Francisco M. Gonzalez-Longatt Juan Manuel Roldan Jose Luis Rueda. Line 5: City, Country

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1 Impact of DC Control Strategie on Dynamic Behaviour of Multi-Terminal Voltage-Source Converter-Baed HVDC after Sudden Diconnection of a Converter Station Francico M. Gonzalez-Longatt Juan Manuel Roldan Joe Lui Rueda Line : Coventry Author Univerity Name/ per t Affiliation Univeridad de Sevilla Line : Author Univerity Name/ Duiburg-Een, per 2nd Affiliation Line Faculty 2: Author of Engineering Name/ and per t Affiliation Ecuela de Ingeniería Eléctrica Line 2: Author Intitute Name/ of Electric per ower t Affiliation Sytem Line 3 (of Affiliation): Computing Dept. name of organization Sevilla, Spain Line 3 (of Affiliation): Duiburg, Dept. name Germany, of organization Line 4: Coventry, name of United organization, ingdom acronym acceptable jmroldan@u.e Line 4: name of organization, joe.rueda@uni-due.de acronym acceptable fglongatt@ieee.org Line 5: City, Country Line 5: City, Country Line 6: addre if deired Line 6: addre if deired Abtract Multi-terminal HVDC (MTDC) tranmiion ytem uing Voltage Source Converter (VSC) provide an increaed tranmiion network capacity and generally enhanced ytem reliability, ecurity and controllability. The aim of thi paper i to evaluate the impact of dc-voltage control trategie on dynamic behaviour of MTDC VSC-Baed HVDC after the udden diconnection of a converter tation. Two dc voltage control method are conidered in thi paper: (i) voltage margin method and (ii) tandard voltage-droop method. The impact i evaluated uing time-domain imulation on imple tet ytem uing. The udden diconnection of a converter-tation i ued a diturbance. Simulation reult demontrate there i a "collaborative cheme" for the dc voltage upport when two converter on the MTDC operate with dc voltage droop characteritic. Index Term Control ytem, high voltage direct current, multi-terminal HVDC, voltage ource converter. I. INTRODUCTION The Multi-terminal HVDC (MTDC) ytem are being tudied a flexible olution for the future maive integration of offhore wind power, including potential benefit to provide additional upport connecting non-ynchronou power ytem [], [2]. The mot appropriate technology in MTDC i HVDC ytem baed on Voltage Source Converter (VSC) a it provide an increaed tranmiion network capacity and generally enhanced ytem reliability, ecurity and controllability. Outtanding effort on the reearch on MTDC have been developed in everal area in recent time. A quite a number of publication are devoted to everal ubject of MTDC: teady tate performance [2], [3], [4], [5], until modelling and imulation of dynamic behaviour [6], [7]. However, there i one important apect that require evaluation, the traditional reliability and availability related to outage a to tranient reliability related to performance during and recovery after temporary fault and diturbance. It i an important apect due to the large amount of power tranmitted by MTDC ytem. The aim of thi paper i to evaluate impact of the dc voltage control trategy on the dynamic behaviour of multiterminal VSC-baed HVDC following a udden diconnection of a converter tation, two trategie are analyzed in thi paper: Voltage Margin Method (VMM) and Standard Voltage Droop Method (SVDM). II. CONTROL SCHEMES FOR A MTDC SYSTEM The control ytem for a MTDC ytem can be divided into two type: (a) central mater controller which i a global area controller and (b) local terminal controller ued locally at each converter tation [7]. The general picture of the control cheme i depicted in Fig.. Fig.. Schematic repreentation of MTDC control ytem hierarchy [7]. The terminal controller control the pecific converter by calculating the WM pule for the converter bridge. Firing control i the lowet level on it and it act very fat. Inner control, outer control and upplementary control are ued for increaingly higher level function, and have increaingly

