Distributed PLL-based Control of Offshore Wind Turbines Connected with Diode-Rectifier based HVDC Systems

Transcription

1 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of Ditributed PLLbaed Control of Offhore Wind Turbine Connected with DiodeRectifier baed HVDC Sytem Lujie Yu, Rui Li, and Lie Xu, Senior Member, IEEE Abtract A ditributed PLLbaed frequency i propoed in thi paper for offhore wind turbine converter connected with dioderectifier baed highvoltagedirectcurrent (HVDC) ytem. The propoed enable a large number of wind turbine to work autonomouly to contribute to the offhore AC frequency and voltage regulation. The propoed alo provide automatic ynchronization of the offline wind turbine to the offhore AC grid. Stability of the propoed frequency i analyzed uing root locu method. Moreover, an active dc voltage of the onhore modular multilevel converter (MMC) i propoed to ridethrough onhore AC fault, where the onhore MMC converter quickly increae the dc voltage by adding additional ubmodule in each phae, in order to rapidly reduce wind farm active power generation o a to achieve quick active power rebalance between the offhore and onhore ide. Thu the overvoltage of the ubmodule capacitor i alleviated during the onhore fault, reducing the poibility of ytem diconnection. Simulation reult in PSCAD verify the propoed trategy during tartup, ynchronization and under onhore and offhore fault condition. Index Term dioderectifier baed HVDC, ditributed PLLbaed frequency, offhore wind power integration, onhore AC fault, ynchronization of wind turbine converter I I. INTRODUCTION N Europe, onhore wind farm have already been well developed, and more attention ha been drawn to offhore ite. Many large offhore wind farm will be developed in the near future, thu reliable and efficient technology for tranmitting the offhore wind power i one of the main development focue. High voltage direct current (HVDC) technology ha been identified a a preferred choice for longditance offhore wind power tranmiion [][7]. To date, voltageourceconverter baed HVDC (VSC HVDC) link have been uccefully ued in offhore wind farm connection project, while linecommutatedconverter baed HVDC (LCCHVDC) link are largely ued for bulky power tranmiion in trong onhore ac grid. Recently, connection of offhore wind farm uing dioderectifier baed HVDC (DRHVDC) ytem where diode rectifier are ued at Thi project ha received funding from the European Union Horizon 2020 reearch and innovation programme under grant agreement No The author are with the Department of Electronic and Electrical Engineering, Univerity of Strathclyde, Glagow, G XW UK ( lujie.yu@trath.ac.uk, rui.li@trath.ac.uk, lie.xu@trath.ac.uk). the offhore wind farm ite and connected to a conventional MMC at onhore ha received noticeable interet [8][5]. Thi approach wa firtly uggeted in [8][], and ha been further developed in indutry for offhore wind farm connection [2], [3]. The main benefit of DRHVDC when compared to VSCHVDC are lower invetment and pace requirement, higher efficiency and robutne. A reported in [2], [3], compared with VSCHVDC for wind farm connection, the volume and tranmiion loe are reduced by 80% and 20% repectively, while the total cot can be reduced by 30%. However, the unled dioderectifier i unable to offhore frequency and voltage a VSCHVDC ytem do. Thu, in DRHVDC connected offhore wind farm, wind turbine (WT) converter have to the offhore ac voltage and frequency. In [8], a voltage and frequency i propoed, while communication for a centralized voltage i required and all the wind turbine tartup from the beginning, with the ynchronization proce not being addreed. The fault ridethrough performance and the operation with reduced diode rectifier filter are analyzed in [0], [] uing the ame trategy. In [3], umbilical ac cable are ued to tartup the unidirectional DRHVDC and a reactive power frequency droop i preented. With no inner current loop for the WT converter, the dynamic repone of the ytem during fat tranient ha yet to be approved. A fixed frequency uggeted in [4] can maintain a table offhore frequency and can be a potential olution for ynchronization, but global poitioning ytem (GPS) i required for each WT. In fact, for the converter ued in the power ytem and indutry, phaelocked loop (PLL) i a much impler and more efficient tool for ytem ynchronization [6], [7]. Therefore, in thi paper, a new ditributed PLLbaed frequency i propoed for the operation of the WT converter connected with DRHVDC ytem. Unlike in [8] [], communication i not required for the propoed ditributed PLLbaed frequency. And a major difference of the propoed method in relation to the method in [4] i that the ynchronization tool of the propoed i baed on PLL, which i much impler and more efficient than GPS ued in [4]. In thi paper, automatic ynchronization of offline wind turbine to the network i tudied in detail, which ha not been analyzed before. Furthermore, the onhore modular multilevel converter ha not been properly tudied in all the previou DRHVDC reearch

