In the early morning hours of Nov. 8, 1998, major tripping started to occur on the 400-kV transmission line routed from Aqaba (a port on the Red Sea in Jordan) to Amman (the capital city of Jordan), causing a 30-minute shutdown of the country's entire electricity network. A thorough investigation conducted by the transmission system operator, the National Electric Power Co. (NEPCO; Amman), determined the location of the fault causing the circuit to trip was in the southern region of Jordan.
The transmission line, constructed for 400-kV operation, was built in 1985 and operated at 132 kV until December 1997, when the voltage was upgraded to 400 kV to prepare for an interconnection between Jordan and Egypt. Since the 400-kV transmission line was erected in 1985, this circuit from Aqaba to Amman — a route length of 326 km (203 miles) — had not been cleaned because the aerodynamic insulators were designed to withstand desert and dust pollution. However, over the years, thick layers of sand were deposited on the surface of the insulators.
The investigations undertaken by NEPCO and the insulator manufacturers found that the equivalent salt deposit density (ESDD) on the suspension insulators collected from towers in the area of the line outages was 0.2 mg/cm2 and the nonsoluble deposit density (NSDD) as 1 gm/cm2. The creepage distance when the line was constructed was not enough to withstand this amount of pollution, which should be an insulation level of 35 mm/kV. Increasing the creepage distance would be difficult, so NEPCO decided it must instead wash the insulators to prevent further system outages.
Transmission Line Pollution
Jordan suffers from four types of pollution: industrial, marine (salt), desert and dust pollution. The dust pollution occurs in semi-arid climates where the ground is not completely covered by grass or trees, and the topsoil becomes very dry and is scattered by the wind. It also occurs during construction.
Pollution accretion on dry insulators can result in an insulator flashover. Dry insulators normally have low conductivity, but rain or early morning dew can cause contaminated insulator surfaces to be conductive. The contaminated areas heat up, creating dry bands that occur due to the heating effect of the leakage current. These dry bands interrupt the continuity of the leakage current, and when the voltage gradient exceeds the air breakdown voltage levels, arcing takes place bridging the dry bands leading to a flashover.
Jordan's Climatic Conditions
In Jordan, rainfall is not abundant. However, before the rainy season starts — beginning usually between mid-October and early November — there are many days when the humidity is high, and the early morning dew causes insulator flashovers. The rainfall is insufficient to wash away the pollution, as the airborne pollutants tend to build up on the underskirt of the insulator rather than the topside of the main insulator shed. Experience has shown that even with tension and post insulators positioned in a horizontal configuration, the contamination occurs in the same area of the insulator.
Prior to upgrading the Aqaba — Amman line from 132 kV to 400 kV, these small outages had no major effect on the line. But, following the upgrade to 400 kV in 1998, the voltage stress was significantly higher. As soon as the insulators became wet, bridging of the dry bands occurred, resulting in significant reductions in the overall effective creepage distance. Hence, on the morning of Nov. 8, 1998, moisture on the insulators caused significant flashovers to occur along the line, which led to the major 30-minute blackout. Flashovers and resulting blackouts continued to plague the 400-kV line, leaving NEPCO no other option but to find a practical, efficient and economical way to clean the insulators.
For many years, NEPCO greased insulators or used manual dry cleaning to reduce pollution buildup, but these methods were expensive in terms of manpower and outage time. Furthermore, in the last decade, a number of 400-kV and 132-kV transmission lines have been built in Jordan. Most of the transmission lines built near Amman and in the hilly areas of Jordan have anti-fog insulators, whose inner surfaces are difficult to clean using manual methods. Figure 1 shows a polluted insulator before and after cleaning.
Live-Line Insulator Washing
For a number of years, NEPCO had considered adopting live-line insulator washing for its network. Following the November 1998 blackout, NEPCO decided it was time and floated an international tender for two mobile skid-mounted insulator-washing units. NEPCO awarded the tender in mid-1999, and in May 2000, the company received its first two insulator-washing units.
A weeklong intensive training course was held for NEPCO personnel at the NEPCO Electric Training Centre, which was established with a grant from the Japanese government in 1986. Commissioning of the washing units was completed, and in June 2000, the trained NEPCO personnel began live-line washing the insulators on the 400-kV line from Aqaba to Amman. The live-line insulator washing on a 400-kV insulator string undertaken by a lineman is shown in Fig. 2. In three short months, NEPCO cleaned the line insulators positioned on the 808 towers on the 400-kV energized line. After completing that line, the trained teams started insulator washing on further circuits on NEPCO's 132-kV network, continuing until the beginning of Jordan's rain season in October 2000.
In the first season of live-line insulator washing, NEPCO cleaned the insulators on 324 km (201 miles) of double-circuit 400-kV transmission lines and a further OHTL and 500 km (311 miles) of double-circuit 132-kV transmission lines OHTL. As a result, in 2000 no outages attributable to insulator pollution were recorded.
Expansion of the Live-Line Insulator-Washing Activity
The live-line insulator-washing season in Jordan starts in April and ends in October. In April 2001, NEPCO resumed cleaning the remaining transmission lines on the network that had not been cleaned the previous season. By the end of August 2001, NEPCO had completed cleaning the entire 400-kV and 132-kV transmission system comprising 1000 400-kV and 1000 132-kV towers. With two months left before the rain season in 2001, the company secured a contract with Irbid District Electricity Co. (IDECO; Irbid, Jordan) to begin insulator washing NEPCO's transmission lines. In 2001, NEPCO floated a tender for one telescopic insulator washer with remote nozzles some 50 m (164 ft) high. This unit was purchased for use on flat agricultural terrains. And in 2002, the company purchased three more mobile skid-mounted units.
