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
Tertiary Winding of Transformer | Three Winding Transformer
Under Electrical Transformer
What is Tertiary Winding ? What is Three Winding Transformer ?In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformer or 3 winding transformer.
Advantages of Using Tertiary Winding in TransformerTertiary winding is provided in electrical power transformer to meet one or more of the following requirements-
- It reduces the unbalancing in the primary due to unbalancing in three phase load.
- It redistributes the flow of fault current.
- Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.
- As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.
Stabilization by Tertiary Winding of TransformerIn star-star transformer comprising three single units or a single unit with 5 limb core offers high impedance to the flow of unbalanced load between the line and neutral.
This is because, in both of these transformers, there is very low reluctance return path of unbalanced flux.
If any transformer has N turns in winding and reluctance of the magnetic path is RL, then,
Where I and Φ are current and flux in the transformer.
Now, from equation (1) & (2), it can be rewritten as,
From this above mathematical expression it is found that, impedance is inversely proportional to reluctance. The impedance offered by the return path of unbalanced load current is very high where very low reluctance return path is provided for unbalanced flux. In other words, very high impedance to the flow of unbalanced current in 3 phase system is offered between line and neutral. Any unbalanced current in three phase system can be divided into three sets of components likewise positive sequence, negative sequence and zero sequence components. The zero sequence current is actually co-phasial current in three lines. If value of co-phasial current in each line is Io, then total current flows through the neutral of secondary side of transformer is In = 3.Io. This current cannot be balanced by primary current as the zero sequence current cannot flow through the isolated neutral star connected primary. Hence the said current in the secondary side set up a magnetic flux in the core. As we discussed earlier in this chapter, low reluctance path is available for the zero sequence flux in a bank of single phase units and in the 5 limb core consequently; the impedance offered to the zero sequence current is very high. The delta connected tertiary winding of transformer permits the circulation of zero sequence current in it. This circulating current in this delta winding balances the zero sequence component of unbalance load, hence prevents unnecessary development of unbalance zero sequence flux in the transformer core. In few words it can be said that, placement of tertiary winding in star - star-neutral transformer considerably reduces the zero sequence impedance of transformer.
Rating of Tertiary Winding of TransformerRating of tertiary winding of transformer depends upon its use. If it has to supply additional load, its winding cross - section and design philosophy is decided as per load, and three phase dead short circuit on its terminal with power flow from both sides of HV & MV.
In case it is to be provided for stabilizing purpose only, its cross - section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line -to-ground fault being the most onerous.