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.
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