DRU-LFAC system and wind turbine generator networking type control and black-start control method

Abstract

The invention provides a DRU-LFAC system and a wind turbine generator network construction type control and black-start control method, and the method comprises the steps: an offshore wind plant is a permanent magnet direct-drive full-power wind turbine generator, and low-frequency alternating current is transmitted to a shore back-to-back converter through a low-frequency side subsystem; the ashore back-to-back converter converts the low-frequency alternating current into high-frequency alternating current and sends the high-frequency alternating current into an ashore power grid; the ashore back-to-back converter comprises a twelve-pulse diode rectification unit and a modularized multi-level converter; the filter is used for reactive compensation. The DRU-LFAC system has an active power-voltage coupling characteristic and a reactive power-frequency coupling characteristic. The method solves the problems that the low-frequency side power characteristic of the current DRU-LFAC system is different from that of a traditional power system and cannot be effectively analyzed, the current traditional wind turbine generator control strategy is not suitable for the DRU-LFAC system, and the offshore wind plant cannot be started through a conventional process due to the one-way conduction characteristic of the DRU.

Description

Technical Field

[0001] The present invention relates to the field of power grid technology, and in particular to a DRU-LFAC system and a wind turbine generator system grid control and black start control method. Background Art

[0002] With the continuous deepening of offshore wind energy development, offshore wind farms are gradually developing in the direction of large capacity and deep sea. With the increase of transmission distance and rated capacity, a series of shortcomings of the current common modular multilevel converter (MMC based High Voltage Direct Current, MMC-HVDC) flexible direct current transmission solution have begun to emerge, including the high cost of MMC converter valves; high cost of offshore converter stations; high maintenance cost and low reliability in the deep sea. Therefore, it is urgent to study a new type of transmission method with large capacity, long distance and high economy to meet the demand for power transmission from large-capacity wind farms in the deep sea.

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

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

[0005] In addition, the power coupling relationship in the low-frequency side system is quite different from that in the traditional power system, and new control strategies need to be studied based on the actual characteristics of the system. Finally, the DRU-LFAC power transmission system has a long AC transmission line, and the power demand at startup is large. If the traditional method of installing energy storage is still used for black start, the offshore energy storage system will be too large, difficult to build and maintain, and have poor economic benefits.

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

[0007] In view of the defects in the prior art, the object of the present invention is to provide a DRU-LFAC system and a wind turbine group network control and black start control method.

[0008] According to one aspect of the present invention, there is provided a DRU-LFAC system, comprising an 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, which are connected in sequence;

[0009] The offshore wind farm adopts a permanent magnet direct-drive full-power wind turbine set, and the low-frequency alternating current LFAC generated by the wind turbine set 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 alternating current LFAC into high-frequency alternating current and sends it to the onshore power grid;

[0011] The filter is used for reactive power compensation to provide reactive power required for 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 power-voltage coupling characteristics and reactive power-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 P input to the diode rectifier unit DRU is DR Determined uniquely by the wind speed; in the active power P DR Under certain conditions, the voltage u at the common connection point PCC voltage point in the low-frequency side subsystem is pcc It can be determined and the range varies between 0.9pu-1.08pu, where pu is the rated voltage value; at this time, the DRU-LFAC system has an active power-voltage coupling characteristic;

[0017] The voltage u at the common connection point PCC voltage point in the low-frequency side subsystem pcc Under certain conditions, the total reactive power Q output by all wind turbine line-side converters is controlled WT To control the frequency f of the low-frequency side subsystem, and f is related to the reactive power Q input to the DRU WTis negatively correlated. At this time, the DRU-LFAC system has a reactive-frequency coupling characteristic.

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

[0019] P DR =P WT -P tf -P net -P line =P WT -ΔP (1)

[0020]

[0021] Where P DR is the active power input to the DRU, P WT is the total active power output by all wind turbine line-side converters, P ut , P net , P line are the total active power lost in the first or second transformer, offshore wind farm cluster feeder and AC cable, u dder is the voltage at the AC side of the DRU, u pcc is the PCC voltage of the low-frequency side subsystem, X line is the AC cable reactance; ΔP represents power loss, and Δu represents voltage loss;

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

[0023]

[0024] In the formula, u dc is the DC side voltage of DRU, i dc is the DC side current of DRU, N is the number of cascaded six-pulse DRUs, X t0 and P t0 They are the leakage reactance and active power loss of the DRU converter transformer respectively;

[0025] The system DRU-LFAC runtime u dc Controlled by MMC, u dc is a constant value, and the voltage on the DRU AC side is:

[0026]

[0027] The voltage of the common connection point PCC of the low-frequency side subsystem is about P WT The relationship is:

[0028]

[0029] The permanent magnet direct-drive full-power wind turbine adopts maximum power point tracking control MPPT. The total active power P output by the line-side converter of all wind turbines is WT It is determined only by the current wind speed and is a fixed quantity;

[0030] The u PCC It is also a fixed quantity, and its magnitude changes within a fixed range as the output active power of the converter changes;

[0031] At this time, the DRU-LFAC system has active power-voltage coupling characteristics.

