A Diode-MMC AC/DC Hub for Connecting Offshore Wind Farm and Offshore Production Platform

Abstract

A diode rectifier-modular multilevel converter AC/DC hub (DR-MMC Hub) is proposed to integrate offshore wind power to the onshore DC network and offshore production platforms (e.g., oil/gas and hydrogen production plants) with different DC voltage levels. The DR and MMCs are connected in parallel at the offshore AC collection network to integrate offshore wind power, and in series at the DC terminals of the offshore production platform and the onshore DC network. Compared with conventional parallel-connected DR-MMC HVDC systems, the proposed DR-MMC hub reduces the required MMC converter rating, leading to lower investment cost and power loss. System control of the DR-MMC AC/DC hub is designed based on the operation requirements of the offshore production platform, considering different control modes (power control or DC voltage control). System behaviors and requirements during AC and DC faults are investigated, and hybrid MMCs with half-bridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operation during DC faults. Simulation results based on PSCAD/EMTDC validate the operation of the DR-MMC hub.

1. Introduction

The global energy transition from fossil fuels to renewable and sustainable alternatives is accelerating, with increased interest in developing offshore wind energy. In Europe, the total installed offshore wind capacity has reached 22.1 GW in 2019 [1]. There has also been increased interest in offshore production platforms (e.g., oil and gas production plants), and the benefits of utilizing offshore wind power and onshore HVDC grid to supply the offshore oil/gas platform have been addressed to replace power generated by gas turbines for carbon reduction commitment [2]. As an efficient and clean form of storing energy, offshore hydrogen can be produced from offshore wind power through power-to-gas technology to accommodate a large amount of intermittent renewable energy in power network [3,4], which could be deployed at unused oil/gas platforms as a financially attractive solution [5]. The interests in offshore production platforms potentially lead to the needs of offshore converter stations with the capability of transmitting wind power to the electrical grid connected to onshore and offshore production platforms with different DC voltages. For example, a concept for integrating offshore wind farm and offshore hydrogen production (delivering power up to 400 MW) is proposed in [3], where the offshore wind power is transmitted to both the onshore power grid and an offshore hydrogen platform by the offshore converter stations at different DC voltage levels.

Due to lower investment, footprint and higher efficiency than other converters, modular multilevel converters (MMCs) and the diode rectifier (DR)-based HVDC transmission systems have been proposed for integrating offshore wind farms [6,7,8,9,10]. However, for the connection of offshore wind/production platform and onshore grid system, using only DRs is not suitable as they cannot separately control the power transmissions to multiple terminals. Thus, a hybrid configuration combining the uncontrolled DR and the fully-controlled MMC is likely required.

Parallel operation of MMC-HVDC and DR-HVDC systems to transmit power from offshore wind farms has been analyzed in [6]. The MMC regulates the offshore AC voltage to support the DR, rather than using distributed wind turbine converters with specific grid-forming control schemes [6,9]. However, the converter power rating of the MMC in this scheme can be quite high, considering the different operation requirements and conditions, leading to an increased cost.

Several hybrid HVDC solutions have been proposed to combine the advantages of different converter topologies. A hybrid scheme with a series-connected line-commutated converter (LCC), or a DR and MMC, has been proposed in which the MMC is used to maintain the AC voltage and frequency [11,12]. However, the hybrid is only dedicated to a single DC network. An MMC-based DC autotransformer (DC AUTO) has been considered for connecting the inner AC bus of the DC AUTO to an external AC system to achieve power exchange between the two DC networks and the AC system [13]. However, the operation control and fault ride-through operation when connecting with an offshore wind farm have not been investigated. A hybrid AC/DC hub composed of the LCC and MMC in series connection has been proposed for onshore wind-power integration and interconnection of two DC networks with different DC voltages [14], but this hub is not suitable for offshore applications due to the large footprint and heavyweight of the LCC, which also needs a strong AC grid for commutation.

