Understanding Transformer Parameters: The Per-Unit System and Testing Methods

Transformers are critical components in electrical power systems, and understanding their parameters is essential for effective operation and maintenance. One of the key concepts in transformer analysis is the per-unit system, which simplifies the comparison of transformer characteristics across different voltage and power ratings. For instance, if a transformer with a ratio of 1000/100 V has a per-unit equivalent impedance of 0.1, the voltage drop across the leakage impedance on the low-voltage side would be 10 volts, while the corresponding drop on the high-voltage side would be 100 volts. This method allows for clearer insights into the transformer's behavior without needing to convert between different units.

In addition to simplifying calculations, the per-unit system also provides a standardized method for expressing transformer impedances. This is particularly useful when comparing transformers of varying sizes and ratings, as their impedances typically fall within a small range of per-unit values, even though their actual ohmic values may differ significantly. For large power transformers, base values are often expressed in kilovolt (kV) for voltage and megavolt-amperes (MVA) for power, enabling consistent calculations of base impedances.

To determine the parameters of a transformer’s equivalent circuit, two essential tests are typically employed: the open-circuit test and the short-circuit test. The open-circuit test is conducted by applying the rated voltage to one winding while keeping the other winding open. This test primarily measures the shunt parameters of the transformer. The no-load current during this test is usually a small percentage of the full load current—ranging from 0.2% to 2%—with larger transformers exhibiting even lower percentages.

During the open-circuit test, the wattmeter reading primarily reflects core losses, as the primary winding's ohmic losses are negligible in comparison. The equivalent circuit simplifies under these conditions, allowing for accurate calculations of the core loss and shunt parameters. The no-load power factor can also be calculated, providing further insight into the transformer’s performance when not under load.

The short-circuit test serves a different purpose by evaluating the transformer's impedance under load conditions. By applying voltage to one winding while shorting the other, this test measures the equivalent circuit parameters associated with the load. The results from both tests enable engineers to analyze transformer efficiency, voltage regulation, and overall performance within the power system.

Understanding the per-unit system and the methods for conducting open-circuit and short-circuit tests equips engineers and technicians with the tools necessary for effective transformer analysis and optimization, ensuring reliable operation in electrical power systems.