Abstract
This paper proposes two novel dc/dc choppers with dc circuit breaker functionality for HVDC transmission lines. These converters feature step-up and/or step-down functions with instant interruption of short-circuit faults. Compared to other state-of-the-art converter topologies described in the literature, the proposed topologies have better control functions, lower manufacturing costs and reduced energy losses. A double closed-loop controller is designed to regulate the inductor current and output voltage of the converters. An auxiliary controller is also proposed to ensure proper shut-down of the circuit breaker in the event of dc short-circuit faults. The simulation and experimental results are presented to validate the effectiveness of the proposed converter configurations and control scheme.
Keywords
• HVDC circuit breakers;
• dc/dc chopper;
• HVDC transmission;
• Multi-terminal HVDC;
• Closed-loop control
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1. Introduction
1.1. Research objective
As prices of fossil fuels rise and as concern about the dangers of global warming grows, electricity production from renewable energy sources such as wind and photovoltaic (PV) energy conversion systems is a subject of great interest nowadays. Many megawatt-level PV plants and offshore wind farms have been installed around the globe and . The high voltage direct current (HVDC) transmission is a promising solution to integrate the power generated from these renewable energy sources into the existing power grid. The technological advancements in power electronics have enabled the lower cost and higher performance operation of HVDC transmission lines and .
Earlier HVDC transmission lines were mainly configured as two terminal systems with limited applications, and recently there has been significant interest for multi-terminal HVDC and HVDC grids , and . They allow integration of many energy sources and the tapping of industrial and consumer loads and . A typical multi-terminal HVDC system with such a flexible power integration and allocation is shown in . The important technical and operational requirements for multi-terminal HVDC system are summarized below:
•Depending on the different power requirements, the tapped loads demand step-up or step-down of the transmission-level dc voltages. When the tapped loads need more power, the output voltage should be forced to step-up, and when some of the tapped loads shut down, the output voltage should be forced to step-down. For example, a 200 kV dc voltage should be stepped-up to 300 kV or stepped-down to 100 kV.
•In the event of short-circuit fault at the low voltage terminal of the HVDC transmission line, the main HVDC transmission lines should be isolated from the fault instantly to ensure the whole grid safety . To fulfill the above two important requirements, dc/dc choppers as depicted in are used. They perform step-up and step-down functions in addition to the dc circuit breaker (DCCB) functionality.