Analysis of Common-Mode Voltage in Utility-Interactive Fuel Cell Power Conditioners

20 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 1, JANUARY 2009

Analysis of Common-Mode Voltage inUtility-Interactive Fuel Cell

Power ConditionersLeonardo Palma, Member, IEEE, Maja Harfman Todorovic, Student Member, IEEE, and

Prasad N. Enjeti, Fellow, IEEE

Abstract—Fuel cell stacks produce a dc output with a 2 : 1voltage variation from no load to full load. It is customary for autility-scale fuel cell stack to consist of several hundreds of series-connected cells, each producing 0.6 V at full load. A power con-ditioner consisting of dc–dc and dc–ac converters is required forutility interface, which are operated in high frequency, employingpulsewidth-modulation control for voltage and current regulation.Due to their switch-mode nature, a common-mode voltage withrespect to ground is generated. The common-mode voltage, inturn, contributes to the circulating ground current, which can in-terfere with the ground fault protection system. In addition, it alsocontributes to the neutral shift and electromagnetic interference.Moreover, the electrostatic potential to ground within the fuel cellstack needs to be limited for safe operation. This paper presentsan analysis of common-mode voltage in several fuel-cell-poweredconverter topologies connected to the electric utility and discussesseveral mitigation methods suitable for utility-scale generation.

Index Terms—Common-mode current, common-mode voltage,fuel cell, utility interactive.

I. INTRODUCTION

FUEL CELLS have been recognized as one of the mostpromising energy sources for power generation in the

near future, particularly high-temperature fuel cells such assolid oxide fuel cells (SOFCs) and molten carbonate fuel cells(MCFCs), which have sufficient potential in terms of overallsystem efficiency and operation costs to compete with con-ventional power plants in the megawatt power range [1]. Thetypical efficiency of conventional power plants ranges from38% to 40%, whereas the efficiency of an SOFC is in therange of 55% to 60%. That is, fuel-cell-based plants have anefficiency that is around 20% higher than conventional systems.On the other hand, the cost of generating power in a fuel-cell-based plant is slightly higher than in conventional systems($0.12/kWh) [1].

Fuel cells are constructed by stacking many cells in series.Since the voltage produced by each cell is around 0.6 Vdc at full

Manuscript received January 31, 2008; revised July 31, 2008. First publishedAugust 26, 2008; current version published December 30, 2008.

L. Palma is with the Universidad de Concepción, 4050030 Concepción, Chile(e-mail: [email protected]).

M. H. Todorovic and P. N. Enjeti are with the Power Electronics and FuelCell Power Conversion Laboratory, Department of Electrical and ComputerEngineering, Texas A&M University, College Station, TX 77840 USA (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/TIE.2008.2004382

Fig. 1. System parasitic capacitances and common-mode coupling paths.

load, many cells need to be stacked to obtain a higher voltage.Considering the aforementioned reasons, the maximum voltagethat a fuel cell stack can safely produce is around 350 V [2]. Toachieve a higher dc-link voltage, two stacks can be connectedin series, and their midpoints are to be tied to ground.

Since fuel cells have a wide variation in their output voltagefrom no load to full load, a power converter is required tointerface the fuel cell stack with the utility. The power converteris usually constructed using a two-stage approach (Fig. 1) witha dc–dc converter connected in series with an inverter. Bothconverters are normally operated by pulsewidth modulation(PWM).

The operation of a power converter produces two types ofvoltage outputs: a differential voltage output and a common-mode voltage output [3]. The differential output is the intendedoutput voltage of the system. On the other hand, the common-mode voltage is a voltage measured between the output lines ofthe converter and ground and is usually referred to as “neutralshift” since the voltages in the converter elevate with respect toground [3].

Since the converter is operated using PWM, the common-mode output voltage has abrupt transitions to and from the dcbus voltage. These transitions have the effect of inducing largecurrent spikes due to dV/dt across stray capacitances to grounddistributed throughout the circuit (Fig. 2). These current spikeshave a short duration and a larger amplitude, and they are afunction of the parasitic capacitance to ground and the rise andfall times of the common-mode voltage (Fig. 2). The time ittakes for the voltage to change from one voltage level to anotheris essentially controlled by the semiconductor switching time(rise and fall times), which is inherent to the technology usedfor the particular transistor.

0278-0046/$25.00 © 2008 IEEE

PALMA et al.: ANALYSIS OF COMMON-MODE VOLTAGE IN UTILITY-INTERACTIVE FUEL CELLS 21

Fig. 2. Effect of common-mode dv/dt in stray capacitances.

