25-08-2017, 09:32 PM
Synchronous Voltage Source Inverter Using FCMLI
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Abstract :
The structure and control schemes of a STATic
synchronous COMpensator (STATCOM) using Flying
Capacitor Multi-Level Inverter (FCMLI) have been discussed in
this paper. The STATCOM is realized by a synchronous
Voltage Source Inverter (VSI), which generates three-phase ac
voltages in phase with the transmission system voltage.
Multilevel inverter structure of the VSI is used for the realization
of the STATCOM. Three multilevel inverter structures (Diode
clamped, cascade and flying voltage source) have been
described in this work. The operation of these structures for
the general inverter operation and compensation purposes
has been studied. FCMLI is a special structure of the flying
voltage source topology. A method for controlling the FCMLI is
used which ensures that the flying capacitor voltages remain
constant. The above inverter structures and control schemes
of the STATCOM have been verified through digital computer
simulation studies using PSCAD/EMTDC software package.
INTRODUCTION
N this paper, the FCMLIs are used to implement an
STATCOM. Three-level and Five-level structures have
been used separately to simulate the STATCOM. Two
control approaches i.e., Direct Control and Indirect
Control are used and comparisons are made based on
the results.
For an STATCOM to operate, several important
requirements listed below, have to be made.
1. The transformers, interconnecting the inverter and
the transmission line, should provide galvanic
isolation of the inverter from the line so that the three
legs of the multilevel inverter can be connected to a
common dc link.
2. To allow the zero sequence currents to flow during
faults, it is necessary to connect the secondary
windings of the three phase transformers in delta,
so that the zero sequence secondary currents flow
as the circulating current of the delta.
3. The transformer primary to secondary turns ratio
should be such that, it insures that the voltage and
currents across the power semiconductor devices
do not exceed their ratings.
TRANSMISSION SYSTEM UNDER
CONSIDERATION
A three-phase, Single Machine Infinite Bus
(SMIB) transmission system is being considered for the
simulation studies. A coupled pi-section model of the
transmission line is being considered. The transmission
line data is given in Table 1. The phase of the sending
end voltage is fixed at 0° while the phase of the
receiving end voltage is varied. Initially, receiving end
voltage has 30° lag with respect to the sending end
voltage phase. At 1.0 second, the phase angle of the
receiving end is changed to 0° such that the power
transferred over the transmission line is reduced to zero.
The phase angle is again changed to − 60° at 1.5
second.
The simulation results for an uncompensated
transmission are shown in Fig. 1. The real power
transferred from the sending end and the power
received at the receiving end is shown in Fig. 1 (a) and
the reactive power is shown in Fig. 1 (b). Note that in
this figure and the following figures in this paper, the
receiving end real power shown with multiplying it by − 1
as this power is entering at the receiving end terminals.
Indirect Control
A STATCOM is connected at the midpoint of the
transmission line. The transmission line discussed in the
previous section has been again taken in this case. The
PSCAD/EMTDC simulation diagram of a STATCOM
connected to transmission line at the midpoint line
through a coupling transformer and a smoothing reactor
is shown in Fig. 5. The data of the coupling transformer
is given in Table4. The VSI is connected to the
transformer through a coupling inductor whose
inductance is 1 mH. Main capacitor and capacitors of
clamping leg are 20000 μF each. Only one capacitor in
each clamping leg is taken for simulation but it is of
different voltage rating. The instantaneous value of the
main capacitor voltage is passed through a moving
average (MA) filter [5]. The output of the MA filter is
compared with the actual value of the other clamping
capacitor to get the error. This error is used to get the
proper switching combination for different voltage levels.
CONCLUSION
The simulation results of a STATCOM are
discussed. It has been observed that the real power
transfer is increased with the shunt compensation while
the reactive power supplied by the sending end and the
receiving end is decreased. The midpoint terminal
voltage remains near to its specified value under low as
well as high transients. The response of the direct
control is faster than indirect control. It can therefore be
concluded that the direct control of the STATCOM is
much more superior to indirect control.