DC-DC MMC for HVDC Grid Interface of Utility-Scale Photovoltaic Conversion Systems

Abstract

This paper explores a utility-scale photovoltaic (PV) plant configuration based on a dc-dc stage interfaced directly to a Modular Multilevel Converter (MMC) used for HVDC power station. Since PV systems are dc by nature, the proposed solution has several advantages such as full dc operation, step-up transformers avoidance with modular and reliable configuration. The contributions of this paper are the experimental validation of the proposed utility-scale configuration, while the conventional modulation stage has been further enhanced by including an adaptive phase-shifted modulation allowing improved voltage and current waveforms injected to dc-grid under unbalanced power string operation. The enhanced performance is obtained by reducing the carrier frequency component of the converter voltage generating an improved voltage ripple.

Existing System

Hardware-Based Cascaded Topology and Modulation, Modulation Techniques.

Proposed System

In this paper a single-phase MMC used for HVDC stations is adapted with an additional dc-dc stage for the USPV integration at string or multistring level as has been introduced. The corresponding control schemes are experimentally validated for a downsized USPV power plant. Despite its conceptual simplicity and advantages, the proposed configuration has some technical challenges inherited by the power distribution among the power cells, thus the carrier angles disposition must be comprehensively changed to reduce the output voltage ripple. To compute each carrier, angle an adaptive phase-shifted modulation is proposed and experimentally validated, giving arise to the main contribution of this paper. An USPV plant based on a dc-MMC has been presented. The proposed configuration consists of a two-stage dc-dc system combining the dc-dc MPPT stage with the HVDC substation converter. The proposed concept is particularly attractive for places where high solar irradiance conditions are located far away from consumption centers, or remote areas where no existing ac grid is available. The programmed control schemes are very straightforward and less complex than ac system which requires synchronization and coordinate transformations. This control scheme and its modulation are experimentally proved in a downsized setup with commercial solar modules and PV emulators under different irradiance conditions. In this fact, the proposed PST-PWM algorithm is a simple solution to perform high quality current waveforms injected to dc-grid when unbalanced power string operation is presented.

Architecture

Configuration of USPV farm with HVDC interface