Virtual Space Vector Pulse Width Modulation for Asymmetric T-Type Neutral Point Clamped 3-Level Inverter
This paper proposes a novel 3-phase asymmetric 3-level T-type NPC inverter and studies its PWM performance using a virtual space vector pulse width modulation control strategy. Firstly, the mathematical model and characteristics of this economical topology are described. Then, a virtual space vector approach is proposed to build a space vector diagram for designing SVPWM control. Similar to the conventional 3-level NPC inverter, the asymmetric inverter can also work with the neutral point voltage self-balancing in a fundamental period, which enables employment of this topology in various applications. Finally, simulation and experiment results under different load conditions have shown good output performance of the asymmetric 3-level topology. Similar tests are also performed on both conventional 2-level and 3-level inverters for comparison. For an almost similar number of different voltage vectors in the space vector diagram, the asymmetric 3-level topology can compete with conventional 3-level inverters for low-cost applications. The obvious benefit of the asymmetric 3-level inverter is a smaller number of switches devices while it can achieve output performance similar to that of the conventional 3-level. The comparative investigation also shows that the total loss given by SVPWM for the asymmetric 3-level configuration is lower than that of the traditional 3-level inverter.
Three-level converters have been widely used in industrial applications, which includes high-power motor drivers , grid connection for renewable energy systems [2, 9], and electric vehicles . In particular, in high-power and medium-voltage applications, it has outstanding advantages compared with 2-level converters, such as better total harmonic distortion (THD), lower switching losses, and reduced voltage stress dv/dt across the power devices [4–6]. These most common topologies are neutral point clamped (NPC), flying capacitor (FC), and cascaded H-bridge (CHB) types. In fact, NPC converters are being widely used. Among them, the T-type NPC was shown to be more efficient than the traditional NPC up to the medium switching frequency range [7–13]. This topology does not use clamping diodes, and the number of power switches is the same as the NPC converters.
For overall good performances in relation to loss distribution, high efficiency compared with conventional 3L-NPC, T-type NPC legs have been commercialized by many manufacturers such as Semikron, Infineon, Mitsubishi, and Fuji [14, 15]. Many modified T-NPC structures have been introduced to satisfy different purposes. For example, an advanced T-NPC (AT-NPC) structure using a reverse blocking IGBT (RB-IGBT) instead of two conventional IGBTs at the T-leg connected to the neutral point has been studied [16–18]. This structure reduces the conduction voltage drop and related device loss when the neutral point is connected to outputs. Another topology named combined AT-NPC and quasi-Z source impedance converter presents an attractive and economic solution for renewable energy applications with low DC input voltage . Recently, numerous research studies on utilizing wide band gap devices for T-type converters also draw much intention from researchers [20, 21]. One of the major trends in modern power converters is to design higher multilevel converters for high-power and high-voltage applications. Because there are no topologies from NPC, FLC and cascaded inverters can meet various demands in practice and many hybrid multilevel topologies were designed and shown as good solutions for industry. For this aim, several five-level hybrid T-type NPC inverters were presented in the recent literature [22–25].
Although 3-level NPC inverters have many advantages compared with 2-level inverters, as mentioned above, they have some disadvantages, such as an increased system volume, higher cost, and reduced reliability due to more switches. Many recent studies focus on developing reduced switched topologies [26–32] to improve system reliability, reducing size and cost. In [26–29], the diode NPC 3-level 2-leg topology was proposed in which the required number of switches is reduced from 12 IGBTs and 6 diodes to 8 IGBTs and 4 diodes. A similar structure for the T-type has been proposed to eliminate the diodes . These topologies only need two legs for a three-phase 3-level inverter, so the number of components is reduced by one-third, as shown in Figure 1(a). However, a drawback is that the linear output voltage is limited to half, as shown in Figure 1(b).
This paper proposes a so-called asymmetric 3-level T-type NPC inverter by adding a half-bridge leg to the 2-leg 3-level NPC inverter. This new configuration, as shown in Figure 2, enables twice the output voltage range compared with Figure 1. In order to evaluate and compare the PWM performance of the novel topology with conventional 2-level and 3-level inverters, virtual space vectors and the related virtual space vector pulse width modulation (VSVPWM) control will be proposed and designed. Simulations and experiments are performed to demonstrate the effectiveness and feasibility of the asymmetric T-type NPC 3-level inverter.