To overcome the above challenges, a DR-MMC AC/DC Hub (DR-MMC Hub) with the DR and MMCs connected in parallel at the AC side, and in series at the DC side, is proposed to transmit offshore wind power to an onshore DC network and an offshore production platform with different DC voltage levels. The main contributions of this paper are as follows:

• The proposal of a new DR-MMC Hub which enables part of the power from the DR to be transmitted to the onshore DC network directly, and thus reduces the size of the MMC and lowers the cost and power loss of the overall converter system when compared with the conventional approach using parallel DR and MMC.
• Based on the operation requirement of the offshore production platform, comprehensive operating conditions of the proposed DR-MMC Hub are investigated, considering the different control modes (power control or DC voltage control).
• Detailed fault ride-through of the DR-MMC Hub and system design are analyzed. For AC faults, the hub can ride through them without adopting any specific protection schemes. For DC faults on either DC network, the hub can isolate them by introducing a hybrid MMC configuration.

The rest of this paper is organized as follows. The topology analysis of the DR-MMC Hub is depicted in Section 2, while Section 3 describes the system control principle of the DR-MMC Hub. A comprehensive AC/DC fault ride-through and converter power loss estimation of the DR-MMC Hub are analyzed in Section 4. Simulation verifications of the DR-MMC Hub during normal operation, and AC and DC faults are given in Section 5. Finally, conclusions are summarized in Section 6.

2. Topology and Efficiency Analysis

2.1. Envisaged Operation Scenario

The envisaged operation scenario is illustrated in Figure 1a, where the offshore converter with parallel-connected MMC and DR, transmits wind power to the onshore DC network (S₁) and the offshore production platform DC network (S₂), with different DC voltages of E₁ and E₂ (E₁ > E₂) respectively. The MMC transmits the generated wind power P_dc1 to S₁ and reverses power to feed S₂ under low/no wind conditions. The wind power P_dc2 is transmitted to S₂ by the DR. The produced oil/gas or hydrogen is transported through pipelines or shipped to land.

2.2. DR-MMC Hub Configuration

Figure 1b depicts the topology of the DR-MMC Hub in a symmetrical monopole setup. The proposed topology is mainly composed of a DR and two MMCs (MMC_P and MMC_N) in series connection.

If power transferring from the offshore wind farm to S₁ and S₂ is defined as positive, the ratios of the DC voltages E₂ and E₁ and the power transfer P_dc1 and P_dc2 can be expressed as:

$$\{\begin{array}{c}m=E_2/E_1, 0<m<1,\\ \alpha =P_{dc1}/P_{dc2}, \alpha >0,\end{array}$$

(1)

where P_dc1 and P_dc2 are the active power transmitted to S₁ and S₂, respectively.

Assuming the power losses on converters and transmission lines are negligible, the total transmitted power from the offshore wind farm is:

$$P_{WF}=P_{dc1}+P_{dc2}=\left(1+\alpha \right)P_{dc2}.$$

(2)

The DC current at the DR (I₂) is the sum of the DC currents of DC networks S₁ (I₁) and S₂ (I₃), as:

$$I_2=I_1+I_3=\left(P_{dc1}/E_1+P_{dc2}/E_2\right).$$

(3)

Assuming the voltage drops on the DC lines are low, and can be neglected, the DC voltage of the DR and the sum of the two MMCs’ DC voltages are:

$$V_{DR}=E_2, V_{MMC}=V_{MM{C}_P}+V_{MM{C}_N}=E_1-E_2.$$

(4)

Consequently, the total active power of the two MMCs in the DR-MMC Hub is:

$$\begin{array}{cc}\hfill P_{MM{C}_{hybrid}}& =V_{MMC}{I}_1=\left(E_1-E_2\right)\frac{P_{dc1}}{E_1}\hfill \\ & =\left(1-m\right)P_{dc1}=\left(\alpha -\alpha m\right)P_{dc2}\hfill \\ & =\frac{\alpha - \alpha m}{1 + \alpha }{P}_{WF}.\hfill \end{array}$$

