DR1 、U DR2 、U MMC U1, U2 and U3 are the voltages at the output terminals of each converter respectively; U' DR1 、U' D ' R1 , U'2 and U'2' are the outlet voltages of the DC power flow controller respectively.

[0155]

[0156] Where U' DR1 、U″ DR1 , U'2 and U″2 are the output voltages of the DC power flow controller respectively; M1, M2, M3 and M4 are Figure 3 The transformation ratio of the DC line power flow controller is shown as follows; U2 is the voltage at the converter outlet of the fixed active power on the shore; U DR1 is the voltage at the outlet of offshore converter DR1;

[0157]

[0158] Where, U2 is the voltage at the converter outlet of the fixed active power on shore; U DR1 is the voltage at the outlet of offshore converter DR1; M1, M2, M3 and M4 are Figure 3 Transformation ratio of the center line power flow controller; I 12 , I 13 , I 24 , I 35 and I 45 are the DC line currents respectively.

[0159] Solve the DC power flow equation using Gaussian elimination to find the DC voltage and DC current at each node;

[0160] ⑥ Calculate the new value of the DC voltage. If the convergence criterion (9) is satisfied, the DC voltage matrix and current matrix and other variables are calculated; otherwise, start the next iteration from step ⑤;

[0161]

[0162] Where ε2 and ε3 are pre-given thresholds; ΔI MMCi , ΔI DRi are the DC current differences between the i-th MMC and DR nodes respectively.

[0163] The mathematical model of the sensitivity of the offshore wind power multi-terminal DC grid loss to the line power flow controller ratio M in step 03 provided in this embodiment has the following steps:

[0164] The converter transmission power and line transmission expressions are as follows:

[0165]

[0166] Where, P i is the total power of converter port i; P ij For line l ij Transmission power; n is the total number of nodes; I i is the current injected into node i; U i is the voltage at node i; U j is the voltage at node j.

[0167] Derivative of formula (10) with respect to the network loss of offshore wind power multi-terminal DC grid with respect to the line power flow controller ratio M can be obtained:

[0168]

[0169] Where U i and U j is the partial derivative of the DC node voltage amplitude; M is the partial derivative of the power flow controller ratio.

[0170] Since the power of the converter station using constant active power does not change with the change of IDCPFC transformation ratio M, the voltage of the converter station using constant DC voltage operation does not change with the change of IDCPFC transformation ratio M, that is:

[0171]

[0172] Where, P i_C is the converter node power with known active power; U i_C is the converter node voltage using constant DC voltage control.

[0173] According to the previous AC and DC power flow iterative calculation results, the power flow of the DC transmission system is known. ij , U i , U j As is known, substitute into formula (10) and we can get: and

[0174] according to and Substitute the value of into formula (13):

[0175]

[0176] Where, is the partial derivative of the line current between DC nodes i and j.

[0177] Therefore, the partial derivative of the line loss of the multi-terminal DC grid with respect to M is expressed as follows:

[0178]

[0179] Where, P loss is the line loss power of the DC system; R ij is the resistance of the line node ij.

[0180] The steps for selecting the DC power flow control node in step 04 provided in this embodiment are as follows:

[0181] Offshore wind power is transmitted through the DR-MMC multi-terminal hybrid DC grid. The DC grid contains multiple DC lines. The different installation locations of the DC line flow controllers result in different DC flow control capabilities.

[0182] When the line-to-line power flow controller ratio M = 1, the network loss is the same regardless of which line the DC line-to-line power flow controller is connected to. Therefore, when M = 1, the absolute value of the sensitivity of the network loss to the line-to-line power flow controller ratio |S| is calculated for each DC line.

[0183] When the line power flow controllers are connected in series in different DC lines, the higher the sensitivity of the grid loss to the DC line power flow controller transformation ratio M, the lower the minimum value of the DC grid loss; thus, the optimal (minimum loss) line power flow controller connection position is obtained.

[0184] To further reduce line losses, this embodiment selects two groups of lines with the highest sensitivity, connects two groups of DC line power flow controllers in series to the above DC lines, and promptly adjusts the DC power flow equation expression in step 02.

[0185] The principle of adjusting the M value in step 05 provided in this embodiment is:

[0186] The DC grid loss will change with the change of M. During the adjustment of M value, there must be a point where M minimizes the grid loss; that is, when the sensitivity S=0, the grid loss is minimized.

[0187] Set a minimum value ε. When |S|>ε, it means that the value of M fails to minimize the system network loss. Therefore, if S<0 and , reduce the value of M, otherwise increase the value of M.

