*3.3. Costs Calculation of GIL Transmission Concept*

The GIL transmission concept is similar to the AC transmission concept, but there is no reactive power compensation costs.

$$\mathbb{C}\_{GIL} = \mathbb{C}\_{cap.GIL} + \mathbb{C}\_{\text{open.GIL}} + \mathbb{C}\_{loss.GIL} \tag{22}$$

#### 3.3.1. Capital Costs

The capital expenditure *Ccap.GIL* is dependent on the transformer substation foundation cost *Cstation.GIL* and cable foundation and installation cost *Ccable.GIL*.

$$
\mathbb{C}\_{cap.GIL} = \mathbb{C}\_{subfund.GIL} + \mathbb{C}\_{cable.GIL} \tag{23}
$$

The foundation cost *Cstation.GIL* is similar to *Cstation.AC* in Equation (3); similarly, the calculation of *Ccable.GIL* is as Equation (4).

#### 3.3.2. Operation and Maintenance Costs

Based on the OPEX in HVAC transmission system, *Copex.GIL* is expressed by the percentage *A* as in Equation (7).

#### 3.3.3. Costs of Loss

The loss costs *Closs.GIL* in the GIL transmission system comprises of *Csub.loss* and line loss *Cline.loss* as well.

$$\mathcal{C}\_{loss.GIL} = \mathcal{C}\_{sub\text{-}loss} + \mathcal{C}\_{advir} + \mathcal{C}\_{cum.loss} \tag{24}$$

where *Csub.loss* is dependent on the *Psub.loss*, *Cline.loss* comprises of conductor losses *Ccon.loss*, as computed by Equation (10), and the eddy current and circulating current loss *Cedcir.loss* of the shell.

#### *3.4. Costs Calculation of Hybrid HVDC Transmission*

#### 3.4.1. Capital Costs

Capital costs in the hybrid HVDC transmission *Ccap.HybDC* covers the converter station foundation cost *Cstation.HybDC*, cable foundation cost, and installation cost *Ccable.HybDC*.

$$
\mathcal{C}\_{cap.Hyb\mathcal{DC}} = \mathcal{C}\_{station.Hyb\mathcal{DC}} + \mathcal{C}\_{cable.Hyb\mathcal{DC}} \tag{25}
$$

where *Cstation.HybDC* is dependent on the sum of the investment costs of the different types of converter stations on both sides.

#### 3.4.2. Operation and Maintenance Costs

Since the OPEX in the hybrid HVDC transmission system is in the same way in the VSC-HVDC transmission system, *Copex.HybD* can be calculated by Equation (8).

#### 3.4.3. Costs of Loss

Similarly, *Closs.HybDC* consists of substation loss *Csub.loss* and line loss *Cline.loss*. *Csub.loss* is dependent on the converter station loss rate *Psub.loss*, which is 1.75% for VSC-HVDC and 0.8% for LCC-HVDC. The average value of *Psub.loss* is 1.275%. The transmission line loss *Cline.loss* is the same as in Equation (20).

#### **4. Results and Discussion**

The empirical component-resolved evaluation models give a crucial message to stakeholders that the economic costs are sensitive to transmission distance and capacity. The cost comparisons of the four electrical transmission options for wind farms with different distances and transmitted power were carried out. The rated voltage is 220 kV, and the frequency is 50 Hz, and the operation hour of full capacity per year is 2500 h. If the capacity is 300 MW, 600 MW, and 900 MW, respectively, the economic evaluations from 25 km to 75 km were conducted.

#### *4.1. Essential Evaluation Data*

Based on the DCF model, the costs evaluation results can be converted to cash value. Unlike an onshore power grid, the specific environment and operational conditions of the offshore substation are more complicated; it is necessary to adopt more strict standards for long-term stability. For the AC cable, one line is needed for 300 MW, two lines for 600 MW, and three lines for 900 MW wind power. However, it is important to point out that in the GIL transmission concept, the rated current is 3.15 kA, so the transmission capacity of a single line is 1200 MVA, and there is no need to install additional lines with different capacity. The data are given in Table 2.


[a] Design Control Index of Power Grid Project in China (2014). [b] Presented in this paper considering both LCC-HVDC and VSC-HVDC cost. [c] Materials provided by Dongfang Cable Factory in Ningbo city, China. [d] Materials of the project of 66kV Xin-Guang underwater cable in Dalian city, China.

#### 4.1.1. Capital Costs Evaluation

The *Cstation* of the AC cable varies among different projects. For example, according to the materials in Design Control Index of Power Grid Project (2014 standard) provided by the Electric Power Planning and Engineering Institute of China [39], the investment of the 220 kV Yucai substation project (indoor) is 0.303 million RMB per megavolt-ampere (MVA), and the cost of the 220 kV Pingli substation project (Laizhou, Shandong) is CNY 0.435 million/MVA. In this study, for 35 kV wind farms with a 220 kV single-core underwater cable, the foundation costs of substation *CperMVA* is CNY 0.45 million/MVA. Procurement materials provided by Dongfang Cable Factory in Ningbo city indicates that the expense basis of a 220 kV single-core underwater cable with the 1200 mm2 cross-sectional area of copper core is CNY 3.732 million/km. The installation costs refer to cable crossing barge, sea sweeping, and trench laying. The project of the 66 kV Xin-Guang submarine cable in Dalian city gives the cost *P*<sup>2</sup> around CNY 0.3 million/km. As for the reactive power compensation, the rated power of the AC cable is 427 MVA and the capacitance for the 1200 mm2 cable is 0.179 μF/km. The maximum DC resistances of the 20 ◦C and AC resistance of 90 ◦C are 0.0151 Ω/km and 0.02 Ω/km, respectively.