(5)

Similarly, the active power of the DR is:

$$\begin{array}{cc}\hfill P_{D{R}_{hybrid}}& =V_{DR}{I}_2=E_2\left(\frac{P_{dc1}}{E_1}+\frac{P_{dc2}}{E_2}\right)\hfill \\ & =m{P}_{dc1}+P_{dc2}=\left(m\alpha +1\right)P_{dc2}\hfill \\ & =\frac{m\alpha + 1}{1 + \alpha }{P}_{WF}.\hfill \end{array}$$

(6)

If the conventional parallel system shown in Figure 1a is used, the converter power ratings of the MMC and the DR are:

$$\{\begin{array}{c}{P}_{MMC\_con}=P_{dc1}=\alpha P_{dc2}>P_{MMC\_hybrid},\\ P_{DR\_con}=P_{dc2}<P_{DR\_hybrid}.\end{array}$$

(7)

The converter power ratings of the MMC and DR in different system configurations are shown in Figure 2. Figure 2a indicates that the converter power rating of the MMCs in the DR-MMC Hub is smaller than that in the parallel-connected configuration shown in Figure 1a, especially when the voltage ratio m is high (close to 1). In contrast, as shown in Figure 2b, the DR power rating of the proposed system is higher than that of the conventional parallel system. For example, in a ±100 kV/±320 kV DR-MMC Hub transferring 400 MW P_dc1 and 400 MW P_dc2 (i.e., m = 0.3125 and α = 1), the required power ratings of the MMC and DR are 275 MW and 525 MW, respectively, compared to 400 MW and 400 MW in the parallel connection design. Therefore, the overall efficiency and cost of the DR-MMC Hub are superior to the conventional parallel system as the power loss and cost of the DR for high power schemes are much lower than that of the MMC [8,9].

3. System Control Principle

Figure 3 shows the control structure of the DR-MMC Hub. An aggregated wind turbine model is considered, where the rotor-side converter (RSC) controls the DC-link voltage while the grid-side converter (GSC) regulates the active and reactive power [7]. The reactive power of the wind farm is set to be zero in the study, and reactive power compensation is provided by the MMCs of the offshore hub.

For the offshore hub shown in Figure 1b, a 12-pulse DR converter with AC filters is used, while hybrid MMCs with HBSMs and FBSMs are adopted for MMC_P and MMC_N. The details on the need for FBSMs will be discussed later in the paper. The MMCs in the offshore hub operate in grid-forming mode to control the offshore AC network with desirable voltage amplitude and frequency [15]. As shown in Figure 3, the reference of the q-axis voltage $v_q^{*}$ is set to be zero, and the local frequency at the point of common coupling (PCC) is set to be a constant value (e.g., 50 Hz), while the reference of the d-axis voltage $v_d^{*}$ is regulated as described late in the paper.

The control target of the DR-MMC Hub is to distribute the offshore wind farm power (P_WF) to the two DC systems (i.e., P_dc1 and P_dc2). If P_dc2 is determined by the offshore production platform, the DC voltage of the DR should be maintained at a constant value, which is given as [6]:

$$V_{DR}=\frac{6}{\pi }\left(\sqrt{3}{T}_{DR}{v}_{PCC}-\frac{P_{DR}{X}_{DR}}{V_{DR}}\right),$$

(8)

where the turn ratio and reactance of the DR transformer are denoted as T_DR and X_DR. V_DR and V_PCC are the DC voltage of the DR and the PCC voltage, respectively.

