Active power calculation method and system for doubly-fed wind turbine based on fault ride-through process

Abstract

The present application relates to a kind of active power calculation method and system of double-fed fan based on fault ride-through process, belong to power system fault analysis technical field, solve the problem that the active power in the fault ride-through process obtained is not accurate in the transient characteristic research of double-fed fan in prior art ignores the influence of fan terminal voltage phase angle jump.For example: when detecting that symmetric fault occurs at the outlet of double-fed fan, obtain rotor current before fault, and fault point voltage, voltage drop rate and voltage phase angle jump angle after fault;According to the information obtained, the voltage jump angle after fault is obtained, and then the current and voltage of the stator and rotor of double-fed fan in the fault ride-through process are obtained;Based on the current and voltage of the stator and rotor of double-fed fan in the fault ride-through process, the active power outputted by the stator and rotor side in the fault ride-through process is obtained, and then the active power outputted by double-fed fan in the fault ride-through process is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system fault analysis, and particularly relates to a method and system for calculating active power of a doubly-fed wind turbine based on a fault ride-through process. BACKGROUND

[0002] With the increase of the grid-connected capacity of the doubly-fed wind turbine, the accuracy requirement of the short-circuit current for the power grid protection is higher and higher. Inaccurate short-circuit current characteristics will affect the results of fault analysis, and further cause errors in the evaluation of the protection action characteristics. Therefore, it is of great significance to study the transient characteristics of the wind turbine after a fault.

[0003] However, most of the existing researches on the fault current of the doubly-fed wind turbine ignore the influence of the phase angle jump of the wind turbine terminal voltage and the typical phase-locked loop control link. Ignoring these links will affect the transient characteristics of the fault current, so that the active power in the fault ride-through process obtained is not accurate, which is inconsistent with the engineering practice, and thus the subsequent research and discussion based thereon will also have greater errors. SUMMARY

[0004] In view of the above analysis, the embodiments of the present application aim to provide a method and system for calculating active power of a doubly-fed wind turbine based on a fault ride-through process, so as to solve the problem that the existing research on the transient characteristics of the doubly-fed wind turbine ignores the influence of the phase angle jump of the wind turbine terminal voltage, and the active power in the fault ride-through process obtained is not accurate.

[0005] In one aspect, the embodiments of the present application provide a method for calculating active power of a doubly-fed wind turbine based on a fault ride-through process, comprising the following steps:

[0006] When a symmetrical fault at the outlet of the doubly-fed wind turbine is detected, the rotor current before the fault, the fault point voltage after the fault, the voltage drop rate and the voltage phase angle jump angle are obtained;

[0007] According to the obtained information, the voltage jump angle after the fault is obtained, and then the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process are obtained.

[0008] Based on the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process, the active power output by the stator and rotor side in the fault ride-through process is obtained, and then the active power output by the doubly-fed wind turbine in the fault ride-through process is obtained.

[0009] Further, the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process are obtained by the following way:

[0010] Based on the obtained fault point voltage after the fault and the phase-locked loop control, the voltage jump angle after the fault is obtained, and then a mathematical model of the doubly-fed wind turbine is established based on the motor convention;

[0011] Based on the obtained post-fault fault point voltage drop rate and voltage phase angle jump angle, the stator voltage in the fault ride-through process is obtained;

[0012] Based on the mathematical model of the double-fed wind turbine and the stator voltage in the fault ride-through process, the induced electromotive force of the post-fault stator flux linkage on the rotor of the double-fed wind turbine is obtained, and then the rotor voltage in the fault ride-through process is obtained.

[0013] Based on the obtained rotor voltage in the fault ride-through process, the rotor current before the fault, and the stator flux linkage oriented vector control mode of the rotor side converter, the rotor current equation is obtained, and then the rotor current in the fault ride-through process is obtained, and then the stator current in the fault ride-through process is obtained.

[0014] Further, taking any time before the fault occurs as the initial time, the fault occurs at time t1, and the double-fed wind turbine is in the fault ride-through process at time t, wherein t≥t1; the active power P(t) and P(t) output by the stator and the rotor at time t are respectively represented as: s r

[0015]

[0016]

[0017] In the formula, u(t) and u(t) respectively represent the stator and rotor voltages in the fault ride-through process at time t, s2 r s r

[0018] Further, the active power P(t) output by the double-fed wind turbine in the fault ride-through process at time t is represented as:

[0019] P(t)=P s (t)+P r (t).

