Advanced
Tolerance Control for the Inner Open-Switch Faults of a T-Type Three-Level Rectifier
Tolerance Control for the Inner Open-Switch Faults of a T-Type Three-Level Rectifier
Journal of Power Electronics. 2014. Nov, 14(6): 1157-1165
Copyright © 2014, The Korean Institute Of Power Electronics
  • Received : May 06, 2014
  • Accepted : October 04, 2014
  • Published : November 20, 2014
Download
PDF
e-PUB
PubReader
PPT
Export by style
Share
Article
Author
Metrics
Cited by
TagCloud
About the Authors
June-Seok Lee
Department of Electrical and Computer Engineering, Ajou University, Suwon, Korea
Kyo-Beum Lee
Department of Electrical and Computer Engineering, Ajou University, Suwon, Korea
kyl@ajou.ac.kr

Abstract
The T-type topology is a three-level topology that has an advantage in terms of its number of switching device and its efficiency when compared to the neutral-point clamped (NPC)-type topology. With the recent increase in the usage of the T-type topology, the interest in its reliability has also increased. Therefore, a tolerance control for a T-type rectifier is necessary to improve the reliability of applications when an open-switch fault occurs. NPC-type rectifiers cannot eliminate input current distortion completely. However, the T-type rectifier is able to restore distorted current. In this paper, a tolerance control for the S x2 and S x3 open-switch faults of a T-type rectifier is proposed where it is advantageous in terms of efficiency when compared with other tolerance controls. The performance of the proposed tolerance control is verified through simulation and experimental results.
Keywords
I. INTRODUCTION
Multi-level topologies have outstanding performance in terms of total harmonic distortion (THD) and efficiency [7] - [9] . Three-level topologies have been used in applications of wide-power range [1] - [6] .
There are two types of three-level rectifier topologies. In the neutral-point clamped (NPC)-type, four switches are connected in series and two clamping diodes are connected to the neutral point. Therefore, the NPC-type rectifier can reduce the switching device’s collector-emitter voltage (V CE ) in half. The other type of topology is the T-type rectifier as shown in Fig. 1 . Although the switches used in the T-type rectifier have a V CE which is the same as that of the conventional two-level rectifier, the T-type rectifier has lower conduction and switching losses, and it does not need clamping diodes when compared to the NPC-type rectifier [7] .
PPT Slide
Lager Image
Grid connected T-type rectifier.
Research on improvements in the reliability of systems using multi-level topologies has become an important issue. Therefore, systems using switching devices need to diagnose switching device faults and to maintain their operation by tolerance controls. Diagnosis methods for detecting the open-switch faults of NPC-type inverters and rectifiers are proposed in [8] - [11] . To detect open-switch faults, additional devices such as voltage sensors or current patterns are used.
In [12] , an imperfect tolerance control for an NPC-type rectifier is proposed without additional devices. This tolerance control considers inner open-switch faults (S x2 and S x3 ) but not outer open-switch faults (S x1 and S x4 ) because the outer switches do not have an effect on rectifier operation with unity power factor [13] . These tolerance controls do not completely eliminate current distortion because of structural limitations of the NPC-type topology. In [14] , there are two tolerance controls, which do not require additional devices, for the inner open-switch faults (S x2 and S x3 ) of an T-type rectifier with a high modulation index. These two methods are the replacement two-level switching (R2LS) and the maintenance three-level switching (M3LS) tolerance controls. These controls use a two-level switching method to completely restore distorted currents. They have opposite characteristic in terms of the current THD and the dc-link voltage ripple. The R2LS tolerance control maintains the two-level switching method in four of the six sectors. Therefore, the current THD in this control is higher than that of M3LS tolerance control. However, the M3LS tolerance control, which considers the neutral-point balance, has a larger dc-link voltage ripple than the R2LS tolerance control.
This paper proposes a new tolerance control based on the nearest space-vector PWM (SVM) method [15] for the S x2 and S x3 open-switch faults of a T-type rectifier. The proposed tolerance control guarantees operation in the whole modulation index and is advantageous in terms of efficiency when compared with other tolerance controls. Simulations and experiments are conducted to demonstrate the performance and characteristics of the proposed tolerance control.
