A Multi-Terminal HVdc Grid Topology Proposal for Offshore Wind Farms

Abstract

Although various topologies of multi-terminal high voltage direct current (MT-HVdc) transmission systems are available in the literature, most of them are prone to loss of flexibility, reliability, stability, and redundancy in the events of grid contingencies. In this research, two new wind farms and substation ring topology (2WF-SSRT) are designed and proposed to address the aforementioned shortcomings. The objective of this paper is to investigate MT-HVdc grid topologies for integrating large offshore wind farms with an emphasis on power loss in the event of a dc grid fault or mainland alternating current (ac)grid abnormality. Standards and control of voltage source converter (VSC) based MT-HVdc grids are defined and discussed. High voltage dc switch-gear and dc circuit topologies are appraised based on the necessity of dc cables, HVdc circuit breakers, and extra offshore platforms. In this paper, the proposed topology is analyzed and compared with the formers for number and ratings of offshore substations, dc breakers, ultra-fast mechanical actuators, dc circuits, cost, flexibility, utilization, and redundancy of HVdc links. Coordinated operation of various topologies is assessed and compared with respect to the designed control scheme via a developed EMTDC/PSCAD simulation platform considering three fault scenarios: dc fault on transmission link connecting the wind farm to mainland power converters, dc fault within substation ring of VSC-HVdc stations, and ultimate disconnection of grid side VSC station. Results show that 2WF-SSRT is a promising topology for future MT-HVdc grids.

1. Introduction

In recent years, multi-terminal high voltage direct current (MT-HVdc) systems topologies have attracted great attention for the integration of offshore wind farms to ac grids [1]. In this emerged paradigm, the main objectives are improving the stability and reducing the cost of multi-terminal HVdc grid as a whole and particularly the cost of dc circuits, control systems, the number of offshore substations, and HVdc circuit breakers (DCCB), respectively.

Currently, more than 200 point-to-point HVdc transmission systems have been launched around the world. Actually, the parallel connection of HVdc systems was investigated in 1963 [2], while series HVdc was discussed in 1965 [3]. However, in order to get the first working parallel MT-HVdc system, some existing HVdc applications have been extended with more than two terminals over the last two decades [4,5]. Provision of the MT-HVdc grid, comprised of multiple voltage-source converters (VSCs), has become an encouraging choice with the increased advancements and rapid availability of high-power VSCs from multiple vendors [5]. In this regard, MT-HVdc grid applications and its significant advantages have been identified and proposed in [6,7,8,9,10,11]. Moreover, MT-HVdc systems are derivers of the developments of European super-grid [12,13,14,15,16], in order to interconnect North Sea wind farms with the Scandinavian hydropower and Mediterranean solar power plants. Predominantly, MT-HVdc grids emerged as a strong contender that can meet the future needs of electrical power systems. Therefore, the topological evaluation of MT-HVdc grid circuits is the need of time.

Naeem et al. studied the impacts of HVdc grid topology on transient stability on the HVdc segmented electric grid [17,18]. HVdc grid converter topologies are discussed in [19]. Connection schemes for integration of offshore wind farms with mainland grids via VSC-HVdc grid are discussed in [20] but lack protection and control strategy. Schön et al. worked on the connection of HVdc link with different circuit topologies [21]. Examination of different communication topologies for distributed control of multi-terminal HVdc grids is carried out in [22]. Haileselassie studied the impact of dc-link voltage drop on the power flow of the MT-HVdc grid using droop control [23]. Therefore, in this paper, the tolerance limit of ±10% of dc-link voltage has been selected as a stability evaluation criterion for the topologies [24]. Kontos et al. studied the impacts of HVdc grid topology on MT-HVdc grid fault, but this study is limited to star connection topology [25]. The dc transmission circuit topology affects the protective operations in MT-HVdc networks [26]. Thus, it is important to lay out a detailed techno-economic comparison of HVdc grid topologies to address the shortcomings. Finally, a solution is proposed in terms of more dynamic MT-HVdc grid topology to cope with shortcomings.