If the offshore production platform controls the DC voltage of the network S₂, P_dc2 should be controlled by the DR. Based on (2) and (6), the relationships between active power, m and α are:

$$\{\begin{array}{c}{P}_{dc1}=P_{WF}-P_{dc2},\\ P_{dc2}=\frac{P_{DR}}{1 + ma},\\ a=\left(P_{WF}/P_{dc2}\right)-1.\end{array}$$

(9)

From (9), the DC voltage ratio is fixed and the power transfer ratio is varied with the changes of P_WF and P_dc2. Under low wind conditions, when the wind farm output is insufficient for the offshore production platform, the power from the onshore HVDC network is reversed through the MMCs. The active power of the DR is expressed as [16]:

$$P_{DR}=\frac{\sqrt{2}{T}_{DR}{E}_2}{X_{DR}}{v}_{PCC}-\frac{\pi E_2{}^2}{6X_{DR}}.$$

(10)

From (8) and (10), the DR’s DC voltage and active power are largely determined by the AC voltage V_PCC.

Based on the alternative control targets of the offshore production platform as previously described, i.e., DC voltage (V_DR) control or active power (P_dc2) control, the offshore voltage amplitude $v_d^{*}$ is generated with different outer loops. When the offshore production platform behaves like a passive load, the DC voltage of S₂ is determined by the DR through the control of the MMCs. In this case, the V_dc-V_ac loop controls the DC voltage of the DR to the reference value E₂, where the DC voltage error at S₂ sets the offshore AC voltage reference. When the DC voltage of the offshore production platform is controlled by S₂, e.g., by other energy storage devices in S₂, the power transmitted to S₂ needs to be controlled. In this event, the P-V_ac control loop is implemented to regulate P_dc2, where the power output $P_{DR}^{*}$ is dynamically regulated by the desired power reference $P_{dc2}^{*}$ and the variation of P_WF to produce the offshore AC voltage reference. A set-point v₀ is added to keep the offshore AC voltage in the range of 0.9 to 1 p.u. for both cases, as shown in Figure 3. The circulating current suppression control (CCSC), capacitor voltage control and modulation methods which were investigated in [17,18] will not be discussed in this paper.

4. Fault Ride-Through and Power Loss Estimation

4.1. AC Fault Ride-Through

The response of the DR-MMC Hub and the offshore wind farm during various AC faults in different operation scenarios are presented in this subsection.

An offshore AC fault leads to a significant reduction of the offshore AC voltage and current-limiting operation of the wind turbine converters [19]. There is no active power transmitted from the DR station, of which conduction is blocked when the AC voltage becomes lower than the minimum DR conduction voltage (i.e., ${\pi E}_2/6\sqrt{2}{T}_{DR}$).

If the offshore production platform behaves as a passive load, and E₂ is controlled by the MMCs, the DC voltage collapses with the decrease of the offshore AC voltage, behaving like a pole-to-pole (p2p) DC fault at the S₂ network. Therefore, the MMCs should be blocked immediately to support the DC voltage E₁, as detailed in Section 4.2. After fault clearance, the system recovers to the normal operation quickly once the MMCs are re-enabled.

If the MMCs control the DC power of S₂, although the offshore AC voltage decreases quickly during the AC fault, E₂ is maintained at the rated value by the offshore production platform. No active power is being transmitted through the MMCs or the DR. For the MMCs in the offshore hub, the fault current in the AC fault cases is limited without disturbing DC side performance due to the inner current control loop, which has been well researched in [20]. After the AC faults, the system is restored rapidly with the recovery of the AC system voltage.

4.2. DC Fault Ride-Through

P2p DC faults in DC networks of S₁ and S₂ are considered. If a DC fault occurs on either DC network, the healthy DC side feeds the fault current into the faulty DC side through the MMC due to its direct electrical connection [13,14]. On account of the expensive cost and large volume relative to ACCBs at comparable voltages, high-voltage DC circuit breakers (DCCBs) are not considered in the offshore scheme to interrupt the DC faults [21,22].

