Direct power control method for brushless doubly-fed generator under unbalanced grid

Abstract

The application discloses a direct power control method of a brushless doubly-fed generator under an unbalanced power grid, and aims to solve the technical problems of great limitations of vector control and great steady-state error of a system in the prior art. The application establishes a mathematical model of positive and negative sequence coordinate axes of the brushless doubly-fed generator by positive and negative sequence voltage decomposition under unbalanced power grid voltage; and establishes a relational equation between the voltage at the control winding side and active power and reactive power at the power winding side based on the positive and negative sequence voltage vectors to realize decoupling control of the active power and the reactive power; and improves a sliding mode control approach law to overcome the shortcomings of traditional sliding mode control chattering and discontinuous high-frequency switching control, adopts an extended state observer to suppress system disturbance, and reduces the steady-state error of the system; and given negative sequence current commands are regulated by PI regulation to realize multi-target control and make the brushless doubly-fed generator achieve stable operation under the unbalanced power grid, can effectively reduce power fluctuation caused by unbalanced power grid voltage, has strong robustness, and improves the stability of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of motor control, in particular to a direct power control method of brushless doubly-fed generator under unbalanced power grid. BACKGROUND

[0002] Brushless doubly-fed generator (BDFG) has broad application prospects in the field of wind power generation due to its advantages of brushless, small capacity of frequency converter, controllable power factor, reliable operation, etc. Brushless doubly-fed generator has two sets of stator windings, namely power winding and control winding. Since this motor realizes brushless doubly-fed operation on the stator, it not only has a simple rotor structure, but also has excellent characteristics of wound rotor induction motor and synchronous motor, and can be used as an AC speed-regulating motor and a variable-speed constant-frequency generator.

[0003] When the brushless doubly-fed generator operates under unbalanced power grid, the existence of negative sequence components will lead to the introduction of twice frequency components of active power and reactive power of the brushless doubly-fed generator, torque pulsation and power winding current imbalance. Domestic and foreign research teams have proposed corresponding control strategies for brushless doubly-fed generators under unbalanced power grid to reduce the adverse effects caused by unbalanced power grid. Vector control strategy is mainly used to suppress the adverse effects caused by unbalanced power grid.

[0004] However, the present inventors found at least the following technical problems in the process of implementing the technical solutions in the embodiments of the present application: the vector control is limited, and the derivation process and results are quite complex; and the steady-state error of the direct power control system of the brushless doubly-fed generator under unbalanced power grid is large.

[0005] The information disclosed in this section of the background art is only intended to deepen the understanding of the background art of the present disclosure, and should not be regarded as acknowledging or implying in any form that the information constitutes prior art known to those skilled in the art. SUMMARY

[0006] In view of at least one of the above technical problems, the present application provides a direct power control method of brushless doubly-fed generator under unbalanced power grid, which mainly uses at least one of the following technical means: based on positive and negative sequence voltage vectors to establish the relationship equation between the control winding side voltage and the active power and reactive power of the power winding side to realize the decoupling control of the active power and the reactive power; improve the new sliding mode control approach law to overcome the disadvantages of traditional sliding mode control chattering and discontinuous high-frequency switching control; use an extended state observer to suppress system disturbance to reduce the steady-state error of the system.

[0007] According to one aspect of the present application, a brushless doubly-fed generator direct power control method under unbalanced power grid is provided, mainly comprising the following steps:

[0008] S1, obtaining three-phase stator voltage, current and power winding side stator flux amplitude, phase angle and frequency of the power winding and control winding in the stationary coordinate system;

[0009] S2, using positive and negative sequence fast separation method to decompose the unbalanced grid voltage into positive and negative sequence components and obtain balanced positive and negative sequence components;

[0010] S3, establishing two-phase synchronous rotating coordinate system and positive and negative sequence mathematical model of brushless doubly-fed generator under unbalanced power grid; and based on positive and negative sequence voltage vector, establishing the relationship equation between control winding side voltage and active power and reactive power of power winding side, to realize decoupling control of active power and reactive power;

[0011] S4, constructing a controller of integral sliding mode variable structure and PI regulation together; and establishing integral sliding mode variable structure mathematical model of brushless doubly-fed generator under unbalanced power grid;

[0012] S5, performing stability analysis based on Lyapunov function for the integral sliding mode variable structure controller, and obtaining parameters of the stable control model;

[0013] S6, constructing a corresponding extended state observer, estimating the system disturbance and compensating the estimated disturbance value to the system, to improve the stability of the system;

[0014] S7, obtaining stator current and stator voltage in the negative sequence dq rotating coordinate system of the control winding side, obtaining the negative sequence dq rotating coordinate system stator current through PI regulation to obtain the negative sequence dq rotating coordinate system control winding side stator voltage given command signal; and applying the output voltage of the control winding side converter to the motor control side, so as to suppress the power disturbance caused by the unbalanced power grid and make the system work in a stable operating state.

[0015] In some embodiments of the present application, the main steps of establishing two-phase synchronous rotating coordinate system and positive and negative sequence mathematical model of brushless doubly-fed generator under unbalanced power grid include:

[0016] Convert the three-phase voltage in the stationary coordinate system to the two-phase stationary coordinate system, and according to the voltage in the two-phase αβ stationary coordinate system, use the positive and negative sequence fast separation method to construct the positive and negative sequence voltage, and then respectively obtain the voltage equation in the two-phase dq rotating coordinate system, and the derivation process is as follows:

[0017]

[0018]

[0019]

[0020]

[0021] The time delay period variable n / m is introduced, where m=2π, 0≤n≤2π, and the voltage equation after the time delay period variable change is:

[0022]

[0023] When the time delay angle n<<m, the above formula can be approximated as:

[0024]

[0025]

[0026]

[0027] Substituting formula (2) and (3) into formula (8) gives:

[0028]

[0029]

[0030]

[0031] If n / m=T / 4, the above expression can be converted into the standard form of T / 4 time delay positive and negative sequence decomposition method, but the T / 4 time delay decomposition method has a certain period delay in positive and negative sequence decomposition, resulting in inaccurate positive and negative sequence decomposition vectors, so n / m=T / 16, i.e., T / 16 time delay positive and negative sequence decomposition, which can effectively suppress the period delay problem existing in the T / 4 decomposition method, and can quickly decompose the positive and negative sequence vectors. m = T / 16, i.e., T / 16 time delay positive and negative sequence decomposition, which can effectively suppress the period delay problem existing in the T / 4 decomposition method, and can quickly decompose the positive and negative sequence vectors.

