Online Electrical Engineering
Sweep Frequency Response Analysis Test | SFRA Test
Electrical Power Transformer | Definition and Types of Transformer
What is transformer? Definition & Working Principle of Transformer
Theory of Transformer on load and no load operation
EMF Equation of Transformer | Turns Voltage Transformation Ratio of Transformer
Resistance & Leakage Reactance or Impedance of Transformer
Equivalent Circuit of Transformer referred to Primary and Secondary
Voltage Regulation of Transformer
Hysteresis Eddy Current Iron or Core Losses and Copper Loss in Transformer
Open and Short Circuit Test on Transformer
What is Auto Transformer ?
Tertiary Winding of Transformer | Three Winding Transformer
Parallel operation of Transformers
Transformer Cooling System and Methods
Core of Transformer and Design of Transformer Core
Transformer Insulating Oil and Types of Transformer Oil
Transformer Accessories | Breather and Conservator Tank | Radiator
DGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfuraldehyde Analysis
Over Fluxing in Transformer
Single Three Phase Transformer vs bank of three Single Phase Transformers
Current Transformer CT class Ratio Error Phase Angle Error in Current Transformer
Voltage Transformer or Potential Transformer Theory
Accuracy Limit Factor and Instrument Security Factor of Current Transformer
Knee Point Voltage of Current Transformer PS Class
What is Earthing Transformer or Grounding Transformer
Differential Protection of Transformer | Differential Relays
Restricted Earth Fault Protection of Transformer | REF Protection
Buchholz Relay in transformer | Buchholz Relay operation and principle
Transformer Testing | Type Test and Routine Test of Transformer
Transformer Winding Resistance Measurement
Voltage and Turn Ratio Test of Transformer
Vector Group Test of Power Transformer
Insulation Dielectric Test of Transformer
Transformer Oil and Winding Temperature Rise Test
Impulse Test of Transformer
Magnetizing Inrush Current in Power Transformer
Air Core Transformer
High Voltage Transformer
Silica Gel Breather of Transformer
Conservator Tank of Transformer
Maintenance of Transformer
Radiator of Transformer | Function of Radiator
Magnetic Oil Gauge or MOG | Magnetic Oil Level Indicator of Transformer
Oil Winding and Remote Temperature Indicator of Transformer
On Load and No Load Tap Changer of Transformer | OLTC and NLTC
Distribution Transformer | All Day Efficiency of Distribution Transformer
Installation of Power Transformer
Commissioning of Power Transformer
Design of High Frequency Pulse Transformer
Core of Transformer and Design of Transformer Core
Under Electrical Transformer
Purpose of Transformer CoreIn an electrical power transformer, there are primary, secondary and may be tertiary windings. The performance of a transformer mainly depends upon the flux linkages between these windings. For efficient flux linking between these windings, one low reluctance magnetic path common to all windings should be provided in the transformer. This low reluctance magnetic path in transformer is known as core of transformer.
Influence of Diameter of Transformer CoreLet us consider, the diameter of transformer core be ′D′
Then, cross-sectional area of the core,
Now, voltage per turn,
Where Bm is the maximum flux density of the core.
E is proportional to D2.
Therefore voltage per turn is increased with increase in diameter of transformer core.
Again if voltage across the winding of transformer is V.
Then V = eN, where N is the number of turns in winding.
If V is constant, e is inversely proportional to N. And hence, D2 is inversely proportional to N. So, diameter of the core is increased, the number of turns in the transformer winding reduced. Reduction of number of turns, reduction in height of the core legs in-spite of reduction of core legs height increased in core diameter, results increase in overall diameter of magnetic core of transformer. This increased steel weight ultimately leads to increased core losses in transformer. Increased diameter of the core leads to increase in the main diameter on the winding. In – spite of increased diameter of the winding turns, reduced number of turns in the windings, leads to less copper loss in transformer.
So, we go on increasing diameter of the transformer core, losses in the transformer core will be increased but at the same time, load loss or copper loss in transformer is reduced. On the other hand, if diameter of the core is decreased, the weight of the steel in the core is reduced; which leads to less core loss of transformer, but in the same time, this leads to increase in number of turns in the winding, means increase in copper weight, which leads to extra copper loss in transformer. So, diameter of the core must be optimized during designing of transformer core, considering both the aspects.
