Discontinuous PWM Strategies Research Articles

An Optimal PWM Technique for Dual-Output Nine-Switch Boost Inverters With Minimum Passive Component Count

Nine-Switch inverter (NSI) has been recently introduced for independent control of dual ac motors. Although this topology reduces the number of active switches by 25% compared to conventional inverters, it has a main conceptual shortcoming due to voltage gain limitations. In NSI, the voltage gains of both outputs are relatively low, while their maximum achievable magnitudes depend on the phase angle between the two sets. In order to mitigate this problem, this article introduces a new dual output nine-switch boost inverter (NSBI) topology with a minimum number of passive elements. The detailed analysis of NSBI is presented, while the required modifications to conventional pulsewidth modulation (PWM) strategies of the NSI are explained to ensure an acceptable voltage boosting ratio while a high-quality voltage output is obtained. Among different PWM schemes, a modified discontinuous PWM scheme is found optimum with this proposed topology to minimize the voltage stress across switches and improve the input current and output voltage quality. The proposed topology is also validated under both common-frequency and different-frequency modes of operations. Moreover, a clear picture of the NSBI performance compared with other possible counterparts is introduced. The theoretical findings have been corroborated using simulation and experiments.

A High Performance Interleaved Discontinuous PWM Strategy for Two Paralleled Three-Phase Inverter

This article aims to obtain the optimal combination of the switching loss, maximum zero-sequence circulating current (ZSCC), and line current ripple, which benefits most of the applications. The analysis shows that the interleaved discontinues pulsewidth modulation (IDPWM) maintains the minimum switching loss and line current ripples, but it introduces a larger ZSCC. Therefore, to obtain the optimal combination of the switching loss, maximum ZSCC peak, and line current ripple, it is essential to retain the minimum switching times and the optimal line current ripple of IDPWM but further reducing its overly large ZSCC. Further analysis reveals that IDPWM includes the vector combinations with medium ZSCC change rates, resulting in the larger ZSCC. Given the redundancies of the vector combinations, this article proposes a simple matrix to modify the original modulation signals of IDPWM, eliminating the medium ZSCC change rates. The proposed PWM scheme retains the minimum line current ripple and switching times as those of IDPWM while further reducing its overly large ZSCC, as validated by analytical results as well as experimental results. Since the proposed method obtains the optimal combination among switching loss, line current ripple, and maximum ZSCC peak, we name the proposed PWM scheme a high-performance DPWM.

Discontinuous Space Vector PWM Strategy for Three-Phase Three-Level Electric Vehicle Traction Inverter Fed Two-Phase Load

Discontinuous pulse width modulation (DPWM) strategies are usually adopted to reduce the switching loss and output current ripple of three-phase three-level traction inverters under three-phase load conditions. However, if there is a short circuit in any arbitrary phase or the inverter is used to feed a two-phase load, the output performance of conventional DPWM strategies will be deteriorated. Here, four improved DPWM (IDPWM) strategies for three-phase three-level neutral-point-clamped (NPC) traction inverter fed two-phase load are proposed. Unlike three-phase load conditions, the phase angle and the amplitude of each basic voltage vector in the space vector diagram are modified under two-phase load conditions. Consequently, sectors are re-divided and duty cycles of basic vectors during synthesis are recalculated. Clamping intervals of each phase for the four type discontinuous PWM (DPWM) strategies are rearranged according to the modified space vector diagram; then, the proposed DPWM strategies can be obtained. Compared with the conventional DPWM strategies, the output current waveform quality of the proposed strategy is significantly improved. Meanwhile, the amplitude of the neutral-point voltage ripple is also reduced.

Modified Discontinuous PWM Strategy for Three-Phase Grid-Connected PV Inverters With Hybrid Active–Passive Damping Scheme

This article proposes a modified discontinuous pulsewidth modulation (MDPWM) strategy for transformerless three-phase grid-connected photovoltaic (PV) inverters. MDPWM reduces the leakage current from PV inverter by reducing the magnitude of low-frequency harmonics presented in the common-mode voltage in comparison to classical discontinuous PWM technique. To enable the application of the MDPWM, a new hybrid active–passive damping scheme is proposed where common-mode and differential-mode dynamics are damped by passive and active schemes, respectively. Theoretical and experimental results show that the proposed scheme results in higher system efficiency in comparison to classic continuous PWM scheme keeping the leakage current reduced.

