Power Transformers:Transformer Failure Modes.

Transformer Failure Modes

The failure of a power transformer is almost always a catastrophic event that will cause the system to fail. The two primary enemies of power transformers are transient overvoltages and heat. Power input to a transformer is not all delivered to the secondary load. Some is expended as copper losses in the primary and secondary windings. These I 2R losses are practically independent of voltage; the controlling factor is current flow. To keep the losses as small as possible, the coils of a power transformer are wound with wire of the largest cross section that space will permit. A medium-power, three-phase power transformer is shown in Figure 4.24.

A practical transformer also will experience core-related losses, also known as iron losses. Repeated magnetizing and demagnetizing of the core (which occurs naturally in an ac waveform) results in power loss because of the repeated realignment of the magnetic domains. This factor (hysteresis loss) is proportional to frequency and flux density. Silicon steel alloy is used for the magnetic circuit to minimize hyster- esis loss. The changing magnetic flux also induces circulating currents (eddy currents) in the core material. Eddy current loss is proportional to the square of the frequency and the square of the flux density. To minimize eddy currents, the core is constructed of laminations or layers of steel that are clamped or bonded together to form a single magnetic mass.

Thermal Considerations

Temperature rise inside a transformer is the result of power losses in the windings and the core. The insulation within and between the windings tends to blanket these heat sources and prevents efficient dissipation of the waste energy, as illustrated in Figure 4.25. Each successive layer

acts to prevent heat transfer from the hot core to the local environment (air).

The hot spot shown in the figure can be dangerously high even though the outside trans- former case and winding are relatively cool to the touch. Temperature rise is the primary limit- ing factor in determining the power-handling

Windings

capability of a transformer. To ensure reliable operation, a large margin of safety must be designed into a transformer. Design criteria include winding wire size, insulation material, and core size.

Life Expectancy and Temperature

The nameplate kVA rating of a transformer represents the kVA that will result in the rated average wind- ing temperature rise when the unit is operated at 100% of rated kVA under normal service conditions [8]. When operating under these conditions, the result should be a normal life expectancy for the trans- former.

A general rule of thumb says that a 30-year life can be expected for a transformer with a 220°C insulating system that has a winding hot-spot temperature allowance of 30°C. Operating such a transformer at rated kVA on a continuous basis with a 30°C, 24-hr average ambient (40°C maximum ambient) should equate to a normal useful life.

It should be recognized that the life expectancy of transformers operating at varying temperatures is not accurately known. Fluctuating load conditions and changes in ambient temperatures make it diffi- cult, if not impossible, to arrive at such definitive information. However, if a transformer is operated under normal conditions, it could easily last longer. A 40-year life span is not unusual, and some trans- formers have exceeded that.

Voltage Considerations

Transformer failures resulting from transient overvoltages typically occur between layers of windings within a transformer. (See Figure 4.26.) At the end of each layer, where the wire rises from one layer to the next, zero potential voltage exists. However, as the windings move toward the opposite end of the coil in a typical layer-wound device, a potential difference of up to twice the voltage across one complete layer exists. The greatest potential difference, therefore, is found at the far opposite end of the layers.

switched on or when a transient overvoltage is impressed upon the device, the voltage distribution from one hot layer to the next can increase Cs Cg dramatically, raising the possibility of arc-over.

This effect is caused by the inductive nature of the transformer windings and the inherent distributed capacitance of the coil. Insulation break- down can result from one or more of the following:

of a power transformer.

• Puncture through the insulating material of the device

• Tracking across the surface of the windings

• Flashing through the air

Any of these modes can result in catastrophic failure. Figure 4.27 illustrates the mechanisms involved. A transformer winding can be modeled as a series of inductances and shunt capacitances. The interturn and turn-to-ground capacitances are shown by Cs and Cg, respectively. During normal opera- tion, the applied voltage is distributed evenly across the full winding. However, if a steep front wave is impressed upon the device, the voltage distribution radically changes. For the voltage wave to start dis- tributing itself along the winding, the line-to-ground capacitance (Cg) must be charged. This charging is dependent upon the transformer winding-to-ground capacitance and the impedance of the supply line.

Mechanical Considerations

Current flow through the windings of a transformer applies stress to the coils. The individual turns in any one coil tend to be crushed together when current flows through them. There also may be large repulsion forces between the primary and secondary windings. These mechanical forces are proportional to the square of the instantaneous current; they are, therefore, vibratory in nature under normal operating conditions. These forces, if not controlled, can lead to failure of the transformer through insulation break- down. Vibration over a sufficient period of time can wear the insulation off adjacent conductors and create a short circuit. To prevent this failure mode, power transformers routinely are coated or dipped into an insulating varnish to solidify the windings and the core into one element.

Dry-Type and Liquid-Filled Transformers

The advantages and disadvantages of dry-type transformers vs. liquid-filled units depend upon the application [9]. Dry-type transformers can usually be located closer to the load, resulting in cost savings because of shorter cable runs and lower electrical losses. A liquid-filled transformer, on the other hand, may require special con- struction features for the room in which it will be placed because of fire-safety considerations. This may dictate a location some distance from the load. In addition, periodic testing must be conducted on the fluid to deter- mine its dielectric strength, water content, dissolved gases, and other parameters.

