Active support coordination control method for sending end frequency of wind power passing through high-voltage direct-current delivery system

Abstract

The invention relates to a sending end frequency active support coordination control method for wind power through a high-voltage direct-current delivery system, and belongs to the field of frequency stabilization of large-scale wind power through the high-voltage direct-current delivery system. The method comprises the following steps: firstly, according to unbalanced power caused by power regulation margins and disturbance of various frequency modulation resources, analyzing high-voltage direct current system sending end system frequency support requirements in different scenes; secondly, providing a wind power plant multi-time scale frequency modulation control strategy based on the doubly-fed wind turbine generator and a doubly-fed wind turbine generator and high-voltage direct current system frequency support coordination control strategy considering a frequency modulation dead zone; finally, key control parameters in the control link of the doubly-fed wind turbine generator and the high-voltage direct-current system are designed according to the power regulation margin of the synchronous generator set and the wind power plant, so that inertia support and primary frequency modulation effects can be adaptively switched according to different scenes; the quick response capability of the doubly-fed wind turbine generator and the high-voltage direct-current system is exerted, and meanwhile, the power adjusting capability of each unit is fully utilized.

Description

Technical Field

[0001] The present invention relates to the field of frequency stability of a large-scale wind power grid-connected system via a high-voltage direct current system, and in particular to a system frequency active support coordinated control method that takes into account the frequency regulation characteristics of synchronous units, doubly-fed wind turbine units and high-voltage direct current systems, and in particular to a coordinated control method that improves the frequency stability of a large-scale wind power transmission system at the sending end via a high-voltage direct current system, so as to ensure the frequency stability of the AC power grid at the sending end. Background Art

[0002] As a new type of green energy, wind energy has a huge total amount of resources and gradually occupies an important position in the global energy structure. Wind power generation has gradually become the main form of wind energy utilization; the high-voltage direct current system transmission based on the grid-commutated converter has the characteristics of fast flexibility, high controllability, and high economy, and is widely used in long-distance power transmission from large energy bases to load centers. As the proportion of wind power in the sending-end power grid continues to increase, the sending-end AC system presents the characteristics of low inertia level and weak frequency regulation capability. When it is subjected to different degrees of disturbance, if the power regulation margin of the traditional synchronous unit cannot meet the grid frequency regulation requirements, it may lead to problems such as excessive system frequency change rate and frequency over-limit. Therefore, it is urgent to tap the frequency regulation potential of doubly-fed wind turbines and high-voltage direct current systems. Under the coordinated cooperation of multiple frequency regulation resources, they jointly support the system inertia, participate in the frequency stability regulation, and improve the safe and stable operation capability of the sending-end system.

[0003] At present, the doubly-fed wind turbine generator system based on grid-following control mainly uses additional frequency control to enable it to actively support the system frequency. Its control strategies are mainly divided into virtual inertia control, droop control and load reduction control. The HVDC system can change the size of its transmission power through additional frequency control, and then participate in the frequency regulation task of the sending-end AC system. Among them, the use of frequency limit controller and reasonable design of its frequency regulation dead zone can improve the problem of periodic oscillation of system frequency. In terms of the coordination and cooperation between doubly-fed wind turbine generator systems, HVDC systems and other frequency regulation resources, only the goal of different frequency regulation resources jointly supporting the stability of system frequency is achieved, and the coordinated and optimized allocation of inertia support energy and frequency regulation power of traditional synchronous units, wind farms and HVDC systems under different disturbance conditions is not fully considered, and the frequency regulation capacity of each frequency regulation resource is fully utilized while maintaining the stability of system frequency, resulting in problems such as a large disturbance impact range and poor frequency safety and stability of the AC power grid. Summary of the invention

[0004] The purpose of the present invention is to provide a method for actively supporting and coordinating the frequency of the wind power transmission system at the sending end through a high-voltage direct current transmission system, which solves the problems of the existing frequency modulation resources not being fully utilized and the coordination with the wind farm and the high-voltage direct current system in the prior art, resulting in a large disturbance impact range and poor frequency safety and stability of the AC power grid. The present invention first divides the working conditions according to the maximum power regulation margin of the primary frequency modulation of the traditional synchronous unit and the wind farm and the size of the unbalanced power after the disturbance, and analyzes the frequency support demand; secondly, the frequency modulation advantages of the synchronous machine, the double-fed wind turbine unit and the high-voltage direct current system are comprehensively considered, and a multi-time scale frequency modulation control strategy and a frequency support coordination control strategy of the double-fed wind turbine unit and the high-voltage direct current system considering the dead zone of the frequency modulation are designed; and the dead zone value is designed according to the maximum power regulation margin of the synchronous unit and the wind farm. On this basis, according to the limit values ​​of the rotor speed and pitch angle of the double-fed wind turbine unit, the droop control coefficient of the rotor speed control and the additional pitch angle control of the double-fed wind turbine unit is designed, so that the double-fed wind turbine unit presents different frequency modulation characteristics under different frequency regulation requirements, and the frequency of the system is maintained stable while the frequency modulation resources are fully utilized.