Low Frequency Harmonics Reduction Discontinuous PWM Strategy With Active Neutral Point Voltage Control for Neutral Point Clamped Three-Level Inverter

The traditional discontinuous pulse width modulation (DPWM) strategy is very suitable for reducing the switching losses of three-level inverter (TLI). However, large neutral point voltage (NPV) fluctuation is usually caused, which results in low frequency harmonics (LFHs) in the output line voltage. Therefore, a DPWM modulation strategy that can suppress LFHs is presented in this paper, and it is recorded as low frequency harmonic reduction DPWM (LFHRDPWM). The proposed LFHRDPWM can be seen as an improvement of traditional DPWM, which modifies the duty ratios according to voltages of the two DC-link capacitors. For LFHRDPWM, due to the suppressed LFHs, the self-balancing ability of NPV is thus lost, and gradually increased DC offset on NPV is caused. To overcome that, an active neutral point voltage control (ANPVC) method is also proposed. The proposed methods not only can achieve switching loss (SL) reduction almost as the same as traditional DPWM strategy, but also can realize NPV control and LFHs reduction. The effectiveness of the proposed method is verified by experiments.

A Decoupled Hybrid Current Control for Improving the Performance of 60° DPWM-Based Three-Phase Grid-Connected Inverter

Nonlinear boundary control technique can offer superior dynamic behavior and eliminate the LCL-filter resonance in the grid-connected inverter. However, the chattering might be aroused by the distorted intracycle voltage information in the control law. It becomes worse when the nonlinear boundary controller is directly applied in three-phase three-wire inverter configuration. A hybrid current control scheme utilizing the decoupled predictive hysteresis capacitor current controller integrated with the linear grid current controller is proposed in the paper. By applying the 60° discontinuous pulse-width modulation scheme and using the virtual dual-buck decoupled structure, two separate sets of switching criteria for dictating the switching states of two specific half-bridge legs are formulated to avoid the interactions amongst phases. A small-signal ac model is also built to study the transfer characteristics of such hybrid current controlled inverter system. Simulation and experimental results have been offered to evaluate the effectiveness of the proposed method. Both results meet the theoretical predictions.

Minimum Loss Discontinuous Pulse-Width Modulation Per Phase Method for Three-Phase Four-Leg Inverter

The three-phase four-leg inverter (TPFLI) provides unbalanced voltage to a load or injects unbalanced current to a grid to compensate unbalanced current. However, it has a high switching loss because of the fourth leg. This paper presents a discontinuous pulse-width modulation (DPWM) method that minimizes the per-phase switching loss of the TPFLI. The discontinuous three-dimensional space-vector pulse-width modulation (3D SVPWM) can be implemented in the TPFLI by injecting the same offset voltage as the conventional DPWM of three-phase three-leg inverter (TPTLI). However, the conventional DPWM is unsuitable for TPFLI because of its unbalanced current. We demonstrate that the discontinuous phase can be chosen according to the offset direction when a voltage reference vector is specified. Based on this result, a new per-phase minimum-loss discontinuous PWM strategy is developed and compared with the conventional DPWM. This method can also be applied to the TPTLI and implemented by injecting an offset voltage generated using the inverter phase voltage reference and phase current. Furthermore, the lifespan of a TPFLI can be extended by preventing deterioration of a specific leg. The validity of the proposed method is verified through simulation and experiments.

