analysis of converter transformer failure in hvdc systems

814 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 2, APRIL 2009

Analysis of Converter Transformer Failurein HVDC Systems and Possible Solutions

G. Bhuvaneswari, Senior Member, IEEE, and B. C. Mahanta

Abstract—An HVDC transmission system has a converter trans-former as one of its main components. The failure of the convertertransformer is one of the major concerns for electric power utili-ties all over the world. Invariably, the top portions of the secondarywindings of the converter transformers fail whereas the primariesare left unaffected. In this paper, an effort has been made to an-alyze the causes for these failures by means of modeling a prac-tical HVDC system existing in India which ties up Talcher andKolar and has a length of 1368 km. The modeling and analysishave been carried out in the MATLAB/SIMULINK environment.Based on the analysis, possible solutions for this problem have beensuggested, such as providing passive filters on the secondary wind-ings of the converter transformer, connecting a parallel capacitoron the dc side of the converter and R-C snubbers across the sec-ondary windings. The suggested solutions have been compared tobring out their relative merits and demerits.

Index Terms—Commutation overlap, converter transformerfailure, harmonics, HVDC system.

I. INTRODUCTION

D UE to the evolution of power semiconductor devices,HVDC transmission has been gaining popularity ever

since its first commercial operation in 1954. The advent ofHVDC technology [1] has been so rapid that it is widelyapplied all over the world for bulk power transmission overlong distances. It is popularly employed for interconnectingtwo asynchronous systems not only through overhead linesbut also through submarine cables. The power supply is madeavailable to islands and remote places by means of HVDCtransmission. Due to revolutionary progress in flexible actransmission systems (FACTS) devices [2], HVAC is emergingas a tough competitor of the power carrier to HVDC. But still,the HVDC transmission system has an edge over HVAC, due tothe advent of higher capacity power-electronics devices, suchas insulated-gate bipolar transistors (IGBTs), integrated-gatecommutated thyristors (IGCTs), metal–oxide semiconductorfield-effect transistors (MOSFETs), and gate turnoff thyristors(GTOs). But most of the HVDC stations still use the thyristor

Manuscript received July 16, 2007; revised October 25, 2007. Current versionpublished March 25, 2009. Paper no. TPWRD-00413-2007.

G. Bhuvaneswari is with the Department of Electrical Engineering, In-dian Institute of Technology, New Delhi, New Delhi 110016, India (e-mail:[email protected]).

B. C. Mahanta is with NTPC Ltd., Talcher, Orissa 759101, India (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRD.2009.2014271

as the switching device for the converter-inverter circuit.Voltage-source converters (VSCs) [3], which use self-commu-tating devices such as IGBTs or GTOs, are increasingly beingused for HVDC transmission systems with limited power-han-dling capacity known as “HVDC Lite.” These do not cause anypower-quality (PQ) problems. HVDC systems are also gainingimportance because of the increased use of renewable energyresources for power generation. HVDC systems are used totransmit electricity from remotely located nonconventionalenergy sources to thickly populated cities. Even in conventionalpower stations, it is easier to transmit electricity by HVDCfrom a remotely located power station closer to the locationof the coal or natural gas, rather than to set up a power stationcloser to the thickly populated city and to transport coal fromremotely located coal mines.

The converter transformer is one piece of vital equipment inthe HVDC transmission system. It serves as the isolating de-vice between the power transformer and the dc link and stepsdown the voltage as required by the thyristor converters. It isequipped with onload tap changers on the primary side to main-tain the ac voltage supplied to the thyristor converters constantat all conditions. But frequent failure of the converter trans-former is a major cause for concern to the electric power utili-ties all over the world. Invariably, it is found that the secondarywinding of the converter transformer is the one that fails. Gener-ally, 12 pulse thyristor converters are employed in HVDC trans-mission to eliminate the fifth and seventh harmonics and this isachieved by connecting the secondary windings in Y- fashionto introduce a 30 phase shift. But the fifth and seventh har-monics are very much present in the secondary windings al-though they are absent in the primary side. The secondary wind-ings should have been designed to withstand these harmoniccontents. The failure analysis of HVDC systems reported byCIGRE [4] states that out of 22 failures in the last few years,14 failures were secondary winding failures. In India, almost allfailures have taken place on the secondary windings of the con-verter transformers. Such failures have been attributed to corro-sive oil-forming copper sulphide sediments, voltage transientsarising during the commutation process, and temperature rise. Ithas also been suggested that repetitive voltage transients initiatepartial discharge and eventually result in the failure of the con-verter transformer [5], [6]. The harmonic leakage fluxes causethermal problems in the converter transformer which may leadto its failure as discussed in [7]. Grant and McDermid in [8]stated that a converter transformer shows signs of insulationdegradation due to thermal aging after a few decades of normaloperation. In this paper, an effort has been made to analyze theHVDC system from an electrical power engineer’s point of view