Various topologies for MT-HVdc grids are proposed in [27,28,29,30], namely: point-to-point topology (PPT), star topology (ST), general ring topology (GRT), the star with central switching topology (SGRT), substation ring topology (SSRT), and wind farm ring topology (WFRT). However, these topologies lose their flexibility, stability, and redundancy in the event of an ac network fault [29,30]. In [30], WFRT and SSRT are analyzed and compared in detail and, based on that, a more efficient 2WF-1SST topology is proposed. However, 2WF-1SST of [30] also loses its flexibility, redundancy, and does not follow the maximum power loss criterion of the Great Britain’s security and quality of supply standards (GBSQSS) [31] during (i) mainland ac grid fault (grid side VSC) and (ii) dc-link fault on either of the extreme ends (end sectioned lines), simulations proved these drawbacks in Section 6. Thus, a new better circuit topology/variant is needed. Considering this, a more stable and promising topology for the MT-HVdc grid to connect offshore wind farms with the mainland grid is proposed in this paper and termed as two wind farms and substation ring topology (2WF-SSRT). This new MT-HVdc grid topology offers benefits of reduced length, rating, and the number of dc circuits; no extra offshore substation is required with the least number of dc breakers. 2WF-SSRT exhibit features of 2WF-1SST [30] and SSRT [28]. Furthermore, the proposed topology is able to improve the stability, utilization, and redundancy while fulfilling the maximum loss criterion in the normal and abnormal conditions by providing an alternate path for the power flow.

The main contributions of this paper are:

• A new promising topology for the MT-HVdc grid is proposed and named as two wind farms and substation ring topology (2WF-SSRT).
• MT-HVdc transmission grid topologies are investigated with stress on wind farm power (P_WF) loss during dc grid and mainland ac grid faults via simulations.
• The core contribution of this paper is a comparison of different topologies of [27,28,29,30] with the proposed 2WF-SSRT topology based on the techno-economic factors such as length, number, and ratings of dc cables, number, and ratings of VSCs and DCCBs, additional offshore substation requirements, flexibility, capital, and running costs, stability, redundancy, and loss of VSC station in case of a fault. Such a comprehensive comparison was missing in the literature.
• Topology with the least number of offshore stations, DCCBs and reduced dc-link length with maximum flexibility, stability, utilization, and redundancy is checked to endure the MT-HVdc grid codes without a communication system, via simulations—a step to meet the HVdc grid codes by following the standards recommended by the GBSQSS [31].
• EMTDC/PSCAD tool is used to analyze and compare the transmission circuit topologies. In [27,28,29], the authors just proposed the circuit layout for MT-HVdc grids without simulations. However, in this paper, time-domain simulated configurations are experienced with the dc line–line faults and ultimate disconnection of the grid side VSC (GS-VSC) to assess the real-time evaluation of topologies.
• Finally, annotations on MT-HVdc grid topological evaluation are provided which may serve as a guideline for the researchers to understand different norms in this field.

The rest of the paper is organized as follows: multi-terminal HVdc transmission systems and its necessities are described in Section 2. Various topologies for the MT-HVdc grid are discussed from the literature in Section 3. A new topology is proposed in Section 4 for MT-HVdc transmission systems and stability of this newly proposed topology is assessed via two dc fault scenarios. A general case study based on techno-economic analysis is presented in Section 5. Simulations are conducted in Section 6 and each topology is tested for (i) GS-VSC disconnection and (ii) dc line–line fault. In Section 7, annotation on the topological evaluation of HVdc circuits is provided. Finally, remarks and conclusions are drawn in Section 8.

2. MT-HVdc Transmission Systems and Necessities

Offshore platforms are required for each offshore wind farm to install a VSC converter and a number of connections to connect the HVdc links depending upon the MT-HVdc application. The fundamental design of an MT-HVdc system relies both on economic and technical factors imposed by both society and utility. Further economic factors include geological position, length of dc circuits, and its rating, ultra-fast mechanical actuator (UFMA), and the number of HVdc circuit breakers, VSC converters, and its ratings, need of communication between converters and additional offshore substations. Moreover, technical perspectives can be successful usage of dc circuits, availability of security to the MT-HVdc system under abnormal conditions, dc grid flexibility, and inertia sharing with mainland ac grid and redundancy.

Great Britain’s security and quality of supply standards have proposed principles for offshore wind farms’ connection with onshore ac networks [31]. Hence, an MT-HVdc grid needs to ensure the following:

(1)

Direct voltage must be regulated during both faulty and normal operating conditions.

(2)

In the event of fault occurrence, the MT-HVdc system should provide support to the mainland ac grid.

(3)

In case of any VSC station failure, an MT-HVdc system needs to guarantee that power transferred to the ac network will not be reduced more than the maximum power failure (P_max-fail e.g., for Great Britain its 1320 MW [32]).

The performance of MT-HVdc greatly depends on the employed control strategy, while control mainly relies on a kind of ac grid connection and the dc network topology [30]. This paper does not investigate control strategies in detail; however, operation and control of the MT-HVdc grid are discussed in our published research [33,34,35]. Accordingly, P−V_ac control is deployed to regulate the ac voltages of WFs at a precise level [33]. Proportional integral (PI) control is used for constant voltage generation at 50 Hz. PI controller diminishes the voltage error (e = V_WF * − V_WF), which was then employed as a performance index [33,34]. MT-HVdc system specifications and parameters for PI controllers are enlisted in

Table 1. Control parameters and systems specifications.

