MAINTENANCE OF HIGH-VOLTAGE CIRCUIT BREAKERS

FREQUENCY OF INSPECTIONS.-Most manufacturers recommend complate inspections, external and internal, at intervals of from 6 to 12 months. Experience has shown that a considerable expense is involved, some of which maybe unnecessary, in adhering to the manufacturer's recommendations of internal inspections at 6- to 12-month intervals. With proper external checks,part of the expense, delay, and labor of internal inspections may be avoided without sacrifice of dependability.

Inspection schedule for new breakers.- A temporary schedule of frequent inspections is necessary after the erection of new equipment, the modification or modernization of old equipment, or the replication of old equipment under different conditions. The temporary schedule is required to Correct internal defects which ordinarily appear in the first year of service and to correlate external check procedures with internal conditions as a basis for more conservative maintenance program there after. Assuming that a circuit breaker shows no serious defects at the early complete inspections and no heavy interrupting duty is imposed, the following inspection schedule is recommended:

6 months after erection - Complete inspection and adjustment

12 months after previous inspection- Complete inspection and adjustment

12 months after previous inspection- Complete inspection and adjustment

12 months after previous inspection - External checks and inspection; if checks are satisfactory, no internal inspection

12 months after previous inspection - Complete inspection and adjustment

Inspection schedule for existing breakers. - The inspection schedule should be based by the interrupting duty imposed on the breaker. It is advisable to make a complete internal inspection after the first severe fault interruption. If internal conditions are satisfactory, progressively more fault interruptions may be allowed before an internal inspection is made. average experience indicates that up to five fault interruptions are allowable between inspections on 230 kV and above circuit breakers, and up to 10 fault interruptions are allowable on circuit breakers rated under 230 KV.Normally, no more than 2 years should elapse between external inspections or 4 years between internal inspections.

EXTERNAL INSPECTION GUIDE-LINES.- The following items should be included in an external inspection of a high-voltage breaker.

Visually inspect PCB externals and operating mechanism. The tripping latches should be examined with special care since small errors in adjustments and clearances and roughness of the latching surfaces may cause the breaker to fail to latch properly or increase the force necessary to trip the breaker to such an extent that electrical tripping will not always be successful, especially if the tripping voltage is low. Excessive "opening" spring pressure can cause excessive friction at the tripping latch and should be avoided. Also, some extra pressure against the tripping latch may be caused by the electromagnetic forces due to flow of heavy short-circuit currents through the breaker. Lubrication of the bearing surfaces of the operating mechanism should be made as recommended in the manufacturer's instruction book, but excessive lubrication should be avoided as oily surfaces collect dust and grit and get stiff in cold weather, resulting in excessive friction.

Check oil dielectric strength and color for oil breakers. The dielectric strength must be maintained to prevent internal breakdown under voltage surges and to enable the interrupter to function properly since its action depends upon changing the internal arc path from a fair conductor to a good insulator in the short interval while the current is passing through zero. Manufacturer's instructions state the lowest allowable dielectric strength for the various circuit breakers. It is advisable to maintain the dielectric strength above 20 kV even though some manufacturer's instructions allow 16 kV.

If the oil is carbonized, filtering may remove the suspended particles, but the interrupters, bushings, etc., must be wiped clean. If the dielectric strength is lowered by moisture, an inspection of the fiber and wood parts is advisable and the source of the moisture should be corrected. For these reasons, it is rarely worthwhile to filter the oil in a circuit breaker while it is in service.

Observe breaker operation under load.

Operate breaker manually and electrically and observe for malfunction. The presence of excessive friction in the tripping mechanism and the margin of safety in the tripping function should be determined by making a test of the minimum voltage required to trip the breaker. This can be accomplished by connecting a switch and rheostat in series in the trip-coil circuit at the breaker (across the terminals to the remote control switch) and a voltmeter across the trip coil. Staring with not over 50 percent of rated trip-coil voltage, gradually increase the voltage until the trip-coil plunger picks up and successfully trips the breaker and record the minimum tripping voltage. Most breakers. Should trip at about 56 percent of rated trip-coil voltage. The trip-coil resistance should be measured and compared with the factor test value to disclose shorted turns.

Most modern breakers have trip coils which will overheat or burn out if left energized for more than a short period. An auxiliary switch is used in series with the coil to open the circuit as soon as the breaker has closed. The auxiliary switch must be properly adjusted and successfully break the arc without damage to the contacts.

Tests should also be made to determine the minimum voltage which will close the breaker and the closing coil resistance.

Trip breaker from protective relays.

Check operating mechanism adjustments. Measurements of the mechanical clearances of the operating mechanism associated with the tank or pole should be made. Appreciable variation between the value found and the setting when erected or after the last maintenance overhaul is erected or after the last maintenance overhaul is usually an indication of mechanical trouble. Temperature and difference of temperature between different parts of the mechanism effect the clearances some. The manufacturers' recommended tolerances usually allow for these effects.

