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.



Ferroresonance is the name given to the phenomenon where the exciting reactance of the transformer can become nearly equal to the capacitive reactance of the line to ground, forming a resonant circuit. Such a resonant circuit can distort the normal line impedance to ground so that one line of a 3-phase circuit can rise to a destructive voltage.

Distribution transformers are generally considered as transformers of 500 kVA, and smaller 67,000 V and below, both single-phase and 3-phase. Older installations are primarily pole-/platform-mounted units. Newer installations are frequently pad-mounted units.

Typical applications are for supplying power to farms, residences, public buildings or stores, workshops, and shopping centers. Distribution transformers have been standardized as to high- and low-voltage ratings, taps, type of bushings, size and type of terminals, mounting arrangements, nameplates, accessories, and a number of mechanical features, so that a good degree of interchangeability results for transformers in a certain kVA range of a given voltage rating. They are now normally designed for 65 C rise.

Such a ferroresonance practically never occurs in a normal circuit configuration with the transformers loaded, but it can exist under a combination of the following circumstances which usually occur only during switching of a 3-phase bank or blowing of a fuse in one line:

1. System neutral grounded, ungrounded transformer neutral

2. No load on the transformer

3. Relatively large capacitance line-to-ground such as may exist in cable circuits (underground distribution) or very long overhead lines (although ferroresonance can be and has been corrected by adding still more capacitance which presumably throws the combination out of resonance again)

Although ferroresonance has been studied at some length, it still does not seem possible to reliably predict its occurrence. Experience indicates that it is possible to prevent ferroresonance during switching on a transformer bank if all three transformers are resistance-loaded to 15% or more of their rating, or if special switches are used to assure that the three lines close simultaneously.



Low-temperature superconducting (LTS) transformers were first proposed in the 1970s, and designed to operate at 6◦K to 14◦K (−268◦C to −260◦C). The invention of high temperature superconducting (HTS) materials increased the prospects for superconducting units designed to operate between 20◦K to 77◦K. A three-phase 630 kVA, 18.7 kVl−−l/420 Vl−−l demonstration transformer based on HTS winding technology is presently under test on the power grid.

Superconducting transformers have about half the weight of conventional oil-filled transformers, and they require less space due to their reduced size, which is important for urban locations. They are nonflammable and employ environmentally benign liquid nitrogen as the cooling medium.

But perhaps the key advantage is their capability for overcapacity operation, due in part to the low temperatures at which HTS windings operate. Heat is the principal enemy of the paper-oil electrical insulation system of conventional power transformers.

HTS transformers operate in the ultra cold range of 20◦K to 77◦K (−253◦C to −196◦C), where insulation materials will not degrade. They can operate up to twice rated power, and they have a low series impedance, improving voltage regulation.

Conventional transformers typically have ηpower = 99.3% to 99.7% for the 30 MVA class. HTS transformers have a higher efficiency, to the extent that the reduced loss in a HTS unit can more than pay for its initial capital cost over its lifetime.

HTS units have a similar construction to the liquid-filled conventional transformer: the magnetic core carries super conducting windings cooled by liquid nitrogen, which is the only safe and low-cost cryogen available in liquid form in the 20◦K to 77◦K temperature range.

The superconducting windings are manufactured either as wires or as flat tapes using BSCCO-2223 material. To date there are not many data available concerning the reliability of HTS units. Most publications concede that a superior, cost-effective HTS transformer technology might take two decades to become available.



The selection of a cooling system based on liquids permits a greater overload capability. Liquid-filled units are cooled in a variety of ways. Some of them protect the coolant from oxidation by sealing the transformer and inserting inert gas in the air space.

(1) Oil-Immersed Self-Cooled The insulating mineral oil circulates by natural convection within the tank, which has either smooth sides, corrugated sides, integral tubular sides, or detachable radiators.

(2) Oil-immersed self-cooled and forced-air cooled The same as type 1, but the addition of fans increases the rate of heat transfer from the cooling surfaces, thereby increasing the permissible transformer output.

(3) Oil-Immersed Self-Cooled and Forced-Oil–Forced-Air Cooled The rating of an oil-immersed transformer may be further increased by the addition of some combinations of fans and oil pumps.

(4) Oil-Immersed Forced-Oil-Cooled with Forced-Air Cooler Heat transfer from oil to air is accomplished in external oil-to-air heat exchangers with oil pumps and fans.

(5) Oil-Immersed Water-Cooled Cooling water runs through pipes that are in contact with the cooling oil of the transformer. The oil flows around the outside of these pipe coils by natural convection, thereby effecting the desired heat transfer to the cooling water.

(6) Oil-Immersed Forced-Oil-Cooled with Forced-Water Cooler External oil-to-water heat exchangers are used in this type of unit to transfer heat from oil to cooling water.

Depending upon the geometric duct dimensions and the pressure applied by the oil pumps, the oil velocities for laminar flow range from 0.005 m/s to 0.05 m/s. A great disadvantage of mineral oil is its flammability.

For this reason nonflammable synthetic oils were developed, such as those with the brand names Askarel, Inerteen, Pyranol (USA), Permitol (England), Aroclor (France), and Clophen (Germany). Unfortunately, most of these have proven to be undesirable from an environmental and health point of view, and are not used in new transformer designs.



