X International Symposium on                                                    Lightning Protection                      ...
steady state is emphasized in the planning of thegrounding, the transient characteristics are oftenneglected.When a wind t...
was connected between the remote end of the voltage                                                                       ...
As mentioned above, the grounding characteristics of thesystem showed strong inductiveness at the wave frontbecause the st...
responses, the time response to lightning of several wave                                              4 CONCLUSIONSshapes...
a Wind Turbine”, 2008 Annual Meeting of the Institute of           10th     Mediterranean     Electrotechnical   Conferenc...
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A study of transient characteristics of an actual wind turbine grounding system yamamoto et al (sipda 2009)

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A study of transient characteristics of an actual wind turbine grounding system yamamoto et al (sipda 2009)

  1. 1. X International Symposium on Lightning Protection 9th-13th November, 2009 – Curitiba, Brazil A STUDY OF TRANSIENT CHARACTERISTICS OF AN ACTUAL WIND TURBINE GROUNDING SYSTEM Kazuo Yamamoto1, Shunichi Yanagawa2 Koichi Yamabuki3, Shozo sekioka4, Shigeru Yokoyama5 1 Kobe City College of Technology, Japan – kyamamoto@mem.iee.or.jp 2 Shoden Company, Japan – yanagawa@sdn.co.jp 3 Wakayama National College of Technology, Japan – yamab@wakayama-nct.ac.jp 4 Shonan Institute of Technology, Japan – sekioka@elec.shonan-it.ac.jp 5 CRIEPI, Japan – yokoyama@criepi.denken.or.jpAbstract - In order to exploit high wind conditions, wind transient characteristics of the grounding byturbine generator systems are often constructed in places experimental and analytical methods using a reduced-where few tall structures exist; therefore, they are often size model of current wind turbine foundations [8-11].struck by lightning. Much of the damage caused by Research using simulations of the transient and steady-lightning is from the resulting breakdown and malfunction state grounding characteristics of wind turbineof the electrical, communications, and control systemsinside the wind turbine generator system; these foundations has already been presented [12-22]. However,breakdowns can be attributed to a rise in electric potential papers that report using an actual wind turbine generatorboth within the system and in the surroundings due to system to study transient grounding characteristics arelightning. Impulse tests were conducted on an actual wind very few in numbers [23].turbine generator system. The rise in ground potential ofthe system, and that around its foundation was measured. In this paper, we present experimental studies of theThe frequency characteristics were calculated using the impulse tests conducted on an actual wind turbineLaplace transform to get voltage responses for all types of generator system. The ground potential rise of the systemlightning current waveforms. As a typical potential rise itself, and around its foundations, was measured. Theresponse, the response to the step current which has thepeak value of 1 A was calculated. frequency characteristics were calculated using the Laplace transform [24] to get voltage responses to all 1 INTRODUCTION types of lightning current waveforms. As a typical potential rise response, the response to the step currentA Based on the diffusion of wind turbine generation which has the peak value of 1 A was calculated. Whensystems, many accidents caused by natural disasters such lightning strikes the wind turbine generator systemas lightning and typhoons have occurred in recent years. constructed at a site where the grounding resistivity isEspecially, the damages caused by lightning become so very low, the potential rise at the wave front typicallyserious [1-5]. Wind turbine generation systems are built becomes larger than that of the steady state. This isat locations where few tall structures are found nearby so because of the inductivity of the grounding system.as to obtain good wind conditions, and thus, they are Therefore, the transient characteristics of the groundingoften struck by lightning. To promote wind power system become important, in comparison to its steady-generation, lightning protection methodologies for such state characteristics.wind turbine generation systems have to be established. 