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Contents
1. Power transformer
1.1. Difference between Power Transformer and Distribution Transformer.
1.2. power transformer protection.
1.3. Power transformer specification.
2. Instrument transformer
2.1. Potential / voltage transformer.
2.2. Current transformer.
3. Autotransformer
4. Audio frequency transformer
5. Grounding transformer
6. Welding transformer
7. Isolation transformers
8. DRY TYPE TRANSFORMERS
9. Oil Filled Transformers
1 Power transformer
Generation of
electrical power in low voltage level is very much cost effective. Hence electrical
power is generated in low voltage level. Theoretically, this low voltage level
power can be transmitted to the receiving end. But if the voltage level of a
power is increased, the current of the power is reduced which causes reduction
in ohmic or I2R losses in the system, reduction in cross sectional
area of the conductor i.e. reduction in capital cost of the system and it also
improves the voltage regulation of the system. Because of these, low level
power must be stepped up for efficient electrical power transmission. This is
done by step up transformer at the sending side of the power system network. As
this high voltage power may not be distributed to the consumers directly, this
must be stepped down to the desired level at the receiving end with the help of
step down transformer. These are the uses of electrical
power transformer in the electrical power system.
1.1.Difference between Power Transformer and Distribution Transformer
Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.
1.2. power transformer protection
The following factors affect the differential current in transformers and should be consideredwhile applying differential protection. These factors can result in a differential current even underbalanced power in & out conditions: 1.Magnetizing inrush current– The normal magnetizing current drawn is 2–5% of therated current. However during Magnetizing inrush the current can be as high as 8–30times the rated current for typically 10 cycles, depending upon the transformer andsystem resistance. 2. Overexcitation–This is normally of concern in generator–transformer units.Transformers are typically designed to operate just below the flux saturation level. Anyfurther increase from the max permissible voltage level (or Voltage/Frequency ratio), could lead to saturation of the core, in turn leading to substantial increase in theexcitation current drawn by the transformer. 3. CT Saturation – External fault currents can lead to CT saturation. This can cause relayoperating current to flow due to distortion of the saturated CT current.1.3. Power transformer specification.
2. Instrument transformer
Electrical instrument transformers transform high currents and voltages to standardized low and easily measurable values that are isolated from the high voltage. When used for metering purposes, instrument transformers provide voltage or current signals that are very accurate representations of the transmission line values in both magnitude and phase. These signals allow accurate determination of revenue billing.
When used for protection purposes, the instrument transformer outputs must accurately represent the transmission line values during both steady-state and transient conditions. These critical signals provide the basis for circuit-breaker operation under fault conditions, and as such are fundamental to network reliability and security.
Instrument transformers used for network control supply important information for determining the state of the operating conditions of the network.
2.1.Potential or voltage transformers
Voltage and potential transformers are used to measure voltage
(potential). The secondary voltage is substantially proportional to the primary
voltage and differs from it in phase by an angle that is approximately zero.
Voltage and potential transformers that are designed for monitoring
single-phase and three-phase line voltages in power-metering applications are
used mainly as step-down devices. They are designed for connecting line-to-line
or line-to-neutral in the same way as ordinary voltmeters. The secondary
voltage has a fixed relationship to the primary voltage so that a change in
potential within the primary circuit is monitored accurately by meters
connected across the secondary terminals.
2.2. Current transformer.
The Current Transformer ( C.T. ),
is a type of “instrument transformer” that is designed to produce an
alternating current in its secondary winding which is proportional to the
current being measured in its primary.
Current transformers reduce high voltage currents to a much lower
value and provide a convenient way of safely monitoring the actual electrical
current flowing in an AC transmission line using a standard ammeter. The
principal of operation of a current transformer is no different from that of an
ordinary transformer.Typical Current Transformer
Unlike the voltage or Power Transformer looked at previously, the current transformer consists of only one or very few turns as its primary winding. This primary winding can be of either a single flat turn, a coil of heavy duty wire wrapped around the core or just a conductor or bus bar placed through a central hole
3. Auto Transformer
An autotransformer or auto step Transformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased KVA rating.
Autotransformers are often used to step up or step down voltages in the 110-115-120 V range and voltages in the 220-230-240 volt range—for example. providing 110 V or 120 V (with taps) from 230 V input, allowing equipment designed for 100 or 120 volts to be used with a 230 volt supply (as in using US electrical equipment with higher European voltages).
4. Audio frequency transformer
An iron-core transformer that is
used for coupling audio-frequency circuits. Also known as audio transformer.
5. Grounding transformer
earthing transformer
on the Delta Side is outsides the Zone of protection the Earth Fault(E/F)in the
delta system outside Current Transformer(CT) locations would produce current
distributions as shown which circulate within the differential CT secondaries
and is kept out of operating coils.
Zig-Zag or inter connected star grounding transformer has
normal magnetising impedance of high value but for E/F, currents flow in
windings of the same – core in such a manner that the ampere turn cancel and
hence offer lower impedance.
In cases where the neutral point of three phase system is
not accessible like the system connected to the delta connected side of a electrical
power transformer, an artificial neutral point may be created with help of a
zigzag connected earthing transformer.
