CN106842021B - Method and device for judging transient voltage stability - Google Patents
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Abstract
The invention provides a method and a device for judging the stability of transient voltage, wherein the method comprises the steps of calculating the minimum terminal voltage and the minimum terminal voltage of an induction motor during stable operation according to the slip of the induction motor and the terminal recovery voltage collected before and after fault removal; judging the voltage stability of the induction motor according to the slip difference between the moment of fault removal and the stable operation of the induction motor; determining whether the voltage stability condition of the induction motor is met or not according to the difference between the critical instability slip of the induction motor and the slip of the induction motor at the fault removal moment, if so, calculating the terminal voltage when the electromagnetic torque is equal to the mechanical torque, comparing the terminal voltage with the terminal recovery voltage, and judging the voltage stability of the induction motor; the apparatus includes a first comparing unit and a determining unit. The technical scheme provided by the invention provides powerful theoretical support for preventing the voltage instability and making a voltage instability solving measure.
Description
Technical Field
The invention belongs to the field of power system stability judgment, and particularly relates to a method and a device for judging transient voltage stability.
Background
The development of the power industry in some regions and the continuous expansion of the scale of the receiving end system enable long-distance large-capacity power transmission to be a normal state, and the large demand of the receiving end system for external power also makes the problem of voltage stability more prominent. The induction motor load is an important component of the power system load, the proportion of the induction motor load in the industrial load is up to more than 90%, the load characteristic of the induction motor in the power grid interacts with the apparent power transmission characteristic of the system network, and the transient voltage stability is affected in a complex way, so people pay attention to the voltage stability problem containing the induction motor load.
The inventor has realized from extensive research that when a system with an induction motor load is disturbed, the induction motor load characteristic and the network apparent power transmission characteristic have an interaction process to form a voltage instability positive feedback dynamic process taking the induction motor as a core, and the interaction characteristic is called induction motor network charge mutual feedback characteristic.
For the consideration of the transient voltage stability problem of the load of the induction motor, the existing method is mostly an analysis method for calculating the limit cutting slip of the induction motor by combining an induction motor model and the change of the mechanical torque of the load of the induction motor to judge whether the induction motor is unstable or not according to whether the slip at the fault cutting moment is larger than the slip at the limit cutting or not. The calculation and the judgment are simple, convenient and inaccurate.
In order to overcome the defects of the prior art, the voltage stability determination method of the induction motor, which can fully consider the network charge mutual feedback characteristic of the induction motor, needs to be provided to meet the requirements of the prior art.
Disclosure of Invention
In order to meet the development requirement of the prior art, the invention provides a method for judging the stability of the transient voltage.
The invention provides a transient voltage stability judging method, which is improved in that the method comprises the following steps:
calculating the minimum terminal voltage and the induction motor slip when the induction motor stably operates according to the induction motor slip and the terminal recovery voltage collected before and after the fault is removed;
and judging the voltage stability of the induction motor according to the difference between the slip at the fault clearing moment and the slip when the induction motor stably operates and/or the difference between the critical instability slip of the induction motor and the slip of the induction motor at the fault clearing moment.
Further, the slip S of the induction motor at the moment of fault removal is collectedcAnd the instantaneous induction motor end recovery voltage U after fault removalre；
Electromagnetic power curve T when induction motor operates stablyeWith mechanical power curve TmSlip S of induction motor at phase cutmAnd a minimum terminal voltage ULminCalculated as follows:
in the formula, T0: induction electricityMotive initial mechanical torque;
R1: representing the equivalent resistance, X, after simplification of the induction motor model1: representing the equivalent reactance after the induction motor model is simplified; rR: an induction motor rotor resistance; rs: an induction motor stator resistance; xRAnd Xs: respectively an induction motor rotor reactance and a stator reactance; xM: a field reactance of the induction motor; omegas: synchronous angular velocity of the induction motor load; A. b, C are coefficients of mechanical torque, respectively.
Further, the judgment based on the difference between the slip at the time of the fault removal and the slip at the time of stable operation of the induction motor includes:
if Sc≤SmComparing the induction motor end recovery UreAnd minimum terminal voltage ULmin；
Otherwise, calculating the critical instability slip Su。
Further, according to the comparison, the induction motor terminal recovery voltage UreAnd minimum terminal voltage ULminThe determination made of the difference of (a) includes:
if U isre＞ULminThe voltage of the induction motor is stable; otherwise, the voltage instability of the induction motor is judged.
