An Improved Direct AC-AC Converter for Voltage Sag Mitigation

IEEE Transactions on Industrial Electronics, 2013 


Dynamic Voltage Restorer (DVR) is a definitive solution towards compensation of voltage sag with phase jump. Conventional DVR topologies however have dc-link and two stage power conversion. This increases its size, cost and associated losses. Therefore topologies without the dc-link, mitigating sag by utilizing direct ac-ac converters, are preferable over the conventional ones. As no storage device is employed, compensation by these topologies is limited only by the voltages at the point of common coupling that is feeding the converters. In this paper, a direct ac-ac converter based topology fed with line voltages is proposed. The arrangement provides increased range of compensation in terms of magnitude and phase angle correction. Detailed simulations have been carried out in MATLAB to compare the capability of the proposed topology with other similar topologies.



  1. Dynamic voltage restorer (DVR)
  2. Voltage source inverter (VSI)
  3. Voltage sag compensation
  4. Voltage phase jump compensation.
  5. AC-AC converter





Fig. 1. Interphase ac-ac converter topology


Fig. 2. Proposed converter topology.



Fig. 3. Compensation of a sag type Ba. (a) Three phase voltage at the PCC with sag of 0.3 p.u. magnitude and 􀀀100 phase jump. (b) Three phase load voltage(c) Injected voltage. (d) The duty cycle of choppers in phase a sag supporter.

Fig. 4. Compensation of a sag type Ca. (a) Three phase voltage at the PCC with sag of 0.4 p.u. characteristic voltage magnitude and 􀀀200 phase jump. (b) Three phase load voltage at the PCC. (c) Injected voltages. (d) The duty cycle of voltages in phase b sag supporter. (e) The duty cycle of choppers in phase c sag supporter.

Fig. 5. Compensation  of  symmetrical  sag. (a) Three phase voltage at the PCC with sag of 0.5 p.u. magnitude and 􀀀600 phase jump. (b) Three phase load voltage at the PCC. (c) Injected voltages. (d) The duty cycle of voltages in all sag supporters.



In this paper, an ac-ac converter based voltage sag supporter fed with line voltage has been proposed to compensate voltage sag with phase jump. The operation and switching logic of this topology are explained in detail. The capability of the topology is tested for different types of voltage sags are compared with other topologies. This topology has the advantage of eliminating storage device and providing increased range of compensation. The efficacy of the proposed topology is validated through simulation and experimental studies. An intuitive method of classification of voltage sags [2], assorts sag into four basic types as shown in Fig. In the figure, the dashed lines represent the pre-sag voltage, and the solid lines represent the voltages during sag. The pre-sag voltages are given by V j , and during sag voltages by V0 j ,where j = a, b, and c. A single phase fault causes voltage sag in one phase (type B) at the terminals of a star connected load and in two phases (type C) at the terminals of a delta connected load. A phase-to-phase fault causes type C sag at the terminals of a star connected load and type D sag at the terminals of a delta connected load. A three phase symmetrical sag (type A) is caused by three phase fault. Further, voltage sag gets transformed into other sag types as it propagates in power system to lower voltage levels through transformers. Transformation of a voltage sag due to single phase fault i.e. type B sag, is illustrated in Fig. The type B sag when propagates through a star-delta transformer it transforms to a type C sag. When type C sag in-turn propagates through a star-delta transformer, it transforms to a type D sag. Each sag type is further classified into three subtypes based on the phase(s) that is/are affected. The subtypes are represented by a, b or c subscript, for easy reference. For instance, sag type Ba and Da have voltage sag in phase-a; while for sag type Ca, the line voltage bc is faulty and phase- a is healthy. Characterization of each type of sag is done in terms of the type and the complex characteristic voltage (V0 ch). The characteristic voltage defines three phase voltage sag. The phase voltages as a function of the characteristic voltage and the pre-fault voltage (which is usually 1 p.u.) is given in Table IV for the basic four types [2].