6. Welding transformer
Welding Transformers are used in AC machines to change
alternating current from the power line into a low-voltage, high amperage
current in the secondary winding. A combination of primary and/or secondary
taps on the welding transformer are
commonly used to provide a macro adjustment of the welding current, as
well as adjustment of secondary voltage. Transformer ratings for AC machines
are expressed in KVA (kilovolt-amperes) for a specified duty cycle. This duty
cycle rating is a thermal rating, and indicates the amount of energy that the
transformer can deliver for a stated percentage of a specific time period,
usually one minute, without exceeding its temperature rating. The RMS Short Circuit
Secondary Current specification indicates the maximum current that can be
obtained from the transformer. Since heating is a function of the welding
current, this parameter gives an indication of the thickness of the materials that can be welded.
7. Isolation transformers
isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings.
Isolation transformers block transmission of the DC component in signals from one circuit to the other, but allow AC components in signals to pass. Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks. Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments
Transformer is an electrical device which transfers
electrical power from one coil (primary) to another (secondary) by the
principle of mutual induction. The input is given through the primary and the
output is tapped from the secondary.
If the secondary coil has more number of turns than that of the primary it is called "STEP UP" transformer. Because , the secondary voltage will be more than that of the primary voltage. If it is the other way it is called "STEP DOWN" transformer. Here the secondary voltage will be less than that of the primary.
In an ideal transformer, the power input will be equal to the power output. But practically it is impossible due to certain transformer losses.
Actually the transformer increases or decreases the voltage. So the current will be inversely proportional. ie. if the voltage is increased current will be decreased. and vice versa.
If the secondary coil has more number of turns than that of the primary it is called "STEP UP" transformer. Because , the secondary voltage will be more than that of the primary voltage. If it is the other way it is called "STEP DOWN" transformer. Here the secondary voltage will be less than that of the primary.
In an ideal transformer, the power input will be equal to the power output. But practically it is impossible due to certain transformer losses.
Actually the transformer increases or decreases the voltage. So the current will be inversely proportional. ie. if the voltage is increased current will be decreased. and vice versa.
Difference between step-up
transformer and voltage amplifier
Than a very strange but thinkable question comes what is the difference between the two and can we use a small step up transformer in place of voltage amplifier and vice-versa?
Differences
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Transformer
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Amplifier
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Transformers are unable to amplify (step up) an ac input
Voltage without reducing (stepping down) it`s current capability.
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Amplifier can amplify both current and Voltage at the same
time. We can have 1V at 1uA to drive the input but might also get many volts
at many Amps at the output.
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Transformer`s coil windings never requires a dc Voltage to
operate. Sometimes a dc Voltage might be present in a transformer winding for
auxiliaries but the dc is not required for the operation of the transformer.
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Amplifier almost always requires a dc working supply
Voltage to operate.
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Transformer has more winding added to its secondary
winding to obtain Voltage amplification.
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An Amplifier actually modulates a fixed dc source Voltage
in response to an ac input Voltage to obtain output Voltage amplification.
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A transformer`s input current is proportional to its load
current.
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Amplifier’s input current is normally almost independent
of its load current.
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A transformer is like a gearbox, whereas an amplifier is
like an engine. The gearbox converts energy like a transformer.
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Amplifier is like an engine, which consumes fuel to give
output. Similarly amplifier consumed DC supply to give output.
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A step up transformer can amplify a specified type of
input which is the sinusoidal input or time varying input and add to that the
range of input the transformer is very flexible in range.
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Amplifier can amplify any signal and while the amplifier
would have a limited range then in the saturation state.
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In an ideal transformer output impedance is equal to the
source impedance times the square of the turns ratio.
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An amplifier can have output impedance that is independent
of the source impedance.
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Rule for a Step-Up Transformer
Electrical Codes
·
Under the National Electrical Code, a step-up
transformer must have certain current carrying protections in place. There must
be a main circuit breaker installed in a load center served by the transformer,
and the main breaker can be no larger than 70 percent of the maximum current
capacity of the transformer.
KVA Rating
·
All transformers have a KVA rating. This rating
represents the transformer's maximum capacity in kilowatts (thousands of
watts). In calculating what KVA means in practical terms, Watt's Law is
applied. In Watt's Law, power (P) equals the output voltage (E) times the
ampere capacity (I). Using this formula, P = E x I, and its direct derivatives,
I = P / E and E = P / I, all transformer attributes can be calculated. For
example, if the transformer's rating is 10 KVA and has a 240-volt output, it
has a current capacity of 41.67 amperes (10,000 watts / 240 volts = 41.67
amps).
Application
·
Watt's Law also applies when choosing a step-up
transformer for an application. If a transformer needs to step-up 240 volts to
480 volts, and you need a maximum current capacity of 40 amps, you must first
calculate the number of amps needed to comply with electrical codes. If the
codes require a maximum usage of 70 percent of the transformer's capacity,
multiply 40 times 1.43. The product will be 57.2 amps, 70 percent of which is
40.0 amps. Knowing you need 57.2 amps (I) and the output of 480 volts (E)
Watt's law can be applied as P = I x E. Therefore, 57.2 x 480 = 27,456 watts,
or 27.456 KVA. This would be the minimum size transformer required for this
application
How Calculate turn ratio on step up transformers?
For example: if the primery winding on the transformer were 250 turns and the 1250 turns on the secondary, what is the turn ratio? Well what you do is you take 1250 divide that by 250 will give you 5. So the answer would be 1:5 since 1 will be on the top of the 250 and the 5 would be on the top of the 1250. IN ALL STEP UP TRANSFORMERS HAVING A TURNS RATIO MORE THAN ONE...BECAUSE IT STEPS UP VOLTAGE.
Step up Transfromer Calculation