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BHUVANESWARI AND MAHANTA: ANALYSIS OF CONVERTER TRANSFORMER FAILURE 815

Fig. 1. Schematic of the converter transformer connection.

to find out the root cause for the failure of the converter trans-former secondary. Some solutions have also been suggested toeliminate these problems.

II. SYSTEM CONFIGURATION

In the present scenario, converter transformers are made upof single-phase—three winding transformer units. The primarysides are connected in star with grounded neutral. Secondarywindings of the converter transformers are connected in Y-to achieve a 30 phase shift so that the 12-pulse thyristor con-verters can be fed. Fig. 1 shows the schematic of the convertertransformer connections. Two six-pulse thyristor groups areconnected in series so that each converter carries the full-loaddc-link current and develops half the dc-link voltage. Due tothe 12-pulse thyristor converter operations, the 11th, 13th,23rd, and 25th harmonics are generated in the source side ofthe converter. Hence, the converter acts as a harmonic currentsource, thus polluting the power supply. But the convertersgenerate 12th, 24th, and 36th harmonic voltages on the dc side.Generally, passive harmonic filters are installed on the sourceside to mitigate these harmonics and to supply the reactivepower required by the converters at fundamental frequency.Similar passive filters are also connected on the dc side tomitigate harmonics. These filters are generally of the single ordouble tuned type. Some of the stations are employing activefilters on the dc side of the converters. But to the authors’ bestknowledge, only one station (the Tjele converter station inDenmark) has employed active filters on the ac side all over theworld [9].

III. MODELING AND ANALYSIS

The Talcher–Kolar HVDC line has been taken as the refer-ence model for analysis of the secondary winding harmonicsand their effects on the converter transformer. In this HVDCsystem, 2000 MW of power is transmitted through the bipolarlink. Talcher, situated on the eastern coast of India, is the sourceside of this bipolar link and Kolar on the south of India is thereceiving end of the link. The total length of this HVDC trans-mission line is 1368 km and it is the second longest HVDC

line to date in the world. The NTPC Kaniha Station, whichis the biggest power station in India, has a total capacity of3000 MW from which 2000-MW power is being tapped by thisHVDC line. The voltage rating is 500 kV and the full-loadcurrent is 2000 A. Three single-phase transformers, each of thethree winding type, are connected in Y- -Y and two thyristorconverters are connected in series to achieve 12-pulse opera-tions. Each leg in the bridge rectifier consists of 84 thyristorsin series for the upper portion and another set of 84 thyristorsfor the lower portion. Each thyristor has an R-C snubber con-nected across it with the values being 45 and 1.6 F, re-spectively. So, each 12-pulse converter is made up of 84 12

1008 thyristor devices. Passive filters of the double tunedtype are connected in the source side to eliminate the 11th-,13th-, 23rd-, and 25th-order current harmonics. In the dc side,double-tuned passive filters are connected to eliminate the 12th-and 24th-order voltage harmonics. The capacitors in the pas-sive filters on the ac side act as the source of reactive powerfor the converters at the fundamental frequency. The system ismodeled in Simulink using the power system blockset and var-ious analyses are carried out. Fig. 2 depicts the schematic of thisSimulink [10] model.

IV. SIMULATION OF THE SYSTEM AT FULL LOAD

The system has been simulated at full load along with pas-sive filters in service. The system parameters are presented inthe Appendix. It is observed that the source-side voltage andcurrent total harmonic distortions (%THD) are within 2%. Butwhen the voltages and currents in the secondary winding of theconverter transformer are analyzed, it is found that voltage andcurrent total harmonic distortion (THD) are greater than 20% ata firing angle of 13 at the sending end rectifier. Hence, it canbe concluded that secondary windings are highly stressed withthese harmonic burdens of load. Also, it is observed from thesecondary winding voltage waveforms that the rate of changeof voltage with respect to time (dv/dt) across the secondarywinding is around 1.5 kV/ s and it is repetitive in nature.These voltage transients are always present in the secondarywinding of the converter transformer and increase as the firingangle increases. While designing the converter transformer,these harmonics should have been taken into consideration andgraded insulation for the secondary winding should have beenput in place. However, the stress level to which these windingsis being subjected to is quite large as the voltage waveformsindicate. This leads to the partial discharge phenomenon and,thus, the failure of the converter transformer eventually be-comes inevitable.

