Section 1

Power Transformer

Transformer is a vital link in a power system which has made possible the power

generated at low voltages (6600 to 22000 volts) to be stepped up to extra high

voltages for transmission over long distances and then transformed to low voltages for

utilization at proper load centers. The power transformer adjusts the voltage to a

suitable level of power transmission from generation to the end-user. This means

stepping up the voltage at generation for long transmission with low losses and down

to domestic voltage manageable in the power outlet for home and industry. Power

transformers are used to convert from one voltage to another, at significant power

levels.

Types:

(i) Closed - Core type

The close-core transformer ha s heavily insulated coils wound around a

square metal doughnut. The magnetic field is provided with a continuous

path so that a relatively small amount of magnetic energy is lost. The

transformer core is laminated (made up of thin strips of steel pressed

together). Each strip or layer is insulated by paint or enamel. A laminated

core offers a much higher resistance to t he flow of eddy currents t han a core

of solid construction. The doughnut type of closed-core transformer is

efficient and is commonly utilized in x-ray generating equipment. Usually,

t he high voltage transformer is submerged in a special type of oil to ensure

maximum insulation and cooling.

Core type transformer

(ii) Shell type

The shell-type transformer is considered t he most efficient. Such

transformers are used in transmitting commercial power. The core of t he

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shell-type transformer is made of laminated silicon steel sheets placed on top

of one another. The coils are wound around t he central section of t he core.

Since t he primary and secondary coils are wound close together around t he

core, t he windings must be highly insulated. A special insulating material is

coated on t he wires of both coils. For t he high voltage used in x-ray, t he

entire transformer is immersed in a container filled with a special insulating

oil or gas. The insulating oil also helps to cool t he transformer during

operation.

Shell type transformer

Power and water Corporation use Core type transformers.

Another classification is,

(i) Step Up or Step Down

A step up transformer works due to the secondary voltage being greater than the

primary voltage.  When the secondary voltage is greater, it steps up the voltage input

at the power source. If the primary winding has fewer turns than the secondary

winding, the device becomes a step up transformer that will increase electrical

voltage.

Step up transformer

A step-down transformer is one whose secondary voltage is less than its primary

voltage.  The step down transformer is designed to reduce the voltage from the

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primary winding to the secondary winding.  This kind of transformer "steps down" the

voltage applied to it.

Step down pole mounted transformer

(ii) Oil-Filled or Dry

Oil filled transformers are transformers filled with a highly refined mineral oil that is

used to insulate internal live parts of the transformer.  The oil prevents corona and

manages temperature control inside the transformer for the prevention of equipment

and machinery overheating during the operation of large job applications.  Because of

the oil inside the transformer being of non-combustible properties, these transformers

are very safe and can operate machinery for long periods of time.

Oil Filled transformer

Dry-type transformers have high sound levels, but oil filled transformers are much

quieter due to the immersion of its mechanisms in liquid.  This results in less vibration

as well.  Oil filled transformers require low maintenance and are quite

environmentally friendly. Dry type means it is cooled by normal air ventilation. The

dry type transformer does not require a liquid such as oil or silicone or any other

liquid to cool the electrical core and coils. Dry type transformers are voltage changing

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(Step-up or Step-down) or isolation device that is air cooled rather than liquid cooled.

The transformer case is ventilated to allow air to flow and cool the coils.

Dry type transformer

(iii) Pole Mounted or Stationary

Pole mounted transformers are mounted on an electrical service pole, usually at

the level of the overhead cables but occasionally at ground level. Pole-mounted

transformers are the common breadbox transformers used for converting

distribution voltage to the 120/240 volt power used by homes and low-volume

commercial installations. Pole mounted transformers are used in extensive rural

area. These transformers range from 16 kVA to 500kVA and transform 11,000 to

33,000 volts down to a low voltage of 400 volts. Pole mounted transformers are

reasonably small in size and mass so that it is easy to install them on single pole

structures, and larger units on two-pole structures, approximately 5 meters above

the ground. This makes the transformers inaccessible, reducing the risk of injury

to animals and people and minimizing vandalism.

Pole mounted transformer

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Most other transformers are ground-mounted. It is the broad category of transformers

that are neither pole mounted nor pad mounted, but that has exposed bushings and

therefore must be installed within some kind of enclosure to protect the public. Often,

the enclosures are simply gated chain-link fencing.

