Simulation of STATCOM based control scheme for grid connected wind powersystem.

K.NAGARAJU1, B.RAMESH BABU2 ,1Assistant Professor Dept. of EEE, Geethanjali College of Engineering, Hyderabad, T.S, India,

2Assistant Professor Dept. of EEE, Geethanajali College of Engineering, Hyderabad, T.S, India,

effect of turbulence, wind shear and tower-shadow and of

Abstract: [1]. The integration of wind energy into existing power

A Power quality problem is an occurrence of control system in the power system.

nonstandard voltage, current or frequencythat results in a failure or a disoperation of end

Thus, the network system presents a technical challenges

sensitive industrial loads and critical commercialregulation, stability, power issues can be

operations suffer from various types of outages and viewed with respect to the wind generation, qualityservice interruptions which can cost significant problems. The power quality is an essential transmissionfinancial losses. With the increase in load demand, and distribution network, such as voltage customer-the Renewable Energy Sources (RES) are focused measure and is greatly affected by the sag, swells,increasingly connected in the distribution systems flickes, harmonics etc. However the wind operation of awhich utilizes power electronic Converters/Inverters distribution and transmission network. generatorin cups (custom power devices). This paper presents introduces disturbances into the distribution The issue ofnovel power electronics for the integration of wind power quality is of great importance to the network. Oneand photovoltaic (PV) control strategy for achieving of the simple methods

maximum benefits from these grid-interfacing of running a wind turbine [2]. There has been an extensive

inverters using cascaded current voltage control growth and generating system is to use the induction

strategy is proposed for inverters to simultaneously generator quick development in the exploitation of wind

improve the power quality of the inverter local load energy in connected directly to the grid system. The

voltage and the current exchanged with the grid induction recent years. The individual units can be oflarge capacity generator has inherent advantages of cost

INTRODUCTION up to 2 MW, feeding into distribution network,

effectiveness and robustness. However; induction

To have sustainable growth and social progress, it generators require reactive power for magnetization.

successfully operating all over the world. In the fixed is When from the wind velocity and generator torque. The

necessary to meet the energy need by utilizing the speed voltage the generated active power of an induction

wind turbine operation, all the fluctuation in the generator is varied due to wind, absorbed reactive power

renewable energy resources like wind, biomass, hydro, and terminal voltage of an induction generator can be

co- wind speed are transmitted as fluctuations in the significantly affected. In the event of increasing grid

generation, etc In sustainable energy system, energy disturbance, a battery energy storage system for wind

mechanical torque, electrical power on the grid and leads energy generating system is generally required to

conservation and the use of renewable source are the key compensate the fluctuation generated by wind turbine. A

to large voltage fluctuations. During the normal operation, STATCOM based control

paradigm. The need to integrate the renewable energy likewind turbine produces continuous variable output windenergy into power system is to make it possible to power.These power variations are mainly caused by theminimize the environmental impact on conventional plant

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user equipments. Utility distribution networks,and that requires needs to manage for suchfluctuations. The power quality consideration of voltage

power factor at the source side. Reactive powersupport only from STATCOM to wind Generatorand Load. Simple bang-bang controller forSTATCOM to achieve fast dynamic response.The Increasing number of renewable energysources and distributed generators requires newstrategies for the operation and management ofthe electricity grid in order to maintain or even toimprove the power-supply reliability and quality.In addition, liberalization of the grids leads to newmanagement structures, in which trading ofenergy and power is becoming increasinglyimportant. The power-electronic technology playsan important role in distributed generation and inintegration of renewable energy sources into theelectrical grid, and it is widely used and rapidlyexpanding as these applications become moreintegrated with the grid-based systems. During thelast few years, power electronics has undergone afast evolution, which is mainly due to two factors.The first one is the development of fastsemiconductor switches that are capable ofswitching quickly and handling high powers. Thesecond factor is the introduction of real-timecomputer controllers that can implement advancedand complex controlalgorithms. These factors together have led to thedevelopment of cost-effective and grid-friendlyconverters. In this paper, new trends in power-electronic technology for the integration ofrenewable energy sources and energy-storagesystems are presented. This paper is organized asfollows.In Section II, we describe the current technologyand future trends in variable-speed wind turbines.Wind energy has been demonstrated to be bothtechnically and economically viable. It is expectedthat current developments in gearless energytransmission with power-electronic grid interfacewill lead to a new generation of quiet, efficient,and economical windturbines. In Section III, we present power-conditioning systems used in grid-connectedphotovoltaic (PV) generation plants. Thecontinuously decreasing prices for the PV

modules lead to the increasing importance of costreduction of the specific PV converters. Energystorage in an electricity generation and supplysystem enables the decoupling of electricitygeneration from demand.In other words, the electricity that can beproduced at times of either low-demand low-generation cost or from intermittent renewableenergy sources is shifted in time for release attimes of high-demand high-generation cost orwhen no other generation is available.Appropriate integration of renewable energysources with storage systems allows for a greatermarket penetration and results in primary energyand emission savings. In Section IV, we presentresearch and development trends in energy-storage systems used for the grid integration ofintermittent renewable energy sources.

