RU2827055C1 - Method of controlling success of direct start of asynchronous motor - Google Patents

Abstract

FIELD: electric power industry.

SUBSTANCE: invention can be used in electric power systems, power supply systems, electric networks of mainly comparable generation power and the largest asynchronous motors to control their direct starts. Method consists in the fact that the induction motor start-up process is divided into five stages, on each of which instantaneous values of motor current and voltage are continuously measured, based on the measurement results, the motor electrical parameters and the critical mode parameters for the motor and the supply network for each stage are determined, at the end of the stages, compliance with the start-up conditions is checked, in case of non-fulfillment of which, emergency shutdown of the motor from the supply mains is performed.

EFFECT: prevention of failures of supply network equipment and asynchronous motor at direct start-up due to step-by-step control of start-up dynamics, speed and value of voltage change, supply mains frequency with emergency interruption.

1 cl, 5 dwg, 1 tbl

Description

The proposed invention relates to the field of electric power engineering and can be used in electric power systems, power supply systems, and electrical networks of predominantly commensurate generation and unit consumption power for monitoring the success of direct starts of general-purpose asynchronous motors (AM).

A method for the smooth start of an induction motor with a squirrel-cage rotor is known (Patent RU 2 625 807 C2), characterized by the following set of features: a three-phase voltage is supplied to the induction motor stator windings from the network buses through three pairs of anti-parallel connected thyristors controlled by a synchronized pulse-phase control system, to the input of which, upon the "Start" command, a pre-formed control signal equivalent to the thyristor opening angle is supplied, wherein the control signal is formed by continuously measuring the residual voltage on the network buses and comparing it with a pre-set value of this voltage, upon disconnection from which the control signal is adjusted, maintaining the residual voltage on the network buses constant during the start-up process, additionally measuring the voltage drop on the thyristors and comparing it with a pre-set value, upon exceeding which, after a pre-set time, a "Stop" command is formed, ensuring the motor is disconnected during a prolonged or failed start.

However, this method has the following disadvantages: it requires expensive equipment; it does not take into account the influence of the current values of the operating parameters on the IM buses before starting, the frequency of the power supply network on the starting process, and it also does not take into account the influence of the IM starting mode on the balance of active power in a small power system, and, as a consequence, on the operating mode of generating units; exceeding the permissible starting duration is determined by the actual excess of a predetermined value, which increases the thermal load on the IM and the duration of the effect of the engine starting mode on the power supply network.

A device for estimating the parameters and processes of an induction motor is known (Patent RU 2 543 495 C1), intended for determining the parameters of an induction motor based on recordings of a transient process, characterized by the following set of features: it contains phase voltage sensors, stator phase current sensors, a converter of phase voltages and stator phase currents, allowing the phase voltages and stator currents to be converted into voltages and currents in the d, q coordinate system of a generalized machine, five blocks for calculating parameters, adders, a customizable induction motor model, which allow the calculation of parameters, variables, and the rotation frequency of an asynchronous motor without using rotation frequency sensors, angular acceleration, and differentiation devices when using a method based on generating a sensitivity function of the stator phase currents of a generalized machine based on the induction motor parameters.

However, the said device cannot be used to control the start-up of the AM at the process rate, since the identification method used in it requires pre-set initial values of the AM parameter estimates, and the duration of the parameter identification process, which is 40 seconds, is not acceptable for real-time process control.

In addition, a method of direct starting of an asynchronous motor is known (Voldek A.I., Popov V.V. Electrical machines. AC machines: Textbook for universities. - SPb: Piter, 2008. - 350 p., pp. 134-140.) which is a prototype of the proposed invention and consists in the fact that the full three-phase voltage is supplied to the stator windings of the induction motor after checking the normality of the voltage of the power supply network and, in the event of abnormal voltage after switching on or a prolonged excess of the overload capacity for current, the motor is disconnected from the power supply network.

However, this method has the following disadvantages: during start-up, the starting current consumed is an order of magnitude higher than the nominal current, which causes a decrease in voltage in the adjacent network with the risk of developing a voltage avalanche when other operating engines overturn and a critical thermal effect on the induction motor during prolonged starts; the influence of the power supply network frequency on the start-up process is not controlled or taken into account, and the duration of the engine start-up is not predicted.

