WITH THE increasing size of offshore power parks and

IEEE TRANSACTIONS ON POWER DELIVERY 1 Multiport High-Power LCL DC Hub for Use in DC Transmission Grids Dragan Jovcic, Senior Member, IEEE, and Weixing Lin, Member, IEEE Abstract This paper proposes an LCL dc hub concept which plays the role of a dc substation in a dc grid. The hub is capable of connecting multiple dc transmission lines with different dc voltages. Each dc transmission line can be added to or isolated from the hub without affecting the operation of the other dc transmission lines. The hub is based on multiple ac/dc insulated-gate biploar transistor-based converters and an internal passive LCL circuit without internal ac transformers. The LCL circuit is designed to interconnect multiple ac/dc bridges with different dc voltages and achieving minimal reactive power circulation at each converter bridge. Each ac/dc bridge controls its active power independently. Thedesignedhubhastheabilitytoridethroughthedcfaultsby keeping the fault current within the order of its rated value. Detailed PSCAD/EMTDC simulations of a seven-port test system are presented to validate the proposed topology. Index Terms DC power systems, DC power transmission, DC DC power conversion, HVDC converters, HVDC transmission, wind energy. I. INTRODUCTION WITH THE increasing size of offshore power parks and the need for new interconnectors, there has be growing interest in developing dc grids in the Europe. [1] [5]. The Future European Super Grid will be developed using dc transmission as the ac transmission is already highly congested and incapable of integrating the intermittent nature of renewable energy sources [1] [5]. In other countries such as China, large numbers of HVDC lines are now operating as point-to-point links. There would be significant operational and cost benefit if these lines could be interconnected or tapped on dc side [6]. Though there is significant interest for dc grids, the technology is still unknown because of technical challenges such as the lack of appropriate dc circuit breakers[1] [5], low rating of XLPE dc cables [2], power flow control difficulty in meshed dc grid [7], interconnecting dc transmission lines with different dc voltages [4] and protection and reliability issues [1] [5]. The protection of the dc grid is especially challenging. A prototype dc circuit breaker that can detect and isolate faulted dc transmission lines within several milliseconds is developed in Manuscript received March 08, 2013; revised May 17, 2013; accepted September 02, 2013. This work was supported by the European Research Council under the Ideas Program in FP7 under Grant 259328, 2010. Paper no. TPWRD-00267-2013. The authors are with the School of Engineering, University of Aberdeen, Aberdeen, AB24 3UE, U.K. (e-mail: d.jovcic@abdn.ac.uk; weixinglin@abdn.ac. uk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. DigitalObjectIdentifier 10.1109/TPWRD.2013.2280759 [8]. As there is no reactance in the dc grid, the small dc line resistances bring a range of issues during dc faults. Firstly, the whole large dc grid will see the dc fault and experience power interruption at all terminals. This implies urgency to rapidly clear the fault and raises importance of protection reliability. Also, small impedances make it difficult to locate fault in dc grid. It is likely that some form of (pilot wire) communication will be required between breakers in the grid. However, communication along dc cables requires time and complex processing logic. One-way communication and processing delay for a 200 km dc transmission line takes up several milliseconds. Furthermore, small dc impedances imply large fault currents and increase protection costs.theintegrityofadcgrid, which might spread over 1000 km, hinges on the fast operation of dc circuit breakers and accurate discrimination of fault location within few milliseconds. A dc grid requires significant investment and will likely be completed in steps over many years [1] [4]. A possible scenario is to first build point-to-point HVDC and regional multi-terminal HVDC systems which later may be connected and integrated to form a large dc grid [1] [4]. As technology progresses, the planned HVDC links will be operating at different dc voltages from the existing ones [9], [10]. Different manufacturers will also use different dc voltage and challenges arise how to interconnect different dc voltages. A solid-state dc-dc multiport converter based on dual active bridges which is able to interconnect multiple dc systems at different voltage levels can