DC Grid Specifications
Parameters	Values
DC grid voltage	400 kV
Droop coefficients k1, k2, k3, k4, k5, k6	5, 7, 4, 5, 3, 6
Max. power of each VSC	1000 MW
Converter capacitance	1400 µF
Coupling inductance	2.5 mH
DC lines	Inductance	Resistance
All DC links (Lij)	0.50 mH	0.10 Ω
PI Control Parameters
PI controller	Ti	Kp
q-axis PI	0.0067	0.48
d-axis PI	0.0067	0.48
P controller	0.0400	0.01
WF ac voltage PI	1.0000	1.00

.

As shown in Figure 1, for assessment of MT-HVdc topology, V_dc and Q are controlled at GS-VSCs. Furthermore, Figure 2 shows a scheme of droop controller, employed to coordinate the direct voltage between GS-VSCs, where P * pu and V_dc * pu are the real power and direct voltage references, respectively. k is droop characteristic slope. Control of Figure 2 is more proficient than customary control and gives the least PI error [33] by adding limiters.

For modeling of the MT-HVdc grid, an average VSC value model is used, while offshore WFs are demonstrated as fixed power sources. The dc transmission lines are modeled via the π model.

3. MT-HVdc Grid Topologies

A number of MT-HVdc grid topologies are evaluated and analyzed by considering the ratings of VSC stations and dc breakers, length and capacity of dc cables, additional offshore substation requirements, flexibility, capital and running costs, stability, and redundancy for topological assessment. The authors of [27,28] suggested that wind farm ring topology is the best topology among general ring topology (GRT), substation ring topology (SSRT), star with central switching ring topology (SGRT), wind farm ring topology (WFRT), point-to-point topology (PPT), and star topology (ST) for a MT-HVdc system. However, authors have not investigated the impact of dc fault as the WFRT loses flexibility and stability under a fault on GS-VSC station or on permanent disconnection of GS-VSC. In the event of such a dc fault, the dc breaker will operate on either side of the fault [28] and, as a result, a substation and two WFs will be disconnected from the MT-HVdc grid, which could not fulfill the third requirement of GBSQSS. Therefore, 2WF-1SST is suggested in [30], which can sustain the effects of anomalies of the system. The proposed topology and the two prominent topologies (WFRT and 2WF-1SST) from literature are described in the subsequent section with their merits and demerits, and then these are simulated.

3.1. Wind Farm Ring Topology

The wind farm ring topology (WFRT) contains offshore WFs accompanied in a ring, possesses the equal number of WFs and dc breakers, and each WF is linked to an associated mainland network as shown in Figure 3 [27,28]. In the event of a dc fault, DCCBs operate on either side of the fault [28], thus a WF side VSC (WF-VSC) and a GS side VSC are disconnected. As a result, the maximum power loss criterion of GBSQSS has not complied. An isolator segregates the faulty region; once the fault current is zero, HVdc breakers reclose its contacts and hence onshore SS and the offshore WF are again functional. WFRT does not permanently cut off the WF under the failure in connecting lines from GS-VSC to WF-VSC, an advantage of WFRT. However, WFRT loses its stability and flexibility when the failure ensues within the GS-VSC, as this leads to a rise of dc-link voltage [30]. Furthermore, in the event of a dc fault within a ring, dc breakers operate on either side of the faulty region [28] and an onshore VSC and an offshore WF are disconnected from the MT-HVdc grid. As a result, the maximum power loss criterion of GBSQSS does not comply.

3.2. Substation Ring Topology

SSRT is similar to a WFRT with a ring on GS-VSCs. Each WF-VSC is integrated with a corresponding mainland power converter station as shown in Figure 4 [28]. The difference between WFRT and SSRT is that GS-VSC is isolated in WFRT while WF-VSC in SSRT, from which a dc fault persists in a dc-link. This configuration offers more flexibility under both faulty and maintenance operations on the mainland ac grid than the wind farm side [30]. The third condition of GBSQSS is not satisfied as, during dc fault in SS ring, an onshore SS and an offshore WF are disconnected (dc breaker operation on either side of the fault), which leads to maximum power loss.

3.3. Two Wind Farm and a Substation Topology

The 2WF-1SST is comprised of two WF side VSCs linked to a GS side VSC via a dc-link such that each WF-VSC is connected with the adjacent unit’s WF-VSC through UFMA as depicted in Figure 5 [25]. The application of the MT-HVdc system decides the number of such units. It exhibits better stability, flexibility, and efficiency with the reduced number of DCCBs, but operation of 2WF-1SST will be greatly affected if the fault persists on end sectioned lines (i.e., L₁₂ and L₅₆) or permanent fault persist within sectioned GS-VSCs (SS₁ or SS₃). Thus, a new better variant is needed.

