| Point$ About Transformer Products
Each of these ValuePoints Bulletins is a handy reference that discusses
specific points about transformer product applications. Click on the
bulletin number at the end of each description to open the pdf file
containing the full document.
Copper vs. Aluminum Conductor
There is a common misconception that a distribution transformer with copper windings is in some way more efficient, more reliable, or has higher short circuit strength when compared to a transformer with aluminum windings.
Bulletin 99028
Rectangular vs. Round Coils
Manufacturers promote product design features to give themselves competitive advantage in the marketplace. Potential customers may be influenced by these promotions, and act without careful consideration of alternatives. In transformers, specifying round coil designs does not necessarily result in benefit to the customer. In some cases, it may even cost the customer more.
Cooper Power Systems' transformer designs meet all ANSI Short Circuit and other performance requirements, and may provide significantly better value than competitive designs featuring round transformer coils.
Bulletin 01001
Bay-O-NetTM with ELSP Fuse vs. Drywell Canister Fusing
To protect against available fault currents above 3500 amps, there are two functional choices for overcurrent protection of pad-mounted transformers: 1) the Bay-O-Net system, (consisting of an under-oil expulsion fuse in a loadbreak holder in combination with a back-up partial-range current-limiting fuse mounted under oil on a panel inside the transformer tank), or 2) a drywell canister fuse system (consisting of a single sand-filled fuse with high-current and low-current filaments). Because each system protects against fault currents up to 50 kA, limits the energy allowed into the transformer tank, and offers the features of loadbreak/loadmake capability, deadfront construction, and hotstick fuse removal, there is a common misconception that these two choices provide the same protection at the same price.
Bulletin 99026
Voltage Regulators vs. Load Tap Changers
As a means to regulate substation voltage, some utilities use a substation transformer with a Load Tap Changer (LTC) in one package. However, if one phase of the tap changer fails or requires maintenance, the entire unit is removed from service, affecting all three phases. Longer service outages, decreased load-control flexibility and increased cost can result.
Bulletin 99032
VFI Transformer vs. Switchgear
A transformer equipped with an integral Vacuum Fault Interrupter (VFI) can be a superior alternative to a non-VFI transformer in combination with stand-alone switchgear.
Bulletin 99031
Transformer Coating System
Keeping a transformer painted in the field is important, not only because it extends the operational life of the unit but also because the public expects utilities and industrial facilities to maintain the appearance of equipment installed near their property. With the cost of repainting a transformer in the field amounting to hundreds of dollars, and many transformers requiring repainting every 7 to 10 years, the durability of the factory finish can have a significant impact on maintenance budgets.
Bulletin 99029
Power Factor
The insulation power factor of a transformer is the ratio of the resistive current to the total current flowing in the transformer's insulation structure when tested under a sinusoidal voltage and prescribed conditions. (Another commonly used measurement, dissipation factor, divides the amount of resistive current by the amount of capacitive current flowing through the insulation.) A particular power (or dissipation) factor value is not necessarily significant. According to ANSI C57.12.90, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers, "A factory power-factor test will be of value for comparison with field power-factor measurements to assess the probable condition of the insulation. It has not been feasible to establish standard power-factor values for liquid-immersed transformers."
Bulletin 01027
Bolt-On vs. Weld-On Tank Covers
Many pad-mounted transformer specifications require welded-on covers because bolt-on covers are sometimes considered suspect in terms of their ability to maintain reliable seal integrity and tamperproof security, and prevent contamination of the insulating fluid.
Bulletin 99025
Isolated vs. Solidly Grounded Core
When large transformers with round cores are transported, they may be vulnerable to shipping damage. If a round core is not tightly clamped or secured, it has the potential to shift and be damaged during shipment. An isolated core — versus a solidly grounded core — is sometimes requested to enable testing to detect such damage.
Bulletin 00069
Bolt-On vs. Weld-On Radiators
The use of radiators to reduce operating temperatures of distribution transformers is a standard design practice. Most transformer radiators are attached by welding the radiator header to the tank wall. However, a more efficient alternate design may call for radiators bolted to a valve assembly which, in turn, is welded to the tank wall.
Bulletin 00057
Harmonic Loads
In recent years, harmonic loads on distribution systems (in which currents at higher-frequency multiples of the fundamental 60Hz frequency add to the fundamental current) have increased dramatically due to the increased use of ferromagnetic devices (motors, transformers), arcing devices (fluorescent lighting, welders) and electric power converters (adjustable-speed drives, UPS, SCR devices). Such higher frequency loads increase the heat generated within transformer windings and leads by as much as 300%. This can lead to premature failure of standard-design distribution transformers.
Bulletin 00071
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