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Abstract--The continuous increase in wind power

penetration level brings new requirements for wind turbine integration into the network. The grid code requires that after clearance of an external short-circuit fault, grid-connected wind turbines should restore their normal operation without power loss caused by disconnections. This paper presents a transient performance study of an offshore wind farm with HVDC transmission for grid connection, where the wind turbines in the offshore wind farm are also connected with dc collection network. A power-reduction control strategy (PRCS) for transient performance improvement is proposed for the offshore wind farm that allows it to withstand severe voltage dips. A simulation model of a 400 MW offshore wind farm developed in PSCAD/EMTDC is presented. The transient performance of the offshore wind farm is studied, and the results show the effectiveness of the proposed control strategy.

Index Terms—Offshore wind farm, high-voltage direct current (HVDC), permanent magnet synchronous generator (PMSG), dc grid.

I. INTRODUCTION ind turbine technology has being undergoing a dramatic development and now is the world’s fastest growing

energy [1]. With large-scale exploration and integration of wind sources, Global Wind Energy Council (GWEC) predicts the global wind market would grow by over 155% from the 240 GW in 2007 of total installed capacity until 2012. This means a 146 GW increase in just five years [2].

Along with the increase in the capacity of offshore wind farms and the distance between the offshore wind farm and the load, the high-voltage direct current (HVDC) becomes a favorable solution from the view of economics and technology [3], [4]. The dc transmission and distribution systems offer some advantages compared with ac systems, in particular, issues related to reactive power and harmonics. Just because of this, the dc systems are being used for point-to-point transmissions via HVDC [5]. Meanwhile, various connection configurations for offshore wind farm are proposed, for

Fujin Deng is with the Department of Energy Technology, Aalborg

University, Aalborg, 9220, Denmark (e-mail: fde@et.aau.dk). Zhe Chen is with the Department of Energy Technology, Aalborg

University, Aalborg, 9220, Denmark (e-mail: zch@et.aau.dk).

example, as shown in Fig. 1, the internal dc connection configuration for the wind turbines inside the offshore wind farm associated with a dc transmission system [5-7]. Most grid codes now require that, in case of a short-circuit fault in the external grid, wind turbines should keep connection to the grid and restore their normal operation after fault clearance [8]. The reason is that, when the wind power penetration level is high, the protective disconnection of a large amount of wind power will be an unacceptable consequence that may threaten the power system stability [9].

In this paper, an offshore wind farm with dc grid connection is presented. The dc grid in the offshore wind farm consists of several clusters, where a number of wind turbines with dc output are connected in parallel in each cluster. These clusters are collected by an offshore dc/dc converter which increases the voltage to a transmission level for the HVDC link connecting to the ac grid on shore. A power-reduction control strategy (PRCS) with improved transient performance is proposed for the offshore wind farm, which allows the wind farm to withstand severe grid disturbance.

II. OFFSHORE WIND FARM CONFIGURATION The studied offshore wind farm with dc grid connection in

this paper is shown in Fig. 2, which can be divided into wind turbine level, collection level and transmission level.

Direct drive permanent magnet synchronous generator (PMSG), which are preferred for large, remote offshore wind farms [10], are assumed as the wind power generator in the wind farm. The wind turbine topology is shown in Fig. 3. A voltage source converter (VSC) is used as generator-side converter to transfer the ac to dc. A full bridge isolated boost (FBIB) converter is used to step up the dc voltage to the

Fujin Deng, Student Member, IEEE, and Zhe Chen, Senior Member, IEEE

An Offshore Wind Farm with DC Grid Connection and Its Performance under Power

System Transients

W

Fig. 1. Block diagram of an example offshore wind farm with dc grid connection.