[0020] Further, the stator and rotor voltages u(t) and u(t) in the fault ride-through process at time t are respectively represented as: s2 r

[0021]

[0022] u r (t)=(R r +jω(t)σL r ​​​​​​​​​i r (t)+σL r Di r (t)+e r (t)

[0023] wherein,

[0024] ω(t) = ω e (t)-ω r ,

[0025]

[0026]

[0027] wherein, k represents a fault point voltage drop rate, U s represents a stator voltage amplitude when the doubly-fed wind turbine is in steady state, ω1 represents a synchronous angular velocity before the fault, ω r represents a rotor electric angular velocity, represents a voltage phase angle jump angle after the fault, R r represents a wind turbine rotor side resistance, L s , L r respectively represent stator and rotor equivalent two-phase winding self-inductances in the dq coordinate system, L m represents a mutual inductance between the stator and rotor coaxial equivalent windings in the dq coordinate system, D represents a differential operator, e r (t) represents an induced electromotive force generated by the stator flux linkage at the tth moment on the doubly-fed wind turbine rotor; and Δθ(t) represents a voltage jump angle at the tth moment.

[0028] Further, the induced electromotive force e r (t) generated by the stator flux linkage at the tth moment on the doubly-fed wind turbine rotor is represented as:

[0029]

[0030] wherein,

[0031]

[0032]

[0033]

[0034]

[0035]

[0036] wherein, R s represents a wind turbine stator side resistance.

[0037] Further, the voltage jump angle Δθ(t) at the tth moment is expressed as:

[0038]

[0039] wherein,

[0040]

[0041]

[0042] wherein, U m represents the fault point voltage amplitude, k ppll , k ipll respectively represent the proportional and integral constants of the phase-locked loop PI controller, Δθ (0) represents the stator terminal voltage phase angle jump value at the fault moment.

[0043] Further, the stator and rotor currents i s (t) and i r (t) during the fault ride-through process at the tth moment are respectively expressed as:

[0044] i s (t) = i sf1 + i sf2 (t) + i sf3 (t) + i sn (t)

[0045] i r (t) = i rf1 + i rf2 (t) + i rn (t)

[0046] wherein,

[0047]

[0048]

[0049] wherein, i r_ref represents the rotor current reference value of the doubly-fed wind turbine during the fault, k p , k i respectively represent the proportional and integral constants of the PI controller, i r0 represents the rotor current during normal operation.

[0050] Further, the rotor current reference value i r_ref of the doubly-fed wind turbine during the fault is expressed as:

[0051] i r_ref = i rd_ref + j i rq_ref

[0052] wherein,

[0053]

[0054] wherein, i rd_ref , i rq_ref respectively represent the reference value of rotor d, q axis current component, I rN , i rmax respectively represent the rated current and the maximum current limiting current of rotor, K d represents the reactive current gain coefficient, P0 and Q0 respectively represent the active and reactive power when the wind turbine is normally operated.

[0055] On the other hand, the embodiment of the present application provides an active power calculation system of a doubly-fed wind turbine based on a fault ride-through process, comprising:

[0056] a data acquisition module, which acquires the rotor current before the fault, and the fault point voltage, voltage drop rate and voltage phase angle jump angle after the fault;

[0057] a stator and rotor current and voltage calculation module, which is used to obtain the voltage jump angle after the fault, and then obtain the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process according to the acquired information;

[0058] an active power calculation module, which is used to obtain the active power output on the stator and rotor sides based on the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process, and then obtain the active power output of the doubly-fed wind turbine after the fault.

[0059] Compared with the prior art, the present application can achieve the following beneficial effects:

[0060] The active power calculation method and system of a doubly-fed wind turbine based on a fault ride-through process provided by the present application add a typical phase-locked vector control strategy in the analysis process, the jump angle is analyzed through the control link, then the stator and rotor currents considering the influence of voltage phase angle jump under the fault ride-through condition are deduced, and then the transient active power output of the doubly-fed wind turbine in the fault ride-through process is obtained in combination with the internal energy flow and structure of the doubly-fed induction wind turbine, which is closer to the transient characteristics of the doubly-fed wind turbine and also more in line with the engineering practice.