In the event of a p2p fault happening at S₂ (F₂), the MMCs must provide the full DC voltage E₁ to break the fault current from S₁. During an F₂ fault, the fault current path of the blocked MMCs in the DR-MMC Hub is shown in Figure 4a. The fault current path (shown in red) indicates that all SM capacitors of the MMCs are charged by the DC voltage E₁. The fault current during F₂ fault can be isolated as long as the total charged SM capacitor voltages in both MMC_P and MMC_N are higher than E₁/2.

The required SM capacitor voltage ($V_{arm\_MMC}^{*}$) in each arm for isolating an F₂ fault, and the nominal SM capacitor voltage ($V_{arm\_MMC}^{*}$) of the MMCs are calculated as:

$$\{\begin{array}{c}{V}_{arm\_MMC}^{*}=E_1/4,\\ V_{arm\_MMC}=\left(1-m\right)E_1/2.\end{array}$$

(11)

If $V_{arm\_MMC}^{*}$ > $V_{arm\_MMC}^{*}$ (i.e., m > 1/2), additional HBSMs are required for each arm of the MMCs to withstand the voltage of E₁/4 to avoid overcharging SM capacitors.

If a p2p fault happens at S₁ (F₁), the full DC voltage E₂ should be withstood by the MMCs. Thus, FBSMs are required to replace some HBSMs in MMCs to isolate the fault current from S₁. As shown in Figure 4b, the HBSMs will be by-passed during F₁ fault. The fault current from S₂ during F₁ fault can be blocked as long as the total charged capacitor voltages of the FBSMs in MMC_P and MMC_N are higher than E₂/2. The required capacitor voltage ($V_{arm\_FBSM}^{*}$) of the total FBSMs in each arm to isolate F₁ fault is given as:

$$V_{arm\_FBSM}^{*}=E_2/4=m{E}_1/4.$$

(12)

If the SM number of each MMC arm without fault considerations is denoted as $N_{SM}$, according to (11) and (12), the required FBSMs ($N_{FB}^{*}$) and HBSMs ($N_{HB}^{*}$) numbers in the hybrid MMCs for isolating DC faults can be calculated as:

$$N_{FB}^{*}=\frac{m}{2-2m}{N}_{SM},N_{HB}^{*}=\{\begin{array}{c}\frac{2-3m}{2-2m}{N}_{SM},m\le 1/2,\hfill \\ \frac{1}{2}{N}_{SM},1/2<m<1.\hfill \end{array}$$

(13)

From (13), the insulated-gate bipolar transistor (IGBT) device cost per MVA in the MMCs increases compared to the HB-MMCs without fault consideration. Taking a ±100 kV/±320 kV DR-MMC Hub as an example, to safely isolate the DC faults, the required number of FBSMs and HBSMs in the hybrid MMCs are approximately 0.23N_SM and 0.77N_SM, respectively. Considering the cost of a FBSM is 1.5 times that of an HBSM, the IGBT device cost per MVA of the hybrid MMCs with DC fault blocking capabilities is 1.115 times the HB-MMC base value. However, even taking this into consideration, the proposed DR-MMC Hub still requires fewer switching devices (i.e., IGBTs) when compared to the conventional design outlined in Section 2.

Once a DC fault is detected, the MMCs that regulate the offshore AC voltage and frequency are blocked immediately to isolate the fault current from the AC to the DC side. Consequently, power transmission from the offshore AC network to the DC side is interrupted. If the offshore wind turbines are controlled in such a way that they continue generating active power, the surplus wind power increases the offshore AC voltage. Consequently, overcurrent could occur in the DR. Thus, after blocking the MMCs, the ACCB that connects the offshore wind farm and the DR-MMC Hub is opened to interrupt the potential overcurrent of the DR from the AC side.

When the DC current is reduced to near zero (<10 A) by blocking the MMCs, the fast DC switches on and the faulty part can be opened with the help of a selected DC fault detection algorithm. The DR-MMC Hub and offshore wind farm are then isolated from the DC fault point until fault clearance.

