Introduction
In a typical UK wind farm a series of radial 33kV collector circuits run from the main switchboard and link together individual wind turbine generator (WTG) transformers. At the design stage it is necessary to determine the maximum number of WTG transformers that can be energised simultaneously from the 33kV system.

One of the factors to be considered is the voltage dip experienced at the point of common coupling (PCC) or interface between the electrical system of the wind farm and the utility company. The UK standard applied is the Electricity Council’s Engineering Recommendation P28, which allows a 3% voltage dip. This article describes wind farm transformer inrush analysis studies the Glasgow based power systems consultants Mott MacDonald have undertaken using PSCAD to demonstrate compliance with P28.

Transformer inrush
When a transformer is energised, it may draw a high magnitude transient current from the supply causing a temporary voltage dip. This current, characterised as being almost entirely unidirectional, rises abruptly to its maximum value in the first half-cycle and then decays until the normal steady-state magnetizing conditions are reached. The magnitude and duration of the inrush current depends upon the following all of which can be represented using a PSCAD model:

the point on the voltage wave at the instant the transformer is energised (i.e. switching angle);
the impedance of the supply circuit;
the value and sign of the residual flux linkage in the core;
the non-linear magnetic saturation characteristic of the core.

2.1 All equipment and material shall be designed manufactured and tested in accordance with the latest applicable Indian Standard. IEC standard and CBIP manuals except where modified and / or supplemented this specification.

2.2. Equipment and material confirming to any other standard, which ensures equal or better quality, may be accepted. In such case copies of English version of the standard adopted shall be submitted.

2.3. The Transformer offered shall in general comply with the latest issues including amendments of the following Indian standards but not restricted to it.

3. System Description:
The distribution transformers shall be installed at outdoor/indoor location along 11 kV distribution networks, which consists of both underground and overhead network. The HV winding would be connected to SFU/OCB/VCB of the ring main unit through 11 kV (E) XLPE/PILC cable. LV winding would be connected to switch type Fuse section pillar through 1.1 kV 1c, 400 sq mm XLPE cable.

4.2. The transformers shall be capable of continuous operation of rated output under the operating conditions of voltage and frequency variations as per statutory limits governed by relevant Indian Standard and Electricity Act-2003 and its amendments in force.

4.3. The transformer shall conform to best engineering practice.

4.4. The design material construction shall be such that to secure reliability, economy, safe and convenient operation and shall include all specified or unspecified incidental items necessary for similar equipment for convenient working in every respect.

4.5. The transformers shall be capable of withstanding the short circuit stresses due to terminal fault between phase to phase and phase to ground on one winding with full voltage maintained on the other windings for a minimum period of three seconds.

4.6. The transformers shall be free from annoying hum or vibration. The design shall be such as not to cause any undesirable interference with radio or communication circuits.

Tap-Changer Designs for Moderate kVA and Current. In the smaller ratings, where both the voltage and the current are moderate, the energy to be ruptured in switching from tap to tap becomes relatively so small that light and simple equipments are feasible.

A variety of mechanical designs, together with special circuits, has been evolved with the purpose of providing simpler, smaller, and inherently less expensive equipments. The following may be noted:

1. Designing the tap changer so that it is capable of rupturing the current directly on the same switches which select the taps

2. Designing the circuit so that the tapped winding is reversed in going from maximum to minimum range, thereby securing a substantial reduction in the rating of core and coils for a given output

3. Using higher switching speed, by means of which the life of the arcing contacts is increased.

Tap Changers Designed to Interrupt Current. The contactors C (Fig. 10-27) operate to open the switching circuits so that there is no interrupting duty on the selector contacts which connect to the transformer taps.

When the rated current is moderate, it becomes possible to rupture the current directly on the tap-selector switches and thus obtain a major economy in the cost of the mechanical equipment.

The "fusing factor" is used to determine the K, or T fuse link rating that will strike a suitable balance between operation on secondary fault currents and operation on expected overload currents, such as motor starting currents.

It is obtained by using a rule of thumb such as one of the following: (The current obtained by the selected rule of thumb becomes the "fusing factor.")

1. 1.5 times the rated full-load current of the transformer (Generally used on transformers 25 kva and larger where motor starting currents are not the controlling factor)

2. 2.0 times the rated full-load transformer current

3. 2.4 times the rated full-load transformer current (This rule is frequently expressed as, “1 ampere per kva rating of transformers at 2400 volts, ½ ampere per kva at 4800 volts, and 1/3 ampere per kva at 6900 to 7600 volts.”)

4. 3.0 (or above) times the rated full-load transformer current.

Example:
If the selected rule of thumb is 2.4 times rated full-load current, the system voltage is 4800 volts and the transformer is rated 50 kva, what fuse link should be used?

Core earthing
Before concluding the description of core construction, mention should be made of the subject of core earthing. Any conducting metal parts of a transformer, unless solidly bonded to earth, will acquire a potential in operation which depends on their location relative to the electric field within which they lie.

In theory, the designer could insulate them from earthed metal but, in practice, it is easier and more convenient to bond them to earth. However, in adopting this alternative, there are two important requirements:

ž The bonding must ensure good electrical contact and remain secure throughout the transformer life.

