4. Underground Distribution

Questions

1. Outage frequency
2. Outage duration
3. Cost
4. Safety
6. Aesthetics

1. Outage frequency
2. Outage duration
3. Cost
4. Safety
6. Aesthetics
3. Which type of cable is most susceptable to water treeing?

1. EPR
2. HMWPE
3. TR-XLPE
4. XLPE
4. What options should reduce problems with water treeing? Mark all that apply.

1. Improved lightning protection
2. A larger conductor size
3. Thicker insulation
4. Water-blocked conductor
5. Cable injection
6. Jacketed cable
5. Faults on underground systems are generally permanent rather than temporary. What is the most common exception? Faults in:

1. Cable splices
3. PILC
4. Solid-dielectric cables
6. Which type of cable has the lowest dielectric losses?

1. EPR
2. HMWPE
3. PILC
4. TR-XLPE
5. XLPE
7. True/false: Dielectric losses are higher at higher voltages.

1. True
2. False
8. Which soil is the worst for cable ampacity?

1. Clay at 2% moisure
2. Clay at 5% moisure
3. Sand at 2% moisture
4. Sand at 5% moisture
9. On a single-phase cable, if you decrease the resistance of the neutral, what will happen to the loop resistance of the cable?

1. It increases.
2. It decreases.
3. It depends on other factors.
10. For a three-phase cable system with 350-kcmil aluminum cables in a flat configuration with a separation of 7.5” and 1/3 neutrals, what has the most impact to reduce zero-sequence resistance?

1. Increase cables to 500 kcmil.
2. Decrease cable separation to 1”.
3. Decrease neutral resistance by 50%.
11. True/false: Ampacities are lower in direct-buried cable than for cable in conduit.

1. True
2. False
12. For a 350-kcmil cable in a triplex configuration, which of the following is best for cable ampacity?

1. Shield resistance = 2X the phase resistance
2. Shield resistance = 4X the phase resistance
3. Shield resistance = 6X the phase resistance

Problems

1. Find the maximum electric field on a single-phase cable for the following combinations:

1. System voltage = 13.2 kV, 2/0 aluminum cable with 175-mil insulation, XLPE
2. System voltage = 34.5 kV, 2/0 aluminum cable with 345-mil insulation, XLPE
3. System voltage = 13.2 kV, 500-kcmil aluminum cable with 175-mil insulation, XLPE
4. System voltage = 34.5 kV, 500-kcmil aluminum cable with 345-mil insulation, XLPE
2. If a cable has a resistance of 0.1 ohms/kft at 20°C, find the resistance at 90°C.

3. Find the capacitance from line to ground of 1 mile of a 500-kcmil copper cable with 220-mil insulation.

4. Using the Anders approximation, find the resistance at 1000 Hz as a multiplier of the dc resistance for a 500-kcmil copper cable with EPR insulation.

5. If a cable has an ampacity of 500 A at an earth temperature of 20°C and an allowed conductor temperature of 80°C, find the new ampacity if the allowed conductor temperature is increased to 95°C.

6. For a fault current of 5000 A, find the minimum clearing time needed to prevent damage on a 500-kcmil copper 175-mil EPR cable. If the cable has a 4-mil copper sheath, also find the minimum clearing time to prevent damage (assume all fault current flows in the cable sheath).

Projects

1. Write a cable ampacity program in R, Matlab, Excel, or other tool.

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.