WO2015035892A1

WO2015035892A1 - Method for calculating transient short circuit current of dfig set

Info

Publication number: WO2015035892A1
Application number: PCT/CN2014/086047
Authority: WO
Inventors: 王耀函; 刘辉; 吴林林; 刘京波; 王皓靖
Original assignee: 国家电网公司; 华北电力科学研究院有限责任公司
Priority date: 2013-09-10
Filing date: 2014-09-05
Publication date: 2015-03-19
Also published as: CN103500269A; CN103500269B

Abstract

A method for calculating a transient short circuit current of a double fed induction generator (DFIG) set comprises: establishing a transient mathematical model of the DFIG set when there is a deep drop failure of power grid voltage, and establishing a transient equivalent circuit of the DFIG set according to a transient electrical relationship in the mathematical model; analyzing the magnetic linkage relationship of the transient equivalent circuit of the DFIG set, to obtain a first stator transient current appearing when a terminal of the DFIG set experiences a voltage drop; correcting the transient equivalent circuit of the DFIG set according to a crowbar circuit activation mechanism, to obtain, after correction, a second stator transient current appearing when the terminal of the DFIG set experiences a voltage drop in the case of crowbar circuit activation; and acquiring a transient short circuit current of the DFIG set according to the second stator transient current.

Description

Method for calculating transient short-circuit current of doubly-fed asynchronous generator set

Technical field

The invention relates to the field of wind power generation, in particular to a method for calculating transient short-circuit current of a doubly-fed asynchronous generator set.

Background technique

Among all types of wind turbines, the Double Fed Induction Generator (DFIG) is one of the mainstream models of the current MW-class wind turbines. Since the generator stator of the doubly-fed wind turbine is directly connected to the grid, when the system has a short-circuit fault, the voltage change on the grid side will cause a large current on the rotor side through the induction between the stator and the rotor, that is, a large rotor is generated on the rotor side. The short-circuit current will have an impact on the converter and fan of the wind turbine. In addition, the protection setting of the wind turbine needs to consider the influence of the short-circuit current of the wind turbine. Therefore, quantitative calculation of the short-circuit current of the doubly-fed fan is of great significance for the selection of the doubly-fed fan equipment and the setting of the protection settings.

At present, the research results of the short-circuit current of the doubly-fed fan are mostly concentrated on the three-phase short circuit at the machine end, and the stator and rotor transient currents when the terminal voltage drops to 0. The analysis methods are mostly mathematical analytical methods based on flux linkage analysis. There is less analysis of the residual voltage after the three-phase voltage drop; in addition, the analysis of the CrowBar input situation assumes that the CrowBar is immediately invested in the moment of the failure, and the CrowBar input strategy is considered in the transient analysis. Few. For example, the Chinese patent application with the publication number 102867085A is the closest prior art of the present invention, which discloses a short-circuit current calculation method for a power system including a doubly-fed wind turbine, in which the attenuation of the flux linkage in the transient process is analyzed. The calculation result is obtained by the relationship between the flux linkage and the current, and the actual input mechanism of the protection is not considered in the calculation process, and the transient characteristics of the low voltage crossing process of the wind turbine under the protection of the crowbar circuit cannot be obtained.

Summary of the invention

The embodiment of the invention provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, which is used for solving the technical problem that the transient short-circuit current of the doubly-fed asynchronous generator set cannot be accurately calculated in the prior art.

Embodiments of the present invention provide a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, where the method includes:

Establishing a transient mathematical model of the doubly-fed asynchronous generator set under the grid voltage deep drop fault, and establishing a transient equivalent circuit of the doubly-fed asynchronous generator set according to the transient electrical relationship in the mathematical model;

The flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set is analyzed, and the first stator transient current when the terminal voltage drop of the doubly-fed asynchronous generator set is obtained;

According to the input mechanism of the crowbar circuit, the transient equivalent circuit of the doubly-fed asynchronous generator set is corrected, and the second stator transient state of the double-fed asynchronous generator set voltage drop is obtained after the correction is obtained. Current

Obtaining a transient short circuit current of the doubly-fed asynchronous generator set according to the second stator transient current.

