Multiphase Multi GW LCL DC hub With High Security and Redundancy

Transcription

1 1 Multiphase Multi GW LCL DC hub With High Security and Redundancy Wei-Xing Lin and Dragan Jovcic School of Engineering, King s College, University of Aberdeen, Aberdeen AB24 3UE, UK Abstract Future DC transmission grids may include a central DC hub which will be connecting numerous DC lines and transferring multi GW power. The LCL DC hub is a multiport high power DC/DC converter that incorporates an inner LCL circuit to interconnect DC systems with different DC voltages through DC/AC ports. The LCL DC hub has an intrinsic attribute to limit the propagation of DC faults such that DC fault at one DC transmission line will not bring down the DC voltage or cause DC over current on other DC lines. As the operating security of the hub is of top concern, this paper proposes a reconfigurable multiphase method to avoid a common failure mode. The n-1 redundancy and expandability using multi-phase approach are analyzed and compared with other methods. A fast, on-line phase reconfiguration and phase replacing control of the DC hub are proposed and verified, enabling the hub to continue operating under single-phase faults. Common mode current of the LCL DC hub is analyzed to provide guidelines on structure and operation of the ports. It is concluded that 4-phase topology, possibly with a redundant 5th phase might offer best overall performance and with high redundancy/security required for large DC grids. Key Words Super Grid; DC power systems; DC power transmission; DC-DC power conversion; HVDC transmission system security *Corresponding author. Tel.: ; fax: E-main addresses:.( weixinglin@abdn.ac.uk, *d.jovcic@abdn.ac.uk).

2 2 1 INTRODUCTION With the increased installation of offshore wind farms and the advances in the VSC (Voltage Source Converter) HVDC technology, there has been an increasing interest in building DC grids [1]-[2]. The European DC Super Grid is being studied since the AC transmission is already highly congested and incapable to balance the intermediate nature of renewable energy sources [1]-[2]. A conventional approach to build DC grid is to interconnect different DC transmission lines using DC circuit breakers (CB) [3]. This method confronts the significant challenge of coordination of protection for a large DC grid [4]. Because of very small DC cable impedance, the DC faults will cause widespread voltage collapse and require fast discriminative protection action in the time scale of few milliseconds, which is technically very challenging [4]. Also the DC grid based on DC CB is not able to interconnect DC lines at different DC voltages. The existing and planned HVDC lines do not have a unified DC voltage level [5] and therefore the DC voltage stepping function becomes important in building DC grids. There is significant benefit in developing DC grids using hub-type components which provide opportunity for power exchange between multiple DC terminals having different DC voltages. A solid-state dc-dc multiport converter based on dual active bridges can be developed using inner AC transformers [6]. However, the DC-DC converter based on dual active bridge requires lossy high-power high-frequency transformers and still confronts the challenge of 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 [7]. To resolve DC fault propagation issue, and therefore provide required security for the future DC grid, a LCL DC/DC converter is proposed in [8] to connect two different DC systems. The future DC grids like North Sea grid will have 2-1GW capacity and will be connecting several countries [1]-[2], and it would be impractical to use numerous DC/DC converters. The LCL

3 3 DC/DC converter can be expanded to a multiport LCL DC hub to interconnect more than two DC systems [9]. A DC hub enables start (or radial) connection of DC terminals, and multiple DC hubs can be located in the same DC grid. As the LCL DC hub is located centrally between multiple DC terminals, it plays crucial role in DC grid security. The LCL DC hub should be designed in such way that any internal fault within the hub will not bring down the whole hub (no common failure modes), since this would imply loss of DC grid terminals. In this paper, the security is considered at a component level from a technical standpoint. Redundancy is not normally used with HVDC systems (except switch redundancy at valve level) because of costs of power electronics and limited power levels [1]. However, since the power exchange within a DC grid is large, we are arguing for introducing phase redundancy in the LCL DC hub in order to ensure that the hub can still provide power exchange between grid terminals when one segment/component is unavailable [2]. While in AC systems a single phase fault implies loss of component/capacity, the power electronics units with fast control provide opportunity to reduce/eliminate loss in capacity. This paper will explore reconfigurable multiphase and multi block topologies within DC hub using redundant converter arms and passive components. We aim at developing a component which retains either partial or full power transfer under single-phase faults. Since the inner DC hub circuit is an islanded AC system, the number of phases can be selected to enable best utilization of components (DC cables and switch ratings) [11]-[12], to provide redundancy [13] and to minimize ground current circulation[14]. The link between redundancy and reliability has been established for AC systems [15] and similar methods can be used for DC systems. The topology flexibility and expandability are also important for developing future DC grids since new terminals can be added and upgrades are expected on the existing terminals. The ground harmonic current circulation may not be big issue with HVDC but will become more important as number converters increase in a large DC grid. We will further explore DC hub

