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  • 1. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645 Fault Analysis And Improve Power Quality By Multilevel Statcoms Pragallapati Manikanta (M.Tech), K. Anand. M.E *(Department of Electrical Engineering, GIET, JNTUK, Rajahmundry, A .P, INDIA )ABSTRACT The static synchronous compensator potential failure point. Therefore, it is important to(STATCOM) has been well accepted as a power design a sophisticated control to produce a fault-system controller for improving voltage tolerant STATCOM. A faulty power cell in aregulation and reactive compensation [1]–[5]. cascaded H-Bridge STATCOM can potentiallyThere are several compelling reasons to consider cause switch modules to explode [10] leading to thea multilevel converter topology for the fault conditions such as a short circuit or anSTATCOM [6]–[8]. In this project, the method overvoltage on the power system resulting in anwe propose requires only that the output dc link expensive down time [11]. Subsequently, it is crucialvoltage of each phase be measured. This to identify the existence and location of the fault formeasurement is typically accomplished anyway it to be removed. Several fault detection methodsfor control purposes. If a fault is detected, the have been proposed over the last few years [10]–module in which the fault occurred is then [18]. Resistor sensing, current transformation andisolated and removed from service. This VCE sensing are some of the more commonapproach is consistent with the modular design of approaches. For example, a method based on thecascaded converters in which the cells are output current behavior is used to identify IGBTdesigned to be interchangeable and rapidly short circuits [12]. The primary drawback with theremoved and replaced. proposed approach is that the fault detection time depends on the time constant of the load. Therefore,Keywords - Fault detection, power quality, for loads with a large RL time constant, the faultymultilevel converter, static Synchronous power cell can go undetected for numerous cycles,compensator (STATCOM). potentially leading to circuit damage. Another fault detection approach proposed in [13] is based on aI. INTRODUCTION switching frequency analysis of the output phase Many static synchronous compensators voltage. This method was applied to flying capacitor(STATCOMs) utilize multilevel converters due to converters and has not been extended to cascadedthe following: 1) lower harmonic injection into the converters. AI-based methods were proposed topower system; 2) decreased stress on the electronic extract pertinent signal features to detect faults incomponents due to decreased voltages; and 3) lower [14]. In [15], sensors are used to measure each IGBTswitching losses. One disadvantage, however, is the current and to initiate switching if a fault is detected.increased likelihood of a switch failure due to the A fault-tolerant neutral point-clamped converter wasincreased number of switches in a multilevel proposed in [16]. In [17], a reconfiguration systemconverter. A single switch failure, however, does not based on bidirectional switches has been designednecessarily force an (2n + 1)-level STATCOM for three-phase asymmetric cascaded H-bridgeoffline. Even with a reduced number of switches, a inverters. The fundamental output voltage phaseSTATCOM can still provide a significant range of shifts are used to rebalance a faulted multilevelcontrol by removing the module of the faulted cascaded converter in [18].switch and continuing with (2n − 1) levels. In this paper, the method we propose This project introduces an approach to requires only that the output dc link voltage of eachdetect the existence of the faulted switch, identify phase be measured. This measurement is typicallywhich switch is faulty, and reconfigure the accomplished anyway for control purposes. If a faultSTATCOM. This converter uses several full bridges is detected, the module in which the fault occurred isin series to synthesize staircase waveforms. Because then isolated and removed from service. Thisevery full bridge can have three output voltages with approach is consistent with the modular design ofdifferent switching combinations, the number of cascaded converters in which the cells are designedoutput voltage levels is 2n + 1 where n is the number to be interchangeable and rapidly removed andof full bridges in every phase. The converter cells replaced. Until the module is replaced, the multilevelare identical and therefore modular. As higher level STATCOM continues to operate with slightlyconverters are used for high output rating power decreased, but still acceptable, performance.applications, a large number of power switching In summary, this approach offers thedevices, will be used. Each of these devices is a following advantages: 1638 | P a g e