The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, wherein the method further comprises: calculating, according to the first stator transient current and the second stator transient current, when the crowbar circuit is input delayed The doubly-fed asynchronous generator set transient short-circuit current.

The above method for calculating the transient short-circuit current of the doubly-fed asynchronous generator set, wherein the flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set is analyzed: the stator transient state after the grid voltage deep drop fault occurs The change of current is the full response process of the transient equivalent circuit of the doubly-fed asynchronous generator set.

The method for calculating a transient short-circuit current of the doubly-fed asynchronous generator set, wherein the transient equivalent circuit of the doubly-fed asynchronous generator set according to the transient electrical relationship in the mathematical model includes:

The stator-rotor voltage equation (1) and the stator-coil flux equation (2) of the doubly-fed asynchronous generator set vector in the synchronous speed rotation dq coordinate system are as follows:

L _s =L _sσ +L _m (3)

L _r =L _rσ +L _m (4)

Where: u _s is the stator voltage vector, u _r is the rotor voltage vector, R _s is the stator resistance, R _r is the rotor resistance, i _s is the stator current vector, i _r is the rotor current vector, ψ _s is the stator flux vector, ψ _r is the rotor flux vector, ω _e is the synchronous angular velocity, ω _slip =ω _s -ω _r is the stator and rotor slip angular velocity, L _sσ is the stator leakage inductance, L _rσ is the rotor leakage inductance, and L _m is the stator and rotor mutual inductance , L _s is the stator inductance, and L _r is the rotor inductance;

According to the formulas (1), (2), (3) and (4), the transient equivalent circuit of the doubly-fed asynchronous generator set is obtained.

The above method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, wherein analyzing the flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set includes:

Transform the equation (2) to obtain the stator and rotor current vector equation (5):

among them:

Assuming that the doubly-fed asynchronous generator set operates at synchronous speed, the corresponding stator transient current at synchronous speed is obtained according to equation (6):

Then the synchronous rotor speed flux equation is (7):

Suppose that the initial phase angle α=0 at the time of voltage drop at t=0, and the residual voltage is Δu, then the stator and rotor flux equation (8) at t=0 ⁺ and the steady-state moment of the voltage drop are fixed. Equation (9) is:

The method for calculating the transient short-circuit current of the doubly-fed asynchronous generator set, wherein the method further comprises: obtaining the first stator transient current i when the terminal voltage drop of the doubly-fed asynchronous generator set is obtained according to formula (10) _s :

Wherein, cos (ω _s t _∞) is a periodic _{_{component, t ∞ = t, ω s}} t ∞ '= ω s t ∞ + β, β∈ (-π, π), T r is the decay time constant and rotor flux The charging time constant, T _s is the stator side decay time constant, the coefficient k _s =L _m /L _s , k _r =L _m /L _r .

The above method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, wherein the stator-side transient equivalent impedance X _{s is} obtained according to formula (11):

X _s =ω _e L _s ' (11).

The above-mentioned calculation method of transient short-circuit current of the doubly-fed asynchronous generator set, wherein the second stator transient current when the terminal voltage drop of the doubly-fed asynchronous generator set is obtained under the input condition of the crowbar circuit includes:

Correcting the stator-side transient equivalent impedance X _s in equation (11) to equation (12):

Wherein, R _cb is the crowbar circuit resistance, the rotor side damping time constant and the charging time constant is corrected to

Therefore, the second stator transient current (12) after the crowbar circuit is input is:

The above method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, wherein, according to the first stator transient current and the second stator transient current, calculating a transient short-circuit current when the crowbar circuit is input delayed comprises :

The equation (14) is established by the direct current component and the alternating current component of equations (10) and (13), respectively, and the correction time of the switching time is obtained:

Where t _c is the time when the crowbar circuit is put into the circuit, t _c1 is the time correction of the crowbar circuit input to the DC component, and t _c2 is the time correction of the crowbar circuit input to the AC component.

