Drul-fac system and wind turbine grid connection control and black start control method

Abstract

The application provides a DRU-LFAC system and a wind turbine generator network type control and black start control method, and relates to the technical field of wind power generation. The offshore wind farm is a permanent magnet direct drive full power wind turbine generator, low frequency alternating current is sent to the onshore back-to-back converter through a low frequency side subsystem, the onshore back-to-back converter converts the low frequency alternating current into high frequency alternating current and sends the high frequency alternating current into an onshore power grid, the onshore back-to-back converter comprises a twelve-pulse diode rectifier unit and a modular multilevel converter, and a filter is used for reactive power compensation. The DRU-LFAC system has active power-voltage coupling characteristics and reactive power-frequency coupling characteristics. The application solves the problems that the low frequency side power characteristics of the current DRU-LFAC system are different from those of the traditional power system and cannot be effectively analyzed, the current traditional wind turbine generator control strategy is not applicable to the DRU-LFAC system, and the offshore wind farm cannot be started through a conventional process due to the unidirectional conduction characteristics of the DRU.

Description

Technical Field

[0001] This invention relates to the field of power grid technology, specifically to a DRU-LFAC system and a wind turbine grid configuration control and black start control method. Background Technology

[0002] With the continuous development of offshore wind energy, offshore wind farms are gradually expanding towards larger capacity and deeper waters. As transmission distance and rated capacity increase, a series of shortcomings of the currently common flexible DC transmission scheme based on Modular Multilevel Converters (MMC) (MMC-based High Voltage Direct Current, MMC-HVDC) are becoming apparent, including high cost of MMC converter valves; high cost of offshore converter stations; high maintenance costs in deep waters; and low reliability. Therefore, there is an urgent need to research a new transmission method with large capacity, long distance, and high economic efficiency to meet the power transmission needs of large-capacity wind farms in deep waters.

[0003] Diode Rectifier Unit based Low Frequency Alternating Current (DRU-LFAC) technology combines the advantages of LFAC and DRU, possessing outstanding advantages such as no need for offshore converter stations, simple maintenance, easy AC networking, small submarine cable capacitance effect, small charging current, and low loss; and low cost of onshore back-to-back converters. It is currently a research hotspot for low-cost offshore wind DC transmission systems and has great application prospects in the scenario of transmitting power from large-capacity wind farms in deep-sea areas.

[0004] However, DRU itself is an uncontrolled unit and cannot independently establish an offshore AC power grid. Ordinary grid-connected wind turbines cannot directly transmit power and connect to the grid through DRU.

[0005] Furthermore, the power coupling relationship in the low-frequency side system differs significantly from that in the traditional power system, requiring the development of new control strategies based on the actual characteristics of the system. Finally, the DRU-LFAC power transmission system has long AC transmission lines and requires a large power output during startup. If the traditional method of installing energy storage is still used for black start, the offshore energy storage system will be too large, making it difficult to construct and maintain, and resulting in poor economic benefits.

[0006] Therefore, in order to solve the above problems, it is necessary to study a wind turbine grid control strategy applicable to the DRU-LFAC system and its corresponding black start control strategy. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a DRU-LFAC system and a wind turbine grid control and black start control method.

[0008] According to one aspect of the present invention, a DRU-LFAC system is provided, comprising an offshore wind farm, a low-frequency side subsystem, a shore-based back-to-back converter, a shore-based power grid, and a filter installed in the low-frequency side subsystem, connected in sequence.

[0009] The offshore wind farm uses permanent magnet direct-drive full-power wind turbine generators, and the low-frequency alternating current (LFAC) generated by them is sent to the onshore back-to-back converter through the low-frequency side subsystem.

[0010] The onshore back-to-back converter converts the low-frequency AC power (LFAC) to high-frequency AC power and sends it into the onshore power grid.

[0011] The filter is used for reactive power compensation, providing the reactive power required for the normal operation of the onshore back-to-back converter;

[0012] in:

[0013] The low-frequency side subsystem includes a wind turbine line-side converter, a first transformer, an offshore wind farm cluster feeder, a second transformer, an AC submarine cable, and a converter transformer connected in sequence.

[0014] The onshore back-to-back converter includes a connected twelve-pulse diode rectifier unit (DRU) and a modular multilevel converter (MMC).

[0015] The DRU-LFAC system has active-voltage coupling characteristics and reactive-frequency coupling characteristics.

[0016] Preferably, when the permanent magnet direct-drive full-power wind turbine adopts maximum power point tracking (MPPT) control, the active power input to the diode rectifier unit (DRU) is... P DR Determined uniquely by wind speed; in the active power P DR Under certain conditions, the voltage at the common coupling point (PCC) in the low-frequency side subsystem is... u pcc The voltage can be determined and varies within the range of 0.9 pu to 1.08 pu, where pu is the rated voltage value; at this time, the DRU-LFAC system has active power-voltage coupling characteristics.

[0017] Voltage at the common coupling point (PCC) in the low-frequency side subsystem u pcc Under certain conditions, the total reactive power output of all wind turbine line-side converters is controlled. QWT To control the frequency of the low-frequency side subsystem f ,and f Reactive power input to DRU Q WT The correlation is negative, and at this time, the DRU-LFAC system has reactive power-frequency coupling characteristics.