[0032] Through positive and negative sequence decomposition in the two-phase static and rotating coordinate system, the positive and negative sequence voltage equations can be converted into the two-phase rotating dq coordinate system:

[0033]

[0034]

[0035] where U a (t), U b (t), and U c (t) are the phase voltages in the three-phase static abc coordinate system of the stator power winding, U α (t), U β (t), and U r (t) are the positive and negative sequence voltages in the two-phase rotating dq coordinate system.(t) are the voltage components of the stator power winding in the two-phase stationary αβ coordinate system on the α-axis and the β-axis, are the positive sequence components of the αβ-axis after positive and negative sequence separation in the two-phase stationary αβ coordinate system of the stator power winding, and are the positive and negative sequence d-q axis components in the two-phase rotating d-q coordinate system of the stator power winding.

[0036] The mathematical model of the brushless doubly-fed generator in the two-phase positive and negative sequence (d-q) rotating coordinate system is established, and the voltage and flux linkage equations are as follows:

[0037]

[0038]

[0039]

[0040] wherein, and are the positive sequence voltage components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the stator power winding, and are the positive sequence current components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the stator power winding; and are the positive sequence voltage components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the stator control winding, and are the positive sequence current components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the stator control winding; and are the positive sequence voltage components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the rotor winding, and are the positive sequence current components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the rotor winding; and are the positive sequence flux linkage components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the stator power winding; and are the positive sequence flux linkage components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the control winding; and are the positive sequence flux linkage components of the d-axis and the q-axis in the positive sequence d-q axis rotating coordinate system of the rotor winding; R1, R2 and R r are the resistances of the power winding, the control winding and the rotor winding, L1 is the self-inductance of the power winding, L2 is the self-inductance of the control winding, L r is the self-inductance of the rotor winding, L 1rLm1is the mutual inductance between the power winding and the rotor winding, L 2r Lm2is the mutual inductance between the control winding and the rotor winding; ω + ωp is the angular speed of the power winding in the arbitrary speed positive sequence rotating reference frame, ωc is the angular speed of the control winding in the arbitrary speed positive sequence rotating reference frame, ωr is the angular speed of the rotor winding in the arbitrary speed positive sequence rotating reference frame, p = d / dt is the differential operator.

[0041]

[0042]

[0043]

[0044] Similarly, and are the negative sequence voltage components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the stator power winding, respectively, and are the negative sequence current components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the stator power winding, respectively; and are the negative sequence voltage components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the stator control winding, respectively, and are the negative sequence current components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the stator control winding, respectively; and are the negative sequence voltage components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the rotor winding, respectively, and are the negative sequence current components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the rotor winding, respectively; and are the negative sequence flux linkage components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the stator power winding, respectively; and are the negative sequence flux linkage components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the control winding, respectively; and are the negative sequence flux linkage components of the d-axis and q-axis in the negative sequence d-q axis rotating coordinate system of the rotor winding, respectively; R1, R2 and R r R1, R2 and R are the resistances of the power winding, the control winding and the rotor winding, respectively, L1 is the self-inductance of the power winding, L2 is the self-inductance of the control winding, L r Lr is the self-inductance of the rotor winding, L 1r Lm1is the mutual inductance between the power winding and the rotor winding, L 2r Lm2is the mutual inductance between the control winding and the rotor winding; ω- ωp is the angular speed of the arbitrary speed negative sequence rotating reference frame for the power winding, ωc is the angular speed of the arbitrary speed negative sequence rotating reference frame for the control winding, ωr is the angular speed of the arbitrary speed negative sequence rotating reference frame for the rotor winding, p = d / dt is the differential operator.

[0045] In some embodiments disclosed in the present application, the step of establishing the relationship equation between the control winding side voltage and the active power and the reactive power of the power winding side is as follows:

[0046] Based on the instantaneous power theory, the instantaneous active power P and the reactive power Q of the power winding output of the brushless doubly-fed generator under the unbalanced power grid are defined as follows:

[0047]

[0048]

[0049] wherein, P1 and Q1 are the direct current components of the active power and the reactive power of the stator power winding respectively; P sin , P cos , Q sin , Q cos respectively represent the two-frequency positive and negative sine wave fluctuation components of the active power and the reactive power of the stator power winding.

[0050] The stator flux linkage orientation is performed on d + in the positive sequence synchronous rotating coordinate system, and the transient process of the stator positive sequence magnetic linkage of the power winding is ignored, at this time, d + , q + axis magnetic linkage components and voltage components are respectively The relationship between the power winding current and the control winding magnetic linkage is derived from formula (15) and formula (16) as follows:

[0051]

[0052] respectively, the direct current components of formula (20) and formula (21) are substituted into formula (22), and is substituted, then the following can be obtained:

[0053]

[0054] The derivative of formula (23) is performed, and the derivative of the active power and the reactive power direct current components is expressed as:

[0055]

[0056]

[0057] wherein, P dcand Q dc are the direct components of the stator power winding active power and reactive power respectively, p1 and p2 are the pole pairs of the stator power winding and the pole pairs of the stator control winding respectively, p = d / dt is the differential operator.

[0058] In some embodiments disclosed in the present application, the mathematical model of direct power control of brushless doubly-fed generator under unbalanced grid based on new reaching rate integral sliding mode variable structure is established by the following steps:

[0059] The input of the sliding mode controller is defined as the difference between the system state variable and the reference value:

[0060]

[0061] The sliding surface equation of the system is defined as the sum of the state variable of the sliding mode controller and its integral term:

[0062]

[0063] The initial state of the integrator is defined as:

[0064]

[0065] Therefore, at t = 0, S P = S Q = 0.

[0066] From equation (24), we have:

[0067]

[0068] P ref and Q ref are constants, so According to equation (28), the equivalent control is obtained:

[0069]

[0070] When considering parameter errors and external disturbances, the switching equivalent control is obtained, and at this time:

[0071]

[0072] Substituting (29) into (30) gives:

[0073]

[0074] To ensure that dV / dt < 0, the sliding mode control law with a new reaching rate is designed as:

[0075]

[0076] In the new approach rate, the integral term ρ is used to replace X. When S approaches zero in the system sliding mode stage, the integral of S also approaches zero, and the value of ρ also approaches zero, and finally eliminates the sat(s) term; the introduction of the negative weighted value K in the integral term can greatly reduce the switching gain, and can effectively avoid the phenomenon of large switching gain and severe chattering in the exponential approach rate approach stage.

