A Novel Fast Energy Based Fuzzy Logic Dc-Link Voltage ... · A Novel Fast Energy Based Fuzzy Logic...

A Novel Fast Energy Based Fuzzy LogicDc-Link Voltage Controller For ThreePhase Dstatcom To Compensate Ac

Loads

Dr. G Venu Madhav1, Kothuri Ramakrishna2,1 Professor, Department of EEE,

Anurag Group of Institutions,Hyderabad, India

2Associate Professor, B.V.Raju Institution of Technology,Narsapur Medak(Dist),Telangana, India

April 18, 2018

Abstract

While compensating rapidly varying unbalanced and non-linear loads the transient response of the distribution staticcompensator (DSTATCOM) is very important. If there isany change in load it will affect the dc-link voltage directly.If the load is removed suddenly it will result in an increasein the dc-link voltage above the reference value, where as ifthere is a sudden increase in load it will reduce the dc-linkvoltage below its reference value. For operation of DSTAT-COM the dc-link voltage should be varied within the pre-scribed limits. If we use a conventional PI controller it willuse the deviation of the capacitor voltage from its referencevalue as its input but the transient response using this con-troller is slow. In this paper a fast-acting fuzzy logic-baseddc-link voltage controller is proposed which will achieve thefast-transient response. Simulation results have done usingMATLAB/SIMULINK.

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International Journal of Pure and Applied MathematicsVolume 118 No. 24 2018ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

Key Words: DC-link voltage controller, distributionstatic compensator(DSTATCOM), harmonics, power qual-ity (PQ), fuzzy logic, power factor.

1 Introduction

The Power-Quality (PQ) problems in the power distribution net-work due to nonlinear and unbalanced loads have rapidly been in-creasing. They cause excessive neutral currents, overheating of elec-trical apparatus, poor power factor, voltage distortion, high levelsof neutral-to-ground voltage, and interference with communicationsystems [1],[2]. The flexible ac transmission system and CustomPower are serving the transmission system and the distributionsystem, respectively. The flexible ac transmission system allowsa greater control of power flow and a secure loading of transmissionlines to levels nearer to their thermal limits, among other benefits.

Therefore, the functionalities of the conventional DSTATCOMshould be increased to mitigate the aforementioned PQ problemsand to supply the dc loads from its dc link as well. The load sharingby the ac and dc bus depends upon the design and the rating of theVSI. This DSTATCOM differs from conventional one in the sensethat its dc link not only supports instantaneous compensation butalso supplies dc loads.

However, when the dc link of the DSTATCOM supplies the dcload as well, the corresponding dc power is comparable to the av-erage load power and, hence, plays a major role in the transientresponse of the compensator. Hence, there are two important is-sues. The first one is the regulation of the dc-link voltage with inprescribed limits under transient load conditions. The second oneis the settling time of the dclink voltage controller. Conventionally,a PI controller is used to maintain the dc-link voltage.

In this paper, an energy based fuzzy logic voltage controllerbased on the dc-link capacitor energy is proposed. The detailedmodeling, simulation, and experimental verifications are given toprove the efficacy of this energy based fuzzy logic controller.

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International Journal of Pure and Applied Mathematics Special Issue

2 COMPENSATION OF AC AND DC

LOADS BY USING DSTATCOM

Due to the simplicity, the absence of unbalance in the dc-link volt-age and independent current tracking with respect to other phases,a 3-phase H-bridge VSI topology is shown in Fig. 1.