2 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of WT String WT Cluter4 WT Cluter WT0 BRK0 WT9 BRK9 WT8 BRK8 WT7 BRK7 WT6 BRK6 WT5 BRK5 WT4 BRK4 WT3 BRK3 WT2 BRK2 WT BRK generatoride converter lineide converter WT Cluter5 WT Cluter2 Pdr u off Q dr I dc U dcr P dc DC Cable DC U dc ion u on P mmc AC_GRID Diode Rectifier WT Cluter3 Fig. Single line diagram of the offhore wind farm ytem connected with DR HVDC DC Cable DC MMC work, epecially the onhore fault ride through performance of the ytem. Thi paper propoe an active dc voltage of the onhore modular multilevel converter (MMC), which can quickly rebalance the active power in the DRHVDC ytem during the onhore fault and alleviate the overvoltage of the onhore MMC ubmodule capacitor during the fault without the ue of communication. The ret of thi paper i organized a follow. The DRHVDC tructure and baic are preented in Section II. In Section III, the propoed ditributed PLLbaed frequency i decribed and it tability i analyzed. The propoed active MMC DC voltage for onhore ac fault ridethrough operation i preented in Section IV. Simulation reult are preented in Section V to validate the propoed and finally Section VI draw concluion. A. Structure II. DRHVDC The conidered offhore wind farm connected with DR HVDC i hown in Fig., which conit of five WT cluter. Each cluter i made up of four WT tring with ten fully rated converter baed WT. The dioderectifier i made up of a 2 pule bridge, whoe DC ide are connected in erie by two 6 pule bridge while the ac ide are connected in parallel through a tartar and a tardelta connected tranformer repectively. The onhore MMC with halfbridge ubmodule the dc voltage of the DRHVDC link. B. Baic of WT lineide converter with DRHVDC ) Control requirement When connected with DRHVDC, the WT lineide converter have to work a gridforming converter rather than gridfeeding converter. In addition to balancing tranmitted active power between WT and offhore network, lineide converter alo need to etablih the offhore network frequency and voltage [8], [9]. The requirement of the lineide converter are a follow: o Offhore ac voltage. During the tartup and normal operation, the offhore network voltage hould be led. In addition, overvoltage hould be limited during tranient condition, e.g. onhore ac fault. o power. The lineide converter need to tranmit active power baed on WT operation requirement, e.g. maximum power point tracking. o Reactive power haring. Reactive power need to be hared among WT converter while reactive current circulation among them i avoided. o Frequency. The lineide converter need to regulate the frequency of offhore network and enure offline WT can eaily ynchronize to the offhore ac grid. Fig. 2 how the overall tructure of the ytem for the WT lineide converter. The detailed function and deign of the individual block will be decribed in the following ection. 2) Inner current The inner current loop ha been widely ued for ling VSC with the benefit of fat repone and current limiting during external ac fault. For the converter circuit hown in Fig. 2, the VSC current loop dynamic in the dq reference frame in which the qaxi component of the VSC filter bu voltage UF i approximately 0, are expreed a diwd RIWd L UCd UFd LIWq () dt diwq RIWq L UCq UFq LIWd (2) dt where ω i the angular frequency of the offhore network. With proportionalintegral (PI) regulator, the current loop i illutrated in Fig. 2. 3) Voltage A hown in Fig. 2, the voltage dynamic of the WT lineide converter in the dq reference frame are decribed a du Fd C IWd Id CU Fq (3) dt du Fq C IWq Iq CU Fd (4) dt By regulating the output current of the converter, the voltage at the VSC filter capacitor terminal UF can be led to follow it reference. The diagram of the voltage loop

3 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of can be een in Fig. 2. The output dq current limit are et according to the converter current rating. 4) power A the dc voltage of the DRHVDC link i led at the rated value by the onhore MMC, the tranmitted active power i largely determined by the dc voltage produced by the diode rectifier (U dcr, Fig. 2) [9]. The dc voltage produced by the diode rectifier i given a 3 Udcr 2(.35 Uoff XIdc ) (5) where X i the reactance of the diode rectifier tranformer. Thu, the active power tranmitted from the wind farm to the onhore HVDC i determined by the offhore ac grid voltage U off or VSC filter capacitor terminal voltage UF. Therefore, a WT active power loop can be implemented a hown in Fig. 2 whoe output i the amplitude of the daxi voltage reference, i.e. magnitude of the produced offhore ac voltage. U o i the ac voltage et point of the offhore WT converter. WT lineide converter DC U C I W P Q I I Cable dc abc dq U Fd I Wd kii k ip U cd U cq U Fq R LI Wq LI Wd L kii k ip wt I W I Wq Current wt C abc dq U F U F