ELECTRICAL MAINTENANCE

Transformer Maintenance: The Cheapest Form of Insurance
Part 1: The Transformer Killers

Since the early stages of domestic electricity use the transformer has formed the backbone of the electrical distribution system. Its main purpose is to convert voltage at the generating end to transmission voltages and then to convert it back at the receiving end for utilization voltages. Transformers are used in every facet of our homes and businesses.

The power transformer is very simple in design and construction. In an ideal world it would require very little maintenance, since there are no moving parts to speak of. However, because of the intense voltage stress, mechanical forces and heating that they are subjected to planned maintenance is an absolute must.

Transformers fail without notice because to the untrained eye they rarely display any obvious symptoms of an impending disaster. The transformer in figure 7 was on line for six months after it started to show symptoms of a failure. It eventually suffered a total failure of the center phase.

In most cases, however, the root cause of the failure can be detected, prevented or eliminated. In order to ensure long life of a transformer, it is essential to eliminate the external risk factors by performing regular preventive maintenance inspections.

In this series of articles we will discuss the basic construction of transformers, describe the most common risk factors that cause transformer failure and describe the simple steps to eliminate them through planned predictive maintenance practices.

POWER TRANSFORMER CONSTRUCTION
In order to properly maintain a transformer it is essential to have a working knowledge of how it is constructed and operates. Essentially, there are three basic types of transformers. These are categorized as:

• Dry Type
• Oil Filled
• Fluid Filled

The simplest transformer is the dry type unit. Its construction is based on a grain oriented silicone steel core surrounded by the primary (High Voltage) and secondary (Low Voltage) windings that are typically made of either copper or aluminum.

The current flow through the primary winding induces a magnetic flux in the transformer core that in turn induces a voltage on the secondary winding.

The voltage is converted based on the ratio of the number of coils or "turns" in the primary winding vs. the number of "turns" in the secondary winding. All transformers, regardless of their type ,operate on the same principle. Transformers can be used to step voltages up as high as +700 kV and step down as low as 120 volts.

The primary insulation material in dry type transformers is air, Nomex Aramid™ paper, silicone coated fiberglass, Nomexª sleeving and porcelain.

Oil-filled transformers are essentially the same construction except that the core and coil assembly is placed in a tank and the tank is filled with high dielectric cooling oil. The primary insulation system used in an oil-filled transformer is Kraft paper, wood, porcelain and, of course, oil. Modern units use paper that is chemically treated to improve its tensile strength properties and resistance to decay caused by immersion in oil.

The oil's primary function is to transfer heat from the core and coils to the external radiator banks where the heat is then distributed to the surrounding air.

Connections to the coil assembly are made through bushings that are typically made from epoxy or porcelain. Usually, lower voltage bushings are epoxy and higher voltage units are porcelain.

Liquid-filled units are exactly the same as oil filled transformers except that they use a fluid for cooling other than oil such as Siliconeª, R-Tempª and PCBs like Askeralª or Inerteenª. These types of fluids are used because of their greater flash points, thus allowing large transformers to be installed in areas where fire is a concern.

TRANSFORMER KILLERS
All transformers fail the same way all the time. A failure occurs when the insulation system becomes so badly compromised to the extent that it can no longer act as an insulator to the high voltage that it contains.

Tables 1 and 2 describe some of the reasons why transformers fail.

Table 1 indicates that 52.8 per cent of transformer failures are preventable.

You can see in Table 2 that 73 per cent of transformer failures are caused by insulation breakdown.

Therefore, the key to extending the life of your transformer is to keep the insulation in good condition. In order to maintain the quality of the insulation we must understand what causes its degradation.

The greatest risk factors are:

• Voltage Transients
• Heat
• Moisture
• Dirt
• Insulating Fluid Decay

There is little we can do to protect our transformers from these occurrences except make sure that the lightning arrestors and station grounding systems are in good working order.

External Short Circuits
External Short Circuits will cause damage to transformer windings because they create massive electromechanical forces and currents that subject the winding assemblies to incredible forces.

This type of short circuit is always on the downstream side or secondary of the transformer.

The key to preventing damage from this circumstance is to ensure that the protection and control system is fully operational and the settings are proper to provide full utilization of the transformer's capacity and provide full protection to it against downstream faults.

Typically, utility transformers are more subject to this type of problem because of their extensive distribution system. Industrial, commercial and institutional transformers are also exposed to this risk because the protection system settings are rarely reviewed. This type of protective device coordination review should be done as part of any comprehensive maintenance program at least every five years.

Poor Workmanship
Poor workmanship during the manufacturing process can be detected through the process of factory testing and inspections prior to shipping, then, detailed commissioning tests when the transformer is finally assembled on site.

