Copper strips in chopper transformer
The purpose of this screen is to reduce the capacitive link between the primary and secondary windings of the transformer. 

The strip forms an open turn and is grounded. 

In the low frequency transformers, such a screen is formed by one layer winding with one end grounded.

You can see the difference between a transformer without and with screen on the following schematic:







simulate this circuit &ndash; Schematic created using CircuitLab