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374 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 1, JANUARY 2014

A Space-Vector Modulation Methodfor Common-Mode Voltage Reduction in

Current-Source ConvertersJian Shang, Student Member, IEEE, and Yun Wei Li, Senior Member, IEEE

AbstractThe common-mode voltage (CMV) produced from aconverter system is a source of many problems, e.g., in the mo-tor drive system, CMV might appear at the neutral point of themotor stator windings with respect to the ground and induce de-structive bearing current. Reduced CMV space-vector modulation(RCMV SVM) methods have been proposed in both voltage-sourceconverter (VSC) and current-source converter (CSC) systems. Theavailable RCMV SVMs reduce the CMV by avoiding using zero-state vectors. However, this will lead to some negative effects, suchas shrink of modulation index range, increase of switching fre-

quencies, bipolar line-to-line voltage pulse patterns in VSCs, andpower quality performance deterioration. In this paper, a RCMVSVM method for CSCs is proposed. By allowing the use of zero-state vectors, the proposed RCMV SVM still produces much lowerCMV. However, its other performance indices, such as switchingfrequency and harmonic performance, are unaffected and com-parable to the conventional SVMs. The effectiveness of the pro-posed RCMV SVM for CSCs is verified in the simulations andexperiments.

Index TermsCurrent-source converter (CSC), power quality,reduced common-mode voltage (CMV) SVM, zero-state vector.

I. INTRODUCTION

ALTHOUGH voltage-source converter (VSC) has been

the preferred solution in many industrial applications,

pulsewidth modulated (PWM) CSCs could be a good alterna-

tive to VSC in the wind energy power generation, superconduc-

tor magnetic energy storage (SEMS), and HVdc transmission

systems. Currently, in the medium-voltage drive systems, trans-

formerless PWM CSCs have already become a very popular in-

dustrial solution [1]. Theconfiguration of such a system is shown

in Fig. 1. The transformerless CSC drive system has advantages

over the conventional drive system, such as higher power den-

sity, lower cost, and higher power efficiency. However, due to

the absence of the input isolation transformer, common-mode

voltage (CMV) at the neutral point of the motor stator windings

Manuscript received December 4, 2012; revised February 10, 2013; acceptedFebruary 11, 2013. Date of current version July 18, 2013. This work was pre-sented at the IEEE Applied Power Electronics Conference and Exposition, LongBeach, CA, USA, March 1721, 2013. Recommended for publication by Asso-ciate Editor B. Wu.

The authors are with the Department of Electrical and Computer Engi-neering, University of Alberta, Edmonton, AB T6G 2V4, Canada (e-mail:

[email protected]; [email protected]).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/TPEL.2013.2248025

with respect to the ground can induce bearing current and lead

to the bearing damage of the motor [2][4].

To mitigate CMV in CSCs, the neutral points of the input and

output capacitors are connected to clamp the CMV to nearly

zero.To limit the large circulatingcommon-mode current caused

by this connection, an integrated dc choke with both differential

and common-mode windings is used [5]. The size of the dc

choke can be reduced if the CMV produced by the rectifier and

inverter is minimized. One of the most effective methods forCMV reduction is to modify the PWM patterns.

Most available RCMV PWM schemes are developed for

VSCs and they reduce CMV by avoiding the use of zero-state

vectors [6]. These RCMV space-vector modulations (SVMs)

can be categorized as active zero-state modulation (AZSM)

[7][9], remote-state modulation (RSM) [10], and near-state

modulation (NSM) [11], [12]. Although those nonzero-state

modulation methods can effectively reduce CMV, they are all

subject to some problems, such as the shrink of modulation in-

dex range, bipolar line-to-line voltage pulse patterns, increased

switching frequencies, higher dc-link ripples, and power qual-

ity performance deterioration. Those drawbacks make RCMV-

PWM for VSCs difficult to be applied in the industry.As for the PWM of CSCs, selective harmonic elimination

(SHE) PWM is popular in the medium-voltage high-power drive

systems due to SHE PWMs excellent harmonic performance

with low switching frequencies. However, SVM, as a kind of

online modulation method, has been used to dampLC resonance

[13], [14], minimize dc-link current [15], control input power

factor [16], [17], etc. In [18], the authors introduce the nonzero-

state modulation concept for VSCs into CSCs. However, the

problems observed in RCMV SVMs for VSCs still exist in

those nonzero-state RCMV SVMs for CSCs.

