CN116663297A - Double-fed wind power plant universal equivalence method considering hardware cooperative protection transient characteristics - Google Patents
Double-fed wind power plant universal equivalence method considering hardware cooperative protection transient characteristics Download PDFInfo
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Abstract
The invention discloses a universal equivalent method for a doubly-fed wind farm, which takes account of the transient characteristics of hardware collaborative protection, and belongs to the technical field of wind farms, and the method comprises the following steps: s1: dividing an operation area of a wind power plant unit, and constructing a three-machine equivalent model; s2: determining the cooperative protection action characteristics of a low-voltage ride-through crowbar and direct-current unloading of a fan; s3: and performing equivalence division on the three-machine equivalence model based on the cooperative protection action characteristics of the low-voltage ride-through crowbar and the direct-current unloading of the fan. The invention provides a multi-machine equivalent method aiming at a doubly-fed wind power plant under the cooperative protection of hardware, the units in the wind power plant are divided according to different operation areas and the cooperative protection action characteristics of the hardware, and the multi-machine equivalent method is used for completing the division, and the equivalent method is accurate and has small error.
Description
Technical Field
The invention belongs to the technical field of wind power plants, and particularly relates to a universal equivalent method for a doubly-fed wind power plant, which takes account of the transient characteristics of hardware collaborative protection.
Background
Along with the proposal of the carbon-to-carbon neutralization target, the scale of clean energy source represented by wind and light in China is continuously expanded. The doubly fed induction generator (doubly fed induction generator, DFIG) has become one of the widely used models in wind farms due to its characteristics of good running characteristics, small converter capacity, and separate control of active and nonfunctional power. Because the DFIG stator is directly connected with the grid, the DFIG stator is sensitive to voltage faults, and in order to keep grid-connected operation during grid faults, the crowbar protection is added on the rotor side of the DFIG and the direct current unloading protection is added on the direct current side of the DFIG rotor, so that the DFIG stator becomes the hardware protection commonly used in the existing low-voltage ride through technology.
At present, the doubly-fed wind power plant equivalence research considering the crowbar protection is deeper, and the doubly-fed wind power plant equivalence research considering the crowbar and direct current unloading protection is reported. Most of the existing doubly-fed wind power plants are simultaneously assembled with crowbar and direct current unloading protection, and at the moment, a large equivalent error can be caused by forming wind power plant grouping basis by using one protection. In the prior art, the crowbar and direct current unloading protection are cooperatively matched in the low voltage ride through process of the fan, but only the relation between the protection action and the fault voltage of the machine end is preliminarily described, and the further cooperative protection action characteristic is not known enough.
Disclosure of Invention
The invention aims to solve the problem that the existing wind power plant equivalent modeling method only considers a crowbar, and equivalent errors can be caused to a wind power plant simultaneously provided with two protections, and provides a universal equivalent method for a doubly-fed wind power plant, which takes hardware cooperative protection transient characteristics into account.
The technical scheme of the invention is as follows: a universal equivalent method for a doubly-fed wind farm considering the transient characteristic of hardware cooperative protection comprises the following steps:
s1: dividing an operation area of a wind power plant unit, and constructing a three-machine equivalent model;
s2: determining the cooperative protection action characteristics of a low-voltage ride-through crowbar and direct-current unloading of a fan;
s3: and performing equivalence division on the three-machine equivalence model based on the cooperative protection action characteristics of the low-voltage ride-through crowbar and the direct-current unloading of the fan.
Further, in step S1, the operation area of the wind farm unit includes a start area, a maximum wind energy tracking area, a constant rotation speed area, and a constant power area.
Further, in step S2, the specific method for determining the low voltage ride through crowbar action characteristic of the fan is as follows: acquiring rotor current of a wind power plant unit, determining a ternary relation function of wind speed and a machine end voltage drop coefficient and a rotor current peak value according to the rotor current, and when i is r_max |≥I r_th When the protection device is established, the crowbar protection is started; wherein i is r_max Indicating peak rotor current, I r_th A threshold value representing a crowbar action.
