The output of power transformers may vary according to the load and the on-load tap changer (OLTC) compensates these variations. The protection & control system of a substations includes a voltage regulation function that monitors the voltage level at the output of the power transformer and is connected to the OLTC to increase/decrease this output voltage when the value is out of the programmed limits.
Control devices in substations always have to monitor the position of the OLTC of any power transformer and provide this information to the local SCADA system. Considering the total number of taps in OLTCs (usually, more than 20), several digital inputs of the control device have to be used to perform this task.
However, ABB factory in Sweden (Ludvika) is using by default Weidmüller’s Analogue Signal Conditioner (ACT20P) to provide this information because with just one analogue signal they can indicate to the control device the OLTC position independently of the number of taps.
The ACT20P measures the tap position via a resistor value (potentiometer) with high accuracy (a tap change means a small change in the value of the resistance measured) and the information is converted in a 4-20mA signal that is sent to the control device. Besides the high accuracy in the measurement, considering that the ACT20P is installed on the transformer, the other key reasons why ABB approved the Weidmülelr solution for this application were the robust universal power supply as well as the high 3-way isolation that ensures reliable and long term functionality.