  • 2. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645• No additional sensing requirements;• Additional hardware is limited to two by-passswitches per module;• Is consistent with the modular approach ofcascaded multilevel converters; and• The dynamic performance and THD of theSTATCOM is not significantly impacted.II. STATCOM The STATCOM is a solid-state-basedpower converter version of the SVC. Operating as ashunt-connected SVC, its capacitive or inductiveoutput currents can be controlled independently fromits terminal AC bus voltage. Basically, STATCOMis comprised of three main parts (as seen fromFigure below): a voltage source converter (VSC), astep-up coupling transformer, and a controller. In avery-high-voltage system, the leakage inductances of UT is the STATCOM terminal voltage; Ueqthe step-up power transformers can function as is the equivalent Thevenin voltage seen by thecoupling reactors. The main purpose of the coupling STATCOM; Xeq is the equivalent Thevenininductors is to filter out the current harmonic reactance of the power system seen by thecomponents that are generated mainly by the STATCOM.pulsating output voltage of the power converter. TheSTATCOM is connected to the power system at aPCC (point of common coupling), through a step-upcoupling transformer, where the voltage-qualityproblem is a concern. The PCC is also known as theterminal for which the terminal voltage is UT. Allrequired voltages and currents are measured and arefed into the controller to be compared with thecommands. The controller then performs feedbackcontrol and outputs a set of switching signals (firingangle) to drive the main semiconductor switches ofthe power converter accordingly to either increasethe voltage or to decrease it accordingly. ASTATCOM is a controlled reactive-power source. Itprovides voltage support by generating or absorbingreactive power at the point of common couplingwithout the need of large external reactors orcapacitor banks. III. MULTILEVEL STATCOM The charged capacitor Cdc provides a DC A cascaded multilevel STATCOM containsvoltage, Udc to the converter, which produces a set of several H-bridges in series to synthesize a staircasecontrollable three-phase output voltages, U in waveform. The inverter legs are identical and aresynchronism with the AC system. The synchronism therefore modular. In the eleven-level STATCOM,of the three-phase output voltage with the each leg has five H-bridges. Since each full bridgetransmission line voltage has to be performed by an generates three different level voltages (V, 0, −V)external controller. This reactive power exchange is under different switching states, the number ofthe reactive current injected by the STATCOM, output voltage levels will be eleven. A multilevelwhich is the current from the capacitor produced by configuration offers several advantages over otherabsorbing real power from the AC system. converter types [19]. 1) It is better suited for high-voltage, high-power applications than the conventional converters since the currents and voltages across the individual switching devices are smaller. 2) It generates a multistep staircase voltageWhere Iq is the reactive current injected by the waveform approaching a more sinusoidal outputSTATCOM voltage by increasing the number of levels. 3) It has better dc voltage balancing, since each bridge has its own dc source. 1639 | P a g e
