Critical Technical Issues with a Voltage-Source-Converter-Based High Voltage Direct Current Transmission System for the Onshore Integration of Offshore Wind Farms

Abstract

Long-distance offshore wind power transmission systems utilize multi-terminal high voltage direct current (MT-HVDC) connections based on voltage source converters (VSCs). In addition to having the potential to work around restrictions, the VSC-based MT-HVDC transmission system has significant technical and economic merits over the HVAC transmission system. Offshore wind farms (OWFs) will inevitably grow because of their outstanding resistance to climate change and ability to provide sustainable energy without producing hazardous waste. Due to stronger and more persistent sea winds, the OWF often has a higher generation capacity with less negative climate effects. The majority of modern installations are distant from the shore and produce more power than the early OWF sites, which are situated close to the shore. This paradigm shift has compelled industry and professional researchers to examine transmission choices more closely, specifically HVAC and HVDC transmission. This article conducts a thorough analysis of grid connection technologies for massive OWF integration. In comparison to earlier assessments, a more detailed discussion of HVDC and HVAC topologies, including HVDC based on VSCs and line-commutated converters (LCCs), and all DC transmission systems, is offered. Finally, a selection criterion for HVDC transmission is advised, and its use is argued to be growing.

1. Introduction

High voltage direct current (HVDC) systems play a vital role in facilitating the transfer of huge amounts of electrical power generated by offshore wind farms (OWFs) to the grid. To link offshore wind power (OWP) to the local grid, multi-level voltage source converter (MVSC) topologies have gained increased attention in recent years [1]. An HVDC system with quick and flexible power flow regulation ensures electrical grid reliability and security. Furthermore, OWFs are typically placed far from the local grid. As a result, OWFs require long transmission lines (TLs) to transport the generated electricity to consumers. The adoption of high voltage alternating current (HVAC) technology for such an offshore grid is not feasible, since the reactive power flow increases with transmission distance due to line capacitances, resulting in large line losses. As a result, HVDC TLs are regarded as critical technologies for this purpose. The capacity to independently control active and reactive power flow in each of the AC grids is the key feature of the VSC-HVDC transmission system [2,3].

Power is typically transported from the generation side to the main grid. HVAC and HVDC are two methods for transmitting power to the national grid. In comparison to HVAC, HVDC is considered to be suited for transporting bulk power over vast distances. HVDC TLs based on line-commutated converters (LCCs) or voltage source converters (VSCs) are both thought to be more practical than HVAC. Furthermore, due to technological advancements and long-distance cost comparison, HVDC networks are regarded as economically sustainable [4]. The distance between WFs and coastal areas is also rapidly increasing. Some offshore wind power plants (OWPPs) that are currently operational use underwater cables that extend up to 200 km. According to available studies, HVDC and HVAC cost the same over a distance from 150 to 200 km. HVDC becomes more cost-effective as transmission distance rises. HVAC undersea cables’ cumulative capacitive current also limits their use in long-distance transmission. As wind farms migrate further away from coastal locations, HVDC becomes the only viable option [5,6,7]. Despite the rapid improvement of OWP, it is currently confronted with issues such as WT noise and land availability [8]. OWFs have sparked global interest due to the vast untapped wind potential and improved wind regimes. Currently, OWFs are evolving toward high-capacity and long-distance transmission, while the connection of OWFs to the grid has introduced new difficulties for technology and the economy. As a result, appropriate power transmission systems that can connect big OWFs to onshore power grids over long distances must be investigated [9]. Various transmission schemes for massive OWF integration have been developed and discussed over the last 20 years, with the majority of research focusing on the operational viability and economics of each transmission system [10]. Figure 1 depicts a scenario of a distinctive HVDC system utilized to connect an OWPP with the onshore power infrastructure. Because the larger transformer’s voltage perspective is too huge to fit within a cross-section of a tower, WT transformers normally increase the voltage from 690 V to 25,000–40,000 V. The turbine side converter typically has a power rating from 5 to 10 MW. The majority of collection systems use 33 to 36 kV AC to capture WF energy [11,12]. The offshore platform and the WT output are connected by high-voltage undersea cables. An HV transformer at the OWF elevates the collecting system voltage level from 132,000 to 150,000 V in preparation for the transmission and connection to the onshore grid using a DC line, which is usually 320,000 V [13,14,15].

Table 1. HVDC projects in various nations.