4. Proposed MT-HVdc Grid Topologies

4.1. Two Wind Farm and Substation Ring Topology

In 2WF-SSRT topology, each unit consists of two wind farms side VSCs connected to one onshore VSC station within a ring of substations through dc-link such that each WF-VSC connected to its neighboring-VSC through an ultra-fast mechanical actuator as shown in Figure 6. Proposed topology has features of 2WF-1SST [30] and SSRT [28]. The number of such units in the MT-HVdc system depends upon energy requirements and applications. The proposed scheme aids to increase the stability, flexibility, and efficiency of the system with a reduced number of DCCBs and offshore stations, which minimized the dc-link lengths and GS-VSCs by half to WF-VSCs. In order to analyze the stability of the proposed topology, two dc faults F₁ and F₂ on dc-link L₁₂ and in substation ring (line L₁₃) are considered, respectively, and demonstrated as follows:

4.1.1. Scenario-1, Fault (F₁) on Line L₁₂

Let’s consider a dc line–line fault F₁ on the line L₁₂ and the operation of the proposed topology is described through the following steps:

(1)

At t₀, the system is in steady-state as shown in Figure 7a.

(2)

At t₁, F₁ occurs on L₁₂ as shown in Figure 7b.

(3)

At t₂, CB₁₂ and CB₃₄ open, resulting in loss of SS₁ as shown in Figure 7c.

(4)

At t₃, IS₁₂₂, UFMA₁₂, and UFMA₂₁ open as shown in Figure 7d. When the current through the faulted line L₁₂ becomes zero, WF₁ and WF₂ are disconnected from the system.

(5)

At t₄, first transfer switches UFMA₁₅ and UFMA₂₃ are closed and then CB₁₂ and CB₃₄ reclosed as well. Resulting in WF₁ and WF₂ back into the system through L₃₄ and L₅₆, respectively, as shown in Figure 7e.

(6)

At t₅, on dc-link fault clearance, UFMA₁₂, UFMA_21, and IS₁₂₂ are closed in sequence to restore the 2WF-SSRT topology to its original condition.

4.1.2. Scenario-2, Fault (F₂) on Line L₁₃ within Substation Ring

Again, let us consider a dc line–line fault F₂ on the line L₁₃ but within the substation ring (SSR), and the operation of the proposed topology is explained as:

(1)

At t₀, topology is in steady-state

(2)

At t₁, fault F₂ occurs on the line L₁₃ within the substation ring (SSR).

(3)

At t₂, CB₁₂ and CB₅₆ opened on either side of the fault within the SS ring.

(4)

At t₃, IS₅₆₂ is open. When the current through the faulted line L₁₃ becomes zero, the L₁₃ is disconnected from the SS ring.

(5)

At t₄, CB₅₆ is closed, resulting in SS₃ going back into the system.

(6)

At t₅, on dc-link fault F₂ clearance, IS₁₂₂ and CB₁₂ are closed in sequence to restore the 2WF-SSRT topology to its original state.

For scenario-2, the sectionalized figure is not included to keep paper simple.

5. General Comparison of the Topologies

General comparative case study for several MT-HVdc grid topologies is presented in Figure 8, considering the techno-economic factors such as length, number, and ratings of dc cables, number and ratings of VSCs and DCCBs, redundancy, and loss of VSC station in case of a fault. The geographical positions of VSCs are presented in

Table 2. Geographical positions of WF-VSCs and GS-VSCs for topologies.

Wind Farm	Horizontal (km)	Vertical (km)	Sub Station	Horizontal (km)	Vertical (km)
WF1	50	10	SS1	101	10
WF2	20	20	SS2	128	45
WF3	10	35	SS3	125	70
WF4	11	60	SS4	123	80
WF5	30	82	SS5	110	91
WF6	50	92	SS6	102	95
Centre point for ST				75	45

. PPT is not a multi-terminal topology while SGRT is a combination of GRT and ST. Therefore, these are not considered for comparative analysis.

ST, GRT, SSRT GRT, and ST require the number of DCCBs equal to the number of WF-VSCs plus GS-VSCs while WFRT and SSRT require DCCBs equal to the WF-VSCs. The 2WF-1SST and the proposed 2WF-SSRT need DCCBs equal to half the number of WF-VSCs. The dc grid carries six GS-VSC and offshore WF-VSC stations, each one 400 MW for PPT. The star topology requires six GS-VSCs of 480 MW and six offshore VSC stations of 400 MW, respectively. Twelve VSCs of 2400 MW are required by the GRT. SSRT and WFRT need six offshore VSCs each of them 400 MW and 800 MW, respectively, with 800 MW and 480 MW mainland substations, respectively [30]. Six WF-VSC of 400 MW and three onshore VSCs of 1200 MW are designed for the 2WF-1SST and proposed 2WF-SSRT topology, respectively.