[0005] The above-mentioned object of the present invention is achieved by the following technical solutions:

[0006] The method for actively supporting and coordinating the frequency of the wind power transmission system at the sending end through a high voltage direct current transmission system comprises the following steps:

[0007] Step (1) Frequency support demand analysis: According to the power regulation margin of various frequency regulation resources at the sending end of the HVDC system and the unbalanced power division conditions caused by disturbances, the frequency support demand is analyzed, specifically:

[0008] The maximum power regulation margin of primary frequency regulation of traditional synchronous units and wind farms and the unbalanced power after disturbance are divided into three working conditions. Among them, working condition 1 is that the maximum power regulation margin of primary frequency regulation of traditional synchronous units is greater than the unbalanced power; working condition 2 is that the maximum power regulation margin of primary frequency regulation of traditional synchronous units is less than the unbalanced power, and the sum of the maximum power regulation margin of primary frequency regulation of traditional synchronous units and wind farms is greater than the unbalanced power; working condition 3 is that the sum of the maximum power regulation margin of primary frequency regulation of traditional synchronous units and wind farms is less than the unbalanced power, and then analyzes its frequency support demand.

[0009] Step (2) comprehensively considers the response speed and regulation capability differences of various frequency regulation resources, designs a multi-time scale frequency regulation control strategy and a frequency support coordination control strategy for the double-fed wind turbine generator system and the high voltage direct current system considering the frequency regulation dead zone; specifically:

[0010] On the basis of the analysis in step (1), the advantages of variable speed control and variable pitch control of the doubly fed wind turbine are comprehensively considered, and a multi-time scale frequency regulation control strategy is designed. The frequency information is introduced into the rotor speed control link and the additional pitch angle control link of the doubly fed wind turbine, respectively. At the same time, a coordinated control strategy for the frequency support of the doubly fed wind turbine and the high voltage DC system is designed considering the frequency regulation dead zone, so that both have the ability of inertia support and primary frequency regulation, thereby improving the frequency stability of the sending end system.

[0011] The additional control link expression of the rotor speed of the doubly-fed wind turbine is:

[0012]

[0013] Where, f and Δf are the system frequency and frequency deviation respectively; f wwh It is the dead zone of the droop control link in the additional speed control; Δω D is the output of the virtual inertia control link; k wd is the differential coefficient of virtual inertia control; Δω p is the output of the droop control link; Δω max and Δω min They are Δω p The upper and lower limits of k wp1 and k wp2 are the proportional coefficients of the droop control link; ω ref0 is the initial value of the rotor speed reference; ω ref is the current reference value of the rotor speed; ω opt It is the optimal speed value of the doubly-fed wind turbine generator set operating in the maximum power tracking mode, and is also the lower limit of the speed reference value; ω max It is the upper limit of the speed reference value allowed for the doubly-fed wind turbine generator set.

[0014] The expression of additional pitch angle control link of doubly-fed wind turbine is:

[0015]

[0016] In the formula, k β1 and k β2 is the proportional coefficient of the additional pitch angle control of the doubly-fed wind turbine; f wβH is the dead zone value of the additional pitch angle control; Δβ is the output of the additional pitch angle control.

[0017] The expression of additional control link of high voltage DC system is:

[0018]

[0019] In the formula, f dh is the frequency modulation dead zone value of the additional control of the high voltage DC system; ΔP dcis the power regulation of the HVDC system, ΔP dcmax is its upper limit, ΔP dcmin is its lower limit; y i is the state quantity of the DC system, y max and min are the upper and lower limits respectively; k dp is the proportionality coefficient; k di is the integration coefficient.

[0020] Step (3) designs the control parameters in the control link of the doubly-fed wind turbine and the high-voltage direct current system, so that the doubly-fed wind turbine and the high-voltage direct current system can present different frequency response characteristics according to different frequency support requirements, and fully utilize the frequency regulation capability of each unit while maintaining the frequency stability of the sending end system. Specifically: Based on the analysis in step (2), combined with the maximum power regulation margin of the primary frequency regulation of the synchronous unit and the wind farm, the dead zone value in the coordinated control strategy of the doubly-fed wind turbine and the high-voltage direct current system is designed. On this basis, according to the limit value [ω opt ,ω max ] and pitch angle limits [0°, 27°], and designed the proportional coefficient of the doubly fed wind turbine rotor speed droop control link and the additional pitch angle control respectively, realizing the adaptive switching of the inertia support and primary frequency regulation of the doubly fed wind turbine and the high voltage DC system under different scenarios, minimizing the influence range of the sending-end disturbance while ensuring the system stability.

[0021] The dead zone value of the droop control link in the additional speed control of the doubly fed wind turbine. The frequency deviation expression corresponding to the maximum power regulation margin of the synchronous unit primary frequency regulation is:

[0022] Δf g1 =ΔP gmax B p (3-1)

[0023] In the formula, B p is the differential coefficient of the primary frequency regulation of the synchronous unit, ΔP gmax is the maximum power adjustment margin of the primary frequency regulation of the synchronous unit at the sending end, Δf g1 is the frequency deviation corresponding to the maximum power regulation margin of the synchronous unit frequency regulation, that is, the dead zone value f of the droop control link in the additional speed control of the doubly-fed wind turbine unit of the present invention. wwh , that is, f wwh =Δf g1 .

