Double-fed wind power plant grid-connected system subsynchronous oscillation analysis method based on complex torque coefficient method

Abstract

The invention discloses a double-fed wind power plant grid-connected system subsynchronous oscillation analysis method based on a complex torque coefficient method. Double-fed wind power plant grid connection has the risk of causing subsynchronous oscillation of a system, and meanwhile, the subsynchronous oscillation characteristic of a thermal power generating unit in the system is possibly influenced. The method comprises the following steps: firstly, dividing each generator set in a system into a to-be-researched generator set and a non-researched generator set, and giving a complex frequency domain model general form of the to-be-researched generator set and the non-to-be-researched generator set suitable for a complex torque coefficient method; secondly, a general form complex frequency domain model of a traditional thermal power generating unit suitable for subsynchronous oscillation analysis of the multi-machine system is given, and a general form complex frequency domain model of a double-fed fan is established; and finally, solving a complex torque coefficient by combining the established generator complex frequency domain model with a system complex frequency domain network equation, and further analyzing the subsynchronous oscillation of the double-fed wind power plant grid-connected system.

Description

Technical Field

[0001] The present invention relates to the technical field of power system analysis, and in particular to a subsynchronous oscillation analysis method of a double-fed wind farm grid-connected system based on a complex torque coefficient method. Background Art

[0002] At present, with the rapid development of the economy and society, energy consumption and environmental pollution are increasingly becoming common problems faced by the world and urgently need to be solved. Under the guidance of my country's "dual carbon" strategic goal, my country is in the rising period of transformation from the traditional power system to a new power system with a high proportion of new energy power generation and a high proportion of power electronic equipment. New energy represented by wind power is developing rapidly. According to statistics from the National Energy Administration, as of the end of December 2022, my country's wind turbine installed capacity is about 370 million kilowatts, an increase of 11.2% year-on-year. Among different types of generators, doubly fed wind turbines are widely used due to their low cost, flexible control methods, and high wind energy utilization rate.

[0003] At the same time, the subsynchronous oscillation phenomenon caused by the grid-connected doubly-fed wind farm has gradually become prominent. From 2012 to 2013, the Guyuan wind power-series compensation transmission system in Hebei Province, my country, had subsynchronous oscillations many times, causing many wind turbines to be disconnected from the grid. The reason was the subsynchronous control interaction (SSCI) caused by the interaction between the converter and the series compensation capacitor of the doubly-fed wind turbine unit; in 2015, the wind farm in Hami, Xinjiang, my country, had subsynchronous oscillations, which in turn caused dangerous subsynchronous torsional vibrations in the thermal power unit 300 kilometers away, causing the shaft torsional vibration protection to cause the thermal power unit to shut down. Therefore, it is of great value to analyze the subsynchronous oscillation of the grid-connected doubly-fed wind farm system.

[0004] As one of the main methods for analyzing subsynchronous oscillations, the complex torque coefficient method has clear physical meaning and can avoid the problem of "dimensionality curse" when modeling. However, the application of this method in complex systems is mostly concentrated on the research of traditional thermal power units, and its application in high-proportion new energy and high-proportion power electronic systems still needs further research. Therefore, it is of great significance to establish a subsynchronous oscillation analysis method for doubly fed wind farm grid-connected systems based on the complex torque coefficient method. Summary of the invention

[0005] The technical problem to be solved by the present invention is that, in view of the fact that the subsynchronous oscillation analysis of multi-machine systems based on the complex torque coefficient method was previously mostly used in the research of thermal power units, in today's scenario where the rapid development of the "dual-high" new power system has led to the prone occurrence of subsynchronous oscillations in the grid-connected system of doubly fed wind farms, a subsynchronous oscillation analysis method for the grid-connected system of doubly fed wind farms based on the complex torque coefficient method is studied.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A subsynchronous oscillation analysis method for a doubly-fed wind farm grid-connected system based on a complex torque coefficient method comprises the following steps:

[0008] Step 1: Divide the generator sets in the system into generator sets to be studied and generator sets not to be studied, and give the general form of the complex frequency domain model of the generator sets to be studied and the generator sets not to be studied that are suitable for the complex torque coefficient method, including: dividing the system generator sets, numbering the nodes of the generator sets to be studied as 1, and numbering the remaining generator nodes from 2 to n in sequence, and giving the general form of the complex frequency domain model with the generator node voltage as the output and the current and speed as the input.