[0077] Therefore, the control winding feedback voltage is:

[0078]

[0079] Wherein, X P and X Q are the difference between the active power direct current output by the brushless doubly-fed motor and the active power reference value and the difference between the reactive power direct current output by the brushless doubly-fed motor and the reactive power reference value, S P is the sliding variable of the active power of the system power winding, that is, the sum of the difference between the given value and the actual value of the active power and the integral term, S Q is the sliding variable of the active power of the system power winding, that is, the sum of the difference between the given value and the actual value of the reactive power and the integral term, C i is the sliding surface coefficient, H d and H q are external disturbance, P ref , Q ref are the given values of the active power and the reactive power of the system power winding; S PΔ is the sliding variable of the active power of the power winding when considering system error, that is, the sum of the difference between the given value and the actual value of the active power and the integral term, S QΔ is the sliding variable of the active power of the power winding when considering system error.

[0080] In some embodiments disclosed in the present application, the method of Lyapunov function-based stability analysis for the integral sliding mode variable structure controller is as follows:

[0081] The Lyapunov function is constructed as follows:

[0082] The time derivative of the above Lyapunov function is obtained as follows:

[0083]

[0084] At this time, formula (29) and (30) are substituted into (35) to obtain:

[0085]

[0086] According to formula (36), if S P > 0, then K q |p|SP sat(S P |S p | a > 0, H q > 0, so dVs P / dt < 0; if S P < 0, then K q |p|S P sat(S P )|S p | a > 0, H q > 0, K q > H q , so dVs P / dt < 0; if S Q > 0, then K q |p|S Q sat(S Q )|S Q | a > 0, H d > 0, so dVs Q / dt < 0; if S Q < 0, then K d |p|S Q sat(S Q )|S Q | a > 0, H d > 0, K d > H d , so dVs Q / dt < 0. From the above, dV / dt < 0; from , it can be known that the Lyapunov function V > 0, thus, the system satisfies the Lyapunov asymptotic stability condition, and the actual values of the active power and the reactive power will converge to the given values.

[0087] The obtained stator control winding voltage is fed back to the system, and the power of the brushless doubly-fed motor under the unbalanced power grid can be directly controlled.

[0088] In some embodiments disclosed in the application, a negative sequence current tracking instruction control model and a design of an improved extended state observer are built, including the following steps:

[0089] From equation (20), it can be known that

[0090]

[0091] To suppress the double frequency fluctuation of system output power, the double frequency fluctuation of active power positive and negative sine wave can be made to be zero, i.e. cos P sin = 0; at this time, the given value of negative sequence stator current in negative sequence coordinate system can be obtained:

[0092]

[0093] Similarly, the double frequency fluctuation of reactive power positive and negative sine wave, i.e. cos Q sin = 0, the corresponding negative sequence stator current command value can be obtained:

[0094]

[0095] However, under the condition of grid voltage imbalance, the stator and rotor currents, stator side active power and reactive power, etc. will appear pulsation, and the control winding current negative sequence component has only two control quantities, so it is impossible to suppress all pulsation at the same time, so according to different control requirements, the control target can be divided into four categories: suppressing stator current imbalance, suppressing control winding current imbalance, suppressing stator side active power double frequency disturbance and suppressing reactive power double frequency disturbance.

[0096] As follows:

[0097] Target 1: suppress stator current asymmetry:

[0098]

[0099] Target 2: suppress control winding current asymmetry:

[0100] According to equation (18), the relationship between stator power winding side negative sequence current, stator control winding side negative sequence current and rotor negative sequence current can be deduced;

[0101]

[0102]

[0103] At this time, let The stator power winding negative sequence current command can be obtained:

[0104]

[0105] Target 3: suppress stator side active power pulsation:

[0106]

[0107] Target 4: suppress stator side reactive power pulsation:

[0108]

[0109] The stator power winding negative sequence current command obtained can be used to obtain the stator control winding negative sequence voltage through PI adjustment, and the stator control winding negative sequence voltage fed back to the system can effectively suppress the twice-frequency power disturbance caused by the unbalanced power grid, and the specific derivation steps are as follows:

[0110] According to the relationship between the negative sequence current and the negative sequence flux in formula (18), the following derivation can be made:

[0111]

[0112]

[0113] The relationship between the stator power winding negative sequence current and the rotor negative sequence current in (46) and (47) can be combined to obtain the expression of the stator control winding negative sequence current as follows:

[0114]

[0115] Substituting formula (40) into the control winding voltage in formula (11) can obtain:

[0116]

[0117] Through the relationship between the stator control winding negative sequence voltage and the stator power winding negative sequence current, the stator power winding negative sequence current can be converted into the stator control winding negative sequence voltage by using a PI controller and fed back to the system, so that the corresponding control target is achieved.

[0118] The one or more technical solutions provided in the embodiments of the present application have at least the following technical effects or advantages:

[0119] Under the unbalanced grid voltage, the positive and negative sequence voltage decomposition is performed to establish the mathematical model of the positive and negative sequence coordinate axes of the brushless doubly-fed motor.

[0120] Based on the positive and negative sequence voltage vectors, the relationship equation between the control winding side voltage and the active power and the reactive power of the power winding side is established, so that the decoupling control of the active power and the reactive power is realized.

[0121] The given negative sequence current command is adjusted through a PI to realize multi-target control and make the brushless doubly-fed motor achieve stable operation under the unbalanced grid, which can effectively reduce the power fluctuation caused by the unbalanced grid voltage, has strong robustness, and improves the stability of the system.

[0122] The control method improves the new sliding mode control approach law, overcomes the disadvantages of traditional sliding mode control chattering and discontinuous high-frequency switching control, uses an extended state observer to suppress system disturbance, reduces the steady-state error of the system, has good practicability, and is easy to popularize. BRIEF DESCRIPTION OF DRAWINGS

[0123] Figure 1 Flow chart of the method for adjusting input content in the embodiment of the application.

[0124] Figure 2 BDFG variable-speed constant-frequency wind power generation system in the embodiment of the application.

[0125] Figure 3 Power winding flux linkage directional vector space relation diagram under unbalanced power grid in the embodiment of the application.

[0126] Figure 4 Improved extended state observer system block diagram in the embodiment of the application.

[0127] Figure 5 BDFG direct power control and multi-target PI regulation system block diagram of new approaching rate integral sliding mode variable structure in the embodiment of the application.

[0128] Figure 6 New approaching rate integral sliding mode variable structure control part simulation waveform under unbalanced power grid and constant speed and power conditions in the embodiment of the application, wherein (a) is a comparison of stator power winding currents before and after negative sequence current instruction control; (b) is a comparison of stator control winding currents before and after negative sequence current instruction control; (c) is a comparison of active power before and after negative sequence current instruction control; (d) is a comparison of reactive power before and after negative sequence current instruction control. DETAILED DESCRIPTION

[0129] In the description of the application, "first", "second", etc. are used to distinguish the described objects, and do not have any order or technical meaning. Unless otherwise specified, the "connection" and "coupling" referred to in the application include direct and indirect connections (couplings).

