Transelectrica serves as the transmission system operator of the Romanian power system. It is responsible for the operation of transmission lines that extend to some 9000 km (5592 miles). The system includes 154 km (96 miles) of lines operating at 750 kV, 2876 km (1787 miles) at 400 kV, 4133 km (2568 miles) at 220 kV and 39 km (24 miles) at 110 kV. Several high-voltage overhead lines routed in the mountainous zone are subject to very difficult weather conditions, varying from hoarfrost during the winter to lightning storms during the summer. Some sections of the 400-kV and 220-kV circuits erected in this zone and exposed to lightning have been constructed without a ground wire.
Transelectrica's Northern and Central Moldavia mountainous zone of the grid is short of generating capacity, relying on a single 400-kV overhead transmission line connection from Brasov to Gutinas 400/220/110-kV substation. The Brasov-Gutinas circuit, which is 126 km (78 miles) long, transfers the power to the Moldavia zone from the power plants in Oltenia. Because of the length of this circuit, Transelectrica has given special attention to this 400-kV voltage line, basing its maximum load-transfer capacity on steady-state stability studies. The National Dispatch Center also finds it extremely difficult to program a circuit outage, as this results in energy transfer using the 220-kV transmission lines in the southern zone of the grid.
System reliability of the 400-kV Brasov-Gutinas line, which is routed through the mountainous zone at an altitude exceeding 1000 m (3281 ft), is very exposed and subject to lightning, so the circuit performance in terms of faults is quite poor. For this reason, Transelectrica has invested in system protection measures, including the installation of lightning surge arresters, in order to restore the reliability of supplies to Northern and Central Moldavia to an acceptable level.
Performance of the Line
The 400-kV Brasov-Gutinas line is strung with 973 kcmil (228 sq mm) aluminum conductor, steel-reinforced conductor supported by composite insulator strings. Through operational experience, Transelectrica determined that the section of overhead line subject to lightning has problems due to a lack of a ground conductor and tower footing resistance, which varies from 3.1 ohms to 9.7 ohms. This 2.77-km (1.72-mile) section from tower 130 to tower 145 experiences wind speeds up to 200 kmph (124 mph) and 46.2-mm (1.18-inch) radial accretion of hoarfrost, conditions that are much higher than average.
A succession of operational incidents and failures prompted the following work to be done in advance of installing lightning protection:
- Replacing the previous twin conductors per phase with a single conductor to prevent the hoarfrost sediment joining the twin conductors
- Replacing all the suspension towers with tension (deadend) towers
- Reducing the existing span length by increasing the number of supporting towers
- Dismantling the ground conductors, which were suspended below the phase conductors because of the hoarfrost loading
- Installing counterweights at the end of insulator strings to limit conductor swing due to wind forces.
From 2000 to 2006, this circuit experienced as many as 10 faults per annum. The poor performance of this circuit resulted in Transelectrica seeking a long-term solution by adopting current technology. Based on the recommendations included in an independent consultant report, a tender was issued for the supply and installation of 36 overhead line surge arresters. The contract was subsequently awarded to SMART S.A., a maintenance company of Transelectrica, in association with Eximprod Power Systems S.R.L., the distributor in Romania for Hubbell/Ohio Brass.
Installation of Lightning Arresters
As the transmission line section between tower 130 and tower 145 operated without ground wires to afford protection against lightning, it was decided to install 36 units of Protecta*Lite arresters in parallel with each insulator string. The surge arresters were installed from the tower crossarms and connected to the phase conductors. The line surge arresters are connected electrically in parallel with the line insulation. They are designed to limit the surge voltage across the line insulation by conduction at a voltage below the flashover voltage of the line insulation.
After the surge arrester has successfully discharged the lightning surge, the voltage across the arrester returns to the line-to-ground value. Since the line arresters used in this application are gapless, the arrester is only in the conduction mode during the lightning strike. The timing of this simultaneous operation is too short to be detected by conventional system protection relays. Therefore, the operation of the lightning arrester does not result in circuit interruption. Electrically, these arresters are similar in design to those normally installed in substations. In order to improve the line protection, vertical lightning rods were also installed on the tower peaks in conjunction with the work undertaken to refurbish the tower.
