Single Phase D-Q Theory Based Control of DER Inverters for Power Quality Improvement

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1 Single Phse D-Q Theory Bsed Control of DER nerters for Power Qulity mroement Lin Shil Stnly, Anu P, Diy R, Dr. Mnjul G Nir Dertment of Electricl nd Electronics, Amrit Vishw Vidyeethm, Amrituri, ndi linshil@gmil.com Abstrct This er resents controller for DER inerters in the distribution side to ct s shunt ctie filters nd mitigte the ower qulity issues in the grid without the instlltion of dditionl FACTS deices. The inerter is ble to inject the rel ower ilble t the DC link from renewble energy resource s well s erform ower fctor correction, hrmonic elimintion nd blncing of the lods connected to the system t the couling oint. MATLAB/SMULNK model of the system is deeloed nd erformnce of the controller is tested nd lidted under rying irrdince nd lod conditions. Keywords-distributed genertion; single hse D-Q theory; ower qulity; shunt ctie filter. NTRODUCTON Renewble energy bsed Distributed Genertion (DG) systems re incresing widely, of which solr hotooltic which is clen, inexhustble nd enironmentl friendly is sustinble lternte to the conentionl sources of energy. The rice of the PV modules ws in the st mjor contribution to the cost of PV systems, but grid connection ses the cost of bttery. Moreoer, the worldwide goernment olicies imed t incresing the deloyment of these systems mkes Solr Photooltic romising energy source for the future. n the ower system rious domestic nd industril lods crete ower qulity issues. These lods results in low ower fctor, cretes hrmonics nd system unblnce. Vrious Flexible AC Trnsmission Systems (FACTS) deices like ssie or ctie filters re used for mitigting these drwbcks [1]. When comred to the bulkier nd fixed ssie tye, ctie filters re of more reserch interest. A cost effectie method of using Distributed Energy resource (DER) inerters, referbly of Solr Photooltic system to ct s filters for three hse lods he been deeloed [], [3]. Since in the distribution side were most of the lods re single hse in nture, usully single hse inerters ct s shunt ctie filters. Thus ech hse is in need of serte inerter nd its control. f the 3Ф DER inerter of the DG system cn be mde to oerte s ctie filter for the single hse lods t the DG side the cost of using indeendent filter for ech hse cn be sed. n this er the grid connected distributed genertion system is controlled to ct s three hse shunt ctie filter. The three hse interfcing inerter of the DG system is controlled using single hse D-Q frme theory to feed the ctie ower from PV nd imroe the ower qulity of the distribution side due to rious single hse liner/nonliner lods.. SYSTEM DESCRPTON The block digrm of grid interfced Distributed generting unit is shown in Fig 1. The system consists of solr PV string, inerter nd interfcing inductors. The PV string consists of series nd rllel combintions of PV modules to obtin the desired ower rting. The system is interfced to the three hse grid. The lods connected t the couling oint re the locl lods for the DG system nd my include single hse liner nd non-liner lods. The four leg three hse inerter with slit ccitor s fourth leg cts s filter nd roides rel ower injection nd comenstion for the locl lods The uthors - Published by Atlntis Press 170

2 Figure 1. Block digrm of grid connected PV system. CONTROL ALGORTHM The ulses for controlling the VS is generted by PWM technique bsed on the reference source currents obtined using single hse D-Q frme theory bsed lgorithm [4]. The block digrm of the control is shown in Fig. The single hse signl is trnsformed to two hse signl nd then conerted to rotting frme using the following reltionshi. xd cosθ sinθ xα = xq sinθ cosθ xβ An esy wy of generting the orthogonl system in single-hse structure is using trnsort dely block, which is resonsible for introducing hse shift of 90 degrees with resect to the fundmentl frequency of the reference signls [5]. Assuming blnced three hse source, s sb sb Sin ωt Sin ( ωt 10 ) Sin ( ωt + 10 ) The mlitude of hse oltge is estimted s [6], V (1) ( s + sb + sc ) = () 3 Where s, sb, sc re the instntneous hse oltges. Using the mlitude of hse oltges, the unit in-hse comonents of hse oltges re clculted s, s sb sc U (3) =, Ub =, Uc = V V V 015. The uthors - Published by Atlntis Press 171

3 Figure. Block digrm of controller The oltge nd current of ech hse re trnsformed into sttionry α-β frme, nd then the source oltges in sttionry α-β frme re reresented s Where reltions cosθ nd θ sin re clculted using the = U = U α, bα b, c c β bβ cβ = U = U = U b c T t 4 T t 4 T t 4 α = U (4) Similrly the lod currents in sttionry α-β frme re lso comuted. The lod currents (il, ilb, ilc) re sensed nd trnsformed into sttionry α-β frme using hse shifter. The direct xis comonents corresonding to lod current in ech hse is obtined by trnsforming the signls from α-β frme to synchronously rotting D-Q frme. The direct xis comonent of hse A is obtined by the following reltionshi ld lq cosθ = sinθ sinθ cosθ lα lβ (5) (6) sinθ = cosθ α 1 + β α Similrly D Q comonents of the lod currents in hses B nd C cn be clculted. The equilent D-xis comonent of the totl lod is obtined by dding the D-xis comonents of the three hses together. ld ld lbd β lcd (7) = + + (8) The source current should contin only the ctie comonent of lod current for mintining the grid qulity. Therefore for blnced sinusoidl source currents, the reference source currents generted for controlling the ctie filter will be the erge of the ctie comonent of lod currents for ech hse. Since the energy from DG is ilble t the DC link of the ctie filter, the reference source currents will be reduced by lue of current equilent to the PV ower t ech hse. The instntneous equilent current t the grid side due to PV is obtined by equting the dc side nd c side ower t the inut nd outut of the inerter The uthors - Published by Atlntis Press 17