Further, the critical instability slip S of the induction motoruThe calculation of (a) includes:
1) the apparent power S absorbed by the induction motor is calculated as followsML：
Wherein, PMLAnd QML: the active and reactive power absorbed by the induction motor, respectively the slip s, is calculated by:
in the formula, U: the voltage at the terminal of the induction motor,
US: equivalent terminal voltage of the induction motor;
a conjugate representing an equivalent impedance of the induction motor; equivalent impedance Z of induction motoreCalculated as follows:
2) network transmission characteristic curve SMLAs shown in the following formula:
wherein, X: equivalent line impedance, ES: the equal potential of the sending end is provided,
a load power factor angle;
calculating the load power factor of an induction motor by the following formula
3) Calculating the positive real number as the critical instability slip S of the induction motoruThe value of (c).
Further, according to the critical instability slip S of the induction motoruInduction motor slip S at the moment of fault removalcThe determination of the difference of (a) includes:
if Sc＜SuCalculating the terminal voltage when the electromagnetic torque and the mechanical torque are equal, and judging the voltage stability of the induction motor again;
otherwise, the voltage instability of the induction motor is judged.
Further, the terminal voltage U when the electromagnetic torque is equal to the mechanical torqueEMThe calculation of (a) includes:
calculating the terminal voltage when the induction motor electromagnetic torque is equal to the mechanical torque according to the following formula:
wherein, the equivalent terminal voltage of the induction motor is calculated according to the following formula:
electromagnetic torque T when induction motor stably operateseWith mechanical torque TmEquilibrium relationship of (1)e(US,sc)＝Tm(sc) Calculating the equivalent terminal voltage of the induction motor, wherein the slip is ScThe mechanical power is as follows:
Tm(sc)＝T0[A(1-sc)2+B(1-sc)+C]；
electromagnetic torque TeIs composed of
Further, the determining of the voltage stability of the induction motor includes:
if terminal recovery voltage UreTerminal voltage U greater than when electromagnetic torque and mechanical torque are equalEMJudging that the voltage of the induction motor is stable; otherwise, the voltage instability of the induction motor is judged.
A transient voltage stability discrimination device, the device comprising:
the calculation unit is used for calculating the minimum machine end voltage and the induction motor slip when the induction motor stably runs according to the pre-collected induction motor slip and the machine end recovery voltage before and after the fault is removed;
and the judging unit is used for judging the voltage stability of the induction motor according to the difference between the slip at the fault clearing moment and the slip when the induction motor stably operates and/or the difference between the critical instability slip of the induction motor and the slip of the induction motor at the fault clearing moment.
Further, the device further comprises a criterion formulating unit for formulating a criterion for judging the voltage stability of the induction motor, wherein the criterion is as follows:
wherein s is0: initial slip of the induction motor; sc: any slip of the induction motor; sm: when the induction motor runs stably, the slip of the induction motor corresponding to the minimum terminal voltage is reduced; su: critical instability slip.
Further, the determination unit includes:
the critical calculating subunit is used for calculating the critical instability slip of the induction motor and comparing the critical instability slip with the slip of the induction motor at the moment of fault removal;
the first comparison subunit is used for comparing the fault clearing moment with the slip when the induction motor stably runs and judging the voltage stability of the induction motor;
and a second comparison subunit for judging the voltage stability of the induction motor according to the comparison between the generator end voltage and the generator end recovery voltage when the electromagnetic torque is equal to the mechanical torque.
Compared with the closest prior art, the technical scheme provided by the invention has the following beneficial effects:
(1) the technical scheme provided by the invention calculates the minimum terminal voltage and the induction motor slip when the induction motor stably runs according to the induction motor slip and the terminal recovery voltage collected before and after the fault is removed; the voltage stability of the induction motor is judged according to the difference between the slip at the fault removal moment and the slip when the induction motor stably operates, and/or the difference between the critical instability slip of the induction motor and the slip of the induction motor at the fault removal moment, the stability of the induction motor is judged in a double judgment mode, the judgment step of the stability of the induction motor is simplified, and the accuracy of stable judgment is effectively improved;
(2) the technical scheme provided by the invention judges based on the network charge mutual feedback characteristic of the induction motor, comprehensively considers the network transmission characteristic, the load characteristic of the induction motor and the interaction process of the network transmission characteristic and the load characteristic of the induction motor, can more comprehensively and effectively reflect the voltage instability condition taking the induction motor as a core, and has better accuracy.