Metalwork which becomes inadequately bonded, possibly due to shrinkage or vibration, creates arcing which will cause breakdown of insulation and oil and will produce gases which may lead to Buchholz relay operation, where fitted, or cause confusion of routine gas-in-oil monitoring results y masking other more serious internal faults, and can thus be very troublesome in service.

The core and its framework represent the largest bulk of metalwork requiring to be bonded to earth. On large, important transformers, connections to core and frames can be individually brought outside the tank via 3.3 kV bushings and then connected to earth externally.

This enables the earth connection to be readily accessed at the time of initial installation on site and during subsequent maintenance without lowering the oil level for removal of inspection covers so that core insulation resistance checks can be carried out.

In order to comply with the above requirement to avoid circulating currents, the core and frames will need to be effectively insulated from the tank and from each other, nevertheless it is necessary for the core to be very positively located within the tank particularly so as to avoid movement and possible damage during transport.

It is usual to incorporate location brackets within the base of the tank in order to meet this requirement. Because of the large weight of the core and windings these locating devices and the insulation between them and the core and frames will need to be physically very substantial, although the relevant test voltage may be modest.

Three-Phase Pad-Mounted Transformers
Three-phase pad-mounted transformers are typically applied to serve commercial and industrial threephase loads from underground distribution systems. Traditionally, there have been two national standards that detailed requirements for pad-mounted transformers — one for live front (ANSI C57.12.22) and one for dead front (IEEE C57.12.26). The two standards have now been combined into one for all pad mounts, designated IEEE C57.12.34.

Live Front
Live-front transformers are specified as radial units and thus do not come with any fuse protection. See Figure 2.2.29.

The primary compartment is on the left, and the secondary compartment is on the right, with a rigid barrier separating them. The secondary door must be opened before the primary door can be opened.

Stress-cone-terminated primary cables rise vertically and connect to the terminals on the end of the high-voltage bushings. Secondary cables rise vertically and are terminated on spades connected to the secondary bushings.

Units with a secondary of 208Y/120 V are available up to 1000 kVA. Units with a secondary of 480Y/277 V are available up to 2500 kVA. Although not detailed in a national standard, there are many similar types available.

A loop-style live front (Figure 2.2.30) can be constructed by adding fuses mounted below the primary bushings. Two primary cables are then both connected to the bottom of the fuse. The loop is then made at the terminal of the high-voltage bushing, external to the transformer but within its primary compartment.

Dead Front
Both radial- and loop-feed dead-front pad-mounted transformers are detailed in the standard. Radialstyle units have three primary bushings arranged horizontally, as seen in Figure 2.2.31. Loop-style units have six primary bushings arranged in a V pattern, as seen in Figure 2.2.32 and Figure 2.2.33.

In both, the primary compartment is on the left, and the secondary compartment is on the right, with a rigid barrier between them. The secondary door must be opened before the primary door can be opened.

The primary cables are terminated with separable insulated high-voltage connectors, commonly referred to as 200-A elbows, specified in IEEE Standard 386. These plug onto the primary bushings, which can be either bushing wells with an insert, or they can be integral bushings.

Bushing wells with inserts are preferred, as they allow both the insert and elbow to be easily replaced. Units with a secondary of 208Y/ 120 V are available up to 1000 kVA. Units with a secondary of 480Y/277 V are available up to 2500 kVA.

The major improvement in core materials was the introduction of silicon steel in 1932. Over the years, the performance of electrical steels has been improved by grain orientation (1933) and continued improvement in the steel chemistry and insulating properties of surface coatings.

The thinner and more effective the insulating coatings are, the more efficient a particular core material will be. The thinner the laminations of electrical steel, the lower the losses in the core due to circulating currents. Mass production of distribution transformers has made it feasible to replace stacked cores with wound cores.

C-cores were first used in distribution transformers around 1940. A C-core is made from a continuous strip of steel, wrapped and formed into a rectangular shape, then annealed and bonded together.

The core is then sawn in half to form two C-shaped sections that are machine-faced and reassembled around the coil.

In the mid 1950s, various manufacturers developed wound cores that were die-formed into a rectangular shape and then annealed to relieve their mechanical stresses. The cores of most distribution transformers made today are made with wound cores.

Typically, the individual layers are cut, with each turn slightly lapping over itself. This allows the core to be disassembled and put back together around the coil structures while allowing a minimum of energy loss in the completed core.

Electrical steel manufacturers now produce stock for wound cores that is from 0.35 to 0.18 mm thick in various grades.

In the early 1980s, rapid increases in the cost of energy prompted the introduction of amorphous core steel. Amorphous metal is cooled down from the liquid state so rapidly that there is no time to organize into a crystalline structure.

Thus it forms the metal equivalent of glass and is often referred to as metal glass or “met-glass.” Amorphous core steel is usually 0.025 mm thick and offers another choice in the marketplace for transformer users that have very high energy costs.

This is the general standard that is widely used by
countries in the Western Hemisphere and contains definitions, service
conditions, ratings, general electrical and mechanical requirements, and
detailed descriptions of routine and design test procedures for
outdoor-power-apparatus bushings.

This standard lists the electrical-insulation and
test-voltage requirements for power-apparatus bushings rated from 15 through
800-kV maximum system voltages.

