The OMICRON DIRANA is a dielectric response analyzer that determines moisture in the paper and pressboard of transformers without taking oil samples. It combines frequency domain spectroscopy (FDS) with a time domain measurement (PDC+) and also delivers oil conductivity, power factor, capacitance and insulation resistance.
What it is and where it is used
The OMICRON DIRANA is a dielectric response instrument designed to find out how much water is in the paper and pressboard insulation of a transformer. Moisture is one of the parameters that most affects the remaining life of a transformer: it comes from the aging of the paper itself, or enters through leaking seals and breathing, and it lowers dielectric strength and speeds up aging. Knowing it allows condition-based maintenance and shows whether a new transformer was really left dry.
It is used on substation and industrial power transformers at medium, high and extra-high voltage. The technique itself, what it measures and how it is interpreted, is in the guide on dielectric frequency response. This guide describes the instrument. We use the DIRANA often in our transformer tests.
Variants and scope
| Application | What the DIRANA does | Note |
|---|---|---|
| Power transformers | Moisture in cellulose and oil conductivity | With mineral oil, natural ester or synthetic ester, and also on units filled with air or under vacuum |
| Instrument transformers | The same results | The manufacturer lists them among the most tested assets |
| Bushings | Diagnosis by dielectric response | The manufacturer lists it as a common use |
| Rotating machines, resin-bonded paper bushings and cables | Dielectric measurement for diagnosis | Additional application stated by OMICRON |
There are no model variants under another name: the DIRANA is offered as a single instrument, operated with the Primary Test Manager software.
How it measures
The DIRANA combines two methods and takes the best of each:
- Frequency domain spectroscopy (FDS): measures the insulation power factor over a wide frequency band. It is fast and accurate at high frequencies, down to 0.1 Hz, but slow at low ones.
- Polarization and depolarization current (PDC+): uses a DC voltage step to measure all frequencies at once, although it only works for low frequencies. The ”+” stands for an advanced version that shortens the time and improves noise immunity.
The instrument uses FDS for the high part of the spectrum and PDC+ for frequencies below 0.1 Hz, and it determines the frequency range of each measurement automatically, so exactly what is needed is measured. OMICRON states a minimum frequency of 10 µHz, the lowest available for transformer analysis according to the manufacturer. It also states times of 30 minutes to 1 hour 45 minutes to sweep from 1 kHz to 10 µHz, compared with more than a day with conventional FDS.
The resulting curve carries useful information: the very low frequencies reflect the moisture of the solid insulation, and the position of the slope in the mid range indicates the conductivity of the liquid. The software compares the curve with model curves and calculates the water content of the cellulose. The manufacturer states that the method is endorsed by CIGRE.
Results it delivers
A single measurement delivers, according to the manufacturer’s brochure:
- the water content of the paper and the conductivity of the oil;
- the power factor or dissipation factor at power frequency;
- the capacitance;
- the insulation resistance and the polarization index (PI) and dielectric absorption ratio (DAR).
This makes it possible to tell whether a high power factor value is due to water, to deteriorated oil or to the bushings, or to other factors such as soot, corrosive sulfur or carbonized spots. The assessment follows the moisture categories of IEC 60422, the guide for supervision and maintenance of mineral insulating oils in electrical equipment, or user-defined criteria. OMICRON states that the DIRANA compensates for the influence of aging by-products so as not to overestimate the water in old transformers.
Instrument characteristics
Data stated by OMICRON in its brochure:
| Item | Stated value |
|---|---|
| Measurement voltage | 200 V peak |
| Maximum DC output current | 50 mA peak |
| PDC+ measurement | ±10 mA range |
| Mass of the unit / of the complete case | 2.3 kg / 20 kg |
| Supply | 85 to 265 V, 50 to 60 Hz |
| Operating temperature | −10 °C to +55 °C |
The set comes in a single case with built-in clamps and guard connections. On a power transformer, the output is connected to the high-voltage winding, the input to the low-voltage one and the guard to the tank, to measure the insulation between the two windings. The parameters are set automatically from the nameplate data and the user only enters the oil temperature.
Software and integration
The DIRANA is operated with Primary Test Manager (PTM). It provides a database of locations, assets and jobs, test templates and connection diagrams. It also offers automatic moisture and oil analysis, comparison between measurements of the same asset or of others, and reports with the company logo. PTM also manages other transformer tests, such as SFRA, leakage reactance, excitation current and winding resistance. Measurements made with the earlier software can be imported.
Precautions and errors that skew the reading
- Oil temperature entered wrongly. It is the only data the user must provide and the analysis depends on it; take it with the transformer stable, not just energized or just de-energized.
- Connection with stray leakage currents. The shields and the guard exist to divert unwanted capacitive and resistive currents; a badly connected guard alters the power factor and the capacitance.
- Substation interference. The brochure states an allowable 50/60 Hz interference of up to 10 mA (signal-to-noise ratio 1:10); a measurement in an energized yard may take longer or have to be repeated.
- Comparing under different conditions. Trends between measurements are read with the same connection method and with temperature-corrected values.
- Heavily aged oil. Aging by-products shift the curve and can be mistaken for moisture; the software compensates for them, but the result is checked against the oil analysis.
Frequently asked technical questions
- Manual and software: the manual and the updates are provided by OMICRON to its customers; this sheet is limited to what the manufacturer states in its public brochure.
- Effect of temperature: the analysis compensates for temperature, insulation geometry, oil conductivity and aging by-products. Even so, the oil temperature entered must be reliable.
- When it is done: at commissioning of a new transformer, to check that it is dry; in maintenance, to decide whether drying or an oil change is needed; and after events that may have introduced moisture.
- How it relates to gas analysis: moisture and dissolved gases answer different questions; see insulating oil analysis and the free dissolved gas interpretation tool.
How we help
Do you need this test? We do it for you: based on your case, we recommend the right test and equipment and include it in the quote. That way you get the test your case really needs, without paying for extra tests. We test in Colombia, or elsewhere on request.
See the scope in power transformer testing and in testing and commissioning, or tell us about your case through contact.
Related guides
- Dielectric frequency response test
- Power factor and tan delta test
- Insulating oil analysis
- Insulation resistance and polarization index test
- Power transformer commissioning
- OMICRON FRAnalyzer and FRANEO 800: frequency response
References
- IEC 60422, guide for supervision and maintenance of mineral insulating oils in electrical equipment.
- IEEE Std C57.161-2018, Guide for Dielectric Frequency Response Test.
- CIGRE Technical Brochure 254 (2004), Dielectric response methods for diagnostics of power transformers.
- CIGRE Technical Brochure 349 (2008), Moisture equilibrium and moisture migration within transformer insulation systems.
- OMICRON, brochure “DIRANA: the fastest way of moisture determination of power and instrument transformers and condition assessment of rotating machines” (English).
Frequently asked questions
What is the OMICRON DIRANA used for?
To estimate the water content of the solid insulation (paper and pressboard) of power and instrument transformers, and the conductivity of the insulating liquid, without taking an oil sample. The measurement also delivers the power factor at power frequency, the capacitance and the insulation resistance.
Why isn't measuring the water in the oil enough?
Because most of the water in a transformer is in the cellulose, not in the oil, and how it is split between the two depends on temperature. An oil sample gives the water in the liquid at that moment; dielectric response measures the solid insulation directly. That is why they are often used together.
Can the DIRANA be used on equipment other than power transformers?
According to the manufacturer, it is used with power and instrument transformers and with bushings, and the dielectric measurement is also available for diagnosing rotating machines, resin-impregnated paper bushings, bushings insulated with resin-bonded paper or synthetic resin, and cables.