abc dq I d I Wdref CU Fq I q I Wqref CU Fd U Fd kvi k vp Voltage kli klp 0 PLL U Fq U Fq kvi k vp U Fqref Fig. 2 Diagram of WT lineide converter k f U off Frequency ref kq U dcr DC U Fdref k pi k P pp ref U 0 P wt power Q wt 0 Q 0 Reactive power haring 5) Reactive power haring When a large number of WT are connected to the diode rectifier, the reactive power need to be hared among the WT to avoid overcurrent and reactive current circulation. The adopted reactive power haring [8], [3] i hown in Fig. 2 where a reactive power frequency droop i ued a k Q Q (6) ref q wt 0 0 If Q wt i repreented a per unit value (poitive Q wt defined a WT providing capacitive reactive power to the offhore ac network) where the power rating of the repective WT i ued a the baed power, ame k q and Q 0 can be ued for all the WT to achieve equal reactive power haring (baed on their repective power rating). 6) Startup trategy Diode rectifier cannot upply the energy for offhore network during the tartup, thu additional power ource i required. In [2], [3], umbilical ac cable are ued to tartup the unidirectional DRHVDC. In thi tudy, umbilical ac cable are not conidered and it i aumed that there are energy torage intalled at ome WT to provide elftartup capability. During the tartup, WT converter operate at ac voltage mode to etablih the ac voltage. In order to avoid active power circulation among WT converter, active power / ac voltage droop i adopted, which i realized by etting the integrator coefficient (k pi) in the active power (hown in Fig. 2) at 0. III. DISTRIBUTED PLLBASED FREQUENCY CONTROL Due to the unlability of the diode rectifier, the ytem need to enure that large number of WT work autonomouly to provide frequency of the offhore ytem. To tackle thi problem, a ditributed PLLbaed frequency i propoed for each turbine, a illutrated in Fig. 2. In addition to the frequency lability, the propoed enure plugandplay capability providing automatic ynchronization of the offline WT and minimum impact during diconnection of ome of the turbine with the offhore ac network. Such iue have not been conidered previouly [8], [9]. A. Principle of ditributed PLLbaed frequency In the exiting voltage for iolated converter baed network, the voltage amplitude of the network i regulated by the daxi voltage reference U Fdref while the qaxi reference U Fqref i normally et at zero. On the other hand, the PLL take U Fq a the input and regulate the frequency output to enure the qaxi voltage U Fq to zero, a hown in Fig. 3. For example if the meaured U Fq i lightly larger than zero for a hort time, the detected frequency of the voltage vector will increae, a hown in Fig. 4 and (7) klpu Fq kli UFqdt 0 (7) Since the frequency and phae angle meaured by the PLL will alo drive (ynchronize) the output of the converter for the offhore ac network, the frequency of the offhore ytem will increae under uch condition. Thi indicate that the qaxi voltage reference U Fqref can be ued to the ac frequency and thu an additional PLLbaed frequency loop i propoed to generate the deired U Fqref, a hown in Fig. 2 and expreed a U k (8) Fqref f ref When ω < ω ref (i.e. U Fqref > 0), the PLLbaed frequency produce a poitive U Fqref feeding to the ac voltage ler, a een in Fig. 2. The voltage and current loop enure the converter produce the required U Fq according to it reference value produced by the frequency loop. Conequently the frequency meaured by the PLL i increaed (due to U Fq >0) until becoming identical to the reference (ω=ω ref). Similarly, when ω > ω ref (i.e. U Fqref < 0), the propoed frequency produce a negative U Fq o the frequency i reduced accordingly. Such frequency can be implemented at each lineide converter of the WT and i able to operate autonomouly to contribute to the overall frequency regulation of the offhore ac network. Fig. 5 how the frequency regulation of the propoed PLLbaed frequency enabled at 0.4 for a 5 MW converter (parameter een in Table I). For cae, the initial frequency i higher than the reference frequency of 50 Hz wherea for cae

4 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of 2, it i lower than 50 Hz. A can be een, both frequencie quickly follow the reference after enabling the propoed frequency at 0.4 (k f tep from 0 to 0.0). U F abc dq Fig. 3 Diagram of the PLL 0 U Fq Fig. 4 Voltage vector in dq reference frame Frequency (Hz) q k cae:, U 0 ref ref cae2 :, U 0 ref Fqref Fqref lp k li β U F U Fq U Fd α d PLL baed frequency enabled Time () Fig. 5 Frequency regulation uing the propoed PLLbaed frequency B. Stability analyi of PLLbaed frequency The frequency loop with the propoed i analyzed in thi ubection and the tability range of the frequency gain k f i derived. ref U qref k f U q I q kvi Ck vp I qref kii Lk ip T pwm Fig. 6 Cloed loop of PLLbaed frequency L C kli k lp The cloed loop of the PLLbaed frequency, including the voltage, current loop and PLL, i repreented in Fig. 6. The converter including