TRANSFORMER
PROTECTION
CONTENTS
TRANSFORMER
PROTECTION.....................................................................
GENERAL..............................................................................................................
DIFFERENTIAL PROTECTION FOR TRANSFORMERS.................................
RESTRICTED EARTH FAULT PROTECTION FOR
TRANSFORMERS.........
SPECIAL TRANSFORMER PROTECTION........................................................
'Qualitrol' Protection (Q).........................................................................................
Buchholz Relay.........................................................................................................
Interlocks and lntertrips............................................................................................
Coolant Level...........................................................................................................
Sealing Monitor........................................................................................................
Over-temperature Protection...................................................................................
EARTHING............................................................................................................
TRANSFORMER PROTECTION
GENERAL
All main
transformers which transmit bulk power between the generators and the
low-voltage distribution system of an offshore installation, and between the
Supply Authority's system and the low-voltage equipment in onshore
installations, have their own individual protective systems. This is to protect the transformer against
damage due to electrical faults arising both outside and inside it.
A typical
transformer protection scheme is shown in Figure 8.1 , which also shows
associated instrumentation. Many of the
general protection measures described earlier are applied also to transformers,
but in addition there are some more specific ones.
FIGURE 8.1 -
TYPICAL TRANSFORMER PROTECTION
Points worthy of
note in Figure 8.1 include the following:
·
Overcurrent
protection is on the HV side only. It is
provided by two inverse- time elements combined with an earth-fault element
(2OCIT/E) together with two instantaneous high-set overcurrent elements (2OC),
all in the same case. The relay operates
to trip the HV circuit-breaker directly and both the HV and the LV breakers
through the lock-out relay (TH). The
time and current settings will be determined by the overall discrimination
plan. Overcurrent on the LV side causes
corresponding overcurrent on the HV side, which therefore takes care of both
overloading and LV short-circuits.
·
Restricted
earth-fault protection is used on the secondary side (it is the only
secondary-side protection), with four protective type CTs. The relay operates instantaneously to trip
both the HV and the LV breakers through the lock-out relay.
·
Lock-out
hand-reset relay (TH).
·
There is
interlocking and intertripping from the HV to the LV circuit-breakers (but not
in reverse).
·
Instrumentation
includes a maximum-demand ammeter with an alarm contact.
DIFFERENTIAL PROTECTION FOR TRANSFORMERS
It is explained
in Section 7 that differential protection must be provided for generators
because an internal fault is self-fed and would not be cleared by the generator
supply breaker. Such differential
protection, not forming part of the discrimination ladder, is arranged to
operate instantaneously.
In the case of
transformers however there is a circuit-breaker upstream of the unit, and this
can clear an internal fault by removing the supply that feeds it. If the upstream circuit- breaker protection
has an instantaneous 'high-set' relay (as here), the clearance can be
immediate.
Therefore it is
not usual practice to provide differential protection to offshore, or to
smaller onshore, transformers, but to rely on the HV protection to clear any
internal primary or 'through' fault.
Internal earth faults on the secondary side are within the protected
zone and are dealt with by the REF protection.
Nevertheless
large onshore transformers are often provided with full differential protection
using three primary side and three secondary side current transformers. This gives the same benefits as restricted
earth-fault protection and, in addition, rapid protection against inter-phase
faults in the transformer as well as earth faults on the primary (delta)
winding. In these respects it is far
superior to REF protection.
The difference
between the primary and secondary currents in a transformer because of its
turns ratio does not prevent the necessary balance in the differential relay
circuits so long as the current transformer ratios are in inverse proportion to
that of the power transformer. Where, as
is usually the case, the power transformer has delta/star windings, which
introduce a phase shift between primary and secondary currents, a star/delta
arrangement of the CT secondary windings is necessary to achieve balance in the
secondary circuit.
Allowance has to
be made, in differential protection schemes for transformers, for the
magnetising inrush currents which flow only in the primary windings when the
transformer is switched onto the supply; they are not reflected in the
secondary windings and therefore appear similar to primary fault currents,
which may falsely operate the differential protection. The simplest solution is a short time delay
in the relay - an induction disc relay may be used - although there are more
subtle solutions available in cases where a delayed response is not desirable.
RESTRICTED EARTH FAULT PROTECTION FOR TRANSFORMERS
It should be
noted that, although restricted earth-fault protection will operate
satisfactorily for internal solid-earth faults on most parts of transformer
secondary windings, a high-impedance fault to earth may not give rise to
sufficient fault current to operate the relay, even though it is given a light
setting.
FIGURE 8.2
PROTECTION OF TRANSFORMER WINDING BY
RESTRICTED EARTH FAULT PROTECTION
Another point to
be noted is that, if the fault occurs near the star-point, the voltage at that
point may not be sufficient to cause a fault current high enough to operate the
relay. This situation is shown in Figure
8.2. Thus, although restricted
earth-fault protection is usually installed for transformer secondaries, it
cannot be regarded as one hundred per cent certain to operate.
SPECIAL TRANSFORMER PROTECTION
In addition to
the protection listed above, whose purposes have already been explained, there
are the following additional features special to transformers:
'Qualitrol' Protection (Q)
Qualitrol protection is fitted only on sealed
transformers such as those used on offshore installations. It is a proprietary device fitted at the top
of the transformer. It detects over-
pressure within the transformer and, if it exceeds a certain preset level,
trips both HV and LV circuit-breakers simultaneously through a flag relay (FG)
and the lock-out relay (TH). The device
has a spring-loaded discharge disc to relieve pressure immediately if it builds
up too quickly.
