What is the Voltage Ratio?

The voltage ratio indicated on the nameplate of a transformer refers to the voltage ratio of the transformer under no-load conditions. It serves as a parameter used by the manufacturing factory to assess whether the number of turns in the winding meets the specified requirements. The actual voltage ratio during operation is influenced by the type of load (capacitive or inductive), the magnitude of the load current, and the parameters of the transformer itself, such as resistance and inductance. It is essential to emphasize that the voltage ratio is a concept related to the no-load voltage ratio, not the voltage ratio during actual operation.

When indicating the voltage ratio, for three-phase transformers, the ratio of line voltages should be marked. For example, a Yd-connected transformer can be labeled as 121,000V/10,500V. In the case of tap changes, it can be marked as (110,000±8×1.25%)V/10,500V. The specified voltage values are all in terms of line voltage, and the unit can be expressed in volts or kilovolts. However, for single-phase transformers, the marking is not in terms of line voltage but rather phase voltage. The phase voltage for Y-connected transformers must be marked by dividing the line voltage by √3.

The concept of voltage ratio involves the primary side being Y-connected or YN-connected when forming a three-phase group, while the secondary side is connected in a delta (d) configuration. Therefore, the voltage ratio must be marked according to regulations under no-load conditions. For three-winding transformers, the high voltage should be marked first, followed by the medium voltage, and finally, the low voltage.

When there are positive and negative ranges for tap changes, they can be separately marked, for example,：

(110,000±8×1.25%)V/(35,000+}×2.5%)V/10,500V.

The specific value of the tap voltage is a parameter for determining the number of turns in the tap winding. Therefore, the tap voltage can be higher than the system's maximum voltage (U). For example, taking 121,000±5%V as an illustration, 121,000+5% equals 127,050V, which is greater than the U of the 110kV level (=126kV). This indicates that during operation, if the transformer is at the maximum tap position, the tap voltage is 127,050V, and the voltages applied to various tap positions on the transformer should not exceed 126,000V. At the maximum tap position, the transformer operates in an underexcited state, while at the minimum tap position (minimum tap voltage of 114,950V), it operates in an overexcited state. Overexcitation should comply with standard regulations, allowing for a 10% overexcitation during long-term no-load and a 5% overexcitation during long-term full load. Otherwise, the tap position should be changed to meet the overexcitation requirements, or the voltages applied to various tap positions should be adjusted.

The voltage ratio may not necessarily be equal to the turns ratio; the turns ratio is the measured voltage ratio, and after adding the allowable deviation specified in the standard, it should comply with the voltage ratio. Therefore, the voltage ratio should also be the nominal voltage ratio under no-load conditions. The difference between the measured voltage ratio and the specified allowable deviation should meet the standards.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

Know more about power transformer and reactor :www.lstransformer.com