On load tap changers data

 data of On load tap changers : 

- Referred standard and relevant specification

- Insulation level : The following values occurring on all tapping positions of the transformer should be checked against the applicable technical guide.

1- Power-frequency voltage appearing in the tapchanger during test or in service.

2- Impulse voltage appearing on the tap-changer during test on the transformer or in service

- Highest phase service voltage across the regulating winding

Example

115 kV ± 16 % regulating range Y connected, gives the parameter in question as

115 · 0.16 / √3= 10.6 kV

This value has to be compared with highest permissible phase service voltage across the regulating winding
detailed in the applicable technical guide

parameter may in some rare cases be a limiting factor.

- Rated through current

The rated through current of the tap-changer should be not less than that resulting from the highest value of
tapping current of the tapped winding of the transformer.

- Occasional overloading

- Short-circuit current strength
The tap-changer short-circuit current is given in the applicable technical guide.

- Phase step voltage

If the step voltage varies over the range, full details together with associated currents is needed to calculate
the maximum breaking capacity. Limit values are given in the applicable technical guide.

- Tapping range and tapping arrangement

The number of tapping positions has to be considered when selecting type of tap-changer

- Recovery voltage

If the service voltage and the winding capacitance exceed certain limit values stated in the technical guide, tie-in resistors have to be used in order to avoid difficulties with regard to dielectric stresses within the change-over selector in the tap-changer.

- Contact life

The predicted contact life is a function of the rated through current and has to be compared with the requirements.

- Temperature conditions

The temperature of the oil or ambient air temperature surrounding the tap-changer can be in the range between –25 °C and +105 °C during normal service without any special precautions. The range can be extended to –40 °C provided the oil viscosity is between 200 to 800 mm2/s (cst).