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International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8Voltage Unbalance Correction in a Grid Using InverterK.Jayakumar 1 , N.Sriharish 2 , Ch.Rambabu 31 M.Tech Student in Power Electronics, Dept. of EEE at Sri Vasavi Engineering College, Tadepalligudem, A.P, India2 Assistant Professor, Dept. of EEE at Sri Vasavi Engineering College, Tadepalligudem, A.P, India3 Professor & HOD, Dept. of EEE at Sri Vasavi Engineering College, Tadepalligudem, A.P, IndiaAbstractThis paper presents the control of a “voltage unbalance correction in a grid using inverter”. The inverters are proposedgive additional function to decrease the negative sequence voltage at the point of correction with the utility grid. By usingimproved multi variable filter, the grid inverter absorbs a small amount of negative sequence current from the grid, and whic hbased up on symmetric sequence voltage decomposition, thereby helping to correct the negative sequence voltage. But theamplitude reduction by each individual inverter system is small as compared to the entire negative sequence component, andthese inverter modules can achieve to collect substantial results in the grid. Finally the analyses of the scheme along with thesuitable design are presented by using basic circuit diagram and proposed control has been verified by simulation results areshown.Keywords: PWM inverter, multi variable filter, voltage unbalance, point of correction, negative sequence voltage, distributedgeneration, grid interfacing etc.I. Introduction:In practical three phase power systems, voltage unbalance problems are existing. These problems are mainly caused bysingle phase and non-linear loads, which are unequally distributed. Therefore these unequal voltage drops are mainly occuracross transformers and line impedances. Here the negative sequence voltages are especially troublesome in practicalapplications. Due to this the zero sequence component are not exist in three wire systems. These voltage un balance effect isquite serve for electrical machines, power electronic converters and its drives [1]. So to mitigate this voltage unbalance we cango to design power electronic converters for regulating the reactive power [2, 3]. But in underground cables this approach is notsuitable because in underground cables the resistance of the cable dominates its inductance. To maintain a balanced voltage a tthe load terminals, an often used idea is to inject a series voltage [4, 5]. It is straightforward to mitig ate the voltage unbalanceproblem with such converters, but a disadvantage is that they are unused or only lightly loaded when there are no voltageunbalance problems. For dealing with other power quality problems than voltage unbalance, so -called unified power qualityconditioners (UPQC) are proposed and continuously improved. However, the UPQC has no energy storage capabilities [6], andshould be extended to cope with distributed generation (DG) [7].Facing the emerging application of distributed generation, power electronics-based grid-interfacing inverters are playingan important role interfacing DGs to the utility grid. In addition to conventional delivery of electricity, ancillary functio nalityfor improvement of power quality problems is being introduced into grid-interfacing inverters [8, 9]. In this paper, it isproposed to integrate voltage unbalance correction into the control of grid-interfacing inverters. This does not require morehardware, since the feedback variables for this control are already available. By controlling the negative-sequence currents,which induce opposite negative-sequence voltage drops on the line impedances, the objective of eliminating negative sequencevoltages at the point of connection (PoC) with the grid may be achiev ed. To investigate the effectiveness of the proposedfunction, a three-phase four-wire inverter is used to control voltage unbalance correction. The employed inverter operatesnormally when the utility voltages are balanced, and when unbalanced, performs compensation automatically for negativesequencevoltage, based on utility voltage unbalance factor (VUF) [1]. To this aim, the analysis of negative -sequence currentcontrol and high performance detection for symmetrical sequences are introduced in the following. Then, the inverter controlscheme and reference signal generation are presented. Finally, the proposed control methods are verified by simulations.II. Grid-interfacing inverter with integrated voltage unbalance correction:Fig. (1) shows the structure of a three-phase four-wire grid-interfacing system being connected to the utility grid at thePOC through LCL filters. It normally synchronizes with the utility grid and delivers electrical energy to the grid from the D C-bus when pre-regulated distributed sources are connected. The voltage unbalance correction function is added, whichintentionally regulates negative sequence currents. Note that, in order to obtain a maximum power factor, most grid -interfacinginverters deliver only positive-sequence currents under either balanced or unbalanced conditions. Therefore, the development ofthis proposed controller differs from the conventional one, and its design will be presented in the next sections of this pap er. In||Issn 2250-3005(online)|| ||December||2012|| Page 201
International Journal Of Computational Engineering Research (ijceronline.com) Vol. 2 Issue. 8view of unbalanced situations, a four-leg inverter topology is used as the circuit to eliminate zero-sequence currents. With thetheory of symmetric decomposition for three phase systems [10], unbalanced grid voltages can be divided into three groups,namely positive, negative, and zero sequence voltages. Similarly, current quantities can also be separated. By disregarding themutual coupling between the grid lines in Fig. (1), an equivalent circuit model for each group of sequence components can bederived [11]. The diagram for negative-sequence components is shown in Fig.(2), where the superscript “-” denotes negativesequence. Similarly, the superscript “+” denotes positive sequence. Phasors and are the negative-sequence voltages of theutility grid and at the PoC, respectively. Current is the negative-sequence current equivalent line impedance is representedby Zg, the equivalent impedance of the utility grid when the line impedances of the three phases are assumed symmetrical.Accordingly, a phasor diagram showing the change for negative-sequence fundamental current is drawn in Fig.(3). Bychanging the amplitude and phase of the negative sequence current , the negative-sequence voltage can be regulatedthrough the voltage drops across the line impedance. For a given amplitude , the voltage changes along the dashed circle andreaches a minimum value at the point M where θ− equals the negative of impedance angle of Zg’s. Similarly, zero-sequencevoltages at the PoC can be compensated by regulating the zero-sequence currents within the system. This paper onlyconcentrates on the correction of negative-sequence voltages, considering zero-sequence voltages do not exist in case of threewire systems. Of course, zero-sequence voltages can be isolated by transformers when needed. Furthermore, it is noted thatmeasurements of zero-sequence components can be done simply by adding three phase while accurate positive- and negativesequencecomponents are difficult to be determined. Therefore, zero s equence voltage correction can be trivially added to thecontrol based on the proposed control scheme for negative-sequence voltage correction and is not discussed in this paper.Fig.(1) Three-phase four-wire grid-interfacing four leg inverter at PoC.Fig.(2) Negative-sequence equivalent modelFig.(3) Phasor diagram of the negative-sequence model.III. Control scheme:A. Determination of Negative-sequence CurrentsFig. (3) Illustrates the basic principle of how to correct unbalanced voltage at the PoC with sequence-current control. Itis suggested to determine the negative-sequence currents based on the voltage unbalance factor. To assess unbalanced voltagesat the PoC, the voltage unbalance factor, KVUF is defined as the ratio between the amplitude of the negative-sequence voltageand the amplitude of the positive-sequence voltage . The following constraint equation is proposed to calculate thedesired current amplitude :(1)where is the amplitude of the positive-sequence current. Then, the resulting is derived based on the ratio of unbalancevoltages at the PoC from (1).However, the voltage unbalance factor at the PoC varies with the controlled negative-sequence currents, because thecontroller utilizes feed forward measurements of K VUF and operates in a open-loop. Consequently, this strategy may cause thevalue of K VUF in (1) to vary. To ensure a stable correction, a smooth update method for K VUF is added to the control. The flowchart shown in Fig.(4) illustrates how to derive the final The currently measured quantity is referred to as (n) , and the||Issn 2250-3005(online)|| ||December||2012|| Page 202
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