A review of distribution static compensator

Renewable and Sustainable Energy Reviews 50 (2015) 531–546

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

A review of distribution static compensator Om Prakash Mahela n, Abdul Gafoor Shaik Department of Electrical Engineering, Indian Institute of Technology Jodhpur, 342011, India

art ic l e i nf o

a b s t r a c t

Article history: Received 2 October 2014 Received in revised form 25 March 2015 Accepted 7 May 2015 Available online 29 May 2015

This paper presents a comprehensive review of the distribution static compensator employed for harmonic filtering, power factor correction, neutral current compensation, and load balancing in the distribution network. The intention of this review is to provide a wide spectrum on architecture, topologies, and control techniques to the researchers, designers, and engineers working on power quality improvement. More than 100 research publications on the topologies, configuration, control techniques, and applications of distribution static compensator have been thoroughly reviewed and classified for quick reference. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Control technique Distribution static compensator (DSTATCOM) Power quality Topology Voltage control mode Voltage source converter

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 Distribution static compensator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 2.1. Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 2.2. Major components of DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 2.2.1. Voltage source converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 2.2.2. Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 2.2.3. Ripple filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 2.2.4. AC inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 2.3. Applications of DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 Topologies of DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 3.1. Three-phase three-wire DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 3.1.1. Isolated VSC-based DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 3.1.2. Nonisolated VSC-based DSTATCOM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535 3.2. Three-phase four-wire DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 3.2.1. Isolated two-leg VSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 3.2.2. Isolated three-leg VSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 3.2.3. Isolated three single-phase VSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 3.2.4. Nonisolated VSC without transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537 3.2.5. Nonisolated two-leg VSC using transformer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 3.2.6. Nonisolated three-leg VSC using transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 Control techniques of DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 4.1. Instantaneous reactive power (IRP) theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 4.2. Synchronous reference frame (SRF) theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540

n

Corresponding author. E-mail addresses: [email protected] (O.P. Mahela), [email protected] (A.G. Shaik). http://dx.doi.org/10.1016/j.rser.2015.05.018 1364-0321/& 2015 Elsevier Ltd. All rights reserved.

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4.3. Symmetrical component theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Average unit power factor (AUPF) theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Proportional-integral (PI) controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Adaline based neural network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Sliding mode control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8. Miscellaneous control techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.9. Comparative study of control techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Selection considerations for specific application of DSTATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Power quality problems related to both current and voltage such as poor voltage regulation, high harmonics current burden, load balancing, poor power factor (PF), excessive neutral current, voltage flicker, sag and swell originate in distribution networks [1]. Increasing penetration of renewable energy (RE) sources has further affected the quality of power supplied [2]. Different power quality (PQ) detection and classification techniques have been reported in [3]. Power electronic converters such as distribution static compensator (DSTATCOM), dynamic voltage restorer (DVR), and unified power quality conditioner (UPQC) can eliminate harmonics and unbalancing on both the source and load side of the system [4,5]. DSTATCOM is a synchronous voltage generator capable of supplying rapid and uninterrupted capacitive and inductive reactive powers [6,7]. Many DSTATCOM topologies related to three-phase three-wire (3P3W) and three-phase four-wire (3P4W), isolated and nonisolated, and with and without transformers are reported in the literature. DSTATCOM in current control mode injects harmonic and reactive components of load current addressing power quality [8]. In voltage control mode, it regulates load voltage at a constant value protecting loads from voltage disturbances [9,10]. The performance of DSTATCOM depends on the control algorithm used for extraction of reference current components [11] such as instantaneous reactive power (IRP) theory, symmetrical component (SC) theory, synchronous reference frame (SRF) theory, average unit power factor (AUPF) theory, sliding mode control and adaline based neural network. [12,13]. The optimal location and sizing of the DSTATCOM which plays an important role in PQ improvement has been employed using firefly algorithm [14] and particle swarm optimization technique [15]. This paper aims at presenting a comprehensive review on the configuration, topologies and control techniques of DSTATCOM. Over 100 research publications [1–127] are critically reviewed and classified broadly into four categories. The first category [1–15] is based on general concepts of power quality and DSTATCOM. The second category [16–42] comprises of DSTATCOM configuration, principle of operation and its potential applications. The third category [43–97] is on the DSTATCOM topologies which is further sub-classified in to 3P3W [43–54] and 3P4W [55–97]. The fourth category [98–127] is on DSTATCOM control techniques which is sub-classified into IRP theory [99–105], SRF theory [106–109], SC theory [110,111], AUPF theory [112,113], PI controller [114–116], adaline based neural network [117–119], sliding mode controller [120–122], and miscellaneous control techniques [123–127]. However, some publications include more than one category and have been classified based on their dominant field and some publications are included in more than one category depending on their utility. This paper is divided into seven sections. Section 2 covers principle of operation, major components and applications of DSTATCOM. The DSTATCOM topologies are covered under Section 3. Section 4 describes control techniques. Section 5 relates to the application specific selection criteria of DSTATCOM. The proposed

540 540 541 541 541 541 541 542 543 543 543

future work for DSTATCOM is presented in Section 6. The conclusions are drawn in Section 7.

