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Examination of the 2006 blackout in Kefallonia Island, Greece
Electrical Power and Energy Systems 49 (2013) 122–127
Contents lists available at SciVerse ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Examination of the 2006 blackout in Kefallonia Island, Greece Stavros D. Anagnostatos, Constantinos D. Halevidis ⇑, Aikaterini D. Polykrati, Perikles D. Bourkas, Constantinos G. Karagiannopoulos National Technical University of Athens, School of Electrical and Computer Engineering, High Voltages and Electrical Measurements Laboratory, 9 Iroon Polytechniou Str., GR 157-80 Athens, Greece
a r t i c l e
i n f o
Article history: Received 29 April 2010 Received in revised form 17 November 2012 Accepted 20 December 2012
Keywords: System blackout Transmission line Extreme weather conditions Single circuit tower
a b s t r a c t The blackout on the Greek island of Kefallonia on January 24th 2006 was caused by the fall of 10 towers of the 150 kV power transmission network due to high winds and heavy snowfall. These extreme meteorological phenomena were unprecedented on the island and thus had not been taken into consideration by the Greek installation and maintenance regulations regarding these towers. Due to changes in meteorological conditions in recent years, the above regulations have to be revised so as to propose smaller distances between towers in new transmission lines and improvements to the existing lines to endure severe weather conditions when necessary. This paper provides a short description of the High Voltage (HV) network of the island of Kefallonia, the system conditions before and after the blackout, and identifies the causes of the blackout. In addition, the mechanical strength evaluation of the simple circuit towers (similar type to those that collapsed) is made under the stress of gale-force winds combined with the ice-coating of conductors. Finally, in order to avoid the repetition of such an outage, several methods are proposed. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A blackout, i.e. a power outage, leaves large regions without electricity for large time periods. Consequently, great effort is being put by researchers into identifying the underlying causes and countering them so as to avoid the repetition of such events. An example of such an effort is described in [1]. Transmission networks are designed so as to ride through at least one contingencyfault (N-1 analysis). However, in storm and severe weather conditions the frequency of multiple contingencies can be greater than that of a single contingency in fair weather [2]. Severe weather conditions cost power utilities great amounts of money due to unserviceable loads as well as for the replacement of destroyed equipment [3]. Thus, the planning and design of the overhead transmission and distribution networks, so as that they reliably endure the mechanical stresses caused by meteorological phenomena, is very important [4,5]. The Argostoli1 substation is supplied in a loop configuration by two 150 kV High Voltage (HV) transmission lines, the Aktio line from the north, and the Kyllini line from the south. Each one of them can sufficiently supply the island with electricity. Downstream of the substation, the electric energy is distributed using 15 kV Middle Voltage (MV) lines. Finally, the voltage is stepped down to 400 V ⇑ Corresponding author. Tel.: +30 6932337468. 1
E-mail address: [email protected] (C.D. Halevidis). Argostoli is the largest town of Kefallonia Island.
0142-0615/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijepes.2012.12.003
via Delta-grounded wye distribution transformers. This international practice of designing networks in loop arrangement, with double High Voltage lines, is applied in order to have a backup power supply, as each of the two inputs ensures the total energy supply in case one fails. The existence of a second backup transmission line is not considered, as the power supply of the island follows the N-1 security standard. Furthermore, as the construction is based on past weather conditions and other natural phenomena in the region, only an extreme weather phenomenon which can render inoperable both transmission lines can cause a blackout. This was the case on January 24, 2006 in Kefallonia Island due to coexistence of high winds of Beaufort scale ten (10) and ice sleeves on the conductors which were multiple in size compared to those usually observed in Greece due to heavy snowfall. The ambient temperature recorded by the National Meteorological Service at the meteorological station of Argostoli was 5 °C. The above phenomena caused towers to collapse in both 150 kV lines. It should be noted that the conductors’ ice sleeve was approximately 15 cm in diameter. The population of Kefallonia was 39,488, and 41,365 according to the 2001 and 2011 censuses respectively. The population density is 45.8 people per km2 according to the 2011 census, while the total area is 904 km2. These figures include the island of Ithaki, which is connected to Kefallonia through submarine MV cables. In this paper, a short description of the layout of the High Voltage network of the Kefallonia Island, the state of the network before and after the blackout and the weather condition during the blackout is given. Afterwards, an evaluation of the mechanical
S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127
strength of single-circuit towers, of the type of those that collapsed, is performed, in the case where the mechanical stress is caused by high winds in combination with ice covering the conductors. Additionally, the causes of the blackout are presented and possible solutions are given so as to avoid repetitions of similar outages.