2. Circuit Analysis and Mathematical Model
The asymmetric inverter topology is illustrated in Figure 2. The phase leg B is a half-bridge; phases A and C are 3-level T-type legs. The DC-bus voltage is supplied via two DC-link capacitors in series. Three-phase load R-L is connected to the output terminals of the converter.
Under the condition of balanced DC-link capacitor voltages, the voltage from the output terminal to the negative of the DC-bus can be expressed as follows:where and is the switching state and defined in terms of the IGBT states per phase legs. For 3-level leg phases A and C, can be 0, 1, and 2. For 2-level leg phase B, can be 0, 2. The switching states of the three phases A, B, and C are described in Table 1.
The Clarke formula transforms the voltage vector in the to coordinate system as follows:where is the output voltage vector of the inverter in the αβ coordinate system and k is denoted as the vector index.
The converter generates 18 voltage vectors, including 2 redundant zero voltage vectors (ZV), 6 small voltage vectors (SV), 4 medium-voltage vectors (MV), and 6 large voltage vectors (LV) illustrated in Figure 3. Compared with 7 and 19 difference vectors used in 2-level and 3-level NPC inverters, respectively, the number of difference vectors in the asymmetric NPC is 17, nearly identical to the NPC. So, it is expected that the performance of the asymmetric PWM control will be as good as the 3-level NPC inverter.
Form (1), the line voltage between two terminal legs is rewritten and deduced as follows:
With the switching state as defined in Table 1, applying the above analysis for the 2-level and 3-level conventional converters, the specific characteristics of each type are compared with the asymmetric configuration reported in Table 2. The asymmetric converter generates line voltages at 5 voltage levels , , and 0, similar to traditional 3-level converters.
3. Virtual SVPWM for Asymmetric Inverter
The reference voltage vector can be synthesized in relation with the switching states on the phase legs, with , as follows:
We define as the maximum amplitude of the reference vector in the linear modulation range, corresponding to the radius of the largest circle inscribed in the hexagon. The modulation index is defined as follows:
It can be seen that the maximum amplitude of , and asymmetric 3-level achieves the same maximum voltage as traditional symmetric configurations.
The space vector diagram of the asymmetric inverter in Figure 3 lacks two medium-voltage vectors, and , in sectors I and IV, respectively. To apply the SVPWM technique of the traditional 3-level into asymmetric topology, two virtual vectors are proposed and added to the space vector diagram as illustrated in Figure 4, defined as follows:
3.1. Identify Region of Sectors
As in Figure 4, each sector is divided into 4 regions numbered 1 to 4. The SVPWM technique for will be solved for sector I, and these results will be properly deduced for other sectors.
The reference voltage vector synthesized by the three nearest voltage vectors, which are determined through the region where the reference vector is located, is as follows :
The region identification is related to and as follows: If , then region 1 If , then region 3 I , then region 4 Else region 2
The remaining sectors will be converted to sector I. Therefore, the reference vector will be calibrated so that the modified angle falls between 0 and π/3, that is,where k = 2, …, 6 for sectors II, …, VI, respectively.
3.2. Duty Cycle Calculation
The duty cycle for the voltage vector at the vertices of the triangle where the reference vector is placed essentially represents the dwell time of the selected switching states during the sampling interval . For example, the reference vector in region 3 of sector I, as shown in Figure 5, is synthesized by voltage vectors , , and over a sampling period, defined by the following.where , , and are the duty ratios for the vectors , , and , respectively.
Substituting the values of the voltage vectors into (9) and separating the real and imaginary parts give
Calculating similarly for the remaining regions, the relationship between the location and duty factors is summarized in Table 3.
