RU2297704C1 - Induction motor turn-to-turn fault protection method - Google Patents

Abstract

FIELD: protection of induction motor stator winding against turn-to-turn fault.

SUBSTANCE: proposed method includes measurements of instant current and voltage across motor terminals, their conversion into voltages proportional to current and voltage, measurement of motor slip, recording of obtained signals proportional to current and voltage which are used to evaluate initial phase of transient and initial current, calculation of next instant currents at initial values of equivalent active resistance and equivalent inductance of motor, comparison of calculated instant currents with respective recorded values, evaluation of their difference, and variation of equivalent active resistance and equivalent inductance depending on this difference until difference between recorded and calculated values of instant current is brought to minimum, comparison of equivalent active resistance and equivalent inductance obtained in the process at measured slip with admissible values, and generation of OFF-operation signal in case these values exceed admissible ones.

EFFECT: enhanced protective-gear selectivity and speed of response.

3 cl, 8 dwg

Description

The invention relates to relay protection and is intended to protect an induction motor from coil circuits in the stator winding.

A known method used in devices for protecting engines from coil circuits using an estimate of the excess of phase permissible stator currents to the maximum permissible values. Based on it, current cutoffs and maximum current protection (MTZ) are created [1]. However, such protections have insufficient selectivity, sensitivity, and speed, since the values of the motor current in the normal starting mode differ little from the motor current when a small number of turns are closed. Therefore, such protections make it possible to detect short circuits of only a significant number of turns of the winding with a long delay of the response time.

The closest in technical essence to the proposed method is the method according to which the impedance of the controlled motor is determined by the integrated values of current and voltage and this resistance is compared with the setting [2].

The disadvantage of this method is the low selectivity. This is due to the fact that in transient conditions it is difficult to determine the reliable value of the impedance of a controlled electric motor, since currents and voltages have a non-sinusoidal shape and contain aperiodic components. Under these conditions, reliable determination of the integral values of current and voltage requires long-term monitoring of the processes, which leads to a decrease in the speed of protection.

The purpose of the invention is to increase the selectivity and speed of the method.

This goal is achieved by measuring the instantaneous values of currents and voltages at the terminals of the motor, converting them into a voltage proportional to the current, and into a voltage proportional to the voltage, registering the received signals, in addition, using a speed sensor, measure the slip of the motor. The instantaneous values of currents, voltages and the phase of occurrence of the transient process are determined. Subsequent instantaneous current values are calculated at the initial values of the equivalent active resistance and the equivalent inductance of the electric motor. The calculated instantaneous current values are compared with the corresponding recorded values, their difference is determined and, depending on its value, the values of the equivalent active resistance and the equivalent inductance of the electric motor are changed, and if the obtained difference has a positive value, the values of the equivalent active resistance and equivalent inductance are increased, and with a negative difference is reduced. Correction of the parameters occurs until the difference between the calculated and registered current values reaches the set minimum limit value. The obtained parameters, with the measured value of the slip, are compared with the settings, and if the values of the equivalent active resistance and equivalent inductance are outside the acceptable range, then they form a signal to turn off the electric motor. The analysis of the processes is carried out on the basis of a T-shaped equivalent circuit of an induction motor, shown in figure 1. Given the real ratio of the parameters of the electric motors, for the purpose of relay protection, a controlled electric motor can be represented by an electric circuit consisting of an equivalent active resistance and an equivalent inductance, shown in figure 2 [3]. In this case, the stator current of the electric motor is described by the following equation [4]:

where I _m is the amplitude value of the current;

- фазовый сдвиг;

- phase shift;

i (0) is the initial value of the current at the time of the occurrence of a circuit;

R is the equivalent active resistance of an induction motor;

L is the equivalent inductance of the induction motor;

t is the current transient time.

This calculation method allows you to operate with instantaneous values of currents and voltages, that is, to make a more accurate analysis of transient processes, and an additional measurement of motor slip - to distinguish normal transient from emergency.

Figure 1 and figure 2 shows, respectively, a T-shaped and equivalent circuit equivalent circuit of an induction motor. The designations on the diagrams correspond to the following values, given to the stator winding:

R ₁ , L ₁ - active resistance and inductance of the stator winding;

R ₂ , L ₂ - active resistance and inductance of the rotor winding;

R _m , L _m - active resistance and inductance of the magnetization circuit;

s - slip induction motor;

R _EC , L _EC - equivalent active resistance and equivalent motor inductance.

Figure 3 shows a functional diagram of a device that implements the proposed method.

Figure 4 shows the location of the terminals in the motor winding for simulation of turns.

Figure 5 shows the waveform of the current when the coil circuit under load (26 turns with a total number of turns of the coil 216) induction motor series AIR. The darker line in the waveform shows the calculated current.

