Quality includes aspects of temperature, cooling, insulation thickness, insulation dryness, mechanical properties, paint or coating selection and application, dimensional compliance, auxiliary wiring consistency, acceptance testing results etc. None of which are related to efficiency.
Higher losses will generate more heat, and heat is the number one cause of insulation failure. A properly designed transformer will take into consideration the losses, whatever they are and cool appropriately. When this is done, no additional loss of life will be attributed to the losses.
From a purely operational point of view, the location is the number one consideration. Low flash point fluids should not be used indoors. High flash point fluids can be used outdoor when transformers are used close to buildings or without fire barriers. There are also insurance considerations where high fire point fluids are preferred. Biodegradable fluids are especially desirable around bodies of water or other environmentally fragile sites. In some regions low temperatures will dictate special fluid performance that limits choices of suitable fluids.
Standard operating conditions are defined in ANSI/IEEE Standard C57.12.00. Where Altitude is 3000 feet or less, and average ambient is 30 for any 24 hour period and a maximum of 40°C.
As of 2016, DOE efficiency requirements apply to distribution transformers rated 2500 kVA and below, with HV of 34.5 kV and below with output voltages of 600 volts and below. They are further divided by application, but in general all distribution applications are covered. Special duty transformers, such as welding, grounding, test transformers, transformers with tap ranges greater than 20% and rectifier transformers are examples of transformers which are not included.
Technically they are the same, however, some applications require special construction that differentiates them. For high inrush applications, it is advantageous to have additional inductance between the energized input winding and the core to prevent core saturation. To accomplish this, the low voltage winding can be wound over the high voltage winding thus putting the HV between the LV and the core. Special impedance may also be required to limit inrush currents. Care must also be taken to put adjustment taps in the correct winding to control output voltage.
Local core saturation will heat the core damaging insulation. Saturation also causes distortion of the sinus wave which results in over-voltages. At the molecular level, the magnetic properties of the core steel changes when the material is “over fluxed” (exceeding the designed flux density in the region above the saturation knee point of the core performance curve). In this region the magnetic reluctance of the core increases and the core requires more energy to magnetize. This causes flux which would normally be “trapped” by the core to flow in other non-current carrying parts with a lower reluctance value, such as the core clamp and adjacent tank walls. In addition to increased magnetizing current, tank heating can burn paint off the tank and cause further coil insulation damage.
In addition to damaging cables, harmonics pose a real danger to the transformer coils. Harmonic currents can increase eddy currents flowing in the edge of current carrying conductors. This will increase loses and conductor heat, which poses a danger to the insulation. While harmonic voltage can distort the fundamental sinus wave and cause overvoltage spikes that drive the core into local saturation as discussed above.
Are grounding transformers designed different from conventional distribution or small power transformers?
Distribution and power transformers are designed for relatively steady-state loading and heating. Heat transfer takes place over time. Grounding transformers are designed for high current events that happen over a very short period of time. Heat from these short time events does not have time to dissipate in the typical manner and the coils must be designed to withstand the heat which is considered to be held in the conductors. Furthermore in order to provide a ground path for fault currents, the Grounding Transformer must stay in the circuit during short time events, whereas a distribution transformer is often taken off line by it’s protection scheme during a fault event. Extra consideration must be given to design the Grounding Transformer to withstand the mechanical forces it will see during fault events.
AS the name implies, a current limiting fuse is designed to interrupt current flow during a high current event. The special construction creates a break in the conductor path within one half a cycle. This gap is too great for voltage to ”bridge” once it crosses current zero and attempts to rebuild. This is called “forcing current zero”. In most CL Fuses, there is a material that adsorbs the energy to contain the arcing. A common material is silica. When subjected to high energy electric arcs, the silica forms a glass like material which further insulates the conductor gap to inhibit voltage re-strike.
A Sudden Pressure device reacts to a rapid increase in pressure. That is to say it is sensitive to the rate of pressure change, while a pressure relief device will relieve pressure after it has built up to a predetermined limit, regardless of how quickly it has accumulated. Since the sudden pressure device reacts instantaneously the relay will activate and immediately open again. For this reason, the sudden pressure deice is often coupled with a seal-in relay that once activated remains locked in position until it is manually reset.
Class 1 Division 2 are sites where explosive gasses or dust are present. It is important that any arcing devices be either (1) intrinsically safe or (2) be contained within an explosion proof enclosure. Monitoring devices with dry contacts can be rendered intrinsically safe by using a low voltage relay contained within an EP enclosure and sending a very low voltage signal through the devices exposed dry contacts. Other devise such as strip heaters can be designated as IS if they do not exceed the temperature limits based on the type of gas or dust at the site. This temperature will vary depending on the type of combustible products which are found at the individual site. These devices will carry a “T” code to designate the maximum surface temperature they will attain when operated in an ambient temperature defined by the device manufacturer. An associated concern is for externally located control wiring. All control wiring should be contained within sealed rigid conduit. Special connection boxes are sealed to prevent any possible arcing within the conduit from igniting resident gasses.