[0024] The dead zone value of the additional pitch angle control of the doubly-fed wind turbine. The frequency deviation expression corresponding to the sum of the maximum power regulation margin of the primary frequency regulation of the synchronous unit and the maximum power regulation margin of the primary frequency regulation reserved by the wind farm based on overspeed load reduction is:

[0025] Δfg2 =(ΔP gmax +ΔP wwmax )B p (3-2)

[0026] In the formula, ΔP wwmax The maximum power regulation margin reserved for primary frequency regulation based on overspeed load reduction in the wind farm, Δf g2 The frequency deviation corresponding to the sum of the maximum power adjustment margins of the synchronous unit and the wind farm reserved for primary frequency regulation based on overspeed load reduction is the dead zone value f of the additional pitch angle control of the doubly fed wind turbine of the present invention. wβh , that is, f wβh =Δf g2 .

[0027] The dead zone value of the additional control of the high-voltage DC system. The frequency deviation expression corresponding to the sum of the maximum power regulation margin of the primary frequency regulation of the synchronous unit and the wind farm is:

[0028]

[0029] In the formula, ΔP wmax The maximum power regulation margin reserved for primary frequency regulation of the wind farm, Δf g+w is the frequency deviation corresponding to the sum of the maximum power regulation margin of the synchronous unit and the total primary frequency regulation of the wind farm, f dh is the dead zone value of the additional control of the high voltage DC system of the present invention. g+w ≤0.5Hz, then f dh =Δf g+w Otherwise, f dh =0.5Hz.

[0030] Within the primary frequency regulation capability of variable speed control, the range of rotor speed of the doubly fed wind turbine is [ω opt ,ω max ], the frequency deviation range is [f wwh , f wβh ](or [-f wβh , -f wwh ]). Δω ref Based on the dead-band frequency deviation (Δf ± f wwh ), in order to make full use of the power regulation margin reserved by speed control, based on the rotor speed reference value ω ref The coupling relationship between the frequency deviation and Δω ref and (Δf ± f wwh ) coupling relationship k wp The value of is shown in formula (4-4).

[0031]

[0032] When the rotor speed reaches ω opt (or ω max ), keep the rotor speed of the doubly fed wind turbine unchanged and start the additional pitch angle control. Within the primary frequency regulation capability of the pitch control, the pitch angle range is [0°, 27°], and the frequency deviation range is [f wβh , f dh ](or [-f dh , -f wβh ]). Analogous to the idea of ​​droop control coefficient design, the coupling relationship k in the additional pitch angle control β Can be defined as

[0033]

[0034] The beneficial effects of the present invention are as follows: in response to the frequency stability problem caused by the transmission of large-scale wind power through the high-voltage direct current system, the present invention proposes a multi-time scale coordinated control strategy for the system frequency supported by a doubly-fed wind turbine, a high-voltage direct current system and a traditional synchronous unit. The doubly-fed wind turbine and the high-voltage direct current system have the ability to adaptively switch between inertia support and primary frequency regulation, which can maintain the stability of the system frequency while fully utilizing the frequency regulation resources. The design of key parameters in the method proposed by the present invention can not only realize the reasonable allocation of frequency regulation power between synchronous units, doubly-fed wind turbines and high-voltage direct current systems, but also can minimize the disturbance range while ensuring the stability of the system frequency, and has strong practicality.

[0035] The present invention comprehensively considers the frequency response characteristics of the doubly-fed wind turbine, the high-voltage direct current system and the traditional synchronous unit, realizes the coordination of the high-voltage direct current system and the synchronous unit, improves the system frequency stability and realizes the rational use of frequency modulation resources. First, according to the power regulation margin of various frequency modulation resources and the unbalanced power caused by disturbance, the frequency support requirements of the high-voltage direct current system sending end in different scenarios are analyzed; secondly, considering the advantages of the variable speed control and variable pitch control of the doubly-fed wind turbine, a multi-time scale frequency modulation control strategy is proposed. On this basis, combined with the advantages of each frequency modulation resource, a frequency support coordination control strategy of the doubly-fed wind turbine and the high-voltage direct current system considering the frequency modulation dead zone is proposed, so that the doubly-fed wind turbine and the high-voltage direct current system both have the ability of inertia support and primary frequency modulation; finally, the key control parameters in the control link of the doubly-fed wind turbine and the high-voltage direct current system are designed, so that it can adaptively switch the inertia support and primary frequency modulation according to different scenarios, and fully utilize the power regulation capabilities of each unit while giving full play to the rapid response capabilities of the doubly-fed wind turbine and the high-voltage direct current system. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The drawings described herein are used to provide further understanding of the present invention and constitute a part of this application. The illustrative examples of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0037] Figure 1 This is a structural diagram of a large-scale wind power transmission system via high voltage direct current according to the present invention;

[0038] Figure 2 The multi-time scale frequency modulation control strategy of the present invention;

[0039] Figure 3 Additional frequency control strategy for the high voltage direct current system of the present invention;