(3) The technical scheme provided by the invention has the advantages that the voltage required by the slip recovery of the induction motor after the fault is removed is determined by the slip value of the induction motor during the fault removal, the piecewise function of the recovered voltage relative to the slip during the fault removal is taken as the necessary condition for judging the voltage stability of the induction motor, the criterion method has better applicability, and powerful theoretical support is provided for preventing the voltage instability phenomenon and formulating voltage instability solving measures.
Drawings
FIG. 1 is a flow chart of a determination method provided by the present invention;
FIG. 2 is an equivalent model of an induction motor provided by the present invention;
FIG. 3 is a simplified induction motor model provided by the present invention;
FIG. 4 is a diagram of an equivalent model for calculating the apparent power transmission limit in the embodiment;
FIG. 5 is a graph showing the relationship between the slip of the induction motor and the change of the electromagnetic power at different recovery voltages in the embodiment;
FIG. 6 is a graph of the slip and power variation of an induction motor for an embodiment of the invention;
fig. 7 is a graph of slip and power change for a triple permanent fault induction motor in an embodiment.
Detailed Description
The technical solution provided by the present invention will be described in detail by way of specific embodiments in conjunction with the accompanying drawings of the specification.
Aiming at the characteristics of the interaction of active power and reactive power and the interaction of an induction motor and network transmission in the voltage stabilization process, the invention comprehensively considers the influence of the interaction of the active power and the reactive power transmitted by a system on the voltage stability from the basic theory of apparent power transmission and defines the interaction relation of the induction motor and the network apparent power transmission as the network charge mutual feedback characteristic of the induction motor.
The invention provides a novel voltage stability judging method based on the network charge feedback characteristic of an induction motor by analyzing the network charge feedback characteristic of the induction motor around the transient voltage instability positive feedback dynamic process taking the induction motor as a core so as to further explain the voltage instability process containing the load of the induction motor.
As shown in the flowchart of fig. 1, the method for determining voltage stability based on the grid charge mutual feedback characteristic of the induction motor provided by the invention comprises the following steps:
step A: monitoring fault clearing moment induction motor slip ScAnd the instantaneous induction motor end recovery voltage U after fault removalre。
And B: calculating the minimum terminal voltage U of the induction motor when the induction motor stably operatesLminWhen the electromagnetic power curve of the induction motor is just tangent to the mechanical power curve, the corresponding induction motor slip Sm. Obtaining S from formula (1)mAnd ULmin。
Wherein:
Tm(s)＝T0[A(1-s)2+B(1-s)+C] (2)
in the formula, T0Initiating a mechanical torque for the induction motor;
R1: representing the equivalent resistance, X, after simplification of the induction motor model1: representing the equivalent reactance after the induction motor model is simplified; rR、XR、RS、XSRespectively, rotor resistance, rotor reactance, stator resistance and stator reactance of induction motor, XMThe equivalent circuit parameters of the induction motor are shown in an equivalent model of the induction motor as shown in figure 2 for the excitation reactance of the induction motor, and the simplified induction motor model is shown in figure 3; omegasIs the synchronous angular velocity of the induction motor load; A. b, C is the mechanical torque coefficient.
And C: comparing the slip S of the induction motor at the moment of fault removalcInduction motor slip S corresponding to when the induction motor electromagnetic power curve is tangent to the mechanical power curvem。
If ScD, when inequality (4) is satisfied, turning to step D; otherwise, go to step E.
sc≤sm (4)
Step D: comparing the recovery voltage U at the motor end of the induction motorreMinimum terminal voltage U of induction motorLmin. If the induction motor end recovers the voltage UreIf inequality (5) is satisfied, the voltage of the induction motor is judged to be stable; otherwise, the voltage instability of the induction motor is judged.
Ure＞ULmin (5)
Step E: and calculating the corresponding induction motor operation amount when any slip is s.
