Power Transformer – Principle, Function & Testing

September 12, 2023
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Author: Engr. Jahangir Amir

A transformer is an electrical device used to transfer electrical energy between two or more coils of wire through electromagnetic induction. It operates on the principle of electromagnetic induction and is primarily used for voltage transformation and electrical isolation. Transformers play a crucial role in the transmission and distribution of electrical power. Here’s an overview of the principle, function and Testing of a transformer


The operation of a transformer is based on Faraday’s law of electromagnetic induction and the principle of mutual inductance. Here’s how it works:

  1. Two Coils: A transformer consists of two coils of wire wound around a common iron core. These coils are known as the primary winding and the secondary winding.
  2. Alternating Current (AC): When an alternating current (AC) flows through the primary winding, it creates an alternating magnetic field in the iron core.
  3. Mutual Induction: This changing magnetic field induces an electromotive force (EMF) or voltage in the secondary winding, according to Faraday’s law of electromagnetic induction. The induced voltage in the secondary winding depends on the turns ratio between the primary and secondary windings.
  4. Voltage Transformation: The turns ratio of the windings determines the voltage transformation. If the secondary winding has more turns than the primary, the transformer steps up the voltage (voltage is increased). Conversely, if the secondary winding has fewer turns than the primary, the transformer steps down the voltage (voltage is decreased).
  5. Power Conservation: The power (product of voltage and current) on the primary side is approximately equal to the power on the secondary side, neglecting losses. This conservation of power ensures that power is transferred efficiently from the primary to the secondary.


  1. Voltage Transformation: The primary function of a transformer is to change the voltage level of an alternating current (AC) electrical supply. It can step up (increase) or step down (decrease) the voltage as needed. This voltage transformation is essential for transmitting electricity efficiently over long distances and for providing different voltage levels for various applications.
  2. Electrical Isolation: Transformers provide electrical isolation between the input (primary) and output (secondary) windings. This isolation ensures that there is no direct electrical connection between the two sides, which can be crucial for safety and preventing electrical faults.
  3. Power Transfer: Transformers enable efficient power transfer from one circuit to another without the need for direct electrical connections. They allow the transmission of electrical energy at higher voltages over long distances with reduced losses, making long-distance power distribution more economical.


Testing of power transformers is essential to ensure their proper functioning, reliability, and safety in electrical power systems. These tests are conducted during the manufacturing process, installation, and periodic maintenance to detect any defects, verify performance specifications, and assess insulation integrity. Here are some of the common tests conducted on power transformers:

1. Routine Tests:

Routine tests are performed on every power transformer to check its basic electrical parameters and confirm that it meets design specifications. These tests include:

  • Transformer Turns Ratio Test: This test verifies the turns ratio between the primary and secondary windings, ensuring that it matches the design specifications.
  • Winding Resistance Measurement: It measures the resistance of the transformer windings to ensure that it is within acceptable limits, which helps identify any loose connections or damage.
  • Open-Circuit and Short-Circuit Tests: These tests determine the core and copper losses of the transformer, which are crucial for assessing its efficiency.
  • Polarity Test: The polarity test ensures that the transformer’s primary and secondary windings are correctly connected.
  • Vector Group Determination: It identifies the vector group of the transformer, which is important for proper parallel operation with other transformers.

2. Type Tests:

Type tests are performed on a few representative units of a transformer design to confirm its ability to withstand various abnormal conditions. These tests include:

  • Impulse Voltage Withstand Test: This test evaluates the transformer’s ability to withstand lightning strikes and other voltage surges by applying high-voltage impulses to the windings.
  • Partial Discharge Test: It checks for partial discharges in the insulation, which can indicate insulation defects. This test is essential for assessing long-term insulation performance.
  • Temperature Rise Test: This test measures the temperature rise of the transformer’s windings and oil under full-load conditions to ensure that it operates within safe temperature limits.

3. Special Tests:

Special tests may be conducted based on specific requirements or unique operating conditions:

  • Short-Circuit Withstand Test: This test assesses the transformer’s ability to withstand short-circuit currents, simulating a fault condition. It helps determine the transformer’s short-circuit impedance.
  • Noise Level Measurement: Transformers can produce audible noise due to vibrations in the core and windings. Noise level measurements help ensure that the transformer meets acceptable noise criteria.
  • Load Loss and No-Load Loss Measurement: These measurements provide detailed information about the transformer’s losses under load and no-load conditions, aiding in assessing efficiency.

4. Insulation Tests:

  • Insulation Resistance Test: This test measures the resistance of the insulation between windings and between the windings and the transformer’s core. It helps identify insulation weaknesses.
  • Dielectric Withstand Test: Also known as the high-potential (hipot) test, it checks the dielectric strength of the insulation by applying a high voltage for a specified duration.

5. Oil Tests:

  • Dissolved Gas Analysis (DGA): DGA monitors the gases dissolved in the transformer oil. An increase in specific gases, such as methane and ethylene, can indicate faults or overheating in the transformer.

These tests are crucial for ensuring the reliability and safety of power transformers in electrical networks. Regular maintenance tests are also conducted during the transformer’s service life to assess its condition and plan for maintenance or replacement as needed. Proper testing and maintenance contribute to the longevity and efficiency of power transformers in electrical power systems.

In summary, transformers are vital electrical devices that facilitate voltage transformation and electrical isolation using electromagnetic induction. They are widely used in electrical power generation, transmission, and distribution, as well as in various industrial and commercial applications to meet different voltage requirements.

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