Further, in step S2, a rotor current i of the wind farm unit r The calculation formula of (2) is as follows:
wherein P is s_ref Representing the active power reference value, ω r Represents the angular rotation speed of the rotor, K d Representing the voltage drop coefficient at the machine end, T representing time, f (·) representing the functional expression of the rotor current, T sn The attenuation coefficient of the DC component of the induced potential of the rotor is represented, s represents the slip of the generator, and k s Represents the inductance coupling coefficient of the stator, u s0 Representing the voltage of the machine end before the fault, L r ' represents the rotor winding transient inductance, μ represents the first coefficient of the rotor current differential equation, λ represents the second coefficient of the rotor current differential equation, α represents the initial phase angle of the stator voltage during steady state operation, α 1 A root, alpha, representing the normal differential characteristic equation of the rotor current 2 Indicating rotor currentAnother root of differential characteristic equation, i r0 Representing the initial value of the rotor current vector, i r_ref Representing a rotor current reference value in a dq coordinate system, and e represents a natural constant;
in step S2, the expression of the ternary relationship function between the wind speed and the machine side voltage drop coefficient and the rotor current peak value is:
i r_max ＝g(V wind ，K d )
wherein g (·) represents a functional expression of a peak value of the rotor current, i r_max Representing peak rotor current, V wind Indicating wind speed.
Further, in step S2, the specific method for determining the low voltage ride through dc unloading characteristic of the fan is as follows: and acquiring the rotor voltage of the wind power plant unit, determining a ternary function of the wind speed and the voltage drop coefficient at the machine end and the maximum value of the rotor voltage according to the rotor voltage, and obtaining a direct current unloading protection starting area according to the rotor voltage and the direct current bus voltage.
Further, in step S2, a rotor voltage u of the wind farm unit r The calculation formula of (2) is as follows:
wherein P is s_ref Representing the active power reference value, ω r Represents the angular rotation speed of the rotor, K d Representing the voltage drop coefficient at the machine end, t representing time, h (·) representing the functional expression of the rotor voltage, s representing the generator slip, k i Represents the integral time constant, k, of the PI controller in the rotor current loop p Representing the scaling factor, T, of the rotor current inner loop PI controller sn Represents the attenuation coefficient, K, of the DC component of the induced potential of the rotor d Indicating the voltage drop coefficient of the machine terminal, u s0 Represents the voltage of the machine end before failure, j represents the imaginary number, L r ' represents the rotor winding transient inductance, i r_ref Representing the rotor current reference value, i, in the dq coordinate system r0 Representing the initial value, k, of the rotor current vector s Representing the inductance coupling coefficient of the stator, R r Representation turnChild side resistance omega p Represents the slip angular velocity, μ represents the first coefficient of the rotor current differential equation, λ represents the second coefficient of the rotor current differential equation, α represents the initial phase angle of the stator voltage at steady state operation, α 1 A root, alpha, representing the normal differential characteristic equation of the rotor current 2 Another root representing the rotor current ordinary differential characteristic equation;
in step S2, the expression of the ternary function of the wind speed and the machine side voltage drop coefficient and the maximum value of the rotor voltage is:
u r_max ＝i(V wind ，K d )
where i (·) represents a functional expression of the maximum value of the rotor voltage, u r_max Representing the maximum value of the rotor voltage, V wind Indicating wind speed.
Further, in step S3, the equal-value wind speed V is divided into equal values wind_eq The calculation formula of (2) is as follows:
wherein n represents the number of DFIG wind turbines in the divided operating region, and P i Representing the active power of the ith fan, f 1 Representing the inverse of the wind power function curve.
The beneficial effects of the invention are as follows: the invention provides a multi-machine equivalent method aiming at a doubly-fed wind power plant under the cooperative protection of hardware, the units in the wind power plant are divided according to different operation areas and the cooperative protection action characteristics of the hardware, and the multi-machine equivalent method is used for completing the division, and the equivalent method is accurate and has small error.