Is the phase angle correction of the crowbar circuit input to the AC component;

Then, the transient short-circuit current i _mix of the crowbar circuit is delayed by:

Where t ₁ = tt _c + t _c1 , t ₂ = tt _c + t _c2 . β is the correction of the phase angle in the first stator transient current,

The phase angle correction that satisfies the boundary conditions when the crowbar circuit is input.

The calculation method of the transient short-circuit current of the doubly-fed asynchronous generator set provided by the embodiment of the present invention is obtained by considering the conversion time of the transient equivalent circuit according to the transient current calculation process of the crowbar circuit input mechanism, and finally obtaining the conversion time of the transient equivalent circuit. A method for calculating the transient short-circuit current that is closer to the measured curve in the field and is in agreement with the actual situation.

DRAWINGS

The drawings described herein are provided to provide a further understanding of the invention, and are not intended to limit the invention. In the drawing:

1 is a flow chart of a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to an embodiment of the present invention;

2 is a schematic diagram of a transient equivalent circuit of a doubly-fed asynchronous generator set according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of comparison between the calculated transient short-circuit current and the measured transient short-circuit current in the embodiment of the present invention.

detailed description

In order to make the objects, the technical solutions and the advantages of the present invention more clearly, the embodiments of the present invention are further described in detail below with reference to the accompanying drawings. The illustrative embodiments of the present invention and the description thereof are intended to explain the present invention, but are not intended to limit the invention.

Embodiments of the present invention provide a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. As shown in FIG. 1 , the method includes:

Step 101: Establish a transient mathematical model of the doubly-fed asynchronous generator set under the grid voltage deep drop fault, and establish a transient equivalent circuit of the doubly-fed asynchronous generator set according to the transient electrical relationship in the mathematical model;

Step 102: analyzing a flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set, and obtaining a first stator transient current when the terminal voltage of the doubly-fed asynchronous generator set is dropped;

Preferably, the change of the stator transient current after the grid voltage deep drop fault occurs is the full response process of the transient equivalent circuit of the doubly-fed asynchronous generator set, that is, the superposition of the zero input response and the zero state response.

Step 103: Correct the transient equivalent circuit of the doubly-fed asynchronous generator set according to the input mechanism of the crowbar circuit, and obtain the second voltage drop of the doubly-fed asynchronous generator set when the crowbar circuit is input. Stator transient current; preferably, the correction includes correction of the stator side transient equivalent impedance and the rotor flux decay time constant.

Step 104: Obtain a transient short-circuit current of the doubly-fed asynchronous generator set according to the second stator transient current. Preferably, in the embodiment of the present invention, the second stator transient current is approximately equivalent to the transient short-circuit current of the doubly-fed asynchronous generator set. Specifically, since the stator side of the doubly-fed asynchronous generator set is directly connected to the power grid, the rotor side The dual PWM inverter is connected to the grid, and the inverter capacity is only about 1/3 of the unit capacity. At the moment of failure, the stator current can reach 4 to 5 times of the rated current of the unit. At this time, the contribution of the rotor-side inverter to the current of the machine is very small and can be neglected, so the second stator transient current is approximately equivalent to the final doubly-fed. Asynchronous generator set transient short circuit current.

The calculation method of the transient short-circuit current provided by the embodiment of the present invention considers the influence of the input mechanism of the crowbar circuit when the short-circuit fault occurs, and can obtain an accurate expression of the transient short-circuit current of the wind turbine. The invention can be used for estimating the transient current during the low voltage crossing process, guiding the correction of the fan control strategy, reducing the damage caused by the voltage drop of the terminal to the wind turbine and the power grid, especially for formulating the low voltage ride through hardware protection control strategy. Important guiding significance.

