A Grid-Forming HVDC Series Tapping Converter Using Extended Techniques of Flex-LCC

TL;DR

This paper introduces the Extended Flex-LCC (EFLCC), a novel grid-forming HVDC series tapping converter that integrates advanced control and design techniques to improve efficiency, reduce costs, and facilitate renewable energy integration.

Contribution

The paper presents an innovative extension of the Flex-LCC technology, transforming it into a more efficient, cost-effective GFM HVDC converter suitable for renewable energy sources.

Findings

EFLCC exhibits dc characteristics similar to CSCs and ac characteristics of GFM VSCs.

EFLCC requires fewer switches and less energy storage, reducing losses and costs.

Experimental and simulation results confirm the theoretical analysis.

Abstract

This paper discusses an extension technology for the previously proposed Flexible Line-Commutated Converter (Flex LCC) [1]. The proposed extension involves modifying the arm internal-electromotive-force control, redesigning the main-circuit parameters, and integrating a low-power coordination strategy. As a result, the Flex-LCC transforms from a grid-forming (GFM) voltage source converter (VSC) based on series-connected LCC and FBMMC into a novel GFM HVDC series tapping converter, referred to as the Extended Flex-LCC (EFLCC). The EFLCC provides dc characteristics resembling those of current source converters (CSCs) and ac characteristics resembling those of GFM VSCs. This makes it easier to integrate relatively small renewable energy sources (RESs) that operate in islanded or weak-grid supported conditions with an existing LCC-HVDC. Meanwhile, the EFLCC distinguishes itself by requiring fewer full-controlled switches and less energy storage, resulting in lower losses and costs compared to the FBMMC HVDC series tap solution. In particular, the reduced capacity requirement and the wide allowable range of valve-side ac voltages in the FBMMC part facilitate the matching of current-carrying capacities between full-controlled switches and thyristors. The application scenario, system-level analysis, implementation, converter-level operation, and comparison of the EFLCC are presented in detail in this paper. The theoretical analysis is confirmed by experimental and simulation results.