[0130] In order to better understand the technical solutions of the application, the above technical solutions will be described in detail below in combination with the drawings of the specification and specific embodiments.

[0131] Embodiment

[0132] This example is based on the BDFG variable-speed constant-frequency wind power generation system, combined with Figure 1 The flow chart shown in the figure is used to describe this embodiment.

[0133] The structure of the BDFG variable-speed constant-frequency wind power generation system is shown in Figure 2The wind turbine is mainly composed of a wind turbine, a speed increasing gearbox, a power winding, a control winding, a double converter, a transformer, a filter and the like, as shown in the figure; the power winding is directly connected to a power grid, and the control winding is connected to the power grid through a bi-directional reversible converter and a transformer due to the bi-directional energy flow.

[0134] Specifically, the direct power control method of the brushless doubly-fed generator under an unbalanced power grid mainly includes the following steps:

[0135] S1: Obtain the three-phase stator voltage and current of the power winding and the control winding in the stationary coordinate system, obtain the stator flux amplitude, phase angle and frequency of the power winding side, decompose the unbalanced grid voltage into positive and negative sequences by using the T / 4 period delay method, and obtain the balanced positive sequence, negative sequence and zero sequence components.

[0136] Establish a two-phase synchronous rotating coordinate system and a positive and negative sequence mathematical model of the brushless doubly-fed generator under an unbalanced power grid, and the main derivation process is as follows:

[0137] ①Convert the three-phase voltage in the stationary coordinate system to the two-phase stationary coordinate system, and construct the positive and negative sequence voltages by using the positive and negative sequence fast separation method according to the voltage in the two-phase αβ stationary coordinate system, and then obtain the voltage equation in the two-phase dq axis rotating coordinate system:

[0138]

[0139]

[0140]

[0141]

[0142] Introduce a delay period variable n / m, where m=2π, 0≤n≤2π, and the voltage equation after the delay period variable change is:

[0143]

[0144] When the delay angle n<<m, the above formula can be approximated as:

[0145]

[0146]

[0147]

[0148] Substitute equations (2) and (3) into equation (8) to obtain:

[0149]

[0150]

[0151]

[0152] If n / m = T / 4, the above expression can be converted into the standard form of T / 4 delay positive and negative sequence decomposition method, but T / 4 delay decomposition method has a certain period delay in positive and negative sequence decomposition, which leads to the inaccuracy of positive and negative sequence decomposition vector, so that n / m = T / 16, that is, T / 16 delay positive and negative sequence decomposition, which can effectively suppress the period delay problem existing in T / 4 decomposition method, and can make the positive and negative sequence vector quickly decompose.

[0153] Through the positive and negative sequence decomposition in the two-phase stationary-rotating coordinate system, the positive and negative sequence voltage equations can be converted into the two-phase rotating dq coordinate system:

[0154]

[0155]

[0156] Where U a (t), U b (t) and U c (t) are the phase voltages of the stator power winding in the three-phase stationary abc coordinate system, U α (t), U β (t) are the voltage components of the α axis and the β axis in the two-phase stationary αβ coordinate system of the stator power winding, is the positive and negative sequence αβ axis component in the two-phase stationary αβ coordinate system of the stator power winding after T / 16 period delay, and are the positive and negative sequence d-q axis components in the two-phase rotating d-q coordinate system of the stator power winding.

[0157] The mathematical model of the brushless doubly-fed generator in the two-phase positive and negative sequence (d-q) rotating coordinate system is established, and the voltage and flux linkage equations are as follows:

[0158]

[0159]

[0160]

[0161] Where, and are the positive sequence voltage components of the d axis and the q axis in the positive sequence d-q axis rotating coordinate system of the stator power winding, and are the positive sequence current components of the d axis and the q axis in the positive sequence d-q axis rotating coordinate system of the stator power winding. and are the positive-sequence voltage components of the stator control winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively, and are the positive-sequence current components of the stator control winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively; and are the positive-sequence voltage components of the rotor winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively, and are the positive-sequence current components of the rotor winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively; and are the positive-sequence flux components of the stator power winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively; and are the positive-sequence flux components of the control winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively; and are the positive-sequence flux components of the rotor winding in the d and q axes of the positive-sequence d-q axis rotating reference frame, respectively; R1, R2 and R r are the power winding, control winding and rotor winding resistances, respectively, L1 is the power winding self-inductance, L2 is the control winding self-inductance, L r is the rotor winding self-inductance, L 1r is the mutual inductance between the power winding and the rotor winding, L 2r is the mutual inductance between the control winding and the rotor winding; ω + is the angular speed of the power winding arbitrary-speed positive-sequence rotating reference frame, is the angular speed of the control winding arbitrary-speed positive-sequence rotating reference frame, is the angular speed of the rotor winding arbitrary-speed positive-sequence rotating reference frame, p = d / dt is the differential operator.

[0162]

[0163]

[0164]

[0165] In the above formulae, and are the negative-sequence voltage components of the stator power winding in the d and q axes of the negative-sequence d-q axis rotating reference frame, respectively, and are the negative-sequence current components of the stator power winding in the d and q axes of the negative-sequence d-q axis rotating reference frame, respectively; and are negative sequence voltage components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of stator control winding respectively, and are negative sequence current components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of stator control winding respectively; and are negative sequence voltage components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of rotor winding respectively, and are negative sequence current components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of rotor winding respectively; and are negative sequence flux linkage components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of stator power winding respectively; and are negative sequence flux linkage components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of control winding respectively; and are negative sequence flux linkage components of d-axis and q-axis in negative sequence d-q axis rotating coordinate system of rotor winding respectively; ω - is angular velocity of arbitrary speed negative sequence rotating reference coordinate system of power winding, is angular velocity of arbitrary speed negative sequence rotating reference coordinate system of control winding, is angular velocity of arbitrary speed negative sequence rotating reference coordinate system of rotor winding, p=d / dt is differential operator.

[0166] ②Based on instantaneous power principle, the mathematical model of direct power control is derived in detail, the active and reactive power decoupling control is realized by establishing the relationship equation between control winding side voltage and power winding side active and reactive power, the main derivation process includes:

[0167] Ⅰ) According to instantaneous power theory, the expressions of instantaneous active power P and reactive power Q output by BDFG power winding are respectively:

[0168]

[0169]

[0170] wherein, P dc and Q dc are direct current components of active power and reactive power of stator power winding; P sin , P cos , Q sin , Q cos represent two times frequency positive and negative sine wave fluctuation components of active power and reactive power of stator power winding.