The above figure shows a three-phase, four-wire compensated sys-tem using an H-bridge VSI topology-based DSTATCOM compen-sating unbalanced and nonlinear ac load. In addition to this, a dcload is connected across the dc link. The DSTATCOM consists of 12insulated-gate biploar transistor (IGBT) switches each with an an-tiparallel diode, dc storage capacitor, three isolation transformers,and three interface inductors. The star point of the isolation trans-formers is connected to the neutral of load and source. The Fig.1Three-phase, four-wire compensated system using the H-bridge VSItopology-based DSTATCOM. H-bridge VSIs are connected to thePCC through interface inductors. The isolation transformers pre-vent a short circuit of the dc capacitor for various combinations ofthe switching states of the VSI. The inductance and resistance of theisolation transformers are also included in and the source voltagesare assumed to be balanced and sinusoidal. While compensating,the DSTATCOM maintains the balanced sinusoidal source currentswith unity power factor and supplies the dc load through its dc bus.

During normal condition the source is going to generate bal-anced and sinusoidal voltage. If there is a sudden decrease/increaseof the unbalanced and nonlinear load in this type of system, it

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causes unbalance and harmonics in the load current [4], [5]. Thesecurrents are going to be injected at the PCC with the help ofDSTATCOM. The operation of the compensator is supported withthe help of the dc capacitor and this supplies the required dc load[6] [7].

3 MODELING OF THE DSTATCOM

A DSTATCOM consists of a three-phase voltage source invertershunt-connected to the distribution network by means of a couplingtransformer, as depicted in Fig.2 Its topology allows the device togenerate a set of three almost sinusoidal voltages at the funda-mental frequency, with controllable amplitude and phase angle. Ingeneral, the DSTATCOM can be utilized for providing voltage reg-ulation, power factor correction, harmonics compensation and loadleveling [3]. The addition of energy storage through an appropriateinterface to the power custom device leads to a more flexible inte-grated controller. The ability of the DSTATCOM/ESS of supplyingeffectively extra active power allows expanding its compensating ac-tions, reducing transmission losses and enhancing the operation ofthe electric grid.

To calculate the system actual currents, the state-space equations

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are written by using the system notations as follows:

Using the above state-space model, the system state variables arecomputed at every instant [1]. In order to keep the dc-link volt-age of the inverter in the DSTATCOM at an assigned level duringoperation, the DSTATCOM needs to absorb active power from thepower source to supply the power losses and charge the dc-link ca-pacitor in the DSTATCOM. Hence, use of a Proportional-Integraltype feedback controller in the DSTATCOM controller regulatesthe active current — Ir— of the DSTATCOM [13].and three phasecurrents are required to comparison of base currents. The resultantwill beefed to the controllers in the form of voltages .These currentsare achieved with the help of following diagram as shown in fig.3

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4 DESIGN OF FUZZY LOGIC CON-

TROLLER

The design of Mamdani type Fuzzy PD controller using Fuzzy Logictool box in MATLAB is demonstrated [8]. The membership func-tions of input (error and rate of change of error) and output vari-ables (control action) and rule base are defined in the tool box.The PI controller block in the control scheme of the DSTATCOMwas replaced by the designed Fuzzy Inference System (FIS). TheDSTATCOM was then simulated for the same load with all otherparameters maintaining the same.

A. Fuzzy Control Scheme For D-Statcom.Fuzzy logic (FL) controller is the heart of fuzzy set theory; the

major features are the use of linguistic variables rather than numer-ical variables. This control technique relies on human capability tounderstand the systems behavior and is based on quality controlrules. Fuzzy Logic provides a simple way to arrive at a definiteconclusion based upon vague, imprecise, noisy, ambiguous, or miss-ing input information. The structure of an FLC is represented inFig.4 and comprises of four principal components.

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The Fuzzification interface will converts input data into suitablelinguistic values.

• The Knowledge Base is consists of a data base with the nec-essary linguistic definitions and control rule set.

• A Decision Making Logic will, simulating a human decisionprocess, and interface the fuzzy control action from the knowledgeof the control rules and the linguistic variable definitions.

• Defuzzification interface is yields a non-fuzzy control actionfrom an inferred fuzzy control action.

B. Membership FunctionsThe input and output membership functions and the rule base

are shown below.

