Series Compensator Based on CascadedTransformers Coupled with Three-Phase Bridge Converters
Abstract
This paper proposes a multilevel series compensator(MSC) to deal with: i) voltage sags/swells, ii) harmonic compensationor iii) reactive power compensation. Such a devicecan be considered as a dynamic voltage restorer (DVR) or aseries active power filter (Series-APF). The MSC can improve thepower quality of loads located in stiff systems. The configurationis based on three-phase bridge (TPB) converters connected bymeans of cascaded single-phase transformers. This arrangementpermits to use a single dc-link. A generalization for K-stages,in which K-transformers are coupled with K-TPB converters, ispresented. The topology permits to generate high number of levelsin the voltage waveforms with a low number of power switchesin comparison with a classic topology. The multilevel waveformsare generated by the converters through a suitable PWM strategythat takes into consideration the transformer turns ratios.Modularity and simple maintenance make the proposed MSC anattractive solution compared to some conventional configurations.Model, PWM strategy and overall control are discussed in thispaper. Simulation and experimental results are presented as well.
EXISTING SYSTEM:
The series compensator (DVR or Series-APF) is commonlycomposed of: i) injection transformers, ii) voltage sourceconverter (VSC), iii) energy storage, iv) optional passivefilters and v) protection circuits (e.g., bypass thyristors). TheVSC based on two-level (2L) converter is the most commonsolution used for low-voltage systems . However,for high-voltage levels (i.e., high power applications), 2Lbasedconverters have experienced limitations and difficulty topenetrate in this market. The cost associated for designinga 2L based compensator for more than 690 V(accordingto IEC) makes this solution not feasible for high-voltage applications. In this context, the multilevel-based VSCtechnology has become the most mature and feasible solutionfor this type of applications. Multilevel series compensators(MSCs) have been investigated in different aspects, which make clear the fact that the multilevel convertersensure that voltage waveforms can be synthesized with lowerharmonic content than 2L converters and also operate at higher Some multilevel configurations can be highlighted from thetechnical literature. However, those configurationshave some issues associated to the high number ofdc-link capacitors, inherent in their topology. For instance,cascaded neutral point-clamped (NPC) and cascaded flyingcapacitor topologies have issues with unbalanced dc-link voltagesand power sharing in each cell.
PROPOSED SYSTEM:
To cope with this issue, the concept of using cascaded transformershas been introduced as an alternative solution. In this way, the multilevel features can be guaranteed byusing a single dc-link capacitor. A conventional DVR based oncascaded transformer coupled with H-bridge (HB) converterswas presented in. Usually, injection transformers are takeninto consideration for the series voltage compensator design. In this way, the transformer turn ratio associated witheach transformer can be considered to improve the waveformquality of the output voltage generated by the compensator.This paper proposes a series compensator based on cascadedtransformers coupled with three-phase bridge (TPB) converters, as illustrated. Equivalent multilevel operationis achieved with reduced number of semiconductor devicesin comparison with conventional HB. The multilevel waveformsare generated by TPB converters through a suitablePWM strategy associated with the transformer turns ratio. Themodularity and simple maintenance make the proposed MSCan attractive solution in comparison with some conventionalconfigurations. The model and control are addressed in thispaper. Simulation and experimental results are presented too.
CONCLUSION
This paper has presented a MSC based on cascaded transformerscoupled with three-phase-bridge (TPB) converters. Ageneralization with K-stages was introduced in order to showthat by increasing the number of cells, the performance canbe improved as well as can increase the power rating of thecompensator. The solution is an attractive option because itdoes not need any additional dc-link capacitors as observed insome conventional multilevel compensator, such as NPC ,flying capacitors, cascaded HB , etc.Since TPB modules are available in the market, the modularityis a good feature for this topology. The proposedMSC has two options of operation: (1) as a dynamic voltagerestorer (DVR) or (2) as a harmonic series active power filter(Series-APF). Some analysis and comparisons consideringWTHD, semiconductor losses estimation and number of levelsgenerated per power switches were presented. Compared withconventional HB, the proposed MSC has presented bettervalues for these figures of merit. Simulation and experimentalresults were presented in order to validate theoretical considerations.
REFERENCES
[1] C. Ho and H. Chung, “Implementation and performance evaluation ofa fast dynamic control scheme for capacitor-supported interline DVR,”Power Electronics, IEEE Transactions on, vol. 25, no. 8, pp. 1975–1988,2010.
[2] J. Martinez and J. Martin-Arnedo, “Voltage sag studies in distributionnetworks-part i: system modeling,” Power Delivery, IEEE Transactionson, vol. 21, pp. 1670–1678, July 2006.
[3] M. Newman, D. Holmes, J. Nielsen, and F. Blaabjerg, “A dynamicvoltage restorer (DVR) with selective harmonic compensation at mediumvoltage level,” Industry Applications, IEEE Transactions on, vol. 41,pp. 1744–1753, Nov 2005.
[4] B. Wang, G. Venkataramanan, and M. Illindala, “Operation and controlof a dynamic voltage restorer using transformer coupled H-bridgeconverters,” Power Electronics, IEEE Transactions on, vol. 21, pp. 1053–1061, July 2006.
[5] I. Amariz Pires, S. Silva, and B. de Jesus Cardoso Filho, “Increasingride-through capability of control panels using square-wave series voltagecompensator,” Industry Applications, IEEE Transactions on, vol. 51,pp. 1309–1316, March 2015.[
7] E. Ribeiro and I. Barbi, “Harmonic voltage reduction using a seriesactive filter under different load conditions,” Power Electronics, IEEETransactions on, vol. 21, pp. 1394–1402, Sept 2006.
[8] R. Millnitz dos Santos, J. da Cunha, and M. Mezaroba, “A simplifiedcontrol technique for a dual unified power quality conditioner,” IndustrialElectronics, IEEE Transactions on, vol. 61, pp. 5851–5860, Nov2014
[9] V. Khadkikar and A. Chandra, “UPQC-S: A novel concept of simultaneousvoltage sag/swell and load reactive power compensations utilizingseries inverter of UPQC,” Power Electronics, IEEE Transactions on,vol. 26, pp. 2414–2425, Sept 2011.
[10] H. Akagi, “New trends in active filters for power conditioning,” IndustryApplications, IEEE Transactions on, vol. 32, pp. 1312–1322, Nov 1996.
[11] A. Rauf and V. Khadkikar, “An enhanced voltage sag compensationscheme for dynamic voltage restorer,” Industrial Electronics, IEEETransactions on, vol. 62, pp. 2683–2692, May 2015.
[12] H. Akagi, “The state-of-the-art of active filters for power conditioning,”in Power Electronics and Applications, 2005 European Conference on,pp. 15 pp.–P.15, 2005.