[0040] Figure 4 It is a simulation diagram of the sending end frequency of different disturbance powers under the control mode of the present invention;

[0041] Figure 5 It is a simulation diagram of the output power of the synchronous unit with different disturbance powers under the control mode implemented by the present invention;

[0042] Figure 6 It is a simulation diagram of wind farm output power with different disturbance powers under the control mode implemented by the present invention;

[0043] Figure 7 It is a simulation diagram of the transmission power of a high-voltage direct current system with different disturbance powers under the control mode implemented by the present invention. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0045] See also Figures 1 to 7As shown, the method for actively supporting and coordinating the frequency control at the sending end of a wind power transmission system via a high-voltage direct current system of the present invention belongs to the field of frequency stabilization of a large-scale wind power transmission system via a high-voltage direct current system. The frequency support requirements are analyzed according to the power regulation margin of various frequency regulation resources at the sending end of the high-voltage direct current system and the unbalanced power division conditions caused by disturbances; in addition, a multi-time scale frequency regulation control strategy for wind farms based on doubly-fed wind turbines and a frequency support coordination control strategy for doubly-fed wind turbines and high-voltage direct current systems considering the frequency regulation dead zone are designed by comprehensively considering the response speed and regulation capability differences of each frequency regulation resource; through the reasonable design of control parameters, both the doubly-fed wind turbines and the high-voltage direct current system can present different frequency response characteristics according to different frequency support requirements, and fully utilize the frequency regulation capability of each unit while maintaining the frequency stability of the sending end system. The specific steps are as follows:

[0046] Step 1: Analysis of the structure and frequency support requirements of large-scale wind power transmission via HVDC system:

[0047] (1) Large-scale wind power transmission system structure via high voltage direct current system

[0048] The structure of large-scale wind power transmission system via high voltage direct current system is as follows Figure 1 As shown, the sending-end power generation equipment includes a doubly-fed wind turbine and a traditional synchronous unit; the sending-end converter station of the high-voltage DC system is a rectifier (LCC1), which adopts a constant DC current control, and the receiving-end converter station is an inverter (LCC2), which adopts a constant DC voltage control to maintain the DC voltage stability of the system; Bus1 and Bus2 are the AC buses on the rectifier side and the inverter side, respectively.

[0049] (2) Frequency demand analysis

[0050] For large-scale wind power transmission systems via high-voltage direct current systems, the proportion of power electronic equipment is increasing, and the participation of doubly-fed wind turbines and high-voltage direct current systems in the frequency regulation of the transmission-end power grid is a future development trend. In addition, the unbalanced power generated by different degrees of power disturbance is different. According to the maximum power regulation margin and disturbance power of the primary frequency regulation of traditional synchronous units and wind farms, the frequency support demand is divided into the following three scenarios:

[0051] Scenario 1: If the disturbance power satisfies equation (1-1), that is, the primary frequency regulation power adjustment margin of the traditional synchronous unit is sufficient to absorb the unbalanced power, then the primary frequency regulation task of the sending-end power grid can be undertaken only by the synchronous unit. However, due to the high proportion of power electronic equipment in the sending-end power grid, the primary frequency regulation response speed of the traditional synchronous machine is slow, and the frequency change rate of the sending-end power grid is too large at the initial stage of the disturbance, which may cause the wind power protection to operate, causing the double-fed wind turbine to be disconnected from the grid, threatening the safe and stable operation of the system. Therefore, it is necessary to tap the inertia support potential of the double-fed wind turbine and the high-voltage direct current system, and take advantage of their fast response speed to actively participate in suppressing the frequency change rate and maximum frequency deviation of the sending-end power grid at the initial stage of the disturbance, so as to provide time for the primary frequency regulation response of the traditional synchronous machine. In this scenario, the unbalanced power of the sending-end power grid is borne only by the synchronous unit in the steady state, and the double-fed wind turbine or the high-voltage direct current system only temporarily provides inertia support to suppress the frequency change and maximum frequency deviation of the sending-end power grid.

[0052] ΔP unb ≤ΔP gmax (1-1)

[0053] In the formula, ΔP unb is the disturbance power of the power grid at the sending end; ΔP gmax It is the maximum power regulation margin of primary frequency regulation of traditional synchronous units.

[0054] Scenario 2: If the disturbance power satisfies equation (1-2), that is, the unbalanced power of the power grid at the sending end cannot be completely absorbed by the power regulation margin of the synchronous unit's primary frequency regulation. Considering the need to minimize the impact of the power disturbance on the sending end power grid, the unbalanced power needs to be shared by the synchronous unit and the doubly-fed wind turbine unit. Considering the need to minimize the operating state of the doubly-fed wind turbine unit, after fully utilizing the maximum primary frequency regulation capacity of the synchronous unit, the remaining unbalanced power will be borne by the doubly-fed wind turbine unit. In this scenario, when the power grid at the sending end reaches a steady state, the disturbance power is shared by the synchronous unit and the doubly-fed wind turbine unit. In this process, the doubly-fed wind turbine unit plays the role of inertia support and primary frequency regulation, and the high-voltage DC system can only play the role of inertia support.

