Voltage instrument transformers (VT, sometimes also abbreviated with PT) are used on the primary side of medium- and high voltage distribution transformers.
Transformer operated energy meters need instrument transformers to reduce high voltage- and current values to lower quantities for measurement purpose.
Each conductor wire has a resistance. Resistance means heat development inside the wire. Technically it’s a loss.
Usually the power supply systems are named with single phase, three-phase three wire or three-phase four-wire. The German VDE and the IEC60364-1 have unified the supply systems based on grounding.