Offshore wind power flexible direct current transmission system based on supercapacitor and its inertia coordination method

Abstract

The present invention discloses an offshore wind power flexible direct current transmission system based on supercapacitors, comprising a supercapacitor, and first, second, third, and fourth converters connected in sequence; the supercapacitor is connected to the DC side of the first converter through a bidirectional DC / DC converter; the controller of the third converter comprises a current inner loop module A and an outer loop module A; the outer loop module A comprises a frequency restoration control module and a Parker conversion module A; the frequency restoration control module multiplies the difference between the actual value of the DC side voltage of the fourth converter and the DC voltage rated value of the flexible DC transmission system, and the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds it to the rated frequency of the onshore power grid, and after processing, obtains the frequency reference value of the direct-drive offshore wind farm power grid, which is converted into a phase angle reference value and output to the Parker conversion module A. The present invention also discloses an inertia coordination method of the system. The present invention reduces investment costs and increases system reliability.

Description

Technical Field

[0001] The present invention relates to an offshore wind power flexible direct current transmission system and a control method thereof, and in particular to an offshore wind power flexible direct current transmission system based on a supercapacitor and an inertia coordination method thereof. Background Art

[0002] At present, offshore wind power, as an important technical direction of renewable energy, has the advantages of being rich and stable, not occupying land, and suitable for large-scale development. It has become a development focus in recent years. As of the end of December 2020, the cumulative grid-connected installed capacity of offshore wind power in China reached 9.9 million kilowatts, ranking second in the world, second only to the United Kingdom; the country's new offshore wind power installed capacity was 3 million kilowatts, accounting for 16.8% of the country's new grid-connected wind power installed capacity. The development of offshore wind power is rapid. With the continuous scale and offshore development of offshore wind power, the application of traditional AC transmission technology has gradually encountered bottlenecks. The flexible DC transmission system with superior control performance and networking capabilities provides a new solution for it.

[0003] Large-scale offshore wind farms are connected to the power grid through flexible DC transmission systems. On the one hand, new energy generators with small rotational inertia replace synchronous generators with large rotational inertia, which will reduce the overall inertia of the power grid; on the other hand, due to the decoupling effect of the flexible DC transmission system, offshore wind farms cannot directly respond to the frequency changes of the onshore AC system and provide active frequency support. Existing research uses rotor kinetic energy control to improve the ability of wind turbines to provide inertia support. However, rotor kinetic energy control uses the actual mechanical inertia of the wind turbine, which is prone to excessive energy extraction, causing the speed of the unit to oscillate or even stall. In severe cases, it damages the mechanical structure of the unit and reduces its service life. In order to break the decoupling barrier of the flexible DC transmission system, communication channels are often used to directly transmit the real-time frequency information of the onshore power grid to the wind farm, but the communication delay has a great impact on the dynamic performance and reliability of the control system. The existing non-communication frequency coupling scheme does not take into account the loss of the VSC-HVDC line, which will cause the deviation between the restored frequency information of the wind farm and the actual frequency of the onshore power grid, affecting the effectiveness of inertia simulation. In summary, it is urgent to explore a communication-free method to transmit the main grid frequency change information to the wind turbines in real time and accurately, study the inertia coordination control technology between wind turbines and flexible DC transmission systems, and improve the frequency stability of onshore power grids. Summary of the invention

[0004] The present invention provides a supercapacitor-based offshore wind power flexible direct current transmission system and an inertia coordination method thereof to solve the technical problems existing in the known technology.

[0005] The technical solution adopted by the present invention to solve the technical problems existing in the known technology is: an offshore wind power flexible direct current transmission system based on supercapacitors, including a direct-driven offshore wind farm power grid, a flexible direct current transmission system and an onshore power grid connected in sequence; the direct-driven offshore wind farm power grid includes: a supercapacitor, a wind power generation system, a first converter, and a second converter connected in sequence; the flexible direct current transmission system includes a third converter, a DC bus and a fourth converter connected in sequence; the first and third converters are used to convert alternating current into direct current; the second and fourth converters are used to convert direct current into alternating current; the supercapacitor is connected to the DC side of the first converter through a bidirectional DC / DC converter; the controller of the third converter is a double closed-loop vector controller, which It includes a current inner loop module A and an outer loop module A; the outer loop module A includes a frequency restoration control module and a Parker transformation module A; the frequency restoration control module multiplies the difference between the actual value of the DC side voltage of the fourth converter and the DC voltage rated value of the flexible DC transmission system by the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds it to the rated frequency of the onshore power grid to obtain the frequency reference value of the direct-driven offshore wind farm power grid after processing, and converts the frequency reference value of the direct-driven offshore wind farm power grid into a phase angle reference value of the direct-driven offshore wind farm power grid and outputs it to the Parker transformation module A, and the Parker transformation module A outputs the voltage and current in the dq coordinate system corresponding to the direct-driven offshore wind farm power grid to the current inner loop module A.

[0006] Furthermore, the controller of the third converter also includes a Parker inverse transformation module A; the Parker inverse transformation module A receives the voltage reference signals of the d-axis and q-axis from the current inner loop module A, and the phase angle reference value of the direct-driven offshore wind farm power grid from the frequency restoration control module, and outputs the voltage reference signal corresponding to the direct-driven offshore wind farm power grid in the abc coordinate system to the pulse width modulator of the third converter.

[0007] Furthermore, the controller of the fourth converter is a dual closed-loop vector controller, which includes a current inner loop module B and a voltage outer loop module B; the voltage outer loop module B includes a droop control module and a PI controller B; the droop control module multiplies the difference between the actual frequency of the onshore power grid and the rated frequency of the onshore power grid with the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid, and then adds the product to the rated DC voltage of the flexible DC transmission system, and outputs a reference value of the DC side voltage of the fourth converter after processing; the PI controller B inputs the difference between the reference value and the actual value of the DC side voltage of the fourth converter, and outputs a current reference value corresponding to the current inner loop module B.