978-1-4577-1002-5/11/$26.00 ©2011 IEEE

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collection level voltage [11]. Four aggregated variable speed wind turbines (VSWTs)

with 100MW each are connected to the offshore converter with 40kV collection voltage. In aggregated model, it is assumed that twenty 5MW wind turbines are lumped together to obtain a large wind turbine generator system [12]. The offshore converter, which is built with two series FBIB converters, steps up the collection voltage to transmission voltage as ±150kV, where two 100 km 150kV XLPE transmission cable with the cross section area of 1200 mm2 is applied for power delivery, where the resistance and the inductance of this cable are approximately 0.0151 Ω/km and 0.00049 H/km respectively. Afterwards, two VSCs are used as the onshore converter to convert dc to ac. A three-winding converter transformer is connected with the two converters, an ungrounded star and a delta connection. Also, the ac voltages from the two VSCs have 30o phase difference, which is utilized for eliminating some harmonics. Finally, the offshore wind farm power is integrated into an ac grid through the double circuit transmission lines and a step-up transformer. The offshore wind farm parameters are given in Appendix.

III. WIND TURBINE MODELING

A. Aerodynamic Model The mechanical power extracted from the wind can be

expressed as follows [6],

),(21 32 λθpw CvρπRP = (1)

where Pw is the extracted power from the wind, ρ is air density (kg/m3), R is the blade radius (m), v is the wind speed (m/s) and CP is the power coefficient which is a function of the pitch angle of rotor bladesθ (deg) and of the tip speed ratio λ. The term λ is defined as λ= ωwR/v, with ωw the wind turbine speed.

The power coefficient may be calculated as ieC

ip

λθθλ

/4.1814.2 )2.13002.058.0151(73.0 −⋅−−−= (2)

with

1

003.002.0

113 +

−−

=θθλλi

(3)

The wind turbine power curves shown in Fig. 4 for various wind speed can be derived from (1) ~ (3). For each wind velocity, there is a turbine speed that gives an optimal output power. Normally, in low to moderate wind speeds, the control of wind turbine is to follow the optimal power coefficient to capture the optimal power [13]. In high wind speeds, the pitch angle controller with a model shown in Fig. 5 starts to be active to prevent the rotor speed from becoming too high [14].

B. Drive Train Model A comparative study of wind turbine systems using

different drive train models [15] has shown that the two-mass model in Fig. 6 is suitable for transient stability analysis, which could be described below [1].

⎪⎪⎩

⎪⎪⎨

⎧

−−+=

−−−=

ggwwgg

g

gwwgww

w

TDKdt

dJ

DKTdt

dJ

)(

)(

ωωθω

ωωθω

(4)

where Jw and Jg are the equivalent wind turbine inertia and

generator inertia respectively. Torque Tw and Tg represent the aerodynamic torque of the wind turbine and the generator loading torque respectively. ωw and ωg are the wind turbine and generator rotor speed respectively. θwg is the angle between the turbine rotor and the generator rotor. K is the elastic characteristic of the shaft. D is the mutual damping between the two masses.

Fig. 2. Block diagram of the offshore wind farm with dc grid connection.

Fig. 4. Wind turbine power curves under different wind speeds.

wωmaxω

)11(β

β sTK +

refβ

Ts1 β

Fig. 5. Block diagram of pitch angle controller model.

wT wω

wJ

gJ

gTK

D

gω

Fig. 6. Block diagram of the two-mass drive-train model.

Fig. 3. Block diagram of the wind turbine.

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IV. WIND FARM CONTROL The control system for the offshore wind turbine could be

divided into wind turbine control, offshore converter control and onshore converter control. Based on the VSWT configuration shown in Fig. 3, the wind turbine control contains generator-side converter and grid-side converter control. The control of the offshore wind farm is described below.

A. Generator-Side Converter Control The generator-side converter is used to regulate the wind

turbine, which enables optimal speed tracking for the optimal power capture from the wind. The control block is developed for generator-side converter as shown in Fig. 7, where the d-axis is aligned with the rotor flux. Based on the measured generator speed, the optimal power point tracking algorithm calculates the power Pg_ref, the optimal power can be captured from the wind. A power controller Ggp(s) is used in the outside loop to regulate the generator power Pg so as to follow the command value Pg_ref, and produces corresponding q-axis current command iq_ref. In the inside loop, the current controllers are adopted to regulate d- and q-axis stator current to track the command. Here, the d-axis stator current, which is proportional to the reactive power, is set to zero so as to perform unity power factor operation.