Since voltage THD, current THD, and the waveforms areidentical for star- and delta-connected transformer secondarywindings and for sending and receiving ends of the HVDC line,the waveforms and the corresponding THD of the voltage andcurrents at the sending end are presented here. The voltage andcurrent waveforms and their THD in the primary and secondaryside of the converter transformers at the sending end are shownin Figs. 3–6, respectively. The corresponding THD values havebeen tabulated in Table I.



Fig. 2. Schematic of the Simulink model of the Talcher–Kolar bipolar HVDC system.

Fig. 3. Waveforms at the primary of the converter transformer at the sendingend. (a) Voltage waveform. (b) Current waveform.

V. POSSIBLE CAUSES OF CONVERTER TRANSFORMER FAILURE

From the simulation results, it has been seen that the fol-lowing two phenomena might be primary causes for convertertransformer failures.

A. High Harmonic Contents on the Secondary Winding of theConverter Transformer

Both voltage and current THD are very high % in thesecondary side of the converter transformer. Hence, the insula-

Fig. 4. Harmonic spectrum of the voltage and current at the primary of theconverter transformer at the sending end. (a) Voltage THD. (b) Current THD.



Fig. 5. Waveforms at the secondary of the converter transformer at the sendingend. (a) Voltage waveform. (b) Current waveform.

Fig. 6. Voltage and current THD at the secondary of the converter transformerat the sending end. (a) Voltage THD, (b) Current THD.

tions of the secondary windings are severely stressed due to thepresence of these higher order harmonics.

TABLE IVARIOUS THD AT FULL LOAD WITH THE EXISTING SYSTEM

B. High Commutation Overlap

The converter transformer source impedance is very high and,hence, voltage transients are generated during the commuta-tion process. These commutation voltage spikes that are seenduring the changeover of current from one phase to another,are repetitive in nature. The rate of change of voltage (dv/dt)across the secondary windings is observed to be 1.5 kV/ s.These voltage transients [4] stress locally the part-to-part andturn-to-turn insulation of the secondary windings of the con-verter transformer. These transients increase as the firing angleincreases and degrade the insulations of the secondary wind-ings of the converter transformers. CIGRE analysis [4] of theconverter transformer failure clearly describes that commuta-tion voltage transients have been the major cause for convertertransformer failure in the recent past. As per Greenwood [11],the transient surge effects across the transformer windings areas follows.

1) Uneven voltage distribution across the winding: In the ini-tial part of the transient period which exists for a few mi-croseconds, no significant amount of current can penetratethe winding due to inductance. Currents during this periodflow as capacitive currents in the winding. If there is noproper insulation grading of the individual turns, then theturns adjacent to the line will be highly stressed comparedwith the turns closer to the neutral. As the voltage transientsin the converter transformer are repetitive in nature and ifthere is any possibility of improper insulation grading inthe windings, then insulations will be stressed and prone tofailure in the long run even if this transient period is verysmall.

2) Oscillations of the winding in space: During the secondtime interval, this transient surge voltage distribution givesrise to a complicated system of oscillations within thewinding. Here, inductive and resistive elements come intothe picture. Space harmonics are generated and oscillationof the winding takes place. Since the surge is aperiodicand contains a spectrum of frequencies when it strikes atthe transformer terminals, different frequencies penetratethe winding at different velocities. These phenomenoncontinue until the critical frequency of the winding.Frequencies greater than this critical frequency cannotpenetrate the winding, and the winding behaves like afilter.

Hence, both of these phenomena cause an immense amountof stress to the insulations of the secondary winding of the con-verter transformer. Insulation grading across the length of thewinding is carried out, but, in practice, it is difficult to achieveoptimal grading of the insulation according to the literature.



Fig. 7. Schematic of the capacitors connected to the dc side of the converters.

Fig. 8. Waveforms on the secondary winding of the converter transformer withcapacitors connected across the dc side of the converter. (a) Voltage waveform.(b) Current waveform.