Drawings
FIG. 1 is a flow chart of a universal equivalence method for a doubly-fed wind farm accounting for hardware co-protection transient characteristics;
FIG. 2 is a diagram of the equivalent circuit of DFIG in synchronous coordinate system;
FIG. 3 is a graph of DFIG active versus rotor angular velocity;
FIG. 4 is a graph showing the effect of two protection inputs on key parameters;
fig. 5 is a DFIG equivalent circuit diagram after crowbar input.
Detailed Description
Embodiments of the present invention are further described below with reference to the accompanying drawings.
Before describing particular embodiments of the present invention, in order to make the aspects of the present invention more apparent and complete, abbreviations and key term definitions appearing in the present invention will be described first:
three machine equivalent models: and obtaining the equivalent model of the wind power plant by utilizing the rule that the transient state characteristics of the DFIG fan power have clustering in different operation areas.
Start zone: the DFIG fan speed rises from near 0 to the region of cut-in wind speed.
Maximum wind energy tracking area: and a variable speed operation region in which the maximum wind energy tracking control is performed.
Constant rotation speed region: the area with the maximum rotating speed and the wind turbine output power not reaching the rated output state;
constant power region: the generator output power has reached the limit.
Constant rotation speed region and constant power region: the region where the rotational speed has reached the maximum.
As shown in FIG. 1, the invention provides a universal equivalent method for a doubly-fed wind farm, which takes account of the transient characteristics of hardware collaborative protection, and comprises the following steps:
s1: dividing an operation area of a wind power plant unit, and constructing a three-machine equivalent model;
s2: determining the cooperative protection action characteristics of a low-voltage ride-through crowbar and direct-current unloading of a fan;
s3: and performing equivalence division on the three-machine equivalence model based on the cooperative protection action characteristics of the low-voltage ride-through crowbar and the direct-current unloading of the fan.
In the embodiment of the present invention, in step S1, the operation area of the wind farm unit includes a start area, a maximum wind energy tracking area, a constant rotation speed area, and a constant power area.
In the embodiment of the invention, in step S2, a specific method for determining the action characteristics of the low-voltage ride-through crowbar of the fan is determinedThe method comprises the following steps: acquiring rotor current of a wind power plant unit, determining a ternary relation function of wind speed and a machine end voltage drop coefficient and a rotor current peak value according to the rotor current, and when i is r_max |≥I r_th When the protection device is established, the crowbar protection is started; wherein i is r_max Indicating peak rotor current, I r_th A threshold value representing a crowbar action.
In the embodiment of the invention, in step S2, the rotor current i of the wind farm unit r The calculation formula of (2) is as follows:
wherein P is s_ref Representing the active power reference value, ω r Represents the angular rotation speed of the rotor, K d Representing the voltage drop coefficient at the machine end, T representing time, f (·) representing the functional expression of the rotor current, T sn The attenuation coefficient of the DC component of the induced potential of the rotor is represented, s represents the slip of the generator, and k s Represents the inductance coupling coefficient of the stator, u s0 Representing the voltage of the machine end before the fault, L r ' represents the rotor winding transient inductance, μ represents the first coefficient of the rotor current differential equation, λ represents the second coefficient of the rotor current differential equation, α represents the initial phase angle of the stator voltage during steady state operation, α 1 A root, alpha, representing the normal differential characteristic equation of the rotor current 2 Another root, i, representing the normal differential characteristic equation of the rotor current r0 Representing the initial value of the rotor current vector, i r_ref Representing a rotor current reference value in a dq coordinate system, and e represents a natural constant;
in step S2, the expression of the ternary relationship function between the wind speed and the machine side voltage drop coefficient and the rotor current peak value is:
i r_max ＝g(V wind ，K d )
wherein g (·) represents a functional expression of a peak value of the rotor current, i r_max Representing peak rotor current, V wind Indicating wind speed.
In the embodiment of the invention, in step S2, the specific method for determining the low-voltage ride through dc unloading action characteristic of the fan is as follows: and acquiring the rotor voltage of the wind power plant unit, determining a ternary function of the wind speed and the voltage drop coefficient at the machine end and the maximum value of the rotor voltage according to the rotor voltage, and obtaining a direct current unloading protection starting area according to the rotor voltage and the direct current bus voltage.