[0018] Preferably, the low-frequency side subsystem has the following relationship:

[0019] (1)

[0020] (2)

[0021] In the formula, P DR The active power input to the DRU, P WT This represents the total active power output from all line-side converters of the wind turbines. P ut , P net , P line These represent the total active power losses on the first or second transformer, the offshore wind farm cluster feeder, and the AC cables. u dder This is the AC side voltage of the DRU. u pcc The common coupling point (PCC) voltage for the low-frequency side subsystem. X line For AC cable reactance; Indicates power loss. Indicates voltage loss;

[0022] The DC-side voltage and current of the DRU are:

[0023] (3)

[0024] (4)

[0025] In the formula, u dc This is the DC-side voltage of the DRU. i dc This is the DC-side current of the DRU. N The number of cascaded six-pulse DRUs. X t0 and P t0 These are the leakage reactance and active power loss of the DRU converter transformer, respectively.

[0026] The system DRU-LFAC is runningu dc Controlled by MMC u dc The AC side voltage of the DRU is a constant value:

[0027] (5)

[0028] The common coupling point (PCC) voltage of the low-frequency side subsystem is related to P WT The relation is:

[0029] (6)

[0030] The permanent magnet direct-drive full-power wind turbine adopts maximum power point tracking (MPPT) control, and the total active power output of all wind turbine line-side converters is [data missing]. P WT Determined solely by the current wind speed, it is a fixed quantity;

[0031] The u PCC It is also a definite quantity, the magnitude of which varies within a fixed range as the output active power of the converter changes;

[0032] At this time, the DRU-LFAC system has active power-voltage coupling characteristics.

[0033] Preferably, the relationship within the low-frequency side subsystem is as follows:

[0034] (7)

[0035] In the formula, Q DR The reactive power input to the DRU Q WT This represents the total reactive power output of all wind turbine line-side converters. Q tf , Q net , Q line These refer to the total reactive power losses on the first or second transformer, the offshore wind farm cluster feeder, and the AC cable, respectively.

[0036] The reactive power input to the DRU is:

[0037] (8)

[0038] In the formula, The power factor of the DRU. μ The commutation overlap angle of the DRU;

[0039] The DC-side current of the DRU is:

[0040] (9)

[0041] In the formula, f For the frequency of the low-frequency side subsystem, L t0 For the leakage inductance of the converter transformer, there is L t0 = X t0 / (2 πf );

[0042] The active power of the DRU is negligible. P t0 After, it is represented as P DR = i dc u dc ;

[0043] The frequency of the low-frequency side subsystem f about Q WT The expression is:

[0044] (10)

[0045] voltage at PCC point u pcc Subject to active power control, all wind turbine line-side converters output active power at the converter's output. P WT Under certain conditions, adjust the total reactive power output of all wind turbine line-side converters. Q WT To achieve frequency adjustment of the low-frequency side subsystem, and Q WT It is negatively correlated with the frequency of the low-frequency side subsystem;

[0046] At this time, the DRU-LFAC system has reactive power-frequency coupling characteristics.

[0047] According to a second aspect of the present invention, a wind turbine grid control method suitable for DRU-LFAC systems is provided, comprising:

[0048] The line-side converters of offshore wind farms are controlled according to maximum power point tracking (MPPT).

[0049] Offshore wind farms have three layers of grid-side control, from the outside to the inside: d-axis DC voltage control and q-axis Qf droop control, AC voltage middle loop control, and current inner loop control;

[0050] The frequency and voltage of the low-frequency subsystem of the DRU-LFAC system are established through the maximum power point tracking (MPPT) control and the three-layer control.

[0051] Preferably, the d-axis DC voltage control specifically includes:

[0052] The d-axis controls the active current output by the converter. i Fd This is used to control the DC bus voltage to ensure that all active power generated by the wind turbine can be transmitted to the power grid.

[0053]

[0054] i Fdref It is the reference value of the d-axis current output by the d-axis DC voltage control; K p It is the proportional coefficient of the PI controller; K i It is the integral coefficient of the PI controller; u dc This is the actual value of the DC voltage; u dcref This is a DC voltage reference value;

[0055] The q-axis Qf Sagging control, specifically:

[0056] Current actual frequency on the low-frequency side ω Less than a given frequency ω At 0, the output of the phase-locked loop (PLL) is... Δω <0;

[0057] according to Qf Sag control includes:

[0058] (11)

[0059] Q ref It is the reactive power reference value of the q-axis Qf droop control output; k fq It is the droop coefficient controlled by the q-axis Qf droop.

[0060] Then the output at this time Q ref <0;

[0061] Based on the q-axis outer loop control, we have:

[0062] (12)

[0063] Q ref It is the reactive power reference value of the q-axis Qf droop control output; k fq K is the droop coefficient for q-axis Qf droop control. p It is the proportional coefficient of the PI controller; K iThese are the integral coefficients of the PI controller; s is the Laplace operator;

[0064] because Q The reactive current reference value has not changed, therefore the output reactive current reference value is... i Fqref In Q ref Increased under the influence of;

[0065] Based on the current inner loop control i Fq Will track i Fqref Therefore i Fq Increase;

[0066] According to the reactive power calculation formula:

[0067] (13)

[0068] Determine the active power output of the converter Q It will decrease;

[0069] Based on the aforementioned reactive-frequency coupling characteristics Q The reduction will bring f The increase, even if ω Increase;

[0070] The q-axis Qf droop control and the reactive-frequency coupling characteristic together constitute a system. ω The negative feedback loop can artificially control the frequency. ω To implement control in order to achieve ω For a given frequency ω Follow 0.

