Li Ran, Fang Jun-long*, Wang Jun-fu, and Wang Guang-ya

1 College of Electrical and Information, Northeast Agricultural University, Harbin 150030, China

2 Harbin Power Supply Company, State Grid Heilongjiang Electric Power Company, Harbin 150010, China

Abstract: With the acceleration of agricultural electrification, a lot of nonlinear and shock loads appear in the rural power grid, and the resulting harmonic and reactive currents pollute the rural power grid more and more seriously. To solve the above problem, threelevel neutral point clamped (NPC) inverters have been widely used, but their development is greatly restricted by the defect of neutral point voltage imbalance. In this paper, an improved virtual space vector pulse width modulation (VSVPWM) was proposed. Firstly, the mathematical models of various space vectors were established, and the influence of various space vectors on neutral point voltage was analyzed. The sum of the vector current at the neutral point was zero and the voltage control at the neutral point was completed by introducing the time offset into different switching times of the redundant small vector. This method was simple in design and avoided the redundant calculation of the traditional VSVPWM method and tedious switch sequence design. This balancing control strategy could greatly reduce the influence of virtual vectors on neutral point voltage and effectively control the low-frequency oscillation of neutral point voltage. The validity of the method was verified by establishing a matlab simulation model.

Key words: rural power grid, virtual space vector pulse width modulation, neutral point clamped inverter, neutral point voltage

Introduction

After years of development, power electronics-related technologies in the field of low voltage and small power have been very mature in the rural power grid. However, with the deepening of the agricultural electrification process, the rural power quality problem is increasingly serious, and the original topology can no longer meet the demand for high voltage and high power, so it is particularly necessary to seek a breakthrough in the topology structure, three-level neutral point clamped (NPC) topology comes into being (Fang and Wang, 2018; Huet al., 2009). Three-level NPC topology has been widely used in the rural power grid (Zhanget al., 2017; Qinet al., 2018). Three-level converter technology has attracted wide attention due to its characteristics such as high degree of self-fitting, low harmonic content, fast dynamic response and easy implementation (Choudhuryet al., 2016; Liet al., 2019). However, the neutral point on the DC side of this topology participates in the energy flow and introduces the neutral point current, so there is a problem of voltage imbalance at the neutral point (Fanget al., 2014; Jianget al., 2018).

For a long time, the topic of the neutral point voltage balance of the three-level NPC inverter has been the focus of domestic and foreign researchers, and some control strategies have been proposed (Nguyenet al., 2016; Wanget al., 2019). Space vector modulation is widely used because of its advantages of easy control and high voltage utilization (Zhanget al., 2017; Wanget al., 2017; Wuet al., 2015). Later, some scholars proposed the four-partition virtual space vector pulse width modulation (VSVPWM) modulation method (Liet al., 2019). Although the amount of calculation is reduced by reducing the number of small regions, there is still the problem that the number of space vectors fitting the reference voltage vector is different in different small regions. Besides, the switching state changes two levels at a time. Grigoletto and Pinheiro (2009) proposed a space vector modulation method that does not involve medium vectors. Although this method can eliminate the low-frequency voltage oscillation, because medium vectors do not participate in the vector synthesis, it will increase the output voltage distortion of the converter and increase the harmonic content. Xianget al. (2018) proposed to divide each large sector of space vector region into three subsectors. Although the method can reduce the computation in determining the reference voltage vector region in the modulation process, according to the offset of the neutral point voltage, it is necessary to design several sets of vector switch orders to balance the neutral point voltage. This method is complicated in design, difficult to implement, low instability and slow in dynamic response. In this paper, a virtual space vector was constructed for threepartition VSVPWM. By reallocating the action time of redundant small vectors in different switching states, the balance control of neutral point voltage was completed. Finally, a simulation model was established to verify the correctness and effectiveness of the proposed method by comparing the simulation results.

Materials and Methods

NPC three-level inverter

The topology structure of the three-level NPC inverter is shown in Fig. 1A. Three-level states of P, O and N were output by controlling the opening and closing of the switch tube on the bridge arm. The 27 different switching states were obtained by combining the outputs of three-phase bridge arms, as shown in Fig. 1B. The three-phase currents wereia,ibandic, respectively, and the direction was positive with the outflow inverter (Zenget al., 2019; Choiet al., 2014; Zhaoet al., 2018).

Fig. 1 NPC inverter simplified topology

Although traditional three-partition VSVPWM modulation took into account the influence of small vector and medium vector on neutral point voltage, designed redundant switch sequence, and then selected switch sequence, according to neutral point voltage offset, this method was not only tedious in design, but also had the limited effect of balancing neutral point voltage. In this paper, the effect of the medium vector and the small vector on the neutral point voltage was offset by the reasonable allocation of different switching state action times of a redundant small vector.

Improved VSVPWM modulation

The space vector region was divided according to Fig. 2A. When the reference vectorVrefwas located in I sector, virtual space vectors were constructed, as shown in the image (Fig. 2B).

Where,xwas the serial number of the virtual large vector in the large region;andrespectively represented P and N basic small vectors.

According to the nearest three virtual vectors method (NTV2) synthesis principle, usingVVL1,VVMandVV0,Vrefwas the sampling period, Equation (2) could be obtained as the following:

The specific action times of the virtual space vectorsVVL1,VVMandVV0were respectivelyTVL1,TVMandTV0, as shown in Equation (3):

The influence of neutral point current on neutral point voltage is shown in Equation (4):

Where,i(t) was the relationship between the neutral point current and time,Twas the action time, andCwas the capacitance of the DC side.

