Design of Protective Inductors for HVDC Transmission ... - IEEE Xplore

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 1, JANUARY 2013

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Design of Protective Inductors for HVDC Transmission Line Within DC Grid Offshore Wind Farms Fujin Deng, Student Member, IEEE, and Zhe Chen, Senior Member, IEEE

Abstract—This paper presents fault analysis and protective inductors design for an offshore wind farm, where the power collection system in the wind farm and the power transmission link to the grid adopt high-voltage direct-current (HVDC) technology. This paper focuses on dealing with short-circuit faults in the HVDC link between the offshore station and the onshore station. The transient characteristics of the transmission system are analyzed in detail. The criteria of selecting protective inductors are proposed to effectively limit the short-circuit current and avoid the damage to the converters. A dc grid offshore wind farm is simulated, and the results demonstrate the effectiveness of the proposed protective inductors design. Index Terms—DC fault, high-voltage direct current (HVDC), offshore wind farm, wind power generation.

I. INTRODUCTION

O

FFSHORE wind farms are currently seen as a promising solution to satisfy the growing demand for renewable energy sources [1]. At sea, a huge amount of wind resource is available, and the higher and less fluctuating wind speed leads to greater and smoother power production [2]. Offshore installations offer higher energy yields at the expense of the higher installation and maintenance costs. Along with the increase in the capacity of offshore wind farms and the distance between offshore wind farms and grids, the high-voltage direct-current transmission using voltage-source converters (VSC-HVDC) becomes attractive [3]–[8]. The dc transmission systems have some significant advantages over ac systems, such as reactive power and harmonics and so on [2]. Furthermore, a dc grid may also offer some advantages for interconnecting the wind turbines within the offshore wind farm [9], [10]. In the dc grid, the modern dc/dc converters with medium frequency transformers (MFT) would be used to step up the voltage. Compared with the bulky 50-Hz transformers used in the traditional ac system, the frequency of the MFT in the dc/dc converter would be significantly higher than 50 Hz, which dramatically reduces the size and weight of the MFT. Especially, for the offshore wind farms, the weight of the components is an important issue [2].

Manuscript received July 03, 2011; revised January 17, 2012; accepted October 03, 2012. Date of publication November 27, 2012; date of current version December 19, 2012. Paper no. TPWRD-00567-2011. The authors are with the Department of Energy Technology, Aalborg University, Aalborg, 9220, Denmark (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2012.2224384

In traditional HVDC transmission systems based on the linecommutated converter (LCC) with thyristors, a large smoothing reactance is connected in series with cables [11], [12]. Therefore, the dc link has no overcurrent problem caused by cable faults. However, in a VSC-based HVDC transmission system, a dc fault may result in significant overvoltage or overcurrent in the dc grid; therefore, the protection of a dc grid becomes more challenging for such a dc wind farm, and dedicated protection methods and equipment may be required, for example, dc circuit breakers (CBs) for isolating the faulty sections of a dc system. Recently, both passive and active dc breakers for high-voltage transmission systems have been developed [13], [14]. Unfortunately, there is not much literature about the protection of dc transmission, especially for VSC-HVDC systems. Reference [15] presents a handshaking method, which locates and isolates the faulted dc line and restores the dc grid system without using telecommunication technology. Some research work considers low-voltage dc grid (LVDC) systems, such as overcurrent-based protection schemes presented in [16]. A collection grid internal fault is analyzed in detail for a multiterminal dc wind farm with a brief introduction of some possible protection methods in [10]. However, no detailed protection design for the HVDC transmission system has been presented. This paper presents the fault analysis and protective inductors design for an offshore wind farm, where the HVDC technology is adopted for the power collection system in the wind farm and the power transmission link to the grid. This paper is focused on short-circuit faults in the HVDC link between the offshore station and the onshore station. The transient characteristics of the transmission system under faults are analyzed in detail. The criteria of selecting protective inductors are proposed to effectively limit the short-circuit current and avoid damage to the converters. This paper is organized as follows. In Section II, the studied offshore wind farm with the dc grid connection is presented. The cable-to-ground fault in the HVDC link between the offshore station and the onshore station is analyzed in detail in Section III. The criteria of selecting protective inductors are proposed in Section IV. Finally, system simulations by PSCAD/ EMTDC are presented in Section V to show the effectiveness of the proposed protective inductors design. II. OFFSHORE WIND FARM WITH DC GRID CONNECTION The offshore wind farm with dc grid connection studied in this paper is shown in Fig. 1, which is composed with the wind

