Test Data Reference Book (Secc 7 Rotating Machinery)

POWER FACTORS AND RADIO - INFLUENCE VOLTAGES FOR GENERATOR - STATOR INSULATION

POWER FACTORS AND RADIO - INFLUENCE VOLTAGESFOR GENERATOR - STATOR INSULATION

(A PROGRESS REPORT)

R.J.McGrath and F. J. GryszkiewiczDoble Engineering Company

*

GENERAL

The tests performed by the Doble Engineering Company on generator-stator insulation, their significance, and the equipment employed in performing them have been discussed a number of times during past Client Conferences1-11. 21, 24. The purpose of this report is to present data (which has accumulated as result of testing both by the Doble Company and by our Clients) on both old asphalt-mica and the more recent epoxy-mica insulation systems with voltage ratings from 13.8 to 26 kV, and capacities up to 780 MVA. Plots, included showing power factors and tip-ups in power factor, with voltage and radio-influence voltages records for these units. The data plotted are for nn individual phases, of the units.

Also included is a commment on the significance of electromagnetic and electrostatic probe measurements of generator-stator insulation.

TEST EQUIPMENT AND METHODS

Power-factor test data were recorded using the Doble Type M2H, Type MH(extended range), and Type MHM test sets. In practically every case, the unit under test was completely isolated from all external cable and associated apparatus. Tests were performed on individual phases with grounds applied to other phases not under test. Power-factor versus voltage characteristics of the various units were obtained by making test beginning at 2.0 kV and moving up at least to operating line-to-neutral voltage. Where possible, test potent was extended to 125% of operating line-to-neutral voltage.

Radio-influence voltage measurements were made in conjunction with power-factor tests, using a field adaptation8 of a method recommended by the joint Coordination Committee on radio reception of E.E.I., NEMA, and R.M.A. This method is described in NEMA Publication No.107, and is discussed in reference 12.

POWER-FACTOR TEST DATA

Figures lA and 2A are plots of powerfactors for the 107 generators tested which had been constructed with asphalt-mica insulation. Data are dividedby voltage rating in to two groups.No attempt has been made to correct data for temperature. Tests were performed over a range of temperatures up to 42°C; how has been our experience 6,13-15 that temperatures in this region have little effect on power factor.

Data presented in Figures 1A and 2A are 2 kV (in some instances 2.5 kV) power factors which can be assumed to be free from the effects of corona and representative of the type of insulation and its condition with respect to general deterioration, moisture, and/or dirt. Our data show that power factors of 4% or less can be expected of asphalt-mica generator-stator insulation which is free of contamination. The higher power factors plotted are for older units having inherently higher losses and for more modern units left exposed an unheated in moisture-rich atmospheres for prolonged periods prior to tests.

An indication of the inherent power factor of stator insulating material and its condition with respect general deterioration, moisture, and/or dirt are all that should be expected of a power-factor test at it potentials below corona-starting voltage. To confirm the presence or absence of atmospheric contamination our routine test procedure includes ungrounded-specimen tests between phases of a generator stator. Because of the effect of the stator iron in shielding the slot sections of the phases from one another, the interphase test becomes a test of the exposed end-turn insulation which is affected most by atmospheric contamination. Interphase power factors are generally higher than those recorded from phase-to-ground insulation, being affected not only by contamination but by the type of corona control in the end-turns.

In Figures 3A, 4A, and 5A are plots of power factors for the 122 generators which were tested, and which had been constructed with epoxy-mica insulation. Data are divided by voltage rating into three groups. No attempt has been made to correct the data for temperature. Tests were performed over a range of temperatures between approximately 5°C and 50°C; however, as noted before, it has been our experience6,13,15 that temperatures in this region have little effect on power factor of generator stator insulation.

Data presented in Figures 3A, 4A, and 5A are low-voltage (2-2.5 kV) power factors which can be assumed to be both free from the

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effects of corona and representative of the type of insulation and its condition with respect to general deterioration, moisture and/or dirt. Our data show that power factors of 1.0% or less can be expected for most modern (epoxy-mica) generator-stator insulation in the 13.8-14.4 kV range, 1.5% or less for units in the 15-18 kV range and 2.0% or less for units in the 19-26 kV range, depending upon the material (manufacturer) curing and dryness.

POWER-FACTOR TIP-UP

Figures 1B and 2B are plots of power-factor tip-up in the 107 asphalt-mica wound units tested. Tip-up is in terms of percent power-factor increase as the test potential was increased from 2 (or 2.5) kV up to operating lineto-neutral voltage. The slope of the power-factor versus voltage characteristic between 2 kV and operating voltage is a qualitative indication of the corona characteristics of a generator stator. The magnitude of the tip-up at operating voltage is a quantitative indication of the amount of corona loss available at operating voltage to attack insulation binding materials and varnishes.15

Data plotted in Figures 1B and 2B show that most of the units included in our investigation had less than 2% power-factor tip-up.

In Figures 3B, 4B, and 5B are plots of power-factor tip-up in the 122 epoxy-mica wound units tested. (Tip-up is defined as the increase in power factor as voltage is increased from 2 kV to operating line-to-neutral voltage.) Data plotted show that most of the units included in our investigation had power-factor tip-ups of 1.0% or less.

RADIO-INFLUENCE VOLTAGE (RIV)

Radio-influence voltages measured in generators are a quantitative indication of the amount of corona in a stator resulting from the over stressing and ionization of air in voids and in the contact with coil surfaces in slot sections and end-turns. Figures 6 and 7 are plots of radio-influence voltages measured at operating line-toneutral voltage in 107 asphalt-mica wound generator stators.