[0032] Preferably, the relationship within the low-frequency side subsystem is:

[0033] Q DR =Q WT -Q tf -Q net -Q line =Q WT -ΔQ (7)

[0034] In the formula, Q DR is the reactive power input to the DRU, Q WT is the total reactive power output by all wind turbine line-side converters, Q tf , Q net , Q line are the total reactive power lost in the first or second transformer, the offshore wind farm cluster feeder, and the AC cable, respectively;

[0035] The reactive power input by the DRU is:

[0036]

[0037] In the formula, is the power factor of DRU, μ is the commutation overlap angle of DRU;

[0038] The DRU DC side current is:

[0039]

[0040] Where f is the frequency of the low-frequency side subsystem, L t0 is the leakage inductance of the converter transformer, and L t0 =X t0 / (2πf);

[0041] The active power of the DRU is taken into account by ignoring P t0 After that, it is expressed as P DR =i dc u dc ;

[0042] The frequency f of the low-frequency side subsystem is about Q WT The expression is:

[0043]

[0044] The voltage u at the PCC point pcc Under active power control, all wind turbine line-side converters output active power P WT Under certain conditions, the total reactive power Q output by all wind turbine line-side converters is adjusted WT Realize the adjustment of the frequency of the low-frequency side subsystem, and Q WT Negatively correlated with the frequency of the low-frequency side subsystem;

[0045] At this time, the DRU-LFAC system has reactive power-frequency coupling characteristics.

[0046] According to a second aspect of the present invention, there is provided a wind turbine configuration grid type control method applicable to a DRU-LFAC system, comprising:

[0047] The line-side converters of wind turbines in offshore wind farms are controlled according to the maximum power point tracking (MPPT);

[0048] There are three levels of control on the grid side of an offshore wind farm, which are divided from outside to inside into: d-axis DC voltage control and q-axis Qf droop control, AC voltage middle loop control, and current inner loop control;

[0049] Through the maximum power point tracking MPPT control and the three-layer control, the frequency and voltage of the low-frequency side subsystem of the DRU-LFAC system are established.

[0050] Preferably, the d-axis DC voltage control is specifically:

[0051] The d-axis controls the active current i output by the converter Fd To control the DC bus voltage to ensure that all active power generated by the wind turbine can be delivered to the grid:

[0052]

[0053] i Fdref is the d-axis current reference value of the d-axis DC voltage control output; k p is the proportional coefficient of the PI controller; k i is the integral coefficient of the PI controller; u dc is the actual value of DC voltage; u dcref is the DC voltage reference value;

[0054] The q-axis Qf droop control is specifically as follows:

[0055] When the actual frequency ω on the current low-frequency side is less than the given frequency ω0, the Δω output by the phase-locked loop PLL is less than 0;

[0056] According to Qf droop control, we have:

[0057] Q ref =k fq (ω-ω0)=k fq Δω (11)

[0058] Q ref is the reactive reference value of the q-axis Qf droop control output; k fq is the droop coefficient of the q-axis Qf droop control;

[0059] Then the output Q ref <0;

[0060] According to the q-axis outer loop control, we have:

[0061]

[0062] Q ref is the reactive reference value of the q-axis Qf droop control output; k fq is the droop coefficient k of the q-axis Qf droop control p is the proportional coefficient of the PI controller; k i is the integral coefficient of the PI controller; s is the Laplace operator;

[0063] Since Q has not changed, the output reactive current reference value i Fqref Will be in Q ref Increased under the effect of

[0064] According to the current inner loop control, i Fq Will track me Fqref , so i Fq Increase;

[0065] According to the reactive power calculation formula:

[0066] Q=1.5I d U q -1.5I q U d (13)

[0067] It is determined that the active power Q output by the converter will decrease;

[0068] According to the reactive-frequency coupling characteristics, the reduction of Q will lead to an increase of f, that is, an increase of ω;

[0069] The q-axis Qf droop control and the reactive-frequency coupling characteristic jointly construct a negative feedback loop of ω, which can artificially control the frequency ω to achieve ω following the given frequency ω0.