2 GROUNDING OF WIND TURBINELightning damage to wind turbine generator systems GENERATOR SYSTEMaffects the safety and reliability of these systems. Most ofthe breakdowns and malfunctions of the electrical and 2.1 Importance of Transient Characteristicscontrol systems inside wind turbines are caused by a risein ground potential due to lightning [6, 7]. To understand Both transient and steady-state characteristics becomethis rise in ground potential, we had researched the important for understanding the grounding phenomena of a wind turbine generator system. However, because the 285
  2. 2. steady state is emphasized in the planning of thegrounding, the transient characteristics are oftenneglected.When a wind turbine generator system is constructed in a 8.5 mmountain area where resistivity is comparatively high,the steady-state grounding resistance, in many cases,becomes more important than the transient groundingresistance. A potential rise caused by a lightning strike to 3ma wind turbine generator system is more remarkable atthe wave tail than at the wave front. The potential rise atthe wave tail depends on the steady-state groundingresistance. When a wind turbine generator system is 2mconstructed at a low resistivity site, such as a coastal area,a significant potential rise occurs due to the inductivity of grounding meshthe grounding system. The transient grounding resistance foundation foot : 50 mat the wave front, which depends on the inductivity of thegrounding system, is larger than the steady-stateresistance.2.2 Grounding of an Offshore Wind TurbineThe soil around the actual wind turbine generator system Fig. 1 - Foundation of the actual wind turbine generator systemon the disposal site where the measurements has beenperformed has electrical characteristics similar toseawater, because the soil on the disposal site contained alot of seawater. The target wind turbine generator systemhad four long foundation feet, like those of offshore wind ρ1 = 15 [Ωm] d1 = 3.0 [m]turbines, to increase the bearing capacity of the soil. Thegrounding characteristics of the foundation constructedon the disposal site exhibited inductivity in the wayexplained in the previous section. Construction ofoffshore wind turbine generator systems is prohibited inJapan because of fishery rights, destruction of the ρ2 = 1 [Ωm] ∞environment, and so on. However, there are wind turbinegenerator systems on the coastal area. Depending on the Fig. 2 - Resistivity around the wind turbine generator systemgovernmental energy policy, offshore wind turbine estimated by Wenner methodgenerator systems may be constructed in the future [5].Therefore, the grounding characteristics of the wind Grounding mesh existed underneath the foundation; itsturbine foundation on disposal sites should be researched size was about 8.5 m × 8.5 m. The stratiform resistivityto estimate the grounding characteristics of low around the wind turbine generator system is shown in Fig.resistivity sites. 2. The Wenner method was utilized to measure the resistivity. The steady-state grounding resistance of the 3 MEASURMENTS grounding system of the wind turbine generator system was 0.062 Ω.3.1 Foundations 3.2 Experimental ConditionsFig. 1 shows in detail the foundation of the actual windt ur bi n e gen er a t or syst em t h a t wa s used in our Fig. 3 shows the experimental set up. The current wasmeasurements. The shape was rectangular and parallel- led to the foundation from the impulse generator by usingpiped, and 8.5 m × 8.5 m × 2 m in size. The foundation insulated copper wire (length: 100 m; cross section: 5.5wa s r ei n for ced con cr et e; t h e i ntervals between mm2) as the current lead wire. The height of the currentreinforcing were about 30 cm. The tower was connected lead wire was about 1 m. The fast front current generatedto the foundation at ground level. The depth of the by the impulse generator was injected into the foundationfoundation was 2 m, and the length of the foundation feet through a resistance of 500 Ω from a current lead wire,was 50 m, to enhance the bearing capacity of soil. as shown in Fig. 3. The peak value of the current was 60 286