[0061] The above technical solutions can be combined with each other in the present application to realize more preferred combination solutions. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification, or will be understood by implementing the present application. The purpose and other advantages of the present application can be realized and obtained from the contents specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0062] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and are used to explain the principles of the application, but not to limit the scope of the application.

[0063] Figure 1 A flowchart of a method for calculating active power of a doubly-fed wind turbine based on a fault ride-through process is provided for Embodiment 1 of the application.

[0064] Figure 2 A schematic diagram of a phase-locked loop working principle is provided for Embodiment 1 of the application.

[0065] Figure 3 A schematic diagram of a rotor-side converter current inner loop control principle is provided for Embodiment 1 of the application.

[0066] Figure 4 A schematic diagram of a doubly-fed wind power generation system framework is provided for Embodiment 1 of the application.

[0067] Figure 5 A schematic diagram of a power grid structure of a doubly-fed wind turbine is provided for Embodiment 3 of the application.

[0068] Figure 6 A stator A-phase current when the terminal voltage of the doubly-fed wind turbine drops to 40% is provided for Embodiment 3 of the application.

[0069] Figure 7 A stator A-phase current when the terminal voltage of the doubly-fed wind turbine drops to 60% is provided for Embodiment 3 of the application.

[0070] Figure 8 A stator A-phase current when the terminal voltage of the doubly-fed wind turbine drops to 80% is provided for Embodiment 3 of the application.

[0071] Figure 9 Active power when the terminal voltage of the doubly-fed wind turbine drops to 40% is provided for Embodiment 3 of the application.

[0072] Figure 10 Active power when the terminal voltage of the doubly-fed wind turbine drops to 60% is provided for Embodiment 3 of the application.

[0073] Figure 11 Active power when the terminal voltage of the doubly-fed wind turbine drops to 80% is provided for Embodiment 3 of the application. DETAILED DESCRIPTION

[0074] The preferred embodiments of the present application will be described in detail with reference to the drawings, in which:

[0075] Embodiment 1

[0076] One specific embodiment of the present application discloses a method for calculating active power of a doubly-fed wind turbine based on a fault ride-through process, as shown in the following steps: Figure 1

[0077] S1, when detecting that a symmetrical fault occurs at the outlet of the doubly-fed wind turbine, obtaining the rotor current before the fault, and the fault point voltage, voltage drop rate and voltage phase angle jump angle after the fault.

[0078] S2, obtaining the voltage jump angle after the fault according to the obtained information, and then obtaining the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process.

[0079] In the implementation, the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process are obtained in step S2 by the following method:

[0080] S21, obtaining the voltage jump angle after the fault based on the obtained fault point voltage after the fault and the phase-locked loop control, and then establishing a mathematical model of the doubly-fed wind turbine based on the motor convention;

[0081] S22, obtaining the stator voltage in the fault ride-through process based on the obtained fault point voltage drop rate and voltage phase angle jump angle after the fault;

[0082] S23, obtaining the induced electromotive force of the stator flux linkage on the rotor of the doubly-fed wind turbine after the fault based on the mathematical model of the doubly-fed wind turbine and the stator voltage in the fault ride-through process, and then obtaining the rotor voltage in the fault ride-through process;

[0083] S24, obtaining the rotor current equation based on the obtained rotor voltage in the fault ride-through process, the rotor current before the fault and the stator flux linkage oriented vector control mode of the rotor side converter, and then obtaining the rotor current in the fault ride-through process, and further obtaining the stator current in the fault ride-through process.

[0084] S3, obtaining the active power output by the stator and rotor in the fault ride-through process based on the current and voltage of the stator and rotor of the doubly-fed wind turbine in the fault ride-through process, and then obtaining the active power output by the doubly-fed wind turbine in the fault ride-through process.

[0085] Compared with the prior art, the embodiment provides a method for calculating active power of a doubly-fed wind turbine based on a fault ride-through process, which adds a typical phase-locked vector control strategy in the analysis process, analyzes the jump angle through the control link, and then deduces the stator and rotor currents considering the influence of voltage phase angle jump in the fault ride-through process. In combination with the internal energy flow and structure of the doubly-fed induction wind turbine generator, the transient active power output by the doubly-fed wind turbine in the fault ride-through process is obtained, which is closer to the transient characteristics of the doubly-fed wind turbine generator and also more consistent with the engineering practice.

