Choosing an oil test set (part 3)

In the first two articles in this series, we provided information about the principles of dielectric breakdown testing and practical advice on carrying out breakdown tests. In this third article, we start to explore the factors that need to be considered when selecting a test set.

It would be easy to assume that because all dielectric breakdown voltage test sets have to work to the same strict test standards, there is little to choose between them. This is far from true and a little effort put into selecting the best test set will be amply repaid in terms of convenience as well as time and cost savings over the instrument's life.

The first decision is whether the tests will be performed in a laboratory or on site, as there are differences between laboratory and portable instruments.

On-site versus laboratory testing

The relative merits of on-site and laboratory testing have been debated among engineers for many years. The underlying issue is that contamination greatly influences test results. Some argue that this means it is better to test on site because, if a sample has to be bottled and the bottle sent to a laboratory, there will always be doubt about whether the bottle was cleaned adequately before use and whether it was sealed sufficiently well to guard against contamination in transit.

Others point out that testing on site provides no guarantees against contamination, as the

most likely time for a sample to be contaminated is while it is being collected. They may also argue that, if a sample is sent to a laboratory, it will be tested by a skilled technician working in good conditions. Conversely, tests carried out on site are performed under less than ideal conditions and often under significant time pressure, which may lead to errors.

On-site testing does, however, undeniably have the benefit of immediacy. If a suspicious result is obtained, it is usually possible to repeat the test without delay and if the problem is confirmed, the affected transformer can, at least in principle, be taken out of service immediately.

There is no clear-cut answer to the question of whether on-site or laboratory testing is best; it is a matter for individual users to decide which approach best suits their circumstances. Megger is always happy to provide advice in making this decision and, as a company that manufactures both on-site and laboratory test sets, the impartiality of its advice can be guaranteed.

General instrument selection considerations

An instrument is a long-term investment, so it's important to choose one that's built to last and as future proof as possible. This section looks at considerations that apply to all oil test sets; later sections deal with the specifics of laboratory and portable instruments.

Cleanliness

An instrument is a long-term investment, so it's important to choose one that's built to last and as future proof as possible. This section looks at considerations that apply to all oil test sets; later sections deal with the specifics of laboratory and portable instruments.

Test vessel ease of cleaning

The vessel should have no corners, as these trap dirt and are difficult to clean.

Chemical resistance

The test vessel should be made of a material that is resistant to chemical attack, but glass is not the best choice because of its fragility. A properly specified moulded material will not affect test results.

Ease of pouring

Always look for a test vessel with a spout to aid pouring.

Testing to ASTM D1816

This standard requires the oil sample to be stirred with an impeller, and also specifies that the test vessel must have a cover or be fitted with a baffle to prevent air from coming into contact with the sample. In many instruments, oil is displaced by the lid, resulting in messy spills. Look for a test set that does not have this problem.

Test chamber ease of cleaning

Like the test vessel itself, the test chamber should have no corners to trap dirt and contaminants.

Dealing with spillage

Inevitably, oil is sometimes spilled into the instrument's test chamber. With many instruments, the only way of removing this spilt oil is by using cloth or tissues to mop it up. An instrument that has a test chamber with a means of quickly and easily draining spilled oil is much to be preferred.

Ease of use

An instrument that is difficult to use wastes time and reduces productivity, so it's important to look for an instrument that has a good user interface. Operation should be intuitive and it should not be necessary to refer repeatedly to the user guide. This is particularly important for instruments that are available for hire, and in cases where they will be used by engineers and technicians who do not perform oil tests regularly. Specific points to consider include:

User interface

The user should be able to easily identify and access the main functions of the instrument, such as test method selection, test sequence set up, stored data functions and user settings.

Display

The display should be clear and bright, and preferably in colour.

Test chamber access

Good access will aid instrument operation and cleaning. Deep and dark chambers make it difficult to see if the chamber is dirty. Most users prefer top access instruments.

Automatic testing

Most modern instruments support automatic testing. Low-cost manual instruments are still available, but careful thought is needed before purchasing one of these. They may be appropriate for some users, but a high level of skill is needed to use them successfully.

Ownership costs

Additional and replacement test vessels. IEC 60156 recommends that a separate test vessel is used for each type of fluid to be tested. Many laboratories follow this advice and, as a result, have six or more test vessels. It is important, therefore, to check the price of spare vessels. Some are very expensive and can greatly increase overall costs.

Broken test vessels

The previous comments about the price of test vessels are also relevant to broken test vessels, but also note that moulded test vessels are much less likely to be broken than glass vessels.

In the next part

In the final part of this article, we'll look at more factors that should be considered when purchasing an oil test including provision for test voltage verification and the ability to adapt to new standards that may be produced in the future. The article will conclude with a discussion of factors that affect only laboratory instruments, and those

Insulation resistance testers the next generation

The new generation of transmission, distribution and industrial Insulation Resistance Testers (IRTs) from Megger are smaller and lighter, easy to use, flexible and operate from battery or mains/line power. There are three models; a 5 kV MIT515 with IR, DAR and PI but no memory, a fully featured 5 kV IRT with memory and a top of the range 10 kV model MIT1025. All models are CATIV 600 V safety rated and housed in a rugged, IP65 case.

