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238 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 1, JANUARY/FEBRUARY 2011 Load Compensation for Diesel Generator-Based Isolated Generation System Employing DSTATCOM Bhim Singh, Fellow, IEEE, and Jitendra Solanki, Member, IEEE Abstract—This paper presents the control of distribution static synchronous compensator (DSTATCOM) for reactive power, harmonics and unbalanced load current compensation of a diesel generator set for an isolated system. The control of DSTATCOM is achieved usin
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  238 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 1, JANUARY/FEBRUARY 2011 Load Compensation for Diesel Generator-BasedIsolated Generation System Employing DSTATCOM Bhim Singh, Fellow, IEEE  , and Jitendra Solanki, Member, IEEE   Abstract —This paper presents the control of distribution staticsynchronous compensator (DSTATCOM) for reactive power, har-monics and unbalanced load current compensation of a dieselgenerator set for an isolated system. The control of DSTATCOMis achieved using least mean square-based adaptive linear ele-ment (Adaline). An Adaline is used to extract balanced positive-sequence real fundamental frequency component of the loadcurrent and a proportional–integral (PI) controller is used tomaintain a constant voltage at the dc-bus of a voltage-sourceconverter (VSC) working as a DSTATCOM. Switching of VSCis achieved by controlling source currents to follow referencecurrents using hysteresis-based PWM control. This scheme issimulated under MATLAB environment using Simulink andPSB block-set toolboxes for feeding linear and nonlinear loads.The modeling is performed for a three-phase, three-wire star-connected synchronousgeneratorcoupledtoadieselengine,alongwith the three-leg VSC working as a DSTATCOM. Results arepresented to verify the effectiveness of the control of DSTATCOMfor the load compensation and an optimal operation of the DG set.  Index Terms —Adaline, diesel generator set, distribution staticsynchronous compensator (DSTATCOM), harmonic elimination,load compensation. I. I NTRODUCTION I NSTALLATION OF the diesel engine-based electricity gen-eration unit (DG set) is a widely used practice to feed thepower to some crucial equipment in remote areas [1], [2].DG sets used for these purposes are loaded with unbalanced,reactive and nonlinear loads such as power supplies in sometelecommunication equipment and medical equipment. Thesource impedance of the DG set is quite high, and the un-balanced and distorted currents lead to the unbalanced anddistorted three-phase voltages at point of common coupling(PCC). Harmonics and unbalanced currents flowing throughthe generator result into torque ripples at the generator shaft.All of these factors lead to the increased fuel consumption andreduced life of the DG sets. These forces the DG sets to beoperated with derating, which results into an increased costof the system. Nowadays, small generator units are available Manuscript received February 25, 2010; revised May 10, 2010; acceptedMay 16, 2010. Date of publication November 9, 2010; date of current versionJanuary 19, 2011. Paper 2010-ESC-094.R1, presented at the 2006 InternationalConference on Power Electronics, Drives and Energy Systems for IndustrialGrowth, New Delhi, India, December 12–15, and approved for publicationin the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the EnergySystems Committee of the IEEE Industry Applications Society.The authors are with the Department of Electrical Engineering, IndianInstitute of Technology, New Delhi 110 016, India (e-mail: bhimsinghiitd@gmail.com; ejitendra@yahoo.com).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TIA.2010.2090847 with full conversion (inverter-converter) units to meet stringentpower quality norms [3]. Instead of using these, a DSTATCOM[2] can be used with a three-phase DG set to feed unbalancedloads without derating the DG set and to have the same costinvolved. For example, a 24-kW lagging power factor load of 0.8 pf will consume 18 kVAR which is 60% of total kVArating of a 30 kVA generator. The market price of an inverteris $ 50–70 per kVA which can be easily be configured to work as a DSTATCOM. However, the capital cost of the dieselgenerator is approximately § 500 per kVA rating. Moreover, theDSATCOM can provide compensation for harmonics whichfacilitates to load the DG set up to its full kVA rating.The performance of DSTATCOM is very much dependenton the method of deriving reference compensating signals.Instantaneous reactive power theory, modified p-q theory, syn-chronous reference frame theory, instantaneous i d − i q theory,and method for estimation of reference currents by maintainingthe voltage of dc link are generally reported in the literature foranestimationofreferencecurrentsfortheDSTATCOMthroughthe extraction of positive-sequence real fundamental currentcomponent from