Ground-mounted transformer

Pad-mounted transformers are designed to be mounted on concrete pads. Pad mounted

transformers are usually three-phase (weight is less of a concern), and they are

generally intended for use in applications with the high and low voltage cables enter

the transformer from below such as in underground distribution applications. Pad

mounts are almost always contained within grounded metal housings that protect the

public from high voltage.

Pad-mounted transformer

(iv) Tapped or Center Tapped

A center tapped transformer is a transformer with a tap in the middle of the secondary

winding, usually used as a grounded neutral connection, intended to provide an option

for the secondary side to use the full available voltage output or just half of it

according to need. This is often done in North American residential wiring.

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A tap on a transformer is an additional connection somewhere in the middle of the

winding, used either in conjunction with or instead of the connections at the ends of

the winding to provide different winding ratios. A center tap is a special case in which

the tap is halfway between the ends of the secondary winding, such that it can be used

to split the voltage output in half.

Center tapped pole mounted transformer

(v) Three Phases, Delta or Wye

Three-phase transformers are connected in delta or wye configurations. A delta

system is a good short-distance distribution system. It is used for neighbourhood and

small commercial loads close to the supplying substation. Only one voltage is

available between any two wires in a delta system. The delta system can be illustrated

by a simple triangle. A wire from each point of the triangle would represent a three-

phase, three-wire delta system. The voltage would be the same between any two

wires.

Delta Connection

In a wye system the voltage between any two wires will always give the same amount

of voltage on a three phase system. However, the voltage between any one of the

phase conductors (X1, X2, X3) and the neutral (X0) will be less than the power

conductors. For example, if the voltage between the power conductors of any two

phases of a three wire system is 208v, then the voltage from any phase conductor to

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ground will be 120v. This is due to the square root of three phase power. In a wye

system, the voltage between any two power conductors will always be 1.732 (which is

the square root of 3) times the voltage between the neutral and any one of the power

phase conductors. The phase-to-ground voltage can be found by dividing the phase-

to-phase voltage by 1.732.

Wye Connection

Section 2

Instrumental Transformer

Instrument transformers are used for measuring voltage and current in electrical

power systems, and for power system protection and control. Where a voltage or

current is too large to be conveniently used by an instrument, it can be scaled down to

a standardized, low value. Instrument transformers isolate measurement, protection

and control circuitry from the high currents or voltages present on the circuits being

measured or controlled. There are two types of instrumental transformers:

(i) Voltage transformer and

(ii) Current transformer

1. Voltage Transformer:

Voltage transformers (VT) or potential transformers (PT) are another type of

instrument transformer, used for metering and protection in high-voltage circuits.

They are designed to present negligible load to the supply being measured and to have

a precise voltage ratio to accurately step down high voltages so that metering and

protective relay equipment can be operated at a lower potential.

Types:

Two types of voltage transformer are used for protective-relaying purposes. They are

(i) Instrument potential transformer or potential transformer and

(ii) Capacitance potential device.

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A potential transformer is a conventional transformer having primary and secondary

windings. The primary winding is connected directly to the power circuit either

between two phases or between one phase and ground, depending on the rating of the

transformer and on the requirements of the application.

Potential transformer

A capacitance potential device is voltage-transforming equipment using a capacitance

voltage divider connected between phase and ground of a power circuit.

Two types of capacitance potential device are used for protective relaying:

(1) Coupling-capacitor potential device and

(2) Bushing potential device.

The two devices are basically alike, the principal difference being in the type of

capacitance voltage divider used, which in turn affects their rated burden.

The coupling-capacitor device uses as a voltage divider a coupling capacitor

consisting of a stack of series-connected capacitor units, and an auxiliary capacitor.

The bushing device uses the capacitance coupling of a specially constructed bushing

of a circuit breaker or power transformer. Both of these relaying potential devices are

called "Class A" devices. They are also sometimes called "In-phase" or "Resonant"

devices.

Other types of potential devices are "Class C" or "Out-of-phase" or "Non-resonant".

But they are not generally suitable for protective relaying.

Coupling Capacitor

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Coupling-capacitor potential device Bushing potential device

2. Current Transformer:

Current transformer which is widely known as 'CT' converts line current (primary

current) in to small standard current values which are suitable for the devices to which

it is connected e.g. measuring instruments, meters, protection relays. Basic functions

of CTs are,

To reduce line current to a value which is suitable for standard measuring

instruments, relays etc.

To isolate the measuring instruments, meters, relays etc. from line voltage of an

installation.

To protect measuring instruments against short circuit currents.

To sense abnormalities in current and give current signals to protective relays to

isolate the defective system.