Title of Project	Distance in Miles	Power	Voltage	Model	Nations	Year
Zhoushan	$83.2637$	$400\times {10}^6$	$200\times {10}^3$	IGB	China	2014
Estlink	$65.244$	$350\times {10}^6$	$150\times {10}^3$	IGB	Estonia to Finland	2006
NordBalt	$279.617$	$700\times {10}^6$	$300\times {10}^3$	IGB	Sweden to Lithuania	2015
HVDC Link in the West	$262.219$	$2200\times {10}^6$	$600\times {10}^3$	Thy	UK	2017
Yunnan-Guangdong	$881.1044$	$5000\times {10}^6$	$800\times {10}^3$	Thy	China	2010
NorNed	$360.395$	$700\times {10}^6$	$450\times {10}^3$	Thy	Netherlands to Norway	2008
Shanghai’s Three Gorges	$658.6535$	$3000\times {10}^6$	$500\times {10}^3$	Thy	China	2006
Mundra–Haryana	$596.516$	$2500\times {10}^6$	$500\times {10}^3$	Thy	India	2012
Western Transmission Line in Alberta	$217.4799$	$1000\times {10}^6$	$500\times {10}^3$	Thy	Canada	2015
Skagerrak 4	$151.615$	$700\times {10}^6$	$500\times {10}^3$	IGB	Denmark to Norway	2015
East China’s Jinsha River II	----	$6400\times {10}^6$	$800\times {10}^3$	Thy	China	2016
DolWin2	$83.8851$	$900\times {10}^6$	$320\times {10}^3$	IGB	Germany	2016
SydVastlanken	$161.557$	$720\times {10}^6$	$300\times {10}^3$	IGB	Sweden	2016
SAPEI	$270.296$	$1000\times {10}^6$	$500\times {10}^3$	Thy	------	2011
BroWin1	$124.274$	$400\times {10}^6$	$150\times {10}^3$	IGB	Germany	2012
Xinjiang-Anhui	$2071.03$	$\mathrm{10,000}\times {10}^6$	$1100\times {10}^3$	Thy	China	2017
AL link	$98.1766$	$10\times {10}^6$	$80\times {10}^3$	IGB	Aland to Finland	2015

summarizes the installation of some HVDC-TNs based on a couple of technologies in various parts of the globe and discusses further projects [4,16].

In the work of Bresesti et al. [17], the authors have assessed the advantages and disadvantages of linking OWFs to the grid through direct current link, and this article also provides a technical and economic analysis. In the work of Pizano-Martinez et al. [18], the authors have proposed the model of an optimal power flow (OPF) solution utilizing Newton’s method for a VSC-HVDC. The work of Haghi and Riahimi [1] discusses the relationship between onshore converter control and HVDC transmission line dynamics and extracts the VSC-HVDC system dynamics on the dc side. The study also provides a controller design for the onshore converter’s dc-link voltage regulation and investigates the effects of the HVDC line length and dc voltage control bandwidth on system stability, using time domain simulations and modal analysis. Numerous HVDC authors have been published in the literature, focusing on various facets of the adoption of the technology. While many assessments concentrate on specific system elements (such as converter stations, cables, and protective equipment) [19,20,21], some carry out thorough comparisons at the system level with other options of transmission [22,23,24]. Other assessments focus on the implementation perspective and the difficulties for HVDC experts in terms of geographical challenges [14,25,26].

The majority of future WFs are planned to be located in deep sea areas offshore due to the improved wind conditions in terms of amplitude and consistency in such locations. Power transmission over great distances is undoubtedly a difficulty that must be handled while considering a trade-off between investment cost and efficiency. In these situations, HVDC transmission is a cost-effective option, as illustrated in Figure 2. The boundary between HVDC and HVAC is not clearly distinct and is affected by a variety of parameters, including the cable utilized and HVDC technology classification. For submarine cables, a breaking point of 40 km is acceptable. The majority of future OWFs are likely to be situated outside this range [17,27].

Despite the steady advancement of HVDC technology over the last decade, it has to be tailored to the unique requirements of offshore projects. HVDC has additional issues, particularly for OWF grid integration, for instance:

(1)

Efficiency: Offshore projects can be more affordable, minimizing power losses in the system.

(2)

Harsh environment: There is only restricted maintenance access.

(3)

Footprint: The size and weight of HVDC stations have a substantial impact on the costs of investment.

(4)

Reliability: The cost of unharvested energy due to transmission system outages can be critical in determining the viability of an offshore project [27].

In contrast, many existing power grids throughout the world, especially in Europe, find it difficult to accommodate $8000\times {10}^{6 }\mathrm{W}$ DC electricity at AC grids’ single point via LCC HVDC. Future VSC HVDC lines of this rating, on the other hand, are more capable of connecting to weak networks [12].

The gas-insulated line (GIL), as derived from geographic information system (GIS), requires only the basic performance of the electrical grid, i.e., there is no switchgear, dynamic thermal stability, and insulation; hence, it has observable consistency merits over both overhead lines with a high capability for power transmission and long-distance lines. The high prices and technical requirements of the manufacture design, as well as the protracted project term, are significant difficulties for the actual project. A new hybrid DC transmission model in China combines the improved performance of VSC-HVDC and LCC-HVDC technologies while being less expensive than current VSC HVDC transmission [28]. However, the existing transmission power is decided by the VSC HVDC side, and power flow reversal is difficult to achieve because of the voltage polarity that must be altered in the LCC converter station [12]. Nonetheless, it is a new trend of innovation that is becoming an alternative to OWP transmission, though it has not been used due to a lack of research, except in China.

Table 2. Evaluation of four transmission choices on a technical level.

Merits	Potential	Techniques	Limitations
Capacity expansion is simple. Improved stability. Construction. Lower line loss and cost. Limits propagation faults.	Rapidly growing. Long-distance transmission. High-capacity transmission.	VSC-HVDC	Converter station layout. Extra offshore platform.
High dependability. Simple layout. Extensive experience.	Accepted for OWFs near the sea.	HVAC	Additional reactive power correction. Large distributed capacitance. Multiple lines for increased capacity. Propagation of synchronous faults.
Lower cost than VSC-HVDC. Improved performance over VSC-HVDC or LCC-HVDC.	A new transmission trend is worth developing.	Hybrid HVDC	The accessible transmission power is decided by VSC side. It is difficult to reverse the power flow. There is a lack of research.
Best operation dependability. Large single-line transmission capacity. High ampacity. Low loss.	Restricted use for high-capacity transmission.	GIL	High cost. Stringent technical requirements. A lengthy project timeline.

summarizes the technical potential of the four techniques [29].

