- Single Three Phase Transformer Definition: A single three-phase transformer is a single unit that manages three-phase electrical power, costing less and taking up less space than multiple single-phase transformers.
- Bank of Three Single Phase Transformers: This setup uses three single-phase transformers together, offering easier transportation and operational flexibility if one transformer fails.
- Transformer Banking: The practice of using multiple transformers together to manage three-phase power, offering advantages like cost-effectiveness and ease of maintenance.
- Connection Methods: Various connection methods, including star-star, delta-delta, star-delta, and delta-star, are used to manage phase relationships and voltage transformations.
- Economic Considerations: Star-delta connections are economical for step-down purposes, while delta-star connections are economical for step-up purposes due to insulation cost and potential stress differences.
Comparison between Single Three Phase and Bank of Three Single Phase Transformers for Three Phase System
Generation, transmission and distribution of electrical power are more economical in a three phase system than in a single-phase system. A three-phase system needs three single-phase windings. That transformation can be done in two ways: with one three-phase transformer, or with a bank of three single-phase transformers. Each option has advantages and disadvantages.
A single three-phase transformer is often quoted as costing about 15% less than a bank of three single-phase transformers and takes up less space. Transporting a large three-phase transformer can be difficult, so three single-phase transformers are easier to move. They can be transported separately and then assembled on site to form a three-phase unit.
Another advantage of a bank of three single-phase transformers is that if one unit fails, the remaining two can still operate as an open delta transformer.
Connection of Three Phase Transformer
Several connection methods are possible for both a single three-phase transformer and a bank of three single-phase transformers.
Marking or Labeling the Different Terminals of Transformer
Terminals of each phase of the HV side should be labeled as capital letters A, B, C and those of the LV side as small letters a, b, c. Terminal polarities are indicated by suffixes 1 and 2. Suffix 1 marks similar polarity ends, and so do suffix 2 marks.
Star-Star Transformer
A Star-star transformer is formed in a 3-phase transformer by connecting one terminal of each phase of each side together. The common terminal is indicated by suffix 1 in the figure below. If the terminals with suffix 1 on both primary and secondary are used as the common terminal, primary and secondary voltages are in the same phase. This connection is therefore called a zero-degree connection or 0o – connection.
If the terminals with suffix 1 are connected together on the HV side as the common point and the terminals with suffix 2 on the LV side are connected together as the common point, the voltages in primary and secondary will be in opposite phase. Hence a star-star transformer connection is called a 180o-connection of a three-phase transformer. You can learn more about transformers by studying our MCQ on transformers.
Delta-Delta Transformer
In a delta-delta transformer, 1-suffixed terminals of each phase primary winding are connected with the 2-suffixed terminal of the next phase primary winding.
If the primary is the HV side, then A1 will be connected to B2, B1 will be connected to C2 and C1 will be connected to A2. On the LV side, 1-suffixed terminals of each phase winding are connected with 2-suffixed terminals of the next phase winding. So a1 will be connected to b2, b1 will be connected to c2 and c1 will be connected to a2.
If transformer leads are taken out from primary and secondary 2-suffixed terminals of the winding, there is no phase difference between similar line voltages in primary and secondary. This delta delta transformer connection is a zero-degree connection or 0o-connection. But on the LV side, if a2 is connected to b1, b2 is connected to c1 and c2 is connected to a1, and the secondary leads are taken from 2-suffixed terminals of the LV windings, similar line voltages in primary and secondary will be in phase opposition. This connection is called a 180o-connection of a three-phase transformer.
Star-Delta Transformer
Here in a star-delta transformer, the star connection on the HV side is formed by connecting all the 1-suffixed terminals together as the common point, and the transformer primary leads are taken from 2-suffixed terminals of the primary windings.
The delta connection on the LV side is formed by connecting 1-suffixed terminals of each phase LV winding with the 2-suffixed terminal of the next phase LV winding. So a1 is connected to b2, b1 is connected to c2 and c1 is connected to a2. The secondary (here taken as LV) leads are taken from 2-suffixed ends of the secondary windings. The transformer connection diagram is shown in the figure. The sum of the voltages in the delta side is zero. That must hold, otherwise a closed delta would be a short circuit. From the phasor diagram, phase-to-neutral voltage (equivalent star basis) on the delta side lags by − 30o to the phase-to-neutral voltage on the star side. The same phase shift appears between the respective line-to-line voltages. This star delta transformer connection is therefore known as a − 30o-connection. A star-delta + 30o-connection is also possible by connecting secondary terminals in this sequence: a2 is connected to b1, b2 is connected to c1 and c2 is connected to a1. The secondary leads are taken from 2-suffixed terminals of the LV windings.
Delta-Star Transformer
A Delta-star transformer connection of a three-phase transformer is similar to a star-delta connection. If HV and LV are swapped in the star-delta diagram, it becomes a delta-star connected 3-phase transformer. All small letters of the star-delta connection should be replaced by capital letters in the delta-star transformer connection.
Delta-Zigzag Transformer
The winding of each phase on the star-connected side is divided into two equal halves in a delta zig zag transformer. Each leg of the core of transformer is wound by a half winding from two different secondary phases in addition to its primary winding.
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 a 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 a star connection with an earthed neutral, phase voltage (phase to neutral) is 1/√3 times line voltage (line to line). In a delta connection, phase voltage equals line voltage. A star-connected high-voltage-side electrical power transformer is often quoted as about 10% cheaper than a delta-connected high-voltage-side transformer.
Take the following symbols.
Let the voltage ratio of transformer between HV and LV be K, the voltage across the HV winding be VH and the voltage across the LV winding be VL, and the voltage across the transformer leads on the HV side be Vp and on the LV side Vs.
In Star-Star Transformer
Voltage difference between HV & LV winding,
In Star-Delta Transformer
Voltage difference between HV & LV winding,
In Delta-Star Transformer
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 the voltage difference between HV and LV winding is minimum, so potential stress between them is minimum and insulation cost between those windings is also lower. For a step-down duty a star-delta transformer connection is therefore the cheapest of the three layouts. By the same argument, for step-up duty a delta-star three phase transformer connection is the cheapest.