It also lists dimensions for standard-dimensioned bushings,
cantilever-test requirements for bushings rated through 345-kV system voltage,
and partial-discharge limits as well as limits for power factor and capacitance
change from before to after the standard electrical tests.

C57.19.00 for bushings for direct-current equipment,
including oil-filled converter transformers and smoothing reactors. It also
covers air-to-air dc bushings.

4. IEEE Std. C57.19.100, Guide for Application of Power
Apparatus Bushings [8]. This guide recommends practices to be used (1) for
thermal loading above nameplate rating for bushings applied on power
transformers and circuit breakers and (2) for bushings connected to isolated-phase
bus.

It also recommends practices for allowable cantilever
loading caused by the pull of the line connected to the bushing, applications
for contaminated environments and high altitudes, and maintenance practices.

5. IEC Publication 137 [9], Bushings for Alternating
Voltages above 1000 V. This standard is the IEC equivalent to the first
standard listed above and is used widely in European and Asian countries.

Another classification relates to the insulating material
used inside the bushing. In general, these materials can be used in either the
solid- or capacitance-graded construction, and in several types, more than one
of these insulating materials can be used in conjunction.

The following text gives a brief description of these types:

Air-Insulated Bushings

Air-insulated bushings generally are used only with
air-insulated apparatus and are of the solid construction that employs air at
atmospheric pressure between the conductor and the insulators.

Oil-Insulated or Oil-Filled Bushings

Oil-insulated or oil-filled bushings have electrical-grade
mineral oil between the conductor and the insulators in solid-type bushings.
This oil can be contained within the bushing, or it can be shared with the
apparatus in which the bushing is used.

Capacitance-graded bushings also use mineral oil, usually
contained within the bushing, between the insulating material and the
insulators for the purposes of impregnating the kraft paper and transferring
heat from the conducting lead.

Oil-Impregnated Paper-Insulated Bushings

Oil-impregnated paper-insulated bushings use the dielectric
synergy of mineral oil and electric grades of kraft paper to produce a
composite material with superior dielectric-withstand characteristics.

This material has been used extensively as the insulating
material in capacitance-graded cores for approximately the last 50 years.

Resin-Bonded or -Impregnated Paper-Insulated Bushings

Resin-bonded paper-insulated bushings use a resin-coated
kraft paper to fabricate the capacitance graded core, whereas resin-impregnated
paper-insulated bushings use papers impregnated with resin, which are then used
to fabricate the capacitance-graded core.

Cast-insulation bushings are constructed of a solid-cast
material with or without an inorganic filler. These bushings can be either of
the solid or capacitance-graded types, although the former type is more
representative of present technology.

Gas-Insulated Bushings

Gas-insulated bushings use pressurized gas, such as SF6 gas,
to insulate between the central conductor and the flange. It uses the same
pressurized gas as the circuit breaker, has no capacitance grading, and uses
the dimensions and placement of the ground shield to control the electric
fields.

Other designs use a lower
insulator to enclose the bushing, which permits the gas pressure to be
different than within the circuit breaker. Still other designs use
capacitance-graded cores made of plastic-film material that is compatible with
SF6 gas.

PARAMETERS THAT AFFECT THE DEGRADATION OF TRANSFORMER OIL
BASIC INFORMATION

What Are The Parameters That Affect The Transformer Oil?

Heat

Just as temperature influences the rate of degradation of
the solid insulation, so does it affect the rate of oil degradation. Although the rates of both processes are different, both are
influenced by temperature in the same way. As the temperature rises, the rates of degradation reactions
increase. For every 10° (Celsius) rise in temperature, reaction rates double!

Oxygen

Hydrocarbon-based insulating oil, like all products of
nature, is subject to the ongoing, relentless process of oxidation. Oxidation
is often referred to as aging.

The abundance of oxygen in the atmosphere provides the
reactant for this most common degradation reaction. The ultimate products of
oxidation of hydrocarbon materials are carbon dioxide and water.

However, the process of oxidation can involve the production
of other compounds that are formed by intermediate reactions, such as alcohols,
aldehydes, ketones, peroxides, and acids.

Partial Discharge and Thermal Faulting

Of all the oil degradation processes, hydrogen gas requires
the lowest amount of energy to be produced. Hydrogen gas results from the
breaking of carbon–hydrogen bonds in the oil molecules.

All of the three fault processes (partial discharge, thermal
faulting, and arcing) will produce hydrogen, but it is only with partial
discharge or corona that hydrogen will be the only gas produced in significant
quantity.

In the presence of thermal faults, along with hydrogen will
be the production of methane together with ethane and ethylene. The ratio of
ethylene to ethane increases as the temperature of the fault increases.

Arcing

With arcing, acetylene is produced along with the other
fault gases. Acetylene is characteristic of arcing.

Because arcing can generally lead to failure over a much
shorter time interval than faults of other types, even trace levels of
acetylene (a few parts per million) must be taken seriously as a cause for
concern.

Acid

High levels of acid (generally acid levels greater than 0.6
mg KOH/g of oil) cause sludge formation in the oil. Sludge is a solid product
of complex chemical composition that can deposit throughout the transformer.
The deposition of sludge can seriously and adversely affect heat dissipation
and ultimately

In an HVDC system, reactors are used for various functions,
as shown, in principle, in Figure 2.9.25.