In the embodiment of the invention, in step S2, the rotor voltage u of the wind farm unit r The calculation formula of (2) is as follows:
wherein P is s_ref Representing the active power reference value, ω r Represents the angular rotation speed of the rotor, K d Representing the voltage drop coefficient at the machine end, t representing time, h (·) representing the functional expression of the rotor voltage, s representing the generator slip, k i Represents the integral time constant, k, of the PI controller in the rotor current loop p Representing the scaling factor, T, of the rotor current inner loop PI controller sn Represents the attenuation coefficient, K, of the DC component of the induced potential of the rotor d Indicating the voltage drop coefficient of the machine terminal, u s0 Represents the voltage of the machine end before failure, j represents the imaginary number, L r ' represents the rotor winding transient inductance, i r_ref Representing the rotor current reference value, i, in the dq coordinate system r0 Representing the initial value, k, of the rotor current vector s Representing the inductance coupling coefficient of the stator, R r Represents rotor side resistance ω p Represents the slip angular velocity, μ represents the first coefficient of the rotor current differential equation, λ represents the second coefficient of the rotor current differential equation, α represents the initial phase angle of the stator voltage at steady state operation, α 1 A root, alpha, representing the normal differential characteristic equation of the rotor current 2 Another root representing the rotor current ordinary differential characteristic equation;
in step S2, the expression of the ternary function of the wind speed and the machine side voltage drop coefficient and the maximum value of the rotor voltage is:
u r_max ＝i(V wind ，K d )
wherein i (·) represents the maximum rotor voltageFunctional expression of the value, u r_max Representing the maximum value of the rotor voltage, V wind Indicating wind speed.
In the embodiment of the present invention, in step S3, the equivalent wind speed V of the equivalent division is performed wind_eq The calculation formula of (2) is as follows:
wherein n represents the number of DFIG wind turbines in the divided operating region, and P i Representing the active power of the ith fan, f 1 Representing the inverse of the wind power function curve.
The equivalent parameters of the wind turbine generator are calculated according to a capacity weighting method, and the equivalent cable parameters are calculated according to the principle that the power loss of the current collection network is equal before and after the equivalent.
The following describes a specific determination process of the low voltage ride through crowbar operation characteristic of the blower.
In the embodiment of the present invention, the electromagnetic transient equation of the DFIG in the synchronous rotation coordinate system can be described as:
ψ s ＝L s i s +L m i r (3)
ψ r ＝L m i s +L r i r (4)
wherein u is s And u r Respectively stator and rotor voltage vectors; psi phi type s Sum phi r Respectively a stator flux linkage vector and a rotor flux linkage vector; i.e s And i r Respectively a stator current vector and a rotor current vector; omega s 、ω p Synchronous angular velocity and slip angular velocity respectively; r and L componentsThe parameters are resistance and inductance; subscripts's' and 'r' represent stator side and rotor side parameters; l (L) m Is an excitation inductance.
Based on the above equation, a DFIG fan transient equivalent circuit before the protection action can be established, as shown in fig. 2.
Whether the peak value of the rotor current is out of limit or not determines the action condition of the pry bar, and the threshold value of the pry bar action is generally set as I r_th ＝2|i rn |，i rn Is the rotor current rating. The invention assumes that the crowbar circuit in the wind power plant is started at the same time, and is cut out after 60ms of input.
In order to ascertain the action characteristics of the crowbar during the fault, firstly, an action criterion is formed by accurately calculating the overcurrent peak value of the rotor after the fault occurs. And on the premise of instantaneous drop of faults, the stator phase jump is ignored. Assuming that the fault occurs at time t=0, t=0 + When the voltage of the stator side of the generator drops to u s1 This can be expressed as:
u s1 ＝(1-K d )u s0 (5)
wherein K is d Is the voltage drop coefficient of the machine terminal; u (u) s0 Is the voltage of the machine terminal before the fault.
