PEDS 2007

Compensation ofDC-Link Oscillations ofCascaded H-Bridge Converters

M. Tavakoli Bina* and B. Eskandari** Faculty of Electrical Engineering, K. N. Toosi University of Technology, P. 0. Box 16315-1355, Tehran 16314, Iran,

E-mail: tavakoli@,ieee.org

Abstract-Single-phase AC applied voltage of an H-bridge converter produces second harmonic on top of theDC-link voltage. Three-phase unbalanced voltages makesimilar effects on the DC-link voltages, as well.Nevertheless, for applications that need higher voltages,series connection of H-bridges could lower the amplitude ofthe oscillations. Low-frequency oscillations are considerablewhen the number of cascaded H-bridges is less than four,introducing the worst case oscillations for a single H-bridgeconverter. This paper proposes various external DC activefilter circuits, aiming at cancelling these oscillations ofcascaded H-bridges up to three. Proposed circuits aresimulated, and their performances on compensation ofoscillations are compared to select the best choice.Simulation results confirm that the proposed methods limitthe DC-link oscillations on DC-link of H-bridges. Also, thepresented methods are compared in terms of both theiradvantages and disadvantages.

Index Terms-Active-filtering, auxiliary compensation,DC-link Oscillations, H-bridge, S-bridge.

I. INTRODUCTION

CASCADED H-bridge multilevel converters canpotentially be used as an alternative to the series

connection of semiconductor switches to increase thesystem voltage [1]-[4]. Figure 1 shows a typical cascadeH-bridge converter in which the harmonic performance isexpected to be improved compared to the converters withseries connected switches. This topology has found high-voltage high-power applications such as modularmultilevel AC-AC converters (M2LC) [5]-[6]. TheM2LC includes four modules of cascaded H-bridgeconverters of type shown by Fig. 1. However, each H-bridge sub-module of Fig. 1 exchanges active powerbetween the electrical network and the load through theother H-bridge converters. This power exchange dependson the magnitude of the fundamental voltage of each H-bridge converter as well as the magnitudes of low orderharmonics. The exchanged power would influenceconsiderably on the DC-link voltage of the H-bridgeconverter, causing low frequency oscillations. Further,when H-bridge converters introduce different powerexchanges from each other, then balance of capacitorvoltages is a major concern that could possibly lead toinstability. Figure l(b) depicts the SPWM technique thatis used to modulate two cascaded H-bridge converters[7]. It can be seen that the difference in the output voltage

This work was performed in the Research Laboratory of K. N. ToosiUniversity of Technology.

magnitudes of the two H-bridge converters can affect thebalancing of the two capacitor voltages.

(a)

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4 .- - - - - - - -r - - - - - - - -E - - - - - - - - - - - - -- - 'E_______________

-2

o 0.5 I 15 2 2.5 3 x 10

(b)Figure 1 (a) General topology of Cascade H-bridge converters, and (b)the effect of difference in fundamental voltage magnitudes of two H-

bridge converters on the DC-link voltage balancing.

This paper examines various methods to compensatethe low-frequency oscillations that appear on top of theDC-link voltages of H-bridge multilevel converters.Different passive/active filtering circuits are proposedthat is connected to the DC-link capacitor, resulting in

1-4244-0645-5/07/$20.00©2007 IEEE 855

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(b)

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150

149

148

Z 147-

146-

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(c) (d)Figure 2: (a) Cascaded two H-brige converter without any compensators, (b) DC-link oscillati.ons excluding any DC-link filters, (c) atuned passive LC filter compensates the DC-link oscillations, and (d) simulation results when DC capacitor oscillations are absorbed by the passive

filter.

compensation of the voltage oscillations. All DC-filters

are examined and simulated with MATLAB to compare

their performances along with suitability for the cascade

H-bridge converters. Simulation results confirm that the

DC active-filtering proposals compensate the DC-link

oscillations much effective than other introduced

methods.