The data plotted show considerable spread; however, the large majority of units investigated had radio-influence voltages below 1000 microvolts at one megacycle when operating line-to-neutral voltage was impressed upon them. The spread noted is among different generators but not among different phases of any particular unit. It has been our experience that, under normal conditions, there should be good agreement in the radioinfluence voltages recorded for individual phases of a particular generator stator. It has also been our experience that, barring a change in the condition of any one phase of the entire winding, agreement should also be expected between results recorded for periodic tests on the same winding.

Temperature in the range encountered, by itself, appears to have little effect on the results of RIV measurements; however, moisture and surface contamination have very definite effect. These factors should be considered in comparing the results of tests on individual phases of a machine or results of a series of tests on the same machine. In at least two instances, it was found necessary to clean terminal-bushing surfaces carefully before acceptable readings could be recorded.

It does not appear possible, at present, to compare RIVs recorded for different types and sizes of machines. This is due to differences in materials and designs, and the attenuating or shunting effects of specimen capacitance in the present test method.

Due to the fact that only a few clients routinely perform RIV measurements, plots of data on epoxy-mica insulation systems were not attempted. However, it is our experience that the large majority of unto investigated by Doble had radio-influence voltages below 1000 microvolts at one megahertz when operating line-to-neutral voltage was impressed upon them.

ELECTROSTATIC PROBE

When end bells are removed from a machine, the foregoing tests are supplemented by tests with an electrostatic probe. This device was described at a previous Client Conference9.

End-turn corona along individual coil surfaces is suppressed by the application of a high resistivity coating overlapping the low-resistivity coating in the straight portion and extending into the end turns a short distance. The resistivity of this coating on the end turns is designed to control the voltage gradient along the coil surface so as to keep it below the critical voltage of the gas during operation. Sufficient space is provided between coil, and end connections to prevent corona elsewhere.

Electrostatic probe tests are helpful in detecting discontinuities that may exist in this coating and other conditions which cause corona discharges in the end-turn insulation. The results of such tests arc often helpful in the interpretation of results recorded for power-factor or RIV measurements.

ELECTROMAGNETIC PROBE



When the rotor is removed from a generator, the foregoing tests are further supplemented by tests with an electromagnetic probe. This device was described at previous Client Conferences 19,21,22,23.

Slot corona and slot discharge are prevented by the application of a low-resistivity conducting coating applied. to the straight portion of all high-voltage stator coils. This extends beyond the end of the core.

The detection of discharges in the slot portion of a winding by probe methods is complicated by the fact that the coils are, to a large extent, electrostaticallv shielded by the stator iron and conducting coil surfaces. This, in general, limits the effectiveness of the electrostatic probe, and for this reason attention has been directed to the electromagnetic probe.

The electromagnetic probe measures partial discharges in the Slot sections of individual coils of a stator winding.

When interpreting power-factor tip-up and EM probe readings, it should be kept in mind that the void, supporting internal partial discharge activity at operating voltage may have formed from relaxation of the bond between groundwall tapes. Continuous partial discharge in these voids at operating voltage does not necessarily result in an increase in tip-up and in EM probe readings; indeed even reductions due to shorting of the voids with the products of discharge may be noted eventually.

In the case of hard insulation systems, high PD on the electromagnetic probe test is frequently due to surface or slot discharge. Surface discharges external to the slot may reflect deterioration of the stress grading system. Slot discharges may reflect the early stages of the deterioration of the semiconducting coating, or more advanced deterioration of this coating together with loose wedges and/or increased clearance between coil and slot.

As with any diagnostic test, there is need for careful data analysis and correlation with result, of other tests and visual inspection if maximum benefits are to be obtained.

ANALYSIS OF TEST RESULTS

The analysis of results recorded for the tests described involves a comparison of power factors, power-factor tipup and RIV recorded for individual phases and for previous tests, if the latter data are available. Where previous test data are not available, the test results may be compared among phases and with those recorded for other units of similar manufacture and rating.

Significant information is also available in a comparison of the power factor-versus-voltage and RIV-versus-voltage characteristics recorded for each phase. Generally speaking, for conditions of corona distributed throughout the winding, both characteristics have the same general pattern - not in the sense that rate of increase in both is the same but rather that the first measurable indication of both occurs at approximately the same test potential and both increase with test potential at rates that are somewhat comparable. In a purer local corona condition, this relationship may exist until some critical test potential is reached, at which a more rapid increase in RIV is noted. This may be explained by considering the magnitude of the overall Watts-loss in one phase of a large generator and its masking effect on that dissipated in one spot of even severe discharge.

In addition, a method to develop a measurement system to determine which stator bars are in imminent danger of failure is described in Reference 25. This was necessitated by a steadily increasing number of stator winding groundwall insulation failures during routine maintenance a-c high-potential tests at 21 kV rms on a 15 kV water-cooled stator. A set of 30 top stator bars was selected for expanded tests. The normal power factor for the subject bars at 9.0 kV was found to range from 1.0 to 2.0 percent. All bars having power factor, over 6.0 percent failed high-potential tests; however, those bars that did not exceed 3.0 percent did not fail. These expanded tests revealed that the most sensitive test for detecting bars close to failure appears to be the Doble power-factor test.

The results of probe tests are helpful in locating points at which corona discharges occur. Unfortunately, the electrostatic probe is practically applicable only to exposed end-turn insulation. It is necessary to remove the rotor in order to perform electromagnetic-probe measurements.

