The 2014 International Power Electronics Conference

A Front-to-Front (FTF) System Consisting of Multiple Modular Multilevel Cascade Converters

for Offshore Wind Farms Firman Sasangka, Makata Hagiwara and Hirofumi Akagi

Department of Electrical and Electronic Engineering Tokyo Institute of Technology

NE-11, 2-12-1, O-okayama, Meguro, Tokyo, JAPAN E-mail: akagi@ee.titech.ac.jp

Abstract-This paper presents a front-to-front (FTF) system

based on modular multilevel cascade converters (MMCC). It

is called an FTF system because the ac sides of the MMCCs

are connected together making a front-to-front configuration

via a medium-frequency transformer for voltage matching and

galvanic isolation. The system configuration is applicable to dc

power collections. Moreover, it is suitable as a power converter for

multi-terminal dc power networks since it can handle dc faults

inherently without using costly dc circuit breakers. Simulated

results using a "PSCADIEMTDC" software package verify the

operating principles and control method of the FTF system.

Keywords-Medium-voltage high-power dc-dc converter, modu­

lar multilevel cascade converter, multi-terminal dc power network.

I. INTRODUCTION

Offshore wind energy reserves an enormous potential for future large-scale sustainable energy resources. Offshore winds have several favorable characteristics compared to onshore winds, e.g. higher wind speeds, less turbulence, and large areas availability, thus leading to a higher energy yield and higher capacity factor. Moreover, many problems associated with installation of onshore wind farms, such as acoustic noises, visual impacts, and land conflicts, are less relevant to offshore wind farm projects. Recently, a substantial shift towards more large offshore wind farms has been made [1], [2]. Most offshore wind farms today are less than 30 km from shore and using ac interconnections for power collection as well as transmission to inland grids. Since submarine ac cables can produce large charging current, the upper limits of transmission voltage and distance are restricted. Future offshore wind farms are likely to be further away from shore to increase the size and to reduce visual impacts. However, as the distance from shore increases, the requirements for the wind turbines and their foundations, the transmission distance and its capacity, and also the collection layout and grid interconnection will need to be designed to suit the far offshore conditions [1]-[9].

For longer distance and higher power transmission, high­voltage direct current (HVDC) transmission is a preferred op­tion for future large-scale offshore wind farms. Since the space requirement is critical in offshore installation, an HVDC based

978-1-4799-2705-0/14/$31.00 ©2014 IEEE 1761

on line-commutated converters (LCC-HVDC) is unfavorable because it requires heavy and bulky auxiliary filters. On the other hand, an HVDC based on voltage-sourced converters (VSC-HVDC) offers attractive advantages such as fast control of active and reactive power, fewer auxiliary filters, and reactive power support during grid faults. However, the VSC­HVDC system suffers from higher converter cost and losses. Nevertheless, to comply with grid requirements imposed to the integration of wind farms to grid networks, the VSC­HVDC may be the preferred solution as the converters for offshore power transmissions [2]-[6], [9]-[13]. For high-power and high-voltage applications, compared to two-level or three­level converters, the modular multilevel converter (MMC) is more attractive since it has a modular structure and redundant operation, control and power/voltage rating flexibility, and also low voltage- and current-harmonic contents which comply with the power quality standard [9], [14]-[16].

The concept of using dc power systems for power collec­tion and transmission could be an alternative solution for future offshore wind farms [5], [6], [8], [10], [11], [17]-[19]. The dc power systems show higher reliability and flexibility than the conventional ac power systems. Moreover, the transmissible power is not limited by the transmission length as in the ac power systems. In order to realize dc power transmissions, medium- and high-voltage dc-dc converters are required. Fig. 1 shows an example of a large wind farm configuration based on dc-dc layout. Each power generation unit can be a sin­gle connection of a wind turbine (WT) or multiple series­parallel connections of wind turbines (WTs). Furthermore, interconnection of distant renewable offshore power generators to the inland grid via a multi-terminal dc network will improve the system reliability and stability [20]-[22]. Since the multi­terminal dc transmission requires a robust protection system to deal with dc faults, instead of using costly dc breakers, other alternatives have to be found [14], [23], [24]. The conventional MMC is incapable to handle dc faults in such that when a dc­side short circuit occurs, the fault current continues to flow through the anti-parallel diodes across the switching devices. However, the anti-parallel diodes can not withstand the large surge current and should be equipped by using press-pack thyristors in parallel with the diodes. Fast interruption of the

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Fig. l. Large offshore wind farm configuration based on dc-dc layout.

fault currents is essential for the grid reliability.