[0009] The complex torque coefficient of the generator is divided into mechanical complex torque coefficient and electrical complex torque coefficient. The mechanical complex torque coefficient can be directly calculated from the shaft system equation of the mechanical part. The solution process is relatively simple and will not be repeated here. The solution of the electrical complex torque coefficient requires the connection between the generator and the electrical part of the system network. The solution process is relatively complicated. Here, a group of complex frequency domain models of the generator set that are convenient for solving the electrical complex torque coefficient are given, as described below.

[0010] The general forms of the complex frequency domain models of the generator set to be studied and the non-generator set to be studied are as follows: Figure 2 The structures are shown in the red box and the blue box.

[0011] The electrical complex torque coefficient of the generator to be studied is only related to the electrical part, and its general form of complex frequency domain model can be expressed as:

[0012]

[0013] Where: Δu Gdq1 , Δi Gdq1 are the node voltage and current of the generator to be studied respectively; Δω r1 is the speed of the generator set to be studied; ΔT e1 is the electromagnetic torque of the generator set to be studied; Z1(s), D1(s) and α1(s) are the corresponding coefficient matrices when deriving the general form of the complex frequency domain model of the generator set to be studied.

[0014] When obtaining the electrical complex torque coefficient of the generator set, it is hoped that the influence of the mechanical parts of other generator sets will be considered. Therefore, based on the complex frequency domain model of the generator set to be studied, the mechanical parts of the generator sets not to be studied need to be taken into account. Taking the generator set numbered 2 as an example, the general form of its complex frequency domain model can be expressed as:

[0015]

[0016] Where: Δu Gdq2 , Δi Gdq2 are the node voltage and current of the non-studied generator respectively; Δωr2 is the speed of the generator set not to be studied; ΔT e2 is the electromagnetic torque of the generator set not to be studied; ΔT m2 is the mechanical torque of the generator set not to be studied; Z2(s), D2(s), α2(s) and K M2 (s) are the corresponding coefficient matrices when deriving the general form of the complex frequency domain model of the generator set not to be studied.

[0017] Step 2: Give a general form of complex frequency domain model of traditional thermal power units suitable for subsynchronous oscillation analysis of multi-machine systems, and establish a general form of complex frequency domain model of doubly fed wind turbines, including: establishing a general form of complex frequency domain model of generator sets suitable for subsynchronous oscillation analysis of multi-machine systems. For thermal power units, directly give the general form of their complex frequency domain model; for doubly fed wind turbines (DFIGs), respectively establish small signal models of stator and rotor flux, rotor voltage, RSC control circuit, MPPT control, stator voltage, power and electromagnetic torque of doubly fed wind turbines, and then establish a small signal connection diagram of doubly fed wind turbines based on the connection relationship between the small signal models of each part, and then obtain the complex frequency domain model when DFIG is the generator set to be studied, and then combine the small signal model after the linearization of the DFIG mechanical system shaft system equation to construct the complex frequency domain model when DFIG is not a generator set to be studied.

[0018] The general form of the complex frequency domain model of a thermal power unit can be expressed as:

[0019] When the thermal power unit is the power generating unit to be studied:

[0020]

[0021] in:

[0022]

[0023] Where: r a is the generator armature resistance; ψ d0 , q0 are the steady-state values ​​of the dq-axis components of the flux linkage, respectively; T d '、T q ' is the transient short-circuit time constant; T d0 '、T q0 ' is the transient open circuit time constant; T d ”、T q " is the subtransient short-circuit time constant; T d0 ”、T q0 ” is the subtransient open circuit time constant.