be developed using inner ac transformers [11]. This concept requires lossy high-power high-frequency transformers and still confronts the widespread voltage collapse if any point is at dc fault. Large protective inductors are needed to limit the short circuit currents before dc CB isolates the faulted dc transmission line [12], [13]. ALCLdc/dcconverter proposed in [14] has the abilities of interconnecting two dc transmission lines with different dc voltages, provide power flow control and inherently prevent dc fault propagation from one zone to another [14]. They however connect only two dc lines and the cost would prohibit widespread placement indcgrids. In order to provide single connection point for multiple dc lines and to overcome the protection challenge in dc grid, a multiport dc hub concept is studied in this article. The aim is to facilitate power exchange between numerous dc lines and to restrict thedcfault to only the faulted dc transmission line. The main contribution of this paper is design study of a LCL dc hub that combines the function of dc fault current decoupling, interconnecting of dc systems at different voltage levels and power flow control in one single component. The LCL dc hub also has the potential use in dc wind farm collection grid [15], [16] and in the power distribution field [11]. 0885-8977 2013 IEEE

2 IEEE TRANSACTIONS ON POWER DELIVERY Fig. 2. Two phase -port dc hub topology. B. ControlofDCGridsWithDCHubs Fig. 1. Five-terminal dc grid with a 5-port dc hub. II. DC GRIDS WITH LCL DC/DC HUBS A. Role and Benefits of the DC HUB in a DC Grid Fig. 1 shows a proposed topology of dc grid and the position of the hub. The dc grid has 5 terminals (ac/dc stations), and the hub has 5 ports which connect to dc lines. The term ports is used with hubs to avoid confusion with dc grid terminals. Themainobjectivesofthedchubinthedcgridare: 1) Provide opportunity for any terminal to exchange power with any other terminal in the dc grid. 2) Facilitate connection and disconnection of any dc line on the fly without affecting operation of the grid. Ideally the capability to connect unlimited number of new dc lines to the hub (flexible expansion) is desired. 3) In case of any dc cable fault, isolate the faulted dc cable and provide undisturbed operation of the remaining grid. 4) Ability to connect dc lines of different dc voltage levels. 5) Ability to control power in each dc line. As the dc hub is located centrally in the dc grid, it will have crucial role in the security of dc grid power transfer. The design and operation of the hub should ensure that no single fault will bring down the whole grid. Since dc cable faults are most common, we will design the hub that the faulted dc cable can be readily isolated without affecting operation of the remaining ports and the remaining segments of the dc grid. A dc grid with dc hub has concentric layered structure (passive circuit, converter, passive circuit, ) where the inner passive circuit of dc hub is in the center. The power transfer values have the most relevance at the outer layer, i.e., at terminals, which are the trading points for ac network operators. It is assumed that each terminal can set independently and arbitrarily the local desired power. The desired power at each terminal will be moderated by the local droop feedback which ensures power balance within the whole dc grid. This approach ensures grid integrity and this (maintaining all dc variables bounded) is the primary control aim of dc terminals. The primary control aim of the dc hub ports is maintaining all internal ac variables bounded (hub integrity). This is achieved by balancing internal power flow (the sum of powers on all ports should be zero). The desired power at each port is obtained either directly or indirectly. It can be obtained directly by communicating power order from the connecting terminal at higher layer or between two hubs. Alternatively, it can be obtained indirectly by detecting local dc voltage which will in turn respond to power changes at the connecting terminal. Within the above control framework there are numerous possible control implementations using cascaded controls with limiters and additional droop feedback control loops. A. Topology of the Hub III. LCL DC HUB Topology of the proposed LCL hub is shown in Fig. 2 where a 2 phase hub is illustrated. The hub comprises ports and 2 common ac buses. Each of the ports is comprised of 2 inductors, 2 capacitors and one dc/ac bridge with switches ( ). Each port is connected to the common ac buses through ac circuit breaker.

