techniques for improved predictive maintenance testing of industrial power systems.pdf.pdf

992 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 25, NO. 6, NOVEMBERIDECEMBER 1989

Techniques for Improved Predictive Maintenance Testing of Industrial Power Systems

Absfmcf- Periodic testing of industrial power system components can result in the early detection of problems, allowing corrective actions to be taken before a failure occurs. The positive impact of this on system availability and safety is beginning to be appreciated by both maintenance and production personnel in the mining industry. A variety of procedures are available for periodic testing, although they have some inherent limitations which must be recognized if a meaningful interpretation of the test data is to be achieved. These tests, and their inherent limitations, are reviewed in this paper; appropriate applications for these tests are suggested, and the use of negative-sequence monitoring, as an inexpensive but powerful detector of incipient failure in industrial power systems is proposed.

INTRODUCTION

N unexpected failure of a power-system component may A, esult in a loss of production. Often the cost of lost production far exceeds the cost to replace the failed component. Safety can be compromised as well. The failure may result in an exposure hazard, or the pressure to restore power may result in unsafe worker practices. In some cases the unexpected failure may result in damage that goes beyond the failed component itself. These problems are well-known, and a variety of techniques have been developed to aid in detecting deterioration, which signals a future failure. When detected, a corrective action can be scheduled, thereby avoiding production delays, safety problems associated with emergency maintenance, and additional damage to the power system.

It would seem that with the existence of these techniques and the well-understood benefits of detecting incipient failures, they would be widely used. In some industries they are used, but in mining they generally are not. There are three apparent reasons for this: first, interpreting the results of such tests is sometimes problematic, leading to a loss of confidence in the method; second, a knowledge of these techniques is not well-understood throughout the mining industry; and third, many of the existing test techniques are not easily performed in mines, and particularly in underground mines.

As part of an ongoing effort to develop continuous monitoring techniques for incipient-failure detection, it was recently

Paper PID 89-42, approved by the Mining Industry Committee of the IEEE Industry Applications Society for presentation at the 1987 Mining Industry Committee Technical Conference, Pittsburgh, PA, June 9-1 1 .

J. Sottile is currently a Graduate Research Fellow at the Pennsylvania State University Mine Electrical Laboratory, 421 Academic Utilities Building, Uni- versity Park, PA 16802.

J. L. Kohler is Director of the Mine Electrical Laboratory, Pennsylva- nia State University, 41 1 Academic Activities Building, University Park, PA 16802.

IEEE Log Number 8930229.

necessary to compare these techniques to existing periodic test methods. While collecting information to perform this comparison, two things became evident. First, the limited application of periodic testing being conducted in the mining industry is fraught with problems, primarily due to a misun- derstanding of the test techniques. Second, there is a strong need for a reliable technique that can be easily and inexpen- sively implemented. The objective of this paper is to address both of these observations by reviewing the tests and then presenting an alternative method, which was developed from research findings of the ongoing incipient-failure prediction research program.

PERIODIC TEST METHODS

Periodic test methods, i.e., manually acquired measurements taken from time to time rather than continuously, can be divided into three groups based on the characteristic of the ex- citation source used in the test. These are direct current, alternating current, and surge tests. Applicable techniques within each of these groups will be discussed in this paper.

DC TESTS

Direct-current methods are widely used in motor insulation testing, probably because of their convenience of use as much as anything else. The test apparatus is lightweight and portable, the tests are relatively safe, and very large values of resistance (megohms) can be measured. A megohmmeter is typically used for performing the tests although hypots (i.e., high-potential testers) may also be used.

The dc test methods are based on measuring total current as a function of test voltage and specimen resistance. Actually, this total current is composed of three separate components.

The first component of the total current, commonly known as geometric capacitance current, is defined as

i l = (V/R)e-'/RC (1)

where

C the geometric capacitance (sometimes referred to as the infinite frequency capacitance),

V the potential of the dc source, and R the source's internal resistance.

It is generally believed that the geometric capacitance current gives no indication of the condition of the insulation. How- ever, erroneous results will occur if this current is not allowed to decay to an insignificant value before readings are taken.