II. T-TYPE RECTIFIER AND INNER-OPEN SWITCH FAULTS
- A. Description of a Three-Level T-type Rectifier
In a three-level T-type rectifier, the pole voltage V xz (x = a, b, c) with respect to the neutral-point Z can be either V dc /2, 0, or -V dc /2 depending on the operating state. The first operating state “P” has V xz of V dc /2. In “P”, the switches S x1 and S x2 are ON and the switches S x3 and S x4 are OFF. When S x2 and S x3 are ON and S x1 and S x4 are OFF, the operating state is “O” and V xz is 0. When S x1 and S x2 are OFF and S x3 and S x4 are ON, V xz is –V dc /2 at the operation state “N”. The switch states and pole voltages depending on the operating states are shown in Table Ι .
OPERATION STATE AND POLE VOLTAGE
PPT Slide
Lager Image
OPERATION STATE AND POLE VOLTAGE
According to the switching states of each phase, there are six large-, six medium-, twelve small- and three zero-vectors in the three-level space vector diagram of a three-level T-type rectifier shown in Fig. 2 .
PPT Slide
Lager Image
Space vector diagram of three-level T-type rectifier.
- B. Inner Open-Switch (Sx2and Sx3) Faults
In a rectifier, according to the operating states and current direction, there are six current paths as shown in Fig. 3 . The current of the rectifier is generated by the operating state “O” and the current continuously flows through a diode of the outer switches if the operating state is changed to “P” or “N”. Therefore, the impossibility of the “O” operating state only lead to a zero current like the current distortion.
PPT Slide
Lager Image
Current paths depending on operating state and current.
In the positive current part, most of the current of the rectifier, which has a unity power factor, flows through paths (a) and (b). Therefore, the range of path (c) can be ignored. This means that the “N” operating state does not appear in the positive current part. An S x3 open-switch fault in the “O” operating state is fatal to rectifier operation. On the other hand, on a unity power factor, an S x1 open-switch fault does not lead to current distortion in the “P” operating state because the current flows through a diode. S x1 also does not affect the current in the negative current part. Based on the fact that S x4 and S x1 are not needed, a reduced part topology was proposed in [15] . Therefore, in this paper, S a2 and S a3 open-switch faults are considered to explain the effect of an open-switch fault.
Fig. 4 shows a rectifier’s input three phase currents, a-phase pole voltage, and dc-link voltage when S a2 and S a3 open-switch faults occur. Inner open-switch faults have a fatal effect on the a-phase input current. In the negative part of the a-phase current, as shown in Fig. 4 (a), the valid operating state “O”, shown in Fig. 3 (e), becomes infeasible. Therefore, the negative part of the a-phase current cannot be generated and the dc-link voltage has ripple. Like an S a2 open-switch fault, an S a3 open-switch fault has a fatal effect on the positive part of the a-phase current.
PPT Slide
Lager Image
Rectifier waveforms: (a) Sa2. (b) Sa3 open-switch fault.
III. PROPOSED TOLERANCE CONTROL FOR THE INNER OPEN-SWITCH FAULTS OF T-TYPE RECTIFIERS
The proposed tolerance control does not use the “O” operating vectors of a phase which contains an open-switch fault. Furthermore, in the proposed tolerance control, the two-level switching method is applied for an a phase containing an open-switch fault and two pole voltages ( Vbz , and Vcz ) are maintained to three levels (V dc /0, 0, -V dc /2) at a high modulation index. As a result, the proposed tolerance control is advantageous in terms of efficiency when compared with the other tolerance controls using the two-level switching method. In this section, an S a2 open-switch fault is considered as the fault case. The proposed tolerance control is developed based on the nearest space-vector PWM (SVM) method. Fig. 5 shows the infeasible vectors when an S a2 open-switch fault occurs.
PPT Slide
Lager Image
Space vector diagram of Sa2 open-switching fault.
- A. Tolerance Operation at a High Modulation Index
An S a2 open-switch fault leads two medium-vectors (OPN, ONP), five small-vectors (OPO, OPP, OOP, OON, ONO) and one zero-vector (OOO) being infeasible when the a-phase current is negative. At a high modulation index, which is shown in the white part of Fig. 5 , the OPN, ONP, OPO, OPP, OPP, OON, and ONO vectors should be changed to other feasible vectors during negative current.