[0171] Ⅱ) In the positive sequence synchronous rotating coordinate system, d +Stator flux linkage orientation is performed, and the transient process of the power winding stator positive sequence flux linkage is ignored. At this time, d + , q + The flux linkage components and voltage components on the d The relationship between the power winding current and the control winding flux linkage is derived from equation (15) and equation (16):

[0172]

[0173] Substitute equation (22) into the direct current components of equation (20) and equation (21) respectively, and substitute , then the following equation can be obtained:

[0174]

[0175] Wherein, L1 is the self-inductance of the power winding, L2 is the self-inductance of the control winding, L r is the self-inductance of the rotor winding, L 1r is the mutual inductance between the power winding and the rotor winding, and L 2r is the mutual inductance between the control winding and the rotor winding.

[0176] Ⅲ) Differentiate equation (23), and the derivatives of the direct current components of the active power and the reactive power are expressed as:

[0177]

[0178]

[0179] Wherein, P dc and Q dc are the direct current components of the active power and the reactive power of the stator power winding, and are the positive sequence voltage components of the d-axis and the q-axis in the stator control winding positive sequence d-q axis rotating coordinate system, is the positive sequence flux linkage vector modulus of the q-axis in the stator power winding positive sequence d-q axis rotating coordinate system, and are the positive sequence flux linkage vector modulus of the d-axis and the q-axis in the control winding positive sequence d-q axis rotating coordinate system, and are the positive sequence flux linkage components of the d-axis and the q-axis in the rotor winding positive sequence d-q axis rotating coordinate system, ω r is the rotor angular velocity in the system, ω + is the angular velocity of the power winding arbitrary speed positive sequence rotating reference coordinate system, is the angular velocity of the control winding arbitrary speed positive sequence rotating reference coordinate system, The angular velocity of the arbitrary speed positive sequence rotating reference frame of the rotor winding, p1 and p2 are the pole pair numbers of the stator power winding and the stator control winding respectively, and p=d / dt is a differential operator.

[0180] S2: Design a new type of reaching rate integral sliding mode variable structure mathematical model of brushless doubly-fed generator direct power control under unbalanced power grid and an improved extended state observer:

[0181] ①Integral sliding mode variable structure control is one of the traditional sliding mode control methods, and a continuous time system is as follows:

[0182]

[0183] Wherein is the ideal mathematical model of the system, and h(x, t) contains parameter errors and external disturbances.

[0184] The integral variable structure controller is designed for the state equation, and the input of the sliding mode controller is defined as the difference between the system state variable and its reference value:

[0185] x e =x ref -x (26);

[0186] The sliding surface equation of the system is defined as the sum of the state variable of the sliding mode controller and its integral term:

[0187]

[0188] Wherein c is the settable sliding surface coefficient, and z is the auxiliary variable of the integrator.

[0189] In order to make the system at the initial time on the sliding surface s, the initial state of the integrator is defined as:

[0190]

[0191] Wherein x e0 is the deviation at the initial time of the system.

[0192] Therefore, at t=0, there is:

[0193]

[0194] Formula (21) shows that the system is in sliding mode motion state at the initial time, and there is no reaching phase; The control objective of the integral variable structure controller is to ensure that the system always moves on the sliding surface, and is not affected by system parameter errors and external disturbances; When the system is disturbed by external disturbance or system parameter error, it will deviate from the sliding surface, in order to pull it back to the sliding surface, the output of the sliding mode variable structure controller is decomposed into equivalent control and switching control as follows:

[0195] u = u eq + Δu (30) ;

[0196] where u eq is the equivalent control and Δu is the switching control.

[0197] The role of the equivalent control is to make the system state variable move along the sliding surface in ideal conditions; according to formula (29), the system state variable is on the sliding surface at the initial time, and the equivalent control only needs to control the ideal model of the system to make the derivative of the sliding surface function zero, so as to restrict the system movement from exceeding the sliding surface, that is, to make the system state move along the sliding surface, that is, to satisfy:

[0198]

[0199] From (31), the equivalent control is:

[0200]

[0201] Therefore, when designing the equivalent control, only the ideal model of the system needs to be considered, without considering the system parameter changes and external disturbances; therefore, the equivalent control only needs to ensure that the system state moves along the sliding surface in the ideal model; when the system parameter changes and external disturbances occur, the system state will leave the sliding surface, and the switching control will start to work to pull it back to the sliding surface; the design of the control Δu needs to make the sliding mode exist condition hold, that is, to satisfy:

[0202]

[0203] Substitute the equivalent control (32) into formula (33) to get:

[0204] s{-B(x)Δu-Bh(x,t)]}<0 (34) ;

[0205] Therefore, the selection of the switching control needs to make (34) hold.

[0206] 2. Define the input of the sliding mode controller as the difference between the system state variable and the reference value:

[0207]

[0208] Define the sliding surface equation of the system as the sum of the state variable of the sliding mode controller and its integral term:

[0209]

[0210] Define the initial state of the integrator:

[0211]

[0212] So at t = 0, we have: S P = S Q = 0.

[0213] From equation (24), we have:

[0214]

[0215] P ref and Q ref are constants, so

[0216] According to equation (38), we have its equivalent control:

[0217]

[0218] Considering parameter errors and external disturbances, we have its equivalent control, and at this time:

[0219]

[0220] Substituting equation (39) into equation (40), we have:

[0221]

[0222] To ensure that dV / dt < 0, a new reaching rate sliding mode control law is designed as:

[0223]

[0224] In the new reaching rate, the integral term ρ is used to replace X. When S approaches zero in the system sliding mode, the integral of S also approaches zero, and the value of ρ also approaches zero, and finally eliminates the sat(s) term. The introduction of the negative weight value K in the integral term can greatly reduce the switching gain, which can effectively avoid the phenomenon of large switching gain and severe chattering in the exponential reaching rate approaching stage.

[0225] Therefore, the control winding feedback voltage is:

[0226]

[0227] where X P and X Q are the differences between the active power direct current output by the brushless doubly-fed motor and the active power reference, and the difference between the reactive power direct current output by the brushless doubly-fed motor and the reactive power reference, S PΔ is the sliding variable of the power winding active power when considering system errors, which is the sum of the difference between the active power given value and the actual value and the integral term, S QΔ is the sliding variable of the power winding active power when considering system errors, which is the sum of the difference between the reactive power given value and the actual value and the integral term, C i is the sliding surface coefficient, and is the equivalent feedback voltage of stator control winding in positive sequence d-q axis rotating coordinate system, ΔU d and ΔU q is the equivalent control voltage of stator control winding in positive sequence d-q axis rotating coordinate system, H d and H q is the external disturbance.