B. Grid-Side Converter Control The grid-side converter is a FBIB converter [11], which is

used to keep the input dc-link voltage Vin constant. The voltage Vin control using FBIB converter can be realized through the control structure as shown in Fig. 8. A simple PI controller Gu(s) is used in the outside loop as the dc-link voltage controller to generate the required current reference iL_ref. In the inside loop, the PWM regulation system controlled by the other PI current controller Gi(s) would produce the corresponding pulses to drive the FBIB converter so as to track the current reference.

C. Offshore Converter Control Two series FBIB converters make up of the offshore

converter, which is used to maintain the collection line voltage close to the specified reference level. The control of the offshore converter is very similar to the grid-side converter control of the wind turbine shown in Fig. 8. It adopts double control loops. One is current control inside loop and the other one is voltage control outside loop. The collection line voltage is maintained at the reference by controlling the injected current of FBIB converter.

D. Onshore Converter Control The onshore converter is composed with two VSCs, which

is used to maintain the HVDC link voltage close to the reference by adjusting the active power transmitted to the onshore ac network to match is the power received from the offshore HVDC converter.

Vector control technique has been developed for the onshore current regulated VSC. It is illustrated in Fig. 9, where the subscripts d and q refer to the d, q-axis quantities. The VSC is controlled in a synchronous rotating d, q-axis frame with the d-axis oriented along the grid voltage vector position, which ensures the decoupling control of active and reactive powers into the ac grid [13]. The onshore converter adapts double control loops as well. The inside loop is used to control the current. The outside loop is used to maintain the transmission level dc voltage Vdc_t at the rated value. In addition, the VSC can improve grid power factor and inject a current with very low harmonic into the utility grid.

V. WIND FARM RESPONSED UNDER GRID DISTURBANCE Wind farm response under grid disturbance is studied in

the section, which mainly focuses on the voltage dip situation at the point of common coupling (PCC).

The power flow in the offshore wind farm could be described as shown in Fig. 10. Neglecting the power electronics loss, the relationship of the power in the offshore wind farm could be obtained below.

gkw PPP += (5)

θ

Fig. 9. Block diagram of the control for onshore converter.

Fig. 8. Block diagram of the control for grid-side converter.

Fig. 7. Block diagram of the control for generator-side converter.

Fig. 10. Power flow in the offshore wind farm.

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1cacg PPP += (6)

2catc PPP += (7) 3caact PPP += (8) The generator controlled by generator-side converter

captures the power Pg from the wind power Pw, the power Pk is stored as the kinetic energy. And then, the grid-side converter transmits the power Pc from the generator to the collection level, while power Pca1 is stored in the capacitor between the generator-side converter and the grid-side converter. Afterwards, most of the collection level power is sent to the transmission level as Pt by the offshore converter, except that some is stored in the collection level capacitors as Pca2. Finally, the ac grid extracts the power Pac from the onshore converter with some power stored in the transmission level capacitors as Pca3.

When a disturbance occurs in the external grid, a voltage dip occurs at the ac output terminals of the offshore wind farm. The maximum active power that the offshore wind farm can export to the ac grid is reduced in proportion to the terminal voltage reduction. At this time, the output power of the onshore converter is quickly reduced. However, the input power of the onshore converter is nearly not changed. Therefore, there is a power imbalance in the transmission level under the grid disturbance. It results in the increase of the transmission level voltage, which may be harmful for the offshore wind farm. A power-reduction control strategy is proposed to emphasize the capacity of handling faults for the offshore wind farm as follows.

A. Improved Offshore Converter Control The improvement for the offshore converter to avoid

overvoltage could be realized by embedding a voltage limit loop printed in the gray area shown in Fig. 8. The voltage limit loop could effectively limit the overvoltage in the transmission level under the grid faults.