[0070] Preferably, the AC voltage mid-loop control includes capacitor feed-forward decoupling and AC voltage dynamic control;

[0071] The capacitor feedforward decoupling considers the capacitor effect in the d-axis current loop to improve the dynamic characteristics of the voltage, and the AC voltage dynamic control introduces the voltage rating u Fd0 and current feedforward i Fd2 , in order to limit the change range of AC voltage and speed up the response speed, the control equation is:

[0072]

[0073] Where: i Fdcref i generated for the DC voltage control loop Fd Reference value, u Fd0 is the rated value of the voltage d-axis component, i Fd2 is the d-axis component of the current flowing into the step-up transformer, u Fd is the actual value of the voltage d-axis component, ω0 is the given frequency, C F is the filter capacitor value of the wind turbine line side converter, u Fq is the actual value of the voltage q-axis component; -ω0C F u Fq Decoupling for the capacitor feedforward.

[0074] Preferably, the current inner loop control is specifically:

[0075] The current inner loop generates a voltage reference value u Fdref with u Fqref ; Its control equation is:

[0076]

[0077] According to a third aspect of the present invention, there is provided a black start control method applicable to a DRU-LFAC system, comprising:

[0078] An auxiliary converter is added to the DRU-LFAC system, and the auxiliary converter is connected in parallel with the back-to-back converter on shore;

[0079] The offshore wind farm is black-started by the auxiliary converter, and the wind turbines in the offshore wind farm are controlled to switch between the grid-following mode and the grid-building mode to cooperate with the auxiliary converter to perform the black-start.

[0080] Preferably, performing a black start on the offshore wind farm by using the auxiliary converter, and controlling the wind turbines of the offshore wind farm to switch between a grid-following type and a grid-building type to cooperate with the auxiliary converter to perform a black start, comprises:

[0081] Phase 1: Start the auxiliary converters on the MMC and DRU sides, and control the voltages on the DC sides of the onshore back-to-back converter and the auxiliary converter to the rated values ​​respectively; at the same time, except for the wind turbines participating in the black start, the grid-side converters of the other wind turbines are all in the cut-off state, and the filters on the AC side of the DRU are also in the cut-off state;

[0082] 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 works in the grid-following Udc / Q control mode, and synchronizes 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.

[0083] The third stage: The synchronous generator is connected, the wind turbine side converter is started and works in the MPPT control mode to start the wind turbine;

[0084] Stage 4: When the wind turbine starts to reverse power to the auxiliary converter, the wind turbine grid-side converter is switched to Qf droop control, and the auxiliary converter on the DRU side is cut off; 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 conduction voltage, causing the DRU to conduct;

[0085] Phase 5: After the DRU is turned on, power is transmitted through the DRU and the system enters a 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 DRU AC port is connected; the remaining fans that have not yet started are started in turn and output active power until all fans are started.

[0086] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0087] The DRU-LFAC system in the embodiment of the present invention has the advantages of both LFAC and DRU, including no offshore converter station, simple maintenance, easy AC networking; small submarine cable capacitance effect, small charging current, small loss; low cost of onshore back-to-back converters, etc.

[0088] The DRU-LFAC system in the embodiment of the present invention obtains the power coupling relationship in the low-frequency side system, solving the problem that the power characteristics of the low-frequency side of the current DRU-LFAC system are different from those of the traditional power system and have not been effectively analyzed;

[0089] The wind turbine network control method applicable to the DRU-LFAC system in the embodiment of the present invention can effectively control the DRU-LFAC system and solve the problem that the DRU itself is an uncontrolled unit and cannot independently establish an offshore AC power grid.

[0090] The black start control method applicable to the DRU-LFAC system in the embodiment of the present invention solves the problem that the offshore wind farm cannot be started through the conventional process due to the unidirectional conduction characteristic of the DRU. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0092] Figure 1 This is a structural diagram of an offshore wind farm DRU-LFAC power transmission system in one embodiment of the present invention;

[0093] Figure 2 is a detailed structural diagram of a low-frequency side subsystem in a preferred embodiment of the present invention;

[0094] Figure 3 The active power-voltage relationship curve of the DRU-LFAC system at the low frequency side in a preferred embodiment of the present invention;

[0095] Figure 4 The frequency-reactive power relationship curve of the low-frequency side of the DRU-LFAC system in a preferred embodiment of the present invention;

[0096] Figure 5 A block diagram of a grid-type control strategy based on a phase-locked loop and power droop in a preferred embodiment of the present invention;

[0097] Figure 6 A schematic diagram of a frequency-reactive power control mechanism in a preferred embodiment of the present invention;

[0098] Figure 7 A schematic diagram of black start timing logic in a preferred embodiment of the present invention;

[0099] Figure 8 It is a simulation result diagram in a specific embodiment of the present invention. DETAILED DESCRIPTION

