Onload Tap Changer (OLTC) is used with higher capacity transformers where HT side voltage variation is frequent and a nearly constant LT is required. OLTC is fitted with the transformer itself. Multiple tappings from HV windings are brought to the OLTC chamber and conacted to fixed contacts.

Moving contacts rotates with the help of rotating mechanism having a spindle. This spindle can be rotated manually as well as electrically with a motor. Motor is connected in such a way that it can rotate in both the directions so as to rotate the OLTC contacts in clockwise and anticlock-wise direction.

Two push buttons are fitted on the LCP (local control panel) to rotate the motor and hence the OLTC contacts in clockwise and anti-clockwise direction. This movement of contacts thus controls the output LV voltage of the transformer.

So rotating of OLTC contacts with spindle or push buttons in this way is a manuall process. In case this process of rotating the OLTC contacts and hence controlling the LV side voltage is to be done automatically then a RTCC (Remote Tap Changer Controller) is installed with the transformer HT Panel.

The RTCC sends signals to LCP and LCP in turn rotates the motor as per the signals received from the the RTCC.

The Automatic Voltage Regulator Relay – SAG-E44, is a totally microprocessor based relay that controls the arm movement with the help of high torque motor (OLTC and motor are not parts of the AVR supply). It also indicates the line voltage, line current, load on the transformer, apparent and reactive powers, system frequency and power factor. The AVR can be set to “Auto” or “Manual” mode. In “Auto” mode the AVR controls and maintains the transformer LV voltage as per the pre set value automatically.

It sences the variations in LV voltage for a pre-set time delay and brings the voltage to normal value by changing the tap position of OLTC with the help of high torque motor. In “Manual” mode the LV voltage can be controlled through low/high push buttons which are fitted along with OLTC – LCP (Local Control Panel). Up and down movement of OLTC motor is indicated with the glow of LEDs fitted on the AVR and on the OLTC – LCP.

This AVR can be used with any type of OLTC as it has to control the movement of motor which is connected with the arm that moves the contacts of the OLTC. The LCP is generally fitted near the OLTC or upon the OLTC box while the RTCC is fitted with the HT panel of the transformer.