Several studies on grid connection methods for the integration of OWFs have been presented to date, and their key limitations and implications are shown in

Table 3. Comparison of past reviews.

Key Implication	Methods	Limitations	Ref
VSC-based HVDC converter topologies based on grid integration of OWFs. VSC-based HVDC control techniques.	VSC-HVDC	VSC-based HVDC classification is not clear and complete. VSC-based HVDC topologies for OWF connections are not explored and classified.	[30]
Current LFAC investigation. LFAC transmission structure elements.	LFAC	Insufficient analysis of economics. Narrow content range.	[31]
VSC-based HVDC topologies. FRT technologies.	VSC-HVDC	The application of multiple techniques to the environment is not discussed. A scarcity of novel VSC-HVDC technologies and topologies. Insufficient realistic perspectives for future work.	[32]
MMC. Key technology and operation for big OWFs integration.	MT-HVDC	Inadequate evaluation topologies of MTDC. Lack of innovative MMC technologies.	[33]

[8].

The worldwide development toward clean energy motivates the increase in wind-based electricity integration in power systems [34,35]. Wind turbines, both large and tiny, generate electricity for networks and utilities, as well as for isolated distant places [36]. When the distance between the shore and the point of connection with the onshore grid is significant, the charging current inserted along the HVAC cables can become significant. Although compensating for the consequent reactive current at the ends of HVAC cables can enhance operating conditions, the cables must still be sized for the maximum current passing through them at bottlenecks [37].

The first novelty of this article is the comparison of different converters such as LCCs, VSCs, LCC-based turbine converters, DC-to-DC buck interface converters between HVDC systems, and VSC turbines for offshore and onshore wind farm integration with grid connection for HVDC. The second is to meet the challenges around the number of wind farms, number of connected onshore stations, total installed power, and other selection criteria of the DC circuit breaker and DC transmission line, and suggesting and providing a very flexible solution. For example, for generating 4000 MW of power, the cost/km will be approximately USD 1.9 M to connect from Matiari to Lahore, whereas to generate 5000 MW and 8000 MW of power in Chongqing-Hubei and SHZ, China, costs/km were USD 1498.36 M and USD 1498.36 M, respectively.

The paper contains eight sections. The literature on VSC-based HVDC transmission for onshore integration of OWFs, energy from OWFs, and HVDC offshore, an overview and comparison these, and the motivation for the study have been discussed in Section 1. The LCC- and VSC-based HVDC converter technology has been discussed in Section 2. A VSC and LCC comparison in terms of HVDC has been presented in Section 3. A VSC-based HVDC scheme for OWFs and MT-HVDC-based transmission systems’ present and future have been discussed in Section 4 and Section 5. Cost calculations for HVDC OWFs have been presented in Section 6, an IEEE- and IEC-based discussion has been presented in Section 7, whereas the conclusion and future work have been presented in Section 8.

2. HVDC Converter Technology

Due to its inherent drawbacks, such as the inability to change voltage without transformers, the need for conversion equipment, the requirement for reactive power, harmonics, and the complexity of control, direct current transmission has only recently found widespread application [38]. However, due to the development of remote wind energy technology and the need for linking isolated asynchronous HVAC systems, HVDC transmission is currently regaining prominence [17,39,40,41,42,43]. HVDC transmission installations are happening more quickly than ever before. The following are just a few possible advantages of HVDC transmission:

• Compared to AC cables, DC cables do not exhibit the skin effect or the proximity effect.
• Notably, in long-distance, high-voltage applications, transmission losses are reduced by a DC cable’s lack of a large reactive charging current.
• Due to the substantial transmission loss, it is challenging for an AC system to utilize distant resources like offshore wind generation.
• Effective active power control is present in DC systems [44].

Because they may be utilized to connect several AC grids [45] using DC networks, VSC-HVDC systems enable us to take advantage of a multidirectional power flow [46,47]. Since HVDC systems have several advantages over HVAC schemes, including the asynchronous connection of multiple independent AC networks, the absence of reactive power in the DC link, and lower power losses [48,49], they can help to build a more dependable power network and smarter grids [50,51]. Compared to MVAC systems, DC-collecting systems provide more benefits. DC cables lead to reduced losses and do not require reactive power correction, as was previously indicated for HVDC transmission. The substitution of large, heavy, low-frequency transformers with DC to DC converters outfitted with lighter, smaller, and medium-frequency transformers has another benefit [52]. The possible reduction in weight and size is crucial in applications of offshore wind, since this is somewhere where supporting structures and storage space for the equipment are expensive and limited. Although there are currently no fully operating all-DC wind power plants (WPPs) with collection of the DC systems deployed, the introduction of DC technology still faces many hurdles [53,54].

2.1. LCC

LCC, often known as “classical HVDC”, are the “oldest” and most used thyristor-based systems [55]. The well-known and advanced LCC converter technology is depicted in Figure 3. LCC converters have the advantage of having a lot of operating expertise and being reasonably priced. The high voltage of up to 600 kV and high power ratings of thyristor-based converters are their distinguishing features. Active power control and a robust AC grid were necessities for the thyristor technology. Transistors were widely used, which changed HVDC technology and made LCC less practical for use in offshore applications.

