Analysis and characterization of DC Bus ripple current of two-level inverters using the equivalent centered harmonic approach

Abstract

The dc bus PWM ripple current of three-phase two-level voltage source inverters is a function of the PWM method, the load current magnitude, power factor angle, and the modulation index. Thus, the ripple current characteristics are highly involved and difficult to understand. Using the double Fourier integral approach, this paper investigates the ripple current characteristics thoroughly for a wide range of operating conditions and PWM methods. Then, the equivalent harmonic approach is used to lump the ripple current carrier frequency component and its sidebands to the center as a single frequency equivalent (and the same done for the multiples of the carrier). With this approach the capacitor ESR based losses are predicted easily and the dominant frequency range can be understood better. Thus, a better inverter and dc bus capacitor design can be achieved. Analysis is supported by means of simulations and a high degree of correlation is obtained. The paper additionally evaluates and suggests PWM methods for various inverter applications with the dc bus ripple performance being the main constraint. Thus, it is beneficial for the design engineer.

Analysis of dc-Link Voltage Switching Ripple in Three-Phase PWM Inverters

“…When the load is balanced, the low-frequency component is zero. Consequently, the inverter input current only contains the dc (average) I = Idc, which comes from the dc supply, and the high-frequency component ∆i(t) [15], which is bypassed through the dc-link capacitor. Thus, the instantaneous input current can be expressed as: For a PWM inverter, by neglecting the dc-link voltage oscillations (v ≈ V) and by considering only a linear modulation range, the inverter output phase voltages averaged over the switching period correspond to the reference phase voltages:…”

“…When the load is balanced, the low-frequency component is zero. Consequently, the inverter input current only contains the dc (average) I = I dc , which comes from the dc supply, and the high-frequency component ∆i(t) [15], which is bypassed through the dc-link capacitor. Thus, the instantaneous input current can be expressed as:…”

“…The analytical expression for the current stress on the dc-link capacitor and the experimental verification has been presented in [13] for a three-phase converter system. Based on the Fourier analysis and considering the three-phase balanced load, the dc-link current ripple characteristics have been investigated in [14][15][16]. Similar analyzes under balanced and unbalanced loads are presented in [17], including an investigation of the second harmonic dc-link voltage ripple component.…”

Investigation into the control methods to reduce the DC-link capacitor ripple current in a back-to-back converter

“…Actually, the baseband harmonics of the DC link current are much smaller than the carrier frequency and carrier sideband harmonics, therefore they are neglected. It should be noted that the RMS value of carrier-sideband group can be calculated at the carrier frequency multiplier since their frequencies are quite close [13], but this is only valid for the DC capacitor current in a single stage converter and not the BTB converter.…”

“…The research focusing on the carrier frequency ripple current has therefore be conducted [9][10][11][12][13][14][15]. The RMS value of the DC capacitor current is derived for a AC-DC-AC system with a diode rectifier in front [9].…”