Compensation of DC-Link Oscillation in Single-Phase to Single-Phase VSC/CSC and Power Density Comparison
This study proposes a technique to reduce the low-frequency DC link energy oscillation that occurs due to pulsating single-phase energy flow in a single-phase to single-phase voltage source converter (VSC) and current source converter (CSC) which operate connected to the grid. That pulsating energy is the major contributor to an increase in size of the passive components and the power losses in the converter. The pulsation reduction is achieved by incorporating two auxiliary active switches and one passive energy storage element to the rectifier and inverter side. Here, they will constitute the oscillating power compensator. The energy storage element will be responsible for absorbing and delivering the oscillating power, which is naturally required by a single-phase converter. The proposed (VSC/CSC) single-phase to single-phase converters, with power compensators, are presented in Fig. 1. Fig. 1(a) illustrates the topology for the VSC. In this case, the energy storage elements are the inductors LRr and LRiFor the CSC, shown in Fig. 1(b), the storage elements are the capacitors CRr and CRi. Additionally, these convertes are compared in terms of efficiency and power density.
In Fig. 2 are a comparison and simulation results for the converters. The volume comparison results are shown in Fig. 2(a). The inductor size is mainly determined by maximum flux density and loss. Different ripple energy and supply frequency applications will lead to different materials to achieve minimum volume. In terms of energy density, the film capacitor is more compact for low ripple energy, high-frequency applications. Otherwise, an aluminum capacitor would be better. For most of the application range, the inductive ripple energy storage method is bulkier and heavier than capacitive energy storage.
For the VSC, shown in Fig. 2(b), the system operates at 1kW with a Vdc*=320V, and 110V/50Hz and 220V/60Hz for the rectifier and inverter sides, respectively. For the CSC, shown in Fig. 2(c), the system also operates at 1kW with an Idc*=14A, and 220V/60Hz and 110V/50Hz for the rectifier and inverter side, respectively. In Figs. 2(b) and 2(c), (top), are shown the Vdc*/Idc* for the VSC/CSC using a bus capacitor of 40µF (VSC) and a bus inductor of 30mH (CSC). With the DC capacitor and inductor values used, a DC ripple of 3.77V (VSC) and 0.22A (CSC) were reached. To obtain the same ripple seen for the proposed system, it would be necessary a capacitor Cdc of 2,200µF (VSC) and a inductor Ldc of 1,400mH (CSC) in a conventional converter. Thus it was possible to achieve a reduction of 55 in the DC capacitor of the VSC and 46.67 in the DC inductor of the CSC. In Figs. 2(b) and 2(c), (bottom), are shown the waveforms of the signals in the AC side of the VSC and CSC, respectively. It is possible to observe that it was reached the unity power factor for the current iacr provided for the grid and a good waveform for the Vaci.