0093-9994/89/11OO-0992$01 .OO 0 1989 IEEE

SOTTILE, JR. AND KOHLER: IMPROVED PREDICTIVE MAINTENANCE TESTING 993

The second current component is the absorption current, which is a consequence of the various polarizations taking place in the dielectric under the influence of an impressed dc voltage. Interfacial polarization, which occurs at the interface of dissimilar materials, constitutes the major effect. Current flow due to interfacial polarization is described as

i 2 = dVCkt-” (2) where

d V the incremental applied voltage, C the specimen capacitance, k a proportionality factor, dependent on type of insula-

tion, its condition, and temperature, t time, and n a constant, known as the storage coefficient.

The specifics concerning the parameters of this equation are not of interest because they are rarely computed in an industrial application. Rather, it is the existence of this current and its general behavior which is of interest. For example, the “dielectric absorption ratio test” was designed to take advan- tage of changes in the parameters of (2) , which can occur as a dielectric ages [ 11.

The third component of the total current is the conduction or leakage current, which remains once the geometric capacitance and absorption currents have decayed to zero. This component is defined by Ohm’s law:

i3 = VIR (3) where

V the applied voltage, and R the resistance of the insulation.

It is typically the conduction current that is considered most important during dc insulation tests. This current is free to follow a path either along the surface of the insulation or through the insulation.

Most field tests of machinery insulation do not attempt to distinguish the various current components. Instead, different test procedures are designed to reflect the effects of these components on the measured resistance.

The time-resistance method is an example of one such test. In this method, two or more readings are taken over the dura- tion of the test time (i.e., 5 or 10 min). If the insulation is in good condition, there should be a gradual increase in resistance. This effect is caused by the decrease in the absorption component. If the insulation is contaminated, the resistance will not increase during the test period.

The dielectric absorption ratio is a ratio of two time- resistance readings, for example 60 s/30 s. This ratio is uti- lized to show the effects of absorption current in good insulation. Reference [2] includes a table that gives insulation condition for various dielectric absorption ratios. However, these types of tables are only to be used as guidelines; maintenance personnel must develop their own guidelines based on experience over years.

Some maintenance personnel establish a step-voltage test for testing insulation well above the rated ac voltage of the

equipment. It is believed that such dc tests are capable of re- vealing incipient problems that could not otherwise be found. The technique for performing this type of analysis consists of applying two or more dc voltages and observing any reduction in insulation values at the higher voltage levels. It is important to discharge the specimen between steps during this test. Otherwise the absorption current at each voltage level will be superimposed, resulting in a cumulative value that would not be useful for field applications [l].

Factors Affecting DC Tests Because dc methods measure such high values of resistance,

many factors involving both the condition of the insulation and the test procedure affect the results. As will be seen, many of these problems are associated with the absorption current.

The resistance of insulating materials decreases substan- tially with increasing temperature. In many cases, a rule of thumb has been to halve the resistance for every 10°C increase in temperature. In addition, tables of correction factors have been developed to correct resistance readings to a base temperature [2]. However, problems exist with these simple correction factors because the correction factor for leakage current is not necessarily the same as that for absorption current. Thus, it is not always accurate to compare results obtained at two different temperatures, particularly when the absorption component is significant.

Another temperature-related problem is the uneven cooling that occurs within a motor once it has been deenergized. It is very difficult to estimate an average temperature for the motor windings, unless the motor can be kept off-line long enough to reach ambient temperature. This is often not practical, and even if it were, condensation within the motor can lead to erroneous results. It should be noted that step-voltage tests are particularly susceptible to error in motors that are not at thermal equilibrium, because the winding temperature will continue to drop between tests.

The second major factor affecting the accuracy of dc tests is moisture. Insulation resistance decreases dramatically with increasing moisture, and in many cases, dc tests reflect the moisture content of the insulation. This is not necessarily a problem as long as the operator is aware of the presence of moisture. If damp or wet windings are suspected, the motor can be dried out and retested before it is energized.

Problems with the execution of test procedures are much more difficult to identify. Probably the most common problems are associated with improper timing of the test, insuf- ficient wait time between voltages during step-voltage tests, and a fluctuating test voltage.

Evaluation The results of dc tests are relative, and therefore evaluation

is based on trends over time rather than on the specific value of one reading. For this reason, it is best to observe the trend of the readings by plotting the resistance over time for each piece of equipment.