Performance study of various discontinuous PWM strategies for multilevel inverters using generalized space vector algorithm

The space vector (SV) pulse-width modulation (PWM) approach is extensively preferred in the control of multilevel inverters (MLI). In this paper, the implementation of various advanced discontinuous pulse-width modulation (ADPWM) strategies using a generalized space vector algorithm is presented. The ADPWM MIN, ADPWM MAX, and ADPWM 0 ~ ADPWM 3, strategies are developed using a combination of advanced switching sequences 1012-2721 and 0121-7212. The output performance in terms of voltage THD, current THD, and reduction of common mode voltage (CMV) of a three-level inverter-fed induction motor for different ADPWM strategies is investigated initially through MATLAB simulation and comparison of the obtained results is presented. It can be observed that the ADPWM 1 strategy with the 0121–7212 switching sequence offered the best performance in terms of the voltage and current THD at the modulation range of 47.5 Hz to 50 Hz. In addition, it is validated experimentally. Experimental work is done on a V/f-controlled three-level inverter-fed induction motor with the help of dSPACE. To validate the study, experimental results of the line voltage, stator current, and CMV waveforms for the ADPWM 1 (optimal performance PWM strategy) are presented in comparison with the 0127 sequence at fundamental frequencies of 49 Hz and 30 Hz.

Carrier-Based Discontinuous PWM for a Five-Level Unidirectional Rectifier

In applications, where bidirectional power flow is not required, unidirectional pulsewidth modulated (PWM) rectifiers are advantageous in terms of power density, hardware simplicity, and reliability compared to the conventional PWM rectifiers. For high-power medium-voltage applications, a discontinuous pulsewidth modulation (DPWM) strategy is useful in reducing the switching loss. This article proposes a DPWM strategy for a recently proposed five-level unidirectional rectifier. The rectifier is analyzed in the space vector (SV) domain to determine the switching sequences for DPWM. Two such DPWM sequences (i.e., DPWMA and DPWMB) are identified. Implementation of these DPWM sequences in the SV domain is known to be very computation intensive. Therefore, carrier-based implementation of both the DPWM strategies is proposed, which are derived following the SV approach. The proposed strategies are found to be very simple with very few computations. A new voltage-balancing method for the dc-link capacitors is also proposed in this article. Performances of these DPWM strategies over the entire operating range are predicted analytically and verified experimentally. This article is accompanied by MATLAB files demonstrating various performance indexes of the proposed DPWM sequences.

An Extended DPWM Strategy With Unconditional Balanced Neutral Point Voltage for Neutral Point Clamped Three-Level Converter

According to different clamping methods, nine modes (C_MODE1-9) of the traditional discontinuous pulsewidth modulation (DPWM) for neutral-point-clamped three-level converter have been revealed. Since traditional DPWM cannot balance neutral point (NP) voltage under the full range of modulation index (MI) and power factor (PF), two extended modes (E _MODEs) are proposed in this paper, which can balance NP voltage. The proposed extended discontinuous pulsewidth modulation (EDPWM) is characterized by using C_MODEs and E_MODEs alternately.Therefore, EDPWM can balance NP voltage under the full range of MI and PF. Since the switching numbers under C_MODE and E_MODE in one switching cycle are two and three, respectively, the switching loss under EDPWM is lower than that under other pulsewidth modulation (PWM) strategies. The performance of EDPWM is analyzed and compared with traditional DPWM, space vector PWM (SVPWM) and virtual SVPWM (VSVPWM) in terms of switching loss, waveform quality, and the ability of NP voltage control. Finally, the feasibility and superiority of the proposed method are verified by experiments.

A Novel Discontinuous PWM Strategy to Control Neutral Point Voltage for Neutral Point Clamped Three-Level Inverter With Improved PWM Sequence

In order to reduce switching loss of neutral point clamped three-level inverter (NPC TLI), generally discontinuous pulsewidth modulation (DPWM) is used. But it can result in dc offset and ac ripple on neutral point (NP) voltage. So a novel pulse sequence DPWM (NPSDPWM) is proposed to reduce switching loss and control NP voltage simultaneously in this paper. NP voltage is controlled by choosing proper clamping modes. To avoid unexpected switching action during changing clamping mode, an improved pulse sequence is also presented. The switching loss and NP voltage ripple of NPSDPWM, traditional and proposed DPWM in previous literature are compared, respectively. The experimental results show that NPSDPWM has well NP voltage control ability and the switching losses are reduced effectively.