Active power P absorbed by induction motor when slip is sMLAnd reactive power QMLComprises the following steps:
in the formula, U: the voltage at the terminal of the induction motor,
US: equivalent terminal voltage of the induction motor;
a conjugate representing an equivalent impedance of the induction motor; zeThe equivalent impedance for an induction motor is calculated as follows:
thereby obtaining the apparent power S absorbed by the induction motorMLComprises the following steps:
calculating critical instability slip Su。
The network transmission characteristic curve equation is shown as follows:
wherein X is the equivalent line impedance, ESIs the equal-value potential of the sending end,
is the load power factor angle.
Load power factor of induction motor
The relationship to slip is:
the combined vertical type (5), (6), (7), (8) and (9) are solved to obtain positive real root as the critical instability slip S of the induction motoruValue of (A)。
F, performing a step; comparing the slip S of the induction motor at the moment of fault removalcSlip with critical instability Su. If ScIf inequality (10) is satisfied, go to step G; otherwise, the voltage instability of the induction motor is judged.
sc＜su (10)
Step G: calculating terminal voltage U when electromagnetic torque and mechanical torque are equalEM。
For the induction motor to be studied, the slip is calculated as scThe mechanical power is as follows:
Tm(sc)＝T0[A(1-sc)2+B(1-sc)+C] (11)
induction motor electromagnetic torque T in view of stable operationeWith mechanical torque TmThe balance relationship of (a) can be obtained:
Te(US,sc)＝Tm(sc) (12)
thereby solving for slip scLower induction motor equivalent terminal voltage USAs shown in the following formula:
further, a terminal voltage U when the electromagnetic torque and the mechanical torque of the induction motor are equal is obtainedEMAs shown in the following formula:
step H: u shapereAnd UEMAnd (6) comparing. If the induction motor end recovers the voltage UreIf inequality (13) is satisfied, the induction motor is judged to be stable; otherwise, the voltage instability of the induction motor is judged.
Ure＞UEM (15)
Examples
The invention uses infinite single load system calculation example to verify the method correctness, uses PSD-BPA transient simulation software, and has the following specific implementation process:
an infinite single load system model shown in fig. 4 was established using typical parameters of the induction motor shown in table 1, and a three-phase short circuit fault was used. The load adopts a 100% induction motor model, and the initial slip s of the induction motor0When s is 0.0116, s can be obtainedm＝0.08，ULmin＝0.554p.u，su＝0.242。
TABLE 1 typical parameters of induction motors
| RR | XR | XM | RS | XS |
| 0.02 | 0.12 | 3.49 | 0.02 | 0.18 |
If the fault is removed 0.12s after the short circuit occurs, the slip s of the induction motor is removed at the momentc0.042, when sc<sm，UEM0.603 p.u. When the induction motor recovers the voltage U as shown in FIG. 5 (a)reAt about 0.582p.u, U is presentre<UEMBut satisfy Ure>ULminThe induction motor slip can be recovered; when recovering voltage UreAround 0.538p.u, Ure<ULminIn time, the induction motor slip continues to increase and the induction motor tends to be unstable.
When the short circuit cut-off time is prolonged, the cut-off time is induced by the motor slip sc0.104, when sm<sc<su，UEM0.561 p.u. When the voltage U is restored in the induction motor, as shown in FIG. 5 (b)reAt about 0.555p.u, there is UreHigher than ULminThe slip of the induction motor cannot be recovered; when the induction motor recovers the voltage UEM0.567p.u, higher than the voltage value U corresponding to slip 0.104EMAt 0.561p.u, the induction motor slip can be recovered. Thus verifying the rationality of the method.
If three-phase short circuit instantaneous fault simulation is adopted, the slip and power change of the induction motor are shown in figure 6, when the short circuit is cut off, the slip of the induction motor is increased to 0.241, the slip of the induction motor starts to decline after the fault is cut off, and after transition, the running slip of the induction motor can be finally recovered; when the short circuit is cut off, the slip is increased to 0.245, and the slip is continuously increased after the fault is cut off until the induction motor is locked. It can be seen that the critical slip s of the induction motorcShould be between 0.241 and 0.245 (see fig. 6). The critical slip s at this point is obtained from the method hereinc0.251, which is closer to the simulation value.
