This article proposes the topology and space vector modulation (SVM) method of a reduced switch count (RSC) quasi-Z-source (QZS) three-level inverter, which can realize voltage buck-boost operation ...by adopting ten power switches. Compared with the QZS three-level T-type inverter, the presented inverter can further reduce the number of power switches. The SVM method for the proposed topology is designed, which introduces a distribution factor to balance the neutral-point voltage (NPV) actively. In addition, the up-shoot-through and down-shoot-through states are inserted within the durations of redundant small vectors to boost the dc input voltage, while the normal ac output voltage is not affected. Detailed comparisons of the RSC QZS three-level inverter and the existing impedance-source-based three-level inverters are conducted, and design guidelines and recommendations for selecting different components are given as well. Simulated tests based on MATLAB software and experimental tests based on a hardware-based test rig are conducted to validate the operating performance of the proposed topology and modulation method, which reveal that single-stage power conversion is achieved. Moreover, the NPV balance is actively controlled by using the proposed method.
A single-stage high-voltage gain boost inverter is getting popularity in applications like solar photovoltaic, fuel cell, uninterruptible power system (UPS) systems, etc. Recently, single-stage ...voltage boost multilevel Z-source inverter (ZSI) and quasi-Z-source inverter (QZSI) have been proposed for dc-ac power conversion with improved power quality. Multilevel ZSI uses more number of high-power passive components in the intermediate network, which increase the system size and weight. Also, its input current is discontinuous in nature which is not desirable in some of the applications like fuel cell, UPS systems, hybrid electric vehicle, etc. In this paper, a continuous current input three-level LC-switching-based voltage boost neutral-point-clamped inverter is proposed, which uses comparatively less number of high-power passive components at the same time retains all the advantages of multilevel QZSI/ZSI. It is able to boost the input dc voltage and give required three-level ac output voltage in a single stage. Steady-state analysis of the proposed inverter is discussed to formulate the relationship between the input dc voltage and three-level ac output voltage. A unipolar pulse width modulation technique devised for the proposed inverter to eliminate first center band harmonics is also presented. The proposed converter has been verified by simulation in MATLAB Simulink as well as performing experiment with the help of a laboratory prototype.
This paper presents an optimized switching strategy to reduce the dc-link ripple current for three-level photovoltaic (PV) inverters. The large electrolytic capacitors are commonly used for the dc ...link of power electronics applications to stabilize the dc-link voltage. The most important factor of designing the dc-link capacitor is the allowable current ripple. The over-ripple current flowing through the capacitor causes a high heat loss, shortened lifespan, low stability, and reliability. The proposed switching scheme selects voltage vectors and reconfigures dwelling order of the vectors to reduce the capacitor ripple current. This switching method is able to extend the lifetime of the dc-link capacitors by simple software programming. In addition, this switching method reduces the common-mode voltage and leakage current that represent high reliability and safety of the system. The effectiveness of the proposed method is verified with simulations and experimental results.
A sensorless three-level neutral-point-clamped inverter-fed induction motor drive is proposed in this paper. The conventional direct torque control (DTC) switching table fails to consider the circuit ...limitations, such as neutral-point-balance and smooth vector switching, caused by the topology of a three-level inverter. Two kinds of modified schemes for three-level DTC are proposed to solve these problems. They also provide performance enhancement while maintaining robustness and simplicity. Fuzzy logic control and the speed-adaptive flux observer (with novel gain and load toque observation) are introduced to enhance the performance of the system. The issue of large starting current is investigated and solved by introducing the technique of preexcitation. A 32-bit fixed-point DSP-based motor drive is developed to achieve high-performance sensorless control over a wide speed range. The effectiveness of the proposed schemes is confirmed by simulation implementation and experimental validation.
In this letter, space vector pulse width modulation (SVPWM) and virtual SVPWM (VSVPWM) for the neutral-point-clamped (NPC) three-level inverter (TLI) are reviewed briefly. The relationship between ...VSVPWM and carrier-based PWM is revealed based on the PWM model of TLI with NP current equation. Then, carrier-based VSVPWM (CB_VSVPWM) is proposed. The CB_VSVPWM is implemented by comparing double modulation waves with a single carrier, and the two modulation waves for each phase are deduced. The CB_VSVPWM can simplify the implementation of VSVPWM to a great extent. Moreover, comparisons between SVPWM and CB_VSVPWM are provided in detail. At last, the simple method proposed in this letter is verified by experiments.