Current transformer

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TYPES:

Current transformers are broadly classified as,

1. RING TYPE CTs: A CT which has an opening in the center to accommodate a

primary conductor (busbar or cable) through it.

Ring type

High grade silicon steel toroidal cores are carefully selected, then insulated and

protected by resin impregnated cotton/epoxy resin coating on which the secondary

winding is wound. Insulation taping and winding is done by computer controlled

machines. The enamel coated secondary winding wire is protected by self bonding

PVC insulation. Insulation levels, construction and treatment ensure maximum

reliability.

2. BAR - PRIMARY CTs: A CT in which the primary winding consists of bar of

suitable size and, material forming an integral part of the CT.

Bus - bar Primary

3. WOUND TYPE CTs: A CT having a primary winding of more than one full turn

wound on the core.

Wound type

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Wound primary current transformers have primary windings that usually consist of

more that one turn. Wound primary transformers have an internal primary winding

and have no windows for the primary conductor to pass through. They have an

advantage in that the designer can make units with low current ratios that therefore

have better accuracy and burden capabilities. The primary of the transformer is

inserted in series with the conductor that is being monitored. For this reason there is

some hesitation to use wound primary transformers even though they may be the most

effective way to achieve the desired performance, especially for the low current ratio

requirements. The wound primary types may be designed using the toroidal cores

because of its excellent efficiency. The wound primary current transformers are more

common in the higher (greater than 600V class) voltage class current transformers as

it makes the problems of the higher voltages easier for the design engineer to cope

with, while keeping the design as cost effective as possible.

Ring type (or rectangular type) CTs are normally preferred over other types of CTs

because they are simple in construction, mechanically stronger and cheaper. In a ring

type/bar primary type CTs the working ampere-turns are determined by the primary

current and therefore the accuracy of these CTs becomes higher as the rated primary

current decreases. If higher accuracy and burdens are required for CTs of low primary

current, wound types CTs are used.

Current transformers are also classified in accordance with their nature of application,

i.e.

1. MEASURING CURRENT TRANSFORMER and

2. PROTECTIVE CURRENT TRANSFORMER.

MEASURING CT PROTECTIVE CT

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CTs can be manufactured to suit above individual application or combined dual

purpose application. Following are general type of CTs;

1) INTERPOSING CTs: These CTs are used in conjunction with main CTs to alter

the ratio of main CT or to provide isolation to meters or relays from main CTs

secondary circuit. Primary current of these CTs are generally lower than 10 amps.

Due to which they are always wound primary types.

Interposing CT

2) SUMMATION CTs: When current flowing, in more than one feeder is required to

be metered summation current transformer are used. These CTs are provided with

more than one primary winding and one common secondary. The standard primary &

secondary currents are 5 or 1 amp.

Summation CT

3) BUSHING TYPE OR BUSDUCT CTs: These CTs are Ring type construction &

can be mounted on Bus-duct or Bushing turret of power transformer.

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Siemens SF6 Circuit Breaker With externally mounted bushing CTs

4) CORE BALANCE CTs: These are ring type CTs & suitable for the measurement

of the sum of three phase currents in a 3-ph cable. Under normal operating conditions

this sum is zero. In the event of an earth-fault the sum of the current is equal to the

zero sequence current. These CTs are generally used with English Electric make

CTUM 15 static relay.

Core Balance CT

5) FURNACE CTs: These transformers are generally split core type which can be

easily mounted on the bus of furnace transformer. Primary current of these

transformers are o value 10,000 Amp & above. Special precaution is necessary to

reduce the heat developed in the transformers due to eddy current during design.

Furnace CT

6) PRECISION GRADE CTs: These CTs are of accuracy of 0.1, 0.2 or 0.5 and used

as a standard current transformer to check accuracies of other transformer. These

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current transformers are either wound primary or ring type and manufactured in

teakwood cases.

Precision Grade CT

Section 3

Difference between Power and Instrument transformers for

oil sampling

Power transformer Instrument transformer

Tests conducted are more (DGA and

FURANS)

Tests conducted are less

Volume of oil content is more Volume of oil content is less

No pressure is applied for power

transformers

Instrument transformers may be

pressurised

No need to apply high purity nitrogen Need to apply high purity nitrogen if the

pressure is below its rated value

No need to check for Positive or Negative Pressure

Need to check for Positive or Negative Pressure

Need to do test every year Need to do test only within 2years or more

Power transformers can be live whilst sampling

Instrument transformers are not sampled live

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