OVERCURRENT PROTECTION TRIPPING METHODS ON POWER SYSTEM TUTORIALS

The substation circuit-breaker tripping power may be from either a d-c or an a-c source. A d-c tripping source is usually obtained from a tripping batttery, but may also be obtained from a station service battery or a charged capacitor.

The a-c tripping source is obtained from current transformers located in the circuit to be protected.

DC Battery Trip
When properly and adequately maintained, the battery offers the most reliable tripping source. It requires no auxiliary tripping devices, and uses single-contact relays that directly energize a single trip coil in the breaker.

A battery trip supply is not affected by the power-circuit voltage and current conditions during time of faults, and therefore is considered the best source for all types of protective relay tripping. An additional advantage is that only one battery is required for each substation location and it may be used for other equipment; e.g., highvoltage breaker trip circuits and ground switches.

A tripping battery is usually the most economical source of power for tripping a number of breakers. When only one or two breakers are involved, however, it may be more economical to use a-c current or capacitor trip.

Long service can be obtained from batteries when they receive proper maintenance and when they are kept fully charged and the electrolyte is maintained at the proper level and density. When lead acid batteries are subjected to extremely low ambient temperatures, their output is considerably
reduced.

In outdoor unit substations, this necessitates larger ampere-hour capacities. For substations in outlying locations where periodic maintenance is difficult, such as many single-circuit substation applications, other types of tripping sources may be more satisfactory.

Capacitor Trip
An a-c potential source is required for charging the capacitors used in the capacitor trip unit. This source may be either a control power transformer or a potential transformer connected where voltage is normally present.

A control power transformer is usually used because it is required for a-c closing of the circuit breakers. Capacitor trip uses the same standard single-closing contact relays as d-c battery trip.

A separate capacitor trip unit is required for each breaker in the substation. The charging time for the unit is approximately 0.04 second and any failure in the charging source for a period longer than 30 seconds renders the trip inoperative.

This time must be factored into time-delay settings of relays. The capacitor trip unit can be used only with low energy tripping devices such as the impact trip device used on modern breaker operating mechanisms.

Due to the limited amount of energy available from this device, the breaker must be well maintained to assure successful operation. This unit provides tripping potential independent of the magnitude of fault current, which makes it particularly applicable on lightly loaded, high impedance circuits where a-c current trip cannot be used and a battery cannot be justified.

AC Current Trip
If adequate current is always available during fault conditions, the current transformers in the protected circuit provide a reliable source of tripping energy which is obtained directly from the faulted circuit. The tripping may be either instantaneous or time delay in operation; but in all cases, it is applicable only to overcurrent protection.

The trip circuit is more complex than for d-c tripping because three trip circuits, complete with individual trip coils and auxiliary devices, are required for each breaker for overcurrent tripping. A potential trip coil is also required for each breaker for normal switching operations.

This permits manual tripping of the breaker by means of the breaker control switch. The three trip coils are normally connected in each phase circuit, rather than two phase coils and one residual coil.

This is because adequate trip current may not be available under all ground-fault conditions – e.g., when a ground fault occurs at some distance out on the feeder so that there is sufficient neutral impedance to limit the fault current to a value insufficient to cause tripping, or when applied to a system grounded through a neutral impedance.

A residual relay, which trips the breaker by means of a potential trip coil, is used to provide ground fault protection under conditions such as these. A minimum of three or four amperes CT secondary current is required to energize the three-ampere current-trip coils used for this method of tripping.

The use of 0.5- to 4.0-ampere range time-overcurrent relays is not recommended because they are more sensitive than the a-c trip coils.

A-c current trip may be by means of reactor trip (circuit closing relays) or auxiliary relay trip (circuit-opening relays). The reactor trip method is usually recommended because of its simplicity and because it uses the more standard type overcurrent relays.

Related post



No comments:

PREVIOUS ARTICLES