[0086] ​In implementation, any time before the fault occurrence time is taken as the initial time, and the fault occurrence time is t1, the doubly-fed wind turbine is in the fault ride-through process at the t time, where t≥t1; the active power P(t) output by the doubly-fed wind turbine in the fault ride-through process at the t time is expressed as:

[0087] P(t) = P s (t) + P r (t).

[0088] In the formula, P s (t) and P r (t) represent the active power output at the stator side and the rotor side at the t time, respectively.

[0089] In implementation, the active power P s (t) and P r (t) output at the stator side and the rotor side at the t time are respectively expressed as:

[0090]

[0091]

[0092] In the formula, u s2 (t) and u r (t) represent the stator voltage and the rotor voltage in the fault ride-through process at the t time, respectively, and represent the conjugate vectors of the stator current i s (t) and the rotor current i r (t) in the fault ride-through process at the t time, and Re() represents the real part of a complex number.

[0093] Specifically, the stator voltage u s2 (t) and the rotor voltage u r (t) in the fault ride-through process at the t time are respectively expressed as:

[0094]

[0095] u r (t) = (R r + jω(t)σL r )i r (t) + σL r Di r (t) + e r (t)

[0096] wherein,

[0097] ω(t) = ω e (t) - ω r ,

[0098]

[0099]

[0100] where k represents the fault point voltage drop rate, U s represents the stator voltage amplitude of the doubly-fed wind turbine in steady state, ω1 represents the synchronous angular velocity before the fault, ω r represents the rotor electric angular velocity, represents the voltage phase angle jump angle after the fault, R r represents the wind turbine rotor side resistance, L s , L r respectively represent the dq coordinate system equivalent two-phase winding self-inductance of the stator and the rotor, L m represents the dq coordinate system equivalent winding mutual inductance between the stator and the rotor, D represents the differential operator, e r (t) represents the induced electromotive force generated by the stator flux linkage at the tth moment on the doubly-fed wind turbine rotor; and Δθ(t) represents the voltage jump angle at the tth moment.

[0101] More specifically, the induced electromotive force e r (t) generated by the stator flux linkage at the tth moment on the doubly-fed wind turbine rotor is represented as:

[0102]

[0103] wherein,

[0104]

[0105]

[0106]

[0107]

[0108]

[0109] wherein R s represents the wind turbine stator side resistance, u s1 represents the stator voltage before the fault.

[0110] More specifically, the voltage jump angle Δθ(t) at the tth moment is represented as:

[0111]

[0112] wherein,

[0113]

[0114]

[0115] wherein U m represents the fault point voltage amplitude, k ppll , k ipll represent the proportional and integral constants of the phase-locked loop PI controller, respectively, Δθ (0) represents the phase angle jump value of the machine terminal voltage at the fault moment.

[0116] In the embodiment, the stator and rotor currents i s (t) and i r (t) in the fault ride-through process at the tth moment are respectively represented as:

[0117] i s (t) = i sf1 + i sf2 (t) + i sf3 (t) + i sn (t)

[0118] i r (t) = i rf1 + i rf2 (t) + i rn (t)

[0119] wherein,

[0120]

[0121]

[0122] wherein i r_ref represents the rotor current reference value of the doubly-fed wind turbine during the fault, k p , k i represent the proportional and integral constants of the PI controller, respectively, i r0 represents the current when the rotor is normally operated.

[0123] In particular, the rotor current reference value i r_ref of the doubly-fed wind turbine during the fault is represented as:

[0124] i r_ref = i rd_ref + j i rq_ref

[0125] wherein,

[0126]

[0127] wherein i rd_ref , i rq_ref represent the reference values of the rotor d-axis and q-axis current components, respectively, I rN , i rmaxK represents the rotor rated current and the maximum current limiting current, respectively. d The value represents the reactive current gain coefficient, and P0 and Q0 represent the active and reactive power of the wind turbine during normal operation, respectively.

[0128] It should be noted that the active power obtained in this embodiment during fault ride-through is based on the following derivation:

[0129] It should be noted that in the following derivation process, any time before the fault occurs is taken as the initial time, so the fault occurs at time t1. At time t, the doubly fed wind turbine is in the fault ride-through process, where t≥t1.

[0130] First, considering the influence of the phase-locked loop, a mathematical model of the doubly fed fan after a fault is established based on the motor convention.