VI. PROPOSED SOLUTIONS TO OVERCOME THE PROBLEMS

ASSOCIATED WITH THE SECONDARY WINDINGS

Due to the higher source impedance of the converter trans-former, voltage transients are introduced during the changeoverof current from one phase to another. A reduction of thesevoltage transients can be achieved in the following ways:

1) reduction of commutation overlap;2) inclusion of the filters on the secondary of the converter

transformer;3) introducing RC snubbers on the secondary of the converter

transformer.1) Reduction of Commutation Overlap: Commutation

overlap can be reduced by introducing the capacitor [12], [13]on the dc side of the converter as depicted in Fig. 7. It willprevent the dc-link voltage from sagging. The capacitor willalso counter the effect of source inductance to decrease thecommutation overlap. Thus, the commutation process becomesfaster and voltage transients during commutation will be con-tained. When the system is simulated with this configuration,there is a reduction of voltage spikes. The maximum voltagespike is observed to be 150 kV during commutation which wasoriginally 200 kV without the capacitor. Voltage THD is ob-served to be reduced to 17% from 20%. Only the fifth harmonic

Fig. 9. THD of the voltage and current on the secondary winding of the con-verter transformer with capacitors connected across the dc side of the converters.(a) Voltage THD. (b) Current THD.

TABLE IIVARIOUS THD AT FULL LOAD WITH CAPACITORS CONNECTED

ON THE DC SIDE OF THE CONVERTERS

current is observed to be predominant in the secondary windingof the converter transformer. The R-C values considered forsimulation are 65 and 89.54 F. The capacitancevalue is decided with an aim to nullify the reactance effect ofthe converter transformer connected to each 6-pulse thyristorconverter. The value of is decided by a hit-and-trial byrepeating computer simulations with an aim to minimize thelosses in the resistance and to keep the source current withinlimits. The corresponding voltage and current waveforms andthe respective harmonic spectrums are shown in Figs. 8 and 9and in Table II.

2) Inclusion of Filters on the Secondary of the ConverterTransformer: Generally, filters are included in the source sideand in the present configuration, there is no space to add filtersin the secondary of the converter transformer. But if there is away to introduce filters on the secondary side, then it will help



Fig. 10. Waveforms on the secondary winding of the converter transformerwith capacitors connected across the dc side of the converters and fifth and sev-enth harmonic passive filters in service. (a) Voltage waveform. (b) Current wave-form.

to reduce the secondary winding harmonics and the cost on theprimary side filters can also be reduced. The system has beensimulated with fifth harmonic filters on the secondary side ofthe converter transformer along with capacitor on the dc sideof the converters. It is observed that current THD on the sec-ondary side is getting reduced to 7% from 21% and voltageTHD is observed to be coming down to 14% from 21%. Whenthe fifth and seventh harmonic passive filters are put on the sec-ondary side, further improvement in current harmonics is ob-served. In both cases, the maximum voltage spike is observedto reduce to 125 kV from 200 kV. Hence, it will definitely beuseful to include passive filters in the secondary side to reducethe voltage stress across the secondary windings of the convertertransformer. Here, the fifth and seventh harmonic filters willmitigate the fifth and seventh harmonic current, and the reac-tive power requirement by the converters will be supplied bythe filters on the source side. However, space has to be createdfor introducing the fifth and seventh harmonic filters on the sec-ondary side of the transformer. Also, the filters on the sourceside can be further simplified if part of the reactive power re-quirement can be met by the secondary side filters. The capac-itor connected across the dc side of the converter will help toreduce the commutation overlap. The voltage and current wave-forms and corresponding THDs are depicted in Figs. 10, 11, andTable III, respectively.

Design of Fifth and Seventh Harmonic Passive Filters:Single-tuned passive filters are designed according to [12]. Thevalue of has been taken as 0.5 F; accordingly, the value of

has been calculated by using LC and a qualityfactor of 150 has been chosen. A smaller value of is taken inorder to reduce the reactive power generation as it has alreadybeen supplied by passive filters present on the source side.

3) Provision of RC Snubbers on the Secondary of the Con-verter Transformer: Excessive overvoltages can occur in thetransformer windings if the exciting overvoltage has the samefrequency as the natural frequency [13], [14] of a winding or

Fig. 11. Voltage and current THD on the secondary winding of the convertertransformer with capacitors connected across the dc side of the converters andfifth and seventh harmonic passive filters in service. (a) Voltage THD. (b) Cur-rent THD.