RECOMMENDATIONS

High-potential acceptance tests, preferable ac at a specified voltage (2E, plus I ,000), should be performed on new or rewound generator stators to establish the adequacy of the insulation design and installation. At the same time, nondestructive medium-voltage ac tests of the type described in this report should also be performed to obtain baseline data. From then on, routine maintenance tests should not be of a destructive type in which a machine either passes or fails, for insulation damage is not only certain in the latter event but is likely in the former.



Routine maintenance tests should be of the nondestructive type and of a nature that would detect causes and rates of deterioration. On the basis of the results recorded, insulation Subjected to such tests should he graded good, bad, or otherwise. Unless a serious departure from the normal is noted in the test results, the machine may be returned to service following tests, and any maintenance indicated as necessary may be performed at some convenient time. It has been our experience that the medium voltage ac tests described in this report meet this requirement.

The Doble Company is prepared to perform tests of the type described on a day-work basis or to provide equipment so that a client may carry out his own test program.

REFERENCES

1. Fawcett, O. E. "Testing of Generator Windings," Minutes of the Third Annual Conference of Doble Clients, 1936, Sec. 5-3. 2. Fawcett, O. E. and Johns, G. J. "Report of Insulation Tests on Old Horizontal Turbo-Generator Ho. 4 at Connellsville Power Station

of West Penn Power Company, Pittsburgh, Pa.," Minutes of the Fifth Annual Conference of Doble Clients, 1938, Sec. 6-3. 3. Fawcett, O. E. and Johns, G. J. "Report of Insulation Tests on Ho. 1, 2300 Volt 2000 kW. G. E. Horizontal Turbo-Generator at the

Ridgway Power Station of West Penn Power Company, Pittsburgh, Pa.," Minutes of the Sixth Annual Conference of Doble Clients, 1939, Sec. 7-5.

4. Fawcett, O. E. "Generator Insulation Testing," Minutes of the Sixth Annual Conference of Doble Clients, 1939, Sec. 7-37. 5. Browning, G. H. "A Generator Power-Factor Tester," Minutes of the Seventh Annual Conference of Doble Clients, 1940, Sec. 7-19. 6. Smith, L. W. "Preliminary Report on East Peoria Generator Tests," Minutes of the Seventh Annual Conference of Doble Clients,

1940, Sec. 7-23. 7. Smith, L. W. "A Review of A-C Generator Insulation Failures and Maintenance Testing Technique." Minutes of the Ninth Annual

Conference of Doble Clients, 1942, Sec. 7-101. 8. Oliver, F. S. and Dolbear, B. L. "Progress Report on Generator Testing," Minutes of the Seventeenth Annual Conference of Doble

Clients, 1950, Sec. 7-301.9. Oliver, F. S. and Povey, E. H. "Generator Testing, A Progress Report," Minutes of the Eighteenth Annual Conference of Doble

Clients, 1951, Sec. 7-501. 10. Povey, E. H. "Generator Testing, A Progress Report," Minutes of the Twenty-First Annual Conference, of Doble Clients, 1954, Sec.

7-301. 11. Oliver, F. S. "Developments in the Testing of Rotating Machinery," Minutes of the Twenty-Fourth Annual Conference of Doble

Clients, 1957, Sec. 7-401. 12. Povey, E. H. "Corona Measurements by the R1V Method," Minutes of the Twenty-Fifth Annual Conference of Doble Clients, 1958,

Sec. 3-201. 13. Cornelius, H. A. "Generator Stator Insulation Temperature Conversion Data," Minutes of the Eighth Annual Conference of Doble

Clients. 1941, Sec. 7-17. 14. Povey, E. H. "Generator Insulation Testing, A Progress Report," Minutes of the Twenty-Second Annual Conference of Doble Clients,

1955, Sec. 7-401. 15. Rey, W. A. "Increased Deterioration of Generator Insulation by Corona Action," Minutes of Ili, Eighteenth Annual Conference of

Doble Clients, 1951, Sec. 7-101. 16. Smith, L. W. "Effect of Corona and Ionization on the Maintenance of Electrical Apparatus," Minutes y the Eighteenth Annual

Conference of Doble Clients, 1951, Sec. 2-201. 17. Smith, L. W. "Ten Years Progress in A-C Generator Insulation Maintenance Tests," Minutes of the Nineteenth Annual Conference of

Doble Clients, 1952, Sec. 7-601. 18. Povey, E. H. and Dolbear, B. L. "Progress on Generator Coil Study," Minutes of the Nineteenth Annual of Doble Clients, 1952, Sec.

7-401.19. Smith, L. E. "A Peak-Pulse Ammeter-Voltmeter Suitable for (Corona) Measurement in Electrical Equipment," Minutes of the Thirty-

Seventh Annual International Conference of Doble Client, 1970, Sec. 3-401. 20. Goodwin, T. A. "Corona Probe Measurements Taken on Hydro Machines at Grand Coulee Dam." Minutes of the Thirty-Eighth

Annual International Conference of Doble Clients, 1971, Sec. 7-501. 21. Rickley, A. L. "Power Factor and Radio-Influence Voltages for Generator-Stator Insulation," Minute of the Twenty-Fifth Annual

Conference of Doble Clients, 1958, Sec. 7-201. 22. Cosby, J. R., Menard, P., and Bickerdike, C. G. "Electromagnetic Corona Probe Tests for the Evaluation of Generator Stator

Windings," Minutes of the Fortieth Annual International Conference of Doble Clients, 1973, Sec. 7701. 23. Dawes, C. L. "Corona Discharge in Generator Slots and Its Measurement," Minutes of the Forty-First Annual International

Conference of Doble Clients, 1974, Sec. 7-301. 24. Armstrong, Jr., G. W. and McGrath, R. J. "Power Factors and Radio-Influence Voltages for GeneratorStator Insulation, A Progress

Report", Minutes of the Forty-First Annual International Conference o/ Doble Clients, 1974, Sec. 7-901. 25. Arbour, R. C. and Milano, B. "Diagnosing High-Potential Test Failures in Large Water-Cooled- Hydro Generators," Proceedings of

the 19th Electrical Electronics Insulation Conference, 1989.