Fig. 3. Fuel cell power converter topology 1 with a line frequency isolationtransformer.

Power converters are enclosed in metal cabinets; thus, mostof the electromagnetic interference (EMI) generated is due tothe current circulating through ground (Fig. 1), which can alsocause interference in ground fault protection systems [3]. Theeffect of the common-mode noise induced by these currents onother equipment is a function of the distance separating thenoise generation and reception. Therefore, it is important tokeep the circulation paths of common-mode currents as shortas possible.

In this paper, the generation of the common-mode voltage inthree types of power converter topologies for utility-connectedfuel cell systems is analyzed. In addition, methods to reduce themagnitude of the common mode are proposed.

II. ANALYSIS OF COMMON-MODE VOLTAGE

IN FUEL CELL SYSTEMS

Multiple converter configurations for the power conditionerare possible; however, there are three basic structures fromwhich other topologies can be derived. In this paper, analysison those three main configurations is carried out.

A. Nonisolated DC–DC Converter, Inverter, andLine Frequency Transformer

Fig. 3 shows the topology of a fuel cell power converter em-ploying two fuel cell stacks, with their center point grounded.

Fig. 4. Common-mode equivalent circuit for the fuel cell topology 1 (Fig. 3).

A nonisolated dc–dc boost converter is utilized along with aninverter stage and a line frequency transformer. The function ofthe dc–dc conversion stage is to regulate the dc-link voltage Vo,and the inverter operates in PWM to generate a three-phaseoutput to interface with the utility.

The analysis of the generation of the common-mode voltageand current in this system is simplified if an equivalent circuitis used.

From the common-mode voltage point of view, each of thelegs in the three-phase inverter can be modeled as a switch-mode voltage source from the midpoint of the dc link “o” toone of the output lines of the inverter. On the other hand, thecommon-mode voltage generated at the output terminals of theboost converter can be calculated as follows:

Vdg = Vo − Vfc2 (1)

Veg = −Vfc2. (2)

Based on this, the common-mode equivalent circuit for thistopology is given by the schematic shown in Fig. 4. The PWMvoltages Vao, Vbo, and Vco are generated by the three inverterlegs (Fig. 3). In addition, the voltages Vao, Vbo, and Vco canonly occupy ±Vo/2 voltage levels. Thus, the sum of the threevoltages is nonzero and results in a common-mode voltagebetween points “f” and “g.” Furthermore, the utility transformerwinding to ground capacitance is lumped in the equivalentcapacitance Cfg.

In Fig. 4, it can be seen that the common-mode current Icm isdue to the common-mode voltage produced by the inverter andthat appears between point f and ground, i.e.,

Vfg =Vao + Vbo + Vco

3+ Vog (3)

where Vog is given by

Vog =Vde

2− Vfc2 =

Vo

2− Vfc2. (4)

From (3) and (4), the common-mode current can be calcu-lated by

Icm = Cfgd

dt

(Vao + Vbo + Vco

3+ Vog

). (5)


Fig. 5. Simulation results for topology 1. (a) Common-mode voltage Vfg.(b) Common-mode current Icm.

Equation (4) shows that the voltage Vog between the centerpoint of the dc link and ground is due the voltage of the lowersection of the fuel cell and the output voltage of the dc–dcconverter, which are dc quantities. Thus, the voltage Vog in(5) does not contribute to the generation of the common-modecurrent. Therefore, the circulating current is only due to theoperation of the three-phase inverter.

Fig. 5 shows the results obtained from simulations on PSimof the nonisolated system with a 700-V dc link suppliedfrom a stack constructed by two 330-V fuel cells and witha transformer capacitance to ground Cfg of 550 pF. As canbe observed from these results, the common-mode currentcirculating through ground has a peak value of 4 A. In addition,as shown in (3), the common-mode voltage has an offset due tothe voltage that appears from the center point of the dc link toground.

From (5), it can be seen that the peak amplitude of thecommon-mode current can be reduced if the rise and fall timesof the voltage Vfg are increased. It has been shown in [6]that this can be accomplished by connecting a low-pass filterbetween the inverter output terminals and the dc link, as shownin Fig. 6. Filter components can be designed according to theequations developed in [6], where the design criterion is basedon increasing the rise time tr of inverter output pulses suchthat the filter time constant is larger than a critical rise time.The value of the critical rise time is a function of all theparasitic capacitances in the system and can experimentally bedetermined as follows:

√LfCf ≥ tr,critical. (6)

The effectiveness of the common-mode filter can be verifiedin Fig. 7. Both the rise and fall times of the common-modevoltage Vfg are increased. Thus, it can be seen that the peakamplitude of the common-mode current is considerably re-duced to 0.25 A.

