07-12-2012, 06:24 PM
Transformer Protection Application Guide
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Transformer Protection Application Guide
This guide focuses primarily on application of
protective relays for the protection of power
transformers, with an emphasis on the most
prevalent protection schemes and transformers.
Principles are emphasized. Setting procedures
are only discussed in a general nature in the
material to follow. Refer to specific instruction
manuals for your relay. The references provide a
source for additional theory and application
guidance.
The engineer must balance the expense of
applying a particular protection scheme against
the consequences of relying on other protection
or sacrificing the transformer. Allowing a protracted
fault would increase the damage to the
transformer and the possibility of tank rupture
with a consequent oil fire and consequent
personnel safety risks. There is no rule that says
what specific protection scheme is appropriate
for a given transformer application. There is
some tendency to tie protection schemes to the
MVA and primary kV of a transformer. While
there is some validity to this approach, there are
many other issues to be considered. Issues to
be considered include:
Failure Statistics
Table I lists failures for six categories of faults
(IEEE C37.90, “Guide for Protective Relay
Applications to Power Transformers, Ref. 1).
Winding and tap changers account for 70% of
failures. Loose connections are included as the
initiating event, as well as insulation failures. The
miscellaneous category includes CT failure,
external faults, overloads, and damage in
shipment. An undisclosed number of failures
starts as incipient insulation breakdown problems.
These failures can be detected by sophisticated
online monitoring devices (e.g. gas-in-oil
analyzer) before a serious event occurs.
Differential Relaying (87)
Differential relays sense the unbalance in the flow
of currents in various apparatus or buses. In the
absence of a fault in the protected zone, this
unbalance tends to be small and the flows into
the zone are closely matched to the flows
leaving. Accordingly, such relays can be more
sensitive than phase overcurrent relays and need
not be delayed to coordinate with other relays
during external faults, except for some issues
associated with transient CT saturation, to be
discussed below.
Energizing Inrush
Energization inrush is caused by remanence
(residual flux) in the core and the point in the
voltage waveform when a transformer breaker
closes. If the instantaneous voltage at
energization calls for flux of the same polarity as
the remanence, the core is driven into saturation,
creating peak exciting currents that can exceed
ten times rated exciting current. As a comparison,
normal steady-state excitation current is
about 0.01 to 0.03 times rated.