the load current [4]–[7]. These techniques arebased on complex calculations and generally incorporate a setof low-pass filter which results in a delay in the computation of referencecurrentsandthereforeleadstoslowdynamicresponseof the DSTATCOM. In this paper, a fast and simple neuralnetwork-based control scheme is used to estimate referencesource currents for the control of the DSTATCOM.This paper presents a DSTATCOM for the load compensa-tion of a diesel generator set to enhance its performance. Thecontrol of DSTATCOM with capabilities of reactive power,harmonics and unbalanced load compensation is achieved byLeast Mean Square (LMS) algorithm [8], [9] based adap-tive linear element (Adaline). The Adaline is used to extractpositive-sequence fundamental frequency real component of the load current. The dc-bus voltage of voltage source converter(VSC) is supported by a proportional–integral (PI) controllerwhich computes current component to compensate losses inDSTATCOM. The extraction of reference currents using Ada-line involves an estimation of weights. These weights are mea-sure of peak of fundamental frequency real current componentof the load current. The life of a DG set is enhanced in theabsence of unbalanced and harmonic currents. The modeling of the DG set is performed using a synchronous generator, a speedgovernor, and the excitation control system. This proposed sys-tem is simulated under MATLAB environment using Simulink and PSB Block-set toolboxes. The results for a 30-kVA DGset with the linear load at 0.8 lagging pf and a nonlinear loadwith different load dynamics and unbalance load conditions are 0093-9994/$26.00 © 2011 IEEE  SINGH AND SOLANKI: LOAD COMPENSATION FOR DIESEL GENERATOR ISOLATED SYSTEM EMPLOYING DSTATCOM 239 Fig. 1. Basic configuration of the DG set with DSTATCOM.TABLE IS YSTEM S PECIFICATIONS presented to demonstrate the effectiveness of DSTATCOM-DGset system.II. S YSTEM C ONFIGURATION Fig. 1 shows the configuration of the system for a three-phase three-wire DG set feeding to variety of loads. A30 kVA system is chosen to demonstrate the work of thesystem with the DSTATCOM. The DSTATCOM consistsof an insulated gate bipolar transistors-based three-phasethree-leg VSC system. The load current is tracked usingAdaline-based reference current generator, which in conjunc-tion with the hysteresis-based PWM current controller thatprovides switching signals for VSC-based DSTATCOM. Itcontrols source currents to follow a set of three-phase ref-erence currents. The parameters of a salient pole synchro-nous generator are 415 V, 30 kVA, 4 pole, 1500 rpm,50 Hz, X  d = 1 . 56 pu, X   d = 0 . 15 pu X   d = 0 . 11 pu, X  q =0 . 78 , X   q = 0 . 17 , X   q = 0 . 6 , H  s = 0 . 08 . The other criticalparameters are given in Table I.III. C ONTROL A LGORITHM The operation of this system requires a DG set to supply realpower needed to the load and some losses (switching losses of devices used in VSC, losses in the reactor, and dielectric losses Fig. 2. (a), (b). Control block diagram of the reference current extractionscheme. of the dc capacitor) in DSTATCOM. Therefore, the referencesource current used to decide the switching of the DSTATCOMhas two parts. One is real fundamental frequency componentof the load current, which is being extracted using Adalineand another component, which corresponds to the losses inthe DSTATCOM, are estimated using a PI controller over dcvoltage of DSTATCOM. Fig. 2(a) shows the control schemefor the implementation of reactive, unbalanced and harmoniccurrents compensation. The output of the PI controller is addedto the weight calculated by the Adaline to maintain the dc-busvoltage of the DSTATCOM.  A. Extraction of Real Positive-Sequence FundamentalFrequency Current from Load Current  The basic theory of the proposed decomposer is based onLMS algorithm [9] and its training through Adaline, whichtracks a unit voltage vector templates to maintain minimumerror. The basic concept of theory used here can be under-stood by considering the analysis in single-phase systemwhich is given. For an ac system, the supply voltage may beexpressed as v s = V  sin ωt (1)where v s is the instantaneous ac terminal voltage, V  is anamplitude and ω is the angular frequency of the voltage.The load current ( i L ) consists of active current ( i +  p ) , reac-tive current ( i + q ) for the positive sequence, negative-sequence  240 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 1, JANUARY/FEBRUARY 2011 Fig. 3. MATLAB based simulation Model. current ( i − ) , and harmonic frequency current ( i h ) can bewritten as summation of different parts as i L = i +  p + i + q + i − + i h . (2)The control algorithm is based on the extraction of the cur-rent component in phase with unit voltage template. To estimatethe fundamental frequency positive-sequence real componentof load current, the unit voltage template should be in phasewith the system voltage and should