TERTIARY-WINDING OVERCURRENT PROTECTION BASIC INFORMATION

The tertiary winding of an autotransformer, or three-winding transformer, is usually of much smaller kVA rating than the main windings. Therefore, fuses or overcurrent relays set to protect the main windings offer almost no protection to tertiaries.

During external system ground faults, tertiary windings may carry very heavy currents. Hence, to guard against failure of the primary protection for external ground faults, separate tertiary overcurrent protection may be desirable.

The method selected for protecting the tertiary generally depends on whether or not the tertiary is used to carry load. If the tertiary does not carry load, protection can be provided by a single overcurrent relay connected to a CT in series with one winding of the Δ.

This relay will sense system grounds as well as phase faults in the tertiary or in its leads. When tertiary windings are connected by cables, the overcurrent protection provided to the tertiary winding should account for the thermal withstand of the cables.

Alarming and tripping as a result of a prolonged unbalance condition or load tap changer malfunction should prevent damage to cables. If the tertiary is used to carry load, partial protection can be provided by a single overcurrent relay supplied by three CTs, one in each winding of the Δ and connected in parallel to the relay.

This connection provides only zero sequence overload protection and does not protect for positive and negative sequence overload current. In this case, the relay will operate for system ground but will not operate for phase faults in the tertiary or its leads.

Where deemed necessary, separate relaying such as differential type should be provided for protection for phase faults in the tertiary or its leads. The setting of the tertiary overcurrent relay can normally be based on considerations similar to those in line time overcurrent.

However, if the tertiary does not carry load, or if load is to be carried and the three CT, zero sequence connection is used, the associated overcurrent relay can be set below the rating of the tertiary winding. This relay should still be set to coordinate with other system relays.

PROTECTIVE RELAY CURRENTS EXPERIENCED BY POWER TRANSFORMERS DURING FAULT CONDITIONS

Two characteristics of power transformers combine to complicate detection of internal faults with current operated relays

a)The change in magnitude of current at the transformer terminals may be very small when a limited number of turns are shorted within the transformer.

b)When a transformer is energized, magnetizing inrush current that flows in one set of terminals may equal many times the transformer rating. These and other considerations require careful thought to obtain relay characteristics best-suited to the particular application.

Minimum internal faults

The most difficult transformer winding fault for which to provide protection is the fault that initially involves one turn. A turn-to-turn fault will result in a terminal current of much less than rated full-load current.

For example, as much as 10% of the winding may have to be shorted to cause full-load terminal current to flow.  Therefore, a single turn-to-turn fault will result in an undetectable amount of current.

Maximum internal faults

There is no limit to the maximum internal fault current that can flow, other than the system capability, when the fault is a terminal fault or a fault external to the transformer but in the relay zone. The relay system should be capable of withstanding the secondary current of the CT on a short-time basis.

This may be a factor if the transformer is small relative to the system fault and if the CT ratio is chosen to match the transformer rating.

Through-faults

Fault current through a transformer is limited by the transformer and source impedance. While current through a transformer thus limited by its impedance can still cause incorrect relay operations or even transformer failure, CT saturation is less likely to occur than with unlimited currents.

The above favorable aspect may disappear if the transformer protective zone includes a bus area with two or more breakers on the same side of the transformer through which external fault current can flow with no relationship to the transformer rating. An example is a transformer connected to a section of a ring bus with the transformer protection including the ring bus section.

TYPES OF POWER TRANSFORMER FAULTS BASIC INFORMATION

The electrical windings and the magnetic core in a transformer are subject to a number of different forces during operation, for example

a)Expansion and contraction due to thermal cycling

b)Vibration

c)Local heating due to magnetic flux

d)Impact forces due to through fault current

e)Excessive heating to to overloading or inadequate cooling

These forces can cause deterioration and failure of the winding electrical insulation. Table below summarizes failure statistics for a broad range of transformer failure causes reported by a group of U.S. utilities over a period of years.

This guide deals primarily with the application of electrical relays to detect the fault current that results from an insulation failure. The current a relay can expect to see as a result of various types of winding insulation failures.

The detection systems that monitor other transformer parameters can be used to indicate an incipient electrical fault. Prompt response to these indicators may help avoid a serious fault. For example

a)Temperature monitors for winding or oil temperature are typically used to initiate an alarm requiring investigation by maintenance staff.

b)Gas detection relays can detect the evolution of gases within the transformer oil. Analysis of the gas composition indicates the mechanism that caused the formation of the gas; e.g., acetylene can be caused by electrical arcing, other gases are caused by corona and thermal degradation of the cellulose insulation.

The gas detection relays may be used to trip or alarm depending on utility practice. Generally, gas analysis is performed on samples of the oil, which are collected periodically. Alternatively, a continuous gas analyzer is available to allow on-line detection of insulation system degradation.

c)Sudden-pressure relays respond to the pressure waves in the transformer oil caused by the gas evolution associated with arcing.

d)Oil level detectors sense the oil level in the tank and are used to alarm for minor reductions in oil level and trip for severe reductions.

POWER TRANSFORMER RELAYING PHILOSOPHY AND ECONOMIC CONSIDERATIONS

Protective relaying is applied to components of a power system for the following reasons:

a)Separate the faulted equipment from the remainder of the system so that the system can continue to function

b)Limit damage to the faulted equipment

c)Minimize the possibility of fire

d)Minimize hazards to personnel

e)Minimize the risk of damage to adjacent high voltage apparatus

In protecting some components, particularly high-voltage transmission lines, the limiting of damage becomes a by-product of the system protection function of the relay. However, since the cost of repairing faulty transformers may be great and since high-speed, highly sensitive protective devices can reduce damage and therefore repair cost, relays should be considered for protecting transformers also, particularly in the larger sizes.

