2. Objective:
Classification of cables
voltage drop
Short circuit current
Some of Cables problems
Protection on cables
Fuses and circuit breaker
Cable Fault Localization
Tests on cables
Ring Main Unit , RMU
Types of earthing
5. The relation between (R and L):
- Ac current resistance differ about dc current for same
material for two reasons :
1- skin effect
2- proximity effect
6.
7. Cable capacitance:
C=(q/v)
- Produce capacitance charges between core and sheath
(earth) or between cores and earth
- Must discharge the cable after the disconnection
- Occurs some losses caused by these charges but it is
useful for power factor correction
8.
9. 3- electrical field:
V=K (q/d) E= v/d v/m
- Max electrical field at conductor surface
- Min electrical field at inner sheath surface
-
10. E = V / [x ln (R/r)] v/m
- Max (E) at (x=r)
- Min (E) at (x=R)
- Max (E) at conductor surface so that use
semiconducting material on core for field regulating
11. 4- insulation resistance :
- (not Rins = ∞ )
- use insulator has high resistance to reduce (tan δ )
13. - To connect cable shield must cut part of shield of cable
end to prevent cable fault
- This causes un continuous of electrical field and
concentration on shield surface lead to insulation
failure
- Use tube to reduce pressure
14.
15. Wires Scheduled causes increasing in electrical field
on conductor surface about (20%)
so that semiconducting on conductor reduce
electrical field on surface
16. conductor shield is made of plastic with black carbon
to give semiconducting
Where semiconducting resistance higher than
conductor resistance
18. advantages of semiconducting material:
1- regulate the electrical field
2- plastic property make adequateness (expansion
and shrinkage) with insulator in periods (loading
and unloading )
- This to prevent bubbles between conductor
insulator
19. sheath effect :
when flow current in conductor produce field and
causes emf in cable sheath and behind cables
sheath
These emf causes (eddy current )
20. 1- eddy current losses in sheath :
It about (2% of conductor losses ) where appear in metallic
sheath cables
21. 2- sheath losses :
when connect cable sheath to earth or other cable sheath
produce (closed loop ) “emf” where current flow in sheath
in single core cables causes losses
These losses reduce current capacity for cable
Solution : use cross bonded
24. Breakdown voltage for cable:
1- voltage effect time
2- conductor shape
3-temperature
4- spaces quantity in cable
Cable (A) Void between the Conductor and the Insulation
(B) Tracking In Cable
25. At testing:
- Used voltage in old cable is less than Used voltage in
new cable ‽
Solution:
- At loading and unloading increase and decrease
temperature
- lead to expansion and shrinkage in cable causes
spaces and ionization causes faliure
29. 1- PVC:
Advantages:
1- used in voltages (3.3kv)
2-used at low temperature
3- high hardness
4- moisture resistance
5- resistance to ozone gas, acids
6- resistance to flame
30. disadvantages:
1-In high temperature reduce the insulation resistance
2-not exposed to continuous temperature exceeds about
(105 c) or reduce to zero
3- not exceeds conductor temperature about (150c -160c)
4- at high temperature (insulator cracking),
At low temperature (insulator stiffen)
31. 2- XLPE:
Advantages:
1-used in high voltages (up to 275kv)
2- carry high temperature during continuous loading
“90c” or S.C at “250c”
3- tan δ is small
4- breakdown voltage (50-70kv/mm)
5-high resistance moisture
33. Curing processing:
Used water steam or (inert gas “nitrogen”) to prevent
impurities and gap or moisture
34. Must be subject to the following when using cables
XLPE:
1- not exceeds max continuous loading temperature (90c
) for conductor
2- not exceeds max over loading temperature (130c for
conductor )
3- not exceed max temperature about (250 c ) for
conductor during short circuit
disadvantages:
1- not used at Bending because it has rigid
35. 3- EPR:
1- used at voltages (5-35kv )
2- carry high temperature at (90c )
3- good elastic properties
4- ozone resistance