Material for Transformer CoreThe main problem with transformer core is, its hysteresis loss and eddy current loss in transformer. Hysteresis loss in transformer mainly depends upon its core materials. It is found that, a small quantity of silicon alloyed with low carbon content steel produces material for transformer core, which has low hysteresis loss and high permeability. Because of increasing demand of power, it is required to further reduce the core losses and for that, another technique is employed on steel, which is known as cold rolling. This technique arranges the orientation of grain in ferromagnetic steel in the direction of rolling.
The core steel which has under gone through both the silicon alloying and cold rolling treatments is commonly known as CRGOS or Cold Rolled Grain Oriented Silicon Steel. This material is now universally used for manufacturing transformer core.
Although this material has low specific iron loss but still; it has some disadvantages, like, it is susceptible to increase loss due to flux flow in direction other than grain orientation and it also susceptible to impaired performance due to impact of bending and blanking the cutting CRGOS sheet. Both the surfaces of the sheet are provided with an insulating of oxide coating.
Optimum Design of Cross – Section of Transformer CoreThe maximum flux density of CRGO steel is about 1.9 Tesla. Means the steel becomes saturated at the flux density 1.9 Tesla. One important criteria for the design of transformer core, is that, it must not be saturated during the transformer’s normal operation mode. Voltages of transformer depend upon its total magnetizing flux. Total magnetizing flux through core is nothing but the product of flux density and cross – sectional area of the core. Hence, flux density of a core can be controlled by adjusting the cross sectional area of the core during its design.
The ideal shape of cross-section of a transformer core is circular. For making perfect circular cross section, each and every successive lamination steel sheet should be cut in different dimension and size. This is absolutely uneconomical for practical manufacturing. In reality, manufacturers use different groups or packets of predefined number of same dimension lamination sheets. The group or packet is a block of laminated sheets with a predefined optimum height (thickness). The core is an assembly of these blocks in such a successive manner as per their size from core central line, that it gives an optimum circular shape of the cross-section. Such typical cross-section is shown in the figure below.
Oil ducts are needed for cooling the core. Cooling ducts are necessary because hot-spot temperature may rise dangerously high and their number depends on the core diameter and materials that get used for core. In addition to that, clamp plates made of steel are needed on either sides of the core to clamp the lamination. The steel sheet lamination blocks, oil ducts, and clamping plates; all should lie within the peripheral of optimum core circle.
The net sectional area is calculated from the dimensions of various packets and allowance is made for the space lost between lamination (known as stacking factor) for which steel sheet of 0.28 mm thickness with insulation coating is approximately 0.96. Area is also deducted for oil ducts. The ratio of net cross sectional area of core to the gross cross - sectional area inside the imaginary peripheral circle is known as Utilization factor of transformer core. Increasing numbers of steps improve the Utilization factor but at the same time, it increases manufacturing cost. Optimum numbers of steps are between 6 (for smaller diameter) to 15 (larger diameter).
Manufacturing of Transformer CoreDuring core manufacturing in factory some factors are taken into consideration,
- Higher reliability.
- Reduction in iron loss in transformer and magnetizing current.
- Lowering material cost and labor cost.
- abatement of noise levels.
Corner Jointing of Limbs with YokesCore losses in transformer happen mainly due to,
- Magnetic flux flow along the direction of the grain orientation,
- Magnetic flux flow perpendicular to the direction of the grain orientation, this is also known as cross grain iron losses. The cross grain loss mainly occurs in the zones of corner jointing of limbs with yokes and it can be controlled to some extent by applying special corner jointing techniques. There are normally two types of joints used in transformer core,
- Interleaved joints
- Mited joints
Interleaved Joints in Transformer CoreInterleaved joint in transformer core is the simplest form of joints. This joint is shown in the figure. The flux leaves and enters at the joint in perpendicular to grain orientation. Hence cross grain losses are high in this type of joints. But considering the low manufacturing cost, it is preferable to use in small rating transformer.