Problem Set (3-B) - Solution

7/25/2019 Problem Set (3-B) - Solution

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ECE 462 Problems Set (3-B)

Solution

Problem (1):

It is required to design a 4.16 kV, three phase feeder (Figure 1) that will have the following

specifications:

I. Total length of feeder: 5000 ft.

II. Load: 10 loads each one is 500 kVA (three phase), 0.9 lagging power factor spaced every

500 ft with the first load 500 ft from the substation as shown in the following figure.III. Voltage drop: not to exceed 5% from the substation to the last load.

Figure 1

It has been decided to use 336,400 26/7 ACSR conductors on 45-ft poles with 8 ft cross-arms.

The spacing of the conductors on the cross-arms are 2.5 ft, 4.5 ft and 7.0 ft.

a) Determine the percent voltage drop to the last load point.

b)

Using the lumped model in the middle of the feeder for uniformly distributed loadcalculate the voltage drop.

c) Is the answer in (a) different than (b), why?

d) Calculate the total three-phase power loss for the feeder.

Solution


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(1.a):

(1.b):

(1.c):

The two answers in (2.a) and (2.b) are different because nin part (2.a) is a finite number,

whereas the solution of (2.b) is based on assuming nas infinite.

(1.d):

Solve it using the exact lumped model as following,


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Problem (2):

A rectangle-triangle area (Figure 2) is being fed from a source at point X. Both areas have a load

density of 6000 kVA/mile2, with the loads being uniformly distributed. In addition to the

uniformly distributed loads, there is a spot load at point Z that is 2000 kVA. The Kdropfactorfor the primary main conductors is 0.00022626% drop/kVA.mile. Determine the percent voltage

drop to point Z.

Figure 2

Solution


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Problem (3):

A square area of 20,000 ft has a load density of 2000 kVA/mile2on a side, and 0.9 lagging power

factor is to be served from a 12.47 kV substation that is located in the center of the square. Two

different plans are being considered for serving the area. The two plans are shown in Figure 4.

Plan Aproposes to break the area into four square areas and serve it as shown. The big black

line will be the three-phase primary main consisting of 336,400 26/7 ACSR conductors and the

dotted lines will be the three-phase laterals consisting of 4/0 ACSR conductors. Both the mainand laterals are constructed such that Deq= 4.3795 ft. The three-phase laterals will be spaced

every 500 ft.

Plan Bproposes to serve the area with four triangularly shaped feeders. Again the primary main

is shown in the dark black line and the laterals are spaced every 500 ft and shown as dotted lines.The same conductors and Deqwill be used in this plan.

Determine the percent voltage drop to the last customer (points a and b) for the two plans.

Figure 3

Solution


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Plan (A)


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Plan (B)

Plan (B) gives the lower voltage drop


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Problem Set (3-B) - Solution