The nominal power efficiency ηpower of a transformer is the ratio of rated real power output to rated real power input: ηpower = Pout/Pin = 1− (Ploss/Pin). Total losses Ploss are the sum of the no-load and load losses. No-load losses consist of eddy-current and hysteresis losses within the core (|˜ic|2 Rc, the loss caused by the core-loss component ic of the exciting current iφ;), ohmic loss |˜iφ|2 Rp, and dielectric loss: that is, all losses that occur at full voltage with the secondary circuit open.

Load losses are |˜ip(t)|2 Rp+|˜is(t)|2 Rs caused by the primary [ip(t)] and secondary [is(t)] load currents. Eddy-current losses also occur, induced by stray fluxes within the solid transformer structure, and similar losses are generated in the windings, varying with the load current.

No-load losses are measured at rated frequency and rated secondary voltage (if the secondary side is the low-voltage side) and are considered to be independent of load. Load losses are measured at rated frequency and rated secondary current, but with the secondary short-circuited and with reduced voltage applied to the primary, the high-voltage side. Load losses can be assumed to vary as the square of the load current.

Most units are not fully loaded all the time, and therefore one defines the energy efficiency of a transformer, where lightly loaded periods are also taken into account during a load cycle. For low-power-efficiency transformers (ηpower < 96%) the loss can be measured from the relatively large difference between the input power Pin and the output power Pout.

However, for high power efficiency units (ηpower > 96%), the errors in measuring Pin and Pout and the small difference between the two make an efficiency determination meaningless. If two current transformers (CTs, maximum errors εCT1 = εCT2 = 5 mA, CT ratio = 20) and two potential transformers (PTs, εPT1 = εPT2 = 0.24 V, PT ratio = 30) as well as two ammeters (εA1 = εA2 = 5 Ma) and voltmeters (εV1 = εV2 = 0.3 V) with full-scale errors of 0.1% are used, then the maximum error in the measured losses for a 25 kVA, ηpower = 98.44%, 240 V/7200 V single-phase transformer at cos φ1 = 1 is #Ploss = (240 V ± εPT1 ± εV1)(5.20835 A ± εCT1 ± εA1) × 20 − 30 (240 V ± εPT2 ± εV2 (3.472 A ± εCT2 ± εA2) = (240.54 V) × (104.367 A) − (7183.8 V) × (3.462 A) = 234.1 W, so that #Ploss/Ploss = ± (234.1/390)100% ≈ 60%.

This means the conventional method of measuring the losses and therefore the power efficiency of high-efficiency units does not produce accurate results, and other methods must be used.



Effects of Overcurrent.
A transformer may be subjected to overcurrents ranging from just in excess of nameplate rating to as much as 10 or 20 times rating. Currents up to about twice rating normally result from overload conditions on the system, while higher currents are a consequence of system faults.

When such overcurrents are of extended duration, they may produce either mechanical or thermal damage in a transformer, or possibly both. At current levels near the maximum design capability (worst-case through fault), mechanical effects from electromagnetically generated forces are of primary concern.

The pulsating forces tend to loosen the coils, conductors may be deformed or displaced, and insulation may be damaged. Lower levels of current principally produce thermal heating, with consequences as described later on loading practices. For all current levels, the extent of the damage is increased with time duration.

Protective Devices. 
Whatever the cause, magnitude, or duration of the overcurrent, it is desirable that some component of the system recognize the abnormal condition and initiate action to protect the transformer. Fuses and protective relays are two forms of protective devices in common use.

A fuse consists of a fusible conducting link which will be destroyed after it is subjected to an overcurrent for some period of time, thus opening the circuit. Typically, fuses are employed to protect distribution transformers and small power transformers up to 5000 to 10,000 kVA.

Traditional relays are electromagnetic devices which operate on a reduced current derived from a current transformer in the main transformer line to close or open control contacts, which can initiate the operation of a circuit breaker in the transformer line circuit. Relays are used to protect all medium and large power transformers.

All protective devices, such as fuses and relays, have a defined operating characteristic in the current-time domain. This characteristic should be properly coordinated with the current-carrying capability of the transformer to avoid damage from prolonged overloads or through faults.

Transformer capability is defined in general terms in a guide document, ANSI/IEEE C57.109, Transformer Through Fault Current Duration Guide. The format of the transformer capability curves is shown in Fig. 10-35.

The solid curve, A, defines the thermal capability for all ratings, while the dashed curves, B (appropriate to the specific transformer impedance), define mechanical capability. For proper coordination on any power transformer, the protective-device characteristic should fall below both the mechanical and thermal portions of the transformer capability curve.

(See ANSI/ IEEE C57.10-38 for details of application.)



It is a general practice to have some means of adjustment to maintain constant voltage at the output terminals by compensating for the variations of the input voltage. This is done by tapping out or adding turns to the primary or input winding and maintaining the volts per turn, and thus the output voltage.

This operation is usually performed when the transformer is de-energized; this is called off-circuit tap changing. In dry type transformers, the usual method is to bring out the tap terminals on the outer surface of the coil or on a terminal board, where the linking to obtain the required turns is done manually with the unit de-energized.