The HVDC-smoothing reactors are connected in series with an
HVDC transmission line or inserted in the intermediate dc circuit of a
back-to-back link to reduce the harmonics on the dc side, to reduce the current
rise caused by faults in the dc system, and to improve the dynamic stability of
the HVDC transmission system.

Filter reactors are installed for harmonic filtering on the
ac and on the dc side of the converters. The ac filters serve two purposes
simultaneously: the supply of reactive power and the reduction of harmonic
currents.

The ac filter reactors are utilized in three types of filter
configurations employing combinations of resistors and capacitors, namely
single-tuned filters, double-tuned filters, and high-pass filters.

A single tuned filter is normally designed to filter the
low-order harmonics on the ac side of the converter.

A double-tuned filter is designed to filter multiple
discrete frequencies using a single combined filter circuit.

A high-pass filter is essentially a single-tuned damped
filter. Damping flattens and extends the filter response to more effectively
cover high-order harmonics. The dc filter reactors are installed in shunt with
the dc line, on the line side of the smoothing reactors.

The function of these dc filter banks is to further reduce
the harmonic currents on the dc line (see Figure 2.9.24 and Figure 2.9.25).

PLC (power-line carrier)
and RI (radio interference) filter reactors are employed on the ac or dc side
of the HVDC converter to reduce high-frequency noise propagation in the lines.

Capacitor switching can cause significant transients at both
the switched capacitor and remote locations.

The most common transients are:

• Overvoltage on the switched capacitor during energization

• Voltage magnification at lower-voltage capacitors

• Transformer phase-to-phase overvoltages at line
termination

• Inrush current from another capacitor during back-to-back
switching

• Current outrush from a capacitor into a nearby fault

• Dynamic overvoltage when switching a capacitor and
transformer simultaneously

Capacitor inrush/outrush reactors (Figure 2.9.15) are used
to reduce the severity of some of the transients listed above in order to
minimize dielectric stresses on breakers, capacitors, transformers, surge
arresters, and associated station electrical equipment.

High-frequency-transient interference in nearby control and
communication equipment is also reduced. Reactors are effective in reducing all
transients associated with capacitor switching, since they limit the magnitude
of the transient current (Equation 2.9.5), in kA, and significantly reduce the
transient frequency (Equation 2.9.6), in Hz.

where

Ceq = equivalent capacitance of the circuit, F

Leq = equivalent inductance of the circuit, H

VLL = system line-to-line voltage, kV

Therefore, reflecting the information presented in the
preceding discussion, IEEE Std. 1036-1992, Guide for Application of Shunt Power
Capacitors, calls for the installation of reactors in series with each
capacitor bank, especially when switching back-to-back capacitor banks.

A well-known solution for
electrical “noise” in industrial plants has been the constant-voltage
transformer, or CVT.

The typical components of a CVT are shown in Figure 2.8.2. The magnetic shunt
on the central core has the following effects on the core’s reluctance. It
reduces the reluctance of the core.

This can be thought of as introducing more resistance in
parallel to an existing resistance. The magnetic shunt in the CVT design allows
the portion of the core below the magnetic shunt to become saturated while the
upper portion of the core remains unsaturated.

This condition occurs because of the presence of the air-gap
between the magnetic shunt and the core limbs. Air has a much higher reluctance
than the iron core.

Therefore, most of the flux passes through the lower portion
of the core, as shown by the thick lines in Figure 2.8.2.

In terms of an electrical analogy, this configuration can be
thought of as two resistances of unequal values in parallel.

The smaller resistance carries the larger current, and the
larger resistance carries the smaller current.

The CVT is designed such that:

• The lower portion of the central limb is saturated under
normal operating conditions, and the secondary and the resonating windings
operate in the nonlinear portion of the flux-current curve.

• Because of saturation in the central limb, the voltage in
the secondary winding is not linearly related to the voltage in the primary
winding.

There is consonance between the resonating winding on the
saturated core and the capacitor. This arrangement acts as a tank circuit,
drawing power from the primary. This results in sustained, regulated

Polarity tests on single-phase transformers shall be made in
accordance with one of the following methods:

a) Inductive kick

b) Alternating voltage

c) Comparison

d) Ratio bridge

Polarity by inductive kick

The polarity of transformers with leads arranged as shown in
may be determined when making resistance measurements as follows:

a) With direct current passing through the high-voltage
winding, connect a high-voltage direct-current voltmeter across the
high-voltage winding terminals to obtain a small deflection of the pointer.

b) Transfer the two voltmeter leads directly across the
transformer to the adjacent low-voltage leads, respectively.

NOTE—For example, in Figure 5, the voltmeter lead connected
to H1 will be transferred to X2 as the adjacent lead,and that connected to H2to
X1.

c) Break direct-current excitation, thereby inducing a
voltage in the low-voltage winding (inductive kick), which will cause
deflection in the voltmeter. The deflection is interpreted in d) and e) below.

d) When the pointer swings in the opposite direction
(negative), the polarity is subtractive.

e) When the pointer swings in the same direction as before
(positive), the polarity is additive.

Polarity by alternating-voltage test

For transformers having a ratio of transformation of 30 to 1
or less, the H1 lead shall be connected to the adjacent low-voltage lead (X1 in
Figure 6).