JOVCIC AND LIN: MULTIPORT HIGH-POWER LCL DC HUB FOR USE IN DC TRANSMISSION GRIDS 3 Fig. 4. versus. Fig. 3. Bipolar Modulation. In a frame which is aligned to the global reference angle, the ac voltage vector of the instantaneous voltage is A fixed frequency central Voltage Controlled Oscillator (VCO) is used to generate common reference angle for all ports. Each port generates ac voltage given by where is the voltage RMS value with its rated (maximum) value of, is the control signal, is operating frequency (of all ac variables), is phase angle of. is fixed and common for all ports. There are numerous methods of generating ac voltage from a given dc voltage, and Fig. 3 shows the bipolar modulation that is used in the 7-port test model. In the top graph the firing pulses for, pair are shown and those for, in the middle graph. The bottom graph shows the resulting ac voltage. The switching frequency is selected as which is the lowest possible value giving symmetrical ac waveform, as seen in Fig. 3. In practical applications, modular multilevel converter [1] [4] should be one of the best choices for converting the dc voltage to ac voltage, and it is expected that the power loss of each converter bridge should be around 0.5 1% of its rated capacity. (1) The subscripts and denote corresponding vector components in the frame, which are calculated as where, are - components of PWM modulated control signals, is the maximum line-neutral RMS value of. The equation for current of the inductor is The equation for the capacitor voltage is For simplicity, is maintained aligned with the axis in steady state and, therefore, (5) (6) (7) (8) B. Basic Circuit Equation (9) The fundamental RMS line-neutral ac voltage value in Fig. 3 is of (2) where voltage of port is the RMS line-neutral ac voltage of the capacitor. On assumption of (9), from (5) and (7), the ac current is expressed as It can be shown that in Fig. 3 satisfies the following: (3) where is magnitude of sine reference. From (2) and (3), can be calculated on any given. However, due to the high nonlinearity of (3), cannot be expressed explicitly using. We approximate with the ideal waveform: (4) Fig. 4 compares the ideal and actual values of which indicates that it is acceptable to use (4) to approximate (2). Considering (9), the capacitor voltage in (8) becomes (10) (11) Fig. 5 shows the phasor diagram of a 3-port test system with port1 injecting power to the hub and port2, port3 absorbing power. In this figure, is controlled to be aligned with the axis, and current vectors are assumed to be in phase(or opposite) with voltage vectors.

4 IEEE TRANSACTIONS ON POWER DELIVERY Substitute the current (10) into the apparent power (12) and taking the imaginary part of (12), the reactive power of each converter bridge is expressed as: (17) Fig. 5. Phasor diagram for a 3-port hub. C. Design of the Inner LCL Circuit Considering (10), the per phase apparent power at port is (12) The total apparent power of each port is,where is the number of phases. The real power ( ) is the real part of (12) (13) The condition for zero reactive power at converter bridge ( ), from (17) implies: B. Calculation of (18) The inductor is designed to enable rated (maximum) power transferring of port and, therefore, we substitute with and with in (13) and (18). For maximum utilisation of the converter, lowest current and conduction loss, we operate at largest modulation index and, therefore, substitute with. Taking square of (18) and adding to the square of (13) results: From (13) a general formula for designing the inductor per phase ( ) is deduced (14) We can further rearrange (19) in the following form (19) (20) where is the rated value of, is the port rated power per phase; is preselected according to different requirements. From this point it will be assumed that, which can be achieved with appropriate hub controls. The capacitor ( ) is designed to compensate the reactive current generated by at rated power. From (10) and (11) (15) (16) where is preselected according to different design requirements. IV. OPTIMAL DESIGN OF THE INNER LCL CIRCUIT A. Zero Reactive Current Operation at Each Port The proposed optimal design is to achieve zero reactive power at the converter bridge of each port atmaximum power condition in order to minimize the converter current, implying minimal component size and losses. Herein the reactive power is measured at each dc/ac converter bridge. The reactive power viewed at the capacitor side is of no concern as it does not have significant consequences in losses. The ability to reach zero reactive power at both the converter bridge and the coupling point is one of the advantages of introducing LCL circuit [17]. A conventional VSC converter can only reach zero reactive power at the coupling point. Equation (20) indicates that should be higher than any, or otherwise the term is less than zero, which never holds true. Equation (20) enables calculating under the condition of zero reactive power at port,as: C. Calculation of (21) Multiply both sides of (15) by and considering the zero reactive power condition (18) at maximum power, the formula for calculating is achieved as: (22) Replacing the from (21) into (22) results in the following formula that gives capacitor for given port power,port voltage and for selected. (23) By summing all capacitors in (23) for to,the total capacitor of the LCL hub is: (24)