2 higher cycle time. The inner control or current control loop i deigned to be much fater than the outer controller. The outer controller are the one reponible for providing the current reference ignal for the inner current controller. The terminal controller determine the behavior of the converter at the ytem bu. They are deigned for the main function for controlling: active power, reactive power, AC and the dc voltage. The mater control optimize the overall performance of the MTDC by regulating the dc ide voltage. It i provided with the minimum et of function neceary for coordinated operation of the terminal in the dc circuit, i.e. tart and top, minimization of loe, ocillation damping and power flow reveral, black tart, ac frequency and ac voltage upport. A general overview about the terminal controller i depicted on Fig. 2. V ac, Ctrl Q Ctrl Q ref Q V ac V, ref ac i q U dc, Ctrl Ctrl U dc U, ref dc Fig. 2. Repreentative block diagram of a Terminal Controller in a MTDC ytem [7]. A. Current Controller The current controller loop i the inner part of the cacaded control trategy. It need to be very fat a compared to the outer controller o a to achieve control ytem tability. It i upplied current reference value from the outer controller and dq tranformed current from tranducer. The objective of the inner controller are to track the current reference value given by the outer controller and to generate voltage reference value i.e. u d and u q fed to the controlled voltage ource (ee Fig. 3). B. ower Controller The active power controller i deigned to regulate the active power () exchanged at the common bu to match the given reference value ( ) by modifying i d. The output of the active power controller (i d ) i the reference input to the d- axi current controller of the inner current loop. In order to limit the magnitude of current in the VSC-HVDC to a maximum limit, the output of the active power controller i followed by a limiter function of i max limit, where: i max = i rated (ee Fig. 4). C. Reactive ower Controller The objective of thi controller i to govern the reactive power (Q) exchanged at the common bu to match the given reference value (Q ref ) by modifying i q (ee Fig. 5). ref, i q i qref, v d iid, pid, L L iiq, piq, Fig. 3. Baic cheme for inner-current Controller. p, Fig. 4. Baic cheme for active power controller. Q ref Q pq, i, iq, v q i max i max i max i max v d i q v q Fig. 5. Baic cheme for reactive power controller. The output of the reactive power controller (i q ) i the reference input for the reactive current controller of the inner current loop. i q i limited to I q-max in uch a way that the total converter current hould not exceed the rated current (I max =I rated ). Thi take the aumption that that priority i given to tranfer of active power. Hence: q max 2 d i I i () 2 max D. AC voltage controller Thi controller i deigned to regulate the amplitude of the ac voltage (V ac ) at the common bu to be equal to the given reference value by modifying i q. Thi implie that the controller govern the converter to generate an amount of reactive power o that the voltage at the common bu matche the given reference value (V ac,ref ) (ee Fig. 6). V ac V ac, ref i max ivac, pvac, i q i max Fig. 6. Baic cheme for AC voltage controller. III. DC VOLTAGE CONTROLLER Conidering the operational requirement for dc voltage on MTDC, the literature provide two control trategie which

3 poibly can be applied in future trannational network [7]: (i) the tandard voltage droop method (SVDM) and the (ii) voltage-margin method (VMM). Thee control method are explained on the general cheme for HVDC ytem conidering only two converter ubtation (ee Fig. 7). Terminal controller A Mater Controller Terminal controller B Fig. 7. General cheme for two converter tation HVDC ytem. VSC i operate a inverter ( i <) or rectifier ( i >) depending in power direction. A. Standard Voltage Droop Method (SVDM) The voltage margin i defined a the difference between the dc reference voltage of the two terminal [8]. Fig. 8 how the U dc - characteritic of both terminal at Terminal A, the interection U dc - of the characteritic of each terminal i the operating point "a". U dc,a U dc upper U dc v d lower, upper lower, Fig. 9. Baic cheme for VMM controller with adjutable limit. Frequency droop control i a well etablihed method and the bai for table operation in all ac grid. The ytem frequency i ued a a global meaure for the intantaneou balance between power generation and demand [9]. The dc voltage-droop method i a coordinated control to maintain a power balance and a deired power exchange in the MTDC. Thi control i a modification of the VMM control where the horizontal line ection ( lower < A < upper ) of the U dc - characteritic curve i replaced by a line with mall lope ( c ) []. The dc voltage-droop, c, indicate the degree of compenation of power unbalance in the dc grid at a cot of reduction in the dc bu voltage. Thi principle of VMM control i hown in Fig.. U dc,a a b U ref U dc upper lower Fig. 8. U dc - characteritic howing the operating point "a" in VMM for one terminal. When the active power i to be tranmitted from Terminal B to Terminal A ( A <, B >), the voltage margin (U dc ) i ubtracted from the dc reference voltage for Terminal A. Terminal B (rectifier) determine the dc ytem voltage and Terminal A (inverter) control the active power ( A ) determined by the lower limit of the dc voltage regulator. The dc voltage controller trie to keep the dc voltage to the reference value U dc,ref by adjuting A, until A reache the upper limit or the lower limit [7] (ee Fig. 9). The voltage margin method give reliable way of controlling MTDC without the need for communication between terminal and i capable of keeping the teady tate voltage with in preet limit even after load witching and diconnection of ome converter terminal. But on the other hand, thi method implie allocation of only one terminal at a time for the regulation of dc voltage and the other terminal do not experience ignificant change during change in power flow of the dc network [7]. A lower a b upper Fig.. U dc - characteritic howing the operating point "a" in VMM for one terminal. When U dc,a drop (e.g. due to large withdrawal of power omeplace ele in the dc network, operation point move from "a" to "b") the lack converter tation (VSC A ) will increae the active power injection in the dc grid A until a b b new equilibrium point (, ), at a lower dc voltage, Aref, b a i reached (Udc, ref U dc, ref U dc ). The ue of a proportional dc voltage-controller allow multiple converter to regulate the voltage at the ame time and the concept of ditributed lack bu i poible. U dc ref iudc, pudc, c A i max imax Fig.. Baic cheme for SVDM controller. Fig. how how i implemented the droop characteritic baed on the power active controller. When

4 voltage droop control i ued in the abence of a I controller, the voltage controller active power will change when the value of the dc bu voltage change IV. SIMULATION AND RESULTS Time domain-imulation are ued to evaluate dynamic behaviour of dc voltage and power flow in a tet MTDC network. The ac network i baed on claical 5-node tet network from the book of Stagg and El-Abiad [] and 4- node VSC MTDC network i included. Detail of the teady tate unditurbed condition are hown in Fig. 2. Fig node tet ac network and 4-node VSC MTDC ytem between node 2, 3, 4, and 5. DIgSILENT owerfactory TM i ued a imulation tool, the model of all controller are developed uing DIgSilent Simulation Language (DSL). All imulation are performed uing a peronal computer baed on Intel, Core TM i7 CU 2.GHz, 8 GB RAM with Window 7 Home Edition 64-bit operating ytem. Three cae are evaluated in thi paper - Cae I: The converter tation VSC37 i choen a dc lack-bu when the VMM i ued, thereby controlling the voltage on the dc network. The other converter tation (VSC26, VSC58, VSC49) are directly controlling their reactive power injection (contant Q-mode). The converter tation VSC37 i alo ued to control the voltage at bu 3 (.998 p.u). - Cae II: Thi cae conider the ue of multiple dc lack bu, in thi cae, all converter tation are uing controller baed on SDVDM conidering c = Cae III: It i imilar to Cae II but it conider c = Two imple contingencie are imulated in thi paper. A. Scenario A: Sudden Diconnection of VSC37 Fig. 3 how the time-domain repone of dc voltage and power flow after the udden diconnection of one converter tation VSC37 (Scenario A). Conidering the maximum intantaneou value on dc voltage, the Cae II provide the lowet value (min: p.u. at bu 6) and Cae III provide the highet (max: 6 p.u at bu 6). The voltage droop characteritic U dc - and it lope c have trong influence during the tranient. A low c -value produce a low peak on the maximum intantaneou value on the dc voltage profile compared with it produced by a high c value. Thi contingency produce a power imbalance on the dc ytem ( VSC35 = 43.2MW), and it mut be re-ditributed between the remaining converter tation. The Cae I produce the lowet change on the teady-tate dc voltage and the highet change on the power flow following the contingency (ee Fig. 4). B. Scenario B: Sudden Diconnection of VSC58 The udden diconnection of converter tation VSC58 produce an infeed lo of VSC58 = 55 MW, a conequence, there are dynamic change on dc voltage and power flow in order to reach the new operational condition. Thi dynamic i depicted on Fig 5. The maximum intantaneou dc voltage on Cae II are the lowet value (min:.75 p.u. at bu 6) and Cae III provide the highet value (max: 39 p.u at bu 6). It i conequence of the lope c ued on the characteritic U dc -. The final teady-tate dc voltage i highly modified from the initial tate. Thee change are detailed per cae on Fig. 6. The VSC37 kept contant the voltage at bu 3 (.998 p.u) during the Cae I but the ytem i not capable to recover the initial power flow (ee Fig. 6) becaue the converter VSC26 and VSC49 are operating contant Q-mode (Q VSC49 =, Q VSC26 = 4 Mvar).