2. Distribution static compensator A static synchronous compensator (STATCOM) with a coupling transformer, an inverter, and energy storage device used in distribution system is called DSTATCOM and has configuration as the STATCOM [16]. 2.1. Principle of operation A typical DSTATCOM connected to the point of common coupling (PCC) in distribution system having unbalanced and nonlinear loads is shown in Fig. 1. The main function of DSTATCOM is to supply reactive power (as per requirement) to the system in order to regulate the voltage at the PCC. Active power can also be supplied if a storage battery or fly wheel is available on dc-side of the DSTATCOM [17,18]. Equivalent circuit of the DSTATCOM as shown in Fig. 2 is represented by a controlled voltage source (VVR) in series with transformer impedance ZVR. The voltage VVR can be regulated to control voltage ðV k Þ of the bus k. Fig. 3 represents phasor diagram related to the DSTATCOM operation under both lagging and leading power factor modes. 2.2. Major components of DSTATCOM The various components of DSTATCOM include voltage source converter (VSC), dc bus capacitor, transformer and ripple filter as shown in Fig. 4. The VSC converts a dc voltage into a three-phase ac voltage and synchronized with PCC through a tie reactor and capacitor. The transformer is used to match the inverter output to the line voltage [19,20]. The important components are described in the following subsections. 2.2.1. Voltage source converter The VSC allows bidirectional power flow and realized using devices such as insulated gate bipolar transistors (IGBT) and metal

Fig. 1. Single-line diagram of the DSTATCOM.

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the neutral current depends on the system impedance and location of the compensator [27,28]. Transformers are used either in nonisolated condition for compensating the neutral current only or for providing isolation of the VSC along with neutral current compensation. The transformer topologies commonly used with DSTATCOM are zig-zag, star/delta, T-connected, and star/hexagon [29]. 2.2.3. Ripple filter A first-order high-pass filter tuned at half the switching frequency is used to filter the high-frequency noise from the voltage at the PCC [30]. It consists of a series resistance Rf in series with the capacitor Cf [31,32]. The time constant of the filter should be very small compared to the fundamental time period (T) [33]:

Fig. 2. Equivalent circuit of the DSTATCOM.

Rf C f 5 ðT=10Þ

ð3Þ

2.2.4. AC inductor For reducing ripple in compensating currents, the tuned values of interfacing inductors are connected at the ac output of VSC [34]. The ac inductance ðLf Þ of VSC depends on the current ripple icr;p  p , switching frequency fs, and dc bus voltage Vdc and its value is given as [35] pffiffiffi 3mV dc ð4Þ Lf ¼ 12af s icr;p  p where m is the modulation index and a is the overloading factor.

Fig. 3. Phasor diagram: (a) lagging operation and (b) leading operation.

2.3. Applications of DSTATCOM DSTATCOM injects current into the system at PCC which helps in achieving harmonic filtering, power factor correction, neutral current compensation, and load balancing. Potential applications of DSTATCOM such as reactive power compensation in singlephase operation of microgrid [36], voltage support strategy in low voltage (LV) networks [37], a dynamic hybrid VAR compensator along with thyristor switched capacitor (TSC) in distribution system [38], system impact study [39], reduction of photovoltaic power fluctuations [40], enhancement of PV penetration in distribution system [41], and mitigation of voltage sag/swell/flicker [42] are reported in the literature.

Fig. 4. Block diagram of the DSTATCOM.

oxide field effect transistors (MOSFET). The switching of these devices is based on pulse-width modulation (PWM) technique [21,22]. In addition to switching devices, VSC also has components like dc bus capacitor and interfacing inductor [23,24]. The minimum dc bus voltage should be greater than twice the peak value of the phase voltage of the system. The value of dc capacitor depends on the instantaneous energy available to the DSTATCOM during transients. The dc bus capacitor may also be used with two split sections having equal or unequal values [25,26]. The dc capacitor Cdc and dc bus voltage Vdc are calculated as pffiffiffi 2 2V LL ð1Þ V dc ¼ pffiffiffi 3m C dc ¼

6aVIt ½V 2dc  V 2dc1 

3. Topologies of DSTATCOM The DSTATCOM topologies can be classified based on the application in 3P3W and 3P4W distribution systems. Further topological classification can be based on the use of transformer for isolation and neutral current compensation, number of switching devices, type of converter, etc. The classification of DSTATCOM topologies is shown in Fig. 5. In topological diagrams the unbalanced/non-linear loads as well as supply source are not shown, only the converter topologies and transformer configurations are included for the sake of clarity. 3.1. Three-phase three-wire DSTATCOM

ð2Þ

where m is the modulation index; VLL is the ac line voltage; a is the overloading factor; V is the phase voltage; I is the phase current; Vdc1 is the phase voltage; t is the time by which the dc bus voltage is to be recovered.

Three-phase three-wire DSTATCOMs are used for reactive power compensation, harmonic elimination, PQ improvement and load balancing in 3P3W distribution system. The topologies for three-phase three-wire DSTATCOMs include isolated VSC and nonisolated VSC-based DSTATCOM.

2.2.2. Transformer Unbalanced and nonlinear loads in distribution system cause problems of excessive neutral current. Transformers are used for neutral current compensation. Its effectiveness for compensating

3.1.1. Isolated VSC-based DSTATCOM The VSC is isolated from supply system through a transformer. Isolated VSC-based three-phase three-wire DSTATCOM topology reported in [43] is shown in Fig. 6(a). It contains a bank of three

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Fig. 5. Topological classification of DSTATCOM.

Fig. 6. Isolated VSC based 3P3W DSTATCOM topology: (a) three single-phase VSC, (b) three-leg VSC, (c) two-leg VSC and (d) three-leg VSC with flywheel.

Fig. 7. Nonisolated VSC-based 3P3W DSTATCOM: (a) three-leg VSC and (b) two-leg VSC with split capacitor.

