Power Transformer Dispersion Testing 3

[JBirch]

G'day all,

We have been asked to perform 'dispersion testing' on a power transformer by a client. However, until today we had not even heard of this?

I have tried to search for some idea of what is involved but cannot find any relevent information.

Does anyone know what this is and what it involves?

Thanks in advance.

 

[Zogzog]

dissapation factor testing maybe????

 

[ausphil]

A dispersion test is an old test that was designed by ERA in the UK, which people now know as a recovery voltage test. The difference is that the dispersion test is performed at one charging and discharging time only, whereas recovery voltage tests are performed over a range of changing and discharging times.

It is a measurement that gives an indication of the amount of ionic polarisation in the insulation, which gives a good indication of moisture internal to the insulation.

The difference between DDF (or power factor or dissipation factor or tan delta tests) and dispersion, is that DDF tests tell you resitive losses of the whole of the insulation, including the surface of the material, which can be significant if the surface IR is low, whereas dispersion is only looking at the internal condition of the insulation, and how the molecules are polarised when an electric field is applied.

The test involves applying a dc voltage to the insulation for a specifc time, then removing the test supply and shorting the terminals of the insulation for a specific time. The short is then removed and the "recovery" voltage is measured, together with a value for the leakage time constant of the insulation.

I would think nowadays that a request for a dispersion test would be very rare, and that they would possibly mean a dielectric dissipation factor (DDF) test, or a time domain (recovery voltage) or frequency domain (IDA or PAX) spectroscopy test.

ausphil

 

[prc]

ausphil,you have given an excellent explanation.Let me add a little further.Today there are three dispersion or dielectric response test methods followed for transformers.
RVM (return or recovery voltage measurement ie measurement of recovery voltgae after different times of DC voltage application and discharging 10E-02 to 10E+04 seconds)), FDS -Dielectric frequency domain spectroscopy (measurement of capacitance and dissipation factor from 0.1mHz to 1 kHz), PDC( Dielectric spectroscopy in time domain ie measurement of polarisation and depolarisation currents with same DC voltage application for time durations of 1 to 10000 seconds)All three methods reflect the same fundamental polarisation and conduction phenomena in transformer insulation which consists of oil gaps and solid insulation.Measurements are benchmarked with results on ideal models to find out the moisture content in solid insulation of oil filled power transformers.

Today RVM is not preferred due to certain limitations in interpretation. PDC and FDS are getting popular.Certain companies have come out with models combining both PDC and FDS.

 

[JBirch]

Ah ha. Thankyou very much for the answers!

Given our client has requested dispersion testing be performed only if other testing fails, and the other testing includes DDF (or DLA) I guess they are really asking for either RVM or PDC/FDS.

This is good news as we now know we can indeed to the testing required, despite having never heard of dispersion testing until this week!

Thanks again.

 

[rbulsara]

LPSs and thank you for ausphil and prc.

Rafiq Bulsara

http://www.srengineersct.com