the PWM modulator i approximated uing a firt order delay. The cloed loop tranfer function i thu expreed a G H. (9) G In (9), G() i the open loop tranfer function and i written a 3 2 k f ( a b c d) G () (0) T e f g h where pwm a kipkvpklp; b kiikvpklp kvikipklp klikipkvp; c kiikviklp kiiklikvp kviklikip; d kiikvikli; e kip; f kii kipkvp; g k k k k ; h k k ; ii vp vi ip ii vi () For the lineide converter of a 5MW WT converter with parameter lited in Table I, the PI parameter of the current loop, voltage loop, and PLL are (k ip=256 Ω, k ii=98696 Ω./); (k vp=25 Ω, k vi=5775 Ω ); and (k lp=223.2 rad./(v), k li= rad./( 2 v)) repectively. L and C are lited in Table I. T pwm i half of VSC witching period of 0.5m. From (9)(), the correponding root locu i hown in Fig. 7. A illutrated in Fig. 7, the PLLbaed frequency i table when k f < in thi cae. Although the increae of k f lead to fat frequency repone, the damping i reduced. Imaginary Axi (econd ) j2090 kf=0.057 j2090 kf= Real Axi (econd ) Fig. 7 Cloed loop rootlocu of PLLbaed frequency IV. FAULT RIDETHROUGH DURING ONSHORE AC FAULT A. Onhore ac fault characteritic During an onhore ac fault, it i deirable that the offhore ytem and the DRHVDC can remain operational, and continue tranmitting active power from WT and upport the onhore ac grid. With the udden drop of the ac voltage during a evere threephae fault, the onhore MMC will operate in current limit mode and active power that can be tranmitted will be ignificantly reduced. However, in the meantime the WT and diode rectifier till try to tranfer the generated power to the dc link, leading to active power unbalance in the DRHVDC ytem. The urplu power will have to be tored in the dc cable and ubmodule capacitor of the onhore MMC tation reulting in dc ytem overvoltage. The amount of power that can be tranmitted from the offhore wind farm after the onhore fault depend largely on the dc voltage of the HVDC ytem: o If the dc voltage i led at a contant value during the onhore fault by the MMC tation, the WT converter will continue delivering generated active power. The contant active power unbalance due to the reduced output power to the onhore ac ytem can reult in exceive overvoltage in the MMC ubmodule and potentially lead to ytem diconnection. Fat communication may have to be ued to reduce the offhore wind power generation to achieve active power rebalance. o dc voltage increae during the onhore fault. WT converter active power will puh up the ac voltage until the maximum ac voltage limit i reached. Conequently, the offhore active power tranmiion i reduced and power rebalance between the offhore and onhore ide i achieved automatically without the ue of communication. If the dc voltage can be increaed quickly at the initial tage of the fault, active power rebalance can be achieved fater leading to reduced capacitor

5 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of overvoltage in the MMC ubmodule. Two different method (paive and active) for increaing the dc voltage during an onhore fault will be further decribed in the following ection. B. Paive MMC dc voltage With the paive MMC dc voltage, the dc voltage i directly linked to the ubmodule capacitor voltage in which the total number of inerted ubmodule in the upper and lower arm in each phae (N) keep contant, even during the onhore fault. The relationhip between the increae of dc link voltage (ΔU dc) and the ubmodule capacitor voltage increae (ΔU c) can be expreed a Udc N Uc (2) where ubmodule capacitor voltage increae i caued by the energy unbalance between offhore generation and onhore tranmiion. The increae of the dc link voltage, in turn, reduce the offhore tranmitted active power until the active power i rebalanced between the offhore and onhore ide. For the DRHVDC ytem with offhore ac voltage limit, there i an intrinic negative feedback loop after onhore fault that the unbalanced active power can alway be automatically driven to 0. The urplu energy at each WT i dealt with a part of WT fault ride through requirement. C. MMC dc voltage A dicued before, fater dc voltage increae lead to fater active power rebalance. Thu, the energy aborbed by the MMC ubmodule capacitor can be reduced alleviating exceive overvoltage of the ubmodule. Baed on thi obervation, an active MMC dc voltage i propoed to automatically increae the dc voltage reference after the onhore fault, and the increae of the dc voltage and ubmodule voltage can be expreed a Udc knu c, k (3) A limiter on ΔU dc can be ued in the propoed to limit the HVDC link overvoltage in an acceptable range. After the dc link voltage hit the limit, the number of inerted ubmodule per phae tart to decreae with the increae of the ubmodule capacitor voltage. In thi way, the HVDC link voltage i regulated at the preet maximum value to enure afe operation of the HVDC link. After the fault clearance, the onhore ac voltage retore and more active power can be exported from the