IEEE C62.92.3-2012 pdf free
IEEE C62.92.3-2012 pdf free.IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part I-Generator Auxiliary Systems.
Whether an auxiliary power system is intentionally grounded or not, it is presumed that generating station systems are equipped with ground detectors and alarms so that grounds will not be allowed to remain on the system unintentionally. Service continuity of ungrounded systems, or systems grounded through high resistance without selective relaying. may be continued for a limited period after the fault to allow an orderly procedure for locating the fault. This can contribute to high service continuity. Iong search times and successive removal of several circuits from service for fault location can result in decreased continuity. Some nuclear plant safeiy-related” systems are normally grounded and operate ungrounded only during emergency conditions so that service continuity will not be jeopardized by tripout on a single-phase ground fault. Two situations are critical and have influenced the choice to provide intentional grounding. If a ground fault is not located promptly and subsequently a second ground fault occurs somewhere on the system. a short circuit will exist between these two points, and substantial portions of the system, usually at least two feeders, will be forced out of service without notice. The second situation arises from a high transient overvoltage. This can cause several simultaneous insulation failures to ground on one phase in several machines or at separated locations. Locating these faults requires pulse current injection and tracing. Otherwise, the difficulty of removing circuits from service in various combinations of pairs, triplets. etc.. to disconnect all faults simultaneously may be prohibitive. However, ungrounded low-voltage systems that have high-quality system insulation with generous margins can provide a high level of service continuity if ground faults are infrequent and maintenance and repair are prompt and well executed. Users whose experience shows that ground faults may be kept sufficiently infrequent may prefer to operate low- voltage systems ungrounded, or with high resistance and without ground-fault relaying, to allow freedom from trip-outs without notice. Some sensitive electronic equipment such as computers and microprocessors may not be suitable for connection to ungrounded or high-resistance grounded systems. (iround-fault transient or temporary ovcrvoltages may, for example, saturate power supplies, burn out surge suppressors. and cause tires. Sensitive loads need to be selected or equipped to he compatible with the grounding.
Grounded and relayed systems achieve high continuity by controlling and limiting the damage caused by ground faults, and by selective fast disconnection of faulted auxiliaries and system elements to minimize interruption of the system. The success of this approach depends upon at least the following factors:
adequately redundant auxiliaries, quick transfer of function to alternate auxiliaries, adequacy of preventive and restorative maintenance, and speedy repair.
5.3 Damage criterion
The ranges of ground-fault currents associated with each class of grounding arc displayed in Table I of IEEE Sid (‘62.92.1 TM, It may be noted that the grounding impedance of all non-etTectively grounded systems limits the ground-fault currents to a small fraction of the three-phase short-circuit current. Ground faults may occur where significant amounts of fault resistance or of fuult arc voltage drop occur, which is an appreciable portion of the driving voltage. This is particularly true of systems operating at 600 V or less. Consequently, ground-fault currents may occur that are less than normal load currents or that are so low as not to be readily detectable by the usual phase-protective devices. IEEE C62.92.3 pdf download.
