System intertie transformers

[ali hassan]

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System intertie transformers

Function:
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

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

Types:
1- Separate windings transformers:
This type has separate windings for HV and LV levels as a normal
transformer construction.

2-Auto-connected transformers (auto transformers):
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 windings as follows:
1- 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
2- 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.

Sometimes, the tappings are located at the neutral point of auto-
connected transformers 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.

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 transformers special types:
1-Phase- shifting transformers
2- HVDC transformers


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