4.3. Valve Power Losses Estimation

The power losses in the proposed DR-MMC Hub are estimated considering the losses of the DR, HBSMs and FBSMs denoted as ${\eta }_{DR}$, ${\eta }_{HB}$ and ${\eta }_{FB}$. Thus, the estimated power losses of the conventional parallel system can be obtained as:

$${\eta }_{con}=\frac{{\eta }_{HB}{P}_{dc1}+{\eta }_{DR}{P}_{dc2}}{P_{WF}}.$$

(14)

The estimated power losses of the DR-MMC Hub are:

$${\eta }_{hybrid}=\frac{\left({\eta }_{FB}\frac{N_{FB}^{*}}{N_{SM}}+{\eta }_{HB}\frac{N_{HB}^{*}}{N_{SM}}\right)P_{MMC\_hybrid}+{\eta }_{DR}{P}_{DR\_hybrid}}{P_{WF}}.$$

(15)

References [8,23] indicate that the valve power losses of the DR, HBSMs and FBSMs are approximately 0.11%, 0.6% and 1.1%, respectively.

Table 1. Comparison between the two different configurations.

Configuration	Parallel System	DR-MMC Hub
DR power rating	400 MW	525 MW
MMC power rating	400 MW	275 MW
FBSM ratio	0%	23%
Valve power loss	0.355%	0.318%
DC fault blocking	No	Yes

compares the two different configurations in a ±100 kV/±320 kV system transferring 400 MW P_dc1 and 400 MW P_dc2. Although FBSM-based MMCs have higher power losses than MMCs with HBSMs only, the estimated power loss of the DR-MMC Hub is only 0.3175%, whereas the power loss of the offshore converter system in parallel connection is approximately 0.355%. Furthermore, the ability of the DR-MMC Hub to block DC fault indicates the potential use of low-cost DC switch/disconnectors for DC line protection, rather than using expensive DC circuit breakers.

5. Simulation Verifications

The system shown in Figure 1b is modeled using PSCAD/EMTDC. The DC voltages are ±100 kV/±320 kV, and the DC power transmitted to S₁ and S₂ (P_dc1 and P_dc2) are both 400 MW. The DC cables in S₁ (100 km) and S₂ (50 km) are modeled using the frequency-dependent model in PSCAD/EMTDC. Fast DC switches are installed between the converters and DC cables.

The simulated system parameters are shown in

Table 2. Simulated system parameters.

Parameters		Nominal Value
MMCP & MMCN	Power rating	137.5 MW
	Rated DC voltage	±220 kV
	SM capacitor voltage	1.83 kV
	SM capacitance	7.5 mF
	Arm inductance	0.0241 H
	SM number per arm	125
	FBSM number per arm	35
	Interfacing transformer voltage ratio	66 kV/110 kV
	DC smoothing reactance	0.1 H
12-pulse DR bridge	Power rating	525 MW
	Rated DC voltage	±100 kV
	Reactive power compensation	0.4 p.u.
	Interfacing transformer voltage ratio	66 kV/87.3 kV/87.3 kV
	DC smoothing reactance	0.1 H
Wind farm aggregated model	Power rating	800 MW
	Interfacing transformer voltage ratio	0.69 kV/66 kV
	AC cable length	10 km

. The hybrid MMC adopts the equivalent averaged model to improve simulation efficiency [24]. The simulation results of the MMC_P and MMC_N are the same due to the symmetrical monopole topology of the proposed system and, therefore, only the results of the MMC_P are provided here. The DC voltages of S₁ and S₂ are given at the rated value when the MMCs operate in P-V_ac control mode. If the MMCs operate in V_dc-V_ac control mode, the DC voltage source in S₂ is replaced by a passive load.

