Part 2 of this article by Contributor and T&D Specialist, Raouf Znaidi of Tunisia, reports on how Pakistan’s National Transmission & Dispatch Company (NTDC) and National Engineering Services Pakistan (NESPAK) are dealing with challenges to construction and reliable operation of the overhead EHV grid. Apart from heavy pollution and corrosion, these include snow and icing in the steep, mountainous terrain of the north.
Challenges in Northern Pakistan: Mountains, Icing & Lightning
To help alleviate power shortages and explore the country’s full potential for greater hydroelectric generation, the Government of Pakistan has planned one of the country’s largest hydropower plants, in co-operation with an independent power producer, on the Kunhar and Jhelum Rivers and tributaries. Ikram Latif Wain, General Manager at the Power & Mechanical Div. and Shakir Hafeez, a Project Manager at NESPAK, are currently involved in supplying support for design of some 575 km of new double circuit 500 kV lines. This grid expansion will be part of the Interconnection Project with the Suki Kinari Power Plant which, when commissioned, is expected to have a capacity of 870 MW.
While NESPAK’s Power & Mechanical Div. has plenty of experience in designing and building EHV systems across rugged and remote areas of Pakistan, Wain still anticipates significant new challenges given the snow and icing in the northern region combined with the steep mountainous terrain. Another issue that will impact the new transmission line from Suki-Kinari to the Maira Switching Station in particular will be design of suitable tower structures given the rocky ground, poor tower footing resistance and frequent localized lightning activity. Wain explains that, based on an optimization and minimum life cycle cost study, a decision has been made to equip the new double circuit quad bundle 500 kV line with ACSR ‘Bunting’ conductor. He also notes that every effort was made to optimize the new line’s routing in terms of such factors as ensuring tower locations are as close as possible to accessible roads and paths while also respecting the need to control construction costs and ensure that future maintenance access would be as easy as possible. For example, suspension bridges and access roads were included in the bidding documents. Because the topography of the proposed transmission line route consists of mountains with numerous ridges, steep slopes and valleys, access to the sites of angle towers was a prime concern. The rugged terrain has few roads, making it a challenge to summit each proposed tower location to sample rock and soil as well as to assess possible geological implications for foundations.
In the end, selecting final angle tower markers had to take into account a wide range of aspects, including: ground clearance, availability of locations, center line of the route, right-of-way issues, possible required adjustments in span length, soil stability, rock type and depth, degree of erodible soil, surface run off and drainage as well as risk of landslide. Adjustments to the field survey also had to be made to respect preserved areas such as protected and private forests, prohibited areas, crossings with other transmission & distribution lines, roads, rivers and telecommunication lines. The detailed survey was carried out using a Real Time Kinematic (RTK) system covering a radius of two to three kilometers around the route’s established center-line. All relevant data such as UTM coordinates, elevations, major crossing points, geological features and slopes on and along both sides of the center line were collected, as per requirements of the circa 50 m line corridor and RoW. The digital ground profile was prepared with the help of Power Line Systems Computer Aided Design and Drafting (PLS-CADD) software that shows the locations of towers together with preliminary data indicating tower type, wind and span load, angle of deviation, crossings, reference levels and other key details. These included nearby permanent landmarks such as trees, homes, other towers or poles, rivers, roads, etc. Finally, towers were spotted based on ground profile while also taking into account leg & body extensions, section length, loading, angles of deviation and all types of crossings.
Lightning Activity In Northern Pakistan
The new 500 kV Suki Kinari transmission line will be situated in a region of Pakistan that experiences high lightning activity. For example, lightning strike data during recent years registered at three meteorological stations located in the vicinity of the new line route, showed up to 108 thunderstorm days per year – equivalent to a ground flash density (GFD) of 13.93 flashes/km2/year.
