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ISO166/ISO176

PARTIAL DISCHARGE

When an insulation defect such as a void occurs within an

insulation system, the defect will display localized corona or

ionization during exposure to high-voltage stress. This ion-

ization requires a higher applied voltage to start the dis-

charge and lower voltage to maintain it or extinguish it once

started. The higher start voltage is known as the inception

voltage, while the extinction voltage is that level of voltage

stress at which the discharge ceases. Just as the total insula-

tion system has an inception voltage, so do the individual

voids. A voltage will build up across a void until its incep-

tion voltage is reached, at which point the void will ionize,

effectively shorting itself out. This action redistributes elec-

trical charge within the dielectric and is known as partial

discharge. If, as is the case with AC, the applied voltage

gradient across the device continues to rise, another partial

discharge cycle begins. The importance of this phenomenon

is that, if the discharge does not occur, the insulation system

retains its integrity. If the discharge begins, and is allowed

to continue, the action of the ions and electrons within the

defect will eventually degrade any organic insulation system

in which they occur. The measurement of partial discharge

is still useful in rating the devices and providing quality

control of the manufacturing process. The inception voltage

for these voids tends to be constant, so that the measurement

of total charge being redistributed within the dielectric is a

very good indicator of the size of the voids and their

likelihood of becoming an incipient failure. The bulk incep-

tion voltage, on the other hand, varies with the insulation

system, and the number of ionization defects and directly

establishes the absolute maximum voltage (transient) that

can be applied across the test device before destructive

partial discharge can begin. Measuring the bulk extinction

voltage provides a lower, more conservative voltage from

which to derive a safe continuous rating.

PARTIAL DISCHARGE TESTING

Not only does this test method provide far more qualitative

information about stress-withstand levels than did previous

stress tests, but it provides quantitative measurements from

which quality assurance and control measures can be based.

Tests similar to this test have been used by some manufac-

turers, such as those of high-voltage power distribution

equipment, for some time, but they employed a simple

measurement of RF noise to detect ionization. This method

was not quantitative with regard to energy of the discharge,

and was not sensitive enough for small components such as

isolation amplifiers. Now, however, manufacturers of HV

test equipment have developed means to quantify partial

discharge. VDE in Germany, an acknowledged leader in

high-voltage test standards, has developed a standard test

method to apply this powerful technique. Use of partial

discharge testing is an improved method for measuring the

integrity of an isolation barrier.

To accommodate poorly-defined transients, the part under

test is exposed to voltage that is 1.6 times the continuous-

rated voltage and must display less than or equal to 5pC

partial discharge level in a 100% production test.

APPLICATIONS

The ISO166 and ISO176 isolation amplifiers are used in

three categories of applications:

• Accurate isolation of signals from high voltage ground

potentials.

• Accurate isolation of signals from severe ground noise and,

• Fault protection from high voltages in analog measure-

ments.