Two most importantperformanceindices forPWM patterns in

the medium-voltage drives and grid-tied converters are switch-ing frequency [19][21] and harmonic performance [22][24],

which are also two important considerations in the design of

RCMV SVM for CSCs. A novel RCMV SVM method for CSR

and CSI is proposed in this paper to reduce the CMV by se-

lecting proper zero-state vectors instead of completely avoiding

using them. To better illustrate the working principles of the pro-

posed RCMV SVM for CSCs, Section II presents the definition

of CMV in a CSC drive system and the CMV values associated

with all the space vectors. Section III describesthe working prin-

ciples of the proposed RCMV SVM in detail, including zero-

state vectors selection, sequences selection for switching fre-

quency minimization, and single-sequence rule for harmonic

0885-8993/$31.00 2013 IEEE


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SHANGANDLI:SPACE-VECTORMODULATIONMETHODFORCOMMON-MODEVOLTAGEREDUCTIONIN CURRENT-SOURCE CONVERTERS 375

Fig. 1. Configuration of transformerless current-source drive system.

performance optimization. To verify the effectiveness of the

proposed RCMV SVM for CSCs, Sections IV and V present

the simulation and experimental results for the RCMV SVM

and the comparison results with conventional three-segment and

five-segment SVMs. The results show that the CMV can be re-duced up to 50% compared to the conventional SVM methods.

The switching frequency and harmonic performance of the pro-

posed RCMV SVM are comparable to the conventional SVMs.

II. CMVIN CURRENT-SOURCECONVERTERS

A. CMV in Current-Source Converters

The CMV produced in the CSC system, as shown in Fig. 1,

consists of CMVin the current-source rectifier (CSR) side vcm rand CMV in the current-source inverter (CSI) sidevcm i , whichcan be defined, respectively, by

vcm r = vp1g +vn1g2

(1)

vcm i =vp2o + vn 2o

2 (2)

where vp1g and vn1g are the voltages at points p1 and n1,respectively, with respect to the ground, and vp2o andvn 2oare the voltages at points p2 andn2, respectively, with respectto the neutral of the induction motor.

If the differential inductance in the positive dc rail is equal to

that in the negative dc rail, the CMV in the whole drive system

vog can be given by

vog =vcm r vcm i = vp1g +vn 1g2

vp2o+ vn2o2

.

(3)

When PWM control is applied to the CSC drive systems,

vcm r , vcm i , andvog can be expressed as

vcm r = [ S1 + S4 S3 + S6 S5 + S2].

0.5va

0.5vb

0.5vc

(4)

vcm i = [ S1 + S

4 S

3 + S

6 S

5 + S

2].

0.5vu

0.5vv

0.5vw

(5)

Fig. 2. Operating principle of SVM for CSCs and space vectors definition.

vog = [ S1 + S4 S3 + S6 S5 + S2].

0.5va

0.5vb

0.5vc

[ S1 + S

4 S

3 + S

6 S

5 + S

2].

0.5vu

0.5vv

0.5vw

(6)

whereS1S6 are the switching states of CSRs switching de-vices,S1S

6 the switching states of CSIs switching devices,

va , vb andvc the phase voltages of the grid, andvu , vv , andvware the phase voltages of the motor stator [25], [26].

B. CMV Associated With Space Vectors

Fig. 2 illustrates the operating principle of SVM for CSCs

and the space vectors definition. According to (4) and (5), the

instantaneous value of CMV associated with the space vectors

can be obtained and summarized in Table I using CSR as an

example. The CMV peak value is determined by many factors,

such as delay angle for CSR (in a current-source drive, the dc-

link current is usually regulated by the delay angle control in the

CSR side [1]),LC filter capacitance, dc-link current, modulation

index, and CSI load power factor. In the conventional SVMs,

the maximum CMV peak value produced by zero-state vectors

in the SVM for CSR or CSI can be as high as the peak value of


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TABLE ICMV ASSOCIATED WITHSPACEVECTORS INSVM FOR CSCS

phase voltage of grid side|Vg |or that of motor stator side|Vm |,while the maximum CMV peak value produced by active-state

vectors is equal to |Vg | /2or |Vm | /2. Thus, the maximum CMVpeak value in the drive system|Vog |is|Vg |+|Vm |.

