Multinivel

2398

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 57, NO. 7, JULY 2010

Evaluation of a Multilevel Cascaded-Type Dynamic Voltage Restorer Employing Discontinuous Space Vector Modulation
Ahmed M. Massoud, Member, IEEE, Shehab Ahmed, Member, IEEE, Prasad N. Enjeti, Fellow, IEEE, and Barry W. Williams

Abstract—In this paper, the application of a cascaded multilevel inverter as adynamic voltage restorer (DVR) is investigated. Two discontinuous multilevel space vector modulation (SVM) techniques are implemented for DVR control and are shown to reduce inverter switching losses while maintaining virtually the same harmonic performance as the conventional multilevel SVM at a high number of levels. This paper also presents a mathematical relationship for computing the distortion atthe point of common coupling (PCC) as a function of the distortion of the DVR. This enables the selection of the number of levels required for a certain application. An extended sag duration support compared to the two-level DVR is another advantage of the DVR with a cascaded multilevel inverter. The common-mode voltage (CMV) at the PCC has been evaluated for the three SVM techniques (theconventional multilevel SVM and the two discontinuous SVM), presenting a lower CMV for the second discontinuous SVM. A design example is presented for an 11-kV 5-MVA DVR multilevel cascaded inverter for up to 17 levels, employing the conventional multilevel SVM and the two discontinuous SVM techniques. Index Terms—Dynamic voltage restorer (DVR), multilevel inverter, space vector modulation (SVM).

Fig.1.

Proposed DVR in a distribution system.

I. I NTRODUCTION OLTAGE SAG is defined as a sudden reduction of the supply voltage to less than 90% of the rated voltage, according to EN 50160 [1]. Voltage sag is considered an important factor in electric system power quality particularly with the proliferation of electronic devices sensitive to electrical disturbance [2]. In textile and papermills, a brief voltage sag may potentially cause an adjustable speed drive (ASD) to introduce speed fluctuations which can damage the end product. Furthermore, a brief voltage sag also causes a momentary decrease in dc-link voltage, triggering an undervoltage trip or resulting in an overcurrent trip. Such nuisance tripping of ASD equipment employed in continuous-process industries contributes to loss inrevenue and can incur other costs.
Manuscript received March 15, 2009; revised August 8, 2009 and November 21, 2009; accepted December 19, 2009. Date of publication February 8, 2010; date of current version June 11, 2010. A. M. Massoud is with Qatar University, Doha 2713, Qatar (e-mail: ahmed.massoud@qu.edu.qa). S. Ahmed and P. N. Enjeti are with Texas A&M University at Qatar, Doha 23874, Qatar(e-mail: Shehab.ahmed@qatar.tamu.edu; Prasad.enjeti@ qatar.tamu.edu). B. W. Williams is with the University of Strathclyde, G1 1XQ Glasgow, U.K. (e-mail: barry.williams@eee.strath.ac.uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2010.2041732

V

As a remedy, the uninterruptible powersupply (UPS) is used for protecting loads from voltage disturbances such as voltage sags or outages. However, in high power ratings, the UPS is not able to support the load. Therefore, the need exists to find a device that supports the load without replacing the supply. Connecting a dynamic voltage restorer (DVR) in series achieves the objective. The DVR (shown in Fig. 1) is a power-electronicbaseddevice used for supporting the load voltage during supply voltage sag (and can be extended to include voltage swell compensation). The DVR has been discussed in literature [3]–[30]. The first DVR was installed in 1996 for the Electric Power Research Institute on the Duke Power Company in North Carolina by Westinghouse [3]. Table I summarizes the salient features of the DVR in the literature. Although...