Asphalt-mica or Equivalent Insulation

FIGURE 1



Asphalt-mica or Equivalent Insulation

FIGURE 2



Epoxy-mica or Equivalent Insulation FIGURE 3



Epoxy-mica or Equivalent Insulation

FIGURE 4



Epoxy-mica or Equivalent Insulation

FIGURE 5

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POWER-FACTOR TEST DATA FOR MOTOR-STATOR INSULATION


(A Progress Report)

R. J. McGrath and E. J. Marottoli Doble Engineering Company

*

INTRODUCTION

The tabulated data presented in Table I supersedes the data presented at the 1980 Doble Client Conference. This data reflects the experience of those clients who utilize power factor and power factor tipup tests as part of their preventative maintenance test program for motor-stator insulation.

POWER FACTOR DATA

A review of the tabulated data indicates some questionable data. Out of necessity, however, these units were returned to service. Doble will attempt to document the service experience of these motors.

Doble clients are well aware that a change of power factor or an increase in power factor tip-up indicates a change in the insulation system of the motor.

It is most important that power factor tests be performed on the insulation system as a part of any acceptance program for new or rewound motors to insure the following:

1. Conforming to purchase specifications.

2. Baseline data for future reference in the event questionable test data is obtained.

3. Power factor versus temperature tests should be performed if the operating environment of the motor is at a high temperature.

CLIENT EXPERIENCE

Recommendation Number 3, above, is a direct response to the experience of a Doble client dealing with failures of several large motors operating at high temperature. Table III outlines this experience.



TABLE II

Temperature % Power FactorMotor No.* °F at 1.0 kV Comment

1 108 1.4 Rewound 1982139 2.1 No Service Life146 2.6162 3.2174 4.0200 6.3

2 91 2.8 In Service130 3.8 10 Years155 7.3210 20.2

3 110 3.0 In Service10 Years

4 100 1.1 Rewound 1971In Service11 Years

*6000 Horsepower rated for 4.0 kV Epoxy-Mica Insulation.

Motor Nos. 2 and 3 were rated for Investigation.



Table III lists manufacturers' references to power factor data for motor insulation. We will be pleased to expand this listing upon receipt of any copies of other bulletins members of the Client group may submit to us.

TABLE III MANUFACTURERS' REFERENCES TO POWER FACTOR

DATA FOR MOTOR INSULATION

kV Rating Manufacturer Bulletin Information4.16 Westinghouse SA-8362 (3/59) Thermalastic PF vs kV

- Allis Chalmers 05-B8913 (1/58) Silco-Flex InsulationSystem PF vs kV

- Electric Machinery ABC of Motor and Quality Control Limit Manufacturing Company Generator Insulation (full coil 2%

100-SYN-73 Power-Factor Tip-Up) - Armet Industries Silicone Tape PF vs kV

Corporation Motors

REFERENCE

1. McGrath, R. J. and Marottoli, E. J. "Power-Factor Test Data for Motor-Stator Insulation," Minutes of the Forty-Seventh Annual International Conference of Doble Clients, 1980, Sec, 7-401.

© 1985 Doble Engineering Company 85 ROTATING MACHINERY Sec. 7-801


POWER-FACTOR MEASURING TECHNIQUES ON STATOR COILS David Train and Lawrence Melia Doble Engineering Company

POWER-FACTOR MEASURING TECHNIQUES ON STATOR COILS

David Train and Lawrence MeliaDoble Engineering Company

Jan Capek Alcan Smelters and Chemicals, Ltd.

Denis Pelletier National Electric Coil Ltd.

*

INTRODUCTION

Voltage-endurance tests on stator coils performed according to IEEE Std. 1043-1989 require a number of pretest evaluations to be carried out prior to the commencement of the endurance tests. A power-factor measurement is included in the pretest series and the Standard specifies that for this test the resistive grading paint at each end of the slot portion of the coil must be short-circuited by wrapping aluminum foil around it. A guard electrode must also be wrapped around the conductor about one tenth of an inch beyond the foil. The intention of the foil wrapping is to eliminate the resistive losses which would otherwise be produced in the resistive grading paint and which would therefore influence the apparent power factor of the stator coil. This method is also described in the Doble Rotating Machinery Insulation - Test Guide.

Some users also perform Power-Factor and Tip-up measurements on new coils before they are installed in machines either during repair or rewinds. The circuit connections including guard techniques for these tests are not clearly specified in IEEE Standards and, consequently, many users tend to use either the foil wrapping over the resistive grading paint or some other method. As a result, several different techniques are in use and therefore comparisons are difficult or even impossible to make.

This paper describes the results of an investigation of five different measurement and/ or guard arrangements in order to determine the most suitable method to be used on stator coils during power factor measurement. The study was carried out on two coils for a 35 MVA, 13.8 kV generator. These coils had already been subjected to a voltage-endurance test.

CIRCUIT ARRANGEMENTS INVESTIGATED

The various methods investigated ranged from no special precautions whatsoever to an arrangement which permitted separate measurements of power-factor in the slot and overhang portions of the coil respectively. Details of the different methods are described below.