An extra offshore platform is required for only ST and SGRT to install circuit breakers while dc circuits have the same power ratings as an associated VSC station. The dc circuits in WF/SS rings need a rating equal to the power of two adjacent VSCs while the rating of the dc circuit connecting GS/WF-VSCs from WF/SS ring depends upon the rating of the respective GS/WF-VSC. For 2WF-1SST and proposed 2WF-SSRT, dc-links connecting WFs to the common point need to be rated at double capacity than the WF-VSCs while the dc circuits connecting neighboring-WF’s are rated equal to the WF-VSC. Rating of the lines to the mainland grid from common point depends upon the rating of the GS-VSC for 2WF-1SST, while, for 2WF-SSRT, it is equal to the rating of the respective unit. The rating of the cables within the SS ring of 2WF-SSRT would be equal to the rating of two adjacent GS-VSCs.

In the event of a dc-link fault, connected WF will be lost for ST and SSRT. However, in general, it will not affect the operation of GRT, WFRT, 2WF-1SST, and 2WF-SSRT topologies. However, the operation of 2WF-1SST will be greatly affected if the fault persists on end sectioned lines (i.e., L₁₂ and L₅₆). Thus, 2WF-SSRT topology shall support in this regard. Conversely, if the permanent fault is within GS-VSC station, ST, GRT, and WFRT will lose the GS-VSC and hence lack in supplying 400 MW to the connected ac system, while 2WF-1SST and 2WF-SSRT can deal with this situation by providing an alternate path to power flow.

However, if the permanent fault persists within extreme end GS-VSC (i.e., GS-VSC₁ or GS-VSC₃), then again power from WF₁ and WF₆ will be lost for 2WF-1SST, while 2WF-SSRT will aid with overcoming this shortcoming of 2WF-1SST by providing intra-connections between units.

PPT and ST require the same ratings (400 MW each) of dc cables as their converters. The dc cables of the central ring in GRT demand for power rating equivalent to the total P_WF (2400 MW). Rating of dc lines within the SS/WF ring is equal to the sum of the power ratings of the two adjacent VSCs i.e., 800 MW while the rating of cables linking WFs to onshore grids needs to be equal to the ratings of WF-VSCs and GS-VSCs, respectively. For 2WF-1SST and the proposed 2WF-SSRT, lines integrating the WFs to the common point needs to be rated at double capacity of the WF side VSCs while the rating of the link (cable) between neighboring WF-VSCs should be equal to the WF-VSC. However, dc cables rating in SS ring of 2WF-SSRT needs to be equal to the summation of two adjacent VSCs, while dc cables are rated equal to the onshore converter for 2WF-1SST.

Likewise, no dc breaker is required for PPT. Six DCCBs of 400 MW are needed at the offshore site, while six HVdc breakers of 480 MW are required at the onshore station for star topology. Twelve HVdc circuit breakers are required for the central ring in GRT, each of them being 2400 MW. Six DCCBs are demanded in the SS/WF ring of SSRT/WFRT, each of them 800 MW. The 2WF-1SST and 2WF-SSRT topologies require only three dc circuit breakers, each of them 1200 MW, respectively.

The results of the topological evaluation are summarized in

Table 3. Summary of the general case study.

Topology		No. of CBs		Rating of CBs (MW)		No. of Cables		Total Length of Cables (km)	Rating of Cables (MW)		No. of VSCs		Rating of VSCs (MW)		Utilization
		WF	GS	WF	GS	WF	GS		WF	GS	WF	GS	WF	GS
PPT		-		-		6		531	400		6	6	400	400	100%
GRT		7	5	2400	2400	7	5	332	2400	2400	6	6	2400	2400	20–50%
ST		6	6	400	480	6	6	671	400	480	6	6	400	480	80–100%
WFRT	Ring	6	-	800	-	6	-	208	800	-	6	-	800	-	50%
	Line	-	-	-	-	-	6	519	-	480	-	6	-	480	80%
	Total					12		727			12				-
SSRT	Ring	-	6	-	800	-	6	190	-	800	-	6	-	800	50%
	Line	-	-	-	-	6	-	519	400	-	6	-	400	-	80%
	Total					12		709			12				-
2WF-1SST		-	3	-	1200	5	3	403	800−400	1200	6	3	400	1200	67–100%
2WF-SSRT		-	3	-	800	5	3		800−400	800	6	3	400	800	80–100%