The representation of OWFs and the LCC HVDC link connection [56,57,58] is illustrated in Figure 4. Because an LCC requires a commutation voltage to operate, it cannot power a passive network and lacks black start capability. Due to the lack of commutation voltage before wind farm startup, an external device, provide a consistent AC voltage for the converter using of STATCOM is necessary [59]. The work of Liu and Sun [60] contains models of impedance for WT inverters, LCC HVDC rectifiers, and STATCOM.

2.2. VSC

The VSC using IGBT transistors has been a common terminal for HVDC since 1999. This approach was used in the first HVDC commercial project using an OWF [61,62]. The numerous benefits of the VSC technology used in the HVDC systems depicted in Figure 5 include compactness, the ability to join the weak AC system, separate regulation of reactive and active power, and black start capabilities [63,64,65,66,67]. As a result, it has taken over as the primary converter technology of HVDC for linking OWPPs, such as Borwin 1 by ABB in 2009, 400 MW, 75 onshore km + 125 offshore, Sylwin1 by Siemens in 2014 (45.5 onshore km 159 offshore, 864 MW), and DolWin3 by Alstom (2017, $900\times {10}^6, 76.5 \mathrm{o}\mathrm{n}\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}+84.5 \mathrm{o}\mathrm{f}\mathrm{f}\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}$). The VSC scheme outperforms LCC systems for the two separate DC transmission methods in terms of independent reactive power regulation, not having a requirement for an external voltage source, and quick system control. VSC-based HVDC technology is appealing for use in WF grid connection because of these characteristics [43]. VSC HVDC transmission technology is superior to LCC HVDC builds and is better suited for OWF grid connection. Furthermore, the use of VSC-HVDC promotes the implementation of multi-terminal grids and future worldwide power interconnection [32,68].

2.3. Offshore Turbine Converter Topologies

Many turbine topologies have been suggested to fit the series architecture, which is divided into two categories as follows [5]:

(1)

LCC-based turbine converters or CSC with PWM.

(2)

Interface DC/DC buck between an HVDC system and turbine VSC

DC/DC buck interface between the turbine VSC and HVDC system.

2.3.1. LCC-Based Turbine Converter

Figure 6a,b illustrate the first type of turbine converter based on small-scale PWM-CSC or LCC. Both are naturally CSC converters and can regulate dc volt across a large range. PWM CSC and LCC topologies are less complex than topologies of turbine converters [69,70,71].

2.3.2. DC-to-DC Buck Interface between an HVDC System and VSC Turbine

Figure 7a,b illustrate how variable DC voltage is added using a dc-to-dc buck converter in-between the VSC and dc line, while still retaining the turbine converter’s core converter topologies. VSC, which is the most common converter architecture, is maintained by the dc-to-dc buck converter, which simply adds a basic interface between the HVDC system and original turbine converter and has two or three DC voltage levels. This configuration would introduce voltage harmonics into the DC system [72,73].

3. VSC-HVDC vs. LCC-HVDC

Current-source converters (CSCs), as well known as the LCC-utilizing thyristor, and VSC HVDC, using IGBT transistors, are now the two main HVDC technologies. Both are suited for a variety of applications. CSC-HVDC technology outperforms conventional AC alternatives in terms of efficiency and power transfer capacity for long-distance, high-capacity TSs. VSC-based HVDC is the preferred technology for power transmission (PT) from OPPs with constrained space because it offers superb reactive and active power regulation capabilities. Mercury-arc valves, a technology predominantly developed in Sweden and Germany during the 1930s, are a component of modern HVDC systems. In the past, there have been commercial uses for an HVDC system between Moscow and Kashira in the Soviet Union in $1951$, and a $20 \mathrm{M}\mathrm{W}$, $100 \mathrm{k}\mathrm{V}$ link between the island of Gotland and the Swedish mainland [74]. Since the 1970s, thyristor valves have only recently been utilized in HVDC applications. The mercury-arc technology has largely overcome its drawbacks. The Eel River Converter Station in Canada became the first LCC-HVDC to be operational in 1972. The IGBT valves used in VSCs provided an upgrade over thyristor valves. The first commercial VSC HVDC connection between the Gotland Islands and mainland Sweden was launched by ABB in 1999. It was made up of underwater cables with a $50 \mathrm{M}\mathrm{W}$ rating and an 80 kV rating [75,76]. The VSC-based HVDC TS emerged as a viable solution for bulk power transmission over great distances after many years of research and development. As VSCs provide independent control of both reactive and active power, they are now preferable to LCC-HVDC. Additionally, whereas LCCs required external mechanical switches to push off the switches, VSCs do not. Compared to LCC HVDC, VSC HVDC-based networks provide some benefits, some of which are listed in

Table 4. LCC HVDC versus VSC HVDC comparison.

Task	LCC HVDC	VSC HVDC
Reversal of direction.	Requires external switches to change the current direction.	The current direction shift is feasible in the converter without the requirement for outside mechanical switches.
Influences of AC.	Possible commutation failure; HVDC grid short circuit.	Offers the capacity to ride through faults, there are no AC disruptions, and there is minimal active power transfer loss.
Controlling AC voltage.	Uses half the VAR required for AC filters to provide VAR compensation.	Controls both reactive and active power.
Power outlets.	Connection restricted to circuits of medium and high capacity.	AC circuits might be connected either electrically, weak, or black.
Multi-terminal.	Only allows for 3 terminals.	No restrictions.
Schedule of delivery.	36 Months.	24 Months.

.