When the output voltage Vout of the offshore converter (FBIB converter) is increased and over the limit value Vout_limit, which is set as 1.1 p.u. under grid faults, the voltage limit loop starts to be active to produce a current compensation component iL_comp, which is embedded into the voltage control loop to reduce the current reference value. Hence, the input current of FBIB converter is reduced, which effectively decreases the power Pt transmitted to the transmission level. Based on (8), although the ac grid power Pac is reduced owing to the voltage dip at PCC, the decrease of the transmission level power Pt is also fast reduced by the improved control strategy, which could effectively reduce the capacitor power Pca3 and limits the increase of the capacitor voltage in the transmission level.

On the other hand, based on (7), the decrease of the power Pt sent to the transmission level by the offshore converter could results in the increase of power Pc. Consequently, the overvoltage may appear in the collection level. The following improved grid-side converter control could effectively handle this problem.

B. Improved Grid-Side Converter Control The overvoltage in the collection level could be limited by

the improved grid-side converter control. Based on the description before, the topology and the control strategy for the grid-side converter is similar to that of the offshore converter. In order to emphasize the fault handling capacity for the offshore wind farm, the improvement for the grid-side converter control is nearly the same as that of the offshore converter.

The output voltage of the wind turbine is measured by the control system and compared with the set value, which is set as 1.1 p.u. in this study. Once the output voltage Vout is over the set value, the reference current is reduced by the voltage limit loop, which results in the decrease of the output power Pc of the wind turbine. Hence, according to (7), the capacitor energy in collection level could not be increase so much, and the voltage in the collection level is effectively limited.

C. Improved Generator-Side Converter Control Along with the improvement of the grid-side converter

control, the output power Pc of the wind turbine is reduced. It means that the more energy may be stored in the capacitor in the wind turbine converter, which may result in a voltage increase and damage the wind turbine converter. Hence, the generator-side converter control should be improved too so as to prevent the possible damage.

The generator-side converter control is improved with the gray area in Fig. 7. When the dc-link voltage in the wind turbine is higher than the reference value as 1.1 p.u., the compensation is active in the power control loop, which is used to reduce the active power Pg of the generator. Based on (6), it can be seen that with the decrease of the generator power Pg, the capacitor power in the wind turbine is effectively limited. Hence, the overvoltage on the capacitor of the wind turbine converter is avoided. On the other hand, from (5), it can be realized that more power is transferred to kinetic energy because of the decrease of the generator power. Consequently, the kinetic energy is increased with the increase of the wind turbine speed. The typical inertia constants for the generators of the large power plants are in the range of 2-9s [16]. According to the drive-train system equation (4), the bigger inertia would result in smaller increase of the wind turbine speed during the short faults period. Besides, the pitch angle control system would be active if the wind turbine speed is over its rated value to effectively limit the wind turbine speed.

VI. SIMULATION The simulation study is conducted with the software

PSCAD/EMTDC. A 400 MW offshore wind farm is modeled as shown in Fig. 1.

A three-phase short-circuit fault happened on one of the 110kV double circuit transmission lines, which result in that the voltage Vpcc at the PCC is dipped to 15% of the rated value. After 150ms, the fault is cleared from the power system and the voltage of PCC is recovered. Fig. 11 shows the offshore

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wind farm response with the proposed control strategy under the ac grid disturbance.

The wind speed for the wind turbines are shown in Fig. 11(a). The wind turbines in the offshore wind farm are controlled by the generator-side converter to capture the maximum power from the wind, with the corresponding wind turbine speed and pitch angle. The grid-side converters keep the dc-link capacitor voltage constant as 6.4kV for the normal operation of the generator-side converter. The collection level voltage is kept by the offshore converter as 40kV and the transmission level voltage is maintained as 150kV by the onshore converter before grid fault.

