Energy Efficiency
Reliability

A Refresher on Reducing Distribution System Loss

At every stage of the electric system – from the generator to the outlet – there are energy losses. This inefficiency has a price: When energy is lost, utilities must generate or purchase additional energy to meet demand. Efficiency isn’t just about cost – it is also a gauge of system performance, and monitoring items such as excess heat from transformers can support increased reliability.

Some system losses are inevitable, and loss cannot be eliminated altogether. Almost two-thirds of energy is lost in the generation and transmission of electricity. At the distribution level, most losses occur in lines and regulators (about half of losses) and transformers (about 27%). Losses in other devices, such as switches and breakers, typically make up a lower portion of losses but might be significant in system secondaries where currents tend to be high.

Here’s a brief refresher on ways that public power utilities can work to reduce losses throughout the distribution system.

Calculating Loss

A simple way to calculate the cost of losses is by multiplying the average cost of energy per megawatt-hour by the total energy losses. Another way is to find out the utility’s loss percentage, which is the ratio of total energy losses to total sources of energy. The median loss percentage for public power is 4.07%. Losses of more than 6% for public power utilities may suggest excessive physical losses.

Reducing Conductor Losses

Refurbishing or replacing old conductors is an important loss reduction technique and can provide increased capacity on the system. Conductors allow the flow of electrical current. Conductors also offer resistance to the flow of current, which results in power loss. The loss of power (in watts) is represented by the familiar relationship:

P=I2R

The current carried by the conductor in amperes (A) and the electrical resistance in ohms (Ω) are symbolized as I and R, respectively.

Resistance, R, for a conductor is determined by the following equation:

R=ρL/A

The resistivity of an object is represented by ρ (rho) and is measured in Ω m (ohmmeters). L represents the length, and A represents the cross-sectional area of the material. Resistance increases with the length of the conductor and decreases with the cross-sectional area of the conductor. Just as more water can flow through a wide pipe compared to a narrow one, electrical charge is higher and resistance is lower on wires with greater cross-sectional areas.

The following example shows how reconductoring older #4 AWG copper conductors to newer 336.4 kcmil 26/7 aluminum and steel conductors can reduce line losses by a factor of nearly five.

Conductor Stranding Circular mils Allowable Ampacity Resistance ohms/mile Line losses for 100 Amp load at the end of a 1-mile line
4 AWG Solid 41740 170 1.314 13.14 kW
336.4 26/7 336400 510 0.273 2.73 kW

While reconductoring is theoretically a great option for reducing losses, the process, including new hardware, is costly – and can be constrained by disruptions in the supply chain for certain materials.

Other Ways to Reduce Loss

Regularly examining system performance, and having an accurate picture of load factor, can help utilities to pinpoint problem areas and prioritize upgrades based on biggest cost of loss. When supply chain constraints ease, utilities can deploy strategies such as building in guarantees against transformer loss values to purchase agreements with manufacturers, such as by requiring expanded testing for large lots of transformers or on-site visits by utility personnel during manufacturer testing, or price adjustments for transformers not meeting the guaranteed loss performance.

There are many more ways to measure and reduce distribution system loss. Some are easier to implement and others are associated with higher expenses.

  • Maintain balanced currents on all three feeder circuit phases as much as is practical.
  • Keep secondary circuits as short as possible.
  • Use the smallest capacity transformer feasible for each installation, considering factors such as ambient temperature during peak load, duration of expected peak load, and expected load growth.
  • Evaluate the benefits of three-phase versus single-phase construction; avoid using voltage regulators downstream from the substation where possible.
  • Ensure all abandoned transformers have been disconnected from the primary line.
  • Analyze capacitor banks to verify that capacitor size and location are properly matched to feeder load.
  • Check every meter multiplier recorded on the billing system against the corresponding multipliers marked on the meters every two years.
  • Perform regular meter testing and calibration. Test single-phase customer meters every eight years, polyphase meters every six years, and high-use meters annually.
  • Install substation metering/supervisory equipment for each feeder to obtain profiles of voltage, current, and power factor versus time.
  • Convert long, substantially loaded single-phase circuits to three-phase.
  • Convert feeders to a higher voltage level.
  • Re-conductor the trunks of heavily-loaded circuits, beginning at the source end.

Increasing efficiency helps to continue to maintain public power’s edge in reliability and affordability compared to our peers. Utilities with outstanding energy efficiency efforts should consider applying for the Smart Energy Provider designation.