PHASE SHIFTING TRANSFORMER UNUSUAL SERVICE CONDITIONS

The unusual conditions shall be the same as those listed in IEEE Std C57.12.00-2000, 4.3.1 through 4.3.3. Additional unusual service conditions that may apply to PSTs are as follows:

Operation with two or more PSTs in parallel or in series
The purchaser shall ensure the manufacturer has all nameplate data, test data, and applicable system information necessary to design the PSTs for proper load sharing. The purchaser must specify in detail to the manufacturer the LTC’s controls that will be provided by the purchaser.

If the manufacturer provides the LTC’s controls, the purchaser shall provide the control scheme used with any existing PSTs to the manufacturer to ensure a compatible system.

Operation of PSTs in series with series capacitor banks
If the PST is, or may be, operated in series with a series capacitor bank, this operating condition shall be pointed out to the manufacturer by the purchaser. The operating conditions shall be specified and the protection scheme used by the purchaser to prevent series resonance shall be provided to the PST manufacturer for review and for considerations in design.

Unbalanced current flow through the PST
The purchaser must provide details of operating conditions that will subject the PSTs to unbalanced phase currents and voltages that may exceed allowable standard limits. The manufacturer will provide for these conditions during the design of the PSTs. The following are examples of operating conditions that could produce such problems:

a) Unbalances resulting from operation of parallel transmission lines in close proximity to the PST connected lines, where line transpositions are unequal resulting in unbalanced voltage at the PSTs and unequal current flow through the series windings.

b) Single-pole operation of the circuit breakers following line faults where single-pole reclosing is utilized


Transient recovery voltages
Transient voltage may exist circuit breakers are operated. These conditions may be between the PST and the circuit breaker.

Surge protection
Any condition where the PST may operate without surge protection applied at all S and L terminals.

USUAL SERVICE CONDITION OF PHASE SHIFTING TRANSFORMER

Usual service conditions
These conditions shall be as stated in IEEE Std C57.12.00-2000, 4.1.1 through 4.1.7, and 4.1.9; 4.1.8 shall not apply. In 4.1.6.1 (a), the word secondary shall mean the L terminals of the PST.

a) The purchaser of the PST shall specify the switching arrangements that will be used to place the PST in and out of service. This shall include breaker or switch operations resulting from faults external and internal to the PST.

b) The PST shall be suitable for energization by voltage applied to either the S or L terminals.

c) The PST shall be capable of transferring rated kVA with the electrical source of power connected to the S or L terminals. Limited power transfer in the retard position has to be considered.

d) Seismic requirements shall be as specified in IEEE Std 693-1997. The seismic zone shall be provided by the purchaser. The foundation design shall be provided to the PST manufacturer by the purchaser.

The manufacturer shall provide for differential motion between the two tanks, if used, and in the case of remotely mounted radiators provide for their differential motion.

e) The manufacturer of the PST shall make provisions for differential alignments that will occur when two tanks are connected. The foundation tolerance shall be defined by agreement between purchaser and manufacturer.

f) Unless specified otherwise, the PST shall be manufactured for operation in the bypassed state with the source and load bushing connected through bus work. This shall require special consideration in design for lightning impulse and switching surges.

This condition will require additional testing with the terminals connected, as in operation, to demonstrate that the insulation level meets the specified BIL.

Loading at other than rated conditions
This subclause shall be the same as IEEE Std C57.12.00-2000, 4.2, with the exception that additional limits must be observed for retard operation under overload. These limits must be defined by the manufacturer and agreed upon by the purchaser prior to completion of the PST design.