2. Description of the High Voltage network (150 kV) in the Kefallonia Island Fig. 1 shows a simplified diagram of the High Voltage (150 kV) transmission network in the Kefallonia Island. Furthermore, the two northern-side towers (I and II) and the eight southern-side towers (1–8), which collapsed and caused the blackout of the island, are shown. The overhead line conductors of the two lines were aluminum conductors steel reinforced (ACSR) of LINET type [6]. Their characteristics are the following: diameter of 18.31 mm, equivalent copper cross section of 107.48 mm2, and nominal current of 483 A. The protection of the transmission lines was achieved through a current differential scheme, with distance protection acting as backup. This protection scheme was necessary, as the transmission lines are composite; i.e., overhead line combined with submarine cables. Additionally, directional over-current relays are used. At
123
this point it should be noted that steps are being undertaken in the direction of fault location in composite transmission lines [7]. Furthermore, significant effort has been dedicated to the development of new features for distance relays [8,9]. The Argostoli substation consisted of one 40/50 MVA (150 kV/ 15 kV) and two 20/25 MVA (150 kV/15 kV) power transformers. Consequently, the total installed power was equal to 100 MVA. Furthermore, three inductor banks are installed at the High Voltage side, with a combined rating of 44.5 MVAr, in order to balance the cable capacitance. The single line diagram of the substation is shown in Fig. 2. Closed Cycle Gas Turbines (CCGTs) were installed in the island of Kefallonia until 2002, when they were removed due to the transmission line from Kyllini being commissioned. Furthermore, at the time of the blackout, a wind farm with a nominal rating of 13.6 MW was connected to the electrical grid of Kefallonia. From the Greek Regulations regarding the installation and maintenance practice of overhead lines [10], the Public Power Corporation specifications for 150 kV single-circuit line towers such as the ones under consideration [11], the following can be extracted: (1) All towers of the southern side were steel lattice alignment towers of the S2 type with an allowed maximum transverse load per phase conductor equal to 7300 N.
Fig. 1. Simplified diagram of the High Voltage transmission network. The damaged towers are: the two towers of the northern side (I and II) and the eight towers of the southern side (1–8).
Fig. 2. Single line diagram of the Argostoli substation.
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S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127
From the inspection of the areas where the High Voltage towers collapsed, the following points were established: On the conductor surfaces there was an ice-sleeve approximately 15 cm in diameter (see Fig. 5). This caused an excessive increase in frontal surface area, resulting in much greater wind pressure. It should be noted that design studies in Greece for the 150 kV power transmission lines assume an ice sleeve of 6.5 mm for simple circuit towers [12]. The High Voltage line’s towers (from Aktio and from Kyllini) had collapsed towards the wind direction, almost vertically to the line’s direction. A collapsed tower is shown in Fig. 6. It should be noted that the tower failed at the body extension. Tower torsion phenomena were observed because of the conductors’ abruption, before or during their collapse. The collapsed towers were erected from galvanized steel, and no trace of corrosion was found. The concrete foundations of the towers were not damaged. Generally, there was no discernible sign of conductor wear which would have weakened their mechanical strength. Fig. 3. Steel lattice tower dimensions of 150 kV single-circuit power transmission lines (the ones under consideration).
(2) Tower I of the northern side was a steel lattice tower for small direction angle changes of the R2 type with an allowed maximum transverse load per phase conductor of 14,650 N. (3) Tower II of the northern side was a steel lattice tower for a 45° direction angle change of the T2 type with an allowed maximum transverse load per phase conductor of 39,500 N. (4) The dimensions of the towers as well as the insulation distances between the conductors are given in Fig. 3.
3. The system condition before and after the blackout
4. Single-circuit towers mechanical strength evaluation, according to EN 50341-3-10 The diameter D of the ACSR LINET type conductor, that was used in the HV lines, was equal to 18.31 mm [6]. According to EN 50341-3-10, the combined wind and ice load for the country of Greece is given as an ice sleeve radius Rsleeve equal to 6.5 mm and wind pressure Pen equal to 190 N/m2 [12]. Consequently, the frontal surface A per unit length of the aforementioned conductor is:
A ¼ ðD þ 2Rsleev e Þ ¼ 31:31 103 m2 =m
ð1Þ
Thus, for ice sleeves with a radius equal to 6.5 mm, the transverse force F (wind load) per unit meter, applied on the conductor, is equal to:
F ¼ A Pen ffi 5:94 N=m On 24-01-2006 prior to the blackout, the load of the Argostoli substation was equal to 29.8 MW, and on 29-01-2006, after the complete power supply restoration, 26.3 MW. The monthly maximum demand of the above substation for the years 2002–2005 is given in Fig. 4. The operation of these two lines with such a demand load effectively rules out the possibility of conductor overheating, as each transmission line has a 125 MVA capacity. Initially, at 21:57 on 23-01-2006, a mechanical failure occurred in the High Voltage line of 150 kV from Aktio (northern side). Namely, two towers collapsed and a third was damaged. This, in turn, caused the tripping of the line protection from both ends. Despite this, the power supply of the island was not interrupted, as the power supply was continued from the 150 kV High Voltage line from Kyllini (southern side). Then, at 01:53 on 24-01-2006 another mechanical failure occurred in the 150 kV Kyllini line, with the collapse of eight towers. Similarly, the circuit breaker located at the Zakinthos substation2 was tripped. Consequently, the voltage collapsed, as the load could not be fed, resulting in the blackout of the island. At this point it should be noted that the stability of the main grid was not impacted, as the faults were cleared by the respective circuit breakers. Due to this unprecedented bad weather in Kefallonia, damage was also sustained by the distribution network, in both Middle and Low Voltage networks. Eighty wooden poles, six distribution substations, and approximately 30 km of Middle and Low Voltage lines were destroyed. Additionally, 300 house service cables were cut, and a large number of Middle and Low Voltage fuses were blown, in addition to other small damages.
