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ISO176P Datasheet(PDF) 8 Page - Burr-Brown (TI) |
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ISO176P Datasheet(HTML) 8 Page - Burr-Brown (TI) |
8 / 8 page 8 ® 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. |
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