[0100] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0101] Figure 1 This is the structure diagram of the DRU-LFAC power transmission system of the offshore wind farm. As shown in the figure, it includes the offshore wind farm, the low-frequency side subsystem, the onshore back-to-back converter, the onshore power grid and the filter installed in the low-frequency side subsystem connected in sequence; the offshore wind farm adopts a permanent magnet direct-drive full-power wind turbine, and the low-frequency alternating current LFAC it generates 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 alternating current LFAC (low frequency alternating current) into high-frequency alternating current and sends it to the onshore power grid; the low-frequency side subsystem includes the wind turbine line-side converter, the first transformer, the offshore wind farm cluster feeder, the second transformer, the AC submarine cable and the converter transformer connected in sequence; the onshore back-to-back converter includes a twelve-pulse diode rectifier unit DRU (Diode Rectifier Unit) and a modular multilevel converter MMC (Modular Multilevel Converter) connected; the filter is used for reactive power compensation to provide the reactive power required for the normal operation of the diode rectifier unit DRU.

[0102] The DRU-LFAC system in the above-mentioned embodiment of the present invention has active power-voltage coupling characteristics and reactive power-frequency coupling characteristics.

[0103] Generally, the onshore back-to-back converter converts the 20Hz low frequency alternating current to 50Hz industrial frequency and feeds it into the grid.

[0104] The DRU-LFAC system in the above embodiment has the advantages of both LFAC and DRU, including no offshore converter station, simple maintenance, easy AC networking; small submarine cable capacitance effect, small charging current, small loss; low cost of onshore back-to-back converters, etc.

[0105] The power coupling relationship in the low-frequency side subsystem is quite different from that in the traditional power system. Therefore, in a preferred embodiment of the present invention, the power flow distribution of the low-frequency side subsystem is analyzed, such as Figure 2 As shown in the figure, combined with the external characteristics of DRU, it is pointed out that the low-frequency side subsystem of the DRU-LFAC system has strong active-voltage coupling characteristics and reactive-frequency coupling characteristics.

[0106] Based on the above DRU-LFAC system, the active power characteristics of the DRU-LFAC system are analyzed.

[0107] The power flow distribution of the low-frequency side subsystem is as follows: Figure 2 As shown, Figure 1 The numbered position indicates the installation Figure 2 The corresponding numbered parts in Figure 1 1 in the list indicates that Figure 2 The wind turbine line-side converter and the first transformer at 1; Figure 1 The 2 places in the table indicate that there is Figure 2 Offshore wind farm cluster feeders at 2 locations, Figure 1 3 indicates installation Figure 2 The second transformer at 3, Figure 1 4 in the middle indicates installation Figure 2 The AC submarine cable at No. 4, Figure 1 5 of them indicate Figure 2 The converter transformer is installed at location 5.

[0108] Obviously, considering the loss in the active power transmission process, the following relationship exists in the low-frequency side subsystem:

[0109] P DR =P WT -P tf -P net -P line =P WT -ΔP (15)

[0110]

[0111] Where P DR is the active power input to the DRU, P WT is the total active power output by all converters, P tf , P net , P line are the total active power lost in the transformer (first transformer, second transformer), offshore wind farm cluster feeder and AC cable, u DR is the voltage at the AC side of the DRU, u PCC is the voltage of the common connection point (PCC) of the low-frequency side subsystem, X line is the AC cable reactance.

[0112] According to the external characteristics of DRU, we can get:

[0113]

[0114] In the formula, u dc is the DC side voltage of DRU, i dc is the DC side current of DRU, N is the number of cascaded six-pulse DRUs, X t0 and P t0 are the leakage reactance and active power loss of the DRU converter transformer respectively. Considering that the entire system is running dc It is controlled by MMC, so u can be used in analysis dcis considered as a constant value; at the same time, P t0 Relative to P DR is very small, so it can be ignored in the analysis, and we can further get the information about u DR The expression is:

[0115]

[0116] Combining equations (1), (2), and (5), we can get u PCC About P WT The relationship is:

[0117]

[0118] It can be seen from equation (6) that the voltage u at the PCC point of the offshore power grid is pcc With the fan active output P WT There is a strong forward coupling relationship, with only a small amount of reactive output Q in the two loss terms Δu and ΔP WT impact.

[0119] In actual control, permanent magnet direct-drive full-power wind turbines generally adopt the maximum power point tracking (MPPT) control method, so the converter output active power P WT It is only determined by the current wind speed and can be regarded as a fixed quantity. Therefore, the voltage at the PCC point can also be regarded as a fixed quantity, and its magnitude changes within a certain range as the active power output of the converter changes.