ð2Þ
Consequently the tower’s transverse stress for the maximum span of 397 m of the lines would be 397 F ffi 2360 N. This value is smaller than the maximum allowed load Fmax for a S2 type tower, which is equal to 7300 N [10]. Similarly, it arises that the transverse stresses for R2 and T2 type towers were smaller than those allowed in the Public Power Corporation specifications [11]. Consequently, the mechanical strength of the three types of towers, as used in the transmission lines, would be sufficient to endure the combined ice and wind loading used by EN 50341 and the Greek regulations. 5. The minimum ice sleeve size that can cause tower collapse in case of high winds In the following calculations, the collapse of a S2 type tower for the minimum span Lmin of 238 m is examined. It should be noted that the torsion effect is not taken into account. Thus, the calculations err on the side of caution. As presented in paragraph 2, the allowed transverse stress Fmax for a S2 type tower is equal to 7300 N. Consequently, for a span of 238 m, the allowed wind pressure per unit length is:
P Dlim ¼ F max =Lmin ¼ 7300=238 ¼ 30:67 N=m
ð3Þ
The dynamic wind pressure P applied on the conductor is [13]:
P ¼ C d qu2 =2 ¼ 494 N=m2
ð4Þ 3
2
Zakinthos substation is the substation immediately upstream on the Kyllini line.
where q is the air density taken equal to 1.225 kg/m [13], Cd is the drag coefficient and is assumed equal to unity, and u is the wind
S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127
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Fig. 4. Argostoli substation monthly maximum load for the years 2002–2005.
Fig. 5. Overhead line conductors with ice sleeves.
speed. According to the National Meteorological Service report the wind speed was equal to 28.4 m/s3, measured at a height of 10 m. The real wind pressure is greater than the calculated one as the wind 3
Equivalent to a magnitude of Beaufort scale ten.
Fig. 6. Tower collapsed towards the wind direction (almost vertically to the transmission line direction).
speed increases with increasing height, and the conductors were suspended at a height of 19 m approximately. The wind speed as a function of the height can be approximated as following the power law [14], and is expressed as:
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a u2 h2 ¼ u1 h1
S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127
ð5Þ
where h is the height, and a is the ground surface roughness length. Consequently, for a wind pressure P equal to 494 N/m2 applied on the conductors, the ice sleeve diameter limit for the collapse of the tower is:
Dlim ¼ F max =P ¼ 30:67=494 ffi 6:2 cm
ð6Þ
Consequently, for high winds in the tower region (as reported by the National Meteorological Service in its report) and for an ice sleeve of approximately 15 cm in diameter on the conductors’ surface, the collapse of the towers was expected, even more so in the case of towers with a span bigger than 238 m. Another factor adding to the total mechanical stress is the torsion due to the abruption of one or more conductors. Similarly, it can be concluded that the 15 cm ice sleeve, in combination with the high winds, caused the collapse of the two towers of R2 and T2 type of the northern side. 6. Discussion The Kefallonia Island power blackout happened at 01:53 on 2401-2006, because of the collapse of eight S2 type towers of the 150 kV transmission line coming from Kyllini (southern side). Additionally, great damages were caused to the distribution system. The blackout caused an unserviceable electrical energy of approximately 3000 MWh.4 It arises that the construction and design of the aforementioned collapsed line, was in accordance with the standing Greek Regulations [10], with the Public Power Corporation specifications [11], as well as with the National Normative Aspects for Greece [12] as the transverse stresses, to which the towers are subjected, under high winds and with ice accretion thickness of 6.5 mm are smaller than the allowed ones [11]. This is also valid for the collapsed part of the 150 kV transmission line coming from Aktio (northern side). It should be noted that: 1. A tower collapse had not happened before. 2. Towers of the same type have been installed without particular problems in other regions of Greece; e.g. in the Florina and Kastoria regions which are subject to significant snowfall. 3. Previous service discontinuities, which were caused by ice and wind, were due to damages to the MV network. From the results of the calculations, it can be concluded that the cause of towers collapse was the very big frontal surface of conductors, due to the ice accretions of approximately 15 cm in diameter, in combination with great wind pressure, because of the winds prevailing in the region. The collapsed towers were from galvanized steel and did not have any trace of corrosion. Additionally, no signs of deterioration were observed on the conductors which could reduce their mechanical strength. Consequently, the attribution of the collapses to material failures can be excluded. The concrete foundations of the collapsed towers were not damaged. Thus, the existing foundations were used for the restoration of the power supply of the island using the 150 kV transmission line coming from Aktio (northern side). The simultaneous existence of high winds, that appeared in the towers’ region according to the data of the National Meteorological Service, accompanied with hard snowfall, constituted an extreme meteorological phenomenon, that had not happened previously in the island, and for this reason it was not possible to forecast in 4 As service was interrupted for 5 days, and assuming an average load of 25 MW, the estimate of 3000 kWh was reached as 5 days 24 h 25 MW = 3000 MWh.