[0008] Furthermore, the controller of the bidirectional DC / DC converter is a dual closed-loop controller, which includes a current inner loop module C and a voltage outer loop module C; the voltage outer loop module C includes a PI controller C and an inertia simulation module; the inertia simulation module is used to simulate the inertia of the synchronous generator, and its input is the difference between the actual frequency of the direct-driven offshore wind farm power grid and the rated frequency of the direct-driven offshore wind farm power grid, and its output is a reference value of the supercapacitor voltage; the PI controller C, its input is the difference between the reference value and the actual value of the supercapacitor voltage, and its output corresponds to the current reference value of the current inner loop module C.

[0009] Furthermore, the mathematical expression of the inertia simulation module is as follows:

[0010]

[0011] in,

[0012] is the supercapacitor voltage reference value;

[0013] C sc is the supercapacitor value;

[0014] S n is the rated capacity of the supercapacitor energy storage unit;

[0015] H sc is the inertia time constant simulated for the supercapacitor;

[0016] f owf To directly drive the actual frequency of the power grid of the offshore wind farm;

[0017] f ow0 The rated frequency of the grid for direct-drive offshore wind farms;

[0018] u sc0 The voltage of the supercapacitor is the operating neutral point voltage.

[0019] The present invention also provides an inertia coordination method for an offshore wind power flexible direct current transmission system based on a supercapacitor, the method comprising: setting a direct-drive offshore wind farm power grid, a flexible direct current transmission system and an onshore power grid connected in sequence; the direct-drive offshore wind farm power grid is provided with: a supercapacitor, a wind power generation system, a first converter and a second converter connected in sequence; the flexible direct current transmission system is provided with a third converter, a direct current bus and a fourth converter connected in sequence; the first and third converters are used to convert alternating current into direct current; the second and fourth converters are used to convert direct current into alternating current; the supercapacitor is connected to the direct current side of the first converter through a bidirectional DC / DC converter; the controller of the third converter adopts a double closed-loop vector A quantity controller is provided, which is provided with a current inner loop module A and an outer loop module A; the outer loop module A is provided with a frequency restoration control module and a Park transformation module A; the frequency restoration control module multiplies the difference between the actual value of the DC side voltage of the fourth converter and the DC voltage rated value of the flexible DC transmission system by the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds the product with the rated frequency of the onshore power grid to obtain the frequency reference value of the direct-driven offshore wind farm power grid after processing, and converts the frequency reference value of the direct-driven offshore wind farm power grid into a phase angle reference value of the direct-driven offshore wind farm power grid and outputs it to the Park transformation module A, and the Park transformation module A outputs the voltage and current in the dq coordinate system of the direct-driven offshore wind farm power grid to the current inner loop module A.

[0020] Furthermore, the actual value of the DC side voltage of the third converter is detected on the DC side of the third converter, and the actual value of the DC side voltage of the fourth converter is derived according to the following formula:

[0021]

[0022] in,

[0023] u dc1 is the actual value of the DC side voltage of the third converter;

[0024] u dc2 is the actual value of the DC side voltage of the fourth converter;

[0025] R dc is the equivalent resistance of the flexible DC transmission system;

[0026] L dc is the equivalent reactance of the flexible DC transmission system;

[0027] I dc is the DC current of the DC bus.

[0028] Furthermore, the controller of the third converter is also provided with a Parker inverse transformation module A; the Parker inverse transformation module A receives the voltage reference signals of the d-axis and q-axis from the current inner loop module A, and the phase angle reference value of the direct-driven offshore wind farm power grid from the frequency restoration control module, and outputs the voltage reference signal corresponding to the direct-driven offshore wind farm power grid in the abc coordinate system to the pulse width modulator of the third converter.

[0029] Further, the controller of the fourth converter is a double closed-loop vector controller, which is provided with a current inner loop module B and a voltage outer loop module B; the voltage outer loop module B is provided with a droop control module and a PI controller B; the reference value of the DC side voltage of the fourth converter is obtained through the droop control module, and the PI controller B inputs the difference between the reference value and the actual value of the DC side voltage of the fourth converter, and outputs the current reference value corresponding to the current inner loop module B; wherein the droop control module dynamically adjusts the reference value of the DC side voltage of the fourth converter through the following formula:

[0030]

[0031] in,

[0032] u dc2 * is a reference value of the DC side voltage of the fourth converter;

[0033] f is the actual frequency of the onshore power grid;

[0034] f0 is the rated frequency of the onshore power grid;

[0035] u dc0 is the DC voltage rating of the flexible DC transmission system;

[0036] K dc is the droop control coefficient of the droop control module, which is the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid.

[0037] Furthermore, the controller of the bidirectional DC / DC converter adopts a dual closed-loop controller, which is provided with a current inner loop module C and a voltage outer loop module C; the voltage outer loop module C is provided with an inertia simulation module and a PI controller C; the inertia simulation module is used to simulate the inertia of the synchronous generator, and its input is the difference between the actual frequency of the direct-driven offshore wind farm power grid and the rated frequency of the direct-driven offshore wind farm power grid, and its output is the reference value of the supercapacitor voltage; the PI controller C, its input is the difference between the reference value and the actual value of the supercapacitor voltage, and its output is the current reference value corresponding to the current inner loop module C; wherein, the mathematical expression of the inertia simulation module is as follows:

[0038]

[0039] in,

[0040] is the supercapacitor voltage reference value;

[0041] C sc is the supercapacitor value;

[0042] S n is the rated capacity of the supercapacitor energy storage unit;

[0043] H sc is the inertia time constant s simulated by the supercapacitor;

[0044] f owf To directly drive the actual frequency of the power grid of the offshore wind farm;

[0045] f ow0 The rated frequency of the grid for direct-drive offshore wind farms;

[0046] u sc0 The voltage of the supercapacitor is the operating neutral point voltage.