[0071] Preferably, the AC voltage loop control includes capacitor feedforward decoupling and AC voltage dynamic control;

[0072] Specifically, the capacitor feedforward decoupling considers the effect of capacitance in the d-axis current loop to improve the dynamic characteristics of the voltage, and the AC voltage dynamic control introduces a voltage rating. u Fd0 and current feedforward i Fd2 To limit the amplitude of AC voltage changes and accelerate the response speed, the governing equation is:

[0073] (14)

[0074] In the formula: i Fdcref For DC voltage control loop generation i Fd Reference value uFd0 This represents the rated value of the d-axis component of the voltage. i Fd2 u is the d-axis component of the current flowing into the step-up transformer. Fd This represents the actual value of the d-axis component of the voltage. ω 0 represents a given frequency, C F The capacitance value of the filter capacitor of the converter on the line side of the wind turbine, u Fq This represents the actual value of the q-axis component of the voltage; ω 0C F u Fq This is to decouple the capacitor feedforward.

[0075] Preferably, the current inner loop control specifically includes:

[0076] Reference value of voltage generated by the inner current loop u Fdref and u Fqref Its governing equations are:

[0077] (15)

[0078] According to a third aspect of the present invention, a black-start control method suitable for a DRU-LFAC system is provided, comprising:

[0079] An auxiliary converter is added to the DRU-LFAC system, and the auxiliary converter is connected in parallel with the onshore back-to-back converter.

[0080] The auxiliary converter enables black start of the offshore wind farm, controlling the wind turbines of the offshore wind farm to switch between grid-connected and grid-connected configurations to facilitate black start.

[0081] Preferably, the step of performing black start on the offshore wind farm through the auxiliary converter, controlling the wind turbines of the offshore wind farm to switch between grid-connected and grid-connected configurations to cooperate with the auxiliary converter for black start, includes:

[0082] Phase 1: Start the auxiliary converters on the MMC and DRU sides to control the DC side voltage of the onshore back-to-back converters and auxiliary converters to the rated value; at the same time, except for the wind turbines participating in black start, the grid-side converters of the other wind turbines are disconnected, and the filters on the AC side of the DRU are also disconnected.

[0083] Phase 2: The auxiliary converter on the DRU side is connected with V / f control to establish the AC voltage and frequency of the low-frequency side subsystem; after the AC voltage of the low-frequency side subsystem is established, the wind turbine grid-side converter is enabled and operates in grid-following Udc / Q control mode, and is synchronized with the AC voltage established by the auxiliary converter through PLL to establish the DC voltage of the wind turbine converter on the machine side and the grid side.

[0084] Phase 3: The synchronous generator is connected, the wind turbine's machine-side converter starts and operates in MPPT control mode, thus realizing the start-up of the wind turbine;

[0085] Fourth stage: After the wind turbine starts to backfeed power to the auxiliary converter, switch the wind turbine grid-side converter to Qf droop control and disconnect the DRU-side auxiliary converter; at this time there is no power path in the system, the surplus power will cause the AC voltage at the DRU AC port to rise and reach the DRU's turn-on voltage, so that the DRU turns on.

[0086] Phase 5: After the DRU is turned on, power is transmitted through the DRU, and the system enters the stable operation phase; the auxiliary converter is reconnected with PQ control, and the PQ command values ​​are all 0; at the same time, the filter at the AC port of the DRU is connected; the remaining wind turbines that have not yet started start up in sequence and output active power until all wind turbines have started up.

[0087] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0088] The DRU-LFAC system in this embodiment of the invention combines the advantages of LFAC and DRU, and has outstanding advantages such as no offshore converter station, simple maintenance, easy AC networking; small submarine cable capacitance effect, small charging current, and low loss; and low cost of onshore back-to-back converters.

[0089] The DRU-LFAC system in this embodiment of the invention obtains the power coupling relationship in the low-frequency side system, solving the problem that the low-frequency side power characteristics of the current DRU-LFAC system are different from those of the traditional power system and have not been effectively analyzed.

[0090] The wind turbine grid control method applicable to the DRU-LFAC system in this embodiment of the invention can effectively control the DRU-LFAC system and solve the problem that the DRU itself is an uncontrolled unit and cannot autonomously establish an offshore AC power grid.

[0091] The black start control method for DRU-LFAC systems in this embodiment of the invention solves the problem that offshore wind farms cannot be started through conventional procedures due to the unidirectional conduction characteristics of DRUs. Attached Figure Description

[0092] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0093] Figure 1 This is a structural diagram of a offshore wind farm DRU-LFAC power transmission system according to an embodiment of the present invention;

[0094] Figure 2 This is a detailed structural diagram of the low-frequency side subsystem in a preferred embodiment of the present invention;

[0095] Figure 3 This is the active power-voltage relationship curve on the low-frequency side of the DRU-LFAC system in a preferred embodiment of the present invention;

[0096] Figure 4 This is a frequency-reactive power relationship curve on the low-frequency side of the DRU-LFAC system in a preferred embodiment of the present invention;

[0097] Figure 5 This is a block diagram of a grid-type control strategy based on phase-locked loop and power droop in a preferred embodiment of the present invention;

[0098] Figure 6 This is a schematic diagram of the frequency-reactive power control mechanism in a preferred embodiment of the present invention;