The recently proposed RCMV SVMs for CSCs in [18] re-

duce CMV by avoiding the use of zero-state vectors, resultingin a maximum CMV peak value as high as |Vg | /2or |Vm | /2.However, instead of completely avoiding the use of zero-state

vectors, another effective method for CMV reduction is to se-

lect the zero-state vector producing the lowest CMV from the

three ones in each SVM sample. In this way, the maximum

CMV peak value produced by the proposed RCMV SVM can

be lower than|Vg | /2or|Vm | /2as well, but it will not cause theaforementioned negative effects in those nonzero-state RCMV

SVMs.

III. PROPOSEDRCMV SVM FORCSCS

The design of the proposed RCMV SVM is discussed in

this section. The zero-state vectors selection rule is proposed to

reduce CMV and a sequence selection rule is then applied to

minimize switching frequencies. The single-sequence rule is

also applied in some cases to realize the harmonic performance

optimization.

A. Zero-State Vectors Selection Rule for CMV Reduction

The zero-state vectors selection rule is that the zero-state vec-

tor producing the lowest CMV among the three zero-state vec-tors at any sampling period would be selected. For example, in

Fig. 3, the absolute value of phase B voltage is the lowest among

the three phase voltages during Ta , so the CMV produced byI0b (S3 , S6 )would be the lowest among the three zero-state vec-

tors. Therefore,I0b (S3 , S6 )should be used to minimize CMV

during Ta . According to this zero-state vectors selection rule,the selected zero-state vectors and the corresponding phase volt-

ages in one fundamental period are shown in Fig. 3. Using this

zero-state vectors selection method, the maximum CMV peak

value produced by zero-state vectors would be half of the peak

value of ac-side phase voltages, which is same as that produced

by active-state vectors.

Fig. 3. Selected zero-state vectors in RCMV SVM in each instant of onefundamental period.

Fig. 4. Conventional SVM sequences for CSCs: (a) three-segment sequenceand (b) five-segment sequence.

B. Sequence Selection Rule for Switching Frequency

Minimization

Six types of SVM sequences for CSCs have been compared

in [27], where it shows the five-segment SVM sequence hasthe best harmonic performance in high modulation index range,

while the three-segment SVM sequence has the best in low

modulation index range. The conventional three-segment and

five-segment SVM sequences are shown in Fig. 4. According

to the analysis in [27], the sampling frequencies for SVM of

CSCs have to be set as an integral multiple of 6 f1 (wheref1 isthe fundamental frequency) in order to synchronize the PWM

waveform with the fundamental frequency and eliminate even

and triplen harmonics. In order to obtain low switching frequen-

cies fit for high-power application, the sampling frequencies for

conventional three-segment and five-segment SVMs are 18f1and 24f1 , respectively.

To avoid the increase of switching frequencies compared tothe conventional SVMs, the sequence for RCMV SVM should

be redesigned since the zero-state vectors selection rule has

changed as compared to the conventional SVMs. For instance,

bothI0b and

I0c (instead of

I0a ) could be possibly selected in

sector I according to the zero-state vectors selection rule for

CMV reduction. In this case, there will be extra switching if the

three-segment sequence is applied in RCMV SVM. To avoid

the increase of switching frequency, two types of five-segment

sequences, as shown in Fig. 5, should be applied according to

the selected zero-state vectors. Specifically, ifI0 has one com-

mon on-state switch withIn+ 1 , sequence (a) should be selected.

Otherwise, sequence (b) should be selected. In some cases,

I0


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Fig. 5. Two sequences for RCMV SVM. Sequence (a)applied whenI0 has

one common on-state switch withIn + 1 . Sequence (b)applied when

I0 has

one common on-state switch withIn .

Fig. 6. Selected zero-state vectors and sequences in RCMV SVM for CSR

(= 0

).

may have one common on-state switch with both active-state

vectors in that sector (such asI0a in sector I), so both sequences

(a) and (b) are eligible in terms of switching frequency mini-

mization. To optimize the harmonic performance, the sequence

selection for this type of zero-state vector should abide by the

single-sequence rule (only one type of sequence is applied

during one fundamental period), which would be explained in

detail later. Note that the five-segment sequence with the active-

state vector positioned in the center, as shown in Fig. 4(b), can

be also applied in RCMV SVM. It has a similar performance

with those sequences in Fig. 5, where the zero-state vector ispositioned in the center.

