Study of Diode-Clamped Three-Level Inverter

二极管钳位三电平逆变器研究Study of Diode-Clamped Three-Level Inverter

二极管钳位三电平逆变器研究

孙闯（上海发电设备成套设计研究院，学号P200909071）

Study of Diode-Clamped Three-Level

Inverter

Sun Chuang (Shanghai Power Equipment Research Institute, Minhang District,

Shanghai,200240,China)

ABSTRACT: The basic structure and operating principles of the

diode-clamped three-level inverter are studied. For the deviation of neutral point voltage, this paper presents a novel circuit for neutral-point voltage balance in the three-level inverter.

KEY WORDS: Diode-clamped three-level inverter; neutral-point voltage balancing

With the high-voltage technology, the capacity of the inverter have become increasingly demanding. Due to restrictions on power electronic devices, the capacity of the traditional two-level inverter has been difficult to achieve high-power requirements. Multi-Inverter become a hot research topic. And the multilevel inverter has been studied and used widely in high power applications for medium or high voltage.

Diode-clamped three-level inverter is a simple and practical kind of inverter. But the deviation of neutral point voltage is one of the key aspects that affects the reliability of the three-level inverter and its electric drive system. This paper conduct in-depth study, and presents

二极管钳位三电平逆变器研究Study of Diode-Clamped Three-Level Inverter

a novel circuit for neutral-point voltage balance in the three-level inverter. The experimental results verify the correctness and feasibility of the proposed control scheme.

1. The operating principle of the diode-clamp three-level

inverter and the shifting principle of the mid-point potential

The fig.1 shows the main circuit of the diode-clamp three-level inverter[1].

In the fig.1, VX1～VX4 are the power electronic devices of the X(X = A,B,C) phase——insulated gate bipolar transistor; DX1～DX4 are fly-wheel diodes; DX5 and DX6 are clamping diodes; P is the positive voltage bus of the DC side; N is the negative voltage bus of the DC side; O is the neutral point; C1 and C2 are the divider capacitors of the DC side; UA ~ UC are three-phase output voltage of the inverter; Udc is the DC-side voltage; ic1 is the current flow through C1, and ic2is the current flow through C2; iNP is the current flow through the neutral point. For example, the operating principle of the three-level inverter is: when VX1 and VX2 turn on, the X-phase is positive level; when VX3 and VX4 turn on, the X-phase is negative level; when VX2 and VX3 turn on, the X-phase is zero level. Therefore, relative to the DC-Side Voltage, the each phase output voltage of the AC side of the inverter has three possible

二极管钳位三电平逆变器研究Study of Diode-Clamped Three-Level Inverter

values, which is the origin of the three-level inverter.

When the three-level inverter operate, there is a problem. That is the midpoint potential shift, which due to the existence of the midpoint current iNPat the neutral point of DC side. As shown in Figure 1, when

the output voltage of a phase is zero voltage (when VX2 and VX3 turn on),

the midpoint current makes the DC capacitor voltage divider lose balance: When the iNP flows out the midpoint, C1 is charged; when iNP flows into

the midpoint, C2 is charged. If the charge-discharge process of C1 and C2 do not balance, then the midpoint potential would shift. Thus, the necessary conditions for the midpoint potential shift is ic1≠ic2 or iNP≠ 0, while the zero-voltage play a significant impact in this process. 2. The balance design circuit of midpoint potential and

operating principle

In this paper, the fig.2 shows the main circuit of the midpoint balance circuit. T1、T2 and T3 are IGBT; D1 and D2are the fry-wheeling diodes; L1 and L2 are the energy storage inductor; C1 and C2are the divider capacitors. Compared with the normal suppression circuit, the circuit adds an IGBT(T3). By controlling T3 turn-on and turn-off, it can restrain the voltage change of C1 and C2 under the unchanged of the DC side voltage (Udc). Even if Udc reduce, the circuit is also effective restrain the shift of the midpoint potential. [1]