[0228] ③ Construct Lyapunov function V, which is:

[0229]

[0230] The time derivative of the above Lyapunov function is:

[0231]

[0232] Again, formula (39) (40) is substituted into (43) to obtain:

[0233]

[0234] From formula (46), if S P > 0, then K q |p|S P sat(S P )|S p | a > 0, H q > 0, so dVs P / dt < 0; if S P < 0, then K q |p|S P sat(S P )|S p | a > 0, H q > 0, K q > H q , so dVs P / dt < 0; similarly, if S Q > 0, then K q |p|S Q sat(S Q )|S Q | a > 0, H d > 0, so dVs Q / dt < 0; if S Q < 0, then K d |p|S Q sat(S Q )|SQ | a > 0, H d > 0, K d > H d , so dV / dt < 0. Q / dt < 0.

[0235] From above, dV / dt < 0; from , we know that V > 0, so the system satisfies the Lyapunov asymptotic stability condition, and the actual values of active power and reactive power will converge to the given values.

[0236] ④ Build a negative sequence current tracking instruction control model and design an improved extended state observer, including the following steps:

[0237] From equation (20), we know that

[0238]

[0239] To suppress the double-frequency fluctuations of the system output power, the active power double-frequency sine wave fluctuation can be made to be zero, that is, P cos = P sin = 0; At this time, the given value of the negative sequence stator current in the negative sequence coordinate system can be obtained:

[0240]

[0241] Similarly, to suppress the double-frequency sine wave fluctuation of the reactive power, that is, Q cos = Q sin = 0, the corresponding negative sequence stator current instruction value can be obtained:

[0242]

[0243] However, under the condition of unbalanced grid voltage, the stator and rotor currents, the active power and reactive power on the stator side, etc. will all appear pulsations, and there are only two control quantities for the negative sequence component of the control winding current. Therefore, it is impossible to suppress all pulsations at the same time. Therefore, according to different control requirements, the control targets can be divided into four categories: suppressing stator current imbalance, suppressing control winding current imbalance, suppressing stator-side active power double-frequency disturbance, and suppressing reactive power double-frequency disturbance.

[0244] As follows:

[0245] Target 1: Suppress stator current asymmetry:

[0246]

[0247] Target 2: Suppress control winding current asymmetry:

[0248] The relationship between the stator power winding side negative sequence current, the stator control winding side negative sequence current and the rotor negative sequence current can be derived according to formula (18);

[0249]

[0250]

[0251] At this time, let The stator power winding negative sequence current instruction can be obtained:

[0252]

[0253] Target 3: suppress stator side active power pulsation:

[0254]

[0255] Target 4: suppress stator side reactive power pulsation:

[0256]

[0257] The stator control winding negative sequence voltage can be obtained by PI regulation through the obtained stator power winding negative sequence current instruction. Feedback of the stator control winding negative sequence voltage to the system can effectively suppress the twice-frequency power disturbance caused by the unbalanced power grid. The specific derivation steps are as follows:

[0258] According to the relationship between the negative sequence current and the negative sequence flux in formula (18), the following derivation can be made:

[0259]

[0260]

[0261] The expression of the stator control winding negative sequence current can be obtained according to the relationship between the stator power winding negative sequence current and the rotor negative sequence current in formulas (46) and (47):

[0262]

[0263] By substituting formula (48) into the control winding voltage in formula (19), the following can be obtained:

[0264]

[0265] Through the relationship between the stator control winding negative sequence voltage and the stator power winding negative sequence current, the stator power winding negative sequence current can be converted into the stator control winding negative sequence voltage by a PI controller and fed back to the system, thereby achieving the corresponding control target.

[0266] In the operation of the direct power control system of the brushless doubly-fed machine under unbalanced power grid, the control winding flux linkage, friction coefficient and the parameter variation of the internal system of the machine cannot be measured and will appear in the form of disturbance in the control system, which will affect the stable operation of the system. In this case, a new improved extended state observer is used to estimate the disturbance on line and compensate the estimated disturbance value to the system, so as to improve the stability of the system.

[0267] The state equations of the active power and the reactive power can be obtained from formula (23) and (24):

[0268]

[0269]

[0270] Therefore, the following can be obtained:

[0271]

[0272] wherein, r P and r Q are the variation rates of the disturbance d P and d Q of the system, respectively;

[0273] According to formula (61), the improved extended state observer is designed as (see Figure 4 ):

[0274]

[0275] In the formula, and are the estimated values of the active power and the reactive power output by the brushless doubly-fed machine, respectively; and are the estimated values of the disturbance d p and d Q , respectively; u Psmo = μ p sgn(x P ), u Qsmo = μ Q sgn(x Q ), μ p , μ Q are the observer parameters; wherein, μ p , μ Q need to be selected according to the sliding mode dynamic condition of the power state of the system, that is, the power sliding surface satisfies .

[0276] The state equations of the active power and the reactive power are:

[0277]

[0278] The error equation is obtained by subtracting equation (22) from equation (23):

[0279]

[0280] In the formula is the error between the active power and reactive power estimated value and the actual value, is the error between the disturbance estimated value and the actual value.

[0281] Substituting equation (64) into equation (62) gives:

[0282]

[0283] According to the positive and negative of e P , e Q , equation (65) can be expressed as follows:

[0284]

[0285]

[0286] The parameters μ P , μ Q in the extended state observer can be designed as follows according to equations (66) and (67):

[0287]

[0288] The specific values of μ P , μ Q in the extended state observer can be determined by equation (68) to complete error compensation.

[0289] S3: The mathematical model is simulated and verified by using Matlab / simulink simulation software. The results meet the requirements of direct power control, that is, the power given value should be stably tracked and the error should be within 100 W. The power double-frequency fluctuation caused by negative sequence voltage is suppressed. If it does not meet the requirements, the parameters are continuously debugged and optimized.

[0290] The simulation results of the new BDFG approach rate integral sliding mode variable structure direct power control when the speed is constant at 550 r / min, the reactive power given value is 5 kVar, and the active power given value is -10 kW are shown in Figure 6 .

[0291] Figure 6 The simulation results of the new BDFG approach rate integral sliding mode variable structure direct power control when the speed is constant at 550 r / min, the reactive power given value is 5 kVar, and the active power given value is -10 kW are shown in Figure 6It can be seen that both active and reactive power can track the set reference value well, and the power double frequency fluctuation is suppressed, which shows excellent static performance and tracking ability.

