Keywords - Hysteresis Controller, DSTATCOM, Power Quality, Single Phase SRF theory, Voltage Unbalance, Current Controlled Mode, Voltage Controlled Mode.
  1. 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.