[0120] In some specific embodiments, the above system is simulated and it is found that when the frequency of the offshore power grid is maintained at 20 Hz and the active power output of the converter varies within 0-1 pu, the voltage change at the PCC point is as follows: Figure 3 shown.

[0121] Obviously, due to the active power-voltage coupling characteristics in the system, the voltage at the PCC point is clamped by the active power between 0.9pu and 1.08pu.

[0122] Furthermore, the reactive power characteristics of the DRU-LFAC system are analyzed.

[0123] Considering the loss during reactive power transmission, the following relationship exists in the low-frequency side subsystem:

[0124] Q DR =Q WT -Q tf -Q net -Q line =Q WT -ΔQ (21)

[0125] In the formula, Q DR is the reactive power input to the DRU, Q WT is the total reactive power generated by all converters, Q tf , Q net , Q line are the total reactive power lost in the transformer, offshore wind farm cluster feeder and AC cable respectively. According to the external characteristics of DRU, Q DR The expression is:

[0126]

[0127] In the formula, is the power factor of DRU, μ is the commutation overlap angle of DRU. It can be known from formula (3):

[0128]

[0129] Where f is the frequency on the low-frequency side, L t0 is the leakage inductance of the converter transformer, and L t0 =X t0 / (2πf). Considering that P is ignored t0 After that, P DR =i dc u dc , then combining equations (2), (7), (8), and (9) we can obtain:

[0130]

[0131] This gives the frequency f with respect to Q WT expression.

[0132] According to the previous analysis, the voltage u at the PCC point is pcc It is controlled by active power. Therefore, from formula (10), it can be seen that when the converter outputs active power P WT If the condition is determined, the reactive power Q can be adjusted WT To achieve the regulation of the offshore power grid frequency, the greater the output reactive power, the lower the offshore power grid frequency.

[0133] In some specific embodiments, the above system is simulated. When the active power output of the converter is maintained at 1 pu, the frequency of the offshore power grid is proportional to the reactive power output of the converter. WT The relationship is as Figure 4 shown.

[0134] The above embodiment analyzes 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 pccBasically determined and with a very small range of variation; Under the condition that upcc is basically determined, it can be controlled by Q WT To control the frequency f on the offshore grid side, and f is related to Q WT Negatively correlated.

[0135] 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:

[0136] The wind turbine side converter is controlled according to the conventional MPPT control method;

[0137] The wind turbine grid-side converter 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.

[0138] Under the above control, the offshore wind farm can autonomously establish the system low-frequency side voltage (u pcc ) and frequency (f or ω), and feeds power into the grid through the DRU-LFAC system.

[0139] In a preferred embodiment, Figure 5 As shown in the figure, the specific control process of d-axis DC voltage control and q-axis Qf droop control is provided. The d-axis controls the active current i output by the converter. Fd To control the DC bus voltage to ensure that all active power generated by the wind turbine can be delivered to the grid, specifically:

[0140] In the formula, i Fdref is the d-axis current reference value of the d-axis DC voltage control output; k p is the proportional coefficient of the PI controller; k i is the integral coefficient of the PI controller; u dc is the actual value of DC voltage; u dcref is the DC voltage reference value;

[0141] The q-axis is controlled by Qf droop and a phase-locked loop (PLL) is used to establish and synchronize the frequency. Figure 6 As shown:

[0142] Assuming that the actual frequency ω on the current low-frequency side is less than the given frequency ω0, then the Δω output by the PLL is less than 0. According to the Qf droop control, we have:

[0143] Q ref =k fq (ω-ω0)=k fq Δω (25)

[0144] Obviously, the output Q ref <0. According to the q-axis outer loop control, it is obvious that:

[0145]

[0146] Since Q has not changed, the output reactive current reference value i Fqref Will be in Q ref According to the current inner loop control, i Fq Will track me Fqref , so i Fq Increase. According to the reactive power calculation formula:

[0147] Q=1.5I d U q -1.5I q U d (27)

[0148] It can be seen that the converter output active power Q will decrease. According to formula (10), the decrease of Q will lead to the increase of f, that is, ω will increase. In this way, a negative feedback is introduced between ω and Q through control to achieve ω following the given frequency ω0.

[0149] As mentioned above, considering that the variation range of U is small in PU coupling, the output voltage u of the converter can be ignored in steady state. Fabc However, considering that the wind turbine output active power suddenly changes, it is necessary to control the size of u Fabc In some embodiments, the AC voltage mid-loop control includes two parts: capacitor feedforward decoupling and AC voltage dynamic control. Figure 5 As shown:

[0150] Capacitor feedforward decoupling considers the capacitor effect in the d-axis current loop to improve the dynamic characteristics of the voltage. AC voltage dynamic control is achieved by introducing the voltage rating u Fd0 and current feedforward i Fd2 In order to limit the change range of AC voltage and speed up the response speed, the control equation is as follows:

[0151]

[0152] In the formula, i Fdc i generated for the DC voltage control loop Fd Reference value, u Fd0 is the rated value of the voltage d-axis component, i Fd2 is the d-axis component of the current flowing into the step-up transformer.