Any convenient value of alternating voltage shall be applied
to the full high-voltage winding and readings taken of the applied voltage and
the voltage between the right-hand adjacent high-voltage and low-voltage leads.

When the latter reading is greater than the former, the
polarity is additive. When the latter voltage reading is less than the former
(indicating the approximate difference in voltage between the high-voltage and
low-voltage windings), the polarity is subtractive.

Polarity by comparison

When a transformer of known polarity and of the same ratio
as the unit under test is available, the polarity can be checked by comparison,
as follows, similar to the comparison method used for the ratio test.

a) Connect the high-voltage windings of both transformers in
parallel by connecting similarly marked

c) With these connections, apply a reduced value of voltage
to the high-voltage windings and measure the voltage between the two free
leads.

A zero or negligible reading of the voltmeter will indicate
that the relative polarities of both transformers are identical.

An alternative method of checking the polarity is to
substitute a low-rated fuse or suitable lamps for the voltmeter. This procedure
is recommended as a precautionary measure before connecting the voltmeter.

Polarity by ratio bridge

The ratio bridge can also
be used to test polarity. A bridge using the basic circuit below may be used to
measure ratio.

Dry-type transformers may be loaded above rated kilovolt-amperes
under conditions other than those specified in the preceding clauses, with a
sacrifice of life expectancy dependent on the load capability of the
transformer and on the actual operating conditions.

The overload capability of dry-type transformers varies
widely and is affected by the following characteristics:

a) Hottest-spot winding conductor rise over ambient;

b) Ratio of load losses to no-load losses;

c) Time constant;

d) Ambient temperature.

Operating conditions for dry-type transformers are so
variable that no single set of practical loading data can be presented for all
possible combinations of conditions and loading.

However, methods are outlined in this clause whereby the
user can estimate allowable loads for the user’s own conditions by taking into
account the probable number and nature of such loads during the life of the
transformer, and the approximate percentage of life expectancy that the user is
willing to sacrifice.

In general, permissible temperatures and loads calculated by
the means outlined here will be higher than those loading for life expectancy
which are necessarily conservative in order to cover the wide variation in
sizes and makes of dry-type transformers.

The necessary curves, tables, equations, and definitions used
as a basis for the methods given here are presented in Clause 6 of IEEE
STD-C57.56. Information given is considered to be the best that can be produced
from the present knowledge of the subject.

In spite of its approximate nature, it is believed that it
will be of value as a guide to the user.

Rogowski coil, is an air-core current transformer that is especially well suited to measuring ripple currents in the presence of a DC component or measuring pulsed currents. The raw output is proportional to the derivative of the current, and the current can be recovered by an integrator or a low-pass filter.

The output voltage is given by:

Rogowski Coil

where
n
is the number of turns,
A
is the cross sectional area of the toroid,
and
s
is the centerline circumference.

The coil is wound on an air-core form of suitable size for the current conductor. The winding should be applied in evenly spaced turns in one direction only—not back and forth—so that capacitive effects are minimized.

The far end of the winding should be brought back around the circumference of the coil to eliminate the turn formed by the winding itself. The winding must generally be shielded, since the output voltage is relatively low.

The shield should be applied so that it does not form a shorted turn through the opening, and the coil should be equipped with an integral shielded output lead with the ground side connected to the coil shield.

Output from the Rogowski coil can either be integrated with a passive network as an R/C low-pass filter or with an operational amplifier. The advantage of the R/C network is that no power is required for operation.

The disadvantages are that it cannot be gated and that the output voltage becomes very low if low-frequency response is required. Although a toroidal form is shown in the sketch, Rogowski coils are commercially available that are wound in the form of a very long, flexible solenoid that can be wrapped around a conductor and then secured mechanically.

Rogowski coils are largely unaffected by stray fields that have a constant amplitude across the coil. A field gradient across the coil, however, will introduce a spurious output if the field is time varying. It is good practice to make the coil as small as possible within the electrical and physical constraints of the equipment.

The Rogowski coil can be calibrated from a 50/60-Hz current assuming, of course, that the bandpass of the filter or integrator extends down to those frequencies.

A dry-type transformer is one in which the insulating medium surrounding the winding assembly is a gas or dry compound. Basically, any transformer can be constructed as “dry” as long as the ratings, most especially the voltage and kVA, can be economically accommodated without the use of insulating oil or other liquid media.

Many perceptions of dry-type transformers are associated with the class of design by virtue of the range of ratings or end-use applications commonly associated with that form of construction Of course, the fundamental principles are no different from those encountered in liquid-immersed designs.

Dry-type transformers compared with oil-immersed are lighter and nonflammable. Increased experience with thermal behavior of materials, continued development of materials and transformer design have improved transformer thermal capability.

Upper limits of voltage and kVA have increased. Winding insulation materials have advanced from protection against moisture to protection under more adverse conditions (e.g., abrasive dust and corrosive environments).

Cooling Classes for Dry-Type Transformers
American and European cooling-class designations are indicated in Table 2.5.1. Cooling classes for drytype transformers are as follows (IEEE, 100, 1996; ANSI/IEEE, C57.94-1982 (R-1987)):

Ventilated — Ambient air may circulate, cooling the transformer core and windings
Nonventilated — No intentional circulation of external air through the transformer
Sealed — Self-cooled transformer with hermetically sealed tank
Self-cooled — Cooled by natural circulation of air
Force-air cooled — Cooled by forced circulation of air
Self-cooled/forced-air cooled — A rating with cooling by natural circulation of air and a rating with cooling by forced circulation of air.

