Transformer phase displacement measurement

Transformers phase displacement measurement for vector group verification

A three-phase transformer consists of three primary and three secondary windings. Each winding corresponds to a phase of the three-phase power system. When three-phase voltage is applied, the primary windings generate a magnetic field, which induces voltage in the secondary windings. The specific phase angle results from the winding configuration and the relative physical arrangement of the windings within the core. Depending on the installation, each arrangement can offer specific advantages and disadvantages. The resulting vector group of a transformer is determined through phase displacement measurements.

For a transformer vector group test, three-phase voltage is applied to the primary winding. Voltage and phase displacement are measured on the secondary winding. The measured phase displacement, together with the winding connection, confirms the correct specified vector group.

Reading vector group verification results

A transformer vector group designation consists of:

• A capital letter indicating the configuration of a high-voltage winding
• A lowercase letter indicating the configuration of a low-voltage winding
• A lowercase n indicating accessible neutral point on the low-voltage side
• A number representing the phase shift angle between the high-voltage and low-voltage sides, represented using clock notation, with each hour past 12 on the clock representing a 30-degree lag

As an example, here’s vector group name “Dyn11” explained in detail:

• “D” for high-voltage Delta connection
• “y” for low-voltage Star/Wye connection
• “n” for accessible neutral point
• “11” for 330° (or −30°) phase shift (1 hour before 12 on the clockface)

Dyn11 is the most common type of three-phase transformer, and it’s widely used in energy networks. Following phase displacement measurement and vector group verification, a transformer can safely operate in parallel with transformers with the same vector group. For example, Dyn11 can be operated in parallel with other Dyn11-certified transformers, if the other requirements for parallel operation are also fulfilled.

Risks of mismatched transformer

The phase angle difference between transformers with mismatched vector groups cannot be compensated. If they are operated in parallel, differences in phase displacement create circulating currents between the transformers. These currents can become considerably high, potentially resulting in excessive heating, increased losses, protection trips and equipment damage.

Your transformer vector group testing challenges

Power networks and industries routinely rely on multiple three-phase transformers running in parallel. This standard procedure is cost efficient and allows for flexible expansion, reduction or adaptation of capacities. Accurate phase displacement measurement and vector group verification are essential to ensure transformers are in sync.

Transformer vector group verification can be achieved by applying voltage to the primary winding and measuring the voltage and phase displacement on the secondary winding with a phase-angle meter, voltmeter or oscilloscope. The clock notation method or phase vector diagram then requires manual analysis to identify winding configuration and phase angle to confirm the vector group.

Standard power analyzers cannot directly determine this phase displacement. This is because they synchronize the primary-side and secondary-side channels independently, each to just one voltage or current signal from its respective three-phase system. Without a shared timing reference between the two sides, it is not possible for the analyzer to directly compare their phase angles.

Our solution for transformation vector group testing

An advanced precision power analyzer like the LMG671 measures phase displacement between primary and secondary windings while simultaneously determining and displaying a transformer’s vector group. With its high-resolution harmonic analysis, it displays the measured current, voltage and power harmonics (both in RMS value and phase angle). For checking signal quality and phase symmetry, it delivers a graphical representation of voltages and currents in a vector diagram for the respective harmonic order. The fundamental frequency is the most important for vector group verification.

It also shows phase displacement during direct, comprehensive efficiency measurements. It does this using six power measurement channels and does not require any additional rewiring. The LMG671 has unique script that directly calculates phase displacement based on the fundamental phase angles obtained from the aliasing-free harmonic analysis.

Vector group verification through one phase displacement measurement eliminates the need for manual calculations from separate measurements. Multi-channel testing enables sophisticated and time-efficient measurements, resulting in reliable data. This comprehensive measurement setup provides immediate verification of phase quality and phase relationships in real time.

Our power transformer testing solution is customizable, reliable and both application- and user-oriented. Manufacturers and test laboratories rely on it for its precise and comprehensive no-load loss measurement and analysis.