  • 3. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645To achieve a high-quality output voltage waveform, Where n is the number of cells in each phase. Figurethe voltages across all of the dc capacitors should illustrates the carrier and reference waveforms for amaintain a constant value. Variations in load cause phase leg of the eleven-level STATCOM. In thisthe dc capacitors to charge and discharge unevenly figure, the carrier frequency has been decreased forleading to different voltages in each leg of each better clarity. Normally, the carrier frequency forphase. However, because of the redundancy in PWM is in the range of 1–10 kHz.switching states, there is frequently more than onestate that can synthesize any given voltage level. IV. FAULT ANALYSIS FOR THETherefore, there exists a “best” state among all the MULTILEVEL STATCOMpossible states that produces the most balanced A converter cell block, as shown in Figure,voltages [20]. can experience several types of faults. Each switch in the cell can fail in an open or closed state. The closed state is the most severe failure since it may lead to shoot through and short circuit the entire cell. An open circuit can be avoided by using a proper gate circuit to control the gate current of the switch during the failure [23]. If a short circuit failure occurs, the capacitors will rapidly discharge through the conducting switch pair if no protective action is taken. Hence, the counterpart switch to the failed switch must be quickly turned off to avoid system collapse due to a sharp current surge. Nomenclature for the proposed method is given in Table I. The staircase voltage waveform shown in Fig. 3 is synthesized by combining the voltages of the various cells into the desired level of output voltage. At the middle levels of the voltage waveform, due to the switching state redundancy, there are more than one set of switching Since there are multiple possible switching combinations that may be used to construct thestates that can be used to synthesize a given voltage desired voltage level. Therefore, by varying thelevel, the particular switching topology is chosen switching patterns, the loss of any individual cellsuch that the capacitors with the lowest voltages are will not significantly impact the middle voltages ofcharged or conversely, the capacitors with the the output voltage. However, the peak voltageshighest voltages are discharged. This redundant state require that all cells contribute to the voltage;selection approach is used to maintain the total dc therefore, the short circuit failure of any one cell willlink voltage to a near constant value and each lead to the loss of the first and (2n + 1) output levelsindividual cell capacitor within a tight bound. and cause degradation in the ability of theDifferent pulse width modulation (PWM) techniques STATCOM to produce the full output voltage level.have been used to obtain the multilevel converter Consider the simplified eleven-level converteroutput voltage. One common PWM approach is the shown in Figure. The process for identifying andphase shift PWM (PSPWM) switching concept [21]. removing the faulty cell block is summarized inThe PSPWM strategy causes cancellation of all Figure. The input to the detection algorithm is ˆEoutcarrier and associated sideband harmonics up to the for each phase, where ˆEout is the STATCOM(N − 1)th carrier group for an N-level converter. filtered RMS output voltage.Each carrier signal is phase shifted by 1640 | P a g e
  • 4. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645 V. METHOD OF COMPARISON Each fault detection method has its own advantages and disadvantages. Most of the methods in the literature are applicable to neutral point-clamped converters and are therefore not directly applicable to cascaded converters. In this section, each applicable approach is succinctly summarized and compared with other methods. Two recent methods are briefly reviewed below. 1) Voltage Frequency Analysis [23]. In this scheme, the basic approach is to use SPWM to produce the converter output voltage. By using If the STATCOM RMS output voltage SPWM, voltages with different phase angles will bedrops below a preset threshold value (E_), then, a produced at each cell of the multilevel converter.fault is known to have occurred (see Fig. 6). Once a The sum of the three phase voltages is zero infault has been detected to have occurred, then, the normal operation, but that is not zero if there is anext step is to identify the faulty cell. By utilizing faulty cell. This condition is used as the criteria forthe switching signals in each converter cell, (i.e., S1 identifying the faulty cell. The phase angle of theand S2), it is possible to calculate all of the possible voltage sum indicates the location of the fault.voltages that can be produced at any given instant asillustrated in Table II (terminology adopted from[23]): Thus, the