[0024] When the thermal power unit is not a generator unit to be studied, its mechanical complex torque coefficient can be obtained through the mechanical system shaft system equation of the thermal power unit. The complex frequency domain model of the thermal power unit when it is not a generator unit to be studied can be obtained by combining the mechanical complex torque coefficient of the thermal power unit and the complex frequency domain model when the above generator is a generator unit to be studied.

[0025] The general form of the double-fed wind turbine in the complex frequency domain is derived as follows:

[0026] The flux equation and rotor voltage equation of the doubly fed wind turbine are:

[0027]

[0028]

[0029] Where: u rd 、u rq are the dq axis components of the rotor voltage respectively; i rd 、i rq are the dq axis components of the rotor current respectively; R r is the rotor winding resistance; ω e ,ω r are synchronous speed and rotor speed respectively; ψ sd , sq are the dq axis components of the DFIG stator flux respectively; ψ rd , rq are the dq axis components of the DFIG rotor flux respectively; i sd 、i sq are the dq axis components of the stator current respectively; L s , L r , L m are the self-inductance and mutual inductance of the stator and rotor windings respectively;

[0030] The rotor converter (RSC) of the doubly fed wind turbine can be Figure 3 The control block diagram shown is shown.

[0031] Combining the above flux equation, rotor voltage equation and RSC control block diagram, we can get:

[0032]

[0033] in:

[0034]

[0035] Where: k p3 , k i3 are the proportional and integral constants of the RSC current inner loop respectively; i rd-ref 、i rq-ref are the reference values ​​of the dq-axis components of the rotor current respectively.

[0036] DFIG implements MMPT, and the reactive power reference is set to 0, so the RSC active power and reactive power reference values ​​can be given by the following formula:

[0037]

[0038] Where: P s-ref , Q s-ref are the reference values ​​of active and reactive power output of DFIG respectively; k opt is the maximum power tracking curve proportional coefficient.

[0039] The stator voltage equation, power equation and electromagnetic torque equation of DFIG are:

[0040]

[0041]

[0042] Where: u sd 、u sqq are the dq axis components of the stator voltage respectively; R s is the stator winding resistance; P s , Q s is the output active and reactive power; T e is the electromagnetic torque; ψ s is the stator flux vector amplitude after stator flux orientation.

[0043] Combining the above equations and Figure 3 The control block diagram shown in the figure is linearized to obtain the connection diagram of each DFIG signal, as shown in Figure 4 shown.

[0044] Figure 4 The values ​​of the transfer functions are as follows:

[0045]

[0046] G2(s)=3k opt ω r0 2

[0047]

[0048]

[0049]

[0050] Where: k p1 , k i1 are the proportional and integral constants of the RSC reactive outer loop respectively; k p2 , k i2They are the RSC active outer loop proportional and integral constants respectively; ω r0 is the steady-state speed value.

[0051] Depend on Figure 4 The relationship between the small signals shown in the DFIG signal connection diagram can be deduced as follows: Figure 5 The DFIG shown is the general form of the complex frequency domain model of the generator set to be studied.

[0052] If the DFIG is not a research generator, its mechanical system needs to be combined. The DFIG mechanical part adopts a two-mass block model, and the dynamic process of its shaft system can be expressed by the following formula:

[0053]

[0054] Where: H t , H g is the inertia time constant of the wind wheel and generator; K s is the total stiffness of the shaft system; ω t ,ω r is the wind wheel speed and generator speed; θ s is the relative displacement angle between the two mass blocks; T m 、T e is the mechanical torque and electromagnetic torque; D t , D g Ask the damping coefficients of wind turbine and generator rotors respectively; D tg is the mutual damping coefficient between the wind turbine and the generator rotor; f e is the rated frequency of the power grid; p n is the pole pair number.

[0055] Combining the above expressions and linearizing them, we can get:

[0056]

[0057] in:

[0058]

[0059] Combining the above formula and Figure 5 You can get Figure 6 The DFIG shown is a complex frequency domain model when it is not a generator set under study.