11. DC ACTIVE -FILTERING OF OSCILLATIONS

Here we examine connection of various DC

passive/active filtering circuits across the DC capacitor to

compensate the low-frequency oscillations imposed bythe exchange of active power. Predominant frequency of

these oscillations is 100/120 Hz when power system

operates at 50/60 Hz. Figure 2(a) shows a cascaded two

H-bridge converter, excluding any DC-link compensator,

which is simulated with SIMULINK. A PI controller is

employed to control the phase of the H-bridges output

voltages such that the average DC-voltage remains fixed

at 150 V [8]-[9]. Switching pulses are swapped between

the two H-bridges to force both capacitor voltages vary

similarly. Figure 2(b) illustrates the DC capacitorvariations when no filter is designed to absorb the

oscillations. It is clear that the oscillations cannot be

damped (peak-to-peak variation is about 8V or 5.330o).

Hence, the following sub-sections suggest and examine

various topologies that use passive/active filters to dampmore efficiently the oscillations.

A. Passive LC-filtier compensation (PLC): proposition

The low-frequency oscillations of the DC-link can be

filtered using a tuned LC passive filter that its natural

frequency is 100 Hz. Figure 2(c) shows a cascade

converter consisting of two H-bridges as well as two LC

passive filters, which is simulated by SIMULINK. This

would effectively reduce the oscillations, needs no extra

semiconductor switches and control circuits.

Nevertheless, the disadvantage of the PLC method is that

the 100 Hz passive LC filter needs big parameters as well

as occupying huge space [10].

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B. Auxiliary H-brige compensation (AHC): proposition 2Here an H-bridge converter (with DC capacitor Cf ) is

connected across the DC-link capacitor C of each mainH-bridge converter through the inductance L like it isshown by Fig. 3(a) [11]. Capacitance Cf is much bigger

than that of C , while the average voltage of bothcapacitors is considered identical. The two H-bridgesoperate in a way that when the voltage of capacitorCstarts rising, the capacitor Cf is charged by its H-bridge

converter. When voltage of capacitor C starts gettinglower, then the capacitor Cf is discharged to prevent the

decrease in voltage of capacitorC . It is noticeable thatthe voltage variations in capacitor Cf is lower than

capacitor C because Cf is bigger than C .

Any oscillations on the capacitor C is transferred tothe capacitor Thus, the low-frequency oscillations on thecapacitor voltage Vc are sampled from the capacitorvoltage VC . A PI controller is used to regulate the

average voltage of the capacitor Cf. Also, the capacitorvoltage is considered to be slightly bigger than Vc. Thiswould prevent the flow of current from Cf through theparallel diodes to the capacitor C.

It should also be mentioned that the switching signalsare swapped between the two H-bridge converters of Fig.

3(a) symmetrically. This implies that the two capacitors(Cf) have similar variations. Hence, taking sample fromone of the capacitors is compared to a reference voltage,and the resultant error is sent to a PI controller.

One disadvantage over the AHC is the bigcapacitance Cf, which causes dynamically slow tracking

of the DC-link voltage. Also, since the average DCvoltages of both C and Cf are identical, a voltage

difference between these two capacitors is needed tocontrol the current exchange through the inductance L .

Therefore, the AHC is unable to damp completely theoscillations of the DC-links. Figure 3(b) providessimulated oscillations of the two DC-links using theAHC, which is much lower than those of theuncompensated case.

C. Auxiliary S-bridge compensation (ASC): proposition3

This proposal uses two identical capacitors(Cfl and Cf2) along with three switches for each DC-

link like it is illustrated by Fig. 4(a). CapacitancesCf1 and Cf 2 do not have to be big, while their average

DC voltages are equal to three quarters of the DC-linkvoltage of capacitor C. Three switches are operated in a

way that when the DC-link voltage is increased beyondthan a positive band (AV ), the two vertical switches are

turned on and the horizontal switch is turned off. Thismakes both capacitances Cfl and Cf2 to operate in

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(b) x lo'Figure 3: (a) The DC-links oftwo cascaded H-bridge converters are compensated using two auxiliary H-bridge

converters, and (b) the compensated oscillations of the two DC-link capacitor voltages.

