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Research ArticleExperimental Evaluation on the Influence of RCD Snubbers ina 3-Level Thyristor Based MLCR CSC
Bhaba P. Das,1 Neville R. Watson,2 and Yonghe Liu2,3
1The Engineering Department, ETEL Ltd., Auckland 0640, New Zealand2The Department of Electrical and Computer Engineering, University of Canterbury, Christchurch 8140, New Zealand3Inner Mongolia University of Technology, Hohhot, China
Correspondence should be addressed to Bhaba P. Das; bhaba.das@eteltransformers.co.nz
Received 27 September 2015; Accepted 18 May 2016
Academic Editor: Jiun-Wei Horng
Copyright Β© 2016 Bhaba P. Das et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Multilevel current reinjection (MLCR) concept provides self-commutation capability to thyristors, enabling thyristor based currentsource converters (CSC) to operate under negative firing angle. It also lowers the input current harmonic distortion.This is achievedby using an auxiliary reinjection bridge. Extensive experimental results are presented in this paper to analyse the performance ofthe 3-level MLCR CSC for different snubber components across the reinjection bridge. The trade-off in the choice of the snubbercircuit is illustrated, with its influence on the AC side line current and DC side output voltage of the 3-level MLCR CSC.
1. Introduction
In order to reduce the harmonic content in the line currentcaused by High Voltage Direct Current (HVDC) converters,different strategies have been proposed. These are (i) multi-pulse methods, (ii) multilevel methods, and (iii) pulse-widthmodulation (PWM) methods.
Different methods for increasing the number of pulsesusing multipulse converters have been investigated [1]. Thesimplest method is by increasing the pulse number by usingphase-shifting transformers. Multipulse converters such as12-, 18-, 24-, and 36-pulse converter are used nowadaysand the total harmonic distortion (THD) of the input linecurrent of these converters has been reduced to around15.2%, 10.1%, 7.5%, and 5.3%, respectively. Many modellingtechniques for phase-shifting converter transformers havebeen also proposed in [2β4] to meet the IEEE 519 standards.However, the weakness of these methods is the increase insize and complexity of the transformer and the high costsinvolved due to multiple 6-pulse bridges. A DC side currentinjection method [5] improves the quality of the AC side linecurrents by using a three-phase current-controlled inverterwhich injects compensation currents in the AC side of theconverter. However, this method uses a complex 18-pulsephase-shifting transformer.
Multilevel converters give significantly lower switchinglosses and better harmonic performance [6β8] than equiva-lent series-connected converters operating at the same ratingand carrier frequency but advantages of multilevel con-verter are counterbalanced by the need for self-commutatedswitches instead of thyristors, balancing of capacitors, andso forth. Modular current source converters (MCSC) utilisePWM techniques just like the PWM-voltage source con-verters (VSC). Two-level PWM-VSC or Modular MultilevelConverter (MMC) [9] has been used by the industry, forexample, HVDC-light and HVDC-plus, respectively. How-ever, they are only being used for medium voltage HVDCtransmission. MCSC utilise selective harmonic elimination[10] as the PWM strategy but the use of thyristors in the mainbridge is still not possible. Ongoing research in the field ofMMC converters includes direct power control [11], neuralnetwork based sliding mode control [12], and fuzzy logicbased control [13].
It is now established that PWM-VSC/MMC/MCSC tech-nology is well suited for the medium voltage levels but willnot be able to catch up with the thyristor based technology interms of high power handling capacity in the very near future.Thus, there is a clear incentive for research in the thyristorbased HVDC technology to achieve the following:
Hindawi Publishing CorporationJournal of EngineeringVolume 2016, Article ID 2490926, 18 pageshttp://dx.doi.org/10.1155/2016/2490926
2 Journal of Engineering
Load
Va
Vb
Vc
Ia
I1
Ib
Ic
YY
YD
IaY
IcaD
VabD
Vy
Vd
Vm
I2
VabY
VxVz
n0
n0
n1
n1
C
C
Iinj
Vyac
Vdac
Y1 Y3 Y5
Y4 Y6 Y2
D1 D3 D5
D4 D6 D2
Ij2
Ij1
Sp1
g1
g2
ISπ1
Sn2
Sp0
g0
g0
ISπ0
Sn0
Sp2
g2
g1
ISπ2
Sn1
IDC
VDC
LDC
gy1
gy2
gy3
gy4
gy5
gy6
gd1
gd2
gd3
gd4
gd5
gd6
Figure 1: Three-level thyristor based MLCR CSC with Linear Reinjection.
(i) Achieve force commutation with thyristors.(ii) Operate at negative firing angles enabling reactive
power export.(iii) Operate the thyristors under zero-current switching
lowering the switching losses, voltage stress, andcosts.
(iv) Reduce harmonic distortion caused by thyristor basedsystems.
The new concept of multilevel current reinjection(MLCR) [14] combines the advantages of DC-ripple rein-jection [15, 16] and multilevel conversion and soft switchingtechnique. Thyristor based MLCR current source converter(CSC) was proposed in [17] where the magnitude andduration of the reinjection current, used to minimize theharmonic content, were adjusted to ensure that the currentsthrough the main bridge thyristors are forced to zero duringthe main bridge commutations instants. This possibility hadthe important implication that the thyristors do not needto rely on the line voltage to commutate and are capable ofoperatingwith firing angles that provide leading power-factorjust like self-commutating switches. Based on this, the self-commutated HVDCMLCR CSC was introduced in [18].
