**1. Introduction**

Deployment of variable renewable energy resources are technical solutions driving global climate change. In order to sharply decrease the carbon emission and accelerate the global energy transition [1], wind power has experienced a rapid development in the last 20 years, which has become the mainstream renewable energy around the world now [2]. In 2019, China maintains the first place in terms of cumulative installed capacity of wind power and is vigorously promoting wind power on a priority basis [3]. Compared with onshore wind power, offshore wind farms have much less negative impacts on humankind as no land resource is needed, which also makes them usually have a larger scale and the offshore turbines have a larger capacity, which means a fall in the capital costs [4].

Because of the above advantages, plenty of studies have been conducted in the cost assessment area of the offshore wind farms (OWFs), which concentrates on the cost evaluation methodologies, potential economical technologies, and cost reduction. The infrastructure costs of OWFs are strongly related to the spatial condition [5,6]. Myhr et al. presented a cost sensitivity analysis and pointed out that the results suffer significant spatial bias and may differ in various countries [7], such as spatially-explicit assessment for the United Kingdom (UK) [8,9], Australia [10], Thailand [11], India [12], and Nigeria [13]. Thus, a Geographic Information System (GIS) makes costs and energy potential estimations possible based on spatially clustered data [14,15]. To obtain the cost reduction potential, the GIS-based levelized production cost (LPC) methodology is a common analysis model [16–18]. Furthermore, some assessments take the impacts of marine ecosystem and weather or climate variance into consideration [19,20].

**Citation:** Jiang, Q.; Li, B.; Liu, T. Tech-Economic Assessment of Power Transmission Options for Large-Scale Offshore Wind Farms in China. *Processes* **2022**, *10*, 979. https:// doi.org/10.3390/pr10050979

Academic Editors: Eugen Rusu, Kostas Belibassakis and George Lavidas

Received: 11 April 2022 Accepted: 11 May 2022 Published: 13 May 2022

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**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/).

The costs of the OWFs are more expensive than the onshore farms due to its complex foundation, installation, and submarine cabling; with the construction of the marine economy, the transmission vehicle becomes an important part [21]. Furthermore, the costs of different transmission methods are distance- and capacity-dependent functions [22], because the required diameter and number of cables are capacity-resolved, especially for projects with GWs capacity, and there exists a "breakeven" distance [23]. Two prevailing approaches are conventional: alternating current (AC) transmission, which is effective for near shore farms [24], and extensive voltage source converter-based high-voltage direct current (VSC-HVDC) transmission, which is the preferred solution for long-distance transmission. The HVDC transmission technology has many advantages, such as a fast power control speed [25] and oscillation damping control [26], and can be used in ultra-high-voltage occasions [27–29]. However, the HVDC implementation also has disadvantages. The first disadvantage compared to AC transmission is the cost, as the VSC is based on so many IGBT components that finally lead a relatively high investment. The second disadvantage is the stability problem, where the VSC often suffers from oscillation risk, especially when the power fluctuates. The third disadvantage is that the IGBT is very sensitive to the fault current, and it requires a fast protection scheme. However, the advantage of VSC makes it still be suitable for offshore wind farm integration. Offshore wind power is often located in the far sea area and the transmission cable also decreases the fault possibilities. The same as with HVDC transmission, a gas-insulated transmission Line (GIL) provides another way due to its advantage of considerably larger capacity, but its exaggerated expense makes it less competitive. Consequently, VSC-HVDC becomes more eye-catching for investors with predominant capability and desirable loss, which is suitable for crossing long-distance water transmission, such as in the North Sea of Germany [30]; but, the terminal converter stations are more expensive. However, the choice of electrical transmission ultimately depends on both technological potential and economic potential [31]. For future technical development of OWFs, there is another competitive option—hybrid VSC/LCC-HVDC technology—which is a novel form of HVDC transmission not widely applied, but it greatly decreases the costs and is planned for use China, possessing huge technological potential.

In China, there are many large-scale blocks with a capacity of more than hundreds of MW planned for OWFs [32]. The existing research has mostly focused on a single offshore wind farm project [33], lacking the overall research on regional offshore transmission systems [34]. There is a need to explore the optimal technical transmission method of the regional offshore transmission network for wind farms, which is conducive to wind energy utilization. This paper conducted regional cost analysis and economic feasibility comparisons of four electricity transmission options for offshore wind power in Guangdong Province using component-resolved evaluation models. Economic costs and sensitivity have been derived using the Discounted Cashflow Model (DCF). This contributes to determining the reasonable scope of technical and economic application of various transmission modes, giving perceptible information for stakeholders for offshore wind transmission infrastructure under indigenous development, economic perspective of relevant technologies, and possible potential to future deployment and implementation of marine projects.