Fig. 2 Improved VSVPWM vector region division diagram and vector synthesis diagram

Because the sampling timeTswas short, the threephase current in this process could be regarded as a fixed value. I-1 subsector, for example, the neutral point currents generated by [OON] and [PON] wereicandib, respectively, and the effect on the neutral point voltage was as the followings:

The offset time introduced for the basic small vector and the basic medium vector were respectivelyToff1andToff2. The time deviation introduced for [OON] was as the followings:

Namely,

The time deviation introduced for [PON] was as the followings :

Namely,

The opening time of each state was as the followings:

Meanwhile, to obtain excellent output characteristics and reduce additional distortion,TPOO,should be guaranteed. Similarly, the switching sequence of I-2 and -3 subsector is shown in Table 1. Finally, achieved the modulation in all the subsectors.

Table 1 State order of space vectors

Results

The three-level NPC inverter model based on virtual space vector modulation was built under the MAT LAB/Simulink simulation platform. The traditional VSVPWM and improved VSVPWM modulation strategies were compared under different modulations in dexes(M=0.1, 0.9) to verify the orrectness and effectiveness of the improved VSVPWM modulation strategy proposed in this paper to suppress the neutral voltage imbalance. The simulation parameters are shown in Table 2.

Table 2 Simulation parameters

M=0.9

By observing the simulation results as shown in Fig. 3A, the traditional VSVPWM modulation strategy stabilized the neutral point voltage fluctuation within ±5 V. As shown in Fig. 3B, the improved VSVPWM modulation strategy described in this paper stabilized the neutral point voltage fluctuation within ±1 V.

Fig. 3 Voltage of C1, C2, on DC side (M=0.9)

By comparing Fig. 4A and B, it could be seen that the output current waveform of the improved modulation method was better than that of the traditional method. The FFT analysis of the two output waveforms was carried out, and the results are shown in Fig. 5. The harmonic content THD of the traditional method was 0.45%, while the harmonic content THD of the inverter using the improved method proposed in this paper was 0.31%.

Fig. 4 Output current simulation results (M=0.9)

Fig. 5 FFT analysis of output voltage waveform (M=0.9)

M=0.1

Fig. 6 showed the neutral point voltage fluctuation of the three-level inverter modulated by the traditional method and the improved method. As shown in Fig. 6A, the fluctuation range of neutral point voltage under the traditional method was ±5 V, while the neutral point voltage under the improved method proposed in this paper had no obvious fluctuation, as shown in Fig. 6B. By observing Fig. 7, it could be seen that the current waveform of the three-level NPC inverter modulated by the two methods had no obvious distortion. Fig. 8 showed whenM=0.1, the THD of the two modulation methods was 0.42% and 0.34%, respectively.

Discussion

By comparing the simulation results, it could be observed that the neutral point voltage balance control reached the design expectation and the output characteristics of the inverter were improved, which proved the effectiveness of the improved VSVPWM modulation strategy proposed in this paper for the neutral point voltage balance.

Fig. 6 Voltage of C1, C2, on DC side (M=0.1)

Fig. 7 Output current simulation results (M=0.1)

Fig. 8 FFT analysis of output voltage waveform (M=0.1)

Conclusions

In this paper, the neutral point current generated by the DC side neutral point participating in the energy flow during the operation of the three-level NPC inverter led to the unbalanced neutral point voltage. Based on the three-partition VSVPWM modulation strategy, the method of reasonably distributing the redundant small vector in different switching state action times was proposed to solve this problem. The comparison of simulation results showed that the method described in this paper had the following characteristics.

It had a positive effect on balancing the neutral point voltage. The improved VSVPWM modulation method proposed in this paper could stabilize the neutral point voltage fluctuation within ±0.6 V after 0.06 s even under the condition of a high modulation index and low load power factor, thus achieving the purpose of balancing the neutral point voltage.

Easy to implement. Based on traditional VSVPWM, this method eliminated the step of designing a redundant switch state, simplified the design process, and was easier to realize. The simulation results showed that the improved VSVPWM modulation strategy proposed in this paper could effectively balance the neutral point voltage, reduce the output distortion rate of the inverter, improve the overall performance of the inverter and help improve the power quality in the rural power grid.

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Oatley J M, de Avila D M, McLean D J,et al. 2002. Transplantation of bovine germinal cells into mouse testes.J Anim Sci, 80(5): 1925-1931.

Ives J D. 1989.The Himalayan dilemma:reconciling development and conservation. Routledge, London. pp. 14-17.

Martin G. 1996. Control of electronic resources in Australia. In: Pattle L W, Cox B J.Electronic resources:selection and bibliographic control. The Haworth Press, New York. pp. 85-96.

Yufin S A. 2000. Geoecology and computers. In:Proceedings of the third international conference on advances of computer methods in geotechnical engineering. A. A. Balkema, Rotterdam. pp. 151-158.

Pan G J. 2005.The epidemic factor analysis and cultivation regulation of rice blast in early Japonica in Heilongjiang Province. Shenyang Agricultural University, Shenyang.

Ambion, Inc. 2004. Cancer-related miRNA uncovered by themirVanaTMmiRNA microarray.http://www.ambion.com/techlib/tn/121/4.html.