In addition, from the results obtained, it is clear that inthis topology, the neutral of the line transformer cannot beconnected to ground. If this point is grounded, the common-mode impedance is reduced to zero, resulting in a short circuitthrough ground.

B. Isolated DC–DC Converter and Inverter Topology

The second possible topology for the power converter isusing an isolated dc–dc converter such as a full bridge, which iscomposed of an H-bridge inverter, a transformer, and a dioderectifier instead of a boost converter, as shown in Fig. 8. Inthis case, the line frequency transformer is not needed, andthe inverter stage can directly be connected to the utility. Asdone in the previous section, an equivalent circuit is obtained bymodeling each inverter leg as a common-mode voltage source(Fig. 9). The transformer in the dc–dc converter is modeled bylumped capacitances from primary and secondary to ground[Fig. 11(a)], which represent the distributed parasitic capaci-tances of each turn to ground in both primary and secondary [7].

An additional capacitance from secondary to primary mod-els the parasitic capacitance between primary and secondary[Fig. 11(a)].

In Fig. 9(a), the voltage from point “o” to ground can becalculated by

Vog =Vao + Vbo + Vco

3. (7)

In the same way, the common-mode voltage generated by thedc–dc converter and its rectifier are given by

Vpg =Vpx + Vpy

2(8)

Vso =Vsd + Vsf

2. (9)

The equivalent circuit can further be simplified using (7)–(9),resulting in the model shown in Fig. 9(b). The common-mode current Icm can then be calculated using the followingequation:

Icm = Cpsd(Vog + Vso − Vpg)

dt+ Cpg

dVpg

dt. (10)

A three-phase fuel cell inverter system connected to a460-V utility bus was simulated on PSim in order to verifythe circulation of the common-mode current through groundand validate the preceding analysis. The voltage supplied byeach fuel cell stack in this case is 330 V, and the output of thesystem is 460 Vrms, 60 Hz. The high-frequency transformerparasitic capacitances for the simulation are Cpg = 100 pF,Csg = 200 pF, and Cps = 50 pF. The common-mode currentobtained from the simulation is shown in Fig. 10, where it canbe observed that the peak value of the common-mode current inthis case is 3.3 A and that it has a root-mean-square (rms) valueof 0.74 A. As can be seen from the equivalent circuit and from(10), this value is given by a component circulating through thecapacitor Cps due to the inverter operation and a componentgiven by the operation of the full-bridge dc–dc converter.

From (10), the component due to the operation of the invertercan be reduced by reducing Cps. A practical way of reducingthe value of this capacitance is by introducing a shield in thedc–dc converter isolation transformer. The shield is then con-nected to ground [Fig. 11(b)], reducing the capacitive couplingbetween primary and secondary.


Fig. 6. Use of a common-mode filter in topology 1.

Fig. 7. Use of a common-mode filter in topology 1. (a) Voltage. (b) Current.

Fig. 8. Fuel cell power converter topology 2.

Fig. 9. Common-mode equivalent circuit for topology 2 (Fig. 8). (a) Detailedequivalent. (b) Simplified equivalent.

Fig. 10. Common-mode current Icm and voltage Vog in topology 2 (Fig. 8).

Fig. 11. Conventional and shielded transformers.

Fig. 12. Use of a shielded transformer in topology 2 (Fig. 8).

As can be observed in Fig. 12, the effect of introducing ashield in the high-frequency transformer of the dc–dc converteris splitting the primary-to-secondary coupling capacitance,which is then connected to ground. The end effect of this isbreaking the circulation path for the common-mode current.

Fig. 13 shows the results obtained for the system when ashielded transformer is used in the dc–dc converter. In this case,the peak value of the common-mode current is reduced to 2.3 A,


Fig. 13. Common-mode current reduction in topology 2 with a shieldedtransformer (Fig. 8).

Fig. 14. Common-mode current reduction in topology 2 with a shieldedtransformer and a common-mode filter.

