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  1. 1. K Srinivas, B Subash / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 6, November- December 2012, pp.1636-1640Power Quality Improvement Features of the Grid at Distribution Side With Wind Energy Source 1 2 K Srinivas B Subash (M.Tech SR Eng College) (Sr.Asst Prof SR Eng College)Abstract ELECTRIC utilities and end users of network in terms of stability, voltage regulation andelectric power are becoming increasingly power-quality (PQ) issues. Therefore, the DGconcerned about meeting the growing energy systems are required to comply with strict technicaldemand. Seventy five percent of total global and regulatory frameworks to ensure safe, reliableenergy demand is supplied by the burning of and efficient operation of overall network.fossil fuels. But increasing air pollution, global With the advancement in power electronicswarming concerns, diminishing fossil fuels and and digital control technology, the DG systems cantheir increasing cost have made it necessary to now be actively controlled to enhance the systemlook towards renewable sources as a future operation with improved PQ at PCC. However, theenergy solution. Since the past decade, there has extensive use of power electronics based equipmentbeen an enormous interest in many countries on and non-linear loads at PCC generate harmonicrenewable energy for power generation. currents, which may deteriorate the quality ofRenewable energy resources (RES) are being power.increasingly connected in distribution systems Generally, current controlled voltageutilizing power electronic converters. This paper source inverters are used to interface the intermittentpresents a novel control strategy for achieving RES in distributed system. Recently, a few controlmaximum benefits from these grid-interfacing strategies for grid connected inverters incorporatinginverters when installed in 3-phase 4-wire PQ solution have been proposed. In an inverterdistribution systems. The inverter is controlled to operates as active inductor at a certain frequency toperform as a multi-function device by absorb the harmonic current. But the exactincorporating active power filter functionality. calculation of network inductance in real-time isThe inverter can thus be utilized as: 1) power difficult and may deteriorate the controlconverter to inject power generated from RES to performance. A similar approach in which a shuntthe grid 2)shunt APF to compensate current active filter acts as active conductance to damp outunbalance 3) load current harmonics 4) load the harmonics in distribution network. . Proposed. Areactive power demand and load neutral current. control strategy for renewable interfacing inverterAll of these functions may be accomplished either based on – theory is proposed. In this strategy bothindividually or simultaneously. With such a load and inverter current sensing is required tocontrol, the combination of grid-interfacing compensate the load current harmonics.inverter and the 3-phase 4-wire Linear/nonlinear The non-linear load current harmonicsunbalanced load at point of common coupling may result in voltage harmonics and can create aappears as balanced linear load to the grid. This serious PQ problem in the power system control concept is demonstrated with Active power filters (APF) are extensively used toextensive MATLAB/Simulink simulation studies compensate the load current harmonics and loadand validated through digital signal processor- unbalance at distribution level. This results in anbased laboratory experimental results additional hardware cost. However, 1) active power harvested from the renewable resources (wind,Index Terms- Power quality (PQ), distributed solar, etc.); 2) load reactive power demand support;generation (DG), grid inter connection, active power 3) current harmonics compensation at PCC; and 4)filter (APF), renewable energy. current unbalance and neutral current compensation in case of 3-phase 4-wire system. Moreover, withI. INTRODUCTION adequate control of grid-interfacing inverter, all the Renewable energy source (RES) integrated four objectives can be accomplished eitherat distribution level is termed as distributed individually or simultaneously. The PQ constraintsgeneration (DG). The utility is concerned due to the at the PCC can therefore be strictly maintainedhigh penetration level of intermittent RES in within the utility standards without additionaldistribution systems as it may pose a threat to hardware cost. In this paper authors have incorporated the maximum utilization of inverter rating which isfeatures of APF in the, conventional inverter most of the time underutilized due to intermittentinterfacing renewable with the grid, without any nature of RES.additional hardware cost. Here, the main idea is the 1636 | P a g e
  2. 2. K Srinivas, B Subash / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 6, November- December 2012, pp.1636-1640Fig.1. Schematic of proposed wind based distributed generation systemII. SYSTEM DESCRIPTION needs power conditioning (i.e., dc/dc or ac/dc) The proposed system consists of RES before connecting on dc-link. The dc-capacitorconnected to the dc-link of a grid-interfacing decouples the RES from grid and also allowsinverter as shown in Fig. 1. The voltage source independent control of converters on either side ofinverter is a key element of a DG system as it dc-link.interfaces the renewable energy source to the gridand delivers the generated power. The RES may be A. Transfer of variable power from wind sourcea DC source or an AC source with rectifier coupled to gridto dc-link. Usually, the fuel cell and photovoltaic Due to the intermittent nature of RES, theenergy sources generate power at variable low dc generated power is of variable nature. The dc-linkvoltage, while the variable speed wind turbines plays an important role in transferring this variablegenerate power at variable ac voltage. Thus, the power from renewable energy source to the grid.power generated from these renewable sources RES are represented as current sources connected to the dc-link of a grid-interfacing inverter. Fig. 2. DC-Link equivalent diagram.Fig. 2 shows the systematic representation of The current flow on the other side of dc-link can bepower transfer from the renewable energy represented as,resources to the grid via the dc-link. The currentinjected by renewable into dc-link at voltage level can be given …………. (2) 𝑃𝑊Idc1= 𝑉𝑑𝑐 as..................... (1) Where and are totalWhere is the power generated from RES. power available at grid-interfacing inverter side, active power supplied to the grid and inverter 1637 | P a g e