A grid fault happened at 7s causes ac grid voltage Vpcc dip to 15% of the rated value and lasting for 150ms shown in Fig. 11(b). The power sent into the grid is reduced in proportional to the decrease of the ac grid voltage shown in Fig. 11(d). At this time, the onshore converter loses its controllability, and could only send little power to the ac grid. However, the power transferred from the offshore converter is nearly unchanged. Consequently, more energy is stored on the transmission level, which results in the increase of the

transmission level voltage. It can be seen that the transmission level voltage is nearly increased by four times as shown in Fig. 11(j). Owing to that the generation power by wind turbine 1 and 2 is bigger than that of wind turbine 3 and 4 as shown in Fig. 11(g), the Vdc1 is made higher than Vdc2. Nevertheless, the offshore converter, grid-side and generator-side converters in the wind turbine are nearly not affected and still work as normal as shown in Fig. 11(g ~ i).

Based on the proposed power-reduction control strategy, if the transmission voltage is measured to be over the limit value as 1.1 p.u. due to the grid fault, the offshore converter starts to reduce its input power so as to limit the increase of transmission level voltage. Fig. 11(o) shows that the transmission level voltage, which is only increased to 1.2 p.u.. Because of the decrease of the input power for the offshore converter, the imbalance appears in the collection level, which causes the increase of the collection level voltage. With the proposed control strategy, the grid-side converter in the wind turbine reduces its output power if its output voltage is over the reference value as 1.1 p.u.. Hence, the collection level voltage is effectively limited with the maximum value as 1.2 p.u. shown in Fig. 11(n). Also, the dc-link voltage in the wind turbine converter is increased because of the decrease of its output power. With the proposed control strategy, the generator-side converter starts to reduce its output power shown in Fig. 11(l) to keep the dc-link voltage in the wind turbine if the dc-link voltage is over the set point as 1.1 p.u. shown in Fig. 11(m). Finally, the decrease of the generator power causes the increase of the kinetic energy, which results in the increase of the wind turbine speed shown in Fig. 11(e). Nevertheless, owing to the action of pitch angle control system as shown in Fig. 11(k) and the big inertia of the wind

Fig. 11(b). Grid voltage at point of common coupling.

Fig. 11(c). Wind turbine speed without PRCS.

Fig. 11(a). Grid voltage.

Fig. 11(d). Grid power.

Fig. 11(e). Wind turbine speed with PRCS.

Fig. 11(a). Wind speed.

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Fig. 11(f). Pitch angle without PRCS.

Fig. 11(g). Generator power without PRCS.

Fig. 11(h). DC-link voltage in wind turbine without PRCS.

Fig. 11(i). DC-link voltage in collection level without PRCS.

Fig. 11(j). DC-link voltage in transmission level without PRCS.

Fig. 11(k). Pitch angle with PRCS.

Fig. 11(l). Generator power with PRCS.

Fig. 11(m). DC-link voltage in wind turbine with PRCS.

Fig. 11(n). DC-link voltage in collection level with PRCS.

Fig. 11(o). DC-link voltage in transmission level with PRCS.

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turbine, the wind turbine speed could not have a big change during the short time period. As a result, the offshore wind farm with the dc grid connection could have a good transient performance under the proposed control strategy.

VII. CONCLUSION This paper concentrates on the transient performance study

of an offshore wind farm with dc grid connection. The offshore wind farm configuration is presented. The HVDC technology is used in transmission level, a dc grid is applied in the collection level. The wind turbines with PMSG, VSC and FBIB converter, can directly output the dc, which makes the wind turbines could be connected and collected with the dc network. A power-reduction control strategy is proposed as well, which effectively improve the transient performance of the offshore wind farm under the grid disturbance. Finally, the model of a 400 MW offshore wind farm is built in PSCAD/EMTDC, and the simulation results shown the effectiveness of the proposed control strategy.

VIII. ACKNOWLEDGMENT The authors gratefully acknowledge the AAU (Aalborg

University) and CSC (China Scholarship Council) for providing a scholarship to support the study.

