 A Simulation Study of the PWM Strategy for Inverters

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A Simulation Study of the PWM Strategy for Inverters
808-PowerLab. NCTU Power Electronic Systems & Chips Lab., NCTU, Taiwan A Simulation Study of the PWM Strategy for Inverters 鄒 應 嶼 教 授 國立交通大學 電機控制工程研究所 2014年11月21日 電力電子系統與晶片實驗室 Power Electronic Systems & Chips Lab. 交通大學 • 電機控制工程研究所 台灣新竹‧交通大學‧電機控制工程研究所‧電力電子實驗室~鄒應嶼 教授

A Simulation Study of the PWM Strategy for Inverters
Gate Driver Switching logic Two-level three-phase inverters are widely used in industrial applications, home appliances, and distributed power generation systems, such as motor drives, PV inverters, UPS, etc. We can make a study of characteristics of PWM strategy for a grid-connected three-phase inverter by using computer simulation. The load is a three-phase R-L load in series connected with a three-phase ac source. Natural sampling sinusoidal PWM is adopted. The modulation signals has a phase shift of 120º between each other.

Define the Load Parameters
Vdc = 400V Phase voltage of the three-phase utility (ea, eb, and ec): 110VRMS, 60 Hz La = Lb = Lc = 10 mH ra = rb = rc = 0.2 W Inverter switching frequency fs = 12 kHz Rated output power: 4 kW All power switches are assumed ideal. Make a study of the three-phase PWM inverter with natural sampling sinusoidal PWM.

Study Issues: PWM Principle & Power Factor Control
Illustration of PWM Principle: Set the modulation index with 0.75 and modulation frequency of 60 Hz, apply SPWM to the inverter Complete the simulation of (1) by using SPWM and show some selected variables shown in the following figure to illustrate the operation principle of PWM strategy. You may show its effect with a much lower switching frequency so that we can make observations of the corresponding relationships between the selected variables, such as the switching of the power switches and the corresponding current waveforms. Load Power Factor Control: Control the phase voltages in open loop so that the inverter output phase currents ia, ib, and ic will be in phase with the corresponding phase voltages ea, eb, and ec of the three-phase utility voltages. To study the loading effect, you may set the output phase current to be 1, 5, and 10 amperes, respectively. Make a description of the determination of the generation of the modulating signal and make a computer simulation verification.

1. Illustration of PWM Principle
1.0 90 180 270 360 -1.0 1.0 -1.0 2.0 -2.0 1.0 -1.0 The van is the voltage applied to the phase voltage with a common neutral of a Y-connected ac motor.

2. Load Power Factor Control
Keep phase current ia in phase with ea: To get the maximum output power with the minimum output voltage, the phase current must be in phase with the utility. Generate the phase output voltages va, vb, and vc so that the phase currents ia, ib, and ic will be in phase with the corresponding utility phase voltages ea, eb, and ec.

2. Load Power Factor Control
Calculate the magnitude of phase current at rated output power. where a is the angle between ea and ia1, for get the maximum output power with minimum output voltage, the phase current must be in phase with the voltage, therefore, a =0. ea represents the magnitude of the utility phase voltage. We need to generate a three-phase voltage with its phase-a voltage van1 has a leading phase angle of f relative to ea.

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