Sector Subdivision Based SVPWM Strategy of Neutral-Point-Clamped Three-Level Inverter for Current Ripple Reduction

This paper presents a sector subdivision based space vector pulse width modulation (SVPWM) strategy with reduced current ripple for a three-level inverter. Using the output current ripple theory, closed-form expressions of average current ripple vectors for both continuous and discontinuous switching sequences are derived. Based on the sector and triangle distributions of conventional SVPWM strategy, each triangle is further divided into three small-regions. Then the switching sequence with the lowest magnitude of the average current ripple vector is applied in every small-region, so that the advantages of continuous pulse width modulation (CPWM) and discontinuous pulse width modulation (DPWM) under different modulation index conditions are combined to reduce the current ripple in the whole modulation range. The output performance of the proposed strategy is compared with the conventional CPWM and DPWM strategy, and experimental results verify that the proposed strategy could reduce the current ripple of three-level inverters effectively.

Discontinuous PWM Strategy for Neutral Point Clamped Three-Level Inverter to Achieve Multiple Control Objectives

In order to reduce switching loss of neutral-point clamped (NPC) three-level inverter (TLI), a switching loss minimized discontinuous PWM (SLMDPWM) strategy is proposed firstly. There are DC offset and AC ripple on the neutral-point (NP) voltage when SLMDPWM is used, which causes an appearance of low frequency harmonics (LFHs) in the output voltage. Aiming at this drawback, reduced LFHs (RLFHs) method is then proposed. While using RLFHs, the self-equilibrium ability of NP voltage is thus lost. So, active neutral-point voltage control (ANPVC) is also proposed. The performances of NPC TLI with the proposed DPWM and other DPWMs are compared by simulation and experimental results. The results show that switching loss, LFHs can be reduced significantly and the NP voltage can be controlled within an allowable range. Therefore, the multiple control objectives of reduced switching loss, no LFHs and allowable NP voltage are achieved.

A Generalized Discontinuous PWM Scheme for Three-Level NPC Traction Inverter With Minimum Switching Loss for Electric Vehicles

A discontinuous pulsewidth modulation (DPWM) strategy for three-level neutral-point-clamped (NPC) traction inverter drive for electric vehicles (EVs) with minimum switching loss for the full power factor (PF) range is developed and presented in this paper. Considering the dynamic loading conditions in an EV propelled by an induction motor (IM), the discontinuous modulation strategy is generalized to obtain minimum switching loss for a wide variations in load PF and modulation depth. The dynamic operating conditions in EV may render frequent unbalances in dc-link capacitor voltages in NPC inverter. It is demonstrated that the addition of an appropriate offset signal with the reference discontinuous signals generates compensating neutral current in the right direction, which can be used to mitigate existing unbalance in two dc-link capacitor voltages. Minimum switching loss at variable PF is attained by positioning the nonswitching clamped regions of the reference auxiliary signals according to the load PF. The effectiveness of the proposed scheme for a wide range of torque/speed variations is studied through simulation by developing a MATLAB/SIMULINK model of an IM traction inverter drive with field-oriented control strategy for closed loop control. Finally, the modulation scheme is validated through experimentation in a small-scale prototype NPC inverter with motor loads.

An Overall Minimized Switching Loss Discontinuous PWM Strategy for Neutral Point Clamped Three Level Inverters

Aim at reducing switching loss (SL) of neutral-point clamped three-level inverter (NPC TLI) as much as possible, a novel SL minimized discontinuous pulse width modulation (SLMDPWM) strategy is proposed in this paper. The core of SLMDPWM is that the specific maximum or middle phase current is clamped to positive bus, NP or negative bus according to modulation index and power factor angle. The realization of SLMDPWM based on carrier-based PWM (CBPWM) is revealed to simplify the calculation process. The performance characteristics of SLMDPWM and DPWM I-IV in term of the output voltage, SL and NP voltage ripple are compared by simulation and experimental results, and conclusion can be acquired that the SL of SLMDPWM is reduced significantly.