It is also important for the person making the evaluation to be aware of any possible test problems or conditions that may cause sudden changes in the insulation resistance. In many


cases, low values are caused by the effects of humidity; this can be remedied if the windings can be dried out. In other cases, anomolous readings may occur because of temperature effects or problems with measurement methodology.

Often, results from properly performed tests are difficult to evaluate, because it is not always clear when a motor should be replaced. A quantitative means of assessing individual insulation resistance (IR) readings is offered in [3]. This reference defines an acceptable minimum insulation resistance as

Rmin = kV + 1, (4)

where

Rmin

kV

This rule implicitly accounts for the fact that test intervals are typically several months apart. Thus, it yields very conser- vative values that will almost always allow adequate lead time to replace a failing motor. However, some maintenance personnel would argue that such an approach leads to unnecessary equipment changeout. For example, this standard indicates that a low voltage motor should have an IR value in excess of 1.5 MR, yet experience has shown that values of 25 kR do not present operational or safety problems [4]. There are cases of motors operating for years with IR values of 5000 R. It is this problem with interpretation that has led many maintenance personnel to limit IR testing to high-voltage motors, for the purpose of identifying arcing or tracking paths.

DC tests have several advantages, if properly performed. The insulation can be evaluated in terms of a spot resistance, resistance versus voltage, ratios of resistances, and so forth. Each of these tests will yield specific information about the machine insulation. In addition, the equipment is portable, and the tests are relatively safe. These points make dc tests very attractive for testing low voltage motors if the user can establish a meaningful procedure to interpret the results.

The technical problems associated with dc tests mainly occur as a result of the application. Informative tests require time and carefully executed test procedures. For example, the proper testing and evaluation of step-voltage tests can become quite involved [ 5 ] , [6]. Even the determination of insulation resistance due to conduction current is quite time consuming because the absorption current is present for such extended times. Tests on small motors have indicated the presence of detectable absorption current for extended times (2-45 min). Beyond this time, the absorption current may still be present but may be masked by the leakage current.

For most small-motor applications, the informative and time-consuming tests can rarely be justified, except on the most critical machines. In fact, many mines do not have maintenance policies that include insulation tests on many of the mine motors. Reasons for lack of testing are understandable. In the course of a production shift, it is unlikely that a mining machine will ever be disconnected from its power source for testing individual motor insulation. Even during maintenance shifts, crews are typically very busy with more immediate

the recommended minimum insulation resistance in MR at 40"C, and the machine terminal potential, kV.

6 Fig. 1. Circuit model of dielectric.

problems. In addition, the prospect of accessing the individual motor leads for testing is not attractive.

At longwall faces, motor maintenance is more easily justified, particularly with the longer faces and the introduction of high voltage at the face. The ability to predict motor failure so that changeouts could be made during moves or on a week- end would be very valuable. Also, by the nature of longwall mining systems, there are opportunities for performing more thorough tests at surface facilities as opposed to underground. These tests could include ac tests or surge tests.

AC TESTS

In principle, ac test methods are quite different than dc methods. The basic differences can be illustrated by looking at the model for a dielectric and reviewing the major losses that occur in a dielectric.

Fig. 1 illustrates the circuit model for a dielectric. If this dielectric were "ideal," Rs = 0 and Rp = 00, and the phase angle 0 would be 90". Of course, insulating materials are not ideal, and the phase angle between voltage and current is less than 90". Probably the most common method of expressing this phasor relationship is the dissipation factor or tan 6 (6 = 90 - 0). Tests that measure this phase angle can be used to obtain information about the losses in the dielectric that relate to the insulation's condition.

Losses

Partial discharges can represent significant losses in large high-voltage machinery. It is doubtful that partial discharges represent a source of concern in most underground mining machinery; therefore, it is only mentioned here for complete- ness. It is, however, possible that partial discharges could occur in some of the high-voltage equipment on longwall faces, particularly if there are many voids in the insulation [7].

