Abstract
In this paper, a modular multiple DC (MMDC) transformer based DC transmission system, which is used for permanent magnetic synchronous generator (PMSG) based offshore wind farm grid connection, is proposed. The MMDC transformer is composed of input-parallel output-series (IPOS) DC/DC submodule groups, each of which consists of several input-series output-series (ISOS) LLC resonant converters (i.e., submodules). In the offshore wind farm, the electrical power generated by PMSG is converted to DC power by diode rectifier. Compared with the existing offshore wind farm grid-connection schemes, the proposed system has advantages of fewer conversion stages, higher efficiency and lower cost. The mathematical relationship of the system is studied, and then one kind of control strategy is proposed. Furthermore, a detailed DC transmission system simulation model containing a 160 submodules (LLC resonant converters) based MMDC transformer is built in PSCAD/EMTDC. Finally, a down-scaled experimental prototype of the DC system, which includes 9 submodules, is presented. Both simulation and experimental results validate the feasibility of the proposed DC system and effectiveness of the control strategy.
Highlights
As the most mature renewable energy source (RES), more than 500 GW of capacity has been installed until year 2019
The proposed Modular Multiple DC (MMDC) transformer based DC transmission system for Permanent magnet synchronous generator (PMSG) based offshore wind farm integration is shown in Fig.5 (a), which consists of one or several offshore wind power generation units (WPGU), an onshore modular multilevel converter (MMC), high voltage direct current (HVDC) bus and cables
EXPERIMENTAL RESULTS To further verify the proposed topology and control strategy, a down-scaled modular multiple DC (MMDC) transformer based DC transmission system is built in laboratory
Summary
As the most mature renewable energy source (RES), more than 500 GW of capacity has been installed until year 2019. Some scholars proposed to use different types of HVDC converters for the grid-connecting of offshore wind farms, and their control strategies were studied as well. The proposed Modular Multiple DC (MMDC) transformer based DC transmission system for PMSG based offshore wind farm integration is shown in Fig. (a), which consists of one or several offshore wind power generation units (WPGU), an onshore modular multilevel converter (MMC), HVDC bus and cables. The multiple series and parallel connection configuration increases the withstanding voltage and flowing-through current of the MMDC transformer, making it suitable for high-voltage and high-power applications, such as offshore wind power integration. B. TOPOLOGY SELECTION OF MODULAR DC/DC CONVERTER For the PMSG and diode rectifier based offshore wind farm integration application, the main features are high voltage and high power, unidirectional power transfer and output voltage variation. According to eqns. (13) and (16), the input voltage and power of the LLC submodule are dependent on the excitation electromotive force (es)
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