However, questions have been raised about the ability toforce the main thyristors off using the reinjection bridge. In areal world implementation, there are inevitable stray capac-itance and inductances which may influence the thyristorturn-off and simulation switching model may not fully rep-resent the switching characteristics accurately. The questionthat needed answering was whether these would hinder theself-commutation ability with thyristors. For this proof ofconcept, a small-scale prototype of a 3-level MLCR CSC has
been built in the laboratory. The experimental investigation(under steady-state conditions), presented here, shows thatthe line currents in AC side ofMLCRCSC follow the theoret-ical current wave-shape well. This has clearly demonstratedthat self-commutation ability is achievable in a practicalsystem by the use of an auxiliary reinjection bridge.
RCD snubber is necessary across the reinjection bridgeswitches to limit the sharp rise in voltage across it due tothe sudden interruption of current flowing through it. Thispaper presents the experimental investigation of the influenceof the RCD snubber across the reinjection switches in a 3-level thyristor based MLCR CSC performance. This paperis divided into the following sections: Section 2 providesthe brief introduction to the 3-level thyristor based MLCRCSC; Section 3 provides the PSCAD/EMTDC simulationresults, with and without the RCD snubber circuit, of theperformance of the 3-level MLCR CSC. Section 4 presentssome of the main hardware modules while Section 5 presentsthe experimental results obtained. The trade-off in choice ofthe snubber circuit is explained clearlywith respect toAC sideline current and DC side voltage waveforms.
2. Three-Level Thyristor Based MLCR CSC
Figure 1 shows the 3-level thyristor based MLCR CSC alongwith the reinjection circuit. The reinjection transformeris a single-phase two-winding transformer with 1 : 1 turnsratio. The primaries of the two single-phase transformersare connected across the DC bus through DC blockingcapacitors (πΆ). The DC current (πΌDC) flows through thereinjection IGBTs and load inductance (πΏDC) and the load.It is chopped into AC waveforms in the secondary windings
Journal of Engineering 3
β22
400
0.05
Harmonic order
150 210 30060 240 330 3600 180 27090 12030
60 80 100 120 140 160 180 2000 20
(β)
I 1/I
DC
I 2/I
DC
I aY/I
DC
I caD/I
DC
I a/I
DC
02
02
β22
β22
MLCR CSC output current spectrum THD3-level = 7.77%
I aπ/I
a1
Figure 2: Current waveforms for 3-level thyristor based MLCR CSC with Linear Reinjection.
150 210 30060 240 330 3600 180 27090 12030(β)
β11
0
2
02
β2
2
β1
1
02
Vz
Vm
Vx
Vy,V
dVa,V
b,V
cV
DC
Figure 3: Theoretical DC voltage waveforms for 3-level thyristor based MLCR CSC for πΌ = β45β.
of the reinjection transformer with the help of reinjectionswitches (π
π1
/ππ1
, etc.). These currents are coupled to thereinjection transformer primary winding to form multilevelcurrents πΌ
π1
and πΌπ2
which combine with πΌDC to shape theDC bus currents πΌ
1and πΌ2into multilevel waveforms. This
reinjection circuit generates three current steps in πΌ1and πΌ2.
Two levels are generated due to reverse connected switches(ππ1
/ππ1
and ππ2
/ππ2
) and one additional level is obtained byfiring π
π0
/ππ0
when πΌπ1
and πΌπ2
are both zero.
2.1. AC Side Current Waveforms. The theoretical analysis ofthe circuit shown in Figure 1 allows πΌ
π(ππ‘) to be determined
from the time domain components of the AC side secondarycurrents πΌ
ππ(ππ‘) and πΌ
πππ·(ππ‘). This gives
πΌπ(ππ‘) =1
ππ
[πΌππ(ππ‘) + β3πΌ
πππ·(ππ‘)] , (1)
where ππis interface transformer turns ratio. The corre-
sponding current waveforms are shown in Figure 2 where theresulting current (πΌ
π) THD3 level of 7.77% is obtained.
2.2. DC Side Voltage Waveforms. The DC side voltage wave-forms (Figure 3) are time referenced with respect to the ACside current waveform πΌ
π(Figure 2). The DC side voltages
across the individual bridges (ππ¦(ππ‘) and π
π(ππ‘)) are plotted
in Figure 3with respect to the peak phase source voltage (πππ
)and can be expressed as for the first π/6 interval as
ππ¦(ππ‘) =β3
ππ
πππ
cos (ππ‘ + πΌ) ,
ππ(ππ‘) =β3
ππ
πππ
cos(ππ‘ + πΌ β π6) ,
ππ₯(ππ‘) = π
π¦(ππ‘) + π
π(ππ‘) ,
(2)
4 Journal of Engineering
40
3-level thyristor based MLCR CSC output DC voltagespectrum
0
0.01
0.02
0.03
0.04
0.05
60 80 100 120 140 160 180 2000 20Harmonic order
VD
Cπ/V
DC
0
Figure 4: Harmonic spectrum of πDC of 3-level thyristor based MLCR CSC for πΌ = β45β.
n1
n1n0
n0
Iinj
Ij1
Ij2
IS1
IS0
IS2
VSπ1
Sp1
Sp0
Sp2
ISπ1
Sn2
Sn0
Sn1
ISπ0
ISπ2
ISπ0
ISπ1
ISπ2
IDC
IDC
Figure 5: Reinjection current flow for a 3-level MLCR CSC.
where πΌ is firing angle. The DC load voltage (πDC) iscalculated as
πDC (ππ‘) = ππ₯ (ππ‘) + ππ§ (ππ‘) β 3.39πππ
ππ
cosπΌ. (3)
The harmonic spectrum of πDC is shown in Figure 4.It can be clearly seen that the dominant harmonics are24th, 48th, 72nd, 96th, and 120th which represents 24-pulseoperation. A small amount of 12-pulse related harmonicssuch as 12th, 36th, and 60th can also be seen but themagnitude is negligible.