[0099] Figure 7 This is a schematic diagram of the black-start timing logic in a preferred embodiment of the present invention;

[0100] Figure 8 This is a simulation result diagram from a specific embodiment of the present invention. Detailed Implementation

[0101] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0102] Figure 1This diagram illustrates the structure of an offshore wind farm DRU-LFAC power transmission system. As shown, it includes, in sequence, the offshore wind farm, a low-frequency side subsystem, an onshore back-to-back converter, an onshore power grid, and a filter installed in the low-frequency side subsystem. The offshore wind farm uses permanent magnet direct-drive full-power wind turbines, and the low-frequency alternating current (LFAC) generated is transmitted to the onshore back-to-back converter via the low-frequency side subsystem. The onshore back-to-back converter converts the low-frequency alternating current (LFAC) to high-frequency alternating current and feeds it into the onshore power grid. The low-frequency side subsystem includes, in sequence, a wind turbine line-side converter, a first transformer, an offshore wind farm cluster feeder, a second transformer, an AC submarine cable, and a converter transformer. The onshore back-to-back converter includes a connected twelve-pulse diode rectifier unit (DRU) and a modular multilevel converter (MMC). The filter is used for reactive power compensation, providing the reactive power required for the normal operation of the diode rectifier unit (DRU).

[0103] The DRU-LFAC system in the above embodiments of the present invention has active-voltage coupling characteristics and reactive-frequency coupling characteristics.

[0104] Typically, onshore back-to-back converters convert 20Hz low-frequency alternating current to 50Hz power frequency and feed it into the power grid.

[0105] The DRU-LFAC system in the above embodiments combines the advantages of LFAC and DRU, and has outstanding advantages such as no offshore converter station, simple maintenance, easy AC networking; small submarine cable capacitance effect, small charging current, and low loss; and low cost of onshore back-to-back converters.

[0106] The power coupling relationship in the low-frequency subsystem differs significantly from that in traditional power systems. Therefore, in a preferred embodiment of the present invention, the power flow distribution of the low-frequency subsystem is analyzed, such as... Figure 2 As shown, based on the external characteristics of the DRU, it is pointed out that the low-frequency subsystem of the DRU-LFAC system has strong active-voltage coupling characteristics and reactive-frequency coupling characteristics.

[0107] Based on the above DRU-LFAC system, the active power characteristics of the DRU-LFAC system are analyzed.

[0108] Power flow distribution of low-frequency side subsystems, such as Figure 2 As shown, Figure 1 The numbered position indicates that it is installed Figure 2 The component with the corresponding number in the middle, Figure 1One of the locations indicates that it is installed. Figure 2 The wind turbine line-side converter and the first transformer at point 1; Figure 1 The two places in the text indicate that they are installed Figure 2 Two offshore wind farm cluster feeders in the middle, Figure 1 The three locations in the middle indicate that they have been installed. Figure 2 The second transformer at point 3 Figure 1 The four locations in the middle indicate installation. Figure 2 The four submarine cables in the middle, Figure 1 Five places in the middle indicate that there is Figure 2 Install the converter transformer at point 5.

[0109] Obviously, considering the losses during active power transmission, the following relationship exists within the low-frequency subsystem:

[0110] (15)

[0111] (16)

[0112] In the formula, P DR The active power input to the DRU, P WT The total active power output of all converters. P tf , P net , P line These represent the total active power losses on transformers (first transformer and second transformer), offshore wind farm cluster feeders, and AC cables. u DR This is the AC side voltage of the DRU. u PCC For the point of common coupling (PCC) voltage of the low-frequency side subsystem, X line For AC cable reactance.

[0113] Based on the external characteristics of DRU, we can conclude that:

[0114] (17)

[0115] (18)

[0116] In the formula, u dc This is the DC-side voltage of the DRU. i dc This is the DC-side current of the DRU. N The number of cascaded six-pulse DRUs. Xt0 and P t0 These are the leakage reactance and active power loss of the DRU converter transformer, respectively. Considering the operation of the entire system... u dc Controlled by MMC, therefore, during analysis, u dc It is considered a constant value; at the same time, P t0 Compared to P DR It is extremely small, so it can be ignored in the analysis, and then we can further obtain information about... u DR The expression:

[0117] (19)

[0118] Combining equations (1), (2), and (5), we can obtain u PCC about P WT Relationship:

[0119] (20)

[0120] It can be seen from equation (6) that the voltage at the PCC point of the offshore power grid is... u pcc Power output of wind turbine P WT They exhibit a strong positive coupling relationship, only in the two loss terms Δ u and Δ P There is a small amount of reactive power output. Q WT The impact.

[0121] In practical control, permanent magnet direct-drive full-power wind turbine generators generally adopt the maximum power point tracking (MPPT) control method, therefore the converter output has active power. P WT Determined solely by the current wind speed, it can be considered a fixed quantity. Therefore, the voltage at the PCC point can also be considered a fixed quantity, its magnitude varying within a certain range as the active power output of the converter changes.

[0122] In some specific embodiments, simulations of the above system revealed that when the offshore power grid frequency is maintained at 20Hz and the active power output of the converter varies within the range of 0-1 pu, the voltage change at the PCC point is as follows. Figure 3 As shown.

[0123] Obviously, due to the active power-voltage coupling characteristics in the system, the voltage at point PCC is clamped by the active power between 0.9 pu and 1.08 pu.

