A T-Connected Transformer and A Three-leg VSC with Fuzzy Based DSTATCOM for Power Quality Improvement Y. V. Jaya Krishna 1 , K. Kiran Kumar 2 1. PG Student, Dept. of EEE, SRKREC, Bhimavaram, email: yvjayakrishna@gmail.com 2. Asst. Professor, Dept. of EEE, SRKREC, Bhimavaram, email: friendlykiran31@gmail.com Abstract- Power quality is the major problem which needs a lot of research work to mitigate this problem. Since load cur- rent is high in magnitude in distribution system, the voltage drop and current harmonics are also high. So we need to min- imize this problem in the distribution system itself. The STATCOM circuit used in distribution system is called DSTATCOM. For more benefits we use a T-Connected Trans- former here. The T-Connected transformer connection miti- gates the neutral current and the three-leg VSC based DSTATCOM compensates harmonic current, reactive power and balances the load. I. INTRODUCTION Poor voltage regulation, high reactive power and harmon- ics current burden, load unbalancing, excessive neutral cur- rent are some of the power quality problems in three-phase four-wire distribution systems.[1]–[6]. All the commercial buildings, office buildings, hospitals, etc. use three-phase four-wire distribution systems. Most of the loads in these locations are nonlinear loads and are mostly unbalanced loads in the distribution system. This creates excessive neu- tral current both of fundamental and harmonic frequency and the neutral conductor gets overloaded. The voltage regulation is also poor in the distribution system due to the unplanned expansion and the installation of different types of loads in the existing distribution system. In order to con- trol the power quality problems, many standards are pro- posed, such as the IEEE-519 standard [7]. Custom power devices (CPD) are the devices used for mitigating the power quality problems [2]. One of the shunt-connected CPD is distribution static compensator (DSTATCOM) which is capable of compensating power quality problems in the load current. Some of the topologies of DSTATCOM for three phase four-wire system for the mitigation of neutral current along with power quality compensation in the source current are four-leg voltage source converter (VSC), three single- phase VSCs, three-leg VSC with split capacitors [4], three- leg VSC with zig-zag transformer [8]–[10], and three-leg VSC with neutral terminal at the positive or negative of dc bus [11]. A shunt compensator improves the voltage regula- tion in the distribution feeder [12]. There are many control schemes reported in the literature for control of shunt active compensators such as instantaneous reactive power theory, power balance theory, synchronous reference frame theory, symmetrical components based, etc. [13], [14]. The syn- chronous reference frame theory [13] is used for the control of the proposed DSTATCOM. In this paper, a new topology of DSTATCOM is proposed for a three-phase four-wire distribution system, which is based on three-leg VSC along with fuzzy and a T-connected transformer. The control of VSC is generally done with analog components but fuzzy control will help in improving the accuracy of the control network. There are various applications for T-connected transformer which is used in three-phase distribution system [15]–[17].But for the first time, the application of T- connected transformer for neutral current compensation is demonstrated. Moreover, the T-connected transformer is suitably designed for magnetic motive force (mmf) balance. The T-connected transformer mitigates the neutral current and the three-leg VSC compensates the harmonic current and reactive power, and balances the load. The insulated gate bipolar transistor (IGBT) based VSC is self-supported with a dc bus capacitor and is controlled for the required compensation of the load current. The DSTATCOM is de- signed and simulated using MATLAB software with its Simulink and power system block set (PSB) toolboxes for power factor correction and voltage regulation along with neutral current compensation, harmonic elimination, and load balancing with linear loads as well as nonlinear loads. II. SYSTEM CONFIGURATION AND DESIGN The single-line diagram of the shunt–connected DSTAT- COM based distribution system is shown in Fig. 1(a). For the normal operation of the DSTATCOM system the dc capacitor connected at the dc bus of the converter acts as an energy buffer and establishes a dc voltage. The operating modes of DSTATCOM can be used for reactive power compensation, power factor correction and voltage regula- tion. The phasor diagram of unity power factor operation is shown in fig 1(b). The DSTATCOM injects a current I c such that the source current is only I s , and this is in-phase with voltage. The voltage regulation operation of DSTATCOM is depicted in the phasor diagram of Fig. 1(b). The DSTAT- COM injects a current I c such that the voltage at the load (V S ) is equal to the source voltage (V M ). Fig 2. Shows the proposed DSTATCOM consisting of a three-leg VSC and a T-connected transformer, where the T- connected transformer compensates the neutral current. The design of T-connected transformer windings is such that the mmf is balanced properly in the transformer. A three-leg VSC is used as an active shunt compensator along with a T- connected transformer, as shown in Fig. 2, and this topology has six IGBTs, three ac inductors, and one dc capacitor. The