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Fig. 1. Block diagram of the offshore wind farm with dc grid connection. Fig. 2. HVDC transmission system with positive pole cable-to-ground fault.

turbines, collection and transmission systems, and converter stations. The relevant parameters are given in the Appendix. The studied wind farm is represented by four aggregated wind turbines with 100 MW each, respectively, as shown in Fig. 1. The direct-drive permanent-magnet synchronous generators (PMSG), which has some preferred features for large offshore wind farms [17], are used. It is assumed that each aggregated model has twenty 5-MW wind turbines lumped together. The ac output of the generator is converted into the low dc voltage with a VSC. A full-bridge isolated boost (FBIB) converter is used as a dc/dc converter to step up the low dc voltage to a medium-voltage level, which effectively reduces the cable losses at the collection system [2]. Two FBIB converters are configured into positive and negative poles of HVDC transmission line. The FBIB converters have the benefit of only a few components, including diodes and capacitors on the high-voltage side of the transformer [18], [19]. The output of the dc/dc converters would withstand a highvoltage level, and the use of the diode bridges in the dc/dc converters could be advantageous. In addition, the loss for the FBIB converter is less than 3% [19], and is smaller than that for the two-level VSC, whose loss is more than 3% [20]. The collection voltage is boosted to the transmission voltage level by the offshore converters. The 2-level VSCs at the onshore station convert dc into ac. Finally, the offshore wind power is sent into the grid through the double-circuit transmission lines. III. HVDC TRANSMISSION FAULT ANALYSIS The HVDC transmission system is shown in Fig. 2, which consists of the positive and negative onshore converters (PONC and NONC), offshore converters (POFC and NOFC), and transmission cables. In case where any converter breaks downs, the transmission system can still operate with the other healthy converters, which can provide a high degree of redundancy for the HVDC transmission system. Normally, the fishing activities, anchors, aging phenomena, and so on, would cause the cable faults with the possibility as approximate 1 fault/100 km/year [21], which may result in the faults in the HVDC transmission system. The repair for the damaged system in the offshore wind farm is costly, and often takes considerable time, even several months, which results in the possible loss of income becoming enormous [22]. In this paper, a positive pole cable-to-ground fault is analyzed and the fault impacts on the onshore and offshore converters are discussed. The negative pole fault can be treated similarly.

Fig. 3. Equivalent circuit for the onshore converter under faults.

A. Fault Analysis for the Onshore Converter The equivalent circuit for the onshore converter under faults can be presented as Fig. 3, where , and are equivalent transformer voltage. and are the filter inductance and resistance. and are the equivalent inductance and resistance. is the distance from PONC to the fault point . When a dc fault occurs, the cable current at receiving end reduces to 0, and reverses the current direction while the capacitors at the terminal start to discharge into the fault point. The PONC IGBTs may be blocked for protection purposes. The discharge of capacitor may result in a high short-circuit current. After the capacitor voltage collapses and is reduced to 0, the capacitor is bypassed by the diodes. And then, the cable inductance starts to discharge and the cable current circulates in the freewheel diodes with an initial value. Here, each phase leg carries a third of the cable current. The bypass of the capacitor by the diodes results in the abrupt appearance of a huge current in the diodes, which may damage the diodes. At the ac side, the short-circuit fault causes a transient performance, where the ac current , , and are fed into the dc link via diodes. After the capacitor voltage collapses within a very short time and the capacitor is bypassed by the diodes, the ac side is short circuit at the points , , and . Assume that the transformer phase voltage after the short circuit occurs is expressed as (1) where is the amplitude of the transformer phase voltage. is the synchronous angular frequency. is the phase angle. The