In regions 2, 3, and 4 of sectors I and IV, the reference voltage vector is synthesized with the participation of the virtual vector. For example, in region 3 of sector I is synthesized by 3 voltage vectors , , and with duty coefficients , , and , respectively, in which is performed by and , as defined in (6), with duty for each. The duty factors for available vectors , , and are adjusted to , , and , rewritten as , , and , respectively. All the adjusted duty coefficients of the voltage vectors of sectors I and IV are detailed in Table 4.
3.3. Switching Sequence
After calculating the duty for the selected voltage vectors to synthesize into the reference vector, the next step is to arrange the switching sequence. In this section, the switching sequence for is designed to minimize the switching frequency according to required criteria is as follows. ➀ The transition between switching states involves only two switches in the same phase leg. It is satisfied if there is no switching from state 0 to state 2 or vice versa in phase leg A and C. ➁ When the reference vector moves from one sector (or region) to the next, the transition requires no or a minimum number of switching. ➂ The switching sequence is arranged in a half-wave symmetrical pattern to eliminate even-order harmonics on the output line voltages.
For example, is located in region 2 of sector I. A typical seven-segment switching sequence and output line-voltage waveforms are presented in Figure 6(a). It can be seen that requirement ➀ is satisfied. The starting and ending point is state 100, so the switching sequence in the remaining regions of sector I must also start from state 100 or 200 to satisfy requirement ➁. For the requirement ➂ to be also satisfied, then region 2 of sector IV must be arranged in the opposite order of sector I, as shown in Figure 6(b).
The typical switching sequence for the asymmetric T-type NPC inverter is shown in Figure 7.
3.4. The Self-Balancing Mechanism
Kirchhoff’s current law is applied at the neutral point as follows:
Assuming , the difference between and voltages per fundamental cycle is calculated as follows:
When a zero or large vector is applied, as shown in Figures 8(a)and 8(d), the difference voltage is not affected because of In contrast, small and medium vectors can contribute to the variation of , as shown in Figures 8(b) and 8(c), in which the polarity and magnitude of are determined by the load current generated by the switching state.
For the selected switching sequence, as in Figure 7, the voltage difference generated by the switching states at the low modulation index in one fundamental period is presented in Figure 9. For example, when the reference vector moves in region 1 of sector I, the switching states 100, 221, and 222 are applied to synthesize the reference vector. The voltage difference caused by the switching states is calculated as follows:
Similarly for region 1 of sector IV,
The general equation of the load currents is as follows:where is the max amplitude of the phase load current and is the load power factor angle; .
Expression (18) and Figure 9 show that when applying the switching state 100, it causes a capacitor imbalance, by the red area marked ➀ in sector I. Meanwhile, the switching state 122 in sector IV makes the capacitor unbalanced by an amount equal to that caused by switching state 100 but a different sign (area marker ➀ in sector IV). Similarly, with the remaining definite integrals, we have
In one fundamental cycle, the switching state in the first half causes an imbalance. However, it quickly returns to balance in the next half-cycle by switching states on the opposite side, which have the same neutral current characteristics but a different sign, sowhere is the difference voltage caused by the switching states in sector ; .
The self-balancing mechanism for the high modulation index also occurs similarly, as presented in Figure 10.
3.5. Harmonic Distortion
The harmonic distortion performance of SVPWM strategies is evaluated by the THD of the load current and voltage output as follows :where and are the fundamental magnitude and the nth harmonic magnitude of phase current output, respectively. and are the fundamental magnitude and the nth harmonic magnitude of line-voltage output, respectively. In this paper, the THD spectrum of load current and output voltage is calculated to the 1000th harmonic.
4. Simulation and Experimental Results
4.1. Simulation Results
Figure 12 shows the steady-state of the VSVPWM method for the asymmetric 3-level inverter with ma = 0.9. The load current and the balance of DC-link capacitor voltages are depicted in Figures 12(c) and 12(d). Let be a line voltage between outputs of 3-level and 2-level legs and be a line voltage between outputs of two 3-level legs. The waveforms of and are illustrated in Figures 12(a) and 12(b) with THDV of about 44.4% and 32.8%, respectively.
