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.



Some utilities provide protection for large high-voltage and extra-high-voltage autotransformers by using voltage-operated bus-type high-impedance differential relays. Typical connections of this protective system for autotransformers, with the neutral point of the wye winding solidly grounded, are shown below.

Typical schematic connections for high-impedance differential protection of a Y autotransformer with unloaded tertiary

This arrangement provides protection for all types of phase faults and ground faults, but not turn-to turn
faults. In this application, three sets of three-phase CTs are required, one set on the high-voltage side, another set on the low-voltage side, and the third set in the neutral ends of the winding.

All CTs should have the same turns ratio and should be reasonably matched in accuracy class. A single high-impedance relay connected in a ground differential scheme is also applicable for autotransformer protection.

This protection is immune to the effects of magnetizing inrush current because inrush current is cancelled by the neutral CTs. Also, there is no imbalance current in the relay circuit due to the load tap changing equipment.

Thus a high-impedance differential relay can be applied without any harmonic restraint, load bias, or time delay. Autotransformers are often provided with a Δ tertiary winding. It should be noted that with this type of scheme no protection is afforded for faults occurring in the Δ tertiary winding.

Where the terminals for this winding are not brought out to supply load, one corner of the Δ can be connected between the end of one phase of the main winding and its neutral CT. This connection is shown above.

In such an arrangement, the tertiary winding is included in the differential protection zone, and the relay would sense ground faults in the tertiary winding. This scheme does not provide protection for phase faults or turn-to-turn faults in the tertiary winding.

Where the tertiary winding is used to supply load, the Δ winding corner connection cannot be used. Hence, separate protection is required.

Information can be found here.



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.

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.



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
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.



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. 



Short Circuit Requirements 

PSTs shall comply with the short circuit requirements of IEEE Std C57.12.00-2000, unless otherwise agreed upon by the purchaser and manufacturer.

Transformer categories
The kVA rating to be considered for determining the category should be the equivalent to the rating according to IEEE Std C57.12.00-2000.

Short-circuit current magnitude
The manufacturer shall determine the most onerous conditions for short circuit on every winding or active part in accordance with IEEE Std C57.12.00-2000.

These conditions should take into account the large impedance swings that can occur as the tap position is changed from the extreme positions to the mid position.

Since the system short-circuit levels are critical to the design of PSTs, the user shall specify the maximum system short-circuit fault levels expected throughout the life of the unit.

If a short-circuit test is performed, it shall be done in accordance with IEEE Std C57.12.90-1993.

The test shall be carried out on the tap position that produces the most severe stresses in each winding. This may require more than a single test depending on the type of construction.

For two-core PSTs this usually requires a test on the zero phase-shift position, as this position involves only the series transformer, and a second test on a position to be agreed upon between customer and manufacturer.



In general, construction requirements for PSTs should be in accordance with the requirements for power transformers, as covered in IEEE Std C57.12.00-2000 and other applicable IEEE standards based on kV and kVA ratings, with the following exceptions or additions.

Enclosed throat connections
Enclosed throat connections in fully assembled condition must meet the pressure and vacuum requirements of PST tanks for all designs that subject the enclosed throat connection to the same operating pressures and vacuum levels as the transformer tank.

Liquid insulation and preservation system 
Liquid insulation and preservation systems shall be in accordance with IEEE Std C57.12.00-2000 with the following addition.

Two tank designs with enclosed liquid-filled throat connection
Enclosed liquid-filled throat connections may be either sealed from each tank or opened to the insulating fluid from one or both tanks. Enclosed throat connections shall be designed for installation or removal without the need to jack or move either or both of the transformer tanks and shall accommodate thermal expansion and contraction of the throat assembly and both tanks. For a sealed throat system that isolates the insulating fluid, the throat connections require a separate conservator system.

For a system where the throats are not directly connected to a main tank and the isolation of the insulating fluid in different compartments is not important, the throats may be connected to the conservator system of the main tank. If this approach is used, the user should be aware that the use of oil and gas analysis to isolate problems will be complicated.

For throat connections that place barriers between both the tanks and the throat, the throat shall be equipped with the following accessories:

— Gas accumulation relay
— Pressure relief device/relay
— Liquid filling and draining valves
— Rapid rate of rise relay
— Liquid level gauge



In general, rating data for PSTs should be in accordance with the requirements for power transformers as covered in IEEE Std C57.12.00-2000 with the following exceptions or additions.

Polarity, angular displacement, and terminal markings
Terminal markings unique to PSTs
The designations H and X shall not be used and shall be replaced by S and L to indicate the source and load. The S terminals shall be marked S1, S2, S3, and (if applicable) S0. The L terminals shall be marked L1, L2, L3, and (if applicable) L0. Y and Z designations shall be used for additional windings that are brought out of the tank.

Enclosed throat connection terminal markings
Enclosed throat winding terminal connections shall be marked in any manner that will permit convenient reference and cannot be confused with the markings of the external transformer terminals.

Impedance shall be in accordance with IEEE Std C57.12.00-2000 with the following additions.

Rated impedance shall be at zero phase-shift connections.

Change in impedance with phase-angle regulation
The impedance of PSTs can vary substantially over its range of phase-angle regulation. The user must specify the acceptable ranges of impedances and the manufacturer shall calculate and provide a matrix of impedances as required by the user. The extent of test verification of impedance values other than rated impedance should be specified and agreed upon by the purchaser and manufacturer.

Nameplates shall be in accordance with IEEE Std C57.12.00-2000 with the following addition: The nameplate of the PST shall show the phase shift in degrees from the S to the L terminals starting at the zero phase-shift tap and for each tap position in the advance and retard direction while operating at no-load.

The nameplate shall also show the phase shift in degrees from the S to the L terminals while operating at maximum rated kVA output at unity power factor at the S terminal for all tap positions which result in acceptable service conditions.