MMC to the onhore grid. The DRHVDC ytem and offhore wind generation can recover quickly. The propoed alo work in normal operation (a ΔU c=0 and ΔU dc=0) and doe not have any negative influence on the normal operation. The paive hown in Section IV B i a pecific cae for the propoed active where k i et at unity, a depicted by (2) and (3). Hence, no fault detection, mode witching or communication i required for the propoed. V. SIMULATION RESULTS The ytem imilar to the one hown in Fig. i imulated in PSCAD/EMTDC environment to verify the propoed of the DRHVDC ytem with parameter lited in Table I. Detailed model i ued for the diode rectifier, and equivalent MMC model from PSCAD library containing 256 ubmodule per arm i ued for the onhore converter [8][20]. The wind farm model include the following: ten 5 MW WT repreenting one WT tring, an aggregate 50 MW WT (equivalent to 3 WT tring), an aggregate 200 MW WT (equivalent to a WT cluter with 4 tring) and an aggregate 600 MW WT (equivalent to 3 WT cluter). WT mechanical part and generateide converter are not modelled and the lineide converter are connected to contant dc voltage ource [8], [9], and ue detailed witching model. TABLE I PARAMETERS OF THE TESTED DRHVDC SYSTEM Component Parameter Value DRHVDC link 2pule diode rectifier Onhore MMC WT converter Power 000 MW dc voltage ±320 kv Tranformer (Y/Y/ ) 66/26.8/26.8 kv, Leakage inductance 0.8 pu Reactive power compenation 0.3 pu Submodule capacitance 8000 uf Submodule number per arm N 256 Submodule capacitor voltage 2.5 kv Arm inductance pu Tranformer (Y/ ) 400/330 kv Rating of individual WT 5 MW Rating of a WT tring 5 MW 0 Rating of a WT cluter 200 MW Tranformer (Y/ ) 0.69/66 kv Leakage inductance 0.08 pu Filter capacitor C 0.pu Filter reitance 0 pu Converter reactance L 0.5 pu Switching frequency 2 khz Ac cable length between two 0.5 km 5MW WT Ac cable length (for each 5 km, 0 km, aggregate converter) 20 km A. Startup and ynchronization The performance of the propoed PLLbaed i verified firt during tartup, with the aumption that the WT dc link initial energy i provided by WT energy torage. The time line of the imulation i lited in Table II. At the beginning, the firt 5 MW WT (een in Fig. ) i connected with it WT tring, while the DRHVDC dc link voltage i etablihed by the onhore MMC. During 00.5, the voltage and frequency of WT increae it output ac voltage (U F) and the offhore voltage (U off) to 0.86 pu with ac frequency at 50Hz, a hown in Fig. 8 (a), (h) and (g). At, breaker BRK2 i cloed to connect WT2 to the WT tring, with the WT2 filter and tranformer being energized. At.2, the propoed PLLbaed enable WT2 to ynchronize automatically with the WT tring. A een from Fig. 8 (c) and (d), each WT active power output i around 0 (only need compenate the power lo of the cable) while reactive power i well hared between thee two WT (0.33 MVAr each). During 29.2, WT3WT0 are enabled and ynchronized with the WT tring equentially after cloing each ac breaker. A een in Fig. 8 (b), no overcurrent i oberved

6 (b)iw(ka) I3 I4 V7 V8 I5 I6 I7 I8 V9 V0 V50 V200 (d)qwt(mvar) Q5 Q6 Q7 Q8 I4I0 PP2P3 P4P0 P600 P200 P50 Q600 Q50 Q200 (j)qdr(mvar) (i)pdr(mw) (h)uoff(pu) (g)f(hz) (f)qwt(mvar) I I2 I3 Q9 Q0 (e)pwt(mw) Q Q2 Q3 Q4 V600 I9 I0 (c)pwt(mw) I2 V6 (k)pmmc(mw) At 2, the breaker of the diode rectifier i cloed. Thu, the diode rectifier, tranformer and the 50 MVAr filter are energized. From 2.5 to 2.8, firt WT tart power production, ramping active power from 0 to it rated value of 5 MW a een in Fig. 8 (c). After the conduction of the diode rectifier, the offhore ac voltage increae to 0.9 pu in order to tranmit active power. From 3 to 3.3 and 3.5 to 3.8, WT2 and WT3 increae active power production to it rated value repectively. From 44.3, WT4WT0 increae active power production to rated value at the ame time. After 4.3, all the ten WT in thi tring operate at the rated active power, I V5 (l)idc(ka) TABLE II SEQUENCE OF STARTUP AND SYNCHRONIZATION Time Event 0 Breaker BRK (een in Fig. ) i cloed 00.5 WT etablihe ac voltage and frequency, operating in ilanded mode.0 Breaker BRK2 cloed, filter and tranformer of WT2 energized.2 WT2 enabled, operating in ilanded mode 9 Breaker BRK0 cloed, filter and tranformer of WT0 energized 9.2 WT0 enabled, operating in ilanded mode 0 Breaker of aggregate 50 MW WT cloed, filter and tranformer of WT energized 0.2 Aggregate 50 MW WT enabled, operating in ilanded mode Breaker of aggregate 200 MW WT cloed, filter and tranformer of WT energized.2 Aggregate 200 MW WT enabled, operating in ilanded mode 2 Diode rectifier and tranformer energized and connected, 50 MVAr filter energized and connected WT tart power production 33.3 WT2 tart power production WT3 tart power production 44.3 WT4WT0 tart power production Aggregate 50 MW WT tart power production 55.3 Aggregate 200 MW WT tart power production 5.5 Breaker of aggregate 600 MW WT cloed 66.3 Aggregate 600 MW WT enabled and