Figure 6 shows in relative units the curve of the change in inductance versus the number of closed turns when the turns are closed in an induction motor. This dependence was obtained experimentally for engines of the AIR and 4A series.

Figure 7 shows the magnetization curve in relative units for the engine series AIR.

On Fig depicts the dependence of the inductance of the motor on the current, constructed using a magnetization curve.

The proposed method is implemented as follows.

Created a protection device, a functional diagram of which is shown in figure 3. The device has been tested on a 3 kW electric motor of the AIR series. When testing a protective device, various modes of operation of the electric motor are implemented. The protection operation was checked in normal conditions: at start-up, at idle and under load. For the simulation of coil faults in the winding of one of the phases of the electric motor, 6 taps were made. For one pair of poles in each phase of the motor, there is one section consisting of three coils with 36 turns.

Solder coordinates:

K ₁ - 26 turns of the first coil;

To ₂ - 36 turns (end of the first coil);

To ₃ - 72 turns (end of the second coil);

To ₄ - 108 turns (end of the third coil);

To ₅ - 144 turns (end of the fourth coil);

To ₆ - 180 turns (the end of the fifth coil).

The layout of the findings in the motor winding for simulation of coil faults is presented in figure 4. Circuit closures were modeled using closures of the tap to the beginning of the winding. The tests were carried out at a nominal phase voltage of 220 V.

The protection device comprises a current to voltage converter unit 1, a voltage to voltage converter unit 2, a current registration unit 3 and a voltage registration unit 4, a disturbance phase determination unit 5, an instantaneous current value calculation unit 6, a timer 7, a current comparison unit 8, unit 9 setting the minimum current difference, unit 10 for determining the equivalent active resistance and equivalent inductance, unit 11 for comparing parameters, unit 12 for setting the settings, sensor 13 for the rotation speed of the motor shaft, asynchronous motor 14.

The output of block 1 is connected to the input of block 3, the output of block 2 is connected to the input of block 4. The output of block 3 is connected to the first input of block 5, the first input of block 6 and the first input of block 8, the output of block 4 is connected to the second input of block 5. The output of the block 5 is connected to the second input of block 6. The output of block 6 is connected to the second input of block 8. The outputs of block 7 are connected to the second inputs of blocks 3 and 4, as well as to the third inputs of blocks 5 and 6. The output of block 8 is connected to the input of block 10. The output block 9 is connected to the third input of block 8. The first and second outputs of block 10 are connected to the fourth and fifth th input unit 6, and the third output of block 10 is connected to the first input unit 11. The second and third inputs of block 11 are connected to the outputs of blocks 12 and 13, respectively. The output of block 11 is connected to the input of block 14.

The device operates as follows. The inputs of the current-voltage converter 1 and voltage-voltage converter 2 are supplied with the current and voltage of the protected object, respectively. The signals from the outputs of the converters are recorded by the current recorder 3 and the voltage recorder 4. The signals from the registrars are fed to the disturbance phase determination unit 5. The output signal from the perturbation phase determination unit 5 is fed to the input of the instantaneous current value calculation unit 6, the second input of which contains a signal from the current recorder 3, which sets the initial current when a transient occurs. In addition, a signal from timer 7 is applied to the inputs of recorders 3 and 4 to register instantaneous values of current and voltage at the same time instants. The signal from the timer 7, which counts the current time, is also fed to the input of the disturbance phase determination unit 5 and to the input of the instantaneous current value calculation unit 6. The output signal from unit 6 for calculating the instantaneous current values is supplied to unit 8 for comparing currents, to the second input of which the measured current is supplied from current recorder 3, and to the third input, a signal from unit 9 for setting the minimum current difference is supplied. Using block 8, the difference between the calculated instantaneous current values and the corresponding recorded values is determined and compared with the minimum allowable current difference. From the output of the comparison unit 8, the signal is supplied to the equivalent active resistance and equivalent inductance determination unit 10. The output signals of block 10 are sent to block 6 for calculating the instantaneous values of currents, where, if necessary, re-calculation of instantaneous values of currents is carried out, as well as to block 11 for comparing parameters. In the unit 11 for comparing the parameters, the obtained values of the equivalent active resistance and equivalent inductance are compared with the set values that are received at the input of the comparison unit 11 from the setting unit 12. In addition, the motor slip detected by the speed sensor 13 is supplied to the third input of the comparison unit. As a result, an output signal is generated to turn off the induction motor.

For each mode, using the described method, the curves of the calculated currents are built and the equivalent active resistance and inductance are determined. One of these current curves with a turn circuit of 26 turns is shown in Fig.5. For start, idle and load modes, the values of the obtained parameters are confirmed using calculations based on the T-shaped equivalent circuit.

