China Mitigates Lightning Damage
The Chinese power system has tens of thousands of kilometers of optical-fiber composite overhead ground wire (OPGW) that have been in commission since the 1990s. In recent years, there have been several occurrences where the outer wires of the OPGW have been broken by lightning strikes, thereby impacting the reliability of both the transmission line and the communications system. This phenomena has resulted in a special investigation to design a new OPGW with improved characteristics and to improve construction or installation methods.
OPGW is used extensively throughout the world for communications systems and serves two functions: the circuit ground wire and optical-fiber communications cable. As the circuit ground wire, it shields the phase conductors from lightning. OPGW also has the significant advantage of a large communications capacity.
OPGW LIGHTNING DAMAGE EXAMPLES
The Dou-Bay 500-kV transmission line in Hubei Province is 275 km (171 miles) long. Within the first four months following commissioning, the line was subject to five lightning strikes and a thunderbolt, resulting in the outer wires of the aluminum alloy (AA) OPGW being broken.
In most instances, some three to six wires were broken at each location; however, at one location, 10 wires were broken. All of the damage occurred on the OPGW along a 100-km (62-mile) section of the transmission line. The adjacent ground wire on this 500-kV line experienced no damage. The construction of the damaged OPGW was the center aluminum tube type. The inside layer consists of eight 3.3-mm (0.13-inch)-diameter aluminum-wrapped steel (AS) wires, and the outside layer consists of 16 2.5-mm (0.10-inch) AA wires.
In Zhejiang Province, a 500-kV transmission line, in operation since 1998, experienced lightning damage to the OPGW every year from 1999 to 2004; and in September 2002, 10 wires were broken at one fault position. Inspection revealed that the breaks in the outer layer of 13 3.25-mm (0.13-inch) AA wires looked as if the wires had melted like a fuse.
Similarly, in Guangdong Province, the OPGW on one 500-kV transmission has been subject to lightning damage for three consecutive years. Furthermore, damage caused by lightning strikes has occurred on the OPGW on 500-V transmission lines in Hebei Province in 2003 and 2004 in locations where thunderstorms are relatively infrequent. One fault caused damage to the ground wires and the 24 fiber-optic bundle; and on another circuit, severe damage left the OPGW with only two steel-tube (ST) wires intact.
These incidents indicate the severity of the damage to the OPGW caused by lightning strikes and served as justification for a research project launched and funded on behalf of Chinese power utilities and OPGW manufacturers.
RESEARCH INTO REASONS FOR DAMAGE
The design of the damaged OPGWs include wires with AA, AS and ST wires with diameters ranging from 2.33 mm (0.09 inches) to 3.72 mm (0.15 inches). These OPGWs include conductors manufactured and supplied by Chinese and international manufacturers.
The commonly held view is that the heat generated by the lightning arc melts the outer wires and causes the external damage on the OPGW. However, lightning has two basic forms: a pulse current that is very large with a short time duration and a lower magnitude current (several hundred amperes) that continues for several milliseconds. The latter type of lightning strike likely causes the outer wires to melt.
The Zhongtian Technologies Co. (Nantong, Jiangsu, China) has developed a formula that calculates the quantity of heat in fusion generated by a lightning arc to melt aluminum and steel:
E = [C
where, E is the quantity of demand heat in fusion, C is the specific heat of the aluminum or steel, T is the environment temperature or the melting point of the aluminum or steel, δ is the quantity of heat in demand at which a unit weight solid becomes a liquid at melting point and σ is the unit weight of the aluminum or steel in the OPGW.
Using this formula, the quantity of heat in fusion required to melt the wires used in the manufacturing of OPGW can be determined. For example, for a 3.25-mm (0.13-inch)-diameter 23% International Annealed Copper Standard (IACS) AS wire, the calculated quantity of demand heat in fusion is 16,932 cal/m (5162 cal/ft).