[0047] The advantages and positive effects of the present invention are:

[0048] (1) The present invention accurately restores the frequency of the onshore power grid to the converter station on the wind farm side by detecting the DC voltage change on the third converter side, thereby eliminating the need for a remote communication system between converter stations, reducing investment costs, and increasing system reliability;

[0049] (2) The method proposed in the present invention takes into account the line loss of the DC bus and can more accurately restore the onshore power grid frequency to the direct-drive offshore wind farm power grid frequency by controlling the third converter, thereby improving the effectiveness of inertial support;

[0050] (3) The method proposed in the present invention uses a supercapacitor connected to the DC side of the wind turbine through a bidirectional DC / DC converter as an inertia energy source, does not use the inherent mechanical inertia of the wind turbine, and does not have the risk of over-response or even stall oscillation of the unit speed. In addition, the DC / DC converter can achieve a larger voltage variation range of the supercapacitor, and has a high energy utilization rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 The present invention discloses a topological structure of an offshore wind power flexible direct current transmission system based on supercapacitors and a coordinated control diagram of the overall inertia of the system.

[0052] Figure 2 This is the principle diagram of frequency restoration control of onshore power grid without communication in flexible DC transmission system.

[0053] Figure 3 This is the schematic diagram of the grid inertia simulation control of a direct-drive offshore wind farm based on supercapacitors.

[0054] Figure 4 It is a three-dimensional relationship diagram of the supercapacitor value, the maximum simulated inertia time and the maximum frequency deviation in the system of the present invention.

[0055] Figure 5 It is a comparison diagram of the onshore power grid frequency and the wind farm restoration frequency under the system load variation condition of the present invention.

[0056] Figure 6 It is a comparison diagram of the onshore power grid frequency and the wind farm restoration frequency under the system wind speed variation condition of the present invention.

[0057] Figure 7 It is a comparison diagram of the onshore power grid frequency and the wind farm restoration frequency under the system power grid fault condition of the present invention.

[0058] Figure 8 The figure is a comparison diagram of transient response simulation curves of the system of the present invention and the prior art under load variation conditions.

[0059] Fig. 9 The figure is a comparison diagram of transient response simulation curves of the system of the present invention and the prior art under the condition of varying wind speed.

[0060] Fig.10 The figure is a comparison diagram of transient response simulation curves of the system of the present invention and the prior art under power grid fault conditions.

[0061] Fig.11 It is a simulation test diagram for verifying the effectiveness and superiority of the system and inertia coordination method of the present invention.

[0062] In the figure:

[0063] θ * is the reference value of the grid phase angle of a direct-drive offshore wind farm, in rad / s.

[0064] θ is the actual phase angle of the onshore power grid, in rad / s.

[0065] f0 is the rated frequency of the direct-drive offshore wind farm power grid and the onshore power grid, in Hz.

[0066] f * It is the frequency reference value of the direct-drive offshore wind farm power grid, in V.

[0067] f is the actual frequency of the onshore power grid, in Hz.

[0068] f owf It is the actual frequency of the power grid of the direct-drive offshore wind farm, in Hz.

[0069] f ow0 is the rated frequency of the direct-drive offshore wind farm power grid, in Hz.

[0070] f owf * It is the reference value of the grid frequency of direct-drive offshore wind farms, in Hz.

[0071] s is the Laplace transform factor.

[0072] u dc2 * is the reference value of the DC side voltage of the fourth converter, in V.

[0073] u dc0 It is the rated DC voltage of the flexible DC transmission system, in V.

[0074] K dc is the droop control coefficient of the droop control module, which is the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid.

[0075] is the supercapacitor voltage reference value, in V.

[0076] u sc is the actual value of the supercapacitor voltage, in V.

[0077] C sc is the supercapacitor value, in F.

[0078] i sc * It is the current reference value of the current inner loop module C of the DC / DC converter.

[0079] i sc is the actual value of the current flowing through the supercapacitor.

[0080] V DC is the DC side voltage reference value of the first converter.

[0081] D is the duty cycle.

[0082] S n It is the rated capacity of the supercapacitor energy storage unit, in VA.

[0083] H sc is the inertia time constant simulated by the supercapacitor, in s.

[0084] u sc0 is the voltage operating neutral point voltage of the supercapacitor, in V.

[0085] u dc1 is the actual value of the DC side voltage of the third converter.

[0086] u dc2 is the actual value of the DC side voltage of the fourth converter.

[0087] R dc is the equivalent resistance of the flexible DC transmission system.

[0088] L dc is the equivalent reactance of the flexible DC transmission system.

[0089] I dc is the DC current of the DC bus.

[0090] Δu is the change value of DC bus voltage;

[0091] Δt is the time change value;

[0092] u abc1 It is the bus voltage in the grid connection point 1abc coordinate system.

[0093] u abc1 * It is the voltage reference signal of the direct-drive offshore wind farm power grid in the abc coordinate system.

[0094] i abc1 It is the bus current in the grid-connected point 1abc coordinate system.

[0095] u abc2 It is the bus voltage in the grid connection point 2abc coordinate system.

[0096] u abc2 * It is the voltage reference signal of the onshore power grid in the abc coordinate system.

[0097] i abc2 It is the bus current in the grid-connected point 2abc coordinate system.

[0098] u dq1 is the bus voltage in the 1dq coordinate system of the grid connection point.

[0099] u dq2 is the bus voltage in the 2dq coordinate system of the grid connection point.

[0100] i dq1 is the bus current in the 1dq coordinate system of the grid-connected point.

[0101] i dq2 is the bus current in the 2dq coordinate system of the grid-connected point.

[0102] u d1 * is the d-axis voltage reference value of the third converter.

[0103] u q1 * is the q-axis voltage reference value of the third converter.

[0104] i dq1 * is the dq axis current reference value of the third converter.

[0105] u d2 * is the d-axis voltage reference value of the fourth converter.

[0106] u q2 * is the q-axis voltage reference value of the fourth converter.

[0107] i d2 * is the d-axis current reference value of the fourth converter.

[0108] i q2 * is the q-axis current reference value of the fourth converter.

[0109] Q ref is the reactive power reference value of the fourth converter;

[0110] Q is the actual value of reactive power of the fourth converter;

[0111] g1 is the power tube 1 of the DC voltage converter.