The embodiment of the present invention further provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. Preferably, the method further includes: when the crowbar circuit is input delayed, according to the first stator transient current And calculating a transient short circuit current of the doubly-fed asynchronous generator set by the second stator transient current. Preferably, when the crowbar circuit is delayed, the second stator transient current is approximately equivalent to the calculation of the transient short-circuit current of the final doubly-fed asynchronous generator set, which is not in accordance with the actual situation, and the actual doubly-fed asynchronous generator set rotor The input of the side boring bar circuit needs to consider the delay of denoising, the delay of the detecting device and the actuator. In order to better reflect the actual situation of the site, it is necessary to set the trigger mechanism of the crowbar circuit input according to the actual situation, and then modify the transient current of the stator as the transient short-circuit current when the crowbar circuit inputs the delay.

The embodiment of the invention further provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. Preferably, the transient state of the doubly-fed asynchronous generator set is established according to the transient electrical relationship in the mathematical model. Equivalent Circuit. Specifically, the step of establishing a transient equivalent circuit includes:

L _s =L _sσ +L _m (3)

L _r =L _rσ +L _m (4)

According to the formulas (1), (2), (3) and (4), the transient equivalent circuit of the doubly-fed asynchronous generator set is obtained, and FIG. 2 is a specific equivalent circuit diagram.

The embodiment of the invention further provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. Preferably, the flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set is analyzed:

among them:

It is assumed that the doubly-fed asynchronous generator set operates at synchronous speed. At this time, the rotor current only acts as a DC excitation, and the rotor current amplitude I _r =0, so the stator and rotor resistance are much smaller than the leakage reactance and can be ignored. Equation (6) can obtain the corresponding stator transient current at synchronous speed:

Then the synchronous rotor speed flux equation is (7):

It is assumed that the initial phase angle α=0 when the voltage is dropped at time t=0, and the residual voltage is Δu. Preferably, for example, when the terminal voltage of the doubly-fed asynchronous generator falls to the rated voltage of 20%, Δu=0.2, according to the magnetic The law of chain conservation, the flux linkage does not abruptly change at the moment of failure, then the stator-torque flux equation (8) at t=0 ⁺ and the steady-state moment of the voltage drop are (4):

In the above embodiment of the present invention, the influence of residual voltage after voltage drop is considered on the basis of the traditional analytical method, and the stator current change after the fault occurs is regarded as the full response process of the transient equivalent circuit of the doubly-fed asynchronous generator set, which is consistent with The actual situation.

The embodiment of the invention further provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. Preferably, the method further comprises: the change of the transient current of the stator after the fault occurs can be regarded as a doubly-fed asynchronous generator set. The full response process of the transient equivalent circuit, including ψ _s0 and ψ _r0 as initial values, and the decay time constant is

Corresponding to zero-input response components and _{_ψ} s∞, ψ r∞ steady state value, the charging time constant

The superposition of the corresponding zero-state response components, so that it is brought into equation (5) to obtain equation (10), and according to equation (10), the first stator transient state when the terminal voltage drop of the doubly-fed asynchronous generator set is obtained Current i _s :

Where cos(ω _s t _∞ ) is a periodic component, t _∞ =t, ω _s t _∞ ′′=ω _s t _∞ +β, β∈(-π,π), and T _r is the rotor flux decay time constant and The charging time constant, T _s is the stator side decay time constant, the coefficient k _s =L _m /L _s , k _r =L _m /L _r .

The embodiment of the invention further provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. Preferably, the stator-side transient equivalent impedance X _{s is} obtained according to formula (11):

X _s =ω _e L _s ' (11).

