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

: 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.


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 powerto-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

•
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.

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 (S 1 ) and the offshore production platform DC network (S 2 ), with different DC voltages of E 1 and E 2 (E 1 > E 2 ) respectively. The MMC transmits the generated wind power P dc1 to S 1 and reverses power to feed S 2 under low/no wind conditions. The wind power P dc2 is transmitted to S 2 by the DR. The produced oil/gas or hydrogen is transported through pipelines or shipped to land. Pdc2 is transmitted to S2 by the DR. The produced oil/gas or hydrogen is transported through pipelines or shipped to land.

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 (MMCP and MMCN) 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 Pdc1 and Pdc2 can be expressed as: where Pdc1 and Pdc2 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:  Figure 1. Topologies of the offshore converter station: (a) Envisaged scenario for DC network interconnection; (b) symmetrical monopole configuration of the proposed DR-MMC Hub.

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 S 1 and S 2 is defined as positive, the ratios of the DC voltages E 2 and E 1 and the power transfer P dc1 and P dc2 can be expressed as: where P dc1 and P dc2 are the active power transmitted to S 1 and S 2 , respectively. Assuming the power losses on converters and transmission lines are negligible, the total transmitted power from the offshore wind farm is: The DC current at the DR (I 2 ) is the sum of the DC currents of DC networks S 1 (I 1 ) and S 2 (I 3 ), as: 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: (4) Consequently, the total active power of the two MMCs in the DR-MMC Hub is: Similarly, the active power of the DR is: If the conventional parallel system shown in Figure 1a is used, the converter power ratings of the MMC and the DR are: 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].
The DC current at the DR (I2) is the sum of the DC currents of DC networks S1 (I1) and S2 (I3), as: 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: Consequently, the total active power of the two MMCs in the DR-MMC Hub is: Similarly, the active power of the DR is: If the conventional parallel system shown in Figure 1a is used, the converter power ratings of the MMC and the DR are: 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 Pdc1 and 400 MW Pdc2 (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].  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.  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 MMCP and MMCN. 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.

System Control Principle
The control target of the DR-MMC Hub is to distribute the offshore wind farm power (PWF) to the two DC systems (i.e., Pdc1 and Pdc2). If Pdc2 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]: where the turn ratio and reactance of the DR transformer are denoted as TDR and XDR. VDR and VPCC 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, Pdc2 should be controlled by the DR. Based on (2) and (6), the relationships between active power, m and α are: From (9), the DC voltage ratio is fixed and the power transfer ratio is varied with the changes of PWF and Pdc2. 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]:  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]: 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 S 2 , P dc2 should be controlled by the DR. Based on (2) and (6), the relationships between active power, m and α are: 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]: Energies 2021, 14, 3759 6 of 16 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 S 2 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 E 2 , where the DC voltage error at S 2 sets the offshore AC voltage reference. When the DC voltage of the offshore production platform is controlled by S 2 , e.g., by other energy storage devices in S 2 , the power transmitted to S 2 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 v 0 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.

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., πE 2 /6 √ 2T DR ). If the offshore production platform behaves as a passive load, and E 2 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 S 2 network. Therefore, the MMCs should be blocked immediately to support the DC voltage E 1 , 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 S 2 , although the offshore AC voltage decreases quickly during the AC fault, E 2 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.

DC Fault Ride-Through
P2p DC faults in DC networks of S 1 and S 2 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 S 2 (F 2 ), the MMCs must provide the full DC voltage E 1 to break the fault current from S 1 . During an F 2 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 E 1 . The fault current during F 2 fault can be isolated as long as the total charged SM capacitor voltages in both MMC P and MMC N are higher than E 1 /2.
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 MMCP and MMCN 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:  The required SM capacitor voltage (V * arm_MMC ) in each arm for isolating an F 2 fault, and the nominal SM capacitor voltage (V * arm_MMC ) of the MMCs are calculated as: 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 E 1 /4 to avoid overcharging SM capacitors.
If a p2p fault happens at S 1 (F 1 ), the full DC voltage E 2 should be withstood by the MMCs. Thus, FBSMs are required to replace some HBSMs in MMCs to isolate the fault current from S 1 . As shown in Figure 4b, the HBSMs will be by-passed during F 1 fault. The fault current from S 2 during F 1 fault can be blocked as long as the total charged capacitor voltages of the FBSMs in MMC P and MMC N are higher than E 2 /2. The required capacitor voltage (V * arm_FBSM ) of the total FBSMs in each arm to isolate F 1 fault is given as: 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: 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.