The voltage drops instantly after t=0, the voltage drop at the two ends of the stator resistor of the stator (1) can be ignored, and the stator flux linkage can be obtained according to the flux linkage conservation law:
t in s For stator decay time constant, T s ＝L s ′/R s ，L s ' represents the transient inductance of the stator winding,
simultaneous (2) - (4), the rotor voltage can be expressed as:
k in s Represents the inductance coupling coefficient, k of the stator s ＝L m /L s ；L r ' represents the rotor winding transient inductance,
the last term of equation (7) is the rotor induced potential for the stator voltage step drop, which can be expressed as:
wherein s is the slip of the generator, T sn For the attenuation coefficient, T, of the DC component of the induced potential of the rotor sn ＝jω s +1/T s 。
DFIG rotor side converters typically employ grid voltage directional vector control, and rotor side converter (rotor side converter, RSC) inner loop current control can be expressed as:
i in r_ref 、i s_estim The reference value is a rotor current reference value and the estimated value is a stator current in a dq coordinate system; k (k) p 、k i The proportional coefficient and the integration time constant of the PI controller are used for the rotor current.
Grid-tie criteria dictate that the longest time interval from the moment of failure to the onset of reactive current rise does not exceed 60ms, whereas prior studies generally assumed t=0 + The DFIG fan starts reactive power control immediately at the moment. Since the rotor current does not suddenly change and peaks generally in the latter half period of the fault, reactive current increase affects the subsequent rotor current peak calculation. The reactive current that DFIG needs additional output during low voltage ride through is:
i sq ＝K q (0.9-u s1 )，(0.2≤u s1 ≤0.9) (10)
in previous studies, the calculation of the reference value of the active current component usually only considers the reactive priority control or the generator vector control principle, which may cause the problem of excessive active component of the rotor current in case of a slight fault and a serious fault, respectively. Therefore, reactive power priority and grid voltage directional control are comprehensively considered, and then a rotor current reference value can be obtained according to the relation between stator and rotor currents:
i in rd_ref 、i rq_ref Respectively rotor current reference value vector i r_ref The dq-axis component of (2); p (P) s_ref Outputting a reference value of active power for the stator; i r_max A safety threshold is continuously operated for the DFIG.
The rotor current expressions can be obtained by combining (7) to (9):
wherein μ= (R) r +K p +jω p L r ′)/L r ′；λ＝k i /L r ′；i r0 Is the initial value of the rotor current vector.
It can be seen that the rotor current is a function of the active power reference, the rotor angular speed, the machine side voltage drop coefficient and time. In order to obtain the peak value of the rotor current, t is set to be a half-period time constant value of 0.01, and meanwhile, according to the operation characteristics of the DFIG, the relation between the active power and the rotating speed of the fan and the wind speed outside the fan exists as shown in figure 3.
For the DFIG power curve, when the fan is operating in the start-up region (between points a-B), constant speed region (between points C-D), and constant power region (between points D-E), the active power can be approximately considered as a linear relationship with wind speed, while in the maximum wind energy tracking region (maximum power point tracking, MPPT) (between points B-C), the active power is proportional to the speed to the third power.
For rotor angular velocity, the angular velocity remains unchanged when the fan is operating in the start zone, the constant speed zone, and the constant power zone, and the angular velocity can be considered approximately as a linear relationship with wind speed within MPPT. Thus wind speed V wind The relationship with rotor angular velocity can be expressed as:
and then the relation between the wind speed and the active power can be obtained:
k in 1 、k 2 、k 3 、λ 3 Is constant.
Active power reference values and rotating speeds in rotor current calculation formulas can be eliminated in the simultaneous formulas (12) - (14) to obtain a ternary function relation of wind speed, machine end voltage drop coefficient and rotor current peak value, namely:
i r_max ＝g(V wind ，K d ) (15)
substituting rotor current reference values (10) and (11) on the basis of (15), and obtaining a low-voltage ride-through reactive current coefficient K in the formula (10) q Usually 1.5 to 2.
The specific determination process of the low-voltage ride through DC unloading action characteristic of the fan is described below.
