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.

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