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Power System Protection Course- TRANSFORMER PROTECTION

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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