This paper presents a front-to-front (FTF) system as a building block for the interconnection of several dc power generation based on multi-terminal HVDe. The dc power collection in one cluster of power generation is achieved by interconnecting several converters at their ac sides, forming front-to-front systems consisting of modular multilevel cas­cade converters based on double-star chopper-cells (MMCC­DSCC) and double-star bridge-cells (MMCC-DSBC). In this paper the MMCC-DSCC will be called as DSCC while the MMCC-DSBC will be called as DSBC for simplicity. The circuit configuration for an FTF system based on MMCC will be explained in the next section. The FTF system fea­tures the flexibility of dc power collection and inherent dc faults protection. The circuit operation of the DSCC is also explained in particular. Later in this paper, the control strategy of the MMCC-based FTF system is presented. The control method for the MMCC will be separated into two categories, the power-flow control and the capacitor-voltage balancing­control. The power-flow control enables a bi-directional power flow for the FTF system while the capacitor-voltage balancing­control ensure the capacitor voltages in all cells maintained at the desired value. Finally, simulation results are presented to verify the proposed circuit configuration and control method for the front-to-front system under steady-state, transient, and fault conditions.

II. MMCC-BASED FTF POWER COLLECTION

The interconnection of two ac grids having different phases or frequencies to improve power system reliability can be fulfilled by using a back-to-back (BTB) system. Fig. 2(a) shows the basic configuration of a BTB system as an interface between two ac networks. The BTB systems can be found in several applications such as HVDC transmissions, frequency changers and asynchronous power-flow controllers.

As opposed to the BTB system, a front-to-front (FTF) system ties two dc grids by using two bi-directional power converters via a transformer. Fig. 2(b) shows the basic con­figuration for an FTF system. One of the applications of the FTF system is the dc power collection. The dc end of the

1762

�c Grid � AC Grid AC :;: DC �

DC ' AC-vv�

DC Grid

0Dc/I �

(a)

(b)

DC Grid ,------" AC ----=---"

Fig. 2. Basic configurations of back-to-back (BTB) and front-to-front (FTF) systems. (a) BTB system between two ac grids. (b) FTF system between two dc grids.

converters are connected either to the power-collecting side or transmission side. The transmission-side converter will have a higher dc voltage rating than the collecting-side converter. In this case, the voltage elevation can be done within each converter and/or by adjusting the transformer ratio. For dc transformer applications, only a few literatures address the use of FTF system [24], [25]. However, no literature has discussed the three-phase FTF system configuration using several MMC converters at the collecting side yet.

Several generator types have been used for variable-speed wind turbine applications. The market share of doubly-fed induction generator (DFIG) for wind turbine application is approximately 50%. However it may not be suitable for future high-power offshore application since it needs gearbox, slip rings and brushes which need regular maintenances. It is reported that the low avaibility of wind turbines in offshore wind farms is mainly due to slip-rings and gearbox failures combined with low access to the offshore sites [1], [9]. A direct-drive multi-pole permanent magnet synchronous gen­erator (PMSG) for variable-speed wind turbine offers better reliability and noise reduction since no slip rings and gearbox are used, thus is believed to be suitable for future multi­megawatt offshore generator [9], [26]-[30]. Although the multi-pole PMSG produces less losses and has ride-through capability, it has larger size and weight compared to DFIG. A good compromise between size and reliability may be achieved by using a single-stage gearbox with PMSG which enable the generator to operate at medium-speed operation [9], [27].

The PMSGs are usually equipped with multipulse diode rectifiers with dc choppers, or back-to-back (BTB) converters either using two-level or neutral point clamped (NPC) topology for ac grid interconnection or used in standalone power gen­eration [9], [27], [29]-[33]. For multi-megawatt applications, the NPC converters are the most adopted topology because of their good performances to extract the maximum power from wind as well as to comply with stringent grid requirements. For offshore applications where the power converters should be highly reliable and less maintenance, the diode rectifier may become more preferred option than the VSC-based rectifier. However, the diode rectifier produces high harmonic distortion

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Fig. 3. System configuration overview for offshore wind power collection based on multiple MMCCs.

on the generator side leading to increased generator losses and torque pulsations. Nevertheless, multipulse diode rectifiers have higher efficiency and reliability, and also have lower cost compared to VSC-based rectifiers.

