The "Y2K" panic a decade ago called attention as never before to the crucial position of generators in the electrical power chain. Stores sold out of them for on-site backup as the public feared a loss of power when computers controlling the grid failed to properly recognize the date stamp of the new century. The widespread loss of power, of course, never occurred. But it served to put the critical role of generators briefly into the public conscience. Today, they continue to churn away, often out of sight and out of mind, except to a strategic maintenance crew that has to keep them running.
The failure of power is, of course, unacceptable, as the Y2K uproar made plain. But first-line electrical maintenance can be performed relatively easily and readily, once armed with a good piece of test equipment and a proper knowledge of how to test and what to look for. The starting point is to recognize the usefulness and establish a program of preventive and predictive maintenance. Electrical maintenance is typically reactive, in response to a "problem" at or near the end of the equipment's life cycle. The most frequent expression of such a maintenance program is to run until it breaks, and then either change out or repair. Generators, however, are typically too costly and too critical for this to be practical. A better plan is to test regularly and plan accordingly...be proactive rather than reactive.
The idea is to use electrical test data to gain a lead time, most desirably in months, to actual failure. Repair and maintenance can then be performed when convenient, rather than imperative, such as during shutdowns or when alternate equipment is in operation. Maintenance performed at scheduled intervals is considered preventive. This includes such simple procedures as cleaning, drying, and checking bonds and connections. Insulation test data enables these intervals to be set judiciously so extra time isn't spent doing redundant operations before they are fully productive. Alternatively, all the "eggs" can be put in "one basket" by constructing a time line that indicates when the equipment approaches a final stage before impending failure, and service is put off until that time. This is predictive maintenance. For these programs, the required testing equipment will be reviewed, some testing procedures described, and some tips on evaluating equipment offered.
The tester of choice for performing electrical preventive and predictive maintenance is the insulation tester or megohmmeter. The basic data collected will be insulation resistance readings. These are ideal because they are relatively easily collected. The testers are not terribly expensive and can be downright economical. But one must be aware of the possible limitations of inexpensive models. The tester is simple in concept, but can be complex in detail. In order to get enough current to measure, a high voltage is applied across the insulation. Test voltage is based on operating voltage and typical selections are "as rated" and "twice rated," but there is no fixed standard. For routine maintenance, a test voltage approximating operating voltage is sufficient, but in troubleshooting, twice-rated tests can reveal problems that might not appear at a lower test voltage. The tester measures the leakage current and converts it, by Ohm's Law, into its reciprocal, resistance. Since resistance must be extremely high in order to render equipment safe and efficient, the unit of measurement is the megohm, or million ohms. Don't be dismayed by "giga-ohms" (thousands of megohms) or "tera-ohms" (thousands of gigs). These higher ratings are becoming more common with newer insulating materials.
So applied, insulation resistance readings act like the odometer on an automobile (but in reverse). When new, insulation resistance will typically overrange all but the highest-range testers. But as soon as it goes into service, the effects of operation impact on the physical and molecular structure of the insulating material create wear. Any imperfection or stress point can be exploited by electrical current to go where it's not supposed to...through the insulation instead of along the conductor. Minimal amounts of this "leakage" current are tolerated, typically on a nano- to micro-amp level. There is no "perfect" or "infinite" insulation. But as equipment wears in service, this unwanted current increases as it finds or creates more paths to exploit. Resistance drops accordingly and, like the miles on a car, can be used as a reasonably reliable indicator of the overall electrical condition of the equipment and an approximation of its position on its life cycle. This provides the basic tool of preventive/predictive maintenance.
Resistance readings are collected at regular intervals and graphed against time. This quickly establishes two significant guidelines: the rate of decline and the expected time of failure. Actual numbers may be high, but if the line is steep, it indicates that the life of the generator may not be up to expectations. Forewarned, an inspection might reveal something in the environment, such as a source of moisture or contamination that can be removed or diverted. If the time line subsequently flattens out, the corrective action has been effective. Standards agencies like IEEE provide recommendations, by type of equipment, for acceptable parameters of various measurements including insulation resistance. The graphic time line can then indicate approximately when the minimum acceptable value will be reached, and the generator can be serviced at some convenient time shortly before (Fig. 1). Conversely, such knowledge can also be used to select equipment to be passed over due to comfortably high readings, thereby making more efficient use of valuable time.
Megohmmeters all have dc output and come with positive and negative terminals. The test leads can be connected across any insulation barrier and a meaningful measurement taken (Fig. 2). It is important for the operator to have at least a mental picture, if not an actual schematic, of what is being tested in order to clearly understand the resultant data and maximize efficiency. Windings can be tested phase-to-phase, phase-to-ground or generator casing. Time can be saved, for instance, by testing all phases together to ground or any other convenient combination. The more that is connected to the megohmmeter, the lower the reading will be, because the more insulating material there is in the path of the test, the more leakage current will flow. Therefore, if multiple elements are combined in a single test and a satisfactory reading is obtained, then each individual element will read higher still. If the collective reading is low, then additional time can be spent sectionalizing the test item to break out the possible problem area.
