# 1. Fundamentals of Distribution Systems

## Questions

1. What is the most load you would typically have on one 12.5-kV circuit?

1. 20 MVA
2. 10 MVA
3. 5 MVA
4. 2 MVA
2. What is the most load you would typically have on one 25-kV circuit?

1. 20 MVA
2. 10 MVA
3. 5 MVA
4. 2 MVA
3. Which of the following are incorrect? Mark all that apply.

2. Distribution primary fault currents are lower than transmission fault currents.
3. Distribution primary voltages are lower than transmission voltages.
4. In an area with relatively high load densities, circuits are generally limited by current. How much would doubling the system voltage increase the circuit coverage?

1. 1X
2. 2X
3. 4X
5. In an area with relatively low load densities, circuits are generally limited by voltage drop. How much would doubling the system voltage increase the circuit coverage?

1. 1X
2. 2X
3. 4X
6. Which areas are more of a concern at higher voltages? Mark all that apply.

1. Reliability
2. Lightning protection
3. Aesthetics
4. Live-line work
5. Equipment reliability
6. Momentary interruptions
7. Voltage sags
8. Harmonics
9. Ferroresonance
7. What is the main reason that an open bus tie in substations is more common?

1. Lower fault currents
2. Fewer voltage sags
3. Higher reliability
4. Better transformer usage
5. No circulating current
8. Line losses increase as a function of what? Mark all that apply.

1. I
2. I2
3. Vpu
4. Vpu2
9. Which of the following is likely to have the highest line losses?

1. 12.5-kV circuit, 12-miles long, peak load = 7.5 MVA
2. 25-kV circuit, 12-miles long, peak load = 7.5 MVA
3. 12.5-kV circuit, 6-miles long, peak load = 10 MVA
10. Where must a 100-A lumped load be placed on a line to cause the same losses as a 100-A uniformly distributed load?

1. 1/3
2. 1/2
3. 2/3
4. End of feeder

## Problems

1. If a 25-kV distribution line carrying 10 MW is stepped down to 12.5-kV, how much power is the 12.5-kV line carrying?

2. For the previous question, find the current on each phase for both the 12.5- and 25-kV circuits.

3. On a 12.5-kV circuit with an average current of 200 A and a load factor of 0.5, what is the peak current?

4. Three loads have peak loads of 300 kW, 200 kW, and 225 kW respectively. Approximately what is the combined peak load of the three loads?

## Projects

1. Look around the distribution circuits in your area. Take pictures of each of the following:

• Distribution primary mainline line
• Distribution substation
• Recloser
• Capacitor bank
• Fuse cutout
• Distribution secondary
2. From your house or other convenient location with overhead distribution. Trace the circuit back to the substation.

3. Using a university technical library or IEEE Xplore, find three historical papers or articles discussing a particular (technical or economic) issue associated with higher-voltage distribution circuits. Issues could include reliability, reach, grounding, equipment performance, or cost. Write a short essay summarizing the three papers and the issue identified.

To the extent possible under law, Tom Short has waived all copyright and related or neighboring rights to these study questions. This work is published from the United States. Please use this material however you want.