Subfundamental Cycle Switching Frequency Variation Based on Output Current Ripple Analysis of a Three-Level Inverter

The output current ripple of an inverter shows different variations for sub-fundamental operating cycle and over few fundamental cycles. This paper investigates subfundamental cycle peak-peak current ripple variation of a three-level three-phase inverter over the entire operating modulation index range for the three-level continuous and discontinuous pulsewidth modulation strategies. Furthermore, based on the analytical current ripple variation, a strategy is presented to change the switching frequency within a fundamental cycle while maintaining the peak-peak current ripple similar to the conventional fixed switching frequency strategies. The strategy has been implemented and experimentally validated on a permanent-magnet synchronous motor drive. The results demonstrate that the proposed strategy reduces inverter switching instances and associated losses around 10–30%.

Discontinuous PWM Scheme for Switching Losses Reduction in Modular Multilevel Converters

Discontinuous PWM Scheme for Switching Losses Reduction in Modular Multilevel Converters

Discontinuous Space Vector PWM Strategy of Neutral-Point-Clamped Three-Level Inverters for Output Current Ripple Reduction

In order to meet the high performance and efficiency requirements, a new space vector-based discontinuous modulation strategy for neutral-point clamped three-level inverter is presented in this paper. Different from the conventional discontinuous PWM (DPWM) strategies, a new clamping state is applied to synthesize the reference voltage under the condition of low modulation index. The rms value of output current ripple for different clamping states are calculated, and then the distribution of the new clamping state and the other two clamping states adopted in conventional DPWMs is optimized based on the calculation results. The switching sequences are redesigned according to the new distribution of the three clamping states. The output voltage and current, switching loss, neutral-point voltage ripple and the common mode voltage of the proposed DPWM strategy and DPWM0–DPWM3 are compared by simulation and experimental results. The results show that by the use of the proposed strategy, the values of total demand distortion of output current ( I TDD) and weighted total harmonic distortion of output voltage $({\boldsymbol{V}_{{\mathbf{WTHD}}}})$ are the lowest. Furthermore, the product of switching loss and $I_{\rm TDD}$ of the proposed strategy maintains the lowest within a large range of modulation index-load angle plane. It can be concluded that the utilization of the new clamping state and optimal distribution of different clamping states could enhance the output performance of the inverter. Moreover, the high-efficiency requirement can also be realized at the same time.

Direct Current Tracking Using Boundary Control With Second-Order Switching Surface for Three-Phase Three-Wire Grid-Connected Inverter

Cascaded boundary-deadbeat controller can ensure system stability of single-phase grid-connected inverter operated under different grid conditions. However, it will require using numerous sensors and may experience performance degradation caused by unbalanced filter parameters in controlling a three-phase three-wire inverter. Furthermore, the system is in variable frequency operation. A boundary controller using second-order switching surface with direct current tracking capability, reduced number of current sensors, and fixed frequency operation of three-phase three-wire grid-connected inverter with inductive-capacitive-inductive ( LCL ) filter is presented. By applying the 60° discontinuous pulsewidth modulation scheme for a fictitious decoupled dual-buck structure in each operation sector, two separate sets of switching criteria for dictating the states of the switches of two half-bridge legs are formulated. Such technique can avoid dealing with the challenges caused by the interactions among three independent current regulators. A switching table with null voltage vectors excluded is designed to dictate the switching actions. Sensitivities of the system transfer characteristics to the parametric variations are investigated. A 3 kW prototype has been built and evaluated under stiff- and weak-grid conditions. The experimental results are favorably compared with theoretical predictions.

Space-Vector-Based Generalized Discontinuous Pulsewidth Modulation for Three-Level Inverters Operating at Lower Modulation Indices

Conventional three-level discontinuous pulsewidth modulation (DPWM) templates typically provide maximum switching loss reduction capability for load power factor angles in the range from 30° lagging to 30° leading. This paper proposes three space-vector-based DPWM templates for lower power factors and a generalized DPWM strategy for three-level inverters operating with modulation indices lower than 0.5. For different power factors, the proposed strategy adapts the inverter pulse sequence by combining different portions of the proposed and conventional DPWM templates within one operating fundamental cycle and ensures minimum switching instances when changing templates. Consequently, the strategy aligns the PWM clamping intervals with the respective load-current peaks, achieving around 50% switching loss reduction capability extended to all operating power factor angles (90° lagging to 90° leading). The simulation and experimental results demonstrate the effectiveness of the proposed three-level generalized DPWM strategy over the conventional three-level modulation strategies.