If a three-phase short circuit permanent fault is adopted, the slip and power change of the induction motor are shown in figure 7, and the critical slip s is verified through simulationcBetween about 0.091 and about 0.092 (see fig. 7). Calculating the critical slip s at this time by the method in the textc0.096, which is also closer to the corresponding simulated critical slip value.
The voltage transient stability criterion of a contrast meter and an induction motor is generally accepted at present, and the slip corresponding to the intersection point of an electromagnetic torque curve and a mechanical torque after a fault is taken as the critical slip sc. When T ism(s) when the expression of the secondary torque is adopted as shown in the formula (2), scFrom equation (14) to。
sc＞sm
The critical slip at the corresponding recovery voltage was also calculated using typical parameters for induction motors as shown in table 1. If a quadratic mechanical torque model is used, when the recovery voltage after the fault is removed is 0.64p.u, s is obtained from equation (14)c0.188; when the recovery voltage is equal to 0.56p.u, s is obtained from equation (14)c0.130, each deviates significantly from the actual value.
Therefore, the method has certain limitation when the bus voltage of the induction motor is changed by adopting a general critical cutting slip judgment method, and the deviation of the calculation result is larger; the transient voltage stabilization criterion proposed herein, which takes into account the induction motor, has better accuracy and applicability.
A transient voltage stability discrimination device, the device comprising:
the calculation unit is used for calculating the minimum machine end voltage and the induction motor slip when the induction motor stably runs according to the pre-collected induction motor slip and the machine end recovery voltage before and after the fault is removed;
and the judging unit is used for judging the voltage stability of the induction motor according to the difference between the slip at the fault clearing moment and the slip when the induction motor stably operates and/or the difference between the critical instability slip of the induction motor and the slip of the induction motor at the fault clearing moment.
Further, the device further comprises a criterion formulating unit for formulating a criterion for judging the voltage stability of the induction motor, wherein the criterion is as follows:
wherein s is0: initial slip of the induction motor; sc: any slip of the induction motor; sm: stabilizationWhen the induction motor runs, the slip of the induction motor corresponding to the minimum terminal voltage; su: critical instability slip.
Further, the determination unit includes:
the critical calculating subunit is used for calculating the critical instability slip of the induction motor and comparing the critical instability slip with the slip of the induction motor at the moment of fault removal;
the first comparison subunit is used for comparing the fault clearing moment with the slip when the induction motor stably runs and judging the voltage stability of the induction motor;
and a second comparison subunit for judging the voltage stability of the induction motor according to the comparison between the generator end voltage and the generator end recovery voltage when the electromagnetic torque is equal to the mechanical torque.
Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.
Claims (9)
1. A method for determining transient voltage stability, the method comprising:
calculating the minimum terminal voltage U of the induction motor in stable operation according to the slip of the induction motor and the terminal recovery voltage collected before and after the fault is removedLminSlip S corresponding to minimum terminal voltage during stable operation of induction motorm；
Induction of motor slip S according to fault clearing timecAnd slip S during steady operation of the induction motormDifference between, and slip S according to critical instability of the induction motoruInduction motor slip S at the moment of fault removalcDetermining the voltage stability of the induction motor according to the difference;
induction motor slip S at fault removal timecAnd instantaneous induction motor end recovery after fault removalVoltage Ure；
Minimum terminal voltage U when electromagnetic power curve is tangent to mechanical power curve when induction motor operates stablyLminSlip S during stable operation of corresponding induction motormAnd a minimum terminal voltage ULminCalculated as follows:
in the formula, T0: an induction motor initial mechanical torque;
R1: representing the equivalent resistance, X, after simplification of the induction motor model1: representing the equivalent reactance after the induction motor model is simplified; rR: an induction motor rotor resistance; rs: an induction motor stator resistance; xRAnd Xs: respectively an induction motor rotor reactance and a stator reactance; xM: a field reactance of the induction motor; omegas: synchronous angular velocity of the induction motor load; A. b, C are coefficients of mechanical torque, respectively;
inducing motor slip S based on said fault clearing timecAnd slip S during steady operation of the induction motormThe determination of the difference between includes:
if Sc≤SmThen comparing the recovery voltage U at the end of the induction motorreAnd minimum terminal voltage ULminA difference of (d);
otherwise, according to the critical instability slip S of the induction motoruInduction motor slip S at the moment of fault removalcAnd determining the voltage stability of the induction motor according to the difference.
