Mathematical Problems in Engineering

Volume 2015, Article ID 736828, 12 pages

http://dx.doi.org/10.1155/2015/736828

## A Novel Neutral-Point Potential Balance Strategy for Three-Level NPC Back-to-Back Converter Based on the Neutral-Point Current Injection Model

School of Information and Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, China

Received 15 October 2014; Revised 2 February 2015; Accepted 2 February 2015

Academic Editor: Kim M. Liew

Copyright © 2015 Yingjie Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The neutral-point (NP) potential balance control in three-level neutral-point-clamped (NPC) back-to-back converter is a research nodus. Its current strategies are the same as the strategies of a single three-level NPC converter. But the strategies do not give full play to its advantages that the neutral-point current can only flow through the connected midlines in both sides of the converter but does not flow through the DC-bus capacitors. In this paper, firstly the NP potential model based on the NP current injected is proposed. It overcomes numerous variable constraints and mutual coupling in the conventional model based on the zero-sequence voltage injected. And then on this basis, three NP-potential balance control algorithms, unilateral control, bilateral independent control, and bilateral coordinated control, are proposed according to difference requirements. All of these algorithms use the midlines rather than the DC-bus capacitors to flow the NP current as much as possible. Their control abilities are further quantitatively analyzed and compared. Finally, simulation results verify the validity and effectiveness of these algorithms.

#### 1. Introduction

The three-level neutral-point-clamped (NPC) back-to-back converter is composed of two three-level NPC topologies. The converter has the advantages of high power factor, two-way flow of energy, good control performance, and less pollution to the power grid. So in some medium-voltage and large-capacity applications, the converter has been a research focus in recent years [1–3].

Due to the inconsistent energy transmission to the upper and lower DC-bus capacitors in the three-level NPC topology, the voltages on two sets of capacitors might change so as to make NP potential unbalanced and fluctuated [4, 5]. This will eventually result in uneven voltage on its power devices and low-order current harmonics. The power device can be damaged by too high voltage. The current harmonics can cause torque ripple of electric machinery and lower efficiency. The frequent fluctuations of NP potential also reduce the lifetime of the capacitors. So the NP potential balance control in three-level NPC topology is widely studied.

The NP potential balance strategy in three-level NPC back-to-back converter commonly uses the strategy of the single three-level NPC topology [6–8] and works in the rectifier side or inverter side. Based on the space vector modulation, there are two kinds of the strategies: precise control [9] and hysteresis control [10]. It is relatively simple for the hysteresis control only to select the appropriate large or small switching vectors, but NP potential has to fluctuate within a certain range. In precise control, NP potential can remain stable in the controllable range, namely, low modulation ratios and high power factors. But it is more complicated to calculate the precise work time of the switching vectors. In order to reduce the complexity, A P (proportional) or PI (proportional integral) controller in [11] is used to control NP potential. But these controllers cause the limitations of the response speed and accuracy. In order to control NP potential in all modulation ratios and power factors, a virtual-vector control strategy is proposed in [12]. But the strategy is at the expense of increasing the switching frequency of power devices. Besides these control strategies, many strategies based on the carrier modulation are also proposed. In [13], the NP potential fluctuation is inhibited by injecting zero-sequence voltage. But this strategy does not take into consideration numerous variable constraints and mutual coupling between zero-sequence voltage and the modulation voltage, which weakens its control ability. In [14, 15], the strategy is further improved by predicting the appropriate zero-sequence voltage.

NP current is required to be zero at any time for NP potential stabilization in these above strategies used in the single three-level NPC topology. However, the inflow and outflow of NP currents are just required to be equal rather than being zero in two parts of the three-level NPC back-to-back converter, because two midlines are connected at NP. So these strategies cannot fully play the advantage of the converter. Moreover, the NP current in the rectifier side or inverter side where NP potential is not controlled could generate adverse effect to the NP potential [16–18].

This paper firstly improves NP potential control model based on zero-sequence voltage injected and further builds the NP potential model based on the NP current injected in order to avoid the numerous variable constraints and mutual coupling in Section 2. The NP-current injection range in a single switching cycle is obtained from the new model. According to different control needs, three algorithms, unilateral control, bilateral independent control, and bilateral coordination control, which are suitable for the characteristic of the converter, are, respectively, proposed in Section 3. Section 4 clarifies their control abilities and applicable occasions through comparative analysis of these three algorithms. Finally, the authors demonstrate the correctness and validity of the proposed algorithms by the simulation experiments in Section 5.

#### 2. The NP Potential Model Based on the NP Current Injected

The topology of the three-level NPC back-to-back converter is shown in Figure 1. It is made up of two three-level NPC topologies connected to the communal DC bus. Rectifier side and inverter side, respectively, connect to the grid and three-phase loads. If the inflow and outflow of NP currents are not equal, the NP potential would fluctuate. Because the converter is symmetrical, this paper only analyzes the control mechanism of NP potential by the rectifier side.