Power quality has become animportant topic to electricity consumers at all levels of usage. Nowadays thedemand for receiving the high quality electrical energy is being increasing asconsumer wants not only reliable but also quality power. Power quality can beimproved in distributed system by using shunt compensation device known asDistribution Static Compensator (DSTATCOM).
In this paper, three differentsystem topologies & control strategies for Distribution Static Compensators(DSTATCOMs) are modeled and tested using Simulink’ SimPowerSystem Toolbox forpower system quality studies. Simulation tests on a distribution system,equipped with the unbalanced and non-linear load. The modeled DSTATCOMtopologies can be used to develop and test different, control strategies andmethods for the DSTATCOM. Thisis intended to present a broad perspective on the status of DSTATCOM technologyto researchers dealing with compensation of PQ problems in distribution systems IntroductionThe development of technology andconsequent up gradation of the loads of power system has brought about aparadigm change in the customer’s outlook for the electrical power he iswilling to receive. Adding to the problem of reactive power compensation, theproliferation of nonlinear loads is causing a higher level of harmonics in thereceived voltage. An alert customer now asks for a power supply that is voltageregulated, balanced, flickers free, without harmonics and without any outages.Of the many CP devices available, DSTATCOM can solve most of the customer’sload related power quality problems. The concept of compensation has itsgenesis to reactive power compensation, which initially was conceived withfixed or passive capacitors.
STATCOM systems are used in distribution andtransmission systems for different purposes. D-STATCOMs have faster responsewhen compared with transmission STATCOMs 11. Literature SurveyTheconcept of D-STATCOM was disclosed by Gyugyi 3 in 1976.
Instead of directlyderiving reactive power from the energy-storage components, the D-STATCOMbasically circulates power with the connected network. The reactive componentsused in the D-STATCOM, therefore, can be much smaller than those in the SVC.Avast literature (Ghosh and Ledwich, 2002; Mathur, 1984; Miller, 1982 4) hasappeared on electric power quality problems and many solutions have beensuggested to improve power quality.
A number of compensators have been reportedfor load balancing using lossless passive elements (L and C) and activeelements (solid state CSI and VSI). Simulation work on some FACTS devices hasbeen performed using PSCAD, EMTDC (Lara and Acha, 2002), MATLAB (Giroux et al.,2001) 5, SABER (Muni et al., 2003), etc. Some experimental work (Clouston andGurney, 1999; Ekanayake et al., 1995; Jung et al., 2002 6; Muni et al.
, 2003)is reported on prototype D-STATCOM models for line voltage regulation, voltageflicker mitigation and power quality improvement. Operationand control of a D-STATCOM in the presence of non-integer harmonics is alsoreported (Ghosh et al., 2003) 7. This paper attempts to highlight the behaviorof D-STATCOM connected to balance unbalanced linear as well as non-linearloads.Variouspossible configuration of DSTATCOM (depending upon the particular application)is also reported in literature bhimsingh , mahela et al ranges from 3 phase 3wire or 3 phase 4 wire system, isolated or non-isolated VSC, with transformeror without transformer connection to the grid, type of transformer used i.estar/delta, zigzag, star/hexagon, T connected transformer etc. Many controltechniques are reported such as instantaneous reactive power theory (Akagi etal., 1984), power balance theory, etc.
an indirect current control technique(Singh et al., 2000), hysteresis current controller (Bhim Singh et al ,1998) isemployed to obtain gating signals for the Insulated Gate Bipolar Transistor(IGBT) devices used in current controlled voltage source inverter (CC-VSI)working as a D-STATCOM. Some authors have considered biologically inspiredmethods of tuning controllers, such as Safari et al., who considered using honeybee mating optimization (HBMO) in order to ?nd an optimal controller gains andcompared results for controllers tuned using a genetic algorithm (GA).Kumaravel and Kumar use bats echolocation algorithm to tune STATCOM PIcontrollers.
The authors note the importance of system impedance, particularlythe variable load impedance, on the controller gains. As with the GA and HBMOcontroller tuning methods, the echolocation method requires evaluation of alarge number of possible controllers which is a computationally intensiveprocess. A paper by Wang presents a model of a power system with a STATCOMinstalled based on the 8Phillips-Heffron model, originally proposed formodeling of synchronous generators. Wang’s paper is of interest for threereasons. First, the model presented shows that the STATCOM and power systembehavior are both non-linear and highly interdependent.
