Tap changer for distributed power

United Utilities have identified that the introduction of combined heat and power plant (CHP) into a new estate within their LV distribution network will give rise to the possibility of reversal of power flow through the distribution transformer at periods of low loading with consequent rise in line voltage. Since such a voltage might be outside the statutory limits, a cost effective method of limiting this voltage is essential. The solution identified is to switch between two predefined taps on the distribution transformer at preset times of the day based on historical loading data. Areva T&D have been studying advanced techniques for on-load tap changers (OLTC) resulting in several novel designs being demonstrated in the laboratory. The techniques are principally aimed at OLTC diverters for power transmission and have considered a range of technologies. The present tap changer uses a vacuum switch arrangement combined with a commutation circuit powered from the tap voltage. Modern control principles are required to obtain accurate point on wave switching, however the tap changer is operated from a single armature to give a simple and reliable scheme. The power circuit fits within a casing that integrates with the transformer tank, raising its height by 300 mm. The control is separate from the power circuit and fits into wall-mounted cabinet. Either a preset time setting or the secondary RMS voltage level can initiate a tap change according to a switch setting on the control unit. In both cases care has been taken to prevent the possibility of the tap change operation "hunting". The development is well advanced and should be ready to go into trial service in late 2007.