[0153] As described above, the inner loop of the present invention is a current inner loop control. In some embodiments, the current inner loop control process is as follows: Figure 5 As shown, the current inner loop is controlled according to the conventional strategy under the dq axis to generate a voltage reference value u Fdref with u Fqref For example Figure 3 The dynamic characteristics of the current loop of the system shown are:

[0154]

[0155] So obviously we can follow Figure 5 The current loop shown in Fdref with u Fqref Take control.

[0156] Since the DRU is unidirectional, the power grid cannot transmit the power required for the wind farm to start up before the voltage on the low-frequency side is established. Therefore, based on the same inventive concept, in one embodiment of the present invention, a black start control method for the DRU-LFAC system in any of the above embodiments is provided, which can adopt the following steps:

[0157] First, the DRU-LFAC system adds a set of auxiliary converters, which are connected in parallel with the RU-MMC.

[0158] Then, the offshore wind farm is black-started through the auxiliary converter, and the wind turbines are controlled to switch between the grid-following mode and the grid-building mode to cooperate with the auxiliary converter for black-start.

[0159] In this black start method, the auxiliary converter is first used to provide the energy required for starting the system low frequency side, so that some wind turbines are started first. After the start of these wind turbines is completed, the auxiliary converter no longer provides starting power, and the started wind turbines are used to provide energy instead, so that the remaining wind turbines are started in sequence. Under this black start control strategy, the offshore wind farm can achieve a relatively smooth start. In a preferred embodiment, the black start process is divided into five stages, such as Figure 7 As shown, specifically:

[0160] (1) Pre-charging stage. In this stage, the MMC and the auxiliary converters on the MMC side are started to control the voltages on the DC side of the main system and the auxiliary system to the rated values. At the same time, in order to reduce the reactive power demand during black start, except for the wind turbines involved in the black start, the grid-side converters of the remaining wind turbines are in the cut-off state, and the filters on the AC side of the DRU are also in the cut-off state.

[0161] (2) Offshore wind farm voltage establishment stage. This stage includes the establishment of AC and DC voltages in 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. In order to avoid the formation of power circulation due to the simultaneous operation of the DRU and the auxiliary converter during the black start process, the voltage set value of the V / f control should be less than the conduction voltage of the DRU, which can be taken as 0.9 times the rated value of the AC voltage.

[0162] After the AC voltage of the offshore power grid is established, the wind turbine grid-side converter is enabled and operates in the grid-following Udc / Q control mode, and synchronizes with the AC voltage established by the auxiliary converter through PLL to establish the DC voltage of the wind turbine converter.

[0163] (3) Wind turbine startup phase: In this phase, the wind turbine is connected, and the wind turbine-side converter is started and operates in the MPPT control mode to start the wind turbine.

[0164] (4) DRU startup phase. In this phase, when the wind turbine starts to reverse power to the auxiliary converter, the wind turbine grid-side converter is switched to the proposed Qf droop control, and the auxiliary converter on the DRU side is cut off. 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.

[0165] (5) Stable operation stage. After the DRU is turned on, power is transmitted through the DRU and the system enters the stable operation stage. The auxiliary converter is reconnected with PQ control, and the PQ command value is 0. At the same time, the filter at the AC port of the DRU is connected. The remaining fans that have not started are started one by one and output active power until all fans are started.

[0166] Figure 7 In the equation, Udc / Qf droop control means DC voltage control and reactive power-frequency droop control; Udc / Q control corresponds to DC voltage control and reactive power control; V / f control corresponds to voltage control and frequency control;

[0167] P / Q control corresponds to active power control and reactive power control; Udc / Q control corresponds to DC voltage control and reactive power control

[0168] In a 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 simulate and verify the proposed grid-type control method and black start method. The parameters of the simulation system are shown in Table 1, where 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.

[0169] During the simulation, the wind speed is set to 12 m / s.

