In power transformer as the most important induction power transfer device when flux flows in the steel core, losses occur in the steel. There are two components of this loss, which are termed “eddy” and “hysteresis” losses. An explanation of these losses would require a full chapter.
For the purpose of this text, it can be stated that the hysteresis loss is caused by the cyclic reversal of flux in the magnetic circuit and can be reduced by metallurgical control of the steel. Eddy loss is caused by eddy currents circulating within the steel induced by the flow of magnetic flux normal to the width of the core, and it can be controlled by reducing the thickness of the steel lamination or by applying a thin insulating coating.
Eddy loss can be expressed as follows:
W = Kw2
K = constant
w = width of the core lamination material normal to the flux
B = flux density
If a solid core were used in a power transformer, the losses would be very high and the temperature would be excessive. For this reason, cores are laminated from very thin sheets, such as 0.23 mm and 0.28 mm, to reduce the thickness of the individual sheets of steel normal to the flux and thereby reducing the losses. Each sheet is coated with a very thin material to prevent shorts between the laminations. Improvements made in electrical steels over the past 50 years have been the major contributor to smaller and more efficient transformers. Some of the more dramatic improvements include:
- Development of cold-rolled grain-oriented (CGO) electrical steels in the mid 1940s
- Introduction of thin coatings with good mechanical properties
- Improved chemistry of the steels, e.g., Hi-B steels
- Further improvement in the orientation of the grains
- Introduction of laser-scribed and plasma-irradiated steels
- Continued reduction in the thickness of the laminations to reduce the eddy-loss component of the core loss
- Introduction of amorphous ribbon (with no crystalline structure) — manufactured using rapid cooling technology — for use with distribution and small power transformers
The combination of these improvements has resulted in electrical steels having less than 40% of the noload loss and 30% of the exciting (magnetizing) current that was possible in the late 1940s.
The effect of the cold-rolling process on the grain formation is to align magnetic domains in the direction of rolling so that the magnetic properties in the rolling direction are far superior to those in other directions. A heat-resistant insulation coating is applied by thermochemical treatment to both sides of the steel during the final stage of processing. The coating is approximately 1μm thick and has only a marginal effect on the stacking factor. Traditionally, a thin coat of varnish had been applied by the transformer manufacturer after completion of cutting and punching operations. However, improvements in the quality and adherence of the steel manufacturers’ coating and in the cutting tools available have eliminated the need for the second coating, and its use has been discontinued.
Guaranteed values of real power loss (in watts per kilogram) and apparent power loss (in volt-amperes per kilogram) apply to magnetization at 0؛ to the direction of rolling. Both real and apparent power loss increase significantly (by a factor of three or more) when CGO is magnetized at an angle to the direction of rolling. Under these circumstances, manufacturers’ guarantees do not apply, and the transformer manufacturer must ensure that a minimum amount of core material is subject to cross-magnetization, i.e., where the flow of magnetic flux is normal to the rolling direction. The aim is to minimize the total core loss and (equally importantly) to ensure that the core temperature in the area is maintained within safe limits. CGO strip cores operate at nominal flux densities of 1.6 to 1.8 tesla (T). This value compares with 1.35 T used for hot-rolled steel, and it is the principal reason for the remarkable improvement achieved in the 1950s in transformer output per unit of active material. CGO steel is produced in two magnetic qualities (each having two subgrades) and up to four thicknesses (0.23, 0.27, 0.30, and 0.35mm), giving a choice of eight different specific loss values. In addition, the designer can consider using domain-controlled Hi-B steel of higher quality, available in three thicknesses (0.23, 0.27, and 0.3 mm).
The different materials are identified by code names:
- CGO material with a thickness of 0.3 mm and a loss of 1.3 W/kg at 1.7 T and 50 Hz, or 1.72 W/ kg at 1.7 T and 60 Hz, is known as M097–30N.
- Hi-B material with a thickness of 0.27 mm and a loss of 0.98 W/kg at 1.7T and 50 Hz, or 1.3 W/ kg at 1.7 T and 60 Hz, is known as M103–27P.
- Domain-controlled Hi-B material with a thickness of 0.23 mm and a loss of 0.92 W/kg at 1.7T and 50 Hz, or 1.2 W/kg at 1.7 T and 60 Hz, is known as 23ZDKH.
The Japanese-grade ZDKH core steel is subjected to laser irradiation to refine the magnetic domains near to the surface. This process considerably reduces the anomalous eddy-current loss, but the laminations must not be annealed after cutting. An alternative route to domain control of the steel is to use plasma irradiation, whereby the laminations can be annealed after cutting.
The decision on which grade to use to meet a particular design requirement depends on the characteristics required in respect of impedance and losses and, particularly, on the cash value that the purchaser has assigned to core loss (the capitalized value of the iron loss). The higher labor cost involved in using the thinner materials is another factor to be considered.