[0292] According to the control method, a BDFG direct power control system block diagram of a new type of approaching rate integral sliding mode variable structure control is drawn as shown in the figure. Figure 5

[0293] In the method of the brushless doubly-fed generator direct power control provided in the example, the decoupling control of active power and reactive power is realized by establishing the relationship equation between the control winding side voltage and the active power and reactive power of the power winding side, the power double frequency fluctuation is suppressed by adopting the negative sequence stator current instruction PI regulation, the control structure is simple, the mode is flexible, and the reaction speed of the system is improved; the control method adopts the new type of approaching rate integral sliding mode variable structure control, effectively reduces the chattering phenomenon in the sliding mode control, the whole control system overcomes the defects of the traditional direct power control, such as the unfixed frequency, the serious control current distortion, and the large static error, can realize the real-time stable tracking control of power, has high control precision, small overshoot, fast response speed, good dynamic and static performance, and high robustness, is suitable for the variable speed constant frequency power generation system, has good practicability, and is worth promoting.

[0294] Although some preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all the preferred embodiments and all the changes and modifications falling within the scope of the present application.

[0295] Obviously, various modifications and changes can be made to the present application by those skilled in the art without departing from the spirit and scope of the present application. Thus, if these modifications and changes fall within the scope of the claims and their equivalents, they are intended to be included in the present application.​

Claims

1. A direct power control method for a brushless doubly-fed generator under an unbalanced power grid, characterized in that, Includes the following steps: S1: Collect the three-phase stator voltage and current of the power winding and control winding in the stationary coordinate system, and obtain the stator flux amplitude, phase angle and frequency on the power winding side; S2, based on the positive and negative sequence fast separation method, the unbalanced grid voltage is decomposed into positive and negative sequence to obtain balanced positive, negative and zero sequence components; S3. Establish a positive and negative sequence mathematical model of a brushless doubly fed generator under a two-phase synchronous rotating coordinate system and an unbalanced power grid; and establish a relationship equation between the voltage on the control winding side and the active and reactive power on the power winding side based on the positive and negative sequence voltage vector, so as to realize the decoupled control of active and reactive power. S4, Construct a controller that combines integral sliding mode variable structure and PI regulation, and establish a mathematical model for direct power control of a brushless doubly-fed generator under an unbalanced power grid based on integral sliding mode variable structure; the mathematical model of the integral sliding mode variable structure is as follows: Wherein, ΔU d and ΔU q It is the equivalent control voltage of the switch in the positive sequence dq-axis rotating coordinate system of the stator control winding, S p S is the sliding variable of the active power of the system power winding, that is, the sum of the difference between the given value and the actual value of the active power and the integral term. Q ρ is the sliding variable of the active power of the system power winding, that is, the sum of the difference between the given value and the actual value of reactive power and the integral term, where ρ is the integral term and K is the weighting value. S5. Perform a stability analysis based on the Lyapunov function on the integral sliding mode variable structure controller to obtain the parameters of the stable control model; S6. Construct the corresponding extended state observer to estimate the disturbance of the brushless doubly fed motor direct power control system under unbalanced grid, and compensate the estimated disturbance value into the system. S7 sets the second harmonic fluctuation of active power and the second harmonic fluctuation of reactive power to zero, and then obtains the stator current and stator voltage in the negative sequence dq coordinate system of the control winding side. The obtained stator current in the negative sequence dq rotating coordinate system is used to obtain the given command signal of the stator voltage in the negative sequence dq rotating coordinate system of the control winding side through PI regulation. The output voltage of the converter on the control winding side is applied to the motor control side to suppress the power disturbance caused by the unbalanced power grid and make it work in a stable operating state.

2. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 1, characterized in that, In step S2, the positive and negative sequence voltage equations are obtained by decomposing the two-phase stationary rotating coordinate system into a two-phase rotating dq coordinate system: Among them, U a (t), U b (t) and U c (t) is the phase voltage of the three-phase stator power winding in the stationary abc coordinate system, U α (t), U β (t) represents the voltage components along the α and β axes of the two-phase stationary αβ coordinate system of the stator power winding. These are the positive and negative sequence αβ axis components of the stator power winding in the two-phase stationary αβ coordinate system after a T / 16 period delay. and These are the positive and negative sequence dq-axis components of the stator power winding in the two-phase rotating dq coordinate system.

3. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 1, characterized in that, In step S3, a mathematical model of a brushless doubly-fed generator is established in a two-phase positive-sequence (dq) rotating coordinate system. The voltage and flux linkage equations are shown below: in, and These represent the positive-sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the stator power winding, respectively. and These are the positive sequence current components of the stator power winding along the d-axis and q-axis in the rotating coordinate system. and These represent the positive-sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the stator control winding, respectively. and These are the positive sequence current components of the stator control winding along the d-axis and q-axis in the rotating coordinate system. and These represent the positive sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the rotor winding, respectively. and These are the positive sequence current components of the rotor winding along the d-axis and q-axis in the rotating coordinate system. and These are the positive sequence flux linkage components of the stator power winding along the d-axis and q-axis in the rotating coordinate system of the stator power winding, respectively. and These are the positive sequence flux linkage components of the d-axis and q-axis in the positive sequence dq-axis rotating coordinate system of the control winding, respectively. and R1, R2, and Rq are the positive-sequence flux linkage components along the d and q axes in the rotating coordinate system of the rotor windings, respectively. r These represent the resistances of the power winding, control winding, and rotor winding, respectively. L1 is the self-inductance of the power winding, and L2 is the self-inductance of the control winding. r For the rotor winding self-inductance, L 1r For the mutual inductance between the power winding and the rotor winding, L 2r To control the mutual inductance between the rotor winding and the rotor winding; ω + Let be the angular velocity of the reference coordinate system for arbitrary positive-sequence rotation of the power winding. To control the angular velocity of the reference coordinate system for arbitrary positive-sequence rotation of the winding, Let ω be the angular velocity of the reference coordinate system for the arbitrary speed positive sequence rotation of the rotor winding, and p = d / dt be the differential operator. in, and These represent the negative sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the stator power winding, respectively. and These are the negative sequence current components of the stator power winding along the d-axis and q-axis in the rotating coordinate system of the stator power winding, respectively. and These represent the negative-sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the stator control winding, respectively. and These are the negative sequence current components of the stator control winding along the d-axis and q-axis in the rotating coordinate system of the stator control winding, respectively. and These represent the negative sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the rotor winding, respectively. and These are the negative sequence current components of the rotor winding along the d-axis and q-axis in the rotating coordinate system of the d-q axis, respectively. and These are the negative sequence flux linkage components of the stator power winding along the d-axis and q-axis in the rotating coordinate system of the stator power winding, respectively. and These are the negative sequence flux linkage components of the d-axis and q-axis in the negative sequence dq-axis rotating coordinate system of the control winding, respectively. and R1, R2, and Rq are the negative-sequence flux linkage components along the d-axis and q-axis in the negative-sequence dq-axis rotating coordinate system of the rotor winding, respectively. r These represent the resistances of the power winding, control winding, and rotor winding, respectively. L1 is the self-inductance of the power winding, and L2 is the self-inductance of the control winding. r For the rotor winding self-inductance, L 1r For the mutual inductance between the power winding and the rotor winding, L 2r To control the mutual inductance between the rotor winding and the rotor winding; ω - Let be the angular velocity of the reference coordinate system for arbitrary negative-sequence rotation of the power winding. To control the angular velocity of the reference coordinate system for arbitrary negative sequence rotation of the winding, Let ω be the angular velocity of the reference coordinate system for the arbitrary speed negative sequence rotation of the rotor winding, and p = d / dt be the differential operator.

4. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 1, characterized in that, In step S3, the following equations are established to relate the voltage on the control winding side to the active and reactive power on the power winding side: Among them, P dc and Q dc These are the DC components of the active and reactive power of the stator power windings, respectively. and These represent the positive-sequence voltage components along the d-axis and q-axis in the rotating coordinate system of the stator control winding, respectively. Let be the positive-sequence flux linkage vector magnitude along the q-axis in the rotating coordinate system of the stator power winding. and These represent the positive-sequence flux linkage vector magnitudes along the d-axis and q-axis in the positive-sequence dq-axis rotating coordinate system, respectively. and Let ω represent the positive-sequence flux linkage components along the d-axis and q-axis in the positive-sequence dq-axis rotating coordinate system of the rotor winding, respectively. r Let ω be the rotor angular velocity in the system. + Let be the angular velocity of the reference coordinate system for arbitrary positive-sequence rotation of the power winding. To control the angular velocity of the reference coordinate system for arbitrary positive-sequence rotation of the winding, Let ω be the angular velocity of the reference coordinate system for the arbitrary speed positive sequence rotation of the rotor winding, p1 and p2 be the number of pole pairs of the stator power winding and the stator control winding, respectively, and p = d / dt be the differential operator.

5. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 4, characterized in that, In step S4, Define the input to the integral sliding mode controller as the difference between the system state variable and the reference value: The sliding surface equation of the system is defined as the sum of the state variables of the sliding mode controller and its integral terms: Define the initial state of the integrator: Therefore, at t=0, we have: S P =S Q =0; From equation (9), we can obtain: P ref and Q ref All are constants, therefore Among them, X P and X Q These represent the differences between the active power DC output of the brushless doubly-fed motor and the active power reference value, and the reactive power DC output and the reactive power reference value, respectively. P S is the sliding variable of the active power of the system power winding, that is, the sum of the difference between the given value and the actual value of the active power and the integral term. Q C is the sliding variable of the active power of the system power winding, i.e., the sum of the difference between the given and actual reactive power and the integral term. i It is the sliding surface coefficient, P ref Q ref P represents the given values ​​for the active power and reactive power of the system power winding, respectively. dc Q dc These are the DC quantities of the active and reactive power outputs of the power windings of a brushless doubly fed motor under an unbalanced power grid. The equivalent control can be obtained according to equation (13): in, L1 is the self-inductance of the power winding, L2 is the self-inductance of the control winding, L r For the rotor winding self-inductance, L 1r For the mutual inductance between the power winding and the rotor winding, L 2r To control the mutual inductance between the rotor winding and the rotor winding; When considering parameter errors and external disturbances, the equivalent switching control is calculated. Substituting equation (14) into equation (15), we get: To ensure dV / dt < 0, a sliding mode control law for the integral sliding mode variable structure with the following approach rate is constructed: In the approach rate, the integral term ρ replaces X. During the sliding mode phase of the system, when S approaches zero, the integral of S also approaches zero, the value of ρ also approaches zero, and the sat(s) term is eventually eliminated; K is the weighting value. Therefore, the control winding feedback voltage is: Among them, S PΔ To account for system errors, the sliding variable of the active power of the power winding is the sum of the difference between the given and actual active power values ​​and the integral term, S QΔ To account for system errors, the sliding variable of the active power of the power winding is the sum of the difference between the given and actual reactive power values ​​and the integral term, C i The sliding surface coefficient, and The equivalent feedback voltage ΔU is the positive sequence dq-axis rotating coordinate system of the stator control winding. d and ΔU q It is the equivalent control voltage of the switch in the positive sequence dq-axis rotating coordinate system of the stator control winding, H d and H q It is an external disturbance. The positive sequence flux linkage vector magnitude of the stator power winding on the q-axis in the rotating coordinate system of the positive sequence dq-axis.

6. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 5, characterized in that, In step S5, the following Lyapunov function V is constructed: Taking the time derivative of the Lyapunov function, we get: Substituting equations (14) and (15) into equation (20), we get:

7. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 1, characterized in that, In step S6, the following extended state observer is constructed: In the formula, and These are the estimated values ​​of the active power and reactive power output of the brushless doubly fed motor, respectively. and These are the disturbance quantities d p and d Q The estimated value; u Psmo =μ p sgn(e P ), u Qsmo =μ Q sgn(e Q ), μ p μ Q These are the observer parameters.

8. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 7, characterized in that, The μ p μ Q The selection is based on the sliding mode dynamics of the system's power state, i.e., the power sliding surface satisfies... condition; The state equations for active power and reactive power are: Taking the difference between equation (23) and equation (22), we obtain the error equation: In the formula This represents the error between the estimated and actual values ​​of active and reactive power. This represents the error between the estimated and actual disturbance values. Substituting equation (24) into equation (22), we get: According to e P e Q The sign of the positive or negative sign will be expressed in equation (25) as follows: Based on equations (26) and (27), the parameter μ is... P μ Q The design is as follows: The μ in the extended state observer is determined by equation (28). P μ Q The specific values ​​are used to complete error compensation.

9. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 1, characterized in that, In step S7, a stator power winding negative sequence current command is obtained based on at least one of the following control objectives: Control objective ①: Suppress stator current asymmetry: Control objective ②: Suppress control winding current asymmetry: Based on the relationship between the negative sequence current on the stator power winding side, the negative sequence current on the stator control winding side, and the negative sequence current on the rotor. make Stator power winding negative sequence current command received: Control objective ③: Suppress stator-side active power pulsation: Control objective ④: Suppress stator-side reactive power pulsation:

10. The direct power control method for a brushless doubly-fed generator under an unbalanced power grid according to claim 1, characterized in that, In step S7, based on the relationship between the stator power winding negative sequence current and the rotor negative sequence current, the expression for the stator control winding negative sequence current is obtained as follows: Substituting equation (35) into the control winding voltage in equation (8), we get: By leveraging the relationship between the negative sequence voltage of the stator control winding and the negative sequence current of the stator power winding, a PI controller converts the negative sequence current of the stator power winding into the negative sequence voltage of the stator control winding and feeds it back into the system, thereby achieving the corresponding control objective.

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