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 η DR , η HB and η FB . Thus, the estimated power losses of the conventional parallel system can be obtained as: The estimated power losses of the DR-MMC Hub are: 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 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 lowcost DC switch/disconnectors for DC line protection, rather than using expensive DC circuit breakers.

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 S 1 and S 2 (P dc1 and P dc2 ) are both 400 MW. The DC cables in S 1 (100 km) and S 2 (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. 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 S 1 and S 2 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 S 2 is replaced by a passive load.  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 E 2 .  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 Pdc2 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 Pdc2, the surplus power is transmitted to S2, as can be seen in Figure 5b.

Operation in V dc -V ac Control Mode
The active and reactive power of the DR and MMCP shown in Figure 5c,d (MMCN 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 PDR).  Figure 5a shows that the DC voltage of S 2 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 S 1 or S 2 initially. When the passive load at S 2 is connected at 1.5 s, the DC power P dc2 is stepped to 400 MW, which is provided by the power reversed from S 1 through the MMCs. When the active power of the offshore wind farm is gradually increased at 2.0 s, the infeed power from S 1 to S 2 is reduced accordingly. After the wind power becomes higher than the DC power P dc2 , the surplus power is transmitted to S 2 , 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 S 2 , 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 ). 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.  Figure 6 illustrates the normal operation of the DR-MMC Hub with the MMCs operating in P-Vac control. Pdc2 is ramped up from 0 MW at 1 s, and to the rated value of 400 MW at 1.5 s, while PWF 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, Figures 6a and 6b show that Pdc2 is well controlled throughout the power variations. While PWF 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. PDR is decreased whereas PMMCp and Pdc1 are increased, so that the overall power is balanced. Similar to Figure 5d, the smooth reactive power exchanges of the DR and MMCP 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 MMCP, where P * DR is varied due to the variation of P * dc2 and PWF. As shown in Figure 6e, the common bus AC voltage is controlled by the MMCs in accordance with the required PDR transmission. 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. 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. Figure 7 shows the system performance during offshore AC faults in Vdc-Vac operation, while Figure 8 shows the corresponding response in P-Vac 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 Vdc-Vac control, the DC voltage of the DR is quickly decreased to zero after the occurrence of the AC fault. MMCP and MMCN 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 MMCP (I1) and the DR (I2) are rapidly reduced to zero during the AC fault. Once the fault is cleared, MMCP and MMCN 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 MMCP, and no overcurrent is observed during the AC fault. When the DR-MMC Hub operates in P-Vac 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 MMCP due to the current control, as shown in Figure 8c. The DC voltages of MMCP 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  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 E 1 . 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 (I 1 ) and the DR (I 2 ) 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.

AC Fault Ride-Through
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 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.  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 MMCP 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 MMCP 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 MMCP (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.  The MMC P DC voltage shown in Figure 9b increases to half of E 1 (320 kV) during a F 2 fault to break the fault current contributed by S 2 . For the F 1 fault, the MMC P DC voltage appears negative to half of E 2 (−100 kV) due to the use of FBSMs to handle the voltage applied by S 1 . The DC currents of MMC P (I 1 ) and the DR (I 2 ), and the current at the DC terminals of S 2 (I 3 ) are shown in Figure 9c. As can be seen, I 1 drops to zero during the DC fault due to the hybrid MMCs' fault blocking capability. Both I 2 and I 3 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 E 2 during the F 2 fault is quickly detected, MMC P is blocked immediately and there is no arm overcurrent. In the case of the F 1 fault, the drop of E 1 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 F 2 fault. For the F 1 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.).

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.