It is possible, though not usual, to have tap switches similar to those used in liquid- filled units. Until recently, dry-type transformers were never supplied with under-load-tap-changing equipment. This was due to the fact that under-load tap changing involves breaking of load current at full voltage, thereby requiring switching equipment with capabilities comparable to those of circuit breakers.

To do this in air was cumbersome, bulky, and extremely expensive. But with the increased capacities and voltages of dry- type transformers, the demand for such equipment has increased, and recently voltage regulators became commercially available.

Two different approaches are used to provide underload voltage regulation. One takes the traditional approach of the liquid-filled units by providing motor-driven selector switches combined with a spring activated vacuum diverter switch.

The other approach uses a separate regulator winding feeding a buck/boost transformer connected in series with the primary winding. Voltage regulation is achieved by means of low-voltage vacuum contactors that modify the tap settings of the regulating winding of the buck/boost transformer, circumventing high-voltage switching equipment.

The contactors are usually controlled by programmable logic controllers (PLC). In cases where high speed response is required, the second approach has successfully used thyristors in place of vacuum contactors, thereby achieving a cycle switching.



Oriented (anisotropic) silicon-steel laminations.
The iron cores of conventional transformers consist of anisotropic silicon-steel laminations with lamination thickness ranging from 0.1 mm to 0.4 mm. In a transformer, the flux travels mostly within the limbs in the with-grain direction, and in the cross-grain direction only near the corners and lamination joints of transformer cores; thus oriented steel sheets are used.

The with- and cross-grain structure of oriented steel is determined by the rolling direction of the sheets during manufacture. Each side of a lamination is coated with insulating material so that no eddy currents can flow between laminations.

The coating does not significantly interfere with the passage of flux. The magnetic resistance, or reluctance, is only slightly increased and is taken into account via the iron-core stacking factor ϕFe = #(iron cross section of all laminations of core)/(cross section of entire core including insulation between laminations).

The stacking factor is in the range of 0.93 ≤ ϕFe ≤ 0.97 for 60 Hz units. For anisotropic electrical silicon steel the relative permeability is larger (and thus the magnetization required is smaller) in the with-grain direction (direction of rolling) than in the cross-grain direction. Similarly, the core losses are small in the with-grain direction and relatively large in the cross-grain direction.

Amorphous (glass-type) cores.
Amorphous magnetic materials either are obtained by quenching the molten material at high cooling rates or are manufactured by deposition techniques in a vacuum. The quenching process does not permit the forming of a crystalline structure, and therefore amorphous magnetic materials have a structure similar to glass.

The cores of transformers with amorphous alloy (AMTs) can be fabricated in the same manner as those made of oriented-silicon-steel. METGLAS (trademark of the Allied Signal Co) cores are 30% heavier than comparable oriented silicon-steel cores, but the no-load losses in amorphous alloy wound cores are only 30% of those in comparable oriented-siliconsteel wound cores.

However, the rated power efficiencies of present-day designs of AMTs and silicon-steel pole transformers with wound cores are about the same. For example, the rated power efficiencies of 20 kVA and 50 kVA wound-core AMTs at unity power factor are ηpower = 98.26% and 98.59%, respectively, while that of a 25 kVA oriented-silicon-steel wound core (10) at unity power factor is ηpower = 98.31%. The fabrication cost for AMTs with wound cores is higher than that for oriented silicon-steel wound cores.



Single-phase transformers can be connected to form three-phase transformer banks for stepping voltages up or down in three-phase systems. Four common configurations for connecting transformers in three-phase systems are delta–delta, wye–wye, wye–delta, and delta–wye.

The first three are shown in Fig. 3-9. The delta–wye is not shown because it is simply the reverse of the wye–delta connection.

Delta–delta connection
The delta–delta connection, shown in Fig. 3-9a, is widely used for moderate voltages. This connection has the advantage of remaining operational in what is known as the open delta or V connection if one transformer is damaged or taken out of service, leaving the remaining two functional.

If it is operated this way, the bank still delivers three-phase currents and voltages in their correct phase relationships. However, the capacity of the bank is reduced to 57.7 percent of the value obtained with all three transformers in service.

Wye–wye connection
In the wye–wye connection, shown in Fig. 3-9b, only 57.7 percent (or 1/1.73) of the line voltage is applied to each winding, but full line current flows in each transformer winding. The drawback to this connection is that power circuits supplied from a wye–wye bank generate serious electromagnetic interference, which could interrupt nearby communications circuits.

Because of this and other disadvantages, the wye–wye connection is seldom used. However, the wye–wye connection can be used to interconnect two delta systems and provide suitable neutrals for grounding both of them.

Delta–wye and wye–delta connections 
The delta–wye connection (not shown) is suitable for stepping up voltages because the voltage is increased by the transformer ratio multiplied by a factor of 1.73. Similarly, the wye–delta connection, shown in Fig. 3-9c, is used for stepping down voltages.

The high-voltage windings of most transformers operating at more than 100 kV are wye-connected. To match the polarities correctly in a wye connection, the H and X markings must be connected symmetrically.

In other words, if an H1 or X1 terminal is connected to the neutral, then all of the H1 or X1 terminals must be connected to the neutral and the remaining H2 or X2 terminals must be brought out as the line connections, as shown in Fig. 3-9b.