[0188] The minimum line loss under the line power flow controller ratio M in step 06 provided in this embodiment is:

[0189]

[0190] Where, P loss is the line loss power of the DC system; R ij is the resistance of line node ij; n is the number of branches in the DC system.

[0191] The network loss analysis system for offshore wind power transmission via DR-MMC multi-terminal DC transmission provided in this embodiment includes:

[0192] AC / DC power flow calculation model building module, used to design control strategies for DR-MMC multi-terminal hybrid DC transmission systems suitable for offshore wind power transmission; combining the equivalent mathematical models of DR and MMC converters to establish the AC / DC power flow calculation model of the DR-MMC multi-terminal hybrid DC transmission system;

[0193] A sensitivity model building module is used to combine the external characteristics of DC line power flow controllers at different installation locations, introduce control parameters into the DC grid power flow calculation, and establish a mathematical model of the sensitivity of the DC grid branch losses to the power flow controller ratio;

[0194] The system operation network loss analysis module is used to select the string insertion position of the line power flow converter based on the sensitivity formula of the network loss to the operating ratio of the line power flow controller, and then obtain the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding line power flow controller operating ratio M.

[0195] This embodiment 1 has achieved some positive effects during the development or use process, and does have great advantages compared to the existing technology. The following is described through embodiment 2 combined with data, charts, etc. of the test process.

[0196] Example 2

[0197] Example 2 is an example of a DR-MMC multi-terminal DC transmission simulation model including two sets of DC line-to-line power flow controllers, as shown in Figures 3 to 5.

[0198] Offshore wind power with DC line flow controller is connected to DR-MMC five-terminal DC grid. Figure 3 The operating parameters of the DC line power flow controller IDCPFC are: C = 7mF, L = 2mH, and the switching frequency is 2kHz. Figure 4 When both IDCPFC groups are not put into operation (M1 and M3 are both equal to 1), the initial network loss of the system is P loss =31.45MW.

[0199] IDCPFC is controlled by a ratio method. The IDCPFC duty cycle expression is:

[0200]

[0201] Where U 1x 、U 2x are the input and output voltage differences of the x-terminal of the two groups of line power flow controllers respectively.

[0202] The following is the specific case implementation process and calculation process using MATLAB:

[0203] Formulas (1) to (8), (10), and (16) contain 28 independent variables and 28 equation systems. The 28 independent variables are as follows:

[0204] Converter DC side current (I DR1 , I DR2 , I MMC , I ON1 , I ON2 ); Line current (I 12 , I 13 , I 23 , I 24 , I 35 , I 45 ); independent variable on the offshore AC side (P 12 、P 23 、U m1 , Q DR1 , Q DR2 ); Line power flow controller related variables (M2, M4, U' DR1、U″ DR1 , U'2, U″2).

[0205] Two independent variables (M1, M3), assign values to M1 and M3, and calculate the remaining dependent variables, and then solve the DC grid loss P through formula (15): loss Through MATLAB programming, the grid loss P loss The minimum point, continuously narrowing the value range of M1 and M3, can be made M1∈[0.997, 1.003], M3∈[0.996, 1.003], P loss Regarding the surface plots of M1 and M3, such as Figure 5 shown.

[0206] Depend on Figure 5 It can be seen that there is a point P with minimum network loss in the figure. loss_min At this time, the system parameters such as the two sets of IDCPFC ratios, voltages, and currents are shown in Table 2.

[0207] Table 2 Operating parameters of two IDCPFC series systems

[0208] parameter <![CDATA[U2 / kV]]> <![CDATA[U DR1 / kV]]> <![CDATA[I 12 / kA]]> <![CDATA[I 13 / kA]]> <![CDATA[I 24 / kA]]> Numerical 638.56 651.72 1.30 0.29 0.17 parameter <![CDATA[I 45 / kA]]> <![CDATA[U 1x / kV]]> <![CDATA[U 1y / kV]]> <![CDATA[U 2x / kV]]> <![CDATA[U 2y / kV]]> Numerical 1.24 -1.92 2.68 -1.69 1.173 parameter <![CDATA[M1]]> <![CDATA[M2]]> <![CDATA[M3]]> <![CDATA[M4]]> <![CDATA[P loss / MW]]> Numerical 0.9970 1.0042 0.9974 1.0018 31.03

[0209] As can be seen from Table 2, two groups of IDCPFC operate simultaneously. 13 and I 24 There is a significant decrease in I 23 Slightly increased, thus, the total network loss P loss Compared with the case without IDCPFC (M1=M3=1), the grid loss is reduced from 31.45MW to 31.03MW, with a maximum reduction of 1.34%.