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single-phase VSC units connected to a common dc storage capacitor. Each VSC unit is connected to the system supply through an isolating transformer which provides isolation between the converters. Transformer also prevents the dc capacitor storage being shorted through controlled switches in different converters. Controlled switch is a power semiconductor device and anti-parallel diode combination. Three-leg VSC and two-leg VSC-based isolated topologies of 3P3W DSTATCOM using star/delta transformer are shown in Fig. 6(b) and (c) respectively and have been reported in [44]. The transformer topology such as T-connected, zig-zag, and star/hexagon may also be used. Three-leg VSC based isolated topology incorporated with flywheel energy storage system (FESS) is shown in Fig. 6(d) and for wind applications reported in [45–47]. Permanent magnet synchronous machine (PMSM) allows power exchange between flywheel and power electronic interface connected to dc bus of the DSTATCOM. The available energy stored by FESS is given by the relation:

ΔE ¼ Jðω2  ω2min Þ

ð5Þ

where ΔE is the energy of flywheel; ω is the operation speed; ωmin is the minimum operation speed; and J is the moment of inertia of the flywheel.

3.1.2. Nonisolated VSC-based DSTATCOM VSC is connected to the supply system through inductive reactor. This topology is classified into three-leg VSC or two-leg

535

VSC with split capacitor. Three-leg VSC-based topology, has three legs in the bridge each comprising of two IGBTs as shown in Fig. 7 (a). The midpoint of each half bridge is connected at the point of common coupling (PCC) through an interface inductor. Threephase loads are connected at the PCC. A three-phase star connected RC filter is used at PCC to absorb voltage switching ripples [48,49]. In [50], authors presented the use of three-leg DSTATCOM for reactive power, harmonics and unbalanced load current compensation of a diesel generator set for an isolated system. A nonlinear controller design for a three-leg DSTATCOM connected to distribution system with distributed generation (DG) to regulate the voltage by reactive power compensation is reported in [51]. In [52], the self supporting voltage of dc bus is used for mitigation of current harmonics and load balancing. Implementation of threeleg VSC based DSTATCOM using self tuning filter (STF) based IRP control algorithm for PQ improvement is presented in [53]. The two-leg VSC based topology with split capacitor is shown in Fig. 7(b) and reported in [54]. The main difference between two-leg and three-leg topologies is that of switching devices and use of split capacitor in two-leg topology. A comparative study of isolated and nonisolated VSC-based 3P3W DSTATCOM topologies is carried out based on critical reviews of publications [43-54] and presented in Table 1. The numerical data included in the table are just indicative of the approximate values of the elements. However, actual values may be different depending on the specific application.

Table 1 Comparative study of 3P3W DSTATCOM topologies. DSTATCOM topology

Semiconductor devices (in nos.)

Isolated three single-phase VSC (Fig. 6 12 (a)) Isolated three-leg VSC (Fig. 6(b)) 6 Isolated two-leg VSC (Fig. 6(c)) 4 Nonisolated three-leg VSC (Fig. 7(a)) 6 Nonisolated two-leg VSC (Fig. 7(b)) 4 a

Transformer

3 Single-phase units Star/deltaa Star/deltaa Not required Not required

Interfacing inductance (mH)

Dc bus voltage (V)

Capacitor ðμf Þ

kVA rating of transf.

6

600

1000

2.5

2.5 7 2.5 4.3

700 1400 700 1400

1000 5000 3000 6000

7.2 7.2 Nil Nil

T-connected, zig-zag, and star-hexagon transformers may also be used.

Fig. 8. Isolated two-leg VSC based 3P4W DSTATCOM topology using (a) star-delta, (b) T-connected, (c) zig-zag and (d) star-hexagon transformer.

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3.2. Three-phase four-wire DSTATCOM Three-phase four-wire DSTATCOM is used in three-phase fourwire distribution system to filter load current to meet out the specifications for the utility connection [55,56]. This can be used to cancel the effect of poor load power factor (PF) such that source current has near unity PF, provide harmonic compensation in loads such that source current become sinusoidal, provide compensation for unbalanced loads such that source current become balanced, cancel dc offset in loads and for PQ improvement. The three-phase four-wire DSTATCOM topologies are mainly classified into two categories, with transformers and without transformers. Important topologies of 3P4W DSTATCOM are discussed as detailed below. 3.2.1. Isolated two-leg VSC Two-leg VSC having split capacitor with a transformer is used as three-phase four-wire DSTATCOM. Transformer provides isolation

from the system. Two-leg three-phase four-wire DSTATCOM topologies using star-delta, T-connected, star/zig-zag, and star/hexagon transformer are shown in Fig. 8(a), (b), (c) and (d) respectively. VSC side winding of transformer is three-phase three-wire with two phase windings connected to the two legs of VSC and third phase winding connected to middle point of the split capacitor. Threephases of the system side windings are connected to the threephases of the supply and neutral of the winding is connected with the neutral of the 3P4W supply system. Ripple filter is separately connected to the supply system. Generally, ripple filter with neutral is used in conjunction with zig-zag transformer and without neutral is used with other topologies. H-bridge VSC and star-delta transformer for PQ improvement in 3P4W distribution system is proposed in [57]. In [58], authors presented T-connected transformer topology for voltage and frequency control of wind power generation system feeding three-phase four-wire loads. The voltage and frequency controller of an isolated wind energy conversion system using two-leg VSC

Fig. 9. Isolated three-leg VSC based 3P4W DSTATCOM topology using (a) star-delta, (b) T-connected, (c) zig-zag and (d) star-hexagon transformer.

Fig. 10. Isolated three single-phase VSC 3P4W DSTATCOM.

