Open-circuit testing of transformers

No-load test transformer setup

To ensure highest efficiency and minimum power loss, transformers are tested prior to series production to assess whether they are suitable for their intended applications (type testing) and again after manufacturing to confirm their quality, reliability and compliance with applicable normative and regulatory requirements (acceptance testing).

First, the no-load loss test measures iron losses in a power transformer when operating under realistic conditions without any load connected. The power analyzer is connected on the primary and high-voltage sides of the transformer, while the secondary side remains open. This is why the no-load loss test is also referred to as the open-circuit test. Precise transformer iron loss measurement is a fundamental part of every type and acceptance test.

Iron loss vs. copper loss: Transformer short-circuit testing

Following the no-load test, a short-circuit test follows an inverse approach to obtain a transformer copper loss measurement. The primary side of the power transformer is connected to the power analyzer, while the secondary side (all phases) is short-circuited.

Learn more about transformer short-circuit testing.

No-load current, corrected power and iron loss measurement in power transformer tests

No-load losses are approximately constant and occur whenever a power transformer is energized. To quantify these losses, multi-channel testing captures the following in a single measurement setup, for each phase and the overall system:

• Power: Active power and loss measurement determines the transformer’s iron loss.
• Current: Magnetizing current measurement characterizes the current needed to build and maintain the magnetic flux in the core.
• Voltage: Test conditions are verified under nominal voltage operation.
• Frequency: Frequency at the test operating point is measured to enable corrected power determination, taking the influences of the test environment into account.

Reasons for iron loss measurement

Separated core losses measured during an open-circuit test can mainly be traced back to hysteresis loss and eddy current loss:

• Hysteresis loss is caused by magnetic friction generating heat as the alternating current reverses direction, repeatedly magnetizing and demagnetizing the core. An inferior steel core with increased hysteresis can increase power loss. High-performance electrical steel shows higher electrical resistivity, higher magnetic permeability and lower core losses. The core design and frequency of the supplied alternating current supply can also affect hysteresis loss.
• Eddy current loss is caused by circulating currents induced in the core laminations generating heat. Thick laminations increase eddy current loss, while thinner laminations lower power loss. Insulation between the laminations should be adequate, and damaged insulation parts must be eliminated for optimal performance. An optimized flux path also improves magnetic efficiency and helps to lower iron loss.

Challenges in transformer no-load testing

Transformer technology is constantly evolving and the corresponding regulations are updated regularly. When open-circuit testing transformers, manufacturers and test laboratories are faced with a series of challenges:

• Acquiring all required electrical parameters for advanced testing: Power, current, voltage and frequency should ideally be captured by one precise, multi-channel, high-functioning instrument in a single measurement setup.
• Complying with standards from the International Electrotechnical Commission (e.g., IEC 60076): The international series of standards for power transformers specifies measurement methods, rules and tolerances that transformer testing must adhere to.
• Accuracy of low power factor measurement (amplitude accuracy and phase accuracy): Low current and low power factor during no-load testing amplify the influence of error tolerances and therefore require high instrument accuracy.
• Corrected power measurement: Standardized and comparable measurement values must account for test environment influences as well as deviations from nominal supply conditions.
• Compatible data export and audit-compliant test documentation: The measured parameters need to be stored, processed and exported in traceable formats compatible with third-party applications and reporting .

Our power measurement solution for transformer no-load testing

The LMG671 precision power analyzer, with its S-type current channels, delivers accurate and efficient testing across the full transformer test sequence. During open-circuit (no-load) tests, when power factor is low, it maintains measurement accuracy. During short-circuit tests, where currents are high, it maintains accuracy at high power - all without needing to trade off one capability for the other.

The LMG671 combines the mandatory functionalities and analysis capabilities of a precision power meter, an oscilloscope, a digital multimeter and a data logger - all in one instrument. It is a multifunctional power analyzer that provides flexible multi-channel measurement and delivers standards-compliant data.

• Full connectivity: Up to seven slots equippable with power measurement channels and/or process signal interfaces (PSI) to measure the power of all phases
• Additional sensor signals: A PSI integration for collecting analog and/or digital signals relevant to the operating point, such as ambient and unit-under-test conditions including temperature, humidity and vibration
• Low power factor measurement accuracy: Nanosecond-level internal timing adjustment between current and voltage measurement inputs across all channels for unambiguous and highly accurate power measurements even at extremely low power factors down to ≤ 0.001
• Power measurement tolerances: Full power factor range coverage from PF = 0 to 1, avoiding the complex error terms or hidden error components that can otherwise arise under low-power-factor conditions
• Customizable application-oriented measurement menu: Comprehensive overviews with customized measurement menus that consolidate all numerically or visually available measurement qualities
• Built-in star-delta computation: Accurate delta-to-star voltage conversion, delivering correct individual phase powers via the software option L6-OPT-SDC.
• Standardization: Precise, reliable and comparable test results in accordance with IEC 60076