4. VSC-Based HVDC Scheme

The IGBT technology is used by VSC. Rainer Marquardt developed this idea in 2003 [77]. In the event of a black start, it generates its AC voltages, enabling the current to be turned on and off as necessary regardless of the AC voltage. Its converters use pulse width modulation, which allows for simultaneous amplitude and phase angle modification while maintaining a constant voltage. Due to its high level of flexibility and inherent capacity to adjust both its reactive and active power, it is more beneficial in locations with metropolitan power networks. The majority of converter stations for the VSC-HVDC technology employ multilayer converter circuits [78,79], as seen in Figure 8 and Figure 9 [80].

VSC-Based HVDC for OWFs

One of the solutions to the problem of fulfilling rising urban and industrial demands is offshore wind energy systems [42,81,82]. OWFs can generate more power and operate more consistently than land-based systems because they have more consistent characteristics and stronger winds than on land [83]. Larger wind tribunes are brought on by the viability of OWFs, but the complexity of maintenance and high installation costs present challenges to reducing O&M expenses [83,84,85]. With growing distances from the shore, wind turbine technology is facing additional difficulties like long-distance HVDC undersea cables and the security of turbine equipment [86]. Some studies have concentrated on economic feasibility [22]. When the OWF exceeds $100 \mathrm{M}\mathrm{W}$ and the distance is greater than $\mathrm{90,000} \mathrm{m}$, it is decided that VSC-based HVDC is the more cost-effective choice. Several others have concentrated on control tactics. Figure 6 shows that VSC technology has several advantages in HVDC systems, including black-start capability, compactness, the ability to connect to weak AC networks, and separate regulation of reactive and active power. As a result, it has become the primary HVDC converter technology for connecting OWPPs, such as ABB’s Borwin1 ($400\times {10}^6 \mathrm{W},75 \mathrm{o}\mathrm{n}\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}+125 \mathrm{o}\mathrm{f}\mathrm{f}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{k}\mathrm{m})$, Siemens’ Sylwin1 ($864\times {10}^6$, $45.5 \mathrm{o}\mathrm{n}\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}+59 \mathrm{o}\mathrm{f}\mathrm{f}\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}$), and Alstom’s DolWin3 ($900\times {10}^6 \mathrm{W}, 76.5 \mathrm{o}\mathrm{n}\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}+84.5 \mathrm{o}\mathrm{f}\mathrm{f}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{e}\mathrm{k}\mathrm{m})$.

5. MT-HVDC Transmission Systems

Several converters are coupled to a single HVDC circuit to form a VSC-based MT-HVDC system [87]. Squirrel cage induction generators (SCIGs) [88] are examples of central power converters used for a group of WTs, while double-fed induction generator (DFIG) WFs use individual power converters in each WT [89,90]. The so-called WF rectifiers are used to link WFs to the shared HVDC circuit. Through grid-side inverters, the HVDC circuit power is introduced into the terrestrial AC grid [91]. The MT DC system’s single-line diagram is depicted in Figure 10. The system includes two grid-side VSCs (GSVSC) and four terminals with wind farm VSCs (WFVSC). The suggested MT DC system, however, can be used with any quantity of terminals and any mix of GSVSCs and WFVSCs. Although other WT technologies, such as those based on the PMG, can also be employed, the two wind farms that are being discussed here are all based on DFIGs and separated from one another. The four terminals are connected at a single point in Figure 11, but other connection patterns are equally applicable.

Energy from the WFs is gathered and converted to DC by the WF VSCs. DC wires are then used to deliver the DC power to the GSVSCs. The two WFVSCs also regulate the respective wind farm networks’ AC voltages and frequencies. According to a predetermined arrangement, the two GSVSCs convert the DC power back to the respective AC grid. They can also control the reactive power and AC voltage for the connected grids. As the grid network at the site of connection occasionally has a low short circuit ratio, this function may be helpful. To eliminate the high-frequency harmonics that the converters produce, a high-frequency filter is connected to each output terminal of the VSC [92].

5.1. VSC HVDC (Two-Terminal)

A typical VSC-HVDC (two-terminal) TS for integrating OWFs is seen in Figure 11 [32]. VSC-HVDC converter stations come in a range of configurations, with two- and three-level converters being used on small OWFs [33,93]. The technological advancements in power electronics, particularly the widespread usage of MMC in VSC-HVDC systems, have considerably increased the economic benefits and efficiency of VSC-HVDC systems [94,95].

5.2. The Present and Future of MT-HVDC

An MTDC system, in which three (03) or more DC converters are joined by a DC transmission network (DCTN), may achieve greater economic and technological advantages, based on the successful implementation of two-terminal DC links all over the globe [96]. When examining the architecture of a point-to-point system, it can be seen that it is more sophisticated. Radial MTDC systems, where each converter station is connected to a single DC line, are one of three major groups [97] that these systems can be categorized under. Each converter station in a mesh or ring MTDC system is connected to multiple DC line systems with all converter stations connected in a series, such as an MTDC. The only VSCs that can be used to implement future meshed HVDC grids are those that can be divided into two-, three-, and multilevel converters. Future HVDC grids will certainly be based on multilevel converters since they have many benefits over competing technologies [98].