Second, it can be seenthat the interactions between the STATCOM and power system are highly dependenton the system operating point and that the modeling relies on linearization ofequations about an assumed operating point.Thereviewed literature has revealed that there is no standard way in which todetermine the controller gains for a STATCOM. The high level of interdependencybetween multiple system variables and the variability of performance atdifferent operating points makes the design of a generic STATCOM voltagecontroller difficult. The use of particle swarm optimization and similarmethods has been proposed because of their usefulness in ?nding workingsolutions from a large initial set of options; however, these methods arecomputationally intensive and therefore may take a very long time to completeand this may also make them unsuitable for use in an embedded system.
C. Rose et.al9 in his paper explores the impedance sensitive STATCOM control for systemssupported by renewable generation. There are many possible reasons for changesin the system impedance. Impedance estimation has proved useful in identifyingsuch changes in system con?guration.
The use of on-line impedance estimation inorder to tune the STATCOM controller has therefore been considered as analternative to using ?xed controller gains based on the limited informationprovided by one-off measurements.Anincreasing the level of penetration of SPV (solar photo voltaic) systems in EPS(electrical power system) applies additional amount of stress on utilitydevices and systems arising complications such as difficulty in voltageregulation, hampering the standard of power delivered, malfunction operation ofprotective equipment, variation in reactive power transfer and posing threat tosecurity, safety and reliability of the grid 3. To integrate distributedgeneration systems (such as wind, PV, etc) with reactive power compensation andharmonics elimination capabilities to grid at PCC, various converter topologies11-12 and advanced control algorithms 13 have been reported in theliterature.
To attain an efficient SPV conversion system, sometimes additionalstages of converter are introduced before a converter for extracting maximumpower from a SPV system as well as to maintain required voltage level at DClink of the converter. A comparative study is reported in between a singlestage and dual-stage converter topologies, which confirms that a single-stagetopology is better for a three-phase network in terms of reduced power losses,cost, system size and efficiency. Moreover, Varma et. al. have demonstrated aPV-STATCOM system which improves the transmission transferring capabilitiesusing STATCOM while transferring active power to grid from same converter.Singh et. al.
have demonstrated and implemented a SPV grid tied system withDSTATCOM capabilities and advanced control schemes thereby improving powerquality issues in the distribution system. Mishra et. al. have implemented PVfed DSTATCOM using optimized control algorithm. Modeling of controller is to bedone using MATLAB software and performing the simulations helps to achieve abetter understanding of the overall system.
Much research confirms severaladvantages of DSTATCOM compared to other custom power devices. These advantagesinclude Size, weight, and cost reduction. – Equality of lagging and leadingoutput. – Precise and continuous reactive power control with fast response. -Possible active harmonic filter capability. Distribution Static Compensator(DSTATCOM) has the ability to overcome the problem of limited bandwidth, higherpassive element count which causes increased size and losses, and slower responseof Static Var Compensators (SVC) and it is done by precise control and fastresponse during transient and steady state, with lower foot print and weight.The DSTATCOM has emerged as a promising device to provide solution for voltagerelated issues and also serving a host of other current related power qualityproblem’s solutions such as voltage regulation, load balancing, reactive powercompensation, power factor correction & improvement and current harmoniccontrol 2.
In this paper, various topologies and different control techniquesof DSTATCOM is demonstrated for voltage regulation or power factor correctionby reactive power compensation along with harmonics elimination and loadbalancing. 1 . CLASSIFICATION OFDSTATCOM TOPOLOGYThe DSTATCOM can be classified onthe bases of different topologies, number of switching devices and on the basesof neutral current compensation etc. These DSTATCOMs are developed to meet therequirements of different applications in distribution system. Converter BasedClassification . DSTATCOM utilizes either a voltage-source inverter (VSI) or acurrent-source inverter (CSI). Voltage source inverter use capacitive energystorage, while Current source inverter use inductive energy storage in theirrespective dc links for voltage and current. However, the voltage sourceinverters are broadly used because of the less heat dissipated, smaller size,and the less cost of the capacitor compared to the inductor, used in the CSI,for the same power rating 4.