increae power production to 300 MW 6.5 A 50 MVAr diode rectifier filter added 77.3 Aggregate 600 MW WT increae power production from 300 MW to 600 MW V V2 V3 V4 (m)udc(kv) during the WT connection and ynchronization. At 0, the breaker of the aggregate 50 MW WT (equivalent to 3 WT tring) i cloed and the correponding WT filter and tranformer are energized. At 0.2, the aggregate 50 MW i enabled and ynchronized with the offhore network. At, the breaker of the aggregate 200 MW WT (equivalent to a WT cluter) i cloed to energize the correponding WT filter and tranformer. At.2, the aggregate 200 MW WT i enabled and ynchronized with the offhore network. A can be een in Fig.8 (a), (b), (g) and (h), the ynchronization i very mooth even with uch large lumped converter and the offhore ac voltage and frequency are well led. (a)uf(p.u) Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of Fig. 8 Performance of tartup and ynchronization: (a) WT voltage (RMS); (b) WT current (RMS); (c) WT active power; (d) WT reactive power; (e) aggregate WT power;(f) aggregate WT reactive power; (g) offhore frequency; (h) offhore voltage; (i) diode rectifier tranmitted active power; (j) diode rectifier reactive power conumption; (k) onhore tranmitted active power; (l) dc current; (m) dc voltage

7 (h)pmmc(pu) (g)pdc(p.u) (f)pdr(pu) (e)ucav(pu) (d)udc(pu) (c)uoff(pu) (b)ion(pu) (a)uon(pu) Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of with offhore frequency being regulated at around 50 Hz and reactive power well hared among WT (.8 MVAr each). During , the generated active power from the aggregate 50 MW WT increae from 0 to 50 MW, a hown in Fig. 8 (e). Meanwhile, the diode rectifier reactive power conumption grow to 33 MVAr, a hown in Fig. 8 (j). During 55.3, the generated active power from the aggregate 200 MW WT increae from 0 to 200 MW with the diode rectifier reactive power conumption of 97 MVAr. A a reult, the reactive power aborbed by the WT decreae accordingly to achieve the offhore network reactive power balance, a hown in Fig. 8 (d) and (f). When thee WT operate at the full rated power (at t=5.3 in Fig. 8), the total tranmitted active power i 400 MW, a hown in Fig.8 (i) and the offhore voltage U off i increaed to 0.94 pu with frequency largely remaining at 50 Hz. At 5.5, the breaker of the aggregate 600 MW WT (equivalent to 3 WT cluter) i cloed and the correponding WT filter and tranformer are energized. During 66.3, it output power i increaed from 0 to 300 MW. Meanwhile, all the wind turbine tart to export reactive power to compenate the increaed reactive power conumption by the diode rectifier (227 MVAr). At 6, a 50 MVAr filter i added. During 7 7.3, the aggregate 600 MW WT further increae active power from 300 MW to 600 MW. From 7.3, the DRHVDC ytem operate at the rated active power with dc current and onhore active power at rated value, a hown in Fig. 8 (l) and (k). The offhore voltage U off i now pu, with the total harmonic ditortion (THD) at around % and frequency led at around 50 Hz. 400 MVAr reactive power i conumed by the diode rectifier. A can be een from Fig. 8 (d) and (f), under teady tate, the reactive power are hared equally (in per unit term with repect to their repective rated power). During the whole tartup and WT connection proce, the dc voltage i well led by the onhore MMC, a hown in Fig. 8 (m). The THD of the offhore voltage U off i lower than.5% throughout the wind power range of 0 to pu, a hown in Table III. A een from Fig. 8, the offhore network how an excellent behavior during the tartup and ynchronization. Offline WT converter ynchronize eamlely with the offhore network during the connection proce, and the overall offhore network frequency i alo well led by the ditributed WT converter. TABLE III THD OF OFFSHORE VOLTAGE Power (pu) THD 0.55% 0.68% 0.7% 0.8% 0.9%.37% Power (pu) THD.06%.% 0.84% 0.96%.% B. Onhore fault ridethrough operation The performance of the active dc voltage during a olid onhore fault i compared to that of the paive dc voltage method and the imulation reult are hown in Fig. 9. During normal operation, both method operate atifactorily. At 0.05, a olid threephae onhore fault occur at the tranformer primary ide and the onhore ac voltage rapidly decreae to 0, a hown in Fig. 9 (a). The onhore tranmitted active power quickly reduce to 0 a hown in Fig. 9 (h), while the WT and diode rectifier till try to tranmit the Paive Paive Paive Paive Paive Fig. 9 Performance of the DRHVDC ytem during onhore olid fault: (a) onhore ac voltage; (b) onhore ac current; (c) offhore ac voltage; (d) dc voltage; (e) average MMC ubmodule capacitor voltage of ix arm; (f) total WT tranmitted active power; (g) dc power; (h) onhore tranmitted active power generated active power, a hown in Fig. 9 (f) and (g). During the fault, the active current of the MMC i reduced uing a voltage dependent current order limit (VDCOL) while it reactive current i increaed [2]. Thi unbalanced active power lead to the charge of MMC ubmodule capacitor and conequently their voltage (hown a the average value of all the ubmodule) increae a hown in Fig. 9 (e). With the conventional paive dc voltage method, both the average ubmodule voltage of the 6 arm and the dc voltage increae from.0 pu to.29 pu in 0.04, a hown in Fig. 9 (d) and (e). The increae of the dc voltage reduce the power output from the WT and tranmitted to dc by the offhore diode rectifier a can be een in Fig. 9 (f) and (g). When the dc

