Dc-dc converters for renewable energy integration with medium-voltage dc grids

Abstract

Continuous rise in global energy consumption coupled with the growing environmental and health concerns necessitate power generation using clean renewable energy sources such as solar photovoltaic (PV) and wind. With large-scale integration of renewable power generation, battery energy storage, and dc loads such as electric vehicles in the power system, employing medium-voltage dc (MVDC) grids as the power exchange medium is beneficial over traditional ac grids. Voltage step-up dc-dc converters with high-frequency isolation are the key interfacing elements between renewable energy sources and MVDC grids, and current-fed full-bridge zero-current-switching (FB-ZCS) dc-dc converters have strong application potential. However, the circuit elements used to realize soft-switching often restrict the converter operation range.

Thirdly, a new FB-ZCS dc-dc converter with series-connected resonant capacitor is proposed to address the limitations with shunt-connected resonant capacitor. Series-capacitor charges during energy transfer from input to output, which eliminates separate capacitor charging interval and significantly minimizes duty-cycle loss. Current-dependent adaptive variation of resonant energy is also achieved, as the series-resonant capacitor is directly in the path of input current. However, at light-load conditions with reduced input current, the capacitor cannot charge up to sufficient voltage required for successful L-C resonance. This issue is addressed by adopting a new control strategy called hybrid phase-shift frequency modulation. At light-loads, the switching frequency is reduced in steps to increase the charging time available for resonant capacitor.

Shunt-connected resonant capacitor has the drawbacks of constant full-load-rated resonant energy at all load conditions and large duty-cycle loss at light-loads. Series-connected resonant capacitor has the drawbacks of increased peak-voltage stress across primary-side components and insufficient resonant capacitor voltage at light-loads. To simultaneously address these limitations, a dual-capacitor resonant circuit based FB-ZCS dc-dc converter is finally proposed. Shunt-capacitor is rated for a fraction of full-load-rated resonant energy, which reduces duty-cycle loss and maintains soft-switching at light-loads to support the series-capacitor. Shunt-capacitor also adds an additional degree of freedom in resonant circuit design, resulting in reduced voltage requirement in series-capacitor to satisfy the resonant condition and reduced peak-voltage stress across primary-side components. Proposed solution utilizes benefits from both the capacitor configurations, resulting in reduced duty-cycle loss as well as reduced peak-voltage stress simultaneously.

Steady-state operation analysis, design, modeling, and control aspects are presented for all proposed converters and successfully validated by simulations and experiments.

FB-ZCS dc-dc converters achieve soft-switching and smooth current commutation by utilizing quasi-resonant operation between the leakage inductance of transformer and a resonant capacitor shunt-connected across the transformer-secondary. In every switching half-cycle, the resonant capacitor is charged to its full-voltage irrespective of converter loading by nature of its shunt-connection. This requires a dedicated charging interval which results in duty-cycle loss. As the converter loading reduces, duty-cycle loss increases as the reduced input current takes longer time to charge the resonant capacitor to its full-voltage. To satisfy the resonant condition for a given leakage inductance, a larger resonant capacitor will be required for higher current ratings and the charging interval of this capacitor results in significant duty-cycle loss, restricting the operation range of converter.

The objective of this thesis is to address these limitations through new converter architectures, topologies, and control techniques. Proposed solutions emphasize on simplicity without using additional switches or complex auxiliary circuits. Firstly, an FB-ZCS dc-dc converter with voltage-doubler output is proposed, which results in reduction of voltage stress across converter components and input current ripple by 50%. This idea is subsequently used in all the proposed converters. Secondly, an input-parallel output-parallel (IPOP) converter configuration is proposed, with proposed FB-ZCS dc-dc converter as the fundamental unit. With a single centralized converter for high-power applications, operational and design constraints imposed by the large resonant capacitor result in significant duty-cycle loss, restricting the operation range. Reliability concerns are also higher with a single centralized converter. An IPOP configuration is proposed to address these limitations - Phase-shedding at reduced loading ensures that individual modules are operated optimally and interleaving of input current reduces the filter inductance requirement.