PV TECHNOLOGY

This section focuses on the review of the recentdevelopments of power-electronic converters andthe state of the art of the implemented PVsystems. PV systems as an alternative energyresource or an energy-resource complementary inhybrid systems have been becoming feasible dueto the increase of research and development workin this area. In order to maximize the success ofthe PV systems, a high reliability, a reasonablecost, and a user-friendly design must be achievedin the proposed PV topologies. Several standardsgiven by the utility companies must be obeyed inthe PV-module connection. Nowadays, thestandards EN61000-3-2 , IEEE1547, and the U.S.National Electrical Code (NEC) 690 , and thefuture international standard (still a CommitteeDraft for Vote- CDV) IEC61727 are beingconsidered. These standards deal with issues likepower quality, detection of islanding operation,grounding, etc. They define the structure and thefeatures of the present and future PV modules.

A. Market ConsiderationsSolar-electric-energy demand has grownconsistently by 20% 25% per annum over the

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past 20 years, which is mainly due to thedecreasing costs and prices. This decline has beendriven by1) an increasing efficiency of solar cells;2) manufacturing-technology improvements; and3) economies of scale.

In 2001, 350 MW of solar equipment was sold toadd to the solar equipment already generating aclean energy. In 2003, 574 MW of PV wasinstalled. This increased to 927 MW in 2004. TheEuropean Union is on track to fulfilling its owntarget of 3 GW of renewable electricity from PVsources for 2010, and in Japan, the target is 4.8GW. If the growth rates of the installation of PVsystems between 2001 and 2003 could bemaintained in the next years, the target of the

European Commission s White Paper for a

Community Strategy and Action Plan onRenewable Sources of Energy would already beachieved in 2008. It is important to notice that thePV installation growth-rate curve in the EuropeanUnion exactly mirrors that of wind power, with adelay of approximately 12 years. This factpredicts a great future for PV systems in thecoming years.B. Design of PV-Converter Families

An overview of some existing power invertertopologies for interfacing PV modules to the gridis presented. The approaches are further discussedand evaluated in order to recognize the mostsuitable topologies for future PV converters, and,finally, a conclusion is given. Due to advances intransistor technology, the inverter topologies havechanged from large thyristor-equipped gridconnected inverters to smaller IGBT-equippedones. These transistors permit to increase thepower switching frequency in order to extractmore energy and fulfill the connecting standards.One requirement of standards is that the invertersmust also be able to detect an islanding situationand take appropriate measures in order to protectpersons and equipment . In his situation, the gridhas been removed from the inverter, which thenonly supplies local loads. This can be troublesomefor many high-power transformer less systems,

since a single phase inverter with a neutral-to-linegrid connection is a system grounded on the gridside. In general, PV cells can be connected to thegrid (grid connection application), or they can beused as isolated power supplies. Severalclassifications of converter topologies can be donewith respect to the number of power processingstages, location of power-decoupling capacitors,use of transformers, and types of grid interface.However, before discussing PV convertertopologies, three designs of inverter families aredefined: central inverters, module-oriented ormodule-integrated inverters, and string inverters.The central converters connect in parallel and/orin series on the dc side. One converter is used forthe entire PV plant (often divided into several

units organized in master slave mode). The

nominal power of this topology is up to severalmegawatts. The module-oriented converters withseveral modules usually connect in series on thedc side and in parallel on the ac side. The nominalpower ratings of such PV power plants are up toseveral megawatts. In addition, in the module-integrated converter topology, one converter perPV module and a parallel connection on the acside are used. In this topology, a central measurefor main supervision is necessary. Although thistopology optimizes the energy yield, it has a lowerefficiency than the string inverter. This conceptcan be implemented for PV plants of about 50

100 W. The multistring topology permits theintegration of PV strings of different technologiesand orientations (north, south, east, and west).C. PV Topologies