  3. 3. was connected between the remote end of the voltage measuring wire and grounding rod, which had about 120 Ω grounding resistance. That was how the noise induced on the voltage measuring wire was discharged to the current lead wire ground readily. 150 m ground I.G. 3.3 Experimental Data I Voltage measuring wire The current into the foundation and potential rise at the foundation 70 m foundation was recorded to study the transient and V steady-state characteristics of the foundation. The potential rises around the wind turbine generator system were measured at intervals of 1 m (0 to 10 m from the edge of the foundation, as shown in Fig. 4) around the foundation, and from 2 m to 4 m (over 10 m from the Fig. 3 - Experimental set up edge of the foundation, as shown in Fig. 4). The potentialTop view rise was measured at 21 locations. An additional rod was buried about 0.1 m deep, at each measured point, to measure potential rise. 1m 3m 5m 7m 9m 12 m 16 m 0m 2 m 4 m 6 m 8 m 10 m 14 m 18 m 3.4 Measuring Instruments foundation The impulse generator had a capacitance of 1.5 μF, andSide view was discharged by using a gap switch. The charging 1m 3m 5m 7m 9m 12 m 16 m voltage was 30 kV for these measurements. 0m 2 m 4 m 6 m 8 m 10 m 14 m 18 m A TDS3054C oscilloscope (Tektronix) was used to measure the voltage and current waveforms; its bandwidth was DC–500 MHz. A P6139A passive probe (Tektronix) was used for voltage measurements; its bandwidth was DC–500 MHz and its input capacitance was up to 8 pF. A PEARSON 150 was used as the current probe; its bandwidth and usable rise time were in Fig. 4 - Measuring point of the potential rise around the the range of 40 kHz to 20 MHz and over 20 ns foundation respectively. The measurements performed using these instruments were accurate, with a rise time of severalA and the wave front was about 0.4 μs. A comparatively hundred nanoseconds.large resistance of 500 Ω was connected in series withthe impulse generator; it can therefore be considered a 3.5 Measured Resultscurrent source. The measurement results are shown in Fig. 5. Figs. 5 (a)The injected current was measured at the end of the and (b) show, respectively, the injected current I and thecurrent lead wire near the foundation by using a current potential rise V at the top of the foundation. The injectedprove as shown in Fig. 3. The potential rise of the current showed a ramp wave, and its peak and rise timefoundation was measured as the voltage difference were, respectively, approximately 60 A and 0.4 μs. Thebetween the top of the foundations and the voltage voltage was inductive at the wave front. The ratio of themeasurement wire. The height of the wire was 1 m, and maximum voltage at the wave front to the current at theit was grounded at the remote end. The potential rise same time was approximately 13 V/A. This value wasaround the wind turbine generation system was measured greater than the steady-state grounding resistance. Theas the voltage between the conductive rods placed at the voltage waveform oscillated after the wave front. Themeasurement points, as shown in Fig. 4, and the voltage medium value of the voltage gradually decreased to themeasurement wire. As shown in Fig. 3, the current lead value of the steady-state grounding resistance. It iswire and the voltage measurement wire were believed that the oscillations were caused by theorthogonalized to decrease their mutual electromagnetic inductance and capacitance of the grounding system. It isinduction. The surge impedance of the voltage measuring possible that the steady-state grounding resistance ofwire was about 500 Ω; therefore, the 400 Ω resistance 0.062 Ω was the convergence value. 287
  4. 4. As mentioned above, the grounding characteristics of thesystem showed strong inductiveness at the wave frontbecause the steady state grounding resistance was as lowas 0 0 6 2 In the case of offshore wind tu bine . Ω. rgenerator systems, similar grounding characteristicsshould be observed. Transient phenomena obviouslybecome more important than steady-state phenomena forlightning protection design.The potential around a wind turbine generation systemincreases when it is struck by lightning. To investigatethe potential rise, the fast-front current was injected intothe grounding system, as shown in Figs. 3 and 4. Theinjected current was very similar to the results shown inFig. 5 (a), the peak value was 60 A and the wave frontwas about 0.4 μs.Fig. 6 (a) shows the measured potential rise around thewind turbine generator system. Fig. 6 (b) shows therelationship between the maximum potential rise and thedistance.The wave shape, shown in Fig. 6 (a), was almostanalogous to the potential rise shown in Fig. 5 (b). If theskin effect of the ground is not considered, and the 80 60 current [A] 40 20 0 0 1 2 3 4 5 time [µs] (a) Injected current into the foundation 500 400 voltage [V] 300 200 100 0 -100 0 1 2 3 4 5 time [µs] (b) Potential rise of the foundationFig. 5 - Transient characteristic of the grounding system of the actual wind turbine generator system