By contrast, in a delta connection, H1 must always be connected to H2 and X1 to X2, and the line connections must be made at these junctions, as shown in Fig. 3-9a.

When a large number of single-phase loads are to be served from a three-phase transformer bank, the wye connected low-voltage winding is recommended because the single-phase loads can be balanced evenly on all phases.


The following technical terms apply to transformers.

BIL: An abbreviation for basic impulse level, a dielectric strength test. Transformer BIL is determined by applying a high-frequency square-wave voltage with a steep leading edge between the windings and between the windings and ground.

The BIL rating provides the maximum input kV rating that a transformer can withstand without causing insulation breakdown. The transformer must also be protected against natural or man-made electrical surges. The NEMA standard BIL rating is 10 kV.

Exciting current: In transformers, the current in amperes required for excitation. This current consists of two components: (1) real in the form of losses (no load watts) and (2) reactive power in kvar. Exciting current varies inversely with kVA rating from approximately 10 percent at 1 kVA to as low as 0.5 percent at 750 kVA.

Eddy-current losses: Contiguous energy losses caused when a varying magnetic flux sets up undesired eddy currents circulating in a ferromagnetic transformer core.

Hysteresis losses: Continuous energy losses in a ferromagnetic transformer core when it is taken through the complete magnetization cycle at the input frequency.

Insulating transformer: A term synonymous with isolating transformer, to describe the insulation or isolation between the primary and secondary windings. The only transformers that are not insulating or isolating are autotransformers. Insulation system temperature: The maximum temperature in degrees Celsius at the hottest point in the winding.

Isolating transformer: See insulating transformer. Shielded-winding transformer: A transformer with a conductive metal shield between the primary and secondary windings to attenuate transient noise.

Taps: Connections made to transformer windings other than at its terminals. They are provided on the input side of some high-voltage transformers to correct for high or low voltages so that the secondary terminals can deliver their full rated output voltages.

Temperature rise: The incremental temperature rise of the windings and insulation above the ambient temperature.

Transformer impedance: The current-limiting characteristic of a transformer expressed as a percentage. It is used in determining the interrupting capacity of a circuit breaker or fuse that will protect the transformer primary.

Transformer voltage regulation: The difference between the no-load and full-load voltages expressed as a percentage. A transformer that delivers 200 V at no load and 190 V at full load has a regulation of 5 percent.



Although transformer oil is a highly refined product, it is not chemically pure. It is a mixture principally of hydrocarbons with other natural compounds which are not detrimental. There is some evidence that a few of these compounds are beneficial in retarding oxidation of the oil.

Although oil is not a “pure” substance, a few particular impurities are most destructive to its dielectric strength and properties. The most troublesome factors are water, oxygen, and the many combinations of compounds which are formed by the combined action of these at elevated temperatures.

A great deal of study has been given to the formation of these compounds and their effects on the dielectric properties of oil, but there apparently is no clear relation between these compounds and the actual dielectric strength of the transformer insulation structure.

Oil will dissolve in true solution a very small quantity of water, about 70 ppm at 25 C and 360 ppm at 70 C. This water in true solution has relatively little effect on the dielectric strength of oil. If, however, acids are present in similar amounts, the capacity of oil to dissolve water is increased, and its dielectric strength is reduced by the dissolved water. Small amounts of water in suspension cause severe decreases in dielectric strength.

The primary reason for concern over moisture in transformer oil, however, may not be for the oil itself but for the paper and pressboard which will quickly absorb it, increasing the dielectric loss and decreasing the dielectric strength as well as accelerating the aging of the paper.

It is generally recognized today that the best answer to the problem of air and water is to eliminate them and keep them out. For this purpose, in American practice, transformer tanks are completely sealed. About three basic schemes are used in sealed transformers to permit normal expansion and contraction of oil (0.00075 per unit volume expansion per degree Celsius) as follows:

1. A gas space above the oil large enough to absorb the expansion and contraction without excessive variation in pressure. Some air may unavoidably be present in the gas space at the time of installation but soon the oxygen mostly combines with the oil without causing significant deterioration, leaving an atmosphere which is mostly nitrogen.

2. A nitrogen atmosphere above the oil maintained in a range of moderate positive pressure by a storage tank of compressed nitrogen and automatic valving. This scheme has the advantage that the entrance of air or moisture is prevented by the continuous positive internal pressure, and the disadvantage of somewhat higher cost.

3. A constant-pressure oil-preservation system consisting of an expansion tank with a flexible synthetic rubber diaphragm floating on top of the oil. This scheme has the advantages that the oil is never in contact with the air and there is always atmospheric pressure and not a variable pressure on the oil. The disadvantage is the higher cost. A number of mechanical variations and elaborations of this general idea have been devised.

It is now generally recommended that the constant-pressure oil-preservation system of item 3 be employed on all high-voltage power transformers (345 kV and above) and on all large generator step-up transformers. This is a consequence of unfavorable experience with transformers having gascushion systems, which inherently operate with large quantities of the cushion gas in solution in the hot oil under load.

If the oil is suddenly cooled (reduction of ambient temperature or load), the oil volume contracts and the static pressure of gas over the oil drops rapidly, allowing free gas bubbles to come out of solution throughout the insulation system. The dielectric strength of the oil and cellulose insulation system is drastically weakened when it has free gas inclusions, and this has occasionally led to electrical failure of operating transformers.