4 4 topology impact on ground harmonics. 2 The LCL DC hub 2.1 Basic attributes of the hub Fig. 1 shows the topology of a 4-phase four port LCL DC hub. Each port comprises a 4-phase DC/AC converter, 4 inductors L i, 4 capacitors C i and 4 single phase AC circuit breaker CB ia CB id. Each phase of each port is connected to a common AC bus Bus_A Bus_D through corresponding circuit breakers. Each inductor L i is designed to enable transmitting of rated power while each capacitor C i is designed to locally compensate the reactive current generated by L i. A circuit breaker is placed between each of the capacitors and the common AC bus such that on tripping a phase or tripping a port, the inductor together with its associated capacitor will be tripped from the hub. The remaining inner LCL circuit still maintains reactive power balance and can operate without interruption. Each inductor and capacitor for a port i is designed according to the following two equations. M V ( V ) ( M ) ( V ) L (1) i r r r 2 r 2 r 2 i i c i i r Pi o P ( V ) ( M ) ( V ) C (2) r r 2 r 2 r 2 1 i c i i i r 2 r r o ( Vc ) Mi Vi Where r M i is the rated modulation index typically selected to be.95 to ensure control margin. r r P i is the rated power of port i per phase, V c is rated RMS line-neutral magnitude of capacitor voltage, r V i is the maximum RMS line-neutral magnitude of the terminal voltage of port i. In a dq frame, the capacitor voltage is controlled such that V V r, V. Angle of the d axis comes from a centrally located voltage controlled oscillator(vco). The real power per phase of each port is cd c cq

5 5 r P V V / ( L ) M V V / ( L ) (3) i iq c o i iq i c o i Where M iq is the q component modulation index of port i. From (3), we can see that M iq could be used to control the active power P i. Equations for capacitor voltage are: KV c KV c cq cd Where N r iq i i 1 o i N M V L MidV L r i i 1 o i K c N 1 oc is a positive value whose positive sign is guaranteed by the design of the L i 1 o i (4) hub. One port in the hub (suppose the k th port) will be used to maintain power balance of the hub employing M kq control signal. The d component modulation index of each port M id will directly influence the capacitor voltage. A control strategy to maintain each M id (except M kd ) to its rated value and to use M kd with a close loop controller is adopted to control the capacitor voltage. More detailed design and control principles of the LCL DC hub are presented in [9]. If conventional 3-phase AC/DC converters and controls are used at DC hub ports, then any internal fault (on LCL circuit or ports) will result in loss of entire hub. 2.2 Fast phase reconfiguration control The traditional 3-phase AC systems cannot operate if one phase is faulted. However in a DC hub, regardless of the number of ports, a phase can be disabled with fast control action on the corresponding converter leg at all ports. The fast phase disconnection would enable possibility to operate at reduced power under a loss of one phase. Fig. 2 shows the phase reconfiguration control of a port i, while the above presented control strategy is used. For the 2-level VSC converters shown in Fig. 1, the bipolar modulation shown in Fig. 3 with triple carrier wave will be used to generate the firing pulses. The reference angle for firing logic of phase A comes from a voltage controlled oscillator, namely

