A hybrid process control system including electrical transmission of power to a sub-sea hydraulic power unit, which in turn provides hydraulic power for control of hydraulic actuators. A circulation system using small bore tubing in the umbilical cord in combination with a traditional topside hydraulic power unit provides for active control of hydraulic fluid quality with respect to contamination caused by the sub-sea hydraulic actuators, especially process gas from down hole safety valves. Thus, a more economical power transmission is achieved without reduction of fluid quality, which is essential to system integrity and reliability. Also, a significant enhancement of power transmission without a dramatic increase in the size of hydraulic supply and return lines is achieved. Fluid environmental issues are reduced to a negligible aspect.