Site testing and commissioning also provides and excellent baseline of test data for future maintenance testing. Defects of this nature will usually cause a transformer to fail in the very early stages of its life.

Heat and Overloading
Heat and overloading are slow killers which reacts against the insulation by causing the physical strength to decay over a long period of time. Eventually the insulation material will break, crack or simply fall apart. This will be accelerated when the core and coil assembly expands and contracts through normal temperature variations or from the inherent vibration found in a transformer. Once a void is created, it will eventually expose an energized conductor. If the conductor is exposed in a critical area, an electrical arc will be created and a winding failure will result.

Moisture
Moisture will enter the transformer during its normal breathing process as the oil expands and contracts and will eventually contaminate the cooling fluid and, eventually, the insulation.

Excessive build up of moisture will saturate the insulation and allow it to become conductive.

Water will always tend to migrate to the paper insulation rather than remain in the oil. If you have test results that show high dissolved water levels in your oil, you can be sure that there are far higher water levels in the paper insulation. It is not uncommon to see failed transformers containing gallons of water in the paper.

Dirt
Dirt will contaminate insulation surfaces allowing the formation of conductive paths along the surfaces and eventually to ground.

Dirt is a large concern in dry type units simply because their coils are exposed. Periodic inspections and cleaning will eliminate this risk as will the addition of dust filters in extreme conditions. Oil transformers are also at risk from dirt due to contamination on exterior bushing surfaces and contamination of the internal systems resulting from the build-up of sludge that we will discuss later.

Insulating Fluid Decay
Insulating fluid decay is the single greatest cause for power transformer failure. The insulating fluid serves two major purposes. First it transfers heat from the core and coil to the exterior tank radiators. Second it acts as an insulating medium.

Over the course of the oil's life it is exposed to high temperatures, oxygen and water. Its interaction with the steel of the tank and core plus the copper and aluminium of the windings will eventually cause the chemical properties of the oil to decay. This process is known as Oxidation.

Oxidation is the chemical combination of unstable hydrocarbons and oxygen. Heat and water are catalysts to this reaction. Vibration, electrical stress, copper and aluminium are accelerators.

As the oil becomes further oxidized it will form fatty acid compounds that settle out of suspension in the oil and rest on the coils, coil cooling ducts, radiator fins and tank floor.

Eventually this sludge will impede the cooling capabilities of the oil because its ability to circulate through the core and coils is greatly reduced and can therefore no longer transfer heat away from them.

Chemically speaking, the fatty acid build-up will cause the insulating paper to weaken and loose its tensile strength.

By looking at these risk factors and breaking each one down into its simplest terms, one common denominator reveals itself in that the insulation system is the key to a healthy transformer.

Since the internal workings of an oil filled transformer are not exposed for easy inspection as is the case with a dry type transformer, the warning signs and symptoms are harder to detect. All of the risk factors will cause damage that, in many cases, is slow acting to the point of failure. This means oil filled transformers operating in a high risk environment can be a ticking time bomb unless predictive and preventive action is taken.

Planning Regular Maintenance
When setting up a maintenance program the key is to get the most efficient use of your maintenance dollar based on the risk factors and importance of the transformer.

Therefore, when planning a maintenance program consideration should be given to these key questions:

Q: How important is this transformer to my operation and what is affected if it is lost?
A: If a transformer has a global effect on your operation, directly affects a key process and has no backup available then it should be considered a high priority

Q: How old is the transformer?
A: Older transformers tend to be more robust in their design as far as withstanding overloading and heat because they contain more oil than newer units. However, they are more subject to failures of external components and gaskets because of their age.

Q: What is the shape of the load profile?
A: If the load is steady state, then there will be less internal mechanical stresses. A load profile that has drastic fluctuations will cause heavy mechanical stress on the coil assembly. Processes like arc furnaces and welding cause the greatest wear and tear.

Q: What is the surrounding environment like?
A: Units that operate inside tend to suffer more from overheating because of poor air flow. They may also be exposed to harsh environments depending of the process being operated at the facility. Outdoor units tend to be less sensitive to the environment but are more at risk because of things like vandalism, animals and weather conditions.

Start the planning stage by identifying the most important transformers in your system. These will be easy to find since they are the ones that if lost, will cause the greatest effect on your plant or production process.

Once you have a list of the important transformers defined, then determine what is their largest risk based on:

• Importance
• Age
• Loading
• Environment

In an ideal world it would be great if we could totally eliminate the risk factors but this is often impossible because it usually means making large changes to how your facility operates. This can involve large capital expenditures that are often prohibitive.

Fred G. Tanguay is Field Service Division Manager, Black & McDonald Limited. Next Month: The Cost of Failure. ET