H-BRIDGE VSC WITH A T-CONNECTED TRANSFORMER BASED
THREE-PHASE FOUR-WIRE DSTATCOM FOR POWER QUALITY
IMPROVEMENT

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I. INTRODUCTION


THE present day ac power distribution systems is
suffering from severe power quality problems. These
power quality problems include high reactive power
burden, harmonics currents, load unbalance, excessive
neutral current etc. [1-7]. The power quality at the point
of common coupling (PCC) with the utility grid is
governed by the various standards and the IEEE-519
standard is widely accepted [1]. Some remedies to these
power quality problems are reported in the literature [2,5-
7]. A group of controllers together called Custom Power
Devices (CPD), which include the DSTATCOM
(distribution static compensator), DVR (dynamic voltage
restorer) and UPQC (unified power quality conditioner).
The DSTATCOM is a shunt-connected device, which
takes care of the power quality problems in the currents.



PROPOSED DSTATCOM
Fig. 1(a) shows the power circuit of proposed H-bridge
VSC based DSTATCOM along with a T- connected
transformer connected in the three-phase four-wire
distribution system. The linear and non-linear, balanced
and unbalanced loads are connected at the PCC.


IV. MATLAB MODELLING OF DSTATCOM SYSTEM
The H-bridge VSC based DSTATCOM and the Tconnected
transformer interfaced to a three phase four
wire system shown in Fig. 1(a) is modelled and simulated
using the MATLAB and its Simulink and Power System
Blockset toolboxes. The load considered is a lagging
power factor linear load and nonlinear load. The ripple
filter is connected to the VSC of the DSTATCOM for
filtering the ripple in the terminal voltage. The system data
are given in Appendix.


V. RESULT AND DISCUSSIONS
The dynamic performance of DSTATCOM using Hbridge
VSC and the T-connected transformer in the unity
power factor (UPF) mode of operation is depicted in Fig. 4
and Fig. 5. The load balancing and neutral current
compensation are demonstrated in Fig. 4 and the harmonic
elimination, load balancing and neutral current
compensation are demonstrated in Fig. 5. The voltages at
PCC (vs), balanced source current (is), load current (iLa, iLb,
iLc), compensator currents (ic), source neutral current (iSn),
load neutral current (iLn), T-connected transformer neutral
current (iCn), dc bus voltages (vdc, vdc1, vdc2) and rms value
of PCC voltage (VS) of DSTATCOM are shown in both
the cases.


VI. CONCLUSION
The modelling and simulation of a new topology of
DSTATCOM consisting of H-bridge VSC and a Tconnected
transformer has been demonstrated for neutral
current compensation along with reactive power
compensation, harmonic elimination and load balancing.
The voltage regulation and power factor correction modes
of the DSTATCOM have been observed to be satisfactory.
The T-connected transformer has been effective for
compensating the zero sequence fundamental and
harmonic currents.