Whether the voltage of the direct current bus is out of limit after the fault occurs determines the starting condition of the Chopper protection, and the Chopper is usually controlled by hysteresis. In order to ascertain the action characteristics of the Chopper during the fault period, firstly, whether the voltage of the direct current bus after the fault occurs is out of limit is accurately judged to form a starting criterion.
Simultaneously (7), (8) and (12) to obtain the rotor voltage calculation result
Also, since the expression of the rotor voltage contains i r_ref The rotor voltage is thus still a function of the four variables. And (3) replacing the active power reference value and the rotor angular speed in the formula (16) to obtain a functional relation of wind speed, time, machine side voltage drop coefficient and rotor voltage.
U in FIG. 2 r，eq To approximate neglect of the leakage flux effect in terms of the rotor equivalent voltage seen from the stator side (to the stator side), according to the theory of electromechanical correlation, the expression can be written as:
in N stator /N rotor The turns ratio of the stator winding to the rotor winding; k is the turns ratio of stator and rotor fundamental wave windings.
Therefore, under the condition of no protection input, the rotor voltage can be used for estimating whether the DC voltage is out of limit, so as to judge the DC unloading protection action condition.
In the calculation, traversing the first n periods after the occurrence of faults by using an electromagnetic transient simulation step length to obtain a maximum value of the rotor voltage, and finally obtaining a ternary function relation between the wind speed, the machine end voltage drop coefficient and the maximum value of the rotor voltage, namely:
u r_max ＝i(V wind ，K d ) (18)
in fact, when the crowbar resistance does not meet the tuning conditions, the dc off-load protection may be activated during the crowbar input. The impact of two protection action cases on the key parameters is shown in fig. 4.
Through a plurality of groups of simulation experiments, both protections can enable the rotor current to recover to the vicinity of the steady state value before the fault, but Crowbar can obviously inhibit the rotor current peak value, and the influence of Chopper on the rotor current peak value is small, as shown in (a) of fig. 4; in FIG. 4 (b), crowbar protection with Chopper and resistor tuningCan obviously inhibit direct-current overvoltage, but when the resistance value R of the pry bar is c When the voltage is too large, the direct current still has the overvoltage risk, and the Chopper circuit is put into operation. Therefore, further studies on the dc unloading protection operating characteristics after the crowbar is put into operation are necessary.
The transient equivalent circuit of the DFIG fan after the crowbar is put into the device is shown in figure 5.
The rotor current at this time is calculated as:
the calculated expression of the rotor voltage at this time is:
u r ＝R c i r (20)
those of ordinary skill in the art will recognize that the embodiments described herein are for the purpose of aiding the reader in understanding the principles of the present invention and should be understood that the scope of the invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific modifications and combinations from the teachings of the present disclosure without departing from the spirit thereof, and such modifications and combinations remain within the scope of the present disclosure.
Claims (7)
1. A universal equivalent method for a doubly-fed wind power plant considering the transient characteristic of hardware cooperative protection is characterized by comprising the following steps:
s1: dividing an operation area of a wind power plant unit, and constructing a three-machine equivalent model;
s2: determining the cooperative protection action characteristics of a low-voltage ride-through crowbar and direct-current unloading of a fan;
s3: and performing equivalence division on the three-machine equivalence model based on the cooperative protection action characteristics of the low-voltage ride-through crowbar and the direct-current unloading of the fan.
2. The method according to claim 1, wherein in step S1, the operation area of the wind farm set includes a start-up area, a maximum wind energy tracking area, a constant rotation speed area, and a constant power area.
3. The universal equivalent method for the doubly-fed wind farm, which takes account of the transient characteristics of the hardware cooperative protection, according to claim 1, is characterized in that in the step S2, the specific method for determining the low voltage ride through crowbar action characteristics of the wind turbine is as follows: acquiring rotor current of a wind power plant unit, determining a ternary relation function of wind speed and a machine end voltage drop coefficient and a rotor current peak value according to the rotor current, and when i is r_max |≥I r_th When the protection device is established, the crowbar protection is started; wherein i is r_max Indicating peak rotor current, I r_th A threshold value representing a crowbar action.