Faults internal to a transformer quite often involve a magnitude of fault current that is low relative to the transformer base rating. This indicates a need for high sensitivity and high speed to ensure good protection. There is no one standard way to protect all transformers, or even identical transformers that are applied differently.

Most installations require individual engineering analysis to determine the best and most cost-effective scheme. Usually more than one scheme is technically feasible, and the alternatives offer varying degrees of sensitivity, speed, and selectivity.

The plan selected should balance the best combination of these factors against the overall economics of the situation while holding to a minimum

a)Cost of repairing damage

b)Cost of lost production

c)Adverse effects on the balance of the system

d)The spread of damage to adjacent equipment

e)The period of vulnerability of the damaged equipment

In protecting transformers, backup protection needs to be considered. The failure of a relay or breaker during a transformer fault may cause such extensive damage to the transformer that its repair would not be practical.

When the fault is not cleared by the transformer protection, remote line relays or other protective relays may operate. Part of the evaluation of the type of protection applied to a transformer should include how the system integrity may be affected by such a failure.

In this determination, since rare but costly failures are involved, a diversity of opinion on the degree of protection required by transformers might be expected among those familiar with power system relay engineering.

The major economic consideration is not ordinarily the fault detection equipment but the isolation devices. Circuit breakers often cannot be justified on the basis of transformer protection alone.

At least as much weight should be given to the service requirements, the operating philosophy, and system design philosophy as to the protection of the transformer. Evaluations of the risks involved and the cost-effectiveness of the protection are necessary to avoid going to extremes. Such considerations involve the art rather than the science of protective relaying. 

BUCHHOLZ RELAY OR POWER TRANSFORMER BASICS AND TUTORIALS

BUCHHOLZ RELAY FOR POWER TRANSFORMER BASIC INFORMATION

What Are Buchholz Relay? How Buccholz Relay Works?


The Buchholz Relay (Gas Relay) is designed to protect equipments submerged in insulating liquid, by means of supervision of the oil abse nce or abnormal flow, and abnormal gassing caused by the equipment. Buchholz relay is usually fitted on transformers provided with an expansion tank for the insulating liquid.

Buchholz Relay

Buchholz relay is capable to accurately detect, for example, the following problems: Leakage of insulating liquid, short - circuit inside the equipment causing a great displacement of insulating liquid, inside gassing due to intermittent or continuous failures occurring inside the equipment.

Buchholz relay is usually installed between the main tank and the oil expansion tank of the transformer.

Buchholz relay housing is made of cast iron, having two flanged openings and two sight glasses showing a graduated scale of gas volume. There are two inside floats, being that the upper float is fo rced to move downwards (this also happens in case of oil leakage).

On the other hand, in case an excessive gassing causes an oil ci rculation through the relay, the lower float reacts, even before the gas reaches the relay. In both cases, the floats make contacts when they are displaced.

The Buchholz Relay has a device for the inside float testing and locking. To check for proper operation of the relay contacts, when it is installed in the transformer, proceed as follows:

Alarm:
· Connect an Ohmmeter to terminal s + C - D. It should indicate an open circuit.

· Remove the testing device plug and introduce it upside down into the device, lowering it as much as possible in all of its length. The Ohmmeter should indicate a closed circuit.

Shutdown
· Connect an Ohmmeter to terminals + A - B. It should indicate an open circuit.

· Remove the testing device plug and introduce it upside down into the device, lowering it as much as possible in all of its length. The Ohmmeter should indicate a closed circuit.

Before supplying po wer to the transformer, the following items should be checked:

· Remove the lid of the relay -testing device.

· Remove the float -locking pin from the inside of the testing device. Both floats should be free to move.

· Replace the cover of the relay -testing device.

· Purge the air from the relay by means of the 1/8” air valve located on the relay lid.

· Check the relay for possible leakage that might have occurred during the installation on the transformer and fix it.

· Check the relay for proper fitting wi th regards to the oil float direction, which arrow should be pointing towards the transformer’s oil expansion tank.

If the alarm sounds without turning off the transformer, it is necessary to turn it off immediately and then test the gas removed from the inside of the relay. In this case, the origin of the failure can be assessed according to the gas testing result, i.e.:

· Combustible gas (contents of acetylene): In this case there must be a failure to be repaired on the electrical part;

· Non-combustible gas (without acetylene) : in this case, it means there is pure air. The transformer can be turned on again without danger after the air is bled out from the relay. When the alarm sounds repeatedly, it indicates that air is penetrating into the transformer. Tur n it off and repair the failure.

· No gassing (the gas level inside the relay is getting lower and an amount of air is being drawn through the open valve), in this case, the oil level is too low, possibly due to a leakage. Top up with oil until the control level and carry out the air tightness essay.

The transformer is turned off without a previous alarm. In this case, the transformer must have been thermally overloaded. Turn it on again after a considerable time interval for cooling. The failure can be found atcthe short-circuit contact in the protection relay system.

The alarm sounds and the transformer is shutdown immediately before or after the alarm sounds. In this case, one of the above mentioned failures must be the cause. Make the gas testing and proceed as described above.

ATTENTION!
Float locking device for transport purpose and testing of contacts : After installing the relay, remove the insert used to lock the floats.

Operation: To test the contacts, press the internal part with the lid pin. The contacts should actuate automatically. If everything is properly working, close the device again in order to prevent any leakage. Now the relay is ready to be put in operation.


NOTE: The insert is used for transport purposes only.