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based topology, isolated with zig-zag transformer is presented in [59]. Isolated two-leg VSC-based DSTATCOM with star/hexagon transformer for PQ improvement is reported in [60]. 3.2.2. Isolated three-leg VSC Three-leg VSC having dc bus with a transformer is used as threephase four-wire DSTATCOM. Transformer provides isolation from the system. Three-leg, three-phase four-wire DSTATCOM topologies using star-delta, T-connected, zig-zag, and star-hexagon transformer are shown in Fig. 9(a), (b), (c) and (d) respectively. VSC side winding of the transformer is three-phase three-wire which is connected to the three legs of VSC. Three-phases of the system side windings are connected to the three-phases of the supply and neutral of the winding is connected with neutral of the 3P4W supply system. Ripple filter is separately connected to the supply system. Generally, ripple filter with neutral is used in conjunction with zig-zag transformer and without neutral is used with other topologies. The isolated three-leg VSC using star-delta and T-connected transformer-based topologies for voltage and frequency controllers for an asynchronous generator-based isolated wind energy conversion system is reported in [61]. Three-leg VSC using star/ hexagon transformer is demonstrated in [62] for PQ improvement in 3P4W distribution system. Three-leg VSC based topology using star/zig-zag transformer for PQ improvement is reported in [63] and for voltage regulation, load balancing, neutral current compensation and elimination of harmonics is reported in [64]. 3.2.3. Isolated three single-phase VSC Three-phase four-wire DSTATCOM topology using isolated three single-phase VSCs having dc bus is shown in Fig. 10 and reported in [33]. Each phase has H-bridge incorporated with

537

single-phase transformer. One terminal of the secondary winding of each transformer is connected to a phase and other terminal to the neutral of supply system. In [65], authors presented stability analysis based on bifurcation theory of isolated three single-phase VSC operating in current control mode. Comparison of control strategies of isolated three single-phase VSC based DSTATCOM for PQ improvement under various source voltage and load conditions are presented in [66] and topology as well as control that can be flexibly operated in the voltage or current control mode are presented in [67]. In [68], authors proposed an energy-based fast-acting dc-link voltage controller for isolated three single-phase VSCs based DSTATCOM to ensure the fast transient response. A comparative study of isolated two-leg, three-leg, three single-phase VSCs-based 3P4W DSTATCOM topologies is carried out based on critical reviews of publications [55–68] and presented in Table 2. Here, again also the numerical data included in the table are just indicative of approximate values of the elements. However, actual values may be different depending on specific application of the DSTATCOM.

3.2.4. Nonisolated VSC without transformer Non-ioslated VSC-based DSTATCOM topologies using no transformer are classified as four-leg and three-leg VSC-based topology. The four-leg VSC-based 3P4W DSTATCOM topology is shown in Fig. 11(a) and reported in [69]. An implementation of a four-leg VSC using an adaptive neural network-based control algorithm for compensation of linear/non-linear loads is presented in [70]. The application with PV for improvement of penetration level with low voltage distribution system is reported in [71] and PQ improvement is reported in [72].

Table 2 Comparison of isolated VSC based 3P4W DSTATCOM topologies. DSTATCOM topology

Semiconductor devices

Two-leg VSC with star/delta transf. (Fig. 8(a)) 4 Two-leg VSC with T-connected transf. (Fig. 8 4 (b)) Two-leg VSC with zig/zag transf. (Fig. 8(c)) 4 Two-leg VSC with star/hexagon transf. (Fig. 8 4 (d)) Three-leg VSC with star/delta transf. (Fig. 9 6 (a)) Three-leg VSC with T-connected transf. (Fig. 9 6 (b)) Three-leg VSC with zig/zag transf. (Fig. 9(c)) 6 Three-leg VSC with star/hexagon transf. 6 (Fig. 9(d)) Three single-phase VSC (Fig. 10) 12

Transformer

Interfacing inductance (mH)

Dc bus voltage (V)

Capacitor ðμf Þ

kVA rating of transf.

Star/delta T-connected

3.3 2.5

400 700

1200 7600

15 12

Star/zig-zag Star/hexagon

4 3.5

400 400

4000 6600

12 12

Star/delta

1.5

700

5500

32

T-connected

2.3

400

6600

12

Star/zig-zag Star/hexagon

2.5 1.5

400 200

2200 1650

12 6

3 single-phase units

2.5

300

5000

5

Fig. 11. Nonisolated VSC based 3P4W DSTATCOM without transformer based topology: (a) four-leg and (b) three-leg.

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Nonisolated three-leg VSC-based 3P4W DSTATCOM without transformer topology with split capacitor and three ac capacitors is shown in Figs. 11(b) and 12 respectively and reported in [73] and [74] respectively. Three-leg VSC with split capacitor based topology has been proposed in [75] for addressing practical issues such as power rating, filter size, compensation performance, and power loss. Its application for PQ improvement is reported in [76,77]. Three ac capacitor based topology for load compensation and energy conservation as well as PQ improvement is reported in [78,79]. Split capacitor topology with LCL filter is reported in [80]. Chopper technique with dc bus in split capacitor topology for harmonic compensation, power factor correction, and load balancing is reported in [81,82]. A comparative study of different topologies of nonisolated VSCbased 3P4W DSTATCOM without transformer is carried out based on critical reviews of publications [69–82] and provided in Table 3. The detailed comments are included to highlight the important aspects of each topology.