On large oil-filled grid and similar
transformers internal pressure is normally relieved into the conservator. Nevertheless it is customary to fit such
transformers with a pressure relief diaphragm on the tank top.
Buchholz Relay
Although termed a 'relay', this is in reality
a mechanical device named after its inventor.
FIGURE
8.3 -
BUCHHOLZ RELAY
The device is fitted in a horizontal section
of the pipe running between the main tank and the conservator in large
oil-filled transformers.
It consists of two parts as shown typically
in Figure 8.3, a gas trap and a surge section.
If an insulation weakness begins to develop under oil in any part of the
transformer winding, small discharge currents start and create tiny bubbles of
gas. As the breakdown slowly progresses,
the rate at which gas is evolved increases.
The bubbles rise slowly to the tank top and pass on, through the
connecting pipe, towards the conservator.
On the way they pass through the Buchholz relay and are caught in the
gas trap. Over a period of time enough
gas is accumulated to cause the oil remaining there to have a free surface, and
a float gradually lowers until, on reaching a preset level, it actuates a
mercury switch. This is usually arranged
to give an alarm, since the process is gradual and has not yet reached
breakdown stage calling for immediate disconnection.
The lower part is the surge section. Here a vane is suspended vertically across
the flow of oil between the tank and conservator and is held firmly against a
stop by a counterweight. Normally the
oil flow is very slight, depending only on temperature changes in the transformer,
and the vane does not move. But if there
is a complete electrical breakdown in any winding under the oil a power arc
will develop inside the tank, causing an expanding, high- pressure bubble of
oil vapour round the arc. This will
rapidly displace oil from the tank into the conservator, causing a surge of oil
past the vane, which will swing against the action of the counterweight and
actuate another mercury switch. Because
an actual breakdown will have occurred, this contact is always arranged to trip
the supply side of the transformer.
The above describes the operation of a
typical Buchholz relay in principle.
Different manufacturers have added many refinements to this basic
design.
Interlocks and lntertrips
Interlocking and intertripping is provided
between the HV and LV breakers. If the
HV breaker opens for any reason, whether tripped by a fault or operated
manually, the LV breaker (if closed) trips in sympathy and cannot be reclosed
until the HV breaker has been closed first.
It will be seen from Figure 8.1 that a fault,
whether on the HV or LV side, operates through the lock-out relay and trips
both the HV and the LV circuit-breakers simultaneously. This is to ensure that, after such a fault,
not only is the transformer isolated from its normal supply side but also that
it cannot be back-fed from the LV side.
The intertrip acts as a back-up for this, but
it is also needed to ensure sympathetic opening of the LV breaker when the HV
breaker is opened by hand, as distinct from by a fault.
Coolant Level
A sight-glass is provided to check the
coolant level within the tank of a sealed transformer. The level varies with temperature, and
allowance must be made for this; level marks for 15C and
45C may be given.
Conservators of large oil-filled transformers
usually have a sight-glass to indicate oil level.
Sealing Monitor
A centre-zero pressure/vacuum gauge may be
provided to indicate pressure in the vapour space over the liquid coolant of a
sealed transformer. The transformer is
filled to a level marked on the sight-glass and sealed at a specified
temperature - say 45°C. In service any variation
above or below this temperature, due either to change of ambient temperature or
to transformer loading, causes the liquid level to fall or rise slightly and a
consequent small vacuum or pressure to be indicated on the gauge.
If the pressure shown by the gauge moves over
a range less than its normal one, it may indicate a failure of the tank sealing
allowing air to be 'breathed' in and out.
Such a situation should be investigated.
Over-temperature Protection
Whereas winding temperature can be monitored
by normal temperature-sensing, a
special arrangement is sometimes used in large liquid-filled transformers.
In this application
Negative-Temperature-Coefficient (NTC) thermistors are used in
temperature-monitoring instruments. They
are suspended in the oil in a housing with a heating element and employ the
technique of 'thermal imaging'. The
thermistor is connected into a resistance bridge, whose output may operate
indicating instruments as well as actuating alarms and trips through an
electronic detector circuit.
Whereas NTC thermistors can operate over a
range of temperatures by adjustment of the associated measuring circuits, a PTC
thermistor is made to change its resistance at a particular temperature,
subject to a small tolerance. It is more
suitable for detecting overtemperature at particular locations in equipment -
for example, at hot spots in generator or motor windings into which they can be
embedded during manufacture. As the PTC
thermistor passes through its critical temperature, the sudden change of
resistance can be made to actuate an alarm or even to give a trip signal.
EARTHING
On all offshore
and onshore installations the transformer secondary star-point is usually
solid-earthed either through a link or through the neutral bar of the LV
switchboard which it feeds as a 4-wire system.
The earth connection can be isolated when desired (for example when
megger-testing the secondary) by means of a link at the switchboard, or, where
the earth connection is made through a link in the 3-pole circuit-breaker, by
withdrawing and isolating the circuit-breaker unit itself.
Care must be
taken, after opening an earth link for any reason, to ensure that it is
replaced immediately after the test. The
whole protection of the transformer may depend on it.
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