have unit amplitude. Theunit voltage template ( u  p ) derived from the system phasevoltage can be represented as: u  p = v s /V. (3)For proper estimation of the current components of the loadcurrent, the unit voltage templates must be undistorted. Incase of the voltage being distorted, the zero crossing of phasevoltage is detected to generate sinusoid (sin ωt ) vector tem-plate, synchronized with system terminal voltage. This signalis generated from the look-up table by adjustment of the delayto track the change in the frequency of the system.An initial estimate of the active part of load current forsingle-phase can be chosen as i +  p = W   p u  p (4)where weight ( W  p ) is estimated using an Adaline. This weightis variable and changes as per the load current. The schemefor estimating weights corresponding to fundamental frequencyreal component of load current (for three-phase system), basedon LMS algorithm-tuned Adaline tracks the unit vector tem-plates to maintain minimum error. The estimation of the weightis given as per the following iterations: W   p ( k +1) = W   p ( k ) + η  i L ( k ) − W   p ( k ) u  p ( k )  u  p ( k ) (5)where subscript k and k  + 1 represent sample instant and η is the convergence coefficient. The value of convergence co-efficient decides the rate of convergence and the accuracy of the estimation. The practical range of convergence coefficientlies in between 0.1 to 1.0. Three-phase reference currentscorresponding to positive-sequence real component of the loadcurrent may be computed as i +  pa = W  +  p u  pa ; i +  pb = W  +  p u  pb ; i +  pc = W  +  p u  pc (6) W  +  p =  W  +  pa + W  +  pa + W  +  pc  / 3 (7)where W  +  p is averaged weight. Weights of phase a, b and care averaged to compute the equivalent weight for positive-sequence current component in the decomposed form. Theaveraging of weights helps in removing the unbalance in loadcurrent components.  B. PI Controller for Maintaining Constant DC-BusVoltage of DSTATCOM  To compute the second component of reference active powercurrent, a reference dc-bus voltage is compared with sensed dc-bus voltage of DSTATCOM. This comparison of sensed dc-busvoltage ( v dc ) to the reference dc-bus voltage ( v ∗ dc ) of VSC,  SINGH AND SOLANKI: LOAD COMPENSATION FOR DIESEL GENERATOR ISOLATED SYSTEM EMPLOYING DSTATCOM 241 Fig. 4. Dynamic performance of the DSTATCOM-DG isolated system with linear load. results in a voltage error ( v dcl ) , which in the nth samplinginstant is expressed as v dcl ( n ) = v ∗ dc ( n ) − v dc ( n ) . (8)This error signal is processed in a PI controller and output { I  p(n) } at the nth sampling instant is expressed as: I   p ( n ) = I   p ( n − 1) + K   pdc  v dcl ( n ) − v dcl ( n − 1)  + K  idc v dcl ( n ) (9)where K  pdc and K  idc are proportional and integral gains of thePI controller.The output of the PI controller accounts for the losses inDSTATCOM and it is considered as the loss component of the current, which is added with the weight estimated bythe Adaline corresponding to fundamental frequency positive-sequence reference active current component. Therefore, thetotal real reference current has component corresponding tothe load and component corresponding to feed the losses of DSTATCOM, is expressed as i ∗ sa =  W  +  p + I   p  u  pa ; i ∗ sb =  W  +  p + I   p  u  pb ; i ∗ sc =  W  +  p + I   p  u  pc . (10)These three-phase currents are considered reference sourcecurrents i ref  ( i ∗ sa , i ∗ sb and i ∗ sc ) and along with sensed sourcecurrents i act ( i sa , i sb and i sc ), these are fed to the hysteresis-based PWM current controller to control the source currentsto follow these reference currents. The switching signals gener-ated by the PWM current controller force actual source currentsto acquire shape close to the reference source currents. Thisindirect current control results in the control of the slow varyingsource current (as compared to DSTATCOM currents) andtherefore requires less computational efforts. Switching signalsare generated on the following logic:if  ( i act ) < ( i ref  − hb/ 2) upper switch of the leg is ON andlower switch is OFF if  ( i act ) > ( i ref  + hb/ 2) upper switch of the leg is OFF andlower switch is ON where hb is hysteresis band around the reference current i ref  .Theweightsarecomputed onlinebyLMSalgorithm.Theup-date equation of weights based on LMS algorithm is describedin (5) for each phase. The structure of such Adaline is depictedin Fig. 2(b). Weights are averaged not only for averaging atfundamental frequency but to cancel out sinusoidal oscillatingcomponents in weights present due to harmonics in the sourcecurrent. The averaging of weights in different phases is shownin Fig. 2(a). Thus Adaline is trained at fundamental frequencyof a particular sequence in-phase with voltage. Fig. 2(a) and(b) show the detailed scheme implemented for control of DSTATCOM.
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