the regulations and specifications. Specifically, EN 50314 states that under combined wind and ice load, the wind loads are reduced compared to just wind loading. In the case at hand, the opposite was true. The conductors were greatly loaded with ice and the winds blowing were of great intensity. This resulted in the collapse of the towers. Since the blackout of 2006, an additional substation (Myrtos substation) has been commissioned, and another one is planned to be constructed by 2015 (Kefallonia II substation). Additionally, the backup generators, which had been sent to assist the power system restoration, remain at the island. Finally, at present, four wind farms are operating, with a total installed power of 73.5 MW. As the climatic conditions have changed over the last years, the revision of the Greek Regulations in particular and the National Normative Aspects in general, regarding the overhead electric power transmission systems, is proposed, so as to establish smaller spans between towers in new lines as well as to upgrade the existing lines, when the recent collected meteorological data, render this necessary. The proposed upgrades of existing or new lines can be categorized in two strategies. The first strategy encompasses the antiicing methods and aims at the reduction of ice accretion or at the increase of ice shedding. Some examples of these techniques are the use of ice phobic materials, the use of freezing point depressant liquids etc. The second strategy encompasses the de-icing methods and aims at the removal of the accumulated ice when the volume of the ice surpasses a threshold. Some examples of these methods are the short circuit method, the use of mechanical stresses so as to remove the ice, etc. [15]. Characteristically, Zhong in [16] proposed an expert system which can help establish the order of short-circuit operations, in order to de-ice the conductors of a High Voltage network. Another de-icing method can be developed based on the operation principle of the recloser. Utilizing three short-circuits of shorter duration instead of a larger one, the same thermal effect can be accomplished. Additionally, the added mechanical stress due to the three operations can contribute in the ice removal due to conductor vibration. Future work could quantify the added benefit of this proposed modification to the short-circuit method. 7. Conclusions Based on the site visit, the investigation and processing of existing data, as well as information collected regarding the 24-01-2006 power blackout in the Kefallonia Island, it can be concluded that the blackout was caused due to the simultaneous existence of high winds and hard snowfall which had never happened before. It was an extreme phenomenon that is not taken into account in the European regulations regarding the installation and maintenance of existing power lines. Due to the fact that the climatic conditions have changed in the last years, it is recommended that the aforementioned regulations are revised, so as to demand smaller spans between towers in new transmission lines. Furthermore, the existing lines should be upgraded when it becomes necessary due to heavy weather conditions. References [1] Wong Ji-Jen, Su Ching-Tzong, Liu Chi-Shuan, Chang Chung-Liang. Study on the 729 blackout in the Taiwan power system. Electrical Power and Energy Systems 2007;29:589–99. [2] Chen RH, Malik OP, Hope GS. On-line multi contingency preprocessing of security assessment for severe weather based on qualitative reasoning with probability statistics classification. Electrical Power and Energy Systems 1995;17:3–10. [3] Zhu Dan, Cheng Danling, Broadwater Robert P, Scirbona Charlie. Storm modeling for prediction of power distribution system outages. Electric Power Systems Research 2007;77:973–9.
S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127 [4] Castro RD. Overview of the transmission line design process. Electric Power Systems Research 1995;35:109–18. [5] Foschi RO. Reliability theory and applications to risk analysis of power components and systems. Electrical Power and Energy Systems 2004;26:249–56. [6] ASTM B 232/B 232M-01. Standard Specification for Concentric-Lay-Stranded Aluminum Conductors, Coated-Steel Reinforced (ACSR). West Conshohocken (US): ASTM International; 2001. [7] Niazy I, Sadeh J. A new single ended fault location algorithm for combined transmission line considering fault clearing transients without using line parameters. Electrical Power and Energy Systems 2013;44:816–23. [8] Bakar Ab Halim Abu, Yatim Fazilah Mat, Yusof Sallehuddin, Othman Mohd Ridzal. Analysis of overload conditions in distance relay under severe system contingencies. Electrical Power and Energy Systems 2010;32:345–50. [9] Al-Kandari Ahmad, Gilany Mahmoud, Madouh Jamal. An accurate technique for locating faults by distance relays. Electrical Power and Energy Systems 2011;33:477–84.
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[10] Greek Official Journal: No. 608-part B. Regulations for Installation and maintenance of outdoor electric power lines, Athens, Greece, 6/10/1967. [11] Public Power Corporation S.A. Athens-Greece, Specification No. TR-8, Steel Reticular Type towers for transmission lines and Annex of Specification TR-8 150 kV Light Type towers for Simple Circuit lines, Athens, Greece, April 1965. [12] EN 50341-3-10:2001. National Normative Aspects (NNA) for Greece, Brussels, Belgium; 2001. [13] Kiessling F, Nefzger P, Nolasco JF, Kaintzyk U. Overhead power lines – Planning, design and construction. Berlin: Springer-Verlag; 2003. [14] Thapar V, Agnihotri G, Krishna Sethi V. Critical analysis of methods for mathematical modeling of wind turbines. Renewable Energy 2011;36:3166–77. [15] Farzaneh M. Atmospheric Icing of Power Networks. Berlin: Springer Science; 2008. [16] Zhong T. Graph theory based expert system to form de-icing route in Changsha power grid. Electrical Power and Energy Systems 2012;43:1318–21.