IX. APPENDIX

TABLE I WIND TURBINE AND GENERATOR CHARACTERISTIC

Wind turbine rated power (MW) 5 Rotor diameter (m) 126 Rotating speed (r/m) 6.9~11.94 Nominal wind speed (m/s) 11.4 Generator Rated power (MW) 5 Stator rated line voltage (kV) 3 Rated frequency (Hz) 20 Number of pole pairs 100 Stator winding resistance (p.u.) 0.001 Unsaturated induction Xd (p.u.) 0.15 Unsaturated induction Xq (p.u.) 0.1 Magnetic strength (p.u.) 1 Generator inertia (s) 0.84 Equivalent wind turbine inertia (s) 5.54 Shaft stiffness K (p.u.) 2.15 Shaft damping D (p.u.) 0.015

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[3] R. O’Donnell, N. Scho field, A. C. Smith, J. Cullen, “Design Concepts for High-Voltage Variable-Capacitance DC Generators,” IEEE

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[4] Risheng Li, G. Bozhko, G. Asher, “Frequency Control Design for Offshore Wind Farm Grid With LCC-HVDC Link Connection,” IEEE Transaction on Power Electronics, vol. 23, Issue 3, pp. 1085-1092, May, 2008.

[5] C. Meyer, M. Hoing, A. Peterson, R. W. De Doncker, “Control and Design of DC Grids for Offshore Wind Farms,” IEEE Transactions on Industry Applications, vol. 43, Issue 6, pp. 1475-1482, Nov-Dec. 2007.

[6] F. Blaabjerg, Zhe Chen, S. B. Kjaer, “Power Electronics as Efficient Interface in Dispersed Power Generation Systems”, IEEE Transactions on Power Electronics, vol. 19, Issue 5, pp.1184- 1194, September 2004.

[7] J. Robinson, D. Jovcic, G. Joos, “Analysis and design of an offshore wind farm using a MV dc grid,” IEEE Transactions on Power Delivery, vol. 25, issue. 4, pp. 2164-2173, 2010.

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[9] Tao Sun, Zhe Chen, F. Blaabjerg, “Transient Stability of DFIG Wind Turbines at an External Short-circuit Fault,” Wind Energy, vol. 8, pp. 345-360, 2005.

[10] M. Chinchilla, S. Arnaltes, J. Burgos, “Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the grid,” IEEE Transaction on Energy Conversion, vol. 21, no. 1, pp. 130-135, March, 2006.

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XI. BIOGRAPHIES

Fujin Deng received the B.Eng. degree in electrical engineering from China University of Mining and Technology, Jiangsu, China, in 2005. He received the M. Sc. Degree in electrical engineering in 2008 from Shanghai Jiao Tong University, Shanghai, P.R. China. He is currently working toward the Ph.D. degree with the Department of Energy Technology, Aalborg University, Aalborg, Denmark.

His current research interests include wind power generation, control of permanent magnet synchronous generator, and offshore wind farm-power systems dynamics.

Zhe Chen (M’95, SM’98) received the B.Eng. and M.Sc. degrees from Northeast China Institute of Electric Power Engineering, Jilin City, China, and the Ph.D. degree from the University of Durham, Durham, U.K.

He was a Lecturer and then Senior Lecturer with De Montfort University, Leicester, U.K. Since 2002, he has been a Research Professor and now a Professor with the Institute of Energy Technology, Aalborg University, Aalborg, Denmark, where he is the coordinator of the Wind Power System Research program at the Institute of Energy Technology. His research areas are power systems, power electronics, and electric machines, with specific

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interest in wind energy and modern power systems. He has more than 200 publications in his technical field.

Dr. Chen is an Associate Editor (Renewable Energy) of the IEEE TRANSACTIONS ON POWER ELECTRONICS, Guest Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS (Special Issue on Power Electronics for Wind Energy Conversion). He is a Member of the Institution of Engineering and Technology (London, U.K.) and a Chartered Engineer in the U.K.