June 2014

A number of catastrophic failures have occurred in the past when synchronous generators have been accidentally energized while at standstill. Among the causes for such incidents were human errors, breaker flashover, or control circuitry malfunction. A number of protection schemes have been devised to protect the generator against inadvertent energization. The basic principle is to […]

In an apparatus protection perspective, generators constitute a special class of power network equipment because faults are very rare but can be highly destructive and therefore very costly when they occur. If for most utilities, generation integrity must be preserved by avoiding erroneous tripping, removing a generator in case of a serious fault is also

Any numbers of conditions have been the reason for an electrical transformer failure. Statistics show that winding failures most frequently cause transformer faults (ANSI/IEEE, 1985). Insulation deterioration, often the result of moisture, overheating, vibration, voltage surges, and mechanical stress created during transformer through faults, is the major reason for winding failure. Voltage regulating load tap

Switching of shunt reactors (and other devices characterized as having small inductive currents such as transformer magnetizing currents, motor starting currents, etc.) can generate high phase-to-ground over-voltages as well as severe recovery voltages, especially on lower voltage equipment such as reactors applied on the tertiary of transformers. Energizing the devices seldom generates high overvoltages, but

SERIES CAPACITOR BANK APPLICATIONS Installation of a series capacitor bank in a transmission line (standard or thyristor controlled) has the potential for increasing the magnitude of phase-to-ground and phase-to-phase switching surge over-voltages due to the trapped charges that can be present on the bank at the instant of line reclosing. In general, surge arresters limit

Surges associated with switching transmission lines (overhead, SF6, or cable) include those that are generated by line energizing, reclosing (three phase and single phase operations), fault initiation, line dropping (de-energizing), fault clearing, etc. During an energizing operation, for example, closing a circuit breaker at the instant of crest system voltage results in a 1 pu

Switching surges occur on power systems as a result of instantaneous changes in the electrical configuration of the system, and such changes are mainly associated with switching operations and fault events. These over-voltages generally have crest magnitudes which range from about 1 per unit to 3 pu for phase-to- ground surges and from about 2.0

A direct stroke is defined as a lightning stroke when it hits either a shield wire, tower, or a phase conductor. An insulator string is stressed by very high voltages caused by a direct stroke. An insulator string can also be stressed by high transient voltages when a lightning stroke hits the nearby ground. An indirect

The level reached by VFT overvoltages originated by disconnector switching or line-to-ground faults inside a GIS is below the BIL of substation and external equipment. However, aging of the insulation of external equipment due to frequent VFT must be considered. TEV is a low energy phenomenon and it is not considered dangerous to humans; the

A) FUSES The most commonly used protective device in a distribution circuit is the fuse. Fuse characteristics vary considerably from one manufacturer to another and the specifics must be obtained from their appropriate literature. Figure 9.28 shows the time-current characteristics which consist of the minimum melt and total clearing curves. FIGURE 9.28 Fuse time-current characteristic.