Intermediary phase shifts at varying loads may be specified by the purchaser for inclusion on the nameplate. The user may request impedance changes be indicated on the nameplate for any tap position.



The unusual conditions shall be the same as those listed in IEEE Std C57.12.00-2000, 4.3.1 through 4.3.3. Additional unusual service conditions that may apply to PSTs are as follows:

Operation with two or more PSTs in parallel or in series
The purchaser shall ensure the manufacturer has all nameplate data, test data, and applicable system information necessary to design the PSTs for proper load sharing. The purchaser must specify in detail to the manufacturer the LTC’s controls that will be provided by the purchaser.

If the manufacturer provides the LTC’s controls, the purchaser shall provide the control scheme used with any existing PSTs to the manufacturer to ensure a compatible system.

Operation of PSTs in series with series capacitor banks
If the PST is, or may be, operated in series with a series capacitor bank, this operating condition shall be pointed out to the manufacturer by the purchaser. The operating conditions shall be specified and the protection scheme used by the purchaser to prevent series resonance shall be provided to the PST manufacturer for review and for considerations in design.

Unbalanced current flow through the PST
The purchaser must provide details of operating conditions that will subject the PSTs to unbalanced phase currents and voltages that may exceed allowable standard limits. The manufacturer will provide for these conditions during the design of the PSTs. The following are examples of operating conditions that could produce such problems:

a) Unbalances resulting from operation of parallel transmission lines in close proximity to the PST connected lines, where line transpositions are unequal resulting in unbalanced voltage at the PSTs and unequal current flow through the series windings.

b) Single-pole operation of the circuit breakers following line faults where single-pole reclosing is utilized

Transient recovery voltages
Transient voltage may exist circuit breakers are operated. These conditions may be between the PST and the circuit breaker.

Surge protection
Any condition where the PST may operate without surge protection applied at all S and L terminals.



Usual service conditions
These conditions shall be as stated in IEEE Std C57.12.00-2000, 4.1.1 through 4.1.7, and 4.1.9; 4.1.8 shall not apply. In (a), the word secondary shall mean the L terminals of the PST.

a) The purchaser of the PST shall specify the switching arrangements that will be used to place the PST in and out of service. This shall include breaker or switch operations resulting from faults external and internal to the PST.

b) The PST shall be suitable for energization by voltage applied to either the S or L terminals.

c) The PST shall be capable of transferring rated kVA with the electrical source of power connected to the S or L terminals. Limited power transfer in the retard position has to be considered.

d) Seismic requirements shall be as specified in IEEE Std 693-1997. The seismic zone shall be provided by the purchaser. The foundation design shall be provided to the PST manufacturer by the purchaser.

The manufacturer shall provide for differential motion between the two tanks, if used, and in the case of remotely mounted radiators provide for their differential motion.

e) The manufacturer of the PST shall make provisions for differential alignments that will occur when two tanks are connected. The foundation tolerance shall be defined by agreement between purchaser and manufacturer.

f) Unless specified otherwise, the PST shall be manufactured for operation in the bypassed state with the source and load bushing connected through bus work. This shall require special consideration in design for lightning impulse and switching surges.

This condition will require additional testing with the terminals connected, as in operation, to demonstrate that the insulation level meets the specified BIL.

Loading at other than rated conditions
This subclause shall be the same as IEEE Std C57.12.00-2000, 4.2, with the exception that additional limits must be observed for retard operation under overload. These limits must be defined by the manufacturer and agreed upon by the purchaser prior to completion of the PST design.



Phase angle
The rated phase angle is defined under no-load conditions. However, it should be noted that the unit is unlikely to operate at this phase angle under load in the advanced position due to the effect of the voltage drop in the unit.

In the retard position the no-load phase angle should not be exceeded (unless the unit has been designed for that), as overexcitation will occur in parts of the PST. In the retard position the power that can be transferred is usually lower than the rated power in the advanced position.

Dielectric design of the two-core type
The transmission of transient voltages in the two-core design is rather complex. When applying impulse tests to either the S or the L terminals of the series transformer, the connected exciting winding of the main transformer will also be exposed to a high voltage.

There may be high-voltage oscillations of the connecting leads, depending on the capacitive voltage control of the series winding. High voltages may be transferred to other windings coupled to the series winding or to the excitation winding. Therefore, rather complex computer models may be required to compute the transient voltages for this configuration.

Special considerations for a two-tank design
When the two-core design is used with two tanks, special precautions must be taken to design connections between the two tanks. The connection operates at the system voltage level so that the leads must be insulated for the overvoltages that may occur under both transients and power frequency conditions.

A short-circuit between the connections of the two units has to be considered as an internal fault, which would cause severe damage or even destroy the PST. A short-circuit proof design for this special case would result, if possible at all, in a significant increase in cost. Therefore, it is strongly recommended to use metal enclosures to protect the connections against lightning strikes and other possible sources of a short circuit.

Overload conditions (loading above nameplate rating)
Overloading of a PST in the sense of operating it with a current beyond the name-plate rating increases the internal phase angle β [see Equation (2)] and consequently also the load phase-shift angle α∗ (r) in the retard position.

This may result in a load phase angle that exceeds the maximum rated no-load phase angle. The voltage across the regulating winding and consequently also the voltage per step of a single-core type, as well as the voltage across the series winding of a two-core type will, in this case, exceed the rated voltage.

Furthermore, in a two-core design, the main transformer also will experience a certain degree of overexcitation with the same consequences for the regulating winding. The degree depends on the ratio of the impedances of series and main transformer.

It must—beside the effect that parts of the core(s) may be overfluxed—therefore also be checked whether the parameters’ voltage per step, current, and switching capability are still within the limits of the LTC design.

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