No-load loss of a transformer refers to the power consumed by the transformer when the secondary side is open-circuited, and the primary side is supplied with a sinusoidal voltage at rated voltage and rated frequency. Generally, attention is focused on the rated frequency and voltage, while sometimes overlooking tap voltages, voltage waveforms, measurement system accuracy, and testing instruments and equipment. There is confusion regarding calculated values, standard values, measured values, and guaranteed values for losses.
When applying voltage to the primary side with taps, in the case of transformers with constant magnetic flux regulation, the applied voltage should correspond to the tap position on the primary side. For transformers with variable magnetic flux regulation, as the no-load loss varies at each tap position, it is essential to select the correct tap position according to technical requirements and apply the specified rated voltage. In variable magnetic flux regulation, a voltage is consistently applied to each tap position on the primary side.
It is generally required that the applied voltage waveform approximates a sine wave. Therefore, harmonic components in the voltage waveform are analyzed using a harmonic analyzer. Alternatively, a simple method involves using an average voltage meter with a scale for effective values to measure the voltage and compare it with the reading from an effective value voltage meter. If the difference between the two readings exceeds 3%, it indicates that the voltage waveform is not a sine wave, rendering the measured no-load loss invalid according to new standards.
For the measurement system, suitable test circuits, equipment, and instruments must be selected. Due to advancements in magnetic materials, the watts per kilogram of loss have significantly decreased. Manufacturers now use high-quality, high-permeability grain-oriented silicon steel or even non-crystalline alloys as magnetic materials. Structural developments, such as step lap joints and fully inclined no-hole structures, and manufacturing processes, such as non-stacked core processes, have contributed to developing low-loss transformers. Particularly, no-load losses have seen a substantial reduction. Hence, new requirements are imposed on measurement systems. With constant capacity and decreasing no-load losses, it implies a decrease in power factor during no-load operation. A low power factor necessitates changes and modifications to the measurement system by the manufacturer. It is advisable to use the three-wattmeter method with 0.05-0.1 class current transformers and wattmeters with low power factor. Only in this way can measurement accuracy be ensured. At a power factor of 0.01, a phase difference of 1 minute in current transformers can cause a power error of 2.9%. Therefore, the correct selection of the current ratio and voltage ratio of current transformers and voltage transformers is crucial during actual measurement. When the actual current is significantly lower than the rated current of the current transformer, a large phase difference and current error may occur, leading to substantial measurement errors. Therefore, the current drawn by the transformer should be close to the rated current of the current transformer.
In design, the calculated no-load loss is determined based on the specified procedure, referring to the unit loss of the selected silicon steel and the process coefficient. This calculated value is generally referred to as the calculated value. This value should be compared with the standard value specified in the standard or the guaranteed value specified in the contract. The calculated value must be smaller than the standard value or guaranteed value, and there should be no leniency in calculations, especially for transformers produced in large quantities. Additionally, the calculated value is only effective within the design department and has no legal implications. It cannot be used to judge the loss level of the product. The standard specified in the standard or the guaranteed value specified in the contract holds legal significance. Products exceeding the standard value plus the allowable deviation, or the guaranteed value (which equals the standard value plus the allowable deviation), are considered non-compliant. In cases where a loss evaluation system exists, the contract may stipulate fines for exceeding the specified loss values, with the highest penalties for no-load losses. The Transformer magazine, November 1994 issue shows loss evaluation values in European countries. Fines can amount to thousands of dollars per kilowatt. This has legal implications directly tied to economic benefits.