C. Selected Zero-State Vectors and Sequences for SVM in CSR

Control

According to the aforementioned zero-state vectors and se-

quence selection rules, the selected zero-state vectors and se-

quences in RCMV SVM in CSR control are shown in Figs. 68

when the delay angle is 0, 30, and 60, respectively.Fig. 6 shows the selected zero-state vectors and sequences

in all the sectors when the delay angle is 0. The phase A

voltage is around the peak value in sector I when the delay

angle is 0

, so the peak value of CMV produced by

I0a in the

Fig. 7. Selected zero-state vectors and sequences in RCMV SVM for CSR(= 30).

Fig. 8. Selected zero-state vectors and sequences in RCMV SVM for CSR(= 60).

conventional SVMs would be as high as the peak value of phase

voltages. According to the proposed zero-state vectors selection

method for CMV reduction,I0b and

I0c (instead of

I0a in the

conventional SVMs) should be applied successively to reduce

the CMV in sector I. Both types of five-segment sequence inFig. 5 are used according to the sequence selection rule for

switching frequency minimization.

Fig. 7 shows the selected zero-state vectors and sequences in

all the sectors when the delay angle is 30. According to the

zero-state vector and sequence selection rules discussed previ-

ously, the sequence (b) of RCMV SVM is always used in one

fundamental period.

Fig. 8 shows the selected zero-state vectors and sequences in

all the sectors when the delay angle is 60. Different from the

two previous cases, here one of the desired zero-state vectors

in one sector has one common on-state switch with both two

active-state vectors in the same sector. For example,

Ioa (S1 , S4 )


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Fig. 9. 5th and 7th harmonics comparison of the switching current Iw and THD comparison of line currentIs when Sequence (a) and (b) and Single-sequence(b) is applied, respectively (= 60): (a) 5th harmonics ofIw, (b) 7th harmonics ofIw, and (c) THD ofIs .

TABLE IISELECTEDSEQUENCES FOR ALL THEPOSSIBLEDELAYANGLESUNDER THESEQUENCESELECTIONRULE FORSWITCHINGFREQUENCYMINIMIZATION

selected in sector I has the common on-state switch S1 withboth of the active-state vectors,

I1 (S1 , S6 )and

I2 (S1 , S2 ). A s a

result, this zero-state vector can be used to build either sequence(a) or sequence (b). On the other hand, the other selected zero-

state vectorIob (S2 , S5 ) in sector I will require sequence (b)

to avoid additional switching. Therefore, there are two possible

sequence combinations in this case: 1) only sequence (b) is used

and 2) both sequences (a) and (b) are used. Here, we name these

two possibilities as Single-sequence (b) and Sequence (a)

and (b), respectively.

Fig. 9(a) and (b) shows the 5th and 7th harmonics of the

switching current Iw , when Sequence (a) and (b) and Single-sequence (b) are used, respectively. Fig. 9(c) shows the THD

of line currents Is with an LC cutoff frequency of 3.33 p.u.(This cutoff frequency of CSRs LC filter is selected to avoid

amplification of the 5th harmonic current [27].) As shown, 5thand 7th harmonics ofIw are lower and THD ofIs is better ifSingle-sequence (b) is applied. It means that rotational use of

sequences (a) and (b) in one fundamental period will deteriorate

the harmonic performance. Moreover, the switching frequency

is lower if Single-sequence (b)is applied, because switching

between sequences (a) and (b) involves more devices. As a

consequence, Single-sequence (b) will be a better option in

this case. Here, the rule that only one type of sequence in one

fundamental period is selected to maintain a better harmonic

performance is named as single-sequence rule.

Table II shows the selected sequences for all the possible

delay angles under this sequence selection rule.

D. Single-Sequence Rule for Harmonic Performance

Optimization

As discussed in the previous section, the use of sequences(a) and (b) in combination would deteriorate the harmonic per-

formance. From Table II, we can find that the single-sequence

rule is violated in the delay angle ranges of30


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Fig. 10. Selected zero-state vectors and sequences based on the single-

sequence rule (= 15

).

TABLE IIISELECTEDSEQUENCES FOR ALL THEPOSSIBLEDELAYANGLES INRCMV

SVM USINGSINGLE-SEQUENCERULE

sequence is applied in one fundamental period, when the delay

angle is in those two ranges. With this method, the switching fre-

quency would increase, but CMV would be kept lower than half

the peak value of ac-side phase voltage and harmonic perfor-

mance would not be deteriorated. Since the increased switching

frequency is not favorable in high power CSC drives, this alter-

native method will not be discussed in detail.