6. Economic Assessment of HVDC OWFs

The objective of the calculation of cost is to evaluate the capital expenditure (CAPEX) for the configurations under investigation. OWFs are made up of a sizable number of parts. Only the major cost drivers are taken into account when estimating CAPEX. These are the onshore substation, the collection cables, the offshore substation with transformers and switchgear, the high-power converters (HPEs), the export cables, and the wind turbines, including the drive train and foundation. Reactive compensation is necessary for all AC wind farms, and expenses also included are any extra platforms and shunt reactors. All cost data must be normalized because they are accessible for multiple years and are provided in different currencies. The usual interchange rate from the beginning of the year, as acquired from the cost data, is used to convert currencies. After that, costs are modified depending on the historical inflation rate, which was calculated to reflect the Euro’s worth [99].

6.1. Operational Costs

Based on the indicated median values, the yearly operational cost (OPEX) of every component was calculated as a percentage of the CAPEX. The net present value of the OPEX is obtained by applying the equation to discount the yearly OPEX over the lifetime of the wind farm (WF) [100].

$${\mathrm{O}\mathrm{p}\mathrm{e}\mathrm{x}}_{\mathrm{N}\mathrm{P}\mathrm{V}}=\frac{{\sum }_{\mathrm{n}=1}^{\mathrm{N}}{\mathrm{O}}_{\mathrm{n}}}{\mathrm{d}}\left(1-\frac{1}{{\left(1+\mathrm{d}\right)}^{{\mathrm{L}}_{\mathrm{T}}}}\right)$$

(1)

where ${\mathrm{O}}_{\mathrm{n}}$ is the yearly $\mathrm{O}\mathrm{P}\mathrm{E}\mathrm{X}$ of component $\mathrm{n},\mathrm{d}$ is the concession rate, ${\mathrm{L}}_{\mathrm{T}}$ is the lifetime in existence, and ${\mathrm{O}\mathrm{p}\mathrm{e}\mathrm{x}}_{\mathrm{N}\mathrm{P}\mathrm{V}}$ is the net present value of the operational costs. A concession charge of 6% and a lifespan of 324.222 months are taken into account in the basic scenario [100,101].

6.2. VSC-Based HVDC Transmission Cost Calculation

Regarding VSC-based HVDC transmission, the calculation performed to gain an idea of the overall cost of ${\mathrm{K}}_{\mathrm{V}\mathrm{S}\mathrm{C}}$ consists of ${\mathrm{K}}_{\mathrm{c}\mathrm{a}\mathrm{p}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$ for capital expenditures, ${\mathrm{K}}_{\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{x}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$ for operation and maintenance, and ${\mathrm{K}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$ for less cost [101].

$${\mathrm{K}}_{\mathrm{V}\mathrm{S}\mathrm{C}}={\mathrm{K}}_{\mathrm{c}\mathrm{a}\mathrm{p}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}+{\mathrm{K}}_{\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{x}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}+{\mathrm{K}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$$

(2)

6.2.1. Capital Costs

${\mathrm{K}}_{\mathrm{c}\mathrm{a}\mathrm{p}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$ is made up of two costs: ${\mathrm{K}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$ for the foundation of the converter station and ${\mathrm{K}}_{\mathrm{c}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$.

$${\mathrm{K}}_{\mathrm{c}\mathrm{a}\mathrm{p}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}={\mathrm{K}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}+{\mathrm{K}}_{\mathrm{c}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$$

(3)

6.2.2. Operation and Maintenance Costs

Equation (3), which is used in this study, yields the ${\mathrm{K}}_{\mathrm{o}\mathrm{p}\mathrm{e}\mathrm{x}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$; the B of the DC submarine cable equals 0.5%, I is 5%, and n is 240 months.

6.2.3. Cost of Losses

The converter station loss ${\mathrm{K}}_{\mathrm{s}\mathrm{u}\mathrm{b}\left(\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\right)}$ and line loss ${\mathrm{K}}_{\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e}\left(\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\right)}$ make up the loss expenses for the ${\mathrm{K}}_{\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}$ loss rate for converter stations. Psub loss measures how much of the transmitted power is lost at the station. Two converter stations’ ${\mathrm{P}}_{\mathrm{s}\mathrm{u}\mathrm{b}\left(\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\right)}$ ranges from 1.6 to 2.4%, and Zhen notes that ${\mathrm{P}}_{\mathrm{s}\mathrm{u}\mathrm{b}\left(\mathrm{l}\mathrm{o}\mathrm{s}\mathrm{s}\right)}$ is between 1 and 2 percent [102].

6.2.4. Foundation Costs for Converter Stations

The entire infrastructure investment for each converter station is the VSC-based converter station cost. In addition, the expected additional expenses for IGBT technology are the converter station layout’s civil construction costs, the converter station’s DC capacitor and AC filter costs, and the converter station’s converter controller and reactor costs. The cost of a converter station based on VSC is then calculated as a percentage of the capacity of each converter station, denoted by P.

$${\mathrm{C}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}={\mathrm{C}}_{\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{M}\mathrm{W}}.2\mathrm{P}$$

(4)

6.2.5. Cost of Cable Installation and Foundation

The DC cable’s VSC-based cost is determined by transmission distance, much like the cost for the HVAC cable is.

$${\mathrm{C}}_{\mathrm{c}\mathrm{a}\mathrm{b}\mathrm{l}\mathrm{e}\left(\mathrm{V}\mathrm{S}\mathrm{C}\right)}=2({\mathrm{P}}_1+{\mathrm{P}}_2)\mathrm{L}$$

