The do's and don'ts of insulation power factor testing – Part 3

Jill Duplessis - Global Technical Marketing Manager and Editor

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This is the third in a short series of articles dealing with power factor (PF)/ dissipation factor (DF) tests, which are widely used to assess insulation condition in transformers and other electrical assets. The first two articles, which are still available online, looked at the theoretical background for these tests, safety and general insulation knowledge. This instalment looks at terminology associated with PF/ DF testing, and at general testing knowledge as it applies to these test techniques.

NOMENCLATURE

In any technical area, it’s easy to become confused by unfamiliar nomenclature and terminology. This is certainly the case for power factor/ dissipation factor tests, so we’ll start by looking at some of the most common sources of confusion using the  same Do’s and Don’ts format we adopted in the previous article.

Nomenclature Don’ts

DON’T get bogged down by DF versus PF

     Figure 1:

Dissipation factor versus power factor – what are the differences and do they matter? In reality, DF and PF are calculated differently (Figure 1); however, both are calculated from the measurements made in the same test and both describe the inefficiency – or efficiency – of an insulation system. And, for most asset insulation systems that engineers will need to test, the numerical values of DF and PF will be almost the same. That means deciding between DF and PF is largely a matter of personal preference.

It is relevant to note, however, that while PF and DF insulation test results may be largely interchangeable, the phrase ‘power factor’ has multiple meanings in our industry. In generator and load studies, for example, power factor is the ratio of the active power in a circuit to the total power in that circuit. For this reason, some power plant engineers – and others – feel that talking about power factor testing in relation to insulation condition is confusing. Dissipation, on the other hand, usually refers to using something wastefully. Since PF/ DF testing actually does report the amount of energy that is lost as heat, it can be argued that dissipation factor is more accurate and reflective of the measurement being made. In fact, the δ angle used to calculate dissipation factor (Figure 1) is also referred to as the loss angle. Nevertheless, there is once again room for personal preference.

DON’T get bogged down with dielectric versus insulation

The terms dielectric and insulation are often used interchangeably, which is reasonable because a dielectric is an electrical insulator. Bear in mind though that there are many other types of insulation – thermal insulation, acoustic insulation and more. So if you’re talking about insulation it’s important to make sure that it’s clear from the context exactly what sort of insulation you mean. Dielectric is a more specific term – its only meaning is a material that will support an electrostatic field without allowing conduction.

Nomenclature Do’s

DO be aware of slang

A dissipation factor test is often referred to as a “tan delta” test, while a power factor test is sometimes called a “doble” test. These terms have no special meaning. They’re just informal ways of referring to those specific tests.

DO keep test frequency in mind

The name ‘power factor test’ implies that the test is carried out at or near line frequency (50 or 60 Hz). PF/DF tests that are carried out over a range of frequencies show how PF/DF and capacitance change with frequency but such tests fall into the category of dielectric frequency response (DFR) testing, which is beyond the scope of this article.

GENERAL TESTING KNOWLEDGE

General Testing Knowledge Do’s 

DO familiarise yourself with history                                                                                                                   Figure 2:

A test’s history, particularly one as old as that of a PF/DF test, may provide valuable insights into the hows and whys of a test, including how common misconceptions about a test can be created and perpetuate. For PF/DF testing, there are publications from the early 1900s that deal with PF testing of cable insulation, but capacitance and PF testing really took off with the popularisation of capacitance graded bushings. Manufacturers of these bushings found that they couldn’t consistently identify localized problems in the bushings’ insulation systems using DC insulation resistance testing, so they turned to PF/ DF tests as a better alternative. Because of their construction (Figure 2), capacitance-graded bushings are the perfect specimens for these tests. As individually insulated conductive layers within the bushing short circuit (i.e., a localized problem), C1 capacitance increases, providing a warning of a potentially imminent failure. Meanwhile PF/DF testing is an averaging measurement so deteriorating insulation of one layer, even if surrounded by layers in excellent health, will influence the PF/DF test result. The increase of PF/DF will alert of the problem.

DO remember that PF/ DF testing was first popularised for bushings

This is important because as tests started to be used more widely, exaggerated claims for the benefits of PF/DF testing followed. ‘Earlier detection of problems in bushings’ (meaning earlier than was possible with DC testing) became, for example, ‘early detection of problems in insulation’ which in many cases is grossly misleading. And ‘very sensitive to moisture contamination in the tap compartment of the bushing’ became ‘very sensitive to moisture contamination’ but, in many cases, this is only true if the PF tests are repeated at many  different frequencies or, in other words, if a DFR test is carried out.

DO simplify testing by understanding dielectric representations and test modes

   Figure 3:

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An understanding of dielectric representations and test modes will make PF/DF and capacitance testing much easier. It’s always beneficial to test the smallest possible section of insulation – smaller is always better, but how can you segment an asset’s insulation system so that you can test one segment at a time? The dielectric representation (e.g., as given in Figure 3) shows how this can be done, and the test modes provide a tool for segmentation. Unfortunately, it often happens that when you’re doing a test, you have to energize more than one segment (sometimes the whole insulation system) but you’ll still need to measure just one segment at a time, and test modes make this possible. As an aside, this means that when you’re considering the capability of an instrument to charge a test specimen, it’s essential to keep in mind that (b)that you may be charging much more than the segment you’re actually measuring.

DO pay attention to the dielectric representation

As we’ve already said, smaller insulation segments are better but the test engineer may not have total control over how an insulation system can be segmented for testing. For example, with capacitance-graded bushings, it’s impossible to stick a test lead into the bushing and test every layer individually. The dielectric representation is a diagram that identifies each component or group of components that will take up a unique voltage when the asset is energised. It is left to the engineer carrying out the test to determine whether or not a component is accessible for a test lead to be attached. Note that the number of unique voltages/component groups present in an asset determines the number of insulation systems needed.