In sectors ΙΙΙ and ΙV, the infeasible P-type small-vectors OPO, OPP, and OOP are changed to the N-type small vectors NON, NOO, and NNO. The increased use of N-type small vectors in sectors ΙΙΙ and ΙV can cause a neutral-point unbalance. Therefore, in sectors Ι and VΙ, the N-type small-vectors OON, ONN, and ONO are changed to the P-type small-vectors PPO, POO, and POP. When an S a2 open-switch fault occurs, the changing vector methods in sectors Ι, ΙΙΙ, ΙV, and VΙ are the same as those of the M3LS tolerance control in [14] . The M3LS tolerance control uses two-level switching in sectors ΙΙ and V. However, the two-level switching increases the switching loss of the three-phase switches.
Sector ΙΙ is divided into four parts as shown is Fig. 5 . In sector ΙΙ-A, there are infeasible OPN and OPO vectors. The OPN vector can be changed to NPN and PPN vectors, and the PPO vector can be also changed to a PPO vector as shown in Fig. 6 . As a result, the NPN, PPN, and PPO vectors retain the b- and c- phase pole voltages Vbz and Vcz . However, the a-phase pole voltage Vaz has V dc /2 and -V dc /2 without 0. The changing vector is easily performed by recalculating the switching time of the newly selected NPN, PPN, and PPO vectors, based on the SVM method as shown in Fig. 6 . It has three on-times for each phase which are calculated from the reference voltage and are expressed as:
PPT Slide
Lager Image
PPT Slide
Lager Image
Tolerance control in sector ΙΙ-A.
To use the NPN, PPN, and PPO vectors, average Vaz in Fig. 6 (a) should be equal to average Vaz in Fig. 6 (b) for a switching period Ts . This is satisfied by adding the offset value to the original on-time Ta,on of the a-phase and it is expressed as:
PPT Slide
Lager Image
Fig. 7 shows the switching sequence of the proposed tolerance control in sector ΙΙ-B. Like sector ΙΙ-A, Vbz and Vcz are not changed. The on-time Ta,on,TC for the proposed tolerance control can be calculated by (2).
PPT Slide
Lager Image
Tolerance control in sector ΙΙ-B.
In sectors ΙΙ-C and II-D of Fig. 8 , the on-time Ta,on,TC for the proposed tolerance control is calculated by using the off-time Ta,off . Ta,off is expressed as:
PPT Slide
Lager Image
PPT Slide
Lager Image
Tolerance control in sector ΙΙ-C and -D.
To make the average Vaz of the proposed tolerance control equal to the original average Vaz , Ta,on,TC is defined as:
PPT Slide
Lager Image
Likewise, in sector V, equations (2) and (4) are applied according to parts of sector V.
Consequently, the proposed tolerance control always keeps Vbz and Vcz as three levels and it makes the level of Vaz , which contains the faulty switch, two in sectors ΙΙ and V when an S a2 open-switch fault occurs. The whole principle of the tolerance control method at a high modulation index is shown in Table ΙΙ .
PROPOSED TOLERANCE CONTROL AT HIGH MODULATION INDEX: SA2OPEN-SWITCH FAULT
PPT Slide
Lager Image
PROPOSED TOLERANCE CONTROL AT HIGH MODULATION INDEX: SA2 OPEN-SWITCH FAULT
- B. Tolerance Operation at a Low Modulation Index
Like the method mentioned in section ΙΙΙ-A, the proposed tolerance control at a low modulation index, which is shown in the blue part of Fig. 5 , changes the infeasible vectors to other vectors. This principle is also different depending on the sectors.
In sectors ΙΙΙ and ΙV, the infeasible P-type small-vectors OPO, OPP, and OOP are changed to the N-type small-vectors NON, NOO, and NNO. Moreover, the zero-vector OOO cannot be used. Therefore, it is should be substituted with PPP and NNN.