Fig. 3 shows the proposed dc power collection based on a front-to-front system applicable for the interconnection of offshore wind-farms to the inland grids. The power from the wind turbines are collected first and then a large MMCC system steps up the voltage to the transmission level. Offshore medium-voltage dc-dc power converters are not needed since the dc-output voltage from each turbine is directly connected to a large power collection and transmission system, This power collection and transmission design could be advantageous since a two-voltage-level system is employed [6]. The idea of using series connection of wind turbines may improve efficiency and remove completely the offshore platforms [18], [19]. However, the insulation level of the equipments represents a major practical issue for the proposed configuration,

A combination of a mUlti-pole permanent magnet syn­chronous generator (PMSG) with a three-phase six-pulse diode rectifier for dc power generation shows a promising candidate for offshore wind farm applications, The multi-pole permanent magnet generator enables the variable-speed power generation to operate at its maximum power coefficient over a wide range of wind speed, while at the same time achieving reliability improvement and reducing maintenance expenses since the gearbox and slip-ring parts can be eliminated. Moreover, although a variable dc voltage will be produced by the three­phase six-pulse diode rectifier, the FTF system can still extract maximum power from wind by varying the input power based on the speed signal from the generator [10], [30].

The proposed system consists of a transmission-side con­verter and power-collecting-side converters, For offshore wind farm applications, the system could be located in the offshore platform several kilometers away from the wind farm location,

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The proposed system collects the distributed dc power from wind turbines and transmits the power to the inland grid directly through the HVDC transmission without intermediate converters for medium-voltage elevation, Furthermore, the system offers bi-directional power flow which can provide the wind towers with emergency power from inland grid instead of using diesel generators when on-site maintenance works are needed. The double-star bridge-cells (DSBC) and double-star chopper-cells (DSCC) are applicable to the front­to-front application since they have five-terminal circuits to interconnect the dc terminals to the three-phase ac circuit [34]. However, since the dc-link voltages from wind turbines vary widely depending on wind conditions, the DSBC configuration is more suitable for the power-collecting converter as it can produce a constant ac voltage [35]. Either the DSCC or DSBC configuration can be used for the transmission-side converter. Note that when DSCC converter configuration is utilized, ac circuit breakers should be put at the ac-Iink of the converter for protective measure, On the contrary, the DSBC can be utilized without the need of ac breakers. Nevertheless, the DSCC configuration offers less switching devices, compared to the DSBC configuration,

The system can be expanded into several clusters which may be located in different areas or platforms. Each cluster can have any number of branches of the power-collecting side (lower-voltage side) which connected at the ac link of the transmission-side converter. Each collecting-side converter may have a different power rating depending on the generated power at that point. With this configuration, the transmission­side converter will have a power rating equal to the total power rating in the collecting side. The frequency of the transformer used here can be 50/60 Hz or higher to reduce the size of the transformer and passive components. Moreover, the gal­vanic isolation can be achieved by using a single transformer with line inductances or a multi-winding transformer. The flexibility of the proposed configuration is very favorable for interconnecting large offshore wind farms or other distributed power generations using renewable energy sources such as photovoltaic.

For handling dc faults, the proposed system can use the safety-procedure operation of the FTF system with or without the common ac breakers, depending on the DSBC-DSCC or DSBC-DSBC combination, When a dc fault on the transmis­sion side occurs, all converters in each end will be turned off, avoiding short-circuit current to flow into the converters. Since the fault is handled directly by the converter operation, the rating of the ac circuit breaker may be lower than the nominal load current. Moreover, when a dc fault occurs in one of the power-collecting feeders, other healthy feeders may still operate normally if the DSBC configuration is used at the feeders. The DSBC configuration can block fault current without turning off the other converters. This feature improves the reliability of the dc power-collecting system.

Fig. 4 shows the basic circuit diagram of an FTF system us­ing two modular multilevel cascade converters (MMCCs), The MMCC can be seen as the building block converter for each

1763

Fig. 4. Circuit diagram of front-to-front system based on two modular multilevel cascade converters (MMCCs).

FTF cluster in the multi-terminal dc power network. Two three­phase MMCCs are connected together at their ac sides via a transformer for galvanic isolation. Each phase leg composed of series-connected chopper-cell or bridge-cell circuits and a center-tapped inductor. The number of cells in each MMCC can be adjusted to withstand the respective system voltage at their dc sides. In this case, the transmission-side MMCC will have more cells than the collecting-side MMCCs to withstand high-power and high-voltage transmission operation. Moreover, for multiple collecting-side MMCCs operation, the number of cells in each converter may differ depending on the power rating of each converter. Since the ac-side voltages nearly resemble sinusoidal waveforms for large number of the cells, the interface inductances can be made smaller, thus reducing the size and cost. The coupled inductances and the interface inductances can be reduced further by applying a higher frequency operation.

