An arrangement for cooling a high voltage converter including a major loop with a pump for making a coolant liquid to pass power semiconductor devices of the converter and a heat exchanger for lowering the temperature of the coolant liquid before passing the power semiconductor devices again. An extra loop is connected to the major loop. The extra loop has a cooling apparatus containing a volume of a cooling medium and adapted to cool the medium to a temperature substantially lower than the temperature of the coolant liquid after having passed the heat exchanger in the major loop. A control unit is adapted to divert at least a part of the coolant liquid to flow through the extra loop when the need of cooling the power semiconductor devices of the converter is extremely high.