Fig. 9 (a) shows the switching sequence in sector ΙΙΙ-1 when the rectifier operates normally without any faults. This switching sequence consists of the NON, NOO, OOO and OPO vectors. The OPO and OOO vectors are infeasible. The P-type small-vector OPO can be replaced with NON, and the OOO zero-vector can be replaced with the NNN zero-vector as shown in Fig. 9 (b). The switching sequence in Fig. 9 (b) causes switching to occur two times in the b- and c- phases. Therefore, the switching sequence should be changed to NNN-NON-NOO-NON-NNN, as shown in Fig. 10 , if the SVM method is used. Its switching sequence can be performed by adding the offset value as shown in Fig. 9 (c). This is expressed as:
PPT Slide
Lager Image
PPT Slide
Lager Image
Tolerance control in sector ΙΙΙ-1.
PPT Slide
Lager Image
Performance of tolerance control at high modulation index. (a) Sa2 open-switch fault. (b) Sa3 open-switch fault.
The positive voltage Vbz in Fig. 9 (a), is changed to a negative voltage as shown in Fig. 9 (c). Vaz and Vcz are maintained as negative voltages. Consequently, when the sign of Vxz is changed, – Ta,on is added to Tx,on for Vxz . Ts Tx,on is added to Tx,on for Vxz when the sign of Vxz is not changed. This method is applied in sectors ΙΙΙ and ΙV.
For neutral-point balancing control, the N-type small vectors are changed to P-type small-vectors and the zero-vector OOO is changed to the zero-vector PPP in sectors Ι and VΙ. In sectors Ι and V, the two-level switching method is used. The whole principle of the tolerance control method at a low modulation index is shown Table ΙΙΙ .
PROPOSED TOLERANCE CONTROL AT LOW MODULATION INDEX: SA2OPEN-SWITCH FAULT
PPT Slide
Lager Image
PROPOSED TOLERANCE CONTROL AT LOW MODULATION INDEX: SA2 OPEN-SWITCH FAULT
IV. SIMULATION RESULTS
Simulations were performed using PSIM. The simulation circuit is the same as the one shown in Fig. 1 . The simulation parameters are shown in Table ΙV .
SIMULATION AND EXPERIMENT PARAMETERS
PPT Slide
Lager Image
SIMULATION AND EXPERIMENT PARAMETERS
Fig. 10 shows the three-phase currents, dc-link voltage, and line-to-line voltage of the rectifier when the proposed tolerance control is applied at a high modulation index (V line-to-line is 126 V rms and the load is 33 Ω). Due to an inner open-switch fault, the current containing an open-switch fault becomes zero for a half period and the dc-link voltage has a large ripple. After 0.6 s, which means that the proposed tolerance control is applied, the three-phase currents are restarted as a sinusoidal waveform and the dc-link voltage ripple is eliminated. Additionally, the line-to-line voltage (V ab ) between the a- and b- inputs of the proposed tolerance control is different from that of the conventional SVM method [15] .
Fig. 11 shows the three-phase pole voltages and currents of the proposed tolerance control at a high modulation index. Vbz and Vcz maintain three levels. However, Vaz has only V dc /2 in sectors Ι and VΙ or –V dc /2 in sectors ΙΙΙ and ΙV. This is the same as that of the two-level switching method in sectors ΙΙ and V. Therefore, the a-phase current ripple increases in sectors ΙΙ and V due to the high dV/dt of the two-level switching method. The proposed tolerance control only uses P-type small vectors for the two sectors and N-type small vectors for the other two sectors. Therefore, the neutral-point voltages (V top and V bottom ) have a low frequency ripple which is the same as the fundamental frequency of the input current as shown in Fig. 11 .
PPT Slide
Lager Image
The neutral-point and three-phase pole voltages of the proposed tolerance control at high modulation index.
Fig. 12 shows the three-phase currents, dc-link voltage, and line-to-line voltage of the rectifier when the proposed tolerance control is applied at a low modulation index (V line-to-line is 63 V rms and the load is 67 Ω). Similar the case of a high modulation index, the current containing an open-switch fault becomes zero for a half period and the dc-link voltage has a large ripple. After the tolerance control is applied, the three-phase currents are restarted as a sinusoidal waveform and the ripple of the dc-link voltage is eliminated. Additionally, the line-to-line voltage (V ab ) between the a- and b- inputs of the rectifier has V dc and 0 or –V dc and 0 in sectors ΙΙ and V when the two-level switching method is applied.