[0060] Step 3: The complex torque coefficient is obtained by combining the established generator complex frequency domain model with the system complex frequency domain network equation, and then the subsynchronous oscillation of the doubly fed wind farm grid-connected system is analyzed, including: numbering the generator nodes as mentioned above, then numbering the non-generator nodes, establishing the complex frequency domain port admittance model of each component of the system, thereby establishing the system complex frequency domain network node admittance matrix, and then forming the system complex frequency domain network equation, and eliminating the non-generator nodes and the non-unit nodes to be studied in combination with the established generator complex frequency domain model, and obtaining the complex torque coefficient of the unit to be studied. For the doubly fed wind farm grid-connected system, the subsynchronous oscillation effect of the doubly fed wind farm can be analyzed by the established DFIG complex torque coefficient, and the influence of the doubly fed wind farm grid-connected on the subsynchronous oscillation of the thermal power unit can be analyzed by the established complex torque coefficient of the thermal power unit.

[0061] Assuming that the system has N+1 nodes, among which there are n generator nodes, the numbering of the generator nodes is as mentioned above, the reference node is numbered N+1, and the remaining non-generator nodes are numbered arbitrarily, the system complex frequency domain network node admittance matrix can be established, as follows: Figure 2 As shown in the green box diagram. The complex frequency domain network equation of the system is formed by the established complex frequency domain node admittance matrix of the system. Combined with the established complex frequency domain model of the generator, the complex torque coefficient of the generator to be studied can be obtained. The subsynchronous oscillation effect of the doubly fed wind farm can be analyzed through the DFIG complex torque coefficient. The influence of the grid-connected doubly fed wind farm on the subsynchronous oscillation of the thermal power unit can be analyzed through the complex torque coefficient of the thermal power unit, and the subsynchronous oscillation analysis of the grid-connected doubly fed wind farm system can be realized.

[0062] Compared with the existing technology, this technology has the following beneficial effects:

[0063] The present invention comprehensively considers the dynamic process of the doubly fed wind turbine, derives the general form of the DFIG complex frequency domain model in the same form as that of the thermal power unit, and studies a subsynchronous oscillation analysis method for the doubly fed wind farm grid-connected system based on the complex torque coefficient method, which is used mostly in the research of thermal power units. In the scenario where the rapid development of the new "dual-high" power system leads to the prone occurrence of subsynchronous oscillations in the doubly fed wind farm grid-connected system, a method for analyzing subsynchronous oscillations in the doubly fed wind farm grid-connected system based on the complex torque coefficient method is studied. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 A diagram for the abstract;

[0065] Figure 2 It is a schematic diagram of the system model;

[0066] Figure 3 This is the RSC control block diagram of the doubly-fed wind turbine;

[0067] Figure 4 This is a schematic diagram of DFIG signal connection;

[0068] Figure 5 DFIG is the complex frequency domain model structure diagram of the generator set to be studied;

[0069] Figure 6 The complex frequency domain model structure diagram of DFIG is not a generator set to be studied;

[0070] Figure 7 This is the structure diagram of the series compensation transmission system of the doubly-fed wind farm;

[0071] Figure 8 The electrical damping of the grid-connected system of the doubly-fed wind farm under different series compensation degrees;

[0072] Fig. 9 The electrical damping of the grid-connected doubly-fed wind farm system under different RSC active outer loop proportional coefficients;

[0073] Fig.10 The electrical damping of the grid-connected doubly-fed wind farm system under different RSC active outer loop integral coefficients; DETAILED DESCRIPTION

[0074] In order to describe the present invention in more detail, the present invention is further illustrated below with reference to the accompanying drawings and specific implementation examples.