[0131] When a symmetrical fault occurs at the fault point of a doubly-fed induction generator (DFIG), let the voltage jump angle be Δθ(t) and the phase angle before the fault be θ1. Then, the voltage jump angle after the fault is θ(t) = Δθ(t) + θ1. The working principle of the phase-locked loop (PLL) is as follows: Figure 2 As shown, its function is to correct the voltage jump angle. The control loop is represented as follows:

[0132]

[0133] In the formula, Δω(t) represents the angular velocity difference caused by the voltage jump angle at time t, u sq (t) represents the q-axis component of the stator voltage of the wind turbine.

[0134] Combining the equations and performing a Taylor expansion, we obtain the second-order differential equation concerning the jump angle:

[0135]

[0136] Therefore, the analytical expression for the voltage jump angle after a fault is obtained:

[0137]

[0138] in,

[0139]

[0140]

[0141] In the formula, U m k represents the voltage amplitude at the fault point. ppll k ipll These represent the proportional and integral constants of the phase-locked loop PI controller, respectively, and Δθ (0) This indicates the voltage phase angle jump value at the moment of the fault.

[0142] The doubly-fed wind turbine in this embodiment is a doubly-fed induction generator (DFIG), and its electromagnetic transient model is a high-order, nonlinear, strongly coupled multivariable model. Based on the mathematical model of the motor convention, the mathematical model of the doubly-fed wind turbine in the fault ride-through process is obtained after considering the influence of the phase-locked loop, and is expressed as:

[0143]

[0144] wherein, ω(t) = ω e (t) - ω r ,

[0145] In the formula, u s (t) and u r (t) are the stator and rotor voltages of the wind turbine, i s (t) and i r (t) are the stator and rotor currents of the wind turbine, R s and R r are the stator and rotor side resistances of the wind turbine, L s and L r are the equivalent two-phase winding self-inductances of the stator and rotor in the dq coordinate system, L m is the mutual inductance between the coaxial equivalent windings of the stator and rotor in the dq coordinate system, ω1 is the synchronous angular velocity before the fault, D is a differential operator, ψ s (t) and ψ r (t) are the stator and rotor flux linkage vectors, ω(t) is the slip angular velocity, and ω r represents the rotor electric angular velocity.

[0146] It should be noted that when the wind turbine is in steady operation, the size and direction of the stator and rotor voltage and current vectors in the synchronous coordinate system remain unchanged.

[0147] Second, according to the fault point voltage drop degree and voltage phase angle jump angle when the symmetric fault occurs at the outlet of the wind turbine, the stator voltage before and after the fault is obtained.

[0148] When the symmetric fault, i.e., three-phase symmetric short-circuit fault, occurs in the doubly-fed wind turbine at t1, the wind turbine terminal voltage amplitude drops, and the rotor speed is constant during the drop, so the stator voltage before and after the fault is obtained:

[0149]

[0150] In the formula, u s1 represents the stator voltage before the fault, and u s2 (t) represents the stator voltage in the fault ride-through process at the tth moment, i.e., the stator voltage after the fault.

[0151] Third, combined with the control link of the doubly-fed wind turbine and the fault parameter information, the stator and rotor currents in the fault ride-through process are obtained.

[0152] Neglecting the stator resistance voltage drop, first assume that the DFIG rotor is open-circuited, i.e. the rotor side does not provide excitation voltage, then the first-order differential equation about the stator flux linkage is obtained from equation (4):

[0153]

[0154] wherein, τ1(t) represents the stator flux linkage decay time constant.

[0155] The general solution of the above equation is obtained:

[0156]

[0157] Further, the particular solution is obtained, which is expressed as:

[0158]

[0159] In the formula, A is a coefficient, and p is an exponential coefficient;

[0160] Let

[0161]

[0162] Substitute equation (8) into equation (9) to obtain:

[0163]

[0164] Simplify to obtain:

[0165]

[0166] Then p = -τ1(t).

[0167] Since the stator flux linkage does not change abruptly, the stator flux linkage before and after the fault is equal, i.e.

[0168]

[0169] Then

[0170]

[0171] That is, the stator flux linkage after the fault is:

[0172]

[0173] Therefore, the induced electromotive force generated by the stator flux linkage on the DFIG rotor after the fault is:

[0174]

[0175] where, s(t) is the slip of the induction generator, e'(t) is the periodic component of the induced voltage, e d (t) is the initial value of the transient induced voltage.

[0176] The rotor voltage equation is obtained according to equation (4) as follows:

[0177]

[0178] where, σ is the leakage coefficient of the generator, σL r is the transient inductance of the rotor.