The effect of polarization from an alternating-voltage source is quite different from that of a direct voltage. For dc tests, the current component that resulted from the impressed voltage was termed absorption current, and it decayed to zero after a short time. Under the influence of an alternating voltage field, this polarization effect is continuously occurring because of the effect of the changing field. This polarization process now represents a loss in the dielectric. It is generally believed that the values of dissipation factor are largely determined by the existence of polarization.

Two types of polarization are known to occur: dipole and interfacial. Dipole polarization is produced by polar molecules, i.e., the electrical centers of the positive and negative charges do not coincide. Under the influence of an alter-

SOTTILE, JR. AND KOHLER: IMPROVED PREDICTIVE MAINTENANCE TESTING 995

nating voltage, the dipoles attempt to follow the direction of the field. If the frequency of the alternating field is sufficiently slow, dipole polarization can be carried to completion.

This rotation of the dipole represents a definite amount of energy loss at each cycle and is in the form of heat. At zero frequency and also at ultrahigh frequencies, this loss is neg- ligible. The maximum loss occurs between these boundaries and is somewhat dependent on the structure of the molecules

Interfacial polarization occurs at the interfaces of materials that have different dielectric constants. These interfaces can occur as very few large surfaces or as many small surfaces. The free electrons move through the dielectric and “bunch up” at the interfaces. The charge that accumulates at the interfaces during each half-cycle contributes to a polarization similar to that of dipole polarization.

Tests Tests that detect partial discharges essentially measure the

charge transfer that takes place in the voids during discharges. This charge transfer may be represented in loop traces using bridge techniques or pulse-counting techniques, among others. Details of these tests are not given because their applicability for mining operations is questionable.

Power-factor tests may find application in some surface facilities or repair shops. The principle behind power-factor testing of equipment is straightforward. The insulation under test is essentially the dielectric of a capacitor. With a perfect dielectric and infinite resistance, the in-phase leakage current across the capacitor is zero, no real power is consumed, and the power factor would be zero. This is of course for a the- oretical case; most insulation systems have power factors in the range of fractions of a percent to a few percent. This power factor represents the power lost through the insulation in phase with the applied voltage. As insulation systems dete- riorate, power factor increases.

Dielectric loss is the property of insulation most closely associated with power-factor testing (although dielectric loss is not actually measured). This loss occurs because of the time- varying polarization of the molecules in the insulation when an ac voltage is applied to the insulating material. Dielectric loss is a function of applied frequency and temperature and generally decreases with these two parameters [9].

A common test performed is the power-factor tip-up test. Power-factor tip-up has been used as a test on individual coils or groups of coils. Typically, measurements are made on each coil to determine the power-factor increment between two des- ignated voltages, with the data being analyzed on a statistical basis. A change in the tip-up value over a period of time is an indication of a change in the condition of the coil insulation. A limiting value of tip-up may be selected on the basis of the deviation from the statistically determined normal tip-up.

To determine power-factor tip-up, tests should be made at predetermined voltages. The recommended voltages for determining tip-up are 25 percent and 100 percent of operating line-to-ground voltage. The value of tip-up is then calculated by subtracting the low-voltage power factor from the high- voltage power factor [lo].

VI.

As with dc tests, several parameters can affect power-factor tests. The two major parameters are temperature and humidity. The dielectric loss of most insulations increases with temperature; therefore, to make accurate analyses of power-factor tests, corrections for temperature must be made. As with dc tests, these factors correct to 20°C.

The presence of moisture is another condition which af- fects power-factor tests. Recall that dielectric absorption is the charge placed on the insulation when a dc voltage is applied. In the case of an alternating voltage, the charge is never fully established before the current direction is reversed. This dielectric-absorption loss increases greatly with the amount of moisture present in the insulation. Power-factor tests are so sensitive to moisture that they may be used to test insulation moisture content [9].

In summary, ac tests provide an extension to dc tests in the evaluation of insulation integrity. Typically, they are more difficult to employ and require larger, more expensive equipment than their dc counterparts. They do, however, provide more complete information about the insulation system of the machine.

