28-11-2012, 04:33 PM
Application of D-STATCOM to Mitigate Voltage Sag Due to DOL Starting of Three Phase Induction Motor
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INTRODUCTION
THREE phase Induction Motor (IM) is a work horse of
of industrial and agricultural sector. Due to its low
cost and rugged construction, IM is always preferred
ferred over other types of electric motors. In many of
these applications IM is started using DOL starter. Thus,
IM draws high current during its starting period which
not only heats the motor, but also causes voltage drop in
supply lines. Thereby affecting the working of other
equipments supplied from the same lines [1]. The starting
period of IM depends upon the factors such as, motor
construction, its condition and its loading condition.
Typically, the starting period is about 1 sec.; therefore
there is a voltage sag in the supply voltage during this
period. Voltage sag is an rms reduction in the AC voltage,
at the power frequency, for durations from a half
cycle to a few seconds [2].
Now a days due to wide usage of sensitive electronic
equipments such as computers, even voltage sag
which lasts for only 0.1 sec may severely affect their
working [3]. Also, there are severe power crisis in India,
especially in Maharashtra state. Therefore, the voltage
profile of the supply lines is always at its lowest. The voltage
sag caused by IM starting leads to further lowering
down the voltage profile which results in deterioration of
power quality. Making this problem worth for giving attention
by researcher to find out its effective and economical
solution.
CHARACTERISTICS OF VOLTAGE SAG
According to IEC Standard 61000-2-8, 2000, IEEE
standard 1346, 1998, a voltage sag is described as a “two
dimensional (2D) electromagnetic (EM) disturbance, the
level of which is determined by both voltage magnitude
and time”.
Following are the important characteristics of voltage sag.
i) Voltage sag Magnitude
ii) Voltage sag Duration
Voltage sag magnitude is defined as the remaining
voltage during the event. The voltage sag duration is
the time interval of presence of sag. [7]. Fig. 1 shows magnitude
and duration of voltage sag.
Basic Operation
The basic electronic block of the D-STATCOM is
the voltage Source Converter (VSC) that converts an input
DC voltage into a three phase output voltage at fundamental
frequency [9].
In its most basic form, the D-STATCOM configuration
consists of a Voltage Source Converter (VSC), a DC
capacitor for energy storage; a coupling transformer
connected in shunt with the AC system, and associated
control circuits [10]. In this arrangement, the steady-state
power exchange between the device and the AC system
is mainly reactive. Fig. 2 shows the schematic representation
of the D-STATCOM.
SIMULATION OF D-STATCOM
The simulation is especially important for a concept
validation. Here the D-STATCOM test system is simulated
in MATLAB/Simulink [11, 12]. Fig. 3 shows the
MATLAB/Simulink model of a D-STATCOM test system.
The test system comprises three phase 5 kw, 415
V, 50 Hz, 4 pole, squirrel cage IM supplied from balanced
three phase supply. The line and source impedance is assumed
to be purely resistive. Three phase CB is used for
DOL starting of IM.
Three phase uncontrolled rectifier connected to
supply through coupling transformer is used to charge the
DC capacitor. The heart of the D-STATCOM is thyristorized
VSC. This is triggered by synchronized six pulse generator.
Coupling transformer is used to couple the DSTATCOM
to the supply lines.
CONCLUSION
In this paper, to validate the capability of DSTATCOM
to mitigate the voltage sag due to DOL starting
of IM; a test system is simulated in MATLAB/
Simulink. The system is operated twice; without
and with D-STATCOM. In both the cases; line voltage,
IM current, electromagnetic torque developed by IM and
the D-STATCOM current are recorded.