A study on performances of carrier-based pulse-width modulation techniques for three-phase three-level t-type neutral-point-clamped inverter under switch-open-circuit fault on two neutral-point-connected legs

Science & Technology Development Journal – Engineering and Technology, 3(3):472-487
Open Access Full Text Article Research Article
Faculty of Electrical and Electronics
Engineering, Ho Chi Minh City
University of Technology, VNU-HCM
Correspondence
Nho-Van Nguyen, Faculty of Electrical
and Electronics Engineering, Ho Chi
Minh City University of Technology,
VNU-HCM
Email: nvnho@hcmut.edu.vn
History
 Received: 10-01-2020 
 Accepted: 09-12-2020 
 Published: 18-12-2020
DOI : 10.32508/stdjet.v3i3.659 
Copyright
© VNU-HCM Press. This is an open-
access article distributed under the
terms of the Creative Commons
Attribution 4.0 International license.
A study on performances of carrier-based pulse-widthmodulation
techniques for three-phase three-level t-type
neutral-point-clamped inverter under switch-open-circuit fault on
two neutral-point-connected legs
Phong Nguyen-Hong Le, Nho-Van Nguyen*
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QR code and download this article
ABSTRACT
Multilevel voltage source inverters (VSIs) have been used for several decades thanks to their advan-
tages comparedwith traditional two level VSI. Among various types ofmultilevel configuration, the
T-type neutral-point-clamped VSI (3L TNPC VSI or 333-type VSI) has gained the attention in recent
years. Due to the unique structure, the 333-type VSI has critical issues in reliability in operation
such as switch-open-circuit (SOC) and switch-short-circuit (SSC), which lead to several unrequired
issues, for instance, reduction of system performance, distorted and unbalanced output voltages
and currents, or triggering the protection circuits. In some applications, the amplitude reduction
and harmonics distortion of output voltages in SOC faults are not acceptable. Therefore, it is nec-
essary to develop a pulse-width modulation (PWM) algorithm for 333-type VSI working under SOC
fault which guarantees the desired output fundamental component voltage. The simultaneous
SOC fault on two neutral-point-connected legs in the 333-type VSI may cause a large reduction in
the output voltage. Under this circumstance, the 333-type VSI becomes an asymmetrical one called
322-type VSI. Certain studies regarding to the operation of 333-type VSI under SOC faults have been
carried out. However, these studies requiremore semiconductor devices in order to create a redun-
dant switching circuit. This leads to higher system cost with reduced inverter efficiency due to the
additional loss. In this study, two carrier-based pulse-width modulation (CBPWM) techniques, i.e.
322-sinusoidal PWM (322-SPWM) and 322-medium offset CBPWM (322-MOCBPWM) are proposed
for 322-type VSI. The proposed techniques are firstly simulated inMATLAB/Simulink and then imple-
mented on a hardware setup. Performances of the proposed techniques are evaluated in terms of
total harmonic distortion (THD) and weighted-THD (WTHD) of output voltages. Simulation results
show that considering the worst output voltage under SOC fault, vBC, the proposed 322-SPWM
technique could improve the THD by 40% and the WTHD by 94% compared with the uncompen-
sated case with m=0.8. The corresponding results of 322-MOCBPWM technique are 42% and 96%,
respectively. Characteristics of THD and WTHD values are also presented for demonstration the
effectiveness of the proposed algorithm.
Keywords: Carrier-based pulse-width modulation, t-type neutral-point-clamped inverter, switch-
open-circuit fault, voltage source inverter, weighted-total harmonic distortion
INTRODUCTION
Multilevel inverters (MLIs) have been being re-
searched for nearly forty years since the first introduc-
tion of three-level neutral-point-clamped (3L NPC)
voltage source inverter (VSI) in 1981 1,2. Compared
with traditional two-level VSI (2L VSI), 3L NPC VSI
offers a larger number of benefits. For instance,
3L NPC VSI has lower distortion of output voltage
and dv/dt, lower distortion of input current, smaller
magnitude of common-mode voltage (CMV) or even
elimination of CMV by using some sophisticated
modulation methods, and ability in operation with
a lower switching frequency 1–7. Among with many
types of 3LNPC topology, 3L T-type NPC (3L TNPC)
topology has an advantage in terms of efficiency com-
pared to 3L NPC 5. For example, 3L TNPC VSI pos-
sesses higher efficiency in low-voltage applications for
the switching frequency between 4-25 kHz such as
photovoltaic (PV) system, uninterruptible power sup-
plies (UPSs), and automotive converter system5,8,9.
3L TNPC VSI combines the benefits of both 2L VSI
and 3L NPC VSI, such as lower conduction losses,
lower switching losses and higher output power qual-
ity8. The TNPC topology also allows the usage of
lower voltage rating power switches and offers consid-
erably power losses comparedwith that of NPC topol-
ogy5,8.