To make clear the characteristics of the different wind power transmission technologies, this paper compares various offshore wind farms with the HVAC, HVDC, GIL, and hybrid HVDC output channels. The novelties of the paper are as follow.


This paper is structured as follows: The study area and technical potentials of the four transmission methods are introduced in Section 2. Section 3 proposes specific evaluation models of the various transmission solutions. Then, the techno-economic costs and sensitivity to transmission distance and capacity analysis results are investigated using the DCF approach in Section 4. The conclusions and areas of further work are discussed in Section 5.

#### **2. Study Area and Methodology**

#### *2.1. Study Area*

Guangdong is located on the eastern coast of China, which is rich in offshore wind energy; it is also the core area of economic development in southern China. To reach the new goal of deployment of the Guangdong–Hong Kong—Macao Greater Bay area (GBA). It has made great efforts to develop offshore wind power, which is effective to adjust coastal resources in line with a prosperous economy. By 2030, more than 1000 km of transmission lines will be built for grid connections for offshore wind power. For this trend of future planning of OWFs in Guangdong, policymakers are concerned with the cost assessments of efficient electrical transmission options to transport large quantities of offshore energy across great distances.

Offshore wind resources of Guangdong Province are in western and eastern Guangdong. Based on the Notice of Guangdong Development and Reform Commission on Guangdong offshore wind power development plan (2017–2030), 15 offshore wind farm sites are located in the offshore shallow water area, and 8 sites are in the offshore deep-water area. Yangjiang city is the closest with a stable wind power supply base in the west to the GBA. There are three regions for offshore wind farms in the plan: Nanpeng Island OWFs, Hailing Island OWFs, and Shapa OWFs. The total planned installed capacity of the renewable energy is about 36 GW, as indicated in Figure 1.

**Figure 1.** Overview of the study area.

#### *2.2. Technical Evaluation of the Transmission Solutions*

Four transmission solutions are studied in this paper, as shown in Figure 2. The offshore wind power from each farm is collected and transmitted to the offshore stepup transformer station. Then the voltage will be raised and the electrical power will be delivered to the onshore step-up transformer station via a submarine high-voltage transmission line (AC/DC cable or GIL line) and delivered to the onshore booster station.

**Figure 2.** Structure of the four transmission methods for OWFs.

As mentioned above, AC transmission is widely used in near sea OWFs, compared with others. The distributed capacitance of the AC cable will become larger and larger, and the ampacity will decrease with the increase in length. This significantly reduces the transmission ability; also, multi lines are needed to transfer the large amount of wind power, which means more investment cots in the AC cable. In addition, due to the close electrical connection between the wind farm and the onshore power grid, the fault of either side will quickly spread to the other side; this will cause voltage oscillation and power instability, which reduces the power quality. It is necessary to install dynamic reactive power compensation devices to improve the stability and available transfer capability. A DC cable is cheaper and able to transfer more capacity with lower loss, which is popular in OWF transmission, but an offshore converter station needs to be assembled and a large DC platform should be built for it, which makes the economic investment of VSC-HVDC higher in the early infrastructure. However, it is convenient to build and expand by stages, and the asynchronous connection to onshore grid can suppress the synchronous transmission of faults.

With regard to GIL lines, as derived from GIS, GIL only needs to have basic electrical performance, such as insulation and dynamic thermal stability, and there is no switchgear; it thus has obvious reliability advantages over either AC/DC cable or overhead lines in long-distance and large-capacity power transmission. However, the high costs and high technical requirements of the construction design and the long project period are difficult problems for the actual project. In China, a new hybrid DC transmission mode combines the superior performance of LCC-HVDC and VSC-HVDC technologies and has a lower cost than current VSC-HVDC transmission. Yet, the available transmission power is determined by the VSC-HVDC side, and it is hard to realize power flow reversal due to the voltage polarity that needs to be changed in the LCC converter station. Still, it is a new trend of innovation and becomes an alternative for offshore wind power transmission though it has not been applied due to a lack of research, except in China. In summary, the technical potential of the four methods is in Table 1.


**Table 1.** Technical comparisons of the four transmission options.

#### **3. Methodology**

The costs evaluation can be broken down into multiple components, such as sitedependent variables, fixed water depth, the distance to grid connection point, and fixed costs [35]. Total investment cost equals the summation of the capital cost components, calculated as suggested by Dicorato et al. [35] and Hong and Möller [14,33].

The methodology establishes an empirical component-resolved evaluation model from an industry standard or outline to evaluate four electrical transmission concepts. The economic costs under each concept are intricate, so the main resolved components, including capital costs, OPEX, and loss costs, are considered and calculated in this paper.