[0170] Table 1 Main circuit parameters of DRU-LFAC power transmission system

[0171]

[0172] The simulation results are as follows Figure 8 As shown. The MMC is charged first, and after the MMC DC voltage rises to 500kV, the auxiliary VSC is started and power is fed into the low-frequency side to build up the PCC point voltage. After the PCC point voltage and the DC voltage of the first wind farm are established, the first wind farm is started, the wind turbine speed rises from 0 to around 1pu, and its output power rises from 0 to about 0.8pu. At the same time, the VSC switches from V / f control to PQ control, and no longer feeds power into the low-frequency side. After the first wind farm runs stably, the remaining wind farms are started in sequence. The simulation results show that the proposed black start strategy based on auxiliary converters can effectively realize the black start of the offshore power grid. At the same time, during the startup process, the proposed grid-type control can better maintain the frequency of the offshore power grid at a given frequency of 20Hz, and the required reactive power is shared by each wind farm. After the system enters a steady state, each wind farm bears about 0.12pu of reactive power.

[0173] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various modifications or variations within the scope of the claims, which does not affect the essence of the present invention. The above preferred features can be used in any combination without conflicting with each other.

Claims

1. A DRU-LFAC system, characterized in that: It includes an 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 connected in sequence; The offshore wind farm adopts a permanent magnet direct-drive full-power wind turbine set, and the low-frequency alternating current LFAC generated by the wind turbine set 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 alternating current LFAC into high-frequency alternating current and sends it to the onshore power grid; The filter is used for reactive power compensation to provide reactive power required for 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 power-voltage coupling characteristics and reactive power-frequency coupling characteristics.

2. A 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 P input to the diode rectifier unit DRU is DR Determined uniquely by the wind speed; in the active power P DR Under certain conditions, the voltage u at the common connection point PCC voltage point in the low-frequency side subsystem is pcc It can be determined and the range varies between 0.9pu-1.08pu, where pu is the rated voltage value. At this time, the DRU-LFAC system has an active power-voltage coupling characteristic; The voltage u at the common connection point PCC voltage point in the low-frequency side subsystem pcc Under certain conditions, the total reactive power Q output by all wind turbine line-side converters is controlled WT To control the frequency f of the low-frequency side subsystem, and f is related to the reactive power Q input to the DRU WT is negatively correlated. At this time, the DRU-LFAC system has a reactive-frequency coupling characteristic.

3. A DRU-LFAC system according to claim 2, characterized in that: The low-frequency side subsystem has the relationship: P DR =P WT -P tf -P net -P line =P WT -ΔP (1) Where P DR is the active power input to the DRU, P WT is the total active power output by all wind turbine line-side converters, P ut , P net , P line are the total active power lost in the first or second transformer, offshore wind farm cluster feeder and AC cable, u dder is the voltage at the AC side of the DRU, u pcc is the PCC voltage of the low-frequency side subsystem, X line is the AC cable reactance; ΔP represents power loss, and Δu represents voltage loss; The voltage and current on the DC side of the DRU are: In the formula, u dc is the DC side voltage of DRU, i dc is the DC side current of DRU, N is the number of cascaded six-pulse DRUs, X t0 and P t0 They are the leakage reactance and active power loss of the DRU converter transformer respectively; The system DRU-LFAC runtime u dc Controlled by MMC, u dc is a constant value, and the voltage on the DRU AC side is: The common connection point PCC voltage u of the low-frequency side subsystem PCC About P WT The relationship is: The permanent magnet direct-drive full-power wind turbine adopts maximum power point tracking control MPPT. The total active power P output by the line-side converter of all wind turbines is WT It is determined only by the current wind speed and is a fixed quantity; The u PCC It is also a fixed quantity, and its magnitude changes 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 in the low-frequency side subsystem is: Q DR =Q WT -Q tf -Q net -Q line =Q WT -ΔQ (7) In the formula, Q DR is the reactive power input to the DRU, Q WT is the total reactive power output by all wind turbine line-side converters, Q tf , Q net , Q line are the total reactive power lost in the first or second transformer, the offshore wind farm cluster feeder, and the AC cable, respectively; The reactive power input by the DRU is: In the formula, is the power factor of DRU, μ is the commutation overlap angle of DRU; The DRU DC side current is: Where f is the frequency of the low-frequency side subsystem, L t0 is the leakage inductance of the converter transformer, and L t0 =X t0 / (2πf); The active power of the DRU is taken into account by ignoring P t0 After that, it is expressed as P DR =i dc u dc ; The frequency f of the low-frequency side subsystem is about Q WT The expression is: The voltage u at the PCC point pcc Under active power control, all wind turbine line-side converters output active power P WT Under certain conditions, the total reactive power Q output by all wind turbine line-side converters is adjusted WT Realize the adjustment of the frequency of the low-frequency side subsystem, and Q WT 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 network control method applicable to the DRU-LFAC system of claim 1, characterized in that: include: The line-side converters of wind turbines in offshore wind farms are controlled according to the maximum power point tracking (MPPT); There are three levels of control on the grid side of an offshore wind farm, which are divided from outside to inside into: d-axis DC voltage control and q-axis Qf droop control, AC voltage middle loop control, and current inner loop control; Through the maximum power point tracking MPPT control and the three-layer control, the frequency and voltage of the low-frequency side subsystem of the DRU-LFAC system are established.