6 6 θ A =2πf o t. f o is the operating frequency of the LCL DC hub and is constant for all the ports. The reference angle for each of the other phases lags the previous phase by φ=36 /p, where p is the number of phases. φ is 9 for a 4-phase LCL inner circuit and 12 for a 3-phase LCL circuit. On detecting a failure on one phase in the hub, the Flag_Trip in Fig. 2 is set to 1, φ is gradually increased from 9 to 12 during the time span T. The inner LCL circuit in the whole hub is reconfigured from a 4-phase to a 3-phase balanced system. The inner LCL circuit can also be reconfigured from a 3-phase system to a 4-phase system if φ is decreased from 12 to 9. A finite reconfiguration time span ( T) is applied to avoid large transients during phase reconfiguration. Otherwise there would be abrupt phase difference between the AC voltage of each port and the central capacitor voltage which would cause large disturbance to the hub. 2.3 Fast phase replacing control To improve the security of power transfer through the multi-phase LCL hub, it is recommended to add a redundant standby phase. On scheduled maintenance or failure of one phase, the standby phase can be used to replace the unavailable phase so that the power transfer capability of the hub is unaffected. Fig. 4 shows the control logic for the replacement of phase B with a standby phase E. On setting the signal Flag_Replace to 1, firing pulses of the operating phase B are blocked while firing pulses of the standby phase E are enabled. To enable a controlled charging of the currents and capacitor voltage of the standby phase, M ie is gradually increased from zero to M i during a time span of T. 3 Three structures for building LCL DC hub 3.1 Multiphase hub Fig. 1 depicts a multiphase implementation of the LCL DC hub. The multiphase implementation has the advantage that the capacity of the hub could be expanded at a step of one phase, instead of

7 7 adding new ports (converters). Assuming that the average DC current of the largest IGBT is 1.4kA, the switch RMS AC current is 1.4*π/ 2 /2=1.56kA, giving phase current approximately kA (assumed as 1.6kA). In a ±32kV DC system, suppose the rated modulation index is.95, the RMS AC voltage per phase is 32/ 2 *.95=215kV. Thus the active power per phase is 347MW. The increase of DC current of transmission line due to adding of one phase is calculated by 1.6*215/(2*32) =.54kA. Fig. 5 depicts the changes of DC current and DC power versus number of phases for a port connecting to ±32kV DC transmission line. Capacity of the hub could be expanded at a step of 347MW. This implies that there is more flexibility in converter design to match the power levels of DC cables and transmission lines [12]. The option of adding a new phase also provides flexibility for future dc hub power upgrades. A redundant standby phase could be added in the multiphase hub to provide phase redundancy. All the circuit breakers of the redundant phase are connected to the common AC busses in normal operation while the firing pulses of the redundant phase valves are blocked. On detecting a single phase fault, the standby redundant phase will replace the faulted phase using the control strategy in Fig. 4 while the faulted phase will be isolated from the DC poles using the procedure described below. The firing pulses of the faulted phase will be blocked first. The capacitor will discharge through the diodes until the capacitor voltage is clipped to the lowest DC voltage of the ports. Subsequently, the capacitor can further be discharged using a dissipation resistor to bring down the capacitor voltage to zero. An isolation switch will then isolate the phase leg from the DC poles. Note that all ports must have same number of phases, which may be disadvantage of multiphase hub as low power ports also need to implement multiphase converter bridge using lower current switching devices.

8 phase Multi-block hub Fig. 6 gives the implementation of LCL DC hub using 2-phase blocks. In this topology, a DC cable supplies two 2-phase blocks which then internally connect to different inner Ac circuits in the hub. Note that existing DC cables can supply power for two 2-phase block built of largest IGBTS. To expand the capacity of the hub, a new 2-phase block needs to be added. Similar to Fig. 5, Fig. 7 gives the changes of DC current and active power versus the change of the number of 2-phase blocks. It is seen that the capacity of the hub is expanded in steps of 694 MW. The advantage of having multiple blocks is in security. Since multiple blocks are used at one single port, there are separate AC buses for the same port. These AC buses can be located in separate buildings to minimize common failure mode. Not all ports need to be connected to two blocks, but those that have high power and high security/availability requirements will be structured to connect to two or more blocks. Different blocks of the same port do not need to be connected to the common AC buses with the same voltage rating. Taking a case of transmitting wind power as an example, where we use DC hub to connect wind farm local DC grid to high voltage DC lines for long distance power transmission. If some of the power is used to supply a nearby regional DC system with lower DC voltage then AC voltage of one of the blocks can be low in order to minimize component costs. An additional advantage of multiple blocks is that small ports need not have large number of phases. In Fig. 6, port 4 has smaller power rating and it only connects to one 2-phase block phase Multi-block hub Fig. 8 gives the 2-block, 3-phase implementation of LCL DC hub, which follows the tradition of AC power transmission and common AC/DC converter topologies. Each block of each port comprises 3 phase legs, but various blocks on one port may not have same current or AC voltage.