Method 1

This method is described with reference to Figure 1. This is the simplest technique of all and in it the measuring lead from the instrument was connected directly to the center of the slot portion of the stator coil. The connection was made by wrapping two turns of copper braid around the center of the slot portion. The braid was wound directly on top of the semi-conducting paint and no dummy slots were used. In addition, the resistive grading paint was not short-circuited by wrapping it in aluminum foil and there were no guard electrodes.



Method 2

This method was similar to that of Method 1 except that this time a pair of metal plates equal in length to the stator core were clamped onto the slot portion. These plates were intended to simulate the stator core and effectively short-circuit the semiconducting paint. The metal plates are referred to as a dummy slot. The arrangement is shown in Figure 2.

Circuit Arrangement According to Method 3

FIGURE 3

Method 3

This method was identical to that prescribed by IEEE Std 1043-1989. The resistive grading paint was completely short-circuited by wrapping it in aluminum foil. Guard electrodes were installed beyond the foil as shown in Figure 3. This method is referred to as the IEEE method.



Circuit Arrangement According to Method 4

FIGURE 4

Method 4

This arrangement was similar to Method 2 with the addition of guard electrodes wrapped around the inward ends of the resistive grading paint. This method is shown in Figure 4.

Circuit Arrangement for Measurement of Power-Factor of Slot Portion According to Method 5



FIGURE 5a

Circuit Arrangement for Measurement of Power-Factor of Overhang Portion According to Method 5

FIGURE 5b

Method 5

In this method, a narrow strip of semi-conducting paint was removed near each end of the slot portion. The remaining semi-conducting paint and part of the resistive grading paint were wrapped in aluminum foil.

The advantage of this technique is that it allows the power factors of the slot and overhang portions of the coil to be determined independently. The power factor of the slot portion is measured by connecting it to the instrument and using the overhang portions as guard electrodes. This arrangement is shown in Figure 5a. The power factor of the overhang portion underneath the resistive grading paint is measured by connecting the aluminum foil to the instrument and using as guard electrodes the slot portion and a small auxiliary guard installed about one tenth of an inch beyond the extremities of the resistive grading paint. This arrangement is shown in Figure 5b.



Power-Factor Characteristics Measured on Coil 1

FIGURE 6



Power-Factor Characteristics Measured on Coil 2

FIGURE 7

TEST PROCEDURE

Test voltages starting at 2 kV rms, 60 Hz and increasing in increments of 2 kV to 16 kV were applied to the coils. Measurements of capacitance and power factor were made at each voltage level for each of the circuit arrangements described above. The corresponding portions of each coil side were connected in parallel during the measurements. Therefore, the measured values apply to the complete coil in each case.

The power-factor and capacitance values measured for each arrangement are given in Tables 1 through 4. The power factors for each arrangement are also plotted in Figures 6 and 7 for coils 1 and 2, respectively.

DISCUSSION OF RESULTS

Method 1

The total watts-loss measured using this technique include not only the dielectric losses in the insulation but also the additional losses produced by the bar capacitance current flowing through the resistive-grading and semi-conducting paints. The equivalent circuit shown in Figure 8 illustrates the currents and corresponding areas which contribute to the additional losses. Graphs 1 in Figures 6 and 7 indicate the power factors measured using this technique. In the coils tested, the additional losses developed in the semi-conducting and resistive grading paints are approximately equal to the dielectric losses in the slot portions of the coils. Also, these additional losses will be dependent on the surface resistivities of the semi-conducting and resistive grading paints and since these resistivities cannot be accurately controlled, the additional losses may vary significantly from one coil to another. This variation, together with the fact that the total measured losses are about double the dielectric losses of interest, will tend to mask the true power factor of the insulation being investigated. This technique is not recommended.



Equivalent Circuit of Stator Coil Showing Areas of 12R Losses in Surface Points

FIGURE 8

Method 2

Graphs 2 in Figures 6 and 7 indicate the power factor characteristics obtained by this method.

The losses dissipated in the semi-conducting paint are eliminated resulting in slightly lower power factors. However, dielectric losses in the overhang section underneath the resistive grading paint together with the resistive losses in the grading paint are still present and are included in the total losses measured. These additional losses will result in a significant error when determining the power-factor of the slot portion insulation and, consequently, this technique is not recommended.

Method 3

This technique, which is the one prescribed by the IEEE, results in the measurement of the total dielectric losses in the slot portion plus those in the overhang insulation underneath the resistive grading paint. Graphs 3 in Figures 6 and 7 indicate the corresponding power factors measured using this technique. The dielectric losses per unit length of coil will be greater in the overhang portion than in the slot portion because: (a) the insulation is less densely compacted there, (b) higher local stresses maybe present due to physical displacement of conductor strands, and (c) possible delamination of some mica paper layers may be present. Consequently, the losses in this overhang portion may contribute a significant percentage (and hence power factor) of the total measured losses. The losses dissipated in this overhang region as a percentage of those dissipated in the slot portion are obviously a function of the relative lengths of the two portions. For short coils, this overhang portion represents a much greater percentage of the slot portion than is the case for long coils. Therefore, the contribution of the losses in the overhang portion to power factor will be much less in the case of long coils than for short coils. However, using this technique, it is not possible to discriminate between the contributions which each portion makes to the total losses. For this reason, this technique is not recommended.