(5)

where ${\mathrm{P}}_1$ and ${\mathrm{P}}_2$ represent the cost and installation expenses for a DC cable per kilometer [17]. Since the DC voltage waveform is not susceptible to peak/effective ratio underutilization, the cost of the cables in the VSC-HVDC option is significantly lower than that of AC alternatives. The transmission capability ratio of DC cables to AC cables can be calculated using Equation (6).

$$\frac{{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}.-\mathrm{D}\mathrm{C}}}{{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}.-\mathrm{A}\mathrm{C}}}=\frac{2{\mathrm{V}}_{\mathrm{d}\mathrm{c}}{\mathrm{I}}_{\mathrm{d}\mathrm{c}}}{3{\mathrm{V}}_{\mathrm{a}\mathrm{c}}{\mathrm{I}}_{\mathrm{a}\mathrm{c}}\mathrm{p}\mathrm{f}}=\frac{2\sqrt{2}{\mathrm{V}}_{\mathrm{a}\mathrm{c}}{\mathrm{I}}_{\mathrm{a}\mathrm{c}}}{3{\mathrm{V}}_{\mathrm{a}\mathrm{c}}{\mathrm{I}}_{\mathrm{a}\mathrm{c}}\mathrm{p}\mathrm{f}}=\frac{2\sqrt{2}}{3}\approx 1$$

(6)

As observed in (6), DC solutions only require two polar wires for a given power transmission, but AC solutions require three. Compared to AC choices, the cost of the cable would be substantially lower with VSC-HVDC. This benefit might be more apparent if the reactive power and skin impact are factored, as in the work of Xiang [103].

6.3. An Economic Comparison of HVAC and HVDC Systems

Since every project has unique situations and features, including line distance, rated power, topography, and utilized technology, it is challenging to estimate the accurate price of HVDC. On the other hand, a broad estimate can be derived using the information from earlier initiatives. The DC grid interconnection back-to-back plan of Chongqing-Hubei in China was the first clarified VSC HVDC line, encompassing a total distance of 1711 km with a maximum voltage capability of 420,000 V and a main power transmission capacity of $5000\times {10}^6$ W [104]. The project cost USD 1498.36 million per kilometer. For the HVDC line in Australia (South Australia to Queensland) with a $1450 \mathrm{k}\mathrm{m}$ span, the cost was therefore estimated to be 1.3 M USD/km for $400\times {10}^3$ V [105]. A large-scale HVDC project in China is the 800 kV, 8000 MW Southern Hami–Zhengzhou (SHZ) HVDC line, which runs from the northwest region of Xinjiang to the central province of Henan. Approximately 1.8 M USD/km and USD 3500 M were spent on the project, while the Lingzhou–Shaoxing HVDC line experienced a minor reduction in cost of $1.7 \mathrm{M} \mathrm{U}\mathrm{S}\mathrm{D}/\mathrm{k}\mathrm{m}$ because of its shorter line exposure of 1722 km as opposed to the line route of SHZ, which is 2200 km. Additionally, the Ministry of Water and Power Pakistan (WAPDA) and the State Grid Corporation of China (SGCC) are working together on the Matiari to Lahore HVDC line, the first ever in Pakistan, which spans 878 km and has a rated power of $4000\times {10}^6$ W and a voltage of 660,000 V. The project will cost around USD 1.9 M per km, or USD 1658.34 M. The planned HVDC line’s anticipated cost is given in this study as $1.7 \mathrm{M} \mathrm{U}\mathrm{S}\mathrm{D}/\mathrm{k}\mathrm{m}$, which is about equivalent to the costs of other HVDC projects that were previously chosen. As a result, the suggested HVDC approach is more advantageous and economically viable for Pakistan’s upcoming transmission projects [4].

For both HVDC and HVAC alternatives,

Table 5. HVDC vs. HVAC investment cost comparison for offshore wind farm transmission.

Ref.	Component	HVAC			VSC HVDC
[106,107,108]		Korea (M EUR)	Germany (M EUR)	Pakistan (M EUR)	Korea (M EUR)	Germany (M EUR)	Pakistan (M EUR)
	Substation	50	10	282.8	630	45	800
	Cable	720	1500		240	600
	Cable installation	288	340		192	215
	Offshore substation ring		13			24
	Onshore land use		50	70		125	70

breaks down the cost of TS into numerous separate components, including cable, substation, offshore rig, cable installation, and land use charges. VSC HVDC converter stations are much less thoroughly researched in terms of their economic implications [106]. Due to the IGBT-based DC-to-AC converters, the substation cost is higher for VSC HVDC compared to HVAC. In each technology, a substation is needed at both ends of the transmission cable. The bipolar HVDC cable pair costs less than the two-parallel, three-core HVAC cables and can convey an equivalent amount of active power [107].

Three cost factors come into play when comparing HVAC and HVDC economically:

• Cost of line;
• Cost of losses;
• Cost of terminal.