The objective of the proposed invention is to prevent damage to asynchronous motors or power supply network equipment, and to preserve their service life in the process of unsuccessful direct motor starts by identifying them at early stages and interrupting them.

The stated problem is solved due to the fact that in the known method, which consists of a preliminary check of the normality of the voltage of the power supply network, supplying voltage to the stator windings of the motor and, in the event of abnormal voltage after switching on or a long-term excess of the overload capacity for current, disconnecting the motor from the power supply network, additionally measuring the frequency of the power supply network and the active power of the motor, monitoring the permissibility of the voltage, frequency and duration of the current overload of the motor after switching it on at four stages, at which the electrical parameters of the motor, critical parameters of the mode for the motor and the power supply network are determined, checking the observance of intermediate conditions for the success of the start, if they are not met at any of the stages, disconnecting the motor from the power supply network.

The implementation of the method is explained by a structural diagram of the connection of the motor to the power supply network with a device for monitoring the success of the start (Fig. 1), a generalized structure of the process of monitoring the success of the start of the AM (Fig. 2), graphs of the change in the operating parameters of the network over time during the successful start of the AM (Fig. 3) and an L-shaped equivalent circuit of the AM (Fig. 4).

The structural diagram (Fig. 1) contains: 1 - power supply network (PS); 2 - voltage measuring unit (VMU); 3 - switching apparatus (SA); 4 - control unit (CU) for starting the motor, which receives an external command (EC) to turn the motor on or off; 5 - current measuring unit (CMU); 6 - MO.

The generalized structure of the process of monitoring the success of the start-up of the motor (Fig. 2) and the graphs of the change in the operating parameters of the power supply network during the successful start-up of the motor (Fig. 3) are divided into five stages: 1 - blocking the start of the motor; 2 - express assessment of its admissibility according to the necessary conditions; 3 - assessment of admissibility according to sufficient conditions; 4 - assessment of the admissibility of the start-up duration; 5 - prohibition of repeated switching on.

The method is implemented as follows: at time A, the IM is in the standby mode for the VK to turn on. After receiving it, at stage 1, up to time B, the BU evaluates the possibility of starting the IM based on the ICN data. The voltage in the load node and the frequency of the power supply network for performing the primary direct start of the IM must satisfy the following conditions:

(1)

(2)

Where - voltage on the AD buses before starting, - minimum permissible voltage in the load node, - frequency value before engine start, - minimum permissible frequency of the power supply network.

For subsequent launches, the following conditions are taken into account together:

Where - minimum voltage for successful start of the motor, - the maximum value of the network frequency for successful start-up of the motor, - the maximum permissible minimum value of frequency in the network according to the condition of operation of the technological automation of generating units.

If the conditions for success before starting are met, the control unit permits the execution of the command to turn on the spacecraft, and the engine is started directly. At stage 2, the control unit determines the parameters of the mathematical model of the induction motor (Fig. 4) using a known method, and the critical values of the parameters (voltage and frequency) of the power supply network mode for a successful engine start, the parameters are checked for going beyond the critical and normatively permissible values, and if they go beyond the normatively permissible values, the engine is turned off, and the critical parameters are used at stage 3.

The electrical parameters of the mathematical model of the induction motor are determined using the least squares method using the expressions:

Where - stator reactance, - the reactance of the rotor reduced to the stator, - active resistance of the stator, - active resistance of the rotor reduced to the stator, - active power consumed by the engine, - reactive power consumed by the engine, - effective value of the phase current of the motor.

The critical stress is determined by the expression:

Where - the moment of resistance of the drive mechanism on the AD shaft, - total reactance of the rotor and stator, - total active resistance of the rotor and stator, - angular velocity of the stator magnetic field.

If retrospective data are available, the moment of resistance of the drive mechanism is taken to be equal to the electromagnetic moment in the normal quasi-steady-state mode preceding the current start of the engine; otherwise, it is taken to be nominal.

The critical frequency for starting an induction motor is determined by the following expression:

Where, - critical angular velocity of the stator magnetic field for successful starting of the induction motor, - the value of the angular velocity of the magnetic field of the induction motor stator at the moment of starting.

At time C, the BU checks that the following conditions are met:

Where - frequency value when starting the engine, - voltage on the AD buses during start-up.

