I'm sorry could you provide more details such as you are in the USA and have purchased a german made 400 watt lighting system to be used on American voltages? You mention a voltage converter? Are you sure this isn't the remote ballast? My guess also that this is a single bulb HPS 400 watt grow light?
It could be a switchable ballast which is why it has the black wire but it is hard to say.
The plug with those wiring colors would be a schuko plug like this...... brown is hot, blue is neutral and green is earth.