PPT Slide
Lager Image
Performance of tolerance control at low modulation index: (a) Sa2 open-switch fault (b) Sa3 open-switch fault.
Fig. 13 shows the three-phase pole voltages and currents of the proposed tolerance control at a low modulation index. In sectors ΙΙ and V, the three-phase pole voltages have V dc /2 or –V dc /2. Therefore, the current ripple of the three-phase increases in sectors ΙΙ and V due to a high dV/dt. The switching operation of the remaining sectors is the same as that in the case of a high modulation index. At a low modulation index, the neutral-point voltages has a low frequency ripple which is the same as the fundamental frequency of the input current
PPT Slide
Lager Image
Neutral-point and three-phase pole voltages of proposed tolerance control at low modulation index.
V. EXPERIMENTAL RESULTS
Experiments are conducted to verify the validity of the proposed tolerance control. Fig. 14 shows the hardware setup which consists of a control board, sensors, gate drivers, and T-type IGBT modules. This module is 4MBI300VG-120R-50 from Fuji. The parameters for the experiments are the same as those used in the simulations.
PPT Slide
Lager Image
Experiment setup of T-type rectifier.
Fig. 15 shows the performance of the proposed tolerance control at a high modulation index (V line-to-line is 126 V rms and the load is 33 Ω) when an S a2 open-switch fault occurs. Due to the S a2 open-switch fault, the positive a-phase current (I a ) becomes zero and the dc-link voltage has a large ripple. After the proposed tolerance control for a high modulation index is applied, the a- and b-phase currents (I a and I b ) are restored as the sinusoidal waveforms and the dc-link voltage ripple decreases as shown in Fig. 15 . In the experimental results, the V ab of the proposed tolerance control, which is different from the V ab of the conventional SVM in two sectors, is the same as that in simulation.
PPT Slide
Lager Image
Experimental results of tolerance control at high modulation index when Sa2 open-switch fault occurs.
Fig. 16 shows the experimental results of the proposed tolerance control for a low modulation index when an S a2 open-switch fault occurs. Like the results at a high modulation index, the current distortion is eliminated and the dc-link ripple decreases. The V ab of the proposed tolerance control for a low modulation index has V dc and –V dc in spite of the low modulation index. This is the same result as the simulation.
PPT Slide
Lager Image
Experimental results of tolerance control at low modulation index when Sa2 open-switch fault occurs.
VI. CONCLUSION
This paper proposes a tolerance control based on the nearest SVM methods for the S x2 and S x3 open-switch faults of a T-type rectifier. The proposed tolerance control does not need additional devices and restores the distorted input currents caused by S x2 and S x3 open-switch faults. In addition, it guarantees the operation at the whole modulation index with inner open-switch faults. In comparison with the R2LS and M3LS tolerance controls, the switching loss is decreased at a high modulation index because only the input voltage of the phase containing an open-switch fault has two levels in two sectors. The performance of the proposed tolerance control is verified by the simulation and experimental results.
Acknowledgements
This work was supported by the 20134030200310 of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy.This work was supported by KETEP (G031462812) which is funded by MKE (Ministry of Knowledge Economy).
BIO
June-Seok Lee received his B.S. and M.S. degrees in Electronic Engineering from Ajou University, Suwon, Korea, in 2011 and 2013, respectively, where he is currently working toward his Ph.D. degree. His current research interests include grid-connected systems, multilevel inverters and reliability.
Kyo-Beum Lee received his B.S. and M.S. degrees in Electrical and Electronic Engineering from Ajou University, Suwon, Korea, in 1997 and 1999, respectively. He received his Ph.D. degree in Electrical Engineering from Korea University, Seoul, Korea in 2003. From 2003 to 2006, he was with the Institute of Energy Technology, Aalborg University, Aalborg, Denmark. From 2006 to 2007, he was with the Division of Electronics and Information Engineering, Chonbuk National University, Jeonju, Korea. In 2007, he joined the Department of Electrical and Computer Engineering, Ajou University, Suwon, Korea. His current research interests include electric machine drives, electric vehicles, and renewable power generation.