Contents lists available at SciVerse ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Examination of the 2006 blackout in Kefallonia Island, Greece Stavros D. Anagnostatos, Constantinos D. Halevidis ⇑, Aikaterini D. Polykrati, Perikles D. Bourkas, Constantinos G. Karagiannopoulos National Technical University of Athens, School of Electrical and Computer Engineering, High Voltages and Electrical Measurements Laboratory, 9 Iroon Polytechniou Str., GR 157-80 Athens, Greece
a r t i c l e
i n f o
Article history: Received 29 April 2010 Received in revised form 17 November 2012 Accepted 20 December 2012
Keywords: System blackout Transmission line Extreme weather conditions Single circuit tower
a b s t r a c t The blackout on the Greek island of Kefallonia on January 24th 2006 was caused by the fall of 10 towers of the 150 kV power transmission network due to high winds and heavy snowfall. These extreme meteorological phenomena were unprecedented on the island and thus had not been taken into consideration by the Greek installation and maintenance regulations regarding these towers. Due to changes in meteorological conditions in recent years, the above regulations have to be revised so as to propose smaller distances between towers in new transmission lines and improvements to the existing lines to endure severe weather conditions when necessary. This paper provides a short description of the High Voltage (HV) network of the island of Kefallonia, the system conditions before and after the blackout, and identifies the causes of the blackout. In addition, the mechanical strength evaluation of the simple circuit towers (similar type to those that collapsed) is made under the stress of gale-force winds combined with the ice-coating of conductors. Finally, in order to avoid the repetition of such an outage, several methods are proposed. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A blackout, i.e. a power outage, leaves large regions without electricity for large time periods. Consequently, great effort is being put by researchers into identifying the underlying causes and countering them so as to avoid the repetition of such events. An example of such an effort is described in [1]. Transmission networks are designed so as to ride through at least one contingencyfault (N-1 analysis). However, in storm and severe weather conditions the frequency of multiple contingencies can be greater than that of a single contingency in fair weather [2]. Severe weather conditions cost power utilities great amounts of money due to unserviceable loads as well as for the replacement of destroyed equipment [3]. Thus, the planning and design of the overhead transmission and distribution networks, so as that they reliably endure the mechanical stresses caused by meteorological phenomena, is very important [4,5]. The Argostoli1 substation is supplied in a loop configuration by two 150 kV High Voltage (HV) transmission lines, the Aktio line from the north, and the Kyllini line from the south. Each one of them can sufficiently supply the island with electricity. Downstream of the substation, the electric energy is distributed using 15 kV Middle Voltage (MV) lines. Finally, the voltage is stepped down to 400 V ⇑ Corresponding author. Tel.: +30 6932337468. 1
E-mail address: [email protected] (C.D. Halevidis). Argostoli is the largest town of Kefallonia Island.
0142-0615/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijepes.2012.12.003
via Delta-grounded wye distribution transformers. This international practice of designing networks in loop arrangement, with double High Voltage lines, is applied in order to have a backup power supply, as each of the two inputs ensures the total energy supply in case one fails. The existence of a second backup transmission line is not considered, as the power supply of the island follows the N-1 security standard. Furthermore, as the construction is based on past weather conditions and other natural phenomena in the region, only an extreme weather phenomenon which can render inoperable both transmission lines can cause a blackout. This was the case on January 24, 2006 in Kefallonia Island due to coexistence of high winds of Beaufort scale ten (10) and ice sleeves on the conductors which were multiple in size compared to those usually observed in Greece due to heavy snowfall. The ambient temperature recorded by the National Meteorological Service at the meteorological station of Argostoli was 5 °C. The above phenomena caused towers to collapse in both 150 kV lines. It should be noted that the conductors’ ice sleeve was approximately 15 cm in diameter. The population of Kefallonia was 39,488, and 41,365 according to the 2001 and 2011 censuses respectively. The population density is 45.8 people per km2 according to the 2011 census, while the total area is 904 km2. These figures include the island of Ithaki, which is connected to Kefallonia through submarine MV cables. In this paper, a short description of the layout of the High Voltage network of the Kefallonia Island, the state of the network before and after the blackout and the weather condition during the blackout is given. Afterwards, an evaluation of the mechanical
S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127
strength of single-circuit towers, of the type of those that collapsed, is performed, in the case where the mechanical stress is caused by high winds in combination with ice covering the conductors. Additionally, the causes of the blackout are presented and possible solutions are given so as to avoid repetitions of similar outages.
