Download Forming Mining Processing Plant Ore-Pulverizing Milldrive Starting

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 21 (2016) pp. 10559-10562
© Research India Publications. http://www.ripublication.com
Forming Mining Processing Plant Ore-Pulverizing Milldrive Starting
Characteristics
Denis Anatolyevich Ustinov and Yuri Vladimirovich Gulkov
Saint-Petersburg Mining University,
2, 21-st Line, Vasilyevsky Island, 199106, St. Petersburg, Russian Federation.
Abstract
There are continuous ongoing technological production
processes, where a break in power supplies results in
substantial financial losses. Mining processing plants widely
use electric mechanic complexes with heavy-load drive
synchronous engines. Starting ore-pulverizing mill drives
may significantly affect the electric equipment operation
mode. The article relates to various synchronous engine
startup options to propose the combined one, which
minimizes the startup electric current, improves the pull-in
torque and limits heating. Matters are discussed, related to
matching the frequency converter output voltage and the
electric power line voltage during switching a synchronous
engine from the frequency converter to the power line and
back..
plants are usually remote from the power supply sources,
getting the electric power they need via long overhead
cables, the voltage losses on substation busses during heavyload synchronous engines startups might exceed 20%, and
this is just unacceptable. Tables 1 and 2 represent statistical
data in connection with the voltage sags at the crushing and
preparation at power substation №53 (PS-53), “Karelian
Okatysh” plant and Oskol electrometallurgy plant [1].
Table 1: Voltage sags at the crushing and preparation
“Karelian Okatysh” plant, (PS-53)
Amount Deepness, % Duration, seconds
3
Above 16
0.01 – 0.02
4
10 – 15
0.17 – 0.86
4
4.6 – 5.0
0.10 – 0.81
5
5.4 – 10
0.21 – 0.86
Keywords: synchronous engines startup, frequency
converter, excitation system, two-way conductivity converter,
harmonic composition.
Table 2: Voltage sags at Oskol electrometallurgy plant
INTRODUCTION
Mining processing plants widely use electric mechanical
complexes with heavy-load drive synchronous engines. The
driving power of only one unit may easily exceed thousands
kilowatt. For example, the installed power of synchronous
drives on ore-pulverizing mills may reach 10 Megawatt or
even more. The operation conditions of such drives are
severe, and include humid environment, extreme operation
loads, strong vibration, and the engines are run up twice in a
sequence from the cold state, or once from the hot state, the
interval between startups may be at least two hours, and the
operation time may be long. The startup of those engines is
usually asynchronous, direct, by the full voltage of the power
line, and with a discharge resistor in the winding circuit of
the excitation system. Therefore, starting up ore-pulverizing
mill drives might greatly affect the operation mode of mining
processing plant electric equipment. Defining the optimal
startup mode for such complexes represents an important
scientific and technical task. Choosing the specific startup
method may be based on comparable analysis of the electric
mechanical complex parameters (operating machine, driving
engine, power supply system).
RESEARCH RESULTS
The simplest to be practically implemented is the direct
startup of a synchronous engine with nominal voltage.
However, startup current of such startup method affects the
power line to the great extent. Because mining processing
Amount Deepness, % Duration, seconds
1
10.5
0.12
1
15.3
0.11
5
11.2 – 27.6
0.08 – 0.09
6
8
10.1 – 11.0
15.7 – 28.1
0.04 – 0.07
0.10 – 0.13
To limit the startup current, a reactor may be used, which
would be included between the power line and the engine
stator winding. A device to limit the synchronous engine
startup current may also be an autotransformer. The
autotransformer reduces the voltage on the engine in
proportion to the transformation coefficient; therefore, the
current to be consumed from the power line would be
reduced in proportion to the square of the transformation
coefficient.
Using the startup autotransformer allows significant
reduction of the current consumed, with the pull-in torque
degrading less, than it would be with the reactor startup. The
reactor startup suggests significant reduction of the current
consumed from the power line would be possible, should the
induced impedance of the reactor be comparable with the
subtransient inductance of the engine.
A promising approach nowadays might be the startup on
reduced voltage via thyristor voltage regulator or via socalled soft-start devices. However, reduced voltage on the
driving synchronous engine means smaller torque moment
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 21 (2016) pp. 10559-10562
© Research India Publications. http://www.ripublication.com
thereon, resulting in longer startup, but it practically does not
reduce the power needed for startup.
The problem of the reduced torque on startup might be
eliminated via starting by under-frequency relay. Power
losses of the engine in this case, with frequency converter
used, would be less than those in case of the constant
frequency of the feeding voltage [2].
Mining processing plant ore-pulverizing mills may employ
multiple synchronous engines, run up with one frequency
converter capable of working then with any synchronous
engines of the group to implement the mode of lasting
rotation frequency regulation. In this case, to limit
electromagnetic interference in feeding power line, converter
and engine, such switching should be carried out, strictly
matching the frequency converter output voltage and the
voltage of the feeding power line. The allowed level of
matching the abovementioned voltage amplitude, frequency
and phase shift during switching a synchronous engine from
the frequency converter to the power line and back would
depend on both electric technical parameters and
technological modes of operation.
