Type S Automatic Load-Transfer Control Installation, Operation, and Maintenance Instructions

Description of Operation

The Type S control is designed for use primarily with Kyle Type VR, VLR, VRV, TSC, and C three-phase, load-break switches in automatic load-transfer schemes. In a typical scheme, service to a critical load is normally supplied from a preferred source. It is automatically switched to an alter- nate, standby source if the preferred source voltage is lost for any reason for a preset period of time. Upon restoration of the preferred source voltage, the load is automatically switched back to the preferred source, again after a predetermined time delay.

Typical Automatic Load-transfer Sequence using the Type S Control

•The load is transferred to the alternate source after a preset time delay, when the preferred source voltage is lost and normal voltage is present on the alternate source. Either Source I or Source II can be selected as the preferred source by a setting on the SOURCE PREFERENCE Switch, S4.

•The load is transferred back to the preferred source— after another preset time delay—when normal volt- age is restored to the preferred source. The return transfer (from Source II to Source I) can be either non-parallel (alternate-source switch opens before preferred-source switch closes) or parallel (preferred- source switch closes before alternate-source switch opens). With parallel return the second interruption is eliminated; however, both sources must be in syn- chronism. The return transfer mode is selected by a setting on the SOURCE PREFERENCE switch, S4.

IMPORTANT: If Control Mode Switch S3 is placed in the “AUTO” position, the S-control will place the high- voltage transfer switches into the configuration that is currently selected by the Source Preference Switch S4. This may result in an unintended transfer. Make sure the desired state of the high-voltage transfer switches match the setting of S4 before returning S3 to the “AUTO” position.

•Manual Operation of the S control. The Source I and Source II high-voltage switches can be opened and closed independently to effect manual transfer from one source to the other. (Accomplished by setting OPERATION SELECTOR switch, S3, to MANUAL and operating MANUAL OPER. SOURCE I (S1) and MAN- UAL OPER. SOURCE II (S2) switches as required.)

In addition, a factory-installed fault block accessory will Block Transfer if loss of voltage is due to a fault on the load side of the high-voltage switches. When preferred source voltage is lost (due to the opening of the backup protective device)—and after the preset time delay—the preferred source switch will open and the fault block accessory will disable the S control to prevent closing either high-voltage switch into the fault. The S control must be manually reset before service to the load can be restored. The fault block option is a factory-installed accessory. The accessory is activated by over-current signals supplied by load-sensing current transformers built into special factory-modified Type VR, VLR, VRV, TSC, and CS high-voltage switches. It is also compatible with the PST-6 switchgear. The PST-9 switchgear does not require fault block.

Variations in the Operation of the Type S Control

•No-Preference Operation. When either source is acceptable for continuous critical load supply. Upon loss of Source I voltage—and after a preset time delay—the load is automatically transferred to Source II, provided normal voltage is present on Source II. However, the load is not transferred back to Source I when voltage is restored, but remains on Source II until such time as Source II voltage is lost. Then an automatic nonparallel transfer to Source I is per- formed. (Accomplished by setting SOURCE PREF- ERENCE switch, S4, to NO PREF position.)

•Hold on Alternate Source. When placed to the HOLD ON ALTERNATE position, the S control will not automatically return to the preferred feeder. If the pre- ferred feeder is energized, the S control can be man- ually transferred to the preferred feeder by momen- tarily moving S3 to the NORMAL position.

Time Delay Selection

The time delay setting for preferred to alternate source transfer must be long enough to allow discrimination between permanent loss of voltage and temporary loss of voltage due to transient effects or reclosing intervals of backup protective reclosers or breakers. The time-delay required to override reclosing intervals is difficult to determine since voltage may be subnormal during the retarded timing operations of the backup device due to the presence of the fault. It is recommended that the time delay before transfer from preferred to alternate source is set to exceed the maximum cumulative time to lockout of the backup protective device. This approach assures that the transfer switch will not interrupt the current of a fault occurring on the load side of the switch.

The time delay for return from alternate to preferred source upon restoration of preferred source voltage should be set for an interval long enough to assure that service on the preferred source has been permanently restored.

When the control is programmed to operate in the “no- preference” mode, there are no preferred or alternate sources. Time delay intervals for transfer from Source I to Source II are governed by the left-hand Preferred to Alternate timer and for transfer from Source II to Source I by the right-hand Alternate to Preferred Timer. Settings of the two timers may be different because of difference in backup protection on the two sources.

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