SURGE TESTS

Surge comparison testing is used to test a motor’s major as well as its minor (or turn and coil) insulation. The surge comparison tester uses the principle of impedance balance to test motor windings. By discharging a capacitor, the instrument applies a very brief surge to the windings being tested. The resulting voltage decay pattern of each of the windings under test is then displayed on an oscilloscope screen. If the windings have equal impedances, the two patterns will be identical and appear as one waveform on the screen. If one of the windings has an insulation defect or fault, the pattern displayed will be a double line. In addition, each winding has a characteristic trace or signature that can be used for future comparison [ 1 11.

In small three-phase machines, surge comparison tests have been documented as being able to detect a one-turn short circuit in as many as 600-series turndphase with test connections at the winding terminals. However, in windings with many parallel circuits, the detection of short-circuited coils by this method is unsatisfactory [ 121.

To test motor windings composed of many parallel circuits, it is most convenient to induce the voltage in each individual coil to be tested. This is best accomplished when the individual coils are available but may still be accomplished when only the phase terminals are available [ 131.

Test voltages should be high enough to stress the insulation adequately, but not too steep-fronted or high in amplitude to cause damage to the windings. In most cases, the time to crest of the impulse varies from 0.5 to 1.5 ps [14]. Table I [14] illustrates voltage levels for various voltage and power ratings of machines.

Although the applicability and utility of surge comparison tests depend on the theory of the distribution of the impulse through the motor windings, this type of analysis is beyond the scope of this paper and also beyond most practical applications. (For an analysis of this phenomenon, see [15].) Therefore, the manufacturer should be consulted for recom-


TABLE I COMMON SURGE IMPULSE TESTERS FOR ROTATING MACHINERY

Maximum Machine Impulse Rating

2.5 kV 100 hp 440 v

6.0 kV 500 hp 2300 V

12 kV 1500 hp 4160 V

24 kV 8000 hp 13.2 kV

mended voltage levels for maintenance testing of equipment utilizing the surge test [16]. For form-wound stator coils, [16] lists the following recommendations:

V , = kVL ( 5 )

where

V , momentary peak test voltage across the coil (not to exceed peak value of voltage applied as ground test, nor to be less than 350 times the number of turns in the coil),

an emperical factor (1.0 suggested for trial use), and recommended voltage for tests made in the field.

This standard also recommends approval from the manufacturer of the coils on any test program.

In summary, surge tests provide a very good means of lo- cating insulation weaknesses in motor windings. These tests are best employed on series-wound motors or on individual coils of larger motors with parallel circuits. Some maintenance people have expressed a desire to use surge tests to help locate turn-to-turn or phase-to-phase faults, which dc methods generally cannot detect. However, these applications would be limited to surface facilities and employed when new or rebuilt equipment is being tested.

A general concern about the application of surge tests is the effect of the surge on the motor windings. Although the energy in the surge is small (a few joules), many people feel that the voltage spike will damage motor windings. This is especially true in random-wound stators where the turn-to- turn voltages cannot be predicted as with form-wound stators. Because of this concern, surge testing has not been readily accepted by maintenance personnel.

V L rated rms line-to-line voltage, k V f

CONTINUOUS MONITOFUNG General

The test methods mentioned thus far are the most common techniques used for machine insulation testing. Most of these methods are best applied to large rotating machinery or on motors at critical locations. Although each method has many strengths in detecting incipient failures, they all have limita-

F

ZMYP ZMYN

'a

'Ik

Fig . 2. Three-phase model for line-line fault in cable

Fig . 3. Sequence networks for line-line cable fault.

tions when applied to industrial operations. Because of these limitations, many researchers have attempted to develop new techniques that would overcome the inherent disadvantages of the aforementioned tests. The most promising efforts are those which relate to actual motor performance and monitor the machine continuously.

Recent research has led to the development of an on-line continuous monitoring technique, which is particularly well suited to the process industries. This technique will provide an economical and reliable means of predicting incipient failures [17]. This work is still ongoing in the laboratory and not ready for field implementation. However, one recent research finding from this project suggests that a negative-sequence monitor could have immediate utility in the field.

Negative-Sequence Monitoring

The utility of the negative-sequence current in protective relaying applications has been known and practiced for years. Its use in a condition monitoring system to detect deterioration in electrical cables was suggested over ten years ago [ 181. The recent finding, based on data presented in [ 191, is that the negative-sequence current can be used to assess the condition of a motor, and that it could be used to detect electrical deterioration within the motor.