References
Alepuz S. , Calle A. , Busquets-Monge S. , Kouro S. , Wu B. 2013 “Use of stored energy in PMSG rotor inertia for low-voltage ride-through in back-to-back NPC converter-based wind power systems” IEEE Trans Ind. Electron. 60 (5) 1787 - 796    DOI : 10.1109/TIE.2012.2190954
Portillo R. C. , Prats M. M. , León J. I. , Sánchez J. A. , Carrasco J. M. , Galván E. , Franquelo L. G. 2006 “Modeling strategy for back-to-back three-level converter applied to high-power wind turbines” IEEE Trans. Ind. Electron. 53 (5) 1483 - 1491    DOI : 10.1109/TIE.2006.882025
Park Y. S. , Sul S. K. , Lim C. H. 2013 “Asymmetric control of DC-link voltages for separated MPPTs in three-level inverters” IEEE Trans. Power Electron. 28 (6) 2760 - 2769    DOI : 10.1109/TPEL.2012.2224139
Lee J. S. , Lee K. B. 2014 “New modulation techniques for a leakage current reduction and a neutral-point voltage balance in transformerless photovoltaic systems using a three-level inverter” IEEE Trans. Power Electron. 29 (4) 1720 - 1732    DOI : 10.1109/TPEL.2013.2264954
Barros J. D. , Silva J. F. A. , Jesus É. G. A. 2013 “Fast-predictive optimal control of npc multilevel converters” IEEE Trans. Ind. Electron. 60 (2) 619 - 627    DOI : 10.1109/TIE.2012.2206352
Choi U. M. , Lee K. B. 2013 “Neutral-point voltage balancing method for three-level inverter systems with a time-offset estimation scheme” Journal of Power Electronics 13 (2) 243 - 249    DOI : 10.6113/JPE.2013.13.2.243
Schweizer M. , Kolar J. W. 2013 “Design and implementation of a highly efficient three-level T-type converter for low-voltage applications” IEEE Trans. Power Electron. 28 (2) 899 - 907    DOI : 10.1109/TPEL.2012.2203151
Choi U. M. , Jeong H. G. , Lee K. B. , Blaabjerg F. 2012 “Method for detecting an open-switch fault in a grid-connected NPC inverter system” IEEE Trans. Power Electron. 27 (6) 2726 - 2739    DOI : 10.1109/TPEL.2011.2178435
Ku K. , Im W. S. , Kim J. M. , Suh Y. S. “Fault detection and tolerant control of 3-phase NPC active rectifier” in Proc. ECCE 2012 4519 - 4524
Im W. S. , Kim J. S. , Kim J. M. , Lee D. C. , Lee K. B. 2012 “Diagnosis methods for IGBT open switch fault applied to 3-phase AC/DC PWM converter” Journal of Power Electronics 12 (1) 120 - 127    DOI : 10.6113/JPE.2012.12.1.120
Choi U. M. , Lee K. B. , Blaabjerg F. 2014 ”Diagnosis and tolerant strategy of an open-switch fault for T-type three-level inverter systems” IEEE Trans. Ind. Appl. 50 (1) 495 - 508    DOI : 10.1109/TIA.2013.2269531
Lee J. S. , Lee K. B. , Blaabjerg F. “Open-switch fault detection method of an NPC converter for wind turbine systems” in Proc. ECCE 2013 1696 - 1701
Corzine K. A. , Baker J. R. 2002 “Reduced-parts-count multilevel rectifiers” IEEE Trans. Ind. Electron. 49 (4) 766 - 774    DOI : 10.1109/TIE.2002.801077
Lee J. S. , Lee K. B. 2014 “An open-switch fault detection method and tolerance controls based on SVM in a grid-connected T-type rectifier with unity power factor” IEEE Trans. Ind. Electron. 61 (12) 7092 - 7014    DOI : 10.1109/TIE.2014.2316228
Seo J. H. , Choi C. H. , Hyun D. S. 2001 “A new simplified space–vector PWM method for three-level inverters” IEEE Trans. Power Electron. 16 (4) 545 - 550