  3. 3. K Srinivas, B Subash / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 6, November- December 2012, pp.1636-1640losses, respectively. If inverter losses are negligible PW=PG.thenB. Control of Grid Interfacing Inverter:Fig.3. Block diagram representation of grid-interfacing inverter control. The control diagram of grid- interfacing The actual dc-link voltage (Vdc) is sensed andinverter for a 3-phase 4-wire system is shown in Fig. passed through a first-order low pass filter (LPF) to3. The fourth leg of inverter is used to compensate eliminate the presence of switching ripples on thethe neutral current of load. The main aim of dc-link voltage and in the generated referenceproposed approach is to regulate the power at PCC current signals. The difference of this filtered dc-during: 1) ; 2) PRES< total load power link voltage and reference dc-link voltage (Vdc*) is given to a discrete- PI regulator to maintain a(PL) ; and 3) PRES > PL. While performing the powermanagement operation, the inverter is actively constant dc-link voltage under varying generation and load conditions. The dc-link voltage error (Vdcerrcontrolled in such a way that it always draws/ th (n)) at n sampling instant is given as:supplies fundamental active power from/ to the grid.If the load connected to the PCC is non-linear or ………….. (6)unbalanced or the combination of both, the given The output of discrete-PI regulator at nth samplingcontrol approach also compensates the harmonics, instant is expressed asunbalance, and neutral current. The duty ratio ofinverter switches are varied in a power cycle suchthat the combination of load and inverter injectedpower appears as balanced resistive load to the grid. ……… (7)The regulation of dc-link voltage carries theinformation regarding the exchange of active power Where and arein between renewable source and grid. Thus the proportional and integral gains of dc-voltage regulator. The instantaneous values of referenceoutput of dc-link voltage regulator results in anactive current Im. The multiplication of active three phase grid currents are computed ascurrent component (Im) with unity grid voltage ……………. (8)vector templates (Ua, Ub and Uc) generates thereference grid currents (Ia* , Ib* and Ic*). The ………….. (9) *reference grid neutral current (In ) is set to zero,being the instantaneous sum of balanced grid …………. (10)currents. The grid synchronizing angle (θ) obtained The neutral current, present if any, due to the loadsfrom phase locked loop (PLL) is used to generate connected to the neutral conductor should beunity vector template. compensated by forth leg of grid-interfacing inverter and thus should not be drawn from the grid. In other ……………. (3) words, the reference current for the grid neutral current is considered as zero and can be expressed as ……………. (4) ………… (11) ……………. (5) The reference grid currents (Ia* , Ib* ,Ic* and In*)are compared with actual grid currents 1638 | P a g e
  4. 4. K Srinivas, B Subash / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 6, November- December 2012, pp.1636-1640(Ia* , Ib* ,Ic* and In*) to compute the current errors as The switching pattern of each IGBT inside inverter can be formulated On the basis of error between ……………. (12) actual and reference current of inverter, which can be explained as: ……………… (13) If , then upper ………………. (14) switch will be OFF and lower ………………. (15) switch will be ON in the phase “a” legThese current errors are given to hysteresis currentcontroller. The hysteresis controller then generates of inverter. If , then upperthe switching pulses (P1 to Pg ) for the gate drives of switch will be ON and lower switchgrid-interfacing inverter. The average model of 4-leg inverter can be obtained by the following state will be OFF in the phase “a” leg ofspace equations inverter. Where hb is the width of hysteresis band. On the same principle, the switching pulses for the ……………….. (16) other remaining three legs can be derived. III.SIMULATION RESULTS …………….. (17) ………………… (18) ……………. (19)…………. (20)Where , and are thethree-phase ac switching voltages generated on theoutput terminal of inverter. These inverter outputvoltages can be modeled in terms of instantaneousdc bus voltage and switching pulses of the inverteras ………………. (21) …………….. (22) ……………….. (23) ……………… (24) Similarly the charging currents , and on dc busdue to the each leg of inverter can be expressed as ……….. (25) Fig. simulation model ………….. (26) ……………. (27) ………………. (28) 1639 | P a g e
  5. 5. K Srinivas, B Subash / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 Vol. 2, Issue 6, November- December 2012, pp.1636-1640 Fig. Simulation results: (a) PQ-Grid, (b) PQ- Load, (c) PQ-Inverter,(d) dc-link voltage. REFERENCES [1] J. M. Guerrero, L. G. de Vicuna, J. Matas, M. Castilla, and J. Miret,“A wireless controller to enhance dynamic performance of parallel invertersin distributed generation systems,” IEEE Trans. Power Electron.,[2]vol. 19, no. 5, pp. 1205–1213, Sep. 2004. (2) J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, “3-phase4-wire shunt active power filter with renewable energy interface,” presented at the Conf. IEEE Rnewable Energy & Power Quality, Seville, Spain, 2007. [3] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006 [4] M. Singh and A. Chandra, “Power maximization and voltage sag/swell ride-through capability of PMSG based variable speed wind energy conversion system,” in Proc. IEEE 34th Annu. Conf. Indus. Electron. Soc., 2008, pp. 2206–2211 .Fig. Simulation results: (a) Grid voltages, (b) Grid .Currents (c) Unbalancedload currents, (d) Inverter Current 1640 | P a g e