How single-phase to three-phase conversion affects distribution line losses?
Electric Utilities around the world typically uses three-phase system in distributing power. Three-phase system is said to be relatively more superior compared to using a single-phase. The idea of a three-phase power was first conceived by non other than the founder of Alternating Current electricity, Nikola Tesla.
Compared to a single-phase system with similar range of power, a three-phase power is found to be more efficient since it generates a power of about 150% more. Also, three-phase system never drops down its power delivered with a constant power is been delivered all the time, unlike in a single-phase system where power falls to zero three times during the cycle. This feature is very beneficial to most dynamic equipment like a motor where this type of equipment needs a balance load to run properly.
Another known beneficial feature that three-phase offers compared to a single-phase distribution system is the advantage of having the opportunity of using conductors that are 75% the size of a single phase for same output power. Also current levels in each of the conductors are lowered in a three phase system since it allows the utility to use three conductors rather then two, as compared to single phase system. Three phase power is a more efficient means of supply power to large electrical loads.
Consider, for example, the effect of upgrading a single-phase line to three-phase. Assuming the system is grounded wye, which is most common, the load current that previously flowed through the single-phase conductor and returned through the neutral and ground will be divided among the three-phase conductors of the new line, and the return current will be negligible if the load is reasonably balanced.
Application of the I squared R law shows that losses on each phase conductor of the upgraded line will be one-ninth of the previous amount, which reduces total conductor losses for all three phases to one-third of the original amount. In addition to this reduction, the losses on the return circuit are reduced virtually to zero. This amount of loss reduction will result in large money savings.
Please find an example below from TVPPA about the effect of single-phase to three-phase conversion especially when it comes to the distribution losses it can generate.
Line losses of 24 kW occur on a single-phase distribution line during peak load periods. If the line is upgraded to three-phase construction with the same conductor size, how much will the losses are reduced? What annual savings will result, assuming the loss factor and cost of losses are 0.3 and $13.2/kW/yr, $0.045kWh/yr respectively?
SOLUTION:
For single-phase lines, three-quarters of the line losses typically occur on the phase conductors and one-quarter on the return circuit. Phase conductor losses are therefore, approximately 18kW. The replacement three-phase line will experience only one-third of the previous phase conductor losses and negligible return circuit losses. This amounts to a loss of 6 kW on the three-phase line. Therefore, the net reduction in losses is 24kW-6kW = 18kW. The annual savings can be calculated as follows;
Annual Demand Cost Savings = $ 13.20/kW/tr x 18kW = $238
Annual Energy Cost Savings = 0.3 x 8,760 hrs x 18kW x $0.045/kWh = $2,129
Total Annual Savings = $238 + $2,129 = $2,367
The savings involved in this example probably will not justify the total cost of upgrading the line, however, other benefits associated with upgrading, such as greater load-carrying capability and voltage improvement, may combine with the loss reduction savings to make the upgrade economically justifiable.
Source: TVPPA Distribution System Loss Reduction Manual November 1994
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