Transformer
A transformer is a static piece of apparatus by means of which electric power is transferred from one circuit to another without a change in frequency. It can raise or lower the voltages with a corresponding decrease or increase in current. It is accomplished by Faraday’s law of electromagnetic induction. In the simplest form, a trafo consists of two conducting coils having mutual inductance.
Table of Contents
COMPONENTS OF A TRANSFORMER
Core:
The core of a trafo is made up of very thin sheets of ferrous metal, commonly cold rolled grain oriented silicon steel, densely stacked together. The core is composed of yokes and limbs joined together to form a single structure. The longer section of the core is the yoke and the comparatively shorter is the limb. The Low-voltage and High-voltage windings are basically wrapped around the core.
The core of the trafo functions as efficient magnetic coupling between the windings for the transfer of energy from primary to secondary. As silicon steel has higher permeability than that of air, it helps to concentrate the flux promoting much more efficient energy transfer. Reduction in the air gap in the core yields higher efficiency of the trafo.
According to design, the core may be of two types, shell type and core type. In shell-type configuration the core surrounds the winding this increases the pathway for the flow of magnetic flux. It results in less energy loss compared to the core-type design. In a core type design, the use of winding material is more and it has basically more losses than a shell type. The core can also be classified in terms of several limbs it has into three limb, four limb, and five-limb designs.
Winding:
It is the main part of the trafo which carries the current. Windings are mainly of two types primary and secondary winding. The primary winding is connected to the source and the latter to the load. Other than the rating of a few KVA most of the trafos utilize rectangular-shaped winding conductors because of compactness and reliability. The round shaped conductors have greater shortcircuit force withstanding capability.
Individual strands are insulated from each other by wrapping helically with insulating paper strips and varnish. This insulation protects against short-term overvoltage stress, overcurrent, and overheating. It is important to note that winding is the most intricate part of the trafo as no repair and maintenance is possible during the service life.
Insulation:
Interwinding insulation normally has a number of oil ducts which the suitable spaced insulating cylinders. The use of pre compressed pressboards for insulation in trafos with profiled rings and angle caps are widely used type of insulation for insulating the winding from the core and other metal parts.
Mineral oil is traditionally used as an insulating medium in trafos. Proper use of insulation minimizes the risk of arcing during electrical faults which can cause serious damage and lead to failure.
WORKING PRINCIPLE OF THE TRANSFORMER
A trafo works on the basis of electromagnetic induction and mutual induction
Electromagnetic Induction:
Whenever a conductor is placed in a varying magnetic field, an electromotive force emf will be generated in the conductor and if the circuit is closed, induced current will flow through it.
In the case of the trafo, the primary is the alternating current carrying winding which generates the varying magnetic field. This varying magnetic field is channeled through the core to the secondary winding, where the varying magnetic field induces an emf because of electromagnetic induction. Since the winding is arranged in series, the total induced emf will be the sum of induced emf in each turn of the secondary.
The primary winding is one, which receives electric power from the supply, and the secondary winding is the one, which delivers to the load. The coils are wound on a laminated core of magnetic material. It operates the mutual inductance between the two circuits linked by a common magnetic flux through a path of low reluctance as shown in the figure.
The EMF equation for the windings of the trafo is given as,
E = 4.44FNBm A
N – Number of turns in primary/secondary
F – Frequency in hertz
E – E.m.f. induced in primary/ secondary
Bm – Maximum flux density in Weber/m2
A – Area of cross-section of the core in m2
Transformation Ratio,
= K
1. Ns > Np, K > 1 is known as step-up trafo.
2. Ns < Np, K < 1 is known as step-down trafo.
In an ideal trafo,
Input VA = Output VA
VpIp = VsIs
TYPES OF TRANSFORMERS
The trafos are also classified as given below based on their function and application: –
Generator transformers:
The generated power at a generating station is usually at a voltage level of 11 to 25 kV which is stepped up by a generator trafo to a higher voltage 220, 345, 400 or 765 kV for transmitting over long distances. They have uniform loads and are designed with higher losses since it is economically viable. Lower noise levels are usually not specified because the generators supplying the trafos are even noisier.
Unit auxiliary transformers:
These are step-down trafos with primary winding connected to the generator output often directly. The secondary voltage is of the order of 6.9 kV for supplying power to various auxiliary equipment and pumps in the generating station.
