Phase Rotation on Delta-Delta Transformer

  A recent project of mine involved installing a 34.5/4.8kV delta-delta mobile transformer in order to remove from service a distribution transformer from service and repair the metering.  The mobile was parked directly under the 34.5kV line to tap from.  The substation crew preferred to bring the high side leads straight down to avoid any crossing of phases without taking into consideration which phase gets connected to each H terminal.  Surely phase rotation of the sub-transmission system should dictate how the three phase power is connected, I thought.  When putting the mobile into service, it would momentarily be in parallel to the distribution transformer, so it would be prudent to determine proper high side connections of the mobile.  Apparently, it does not matter how you wire the high side of delta-delta transformers.  Whatever phase conductor gets connected to a H terminal then that phase is assigned to the corresponding X terminal.  To understand why you can get away with this, you need to ask just what really is phase rotation?

   There are many analogies to explaining phase rotation on three phase power systems but my favorite is the playground, merry-go-round.  Imagine placing three kids on, evenly spaced apart around the edge or 120 degrees apart.  Pretend each kid is a particular phase of a three phase transmission line.  As you are facing the center of the merry-go-round, if a kid is directly in front of your view, then that phase is at zero potential.  If a kid is to your left, then that phase is at negative potential.  If a kid is to your right, then that phase is at positive potential.  When you go to spin the merry-go-round, you are going to experience a certain sequence of kids passing you.  This is called phase sequence and can only be one of two possibilities, A-B-C or A-C-B, depending on whether you spun the merry-go-round in clockwise or counter-clockwise direction.

   This is not to be confused with the actual spin direction of generators, which drives three phase power on the transmission grid.  Obviously, a generator wouldn't stop and spin in the other direction for the sake of obtaining a different phase sequence.  Phase sequence depends on how phases are marked.  For example, Hosensack Substation in PA is an interface between two different transmission owners, Met-Ed and PP&L.  What is phase marked on one side of the interface is not necessarily phase marked the same on the other.  The conductor that is marked A on one side is marked A on the other.  However, the conductor that is marked B on one side is marked C on the other.  Also, the conductor that is marked C on one side is marked B on the other.  The actual physical conductors and equipment that run through the interface are the same but the labeling of them is what is different.

   Now imagine the merry-go-round again and use to picture how the vector groups of transformers rotate.  Met-Ed assigns phase labels so that an A-B-C phase sequence is always experienced.  Therefore, according to the picture, rotation of the vector groups of the H terminals on a transformer is dictated by the phase sequence of the system that the transformer is connected to.  The picture shows what would happen if A and C phase got rolled.  Although the rotation of the vector group would change, the sine waves are identical.  Therefore, phase would work when comparing the low side voltages of both transformers and customer load would receive the correct phase sequence to ensure that three-phase loads such as motor would spin in the proper direction.  X terminals are not shown because in this example, the secondary voltages of this particular delta-delta transformer are in phase with the primary voltages.

   Article by D Scrobe III