Method 4

The use of guard electrodes wrapped around the inward ends of the resistive grading paint will significantly reduce errors caused by dielectric losses in the insulation underneath the paint and resistive losses in the paint itself. Graphs 4 in Figures 6 and 7 indicate the corresponding power factors obtained using this technique. Caution must be exercised when using this technique because there will always be some resistance between the guard electrode and the measuring electrode due to the presence of some resistive-grading paint between them. Providing this resistance is always much higher (i.e., greater than 10x) than that of the range resistor in the M2H (or other) test set, the resultant error will be negligible (less than 10%). However, this has to be checked each time a power-factor measurement is made. Errors due to this source will result in the measured power factor being lower than the actual value. Also, if the guard electrode is not wrapped tightly enough around the resistive grading paint, it will not intercept all of the current flowing to ground from the overhang section. Consequently, the measured power factor may be slightly higher than the actual value. It is difficult to control and hence determine the accuracy of this technique. However, satisfactory results can be obtained providing proper caution is exercised. Systematic errors in power-factor measurement of about 0.1% (i.e. 10%



error) were obtained for the two coils investigated, indicating that the guard electrodes were not intercepting 100% of the currents from the overhang sections. However, if power-factor measurements are used for screening purposes when large numbers of coils are involved, it is recommended that this technique be used.

Method 5

This method permits the separate measurement of the losses in the slot portion and overhang region underneath the resistive grading paint. Since the total power-factor can readily be determined (if required) either by calculation or measurement, this represents the most useful technique of all the methods investigated. Graphs 5s and 5o on Figures 6 and 7 indicate the power factor characteristics obtained separately for the slot and overhang portions, respectively. It is recommended that this technique be used when relatively small numbers of coils are involved, for example, during voltage-endurance tests. In such cases the additional information obtained could be useful during subsequent dissections of the coils.

The calculation of the total power factor involves the measured values of capacitance and watts-loss of each portion of the coil. The total power factor is given by:

P.F. total = C1 PF1 + C2 PF2

C1+C2

Where C1 = capacitance of slot portion C2 = capacitance of overhang portion PF1 = power factor of slot portion PF

2 = power factor of overhang portion

The total power factor may be measured by bridging the gap in the semi-conducting paint and proceeding according to the IEEE method. The bridging of the gap can be accomplished either by touching up the area with semi-conducting paint or by wrapping it with aluminum foil.

Both the slot and overhang regions are electrically stressed during voltage-endurance tests but, in addition, the slot portion is thermally stressed. Consequently, useful information may be learned about the insulation systems in both regions by performing power-factor measurements before and after the voltage-endurance tests using the technique of Method 5.

Tests on Complete Stator Windings

When power factor and tip-up measurements are performed on complete stator windings in the field, it is impossible to employ any of the guard circuit arrangements described above. Consequently, the measurements obtained will be equivalent to the characteristics shown in Graphs 2 in Figures 6 and 7. Since the total watts-losses measured will include those produced in the resistive grading material together with the dielectric losses in the insulation underneath, the power factors and tip-ups may vary over a wide range, particularly if the physical dimensions of the coils and the surface resistivities vary from one machine to another. This fact is confirmed in the test results presented in Ref. [1].



Since information regarding physical dimensions is not normally reported with power factor and tip-up test data, meaningful comparison and interpretation of data is difficult if not impossible. It is therefore recommended that, in future, when power factor and tip-up data are reported, the stressed ratios Rs of the windings also be reported. Rs is defined as:

Rs = slot length________________ lenght under resistive grading material

Presentation of the test measurements grouped into categories of different ranges of Rs should then give more consistent information leading to more reliable interpretation of the test data.

Specification Considerations

Many users specify maximum permissible levels of power factor and tip-up when ordering new coils. It is obvious from the results obtained during this investigation that wide variations in both parameters may be obtained depending on the circuit arrangement used. Therefore, confusion may arise if power factor and tip-up measurements which are performed by manufacturers are repeated by the purchaser using a different circuit arrangement. In order to avoid any misunderstanding or conflict, it is recommended that users who specify maximum permissible levels of power factor and tip-up should also clearly define the technique to be used during the measurements.

CONCLUSIONS

1. During power factor measurements on individual stator coils, the power factor depends not only on the dielectric losses in the stressed insulation, but may also be influenced by I2R losses in semiconducting and resistive grading paints depending on the circuit arrangement being used.

2. The dielectric losses in the overhang section underneath the resistance grading point are about 4 times higher than those in the slot portion. This is due to the fact that; (a) the insulation in the overhang portion is less densely compacted than in the slot portion, (b) higher local stresses occur due to physical displacement of strands and (c) some delamination of the insulation may be present.

3. The IEEE method eliminates the I2R losses in the semiconducting and resistive grading paints. However, the high dielectric losses in the overhang region underneath the resistive grading paint tend to mask the overall power factor of the complete coil.

4. It is recommended that when large numbers of coils are involved (ex. tens or hundreds), power-factor measurements be made with a dummy slot on the cell portion and guard electrodes be placed around the inward ends of the resistive grading point. The resistance between the guard electrode and the dummy slot should be checked and should be significantly higher than the input resistance of the measuring instrument.

5. When small numbers of coils are involved (ex. 2-4) during voltage-endurance tests, it is recommended



that separate measurements be made of the power factors of the slot and overhang portions, respectively. This may provide additional information which could be important during subsequent dissections.

6. Power-factor measurements of the slot and overhang portions can be determined independently by making a narrow interruption (about 0.1 inches wide) in the semi-conducting paint at each end of the slot portion. Both portions are then used as measuring and guard electrodes alternatively.

7. In the future, power-factor measurements reported from tests on complete stator windings should be accompanied by information regarding the relative lengths of slot and overhang portions. By collecting many sets of such data from different machines and sorting it into ranges of different length ratios, the dispersion of power factor measurements will be reduced making analysis and interpretation easier.

8. Users who specify maximum permissible levels of power factor and tip-up in their purchase specifications should also define the arrangement to be used during the measurements of these parameters.