[0075] by Figure 7 The specific implementation method is described by taking the double-fed wind farm as an example of a series compensation transmission system. According to step 1, the double-fed wind farm is used as the generator set to be studied, and its node number is 1. The general form of its complex frequency domain model is as follows: Figure 2 As shown in the red box. Then, according to the DFIG complex frequency domain model derived in step 2, the various parameters are substituted into the complex frequency domain model of the double-fed wind farm; then according to step 3, the non-generator nodes are first numbered, and the specific numbers are as follows Figure 7 As shown; then the complex frequency domain port admittance model of the system transmission line and series compensation capacitor is established, which can be expressed as follows:

[0076] transformer:

[0077]

[0078] Where: r T 、x T are the transformer resistance and reactance respectively.

[0079] Transmission lines:

[0080]

[0081] Where: r L 、x L are the transmission line resistance and reactance respectively.

[0082] Series compensation capacitor:

[0083]

[0084] Where: x C are the capacitive reactance of the series compensation capacitor respectively.

[0085] Then, the complex frequency domain port admittance model of each component mentioned above can be used to establish the complex frequency domain network node admittance matrix of the system, and then form the complex frequency domain network equation of the system:

[0086]

[0087] Where: I G , U G are the generator node current and voltage respectively; U S is the non-generator node voltage.

[0088] The complex torque coefficient of the doubly-fed wind farm can be calculated by combining the complex frequency domain network equation of the system and the complex frequency domain model of the DFIG obtained above.

[0089] Taking the series compensation and the RSC active outer loop control parameters of DFIG as the influencing factors, the subsynchronous oscillation of the grid-connected system of the doubly fed wind farm is analyzed. Since the natural torsional vibration frequency of the DFIG shaft system is very low, the series compensation in actual engineering is difficult to excite the subsynchronous oscillation of the DFIG shaft system, so the possibility of subsynchronous control interaction (SSCI) of DFIG is relatively high. Therefore, the mechanical system of DFIG is not considered here, and the subsynchronous control interaction (SSCI) of DFIG is analyzed through the electrical damping of DFIG.

[0090] Figure 8 is the electrical damping of the doubly-fed wind farm when the series compensation is 50%, 40%, and 30% respectively. Figure 8 It can be seen that as the system series compensation increases, the absolute value of the electrical negative damping gradually increases, and the possibility of SSCI increases; the lowest point of the electrical negative damping moves to the left, and the frequency of SSCI gradually decreases. Therefore, the series compensation not only affects the size of the system electrical damping, but also affects the oscillation frequency.

[0091] Fig. 9 k is the RSC active power outer loop proportional coefficient of DFIG p2 The electrical damping of the double-fed wind farm is 1, 1.5, and 2 respectively. Fig. 9 It can be seen that with the system k p2 As the value of the electrical negative damping increases, the absolute value of the electrical negative damping decreases first and then increases; the lowest point of the electrical negative damping moves to the left, and the frequency of SSCI gradually decreases. Therefore, the difference in the proportional coefficient of the RSC active outer loop will lead to different sizes of the system electrical damping and affect the oscillation frequency.

[0092] Fig.10is the RSC active power outer loop integral coefficient k of DFIG i2 The electrical damping of the double-fed wind farm is 100, 80, and 67 respectively. Fig.10 It can be seen that with the system k i2 As the value of increases, the absolute value of the electrical negative damping increases; the lowest point of the electrical negative damping moves to the left, and the frequency of SSCI gradually decreases. Therefore, the difference in the RSC active outer loop integral coefficient will also lead to the difference in the size of the system electrical damping and affect the oscillation frequency.

[0093] comprehensive Fig. 9 and Fig.10 It can be seen that the control parameters of DFIG will affect the electrical damping of the system, thereby affecting the stability of the system. Different control parameters may excite SSCI of different frequencies.

[0094] Similarly, the influence of other influencing factors of the system on the subsynchronous oscillation of the doubly-fed wind farm grid-connected system can be analyzed by the above method. If there are thermal power units in the system, the complex torque coefficient of the thermal power units can also be obtained to analyze the influence of the doubly-fed wind farm grid-connected system on the subsynchronous oscillation characteristics of the thermal power units, which will not be repeated here.