JOVCIC AND LIN: MULTIPORT HIGH-POWER LCL DC HUB FOR USE IN DC TRANSMISSION GRIDS 5 Equations (21) and (23) give the formula of and assuming the maximum value of. It should be noted a lower than the value given in (14) will also be able to transmit the rated power. However such operation would require lower in numerator of (14). Furthermore, from (22) will be increased when decreases. In practice, to ensure control margin, the rated value of could be set as,thus the final and the final are given by (25) (26) D. Design Procedure Summary Design of the inner LCL circuit of the hub is summarized as: 1) Given are the rated power and the dc voltage of each port; 2) Select the operating frequency and the number of phases ; 3) Select the capacitor voltage. should be higher than the highest of according to (20), is typically selected around 20% higher than the highest. In general smaller implies higher fault currents, whereas larger brings natural LCL frequency closer to operating frequency. 4) Calculate and according to (25) and (26); 5) Test the fault current during dc fault to make sure the fault current is within acceptable range. V. OTHER ATTRIBUTES OF THE HUB A. Expandability and Flexibility of the Hub This section examines the possibility of expanding the hub with an additional port and the operation in case of tripping one or more dc ports. If a dc hub is designed according to previous methods, (21) shows that depends on and but it does not depend on parameters of any other ports ( is constant and assume that controller maintains at rated value). No matter how many other ports are connected or disconnected, the local power flow at port is solely dependent on, and. Thus, additional dc ports can be connected/disconnected to the dc hub readily without the need to change inductor or capacitor at any of the other ports. The required additional capacitor can be determined using (23). It is only required that ac voltage magnitude of the new port is lower than the magnitude of the capacitor voltage (according to (20)). If a port is tripped from the dc hub, the required reduction in capacitor will be according to (26). Therefore each port includes a circuit breaker which is located at bus side of.itenables that port is connected or disconnected without affecting operation of the remaining ports. It is assumed that mechanical ac circuit breakers will be adequate to operate at several khz, since this has been studied with development of high-power mechanical dc CBs in the 1980s [18]. Fig. 6. Simplified equivalent circuits of -port LCL dc hub. B. Multiphase Topology Fig. 2 shows a 2-phase dc hub topology. However there would be some advantages in using more than 2 phases for the inner ac system: 1) IGBTs have current limitation of around 1400 A and they are difficult for paralleling [19]. Multiple phases would increase power handling capability of the hub. Typically dc cables have higher current capability than IGBTs and multiphase ports would enable better use of dc cables. 2) Reliability will be improved. A -phase ( ) system can be readily reconfigured to operate as phase system by disconnecting one phase on all ports. This will be beneficial in case of internal faults or for hub maintenance. C. Possibility of LC Resonance As LCL circuit is used in the hub, there exists LCL resonant frequency which may cause prolonged oscillations. A particular problem occurs if resonant frequency is close to the operating frequency. The simplified equivalent circuit for one phase of the internal LCL circuit of Fig. 2 is shown in Fig. 6, where is the sum of the capacitor of all the ports. The impedance of the hub viewed from each port is Setting to zero results in natural LCL frequency : (27) (28) If ac voltage at the above frequency is supplied at port,then currentatthatportwillbeinfinity. It is seen that (28) is identical for all ports and represents global hub resonance. Substituting (22) into (28) results: (29) Equation (29) indicates that is always higher than the operating frequency.as increases approaches and only for,. Smaller implies that is at higher frequency and damping naturally improves. Also (29) indicates that always holds true regardless whether some ports are tripped or new ports are added to the hub.