5 ower i-j (MW).2 Bu 6 Cae I Time () Bu 8.2 Cae III Cae II Cae I.9 Time () DC Line Cae III Cae II ower i-j (MW).2 Bu 7.9 Time () Bu 9 Cae I Time () DC Line MW Cae III Cae II Cae I Cae III Cae II Bu 6 Cae I Time () Bu 8 Cae III Cae II.95 Time () DC Line MW 38 ower i-j (MW) Cae I Cae III Cae II ower i-j (MW) Bu 7 Cae I.95 Time () Bu 9 Cae III Cae II Cae I Time () DC Line Cae III Cae II ower i-j (MW) 5 Time () DC Line ower i-j (MW) ower i-j (MW) 24 Time () DC Line Cae I - Cae II Cae III -5 Time () Time () DC DC Line MW Time () Fig. 3. Time-domain repone of dc Voltage and dc ower Flow: Scenario A. (a) Cae III Cae I Bu 8 Bu 6 Bu 9Bu 7 -. ercentage DC Voltage (%) VSC58 VSC26 VSC49 Cae II Cae I Cae III Fig. 4. (a) Change (%) on dc teady-tate voltage and (b) Converter tation loading (%) condition: Scenario A. (b) 6.89MW ercentage DC ower (%) ower i-j (MW) 32 Time () DC Line MW -6 Time () ower i-j (MW) ower i-j (MW) Time () DC Line DC Line Time () Time () Fig. 5. Time-domain repone of dc Voltage and dc ower Flow: Scenario B. (a) Cae III Cae I Bu 8 Bu 6 Bu 9Bu 7 -. ercentage DC Voltage (%) VSC26 VSC37-7.6MW Cae II Cae I Cae III Fig. 6. (a) Change (%) on dc teady-tate voltage and (b) Converter tation loading (%) condition: Scenario B. (b) VSC MW ercentage DC ower (%) Cae I Cae II Cae III

6 Cae II and Cae III tet two different value of voltagedroop lope ( c = -2., -5.) and the dynamic repone on the power flow tranfer inide the MTDC how how the power imbalance i hared between the active converter tation baed in the droop lope. There i a collaborative cheme in thi hare proce. The power tranfer on the underea cable 7-8 help to etablih thi power ditribution between converter. SDVDM provide dc voltage change baed on the change on active power; it allow the MTDC urvive a converter outage. V. CONCLUSIONS Simulation reult how the effect of dc Voltage control trategy on the dynamic behavior of but voltage and power flow in a MTDC ytem following a converter-tation outage. Two different dc voltage control method are imulated in thi paper: voltage margin method and voltagedroop method. Time-domain imulation on imple tet ytem uing DigSILENT owerfactory TM are ued to evaluate the repone of ac/dc bu voltage conidering imple contingencie baed on udden converter-tation diconnection. Three cae conidering location and type of dc voltage controller have been conidered. Two value of voltage droop lope have been teted howing that the tranient repone i clearly influenced by the voltage droop characteritic. When two converter on the MTDC operate with dc voltage droop characteritic, it appear a "collaborative cheme" for the dc voltage upport, haring the tak of controlling the dc voltage. Simulation reult demontrate the voltage margin control i capable to urvive a converter outage jut if thi converter i operating on contant power mode. REFERENCES [] T. M. Haileelaie, "Control of Multi-terminal VSC-HVDC Sytem," Mater of Science in Energy and Environment, Department of Electrical ower Engineering, Norwegian Univerity of Science and Technology, Trondheim Norway, 8. [2] F. Gonzalez-Longatt, J. Roldan, and C. A. Charalambou, "ower Flow Solution on Multi-Terminal HVDC Sytem: Supergrid Cae," preented at the International Conference on Renewable Energie and ower Quality (ICREQ 2), Santiago de Compotela (Spain), 2. [3] T.. Vrana, R. E. Torre-Olguin, B. Liu, and T. M. Haileelaie, "The North Sea Super Grid - a technical perpective," in AC and DC ower Tranmiion,. ACDC. 9th IET International Conference on,, pp. -5. [4] E. Acha, B. azemtabrizi, and L. M. Catro, "A New VSC-HVDC Model for ower Flow Uing the Newton-Raphon Method," ower Sytem, IEEE Tranaction on, vol., pp. -, 3. [5] F. Gonzalez-Longatt, J. M. Roldan, and C. A. Charalambou, "Solution of ac/dc power flow on a multiterminal HVDC ytem: Illutrative cae upergrid phae I," in Univeritie ower Engineering Conference (UEC), 2 47th International, 2, pp. -7. [6] J. Beerten, S. Cole, and R. Belman, "Generalized Steady-State VSC MTDC Model for Sequential AC/DC ower Flow Algorithm," ower Sytem, IEEE Tranaction on, vol. 27, pp , 2. [7] F. Gonzalez-Longatt and J. M. Roldan, "Effect of dc voltage control trategie of voltage repone on multi-terminal HVDC following a diturbance," in Univeritie ower Engineering Conference (UEC), 2 47th International, 2, pp. -6. [8] T. Nakajima and S. Irokawa, "A control ytem for HVDC tranmiion by voltage ourced converter," in ower Engineering Society Summer Meeting, 999. IEEE, 999, pp. 3-9 vol.2. [9] T. M. Haileelaie and. Uhlen, "rimary frequency control of remote grid connected by multi-terminal HVDC," in ower and Energy Society General Meeting, IEEE,, pp. -6. [] R. L. Hendrik, G. C. aap, and W. L. ling, "Control of a multiterminal VSC tranmiion cheme for connecting offhore wind farm," in European Wind Energy Conference, Milan, Italy, 7. [] G. W. Stagg and A. H. El-Abiad, Computer method in power ytem analyi. New York: McGraw-Hill, 968.