This paper presents a novel duty cycle-based direct torque control (DDTC) strategy for a three-level neutral point clamped inverter fed interior permanent magnet synchronous motor. DDTC methods ...proposed in the literature for three-level inverter fed DTC (3L-DTC) are complicated, parameter dependent, and pay little attention to inverter inherent switching constraints. In this paper, a simple yet robust duty ratio calculation method, which respects the inverter switching constraints, is proposed. Additionally, a novel switching strategy is adopted to minimize the effects of flux drooping phenomena seen in 3L-DTC drives during low-speed operations. The proposed 3L-DTC method is efficient in reducing both torque and flux ripples while operating at low switching frequencies. Therefore, it is suitable for medium-high power ac drives. Experimental results on a scaled down test rig and comparative analysis are provided to validate the effectiveness of the proposed strategy.
The conventional three-level T-type inverter (3L-T2I) can solve open-circuit fault (OCF) at any switching devices by itself. However, the conventional fault-tolerant (FT) methods for upper switch of ...3L-T2I have a low voltage gain which cannot ensure the inverter operation in low voltage range. In order to improve the voltage-gain of the inverter under faulty condition, this brief presents a new topology of FT-3L-T2I by adding one normally-closed relay. This extra relay connects the mid-point of DC-link voltage and the common-point of bi-directional switches, in normal operating. In post-fault operating, this relay is opened. A new FT 2-level space-vector modulation (SVM) technique is introduced to control the proposed inverter. Under this SVM method, the operation of faulty upper switch is recovered by bi-directional switches and upper switches of other healthy phases. By this way, the voltage gain of the FT-3L-T2I can increase <inline-formula> <tex-math notation="LaTeX">2/\sqrt {3} </tex-math></inline-formula> times than conventional methods. Unlike any traditional method, under this approach, the unbalanced neutral-point voltage cannot affect the output voltage. The experimental results based on 900-kVA laboratory prototype are introduced to verify the effectiveness of the proposed inverter and SVM control strategy.
In this paper, a novel configuration of a three-level neutral-point-clamped (NPC) inverter that can integrate solar photovoltaic (PV) with battery storage in a grid-connected system is proposed. The ...strength of the proposed topology lies in a novel, extended unbalance three-level vector modulation technique that can generate the correct ac voltage under unbalanced dc voltage conditions. This paper presents the design philosophy of the proposed configuration and the theoretical framework of the proposed modulation technique. A new control algorithm for the proposed system is also presented in order to control the power delivery between the solar PV, battery, and grid, which simultaneously provides maximum power point tracking (MPPT) operation for the solar PV. The effectiveness of the proposed methodology is investigated by the simulation of several scenarios, including battery charging and discharging with different levels of solar irradiation. The proposed methodology and topology is further validated using an experimental setup in the laboratory.
In this article, two constant switching frequency direct torque control (CSF-DTC) techniques are proposed for H-bridge three-level inverter (HB-TLI) fed induction motor (IM) for reducing torque ...ripple at low-speed operation. Both the methods use modulators and characteristics of the pulsewidth modulation (PWM) mainly determine the motor drive performance. The first method uses two level-shifted carriers signals, whereas the second method uses carrier signals with appropriate offset voltages. Detailed analytical expressions describing complete harmonic characteristics of HB-TLI are also presented using double Fourier integral approach. Superior harmonic performance is obtained with the proposed PWM methods compared to the existing ones. Drawback of one of the envisaged CSF-DTC techniques entering into overmodulation is rectified at higher motor speeds and a smooth slide-over using zero offset voltage to carrier signals is recommended to overcome the issue. The proposed CSF-DTC techniques are simulated and experimentally validated. The dynamic and steady-state torque performance obtained with the proposed CSF-DTC techniques for IM are compared with the existing DTC methods, which show a significant reduction in the steady-state torque and current ripple, whereas the torque dynamic performance is almost preserved thus, demonstrating its usefulness especially at lower motor speeds.
The quasi-Z-source (QZS) asymmetrical three-level inverter (ATLI) can be regarded a post-fault topology of the QZS T-type three-level inverter. Due to the inherent limitation of this topology, the ...output states are restricted, and the conventional modulation methods are not applicable any more. This article presents a novel space vector modulation (SVM) for the normal operation of the QZS ATLI. Depending on whether the medium vector is included, the sectors of the space vector diagram (SVD) are categorized into two types. According to the sector type, different combinations of basic vectors are selected to generate the correct ac output voltage, and the appropriate shoot-through states are properly adopted to implement the dc voltage boosting. Additionally, to guarantee the neutral-point voltage (NPV) balance, the effective small vectors are selected, whose duty cycles are adjusted in different sectors. The detailed calculating process of duty cycles and arrangement of switching sequences are provided. The effectiveness of the proposed method is verified by experimental results.
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