[0112] g2 is the power tube 2 of the DC voltage converter.

[0113] L1 is the constant load of the onshore power grid.

[0114] L2 is a switchable load of the onshore power grid.

[0115] SC is a supercapacitor.

[0116] PLL is a phase-locked loop.

[0117] PI is a proportional-integral controller.

[0118] TC is a control method in the prior art. DETAILED DESCRIPTION

[0119] In order to further understand the content, features and effects of the present invention, the following embodiments are listed and described in detail with reference to the accompanying drawings:

[0120] The Chinese meanings of the following English words or English abbreviations in this application are as follows:

[0121] PMSG OWF: Direct drive offshore wind farm grid.

[0122] VSC-HVDC: Flexible direct current transmission system.

[0123] WF-VSC: Wind farm side converter station.

[0124] GS-VSC: Grid-side converter station

[0125] SC: Supercapacitor.

[0126] PLL: Phase-locked loop.

[0127] PI controller: Proportional-Integral controller.

[0128] DC / DC: Direct current voltage converter.

[0129] HVDC: direct current bus.

[0130] PCC: connection point.

[0131] PMSG: Permanent magnet direct drive wind power generation system.

[0132] PWM: Pulse Width Modulator.

[0133] See also Figures 1 to 11 , a supercapacitor-based offshore wind power flexible direct current transmission system, comprising a direct-drive offshore wind farm power grid, a flexible direct current transmission system and an onshore power grid connected in sequence; the direct-drive offshore wind farm power grid comprises: a supercapacitor, a wind power generation system, a first converter, a second converter and a first transformer connected in sequence; the flexible direct current transmission system comprises a second transformer, a third converter, a DC bus and a fourth converter connected in sequence; the first converter and the third converter are used to convert alternating current into direct current; the second converter and the fourth converter are used to convert direct current into alternating current; the first transformer and the second transformer are used to adjust the voltage of the direct-drive offshore wind farm power grid to be consistent with the voltage of the onshore power grid. The wind power generation system can be a permanent magnet direct-drive wind power generation system.

[0134] The supercapacitor is connected to the DC side of the first converter through a bidirectional DC / DC converter; the controller of the third converter is a double closed-loop vector controller, which includes a current inner loop module A and an outer loop module A; the outer loop module A includes a frequency restoration control module and a Parker transformation module A; the frequency restoration control module multiplies the difference between the actual value of the DC side voltage of the fourth converter and the DC voltage rated value of the flexible DC transmission system by the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds it to the rated frequency of the onshore power grid, and obtains the frequency reference value of the direct-driven offshore wind farm power grid after processing, and converts the frequency reference value of the direct-driven offshore wind farm power grid into a phase angle reference value of the direct-driven offshore wind farm power grid and outputs it to the Parker transformation module A, and the Parker transformation module A outputs the voltage and current in the dq coordinate system of the direct-driven offshore wind farm power grid to the current inner loop module A. The current inner loop module A, the Parker transformation module A, etc. can all adopt corresponding modules in the prior art; they can also be constructed by using software or components in the prior art and conventional technical means.

[0135] Preferably, the controller of the third converter may further include a Parker inverse transformation module A; the Parker inverse transformation module A may receive voltage reference signals of the d-axis and q-axis from the current inner loop module A, and the direct-drive offshore wind farm power grid phase angle reference value from the frequency restoration control module, and may output a voltage reference signal corresponding to the direct-drive offshore wind farm power grid in the abc coordinate system to the pulse width modulator of the third converter. The Parker inverse transformation module A, the pulse width modulator, etc. may all adopt corresponding modules in the prior art; they may also be constructed by adopting software or components in the prior art and conventional technical means.

[0136] The frequency restoration control module may adopt the frequency restoration control module in the prior art; or it may adopt the software or components in the prior art and be constructed by conventional technical means to realize the above functions. Preferably, the mathematical expression of the frequency restoration control module may be as follows:

[0137]

[0138] in,

[0139] θ * is the reference value of the grid phase angle of the direct-drive offshore wind farm, in rad / s;

[0140] f0 is the rated frequency of the direct-drive offshore wind farm power grid and the onshore power grid, in Hz;

[0141] s is the Laplace transform factor;

[0142] u dc2 is the DC side voltage of the fourth converter, in V;

[0143] u dc0 is the DC voltage rating of the flexible DC transmission system, in V;

[0144] K dc is the droop control coefficient of the droop control module, which is the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid.

[0145] Preferably, the controller of the fourth converter may be a double closed-loop vector controller, which may include a current inner loop module B and a voltage outer loop module B; the voltage outer loop module B may include a droop control module and a PI controller B; the droop control module may multiply the difference between the actual frequency of the onshore power grid and the rated frequency of the onshore power grid by the ratio of the real-time voltage deviation of the DC bus and the real-time frequency deviation of the onshore power grid, and then add the difference to the rated value of the DC voltage of the flexible DC power transmission system, and after processing, may output a reference value of the DC side voltage of the fourth converter; the PI controller B may input the difference between the reference value and the actual value of the DC side voltage of the fourth converter, and output a current reference value corresponding to the current inner loop module B. The current inner loop module B, the PI controller B, etc. may all adopt corresponding modules in the prior art; or may adopt software or components in the prior art and be constructed by conventional technical means.

[0146] The droop control module may adopt the droop control module in the prior art; or it may adopt the software or components in the prior art and be constructed by conventional technical means to realize the above functions.

[0147] Preferably, the controller of the bidirectional DC / DC converter may be a dual closed-loop controller, which may include a current inner loop module C and a voltage outer loop module C; the voltage outer loop module C may include a PI controller C and an inertia simulation module; the inertia simulation module may be used to simulate the inertia of the synchronous generator, which may input the difference between the actual frequency of the direct-drive offshore wind farm power grid and the rated frequency of the direct-drive offshore wind farm power grid, and may output a reference value of the supercapacitor voltage; the PI controller C may input the difference between the reference value and the actual value of the supercapacitor voltage, and output a current reference value corresponding to the current inner loop module C. The current inner loop module C, the PI controller C, etc. may all adopt corresponding modules in the prior art; they may also be constructed by using software or components in the prior art and conventional technical means.