Given the high lightning incidence along the new line route, two overhead ground wires will be used to shield phase conductors from direct lightning strokes. The size of these ground wires was determined by prospective short circuit current duty. Numerical shielding failure calculations were performed using the classical electro-geometric model approach and an optimal shielding angle of 5° was selected on the basis of a shielding failure flashover rate (SFFOR) of 0.05 flashovers/100 km/year. Backflashovers represent another hazard associated with high lightning activity. Unlike shielding failure flashovers that result in flashover of a single phase, backflashovers can cause flashovers of multiple phases and circuits, as here in the case of the double-circuit towers. Based on previous experience as well as on CIGRE Technical Brochure #63, “Guide to Procedures for Estimating the Lightning Performance of Transmission Lines”, it was concluded that a minimum strike distance of 4.0 meters and a tower footing resistance of 15 Ω will be required to achieve the targeted backflashover rate of 0.6 flashovers/100km/year. This 15 Ω footing resistance will be achieved through use of vertically driven ground rods and horizontal counterpoise electrodes. While a strike distance of 3.7 meters was found to be sufficient to fulfill insulation requirements with respect to switching overvoltages, tower insulation for this new EHV transmission line will be governed by lightning rather than the more usual switching surges.
Insulation on the new 500 kV line will particularly vulnerable to the impact of severe service conditions including heavy conductive icing and melting. Selection of suitable insulator geometry, material and string design for this environment is therefore regarded as critical. Traditionally, NTDC 500 kV lines have been insulated using anti-fog profile porcelain insulators with Clevis and Tongue couplings, as found on most existing 132 kV and 500 kV lines in this part of Pakistan.
But now, following roundtables and technical meetings between NTDC transmission line design staff and international experts, focused on outdoor insulation of EHV systems, alternatives to porcelain are being investigated. This also includes possible new string design, shed profile and couplings. Tables 3 & 4 show the characteristics of insulators and towers generally used on NTDC 500 kV systems.
In spite of traditional proven design of most 500 kV lines in Pakistan, the challenging environment in the north highlights the importance of mitigating the impact of lightning through measures such as small shielding angle and low footing resistance. One of the special concerns is that lightning flashes could lead to electrical puncture, cracks or other invisible perforation in porcelain discs, making detection of damaged strings difficult if relying only on visual observation using binoculars. Moreover, foot patrols themselves will also be problematic, especially during the snowy winter. The constraint of mostly inaccessible terrain combined with the wintry climate will limit the effectiveness of ground-based inspection and, according to NTDC engineers, has opened the door to application of other types of insulators that are either easier to inspect from afar or where any damage is easier to detect using other methodologies.
New NTDC Outdoor Insulation Strategy & Future Vision
Deputy Managing Director, Rana Wajahat Saeed, explains that NTDC’s entire philosophy in regard to outdoor insulation is in the midst of re-examination and possible change. For example, RTV silicone coatings are being applied for the first time with the goal of improving insulator performance in severely polluted regions of the south. Moreover, toughened glass insulator strings, designed with an anti-swing V-configuration, are being actively considered for application on new 500 kV transmission lines being planned for northern Pakistan. In addition, he states that the NTDC has now also decided to conduct a global pollution assessment and mapping effort, while also reviewing the effectiveness of current programs for mitigating corrosion on towers and hardware. An audit of existing outdoor insulation practices and designs will be part of this new approach to selection of insulation and include construction of three naturally polluted insulator test stations. The goal will be to identify the most suitable RTV coating material as well as types of insulators for local service environments and to develop string designs that offer more efficient pollution performance – especially in contaminated areas of the south.
Mr. Znaidi and INMR extend deep appreciation to the following NTDC and NESPAK staff whose co-operation was invaluable in realizing this article:
Rana Wajahat Saeed, Deputy Managing Director, NTDC
Khawaja Riffat Hussain, General Manager, Design & Engineering, NTDC
Manzoor Ahmad, General Manager at ‘Project Delivery South, NTDC
Muhammad Iqbal, Chief Engineer, Line Design, NTDC
Aamer Latif, Manager Design, Transmission Lines, NTDC
Anees Ahmed, Manager Design, Transmission Lines, NTDC
Zulfiqar Ali Solangi, Executive Engineer 500 kV T/L Maintenance Div., Southern Region, NTDC
Ikram Latif Wain, General Manager, Civil Power & Mechanical Div., NESPAK
Shakir Hafeez, Project Manager, NESPAK
Usama Ahmed, Sr. Transmission Engineer, NESPAK