In this paper, different Distribution Static Compensators (DSTATCOMs) topologies, state of the art, their performance, design considerations, future developments, and potential applications are investigated for power quality improvement. These DSTATCOMs for three-phase three-wire systems and three-phase four-wire systems are developed and installed in the distribution system for many functions, such as reactive power compensation, harmonics elimination, load balancing, and neutral current compensation. This paper is aimed to explore a broad perspective on DSTATCOMs to researchers, engineers, and the community dealing with the power quality improvement. A classified list of some latest research publications is also provided for quick reference.
THREE-PHASE ac power is used unanimously for distribution systems and is used in residential buildings, commercial buildings, office buildings, hospitals, etc. Typical loads in a three-phase distribution system may be computer loads, lighting ballasts, small rating adjustable speeds drives (ASDs) in air conditioners, fans, refrigerators, other domestic and commercial appliances, etc. Almost all these applications use switchedmode power supplies (SMPS), which draw excessive harmonic currents. Three-phase distribution systems are facing severe power quality problems such as poor voltage regulation, high reactive power burden, harmonics current, and load unbalancing. The power quality problems and their mitigation techniques are reported in the literature over the years –. The power quality improvement devices such as three-phase three-wire Distribution Static Compensators (DSTATCOMs) provide reactive power for improving voltage regulation, to eliminate harmonics in the supply currents, and to balance the supply currents when the load currents are unbalanced. There are many standards – proposed to control the quality of electric supply in the distribution system such as IEEE standard 519  and IEEE 1531 . There are surveys and studies on the power quality causes, effects, and analyses –. Many studies and review articles on active shunt compensators for power conditioning are reported in the literature –. The topologies, application, and control of active filters are reported in  and . The voltage source type of harmonic loads and their compensation using series active filters is reported by Peng . The power quality improvement in the three-phase three-wire systems using three-phase three-wire shunt active compensators and their topologies, control techniques, field tests, etc., is reported extensively in the literature over the years and the recent publications are listed in –.
One of the major problems in three-phase four-wire distribution systems is excessive neutral current along with other power quality problems such as poor voltage regulation, high reactive power burden, harmonics current injection, and load unbalancing –. The excessive neutral current is of both fundamental and harmonics, and the neutral conductor is overloaded resulting in busting of it. The major reason for excessive neutral current in the three-phase four-wire distribution systems is the proliferation of nonlinear loads as well as unbalanced loads. In a survey in the United States, observations on computer power systems have indicated harmonic neutral currents from 0 to 1.73 times the phase current . It has also been revealed that 22.6% of the sites have neutral currents exceeding the full-load phase currents, and this scenario is becoming worst in the recent years due to the proliferation of such nonlinear single-phase loads. The iron-cored inductive ballasts as well as electronic ballasts in fluorescent lighting also contribute to third harmonic currents . The topologies, design, and control techniques for many three-phase four-wire DSTATCOMs for power quality improvement are reported in the literature in recent years –. There are applications of DSTATCOM for aircraft electrical systems , wind generation , and offshore oil fields .
In this paper, various topologies and different control techniques of three-phase three-wire and three-phase four-wire DSTATCOMs are explored for load compensation. The performance of some topologies of three-phase three-wire DSTATCOMs is demonstrated for voltage regulation or power factor correction by reactive power compensation along with harmonics elimination and load balancing. A number of topologies of DSTATCOMs for compensation in three-phase four-wire distribution system are classified, designed, and modeled to simulate their performance for voltage regulation or power factor correction by reactive power compensation along with harmonics elimination, load balancing, and neutral current compensation.
II. STATE OF THE ART
The concept of static compensation is first given by Gyugyi and Strycula in 1976 . A voltage source converter (VSC) with a capacitor at its dc bus is able to inject reactive power of which the quantity of reactive power is limited by the specification of power semiconductor devices. The concept of custom power technology for the distribution system is coined in the literature . Various devices such as DSTATCOM, Dynamic Voltage Restorer (DVR), Unified Power Quality Conditioner (UPQC), etc., are proposed and installed under the name of custom power devices for the enhancement of power quality in distribution systems , .
The various aspects such as modeling, design, and simulation for reactive power compensation, unbalanced and harmonic compensations, and voltage regulation are reported in –. Monitoring of electric power quality based on different techniques such as wavelet and neural networks is also reported in  and . The modeling of the DSTATCOM system ,  is necessary for feasibility and validating the design. The review of the present technology and concept of custom power park is discussed by Ghosh and Ledwich . The voltage regulation function of the DSTATCOM is discussed in  and . The concept of constant voltage at point of common coupling (PCC) is realized by pumping extra amount of reactive power into the source side, so that the line drop can be compensated dynamically. The concept of battery energy storage system (BESS) for DSTATCOM is presented , . The operation of the DSTATCOM for weak or isolated generation is also important . The DSTATCOM is proposed for compensating voltage quality problems such as sag and swell – and flicker . The reactive power demand in isolated power generation for voltage regulation is achieved using STATCOM .
The control schemes of static compensators are developed using the well-known theory proposed by Akagi et al. . The extraction of fundamental active and reactive components of currents is demonstrated in this theory. Another widely accepted control theory is synchronous reference frame (SRF) theory reported by Divan et al. . This theory is based on the transformation of currents from frame to synchronous rotating frame and then extracting the fundamental frequency components. Many other control techniques for shunt compensators have been reported such as sliding mode control , voltage template and PI controllers , instantaneous symmetrical component theory , and neural network theory , .
III. TOPOLOGIES OF DSTATCOM The DSTATCOM
topologies can be classified based on the number of switching devices, use of transformers for isolation, use of transformers for neutral current compensation, etc. These DSTATCOMs are developed to meet the requirements of different applications such as three-phase three-wire and threephase four-wire distribution systems.
A. Three-Phase Three-Wire DSTATCOM
Three-phase three-wire DSTATCOMs are used for the power quality improvement in three-phase three-wire distribution system for the compensation of consumer loads. The topologies for the three-phase three-wire DSTATCOMs are classified as shown in Fig. 1(a). There are nonisolated VSC- and isolated VSC-based DSTATCOMs.