[0148] The inertia simulation module can adopt the inertia simulation module in the prior art; it can also adopt the software or components in the prior art and adopt conventional technical means to realize the above functions. Preferably, the mathematical expression of the inertia simulation module can be as follows:

[0149]

[0150] in,

[0151] is the supercapacitor voltage reference value, in V;

[0152] C sc is the supercapacitor value, in F;

[0153] S n is the rated capacity of the supercapacitor energy storage unit, in VA;

[0154] H sc is the inertia time constant simulated by the supercapacitor, in s;

[0155] f owf is the actual frequency of the power grid of the direct-drive offshore wind farm, in Hz;

[0156] f ow0 is the rated frequency of the direct-drive offshore wind farm grid, in Hz;

[0157] u sc0 is the voltage operating neutral point voltage of the supercapacitor, in V.

[0158] The present invention also provides an inertia coordination method for an offshore wind power flexible direct current transmission system based on a supercapacitor, the method comprising: setting a direct-drive offshore wind farm power grid, a flexible direct current transmission system and an onshore power grid connected in sequence; the direct-drive offshore wind farm power grid is provided with: a supercapacitor, a wind power generation system, a first converter, a second converter and a first transformer connected in sequence; the flexible direct current transmission system is provided with a second transformer, a third converter, a DC bus and a fourth converter connected in sequence; the first and third converters are used to convert alternating current into direct current; the second and fourth converters are used to convert direct current into alternating current; the supercapacitor is connected to the DC side of the first converter through a bidirectional DC / DC converter; the controller of the third converter adopts a dual closed-loop vector controller , which is provided with a current inner loop module A and an outer loop module A; the outer loop module A is provided with a frequency restoration control module and a Parker transformation module A; the frequency restoration control module multiplies the difference between the actual value of the DC side voltage of the fourth converter and the DC voltage rated value of the flexible DC transmission system by the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds it to the rated frequency of the onshore power grid, and obtains the frequency reference value of the direct-driven offshore wind farm power grid after processing, and converts the frequency reference value of the direct-driven offshore wind farm power grid into the phase angle reference value of the direct-driven offshore wind farm power grid and outputs it to the Parker transformation module A, and the Parker transformation module A outputs the voltage and current in the dq coordinate system of the direct-driven offshore wind farm power grid to the current inner loop module A. The wind power generation system can be a permanent magnet direct-driven wind power generation system.

[0159] Preferably, the actual value of the DC side voltage of the third converter can be detected on the DC side of the third converter, and the actual value of the DC side voltage of the fourth converter can be derived according to the following formula:

[0160]

[0161] in,

[0162] u dc1 is the actual value of the DC side voltage of the third converter; the unit is V;

[0163] u dc2 is the actual value of the DC side voltage of the fourth converter; the unit is V;

[0164] R dc is the equivalent resistance of the flexible DC transmission system; the unit is Ω;

[0165] L dc is the equivalent reactance of the flexible DC transmission system; the unit is H;

[0166] I dc is the DC current of the DC bus, in A.

[0167] Preferably, the controller of the third converter may also be provided with a Parker inverse transformation module A; the Parker inverse transformation module A may receive the d-axis voltage reference signal and the q-axis voltage reference signal from the current inner loop module A, and the direct-drive offshore wind farm power grid phase angle reference value from the frequency restoration control module, and may output the voltage reference signal corresponding to the direct-drive offshore wind farm power grid in the abc coordinate system to the pulse width modulator of the third converter.

[0168] Preferably, the controller of the fourth converter may be a dual closed-loop vector controller, which may be provided with a current inner loop module B and a voltage outer loop module B; the voltage outer loop module B may be provided with a droop control module and a PI controller B; the reference value of the DC side voltage of the fourth converter may be obtained through the droop control module, and the PI controller B may input the difference between the reference value of the DC side voltage of the fourth converter and the actual value of the DC side voltage of the fourth converter, and output the current reference value corresponding to the current inner loop module B; wherein the droop control module may dynamically adjust the reference value of the DC side voltage of the fourth converter through the following formula:

[0169]

[0170] in,

[0171] u dc2 * is the reference value of the DC side voltage of the fourth converter, in V;

[0172] f is the actual frequency of the onshore power grid, in Hz;

[0173] f0 is the rated frequency of the onshore power grid, in Hz;

[0174] u dc0 is the DC voltage rating of the flexible DC transmission system, in V;

[0175] K dc is the droop control coefficient of the droop control module, which is the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid.

[0176] Preferably, the controller of the bidirectional DC / DC converter can adopt a dual closed-loop controller, which can be provided with a current inner loop module C and a voltage outer loop module C; the voltage outer loop module C can be provided with an inertia simulation module and a PI controller C; the inertia simulation module can be used to simulate the inertia of the synchronous generator, which can input the difference between the actual frequency of the direct-drive offshore wind farm power grid and the rated frequency of the direct-drive offshore wind farm power grid, and output the reference value of the supercapacitor voltage; the PI controller C can input the difference between the reference value and the actual value of the supercapacitor voltage, and output the current reference value corresponding to the current inner loop module C; wherein, the mathematical expression of the inertia simulation module can be as follows:

[0177]

[0178] in,

[0179] is the supercapacitor voltage reference value; the unit is V;

[0180] C sc is the supercapacitor value; the unit is F;

[0181] S n is the rated capacity of the supercapacitor energy storage unit, in VA;

[0182] H sc is the inertia time constant simulated by the supercapacitor, in s;

[0183] f owf is the actual frequency of the power grid of the direct-drive offshore wind farm, in Hz;

[0184] f ow0 is the rated frequency of the direct-drive offshore wind farm grid, in Hz;

[0185] u sc0 is the voltage operating neutral point voltage of the supercapacitor, in V.

