Archive for November 2014
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
Step down transformer: is one whose secondary voltage is less than its primary voltage. It is designed to reduce the voltage from the primary winding to the secondary winding. This kind of transformer “steps down” the voltage applied to it.
As a step-down unit, the transformer converts high-voltage, low-current power into low-voltage, high-current power. The larger-gauge wire used in the secondary winding is necessary due to the increase in current. The primary winding, which doesn’t have to conduct as much current, may be made of smaller-gauge wire.
Step-down transformers are
commonly used to convert the 220 volt electricity found in most parts of the
world to the 110 volts required by North American equipment.
How
to Wire a Step Down Transformer
- Observe and identify the schematic and rating of the step down transformer to be installed. Remove the terminal connection box cover placed at the lower side of the transformer. Only the high amperage types will have this enclosure, while lower powered transformers will have an exposed screw terminal.
- Know termination identification follows for all step down transformers: H1, H2, H3 and H4 signify the high voltage side or power feed end of the transformer. This holds true regardless of the size of the transformer. Interconnection of the transformer will vary depending on the manufacturer and voltage used for feeding the transformer.
- Terminate the feed power wires first by cutting the wires to length. If you are using large wire lugs be sure to take into consideration the length of the lug and the amount of wire that can be inserted into the female crimp area.
- Strip back the outer insulating of the wires with the pocketknife or wire strippers. Insert the eye ring or wire lug over the bare copper wire and crimp the connection device, using the appropriate-size crimper, permanently to the wire.
- Terminate the high side, high voltage of the step down transformer. If the high side terminals are bolts, be sure to follow any torque requirements that are listed by the manufacturer.
- Terminate the low side, low voltage of the transformer. Note these terminals will be identified by X1, X2, X3 and X4. Again follow the manufacturer’s individual schematics for that particular type of transformer. Note that on small control transformers there will only be an X1 and X2. X1 is the power or “hot” side and X2 is generally the grounding and neutral portion of the low voltage.
- Terminate the small control transformer for X1 and X2. X1 will go directly to the control circuit after passing through a small fuse that is rated for the circuit. X2 will be terminated not only to the neutral side of the control circuit, but the grounding safety as well. In other words, the X2 side of the small control transformer must be tied to the grounding system of the electrical circuit.
- Replace all covers on the transformer and any enclosures that protect you from electricity. Apply the high voltage to the transformer by switching on the feeder power circuit. Turn on the low side safety circuit control.
- Use a volt meter to test for proper voltage on the step down side of the transformer. It should be the same that is listed on the specs tag provided by the manufacturer.
- Remove all wires from the transformer terminals using the screwdriver. Identify the wires if they are not already identified. Use a clear tape and pen. Write the terminal that the wires are attached to and place the identified tape on the wire’s end.
- Turn the volt ohmmeter to the “Ohms” position and place the red lead into the connector identified as “Ohms.” Touch the black lead to the metal frame of the transformer.
- Touch the red lead to the transformer’s terminals in the following order: H1, H2, X1 and then X2. The meter should read infinite ohms or wide open. Infinite ohms on a digital meter will be identified as a blank screen or a wide open will have the word “Open” displayed. If the meter registers any form of resistance, there is an internal problem with the windings. The copper coils may be shorted to the metal frame of the transformer. The transformer will have to be replaced.
- Check the continuity of each separate coil using the ohmmeter. Touch the black lead to H1 and the red lead to H2. The meter should give a resistance reading. Generally, it should read in the range of 3 to 100 ohms, depending on the style and type of transformer. Perform the same test to the X1 and X2 terminals. You should receive the same results. If the meter reads infinite ohms or a wide open when checking between the terminals of the same coil, the wires are broken. Replace the transformer.
- Use the ohmmeter to conduct the transformers isolation circuit. Touch the red lead to H1 and the black lead to X1. The meter should read infinite ohms or a wide-open circuit. Perform the same test, but to H2 and X2 respectively. If any resistance at all is read on the meter other than a wide-open circuit, the isolation of the transformer has been compromised and must be replaced.
The problem with surge protectors
Usually, having 220 volts between neutral and ground in an appliance designed for 110 volts is not a problem the insulation has a large safety margin. However, if you connect a surge protector (or a piece of equipment with built-in surge protection) on the 110 volt side, bad things can happen.Surge protectors contain varistors components which protect against surges by effectively shorting out any excess voltage. Some surge protectors contain only a single varistor connected between hot and neutral; those will work fine with a step-down transformer. However, many surge protectors have additional varistors connected between hot and ground and between neutral and ground. When a surge protector of this kind is used with a step-down tranformer, one of these varistors can be subjected to the full 220 volts. This is enough to trigger the varistor into its conducting mode, effectively treating the 220 volts as a surge.
Varistors are designed to absorbed short-lived surges, but they can't handle a persistent overvoltage. A varistor subjected to twice its rated voltage will quickly be destroyed, usually causing a short circuit and a blown fuse.
step down transformer
working
The function of any transformer is to change one AC voltage
value to another AC voltage value. A step down transformer will transform a
higher AC voltage to a lower AC voltage. A step up transformer will transform a
lower AC voltage to a higher AC voltage. The transmission of electrical power
uses both of these types of transformers. From the generation station the
voltage is stepped up to a very high transmission voltage and at the end of the
transmission line it is stepped down to a voltage that consumers can utilize.
step down transformer
calculation
EMF Equation of transformer
can be established in a very easy way. Actually in electrical power transformer
one alternating electrical source is applied to the primary winding and due to
this, magnetizing current flowing through the primary winding which produces
alternating flux in the core of transformer. This flux links with both primary
and secondary windings. As this flux is alternating in nature, there must be a
rate of change of flux. According to Faraday’s law of electromagnetic induction
if any coil or conductor links with any changing flux, there must be an induced
emf in it. As the current source to
primary is sinusoidal, the flux induced by it will be also sinusoidal. Hence,
the function of flux may be considered as a sine function. Mathematically,
derivative of that function will give a function for rate of change of flux
linkage with respect to time. This later function will be a cosine function
since d(sinθ)/dt = cosθ. So, if we derive the expression for rms value of this
cosine wave and multiply it with number of turns of the winding, we will easily
get the expression for rms value of induced emf of that winding. In this way,
we can easily derive the emf equation of transformer
Instructions
1.Obtain
the windings ratio of the transformer. This number is often printed on the
transformer case, and will take the form of "primary:secondary." For
example, a "2:1" transformer will have twice as many windings in the
primary as it has in the secondary.
2.Divide
the secondary number by the primary number. For a 2:1 transformer, this number
is 1/2, or 0.5.
3.Multiply
the input voltage you intend to apply to the transformer by the number you
calculated in the previous step. For example, if you apply 12 volts to a 2:1
transformer's primary, you will obtain 6 volts across the transformer's
secondary.