2.3. Reinjection Current Waveforms. Figure 5 labels the vari-ous current waveforms in the reinjection circuit. The βchop-pingβ of load current πΌDC occurs with the help of the reinjec-tion IGBTs. The injection current πΌinj is a 3-level waveformcomposed of the following:
(i) When reinjection IGBT pair ππ1
β ππ1
conducts, πΌπ1
=
(π1/π0)πΌDC = πΌDC.
(ii) When reinjection IGBT pair ππ0
β ππ0
conducts, πΌπ1
=
0.
(iii) When reinjection IGBT pair ππ2
β ππ2
conducts, πΌπ1
=
β(π1/π0)πΌDC = βπΌDC.
Reinjection currents πΌπ1
and πΌπ2
add up to form πΌinj whichis injected back to themidpoint of the 12-pulse converter.Thetheoretical current waveforms in the reinjection circuit areshown in Figure 6.
3. PSCAD/EMTDC Verification
This section presents the PSCAD/EMTDC simulation resultsof the thyristor based 3-level MLCR CSC. The converteris directly connected to a balanced 3-phase voltage sourcewith series resistance and series inductance. The simu-lation is carried out under the conditions listed in theAppendix.
3.1. Waveforms without RCD Snubber. The voltage waveformacross the reinjection IGBT π
π1
as well as the current flowingthrough it, without the RCD snubber, is shown in Figure 7.It can be clearly seen that there is a sharp rise in voltageacross π
ππ1
whenever there is a current transition through
Journal of Engineering 5
60 3300 30 90 300270180 360240210150120
00.5
1
00.5
1
00.5
1
(β)
I Sπ2/I
DC
I Sπ0/I
DC
I Sπ1/I
DC
(a) πΌππ1
, πΌππ0
, and πΌππ2
waveforms
30 2100 150 18060 240 330120 300270 36090
β101
β101
β101
(β)
I S1/I
DC
I S0/I
DC
I S2/I
DC
ISπ1ISπ2
ISπ1ISπ2
(b) πΌπ1, πΌπ0, and πΌ
π2waveforms
30 120 210180 36060 240 330900 300270150
β101
β101
β202
(β)
I inj/I
DC
I j2/I
DC
I j1/I
DC
(c) πΌπ1, πΌπ2, and πΌinj waveforms
Figure 6: Theoretical waveforms in the reinjection circuit.
β1
0
1
β500
β250
0
330300270210 36030 90 24018060 120 1500(β)
Vz
VS π
1,V
z(V
)I D
C,I
j 1(A
)
VSπ1= 0 VSπ1
VSπ1
= Vz = βVm
ISπ1
ISπ2
Ij1IDC
Figure 7: πππ1
and πΌπ1with no snubber.
it, that is, current transition from 0 to βπΌDC or 0 to πΌDC.This effect was not considered while deriving the theoreticalwaveforms. As seen from Figure 8, these voltage spikes occuracross the individual DC voltage waveform of the two 6-pulse bridges which in turn affect the voltages π
πand π
π§.
The spikes occur exactly 7.5β before and after the theoreticalmain bridge switching instant. Thus there is a need to limitthe voltage πV/ππ‘ using RCD snubber to an acceptable valueso that the voltage across the main bridge switches and DC
blocking capacitor and the reinjection transformer are withintheir rated values.
The simulated AC side current waveforms are shown inFigure 9.The simulated line current THDof 7.8% is very closeto the calculated theoretical value.
3.2. Waveforms with RCD Snubbers. As illustrated inFigure 10(b), the snubber capacitor size is a trade-offbetween πV/ππ‘ across the reinjection switches and switching
6 Journal of Engineering
0 60 120 180 240 3603000
50100
β1000
100
β1000
100
50100150
Vy,V
d(V
)
050
100
0
500
0
500
Vx
(V)
β5000
500
Vm
(V)
β500
0
Vz
(V)
0
150
(β)
VD
C(V
)
Figure 8: Simulated DC voltage waveforms for 3-level thyristor based MLCR CSC for πΌ = β45β without RCD snubber.