[0095] Finally, it should be noted that the above implementation cases are only used to illustrate the technical solution of the present invention rather than to limit it, but the protection scope of the present invention is not limited thereto, and any technician in the relevant field can still modify or make equivalent substitutions for the specific implementation modes of the present invention, which should all be covered within the protection scope of the claims of the present invention.

Claims

1. A method for analyzing subsynchronous oscillation of a doubly-fed wind farm grid-connected system based on a complex torque coefficient method, characterized in that: The following steps are involved: Step 1: Divide the generator sets in the system into generator sets to be studied and generator sets not to be studied, and give the general form of the complex frequency domain model of the generator sets to be studied and the generator sets not to be studied applicable to the complex torque coefficient method; Step 2: Provide a general form complex frequency domain model of a traditional thermal power unit suitable for subsynchronous oscillation analysis of a multi-machine system, and establish a general form complex frequency domain model of a doubly fed wind turbine; Step 3: The complex torque coefficient is obtained by combining the established complex frequency domain model of the generator with the complex frequency domain network equation of the system, and then the subsynchronous oscillation of the doubly fed wind farm grid-connected system is analyzed.

2. The method for analyzing subsynchronous oscillation of a double-fed wind farm grid-connected system based on complex torque coefficient method according to claim 1 is characterized in that: The step 1: divide the generator sets in the system into generator sets to be studied and generator sets not to be studied, and give a general form of the complex frequency domain model of the generator sets to be studied and the generator sets not to be studied that are suitable for the complex torque coefficient method, specifically including: dividing the system generator sets, numbering the nodes of the generator sets to be studied as 1, and numbering the remaining generator nodes from 2 to n in sequence, and giving a general form of the complex frequency domain model in which the generator node voltage is the output quantity and the current and speed are the input quantities.

3. The method for analyzing subsynchronous oscillation of a double-fed wind farm grid-connected system based on complex torque coefficient method according to claim 1 is characterized in that: The step 2: provides a general form of complex frequency domain model of a traditional thermal power unit suitable for subsynchronous oscillation analysis of a multi-machine system, and establishes a general form of complex frequency domain model of a doubly fed wind fan; specifically includes: establishing a general form of complex frequency domain model of a generator unit suitable for subsynchronous oscillation analysis of a multi-machine system, and for a thermal power unit, directly providing the general form of its complex frequency domain model; for a doubly fed wind fan (DFIG), respectively establishing small signal models of stator and rotor flux, rotor voltage, RSC control circuit, MPPT control, stator voltage, power and electromagnetic torque of the doubly fed wind fan, and then establishing a small signal connection diagram of the doubly fed wind fan based on the connection relationship between the small signal models of each part, and then by obtaining the complex frequency domain model when DFIG is the generator unit to be studied, and then combining the small signal model after the linearization of the DFIG mechanical system shaft system equation, the complex frequency domain model of DFIG when it is not a generator unit to be studied can be constructed.

4. The method for analyzing subsynchronous oscillation of a doubly-fed wind farm grid-connected system based on complex torque coefficient method according to claim 1, characterized in that: The step 3: using the established generator complex frequency domain model in combination with the system complex frequency domain network equation to obtain the complex torque coefficient, and then analyzing the subsynchronous oscillation of the grid-connected system of the doubly fed wind farm, specifically includes: numbering the generator nodes as mentioned above, then numbering the non-generator nodes, establishing the complex frequency domain port admittance model of each component of the system, thereby establishing the system complex frequency domain network node admittance matrix, and then forming the system complex frequency domain network equation, combining the established generator complex frequency domain model to eliminate the non-generator nodes and the nodes of the non-units to be studied, and obtaining the complex torque coefficient of the unit to be studied. For the grid-connected system of the doubly fed wind farm, the subsynchronous oscillation effect of the doubly fed wind farm can be analyzed by the established DFIG complex torque coefficient, and the influence of the grid-connected doubly fed wind farm on the subsynchronous oscillation of the thermal power unit can be analyzed by the established complex torque coefficient of the thermal power unit.

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