4 3V rms rms V 3 = P P 3 = V The net ctie comonent of the grid current, ld 3 = (10) 3 (9) For obtining equl reference current in ech hse the resulting current is diided by three nd conerted bck in to the ctul comonent using the reltions gien in eqution 11. i i i sbref scref = = = cosθ cosθb cosθc (11) The reference nd ctul lues of source currents re comred nd the error is gien to PWM genertor for generting the control ulses for switching the inerter. V. SMULATON RESULTS AND DSCUSSON Figure 3. Simulink digrm of the test circuit The erformnce of the control is tested in MATLAB/SMULNK. The simulink model of the system deeloed for testing is shown in Fig 3. t consists of 415V, 50Hz three hse four wire source with single hse lods. A dc source t the DC link of the inerter roides the rel ower suort equilent to PV for the locl liner/nonliner lods t PCC. The inerter feeds the rel ower into grid nd erforms ctie filtering under rying PV ower nd chnging lod conditions The uthors - Published by Atlntis Press 173

5 TABLE. SYSTEM SPECFCATONS FOR SMULATON Grid 415V,50Hz, 3hse 4wire Single hse Liner Lod 1kW,0.8f Single hse Non- Liner 750W Lod PV ower Vritions Rile nductor other lods t grid side. At t=0.s the lod in the B hse is cut off by oening the circuit breker to bring unblnce. With the PV ower remining constnt the ower fed to grid increses due to decrese in lod. At t=0.4s the PV ower is reduced to zero.the totl lod ower required is tken from the grid. At t=0.6s lod is introduced bck in hse B nd system is blnced. During this eriod since the PV ower is zero, the ower tken from the grid increses due to increse in ower demnded by lod. The inerter roides rectie ower comenstion, hrmonic elimintion nd lod blncing under different test conditions nd mintins the grid side current sinusoidl nd in hse with the source oltge. The enlrged iew of the inerter currents t different hses under different test conditions is shown in Fig 6. The inerter current contins more of rel rt when PV ower is ilble from t=0s to t=s. When the PV ower is zero fter t=0.s, the inerter contins only the comensting current. The current t hse B is ure sine we when the lod t B hse is zero to mintin blnced sinusoidl currents t the grid side. Fig 7 is the enlrged iew of Phse A oltge nd current. Unity ower fctor is mintined under ll test conditions 1. 10kW. Zero 5mH Fig 4 shows the oltge nd current weforms. The weforms clerly deict the erformnce of the system. Fig 5 shows the ritions in PV ower, lod ower nd ower t source side. t cn be seen tht initilly from time t=0 to t=0.s the single hse lods re blnced, nd PV ower of 10kW is used to feed these lods. The extr ower ilble fter feeding the locl lods is fed to the grid nd used by Figure 4. Weforms of oltge nd currents Figure 5. Weforms showing ower flow 015. The uthors - Published by Atlntis Press 174

6 Figure 6. Enlrged iew of the inerter currents Results of the FFT nlysis of lod nd source currents re shown in Fig 8. The THD of lod current is 1.61% wheres the source side THD is 4.80% nd within the limits. Figure 7. Enlrged iew of hse A oltge nd current () 015. The uthors - Published by Atlntis Press 175

7 (b) Figure 8. () THD of lod current (b) THD of source current V. CONCLUSON The erformnce of DG system to oerte s ctie filter is demonstrted in this er. The inerter of the system comenstes the rious ower qulity issues on the grid side due to lods t the couling oint like low ower fctor, hrmonics, nd unblnce. The system when tested under different lod nd PV ower conditions shows stisfctory results mintining blnced sinusoidl source currents inhse with the system oltge. Mximum ower oint trcking of the distributed resource, wind/solr cn be done by modifying the controller to mke the system more efficient nd suitble for smrt grid enironment. REFERENCES [1] H.L Jou, Performnce Comrison of Three hse Actie ower- Filter lgorithms EEE roceedings [] A. K. Verm, B. Singh, nd D. T. Shni, Grid interfced solr hotooltic ower generting system with ower qulity imroement t AC mins, resented t the EEE 3rd nt. Conf. Sustinble Energy Technol.,Kthmndu, Nel, Oct. 01. [3] Bhim Singh, Fellow, EEE, Chinmy Jin, Member, EEE, nd Sgr Goel, Student Member, EEE LST Control Algorithm of Single- Stge Dul Purose Grid Connected Solr PV System EEE Trns. Power electronics, ol. 9, no. 10, october 014. [4] Bhim Singh, Fellow, EEE, S. S. Murthy, Life Fellow, EEE, nd Rj Sekhr Reddy Chilii STATCOM-Bsed Controller for Three- Phse SEG Feeding Single-Phse Lods EEE Trns. Energy conersion, ol. 9, no., june 014. [5] Schi Shrm, Single Phse d-q Trnsformtion using s indirect Control Method for Shunt Actie Power Filter nterntionl Journl of Engineering Reserch nd Generl Science Volume, ssue 3, Aril-My 014. [6] John S. Hsu, nstntneous Phsor Method for Obtining nstntneous Blnced Fundmentl Comonents for Power Qulity Control nd Continuous Dignostics EEE Trns. Power Deliery, ol.13, no.4, Oct The uthors - Published by Atlntis Press 176