The VSI connected in shunt with the AC systemprovides multifunctional topology which can be used for different aims such asvoltage regulation and compensation of reactive power, correction of powerfactor, and elimination of current harmonics. Voltage source inverter (VSI)topology is popular because it can be expandable to multilevel, multistep chain topology to enhance the performance with lower switching frequencyand increased power handling capacity. Various multilevel topologies are Diodeclamp multilevel inverter, Cascaded H-bridge & Flying capacitor multilevelinverter. 1.1 Cascade H -bridgemultilevel inverter: In Cascaded H-bridge invertersseparate dc sources (SDC’s) are introduced. This new converter can avoid extraclamping diodes or voltage balancing capacitors.
Fig1 Shows the basicarrangement of the 5-level cascaded-inverters with SDC’s, shown in a singlephase configuration. Each SDC is related with a single phase full-bridgeinverter. To synthesize a multilevel waveform, the ac output of each of thedifferent level H- bridge cells is connected in series.
The number of outputphase voltage levels in a cascaded inverter is defined by m = 2H +1, where H =no. of H-bridges; while the relation between phase voltage and line voltages issame as diode –clamped inverter. Fig1 1.2.
Flying capacitormultilevel inverter: A quite well-known topology ofmultilevel inverter is Flying Capacitor Multilevel Inverter. It is quitesimilar to diode clamped multilevel inverter. The capacitor has to be pre-chargedin this type of multilevel inverter. The topology consists of diodes,capacitors and switching devices as shown in fig.
This has been designed onlyup to six levels of voltage because of the practical restrictions. Each legconsists of switching devices which are in general transistors. Every inverterlimb consists of cells connected in inward nested series.
Every cell has twopower switches and a single capacitor. Power switch is a combination of atransistor connected with an anti-parallel diode. Unlike diode clampedinverter, this topology uses capacitors for clamping.
An inverter with N cellwill have 2N switches and N+1 different voltage levels including zero. We canalso have negative voltage levels, and so all in all we can say that N cellmultilevel inverter can give 2N+1 voltage levels. The voltage level isdecreased as we move towards the load. The number of level depends upon thenumber of conducting switches in each limb. It is also known as Imprecated CellInverter. Since the capacitors floats with respect to earth’s potential, theyare called Flying Capacitor Fig2 1.3.
Three phase fourleg VSI topology : VSI with four legs that are used and requires only one dc storage unit.Three of its legs are used for phase connection while the fourth leg isconnected to the load neutral and the supply neutral, if available, through areactance 6-8. The reference current for the fourth leg is the negative sumof three phase load currents. This nullifies the effect of dc component of loadcurrent. To maintain the adequate charge on dc-side capacitor a PI regulator isused to control the flow of real power from ac side towards dc side of theconverter.
When the compensator is working, zero sequence current is routed topath n-n’ containing switching frequency harmonics. Using fourth leg ofinverter, the negative of zero sequence current – iois is tracked. Certainly itneeds a higher bandwidth VSI to track negative of neutral current (- io) as iocontains harmonics due to non-linear loads.
This increases the switchinglosses. If this current is not tracked properly, it will leave high switchingfrequency current components in the N-n path, which is not desirable. Theadvantage of the topology is that it requires one less capacitor. 1.4. Neutral-ClampedThree-Phase VSI topology: This topology consist of a choppercircuit which is represented by the switches Schl and Sch2 , a diode Dch1 andDch2 in parallel and inductance and resistance which are denoted by Lp and Rp.The purpose of this chopper circuit is to balance the voltages in the twocapacitors as shown in figure3(c).
Let the voltage across Cdc1 be Vdc1 andvoltage across Cdc2 is Vdc2. Normally the two switches are left open and thustwo voltages Vdc1 and Vdc2 are equal. Now suppose there is a voltage drop inVdcl due to this there is rise of voltage in V dc2. Current is built up in theinductor Lp due to closing of switch Sch2 and once the current reaches adefinite level, the switch Sch2 is opened, hence the inductor current get dischargedthrough the diode Dchl to bring up the voltage Vdcl to the desired level. 2 . CLASSIFICATION OFDSTATCOM CONTROL STRATEGY The reactive power needed by the load isprovided by the DSTATCOM and only real power is supplied by the source suchthat source current remains at unity PF. Load balancing is achieved by makingreference source current balanced. It has real fundamental frequency componentof the load current and used to decide switching of the VSC and being extractedby control techniques.
