Citation: Song, Y.; Zhang, Z.; Xu, Z.
Modular Combined DC-DC
Autotransformer for Offshore Wind
Power Integration with DC
Collection. Appl. Sci. 2022, 12, 1810.
https://doi.org/10.3390/app12041810
Academic Editor: Giovanni Petrone
Received: 22 November 2021
Accepted: 8 February 2022
Published: 10 February 2022
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Article
Modular Combined DC-DC Autotransformer for Offshore
Wind Power Integration with DC Collection
Yuanjian Song, Zheren Zhang * and Zheng Xu
Department of Electrical Engineering, Zhejiang University, Hangzhou 310058, China;
yuanjiansong18@163.com (Y.S.); xuzheng007@zju.edu.cn (Z.X.)
* Correspondence: 3071001296zhang@zju.edu.cn
Abstract:
Offshore wind farms (OWFs) integration are attractive extensively for furnishing more
robust power than land wind farms. This paper introduces a modular combined DC-DC autotrans-
former (MCAT), which contributes to the offshore wind power integration of DC grids with different
voltage levels. Traditional DC transformers contains medium- or high-frequency converter transform-
ers, which have the disadvantages of high manufacturing difficulty and cost. These shortcomings
seriously affect the progress of commercial application of DC transformers. To solve these problems,
in the proposed MCAT, converter transformers are replaced with a DC-isolation capacitor and a
compensation inductor in series to reduce the footprint of offshore platforms and improve economy.
Theoretical analysis is carried out for the MCAT operation principle. Selection methods of main
circuit parameters for the MCAT are discussed in detail. Then, corresponding control strategies of the
MCAT are proposed. Finally, the effectiveness of the proposed MCAT and its control strategies are
validated by time domain simulations in PSCAD/EMTDC. The time-domain simulation results show
the correctness of the main circuit parameters and the rationality of the MCAT control strategies.
Keywords:
offshore wind farm (OWF); DC collection grid; modular combined DC-DC autotrans-
former (MCAT); DC-isolation capacitor; compensation inductor
1. Introduction
Offshore wind energy has drawn wide interest due to its distinctive advantages, such
as rich wind resources, high utilization hours, and no need to occupy land resources [
1
].
Recently, China has proposed to build new power systems dominated by renewable energy,
which facilitates the rapid development of offshore wind farms (OWFs).
The high voltage direct current (HVDC) transmission system is preferred to the tra-
ditional AC transmission system for the grid integration of large-scale OWFs. In the
conventional HVDC transmission system for OWFs, wind turbines (WTs) are integrated
together with an AC collection grid and then connected to an offshore substation containing
a step-up transformer and a rectifier [
2
]. However, OWFs based on AC collection grids
need lots of AC cables, bulky step-up transformers, and large expensive offshore platforms.
To address this problem, OWFs based on DC collection grids are researched broadly [
3
,
4
].
In DC-collection-grids OWFs, WTs are interconnected with DC cables and connected to
DC-DC converters, and the low DC voltage is raised to the HVDC transmission voltage.
Compared with AC-collection-grids OWFs, DC-collection-grids OWFs could reduce the
footprint requirement and get the higher power transmission efficiency [5,6].
According to interconnection schemes of WTs, DC-collection-grids OWFs are classified
into DC parallel-connection and DC series-connection schemes [
7
], as shown in Figure 1.
In the series-connection scheme, WTs at two ends have to withstand a high DC voltage. In
addition, lots of WTs will be bypassed when unexpected faults happen. If the DC voltage of
the OWF falls below the threshold limit, the entire OWF will be shut down undesirably [
8
].
These defects definitely limit the application of DC series-connection scheme. Therefore,
the DC parallel-connection scheme is adopted in this paper.
Appl. Sci. 2022, 12, 1810. https://doi.org/10.3390/app12041810 https://www.mdpi.com/journal/applsci