ANSI/IEEE C57.12.90 specifies the method for measuring the average sound level of a transformer. The measured sound level is the arithmetic average of a number of readings taken around the periphery of the unit. For transformers with a tank height of less than 8 ft, measurements are taken at one-half tank height.

For taller transformers, measurements are taken at one-third and two-thirds tank height. Readings are taken at 3-ft intervals around the string periphery of the transformer, with the microphone located 1 ft from the string periphery and 6 ft from fan-cooled surfaces.

The ambient must be at least 5 and preferably 10 dB below that of the unit being measured. There should be no acoustically reflecting surface, other than ground, within 10 ft of the transformer. The A weighting network is used for all standard transformer measurements regardless of sound level.

NEMA Publication TR 1 contains tables of standard sound levels. For oil-filled transformers, from, 1000 to 100,000 kVA, self-cooled (400,000 kVA, forced-oil-cooled) standard levels are given approximately by

Equation L = 10 log E K

where E equivalent two-winding, self-cooled kVA (for forced-oil-forced-air-cooled units, use 0.6  kVA), K constant, from Table 10-3, and L = decibel sound level.

Example. A transformer rated 50,000 kVA self-cooled, 66,667 kVA forced-air-cooled, 83,333 kVA forced-oil-forced-air-cooled, at 825 kV BIL, would have standard sound levels of 78, 80, and 81 dB on its respective ratings.

Public Response to Transformer Sound.
The basic objective of a transformer noise specification is to avoid annoyance. In a particular application, the NEMA Standard level may or may not be suitable, but in order to determine whether it is, some criteria must be available.

One such criterion is that of audibility in the presence of background noise. A sound which is just barely audible should cause no complaint.

Studies of the human ear indicate that it behaves like a narrowband analyzer, comparing the energy of a single frequency tone with the total energy of the ambient sound in a critical band of frequencies centered on that of the pure tone. If the energy in the single-frequency tone does not exceed the energy in the critical band of the ambient sound, it will not be significantly audible.

This requirement should be considered separately for each of the frequencies generated by the transformer core. The width of the ear-critical band is about 40 Hz for the principal transformer harmonics. The ambient sound energy in this band is 40 times the energy in a 1-Hz-wide band.

The sound level for a 1-Hz bandwidth is known as the “spectrum level” and is used as a reference. The sound level of the 40-Hz band is 16 dB (10 log 40) greater than the sound level of the 1-Hz band. Thus, a pure tone must be raised 16 dB above the ambient spectrum level to be barely audible.

The transformer sound should be measured at the standard NEMA positions with a narrow-band analyzer. If only the 120- and 240-Hz components are significant, an octave-band analyzer can be used, since the 75- to 150-Hz and 150- to 300-Hz octave bands each contain only one transformer frequency. The attenuation to the position of the observer can be determined.

The ambient sound should be measured at the observer’s position. For each transformer frequency component, the ambient spectrum level should be determined. An octave-band reading of ambient sound can be converted to spectrum level by the equation

S = B - 10 log C

where B decibels octave-band reading, C hertz octave bandwidth, and S decibels spectrum level.



The delta-delta, the delta-Y, and the Y-Y connections are the most generally used; they are illustrated in figure below. The Y-delta and delta-delta connections may be used as step-up transformers for moderate voltages.

                      Standard 3-phase/3-phase transformer systems.
The Y-delta has the advantage of providing a good grounding point on the Y-connected side which does not shift with unbalanced load and has the further advantage of being free from third-harmonic voltages and currents; the delta-delta has the advantage of permitting operation in V in case of damage to one of the units.

Delta connections are not the best for transmission at very high voltage; they may, however, be associated at some point with other connections that provide  means for properly grounding the high voltage system; but it is better, on the whole, to avoid mixed systems of connections. The delta-Y step-up and Y-delta step-down connections are without question the best for high voltage transmission systems.

They are economical in cost, and provide a stable neutral whereby the high-voltage system may be directly grounded or grounded through resistance of such value as to damp the system critically and prevent the possibility of oscillation.

The Y-Y connection (or Y-connected autotransformer) may be used to interconnect two delta systems and provide suitable neutrals for grounding both of them. A Y-connected autotransformer may be used to interconnect two Y systems which already have neutral grounds, for reasons of economy.

In either case, a delta-connected tertiary winding is frequently provided for one or more of the following purposes. In stabilization of the neutral, if a Y-connected transformer (or autotransformer) with a delta connected tertiary is connected to an ungrounded delta system (or poorly grounded Y system), stability of the system neutral is increased.

That is, a single-phase short-circuit to ground on the transmission line will cause less drop in voltage on the short-circuited phase and less rise in voltage on the other two phases. A 3-phase three-leg Y-connected transformer without delta tertiary furnishes very little stabilization of the neutral, and the delta tertiary is generally needed.

Other Y connections offer no stabilization of the neutral without a delta tertiary. With increased neutral stabilization, the fault current in the neutral on single-phase short circuit is increased, and this may be needed for improved relay protection of the system.

