The Low DC-Link Capacitance Design Consideration for Cascaded H-Bridge STATCOM

Abstract

To achieve low cost and high power density, low dc-link capacitance is preferred in the Cascaded H-Bridge (CHB) STATCOM. However, low dc-link capacitance will result in the increase of dc-link ripples, which will introduce voltage harmonics through modulation. According to the operation principle of the inverter, the output voltage will not exceed the input voltage at any time. Hence, the output voltage range of STATCOM will be affected by large voltage oscillation across the DC link, leading to affect the output capacity of STATCOM. Thus, to guarantee operation range of STATCOM, system capacity should be a key design consideration in the design of low dc-link capacitance, which has not been taken into consideration in the existing design methods. To address this issue, this paper establishes the relationship between the range of compensable current and the dc-link capacitance based on the Kirchhoff's law. Based on this relationship, a new design consideration is proposed to achieve the minimum dc-link capacitance, which is as small as possible under certain capacity of STATCOM. The designed capacitance can satisfy the requirements of system capacity and support the proper operation of STATCOM. Meanwhile, the designed capacitance is small enough to achieve low cost and high power density. A 400V/7kVA STATCOM prototype is built to verify the derived relationship and the proposed design method.