DENG AND CHEN: DESIGN OF PROTECTIVE INDUCTORS FOR HVDC TRANSMISSION LINE

Fig. 4. Most serious transient current

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at the ac grid under faults.

phase current under the ac grid three-phase short circuit can be obtained as (2) [10]

(2) with , , , , , , and being the grid current amplitude and phase angle at the beginning of the ac grid three-phase short circuit. The phase current and the phase current can be obtained with the same method, which are not repeated here. Based on (2) and the parameters in the Appendix, suppose the initial values of are 0, the most serious transient current at the ac side with the maximum value of approximately 11.1 kA is plotted as shown in Fig. 4 under the condition that the grid voltage phase angle is zero at the short-circuit initiation [10]. Although the ac CB is equipped at the ac side as shown in Fig. 2, it may not be fast enough to avoid the large current because approximately tens of milliseconds is required for the ac CB to interrupt the circuit [23]. The onshore converter performance under faults is simulated with Fig. 1, where the different fault distances including 0.1, 1, and 10 km are conducted, respectively. Suppose the positive onshore converter is initially operated with the rated power as 200 MW, the cable initial current is the rated value as 200 150 kV 1.33 kA. The simulation results show that the voltage collapses fast in less than 1 ms as shown in Fig. 5(a). The shorter the fault distance , the faster for the capacitor voltage collapse, and the bigger for the maximum value of the capacitor discharge current and cable current as shown in Fig. 5(a)–(c). In Fig. 5(c), the maximum cable current nearly reaches 68 kA under 0.1-km fault distance. When the voltage is reduced to 0, the capacitor is bypassed by the diodes. Consequently, the huge cable current abruptly flows through the freewheel diodes as shown in Fig. 5(d)– (f), which may damage the diodes. The most serious transient current under as 0.1 km is shown in Fig. 5(g) with the maximum value of approximately 11 kA, which is quite close to the calculation value as shown in Fig. 4. B. Fault Analysis for an Offshore Converter Overcurrent may also occur at the offshore converter during a fault, which makes the POFC block its insulated-gate bipolar transistors (IGBTs) when the cable current is over the IGBT

Fig. 5. Onshore converter performance under the fault distances as 0.1, 1, . (b) Capacitor discharge current . (c) and 10 km. (a) DC bus voltage . (d) Diode current under as 0.1 km. (e) Diode current under Cable current as 1 km. (f) Diode current under as 10 km. (g) The most serious transient current at the ac grid under as 0.1 km.

protection value. As a consequence, the H-bridge inverter in the POFC becomes a diode rectifier, which totally blocks the power flow from the wind farm to the HVDC transmission system. In this case, only the diode rectifier at the output side reacts to the fault as shown in Fig. 6, where is the distance from the POFC to the fault point . The offshore converter performance only contains capacitor discharge and diode freewheel, where the capacitor discharges very fast under faults. Until the voltage is reduced to 0 and the capacitor is bypassed by the diodes, the

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Fig. 6. Equivalent circuit for the offshore converter under faults.