In order to evaluate the dynamic response performance, a step change in modulation index from 0.4 to 0.8 is examined in this study. The diagram of the load current is shown in Figure 13(c). The voltage of the capacitors is maintained in balance with around 20 V and 32 V for the modulation index of 0.4 and 0.8, respectively, as shown in Figure 13(d). For ma = 0.4, the line-voltage quality and are almost the same, with THDV about 76%; for ma = 0.8, THDV of is about 38% while that of is 49.3%, as illustrated in Figures 13(a) and 13(b).
THDV of the asymmetric 3-level inverter for different modulation indices is described in Table 6. For , THDV on both and is almost the same. For , when the reference voltage vector appears in regions 2, 3, and 4, THDV of is higher than . For example, at , THDV of and is 44.4% and 32.8%, respectively.
The behavior of the system is also examined under a sudden change of the load at ma = 0.9, as presented in Figure 14(a). In the initial state, the system operates with the load parameters R = 12 Ω and L = 20 mH; then, another resistor R’ = 12 Ω is connected parallel with R at t = 0.5 s. This results in a load current change from 23 A to 36 A with THDI about 1.32% to 0.98%, respectively, as shown in Figure 14(b). As depicted in Figure 14(c), the capacitor voltages are maintained to be well balanced with around 22 V and 37 V. The output voltage quality of the asymmetric T-type NPC inverter is not influenced by the load parameters with THDV maintained about 44% and 32% for and , as shown in Figure 14(d).
To evaluate the influence of the load power factor on , simulation is examined with a fixed load impedance Z = 13.55 Ω and a variable power factor between 0.4 and 0.95 for the different modulation indexes. The results in Figure 15 show that the neutral point voltage maintains a relatively good balance at different load power factors and modulation indices. The region with ma ≤ 0.4 gives a small capacitor voltage difference in the range , i.e., is about 24 V, with all load power factors. For 0.4 < ma ≤ 0.8, the capacitor voltage difference gradually increases with the modulation index and load power factor and ∆Vc ranges from 3%–7% and reaches the maximum value of about 43 V at (ma; cos φ) = (0.6; 0.95). In the remaining region with ma > 0.8, tends to decrease in the range of 3%–5% when the power factor increases. For example, at ma = 1, = 30 V for power factor 0.55 and = 16 V for power factor 0.95.
A test with unbalanced load parameters depicted in Figure 16(a) is also performed in this study. Figures 16(b) and 16(c) show that even though the 3-phase load currents are unbalanced, the capacitor voltages are maintained balanced with about 28 V.
To verify the behavior of the self-balancing mechanism of neutral point voltage, the simulation is performed at , and capacitor is discharged during by connected a resistor connected in parallel, as shown in Figure 17(a). The difference of capacitor voltages at is about 300 V. Under the proposed VSVPWM, it rapidly decreases and attains a steady-state value at 24 V after 0.3 s, as shown in Figure 17(b).
The graph in Figure 18 compares voltage THD characteristics of the asymmetric T-type NPC, 3-level, and 2-level inverter. As observed, for , the THD characteristic of the asymmetric T-type NPC topology is similar to that of the conventional 3-level. For , the THDV characteristic of the voltage is higher than that of 3-level inverter, while the THDV characteristic of the voltage is similar to that of 3-level inverter. THDV of the asymmetric T-type NPC inverter is significantly better than that of the 2-level inverter for all modulation indices.
Another benefit of the asymmetric inverter can be demonstrated in loss comparison. The total loss is calculated as the sum of conduction loss and switching losses of all IGBTs and diodes, whose device datasheets are given in Table 7. The principle of power loss calculation is explained in detail in . The calculation is realized with the use of SimPowerSystems and Simscape in Matlab, with parameters Vdc = 300 V, ma = 0.9, and load .
The comparison chart of conduction loss, switching loss, and total loss between the asymmetrical 3-level inverter and the conventional 3-level inverter is shown in Figures 19–21, respectively. The conduction loss is almost unchanged at various switching frequencies, about 284 W for the conventional 3-level inverter and 277 W for the asymmetric 3-level inverter, as shown in Figure 19.
