Abstract
The Contents
- Introduction
- 1. Theme urgency
- 2. Goal and tasks of the research
- 3. Analysis of the previous researches
- 4. Material and results of research
- Conclusions
- References
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:
- With the help of a mathematical model to study the different modes of induction motor with temperature control of stator and rotor.
- Development of the operating principles and algorithms for better protection.
- 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:
- 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.
- 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
The digital filtering block is executed on the basis of the first harmonic allocation using the Fourier filter.
The active and reactive components of the filtered quantity X are calculated by (1) and (2), depending on the number of measured values of N over the period T.
where N is the number of measurement points directed to filtration; X - value of the filtered value (phase current or voltage). Thus, in the digital filtering block, active and reactive components of phase voltages and currents, amplitude and effective (acting) values of phase voltages and currents are calculated in real time. The resulting array of calculated quantities is sent to the input of the computation block with a discreteness of the calculation step h = T / N.
The protection of the short-circuited winding of the rotor against thermal overload (TZR) is the key author's design and complements the standard upgraded protection of the asynchronous machine. It is performed on the basis of monitoring the value of the heating temperature of the short-circuited winding of the AED rotor. Calculation of the heating temperature is performed in real time on the basis of determining the active resistance of the rotor and its comparison with the known value of the cold state.
Calculation of the heating temperature of the rotor is based on the use of the circuit for replacing the AED with two rotor circuits (to account for the skin effect) and the loss profile in the steel (see Fig.3.).
The process of setting up the developed defense includes preliminary operations. These include:
- Сarrying out tests of idling and short circuit, feeding three-phase voltage of various frequencies to the stator with a stationary machine, measuring the initial temperature of the cold state [9];
- Determination of resistance of stator windings (RS, XσS), loss of steel (RFE, XσFE), resistance of mutual inductance (Xμ), resistance of two rotor circuits (RR1, XσR1, RR2, XσR2) according to the data of experiments.
The results of preliminary operations (parameters of the replacement circuit, catalog data of the asynchronous electric motor) are recorded in a block of constants for subsequent use in the process of necessary calculations.
- Operations performed in real time in one working cycle:
- measurement of phase currents (ia, ib, ic) and voltages (ua, ub, uc);
- calculation of the slip s based on the measurement of the rotor position angle γ by the DWD;
- measurement of the heating temperature of the stator winding TS by means of a TD;
- Filtering of measured phase currents and voltages in the digital filtering unit;
Calculation of the resultant active resistance of the rotor RRexx (s) depending on the current sliding value s.
- Determination of the phase values of the input active (RBX.A, RBX.B, RBX.C) and inductive (XBX.A, XBX.B, XBX.C) impedances of the IM.
where IA, UA, - active components of phase current and voltage calculated; IP, UP, - reactive components of phase current and voltage calculated.
- Correction of the active resistance of the stator winding IM:
- Calculation of the current value of the heating temperature of the IM rotor:
After calculating the rotor temperature of the asynchronous machine in the calculation unit, the calculated value is compared with the set point of the thermal protection in the setting block. In case of exceeding the temperature setpoint, the protection is triggered by starting the time delay and light-sounding alarm of the IM rotor winding overheating. After the time delay has elapsed, the motor is automatically disconnected from the mains.
An example of simulating the operation of the thermal protection of an IM with a squirrel-cage rotor is made for an AB-series engine with a power of 630 kW and a stator voltage of 6 kV.
The results of the mathematical modeling of the load throwing process are shown in Fig. 2 in the form of oscillograms of the current of the phase A of the IM, the angular frequency of rotation, the heating temperature of the rotor and the signal of the TZR response from time.
