NEMA issued a Standard TP-1 in support of the U.S. Dept. of Energy’s guidelines for more energy efficient electrical devices. This standard was based on a previous U.S. Environmental Protection Agency study showing that the typical dry type transformer under normal operating conditions was loaded to approximately 35% of its nameplate rating. Therefore, TP-1 established a table of minimum efficiencies for various sized transformers when operating linear loads (see Table 1). These efficiencies are really quite incredible as they range from 97% to 98.8%. What TP-1 does not tell you is that it is very unlikely that you will ever see such efficiencies in actual installations. In addition, TP-1 does not tell you that using these very efficient transformers will impact your electrical designs significantly.
Because of the differences among the efficiencies shown in TP-1 and what really happens with real transformers in real applications, the approach you take in your electrical design could be significantly different when attempting to design an electrical system with minimized losses. This article offers suggestions regarding how you approach your electrical designs to maintain minimum losses in the system transformers (see Figure 1). It will also show areas in which you will have greater losses than those shown in TP-1—no matter which design direction you might choose.
Linearity
TP-1 was developed using linear loads. However, in today’s business environment, most of the loads are nonlinear (rich in harmonic content). Computers, fluorescent light fixtures, printers, elevators, or variable frequency drives for motors generate harmonics. Applying harmonically rich loads to transformers can double or triple their total losses. For example, a 75 kVA transformer that would normally have 2% losses at 35% loading would actually have 4% to 6% losses. Therefore, the 26 kVA load (35% of the 75 kVA) would have losses totaling more than 1.5 kW.
Core and coil losses
Transformer losses are a combination of core losses and coil losses. The core losses consist of those generated by energizing the laminated steel core. These losses are virtually constant from no-load to full-load, and for the typical 150 C rise transformer are about 0.5% of the transformer’s full-load rating. The coil losses are also called load losses because they are proportional to the load on the transformer. These coil losses make up the difference between the 0.5% losses for the core and range from 1.5% to 2% of the total load.
Typically, the total losses for a 75 kVA transformer are about 1,000 W at 35% loading or 1.3%. The actual losses when the transformer is fully loaded can be more than 3,000 W for linear loads and 7,000 W for nonlinear loads. This amounts to 4% and 9.3% respectively—considerably more than the NEMA TP-1 table for minimum efficiencies for a 75 kVA transformer. While the overall concept for requiring more energy-efficient transformers is quite good, engineers may want to be very careful about transformer selection when the anticipated operating conditions do not match the base criteria that were used in developing the TP-1 table.
By selecting transformers with lower temperature ratings, that is, 115 and 80 C rise instead of the standard 150 C rise transformers, the core and load losses will change. To reduce the temperature rise, the core is increased in size. This increases the core losses but reduces the load losses, so, according to the anticipated operating point, the total losses may be higher or lower than the standard transformer. Due to the smaller core losses, the total losses for the 150 C transformer are less than the total losses of the 80 C transformer up to about 60% loading. With transformer loading above 60%, the total losses are less than those of a 150 C transformer of the same size (see Figure 2).
A good compromise between core and load losses is the 115 C rise transformer. While the core losses are somewhat higher than those in the 150 C transformer, they are less than the 80 C transformer core losses. Correspondingly, the load losses are less than the 150 C transformer, allowing the total losses to be less than those of the 150 C transformer under normal operating conditions (see “Know the loss data, loading when specifying transformers”).
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