Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 315 (504
)
A996A
and faults in cooling systems. These reasons can cause transformer windings earth faults,
interturn faults or even phase to phase faults.
1.1.1.3 W
HY DIFFERENTIAL PROTECTION IS NEEDED IN TRANSFORMER
PROTECTION
Transformer differential function is based into calculation of ingoing and outgoing current
difference, e.g. in normal operating status of the transformer the power which goes in
must come out as well. If this is not the case then the transformer has internal fault which
should be de-energized as soon as possible to avoid extensive damage into the
transformer. Mostly can be said that if differential function operates the transformer which
is faulty is going to be offline for a long time, if the fault is de-energized fast that can still
save a lot of money since in most cases the transformer can be still fixed and the cost is
significantly lower than buying new transformer. Exceptions to this are the faults which
occur in the differential protection zone but outside of the transformer like in the bus or
cables connected into the transformer. Faults of this type are easily fixed and the
transformer can be energized quickly after the fault is cleared
If transformer should be considered to be protected only with conventional overcurrent
and earthfault protection, operating time should be set to delayed tripping characteristics
coordinated to the low voltage side relays due to the fact that transformer normal condition
energizing and short circuit supply to high/low voltage side shall be directly seen on both
sides of the transformer and overcurrent in instant operation would cause timing
coordination problems or then sensitivity problems if the instant protection should be set
on high current starting criteria. This is not considered big problem in smaller transformers
in which the installation and maintenance of differential protection is considered more
expensive than possibly not full coverage of protection.
Also differential protection is very sensitive and internally scaled to the loading/fault
current flowing through the transformer. When considering interturn faults in the
transformer windings, overcurrent relay not necessarily even pick up for the fault which
could have been already tripped with differential relay in first power cycle. Same goes for
the transformer internal earth faults which with conventional earth fault protection in some
cases are impossible to be noticed before the fault evolves so that it will cause heavier
fault currents e.g. in cases where the fault location is close to the neutral inside the star
winding.
These are the main arguments of using differential protection, sensitive and fast operation
in internal in-zone faults and high stability on the out zone faults guarantee minimum
unwanted power outages and minimized and reduced damage to the transformer itself.