(a)

1 51 5

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Figure 4: (a) The DC-links of two cascaded H-bridge converters are regulated using the second suggested topology, (b) simulation results concernedwith voltages of the two DC-link capacitors.

parallel, asorbing chaging curre ts Ihog ithesparallel, absorbing charging currents through theinductance L from the DC-link C by the following slope:

VDC VDCdiL VL 4Idt L L

VDC4L

Where the voltage VL is the drop on inductance L and

iL is the absorbed current. This relatively big slopeforces the main DC-link voltage to come down morerapidly. Accordingly, when the DC-link voltage isdropped below a negative band (-AV), the two verticalswitches are turned off and the horizontal switch is turnedon. The two capacitors Cfl and Cf 2 operate in series,injecting current to the DC-link C through the inductanceL as follows:

diL VL VDC 4Idt L L

VDC2L

Here the negative slope reverses the current from

Cf andCf 2 to charge the DC-link capacitor C. Note

that here the positive slope is different from the negativeone as it is illustrated by Fig. 4(b). Simulation resultsshow that peak-to-peak of the oscillations is smaller thanIV (or 0.6%). In practice, the hysteresis band could limitthe performance of the proposal because the switchingfrequency could be very high when the chosen AV issmall.

Like the PI controller of the AHC, the controller of theASC takes DC-voltage sample from either Cfi or Cf2;because voltage variations of the two capacitors areidentical. Table I summarizes the simulation resultsobtained by all suggested methods, including the AHC,the ASC, the PLC, and the uncompensated case.

It can be seen from the results gathered in Table I thatwhile the uncompensated case contains considerable low-order oscillations, other suggested methods lower theoscillations from 11.1I% well below 3.5%. Amongst theanalyzed methods, the ASC introduces the lowest level ofoscillations (smaller than 0.6%). Then, the AHC achieves2.6% maximum oscillations, and the PLC up to 3.3%.Since the proposed circuits use low-power elements, totalcost of the added device could be reasonable enough toapply the suggested methods to the H-bridge converters.

858

I

TABLE I: SIMULATION RESULTS CONCERNED WITH THE PEAK-TO-PEAK OSCILLATIONS OF THE DC-LINK OF H-BRIDGE CONVERTERS ALONG WITH THEADVANTAGES AND DISADVANTAGES OF ALL SUGGESTED METHODS

Compensation Method Oscillations (maximum Peak-to-Peak) Oscillations (average Peak-to-Peak)AHC 2.6% 2.6%ASC 0.6% 0.6%PLC 3.13% 1.82%

Uncompensated 11.1%0 8.9%0

III. CONCLUSIONS

The DC-link oscillations related to the H-bridgecascade converters is significant when the number of H-bridges is smaller than four. This situation is worse whenthe three-phase applied voltages are unbalanced. Theoscillations on the DC-link are modulated through the H-bridge converter, and enter the AC system. This hasconsiderable impacts on harmonic performance as wellasthe efficiency of the converter. While an uncompensatedsimulation with two H-bridges show considerableoscillations on the DC-links, three methods are proposedand examined to lower the peak-to peak oscillations.These methods are the passive LC-filter compensation(PLC), the auxiliary H-bridge compensation (AHC) andthe auxiliary S-bridge compensation (ASC). Thesemethods are also simulated with SIMULINK to comparetheir effectiveness on lowering the DC-link oscillations.Simulation results show that these three methodssuccessfully control the oscillations, amongst them theASC performs as the best solution. Nevertheless, thereare both advantages and disadvantages for each methodthat are needed to be taken into account in practicalimplementations.

ACKNOWLEDGEMENT

The authors would like to thank the support of theresearch Laboratory of power quality and reactive powercontrol in K. N. Toosi University of Technology.

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