Table III shows the selected sequences for all possible delay

angles in RCMV SVM using single-sequence rule.

E. Switching Frequency Analysis of the Proposed RCMV SVMfor CSCs

According to the previous analysis, the proposed RCMV

SVM, compared with the conventional five-segment SVM, has

no extra switching during switching states transitions in one

sector if their sampling frequencies are the same. The extra

switching can only happen during the sector crossing. Fig. 11

illustrates the possible device switching of the proposed RCMV

SVM in the sector crossing from sector I to sector II. If the

sampling frequency of RCMV SVM is an even multiple of 6 f1(just like conventional five-segment SVM), there are three pos-

sible cases during the sector crossing as shown in Fig. 11(a)(c),

respectively, depending on the selected zero-state vectors in the

Fig. 11. Possible device switching of the proposed RCMV SVM in the sectorcrossing from sector I to sector II.

sector crossing and the moment of the sector crossing. The sec-

tor crossing in Fig. 11(a) and (b) happens in the center of the

sequence, while that in Fig. 11(c) happens in the end of the

sequence.

Fig. 11(a) shows that the selected zero-state vectors for the

last sample in sector I and the first sample in sector II are bothI0c . This zero-state vector selection method, which has beenapplied in the conventional five-segment SVMs in [27], leads

to minimum device switching. This scenario in Fig. 11(a) couldpossibly happen in the proposed RCMV SVM ifI0c is thezero-state vector needed for CMV reduction during the sector

crossing. The switching frequency in this case is 8f1 if thesampling frequency is 24f1 .

Since the freedom of the zero-state vectors selection has been

used for CMV minimization in RCMV SVM, the device switch-

ing minimization, like in the case shown in Fig 11(a), cannot

be guaranteed all the time. However, the increase of switching

frequency, which is one fundamental frequency, is not signifi-

cant. Other zero-state vectors, likeI0a in Fig. 11(b), could be

selected to reduce CMV during the sector crossing. Compared

to the case in Fig. 11(a), one extra device switching could hap-

pen in Fig. 11(b) between I0 and In+ 1 in the first sample ofsector II. As a result, the switching frequency in this case is 9f1if the sampling frequency is 24f1 .

As shown in Fig. 11(c), the sector crossing can also happen in

the end of the sequence. There is one switching during the sector

crossing under this scenario (which is similar to the situation that

conventional five-segment SVM in Fig. 4(b) with sector crossing

at the center of the sequence). The switching frequency in this

case is also 9f1 if the sampling frequency is 24f1 . Furthermore,if two types of sequences in RCMV SVM are used in one

fundamental period, this extra switching would happen in the

moment of sequences (a) and (b) transition instead of sector

crossings.


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TABLE IVMAINCIRCUITPARAMETERS OFCSR SYSTEM IN THE

SIMULATION ANDEXPERIMENT

Therefore, the switching frequency of RCMV SVM is 8f1or 9f1 depending on the selected zero-state vectors in the sectorcrossing and the moment of sector crossing if the sampling fre-

quency is 24f1 . Thus, under the same sampling frequencies, theswitching frequency of RCMV SVM is equal to or f1 different

from conventional five-segment SVM.

F. Implementation of RCMC SVM in the CSR and CSI control

Similar to the conventional five-segment SVM, each five-

segment sequence of the proposed RCMV SVM needs two

samples to complete. The sectors and active-state vectors are

updated every sample, whereas the zero-state vectors are up-

dated before the beginning of the sequence (two samples) and

kept unchanged during the two samples. In this way, good con-

trol accuracy could be obtained and the switching frequency

would not increase.

In the CSI application, the number of samples in one cycle of

SVM (or the sampling frequency) is changing with the funda-mentalfrequency, which is related to themotor speed.Moreover,

there is no delay angle control in the CSI control. However, these

two differences from CSR control would not affect the RCMV

SVMs application in the CSI side. In the application in the CSI

side, phase voltages of the motor stator or CSI output capacitor

Vm can be detected to determine which zero-state vector andsequence should be applied.

To determine whether the single-sequence rule for har-

monic performance optimization should be applied, the equiv-

alent delay angle (or voltage current displacement angle) in the

CSI control should be known. It depends on the CSI output ca-

pacitance, motor parameters, and motor operating conditions.This equivalent delay angle in CSI control should be defined as

the phase displacement angle ofVm (jw)/Im w (jw), whereVmis the phase voltage of motor stator andIm w is the CSI outputswitching current.