9 9 The blocks may connect to the same port in case that one port can handle high power demanded by two 3-phase blocks, or they connect to different ports. Fig. 9 gives the changes of DC current and active power versus the change of number of 3-phase blocks, indicating the capacity of the hub is expanded at a step of 14MW. 4 Security and Redundancy of the LCL DC hub Each port of a DC hub will have approximately 1GW power and there could be numerous ports in a large DC grid (like the North Sea Super Grid in [2]). The power transfer security of the hub is of utmost importance. The topology and operation of the hub should ensure that no common mode fault will bring down the whole hub. 4.1 DC faults If a DC fault happens on any connected DC line, the LCL DC hub has an intrinsic property to limit current on the faulted DC transmission line (prior to any control action), and the impact on other ports will be minimal. If a DC line is in permanent fault, then IGBTs on respective port of all blocks will be blocked and the associated CBs on all phases will be tripped. All the remaining ports will be able to operate unrestricted on all phases without need to change power level or operating condition. If a DC cable redundancy is required, then new DC cables and ports should be installed. 4.2 Internal AC faults In case of internal AC faults, two options are presented for a multiphase hub: 1. Trip the whole faulted block which might be suitable with multi-block structures. 2. Trip the faulted phase (on all ports) and reconfigure the remaining phases to a new balanced system with reduced number of phases, as presented in section II. The power on all ports will be reduced by one-phase power. If there is a redundant phase it can be inserted in place of faulted phase in a fast manner, enabling n-1 redundancy inside DC hub.

10 1 In case of multi-block 2-phase structure, there is an option is to trip the faulted phase and merge the remaining healthy phase with another un-faulted 2-phase block to make a 3-phase balanced LCL circuit, or trip the faulted 2-phase block. 5 Common mode voltage of the LCL DC hub It is known that there will be common mode voltage and ripple current in paralleled 3-phase inverters [14], and the problem becomes more pronounced with DC grids. The common mode voltage is undesirable since insulation level will increase and also if hub is grounded there will be ground current exchange with other terminals in DC grid. This section will study the cause and the magnitude of the common mode voltage to provide guides for structure and operation of DC hubs. 5.1 Common voltage of the DC hub with 2-level VSC Fig. 1 displays the equivalent circuit of a 3-phase hub where the AC voltage of each 2-level DC/AC converter is expressed using switching function. The switching function of the M th phase is defined as: S im 1, upper switch on 1, lower switch on (5) Where i is the number of port and M=A,B,C. Current equations of inductor L i are: S V v L ( di / dt) ia idc ca i ia S V v L ( di / dt) ib idc cb i ib S V v L ( di / dt) ic idc cc i ic (6) Summing the left and right sides of (6) results: S V - v L ( di / dt) (7) icom idc ccom i icom where S icom =S ia +S ib +S ic, i com =i ia +i ib +i ic and v ccom =v ca +v cb +v cc are defined as the total switch states, common current and common capacitor voltage. Equations for the capacitor voltages are:

11 11 N ia i 1 N ib i 1 N ic i 1 i C( dv / dt) ca i C( dv / dt) cb i C( dv / dt) cc (8) where C is total capacitance on all ports. Sum of the left and right parts of (8) results N iicom C( dvccom / dt) (9) i 1 From (7) and (9), the equivalent circuit for common current and common capacitor voltage is constructed and shown in Fig. 11. If the bipolar modulation shown in Fig. 3 is applied for generation of AC voltage from DC voltage, S icom is a series of discrete pulses whose value could be -3, -1, 1 and 3. Relationship of S icom and the conduction states of the switches are: S icom 3, three lower switches on 1, two lower and one upper switches on 1, one lower and two upper switches on 3, three upper switches on (1) Each switching function S im is a series of PWM bars with a period of 1/f o. S ib and S ic lags S ia by 2π/3 and -2π/3 respectively. Thus, S ( t) S ( t 2 ) ia o ia o S ( t) S ( t 2 / 3) ib o ia o S ( t) S ( t 2 / 3) ic o ia o (11) Where ω o =2πf o. Sum of both sides of (11) results S ( t) S ( t) S ( t 2 / 3) S ( t 2 / 3) (12) icom o ia o ia o ia o Shift S icom by 2π/3 results S ( t 2 / 3) S ( t 2 / 3) S ( t) S ( t 4 / 3) S ( t) (13) icom o ia o ia o ia o icom o

