Design of NPC Three-level phase leg building block with EMC consideration Jie Yang, Xinmin Jin, Xuezhi Wu, Weiwei Zhang, Jingyuan Yin, Jinke Li National Active Distribution Network Technology Research Center, Beijing Jiaotong University, Beijing, China
[email protected]
Abstract IGBT overshoot voltage is an EMC problem in high power converter. This paper discusses four CCLs of Neutral-point clamped three level (NPC-TL) topology and reveals module layout as a factor of stray inductance. To optimize the switch performance, layout strategy suited for NPC-TL is determined through theoretical and simulation comparison. Finally, a high power NPC-TL phase leg building block is implemented.
1. Introduction Compared with traditional two-level counterpart, multilevel converters have received more attention in past three decades due to its outstanding performance in high-power medium- and high-voltage applications
[1]
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Among all the multilevel topologies, NPC-TL is becoming main trend topology since its simplicity and maturity. High power converter can realize high power density and low energy losses thanks to the increase of IGBT module current and voltage ranges and reduction switching time, but it gives birth to intensive increase of di/dt and the voltage overshoot which can raise relevant problems of electromagnetic compatibility (EMC) [2]. High power converters prefer adopting Power electronics building block (PEBB) to deal with the EMC issues[3] because its compact structure and laminated busbar can effectively decrease the current commutation loop (CCL) stray inductance and restrain the switching voltage overshoot. In this paper, a NPC-TL power conversion system is build with three modularized NPC-TL phase leg building blocks. The power range of each phase leg building block is 130kVA so they can make up a 390kVA three-phase NPC-TL converter. For a NPC-TL phase leg with high power rating, a very little difference in CCL stray inductance will lead tremendous change in voltage overshoot on IGBT, the CCL of NPC-TL need to be investigated completely and stray inductance should be minimized. Literatures such as [4, 5] make same conclusion as two-level that the CCL stray inductance is only related to semiconductor devices and laminated busbar. The switching performance and CCL stray inductance for a NPC-TL phase leg have not been studied before and no one investigates the influence of phase leg structure and layout on CCL stray inductance. In this paper, above mentioned issues are analyzed for the purpose of optimizing the NPC-TL phase leg building block.
2. Main circuit configuration and system description As shown in Fig.1, the structure of the NPC-TL power conversion system consists of DC capacitors, three NPC-TL phase leg building blocks and six terminals stretch out as power interfaces, of which A, B, C as ac outputs and P, O, N as dc inputs. The input dc voltage is 1500V, and rated output voltage and current are 750Vrms and 300Arms respectively with 300kVA rated power.
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P
AC Bus
A B C
DC Bus O
N
Fig.1 Structure of the NPC-TL power conversion system
3. NPC-TL phase leg building block design consideration The NPC-TL phase leg building block plays an important role in the power conversion system but in high power application it may cause EMC problems due to the existence of CCL stray inductance. So NPC-TL phase leg is need to be designed completely to minimize the CCL stray inductance and optimize the switching performance for the purpose of EMC improvement. Making block structure compact and using laminated busbar are known to be good traditional solutions for these issues, but following research shows that phase leg switches layout also has great influence on CCL stray inductance.
3.1 Phase leg structure and layout Each NPC-TL phase leg is composed of four main switches (T1~T4) and two clamping switches (D5, D6), to implement 130kVA phase leg with 1500V dc link voltage and 300A rated current, three 1700V/650A dual IGBT modules (M1~M3) are used. Each dual IGBT module can act as any two of six switches so there are different layout strategies, which are presented in Table 1. Laminated busbars are used as P, O, N rails, and copper bars are adopted as connection between any two modules. Laminated busbar and copper bar have low stray inductance, which is shown in detail in Fig.2. The direction of current iLd is from dc side to ac side. Tab.1 Module layout strategies of NPC-TL phase leg Layout strategies
M1
M2
M3
Strategy 1
T1，T2
D5，D6
T3，T4
Strategy 2
T1，D5
T2，T3
T4，D6
LP
LP
L1
C1
T1 D1
T1 D1
M1
L1 D5
D5
T2 D2
T2 D2
M2 L2
LO
M1
C1
T3 D3
D6 L3
iLd
D6
T3 D3 C2
M2
LO
iLd
M3
L3
C2
M3
LN
a. strategy 1
T4 D4
LN
T4 D4
b. strategy 2
Fig.2 Stray inductance of different layout strategies
3.2 Effect of stray inductance of communication loop In the NPC-TL topology, there are six natural commutation modes and eight forced commutation modes. Because of the symmetry of the NPC-TL, four forced commutation modes are representative while four main IGBTs (T1~T4) are being turned off. Fig.3 describes turn-off transients of four IGBTs in NPC-TL phase leg, taking T1 turn-off process as an example, the red line current iLd is from DC side to AC side. During commutation the blue current iT1 is reducing and green current iD5 is increasing, the fast current changing speed induce voltage on the stray inductance of Loop A, which leads to an overshoot on T1.
