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Abstract

The Contents

Introduction

Induction motors (IM) are one of the most common types of electrical equipment, more than 85% of all electric cars - this three-phase IM. With a fairly simple and robust design, they are widely used as drive mechanisms for industrial facilities, systems and auxiliary power plants consume more than half of all energy. Despite the relative simplicity of design and high reliability, the defectiveness of asynchronous motors is quite high and amounts to 20-25% of the total for the year, which is costly to repair. Currently, many companies that specialize in the manufacture of relay protection and automation, are paying enough attention to the development, debugging and production of microprocessor terminals and relays. Virtually no branch of engineering and everyday life, where there used to induction motors. According to statistics, now in the social production of the CIS countries is not less than 50 million units of engines of 0.4 kV.

Failure the IM leads to severe accidents and the big material damage connected with idle time of technological processes, elimination of consequences of accident and repair failed the IM. For this reason there is a need for protection and monitoring devices behind a condition of the equipment and the mechanisms put in action by asynchronous electric motors.

Modern standards of most countries, including Ukraine, are placing ever greater demands on the technical operation of electrical installations. The need for constant monitoring of the expensive equipment reinforces the need to use high-quality, reliable, and comprehensive motor protection.

1. Theme urgency

Induction motors (IM) with squirrel-cage rotor (SCR) are the main type of AC machines used to drive machinery auxiliary power plants and industrial plants. This type of IM is nearly 95% of all electric drive motors in the industry and thus consumes more than half of all energy. The advantages of IM are simple structure, reliable operation. However, despite the advantages of induction motors compared to other electric machines, the percentage of defectiveness still quite high at 20-25% per annum of total EDA in operation. About half of the injuries associated with inadequate perfection of existing relay protection and automation, which are particularly deficient in protecting the motor disturbance associated with thermal overheating. In this regard, the development of improved protection with the use of microprocessor-based systems is urgent.

2. Goal and tasks of the research

This work is devoted to the improvement of thermal protection cage rotor induction motor on the basis of the control parameters of the current regime (instantaneous values of the phase currents, phase voltages and bearing) that is suitable for correct operation with input voltage unbalance objectives:

Main tasks of the research:

  1. With the help of a mathematical model to study the different modes of induction motor with temperature control of stator and rotor.
  2. Development of the operating principles and algorithms for better protection.
  3. Checking with a mathematical model of the effectiveness of security algorithms.

3. Analysis of the previous researches

Now to microprocessor devices of relay protection and automatic equipment pay a lot of attention. In released terminals the standard protective logic recommended by PUE [1] generally is used. However in relation to IM, it doesn't provide reliable protection at damage of cores of a short-circuited rotor, at an overheat of windings of a stator and a rotor caused by asymmetry of the feeding tension, at malfunctions in the cooling system and technological overloads, at asymmetry of the air gap caused by development of bearings, when jamming rotor, at overheats from the harmonicas caused by thyristor converters of frequency. Also low sensitivity takes place at short circuits on the earth in a stator winding.

As protection against IM overload in the majority modern microprocessor protection devices or terminals integrated dependence of current of a stator as time [2] is used. Used integrated dependence of current of a stator as time doesn't allow to supervise temperature of heating of windings of a stator and IM rotor, and also to disconnect the car in case of the overheat of windings caused, for example, by repeated start-up in a row.

The microprocessor devices based on application of pseudo-thermal model, also tolerant at asymmetry of the feeding tension caused by uneven loading of phases of a network or break of a phase through transitional resistance [3] . Addition of these protection with signals from thermal sensors or the thermal sensors (TD) which are built in a winding, don't eliminate defects because of their lag effect.

The main lack of the protection released by leading enterprises, lack of control of temperature of heating of a winding of a short-circuited rotor is. The facts of are known that in an operating mode (S4) temperature of cores of a rotor can reach melting temperature with the winding of a rotor executed from aluminum [4]. Melting of cores of a winding of a rotor of IM it is caused by impossibility of existing protection in due time to reveal and eliminate such type of damages.

Direct control of temperature of heating of a winding of a rotor with use of TD is difficult also expensive in a type of difficulties of their installation, and also take a signal in the course of rotation of a shaft of the engine. For this reason the greatest distribution was gained by algorithms of indirect determination of temperature of a rotor. Such algorithms treat:

  1. The protection based on pseudo-thermal models with use of thermal equivalent circuits of AED and signals of TD, allowing to count temperature of heating of elements of the car (a winding of a stator, a rotor, the case) knowing the size of current of a stator [5]. It is necessary to refer the difficulties connected with determination of parameters of thermal equivalent circuits to shortcomings of such protection, and also their adjustment in the course of work that is reflected in an error and adequate work of protection. Also it should be noted the fact of absence of the accounting of malfunctions in the AED cooling system in thermal model that is reflected in inaction or the slowed-down reaction of protection at such types of damages.
  2. The protection considered in [6,7], are intended for control of temperature of heating of the rotor executed with cores in which there is no effect of replacement of current. Approach [6] is based on the solution of the equation of thermal balance according to calculation of the electromagnetic moment on the basis of measurement of phase currents and stator tension, and in [7] same sizes based on measurement and additional control of angular frequency of rotation of a rotor. Key lack of thermal protection [6] , the big error in determination of temperature of heating of a winding of a rotor in a type of indirect calculation of sliding is. Shortcomings of two approaches is absence of the accounting of effect of replacement of current in a rotor, and consequently impossibility of their use by the 200 kW cars and of stator 6 or 10 kV.

