A method for coordinated control of wind turbine generators and multi-terminal flexible transmission system considering frequency recovery

Abstract

The application belongs to the field of power transmission and distribution, and provides a wind turbine and multi-terminal flexible AC / DC system coordinated control method considering frequency recovery. The method reflects frequency deviation to DC voltage by using adaptive virtual inertia control, coordinates power distribution in the power grid, supports power provided by the power grid connected to each receiving terminal converter station, and if the DC voltage deviation exceeds the safety constraint, the sending terminal converter station starts the additional frequency control, the wind turbine participates in frequency modulation, in the initial stage of frequency disturbance, the virtual inertia coefficient is kept at a certain virtual inertia coefficient to achieve the effect of frequency support, and in the frequency recovery stage, the control strategy is switched, the virtual inertia time coefficient gradually tends to zero, thereby reducing the influence of inertia control on frequency recovery, and achieving the effect of accelerating frequency recovery.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of power transmission and distribution, and particularly relates to a wind turbine and multi-terminal flexible system coordinated control method considering frequency recovery. BACKGROUND

[0002] As a kind of wind power, offshore wind power has the advantages of renewable and abundant resources, but with the increasing penetration of wind power, due to the influence of the scattered distribution of offshore wind resources, the traditional double-terminal point-to-point power transmission system cannot meet the demand of large-scale offshore wind power grid connection, and the multi-terminal DC power transmission system for offshore wind power grid connection has become a trend at home and abroad. Due to the decoupling effect of the flexible system, when the frequency disturbance occurs in the onshore AC system, the offshore wind farm cannot respond to the frequency change of the onshore power grid, and the onshore AC power grids cannot support the frequency.

[0003] To solve the frequency stability problem of wind power through multi-terminal flexible system grid connection, the commonly used method at present is to add frequency control on the basis of droop control of multi-terminal flexible system, which is characterized by superimposing the frequency deviation on the reference power through fixed frequency droop coefficient, so that the frequency signal is transmitted to the flexible system through the DC voltage. However, the fixed frequency droop coefficient cannot be adjusted according to the system operating state, and if the parameter selection is not appropriate, it will cause the frequency deviation of the normal side power grid to be too large during the support power process, and at the same time, the DC voltage deviation of the system will be too large. In addition, the sending end converter station obtains the frequency change of the disturbance side receiving end power grid through the DC voltage of the flexible system, so that the wind turbine adjusts the operating state to participate in frequency modulation by releasing the rotor kinetic energy, which is characterized by introducing the frequency change rate into the fixed power control link through virtual inertia control to simulate the inertia response of synchronous generator. However, in the initial stage of frequency disturbance, the larger inertia coefficient provides more inertia, which hinders the effect of frequency change, but in the frequency recovery stage, the larger inertia coefficient prolongs the frequency recovery time, so the virtual inertia coefficient of the wind turbine should be adjusted in real time according to different time periods. SUMMARY

[0004] The present application aims to solve the problems existing in the prior art and ensure that when the onshore power grid fails, the offshore wind power multi-terminal flexible system can avoid damage caused by excessive DC voltage deviation due to frequency disturbance, and in the frequency recovery stage, the inertia control can reduce the influence on frequency recovery, thereby achieving the effect of accelerating frequency recovery.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows.

[0006] A kind of wind turbine and the coordinated control method of multi-terminal flexible system considering frequency recovery, this method is applied to wind power through multi-terminal flexible grid-connected system, this method includes a wind farm WPP, a sending terminal converter station MMC4, three receiving terminal converter stations are receiving terminal converter station MMC1, receiving terminal converter station MMC2, receiving terminal converter station MMC3, three AC power grids are AC power grid S1, AC power grid S2, AC power grid S3;The electric energy sent by wind farm WPP is sent into sending terminal converter station MMC4 by booster transformer, sending terminal converter station MMC4 converts the electric energy transported into direct current by rectification, then the electric energy is transported to receiving terminal converter station MMC1, receiving terminal converter station MMC2 and receiving terminal converter station MMC3 by direct current cable, receiving terminal converter station MMC1, receiving terminal converter station MMC2 and receiving terminal converter station MMC3 convert the electric energy transported into alternating current, and are transported to AC power grid S1, AC power grid S2 and AC power grid S3.