cable current starts to circulate in the freewheel diodes. Owing to only two diode phase-legs in the offshore converter, each phase-leg takes half of the cable current. The offshore converter performance under faults is simulated with Fig. 1, where the different fault distances including 0.1, 1, and 10 km are conducted, respectively. Suppose the POFC is initially operated with the rated power as 200 MW, the capacitor initial voltage is approximately , where is the cable resistor , and is the cable length (in kilometers). The cable initial current is the rated value at 1.33 kA. Fig. 7 shows the simulation results, where the dc voltage collapses in less than 1 ms as shown in Fig. 7(a). Along with the increase of the fault distance , the speed for the dc bus voltage collapses becomes slow and the maximum value of the capacitor discharge current and cable current becomes small as shown in Fig. 7(a)–(c). After the voltage is reduced to 0 and the capacitor is bypassed by diodes, the cable current starts to circulate in the diodes with a huge initial value. Although each diode phase-leg takes half of the cable current, it is still very huge as shown in Fig. 7(d)–(f), which may damage the diodes. IV. INDUCTORS DESIGN FOR THE HVDC TRANSMISSION LINE The protection for the offshore wind farm is very important to enhance the reliability of the transmission system. The overcurrent caused by the fault should be avoided in order to protect the diodes in the converters. Although the dc circuit breaker (CB) based on the ETO thyristor has fast switching speed in less than 10 s, which could be used in series with a capacitor to limit and interrupt the capacitor discharge current during a fault [10], [16], it may not be used in this high-voltage dc system because of its low-voltage capacity. The new type of active dc CBs based on standard ac CBrs with auxiliary circuits is presented in [13] and [14], which normally needs tens of milliseconds to interrupt the dc circuit, but it may not be fast enough to protect the transmission system as the capacitor discharges very fast. Based on the fault analysis in Section III, the corresponding protective inductors are designed to avoid overcurrent in this section. A. Inductor Design for Onshore Side During a fault, the IGBTs of the onshore converters will be switched off, and only diodes may carry current. As for each phase-leg in Fig. 3, there are two situations: the first situation is that both of the diodes are conducted, the second situation is

Fig. 7. Offshore converter performance under the fault distances as 0.1, 1, . (b) Capacitor discharge current . (c) and 10 km. (a) DC bus voltage Cable current . (d) Diode current under as 0.1 km. (e) Diode current under as 1 km. (f) Diode current under as 10 km.

that only one diode is conducted. Hence, the diode current in these two situations is, respectively, analyzed as follows. 1) Both Diodes in One Leg in Conduction: In Fig. 3, if the two diodes in phase are both conducted, the diodes current in this phase-leg can be expressed as (3) The condition for the first situation can be presented as (4) Hence, the maximum diode current tion can be presented as

in the first situa-


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two diodes in the phase would be blocked, and the other one takes the current as shown in Fig. 3. Hence, the possible maximum diode current in the second situation can be obtained under the most serious transient situation as (11) In order to reduce the diode current and protect converters under faults, (12) should be satisfied, and the is considered as the possible maximum diode current under faults. Fig. 8. Protection design for the onshore converter.

(12)

Fig. 9. Possible maximum values for capacitor under different inductance .

is designed and installed at the In this paper, an inductor terminal of the onshore converter as shown in Fig. 8. It is used to prevent overcurrent and makes (12) satisfied under faults, in order to protect the diodes in the converters. Suppose that the initial capacitor current is 0 in Fig. 8, if the fault distance is 0, the possible maximum cable current can be obtained as (13) when the capacitor voltage is reduced to zero

discharge current

(13) (5) Besides, if the diodes in any two different phase and are all conducted in Fig. 3, there will be the current relationship as (6) Based on (3) and (6), the possibly gets its maximum value when the diodes in the three phase-legs are all conducted, which is a third of the cable current and can be expressed as (7) Substituting (7) into (5), the possible maximum diode current in the first situation can be rewritten as

(8) The may be reached when the capacitor voltage is reduced to zero under faults, which can be expressed as and discussed later. The can be derived from (2) and Fig. 4 under the most serious transient situation, which can be presented as

