Keywords - Hysteresis Controller, DSTATCOM, Power Quality,
Single Phase SRF theory, Voltage Unbalance, Current Controlled Mode,
Voltage Controlled Mode.
- INTRODUCTION
In recent years, due to increasing use of non-linear loads at consumers’
end, the quality of power is highly degraded. It results into poor power
factor, increased value of THD in source current, increase in current
flow through neutral conductor in a distribution system \cite{Arias_Guzman_2015,Xu_2007} . These
problems can be termed as a current related PQ problem. Voltage related
power quality problems like voltage unbalance, voltage sag and swell
etc. are also increasing. Much attention has been drawn by this and
research work has been performed in this area. Voltage unbalance is
generally not as severe as compared to current unbalance but it also has
an impact on both loads and power system equipments \cite{Habijan_2013,Akagi,Karanki_2013} . The
asymmetry in voltage in a network typically appears as a result of the
connection of single-phase customer, which creates uneven load among
phases. The asymmetry increases further when single-phase DG is
integrated into the existing distribution networks\cite{Yan_2013,Campos} . Many
literature studies and research work has been carried out that is
concerned with the unbalanced loads and asymmetric lines\cite{Khadkikar_2010,Neto_2012,Xu_2007,Hrishikesan_2014}.
Unsymmetrical distribution of three phase system, impedance unbalance,
un-transposed transmission line, heavy load connected to a particular
phase, etc. are various causes for voltage unbalance\cite{Venkatraman_2016,Venkatraman_2016a}. If this
unbalance exists in the system, there would be unbalance current flowing
which causes machine to overheat and thereby reducing its efficiency \cite{Tsai_2008,Dheepanchakkravarthy_2016,Osborne_1995}. To overcome both current and voltage related PQ problems
DSTATCOM is used which is connected in shunt at PCC\cite{Singh_2009}.
The core part of DSTATCOM is the Voltage Source Converter (VSC). The
controller of DSTATCOM mainly consists of two parts. First part measures
the load current/source voltage and determines the reference
compensating current that is fed to the system by DSTATCOM and second
part generates the necessary switching signal such that the actual
current that is fed to the system is same as the reference signal
generated\cite{Singh_2014}. To obtain suitable switching sequence for the Voltage Source
Converter (VSC), hysteresis current control method is widely used\cite{Kanjiya_2011,Khadkikar_2009}. This
is because it has an advantage of simplest design, fast current control
response, its inherent peak current limiting capability \cite{Yao,Marei_2006}. The
basic principle of hysteresis current control strategy is switching leg
of each phase to opposite voltage polarity when the measured current
goes above or below certain predefined limits i.e. hysteresis band.
In this paper, a dual functional algorithm is proposed so that a single
DSTATCOM can be used to mitigate both current and voltage related PQ
problems. The CCM mode of operation is simulated using traditional SRF
theory while using the concept of single phase d-q reference frame
theory, mentioned in \cite{Jouanne_2001,Bajaj_2016,Tarafdar_Haque}, a novel concept is proposed and is applied
to a three phase four wire system to compensate the unbalance in voltage
at load side (VCM).
2. DSTATCOM STRUCTURE
Fig. 1 shows the schematic diagram of DSTATCOM connected to power
distribution network. As seen from Fig. 1, Vsarepresents source voltage of phase-a, load voltage at PCC of phase-a is
denoted by Vta. Similarly, source current, load current
and injected current of phase-a are represented by Isa,
Ila, i1a. Zsa represents
line impedance of phase-a. Zla is the load impedance of
phase-a and ZNL is the impedance of the three phase
rectifier connected in parallel to the linear load. Rfand Lf represents the interfacing resistance and
inductance respectively. C1 is a used as a capacitive
ripple filter to remove high frequency switching noise of PCC voltage.
Vdc1 and Vdc2 are the voltages across
the two constant DC sources. When switches Sa,
Sb, Sc are switched ON, the output of
VSC is equal to +Vdc while switching ON
S’a, S’b and S’ccorresponds to –Vdc at output terminal of VSC. ‘u’ is
the switching state which changes between +1 and -1.