The most perspective is the approach [8] based on indirect determination of temperature of heating of a rotor according to measurements of parameters of the current mode (instant values of phase currents, phase tension and sliding). The method is suitable for electric motors in a type of taking note of effect of replacement of current in a rotor on its resistance. However the questions connected with correct work of this algorithm of protection in asymmetrical modes, demand additional consideration.

4. Material and results of research

As it was already noted, indirect measurement of temperature of a rotor is based on calculation in real time of active resistance of a rotor in a hot condition according to measurements of instant values of phase currents and tension and the size of sliding and by results of comparison of active resistance of a rotor in cold and hot conditions temperature of heating of a rotor is defined. Calculation of resistance of a rotor is made on the basis of IM equivalent circuit (EC). As shown in [8], with current replacement in a rotor it is most expedient to apply a double-circuit equivalent circuit to cars with a contour of losses in steel of a stator (fig. 1) for which the knowledge of its parameters is necessary. The method of determination of the EC parameters with a contour of losses is given in steel in [9].

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Figure 1 – IM equivalent circuit with KZR with two contours on a rotor and a contour of losses in steel

However at asymmetry of the feeding tension in active resistance found according to measurements entrance IM (RBX) and equivalent resistance of a rotor chain (RR/s), and also temperature of heating of a rotor there are the fluctuations, proportional degrees of asymmetry of tension. Nature of fluctuations at asymmetry voltage of 4% is shown in fig. 2.

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Figure 2 – Dependences of the entrance active IM and resistance resistance rotor chain from time at asymmetry the feeding tension (U2=0,04 )

It is connected with that, as appears from [10], in the specified modes the measured input instant power can be presented as consisting of average values of active capacities of systems of the direct sequence (DS) and the return sequence (RS), and also from a sign-variable network pulsing with a doubled frequency about zero average value.

Average values of power of direct sequence (Р1) and modules of the generalized vectors of current and tension of direct sequence (I1, U1) we can find on dependences of value.

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5. The theory of calculation of temperature of windings of a rotor on the basis of comparison of active resistance of a rotor and its comparison with known value for a cold condition

On each discrete step of real time the microprocessor device makes the following operations:

• Measurement of instant values of phase currents and tension;

• Measurement of a corner of provision of a rotor by the sensor of provision of a rotor (SPR) installed on a shaft of the engine and definition on these data of sliding;

• Measurement of temperature of heating of a winding of a stator by means of TD established in grooves of a winding of a stator of the electric motor;

• Calculation of modules of the generalized vector of current and tension of a stator of direct sequence;

• Calculation of power of direct sequence;

• Determination of entrance active and inductive resistance of AED through the capacity, tension and current of direct sequence;

• Correction of active resistance of a winding of a stator and contour of losses in steel depending on the actual temperature of heating taken by TD;

• Calculation of resistance of a rotor and magnetization branch;

• Calculation of active resistance of a winding of a short-circuited rotor of AED in a hot condition;

• Calculation of active resistance of a rotor in cold (an initial condition) for the current value of sliding;

• Calculation of the current value of temperature of heating of a rotor of AED on the basis of comparison of active resistance of a rotor in a hot condition and known resistance in a cold (initial) condition;

Operability of the above algorithm is checked at various values of coefficient of asymmetry with use of methods of mathematical modeling on PEVM [11] in the established modes, and also in modes of start-up and loadings. The AED mathematical model on the basis of the full differential equations [9] was used.

In these modes change of temperature of a winding of a rotor was accepted on the basis of the solution of the equation

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where М(t) – the current calculated value of the electromagnetic moment on a shaft; S(t) – current value of sliding; Та – is a constant of time of the unit, (sec), t – time, (sec).

The problem of algorithm of protection included determination of the specified temperature of heating of a winding of a rotor.

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As an example I was modelating an operating mode at a rated load on a shaft and a mode following it loadings of the 4AZM-8000/6000 electric motor being the drive of the nutritious pump (PRPEN) on the 300 MWt power units of thermal power plants. Catalog data of this AED are presented in table 1.

Table 1 – Catalog data of IM tipe 4AZM-8000/6000

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Calculated on [9], parameters of a double-circuit equivalent circuit of AED with a contour of losses in steel of a stator are presented in table 2.

Table 2 – Parameters of the double-circuit equivalent circuits with a contour losses in steel of a stator of IM type 4AZM-8000/6000

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Results of mathematical modeling of the set mode at asymmetry of tension of a network of 4% are shown in fig. 5 in the form of dependences of current of a phase A of a stator and temperature of heating of a winding of a rotor on time.

Calculated values of temperature of heating of a short-circuited winding of a rotor of the asynchronous electric motor are given in table 3 at various coefficients of asymmetry of the feeding tension calculated on algorithm [8] offered in work. Thus value VRprimary = 30°C.

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Figure 4 – Results of determination of temperature of heating of a short-circuited winding of a rotor of IM

Table 3 – Results of comparative calculation of temperature of heating of a winding of a rotor of IM

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Conclusions

In work the way of improvement of algorithm of indirect determination of temperature of heating of a short-circuited winding of a rotor of AED with asymmetry of the feeding tension is offered. In a basis of a way allocation of components of direct and return sequences by which active resistance of a rotor in the current mode are determined is necessary. Such approach allows to exclude influence of sign-variable components of the doubled frequency in measured sizes that allows to specify determination of temperature of heating of a rotor. Work of algorithm of thermal protection of a short-circuited winding of a rotor of AED is checked on PEVM with use of methods of mathematical modeling on the example of the 8000 kW asynchronous 4AZM-8000/6000 electric motor and of stator of 6 kV.

At a writing of the given abstract master's work is not finished yet. Definitive end: January, 2014. The full text of work and materials of a theme can be received from the author or his supervisor after the named date.

References

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