In order to obtain the relationship between the current and the inductor , the initial capacitor voltage is set as 165 kV with a margin of 1.1 for the 150-kV transmission system because of the possible voltage variation. With the increase of the inductance value , the possible maximum cable current can be reduced as shown in Fig. 9. Consequently, the diode freewheel current is also decreased. Based on (13) and Fig. 9, the protection inductor can be designed to limit the cable current and satisfy (12) in order to protect the diodes. In this paper, a 6.5-mH inductor is selected for the studied transmission system. It can limit the maximum capacitor discharge current approximately as 12.2 kA as shown in Fig. 9, and makes the system satisfy (12). From (2) and Fig. 4, the possible maximum diode current is approximately 11.1 kA. The capacity of the diodes in the onshore converters is designed as 12.2 kA with a margin of 1.1. B. Inductor Design for the Offshore Side In order to prevent overcurrent and protect the diodes in the offshore converters, an inductor is designed and installed at the terminal of the offshore converter as shown in Fig. 10. In Fig. 10, the current can be expressed as (14), with the condition that the fault distance is 0

(9) where is the frequency of the ac grid. Substituting (9) into (8), the possible maximum diode current in the first situation can be presented as (10) 2) One of the Diodes in One Leg in Conduction: If the (4) , one of the is not true and

(14) Suppose that the POFC is operated with the rated power of 200 MW, and the cable initial current is the rated value as 1.33 kA. Owing to the possible voltage variation in the transmission system, the capacitor initial voltage is set with the 1.1 margin as 167.2 kV. The possible maximum capacitor discharge current values under different protective inductance are obtained as shown in

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Fig. 10. Protection design for the offshore converter.

Fig. 11. Possible maximum value for capacitor under different inductance .

discharge current

Fig. 11. It is easy to see that the bigger the selected protection inductance value, the smaller the possible maximum capacitor discharge current. Consequently, the diode freewheel current can be limited in a small range. According to (14) and Fig. 11, the offshore protection inductor can be designed in order to limit the capacitor discharge current and diode current, which could effectively protect the diodes in the converters. In this paper, the is selected as 6.5 mH, which can limit the maximum capacitor discharge current approximately as 12.4 kA as shown in Fig. 11. Hence, the possible maximum diode current is half of the maximum capacitor discharge current and equal to 6.2 kA. The capacity for the offshore diodes can be designed as 6.8 kA with a margin of 1.1. V. SIMULATION STUDIES The offshore wind farm as shown in Fig. 1 is modeled using PSCAD/EMTDC. The system parameters are given in the Appendix. The positive pole cable-to-ground fault is studied here, where the fault resistance is generally very small, which is considered as 0 in this study. The designed protective inductors are applied to the offshore wind farm system and verified by simulation results. A. Onshore Converter Performance Fig. 12 shows the onshore converter performance with the 6.5-mH protection inductor under faults at 0.4 s, where a few fault distances including 0, 0.1, 1, and 10 km are conducted, respectively. Compared with Fig. 5, the voltage collapses slowly with the protection inductor as shown in Fig. 12(a). The capacitor discharge current and cable current are limited in a small range as shown in Fig. 12(b) and (c), which effectively

Fig. 12. Onshore converter performance under the fault distances as 0, 0.1, . (b) Capacitor discharge current . (c) 1 and 10 km. (a) DC bus voltage . (d) Diode current under as 0 km. (e) Diode current under Cable current as 0.1 km. (f) Diode current under as 1 km. (g) Diode current under as 10 km. (h) AC current under as 0 km.

reduces the diode current and protects the diodes as shown in Fig. 12(d)–(g). The ac current under different fault distances is


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Fig. 13. (a) Current components under as 0 km. (b) Diode current and under as 0 km. (c) Diode current and under as 0 km. (d) and under as 0 km. Diode current

similar, and only the ac current under as 0 km is shown in Fig. 12(h). Fig. 12(d) and Fig. 13(a) show the current components under the fault distance as 0 km. It can be seen that the between 0.4057 and 0.408 s. In this short time, the is blocked and the takes the maximum diode current . From 0.408 to 0.4145 s, the , which makes the diode blocked and the diode flow through the maximum diode current . Besides, the appears as the maximum value of 5.5 kA at 0.411 s as shown in Fig. 13(a), which causes the maximum diode current as 11 kA. It could be tolerated by the selected onshore diodes in Section IV. Besides, Fig. 13(b)–(d) gives the diode current in a short time, when the diodes in the three phase-legs are all conducted. It can be seen that the simulation results are the same with the calculation results based on (3) and (6).