2. Description of the High Voltage network (150 kV) in the Kefallonia Island Fig. 1 shows a simplified diagram of the High Voltage (150 kV) transmission network in the Kefallonia Island. Furthermore, the two northern-side towers (I and II) and the eight southern-side towers (1–8), which collapsed and caused the blackout of the island, are shown. The overhead line conductors of the two lines were aluminum conductors steel reinforced (ACSR) of LINET type [6]. Their characteristics are the following: diameter of 18.31 mm, equivalent copper cross section of 107.48 mm2, and nominal current of 483 A. The protection of the transmission lines was achieved through a current differential scheme, with distance protection acting as backup. This protection scheme was necessary, as the transmission lines are composite; i.e., overhead line combined with submarine cables. Additionally, directional over-current relays are used. At
123
this point it should be noted that steps are being undertaken in the direction of fault location in composite transmission lines [7]. Furthermore, significant effort has been dedicated to the development of new features for distance relays [8,9]. The Argostoli substation consisted of one 40/50 MVA (150 kV/ 15 kV) and two 20/25 MVA (150 kV/15 kV) power transformers. Consequently, the total installed power was equal to 100 MVA. Furthermore, three inductor banks are installed at the High Voltage side, with a combined rating of 44.5 MVAr, in order to balance the cable capacitance. The single line diagram of the substation is shown in Fig. 2. Closed Cycle Gas Turbines (CCGTs) were installed in the island of Kefallonia until 2002, when they were removed due to the transmission line from Kyllini being commissioned. Furthermore, at the time of the blackout, a wind farm with a nominal rating of 13.6 MW was connected to the electrical grid of Kefallonia. From the Greek Regulations regarding the installation and maintenance practice of overhead lines [10], the Public Power Corporation specifications for 150 kV single-circuit line towers such as the ones under consideration [11], the following can be extracted: (1) All towers of the southern side were steel lattice alignment towers of the S2 type with an allowed maximum transverse load per phase conductor equal to 7300 N.
Fig. 1. Simplified diagram of the High Voltage transmission network. The damaged towers are: the two towers of the northern side (I and II) and the eight towers of the southern side (1–8).
Fig. 2. Single line diagram of the Argostoli substation.
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S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127
From the inspection of the areas where the High Voltage towers collapsed, the following points were established: On the conductor surfaces there was an ice-sleeve approximately 15 cm in diameter (see Fig. 5). This caused an excessive increase in frontal surface area, resulting in much greater wind pressure. It should be noted that design studies in Greece for the 150 kV power transmission lines assume an ice sleeve of 6.5 mm for simple circuit towers [12]. The High Voltage line’s towers (from Aktio and from Kyllini) had collapsed towards the wind direction, almost vertically to the line’s direction. A collapsed tower is shown in Fig. 6. It should be noted that the tower failed at the body extension. Tower torsion phenomena were observed because of the conductors’ abruption, before or during their collapse. The collapsed towers were erected from galvanized steel, and no trace of corrosion was found. The concrete foundations of the towers were not damaged. Generally, there was no discernible sign of conductor wear which would have weakened their mechanical strength. Fig. 3. Steel lattice tower dimensions of 150 kV single-circuit power transmission lines (the ones under consideration).
(2) Tower I of the northern side was a steel lattice tower for small direction angle changes of the R2 type with an allowed maximum transverse load per phase conductor of 14,650 N. (3) Tower II of the northern side was a steel lattice tower for a 45° direction angle change of the T2 type with an allowed maximum transverse load per phase conductor of 39,500 N. (4) The dimensions of the towers as well as the insulation distances between the conductors are given in Fig. 3.
3. The system condition before and after the blackout
4. Single-circuit towers mechanical strength evaluation, according to EN 50341-3-10 The diameter D of the ACSR LINET type conductor, that was used in the HV lines, was equal to 18.31 mm [6]. According to EN 50341-3-10, the combined wind and ice load for the country of Greece is given as an ice sleeve radius Rsleeve equal to 6.5 mm and wind pressure Pen equal to 190 N/m2 [12]. Consequently, the frontal surface A per unit length of the aforementioned conductor is:
A ¼ ðD þ 2Rsleev e Þ ¼ 31:31 103 m2 =m
ð1Þ
Thus, for ice sleeves with a radius equal to 6.5 mm, the transverse force F (wind load) per unit meter, applied on the conductor, is equal to:
F ¼ A Pen ffi 5:94 N=m On 24-01-2006 prior to the blackout, the load of the Argostoli substation was equal to 29.8 MW, and on 29-01-2006, after the complete power supply restoration, 26.3 MW. The monthly maximum demand of the above substation for the years 2002–2005 is given in Fig. 4. The operation of these two lines with such a demand load effectively rules out the possibility of conductor overheating, as each transmission line has a 125 MVA capacity. Initially, at 21:57 on 23-01-2006, a mechanical failure occurred in the High Voltage line of 150 kV from Aktio (northern side). Namely, two towers collapsed and a third was damaged. This, in turn, caused the tripping of the line protection from both ends. Despite this, the power supply of the island was not interrupted, as the power supply was continued from the 150 kV High Voltage line from Kyllini (southern side). Then, at 01:53 on 24-01-2006 another mechanical failure occurred in the 150 kV Kyllini line, with the collapse of eight towers. Similarly, the circuit breaker located at the Zakinthos substation2 was tripped. Consequently, the voltage collapsed, as the load could not be fed, resulting in the blackout of the island. At this point it should be noted that the stability of the main grid was not impacted, as the faults were cleared by the respective circuit breakers. Due to this unprecedented bad weather in Kefallonia, damage was also sustained by the distribution network, in both Middle and Low Voltage networks. Eighty wooden poles, six distribution substations, and approximately 30 km of Middle and Low Voltage lines were destroyed. Additionally, 300 house service cables were cut, and a large number of Middle and Low Voltage fuses were blown, in addition to other small damages.