[0007] The method comprises the following steps:

[0008] Step 1: detect the frequency change of each converter station, judge whether the frequency deviation exceeds the trigger threshold T, if yes, enter step 2, if no, the frequency control is not started, and the frequency change is continuously detected.

[0009] Step 2: when the frequency deviation exceeds the set threshold, the frequency deviation is reflected to the direct current voltage by using adaptive virtual inertia control, the power distribution in the power grid is coordinated by the droop control of adaptive reference power, and the direct current voltage deviation is checked at the same time.

[0010] Step 3: judge whether the direct current voltage deviation is within the safe operation range, if yes, the support power is provided by the power grid connected by each receiving terminal converter station, and the wind turbine does not participate in frequency regulation, if not, enter step 4.

[0011] Step 4: when the direct current voltage exceeds the safety constraint, the sending terminal converter station additional frequency control is started, the wind power participates in frequency regulation, until the direct current voltage meets the safety constraint, and the control is ended.

[0012] The beneficial effects of the wind turbine and the coordinated control method of multi-terminal flexible system considering frequency recovery of the application are as follows:

[0013] The wind turbine and multi-terminal flexible direct system coordinated control method considering frequency recovery is constructed by the application, the frequency deviation is reflected to the direct current voltage by using adaptive virtual inertia control, and the power distribution in the power grid is coordinated, the power is provided by the power grid connected to each receiving terminal converter station, if the direct current voltage deviation exceeds the safety constraint, the sending terminal converter station additional frequency control is started, the wind turbine participates in frequency modulation, in the initial stage of frequency disturbance, the virtual inertia coefficient is kept to a certain virtual inertia coefficient to achieve the effect of frequency support, in the frequency recovery stage, the control strategy is switched, the virtual inertia time coefficient gradually tends to zero, thereby reducing the influence of inertia control on frequency recovery, and the effect of accelerating frequency recovery is achieved. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 It is a schematic diagram of wind power through MMC-MTDC grid connection structure.

[0015] Figure 2 It is a flow chart of the wind turbine and multi-terminal flexible direct system coordinated control method considering frequency recovery of the embodiment of the application.

[0016] Figure 3 It is a control strategy diagram of the receiving terminal converter station of the embodiment of the application.

[0017] Figure 4 It is a control strategy diagram of the sending terminal converter station of the embodiment of the application.

[0018] Figure 5 It is a block diagram of the wind turbine additional frequency control considering frequency recovery of the embodiment of the application.

[0019] Figure 6 It is a system direct current voltage simulation diagram when the converter station MMC1 exits operation of the embodiment of the application.

[0020] Figure 7 It is a wind power output power diagram when the converter station MMC1 exits operation of the embodiment of the application.

[0021] Figure 8 It is an S2 side alternating current power grid frequency diagram when the converter station MMC1 exits operation of the embodiment of the application.

[0022] Figure 9 It is an S3 side alternating current power grid frequency diagram when the converter station MMC1 exits operation of the embodiment of the application. DETAILED DESCRIPTION

[0023] The technical solutions of the application will be further specifically described below by embodiments, and in combination with the drawings.

[0024] The embodiment of the application provides a wind turbine generator and multi-terminal flexible system coordinated control method considering frequency recovery, which is applied to wind power through a multi-terminal flexible grid connection system, and comprises a wind farm WPP, a sending terminal converter station MMC4, three receiving terminal converter stations MMC1, MMC2 and MMC3, and three alternating current grids S1, S2 and S3; the wind farm WPP sends electric energy to the sending terminal converter station MMC4 through a step-up transformer, the sending terminal converter station MMC4 converts the transmitted electric energy into direct current through rectification, and then transmits the electric energy to the receiving terminal converter stations MMC1, MMC2 and MMC3 through a direct current cable, the receiving terminal converter stations MMC1, MMC2 and MMC3 convert the transmitted electric energy into alternating current and transmit the alternating current to the alternating current grids S1, S2 and S3, and the structure is as shown in Figure 1 .

[0025] The method comprises the following steps, and the overall coordinated control method flow chart is as shown in Figure 2 .