60 15030 330270 36090 180120 300210 2400(β)
02
02
β22
β22
β0.5
0.5
I 1(A
)I 2
(A)
I aY
(A)
I caD
(A)
I a(A
)
THD = 7.8%
Figure 9: Simulated AC current waveforms for 3-level thyristor based MLCR CSC for πΌ = β45β without RCD snubber.
time. A βnormalβ snubber is defined as the one which allowsthe current to reach the rated level at the same time thevoltage reaches zero. A small snubber allows the current torise faster while a large snubber slows the current rise rate.However, large capacitor values will provide overclamping.When the RCD snubber is used to control the rate of voltagerise at the IGBT, the capacitor must be completely chargedand discharged during each cycle to be able to control therate-of-rise of the drain voltage. The RC time constant of thesnubber should be much smaller than the switching period.Usually, the time constant (πRC) should be (1/10)th of theswitching period. The value of snubber capacitor πΆsn is givenby the following equation [19]:
πΆsn =πΌπ‘π
2πΈ, (4)
where πΌ is maximum IGBT current (assumed) = 2A, π‘π :
β1.65 πs (3 times the value given in the datasheet of theIGBT), and πΈ is the maximum expected voltage acrossIGBT: β0.9(π
ππ
/ππ) β 36.72V for the present application.
Substituting these values in (4) gives πΆsn = 0.045 πF. Thevalue of π sn is determined by allowing the switching timeconstant (πRC) to be less than 166.66 πs ((1/10)th of 600Hz).With πΆsn = 0.05 πF and π sn = 1 kΞ©, πRC = 50 πs.
Three different cases are considered in this study, smallπΆsn = 0.01 πF, large πΆsn = 0.1 πF, and very large πΆsn =1 πF. The PSCAD/EMTDC results are presented in Figures11β15. From Figure 11, with πΆsn = 1 πF the voltage acrossπππ1
is closest to its calculated value; however the rate ofcurrent rise for the switch π
π1
is slowest. πΆsn = 0.01 πFprovides higher rate of current rise in π
π1
and the currentreinjection current resembles the theoretical current moreclosely. THD for line current πΌ
πis obtained as 7.9%, 8.1%,
and 8.7%, respectively, with πΆsn = 0.01 πF, 0.1 πF, and1 πF (Figures 12β14). The effectiveness of the MLCR schemedepends on the modification of the constant DC bus currentinto a 3-level DC bus current using reinjection currentπΌinj. The use of small πΆsn is found to be the best eventhough it requires overrated reinjection switches as shown inFigure 11.
Journal of Engineering 7
IGBTRs Ds
Cs
(a) RCD snubber for rein-jection IGBT
Voltage
Turn-on
Turn-off
(1)
(2)
(3)
(4)
Current
ts
(b) Voltage and current waveforms for (1) idealand (2) small snubber and (3) normal snubberand (4) large snubber
Figure 10: Snubber consideration for reinjection switches.
4. Hardware Modules
The major components of the experimental set-up consist ofvoltage sensing, thyristor driver circuit, IGBT driver circuit,and so forth. Each will be discussed in the following sections.
4.1. Voltage Transducer Circuit. Voltage sensors LEM LV25-P[20] are used to convert power level voltage into low voltagesignals by scaling down the measured voltage to 5 πRMSThe Hall effect transducers provide the isolation betweenthe high power system and the control electronic circuit.For a 12-pulse converter, six voltage sensors are used. Theline voltages sensed are π
ππ, πππ, πππ, πππ, πππ, and π
ππ. The
triggering signals for the π-π and π-π· connected convertersare derived from the low voltage sensed signals π
πππ
andβππππ
, respectively.The LV25-P scales down the power level voltage into low
level signal when a current proportional to the measuredvoltage is passed through the resistor π in (Figure 16(a)). Forthe best accuracy of LV25-P,π in should be so selected that thevoltagemeasured corresponds to a primary current (πΌinRMS
) of10mA.The current conversion ratio is 2.5; hence, the nominalsecondary current is 25mA. This secondary current flowsthrough an output series resistance π out to give the scaleddown voltage. The fabricated PCB for this purpose is shownin Figure 16(b). The maximum and minimum value of theseries resistance π out with a Β±15 V supply are between 100Ξ©and 350Ξ©.
The 12-pulse interface transformer rating is (πLLRMS)
400V : 50V. The primary side input resistance is calculatedas
π in =400V10mAβ 40 kΞ©. (5)
A fixed value resistor of 40 kΞ©/5W is used.
Theoutput voltage is fixed at 5πRMS.Therefore, the outputresistance π out is calculated as
π out =5
25mAβ 200Ξ©. (6)
A variable 500Ξ©/0.6W resistor is used.
4.2. Forward Converter Based Thyristor Driver Circuit. Thethyristor driver circuit is based on the forward convertertopology with isolation between the control and powercircuit being provided by the 77205C pulse transformer [21](Figure 17(a)).
When the MOSFET is on, diode π·2conducts and πsec =
5V is available across the secondary winding of the pulsetransformer. πsec drives gate current πΌ
πin the gate-cathode
junction of the thyristor to turn it on. For πΌπβ 200mA, the
gate drive resistor π π= 12.5Ξ©. When the MOSFET is off,
diode π·1returns the energy stored in the pulse transformer
back to the supply. The 6-pulse thyristor PCB is shown inFigure 17(b) which uses BT152800R thyristors [22].