The motivation for negative-sequence monitoring can be exemplified by studying the sequence networks for a line-to- line fault in a cable-connected motor circuit. Fig. 2 shows the three-phase diagram, and the sequence networks are shown in Fig. 3.

Fig. 3 illustrates that the current flowing to the negative- sequence network is largely determined by the equivalent impedance of the negative-sequence network and ' lk , the fault impedance. Of this current, Z,2 represents the negative- sequence current flowing between the source and the fault; Zom2 is the negative-sequence current flowing between the

SOTTILE, JR. AND KOHLER: IMPROVED PREDICTIVE MAINTENANCE TESTIN1 G 997

fault and the load. For this model,

I02 = - V x 1 - 1 o m 2 ) . (7)

If the negative-sequence impedance of the motor is signifi- cantly larger than the impedance of the cable from the source to the fault, which is usually the case in cable-connected motors, i.e.,

then

and

where I, is the fault current, A. In other words, the negative- sequence current will equal one-half of the measured fault current. This result has also been tested experimentally on cable-connected motors.

With circuit models such as those shown in Figs. 2 and 3, many relationships between negative-sequence current and fault current, fault impedance, etc., can be derived; however, these relatively simple models do not exist for motors with internal winding faults. Nonetheless, the basis for negative- sequence monitoring has been developed from this foundation, and relationships between fault current and negative-sequence current have been discovered on internal motor faults. The ideas have been developed mostly through experimentation on a motor which allows access to the stator coils. This machine allows various stator coil connections, and it also allows the insertion of fault paths within the windings.

Tests performed on this motor have indicated that negative- sequence current is a good indicator of winding fault severity. In addition, for points of known fault potential, the negative- sequence current can be used as an estimate of the internal fault current, just as it could for the cable-connected model. A summary of results is shown in Fig. 4. This figure illustrates the relationship between negative-sequence current and fault current at three different fault potentials for several fault impedances. It is noted here that many different types and locations of the faults were simulated. In addition, both wye- and delta-connected windings were used. Despite these differences, there is a direct relationship between the measured negative-sequence current and the fault current at a constant fault potential.

Of course, internal fault potentials cannot be measured; therefore, the relationships illustrated in Fig. 4 cannot be used directly. However, if fault severity is determined by the fault potential and current (because these values represent the power consumed by the fault) then the negative-sequence current should reflect the increasing fault severity with potential. This effect is clearly represented in Fig. 4. The level of negative-sequence current increases with potential for constant values of leakage current.

The motivation behind this work is to find a single parameter that can be used to help evaluate motor-winding integrity.

. , L, l z

ru,

= OO W

I 2 LEAKAGE CURRENT, A

(a)

' t LEAKAGE CURRENT, A

(b)

W I

00 'Z LEAKAGE CURRENT, A

(C)

Fig. 4. (a) Fault potential 30 V. Faults: delta-connection: phase A-phase C; wye connection: phase c-ground, phase c , turn-turn; all at no load. (b) Fault potential 56-59 V. Faults: wye-connection: phase c ground, delta- connection: phase B-phase C, phase C, turn-turn, motor terminal a-phase C; all at no load. (c) Fault potential 90 V. Faults: wye connection: phase c-ground, at no load, 75 percent full load, 125 percent full load, phase c , turn-turn, no load.

With almost an infinite number of fault locations, paths, and severities possible within a motor, specifically determining the mode of failure is an extremely difficult task, regardless of the test method used. However, if one parameter can be used which will flag all types of failures, additional testing can be performed, or repairs can be scheduled, thus minimizing downtime. The detection of negative-sequence current is seen as such a test. Presently, guidelines are being developed to relate the level of negative-sequence current to deterioration severity.

IMPLEMENTATION

The implementation of negative-sequence monitoring into a preventive, or predictive maintenance program requires only that the negative-sequence current be monitored at various points throughout the power system. The specific monitoring points will depend on the design of each power system. In general, each branch that contains vital equipment should be monitored. For example, in a coal mining application, the monitoring of longwall shearers, continuous miners, and belt conveyors is justified.

The value of the negative sequence current can be determined by calculation if the current phasors can be measured.