TABLE ICOMMON-MODE CURRENT ISOLATED DC–DC CONVERTER

SYSTEM (FIGS. 9 AND 11)

whereas its rms value drops to 0.37 A. Although the common-mode current is reduced, there is still an important componentdue to the operation of the dc–dc converter. This componentcan further be reduced if a common-mode filter, as discussedin the previous section, is connected at the output of the fullbridge. Fig. 14 shows the result obtained for the system whenthe common-mode filter and shielded transformer are used.As can be observed for this case, the common-mode currenthas a peak value of 0.1 A and an rms value of 30 mA.

Comparison of the results obtained for the two cases thatwere studied is shown in Table I. As can be seen, the useof a shielded transformer in the dc–dc converter reduces thepeak value of the common-mode current by 30%. On the otherhand, using a shielded transformer and a common-mode filterpractically eliminates the problem.

C. Multifuel Cell System

The third case considered in this paper is the use of two fuelcell stacks to supply power to the utility (Fig. 15).

Fig. 15. Fuel cell power converter topology 3.

Fig. 16. Common-mode equivalent circuit for topology 3.

For this system, the use of isolated dc–dc converters isrequired in order to interface the two fuel cell stacks with theinverter. As in the case discussed in Section II-B, this multistacksystem does not require a line frequency transformer at theoutput of the inverter. The common-mode equivalent circuit forthis system can be obtained in the same fashion as was donefor the other two cases that were analyzed, and this is shownin Fig. 16. From this common-mode equivalent circuit, one canobtain that the voltage that appears from point “o” to ground isgiven by

Vog =Vao + Vbo + Vco

3. (11)

Moreover, the voltage at the midpoints of the secondary wind-ings is then calculated using

Vns1 = Vog +Vdc

4(12)

Vns2 = Vog − Vdc

4. (13)

Using these equations, the common-mode current can be foundto be

Icm = Cps1d(Vnp1 − Vns1)

dt+ Cpg1

dVnp1

dt

+ Cps2d(Vnp2 − Vns2)

dt+ Cpg2

dVnp2

dt. (14)


Fig. 17. Common-mode current and voltage. (a) Topology 3. (b) Topology 3and shielded transformers. (c) Topology 3, shielded transformers, and common-mode filters.

From (14), one can see that the magnitude of the common-modecurrent circulating through ground is due to the operation ofboth the dc–dc converters and the inverter. As discussed in thepreceding sections, the common-mode current can be reduced ifthe parasitic capacitance between primary and secondary of thehigh-frequency transformer is reduced using a shielded trans-former and by adding a common-mode filter. In this case, sincethe system is constructed using two dc–dc converters, bothtransformers in the power conditioner will need to be shielded.In addition, common-mode filters will need to be added to theoutputs of the two full-bridge converters used for the dc–dcconverters. The waveforms obtained from simulations on PSimfor the system in the case of using regular transformers in thedc–dc converters are shown in Fig. 17(a).

It can be observed that in this case, the peak amplitude ofthe common-mode current reaches 6.4 A, and its rms value is1.65 A. The effect of using shielded transformers for the imple-mentation of the dc–dc converters is shown in Fig. 17(b), wherethe peak amplitude of the common-mode current is reduced to4.8 A, and its rms value is 1 A. In addition, in Fig. 17(b), itcan be observed that the frequency of the current pulses hasdecreased as the component due to the operation of the inverter

TABLE IICOMMON-MODE CURRENT IN MULTIFUEL CELL SYSTEM (FIG. 15)

Fig. 18. Inverter output voltage and motor terminal voltage with the proposedfilter.

has been blocked due to the shield in the high-frequency trans-formers. The effect of using shielded transformers along withcommon-mode filters connected at the outputs of the dc–dcconverters is shown in Fig. 17(c). As can be seen, the peakamplitude of the common-mode current has now been reducedto 217 mA, and its rms value is 75 mA.

The results obtained for the three cases that were studied inthis section are compared in Table II. As can be seen, the use ofa shielded transformer reduces the circulation of the common-mode current by 25%. On the other hand, the best results areobtained when both approaches, i.e., the shielded transformerand common-mode filter, are used.

III. EXPERIMENTAL RESULTS

To validate the theoretical analysis and simulations, an ex-perimental prototype of the inverter system shown in Fig. 6was built and tested. The inverter system specifications andfilter values in the test setup are as specified in Section II-A.The common-mode current and voltage were measured for thesystem operating with and without the common-mode filter.

The increase in the rise time of the inverter output voltagepulses achieved by the connection of the common-mode filteris shown in Fig. 18; as can be seen, the rise time of the voltagepulses generated by inverter operation increases from 134 ns to2.2 µs when the filter is used. Similarly, the rise and fall times ofthe common-mode voltage measured at the output transformerincrease to 2.2 µs, as shown in Fig. 19.