TABLE IIIVARIOUS THD AT FULL LOAD WITH FIFTH AND SEVENTH HARMONIC FILTERS

AND CAPACITORS CONNECTED ON THE DC SIDE OF THE CONVERTERS

a winding section. In these cases, the overvoltages set up aresonance oscillation inside the winding with high amplitudethat can cause damage to the insulation. To avoid these, RCsnubbers can be connected from terminals to ground or inparallel to the winding section. Soyal [15] has stated thathigh-frequency transient voltages below the protection level ofthe surge arrestor can cause severe internal voltage stresses thatcan result in dielectric breakdown of the winding section. Theseswitching overvoltages can be suppressed by RC snubbers.However, there is no straightforward procedure for decidingthe values of and . These have to be checked by meansof computer simulations, keeping the losses to a minimum[14]. It is observed that there is no provision for the surgearrestor in the secondary of the converter transformer in theTalcher–Kolar system, although lightning arrestors are installed



Fig. 12. Waveforms with the RC snubbers connected between the winding ter-minals at the secondary of the converter transformer. (a) Voltage waveform. (b)Current waveform.

on the primary of the converter transformer. Zinc–oxide varis-tors are connected across the converters to protect them againstovervoltages. To check the validity of the proposed solution,the HVDC system has been simulated by connecting RC snub-bers (a) across the secondary windings and (b) between phaseterminals and ground. The voltage THD comes down to 15%in this case. The rate of change of voltage—dv/dt also reducesconsiderably. Hence, if the snubbers are properly chosen, theywill help to reduce insulation stresses in the secondary windingof the converter transformers.

The design of the capacitor is based on the energy balancemethod [13]. But there is no definite procedure to calculate thevalue of resistance. Here, the capacitance value is taken as 3

F and 750 . Fine tuning of the R-C parameters will re-duce the losses. It is found that whether snubbers are connectedacross the winding terminals or they are connected between lineand ground, the results obtained are identical. The waveformsobtained by connecting snubbers across winding terminals andthe corresponding THDs are presented in Figs. 12 and 13 and inTable IV.

Thus, the analysis on the stresses of the converter trans-former secondary can be summarized as follows: The stressesare caused invariably by harmonics present in the voltages andcurrents in the secondary winding. These are predominantlythe fifth and seventh order. The commutation overlap causedby large reactance of the transformer results in a huge rateof the change of voltage. These ill effets can be minimizedby the following techniques: 1) connecting a capacitor in thedc link, 2) connecting passive filters on the secondary side,and 3) connecting RC snubbers across the phase windings orwinding terminal to ground. Out of these three solutions, acombination of the dc-link capacitor and passive filters seems tobe yielding the best results. However, due to the limitations ofthe Simulink/Matlab software, transient modeling of the trans-former winding could not be performed accurately. To obtainaccurate results on the stress reduction with the introduction of

Fig. 13. Voltage and current THDs with RC snubbers connected between thewinding terminals at the secondary of the converter transformer. (a) VoltageTHD. (b) Current THD.

TABLE IVVARIOUS THD WITH RC SNUBBERS CONNECTED

BETWEEN THE WINDING TERMINALS

secondary side filters and R-C snubbers, an accurate transientmodeling of the transformer winding is very much essential.

VII. CONCLUSION

This paper presented the analysis of the HVDC transmis-sion system of the 2000-MW capacity that exists betweenTalcher and Kolar in the Indian subcontinent. It is found thatalthough the filters installed in the primary side of the convertertransformer eliminate the harmonics in the source end, thesecondary windings are very much affected by the harmonicsand the voltage spikes caused by commutation overlap. Threesolutions have been proposed by the authors to reduce theintensity of problems caused by voltage transients. From thesimulation results obtained, it is found that a combination ofpassive filters of the fifth and seventh order installed in thesecondary side of the transformer along with an R-C snubber inthe dc link yields maximum reduction in the current harmonicsand commutation overlap voltage spikes. It is envisaged that the



installation of filters on the secondary side will also simplify thedesign of secondary windings of the converter transformer andthe requirement of filters on the source side will be reduced.RC snubbers with a suitable design on the secondary of theconverter transformer can provide relief to insulation stressesacross the winding.

APPENDIX

Technical parameters of the Talcher–Kolar HVDC system.Generator: 588 MVA, 21 kV, Yg, % %.Generator transformer: 600 MVA, 21/420 kV, Yg,

% %.Converter transformer: 397/198.5/198.5 MVA, 400/210/210 kV, 1700/1635/1635 A, Impedance %.Filters in service: double-tuned passive filters.Resistance of the transmission line: 12.7–20.1 /pole.