In this paper, only the DSCC circuit configuration will be further discussed, since the DSBC circuit configuration and control system are almost the same [35]. The DSCC was firstly introduced as modular multilevel converter (MMC) [36] and then named as modular multilevel cascade converter double­star chopper-cells (MMCC-DSCC) [34] as one of the family members of the MMCC which applicable for grid intercon­nections and motor drives. Each DSCC has three legs and two arms per leg for three-phase implementation. The output voltages for the upper and lower arms in the u-phase leg can be expressed as

Vdc Vpu = 2 -Vu, (1)

Vdc VNu = 2 + vu, (2)

where Vdc and Vu are the dc and u-phase output voltage respectively. The upper- and lower-arm voltages for v-phase and w-phase can be expressed in a similar way. Within one

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decoupled current control:

ip" iN" iNw

vz" vj" 11,* VZw Eq. (10) C vjv VCI" vj)" Eq. (11)

VCnw vjnw vjw

Fig. 5. Overall control block diagram for the collecting-side converters.

leg of the DSCC, dc and ac currents will flow in each arm and can be written as

. . iu tpu = tzu - 2' (3)

. . iu tNu = tzu + 2' (4)

where izu and iu are the circulating and u-phase output current respectively. The arm currents for v-phase and w-phase can be derived in a similar manner. Equations (3) and (4) show that the arm currents consist of two independent variables, i.e., the circulating current and the output phase current. Since a center­tapped inductor is used instead of two single inductors in each leg, the voltage drop caused by the ac components of the arm currents will cancel each other out [34].

III. CONTROL STRATEGY

A. Power Flow Control

Similar to other VSC topologies, the DSCC can act as an inverter or a rectifier, thus capable of injecting or absorbing power to or from the grid respectively. The power flow will be controlled indirectly by controlling the currents in rotating dq frame, i.e., controlling the d-axis current for active power and the q-axis current for reactive power respectively. The decoupled current control as depicted in Fig. 5 is implemented and the control response depends on the gain parameters used. Since this control method needs a voltage reference for synchronization, the DSCC converter at the transmission side should produce a voltage reference for all converters using power control at the collecting side.

The ac voltage command in the transmission-side DSCC can be written as

v; = hVT sin wt, v; = hVT sin(wt _ 2;), v; = hVT sin(wt + 2;),

(5)

(6)

(7)

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B

150 ∼ 500 Hz

(a)

(b)

Fig. 6. Capacitor voltage balancing control block diagram.

where VT is the line-to-Iine rms voltage at the ac-Iink of transmission-side DSCC and w is the desired ac frequency of the ac-link. By using these voltage commands as a feedforward control for the decoupled current control, only the line currents need to be acquired for feedback purpose without the need of a phase-locked loop (PLL) and ac voltage sensors.

B. Capacitor Voltage Balancing Control

Controlling the capacitor voltages for the modular multi­level converter comprising many floating dc capacitors is one of the common issues to be solved. The capacitor voltage in each individual cell may deviate from the desired value because of the disturbances such as parasitic resistances, tran­sients, and harmonics. An appropriate control method should be applied to regulate the capacitor voltages.

Since the upper- and lower-arm voltages and currents have dc and ac components as expressed in (1 )-( 4), the upper- and lower-arm average power will also have dc and ac part as follows: ltO+T

Ppu = vPuipu dt to VIcosB Vdclzo VIZ1

2V3 + -2

- - V3 ' (8)

lto+T PNu = VNuiNu dt to

VIcosB Vdclzo VIz1 2V3

+ -2 - +

V3 ' (9)

where V, I, and B are the line-to-Iine rms voltage, the line rms current and the power factor angle respectively, while Izo and IZI are dc and fundamental ac component of the circulating current respectively. The first term of each equation above is related to ac power, the second term is related to dc power and the third term is related to the circulating power in each arm. Note that the dc power within one leg is directly related to the dc component of the circulating current. The total dc

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power is equal to the summation of dc powers in all legs. By controlling the dc term of the circulating current, we can regulate the average capacitor voltage in each leg [37].

Because of the circuit topology nature, the ac power will fluctuate between the upper and lower arms and can cause imbalance in each arm capacitor voltage. This problem can be solved by controlling the third term of (8) and (9), i.e., by controlling the fundamental component of the circulating current which has the same phase as the output voltage. Fig. 6(a) shows the control block diagram for balancing arm average capacitor voltages.

