Wednesday, December 3, 2014

DC Voltage Testing of Insulation


When DC voltage is applied to an insulation, the electric field stress gives rise to current conduction and electrical polarization. Consider an elementary circuit as shown in Figure 1.1, which shows a DC voltage source, a switch, and an insulation specimen. When the switch is closed, the insulation becomes electrifi ed and a very high current fl ows at the instant the switch is closed. However, this current immediately drops in value, and then decreases at a slower rate until it reaches a nearly constant value. The current drawn by the insulation may be analyzed into several components as follows:
Capacitance


 • Charging current
• Dielectric absorption current
• Surface leakage current
• Partial discharge current (corona)
• Volumetric leakage current
Capacitance charging current: The capacitance charging 

current is high as the DC voltage is applied and can be calculated by the formula:


FIGURE 1.1
Electrical circuit of insulation under DC voltage test.

where
ie is the capacitance charging current
E is the voltage in kilovolts
R is the resistance in megohms
C is the capacitance in microfarads
t is the time in seconds
e is Napierian logarithmic base


The charging current is a function of time and will decrease as the time of the application of voltage increases. It is the initial charging current when voltage is applied and therefore not of any value for test evaluation. Test readings should not be taken until this current has decreased to a sufficiently low value.

Dielectric absorption current: The dielectric absorption current is also high as the test voltage is applied and decreases as the voltage application time increases, but at a slower rate than the capacitance charging current. This current is not as high as the capacitance charging current. The absorption current can be divided into two currents called reversible and irreversible charging currents. This reversible charging current can be calculated by the formula:

where
ia is the dielectric absorption current
V is the test voltage in kilovolts
C is the capacitance in microfarads
D is the proportionately constant
T is the time in seconds
n is a constant
 

The irreversible charging current is of the same general form as the reversible charging current, but is much smaller in magnitude. The irreversible charging current is lost in the insulation and thus is not recoverable. Again, sufficient time should be allowed before recording test data so that the reversible absorption current has decreased to a low value. be eliminated by the use of stress control shielding at such points during tests.

Surface leakage: The surface leakage current is due to the conduction on the surface of the insulation where the conductor emerges and points of ground potential. This current is not desired in the test results and should therefore be eliminated by carefully cleaning the surface of the conductor to eliminate the leakage paths, or should be captured and guarded out of the meter reading.


Partial discharge current: The partial discharge current, also known as corona current, is caused by overstressing of air at sharp corners of the conductor due to high test voltage. This current is not desirable and should be eliminated by the use of stress control shielding at such points during tests. This current does not occur at lower voltages (below 4000 volts), such as insulation resistance test voltages.
Volumetric leakage current: The volumetric leakage current that flows through the insulation volume itself is of primary importance. This is the current that is used to evaluate the conditions of the insulation system under test.

 Sufficient time should be allowed for the volumetric current to stabilize before test readings are recorded. The total current, consisting of various leakage currents as described above, is shown in Figure 1.2.

Figure 1.2:
Various leakage currents due to the application of DC high voltage to an insulation system.

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