At stage 3, the control unit makes a prognostic calculation of the quasi-steady-state value of the voltage frequency, taking into account the operation of the speed regulators of the generating units, by extrapolating the frequency dependences on time, the nature of the change of which (in Fig. 3 in the interval from 4 to 4.6 s) is described by the approximating function:

(3)

Where - frequency of the PS at the moment of start-up, - minimum value of the PS frequency when starting the AM, - frequency attenuation decrement, - time relative to the start of determining the resulting frequency value when starting the AM, - actual value of the PS frequency

The minimum value of the network frequency when starting the motor is determined by expression (3):

Where

The voltage value, taking into account the action of the excitation regulators, as the least inertial mode parameter, is taken to be equal to the actual value at the end of the probing interval. At moment D, the BU checks the compliance with the following conditions, taking into account the influence of the regulators of the generating units:

(4)

(5)

At stage 4, the BU determines the predicted start duration and the sign of the rate of change of the consumed active power, upon detection of a positive value of which, by extrapolation of the dependence of the active power on time, it determines the duration of acceleration to the normal steady-state speed. Upon detection of a stable sign of the rate of change of the active power, it calculates the start duration:

Where - the time it takes for the engine to reach normal operating speed, - the value of the consumed active power corresponding to the critical slip, - the value of active power at the moment of detecting stable engine acceleration, - time from the start until the moment of detection of stable engine acceleration.

At moment E, the control unit checks the admissibility of the start-up duration of the motor according to the following condition:

Where - the maximum permissible start-up time of the asynchronous motor under the condition of permissibility of voltage reduction in adjacent load nodes or under the condition of maximum thermal impact.

At stage 5, until time F, the CU is in the waiting mode for a command to prohibit restarting the engine in order to prevent the infinity of the motor starting cycles or to exclude switching on after a forced start.

If the conditions for successful start-up of the motor are not met at the given moments, the control unit issues a command to turn off the motor, followed by a prohibition on restarting the motor without improving the conditions for its start-up.

The method was verified by digital modeling of the processes of starting an asynchronous motor in a system of comparable generation power and an asynchronous motor with variations in their conditions in the MATLAB/Simulink software package.

The digital model of the system of comparable generating power and AM (Fig. 5) consists of: 6 - AM (nominal power - 75 kVA; nominal voltage - 400 V; nominal frequency - 50 Hz; stator active resistance - 0.036 Ohm; rotor active resistance - 0.021; stator and rotor leakage inductance - 0.00034 H; inertia - 1.25 kg*m^2; number of pole pairs - 1); 7 - communication lines; 8 - shunt load with a capacity of 1.11+j0.01 MVA; 9 - synchronous generator (nominal power - 2 MVA; rated voltage - 400 V; rated frequency - 50 Hz; inductive reactance along the longitudinal axis (d) - 2.11 p.u.; transient inductive reactance along the longitudinal axis (d) - 0.17 p.u.; subtransient reactance along the longitudinal axis (d) - 0.13 p.u.; inductive reactance along the transverse axis (q) - 1.56 p.u.; subtransient reactance along the transverse axis (q) - 0.23 p.u.; transient time constant of the rotor with a short-circuited stator - 0.33 s; subtransient time constant of the rotor with a short-circuited stator along the longitudinal axis (d), along the transverse axis (q) -0.03 s; coefficient of inertia - 0.3072 s; number of pole pairs - 2); 10 - automatic speed controller (ASC) of a synchronous generator; 11 - astatic automatic excitation controller (AEC) of a synchronous generator.

The conditions for starting the asynchronous motor (variations: the moment of resistance of the drive mechanism ( M _res ); the voltage on the asynchronous motor buses before starting ( U ₀ ); the coupling resistance ( Z _cv ) of the motor and generator; the frequency of the PS before starting the motor ( f ₀ )) and the results of the method are presented in Table 1.

