- Let us consider the three core belted cable.
- As there is potential difference exists between core to core of cable and core to sheath of cable, electrostatic field set up between them.
- There is core to core capacitance Cc and core to sheath capacitance Ce exists in the three core cable.
- The core to core capacitance is delta connected whereas core to sheath capacitance is star connected.
Core to core capacitance = Cc
Core to sheath capacitance = Ce
- The three core to core delta connected capacitance is converted into equivalent star connection.
Ceq = 3Cc
Where
Ceq = Equivalent star connection of delta connected core to core
Capacitance
- There, the total capacitance is equal to parallel combination of core to sheath capacitance and equivalent star connection of core to core capacitance of cable.
- The capacitance between core to neutral
CN = Ce + Ceq
= Ce
+ 3Cc
Measurement of capacitance of cable
( 1 ) Three cores bunched together
- The three cores of cable are bunched together and capacitance is measured between cores and sheath of cable.
- As the three cores are bunched together, there is no capacitance between cores. There exists capacitance between core and sheath Ce which are in parallel.
- Let us consider that this capacitor is C1 therefore
C1 = 3Ce
Ce = C1 / 3…….. ( 1 )
( 2 ) Capacitance between two cores is measured with third core connected to the sheath
- The capacitance between two cores is measured with the third core connected to the sheath.
- This eliminated one core to sheath capacitance. Let us consider that this capacitance is C2.
C2 = Cc + Cc / 2 + Cs
/ 2
= ( Cc + Cc / 2 ) + Cs
/ 2
= ( 3Cc
/ 2 ) + Cs / 2
= ½ ( 3Cc
+ Cs )
= ½ CN
( 3 ) Capacitance between two cores shorted with the sheath and third core
Let us consider that this capacitor is equal to C3.
C3 = 2Cc + Cs
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