[0179] Substituting equation (15) into equation (16) gives the rotor voltage after the fault:

[0180] u r (t) = (R r +jω(t)σL r )i r (t) + σL r Di r (t) + e r (t) (17)

[0181] In order to simplify the analysis, this embodiment only considers the effect of the rotor-side converter. The stator flux linkage oriented vector control method is often used in the control of the rotor-side converter of a DFIG, and the active and reactive power decoupling control is realized by using the feedforward compensation of the coupling term. The current inner loop control circuit of the rotor-side converter is shown in Figure 3 , i rd_ref , i rq_ref are the reference values of the rotor d-axis and q-axis current components, which are determined by the reference values of the active and reactive power, or are used to realize maximum wind speed tracking and terminal voltage control, u rd_ref , u rq_ref are the rotor voltage reference values required for tracking the rotor current.

[0182] Assuming that the closed-loop bandwidth of the current control circuit is large enough, the AC side voltage of the converter can well track the reference value, and the rotor voltage vector in the synchronous rotating coordinate system during the grid short circuit can be written as:

[0183] u r (t) = k p (i r_ref -i r (t)) + k i ∫(i r_ref -i r(t)dt - jω(t)σL r i r (t) (18)

[0184] where k p and k i are the proportional and integral constants of the PI controller respectively.

[0185] When the DFIG is in the process of fault ride-through, i.e. the voltage at the point of interconnection of the wind turbine drops to 20% to 90% of the rated voltage, the wind farm should provide dynamic reactive current to support the grid voltage. According to the LVRT standard, the command values of the DFIG rotor active and reactive currents during the grid fault should be:

[0186]

[0187] where I rN and i rmax are the rated current and the maximum current limiting current of the rotor respectively, P0 and Q0 are the active and reactive power of the wind turbine in normal operation, and K d is the reactive current gain coefficient.

[0188] According to equation (19), the reference value of the DFIG rotor current during the fault is:

[0189] i r_ref (t) = i rd_ref + ji rq_ref (20)

[0190] Substituting equation (18) into equation (17), the rotor current equation is:

[0191]

[0192] where,

[0193] Solving the second-order differential equation of the rotor current in equation (21), the rotor current expression after the grid symmetrical short circuit is:

[0194] i r (t) = i rf1 + i rf2 (t) + i rn (t) (22)

[0195] where,

[0196] where i r0 is the current of the rotor in normal operation.

[0197] According to equation (4), the stator current is:

[0198]

[0199] Substituting equations (14) and (22) into equation (23) and considering the rotor-side converter control, the expression of the stator current of the DFIG during symmetrical short circuit is:

[0200] i s (t) = i sf1 i sf2 (t) + i sf3 (t) + i sn (t) (24)

[0201] where,

[0202] The short circuit current instantaneous expression is obtained by transforming to the stator three-phase stationary coordinate system:

[0203]

[0204] where, C 2r / 3s is the two-phase rotating to three-phase stationary coordinate transformation matrix, θ1' is the angle of the d-axis leading the a-axis;

[0205] where, i sA , i sB , i sC represent the stator three-phase currents after the fault in the three-phase stationary coordinate system.

[0206] Fourth, the active power output by the doubly-fed wind turbine after the fault is obtained according to the structure and fault condition inside the wind turbine.

[0207] The schematic diagram of the energy flow inside the wind turbine is shown in Figure 4 The active power delivered to the grid by the doubly-fed wind turbine is equal to the sum of the active power of the stator side and the active power on the rotor-side converter. In the ideal case, the active power of the rotor-side converter is equal to that of the grid-side converter, i.e., P r = P g , so the active power delivered to the grid is equal to the sum of the active power of the stator side and the rotor side, which is expressed as:

[0208] P(t) = P s (t) + P r (t) (26)

[0209] The active power output by the stator side during the fault ride-through process is obtained from equation (5) and equation (24):

[0210]

[0211] It should be noted that the negative sign in the coefficient is because the stator winding adopts the motor convention to define the flow direction of power. Because the coordinate transformation adopts the amplitude constant principle, is the conjugate vector of the stator current.