5.1. Operation in V_dc-V_ac Control Mode

Figure 5 shows the normal operation of the DR-MMC Hub during power variation between the offshore production plant (P_dc2) and offshore wind farm (P_WF). Initially, P_WF is 0 and ramped up to the rated value of 800 MW from 2.0 s to 2.5 s. The initial power demand of the offshore production platform (P_dc2) is set at zero, and is stepped to the rated value of 400 MW at 1.5 s by connecting a passive load to E₂.

Figure 5a shows that the DC voltage of S₂ is well controlled by the MMCs throughout the power and load variations. According to Figure 5b,c, no power is transmitted from the offshore wind farm to S₁ or S₂ initially. When the passive load at S₂ is connected at 1.5 s, the DC power P_dc2 is stepped to 400 MW, which is provided by the power reversed from S₁ through the MMCs. When the active power of the offshore wind farm is gradually increased at 2.0 s, the infeed power from S₁ to S₂ is reduced accordingly. After the wind power becomes higher than the DC power P_dc2, the surplus power is transmitted to S₂, as can be seen in Figure 5b.

The active and reactive power of the DR and MMC_P shown in Figure 5c,d (MMC_N is identical, thus not shown here) follow the system power change smoothly, and the offshore wind farm reactive power is well regulated at zero. To maintain the DC voltage of S₂, the MMCs regulate the common bus AC voltage (shown in the RMS value in Figure 5e, and is varied with the change of P_DR).

5.2. Operation in P-V_ac Control Mode

Figure 6 illustrates the normal operation of the DR-MMC Hub with the MMCs operating in P-V_ac control. P_dc2 is ramped up from 0 MW at 1 s, and to the rated value of 400 MW at 1.5 s, while P_WF is ramped up from 0 MW at 2.0 s, to the rated value at 2.5 s and then ramped down by 0.15 p.u. (120 MW) from 3 s to 3.1 s. As a result, Figure 6a,b show that P_dc2 is well controlled throughout the power variations. While P_WF remains at 0.85 p.u. (680 MW), and the power reference $P_{dc2}^{*}$ shown in Figure 6d is dropped by 0.25 p.u. (100 MW) from 3.5 s to 3.6 s. P_DR is decreased whereas P_MMCp and P_dc1 are increased, so that the overall power is balanced.

Similar to Figure 5d, the smooth reactive power exchanges of the DR and MMC_P are shown in Figure 6c, and the zero reactive power from the offshore wind farm is set by the control objective in the simulation study.

Figure 6d shows the power references $P_{DR}^{*}$ and $P_{dc2}^{*}$ from the outer controller of the MMC_P, where $P_{DR}^{*}$ is varied due to the variation of $P_{dc2}^{*}$ and P_WF. As shown in Figure 6e, the common bus AC voltage is controlled by the MMCs in accordance with the required P_DR transmission.

5.3. AC Fault Ride-Through

Figure 7 shows the system performance during offshore AC faults in V_dc-V_ac operation, while Figure 8 shows the corresponding response in P-V_ac control. During the studies, a 200 ms three-phase to ground fault occurs at 3.0 s.

As shown in Figure 7a,b, when the DR-MMC Hub operates in V_dc-V_ac control, the DC voltage of the DR is quickly decreased to zero after the occurrence of the AC fault. MMC_P and MMC_N are blocked immediately and their SM capacitors support the DC voltage E₁. Consequently, each of the MMC DC terminal voltage increase from 220 kV to 320 kV, as can be seen in Figure 7b, while Figure 7c shows that the DC currents of MMC_P (I₁) and the DR (I₂) are rapidly reduced to zero during the AC fault. Once the fault is cleared, MMC_P and MMC_N are re-enabled to restore the DC voltages of the MMCs and the DR, and the DC currents return to the prefault values. Figure 7d shows the upper arm current of MMC_P, and no overcurrent is observed during the AC fault.