REFERENCES

[1] J. Arrillaga, High Voltage Direct Current Transmission. London,U.K.: Inst. Elect. Eng. Press, 1998.

[2] Hingorani and Gyugyi, “Understanding FACTS,” in Standard Pub-lishers and Distributors. Piscataway, NJ: IEEE Press, 2001.

[3] T. Larsson, A. Edris, D. Kidd, and F. Aboytess, “Eagle pass back-to-back tie: A dual purpose applications of voltage source converter tech-nology,” in Proc. IEEE Power Eng. Soc. Summer Meeting, Jul. 15–19,2001, vol. 3, pp. 1686–1691.

[4] G. M. Bastos, J. C. Brandao, J. Santelli, R. Albuquerque,S. Arenare, J. C. Mendes, and J. Hajek, “HVDC convertertransformer performance on ITAIPU system,” presented at theB4-201, CIGRE Paris Symp., Paris, France, 2006. [Online].Available: http://library.abb.com/global/scot/scot221.nsf/verity-display/6c9b4a9ad68589f4c125708a002ec4f0/$File/Active%20Fil-ters%20In%20Hvdc%20Transmission.pdf.

[5] W. McDermid, A. Glodjo, and J. C. Bromley, “Analysis of windingfailures in HVDC conveter transformer,” in Proc. Electrical InsulationElectrical Manufacturing Coil Winding Conf., Oct. 1999, pp. 653–657.

[6] W. McDermid et al., “Analysis of converter transformer failures andapplication of periodic on-line partial discharge measurements,” inProc. Electrical Insulation Electrical Manufacturing Coil WindingConf., Oct. 2001, pp. 577–582.

[7] J. Alan, C. Forrest, and B. Allard, “Thermal problems caused by har-monic frequency leakage fluxes in three-phase three-winding convertertransformers,” IEEE Trans. Power Del., vol. 19, no. 1, pp. 208–213,Jan. 2004.

[8] D. H. Grant and W. McDermid, “Assessment of thermal aging ofHVDC converter tyransformer insulation,” in Proc. IEEE Conf. Int.Symp. Electrical Insulation, Sep. 2004, pp. 230–232.

[9] S. Gunnarsson et al., Active filters in HVDC transmission. [Online].Available: http://www.abb.com/hvdc.

[10] Simulink/MATLAB Manual—On-Line Documentation. Natick,MA, MathWorks. Inc.

[11] A. Greenwood, Electrical Transients in Power Systems. New York:Wiley, 1991.

[12] J. Arrillaga and N. R. Watson, Power System Harmonics, 2nd ed.New York: Wiley, 2003.

[13] C. Zhao, H. Cui, and G. Li, “A novel HVDC transmission system withparallel large capacitor connected in the DC side of the rectifier andits technical feasibility,” in Proc. IEEE/Power Eng. Soc. TransmissionDistribution Conf.: Asia and Pacific Dalian, China, 2005, pp. 1–5.

[14] P. Heinzig, W. Knorr, C. Ploetner, A. Kramer, and M. Kolbol, “Long-time experiences of ZnO varistor application in power transformers andOLTC’s,” in CIGRE Paris Symp., 2006.

[15] O. A. Soyal, “Voltage stresses in a distribution transformer under non-ideal switching conditions,” in Proc. IEEE Power Eng. Soc. WinterMeeting, 1999, vol. 2, pp. 1031–1035.

G. Bhuvaneswari (SM’99) received the M.Sc. andPh.D. degrees in electrical engineering from the In-dian Institute of Technology (IIT), Madras, India.

She was a faculty member with Anna University,Madras, for about two years after which she waswith the Electrical Utility ComEd, Chicago, IL.Since 1997, she has been a faculty member in theDepartment of Electrical Engineering, IIT, Delhi,where she is currently an Associate Professor. Herareas of interest are power electronics, electricalmachines, drives, and power quality.

Dr. Bhuvaneswari is a Life Fellow of the Institution of Electronics andTelecommunication Engineers.

B. C. Mahanta received the M.Tech. degree in power generation technologyfrom the Indian Institute of Technology, Delhi, India, in 2007.

He has more than a decade of experience in the power generation sector. Cur-rently, he is with NTPC Ltd., Talcher, India. His areas of interest are powersystems, power quality, and drives.


analysis of converter transformer failure in hvdc systems

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