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Abstract

Content

Introduction

One of the main objectives solved at the organization of work of a power system, ensuring high-quality and uninterrupted power supply of consumers is. Failures, interruptions and emissions of tension of an electric network are the most common causes of failures in work of the industrial enterprises and damages of the household equipment, causing essential economic damage, both to consumers, and suppliers of the electric power.

This work is devoted to a question of increase of reliability of systems of power supply with asynchronous engines at the expense of application of high-speed automatic inclusion of a reserve in microprocessor execution. The offered device allows to reduce break time in a food and to prevent nonsynchronous giving of tension on engines which lost a food.

1. Relevance of the topic

Power supply systems with groups of induction motors are widely used in various industries are. This is due to the fact that asynchronous drives are reliable and simple structurally efficient, but at a reduced voltage due to short-circuit and short interruptions are not always ensured their successful self-starting. This leads to disruption of the process setpoint economic damage. So is the actual development of microprocessor AVR, ensuring a successful self-starting engine through the use of techniques to reduce interruption of power supply.

2. The purpose and objectives of the study, appointment

The purpose of this work is improvement of way BAVR in power supply systems with asynchronous engines, is based only on control of tension which doesn't demand measurement of active power. The way of realization of AVR by means of microprocessors is considered also.

The successful self-start of electric motors caused by repeated giving of tension requires the solution of the following tasks:

  1. To consider ways of reduction of duration of a break of a food that will allow to reduce fall of speed of rotation and to provide faster subsequent self-start of electric motors.
  2. To develop ways of prevention of nonsynchronous giving of reserve tension for prevention of emergence of currents of the engine exceeding maximum permissible.

Appointment

High-speed AVR is intended for ensuring continuous power supply of consumers by their switching on a reserve source at power supply breaks, providing thus:

  1. Preservation of dynamic stability of the motive loading connected to tires of substation;
  2. Reduction of number of shutdowns of electric motors because of falling off of actuators and contactors at tension fall;
  3. Increase in a resource of engines at the expense of restriction of currents of inclusion;
  4. Preservation of power supply of other consumers, including systems and devices of automatic control of substations, providing that increase of reliability of power supply of object as a whole.

3. Scientific importance

Today one of ways of increase of reliability of power supply of systems with groups of asynchronous engines is application of the high-speed automatic inclusion of a reserve (HSAIR). BAVR use at short-term loss of a food for any reason allows to provide restoration of a doavariyny operating mode of electric motors and to prevent violation of difficult technological process of plants, and also to reduce number of start-up of electric motors that substantially increases their resource.

4. A review of research on the topic

Development of microprocessor equipment and the power equipment allows to improve known ways BAVR. So, for example, the way of automatic inclusion of reserve power supply of consumers [1] includes measurement on tires of the main and reserve power supplies of tension of direct sequence, a corner between vectors of tension of direct sequence, the minimum current of input of each section and definition of the direction of active power on input of tires of the main source of a food.

Switching of the tire of the main source of a food agrees to the reserve occurs at tension reduction on tires of the main source of a food below the set level or at corner excess between a vector of tension of direct sequence more set and at the direction of active power from tires to the main source of a food and in addition measure the minimum current of input of each section, comparing it to a preset value of current of input, and at excess of the set current also carry out switching on the reserve power supply [1].

However in the presence of only asynchronous engines work of this way BAVR doesn't ensure of reliable functioning because of fast attenuation of tension on tires of the working power supply in size and a corner. Therefore for an assessment of possibility of work of such BAVR it is necessary to conduct additional researches by means of modeling on the COMPUTER or experiments.

The described method does not provide a credible job with asymmetric short-circuit in the supply chain as well as in the single-phase, two-phase or two-phase to earth short circuits direction of the active power at the input of tires can not be changed.

In a way of automatic inclusion of reserve power supply of consumers [2] measure on tires of the main and reserve power supplies of tension of direct sequence, a corner between vectors of tension of direct sequence. Define the direction of active power on input of tires of the main source of a food on the basis of phase currents and opposite linear tension taking into account a corner of the maximum sensitivity and the minimum current of input of each section.