二极管钳位三电平逆变器研究Study of Diode-Clamped Three-Level Inverter

2.1 The balance of the midpoint potential on Udc remaining the

same value

If the Udc remain unchanged, Udc Uc1 Uc2 is a constant. The addition of Uc1must lead to the decline of Uc2, and the decline of Uc1must lead to the increase of Uc2. Thus it can adjust voltage of the

two separate capacitors of the DC side to balance the neutral point voltage. To achieve this goal, T3 is always in on-state. The fig.3 shows the equivalent circuit. [2]

This circuit consists of Boost converter and Buck converter. Buck converter consists of T1、D1、L1 and C2; Boost converter consists of

The operating modes of the circuit accordingly divide T2 、D2、L2 and C3.

into Buck transform mode and Boost mode transformation. The working condition of the two change models should be complement. When Uc1 Uc2, Buck conversion circuit (T1,D1,L1,C1) begin work. At the same time, Boost conversion circuit stop working. In Buck transform mode, it can restrain

[3]

二极管钳位三电平逆变器研究Study of Diode-Clamped Three-Level Inverter

the shift of the midpoint potential by adjusting the voltage of C2. When T1 turns on, the one hand, the current flows through T1、L1and C2 under the action of the Udc, the other hand, the voltage Uc1of the capacitor

C1 through T1 and L1 constitute the circuit, which can make L1 charge ;

When T1turns off, the energy of L1convert toC2though the loop circuit

consist of C2、And C2is charged, the charging voltage increase D1and L1.

until the voltage balance of C1and C2. When Uc2 Uc1, Buck converter is

no longer work and Boost conversion circuit begin to work. Because Uc2 Uc1, the energy of C2will be transferred to C1 indirectly. When T2 turns on, the one hand, the voltage Uc2of C2discharge by L2and T2, the energy store in L2; the other hand, Udcdistribute voltage by C1 and C2. When T2 turns off, the diode D2 conducts, the energy of L2 can be transferred to C1 through D2. In this way, Boost transform mode, it can

restrain the shift of the midpoint potential by adjusting the voltage of C1, until Uc2 Uc1.

2.2 The balance of the midpoint potential on the decline of

DC side voltage Udc When Udc reduce to below the voltage protection setting value, T3 turn off in Fig.2. The fig.4 shows the equivalent circuit. Boost converter and Buck converter work simultaneously. The voltage of C1and C2 not only counterbalance, but also the sum equal to the setting value of Udc. Buck converter adjust the voltage of the capacitor C2. When T1 turns on, the current flows from Udc to T1、 L1and C2,so that L1is charged; when T1 turns off, the energy of L1 should be transferred to C2. At the same time, Boost converter convert energy from C2to C1, and it can adjust the voltage of C1. Its work process and the above-mentioned Boost transform mode is identical congruent. [2]

二极管钳位三电平逆变器研究Study of Diode-Clamped Three-Level Inverter

3. Conclusion

This paper analyses the midpoint balance problem of the three-level inverter, and proposes a design of the hardware circuit. This method provides a good reference for the evenly distributed capacitor voltage, and could be considered for the performance improvement of the low-voltage system, and provides a theoretical and experimental basis for the application of the three-level inverter at the high-voltage power occasions.

REFERENCES

[1] TAO Sheng-gui, GONG Xi-guo, YUAN Deng-ke. Design and Simulation of

Novel Circuit for Neutral-Point Voltage Balance in Three-Level Inverter. JOURNAL OF TONGJI UNIVERSITY (NATURAL SCIENCE), 2005

[2] LI Song, LIU Zhi-hong. Simulation Study of Diode Clamping Three-Level

Inverter. Journal of Nanchang Institute of Technology, 2006

[3] LIN Lei, ZOU Yun-ping, ZHONG He-qing, ZOU Xu-dong, DING Kai. STUDY

OF CONTROL SYSTEM OF DIODE-CLAMPED THREE-LEVEL INVERTER. Proceedings of the CSEE, 2005

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