Ziyou Lim

Modular-Cell Inverter Employing Reduced Flying Capacitors With Hybrid Phase-Shifted Carrier Phase-Disposition PWM

This paper proposes a new approach of modular-cell inverter with a reduced number of flying capacitors (RFCI) using hybrid carrier-based pulsewidth modulation (PWM) named as phase-shifted carrier phase-disposition PWM (PSC-PD-PWM). The novel multilevel inverter topology is designed by clamping a series of modular cell onto a low-operational-frequency cell. The proposed transformerless RFCI reduces significantly the component counts, cost, and size of the converter. The overall performances of RFCI are greatly enhanced as compared with the classical multilevel flying capacitor inverter (MFCI). Hence, a comparative analysis of the proposed RFCI is made against the classical MFCI in this paper based on seven-level output phase voltages. The feasibility of the RFCI with PSC-PD-PWM is validated through experimental results under dynamic operating conditions.

Five-Level Multiple-Pole PWM AC–AC Converters With Reduced Components Count

This paper proposes novel multilevel ac/dc/ac converters with reduced number of semiconductor devices to achieve light weight, efficiency, and better input current quality. The proposed three-phase front-end bidirectional five-level (5L) rectifier and rear-end 5L inverter are constructed based on multiple-pole multilevel diode-clamped (M2DCC) approach, which consist a total number of 24 diode components clamped. Based on the M2DCC concept, the number of switching devices can be further reduced to construct a new unidirectional front-end rectifier (M2SCR). Comparative studies are conducted to analyze the performances of the two proposed front-end rectifiers with supportive mathematical derivations. Experimental prototypes have proven the feasibility of the proposed ac/dc/ac configurations. Results obtained show that low current harmonic distortions of 5% and high power factor of 0.99 are achieved with small input inductors despite low-switching-frequency operation at 1 kHz.

NPC photovoltaic grid-connected inverter with ride-through capability under grid faults

Fault ride through (FRT) capability is one of the challenges faced in the medium to high voltage grid-tied large-scale photovoltaic (PV) power plants. This paper proposes a novel control strategy to enhance the FRT capability of a two-stage multi-string PV plant which consists of DC/DC converters and NPC inverter. The proposed control is implemented to the DC/DC converter to maintain the DC-link voltage under any grid fault conditions by adjusting the extracted PV power to the inverter output power. This is achieved by changing the PV string reference voltage from its maximum power point voltage to a new voltage point with less power. On top of that, a novel voltage-oriented control (VOC) control structure using proportional-resonant (PR) controller is proposed for the NPC inverter. Additionally, the DC-link capacitor voltages are remained balanced at all times due to the adaptive space vector modulation (ASVM) scheme. The evaluation results have verified the feasibility and the FRT capability of a 14kW PV plant using the proposed control scheme under different grid fault conditions.

Proportional-resonant controlled NPC converter for more-electric-aircraft starter-generator

More electric aircraft (MEA) technology is achieved by exchanging various mechanical and pneumatic elements of a conventional aircraft with their equivalent electrical devices in order to increase the reliability and decrease the maintenance. This paper proposes a proportional resonant (PR) controller together with the adaptive space vector modulation (ASVM) technique for the more electric aircraft (MEA) starter-generator neutral-point clamped (NPC) converter. The proposed controller is implemented in the stationary (αβ) frame where the calculated reference voltages of the PR controller can be directly fed into the ASVM. Hence, the main objectives of the proposed controller are to reduce the computational complexity and the steady state error by decreasing the required number of framework transformations units. On top of that, the ASVM technique provides a proper DC-link capacitor voltage balancing with improved output power quality. The dynamic performance of the proposed controller is evaluated under both initial starting interval and generating mode.

Common-Mode Reduction for ANPC With Enhanced Harmonic Profile Using Interleaved Sawtooth Carrier Phase-Disposition PWM

Multilevel active neutral-point-clamped (ANPC) converter is popularly known for its higher medium voltage drives up to 6.9 kV. However, common-mode voltages (CMV) are generated when using the well-established carrier-based pulse width modulation (PWM) modulation technique. As a result, CMV will become the major cause of the premature bearing failures and the conducted electromagnetic interferences. In this paper, a reduced CMV strategy is proposed for the ANPC which employs an alternatively new hybrid carrier-based modulation named as interleaved sawtooth carrier phase-disposition PWM (ISC-PD-PWM). By doing so, enhanced harmonic profile is achieved as compared to the typically used classical phase-shifted carrier PWM (PSC-PWM). Therefore, a comparative evaluation of the seven-level (7L) ANPC performance using the proposed reduced CMV approach is made between the novel hybrid ISC-PD-PWM and the conventional PSC-PWM in terms of harmonic analysis. The experimental results have proven the feasibility of the proposed concepts with the CMV effectively reduced. Such perceived merits make ANPC be more attractive for its motor drives.

Study on the unbalanced current injection capability of grid-connected photovoltaic neutral-point-clamped inverter

Due to high penetration of distributed generation units in power system, fault ride through (FRT) capability is one of the new requirements of the medium-scale grid-tied photovoltaic power plants (PVPPs). This paper proposes and investigates a control strategy for the neutral-point-clamped (NPC) inverter in order to inject proper unbalanced reactive currents to the grid during unbalanced grid faults. The proper unbalanced current injection reduces negative sequence of grid voltages and currents. The current references are scaled up/down individually, based on the grid phase rms voltages and inverter nominal current. The performance of the implemented control algorithm is investigated on a 150-kVA PVPP connected to 12.47-kV medium-voltage grid simulation model under various voltage sag conditions. Results from an experimental setup of grid-tied NPC inverter are presented in order to demonstrate the effectiveness of the proposed unbalanced current injection algorithm.

Voltage balancing control for reduced flying capacitor converters using hybrid phase-shifted carrier phase-disposition PWM

This paper proposes a new approach of modular cell converter with reduced flying capacitors (RFCC) using hybrid phase-shifted carrier phase-disposition PWM (PSC-PD-PWM). The proposed topological design is achieved by clamping a series of modular cell onto a low operational frequency cell. By doing so, the transformerless RFCC significantly reduces the component counts, cost and size. The voltage balancing of the flying capacitors are importantly concerned which may affect both reliability and stability of the system. Thus, a closed-loop flying capacitor voltage balancing control is proposed as well. The hardware prototype is also developed to prove the feasibility of the proposed RFCC and the voltage balancing control based on the seven-level (7L) pole voltage waveforms under various dynamic operating conditions.