Strip heaters are used to prevent humidity from condensing within enclosures and conduits where exposed connections or contacts can corrode. Many times moisture will accumulate in conduit runs and drain into terminal connection boxes or wiring cabinets. In this case heaters are employed to protect those sensitive areas. It is acceptable practice to not use heaters unless there are relay contacts or terminal strips inside the enclosure.
While we tend to consider all stainless steel equal, it is in fact not the case. Some forms of SS are prone to surface staining with oxidation. Additionally, , if metal shot blasting is used when the device is prepped for painting, particles of ferrous metal can be imbedded into the surface and appear to rust later. Regardless of the source, surface staining is just that, surface discoloring and is not going to lead to metal failure.
According to ANSI/IEE C57.12.26, lifting devices should carry a security factor of 5.
Partial discharge is a condition where local dielectric stress exceeds the dielectric strength of the insulation, resulting in an ionization which can be measured. It is not a complete insulation flash-over but will result in insulation breakdown if allowed to continue. If the cause of the discharge is not found and remedied, it will result in transformer failure.
Recent studies indicate the average life of a transformer installed on the USA grid is around 40 years. This includes all transformers even though some applications, such as wind farm step deigns are subject to a much higher mortality. A common evaluating technique uses 20 years as the expected life span.
From a transformer design point of view both materials are equal. Copper can operate at a higher current density than Aluminum but with proper design, similar losses and thus similar cooling can be achieved. The copper design might be smaller, but with today’s pricing, it will also be more expensive. Taking the size difference into account, the weights will be similar also. Where copper gains an advantage is when the current is small and the appropriate aluminum wire size is also very small, copper conductor will offer better mechanical strength. There are also applications such as Grounding Transformers where repeated high current faults place emphasis on mechanical strength and higher temperature withstand limits that copper can offer.
There are benefits to each design and applications where each is better suited. Rectangular coils are well suited to steady loading normally associated with distribution loads. They are less expensive to build and lend themselves to mass production. These reasons make rectangular designs the popular choice for smaller distribution transformers. Circular designs are more mechanically resistant and offer more fault duty withstand and better cooling than rectangular designs. Medium voltage severe duty applications with varying loading are where these are well suited. Disc windings are the strongest , mechanically, of the three styles and offer the best cooling. This makes them ideal for very large transformers with high current loading. However, more importantly, disc windings have the lowest dielectric stress and are universally used for voltages of 69 KV and above . The choice of coil design for your next transformer should take these points into account.[/accordion_item]
This is a type of core steel which Is processed in such a manner to have the magnetic domains aligned to enhance it’s magnetic properties Grain-oriented electrical steel usually has a silicon level of 3% (Si:11Fe). It is processed in such a way that the optimum properties are developed in the rolling direction, due to a tight control of the crystal orientation relative to the sheet. The magnetic flux density is increased by 30% in the coil rolling direction.
Loss in the core is related to fringing flux and eddy currents in the core sheet edges. One effective way to control eddy current loss is to minimize the thickness of each sheet. For a given quality of steel, the thinner the sheet, the better the performance and the lower the losses.
Previously, I stated that heat is the number one enemy of transformer insulation. One reason is that heat breaks down the insulation and moisture is released which further degrades the insulation. Consequently removing moisture from the insulation prior to impregnation is critical to long insulation life. Vacuum processing when coupled with heat will lower the moisture content from 8% normal shelf value to less than 1% moisture content in the paper when fully dried. The vacuum is needed to reduce the vapor pressure curve for final drying. Without vacuum, the moisture content is around 2% using heat alone. Vacuum further enhances the filling and impregnation cycle. Air is removed from the dielectric fluid during filling by slowly “splashing” the fluid in the transformer vessel and trapped air is removed from the coils before impregnation to enhance the dielectric qualities of the coils.
ANSI/IEEE C57.12.13 1982 Table 1 gives minimum external clearances for live parts phase-to-phase and live parts live-to-ground for transformers.
Power factor is used as an indicator of moisture content in the coils. Since moisture is a major contributor of insulation failure, having a low power factor is significant for assuring transformer insulation quality. The absolute value of acceptable power factor varies from a new transformer to one that has been installed for sometime. ANSI/IEEE does not publish a value due to the complexity if contributing factors. Most manufactures will guarantee 1% or less on new transformers, but it is not uncommon to have requests for .5% for new transformers with mineral oil.
The 120°C rating is the total temperature which includes ambient temperature, coil average temperature rise, and a hot spot allowance as defined in ANSI/IEEE C57.12.00. ANSI/IEEE defines standard ambient temperature of 30°C average for 24 hours with 40°C maximum. So you must consider the 40°C +65°C coil rise above ambient=105°C plus 15°C hot spot allowance to total 120°C.