[0055] ΔP gmax <ΔP unb ≤ΔP gmax +ΔP wmax (1-2)

[0056] In the formula, ΔP wmax It is the maximum power regulation margin of primary frequency regulation in wind farm.

[0057] Scenario 3: If the disturbance power satisfies equation (1-3), that is, the traditional synchronous units and wind farms at the sending-end power grid have reached the upper limit of the primary frequency regulation capacity and cannot completely absorb the unbalanced power, the high-voltage DC system needs to jointly undertake the frequency regulation task. Since the high-voltage DC system will increase or decrease its steady-state transmission power when participating in the primary frequency regulation task of the sending-end power grid, thereby affecting the receiving-end power grid and expanding the power disturbance range, after fully utilizing the maximum frequency regulation power adjustment margin of the synchronous units and wind farms, the remaining unbalanced power is borne by the high-voltage DC system, maintaining the safe and stable operation of the sending-end power grid while reducing the impact on the receiving-end power grid. In this scenario, when the frequency of the sending-end power grid reaches a steady state, the unbalanced power is jointly borne by the synchronous units, wind farms and high-voltage DC systems. The high-voltage DC system plays the role of inertia support and primary frequency regulation in this process.

[0058] ΔP unb >ΔP gmax +ΔP wmax (1-3)

[0059] Step 2: Coordinated control strategy to improve the frequency stability of large-scale wind power transmission system through high voltage direct current system:

[0060] Combining the frequency regulation capabilities and response speeds of synchronous units, doubly-fed wind turbines and high-voltage DC systems, a frequency support coordination control strategy based on the three is designed to meet the frequency support requirements under the above three scenarios, improve the frequency stability of the system, and make full use of its frequency regulation advantages, optimize the use of frequency regulation resources, and reduce the impact range of disturbances.

[0061] (1) Frequency support control strategy for doubly-fed wind turbines

[0062] In order to enable wind power to adaptively switch according to the size of unbalanced power when it presents inertia support and primary frequency regulation, the following is proposed Figure 2 The control method shown participates in the frequency regulation task of the weak sending-end power grid, and includes three control links: virtual inertia control, droop control, and additional pitch angle control. The design logic of this control method is: after a power disturbance occurs, the frequency change of the sending-end power grid is introduced into the speed reference value through a differential link; after the frequency deviation exceeds its frequency regulation dead zone, the droop control is started, and its adjustment amount is added to the speed reference value; if the rotor speed reaches the optimal speed value or the maximum speed value of the maximum power tracking control operating point, the additional pitch angle control is started, and its adjustment amount is added to the pitch angle. In order to enable the doubly fed wind turbine to quickly support the system inertia, the virtual inertia control link does not have a dead zone; but the droop control and additional pitch angle control set dead zones, making full use of the primary frequency regulation power regulation margin of the traditional synchronous unit, and enabling the doubly fed wind turbine to have the ability to adaptively switch between the inertia support function and the primary frequency regulation function. The principle expression of the additional frequency regulation control strategy of the doubly fed wind turbine is shown in equations (2-1) and (2-2):

[0063]

[0064] Where, f and Δf are the system frequency and frequency deviation respectively; f wwh It is the dead zone of the droop control link in the additional speed control; Δω D is the output of the virtual inertia control link; k wd is the differential coefficient of virtual inertia control; Δω p is the output of the droop control link; Δω max and Δω min They are Δω p The upper and lower limits of k wp1 and k wp2 are the proportional coefficients of the droop control link; ω ref0 is the initial value of the rotor speed reference; ω ref is the current reference value of the rotor speed; ω opt It is the optimal speed value of the doubly-fed wind turbine generator set operating in the maximum power tracking mode, and is also the lower limit of the speed reference value; ω max It is the upper limit of the speed reference value allowed for the doubly-fed wind turbine generator set.

[0065]

[0066] In the formula, k β1 and k β2 is the proportional coefficient of the additional pitch angle control of the doubly-fed wind turbine; f wβH is the dead zone value of the additional pitch angle control; Δβ is the output of the additional pitch angle control.

[0067] (2) Frequency support control strategy of HVDC system

[0068] Frequency support control strategy of HVDC system Figure 3 As shown, its working principle is: after the power disturbance, when the frequency deviation of the sending-end power grid exceeds the dead zone value of the control strategy, the sending-end converter station will change the transmission power of the high-voltage DC system, reduce the unbalanced power after the sending-end power grid disturbance, and suppress the frequency change of the sending-end power grid. Its control principle expression is shown in formula (2-3). In order to make the system operate safely and stably and prevent the sending-end disturbance from affecting the receiving-end power grid as much as possible, the setting of the dead zone value in the frequency support control strategy of the high-voltage DC system must meet the requirements of fully utilizing the primary frequency regulation power adjustment margin of the synchronous unit and the wind farm, as well as the adaptive switching function of the inertia support and primary frequency regulation of the high-voltage DC system.

[0069]

[0070] In the formula, f dh is the frequency modulation dead zone value of the additional control of the high voltage DC system; ΔP dcis the power regulation of the HVDC system, ΔP dcmax is its upper limit, ΔP dcmin is its lower limit; y i is the state quantity of the DC system, y max and min are the upper and lower limits respectively; k dp is the proportionality coefficient; k di is the integration coefficient.

