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Introduction
Strength
Electrical Power
Electrical Communication
Temperature Rating
Special Cable Problems
Cable Service
Reversing a Line
Attenuation VS Freq
Graph
Electrical Fault
Location
Derivation of Fault
Detection Formula
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Cased Hole
ELECTRICAL COMMUNICATION
A variety of signals
are transmitted from down hole
instruments to the surface by means of the cable conductors and armor. These
signals vary in frequency from DC to 60 - 100 khz. At 100 khz the attenuation
of a 1N32PP cable is in the range of 0.68 db/kft @ 10khz and 2.28 db/kft at 60
hz. A plot of attenuation versus frequency is
shown in the attached graph.
- Capacitance and Resistance.
The cable is basically a R-C network for most of the frequencies used on the
cable. Therefore to improve signal transmission, it is desirable to reduce capacity and
resistance. Unfortunately as the conductor diameter is increased to reduce
electrical resistance, the electrical capacity of conductor (with respect to armor) increases.
For the 1N32PP cable the D/d ratio of 2.1 results in a nominal characteristic impedance in the range of
30 to 50 ohms for the frequencies normally used. The characteristic impedance varies
with frequency starting at the DC value and. asymptotically approaching a high
frequency value.
- Dielectric Materials. Signal
attenuation in different dielectric materials varies due to the differences in
dielectric constants.
| a. |
Poly Propylene |
2.3 |
| b. |
Tefzel |
2.6 |
| c. |
FEP Teflon |
2.1 |
| d. |
PFA Teflon |
2.1 |
The dielectric losses for all of these
materials is so low over the frequencies involved that it can be ignored.
-
Temperature Effects. For all
practical purposes the dielectric constant of all the plastic materials is
unchanged up to the maximum operating temperature. On the other hand
the resistance of the copper conductors goes up rapidly and therefore so
does the attenuation of the cable. At 526° F. the
resistance of copper is double its value at 68° F. Therefore that
portion of a cable at 526 degree F. will have double the attenuation of that
portion of the same cable at 68 degree F.
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