from Figure 8 and the above discussion. Among them, 2WF-SSRT topology is the best for WF integration with onshore grids because it gives maximum flexibility, stability, reliability, redundancy, and utilization as it can tolerate all faulty conditions with three DCCB and GS-VSCs, respectively. A VSC power converter station of 1000 MW costs 110 M€, while the cost of DCCB is one-sixth of the power VSC converter price [36,37]. Subsea HVdc cable approximate price is in the range of 1.2 M€–1.4 M€ per km [38]. This information proves that 2WF-SSRT topology offers reduced overall capital and operating cost (economical) even better than 2WF-1SST.

6. Simulation Results

MT-HVdc grid and control parameters given in Table 1 are used to evaluate WFRT, 2WF-1SST, and the proposed 2WF-SSRT topologies of the MT-HVdc transmission system. Proportional droop control is employed on GS-VSCs to attain dc voltage control [33,34,35]. Maximum P_WF is extracted via V_ac and P control on WF-VSCs [39]. CIGRE Bologna DCCB is used for fault isolation and dc inductors of 100 mH are added to limit the rate of rising of dc fault current [40,41]. Fault discrimination time is taken as 5 ms [41,42,43]. Permanent VSC disconnection and dc line–line fault tests are performed under the base power of 1000 MW and 400 kV V_dc.

6.1. Simulation Results of WFRT

Figure 3 gives the WFRT topology with four HVdc circuit breakers. This topology is developed in EMTDC/PSCAD with the parameters reported in Table 1.

6.1.1. Disconnection of GS-VSC₁

First trial associates with the disconnection of the grid side converter. The dc voltage and power profiles of WFRT are shown in Figure 9. GS-VSC₁ power dropped to zero from −0.6 pu because of the immediate disconnection as this VSC station experienced a symmetric fault on the ac side, causing excessive power in the dc grid. Promptly, dc-link voltage builds up to 1.25 pu while, at t = 2.1 s, V_dc drops to 1.07 pu as droop control increases power transfer through GS-VSC₂, GS-VSC₃, and GS-VSC₄, supplied by the WF-VSCs. Wind power extraction stays unchanged. The permissible range of ±10% is violated for dc-link voltage. WFRT does not devise any protection to tackle with such circumstances, which is a drawback.

6.1.2. Permanent dc Line–Line Fault on Line L₄₁

During the second test, HVdc circuit breakers CB₁₂ and CB₄₁ operate on both sides of the line–line fault within WF ring on line L₄₁. This situation results in the disconnection of WF-VSC₁ and GS-VSC₁ from the MT-HVdc grid. Dashed lines in Figure 10 represent the power of the detached VSCs after isolation. The permissible range of MT-HVdc grid voltage is not violated as long as power transmission is balanced. However, WFRT may not fulfill the maximum power loss standard during permanent line–line fault within the WF ring as every transmission system operator (TSO) implies its own measures and constraints.

Considering the above-described control and operation during the tests, it is proved that the designed system architecture of WFRT is not applicable for the applications where sudden and permanent GS-VSC disconnection or a permanent dc-link fault within WF ring is expected. Therefore, exploring and developing new topology is the need of the time to meet system needs under steady-state and dynamic situations.

6.2. Simulation Results of 2WF-1SST Topology

Simulations of 2WF-1SST transmission system topology of Figure 5 are developed in EMTDC/PSCAD with values of controls given in Table 1, with three dc breakers.

6.2.1. Disconnection of GS-VSC₁

Grid side VSC disconnection is assessed in the first test. A three-phase permanent fault is experienced on the ac side of GS-VSC₁ and thus P_GS1 drop to zero from −0.6 pu. Instantly, the dc grid voltage rises to 1.013 pu. Power extraction from WFs is unchanged. Power transported via WF-VSC₃ upswings to 0.8 pu via UFMA₂₃, delivered from the WF-VSC₂ while power from WF₁ is wasted as it has lost its connection to the onshore grid. Power and the dc voltage profiles are shown in Figure 11. P_WF1 and P_WF2 are shown by dashed lines after the disconnection of GS-VSC₁. Again, 2WF-1SST is lacking in satisfying the 3rd condition of Great Britain’s security and quality of supply standards.

6.2.2. Permanent dc Line–Line Fault on Line L₅₆

In the second test, dc line–line fault is developed on line L₅₆, which results in the operation of HVdc circuit breakers CB₅₆. This situation results in the disconnection of GS-VSC₃ from WF-VSC₅ and WF-VSC₆. Consequently, power from WF₅ starts flowing through WF-VSC₄ while P_WF6 is lost, not accomplishing the GBSQSS standard—a drawback. The flow of power and the dc-link voltage under this scenario are similar to test-1 (disconnection of GS-VSC₁) as shown in Figure 12.