6 IEEE TRANSACTIONS ON POWER DELIVERY maintained to,andsince, (17) indicates that could be increased by decreasing. All the ports therefore can control local power. However the desired port power will be moderated in response to the internal power unbalance, which is detected using capacitor voltage. B. Controllability of Capacitor Voltage Fig. 7. Resonant frequency versus. Substituting the current (7) into (8) results: The second difficulty arise if local resonance. in (27), resulting in (31) (30) In order to simplify (31), define constant : This will imply that local current is insensitive to local voltage and, therefore, port cannot control local current if applied voltage is at the resulting frequency. Fig. 7 shows the resonant frequencies versus of the 7-port test hub, which is described in detail in Section VIII. From (21) should be higher than any so is varied according to the ratio over which is the highest value among. should be chosen that no resonant frequencies falls at the operating frequency. In Fig. 7, is the global resonant frequency, are the local resonant frequencies and is the operating frequency of the hub. D. Voltage Stepping Equations (13), (17) and optimal design in Section IV indicate possibility for rated power transfer for any,whichin turn is directly dependent on dc voltage. If voltage source converters are used at hub ports, the ac voltage is firmly linked with dc voltage and, therefore, converters are only stressed for local dc voltage. Even in cases when is considerably higher than at port, the port electronics has optimal rating ( and ). Note however that all hub CBs will be rated for. Nevertheless this may not be a significant drawback since low-cost mechanical CBs could be used. VI. CONTROLLABILITY OF THE HUB Since the inner LCL circuit of the LCL dc hub is a passive ac network and all the high frequency capacitors, inductors and CBs are located in one or several nearby buildings, a Voltage Controlled Oscillator is used to generate global reference angle for all the ports. This method is similar to the case of connecting VSC to a dead ac grid, like connecting to an offshore wind farm [20]. No PLL will be required. A. Control of Active Power and Reactive Power Equation (13) indicates can be used to control,which increases following the increase of.insteadystate, is Substitute equation for capacitor (22) into (32) results Thus,. Using (33), (31) is simplified to (32) (33) (34) (35) Therefore (34) and (35) prove that and will respond well to and control signals respectively. Furthermore, if we multiply both sides of (34) by and considering (13) we obtain the hub active power balance equation: (36) To maintain to, (35) indicates that one approach is to let each be fixed to its rated value for all power levels. In such case will be good indicator of hub power unbalance. C. Reactive Power at Partial Power Load According to (13), can be reduced by reducing in partial load condition. Equation (17) implies that each will be negative when is reduced and kept constant. Theoretically, from (17) reactive power can be kept to zero by manipulating at partial load. However will be kept constant in order to maintain in (35) to since constant is of paramount importance for the hub. As a conclusion, zero reactive power is only obtained at rated condition.

JOVCIC AND LIN: MULTIPORT HIGH-POWER LCL DC HUB FOR USE IN DC TRANSMISSION GRIDS 7 VII. FAULT STUDY A. Faults on Connecting DC Lines Combine (34) and (35) in the following form: TABLE I PARAMETERS OF 7-PORT TEST SYSTEM (37) Equation (37) gives the expression of in terms of.if adcfaulthappensatport, becomes zero and this clearly implies that reduces to a new value. The ac current of any fault free port is calculated as TABLE II COMMON PARAMETERS OF 7-PORT TEST SYSTEM (38) The ac current of the faulted port is calculated as: (39) Although direct analytical formula for magnitude of fault currents cannot be derived, it is seen that denominator in (39) is large (both and are larger than with typical VSCs) implying limited fault current magnitudes. The extensive detailed simulations are performed with realistic parameters and the fault current are normally 1.1 1.3 pu of the rated current. This is analytically confirmed for a two-port case in [14]. This is significant finding, since semiconductors can withstand this overcurrent and there is no need to trip ports for dc faults. B. Internal Faults The internal hub faults are only briefly analyzed since HVDC equipment is always located in valve halls and it is known that valve hall faults are extremely rare. The internal faults on port valves, inductors or capacitors can be readily isolated, considering that each port has a CB, as seen in Fig. 2. A fault on one of common ac busses would be a common failure mode for the whole hub and this potentially has