Third-harmonic components of exciting current find a relatively low impedance path in a delta tertiary on a Y-connected transformer, and less of the third-harmonic exciting current appears in the connected transmission lines, where it might cause interference with communication circuits. Failure to provide a path for third-harmonic current in Y-connected 3-phase shell-type transformers or banks of single-phase transformers will result in excessive third-harmonic voltage from line to neutral.

The bank of a 3-phase, three-legged core-type Y-connected transformer acts as a delta winding with high impedance to the other windings. As a consequence, there is very little third-harmonic line-to-neutral voltage and a separate delta tertiary is not needed to reduce it. An external load can be supplied from a delta tertiary. This may include synchronous or static capacitors to improve system operating conditions.



Tap-changing equipment is sometimes used in a loop system, for phase-angle control, for the purpose of obtaining minimum losses in the loop due to unequal impedances in the various portions of the circuit.

Transformers used to derive phase-angle control do not differ materially, either mechanically or electrically, from those used for in phase control. In general, phase-angle control is obtained by interconnecting the phases, that is, by deriving a voltage from one phase and inserting it in another.

The simple arrangement given in figure below illustrates a single core delta-connected autotransformer in which the series windings are so interconnected as to introduce into the line a quadrature voltage.

One phase only is printed in solid lines so as to show more clearly how the quadrature voltage is obtained. The terminals of the common winding are connected to the midpoints of the series winding in order that the in phase voltage ratio between the primary lines ABC and secondary lines XYZ is unity for all values of phase angle introduced between them.

As large high-voltage systems have become extensively interconnected, a need has developed to control the transfer of real power between systems by means of phase-angle-regulating transformers.

The most commonly used circuit for this purpose is the two-core, four-winding arrangement. The high-voltage common winding is Y-connected, with reduced insulation at the neutral for economy of design, and a series transformer is employed so that low-voltage-switching equipment may be used.

Phase-shifting regulating transformers; single core delta-connected common winding for low-voltage systems.



Below shows the voltage relations across an autotransformer and switching contacts during a tap changing cycle using an autotransformer designed for 60% circulating current and with 100% load current at 80% power factor flowing through it.

Perfect interlacing between the autotransformer halves is assumed, and the voltage drop due to resistance of the autotransformer winding is neglected.

A study of the figure will disclose the fact that increasing the magnetizing reactance of the autotransformer to reduce the circulating current will

1. Increase the voltage across the full autotransformer winding
2. Increase the voltage to be ruptured
3. Introduce undue voltage fluctuations in the line

Since B-4 and B-3 represent the voltages appearing across the arcing contacts when the bridging position is opened at A and B, the voltage rupturing duty will increase with

1. Increase in voltage between adjacent taps
2. Increase in load
3. Decrease in power factor of the load
4. Decrease in the magnetizing current for which the autotransformer is designed

Vector relations for bridging position AB—voltage across adjacent taps; A-1 and A-2— reactance volts due to load current in only half the autotransformer winding; A-3 and A-4—induced voltage across full auto transformer winding; B-4— voltage ruptured when bridging position is ruptured
at A; B-3—voltage ruptured when bridging position is ruptured at B.



Three factors must be considered in the evaluation of the dielectric capability of an insulation structures —the voltage distribution must be calculated between different parts of the winding, the dielectric stresses are then calculated knowing the voltages and the geometry, and finally the actual stresses can be compared with breakdown or design stresses to determine the design margin.

Voltage distributions are linear when the flux in the core is established. This occurs during all power frequency test and operating conditions and to a great extent under switching impulse conditions (Switching impulse waves have front times in the order of tens to hundreds of microseconds and tails in excess of 1000 μs.)

These conditions tend to stress the major insulation and not inside of the winding. For shorter-duration impulses, such as full-wave, chopped-wave, or front-wave, the voltage does not divide linearly within the winding and must be determined by calculation or low voltage measurement. The initial distribution is determined by the capacitative network of the winding.

For disk and helical windings, the capacitance to ground is usually much greater than the series capacitance through the winding. Under impulse conditions, most of the capacitive current flows through the capacitance to ground near the end of the winding, creating a large voltage drop across the line end portion of the coil.

The capacitance network for shell form and layer-wound core form results in a more uniform initial distribution because they use electrostatic shields on both terminals of the coil to increase the ratio between the series and to ground capacitances.

Static shields are commonly used in disk windings to prevent excessive concentrations of voltages on the line-end turns by increasing the effective series capacitance within the coil, especially in the line end sections.

Interleaving turns and introducing floating metal shields are two other techniques that are commonly used to increase the series capacitance of the coil.

Following the initial period, electrical oscillations occur within the windings. These oscillations impose greater stresses from the middle parts of the windings to ground for long-duration waves than for short-duration waves.

Very fast impulses, such as steep chopped waves, impose the greatest stresses between turns and coil portions. Note that switching impulse transient voltages are two types— asperiodic and oscillatory. Unlike the asperiodic waves discussed earlier, the oscillatory waves can excite winding natural frequencies and produce stresses of concern in the internal winding insulation.

Transformer windings that have low natural frequencies are the most vulnerable because internal damping is more effective at high frequencies. Dielectric stresses existing within the insulation structure are determined using direct calculation (for basic geometries), analog modeling, or most recently, sophisticated finite-element computer programs.