It is clear from the conducted tests that the 2WF-1SST is not conforming to the GBSQSS and is hence not viable during GS-VSC disconnection and dc-link fault within remote/far-end/the last unit (unit-2 or unit-3 in Figure 5). As a result, a more feasible configuration is required to accommodate shortcomings of literature’s best two topologies i.e., WFRT and 2WF-1SST.

6.3. Simulation Results of the Proposed 2WF-SSRT Topology

Configuration of Figure 6 presents 2WF-SSRT with three dc breakers in SS ring, for six offshore WFs. Simulations are developed in EMTDC/PSCAD with the parameters reported in Table 1.

6.3.1. Disconnection of GS-VSC₁

Grid side VSC₁ is subjected to a permanent three-phase fault and thus it is disconnected from SS ring of 2WF-SSRT following the operation of CB₃₄ and CB₁₂. Power flow and the dc voltage profiles of the proposed topology under this scenario are shown in Figure 13. Power extractions from the WFs remain constant while the power flow through GS-VSC₁ drops to zero as depicted with the dashed line. However, the flow of power via GS-VSC₂ and GS-VSC₃ increased smoothly to −1.0 pu and −1.1 pu from −0.7 pu and −0.8 pu, respectively, because of SS ring beauty. No sharp spikes are experienced. In addition, no violation of Great Britain’s security and quality of supply standards is observed.

6.3.2. Permanent dc Line–Line Fault on Line L₁₂

A permanent dc line–line fault F₁ is experienced on L₁₂ of unit-1 as explained in scenario-1 of Section 4.1.1, which results in the cutoff of power flow to SS ring via L₁₂. Thus, the flow of powers from WF₁ and WF₂ of unit-1 is diverted to WF₅ of unit-3 and WF₃ of unit-2, respectively. Power flow within the SS ring almost remains the same. Power and dc-link voltage profiles are depicted in Figure 14. The dc voltage stabilizes itself to 1.025 pu. Thus, the dc grid parameters are within the safe limit which shows that 2WF-SSRT has even stable topology for dc-link faults within remote/far-end/the last unit.

6.3.3. Permanent dc Line–Line Fault on Line L₁₃ within the Substation Ring

In the third case, again a permanent dc line–line fault is considered but within the SS ring as discussed in scenario-2 of Section 4.1.2. In the event of dc fault on L₁₃, dc breakers on either side of the fault will operate and open the ring. However, the power of units-1, 2, and 3 are smoothly and continuously being exported to ac networks from the MT-HVdc grid via GS-VSC₁, GS-VSC₂, and GS-VSC₃, since WFs are directly attached to respective substations after the operation of DCCBs. The direct voltage and power profiles are shown in Figure 15. Profiles are unaltered before and after the operation of DCCBs in the event of fault F₂, which gives conformability of the proposed topology with all three standards of GBSQSS.

All the above three tests prove the superiority of the proposed 2WF-SSRT over the simulated (literature best) WFRT and 2WF-1SST. 2WF-SSRT fulfills the GBSQSS requirements and has enough protection to cope with the transients that occurred due to faults and disconnection of the GS-VSCs.

7. Annotations on Topological Evaluation of HVdc Circuits

From all of the above discussions, the following remarks are drawn on multi-terminal VSC-HVdc transmission system topologies:

(1)

In the context of dc breakers, ST, GRT, and SGRT require a dc breaker for each WF and onshore SS (N_WF + N_SS). SSRT and WFRT demand that DCCB equals the number of WFs (N_WFs) while PPT does not need dc breakers (not an MT-HVdc topology). 2WF-1SST and proposed 2WF-SSRT acquire dc breakers equal to the number of substations (N_SS). The fewer the requirements of dc breakers, the more economical is the topology. Thus, 2WF-1SST and 2WF-SSRT are more economical configurations than all of them.

(2)

No extra offshore SS is required for all discussed topologies except ST and SGRT for connections and dc breakers’ installations.

(3)

P_max-fail criterion imposed by GBSQSS is satisfied during all cases but for a permanent dc fault within WF/SS ring of WFRT/SSRT, on a central star node of star topology and during onshore converter disconnection and dc-link fault within remote/far-end/the last unit (unit-1 and unit-3 of Figure 5) of 2WF-1SST is not satisfied. Stability and flexibility of the MT-HVdc grid are on the stake in the event of permanent failure of GS-VSC station in WFRT, SSRT, ST, SGRT, and GRT, unlike the proposed 2WF-SSRT. Simulations show that 2WF-SSRT is the most stable configuration as it satisfies all operating conditions of GBSQSS.