[0071] Step 3: Design of key parameters in the control link of doubly-fed wind turbine and high-voltage DC system:

[0072] After the disturbance, the rotational inertia in the synchronous machine rotor automatically participates in the frequency regulation of the sending-end power grid, that is, the rotor kinetic energy is used to first bear the disturbance power and suppress the frequency change trend. The response time at this stage is in milliseconds. In order to ensure the safe operation of the synchronous machine, the disturbance power borne by its primary frequency regulation is limited to 6% of the rated load. The principle of primary frequency regulation is as shown in formula (3). The power supply equipment participating in the primary frequency regulation of the sending-end power grid includes synchronous units, doubly fed wind turbines and high-voltage DC systems. Since the frequency of the sending-end power grid is determined by the rotor speed of the synchronous unit, the primary frequency regulation power regulation margin of the wind farm can be used as a supplement to the primary frequency regulation power regulation margin of the synchronous unit.

[0073] ΔP G =K G ·Δf (3-1)

[0074] In the formula, K G is the adjustment coefficient of primary frequency modulation, ΔP g The unbalanced power regulated by the primary frequency regulation of the synchronous unit.

[0075] (1) Additional frequency control dead zone value design

[0076] The power regulation margin of the wind farm is the power regulation margin ΔP reserved by speed reduction. ww and the power regulation margin ΔP reserved by pitch load shedding wβ According to the frequency support demand analysis, considering the frequency regulation advantages of synchronous units and doubly fed wind turbines and making full use of the power regulation margin of synchronous units, the dead zone value f of the droop control of doubly fed wind turbines is wwh It is determined by the maximum power adjustment margin of the primary frequency regulation of the synchronous unit, as shown in formula (3-1); based on the control logic of the speed control and pitch control of the doubly fed wind turbine, the dead zone value f of the additional pitch angle control is wβh It is determined by the sum of the maximum power regulation margins reserved for synchronous units and wind farms based on variable speed load reduction, as shown in formula (3-2).

[0077] Δf g1 =ΔPgmax B p (3-2)

[0078] Δf g2 =(ΔP gmax +ΔP wwmax )B p (3-3)

[0079] In the formula, ΔP wwmax is the maximum power regulation margin reserved by the wind farm after speed reduction and load shedding, ΔP wβmax is the maximum power regulation margin reserved by the wind farm after pitch reduction, and ΔP wwmax +ΔP wβmax =ΔP wmax ; Δf g1 is the frequency deviation corresponding to the maximum frequency regulation capability of the synchronous unit, that is, f wwh =Δf g1 ; Δf g2 is the frequency deviation corresponding to the synchronous unit and the primary frequency regulation capability of the wind farm based on variable speed control reaching the upper limit, that is, f wβh =Δf g2 .

[0080] When the unbalanced power generated by the disturbance power of the sending-end grid is too large, the HVDC system must participate in the primary frequency regulation task. In order to make full use of the power regulation margin of the synchronous unit and the wind farm, the dead zone value in the HVDC system additional frequency control strategy is determined by the maximum value of the sum of the primary frequency regulation power regulation margins of the synchronous unit and the wind farm, as shown in formula (3-3):

[0081]

[0082] Where Δf g+w It is the frequency deviation corresponding to the sum of the maximum power regulation margin of the synchronous unit and the wind farm. According to the standard of "Power Quality·Power System Frequency Deviation", if the calculated Δf g+w ≤0.5Hz, then Δf g+w =f dh On the contrary, Δf g+w =0.5Hz, to meet the requirement that the grid frequency deviation limit is ±0.5Hz in steady state.

[0083] (2) Design of additional droop coefficient parameters for doubly-fed wind turbines

[0084] Within the primary frequency regulation capability of variable speed control. The range of rotor speed of the doubly fed wind turbine is [ω opt ,ω max ], the frequency deviation range is [f wwh , f wβh](or [-f wβh , -f wwh ]). Δω ref Based on the dead-band frequency deviation (Δf ± f wwh ), in order to make full use of the power regulation margin reserved by speed control, based on the rotor speed reference value ω ref The coupling relationship between the frequency deviation and Δω ref and (Δf ± f wwh ) coupling relationship k wp The value of is shown in formula (3-4).