8 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of voltage reache.29 pu, the offhore ac voltage i limited by the WT converter (et at. pu) and thu no active power can be generated and tranmitted to the dc. The exce power in individual WT need to be dealt with a part of WT fault ride through trategy, e.g. uing dc damping reitor o i not invetigated here. Uing the propoed active dc voltage method, the dc voltage increae to.29 pu within the firt 0.0 and the offhore ac voltage i quickly limited by the voltage limit at. p.u, a hown in Fig. 9 (c). The fater dc voltage increae yield fater reduction of wind power generation and the energy aborbed by the MMC ubmodule i decreaed. A a reult, the ubmodule overvoltage i only.2 pu with the propoed active compared to.29 pu with conventional paive, a hown in Fig. 9 (e). At 0.2, the fault i cleared and the onhore ac voltage recover, leading to the increae of onhore tranmitted active power. A een from Fig. 9 (f), the wind power generation i alo quickly retored. (a)uoff(pu) (g)idc(pu) (f)udc(pu) (e)iq(pu) (d)id(pu) (c)pwt(pu) (b)ioff(pu) the diode rectifier tranformer at A hown in Fig.0 (a), the offhore AC voltage rapidly decreae to 0 after the fault. The drop of the ac offhore voltage lead to the reduction of active power tranmiion through the DRHVDC link and the dc current quickly reduce to 0, a hown Fig.0 (g). The active power from the wind turbine converter alo decreae immediately to 0 a indicated in Fig. 0 (c) for the aggregate 50MW WT. However, the dc voltage of the DRHVDC ytem i till led at pu by the onhore MMC, a hown in Fig. 0 (f). Meanwhile, the WT reactive current increae from 0.09 pu to.28 pu (capacitive) in order to achieve the reactive power rebalance after the offhore fault, a hown in Fig. 0 (e). A the reactive current i et a priority, the active current decreae from pu to 0.2 pu to avoid overcurrent, a can be een in Fig. 0 (d). Fig. 0 (b) how the fault current i well led at around.3 pu which i the maximum current et by the WT ler auming 30% overcurrent capability. At 0.2, the offhore fault i cleared and the offhore ac voltage recover, leading to the recovery of the reactive current from.28 pu to 0.09 pu, and the active current retore to nominal value of pu. A een from the Fig. 0 (c) and (g), the wind power generation i quickly retored. VI. CONCLUSION A ditributed of WT lineide converter for offhore wind farm connected by diode rectifier baed HVDC ytem i propoed in thi paper. The propoed method ue an additional PLLbaed frequency loop to et the reference of the qaxi voltage component for frequency regulation. With the propoed, each WT converter work autonomouly to contribute to the overall offhore frequency regulation, and provide WT with plugandplay capability when offline WT are ynchronized to the offhore network. To ridethrough an onhore fault, an active dc voltage method i propoed for the onhore HVDC MMC converter. By inerting additional ubmodule to temporarily increae the dc link voltage, the power tranmiion from offhore wind farm through the diode rectifier can be quickly reduced thu to achieve fater active power rebalance between the offhore and onhore ide and the MMC ubmodule capacitor overvoltage i thu alleviated due to the reduced urplu energy. Simulation reult verify the propoed trategy of DRHVDC connected WT during tartup, ynchronization and under onhore and offhore fault condition. VII. REFERENCE Fig. 0 Performance of the DRHVDC ytem during offhore olid fault: (a) offhore ac voltage; (b) offhore ac current; (c) aggregate 50 MW WT tranmitted active power; (d) aggregate 50 MW WT active current; (e) aggregate 50 MW WT reactive current; (f) dc voltage of DR; (g) dc current. C. Offhore fault ridethrough operation The performance of wind turbine connected with DR HVDC ytem during offhore fault i hown in Fig. 0 where a olid threephae offhore fault occur at the primary ide of [] S. M. Muyeen, R. Takahahi, and J. Tamura, "Operation and Control of HVDCConnected Offhore Wind Farm," IEEE Tran. Sutain. Energy., vol., pp. 3037, 200. [2] X. Chen, H. Sun, J. Wen, W. J. Lee, X. Yuan, N. Li, et al., "Integrating Wind Farm to the Grid Uing Hybrid Multiterminal HVDC Technology," IEEE Tran. Ind. Appl., vol. 47, pp , 20. [3] O. GomiBellmunt, A. JunyentFerre, A. Sumper, and J. BergaJane, "Control of a Wind Farm Baed on Synchronou Generator With a Central HVDCVSC Converter," IEEE Tran. Power Syt., vol. 26, pp , 20.