output voltage of a cell is and sincethe cells of the STATCOM are serially connected,the total output voltage per phase isWhere „n‟ is the number of blocks. If there is a faulted cell, only one fi will benear the actual STATCOM output phase voltageEout; all of the others will be too high. Therefore, todetermine the location of the fault cell, each fi iscompared against Eout to yield The smallest xi indicates the location of thefaulted block because this indicates the fi whichmost closely predicts the actual Eout. The choice ofthreshold voltage E_ depends on the number of cellsin the converter. The ideal output voltage isDuring a fault, Eout will decrease by Vdc0 yielding Therefore, the threshold voltage E_ should 2) AI-BASED FAULT DETECTION [14].be chosen such that (n − 1/n) Eout,0 ≤ E_ ≤ Eout,0. This scheme is built around a neuralIn an eleven-level converter, n = 5 and the faulted network (NN) classification for fault diagnosis of aRMS voltage will decrease by roughly 20%. multilevel cascaded converter. Multilayer perceptionTherefore, a good choice for E_ is 85% of the rated networks are used to identify the type and locationoutput STATCOM voltage. 1641 | P a g e
  • 5. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645of occurring faults. The principal componentanalysis is utilized in the feature extraction processto reduce the NN input size. Since these methods areall designed to detect and then bypass the faultedcell, the hardware requirements are identical. Thesemethods are compared and contrasted to theproposed method in Table III. Each method has itsown advantages and disadvantages. For example, thevoltage frequency method detects and clears thefaulty cell rapidly, but requires complex frequencyanalysis and may not be suitable for implementationin all applications. The proposed method does notrespond as rapidly, but only requires simplecalculations and can be implemented easily in mostDSPs. Furthermore, the proposed method onlyrequires voltage magnitude measurements which areeasily obtained. VI. EXPERIMENTAL RESULTS To confirm the operation of the fault detection algorithm for cascaded H-bridge multilevel converters, an experimental prototype is constructed for applying and detecting different type of faults. The experimental rack consists of 36 Power ex CM75Du-24F IGBTs rated at 1200 V and 75 A for main switching devices. Passive components include a 1.2-mH, 45-A reactor and 18 electrolytic capacitors rated at 3900 μF and 450 V. The IGBTs are driven by Concept 6SD106E1 gate drivers and controlled by a 320F2812 fixed-point digital signal processor (DSP). 1642 | P a g e
  • 6. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645 improvement in the output waveform of the converter. The above both output of Experimental STATCOM dynamics shows the same fault as in Figure, except the fault bypass signal is intentionally delayed by several cycles to demonstrate the effect of changing the PWM pattern. Note that after the fault and discharge of the corresponding capacitor, the output waveform contains considerable distortion. However, modifying the PWM switching signals based on two cascaded H-bridges, the THD of the output waveform can be significantly The output voltage of the converter during decreased and the filtered output waveform becomethe normal operation, during the fault, and after sinusoidal again.removing the faulty cell is depicted in Figure. Afault is applied to the second cell at point “F” as VII. CONCLUSIONshown in the figure with the dashed line. After In this paper, a fault detection anddetecting the fault and bypassing the faulty H- mitigation strategy for a multilevel cascadedbridge, the modulation index is increased to converter has been proposed. This approach requirescompensate for the lost voltage levels in the output. no extra sensors and only one additional by bypassIn addition, the PWM switching patterns are switch per module per phase. The approach has beenmodified based on existence of two cascaded H- validated on a 115-kV system with a STATCOMbridges instead of three. This causes significant compensating an electric arc furnace load. This application was chosen since the arc furnace 1643 | P a g e