DO take note of the way insulation between components is depicted

In a dielectric representation, the insulation between components is depicted as a single capacitor (Figure 3). This is adequate, because a dielectric representation is simply an aid to testing. In contrast, a dielectric model is used to predict the electrical behaviour of an insulation system, and a single capacitor representation would not be adequate for this.

When you’re deciding how to test a transformer, a helpful rule of thumb is to count the number of separable accessible windings, as demonstrated by these two examples. The first is an autotransformer with an inaccessible tertiary winding. This transformer has three windings – series, common and tertiary – but how many of these are separable accessible windings? The tertiary isn’t because its connections are not brought out of the tank. The series and common windings are accessible, but they’re not separable, so they count as one winding. There is therefore only a single separable accessible winding and the transformer must be tested as a single-winding transformer. Now let’s consider a four-winding Δ-Y-Y-Y transformer that has all winding terminals accessible except for the three secondary neutral terminals which are connected together and grounded within the transformer. In this case, the number of separable windings is two: the delta winding and the three wye-connected secondary windings, which have to be treated as one winding since they can’t be separated. As for accessibility, while the delta winding is fully accessible, the neutral terminals of the secondary group (Y-Y-Y) are not, making it impractical to disconnect them from ground. This means that although this is a four-winding transformer, it will need to be tested using the two-winding transformer test procedure, bearing in mind that the test results may not be entirely meaningful since the secondary winding cannot be lifted from ground.

DO make best use of test modes

The feature of the PF/DF test set that enables it to support test modes is the guard circuit. This is arranged so that all current resulting from the application of the test voltage will seek to return to the guard point. The current return paths to the instrument are provided by up to three test leads – the ground lead and two low-voltage leads (red, R, and blue, B) – that are connected between the instrument and the asset under test. Three test modes are available: GST-Ground, GST-Guard and UST (Figure 4).

     Figure 4:

The GST modes are grounded specimen tests. The “-Ground” or “-Guard” designation indicates where the test instrument will internally connect the LV leads, the use of which is optional. The UST mode is an ungrounded specimen test; current flow in the LV leads is measured. The ground lead is always used and serves to compare the specimen ground to the voltage supply ground. It is also a current-carrying lead used for measuring or guarding.

DO be prepared

Dealing with external variables is one of the biggest challenges when performing PF/ DF tests. These variables include temperature, humidity, surface leakage and the quality of test preparations.  All of these will be dealt with in more detail in  later sections.

General Testing Knowledge Don’ts

DON’T forget that success depends on testing the smallest possible amount of insulation

PF/DF testing provides a result that tells you about the average condition of the insulation system under test. If the system you’re testing is large, a small, localised problem area may not significantly influence the average and will therefore not be detected. This means you should always look at the size of the specimen you are testing, as this will help shape your expectations for the diagnostic capabilities of the test. A real-life example relates to a large single-phase autotransformer with an inaccessible tertiary winding. This had severe tracking on the collar around the lower insulator of one of the bushings. Only one PF/DF test and capacitance measurement could be performed on this transformer. This was done with all winding terminals connected together and energised for a single winding-to-ground insulation measurement. Because of the large size of the test specimen and insulation under test, the localised contamination made no measurable difference to the test results compared with historical measurements.

DON’T rely too much on scalability

As mentioned at the beginning of this series of articles, we evaluate PF/ DF test results rather than relying solely on dielectric loss results because PF/ DF results allow comparisons to be made between insulation systems of different sizes. It is, however, important not to take this too far. It’s certainly permissible to make comparisons of this type, but bear in mind that the test on a larger system will not be as searching as the test on a smaller system. In other words, the results are not completely scalable. Even if both systems give identical PF/DF results, this doesn’t mean it’s safe to assume that the insulation in both is in the same condition. As the earlier example showed, a serious problem can be concealed behind a good PF/DF value if the problem area is small and the system under test is large.

DON’T have unrealistic expectations for the test                                                                            Figure 5:

Increasing levels of some contaminants, particularly moisture, don’t result in a noticeable change in PF/DF until the level of contamination reaches a certain threshold. Figure 5 illustrates this.

On the y-axis, dissipation factor at 50 Hz is plotted on a logarithmic scale. On the x-axis, percentage moisture in cellulose is plotted on a linear scale. The diagram shows DF curves for a new transformer (blue) and for a service-aged transformer (red) as they become increasingly contaminated with moisture. At low moisture levels, the slopes of both curves are relatively flat, which means that, in this region, DF does not increase noticeably as moisture increases. Someone relying on DF measurements would therefore be unaware that moisture contamination is present and is increasing. It is not until the moisture content reaches 2.5% that the slopes increase marginally and not until 3.5% that they increase noticeably. From this point on, it can be said that DF testing has increased sensitivity to moisture. Notice, however, that this increase in sensitivity doesn’t occur until at least 3% moisture content, whereas most operators of power transformers start to become concerned when water content reaches 1.7% and by 2.5% they would typically be scheduling a visit by a processing unit to dry out the transformer. It’s also worth noting the divergence between the curves for the new and service-aged transformers. This shows that the same dissipation factor may represent a range of moisture contamination conditions, depending on the age of the insulation system. For example, a dissipation factor of 0.3% corresponds to a moisture content of 0.5% in the service-aged transformer, but to a moisture content of 2% for the new transformer. This presents a challenge as it’s not easy to quantify ageing in a transformer – years of service is not a reliable indicator of ageing for an insulation system as environmental and system variables, maintenance and many other factors play a role in how insulation ages.

The fourth article in this series, which will appear in the next issue of Electrical Tester, will deal with test preparation, the choice of test voltage and analysis of the test results.

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