Transformer Star-Star Connection Procedure

Source: B.L. Theresa Book

Star-Star Connection of Distribution Transformer

practically reduced to zero whereas EBN and ECN will rise to nearly full primary line voltage. This difficulty of shifting (or floating) neutral can be obviated by connecting the primary neutral (shown dotted in the figure) back to the generator so that primary coil A can take its required power from between its line and the neutral. 

It should be noted that if a single phase load is connected between lines a and b, there will be a similar but less pronounced neutral shift which results in an overvoltage on one or more transformers.

Another advantage of stabilizing the primary neutral by connecting it to the neutral of the generator is that it eliminates distortion in the secondary phase voltages. This is explained as follows. For delivering a sine wave of voltage, it is necessary to have a sine wave of flux in the core, but on account of the characteristics of iron, a sine wave of flux requires a third harmonic component in the exciting current. 

As the frequency of this component is thrice the frequency of the circuit, at any given instant, tends to flow either towards or away from the neutral point in all three transformers. 

If the primary neutral is isolated, the triple frequency current cannot flow. Hence, the flux in the core cannot be a sine wave and so the voltages are distorted. But if the primary neutral is earthed i.e. joined to the generator neutral, then this provides a path for the triple-frequency currents and e.m.fs. and the difficulty is overcome. 

Another way of avoiding this trouble of oscillating neutral is to provide each of the transformers with a third or tertiary winding of a relatively low kVA rating. This tertiary winding is connected to ∆ and provides a circuit in which the triple-frequency component of the magnetizing current can flow (with an isolated neutral, it could not). 

In that case, a sine wave of the voltage applied to the primary will result in a sine wave of phase voltage in the secondary. As said above, the advantage of this connection is that insulation is stressed only to the extent of line to neutral voltage i.e. 58% of the line voltage.

Application of Star-Star Connection Transformer

  • This type of Transformer is rarely used due to problems with unbalanced loads.
  • It is economical for small high-voltage transformers as the number of turns per phase and the amount of insulation required are less.

Three-phase Transformer Line Voltage and Current

Primary-Secondary
Configuration
Line Voltage
Primary or Secondary
Line Current
Primary or Secondary
Delta – Delta
delta-delta line voltage
delta-delta line current
Delta – Star
delta-star line voltage
delta-star line current
Star – Delta
star-delta line voltage
star-delta line current
Star – Star
star-star line voltage
star-star line current


Voltage Ratio of Star and Star Three-Phase Transformer Connection

The ratio of line voltages on the primary side and the secondary side is equal to the transformation ratio of the transformers. 

This phase voltage between the neutral point and any one of the line connections is 1/√3 × VL of the line voltage.


Three-phase Voltage and Current 

ConnectionPhase VoltageLine VoltagePhase CurrentLine Current
Star
VP = VL ÷ √3
VL = √3 × VP
IP = IL
IL = IP
Delta
VP = VL
VL = VP
IP = IL ÷ √3
IL = √3 × IP
Where again, VL is the line-to-line voltage, and VP is the phase-to-neutral voltage on either the primary or the secondary side.

Advantages of Star-Star Connection

The main advantage of the transformer star-star connection is having the neutral terminal in this procedure. Hence, it can be used where the primary and the secondary require a neutral and the voltages are moderate and high.

Major Disadvantages of Star-Star Connection and Methods of Solution

If an unbalanced load is connected and there is a neutral system then the phase voltages tend to become severely unbalanced. Thus, without a neutral connection, the star-star connection is not satisfactory for the unbalanced load.

The magnetizing current of a transformer varies non-sinusoidally and contains a third harmonic. The third harmonic is zero for a three-phase balanced load adding each of three phase magnitude of harmonics at the neutral point of the star connection. But, in an unbalanced load, it does not become zero.

The methods of solving the unbalanced and third Harmonic problems in a star-star connection of a distribution transformer are as below:

  1. Solid Grounding of Neutrals;
  2. Providing Tertiary Windings.

A solid grounding of neutral between the star point of the primary of the transformer and the neutral point of the alternator can provide low impedance grounding resistance.

Providing a third winding in addition to the primary and secondary windings in a star-star connected transformer which is known as Third Winding or the Tertiary Windings.

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