Cite this article : Le P N, Nguyen N. A study on performances of carrier-based pulse-width modu-
lation techniques for thre ... B3, vB3C3, and
vC3A3 are the same, at 32%, while under SOC faults
with conventional PWM, these values obtained from
Figure 11(b) are 194%, 250%, and 170%, respectively.
In terms of WTHD, the normal operation values of
three L2L voltages are the same, at 0.31% yet under
faulty condition, these values are 15.9%, 34.9%, and
15.7%, respectively, are obtained from Figure 12(b).
However, with the proposed 322-SPWM technique
implemented on the VSI under faulty condition, the
harmonic distortion of three output voltages vA3B2,
vB2C2, and vC2A3 shown in Figure 11(c) are improved
to 69%, 71%, and 64% in terms of THD while the
correspondingWTHDs are 0.59%, 0.44%, and 0.59%,
respectively, as regards to m=0.5, as shown in Fig-
ure 12(c).
Simulation and experiment results of output L2L volt-
ages and currents in the proposed 322-MOCBPWM
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Science & Technology Development Journal – Engineering and Technology, 3(3):472-487
Figure 12: Simulated WTHD characteristics of output L2L voltages in SPWM technique: (a) vAB; (b) vBC ; (c) vCA
Figure 13: Simulated waveforms of output L2L voltages and currents in MOCBPWM (m=0.8): (a) in pre-fault op-
eration with conventional MOCBPWM (333-MOCBPWM); (b) in faulty operation with conventional MOCBPWM; (c)
in faulty operation with proposed 322-MOCBPWM. From top to bottom: vAB , vBC , vCA , and iABC .
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Science & Technology Development Journal – Engineering and Technology, 3(3):472-487
technique are illustrated in the following figures from
Figure 13 to Figure 17. The waveforms of output
voltages and currents in Figure 13 corresponding to
MOCBPWM technique are similar to those of SPWM
technique shown inFigure 8. The experiment results
in Figure 14 verify the simulation ones. In the simula-
tion, THD andWTHD of vA3B2 are 36.4% and 0.28%,
respectively while those in experiment are 37.1% and
0.57%, respectively.
Similarly, with the 322-MOCBPWM algorithm,
output voltages and currents in 322-type VSI
are improved, as shown in Figure 15. As an ex-
ample for m=0.8, Figure 15(a) and Figure 15(b)
show significant increasing in harmonic dis-
tortion factor of vAB from THD(vA3B3)=22.5%
to THD(vA3B2_0)=63.1%, and from
WTHD(vA3B3)=0.14% to WTHD(vA3B2_0)=6.45%
, respectively. However, results from Figure 15(c)
show that the proposed 322-MOCBPWM technique
reduces the THD and WTHD of vAB to 36.4%
and 0.28%, respectively, i.e. a reduction of 42% as
regards to THD and 96% as regards to WTHD. The
fundamental voltage vAB which reduced from 80 V in
normal operation to 61.2 V in faulty operation with
conventional MOCBPWM, as shown in Figure 15(a)
and Figure 15(b), is now recovered to the initial
reference value, i.e., VA3B2(1)=80 V which shown in
Figure 15(c). The effectiveness of 322-MOCBPWM
technique on vBC and vCA can be observed in
Figure 15, as well.
The simulated THD and WTHD characteristics
of output voltages in MOCBPWM technique are
illustrated in Figure 16 and Figure 17, respectively.
Characteristics shown in Figure 16 and Figure 17
have the similarity to those in Figure 11 and Figure 12
so that the faulty operation of 333-type VSI leads
to a considerably surge in harmonic distortion
of L2L voltages, especially in the low values of
modulation index. In addition, the voltage vBC has
the worst harmonic distortion compared to that
of vAB and vCA, as regarding to the same value of
m. For instance, under faulty condition for m=0.5,
THD(vB2C2_0)=285% while THD(vA3B2_0)=194%
and THD(vC2A3_0)=170%. With use of the proposed
322-MOCBPWM technique, these values are 72%,
69%, and 64%, respectively. Therefore, there is only
small difference between THD values of output
voltages with the proposed algorithm. As regarding
to WTHD factor, with use of compensating PWM
algorithm, at m=1, they are WTHD(vA3B2)=0.29%,
WTHD(vB2C2)=0.37%, and WTHD(vC2A3)=0.37%
while those values under faulty condition are 4.30%,
4.13%, and 3.58%, respectively.
In terms of vBC , simulated results in two CBPWM
techniques with m=0.8 can be summarized by Ta-
ble 3. Comparison between 322-SPWM technique
and 322-MOCBPWM technique, it can be seen that
the second one has the lower values of harmonic dis-
tortion factors. For instance, under faulty condition,
the proposed 322-SPWM strategy has THD(vB2C2)
value of 50.1% while that of 322-MOCBPWM strat-
egy is 45.7%. The corresponding WTHD(vB2C2) of
these algorithms are 0.45% and 0.36%, respectively.