Allowable stresses are determined from experience, model tests, or published data. For liquidinsulated transformers, insulation strength is greatly affected by contamination and moisture. The relatively porous and hygroscopic paper-based insulation must be carefully dried and vacuum impregnated with oil to remove moisture and gas to obtain the required high dielectric strength and to resist deterioration at operating temperatures.

Gas pockets or bubbles in the insulation are particularly destructive to the insulation because the gas (usually air) not only has a low dielectric constant (about 1.0), which means that it will be stressed more highly than the other insulation, but also air has a low dielectric strength.

High-voltage dc stresses may be imposed on certain transformers used in terminal equipment for dc transmission lines. Direct-current voltage applied to a composite insulation structure divides between individual components in proportion to the resistivities of the material.

In general the resistivity of an insulating material is not a constant but varies over a range of 100:1 or more, depending on temperature, dryness, contamination, and stress. Insulation design of high-voltage dc transformers in particular require extreme care.



The core loss (no-load loss) of a power transformer may be obtained from an empirical design curve of watts per pound of core steel (Fig. below). Such curves are established by plotting data obtained from transformers of similar construction.

The basic loss level is determined by the grade of core steel used and is further influenced by the number and type of joints employed in construction of the core. Figure 10-1 applies for 9-mil-thick M 3-grade steel in a single-phase core with 45” mitered joints.

Loss for the same grade of steel in a 3-phase core would usually be 5% to 10% higher. Exciting current for a power transformer may be established from a similar empirical curve of exciting volt-amperes per pound of core steel.

The steel grade and core construction are the same as for Fig. 10-1. The exciting current characteristic is influenced primarily by the number, type, and quality of the core joints, and only secondarily by the grade of steel.

Because of the more complex joints in the 3-phase core, the exciting volt-amperes will be approximately 50% higher than for the single-phase core. The exciting current of a transformer contains many harmonic components because of the greatly varying permeability of the steel.
For most purposes, it is satisfactory to neglect the harmonics and assume a sinusoidal exciting current of the same effective value. This current may be regarded as composed of a core-loss component in phase with the induced voltage (90DEG ahead of the flux) and a magnetizing component in phase with the flux.

Sometimes it is necessary to consider the harmonics of exciting current to avoid inductive interference with communication circuits. The harmonic content of the exciting current increases as the peak flux density is increased.

Performance can be predicted by comparison with test data from previous designs using similar core steel and similar construction. The largest harmonic component of the exciting current is the third.

Higher-order harmonics are progressively smaller. For balanced 3-phase transformer banks, the third harmonic components



The effects of harmonics on transformers are

• Increased copper losses
• Increased iron losses
• Possibly resonance between transformers
• windings and line capacitance
• Insulation stress
• Neutral overheating due to triplen harmonics

The copper losses and iron losses in the presence of harmonics can be computed. The application
of general equations assumes that the transformer is a linear device which it is not. However, for normal, operating conditions and normal levels of harmonics, this is a reasonable approximation.

However, the increase of hysteresis losses due to harmonics is only a fraction of the eddy current losses. Voltage harmonics result in higher transformer voltage, therefore higher insulation stress. This is not a problem since most transformers are insulated for much higher voltage levels than the overvoltages due to usual levels of harmonics.

There is a certain degree of interaction between voltage and current harmonics for transformers designed to operate near the saturation point (knee of the saturation curve). It is possible a small level of voltage harmonic to generate a high level of current harmonics. This phenomenon depends on specific harmonic and phase relationship to the fundamental.

To address the overheating of transformers due to harmonics, the ANSI/IEEE published a standard C57.110-1998, “Recommended practice for establishing transformer capability when supplying nonsinusoidal load currents,” which was reaffirmed in 2004. This standard establishes methods for determining derating factors for transformer capability to carry nonsinusoidal load currents.

In 1990, Underwriters Laboratory (UL) established the method for testing transformers that serve nonlinear loads. The UL test addresses coil heating due to nonlinear loads and overheating of the neutral conductor by assigning a “K“ factor to the transformer. The K-factor is meant to apply to transformers serving general nonlinear loads. UL has devised the K-factor method for labeling and rating the ability of dry-type transformers to withstand the effects of harmonics.

The K-factor rating indicates the transformer’s ability to tolerate the additional heating caused by harmonics. The K-factor is based on the methodology similar to that discussed in the ANSI/IEEE C57.110 standard. The K-factor can be calculated as the sum of the product of each harmonic current squared and that harmonic number squared for all harmonics from the fundamental to the highest harmonic of consequence.

When K-factor is multiplied by the stray losses of the transformer, the result represents the total stray losses in the transformer caused by harmonic currents. To obtain the total load losses, the total stray losses are then added to the load losses. It should be obvious that the K-factor for linear loads (absence of harmonics) is 1.

Also, the K-factor does not mean that the transformer can eliminate harmonics. Harmonics increase heating losses in all transformers, and some of these losses are deep within the core and windings and some are closer to the surface. Oil-filled transformers react differently to the increased heat and are better able to cool whereas dry-type transformers are more susceptible to the harmonic current effects and are so labeled. The UL test addresses coil heating due to nonlinear loads and overheating of the neutral conductor.