(4)

Proposed 2WF-SSRT cuts the complexities such as operation and control approach and the protection scheme because of the standalone operation of each unit of 2WF-SSRT without mediation and burden of the power converters, UFMA, and DCCBs from the adjoining units during the steady-state operation. However, the impact of all VSC stations, UFMAs, dc cables, and DCCBs of a configuration needs to be considered while designing/selecting control and operation, and a protection approach for all other topologies, which proves that 2WF-SSRT is a practical configuration.

Comparative evaluation and detailed remarks on MT-HVdc grid topologies are summarized in

Table 4. Comparison of MT-HVdc system topologies.

Topology		No. of DCCB	No. of HVdc Link	Extra Offshore Substation	Flexibility	Stability	Redundancy	Comm	Remarks
PPT		Zero	NWF = NSS	No	No	Yes	No	No	Single terminal, not flexible
GRT		NWF + NSS	NWF + NSS	No	Excellent	Moderate	Excellent	Yes	Flexible but the dc-link have to be rated at full system’s power
ST	WF side	NWF + NSS	NWF	Yes	Bad	Excellent	Moderate	No	The rating of dc circuits of GS-VSC and WF-VSC are equal. Fault in the central star node will collapse the system
	SS side	Zero	NSS	No	Excellent	Bad	Moderate	No
	Total	NWF + NSS	NWF + NSS	Yes	Moderate	Moderate	Moderate	No
SGRT	WF side	NWF + NSS	NWF	Yes	Bad	Excellent	Moderate	No	Full power rating dc-link required in the central ring. Moderately flexible and stable
	SS side	Zero	NSS	No	Excellent	Bad	Moderate	No
	Total	NWF + NSS	NWF + NSS	Yes	Moderate	Moderate	Moderate	No
WFRT	WF side	NWF	NWF	No	Excellent	Excellent	Excellent	Yes	Requires a number of DCCBs equal to WFs. Flexible on the WFs side. Poor stability during fault inside the GS-VSC.
	SS side	Zero	NSS	No	Bad	Bad	Moderate	Yes
	Total	NWF	NWF + NSS	No	Moderate	Moderate	Moderate	Yes
SSRT	WF side	Zero	NWF	No	Bad	Bad	Moderate	Yes	Good stability but poor redundancy and flexibility. Number of HVdc CB is equal to NSS
	SS side	NSS	NSS	No	Excellent	Excellent	Excellent	Yes
	Total	NSS	NWF + NSS	No	Moderate	Moderate	Moderate	Yes
2WF-1SS	WF side	Zero	NWF	No	Moderate	Moderate	Moderate	Yes	The number of DCCB is half to WFs. Moderate flexibility and stability
	SS side	NSS	NSS	No	Bad	Bad	Bad	Yes
	Total	NSS	NWF + NSS	No	Moderate	Moderate	Moderate	Yes
WF-SSRT	WF side	Zero	NWF	No	Excellent	Excellent	Excellent	Yes	Excellent flexibility, stability, and redundancy with the same number of HVdc CB as in 2WF-1SS
	SS side	NSS	NSS	No	Excellent	Excellent	Excellent	Yes
	Total	NSS	NWF + NSS	No	Excellent	Excellent	Excellent	Yes

[27,28,29,30].

8. Conclusions

In this study, available MT-HVdc transmission system topologies, as well as the one proposed by this paper, have been compared for offshore wind farms integration to onshore substations. Each topology has been characterized in terms of techno-economic factors, such as number and rating of dc circuit breakers and VSCs, the necessity of extra offshore stations, number, length and rating of dc-link, stability, flexibility, redundancy, cost, and meeting the maximum loss criterion. Simulations are developed in EMTDC/PSCAD to analyze the operation and control of the configurations in terms of (i) permanent GS-VSC disconnection and (ii) dc line–line faults. Results have shown that WFRT and 2WF-1SST are not conforming to the GBSQSS and hence not viable during GS-VSC disconnection and dc-link fault within remote/far-end/the last unit (unit-2 or unit-3 in Figure 5). Therefore, a more dynamic topology for the MT-HVdc grid is proposed and named as two wind farm and substation ring topology to overcome the aforementioned shortcomings. The proposed 2WF-SSRT is able to improve the stability, utilization, and redundancy while fulfilling the maximum loss criterion in the normal and abnormal conditions by providing an alternate path for the power flow with the least number and ratings of HVdc circuit breakers and grid side VSC-HVdc stations. Therefore, the anticipated 2WF-SSRT is a promising topology for the future wind farm MT-HVdc grids.