Independent Control of Active and Reactive Power in Microgrid

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1 IJSRD - Internatonal Journal for Scentfc Research & Deelopment Vol., Issue 03, 014 ISSN (onlne): Independent Control of Acte and Reacte ower n Mcrogrd Densh A. rajapat S 1 Bhak N. Suthar Vnod S Tejwan 3 1, Electrcal Department 3 h.d. Research Scholar 1, LDCE Ahmedabad 3 TU, Ahmedabad Abstract Now a days the ncreasng demand of electrcal energy, renewable energy based dstrbuton generators (Ds) are ncreasngly play a domnant role n electrcty producton. In ths paper t s shown that control of generated power s acheed from the mcrogrd (M) to cater the senste and crtcal load durng dsturbances. The effect of RL load connecton and dsconnecton s shown by MATLAB results. The conerter used s a oltage source nerter (VSI) whch s controlled usng d-q reference frame to nject a controlled current nto the grd. hase lock loop (LL) s used to generate the ectorzed output. Keywords: Source Inerter (VSI), ower Control, p-q control. I. INTRODUCTION The need of reducng emssons n the electrcty generaton feld, and recent technologcal deelopments n the mcrogeneraton doman are the man factors responsble for the growng nterest n the use of mcrogeneraton. In [1], the feasblty of two mcrogrd control strateges, It was shown that both the master/slae approach and droop approach ensure stable mcrogrd operaton subsequent to a fault condton. If there are no synchronous machnes to balance demand and supply, through ts frequency control scheme, the nerters should also be responsble for frequency control durng slanded operaton. In addton, a oltage regulaton strategy s requred; otherwse, the M mght experence oltage and/or reacte power oscllatons []. Mantanng the oltage and frequency wthn the standard permssble leels s an mportant requrement n mcrogrd desgn procedures. Varous mcrogrd control approaches hae been deeloped to mantan the oltage and frequency of the mcrogrd wthn permssble leels. Examples of these control approaches are: 1) master/slae control [1]; ) droop-based control [3], [4]. Under normal operaton, each D system n the mcrogrd usually works n a constant current control mode n order to prode a preset power to the man grd. When the mcrogrd s cut off from the man grd, each D nerter system must detect ths slandng stuaton and must swtch to a oltage control mode. In ths mode, the mcrogrd wll prode a constant oltage to the local load [5]. Control strategy to control grd current usng p-q theory and d-q (Synchronous reference frame) theory wth phase lock loop (LL) control has been dscussed n ths paper. The work presented here s about the smulaton of a mcrogrd wth RL load where the output current of nerter s controlled n d-q reference frame. LL s used to generate the ectorzed output. The desgn of the entre system, and results obtaned are presented n the subsequent sectons. II. BASIC STRUCTURE OF THE OWER CONTROL Basc structure of the nerter connected to grd s shown n Fg. 1. and source Inerter V δ L p,q V δ Fg. 1: Basc structure of the nerter connected to rd Where, Let, Then, and Where, Q sn δ δ (1) cos δ δ rd () V = Inerter output oltage V = rd oltage δ = hase angle of nerter output oltage δ = hase angle of grd oltage = Lne reactance δ = 0, δ = δ Q = Real power Q = Reacte power snδ s the equalent lne reactance where the resstance s gnored. Snce the D unt output oltage control already exsts, the task of the power controller s to generate oltage command (V δ) for the oltage controller based on the desred power alues * and Q* and actual alues and Q. The desred D output oltage V and the power angle δ can be calculated from equaton (3) and (4) gen desred and Q alues and system parameter. (3) (4) All rghts resered by

