9.9  POWER ISOLATION TRANSFORMERS (Grounding and Shielding Techniques by R. Morrison)

The words isolation transformer imply a solution to a problem. Just adding a transformer to a problem rarely works. The problem and the solution must match. An isolation transformer used in instrumentation usually has one or more electrostatic shields wrapped around internal coils. This transformer is inside of the equipment and is not considered a part of the facility. These shields are connected to various points within the instrument and can control the flow of unwanted current (see Section 5.6). When an isolation transformer is used to power a group of instruments, then some new issues arise. The transformer becomes a part of the facility, and the role of the shields is quite different.
   The transformers and associated circuits that are described in this section can be single-phase or three-phase. The single-phase circuit is used as an example because it is simpler to draw. The single shield is shown in Figure 9.1. This transformer is ideally mounted on a ground plane. The single shield is internally connected to the transformer frame. When the transformer is mounted, the shield is connected. The primary and secondary conductors are carried in conduit. The conduit bonds to the transformer frame and is considered a part of the equipment grounding system.
   Transformer action involves the difference signal between the power conductors. There can also be a common-mode signal (interference) on the primary transmission line. The shield in Figure 9.1 reflects any common- mode signals and stops them from coupling to the secondary circuits. Without the shield, capacitance couples common-mode signals directly from the primary to the secondary circuits. In a typical isolation transformer the leakage capacitance out of the shield is about 5 pF. With a shield, most of the common-mode current returns inside the primary conduit and never sees the secondary circuits.
   Some of the common-mode current flows in coils of the primary .This current flow appears in the secondary by transformer action. To limit this effect, a second shield can be added to the transformer. This shield is internally connected to one side of the primary coil. Common-mode current now flows between the shields not in the coils of the primary. This added shield is shown in Figure 9.2.

   If the secondary loads generate any common-mode signals, it is desirable to keep these signals from propagating through the transformer other systems. This path can be closed by adding a third shield that is again connected to one side of the secondary coil.
   The power isolation transformer has internal shields that are not available  to the user. If they are brought out separately, they should be connected at the transformer. Connections to remote grounds are too inductive. These connections make no electrical sense and they violate the isolation processes.