Test leads are accommodated in a lead pouch attached to the case lid.

The next generation

All models have a short circuit test current of 3 mA along with 3 mA noise filtering capability, ensuring operation in electrically noisy MV and HV switchyards. Tests can either be logged to produce a resistance curve or the final result saved in the instrument for on-screen recall. A real time clock time/date stamps readings in memory.

Saved test data can be transferred via a 10 kV isolated USB device interface to PowerDB Lite supplied with MIT525 and MIT1025 Industry standard timer settings are preset for the different test modes and are user settable. A lock voltage range serves as a user selected specific test voltage.

The new MIT515, MIT525 and MIT1025 are a perfect fit for transmission and distribution maintenance, commissioning and industrial/OEM use. Typical applications include insulation testing of transformers, switchyard equipment, cables and industrial/OEM testing and quality control.

Insulation testing is important to ensure the continued operation of transmission and distribution equipment and prevent outages. Resistance results should be trended over time to offer advanced warning of deterioration.

Grounding and the weather (part 2)

In the first part of this article, we looked in general terms at how the weather affects ground resistance testing, and in more

detail at the effects of temperature and the influence of ions in the soil. This time we look at temperature effects, freezing and the reasons why ground testing cannot be treated as a once-and-for-all exercise.

Temperature

The other major weather component is temperature. This doesn't have as profound an effect as does moisture, but still must be taken into account. Temperature effects can vary in opposite directions on different types of material, so it is wise not to generalize. In soil, decreasing temperature slows the movement of ions and decreases conductivity. Again, the battery analogy holds.

The truly critical change occurs over a single degree, when going from liquid to ice. If the ground becomes frozen, ice immobilizes the ion flow and resistivity takes a quantum leap. A typical study is once again illustrative. On sandy loam, a drop from 50° to 32 °F (10° to 0 °C) was found to increase resistivity from 9900 to 13,800 Ω-cm, but in going from liquid to freezing, the same sample increased resistivity to 30,000! For ground electrode installation, this means that the working structure must be below the frost line, whether that be permafrost or the anticipated worst winter case. Deep-driven rods can ultimately achieve the necessary contact with unfrozen soil, but in areas of shallow bedrock, frost can present a double whammy to the use of multiple shallow rods.

Freezing presents another potential problem, and one that is insidious because completely unseen. That is, freezing and thawing can exert a mechanical strain on the grounding electrode apart from the electrical stress applied by fault clearance. Grids can separate and sections become disconnected from the electrical system. The most extreme of weather conditions, lightning, can have a similar effect.

Of course, lightning does not have the long-term effect of moisture or temperature, but for the milliseconds that it lasts, it can disintegrate a grounding electrode below grade. The overall effectiveness of what remains can be readily determined by a routine ground test, but high-current grid testers are also available that can indicate damage to the point-to-point structure of the grid itself. This information can be valuable in repairing the grid in order to head off further disintegration.

Weather averaged out over representative periods produces climate. The one effect to be aware of here is the impact on the water table. This is more likely to be man-made, by activities like well drilling, but whether man-made or climatic, a change in water table will affect ground resistance. If water table drops, because of reduced rainfall or the siphoning off of water from wells, a grounding electrode that was installed in good conductive soil may later be sitting in a much drier environment.

What is the effect of all this on ground testing?

More than anything, it's important to be aware of the possibilities and not treat ground testing as once-and-done. The results of a ground resistance test – an installation test, for instance – will be heavily influenced by recent weather conditions. Carried out at the time of installation, a once-and-done test could leave the electrical system and associated equipment protected for part of the year only.

Around the calendar, the resistance on any given day can vary mightily, and a fault clearance event occurring on the high end of this cycle could result in loss of equipment. Put simply, a ground electrode is only as good as its worst day. In general, grounding conditions are best in spring and autumn, when weather conditions tend to be moist and reasonably warm.

In summer, drought can put the electrical system at risk, and in winter, freezing can present a similar danger. If an installation test is made at a good time and just meets spec, there is a high risk of it being considerably out of spec at another time of year. Specialized grounding equipment is available to mitigate this sawtooth effect by artificially creating a more stable environment around the electrode.

This can be accomplished by appropriate backfills, chemical rods, and similar treatments. But don't overlook the effects these treatments may have on concrete foundations, water table, environmental regulations and even the electrode itself. The additional maintenance that may possibly be required must also be taken into account.

The third and final part of this article will deal with the importance of know-how in ground testing, and explains why properly planned testing may involve working in adverse weather conditions, including snow. It also looks at criteria that affect the selection of test equipment for use in these conditions.