Switching of tires of the main source of a food on reserve, agrees [2], occurs, as well as in way [1] but if value of linear tension on tires of the main source there is less than preset value, as vectors of this tension accept the corresponding tension of a reserve source. If on input of the main source it is less than value of phase currents set, and on input of a reserve food it is more than value of phase currents set or if capacities on input from the main source change a sign on negative at least in one phase, and capacities on input from a reserve source in all phases are positive, switch a food to a reserve source.

The way [2] doesn't ensure of reliable functioning in power supply systems with asynchronous engines for the same reasons, as the way [1].Также doesn't possess this way high speed because of a large number of measurements necessary for definition of the direction of power in each phase of the main and reserve power supplies and doesn't ensure of reliable functioning on the reason of measurement of the "fictitious" power determined on the basis of measurement of currents and tension, changing with a various frequency.

5. The main material

According to [3] after food loss engines reduce the speed of rotation and residual EMF of engines fades in size and frequency. In this case fast giving of a food on engines can be accompanied by considerable currents of self-start if EDS of running-out engines didn't manage to decrease considerably and by the time of giving of reserve tension is with it in an antiphase. The oscillogram of current of the engine at such giving of tension is shown in fig. 1.

Figure 1 – Oscillogram of currents of the asynchronous engine

a) current at nonsynchronous giving of tension; b) starting current

When giving reserve tension on running-out engines when its residual EMF will fade, the size of currents of self-start will be equal to the size of starting currents. Experimental researches of many authors it is established that time of attenuation of tension of the asynchronous engine at run out makes about 0,5÷1 sec., and at short circuit near feeding tires about 0,05÷0,1 sec. The oscillogram of current of the engine at such giving of tension is shown in fig. 2.

Oscillogram of currents of the asynchronous engine

Figure 2 – Oscillogram of currents of the asynchronous engine

a) current when giving tension after a food break; b) starting current

According to [4] self-start occurs, as a rule, at the loaded mechanisms that can lead to increase in duration of dispersal and temperature increase of windings of the engines, caused by increase in current in comparison with its nominal rates.  For reduction of time of dispersal it is necessary, what decrease in speed of rotation of the running-out engine was as it is possible smaller.  Thus, successful self-start of the engine is influenced also by duration of a break of a food during which there is a decrease in speed of rotation, and in certain cases and a full stop of the engine.

Proceeding from the above microprocessor realization of the BAVR device has to be based on two basic provisions:

  1. Repeated giving of tension has to happen at achievement of a corner of shift between vectors of tension of two sections of value 360 ° .
  2. Possibility of change of a corner of operation of BAVR depending on time of operation of the switch has to be provided.

In this work microprocessor realization of the known device for inphase automatic inclusion of a reserve food of consumers with motive loading [5] which is based on corner control between the vectors of tension of mutually reserved sections of the same name is offered.

The following realization of microprocessor BAVR shown in fig. 3, for the scheme consisting of two in parallel working transformers on partitioned system of tires from which asynchronous engines are powered is offered.

The block diagram of realization of the BAVR microprocessor device

Figure 3 – The block diagram of realization of the BAVR microprocessor device

In the scheme presented on rice 3. enter: the analog-digital converters (ADC), blocks of comparison of tension with settings, the block of measurement of a corner between vectors of tension of the 1st and 2nd sections, the block of measurement of a difference between tension of the 1st and 2nd sections, logical blocks "I" and "OR".
ATsP work in this scheme is based on sinusoid representation in the form of a step signal. The signal of tension received after processing is presented to ATsP in fig. 4.

Signal of tension received after processing in ATsP (7 shots, 15 seconds, 7 KB)

Figure 4 – Signal of tension received after processing in ATsP (7 shots, 15 seconds, 7 KB)

Operation of the block of measurement of a corner between tension consists in difference measurement between step signals of the first and second sections and on duration of a signal of a difference determination of size of a corner between tension of both sections. If corner size between sections becomes equal 90 °, at the exit there is a signal "1" if corner size less than 90 °, a signal at the exit "0" Charts of operation of this block are shown in fig. 5. Operation of the block of measurement of a difference between tension is similar to operation of the block of measurement of a corner between tension.