[0026] Step 1: Detect the frequency change of each converter station, judge whether the frequency deviation exceeds the trigger threshold T, if yes, enter step 2, if not, the frequency control is not started, and the frequency change is continuously detected.

[0027] Step 2: When the frequency deviation exceeds the set threshold, the frequency deviation is reflected to the direct current voltage by using the adaptive virtual inertia control, the power distribution in the power grid is coordinated by the droop control of the adaptive reference power, and the direct current voltage deviation is checked at the same time.

[0028] The control strategy diagram of the controller of the receiving terminal converter station is as shown in Figure 3 , and the following steps are adopted when it is working.

[0029] The frequency deviation is superimposed on the reference power through a fixed coefficient by adding a frequency control link; the active power droop can be expressed as:

[0030]

[0031] In the formula, U dc.ref and U dc are the reference value and the measured value of the direct current voltage of the system, K v and K f are the voltage droop coefficient and the frequency droop coefficient respectively, P ref and P are the reference power and the measured power of the direct current side of the converter station, and f ref and f are the frequency reference value and the measured value.

[0032] The frequency variation is coupled with the direct current voltage, and the frequency variation of the single receiving end power grid is spread to the whole HVDC system through the direct current voltage.

[0033] Step 3: whether the direct current voltage deviation is in the safe operation range, if yes, the support power is provided by the power grid connected by each receiving end converter station, the wind turbine does not participate in frequency modulation, if not, step 4 is entered.

[0034] Step 4: when the direct current voltage exceeds the safety constraint, the sending end converter station additional frequency control is started, the wind power participates in frequency modulation, until the direct current voltage meets the safety constraint, and the control is ended.

[0035] The control strategy diagram of the controller of the sending end converter station MMC4 is as shown in Figure 4 When it works, the following steps are adopted:

[0036] The sending end converter station adopts voltage / frequency control, and the deviation of the direct current voltage from the rated value is multiplied by a coefficient N dc The conversion is converted into the frequency variation of the alternating current system, so that the wind turbine can feel the frequency variation of the land grid, and the relationship can be expressed as:

[0037]

[0038] In the formula, U dc.ref and U dc are the direct current voltage reference value and the measured value of the system, f ref and f are the frequency reference value and the measured value, N dc is the conversion coefficient.

[0039] The additional frequency control of the wind turbine considering frequency recovery is as shown in Figure 5 In the frequency recovery stage, the following steps are adopted:

[0040] Since the sending end converter station adopts frequency voltage control, the direct current voltage signal transmitted through the HVDC system is converted into a frequency signal and reflected to the offshore wind farm, and the wind turbine can release the rotor kinetic energy through the additional frequency control, and the traditional virtual inertia control is not flexible enough in frequency regulation, and a large virtual inertia coefficient during frequency recovery is easy to hinder the frequency recovery, so the adaptive inertia coefficient is set on the basis of the traditional virtual inertia control, so as to reduce the influence on the frequency recovery.

[0041] According to aerodynamics, the electromagnetic reference power of the wind turbine under maximum wind power tracking is:

[0042]

[0043] In the formula, p is air density, R is the radius of the wind turbine blade, v w is the wind speed, and λ optTip speed ratio, w0 is the rotor speed of the wind turbine, Cp is the maximum wind energy utilization coefficient of the wind turbine; k opt Cp is the maximum wind energy utilization coefficient of the wind turbine; k

[0044] The rotor speed of the wind turbine can also reflect the relationship between the electromagnetic power and the mechanical power thereof:

[0045]

[0046] In the formula, H W is the inertia time constant of the wind turbine, P wind and P out are the mechanical power and the electromagnetic power of the wind turbine, respectively.

[0047] When the frequency disturbance occurs in the receiving end power grid, the wind turbine releases the rotor kinetic energy to provide power support by adjusting the rotor speed, and when the rotor speed changes from w0 to w1, the rotor kinetic energy released by the wind turbine is:

[0048]

[0049] As can be seen from formula (5), the value of H W determines the size of the power support provided by the wind turbine, the greater H W is, the more power support the wind turbine can provide, and the stronger the frequency support effect is. However, the rate of frequency recovery is also affected by the value of H W . Therefore, a suitable H W needs to be selected during the frequency disturbance period and the frequency recovery period.

