In order to control a voltage using e.g. a tap transformer (5000), the difference V between the actual voltage (V) and a reference voltage (110) is obtained (120) and integrated over time (130). That integral is then modified by at least one parameter generated by fuzzy logic (3000) from control conditions of the voltage. In this way the fuzzy logic (3000) may find the optimum control strategy which keeps the actual voltage closest to the reference voltage without an excessive number of tap position changes. The reference voltage (110) may vary with time, and therefore the control of the voltage with time may be evaluated (2000), and the control conditions varied accordingly, e.g. by fuzzy logic so that the control arrangement can exhibit learning. Normally, the integral is modified by such that ∫ΔVdt ≧ B when B is the parameter determined by the fuzzy logic (3000), but other control strategies may be used. The present invention is also applicable to control of reactive power.