This increase in the rise and fall times, in turn, results ina reduction of the common-mode current, as can be observedfrom the following results (Figs. 20 and 21), which show thecommon-mode current measured in the system without andwith the filter. As can be seen, there is a reduction of 11% onthe rms value of the common-mode current. Likewise, the peak


Fig. 19. Common-mode voltage at inverter and motor terminals with theproposed filter.

Fig. 20. Common-mode current without the proposed filter.

Fig. 21. Common-mode current with the proposed filter.

value of the common-mode current is reduced by 43% when thefilter is connected to the system.

IV. CONCLUSION

In this paper, analysis of the common-mode voltage in fuelcell power conditioners connected to the electric utility hasbeen presented. Three types of fuel cell power converters forutility interface have been considered. Two different methodsof reducing the magnitude of this circulating current were

discussed: 1) the use of a common-mode filter and 2) the intro-duction of a shielded high-frequency transformer. Simulationsand laboratory tests were used to verify the effectiveness ofthese methods, and it was shown that a combination of thesetwo methods has to be used to effectively reduce the common-mode current.

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Leonardo Palma (S’02–M’07) received the B.S.degree in electrical engineering from the Univer-sidad de Concepción, Concepción, Chile, in 1999and the M.S. and Ph.D. degrees from Texas A&MUniversity, College Station, in 2003 and 2006,respectively.

He is currently an Assistant Professor with theUniversidad de Concepción. His research and pro-fessional interests include theory and applications inthe areas of distributed energy systems, green power,power supplies, motor drives, and modern energy

storage devices.

Maja Harfman Todorovic (S’02) received theDiploma degree in electrical engineering from theUniversity of Belgrade, Belgrade, Serbia, in 2001and the M.S. and Ph.D. degrees from Texas A&MUniversity, College Station, in 2004 and 2008,respectively.

Her research interests include converters for fuelcells, green power, switching-mode power supply de-sign, uninterruptible power systems, energy storagedevices, and digital control of power converters.

Prasad N. Enjeti (M’85–SM’88–F’00) receivedthe B.E. degree in electrical engineering fromOsmania University, Hyderabad, India, in 1980, theM.Tech. degree in electrical engineering from theIndian Institute of Technology, Kanpur, India, in1982, and the Ph.D. degree in electrical engineeringfrom Concordia University, Montreal, QC, Canada,in 1988.

In 1988, he joined as an Assistant Professor withthe Department of Electrical Engineering, TexasA&M University, College Station. He was then pro-

moted to an Associate Professor in 1994 and to a Full Professor in 1998.He is the Lead Developer of the Power Electronics/Power Quality and FuelCell Power Conditioning Laboratories, Texas A&M University, and is activelyinvolved in many projects with industries while engaged in teaching, research,and consulting in the area of power electronics, motor drives, power quality, andclean power utility interface issues. He is the holder of four U.S. patents andhas licensed two new technologies to the industry so far. His primary re-search interests are as follows: advanced converters for power supplies andmotor drives; power quality issues and active power filter development; utilityinterface issues; advancing switching power supply designs and solutions tocomplex power management issues in the context of analog and mixed-signalapplications; exploring alternative designs to meet the demands of high-slew-rate load currents at low-output voltages; power conditioning systems for fuelcells, wind, and solar energy systems; and design of high-temperature powerconversion systems with wide band-gap semiconductor devices.

Dr. Enjeti is a Registered Professional Engineer in the State of Texas. In2000, he was elected to IEEE Fellow for “contributions to solutions of utility in-terface problems in power electronic systems and harmonic mitigation.” He wasthe recipient of the IEEE Industry Applications Society (IEEE-IAS) Second andThird Best Paper Awards in 1993, 1998, 1999, 2001, and 1996, respectively; theSecond Prize Paper Award for the best paper published in mid-1994 to mid-1995 in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS; and theIEEE-IAS Magazine Prize Article Award in 1996. He was also the recipientof the select title “Class of 2001 Texas A&M University Faculty Fellow”Award for demonstrated achievement of excellence in research, scholarship,and leadership in the field. He directed a team of students to design and builda low-cost fuel cell inverter for residential applications, which won the 2001Future Energy Challenge Award grand prize from the Department of Energy.

Analysis of Common-Mode Voltage in Utility-Interactive Fuel Cell Power Conditioners

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