4.3. Generating of Reinjection Pulses for 3-Level MLCR. Inorder to produce the firing sequence needed to generate the3-level πΌinj, the firing sequences of the reinjection IGBTs aresynchronised with the main bridge switching. In Figure 18it is shown that each reinjection turn-on pulse is delayedfrom the main bridge switching pulse by 52.5β and the widthof the turn-on pulse is 15β. Based on the six main bridgeswitching pulses, six reinjection turn-on pulses are derivedand these are added together from the reinjection pulse forreinjection IGBT pair π
π1
/ππ1
. Following a similar procedure,the reinjection pulse for reinjection IGBT pair π
π2
/ππ2
is alsoderived as shown in Figure 19. Aminimumdead time of 10πsis selected between π
π1
/ππ1
and ππ0
/ππ0
and ππ2
/ππ2
and the
8 Journal of Engineering
IGBT blocking voltage
30 60 27012090 210 300180 240 3300 360150
β200β100
0100200
β101
(β)
Vz
VSπ1= Vz = βVmVSπ1
= 0
VSπ1
IDC
ISπ1ISπ2
IS1
VS π
1,V
z(V
)I D
C,I
S 1(A
)
(a) πππ1
and πΌπ1
with πΆsn = 0.01 πF
Blocking voltage reduced
9030 60 300240 3602701501200 210180 330β1
01
β100β50
050
100
Vz
(β)
VSπ1= Vz = βVmVSπ1
= 0
VSπ1
IDCIS1
VS π
1,V
z(V
)I D
C,I
S 1(A
)
ISπ1ISπ2
(b) πππ1
and πΌπ1
with πΆsn = 0.1 πF
Blocking voltage reduced further
60 15030 330270 36090 180120 300210 2400β1
01
β100β50
050
100
Vz
(β)
VSπ1= Vz = βVmVSπ1
= 0
VS π
1,V
z(V
)I D
C,I
S 1(A
)
VSπ1
IDC
IS1
ISπ1ISπ2
(c) πππ1
and πΌπ1
with πΆsn = 1πF
Figure 11: Voltage across reinjection IGBT ππ1for different πΆsn.
derivation of the reinjection pulse for ππ0
/ππ0
is shown inFigure 20.
4.4. Optocoupler Based IGBTDriver Circuit. The isolated gatedriver ACPL-312T [23] is used for this purpose.The responseof optocoupler depends on the value of gate resistance (π
π).
π πis calculated from the peak gate current (πΌ
πΊpeak) of 2.5 A
(from ACPL-312T datasheet). πOL is obtained from Figure 6in the ACPL-312T datasheet, assuming temperature = 25β.Consider
π πβ₯πcc β πOLπΌπΊpeak
=15 β 3.5
2.5= 4.6Ξ©. (7)
The value of the gate resistance has a significant impacton the dynamic performance of IGBTs. A smaller gate resistorcharges and discharges the power transistor input capacitancefaster, reducing switching times and switching losses. Thetrade-off is that this could lead to higher voltage oscillations.
At lower π πvalues, the voltage supplied by the ACPL-312T
is not an ideal voltage step. Since the maximum peak gatecurrent of the driver must be equal to or lower than πΌ
πΊpeak,
choosing π π= 10Ξ© is a good trade-off.
Once the logical signals (reinjection pulses for reinjectionswitches) are available, these are fed to the IGBT drivercircuit.Thedriver circuit is shown in Figure 21. Since there aresix reinjection switches (IXGP20N [24]) for a 3-level MLCRCSC, six isolated DC-DC converters are required for theACPL-312T. The isolated DC-DC converter 0515S [25] fromTracopower is used here. πcc = 15V is chosen because theoutput voltage (π
π) of ACPL-312T goes high whenπcc βπee is
between 13.5 V and 30V. Input resistance for ACPL-312T, π in,is calculated as follows (input current πΌin of ACPL-312T needsto be limited between 7mA and 16mA):
π in =15 β 1.5
10mAβ 1.5 kΞ©. (8)
Journal of Engineering 9
60
β202
β101
β0.4β0.2
00.2
180 330 360270210 240 30090300 120 150
THD = 7.9%
I aY
(A)
I caD
(A)
I a(A
)
(β)
(a) πΌππ¦, πΌπππ·
, and πΌπfor πΆsn = 0.01 πF
(β)30 12090 2100 180 240 330 36060 300270150
β202
β2
0
2
β0.5
0
0.5
I aY
(A)
I caD
(A)
I a(A
)
THD = 8.1%
(b) πΌππ¦, πΌπππ·
, and πΌπfor πΆsn = 0.1 πF
THD = 8.7%
0 90 12030 210180 240 330 36060 300270150(β)
I aY
(A)
I caD
(A)
I a(A
)
β202
β2
0
2
β0.4β0.2
00.2
(c) πΌππ¦, πΌπππ·
, and πΌπfor πΆsn = 1 πF
Figure 12: Simulated AC side current and DC bus waveforms for the 3-level thyristor based MLCR CSC.
360180120 270150 33090 240210 3000 30 60
0
1
2
0
1
2
I 1(A
)I 2
(A)
(β)
IDC
IDC
(a) πΌ1and πΌ2for πΆsn = 0.01 πF
120 210 36018090 240 33060 30030 2700 150
0
1
2
0
1
2
I 1(A
)I 2
(A)
(β)
IDC
IDC
(b) πΌ1and πΌ2for πΆsn = 0.1 πF
30 12090 330 360210 300600 180 240 270150(β)
0
1
2
0
1
2
I 1(A
)I 2
(A)
IDC
IDC
(c) πΌ1and πΌ2for πΆsn = 1 πF
Figure 13: Simulated AC side current and DC bus waveforms for the 3-level thyristor based MLCR CSC.
