Ciclo de la excentricidad

is to find the base kVA load for one consumer using Chart 2. The chart row labeled “TE” gives the base total electric load related to house size. The “A/C” row gives the air-conditioning load for the air-conditioner sizes shown. The equiva- lent kVA demands for various resistance strip heaters are listed in Chart 3. To find the load for a group of consumers, multiply the kVA values from Charts 2 and 3 by the appropriate diversity

factors from Chart 1. Diversity factors depend on the number of consumers in the group. To determine whether transformer size is set by the summer or winter load, do the calculation with air-conditioner load and then with resistance heat load.

Equation 4.8 gives the total load (L_X) for X identical consumers.

Example 4.7 clarifies the procedure. Chart 3

Equivalent kVA Demand for Houses With Resistance Heat

kW Rating kVA Demand

5.0 5.0 7.5 6.5 10.0 8.0 15.0 10.5 20.0 14.0 Chart 2

Standard House Loads (kVA)

Typical Residence Size (Square Feet)

Type of Load 1,500 2–3,000 5,000+

TE 4.3 kVA 5.7 kVA 7.5 kVA

Typical Air Conditioner Size (Tons)

Type of Load 3 4 5

A/C 3.8 kVA 5.1 kVA 6.3 kVA

TABLE 4.14: Application of Single-Phase Distribution Transformers to Serve Residential Consumers—Sample Loading Guide.

Chart 1

Diversity Factor D Number of

Consumers Total Electric Air Conditioning

in Group (X) (TE) (A/C)

1 1.00 1.00 2 0.85 0.85 3 0.74 0.83 4 0.66 0.80 5 0.61 0.77 6 0.57 0.75 7 0.54 0.73 8 0.52 0.72 9 0.50 0.71 10 0.49 0.70 11 0.47 0.70 12 0.46 0.69 13 0.45 0.69 14 0.43 0.68 15 0.42 0.68 16 0.41 0.67 17 0.39 0.67 18 0.38 0.66 19 0.38 0.66 20 0.37 0.65

Note. Values in the charts were excerpted from

the South Carolina Public Service Authority (Santee Cooper) Distribution Engineering

Reference Manual dated February 2, 1987.

Example 4.7 assumes the transformer full-load rating, corrected for ambient temperature, can be up to 140 percent of its summer loading. The ability to carry more load in the winter is justified because the heating load factor is much lower than the cooling load factor for the assumed transformer service area. Cooler ambient temper- ature in winter also increases transformer loading capabilities. Each cooperative must set its own percentage loading limit based on experience. Before the transformer is installed, its size should be checked to see if it meets cooperative voltage drop and flicker criteria. These calcula- tions are covered inAppendix B, “Transformer and Secondary Voltage Drop.”

Equation 4.8

LXSummer = X[(TE Load)(DX(TE)) + (A/C Load)(DX(A/C))]kVA LXWinter = X[(TE Load)(DX(TE)) + (Heat Load)(DX(A/C))]kVA

where: L_X = Total load for X identical consumers, in kVA X = Total consumers in group

TE Load = Base total electric house load from Chart 2, in kVA DX(TE) = Diversity factor D for X consumers from Chart 1,

TE column

A/C Load = Base air-conditioner load from Chart 2, in kVA Heat Load = Base resistance heat load from Chart 3, in kVA DX(A/C) = Diversity factor D for X consumers from Chart 1,

A/C column

EXAMPLE 4.7: Pad-Mounted Transformer Sizing for New UD Residential Consumers.

Assume four totally electric, 1,500-sq.-ft. homes are to be fed from the secondary of a pad-mounted transformer in a new subdivision. All homes have identical electrical appliances, three-ton (36,000-Btu) air conditioners, and 7.5-kW resistance heaters. Select the transformer size that will serve the summer and winter loads and has a 20-year life expectancy. Pad-mounted transformers to choose from are rated 25, 37.5, and 50 kVA.

First, select the diversity factors from Chart 1:

Second, choose the base TE load and A/C load for a single house from Chart 2:

From Equation 4.8, the total summer load is 23.52 kVA, as calculated:

A 25-kVA transformer is the proper size to choose, as no new houses will be added to the transformer.

The total winter load is calculated the same way by replacing the air- conditioning load with the strip heater load from Chart 3. The A/C

diversity factor is applied to the heating load in this instance. Because the ambient temperature will be lower in the winter, it is assumed the transformer will carry up to 140 percent of its summer peak load for short periods without undue loss of life.

For the winter peak, the TE load component of the total load is the same as before:

The 7.5-kW strip heating component of total demand is then

Total winter diversified demand is equal to

The ratio of winter to summer load is then

Because the ratio is below 140 percent, the transformer size will be set by the 23.52-kVA summer load. The 25-kVA unit is still the proper trans- former to install. (Note: Keep in mind this example is based on a methodology used by a southeastern U.S. utility and should be modi- fied for use in other climates.)

X = 4 consumers in groups D4(TE) = 0.66

D_4(A/C)= 0.80

TE Load = 4.3 kVA A/C Load = 3.8 kVA

TE Load = (4)(4.3)(0.66) = 11.35 kVA 4(6.5)(0.8) = 20.8 kVA Winter L4= 11.35 + 20.8 = 32.15 kVA = 137% Ratio =32.15 23.52 Summer L4= 4 [(4.3)(0.66) + (3.8)(0.80)] = 4[2.84 + 3.04] = 23.52 kVA

Another important concern is initial loading versus future loading when load growth is expect- ed. For many UD areas, significant load growth is not expected for individual transformers because the number of living units per transformer is set in the development plans. The modern trend in housing construction is to install all heavy appli- ances and heating, ventilating, and air-condition- ing (HVAC) equipment in a dwelling before ini- tial occupancy, so any growth beyond the initial level is expected to be insignificant.

Even when engineers expect load growth, they seldom accurately know the rate of growth. Although complicated formulas exist for eco- nomic sizing of transformers based on load growth, use of these formulas is meaningless if the growth rate is not accurately known. A sim- ple procedure is recommended, such as sizing the transformer for the load that is estimated to be present 10 years in the future.

TRANSFORMER SIZING FOR THREE-PHASE TRANSFORMERS FOR NEW COMMERCIAL AND INDUSTRIAL LOADS

Three-phase transformers—required to render service to commercial and industrial con- sumers—represent a significant investment for the average cooperative. As such, care should be taken in selecting transformers sized to mini- mize cost and losses, while providing reliable service.

Sizing transformers for these type installations is not an exact science and requires sound judg- ment and previous experience, similar to the philosophy involved in sizing single-phase trans- formers. Local geographical and climatological conditions must be considered, as they play a significant role in sizing equipment. This subsec- tion presents three generally accepted methods of sizing transformers that most cooperatives and utilities have used over the years:

1. Previous demands on similar loads, 2. Watts-per-square foot demand factors, and 3. Diversified connected load analysis.

It is suggested that an analysis be made using all three methods, if possible, as a crosscheck to validate the final selection of a properly sized

unit. Further analysis using both knowledge of specific types of loads and experience anticipat- ing the likelihood of growth in consumer de- mand is recommended.

Method I: Previous Demands