Using insulation testing to diagnose electrical problems

By Jeff Jowett, Senior Application Engineer, AVO International, Valley Forge, Pennsylvania

If a critical piece of capital equipment "dies", the obvious response is to attempt to bring it back online through troubleshooting and repair. Is there power? Connect a voltmeter. Check the fuses. Is it drawing too much current? Take a reading with a clamp-on ammeter. Hook up an insulation tester and look for shorts to ground. These commonplace diagnostic procedures are the most familiar applications of electrical test instrumentation to the maintenance of capital equipment.

The sophistication of electronic test instruments, however, has broadened and refined the capabilities of the devices, adding a new dimension to electrical maintenance. Comparatively crude "go/no-go" tests are still as useful as ever. But they can now be supplemented with analytical procedures that furnish the astute maintenance technician with a reliable picture of the equipment's condition with respect to its normal life cycle.

A fortunate property of insulation is that it can be used as a barometer of the overall condition of the electrical system it supports. Insulation deteriorates steadily and gradually over time. Its electrical resistance can be measured and used as the relevant indicator. Circuitry exhibits no such predictive deterioration, while components tend to fail catastrophically and can be difficult or inconvenient to test. Insulation, in addition to its primary function, provides the ideal indicator of the equipment's condition.

New technology = new opportunity

The challenge of electrical instrumentation, then, is to facilitate maximum use of this opportunity. The enhanced performance of newer technology lets established procedures yield greater insights and make new maintenance methods available. Traditional testers commonly generated only 1,000 volts, and the measurement range was often limited to hundreds of, or a few thousand, megohms. The quantum leap to a 5-kV tester was often made on a "take it or leave it" basis, purely to accommodate the demands of high-voltage equipment.

Modern instruments deliver stable voltage, above minimum load requirement, over the full resistance range of the test specimen and feature microprocessor sensitivity in the measuring circuit. The combination of higher voltage and enhanced sensitivity enables the tester to both draw and measure the miniscule current that quality insulation in new, capital equipment passes. Accordingly, a plant can develop and implement sophisticated procedures that rely on precise measurements. The insulation tester isn't limited to values associated with faulty or aged equipment, but can be used to pinpoint the test item's position anywhere along its aging curve. The "infinity" (that is to say, over-range) indication, which is a delight to the repair technician, represents a void to the diagnostician. Improved 5-kV testing fills that void with valuable analytical data.

Standard tests gain increased capability

Familiar standardized test procedures that have been used for years benefit from the improved capabilities of enhanced 5-kV testing. Most basic of these is the time-resistance method. A valuable property of insulation, but one that must be understood, is that it "charges" during the course of a test. The polar DC field the tester applies causes realignment of the insulating material on the molecular level, as dipoles orient themselves with the electric field. This movement of charge, of course, constitutes a current. Its value as a diagnostic indicator is based on two opposing factors: the current dies away as the structure reaches its final orientation, while "leakage" promoted by deterioration passes a comparatively large, constant current. The net result is that in "good" insulation, leakage current is relatively small, and resistance rises dramatically as charging goes to completion. This changing resistance bedevils the unschooled, but is exactly what the diagnostician wants to see. Deteriorated insulation passes relatively large amounts of leakage current at a constant rate for a given applied voltage. This leakage current "floods out" the charging effect.

Polarization index

Time-resistance methods, as they are known, take advantage of this effect. Graphing the resistance reading as a function of time from initiation of the test yields a smoothly rising curve for "good" insulation, but a "flat" curve if the insulation shows signs of deterioration (see Figure 1). The ultimate simplification of this technique is represented by the popular polarization index test that requires only two readings and simple division. The 10-minute reading is divided by the one-minute reading to give a ratio. Obviously, a low ratio indicates little change, hence poor insulation, while a high ratio indicates the opposite. References to typical polarization index values are common in the literature, which makes this test very easy and readily used. Note that resistance readings alone are almost meaningless since they may range from enormous values in new equipment down to a few megohms just before removal from service. A test like the polarization index is particularly useful because it can be performed on even the largest equipment and yields a self-contained evaluation based on relative readings rather than absolute values. No polarization index can be calculated with a tester of limited range, because "infinity" is not a meaningful number that one can use in a calculation. Advanced 5-kV testers readily reach the teraohm range-1012, or thousands of thousands of "megs"-and so do not run off the graph. The largest and newest capital equipment can be readily tested to yield repeatable data for recording and subsequent trend evaluation. Figure 2 shows typical polarization index test results.