Conventionally, a classification of PV topologiesis divided into two major categories: PV inverterswith dc/dc converter (with or without isolation)and PV inverters without dc/dc converter (with orwithout isolation). The isolation used in bothcategories is acquired using a transformer that canbe placed on either the grid or low frequency (LF)side or on the HF side. The line-frequencytransformer is an important component in thesystem due to its size, weight, and price. The HFtransformer is more compact, but special attention

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must be paid to reduce losses . The use of atransformer leads to the necessary isolation(requirement in U.S.), and modern inverters tendto use an HF transformer. However, PV inverterswith a dc/dc converter without isolation areusually implemented in some countries wheregrid-isolation is not mandatory. Basic designsfocused on solutions for HF dc/dc convertertopologies with isolation such as full-bridge orsingle-inductor push pull permit to reduce thetransformer ratio providing a higher efficiencytogether with a smoother input current. However,a transformer with tap point is required. In

addition, a double-inductor push pull is

implemented in other kind of applications(equivalent with two interleaved boost convertersleading to a lower ripple in the input current), butextra inductor is needed. A full-bridge converteris usually used at power levels above 750 W dueto its good transformer utilization. Anotherpossible classification of PV inverter topologiescan be based on the number of cascade powerprocessing stages. The single-stage inverter musthandle all tasks such as maximum-power-point-tracking (MPPT) control, grid-current control, andvoltage amplification. This configuration, whichis useful for a centralized inverter, has somedrawbacks because it must be designed to achievea peak power of twice the nominal power.Another possibility is to use a dual-stage inverter.In this case, the dc/dc converter performs theMPPT (and perhaps voltage amplification), andthe dc/ac inverter is dedicated to control the gridcurrent by means of pulse width modulation(PWM), space vector modulation (SVM), or

bang bang operation. Finally, multistage inverters

can be used, as mentioned above. In this case, thetask for each dc/dc converter is MPPT and,normally, the increase of the dc voltage. Thedc/dc converters are connected to the dc link of acommon dc/ac inverter, which takes care for thegrid-current control. This is beneficial since abetter control of each PV module/string isachieved, and that common dc/ac inverter may bebased on a standard variable speed- drive (VSD)

technology. There is no any standard PV invertertopology. Several useful proposed topologies havebeen presented, and some good studies regardingcurrent PV inverters have been done . The currentcontrol scheme is mainly used in PV inverterapplications. In these converters, the current intothe stage is modulated/controlled to follow arectified sinusoidal waveform, and the task for thecircuit is simply to recreate the sine wave andinject it into the grid. The circuits apply zerovoltage switching (ZVS) and zero-currentswitching (ZCS). Thus, only conduction losses ofthe semiconductors remain. If the converter hasseveral stages, power decoupling must beachieved with a capacitor in parallel with the PVmodule(s). The current control scheme isemployed more frequently because a high-powerfactor can be obtained with simple controlcircuits, and transient current suppression ispossible when disturbances such as voltagechanges occur in the utility power system. In thecurrent control scheme, operation as an isolatedpower source is difficult, but there are noproblems with grid interconnection operation.D. Future Trends

The increasing interest and steadily growingnumber of investors in solar energy stimulatedresearch that resulted in the development of veryefficient PV cells, leading to universalimplementations in isolated locations . Due to theimprovement of roofing PV systems, residentialneighborhoods are becoming a target of solarpanels, and some current projects involveinstallation and setup of PV modules in highbuilding structures. PV systems withouttransformers would be the most suitable option inorder to minimize the cost of the total system. Onthe other hand, the cost of the grid-connectedinverter is becoming more visible in the totalsystem price. A cost reduction per inverter watt is,therefore, important to make PV-generated powermore attractive. Therefore, it seems thatcentralized converters would be a good option forPV systems. However problems associated withthe centralized control appear, and it can be

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difficult to use this type of systems. An increasinginterest is being focused on ac modules thatimplement MPPT for PV modules improving thetotal system efficiency. The future of this type of

topologies is to develop plug and play systems

that are easy to install for non expert users. Thismeans that new ac modules may see the light inthe future, and they would be the future trend inthis type of technology. The inverters mustguarantee that the PV module is operated at themaximum power point (MPP) owing to useMPPT control increasing the PV systemsefficiency. The operation around the MPP withouttoo much fluctuation will reduce the ripple at theterminals of the PV module. Therefore, thecontrol topics such as improvements of MPPTcontrol, THD improvements, and reduction ofcurrent or voltage ripples will be the focus ofresearchers in the years to come . These topicshave been deeply studied during the last years, butsome improvements still can be done using newtopologies such as multilevel converters.