Fig. 14. Offshore converter performance under the fault distances as 0, 0.1, . (b) Capacitor discharge current . (c) 1 and 10 km. (a) DC bus voltage Cable current . (d) Diode current under as 0 km. (e) Diode current under as 0.1 km. (f) Diode current under as 1 km. (g) Diode current under as 10 km.

B. Offshore Converter Performance The offshore converter performance under faults at 0.4 s is shown in Fig. 14, where different fault distances as 0, 0.1, 1, and 10 km are conducted. The maximum capacitor discharge current and cable current are effectively limited by the installation of a 6.5-mH inductor as shown in Fig. 14(b) and (c). The maximum cable current is approximately 11.3 kA under of 0 km as shown in Fig. 14(c). Consequently, the diode current under different fault distances is also decreased as shown

in Fig. 14(d)–(g), which is within the offshore diode capacity selected in Section IV. Hence, the offshore converter is protected. VI. CONCLUSION This paper presents fault analysis and protective inductors’ design for an offshore wind farm, where the power collection system in the wind farm and the power transmission link to the

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TABLE I PROPERTIES OF THE CABLE

TABLE II GENERATOR CHARACTERISTICS

TABLE III HVDC TRANSMISSION SYSTEM CHARACTERISTICS

grid adopt HVDC technology. The transient characteristic of the transmission system under the cable-to-ground fault is analyzed in detail. A huge current may be caused at the onshore and offshore converters during a fault, which may damage the diodes in the converters. The corresponding protections for the onshore and offshore converters are designed by installing the suitable protective inductors at the terminals of the onshore and offshore converters, respectively. Consequently, the capacitor discharge current, cable current, and diode current could be effectively limited in a small range by the protective inductors. In addition, the capacity of the diodes in the onshore and offshore converters is determined based on the designed protection inductors, which is enough to tolerate the current under faults. Simulation studies of a 400-MW offshore wind farm system with positive-pole cable-to-ground fault are conducted, and the results show that the designed protective inductors can effectively prevent overcurrent and protect converters.

APPENDIX The frequency-dependent phase model is applied as the simulation model for cable in PSCAD/EMTDC [24]. The relevant data are listed in Table I, which is based on a cable design that is assembled from [1] and [25].

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Fujin Deng (S’10) received the B.Eng. degree in electrical engineering from China University of Mining and Technology, Jiangsu, China, in 2005, the M.Sc. degree in electrical engineering from Shanghai Jiao Tong University, Shanghai, China, in 2008, and is currently pursuing the Ph.D. degree in energy technology from Aalborg University, Aalborg, Denmark. His current research interests include wind power generation, converter topology, dc grid, control of the permanent-magnet synchronous generator, high-voltage direct-current (HVDC) technology, and offshore wind farm-power systems dynamics.

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Zhe Chen (M’95–SM’98) received the B.Eng. and M.Sc. degrees in electrical engineering from Northeast China Institute of Electric Power Engineering, Jilin City, China, and the Ph.D. degree in electrical engineering from the University of Durham, Durham, U.K. He is a Full Professor with the Department of Energy Technology, Aalborg University, Aalborg, Denmark. He is the Leader of Wind Power System Research program at the Department of Energy Technology, Aalborg University and the Danish Principle Investigator for Wind Energy of Sino-Danish Centre for Education and Research. He has more than 270 publications in his technical field. His research areas are power systems, power electronics, and electric machines. His main current research interests are wind energy and modern power systems. Dr. Chen is an Associate Editor (Renewable Energy) of the IEEE TRANSACTIONS ON POWER ELECTRONICS, a Fellow of the Institution of Engineering and Technology (London, U.K.), and a Chartered Engineer in the U.K.