[0124] Furthermore, the reactive power characteristics of the DRU-LFAC system are analyzed.

[0125] Considering the losses during reactive power transmission, the following relationship exists within the low-frequency subsystem:

[0126] (twenty one)

[0127] In the formula, Q DR The reactive power input to the DRU Q WT The total reactive power generated by all converters. Q tf , Q net , Q line These represent the total reactive power losses on transformers, offshore wind farm cluster feeders, and AC cables, respectively. Based on the external characteristics of the DRU, we can obtain... Q DR The expression is:

[0128] (twenty two)

[0129] In the formula, The power factor of the DRU. μ Let be the commutation overlap angle of the DRU. From equation (3), we can also see that:

[0130] (twenty three)

[0131] In the formula, f The frequency on the low-frequency side, L t0 For the leakage inductance of the converter transformer, there is L t0 = X t0 / (2 πf Considering ignoring P t0 back, P DR = i dc u dc Combining equations (2), (7), (8), and (9), we can obtain:

[0132] (twenty four)

[0133] This yields the frequency. f about Q WT The expression.

[0134] Based on the previous analysis, the voltage at point PCC u pcc Controlled by active power. Therefore, as shown in equation (10), when the converter outputs active power... P WT Under certain circumstances, reactive power can be adjusted. Q WT This is to regulate the frequency of the offshore power grid, and the greater the reactive power output, the lower the frequency of the offshore power grid.

[0135] In some specific embodiments, simulations of the above system show that when the active power output of the converter is maintained at 1 pu, the offshore power grid frequency and the reactive power output of the converter are... Q WT Relationship such as Figure 4 As shown.

[0136] The above embodiments analyzed the low-frequency power characteristics of the DRU-LFAC system, and it can be concluded that: under MPPT control, PDR is uniquely determined by wind speed; under the condition that PDR is determined, u pcc It is basically determined and its range of variation is extremely small; under the condition that upcc is basically determined, it can be controlled by Q. WT To control the frequency f on the offshore power grid side, and f is related to Q WT It shows a negative correlation.

[0137] Based on the same inventive concept, in one embodiment of the present invention, a wind turbine grid control method applicable to the DRU-LFAC system in any of the above embodiments is provided, such as... Figure 5 As shown, the steps are as follows:

[0138] The wind turbine generator-side converter is controlled using the conventional MPPT control method;

[0139] The grid-side converter of the wind turbine has a three-layer control structure: the outer loop is DC voltage control and reactive power-frequency droop control, the middle loop is AC voltage middle loop control, and the inner loop is current inner loop control.

[0140] Under the aforementioned control, offshore wind farms are able to autonomously establish the low-frequency side voltage (u) of the system. pcc ) and frequency ( f or ω And power is fed into the grid through the DRU-LFAC system.

[0141] In a preferred embodiment, such as Figure 5 As shown, d-axis DC voltage control and q-axis... QfThe specific control process of droop control. The d-axis controls the active current output by the converter. i Fd This is used to control the DC bus voltage to ensure that all active power generated by the wind turbine can be transmitted to the power grid. Specifically:

[0142] In the formula, i Fdref It is the reference value of the d-axis current output by the d-axis DC voltage control; K p It is the proportional coefficient of the PI controller; K i It is the integral coefficient of the PI controller; u dc This is the actual value of the DC voltage; u dcref This is a DC voltage reference value;

[0143] q-axis passes through Qf Droop control is achieved using a phase-locked loop (PLL) to establish and synchronize the frequency. Specifically, for example... Figure 6 As shown:

[0144] Assuming the current actual frequency on the low-frequency side ω Less than a given frequency ω 0, then the PLL output at this time Δω <0, according to Qf Sag control includes:

[0145] (25)

[0146] Then obviously the output at this time is Q ref <0. According to the q-axis outer loop control, it is obvious that:

[0147] (26)

[0148] because Q The reactive current reference value has not changed, therefore the output reactive current reference value is... i Fqref In Q ref The effect increases under the influence of [the current]. According to the current inner loop control, i Fq Will track i Fqref Therefore i Fq Increase. According to the reactive power calculation formula:

[0149] (27)

[0150] It can be seen that the converter output has active power. Q It will decrease. According to equation (10), Q The reduction will bringf The increase, even if ω Increase. This is achieved by controlling... ω and Q A negative feedback loop was introduced to achieve this. ω For a given frequency ω Follow 0.

[0151] As mentioned above, considering that PU In coupling, U The variation range is small, so the output voltage of the converter can be left unchanged in steady state. u Fabc The magnitude needs to be controlled. However, considering the sudden change in active power output of the wind turbine, it is necessary to... u Fabc The transient process is controlled. In some embodiments, the AC voltage loop control includes two parts: capacitor feedforward decoupling and AC voltage dynamic control, such as... Figure 5 As shown:

[0152] Capacitor feedforward decoupling considers the effect of capacitance in the d-axis current loop to improve the dynamic characteristics of the voltage. AC voltage dynamic control, on the other hand, introduces a voltage rating. u Fd0 and current feedforward i Fd2 To limit the amplitude of AC voltage changes and accelerate the response speed, the control equation is as follows:

[0153] (28)

[0154] In the formula, i Fdc For DC voltage control loop generation i Fd Reference value u Fd0 This represents the rated value of the d-axis component of the voltage. i Fd2 This represents the d-axis component of the current flowing into the step-up transformer.