3.2.5. Nonisolated two-leg VSC using transformer Nonisolated two-leg VSC-based 3P4W DSTATCOM topologies using star-delta, T-connected, zig-zag, and star-hexagon transformer are shown in Fig. 13(a), (b), (c) and (d) respectively. Two-leg VSC with split capacitor is directly connected to the three-phases of the supply system. Three-phases and neutral of the transformer winding are connected to the three-phases and neutral of the supply system respectively forming a parallel arrangement with two-leg VSC. Star-delta topology for PQ improvement in 3P4W distribution system is reported in [83]. Zig-zag transformer topology for load controller in mini hydro power generating system is presented in

[84]. Star-hexagon topology for PQ improvement in 3P4W distribution system is presented in [85]. Zig-zag transformer topology for PQ improvement in distribution system is presented in [86]. Tconnected transformer based topology of DATSTCOM for PQ improvement is reported in [87]. 3.2.6. Nonisolated three-leg VSC using transformer Nonisolated three-leg VSC-based 3P4W DSTATCOM topology using star-delta, T-connected, zig-zag, and star-hexagon transformer is shown in Fig. 14(a), (b), (c) and (d) respectively. Three-legs of the VSC are directly connected to the three-phases of the supply system. Dc bus is not using the split capacitor and connected in parallel to the legs of VSC. Three-phases and neutral of the transformer winding are connected to the three-phases and neutral of the supply system respectively forming a parallel arrangement with three-leg VSC. In [88], authors presented star-delta transformer based topology for voltage regulation in 3P4W distribution system. Star-delta topology with photovoltaic based DSTATCOM for PQ improvement is proposed in [89]. Star-delta topology for reactive power compensation, harmonic elimination, and load balancing with linear/non-linear loads is presented in [90]. T-connected transformer based topology is used with electronic load controller for a standalone induction generator used in small hydro power plant [91]. T-connected transformer based topology for PF correction, harmonics elimination, load balancing and neutral current compensation of linear/nonlinear, balanced/unbalanced loads is proposed in [30,92]. Zig-zag transformer based topology for PQ improvement in 3P4W distribution system is proposed in [93,94]. In [95], authors presented zig-zag transformer based topology for load balancing and unity PF operation in the presence of unbalanced and distorted voltages. In [96], a 3-leg VSC with a zig-zag transformer is used for compensation of reactive power, harmonic currents, unbalance loads and neutral current in 3P4W distribution system. In [97], authors presented star-hexagon transformer based topology with the application of enhanced phase locked loop technique for the voltage and frequency controller in a stand alone wind energy conversion system using isolated asynchronous generator feeding 3P4W loads. A comparative study of nonisolated three-leg and two-leg VSCbased 3P4W DSTATCOM topologies using transformer is carried out based on critical reviews of publications [83–97] and provided in Table 4. The numerical data included are indicative only and practical values may be different depending on the specific application.

4. Control techniques of DSTATCOM

Fig. 12. Nonisolated three-leg VSC based 3P4W DSTATCOM topology with three ac capacitors and without transformer.

The reactive power needed by the load is provided by the DSTATCOM and only real power is supplied by the source such that source current remains at unity PF. Load balancing is achieved by

Table 3 Comparison of nonisolated VSC-based 3P4W DSTATCOM topologies without transformer. Sr. no.

Topology

1

Four-leg VSC (Fig. 11(a))

1 8

2

Three-leg VSC split capacitor (Fig. 11(b))

2 6

3

Three-leg VSC with ac capacitors (Fig. 12)

n

Nc

2n

Ns Comments

6

It uses maximum number of switches and compensate for unbalanced load currents containing dc components in 3P4W system. Fourth-leg is used for compensation of zero sequence component of load currents which need separate reference currents along with an appropriate switching control strategy for their operation. Voltage of dc bus is easier to control. It can compensate for unbalanced load currents in 3P4W system. The dc component of load current is not compensated. Size of capacitor is large to reduce ripple. The two dc bus capacitors increase size of the DSTATCOM. The zero sequence component of load current injected by the compensator returns at mid-point of the dc capacitor. It uses minimum number of switches. It requires two dc storage devices. Voltage across each dc link capacitor is chosen as 1.6 times the peak value of source voltages. The passive capacitor has the capability to supply a part of reactive power required by the load and reduces dclink voltage as well as switching frequency. The active filter will compensate the balance reactive power and harmonics present in the load.

In addition to 2 dc capacitors, 3 capacitors are also used on ac side of the converter; Nc – number of DC capacitors; Ns – number of power electric switches.

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making reference source current balanced. It has real fundamental frequency component of the load current and used to decide switching of the VSC and being extracted by control techniques [98]. Different control strategies reported in the literature such as IRP theory, SRF theory, adaline-based control algorithm, PI controller for maintaining dc bus voltage. Some important and widely used techniques are detailed below in the subsections as follows.

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4.1. Instantaneous reactive power (IRP) theory IRP theory is also known as p–q theory and proposed by Akagi [99]. In this method, sensed three-phase voltage and load currents are transformed into two-phase quantities in α–β frame using clark's transformation. The instantaneous active and reactive power is calculated in this frame. The reference currents in α–β

Fig. 13. Non-isolated two-leg VSC based 3P4W DSTATCOM topology using (a) star-delta, (b) T-connected, (c) zig-zag and (d) star-hexagon transformer.

Fig. 14. Non-isolated three-leg VSC based 3P4W DSTATCOM topology using (a) star-delta, (b) T-connected, (c) zig-zag and (d) star-hexagon transformer.

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Table 4 Comparative study of nonisolated VSC based 3P4W DSTATCOM topologies. DSTATCOM topology

Semiconductor devices

Transformer Interfacing inductance (mH)

Dc bus voltage (V)

Capacitor ðμf Þ

kVA rating of transf.