Winding Insulation System
General practice is to seal or coat dry-type transformer windings with resin or varnish to provide protection against adverse environmental conditions that can cause degradation of transformer windings. Insulating media for primary and secondary windings are categorized as follows:

Cast coil — The winding is reinforced or placed in a mold and cast in a resin under vacuum pressure. Lower sound levels are realized as the winding is encased in solid insulation. Filling the winding with resin under vacuum pressure eliminates voids that can cause corona. With a solid insulation system, the winding has superior mechanical and short-circuit strength and is impervious to moisture and contaminants.

Vacuum-pressure encapsulated — The winding is embedded in a resin under vacuum pressure. Encapsulating the winding with resin under vacuum pressure eliminates voids that can cause corona. The winding has excellent mechanical and short-circuit strength and provides protection against moisture and contaminants.

Vacuum-pressure impregnated — The winding is permeated in a varnish under vacuum pressure. An impregnated winding provides protection against moisture and contaminants.

Coated — The winding is dipped in a varnish or resin. A coated winding provides some protection against moisture and contaminants for application in moderate environments.

As the winding is not in contact with the external air, it is suitable for applications, e.g., exposure to fumes, vapors, dust, steam, salt spray, moisture, dripping water, rain, and snow.

Ventilated dry-type transformers are recommended only for dry environments unless designed with additional environmental protection. External air carrying contaminants or excessive moisture could degrade winding insulation.

Dust and dirt accumulation can reduce air circulation through the windings (ANSI/IEEE, 57.94-1982 [R 1987]). Table 2.5.2 indicates transformer applications based upon the process employed to protect the winding insulation system from environmental conditions.

Enclosures
All energized parts should be enclosed to prevent contact. Ventilated openings should be covered with baffles, grills, or barriers to prevent entry of water, rain, snow, etc. The enclosure should be tamper resistant.

A means for effective grounding should be provided (ANSI/IEEE, C2-2002). The enclosure should provide protection suitable for the application, e.g., a weather- and corrosion-resistant enclosure for outdoor installations.

If not designed to be moisture resistant, ventilated and nonventilated dry-type transformers operating in a high-moisture or high-humidity environments when deenergized should be kept dry to prevent moisture ingress.

Strip heaters can be installed to switch on manually or automatically when the transformer is deenergized for maintaining temperature after shutdown to a few degrees above ambient temperature.
Operating Conditions
The specifier should inform the manufacturer of any unusual conditions to which the transformer will
be subjected. Dry-type transformers are designed for application under the usual operating conditions
indicated in Table 2.5.3.
Gas may condense in a gas-sealed transformer left deenergized for a significant period of time at low
ambient temperature. Supplemental heating may be required to vaporize the gas before energizing the
transformer (ANSI/IEEE, C57.94-1982 [R1987]).

Limits of Temperature Rise
Winding temperature-rise limits are chosen so that the transformer will experience normal life expectancy for the given winding insulation system under usual operating conditions. Operation at rated load and loading above nameplate will result in normal life expectancy.

A lower average winding temperature rise, 80°C rise for 180°C temperature class and 80°C or 115°C rise for 220°C temperature class, may be designed providing increased life expectancy and additional capacity for loading above nameplate rating.

Accessories
The winding-temperature indicator can be furnished with contacts to provide indication and/or alarm of winding temperature approaching or in excess of maximum operating limits. For sealed dry-type transformers, a gas-pressure switch can be furnished with contacts to provide indication and/or alarm of gas-pressure deviation from recommended range of operating pressure.

Surge Protection
For transformers with exposure to lightning or other voltage surges, protective surge arresters should be coordinated with transformer basic lightning impulse insulation level, BIL.

The lead length connecting from transformer bushing to arrester—and from arrester ground to neutral—should be minimum length to eliminate inductive voltage drop in the ground lead and ground current (ANSI-IEEE, C62.2-1987 [R1994]).

Lower BIL levels can be applied where surge arresters provide appropriate protection. At 25 kV and above, higher BIL levels may be required due to exposure to overvoltage or for a higher protective margin (ANSI/IEEE, C57.12.01-1989 [R1998]).

In a real transformer, some power is dissipated in the form of heat. A portion of these power losses occur in the conductor windings due to electrical resistance and are referred to as copper losses.

However, so-called iron losses from the transformer core are also important. The latter result from the rapid change of direction of the magnetic field, which means that the microscopic iron particles must continually realign themselves—technically, their magnetic moment—in the direction of the field (or flux).

Just as with the flow of charge, this realignment encounters friction on the microscopic level and therefore dissipates energy, which becomes tangible as heating of the material.

Taking account of both iron and copper losses, the efficiency (or ratio of electrical power out to electrical power in) of real transformers can be in the high 90% range. Still, even a small percentage of losses in a large transformer corresponds to a significant amount of heat that must be dealt with.

In the case of small transformers inside typical household adaptors for low-voltage d.c. appliances, we know that they are warm to the touch.

Yet they transfer such small quantities of power that the heat is easily dissipated into the ambient air (bothering only conservatio nminded analysts, who note the energy waste that could be avoided by unplugging all these adaptors when not in use).