  • S. Vedam and M. S. Sarma, Power Quality: VAR Compensation in Power Systems. CRC press, 2009.
  • H. J. Bollen, Understanding Power Quality Problems. New York: IEEE press, 2000.
  • Mohseni, S. M. Islam, and M. A. Masoum, “Impacts of symmetrical and asymmetrical voltage sags on dfig-based wind turbines considering phase-angle jump, voltage recovery, and sag parameters,” IEEE Trans. Power Electron., vol. 26, no. 5, pp. 1587–1598, May 2011.
  • Massoud, S. Ahmed, P. Enjeti, and B. Williams, “Evaluation of a multilevel cascaded-type dynamic voltage restorer employing discontinuous space vector modulation,” IEEE Trans. Ind. Electron., vol. 57, no. 7, pp. 2398–2410, Jul. 2010.
  • W. Li, D. Vilathgamuwa, F. Blaabjerg, and P. C. Loh, “A robust control scheme for medium-voltage-level dvr implementation,” IEEE Trans. Ind. Electron., vol. 54, no. 4, pp. 2249–2261, Aug. 2007.

Performance of Distributed Power Flow Controller on System Behavior under Unbalance Fault Condition



Recently, within the family of FACTS, the distributed power flow controller is an additional device. This paper highlights on voltage sag mitigation which is one of the burgeoning power quality issues. It deals with the working concept of distributed power flow controller for compensating unbalanced three phase line currents in the transmission system. The single phase series converters of DPFC are able to compensate active as well as reactive, negative and zero sequence unbalanced currents. In this paper the performance of the DPFC has been studied by considering line to ground fault near the load end. The MATLAB/SIMULINK results obtained shows an improved performance in voltage sag mitigation, unbalance compensation, remarkable reduction in load voltage harmonics and also enhanced power flow control.


  1. DPFC
  2. Power flow control
  3. Reduction of load voltage harmonics
  4. Reliability improvement
  5. Voltage sag mitigation
  6. Unbalance fault condition



Fig. 1. Basic DPFC structure.


 Fig. 2. Load voltage sag waveform during unbalance fault.

Fig. 3. Mitigation of load voltage sag wave form during unbalance fault with DPFC.



Fig. 4. Load voltage. (a) Signal selected for calculating THD without DPFC.

(b) THD without DPFC.



Fig. 5. Load voltage. (a) Signal selected for calculating THD with DPFC.

(b) THD with DPFC.

Fig. 6. Capacitor dc voltage in dc side of shunt converter within DPFC.


This paper introduces the unbalance compensation and the voltage sag mitigation during unbalance fault condition by utilizing a recent additional FACTS device which is distributed power flow controller (DPFC) adopting sequence analysis technique. The DPFC is designed by employing three control loops. The simulated system has two machine systems, in presence and absence of the DPFC in the system. To examine the capability of the DPFC, an unsymmetrical L-G fault is taken into account near the load end side. In this paper simulation done verifies that the adopted control is able to give unbalance compensation and mitigation of voltage sag.


[1] N. G. Hingorani and L. Gyugyi, Understanding FACTS : Concepts And Technology of Flexible AC Transmission Systems. New York: IEEE Press, 2000.

[2] L. Gyugyi, C. D. Schauder, S. L. Williams, T. R. Rietman, D. R. Torgerson, and A. Edris, “The Unified Power Flow Controller: A New Approach to Power Transmission Control,” IEEE Trans. Power Del., vol. 10, no. 2, pp. 1085–1097, April 1995.

[3] Y. H. Song, and A. Johns, “Flexible AC Transmission Systems (FACTS),” Institution of Electrical Engineers (IEE Power and Energy Series), London, U.K:, vol. 30, 1999.

[4] K. Ramya and C. Christober Asir Rajan, “Analysis And Regulation of System Parameters Using DPFC,” IEEE International Conference on Advances in Engineering, Science And Management (ICAESM), March 2012, pp. 505-509.

[5] M. D. Deepak, E. B. William, S. S. Robert, K. Bill, W. G. Randal, T. B. Dale, R. I. Michael, and S. G. Ian, “A Distributed Static Series Compensator System For Realizing Active Power Flow Control on Existing Power Lines,” IEEE Trans. Power Del., vol. 22, pp. 642-649, Jan. 2007.