The overall cost is split into two parts: the expense of building the infrastructure and the expense of maintaining the system once it is operational. This considers the cost of the investment, the cost of the poles, wires, insulation, converter stations, and the use of the right of way. Financial losses are specifically included in the operating cost. Given that both AC and DC use the same types of insulation and conductors, AC requires three conductors while DC just requires two [109]. DC poles become a less costly route as a result, using less conductor and insulator material [110]. Calculations for AC and DC transmission line losses look like this:

$$\mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r} \mathrm{T}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{s}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{b}\mathrm{y} \mathrm{D}\mathrm{C}={\mathrm{V}}_{\mathrm{d}}{\mathrm{I}}_{\mathrm{d}}$$

(7)

$$\mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r} \mathrm{T}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{s}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{b}\mathrm{y} \mathrm{A}\mathrm{C}=3{\mathrm{V}}_{\mathrm{a}}{\mathrm{I}}_{\mathrm{a}}\cos \mathsf{\theta }$$

(8)

$$\mathrm{D}\mathrm{C} \mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r} \mathrm{L}\mathrm{o}\mathrm{s}\mathrm{s}=2{\mathrm{I}}_{\mathrm{d}}^2\mathrm{R}$$

(9)

$$\mathrm{A}\mathrm{C} \mathrm{P}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r} \mathrm{L}\mathrm{o}\mathrm{s}\mathrm{s}=3{\mathrm{I}}_{\mathrm{a}}^2\mathrm{R}$$

(10)

By combining Equations (9) and (10) we obtain Equation (11).

$$2{\mathrm{I}}_{\mathrm{d}}^2\mathrm{R}=3{\mathrm{I}}_{\mathrm{a}}^2\mathrm{R},{\mathrm{I}}_{\mathrm{a}}{\mathrm{I}}_{\mathrm{d}}=\sqrt{\frac{2}{3}}$$

(11)

If three-phase AC is substituted with DC, the same power transfer, conductor size, and power loss are assumed [24,111].

Figure 12 illustrates a proposed criteria flowchart for different steps to analyze the cost, distance of transmission system, and generation of OWFs. There are three regions that have been analyzed to show which one is best at the rated values of cost, distance, and generation of OWF system.

7. Discussion

Offshore wind farms must comply with safety limits to ensure the stable operation of the system. The IEC and IEEE have established standards for the integration of offshore wind farms into electrical power systems. These standards cover a range of topics including design, installation, operation, and maintenance of OWFs. Additionally, the safe distance between WTs and the coast, shipping lanes, and other infrastructure are considered in the design process. These standards are periodically reviewed and updated to reflect the latest industry developments and advances in technology.

Some specific safety and performance standards for offshore wind farms include:

• IEC 61400-3 [112], which provides guidelines for the design, installation, operation, and maintenance of WT generators and WF control systems.
• IEEE 1547 [113], which covers the interconnection and interoperation of distributed energy resources within the electric power system.
• IEEE P2450 [114], which provides guidelines for the planning, design, installation, operation, and maintenance of WT generator systems, including the wind turbine, electrical equipment, and the wind farm control system.
• IEC 62271-110 [115], which covers the HVDC systems used to transmit power from OWFs to the onshore electrical grid.

It is important to note that compliance with these standards is not mandatory, but it is highly recommended as it ensures the safety of the equipment and the people who operate it, and also helps in the integration of the WFs into the existing power grid.

8. Conclusions

Pakistan is facing an energy crisis as a result of rising demand and aging internal power production plants. A VSC-based HVDC transmission scheme was developed in this study to convey OWF power over great distances. This article conducted a cost-effective investigation of VSC-HVDC vs. HVAC transmission due to its advanced characteristics and technical advantages. As important technical aspects of this research, multiple forms of cost calculations of HVDC OWFs (operational costs, transmission costs) and evaluation of HVAC and HVDC systems were performed. As indicated in Figure 2, the bulk of future WFs will be placed in deep-sea, offshore areas due to improved wind situations in terms of stability and amplitude. Power transmission across long distances is clearly a challenge that must be addressed while balancing efficiency and investment cost. In these cases, the most cost-effective least expensive solution is HVDC transmission. It is found that for the VSC HVDC project in Chongqing-Hubei China, the cost was $1498.36$ million USD/km for the capacity of $5000 \mathrm{M}\mathrm{W}$, and in Australia it was estimated to cost 1.3 M USD/km. Between 1.8 and 3500 M USD/km were spent for $8000 \mathrm{M}\mathrm{W}$ on the large-scale HVDC project of SHZ in China. However, the first ever HVDC line from Matiari to Lahore will cost approximately $1.9 \mathrm{M} \mathrm{U}\mathrm{S}\mathrm{D}/\mathrm{k}\mathrm{m}$ for $4000 \mathrm{M}\mathrm{W}$. The expected cost of the considered HVDC line is indicated in this research as 1.7 M USD/km, which is less expensive than the cost of HVDC projects that were previously preferred. Therefore, the proposed HVDC technique is more favorable and cost-effective for Pakistan’s planned transmission projects.

First, all the cost computations are calculated based on the cost coefficients, which include the cost of lines, the cost of losses, and the cost of HVDC terminals. Then, overall cost function in both CAPEX and OPEX are described in detail. The Authors then introduce the safety and performance standards IEC 61400-3, IEEE 1547, IEEE P2450, and IEC 62271-110 [112,113,114,115]. Secondly, the proposed approach can help the field engineer when considering the optimal choice for HVAC and VSC HVDC for optimal wind farm transmission.

Recommendations for future work and the future direction of offshore wind turbines (OWTs) include:

✓

Deeper ocean installation of wind turbines with larger power ratings to withstand stronger winds.

✓

Integration with various renewable energy sources, such as a converter for wave energy.

✓

Enhancement in performance and control with the use of cutting-edge power electronics and sophisticated control strategies.

✓

The reliability and cyber security aspects of these offshore renewable energy sources are becoming more apparent.

Further research should be conducted to explore various technological factors, societal implications, and regulatory and fiscal needs for this proposed future transmission project for real-time execution.