The insulation was designed to withstand stresses during its useful life, but we must bear in mind that the electrical insulation degrades over time and abnormal stresses can accelerate this natural process, shortening its life. This is why it is important to perform tests regularly to detect accelerated aging, its cause and actions to correct the situation.
The degradation of the insulation is due to 5 causes that interact with each other: electrical solicitation (due to overvoltage or sub-voltages that lead to cracking or delamination), mechanical (due to knocks, frequent stops and starts, vibration,..), chemical attack (by vapors) corrosive, dirt or oil), thermal stress (excessive heat or cold conditions) and environmental pollution (humidity, holes for rodents,..)
For 500 or 1000V voltages, punctual tests are carried out to simply know if the insulation is in good condition, but at voltages higher than 1Kv we use another methodology, always using a stable voltage throughout its resistance range with improved sensitivity.
The diagnostic tests of insulation in the preventive maintenance program are essential to predict and prevent ruptures of electrical equipment. The use of this equipment is not associated with aged equipment but it can be used to accurately locate the position of the test object anywhere along its aging curve.
How is the insulation resistance measured?
Amperis insulation test equipment is a high-range resistor meter with a built-in DC generator, portable equipment that provides a direct reading of the insulation resistance. In good insulation, the resistance will be in the range of megohms or higher.
If we apply a test voltage through an insulation portion, measuring the resulting current and applying the Ohm's Law (R = E / I), the insulation resistance can be calculated, but when more than one current flows, the analysis it gets complicated. You must take into account: the capacitive charge current (short-term large current), absorption current or polarization (composed of 3 components decay to zero for several minutes), surface leakage current (due to moisture or salt contamination) of the insulation surface) and conduction current (stable through insulation). The total current is the sum of these components, and is what Amperis insulation test equipment measures.
The first step in any insulation test is to ensure that the insulation is completely discharged.
When insulation tests are done, the more the operator knows about the results (during and after the test), the better his decision will be on how to implement a possible solution to the problem. Amperis equipment is used to measure the insulation resistance: in a shielded power cable, in circuit breaker or bushing insulators, in a power transformer, in an AC generator ...
By means of the load graph that indicates the characteristics of the output voltage as a function of the load resistance, we ensure that an adequate voltage is applied to the resistance in the range of interest. The equipment of Amperis insulation test equipment is a high-range resistor meter with a built-in DC generator, which provides a direct reading of the insulation resistance. In good insulation, the resistance will be in the range of megohms or higher. Voltage to a level of resistance corresponding to good insulation. A fast growth time ensures an effective measurement.
Protection terminal
The protective terminal must perform well (ability to efficiently eliminate the effects of surface or unwanted leaks) and be well protected (against accidental application of voltage or transients according to IEC61010); for this reason it is an important part of a team of insulation tests> 1Kv.
The equipment offered by Amperis includes a protective terminal which allows to deliver the full spectrum of information obtained through isolation tests. Hence the importance of the protective terminal, that the better its performance, the greater the accuracy of the insulation resistance measurement. Remember that an effective preventive maintenance to obtain an early detection of faults, depends on the results of the tests. Therefore, the user can quickly identify if there is a surface leakage and what is its magnitude: you only have to perform 2 tests, one with the protective terminal and another without it.
The performance of high-voltage insulation tests is an essential tool for fault diagnosis and for the monitoring of conditions, so the use of the protective terminal must not destroy the accuracy of the instrument (since the equipment does not only have that deliver the necessary current for the insulation test, but also the deflected current flowing through the protective terminal).
In the image is the AMIC-5010 insulation resistance equipment, one of the best rated equipment by our customers.
http://www.amperis.com/en/products/insulation-testers/amic-5010/
Results analysis
One of the most important characteristics of an insulation test equipment is the range that the instrument can measure.
The important information is the tendency of the readings in a period of time, which shows a reduction in resistance and warns about future problems.
- In a good insulation the leakage current or conduction current is weak and the measurement is highly influenced by the capacitive charge and dielectric absorption currents. The measurement of the insulation resistance will increase during the time of application of the test voltage, since these eddy currents decrease.
- In an incorrect insulation (deteriorated, dirty and humid), the leakage current is constant and very strong, in addition it exceeds the currents of capacitive load and of dielectric absorption; thus the measurement of the insulation resistance will in this case reach, very quickly, a constant and stable level.
Therefore, by examining the variations in the insulation value as a function of the duration of application of the test voltage, the quality of the insulation is determined. Within the preventive maintenance program, it is recommended to save the periodic checks.
The insulation resistance measurement is based on Ohm's law. If we apply a DC voltage of known value (which is less than the voltage of the dielectric test) and then measure the current in circulation, the resistance is easily determined. The resistance of the insulation has a very high value, therefore, measuring the weak current in circulation, the mega-meter indicates the value of the insulation resistance. This resistance shows the quality of the insulation and possible risks of circulation of leakage currents, being able to be affected the value by the temperature and the humidity (these 2 factors make vary the value of the insulation resistance, the ideal is to make the measurements under a reference temperature above the dew point, otherwise it would be affected by humidity).
The current circulating in the body of the insulator is the sum of 3 components:
- Capacitive load current: transient current corresponding to the load capacity of the tested insulation, raised at the beginning and negligible when the circuit tested is electrically charged.
- Absorption current: contribution of energy to reorient the molecules of the insulator, decreases much more slowly.
- Leakage current or conduction: it is the current that indicates the quality of the insulation, it is stable in time.
The total current flowing through the insulation that is being tested is variable over time.
Isolation diagnostic tests
- Punctual or short-term measurement: it is the simplest method. The test voltage is applied for less than 1 minute and the insulation resistance value is pointed at that instant. It is highly disturbed by temperature and humidity. With this method the trend is analyzed over time, being more representative of the evolution of the insulation characteristics of the installation and equipment, concluding a correct diagnosis, being able to compare the reading with the minimum specifications of the installation.
- Methods based on the influence of the time of application of the test voltage: diagnostic test taking successive readings at determined intervals to compare the graph of a sound insulation and a contaminated isolation (almost no influence on temperature). They are recommended for the preventive maintenance of rotating machines and the control of their insulators.
- Polarization index: consists of making 2 readings at 1 and 10 minutes. By dividing the insulation resistance at 10 minutes between the 1 minute, the polarization index is obtained. Recommendation IEEE 43-2000 "Recommended Practice for Testing Insulation Resistance of Rotating Machinery" defines: that an IP index greater than 4 is a sign of good insulation, but an index of less than 2 indicates a problem.
- Dielectric absorption ratio (DAR): it is similar to the IP but dividing the insulation resistance at 60 seconds between the 30 seconds. If the DAR is greater than 1.6 the isolation condition is excellent and if it is less than 1.25 insufficient.
- Method based on the influence of the test voltage variation (measurement by steps): Time-based measurements (such as PI, DAR views) can reveal the presence of contaminants (dust, dirt) or moisture on the surface of the samples. insulators Doing a scale test distributing in 5 equal steps the maximum applied voltage (the results are independent of the type of insulation and temperature).
- Discharge test method (DD): or reabsorption current test is carried out by measuring the current during the discharge of the dielectric of the equipment to be tested. It is calculated by dividing the current between the product of the test voltage and the overall capacity. If the DD value is greater than 7 the quality of the insulation is bad, if it is less than 2 it is good. This method is temperature dependent.
Amperis insulation test equipment meets the recommendations for tests described in the IEEE standards
More information:
http://www.amperis.com/en/products/insulation-testers/