Several studies provide various foundation costs for the converter station for reference. The costs of the traditional ±500 kV and ±800 kV LCC-HVDC converter stations are around CNY 0.52428 million/MW and CNY 0.56228 million/MW, respectively. There is a lack of reports on the cost of a VSC-HVDC converter station in China, which varies widely across the globe. Reference [36] applied the technical materials of ABB Ltd. to evaluate the costs of a VSC-HVDC station as CNY 1.155 to 1.343 million/MW, and the costs of the ±300 kV converter station are CNY 1.2 million/MW. Taking the development of offshore wind power technology into account, the standard of *CperMW* is CNY 1.1 million/MW. According to the industry date provided by Dongfang Cable Factory, the expense the *P*<sup>1</sup> of XLPE-insulated DC submarine cable (Model: DC200 kV YJQ411 500 + 2 × 12 (core optical cable)) with a cross-sectional area of 500 mm<sup>2</sup> is CNY 1.077 million/km. Moreover, considering the difficulty of hybrid HVDC transmission technology, then *CperMW* is CNY 0.9621 million/MW.

The expense of GIL *P*<sup>1</sup> is CNY 20 million/km. *P*<sup>2</sup> of the four transmission methods equals CNY 0.3 million/km. Thus, the capital costs under different capacity can be obtained.

#### 4.1.2. Operation and Maintenance Costs Evaluation

The annual percentage *A* of the operation and maintenance cost of the AC submarine cable accounts for 1.2%, and 0.5% is adopted in the other three transmission methods.

#### 4.1.3. Costs of Loss Evaluation

It is assumed that the operation hours are 2500 h per year, referred to in [37], and the on-grid price of offshore wind power is CNY 0.0085 million/MW·h [40]. The substation loss rate *Psub.loss* in AC cable and GIL transmission system is 0.4%. The apparent power is 427 MVA, based on Equation (9), and the current of copper core with *Icable* is 0.8287 kA. *Rcu* is 0.006 Ω/km. Some industry gives *Is* is 502.4 A and *IA* is 313.2 A. The resistances of the sheath and armor are 0.21 Ω/km and 0.301 Ω/km. The eddy current loss *Ped.loss* and circulating current loss *Pcir.loss* in the GIL lines are 0.0177 MW/km and 0.0062 MW/km, respectively.

For the XLPE-insulated DC submarine cable, the rated power is 324 MW and the DC resistance is 0.0366 Ω/km. Based on Equation (20), the conductor loss of 300 MW is 0.0412 MW/km, 0.0824 MW/km, and 0.1648 MW/km. The substation loss rate *Psub.loss* in the hybrid HVDC system is 1.275%.

#### *4.2. Evaluation Results*

It can be seen from the above analysis that offshore distance and capacity have an important impact on the capital costs of the four types of transmission. Based on the DCF model, the comparisons with different transmission distances and capacities were calculated, and the results shown in Table 3. Components analysis of the total costs was carried out to acquire an importance view for stakeholders, shown in Table 4. The gradual change in color from green to red represents an increase in costs.


**Table 3.** Economic costs comparisons of different *P* and different *L*.

**Table 4.** Economic costs comparisons of different *P* and different *L*.


#### *4.3. Comparisons of Economic Evaluation*

To obtain the best transmission method for Guangdong offshore wind power, the costs comparisons were calculated.

#### 4.3.1. Total Costs Comparisons with Different *L*

According to the data in Table 2, the relationships between the total costs of transmission distance *L* from 25 km to 75 km are shown in Figure 3.

It is clear that the economic costs of the GIL electrical transmission concept are considerably much more than either the HVAC or HVDC transmission concept. When the transmission distance is not so long, such as 25 km, the costs of the GIL system is more than twice of that in other systems. In addition, the costs of the GIL changes great when *L* increases, which means it is most sensitive to transmission distance; it can even be four times that of the others at a distance of 75 km. On the other hand, the hybrid HVDC transmission concept has economic advantages to the VSC-HVDC system, and when the installed capacity increases, the preferred distance range under the hybrid HVDC technology becomes longer from 50 km (300 MW), 38.6 km (600 MW), to 36.4 km (900 Mw). It is feasible if the technology is developed widely in the future.

**Figure 3.** Total costs of different *L* of 300 MW, 600 MW, and 900 MW wind farms: (**a**) shows the 300 MW offshore wind farm total costs with different distances; (**b**) shows the 600 MW offshore wind farm total costs with different distances; (**c**) shows the 900 MW offshore wind farms total costs with different distances.

If the hybrid HVDC is not taken into present planning consideration due to its limited development, compared with VSC-HVDC electrical transmission, the HVAC concept is more superior when *L* is less than around 50 km in both 300 MW, 600 MW, and 900 MW wind farms. Otherwise, the HVDC transmission concept is preferable with a longer distance. HVDC is also less sensitive to transmission distance than the HVAC system.

#### 4.3.2. Costs Components Comparisons with Different *L*

Taking 300 MW wind farms as an example, as Figure 4 shows, in the GIL and AC transmission system, the cable costs account for a large proportion of the total cost, especially the extravagant cable costs of the GIL transmission concept. That means the capital costs are the most important component to be considered, and the HVDC system has huge technological potential for offshore wind power transmission.

For the VSC-HVDC and hybrid HVDC transmission systems, cable costs are cheaper than for the AC transmission system. In turn, he costs of the converter station are much higher than that of the substation, as well as the costs of the converter station loss. It is important to notice that the capital costs in the AC system and GIL system increase greater than in HVDC systems.