Table 1

No. Variable start conditions parameter Parameter value Digital model Model of control of starting of the AD Stage and time of issuing a command to interrupt the AD 1 M _resistance , N*m 0.1 Successfully Successfully Not issued 2 50 Successfully Successfully 3 101 Successfully Successfully 4 200 Successfully Successfully 5 213 Successfully Successfully 6 215 Successfully Successfully 7 216 Successfully Successfully 8 220 Successfully Unsuccessful Stage 3, condition (4), 0.24 s 9 223 Successfully Unsuccessful 10 224 Unsuccessful Unsuccessful 11 225 Unsuccessful Unsuccessful 12 U ₀ , o.e.
( M _resistance = 101 N*m) 0.9 Successfully Successfully Not issued 13 0.8 Successfully Successfully 14 0.75 Successfully Successfully 15 0.735 Successfully Successfully 16 0.73 Successfully Successfully 17 0.725 Successfully Unsuccessful Stage 3, condition (4), 0.24 s 18 0.72 Unsuccessful Unsuccessful 19 0.715 Unsuccessful Unsuccessful 20 0.71 Unsuccessful Unsuccessful 21 0.709 Unsuccessful Unsuccessful 22 0.708 Unsuccessful Unsuccessful 23 0.706 Unsuccessful Unsuccessful 24 0.704 Unsuccessful Unsuccessful 25 0.701 Unsuccessful Unsuccessful 26 0.7 Unsuccessful Unsuccessful Step 1, condition (1) 27 0.5 Unsuccessful Unsuccessful 28 Z _sv , Ohm
( M _resistance = 101 N*m) 0.101 Successfully Successfully Not issued 29 0.151 Successfully Successfully 30 0.176 Successfully Unsuccessful Step 3, condition (4),
0.24 s 31 0.181 Successfully Unsuccessful 32 0.182 Unsuccessful Unsuccessful 33 0.183 Unsuccessful Unsuccessful 34 0.201 Unsuccessful Unsuccessful 35 f ₀ , Hz
( M _resistance = 200 N*m) 54 Unsuccessful Unsuccessful Step 3, condition (5),
0.23 s 36 53.5 Unsuccessful Unsuccessful 37 52.9 Unsuccessful Unsuccessful 38 52.5 Successfully Successfully Not issued 39 52 Successfully Successfully 40 51 Successfully Successfully 41 50.5 Successfully Successfully 42 50 Successfully Successfully 43 49.5 Successfully Successfully 44 49 Successfully Successfully 45 48.5 Unsuccessful Unsuccessful Step 3, condition (5),
0.3 s 46 48 Unsuccessful Unsuccessful 47 46 Unsuccessful Unsuccessful Step 1, condition (1)

The results confirm the efficiency and effectiveness of the proposed method, since in 89% of cases the control unit correctly identified the outcome of the start-up process and issued/did not issue commands to interrupt it at different stages; in 11% of cases (8, 9, 17, 30, 31 (Table 1)) the interruption was unnecessary.

Thus, the technical result is the detection of failure of the start-up process of the motor at the early stages, followed by the shutdown of the motor to prevent its damage or damage to the equipment of the power supply networks, and to preserve their service life.

Claims (1)

A method for monitoring the direct start of an asynchronous motor, which consists in checking the normality of the supply network voltage, connecting the motor to the network, and in the event of an unacceptable drop in network voltage or a prolonged excess of the permissible current, disconnecting the motor, characterized in that the voltage and frequency of the supply network, the current and active power of the motor are measured, its electromagnetic torque is determined during the entire process of starting the motor, the motor start is blocked or interrupted in stages, where at the first stage, before turning on the motor, the conditions for the admissibility of starting for the voltage and frequency of the supply network are checked and, if they are not met, the motor is blocked, at the second stage, after turning on the motor, based on the results of measurements using the least squares method, the electrical parameters of the mathematical model of the motor mode and the critical voltage and frequency for starting the motor are calculated, taking, in the presence of retrospective data, the moment of the mechanical load of the motor to be equal to its electromagnetic torque in the normal mode preceding the start or nominal in the absence of them, the conditions for the admissibility of the voltage and frequency of the supply network are checked, if the conditions are not met, the motor is disconnected, at the third stage, the predicted quasi-steady-state values of frequency and voltage after the action of automatic speed controllers and excitation of generating units of the power supply network, check the conditions of their admissibility for generating units of the power supply network and the engine, in case of unacceptable values of voltage or frequency of the power supply network for starting the engine or operating the generating units, the engine is switched off, at the fourth stage the dependence of the active power of the engine on time is monitored, if a positive sign of the rate of change of the consumed active power is detected, the predicted duration of the start is calculated, in case of an unacceptable duration of the start or the absence of a positive sign of change in the consumed active power, the engine is switched off, at the fifth stage a prohibition is introduced on repeated switching on of the engine for a specified time.