[0155] As mentioned above, the inner loop of this invention is a current inner loop control. In some embodiments, the current inner loop control process is as follows: Figure 5 As shown, the inner current loop is controlled according to the conventional strategy under the dq axis to generate a voltage reference value. u Fdref and u Fqref For example Figure 3 The dynamic characteristics of the current loop in the system shown are as follows:

[0156] (29)

[0157] Therefore, it is obvious that it can be done according to Figure 5 The current loop pair shown u Fdref and u Fqref To take control.

[0158] Because the DRU is unidirectional, the power grid cannot transmit the power required for wind farm startup to the wind farm before the low-frequency side voltage is established. Therefore, based on the same inventive concept, one embodiment of the present invention provides a black-start control method applicable to the DRU-LFAC system in any of the above embodiments, which can take the following steps:

[0159] First, an auxiliary converter is added to the DRU-LFAC system, and the auxiliary converter is connected in parallel with the RU-MMC.

[0160] Then, the offshore wind farm is black-started through the auxiliary converter, and the wind turbine is controlled to switch between grid-connected and grid-connected types in order to cooperate with the auxiliary converter for black start.

[0161] In this black-start method, an auxiliary converter first provides the energy required for startup to the low-frequency side of the system, enabling some wind turbines to start first. Once these turbines have started, the auxiliary converter stops providing startup power and instead uses the already started turbines to provide energy, allowing the remaining turbines to start sequentially. Under this black-start control strategy, offshore wind farms can achieve relatively smooth startup. In a preferred embodiment, the black-start process is divided into five stages, such as... Figure 7 As shown, specifically:

[0162] (1) Pre-charging stage. In this stage, the MMC and the auxiliary converter on the MMC side are started, and the voltage on the DC side of the main system and the auxiliary system is controlled to the rated value respectively. At the same time, in order to reduce the reactive power demand during black start, except for the wind turbines participating in black start, the grid-side converters of the other wind turbines are disconnected, and the filters on the AC side of the DRU are also disconnected.

[0163] (2) Offshore wind farm voltage establishment stage. This stage includes the establishment of AC and DC voltages for the offshore wind farm. The auxiliary converter on the DRU side is connected with V / f control to establish the AC voltage and frequency of the offshore power grid. To avoid the formation of power circulating current when the DRU and the auxiliary converter work simultaneously during the black start process, the voltage setpoint of V / f control should be less than the DRU's turn-on voltage, which can be taken as 0.9 times the rated AC voltage.

[0164] Once the AC voltage of the offshore power grid is established, the wind turbine grid-side converter is enabled and operates in grid-following Udc / Q control mode. It synchronizes with the AC voltage established by the auxiliary converter through the PLL to establish the DC voltage of the wind turbine converter.

[0165] (3) Fan start-up stage. In this stage, the fan is connected, the fan-side converter is started and operates in MPPT control mode to realize the start-up of the fan.

[0166] (4) DRU Start-up Stage. In this stage, after the wind turbine starts to feed power back to the auxiliary converter, the wind turbine grid-side converter is switched to the proposed Qf droop control, and the DRU-side auxiliary converter is disconnected. At this time, there is no power path in the system, and the surplus power will cause the AC voltage at the DRU AC port to rise and reach the DRU's turn-on voltage, thereby turning on the DRU.

[0167] (5) Stable operation phase. After the DRU is turned on, power is transmitted through the DRU, and the system enters the stable operation phase. The auxiliary converter is reconnected with PQ control, and the PQ command values ​​are all 0. At the same time, the filter at the AC port of the DRU is connected. The remaining wind turbines that have not yet started start up in sequence and output active power until all wind turbines have started up.

[0168] Figure 7 In this context, Udc / Qf droop control represents DC voltage control and reactive power-frequency droop control; Udc / Q control corresponds to DC voltage control and reactive power control; and V / f control corresponds to voltage control and frequency control.

[0169] P / Q control corresponds to active power control and reactive power control; Udc / Q control corresponds to DC voltage control and reactive power control.

[0170] In one specific embodiment of the present invention, based on Figure 1 The system topology is shown, and a simulation model is built in PSCAD / EMTDC to verify the proposed network control method and black start method. The parameters of the simulation system are shown in Table 1. The offshore wind farm consists of four 250MW wind farms, including three 250MW equivalent wind farms and one detailed wind farm consisting of five 50MW equivalent wind turbines.

[0171] During the simulation, the wind speed was set to 12 m / s.

[0172] Table 1. Main Circuit Parameters of DRU-LFAC Power Transmission System

[0173]

[0174] Simulation results are as follows Figure 8As shown, the MMC (Multi-Cycle Controller) is first charged. After the MMC DC voltage rises to 500kV, the auxiliary VSC (Variable Reactive Power Controller) starts up, feeding power to the low-frequency side to establish the PCC (Power Generation Control) point voltage. Once the PCC point voltage and the DC voltage of the first wind farm are established, the first wind farm starts up, with the turbine speed increasing from 0 to approximately 1 pu (pu), and its output power increasing from 0 to approximately 0.8 pu. Simultaneously, the VSC switches from V / f control to PQ control, ceasing power feeding to the low-frequency side. After the first wind farm stabilizes, the remaining wind farms start up sequentially. Simulation results show that the proposed black-start strategy based on the auxiliary converter can effectively achieve black-start of the offshore grid. Furthermore, during startup, the proposed grid-type control can maintain the offshore grid frequency well at the given frequency of 20Hz, with the required reactive power shared by all wind farms. After the system enters steady state, each wind farm bears approximately 0.12 pu of reactive power.