In some applications, there is no option to the use of liquid-filled transformers; dry-types are limited in size and voltage handling capability. Liquid-filled types are available in almost limitless kVA and volt- age ratings. Also, if requirements call for a transformer to be located outdoors, it may be less expensive to purchase a liquid-filled unit. With oil as the liquid, the cost would be lower than for a dry-type of equiva- lent rating; with low-firepoint fluids, the cost would probably be comparable to a dry-type.

For liquid-filled transformers, the main cooling/insulating mediums used today are mineral oil, high-molecular-weight hydrocarbon, and silicone fluid. If a leak occurs in the transformer tank, fire safety becomes an important issue. Because of hazards associated with tank rupture and the possible ignition of the dielectric, a thorough analysis covering fire safety and the possible effects on the environment should be carried out well in advance of device installation.

Some materials are covered under the Federal Resource Conservation and Recovery Act and the Clean Water Act, including requirements for

• Special handling

• Spill reporting

• Disposal procedures

• Record-keeping

These considerations can have an effect on installation costs, long-term operating expenses, and maintenance procedures.

Insulation Materials

Liquid-filled transformers use an insulation system of kraft or aramid paper, pressboard or aramid spac- ers, and a fluid that serves as both an insulating and cooling medium for the transformer [9]. Paper is commonly used for insulation between layers of winding material. It typically has an diamond-patterned adhesive backing that, when cured, solidly contains the winding. Spacers serve as a form (which can be rectangular or cylindrical in shape) for the windings as well as a spacer between layers of the windings. The spacing is necessary to allow the insulating fluid to flow through and cool the windings and the core. Spacers are also used to insulate the windings from the core as well as to support the leads on their path to the bushings.

Any moisture that is present in the finished core and winding assembly is purged by vacuum and oven drying processes. After removal from the oven and while still hot, all connections are tightened and the entire assembly is immersed into its liquid-filled tank. This ensures that moisture will not again pen- etrate into the windings and also allows the insulation to absorb the maximum amount of dielectric fluid.

The particular type of insulation used is rarely specified by the customer for other than large utility substation transformers or for unusual applications. More often, it is the transformer manufacturer’s or rebuilder’s choice based upon the operating conditions the transformer must meet.

Insulating Liquids

Dielectric liquids of various types are used as an insulating medium as well as a means of cooling liquid- filled transformers [9]. Common insulating liquids include the following:

• Mineral oil. A mineral oil-filled transformer is generally the smallest, lightest, and most econom- ical transformer available. Mineral oil has excellent properties for use in transformers, but it has the inherent weakness of being flammable. Its use, therefore, is restricted to outdoor installations or when the transformer is installed within a vault if used indoors.

• Silicone. A wide variety of synthetic polymer chemicals are referred to by the generic term silicone. Silicone transformer liquids are actually known chemically as polydimethylsiloxane (PDMS). PDMS is a water-clear, odorless, chemically stable, nontoxic liquid.

• High-molecular-weight hydrocarbon (HMWH). HMWH is another high-firepoint dielectric that is widely used as a transformer liquid. It has similar values for dielectric strength and dielectric constant, power factor, and thermal conductivity as mineral oil.

Fire properties of dielectric fluids are typically classified by the following characteristics:

• Flash point: The temperature at which vapors from a liquid surface will ignite in the presence of a flame.

• Fire point: The temperature at the surface of a liquid that will sustain a fire.

• Flame spread: A series of consecutive ignitions.

• Ease of ignition: How readily the liquid will generate and maintain a flammable fuel/vapor mixture at the surface.

• Heat release rate: The product of vaporization rate and the heat of combustion of the fluid. The higher this rate in a large-scale fire, the higher the degree of fire hazard.

Selection of the dielectric liquid depends on the transformer application. Normally, the choice is mineral oil if the device is to be located outdoors. The National Electrical Code (NEC) does, however, specify certain limitations regarding the use of oil-filled transformers in particular outdoor locations. The selection of less-flammable liquids (PDMS and HMWH) often depends upon personal preference, the liquid used in other transformers on the site, or the transformer manufacturer’s recommendation.

Cooling

In cooling a liquid-filled transformer, the insulating fluid flows in the transformer through ducts and around the coil ends within a tank that contains the core and coils [9]. Removal of the heat from the fluid takes place in external tubes. These radiators consist of headers extending from the bottom and top of the transformer tank and rows of tubes connected between the two headers. When operating within its self- cooled (OA) rating, natural convection caused by temperature differences within the tank carries the oil up through the windings, down through the cooling tubes, and back into the tank. The transformer fluid, acting as a heat-transfer medium, picks up the heat from the core and coils and dissipates it to the air via the tubes.

Auxiliary cooling fans can be provided if the transformer is to be operated above its self-cooled rat- ings. This is advisable when the transformer is to operate under occasional heavy overloads or high ambi- ent temperatures, or to accommodate new loads beyond its rating. Liquid-filled transformers, because of their double heat-transfer requirement (core/coil-to-liquid and liquid-to-air), have a lower forced air (FA) rating than dry-types. In liquid-filled types, the forced air rating of transformers up to 2500 kVA is raised to 115% of its self-cooled kVA rating, and those of larger units to 125% of their self-cooled VA rating.

Cooling fans can be controlled manually or automatically. Fans can be cycled on automatically based on the top oil temperature, winding temperature, or ambient temperature. Alarm contacts and remote indication are also available options.

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