System Intertie transformers

Purpose of using System intertie transformers : 

 

fig (1) : System intertie transformers

System intertie transformer connect transmission systems with different voltages together with the purpose that active as well as reactive power can be exchanged between the systems

general Features of System intertie transformers :

• They have High power rating up to 1000 MVA. 
• The insulation of the windings is normally graded. 

Types of System intertie transformers :

1- Separate windings transformers: see fig.2

fig (2) : Separate windings , core type system intertie transformer

This type has separate windings for HV and LV levels as a normal transformer construction.

2- Auto-connected transformers (auto transformers): see fig.3

fig (3): 525 KV Auto transformer 

The system intertie transformers may be built as auto-connected windings to minimize Weight and physical dimension, to facilitate the transport from the factory to site, and to minimize the cost of making these transformers with separate windings.

The auto-connected transformer has two types as follows:

a- Common winding: which is connected to a neutral and as the name indicates, the turns of this winding are common for both sides of the transformer

b- Series winding: Which is connected at one end to the common winding and at the other end to the high voltage terminal.

Note:
The common winding and the series winding are arranged as concentric cylindrical shells, and the ampere-turns in the two windings are equal in value and opposite in direction.



System intertie transformers tapping: 

Tapping purpose:
The turns ratio of these transformers is fixed but when there is a need to increase the ability of reactive power exchange between different transmission systems, a tapping will be provided for these transformers.

Note:

In high voltages levels, the tappings may not noticeably influence the voltage on either side of the transformer).

Tappings location:
1- In separate windings transformers:

• tappings are placed in the neutral end of one of the windings 

2- In auto-connected transformers:

• tappings are typically located in the phases of the low voltage side (see fig.4).

fig (4)

• Sometimes, the tappings are located at the neutral point of auto-connected transformers (see fig.5) where the voltage level to earth and the voltage differences between phases are lower than when the tappings are situated at the auto tap. A simpler and cheaper tap changer can then be used. 

fig (5)

• On the other hand tappings at the neutral will need a larger number of turns in the tapping range to achieve the same variation in the turn ratio as when the tappings are situated at the auto tap or at the high voltage terminal. 

System intertie transformer special types:

there are important special types for System intertie transformers as follows :

• Phase- shifting transformers.

• HVDC transformers.

and in the next topic , i will show more information about these special types.