This has been done in the lab using a waveform analyzer, and in the field using an oscilloscope; clamp-on CT’s and resis- tive voltage dividers were used in both cases. The use of a scope requires great care to accurately determine the phase angles for the current and is probably not practical for use in an extensive test program, although it would be fine for eval- uating a few motors. The use of a commercial instrument that directly measures the negative-sequence component would be far more desirable for everyday application.

The efficacy of the negative-sequence current as an early predictor of motor deterioration was discovered during the development of a continuous monitoring system for predicting incipient failure. This system measures the voltages and current phasors, and as such, determination of the negative- sequence component is trivial. The negative-sequence component will be an important predictor in the continuous monitoring system, but equally important, it can and should be used in a periodic test program as well. Admittedly, the user will have to locate or construct a device that is convenient to use, since that aspect was not investigated by the authors.

It is well-known that voltage unbalances can induce the flow of negative-sequence current because induction motors have low negative-sequence impedance. In many installations, unbalanced supplies may not be present. However, in some cases, voltage unbalances will exist. This is problematic because the measured negative-sequence current may be a result of the voltage unbalance and not a sign of deterioration in the motor. Both experimental and mathematical evidence suggest that a voltage unbalance factor of 0.1 percent could obscure significant levels of deterioration within a motor.

The effect of the source unbalance on the magnitude of the negative-sequence current must be separated from the con- tribution, which is due to deterioration, if sensitivity to low levels of deterioration is not to be lost. A method of doing this has been developed and uses linear superposition to identify the portion of the measured negative-sequence component that is due only to deterioration within the motor [20]. Unfortu- nately, this method requires an in-depth knowledge of motor parameters; these parameters may be difficult to obtain from the motor manufacturer. However, there is reason to believe that it will be possible to simplify the method with an acceptable loss of sensitivity such that the user will not require detailed motor design parameters.

The implementation of negative-sequence monitoring into a maintenance program should not preclude the use of other test procedures. For example, megohmmeter tests, if properly run and interpreted, can be very useful for detecting weaknesses in a motor’s major insulation system. However, they are not very useful for detecting phase-phase or turn-turn problems, which usually occur before the major insulation fails. Surge testing, which could detect these problems, is difficult to employ in some locations such as in an underground mine and should be reserved for surface facilities. Negative-sequence monitoring could be very useful for detecting problems in advance of failure so that more complete tests could be run or repairs could be scheduled. In addition, negative-sequence monitoring will not interfere with the normal operation of the motor.

CONCLUSION

The mining industry, like many of the process industries, places severe demands on electric motors. The ability to detect failures in advance would make scheduled repairs possible; however, traditional motor insulation tests are not practical in many cases and have severe limitations in most cases. Negative-sequence monitoring is seen as a viable approach to monitoring motor performance. This method does not interfere with normal operations and is continuous. In laboratory experiments, it has proven to be a sufficient indicator for detecting electrical deterioration within a squirrel-cage induction motor. Despite the effects of an unbalanced voltage supply on the measurement and the present uncertainty over how to com- pensate for these effects, negative-sequence monitoring can be a significant and powerful addition to a plant’s motor testing program.

REFERENCES E. B. Curdts, “Insulation testing by dc methods,” Technical F’ublica- tion 22Tlb, Biddle Ins, 1984. “A Stitch in Time.. .,” Biddle Instruments, 1984. Recommended Practice for Testing Insulation Resistance of Rotat- ing Machinery, IEEE Standard 43-1973, IEEE, 1974. J. L. Kohler, F. C. Trutt, and J. Sottile, Performance and Condi- tion Monitoring of Electrical Machines, Internal Report on Bureau of Mines Contract No. J0338028, The Pennsylvania State University, University Park, Mar. 1987. C. W. Ross and E. B. Curdts, “The recognition of possible measurement errors in dc dielectric testing in the field,” AZEE Tmns., paper no. 55456, 1955. E. B. Curdts, “The Field Testing of Electrical insulation by dc methods,” reprint included in Technical Publication 2 I-PSa, Biddle Ins, 1984. J. L. Kohler, “Corona: mechanisms and applications for underground coal mines,” in Conf. Rec., IEEE-ZAS I975 Ann. Meeting, Oct.