"Sectionalizing" the generator can be accomplished by judicious use of the leads and terminals. If the tester has a third terminal, the testing capabilities take a quantum leap. A third terminal on an insulation tester is a guard terminal. Don't mistake it for a safety ground. The guard is a shunt circuit, its purpose being to remove a parallel leakage from the measurement. If connected to the same element that is under test, such as ground, it will short-circuit the test. All leakage will be diverted, the tester will register an "infinity" reading, and the operator will be falsely assured. Properly used, the guard diverts a parallel leakage path that the operator does not wish to measure, thereby providing a more sharply focused picture of the test item. Guards were devised to eliminate surface leakage–the tracking of current through surface dirt and moisture from one alligator clip to the other, which thereby brought down resistance readings, often dramatically. This was a problem at cable terminations, where surface leakage could be intercepted by an interposed bare copper conductor, connected to the guard and diverted around the measurement function (Fig. 3). Accordingly, the tester would display only the effect of leakage through the body of insulation.
Applied judiciously, the guard can, for instance, identify a problem on a particular winding by testing it to ground with the other windings guarded out. A short between windings can be spotted by testing between the windings while guarding out the rest of the generator. The guard helps to localize problems and reduce time in troubleshooting and repair. But beware! There's a cost. All insulation testers have very limited current output. This is because little current is needed to test insulation. If the insulation is passing more than a couple of milli-amps, it isn't insulating anymore! (The human body typically reacts to about 5 mA as a "shock.") Parallel leakage paths are competing for limited current. If too much is diverted by the guard circuit, there may be considerable voltage drop across the test item, thereby producing confusing results and nullifying comparison to previous tests. To avoid this, the tester should have an acceptable guard terminal error. This is the error in addition to the basic percentage accuracy, when the guard is in use. Make sure that it is acceptable.
Economical megohmmeters may be sufficient to perform basic maintenance and record keeping. But be aware of two considerations that may be critical in the long run, having to do with both the high and low end of range selection. On the high side, increased sensitivity to smaller amounts of current has expanded the range of modern testers. Some models now range high into tera-ohms. (To put in perspective, that's one trillion ohms!) This isn't just a design engineer "showing off." High-range models can perform a commissioning test at time of installation of a new generator and provide more valuable data than the old "infinity" symbol. The ability to record high numbers can provide a "distant early warning" of precipitous change and give the operator flexible time in which to make corrections and adjustments. A meter of limited range may not indicate an impending failure until reaction time has been shortened to the point of inconvenience or even emergency (Fig. 4).
Equipment failures are not always cumulative, but may be catastrophic. Examples are fire and flooding. In such instances, previous readings are undermined all at once, and the low end of the tester's range comes into use. Many insulation testers measure only at opposite ends: high voltage/high range and continuity. An added function that can be critical after catastrophic failure is a kilohm range. This is a low voltage test that closes the gap between continuity and insulation ranges. It is useful in determining the precise condition of failed equipment or suspected failures. While limited current makes megohmmeters non-destructive of good insulation, any additional stress to failed equipment is better avoided. A quick test with a kilohm range will show if the test item has dropped below the megohm range, to "thousands" of ohms. This is not an acceptable reading for operation. A brief drying procedure should then show an increase in resistance, indicating that the equipment can be restored to service. This is a more efficient procedure than getting a high voltage reading off the low end of the scale and then fumbling in the dark.
Finally, the sine qua non of good testing is safety. Safe practice occurs both during and after the test, and involves both test instrument and test item. Insulation tests are always performed on de-energized equipment, but accidents happen. Important safeguards are high voltage warning, test inhibition and IEC61010-1 rating. The IEC rating indicates level of protection from arc flash and arc blast. It is imperative to be familiar with it and to use the instrument in its proper environment and voltage rating. At the conclusion of a test, remember Yogi Berra: "It ain't over 'til it's over." A long dc test on a fairly large generator can store enough static charge to be lethal. The operator does not want to be the discharge path to ground! Testers designed for safety will have a built-in voltage warning and discharge circuit so the operator knows when it is safe to touch the test item. For the sake of time, discharge can be augmented with discharge sticks. But the important thing is for the tester to immediately begin affording protection upon conclusion of the test, without conscious operator involvement.
A quality tester complete with necessary features and functions, and the utilization of a well-maintained system of data collection will keep generators running for a longer life cycle.