The embodiment of the invention also provides a method for calculating the transient short-circuit current of the doubly-fed asynchronous generator set. Preferably, the second stator transient current of the double-fed asynchronous generator set is dropped when the crowbar circuit is input. include:

Where R _cb is the resistance of the crowbar circuit, and the value is 0.1Ω. Correct the rotor side decay time constant and charging time constant to

The embodiment of the invention further provides a method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set. Preferably, the crowbar circuit input is calculated according to the first stator transient current and the second stator transient current. Transient short-circuit currents at delay include:

Before and after the crowbar circuit is input, the current amplitude and phase angle do not change before and after the action moment due to the conservation of the flux linkage. Therefore, the DC component and the AC component of equations (10) and (13) are respectively established in an equation relationship (14). ), get the correction time correction amount:

Is the phase angle correction of the crowbar circuit input to the AC component;

A specific embodiment is used in the embodiment of the present invention to prove that the transient short-circuit current calculated by the method of the present invention conforms to the actual situation. Preferably, the actual parameters measured by a doubly-fed asynchronous generator set in the embodiment of the present invention are as follows: stator resistance R _s = 9.18 mΩ, rotor resistance R _r = 3.24 mΩ, stator leakage inductance L _sσ = 0.3578 mH, Rotor leakage inductance L _rσ =0.3278mH, magnetizing inductance L _m =13.82mH, motor synchronous speed 1500rpm, rated phase voltage V _s =400V, crowbar circuit resistance value 0.3Ω, input rotor current threshold I _rot =1.5pu, denoising The actuator delay is t _c1 =3ms. As shown in FIG. 3, the data calculated according to the embodiment of the present invention draws a transient short-circuit current curve 1 considering the input delay of the crowbar circuit, and curve 2 is a doubly-fed asynchronous generator set corresponding to the doubly-fed asynchronous generator set parameters. The low voltage traverses the phase current curve of the tester. The test environment is three-phase voltage drop, residual voltage 20%, and windy condition (rated power). The curve drawn by the data calculated by the embodiment of the present invention is compared with the field measured curve as shown in Table 1:

Table 1

	曲线1 Curve 1	曲线2 Curve 2
	曲线1 Curve 1	曲线2 Curve 2	峰值出现时间(s)Peak appearance time (s)	0.00650.0065	0.00660.0066
峰值大小(A)Peak size (A)	41314131	41954195	峰值出现时间(s)Peak appearance time (s)	0.00650.0065	0.00660.0066
峰值大小(A)Peak size (A)	41314131	41954195	进入稳态时间(s)Enter steady state time (s)	0.210.21	0.230.23

It can be seen that the calculated curve has a higher fitness fit with the actual situation than the transient peak and decay time, thus verifying the validity of the calculation method.

It can be seen from the above that the calculation method of the transient short-circuit current of the doubly-fed asynchronous generator set provided by the embodiment of the present invention can not only Obtain the transient short-circuit current of the double-fed asynchronous generator set under different voltage drops, and consider the common crowbar circuit protection in the low-through process of the fan to consider the calculation method. The factors considered in the calculation are more in line with the actual situation and pass The comparison with the measured curve proves that the invention can more accurately reflect the transient short-circuit current characteristics of a single fan, and can provide a reference for the selection of the unit equipment and the setting of the fault protection setting of the wind turbine.

The specific embodiments of the present invention have been described in detail with reference to the preferred embodiments of the present invention. All modifications, equivalent substitutions, improvements, etc., made within the spirit and scope of the invention are intended to be included within the scope of the invention.

Claims

A method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set, characterized in that the method comprises:

Establishing a transient mathematical model of the doubly-fed asynchronous generator set under the grid voltage deep drop fault, and establishing a transient equivalent circuit of the doubly-fed asynchronous generator set according to the transient electrical relationship in the mathematical model;

The flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set is analyzed, and the first stator transient current when the terminal voltage drop of the doubly-fed asynchronous generator set is obtained;

According to the input mechanism of the crowbar circuit, the transient equivalent circuit of the doubly-fed asynchronous generator set is corrected, and the second stator transient state of the double-fed asynchronous generator set voltage drop is obtained after the correction is obtained. Current