  • 7. Pragallapati Manikanta, K. Anand. M.E / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1638-1645provides a severe application with its non sinusoidal, IEEE Trans. Ind. Electron., vol. 49, no. 5,unbalanced, and randomly fluctuating load. The pp. 988–997, Oct. 2002.proposed approach was able to accurately identify 11. S. Wei, B. Wu, F. Li, and X. Sun, “Controland remove the faulted module. In addition, the method for cascaded H-bridge multilevelSTATCOM was able to remain in service and inverter with faulty power cells , ” in Proc.continue to provide compensation without exceeding Appl. Power Electron. Conf. Expo., Feb.the total harmonic distortion allowances. 2003, vol. 1, pp. 261–267. 12. S. Li and L. Xu, “Fault-tolerant operationREFERENCES of a 150 kW 3-level neutral point clamped 1. V. Dinavahi, R. Iravani, and R. Bonert, PWM inverter in a flywheel energy storage “Design of a real-time digital simulator for system,” in Conf. Rec. 36th IEEE IAS a D-STATCOM system,” IEEE Trans. Ind. Annu. Meeting, Chicago, IL, Oct. 2001, pp. Electron., vol. 51, no. 5, pp. 1001–1008, 585–588. Oct. 2004. 13. F. Richardeau, P. Baudesson, and T. 2. Atousa Yazdani, Hossein Sepahvand, Meynard, “Failure-tolerance and remedial Mariesa L. Crow, and Mehdi Ferdowsi, strategies of a PWM multicell inverter,” “Fault Detection and Mitigation in IEEE Trans. Power Electron., vol. 17, no. Multilevel Converter STATCOMs” IEEE 6, pp. 905–912, Nov. 2002. Std, Apr 2011. 14. S. Khomfoi and L. Tolbert, “Fault 3. B. Singh, S. Murthy, and S. Gupta, diagnosis and reconfiguration for multilevel “STATCOM-based voltage regulator for inverter drive using AI-based techniques,” self-excited induction generator feeding IEEE Trans. Ind. Electron., vol. 54, no. 6, nonlinear loads,” IEEE Trans. Ind. pp. 2954–2968, Dec. 2007. Electron., vol. 53, no. 5, pp. 1437–1452, 15. M. Ma, L. Hu, A. Chen, and X. He, Oct. 2006. “Reconfiguration of carrier-based 4. A. Luo, C. Tang, Z. Shuai, J. Tang, X. Xu, modulation strategy for fault tolerant and D. Chen, “Fuzzy-PI-based direct- multilevel inverters ,” IEEE Trans. Power output-voltage control strategy for the Electron. vol. 22, no. 5, pp. 2050–2060, STATCOM used in utility distribution Sep. 2007. systems,” IEEE Trans. Ind. Electron., vol. 16. S. Ceballos, J. Pou, E. Robles, I. Gabiola, J. 56, no. 7, pp. 2401– 2411, Jul. 2009. Zaragoza, J. L. Villate, and D. Boroyevich, 5. M. Molinas, J. Suul, and T. Undeland, “Three-level converter topologies with “Extending the life of gear box in wind switch breakdown fault-tolerance generators by smoothing transient torque capability,” IEEE Trans. Ind. Electron., with STATCOM,” IEEE Trans. Ind. vol. 55, no. 3, pp. 982–995, Mar. 2008. Electron., vol. 57, no. 2, pp. 476–484, Feb. 17. P. Barriuso, J. Dixon, P. Flores, and L. 2010. Morán, “Fault-tolerant reconfiguration 6. C.-H. Liu and Y. - Y. Hsu, “Design of a system for asymmetric multilevel self-tuning PI controller for a STATCOM converters using bidirectional power using particle swarm optimization, ” IEEE switches,” IEEE Trans. Ind. Electron., vol. Trans. Ind. Electron., vol. 57, no. 2, pp. 56, no. 4, pp. 1300– 1306, Apr. 2009. 702–715, Feb. 2010. 18. P. Lezana and G. Ortiz, “Extended 7. D. Soto and T. C. Green, “A comparison of operation of cascade multicell converters high-power converter topologies for the under fault condition,” IEEE Trans. Ind. implementation of FACTS controllers,” Electron., vol. 56, no. 7, pp. 2697–2703, IEEE Trans. Ind. Electron., vol. 49, no. 5, Jul. 2009. pp. 1072–1080, Oct. 2002. 19. F. Z. Peng, J. S. Lai, W. McKeever, and J. 8. J. A. Barrena, L. Marroyo, M. Á. Rodríguez VanCoevering, “A multilevel voltage Vidal, and J. R. Torrealday Apraiz, source inverter with separate dc sources for “Individual voltage balancing strategy for static VAr generation,” IEEE Trans. Ind. PWM cascaded H-bridge converter-based Appl., vol. 32, no. 5, pp. 1130–1138, Sep. STATCOM,” IEEE Trans. Ind. Electron., 1996. vol. 55, no. 1, pp. 21–29, Jan. 2008. 20. K. A. Corzine, M. W. Wielebski, F. Z. 9. T. A.Meynard,M. Fadel, and N. Aouda, Peng, and J. Wang, “Control of cascaded “Modeling of multilevel converters,” IEEE multilevel inverters,” IEEE Trans. Power Trans. Ind. Electron., vol. 44, no. 3, pp. Electron., vol. 19, no. 3, pp. 732–738, May 356–364, Jun. 1997. 2004. 10. C. Turpin, P. Baudesson, F. Richardeu, F. 21. B. McGrath and D. Holmes, “Multicarrier Forest, and T. A. Meynard, “Fault PWM strategies for multilevel converters,” management of multi-cell converters,” 1644 | P a g e
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