ONLINE ELECTRICAL ENGINEERING STUDY SITE

Tertiary Winding of Transformer | Three Winding 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 Transformer

Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements-

  1. It reduces the unbalancing in the primary due to unbalancing in three phase load.
  2. It redistributes the flow of fault current.
  3. 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.
  4. 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 Transformer

In 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) and (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. tertiary winding of transformer 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 Transformer

Rating 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 and 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.




Closely Related Articles Transformer Accessories | Breather and Conservator Tank | RadiatorSilica Gel Breather of TransformerConservator Tank of TransformerRadiator of Transformer | Function of RadiatorMagnetic Oil Gauge or MOG | Magnetic Oil Level Indicator of TransformerOil Winding and Remote Temperature Indicator of TransformerTransformer Cooling System and MethodsOn Load and No Load Tap Changer of Transformer | OLTC and NLTCCore of Transformer and Design of Transformer CoreMore Related Articles What is transformer? Definition and Working Principle of TransformerEMF Equation of Transformer | Turns Voltage Transformation Ratio of TransformerIdeal TransformerTheory of Transformer on Load and No Load OperationResistance and Leakage Reactance or Impedance of TransformerEquivalent Circuit of Transformer referred to Primary and SecondaryHysteresis Eddy Current Iron or Core Losses and Copper Loss in TransformerVoltage Regulation of TransformerSingle Three Phase Transformer vs Bank of Three Single Phase TransformersParallel operation of TransformersMagnetizing Inrush Current in Power TransformerInstrument TransformersCurrent Transformer CT class Ratio Error Phase Angle Error in Current TransformerVoltage Transformer or Potential Transformer TheoryKnee Point Voltage of Current Transformer PS ClassAccuracy Limit Factor and Instrument Security Factor of Current TransformerIsolation TransformerTransformer Insulating Oil and Types of Transformer OilDGA or Dissolved Gas Analysis of Transformer Oil | Furfural or Furfuraldehyde AnalysisRestricted Earth Fault Protection of Transformer | REF ProtectionBuchholz Relay in Transformer | Buchholz Relay Operation and PrincipleWhat is Earthing Transformer or Grounding TransformerDifferential Protection of Transformer | Differential RelaysOver Fluxing in TransformerTransformer Testing | Type Test and Routine Test of TransformerTransformer Winding Resistance MeasurementVoltage and Turn Ratio Test of TransformerVector Group Test of Power TransformerOpen and Short Circuit Test on TransformerInsulation Dielectric Test of TransformerTransformer Oil and Winding Temperature Rise TestImpulse Test of TransformerMaintenance of TransformerSweep Frequency Response Analysis Test | SFRA TestInstallation of Power TransformerCommissioning of Power TransformerElectrical Power Transformer | Definition and Types of TransformerStep Up TransformerStep Down TransformerWhat is Auto Transformer ?High Voltage TransformerDistribution Transformer | All Day Efficiency of Distribution TransformerDry Type TransformerAir Core TransformerDesign of Inductor in Switched Mode Power Supply SystemsDesign of High Frequency Pulse TransformerSingle Phase TransformerToroidal TransformerNew Articles Ring CounterDischarging a CapacitorCharging a CapacitorElectric PotentialParity Generator