[0175] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. The above preferred features can be used in any combination without conflict.

Claims

1. A DRU-LFAC system, characterized in that, It includes, in sequence, an offshore wind farm, a low-frequency side subsystem, a shore-based back-to-back converter, a shore power grid, and a filter installed in the low-frequency side subsystem; The offshore wind farm uses permanent magnet direct-drive full-power wind turbine generators, and the low-frequency alternating current (LFAC) generated by them is sent to the onshore back-to-back converter through the low-frequency side subsystem. The onshore back-to-back converter converts the low-frequency AC power (LFAC) to high-frequency AC power and sends it into the onshore power grid. The filter is used for reactive power compensation, providing the reactive power required for the normal operation of the onshore back-to-back converter; in: The low-frequency side subsystem includes a wind turbine line-side converter, a first transformer, an offshore wind farm cluster feeder, a second transformer, an AC submarine cable, and a converter transformer connected in sequence. The onshore back-to-back converter includes a connected twelve-pulse diode rectifier unit (DRU) and a modular multilevel converter (MMC). The DRU-LFAC system has active-voltage coupling characteristics and reactive-frequency coupling characteristics.

2. The DRU-LFAC system according to claim 1, characterized in that, When the permanent magnet direct-drive full-power wind turbine adopts maximum power point tracking (MPPT) control, the active power input to the diode rectifier unit (DRU) is... P DR Determined uniquely by wind speed; in the active power P DR Under certain conditions, the voltage at the common coupling point (PCC) in the low-frequency side subsystem is... u pcc The voltage can be determined and varies within the range of 0.9 pu to 1.08 pu, where pu is the rated voltage value. At this time, the DRU-LFAC system has active power-voltage coupling characteristics. Voltage at the common coupling point (PCC) in the low-frequency side subsystem u pcc Under certain conditions, the total reactive power output of all wind turbine line-side converters is controlled. Q WT To control the frequency of the low-frequency side subsystem f ,and f Reactive power input to DRU Q WT The correlation is negative, and at this time, the DRU-LFAC system has reactive power-frequency coupling characteristics.

3. The DRU-LFAC system according to claim 2, characterized in that, The low-frequency side subsystem is related to: (1) (2) In the formula, P DR The active power input to the DRU, P WT This represents the total active power output from all line-side converters of the wind turbines. P tf , P net , P line These represent the total active power losses on the first or second transformer, the offshore wind farm cluster feeder, and the AC cables. u DR This is the AC side voltage of the DRU. u pcc The common coupling point (PCC) voltage for the low-frequency side subsystem. X line For AC cable reactance; Indicates power loss. Indicates voltage loss; The DC-side voltage and current of the DRU are: (3) (4) In the formula, u dc This is the DC-side voltage of the DRU. i dc This is the DC-side current of the DRU. N The number of cascaded six-pulse DRUs. X t0 and P t0 These are the leakage reactance and active power loss of the DRU converter transformer, respectively. The system DRU-LFAC is running u dc Controlled by MMC u dc The AC side voltage of the DRU is a constant value: (5) The common coupling point (PCC) voltage of the low-frequency side subsystem u PCC about P WT The relation is: (6) The permanent magnet direct-drive full-power wind turbine adopts maximum power point tracking (MPPT) control, and the total active power output of all wind turbine line-side converters is [data missing]. P WT Determined solely by the current wind speed, it is a fixed quantity; The u PCC It is also a definite quantity, the magnitude of which varies within a fixed range as the output active power of the converter changes; At this time, the DRU-LFAC system has active power-voltage coupling characteristics.

4. A DRU-LFAC system according to claim 2, characterized in that, The relationship within the low-frequency side subsystem is as follows: (7) In the formula, Q DR The reactive power input to the DRU Q WT This represents the total reactive power output of all wind turbine line-side converters. Q tf , Q net , Q line These refer to the total reactive power losses on the first or second transformer, the offshore wind farm cluster feeder, and the AC cable, respectively. The reactive power input to the DRU is: (8) In the formula, The power factor of the DRU. μ The commutation overlap angle of the DRU; The DC-side current of the DRU is: (9) In the formula, f For the frequency of the low-frequency side subsystem, L t0 For the leakage inductance of the converter transformer, there is L t0 = X t0 / (2 πf ); The active power of the DRU is negligible. P t0 After, it is represented as P DR = i dc u dc ; The frequency of the low-frequency side subsystem f about Q WT The expression is: (10) voltage at PCC point u pcc Subject to active power control, all wind turbine line-side converters output active power at the converter's output. P WT Under certain conditions, adjust the total reactive power output of all wind turbine line-side converters. Q WT To achieve frequency adjustment of the low-frequency side subsystem, and Q WT It is negatively correlated with the frequency of the low-frequency side subsystem; At this time, the DRU-LFAC system has reactive power-frequency coupling characteristics.

5. A wind turbine grid control method applicable to the DRU-LFAC system of claim 1, characterized in that, include: The line-side converters of offshore wind farms are controlled according to maximum power point tracking (MPPT). Offshore wind farms have three layers of grid-side control, from the outside to the inside: d-axis DC voltage control and q-axis Qf droop control, AC voltage middle loop control, and current inner loop control; The frequency and voltage of the low-frequency subsystem of the DRU-LFAC system are established through the maximum power point tracking (MPPT) control and the three-layer control.