By contrast, suppose a 10-MVA transformer at a distribution substation operates at an efficiency of 99%: A 1% loss here corresponds to a staggering 100 kW.

In general, smaller transformers like those on distribution poles are passively cooled by simply radiating heat away to their surroundings, sometimes assisted by radiator vanes that maximize the available surface area for removing the heat.

Large transformers like those at substations or power plants require the heat to be removed from the core and windings by active cooling, generally through circulating oil that simultaneously functions as an electrical insulator.

The capacity limit of a transformer is dictated by the rate of heat dissipation. Thus, as is true for power lines, the ability to load a transformer depends in part on ambient conditions including temperature, wind, and rain.

For example, if a transformer appears to be reaching its thermal limit on a hot day, one way to salvage the situation is to hose down its exterior with cold water—a procedure that is not “by the book,” but has been reported to work in emergencies.

When transformers are operated near their capacity limit, the key variable to monitor is the internal or oil temperature. This task is complicated by the problem that the temperature may not be uniform throughout the inside of the transformer, and damage can be done by just a local hot spot. Under extreme heat, the oil can break down, sustain an electric arc, or even burn, and a transformer may explode.

A cooling and insulating fluid for transformers has to meet criteria similar to those for other high-voltage equipment, such as circuit breakers and capacitors: it must conduct heat but not electricity; it must not be chemically reactive; and it must not be easily ionized, which would allow arcs to form.

Mineral oil meets these criteria fairly well, since the long, nonpolar molecules do not readily break apart under an electric field.

Another class of compounds that performs very well and has been in widespread use for transformers and other equipment is polychlorinated biphenyls, commonly known as PCBs.

Because PCBs and the dioxins that contaminate them were found to be carcinogenic and ecologically toxic and persistent, they are no longer manufactured in the United States; the installation of new PCB-containing utility equipment has been banned since 1977.

However, much of the extant hardware predates this phase-out and is therefore subject to careful maintenance and disposal procedures (somewhat analogous to asbestos in buildings).

Introduced in the 1960s, sulfur hexafluoride (SF6) is another very effective arcextinguishing fluid for high-voltage equipment. SF6 has the advantage of being reasonably nontoxic as well as chemically inert, and it has a superior ability to withstand electric fields without ionizing.

While the size of transformers and capacitors is constrained by other factors, circuit breakers can be made much smaller with SF6 than traditional oil-filled breakers.

However, it turns out that SF6 absorbs thermal infrared radiation and thus acts as a greenhouse gas when it escapes into the atmosphere; it is included among regulated substances in the Kyoto Protocol on global climate change.

SF6 in the atmosphere also appears to form another compound by the name of trifluoromethyl sulfur pentafluoride (SF5CF3), an even more potent greenhouse gas whose atmospheric concentration is rapidly increasing. This surprising and unfortunate characteristic may motivate future restriction of SF6 use.

PUBLIC RESPONSE TO TRANSFORMER AUDIBLE SOUND BASIC INFORMATIONWhat Is The Public Response To Transformer Audible 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

The 120-Hz transformer sound at the observer’s position exceeds the ambient spectrum level by 19.7 dB. This is 3.7 dB greater than the 16-dB differential which would result in bare audibility; thus the transformer sound will be audible to the observer.

When transformer sound exceeds the limits of bare audibility, public response is not necessarily strongly negative. Some attempts have been made to categorize public response on a quantitative basis when the sound is clearly audible (Schultz and Ringlee 1960).

For a case where specific knowledge of transformer- and ambient-sound-level frequency composition is not available, some more general guidelines are useful. Typical average nighttime ambient-sound levels for certain types of communities have been established.

These are 30 dB for a “quiet suburban,” 35 dB for a “residential suburban,” and 40 dB for a “residential urban” community. All sound levels are based on the A scale of weighing.

Calculations for typical transformer frequency distributions have been made to determine the nighttime transformer noise which will be audible 50% of the time in these communities. The results are 24 dB for quiet suburban, 29 dB for residential suburban, and 34 dB for residential urban.

The NEMA standard sound level can be corrected for attenuation with distance to the nearest observer and checked against the above guides for audibility.

The broadband sound from fans, pumps, and coolers has the same character as ambient sound and tends to blend in with the ambient.

While the noise from cooling equipment may be audible to a
neighboring observer, it will seldom, if ever, cause a complaint.

POWER TRANSFORMER AUTOMATIC CONTROL FOR TAP CHANGERS BASIC INFORMATIONWhat Is Automatic Tap Changer Controls For Power Transformers?

Automatic Control for Tap Changers. It is usual practice to use some sort of voltage measuring device to control the operation of the motor which drives the tap changer.

Such devices may be mechanical, balancing the force of a solenoid actuated by the voltage against weights or springs, or they may be an electrical network, usually a bridge circuit which balances against the voltage of a Zener dioide.

With either type of device, a voltage higher than a desired upper limit will start the tapchanger driving motor to change to the next lower tap voltage; similarly, a voltage lower than the desired lower limit will cause a change to the next higher tap.

The circuit usually includes a time delay to prevent tap changes, which would occur unnecessarily during very short time variations in voltage. It also may include a line drop compensator to facilitate maintaining the voltage within a given band at a point (load center) some distance from the transformer.