Power Quality Standards, Issues And it s

Consequences International Electro TechnicalCommission Guidelines:

The guidelines are provided for measurement ofpower quality of wind turbine. The Internationalstandards are developed by the working group ofTechnical Committee- 88 of the InternationalElectro-technical Commission(IEC)[15], IEC standard 61400-21, describes theprocedure for determining the power qualitycharacteristics of the wind turbine [4-10]. Thestandard norms are specified. IEC 61400-21:Wind turbine generating system, part-21.Measurement and Assessment of Powerquality characteristic of grid connected wind

Turbine. IEC 61400-13: Wind Turbine

measuring procedure in determining the powerbehavior. IEC 61400-3-7: Assessment of emissionlimit for Fluctuating load IEC 61400-12: WindTurbine Performance. The data sheet withelectrical characteristic of wind turbine provides

the base for the utility assessment regarding a gridconnection.Voltage Variation: The voltage variation issueresults variation is directly related to real andreactive power variations. The voltage variation iscommonly classified as under:

Voltage Sag/Voltage Dips.Voltage Swells.Short Interruptions.Long duration voltage variation.

The voltage flicker issue describes dynamicvariations in the network caused by wind turbineor by varying loads. Thus the power fluctuationfrom wind turbine occurs during continuousoperation. The amplitude of voltage fluctuationdepends on grid strength, network impedance andphase-angle and power factor of the windturbines. It is defined as a fluctuation of voltage ina frequency 10 35 Hz. The IEC 61400-4-15

specifies a flicker meter that can be used tomeasure flicker directly.Harmonics: The harmonic results due to theoperation of power electronic converters. Theharmonic voltage and current should be limited tothe acceptable level at the point of wind turbineconnection to the network.To ensure the harmonic voltage within limit, eachsource of harmonic current can allow only alimited contribution , as per the IEC-61400-36guideline. The rapid switching gives a largereduction in lower order harmonic currentcompared to the line commutated converter, butthe output current will have high frequencycurrent and can be easily filter-out.Wind Turbine Location in Power System: Theway of connecting the wind generating systeminto the power system highly influences the powerquality. Thus the operation and its influence onpower system depend on the structure of theadjoining power network.Self Excitation of Wind Turbine Generating

System: The self excitation of wind turbinegenerating system (WTGS) with an asynchronousgenerator takes place after disconnection of windturbine generating system (WTGS) With local

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load. The risk of self excitation arises especiallywhen WTGS is equipped with compensatingcapacitor. The capacitor connected to inductiongenerator provides reactive power compensation.However the voltage and frequency aredetermined by the balancing of the system. Thedisadvantages of self excitation are the safetyaspect and balance between real and reactivepower [5].GRID COORDINATION RULE American WindEnergy Association (AWEA) led the effort todevelop its own grid code for stable operation asper IEC-61400-21 for the interconnection of windplants to the utility systems, after the block out in

United State in August 2003. According to these, operatorof transmission grid is responsible for the organizationand operation of interconnected system.

1) Voltage rise (u) The voltage rise at the point ofcommon coupling can be approximated as a functionof maximum apparent power Smax of the turbine, thegrid impedances R and X at the point of commoncoupling and the phase angle , given in Eq. 1.

Where voltage rise,Smax max. apparent power,

phase difference,U nominal voltage of grid.

The Limiting voltage rise value is <2 %

2) Voltage dips (d) The voltage dips is due to startup ofwind turbine and it causes a sudden reduction ofvoltage. It is the relative % voltage change due toswitching operation of win turbine. The decrease ofnominal voltage change is given in Eq. 2.

D = Ku sn/sk (2)

Where d is relative voltage change, sn is ratedapparent power, sk is short circuit apparent power,and Ku is sudden voltage reduction factor. Theacceptable voltage dips limiting value is <3%.

3) Flicker The measurements are made for maximumnumber of specified switching operation of wind

turbine with 10- min period and 2-h period arespecified, as given in Eq. 3.

P = k ) sn/sk (3)Where P Long term flicker.

c ( k ) Flicker coefficient The Limiting Value

of 2 h.

4) Harmonics The harmonic distortion is assessed forvariable speed turbine with a electronic powerconverter at the point of common connection. Thetotal harmonic voltage distortion of voltage is given

Where Vn is the nth harmonic voltage and V1 is thefundamental frequency (50) Hz.

5) GRID FREQUENCY The grid frequency in India isspecified in the range of 47.5 51.5 Hz, for wind farmconnection.