REFERENCE

[1] McGrath, R. J. and Gryszkiewicz, F. J., "Power Factors and Radio-Influence Voltages for Generator-Stator Insulation (A Progress Report)" Minutes of the Fifty-Seventh Annual International Conference of Doble Clients, 1990, Sec. 7-6.1

TABLE I

Power Factor %

Method

Voltage

kV 1 2 3 4 5s 50

2 0.850 0.829 0.732 0.667 0.531 1.323

4 1.030 0.969 0.825 0.718 0.552 1.871

6 1.162 1.099 0.988 0.785 0.583 2.441

8 1.352 1.335 1.168 0.854 0.670 2.981

10 1.534 1.475 1.286 0.924 0.730 3.164



12 1.648 1.625 1.416 0.964 0.797 3.324

14 1.808 1.755 1.477 1.044 0.847 3.324

16 1.898 1.869 1.507 1.074 0.899 3.447

Power factors measured on coil 1.

TABLE II

Capacitance pF

Method

Voltage

kV 1 2 3 4 5s 50

2 337.92 338.54 404.04 336.12 295.96 105.68

4 338.61 339.10 405.45 336.35 296.02 107.06

6 339.22 339.90 406.85 336.71 296.16 108.37

8 340.25 340.88 408.55 337.32 296.62 110.11

10 341.26 341.78 409.92 337.84 297.04 111.04

12 341.97 342.65 411.60 338.23 297.46 112.30

14 343.01 343.48 412.65 338.90 297.84 112.98

16 343.54 344.18 413.29 339.20 298.25 113.76

Capacitance values measured on coil 1.

TABLE III

Power Factor %



Method

Voltage

kV 1 2 3 4 5s 5o

2 0.739 0.652 0.626 0.502 0.417 1.388

4 0.926 0.862 0.820 0.602 0.456 1.819

6 1.146 1.027 1.061 0.724 0.536 2.609

8 1.327 1.237 1.281 0.799 0.645 3.179

10 1.467 1.427 1.431 0.839 0.695 3.488

12 1.677 1.537 1.524 0.879 0.765 3.668

14 1.787 1.667 1.584 0.919 0.827 3.748

16 1.930 1.786 1.642 0.952 0.868 3.849

Power factors measured on coil 2.

TABLE IV

Capacitance pF

Method

Voltage

kV 1 2 3 4 5s 5o

2 322.87 323.57 389.48 321.17 288.03 101.13



4 323.53 324.38 390.80 321.51 288.18 102.15

6 324.58 325.14 292.54 322.23 288.58 103.80

8 325.54 326.37 394.62 322.84 289.31 105.48

10 326.44 327.63 396.62 323.33 289.75 106.82

12 327.74 328.39 398.36 323.96 290.32 108.05

14 328.52 329.46 399.74 324.35 290.78 108.80

16 329.59 330.30 400.88 324.85 291.25 109.80

Capacitance values measured on coil 2.


91 ROTATING MACHINERY 7-3.1


POWER FACTOR TESTS ON WATER-COOLED GENERATORS


E. H. Povey Doble Engineering Company

*

For some time after the introduction of water-cooled stator windings on large rotating machines, it appeared impossible to obtain useful measurements on the stator insulation of such machines without draining and drying the water-hose connections. Now, however, a method has been devised which permits the alternating voltage characteristics of stator insulation, including power factor tipup, to be obtained with fair accuracy while the cooling water is being circulated. This method originated with the Central Electricity Generating Board in England(1) and was brought to our attention by our Canadian clients.

In this method a direct-voltage resistance measurement is made between a stator winding and ground. Actually, this is a measurement of the stator winding insulation resistance in parallel with the resistance of the water hose connections. It is assumed, however, that the winding insulation has an insignificant effect on the measurement so that the measured resistance is substantially that of the water hose connections. A second assumption is then made that the water hose resistance is essentially constant with test voltage so that from the direct-voltage resistance measurement the hose loss for an applied alternating test voltage can be calculated. Then, when an alternating test voltage is applied between the stator winding and ground and the total losses measured, the hose loss as calculated from the direct-voltage measurement can be subtracted, leaving only the alternating-voltage losses in the stator winding itself. The losses thus found can be used in the calculation of winding power factor.

Winding insulation losses found by this method do not include losses due to continuous leakage paths through the insulation, since these losses are subtracted along with the hose losses. Such losses, however, are probably insignificant in comparison with such alternating-voltage losses as those due to polarizations and ionization in the insulation.

For convenience, formulas for adjusting the direct-voltage measurement to the alternatingvoltage test level are given here. When the direct-voltage measurement is given as a resistance of Rh megohms, the hose loss Wh in watts at an alternating voltage test level of Va kilovolts is

Wh - V2a/Rh (1A)

or for an equivalent 10-kV test level

Wh = 100/Rh (1B)

When the direct voltage measurement is given as a current Id in milliamps at an applied voltage of Vd kilovolts, then the hose losses are

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Wh = Id V2a/Vd (2A)

or for an equivalent 10-kV test

Wh = 100 Id/Vd (2B)

Having now determined the hose losses at an alternating-voltage test level of Va, the alternating-voltage measurements on the stator, showing a charging current, la, and loss, Wa, at test voltage, Va, can now be corrected. Subtracting Wh as found from (IA, B) or (2A, B) leaves substantially the dielectric losses in the stator insulation. That is,

Ws = Wa - Wh (3)

The power factor of the stator insulation is

where Ws from (3) is in watts and Ia is in milliamperes, both referred to the same voltage level, Va, in Kilovolts. While in Formula (4) the loss value has been corrected to exclude the water-hose loss, the current, Ia, which includes the leakage current through the hoses, has not been corrected. Because this leakage current is small in comparison with the stator charging current and is almost at a 90 degree phase angle to it, any correction would be insignificant.