Hence, the harmonic distortionmetrics of vBC in 322-
MOCBPWM are lower than that in 322-SPWM by
8.8% in terms of THD and by 20% in terms ofWTHD,
respectively.
CONCLUSION
This paper has presented the analysis and imple-
mentation of CBPWM techniques on a 333-type
VSI working under simultaneous SOC faults on
two neutral-point-connected phase-legs. Simulation
results show that under the aforementioned SOC
fault condition, the output voltages and currents are
strongly distorted and unbalanced, with a reduction
of fundamental voltages by up to 30%. The use of
the proposed 322-SPWMand 322-MOCBPWMtech-
niques can help attaining required fundamental volt-
ages, and also lowering harmonic content after faulty
condition. For the worst harmonic quality voltage
vBC , the proposed 322-SPWMhas the ability in reduc-
tion of 40% and 94% in terms of THD and WTHD,
respectively, while the corresponding results of 322-
MOCBPWM technique are 42% and 96%, as regard
to m=0.8. The advantages of 322-MOCBPWM tech-
nique compared to 322-SPWM technique are also
presented, which are not only the extension of mod-
ulation range but also the better harmonic quality at
the same operating condition. Compared with other
studies cited, the proposed techniques do not need
any additional hardware but still guarantees the de-
sired values of fundamental voltages and balanced
output currents. The proposed CBPWM algorithms
are simple and easy for implementation, as well.
LIST OF ABBREVIATIONS
CBPWM: Carrier-based pulse-width modulation
SPWM: Sinusoidal pulse-width modulation
MOCBPWM: Medium offset carrier-based pulse-
width modulation
VSI: Voltage source inverter
SOC: Switch-open-circuit
SSC: Switch-short-circuit
THD: Total harmonic distortion
UPS: Uninterruptible power supply
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Science & Technology Development Journal – Engineering and Technology, 3(3):472-487
Figure 14: Experimented waveforms spectra of output L2L voltages and currents of the 322-type VSI with the
proposed 322-MOCBPWM (m=0.8): (a) vAB, (b) vBC , (c) vCA , and (d) iABC . X-axis: 5ms/div; Y-axis: 40 V/div (voltages),
1 A/div (currents)
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Science & Technology Development Journal – Engineering and Technology, 3(3):472-487
Figure15: Simulated spectra of output L2L voltages inMOCBPWM (m=0.8): (a) in pre-fault operationwith conven-
tional MOCBPWM (333-MOCBPWM); (b) in faulty operation with conventional MOCBPWM; (c) in faulty operation
with proposed 322-MOCBPWM. From top to bottom: vAB, vBC , and vCA .
Figure 16: Simulated THD characteristics of output L2L voltages in MOCBPWM technique: (a) vAB ; (b) vBC ; (c) vCA
Table 3: Simulated Harmonic Distortion Results Of Vbc (M=0.8)
Harmonic
distortion
metrics
Normal operation (with
conventional PWM)
Faulty operation (with con-
ventional PWM)
Faulty operation (with pro-
posed PWM)
333-
SPWM
333-
MOCBPWM
333-SPWM 333-
MOCBPWM
322-SPWM 322-
MOCBPWM
THD (%) 24.5 22.5 83.0 80.2 50.1 45.7
WTHD (%) 0.20 0.14 7.54 7.32 0.45 0.36
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Science & Technology Development Journal – Engineering and Technology, 3(3):472-487
Figure 17: Simulated WTHD characteristics of output L2L voltages in MOCBPWM technique: (a) vAB; (b) vBC ; (c)
vCA
PV: Photovoltaic
WTHD: Weighted-total harmonic distortion
2L VSI: Two-level voltage source inverter
322-type VSI: 322-type asymmetrical voltage source
inverter
333-type VSI: Three-level T-type neutral-point
clamped voltage source inverter
L2L: Line-to-line
CONFLICT OF INTERESTS
Theauthor declares that there is no conflict of interests
regarding the publication of this paper.
AUTHORS’ CONTRIBUTIONS
Nho-Van Nguyen: Conceptualization, Methodology,
Validation, Writing—review and editing, Supervi-
sion.
Phong Nguyen-Hong Le: Software, Validation, Data
curation, Writing—original draft preparation.