Station transformers:
These trafos are required to supply power to auxiliary equipment during the setting-up operation of generating stations and subsequently during the start-up operation. With a smaller rating, their primary winding is connected to a high voltage transmission line.
Interconnecting transformers or autotransformers:
These trafos are used to interconnect two systems operating at different system voltages 400 kV and 220 kV, 345 kV and 132 kV. There is no electrical isolation between the primary and secondary windings as some VA are conductively transformed and the remaining are transformed due to induction.
The design of the autotrafo becomes most cost-effective as the ratio of the secondary winding voltage to the primary winding voltage approaches unity. This is the fact for which a lesser number of 132/33 KV trafo are autotrafos.
Autotrafos are characterized by a wide tapping range and a delta-connected tertiary winding. The unloaded tertiary winding can act as a stabilizing winding by providing a path for third-harmonic currents.
Functions of the tertiary winding
Tertiary winding can be used
To reduce the third harmonic contents of the output voltage thereby stabilizing the potential of the neutral point.
To suppress third harmonic current which causes communication interference.
To permit the transformation of an unbalanced 3-phase load.
To reduce system zero sequence impedance for effective grounding where solid grounding is not provided.
To supply the additional auxiliary loads.
Receiving station transformers:
These are step-down trafos that reduce the transmission line voltage to a primary feeder level voltage for example 132 kV/33 kV trafos. They are used to feed industrial plants directly and distribution substations. Loads on these trafos vary in a wide range. Automatic tap changing on load is a must-have for these types, and the tapping range is also kept higher to account for a wide variation in the voltage. A lower noise level is usually specified for trafos that are close to residential areas.
Distribution transformers:
For distribution trafos, the secondary voltage is reduced to the actual utilization voltage of 415 or 440 V for domestic/industrial use. The load on these trafos varies widely, and they run mostly overloaded. The low value of the no-load loss is desirable to improve the all-day efficiency of these trafos. Hence, the no-load loss is usually capitalized with a high rate at the designing stage. Since very little supervision is possible, utilities expect a minimum level of maintenance on these trafos. The cost of supplying losses and reactive power is the highest for these trafos.
Phase shifting transformers:
Used to control the power flow of transmission lines by varying the phase angle between the output and input voltages of the trafo. Through a proper tap-change operation, the output voltage can be made either to lead or lag the input voltage. The rating and size of the trafo are directly based on total phase shift requirements.
The single-core design is used for small phase shifts and lower MVA / voltage ratings, while the two-core design is employed for bulk power transmission with higher ratings of phase shifting. The design makes use of two trafos, one associated with the line terminals and the other with the tap-changer.
Earthing or grounding transformers:
These are used to provide a neutral point that facilitates grounding and detection of earth faults in an ungrounded part of the network example delta-connected systems have no neutral. Their windings are usually connected in a zigzag manner, which helps to get rid of the third harmonic voltages in the lines. They have an additional advantage since they are not affected by the DC magnetization problems normally associated with power electronic converters.
Transformers for rectifier and inverter circuits:
These have design and manufacturing features that enable them to counter harmonic effects. Because of additional harmonic losses, the operating flux density in their core is usually kept lower around 1.6 Tesla. The winding conductor dimensions need to be smaller to reduce eddy losses, and a proper de-rating factor is applied depending on the magnitudes of various harmonic components. Thermal design aspects are carefully looked at to eliminate hot spots. For the trafos used in high voltage direct current (HVDC) systems, the design of their insulation system is a challenging task because of combined AC-DC voltage stresses.
Furnace duty transformers:
These trafos are used to feed the arc or induction furnaces, which are characterized by a low secondary voltage of 80 to 1000 V and a high current of 10 to 60 kA depending upon their MVA rating. Non-magnetic steel material is invariably used for the termination of LV leads and tank portion in the vicinity to eliminate hot spots and minimize stray losses. For applications involving very high currents, LV terminals are in the form of U-shaped hollow copper tubes having suitable inside diameters for cooling by oil/water circulation. A booster trafo is often used along with the main trafo to reduce the rating of the tap-changer.
Freight loco transformers:
These are mounted in the engine compartments of locomotives, the primary winding of these trafos is connected to the overhead line. The primary voltage is stepped down to an appropriate level for feeding the rectifier, and the output DC voltage of the rectifier drives the locomotive. The structural design of the trafos must be designed to take care of locomotive vibrations. The mechanical natural frequencies must be checked to eliminate any resonance effect.
