Transformer
Introduction:
. Transformer is a static device, an essential device in electrical AC power distribution system, which is used to transform AC voltage magnitudes of any value obtained from a source to any desired value for the purpose of distribution and/or consumption. The development of power transformer dates back to 19th century. The main advantage of a transformer is its constant VA rating whether referred to its primary side or the secondary side. With VA being constant (V refers to the voltage magnitude and A refers to the current magnitude in a transformer winding), it is possible to get a higher V with lower A or a lower V with a higher A, by choosing suitable ratio for the transformer. Fig(1.1)
.The major benefit of such a device is its ability to take in the high current produced at relatively low voltage from the electrical generators and transform this power at high voltage level with lower current. This ensures that the power generated in the order of several megavolt amperes (MVA) is being transmitted at low current magnitudes in a cable of practical dimensions over very long distances. Today’s transmission and distribution systems are heavily dependent upon this technology and transformers are used extensively throughout the world.
- If the primary has fewer turns than the secondary, you have a step.up transformer that increases the voltage.
- If the primary has more turns than the secondary, you have a step.down transformer that reduces the voltage.
- If the primary has the same number of turns as the secondary, the outgoing voltage will be the same as what comes in. This is the case for an isolation transformer.
- In certain exceptional cases, one large coil of wire can serve as both primary and secondary. This is the case with variable auto.transformers and xenon strobe trigger transformers.
Types of Transformer
Step Up Transformer & Step Down Transformer
Generally used for stepping up and down the voltage level of power in transmission and distribution power system network.
Three Phase Transformer & Single Phase Transformer
Former is generally used in three phase power system as it is cost effective than later. But when size matters, it is preferable to use a bank of three single phase transformer as it is easier to transport than one single three phase transformer unit
Instrument transformers
Include C.T & P.T which are used to reduce high voltages and current to lesser values which can be measured by conventional instruments.at protection.
Former is generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use later where the ratio between high voltage and low voltage is less than 2.
Outdoor Transformer & Indoor Transformer.
Transformers that are designed for installing at outdoor are outdoor transformers, and transformers designed for installing at indoor are indoor transformers.
Oil Cooled & Dry Type Transformer.
In oil cooled transformer the cooling medium is transformer oil whereas the dry type transformer is air cooled.
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.
- Power transformer is used for the transmission purpose at heavy load, high voltage greater than 33 KV & 100% efficiency. It also having a big in size as compare to distribution transformer, it used in generating station and Transmission substation .high insulation level
Power Transformer construction
Transformer core.
. Construction in which the iron circuit is surrounded by windings and forms a low reluctance path for the magnetic flux set up by the voltage impressed on the primary.
.transformers are generally considered to be either.
core form.
shell form.
berry form.
In core type transformer it has two vertical legs or limbs with two horizontal sections named yoke. Core is rectangular in shape with a common magnetic circuit. Cylindrical coils (HV & LV) are placed on both the limbs. Shell type transformer: It has a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. Double magnetic circuit is present. Berry type transformer: The core looks like spokes of wheels. Tightly fitted metal sheet tanks are used for housing this type of transformer with transformer oil filled inside.
Winding:
Both low and high voltage windings are manufactured from high conductivity electrolyte copper/ aluminum wires. They are provided with vertical ducts to improve the cooling efficiency.
Low Voltage Windings.
For the transformers with ratings up to 200 KVA, paper insulated rectangular copper wires are used. The windings are of the multi-layer helical type. For large transformers with ratings of 300 . 2000 KVA, copper foils are used to obtain multi-layers cylindrical windings that are characterized with complete mechanical balance during short circuit stresses.
High Voltage Windings
The windings of the transformer up to 1500 KVA are manufactured from round copper wires, which are insulated with special type of varnish, which has a good resistance to heat and transformer oil. The windings in that case are of the multi-layers helical shape.
The winding of the transformers of the rating 2000 KVA, are made from paper insulated rectangular copper wire and are of multi-layers helical shape.
Tank.
. The steel side radiators of the transformer tank are corrugated in order to increase the cooling surface area. The tank made from sheet of suitable thickness. The cover is bolted and sealed to the tank and provided with thermometer pockets, lifting lugs, tap changer – operating handle. Terminal bushings and connection pipe to the conservator
Terminals.
.The terminals of the low and high voltage windings are connected through porcelain bushings for the rated voltage and current and for indoor or outdoor mounting. The bushing insulators are fixed on the tank cover such that they can be changed without opening the tank cover
Bushings.
