两层空间矢量图法实现三极NPC逆变器的创新空间矢量调制

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本文主要探讨了在三电平逆变器中应用两电平空间矢量图(Space Vector Diagram, SVD)进行空间矢量调制(Space Vector Modulation, SVM)的新概念。研究集中在中性点钳位(Neutral Point Clamped, NPC)逆变器，尽管方法并不局限于这种特定结构。作者Narendra Babu和Pramod Agarwal来自印度理工学院拉克纳校区的电气工程系，他们提出了一种创新的方法，通过将2L-SVD与3L-SVM相结合，以实现更高效和精确的控制。论文介绍了一种名为"最近三个矢量(N3V)"调制和"最近三个虚拟矢量(N3VV)"调制的两种具体策略。在NPC逆变器中，传统的3L-SVM可能会导致中性点电压波动的问题，特别是当全模态下工作时。N3V调制虽然能够减少这些波动，但在计算导通角或负载比时需要大量的计算，使得这种方法在实际应用中面临挑战。 N3VV调制方法则是针对这个问题提出的解决方案。它有效地解决了中性点电压问题，并能在全范围的功率因数下稳定运行。然而，尽管N3VV具有明显的优势，其复杂度也体现在了计算过程中，需要设计出更为高效的算法来简化这个过程，以便于实时应用。本文的研究对于NPC逆变器的设计者和工程师来说具有重要意义，因为它提供了一种新颖的控制策略，可以优化逆变器性能，提高效率和稳定性。同时，它也揭示了在实际工程实践中需要权衡的 trade-offs，即复杂度与性能之间的取舍。这项研究为进一步发展高效、可靠的三电平NPC逆变器技术奠定了理论基础。

Space Vector Modulation for Three-Level NPC

Inverter Using Two-Level Space Vector Diagram

Narendra Babu A

Department of Electrical Engineering

Indian Institute of Technology Roorkee

Roorkee,Uttarakhand, India

narendra7iitr@gmail.com

Pramod Agarwal

Department of Electrical Engineering

Indian Institute of Technology Roorkee

Roorkee Uttarakhand, India

pramgfee@iitr.ac.in

Abstract— A novel concept that uses two-level (2L) space

vector diagram (SVD) to achieve three-level (3L) space vector

modulation (SVM) is introduced in this paper. Main emphasis

is given on neutral-point (NP) clamped (NPC) inverter, though,

the idea does not depend on the topology. Two interesting

schemes for NPC namely nearest three vector (N3V)

modulation and nearest three virtual vector (N3VV)

modulation are considered for detailed analysis. The main

technical drawback of NP voltage oscillations with N3V is a

effectively addressed by N3VV for full modulation and power

factor range. However, the extensive calculations involved in

the process of obtaining the duty ratios have made this method

difficult to implement. In this paper, first, simple mathematical

relations are established for N3V and N3VV modulation

methods using 2L SVM. Later, appropriate modulation

algorithms are developed for implementation. The simplicity of

the proposed modulation approach has a significant impact on

execution process. No simplifications have made in the Space

vector diagram (SVD) unlike some newly proposed simplified

schemes. The performance of the proposed modulation

algorithm is simulated in MATLAB, Simulink for various

operating conditions.

Keywords—

Neutral point clamped (NPC) inverter, Virtual

vectors (VV), Space vector modulation (SVM)

NTRODUCTION

In neutral point (NP) clamped (NPC) inverter with single

DC-source, flow of neutral current develops a voltage ripple

in the neutral point (NP) depending on the operating point.

As derived in [1], the ripple is negligible for UPF and is

more significant when the power factor decreases. This NP

voltage oscillations will cause overvoltage stress on the

devices lead to failure, decrease the output wave shape

quality (even order harmonics for unequal capacitor

voltages) and reduction of maximum switching frequency

[2],etc.

The nearest three vector (N3V) is a widely used PWM

technique for multilevel inverters in digital environment

[3][4][5]. However, in NPC, it is not possible to attain a zero

averaged NP current in each switching period by using N3V

for entire modulation index and power factors range. The

drawback is eliminated by using an interesting non-nearest

three vector (non-N3V) modulation scheme known as

nearest three virtual vector (N3VV) PWM [6], which can

eliminate NP voltage ripple for all power factors in the linear

modulation range. However, due to inequality of sub-triangle

regions in the space vector diagram (SVD) of these non-

NTV methods, the calculation of duty ratios is quit complex.

Some authors used phase duty ratios to simplify the

computations, yet requires closed loop correction [6].

Another approach is to neglect some least significant

switching states to simplify SVD is presented in [7]. Others

[8] have limitation in the modulation index.

The aim of this study is to establish various duty ratio

expressions of N3V and N3VV in terms of an equivalent 2L

SVM formed by outer most voltage vectors. Later, with the

help of 2L duty ratios, an algorithm is developed for 3L NPC

inverter. No simplifications made to actual three level (3L)

space vector diagram (SVD) and can be calculated

completely online.

(1)

abc

(2)

Fig. 1. Circuit schematic of three-level NPC inverter

II. S

PACE

ECTOR

IAGRAM AND

UTY RATION

EXPRESSIONS

Fig.1 shows the power circuit of 3L NPC operating with

single DC-source. Taking

as reference, the switching

table and corresponding output voltages are given in the

Table I. The aim of the converter is to synthesize the three

phase reference voltages at the ac side collectively

represented as a space vector given by

j2 /3 j4 /3

ab c

.( . . )

ref

Vvveve

ππ

=+ +

(1)

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