IV. SIMULATIONVERIFICATIONS

The proposed RCM SVM method is first tested in MATLAB/

Simulink simulations. The CSR testing system parameters in

the simulation are shown in Table IV.

Figs. 12 and 13 show the comparison of the CMV, switch-

ing current, and line current under the operating conditions

of ma = 0.7 and = 0

and ma = 0.3 and = 0

, re-

spectively. According to previous analysis, the sampling fre-

quency for the conventional three-segment SVM is 1080 Hz,

while that for the conventional five-segment and RCMV SVM

is 1440 Hz. In the figures, 3Seg (such as CMV_3Seg)

and 5Seg (such as CMV_5Seg) represent conven-

tional three-segment and five-segment SVMs, respectively,

while RCMV (such as CMV_RCMV) and RCMV_TF

(such as CMV_RCMV_TF) represent the proposed RCMV

SVM without and with the single-sequence rule applied,

respectively.

Table V summarizes the simulation results. In Table V,

Iw 5/Iw 1 and Iw 7/Iw 1 represent the 5th and 7th harmon-ics over the fundamental current in the switching current. fs represents the switching frequency and CMV_pk represents

the peak value of CMV. It should be noted that the CMV pro-

duced by conventional SVMs is a bit higher than the peak value

of the grid side phase voltage. This is due to the voltage distor-

tion in the CSRs connection point with the LC filter, which is

caused by harmonic current flowing through the line inductor.

From the simulation results, we can find that the CMV producedby the proposed RCMV SVM is almost half of that produced

by conventional SVMs.

As expected, the switching frequency of RCMV SVM is

60 Hz higher than the conventional five-segment SVM andequal

to the conventional three-segment SVM. Since the delay angle

of 0 is in the range where single-sequence rule can be applied

to avoid harmonic performance deterioration, the performance

of the RCMV SVM with and without using single-sequence

rule is also compared. The RCMV SVM using the single-

sequence rule produces less low-order harmonic currents but

a bit higher CMV than that without using the single-sequence

rule.Fig. 14 is the comparison results of THD ofIs versus mawhen is 0, 30, and 60, respectively, which is tested basedon the simulation parameters in Table IV. This comprehensive

harmonic performance comparsion verifies that the proposed

RCMV SVM has a simialr harmonic performance with the con-

ventional five-segment SVM.

V. EXPERIMENTALVERIFICATIONS

The proposed RCMV SVM for CSC is verified in a 10 kVA

CSR prototype system in the lab. The CSR control plat-

form is designed based on a dSPACE (DS1103)-CPLD (Xilinx

XCR3064XL) system. The DS1103 PPC controller generatesthe control signals and the CPLD is used to convert the electrical

signals from the DS1103 to the optic signals for driving inte-

grated gate-commutated thyristors (IGCTs). The experimental

system parameters in the main circuit are shown in Table IV.

The effectiveness of RCMV SVM in the CSR is verified un-

der three operating conditions, i.e., ma = 0.7 and = 0,

ma = 0.7 and = 30, and ma = 0.7 and = 60, re-spectively. Its performance is compared with the conventional

three-segment and five-segment SVM. The sampling frequen-

cies of the conventional three-segment and five-segment SVMs

and RCMV SVM are same as those in the simulation. When

the delay angle is 0

, single-sequence rule can be applied to


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Fig. 12. Simulation results comparison of conventional three-segment SVM, conventional five-segment SVM, RCMV SVM using Sequence (a) and (b) andRCMV SVM using Single-sequence (b) under the operating condition ofma =0.7 and = 0

: (a) CMV, (b) switching current, and (c) line current.

Fig. 13. Simulation results comparison of conventional three-segment SVM, conventional five-segment SVM, RCMV SVM using Sequence (a) and (b) andRCMV SVM using Single-sequence (b) under the operating condition ofma =0.3 and = 0

: (a) CMV, (b) switching current, and (c) line current.

maintain good harmonic performance. As for the 30 and 60

delay angle, the single-sequence rule is already satisfied ac-

cording to the sequence selection rule for switching frequency

minimization.

