Webinar archive - Transformer testing

A collection of past webinars on transformer testing

Megger provides thoughtfully equipped advanced transformer test instruments for commissioning, routine, and diagnostic needs. There are several design characteristics (dielectric, mechanical, electro-magnetic and thermal) and internal or external components in a power transformer which require specific testing procedures to assess transformer's overall health. Every single test in an individual component has characteristic strengths. Therefore, the individual tests are complementary one to each other and together provide a more robust representation of the component's health. Many of the electrical field tests appearing in these suggested groups provide assessment information for several transformer components at once. So, when the individual component test lists are merged, a thoughtful and overall smart test recommendation for transformers emerges. As some transformer tests are better suited as commissioning and diagnostic tests, the reason for testing (e.g., commissioning, routine screening, and diagnostic test situations) will influence the electrical test recommendation for transformers. Transformers should be tested periodically through their lifetime. However, limitless testing is not a good use of budget and manpower and not necessary. Some tests are well suited as screening tools while others make sense only under suspicion of specific problems.

By watching our webinar library, you will learn about numerous transformer tests:

DC Insulation

A high DC voltage (typically from 1 kV to 10 kV) is applied to the winding under test. The leakage current is measured, and the insulation resistance calculated. The measurement cannot be compared to AC losses. IR measurement is temperature dependent.

Core ground test

The test measures the insulation resistance of the core ground connection to be disconnected. The test voltage should not exceed 1000 VDC.

Turns ratio test

A single-phase or three-phase LV AC source is connected to each phase of the HV winding or LV winding. Voltage across HV and LV windings are measured and ratio calculated.

Magnetic balance

The test is performed on three phase transformers to find out the magnetization status of the core. In this test, a LV AC signal is applied to a winding of one phase with all other windings kept open circuited. Voltages induced in the corresponding windings of other two phases are measured.

Winding resistance test

A DC current is injected into the winding(s) to achieve core saturation. Test current and voltage across the winding(s) are measured and resistance is calculated. Demagnetization is required after the winding resistance test.

Leakage reactance

An AC source is used to drive a current through the impedance of a winding. The source is connected to each phase of a winding with the opposite winding short circuited. Excitation voltage and total current across the impedance are measured simultaneously to determine the leakage impedance given by the ratio of the measured voltage and current.

Dynamic resistance measurement

This is a test in which DC current is injected through a winding while an OLTC moves through all its positions. Current signatures are recorded as well as voltages measured on both windings. The options are measurement of Dynamic Current, Dynamic Voltage or “True” Dynamic Resistance.

Frequency response of stray losses

An AC source with variable frequency (typically 20 Hz to 400 Hz) is connected as for a Leakage Reactance test. The stray losses are represented by the AC resistance of the short circuit impedance. 

Frequency response analysis

A LV sinusoidal AC source with variable frequency (20 Hz - 2 MHz) is connected successively to each phase of the HV and LV windings. FRA consists of measuring the admittance or impedance of the transformer/s RLC network. In all FRA measurements, grounding has a significant effect on test results. Types of measurement include open circuit, short-circuit, capacitive inter-winding, and inductive inter-winding.

Exciting / Magnetization current

An AC source is connected to each phase of the HV windings with all other windings floating. The source voltage and the source current are measured, as well as the phase angle.

Capacitance and DF/PF

A variable frequency AC source is connected across windings and ground, and between windings to apply an electric field to each insulating section of the transformer. The measurement bridge of the instrument and its components can determine the capacitance value as well as the loss factor of the insulation under test. Technology allows for line frequency (50/60 Hz) and 1 Hz assessment.

Dielectric frequency response (frequency domain spectroscopy)

A variable-frequency (1 mHz - 1 kHz typical range) AC source is connected across windings and ground, and between windings to apply an electric field to each insulating section of the transformer. Capacitance, PF (DF or tan delta), and/or permittivity vs frequency are plotted and compared against a model database to estimate moisture concentration in the solid insulation and conductivity of liquid insulation.

Polarization/Depolarisation current

A DC voltage is applied across the insulation during a certain time to measure the polarization current. Then the insulation is short-circuited, and the current is measured again (depolarization current).

Dielectric breakdown

An AC voltage is applied to electrodes with a set gap between them, across the liquid insulation, to measure the dielectric strength of the insulating liquid. 

Oil tan delta and resistivity

Tan delta, an AC voltage is applied across the inner and outer electrodes to measure the dielectric losses within the insulating liquid. Resistivity – A DC voltage is applied to measure how strongly an insulating liquid opposes the flow of electric current. 

Electrical Detection of PD

The test method outlined in IEC 60270 (<1MHz) or in frequencies up to some tenth of MHz is a useful tool to detect most internal PD signals on transformers. Modern PD measuring systems can be used to separate PD sources from each other and from external electromagnetic noise (interference).

Acoustical Detection of PD

This method is used to locate (pinpoint) PD during offline tests (SAT or FAT). It is a complementary measurement method to the electrical (HF) or electromagnetic (UHF) methods that needs to be combined with one of both. 

UHF detection of PD

PD measurement methods based on higher measurement frequencies can be applied in strongly disturbed environments to detect internal PD. Limitations are given for PD defects deep inside of windings or hidden in shielded spots.