Experimental work linked to lightning strikes on OPGWs by another company present the results in the form of the probability that a wire will be damaged in the event of a lightning strike. The results for AS wire are shown in the table.
The experiment confirmed that if the outer layer of the OPGW has a 2.55-mm (0.10-inch)-diameter AA wire, then an electric charge of 50 coulombs is sufficient to damage all the outer layer wires.
Based on available research data, it now appears that in areas subject to frequent lightning storms, the outside layer of the OPGW should be AS wire with a diameter greater than 3 mm (0.12 inches). The higher melting point of AS wires should give the OPGW the characteristics to withstand lightning strikes without permanent damage.
PROTECTION FOR THE FUTURE
China has several decades of general ground wire operational experience with very few reported failures caused by lightning strikes. However, worldwide experience suggests problems with OPGWs; therefore, it is interesting to compare the conditions of OPGW used in China and around the world.
Commonly, in comparable transmission lines, the per-unit length resistance of the general ground wire is always larger than that of the OPGW.
On transmission lines, the OPGW will be grounded directly through the tower body on every tower, but different practices are used with regard to the grounding of the general ground wire. Therefore, the total grounding resistance of the OPGW wire will be lower than that of the general ground wire.
The ground wire to the tower body resistance will be higher than that of the OPGW connection resistance to ground.
Transmission line ground wires are used to protect the conductor from lightning strikes, but as the connection resistance from OPGW to ground is less than that of the general ground wire, it encounters lightning strikes more easily.
According to statistical data, 90% of lightning is negative. Laboratory research involving the use of a physics process emulator and the lightning photograph indicates that the flashover from a cloud that has a large number of negative electrons to ground is not consecutive. The flashover can be considered in three stages:
The cloud-to-ground distance is great, so the flashover trigger is mainly subjected to the cloud electric field and milieu space electric field.
Together with the flashover development, because as the flashover leader leaving the cloud and ground are well separated, the development of flashover leader is only subject to the influence of the head milieu space electric charge, so the development is very optional.
The flashover leader approaches the ground, and the ground surface field gradually increases. When the clearance between the object on ground and the lightning flashover leader is pierced, the lightning strike occurs.
On the basis that the passage resistance from OPGW to ground is low, it seems inevitable that the lightning arc melts the OPGW outer layer wires. This phenomenon is comparable to the electric welding process.
Based on the available lightning fault evidence and research data, to improve the ability of an OPGW to withstand lightning strikes, it is recommended that consideration be given to the material used for the outer layers of the OPGW and the method of grounding employed. In areas subject to frequent lightning activity and where the ground resistivity is low, an air gap is introduced between the OPGW and the tower body. This gap will allow the lightning charge in the form of a traveling wave to discharge to ground via the tower body.
Ming Anchi is an engineering professor at the Central Southern China Electric Power Design Institute. Anchi has been involved in the study of underground high-voltage transmission line design and performance for more than 20 years. minganchi@csepdi.com
Xie Banghua is an engineer with the Central Southern China Electric Power Design Institute. For the past five years, Banghua has been engaged with the study of underground high-voltage transmission line design. xiebanghua@csepdi.com
Ming Ming is a mater graduate and an associate still studying at the Wuhan University of Science and Engineering. Currently on study leave, Ming is working for the Wuhan Hongxin Tele-communication Technologies Co. robinsilver@vip.sina.com
| Diameter of outer strands mm (inches) | IACS* | Value of Electric Charge (coulomb) | ||
|---|---|---|---|---|
| 100 | 150 | 200 | ||
| 2.75 (0.108) | 10% 20% 27% 40% |
0% 0% 0.8% 0.8% |
1.0% 1.2% 1.4% 1.8% |
1.8% |
| 2.75 (0.108) | 27% | 0.8% | 1.4% | |
| 3.4 (0.133) | 27% | 0.6% | 1.0% | |
| *International Annealed Copper Standard (IACS) | ||||
Want to use this article? Click here for options!
© 2008 Penton Media Inc.