Since the 400-kV Gutinas-Brasov overhead line is the main source of supply to Northern and Central Moldavia, it is very difficult to program a circuit outage. For this reason, live-line working techniques were used to install the lightning arresters and lightning rods.
Line outage incident statistics since the installation of the lightning arresters in 2006 indicate the line protection solution applied was both technically and economically justified. Also, a reduction in the number of incidents automatically reduces the cost of corrective maintenance.
However, Transelectrica continued to experience operating problems following heavy winds and hoarfrost that caused the arrester connection assembly at the bottom of the insulator to fail. To be more precise, the disconnector that had been installed to protect the arrester had become a weak point. As the result of strong winds, the disconnectors broke, and three of the arresters were disconnected from the phase conductors.
Jointly, representatives from Transelectrica, SMART S.A. and Eximprod Power Systems tried to find a solution to secure the arrester connection to the phase conductors. Engineers from the arrester team at Hubbell Power Systems, together with engineers from SMART S.A., provided the final solution. To avoid damaging the disconnector, Hubbell's engineers tested and replaced the original disconnector with two disconnectors connected in parallel. The after-sales services provided by Hubbell should be noted as the manufacturer accepted full responsibility for all the costs associated with the manufacturing and transportation of the new disconnecting assembly. The two replacement devices were mounted by SMART S.A. using live-line working techniques.
Since the New Arresters
The number of circuit outages have decreased dramatically since 2006, and Transelectrica experienced only one outage in 2008, even though not all of its exposed towers of the transmission line are protected by line arresters. Since March 2007, when the original disconnectors were replaced, there have not been any other mechanical failures.
Line arresters have now become the accepted solution for the protection of transmission lines subject to lightning in Romania and an additional 11 towers on a 220-kV transmission line prone to lightning problems are now protected by Protecta*Lite. This line, also owned by Transelectrica, had the arresters installed in several stages between 2004 and 2007. Furthermore, Hidroelectrica, the Romanian generation company responsible for hydro power plants, also chose to install Protecta*Lite arresters in the first part of 2009 on 19 towers on the 110-kV Lotru-Ciunget transmission line.
Now a Standard Practice
This method of installing lightning arresters on overhead transmission lines constructed without a ground wire provides a modern and viable solution to improve the reliability of the transmission system. Following the installation of surge arresters on the 400-kV Brasov-Gutinas transmission line, this practice is now set to become the standard design practice on all Romanian transmission lines subject to above-average fault outages due to lightning.
Dr. Stelian Iuliu Alexandru Gal ([email protected]) has a MSEE degree and earned his Ph.D. in 1995. His professional experience has included the roles of design engineer, technical manager and manager. Between 2005 and 2009, he served as general manager of C.N. Transelectrica S.A., and he is now director of the Sibiu branch of Transelectrica. He is a member of CIGRÉ and the Romanian Live Working Association.
Dr. Traian Fagarasan ([email protected]) graduated from the Cluj-Napoca Institute of Polytechnics in 1987. He received his Ph.D. in 1997. Working initially as a protection engineer, he is now manager of the Transelectrica's Sibiu subsidiary of SC Smart S.A. He is a member of CIGRÉ and the Romanian Live Working Association.
Marius Oltean ([email protected]) graduated from the Timisoara University of Polytechnics in 1986 and started his career in a heavy water factory in Drobeta-Turnu Severin and IRE Sibiu. Since 2001, Oltean has worked within Transelectrica's Sibiu subsidiary of SC Smart S.A. as technical manager and manager. He is also a member and secretary of the Romanian Live Working Association.
Constantin Matea ([email protected]) graduated from the Bucharest University of Polytechnics in 1972. His experience includes working as design engineer at ICMP and Conel. In 2005, Matea joined CN Transelectrica S.A. as the project manager responsible for overhead line maintenance.
Companies mentioned in this article:
Eximprod Power Systems S.R.L. www.eximprod.ro
Hubbell Power Systems www.hubbell.com
SMART S.A. www.smart-sa.ro