2 III. THEORETICAL CONSIDERATIONS ON -Q CONTROL The basc requrement of oltage source nerters s to control the flow of real and reacte powers between the mcrogrd and AC power system. Therefore, the frst step n studyng the subject of generaton control s to see whch arables nfluence the flow of real and reacte powers. The oltage source nerter controls both the magntude and phase of ts output oltage (V δ n Fg. 1). The ector relatonshp between the nerter oltage (V ) and the power system oltage (V ) along wth the nductor s reactance determnes the flow of real and reacte power from the mcrogrd to the power system. The flow of real and reacte powers can be controlled by generatng the proper alues for two arables of δ * and V * from equaton (3) and (4). Concernng the aboe expressons, the followng ponts can be rased [7] [8]. As a consequence, the control of real and reacte power (* Q*) flow s reduced to the control of the power angle (δ *) and the oltage leel (V *) of the nerter. Therefore, power angle and oltage leel would be the crtcal arables for real and reacte power flow control and can be controlled as shown n Fg.. Although the flow of real and reacte power s not completely decoupled, they are farly ndependent. In other words, the control of each one has only a mnor mpact on the other one. It can be obsered from equaton (3) and (4) that both and Q wll be affected by only adjustng one of V and δ, whch s so called couplng between and Q. Howeer, aratons of V and δ hae dfferent leels of mpact on and Q as descrbed n the followng partal derates. δ δ Q Q snδ snδ (5) (6) (7) When δ s small whch s true for large capacty power systems, s close to δ Q s close to 0 and δ and s (8) close to 0 and Q smlarly s close to. Ths fact ndcates that s more senste to δ and Q s more senste to V especally when the D unt s connected to a large capacty system where the power angle δ s usually small. The oltage and phase angle references can be generated as Independent Control of Acte and Reacte ower n Mcrogrd (IJSRD/Vol. /Issue 03/014/413) * * δ Kp dt * (9) * Kq Q Q dt (10) In the proposed power regulaton approach, and Q controls are decoupled under steady state due to the ntegraton of the errors. Howeer, n the transent, the and Q couplng cannot be elmnated. Both and Q are nonlnear functon of V and δ. Vdc Source Inerter V* δ* V δ V δ I I L p * q * p q -Q Calculaton rd Fg. : Basc structure of the nerter control scheme. The basc structure of the oltage source nerter (VSI) -Q controller n the smplest way s shown n Fg. [9] [10]. Real and reacte powers (p,q) can be controlled ndependently to a good extent. Two I controllers would suffce to control the flow of real and reacte powers by generatng the proper alues for two arables of V * and δ * as shown n Fg. Howeer, n practcal systems, the aboe approach s not feasble. ractcally, the alue of cannot be precsely known and may change due to the operaton of the power system. Once an naccurate alue of s used n the control, the error of wll lead to poor trackng and een nstablty. Therefore, power control solutons requrng knowledge of cannot be practcally used. IV. -Q THEORY In the case of the d-q frame, f the d-axs s algned to the grd oltage, the reference current d-component d * s controlled to manage the power angle (δ*) and hence real power flow, whle the reference current q-component q * s controlled to manage the oltage leel (V*) and hence reacte power flow of the nerter. The reference current q- component should be zero n order to hae the grd currents n phase wth the grd oltage. The p-q theory mplements a transformaton from a statonary reference system n a-b-c coordnates, to a system wth coordnates α-β-0, where coordnates α-β are orthogonal to each other, and coordnate 0 corresponds to the zero-sequence component. [6] α β a 3 b 1 c (11) All rghts resered by

3 Independent Control of Acte and Reacte ower n Mcrogrd (IJSRD/Vol. /Issue 03/014/413) The α-β coordnates are transformed to the d-q coordnates that rotate at the angular fundamental frequency ω of the grd oltage waeform. The state-ectors whch express the nerter electrcal quanttes are projected on the d-axs and q-axs. d α cosωt sn ωt R( ωt ), R(ωt ) q β sn ωt cosωt (1) If the nerter s connected to the grd through Lflter as shown n Fg. 1, then n the d-q frame rotatng at the angular speed ω, d e d d 1 d R(ωt) R(ωt ) q e q dt q L (13) The aboe equaton can be rewrtten as follow: d d 1 d q 1 ed ω dt q L q d L eq Intally measure the DC bus oltage and current, based on these alues calculate the solar. If solar s greater than load then surplus power s delered to grd and grd becomes poste. If solar s less than load then power s taken from grd and grd becomes negate. [1]. V. RESULTS AND DISCUSSIONS The Matlab/Smulnk model of mcrogrd wth RL load consst of followng parameters The desred and Q alues are set for two dfferent cases. The reference d-q components of desred and Q alue are regulated wth I controllers. The gans of I controllers are determned by tral and error for each case. A. Case 1:ref = 1.0 p.u, Qref = 0. p.u The nerter s controlled usng the tme-ntegral of the d-q space ector. The resultng d-q components are tme ntegrated, resultng n the flux ectors n I controller nputs, for the nerter and AC system oltages. The p-q power control s mplemented through the current controller. The block dagram of the VSI p-q controller s shown n Fg 3. Synchronous reference frame control also called d-q control uses a reference frame transformaton abc to dq whch transforms the grd current and oltages nto d-q frame. The transformed current controls the grd current [11]. For the smulaton, battery s used as nput of nerter. V Source Inerter Vdc V δ δ L rd p,q WM Va* Vb* Vc* L L ark Transformaton Ird Id* Iq* p* q* Id Current Controller Iq Reference eneraton p q -Q Vrd Calculaton Ird Fg. 3: Control schemes of the Inerter nterface to the utlty grd. Fg. 5: Smulaton results of Inerter, rd and Load currents. For the frst case study, the reference real power s set to ref = 1.0 p.u and the reference reacte power s set to Qref = 0. p.u. In ths case nerter based mcrogrd s capable to delered the power for the RL load ( = 0 kw, Q = 10 kvar). The amount of power generated s more so surplus power can be delered to grd. At t = 1 sec another RL load ( = 0 kw, Q = 10 kvar) s connect to mcrogrd. For that power generated by nerter s gen to RL load and t = 1. sec RL load s dsconnected. Ths effect s shown n Fg. 5. Fg. 6 shows the Id_ref and Iq_ref whch s generated based on actual and reference acte and reacte power. The reference acte power ref s set to 1.0 p.u. and reference reacte power Qref s set to 0. p.u. Fg. 4: Flow chart for ower Flow between Mcrogrd and rd. All rghts resered by 160