Charts of operation of the block of measurement of a corner between tension

Figure 5 – Charts of operation of the block of measurement of a corner between tension

a) at shift between vectors on 90 °; b) at shift between vectors on 45 °

In a normal mode of section eat from the T1 and T2 transformers and the BAVR device doesn't come into action because of a signal "0" from blocks of comparison of tension with the set setting.

Operation of the device is based on the following algorithm. At food loss on the 1st section the system of vectors of EFM of running-out engines will start rotating concerning system of vectors of tension of the 2nd section and tension on the first section starts decreasing. At achievement by a corner between vectors of tension of two sections of the value equal 90 ° and at a voltage reduction of the first section below the set level at the exit of blocks of comparison of tension with a setting and corner measurements between tension appear signals "1". These signals are given on an entrance of the logical block "I" which gives a signal on shutdown of the switch of the first section Q1 if the signal at its exit is equal "1". At the same time with it control of a difference of tension between two sections is exercised and at achievement of a difference less 1.4Uном at the exit of the block of measurement of a difference of tension there will be a signal "1". For the correct operation of the device the logical OR block which excludes false turning on of the section QK switch at a difference between tension more 1.4Uном is provided. Signals from the block of measurement of a difference of tension and from the logical OR block move on the logical block "I" which gives a signal on turning on of the section QK switch in the presence of a signal "1" at its exit.

In case of three-phase or single-phase short circuits on departing lines the corner between vectors of tension of the 1st and 2nd sections will be less than 90 ° and at the exit of the block of measurement of a corner between tension there will be a signal "0" which doesn't lead to device misoperation. At emergence of two-phase short circuits on departing accessions tension on section won't decrease below a setting and at the exit of the block of comparison of tension with a setting there will be a signal equal "0". Thus, control of level of tension on sections and corner sizes between vectors of tension of both sections allows to exclude device misoperation at different types of short circuits on departing accessions. For the reasons stated above the device won't work and at emergence of short circuits on feeding accessions which have to be disconnected by high-speed protection.

It is most expedient to realize the offered algorithm of BAVR on the microprocessor platform based on base of the modern STM32F4xx microcontroller, the production of the STMicroelectronics [6]company including a 32-bittny kernel of new generation of ARM Cortex-M4 with a clock frequency of 168 MHz [7]. In this controler three 12-bit ATsP on 24 entrance canals, two 12-bit TsAP, the temperature sensor and others are built-in. At STM32F4xx there are all modern interfaces providing possibility of control, management and communication (I2C, USART, SPI, I2S; CAN, USB 2.0, 10/100 Ethernet, the SDIO controler for possibility of work with SD, SDIO, MMC, CE-ATA memory cards). Thanks to a ratio the price quality use of this microcontroller will allow qualitatively, with insignificant financial expenses, to realize the algorithm of high-speed BAVR offered in work in practice. The scheme of connection of the BAVR digital device on the basis of the STM32F4xx microcontroller is submitted in fig. 6.

The scheme of connection of the BAVR digital device on the basis of the STM32F4xx microcontroller

Figure 6 – The scheme of connection of the BAVR digital device on the basis of the STM32F4xx microcontroller

Conclusions

  1. The way of performance of BAVR in power supply schemes with asynchronous engines in which basis tension measurement on working and reserve sections is necessary is considered.
  2. Use of the described device, in combination with modern high-speed switches, at food loss on section for any reason will allow:

    - to execute food switching on a reserve source for minimum possible period;

    - to exclude nonsynchronous giving of tension on running-out engines that substantially reduces level and duration of course of starting currents of the engine.
  3. It is offered to realize algorithm of BAVR on the basis of the STM32F4xx microcontroller

On the moment of writing this abstract the master's work was not completed. Its final variant can be obtained from the author or the scientific adviser after December 2013.

References

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