4. A doubly-fed wind farm general equivalence method according to claim 3 and based on hardware cooperative protection transient characteristics, wherein in step S2, rotor current i of wind farm units is r The calculation formula of (2) is as follows:
wherein P is s_ref Representing the active power reference value, ω r Represents the angular rotation speed of the rotor, K d Representing the voltage drop coefficient at the machine end, T representing time, f (·) representing the functional expression of the rotor current, T sn The attenuation coefficient of the DC component of the induced potential of the rotor is represented, s represents the slip of the generator, and k s Represents the inductance coupling coefficient of the stator, u s0 Representing the voltage of the machine end before the fault, L r ' represents the rotor winding transient inductance, μ represents the first coefficient of the rotor current differential equation, λ represents the second coefficient of the rotor current differential equation, α represents the initial phase angle of the stator voltage during steady state operation, α 1 A root, alpha, representing the normal differential characteristic equation of the rotor current 2 Another root, i, representing the normal differential characteristic equation of the rotor current r0 Representing the initial value of the rotor current vector, i r_ref Representing a rotor current reference value in a dq coordinate system, and e represents a natural constant;
in the step S2, the expression of the ternary relation function of the wind speed and the machine-side voltage drop coefficient and the rotor current peak value is:
i r_max ＝g(V wind ,K d )
wherein g (·) represents a functional expression of a peak value of the rotor current, i r_max Representing peak rotor current, V wind Indicating wind speed.
5. The universal equivalent method for the doubly-fed wind farm considering the hardware cooperative protection transient characteristic according to claim 1, wherein in the step S2, the specific method for determining the low-voltage ride through dc unloading action characteristic of the wind turbine is as follows: and acquiring the rotor voltage of the wind power plant unit, determining a ternary function of the wind speed and the voltage drop coefficient at the machine end and the maximum value of the rotor voltage according to the rotor voltage, and obtaining a direct current unloading protection starting area according to the rotor voltage and the direct current bus voltage.
6. The method according to claim 5, wherein in step S2, the rotor voltage u of the wind farm set is calculated by r The calculation formula of (2) is as follows:
wherein P is s_ref Representing the active power reference value, ω r Represents the angular rotation speed of the rotor, K d Representing the voltage drop coefficient at the machine end, t representing time, h (·) representing the functional expression of the rotor voltage, s representing the generator slip, k i Represents the integral time constant, k, of the PI controller in the rotor current loop p Representing the scaling factor, T, of the rotor current inner loop PI controller sn Represents the attenuation coefficient, K, of the DC component of the induced potential of the rotor d Indicating the voltage drop coefficient of the machine terminal, u s0 Representing the voltage of the machine terminal before the faultJ represents an imaginary number, L r ' represents the rotor winding transient inductance, i r_ref Representing the rotor current reference value, i, in the dq coordinate system r0 Representing the initial value, k, of the rotor current vector s Representing the inductance coupling coefficient of the stator, R r Represents rotor side resistance ω p Represents the slip angular velocity, μ represents the first coefficient of the rotor current differential equation, λ represents the second coefficient of the rotor current differential equation, α represents the initial phase angle of the stator voltage at steady state operation, α 1 A root, alpha, representing the normal differential characteristic equation of the rotor current 2 Another root representing the rotor current ordinary differential characteristic equation;
in the step S2, the expression of the ternary function of the wind speed and the machine side voltage drop coefficient and the maximum value of the rotor voltage is:
u r_max ＝i(V wind ,K d )
where i (·) represents a functional expression of the maximum value of the rotor voltage, u r_max Representing the maximum value of the rotor voltage, V wind Indicating wind speed.
7. The universal equivalence method for doubly-fed wind farm according to claim 1, wherein in step S3, the equivalence divided equivalence wind speed V is performed wind_eq The calculation formula of (2) is as follows:
wherein n represents the number of DFIG wind turbines in the divided operating region, and P i Representing the active power of the ith fan, f -1 Representing the inverse of the wind power function curve.
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