4 Independent Control of Acte and Reacte ower n Mcrogrd (IJSRD/Vol. /Issue 03/014/413) Fg. 6: Smulaton Results of I d _ref and I q _ref. B. Case : ref = load, Q ref = Q load Fg. 9: Smulaton Results of ref and Q ref. From Fg. 9 t s shown that the reference acte and reacte power s changed when load s change at t = 1 sec and t = 1. sec. VI. CONCLUSION From results of smulaton t can be concluded that by controllng the nerter, the power exchanges between the grd, load and dstrbuted generators can be done for arous loadng condtons. ower generated by the nerter can be controlled usng synchronous d-q transformaton. Fg. 7: Smulaton results of Inerter, rd and Load currents. For the second case study, the reference real power s set to ref = load and the reference reacte power s set to Q ref = Q load. In ths case nerter generate the power whch s actually requred for RL load ( = 0 kw, Q = 10 kvar). The amount of power generated s equal to the power requred for the load. At t = 1 sec another RL load ( = 0 kw, Q = 10 kvar) s connect to mcrogrd. For that nerter generate the requred power for RL load. Ths effect s shown n Fg. 7 and also shown that the transent n grd current when load s connect and dsconnect. Fg. 8 shows the I d _ref and I q _ref whch s follow the reference power because reference power s change when there s a change n load. Fg. 8 Smulaton Results of I d _ref and I q _ref. REFERENCES [1] J. A. Lopes, C. Morera, and A. Madurera, Defnng control strateges for mcrogrds slanded operaton, IEEE Trans. ower Syst., ol. 1, no., pp , May 006. [] R. H. Lasseter and. ag, Mcrogrd: A conceptual soluton, n roc. 35th ESC, ol. 6, Aachen, ermany, Jun. 004, pp [3] K. De Brabandere, B. Bolsens, J. Van den Keybus, A. Woyte, J. Dresen, and R. Belmans, A oltage and frequency droop control method for parallel nerters, IEEE Trans. ower Electron., ol., no. 4, pp , Jul [4] C. K. Sao and. W. Lehn, Control and power management of conerter fed mcrogrds, IEEE Trans. ower Syst., ol. 3, no. 3, pp , Aug [5] I. J. Balaguer, L. Qn, Y. Shutao, U. Supatt, and. F. Zheng, Control for grd-connected and ntentonal slandng operatons of dstrbuted power generaton, IEEE Trans. Ind. Electron., ol. 58, no. 1, pp , Jan [6] Hrofum Akag, Edson Hrokazu Watanabe, Maurco Aredes, Instantaneous ower Theory ans Applcatons to ower Condtonng, IEEE ress seres on ower engneerng., A John wley & Sons, ublcaton. 007 [7] C. Wang, M. H. Nehrr, and S. R. Shaw, Dynamc models and model aldaton for EM fuel cells usng electrcal crcuts, IEEE Trans. on Energy Conerson, ol. 0, No., pp , June, 005. [8] Koen J.. Macken, Jeroen Van den Keybus, and Ronne J. M. Belmans, Dstrbuted Control of Renewable eneraton Unts Wth Integrated Acte Flter, IEEE Transactons on ower Electroncs, ol. 19, No. 5, pp , Sep., 004. [9] R.Lasseter and. ag, rodng remum ower through Dstrbuted Resources, roceedngs of the All rghts resered by

5 Independent Control of Acte and Reacte ower n Mcrogrd (IJSRD/Vol. /Issue 03/014/413) 33rd Hawa Internatonal Conference on System Scences, 000. [10]. roux,. Syblle, and H. Le -Huy, Modelng and Smulaton of a Dstrbuton STATCOM usng Smulnk s ower System Blockset, IECON 01: 7th Annual Conference of the IEEE Industral Electroncs Socety, pp , 001. [11] Sangta R Nandurkar, Mn Rajee, Desgn and Smulaton of three phase Inerter for grd connected hotooltc systems roceedngs of Thrd Bennal Natonal Conference, NCNTE- 01, Feb 4-5. [1] V. S. Tejwan, B. N. Suthar, Noel Control Strategy for rd- connected VES for Smart Dstrbuton System Ffth Internatonal Conference on ower and Energy Systems, Kathmandu, Nepal 8-30 October, 013. All rghts resered by