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Fig. 5: Error as input

Fig. 6: Change in error as output

In this fuzzy logic controller rules are designed based on themembership functions. Those rules are represented with followingnotations and rules are arranged in Table.1 as shown in bellow

The two inputs were represented by sets of seven membershipfunctions and expressed in linguistic values as negative large (NL),negative medium (NM), negative small (NS), zero (ZE), positivesmall (PS), positive medium (PM), and positive large (PB). Therange for the error input was set and that for change of error wasset .The AND method used during interpretation of the IF-THENrules was min and the OR method used was max. Also, min wasused as the implication method whereas the max method was usedfor aggregation [9], [10]. The 49 fuzzy IF-THEN weighted rulebase was designed to maintain the capacitor voltage constant byproviding the required reference current amplitude.

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5 SIMULATION RESULTS

To analyze the performance of system we have designed a Mat-lab/Simulink model as shown in the fig. Here a three phase sourceis connected to the unbalanced load and non-linear load, the dstat-com is connected in shunt at the point of common coupling. Themain intention here is to control the dc-link voltage. Whenever ifthere is any change in load, this causes the variation in the dc-linkvoltage from its reference value. In order to bring back the dc-linkvoltage to its reference value here we are considering three cases(i) DC-link voltage control with conventional PI controller (ii) DC-link voltage control with Energy based PI controller (iii) DC-linkvoltage control with Energy based Fuzzy Logic controller. On thesethree conditions we are getting the variation of dc-link voltage att=0.4 sec and at t=0.8 sec. The explanation of control circuit andthe simulation results are as follows.

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Fig. 7 Simulink diagram of the system with D-STATCOM

A. DC-Link voltage control with conventional PI controllerThe controlling circuit with conventional PI controller is shown

in the fig below.

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Here we are going to consider two cases (i) when there is suddenload reduction and (ii) when there is sudden increase in load to fullload condition. In the first case if there is a sudden reduction in theload then the dc-link capacitor will absorb the extra power from thesource. Fig shows the increase in the voltage of DC-Link capacitorabove the reference value. Then the PI controller comes into actionand brings the dc-link capacitor voltage to its reference value in0.04 sec. In the second case if the load comes back to its full loadcondition then the capacitor has to supply the required voltage.Hence there will be a reduction in the voltage from reference value.This will again bring back to its reference value with the help ofconventional PI controller.

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B. DC-Link voltage control with Fuzzy controllerThe controlling circuit with Fuzzy controller is shown below.

Here the conventional PI controller is replaced with Fuzzy con-troller and obtained the results. After the changes in load the small

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spike which was there in the above result at t=0.4 sec and at t=0.8sec is controlled in 0.02 sec only.

C. DC-Link voltage control with Energy based Fuzzy controllerHere we are taking the square of the dc voltage and its refer-

ence and the resultant after comparing these two is passed throughthe fuzzy controller. Then the dc load power is calculated. Herealso we will consider the same two cases as above. By using theenergy based fuzzy controller here we are bringing the voltage toits reference value in 0.002 sec after the changes in load.

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6 COMPARISIONS

The bellow fig.14 shows that comparison between dc link voltagesfor compensating variation of loads. In above, graph1 (refer fig.9)represents the settling time is 0.42sec.graph 2 (refer fig.11) repre-sents the settling time is 0.402sec. Graph 3 (refer fig.13) representsthe settling time is 0.4002sec. The conclusion of the above graphis energy based fuzzy control is better one and it has improvementof power factor is 0.98 that is shown in bellow.

Power factor improvement for PI controller and fuzzy logic con-trollers is as shown bellow

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Fig. 16, 17, 18 shows the FFT analysis of the load voltage forconventional PI, Energy based PI and with Fuzzy Logic controllers.The comparison table between different controllers is shown below.

