Does the Micro-AT Source-Transfer Control use a negative or zero sequence calculation to determine if it should initiate a transfer? Which method is better?

The Micro-AT Source-Transfer Control determines if the preferred or alternate source is abnormal based on a mathematical calculation of negative sequence voltage (V2). V2 is determined using the following equation:

V2 = 1/3 (VA + a2VB + aVC)

where “a” is a mathematical operator with the following characteristic:

a = 1Ð120°

The V2 method of determining abnormal voltage is believed to offer the best indication for initiating a source transfer since zero sequence voltage (V0), determined by the following equation

V0 = 1/3 (VA + VB + VC),

might not be detectable under certain conditions.

One specific system configuration for which V0 voltage measurement is unable to detect abnormal source voltages is where the source transfer gear (ST Gear) feeds a delta/wye, wye/delta or delta/delta transformer (T2).

If a single phase-to-ground fault occurs on the source side of the ST Gear at F1 and is cleared by the fuse, transformer T2’s delta connection (primary or secondary) may prevent the source-transfer control from detecting the single-phase voltage loss unless V2 (calculated by the Micro-AT) is used.

To simplify the explanation, consider that transformer T2 is unloaded. Presume an A-phase fault is cleared at F1 by the A-phase fuse. Under this condition, A-phase and B-phase transformer windings are energized to the same voltage level and phase angle as the C-phase transformer winding. As a result of transformer T2’s delta connection, ½ voltage of opposite polarity develops for A- and B-phase transformer windings.

When the V0 voltage determination is made, the conclusion reached is that no voltage loss is detectable

V0 = 1/3 (-½VC + -½VC + VC) = 0

Whereas when the V2 calculation is made

V2 = 1/3 (-½VC + -½ a2VC + aVC),

and the “a” operator is carried through

V2 = 1/3 (-½VC + -½ VA + VB)

a quantity of V2 can be measured.

S&C’s Micro-AT Source-Transfer Control is used on Source-Transfer Vista® Underground Distribution Switchgear, Source-Transfer PMH and Source-Transfer PME Pad-Mounted Gear, and in a weatherproof enclosure for overhead source-transfer applications.