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On-Load Tap Changer Filter Systems

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AGC REFINING & FILTRATION

ON-LOAD TAP CHANGER FILTER SYSTEMS 2

Contents Problems in Tap Changers 4

The Tap Changer Filter System 4

Equipment Description 5

Operation 6

Conclusion 7



On-Load tap changers are used for changing the transforming ratio of power transformers without supply interruption. To reduce arcing at the contacts, the tap changer operates in a chamber filled with insulating transformer oil. Some arcing is unavoidable and both the tap changer oil and the switch contacts will slowly deteriorate. The arcing produces the same contaminants in the tap changer as in the transformer. These are the fault gases such as hydrogen and acetylene as well as carbon particles. A change in ambient temperature causes condensation to accumulate in the tap changer compartment. This, combined with the other contaminants, slowly degrades the dielectric strength to such a degree that it, if left unattended, could cause a failure of the tap changer.

The process of changing tap positions includes arcing, which generates gases and carbon particles. The movement of contacts also promotes erosion of the metal and generates small metal particles. In the long run, those particles affect the dielectric properties of the oil. Moreover, carbon particles tend to accumulate on contacts and increase contact resistance. They also accumulate on connecting boards and may lead to tracking on the surface of the board. The reliability of a tap changer can be improved by a simple filtering device to keep the oil clean.

In a tap changer, contacts that operate frequently usually remain clean. The ones that do not move tend to oxidize and collect carbon. With time, contact resistance increases, which produces more heat.

The diverter switch and the tap selector are the only moving parts in a transformer. The diverter switch does the entire load making and breaking of currents while the tap selector pre-selects the tap to which the diverter switch will transfer the load current. The tap selector operates off-load and therefore needs no maintenance.

Illustration courtesy of ABB Type UZF Tap Changer



Problems in Tap Changers

Problems in tap changers can occur in a number of areas. Improper contact pressure at the tap contacts can cause hot spots and result in thermal degradation of the switch and the oil. In these hot spots, carbon precipitates out of the oil onto the heated surfaces and forms a granulated, carbon-based deposit called coke. The deposit acts as an insulator and causes more overheating, leading to thermal failure. Over-voltages due to switching or lightning impulses produce voltage stresses at the tap connections, which can cause insulation breakdown. Copper and carbon deposits are often found on horizontal surfaces of the diverter switch due to small convection currents in the oil that are caused by each tap change.

The Tap Changer Filter System

The Allen Tap Changer Filter System is a completely self-contained automatic system with single or multiple filter elements that is designed to remove solids, moisture, and gases on a continuous basis to keep the oil in the tap changer clean. Mechanical abrasion of moving parts on the diverter switch is also reduced with the removal of the hard carbon particles.

Model TP-1



Equipment Description

All Allen Tap Changer Filter Systems are designed to operate unattended and automatically. All components are easily accessible, yet totally enclosed in a stainless steel, weather-proof cabinet. The cabinets are provided with lifting lugs. The design is uncomplicated and cost-effective.

The Allen Tap Changer Filer System is completely enclosed in a stainless steel weather-proof cabinet.

Pump and Motor

The electric motor is energy efficient, totally enclosed, fan-cooled, and close-coupled to the pump. The motor and the circulation pump are reversible so they can be used to fill or drain the tap changer. The pump is a positive displacement gear-type pump with a capacity of 3 or 5 gallons per minute. These low pump flows prevent unnecessary turbulence in the tap changer compartment. The inlet, outlet, and fill lines are .25-inch stainless steel with Swagelok™ compression fittings.

Allen Tap Changer Filter Systems utilize an energy efficient, totally enclosed, and fan-cooled electric motor.

The Filters

The system can be supplied with up to three different types of cartridge filters. The first is generally a 5-micron solids filter. Most Tap Changer Filter Systems use only this type of filter to remove the hard carbon particles that are the result of internal arcing. Additional media filters include water absorbing filters and



fault gas adsorbing elements.

The filter vessel with solids filter element.

Accessories

Standard equipment is a cabinet heater, which—combined with an exhaust fan—is controlled by a thermostat. The exhaust fan is externally protected by an insect screen. An analog or digital timer controls the operating times of the system. A control box contains the electrical wiring and fuses. The system can be provided with remote data transmission combined with a GPS locater. The signal can be transmitted to a central location such as the head office. Alarms can also be provided for high filter differential pressure, motor or temperature malfunctions, or oil leaks. The system has a standard leak detection device that shuts the operation down if an internal oil leak occurs.

Addition Allen Tap Changer Filter System components include a leak detection device, control box, and exhaust fan.

Operation

The dielectric properties in the tap changer tank are kept at optimum conditions by constantly removing:

carbon particles and other types of solids,

moisture with the moisture adsorbing filter element, and

fault gases with the activated carbon filter element.

Periodic sampling of the oil for moisture, solids, acidity, and dissolved gases will keep the tap changer in



operation for a long time.

Conclusion

Load tap changers (LTCs) are a crucial component of power transmission networks. They must operate with great reliability and precision in order to maintain a constant voltage output. LTCs have been a weak link in many electrical power networks as they deteriorate over time due to mechanical problems or contamination of the insulating oil. Recent developments in the power industry are placing more emphasis on reducing out-of-service time and minimizing down time and maintenance activities.

The Allen Tap Changer Filter System is the sentinel that operates 24 hours per day—completely independent of the on-load tap changer—consistently removing solids and moisture, thereby maintaining a high dielectric strength.

www.AGCInternational.com

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Springfield, Missouri 65802, USA

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