  5. 5. responses, the time response to lightning of several wave 4 CONCLUSIONSshapes can be calculated. This paper has presented the results of experimentalIt should be noted that above mentioned measured results studies that investigated the grounding characteristics ofinclude the influence of the surge propagations on the a actual wind turbine generation system, and the voltagetower and blades, the induced voltage on the voltage rise around it. The grounding characteristics of themeasuring wire from the current lead wire and so on. If grounding system showed strong inductivity at the wavewe want to obtain the independent grounding front. The frequency and step responses of the groundingcharacteristic of the foundation, the model of the wind system have been presented to get voltage responses to allturbine with the grounding system should be established types of lightning current waveforms.in the numerical electromagnetic field analyses such asFDTD (Finite-Difference Time-Domain) method, and the The installation features of the wind turbine generatorindependent model of the grounding system should be system that were employed in this paper were verycalculated. similar to those used at sea. The long foundation feet were much like those of an offshore wind turbine generator system. The results given in this paper will be 30 very useful as basic data for lightning protection of wind 25 turbine generator systems at low resistivity sites, impedance [Ω] including those of offshore wind turbine generator 20 systems. 15 10 5 REFERENCES 5 [1] I. Cotton, B. Mcniff, T. Soerenson, W. Zischank, P. 0 Christiansen, M. Hoppe-Kilpper, S. Ramakers, P. 4 5 6 7 10 10 10 10 Pettersson, and E. Muljadi: “Lightning Protection for Wind frequency [Hz] Turbines”, in Proc. 25th International Conference on Lightning Protection, pp. 848–853, Rhodes, Greece (2000- (a) Absolute value of the grounding impedance 9). [2] IEC TR 61400-24: “Wind Turbine Generator Systems–Part 90 24: Lightning protection” (2002). 80 [3] NEDO: “Wind Turbine Failures and Troubles Investigating phase [degree] 70 Committee Annual Report” (2006) (in Japanese). 60 50 [4] NEDO: “Wind Turbine Failures and Troubles Investigating 40 Committee Annual Report”, (2007) (in Japanese). 30 [5] NEDO: NEDO Report, NP-9801 (1999). 20 [6] K. Yamamoto, T. Noda, S. Yokoyama, and A. Ametani: 10 “An Experimental Study of Lightning Overvoltages in 0 Wind Turbine Generation Systems Using a Reduced-size 4 5 6 7 Model”, Electrical Engineering in Japan, Vol. 158, No. 4, 10 10 10 10 frequency [Hz] pp. 22–30 (2007-3). [7] K. Yamamoto, T. Noda, S. Yokoyama, and A. Ametani: (b) Phase value of the grounding impedance “Experimental and Analytical Studies of Lightning Overvoltages in Wind Turbine Generator Systems”, 10 Electric Power Systems Research, Vol. 79, No. 3, pp. 436– 9 442, ISSN:0378-7796 (2009-3). 8 7 [8] K. Yamamoto, T. Noda, S. Yokoyama, and A. Ametani: voltage [V] 6 “Grounding Characteristics of a Wind Turbine Generation 5 System and Voltage Rise around It”, International 4 Conference on Grounding and Earthing (Ground2006), pp. 3 2 415–419, Maceio, Brazil (2006-11). 1 [9] K. Yamamoto, T. Senoo, A. Ametani, T. Noda, and S. 0 Yokoyama: “Grounding Characteristics of the Foundations -1 of a Wind Turbine Generation System”, 2007 Annual 0 2 4 6 8 10 Meeting of the Institute of Electrical Engineers of Japan, 7- time [µs] 094 (2007-3) (in Japanese). (c) Step responce of the grounding impedance [10] K. Yamamoto, T. Senoo, A. Fukuoka, and A. Ametani:Fig. 7 – Frequency and step responses of the grounding system “Effects of Grounding Conductors around the Foundation of on the actual wind turbine generator system 289
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