[0212] Similarly, the active power output of the rotor side during the fault ride-through process can be obtained from formula (17) and (22):

[0213]

[0214] Embodiment 2

[0215] In one specific embodiment 2 of the present application, an active power calculation system of a doubly-fed wind turbine based on a fault ride-through process is provided, comprising:

[0216] A data acquisition module acquires the rotor current before the fault, and the fault point voltage, voltage drop rate and voltage phase angle jump angle after the fault;

[0217] A stator and rotor current and voltage calculation module is configured to obtain the voltage jump angle after the fault based on the acquired information, and further obtain the current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process;

[0218] An active power calculation module is configured to obtain the active power output on the stator and rotor side based on the current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process, and further obtain the active power output of the doubly-fed wind turbine after the fault.

[0219] The specific implementation process of the embodiment of the present application can be referred to the above method embodiment, which will not be described here.

[0220] Since the principle of the present embodiment is the same as that of the above method embodiment, the present system also has the corresponding technical effects of the above method embodiment.

[0221] Embodiment 3

[0222] To verify the correctness of embodiments 1 and 2 of the present application, the scheme in the above embodiments is tested, and the main parameters of the doubly-fed wind turbine are shown in Table 1, and the grid structure diagram of the doubly-fed wind turbine is shown in Figure 5 .

[0223] Table 1 Main parameters of the doubly-fed wind turbine and the wind farm

[0224]

[0225] Assuming that the initial time is 2s before the fault occurs, at t=2s, a three-phase symmetrical short-circuit fault occurs on the high-voltage side of the transformer, and the DFIG terminal voltage drops to 40%, 60% and 80%, respectively. The comparison chart of the calculated value and the simulation value is shown in Figure 6 , Figure 7 , Figure 8 .

[0226] From Figure 6 , Figure 7 , Figure 8 it can be seen that the calculated value of the stator short-circuit current is consistent with the simulation value in size and variation trend, and the transient decay is obvious, which is consistent with the transient characteristics of the stator short-circuit current at the fault moment and during the fault process.

[0227] Suppose that 2s before the fault moment is the initial moment, then at t=2s, a three-phase symmetrical short-circuit fault occurs at the outlet of the doubly-fed wind turbine, and the fault point voltage drops to 40%, 60% and 80% respectively. The comparison chart of the calculated value and the simulation value is shown in Figure 9 , Figure 10 , Figure 11 .

[0228] From Figure 9 , Figure 10 , Figure 11 it can be seen that the calculated result of the active power output of the doubly-fed wind turbine after the symmetrical fault is basically consistent with the simulation result in trend and value, and can reflect the transient characteristics of the active power during the fault.

[0229] Those skilled in the art can understand that all or part of the processes of the above-mentioned embodiment methods can be completed by a computer program instructing related hardware, and the program can be stored in a computer readable storage medium. The computer readable storage medium is a disk, an optical disk, a read-only memory or a random access memory, etc.

[0230] The above is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any changes or replacements within the technical scope disclosed by the present application can be easily thought by those skilled in the art, and should be covered within the protection scope of the present application.

Claims

1. A method for calculating active power of a doubly-fed wind generator based on a fault ride-through process, characterized by, The method comprises the following steps: When a symmetrical fault at the outlet of the doubly-fed wind turbine is detected, the rotor current before the fault, the fault point voltage after the fault, the voltage drop rate and the voltage phase angle jump angle are obtained; According to the obtained information, the voltage jump angle after the fault is obtained, and then the current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process are obtained; Based on the current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process, the active power output by the stator and rotor sides during the fault ride-through process is obtained, and then the active power output by the doubly-fed wind turbine during the fault ride-through process is obtained.

2. The method of claim 1, wherein, The current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process are obtained in the following manner: Based on the obtained fault point voltage after the fault and the phase-locked loop control, the voltage jump angle after the fault is obtained, and then a mathematical model of the doubly-fed wind turbine is established based on the motor convention; Based on the obtained voltage drop rate and voltage phase angle jump angle of the fault point after the fault, the stator voltage during the fault ride-through process is obtained; Based on the mathematical model of the doubly-fed wind turbine and the stator voltage during the fault ride-through process, the induced electromotive force generated by the stator flux linkage of the doubly-fed wind turbine on the rotor after the fault is obtained, and then the rotor voltage during the fault ride-through process is obtained; Based on the obtained rotor voltage during the fault ride-through process, the rotor current before the fault and the stator flux linkage oriented vector control mode of the rotor side converter, the rotor current equation is obtained, and then the rotor current during the fault ride-through process is obtained, and then the stator current during the fault ride-through process is obtained.