[0186] The structure, working process and working principle of the present invention are further described below with a preferred embodiment of the present invention: A supercapacitor-based offshore wind power flexible direct current transmission system, such as Figure 1 As shown in FIG. 1 , the system consists of three parts: a direct-drive offshore wind farm grid based on supercapacitors (hereinafter referred to as PMSG OWF), a flexible direct current transmission system (hereinafter referred to as VSC-HVDC), and an onshore power grid. The flexible direct current transmission system includes a third converter, a DC bus, and a fourth converter.

[0187] The electric energy generated by the PMSG OWF is transformed and connected to the third converter of the VSC-HVDC. The third converter is also called the wind farm side converter station, hereinafter referred to as WF-VSC, and then after long-distance DC transmission, it is connected to the onshore power grid through the fourth converter, also called the grid side converter station, hereinafter referred to as GS-VSC. The supercapacitor (SC) is connected to the DC side of each direct-drive wind turbine through a bidirectional DC / DC transformer as an energy source for inertia simulation. Figure 1 The lower part is the overall inertia coordinated control of the system. Under frequency events, GS-VSC first reflects the frequency information of the onshore power grid to the DC voltage through DC voltage / frequency droop control. Then, after sensing the fluctuation of the DC voltage, WF-VSC converts the changing DC voltage into a frequency signal of the offshore wind farm through frequency conversion control, realizing the artificial coupling of the frequencies of the offshore and onshore power grids and eliminating the communication between the converter stations at both ends. Finally, in response to the frequency change of WF-VSC, the supercapacitor will adjust the supercapacitor voltage reference value so that its charging and discharging simulates the inertia response of the synchronous machine. Through a series of coordinated controls, the offshore wind farm provides inertia support for the onshore power grid through the flexible DC transmission system.

[0188] A non-communication coordinated control method for offshore wind power inertia simulation and flexible direct current transmission system based on supercapacitors specifically includes the following three parts:

[0189] (1) The grid-side converter station (GS-VSC) adopts voltage / frequency droop control. Its working principle is as follows: Figure 2 As shown in the right half, its control goal is to reflect the onshore power grid frequency information to the DC voltage electrical quantity in real time and accurately, and realize the instant transmission of the power grid frequency from the onshore power grid side to the offshore wind farm side without communication.

[0190] Step (1-1): Collect the bus voltage u of the grid connection point 2 abc2 ,u abc2 The onshore power grid frequency f is obtained through the phase-locked loop PLL.

[0191] Step (1-2): GS-VSC uses the traditional double closed-loop vector control strategy, and the outer loop DC voltage reference value u dc2 * According to the real-time frequency f of the onshore power grid, dynamic adjustment is performed through the following formula:

[0192]

[0193] Among them, u dc2 * is the reference value of the DC side voltage of the fourth converter, in V;

[0194] f is the actual frequency of the onshore power grid, in Hz;

[0195] f0 is the rated frequency of the onshore power grid, in Hz;

[0196] u dc0 is the DC voltage rating of the flexible DC transmission system, in V;

[0197] K dc is the droop control coefficient of the droop control module, which is the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid.

[0198] (2) The wind farm-side converter station (WF-VSC) adopts frequency restoration control, and its working principle is as follows: Figure 2 As shown in the left half, its control target is to control the frequency of the direct-drive offshore wind farm grid through WF-VSC based on the DC voltage representing the frequency change information of the onshore grid, so that it is accurately coupled with the onshore grid frequency.

[0199] Step (2-1): Detect the DC voltage u at the WF-VSC port dc1 Considering the line loss of VSC-HVDC, the DC voltage u at the GS-VSC port is derived according to the following formula: dc2 :

[0200]

[0201] Among them, u dc1 is the actual value of the DC side voltage of the third converter; the unit is V;

[0202] u dc2 is the actual value of the DC side voltage of the fourth converter; the unit is V;

[0203] R dc is the equivalent resistance of the flexible DC transmission system; the unit is Ω;

[0204] L dc is the equivalent reactance of the flexible DC transmission system; the unit is H;

[0205] I dc is the DC current of the DC bus, in A.

[0206] Step (2-2): WF-VSC adopts variable frequency control algorithm, according to the derived GS-VSC port DC voltage u dc2 , the frequency reference value f of PMSG OWF is calculated based on the following formula: owf * Adjusted to:

[0207]

[0208] f owf* It is the reference value of grid frequency for direct-drive offshore wind farms.

[0209] The coordination between the voltage / frequency droop control of the grid-side converter station (GS-VSC) and the frequency restoration control of the wind farm-side converter station (WF-VSC) can achieve precise coupling of the frequency of the direct-drive offshore wind farm grid and the frequency of the onshore grid, such as Figures 5 to 7 Therefore, the wind farm can sense the frequency changes of the onshore grid and activate the inertia response of the wind turbines.

[0210] (3) The DC / DC converter of the supercapacitor adopts inertia analog control, and its working principle is as follows Figure 3 As shown, the control goal is to utilize the charging and discharging of supercapacitors to simulate the inertia of synchronous machines, so that the wind turbines have the ability to provide sufficient inertia support under grid frequency events.

[0211] Step (3-1): Collect the bus voltage u of the wind turbine grid connection point 1 abc1 ,u abc1 The direct-drive offshore wind farm grid frequency f is obtained through the phase-locked loop PLL owf .

[0212] Step (3-2): According to f owf Change information changes the supercapacitor voltage reference value u in real time sc * :

[0213]

[0214] in, is the supercapacitor voltage reference value, in V;

[0215] C sc is the supercapacitor value, in F;

[0216] S n is the rated capacity of the supercapacitor energy storage unit, in VA;

[0217] H sc is the inertia time constant simulated by the supercapacitor, in s;

[0218] f owf is the actual frequency of the power grid of the direct-drive offshore wind farm, in Hz;

[0219] f ow0 is the rated frequency of the direct-drive offshore wind farm power grid, in Hz; the rated frequency of the direct-drive offshore wind farm power grid and the onshore power grid frequency may be equal, which is equal to f0;

[0220] u sc0is the voltage operating neutral point voltage of the supercapacitor, in V.

