How to Read ±2 × 2.5% on a Transformer Tap Schedule

On a conventional symmetric schedule, ±2 × 2.5% means two steps above the nominal tapping voltage and two below it, each step equal to 2.5% of nominal. Including the nominal position, that gives five positions. When asking Zisheng Electric to quote a transformer, request the actual voltages alongside the percentages. A five-line table makes a discrepancy easier to spot than a plus sign on a catalogue sheet.

Translate the shorthand into volts

Consider a hypothetical 11,000 V primary winding with the above schedule. One step is 275 V. The winding tapping voltages are shown below in ascending order. These are voltage designations, not instructions to move a switch. The manufacturer’s position numbers and physical sequence must be read from its own approved drawing.

The highest tapping voltage is five percent above nominal. The span between lowest and highest is ten percent of nominal. Recording only “10% range” loses information. A bidder could interpret that as an asymmetric arrangement entirely below nominal. Ask for both endpoints, the nominal position, step size and the winding on which the taps are provided.

Which voltage does the plus sign describe?

In this example the plus sign describes a primary winding tapping voltage. It does not promise a higher secondary output at the same incoming voltage. For a fixed secondary winding, selecting more primary turns gives a lower ideal secondary voltage at unchanged input. The intended use of a higher primary tapping voltage is to accommodate a correspondingly higher supply while maintaining approximately the target secondary voltage.

For example, take an idealized 11,000/400 V ratio. With 11,550 V applied to the 11,550 V tap, the ideal secondary voltage is 400 V. With 11,000 V applied to that same tap it is about 381 V, calculated as 400 × 11,000 / 11,550. Actual loaded terminal voltage depends on impedance, load and power factor. The arithmetic explains the direction of the change. It does not select a field setting or permit operation outside the approved rating.

Schneider Electric’s voltage adjustment tap explanation gives a manufacturer example of the relationship between supply and secondary voltage. Compare the actual position table for the offered equipment before assigning any universal meaning to “position one.”

Separate coverage from the way taps are changed

Two quotations can offer the same five voltages and still serve different operating needs. A de-energized arrangement requires the specified outage and safe isolation. An on-load tap changer is designed to change taps while carrying load within its rating. The tap changer selection guide covers that choice. The percentages alone do not establish which mechanism, controls or operating procedure is included.

Ask whether full rated apparent power is available on every required tap. A percentage schedule establishes the voltage steps. It does not by itself establish current limits or permissible loading at the extreme positions. Obtain the manufacturer’s tapping table with rated voltage, current and power for the required duty. Carry the approved values into the nameplate and rating-record review.

Give the bidder a supply range it can check

State the normal, minimum and maximum incoming voltage on a consistent basis, together with frequency, required secondary voltage and the relevant load cases. Distinguish occasional deviations from sustained operating conditions. If the owner requires a particular secondary voltage band, state where it is measured and under what load. The transformer terminals and a remote motor bus may have different voltages.

For a 110 kV–115 kV power transformer enquiry, give Zisheng Electric the utility voltage schedule and any restrictions on outages. Ask the bidder to return one row per position and identify any project operating case the proposed range cannot cover. Procurement can then resolve the gap before the drawing is released for manufacture.