In 2002, NEPCO expanded this business by securing contracts to wash insulators on the transmission lines of the Electricity Distribution Company (EDCO) as well as the private lines owned by the Potash, Bromide and Magnesium companies. Outages caused by polluted insulators ceased to exist on NEPCO's network and on the distribution lines NEPCO had contracted to be cleaned. In 2002, IDECO was the first distribution company to purchase a telescopic insulator washer, followed by Jordanian Electric Power Co. in 2003. By the end of 2003, NEPCO's overhead line maintenance department's fleet for live-line insulator washing was composed of five mobile skid-mounted units and one telescopic unit with remote nozzle.
Now in its fourth season of live-line insulator washing, NEPCO's overhead line maintenance department has 70 highly trained staff and six insulator-washing units, and each team washes insulators on approximately 18 (400-kV or 132-kV) double-circuit towers a day. Figure 3 shows one of NEPCO's live-line insulator-washing teams on-site with the mobile telescopic insulator washer.
Since adopting live-line washing on the overhead transmission line network (Fig. 4), interruption of supply due to pollution has almost ceased to exist. NEPCO's crews are highly trained and are constantly monitored to ensure correct safety procedures are being enforced during all insulator-washing operations. Also, strict safety policies are being followed by all our crews, whether on the ground or on the tower.
NEPCO's Safety Procedures
NEPCO adheres to strict safety guidelines and procedures during live-line insulator washing, and all line crews wear proper safety clothing, including harnesses, boots, hats and glasses. Washing equipment is grounded before any energized insulator washing commences; this includes the chassis, insulator-washing equipment and wash guns. The crew always checks water conductivity before insulator washing begins. This is of utmost importance because as the temperature rises during the day, the water in the tank will heat up, hence increasing the water conductivity. Water conductivity greater than 400 micro mhos is not allowed for washing insulators on high-voltage lines, 132 kV or higher.
Safe washing distances must be maintained by the linemen to ensure that the leakage current is limited to a safe level. The safe washing distances depend on line voltage; for example, at 132 kV the minimum safe distance is 4.57 m (14.9 ft) and at 400 kV the minimum safe distance is 5.49 m (17.9 ft).
In order to prevent a flashover from occurring during insulator washing, the following points must be taken into consideration:
Insulator washing should start on the lowest phase conductor.
Always undertake the washing so that the spray does not fall on dirty insulators, as this may cause a flashover.
Washing should always start at the bottom of the insulator, finishing at the top of the string.
Always wash then flush the string.
It is extremely important to consider the direction of the wind.
The trained teams encountered additional challenges in Jordan when conducting insulator washing on energized transmission lines:
The wind can be a major deterrent to washing; when it exceeds 30 km/hr (19 miles/hr), insulator washing must be stopped. Cross winds increase the amount of water required to clean the insulators, hence resulting in water wastage.
Jordan does not have lakes or rivers with low-conductivity water; therefore, distilled water is used for live-line insulator washing. The only available source of distilled water in the country is from the thermal power stations, which means the mobile field teams sometimes have to travel hundreds of miles to fill up their water tanks. NEPCO has reduced this problem by setting up permanent storage tanks along its transmission grid. The storage tanks are concealed out of the sun and kept in a cool environment to ensure the water's conductivity does not increase above 400 micro mho/cm, the maximum conductivity allowed for washing transmission lines.
NEPCO's insulator-washing fleet is currently made up of six units in the overhead line maintenance department and two units in the substation maintenance department. By adopting live-line insulator washing, NEPCO has significantly reduced outages, which has allowed its maintenance crew to concentrate on other jobs such as preventive maintenance and other works regarding the improvement of the transmission system.
As NEPCO's insulator-washing fleet expands, so does Jordan's transmission system. The transmission system comprises a 400-kV double-circuit line from Aqaba to Amman North Substation and a 400-kV single-circuit line from the Amman North Substation to the Syrian border, plus multiple 132-kV double-circuit overhead lines that branch out all over the country (Fig. 5).
Khaled Mahmoud Said was awarded a BSEE degree at Alleppo University, Syria, in 1975 and joined NEPCO. Starting out in system design, Said has held several senior management appointments in NEPCO's HV, Overhead Line Maintenance section and in the Overhead Lines department with responsibility for many major transmission projects, including the Jordan-Egypt and Jordan-Syria 400-kV interconnections. Since 2001, Said has been NEPCO's managing director assistant for transmission affairs. email@example.com
Abdel Fattah Ali El Daradkah obtained his BSEE degree from the University of Technology in Baghdad, Iraq. He joined NEPCO in 1979, starting as a site engineer in the EHV Construction Department. Subsequent career appointments include work in NEPCO's maintenance department, including a period as section head and a three-year appointment as resident engineer for the Taiz-Aden power-link project in Yemen. For the last four years, Ali El Daradkah has been NEPCO's transmission lines maintenance manager. He is an active member of Jordan's Engineers Association, and was appointed head of the Electrical Engineers Counsel in 2003. firstname.lastname@example.org
NEPCO's External Business Activities
NEPCO's long-term vision is to widen its services. In August 2004, NEPCO formed another service company called Energized Maintenance Engineering Co. (EMECO), which is 50% owned by NEPCO and 50% owned by Mitsa International Ltd. (Limassol, Cyprus), a company that specializes in providing solutions for live-line insulator washing and energized maintenance. EMECO focuses on energized maintenance training programs in live-line insulator washing, overhead line and substation maintenance, supervision of turnkey projects for overhead lines and substations, plus turnkey contracts for live-line insulator washing throughout the Middle East and Africa.