The above discussion assumed that a complete stator winding was under test. The same procedure applies, however, to tests on the individual phases. When using this procedure on an individual phase or part of the machine, it is important that the terminals to which the test connections are made be the same for both the alternating- and direct-voltage tests.

The Doble Type MH and M2H Test Sets lend themselves especially well to this test procedure, since unlike most bridges, they read out in terms of watts loss. Furthermore, they read in terms of equivalent 10-kV loss, so that only the 10-kV value of the water-hose loss needs to be calculated even when Doble tests are to be made over a range of test voltages.

Before the Doble Company tried this method in the field, a rough check was made on the resistance characteristics of a water hose. For this test we used a few inches of Tygon tubing through which we flowed ordinary tap water. Measurements were made with both direct- and alternating-test voltages. The results, shown in Table I, indicate a good correspondence between the alternating and direct-voltage tests.



We were fortunate to be able to test a generator before and after the installation of water hoses. The machine had seen some service between the ''before" and "after" tests. The "after" test was made with cooling water in circulation, and the procedure described was used to correct the reading for hose losses. Table II is a sample of the results obtained. The hose resistance as measured by a Doble DC Attachment was found to be about 2. 1 megohms.

The accuracy of the results depends on the proportion of hose losses to the total loss, decreasing as hose losses increase. All measurements involved in the procedure should be made with a high degree of accuracy.

While it may be some time before enough data is on hand to permit a firm conclusion, it is our present opinion that this method will provide sufficiently accurate results to permit useful conclusions to be reached as to the condition of the stator insulation.

REFERENCE

1. Central Electricity Generating Board Technical Disclosure Bulletin No. 140, Grindall House, 25



Newgate Street, London E.C. 1, England.


TDRB-291 ROTATING MACHINERY 9-4A.1


POWER-FACTOR TESTING OF WATER-COOLED GENERATORS (A Progress Report)


A. L. Rickley and R. J. McGrath Doble Engineering Company

*

INTRODUCTION

At the 1973 Doble Client Conference, E. H. Povey of the Doble Engineering Company presented a paper1 on the power-factor testing of water-cooled generator stators. He discussed the development of a method which originated with the Central Electricity Generating Board in England.2 The method permits the alternating-voltage characteristics of stator insulation, including power factor and power-factor tip up, to be obtained with reasonable accuracy while the cooling water is being circulated. Prior to the development of this method, it appeared impossible to obtain meaningful test results on the stator insulation of machines of this type without first draining, and drying or disconnecting the water hose connections.

The test method described by Mr. Povey permits successful ground-insulation tests on individual phases in the usual manner, at voltages up to and including 100% of rated line-to-ground voltage. Tests may also be performed at voltages approaching 110-125% of rated line-to-ground voltage with the client's approval. The routine tests are preceded by a direct-voltage measurement of the resistance (or leakage current) between individual phases of the stator winding and ground. This resistance is assumed to be substantially that of the water-hose assembly. The hose-assembly losses are calculated from the measured dc resistance or leakage current and subtracted from the overall losses measured in the routine power-factor measurements, leaving substantially the losses in the stator-winding insulation. Stator-winding power factors are calculated from the corrected watts-loss readings.

FIELD TEST RESULTS

The Doble Engineering Company has tested approximately 30 water-cooled generators utilizing the test method summarized in the foregoing. The following relates an interesting experience we observed during the past year while testing a 24-kV General Electric generator rated 690 MVA, hydrogen and water-cooled. In an effort to avoid redundancy, data on the ground insulation of Phase I only is shown. Interphase power factor and radio-influence voltage (RIV) results, although measured, are also not included. The tests were performed with the Doble Type M2H test set and a resonating inductor. In each case, with the exception of the original tests in 1972, the cooling water was circulating slowly through the stator windings during the tests. The conductivity of the water during each series of tests is also included.

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The conductivity of the water in the cooling system had increased to 6.3 micromhos and the hoseassembly losses increased to 1100 watts. It was concluded, and investigation confirmed, that the stator cooling-water was contaminated.

(C) The cooling system was drained, flushed and refilled with uncontaminated water from a sister unit. Client personnel monitored the flushing and refilling by performing do resistance measurements at 2-kV periodically over the next few days. The results improved as follows:

The conductivity of the water in micromhos decreased appreciably and the insulation resistance had increased to more normal values. Complete and final test results were as follow:

The results of this final test compare favorably with those recorded in 1972 and 1976.

CONCLUSIONS

The test method described by E. H. Povey in 1973 has been used effectively on 30 relatively large watercooled generator stators. The test method, performed at voltages up to and slightly greater than rated lineto-neutral, provides a meaningful,



nondestructive preventive-maintenance test for units of this type, without the need for draining and disconnecting of the cooling-water system. The method also provides useful information regarding the condition of the cooling water and its significance in terms of dielectric loss or leakage in the cooling system.

REFERENCES

1. Povey, E. H. "Power Factor Tests on Water-Cooled Generators," Minutes of the Fortieth Annual International Conference of Doble Clients,1973, Sec. 7-601.

2. Central Electricity Generating Board Technical Disclosure Bulletin No. 140, Grindall House, 25 Newgate Street, London E.C. 1, England.

TDRB-291 ROTATING MACHINERY 9-4B.1


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Test Data Reference Book (Secc 7 Rotating Machinery)

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