12 12 Equation (13) indicates the frequency of S icom is triple of f o if the number of phases is 3. By Fourier series analysis, the common voltage v icom =S icom V idc is expressed as: 4 3 vicom Vidc [ (1 2sin 3 )sin(3 ot ) (1 2sin( ) sin(9 ot) (1 2sin(15 )) sin(15 ot) ] 9 15 (14) where δ is shown in Fig. 2. Equation (14) indicates the common mode voltage of a 3-phase hub contains 3 rd,9 th and 15 th zero sequence components. For a hub with 4-phases, the relationship of the switching functions of all the four phases is: S ( t) S ( t 2 ) S ( t ) ia o ia o ia o S ( t) S ( t / 2) ib o ia o S ( t) S ( t ) S ( t) ic o ia o ia o S ( t) S ( t / 2) S ( t) id o ia o ib o (15) The sum of both sides of (15) is. S ( t) S ( t) S ( t) S ( t) S ( t) (16) icom o ia o ib o ic o id o Equation (16) indicates that the common mode voltage with even number of phases will theoretically be zero. The above analysis using 3-phase hub and 4-phase can be readily generalized. It is concluded that a p-phase hub with even number of phases has zero common mode voltage while a hub with odd number of phases has common mode voltage with frequency of p*f o. 5.2 Ground current of the hub when MMC are used In high voltage high power applications, Modular Multilevel MMC will most likely be used in the LCL DC hub. According to [16], equivalent circuit of one phase of MMC at the AC side can be represented by an internal AC voltage e v in series with RL circuit with the resistor equal to R /2 and inductor equal to L /2 where R and L are arm resistor and arm inductor respectively. The internal AC voltage e v is calculated by [16]

13 13 e ( u u ) / 2 (17) v dn up where u dn and u up are the total inserted capacitor voltages of the down arm and upper arm. Suppose the capacitor voltage of each sub-module is maintained to U c and the modulation index is m(t). Then u dn and u up are calculated by: 1 mt ( ) u N * U round ( N)* U 2 u N * U ( N N )* U up up c c dn dn c up c (18) where N is the total number of sub-modules per arm. N*U c =U dc and U dc is the pole DC voltage of MMC. The modulation index m(t)=mcos(2πf o t+φ m ). Similar to (7), define the common internal AC voltage as: eicom eiva eivb eivc (19) From (17) (19), voltage e icom is calculated as: ma( t)* N mb( t)* N mc ( t)* N eicom [ round ( ) round ( ) round ( )]* Uc (2) where m A (t)=mcos(2πf o t+φ ma ), m B (t)=mcos(2πf o t+φ ma -2π/3) and m C (t)=mcos(2πf o t+φ ma +2π/3). Similar to Fig. 11, Fig. 12 shows the derived equivalent circuit for LCL DC hub if MMC is used as the DC/AC converter bridge. From Fig. 12, the common current is caused by the nonzero e icom. Note that e icom will always stay at zero if the round function in (18) is removed. Thus, the common current is caused by the discontinuity of the values of each e ivm. Since the voltage step of the AC voltage of MMC is the capacitor voltage of each sub-module, it is foreseen that the common voltage will be very small compared with the total output AC voltage of MMC. The voltage e icom has only three discrete values, U c, and U c regardless of the number of sub-modules per arm. Fig. 13 shows the value of e icom for a 9 level MMC. For a MMC such as the one implemented in HVDC transmission in [17] the number of

14 14 sub-modules per arm is 4, thus the magnitude of common mode voltage is in the order of 1/4 of the phase voltage. The magnitude of common mode current should be in the order of 1/(p*4) of the operating current, which is negligible. Fig. 13 also shows the frequency of e icom is triple of the operating frequency. Similar to (16), e icom = if the number of phases is an even number. 5.3 Grounding of the hub Table 1 summarizes the relationship of common mode current with the number of phases and the technology of VSC converter. There is a considerable ground current only if odd number of phases with 2-level VSC converter is used at each port. For other cases, the ground current is theoretically zero or of negligible portion. For high power high voltage applications, modular multilevel converters will be used at the ports of LCL DC hub. Since the ground current will be theoretically zero or of negligible magnitude, it can be concluded that the common ground bus of the inner LCL circuit (Bus_G in Fig. 1) could be grounded to enable safety and simple detection of ground faults. All other inner blocks in Fig. 6 can also be grounded. A grounded hub is also desirable during phase reconfiguration or phase replacement. During these events, there will be temporary phase unbalance and grounding of the neutral bus provides current path for the common mode current of the LCL circuit. Otherwise the voltage of the common neutral bus will be nonzero, which will deteriorate the capacitor voltage and also stress the insulation. 6 Verifications Detailed simulation of a 4-port DC hub in PSCAD/EMTDC is used to demonstrate the high security and redundancy of multiport LCL DC hub. Parameters of one phase of the 4-port hub are shown in