Different control strategies are reported in theliterature such as IRP theory, SRF theory, Adaline-based control algorithm, PIcontroller for maintaining dc bus voltage. 2.1 Synchronous reference frame (SRF) basedcontrol strategy :SRF control technique is based ontransformation of current in synchronously rotating d-q frame. The voltagesignal is sensed and processed by phased lock loop (PLL) to generate sine andcosine signals as shown in figure 9.
The sensed current signal then aretransformed to d-q frame and filtered. After filtering the filtered currentsare back transformed to abc frame and fed to hysteresis currentcontroller for switching plus generation. The mathematical transformationequations. The currents generated in ?–? coordinates are transformed tod-qframe with the help of park’s transformation using ? as transformation angle 2.2Instantaneous p–q Theory :The control of DSTATCOM isimplemented on the basis of instantaneous reactive power theory (IRPT) or p-qtheory to calculate the desired compensation current. The block diagram for thecontrol using IRPT is shown in Figure-10. In this method, the sensedthree-phase PCC voltages and load currents are transformed into ?-?-o axisusing Clark’s transformation. In addition, the source must deliver nozero-sequence active power (so that the zero-sequence component of the voltageat the PCC does not contribute to the source power).
The reference sourcecurrents in the reference ?-?-o frame are converted to the abc frameusing the reverse Clark’s transformation 2.3 Neural Network Based Control System: In this control algorithm, there isrequirement of unit vector template corresponding to fundamental positivesequence component of current in phase with the phase voltage waveform. Forproper estimation of components of load current, a undistorted unit voltagetemplates can be represented by: Vp (t)=Usin?t (4) To generatesinusoid (sincot) vector template, synchronized with ac mains, the zerocrossing of phase voltage is detected in case of voltage being distorted. ip(t) =Wpvp(t) (5) where weight(Wp) is estimated using Adaline. The weight can be represented in terms ofvoltage and current given as: Wp=I1cos?/U(6) To maintainminimum error, the scheme for estimating weights corresponding to fundamentalfrequency real component of current (for three-phase system), based on LMSalgorithm tuned Adaline tracks the unit vector templates. RESULTS: Table1: Comparison of different control techniques CONCLUSIONS AND FUTUREWORK:The DSTATCOM is very effective for improvement of power quality (PQ) (Molinas, Suul and Undeland, Low voltage ride through of wind farms with cage generators: STATCOM versus SVC 2008) problems related toboth current and voltage such as load balancing, Harmonics elimination, powerfactor correction, voltage regulation and neutral current compensation indistribution system.
This paper presents a detailed survey on varioustopologies and control strategies of DSTATCOM used in both 3phase 3 wire and3phase 4 wire distribution systems. Comparative studies of various controlschemes are also presented and from the Table-1 we analyzed that IRP is muchcomplex then the other schemes but excellent in harmonics mitigation. For theload balancing, IRP and NN control schemes are preferred and for reactive powercompensation, SMC control scheme can be implemented. This comparative studywill helps the users in selecting the particular topology and control techniqueof DSTATCOM that suits for specific application. Currently research is going onto reduce the cost of DSTATCOM without affecting the efficiency andeffectiveness in PQ improvement capability.
. The reviewed literature hasrevealed that there is no standard way in which to determine the controllergains for a STATCOM. The high level of interdependency between multiple systemvariables and the variability of performance at different operating pointsmakes the design of a generic STATCOM voltage controller difficult. Thisresearch work mainly focuses on the SPV generation systems connected in thedistribution system.
Multifunctional PV-DSTATCOM system is proposed in thiswhich are classified depending on their connection to AC distribution system(single-phase or three-phase) and number of power conversion stages(single-stage or two-stage). Renewable energy (RE) penetration into theelectrical utility grid is increasing day by day and affects the quality ofsupplied power. The weather conditions such as variable solar insolation andwind speed variations affect the power output of RE sources.
The implementationof DSTATCOM in RE based power system is required to be explored as the DSTATCOMmay be an effective solution for these problems