10 Journal of Engineering
300270150 240120 180300 90 36060 210 330
β101
β101
β202
(β)
I inj
(A)
I j1
(A)
I j2
(A)
(a) πΌπ1, πΌπ2, and πΌinj for πΆsn = 0.01 πF
90 12030 60 2100 180 270 330 360300240150
β101
β101
β202
(β)
I inj
(A)
I j1
(A)
I j2
(A)
(b) πΌπ1, πΌπ2, and πΌinj for πΆsn = 0.1 πF
90 24030 120 3300 18060 270210 300 360150
β101
β101
β202
(β)
I inj
(A)
I j1
(A)
I j2
(A)
(c) πΌπ1, πΌπ2, and πΌinj for πΆsn = 1 πF
Figure 14: Simulated Reinjection current and DC side voltage waveforms for the 3-level thyristor based MLCR CSC.
90 240 30012030 270600 180 330150 360210(β)
0100200
0100200300
050
100
Vy,V
d(V
)Vx
(V)
VD
C(V
)
(a) ππ¦, ππ, ππ₯, and πDC for πΆ = 0.01 πF
33090 2100 30024060 270150 180120 36030
050
100
50100150
050
100
Vy,V
d(V
)Vx
(V)
(β)
VD
C(V
)
(b) ππ¦, ππ, ππ₯, and πDC for πΆ = 0.1 πF
90 300120 27030 60 150 360240180 210 3300
050
100
050
100150
050
100
Vy,V
d(V
)Vx
(V)
(β)
VD
C(V
)
(c) ππ¦, ππ, ππ₯, and πDC for πΆ = 1πF
Figure 15: Simulated Reinjection current and DC side voltage waveforms for the 3-level thyristor based MLCR CSC.
Journal of Engineering 11
+ +Iin
+15V
β15V
VsenseRout
M
LEMLV 25 V
Rin
VinIout
β
β
(a) Hall effect voltage transducer
Y-D voltage sense
Y-Y voltage sense
(b) Voltage sensor PCBs for π-π andπ-π· voltage sensing
Figure 16: Voltage transducer circuit and fabricated PCB.
1
77205C
Cathode (K)
Gate (G)3
4Controlsignal 25
6
S
+5V
Ipri
G
D
2N700
Rg
D2
D1Ig
Vse
c
12.5Ξ©
10Ξ©
(a) Forward converter based thyristor driver circuit
BT152800RPulse transformer77205C
GND rail
2N700
Rg
5V rail
D1
D2
gy1
gy2
gy3
gy4
gy5
gy6
(b) Six-pulse phase controlled thyristor converter PCB
Figure 17: Forward converter based thyristor driver circuit and PCB detail.
A decoupling capacitor of 0.1 πF is placed across πcc andπee, very close to the optocoupler itself to filter out any noisecoming from the isolated DC-DC converter.
4.5. DC Blocking Capacitor. A 400V/1mF (PEH200 Series,Β±20%) electrolytic capacitor is used as the DC blockingcapacitor. This unit is rated for a total of 5.9 A based on anequivalent series resistance (ESR) of 76mΞ© at a frequency of100Hz and equivalent series inductance (ESL) of 16 nHwhichgives π
100Hz = 1.59Ξ©. The maximum average steady-stateDC voltage for this 3-level MLCR CSC prototype is β138V,πΌDC = 1.38A. The RMS value of reinjection currents πΌ
π1
andπΌπ2
= 0.707 Γ 1.38 = 0.975A β 1 A which is well underthe rating of PEH200 capacitor. Figure 22 shows the set-upfor the reinjection transformer along with the DC blockingcapacitors.
5. Experimental Results
Real world capacitors often have Β±1%βΒ±20% variation inthe capacitance values while resistors also have Β±1%βΒ±5%
variation; thus exact DC bus currents (πΌ1and πΌ
2) may
not be possible in hardware implementation. However, forcomparisonwith the theoretical waveforms, the experimentaland theoretical waveforms are overlapped together. Twodifferent types of PCBs with πΆsn: 0.01πF and 1 πF (Figure 23)are fabricated and the results are summarized in this section.The experimental πDC obtained from the prototype is 96.3 Vand πΌDC = 0.97A with πΌ = β45β.
5.1. Current through Reinjection IGBTs. Figure 24 shows thecurrent flowing through reinjection IGBTs π
π1
, ππ0
, and ππ2
.For πΆsn = 0.01 πF (Figure 24(a)), these waveforms followthe theoretical waveform closely with πΌ
ππ1
β πΌDC for almostthe entire theoretical duration of 15β. As observed fromFigure 24(b), with πΆsn = 1 πF, πΌπ
π1
β πΌDC for less than halfof the theoretical duration. These waveforms constitute theβchoppingβ of πΌDC and formation of the reinjection currents.
5.2. Reinjection Currents Waveforms. Figure 25 shows thereinjection currents πΌ
π1
, πΌπ2
, and πΌinj on the primary side of the
12 Journal of Engineering
AdderDelay 6 Width 6
0 60 120 180 240 300 360
15β
15β
52.5β
52.5β
60β
(β)
SD1
SD2
SD3
SD4
SD5
SD6
S p2/S
n2
Figure 18: Generation of reinjection pulse for reinjection IGBT ππ1/ππ1.
0 60 120 180 240 300 360
AdderDelay 6 Width 6
60β
52.5β
52.5β
15β
15β
(β)
SY1
SY2
SY3
SY4
SY5
SY6
S p1/S
n1
Figure 19: Generation of reinjection pulse for reinjection IGBT ππ2/ππ2.