15 15 Table 2. P ir is the rated power per phase. A 2-level VSC topology is used with the operating frequency of 125Hz, and rated RMS line to neutral voltage at the common AC bus is 378.5kV. 6.1 On-line reconfiguration of the inner LCL circuit Fig. 14 shows the simulation of tripping phase C from a DC hub that is originally operating at full power with 3 phases. At 1.s, phase C of all the 4 ports is tripped by corresponding circuit breaker CB 1C CB 3C. The controller reacts to establish a balanced 2-phase system, and in the reconfiguration interval (1.s to 1.1 s) the phase difference φ is gradually increased from 2π/3 to π (Fig. 14(a)). Fig. 14(b) shows that each 3-phase port is absorbing /injecting 3pu DC power from/to the hub before tripping of phase C. Power of each port reduces to 2pu after phase C is tripped. Fig. 14(c) shows that the AC capacitor voltages are balanced before and after phase reconfiguration. The capacitor voltage is only temporarily unbalanced during phase reconfiguration. Fig. 14(d) shows the magnitudes of ground current where magnitudes of the fundamental component, the 3 rd harmonic, and the RMS value of the ground current are shown. The measuring point of the ground current is shown in Fig. 1. Before 1.s, the system is operating as a 3-phase system using the 2-level VSC converters, and Fig. 14(d) shows that there is considerable 3 rd harmonic component of the ground current. This is consistent with conclusions in section 5. The magnitude of 3 rd harmonic is similar to the total RMS magnitude of the ground current indicating that very little other harmonics are present. After 1.s, the hub is operating with even number of phases and there is no ground current. Fig. 14 confirms that the hub can be reconfigured "on the fly" by connecting/disconnecting a phase, while operation of other phases is unaffected. 6.2 Replacing a faulted phase with redundant phase Fig. 15 shows system response when replacing a faulted phase with a redundant standby phase.

16 16 The hub is initially operating as balanced 4-phase system. A permanent AC fault is applied at Bus_B at.5 s. On detecting the AC current increase to 2 times of the rated value, the IGBTs of faulted phase are blocked. Subsequently within 2-5ms, the AC circuit breakers of the faulted phase of all the ports are opened. On blocking firing pulses of faulted phase B, firing pulses of the stand-by phase E are enabled. The reference angle for phase E firing pulses is identical to the replaced phase B. The control signals M ide and M iqe of phase E are gradually increased to M id and M iq in a time span of.1 s. Fig. 15(a) shows instantaneous AC current of port 1. The current i 1acB firstly increases because of the short circuit and reduces to zero after blocking of phase B IGBTs. The current i 1acE is initially zero and gradually increased to rated value after the de-blocking of phase E IGBTs. It is very much desired that the currents of other phases are unaffected. Fig. 15(b) gives the DC power of each port. There is temporary drop of DC power due to the loss of phase B. However, the DC power quickly returns to 4 pu after phase E replaces the faulted phase B. Fig. 15(c) gives the fundamental RMS capacitor voltage of all the five phases. V cpue is initially zero and gradually increases to rated value when it is used to replace phase B. Fig. 15(d) shows the values of the ground current of the hub. There is temporary nonzero ground current due to the temporary phase unbalance from.5s to.6s. The 4-phase hub ground current finally returns to zero. 7 Conclusion Multiphase and multi-block topologies are proposed to meet the high security and redundancy of Multi GW DC hub in large DC grids. It is concluded that the multiphase hub additionally provides flexibility to match DC cable power, to maximise power transfer and to better optimise DC hub components. On internal AC fault, instead of tripping the whole hub or faulted block, only the faulted phase needs to be isolated from the hub and the remaining phases can be reconfigured to a new