A. Operating Condition ofma =0.7 and=0

Figs. 15 and 16 present the CMV, switching current, and line

current waveforms comparison under the operating condition of

ma = 0.7 and = 0. Table VI summarizes the experimen-

tal results of the four types of SVMs. The experimental results

show that the peak value of CMV produced from conventional

three- and five-segment SVMs [as shown in Fig. 15(a) and (b)] is

approximately equal to the peak value of the line phase voltage,

while that produced from RCMV SVM using Sequence (a) and

(b) [as shown in Fig. 15(c)] is approximately half of the peak

value of the line phase voltage. From Table VI, it can be seen that

the RCMV SVMusing Sequence(a) and(b) canproduce more


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TABLE VSIMULATION RESULTSCOMPARISON OFFOURTYPES OFSVM UNDER THEOPERATINGCONDITIONS OFm a = 0.7 AND = 0

ANDm a = 0.3 AND = 0

Fig. 14. THD of line current versusma comparison: (a) = 0, (b) = 30, and (c) = 60.

Fig. 15. CMV waveforms under the operating condition ofm a = 0.7 and = 0. (a) Conventional three-segment SVM. (b) Conventional five-segmentSVM. (c) RCMV SVM using Sequence (a) and (b). (d) RCMV SVM usingSingle-sequence (b).

low-order harmonics and deteriorate the harmonic performance

compared with the conventional five-segment SVM. However,single-sequence rule can be applied to improve its harmonic

performance by sacrificing the CMV reduction capability a lit-

tle. The peak value of CMV produced from RCMV SVM using

Single-sequence (b) [as shown in Fig. 15(d)] is a bit higher

than RCMV SVM using Sequence (a) and (b) but still much

lower than the conventional SVMs. As for the harmonic perfor-

mance,the low-order,i.e., 5th and7th, harmonics produced from

RCMV SVM using Single-sequence (b) are almost the same

as those produced from the conventional five-segment SVM.

This verifies the effectiveness of the single-sequence rule for

harmonic performance optimization.

The experimental waveforms comparison of switching cur-

rent verifies once again that switching frequencies of theconven-

Fig. 16. Switching current Iw and the line current Is under the operatingcondition ofma = 0.7 and = 0

. (a) Conventional three-segment SVM.

(b) Conventional five-segment SVM. (c) RCMV SVM using Sequence (a) and(b). (d) RCMV SVM using Single-sequence (b).


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TABLE VIEXPERIMENTALRESULTSCOMPARISON OFFOURTYPES OFSVM UNDER THEOPERATINGCONDITIONS OFma =0.7 AND = 0

,ma =0.7 AND = 30,AND

ma =0.7 AND = 60

Fig. 17. CMV waveforms under the operating condition ofm a = 0.7 and= 30. (a) Conventional three-segment SVM. (b) Conventional five-segment

SVM. (c) RCMV SVM.

tional three-segment SVM, the RCMV SVM using Sequence

(a) and (b), and the RCMV SVM using Single-sequence (b)

are all equal and only 60 Hz higher than conventional five-

segment SVM.

B. Operating Condition ofma =0.7 and=30


current waveforms comparison under the operating condition of

ma =0.7 and= 30. Table VI summarizes the experimentalresults of the three types of SVM. The proposed RCMV SVMs

CMV reduction capability is also verified in this case. The CMV

produced by RCMV SVM is around 50% of that produced

by conventional SVMs. Moreover, RCMV SVMs switching

frequency is still 540 Hz. The harmonic performance of the

RCMV SVM is also similar with the conventional five-segment

SVM.

C. Operating Condition ofma =0.7 and= 60


current waveforms comparison under the operating condition

ofma = 0.7 and = 60. Table VI summarizes the experi-

mental results of the three types of SVM. The effiectiveness of

Fig. 18. Switching current Iw and the line current Is under the operatingcondition ofm a = 0.7 and = 30

. (a) Conventional three-segment SVM.(b) Conventional five-segment SVM. (c) RCMV SVM.

the RCMV SVM is also verified in this case. The CMV pro-

duced by RCMV SVM is also around half of the peak value of

ac-side phase voltage. Note that CMV produced from conven-

tional three-segment SVM is as low as the CMV produced from

the RCMV SVM. It is because the zero-state vectors utilized

in the conventional three-segment SVM are just the ones pro-

ducing low CMV in this case. Likewise, neither the switching


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Fig. 19. CMV waveforms under the operating condition ofm a = 0.7 and= 60. (a) Conventional three-segment SVM. (b) Conventional five-segmentSVM. (c) RCMV SVM.

Fig. 20. Switching current Iw and the line current Is under the operatingcondition ofm a = 0.7 and = 60

. (a) Conventional three-segment SVM.(b) Conventional five-segment SVM. (c) RCMV SVM.

frequency nor harmonics performance of RCMV SVM becomes

worse in this case.