[0085]

[0086] When the rotor speed reaches ω opt (or ω max ), keep the rotor speed of the doubly fed wind turbine unchanged and start the additional pitch angle control. Within the primary frequency regulation capability of the pitch control, the pitch angle range is [0°, 27°], and the frequency deviation range is [f wβh , f dh ](or [-f dh , -f wβh ]). Analogous to the idea of ​​droop control coefficient design, the coupling relationship k in the additional pitch angle control β Can be defined as

[0087]

[0088] Example:

[0089] See also Figures 4 to 7 As shown in the figure, after the power disturbance occurs, the frequency of the power grid at the sending end drops rapidly. First, the rotational inertia of the synchronous unit rotor naturally responds to the frequency change, and the doubly fed wind turbine also immediately starts the virtual inertia control link to reduce the rotor speed, release the rotor kinetic energy while increasing the captured mechanical power, and jointly suppress the frequency change rate; then, when the frequency deviation of the power grid at the sending end exceeds the frequency dead zone f of the droop control link of the doubly fed wind turbine, the frequency of the power grid at the sending end drops rapidly. wwh , the droop control is immediately put into use. In the process of the frequency deviation from the dead zone value to its maximum value, the rotor speed decreases, the mechanical power captured by the doubly fed wind turbine increases, and the inertial energy is provided to the sending-end power grid. In this process, if the rotor speed reaches the limit, the additional pitch angle control is started to further increase the output power of the doubly fed wind turbine and suppress the frequency change rate and frequency extreme value of the sending-end power grid. If the unbalanced power is less than the maximum power regulation margin of the synchronous unit, the steady-state frequency deviation will be less than f wwh , the adjustment amount of the droop control of the doubly fed wind turbine is equal to zero, and the doubly fed wind turbine returns to the initial operating state. If the unbalanced power is greater than the maximum power regulation margin of the synchronous unit, the steady-state frequency deviation is greater than f wwh, the doubly-fed wind turbine unit participates in inertia support and primary frequency regulation at the same time; similarly, the frequency deviation of the AC grid at the sending end exceeds the frequency regulation dead zone f of the high-voltage DC system dh After that, the additional frequency regulation control of the high-voltage DC system is immediately put into use, and its reduced active power transmission is for the sending-end power grid to perform inertia support or participate in primary frequency regulation. If the unbalanced power is less than the sum of the maximum power regulation margins of the synchronous unit and the wind farm, the high-voltage DC system participates in frequency change suppression in the dynamic process. When the frequency deviation recovers to within the dead zone value, the additional control output of the high-voltage DC system gradually decreases to zero and returns to the initial operating state. If the unbalanced power is greater than the sum of the maximum power regulation margins of the synchronous unit and the wind farm, the high-voltage DC system bears the remaining unbalanced power of the synchronous unit and the wind farm, and finally stabilizes the frequency deviation at f dh .

[0090] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made to the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for actively supporting and coordinating the frequency of a wind power transmission system at the sending end via a high voltage direct current transmission system, characterized in that: The following steps are involved: Step (1) Frequency support demand analysis: Analyze the frequency support demand according to the power regulation margin of various frequency regulation resources at the sending end of the HVDC system and the unbalanced power division conditions caused by disturbances; Step (2) comprehensively considers the response speed and regulation capability differences of various frequency regulation resources, designs a multi-time scale frequency regulation control strategy and a frequency support coordination control strategy for the doubly-fed wind turbine generator system and the high voltage direct current system considering the frequency regulation dead zone; Step (3) designs the control parameters in the control link of the doubly-fed wind turbine and the high-voltage direct current system according to the maximum power regulation margin of the synchronous unit and the wind farm, so that the doubly-fed wind turbine and the high-voltage direct current system can present different frequency response characteristics according to different frequency support requirements, and fully utilize the frequency regulation capability of each unit while maintaining the frequency stability of the sending-end system.

2. The method for actively supporting and coordinating the frequency of the wind power transmission system at the sending end via the high voltage direct current transmission system according to claim 1 is characterized in that: The frequency support demand analysis described in step (1) is specifically as follows: the maximum power regulation margin of primary frequency regulation of traditional synchronous units and wind farms and the unbalanced power after disturbance are divided into three operating conditions; wherein, operating condition one is that the maximum power regulation margin of primary frequency regulation of traditional synchronous units is greater than the unbalanced power; operating condition two is that the maximum power regulation margin of primary frequency regulation of traditional synchronous units is less than the unbalanced power, and the sum of the maximum power regulation margin of primary frequency regulation of traditional synchronous units and wind farms is greater than the unbalanced power; operating condition three is that the sum of the maximum power regulation margin of primary frequency regulation of traditional synchronous units and wind farms is less than the unbalanced power, thereby analyzing the frequency support demand.

