Systems for controlling hydrogen purity in a power plant system are disclosed. In one embodiment, a hydrogen control system for a generator housing includes: a mass flow control system fluidly connected to the generator housing, the mass flow control system being configured to measure a flow rate of a gas mixture extracted from the generator housing and modify the flow rate of the gas mixture across a range of flow rates; and a vent conduit fluidly connected to the mass flow control system, the vent conduit for receiving a portion of the gas mixture from the mass flow control system and feeding the portion of the gas mixture to an outlet.