To ensure each capacitor voltage within one arm is equal to the average value, the individual capacitor control has to be applied. When the arm current flows into the cell, the capacitor will be charged if the cell is activated. Likewise, when the arm current flows out from the cell, the capacitor will be discharged if the cell is activated. The arm current will not flow into or out from the capacitor if the cell is turned off, thus no charging/discharging condition occurs. By using these condition states as a rule, the capacitor voltage in each cell can be maintained at the desired value. Fig. 6(b) shows the individual-balancing-control block diagram.

The overall control for all cell output voltages in the upper arm of u-phase (for j : lrv�) is given by

* 2v� * * Vju = VFF - - + vZu + Vlju' n

(10)

With similar derivation, all cell output voltages in the lower arm of u-phase (for j : � + lrvn) is given by

* 2v� * * vju = VFF + - + vZu + Vlju, n (11)

where VFF is the feed forward control to maintain the voltage at the dc side which is equal to Vdc/n; v� is the power control output; vZu is the circulating-current-control output; and v1ju is the individual-control output. Each cell output voltage command should then be divided by the corresponding cell voltage to produce the modulation for each cell. Fig. 5 shows the overall control block diagram for each DSCC. Note that for transmission-side converter, the voltage-reference command should be used instead of the decoupled current control.

IV. FTF SYSTEM PERFORMANCES

This paper uses a front-to-front system using three DSCC­based converters shown in Fig. 4 for simulation circuit con­figuration. The power-collection side uses two DSCCs while the transmission-side has a single DSCC. The DSBC-DSCCs combination can also be applied with minor changes. Table I summarizes the circuit parameters with all the dc-side voltages to be 13.2 kV and 6.6 kV/150 Hz on the ac link. In the actual system, the rated power of the transmission-side converter should be equal to the total rated power of all collecting­side converters. The ac-Iink frequency could be decided based on a good compromise between switching losses and ac-Iink harmonic contents. The higher ac-Iink frequency will produce

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TABLE I. CIRCUIT PARAMETERS FOR SIMULATION

Parameter Symbol Value

Rated power P to MW

Nominal dc voltage Vdc 13. 2 kV

Ac-Iink voltage reference V' s 6.6 kV

Ac-link frequency reference fs 150 Hz

Transformer voltage ratio 1: 1

Cell count per leg n 16

Dc capacitor e 3 mF

Dc capacitor voltage reference V' C 1.65 kV

U nit capacitance constant H 20 ms at 1.65 kV

Ac-tink inductor LAC 0.37 mH (8%)

Center-tapped inductor Lc l. l mH ( 23.8%)

Switching method Phase-shifted PWM

PWM carrier frequency fc 1350 Hz

Equivalent switching frequency nfc 21.6 kHz

Dead time 4 J.ts

on a three-phase 6.6 -kV, 10-MW, ISO-Hz base

the higher switching losses but also less harmonic contents. In the following discussions, only 150 Hz of frequency is used for the system-operation verification under several conditions. The phase-shifted sinusoidal pulse-width modulation (PWM) technique is applied and 16 triangular-carrier signals with the frequency Ie of 1350 Hz are phase-shifted each other by 22.5°. With this technique, all chopper-cells will have equal switching and conduction power losses. Note that the equiv­alent switching frequency for each leg is 21.6 kHz for 1350-Hz carrier frequency and 16 carrier-signals operation. The simulation is conducted by using "PSCAD/EMTDC" software package and a fully digital control method is implemented with a dead time of 4 f..ls.

A. System Peiformance Under Steady-State

In the first case, the system works under steady-state con­dition with both collecting-side converters deliver power to the transmission-side converter with unequal power distribution with the total of 9 MW at the transmission-side converter. Fig. 7 shows simulated waveforms for FTF system with three DSCCs under steady-state condition. Both the collecting-side DSCCs use decoupled current control with the transmission­side DSCC as the reference. The results show that the line currents distribution in all three converters has the same ratio as the power command for both power directions. Note that the currents iu1 and iu2 from the collecting-side DSCCs have the same phase while the current i,.. from the transmission-side DSCC is 180° out of phase from the other two. The individual capacitor voltage in each cell within one arm is well balanced. The capacitor voltage ripple in each converter has different magnitude due to the power distribution difference.