3. The method for actively supporting and coordinating the frequency of the wind power transmission system at the sending end via a high voltage direct current transmission system according to claim 1 is characterized in that: The multi-time scale frequency modulation control strategy described in step (2) and the frequency support coordination control strategy of the doubly-fed wind turbine and the high-voltage direct current system considering the frequency modulation dead zone are specifically: based on the analysis in step (1), the advantages of the variable speed control and variable pitch control of the doubly-fed wind turbine are comprehensively considered, and a multi-time scale frequency modulation control strategy is designed, and the frequency information is introduced into the rotor speed control link and the additional pitch angle control link of the doubly-fed wind turbine respectively. At the same time, a frequency support coordination control strategy of the doubly-fed wind turbine and the high-voltage direct current system considering the frequency modulation dead zone is designed, so that both have the ability of inertia support and primary frequency modulation, thereby improving the frequency stability of the sending end system; The additional control link expression of the rotor speed of the doubly-fed wind turbine is: Where, f and Δf are the system frequency and frequency deviation respectively; f wwh It is the dead zone of the droop control link in the additional speed control; Δω D is the output of the virtual inertia control link; k wd is the differential coefficient of virtual inertia control; Δω p is the output of the droop control link; Δω max and Δω min They are Δω p The upper and lower limits of k wp1 and k wp2 are the proportional coefficients of the droop control link; ω ref0 is the initial value of the rotor speed reference; ω ref is the current reference value of the rotor speed; ω opt It is the optimal speed value of the doubly-fed wind turbine generator set operating in the maximum power tracking mode, and is also the lower limit of the speed reference value; ω max It is the upper limit of the speed reference value allowed for the doubly-fed wind turbine generator set; The expression of additional pitch angle control link of doubly-fed wind turbine is: In the formula, k β1 and k β2 is the proportional coefficient of the additional pitch angle control of the doubly-fed wind turbine; f wβH is the dead zone value of the additional pitch angle control; Δβ is the output of the additional pitch angle control; The expression of additional control link of high voltage DC system is: In the formula, f dh is the frequency modulation dead zone value of the additional control of the high voltage DC system; ΔP dc is the power regulation of the HVDC system, ΔP dcmax is its upper limit, ΔP dcmin is its lower limit; y i is the state quantity of the DC system, y max and min are the upper and lower limits respectively; k dp is the proportionality coefficient; k di is the integration coefficient.

4. The method for actively supporting and coordinating the frequency of the wind power transmission system at the sending end via a high voltage direct current transmission system according to claim 1 is characterized in that: The control parameters in the control link of the doubly-fed wind turbine and the high-voltage direct current system are designed in step (3), specifically: on the basis of the analysis in step (2), combined with the maximum power regulation margin of the primary frequency regulation of the synchronous unit and the wind farm, the dead zone value in the coordinated control strategy of the doubly-fed wind turbine and the high-voltage direct current system is designed, and on this basis, according to the limit value [ω opt ,ω max ] and pitch angle limit [0°, 27°], respectively, the proportional coefficient of the doubly-fed wind turbine rotor speed droop control link and the additional pitch angle control is designed, and the adaptive switching of the inertia support and primary frequency regulation of the doubly-fed wind turbine and the high-voltage DC system in different scenarios is realized, which reduces the influence range of the sending-end disturbance while ensuring the stability of the system; The dead zone value of the droop control link in the additional speed control of the doubly-fed wind turbine; the frequency deviation expression corresponding to the maximum power regulation margin of the synchronous unit primary frequency regulation: Δf g1 =ΔP gmax B p (3-1) In the formula, B p is the differential coefficient of the primary frequency regulation of the synchronous unit, ΔP gmax is the maximum power adjustment margin of the primary frequency regulation of the synchronous unit at the sending end, Δf g1 is the frequency deviation corresponding to the maximum power regulation margin of the synchronous unit frequency regulation, that is, the dead zone value f of the droop control link in the additional speed control of the doubly fed wind turbine unit wwh , that is, f wwh =Δf g1 ; Dead zone value of additional pitch angle control of doubly-fed wind turbine: The frequency deviation expression corresponding to the sum of the maximum power regulation margin of primary frequency regulation of synchronous unit and the maximum power regulation margin of primary frequency regulation reserved by wind farm based on overspeed load reduction: Δf g2 =(ΔP gmax +ΔP wwmax )B p (3-2) Where ΔP wwmax The maximum power regulation margin reserved for primary frequency regulation based on overspeed load reduction in the wind farm, Δf g2 The frequency deviation corresponding to the sum of the maximum power regulation margin of the synchronous unit and the wind farm reserved for overspeed load reduction is the dead zone value f of the additional pitch angle control of the doubly fed wind turbine unit. wβh , that is, f wβh =Δf g2 ; Dead zone value of additional control of HVDC system: The frequency deviation expression corresponding to the sum of the maximum power regulation margin of the primary frequency regulation of the synchronous unit and the wind farm is: Where ΔP wmax The maximum power regulation margin reserved for primary frequency regulation of the wind farm, Δf g+w is the frequency deviation corresponding to the sum of the maximum power regulation margin of the synchronous unit and the total primary frequency regulation of the wind farm, f dh is the dead zone value of the additional control of the high voltage DC system; if Δf g+w ≤0.5Hz, then f dh =Δf g+w Otherwise, f dh =0.5Hz; Within the primary frequency regulation capability of variable speed control, the range of rotor speed of the doubly fed wind turbine is [ω opt ,ω max ], the frequency deviation range is [f wwh , f wβh ] or [-f wβh , -f wwh ]; Δω ref Based on the dead-band frequency deviation (Δf ± f wwh ) is proportional to the rotor speed reference value ω ref The coupling relationship between the frequency deviation and Δω ref and (Δf ± f wwh ) coupling relationship k wp The value of is formula (3-4): When the rotor speed reaches ω opt or max After that, the rotor speed of the doubly-fed wind turbine is kept unchanged, and the additional pitch angle control is started; within the primary frequency regulation capability of the pitch control, the pitch angle range is [0°, 27°], and the frequency deviation range is [f wβh , f dh ] or [-f dh , -f wβh ]; Coupling relationship k in additional pitch angle control β Defined as:

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