[0050] During the frequency disturbance stage, the frequency change rate and the frequency deviation satisfy and the frequency deviation gradually increases from 0, at this time, a larger virtual inertia coefficient can be taken to provide more power support for the flexible system, and the hyperbolic tangent function as a common S-shaped function changes more smoothly compared with a simple proportional relationship, and the control effect is better. According to the change trend of H W , an adaptive virtual inertia coefficient is established:

[0051]

[0052] During the frequency recovery stage, the frequency change rate and the frequency deviation satisfy The frequency deviation gradually decreases, in order to reduce the influence of the inertia coefficient on the frequency recovery, the virtual inertia coefficient should gradually decrease, and the adaptive virtual inertia coefficient obtained by using the hyperbolic tangent function is:

[0053]

[0054] Integrating formula (6) and formula (7) can obtain that the virtual inertia coefficient is:

[0055]

[0056] The embodiment of the application provides a wind turbine and multi-terminal flexible system coordinated control method considering frequency recovery, which comprises the following steps:

[0057] The method can avoid damage caused by the fact that the offshore wind power multi-terminal flexible system is disturbed by frequency and the DC voltage deviates too much due to the AC fault of the onshore power grid, and can reduce the influence of inertia control on frequency recovery in the frequency recovery stage, thereby accelerating the frequency recovery.

[0058] In order to verify the effectiveness of the method, a simulation model of a wind power MMC-MTDC system connected to a power grid is built in PSCAD / EMTDC as shown in the figure. Figure 1 The parameters are shown in the following table.

[0059] Parameter Value Rated DC voltage / kV 400 MMC1 ~ MMC4 rated capacity / MVA 200、600、500、1000 MMC1 ~ MMC3 initial set power / MW -100、-400、-300 MMC1 ~ MMC3 voltage droop coefficient / p.u. 0.5、0.33、0.4 Single bridge arm sub-module quantity 200 DC cable resistance / Ω 3 Each sub-module capacitance value / mF 10

[0060] The disturbance is set as follows: at t=4.5s, the converter station MMC1 is out of operation.

[0061] Under the steady state operation condition, the active power of the converter station MMC1 is 100MW, and the converter station is out of operation due to the fault at 4.5s, which is equivalent to a sudden increase of 100MW of active power in the DC system. Figure 6 It can be seen that, under the traditional control, the DC voltage is greatly increased, and the deviation reaches 31kV, which exceeds 1.05p.u. Figure 8 It can be seen that the frequency of the AC system S2 changes greatly because of the frequency support, and the peak value of the frequency is 50.56Hz at 8.7s, which exceeds the safe operation range of the frequency.

[0062] Under the proposed coordinated control, because the DC voltage of the system exceeds the set voltage dead zone, the sending end converter station responds to the frequency signal and transmits it to the wind turbine to participate in frequency modulation. Figure 7 It can be seen that the output power is reduced from 800MW to 760MW, thereby reducing the unbalanced power in the DC system, reducing the DC voltage deviation, reducing the frequency modulation pressure of the converter stations MMC2 and MMC3, and reducing the frequency peak value of the normal side AC system S2 from 50.56Hz to 50.46Hz, thereby maintaining the frequency in the safe operation range and improving the stability of the system.

[0063] It can be seen from Figure 8 and Figure 9It can be known that, under the control strategy designed in the application, the proposed coordinated control and the fixed inertia coefficient control are both in the time of about 8.7s when the frequency reaches the peak value, and the time of reaching the maximum deviation is basically consistent under the two control modes, and in the frequency recovery stage, the fan inertia coefficient gradually decreases under the control of the application, so that the frequency recovery rate is improved compared with the traditional fixed coefficient.

[0064] The contents not described in detail in the specification belong to the prior art known to the person skilled in the art.

[0065] Those skilled in the art can understand that the above description is to explain the technical features of the application. Several improvements and modifications made without departing from the application are also protected by the application, so the protection scope of the application should be based on the content defined in the claims of the application.