B. System Peiformance Under Transient

Power-command alterations for an offshore wind farm are needed to extract the maximum power from the varying wind speed. The power command should follow the maximum­power curve based on the corresponding wind speed. In

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transient case, the system runs under power transient from one of the collecting-side converters. The power flow from one of the collecting-side converters increases from 2 MW to 4 MW in 10 ms and finally to 3 MW resulting in power transient on the transmission-side converter from 7 MW to 9 MW and finally to 8 MW. Figs. 8 shows simulated waveforms for a transient operation in the FTF system. As we can see from the results, the line currents follow the power transient smoothly. The average capacitor voltage in each arm within one leg is balanced although the power-flow transients happen. Moreover, the individual capacitor voltage in each cell within one arm has only a slight variation from the arm average capacitor voltage. From these results, it is obvious that the capacitor balancing control is very effective to maintain the capacitor voltages within acceptable values under steady-state and transient conditions.

C. System Peiformance Under DC Fault

DC short circuits may occur both on the transmission side or the power-collection feeders. In the case of a dc fault occurrence on the transmission side, all the converters in the FTF system should be turned off to stop the fault currents to flow from the wind farm. When dc faults occur on one or several of the power-collection feeders, the corresponding collecting-side converters, in this case only the DSBC can be used, should be turned off to allow the other healthy feeders to continue the operation. Note that the transmission­side converter (either DSCC or DSBC) can still be operating unaltered. However, when the DSCC configuration is used for collecting-side converters, all the converters including the transmission-side converter need to be turned off for safety measure.

In this section, only the FTF system using DSCCs is simulated under a dc fault on one of the collecting-side feeders. Figs. 9 shows simulated waveforms for the FTF system performance under a dc fault occurrence. The simulated waveforms show that when a dc fault occurs on the feeder 2 at t = 0, the FTF system can act fast to block the fault current. Since all of the ac-link voltages are zero when the fault occurs, there is no power flow in the FTF system. The overshoot on a dc current from the faulty feeder is occurred because of a discharging process of the reserve energy on the coupled inductors within the converter. This current amplitude depends on the impedances within the converter arms and the dc cable impedance from the converter to the fault location. The discharging time and current-overshoot amplitude will need to be compromised by adjusting the values of the coupled inductors, thereby the ac-link frequency, to get the permissible amplitude of fault current. Moreover, the dc currents on the other converters are blocked almost instantaneously without overshoot as in the faulty feeder. Note that the dc capacitor voltages are slightly different from the reference value after the fault.

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[k V] 1.65T><:><::yc;:><::y:.::><1:y;:':�,.cc�..co�CJ<����

vO ot Clu VCIu V�9u

o [kV] 1.65�=-====-===�>=-=�--=�<?-���="<==

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o IE( 20ms

[kV] 1.65.p-o�?-�=-==-=�==-e==-e=-<l==-===-==�

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Fig. 7. Simulation results for the transmission-side (superscript 0) and the collecting-side DSCCs (superscript I and 2) under steady-state condition.

[kV] 1.65�::><=<::><::><:;:><:�:><:=><::=J<:!==:>.c:::=:>.c:::><=

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Fig. 8. Simulation results for the transmission-side (superscript 0) and the collecting-side DSCCs (superscript 1 and 2) under transient condition.

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[MW] I o�t 1= 0

pO 5 � +---pI

pI 0��2+------------L----------------------2 -5 P

P -10 I +---p o

[kA]1.25� 11

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u ________ -T ______________________ _ tu '2 I +---iO tu I u

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r-tdc '1 -If t t l tdc -2 '0 '2 I ·2 tdc tdc I tdc -3- I

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Fig. 9. Simulation results for the transmission-side (superscript 0) and the collecting-side DSCCs (superscript I and 2) under fault condition.

V. CONCLUSIONS

A front-to-front (FTF) system based on modular multilevel cascade converters (MMCC) topology has been presented in this paper. The system configuration can be implemented for a medium-thigh-voltage high-power dc power collection. A dc-dc power converter based on FTF system has been proposed for multi-terminal dc power network applications. Several DSCCs or DSBCs can be connected together at their ac sides to produce a higher total output power to be transmitted. The simulated results show the effectiveness of the system configuration and control system under steady and transient conditions for dc power collection. The power sharing in the collecting-side MMCCs can easily be controlled by implementing the decoupled current control in the collecting­side MMCCs while the transmission-side MMCC acts as a voltage reference. Furthermore, the system can handle dc faults inherently by turning off the operation of the converters, thus leading to a fast fault protection without the need of dc circuit breakers. To maintain the capacitor voltages in all cells within each converter, the capacitor balancing control is indispensable requirement for stable operation of the converters.

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