When the DR-MMC Hub operates in P-V_ac control, during the AC fault the converters of the offshore wind farm and the MMCs all enter in current limiting operation. As can be seen from Figure 8a,b, the collapse of the offshore AC voltage during the fault quickly reduces the DC currents and power transmission to zero. There is no overcurrent in the arm currents of MMC_P due to the current control, as shown in Figure 8c. The DC voltages of MMC_P and the DR shown in Figure 8d recover to their nominal values after the initial transients when the fault occurs. After fault clearance and the recovery of the offshore AC voltage, the DC currents and voltages return to prefault condition and no overcurrent occurs. As can be seen, AC faults do not affect the secure operation of the DR-MMC Hub.

5.4. DC Fault Ride-Through

Figure 9 illustrates the system performance when a permanent solid p2p fault (F₁ or F₂) is applied at 3.0 s. The waveforms in the first column show the system performance under the F₂ fault, and the system performance under F₁ fault is shown in the second column.

The DC link voltage at the faulty part drops immediately when the DC fault happens, as shown in Figure 9a. The blocking time of the MMCs is set to 2 ms after the fault occurrence, then the offshore ACCB is opened after blocking the MMCs with 60 ms open time delay.

The MMC_P DC voltage shown in Figure 9b increases to half of E₁ (320 kV) during a F₂ fault to break the fault current contributed by S₂. For the F₁ fault, the MMC_P DC voltage appears negative to half of E₂ (−100 kV) due to the use of FBSMs to handle the voltage applied by S₁. The DC currents of MMC_P (I₁) and the DR (I₂), and the current at the DC terminals of S₂ (I₃) are shown in Figure 9c. As can be seen, I₁ drops to zero during the DC fault due to the hybrid MMCs’ fault blocking capability. Both I₂ and I₃ experience overcurrent from the DR and are reduced to zero after the ACCB is opened at 3.062 s. The fast DC disconnectors/switches can then open to disconnect the faulty branch so no DCCB is required.

Figure 9d shows the upper arm current of MMC_P during the DC faults. The collapse of E₂ during the F₂ fault is quickly detected, MMC_P is blocked immediately and there is no arm overcurrent. In the case of the F₁ fault, the drop of E₁ leads to voltage collapse and blocking of MMC_P. The initial arm overcurrent flows through the freewheeling diodes in the MMC and is quickly reduced to zero, as shown in Figure 9d.

Figure 9e shows the averaged upper and lower arm FBSM and HBSM capacitor voltages of MMC_P during the DC faults. All SMs are charged and controlled at around the nominal value during the F₂ fault. For the F₁ fault, the averaged voltages of the bypassed HBSM capacitors are around the rated value, while those of the FBSMs are increased up to 2.08 kV (1.14 p.u.), which is within the safe margin (typically around 1.3~1.4 p.u.).

6. Conclusions

A DR-MMC AC/DC Hub comprised of series-connected DR and MMC for connecting offshore wind farms with onshore DC network and offshore production platform is proposed in this paper. System configuration and efficiency, and control and operation during normal and fault conditions of the proposed DR-MMC hub were studied. This paper considers a ±100 kV/400 MW DC system for offshore production platform, and a ±320 kV/400 MW for onshore DC network. Compared with the conventional approach using paralleled-connected MMC and DR, the proposed DR-MMC Hub reduces the MMC rating from 400 MW for the conventional approach to 275 MW while the DR rating is increased from 400 MW to 525 MW. The proposed hub can also reduce power losses by 10.5% due to lower power losses of the DR compared to MMC, while a smaller MMC also leads to lower investment costs. Considering different operation requirements, two control modes were developed in the proposed DR-MMC Hub to control the voltage or power of the DC system for the offshore production platform. Due to current-limiting control and MMC blocking capabilities, the DR-MMC Hub can securely ride through offshore AC faults in different operation scenarios. The DR-MMC Hub can also isolate DC faults at the two DC networks due to the adopted hybrid MMCs with DC fault blocking capability. PSCAD/EMTDC simulations verified the performance of the proposed hub during normal operations in different operating scenarios and AC/DC fault cases.