تحلیل جریان DC-Link برای اینورتر پل H تحت جریان های خروجی هارمونیک
Abstract
I- Introduction
II- Basic analysis of the dc-link current
III- Dc-link current low order harmonics computation methods
IV- Rms equations of the switching harmonic currents in the dc-link under low order harmonics in ac currents
V- Worst case design of the dc-link currents
VI- Simulation, experimental and computational results
VII- Conclusion
References
Abstract
This paper investigates the dc-link current to facilitate the dc-link capacitor sizing for the single phase H-bridge inverter under harmonic ac currents. As the capacitor equivalent series resistance (ESR) varies significantly until the frequency is higher than 1kHz, the expressions of the dc-link current individual low order harmonics (LOHs) and the switching harmonic currents (SHCs) root mean square (RMS) value are derived respectively to accurately evaluate the dc capacitor losses. The dc-link LOH expressions reveal that each harmonic ac current induces two harmonic currents in the dc-link. When the ac current has multiple combinations of LOHs, the dc-link current envelopes take infinite possible unusual forms, so after somewhat complicated derivation, the SHCs RMS equations are successfully derived and further simplified into one equation for easy worst case design as demonstrated by the actual case study. A single phase H-bridge inverter lab prototype is developed to inject arbitrary combinations of harmonic currents of different orders, amplitudes and angles into the grid. The good match between the computational, simulation and experimental results validate the effectiveness of proposed method. Particularly, the worst case analysis is verified by the experiment and the differences between the experiment and computation results are formulated and explained definitely.Abstract—This paper investigates the dc-link current to facilitate the dc-link capacitor sizing for the single phase H-bridge inverter under harmonic ac currents. As the capacitor equivalent series resistance (ESR) varies significantly until the frequency is higher than 1kHz, the expressions of the dc-link current individual low order harmonics (LOHs) and the switching harmonic currents (SHCs) root mean square (RMS) value are derived respectively to accurately evaluate the dc capacitor losses. The dc-link LOH expressions reveal that each harmonic ac current induces two harmonic currents in the dc-link. When the ac current has multiple combinations of LOHs, the dc-link current envelopes take infinite possible unusual forms, so after somewhat complicated derivation, the SHCs RMS equations are successfully derived and further simplified into one equation for easy worst case design as demonstrated by the actual case study. A single phase H-bridge inverter lab prototype is developed to inject arbitrary combinations of harmonic currents of different orders, amplitudes and angles into the grid. The good match between the computational, simulation and experimental results validate the effectiveness of proposed method. Particularly, the worst case analysis is verified by the experiment and the differences between the experiment and computation results are formulated and explained definitely.
Introduction
Sizing and rating of the capacitor is an important factor for the single phase H-bridge system design. Inaccurate evaluation of the dc-link current would result in capacitor oversizing and increase the system volume. It is well known that the dc-link current of the single phase inverter with the sinusoidal ac current contains a dc component, the 2nd order harmonic and switching harmonic currents (SHCs) with their sidebands components. The 2nd order harmonic current and the switching harmonic currents are absorbed by the dc capacitor. The dc voltage ripple is mostly determined by the low-order harmonic (LOH) currents in the dc-link, and the capacitor power losses are dependent on its equivalent series resistance (ESR) and harmonic currents in the dc-link. A variety of dc-link current analytical methods, either in the time domain (RMS method) or in the frequency domain (Double Fourier Series Analysis method) have been reported in the literature [1]-[7]. The RMS current analytic solution is introduced in [1]-[5], where the dc-link current RMS value is expressed in terms of the modulation index, power factor, and the amplitude of output current. As the inverter output is a pulse width modulated (PWM) voltage, the output current also contains SHC ripples, whose impacts to the dc-link current RMS value are also investigated. Paper [1] and [3] derive the dc-link current RMS value expressions for the three phase and the single phase inverters, respectively. In both [1] and [3], output currents are sinusoidal. Paper [3] further evaluated the influence of the output current PWM switching harmonics (SHC ripples) on the dc-link current RMS value. Herein, the double Fourier analysis is used to get each spectral components of the switching harmonics and then it is concluded in [3] that the SHC ripples impacts can be neglected when they are less than 0.3 times of the fundamental output current. Paper [2] demonstrates that the SHC ripples have very limited influence on dc-link current by numerous simulations.