Obtaining a transient short circuit current of the doubly-fed asynchronous generator set according to the second stator transient current.
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 1, wherein the method further comprises: when the crowbar circuit inputs a delay, according to the first stator transient current and The second stator transient current calculates a transient short circuit current of the doubly-fed asynchronous generator set.
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 1 or 2, characterized in that the flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set is analyzed: The change of the stator transient current after the deep drop fault occurs is the full response process of the transient equivalent circuit of the doubly-fed asynchronous generator set.
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 3, wherein said transient state of said doubly-fed asynchronous generator set is established according to a transient electrical relationship in said mathematical model The circuit includes:

The stator-rotor voltage equation (1) and the stator-coil flux equation (2) of the doubly-fed asynchronous generator set vector in the synchronous speed rotation dq coordinate system are as follows:

L s =L sσ +L m (3)

L r =L rσ +L m (4)

Where: u s is the stator voltage vector, u r is the rotor voltage vector, R s is the stator resistance, R r is the rotor resistance, i s is the stator current vector, i r is the rotor current vector, ψ s is the stator flux vector, ψ r is the rotor flux vector, ω e is the synchronous angular velocity, ω slip =ω s -ω r is the stator and rotor slip angular velocity, L sσ is the stator leakage inductance, L rσ is the rotor leakage inductance, and L m is the stator and rotor mutual inductance , L s is the stator inductance, and L r is the rotor inductance;

According to the formulas (1), (2), (3) and (4), the transient equivalent circuit of the doubly-fed asynchronous generator set is obtained.
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 4, wherein analyzing the flux linkage relationship of the transient equivalent circuit of the doubly-fed asynchronous generator set comprises:

Transform the equation (2) to obtain the stator and rotor current vector equation (5):

among them:

Assuming that the doubly-fed asynchronous generator set operates at synchronous speed, the corresponding stator transient current at synchronous speed is obtained according to equation (6):

Then the synchronous rotor speed flux equation is (7):

Suppose that the initial phase angle α=0 at the time of voltage drop at t=0, and the residual voltage is Δu, then the stator and rotor flux equation (8) at t=0 + and the steady-state moment of the voltage drop are fixed. Equation (9) is:
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 5, wherein the method further comprises: obtaining, according to formula (10), a voltage drop of the terminal of the doubly-fed asynchronous generator set Certain sub-transient current i s :

Where cos(ω s t ∞ ) is a periodic component, t ∞ =t, ω s t ∞ ′′=ω s t ∞ +β, β∈(-π,π), and T r is the rotor flux decay time constant and The charging time constant, T s is the stator side decay time constant, the coefficient k s =L m /L s , k r =L m /L r .
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 6, wherein the stator-side transient equivalent impedance X s is obtained according to formula (11):

X s =ω e L s ' (11).
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 7, wherein the second stator transient current when the terminal voltage drop of the doubly-fed asynchronous generator set is obtained under the input condition of the crowbar circuit comprises: :

Correcting the stator-side transient equivalent impedance X s in equation (11) to equation (12):

Where R cb is the crowbar circuit resistance, and the rotor side decay time constant and the charging time constant are corrected to

Therefore, the second stator transient current (12) after the crowbar circuit is input is:
The method for calculating a transient short-circuit current of a doubly-fed asynchronous generator set according to claim 8, wherein the input delay of the crowbar circuit is calculated according to the first stator transient current and the second stator transient current Transient short-circuit currents include:

The equation (14) is established by the direct current component and the alternating current component of equations (10) and (13), respectively, and the correction time of the switching time is obtained:

Where t c is the time when the crowbar circuit is put into the circuit, t c1 is the time correction of the crowbar circuit input to the DC component, and t c2 is the time correction of the crowbar circuit input to the AC component.
Is the phase angle correction of the crowbar circuit input to the AC component;

Then, the transient short-circuit current i mix of the crowbar circuit is delayed by:

Where t 1 = tt c + t c1 , t 2 = tt c + t c2 , β is the correction of the phase angle in the first stator transient current,
The phase angle correction that satisfies the boundary conditions when the crowbar circuit is input.