6. A wind turbine configuration 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 i output by the converter Fd To control the DC bus voltage to ensure that all active power generated by the wind turbine can be delivered to the grid: i Fdref is the d-axis current reference value of the d-axis DC voltage control output; k p is the proportional coefficient of the PI controller; k i is the integral coefficient of the PI controller; u dc is the actual value of DC voltage; u dcref is the DC voltage reference value; The q-axis Qf droop control is specifically as follows: When the actual frequency ω on the current low-frequency side is less than the given frequency ω0, the Δω output by the phase-locked loop PLL is less than 0; According to Qf droop control, we have: Q ref =k fq (ω-ω0)=k fq See (11) Q ref is the reactive reference value of the q-axis Qf droop control output; k fq is the droop coefficient of the q-axis Qf droop control; Then the output Q ref <0; According to the q-axis outer loop control, we have: Q ref is the reactive reference value of the q-axis Qf droop control output; k fq is the droop coefficient k of the q-axis Qf droop control p is the proportional coefficient of the PI controller; k i is the integral coefficient of the PI controller; s is the Laplace operator; Since Q has not changed, the output reactive current reference value i Fqref Will be in Q ref Increased under the effect of According to the current inner loop control, i Fq Will track me Fqref , so i Fq Increase; According to the reactive power calculation formula: Q=1.5I d U q -1.5I q U d (13) It is determined that the active power Q output by the converter will decrease; According to the reactive-frequency coupling characteristics, the reduction of Q will lead to an increase of f, that is, an increase of ω; The q-axis Qf droop control and the reactive-frequency coupling characteristic jointly construct a negative feedback loop of ω, which can artificially control the frequency ω to achieve ω following the given frequency ω0.

7. A wind turbine configuration grid control method applicable to a DRU-LFAC system according to claim 5, characterized in that: The AC voltage mid-loop control includes capacitor feedforward decoupling and AC voltage dynamic control; The capacitor feedforward decoupling considers the capacitor effect in the d-axis current loop to improve the dynamic characteristics of the voltage, and the AC voltage dynamic control introduces the voltage rating u Fd0 and current feedforward i Fd2 , in order to limit the change range of AC voltage and speed up the response speed, the control equation is: Where: i Fdcref i generated for the DC voltage control loop Fd Reference value, u Fd0 is the rated value of the voltage d-axis component, i Fd2 is the d-axis component of the current flowing into the step-up transformer, u Fd is the actual value of the voltage d-axis component, ω0 is the given frequency, C F is the filter capacitor value of the wind turbine line side converter, u Fq is the actual value of the voltage q-axis component; -ω0C F u Fq Decoupling for the capacitor feedforward.

8. A wind turbine configuration grid control method applicable to a DRU-LFAC system according to claim 5, characterized in that: The current inner loop control is specifically as follows: The current inner loop generates a voltage reference value u Fdref with u Fqref ; Its control equation is:

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 back-to-back converter on shore; The offshore wind farm is black-started by the auxiliary converter, and the wind turbines in the offshore wind farm are controlled to switch between the grid-following mode and the grid-building mode to cooperate with the auxiliary converter to perform the black-start.

10. A black start control method applicable to a DRU-LFAC system according to claim 9, characterized in that: The black start of the offshore wind farm is performed by the auxiliary converter, and the wind turbines of the offshore wind farm are controlled to switch between the grid-following type and the grid-building type to cooperate with the auxiliary converter to perform the black start, including: Phase 1: Start the auxiliary converters on the MMC and DRU sides, and control the voltages on the DC sides of the onshore back-to-back converter and the auxiliary converter to the rated values ​​respectively; at the same time, except for the wind turbines participating in the black start, the grid-side converters of the other wind turbines are all in the cut-off state, and the filters on the AC side of the DRU are also in the cut-off state; 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 works in the grid-following Udc / Q control mode, and synchronizes 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. The third stage: The synchronous generator is connected, the wind turbine side converter is started and works in the MPPT control mode to start the wind turbine; Stage 4: When the wind turbine starts to reverse power to the auxiliary converter, the wind turbine grid-side converter is switched to Qf droop control, and the auxiliary converter on the DRU side is cut off; 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 conduction voltage, causing the DRU to conduct; Phase 5: After the DRU is turned on, power is transmitted through the DRU and the system enters a 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 DRU AC port is connected; the remaining fans that have not yet started are started in turn and output active power until all fans are started.

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