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Sweep Frequency Response Analysis Test | SFRA Test
Electrical Power Transformer | Definition and Types of Transformer
What is transformer? Definition & Working Principle of Transformer
Ideal 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
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

Single Three Phase Transformer vs bank of three Single Phase Transformers

Under Electrical Transformer

Comparison between Single Three Phase and Bank of Three Single Phase Transformers for Three Phase System


Connection of Three Phase Transformer

A verity of connection of three phase transformer are possible on each side of both a single 3 phase transformer or a bank of three single phase transformers.

Marking or Labeling the Different Terminals of Transformer

Terminals of each phase of HV side should be labeled as capital letters, A, B, C, and those of LV side should be labeled as small letters, a, b, c. Terminal polarities are indicated by suffixes 1 & 2. Suffix 1’s indicate similar polarity ends and so do 2’s.


star star three phase transformer Star-star transformer is formed in a 3 phase transformer by connecting one terminal of each phase of individual side, together. The common terminal is indicated by suffix 1 in the figure below. If terminal with suffix 1 in both primary and secondary are used as common terminal, voltages of primary and secondary are in same phase. That is why this connection is called zero degree connection or 0° - connection.

Delta-Delta Transformer

But in LV side of transformer, if, a2 is connected to b1, b2 is connected to c1 and c2 is connected to a1. The secondary leads of transformer are taken out from 2 suffixed terminals of LV windings, and then similar line voltages in primary and secondary will be in phase opposition. This connection is called 180°-connection, of three phase transformer.

Star-Delta Transformer

Here in star-delta transformer, star connection in HV side is formed by connecting all the 1 suffixed terminals together as common point and transformer primary leads are taken out from 2 suffixed terminals of primary windings. star delta three phase transformer The delta connection in LV side is formed by connecting 1 suffixed terminals of each phase LV winding with 2 suffixed terminal of next phase LV winding. More clearly, a1 is connected to b2, b1 is connected to c2 and c1 is connected to a2. The secondary (here it considered as LV) leads are taken out from 2 suffixed ends of the secondary windings of transformer. The transformer connection diagram is shown in the figure beside. It is seen from the figure that the sum of the voltages in delta side is zero. This is a must as otherwise closed delta would mean a short circuit. It is also observed from the phasor diagram that, phase to neutral voltage (equivalent star basis) on the delta side lags by − 30° to the phase to neutral voltage on the star side; this is also the phase relationship between the respective line to line voltages. This star delta transformer connection is therefore known as − 30°-connection.

delta star three phase transformer

Delta-Star Transformer

Delta-star transformer connection of three phase transformer is similar to star – delta connection. If any one interchanges HV side and LV side of star-delta transformer in diagram, it simply becomes delta – star connected 3 phase transformer. That means all small letters of star-delta connection should be replaced by capital letters and all small letters by capital in delta-star transformer connection.

Star-Zigzag Transformer

The winding of each phase on the secondary star in a star-zigzag transformer is divided into two equal halves. Each leg of the core of transformer is wound by half winding from two different secondary phases in addition to its primary winding.

Choice Between Star Connection and Delta Connection of Three Phase Transformer

In star connection with earthed neutral, phase voltage i.e. phase to neutral voltage, is 1/√3 times of line voltage i.e. line to line voltage. But in the case of delta connection phase voltage is equal to line voltage. Star connected high voltage side electrical power transformer is about 10% cheaper than that of delta connected high voltage side transformer.

Let, the voltage ratio of transformer between HV & LV is K, voltage across HV winding is VH and voltage across LV winding is VL and voltage across transformer leads in HV side say Vp and in LV say Vs.

Voltage difference between HV & LV winding,

Voltage difference between HV & LV winding,

Voltage difference between HV & LV winding,

For 132/33KV Transformer K = 4

Case 1 Voltage difference between HV & LV winding,

Case 2 Voltage difference between HV & LV winding,

Case 3 Voltage difference between HV & LV winding,

In case 2 voltage difference between HV and LV winding is minimum therefore potential stresses between them is minimum, implies insulation cost in between these windings is also less. Hence for step down purpose star–delta transformer connection is most economical, design for transformer. Similarly it can be proved that for step up purpose delta-star three phase transformer connection is most economical.

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