3. The method of claim 2, wherein the method further comprises: With any time before the fault occurrence time as the initial time, the fault occurrence time is t1, and the doubly-fed wind turbine is in the fault ride-through process at the t time, wherein t≥t1; the active power P s (t) and P r (t) output by the stator and the rotor at the t time are respectively represented as: wherein u s2 (t), u r (t) and i (t) and i s (t) and i r (t) are the stator and rotor voltages, respectively, during the fault ride-through process at the t-th time instant, and s (t) and i r (t) are the stator and rotor currents, respectively, during the fault ride-through process at the t-th time instant, and s (t) and i r (t) are the stator and rotor currents, respectively, during the fault ride-through process at the t-th time instant, and s (t) and i r (t) are the stator and rotor currents, respectively, during the fault ride-through process at the t-th time instant, and s (t) and i 4. The method of claim 3, wherein the fault ride-through procedure based active power calculation method for a doubly-fed wind turbine is characterized by, The active power P(t) output by the doubly-fed wind turbine during the fault ride-through process at the tth moment is represented as: P(t) = P s (t) + P r (t).

5. The method of claim 3, wherein, The stator and rotor voltage u s2 (t), u r (t), respectively u r (t) = (R r +jω(t)σL r )i r (t) + σL r Di r (t) + e r (t) Wherein, ω(t) = ω e (t) - ω r , where k represents the fault point voltage drop rate, U s represents the stator voltage amplitude of the doubly-fed wind turbine in steady state, ω1 represents the synchronous angular velocity before the fault, ω r represents the rotor electric angular velocity, represents the voltage phase angle jump angle after the fault, R r represents the wind turbine rotor side resistance, L s , L r respectively represent the dq coordinate system equivalent two-phase winding self-inductance of the stator and the rotor, L m represents the mutual inductance between the dq coordinate system equivalent windings of the stator and the rotor, D represents a differential operator, e r (t) represents the induced electromotive force of the stator flux linkage at the tth moment on the doubly-fed wind turbine rotor; and Δθ(t) represents the voltage jump angle at the tth moment.

6. The method of claim 5, wherein the method further comprises: The stator flux linkage at the tth moment generates an induced electromotive force e on the doubly-fed wind turbine rotor r (t), is expressed as: Wherein, In the formula, R s represents the stator-side resistance of the fan.

7. The method of claim 5, wherein the fault ride-through procedure based active power calculation for a doubly-fed wind turbine is characterized by, The voltage jump angle Δθ(t) at the tth moment is represented as: Wherein, In the formula, U m represents the fault point voltage amplitude, k ppll , k ipll respectively represent the proportional and integral constants of the phase-locked loop PI controller, Δθ (0) represents the phase angle jump value of the machine terminal voltage at the fault moment.

8. The method of claim 7, wherein the method further comprises: The stator and rotor currents i s (t), i r (t), respectively i s (t) = i sf1 + i sf2 (t) + i sf3 (t) + i sn (t) i r (t) = i rf1 +i rf2 (t) + i rn (t) Wherein, where i r_ref represents the rotor current reference value during the fault, k p , k i represent the proportional and integral constants of the PI controller, i r0 represents the current when the rotor is operating normally.

9. The method of claim 8, wherein, The fault period double-fed wind turbine rotor current reference value i r_ref is expressed as: i r_ref = i rd_ref + j rq_ref Wherein, where i rd_ref , i rq_ref represent the reference values of rotor d, q-axis current components, I rN , i rmax represent the rated current and the maximum current limiting current of the rotor, K d represents the reactive current gain coefficient, P0, Q0 represent the active and reactive power when the wind turbine is normally operated.

10. A system for calculating active power of a doubly-fed wind generator based on a fault ride-through process, characterized by, The method comprises the following steps: The data acquisition module obtains the rotor current before the fault, the fault point voltage after the fault, the voltage drop rate and the voltage phase angle jump angle; The stator and rotor current and voltage calculation module is configured to obtain the voltage jump angle after the fault according to the obtained information, and then obtain the current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process; The active power calculation module is configured to obtain the active power output by the stator and rotor sides during the fault ride-through process based on the current and voltage of the stator and rotor of the doubly-fed wind turbine during the fault ride-through process, and then obtain the active power output by the doubly-fed wind turbine after the fault.

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