07-08-2014, 11:45 AM
Icosɸ Algorithm Based Control of Zig-zag Transformer Connected Three Phase Four Wire DSTATCOM
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Abstract
In this paper, an Icosɸ algorithm based new
DSTATCOM (Distribution Static Compensator) using three-leg
VSC (voltage source converter) integrated with a zig-zag
transformer is investigated for the power quality improvement in
three-phase four-wire distribution systems. The zig-zag
transformer is used for providing a path to the zero sequence
fundamental as well as harmonics currents. In order to optimize
the voltage rating of the VSCs, the zig-zag transformer is
designed to have a tertiary winding for integrating the VSC. This
new DSTATCOM provides the voltage regulation or power
factor correction by reactive power compensation, harmonics
compensation, load balancing and neutral current compensation
in three-phase four-wire distribution system. The reference
currents for the DSTATCOM is estimated using the Icosɸ
algorithm and this is simple and with reduced computations. The
performance of the proposed DSTATCOM system is
demonstrated under linear and nonlinear balanced and
unbalanced loads by simulation using MATLAB with its
Simulink and simpower systems (SPS) tool boxes.
PROPOSED DSTATCOM TOPOLOGY
The Fig. 1 shows the schematic diagram of three leg VSC
based DSTATCOM for power quality enhancement such as
reactive power compensation, harmonic elimination, load
balancing and neutral current compensation. The VSCs are
connected to the power line using a zig-zag connected primary
windings. The transformer has tertiary star winding to
incorporate the three-leg VSC which is controlled for reactive
compensation, harmonic elimination and balancing of source
currents. The star connected windings may be designed for
optimizing the voltage rating of the VSC. The current rating
depends on the extend of compensation required.
The ripple filter is tuned for filtering the switching
frequency harmonics. The detailed design of the voltage and
current ratings of VSC and the zig-zag transformer are given
in the literature [13]. The DSTATCOM can be operated for
unity power factor operation (UPF) or for zero voltage
COSɸ CONTROL ALGORITHM OF DSTATCOM
The block diagram of the proposed control algorithm is
shown in Fig.2. The reference currents are estimated using
IcosΦ theory to regulate voltage at PCC (point of common
coupling) along with power quality improvement. The load
currents (iLa, iLb, iLc), the terminal voltages (vSa, vSb, vSc) and dc
bus voltage (vdc) of DSTATCOM are sensed as feedback
signals.
A. Active Component of Reference Source Currents
The amplitude of active component of fundamental load
currents are given as,
I Lap1 = ILa1 cos φa1= Real (ILa1 ) (1)
I Lbp1 = ILb1 cos φb1= Real (ILb1 ) (2)
I Lcp1 = ILc1 cos φc1= Real (ILc1 ) (3)
The amplitude of active component (ILa1cosΦa1, ILb1cosΦb1
and ILc1cosΦc1) of fundamental load current is extracted at the
zero crossing of the unit template in-phase of three phase
terminal voltages. The filters are used to extract the
fundamental load current. A zero crossing detector and a
“sample and hold” circuit are used to extract the I1cosΦ1
(amplitude of fundamental load current at zero crossing of
corresponding inphase unit template). For balanced source
currents, the magnitude of active component of reference
source currents can be expressed as,
MODELLING, SIMULATION AND RESULTS
The three-leg VSC and the zigzag transformer based
DSTATCOM connected to a three phase four wire system is
modeled and simulated using the MATLAB and its Simulink
and SimPower System (SPS) toolboxes. The DSTATCOM
system shown in Fig.1 is modeled. The loads considered are
CONCLUSION
The performance of a new topology of DSTATCOM
consisting of isolated three-leg VSC with a zig-zag transformer
controlled using Icosɸ algorithm has been demonstrated for
neutral current compensation along with reactive power
compensation, harmonic elimination and load balancing. The
voltage regulation and power factor correction of the
DSTATCOM have been achieved in satisfactory manner. The
zig-zag transformer has been found effective for compensating
the zero sequence fundamental and harmonic current and the
zig-zag transformer provides isolation to the VSC and
suitability to select its optimum voltage rating of VSC.
Moreover, as the neutral current is compensated by the zig-zag
transformer, the required ratings of VSCs are reduced. The
Icosɸ algorithm is observed to simple to implement and has
less computations compared to other similar algorithms.