6. A wind turbine grid control method applicable to a DRU-LFAC system according to claim 5, characterized in that, The d-axis DC voltage control is specifically as follows: The d-axis controls the active current output by the converter. i Fd This is used to control the DC bus voltage to ensure that all active power generated by the wind turbine can be transmitted to the power grid. ； It is the reference value of the d-axis current output of the d-axis DC voltage control; It is the proportional coefficient of the PI controller; It is the integral coefficient of the PI controller; This is the actual value of the DC voltage; This is a DC voltage reference value; The q-axis Qf Sagging control, specifically: Current actual frequency on the low-frequency side ω Less than a given frequency ω At 0, the output of the phase-locked loop (PLL) is... Δω <0; according to Qf Sag control includes: (11) It is the reactive power reference value of the output of the q-axis Qf droop control; It is the droop coefficient controlled by the q-axis Qf droop. Then the output at this time Q ref <0; Based on the q-axis outer loop control, we have: (12) It is the reactive power reference value of the q-axis Qf droop control output; k fq It is the droop coefficient controlled by the q-axis Qf droop. It is the proportional coefficient of the PI controller; It is the integral coefficient of the PI controller; s is the Laplace operator; because Q The reactive current reference value has not changed, therefore the output reactive current reference value is... i Fqref In Q ref Increased under the influence of; Based on current inner loop control i Fq Will track i Fqref Therefore i Fq Increase; According to the reactive power calculation formula: (13) Determine the active power output of the converter Q It will decrease; Based on the aforementioned reactive-frequency coupling characteristics Q The reduction will bring f The increase, even if ω Increase; The q-axis Qf droop control and the reactive-frequency coupling characteristic together constitute a system. ω The negative feedback loop can artificially control the frequency. ω To control in order to achieve ω For a given frequency ω Follow 0.

7. A wind turbine grid control method applicable to a DRU-LFAC system according to claim 5, characterized in that, The AC voltage loop control includes capacitor feedforward decoupling and AC voltage dynamic control. Specifically, the capacitor feedforward decoupling considers the effect of capacitance in the d-axis current loop to improve the dynamic characteristics of the voltage, and the AC voltage dynamic control introduces a voltage rating. u Fd0 and current feedforward i Fd2 To limit the amplitude of AC voltage changes and accelerate the response speed, the governing equation is: (14) In the formula: i Fdcref For DC voltage control loop generation i Fd Reference value u Fd0 This represents the rated value of the d-axis component of the voltage. i Fd2 u is the d-axis component of the current flowing into the step-up transformer. Fd This represents the actual value of the d-axis component of the voltage. ω 0 represents a given frequency, C F The capacitance value of the filter capacitor of the converter on the line side of the wind turbine, u Fq This represents the actual value of the q-axis component of the voltage; ω 0C F u Fq This is to decouple the capacitor feedforward.

8. A wind turbine grid control method applicable to a DRU-LFAC system according to claim 5, characterized in that, The inner current loop control is specifically as follows: Current inner loop generated voltage reference value u Fdref and u Fqref Its governing equations are: (15)。 9. A black-start control method applicable to the DRU-LFAC system of claim 1, characterized in that, include: An auxiliary converter is added to the DRU-LFAC system, and the auxiliary converter is connected in parallel with the onshore back-to-back converter. The auxiliary converter enables black start of the offshore wind farm, controlling the wind turbines of the offshore wind farm to switch between grid-connected and grid-connected configurations to facilitate black start.

10. A black-start control method for a DRU-LFAC system according to claim 9, characterized in that, The process of performing black start on an offshore wind farm via the auxiliary converter, controlling the wind turbines of the offshore wind farm to switch between grid-connected and grid-connected configurations to cooperate with the auxiliary converter for black start, includes: Phase 1: Start the auxiliary converters on the MMC and DRU sides to control the DC side voltage of the onshore back-to-back converters and auxiliary converters to the rated value; at the same time, except for the wind turbines participating in black start, the grid-side converters of the other wind turbines are disconnected, and the filters on the AC side of the DRU are also disconnected. Phase 2: The auxiliary converter on the DRU side is connected with V / f control to establish the AC voltage and frequency of the low-frequency side subsystem; after the AC voltage of the low-frequency side subsystem is established, the wind turbine grid-side converter is enabled and operates in grid-following Udc / Q control mode, and is synchronized with the AC voltage established by the auxiliary converter through PLL to establish the DC voltage of the wind turbine converter on the machine side and the grid side. Phase 3: The synchronous generator is connected, the wind turbine's machine-side converter starts and operates in MPPT control mode, thus realizing the start-up of the wind turbine; Fourth stage: After the wind turbine starts to backfeed power to the auxiliary converter, switch the wind turbine grid-side converter to Qf droop control and disconnect the DRU-side auxiliary converter; at this time there is no power path in the system, the surplus power will cause the AC voltage at the DRU AC port to rise and reach the DRU's turn-on voltage, so that the DRU turns on. Phase 5: After the DRU is turned on, power is transmitted through the DRU, and the system enters the stable operation phase; the auxiliary converter is reconnected with PQ control, and the PQ command values ​​are all 0; at the same time, the filter at the AC port of the DRU is connected; the remaining wind turbines that have not yet started start up in sequence and output active power until all wind turbines have started up.

Images