17 17 balanced system with reduced number of phases. It is demonstrated that a redundant standby phase can be provided to rapidly replace a faulted phase in multiphase hub enabling thus full power transfer even when internal faults occur. Detailed simulation shows that fast phase reconfiguration is possible within 1ms. Common mode voltage study recommends hubs with even number of phases. In general, a 4-phase hub is a recommend implementation for typical multi GW LCL DC hub, possibly with a 5th redundant phase. Acknowledgements This project is funded by European Research Council under the Ideas program in FP7; grant no , 21. References [1] Friends of Super grid. Roadmap to the Supergrid technologies. URL: (March 212). [2] D. Van Hertem, M. Ghandhari, Multi-terminal VSC HVDC for the European Supergrid: obstacles, Renewable and Sustainable Energy Reviews. 14(9) (21) [3] J. Häfner, B. Jacobson, Proactive Hybrid HVDC Breakers - A key innovation for reliable HVDC grids, in: CIGRE 211 Bologna Symp, 211. [4] M. Hajian, D. Jovcic, B. Wu, Evaluation of semiconductor based methods for fault isolation on high voltage DC grids, IEEE Trans. Smart Grid, 4(2)(213) [5] M. Barnes, A. Beddard, Voltage source converter HVDC links the state of the art and issues going forward, Energy Procedia. 24(212)

18 18 [6] S. Falcones, R. Ayyanar, X. Mao, A DC-DC multiport converter based solid state transformer integrating distributed generation and storage, IEEE Trans. Power Electro. 28(5)(213) [7] F. J. Deng, Z. Chen, Design of protective inductors for HVDC transmission line within DC grid offshore wind farms. IEEE Trans. Power Deli. 28(1) (213) [8] D. Jovcic, J. Zhang. High power IGBT-based DC/DC converter with DC fault tolerance, in: Proc. IEEE PEMC, 212. [9] D. Jovcic, W. Lin, Multiport high power LCL DC hub for use in DC Transmission Grids, in printing in IEEE Trans. Power Del[Manuscript ID: TPWRD R3]. [1] G. T. Son, H. J. Lee, T. S. Nam, Y. H. Chung, U. H. Lee, S. T. Baek, K. Hur, J. W. Park, Design and control of a modular multilevel HVDC converter with redundant power modules for noninterrruptible energy transfer, IEEE Trans. Power Deli. 27(3) (212) [11] ABB, It s time to connect, Technical brochure, 212. [12] D. Huang, Y. Shu, J. Ruan, Y. Hu, Ultra high voltage transmission in China: developments, current status and future prospects, Proc. IEEE. 97(3)(29) [13] E. Levi, Multiphase electric machines for variable speed applications, IEEE Trans. Ind. Electro. 55(5) (28) [14] T. P. Chen, Common-mode ripple current estimator for parallel three-phase inverters, IEEE Trans. Power Electro. 24(5) (29) [15] R. Benato, D. Napolitano, Reliability Assessment of EHV Gas Insulated Transmission Lines: effect of redundancies. IEEE Trans. Power Deli. 23(4) (28), [16] L. Ängquist, A. Antonopoulos, D. Siemaszko, K. Ilves, M. Vasiladiotis, H. P. Nee, Open-loop control of modular multilevel converters using estimation of stored energy, IEEE Trans. Ind. Appl. 47(6)(211)

19 19 [17] J.Peralta, H. Saad, S. Dennetière, J. Mahseredjian, S.Nguefeu, Detailed and averaged models for a 41-level MMC-HVDC system, IEEE Trans. Power Deli. 27(3) (212) Tables and Figures Table 1 Common mode current for VSC technologies VSC topology Number of phases Odd Even 2-level VSC Considerable MMC Negligible Table 2 Parameters of 4-port test system Port V idc (kv) P ir (MW) L i (H) C i (uf) Power direction inject inject absorb absorb