From Table VI, we can see that the CMV produced from

RCMV SVM has been reduced significantly under all the

cases. This CMV reduction will result in the loss reduction

on common-mode inductors, which are integrated in the dc-

link choke. However, the switching frequency is equal to or

60 Hz higher than conventional ones (depending on whether it is

three-segment or five-segment seqeunce). Thus, we can approx-

imately assume that the switching loss of RCMV SVM will not

increase. Moreover, the harmonic perfromance of RCMV SVM

is also similar with the conventional ones. Note that Iw 5/Iw 1increases with the increase of delay anlge . Its because thedc-link current is not ideally constant. The voltage harmonics in

the CSRs connection point with theLCfilter, which are caused

by harmonic current flowing through the line inductor, can, in

turn, produce harmonic current in the dc-link [28]. This dc-link

harmonic current has more obvious effect on ac-side harmonic

current, when the dc-link current is lower (i.e., when the delay

angle is larger).

VI. CONCLUSION

This paper proposes a reduced CMV SVM for CSCs. The

proposed method significantly reduces the CMV without avoid-

ing the use of the zero-state vectors, so that it has superior

performance over the traditional nonzero-state RCMV SVMs.

Unlike the traditional ones, it is not subject to problems suchas the shrink of modulation index range, the increased switch-

ing frequency, lower harmonic performance, etc. Moreover, the

proposed RCMV SVM can be easily implemented in the digi-

tal controller. Although RCMV CMV in some delay angle (or

equivalent delay angle in CSI) ranges needs both sequences (a)

and (b) in one fundamental period, the single-sequence rule

can be applied to improve the harmonic performance by slightly

sacrificing the CMV reduction capability. Note that the RCMV

SVM can be applied in the CSI side as well. In the RCMV

SVMs application in CSI side, the detected voltages used for

zero-state vectors selection are the motor stator voltages instead

of the grid voltages in the CSR application.

The simulation and experimental results show that the RCMVSVM works well under various operating conditions. The peak

value of CMV produced by the proposed RCMV SVM can be

50% lower than that produced from the conventional methods.

Its harmonic performance is very similar to the conventional

five-segment SVM. As for the switching frequency, depending

on the selected zero-state vectors in sector crossing and the mo-

ment of sector crossing, the proposed RCMV SVMs switching

frequency is equal to or a fundamental frequency different from

the conventional five-segment SVM if their sampling frequen-

cies are the same. In comparison with the conventional three-

segment SVM, the proposed RCMV SVM produces a lower

CMV. It also has a better harmonic performance in the highmodulation range when the same switching frequency is used.

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Jian Shang (S12) was born in Shandong, China.He received the B.Eng. degree in electrical engineer-ing from Shandong University, Shandong, China, in2010. He is currently working toward the M.Sc. de-gree in electrical power engineering in the Depart-ment of Electrical and Computer Engineering, Uni-versity of Alberta, Edmonton, Canada.

His current research interests include electricdrives,common-mode voltage mitigation techniques,and renewable energy power generation.

Yun Wei Li(S04M05SM11) received the B.Sc.degree in electrical engineering from Tianjin Univer-sity, Tianjin, China, in 2002, and the Ph.D. degreefrom Nanyang Technological University, Singapore,Singapore, in 2006.

In 2005, he was a Visiting Scholar with theAalborg University, Denmark, where he was in-volved in the medium-voltage dynamic voltage re-storer (DVR) system. From 2006 to 2007, he was aPostdoctoral Research Fellow at Ryerson University,Canada, working on the high-power converter and

electric drives. In 2007, he was also at Rockwell Automation Canada, wherewas engaged in the development of power factor compensation strategies forinduction motor drives. Since 2007, he has been with the Department of Elec-

trical and Computer Engineering, University of Alberta, Edmonton, Canada,where he was initially as an Assistant Professor and then became an AssociateProfessor from 2013. His current research interests include distributed gener-ation, microgrid, renewable energy, power quality, high-power converters, andelectric motor drives.

Dr. Li is currently an Associate Editor for IEEE TRANSACTIONS ONINDUSTRIAL ELECTRONICS and a Guest Editor for the IEEE TRANSACTIONSON INDUSTRIALELECTRONICSSpecial Session on Distributed Generation andMicrogrids.

dc bus voltage

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