The switching loss of the asymmetric 3-level inverter is shown to be smaller than that of the conventional 3-level inverter, as shown in Figure 20. At , the switching loss of the asymmetric inverter is 73.1 W compared with the 104.4 W of the conventional 3-level inverter. It gives a reduction of about 30% switching loss. Similar results are also obtained for other switching frequencies.
As demonstrated in Figure 21, the total loss of the asymmetric inverter is lower than that of a conventional 3-level inverter. For example, at , the total losses of the asymmetric 3-level inverter and conventional 3-level inverter are 326.1 W and 353.8 W, respectively. Likewise, at , the total loss is 374.4 W for the asymmetric inverter and 424.2 W for the conventional 3-level inverter. This translates to a more than 12% improvement in power loss.
4.2. Experimental Results
To verify the effectiveness of the presented topology, the experiments were carried out on an asymmetric T-type NPC, 2-level, and 3-level configuration under both transient and steady-state conditions. A laboratory model is built as shown in Figure 22, including ➀ a digital signal processor TMS320F28379D to perform algorithms built-in Matlab/Simulink environment with Embedded Coder Support Package for TI C2000 Processors; ➁ the inverter made from TOSHIBA’s IGBT GT50J325-type; ➂ IGBT driver circuit which uses QP12W08S-37 type; ➃ DC-link capacitors; ➄ R-L load; and ➅ Tektronix TDS2024C oscilloscope. The experimental parameters are listed in Table 8.
The first test is performed with the modulation index stepped from 0.4 to 0.9. The results illustrated in Figure 23 show that the DC-link capacitor voltages are maintained balanced at both modulation indices. The output voltage qualities and are similar at modulation index , THDV about 76%. For modulation index , the voltages and are distinguished with THDV about 45.32% and 33.85%.
The second experiment is performed to evaluate the system’s response under a step change in load parameters at . Initially, load 1 is with parameters and ; then, another resistor is connected parallel with , as shown in Figure 24(a). The results from Figure 24(b) show that the capacitor voltage is kept balanced for both load parameters. The load current changes from 2.7 A to 5.1 A. The quality of the output voltage is unchanged, THDV about 45% and 33% for and .
The test with an unbalanced load is also performed with the load parameters as in Figure 25 at . The results shown in Figure 26 show that the capacitor voltages are kept balanced, and the output line-voltage qualities are similar to the above-balanced load test case, THDV about 45% and 33% for and .
The experiment results of the self-balancing mechanism using VSVPWM strategy for asymmetric 3-level inverter under the unbalanced capacitor voltage condition at ma = 0.9 are presented in Figure 27. In the initial state in Figure 27(a), switch K is closed, resulting in capacitor c2 discharging and attains about 25 V. Then, K is opened, and rapidly decreases and obtains balancing, as shown in Figure 27(b).
Figure 28 shows the graph of the THDV comparison of the VSVPWM algorithm for the asymmetric 3-level inverter and the SVPWM technique for conventional 2-level and 3-level inverters. The diagram shows that the experimental results are similar to the simulation results obtained in Figure 18.
This paper presents virtual SVPWM control for the asymmetric T-type NPC 3-level inverter topology. A modified space vector diagram is built with the help of the virtual vector. Then, the conventional SVPWM algorithm of the three nearest vectors is implemented. The switching sequence has been designed to reduce the amount of switching. A comparative evaluation between asymmetric T-type, 2-level, and 3-level inverters was performed. Simulation and experiment results show that the output voltage quality of the asymmetric T-type NPC inverter is much improved compared with that of the conventional 2-level inverter and almost similar to that of the conventional 3-level inverter. Besides, the capacitor voltages are also maintained in a good balance. The asymmetric inverter is attractive for applications that require lower cost but own similar output performances of a three-level inverter such as full output voltage range and low harmonic distortion. In another application, the proposed VSVPWM control can be applied for a conventional 3-level T-type NPC in a faulty condition while one T-leg connected to the neutral point is defectively open.
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was funded by the Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number C2021-20-12. The authors acknowledge the support of time and facilities from Ho Chi Minh City University of Technology (HCMUT), VNU-HCM, for this study.
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