Transformer no-load excitation loss, also known as core or iron loss, occurs from a changing magnetic field in the transformer core whenever it is energized. Core loss varies slightly with voltage but is essentially considered constant. Fixed iron loss depends on transformer core design and steel lamination molecular structure. Improved manufacturing of steel cores and introducing amorphous metals (such as metallic glass)
With increased concern for
generation and transmission
efficiencies, utilities will find an
increasingly compelling business
case to adopt distribution loss
reduction strategies.
have reduced core losses. Through faster material cooling, mass-produced metallic glass (or MetGlass ) ribbons were developed by Allied Signal (now Honeywell) in the 1990s that reduced core loss by 60 percent compared to conventional grain-oriented silicon steel cores. Copper losses can be slightly higher in metallic glass core transformers but overall loss at rated capacity is less.
Utilities must determine if reduced energy losses more than offset the price premium for more efficient transformers. In general, early transformer replacement programs are not economically warranted.
Once a distribution system has been built, some of the easiest loss savings comes from balancing current along three-phase circuits. Feeder phase balancing also tends to balance voltage drop among phases giving three-phase customers less voltage unbalance. Amperage magnitude at the substation doesn’t guarantee load is balanced
throughout the feeder length. Feeder phase unbalance may vary during the day and with different seasons. Feeders are usually considered “balanced” when phase current magnitudes are within 10 percent (based on the average current among phases). That is: [(highest phase current – lowest phase current)/average phase current]< 0.1.
Similarly, balancing load among distribution feeders will also lower losses assuming similar conductor resistance. This may require installing additional switches between feeders to allow for appropriate load transfer.
Typical customer power consumption varies throughout the day and over seasons. Residential customers generally draw their highest power demand in the evening hours when they arrive home from work and school and activate the heating or cooling system, turn on lights, and prepare dinner. Conversely, commercial customer load profiles generally peak in the early afternoon. Because current level (hence, load) is the primary driver in distribution power losses, keeping power consumption more level throughout the day will lower peak power loss and overall energy losses. Ideally, peaks should be “shaved” to fill in troughs. A common measurement of load variation is “load factor.” It ranges between zero and one and is defined as the ratio of average load in a specified time period to peak load during that time period. For example, over a 30-day month (720 hours) peak feeder power supplied is 10 MW. If the feeder supplied a total energy of 5,000 MWh, the load factor for that month is 0.69 ( 5,000 MWh/(10MW x 720 hours).
Lower power and energy losses are achieved by raising the load factor, which, evens out feeder demand variation throughout the feeder. Increasing the load factor has been met with limited success by offering customers “ time-of-use” rates. That is, companies use
pricing power to influence consumers to shift electric-intensive activities (such as, electric water and space heating, air conditioning, irrigating, and pool filter pumping) to off-peak times. With financial incentives, some electric customers are also allowing utilities to interrupt large electric loads remotely through radio frequency or powerline carrier during periods of peak use.
Utilities can try to design in higher load factors by running the same feeders through residential and commercial areas.
With increased concern for generation and transmission efficiencies, utilities will find an increasingly compelling business case to adopt distribution loss reduction strategies. The core of these strategies involve reduction in both circuit current and resistance. Distribution feeder design philosophy may be adapted to reduce feeder current by using higher operating voltage and a higher quantity of feeders that are shorter in length. In addition, modifying standards to specify larger conductor sizes will reduce resistance. Current may also be reduced in existing feeders by adding fixed and switched shunt capacitor banks.
Increasing new transformer sizes may reduce peak power loss but may result in higher annual energy loss due to increases in no-load losses. Some utilities may benefit by paying more up front for higher-efficiency transformers and recapture those costs through loss savings over the next 20-plus years. Balancing current on existing feeder phases and redistributing load among feeders will also improve economic operation of existing systems.
Distribution loss reduction takes some engineering analysis, but it will pay off for years to come. ❮❮
Steve Eckles has been a distribution engineer for 14 years at El Paso Electric Company and is a licensed PE in New Mexico and Texas.
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