The line-drop compensator introduces a signal into the voltage regulating relay circuitry. This represents the voltage drop due to line impedance between the transformer and the load center.

The voltage-regulating relay (or contact-making voltmeter) should be adjusted so that the voltage bandwidth, or spread between voltages at which the raising and lowering contacts close, will be not less than the percentage transformer tap plus an allowance for irregular voltage variations.

For example, a tap-changing transformer with 11/4% taps should have a minumum voltage bandwidth of approximately 11/4% 1/2% 13/4%.

In addition, the voltage-regulating relay may contain a component for use when load tap-changing transformers are operated in parallel. In this case, the tap changers must be controlled so that they are approximately on the same tap position.

The component, a paralleling reactor, is used with external circuitry to detect, and generate a signal to minimize, circulating current that results when the tap changers are not on like positions.

Three factors must be considered in the evaluation of the dielectric capability of an insulation structure—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.

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.

When transformers are in operation, many users carry out surveillance testing to monitor operation. The most simple tests are carried out on oil samples taken on a regular basis.

Measurement of oil properties, such as breakdown voltage, water content, acidity, dielectric loss angle, volume resistivity and particle content all give valuable information on the state of the transformer. DGA gives early warning of deterioration due to electrical or thermal causes, particularly sparking, arcing and service overheating.

Analysis of the oil by High-Performance Liquid Chromatography (HPLC) may detect the presence of furanes or furfuranes which will provide further information on moderate overheating of the insulation.

(b) On-line condition monitoring

Sensors can be built into the transformer so that parameters can be monitored on a continuous basis. The parameters which are typically monitored are winding temperature, tank temperature, water content, dissolved hydrogen, partial discharge activity, load current and voltage transients.

The data collection system may simply gather and analyse the information, or it may be arranged to operate alarms or actuate disconnections under specified conditions and limits which represent an emergency.

Whereas surveillance testing is carried out on some distribution transformers and almost all larger transformers, the high cost of on-line condition monitoring has limited the application to strategic transformers and those identified as problem units.

As the costs of simple monitoring equipment fall, the technique should become more applicable to substation transformers.

TAP CHANGERS OF POWER TRANSFORMERS BASIC INFORMATIONWhat Are Power Transformer Tap Changers? How Tap Changers Work?

When a transformer carries load current there is a variation in output voltage which is known as regulation. In order to compensate for this, additional turns are often made available so that the voltage ratio can be changed using a switch mechanism known as a tapchanger.

An off-circuit tapchanger can only be adjusted to switch additional turns in or out of circuit when the transformer is de-energized; it usually has between two and five tapping positions. An on-load tapchanger (OLTC) is designed to increase or decrease the voltage ratio when the load current is flowing, and the OLTC should switch the transformer load current from the tapping in operation to the neighbouring tapping without interruption.

The voltage between tapping positions (the step voltage) is normally between 0.8 per cent and 2.5 per cent of the rated voltage of the transformer. The OLTC mechanisms are based either on a slow-motion reactor principle or a high-speed resistor principle.

The former is commonly used in North America on the low-voltage winding, and the latter is normally used in Europe on the high-voltage winding.

The usual design of an OLTC in Europe employs a selector mechanism to make connection to the winding tapping contacts and a diverter mechanism to control current flows while the tapchanging takes place. The selector and diverter mechanisms may be combined or separate, depending upon the power rating.

In an OLTC which comprises a diverter switch and a tap selector, the tapchange occurs in two operations. First, the next tap is selected by the tap switch but does not carry load current, then the diverter switches the load current from the tap in operation to the selected tap. The two operations are shown in seven stages in Fig. 6.15.

The tap selector operates by gearing directly from a motor drive, and at the same time a spring accumulator is tensioned. This spring operates the diverter switch in a very short time (40 – 60 ms in modern designs), independently of the motion of the motor drive.

The gearing ensures that the diverter switch operation always occurs after the tap selection has been completed. During the diverter switch operation shown in Fig. 6.15(d), (e) and (f), transition resistors are inserted; these are loaded for 20–30 ms and since they have only a short-time loading the amount of material required is very low.

The basic arrangement of tapping windings is shown in Fig. 6.16. The linear arrangement in Fig. 6.16(a) is generally used on power transformers with moderate regulating ranges up to 20 per cent. The reversing changeover selector shown in Fig. 6.16(b) enables the voltage of the tapped winding to be added or subtracted from the main winding so that the tapping range may be doubled or the number of taps reduced.

The greatest copper losses occur at the position with the minimum number of effective turns. This reversing operation is achieved with a changeover selector which is part of the tap selector of the OLTC. The two-part coarse–fine arrangement shown in Fig. 6.16(c) may also be used.

In this case the reversing changeover selector for the fine winding can be connected to the ‘plus’ or ‘minus’ tapping of the coarse winding, and the copper losses are lowest at the position of the lowest number of effective turns. The coarse changeover switch is part of the OLTC.

Regulation is mostly carried out at the neutral point in star windings, resulting in a simple, low-cost, compact OLTC and tapping windings with low insulation strength to earth. Regulation of delta windings requires a three-phase OLTC, in which the three phases are insulated for the highest system voltage which appears between them; alternatively three single-phase OLTCs may be used.