2. The method of claim 1, wherein the induction motor terminal recovery voltage U is based on a comparisonreAnd minimum terminal voltage ULminThe determination made of the difference of (a) includes:
if U isre＞ULminThe voltage of the induction motor is stable; otherwise, the voltage instability of the induction motor is judged.
3. The method of claim 1 wherein said induction motor critical instability slip SuThe calculation of (a) includes:
1) the apparent power S absorbed by the induction motor is calculated as followsML：
Wherein, PMLAnd QML: the active and reactive power absorbed by the induction motor, respectively the slip s, is calculated by:
in the formula, U: the voltage at the terminal of the induction motor,
Us: equivalent terminal voltage of the induction motor;
a conjugate representing an equivalent impedance of the induction motor; equivalent impedance Z of induction motoreCalculated as follows:
2) network transmission characteristic curve SMLAs shown in the following formula:
wherein, X: equivalent line impedance, ES: the equal potential of the sending end is provided,
a load power factor angle;
calculating the load power factor of an induction motor by the following formula
3) Calculating the positive real number as the critical instability slip S of the induction motoruThe value of (c).
4. The method of claim 1, wherein the critical instability slip S of the induction motor is based onuInduction motor slip S at the moment of fault removalcThe determination of the difference of (a) includes:
if Sc＜SuCalculating the electromagnetic torque TeWith mechanical torque TmTerminal voltage U of equalEMJudging the voltage stability of the induction motor again;
otherwise, the voltage instability of the induction motor is judged.
5. Method according to claim 4, characterized in that said electromagnetic torque TeWith mechanical torque TmTerminal voltage U of equalEMThe calculation of (a) includes:
electromagnetic torque T of induction motor is calculated by following formulaeWith mechanical torque TmTerminal voltage U of equalEM：
Wherein, the equivalent terminal voltage of the induction motor is calculated according to the following formula:
electromagnetic torque T when induction motor stably operateseWith mechanical torque TmEquilibrium relationship of (1)e(US，Sc)＝Tm(Sc) Calculating the equivalent terminal voltage of the induction motor, wherein the slip is ScTime mechanical torque TmComprises the following steps:
Tm(Sc)＝T0[A(1-Sc)2+B(1-Sc)+C]；
electromagnetic torque TeIs composed of
6. The method of claim 5, wherein the determination of the voltage stability of the induction motor comprises:
if terminal recovery voltage UreGreater than the electromagnetic torque TeWith mechanical torque TmTerminal voltage U of equalEMJudging that the voltage of the induction motor is stable; otherwise, the voltage instability of the induction motor is judged.
7. An apparatus for applying the method of any of claims 1-6, the apparatus comprising:
a calculating unit for calculating the minimum terminal voltage U of the induction motor during stable operation according to the pre-collected slip and terminal recovery voltage of the induction motor before and after fault removalLminAnd slip S during steady operation of the induction motorm；
A determination unit for inducing motor slip S according to the time of fault removalcAnd slip S during steady operation of the induction motormDifference between, and according to the induction motorCritical instability slip SuInduction motor slip S at the moment of fault removalcAnd determining the voltage stability of the induction motor according to the difference.
8. The apparatus as claimed in claim 7, further comprising a criterion preparation unit for preparing a criterion for determining voltage stability of the induction motor, wherein the criterion is represented by the following formula:
wherein S is0: initial slip of the induction motor; sc: induction of motor slip at the moment of fault removal; sm: the slip of the induction motor corresponding to the minimum terminal voltage during stable operation; su: critical instability slip of induction motors; terminal recovery voltage Ure(ii) a Mechanical torque TmAnd electromagnetic torque TeTerminal voltage U of equalEM(ii) a Minimum terminal voltage ULmin。
9. The apparatus of claim 7, wherein the determination unit comprises:
a critical calculating subunit for calculating the critical instability slip S of the induction motoruAnd induction of motor slip S at the moment of fault removalcComparing;
a first comparison subunit for comparing the induction motor slip S at the moment of fault removalcAnd slip S during steady operation of the induction motormJudging the voltage stability of the induction motor;
a second comparison subunit for comparing the electromagnetic torque TeWith mechanical torque TmTerminal voltage U of equalEMRecovery voltage U of terminal of and machinereAnd comparing to judge the voltage stability of the induction motor.
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