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7 CONCLUSION

Utilities are finding it difficult to maintain the power quality at theconsumer end, and consumers are paying the penalties indirectly inthe form of increased plant downtimes, etc. The Shunt connectedCustom Power device DSTATCOM can compensate for unbalancedand nonlinear load currents. An energy based d.c link voltage con-troller is very efficient in improving the Power Quality. A fuzzycontroller with fast reference voltage generation to regulate Unbal-ance voltage in three-phase system improves Power Quality evenfurther. We conclude that Mamdani type Fuzzy PD controller canperform the same function of conventional PD controller. One ofthe main advantages of Fuzzy control over conventional controlleris the inaccuracies of the sensors on the system performance can bereduced by adding additional rules, this reduces the need for costlysensors and cost of the control system is reduced without a com-promise in performance. The rule base and membership function ofinput and output variables can be obtained easily if we have expert

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knowledge of the system. The simulation results show a very goodperformance of the method and the control scheme under arbitraryfault conditions of the utility supply. Simulation results shown thatsystem limits.

References

[1] N. Hingorani, ”Introducing custom power,” IEEE Spectrum.,vol. 32, no. 6, pp. 4148, Jun. 1995.

[2] W. M. Grady and S. Santoso, Proc. IEEE Power Eng. Rev.Understanding Power System Harmonics, vol. 21, no. 11, pp.811, 2001.

[3] A. Ghosh and G. Ledwich, Power Quality Enhancement UsingCustom Power Devices. Norwell, MA: Kluwer, 2002.

[4] A. Dell Aquila, M. Liserre, V. G. Monopoli, and P. Ro-tondo,”An energy-based control for an n-H-bridges multilevelactive rectifier,” IEEE Trans. Ind. Electron., vol. 52, no. 3, pp.670678, Jun. 2005.

[5] M.K.Mishra, A.Ghosh, and A.Joshi, ”A new STATCOM topol-ogy to compensate loads containing ac and dc components,” inProc. IEEE Power Eng. Soc. Winter Meeting, Singapore, Jan.2327, 2000, vol. 4, pp. 26362641.

[6] Alireza Nami Hoda Ghoreishy, Department of Power, Mazan-daran University, Babol, Mazandaran, Iran”A Novel Ran-dom Hysteresis Current Control for a Single-Phase Inverter.”http://itee.uq.edu.au/ aupec/aupec06/htdocs/content/pdf

[7] Mahesh K. Mishra, Member, IEEE, and K. Karthikeyan,A Fast-Acting DC-Link Voltage Controller for Three-PhaseDSTATCOM to Compensate AC and DC Loads, IEEE Trans-actions On Power Delivery, Vol. 24, No. 4, October 2009.

[8] Jon C. Ervin, Sema E. Alptekin and Dianne J. DeTurris, ”Op-timization of the Fuzzy Logic Controller for an AutonomousUAV,” (EUSFLAT) Conference: Barcelona, Spain (2005).

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[9] ”Neuro-Fuzzy based power quality improvement in a threephase four wire distribution system using D-STATCOM” byE.Babu,R.Subramanyam in International Electrical Engineer-ing Journal(IEEJ) Vol. 4 (2013) No. 1, pp. 953-961 ISSN 2078-2365.

[10] Sheroz Khan, Salami Femi Abdulazeez, Lawal Wahab Ade-tunji, AHM Zahirul Alam,Momoh Jimoh E. Salami, ShihabAhmed Hameed, Aisha Hasan Abdalla and Mohd Rafiqul Is-lam ,”Design and Implementation of an Optimal Fuzzy LogicController Using Genetic Algorithm,” in Journal of ComputerScience 4 (10):2008.

[11] S.S Mortazavi, M.Razaz, and E. Khavari, Department of Elect.Egg, Shahid Chamran University, Ahvaz, Iran.”Power QualityImprovement using a fuzzy logic control of a series active fil-ter.”IEEE Industrial Electronics Society News letter vol.47,no.3, Sept. 2000, ISSN: 0746- 1240,”Active Filters with Con-trol Based on the p-q Theory.”

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