Typically, Dissolved Gas Analysis(DGA) to find any overheating/arcing probelms, and Screening tests, which include dielectric breakdown voltage, color, specific gravity, acid neutralization, water content, and interfacial tension.
Used on the North American continent, excerpted from NETA Maintenance Testing Specifications—2001 §7.2.2 Transformers, Liquid-Filled:
10. Remove a sample of insulating liquid in accordance with ASTM D 923. Sample shall be tested for the following.
[ul]1. Dielectric breakdown voltage: ASTM D 877 and/or ASTM D 1816
2. Acid neutralization number: ANSI/ASTM D 974
*3. Specific gravity: ANSI/ASTM D 1298
4. Interfacial tension: ANSI/ASTM D 971 or ANSI/ASTM D 2285
5. Color: ANSI/ASTM D 1500
6. Visual Condition: ASTM D 1524
*7. Water in insulating liquids: ASTM D 1533. (Required on 25 kV or higher voltages and on all silicone-filled units.)
*8. Measure power-factor or dissipation-factor in accordance with ASTM D 924.[/ul]
11. Remove a sample of insulating liquid in accordance with ASTM D 3613 and perform dissolved-gas analysis (DGA) in accordance with ANSI/IEEE C57.104 or ASTM D3612
I'm not sure that annual testing would be justified except on very important transformers or unless a problem is suspected. Every 5 years is more common in commercial applications.
Given the wealth of data available (very good insulation indications and trending over time of several factors) and the limited impact (low cost and no shutdown), annual oil testing is always recommended.
You will get better and more consistent results if you use a recognised oil test specialist. You sound quite new to oil testing, and the test houses have been doing this work for years, all day, every day. They will pick up subtle trends and indications that you or I, as non specialists, would probably misinterpret, or miss completely. The ability to carry out the test in exactly the same way each time allows accurate trending, but it is important to stay with the same test house and same equipment if at all possible. Two oil samples from the same transformer tested at the same time by two different organisations will normally return slight differences in results, and this can throw out any gradual long-term trend data. Trend data can pick out a developing fault while it is still relatively benign, and an experienced test engineer will probably increase the sample frequency as the fault develops. An experienced test engineer would almost certainly see the fault earlier than a relative layman, increasing the chances of preventing a repairable fault developing into an economic write-off.
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busbar is 100% correct. The ASTM standard describes how the lab is to perform the test (and sometimes how the user requests the test).
Although it's probably not what you are asking, there is one IEEE standard that calls out these same tests, describes the purpose, how much oil needed for each, and discusses limits. IEEE 62-95 (IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus Part 1: Oil Filled Power Transformers, Regulators, and Reactors), section 6.3.
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Also, some of these tests require very expensinsive laboratory type equipment such as a gas chromatagraph and mass spectrometer.
Taking samples correctly also requires some level of skill. Usually an electrical testing company can come to your site, take the samples and send the samples to a proper laboratory for a small fee.
electricpete, I overlooked IEEE Std 62. From ieeexplore.ieee.org:
Std 62-1995 …guide for diagnostic field testing of electric power apparatus - part 1: oil filled power transformers, regulators, and reactors
Abstract: In this IEEE Standard, diagnostic tests and measurements that are performed in the field on oil-immersed power transformers and regulators are described. Whenever possible, shunt reactors are treated in a similar manner to transformers. Tests are presented systematically in categories depending on the subsystem of the unit being examined. A diagnostic chart is included as an aid to identify the various subsystems. Additional information is provided regarding specialized test and measuring techniques