Let us turn to the functional diagram (fig.1) for sequential
smooth starting ore-pulverizing mill synchronous engines by
under-frequency relay, followed by synchronization with the
power line and switching the engine onto the power line. The
said conversion and regulating device includes synchronous
engines M1 – M2, a transformer T, transformation and
regulation device UZ, thyristor exciters UZE1 – UZE2 with
exciters transformers TE1 – TE2, output contactors QF1,
QF2 (a contactor disconnects the engine from the
transformation and regulating device after the engine is
connected to the power line), bypass contactors Q1, Q2
(those connect the engine to the power line), 18-pulse
thyristor rectifier-invertors VUZL, bridge convertor based on
fully controlled semiconductor elements – GTO-thyristors
VUZM, smoothing reactor of the direct current L, threephase capacitor battery of the filter C, microprocessor device
for automated control AC.
This circuitry allows smooth regulation of speed within the
range 10 – 120% of the nominal speed of synchronous
engines (corresponds to the frequency 50 Hz), providing
recuperative deceleration of the synchronous engines;
providing smooth transition from any actual speed to the
defined speed; changing speed assumes restriction of
acceleration and pull-in; torque and driving power of engines
are restricted; maximum short-time driving engine torque is
150% of its nominal value; startup torque from halted state
would be up to 150% of its nominal value (using the rotor
angular position sensor); using converting regulation device
as a startup device allows the unit startup to the nominal
speed, automated synchronization with the power line and
automated switching the synchronous engine to the power
line.
Using modern semiconductor devices allows implementing
excitation in the most favorable moment, for example,
during changing the load angle derivative (Θ) of the
synchronous engine by time from negative value to zero.
This allows using the moment of mass of the rotating parts
during entering the synchronism of the synchronous engine
[3]. It is determined, that restricting the allowed current of
the frequency convertor with 150% of its nominal value
results in allowed voltage mismatches at the output of the
frequency convertor and the power line, with the
synchronous engine under nominal load, during its switching
from the frequency convertor to the power line would be for
amplitude of the voltages ΔU=±1%; for phase shift Δφ=±2
[4].
VUZM converter output currents have the shape of impulses.
High frequency components of those currents pass through
the RC-filter at the output of the convertor. Therefore, the
voltages on the stator windings are smooth, with no surges,
which would be dangerous, as causing early aging of the
windings. Capacitor battery and the smooth shape of the
voltages virtually eliminate restrictions on the length of
cable between the converting and regulation device and the
engine. Stator currents distortion coefficient would be
approximately 8%. With such insignificant difference
between the actual stator currents and the sine wave, any
additional heating of the engine by current harmonics would
be insignificant, if compared to heating reduction by means
of start by under-frequency relay. Studying 4 Megawatt
synchronous engines of mining processing plant ball mills in
startup mode revealed the presence of higher harmonics of
the voltage curve. However, using an 18-pulse rectifier
greatly improves the harmonic composition of the voltage.
The 5th, 7th, 11th and 13th harmonics are virtually eliminated.
On fig.2 shows voltage curves during starting up a
synchronous engine with the 18-pulse rectifier, and the
harmonic composition of the voltage curve is shown at the
fig. 3.
Figure 1: Functional diagram of synchronous electric drive to be
used for start by under-frequency relay for two engines and for
lasting operation of one engine in the regulated electric drive.
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 21 (2016) pp. 10559-10562
© Research India Publications. http://www.ripublication.com
CONCLUSIONS
1. The existing problem with starting up synchronous
engines at mining processing plants might be solved in
the most efficient way via starting by under-frequency
relay, using converting regulation device. In this case,
harmonic composition of the voltage curve meets the
requirements of the relevant state standard, with allowed
voltage, frequency and phase shift mismatch on
switching a synchronous engine from the frequency
converter to the power line and back within the range
allowed for most industrial frequency converter control
systems.
2. Using synchronous engines with excitation system and
two-way conductivity converter, and controlling the
output voltage of the two-way conductivity converter,
allow startup optimization in normal mode and for selfstartup of electric drives in case of a short circuit in
feeding power line. This combined method of starting
up the electric technical complex with synchronous
engines facilitates minimization of the startup current,
improves pull-in torque of the synchronous machine and
limits heating the synchronous engine resulting in
successful synchronization even with reduced feeding
voltage, without additional unloading of the mechanism
(provided the self-startup is allowed by the
technological process).
Analyzing the said harmonic composition revealed the
unsinusoidality coefficient kU to be 4.5%. This meets the
requirements of the relevant state standard ГОСТ
32144-2013 about the quality of electric power supplies.
To improve the startup and self-startup process, it might be
enough to improve the excitation system of the synchronous
engines [3, 5, 7]. For example, using a two-way conductivity
converter, as a part of the excitation system of the
synchronous engines, one might obtain an increased pull-in
torque of synchronous engines via managing the two-way
conductivity converter output voltage as a function of the
angle of the load and the phase/frequency characteristics of
the excitation winding. Figure 4 demonstrates oscilloscope
pattern of changing the feeding voltage U, speed V and the
output voltage of the two-way conductivity converter Uf,
should there be a short circuit in the feeding power line, for
synchronous motor type СДМ4-1250К-32 УХЛ4.
The oscilloscope pattern data were obtained using the math
model of the electric mechanical complex with synchronous
engine and excitation system comprising the two-way
conductivity converter allowing assessing influence by
alternating excitation voltage onto the pull-in torque and the
dynamic stability on interferences by the mechanism driven
and by the power supply system in terms of the
MatLAB/SimuLink math calculation software [3, 6, 8].
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