significant consequence. However the central ac bus can be further split using additional CBs as it is done with common ac substations. Also, if a multiphase inner topology is used, then converter controls can isolate and reconfigure inner phases, in order to provide uninterrupted operation. VIII. VERIFICATIONS Detailed simulation of a 7-port dc hub on PSCAD/EMTDC is used to validate the design, operation and fault current limiting properties of the hub. Table I lists the rated values of line to neutral dc voltage, rated power,, and local resonant frequency of each port. The symbol behind each is the scheduled power direction of each port ( : inject, : absorb). It should be noted that power direction of each port can either be inject or absorb. Table II lists the rated capacitor voltage, number of phases, operating frequency, switching frequency, and global resonant frequency of the 7-port hub. We have deliberately selected each voltage to be different to fully demonstrate the ability of LCL dc hub to interconnect widely different dc systems. The LCL dc hub also works well if all the dc systems have a unified voltage level. Open loop control is used to study the intrinsic behavior of the dc hub without the effect of particular control logic. In the open loop control, each, is manually given. Since the paper focus is on the behavior of the LCL dc hub, each of the dc system is modeled as simple batteries. Current rating and resistance of each dc cable is selected according to [19] considering that length of each dc cable is 100 km. A. Verification on Connecting/Isolating a Port and Zero Reactive Power at Rated Power Fig. 8 shows the steady state operation and the effect of isolating/connecting a dc transmission line (with associated port) to the hub. As shown in Fig. 8(a) and (b), each is increased from zero to rated value from 0 to 0.2 s. Each is increased from zeor to rated value from 0.3 s to 0.5 s. Port 1 is tripped from the hub at 0.6 s and reconected to the hub at 0.7 s. During the trip and reconnecting of port 1, is manually changed to maintain power balance of the hub., and are per unit values with respect to the rated capacitor voltage. and are per unit values with respect to the rated power of corresponding port. Fig. 8(a) and (c) verifies that the capacitor voltage can be controlled by according to (35). With the increase of each, the capacitor voltage increases. stays almost at rated value (1 pu) when each stays at rate value and does not change with the change of each asshowninfig.8(c). stays at zero as power balance is maintained through the tests. Fig. 8(b) and (d) shows that can be used to control active power of each port according to (13). Each increases to rated power when is increased to rated value. Fig. 8(e) verifies that each port can reach zero reactive power at the converter bridge at rated power and each port is

8 IEEE TRANSACTIONS ON POWER DELIVERY Fig. 9. Response to dc faults (at port 4 at 1 s and at port 2 at 1.3 s). (a) DC voltages at dc side of each port. (b) DC currents at dc side of each port. (c) AC currents of each port. (d) Capacitor Voltages. Fig. 8. Steady state operation and isolating/connecting a dc transmission line. (a).(b).(c)capacitor Voltages. (d) Active Power. (e) Reactive Power. absorbing reactive power when in partial load according to(17). Fig. 8(d) and (e) verifies that tripping or reconnecting of a port will not affect the normal operation of the other ports. B. Verification of DC Fault Responses Fig. 9 shows system response to dc cable faults. Permanent dc fault happens at port 4 at 1.0 s and port 4 is tripped from the hub by its ac circuit breaker at 1.05 s (50 ms clearing time). Similar to the previous test, is changed manually to maintain power balance following trip of any port. After the transients settle another permanent dc fault is applied at port 2 at 1.3sandport2istrippedfromthehubbyitscircuitbreaker at 1.35 s. Two successive dc faults are applied to demonstrate that the hub can ride through dc faults with some ports already tripped from the hub. Fig. 9(a) and (b) verifies the LCL dc hub has the ability to limit propagation of dc faults. The dc faults only bring down the dc voltages at the faulted ports, whereas voltages at the other ports remain unaffected. There are no dc overcurrent at the