Two-leg VSC with star/delta transf. (Fig. 13(a)) Two-leg VSC with T-connected transf. (Fig. 13 (b)) Two-leg VSC with zig/zag transf. (Fig. 13(c)) Two-leg VSC with star/hexagon transf. (Fig. 13 (d)) Three-leg VSC with star/delta transf. (Fig. 14(a)) Three-leg VSC with T-connected transf. (Fig. 14 (b)) Three-leg VSC with zig/zag transf. (Fig. 14(c)) Three-leg VSC with star/hexagon transf. (Fig. 14 (d))

4 4

Star/delta 2.5 T-connected 2

1400 700

5000 3000

7.2 5.2

4 4

Zig-zag 3.5 Star/ 2 hexagon Star/delta 2.5 T-connected 2.5

1400 700

3200 3200

9 15

200 400

1650 1650

12 5

200 432

1650 4000

7.5 6

6 6 6 6

Zig-zag Star/ hexagon

frame are converted to abc frame using reverse clark's transformation [100,101]. Positive and negative sequence currents and voltage based IRP theory is reported in [102,103]. The detailed mathematical formulation of IRP theory is reported in [104,105]. The system voltages va ; vb , and vc as well as load currents iLa ; iLb , and iLc are converted into α–β frame using clark's transformation as 2 3 " # rffiffiffi" # v vα  1=2  1=2 6 a 7 2 1 p ffiffiffi pffiffiffi ð6Þ ¼ 4 vb 5 vβ 3=2  3=2 3 0 vc "

iα iβ

#

rffiffiffi" 2 1 ¼ 3 0

 1=2 pffiffiffi 3=2

2 3 # i  1=2 6 a 7 pffiffiffi 4 ib 5  3=2 ic

The instantaneous active and reactive powers are given as #" # " # " vβ vα iα p ¼  vβ vα iβ q

ð7Þ

ð8Þ

Instantaneous active and reactive powers p and q can be decomposed into an average (p) and an oscillatory component ~ The reference source currents insα and insβ used to compensate (p). IRP and oscillatory component of instantaneous active power are calculated as " n # " #  isα p 1 vα  vβ ð9Þ ¼ n isβ Δ vβ vα 0 where Δ ¼ v2α þ v2β . Reference source currents in α–β frame are used to calculate reference source currents in abc frame using reverse clark's transformation as 2 pffiffiffi 32 n 3 2 n 3 i0 1 0 rffiffiffi 1= 2 isa pffiffiffi pffiffiffi 76 n 7 26 6 in 7 6 6 7 i 1= 2  1=2 3 =2 ð10Þ 4 sb 5 ¼ sα 7 34 pffiffiffi pffiffiffi 54 n 5 i insc 1= 2  1=2  3=2 sβ

4.2. Synchronous reference frame (SRF) theory Synchronous reference frame theory (SRF) control technique is based on transformation of currents in synchronously rotating d-q frame [106,107]. Sensed voltage signals are processed by phase locked loop to generate sine and cosine signals. Sensed current signals are transformed to d-q frame and filtered. The filtered currents are back transformed to abc frame and fed to hysteresis current controller for switching pulse generation [108]. The mathematical transformation equations are described in [109]. The currents generated in α–β coordinates are transformed to d-q frame with the help of park's transformation using θ as

3.5 2.5

transformation angle as #" # " # " iα id sin ðθÞ cos ðθÞ ¼ iβ iq  sin ðθÞ cos ðθÞ

ð11Þ

The DC components, iddc and iqdc , are extracted using low pass filter and are transformed back into α–β coordinates using reverse park's transformation as #" " # " # iαdc iddc sin ðθÞ cos ðθÞ ¼ ð12Þ iβdc iqdc  sin ðθÞ cos ðθÞ These currents are transformed to obtain three-phase reference source currents in abc coordinates as 2 3 2 3 " # 1 0 rffiffiffiffi isa pffiffiffi 2 6i 7 6  1=2 7 iαdc 3 =2 : ð13Þ 4 sb 5 ¼ 4 pffiffiffi 5 iβdc 3 isc  1=2  3=2

4.3. Symmetrical component theory The control algorithm is based on prime objective to obtain the balanced source currents for which the positive sequence voltage and currents are considered [110]. Therefore the reference source currents can be considered as isa þ isb þ isc ¼ 0

ð14Þ

Implementation of this control technique is explained by the block diagram in Fig. 15. The power generated from the source is constant and equal to the dc value of the load power. The average load power is computed by using filter. The reference currents as well as sensed currents and voltages are shown in the block diagram. The switching signals generated are used for control of VSC [111]. This technique fails under non-ideal source voltage condition such as unbalanced and distorted source voltage conditions with linear as well as non-linear loads [66]. 4.4. Average unit power factor (AUPF) theory The source must supply the sinusoidal currents in phase with the voltages. The relation between source currents, voltages and average load power is given by the following relation [112]: 2 3 2 3 isa vsa 6 i 7 P lav 6 v 7 ð15Þ 4 sb 5 ¼ 2 4 sb 5 V vsc isc where P lav ¼

1 T

Z ðvsa iLa þ vsb iLb þ vsc iLc Þ dt

ð16Þ

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current. The weights are updated using the LMS algorithm [35,117]. The average weight corresponding to the active and reactive components of load are given as wp ¼ ðwpa þwpb þ wpc Þ=3

ð21Þ

wq ¼ ðwqa þ wqb þ wqc Þ=3

ð22Þ

If u and x represent the active and reactive unit templates respectively for three phases with their subscripts then the real and reactive component of reference source currents are obtained as [118] ð23Þ

iqa ¼ wq xa ; iqb ¼ wq xb ; iqc ¼ wq uc

ð24Þ

The reference source currents are used for the control of VSC. The sensed and reference currents are compared and the error is used to generate the gating signals for switches [93,119]. The reference source currents are given as

Fig. 15. Basic block diagram of symmetrical component theory.