[0221] Step (3-3): The current inner loop of the bidirectional DC / DC converter double closed-loop vector control uses the traditional control strategy, and only the voltage outer loop is modified to: obtain the supercapacitor voltage reference value u according to the grid frequency change sc * And the actual value of supercapacitor voltage u sc The difference between the two is sent to the PI controller to obtain the inner loop current reference command value i sc * , to achieve direct control of the supercapacitor voltage. The expression for this control is:

[0222]

[0223] where k p1 is the proportional coefficient of the DC / DC voltage outer loop PI controller.

[0224] k i1 is the integral coefficient of the DC / DC voltage outer loop PI controller.

[0225] S is the integration operator.

[0226] Through the above-mentioned GS-VSC voltage / frequency droop control and WF-VSC frequency restoration control, the grid frequency can be instantly transmitted from the onshore grid side to the direct-drive offshore wind farm grid without communication. The PMSG OWF can instantly sense the frequency change information of the onshore grid, and then change the supercapacitor voltage to simulate the inertia response of the synchronous machine through the inertia simulation control based on supercapacitors. Through the above coordinated control, the PMSG OWF can finally provide inertia support for the onshore grid through the VSC-HVDC system.

[0227] An inertia coordination method for an offshore wind power flexible direct current transmission system based on supercapacitors is provided. The method adopts a supercapacitor controlled by a DC / DC converter connected in parallel on the DC bus side of a direct-drive wind turbine. The DC / DC converter can increase the allowable variation range of the supercapacitor voltage (for example, ±30%). Figure 4 As shown, the capacitor energy storage is utilized to the maximum extent, and the larger the allowable variation range of the supercapacitor voltage is, the smaller the required supercapacitor value is.

[0228] In MATLAB / Simulink, create Fig.11The simulation model of the offshore wind power flexible direct transmission system based on supercapacitors is shown. Among them, the offshore wind farm is aggregated by 1000MW direct-drive wind turbines, each of which is equipped with a 6.7F supercapacitor with a rated wind speed of 10m / s. The offshore wind farm is connected to the VSC-HVDC system with a rated capacity of 1000MW, a transmission distance of 200km, and a rated DC voltage of ±400kV through a 690V / 35kV bus transformer and a 35kV / 230kV step-up transformer, and then connected to the onshore power grid through a 15km AC transmission line. Among them, the installed capacity of the onshore power grid synchronous generator is 750MW, the constant load is L1=1148MW+j50MVar, and the switchable load is L2=57.4MW+j2.5MVar (accounting for 5% of the total load of the AC system). The effects of this scheme and the traditional control scheme on the stability of the onshore power grid frequency are compared under three different working conditions: load change, wind speed change, and onshore power grid failure. Figure 8 to Fig.10 Under the three working conditions of load change, wind speed change and onshore power grid fault, the method described in this paper can reduce the frequency deviation and frequency change rate of the onshore power grid, and significantly improve the frequency stability of the onshore power grid.

[0229] The embodiments described above are only used to illustrate the technical ideas and features of the present invention, and their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. The patent scope of the present invention cannot be limited only by this embodiment, that is, any equivalent changes or modifications made to the spirit disclosed by the present invention still fall within the patent scope of the present invention.

Claims

1. A supercapacitor-based offshore wind power flexible direct current transmission system, characterized in that: The invention comprises a direct-drive offshore wind farm power grid, a flexible direct current transmission system and an onshore power grid connected in sequence; the direct-drive offshore wind farm power grid comprises: a super capacitor, a wind power generation system, a first converter and a second converter connected in sequence; the flexible direct current transmission system comprises a third converter, a DC bus and a fourth converter connected in sequence; the first and third converters are used to convert alternating current into direct current; the second and fourth converters are used to convert direct current into alternating current; the super capacitor is connected to the DC side of the first converter through a bidirectional DC / DC converter; the controller of the third converter is a double closed-loop vector controller, which comprises a current inner loop module A and an outer loop module A; the outer loop module A comprises a frequency A frequency restoration control module and a Parker transformation module A; a frequency restoration control module, which multiplies the difference between the actual value of the DC side voltage of the fourth converter and the rated value of the DC voltage of the flexible DC transmission system by the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds it to the rated frequency of the onshore power grid to obtain the frequency reference value of the direct-driven offshore wind farm power grid after processing, and converts the frequency reference value of the direct-driven offshore wind farm power grid into a phase angle reference value of the direct-driven offshore wind farm power grid and outputs it to the Parker transformation module A, and the Parker transformation module A outputs the voltage and current in the dq coordinate system of the direct-driven offshore wind farm power grid to the current inner loop module A.

2. The supercapacitor-based offshore wind power flexible direct current transmission system according to claim 1 is characterized in that: The controller of the third converter also includes a Parker inverse transformation module A; the Parker inverse transformation module A receives the voltage reference signals of the d-axis and q-axis from the current inner loop module A, and the phase angle reference value of the direct-driven offshore wind farm power grid from the frequency restoration control module, and outputs the voltage reference signal corresponding to the direct-driven offshore wind farm power grid in the abc coordinate system to the pulse width modulator of the third converter.

3. The supercapacitor-based offshore wind power flexible direct current transmission system according to claim 1 is characterized in that: The controller of the fourth converter is a double closed-loop vector controller, which includes a current inner loop module B and a voltage outer loop module B; the voltage outer loop module B includes a droop control module and a PI controller B; the droop control module multiplies the difference between the actual frequency of the onshore power grid and the rated frequency of the onshore power grid with the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid, and then adds it to the rated value of the DC voltage of the flexible DC transmission system, and outputs a reference value of the DC side voltage of the fourth converter after processing; the PI controller B inputs the difference between the reference value and the actual value of the DC side voltage of the fourth converter, and outputs the current reference value corresponding to the current inner loop module B.