Loop A
T1 D1
T1 D1
T2 D2
D5
T2 D2
iLd T3 D3
T3 D3
Loop B
T2 D2
D6
D6
T3 D3 C2
C2
D5
iLd
D6
D6 T3 D3
C1
T2 D2
iLd
iLd
T1 D1
T1 D1
C1 D5
D5
Loop C
C1
C1
C2
C2
Loop D T4 D4
T4 D4
T4 D4
T4 D4
a. T1 turn-off
b. T2 turn-off
c. T3 turn-off
d. T4 turn-off
Fig.3 CCLs of the NPC-TL during four IGBT turn-off transients To minimize the stray inductance, it is need to take different layout strategies into four CCLs abovementioned and find the one fit the NPC-TL topology, the comparison is presented in Table 2. If it is assumed that stray inductance of bars and switches in strategy 1 is identical to which in strategy 2, the comparison results show strategy 2 is more suited to NPC-TL because its total stray inductance is equals less than strategy 1. Tab.2 Comparison of CCLs stray inductance in different layout strategies Layout strategy 1
Layout strategies 2
Loop A
LP+LO+L1+T1+D5
LP+LO+T1+D5
Loop B
LO+LN+L1+L2+T2+T3+T4+D5
LO+LN+L1+L3+T2+T3+T4+D5
Loop C
L P +LO +L2 +L3 +T1+T2+T3+D6
L P +LO +L1 +L3 +T1+T2+T3+D6
Loop D
L O +LN + L 3+T4+D6
L O +LN + T4+D6
3.3 Switch performance simulation NPC-TL converter models with stray inductance are built using Matlab/Simulink, and its parameters are same as chapter 2. The stray inductance of bars taking value is 15nH and of switches is 5nH. Due to similarity in CCLs, only T1 and T3 switch performances are need to be discussed. Fig.4 presents the simulation waveforms of T1 and T3 at turn-off mode with maximum of rated current 430A in power factor =1 and =-1condition respectively. Simulation results show that the module layout strategy 2 optimize the IGBT switch performance.
a. T1 turn-off
b. T1 turn-off
Tab.4 Switch performance Comparison between different layout strategies
4. Experimental result Fig.5 shows the experimental waveforms of the NPC-TL converter under rated power of 750Vrms and 300Arms. The overshoot voltages are below 200V and the capacitors current is 12.8Arms less than rated value, the maximum temperature of capacitors is 23°C and power devices is 44°C, which illustrated a successful design of NPC-TL phase leg building block.
Vout IT1
IT3
VT1
VT3
Iout Icap
a. T1 turned off performance
b. T1 turned off performance
c. Output voltage and current
Tab.5 Experimental waveforms of NPC-TL converter
5. Acknowledgments This work was supported by “the Fundamental Research Funds for the Central Universities” (2014YJS121) and the National High Technology Research and Development Program (“863” Program) of China (2013AA050901).
6. References 1. Yuan Liqiang, “ IGCT-based multilevel converters for medium voltage drive system,” Beijing: Tsinghua University, 2004. 2. 4. M.C. Caponet, F. Profumo, R.W. De Doncker, A. Tenconi, “ Low stray inductance bus bar design and construction for good EMC performance in power electronic circuit,” IEEE Trans on Power Electronics, 2002, pp.225-231. 3. Yang Jie, Jin Xinmin, Wu Xuezhi, Liang Xiaoguang, Song Gaosheng, Yin Jingyuan, “Power stack design of MW-level of full-power grid-connected converter for wind power generation,” Electric Power Automation Equipment, October 2013, pp. 21-27. 4. T. Bruckner, S. Bernet, H. Guldner, “The Active NPC converter and its loss-balancing control,” IEEE trans. Ind. Electron, June 2005, pp. 855-868. 5. Yang Jiao, Sizhao Lu, Fred C. Lee, “Switching performance optimization of a high power high frequency three-Level active neutral point clamped phase leg,” IEEE trans on Power Electronics, July 2014, pp.3255-3266.