1 V2 ¼ T

ipa ¼ wp ua ; ipb ¼ wp ub ; ipc ¼ wp uc

inSa ¼ ipa þ iqa ; inSb ¼ ipb þ iqb ; inSc ¼ ipc þ iqc :

Z ðvsa vsa þvsb vsb þ vsc vsc Þ dt

The compensator currents are derived as ic ¼ il  is . 2 3 2 3 2 3 ila va ica P 6i 7 6i 7 7 lav 6 4 cb 5 ¼ 4 lb 5  2 4 vb 5 V ilc icc vc

ð17Þ 4.7. Sliding mode control

ð18Þ

The compensator reference currents are compared with actual compensator currents and passed through the hysteresis band controller which generates gate pulses for voltage source converter of DSTATCOM [113]. 4.5. Proportional-integral (PI) controller This technique is used to estimate losses over the dc bus voltage of the DSTATCOM [114,115]. The reference dc bus voltage vndc is compared to the sensed dc bus voltage vdc of the DSTATCOM and produces voltage error, which, in the nth sampling is expressed as vdclðnÞ ¼ vndcðnÞ  vdcðnÞ

ð19Þ

The error voltage signal is processed in a PI controller and the output at the nth sampling is expressed as I pðnÞ ¼ I pðn  1Þ þ K pdc fvdclðnÞ  vdclðn  1Þ g þ K idc vdclðnÞ

ð25Þ

ð20Þ

where Kpdc and Kidc are the proportional and integral gains of the PI controller respectively. The output of PI controller is added with average real power for controlling DSTATCOM using p  q theory. In SRF theory, the output of PI regulator is added with the d-axis component of current signal and in adaline control it is added with equivalent source currents [104,116]. In [116], authors applied genetic algorithm to the optimization of PI coefficients in DSTATCOM nonlinear controller for regulating dc voltage and proposed new PI coefficients. 4.6. Adaline based neural network The positive fundamental frequency component load current is extracted using artificial neural network (ANN) based on least mean square (LMS) algorithm and training through adaline (adaptive linear element) technique. The fundamental active and reactive power components of load current are obtained by estimating the respective weights corresponding to the fundamental active (wpa ; wpb ; wpc ) and reactive (wqa ; wqb ; wqc ) components of the load

For a sliding mode control (SMC) system, the control law usually consists of an equivalent control law and a switching law. Conventionally, equivalent law is deduced from the relationship between sliding mode and its differential on the basis of pertinent mathematical model of the system [120]. Commonly used switching schemes for power converters of SMC are twolevel, three-level, variable hysteresis, and carrier modulation. Tsypkin's locus of linear system and describing function of nonlinear relay is used to determine the stable switching conditions of power converters [121,122]. 4.8. Miscellaneous control techniques Apart from the above techniques mentioned in Sections 4.1–4.7, some other techniques have played a significant role in operation of DSTATCOM for specific application. In [123], authors proposed an adaptive control strategy based on artificial immune system for DSTATCOM applications in an electric ship power system. Coordinated voltage control scheme for regulation of positive and negative sequence voltages utilizing OLTC transformers and DSTATCOM in LV distribution grids is presented in [98]. A hardware implementation of DSTATCOM using adaptive theory based improved linear sinusoidal tracer control algorithm is proposed in [124]. Control schemes for dc capacitor voltages equalization in diode-clamped multilevel inverterbased DSTATCOM are proposed in [125]. Implementation of singlephase enhanced phase-locked loop-based control algorithm for three-phase DSTATCOM is reported in [126,97]. Back-propagation control algorithm for PQ improvement using DSTATCOM is proposed in [34]. Arya et al. [127] proposed implementation of DSTATCOM using a learning-based anti-Hebbian control algorithm for compensation of linear/nonlinear loads. In [49], authors presented DSTATCOM using a Takagi–Sugeno fuzzy-logic controller for harmonic elimination, power factor correction, load balancing and voltage regulation. A fast acting dc-link voltage controller [68], and composite observer-based control algorithm [90] for DSTATCOM in threephase supply system are reported in the literature. 4.9. Comparative study of control techniques Comparative study of DSTATCOM control techniques is carried out based on critical reviews of publications [98–127] and all other publications cited in Section 4. A comparative study of strength and

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weakness of different control techniques for performance and PQ improvement capability of DSTATCOM is analysed and presented in Table 5.

5. Selection considerations for specific application of DSTATCOM The selection of a suitable DSTATCOM topology and control technique for its use in specific application is an important task for users. The performance of DSTATCOM depends on the control algorithm used for extraction of reference current components. Comparative study of control techniques presented in Table 5 is greatly helpful

for selection of these techniques for specific application. The kilovolt ampere (kVA) rating of the transformer is a major consideration for selection of the transformer for specific application [92]. A comparative study of different ratings of transformers is given in Table 6 and advantages and their disadvantages are given in Table 7 which will be helpful in selection of transformer based topology of DSTATCOM for particular application. The star/delta transformer is natural choice because it is simple in design and commonly available in market but has disadvantage of higher rating. Zig-zag transformer has the lowest rating followed by T-connected transformer and with highest rating for star/delta and star/hexagon transformers. The converter topological considerations such as 3P3W, 3P4W, isolated and nonisolated, with and without transformer are critical. The comparative study provided

Table 5 Comparison of DSTATCOM control techniques. Sr. no.

1 2 3 4 5

Attributes

Performance of control technique

Reactive power compensation Harmonic mitigation Load balancing Source neutral current elimination Computational complexity

IRP

SRF

SC

AUPF

PI Controller

NN

SMC

Partial Good Excellent Excellent High

Good Good Average Good Average

Excellent Better Excellent Good Simpler

Excellent Excellent Good Good High

Partial Good Good Average Average

Good Good Excellent Good Simpler

Excellent Excellent Excellent Good High

Table 6 Comparison of transformers used for neutral current compensation. Type of transformer

Winding voltage (V)

Winding current (A)

kVA

Zig-zag

Vl Vl 3 : 3 V ffiffi l p : pVffiffi3l 3 V l pffiffi : Vplffiffi : Vplffiffi, 3 2 3 2 3 Vl Vl 2 : 2 V ffiffi l p : pVffiffi3l : pVffiffi3l 3

I n I n I n

V l In 3 Vpl Iffiffin 3 1 ffiffi 1 ð3 p þ ÞV l In 3 6

I n

Vpl Iffiffin 3

Star-delta T-connected

Starhexagon

Number of Transformers (Nos)

Space Is it a standard requirement transformer

Is it induce circulating currents in the Cost of the secondary winding compensator

3

Low

No

No

Low

3

High

Yes

Yes

High

Lowest

No

No

Lowest

Highest

No

Yes

Highest

11

3

Vl – line-to-line voltage; I=n – neutral current.