R. F. Field, “The basis for nondestructive testing of insulation,” AIEE Trans., vol. 60, Sept. 1941, pp. 890-895. A. T. Nestor, “Determining insulation quality by power factor testing,” Plant Eng., vol. 38, no. 20, Aug. 1984, pp. 46-48. Anon., Recommended Practice for Maasuring Tip-Up of Rotating Machinery Stator Coil Insulation, IEEE Standard 286-1975, IEEE, 1975, p. 1 1 . D. E. Shump, “Applications of surge comparison testing,” presented at the 48th Annual Convention, Elec. Appar. Ser. Assoc., Toroto, ON, Canada, June 23, 1981, p. 2 . G. L. Moses and E. F. Harter, “Winding-Fault Detection and Location by Surge-Comparison Testing,” AIEE Trans., vol. 64, July 1945, pp.

I . A. Oliver, H. H. Woodson, and J. S. Johnson, “A Turn Insulation Test for Stator Coils,” ZEEE Trans. Power Appar. Syst., vol. 87, Mar. 1968, pp. 669-678. R. Lekvre , Vice President Sales, Baker Instrument Company, personal communication, Nov. 1986 M. T. Wright, S. J. Yang, and K. McLeay, “General Theory of Fast- Fronted Interturn Voltage Distribution in Electrical Machine Wind- ings,” IEE Proc., vol. 130, pt. B, no. 4, July 1983, pp. 245-256. Guide for Testing Turn-to-Turn Insulation on Form- Wound Stator Coils for Alternating-Current Rotating Electric Machines-for Trial Use, IEEE standard 522-1977, IEEE 1977, p. 12. J. L. Kohler, F. C. T ~ t t , and L. A. Morley, “Decision Functions for Electric Power System Signals,” Electric Power Systems Research, vol. 1 1 , 1986, pp. 167-169. J. L. Kohler, “A tentative method for the prediction of mine power system component failures by pattern recognition techniques,” M.S. thesis, The Pennsylvania State University, University Park, Mar. 1977, p. 227. J . Sottile, “An experimental analysis of electrical deterioration in three- phase induction motors,” M.S. Thesis, The Pennsylvania State Uni- versity, University Park, Dec. 1986, p. 199.

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499-503.

SOTTILE. JR. AND KOHLER: IMPROVED PREDICTIVE MAINTENANCE TESTING 999

[20] J. Sottile, “Minimizing the effects of voltage unbalances in the feature extraction process,” Internal Report No. MEL 87-13, Mine Electrical Laboratory, The Pennsylvania State University, University Park, 1987, p. 14.

mining engineering.

Joseph Sottile, Jr., (M’86) received the B.S. degree in mining engineering, with highest distinc- tion, and the M.S. degree in mining engineering, both from The Pennsylvania State University, Uni- versity Park, PA, in 1984 and 1986, respectively.

He has worked in both production and engineering for the Barnes and Tucker Company from 1977 to 1983, and he also worked for Consolidation Coal Company in 1987. He is currently a Graduate Re- search Fellow at the Penn State Mine Electrical Lab- oratory, where he is pursuing the Ph.D. degree in

Mr. Sottile is a member of Tau Beta Pi and the Society of Mining Engi- neers.

Jeffrey L. Kohler (S’74-M’76-SM’88) received the B.S. degree in engineering science in 1974 and the M.S and Ph.D degrees in mining engineering in 1977 and 1983, respectively, all from The Penn- sylvania State University, University Park, PA.

He worked as an Electronics Technician and then as an Electrical Engineering Assistant while an un- dergraduate. While studying for the advanced degrees, he served as an Instructor of Mining En- gineer at Penn State. From 1979 to 1982 he was Senior Research Engineer and Senior Associate At

KETRON, Inc., Wayne, PA. In 19833 he became a Faculty Member at Penn State, where he is presently Associate Professor and Director of the Mine Electrical Laboratory. His teaching and research interests of electrical engineering applications in the mineral industries include power system design and analysis

Dr. Kohler is a Certified Mine Electrician by the Mine Health and Safety Administration, and is also a member of the Society of Mining Engineers

techniques for improved predictive maintenance testing of industrial power systems.pdf.pdf

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