Claims

1. A coordinated control method for wind turbines and multi-terminal flexible direct current systems considering frequency recovery, characterized by: The method is applied to a wind power multi-terminal flexible direct current system, which includes a wind farm WPP, a sending-end converter station MMC4, three receiving-end converter stations, namely, receiving-end converter station MMC1, receiving-end converter station MMC2, and receiving-end converter station MMC3, and three AC grids, namely, AC grid S1, AC grid S2, and AC grid S3; the electric energy generated by the wind farm WPP is sent to the sending-end converter station MMC4 through a step-up transformer; the sending-end converter station MMC4 converts the transmitted electric energy into direct current through rectification, and then transmits the electric energy to the receiving-end converter station MMC1, receiving-end converter station MMC2, and receiving-end converter station MMC3 through DC cables; the receiving-end converter stations MMC1, receiving-end converter station MMC2, and receiving-end converter station MMC3 convert the transmitted electric energy into alternating current, and transmit it to the AC grids S1, S2, and S3; The method comprises the following steps: Step 1: Detect the frequency change of each converter station and determine whether the frequency deviation exceeds the trigger threshold T. If so, proceed to step 2. If not, the frequency modulation control is not started and the frequency change detection continues; Step 2: When the frequency deviation exceeds the set threshold, adaptive virtual inertia control is used to reflect the frequency deviation to the DC voltage. Adaptive reference power droop control is used to coordinate power distribution within the grid while checking the DC voltage deviation. Step 3: Determine whether the DC voltage deviation is within the safe operating range. If so, the support power is provided by the grid connected to each receiving converter station, and the wind turbine does not participate in frequency regulation. If not, proceed to step 4. Step 4: When the DC voltage exceeds the safety constraint, additional frequency control at the sending-end converter station is initiated, and wind power participates in frequency regulation until the DC voltage meets the safety constraint, at which point control ends. Considering the additional frequency control of wind turbines for frequency recovery, the following steps are taken during the frequency recovery phase: Step Q1: When the frequency disturbance occurs in the onshore power grid, according to the frequency deviation Δf and the frequency change rate Determine the period of system frequency. In the frequency disturbance stage, the frequency change rate and frequency deviation meet the requirements. The frequency deviation gradually increases from 0. At this time, a larger virtual inertia coefficient should be adopted to provide more power support for the flexible DC system. In the frequency recovery stage, the frequency change rate and frequency deviation meet the requirements. At this time, the frequency deviation gradually decreases, and a larger virtual inertia coefficient will hinder frequency recovery. In order to reduce the impact of the inertia coefficient on frequency recovery, a smaller virtual inertia coefficient needs to be adopted; Step Q2: Use the hyperbolic tangent function to calculate the virtual inertia coefficient H at different periods W Adjustment is made. As a common S-shaped function, the hyperbolic tangent function has a smoother change and better control effect than the simple proportional relationship. W The changing trend of , during the frequency disturbance period, the adaptive virtual inertia coefficient established is as follows: In the frequency recovery stage, the adaptive virtual inertia coefficient obtained by using the hyperbolic tangent function is: It can be concluded that during the frequency change process, the virtual inertia coefficient is:

2. The coordinated control method of wind turbines and multi-terminal flexible direct current systems considering frequency recovery according to claim 1 is characterized in that: In step 2, the controller of the receiving converter station takes the following steps when in operation: Through the additional frequency control link, the frequency deviation is superimposed on the reference power through a fixed coefficient; the active power droop is expressed as: Where U dc.ref and U dc is the DC voltage reference value and measured value of the system, K v and K f are the voltage droop coefficient and frequency droop coefficient respectively, P ref and P are the reference power and measured power of the DC side of the converter station, f ref and f are the reference and measured values ​​of frequency; The frequency change is coupled with the DC voltage, and the frequency change of a single receiving-end power grid is diffused to the entire flexible DC system through the DC voltage.

3. The coordinated control method of wind turbines and multi-terminal flexible direct current systems considering frequency recovery according to claim 1 is characterized in that: In step 4, when the controller of the sending-end converter station is operating, the following steps are adopted: The sending end converter station adopts voltage / frequency control, multiplying the deviation of DC voltage from the rated value by the coefficient N dc Converted into AC system frequency changes, the wind turbine can sense the changes in onshore grid frequency. The relationship can be expressed as: Where: U dc.ref and U dc is the DC voltage reference value and measured value of the system, f ref and f are the reference and measured values ​​of frequency, N dc is the conversion factor.

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