Friday 21 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