Double test bushings and breaker.

Table I - Maximum Contact Resistance

Air Circuit Breakers

KV             AMPERES         MICRO OHMS

5-15              600                    100 Ohms

                     1200                      50 Ohms

                     2000                      50 Ohms      

Oil Circuit Breakers :-

KV                AMPERES         MICRO OHMS

7.2-15             600                    300 Ohms

                        1200                    150 Ohms

                        2000                       75 Ohms      

                  4000                       40 Ohms

23- 24            ALL                       500 Ohms

     

46                   ALL                       700 Ohms

69                  600                        500 Ohms

                     1200                       500 Ohms

                     2000                       100 Ohms

115-230       ALL                       800 Ohms

Measure contact resistance. As long as no foreign material is present,the contact resistance of high pressure, butt-type contacts is practically independent of surface condition.Nevertheless, measurement of the electrical resistance between external bushing terminals of each pole may be regarded as the final "proof of the pudding." Any abnormal increase in the resistance of this circuit may be an indication of foreign material in contacts, contact loose in support,loose jumper, or loose bushing connection. Any one of these may cause localized heating and deterioration. The amount of heat above normal may be readily calculated from the increase in resistance and the current.

Resistance of the main contact circuits can be most conveniently measured with a portable double bridge (Kelvin) or a "Ducter." The breaker contacts should not be opened during this test because of possible damage to the test equipment.

Table 1 gives maximum contact resistances for typical classes of breakers.

Fortunately, these difficulties are most likely to appear early in the use of a breaker and would be disclosed by the early internal inspections. As unsatisfactory internal conditions are corrected and after one or two inspections show the internal conditions to be satisfactory, the frequency of internal inspections may safely be decreased.

INFLUENCE OF DUTY IMPOSED.-

Influence of light duty.- Internal inspection of a circuit breaker which has had no interruption duty or switching since the previous inspection will not be particularly beneficial although it will not be a total loss. If the breaker has been energized, but open, erosion in the form or irregular grooves (called tracking) on the inner surface of the interrupter or shields may appear due to electrostatic charging current. This is usually aggravated by a deposit of carbon sludge which has previously been generated by some interrupting operation. If the breaker has remained closed and carrying current, evidence of heating of the contacts may be found if the contact surfaces were not clean, have oxidized, or if the contact pressure was improper. Any shrinkage and loosening of wood or fiber parts (due to loss of absorbed moisture into the dry oil) will take place following erection, whether the breaker is operated or not. Mechanical operation, however, will make any loosening more evident. It is worthwhile to deliberately impose several switching operations on the breaker before inspection if possible. If this is impossible, some additional information may be gained by operating the breaker several times after it is deenergized, measuring the contact resistance of each pole initially and after each operation.

Influence of normal duty.- The relative severity of duty imposed by load switching, line dropping, and fault interruptions depends upon the type of circuit breaker involved. In circuit breakers which employ an oil blast generated by the power arc, the interruption of light faults or the interruption of line charging current may cause more deterioration than the interruption of heavy faults within the rating of the breaker because of low oil pressure. In some designs using this basic principle of interruption, distress at light interrupting duty is minimized by multiple breaks, rapid contact travel, and turbulence of the oil caused by movement of the contact and mechanism. In designs employing a mechanically driven piston to supplement the arc-driven oil blast, the performance is more uniform. Still more uniform performance is usually yielded by designs which depend for arc interruption upon an oil blast driven by mechanical means. In the latter types, erosion of the contacts may appear only with heavy interruptions. The mechanical stresses which accompany heavy interruptions are always more severe.

These variations of characteristic performance among various designs must be considered when judging the need for maintenance from the service records and when judging the performance of a breaker from evidence on inspection. Because of these variations, the practice of evaluating each fault interruption as equivalent to 100 no-load operations, employed by some companies, is necessarily very approximate although it may be a useful guide in the absence of any other information.

Influence of severe duty.-Erosion of the contacts and damage from severe mechanical stresses may occur during large fault interruption.The most reliable indication of the stress to which a circuit breaker is subjected during fault interruptions is afforded by automatic oscillograph records. Deterioration of the circuit breaker may be assumed to be proportional to the energy dissipated in the breaker during the interruption.The energy dissipated is approximately proportional to the current and the duration of arcing; that is, the time from parting of the contacts to interruption of the current.However, the parting of contacts is not always evident on the oscillograms, and it is sometimes necessary to determine this from indicated relay time and the known time for breaker contacts to part.Where automatic oscillograph records are available, they may be as useful in guiding oil circuit breaker maintenance as in showing relay and system performance.

Where automatic oscillographs are not available, a very approximate, but nevertheless useful, indication of fault duty imposed on the circuit breakers may be obtained from relay operation targets and accompanying system conditions. All such data should be tabulated in the circuit breaker maintenance file.