4. The supercapacitor-based offshore wind power flexible direct current transmission system according to claim 1 is characterized in that: The controller of the bidirectional DC / DC converter is a dual closed-loop controller, which includes a current inner loop module C and a voltage outer loop module C; the voltage outer loop module C includes a PI controller C and an inertia simulation module; the inertia simulation module is used to simulate the inertia of the synchronous generator, and its input is the difference between the actual frequency of the direct-driven offshore wind farm power grid and the rated frequency of the direct-driven offshore wind farm power grid, and its output is the reference value of the supercapacitor voltage; the PI controller C, its input is the difference between the reference value and the actual value of the supercapacitor voltage, and its output corresponds to the current reference value of the current inner loop module C.

5. The supercapacitor-based offshore wind power flexible direct current transmission system according to claim 4 is characterized in that: The mathematical expression of the inertia simulation module is as follows: in, is the supercapacitor voltage reference value; C sc is the supercapacitor value; S n is the rated capacity of the supercapacitor energy storage unit; H sc is the inertia time constant simulated for the supercapacitor; f owf To directly drive the actual frequency of the power grid of the offshore wind farm; f ow0 The rated frequency of the grid for direct-drive offshore wind farms; u sc0 The voltage of the supercapacitor is the operating neutral point voltage.

6. An inertia coordination method for an offshore wind power flexible direct current transmission system based on supercapacitors, characterized in that: A direct-drive offshore wind farm power grid, a flexible direct current transmission system and an onshore power grid connected in sequence are provided; the direct-drive offshore wind farm power grid is provided with: a super capacitor, a wind power generation system, a first converter and a second converter connected in sequence; the flexible direct current transmission system is provided with a third converter, a DC bus and a fourth converter connected in sequence; the first and third converters are used to convert alternating current into direct current; the second and fourth converters are used to convert direct current into alternating current; the super capacitor is connected to the DC side of the first converter through a bidirectional DC / DC converter; the controller of the third converter adopts a double closed-loop vector controller, which is provided with a current inner loop module A and an outer loop module A; the outer loop module A is provided with Frequency restoration control module and Parker transformation module A; the frequency restoration control module multiplies the difference between the actual value of the DC side voltage of the fourth converter and the rated value of the DC voltage of the flexible DC transmission system by the ratio of the real-time frequency deviation of the onshore power grid to the real-time voltage deviation of the DC bus, and then adds it to the rated frequency of the onshore power grid to obtain the frequency reference value of the direct-driven offshore wind farm power grid after processing. The frequency reference value of the direct-driven offshore wind farm power grid is converted into a phase angle reference value of the direct-driven offshore wind farm power grid and outputted to the Parker transformation module A. The Parker transformation module A outputs the voltage and current in the dq coordinate system of the direct-driven offshore wind farm power grid to the current inner loop module A.

7. The inertia coordination method of the supercapacitor-based offshore wind power flexible direct current transmission system according to claim 6 is characterized in that: The actual value of the DC side voltage of the third converter is detected on the DC side of the third converter, and the actual value of the DC side voltage of the fourth converter is derived according to the following formula: in, u dc1 is the actual value of the DC side voltage of the third converter; u dc2 is the actual value of the DC side voltage of the fourth converter; R dc is the equivalent resistance of the flexible DC transmission system; L dc is the equivalent reactance of the flexible DC transmission system; I dc is the DC current of the DC bus.

8. The inertia coordination method of the supercapacitor-based offshore wind power flexible direct current transmission system according to claim 6 is characterized in that: The controller of the third converter is also provided with a Parker inverse transformation module AA; the Parker inverse transformation module AA receives the voltage reference signals of the d-axis and the q-axis from the current inner loop module A, and the phase angle reference value of the direct-driven offshore wind farm power grid from the frequency restoration control module, and outputs the voltage reference signal corresponding to the direct-driven offshore wind farm power grid in the abc coordinate system to the pulse width modulator of the third converter.

9. The inertia coordination method of the supercapacitor-based offshore wind power flexible direct current transmission system according to claim 6 is characterized in that: The controller of the fourth converter is a double closed-loop vector controller, which is provided with a current inner loop module B and a voltage outer loop module B; the voltage outer loop module B is provided with a droop control module and a PI controller B; the reference value of the DC side voltage of the fourth converter is obtained through the droop control module, and the PI controller B inputs the difference between the reference value and the actual value of the DC side voltage of the fourth converter, and outputs the current reference value corresponding to the current inner loop module B; wherein the droop control module dynamically adjusts the reference value of the DC side voltage of the fourth converter through the following formula: in, u dc2 * is a reference value of the DC side voltage of the fourth converter; f is the actual frequency of the onshore power grid; f0 is the rated frequency of the onshore power grid; u dc0 is the DC voltage rating of the flexible DC transmission system; K dc is the droop control coefficient of the droop control module, which is the ratio of the real-time voltage deviation of the DC bus to the real-time frequency deviation of the onshore power grid.

10. The inertia coordination method of the supercapacitor-based offshore wind power flexible direct current transmission system according to claim 6 is characterized in that: The controller of the bidirectional DC / DC converter adopts a dual closed-loop controller, which is provided with a current inner loop module C and a voltage outer loop module C; the voltage outer loop module C is provided with an inertia simulation module and a PI controller C; the inertia simulation module is used to simulate the inertia of the synchronous generator, and its input is the difference between the actual frequency of the direct-drive offshore wind farm power grid and the rated frequency of the direct-drive offshore wind farm power grid, and its output is the reference value of the supercapacitor voltage; the PI controller C, its input is the difference between the reference value and the actual value of the supercapacitor voltage, and its output is the current reference value corresponding to the current inner loop module C; wherein, the mathematical expression of the inertia simulation module is as follows: in, is the supercapacitor voltage reference value; C sc is the supercapacitor value; S n is the rated capacity of the supercapacitor energy storage unit; H sc is the inertia time constant s simulated by the supercapacitor; f owf To directly drive the actual frequency of the power grid of the offshore wind farm; f ow0 The rated frequency of the grid for direct-drive offshore wind farms; u sc0 The voltage of the supercapacitor is the operating neutral point voltage.

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