.The terminals of the low and high voltage windings are connected through porcelain bushings suitable for the rated voltage and current and for indoor or outdoor mounting. The bushing insulators are fixed on the tank cover such that they can be changed without opening the tank cover. The high voltage bushing is provided with arcing horn with gap clearance which depends on the impulse withstand strength of the corresponding winding and the altitude of the installation. The arcing horns serve as coarse protection against external surge voltage and in the case of a flash over. The cable boxes for either the high voltage or the low voltage sides or for both can be provided. specification by value of current and voltage not by rating power of transformer.
and we can know specification
and we can know specification
the side of transformer by see the size of bushing .
Tap Changer.
. The method to change the ratio of Transformers by means of taps on the winding is as old as the Transformer itself. From a very early stage, Transformers with a turn ratio changeable within certain limits have been used for electrical power transmission, since this is the simplest method to control the voltage level as well as the reactive and active power in electrical networks.
.Tap changer also know as tap switches or electronic tap switching transformers regulate output voltage in response to fluctuations in input voltage or load. This is accomplished with Solid state switches which automatically select appropriate taps on a power transformer .
.The H.V. windings have tapping (%25 and ±5% of nominal voltage) which are connected to an off circuit tap changer switch. The tap changer switch is mounted on tank cover. This is
operated by means of an external labeled handle to the required position. This is done only when the transformer is switched off completely .
operated by means of an external labeled handle to the required position. This is done only when the transformer is switched off completely .
The Connection of Power Transformers in the Electrical Network:
As shown in the figure below the power transformer is located in the substation and used to step down the voltage from the higher value transmitted through the transmission line from the source to lower voltage value to be transmitted either to another substation or to the distribution transformers then to the load.
Transformer Connections.
Star-star. Star-delta. Delta-delta. Delta-star. Zig-zag (delta zigzag)
Star-delta-star (Tertiary Winding).
Star.star:
.Least number of windings compared to any other connection and least insulation as the winding voltage is less by a factor or 3 compared to line voltage. Voltage stresses on windings are limited.
.Requires large cross section (thus has high impedance) for the windings compared to other connections as line and phase currents are the same. Large cross section offers the connection good mechanical stability.
.Most economical connection for high voltage and low current transformers Four wire system and offers the availability of a neutral point for earthing .
.Third harmonic component is not cancelled out.
Unbalancing causes the instability of the neutral point .
Used in small power transformer
, interconnecting two delta systems, large
transmission transformers.
In case of a faulted phase, the other two lines
work normally Delta.Delta.
Requires more windings and large insulation for windings as the line voltage is the same as phase voltage thus voltage stresses are high.
Economical in transformers with high current capabilities and low voltages
Least connection in terms of mechanical stability due to small cross section of cables and high number of windings If one of the windings .or one of the transformers in case we use a bank of transformer. is disconnected, the other two remain in service thus producing almost of the actual nominal apparent power.
Third harmonic is eliminated but lacks the availability of neutral point thus requires high insulation due to the high voltage.
No neutral access and thus requires special arrangement for grounding the transformer such as a zigzag transformer.
Not highly used but can be used to supply unbalanced loads. fig(1.20).
Star.Delta.
Delta cancels third harmonic.
Star provides way for grounding the transformer.
Delta ensures the balance of neutral point of the
star Any fault present on any of the sides
requires total disconnection of transformer.
Not preferred to be used as a step up function
as delta winding are weak in terms of mechanical stability.
Very well suited in voltage step down
functionand in transmission systems.
Delta.star.
Delta cancels third harmonic.
Star provides way for grounding the transformer.
Delta ensures the balance of neutral point of the star.
Any fault present on any of the sides requires total disconnection of transformer
Not preferred to be used as a step down function as delta windings are weak in terms of mechanical stability.
Very well suited in voltage step up function and in transmission systems.
Used in distribution transformers for supplying loads that requires 4 wire system, balanced or unbalanced.
(Zig. Zag (delta zigzag) arrangement).
The zigzag connection of transformer is also called the interconnected star connection. It called earthing transformer .
This connection has some of the features of the Y and the ∆ connections, combining the advantages of both.
The zigzag transformer contains six coils on three cores.
Use neutral at connect protection devices.(earthing).
It can cancel triplet (3rd, 9th, 15th, 21st, etc.) harmonic currents.
Different methods of transformer cooling?
v For dry type transformers.
Air Natural (AN)
Air Blast
v For oil immersed transformers.
ü Oil Natural Air Natural (ONAN)
ü Oil Natural Air Forced (ONAF)
ü Oil Forced Air Forced (OFAF)
ü Oil Forced Water Forced (OFWF)
ü Oil directed air forced (ODAF)
ü Oil directed water forced (ODWF)
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