ð2Þ
Consequently the tower’s transverse stress for the maximum span of 397 m of the lines would be 397 F ffi 2360 N. This value is smaller than the maximum allowed load Fmax for a S2 type tower, which is equal to 7300 N [10]. Similarly, it arises that the transverse stresses for R2 and T2 type towers were smaller than those allowed in the Public Power Corporation specifications [11]. Consequently, the mechanical strength of the three types of towers, as used in the transmission lines, would be sufficient to endure the combined ice and wind loading used by EN 50341 and the Greek regulations. 5. The minimum ice sleeve size that can cause tower collapse in case of high winds In the following calculations, the collapse of a S2 type tower for the minimum span Lmin of 238 m is examined. It should be noted that the torsion effect is not taken into account. Thus, the calculations err on the side of caution. As presented in paragraph 2, the allowed transverse stress Fmax for a S2 type tower is equal to 7300 N. Consequently, for a span of 238 m, the allowed wind pressure per unit length is:
P Dlim ¼ F max =Lmin ¼ 7300=238 ¼ 30:67 N=m
ð3Þ
The dynamic wind pressure P applied on the conductor is [13]:
P ¼ C d qu2 =2 ¼ 494 N=m2
ð4Þ 3
2
Zakinthos substation is the substation immediately upstream on the Kyllini line.
where q is the air density taken equal to 1.225 kg/m [13], Cd is the drag coefficient and is assumed equal to unity, and u is the wind
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Fig. 4. Argostoli substation monthly maximum load for the years 2002–2005.
Fig. 5. Overhead line conductors with ice sleeves.
speed. According to the National Meteorological Service report the wind speed was equal to 28.4 m/s3, measured at a height of 10 m. The real wind pressure is greater than the calculated one as the wind 3
Equivalent to a magnitude of Beaufort scale ten.
Fig. 6. Tower collapsed towards the wind direction (almost vertically to the transmission line direction).
speed increases with increasing height, and the conductors were suspended at a height of 19 m approximately. The wind speed as a function of the height can be approximated as following the power law [14], and is expressed as:
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a u2 h2 ¼ u1 h1
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ð5Þ
where h is the height, and a is the ground surface roughness length. Consequently, for a wind pressure P equal to 494 N/m2 applied on the conductors, the ice sleeve diameter limit for the collapse of the tower is:
Dlim ¼ F max =P ¼ 30:67=494 ffi 6:2 cm
ð6Þ
Consequently, for high winds in the tower region (as reported by the National Meteorological Service in its report) and for an ice sleeve of approximately 15 cm in diameter on the conductors’ surface, the collapse of the towers was expected, even more so in the case of towers with a span bigger than 238 m. Another factor adding to the total mechanical stress is the torsion due to the abruption of one or more conductors. Similarly, it can be concluded that the 15 cm ice sleeve, in combination with the high winds, caused the collapse of the two towers of R2 and T2 type of the northern side. 6. Discussion The Kefallonia Island power blackout happened at 01:53 on 2401-2006, because of the collapse of eight S2 type towers of the 150 kV transmission line coming from Kyllini (southern side). Additionally, great damages were caused to the distribution system. The blackout caused an unserviceable electrical energy of approximately 3000 MWh.4 It arises that the construction and design of the aforementioned collapsed line, was in accordance with the standing Greek Regulations [10], with the Public Power Corporation specifications [11], as well as with the National Normative Aspects for Greece [12] as the transverse stresses, to which the towers are subjected, under high winds and with ice accretion thickness of 6.5 mm are smaller than the allowed ones [11]. This is also valid for the collapsed part of the 150 kV transmission line coming from Aktio (northern side). It should be noted that: 1. A tower collapse had not happened before. 2. Towers of the same type have been installed without particular problems in other regions of Greece; e.g. in the Florina and Kastoria regions which are subject to significant snowfall. 3. Previous service discontinuities, which were caused by ice and wind, were due to damages to the MV network. From the results of the calculations, it can be concluded that the cause of towers collapse was the very big frontal surface of conductors, due to the ice accretions of approximately 15 cm in diameter, in combination with great wind pressure, because of the winds prevailing in the region. The collapsed towers were from galvanized steel and did not have any trace of corrosion. Additionally, no signs of deterioration were observed on the conductors which could reduce their mechanical strength. Consequently, the attribution of the collapses to material failures can be excluded. The concrete foundations of the collapsed towers were not damaged. Thus, the existing foundations were used for the restoration of the power supply of the island using the 150 kV transmission line coming from Aktio (northern side). The simultaneous existence of high winds, that appeared in the towers’ region according to the data of the National Meteorological Service, accompanied with hard snowfall, constituted an extreme meteorological phenomenon, that had not happened previously in the island, and for this reason it was not possible to forecast in 4 As service was interrupted for 5 days, and assuming an average load of 25 MW, the estimate of 3000 kWh was reached as 5 days 24 h 25 MW = 3000 MWh.
