And The Results Will Surprise You

The Load Tap Changer (LTC) is a mechanical switching device and is the most expensive and vulnerable accessory of a power transformer. Tap changers cause more failures and outages than any other component of the transformer. They easily account for 40% of all transformer fires and 40-50% of maintenance cost.

The function of an LTC is to change the turns ratio without interrupting the load current. LTC failures are categorized as electrical, mechanical and thermal but most are mechanical in the beginning and develop into electrical faults. This occurs due to problems with the contacts, transition resistors and insulation fluid breakdowns due to contamination. The curious fact is that for maintenance, the tap changer is no more than an afterthought.

Failures and Protective Devices

Failures can generally be grouped as: 1) Mechanical, 2) Erosion of contacts) and 3) Contact coking leading to high resistance and overheating. Most serious faults inside the switching compartment generate large amounts of gas, powerful pressure pulses and a subsequent low oil flow to the conservator tank and trigger a rapid power switch trip of the transformer. In order to minimize the risk of fire or explosion from an internal failure of the diverter or selector switch compartment, several standards protective devices have been installed.

Protection Systems

There are four major protection systems used for on-load tap changers.

The current international standard, IEC 602114-1 states that in order to minimize the risk of fire or explosion resulting from an internal failure, the diverter or selector switch compartment shall be fitted with the following protective devices:

• Over-pressure relay
• Pressure relief device
• Liquid-flow controlled relay

The over-pressure relay responds in the event pressure in the oil exceeds a pre-set value, causing the transformer to be tripped. This is the first option in many transformers.

The pressure relief device opens when a pre-set pressure is exceeded. This system is secondary because it is slower to trip than the over-pressure relay.

The liquid-flow relay is installed in the pipe between the top of the tap changer and the conservator tank. It responds to a predetermined oil flow to trip the transformer.

A fifth protective device is an on-line, continuous oil filtration system, which will be described further below.

Anatomy of a Spark

The mechanical drive mechanism physically moves the position of the electrical contacts to select he appropriate ratio taps of the regulated winding. Differences in voltage between the tap positions cause arcing to take place as the electrical contacts connect and part.

Arc Gas Composition

Note: The most dangerous gas generated is Hydrogen. It is created in the largest quantity with every arc and it is the least soluble in oil and the most explosive of all gases created in an arc.

Continuous On-line Filtration

The Allen Tap Changer Filter System (ATF) is a completely self-contained single or multiple element filtration unit that can be designed to remove solids, adsorb gases as well as absorb small quantities of water on a continuous or timed basis, to keep the oil in the tap changer clean. Mechanical abrasion of the moving parts of the diverter switch is also reduced. Consequently, maintenance cost is minimized.

An electric motor-driven positive displacement gear pump forces contaminated oil through a filter element (or several elements) and returns clean oil to the Tap Changer compartment. The low capacity pump produces little or no turbulence in the OLTC oil compartment.

The Allen ATF’s are designed to operate unattended. They are enclosed in a steel weather-proof (NEMA 4) cabinet and the individual components are positioned for easy access and maintenance. Lifting lugs are provided for ease of installation.

Allen ATF’s are used on Power Transformers for the following applications:

• For applications with a very high number of switching operations (> 40,000) per year at high service currents;
• For applications at high insulation levels e.g. for line-end applications;
• For selector switches in transformers > or = 60 kV in delta connections.

Filtering the diverter tank oil improves the dielectric strength for a long period and reduces mechanical abrasion of the moving diverter switch parts thus prolonging the life of the actual tap changer.

A Typical Allen Tap Changer Filter System (ATF) in operation at CFE, Mexico