Journal of Engineering 13
0 60 120 180 240 300 360
Adder
Dead time
(β)
S p0/S
n0
S p1/S
n1
Sp1/Sn1
S p2/S
n2
Sp2/Sn2
Figure 20: Generation of reinjection pulse for reinjection IGBT ππ0/ππ0.
Inh
GND
N/C
G
E
N/C
DC-DC
Rin
Vcc
Vo
Vo
Vee
Rg
1
2
3
4 5
6
7
8+15V
+5V
Controlsignal
0.1 πF
Figure 21: ACPL-312T IGBT driver circuit.
reinjection transformer. πΌinj is composed of πΌπ1
and πΌπ2
. Again,with πΆsn = 1 πF reinjection current πΌinj is not following thetheoretical πΌinj βperfectlyβ whereas, with πΆsn = 0.01 πF, πΌinjfollows the theoretical wave-shape.
5.3. DC Bus Currents Waveforms. The injected current wave-form πΌinj modifies the DC bus currents as shown in Figure 26.This DC bus current is a 3-level waveform with πΌ
1,2= 0,
πΌ1,2= πΌDC, and πΌ1,2 = 2πΌDC as the three different levels of
DC bus current waveform.
5.4. AC Side Secondary Line Current Waveforms. The modi-fied AC side secondary line currents πΌ
ππand πΌππ·
are shownin Figure 27. With πΆsn = 0.01 πF, experimental πΌ
ππand πΌππ·
follow the theoretical current wave-shape. Current spikes areobserved in πΌ
ππand πΌππ·
with πΆsn = 1 πF. These spikes are
14 Journal of Engineering
Reinjection transformers
DC blocking capacitor
Figure 22: Three-level MLCR CSC prototype and associated auxiliaries: the DC side reinjection transformer.
IL0515S
ACPL-312T
IGBT
Rs Cs
Ds
(a) Three-level reinjection PCB,πΆsn:0.01 πF
IL0515S
ACPL-312T
IGBT
Rs
Cs
Ds
(b) Three-level reinjection PCB,πΆsn:1πF
Figure 23: Two reinjection PCBs used in the prototype.
240150 270 30018060 12030 900 330 360210
00.5
1
00.5
1
00.5
1
(β)
I Sπ1
(A)
I Sπ0
(A)
I Sπ2
(A)
(a) πΌππ1
, πΌππ0
, and πΌππ2
, πΆsn = 0.01 πF
0 27030 60 150 210 30024090 180 360120 330
00.5
1
00.5
1
00.5
1
(β)
I Sπ1
(A)
I Sπ0
(A)
I Sπ2
(A)
(b) πΌππ1
, πΌππ0
, and πΌππ2
, πΆsn = 1 πF
Figure 24: Experimental currents πΌππ1
, πΌππ0
, and πΌππ2
through reinjection IGBTs.
observed at the same instants when themain bridge thyristorsare triggered. However, these spikes are not observed withπΆsn = 0.01 πF.
5.5. AC Side Primary Line Current Waveform. The mod-ified primary side line current πΌ
πis shown in Figure 28.
The effect of using a βvery largeβ πΆsn is clearly observed(Figure 28(b)). πΌ
πwith πΆsn = 1 πF is highly distorted
although a multistep waveform can be observed. With πΆsn =0.01 πF, πΌ
πfollows the theoretical waveform closely and a
24-step πΌπis obtained.
5.6. π-π and π-π· Connected DC Voltage Waveform. TheDC voltage waveforms for both π-π and π-π· connected 6-pulse bridges are shown in Figure 29. With πΆsn = 0.01 πF,the experimental waveform follows the theoretical one while,
Journal of Engineering 15
60 150 27030 210180 33030090 2400 360120
β101
β101
β202
(β)
I inj
(A)
I j1
(A)
I j2
(A)
(a) πΌπ1, πΌπ2, and πΌinj, πΆsn = 0.01 πF
β101
210150 300270600 24012030 180 330 36090
β101
β202
(β)
I inj
(A)
I j1
(A)
I j2
(A)
(b) πΌπ1, πΌπ2, and πΌinj, πΆsn = 1 πF
Figure 25: Reinjected current waveforms πΌπ1, πΌπ2, and πΌinj.
2400 1506030 30090 180 270 330120 360210
0
1
2
0
1
2
I 1(A
)I 2
(A)
(β)
(a) πΌ1, πΌ2, and πΌDC, πΆsn = 0.01 πF
0 30 60 90 120 150 180 210 240 270 300 330 360
0
1
2
0
12
I 1(A
)I 2
(A)
(β)
(b) πΌ1, πΌ2, and πΌDC, πΆsn = 1 πF
Figure 26: Experimental DC bus currents πΌ1
, πΌ2
, and πΌDC.
18090 240 360150 27030 300 33060 1200 210
β2
0
2
β2
0
2
I aY
(A)
I aD
(A)
(β)
(a) πΌππ
and πΌππ·
, πΆsn = 0.01 πF
β2β1
012
β2
0
2
240 270 300180 33060 150900 30 360120 210
I aY
(A)
I aD
(A)
(β)
(b) πΌππ
and πΌππ·
, πΆsn = 1 πF
Figure 27: Secondary side line currents πΌππ
and πΌππ·
.
with πΆsn = 1 πF, the transitions are delayed by about7.5β which is also observed in PSCAD/EMTDC simulationresult. However, very large voltage πV/ππ‘, as predicted byPSCAD/EMTDC, is absent.