For example, for the normal start-up mode, using the described method, the parameters R = 4 Ohm, L = 0.02 GN were obtained. The same parameters are calculated using a T-shaped equivalent circuit in which the slip of the motor is assumed to be unity, and the magnetization branch is not taken into account. As a result, the following parameters were obtained R = 4.23 Ohm, L = 0.02 GN.

For the idle mode, the parameters R = 9.7 Ohm, L = 0.2 H were obtained, and by the T-shaped scheme, close values were obtained by calculation:

R = 9.667 Ohm, L = 0.182 GN.

As can be seen, the scatter of the parameters obtained by the device according to the real current curves and obtained by calculation by the T-shaped circuit does not exceed 10%. Really achievable accuracy is sufficient for relay protection purposes.

As a result of studies of idle turn circuits for the engine of the AIR series, the following equivalent active resistances and equivalent inductances were obtained:

the closure of the first tap to the beginning: R = 16 Ohms, L = 0.1 H,

second tap to the beginning: R = 14 Ohms, L = 0.08 H,

the third tap to the beginning: R = 13 Ohms, L = 0.018 H,

fourth tap to the beginning: R = 6 Ohm, L = 0.008 H,

fifth tap to the beginning: R = 6 Ohms, L = 0.0075 H,

the sixth tap to the beginning: R = 4 Ohms, L = 0.005 GN.

The curve of the inductance in relative units is shown in Fig.6. Analytically, the curve of the change in the inductance of the electric motor from the number of closed turns can be constructed on the basis of the following considerations.

If by the flux linkage of the circuit (ψ) we mean the total magnetic flux that is coupled to this circuit and caused by the current in it, then the equivalent inductance can be expressed as follows [5]:

where ψ is flux linkage, rel.

i - current, rel.

Thus, taking into account the indicated expression, dividing the flux linkage in the magnetization curve depicted in Fig. 7 by current, we will have the inductance versus current shown in Fig. 8.

It follows that with an increase in the number of closed turns, the current in the damaged phase increases, and the inductance decreases.

In addition, it should be noted that the method provides a high sensitivity of protection with a small number of closed turns.

Thus, the use of the proposed method allows to develop on its basis a number of devices for protecting induction motors from internal and including turn faults, providing high selectivity, speed and the possibility of detecting turn faults with a small number of closed turns.

Using the proposed method for protecting induction motors from coil circuits provides the following advantages compared to existing methods.

Firstly, an increase in the selectivity of action in transitional conditions. In contrast to the prototype, the parameters of the equivalent active resistance and the equivalent inductance of the electrical circuit are monitored, which can be used in the strict analysis of electrical circuits in transient conditions.

Secondly, the slip control of the engine allows you to separate the circuit faults from the normal start-up modes and reduce the likelihood of false trips.

Information sources

1. Fedoseev A.M. Relay protection of electrical systems. M., "Energy", 1970, p. 493.

2. Copyright certificate No. 473124, cl. G01R 27/16, 1973.

3. Voldek A.I. Electric cars. L., "Energy", 1974, p. 840, ill.

4. Atabekov G.I. Fundamentals of circuit theory. Textbook for high schools. M., "Energy", 1969, p. 424, ill.

5. Karantarov P.L., Zeitlin L.A. Inductance Calculation: A Reference Book. L .: Energoatomizdat. Leningrad branch, 1986, p. 488, ill.

Claims (3)

1. A method of protecting an induction motor from coil circuits, in which the instantaneous values of currents and voltages at the terminals of the motor are measured, they are converted into a voltage proportional to the current, and into a voltage proportional to the voltage, respectively, characterized in that the slip of the motor is measured, the obtained proportional signals and current and voltage, they determine the initial phase of the transition process and the initial current value, calculate the subsequent instantaneous current values at the initial values According to the values of equivalent active resistance and equivalent inductance of the electric motor, the calculated instantaneous current values are compared with the corresponding recorded values, their difference is determined and, depending on its magnitude, the values of the equivalent active resistance and equivalent electric inductance of the motor are changed until the difference between the registered and calculated current values of the minimum value obtained the values of the equivalent active resistance and equi alentnoy inductor motor at the measured slip value is compared with the permitted values and, if they are beyond the permissible values form a signal for interrupting the motor. 2. The method of protecting an induction motor from coil circuits according to claim 1, characterized in that when receiving a positive difference between the calculated current values and the registered ones, the values of the equivalent active resistance and equivalent inductance are increased, and if the difference is negative, the values of equivalent resistance and equivalent inductance are reduced. 3. The method of protecting an induction motor from coil circuits according to claim 1, characterized in that the maximum permissible values of the equivalent active resistance and equivalent inductance are set for each slip value.

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