Electricity + Control February 2019

TRANSFORMERS + SUBSTATIONS

have cost?The average price difference between a high efficiency transformer (AoAK) and a standard loss (CoBK) transformer is 20%, on a 1 600 kVA transformer this equates to an additional capital outlay of R73 150.00. Consider the losses of a 1 600 kVA CRT AoAK vs CoBK at 50%, 75% and 100% loading, with a unit cost of 89.52c (ZAR c) per kWH and tabulate ( Table 1 ) the cost of wasted power over a one (1) year period. This calculation assumes that the cost per kWH remains constant. It is interesting to note that the PEI of a trans- former falls closely to the 50% power consump- tion mark. From Table 1, the payback period for an AoAk CRT vs CoBK CRT, with the capital price difference being R73 150.00 can be calculated for 50%, 75% and 100% loading as per Table 2 .

Why is this relevant to Africa? In developing countries within Africa power grids are constrained, non-existent or come at a price. Where grids are non-existent, individual com- pany-based generation is the only source of pow- er. Diesel generation is the form most commonly adopted and is already an expensive form of gen- eration. Furthermore, the running costs of such generation are substantial. By reducing the losses of the transformer, the fuel costs and overall effi- ciency of the generating system can be improved. The same principle can be applied to any other form of generation and should be weighted higher espe- cially when compared to solar and wind generation as this form of power generation is never constant. The cost of power is also increasing every year so not only will high efficiency transformers help to maintain a more stable power grid, they will also help the consumer save money. There is no question that the initial capital outlay for a high efficiency transformer will be more than that of a standard loss transformer, and this is due to the more expensive core steel used and the thicker cross-sectional area of the winding conductor. The question arises, if I am paying more for the initial transformer, where is the saving? For that we must start looking at the payback period for such a transformer. In other words, how much power would I have used if the transformer were not high efficiency and what would this power

Medium voltage windings prior to casting.

Table 2

SPY

PBP 3.43 3.89 3.89

Loading

AoAK 50% R21,330.11 AoAK 75% R18,820.68 AoAK 100% R18,820.68

SPY = CPyCoBK-CPyAoAk SPY = Saving Per Year

Transformer core.

PBP = Initial Capital Outlay Difference ÷SPY PBP = Payback Period in years

From Table 2 , a transformer that is loaded at 50% to 100%’s initial capital outlay can be recouped in under four (4) years. The savings begin after this period. Note that the above calculations do not take into consideration the price increases that have occurred over the past few years. Consider- ing that the average life span of a cast resin trans- former is 20 years, the remaining life span will re- sult in significant savings. Another benefit of a high efficiency transformer is the reduction in cooling requirements within the room. As losses are dissipated in the form of heat, the room temperature will naturally begin to rise. If ventilation is not well considered, the ambient room temperature may reach the point at which the transformer will begin to overheat, resulting in damage to the transformer. With lower losses come lower heat, hence the reduction in cooling requirements.

Table 1

TLPh

CPh

CP24h

CPy

Loading Efficiency

CoBk 50% 99.20% 12 800 W R11.46 CoBk 75% 99.10% 14 400 W R12.89 CoBk 100% 98.93% 17 120 W R15.33 AoAK 50% 99.37% 10 080 W R9.02 AoAK 75% 99.25% 12 000 W R10.74 AoAK 100% 99.08% 14 720 W R13.18

R275.01 R100,376.99 R309.38 R112,924.11 R367.82 R134,254.22 R216.57 R79,046.88 R257.82 R94,103.42 R316.26 R115,433.53

TLPh = Apparent – Power-(Efficency%(Apparent Power) ÷100) TLPh = Total losses per hour CPh = TLPH ÷1000× UC CPh = Cost per hour UC = Unit cost 1 kWH of power in Rands CP24h = CPh ×24 CP24h = Cost per 24 hours in Rands CPy = CP24h *365 CPy = Cost per year in Rands

34 Electricity + Control

FEBRUARY 2019