the regulations and specifications. Specifically, EN 50314 states that under combined wind and ice load, the wind loads are reduced compared to just wind loading. In the case at hand, the opposite was true. The conductors were greatly loaded with ice and the winds blowing were of great intensity. This resulted in the collapse of the towers. Since the blackout of 2006, an additional substation (Myrtos substation) has been commissioned, and another one is planned to be constructed by 2015 (Kefallonia II substation). Additionally, the backup generators, which had been sent to assist the power system restoration, remain at the island. Finally, at present, four wind farms are operating, with a total installed power of 73.5 MW. As the climatic conditions have changed over the last years, the revision of the Greek Regulations in particular and the National Normative Aspects in general, regarding the overhead electric power transmission systems, is proposed, so as to establish smaller spans between towers in new lines as well as to upgrade the existing lines, when the recent collected meteorological data, render this necessary. The proposed upgrades of existing or new lines can be categorized in two strategies. The first strategy encompasses the antiicing methods and aims at the reduction of ice accretion or at the increase of ice shedding. Some examples of these techniques are the use of ice phobic materials, the use of freezing point depressant liquids etc. The second strategy encompasses the de-icing methods and aims at the removal of the accumulated ice when the volume of the ice surpasses a threshold. Some examples of these methods are the short circuit method, the use of mechanical stresses so as to remove the ice, etc. [15]. Characteristically, Zhong in [16] proposed an expert system which can help establish the order of short-circuit operations, in order to de-ice the conductors of a High Voltage network. Another de-icing method can be developed based on the operation principle of the recloser. Utilizing three short-circuits of shorter duration instead of a larger one, the same thermal effect can be accomplished. Additionally, the added mechanical stress due to the three operations can contribute in the ice removal due to conductor vibration. Future work could quantify the added benefit of this proposed modification to the short-circuit method. 7. Conclusions Based on the site visit, the investigation and processing of existing data, as well as information collected regarding the 24-01-2006 power blackout in the Kefallonia Island, it can be concluded that the blackout was caused due to the simultaneous existence of high winds and hard snowfall which had never happened before. It was an extreme phenomenon that is not taken into account in the European regulations regarding the installation and maintenance of existing power lines. Due to the fact that the climatic conditions have changed in the last years, it is recommended that the aforementioned regulations are revised, so as to demand smaller spans between towers in new transmission lines. Furthermore, the existing lines should be upgraded when it becomes necessary due to heavy weather conditions. References [1] Wong Ji-Jen, Su Ching-Tzong, Liu Chi-Shuan, Chang Chung-Liang. Study on the 729 blackout in the Taiwan power system. Electrical Power and Energy Systems 2007;29:589–99. [2] Chen RH, Malik OP, Hope GS. On-line multi contingency preprocessing of security assessment for severe weather based on qualitative reasoning with probability statistics classification. Electrical Power and Energy Systems 1995;17:3–10. [3] Zhu Dan, Cheng Danling, Broadwater Robert P, Scirbona Charlie. Storm modeling for prediction of power distribution system outages. Electric Power Systems Research 2007;77:973–9.
S.D. Anagnostatos et al. / Electrical Power and Energy Systems 49 (2013) 122–127 [4] Castro RD. Overview of the transmission line design process. Electric Power Systems Research 1995;35:109–18. [5] Foschi RO. Reliability theory and applications to risk analysis of power components and systems. Electrical Power and Energy Systems 2004;26:249–56. [6] ASTM B 232/B 232M-01. Standard Specification for Concentric-Lay-Stranded Aluminum Conductors, Coated-Steel Reinforced (ACSR). West Conshohocken (US): ASTM International; 2001. [7] Niazy I, Sadeh J. A new single ended fault location algorithm for combined transmission line considering fault clearing transients without using line parameters. Electrical Power and Energy Systems 2013;44:816–23. [8] Bakar Ab Halim Abu, Yatim Fazilah Mat, Yusof Sallehuddin, Othman Mohd Ridzal. Analysis of overload conditions in distance relay under severe system contingencies. Electrical Power and Energy Systems 2010;32:345–50. [9] Al-Kandari Ahmad, Gilany Mahmoud, Madouh Jamal. An accurate technique for locating faults by distance relays. Electrical Power and Energy Systems 2011;33:477–84.
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[10] Greek Official Journal: No. 608-part B. Regulations for Installation and maintenance of outdoor electric power lines, Athens, Greece, 6/10/1967. [11] Public Power Corporation S.A. Athens-Greece, Specification No. TR-8, Steel Reticular Type towers for transmission lines and Annex of Specification TR-8 150 kV Light Type towers for Simple Circuit lines, Athens, Greece, April 1965. [12] EN 50341-3-10:2001. National Normative Aspects (NNA) for Greece, Brussels, Belgium; 2001. [13] Kiessling F, Nefzger P, Nolasco JF, Kaintzyk U. Overhead power lines – Planning, design and construction. Berlin: Springer-Verlag; 2003. [14] Thapar V, Agnihotri G, Krishna Sethi V. Critical analysis of methods for mathematical modeling of wind turbines. Renewable Energy 2011;36:3166–77. [15] Farzaneh M. Atmospheric Icing of Power Networks. Berlin: Springer Science; 2008. [16] Zhong T. Graph theory based expert system to form de-icing route in Changsha power grid. Electrical Power and Energy Systems 2012;43:1318–21.