TOPOLOGY FOR POWER QUALITYIMPROVEMENTThe STATCOM based current control voltage sourceinverter injects the current into the grid will cancel out thereactive part and harmonic part of the load and inductiongenerator current, thus it improves the power factor andthe power quality. To accomplish these goals, the gridvoltages are sensed and are synchronized in generating thecurrent. The proposed grid connected system isimplemented for power quality improvement at point ofcommon coupling (PCC), for grid connected system inFig. 1

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=R s si (1)

as in Eq. 4.

= =

(4)

=

The THD limit for 132 KV is < 3%.

(5)

A.WIND ENERGY GENERATING SYSTEM: Inthis configuration, wind generations are based onconstant speed topologies with pitch controlturbine The induction generator is used in theproposed scheme because of its simplicity, it doesnot require a separate field circuit, it can acceptconstant and variable loads, and has naturalprotection against short circuit. The availablepower of wind energy system is presented as under

area swept out by turbine blade, V wind is the windspeed in mtr/s. It is not possible to extract allkinetic energy of wind, thus it extract a fraction ofpower in wind, called power coefficient Cp of thewind turbine, and is given in Eq.

type and operating condition of wind turbine. Thiscoefficient can be express as a function of tip speedratio and pitch angle. The mechanical power

Where R is the radius of the blade (m).

B.STATCOM STATIC SYNCHRONOUSCOMPENSATOR The STATCOM (or SSC) is ashunt-connected reactive-power compensationdevice that is capable of generating and/ orabsorbing reactive power and in which the outputcan be varied to control the specific parameters ofan electric power system. In general it is solid stateswitching converter which is capable of generatingor absorbing independently controllable real andreactive power at its output terminals when it is fedfrom an energy source at its input terminals.Specifically, the STATCOM considered in this is avoltage-source converter from a given input of dcvoltage produces a set of 3-phase ac-outputvoltages, each in phase with and coupled to thecorresponding ac system voltage through leakage

reactance. The dc voltage is provided by anenergy-storage capacitor.A STATCOM can improve power-system

performance in such areas as the following: 1. Thedynamic voltage control in Transmission anddistribution systems; 2. The power-oscillationdamping in power transmission systems; 3. Thetransient stability; 4. The voltage flicker control;and 5. It also controls real power in line when it isneeded.Advantages1) It occupies small areas.2) It replaces the large passive banks and circuit

elements by compact converters.3) Reduces site work and time.4) Its response is very fast.

BESS-STATCOM: The battery energy storagesystem (BESS) is used as an energy storageelement for the purpose of voltage regulation. TheBESS will naturally maintain dc capacitor voltageconstant and is best suited in STATCOM since itrapidly injects or absorbed reactive power tostabilize the grid system. It also control thedistribution and transmission system in a very fastrate. When power fluctuation occurs in the system,the BESS can be used to level the powerfluctuation by charging and discharging operation.The battery is connected in parallel to the dccapacitor of STATCOM.The STATCOM is a three-phase voltage sourceinverter having the capacitance on its DC link andconnected at the point of common coupling. TheSTATCOM injects a compensating current ofvariable magnitude and frequency component atthe bus of common coupling.System Operation: The shunt connectedSTATCOM with battery energy storage isconnected with the interface of the inductiongenerator and non-linear load at the PCC in thegrid system. The STATCOM compensator outputis varied according to the controlled strategy, so asto maintain the power quality norms in the gridsystem. The current control strategy is included inthe control.VI. SYSTEM PERFORMANCE

The proposed control scheme is simulatedusing SIMULINK in power system block set. The

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=

in Eq.6..

(6)

=

Where is the power coefficient, depends onCp

(7)

produce by wind turbine is given in Eq. 8.

=(8)

system parameter for given system is given TableI. The system performance of proposed systemunder dynamic condition is also presented.

A. Voltage Source Current Control InverterOperation

The three phase injected current into the grid fromSTATCOM will cancel out the distortion causedby the nonlinearload and wind generator. The IGBT based three-phase inverter is connected to grid through thetransformer. The generation of switching signalsfrom reference current is simulated withinhysteresis band of 0.08. The choice of narrowhysteresis band switching in the system improvesthe current quality. The control signal of switchingfrequency within its operating band, as shown in

Fig. 4. The choice of the current band depends onthe operatingvoltage and the interfacing transformer impedance.The compensated current for the nonlinear loadand demanded reactive power is provided by theinverter. The real power transfer form the battery isalso supported by

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A. Power Quality Improvement

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