JOVCIC AND LIN: MULTIPORT HIGH-POWER LCL DC HUB FOR USE IN DC TRANSMISSION GRIDS 9 faulted port 4 and port 2 which indicate that the LCL dc hub will not contribute fault currents to the dc faults. Also, dc currents at the other ports remain almost unchanged. Measuring points of each isshowninfig.2.asecondorderfilter with 100 Hz characteristic frequency is applied in the measurement loop. It can be also deduced from Fig. 9(a) and (b) that active power of the fault-free dc lines remains almost unchanged during the dc faults. Fig. 9(c) and (d) shows that dc faults will not cause ac overcurrent in the hub, and no severe voltage variation happens at the common ac buses. However, the fault disturbance will cause low frequency oscillations (relative to the operating frequency ) at the inner LCL circuit. The low frequency oscillation can be minimized by designing corresponding damping controllers. IX. CONCLUSION This paper presents a LCL dc hub which has capability to limit the propagation of dc faults in a dc grid. The hub is also able to interconnect multiple dc transmission lines at different voltage levels by using a specially designed inner LCL ac circuit. The hub can be labelled as dc substation for dc grid, which is similar to the function of ac substation in ac grids. At rated power on any of the ports the hub can achieve zero reactive power at the converter bridge of the particular port, which reduces losses in the electronics. dc transmission lines can be connected to or isolated from the hub without affecting the operation of other dc transmission lines. Each port is able to control power on local dc transmission independently. Detailed simulations of a 7-port hub in PSCAD/EMTDC validate the proposed topology and confirm that dc fault at any dc line will not propagate to the other ports. REFERENCES [1] D. Van Hertem and M. Ghandhari, Multi-terminal VSC HVDC for the European supergrid: Obstacles, Renew. Sustain. Energy Rev., vol. 14, no. 9, pp. 3156 3163, Dec. 2010. [2]N.Ahmed,A.Haider,D.VanHertem,L.Zhang,andH.P.Nee, Prospects and challenges of future HVDC Super Grids with modular multilevel converters, presented at the 14th Eur. Conf. Power Electron. Appl., Birmingham, U.K. [3] J. De Decker, P. Kreutzkamp, S. Cowdroy, L. Warland, J. Völker, and J. Tambke et al., Offshoreelectricity grid infrastructure in Europe, Final rep. 3E and EWEA, Oct. 2011. [Online]. Available: http://offshoregrid.eu/ [4] D. Jovcic, K. Linden, D. Van Hartem, and J. P. Taisne, Feasibility of DC transmission networks, presented at the ISGT Eur., Manchester, U.K.,Dec.2011. [5] C.M.Franck, HVDCcircuitbreakers: A review identifying future research needs, IEEE Trans. Power Del., vol. 26, no. 2, pp. 998 1007, Apr. 2011. [6] D. Huang, Y. Shu, J. Ruan, and Y. Hu, Ultra high voltage transmission in China: Developments, current status and future prospects, Proc. IEEE, vol. 97, no. 3, pp. 555 589, Mar. 2009. [7] E. Veilleux and B. T. Ooi, Multiterminal HVDC with thyristor power-flow controller, IEEE Trans. Power Del., vol. 27, no. 3, pp. 1205 1212, Jul. 2012. [8] J. Häfner and B. Jacobson, Proactive hybrid HVDC breakers A key innovation for reliable HVDC grids, presented at the CIGRE 2011 Bologna Symp., Bologna, Italy, Sep. 2011. [9] N. Flourentzou, V. G. Agelidis, and G. D. Demetriades, VSC-based HVDC power transmission systems: An overview, IEEE Trans. Power Electron., vol. 24, no. 3, pp. 592 602, Mar. 2009. [10] R. L. Koropatnick, HVDC Projects Listing, Teshmont Consultants LP, Mar. 2012. [Online]. Available: http://www.ece.uidaho.edu/hvdcfacts/ [11] S. Falcones, R. Ayyanar, and X. Mao, A DC-DC multiport converter based solid state transformer integrating distributed generation and storage, IEEE Trans. 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Zhang, and M. Hajian, LCL VSC converter for high-power applications, IEEE Trans. Power Del., vol. 28, no. 1, pp. 137 144, Jan. 2013. [18] S. Tokuyama, K. Arimatsu, Y. Yoshioka, Y. Kato, and K. Hirata, Development and interrupting tests on 250 kv 8 ka HVDC circuit breaker, IEEE Trans. Power App. Syst., vol. PAS-104, no. 9, pp. 2453 2459, Sep. 1985. [19] HVDR Light It s Time to Connect, Tech. Brochure, ABB, Dec. 2012 [Online]. Available: http://www.abb.co.uk/ [20] L.Xu,L.Z.Yao,andC.Sasse, GridintegrationoflargeDFIG-based wind farms using VSC transmission, IEEE Trans. Power Syst., vol. 22, no. 3, pp. 976 984, Aug. 2007. Dragan Jovcic (S 97 M 00 SM 06) received the Ph.D. degree in electrical engineering from the University of Auckland, Auckland, New Zealand, in 1999. Currently, he is a Professor with the University of Aberdeen, Aberdeen, U.K., where he has been since 2004. He was also a Lecturer with the University of Ulster, Ulster, U.K., from 2000 to 2004 and a Design Engineer in the New Zealand power industry from 1999 to 2000. His research interests are flexible ac transmission systems, HVDC, as well as integration of renewable sources and control systems. Weixing Lin (S 11 M 13) received the B.Sc. degree in electrical engineering from Huazhong University of Science and Technology (HUST), Wuhan, China, in 2008, where he is currently pursuing the Ph.D. degree in electrical engineering. Currently, he is a Research Associate with Aberdeen University, Aberdeen, U.K. His research interest is a high-power LCL dc dc hub, LCL dc dc converters, and dc grids.