The study of different unidirectional input parallel output series connected DC‐DC converters for wind farm based multi‐connected DC system

Abstract

Summary There is increasing interest in interconnecting individual wind turbines in offshore wind farms using DC networks rather than AC networks. However, the output voltage from each turbine/generator is normally low (less than 6 kV), so DC‐DC converters are required to step‐up the voltage to an intermediate level to interconnect the turbine outputs. Another DC‐DC converter is required to step‐up this intermediate voltage to a high level for transmission to the shore. The DC‐DC converter applied to step‐up the voltage from an intermediate voltage to a high level not only needs to operate with high voltage at both input and output, but also be capable of transferring hundreds of megawatts power. In these circumstances, Input Series Input Parallel Output Series (ISIPOS) DC‐DC converter structure is a good option for the application. In this paper, three different kinds of Input Parallel Output Series (IPOS) DC‐DC converters are presented and compared, and it is the first time this comparison has been made. Both the simulation and downscaled hardware test results are provided and analysed, while the best option in different situations is suggested.

The Effects of Filter Capacitors on Cable Ripple at Different Sections of the Wind Farm Based Multi-Terminal DC System

“…The voltage value of the receiving end DC Battery is about 12.5 V. The length of the multi-connected cable is selected to be 2 m in the experiment. Based on the study of the IPOS converter in [8], the switching frequencies of the converters in the downscaled model are about 30 times higher than that in the simulation model, so the length of the cable in the experiment should be selected as 600 m/30 = 20 m. However, 20 m of cable at each section is not easily achieved, and can cause a significant voltage drop due to the low voltage level in the experiment. According to the status quo, the length of the cable is reduced further to 2 m, which will not affect the performance of the filter capacitors in the system when different results are compared.…”

“…Instead, a 15 MW system with three sub-blocks connected onto a single 50 kV cable shown in Figure 2 was modelled to evaluate the effect of the filter capacitors. The reduction of the number of sub-blocks does not influence the property of the filter capacitors at different positions, while the use of an Input Parallel Output Series (IPOS) converter can also reflect the performance of the ISIPOS converter in the system [8]. Figures 3 and 4 show the circuit diagram of the 5 MW DC-DC converter and three different structures for a 15 MW IPOS DC-DC converter, respectively.…”

“…Figures 3 and 4 show the circuit diagram of the 5 MW DC-DC converter and three different structures for a 15 MW IPOS DC-DC converter, respectively. The study of the converters themselves is detailed in [8,9]. In the system model, different values of the output capacitors of the 5 MW DC-DC converters (C o in Figure 3) and of the input capacitors of the IPOS converters in Figure 4 are simulated to monitor their effects on the ripple content of the voltages and AC losses caused by the ripple content of currents in the multi-connected section of the system.…”

Input parallel–output parallel connected modular nonisolated DC‐DC converters with current self‐sharing capability operating in discontinuous conduction mode

“…This paper will address the concept of self‐sharing in modular systems composed of nonisolated converters in DCM and in IPOP configuration, which can be used in dc‐dc sources and renewable systems. The mains contributions of this paper are highlighted as follows: Proposed and analyzed nonisolated dc‐dc converters (the literature just has isolated topologies 12,14,17,19,23,31–35 ); Self‐sharing of the currents among the modules without extras control loops; Proposal based on conventional converters; Simple implementation; Increase the power range of the conventional converters; The number of modules can be adjusted depending the applications and specifications. …”

“…• Proposed and analyzed nonisolated dc-dc converters (the literature just has isolated topologies 12,14,17,19,23,[31][32][33][34][35] ); • Self-sharing of the currents among the modules without extras control loops;…”