9 Thi paper i a potprint of a paper ubmitted to and accepted for publication in IEEE Tranaction on Power Delivery and i ubject to Intitution of [4] F. Deng and Z. Chen, "Deign of Protective Inductor for HVDC Lujie Yu received the B.S. and M.S. degree from Tranmiion Line Within DC Grid Offhore Wind Farm," IEEE Tran. North China Electric Power Univerity (NCEPU), Power Del., vol. 28, pp. 7583, 203. Beijing, China, in 202 and 205. He i currently [5] P. Mitra, L. Zhang, and L. Harnefor, "Offhore Wind Integration to a puruing the Ph.D degree in Electronic & Electrical Weak Grid by VSCHVDC Link Uing PowerSynchronization Control: Engineering, Univerity of Strathclyde, Glagow, UK. A Cae Study," IEEE Tran. Power Del., vol. 29, pp , 204. Hi reearch interet include HVDC tranmiion [6] J. Liang, T. Jing, O. GomiBellmunt, J. Ekanayake, and N. Jenkin, ytem and wind power integration. "Operation and Control of Multiterminal HVDC Tranmiion for Offhore Wind Farm," IEEE Tran. Power Del., vol. 26, pp , 20. [7] R. E. TorreOlguin, M. Molina, and T. Undeland, "Offhore Wind Farm Grid Integration by VSC Technology With LCCBaed HVDC Tranmiion," IEEE Tran. Sutain. Energy., vol. 3, pp , 202. [8] R. BlacoGimenez, S. AnoVillalba, J. RodriguezD'Derlee, et al., Rui Li received the M.S. and Ph.D degree in "Ditributed Voltage and Frequency Control of Offhore Wind Farm electrical engineering from Harbin Intitute of Connected With a DiodeBaed HVdc Link," IEEE Tran. Power Technology, Harbin, China, in 2008 and 203, Electron., vol. 25, pp , 200. repectively. Since 203, he ha been working a a [9] R. BlacoGimenez, S. AnoVillalba, J. RodriguezD'Derlee, et al., reearch aociate with Univerity of Strathclyde in "DiodeBaed HVdc Link for the Connection of Large Offhore Wind Glagow, UK. Farm," IEEE Tran. Energy Conver., vol. 26, pp , 20. Hi reearch interet include HVDC tranmiion [0] S. BernalPerez, S. AnoVillalba, R. BlacoGimenez, and J. Rodriguez ytem, grid integration of renewable power, power D'Derlee, "Efficiency and Fault RideThrough Performance of a Diode electronic converter, and energy converion. Rectifier and VSCInverterBaed HVDC Link for Offhore Wind Farm," IEEE Tran. Ind. Electron., vol. 60, pp , 203. [] R. BlacoGimenez, N. Aparicio, S. AnoVillalba, and S. BernalPerez, "LCCHVDC Connection of Offhore Wind Farm With Reduced Filter Bank," IEEE Tran. Ind. Electron., vol. 60, pp , 203. Lie Xu (M 03 SM 06) received the B.Sc. degree in [2] P. Menke, R. Zurowki, T. Chrit, S. Seman, G. Giering, T. Hammer, et Mechatronic from Zhejiang Univerity, Hangzhou, al., "2nd Generation DC Grid Acce for Large Scale Offhore Wind China, in 993, and the Ph.D. degree in Electrical Farm," in Proceeding of the 4th Wind Integration Workhop,Bruel, Engineering from the Univerity of Sheffield, Belgium, 20th 22nd Oct., 205. Sheffield, UK, in [3] S. Seman, R. Zurowki, and C. Taratori, "Interconnection of advanced He i currently with the Department of Electronic Type 4 WTG with Diode Rectifier baed HVDC olution and weak AC & Electrical Engineering, Univerity of Strathclyde, grid," in Proceeding of the 4th Wind Integration Workhop,Bruel, Glagow, UK. He previouly worked in Queen Belgium, 20th 22nd Oct., 205. Univerity of Belfat and ALSTOM T&D, Stafford, [4] C. Prignitz, H. G. Eckel, S. Achenbach, F. Augburger, and A. Schön, UK. Hi reearch interet include power electronic, "FixReF: A trategy for offhore wind farm with different wind energy generation and grid integration, and application of power WTtype and diode rectifier HVDC tranmiion," in 206 IEEE 7th electronic to power ytem. International Sympoium on Power Electronic for Ditributed Generation Sytem (PEDG), 206, pp. 7. [5] T. H. Nguyen, D. C. Lee, and C. K. Kim, "A SerieConnected Topology of a Diode Rectifier and a VoltageSource Converter for an HVDC Tranmiion Sytem," IEEE Tran. Power Electron., vol. 29, pp , 204. [6] V. Kaura and V. Blako, "Operation of a phae locked loop ytem under ditorted utility condition," IEEE Tran. Ind. Appl., vol. 33, pp. 5863, 997. [7] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodríguez, "Control of Power Converter in AC Microgrid," IEEE Tran. Power Electron., vol. 27, pp , 202. [8] U. N. Gnanarathna, A. M. Gole, and R. P. Jayainghe, "Efficient Modeling of Modular Multilevel HVDC Converter (MMC) on Electromagnetic Tranient Simulation Program," IEEE Tran. Power Del., vol. 26, pp , 20. [9] A. Beddard, M. Barne, and R. Preece, "Comparion of Detailed Modeling Technique for MMC Employed on VSCHVDC Scheme," IEEE Tran. Power Del., vol. 30, pp , 205. [20] J. Xu, C. Zhao, W. Liu, and C. Guo, "Accelerated Model of Modular Multilevel Converter in PSCAD/EMTDC," IEEE Tran. Power Del., vol. 28, pp. 2936, 203. [2] L. Xu, L. Yao, and C. Sae, "Grid Integration of Large DFIGBaed Wind Farm Uing VSC Tranmiion," IEEE Tran. Power Syt., vol. 22, pp , 2007.