Step voltage

The step voltage method is another familiar technique that enjoys expanded applicability when a 5-kV tester is available. Quality insulation can be expected to withstand over-voltage stress. Deteriorated insulation, however, exhibits an increase in current passing to ground as higher voltages exploit more structural flaws. Hence, resistance decreases noticeably with voltage. A recognized standard procedure is to increase voltage in five one-minute increments. If the readings deviate by more than 25 percent, the test item is judged to be unfit and in need of service. Obviously, a typical 500- and 1,000-V tester only approximates this test. Advanced 5-kV models perform a rigorous test, typically in increments from 1 to 5 kV or from 0.5 to 2.5 kV, and are fully automated (see Figure 3). Like the polarization index, the step voltage test is a repeatable, self-evaluating test free of extraneous influences such as temperature effect.

Different problems call for different tests

Various procedures not only provide different ways of testing insulation; they also evaluate different problems selectively. The polarization index test is particularly valuable in revealing moisture ingress, oil soaks and similar pervasive contamination sources. These invading molecules provide convenient paths to ground for electrical leakage, which damages the surrounding insulation and eventually burns through as a "short." This type of problem is revealed at almost any test voltage and appears as a characteristically "flat" polarization index. Moisture and contaminants also bring the readings down, but this requires a previous value for comparison; the polarization index test has the advantage of making an internal comparison.

However, other problems may seem to "pass" a polarization index or simple "spot reading" test by yielding high resistance values at a given voltage. Such problems include localized physical damage, such as pinholes, or dry, brittle insulation in aged equipment. Step voltage tests, however, reveal such problems. Increasing numbers of imperfections pass greater current as greater voltage is applied, a phenomenon reflected in a declining resistance. Higher voltage pulls arcs across small air gaps, thereby providing an early warning of an incipient problem. As equipment ages, the accumulation of dirt and moisture narrows such gaps until a short to ground develops.

A new procedure

An example of a still more specialized test, recently developed by EdF, France's national power utility, is the dielectric discharge test. Unlike more familiar tests, this measures the current that flows during discharge of the test sample. It is especially applicable to multilayered insulation. The test item is first charged at high voltage until full absorption has taken place (10 to 30 minutes). This allows the capacitance to be fully charged and the alignment of dipoles (absorption) essentially completed (see Figure 4). Only leakage current continues to flow. When the external voltage field ceases, molecules "relax" and return to their original random configuration, which constitutes a reabsorption current. This discharge current is measured 60 seconds after the insulation test is finished. At this time, capacitance is discharged and voltage has collapsed, so that the charge stored in the dipoles can be viewed independently from the "masking" currents that are dominant during an insulation test. A high reabsorption current indicates that the insulation has been contaminated, while a low current indicates that it is relatively clean. The precise definition of dielectric discharge (DD) is: current in nanoamperes flowing after one minute / (test voltage x capacitance in microfarads).

The dielectric discharge provides a figure of merit that indicates the condition of the insulation. In multilayer insulation, layers are intended to share the voltage stress equally. Upon discharge, the charge in each layer decreases equally until no voltage remains.

The leakage resistance of a faulty layer sandwiched between good layers will decrease while capacitance is likely to remain constant. The result of a standard insulation test is dominated by the good layers, and not likely to reveal this condition. But during dielectric discharge, the time constant of the faulty layer is sufficiently different from the others to yield a higher dielectric discharge value. A low dielectric discharge value indicates that reabsorption current is decaying quickly, and the time constant of each layer is similar. A high value indicates that reabsorption exhibits long relaxation times, which may point to a problem (see Figure 5).

Implications

The dielectric discharge test illustrates the expanded capabilities that can be accomplished with the high sensitivity of new 5-kV models. Reabsorption effects are small compared with typical test currents, requiring a sophisticated tester for meaningful evaluation. But new electronic models perform this and other tests automatically. Test selection in a given situation relies on the judgment of the operator. Each test gives one perspective on the overall condition of the equipment. Their combination forms a more complete picture than any one test provides. In the practical world, there may not be time to completely analyze a piece of equipment. Educated choices must be made to select the most effective test strategy.