[0210] It should be noted that the embodiments of the present invention can be implemented by hardware, software, or a combination of software and hardware. The hardware portion can be implemented using dedicated logic; the software portion can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. Those skilled in the art will appreciate that the above-mentioned devices and methods can be implemented using computer-executable instructions and / or contained in processor control code, for example, such as a carrier medium such as a disk, CD or DVD-ROM, a programmable memory such as a read-only memory (firmware), or a data carrier such as an optical or electronic signal carrier. The devices and modules of the present invention can be implemented by hardware circuits such as very large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., can also be implemented by software executed by various types of processors, or can be implemented by a combination of the above-mentioned hardware circuits and software, such as firmware.

[0211] The above are only preferred embodiments of the present invention and do not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the present invention specification should be included in the protection scope of the present invention.

Claims

1. Offshore wind power DR-MMC DC grid loss analysis method, characterized by: The following steps are involved: Combining the equivalent mathematical models of DR and MMC converters, an AC / DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system is established. By combining the external characteristics of DC line power flow controllers at different installation locations, the control parameters are introduced into the DC grid power flow calculation, and a sensitivity model for the operating network losses of each branch of the DC grid to the line power flow controller operating ratio M is established. The string insertion position of the line power flow converter is selected based on the sensitivity formula for the network losses of the offshore wind power multi-terminal DC grid to the line power flow controller operating ratio, and the minimum operating network loss of the DR-MMC multi-terminal DC transmission system and the corresponding line power flow controller operating ratio are obtained. The specific steps of this method are as follows: Step 1: Determine the control method of the converter of the offshore wind power DR-MMC multi-terminal hybrid DC transmission system; Step 2: Combine the equivalent mathematical models of DR and MMC converters and the external characteristics of the DC line power flow controller to establish an AC and DC power flow calculation model for the DR-MMC multi-terminal hybrid DC transmission system; Step 3: Based on the AC / DC power flow calculation model, a sensitivity model of the operating network loss of each branch of the DC power grid to the working ratio M of the line power flow controller is established; Step 4. Select the nodes for installing the DC line power flow controller: Assume M = 1. Calculate the absolute value of the sensitivity of the network loss to the line power flow controller transformation ratio when the DC line power flow controller is connected in series to each line. Select the two lines with the highest sensitivity and connect the two sets of DC line power flow controllers in series to the DC lines. Step 5. Principle for adjusting the operating ratio M of the line power flow controller: given a minimum value ε, when S < 0 and |S| > ε, reduce the value of M; when S > 0 and |S| > threshold ε, increase the value of M and repeat steps 1 to 3. Step 6: When |S| is less than the threshold ε, solve the line loss under the working ratio M of the line power flow controller, output the M value and line loss, and end the calculation; Otherwise, repeat step 5.

2. A system for analyzing network losses of offshore wind power via DR-MMC DC grid according to claim 1, characterized in that: It includes an AC / DC power flow calculation model construction module, which is used to determine the control method of the offshore wind power transmission system through the DR-MMC multi-terminal hybrid DC transmission system. It combines the equivalent mathematical models of DR and MMC converters to establish the AC / DC power flow calculation model of the DR-MMC multi-terminal hybrid DC transmission system. A sensitivity model building module is used to combine the external characteristics of DC line power flow controllers at different installation locations, introduce control parameters into the DC grid power flow calculation, and establish a sensitivity model of the operating network losses of each DC grid branch to the operating ratio M of the line power flow controller; The system operation network loss analysis module is used to select the string insertion position of the line power flow controller based on the sensitivity formula of the network loss of the offshore wind power multi-terminal DC power grid to the working ratio of the line power flow controller, and to obtain the minimum operating network loss of the DR-MMC multi-terminal hybrid DC transmission system and the corresponding working ratio of the line power flow controller.

3. An electronic device, characterized in that: The computer device comprises a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the device performs the method according to claim 1 .

4. A computer-readable storage medium, characterized in that The storage medium stores a computer program, and when the computer program is executed by a processor, the processor is caused to execute the method according to claim 1 .

5. An information data processing terminal, characterized in that: The information data processing terminal is used to implement the system of the offshore wind power DR-MMC DC grid loss analysis method according to claim 1.

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