20 2 Port 1 Port 3 V1dc C1d S1_1 S3_1 S5_1 S7_1 L1 CB1A CB3A L3 S1_3 S3_3 S5_3 S7_3 C3d V3dc CB1B CB3B V1dc C1d S2_1 S4_1 S6_1 S8_1 CB1C CB1D v 1 v ca v C1 3 v cb S2_3 S4_3 Bus_A Bus_B Bus_C CB3C CB3D C3 S6_3 S8_3 C3d V3dc v cc Bus_D Port 2 v cd Bus_G Port 4 V2dc C2d S1_2 S3_2 S5_2 S7_2 L2 CB2A ignd CB4A L4 S1_4 S3_4 S5_4 S7_4 C4d V4dc CB2B CB4B CB2C CB4C CB2D CB4D V2dc C2d S2_2 S4_2 S6_2 S8_2 v 2 C2 C4 v 4 S2_4 S4_4 S6_4 S8_4 C4d V4dc Fig. 1. Structure of 4-port DC hub using single 4-phase block P i,ref V cq,pu K droop Firing Logic 1. K p1 +K i1 /s -1. Firing Logic M ir M iq M id M i α i M α θ M α θ Pulses A Pulses B P i,pu f o VCO θ A θ B θ C θ D Flag_Trip π/2 T π/6 φ Fig. 2. Phase reconfiguration control of one port of a 4-phase LCL DC hub δ V idc v iac M i *sin(2πf o t+α i ) -V idc π/2 π 3π/2 2π 5π/2 θ=2πf o t+α i Fig level bipolar modulation for port i M iq M id M i α i M α θ Dblk Fire Logic Pulses B Flag_Replace θ B T Mi M α θ Dblk Fire Logic Pulses E Fig. 4. Phase replacing control

21 Idc(kA) Pdc(MW) Idc(kA) Pdc(MW) Idc Pdc Number of phases Fig. 5. Change of DC current and DC power versus number of phases V 1dc Port1 Blockα Bus_A1 Port3 Blockα V 3dc V 1dc Port2 Blockα Bus_B1 V 3dc V 2dc Port1 Blockβ Bus_A2 Port3 Blockβ V 4dc V 2dc DC cable Port2 Blockβ Bus_B2 Port4 DC cable V 4dc DC Side Cd Cd S1 S2 S4 S3 vac iac L L CB C C CB AC Side Fig. 6. Topology of 4-port DC hub using 2-phase blocks Idc Pdc Number of 2-phase blocks Fig. 7. Change of DC current and DC power versus number of 2-phase blocks Port1_Blockα Bus_A1 Port3_Blockα V1dc Bus_B1 V3dc V1dc Port2_Blockα Bus_C1 Port4_Blockα V3dc V2dc Port1_Blockβ Bus_A2 Port3_Blockβ V4dc V2dc Bus_B2 V4dc DC cable Port2_Blockβ Bus_C2 Port4_Blockβ DC cable Cd S1 S3 S5 L1 CBA DC Side vaca vacb CBB CBC AC Side Cd S2 S4 S6 vacc C

22 Idc(kA) Pdc(MW) 22 Fig. 8. Topology of 4-port DC hub using 3-phase blocks Idc Pdc Number of 3-phase blocks Fig. 9. Change of DC current and DC power versus number of 3-phase blocks S 1A* V 1dc ~ L 1 i 1A C S 1B* V 1dc ~ i 1B v CA i com S 1C* V 1dc ~ i 1C v CB S ia* V idc ~ L i i ia v CC S ib* V idc ~ i ib S ic* V idc ~ i ic Fig. 1. Equivalent circuit of hub using bipolar modulation (only port 1 and port i are shown, connection of other ports is similar) S 1com* V 1dc ~ ~ L 1 i 1com S icom* V idc v Ccom L i i icom C i com Fig. 11. Equivalent circuit for common current and common capacitor voltage (only port 1 and port i are shown, connection of other ports is similar) e 1com ~ ~ R 1 /2 L 1 /2 L 1 i 1com e icom R L v Ccom i /2 i /2 L i i icom C i com Fig. 12. Equivalent circuit for common current and common capacitor voltage (only port 1 and port i are shown, connection of other ports is similar)

23 P idc (pu) ignd (ka) Δφ(deg) P idc (pu) eicom 23 Uc -Uc Fundanmental Cycles Fig. 13. Common mode voltage e icom of a 9 level MMC Time(s) Time(s) P1dc P2dc P3dc P4dc (a) Phase angle difference between each phase (b) DC power v ca -v cc (kv) Time(s) vca vcb vcc Time(s) 1st 3rd RMS (c) Capacitor Voltage during phase reconfiguration (d) DC hub ground current Fig. 14. Response to tripping C phase on all ports (3phase -> 2 phase) i 1ac (ka) Time(s) i1aca i1acb i1acc i1acd i1ace Time(s) P1dc P2dc P3dc P4dc (a) AC current of port 1 (b) DC power

24 Vc(pu) Ignd (ka) VcpuA 1 VcpuB.5 VcpuC VcpuD VcpuE Time(s) (c) Capacitor voltage of each phase Time(s) (d) DC hub ground current RMS DC Fig. 15. Response to Replace B phase with redundant E phase on all ports