Excessive swing of the feedwater (31) in power supply apparatus on the occurrence of a transient is suppressed by injecting an anticipatory compensating signal (ΔW fw ) into the control for the feedwater. Typical overshoot occurs on removal of a large part of the load, the steam flow is reduced so that the conventional control system reduces the flow of feedwater. At the same time there is a reduction of feedwater level in the steam generator (13.15) because of the collapse of the bubbles (125) under increased steam pressure. By the time the control responds to the drop in level, the apparatus has begun to stabilize so that there is overshoot. The anticipatory signal is derived from the boiling power (BP) which is a function of the nuclear power (2N) developed, the enthalpy of saturated water (h . ) and the enthalpy of the feedwater injected into the steam generator (h fw ). From the boiling power (BP) and the increment in steam pressure resulting from the transient an anticipatory increment of feedwater flow is derived. Thus increment is added to the other parameters controlling the feedwater.