Voltage control

A dynamic voltage restorer circuit 14 comprises capacitive storage means 16, switch means 18 and control means 20. The switch means 18 includes an array of anti-parallel diodes 34 and switchable uni-directional devices 36. Appropriate control of the switchable devices 36 by a control 20 allows the capacitor 16 to be connected in series into the power line 24a,24b, with either polarity, to inject stored energy into the line, thereby restoring the voltage.