PHASE SHIFTING TRANSFORMER SHORT CIRCUIT CHARACTERISTIC

Short Circuit Requirements 

General
PSTs shall comply with the short circuit requirements of IEEE Std C57.12.00-2000, unless otherwise agreed upon by the purchaser and manufacturer.

Transformer categories
The kVA rating to be considered for determining the category should be the equivalent to the rating according to IEEE Std C57.12.00-2000.


Short-circuit current magnitude
The manufacturer shall determine the most onerous conditions for short circuit on every winding or active part in accordance with IEEE Std C57.12.00-2000.

These conditions should take into account the large impedance swings that can occur as the tap position is changed from the extreme positions to the mid position.

Since the system short-circuit levels are critical to the design of PSTs, the user shall specify the maximum system short-circuit fault levels expected throughout the life of the unit.

If a short-circuit test is performed, it shall be done in accordance with IEEE Std C57.12.90-1993.

The test shall be carried out on the tap position that produces the most severe stresses in each winding. This may require more than a single test depending on the type of construction.

For two-core PSTs this usually requires a test on the zero phase-shift position, as this position involves only the series transformer, and a second test on a position to be agreed upon between customer and manufacturer.

PHASE SHIFTING TRANSFORMER CONSTRUCTION

In general, construction requirements for PSTs should be in accordance with the requirements for power transformers, as covered in IEEE Std C57.12.00-2000 and other applicable IEEE standards based on kV and kVA ratings, with the following exceptions or additions.


Enclosed throat connections
Enclosed throat connections in fully assembled condition must meet the pressure and vacuum requirements of PST tanks for all designs that subject the enclosed throat connection to the same operating pressures and vacuum levels as the transformer tank.

Liquid insulation and preservation system 
Liquid insulation and preservation systems shall be in accordance with IEEE Std C57.12.00-2000 with the following addition.

Two tank designs with enclosed liquid-filled throat connection
Enclosed liquid-filled throat connections may be either sealed from each tank or opened to the insulating fluid from one or both tanks. Enclosed throat connections shall be designed for installation or removal without the need to jack or move either or both of the transformer tanks and shall accommodate thermal expansion and contraction of the throat assembly and both tanks. For a sealed throat system that isolates the insulating fluid, the throat connections require a separate conservator system.

For a system where the throats are not directly connected to a main tank and the isolation of the insulating fluid in different compartments is not important, the throats may be connected to the conservator system of the main tank. If this approach is used, the user should be aware that the use of oil and gas analysis to isolate problems will be complicated.

For throat connections that place barriers between both the tanks and the throat, the throat shall be equipped with the following accessories:

— Gas accumulation relay
— Pressure relief device/relay
— Liquid filling and draining valves
— Rapid rate of rise relay
— Liquid level gauge