Parts Of Pad-Mounted Single-Phase Distribution Transformers

Single-phase pad mounted distribution transformers are used in underground distribution systems where it is preferable to have underground rather than overhead distribution. An example of a single-phase, pad mounted distribution transformer with its cover raised is shown in below.

Single-phase, pad-mounted transformers are manufactured with ratings from 10 to 167 kVA. All of these distribution transformers are oil-insulated, self-cooled, and made with loop or radial feed. They can meet or exceed ANSI and NEMA standards.

Pad-mounted distribution transformers are enclosed in steel tamper-resistant protective cases designed with low profiles. They are usually painted green to blend in

Submersible single-phase distribution transformers
Single-phase submersible underground transformers are enclosed in round vertical stainless steel tanks that are hermetically sealed for protection against repeated flooding and/or immersion. The terminals, ground pads, and nameplates are mounted on the covers for easy access from ground level.

These transformers are made in ratings of 25 to 167 kVA. Where submersible transformers are to be installed in a trench that is not subject to repeated flooding or immersion, they are enclosed in stainless steel tanks. Their terminals, ground pads, and nameplates are mounted on their covers.



Power transformer capacity is rated in kilovolt-amperes (kVA). The output rating for a transformer is determined by the maximum current that the transformer can withstand without exceeding its stated temperature limits.

Power in an AC circuit depends on the power factor of the load and the current, so if any AC electrical equipment is rated in kilowatts, a power factor must be included to make its power rating meaningful. To avoid this, transformers and most AC machines are rated in kVA, a unit that is independent of power factor.

In addition to its kVA rating, the nameplates of transformers typically include the manufacturer’s type and serial number, the voltage ratings of both high- and low voltage windings, the rated frequency, and the impedance drop expressed as a percentage of rated voltage. Some nameplates also include an electrical connection diagram.

Power transformers are generally defined as those used to transform higher power levels than distribution transformers (usually over 500 kVA or more than 67 kV). The kVA terminal voltages and currents of power transformers, defined in ANSI C57.12.80, are all based on the rated winding voltages at no-load conditions.

However, the actual primary voltage in service must be higher than the rated value by the amount
of regulation if the transformer is to deliver the rated voltage to the load on the secondary.

The efficiency of all power transformers is high, but efficiency is highest for large transformers operating at 50 to 100 percent of full load. However, some losses are present in all transformers. They are classified as copper or I2R losses and core losses.

Copper losses, also called load losses, are proportional to the load being supplied by the transformer. These losses can be calculated for a given load if the resistances of both windings are known. As in generators and motors, the core loss is due to eddy-current induction loss and hysteresis (molecular friction) loss, caused by the changing polarity of the applied AC.

If the cores are laminated from low-loss silicon steel, both eddy-current and hysteresis losses will be reduced. Nevertheless, well-designed transformers in all frequency and power ranges typically have efficiencies of 90 percent or more.



A three-phase overhead distribution transformer is shown in the figure below. Where pole mounted overhead distribution is used to supply three-phase power, three-phase transformers occupy less space than a bank of transformers, and they weigh less.

Moreover, the cost of installation and maintenance is lower for a three-phase overhead transformer than for a bank of three single-phase units.

Three-phase overhead transformers are made with ratings from 30 to 300 kVA. Primary voltages range from 4.16 to 34.5 kV, and secondary voltages range from 120 to 480 V.

The basic impulse level (BIL) ratings are 45 to 150 kV. They are available with wye, delta, or T–T connections. These transformers have four output connections, X0, X1, X2, and X3, and their cases are filled with electrical-grade mineral oil.

These transformers are manufactured in ratings from 45 to 7500 kVA with high-voltage ratings from 2.4 to 46 kV. The standard connections are delta–wye, grounded wye–wye, delta–delta, wye–wye, and wye delta.

The transformers are housed in steel cabinets with front-opening, three-point latching steel doors. As in the overhead transformers, the cases of pad-mounted transformers are filled with electrical-grade mineral oil.



For increasing voltage at the end of lines or to step up voltage where line extensions are being added to existing lines, such as from 6900 VAC to 7200 VAC. Cost per kva output is less than a two-winding transformer; losses are low, regulation is good, and exciting current is low. Voltage transformation greater than 3 to 1 is not recommended.

When the ratio of transformation from the primary to secondary voltage is small, the most economical way of stepping down the voltage is by using autotransformers as shown. For the application, it is necessary that the neutral of the auto transformer bank be connected to the system neutral. Brand circuits shall not be supplied by autotransformers.

Susceptive to burnouts if the system impedance is not great enough to limit the short-circuit current to 20 to 25 times the transformer-rated current. The primary neutral should be tied firmly to the system neutral; otherwise, excessive voltages may develop on the secondary side.

A considerable saving in cost may often be experienced by using autotransformers instead of two-winding transformers. When it is desired to affect a small change in voltage, or where both high and low voltages are low, there is usually no reason why an autotransformer cannot be used as successfully as a two-winding transformer.

Autotransformers should not, except under special conditions, be used where the difference between the high-voltage and low-voltage ratings is great. This is because the occurrence of grounds at certain points will subject the insulation on the low-voltage circuit to the same stress as the high-voltage circuit.

Autotransformers are rated on the basis of output KVA rather than the transformer KVA. Efficiencies, regulation and other electrical characteristics are also based on output rating.

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