Coastal utilities like Southern California Edison (SCE; Rosemead, California) experience salt contamination when salt fog condenses on electrical equipment critical to delivering power to customers. The insulator pollution builds up gradually, but does not decrease the insulation strength when the insulators are dry. But when the polluted insulators become wet, a conductive layer forms on the contaminated insulator surface, initiating leakage current. The line voltage flashes over this contamination, causing a line outage or relay operation. In most cases, several arcing periods may precede an actual flashover that results in an outage event. Most flashover outages are unpredictable and take several hours to remediate.
Contamination and Flashovers
SCE has been washing insulators for more than 40 years, so the utility seldom sees trips on its transmission and distribution lines caused by insulator contamination. SCE knows that salt and airborne contaminates such as dust and industrial emissions build up on transmission and distribution system equipment, increasing the potential for conductivity and arc-over at the insulators. SCE line workers participate in an ongoing program, particularly near coastal areas and inland agricultural areas, to mitigate this contamination challenge that goes with maintaining high levels of reliability.
SCE began a more deliberate insulator washing program back in the 1960s to prevent sediment buildup along the coast. Today, insulator contamination buildup and washing is no longer limited to ocean proximity. In other parts of the country, utilities are plagued with different types of insulator contamination, such as compounds used to melt and prevent ice.
With more than 10,000 miles of transmission and distribution lines in central, coastal and Southern California, SCE divides line washing into three strategies: mobile washing crews (called peashooters or mobile washers); structures and towers with quick-disconnect couplers, where spray guns are connected to the structure by the lineman; and permanently mounted deluge washing systems with spray nozzles.
It is standard procedure to wash while the system is energized. Typically, the only time SCE does a “cold wash” is when a contaminant-related failure takes out a line. Rather than running the risk of having multiple flashovers and repeated relay operations, which is hard on the circuit breakers and substation equipment, the utility takes the system out to wash.
SCE journeyman linemen and apprentices in the training program are responsible for the washing, which typically is done between April and October. All SCE transmission linemen are trained to do this work, and they rotate in and out of washer assignments depending on the overall needs of field maintenance operations. More than 15 crews and a fleet of more than 20 washer trucks are dedicated to cleaning insulators.
SCE’s traditional washer fleet includes 75-, 80- and 100-ft peashooters. The utility also has a fleet of pumper tankers and is currently adding more 80-ft washers and new 120-ft units from Altec Industries Inc. (Birmingham, Alabama) to the existing fleet. Most of the older units will be replaced due to the fact that some are as much as 20 years old.
SCE uses the 75- and 80-ft wash units on some of the lower transmission lines, but they also can wash distribution lines. They normally service lines along streets and can wash up to 200 poles a shift. The selection of the washer units is determined based on a combination of meeting pole height, access requirements and setup time. The 80-ft washer units have straight-down outriggers for setup and stabilization at each pole structure to minimize impact into vehicle traffic lanes.
The HW120 model is considered adequate for most of the transmission-tower requirements at SCE. It has a much larger outrigger spread to stabilize the longer boom, and its productivity varies by the terrain it has to negotiate.
Each peashooter typically contains a 1500- to 2000-gal tank and a water pump to deliver high-pressure water to the wash gun attached at the end of the boom.
Many of SCE’s towers are plumbed with pipe and quick connects. The washer ground crew connects the pumper unit to the piping at the base of the structure. The lineman climbs the tower carrying a 10-ft hose and a wash gun, which is connected to a quick-connect coupler at the top of the structure. The lineman then signals the ground crew to turn on the water and proceeds to wash the insulators that support conductors on that structure.
Another type of washer application, called deluge washing, also involves permanently mounted equipment at each tower. At many of SCE’s 230-kV double-circuit towers, 12 sprinkler heads are permanently mounted and strategically located at the top of insulator strings on the tower. In these cases, no climbing is required. The washer crew simply attaches the pumper tanker unit at the base of the tower and turns on the water at a lower pressure. This washing process delivers a steady stream of water, drenching and washing the sediment from the insulators.
Driving along the Southern California coast on a calm morning, you may see a SCE washer crew with nozzles high in the air spraying insulators, but you won’t see arcing and flashovers thanks to SCE’s preventive washer program.
Gregg Patterson is a technical specialist in the Construction Methods department at Southern California Edison, where he is responsible for reviewing new vehicle and equipment products.