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Tạp chí Phát triển Khoa học và Công nghệ – Kĩ thuật và Công nghệ, 3(3):472-487
Open Access Full Text Article Bài Nghiên cứu
Khoa Điện-Điện tử, Trường Đại học
Bách khoa, ĐHQG-HCM
Liên hệ
Nguyễn Văn Nhờ, Khoa Điện-Điện tử,
Trường Đại học Bách khoa, ĐHQG-HCM
Email: nvnho@hcmut.edu.vn
Lịch sử
 Ngày nhận: 10-01-2020
 Ngày chấp nhận: 09-12-2020 
 Ngày đăng: 18-12-2020
DOI : 10.32508/stdjet.v3i3.659 
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mở được phát hành theo các điều khoản của
the Creative Commons Attribution 4.0
International license.
Nghiên cứu kỹ thuật điều chế độ rộng xung sóngmang chomạch
nghịch lưu nguồn áp 3 pha 3 bậc TNPC trong điều kiện hởmạch 2
nhánh nối điểm trung tính
Lê Nguyễn Hồng Phong, Nguyễn Văn Nhờ*
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TÓM TẮT
Mạch nghịch lưu áp (VSI) đa bậc hiện được sử dụng rộng rãi nhờ vào những ưu điểm so với mạch
2 bậc truyền thống. Trong số các sơ đồ VSI, sơ đồ 3 bậc dạng diode kẹp hình T (3L TNPC VSI hay
333-type VSI) đang ngày càng nhận được sự quan tâm nghiên cứu. Do đặc điểm cấu trúc, mạch
333-type VSI cómột số vấn đề về độ tin cậy khi vận hành, ví dụ như sự cố hởmạch khóa đóng ngắt
(SOC) và sự cố ngắn mạch khóa đóng ngắt (SSC), các sự cố này làm giảm hiệu suất hệ thống, gây
mất cân bằng và méo dạng điện áp và dòng điện ngõ ra, làm kích hoạt mạch bảo vệ. Trong một
số ứng dụng, sự sụt giảm biên độ và méo dạng sóng hài trong điện áp ngõ ra dưới sự cố SOC là
không được chấp nhận. Vì vậy, cần phát triểnmột kỹ thuật điều chế độ rộng xung (PWM) chomạch
333-type VSI hoạt động trong điều kiện sự cố SOC để đảm bảo giá trị thành phần cơ bản của điện
áp đầu ra theo yêu cầu. Sự cố SOC đồng thời trên 2 nhánh nối điểm trung tính của mạch 333-type
VSI dẫn đến sự sụt giảm nghiêm trọng đối với điện áp đầu ra. Trong điều kiện này, mạch 333-type
VSI trở thành mạch không đối xứng là 322-type VSI. Nhiều nghiên cứu khác nhau liên quan đến
vận hành mạch 333-type VSI trong sự cố SOC đã được tiến hành. Tuy nhiên các nghiên cứu này
đều sử dụng thêm linh kiện phần cứng để tạo thànhmột nhánh đóng ngắt bổ sung. Điều này làm
tăng chi phí chế tạo và làm giảm hiệu suất của hệ thống do phát sinh thêm tổn hao. Trong bài báo
này, hai kỹ thuật PWM sóng mang (CBPWM) được đề xuất cho mạch 322-type VSI, lần lượt là PWM
dạng sin (322-SPWM) và CBPWM với hàm offset trung bình (322-MOCBPWM). Các kỹ thuật đề xuất
được mô phỏng trước tiên bằng phầnmềmMATLAB/Simulink và sau đó được triển khai trên phần
cứng thực nghiệm. Chất lượng kỹ thuật đề xuất được đánh giá qua các chỉ số độ méo dạng sóng
hài tổng (THD) và độ méo dạng sóng hài tổng có xét đến trọng số (WTHD) của điện áp ngõ ra. Kết
quả mô phỏng cho thấy khi xét điện áp đầu ra vBC là điện áp bị ảnh hưởng nhiều nhất bởi sự cố,
tại hệ số điều chế m=0.8 thông số THD và WTHD của kỹ thuật 322-SPWM được giảm với giá trị lần
lượt là 40% và 94% so với trường hợp không áp dụng phương pháp đề xuất. Độ giảm tương ứng
với kỹ thuật 322-MOCBPWM lần lượt là 42% và 96%. Các đặc tuyến THD và WTHD được trình bày
nhằm xác minh tính hiệu quả của phương pháp đề xuất.
Từ khoá: mạch nghịch lưu nguồn áp, điều chế độ rộng xung sử dụng sóng mang, mạch nghịch
lưu kẹp điểm trung tính kiểu T, sự cố hở mạch khóa đóng ngắt, độ méo dạng sóng hài tổng có xét
đến trọng số
Trích dẫn bài báo này: Phong L N H, Nhờ N V. Nghiên cứu kỹ thuật điều chế độ rộng xung sóng mang 
cho mạch nghịch lưu nguồn áp 3 pha 3 bậc TNPC trong điều kiện hở mạch 2 nhánh nối điểm trung 
tính. Sci. Tech. Dev. J. - Eng. Tech.; 3(3):472-487.
487