5.7. 12-Pulse DC Voltage Waveform. The 12-pulse DC voltageobtained for both the values of πΆsn is shown in Figure 30.
5.8. Reinjection Transformer Secondary Side Voltage Wave-form. The reinjection transformer secondary side voltageπ
π
obtained is shown in Figure 31. ππwith πΆsn = 1 πF follows
the theoretical ππmore closely but is delayed by about 7.5β.
ππwith πΆsn = 0.01 πF has higher πV/ππ‘ during transitions
(Figure 31(a)).
5.9. Three-Level MLCR CSC DC Voltage Waveform. The DCvoltage waveform πDC is shown in Figure 32. Voltage πV/ππ‘observed in π
πwith πΆsn = 0.01 πF is clearly reflected across
πDC whereas the voltage πV/ππ‘ transitions are not observed inπDC with πΆsn = 1 πF. Nevertheless, πDC has 24 pulses in boththe cases which implies that ripple voltage is getting added toππ₯giving it 24-pulse characteristics.
16 Journal of Engineering
15030 180 270240 3000 12060 330210 36090
I a(A
)
β0.5β0.4β0.3β0.2β0.1
00.10.20.30.40.5
(β)
(a) πΌπ, πΆsn = 0.01 πF
β0.5
0
0.5
33030090 210300 270180150120 36024060
I a(A
)
(β)
(b) πΌπ, πΆsn = 1 πF
Figure 28: Primary side line current πΌπ
.
360600 30 21090 270 300120 180 240150 3300
20
40
60
80
100
Vy,V
d(V
)
(β)
(a) ππ¦and ππ, πΆsn = 0.01 πF
Vy,V
d(V
)
360600 30 21090 270 300120 180 240150 330(β)
0
20
40
60
80
100
(b) ππ¦and ππ, πΆsn = 1 πF
Figure 29: DC voltage waveforms ππ¦
and ππ
.
0
50
100
150
0 24030 150 210 36060 270180120 300 33090(β)
Vx
(V)
(a) ππ₯, πΆsn = 0.01 πF
1500 270120 300210 24090 330180 36030 600
20406080
100120140160
Vx
(V)
(β)
(b) ππ₯, πΆsn = 1 πF
Figure 30: 12-pulse DC voltage waveform ππ₯
.
240 270 300120 1506030 180900 330 360210β80β60β40β20
020406080
Vm
(V)
(β)
(a) ππ, πΆsn = 0.01 πF
240 270 300120 1506030 180900 330 360210(β)
Vm
(V)
β80β60β40β20
020406080
(b) ππ, πΆsn = 1 πF
Figure 31: Reinjection transformer secondary side voltage ππ
.
Journal of Engineering 17
30 1200 60 300 33090 240180 210 360150 2700
20
40
60
80
100
120
(β)
VD
C(V
)
(a) πDC, πΆsn = 0.01 πF
0
20
40
60
80
100
120
30 1200 60 300 33090 240180 210 360150 270(β)
VD
C(V
)
(b) πDC, πΆsn = 1 πF
Figure 32: Three-level MLCR CSC DC voltage waveform πDC.
6. Conclusion
The theory, PSCAD/EMTDC simulation results, hardwareprototype, and experimental results of a 3-level thyristorbasedMLCRCSC have been presented in detail in this paper.The reinjection current forces the main bridge thyristor tocommutate under zero current thereby allowing it to beswitched at negative firing angles. It is possible to achieveself-commutation for thyristors using an auxiliary reinjectionbridge.
From experimental investigation, it is observed thatthe deviation of the actual waveforms from the theoreticalwaveforms is mainly due to the use of a βvery largeβsnubber capacitor. With a βvery largeβ snubber capacitorvalue, interface transformer secondary side current showedcurrent ππ/ππ‘ which happened exactly 7.5β before the cur-rent passed through that particular thyristor. This is whenthe thyristor is switched on. The voltage transitions werealso delayed by 7.5β with a βvery largeβ snubber capacitor.With a βsmallβ snubber capacitor value, current waveformfollows the theoretical waveforms very closely; howevervoltage πV/ππ‘ can be observed in the output DC voltagewaveform.
The use of βsmallβ snubber capacitor value is recom-mended to reduce line current distortion, as the modifi-cation of DC bus current is the key to achieve both self-commutation for thyristors and lower harmonic distortion.
Appendix
Experimental/Simulation Parameters
Source specification is as follows:
(i) Voltage rating: 415V @ 50Hz.(ii) Source impedance: 0.1Ξ© + 5mH.
Interface transformer is as follows:
(i) Type: 3-phase, 3-winding @ 50Hz.(ii) Power rating: 2 kVA.(iii) Voltage rating: 415V : 50V (1 : 1 for π-π and
1 :β3 for π-π·).
Reinjection transformer is as follows:
(i) Type: 1 phase and 2 windings @ 300Hz.(ii) Nominal impedance: 5%.(iii) Power rating: 1 kVA.(iv) Voltage rating: 400V : 400V.
Load specification is as follows:
(i) Load resistance: 100Ξ©.(ii) Load inductance: 400mH.
Firing angle (πΌ) = β45β.
Competing Interests
The authors declare that they have no competing interests.
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