Table 7 Advantages and disadvantages of transformers used for neutral current compensation. Type of transformer

Advantages

Disadvantages

Zig-zag

It provides passive compensation, rugged, and less complex over the active compensation techniques. It has the advantage of reduction in load unbalance and reducing the neutral current on the source side. It has lowest kVA rating. Easily available in market, simple design, less costly. Star connected primary winding offers low impedance path for zero sequence currents. The delta connected secondary winding provides a path for the induced zero sequence currents to circulate. Transformer is small in floor space, low in height, and with a lower weight than any other types of transformers. It uses two single-phase transformers which make the core economical to build and easy to assemble. It can be regarded as open-circuit for the positive and negative sequence currents, hence current flowing through the transformer is only zero-sequence component. Star connected primary winding provides a low impedance path for the zero-sequence harmonic currents and hexagon connected secondary winding provides a path for the induced zero sequence currents. It can reduce zero sequence harmonic current to a large extent.

Performance of zig-zag transformer is dependent on the location close to the load. Performance of reducing the neutral current on the source side is affected during the conditions of distorted and unbalanced voltages.

Star-delta

T-connected

Star-hexagon

Its compensation characteristics depend on the impedance of the transformer, location, and source voltage. It will not completely compensate for the neutral current. High kVA rating. Ratings of the transformer depends on the amount of the load imbalance and harmonic content. Its compensation characteristics depend on the imbalance of the transformer, location, and source voltage. Impedance offered for the zerosequence current is a function of the zero-sequence impedances of the utility system.

It will not completely compensate for the zero sequence currents. Its compensation characteristics depends on the impedance of the transformer, location, and source voltage. It has complex design, high cost, and not easily available. It has highest kVA rating.

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in Tables 1–4 is greatly helpful for topological considerations of the DSTATCOM for particular application.

6. Future work The DSTATCOM is very much effective for improvement of both voltage and current related PQ problems such as harmonic elimination, load balancing, voltage regulation, power factor correction and neutral current compensation in distribution system. However, the present day cost of DSTATCOM is on higher side which is main hurdle for its implementation in the system. Therefore, it is highly desirable to carry out extensive research to reduce the cost of DSTATCOM without affecting the efficiency and effectiveness in PQ improvement capability. Renewable energy (RE) penetration into the electric utility grid is increasing day by day and intermittent nature of these resources affects the quality of supplied power. The weather conditions such as wind speed variations and variable solar insolation affect the power output of RE sources. The DSTATCOM may be an effective solution for these problems, hence possibilities of implementation of DSTATCOM in RE based power system are required to be explored.

7. Conclusion A comprehensive literature review of the DSTATCOM is carried out. This paper presents a detailed survey on the topic of DSTATCOM used for PQ improvement in distribution system. Topologies used in both 3P3W and 3P4W distribution systems are analysed critically and a comparative study of different types of topologies is presented. The control techniques such as IRP, SRF, SC, PI controller, SMC, NN, and AUPF are analysed and their performance is presented. A comparative study of transformers used in the DSTATCOM topologies is also presented. Selection considerations of DSTATCOM topologies and control techniques for specific applications have also been outlined. Finally, at the end of paper future scope for research to enhance the performance and suitability of DSTATCOM for specific purpose is presented. According to the developed review, it can be concluded that DSTATCOM is an effective tool for PQ improvement in distribution system. The commonly used DSTATCOM topologies are isolated and nonisolated 3P3W, isolated two-leg and three-leg 3P4W, nonisolated three-leg/two-leg with and without transformer. The commonly used control techniques are SRF, IRP, SC, PI controller, SMC, NN, and AUPF. A comparative study presented will helps the users in selecting the particular topology and control technique of DSTATCOM that suits for specific application. It is hoped that this review on DSTATCOM will be beneficial to the users, designers, manufacturers, researchers and power engineers for enhancing the quality of power. References [1] Ghosh A, Ledwich G. Load compensating dstatcom in weak ac systems. IEEE Trans Power Deliv 2003;18(4):1302–9. http://dx.doi.org/10.1109/ TPWRD.2003.817743. [2] Basak P, Chowdhury S, nee Dey SH, Chowdhury S. A literature review on integration of distributed energy resources in the perspective of control, protection and stability of microgrid. Renew Sustain Energy Rev 2012;16 (8):5545–56. http://dx.doi.org/10.1016/j.rser.2012.05.043 URL 〈http://www. sciencedirect.com/science/article/pii/S1364032112003772〉. [3] Mahela OP, Shaik AG, Gupta N. A critical review of detection and classification of power quality events. Renew Sustain Energy Rev 2015;41(0):495–505. http://dx.doi.org/10.1016/j.rser.2014.08.070 URL 〈http://www.sciencedirect. com/science/article/pii/S1364032114007564〉. [4] Ghosh A, Joshi A. The concept and operating principles of a mini custom power park. IEEE Trans Power Deliv 2004;19(4):1766–74. http://dx.doi.org/ 10.1109/TPWRD.2003.822541.

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